TWI853016B - Metrology apparatus and method using mechanical filter - Google Patents

Abstract

A metrology apparatus includes: a diagnostic apparatus configured to interact a diagnostic probe with a current target at a diagnostic region before the current target enters a target space; a detection apparatus; and a control system in communication with the detection apparatus. The detection apparatus includes: a light sensor having a field of view overlapping with the diagnostic region and configured to sense light produced from the interaction between the diagnostic probe and the current target at the diagnostic region; and a mechanical filter between the diagnostic region and the light sensor. The mechanical filter includes an optical beam reducer and an optical mask defining an aperture positioned between the optical beam reducer and the light sensor. The control system is configured to estimate a property of the current target based on the output from the light sensor.

Description

Metrology device and method using mechanical filter

The disclosed subject matter relates to metrology apparatus and methods for using mechanical filters to distinguish between two types of light in an extreme ultraviolet light source.

In semiconductor lithography (or photolithography), the manufacture of integrated circuits (ICs) involves performing a variety of physical and chemical processes on a semiconductor (e.g., silicon) substrate, also called a wafer. A photolithography exposure apparatus or scanner is a machine that applies the desired pattern to a target portion of the substrate. The wafer is irradiated with a beam extending in an axial direction, and the wafer is secured to a stage so that the wafer extends generally along a transverse plane that is substantially orthogonal to the axial direction. The light beam may have a wavelength in the ultraviolet (UV) range, for example, from about 10 nanometers (nm) to about 400 nm, and specifically in the deep UV (DUV) range from about 100 nm to about 400 nm or in the extreme ultraviolet (EUV) range of less than about 100 nm (or about 50 nm or less, and including 13 nm). The light beam travels in an axial direction (which is orthogonal to the lateral plane along which the wafer extends).

Methods for generating EUV light include, but are not necessarily limited to, utilizing emission lines in the EUV range to convert a material having an element (e.g., xenon, lithium, or tin) into a plasma state. In one such method, often referred to as laser produced plasma ("LPP"), the desired plasma may be generated by irradiating a target material (e.g., in the form of a droplet, slab, ribbon, stream, or cluster of material) with an amplified beam of light, which may be referred to as a drive laser. For this process, the plasma is typically generated in a sealed container, such as a vacuum chamber, and various types of metrology equipment are used to monitor the plasma.

In some general aspects, a metrology device includes: a diagnostic device configured to cause a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; a detection device; and a control system in communication with the detection device. The detection device includes: a light sensor having a field of view overlapping the diagnostic region and configured to sense light generated by an interaction between the diagnostic probe and the current target at the diagnostic region; and a mechanical filter between the diagnostic region and the light sensor. The mechanical filter includes a beam reducer and an optical mask defining an aperture positioned between the beam reducer and the light sensor. The control system is configured to estimate the properties of the current target based on the output from the light sensor.

Implementations may include one or more of the following features. For example, a mechanical filter may be configured to angularly separate diagnostic light emitted from a diagnostic region from non-diagnostic light emitted from a target space. Diagnostic light may be generated by the interaction between a current target at the diagnostic region and a diagnostic probe. Non-diagnostic light may include light emitted from plasma generated by a previous target in the target space. The lateral extent of the aperture may be approximately the same as or greater than the lateral extent of the diagnostic light in the plane of the optical mask, and the lateral extent of the optical mask may be greater than or approximately the same as the lateral extent of the non-diagnostic light in the plane of the optical mask. The optical mask may be positioned so that the non-diagnostic light emitted from the target space is substantially blocked by the optical mask, while the diagnostic light substantially passes through the aperture.

The mechanical filter may include an optical collimator between the diagnostic region and the beam reducer. The beam reducer may be an afocal beam reducer and may be configured to be combined with the optical collimator to project a finite object to infinity. The assembly of the optical collimator and the beam reducer with a positive focal length proximal to the optical collimator may be integrated into a single refractive element. The beam reducer may be configured to maintain the collimated state of the light.

The photo sensor may include one or more of the following: a photodiode, a phototransistor, a photodependent resistor, a photomultiplier tube, a multi-cell photoreceiver, a quad-cell photoreceiver, and a camera.

The diagnostic probe may include at least one diagnostic beam, and the light sensor is configured to sense the diagnostic light generated by the interaction between the current target and the at least one diagnostic beam. The diagnostic light may include the diagnostic beam reflected from the current target, scattered from the current target, or blocked by the current target.

The detection device may include one or more of a spectral filter and a polarization filter.

The diagnostic probe may include a first diagnostic beam and a second diagnostic beam, each of which is configured to interact with a current target before it enters the target space, each interaction occurring at a different region and at a different time.

The beam reducer may include a refractive telescope, a reflective telescope, or a reflective-refractive telescope. The refractive telescope may include: a positive focal length lens configuration and a negative focal length lens configuration, which are separated by the sum of their focal lengths; or a pair of positive focal length lens configurations, which are separated by the sum of their focal lengths.

The beam reducer can be configured to reduce the lateral size of the impinging light by at least five times, at least ten times, at least twenty times, or approximately ten times.

The opening may include a circular opening, an elliptical opening, a polygonal opening, or a slit opening.

The detection device may be positioned outside a chamber of an extreme ultraviolet (EUV) light source, the diagnostic region may be inside the chamber, and the detection device may receive light from the chamber through an optical window in a wall of the chamber. The distance between the diagnostic region and the optical window may be about 200 to 500 times the size of the distance between the diagnostic region and the target space.

The detection device may include a focusing lens at the output end of the aperture, the focusing lens being configured to focus the sensed light onto the light sensor.

The aperture may have a range of at least 2 millimeters (mm). The aperture may be positioned to receive diagnostic light at an orientation where the diagnostic light is collimated or non-convergent and non-divergent.

In another general aspect, the metrology method includes: causing a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; collecting diagnostic light generated by the interaction between the diagnostic probe and the current target at the diagnostic region, the collection also including collecting non-diagnostic light generated by the target space; causing the diagnostic light and non-diagnostic light to interact with each other at a diagnostic region; Collimating the diagnostic light; angularly separating the diagnostic light and the non-diagnostic light from each other including reducing the lateral range of the diagnostic light and the non-diagnostic light; sensing the diagnostic light at a sensing area laterally displaced from the non-sensing area through which the free non-diagnostic light passes after the diagnostic light and the non-diagnostic light have been angularly separated; and estimating the properties of the current target based on the sensed diagnostic light.

Implementations may include one or more of the following features. For example, the diagnostic probe may interact with a current target at a diagnostic region by causing one or more diagnostic beams to interact with the current target at the diagnostic region; and the diagnostic light may be collected by collecting one or more diagnostic beams at the diagnostic region that have been reflected from, scattered from, or blocked by the current target.

The metrology method may further include filtering the diagnostic light based on one or more of its spectral properties and its polarization state.

The diagnostic area may be inside a hermetically sealed chamber of an extreme ultraviolet (EUV) light source and also include collecting non-diagnostic light. The collecting diagnostic light may include receiving diagnostic light including non-diagnostic light transmitted through an optical window in a wall of the chamber.

The lateral range of diagnostic and non-diagnostic light can be reduced by reducing the lateral range of diagnostic and non-diagnostic light by five times, at least ten times, at least twenty times, or approximately ten times.

The metrology method may also include blocking non-diagnostic light at the non-sensing area or redirecting the non-diagnostic light. The lateral range of the diagnostic light and the non-diagnostic light may be reduced by one or more of the following: refracting the light and reflecting the light.

Metrology methods may also include focusing the diagnostic light at the sensing area.

The lateral range of diagnostic and non-diagnostic light can be reduced by maintaining the collimation of diagnostic and non-diagnostic light.

The metrology method may also include passing the diagnostic light through an aperture of an optical mask having a range greater than the range of the diagnostic light after the diagnostic light and the non-diagnostic light are angularly separated from each other and before the diagnostic light is sensed.

10: Shell

20: Part

30: Micro-image control system

100:Weight and measurement equipment

105: Target

105': Current goal

105c: Current goal

105p: Previous target

106: Continuous Flow

110: Target space

115: Environment

120: Diagnostic light

121: Collimated beam

122: Non-diagnostic radiography

123: Collimated beam

125: Diagnostic probe

130: Light sensor

135: Detection device

140: Mechanical filter

142:Optical collimator

145: Diagnosis area

150: Beam reducer

152:Optical path

155:Optical mask

160: Orifice

165: Diagnostic device

170: Control system

175: Target supply device

255A: Mask

255B: Mask

256A: Range

256B: Range

260A: Orifice

260B: Orifice

261A: Range

261B: Range

330:Sensor

335: Detection device

342:Optical collimator

342a: Lens

342b: Lens

350: Beam reducer

351: Positive focal length lens configuration

351a: Convex lens

351b: Concave lens

352:Optical path

353: Negative focal length lens configuration

354: Auxiliary lens

355: Mask

357: Converging lens

360: Orifice

430:Sensor

435: Detection device

442:Optical collimator

442a: Lens

442b: Lens

450: Beam reducer

451: Input positive focal length lens configuration

451a: Convex lens

451b: Concave lens

452:Optical path

453: Output positive focal length lens configuration

455: Mask

457: Converging lens

460: Orifice

535: Detection device

543: Spectral filter

544: Polarization filter

611A: Beam

611B: Beam

611C_1: Beam

611C_2: Beam

620A: Diagnostic light

620B_1: Diagnostic light

620B_2: Light

625A: Probe beam

625B_1: Probe beam

625B_2: Probe beam

625C_1: Probe beam

625C_2: Probe beam

626A: Light source

626B: Light source

626C_1: Light source

626C_2: Light source

627A:Optical components

627B:Optical components

627C_1: Optical components

627C_2: Optical components

665A: Diagnostic equipment

665B: Diagnostic equipment

665C: Diagnostic equipment

700:Weight and measurement equipment

706: Continuous Flow

710: Target space

715: Vacuum environment

716: Chamber

725: Diagnostic probe

735: Detection device

736:Optical window

745: Diagnostic area

765: Diagnostic device

775: Target supply device

776:EUV light source

777:EUV light

778: Radiation Pulse

779: Output device

780:EUV light collector

781: Optical source

782: Actuation system

783: Control device

784:EUV light

785: Plasma

870:Control system

871:Signal processing module

872: Diagnosis control module

873:Memory

874i: Input device

874o: Output device

883: Control device

884: Optical source actuation module

885: Target delivery module

979: Lithography device

1090: Process

1091,1092,1093,1094,1095,1096: Steps

1142: Refractive element

1151: Refractive element

1200:Weight and measurement equipment

1220: Diagnostic light

1222: Non-diagnostic radiography

1225: Diagnostic probe beam

1230: Two-dimensional sensor

1265: Diagnostic device

1350:Reflected beam reducer

1351: Concave reflective element

1352:Optical path

1353: Convex reflective element

1450: Reflective-refractive beam reducer

1451a: Flat reflective element

1451b: Curved reflective element

1453: Convex/convergent refractive element

1555:Mask

1560: Orifice

B: Exposure beam

d: distance

f1: focal length

f2: focal length

IF:Intermediate focus

M:Mask

R1: Reflective optical element

R2: Reflective optical element

R3: Reflective optical element

S: narrow seam

t: time

t1: time

t2: time

TR: Track

V: rate

W: Substrate

x: position

x1: Direction

x2: Direction

△d: distance/separation

FIG. 1 is a schematic diagram of a metrology device including a detection device having a mechanical filter for collecting diagnostic light and non-diagnostic light generated in an environment, wherein the diagnostic light is generated by a target in a diagnostic region and the non-diagnostic light is generated by a target in a target space different from the diagnostic region; FIG. 2A is a schematic diagram of an embodiment of a mask that can be used in the mechanical filter of FIG. 1; FIG. 2B is a schematic diagram of an embodiment of a mask that can be used in the mechanical filter of FIG. 1; FIG. 3 is a schematic diagram of an embodiment of the detection device of FIG. 1 including a beam reducer designed as a refractive Galilean telescope; FIG. 4 is a schematic diagram of an embodiment of the detection device of FIG. 1 including a beam reducer designed as a refractive Galilean telescope; FIG. 5 is a schematic diagram of an embodiment of the metrology device of FIG. 1 , wherein the detector device includes one or more spectral filters and polarization filters; FIG. 6A is a schematic diagram and block diagram of an embodiment of a diagnostic device that generates a single diagnostic beam; FIG. 6B is a schematic diagram and block diagram of an embodiment of a diagnostic device that generates two diagnostic beams from a single light source; FIG. 6C is a schematic diagram and block diagram of an embodiment of a diagnostic device that generates two diagnostic beams from corresponding light sources; FIG. 7 is a schematic diagram and block diagram of an embodiment of a diagnostic device that generates two diagnostic beams from corresponding light sources; and FIG. 8 is a schematic diagram of an embodiment of a diagnostic device that generates two diagnostic beams from corresponding light sources. FIG. 8 is a block diagram of an embodiment of a control device for the metrology device of FIG. 1 ; FIG. 9 is a schematic diagram of an embodiment of a lithography device that receives EUV light output from the EUV light source of FIG. 7 ; FIG. 10 is a flow chart of a process performed by the metrology device of FIG. 1 to estimate one or more properties of a current target; FIG. 11 is a schematic diagram of an embodiment of a detection device of FIG. 1 including a beam reducer designed as a refractive Galilean telescope and in which the positive focal length lens of the optical collimator and the beam reducer are integrated into a single refractive element; FIG. is a schematic illustration of an embodiment of the metrology device of FIG. 1 , wherein the diagnostic probe provides back illumination to the target such that a shadow of the target is formed by the target obscuring at least a portion of the diagnostic probe; FIG. 13 is a schematic illustration of an embodiment of the detection device of FIG. 1 including a beam reducer designed as a reflective off-axis telescope; FIG. 14 is a schematic illustration of an embodiment of the detection device of FIG. 1 including a beam reducer designed as a catadioptric off-axis telescope; and FIG. 15 is a schematic illustration of an embodiment of the metrology device of FIG. 1 , wherein the mechanical filter includes a mask that blocks diagnostic light while allowing non-diagnostic light to pass to the sensor.

Applicants should note that the drawings may not be to scale. For example, the distances between optical elements in the schematic illustrations may be greater or less than shown.

1 , a metrology device 100 is configured to estimate one or more properties of a current target 105c traveling along a trajectory TR toward a target space 110 within an environment 115. The metrology device 100 is configured to estimate at least one property (e.g., speed, position, velocity, direction) of the current target 105c by analyzing light (referred to as diagnostic light 120) generated by the interaction between one or more diagnostic probes 125 and the current target 105c before the current target 105c enters the target space 110. However, non-diagnostic light 122 is generated simultaneously and similarly to the diagnostic light 120. This non-diagnostic light 122 may saturate the photodetector 130 within the metrology device 100 or may interfere with the operation of the photodetector 130 within the metrology device 100. As such, the non-diagnostic light 122 may reduce the accuracy of the analysis performed by the metrology device 100 and thereby cause errors in the estimated properties of the current target 105c.

The metrology device 100 is able to more efficiently and accurately estimate the properties of the current target 105c because it is able to more effectively distinguish between diagnostic light 120 and non-diagnostic light 122. To this end, the metrology device 100 includes a detection device 135, which includes a photo sensor 130 and a mechanical filter 140 between a diagnostic region 145 (where the current target 105c interacts with the diagnostic probe 125) and the photo sensor 130. The mechanical filter 140 includes a beam reducer 150 and a light-impermeable optical mask 155 that defines a light-transmitting aperture 160. The aperture 160 is positioned between the beam reducer 150 and the photo sensor 130. In order to properly sense the diagnostic light 120, the light sensor 130 is positioned so that its field of view overlaps the diagnostic region 145. The metrology device 100 also includes a diagnostic device 165 that generates the diagnostic probe 125 and a control system 170 that communicates with the detection device 135. The control system 170 receives output from the sensor 130 and performs analysis on the output to estimate one or more properties of the current target 105c.

The mechanical filter 140 includes an optical collimator 142 that forms respective collimated beams from the diagnostic light 120 and the non-diagnostic light 122, and then the beam reducer 150 optically reduces the size of these collimated beams to form reduced-size collimated beams 121, 123, respectively. The beam reducer 150 increases the angular separation between the images produced by the collimated diagnostic beam 121 and the collimated non-diagnostic beam 123. The non-diagnostic light 122 originates from a location outside the location of the current target 105c (which produces the diagnostic light 120). For example, the diagnostic light 120 originates from the diagnostic region 145, while the non-diagnostic light 122 originates from outside the diagnostic region 145, such as from the target space 110. As a result, the non-diagnostic light 122 enters the mechanical filter 140 at a slightly different angle than the diagnostic light 120 enters the mechanical filter 140. This fact can be used in the design of the mechanical filter 140, which can further separate the images generated by the diagnostic light 120 and the non-diagnostic light 122 by increasing the angular separation between the corresponding collimated light beams 121, 123. The increase in angular separation allows for greater differentiation between the two images at aperture 160 after the beam has traveled the length of optical path 152 between beam reducer 150 and mask 155. Thus, in this embodiment, mask 155 is positioned so that aperture 160 allows diagnostic beam 121 (formed by diagnostic light 120) to pass to sensor 130, while mask 155 blocks non-diagnostic beam 123 (formed by non-diagnostic light 122) from passing to sensor 130.

The optical collimator 142 forms a corresponding collimated beam for input to the beam reducer 150. A collimated beam is a beam having a beam divergence low enough that the beam radius does not undergo a substantial change over a moderate propagation distance. In this case, in the absence of any additional beam shaping (and therefore, in the absence of the beam reducer 150), the beam radius of each collimated beam output from the optical collimator 142 will not undergo a substantial change over the distance extending to the sensor 130. For example, the beam radius of the collimated beam output from the optical collimator 142 changes by less than 1%, less than 5%, or less than 10% over the distance along the Z direction to the sensor 130 (in the absence of any intermediate optical elements). In some examples, the distance between the optical collimator 142 and the sensor 130 along the Z direction is about one meter, but it can be shorter or longer depending on the design of the beam reducer 150.

The beam reducer 150 is an afocal system (i.e., a system without a focus), meaning that the beam reducer 150 does not produce a net convergence or divergence of a collimated beam input to the beam reducer 150. That is, the beam reducer 150 can be considered to have an infinite effective focal length that preserves or maintains the collimated state of the beam output from the optical collimator 142. This type of system can be formed to have a pair of optical elements where the distance d between the elements is equal to the sum of the focal lengths f1, f2 of each element (i.e., d=f1+f2). Although an afocal system does not change the divergence of a collimated beam, it does change the width of the beam, thereby increasing or decreasing its magnification. In general, the beam reducer 150 can reduce the lateral size (i.e., the size in the XsYs plane) of the collimated beam output from the optical collimator 142 by at least five times, at least ten times, or at least twenty times. In some embodiments, the beam reducer 150 reduces the lateral size of the collimated beam by about 10 or about 20 times.

The beam reducer 150 may be, for example, a refractive telescope, a reflective telescope, or a reflective-refractive telescope.

A refractive telescope uses refractive optics, such as lenses or prisms, to form an image of a corresponding collimated beam at the plane of mask 155. In some embodiments, the refractive telescope is a Galilean telescope including a positive focal length lens configuration and a negative focal length lens configuration separated by the sum of their focal lengths. In other embodiments, the refractive telescope is a Keplerian telescope including a positive focal length lens configuration separated by the sum of their focal lengths. An example of a refractive telescope used as a beam reducer 150 is discussed below with reference to FIGS. 3 and 4.

The reflecting telescope includes a single piece or a combination of curved mirror surfaces that reflect the collimated light beam output from the optical collimator 142 and form a corresponding image at the plane of the mask 155. For example, in some embodiments, the reflecting telescope is a Gregorian telescope, a Newtonian telescope, or a Cassegrain (and its variations) telescope. In other embodiments, the reflecting telescope is an off-axis design, such as a Herschelian or Schiefspiegler (which is a variation of Cassegrain). An example of a reflecting telescope is shown and described with respect to FIG. 13.

A catadioptric telescope is a type of telescope in which refraction and reflection are incorporated into an optical system that typically implements lenses (i.e., refractive optics) and curved mirrors (i.e., catadioptric optics). For example, in some embodiments, catadioptric telescopes include Schmidt-Cassegrain telescopes and Maksutov-Cassegrain telescopes. In other embodiments, a catadioptric telescope is a catadioptric variation of a Herschelian telescope (which uses both lenses and mirrors) or an off-axis variation of a Stevick-Paul telescope. An example of a catadioptric telescope is shown and described with respect to FIG. 14.

The environment 115 may be a vacuum environment in a chamber of an extreme ultraviolet (EUV) light source (such as the EUV light source discussed below with reference to FIG. 7 ). In some embodiments, the detection device 135 is placed outside the chamber of the EUV light source, while the diagnostic region 145 is in the chamber, and the detection device 135 receives the diagnostic light 120 and the non-diagnostic light 122 through an optical window in the wall of the chamber. The optical window is transparent to the wavelength of the diagnostic light 120. This situation is discussed below with reference to FIG. 7 .

As discussed with reference to FIG. 7 , the EUV light source supplies EUV light to an output device, which may be a lithography device. EUV light is formed in an environment 115 by converting the target 105 into a plasma that emits EUV light when the target 105 reaches the target space 110, and this EUV light is collected and transmitted to the lithography device. The target 105 that reaches the target space 110 is converted by the target 105 interacting with a radiation pulse in the target space 110, which provides sufficient energy to the target 105 to convert it into plasma. A continuous stream 106 of targets (each typically designated as 105) is guided from the target supply device 175 along a trajectory TR toward the target space 110.

The trajectory TR extends along a direction that can be considered as a target (or axial) direction, which is located in a three-dimensional X, Y, Z coordinate system defined by the physical state of the environment 115. Therefore, the X, Y, Z coordinate system can be defined by the walls or spots of the chamber that define the environment 115. The axial direction of each target 105 generally has a component parallel to the -X direction of the coordinate system of the environment 115. The axial direction of each target 105 can also have a component along one or more of the directions Y and Z that are perpendicular to the -X direction. In addition, each target 105 released by the target supply device 175 can have a slightly different actual trajectory and the trajectory depends at least in part on the physical properties of the target supply device 175 and the environment 115 when the target 105 is released.

On the other hand, the detection device 135 defines a local three-dimensional Xs, Ys, Zs coordinate system and this local coordinate system can be defined by the image plane of the sensor 130.

Each target 105 includes a component that emits EUV light when converted to plasma. The targets 105 travel from a generation region (e.g., from a target supply device 175) toward a target space 110 (e.g., ballistically). Properties of a current target 105c (e.g., speed, position, velocity, direction, arrival, or motion) are estimated by the metrology device 100 by probing the current target 105c with a diagnostic probe 125 generated by a diagnostic system 165 as the current target 105c travels along a trajectory TR, detecting or sensing diagnostic light 120 generated by an interaction between the diagnostic probe 125 and the current target 105c, and analyzing the detected diagnostic light 120.

As mentioned above, non-diagnostic light 122 may also be present, and this non-diagnostic light 122 may interfere with the accurate estimation of the properties of the current target 105c. This non-diagnostic light 122 may include broadband light radiation emitted by plasma generated by one or more previous targets 105p entering the target space 110 before or when the current target 105c interacts with the diagnostic probe 125. In addition, the intensity of the non-diagnostic light 122 may be much greater than the intensity of the diagnostic light 120.

Non-diagnostic light 122 may include, for example, EUV light emitted from plasma of previous target 105p, light having a wavelength range that overlaps with the wavelength of diagnostic light 120, and/or any light that exists and has a wavelength range that includes a wavelength range detectable by sensor 130.

On the other hand, the diagnostic light 120 is generated by the interaction between the diagnostic probe 125 and the current target 105c and has a spectral bandwidth that is substantially narrower than the spectral bandwidth of the non-diagnostic light 122. For example, the spectral bandwidth of the diagnostic light 120 may be hundreds of times lower than the overall spectral bandwidth of the non-diagnostic light 122. In some embodiments, as shown in FIGS. 6A to 6C, the diagnostic light 120 is generated by a portion of the diagnostic probe 125 that is reflected from or scattered from the current target 105c.

Generally, the sensor 130 may include one or more of a photodiode, a phototransistor, a photodependent resistor, and a photomultiplier tube. In other embodiments, the sensor 130 includes one or more thermal detectors, such as a pyroelectric detector, a radiothermometer, or a calibrated charge coupled device (CCD) or CMOS. In other embodiments, the sensor 130 includes a multi-unit photoreceiver, a four-unit photoreceiver, or a camera.

As shown in the embodiment of FIG. 1 , the optical mask 155 is positioned so that the collimated non-diagnostic beam 123 (generated from the non-diagnostic light 122) is substantially or mostly blocked, while the collimated diagnostic beam 121 substantially passes through the aperture 160. Another embodiment is shown and discussed with respect to FIG. 15 , in which the collimated non-diagnostic beam 123 substantially passes through the aperture, while the collimated diagnostic beam 121 is substantially blocked by the aperture.

Referring to FIG. 2A , in some embodiments, the mask 155 is a mask 255A defining an aperture 260A having a circular shape in the XsYs plane. In other embodiments, as shown in FIG. 2B , the mask 155 is a mask 255B defining an aperture 260B having a slit shape that is not rotationally symmetric and has a range in the Ys direction that is greater than the range in the Xs direction. The design of FIG. 2B may be useful for situations where the collimated diagnostic beam 121 is moving, oscillating, or perturbed so that its image plane moves in the Ys direction. The range in the Ys direction accommodates this fluctuation in the image plane of the collimated diagnostic beam 121. The mask 155 may be configured to define other shapes of the aperture 160 in the image plane (XsYs plane), such as an elliptical aperture and a polygonal opening.

To properly allow the collimated diagnostic beam 121 to pass to the sensor 130, the aperture 160 along the XsYs plane is at least as large as the lateral extent of the collimated diagnostic beam 121 in the XsYs plane. Additionally, to properly block the collimated non-diagnostic beam 123, the optical mask 155 should have an extent along the XsYs plane that is at least as large as the lateral extent of the collimated non-diagnostic beam 123 in the XsYs plane. This means that, referring to FIG. 2A , the range 261A of the aperture 260A in the XsYs plane is as large as the lateral range of the collimated diagnostic beam 121 in the XsYs plane and the range 256A of the mask 255A is large enough to block the entire range of the collimated non-diagnostic beam 123 in the plane XsYs. As another example, referring to FIG. 2B , the shortest range 261B of the aperture 260B in the XsYs plane is as large as the lateral range of the collimated diagnostic beam 121 in the XsYs plane and the range 256B of the mask 255B is large enough to block the entire range of the collimated non-diagnostic beam 123 in the plane XsYs.

In some embodiments, the range 261A, 261B of the corresponding aperture 260A, 260B in the XsYs plane is at least 2 millimeters (mm) or about 4 mm. The range of the collimated diagnostic beam 121 in the XsYs plane at the aperture 260A or 260B is about 3 mm. In some embodiments, the range 256A, 256B of the corresponding mask 255A, 255B is greater than 3 mm.

Unlike a spatial filter, where light is focused at the aperture of a mask, in a mechanical filter 140, light (diagnostic beam 121) passing through aperture 160 (such as aperture 260A or 260B) is collimated and therefore has a greater lateral extent than light focused at the aperture of a spatial filter. Therefore, the size of aperture 160 (such as aperture 260A or 260B) in the XsYs plane can be much larger than the aperture used in a spatial filter to block focused non-diagnostic light and pass focused diagnostic light. Because of this, unwanted particles (such as dirt in the environment 115) have much less effect on the performance of the aperture 160 (such as aperture 260A or 260B) than such particles have on the aperture of a spatial filter. For example, the size of the aperture 160 in the XsYs plane is on the order of a few millimeters, which is much larger than the size of the unwanted particles. On the other hand, the typical size of the aperture of a spatial filter can be a fraction of a millimeter (e.g., in the range of 100 μm) and the unwanted particles can be of comparable size. Because of this, the interference between the unwanted particles and the collimated beam 121 in the mechanical filter 140 is reduced when compared to the spatial filter.

Additionally, it is easier to guide the collimated diagnostic beam 121 through the aperture 160 (260A, 260B) because the tolerance in the relative positioning between the collimated diagnostic beam 121 and the 4 mm aperture 160 (or 260A, 260B) is about 0.2 to 0.4 mm. On the other hand, the tolerance in the relative positioning between the collimated diagnostic beam 121 and the 100 μm aperture of the spatial filter is about 5 to 10 μm.

3, an embodiment 335 of the detection device 135 is shown. In this illustration, the coordinate system XsYsZs of the detection device 335 is such that Ys is the output of the page and Zs extends perpendicular to the imaging area of the sensor 330. The detection device 335 includes an optical collimator 342 that produces a corresponding collimated beam for each of the diagnostic light 120 and the non-diagnostic light 122 for input to the beam reducer 350. As discussed above, in the absence of the beam reducer 350, the beam radius of each collimated beam output from the optical collimator 342 will not undergo a substantial change in the distance extending from the optical collimator 342 to the sensor 330. The optical collimator 342 is a doublet lens (two lenses 342a, 342b) in this example. The focal length or radius of curvature of the doublet lens 342 is selected so that the initially curved wavefronts of the diagnostic light 120 and the non-diagnostic light 122 become flat or substantially flat at least for a distance extending a certain length to the sensor 330.

In this embodiment, the beam reducer 350 is designed as a Galilean-type refractive telescope. The beam reducer 350 optically reduces the size of the collimated beam output from the optical collimator 342 to form the reduced-size collimated beams 121, 123, respectively. The beam reducer 350 includes a positive focal length lens configuration 351 at the input side (which can be a convergent lens configuration including one or more of a positive biconvex lens, a plano-convex lens, or a concave-convex lens). The beam reducer 350 includes a negative focal length lens configuration 353 at the output side (which can be a divergent lens configuration including a concave lens). The positive focal length lens configuration 351 and the negative focal length lens configuration 353 are separated by the sum of their focal lengths. The converging lens configuration 351 in this example is a compound lens (having a convex lens 351a and a concave lens 351b) that can help correct distortion in the beam. The beam reducer 350 lacks an intermediate focus (there is no focus between the converging lens configuration 351 and the diverging lens configuration 353). Although not required, in this embodiment, the diverging lens configuration 353 includes an auxiliary lens 354 between the converging lens 351 and the diverging lens 353. Auxiliary lens 354 can be used in combination with diverging lens configuration 353 to collimate the beam from positive focal length lens configuration 351. Auxiliary lens 354 can be a positive focal length lens, such as a concave-convex lens (as shown), a biconvex lens, or a plano-convex lens. In general, beam reducer 350 can reduce the lateral size (i.e., the size in the XsYs plane) of the collimated beam output from optical collimator 342 by at least five times, at least ten times, or at least twenty times.

The beam reducer 350 outputs a reduced collimated diagnostic beam 121 and a reduced collimated non-diagnostic beam 123, which then travel the length of the optical path 352 to the mask 355. The longer the optical path 352, the greater the separation between the respective images from these beams 121, 123 at the mask 355. In this embodiment, the mask 355 is positioned so that the aperture 360 allows the diagnostic beam 121 (formed by the diagnostic light 120) to pass to the sensor 330, while the mask 355 blocks the non-diagnostic beam 123 (formed by the non-diagnostic light 122) from passing to the sensor 330. The collimated diagnostic beam 121 that has passed through the aperture 360 can be focused onto the imaging area of the sensor 330 by means of the converging lens 357.

Referring to FIG. 4 , another embodiment 435 of the detection device 135 is shown. In this illustration, the coordinate system XsYsZs of the detection device 435 is aligned with the page. The detection device 435 includes an optical collimator 442 that produces a corresponding collimated beam for each of the diagnostic light 120 and the non-diagnostic light 122 for input to the beam reducer 450 of the detection device 435. As discussed above, the beam radius of each collimated beam output from the optical collimator 442 does not undergo a substantial change in the distance extending to the sensor 430. The optical collimator 442 in this example is similar to the optical collimator 342 and includes a doublet lens (two lenses 442a, 442b). The focal length or radius of curvature of the doublet lens 442 is selected so that the initially curved wavefronts of the diagnostic light 120 and the non-diagnostic light 122 become flat or substantially flat at least for a distance extending a certain length to the sensor 430.

In this embodiment, the beam reducer 450 is designed as a Keplerian-type refractive telescope. The beam reducer 450 optically reduces the size of the collimated beam output from the optical collimator 442 to form reduced-size collimated beams 121, 123, respectively. The beam reducer 450 includes an input positive focal length lens configuration 451 at the input side (which may be a converging lens configuration including one or more of a positive biconvex lens, a plano-convex lens, or a concave-convex lens). The beam reducer 450 includes an output positive focal length lens arrangement 453 at the output side (which may be a converging lens arrangement including one or more of a positive biconvex lens, a plano-convex lens, or a concave-convex lens). For example, the positive focal length lens arrangement 453 is shown as an aspheric lens element in FIG. 4 . It is possible that the positive focal length lens arrangement 453 is a compound lens set. The positive focal length lens arrangements 451, 453 are separated by the sum of their focal lengths. The converging lens arrangement 451 in this example is a compound lens (having a convex lens 451a and a concave lens 451b) that can help correct distortion in the beam. The intermediate focus IF (or intermediate focal plane IF) is between the input converging lens arrangement 451 and the output converging lens arrangement 453. In general, the beam reducer 450 can reduce the lateral size (i.e., the size in the XsYs plane) of the collimated beam output from the optical collimator 442 by at least five times, at least ten times, or at least twenty times. The range of the beam reducer 450 along the Zs direction is often larger than the range of the beam reducer 350 along the Zs direction, and therefore, the space requirements can determine which design of the beam reducer 350 or 450 is more suitable. In addition, if the power of the diagnostic light 120 or the non-diagnostic light 122 is too high to apply the intermediate focus IF (such as exists in the beam reducer 450), the beam reducer 350 may be a more appropriate design.

The beam reducer 450 outputs a reduced collimated diagnostic beam 121 and a reduced collimated non-diagnostic beam 123, which then travel the length of the optical path 452 to the mask 455. The longer the optical path 452, the greater the separation between the respective images from these beams 121, 123 at the mask 455. In this embodiment, the mask 455 is positioned so that the aperture 460 allows the diagnostic beam 121 (formed from the diagnostic light 120) to pass to the sensor 430, while the mask 455 blocks the non-diagnostic beam 123 (formed from the non-diagnostic light 122) from passing to the sensor 430. The collimated diagnostic beam 121 that has passed through the aperture 460 can be focused onto the imaging area of the sensor 430 by means of the converging lens 457.

Referring to FIG. 5 , in other embodiments, the detection device 135 is a detection device 535 that also includes one or more spectral filters 543 and polarization filters 544 that can be configured in series or in parallel with the mechanical filter 140. The spectral filter 543 is a filter that transmits light within a specific wavelength range, such as a bandpass filter. The polarization filter 544 is a filter that transmits light with a specific polarization. For example, the diagnostic light 120 may have a different polarization depending on the polarization of the diagnostic probe 125, while the non-diagnostic light 122 may be non-polarized. Therefore, the polarization filter can select the polarization of the diagnostic light 120 to be transmitted.

Referring to FIG. 6A , in some embodiments, the diagnostic device 165 is designed as a diagnostic device 665A. The diagnostic device 665A generates a single probe beam 625A from a light source 626A as one or more diagnostic probes 125. The probe beam 625A is guided as a light curtain to intersect the trajectory TR at a position x so that each of the targets 105 passes through the light curtain on its way to the target space 110. The light source 626A generates a single beam 611A that is guided through one or more optical elements 627A (such as mirrors, lenses, apertures and/or filters) that modify the light beam 611A to form a single probe beam 625A.

The light source 626A may be a solid-state laser, such as a YAG laser, which may be a neodymium-doped YAG (Nd:YAG) laser operating at 1070 nm and at a power of 50 W. In this example, when the current target 105 c passes through the probe beam 625 A at time t, at least some of the probe beam 625 A is reflected or scattered from the current target 105 c to form diagnostic light 620 A detected by the detection device 135. The control system 170 uses information from the sensor 130 to estimate the motion properties of the current target 105, which can be used to estimate the arrival time of an existing target (which may be the current target 105 c or a subsequent target) at the target space 110. This estimate can be used to adjust the characteristics of the radiation pulse directed to the target space 110 to ensure that the radiation pulse interacts with the existing targets in the target space 110. The control system 170 can also rely on some assumptions about the paths of the existing targets to perform calculations to estimate the arrival time of the existing targets at the target space 110.

The probe beam 625A may be a Gaussian beam so that the transverse distribution of its optical intensity may be described as having a Gaussian function. The focus or beam waist of the probe beam 625A may be configured to overlap at the trajectory TR or -X direction. In addition, the optical element 627A may include a refractive optical device that ensures that the focus (or beam waist) of the probe beam 625A overlaps with the trajectory TR.

6B , in some embodiments, the diagnostic device 165 is designed as a diagnostic device 665B. The diagnostic device 665B generates two probe beams 625B_1 and 625B_2 as one or more diagnostic probes 125. The probe beam 625B_1 is guided as a first light curtain to intersect the track TR at a first orientation (e.g., orientation x1 along the X-axis) so that each of the targets 105 passes through the first light curtain on its way to the target space 110. The probe beam 625B_2 is guided as a second light curtain to intersect the track TR at a second orientation (e.g., orientation x2 along the X-axis) so that each of the targets 105 passes through the second light curtain on its way to the target space 110 and after having passed through the first light curtain. The probe beams 625B_1 and 625B_2 are separated by a distance △d equal to x2-x1 at the track TR. This dual-screen diagnostic device 665B can be used not only to determine the position and arrival information of the target 105 but also to determine the speed or velocity of the target 105.

In some embodiments, the diagnostic device 665B includes a single light source 626B that generates a single light beam 611B and one or more optical elements 627B that receive the single light beam and split the light beam 611B into two probe beams 625B_1, 625B_2. In addition, the optical element 627B may include a component for guiding the probe beams 625B_1, 625B_2 toward corresponding positions x1, x2 along the trajectory TR.

In some embodiments, the optical assembly 627B includes a beam splitter that splits a single light beam from a single light source 626B into two probe beams 625B_1, 625B_2. For example, the beam splitter can be a dielectric mirror, a beam splitter block, or a polarization beam splitter. One or more of the optical assemblies 627B can be reflective optical devices positioned to redirect either or both of the probe beams 625B_1, 625B_2 so that both of the probe beams 625B_1, 625B_2 are directed toward the trajectory TR.

In other embodiments, the optical assembly 627B includes a splitting optics (such as a diffraction optics or a binary phase diffraction grating, a birefringent crystal, an intensity beam splitter, a polarization beam splitter, or a dichroic beam splitter) and a refractive optics, such as a focusing lens. The light beam 611B is directed through the splitting optics, which splits the light beam 611B into two beams that travel in opposite directions and are directed through the refractive optics to produce probe beams 625B_1, 625B_2. The splitting optics can split the light beam 611B so that the probe beams 625B_1, 625B_2 are separated by a set distance at the track TR (e.g., 0.65 mm along the X direction). In this example, x2-x1=0.65 mm. In addition, the refractive optical device can ensure that the focus (or beam waist) of each of the probe beams 625B_1, 625B_2 overlaps with the trajectory TR.

As shown in this example, the probe beams 625B_1, 625B_2 are directed so that they intersect the track TR at different orientations x1, x2 but generally at substantially similar angles relative to the X-axis. For example, the probe beams 625B_1, 625B_2 are directed at about 90° relative to the X-axis. In other embodiments, split optics and refractive optics may be used to adjust the angles at which the probe beams 625B_1, 625B_2 are directed relative to the X-axis so that the probe beams fan out toward the track TR and intersect the track TR at different and different angles. For example, the probe beam 625B_1 may intersect the track TR at approximately 90° relative to the -X direction, and the probe beam 625B_2 may intersect the track TR at an angle less than 90° relative to the -X direction.

Each of the probe beams 625B_1, 625B_2 can be a Gaussian beam, so that the transverse distribution of the optical intensity of each probe beam 625B_1, 625B_2 can be described by a Gaussian function. The focus or beam waist of each probe beam 625B_1, 625B_2 can be configured to overlap at the track TR or -X direction.

The light source 626B may be a solid-state laser, such as a YAG laser, which may be a neodymium-doped YAG (Nd:YAG) laser operating at 1070 nm and at a power of 50 W. In this example, the current target 105c passes through the first probe beam 625B_1 at time t1 (and position x1), and at least some of the probe beam 625B_1 is reflected or scattered from the current target 105c to form the diagnostic light 620B_1 detected by the detection device 135 (by means of the mechanical filter 140). Additionally, the current target 105c passes through the second probe beam 625B_2 at time t2 (and position x2), and at least some of the probe beam 625B_2 is reflected or scattered from the current target 105c to form light 620B_2 detected by the detection device 135 (with the aid of the mechanical filter 140).

The separation Δd between the probe beams 625B_1, 625B_2 at the trajectory TR can be adjusted or customized depending on the rate at which the self-target supply device 175 releases the target 105 and the size and material of the target 105. For example, the separation Δd can be less than the spacing between adjacent targets 105. As another example, the separation Δd can be determined or set based on the spacing between adjacent targets 105 to provide greater accuracy of the measurement performed based on the interaction between the probe beams 625B_1, 625B_2 and the current target 105c. To a certain extent and generally speaking, the larger the separation Δd, the higher the accuracy of the measurement performed. For example, the separation Δd may be between approximately 250 μm and 800 μm.

The interaction between the probe beams 625B_1, 625B_2 and the current target 105c enables the control system 170 to determine movement properties, such as the velocity V of the current target 105c in the -X direction. It is also possible to determine the velocity V or the trend of changing velocity V across many targets 105. If some assumptions are made about the motion of the current target 105c, it is also possible to determine changes in the movement properties of the current target 105c in the -X direction using only the probe beams 625B_1, 625B_2.

The wavelength of the diagnostic probe 125 (such as probe beam 625A and probe beams 625B_1, 625B_2) generated by the diagnostic device 165 should be sufficiently different from the wavelength of the radiation pulse directed to the target space 110 (for interaction with the target 105) to help distinguish between diagnostic light 120 and non-diagnostic light 122. In some embodiments, the wavelength of the probe beams 125, 625A, 625B_1 and 625B_2 is 532nm or 1550nm.

In other embodiments such as shown in FIG. 6C , the diagnostic device 665C includes a pair of light sources 626C_1, 626C_2 (such as two lasers) that each generate a light beam 611C_1, 611C_2, respectively, rather than a single light source such as the light source 626B in the diagnostic device 665B. Each of the light beams 611C_1, 611C_2 passes through a corresponding one or more optical elements 627C_1, 627C_2 that can change or adjust the characteristics of the light beams 611C_1, 611C_2. The output of each of the one or more optical elements is a corresponding probe beam 625C_1, 625C_2. Optical components 627C_1, 627C_2 may include components for directing corresponding probe beams 625C_1, 625C_2 toward corresponding positions x1, x2 along trajectory TR. Examples of optical components 627C_1, 627C_1 are discussed above with reference to optical component 608B.

7 , in some embodiments, the metrology device 100 is implemented as a metrology device 700 in an EUV light source 776 to measure one or more properties of a target 105. The EUV light source 776 includes a target supply device 775 that generates a continuous stream 706 of targets (each generally designated as 105) along a trajectory TR toward a target space 710 inside a vacuum environment 715 defined by a chamber 716. The EUV light source 776 supplies EUV light 777 that has been generated by the interaction between the target 105 and the radiation pulse 778 to an output device 779. As discussed above, the metrology device 700 measures and analyzes one or more movement properties (such as speed, velocity, and acceleration) of the current target 105c as it travels along the trajectory TR toward the target space 710. The trajectory TR extends along a direction that can be considered as a target (or axial) direction, which is located in the three-dimensional X, Y, Z coordinate system defined by the chamber 716. As discussed above, the axial direction of the target 105 generally has a component parallel to the -X direction of the coordinate system of the chamber 716. However, the axial direction of the target 105 may also have a component along one or more of the directions Y and Z that are perpendicular to the -X direction. Additionally, each target 105 released by the target supply device 775 may have a slightly different actual trajectory and the trajectory depends on the physical properties of the target supply device 775 and the environment 715 within the chamber 716 when the target 105 is released.

The EUV light source 776 generally includes an EUV collector 780, an optical source 781, an actuation system 782 communicating with the optical source 781, and a control system 170 communicating with the metrology device 700 and a control device 783 communicating with the target supply device 775, the optical source 781, and the actuation system 782.

The EUV concentrator 780 collects as much EUV light 784 emitted from the plasma 785 as possible and redirects that EUV light 784 toward the output device 779 as collected EUV light 777. The concentrator 780 may be a reflective optical device, such as a curved mirror capable of reflecting light having an EUV wavelength (i.e., EUV light 784) to form the generated EUV light 777.

Optical source 781 generates one or more beams of radiation pulses 778 and directs one or more beams of radiation pulses 778 toward target space 710 generally along the Z direction (although the beams of radiation pulses 778 may be at an angle relative to the Z direction). In FIG. 7 , which is a schematic representation, the beams of radiation pulses 778 are shown as being directed along the −Y direction. Optical source 781 includes one or more light sources that generate radiation pulses 778, a beam delivery system including an optical steering assembly that changes the direction or angle of the beams of radiation pulses 778, and a focusing assembly that focuses the beams of radiation pulses 778 toward target space 710. Exemplary optical steering components include optical elements, such as lenses and mirrors, that steer or direct the beam of radiation pulse 778 by refraction or reflection as needed. Actuation system 782 can be used to control or move various features of the optical components and focusing assembly of the beam delivery system, as well as adjust the state of optical source 781 that generates radiation pulse 778.

The optical source 781 includes at least one gain medium and an energy source for exciting the gain medium to generate a radiation pulse 778. The radiation pulse 778 constitutes a plurality of optical pulses separated from each other in time. In other embodiments, the light beam output from the optical source 781 may be a continuous wave (CW) light beam. The optical source 781 may be or include, for example, a solid-state laser (e.g., a Nd:YAG laser, an erbium-doped fiber (Er:glass) laser, or a neon-doped YAG (Nd:YAG) laser operating at 1070 nm and at a power of 50 W.

Actuation system 782 is coupled to components of optical source 781 and is also in communication with and under control of control device 783. Actuation system 782 is capable of modifying or controlling the relative position between radiation pulse 778 and target 105 in target space 710. For example, actuation system 782 is configured to adjust one or more of the timing of the release of radiation pulse 778 and the direction in which radiation pulse 778 travels.

The target supply device 775 is configured to release a stream (or multiple 706) of targets 105 at a specific rate. The metrology device 700 takes this rate into account when determining the total amount of time required to perform measurement and analysis of one or more motion properties of the current target 105c and changes to other states or components of the EUV light source 776 that are affected based on the measurement and analysis. For example, the control system 170 can transmit the results of the measurement and analysis to the control device 783, which determines how to adjust one or more signals to adapt the actuation system 782 to adjust one or more characteristics of the radiation pulse 778 directed to the target space 710.

Adjustment of one or more characteristics of radiation pulse 778 may improve the relative alignment between existing target 105' and radiation pulse 778 in target space 710. Existing target 105' is a target that has entered target space 710 when radiation pulse 778 (which happens to have been adjusted) arrives in target space 710. Such adjustment of one or more characteristics of radiation pulse 778 improves the interaction between existing target 105' and radiation pulse 778 and increases the amount of EUV light 784 produced by such interaction. As shown in FIG. 7 , a previous target 105p has interacted with a previous radiation pulse (not shown) to produce a plasma 785 that emits non-diagnostic light 122 (in addition to EUV light 784).

In some embodiments, the existing target 105' is the current target 105c. In such embodiments, the adjustment of one or more characteristics of the radiation pulse 778 occurs within a relatively short time frame. The relatively short time frame means that one or more characteristics of the radiation pulse 778 are adjusted during the time after the analysis of the motion properties of the current target 105c is completed to the time when the current target 105c enters the target space 710. Because one or more characteristics of the radiation pulse 778 can be adjusted within a relatively short time frame, there is sufficient time to affect the interaction between the current target 105c (whose motion properties have just been analyzed) and the radiation pulse 778.

In other embodiments, the existing target 105' is another target, that is, a target other than the current target 105c and later in time than the current target 105c. In such embodiments, the adjustment of one or more characteristics of the radiation pulse 778 occurs within a relatively long time frame, so that it is not feasible to affect the interaction between the current target 105c (whose motion properties have just been analyzed) and the radiation pulse 778. On the other hand, it is feasible to affect the interaction between another (or later) target and the radiation pulse 778. The relatively long time frame is a time frame greater than the time after the analysis of the motion properties of the current target 105c is completed to the time when the current target 105c enters the target space 710. Depending on the relatively long time frame, the other target may be adjacent to the current target 105c. Alternatively, the other target may be adjacent to an intermediate target that is adjacent to the current target 105c. In these other embodiments, it is assumed that the other target (which is not the current target 105c) is moving with movement properties that are sufficiently similar to the movement properties of the detected or estimated current target 105c.

Each of the targets 105 (including the previous target 105p and the current target 105c, and all other targets generated by the target supply device 775 (or 175)) includes a material that emits EUV light when converted into plasma. Each target 105 is at least partially or mostly converted into plasma by interacting with the radiation pulse 778 generated by the optical source 781 in the target space 710. Each target 105 generated by the target supply device 775 (or 175) is a target mixture that includes a target material and, if appropriate, impurities such as non-target particles. The target material is a substance that can be converted into a plasma state and has an emission spectrum in the EUV range. The target 105 may be, for example, a droplet of a liquid or molten metal, a portion of a liquid stream, a solid particle or cluster, a solid particle contained in a droplet, a foam of the target material, or a solid particle contained in a portion of a liquid stream. The target material may include, for example, water, tin, lithium, xenon, or any material having an emission spectrum in the EUV range when converted to a plasma state. For example, the target material may be elemental tin, which may be used as pure tin (Sn); as a tin compound, such as _SnBr4 , _SnBr2 , _SnH4 ; as a tin alloy, such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or any combination of such alloys. In the absence of impurities, each target 105 includes only the target material. The discussion provided herein is an example in which each target 105 is a droplet made of a molten metal such as tin. However, each target 105 produced by the target supply device 775 (or 175) can take other forms.

The target 105 may be provided to the target space 710 by passing molten target material through a nozzle of a target supply device 775 (or 175) and allowing the target 105 to drift into the target space 710 along a trajectory TR. In some embodiments, the target 105 may be guided to the target space 710 by a force (in addition to or in spite of gravity). As discussed below, an existing target 105' (which may be the current target 105c) interacting with a radiation pulse 778 may also have interacted with one or more previous radiation pulses. Alternatively, an existing target 105' interacting with a radiation pulse 778 may reach the target space 710 without interacting with any other radiation pulses.

In this embodiment, the detection device 735 is positioned outside the chamber 716, and the diagnostic area 745 is inside the environment 715 of the chamber 716. The wall of the chamber 716 is equipped with an optical window 736 that is transparent to the wavelength of the diagnostic light 120 and can withstand any pressure differential at the wall. The optical window 736 can be held in a mount and hermetically sealed in the wall to maintain the pressure within the environment 715. For example, the optical window 736 can be made of a crown glass with a relatively low refractive index and low dispersion, such as borosilicate glass (BK7 or N-BK7) or fused silica. The optical window 736 has an aperture large enough to accommodate the range of the diagnostic light 120. The distance between the diagnostic region 745 and the optical window may be about hundreds of millimeters (or about 600 to 700 mm), while the distance between the diagnostic region 745 and the target space 710 may be about several millimeters (or about 1 to 5 mm). Therefore, the distance between the diagnostic region 745 and the optical window may be 200 to 500 times the distance between the diagnostic region 745 and the target space 710.

The control device 783 communicates with the control system 170 and also communicates with other components of the EUV light source 776, such as the actuation system 782, the target supply device 775, and the optical source 781. Referring to FIG. 8, an embodiment 883 of the control device 783 is shown and an embodiment 870 of the control system 170 is shown. The control device 883 includes the control system 870, but the control system 870 may be physically separated from the control device 883 and still maintain communication. In addition, features or components of the control device 883 may be shared with the control system 870, including features not shown in FIG. 8.

The control system 870 includes a signal processing module 871 configured to receive output from the self-detection device 735 (or 135, 335, 435). The control system 870 includes a diagnostic control module 872 that communicates with the diagnostic device 765 (or 165). For example, the signal processing module 871 receives a signal from the sensor 130 within the self-detection device 735 (135, 335, 435), where the signal is a voltage signal related to the current generated by the light detected at the sensor 130. Generally speaking, the signal processing module 871 analyzes the output from the sensor 730 and determines one or more movement attributes of the current target 105c based on this analysis. The diagnostic control module 872 controls the operation of the diagnostic device 765. For example, the diagnostic control module 872 may provide signals to the diagnostic device 765 for adjusting one or more characteristics of the diagnostic device 765 and for adjusting one or more characteristics of one or more diagnostic probes 725.

The signal processing module 871 also determines whether adjustments need to be made to the subsequent radiation pulse 778 output from the optical source 781 based on the determination of one or more motion properties of the current target 105c. And, if adjustments are required, the signal processing module 871 sends appropriate signals to the optical source actuation module 884 interfaced with the optical source 781 or the actuation system 782. The optical source actuation module 884 can be within the control device 883 (as shown in FIG. 8) or it can be integrated into the control system 870.

The signal processing module 871 may include one or more field programmable hardware circuits, such as a field-programmable gate array (FPGA). An FPGA is an integrated circuit designed to be configured by a customer or designer after manufacturing. The field programmable hardware circuit may be a dedicated hardware that receives one or more values of a timestamp, performs calculations on the received values, and uses one or more lookup tables to estimate the time for an existing target 105' to arrive at the target space 710. In particular, the field programmable hardware circuit can be used to rapidly perform calculations to implement adjustments to one or more characteristics of the radiation pulse 778 within a relatively short time frame, thereby implementing adjustments to one or more characteristics of the radiation pulse 778 interacting with the current target 105c (whose motion properties have just been analyzed by the signal processing module 871).

The control device 883 includes a target delivery module 885 configured to interface with the target supply device 775. In addition, the control device 883 and the control system 870 may include other modules that are specifically configured to interface with other components of the EUV light source 776 that are not shown.

The control system 870 generally includes or has access to one or more of digital electronic circuitry, computer hardware, firmware, and software. For example, the control system 870 has access to memory 873, which may be read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example: semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; disks, such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. The control system 870 may also include or interface with one or more input devices 874i (such as a keyboard, touch screen, microphone, mouse, handheld input device, etc.) and one or more output devices 874o (such as a speaker and a monitor).

The control system 870 may also include or access one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by the programmable processor. One or more programmable processors may each execute a program of instructions to perform a desired function by operating on input data and generating appropriate output. Generally, the processor receives instructions and data from the memory 873. Any of the foregoing may be supplemented or incorporated therein by a specially designed application-specific integrated circuit (ASIC).

In addition, any one or more of the modules may include its own digital electronic circuits, computer hardware, firmware and software as well as dedicated memory, input and output devices, programmable processors and computer program products. Similarly, any one or more of the modules may access and use memory 873, input devices 874i, output devices 874o, programmable processors and computer program products.

Although the control system 870 is shown as a separate and complete unit, each of its components and modules may be a separate unit. The control device 883 may include other components not shown in Figure 8, such as dedicated memory, input/output devices, processors, and computer program products.

Referring to FIG. 9 , an implementation scheme 979 of the lithography apparatus 779 is shown. The lithography apparatus 979 exposes a substrate (which may be referred to as a wafer) W by an exposure beam B. The lithography apparatus 979 includes a plurality of reflective optical elements R1, R2, R3, a mask M, and a slit S all in a housing 10. The housing 10 is an outer shell, a storage tank, or other structure that can support the reflective optical elements R1, R1, R2, the mask M, and the slit S and can also maintain the evacuated space in the housing 10.

EUV light 777 enters housing 10 and is reflected by optical element R1 through slit S toward mask M. Slit S partially defines the shape of the scattered light used to scan substrate W during lithography. The amount of light delivered to substrate W or the number of photons delivered to substrate W depends on the size of slit S and the speed at which slit S is scanned.

The mask M may also be referred to as a zoom mask or a patterned device. The mask M includes a spatial pattern representing features to be formed in the photoresist on the substrate W. The EUV light 777 interacts with the mask M. The interaction between the EUV light 777 and the mask M causes the pattern of the mask M to be imparted to the EUV light 777 to form an exposure beam B. The exposure beam B passes through the slit S and is guided to the substrate W by the optical elements R2 and R3. The interaction between the substrate W and the exposure beam B exposes the pattern of the mask M to the substrate W, and the photoresist features are thereby formed at the substrate W. The substrate W includes a plurality of portions 20 (e.g., grains). The area of each portion 20 in the Y-Z plane is smaller than the area of the entire substrate W in the Y-Z plane. Each portion 20 can be exposed by exposure beam B to include a copy of mask M, so that each portion 20 includes electronic features indicated by the pattern on mask M.

The lithography apparatus 979 may include a lithography control system 30 that communicates with a control device 783 of the EUV light source 776.

10 , process 1090 is performed by the metrology device 100 (or metrology device 700). Before the current target 105c enters the target space 110, the diagnostic probe 125 interacts with the current target 105c in the diagnostic region 145 (1091). The diagnostic light 120 generated by the interaction (1091) is collected, and at the same time, some unwanted non-diagnostic light 122 generated by the target space 110 is also collected (1092). For example, the pupil at the entrance of the mechanical filter 140 collects the diagnostic light 120 and the non-diagnostic light 122. The diagnostic light 120 and the non-diagnostic light 122 are collimated (1093). The optical collimator 142 collimates the diagnostic light 120 and the non-diagnostic light 122 that have entered the mechanical filter 140. The collimated diagnostic light and the non-diagnostic light are angularly separated from each other (1094). The beam reducer 150 reduces the lateral extent (along the XsYs plane) of the collimated diagnostic beam and the collimated non-diagnostic beam, and these reduced beams 121, 123 leave the beam reducer 150, respectively, and their angular separation increases as they travel along the optical path 152 toward the mask 155. The collimated diagnostic beam 121 is sensed at a sensing region (e.g., sensor 130) in optical communication with an open image plane that is laterally displaced (in the XsYs plane) from a closed image plane that blocks the collimated non-diagnostic beam 123 (1095). For example, the collimated diagnostic beam 121 is sensed by sensor 130 that is in optical communication with aperture 160 (open image plane) and aperture 160 is laterally displaced in the XsYs plane from a mask 155 on which the collimated non-diagnostic beam 123 impinges. A property of the current target 105c is estimated based on the sensed diagnostic light (1096). In particular, the control system 170 analyzes the output from the sensor 130 to estimate one or more motion properties of the current target 105c.

As discussed above with reference to FIGS. 6A-6C , the diagnostic probe 125 may be one or more probe beams directed to intersect the trajectory of the target 105. Thus, the interaction (1091) between the diagnostic probe 125 and the current target 105c may be between such probe beams and the current target 105c. In some embodiments, the diagnostic light 120 collected (1092) may be a portion of the probe beam 125 reflected or scattered from the current target 105c. In other embodiments, the diagnostic light 120 collected (1092) is light generated by the current target 105c. In other embodiments, the diagnostic light 120 collected (1092) is light blocked by the current target 105c (as shown in FIG. 12).

The influence of the non-diagnostic light 122 on the analysis of the output from the sensor 130 is reduced by blocking or redirecting the non-diagnostic light 122 with the mechanical filter 140, and thus preventing the non-diagnostic light 122 from being irradiated on the sensor 130 and the non-diagnostic light 122 is actually blocked by the mask 155. In addition, the collimated state of the collimated diagnostic beam and the collimated non-diagnostic beam output from the collimator 142 is always maintained to the plane of the mask 155, and the collimated diagnostic beam 121 is further focused onto the sensor 130 after passing through the aperture 160.

Referring to FIG. 11 , in other embodiments, the optical collimator 142 and the beam reducer 150 having a positive focal length and closest to the optical collimator 142 are integrated into a single refractive element 1142/1151. This integrated single refractive element 1142/1151 can be applied to a Galilean-type refractive telescope (such as the telescope shown in FIG. 3 and FIG. 11 ) or a Keplerian-type refractive telescope (such as the telescope shown in FIG. 4 ).

Referring to FIG. 12 , in other embodiments, the diagnostic light 120 is generated by a portion of the diagnostic probe beam 1225 that is blocked by the current target 105 c. In this embodiment of the metrology device 1200, the diagnostic probe 1225 provides back illumination of the current target 105 c. The sensor 130 is a two-dimensional (e.g., imaging) sensor 1230, such as a camera. Therefore, when the current target 105 c intersects the diagnostic probe 1225, a shadow of the target 105 c is formed by the target 105 c obscuring at least a portion of the diagnostic probe 1225, as shown in the inset of FIG. 12 . This configuration can be considered a shadowgraph configuration. In such embodiments, the sensor 1230 is disposed on a side of the target trajectory TR opposite the side on which the diagnostic device 1265 is disposed. The sensor 1230 is a camera that captures a two-dimensional representation of the diagnostic light 1220, which can be viewed as an image. Thus, for example, the sensor 1230 includes a two-dimensional array of thousands or millions of light locations (or pixels). The diagnostic light 1220 is directed onto the photosensitive area of each pixel, where it is converted into electrons that are collected into voltage signals and the array of such signals forms a two-dimensional image. As discussed above, the non-diagnostic light 1222 is substantially blocked from reaching the sensor 1230.

Referring to FIG. 13 , in other embodiments, the beam reducer 150 is designed as a reflective beam reducer 1350 that receives a collimated beam from an optical collimator such as the collimator 342. The reflective beam reducer 1350 includes a concave reflective element (curved mirror) 1351 that converges the beam toward a convex reflective element (curved mirror) 1353, thereby collimating the light into a collimated diagnostic beam 121 and a collimated non-diagnostic beam 123 that travel in different directions (or angles) along an optical path 1352 toward the mask 155.

14, in other embodiments, the beam reducer 150 is designed as a catadioptric (hybrid) beam reducer 1450 that receives a collimated beam from an optical collimator, such as the collimator 342. The hybrid beam reducer 1450 includes a flat reflective element (flat mirror) 1451a that directs the beam onto a curved reflective element (curved mirror) 1451b, which converges the beam toward an intermediate focus IF between the curved mirror 1451b and a convex or converging refractive element (lens) 1453. Lens 1453 collimates the light into a collimated diagnostic beam 121 and a collimated non-diagnostic beam 123 that travel in different directions (or angles) along the optical path toward mask 155.

Referring to FIG. 15 , in other embodiments, the mask 155 is designed as a mask 1555 that defines an aperture 1560 aligned with the path of the collimated non-diagnostic light beam 123, and the mask 1555 is used to stop or prevent the collimated diagnostic light beam 121 from passing to the sensor 130. Such a design may be useful if it is necessary to analyze patterns associated with the non-diagnostic light 122.

Other aspects of the invention are described in the following numbered clauses.

1. A metrology device, comprising: a diagnostic device configured to cause a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; a detection device, comprising: a light sensor having a field of view overlapping the diagnostic region and configured to sense light produced by the interaction between the diagnostic probe and the current target at the diagnostic region; light generated by the detection device; and a mechanical filter between the diagnostic region and the light sensor, the mechanical filter comprising a beam reducer and an optical mask defining an aperture positioned between the beam reducer and the light sensor; and a control system in communication with the detection device and configured to estimate a property of a current target based on the output from the light sensor.

2. A metrological device as in clause 1, wherein the mechanical filter is configured to angularly separate diagnostic light emitted from a diagnostic region from non-diagnostic light emitted from a target space, wherein the diagnostic light is generated by the interaction between a target present at the diagnostic region and a diagnostic probe.

3. A metrological device as claimed in clause 2, wherein the non-diagnostic light comprises light emitted from plasma generated by a previous target in the target space.

4. A metrological device as in clause 2, wherein the lateral extent of the aperture is approximately the same as or greater than the lateral extent of the diagnostic light in the plane of the optical mask, and the lateral extent of the optical mask is greater than or approximately the same as the lateral extent of the non-diagnostic light in the plane of the optical mask.

5. A metrological device as in clause 2, wherein the optical mask is positioned so that non-diagnostic light emitted from the target space is substantially blocked by the optical mask, while diagnostic light substantially passes through the aperture.

6. A metrological apparatus as claimed in clause 1, wherein the mechanical filter comprises an optical collimator between the diagnostic region and the beam reducer.

7. A metrology apparatus as claimed in claim 6, wherein the beam reducer is an afocal beam reducer and is configured to be combined with an optical collimator to project a finite object to infinity.

8. A metrological device as claimed in claim 6, wherein the optical collimator and the beam reducer with a positive focal length closest to the optical collimator are integrated into a single refractive element.

9. A metrological apparatus as claimed in claim 6, wherein the beam reducer is configured to maintain the collimation of the light.

10. A metrology device as in clause 1, wherein the photodetector comprises one or more of the following: a photodiode, a phototransistor, a photodependent resistor, a photomultiplier tube, a multi-unit photoreceiver, a quad-unit photoreceiver, and a camera.

11. A metrology device as claimed in claim 1, wherein the diagnostic probe comprises at least one diagnostic light beam, and the light sensor is configured to sense the diagnostic light generated by the interaction between the current target and the at least one diagnostic light beam.

12. A metrological device as claimed in clause 11, wherein the diagnostic light comprises a diagnostic beam reflected from, scattered from or blocked by a current target.

13. A metrology device as claimed in clause 1, wherein the detection device further comprises one or more of a spectral filter and a polarization filter.

14. A metrology device as in clause 1, wherein the diagnostic probe comprises a first diagnostic beam and a second diagnostic beam, each of the first diagnostic beam and the second diagnostic beam being configured to interact with a current target before it enters the target space, each interaction occurring at a different region and at a different time.

15. A metrological apparatus as claimed in claim 1, wherein the beam reducer comprises a refracting telescope, a reflecting telescope or a reflecting-refracting telescope.

16. A metrological apparatus as claimed in claim 15, wherein the refractive telescope comprises: a positive focal length lens arrangement and a negative focal length lens arrangement, which are separated by the sum of their focal lengths; or a pair of positive focal length lens arrangements, which are separated by the sum of their focal lengths.

17. A metrology device as claimed in claim 1, wherein the beam reducer is configured to reduce the lateral size of the incident light by at least five times, at least ten times, at least twenty times or about ten times.

18. A measuring device as claimed in clause 1, wherein the opening comprises a circular opening, an elliptical opening, a polygonal opening or a slit opening.

19. A metrology device as claimed in claim 1, wherein the detection device is positioned outside a chamber of an extreme ultraviolet (EUV) light source, the diagnostic region is inside the chamber, and the detection device receives light from the chamber through an optical window in a wall of the chamber.

20. A metrological device as claimed in claim 19, wherein the distance between the diagnostic region and the optical window may be about 200 to 500 times the size of the distance between the diagnostic region and the target space.

21. A metrological device as claimed in clause 1, wherein the detection device comprises a focusing lens at the output end of the aperture, the focusing lens being configured to focus the sensed light onto the light sensor.

22. A metrological device as claimed in clause 1, wherein the orifice has a range of at least 2 millimetres (mm).

23. A metrological apparatus as claimed in clause 1, wherein the aperture is positioned to receive diagnostic light at an orientation where the diagnostic light is collimated or non-convergent and non-divergent.

24. A metrology method, comprising: causing a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; collecting diagnostic light generated by the interaction between the diagnostic probe and the current target at the diagnostic region, and collecting also includes collecting non-diagnostic light generated by the target space; Causing diagnostic light and non-diagnostic light to interact with each other at a diagnostic region; Collimating light; angularly separating diagnostic light and non-diagnostic light from each other including reducing the lateral range of the diagnostic light and the non-diagnostic light; sensing the diagnostic light at a sensing region laterally displaced from a non-sensing region through which free non-diagnostic light passes after the diagnostic light and the non-diagnostic light have been angularly separated; and estimating properties of a current target based on the sensed diagnostic light.

25. The metrology method of clause 24, wherein: causing the diagnostic probe to interact with the current target at the diagnostic region includes causing one or more diagnostic beams to interact with the current target at the diagnostic region; and collecting diagnostic light includes collecting one or more diagnostic beams at the diagnostic region that have been reflected from the current target, scattered from the current target, or blocked by the current target.

26. The metrology method of clause 24, further comprising filtering the diagnostic light based on one or more of its spectral properties and its polarization state.

27. A metrology method as in clause 24, wherein the diagnostic region is inside a hermetically sealed chamber of an extreme ultraviolet (EUV) light source and also includes collecting non-diagnostic light, wherein collecting the diagnostic light includes receiving diagnostic light including non-diagnostic light transmitted through an optical window in a wall of the chamber.

28. The method of measurement as in clause 24, wherein reducing the lateral range of diagnostic and non-diagnostic optometry comprises reducing the lateral range of diagnostic and non-diagnostic optometry by at least five times, at least ten times, at least twenty times, or about ten times.

29. The metrology method of clause 24, further comprising blocking non-diagnostic light at the non-sensing area or redirecting the non-diagnostic light.

30. The method of measurement as in clause 24, wherein the lateral extent of reducing diagnostic light and non-diagnostic light comprises one or more of the following: refracting the light and reflecting the light.

31. The metrological method of clause 24, further comprising focusing the diagnostic light at the sensing area.

32. A method of measurement as in clause 24, wherein reducing the lateral range of diagnostic light and non-diagnostic light includes maintaining the collimation of the diagnostic light and non-diagnostic light.

33. A metrological method as in clause 24, further comprising passing the diagnostic light through an aperture of an optical mask having a range greater than the range of the diagnostic light after the diagnostic light and the non-diagnostic light are angularly separated from each other and before the diagnostic light is sensed.

Other implementations are within the scope of the following patent applications.

100:Weight and measurement equipment

105: Target

105c: Current goal

105p: Previous target

106: Continuous Flow

110: Target space

115: Environment

120: Diagnostic light

121: Collimated beam

122: Non-diagnostic radiography

123: Collimated beam

125: Diagnostic probe

130: Light sensor

135: Detection device

140: Mechanical filter

142:Optical collimator

145: Diagnosis area

150: Beam reducer

152:Optical path

155:Optical mask

160: Orifice

165: Diagnostic device

170: Control system

175: Target supply device

TR: Track

Claims (31)

A metrology device comprising: a diagnostic device configured to cause a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; a detection device comprising: a light sensor having a field of view overlapping the diagnostic region and configured to sense light generated by the interaction between the diagnostic probe and the current target at the diagnostic region; and a mechanical filter. A mechanical filter is provided between the diagnostic region and the light sensor, the mechanical filter comprising a beam reducer and an optical mask defining an aperture positioned between the beam reducer and the light sensor; and a control system in communication with the detection device and configured to estimate a property of the current target based on output from the light sensor, wherein the beam reducer is configured to maintain a collimated state of the light. The metrology device of claim 1, wherein the mechanical filter is configured to angularly separate diagnostic light emitted from the diagnostic region and non-diagnostic light emitted from the target space, wherein the diagnostic light is generated by an interaction between the current target and the diagnostic probe at the diagnostic region. A metrology device as claimed in claim 2, wherein the non-diagnostic light includes light emitted from a plasma generated by a previous target in the target space. A metrological device as claimed in claim 2, wherein the lateral range of the aperture is approximately the same as or greater than the lateral range of the diagnostic light in the plane of the optical mask, and the lateral range of the optical mask is greater than or approximately the same as the lateral range of the non-diagnostic light in the plane of the optical mask. A metrology device as claimed in claim 2, wherein the optical mask is positioned so that the non-diagnostic light emitted from the target space is substantially blocked by the optical mask, and the diagnostic light substantially passes through the aperture. A metrology device as claimed in claim 1, wherein the mechanical filter comprises an optical collimator between the diagnostic region and the beam reducer. A metrology apparatus as claimed in claim 6, wherein the beam reducer is an afocal beam reducer and is configured to be combined with the optical collimator to project a finite object to infinity. A metrology device as claimed in claim 6, wherein the optical collimator and the beam reducer with a positive focal length closest to the optical collimator are integrated into a single refractive element. A metrology device as claimed in claim 1, wherein the photodetector comprises one or more of the following: a photodiode, a phototransistor, a photodependent resistor, a photomultiplier tube, a multi-unit photoreceiver, a four-unit photoreceiver and a camera. A metrology device as claimed in claim 1, wherein the diagnostic probe comprises at least one diagnostic light beam, and the light sensor is configured to sense the diagnostic light generated by the interaction between the current target and the at least one diagnostic light beam. A metrology device as claimed in claim 10, wherein the diagnostic light includes the diagnostic light beam reflected from the current target, scattered from the current target or blocked by the current target. A metrology device as claimed in claim 1, wherein the detection device further comprises one or more of a spectral filter and a polarization filter. A metrology device as claimed in claim 1, wherein the diagnostic probe comprises a first diagnostic beam and a second diagnostic beam, each of the first diagnostic beam and the second diagnostic beam being configured to interact with the current target before entering the target space, each interaction occurring at a phase difference region and a phase difference time. A metrological device as claimed in claim 1, wherein the beam reducer comprises a refracting telescope, a reflecting telescope or a reflecting refracting telescope. A metrological device as claimed in claim 14, wherein the refractive telescope comprises: a positive focal length lens configuration and a negative focal length lens configuration, which are separated by the sum of their focal lengths; or a pair of positive focal length lens configurations, which are separated by the sum of their focal lengths. A metrology device as claimed in claim 1, wherein the beam reducer is configured to reduce a lateral size of an impinging light by at least five times. A measuring device as claimed in claim 1, wherein the opening comprises a circular opening, an elliptical opening, a polygonal opening or a slit opening. A metrology device as claimed in claim 1, wherein the detection device is positioned outside a chamber of an extreme ultraviolet (EUV) light source, the diagnostic region is inside the chamber, and the detection device receives light from the chamber through an optical window in a wall of the chamber. A metrological device as claimed in claim 18, wherein a distance between the diagnostic region and the optical window is about 200 to 500 times the size of a distance between the diagnostic region and the target space. A metrological device as claimed in claim 1, wherein the detection device comprises a focusing lens at an output end of the orifice, the focusing lens being configured to focus the sensed light onto the light sensor. A metrological device as claimed in claim 1, wherein the orifice has a range of at least 2 millimeters (mm). A metrological apparatus as claimed in claim 1, wherein the aperture is positioned to receive the diagnostic light at an orientation where the diagnostic light is collimated or non-convergent and non-divergent. A metrology method, comprising: causing a diagnostic probe to interact with a current target at a diagnostic region before the current target enters a target space; collecting diagnostic light generated by the interaction between the diagnostic probe and the current target at the diagnostic region, the collecting also including collecting non-diagnostic light generated by the target space; collimating the diagnostic light and the non-diagnostic light; angularly separating the diagnostic light and the non-diagnostic light from each other The method comprises reducing the lateral range of the diagnostic light and the non-diagnostic light; sensing the diagnostic light at a sensing area laterally displaced from a non-sensing area through which the non-diagnostic light passes after the diagnostic light and the non-diagnostic light have been separated in angle; and estimating a property of the current target based on the sensed diagnostic light, wherein reducing the lateral range of the diagnostic light and the non-diagnostic light includes maintaining a collimated state of the diagnostic light and the non-diagnostic light. The metrology method of claim 23, wherein: causing the diagnostic probe to interact with the current target at the diagnostic region includes causing one or more diagnostic beams to interact with the current target at the diagnostic region; and collecting diagnostic light includes collecting one or more diagnostic beams at the diagnostic region that have been reflected from the current target, scattered from the current target, or blocked by the current target. The metrology method of claim 23, further comprising filtering the diagnostic light based on one or more of its spectral properties and its polarization state. The metrology method of claim 23, wherein the diagnostic area is inside a hermetically sealed chamber of an extreme ultraviolet (EUV) light source, and also includes collecting non-diagnostic light, wherein collecting the diagnostic light includes receiving the diagnostic light including the non-diagnostic light transmitted through an optical window in a wall of the chamber. The method of measurement as claimed in claim 23, wherein reducing the lateral range of the diagnostic light and the non-diagnostic light comprises reducing the lateral range of the diagnostic light and the non-diagnostic light by at least five times. The metrology method of claim 23 further comprises blocking the non-diagnostic light at the non-sensing area or redirecting the non-diagnostic light. The metrology method of claim 23, wherein the lateral range of reducing the diagnostic light and the non-diagnostic light includes one or more of the following: refracting the light and reflecting the light. The metrology method of claim 23 further comprises focusing the diagnostic light at the sensing area. The metrology method of claim 23 further comprises passing the diagnostic light through an aperture of an optical mask having a range greater than the range of the diagnostic light after the diagnostic light and the non-diagnostic light are angularly separated from each other and before the diagnostic light is sensed.