JPH07321005A - Projection exposure device - Google Patents

Abstract

(57) [Summary] [Object] In a projection exposure apparatus that requires highly accurate imaging characteristics, isotropic image distortion correction is performed without moving the reticle and without affecting other aberrations. In a projection exposure apparatus that image-projects a pattern image of a reticle R onto a wafer W via a bilateral telecentric projection optical system PL, a gas around the reticle R and an optical system PL is provided. An optical path length between the projection optical system PL and the reticle R is formed by forming a gas layer 33 having a different refractive index and being transparent to illumination light and having a variable thickness, and controlling the thickness of the gas layer 33. Is corrected to correct the isotropic image distortion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection exposure apparatus used, for example, in manufacturing a semiconductor integrated circuit, a liquid crystal device or the like in a photolithography process, and more particularly to image forming characteristics such as distortion of a projection image by a projection optical system. The present invention relates to a projection exposure apparatus having a mechanism for correcting

[0002]

2. Description of the Related Art Conventionally, in this type of projection exposure apparatus, a fine pattern of a reticle (or a photomask, etc.) is projected with high resolution onto a wafer (or a glass plate, etc.) coated with a photoresist. In order to project a reticle pattern onto a pattern already formed on a wafer with high overlay accuracy, it is required to always maintain the imaging characteristics of the projection image by the projection optical system with high accuracy. In this case, the ambient pressure around the projection optical system, environmental changes such as temperature, shape changes due to absorption of illumination light of the reticle or projection optical system, switching of the reticle illumination method, or when using a so-called phase shift mask, etc. The image forming characteristics gradually change due to changes in the pattern on the reticle. Here, switching of the reticle illumination method means switching from a normal illumination method to, for example, an annular illumination method or a modified light source method.

Therefore, conventionally, the amount of these environmental changes and the like are measured, the amount of change in the imaging characteristics is predicted from the measurement result, and the imaging characteristics are corrected so as to cancel the predicted amount of change. Was doing. Further, there are mainly two types of conventional correction targets of the image forming characteristics, that is, the defocus of the projected image and the projection magnification. In order to correct these, for example, regarding defocusing, a target value of the focus position is corrected in a mechanism (autofocus mechanism) that keeps the distance between the projection optical system and the wafer constant. Regarding the correction of the projection magnification, there has been proposed a method of sealing between the lenses inside the projection optical system to change the internal pressure, or a method of moving a part of the lenses of the projection optical system in the optical axis direction. There is.

With respect to this, in recent years, defocusing has occurred as the patterns of semiconductor integrated circuits have become finer and finer.
Not only the projection magnification but also the change of isotropic image distortion (so-called pincushion type or barrel type distortion) cannot be ignored.
As means for correcting the isotropic image distortion, a mechanism for moving the reticle in the optical axis direction of the projection optical system, a mechanism for moving some lenses of the projection optical system in the optical axis direction, an exposure light source (laser) A mechanism for changing the emission wavelength of a light source or the like, or a mechanism for sealing the lenses inside the projection optical system to change the internal pressure has been proposed.

The conventional means for correcting isotropic image distortion as described above has the following disadvantages. First, isotropic image distortion is different from projection magnification and is a high-order aberration. Therefore, of the correction means, a mechanism for moving a part of the lens of the projection optical system in the optical axis direction and an exposure light source If correction is performed using a mechanism that changes the laser wavelength or a mechanism that changes the pressure between predetermined lenses inside the projection optical system, there is the inconvenience that other aberrations change. In this case, if the newly generated aberration is corrected by another mechanism, the entire correction mechanism becomes complicated. Further, even if an attempt is made to correct the isotropic image distortion by setting another aberration change within the allowable range, the amount that can be corrected becomes small, and a desired correction amount cannot be obtained.

[0006]

As described above, in order to correct the isotropic image distortion, the method of moving the reticle in the optical axis direction can be said to be a simple method. However, recently, the demand for exposing a wider field area while maintaining the imaging characteristics has increased, and in order to meet this, the scan type in which the reticle and the wafer are scanned relative to the projection optical system to be exposed. An exposure apparatus (step-and-scan exposure apparatus) has been proposed. With this method, the maximum diameter of the effective exposure field of the projection optical system can be used by illuminating the reticle in a slit shape, and by scanning, the exposure field can be expanded without being restricted by the optical system in the scanning direction. There are advantages. Further, since only a part of the projection optical system is used, there is an advantage that accuracy such as illuminance uniformity and distortion can be easily obtained. However, in this scanning type exposure apparatus, the reticle and the wafer must be scanned in high precision in synchronization with each other, and therefore the reticle stage is required to have high rigidity. In order to increase the rigidity of the stage on the reticle side, it is desirable that there is no mechanism for moving the reticle in the optical axis direction. Even in a batch exposure type device such as a stepper, it is desirable that the reticle side stage has high rigidity.

In view of the above problems, an object of the present invention is to provide a projection exposure apparatus which can correct isotropic image distortion without adversely affecting other aberrations and without moving the reticle.

[0008]

A projection exposure apparatus according to the present invention illuminates a mask (R) on which a transfer pattern is formed as shown in FIG. 1 with exposure light, and under the exposure light. In a projection exposure apparatus that image-projects a pattern of the mask (R) onto a photosensitive substrate (W) through a projection optical system (PL) that is telecentric on both sides, the mask (R) and the projection optical system (PL) A gas layer forming means for forming a gas layer (33) having a refractive index different from that of the gas around the projection optical system (PL) and having a variable thickness with respect to the exposure light, in the space between them ( 12, 13) and its projection optical system P
Image distortion detecting means (28, 29, 30, 31, 32) for predicting or measuring the distortion of the projected image due to L, and the gas layer forming means (12, 13) according to the distortion detected by the image distortion detecting means. ) Via the control means (P, 14-17) for controlling the thickness of the gas layer (33).

In this case, one surface of the gas layer (33) is
The surface of the lens on the mask (R) side of the projection optical system (PL) or the lower surface of the mask (R) is desirable. Further, the gas layer forming means (12, 13) causes the gas layer (3) to move due to the pressure of the gas in the gas layer (33).
It is desirable to control the thickness of 3). Further, for example, as shown in FIG. 8, gas layer forming means (12A, 12B, 1
3A and 13B) have two thicknesses in which a gas having a larger refractive index than the gas around the projection optical system (PL) and a gas having a smaller refractive index than the gas around the projection optical system (PL) are enclosed. Independently variable gas layer (33A, 33A
B) may be formed.

[0010]

According to the projection exposure apparatus of the present invention, a gas layer (33) made of a gas having a refractive index different from that of the gas in the environment of the apparatus.
The optical path length between the mask (R) and the projection optical system (PL) can be changed by changing the thickness of the. In the present invention, when at least the mask side of the projection optical system (PL) is telecentric, by changing the optical path length between the mask (R) and the projection optical system (PL), other aberrations are not adversely affected. , Utilizing the ability to correct isotropic image distortion. This is because the chief ray on the mask side is slightly inclined depending on the position (image height) due to the aberration of the pupil of the projection optical system. Therefore, by changing the thickness of the gas layer (33), isotropic image distortion (so-called pincushion type or barrel type distortion) can be changed almost independently from other aberrations. In addition, since the mask (R) and the lens forming the projection optical system (PL) do not have to be physically driven, even when applied to a scan type exposure apparatus, the rigidity of the apparatus does not decrease. . Furthermore,
By forming the gas layer (33) asymmetrically with respect to the optical axis of the projection optical system (PL), anisotropic image distortion can also be corrected.

Further, one surface of the gas layer (33) is provided on the surface of the lens (18) on the mask (R) side of the projection optical system (PL),
Alternatively, when the lower surface of the mask (R) is used, the device space is small and the structure is simple as compared with one having an independent optical container in which the gas layer is sealed. Further, it is possible to reduce the optical influence due to the fluctuation of the environmental gas interposed between the mask (R) and the projection optical system (PL).

Further, the thickness of the gas layer (33) is set to the gas layer (3
In the case of controlling by the pressure of the gas in 3), the parallel plane plate (12), for example, which is one surface of the wall that seals the gas layer (33), can be easily formed by the uniform surface pressure of the gas applied to the surface. Moved in parallel. Further, the structure is simple as compared with the mechanical driving method. Further, the gas layer forming means,
A gas having a variable refractive index and a gas having a larger refractive index than the gas around the projection optical system (PL) and a gas having a smaller refractive index than the gas around the projection optical system (PL). When forming the layers (33A, 33B),
The image distortion can be corrected even when the image distortion cannot be sufficiently corrected with only one gas layer. That is, by providing the two gas layers (33A, 33B) having different refractive indexes, the change width of the optical path length between the mask (R) and the projection optical system (PL) becomes larger.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the projection exposure apparatus according to the present invention will be described below with reference to FIGS. The present invention can be applied to both a batch exposure type (step and repeat system etc.) and a scan exposure type (step and scan system etc.). Hereinafter, a case where the present invention is applied to a scan exposure type in which the effects of the present invention can be more exerted will be described.
However, the application to the batch exposure type is almost the same. In addition,
In FIG. 1, the Z axis is taken parallel to the optical axis IX of the projection optical system PL, the X axis is parallel to the paper surface of FIG. 1 in the plane perpendicular to the optical axis IX, and the Y axis is perpendicular to the paper surface of FIG. I take the.

FIG. 1 schematically shows a projection exposure apparatus according to an embodiment of the present invention. In FIG. 1, the light source 1 is an excimer laser light source such as a KrF excimer laser or an ArF excimer laser, a copper vapor laser or a YAG. A laser high-wave generator, an ultra-high pressure mercury lamp, or the like is used. When the light source 1 is an ultra-high pressure mercury lamp, the light source 1 emits illumination light IL composed of ultraviolet bright lines (g line, i line, etc.). The illumination light IL is incident on the illuminance homogenizing optical system 2 including a collimator lens, a fly-eye lens, and the like, converted into a luminous flux having an almost uniform illuminance distribution, and then guided to a turret 3 for switching illumination conditions.

FIG. 2 is a front view of the turret 3 of FIG. 1. In this turret 3, diaphragms 41 to 44 are arranged at 90 ° intervals, and the turret 3 is rotated to form diaphragms 41 to 44. It is possible to change the light intensity distribution on the Fourier transform surface (pupil surface) of the projection optical system PL by switching between. This method is one of the techniques for improving the resolution of the projection optical system, and the optimum one is selected from these diaphragms 41 to 44 depending on the pattern to be exposed.

In the example of FIG. 2, the circular diaphragm 41 is a normal aperture diaphragm (σ diaphragm), and the ring-shaped diaphragm 42 is a diaphragm for performing ring-shaped illumination. Further, the small circular diaphragm 43 is for narrowing the angle of the light beam, and has a small coherence factor (σ value) in a normal illumination system (for example, σ
The value is about 0.1 to 0.4). A diaphragm 44 composed of four eccentric circles (or having a cross-shaped light-shielding portion) is a diaphragm for a plurality of inclined illuminations (deformed light sources) and is generally used for exposing a line and space pattern with high resolution. It is something. The turret 3 is used while being sequentially changed to the optimum one according to the pattern of the reticle R.

The illumination light IL having passed through the predetermined stop of the turret 3 reaches the beam splitter 4 having photoelectric sensors 31 and 32 on both sides thereof. Beam splitter 4
Means that almost all the light fluxes are transmitted but a part of the light fluxes is reflected. The light flux coming from the turret 3 side and reflected by the beam splitter 4 enters the photoelectric sensor 32 and comes from the wafer W side. The light reflected by the beam splitter 4 enters the photoelectric sensor 31. The detection signals of the photoelectric sensors 31 and 32 are used to calculate the aberration change of the projection optical system PL as described later. The illumination light IL further passes through an illumination optical system 5 including a relay lens, a field stop, a collimator lens, etc., and is reflected by a dichroic mirror 6.
A reticle R on which a semiconductor circuit pattern or the like is drawn is illuminated. The reticle R is vacuum-sucked on the reticle stage RST, and the reticle stage RST is a plane perpendicular to the optical axis (which coincides with the optical axis of the projection optical system PL) IX bent by the dichroic mirror 6 of the illumination optical system 5. The reticle R is positioned by finely moving two-dimensionally in the (XY plane).

Further, the reticle stage RST is provided with a reticle drive section (not shown) composed of a linear motor or the like.
It is possible to move at a predetermined scanning speed in the X direction (scanning direction). Reticle stage RST has a moving stroke that allows the entire surface of reticle R to cross at least the optical axis IL of the illumination optical system. A movable mirror 8 that reflects the laser beam from the interferometer 9 is fixed to the end of the reticle stage RST.
The position of T in the scanning direction is set by the interferometer 9 to, for example, 0.0.
It is constantly detected with a resolution of about 1 μm. Position information of the reticle stage RST from the interferometer 9 is sent to the stage control system 30A, and the stage control system 30A drives the reticle stage RST via a reticle drive unit (not shown) based on the position information of the reticle stage RST. To do.
Since the initial position of the reticle stage RST is determined so that the reticle R is accurately positioned at a predetermined reference position by a reticle alignment system (not shown), the position of the movable mirror 8 can be simply measured by the interferometer 9. The position of the reticle R is measured with sufficiently high accuracy.

Now, the illumination light IL which has passed through the reticle R
Enters the projection optical system PL that is telecentric on both sides,
The projection optical system PL forms a projection image obtained by reducing the circuit pattern of the reticle R with a reduction magnification β (for example, ⅕ or ¼) on the wafer W whose surface is coated with a photoresist (photosensitive material). . The optical axis I is provided near the pupil plane (Fourier transform plane for the reticle R) of the projection optical system PL.
An optical filter for blocking a light beam near X, that is, a central light blocking type pupil filter PF is detachably installed. The pupil filter PF improves resolution and depth of focus particularly when exposing a contact hole pattern. The main control system 30 controls the attachment / detachment of the pupil filter PF via the attachment / detachment device 30B. Further, the projection optical system PL of this embodiment is equipped with mechanisms (12 to 25, P) for correcting the image forming characteristics, which will be described later.

FIG. 3 shows the reticle R and the wafer W of FIG.
It is a perspective view showing a scanning state. Projection exposure apparatus of this embodiment
The scanning direction of the reticle R as shown in FIG.
Set the longitudinal direction in the direction (Y direction) perpendicular to (X direction).
Reticle with the rectangular (slit-shaped) illumination area IAR
The reticle R is illuminated, and the reticle R is exposed in the −X direction (or
V speed in + X direction)_R Scanned in. Illumination area IA
The pattern in R (center is almost coincident with optical axis IX) is projected
It is projected on the wafer W through the optical system PL and has a slit shape.
A projection area IA is formed. Wafer W is reticle R
Are in an inverted image formation relationship, the wafer W has a velocity V_R Direction
To the reticle R in the + X direction (or -X direction) opposite to
Synchronous, speed V_W Scan on the wafer W
The pattern of the reticle R is sequentially exposed on the entire surface of the area SA.
To be done. Scanning speed ratio (V_W / V _R ) Is the projection optical system PL
Is exactly the same as the reduction ratio β of.
The pattern of the pattern area PA of the circle R is on the wafer W.
Accurate reduction transfer is performed on the print area SA. Illumination area I
The width in the longitudinal direction of AR is the pattern area P on the reticle R.
It is wider than A and narrower than the maximum width of the light shielding area ST.
Is set as follows, and scanning the reticle R
The entire turn area PA is illuminated.

Returning to the explanation of FIG. 1, the wafer W is vacuum-sucked on the wafer holder 7, and the wafer holder 7 is held on the wafer stage WST. The wafer holder 7 can be tilted in an arbitrary direction with respect to the best image plane of the projection optical system PL by a driving unit (not shown) and can be finely moved in the optical axis IX direction (Z direction). Further, the wafer holder 7 can also rotate about the optical axis IX. On the other hand, the wafer stage W
ST is a direction perpendicular to the scan direction (Y direction) so that the ST can be moved not only in the scan direction (X direction) described above but also to any shot area in a plurality of shot areas at any time.
And a step of repeating the operation of scanning and exposing each shot area on the wafer W and the operation of moving to the exposure start position of the next shot area.
And-scan operation is performed. A wafer stage drive unit (not shown) such as a motor moves the wafer stage WST to X and Y.
Drive in the direction. A movable mirror 10 that reflects the laser beam from the interferometer 11 is fixed to the end of the wafer stage WST, and the position of the wafer stage WST in the XY plane is constantly fixed by the interferometer 11 at a resolution of, for example, about 0.01 μm. It has been detected. The position information (or speed information) of wafer stage WST is sent to stage control system 30A, and stage control system 30A controls the wafer stage drive unit based on this position information (or speed information).

Further, in the apparatus of FIG. 1, an irradiation optical system 26 for supplying an image forming light beam for forming a pinhole image or a slit image toward the exposure surface of the wafer W in an oblique direction with respect to the optical axis IX. And a light receiving optical system 27 that receives the reflected light flux of the image-forming light flux on the exposed surface of the wafer W through the slit.
An oblique incidence type wafer position detection system (focus position detection system) consisting of and is fixed to a support portion (not shown) that supports the projection optical system PL. A more detailed structure of this wafer position detection system is disclosed in, for example, Japanese Patent Laid-Open No. 60-168112. The wafer position detection system detects the positional deviation of the exposure surface of the wafer in the Z direction from the best image plane of the projection optical system PL, and moves the wafer holder 7 in the Z direction so that the wafer W and the projection optical system PL maintain a predetermined distance. Used to drive to. Wafer position information from the wafer position detection system is sent to the stage control system 30A via the main control system 30. The stage control system 30A drives the wafer holder 7 in the Z direction based on this wafer position information.

In the present embodiment, a parallel flat glass plate (not shown) provided in advance inside the light receiving optical system 27 so that the best image forming plane (image forming plane) of the projection optical system PL becomes the zero point reference. (Parallel) angle is adjusted and the wafer position detection system is calibrated. Further, for example, a horizontal position detection system as disclosed in Japanese Patent Laid-Open No. 58-113706, which irradiates a parallel light flux on the surface to be detected and detects the lateral shift amount of the condensing point of the reflected light,
Alternatively, the wafer W is configured by configuring the wafer position detection system so as to detect the focal positions at arbitrary plural positions in the image field of the projection optical system PL (for example, projecting plural slit images in the image field). The inclination of the upper predetermined area with respect to the image plane may be detected. In this case, leveling is performed by adjusting the inclination angle of the wafer holder 7.

Further, a photoelectric sensor 28 is installed on the wafer stage WST, and an environment sensor 29 for measuring atmospheric pressure, temperature, humidity and the like is provided near the projection optical system PL, and each detection signal is sent. It is used to calculate the change in the imaging characteristics of the projection optical system PL. Details will be described later.
Next, the image forming characteristic correction mechanism including the isotropic image distortion correction mechanism in the present embodiment will be described. As described above, the types of imaging characteristics to be corrected tend to increase as the exposure line width decreases. Therefore, also in this embodiment,
(A) In addition to isotropic image distortion (hereinafter also referred to as "distortion"), (b) projection magnification, (c) defocus,
And (d) an example of correcting four types of field curvature will be described.

First, the distortion (a) is caused by the gas layer 3 provided in the space between the reticle R and the projection optical system PL.
Correction is made by controlling the thickness of No. 3. Here, the mechanism for changing the thickness of the gas layer 33 will be specifically described with reference to FIG. The gas layer 33 has a glass-made parallel flat plate 12 around the periphery thereof and an expandable and contractible bellows-shaped tube 13.
And a surface 18a of the uppermost lens 18 of the projection optical system PL, which is an almost enclosed space. Gas layer 3
In order to change the thickness of No. 3, the pressure difference between the external air pressure and the gas pressure in the gas layer 33 is used in this embodiment.
Since the plane-parallel plate 12 needs to be kept parallel to the reticle R, a gas pressure method for supporting the plane-parallel plate 12 with a uniform pressure is adopted. In this method, the position of the plane parallel plate 12 in the Z direction is detected by the linear encoder 14, and the plane parallel plate 1 is detected.
The pressure controller 16 controls the pressure inside the gas layer 33 via the pipe system P so that 2 reaches the target value. Since the required pressure of the gas layer 33 only needs to support the weight of the plane parallel plate 12, the pressure difference with the external environmental gas is very small. However, since the refractive index of the gas layer 33 also changes depending on the pressure of the internal gas, the pressure sensor 15 that measures the internal pressure is provided in the pipe system P, and the detection result of the pressure sensor 15 is used for calculating the refractive index.

Further, the gas tank 17 is circulated with the gas layer 33 through the pipe system P, and the tank 17 compensates for the loss of gas due to leakage or the like. However, the leak of gas having a different refractive index causes fluctuations in the refractive index of the surrounding air, which adversely affects the image formation and the like, so the gas layer 33 must be tightly sealed. The conditions of the gas enclosed in the gas layer 33 are stable (inert) and inexpensive.
Usually, the difference in the refractive index from air is as large as possible, the transmittance of ultraviolet rays, which is the wavelength band of exposure light, is high, and no photochemical reaction or the like occurs. As a typical gas having a smaller refractive index than air, helium (H
e), neon (Ne), chlorofluorocarbon (F ₂ ), hydrogen (H ₂ ), etc., and benzene (C ₆ H ₆ ) and carbon dioxide are typical gases having a higher refractive index than air. (CO ₂ ), krypton (Kr), xenon (Xe), and the like.

As a method of changing the thickness of the gas layer 33, a motor or the like may be used as long as the parallelism of the plane parallel plate 12 to the reticle R can be maintained. in this case,
The pressure of the gas layer 33 may be the same as that of the external air, and a configuration in which no force is exerted on the piping system P or the like can be adopted. Next, the principle of controlling the isometric image distortion by the thickness of the gas layer 33 will be described in detail with reference to FIGS.

FIG. 4 is a diagram for schematically explaining the principle of correction of isotropic image distortion in this example. FIG. 5 is a diagram showing an example of the correction result of isotropic image distortion in this example, and FIG. 6 is a diagram showing an example of actual image distortion. The projection optical system PL is a telecentric optical system on both sides, and originally the principal rays traveling from the reticle R to the projection optical system PL should all be parallel to the optical axis IX, but in reality they have a slight inclination due to aberration that cannot be eliminated. Normally, the chief ray is adjusted so as to be substantially parallel to the optical axis IX at the center and the periphery of the exposure field, so that the intermediate chief ray is inclined with respect to the optical axis IX as shown in FIG. Therefore, as shown in FIGS. 4A and 4B, the thickness of the gas layer 33 on the optical axis IX is changed from T ₁ to T.
_If changed to ₂ , the position of the chief ray incident on the uppermost lens 18 of the projection optical system PL is left and right only in the intermediate image height (Y direction).
Shift to. Therefore, the thickness of the air layer 33 is T ₁ and T
_{If the} distortion is adjusted to be approximately 0 in the intermediate state between ₂ and ₂ , the distortion curve can be arbitrarily changed to curves a and b as shown in FIG. The vertical axis in FIG. 5 represents the image height (h), which represents the distance from the center of the exposure field as shown in FIG. 4A, and the horizontal axis in FIG. 5 represents the distortion amount (ΔY).
Normally, only the distortion does not change, but the magnification component also changes, so the distortion curve changes as shown by the curve c in FIG. However, in the present embodiment, the magnification component and the distortion component are corrected by independent correction mechanisms, so that good correction can be performed.

Next, other correction items ((b) projection magnification,
The correction mechanism for (C) defocus and (D) field curvature will be briefly described. Projection magnification of (b) and (d)
For the correction of the curvature of field of, the uppermost lens 18 and the next lens 20 among the lenses constituting the projection optical system PL.
In this embodiment, a method of driving the optical axis in the optical axis direction is adopted.
In FIG. 1, the uppermost lens 18 is fixed to a holder 19, and the lens 20 is fixed to a holder 21. The holder 19 and the holder 21 are connected via a drive element 23 that is expandable and contractible. For example, a piezo element is used as the driving element 23, and about 2 to 4 driving elements 23 are arranged on the circumference. The holder 21 is connected to the main body of the projection optical system PL via a drive element 24. The drive controller 25 drives the drive elements 23 and 24 in response to a command from the main control system 30. Usually, drive elements 23, 24
The amount of expansion and contraction of is feedback-controlled by a position sensor (not shown). The projection magnification and the field curvature are changed by the movement of the uppermost lens 18 and the next lens 20 in the optical axis direction. When desired characteristics of the projection magnification and the curvature of field are desired, the driving amount of each of the uppermost lens 18 and the lens 20 is calculated by solving these simultaneous binary equations. Regarding the correction of defocus in (c), the angle of the parallel plate glass in the light receiving optical system 27 may be adjusted to align the wafer W at a desired position.

As described above, the distortion correction of (a) depends on the thickness of the gas layer 33, and the projection magnification of (b) and the field curvature of (d) are corrected by driving the uppermost lens 18 and the next lens 20. Therefore, and (c) defocus correction can be performed by offset adjustment of the light receiving optical system 27. If the defocus caused by correcting (a), (b), and (d) is also corrected by the uppermost lens 18 and the lens 20 in (d), all can be corrected.
Various correction methods (b) to (d) have been devised in addition to this embodiment, and any method may be used, or may not be used if not necessary. As another method, there is a method of changing the air pressure inside a predetermined lens interval of the projection optical system PL, a method of changing the wavelength of the light source 1, or the like.

Next, how to determine the target value for the above-mentioned correction means, that is, the means for detecting the change amount of the correction target will be described. Since the detection means for each correction target are almost the same, the distortion of (a) will be described as an example. Distortion typically changes due to (e) changes in atmospheric pressure, (f) changes in illumination conditions, (g) absorption of illumination light in the projection optical system, and (h) absorption of illumination light in the reticle. In addition to this, when a plurality of exposure apparatuses are mixed and used, the distortion of the exposure apparatus to be used may be changed so as to match the distortion of the exposure apparatus used for exposing the front layer of the wafer.

First, with respect to the change in atmospheric pressure (e), the change in atmospheric pressure is measured by the environment sensor 29, and the measurement result is sent to the main control system 30. Usually, the change in atmospheric pressure and the change in distortion are in a proportional relationship, and therefore the amount of change in distortion can be calculated from the change in atmospheric pressure from the proportional constant obtained in advance by optical simulation, experiment, or the like. In addition to this, with respect to temperature, humidity, etc., the amount of change in distortion can be calculated in the same manner.

Next, regarding the change of the illumination condition of (f), since the optical path of the light beam from the reticle R inside the projection optical system PL differs depending on the illumination condition, it is affected by the aberration remaining in the projection optical system PL. This causes distortion. On the other hand, since the lighting condition can be known by notifying the main control system 30 of the position of the turret 3,
This is also obtained from the amount of change in distortion that has been obtained in advance by experiments or the like. Other conditions for changing the optical path inside the projection optical system PL include the fineness of the pattern of the reticle R, the difference in the angle of the diffracted light depending on the presence or absence of a phase shifter, or the diaphragm (NA diaphragm) of the pupil plane of the projection optical system PL. ) Or the presence or absence of the pupil filter PF on the pupil plane.
Regarding these, similarly, the relationship with the distortion change may be obtained in advance. As another method, a method of directly measuring the light intensity distribution on the pupil plane of the projection optical system PL can be considered. This is a method in which the relationship between the pupil surface light intensity distribution and the distortion change is obtained in advance and the distortion is obtained by comparing the obtained relation with the actual measurement value. As a method of measuring the light intensity distribution on the pupil plane, a method of inserting a sensor into the pupil plane, a method of opening and closing the diaphragm of the pupil plane while measuring the light quantity with the sensor on the image plane, and the like can be considered.

Next, when correcting the illumination light absorption of the projection optical system of (g), the photoelectric sensor 28 on the wafer stage WST determines the transmittance of the reticle R, and the photoelectric sensor 32 determines the light intensity of the light source 1. Then, the amount of light energy incident on the projection optical system PL is obtained. Further, the optical energy reflected from the wafer W and incident on the projection optical system PL again can be measured by the photoelectric sensor 31. Then, if the change characteristic between the incident light energy and the distortion is also obtained in advance by experiments and stored in the form of a differential equation or the like, the amount of distortion due to the absorption of the illumination light can be obtained by calculation.

Next, regarding the illumination light absorption of the reticle in (H), the transmittance of the reticle R, that is, the pattern density of the reticle R can be obtained by the photoelectric sensor 28 on the wafer stage WST, as in (G). , Photoelectric sensor 32
The intensity of light incident on the reticle R can be obtained. Reticle R
Since the illumination light absorption occurs in the pattern portion, not in the transmission portion, if the pattern density and the light absorption rate of the pattern are known, the amount of heat absorbed by the reticle R can be obtained. The light absorptance of the pattern is determined by the material of the pattern, so it may be input in advance. Further, similarly to (g), the change characteristic of the distortion with respect to the absorbed light energy may be obtained in advance by experiments or the like and stored in the form of a differential equation or the like. As described above, the distortion amount generated by (e), (f), (t), and (h) can be obtained. Therefore, the amount of distortion that must be corrected can be obtained by the sum of (e) to (h).

In the above method, the factors that change the distortion are measured and the amount of distortion change is calculated, but a method of directly measuring the distortion is also conceivable. This will be described with reference to FIG. Figure 7
Shows a mark on the reticle R when measuring the image distortion. In FIG. 7, a plurality of position measuring marks MK are drawn outside the pattern area PA of the reticle R in order to directly measure the distortion. Only the mark MK is illuminated in the illumination area IAR, and the position of the image of the mark is measured and obtained by the photoelectric sensor provided on wafer stage WST. As the photoelectric sensor on wafer stage WST, a two-dimensional or one-dimensional image pickup device such as CCD can be used. In this case, the image pickup device measures the position of the image of mark MK by image processing. Also, as its photoelectric sensor,
There is also known a method of using a slit and a light receiving element that receives an image of the mark MK through the slit, and obtaining the relative position between the position of the slit and the position of the mark from the signal of the light receiving element. These methods are more effective when used in combination with the above-mentioned method of calculation, because the methods may take a long time for measurement and cannot be performed frequently.

Since the amount of change in distortion can be obtained from the above, the thickness of the gas layer 33 may be changed so as to cancel it. Next, another embodiment and yet another embodiment of the present invention will be described with reference to FIGS. 8 and 9, respectively. FIG. 8 shows a gas layer formed in another embodiment. In FIG. 8, a transparent plane-parallel plate 12B is attached to the surface 18a of the uppermost lens 18 via a telescopic tube 13B, and a plane-parallel plate 12B is formed. Cylinder 13A that can expand and contract above
The parallel flat plate 12A is attached via. This allows
Lens surface 18a, barrel 13B, and plane-parallel plate 12B
Gas layer 33B surrounded by, parallel plane plate 12B, and cylinder 13
A and a gas layer 33A surrounded by the plane parallel plate 12A are formed, a gas having a higher refractive index than the surrounding air is supplied to the gas layer 33A, and a gas having a lower refractive index is supplied to the gas layer 33B.
(The positions of 33A and 33B may be reversed). The formation position of the gas layer 33B is the same as the formation position of the gas layer 33 of FIG. As in FIG. 1, gas layers 33A and 33A
The pressure of B is controlled by the respective pressure controllers 16A and 16B via the piping systems PU and PD, respectively. When the distortion cannot be sufficiently corrected with only one gas layer, the gas layer forming means of the present embodiment can be used to obtain a large variation width of the optical path length between the reticle R and the projection optical system PL. Therefore, the distortion can be corrected sufficiently.

FIG. 9 shows a gas layer of still another embodiment. In this FIG. 9, a transparent plane-parallel plate 12C is attached to the reticle stage RST side through a telescopic tube 13C, and the lower surface of the reticle R, the tube. 13C and parallel plane plate 12
A gas layer 33C surrounded by C or the like is provided. The pressure of the gas layer 33C is the same as in the above-described embodiment, that is, the piping system P
Is controlled by the pressure controller 16C. According to the present embodiment, since the gas layer 33C is on the side of the reticle stage RST to be scanned, the end of the reticle stage RST does not come into contact with the gas layer during scanning as in the example of FIG. Further, there is an advantage that the gas layer 33C does not get in the way when it is desired to allow the light flux 92 for alignment or the like to enter from the uppermost lens 18 of the projection optical system PL via the mirror 91 of FIG.

Further, in all of the above-mentioned embodiments, the uppermost lens 18 or the reticle R is used as one wall surface substantially perpendicular to the optical axis IX in order to seal the gas layer, and the other surface is a plane parallel plate. However, two parallel plane plates may be used as the opposite wall surfaces that are substantially perpendicular to the optical axis IX. It is also possible to use a lens having a curvature instead of the plane-parallel plate. Further, the gas layer may be made asymmetric with respect to the optical axis IX (for example, the plane parallel plate is inclined), whereby anisotropic image distortion can be corrected.

In each of the above embodiments, the image distortion of the projected image is obtained by calculation (simulation) using, for example, a model function, but various measuring methods conventionally proposed (for example, slitting on a wafer stage). Also, a light receiving element may be provided to detect an actual image, and the image distortion may be measured by employing a measuring method such as detecting the image distortion based on the detection result. As described above, the present invention is not limited to the above-described embodiments, and various configurations can be adopted without departing from the gist of the present invention.

[0041]

According to the projection exposure apparatus of the present invention, a gas layer is formed between the mask and the projection optical system, and a mechanism for easily and efficiently controlling the thickness of the gas layer is provided. ,
With a simple mechanism, the optical path length between the projection optical system and the mask
It can be changed efficiently without moving the mask. Therefore, there is an advantage that isotropic image distortion can be corrected without lowering the rigidity of the stage that holds the mask. Further, other aberration correction can be easily performed.

Further, when the mask side lens or the lower surface of the mask is used as one surface forming the hermetically sealed space enclosing the gas layer, the projection exposure apparatus of the present invention can be constructed simply and efficiently in space. It is also possible to eliminate the optical influence due to the fluctuation of the environmental gas interposed between the mask and the projection optical system. Furthermore, when the thickness of the gas layer is controlled by the pressure of the gas in the gas layer, the thickness of the gas layer can be controlled by a simple mechanism, and at the same time, the parallel flat plates can be easily kept parallel. There are advantages.

Furthermore, when two gas layers having different refractive indexes are used, the change width of the optical path length between the mask and the projection optical system can be widened, and therefore the distortion correction range can be widened. There is an advantage that you can.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an outline of a projection exposure apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a turret 3 for switching illumination conditions of FIG.

3 is a perspective view showing a scanning state of a reticle R and a wafer W in the projection exposure apparatus of FIG.

FIG. 4 is an explanatory diagram for schematically explaining the principle of correction of isotropic image distortion in the present invention.

FIG. 5 is a diagram showing a correction result of isotropic image distortion in the present invention.

FIG. 6 is a diagram showing the actual occurrence of image distortion.

FIG. 7 is a diagram illustrating marks on a mask for measuring image distortion.

FIG. 8 is a view showing another embodiment of the gas layer forming means in FIG.

9 is a view showing still another embodiment of the gas layer forming means in FIG.

[Explanation of symbols]

 R Reticle PL Projection optical system W Wafer IL Illumination light 33 Gas layer 12 Parallel plane plate 13 Tube 16 Pressure controller 17 Tank 30 Main control system 29 Environment sensor 28, 31, 32 Photoelectric sensor

Claims (4)

[Claims] 1. A mask on which a transfer pattern is formed is illuminated with exposure light, and the pattern of the mask is image-projected under the exposure light onto a photosensitive substrate through a projection optical system that is telecentric on both sides. In the projection exposure apparatus, in the space between the mask and the projection optical system, a variable-thickness gas layer having a refractive index different from that of the gas around the projection optical system and transparent to the exposure light is provided. A gas layer forming means for forming, an image distortion detecting means for predicting or measuring the distortion of the projected image by the projection optical system, and the gas layer forming means for the distortion according to the distortion detected by the image distortion detecting means. A projection exposure apparatus comprising: a control unit that controls the thickness of the gas layer. 2. The projection exposure apparatus according to claim 1, wherein one surface of the gas layer is a surface of a lens on the mask side of the projection optical system or a lower surface of the mask. 3. The projection exposure apparatus according to claim 1, wherein the gas layer forming means controls the thickness of the gas layer by the pressure of the gas in the gas layer. 4. The gas layer forming means has two thicknesses in which a gas having a refractive index larger than that of the gas around the projection optical system and a gas having a smaller refractive index than the gas around the projection optical system are enclosed. 2. The projection exposure apparatus according to claim 1, wherein a gas layer having a variable size is formed independently.