WO2025220579A1 - Optical system, projection exposure device, and projection exposure system - Google Patents

Abstract

Provided is an optical system 20 of a projection exposure device 10, , the optical system 20 being disposed between a mask 40 and a wafer W. The optical system 20 comprises: a pair of first mirrors 21 that each receive illumination light L0 from a light source 31 and reflect the illumination light to the mask 40; and a projection system 22, which has two mirrors, receives reflected light L1 reflected by the mask 40, and guides the reflected light to the wafer W. The projection system 22 has a second mirror 221 disposed adjacent to the pair of first mirrors 21, and a third mirror 222 disposed on the wafer W side with respect to the second mirror 221 and having a second optical surface S2 facing a first optical surface S1 of the second mirror 221. A central part C between the pair of first mirrors 21, the second mirror 221, and the third mirror 222 are located on the same straight line.

Description

Optical system, projection exposure apparatus, and projection exposure system

This disclosure relates to an optical system, a projection exposure apparatus, and a projection exposure system. This application claims priority to Japanese Patent Application No. 2024-065769, filed April 15, 2024, the entire disclosure of which is incorporated herein by reference.

Technology relating to projection exposure apparatuses used in semiconductor manufacturing processes has been known. For example, Patent Document 1 discloses that in a projection lithography system equipped with an anamorphic imaging projection optical system in which the entrance pupil is inaccessible, various adaptations of the imaging operation of the pupil facet mirror and/or its pupil facets and/or transfer optical system may be advantageous.

Special Publication No. 2021-503623

However, the conventional technology described in Patent Document 1 uses eight mirrors in the optical system positioned between the mask and wafer, which means that the reflected light obtained when illumination light is reflected by the mask experiences a large power loss before reaching the wafer. In order to direct reflected light of sufficient power onto the wafer, it is necessary to increase the output power of the projection exposure apparatus's light source, which increases the power consumption of the projection exposure apparatus. As described above, there is room for improvement in the energy efficiency of conventional projection exposure apparatuses.

The present disclosure aims to provide an optical system, projection exposure apparatus, and projection exposure system that can improve energy efficiency.

An optical system according to a first aspect for solving the above problems comprises:
An optical system of a projection exposure apparatus, which is arranged between a mask and a wafer,
a pair of first mirrors each receiving illumination light from a light source and reflecting it onto the mask;
a projection system having two mirrors that receives light reflected by the mask and directs it onto the wafer;
Equipped with
the projection system
a second mirror disposed adjacent to the pair of first mirrors;
a third mirror disposed on the wafer side relative to the second mirror and having a second optical surface facing the first optical surface of the second mirror;
and
The center between the pair of first mirrors, the second mirror, and the third mirror are positioned on the same straight line.

A projection exposure apparatus according to a second aspect comprises:
The optical system described above;
the light source;
a collector mirror disposed relative to the light source and configured to receive the illumination light from the light source;
Equipped with
The collector mirror has four segmented optical surfaces.

A projection exposure system according to a third aspect comprises:
A projection exposure system comprising the above optical system or the above projection exposure apparatus and the mask,
The mask is flat along a scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction.

The optical system, projection exposure apparatus, and projection exposure system according to one embodiment of the present disclosure can improve energy efficiency.

FIG. 1 is a design diagram illustrating in detail an example of the configuration of a projection exposure system according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing an example of an optical surface of the collector mirror of FIG. 1 . FIG. 2 is a schematic diagram illustrating the optical system of the projection exposure apparatus of FIG. 1 . FIG. 2 is a first diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 2 is a second diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 3 is a third diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 2 is a schematic diagram showing an example of the configuration of a mask included in the projection exposure system of FIG. 1 . FIG. 8 is a graph illustrating an example of the function of the mask in FIG. 7 . FIG. 4 is a fourth diagram for explaining an example of the function of the projection exposure apparatus of FIG. FIG. 5 is a fifth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; 2 is a schematic diagram showing the state of a diffraction cone on the optical surface of the third mirror in FIG. 1 . FIG. 6 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1. FIG. 7 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1. FIG. 8 is an eighth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 9 is a diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 10 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1; FIG. 11 is an eleventh diagram for explaining an example of the function of the projection exposure apparatus of FIG. FIG. 12 is a twelfth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 13 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1; FIG. 14 is a fourteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 15 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1. FIG. 16 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1; FIG. 17 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1; FIG. 18 is an 18th diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1; FIG. 19 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1; FIG. 20 is a diagram illustrating an example of the function of the projection exposure apparatus of FIG. 1. FIG. 10 is a first diagram for explaining an example of the function of a projection exposure apparatus according to a modified example. FIG. 2 is a second diagram for explaining an example of the function of the projection exposure apparatus according to the modified example. FIG. 3 is a third diagram for explaining an example of the function of the projection exposure apparatus according to the modified example.

The following mainly describes one embodiment of the present disclosure, with reference to the accompanying drawings. The following description of the optical system 20 also applies to the projection exposure apparatus 10 and projection exposure system 1 that include the optical system 20 to which the present disclosure is applied.

FIG. 1 is a design drawing showing in detail an example of the configuration of a projection exposure system 1 according to an embodiment of the present disclosure. In FIG. 1, only the diffraction cones are shown from the mask 40 to the wafer W inside a projector including a projection system 22 (described below), and illumination is omitted. With reference to FIG. 1, an example of the configuration and function of a projection exposure system 1 according to an embodiment of the present disclosure will be mainly described. The projection exposure system 1 has a projection exposure apparatus 10 and a mask 40 for drawing a circuit pattern on the wafer W. The projection exposure system 1 constitutes, for example, an EUV lithography (Extreme Ultraviolet Lithography) system.

The projection exposure apparatus 10 has an optical system 20. The optical system 20 has a pair of first mirrors 21 and a projection system 22. The projection system 22 has a second mirror 221 and a third mirror 222. In addition to the projection system 22, the projection exposure apparatus 10 has an illumination system 30 that includes the pair of first mirrors 21. The illumination system 30 has a light source 31, a collector mirror 32, a collimator 33, a transparent window 34, a fourth mirror 35, and the pair of first mirrors 21.

In this disclosure, "upstream" corresponds to the direction toward the light source 31 along the optical path of the illumination light L0 configured in the projection exposure apparatus 10. "Downstream" is the opposite side of upstream and corresponds to the direction toward the wafer W along the optical path of the illumination light L0 configured in the projection exposure apparatus 10. The projection exposure apparatus 10 has, from upstream to downstream, an illumination system 30 and a projection system 22, in that order. The projection exposure apparatus 10 has, from upstream to downstream, a light source 31, a collector mirror 32, a collimator 33, a transparent window 34, a fourth mirror 35, a pair of first mirrors 21, a second mirror 221, and a third mirror 222, in that order.

The light source 31 includes, for example, an EUV light source such as laser plasma. The laser plasma includes, for example, laser plasma using tin (Tin). The light source 31 irradiates the collector mirror 32 with illumination light L0 having a predetermined spectral width centered around a wavelength of 13.5 nm.

Figure 2 is a schematic diagram showing an example of the optical surfaces of the collector mirror 32 in Figure 1. The collector mirror 32 is positioned relative to the light source 31 and receives illumination light L0 from the light source 31. The collector mirror 32 reflects the illumination light L0 incident from the light source 31 toward the collimator 33. The collector mirror 32 has four segmented optical surfaces. For example, the collector mirror 32 has a third optical surface 32a1 located at the upper right, a fourth optical surface 32a2 located at the lower right, a fifth optical surface 32b1 located at the upper left, and a sixth optical surface 32b2 located at the lower left in Figure 2. Each of these four optical surfaces constitutes, for example, a toroidal mirror.

The projection exposure apparatus 10 further includes actuators 321 arranged for each of the four optical surfaces of the collector mirror 32, which change the angle of the optical surface. For example, as shown in FIG. 11 (described later), the actuators 321 include a first actuator arranged for the third optical surface 32a1, a second actuator arranged for the fourth optical surface 32a2, a third actuator arranged for the fifth optical surface 32b1, and a fourth actuator arranged for the sixth optical surface 32b2.

Referring again to FIG. 1, the illumination light L0 emitted from the collector mirror 32 forms two sheet beams at the intermediate focus IF. The intermediate focus IF is located on the optical axis between the collector mirror 32 and the transparent window 34. The collimator 33 is positioned at the intermediate focus IF and has a double slit that allows the two sheet beams to pass through. The collimator 33 allows the illumination light L0 that has been reflected by the collector mirror 32 and entered as two sheet beams to pass through and guide it to the transparent window 34 located downstream.

The transparent window 34 is positioned on the optical axis downstream of the intermediate focus IF and blocks debris from the light source 31. The transparent window 34 transmits the illumination light L0 that has passed through the collimator 33 and guides it to the fourth mirror 35. The transparent window 34 is made of any material that can transmit the illumination light L0 at a predetermined transmittance.

The fourth mirror 35 is positioned downstream of the transparent window 34 and reflects the illumination light L0 that has passed through the transparent window 34 toward the pair of first mirrors 21. The fourth mirror 35 is positioned upstream of the pair of first mirrors 21 and converges the illumination light L0. As an example, the fourth mirror 35 is a cylindrical mirror.

Each of the pair of first mirrors 21 is positioned downstream of the fourth mirror 35, and further reflects the illumination light L0 reflected by the fourth mirror 35 towards the mask 40. As an example, each of the pair of first mirrors 21 is a cylindrical mirror. As described below, the fourth mirror 35 and the pair of first mirrors 21 form a first exposure field (scan field) and a second exposure field of the illumination light L0 on the mask 40. The first exposure field and the second exposure field are separate from each other.

The projection exposure apparatus 10 further has a line scan slit 36 that is positioned over the mask 40. The illumination light L0 reflected by each of the pair of first mirrors 21 passes through the line scan slit 36 and enters the mask 40. The reflected light L1 reflected by the mask 40 passes through the line scan slit 36 again, passes between the pair of first mirrors 21, and enters the projection system 22. In this disclosure, "reflected light L1" includes the illumination light L0 reflected by the mask 40 and diffracted light that contains structural information of the logic pattern on the mask 40.

Figure 3 is a schematic diagram of the projection exposure apparatus 10 of Figure 1, focusing on the optical system 20. For the purpose of simplifying the illustration, Figure 3 omits the illustration of the components of the illumination system 30 located upstream of the line scan slit 36 and the pair of first mirrors 21 of Figure 1, and instead focuses on the configuration of the optical system 20.

The optical system 20 of the projection exposure apparatus 10 is positioned between the mask 40 and the wafer W. The optical system 20 has a pair of first mirrors 21 that each receive illumination light L0 from the light source 31 and reflect it onto the mask 40, and a projection system 22 that receives reflected light L1 from the mask 40 and directs it onto the wafer W. The projection system 22 has two mirrors.

The projection system 22 includes a second mirror 221 positioned adjacent to the pair of first mirrors 21, and a third mirror 222 positioned on the opposite side of the pair of first mirrors 21 relative to the second mirror 221, and having a second optical surface S2 facing the first optical surface S1 of the second mirror 221. The center C between the pair of first mirrors 21, the second mirror 221, and the third mirror 222 are located on the same straight line. For example, the second mirror 221 and the third mirror 222 are located on the same central axis A. For example, the optical system 20 is configured so that the center C between the pair of first mirrors 21, the second mirror 221, and the third mirror 222 are located on the central axis A, and the central axis A also coincides with the central axes of the mask 40 and the wafer W. The projection system 22 is configured as an inline projector positioned so as to be sandwiched between the mask 40 and the wafer W.

Each of the second mirror 221 and the third mirror 222 may be arranged perpendicular to the central axis A with their centers positioned on the central axis A, or may be arranged at an angle. The mask 40 may be arranged perpendicular to the central axis A. The mask 40 is arranged, for example, so as to face the projection system 22 without being tilted. Similarly, the wafer W may be arranged perpendicular to the central axis A. The wafer W is arranged, for example, so as to face the projection system 22 without being tilted.

The second mirror 221 and the third mirror 222 are, for example, axially symmetric aspherical mirrors. The first optical surface S1 and the second optical surface S2 have substantially the same radius of curvature. In this disclosure, "substantially the same" radius of curvature means that the numerical values of the two radii of curvature are within 0.3%, more preferably within 0.2%, and even more preferably within 0.1% of each other. The second mirror 221 has a first opening H1 that guides reflected light L1 from the outside to the inside of the projection system 22. The first opening H1 includes a first through-hole that penetrates the second mirror 221 along the thickness direction of the second mirror 221. The third mirror 222 has a second opening H2 that guides reflected light L1 from the inside to the outside of the projection system 22. The second opening H2 includes a second through-hole that penetrates the third mirror 222 along the thickness direction of the third mirror 222.

The reflected light L1, which is reflected by the mask 40 and passes between the pair of first mirrors 21, passes through the first opening H1 of the second mirror 221 and enters the projection system 22. The reflected light L1, which passes through the first opening H1 and enters the projection system 22, is reflected by the second optical surface S2 of the third mirror 222 and enters the first optical surface S1 of the second mirror 221. The reflection angle of the reflected light L1 at the second optical surface S2 is represented by θ in FIG. 1. The reflected light L1 is further reflected by the first optical surface S1 of the second mirror 221 toward the second optical surface S2 of the third mirror 222, passes through the second opening H2 of the third mirror 222, and is directed to the wafer W.

As will be described later, the Fourier image at focal plane F in Figure 1, which is located between first optical surface S1 and second optical surface S2, is an image having, for example, a defect in the center caused by first opening H1 and second opening H2, and four bright spots arranged symmetrically with respect to each other outside the center. In the present disclosure, the "four bright spots" correspond to, for example, the zeroth Bragg spot obtained by the Fourier transform of the pattern on mask 40, which is obtained in addition to the bright spot equal to the origin when Fourier transformed, where illumination light L0 is concentrated. The projection system 22 directs reflected light L1 to wafer W, for example, by symmetric off-axis illumination from four directions.

As also shown in FIG. 1, the projection exposure apparatus 10 further includes a light-shielding portion 223 that is located inside the projection system 22 on the central axis A of the projection system 22 and blocks a portion of the reflected light L1. The light-shielding portion 223 is located inside the projection system 22 between the focal plane F and the third mirror 222. The light-shielding portion 223 includes, for example, a rod suspended by a thin wire.

The projection exposure system 1 includes a projection exposure apparatus 10 having the optical system 20 described above, and a mask 40. As will be described later, the mask 40 is flat along the scanning direction of the mask 40, and has a curved surface along a direction perpendicular to the scanning direction. The mask 40 is configured so that the pattern surface onto which the illumination light L0 is incident has a predetermined radius of curvature along a direction perpendicular to the scanning direction.

In FIG. 1, the scanning direction is, for example, along the x-axis, which corresponds to the positive direction of the x-axis. During scanning, the mask 40 moves, for example, to the left side of the paper. At this time, the wafer W moves conversely in the negative direction of the x-axis. During scanning, the wafer W moves to the right side of the paper. The direction perpendicular to the scanning direction is, for example, along the y-axis.

The optical system 20, projection exposure apparatus 10, and projection exposure system 1 according to the embodiment described above can improve energy efficiency. For example, the projection system 22 of the optical system 20 has two mirrors. This provides a cost-effective solution that meets performance requirements using available technology within a reasonable timeframe. Focusing on inline, two-mirror low-NA lithography as shown in Figures 1 and 3 can reduce costs and power consumption.

For example, multilayer mirrors absorb more than 30% of the EUV optical power with each reflection. In a conventional projection exposure apparatus, for example, six mirrors are arranged in the projection optical system and four mirrors in the illumination optical system. The transmission of optical power from the EUV light source to the wafer is very low.

On the other hand, in the present disclosure, a two-mirror projector having a simplified projection system 22 using two mirrors in series provides a dramatic improvement in optical power transfer efficiency.

For example, equation (1) shows the optical power transfer efficiency based on a projection exposure apparatus 10 according to an embodiment of the present disclosure. Equation (2) shows the optical power transfer efficiency based on a conventional projection exposure apparatus. The efficiency of the two-mirror projector of the present disclosure is approximately 13 times higher, enabling a 92% reduction in the power consumption required to generate EUV optical power. This reduces AC power consumption from approximately 1 MW to approximately 80 kW. Furthermore, the cooling water flow rate in the drive laser system is significantly reduced. The EUV optical power required at the intermediate focus IF is, for example, 20 W for a throughput per tool of 100 wafers/hour. The simplified design of the EUV source reduces investment and maintenance costs and improves reliability. At this power level, a thin, transparent window 34, similar to a mask pellicle, can be placed at the intermediate focus IF of the illumination system 30 to block debris from the plasma source and protect the expensive mask and mirrors.

Existing EUV tools typically have slower scanning speeds than optical scanners due to weaknesses in the EUV light source, such as insufficient intensity. However, by using a projection exposure apparatus 10 according to an embodiment of the present disclosure, it is possible to increase the EUV light power on the wafer W, leading to faster scanning speeds and improved productivity.

A low NA allows light rays to pass closer to the axis, making it easier to correct optical aberrations. Only two aspherical mirrors, the second mirror 221 and the third mirror 222, are required to cover a reasonably wide image field. As described below, optical simulations have confirmed that a projector with an NA of 0.2 and a height of 2 m can provide an image field of 20 mm. Compared to immersion iArF, EUV at low NA offers superior resolution, partly due to its wavelength of 13.5 nm, which is 15 times shorter than ArF's 198 nm. The critical dimension, or resolution, is determined by Abbe's equation:

where _k1 is the process coefficient, λ is the wavelength, and NA is the numerical aperture. The spatial resolution is determined in two cases.

Here we assume _k1 is equal to 0.36 and 0.27 for the EUV and iArF cases, respectively. Using low NA EUV, it is also possible to achieve single patterning at a half pitch of 24 nm.

Additionally, the depth of focus (DOF) is also important, and is defined as:
Substituting equation (3) into equation (6) yields a dimensionless relationship.

Equation (7) shows that a lower NA always results in a longer DOF. In the two cases:
In both cases, we assume that k ₂ =1.

It is clear that low NA EUV has the advantage of a longer DOF. Furthermore, compared to typical EUV projectors that use oblique illumination of the mask, inline projectors, due to their average perpendicular illumination, do not exhibit the image variations typical of EUV around the focus. This eliminates image placement errors caused by non-flatness of the mask 40. Therefore, using low NA EUV simplifies requirements such as flatness and focus control of the mask 40 and wafer W.

Axially symmetric optics provide uniform image contrast around the axis, simplifying source-mask optimization (SMO). Conventional quadrupole illumination is sufficient. Furthermore, as described below, the maximum reflection angle of the third mirror 222 is only 5.5° from the surface normal. This minimizes asymmetric pupil apodization, is polarization-independent, and reduces the phase shift associated with multilayer coatings.

As shown in Figure 3, the two-mirror projector can also be mounted in a tube similar to those used in ultraviolet lithography lenses. In the projection exposure apparatus 10, the highly accurate mirrors are enclosed in a tube, offering several advantages, including mechanical stability, ease of assembly, alignment, replacement, etc., and excellent sealing to protect against dust contamination. This reduces capital and maintenance costs and improves reliability.

The projection exposure system 1 according to one embodiment of the present disclosure will be described in further detail below using examples, but the present disclosure is in no way limited to the following examples. The numerical values described in the examples are merely examples and do not limit the scope of the present disclosure. The scope of the present disclosure should be determined solely based on the claims. Below, components similar to those in the embodiment will be assigned the same reference numerals, and duplicate explanations will be omitted.

In order to increase the field size, the length of the projector including the projection system 22 must be increased. We assume that the height of the module including the projector is within the maximum size allowed in an actual semiconductor factory. In other words, OID = 2000 mm. OID is the object-image distance. In Figure 1, the distance from the wafer W to the mask 40 is 2000 mm.

To maintain the well-known Petzval sum law, it is important to position the third mirror 222 sufficiently close to the wafer W. Assuming the same lens-to-wafer gap size as in ArF immersion, the gap between the wafer W and the body of the third mirror 222 is preferably 5 mm. To maintain the rigidity of the body of the third mirror 222, the distance between the wafer W and the second optical surface S2 of the third mirror 222 is preferably greater than 40-50 mm. As shown below, the second mirror 221 and the third mirror 222 have approximately the same radii of curvature, i.e., within 0.3% of each other, resulting in a wider field. The parameters for each of the second mirror 221 and the third mirror 222 are summarized in the table below.

The simulator predicts a 20 mm field with NA = 0.2, which encompasses a 100 mm mask field. The image reduction ratio is 1/5. As described below, a curved mask 40 is used to correct the residual field curvature in the wide field size and reduce wavefront errors. The simulator used was the OpTaLix simulator.

Figure 4 is the first diagram illustrating an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 4 shows the simulation results of an in-line two-mirror projector with an NA of 0.2 and an OID of 2000 mm. The second mirror 221 and the third mirror 222 are each assumed to have a perfect optical surface with 100% reflectivity, and no pupil apodization or aperture stop is applied. It should be noted that in actual lithography, light travels in the opposite direction. To establish the telecentric condition in the simulator, a virtual light beam is initiated from the wafer W side.

The second mirror 221 and the third mirror 222 are each axially symmetric aspherical mirrors. To direct the illumination light L0 toward the projector, ample space is required between the second mirror 221 and the mask 40 to accommodate a pair of cylindrical mirrors, such as the first mirror 21 and the fourth mirror 35. This results in a magnification of 5x, rather than the standard 4x. The mask exposure field size is 100 mm (20 mm x 5), which roughly matches the 104 mm (26 mm x 4) of the current mask design. Simulation results for an NA of 0.2 are summarized in Tables 2 and 3 below. Table 2 lists the parameters for the two-mirror projector. Table 3 lists the aspherical surface specifications.

In the simulation, we assumed that the second mirror 221 and the third mirror 222 each had a perfect mirror surface with a reflectivity of 100%. In reality, the mirrors are made of multi-layer coatings, and reflection occurs due to wave interference between these layers, resulting in amplitude and phase shifts as the reflection angle changes. More careful simulations including multi-layer coatings are required. In reality, the quality of the mirrors would need to be measured with an interferometer.

The wafer W side is telecentric, but the mask 40 side is not. Therefore, the chief ray is tilted by 1.6° (~50 mm/2000 mm radians) at the edge of the field. Taking into account the half angle of the diffraction cone (NA/5 = 0.04 radians = 2.4°), the maximum reflection angle from the multilayer coating at the mask edge is 4°. This angle is smaller than the 12° cutoff angle of the Mo/Si multilayer coating, minimizing contrast loss. Furthermore, pattern shift due to defocus is extremely small; a mask height error of 500 nm only causes a pattern shift of less than 3 nm at the field edge. This shift is considered to be within the acceptable range.

It should be noted that all light rays from different fields intersect at the focal plane, producing a diffraction spot that represents Fourier space. The light must pass through the central apertures of both the second mirror 221 and the third mirror 222, resulting in a central defect in the diffraction signal. The effect of the central defect is evaluated separately at the focal plane using Fourier analysis, as described below.

Figure 5 is a second diagram illustrating an example of the function of the projection exposure apparatus 10 of Figure 1. The graph in Figure 5 shows the optical path difference along the beam height on a vertical scale of 0.05 wavelengths for an EUV wavelength of 13.5 nm. At the exposure field edge (y = 10 mm), the Strehl ratio is high at 0.991, resulting in a diffraction-limited spot at an NA of 0.2. Figure 6 is a third diagram illustrating an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 6 shows the wavefront aberration for the field edge y = 10 mm.

Figures 5 and 6 show the wavefront aberrations. The optical path difference error decreases at smaller image heights. However, at the field edges, residual aberrations reach a limit of 0.05 wavelengths. Due to the low NA, the Strehl ratio at the field edges is still high (0.991). It should be noted that the Strehl ratio is evaluated without central defects and on-axis illumination. As the illumination is tilted, high-frequency components begin to pass through the projector, increasing resolution. Therefore, aberrations become dominant. Fortunately, the optical path difference in Figure 5 is axially symmetric (actually cylindrically symmetric). As also shown in Figure 6, the phase difference between the first-order Bragg diffraction from the thinnest pattern (see Figures 18A and 18B) and the four-fold off-axis illumination becomes smaller, meaning that aberrations are effectively reduced.

Figure 7 is a schematic diagram showing an example of the configuration of a mask 40 included in the projection exposure system 1 of Figure 1. Figure 7 shows the concept of a curved surface mask that corrects the field curvature in the y direction. A very long radius of curvature is applied to the mask 40 in the y direction. The radius of curvature is, for example, several meters to several hundred meters.

In optical system 20, due to the limited number of mirrors, the projected image is not perfectly flat but curved. The optimum focus varies with the field height, resulting in wavefront errors as shown in Figure 5. By introducing a curved mask as shown in Figure 7, the field curvature in the y direction can be corrected. A very long radius of curvature (1000 m) is applied to mask 40 in the y direction, while mask 40 is flat in the x direction (scanning direction). The maximum bending at the field edge is approximately a few micrometers, which is significantly smaller than the mask width of approximately 100 mm. Therefore, it is unlikely to cause mechanical damage to the structure of mask 40.

Figure 8 is a graph illustrating an example of the function of the mask 40 in Figure 7. Figure 8 shows the improved wavefront error using the curved mask in Figure 7. As shown in Figure 8, the wavefront error was improved by introducing a curved mask with a radius of curvature of 450 meters. By limiting the field size to 10 mm, a two-mirror projector with NA of 0.3 can be realized with a module height as low as an OID of 1500 mm.

Figure 9A is the fourth diagram for explaining an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 9A shows the state of beam defects at the third mirror 222. Figure 9B is the fifth diagram for explaining an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 9B shows the state of beam defects at the second mirror 221. The second aperture H2 and first aperture H1 as beam holes are designed to match the beam edge of an NA of 0.2 with a 2 mm gap around the beam. The three circles indicate the on-axis diffraction cone and both field edges. The diameter D in Figure 9A is 163 mm. The diameter D in Figure 9B is 517 mm.

In the two-mirror in-line projector design described above, the beam hole is located in the center of the mirror, which inevitably leads to the problem of missing parts. Although it is difficult to completely avoid this problem through projector design alone, it is possible to substantially reduce the impact on the projection pattern. To solve this problem, the following three measures can be considered:
(a) Make the beam aperture as small as possible.
(b) Optimize off-axis illumination.
(c) Optimizing the partial coherence factor.

In Figures 9A and 9B, the central beam holes are shown as the second aperture H2 and the first aperture H1, respectively. In this disclosure, to distinguish between the obscuration factor and the partial coherence factor, the uppercase Greek letter Σ is used to refer to the obscuration factor and the lowercase Greek letter σ is used to refer to the partial coherence factor.

The central hole is designed to pass a 0.2 NA beam, with a 2 mm gap surrounding the beam edge. The defect in the second mirror 221 is typically smaller than the defect in the third mirror 222, so only the third mirror 222 will be described.

As shown in FIG. 9A, the normalized pore size (defect factor) is:
Σ _x =0.13 (10)
Σ _y =0.26 (11)
Σ=1 represents the diffraction cone (diameter of the mirror). The low NA results in a small horizontal defect. The surface of the third mirror 222 is located close to the wafer W to maintain the Petzval sum law. This decision also helps to reduce the size of the beam aperture.

We introduce quadrupole illumination, which can avoid the central defect. As shown in Figure 18B, the logic pattern is mainly composed of vertical and horizontal lines, and its diffraction is distributed along the horizontal and vertical axes. If the spacing between the quadrupole illumination spots in the horizontal and vertical directions is larger than the size of the defect, the diffraction will not interfere with the defect area. The design of this disclosure clearly satisfies the following conditions:
W _offaxis =0.71>Σ _x =0.13 (12)
H _offaxis =0.71> _Σy =0.26 (13)

In the special case of staggered contact holes, the diffraction pattern has a 60° rotational symmetry, so the diffraction spot can still reach the defect area. The defect spot can be partially avoided by the partial coherence factor. Since the partial coherence factor is larger than the defect factor, i.e., σ _x = 0.25 > Σ _x = 0.13, the diffraction spot spreads out wider than the width of the central hole.

In the projection exposure apparatus 10 using a two-mirror projector shown in Figure 1, the collector mirror 32 of the light source 31 as an EUV source is composed of four segmented toroidal quad mirrors, providing quadrupole off-axis illumination that avoids central loss and improves resolution. By combining the toroidal collector mirror 32 of the illumination system 30 with the fourth mirror 35 and the first mirror 21, which are cylindrical mirrors, two line fields arrive, converge, and coincide on the mask 40. The tilt angles of the four EUV illuminations on the mask 40 are designed to achieve symmetric quadrupole off-axis illumination. The orientation of the collector mirror 32 is individually adjusted using the actuator 321 described above. The illumination is, on average, perpendicular to the surface of the mask 40, eliminating the 3D effect of the mask 40.

The diameter of a tin droplet is 20-30 μm, while the size of the EUV light source 31 is 90 μm or more due to plasma expansion. This is a sufficient size for lateral partial coherence for a 2.5 mm wide scan line. However, this is not sufficient in the scan line direction (length 100 mm), so to increase the size of the virtual source, the fourth mirror 35 must have a ripple mirror surface (described below as a "partially coherent source").

The projector consists of two aspherical mirrors: the second mirror 221 and the third mirror 222. In Figure 1, the maximum reflection angle θ of the third mirror 222 is 11°, which falls within the bandwidth of the Mo/Si multilayer coating. This allows for a high reflection coefficient with a uniform coating and negligible phase variation, resulting in high contrast.

The intermediate focus IF of the EUV light source 31 is not a spot but two sheet beams. Therefore, a double slit is used as a collimator 33 to remove unnecessary stray light and infrared light from the _CO2- driven laser. A transparent window 34 is installed to separate the clean vacuum environment inside the projector from the light source 31, i.e., to block microdebris from the tin (Tin) plasma light source.

Figure 10 is a schematic diagram showing the state of the diffraction cone at the optical surface S2 of the third mirror 222 in Figure 1. As shown in Figure 10, the diameter of the diffraction cone C1 of the reflected light L1 irradiated onto the optical surface S2 of the third mirror 222 is 160 mm. On the other hand, the diameter of the optical surface S2 is 180 mm. The size of the second opening H2 of the third mirror 222 is 22 mm x 42 mm. The diameter of the diffraction cone C2 located inside the second opening H2 is 18 mm. The size of each of the two exposure fields in this case is 20 mm.

The parameters for each mirror included in the illumination system 30 are summarized in a table below.

FIG. 11 is the sixth diagram for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 11 shows the unfolded illumination path from the EUV light source 31 to the mask 40 and wafer W. Quadrupole illumination is achieved using a quad collector mirror 32 (toroidal focusing mirror). The illumination system 30 has two cylindrical mirrors, the fourth mirror 35 and the first mirror 21, which perform optical shaping of the scan line field.

In an actual projection exposure apparatus 10, all mirrors and the mask 40 reflect and redirect light, but in FIG. 11 , each component is virtually represented as transmitting light like a lens. The quad collector mirror 32 of the light source 31 provides four beams _a1 , _a2 , _b1 , and _b2 , generating quadrupole off-axis illumination on the mask 40. This improves resolution and prevents central defects. The four spots on the focal plane F are images of the tin plasma of the EUV light source 31, realizing Köhler illumination. The illumination on the mask 40 is split into two lines (double exposure fields) to avoid shading by the pair of first mirrors 21, which are cylindrical mirrors.

FIG. 12A is the seventh diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 12A shows in detail the optical path around the mask 40. In FIG. 12A, the field is divided into two lines on the mask 40 so that the first mirror 21, which is a cylindrical mirror, does not block the reflected light L1. FIG. 12B is the eighth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 12B shows how, at the focal plane F, four illumination spots are formed within an aperture that is partially limited by the shadow from the first mirror 21, which is a cylindrical mirror. FIG. 12C is the ninth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 12C shows how the field projected onto the wafer W appears as double lines.

As shown in Figure 12A, the field on the mask 40 is divided into two lines, and each reflected light beam L1 of the illumination light beam L0 passes between the pair of first mirrors 21 and at a position close to the first mirror 21. To prevent the first mirror 21, which is a cylindrical mirror, from blocking the reflected light beam L1, the positions of the two first mirrors 21 are moved outward from each other while maintaining the same illumination angle toward the mask 40. Even in this case, the quadrupole illumination spot does not change. In the case of coherent illumination, the Fourier transform of the illumination on the mask 40 is a delta function spot, as it is designed to be uniform on the nanometer scale within the exposure field. However, the phase has a tilt due to the oblique illumination angle, causing a position shift from the origin, resulting in an off-axis spot around the defect B as shown in Figure 12B.

First, the mask 40 is illuminated with a ( _b1 + _b2 ) field, producing spots _b1 and _b2 at the focal plane F. The reticle image is then projected onto the wafer W. The mask 40 is moved in a scanning motion, and the same reticle pattern is illuminated with an ( _a1 + _a2 ) field, producing a reticle image on the wafer W as shown in Figure 12C. Because the EUV light source 31 is not coherent in space or time, the illumination generates independent photoactivations in the resist layer. Therefore, the total activation pattern is equal to the sum of the two exposure fields.

This process corresponds to the mechanism of color offset printing. The CMYK colors are transferred one by one to the paper by rotating a cylinder with an image film. The print image is pre-patterned using photolithography. This process is repeated four times to create a color image. In the EUV lithography of this disclosure, photoactivation is repeated twice in one scan: ( _b1 + _b2 ) and ( _a1 + _a2 ).

Figure 13 is the tenth diagram for explaining an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 13 shows the trajectory of the chief ray (illumination) from the projector including the fourth mirror 35, the first mirror 21, the mask 40, and the second and third mirrors 221 and 222. Figure 13 shows axial illumination without tilt.

Here, we will explain in detail the convergent illumination conditions conceptually shown in Figure 13. In conventional optical lithography, the telecentricity condition is met on both the mask and the wafer. However, in the two-mirror projector disclosed herein, the wafer W side is telecentric, but the mask 40 side is not. Therefore, it is necessary to provide convergent illumination tailored to the projector as follows:

(1) The fourth mirror 35, which is a cylindrical mirror, collects illumination from the EUV light source 31. The first mirror 21 is a mirror that is flat in the y direction.
(2) Because the mask 40 is also a flat mirror, the reflected illumination maintains the same focal angle.
(3) At the focal plane F, the illumination is focused to a central spot.
(4) The illumination reaches the wafer W from the normal direction, i.e., from a telecentric state.
(5) To create off-axis illumination, i.e., to shift the spot outside the focal plane F, tilt the incident light onto the mask 40 without changing the illumination position.
(6) To form four off-axis illumination points (quadrupole) on the focal plane F, the illumination angle of the mask 40 measured from the center must be close to NA/m (m is the image magnification). The angles in the (x, y) directions are 1/1.414 times smaller, with signs _a1 = (+, +), _a2 = (+, -), _b1 = (-, +), _b2 = (-, -).
(7) Details are determined using the OpTaLix simulator with the mirror parameters shown in Tables 1 to 4. It should be noted that the radii of curvature of the toroidal and cylindrical mirrors are represented by two numbers (R _x , R _y ). When R _x = infinity, it means that the mirror is flat around the x-direction.

In the case of off-axis illumination, each illumination may not be perpendicular to the wafer W, which can destroy the telecentric state and cause an image shift due to a shift in the wafer W position in the z direction (defocus). However, this effect is tolerable if the four illuminations are symmetrical. Therefore, in this disclosure, quadrupole illumination arranged symmetrically around the axis is used. Balancing the Fourier components is very important in lithography, as it maximizes spatial resolution and reduces unwanted shadows around the pattern in the created image.

Below, we will explain partially coherent sources.

When using point-source illumination, frequency components close to the edge are abruptly cut off by the aperture (hard edge cutoff), often resulting in ringing tails in the image. To avoid this problem, partially coherent sources are commonly used in optical lithography.

The partial coherence factor is defined as follows:
For a point light source, σ = 0. This is called coherent illumination.

In conventional UV lithography, a partially coherent illumination factor of 0.2 is commonly used for quadrupole illumination, and it is desirable to use the same value for EUV. Note that partially coherent illumination also mitigates the effect of the central defect. As shown in Figure 9A, the defect size in the x-direction is Σ _x = 0.13, and partially coherent light σ = 0.2 eliminates the hole.

We will explain the source size separately in the x and y directions. First, because the exposure field in the x direction is narrow at 2.5 mm, the natural angular spread of the EUV plasma light source satisfies the required angular spread. Figure 14 is an 11th diagram illustrating an example of the function of the projection exposure apparatus 10 in Figure 1. As shown in Figure 14, we assume that the collection angle in the x direction of the segment mirror is 1 radian per mirror. The diameter of the tin plasma is approximately 100 μm, and a 50 μm width is cut from it, expanded 50 times by the illumination system 30, and sent to the mask 40 as a 2.5 mm wide line field.

Since the phase space area is preserved by the linear optics, the angular divergence is adiabatically reduced by a factor of 50, resulting in an angular spread of 20 mrad on the mask 40. Compared with the entrance pupil diameter 2×NA/m=2×0.2/5=80 mrad, the partial coherence factor is σ _x =20/80=0.25, which meets the required value. The acceptance between the first mirror 21 and the line scan slit 36 cuts off the phase space. 60% of the photon flux can reach the wafer W. In FIG. 14(c), the dashed line indicates the pair illumination.

FIG. 15A is a twelfth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 15B is a thirteenth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 15C is a fourteenth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1.

15A to 15C schematically illustrate the optical acceptance between the first mirror 21 and the line scan slit 36. Collimated illumination from the first mirror 21 is slowly focused onto the focal plane F at a distance of 2.5 m and strikes the mask 40 at a nominal angle, generating a quadrupole illumination spot. FIG. 15A shows how collimated illumination from the first mirror 21 strikes the mask 40 at a nominal angle θ _0. The line scan slit 36 has a 5 mm slit width. Reflected light L1 generates a spot at the focal plane F corresponding to the quadrupole illumination. FIG. 15B shows how the maximum angle condition occurs between the left edge of the line scan slit 36 and the edge of the first mirror 21 on the right. FIG. 15C shows how the minimum angle condition occurs between the left edge of the line scan slit 36 and the edge of the first mirror 21 on the left.

The maximum and minimum angles of acceptance are given as follows:
θ _max =θ ₀ +w/L (15)
θ _min =θ ₀ -w/L (16)
Here, w is the scan line width, which is 2.5 mm. L is the distance between the first mirror 21 and the mask 40. Here, we assume that L = 200 mm and w/L = 12.5 mrad, as shown in FIG. 14(c). The acceptance has a triangular shape, and 60% of the photons can pass through the triangular acceptance and reach the wafer W. The remaining 40% are lost by the first mirror 21 and the slit.

In the y-direction, the illumination system 30 expands the light to a wide line width of 100 mm, encompassing the size of the mask 40. However, this results in a very small angular divergence, which does not satisfy the required partial coherence. To increase the divergence in the y-direction, a "ripple mirror" is introduced as the fourth mirror 35. "Ripple mirrors" were originally introduced for curved exposure fields. The mirror surface has periodic undulations arranged on it that mix the light rays, effectively increasing the partial coherence factor without significant light loss.

FIG. 16 is a diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 16 shows a quadrupole off-axis illumination pattern at the focal plane F, taking into account partial coherence factors σ _x = 0.25 and σ _y = 0.2. Because the shadow of the first mirror 21 blurs and some of the illumination and diffracted light pass through, a 10-15% margin must be secured in the mirror to correct aberrations. The four illumination spots are symmetrically distributed around the pupil size at an angle of 45° from the axis. The usable resolution is determined by the frequency bandwidth 2NA·cos(45°) = 1.4NA. The critical dimension is then calculated as follows:

FIG. 17 is a 16th diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 18A is a 17th diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 18B is an 18th diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. In FIGS. 18A and 18B, the intensity of the Fourier component is represented by the shade of color, with the darker the color, the greater the intensity of the Fourier component. FIG. 19A is a 19th diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 19B is a 20th diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1.

To investigate the imaging capability of the two-mirror projector, we performed an FFT-based image analysis, taking into account the central defect and off-axis illumination. In this simulation, we considered partial coherence factors σ _x = 0.25 and σ _y = 0.2, and also took into account the partial blocking effect of diffracted light by the first mirror 21, resulting in a realistic calculation.

Figure 17 shows a simple test pattern. The half pitch is set to 27 nm, wider than the maximum resolution of 24 nm, taking into account the partial blocking effect of diffracted light by the first mirror 21. Figure 18A shows the Fourier components passing through the aperture. Figure 18B shows the sum of the Fourier components as viewed from each illumination spot. Figure 19A shows a back FFT image of the projected line pattern. Figure 19B shows the intensity profile along the x-direction at the center of the pattern. Sufficiently high contrast is obtained.

It will be apparent to those skilled in the art that the present disclosure may be embodied in other forms than the above-described embodiments without departing from the spirit or essential characteristics thereof. Therefore, the foregoing description is illustrative and not limiting. The scope of the disclosure is defined by the appended claims, not by the foregoing description. All modifications that fall within the range of equivalents of any modifications are intended to be embraced therein.

For example, the shape, pattern, size, arrangement, orientation, type, and number of each of the components described above are not limited to those shown in the above description and drawings. The shape, pattern, size, arrangement, orientation, type, and number of each component may be configured arbitrarily as long as it can achieve its function. The components of the optical system 20, projection exposure apparatus 10, and projection exposure system 1 shown in the drawings are functional concepts, and the specific form of each component is not limited to those shown.

In the above embodiment, the third mirror 222 is described as being positioned on the opposite side of the pair of first mirrors 21 relative to the second mirror 221, but this is not limited to this. The third mirror 222 only needs to be positioned on the wafer W side relative to the second mirror 221, and the positions of the pair of first mirrors 21 relative to the second mirror 221 are not limited to the positions shown in Figure 1, etc. In other words, the pair of first mirrors 21 are not limited to being positioned outside the projection system 22, but may be positioned inside the projection system 22. The pair of first mirrors 21 may be positioned between the second mirror 221 and the third mirror 222.

In the above embodiment, the second mirror 221 and the third mirror 222 are described as being located on the same central axis A, but this is not limited to this. As long as the center C between the pair of first mirrors 21, the second mirror 221, and the third mirror 222 are located on the same straight line, the central axes of the second mirror 221 and the third mirror 222 do not have to coincide with each other.

In the above embodiment, the second mirror 221 and the third mirror 222 are each described as an axially symmetric aspherical mirror, but this is not limited to this. The second mirror 221 and the third mirror 222 do not have to be axially symmetric. The second mirror 221 and the third mirror 222 may also be a type of mirror other than an aspherical mirror that can achieve the functions of the present disclosure.

In the above embodiment, the first optical surface S1 and the second optical surface S2 are described as having approximately the same radius of curvature, but this is not limited to this. The first optical surface S1 and the second optical surface S2 may have different radii of curvature. For example, when the NA is increased to 0.3, the first optical surface S1 and the second optical surface S2 may have different radii of curvature. Even in such a case, wavefront error can be improved by using a curved surface mask as described with reference to Figures 7 and 8. By introducing a curved surface mask and limiting the field size to 10 mm, a two-mirror projector can be realized with an NA of 0.3, a resolution of 16 nm, and an OID of 1500 mm, which results in a smaller module height.

In the above embodiment, the second mirror 221 is described as having a first opening H1 that guides the reflected light L1 from the outside to the inside of the projection system 22, but this is not limited to this. The second mirror 221 is not limited to a first opening H1 configuration such as a first through-hole, and may have any other configuration that is capable of guiding the reflected light L1 from the outside to the inside of the projection system 22. For example, the second mirror 221 may have a transparent window, etc.

In the above embodiment, the third mirror 222 is described as having a second opening H2 that guides the reflected light L1 from the inside to the outside of the projection system 22, but this is not limited to this. The third mirror 222 is not limited to a second opening H2 configuration such as a second through-hole, and may have any other configuration that is capable of guiding the reflected light L1 from the inside to the outside of the projection system 22. For example, the third mirror 222 may have a transparent window, etc.

In the above embodiment, the Fourier image at focal plane F is described as an image having a defect B in the center and four bright spots arranged symmetrically with respect to each other outside the center, but this is not limited to this. The Fourier image at focal plane F is not limited to the image shown in FIG. 12B, and may be an image having multiple bright spots in other numbers and/or arrangements. For example, the Fourier image at focal plane F may be an image having two bright spots arranged symmetrically on the x-axis and a total of six bright spots arranged in three sets along the y-axis. This may realize hexapole off-axis illumination.

In the above embodiment, the projection system 22 is described as directing the reflected light L1 to the wafer W by symmetric off-axis illumination from four directions, but this is not limited to this. The projection system 22 may also direct the reflected light L1 to the wafer W by off-axis illumination from a number of directions other than four.

In the above embodiment, each of the pair of first mirrors 21 is described as a cylindrical mirror, but this is not limited to this. Each of the pair of first mirrors 21 may be any other type of mirror that can achieve the functions of the present disclosure.

In the above embodiment, the collector mirror 32 is described as having four segmented optical surfaces, but this is not limited to this. The collector mirror 32 may have optical surfaces that are segmented in a number other than four.

In the above embodiment, the projection exposure apparatus 10 was described as further including an actuator arranged for each of the four optical surfaces of the collector mirror 32 to change the angle of the optical surface, but this is not limited to this. The projection exposure apparatus 10 may also be capable of adjusting the direction of travel of each of the multiple beams of illumination light L0 using any other mechanism other than actuators arranged for the optical surfaces.

In the above embodiment, the optical surface is described as constituting a toroidal mirror, but this is not limited to this. The optical surface may also constitute any other type of mirror that can achieve the functions of the present disclosure.

In the above embodiment, the illumination light L0 emitted from the collector mirror 32 is described as forming two sheet beams at the intermediate focus IF, but this is not limited to this. The illumination light L0 emitted from the collector mirror 32 may form beams of at least one of a different number and shapes at the intermediate focus IF.

In the above embodiment, the projection exposure apparatus 10 is described as further including a collimator 33 with a double slit that is positioned at the intermediate focus IF and allows two sheet beams to pass through, but this is not limited to this. The projection exposure apparatus 10 may also include other optical elements depending on the configuration of the beams at the intermediate focus IF.

In the above embodiment, the projection exposure apparatus 10 is described as further including a transparent window 34 that is positioned on the optical axis downstream of the intermediate focus IF and blocks debris from the light source 31, but this is not limited to this. The projection exposure apparatus 10 does not need to include a transparent window 34 if the impact of debris from the light source 31 on the downstream side is small.

In the above embodiment, the projection exposure apparatus 10 is described as further including a fourth mirror 35 that is arranged upstream of the pair of first mirrors 21 and converges the illumination light L0, but this is not limited to this. If the functions of the present disclosure can be achieved based on another optical arrangement, the projection exposure apparatus 10 does not need to include the fourth mirror 35. Alternatively, the fourth mirror 35 does not need to converge the illumination light L0.

In the above embodiment, the fourth mirror 35 and the pair of first mirrors 21 form a first exposure field and a second exposure field of the illumination light L0 on the mask 40, and the first exposure field and the second exposure field are described as being separate from each other, but this is not limited to this. The exposure fields on the mask 40 are not limited to the double-line configuration shown in FIG. 11, and may be configured in at least one of other numbers, shapes, and arrangements.

In the above embodiment, the fourth mirror 35 is described as a cylindrical mirror, but is not limited to this. The fourth mirror 35 may be any other type of mirror that can achieve the functions of the present disclosure.

In the above embodiment, the mask 40 is described as being flat along the scanning direction of the mask 40 and having a curved surface along a direction perpendicular to the scanning direction, but this is not limited to this. The mask 40 may have other shapes as long as they can achieve the functions of the present disclosure. For example, the mask 40 may also be flat along a direction perpendicular to the scanning direction.

FIG. 20A is a first diagram illustrating an example of the function of a projection exposure apparatus 10 according to a modified example. FIG. 20A corresponds to FIG. 12A and shows in detail the optical path around the mask 40. For the purpose of simplifying the illustration, FIG. 20A omits the fourth mirror 35 and line scan slit 36 shown in FIG. 12A. FIG. 20B is a second diagram illustrating an example of the function of a projection exposure apparatus 10 according to a modified example. FIG. 20B corresponds to FIG. 12B and shows how, at the focal plane F, four illumination spots are formed within an aperture that is partially limited by the shadows from the pair of first mirrors 21. FIG. 20C is a third diagram illustrating an example of the function of a projection exposure apparatus 10 according to a modified example. FIG. 20C corresponds to FIG. 12C and shows how the field projected onto the wafer W appears as double lines.

In the above embodiment, the mask 40 and wafer W are described as moving in opposite directions by scanning, but this is not limited to this. The mask 40 and wafer W may each remain stationary. In this case, the projection exposure apparatus 10 according to the modified example may function as a stepper together with the mask 40 and wafer W, instead of functioning as a scanner.

For example, although the illumination system 30 of the projection exposure apparatus 10 according to the modified example was intended for use in scanner mode in the above embodiment, it may also be designed to function effectively in stepper mode. For example, the illumination system 30 of the projection exposure apparatus 10 may scan a pair of first mirrors 21 while the mask 40 and wafer W are fixed. By scanning the pair of first mirrors 21, the illumination system 30 sequentially directs the illumination light L0 to different positions on the stationary mask 40 to form a predetermined exposure field. The projection exposure apparatus 10 transfers the pattern on the mask 40 onto the wafer W by repeating the above operations by the illumination system 30 in stages.

As shown in Figure 20A, illumination light L0 from the light source 31 is guided to the pair of first mirrors 21 via other components of the illumination system 30. As the pair of first mirrors 21 are scanned and moved in a predetermined direction, illumination light L0 forms two exposure fields ( _a1 + a2 _{, b1} + _b2 ) that move in the predetermined direction on the mask 40. At this time, the mask 40 is fixed. Therefore, the projection exposure apparatus 10 switches the exposure position solely by optical means based on the scanning of the pair of first mirrors 21 during stepper operation. This is a major difference from the scanner method in the above embodiment.

The rectangular writing region R indicates, for example, a unit exposure field illuminated for each step in stepper operation. The writing region R indicates, for example, a rectangular or square exposure field that is ultimately formed by combining two line fields consisting of an ( _a1 + _a2 ) field and a ( _b1 + _b2 ) field. The writing region R indicates the range onto which a pattern is transferred in one go by the illumination light L0, and is formed as a rectangular or square exposure field. The projection exposure apparatus 10 can form the entire two-dimensional pattern on the mask 40 on the wafer W with high precision by precisely overlapping the writing regions R over multiple stages.

Figure 20B shows a schematic illustration of the illumination distribution at the entrance pupil formed at focal plane F. In the projection exposure apparatus 10 according to the modified example, four spots are also arranged using quadrupole off-axis illumination. The Fourier image at focal plane F, located between the first optical surface S1 and the second optical surface S2, has, for example, a defect B caused by the first opening H1 and the second opening H2 in the center, and four bright spots arranged symmetrically with respect to each other outside the center.

As shown in Figure 20C, the reflected light L1 reflected by the mask 40 is irradiated onto the wafer W via the second mirror 221 and the third mirror 222 of the projection system 22. In stepper mode, the wafer W is fixed in place like the mask 40, and the illumination positions of the two line fields are switched by scanning the pair of first mirrors 21.

The projection exposure apparatus 10 according to the above-described modified example does not require a highly accurate scanning mechanism using a mechanical stage mechanism for each of the mask 40 and wafer W. The projection exposure apparatus 10 does not require a scanning mechanism that precisely synchronizes the mask 40 and wafer W, and enables pattern formation on the wafer W with a simple configuration. In addition, even if distortion remains on the image plane in the projection system 22, the projection exposure apparatus 10 can cancel out the distortion in advance by correcting the mask 40 at the design stage before exposure.

Some embodiments of the present disclosure will be described below as examples, however, it should be noted that the embodiments of the present disclosure are not limited to these examples.
[Appendix 1]
An optical system of a projection exposure apparatus, which is arranged between a mask and a wafer,
a pair of first mirrors each receiving illumination light from a light source and reflecting it onto the mask;
a projection system having two mirrors that receives light reflected by the mask and directs it onto the wafer;
Equipped with
the projection system
a second mirror disposed adjacent to the pair of first mirrors;
a third mirror disposed on the wafer side relative to the second mirror and having a second optical surface facing the first optical surface of the second mirror;
and
a center portion between the pair of first mirrors, the second mirror, and the third mirror are located on the same straight line;
optical system.
[Appendix 2]
2. The optical system of claim 1,
the illumination light reflected by the pair of first mirrors forms a first exposure field and a second exposure field on the mask;
the first exposure field and the second exposure field are separate from each other;
optical system.
[Appendix 3]
3. The optical system according to claim 1,
The second mirror and the third mirror are located on the same central axis.
optical system.
[Appendix 4]
4. The optical system according to any one of claims 1 to 3,
each of the second mirror and the third mirror is an axially symmetric aspherical mirror;
optical system.
[Appendix 5]
5. The optical system according to any one of claims 1 to 4,
the first optical surface and the second optical surface have substantially the same radius of curvature;
optical system.
[Appendix 6]
6. The optical system according to any one of claims 1 to 5,
the second mirror has a first opening that guides the reflected light from the outside to the inside of the projection system;
the third mirror has a second opening that guides the reflected light from the inside to the outside of the projection system;
a Fourier image at a focal plane located between the first optical surface and the second optical surface has a defect at the center caused by the first opening and the second opening, and is an image having four bright points that are located outside the center and are symmetrically arranged with respect to each other with respect to the center;
optical system.
[Appendix 7]
7. The optical system according to claim 6,
the projection system directs the reflected light onto the wafer with symmetric off-axis illumination from four directions;
optical system.
[Appendix 8]
8. The optical system according to any one of claims 1 to 7,
Each of the pair of first mirrors is a cylindrical mirror.
optical system.
[Appendix 9]
An optical system according to any one of Supplementary Notes 1 to 8;
the light source;
a collector mirror disposed relative to the light source and configured to receive the illumination light from the light source;
Equipped with
the collector mirror has four segmented optical surfaces;
Projection exposure equipment.
[Supplementary Note 10]
10. The projection exposure apparatus according to claim 9,
further comprising an actuator disposed for each of the four optical surfaces of the collector mirror, the actuator changing the angle of the optical surface;
Projection exposure equipment.
[Appendix 11]
11. A projection exposure apparatus according to claim 9 or 10,
The optical surface constitutes a toroidal mirror.
Projection exposure equipment.
[Appendix 12]
12. A projection exposure apparatus according to any one of claims 9 to 11,
The illumination light emitted from the collector mirror forms two sheet beams at an intermediate focus.
Projection exposure equipment.
[Appendix 13]
13. The projection exposure apparatus according to claim 12,
a collimator having a double slit disposed at the intermediate focus and passing the two sheet beams;
Projection exposure equipment.
[Appendix 14]
14. A projection exposure apparatus according to claim 12 or 13,
a transparent window disposed on the optical axis downstream of the intermediate focus to block debris from the light source;
Projection exposure equipment.
[Appendix 15]
15. A projection exposure apparatus according to any one of claims 9 to 14,
a fourth mirror disposed upstream of the pair of first mirrors and configured to converge the illumination light;
Projection exposure equipment.
[Appendix 16]
16. The projection exposure apparatus according to claim 15,
the fourth mirror is a cylindrical mirror;
Projection exposure equipment.
[Appendix 17]
A projection exposure system comprising an optical system according to any one of Supplementary Notes 1 to 8 or a projection exposure apparatus according to any one of Supplementary Notes 9 to 16, and the mask,
the mask is flat along a scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction;
Projection exposure system.

1 Projection exposure system 10 Projection exposure apparatus 20 Optical system 21 First mirror 22 Projection system 221 Second mirror 222 Third mirror 223 Light shielding section 30 Illumination system 31 Light source 32 Collector mirror 32a1 Third optical surface 32a2 Fourth optical surface 32b1 Fifth optical surface 32b2 Sixth optical surface 321 Actuator 33 Collimator 34 Transparent window 35 Fourth mirror 36 Line scan slit 40 Mask A Central axis B Defect C Center C1 Diffraction cone C2 Diffraction cone D Diameter F Focal plane H1 First opening H2 Second opening IF Intermediate focus L0 Illumination light L1 Reflected light R Writing area S1 First optical surface S2 Second optical surface W Wafer

Claims (17)

An optical system of a projection exposure apparatus, which is arranged between a mask and a wafer,
a pair of first mirrors each receiving illumination light from a light source and reflecting it onto the mask;
a projection system having two mirrors that receives light reflected by the mask and directs it onto the wafer;
Equipped with
the projection system
a second mirror disposed adjacent to the pair of first mirrors;
a third mirror disposed on the wafer side relative to the second mirror and having a second optical surface facing the first optical surface of the second mirror;
and
a center portion between the pair of first mirrors, the second mirror, and the third mirror are located on the same straight line;
optical system. 2. The optical system according to claim 1,
the illumination light reflected by the pair of first mirrors forms a first exposure field and a second exposure field on the mask;
the first exposure field and the second exposure field are separate from each other;
optical system. 3. The optical system according to claim 1,
The second mirror and the third mirror are located on the same central axis.
optical system. 3. The optical system according to claim 1,
each of the second mirror and the third mirror is an axially symmetric aspherical mirror;
optical system. 3. The optical system according to claim 1,
the first optical surface and the second optical surface have substantially the same radius of curvature;
optical system. 3. The optical system according to claim 1,
the second mirror has a first opening that guides the reflected light from the outside to the inside of the projection system;
the third mirror has a second opening that guides the reflected light from the inside to the outside of the projection system;
a Fourier image at a focal plane located between the first optical surface and the second optical surface has a defect at the center caused by the first opening and the second opening, and is an image having four bright points that are located outside the center and symmetrically arranged with respect to each other with respect to the center;
optical system. 7. The optical system according to claim 6,
the projection system directs the reflected light onto the wafer with symmetric off-axis illumination from four directions;
optical system. 3. The optical system according to claim 1,
Each of the pair of first mirrors is a cylindrical mirror.
optical system. The optical system according to claim 1;
the light source;
a collector mirror disposed relative to the light source and configured to receive the illumination light from the light source;
Equipped with
the collector mirror has four segmented optical surfaces;
Projection exposure equipment. 10. A projection exposure apparatus according to claim 9,
further comprising an actuator disposed for each of the four optical surfaces of the collector mirror, the actuator changing the angle of the optical surface;
Projection exposure equipment. 11. A projection exposure apparatus according to claim 9, wherein
The optical surface constitutes a toroidal mirror.
Projection exposure equipment. 11. A projection exposure apparatus according to claim 9, wherein
The illumination light emitted from the collector mirror forms two sheet beams at an intermediate focus.
Projection exposure equipment. 13. A projection exposure apparatus according to claim 12,
a collimator having a double slit disposed at the intermediate focus and passing the two sheet beams;
Projection exposure equipment. 13. A projection exposure apparatus according to claim 12,
a transparent window disposed on the optical axis downstream of the intermediate focus to block debris from the light source;
Projection exposure equipment. 11. A projection exposure apparatus according to claim 9, wherein
a fourth mirror disposed upstream of the pair of first mirrors and configured to converge the illumination light;
Projection exposure equipment. 16. A projection exposure apparatus according to claim 15,
the fourth mirror is a cylindrical mirror;
Projection exposure equipment. 11. A projection exposure system comprising the optical system according to claim 1 or 2 or the projection exposure apparatus according to claim 9 or 10, and the mask,
the mask is flat along a scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction;
Projection exposure system.