Optical system, projection exposure device, and projection exposure system

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

[0001] This application claims priority from Japanese Patent Application No. 2024-065769, filed on April 15, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] Conventionally, techniques related to projection exposure apparatuses used in semiconductor manufacturing processes are 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 by a pupil facet mirror and / or its pupil facets and / or a transfer optical system may be advantageous.

[0003] Special Publication No. 2021-503623

[0004] However, in the prior art described in Patent Document 1, eight mirrors are used in the optical system located between the mask and the wafer, and the power loss of the reflected light obtained when the illumination light is reflected by the mask before reaching the wafer is large. In order to direct reflected light with sufficient power onto the wafer, it is necessary to increase the output power of the light source of the projection exposure apparatus, 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.

[0005] An object of the present disclosure is to provide an optical system, a projection exposure apparatus, and a projection exposure system that are capable of improving energy efficiency.

[0006] An optical system according to a first aspect for solving the above problem is an optical system of a projection exposure apparatus that is arranged between a mask and a wafer, and comprises: a pair of first mirrors that each receive illumination light from a light source and reflect it onto the mask; and a projection system with two mirrors that receives the light reflected by the mask and directs it to the wafer, wherein the projection system comprises: a second mirror arranged adjacent to the pair of first mirrors; and a third mirror that is arranged on the wafer side of the second mirrors and has a second optical surface that faces the first optical surface of the second mirror, and wherein the center between the pair of first mirrors, the second mirror, and the third mirror are located on the same straight line.

[0007] A projection exposure apparatus according to a second aspect comprises the above-described optical system, the light source, and a collector mirror disposed relative to the light source and receiving the illumination light from the light source, wherein the collector mirror has four segmented optical surfaces.

[0008] A projection exposure system according to a third aspect is a projection exposure system comprising the above optical system or the above projection exposure apparatus, and the above mask, wherein the mask is flat along the scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction.

[0009] According to an embodiment of the present disclosure, an optical system, a projection exposure apparatus, and a projection exposure system can improve energy efficiency.

[0010] 1 is a design diagram showing 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 the optical surface of the collector mirror of FIG. 1; FIG. 3 is a schematic diagram showing an example of the optical system of the projection exposure apparatus of FIG. 1; FIG. 4 is a graph showing an example of the function of the mask of FIG. 7; FIG. 5 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 6 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 7 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 8 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 9 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 10 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; FIG. 11 is a diagram showing an example of the function of the projection exposure apparatus of FIG. 1; 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 thirteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 14 is a thirteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 15 is a thirteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 16 is a thirteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 17 is a thirteenth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 18 is a ninth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 20 is a twentieth diagram for explaining an example of the function of the projection exposure apparatus of FIG. 1. FIG. 1

[0011] An embodiment of the present disclosure will be mainly described below 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.

[0012] FIG. 1 is a design diagram 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 from a mask 40 to a wafer W are shown inside a projector including a projection system 22 (described later), and illumination is omitted. An example of the configuration and functions of the projection exposure system 1 according to an embodiment of the present disclosure will be mainly described with reference to FIG. 1. The projection exposure system 1 includes a projection exposure apparatus 10 and a mask 40 for drawing a circuit pattern on a wafer W. The projection exposure system 1 constitutes, for example, an EUV lithography (Extreme Ultraviolet Lithography) system.

[0013] 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.

[0014] 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, in order from upstream to downstream, an illumination system 30 and a projection system 22. The projection exposure apparatus 10 has, in order 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.

[0015] The light source 31 includes an EUV light source such as a 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.

[0016] 2 is a schematic diagram showing an example of the optical surfaces of the collector mirror 32 in FIG. 1. The collector mirror 32 is disposed with respect 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 FIG. 2. Each of these four optical surfaces constitutes, for example, a toroidal mirror.

[0017] The projection exposure apparatus 10 further includes actuators 321 that are arranged for each of the four optical surfaces of the collector mirror 32 and change the angle of the optical surface. For example, as shown in Fig. 11, which will be described later, the actuators 321 include a first actuator that is arranged for the third optical surface 32a1, a second actuator that is arranged for the fourth optical surface 32a2, a third actuator that is arranged for the fifth optical surface 32b1, and a fourth actuator that is arranged for the sixth optical surface 32b2.

[0018] 1 , the illumination light L0 emitted from the collector mirror 32 forms two sheet beams at an 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 disposed at the intermediate focus IF and has a double slit that passes the two sheet beams. The collimator 33 passes the illumination light L0 that has been reflected by the collector mirror 32 and entered as two sheet beams, and guides it to the transparent window 34 located downstream.

[0019] The transparent window 34 is disposed 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.

[0020] The fourth mirror 35 is disposed 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 disposed upstream of the pair of first mirrors 21 and converges the illumination light L0. The fourth mirror 35 is, for example, a cylindrical mirror.

[0021] Each of the pair of first mirrors 21 is disposed downstream of the fourth mirror 35 and further reflects the illumination light L0 reflected by the fourth mirror 35 toward the mask 40. Each of the pair of first mirrors 21 is, for example, a cylindrical mirror. As will be described later, 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.

[0022] The projection exposure apparatus 10 further includes a line scan slit 36 ​​that is arranged to overlap 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 is incident on 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 is incident on the inside of the projection system 22. In the present disclosure, the "reflected light L1" includes the illumination light L0 reflected by the mask 40 and diffracted light that has structural information of the logic pattern on the mask 40.

[0023] Fig. 3 is a schematic diagram mainly showing the optical system 20 of the projection exposure apparatus 10 of Fig. 1. For the purpose of simplifying the illustration, Fig. 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 Fig. 1, and mainly shows the configuration of the optical system 20.

[0024] The optical system 20 of the projection exposure apparatus 10 is disposed 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.

[0025] The projection system 22 includes a second mirror 221 disposed adjacent to the pair of first mirrors 21, and a third mirror 222 disposed on the opposite side of the pair of first mirrors 21 with respect 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 such 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 between the mask 40 and the wafer W.

[0026] Each of the second mirror 221 and the third mirror 222 may be disposed perpendicular to the central axis A with its center positioned on the central axis A, or may be disposed at an angle relative to the central axis A. The mask 40 may be disposed perpendicular to the central axis A. The mask 40 is disposed, for example, so as to face the projection system 22 without being tilted. Similarly, the wafer W may be disposed perpendicular to the central axis A. The wafer W is disposed, for example, so as to face the projection system 22 without being tilted.

[0027] 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" means that 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 the 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 the 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.

[0028] 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 inside of the projection system 22. The reflected light L1, which passes through the first opening H1 and enters the inside of 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 guided to the wafer W.

[0029] 1 located between the first optical surface S1 and the second optical surface S2 is an image having, for example, a defect at the center caused by the first opening H1 and the second opening H2 and four bright spots arranged symmetrically with respect to each other outside the center. In this disclosure, the "four bright spots" correspond to, for example, zeroth Bragg spots obtained by the Fourier transform of the pattern on the mask 40, which are obtained in addition to the bright spot at the origin when the illumination light L0 is Fourier transformed, where the illumination light L0 is concentrated. The projection system 22 guides the reflected light L1 to the wafer W by, for example, symmetric off-axis illumination from four directions.

[0030] 1, the projection exposure apparatus 10 further includes a light-shielding portion 223 that is located on the central axis A of the projection system 22 inside 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.

[0031] 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.

[0032] 1, the scanning direction is, for example, the direction 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, the direction along the y-axis.

[0033] The optical system 20, the projection exposure apparatus 10, and the 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.

[0034] For example, multi-layer mirrors absorb more than 30% of the EUV optical power at each reflection. In a conventional projection exposure apparatus, for example, six mirrors are arranged in the projection optics and four mirrors in the illumination optics. The transmission of optical power from the EUV light source to the wafer is very low.

[0035] 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.

[0036] 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 driving laser system is also 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-film 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.

[0037] 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 the 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.

[0038] A low NA facilitates optical aberration correction because the light rays pass close to the axis. Only two aspherical mirrors, the second mirror 221 and the third mirror 222, are required to cover a reasonably large image field. As described below, optical simulations confirmed that a 2-m-tall projector with an NA of 0.2 can provide a 20-mm image field. 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:

[0039] Here, k １ is the process coefficient, λ is the wavelength, and NA is the numerical aperture. The spatial resolution is determined in two cases.

[0040] Here, k １ is assumed to be 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 with a half pitch of 24 nm.

[0041] Additionally, the depth of focus (DOF) is also important, and is defined as: Substituting equation (3) into equation (6) yields a dimensionless relationship.

[0042] Equation (7) shows that a lower NA always results in a longer DOF. In the two cases: In either case, k ２ Assume that =1.

[0043] It is clear that low NA EUV has the advantage of a longer DOF. Furthermore, compared to a typical EUV projector that uses oblique illumination on the mask, an in-line projector, due to average perpendicular illumination, does not exhibit EUV-specific image variations 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.

[0044] 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.

[0045] 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 precise mirrors are enclosed in a tube, which offers several advantages, such as mechanical stability, ease of assembly, alignment, replacement, etc., and excellent sealing to protect against dust contamination. This reduces capital and maintenance costs and increases reliability.

[0046] The projection exposure system 1 according to an embodiment of the present disclosure will be described in more detail below using examples, but the present disclosure is not 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.

[0047] In order to increase the field size, it is necessary to increase the length of the projector including the projection system 22. It is assumed that the height of the module including the projector is within the maximum size allowed in an actual semiconductor factory. That is, OID = 2000 mm. OID is the object-image distance. In FIG. 1, the distance from the wafer W to the mask 40 is 2000 mm.

[0048] 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.

[0049] The simulator predicted a 20 mm field with NA=0.2, which encompasses a 100 mm mask field. The image reduction ratio is 1 / 5. As will be described later, a curved mask 40 is used to correct the residual field curvature for large field sizes and reduce wavefront errors. The simulator used was the OpTaLix simulator.

[0050] FIG. 4 is a first diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 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 pupil apodization and aperture stop are not 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.

[0051] The second mirror 221 and the third mirror 222 are each an axially symmetric aspherical mirror. 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 closely 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.

[0052]

[0053]

[0054] 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 depending on the reflection angle. More careful simulations including multi-layer coatings are required. In practice, the quality of the mirrors must be measured using an interferometer.

[0055] The wafer W side is telecentric, but the mask 40 side is not. Therefore, the chief ray is tilted by 1.6° (approximately 50 mm / 2000 mm radians) at the edge of the field. Considering 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, the pattern shift due to defocus is extremely small; a mask height error of 500 nm only results in a pattern shift of less than 3 nm at the field edge. This shift is considered to be within the acceptable range.

[0056] It should be noted that all the rays from the 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.

[0057] FIG. 5 is a second diagram for explaining an example of the functioning of the projection exposure apparatus 10 of FIG. 1. The graph in FIG. 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. FIG. 6 is a third diagram for explaining an example of the functioning of the projection exposure apparatus 10 of FIG. 1. FIG. 6 shows the wavefront aberration for the field edge y=10 mm.

[0058] Figures 5 and 6 show the wavefront aberrations. The optical path difference error decreases at smaller image heights. However, residual aberrations at the field edge reach a limit of 0.05 wavelengths. Due to the low NA, the Strehl ratio at the field edge remains high (0.991). Note that the Strehl ratio is evaluated without central defects and on-axis illumination. Tilting the illumination allows high-frequency components to pass through the projector, resulting in higher 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 quadruple off-axis illumination becomes smaller, meaning that aberrations are effectively reduced.

[0059] 7 is a schematic diagram showing an example of the configuration of a mask 40 included in the projection exposure system 1 of FIG. 1. This figure 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. This radius of curvature is, for example, several meters to several hundred meters.

[0060] Due to the limited number of mirrors in the optical system 20, the projected image is not perfectly flat but curved. The optimum focus varies with the field height, resulting in the wavefront error 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 the mask 40 in the y direction, while the mask 40 is flat in the x direction (scanning direction). The maximum bending at the field edge is about a few micrometers, which is significantly smaller than the mask width of ∼100 mm. Therefore, it is unlikely to cause mechanical damage to the structure of the mask 40.

[0061] Figure 8 is a graph illustrating an example of the function of the mask 40 of Figure 7. Figure 8 shows the improved wavefront error using the curved mask of 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 1500 mm OID.

[0062] FIG. 9A is a fourth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 9A shows the state of beam defects at the third mirror 222. FIG. 9B is a fifth diagram illustrating an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 9B shows the state of beam defects at the second mirror 221. The second aperture H2 and the first aperture H1 as beam holes are designed to match the beam edge of an NA 0.2 with a 2 mm gap around the beam. The three circles indicate the diffraction cone on the axis and both field edges. The diameter D in FIG. 9A is 163 mm. The diameter D in FIG. 9B is 517 mm.

[0063] In the two-mirror in-line projector design described above, the beam aperture is located in the center of the mirror, which inevitably leads to the problem of defects. Although it is not easy to completely avoid this problem through projector design alone, it is possible to substantially reduce its impact on the projected pattern. To solve this problem, there are three possible approaches: (a) make the beam aperture as small as possible; (b) optimize the off-axis illumination; and (c) optimize the partial coherence factor.

[0064] 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 for the obscuration factor and the lowercase Greek letter σ is used for the partial coherence factor.

[0065] 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.

[0066] 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). Because the NA is low, the horizontal defect is small. 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.

[0067] 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 > Σ ｙ = 0.26 (13)

[0068] In the special case of staggered contact holes, the diffraction pattern has a 60° rotational symmetry, so the diffraction spots can still reach the defect areas. The partial coherence factor can partially avoid the defect spots. If 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.

[0069] In the projection exposure apparatus 10 using a two-mirror projector shown in FIG. 1 , the collector mirror 32 of the light source 31 serving as an EUV source is composed of four segmented toroidal quad mirrors, providing quadrupole off-axis illumination that avoids central defects 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.

[0070] The diameter of a tin droplet is 20 to 30 μm, whereas 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). Therefore, to increase the size of the virtual source, the fourth mirror 35 must have a ripple mirror surface (described later as a "partially coherent source").

[0071] The projector is composed of two aspherical mirrors, the second mirror 221 and the third mirror 222. In Fig. 1, the maximum reflection angle θ of the third mirror 222 is 11°, which is within the bandwidth of the Mo / Si multilayer coating. This allows a high reflection coefficient with a uniform coating and negligible phase variation, resulting in high contrast.

[0072] The intermediate focus IF of the EUV light source 31 is not a spot but two sheet beams. ２ To remove unwanted stray light and infrared light from the drive laser, a double slit is used as a collimator 33. A transparent window 34 is installed to separate the clean vacuum environment within the projector from the light source 31, i.e., to block microdebris from the tin (Tin) plasma light source.

[0073] 10 is a schematic diagram showing the state of the diffraction cone at the optical surface S2 of the third mirror 222 in FIG. 1. As shown in FIG. 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. Meanwhile, 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 × 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.

[0074] The parameters for each mirror included in the illumination system 30 are summarized in a table below.

[0075] Figure 11 is a sixth diagram for explaining an example of the function of the projection exposure apparatus 10 of Figure 1. Figure 11 shows an unfolded illumination path from the EUV light source 31 to the mask 40 and wafer W. Quadrupole illumination is realized using a quad collector mirror 32 (toroidal focusing mirror). The illumination system 30 has two cylindrical mirrors, a fourth mirror 35 and a first mirror 21, which perform optical shaping of the scan line field.

[0076] In the actual projection exposure apparatus 10, all mirrors and the mask 40 reflect light and change the direction, but in Figure 11, each component is virtually represented as transmitting light like a lens. １ , a ２ , b １ , b ２ , which generates quadrupole off-axis illumination on the mask 40, thereby improving resolution and avoiding central defects. The four spots on the focal plane F are images of the tin plasma of the EUV light source 31, realizing Kohler illumination. The illumination on the mask 40 is split into two lines (double exposure fields) to avoid blocking the pair of first mirrors 21, which are cylindrical mirrors.

[0077] FIG. 12A is a 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 an 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 a 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.

[0078] As shown in FIG. 12A , the field on the mask 40 is divided into two lines, and each reflected light beam L1 of the illumination light L0 passes through a position between and close to the pair of first mirrors 21. To prevent the cylindrical first mirrors 21 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, because 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 FIG. 12B .

[0079] First, the mask 40 (b １ +b ２ ) field and b １ and b ２ Subsequently, the reticle image is projected onto the wafer W. The mask 40 is moved in a scanning motion to project the same reticle pattern (a １ +a ２) fields to produce a reticle image on the wafer W as shown in Figure 12C. Because the EUV light source 31 is not spatially or temporally coherent, the illumination generates independent photoactivations in the resist layer. Therefore, the total activation pattern is equal to the sum of the two exposure fields.

[0080] The 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 performed in one scan (b １ +b ２ ) and (a １ +a ２ ) is repeated twice.

[0081] Fig. 13 is a tenth diagram for explaining an example of the function of the projection exposure apparatus 10 of Fig. 1. Fig. 13 shows the trajectories 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. Fig. 13 shows on-axis illumination with no tilt.

[0082] Here, the convergent illumination condition conceptually shown in FIG. 13 will be described in detail. In conventional optical lithography, the telecentricity condition is satisfied on both the mask and the wafer. However, in the two-mirror projector of the present disclosure, the wafer W side is telecentric, but the mask 40 side is not. Therefore, it is necessary to provide convergent illumination that is tailored to the projector as follows:

[0083] (1) The fourth mirror 35, which is a cylindrical mirror, focuses the 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 arrives at the wafer W from a normal, i.e., telecentric, state. (5) To create off-axis illumination, i.e., to shift the spot outside the focal plane F, the incident light is tilted onto the mask 40, without changing the illumination position. (6) To create 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 angle in the (x, y) direction is 1 / 1.414 times smaller, and the sign is a １ = (+, +), a ２ = (+, -), b １ = (-, +), b ２ (7) Details are determined using the OpTaLix simulator with the mirror parameters shown in Tables 1 to 4. The curvature radius of the toroidal mirror and cylindrical mirror is determined by two numbers (R ｘ , R ｙ It is important to note that the ｘ = infinity means that the mirror is flat around the x direction.

[0084] In the case of off-axis illumination, each illumination may not be perpendicular to the wafer W, which may destroy the telecentric state and cause an image shift due to a deviation (defocus) in the z-direction of the wafer W position. However, this effect is tolerable if the four illuminations are symmetric. Therefore, in this disclosure, quadrupole illumination arranged symmetrically around the axis is used. Balancing the Fourier components is very important in lithography because it maximizes spatial resolution and reduces unwanted shadows around the pattern in the created image.

[0085] In the following, a partially coherent source is described.

[0086] When using point source illumination, the 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.

[0087] The partial coherence factor is defined as follows: For a point light source, σ = 0. This is called coherent illumination.

[0088] 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. It should be noted that partially coherent illumination also mitigates the effect of central defects. 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.

[0089] The source size in the x and y directions will be described separately. First, since 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 eleventh diagram for explaining an example of the function of the projection exposure apparatus 10 of Figure 1. As shown in Figure 14, it is assumed 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.

[0090] Since phase space area is preserved by 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 to 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 satisfies the required value. The phase space is cut by the acceptance between the first mirror 21 and the line scan slit 36. 60% of the photon flux can reach the wafer W. In FIG. 14(c), the dashed line indicates the pair illumination.

[0091] Fig. 15A is a twelfth diagram for explaining an example of the function of the projection exposure apparatus 10 of Fig. 1. Fig. 15B is a thirteenth diagram for explaining an example of the function of the projection exposure apparatus 10 of Fig. 1. Fig. 15C is a fourteenth diagram for explaining an example of the function of the projection exposure apparatus 10 of Fig. 1.

[0092] 15A to 15C show schematic diagrams of the optical acceptance between the first mirror 21 and the line scan slit 36. The 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 the collimated illumination from the first mirror 21 at a nominal angle θ ０ 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 side. 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 side.

[0093] 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) where w is the scan line width and 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 in the first mirror 21 and the slit.

[0094] 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. The "ripple mirror" was originally introduced for curved exposure fields. The mirror surface has periodic undulations that mix the light rays, effectively increasing the partial coherence factor without significant light loss.

[0095] 16 is a fifteenth diagram for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 16 shows the partial coherence factor σ ｘ = 0.25, σ ｙ The quadrupole off-axis illumination pattern at the focal plane F is shown, taking into account λ = 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 on 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. Then, the critical dimension is as follows:

[0096]

[0097] FIG. 17 is a 16th diagram for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 18A is a 17th diagram for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 18B is an 18th diagram for explaining 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 for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1. FIG. 19B is a 20th diagram for explaining an example of the function of the projection exposure apparatus 10 of FIG. 1.

[0098] To investigate the imaging capabilities 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, the partial coherence factor σｘ = 0.25, σ ｙ = 0.2 and also takes into account the partial blocking effect of diffracted light by the first mirror 21, making this a realistic calculation.

[0099] 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.

[0100] It will be apparent to those skilled in the art that the present disclosure may be embodied in other specific forms other 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 not by the foregoing description but by the appended claims. All modifications within the range of equivalents of any modifications are intended to be embraced therein.

[0101] For example, the shape, pattern, size, arrangement, orientation, type, and number of each of the above-mentioned components 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 the function can be realized. The illustrated components of the optical system 20, projection exposure apparatus 10, and projection exposure system 1 are functional concepts, and the specific form of each component is not limited to those shown.

[0102] In the above embodiment, the third mirror 222 is described as being disposed on the opposite side of the pair of first mirrors 21 with respect to the second mirror 221, but this is not limited to this. The third mirror 222 only needs to be disposed on the wafer W side with respect to the second mirror 221, and the positions of the pair of first mirrors 21 with respect to the second mirror 221 are not limited to the positions shown in FIG. 1 and the like. In other words, the pair of first mirrors 21 are not limited to a configuration in which they are located outside the projection system 22, but may be located inside the projection system 22. The pair of first mirrors 21 may be located between the second mirror 221 and the third mirror 222.

[0103] In the above embodiment, the second mirror 221 and the third mirror 222 are described as being located on the same central axis A. However, 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.

[0104] In the above embodiment, each of the second mirror 221 and the third mirror 222 is described as an axisymmetric aspherical mirror, but this is not limited to this. Each of the second mirror 221 and the third mirror 222 does not have to be axisymmetric. Each of the second mirror 221 and the third mirror 222 may be a type of mirror other than an aspherical mirror that can achieve the functions of the present disclosure.

[0105] In the above embodiment, the first optical surface S1 and the second optical surface S2 have been described as having substantially the same radius of curvature, but this is not limited thereto. 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 the curved surface mask described with reference to FIGS. 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.

[0106] In the above embodiment, the second mirror 221 has been described as having the 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 the configuration of the first opening H1, such as a first through-hole, and may have any other configuration that can guide 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 or the like.

[0107] In the above embodiment, the third mirror 222 has been described as having the 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 the configuration of the second opening H2, such as a second through-hole, and may have any other configuration that can guide 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 or the like.

[0108] In the above embodiment, the Fourier image at focal plane F is described as an image having a defect B at the center and four bright spots arranged symmetrically with respect to the center 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.

[0109] 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 limiting. The projection system 22 may direct the reflected light L1 to the wafer W by off-axis illumination from a number of directions other than four.

[0110] In the above embodiment, each of the pair of first mirrors 21 is described as a cylindrical mirror, but this is not limiting. Each of the pair of first mirrors 21 may be any other type of mirror that can achieve the functions of the present disclosure.

[0111] In the above embodiment, the collector mirror 32 has been described as having four segmented optical surfaces, but is not limited to this. The collector mirror 32 may have optical surfaces that are segmented by a number other than four.

[0112] In the above embodiment, the projection exposure apparatus 10 is described as further including actuators 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 limiting. 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.

[0113] In the above embodiment, the optical surface is described as constituting a toroidal mirror, but is not limited to this. The optical surface may be constituting any other type of mirror that can achieve the functions of the present disclosure.

[0114] In the above embodiment, the illumination light L0 emitted from the collector mirror 32 forms two sheet beams at the intermediate focus IF, but this is not limiting. The illumination light L0 emitted from the collector mirror 32 may form at least one of a different number and shape of beams at the intermediate focus IF.

[0115] In the above embodiment, the projection exposure apparatus 10 is described as further including a collimator 33 with a double slit that is arranged at the intermediate focus IF and that allows the two sheet beams to pass through, but this is not limiting. The projection exposure apparatus 10 may also include other optical elements depending on the configuration of the beams at the intermediate focus IF.

[0116] In the above embodiment, the projection exposure apparatus 10 is described as further including a transparent window 34 that is arranged on the optical axis downstream of the intermediate focus IF and blocks debris from the light source 31, but this is not limiting. The projection exposure apparatus 10 does not need to include the transparent window 34 as long as the impact of debris from the light source 31 on the downstream side is small.

[0117] In the above embodiment, the projection exposure apparatus 10 is described as further including a fourth mirror 35 that is disposed upstream of the pair of first mirrors 21 and converges the illumination light L0, but this is not limiting. 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.

[0118] In the above embodiment, the fourth mirror 35 and the pair of first mirrors 21 form the first and second exposure fields of the illumination light L0 on the mask 40, and the first and second exposure fields 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 Figure 11, and may be configured with at least one of other numbers, shapes, and arrangements.

[0119] 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.

[0120] 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 the 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 be flat along the direction perpendicular to the scanning direction.

[0121] 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 the 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.

[0122] In the above embodiment, the mask 40 and the wafer W are described as moving in opposite directions by scanning, but this is not limited to this. The mask 40 and the wafer W may each be 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 the wafer W, instead of functioning as a scanner.

[0123] 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 the 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 stepwise repeating the above operations by the illumination system 30.

[0124] As shown in FIG. 20A, illumination light L0 from a light source 31 passes through other components of the illumination system 30 and is guided to a pair of first mirrors 21. As the pair of first mirrors 21 are scanned and moved in a predetermined direction, illumination light L0 is scanned over two exposure fields (a １ +a ２、 b １ +b ２ ) is formed. 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.

[0125] The rectangular drawing area R indicates, for example, a unit exposure field illuminated for each step in a stepper operation. １ +a ２ ) field and (b １ +b ２ ) fields. The drawing area R indicates the range onto which a pattern is transferred in one step by the illumination light L0, and is formed as a rectangular or square exposure field. The projection exposure apparatus 10 can precisely overlap the drawing areas R in multiple stages, thereby forming the entire two-dimensional pattern on the mask 40 on the wafer W with high precision.

[0126] 20B shows a schematic illustration of the illumination distribution at the entrance pupil formed at the focal plane F. In the projection exposure apparatus 10 according to the modified example, four spots are also arranged by quadrupole off-axial illumination. The Fourier image at the 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 the center outside the center.

[0127] 20C, the reflected light L1 reflected by the mask 40 passes through the second mirror 221 and the third mirror 222 of the projection system 22 and is irradiated onto the wafer W. In the stepper mode, the wafer W is also 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.

[0128] 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 the wafer W. The projection exposure apparatus 10 does not require a scanning mechanism that precisely synchronizes the mask 40 and the 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.

[0129] Some embodiments of the present disclosure are described below. However, it should be noted that the embodiments of the present disclosure are not limited to these. [Supplementary Note 1] An optical system of a projection exposure apparatus arranged between a mask and a wafer, comprising: a pair of first mirrors that each receive illumination light from a light source and reflect it to the mask; and a projection system having two mirrors that receives the light reflected by the mask and directs it to the wafer, wherein the projection system has: a second mirror arranged adjacent to the pair of first mirrors; and a third mirror that is arranged on the wafer side of the second mirrors and has a second optical surface facing the first optical surface of the second mirror, wherein the center between the pair of first mirrors, the second mirror, and the third mirror are located on the same straight line. [Supplementary Note 2] The optical system according to Supplementary Note 1, wherein the illumination light reflected by the pair of first mirrors forms a first exposure field and a second exposure field on the mask, and the first exposure field and the second exposure field are separated from each other. [Supplementary Note 3] The optical system according to Supplementary Note 1 or 2, wherein the second mirror and the third mirror are located on the same central axis. [Supplementary Note 4] The optical system according to any one of Supplements 1 to 3, wherein the second mirror and the third mirror are each an axially symmetric aspherical mirror. [Supplementary Note 5] The optical system according to any one of Supplements 1 to 4, wherein the first optical surface and the second optical surface have approximately the same radius of curvature. [Supplementary Note 6] The optical system according to any one of Supplements 1 to 5, wherein the second mirror has a first opening that guides the reflected light from outside to inside the projection system, and the third mirror has a second opening that guides the reflected light from inside to outside the projection system, and a Fourier image at a focal plane located between the first optical surface and the second optical surface has a defect in the center due to the first opening and the second opening, and has four bright points that are arranged symmetrically with respect to the center and outside the center.[Supplementary Note 7] The optical system according to Supplementary Note 6, wherein the projection system guides the reflected light to the wafer by symmetric off-axis illumination from four directions. [Supplementary Note 8] The optical system according to any one of Supplements 1 to 7, wherein each of the pair of first mirrors is a cylindrical mirror. [Supplementary Note 9] A projection exposure apparatus comprising: the optical system according to any one of Supplements 1 to 8; the light source; and a collector mirror arranged relative to the light source and receiving the illumination light from the light source, wherein the collector mirror has four segmented optical surfaces. [Supplementary Note 10] The projection exposure apparatus according to Supplementary Note 9, further comprising actuators arranged relative to each of the four optical surfaces on the collector mirror and for changing the angles of the optical surfaces. [Supplementary Note 11] The projection exposure apparatus according to Supplementary Note 9 or 10, wherein the optical surfaces constitute toroidal mirrors. [Supplementary Note 12] The projection exposure apparatus according to any one of Supplements 9 to 11, wherein the illumination light emitted from the collector mirror forms two sheet beams at an intermediate focus. [Supplementary Note 13] The projection exposure apparatus according to Supplementary Note 12, further comprising a collimator having a double slit, arranged at the intermediate focus, for passing the two sheet beams. [Supplementary Note 14] The projection exposure apparatus according to Supplementary Note 12 or 13, further comprising a transparent window, arranged on the optical axis downstream of the intermediate focus, for blocking debris from the light source. [Supplementary Note 15] The projection exposure apparatus according to any one of Supplements 9 to 14, further comprising a fourth mirror, arranged upstream of the pair of first mirrors, for converging the illumination light. [Supplementary Note 16] The projection exposure apparatus according to Supplementary Note 15, wherein the fourth mirror is a cylindrical mirror.[Supplementary Note 17] A projection exposure system comprising the optical system according to any one of Supplements 1 to 8 or the projection exposure apparatus according to any one of Supplements 9 to 16, and the mask, wherein the mask is flat along a scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction.

[0130] 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

1. An optical system for a projection exposure apparatus that is positioned between a mask and a wafer, comprising: a pair of first mirrors that each receive illumination light from a light source and reflect it to the mask; and a projection system with two mirrors that receives the light reflected by the mask and directs it to the wafer, wherein the projection system has: a second mirror that is positioned adjacent to the pair of first mirrors; and a third mirror that is positioned on the wafer side of the second mirrors and has a second optical surface that faces the first optical surface of the second mirror, and wherein the center between the pair of first mirrors, the second mirror, and the third mirror are positioned on the same straight line.

2. An optical system according to claim 1, wherein the illumination light reflected by the pair of first mirrors forms a first exposure field and a second exposure field on the mask, and the first exposure field and the second exposure field are separated from each other.

3. An optical system according to claim 1 or 2, wherein the second mirror and the third mirror are positioned on the same central axis.

4. An optical system according to claim 1 or 2, wherein each of the second mirror and the third mirror is an axially symmetric aspherical mirror.

5. An optical system according to claim 1 or 2, wherein the first optical surface and the second optical surface have substantially the same radius of curvature.

6. An optical system according to claim 1 or 2, wherein the second mirror has a first opening that guides the reflected light from the outside to the inside of the projection system, and the third mirror has a second opening that guides the reflected light from the inside to the outside of the projection system, and a Fourier image at a focal plane located between the first optical surface and the second optical surface has a defect in the center caused by the first opening and the second opening, and is an image having four bright points located outside the center and symmetrically arranged with respect to each other with respect to the center.

7. The optical system of claim 6, wherein the projection system directs the reflected light onto the wafer with symmetric off-axis illumination from four directions.

8. An optical system according to claim 1 or 2, wherein each of the pair of first mirrors is a cylindrical mirror.

9. A projection exposure apparatus comprising: the optical system according to claim 1; the light source; and a collector mirror disposed relative to the light source and receiving the illumination light from the light source, wherein the collector mirror has four segmented optical surfaces.

10. A projection exposure apparatus according to claim 9, further comprising an actuator arranged for each of the four optical surfaces in the collector mirror, for changing the angle of the optical surface.

11. A projection exposure apparatus according to claim 9 or 10, wherein the optical surface constitutes a toroidal mirror.

12. A projection exposure apparatus according to claim 9 or 10, wherein the illumination light emitted from the collector mirror forms two sheet beams at an intermediate focus.

13. A projection exposure apparatus according to claim 12, further comprising a collimator having a double slit arranged at the intermediate focus and passing the two sheet beams.

14. A projection exposure apparatus according to claim 12, further comprising a transparent window arranged on the optical axis downstream of the intermediate focus, for blocking debris from the light source.

15. A projection exposure apparatus according to claim 9 or 10, further comprising a fourth mirror arranged upstream of the pair of first mirrors to converge the illumination light.

16. A projection exposure apparatus according to claim 15, wherein the fourth mirror is a cylindrical mirror.

17. 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, wherein the mask is flat along the scanning direction of the mask and has a curved surface along a direction perpendicular to the scanning direction.

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