Reflection-type projection-optical systems, and exposure apparatus comprising same

Projection-optical systems are disclosed that have a large image-side NA of, e.g., ≧0.45, and that allow inspection for surface-shape errors of reflecting surfaces with prescribed precision. A first reflective image-forming optical system forms an intermediate image of a first surface. A second reflective image-forming optical system forms a reduced image, on a second surface, of the intermediate image. The first reflective image-forming optical system has a first concave mirror, a second convex mirror, a third mirror, and a fourth concave mirror. The second reflective image-forming optical system has a fifth concave mirror, a sixth mirror, a seventh convex mirror, and an eighth concave mirror. If the absolute value of the center radius of curvature of the reflective surface of the second mirror is RM2, and the maximum object height on the first surface is H0, then the condition 1<RM2/H0<6 is satisfied.

This application claims priority to Japan Patent Application No. 2006-081856, filed on Mar. 24, 2006, the contents of which are incorporated herein by reference.

FIELD

This disclosure relates to projection-optical systems, exposure apparatus, and device-manufacturing methods using such exposure apparatus. The projection-optical systems are reflection-type projection-optical systems suitable for X-ray projection-exposure apparatus (e.g., lithographic exposure apparatus) that utilize X-rays for  transferring a circuit pattern, defined on a mask or reticle, onto a photosensitive substrate by a mirror-projection method.

BACKGROUND

In the past, exposure apparatus using X-rays has attracted attention for use as lithographic-exposure apparatus used in the manufacture of semiconductor devices and other products. When using X-rays as exposure light, since there are no X-ray-transmissive optical materials or refractive optical materials that can be used for making lenses, a reflective mask is used. Also, the projection-optical system is a reflection-type projection-optical system. One conventional type of reflection-type projection-optical system usable in exposure apparatus employing X-rays as the exposure light comprises eight reflective mirrors, such as disclosed in U.S. Pat. No. 6,710,917 (corresponding to Japanese Laid-Open Patent Document No. 2002-139672).

In the conventional projection-optical system disclosed in the '917 patent, the image-side numerical aperture (NA) is 0.4. But, it would be desirable to increase the image-side NA further to achieve higher resolution. In addition, this conventional projection-optical system comprises reflective mirrors having aspherical reflecting surfaces having extremely large absolute values of center radii of curvature. Effective methods for inspecting such surfaces, for surface-shape errors, do not yet exist. Currently, the most effective inspection method utilizes the principle in which light that has passed through a pinhole is reflected by the aspherically-shaped reflective surface. But, surface-shape errors cannot be inspected and detected with the desired precision unless the absolute value of the center radius of curvature of the reflective surface is less than a certain value.

SUMMARY

This invention was devised in consideration of the problem summarized above. An object of the invention is to provide projection-optical systems having a comparatively large image-side numerical aperture (NA) of, for example, 0.45 or greater, and that can be inspected, with the desired precision, for surface-shape errors in their reflective surfaces. A further object is to provide exposure apparatus that comprise such projection-optical systems for use with, for example, X-rays as the exposure light, and that perform projection-exposures at high resolution.

According to one aspect of the invention, projection-optical systems are provided. An embodiment of a projection-optical system comprises eight reflective mirrors that collectively form a reduced image, on a second surface, of a first surface. The embodiment also comprises a first reflective image-forming optical system that forms an intermediate image of the first surface, based on light from the first surface. A second reflective image-forming optical system forms the reduced image on the second surface, based on light from the intermediate image. The first reflective image-forming optical system comprises, in order of light incidence from the first surface, a first reflective mirror having a concave reflective surface, a second reflective mirror having a convex reflective surface, a third reflective mirror, and a fourth reflective mirror having a concave reflective surface. The second reflective image-forming optical system comprises, in order of light incidence from the intermediate image, a fifth reflective mirror having a concave reflective surface, a sixth reflective mirror, a seventh reflective mirror having a convex reflecting surface, and an eighth reflective mirror M8having a concave reflective surface. The system further satisfies the condition 1<RM2/H0<6, wherein RM2is the absolute value of the center radius of curvature of the reflective surface of the second mirror, and H0is the maximum object height on the first surface.

According to another aspect, exposure apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system situated and configured to illuminate a mask situated on the first surface. The embodiment also includes a projection-optical system (according to any of the embodiments of the first aspect) situated and configured to project the pattern of the mask onto a photosensitive substrate situated on the second surface.

According to yet another aspect, device-manufacturing methods are provided. An embodiment of such a method comprises an exposure process, in which an exposure apparatus according to the second aspect is used to expose a photosensitive substrate to the mask pattern. A development step is conducted after the exposure to develop the photosensitive substrate.

In another embodiment of an eight-mirror reflection-type double-image-forming projection-optical system, one or more specific conditions are satisfied, including a condition relating to the center radius of curvature of the reflective surface of the second mirror. The projection-optical systems can have a comparatively large image-side numerical aperture (numerical aperture on the side of the second surface), of for example 0.45 or greater. This facilitates inspections, with prescribed precision, for surface-shape errors in the reflective surfaces of the mirrors. Further, by including a projection-optical system in an exposure apparatus, X-rays can be used as the exposure light.

In the subject exposure apparatus, the mask and photosensitive substrate are moved relative to the projection-optical system during exposure. Thus, the pattern defined on the mask is projected onto the photosensitive substrate, at high resolution, to expose the substrate. As a result, scanning-type exposure apparatus, providing high resolving power, can be used, under satisfactory exposure conditions, to manufacture high-precision devices.

DETAILED DESCRIPTION

An embodiment of a projection-optical system comprises eight reflective mirrors M1-M8. Referring, for example, toFIG. 3, light from a first surface (called the “object surface”) passes through a first reflective image-forming optical system G1, which forms an intermediate image of the first surface. Light from the intermediate image passes through a second reflective image-forming optical system G2, which forms a final reduced image of the first surface (i.e., an image of the intermediate image) on a second surface (called the “image surface”). The intermediate image of the first surface is formed in the optical path between the first reflective image-forming optical system G1and the second reflective image-forming optical system G2. The first reflective image-forming optical system G1has, in the order of incidence of light from the first surface, a first reflective mirror M1that is concave; a second reflective mirror M2that is convex; a third reflective mirror M3; and a fourth reflective mirror M4that is concave. The second reflective image-forming optical system G2has, in order of incidence of light from the intermediate image, a fifth reflective mirror M5that is concave; a sixth reflective mirror M6; a seventh reflective mirror M7that is convex; and an eighth reflective mirror M8that is concave.

The embodiment of a reflective projection-optical system described above (producing two images using eight mirrors) satisfies the following conditional Equation (1), in which RM2is the absolute value of the center radius of curvature (vertex radius of curvature) of the reflective surface of the second mirror M2, and H0is the maximum object height on the first surface.
1<RM2/H0<6  (1)
If the upper limit of conditional Equation (1) is exceeded, it is difficult to achieve a comparatively large image-side numerical aperture (NA on the second-surface side) of 0.45 or greater, for example. If the absolute value RM2(the center radius of curvature of the second mirror M2) is too large, it is difficult to inspect the reflective surface of the second mirror M2for surface-shape errors with the prescribed precision. It also is difficult to manufacture the second mirror M2with the prescribed precision. If RM2has a value that causes RM2/H0to fall below the lower limit of conditional Equation (1), then it is easy to provide a comparatively large image-size NA (e.g., 0.45 or greater); but, because RM2is too small, various aberrations tend to arise. To achieve even more advantageous results, the upper limit of conditional Equation (1) desirably is 5, and the lower limit desirably is 2.

A projection-optical system according to this embodiment, employing eight reflective mirrors and by having the reflective surface of the second mirror M2satisfy conditional Equation (1), can achieve a comparatively large image-side NA of, e.g., 0.45 or greater. This embodiment also allows inspections for surface-shape errors in the reflective surfaces of the mirrors to be performed with prescribed precision. Such a projection-optical system used in an exposure apparatus achieves very high resolving power when using, for example, X-rays as the exposure light. Such resolution allows projection exposures to be performed with very high resolution.

Providing a power arrangement for the first and second reflective image-forming optical systems G1and G2as described above avoids having to increase the center radii of curvature of the mirrors forming the double-image-forming optical system. This arrangement also allows the angles of incidence of rays on each of the mirrors to be kept comparatively small, which reduces the effective diameters of the mirrors and provides a compact optical system having a large NA.

In the embodiment described above, the second mirror M2is convex. The mirror M2is second from the object side (first-surface side) in the first reflective image-forming optical system G1. Arranging the optical elements such that the convex mirror M2is situated between the first (concave) mirror M1and the fourth (concave) mirror M4allows the effective diameters of certain mirrors (e.g., of the fourth mirror M4or the fifth mirror M5, which normally tend to be large) to be reduced. In addition, the respective angles of incidence of rays on each reflective mirror are kept comparatively small while keeping the absolute values of center radii of curvature of the mirrors within a prescribed range.

In the embodiment described above, the seventh mirror M7is convex. The mirror M7is seventh from the object side and is located in the second reflective image-forming optical system G2. Arranging the optical elements such that the convex mirror M7is situated between the fifth (concave) mirror M5and the eighth (concave) mirror M8allows the effective diameters of certain mirrors (e.g., the fourth mirror M4or the fifth mirror M5, which normally tend to be large) to be reduced. In addition, the respective angles of incidence of rays on each reflective mirror are kept comparatively small while keeping the absolute values of center radii of curvature of the mirrors within a prescribed range.

This embodiment provides a satisfactorily balanced arrangement of the convex mirrors M2and M7in the first and second reflective image-forming optical systems G1and G2, respectively. Consequently, the Petzval sum of the optical system can be maintained at a satisfactory value, which allows various aberrations to be satisfactorily corrected. Thus, a high-performance optical system is provided and angles of incidence of rays on the reflective surfaces of the mirrors are kept comparatively small, which reduces the effective diameters of the mirrors having appropriate center radii of curvature. Also, in this double-image-forming configuration comprising first and second reflecting image-forming optical systems G1and G2, distortion is satisfactorily corrected. Such a configuration providing, for example, a magnitude of projection magnification (image-forming magnification) of ¼ is compact and achieves excellent optical performance.

It is further desirable that this embodiment satisfy conditional Equation (2), in which RM1is the absolute value of the center radius of curvature of the reflective surface of the first mirror M1, and H0is the maximum object height, as explained above.
1<RM1/H0<6  (2)

If the upper limit of conditional Equation (2) is exceeded, the absolute value RM1of the center radius of curvature of the reflective surface of the first mirror M1is too large. Consequently, inspecting the surface for shape errors is difficult to perform at a prescribed precision. Also, such a first mirror M1is difficult to manufacture with a prescribed precision. If RM1/H0falls below the lower limit of conditional Equation (2), then the absolute value RM1of the center radius of curvature of the reflective surface of the first mirror M1is too small, which allows undesirable aberrations to arise in this reflective surface. In conditional Equation (2), it is even more desirable that the upper limit be 5, and that the lower limit be 2.

In another embodiment, the projection-optical system also satisfies conditional Equation (3), in which RM4is the absolute value of the center radius of curvature of the reflective surface of the fourth mirror M4, and H0is the maximum object height, as explained above.
4<RM4/H0<12  (3)

If the upper limit of conditional equation (3) is exceeded, the absolute value RM4of the center radius of curvature of the reflective surface of the fourth mirror M4is too large. Consequently, inspection of the reflective surface for surface-shape errors is difficult to perform at the prescribed precision. Also, manufacture of the fourth mirror M4at the prescribed precision is undesirably difficult. If RM4/H0falls below the lower limit of conditional Equation (3), then RM4is too small, resulting in various undesirable aberrations of the reflective surface of the fourth mirror M4. For even more advantageous performance, the upper limit of conditional Equation (3) desirably is 10, and the lower limit desirably is 6.

In yet another embodiment, the projection-optical system also satisfies the following conditional Equation (4), in which RM8is the absolute value of the center radius of curvature of the reflective surface of the eighth mirror M8, and H0is the maximum object height, as explained above.
0.1<RM8/H0<4  (4)
If the upper limit of conditional Equation (4) is exceeded, then RM8is too large, which increases the difficulty of inspecting this reflective surface for surface-shape errors at the prescribed precision, and undesirably increases the difficulty of manufacturing the eighth mirror M8with the prescribed precision. If RM8/H0falls below the lower limit of conditional Equation (4), then RM8is too small, which causes the reflecting surface to exhibit various undesirable aberrations. For even more advantageous performance, the upper limit of conditional Equation (4) desirably is 3, and the lower limit desirably is 1.

In yet another embodiment, the projection-optical system also satisfies the following conditional Equation (5), in which RM3is the absolute value of the center radius of curvature of the reflective surface of the third mirror M3, and H0is the maximum object height, as explained above.
4<RM3/H0<13  (5)
If the upper limit of conditional Equation (5) is exceeded, then RM3is too large, which increases the difficulty of inspecting this reflective surface for surface-shape errors at the prescribed precision, and undesirably increases the difficulty of manufacturing the third mirror M3with the prescribed precision. If RM3/H0falls below the lower limit of conditional Equation (5), then RM3is too small, which causes the reflecting surface to exhibit various undesirable aberrations. For even more advantageous performance, the upper limit of conditional Equation (5) desirably is 12, and the lower limit desirably is 5.

In yet another embodiment, the projection-optical system also satisfies the following conditional Equation (6), in which RM7is the absolute value of the center radius of curvature of the reflective surface of the seventh mirror M7, and H0is the maximum object height, as explained above.
0.1<RM7/H0<3  (6)
If the upper limit of conditional Equation (6) is exceeded, then RM7is too large, which increases the difficulty of inspecting this reflective surface for surface-shape errors at the prescribed precision, and undesirably increases the difficulty of manufacturing the seventh mirror M7with the prescribed precision. If RM7/H0falls below the lower limit of conditional Equation (6), then RM7is too small, which causes the reflecting surface to exhibit various undesirable aberrations. For even more advantageous performance, the upper limit of conditional Equation (6) desirably is 2, and the lower limit desirably is 0.5.

In yet another embodiment, the projection-optical system, the object height on the first surface is H0. Also, in seven of the eight reflective mirrors M1-M8, the absolute value RMi, wherein i=1, 2, 3, 4, 5, 6, 7, 8, of the center radius of curvature of the respective reflective surface, satisfies the following conditional Equation (7).
RMi/H0≦12  (7)
If conditional Equation (7) is satisfied, then the absolute value RMiof the center radius of curvature of the respective reflective surface of each of the subject mirrors M1-M8is small compared to conventional systems. Consequently, even in an optical system in which the image-side numerical aperture NA exceeds 0.45, beams can be easily separated at each of the reflective surfaces, which is desirable. For even better performance, the upper limit of conditional Equation (7) is 10.

In yet another embodiment, the projection-optical system also satisfies conditional Equation (8), in which TT is the distance (on the optical axis) between the first surface and second surface (i.e., the distance between object and image), and H0is the maximum object height, as explained above.
4<TT/H0<8  (8)
If the upper limit of conditional Equation (8) is exceeded, the interval TT, as the total length of the optical system, is too large, resulting in the optical system being too large in the axial direction (i. e., along the optical axis). If TT/H0falls below the lower limit of conditional Equation (8), then in practice it is extremely difficult to design a reflection-type double-image-forming optical system having eight mirrors as described above. For even better performance and to prevent excessive size of the optical system along the axial direction, the upper limit in conditional Equation (8) is 7.

Desirably, an aperture stop is provided at the position of the reflective surface of the second mirror M2, or at a position in proximity thereto. This configuration is effective for securing a large image-side numerical aperture NA in a reflection-type double-image-forming projection-optical system comprising eight mirrors such as described above. Using the aperture stop, a beam can be limited to an arbitrary size, thereby allowing light quantity to be adjusted, which in turn provides adjustment of the focal depth and depth of field at the second surface (image surface). By situating the aperture stop at approximately the position of the reflective surface of the second mirror M2, undesirable blocking of the beam by the aperture stop can be avoided.

To achieve higher resolution using the projection-optical system, it is desirable that the image-side numerical aperture NA of the system be greater than 0.48. It is also desirable that a reduced (demagnified) image be formed in an arc-shaped effective image-formation region in the image plane. It is also desirable that the width of the arc-shaped effective image formation region be greater than 0.5 mm. This configuration, as used in an exposure apparatus, allows projection-exposures to be performed with higher throughput.

In yet another embodiment, the projection-optical system also satisfies the following conditional Equation (9), in which φMis the maximum value of the effective diameter of the mirrors M1-M8, and H0is the maximum object height, as explained above. That is, φMis the diameter of the circle circumscribing the reflective region (usage region), centered on the optical axis, in the reflective mirror having a reflective region located farthest from the optical axis (i.e., the largest mirror).
φM/H0≦5  (9)
If the upper limit of conditional Equation (9) is exceeded, the effective diameter φMof the largest reflective mirror is too large, which results in the optical system being too large diameter-wise. If φMis below the lower limit of conditional Equation (9), then in practice it is extremely difficult to design a reflective-type double-image-forming optical system having eight mirrors. As a practical matter, to manufacture mirrors with the prescribed precision and to avoid increasing the size of the optical system, regardless of the magnitude of H0, it is desirable that φMof the largest reflective mirror be 700 mm or less. To prevent, even more satisfactorily, increases in the diameter-wise dimensions of the optical system, it is desirable that the upper limit in conditional Equation (9) be 4.

In yet another embodiment of the projection-optical system, the inclination α of the beam from the first surface to the first mirror M1(with respect to the optical axis of the chief ray) satisfies the following conditional Equation (10).
5°<|α|<10°  (10)
If the upper limit of conditional Equation (10) is exceeded, whenever a reflective mask is placed at the first surface, the effect of blocking of the reflected beam by the pattern of the absorbing bodies on the mask is too great, which is undesirable. If |α| falls below the lower limit of conditional Equation (10), whenever a reflective mask is placed at the first surface, light that is incident on the reflective mask cannot be separated from light reflected from the mask, which is undesirable.

In yet another embodiment of a projection-optical system, the respective reflective surfaces of each of the mirrors M1-M8are aspherical, and have rotational symmetry about the optical axis. It is also desirable that the maximum order of asphericity stipulated for each reflective surface be 18th order or higher. By using these high-order aspherical surfaces, aberrations can be satisfactorily corrected, which provides improved optical performance. Aberrations can, to some extent, be satisfactorily corrected if the maximum order is 16th order; but, by imposing a maximum order of 18th or higher, still more complex and minute aspherical shapes can be represented, thereby making possible adequate aberration correction.

It is also desirable that the projection-optical system be substantially telecentric on the image side (second-surface side). With such a configuration, as used for example in an exposure apparatus, satisfactory image formation is possible within the focal depth of the projection-optical system even when there are depressions and protrusions on the wafer or of the wafer.

In the various projection-optical systems described above, an intermediate image is formed in the optical path between the fourth mirror M4and the fifth mirror M5. But, due to the influence of aberrations and the like, a sharp intermediate image may not be formed. However, there is no need to form a sharp intermediate image; it is much more important to form a sharp final image on the second surface. If a foreign object or the like were to be in the vicinity of the intermediate-image position, if a sharp intermediate image is not formed, then the image of the foreign object on the second surface would be greatly blurred, which is a desirable situation. Therefore, it is better that a sharp intermediate image not be formed.

However, if the intermediate image is formed in extremely close proximity to the reflective surface of a mirror, a concern is that images of minute structures, adhering foreign objects, and the like on the reflective surface of the mirror may be superposed on the image projected onto the second surface. Moreover, the partial diameter (the effective diameter of the beam on the reflective surface) of the mirror may be too small, which may cause problems with respect to manufacturing tolerances. Hence, it is desirable that the optical system be configured to form the intermediate image at a position sufficiently distant from the reflective surfaces of the mirrors.

Various aspects are shown in the attached drawings, of whichFIG. 1summarizes an embodiment of an exposure apparatus, andFIG. 2shows the positional relationship of the arc-shaped effective image-formation region, as formed on the wafer, to the optical axis. InFIG. 1, the Z-axis is in the optical-axis direction of the projection-optical system6, that is, the direction normal to the wafer7serving as the photosensitive substrate. The Y-axis (in the wafer plane) is parallel to the plane of the paper inFIG. 1, and the X-axis (in the wafer plane) is perpendicular to the plane of the paper inFIG. 1.

The exposure apparatus shown inFIG. 1comprises a laser-plasma X-ray source1serving as an exemplary light source of exposure light. A discharge-plasma source or other X-ray source can be used as the X-ray source. Light emitted from the X-ray source1passes through the wavelength-selection filter2and is incident on the illumination-optical system3. Here, the wavelength-selection filter2transmits, of the light produced by the source1, only X-rays of a prescribed wavelength (13.5 nm) and blocks light of other wavelengths. X-rays that have passed through the wavelength-selection filter2pass through the illumination-optical system3, which comprises a plurality of reflective mirrors, to illuminate a reflection-type mask4on which is formed the pattern to be transferred. As an alternative to the wavelength-selection filter2, a wavelength-selection film formed on a reflective mirror may be used, or other wavelength-selection means may be employed. Further alternatively, the wavelength-selection means can be omitted.

The mask4is held by a mask stage5, movable along the Y-direction, such that the patterned surface of the mask extends along the XY plane. Movement of the mask stage5is measured by at least one laser interferometer (not shown). On the mask4is formed an arc-shaped illumination region that is symmetrical with respect to the Y-axis. Light from the illuminated mask4passes through the reflection-type projection-optical system6, and forms an image of the mask pattern on the wafer7serving as the photosensitive substrate.

The effective image-formation region is arc-shaped and symmetrical with respect to the Y-axis, and is formed on the wafer7as shown inFIG. 2. Within the circular region (image circle) IF having a radius φ and centered on the optical axis AX, an effective arc-shaped image-formation region ER is established. The length of the arc-shaped region ER in the X-direction is LX and the length in the Y-direction is LY so as to be tangent to the image circle IF. Thus, the arc-shaped effective image-formation region ER is a portion of an annulus centered on the optical axis AX. The length LY is the width of the effective image-formation region ER along a direction connecting the center of the arc-shaped effective image-formation region ER with the optical axis.

The wafer7is held by a wafer stage8that is movable in two dimensions along the X-direction and Y-direction, such that the exposure plane extends in the XY plane. As with the mask stage5, movement of the wafer stage is measured using at least one laser interferometer (not shown). Thus, while moving the mask stage5and the wafer stage8along the Y-direction, that is, while moving the mask4and wafer7along the Y-direction relative to the projection-optical system6, scanning exposure is performed, by which the pattern of the mask4is transferred onto an exposure region of the wafer7.

If the projection magnification (transfer magnification) of the projection-optical system6is ¼, synchronized scanning is performed with the movement velocity of the wafer stage8set to ¼ of the movement velocity of the mask stage5. By repeating the scanning exposures while moving the wafer stage8in two dimensions along the X-direction and Y-direction, the pattern of the mask4is transferred sequentially to each of the exposure regions on the wafer7. Below, exemplary configurations of a projection-optical system6are explained in the context of first, second, and third representative embodiments.

In each representative embodiment, the projection-optical system6comprises a first reflective image-forming optical system G1, to form an intermediate image of the pattern of the mask4, and a second reflective image-forming optical system G2to form a final reduced image of the pattern of the mask4(i.e., an image of the intermediate image) on the wafer7. The intermediate image of the mask pattern is formed in the optical path between the first reflective image-forming optical system G1and the second reflective image-forming optical system G2.

The first reflective image-forming optical system G1comprises four reflective mirrors M1-M4, and the second reflective image-forming optical system G2comprises four reflective mirrors M5-M8. In each of the representative embodiments, the respective reflective surfaces of all of the mirrors M1-M8are aspherical and have rotational symmetry with respect to the optical axis. Also, the projection-optical system6is substantially telecentric on the wafer side (image side).

In each representative embodiment, an aspherical surface is represented by the following Equation (a), in which the height (in a direction perpendicular to the optical axis) is y, the distance along the optical axis from the tangent plane at the vertex of the aspherical surface to the position on the aspherical surface at height y (sag amount) is z, the vertex radius of curvature is r, the conical coefficient is κ, and the nth order aspheric coefficient is Cn.

First Representative Embodiment

A projection-optical system according to this embodiment is depicted inFIG. 3, in which light from the mask4is reflected in succession by the concave reflective surface of the first mirror M1, the convex reflective surface of the second mirror M2, the concave reflective surface of the third mirror M3, and the concave reflective surface of the fourth mirror M4, after which an intermediate image I1of the mask pattern is formed. The mirrors M1-M4comprise the first reflective image-forming optical system G1. Light from the intermediate image I1, formed by the first reflective image-forming optical system G1, is reflected in succession by the concave reflective surface of the fifth mirror M5, the concave reflective surface of the sixth mirror M6, the convex reflective surface of the seventh mirror M7, and the concave reflective surface of the eighth mirror M8, after which a reduced image (second-order image) of the mask pattern is formed on the wafer7. In this embodiment, an aperture stop (not shown) is situated in proximity to the reflective surface of the second mirror M2. The mirrors M5-M8comprise the second reflective image-forming optical system G2

Various parameters for the projection-optical system of this first representative embodiment are listed in Table 1. In the Main Parameters section of the table, λ is the wavelength of the exposure light, β is the projection magnification, NA is the image-side (wafer-side) numerical aperture, φ is the radius of the image circle IF (maximum image height) on the wafer7, LX is the length in the X-direction of the effective image-formation region ER, and LY is the length in the Y-direction of the effective image-formation region ER (i.e., the width dimension of the arc-shaped effective image-formation region ER). In the section of Table (1) listing conditional Equation corresponding values, RMi(i=1, 2, 3, 4, 5, 6, 7, 8) is the absolute value of the center radius of curvature of the reflective surface for the ith reflective mirror Mi, H0is the maximum object height on the mask4, TT is the distance along the axis between the mask4and the wafer7, and φMis the effective diameter of the largest reflective mirror.

The listed surface numbers indicate the order of the reflective surface from the mask side, along the direction of propagation of rays, from the mask surface (which is the object surface) to the wafer surface (which is the image surface); r is the vertex radius of curvature (center radius of curvature, in mm) for each reflective surface; and d is the interval (distance) along the axis between reflective surfaces, in mm. The sign of the surface interval d changes with each reflection. The radius of curvature of a surface that is convex facing the mask side is positive, regardless of the direction of incidence of rays, and the radius of curvature of a concave surface is negative. The above notation is similarly used for Table 2 and Table 3, below.

FIG. 4shows coma aberration in the projection-optical system of the first representative embodiment. InFIG. 4the meridional coma aberration and sagittal coma aberration are shown at image heights of 100%, 99%, and 98%. As is clear from the aberration diagrams of the first representative embodiment, coma aberration is satisfactorily corrected in the region corresponding to the effective image-formation region ER. Although not shown in the figure, it was confirmed that, in the region corresponding to the effective image-formation region ER, aberrations other than coma aberration, such as spherical aberration and distortion, were also satisfactorily corrected.

Second Representative Embodiment

A projection-optical system according to this embodiment is shown inFIG. 5, in which light from the mask4is reflected in succession by the concave reflective surface of the first mirror M1, the convex reflective surface of the second mirror M2, the concave reflective surface of the third mirror M3, and the concave reflective surface of the fourth mirror M4, after which an intermediate image I1of the mask pattern is formed. Light from the intermediate image I1formed by the first reflective image-forming optical system G1is then reflected in succession by the concave reflective surface of the fifth mirror M5, the convex reflective surface of the sixth mirror M6, the convex reflective surface of the seventh mirror M7, and the concave reflective surface of the eighth mirror M8, after which a reduced image (second-order image) of the mask pattern is formed on the wafer7. In the second representative embodiment, similarly to the first representative embodiment, an aperture stop (not shown) is situated in proximity to the reflective surface of the second mirror M2. Various parameters for the projection-optical system of the second representative embodiment appear in Table 2.

FIG. 6shows coma aberration in the projection-optical system of the second representative embodiment, in which the meridional coma aberration and sagittal coma aberration are shown at image heights of 100%, 99%, and 98%. As is clear from the aberration diagram, in this second representative embodiment (similarly to the first representative embodiment), coma aberration is satisfactorily corrected in the region corresponding to the effective image-formation region ER. Although not shown in the figure, it was confirmed that, in the region corresponding to the effective image-formation region ER, aberrations other than coma aberration, such as spherical aberration and distortion, were also satisfactorily corrected.

Third Representative Embodiment

A projection-optical system according to this embodiment is shown inFIG. 7, in which light from the mask4is reflected in succession by the concave reflective surface of the first mirror M1, the convex reflective surface of the second mirror M2, the concave reflective surface of the third mirror M3, and the concave reflective surface of the fourth mirror M4, after which an intermediate image I1of the mask pattern is formed. Light from the intermediate image I1, formed by the first reflective image-forming optical system G1, is then reflected in succession by the concave reflective surface of the fifth mirror M5, the concave reflective surface of the sixth mirror M6, the convex reflective surface of the seventh mirror M7, and the concave reflective surface of the eighth mirror M8, after which a reduced image (second-order image) of the mask pattern is formed on the wafer7. In the third representative embodiment, similarly to the first embodiment, an aperture stop (not shown) is situated in proximity to the reflective surface of the second mirror M2. Various parameters for this embodiment appear in the following Table 3.

FIG. 8shows coma aberration in the projection-optical system of this embodiment. InFIG. 8, the meridional coma aberration and sagittal coma aberration are shown at image heights of 100%, 98%, and 96%. As is clear from the aberration diagram, in this embodiment (as in first and second representative embodiments), coma aberration is satisfactorily corrected in the region corresponding to the effective image-formation region ER. Although not shown in the figure, it was confirmed that, in the region corresponding to the effective image-formation region ER, aberrations other than coma aberration, such as spherical aberration and distortion, were also satisfactorily corrected.

In each of the embodiments detailed above, a comparatively large image-side numerical aperture of 0.5 is provided for X-rays of wavelength 13.5 nm. In addition, an arc-shaped effective image-formation region of 26 mm×2 mm or 26 mm×1 mm, in which various aberrations are satisfactorily corrected, is provided on the wafer7. Hence, in an exposure region of 26 mm×33 mm on the wafer7, scanning exposure can be performed to transfer the pattern of the mask4with a high resolution of 0.1 μm or less.

In the various embodiments the effective diameter of the fifth (concave) mirror M5, which is the largest mirror, is approximately 510 mm to approximately 570 mm. Thus, M5is kept sufficiently small compared to conventional reflective projection-optical systems. Thus, each of these embodiments represents a relatively compact optical system, which allows performance of measurements and machining with high precision during the manufacture of the mirrors. Also, the inclination of the chief ray in these embodiments is substantially zero over the entire arc-shaped effective image-formation region ER, thereby providing optical systems that are substantially telecentric on the image side.

In the various embodiments, the angle α made by the ray group incident on the mask4and the ray group reflected by the mask4, relative to the optical axis AX, can be kept small, at approximately 7.8° to approximately 8.0°. Consequently, even if a reflective mask4is used, the incident and reflected lights can be separated, and blockage of the reflected beam by the pattern of absorbing bodies on the mask can be reduced, thereby preserving good optical performance. In addition, large changes in image magnification do not tend to result from slight errors in the installed position of the mask4.

In exposure apparatus as described herein, a mask is illuminated by an illumination-optical system (illumination process), and a projection-optical system is used to expose a pattern, defined on the mask, onto a photosensitive substrate so as to transfer the pattern (exposure process). In this manner, microdevices (e.g., semiconductor devices, image-capture elements, liquid-crystal-display devices, thin-film magnetic heads, and the like) can be manufactured. An example of a method for fabricating semiconductor devices (as exemplary microdevices) by forming prescribed circuit patterns on a wafer or other photosensitive substrate using an exposure apparatus as disclosed herein is shown inFIG. 9. In step301ofFIG. 9, a metal film is formed by vacuum deposition on a lot of wafers. In the step302, a photoresist is applied to the metal film on the wafers. In step303, exposure apparatus as described herein is used for transfer-exposing, in each of the “shot” regions on the wafers, an image of the pattern on a mask (reticle), via the projection-optical system. In step304the photoresist is developed. In step305, etching of the wafers is performed, using the resist pattern as a mask, to form a circuit pattern corresponding to the pattern on the mask in each of the shot regions on each wafer. By further forming circuit patterns in successive layers, semiconductor devices or other devices are manufactured. By means of this device-manufacturing method, semiconductor devices having extremely fine circuit patterns can be obtained with good throughput.

In certain embodiments described above, a laser-plasma X-ray source is used as a light source to supply X-rays for exposure. However, it will be understood that any of various other sources may be employed. For example, X-rays can be produced from synchrotron radiation (SOR).

Although the embodiments have been described in connection with exposure apparatus of which the source produces X-rays as exposure light, it will be understood that the embodiments are not limited to X-ray sources. Various aspects of the invention are also applicable to exposure equipment of which the light source provides exposure light at wavelengths other than X-ray wavelengths.

Although the embodiments have been described in connection with projection-optical systems as used in exposure apparatus, it will be understood that that various aspects of the invention can be applied to other projection-optical systems in general.