[0035] The catadioptric projection optical system of the present invention will be described in further detail below.
[0036] As shown in Figure 1, the present invention provides a catadioptric projection optical system, the projection optical system is a catadioptric symmetric structure, that is, from the side of the object surface sequentially includes a front (rear) group, provided with an aperture stop Concave spherical mirror M. Among them, under the light reflection of the concave spherical mirror M, the lens components of the front group are the common components of the rear group, that is, the optical structure of the optical elements shared by the front (rear) group is completely symmetrical (surface The radius and interval are equal, and the optical materials are the same), and the magnification is -1. The advantage of the symmetrical structure with a magnification of -1 is that according to the primary aberration theory, its vertical aberrations: coma, distortion, and chromatic aberration of magnification are automatically corrected to zero. The surfaces of all lens components of the catadioptric projection optical system are spherical or flat, and share the same optical axis.
[0037] In the catadioptric projection optical system of the present invention, the front group with positive refractive power includes in order along its optical axis: a right-angle reflecting prism L1, L1*, a first double convex positive lens L2, and a first curved surface whose convex surface faces the aperture stop. The meniscus lens L3 and the second meniscus thickness lens L4, the second biconvex positive lens L5, the biconcave negative lens L6, and the convex meniscus lens L7 facing the aperture stop.
[0038] In this front group, the entrance and exit surfaces of the above-mentioned right-angle reflecting prisms L1 and L1* are all flat, and they are expanded into parallel flat plates for optical design. The resulting spherical aberration, astigmatism, curvature of field, and axial chromatic aberration are used to compare with The corresponding aberration of the rear lens is designed to compensate. In addition, this design can ensure that the object-side field of view and the image-side field of view are respectively placed on both sides of the optical axis of the projection optical system through the reflecting prism, and are parallel to each other.
[0039] A small axial distance between the first double convex positive lens L2 and the first meniscus lens L3 is reserved to correct the high-level spherical aberration of the system.
[0040] The above-mentioned first meniscus lens L3 adopts optical materials with high refractive index and high dispersion coefficient, and its function is to correct the axial chromatic aberration of the optical system on the one hand, and on the other hand to correct together with the above-mentioned second meniscus lens L4 The image plane of this optical system is curved.
[0041] The above-mentioned second biconvex positive lens L5, biconcave negative lens L6, and meniscus positive lens L7 together form a three-piece optical structure, which can effectively compress the total length of the optical system while achieving a longer working distance, and can compress the above The clear aperture of the concave spherical mirror M. The above-mentioned double-concave negative lens L6 generates positive spherical aberration on the one hand to compensate for the negative spherical aberration and negative astigmatism of the first and second double-convex positive lenses L2 and L5, and on the other hand performs axial chromatic aberration compensation. Because the numerical aperture of the projection optical system of the present invention is relatively large, the second double convex positive lens L5 and the meniscus positive lens L7 are used to compensate for high-level spherical aberration and high-level astigmatism.
[0042] In a preferred embodiment of the present invention, the above-mentioned right-angle reflecting prisms L1, L1* and meniscus positive lens L7 are fused silica glass with low refractive index and low dispersion coefficient. The first double convex positive lens L2, the second meniscus thick lens L4, and the second double convex positive lens L5 adopt crown glass with low refractive index, low dispersion coefficient, and high transmittance, such as FK5HT glass. The above-mentioned first meniscus lens L3 adopts flint glass with high refractive index, high dispersion coefficient and high transmittance, such as F2HT glass. The above-mentioned biconcave negative lens L6 adopts crown glass with low refractive index, low dispersion coefficient and high transmittance, such as BK7HT glass.
[0043] Therefore, the lenses L1 (L1*) to L7 of the front group of the projection optical system can correct spherical aberration, curvature of field, astigmatism, and positional chromatic aberration, so that axial aberration can be better corrected.
[0044] The positive curvature of field generated by the concave spherical surface of the concave spherical mirror M can offset the negative curvature of field generated by the positive lens group (the first and second double convex positive lenses L2, L5, and the meniscus positive lens L7). The reflection effect of the concave spherical mirror M causes the imaging beam to pass through the front group (rear group) twice, so that the transmission optical element will participate in the imaging twice, which is equivalent to a doubling of the number of optical elements, which can reduce the projection optics The total number of optical components in the system and the ability to fold the optical path to reduce the total optical length of the projection optical system.
[0045] The aperture stop is located on the concave spherical mirror M, and the optical elements shared by the front group and the rear group take the aperture stop as a symmetry plane to form a completely symmetrical structure to obtain a -1x optical system, thereby Vertical axis aberration: coma, distortion, and chromatic aberration of magnification are automatically corrected to zero.
[0046] At the same time, the design ensures that the rear focus of the front lens assembly is located at the center of the aperture diaphragm, so that the imaging light cones of the object space and the image space are symmetrical with the chief ray, that is, the chief ray of the object space and the image space are parallel to The optical axis forms a double-telecentric projection optical system. In this way, even if the object plane and the image plane are in the defocused position, the height of the object and the image remains unchanged on the plane perpendicular to the optical axis, that is, the magnification is not changed, so as to ensure that the magnification does not vary with the object plane and the image plane along the optical axis. Move and change.
[0047] As shown in FIG. 2, the transfer function MTF of the catadioptric projection optical system of this embodiment reflects the imaging quality of the catadioptric projection optical system. When the working wavelength is 436nm, 405nm, 365nm, according to the analysis and calculation of the professional optical design software CODE_V, it can be known that high imaging quality can be effectively obtained (the RMS value of the wave aberration in the field of view is 7.7nm, and the telecentric angle error is less than 0.18°= 3.1mrad, the maximum distortion is 0.6nm, the transfer function MTF is close to the diffraction limit), at the same time it can achieve a larger object and image field of view, the total optical length is 884.588mm, and the object and image working distances are both 30mm.
[0048] As shown in FIG. 3, the size diagram of the object field of view and the image field of the catadioptric projection optical system of this embodiment. Among them, the radius of the image-side field of view and the object-side field of view of the present invention can reach 70mm, which can provide a square image-side field of view of 44.5mm×44.5mm, which is sufficient for the bumping package lithography machine to be used for 44mm× 44mm field of view size chip packaging technical requirements.
[0049] Table 1 shows the typical design data of the preferred embodiment of the catadioptric projection optical system. Its image side numerical aperture reaches 0.20; working wavelengths are 436nm, 405nm, 365nm, that is, the g-line, h-line, and i-line defined in the optical field; the object-side field of view radius is 70mm, which can provide a square view of 44.5mm×44.5mm Field, the function is to project the image of the object surface on the image surface; the image side field of view radius is 70 mm; due to the symmetrical structure, the working distance between the object side and the image side are both 30mm, and the magnification is -1 times; The total length (that is, from the object surface to the mirror) is 884.588mm.
[0050] Table 1
[0051] Working wavelength
[0052] As shown in Table 2, it is the specific parameter value of each optical element of the preferred embodiment of the catadioptric projection optical system. The column “belonging to” indicates the position of each surface from the object plane to the image plane. Corresponding optical element; the "radius" column gives the spherical radius corresponding to each surface; the "thickness/spacing" column gives the axial distance between two adjacent surfaces, if the two surfaces belong to the same optical Element, the value of "thickness/pitch" indicates the thickness of the optical element, otherwise it indicates the distance from the object plane/image plane to the optical element or the distance between adjacent optical elements. The "optical material" column indicates the material of the corresponding optical element, and the "half aperture" column indicates the 1/2 aperture value of the corresponding surface.
[0053] Taking optical elements L1 and L2 as an example, the spherical radius of the front surface 1 of L1 is 1e+018, which is a plane, the distance between the front surface 1 of L1 and the object surface is 30mm, and the optical material is SIO2, the front surface of L1 The half aperture is 76.219mm; the spherical radius of the rear surface 2 of L1 is 1e+018, the front surface 1 of L1 to the rear surface 2 of L1, that is, the center thickness of the optical element L1 is 88mm, and the half aperture of the rear surface 2 of L1 is 88.450mm, that is, the optical element L1 is a parallel flat plate. The spherical radius and semi-aperture of the front surface 3 of L2 are 6873.292421mm and 88.777mm, respectively, the distance between the front surface 3 of L2 and the rear surface 2 of L1 is 1.000mm, the optical material of lens L2 is FK5HT; the rear surface of L2 is 4 The spherical radius and the semi-aperture are -151.056352mm and 89.968mm, respectively, and the center thickness of the lens L2 is 39.338mm, that is, L2 is a double convex positive lens. Except that the half-aperture of the image surface (surface Image) represents the image field radius, the meaning of the parameter values of the other surfaces is analogous to L1 and L2.
[0054] The catadioptric projection optical system is also provided with a concave spherical mirror M whose spherical radius and semi-aperture are -869.879223 mm and 103.066 mm, respectively. The change of the 1/2 aperture size will affect the imaging effect of the projection optical system.
[0055] Table 2
[0056] Surface
[0057] Table 2 (continued)
[0058] 12
[0059] In summary, the projection optical system of the present invention can achieve a larger object-side and image-side field of view, and its total optical length is 884.588mm, and the working distance between the object-side and image-side is 30mm. It is a bumping light. The structural design of the workpiece table and the mask table of the engraving machine provides enough reserved space for movement. The large working distance brings great convenience to the movement positioning of the mask and the silicon wafer, the design of the transmission structure, etc., and reduces the entire The volume of the projection optical system.
[0060] The maximum numerical aperture of the projection optical system of the present invention can reach 0.20, so the maximum optical resolution of the projection optical system can reach 0.5 μm (for the half-period length of a periodic object with a duty ratio of 1:1), imaging quality and double telecentricity Such indicators have reached the requirements of practical applications. The surface types of all optical components of the catadioptric projection optical system of the present invention are spherical or flat, without any aspherical surface, and therefore no problems in optical processing, optical inspection, cost, etc. will be introduced. The above-mentioned front and rear groups share the same set of optical components, and reflect mirrors are used to fold the optical path, saving half of the lenses, which is beneficial to reduce the cost of the projection optical system.
