Exposure apparatus using a catadioptric projection system

Abstract: A catadioptric optical system includes a polarization beam splitter (3), a concave mirror (6), lens groups (2,5,8) and a quarter-wave plate (4), wherein an additional polarization-charging means (7) such as a quarter-wave plate, half-wave plate or electro-optic modulator is provided to transform the polarization of light from the polarization beam splitter.

Description: This invention relates generally to an imaging optical system such as a reflection and refraction optical system and, more particularly, to a reflection and refraction optical system usable for imaging a fine pattern in manufacture of microdevices such as semiconductor devices (such as ICs or LSIs), image pickup devices (such as CCDs) or display devices (such as liquid crystal panels). In another aspect, the invention is concerned with a projection exposure apparatus using such a reflection and refraction optical system. The degree of integration of a semiconductor device such as IC or LSI is increasing, and the fine processing technology for a semiconductor wafer is being developed considerably. In the projection exposure technique which is the main of the fine processing technology, the resolution has been increased to a level allowing formation of an image of a linewidth not greater than 0.5 micron. The resolution can be improved by shortening the wavelength of light used for the exposure process. However, the shortening of wavelength restricts the glass materials usable for a projection lens system, and correction of chromatic aberration becomes difficult to attain. A projection optical system with which the difficulty of correcting chromatic aberration can be reduced, may be a reflection and refraction optical system comprising a concave mirror and a lens group, wherein the imaging function mainly attributes to the power of the concave mirror. Such reflection and refraction optical system is disclosed in DE-A-4110296 and includes a polarization beam splitter, a quarter waveplate and a concave mirror, disposed in this order from the object plane side. Light from the object plane goes by way of the polarization beam splitter and the quarter waveplate, it is reflected by the concave mirror. After this, the light goes again by way of the quarter waveplate and the polarization beam splitter, and it is imaged upon an image plane. The combination of polarization beam splitter and quarter waveplate reduces the loss of light. However, use of rectilinearly polarized light for the imaging process involves a problem in the formation of a fine image of a linewidth not greater than 0.5 micron that the imaging performance may change in dependence upon the orientation (lengthwise direction) of a (linear) pattern on the object plane. As an example, the contrast of an image of 0.2 micron, which can be formed by using a projection optical system of a numerical aperture (N.A.) of 0.5 and a design wavelength 248 nm together with a phase shift mask (line-and-space pattern), is changeable by about 20 %, depending on whether the direction of polarization of light used for the imaging process is parallel to or perpendicular to the lengthwise direction of the pattern. DE-A-1572195 discloses an exposure method wherein the polarization of the exposure light is aligned with the direction of the narrow lines of the mask pattern or perpendicular to the critical dimensions. It is an object of the present invention to provide an improved reflection and refraction optical system effective to solve the problem described above. It is a further object of the present invention to provide an improved projection exposure apparatus which alleviates the problem described above. The present invention provides as claimed in claim 1 an imaging optical system including a polarization beam splitter, a quarter waveplate and a concave mirror disposed in this order from an object plane, wherein said polarization beam splitter and said quarter waveplate are arranged to project a light beam from the object plane upon said concave mirror said concave mirror is arranged to reflect the projected light beam, and said quarter waveplate and said polarization beam splitter are arranged to project the reflected light beam upon an image plane, characterized by: polarization state changing means disposed between said polarization beam splitter and the image plane for changing the state of polarization of the polarized light beam projected by said polarization beam splitter. The present invention further provides a projection exposure apparatus comprising the imaging optical system (claim 12) and a method of manufacturing a semiconductor device using the imaging optical system (claim 13). A reflection and refraction optical system according to the present invention may suitably be used for manufacture of microdevices such as semiconductor devices (such as ICs or LSIs), image pickup devices (such as CCDs) or display devices (such as liquid crystal panels). Particularly, a reflection optical system of the present invention when arranged to provide a reduction magnification and used as a projection optical system in combination with deep ultraviolet light, may be effective to image a fine device pattern of a linewidth not greater than 0.5 micron. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a reduction projection exposure apparatus for manufacture of semiconductor devices, according to an embodiment of the present invention. Figure 2 is a flow chart of semiconductor device manufacturing processes. Figure 3 is a flow chart, illustrating details of a wafer process. Figure 1 illustrates a reduction projection exposure apparatus according to an embodiment of the present invention, for manufacture of semiconductor devices. Denoted in Figure 1 at 1 is a reticle having a circuit pattern to be transferred to a wafer 9 for manufacture of semiconductor devices. The reticle 1 is held on an object plane of a reflection and refraction optical system 100, by means of a reticle stage (not shown). The circuit pattern of the reticle 1 can be illuminated with deep ultraviolet light of a wavelength λ (<300 (nm)) from an illumination system (not shown), with uniform illuminance. Divergent light from the illuminated reticle 1, including zeroth order and first order diffraction lights, is received by a first lens group 2 having a positive refracting power. The first lens group 2 serves to transform the received divergent light into a parallel light consisting of a flux of light rays parallel to the optical axis AX, and it projects the light on a polarization beam splitter 3. The parallel light incident on the polarization beam splitter 3 passes through the same, and then it passes through a quarter waveplate 4 and enters a second lens group 5 having a negative refracting power. The parallel light passing through the polarization beam splitter 3 and impinging on the lens group 5 is P-polarized light with respect to the dividing plane 3a of the polarization beam splitter 3. Of the parallel light impinging on the polarization beam splitter 3, the light which is S-polarized light with respect to the dividing plane 3a is reflected by that plane 3a upwardly as viewed in the drawing. Further, the quarter waveplate 4 is arranged and disposed so as to transform P-polarized light, entering it from the left hand side in the drawing, into circularly polarized light and also to transform circularly polarized light, entering it from the right hand side in the drawing, into S-polarized light. The second lens group serves to transform the parallel light, passing through the polarization beam splitter 3 and the quarter waveplate 4, into divergent light and to project the same on a concave mirror 6. The concave mirror 6 has a spherical reflection surface which is rotationally symmetrical with respect to the optical axis AX. The concave mirror 6 serves to reflect and converge the received divergent light back to the lens group 5. The light goes by way of the second lens group 5 and the quarter waveplate 4, and it is projected to the polarization beam splitter 3. Due to the function of the quarter waveplate 4, the light reflected and converged by the concave mirror 6 and impinging again on the polarization beam splitter 3 is S-polarized light with respect to the dividing plane 3a. As a consequence, this re-entering light is reflected by the dividing plane 3a of the polarization beam splitter 3 downwardly as viewed in the drawing. Disposed below the polarization beam splitter 3 are a polarization plane changing means 7 and a third lens group 8 having a positive refracting power. Further below the third lens group 3, there is a silicon wafer 9 for manufacture of semiconductor devices which wafer is held by a movable X-Y stage (not shown) so that its surface, to be exposed, coincides with the image plane of the reflection and refraction optical system 100. The polarization plane changing means 7 comprises a quarter waveplate which serves to transform the light, reflected by the dividing plane 3a of the polarization beam splitter 3, into circularly polarized light which in turn is projected on the third lens group 8. The third lens group 8 serves to collect the circularly polarized light from the quarter waveplate of the polarization plane changing means 7, and a reduced image of the circuit pattern of the reticle 1 is formed on the wafer 9. The projection exposure apparatus of this embodiment uses a polarization beam splitter (3), but it is arranged to form an image through circularly polarized light. As a consequence, for imaging fine patterns, there does not occur non-uniformness of resolution between different patterns which might otherwise result from the polarization dependency of pattern. In other words, the projection exposure apparatus of this embodiment assures constant resolution independently of the type (orientation) of the fine pattern of a reticle 1 used. In the projection exposure apparatus of this embodiment, the reticle stage for supporting the reticle 1 may be disposed horizontally and a reflection mirror may be provided between the reticle stage and the lens group 2 so as to deflect the optical axis AX by 45 deg. In that occasion, the overall size of the apparatus can be made small. The projection exposure apparatus of this embodiment may be arranged to execute step-and-repeat exposures according to which the X-Y stage on which the wafer 9 is placed is moved stepwise to form circuit patterns on substantially the whole surface of the wafer 9. Alternatively, it may be arranged to execute step-and-scan exposures wherein the X-Y stage on which the wafer 9 is place is moved stepwise and scanningly. The projection exposure apparatus of this embodiment may be used in combination with a phase shift mask as the reticle 1. In that occasion, it may be possible to image a pattern of a smaller linewidth. Further, the structure of the illumination system (not shown) may be modified into an oblique illumination system by which the reticle 1 is illuminated along a direction inclined with respect to the optical axis AX. Also in that occasion, a pattern of smaller linewidth may be imaged. The projection exposure apparatus of this embodiment may use a light source comprising a KrF excimer laser (λ ≃ 248 nm), an ArF excimer laser (λ ≃ 193 nm) or an ultra high pressure Hg lamp (emission line spectrum: λ ≃ 250 nm), for example. In another embodiment according to the present invention, a projection exposure apparatus such as described above may include a polarization plane changing means 7 comprising a half waveplate being made rotatable about the optical axis AX. When such polarization plane changing means is used, it is possible to change the plane of polarization of light from the polarization beam splitter 3 in accordance with the orientation of a fine pattern of a reticle 1 used. Thus, imaging with polarization light of increased resolution (non-deteriorated resolution) may always be assured. For example, in an occasion where a fine pattern of a reticle 1 has a lengthwise direction laid longitudinally (up and down) as viewed in the drawing, the rotational angle of the half waveplate 7 may be so set as to transform the plane of polarization of the light from the polarization beam splitter 3, from S-polarized light into P-polarized light. In an occasion where a fine pattern of a reticle 1 has a lengthwise direction laid perpendicularly to the sheet of the drawing, the rotational angle of the half waveplate 7 may be set so as to retain the plane of polarization (S-polarization) of the light from the polarization beam splitter 3. In an occasion where a fine pattern of a reticle 1 has an orientation, extending both longitudinally as viewed in the drawing and and perpendicular to the sheet of the drawing, (i.e., a cross pattern), the rotational angle of the half waveplate 7 may be so set as to transform the plane of polarization (S-polarization) of the light from the polarization beam splitter 3, into polarized light of 45 deg. with respect to both of the S-polarization and P-polarization. In a further embodiment according to the present invention, the waveplate of the polarization plane changing means 7 may comprise an electro-optic crystal device (EO optical-modulator) whose birefringence (double refraction) characteristic can be controlled electrically. Next, an embodiment of a method of manufacturing semiconductor devices based on the reticle 1 and the projection exposure apparatus of Figure 1, will be explained. Figure 2 is a flow chart of the sequence of manufacturing a semiconductor device such as a semiconductor chip (e.g. IC or LSI), a liquid crystal panel or a CCD, for example. Step 1 is a design process for designing the circuit of a semiconductor device. Step 2 is a process for manufacturing a mask on the basis of the circuit pattern design. Step 3 is a process for manufacturing a wafer by using a material such as silicon. Step 4 is a wafer process which is called a pre-process wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. Step 5 subsequent to this is an assembling step which is called a post-process wherein the wafer processed by step 4 is formed into semiconductor chips. This step includes assembling (dicing and bonding) and packaging (chip sealing). Step 6 is an inspection step wherein operability check, durability check and so on of the semiconductor devices produced by step 5 are carried out. With these processes, semiconductor devices are finished and they are shipped (step 7). Figure 3 is a flow chart showing details of the wafer process. Step 11 is an oxidation process for oxidizing the surface of a wafer. Step 12 is a CVD process for forming an insulating film on the wafer surface. Step 13 is an electrode forming process for forming electrodes on the wafer by vapor deposition. Step 14 is an ion implanting process for implanting ions to the wafer. Step 15 is a resist process for applying a resist (photosensitive material) to the wafer. Step 16 is an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above. Step 17 is a developing process for developing the exposed wafer. Step 18 is an etching process for removing portions other than the developed resist image. Step 19 is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer. As described hereinbefore, the present invention in an aspect thereof provides an imaging optical system or a reflection and refraction optical system, by which high resolution is assured independently of the type (orientation) of a fine pattern of an object to be projected. Thus, the present invention in another aspect effectively assures an improved projection exposure apparatus having superior projection exposure performance and based on a reflection and refraction optical system, or a method of manufacturing various devices through use of a reflection and refraction optical system. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An imaging optical system including a polarization beam splitter (3), a quarter waveplate (4) and a concave mirror (6) disposed in this order from an object plane (1), wherein said polarization beam splitter (3) and said quarter waveplate (4) are arranged to project a light beam from the object plane (1) upon said concave mirror (6), said concave mirror (6) is arranged to reflect the projected light beam, and said quarter waveplate (4) and said polarization beam splitter (3) are arranged to project the reflected light beam upon an image plane (9), characterized by: polarization state changing means (7) disposed between said polarization beam splitter (3) and the image plane (9) for changing the state of polarization of the polarized light beam projected by said polarization beam splitter (3). An imaging optical system according to claim 1, further being characterized by a first lens group (2) disposed between the object plane (1) and said polarization beam splitter (3) and a third lens group (8) disposed between said polarization beam splitter (3) and the image plane (9). An imaging optical system according to claim 2, characterized in that said first lens group (2) is effective to transform a light beam from the object plane (1) into a parallel light beam and to project the parallel light beam to said polarization beam splitter (3). An imaging optical system according to claim 3, further being characterized by a second lens group (5) disposed between said polarization beam splitter (3) and said concave mirror (6), wherein said second lens group (5) is effective to transform the parallel light beam from said polarization beam splitter (3) into a divergent light beam and to direct the same to said concave mirror (6). An imaging optical system according to claim 2, characterized in that said polarization state changing means (7) is disposed between said polarization beam splitter (3) and said third lens group (8). An imaging optical system according to any one of claims 1 to 5, characterized in that said polarization state changing means (7) comprises a second quarter waveplate effective to transform the polarized light beam from said polarization beam splitter (3) into a circularly polarized light beam, wherein an image is formed on the image plane with the circularly polarised light. An imaging optical system according to any one of claims 1 to 5, characterized in that said polarization state changing means (7) comprises a half waveplate effective to rotate the plane of polarisation of the polarized light beam from said polarization beam splitter (3) by 45°, wherein an image is formed on the image plane with the rectilinearly polarised light being rotated by 45°. An imaging optical system according to any one of claims 1 to 5, characterized in that said polarization state changing means (7) comprises a rotatable half wave plate for rotating the direction of polarization of the polarized light beam from said polarization beam splitter (3) by a desired rotation angle. An imaging optical system according to any one of claims 1 to 5, characterized in that said polarization state changing means (7) comprises an electro-optical crystal device having an electrically controllable double refraction characteristic for changing the state of polarization of the polarized light beam from said polarization beam splitter (3) into a desired state of polarization. An imaging optical system according to any preceding claim, characterized by means for obliquely projecting illumination light onto the object plane (1). An imaging optical system according to any one of claims 1 to 9, further being characterized by a phase shift mask disposed on the object plane (1). A projection exposure apparatus for projecting a pattern of an original (1) onto a substrate (9) comprising an imaging optical system as recited in any preceding claim. A method of manufacturing a semiconductor device, the method comprising the steps of: projecting a polarized light beam onto a substrate (9) to expose the substrate (9) to a pattern of an original (1), wherein the polarization of the light beam is selected in accordance with the pattern of the original (1); and fabricating a semiconductor device using the exposed substrate (9); characterized in that the polarized light beam is generated using the apparatus of any one of claims 1 to 11 by projecting a light beam through the polarization beam splitter (3) and quarter waveplate (4) onto the concave mirror (6), whereby the light beam is reflected by the concave mirror (6) and the polarization beam splitter (3) through the polarization state changing means (7) onto the substrate (9), wherein the polarization of the light beam for exposure of the substrate (9) is selected using the polarization state changing means (7).

Inventors: MURAKI MASATO, TAKAHASHI KAZUHIRO, MURAKI, MASATO, TAKAHASHI, KAZUHIRO, MURAKI, MASATO, C/O KOSUGI JIGYOSHO, TAKAHASHI, KAZUHIRO, C/O KOSUGI JIGYOSHO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G03F7
IPC codes: H01L21, G02B27, G02B17, G03F7