Abstract:
An optical element is provided with a film having a stress effective to sufficiently suppressing unwanted deformation of the optical element. This accomplishes very small aberration of the optical element and assures precise pattern transfer as the same is used in a lithographic process.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     This invention relates to an optical element which is usable in an optical system for a camera, a telescope, a microscope, or a semiconductor manufacturing apparatus, for example. 
     FIG. 16 is a schematic view for explaining gravity (weight) deformation of a certain substrate  1 . In FIG. 16, the substrate  1  is made of quartz having a diameter 150 mm and a thickness t. It is supported by supporting means  2 , at a distance of radius  a . Where the thickness t of the substrate  1  is small, there occurs a deformation W of the substrate  1  by gravity (weight), at a central portion thereof. 
     Here, if the thickness t of the substrate  1  is t=0.8 mm, the radius  a  of the supporting means  2  is a=70 mm, the Young&#39;s modulus E of quartz is E=7.31×10 4 N/mm 2 , the Poisson&#39;s ratio υ is υ=0.17, the density ρ is ρ=2.22×10 −6 Kg/mm 3 , the amount of deformation W can be determined in accordance with equation (1) below, and the deformation amount at the central portion of the substrate  1  is W=7.2 microns. 
     
       
           W= 3(1−υ 2 )9.81ρ ta   2 {(5+υ) a   2 /(1+υ)}/16 Et   3   (1)  
       
     
     FIG. 17 is a schematic view of a binary optics element  11  which is a diffractive optical element having a very small thickness. If the binary optics element  11  is supported by supporting means  2  of a radius 70 mm, then, as shown in FIG. 18, there will occur deformation in the thin binary optics element  11  due to the gravity, like the example of FIG.  16 . 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical element by which deformation such as described above can be avoided or reduced to a level that can be disregarded. 
     The amount of deformation of an optical element due to the gravity (weight) or the like can be predicted, by calculation, in accordance with the shape or material of the optical element or with the holding method for the optical element, for example. Also, any optical measurement device may be used to practically measure the amount of deformation. In accordance with the present invention, an optical element may be provided with a film formed on the surface thereof and having a stress value effective to produce a deformation amount cancelling the deformation amount as described above. 
     More specifically, in accordance with an aspect of the present invention, there is provided an optical element, characterized in that said optical element is provided with a film having a stress for substantially suppressing deformation of said optical element. 
     The deformation of said optical element may be gravity deformation. The film may comprise an anti-reflection film. The center of deformation of said optical may be substantially registered with an optical center of said optical element. The optical center of said optical element is substantially registered with a center of a substrate. 
     In accordance with another aspect of the present invention, there is provided an optical instrument having an optical element such as described above. 
     In accordance with a further aspect of the present invention, there is provided an exposure apparatus having an optical instrument as described above. 
     In accordance a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer to a device pattern by use of an exposure apparatus as described above, and developing the exposed wafer. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic and sectional view of an optical element according to a first embodiment of the present invention. 
     FIG. 2 is a schematic view for explaining the function of the first embodiment. 
     FIGS. 3A-3H are schematic views, respectively, for explaining binary optics element manufacturing processes. 
     FIG. 4 is a schematic and sectional view of an optical element according to a second embodiment of the present invention. 
     FIG. 5 is a schematic view for explaining the function of the second embodiment. 
     FIG. 6 is a fragmentary enlarged view of the optical element of the second embodiment. 
     FIG. 7 is a schematic view of an optical micro-measuring device. 
     FIGS. 8A-8C are schematic views, respectively, for explaining an optical element according to a third embodiment of the present invention. 
     FIGS. 9A-9C are schematic views, respectively, for explaining an optical element according to a fourth embodiment of the present invention. 
     FIGS. 10A-10D are schematic views, respectively, for explaining an optical element according to a fifth embodiment of the present invention. 
     FIGS. 11A-11D are schematic views, respectively, for explaining an optical element according to a sixth embodiment of the present invention. 
     FIGS. 12A-12C are schematic views, respectively, for explaining a plane-convex lens according to a seventh embodiment of the present invention. 
     FIG. 13 is a schematic view of a semiconductor exposure apparatus according to an embodiment of the present invention. 
     FIG. 14 is a flow chart of semiconductor device manufacturing processes. 
     FIG. 15 is a flow chart for explaining details of a wafer process. 
     FIG. 16 is a schematic view for explaining deformation of a substrate due to a gravity. 
     FIGS. 17 and 18 are schematic views, respectively, for explaining deformation of a substrate due to a gravity. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 are schematic and sectional views for explaining an optical element according to a first embodiment of the present invention. As shown in FIG. 1, the optical element comprises a substrate  20  and a stress adjusting thin film  21  having an arbitrary stress and being formed on the bottom face of the substrate  20 . In FIGS. 1 and 2, a diffraction grating or a spherical surface formed on the substrate is not illustrated. 
     With the provision of the stress adjusting thin film  21  and because of the internal stress S of the film  21 , the substrate  20  can be deformed. The amount of deformation is determined in dependence upon the material, thickness and shape of the substrate  20  of the optical element, as well as the stress value of the stress adjusting film  21  formed on the substrate  20 . Further, it is variable in dependence upon the difference in thermal expansion coefficient between the materials of the substrate  20  and the stress adjusting film  21 , and also upon the temperature. 
     Thus, the internal stress S of the stress adjusting film  21  may be determined so that the deformation amount W resulting from the gravity deformation of the substrate  20  and the deformation amount D due to the internal stress S of the stress adjusting film  21  are cancelled with each other. Then, the optical element may be supported by supporting means  22  as shown in FIG.  2 . By doing so, deformation of the substrate  20  can be removed or reduced to a minimum. 
     Here, if the film thickness of the stress adjusting film  21  is Tf, the internal stress S of the stress adjusting film  21  can be given by the following equation (2) wherein E is a Young&#39;s modulus, t is the thickness of the substrate  20 ,  a  is the radius of the supporting means, andυ is a Poisson&#39;s ratio. 
     
       
           S=dEt   2 /3 Tf ( a/ 2) 2 (1−υ)  (2)  
       
     
     Here, if the deformation amount d due to the internal stress S or the stress adjusting film  21  is d=7.2 microns and the film thickness Tf of the stress adjusting film  21  is Tf=1 micron, then, the stress value necessary for the stress adjusting film  21  to cancel the deformation amount W of the substrate  20  resulting from the gravity deformation, as can be calculated on the basis of specifications in FIG. 16, is S=27 N/mm 2 . 
     In FIG. 20, the center of deformation of the substrate  20  is placed substantially in registration with the optic center (optical axis) of the substrate  20 . This is because, if the deformation center and the optical center are not registered with each other, it leads to production of aberration. Usually, deformation by gravity occurs symmetrically with respect to a geometrical center of the substrate  20 . Thus, by forming the substrate  20  so that the geometrical center thereof is aligned with the optical center, the deformation center can consequently be registered with the optical center. 
     As regards measurement of the internal stress of the stress adjusting film  21 , it may be measured on the basis of the amount of warp of the substrate  20 . Alternatively, if the stress adjusting film has a crystalline property, by using an X-ray diffraction method or a Raman spectroscopy method, it can be calculated from a displacement in crystal lattice distance of the stress adjusting film  21 . 
     As regards the stress adjusting film  21  to be used, preferably it may be one having small absorption with respect to a wavelength to be used with the optical instrument. Further, it is desirable that the substrate  20  and the stress adjusting film  21  have the same optical constant with respect to the wavelength to be used, or optical constants which are close to each other as much as possible. If the optical constants differ from each other considerably, then the physical interface between the substrate  20  and the stress adjusting film  21  functions as an optical interface, and there occurs a loss of light quantity due to reflection at that interface. As an example, where quartz is used for the substrate  20 , silicon dioxide (SiO 2 ) may preferably be used as the material for the stress adjusting film  21 . 
     FIGS. 3A-3H show an example of a procedure for manufacturing a binary optics element having a stress adjusting film according to the numerical example described above. FIG. 3A shows a quarts substrate  31  having a thickness 0.8 mm and a diameter 150 mm. At a step shown in FIG. 3B, a photoresist  32  is applied to the substrate  31  in accordance with a spin coating method, to a thickness 0.5 micron. Then, a heat treatment is executed and, subsequently, a smallest pattern of a first reticle  33   a  is printed on the central portion of the substrate  31  by using a stepper and by using light. Then, a heat treatment is executed again and, thereafter, a development process is performed. By this, a ring-like resist pattern of linewidth 0.5 micron, of the smallest pattern, is produced. 
     Then, while using the thus produced resist pattern as a mask, the quartz substrate  31  is etched to a depth 61 nm in accordance with an ion etching method. After this, the photoresist is removed. By this, a binary optics element  35   a  with two levels, as shown in FIG. 3C, is produced. Further, as shown in FIG. 3D, a photoresist  32  is applied onto the two-level binary optics element  35   a  of FIG.  3 C. Then, a second reticle  33   b  having a pitch twice the first reticle  33   a  is used and, after executing alignment with respect to the etching pattern, the above-described processes are repeated. By this, a binary optics element  35   b  having a four-level structure such as shown in FIG. 3E is produced. 
     Thereafter, as shown in FIG. 3F, a third reticle  33   c  having a pitch twice the second reticle  33   b  is used and, after executing the etching pattern alignment, processes including exposure and etching are repeated. By this, a binary optics element  35   c  having an eight-level structure is produced. Subsequently, in accordance with a sputtering method, a stress adjusting film  36  is a thickness 1 micron, being made of silicon dioxide and having a tensile stress 27 N/mm 2 , is formed on the bottom face of the binary optics element  35   c.    
     In a case where an anti-reflection film is to be formed on a very thin binary optics element such as described above, the provision of a stress adjusting film  36  for cancelling gravity deformation as described above may be omitted and, in place of it, the stress value of the anti-reflection film may be controlled to suppress gravity deformation of the optical element. 
     In that occasion, the material for the anti-reflection film to be used should satisfy the reflection prevention condition with respect to a design wavelength of the optical element. For example, alumina, silicon dioxide, hafnium oxide or any other oxides, or calcium fluoride, magnesium fluoride, lithium fluoride, aluminum fluoride or any other fluorides, may be applied to the surface of the binary optics element  35   c  in accordance with a sputtering method, a vapor deposition method or a chemical vapor deposition method (CVD), for example, to form a film thereon. Since the deformation amount in such binary optics element  35   c  is changeable with the internal stress value of the thus formed anti-reflection film as well as the film thickness thereof, during the film forming process the film thickness should also be controlled. While the film thickness can be controlled by a film forming system, a film may be formed once with a thickness larger than a desired thickness and then the film thickness may be adjusted in accordance with an ordinary mechanical polishing process, a CMP (chemical and mechanical polishing) process, a chemical etching process, a RIE (reactive ion etching) process, or an ion-beam etching process or any other dry etching processes. As a further alternative, by using a difference in thermal expansion coefficient between the substrate  31  and the anti-reflection film, a stress may be produced. This provides substantially the same advantageous results. 
     FIGS. 4 and 5 show a second embodiment of the present invention. There is a four-layer film  42  made of silicon dioxide and alumina, which is formed on the bottom face of a binary optics element  41  through a sputtering method. This film  42  has a tensile stress (total stress) of 27 N/mm 2 , and it functions as a stress adjusting film and also an anti-reflection film. As shown in FIG. 5, the binary optics element  41  is supported by supporting means  22 , by which deformation at the central portion due to its gravity can be cancelled. Further, as shown in an enlarged sectional view of FIG. 6, the surface of the binary optics element  41  is formed with a very fine step-like structure, less than the wavelength used with the optical element, in accordance with an Ar sputtering method. There may be an anti-reflection layer  43  formed. 
     Next, adjustment or correction of the stress value of a stress adjusting film  36 , will be explained. 
     FIG. 7 is a schematic view of an optical micro-measuring system. An optical element  52  is supported on a supporting table  51 , and laser light L is projected to the optical element  52  from an optical micro-head  54  which is connected to an indicator  53 . By comparing the optical element  52  before deformation and the optical element  52 ′ after deformation, the amount of deformation can be measured. 
     FIG. 8A is a sectional view of a binary optics element, in a certain state, according to a third embodiment of the present invention. A binary optics element  35   c  with a stress adjusting film  36 , having been produced in accordance with the first embodiment, is supported by supporting means  22 . In this example, the deformation amount of the binary optics element  35   c  at the central portion thereof may be measured by using an optical micro-measuring system such as shown in FIG.  7 . The result may be that, due to insufficient stress control during formation of the stress adjusting film  36 , there is an upward convex deformation of 1 micron produced. 
     In that occasion, as shown in FIG. 8B, in accordance with a dry etching process, the stress adjusting film  36  as a whole may be etched by 0.02 micron, and after that, it may be placed on the supporting means  22  as shown in FIG. 8C, and the deformation amount at the central portion may be measured again. By this, deformation with respect to a reference plane can be removed. 
     FIG. 9A is a sectional view of a binary optics element according to a fourth embodiment of the present invention. A binary optics element  35   c  with a stress adjusting film  36  as manufactured in accordance with the first embodiment is supported by supporting means  22 . In this example, the deformation amount of the binary optics element  35   c  at the central portion thereof, with respect to a reference plane, may be measured. The result may be that, due to insufficient stress control during formation of the stress adjusting film  36 , there is an upward convex deformation of 0.3 micron produced. 
     In that occasion, as shown in FIG. 9B, ions  61  may be injected into the stress adjusting film  36  so that the deformation as the optical element is supported is removed to zero or reduced to a small level that can be disregarded. By this, as shown in FIG. 9C, when it is supported by the supporting means  22 , the deformation is removed. As regards ions to be injected, helium, neon, argon, krypton, xenon, radon or any other inactive rare gases, or hydrogen, oxygen, fluorine or any other reactive series may be used. If, after formation of the stress adjusting film  36 , a stress distribution is produced in this film, for example, ions may be injected in accordance with the stress distribution, to thereby improve the uniformness of stress value. 
     The procedure described above is not limited to a binary optics element, but it can be applied to various optical components such as a prism, a mirror or a CGH (computer generated hologram), or a photomask or reticle, for example. 
     FIGS. 10A-10C show a fifth embodiment of the present invention. In this embodiment, in place of a quartz substrate  20  of the first embodiment, a fluorite substrate  71  is used. Where the Young&#39;s modulus E of fluorite is E=7.58×10 4 N/mm 2 , the Poissons&#39;s ratio is υ=0.26, the density is ρ=3.18×10 −6  Kg/mm 3 , the deformation amount W as the substrate is supported at the position of a radius 70 mm, like the first embodiment, can be calculated in accordance with equation (1) above. The result is that the deformation amount at the center is W=11.3 microns. Where the deformation amount is W=11.3 microns and if the film thickness Tf of the stress adjusting film  74  is Tf=1 micron, the stress value necessary for producing a deformation that cancels this deformation amount is, according to equation (2), S=50 N/mm 2 . 
     FIG. 10A is a sectional view of a fluorite substrate  71  in this embodiment. In accordance with a sputtering method, a silicon dioxide film  72  of a thickness of about 427 nm is formed on the fluorite substrate  71 . Then, as shown in FIG. 10B, a binary optics element  73  is produced on the fluorite substrate  71  through the procedure similar to that of the first embodiment. Subsequently, as shown in FIG. 10C, the face of the binary optics element  73  to be processed is placed down and the substrate is supported by supporting means  22 , at a position of a radius 70 mm, like the first embodiment. Measurement may be made to the central portion of the binary optics element  73 , and the result may be that there is a concave deformation of 10 microns. 
     In that occasion, as shown in FIG. 10D, a stress adjusting film  74  made of magnesium fluoride and having a thickness of about 1 micron may be formed on the bottom face of the binary optics element  73  in accordance with a vapor deposition method. Then, the binary optics element  73  may be held again, and the deformation can be removed. The internal stress of the stress adjusting film  74  in this case can be calculated from the physical properties described above and in accordance with equation (2) above, and it is S=45 N/mm 2 . As regards the stress adjusting film  74 , calcium fluoride or silicon dioxide may be used. 
     FIG. 11A is a sectional view of a photomask  81  comprising a quartz substrate, according to a sixth embodiment of the present invention. The photomask has a size 150 mm square and a thickness 6.35 mm. A chromium pattern (or mask pattern)  82  is formed on the photomask  81 , and it is supported by supporting means  22  at positions of 140 mm square, in the manner similar to a mask chucking method in a semiconductor exposure apparatus. Deformation amount at the central portion of the photomask  81  may be measured, and the result may be that there is a concave deformation of 0.9 micron. In that occasion, the photomask  81  is unloaded from the supporting table  22 , and then a stress adjusting film  83  made of silicon dioxide and having a film thickness of about 5 microns and an internal stress S=43 N/mm 2  (according to the calculation based on equation (2)), is formed on the face of the photomask  81 , remote from the surface where the chromium pattern (mask pattern)  82  is formed. Then, it is held by the supporting table  22  as shown in FIG.  11 D. By this, deformation of the photomask  81  can be removed. 
     FIG. 12A is a sectional view of a plane-convex lens  91  according to a seventh embodiment of the present invention. This lens has a central thickness of 5 mm and a peripheral thickness of 2 mm. As shown in FIG. 12B, when the lens  91  is held by a supporting table  22  and deformation is measured, there may be a convex deformation of 3.5 microns. Thus, as shown in FIG. 12C, a silicon dioxide film  92  having a tensile stress may be formed on the flat face of the lens  91 , while on the other hand, a silicon dioxide film  93  having a compression stress is formed on the upper convex surface of the lens. Then, deformation amount measurement and ion injection may be repeatedly performed, until deformation as the lens is supported is not measured. By this, deformation due to the gravity of the plane-convex lens itself can be removed. 
     FIG. 13 is a schematic view of a semiconductor exposure apparatus into which a binary optics element according to any one of the preceding embodiments is incorporated. An excimer laser output device  101  produces KrF excimer laser light L which is reflected by reflection mirrors  102 ,  103  and  104 . The light is directed to an illumination optical system  105 . The illumination optical system  105  serves to produce illumination light having a uniform light intensity distribution. With this illumination light, a reticle  107  held by a mask chuck  106  is illuminated at uniform illuminance. Pattern light from the reticle  107  is reduced by a projection optical system  109  having a binary optics element whose deformation is suppressed sufficiently, and the light is projected on a wafer  111  which is held on a wafer chuck  110 , mounted on a stage  109 . 
     With the pattern light (exposure light) from the reticle  107  and through the projection optical system  109 , the pattern of the reticle  107  is imaged on the wafer  111  and thus is transferred thereto. Since the semiconductor exposure apparatus of this embodiment uses a binary optics element such as described hereinbefore, deformation of the element is reduced considerably. Therefore, aberration of the optical system  109  is very small, such that the pattern can be transferred to the wafer vary precisely. Although not shown in the drawing, the exposure apparatus of this embodiment is equipped with an alignment optical system for measuring relative deviation between the reticle  107  and the wafer  111 , a laser interferometer system for measuring the position of the stage  109 , and a conveying system for conveying the wafer  111  and the reticle  107 , for example. A stepper having an optical component, comprising a binary optics element, and a KrF excimer laser, such as described above, can be used to produce semiconductor devices. 
     FIG. 14 is a flow chart of procedure for manufacture of semiconductor chips such as ICs or LSIs, liquid crystal panels, or CCDs, for example. 
     Step  1  is a design process for designing a circuit of a semiconductor device. Step  2  is a process for making a mask on the basis of the circuit pattern design. Step  3  is a process for preparing a wafer by using a material such as silicon. Step  4  is a wafer process (called a pre-process) wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. More specifically, a photomask is loaded into an exposure apparatus and is conveyed onto a mask chuck. When the mask is held by the chuck, mask alignment is performed. Subsequently, a wafer is loaded and, by using an alignment unit, any deviation between the photomask and the wafer is detected. A wafer stage is then moved to accomplish alignment between the mask and the wafer. After it is completed, an exposure process is executed. After completion of this exposure, the wafer is moved stepwise for exposure of a subsequent shot, and the procedure from the alignment operation is repeated. 
     Step  5  subsequent to this is an assembling step (called a post-process) wherein the wafer having been processed by step  4  is formed into semiconductor chips. This step includes an assembling (dicing and bonding) process and a packaging (chip sealing) process. Step  6  is an inspection step wherein operation check, durability check and so on for the semiconductor devices provided by step  5 , are carried out. With these processes, semiconductor devices are completed and they are shipped (step  7 ). 
     FIG. 15 is a flow chart showing details of the wafer process (step  3  in FIG.  14 ). 
     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 upon 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, and by using a KrF excimer laser described with reference to FIG.  14 . 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. 
     With these processes, high density microdevices can be manufactured. 
     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 purposes of the improvements or the scope of the following claims.