Abstract:
A method of producing an optical element having a multiple-level step-like structure includes a first process for providing a first mask pattern at a position corresponding to a predetermined boundary among boundaries at steps of the multiple-level step-like structure of a substrate, the first mask pattern having a width narrower than that of a single step, a second process for providing a second mask pattern upon the substrate having the first mask pattern formed thereon, the second mask pattern having a width corresponding to a single step or plural steps of the multiple-level step-like structure and a third process for processing the substrate by use of the first and second mask patterns and thereafter for removing the second mask pattern while leaving the first mask pattern there. After repeating the second and third processes plural times of a predetermined number, the first mask pattern is removed.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     This invention relates to a method of producing an optical element which may suitably be used as, for example, an optical component of a semiconductor manufacturing reduction projection exposure apparatus, such as a phase modulation plate or an optical element having a two-dimensional binary structure or a phase type CGH (Computer Generated Hologram), or an optical interconnection element. 
     FIG. 9A is a fragmentary sectional view of a Fresnel lens  1 , which comprises a Fresnel type diffraction grating. It has a saw-tooth blazed shape  2 . FIG. 9B is a fragmentary sectional view of a binary optics element  3 , which comprises a binary type diffraction grating. It has a step-like shape  4 . 
     An idealistic diffractive optical element may be one having a blazed shape  2  such as shown in FIG. 9A, and it may assure a diffraction grating of 100% with respect to a design wavelength. However, it is very difficult to produce a complete blazed shape  2 . Therefore, while quantizing and approximating the blazed shape  2 , the binary optics element  3  having a step-like shape  4  such as shown in FIG. 9B is used. Although the binary optics element  3  is made on the basis of approximation of the Fresnel lens  1 , a diffraction efficiency for first-order diffraction light can be 90% or more. In consideration of this, much research has been done with respect to such binary optics  3 , by approximating a blazed shape  2  with a step-like shape. 
     In order that such an optical element has an enlarged power and that, through much better approximation, the optical element has an improved performance as a diffractive optical element, the processed linewidth should be made to be very fine, as much as possible. To this end, a lithographic process having been developed in the semiconductor manufacturing technology and being able to accomplish very high precision processing has been used. 
     FIG. 10 is a schematic view which illustrates the procedure for making a diffractive optical element having an eight-level step structure. More specifically, in (a) of FIG. 10, drops of a resist material are applied to a cleaned substrate  11  and, through spin coating, a resist coating of a film thickness of about 1 micron is formed on the substrate. Then, through a baking process, a resist film  12   a  is produced. In (b) of FIG. 10, the substrate  11  is loaded into an exposure apparatus having a performance with which a finest diffraction pattern can be printed. Then, a reticle  13   a  corresponding to a desired diffraction pattern is used as a mask, and exposure light L, with respect to which the resist film  12   a  has a sensitivity, is projected thereto. When a positive type resist is used, zones having been exposed with the exposure light L become solvable by a developing liquid. Thus, in (c) of FIG. 10, a resist pattern  14   a  having a desired size can be produced. Subsequently, in (d) of FIG. 10, the substrate  11  is introduced into an ion beam etching apparatus or a reactive ion etching apparatus by which an anisotropic etching process can be done. While using the produced resist pattern  14   a  as an etching mask, the substrate  11  is etched for a predetermined time period, by which it is etched to a predetermined depth. Then, the resist pattern  14   a  is removed. By this, a pattern  15   a  having a two-level step structure such as shown in (e) of FIG. 10 is produced. 
     Again, as in (a) of FIG. 10, a resist film  12   b  is produced on the substrate  11  having the pattern  15   a  formed thereon. In (f) of FIG. 10, the substrate then is placed in the exposure apparatus, and an exposure process is performed while a reticle  13   b  formed with a pattern having a periodicity two times larger than that of the reticle  13   a  is used as a mask. The mask pattern is transferred onto the pattern  15   a , after the two are aligned with each other at the alignment precision of the exposure apparatus. By a developing process in (g) of FIG. 10, the resist film  12   b  is developed such that a resist patten  14   b  is produced. Then, in (h) of FIG. 10, as in (d) of FIG. 10, a dry etching process is performed to remove the resist pattern  14   b , by which a pattern  15   b  having a four-level step structure is produced. 
     Subsequently, in (i) of FIG. 10, as in (a) of FIG. 10, again a resist film  12   c  is applied to the substrate  11 . Then, an exposure process is performed while a reticle  13   c  formed with a pattern having a periodicity four times larger than that of the reticle  13   a  is used as a mask. Then, in (j) of FIG. 10, the resist film  12   c  is developed, by which a resist pattern  14   c  is produced. Finally, in (k) of FIG. 10, the resist pattern  14   c  is removed, whereby a diffractive optical element with a pattern  15   c  having an eight-level step-like shape is produced. 
     When a diffractive optical element having a multiple-level step-like shape is to be produced while reticles having periodicities in multiples are used as masks, such as described above, as long as no alignment error or no dimensional error occurs, a multiple-level step structure having an idealistic shape can be produced. For example, by using three reticles  21   a - 21   c  shown in FIG. 11, an eight-level structure “A” having an idealistic step height “d” can be produced. 
     A paper in “O plus E”, No. 11, pages 95-100 (1996), discusses a procedure wherein processes of resist application, mask pattern and etching are repeated, and it mentions that a multiple-level phase type CGH (Computer Generated Hologram) having phase levels of 2 L  can be produced where L is the number of masks used. 
     FIGS. 12 a - 12   c  are plan views for explaining a procedure for producing a CGH. More specifically, FIGS. 12 a - 12   c  show patterns of reticles  31   a ,  31   b  and  31   c , wherein zones depicted with hatching are light blocking portions. For example, the reticle  31   a  is used to perform the etching to a depth of 61 nm, the reticle  31   b  is used to perform the etching to a depth of 122 nm, and the reticle  31   c  is used to perform the etching to a depth of 244 nm. Although the order of using these reticles  31   a - 31   c  is not fixed, a better resist patterning precision is attainable when a reticle for a smaller etching depth is used first. 
     The reticle  31   a  is used first to perform resist patterning on a substrate, and it is etched to a depth of 61 nm. As a result, an etching depth distribution such as shown in FIG. 13A is produced, wherein numerals denote etching depths (nm). Thereafter, the resist on the substrate is removed. Then, the reticle  31   b  is used to perform resist patterning, and the substrate is etched to a depth of 122 nm, by which an etching depth distribution such as shown in FIG. 13B is produced. Further, the resist on the substrate is removed, and the reticle  31   c  is used to perform resist patterning. The substrate is etched to a depth of 244 nm, by which an etching depth distribution such as shown in FIG. 13C is produced. FIG. 14 is a sectional view taken along a line E-e in FIG.  13 C. 
     In the examples described above, if an alignment error occurs in registration of reticles, it directly leads to degradation of the performance of a diffractive optical element. More specifically, at the boundary of zones where phase differences are quantized, a very small structure which is not included in the design may be produced due to the alignment error. Such a very small structure causes degradation of the function of the diffractive optical element, and thus, causes a decrease of the diffraction efficiency. Further, the light corresponding to the decrease of diffraction efficiency advances in a direction not intended in the design as unwanted diffraction light, and it causes various undesirable problems. In this manner, light rays not desired may be produced from the diffractive optical element. Such light rays may function as flare light in an optical system wherein the diffractive optical element is used, and the flare light causes degradation of the imaging performance of the optical system. 
     FIG. 15 shows an example. If an alignment error occurs in the registration of reticles  21   a - 21   c  of FIG.  15  and it causes deviations r 1  and r 2  as illustrated, a produced diffractive optical element has a shape “B” different from the idealistic shape “A” of FIG.  11 . Particularly, very small structures being smaller than the wavelength and produced at the deviated portions of the reticles may cause various problems such as increased scattered light or trapping of light which may result in a temperature rise of the optical element, for example. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of producing a diffractive optical element by which a required shape can be formed very accurately. 
     In accordance with an aspect of the present invention, there is provided a method of producing an optical element, including a process for forming a quantized level distribution on a substrate through lithographic technology, characterized in that, in a portion of the substrate about a boundary between unit zones, in each of which a minimum unit of the level distribution is to be defined, a mask having a width including the boundary and being smaller than the width of the unit zone is provided. This enables that, while the portion about the boundary is kept as an unprocessed region, the remaining portion of the substrate is processed, by which the quantized level distribution is produced on the substrate. 
     In accordance with another aspect of the present invention, there is provided a method of producing an optical element, including a process for forming a quantized level distribution on a substrate through lithographic technology, characterized in that, in a portion of the substrate about a boundary between unit zones, in each of which a minimum unit of the level distribution is to be defined, a mask having a width including the boundary and being smaller than the width of the unit zone is formed. This enables that, while the portion about the boundary is kept as an unprocessed region, the remaining portion of the substrate is etched, by which the quantized level distribution is produced on the substrate, and that, after the level distribution is produced, the unprocessed portion is removed. 
     In these aspects of the present invention, the quantized level distribution may have a function as a diffraction grating. 
     The mask may be provided so that a position of an edge of the mask is registered with the boundary. 
     The mask may be provided by a lithographic process using an electron beam. 
     The mask may be provided by a lithographic process using a phase shift mask. 
     The unprocessed region may have a width not less than an alignment precision of a lithographic apparatus to be used for the production. 
     The mask may comprise a light blocking film having an etching rate smaller than that of the substrate. 
     The light blocking film may be substantially made of one of Cr, Al, Ti, Ni, Mo and W. 
     The surface of the light blocking film may have an anti-reflection function with respect to a wavelength to be used. 
     The light blocking film may comprise a CrO 2  film. 
     The mask may be made of a light transmissive material having an etching rate smaller than that of the substrate. 
     The light transmissive material may be substantially made of one of TiO 2  and indium tin oxide (ITO). 
     The unprocessed region may be defined only in a portion of boundaries providing different phase differences. 
     A portion of the unprocessed region may have a central position being registered with the boundary position of the quantized zones. 
     At a boundary where a phase difference of zones across that boundary corresponds to a unit period of a design wavelength of the optical element, the unprocessed region may be defined to be included in a region of the substrate which is juxtaposed to the unprocessed region and which is not processed, such that the position of the boundary may be registered with an edge position of the unprocessed region. 
     The unprocessed region may be defined to be included in one of the adjacent zones in which the amount of processing of the substrate is smaller than the other, and the position of the boundary may be registered with the position of an edge of the unprocessed region. 
     The removal of the unprocessed region may include an isotropic etching process. 
     The substrate may be made of fluorite, and the isotropic etching process may comprise a wet etching process using water or a nitric acid. 
     The substrate may be made of quartz, and the isotropic etching process may comprise a wet etching process using water or a hydrofluoric acid. 
     The substrate may be made of one of quartz and fluorite. 
     In accordance with a further aspect of the present invention, there is provided an optical element produced in accordance with a procedure as discussed above. 
     The optical element may be a computer generated hologram. 
    
    
     These and other objects, features and advantages of the present 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 
     FIGS.  1 (( a )-( h )) is a schematic view for explaining processes for producing a diffractive optical element in accordance with a first embodiment of the present invention. 
     FIG. 2 is a schematic view of a semiconductor exposure apparatus. 
     FIG. 3 is a graph for explaining the relation between a radial position and a pitch. 
     FIG. 4 is a schematic plan view of a substrate in a second embodiment of the present invention. 
     FIG. 5 is a schematic plan view of a phase shift mask. 
     FIG. 6 is a graph for explaining a light intensity distribution to be provided by a phase shift mask. 
     FIGS. 7A and 7B are schematic plan views, respectively, for explaining superposition of a resist pattern and a substrate. 
     FIG. 8 is a schematic view for explaining a boundary of a resist pattern. 
     FIGS. 9A and 9B are schematic sectional views of diffractive optical elements. 
     FIGS.  10 (( a )-( k )) is a schematic view for explaining an example of conventional production of a diffractive optical element. 
     FIG. 11 is a schematic sectional view of a substrate and reticles. 
     FIGS. 12A-12C are schematic plan views of reticles. 
     FIGS. 13A-13C are schematic views for explaining etching depths. 
     FIG. 14 is a schematic sectional view of a substrate and reticles. 
     FIG. 15 is a schematic sectional view of a substrate and reticles. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view for explaining processes for producing a binary optics element in accordance with a first embodiment of the present invention. FIG. 1 in (a) shows the sectional shape of an eight-level optical element to be produced. In this embodiment, as shown in FIG. 1 in (b), first a Cr film  42  on a quartz substrate  41  is patterned by using an electron beam so that unprocessed regions are defined at positions where boundaries of three masks (reticles) such as described hereinbefore are superposed. The unprocessed region is a region in which a film formed thereon by the patterning will not be removed by etching, and also the unprocessed region is a region which does not function as a diffractive optical element. Thus, use of a light blocking film therefor is suitable. The Cr film  42  may not have a complete etching proof. It may be etched to some extent unless the etching action extends to the substrate  41  underlying it. Namely, the film may be one having a thickness by which, even though the film is etched together with processing regions of the substrate during the etching process, the surface of the substrate  41  underlying the film is not exposed finally. 
     Since the Cr film  42  does not function as a schematic view, the area thereof should desirably be made as small as possible. The smallest linewidth thereof may be determined in accordance with the alignment precision of the exposure apparatus used for the patterning. 
     As shown in FIG. 1 in (c), one of the three masks having a finest pattern is used to form a resist pattern  43   a . Subsequently, as shown in FIG. 1 in (d), the quartz substrate  41  is etched to a desired depth, while using the resist pattern  43   a  and the Cr film  42  as an etching mask. Here, the edge of each step of the substrate  41 , which is registered with the mask edge, is in registration with a corresponding edge of the Cr film  42 , as illustrated. 
     Subsequently, as shown in FIG. 1 in (e), the resist pattern  43   a  is removed, and then, while using a second mask having a slightly wider linewidth pattern, a resist pattern  43   b  is patterned in a similar manner as shown in FIG. 1 in (c). Here, since edges of this mark are registered with those of the first mask used first, all the edges of the resist pattern  43   b  are formed on the Cr film  42 . Then, as shown in FIG. 1 in (f), the substrate  41  is etched to a desired depth while using the resist pattern  43   b  and the Cr film  42  as an etching mask. Here, the edge of each step of the substrate  41 , after the etching process, is in registration with a corresponding edge of the Cr film  42 , as illustrated. 
     Subsequently, as shown in FIG. 1 in (g), the resist pattern  43   b  is removed, and then, while using a third mask having a relatively most coarse pattern, a resist pattern  43   c  is patterned. By a subsequent etching process, a shape having protrusions  44  is produced. 
     As shown in FIG. 1 in (h), the resist pattern  43   c  and the Cr film  42  are then removed through separate processes, and finally, the protrusions  44  are removed by a wet etching process using a hydrofluoric acid. Since the wet etching process shows an isotropic property, protrusions  44  having a column structure can be removed simultaneously, regardless of the height thereof. Thus, when quartz having a good transmission factor with respect to a broad wavelength region is used, a wet etching process using hydrofluoric acid is suitable. 
     FIG. 2 is a schematic view of a semiconductor exposure apparatus which can be used for the patterning of the Cr film  42  upon the substrate  41  as described above. The exposure apparatus  51  comprises a single-axis horizontal motion unit  52 , a rotational driving unit  53  and an electron beam projecting unit  54 . 
     FIG. 3 shows the relation between a radius position and a pitch, in the production of a diffractive optical element having a function as a convex lens. The radius position for forming the Cr film  42  can be determined from the design value which is the function of the radius as such. 
     The horizontal motion unit  52  stops as the electron beam irradiation position reaches a radius position to be exposed. Then, the electron beam projection unit  54  starts projection of an electron beam and, simultaneously therewith, the rotary driving unit  53  rotationally moves the substrate  41 . 
     The position where the Cr film  42  is to be formed is limited to the position where the boundaries of plural masks are to be registered in design, for formation of the resist pattern. Similarly, it may be limited to a position where the number of times of mask superposition is large, or it may be limited to such a region which should be etched to a large depth, although the number of times of superposition is the same. 
     The edge of the Cr film  42  at the boundary of its pitch should desirably be placed in registration with the design pitch boundary. The Cr film  42  within the pitch should preferably be disposed while taking the masking edge designed at the center. Here, the Cr film  42  at the pitch boundary should be included in a region wherein the substrate  41  is not to be etched. If it is included in a region to be etched, the design of the mask pattern edge should be made in consideration of the unprocessed region. Further, the pattern edge of the mask pattern may be designed to be the center of the Cr film  42  as an offset is applied thereto. 
     In this embodiment, a semiconductor exposure apparatus is used as a patterning apparatus. Since a general alignment precision of such a semiconductor exposure apparatus is very good when it is on the order of 100 nm or less, the unprocessed region of the pattern may be designed to keep a process selectivity among the material to be used for the patterning, the material for forming the unprocessed region, and the material for the diffractive optical element. Thus, the materials are not limited to those used in this embodiment. Further, while in this embodiment, the patterning process starts with a mask having a finest mask, the order of the masks to be used is not very influential to the result and, therefore, the masks may be used in a reverse order or at random. 
     FIGS. 4-8 show a second embodiment, wherein FIG. 4 is a plan view of a quartz substrate. The substrate has formed thereon a Cr film  61  having a width of about 0.1 micron and zones  62  of a few microns square. In this embodiment, as in the preceding embodiment, a semiconductor exposure apparatus is used as a patterning apparatus. The unexposed region of the pattern is designed with a width of 100 nm. As regards the formation of the Cr film  61 , an electron beam may be used as in the first embodiment. However, in this embodiment, for more efficient operation, a phase shift mask  71  such as shown in FIG. 5, for example, is used to perform the patterning. In the phase shift mask  71 , there are higher regions  72  and lower regions  73  alternately defined to provide phase differences, whereby a check-like pattern is produced. Here, the width of the Cr film  61  can be adjusted by controlling the exposure amount. 
     As shown in FIG. 6, in the phase shift mask  71  (image thereof), the light intensity distribution is largely lowered at the boundary area between the regions where the phases are mutually shifted by a half period. On the other hand, for the processing of a CGH, the patterning is performed by using three masks corresponding to the depths to be defined in respective regions, that is, a first mask having a reference depth, and a third mask having a depth triple the reference depth. By using these masks sequentially to repeatedly perform the patterning process and an etching process, a CGH of an eight-level shape can be produced. 
     FIG. 7A shows the state in which a resist pattern  81   a  formed by using the first mask is superposed on the substrate of FIG.  4 . The regions  82   a  as can be seen through windows in the resist pattern  81   a  are those regions to be processed by using the resist pattern  81   a . Similarly, FIG. 7B shows the state in which a second resist pattern  81   b  is superposed on the substrate. The regions  82   b  as can be seen through windows in the resist pattern  81   b  are those regions to be processed by using the resist pattern  81   b . The third mask is omitted from illustration since the process using it is similar to those of the first and second masks. 
     FIG. 8 is a schematic view of a state in which the boundaries of the resist patterns  81   a  and  81   b  are superposed with each other. The position of the boundary  91  of the pattern as depicted by a solid line corresponds to the edge positions of the resist patterns, respectively, and the state of a largest alignment error is illustrated in the drawing. Thus, the boundaries of the resist patterns are not registered with each other. However, since the region in which the alignment error occurs is covered by the Cr film  61 , there is no possibility that a very small groove structure is produced after the CGH processing through the etching process. 
     Further, an isotropic etching process similar to the wet etching process using hydrofluoric acid in the first embodiment, for example, may be performed to remove protrusions to be formed below the Cr film  61 . Here, the position of the Cr film  61  may preferably be so determined that the edge thereof is registered with the boundary of the region just to be defined and so that the Cr film forming region  61  is included in a region, among juxtaposed regions, when the process amount is smaller. Further, the pattern edge of the mask pattern should be designed while taking the Cr film  61  as an offset applied thereto as a center. 
     The boundary for defining the Cr film may be selected in accordance with the difference in process amount between juxtaposed regions or any other condition such as the number of times of mask superpositions, for example. Thus, the Cr film  61  may be provided with priority, in accordance with those regions when the number of mask superpositions is large. Further, the Cr film  61  may be omitted in a region where edges are not superposed during the sequential processes. 
     The Cr film  61  formed in the unprocessed region may be provided with an anti-reflection film. On that occasion, when a produced CGH is incorporated into an optical system, it can serve to suppress the reflection of light from the boundary where the Cr film  61  is present. It is, therefore, effective to prevent flare light. 
     In place of the Cr film used in the first and second embodiments, a metal film such as Al, Ti, Ni, Mo or W may be used. With such a metal film, a diffractive optical element having a good effect of flare light prevention can be provided. 
     In the etching process to the substrate, for the removal of a structure resulting from the unprocessed region, which process is to be performed at the final stage of the production of the diffractive optical element, the material constituting the unprocessed region functions like the Cr film. 
     In the embodiments described above, if the light blocking film used causes reflection of light, a CrO 2  film may be used as an anti-reflection film. Further, in a case wherein reflection light rather than transmitted light causes inconveniences, the Cr film may be replaced by a light transmissive film such as TiO 2  or ITO, for example. Even when a material having an etching selectivity to the substrate is used, a diffractive optical element having a good effect of flare light suppression can be accomplished. Here, since the reflection light from the unprocessed region is small and also the light transmitted has a small distribution, no inconvenience is caused in relation to the unprocessed region. When the structure resulting from the etching of the substrate using the unprocessed region is removed by an isotropic etching process, which is to be performed at the final stage of the production of the diffractive optical element, the material constituting the unprocessed region functions like the Cr film. 
     Fluorite may be used for the substrate of a diffractive optical element, in place of quartz described above and, on that occasion, a diffractive optical element having a very good property, that is, a higher inside transmission factor, particularly, to the wavelengths of ArF excimer laser light (193 nm) and F 2  excimer laser light (157 nm) is provided. Also, for the etching process to fluorite, a wet etching process using water or nitric acid is suitable. 
     In the method of producing an optical element according to the embodiments described above, an unprocessed region is defined in a pattern. This effectively assures a desired shape even if an alignment error occurs due to deviations of masks used. Thus, degradation of the optical performance can be avoided effectively. 
     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.