Abstract:
A method of manufacturing an element having a multiple-level step-like shape through plural lithographic processes, or a mold for production of such an element is disclosed, wherein a position of at least one step of the step-like shape is determined by an end of at least a portion of a pattern of a first mask to be formed through a first lithographic process of the plural lithographic processes.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a method of manufacturing an element with a multiple-level step-like structure such as a diffractive optical element, a Fresnel lens, a phase type computer hologram (CGH), or a mask for the CGH, for example, more particularly, an element having a very fine surface-step pattern such as a diffractive optical element, for example, usable in manufacture of a semiconductor integrated circuit, for example. In another aspect, the invention is concerned with a method of manufacturing a mold for producing such an element. 
     Fujita, et al. (“Journal of Electronic Communication Association”, (C) J66-CP85-91, January, 1983) and Japanese Laid-Open Patent Application, Laid-Open No. 26560/1987 as well as Japanese Laid-Open Patent Application, Laid-Open No. 42102/1987 disclose a method wherein a step-like shape is formed by using an electron beam while controlling its dose quantity, and wherein a resist is used directly as a circuit pattern. 
     Japanese Laid-Open Patent Application, Laid-Open No. 137101/1986 discloses a method wherein two or more types of films having an etching durability are accumulated with a desired thickness, wherein the layers are etched from the top layer to provide a step-like structure, whereby a mold is formed. 
     Japanese Laid-Open Patent Application, Laid-Open No. 44628/1986 and Japanese Laid-Open Patent Application, Laid-Open No. 160610/1994 disclose a method wherein, while a resist is used as an etching mask, a step-like structure is formed on the basis of a sequential alignment for the steps, whereby a mold is formed. 
     Japanese Laid-Open Patent Application, Laid-Open No. 15510/1996 discloses a method wherein an etching stopper layer is used and, for each step, an etching stopper layer and a transparent layer are accumulated, and wherein a step-like structure is formed through alignment, exposure and etching. 
     Japanese Laid-Open Patent Application, No. 26339/1994 which corresponds to Japanese Laid-Open Patent Application, Laid-Open No. 72319/1995 and U.S. Pat. No. 5,324,600 disclose a method wherein an alignment operation is performed while using a resist as an etching mask, and a step-like structure is formed. 
     In these examples, however, the minimum size is determined by the smallest resolution of a drawing apparatus and, therefore, it is not easy to produce a very fine shape. 
     A multiple-level step-like diffractive optical element can be manufactured as a diffractive optical element having a step-like sectional shape, in accordance with photolithographic processes based on exposure and etching techniques that are used in semiconductor manufacture. In such a multiple-level step-like diffractive optical element, the function of a diffractive optical element is performed by step-like surface level differences (steps) formed on a substrate. 
     Therefore, the optical performance of the multiple-level step-like diffractive optical element, particularly, the diffraction efficiency, depends on the shape of the formed surface step, that is, depth, width, or sectional shape of the step, for example. Specifically, where plural masks are used, an alignment error between them largely influences the diffraction efficiency. 
     For example, where a step-like shape is to be formed by using masks of harmonic frequencies sequentially, an idealistic step-like shape can be formed if there is no alignment error or line width error. Practically, however, it is very difficult to remove the line width error or the alignment error completely and, therefore, the produced shape differs from the idealistic shape. Basically the same problem is involved in other methods. 
     Referring to a specific example, as shown in FIG. 70, idealistically an eight-level (step) shape can be produced by using three masks, that is, masks A, B and C. If any misregistration occurs between the masks A and B, a problem arises. FIG. 70 shows the resultant shape when there are alignment deviations d b  and d c  among the masks A, B and C. If the illustrated shape results, the optical performance of the optical element D such as diffraction efficiency, for example, considerably degrades. 
     Also, if there is a line width error at each layer, the optical performance such as diffraction efficiency further degrades. In the case of electron beam drawing, there may be no alignment error. However, a bulky drawing operation is required and, therefore, a sufficient throughput with respect to the productivity is not attainable. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method of manufacturing an element having an accurate and very fine step-like shape. 
     It is another object of the present invention to provide a method of manufacturing a mode for producing an element having an accurate and very fine step-like shape. 
     In accordance with an aspect of the present invention, there is provided a method of manufacturing an element having a multiple-level step-like shape through plural lithographic processes, wherein a position of at least one step of the step-like shape is determined by an end of at least a portion of a pattern of a first mask to be formed through a first lithographic process of the plural lithographic processes. 
     In accordance with another aspect of the present invention, there is provided a method of manufacturing a mold for production of an element having a multiple-level step-like shape through plural lithographic processes, wherein a position of at least one step of the step-like shape is determined by an end of at least a portion of a pattern of a first mask to be formed through a first lithographic process of the plural lithographic processes. 
     In one preferred form of these aspects of the present invention, the method includes (i) a first process for etching a portion of a base material not covered by the first mask to a predetermined depth, (ii) a second process for forming a second mask so that it covers a particular region of a portion of the base material not covered by the first mask and also that it overlaps with the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask, and (iii) a third process in which, after the second mask is separated, the second process is repeated as required and, after the second mask is separated, a third mask is used to cover the portion not covered by the first mask, in which a fourth mask is formed so that an end of a pattern of the fourth mask lines at an end portion of the pattern of the first mask while an opposite end of the pattern of the fourth mask overlaps with the pattern of the third mask, and in which, after the fourth mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated at least once, as required, after the third and fourth masks are separated. 
     In another preferred form of these aspects of the present invention, the method includes (i) a first process for etching a portion of a base material not covered by the first mask to a predetermined depth, (ii) a second process for forming a second mask so that it covers a particular region of a portion of the base material not covered by the first mask and also that it overlaps with the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask, and (iii) a third process in which the second process is repeated as required after the second mask is separated and, after the second mask is separated, a third mask is formed so that it covers the portion not covered by the first mask and that an end of a pattern of the third mask lies at an end portion of the pattern of the first mask while an opposite end of the pattern of the third mask lies on the first mask, and in which, after the third mask is formed, an exposed portion of the first mask is etched, wherein the third process is repeated as required after the third mask is separated. 
     In a further preferred form of these aspects of the present invention, the method includes (i) a first process for forming a second mask so that it covers a particular region of a portion of a base material not covered by the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask to remove the second mask, (ii) a second process for repeating the first process, at least once, so that the portion of the base material not covered by the first mask is etched to a predetermined depth, and (iii) a third process in which a third mask is used to cover the portion not covered by the first mask, in which a fourth mask is formed so that an end of the fourth mask lies on the first mask while an opposite end of the pattern of the fourth mask overlaps with the third mask, and in which, after the fourth mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated as required after the third mask is separated. 
     In a still further preferred form of these aspects of the present invention, the method includes (i) a first process for forming a second mask so that it covers a particular region of a portion of a base material not covered by the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask to remove the second mask, (ii) a second process for repeating the first process at least once so that the portion of the base material not covered by the second mask is etched to a predetermined depth, and (iii) a third process in which a third mask is formed, after the second mask is separated, so that the third mask covers at least the portion not covered by the first mask and also that an end of a pattern of the third mask lies at an end of a pattern of the first masks while an opposite end of the pattern of the third mask lies on the first mask, and in which, after the third mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated as required after the third mask is separated. 
     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 
     FIGS. 1-19 are sectional views, respectively, for explaining the manufacturing processes in accordance with a first embodiment of the present invention. 
     FIGS. 20-33 are sectional views, respectively, for explaining the manufacturing processes in accordance with a second embodiment of the present invention. 
     FIGS. 34-46 are sectional views, respectively, for explaining the manufacturing processes in accordance with a third embodiment of the present invention. 
     FIGS. 47-61 are sectional views, respectively, for explaining the manufacturing processes in accordance with a fourth embodiment of the present invention. 
     FIGS. 62-65 are sectional views, respectively, for explaining the manufacturing processes in accordance with a fifth embodiment of the present invention. 
     FIG. 66 is a schematic and sectional view for explaining a reflection type element. 
     FIG. 67 is a schematic view of a stepper according to a seventh embodiment of the present invention. 
     FIG. 68 is a schematic view of a step-like diffractive optical element to be incorporated into the stepper of the seventh embodiment. 
     FIG. 69 is a schematic and sectional view of a step-like diffractive optical element to be incorporated into the stepper of the seventh embodiment. 
     FIG. 70 is a schematic view for explaining the background of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to the drawings of FIGS. 1-69. 
     In a first embodiment of the present invention, as shown in FIG. 1, there is a quartz substrate  1  on which a Cr film  2  is formed by sputtering, as shown in FIG. 2, with a thickness of 1000 angstroms. For enhancement of a patterning resolution, an anti-reflection film (not shown) of chromium oxide, for example, of 200-300 angstroms, may be provided on the Cr film  2 . 
     Then, a photoresist is applied to the quartz substrate  1  and, through an exposure process and a development process, a first-time resist pattern is formed thereon. Subsequently, by using the resist pattern as a mask, the Cr film  2  is etched. Here, the etching process may use a parallel plane plate type RIE (reactive ion etching) apparatus, for example, and an etching gas of a chlorine gas or a mixture gas of chlorine gas and oxygen, for example. 
     Then, as shown in FIG. 3, the resist pattern is separated in accordance with an oxygen ashing method or by using a removing liquid, whereby a pattern of Cr film  2  is produced. Subsequently, as shown in FIG. 4, by using the Cr film pattern  2  as a mask, the quartz substrate  1  is etched. Here, the etching process may use a RIE (reactive ion etching) apparatus as described above, for example, and an etching gas of a mixture gas of CF 4  and hydrogen, for example. The etching conditions may be, for example: CF 4  flow rate is 20 sccm, hydrogen flow rate is 3 sccm, pressure is 4 Pa, and RF power is 60 W. 
     Thereafter, a photoresist is applied to the whole surface and, through an exposure process and a development process, patterning of it is performed as shown in FIG.  5 . Then, by using the Cr film  2  and the resist pattern  3  as a mask, the quartz substrate  1  is etched. Here, the etching process may use a RIE (reactive ion etching) apparatus as described, for example, and it may be performed in a similar manner as described above. 
     Subsequently, the photoresist pattern  3  is separated and, thereafter, again a photoresist pattern  4  is applied to the whole surface. Through an exposure process and a development process, the patterning of it is accomplished, as shown in FIG.  7 . Then, by using the Cr film  2  and the resist pattern  4  as a mask, the quartz substrate  1  is etched, as shown in FIG.  8 . As the photoresist pattern  4  is removed, the result such as shown in FIG. 9 is obtained. 
     As shown in FIG. 10, a negative type resist  5  is applied to the whole surface, and an exposure of the substrate is performed from the bottom face side of the substrate  1 . As a development process is performed, the result is such that, as shown in FIG. 11, a resist pattern  7  is formed only at a portion where no Cr film  2  is present. 
     Subsequently, a photoresist is applied to the whole surface, and a pattern  8  is patterned as shown in FIG.  12 . Then, as shown in FIG. 13, the portion of the Cr film  2  not covered by the pattern  7  or the pattern  8  is etched. The etching process may be performed in accordance with a RIE (reactive ion etching) method, using a chlorine gas or a mixture gas of chlorine gas and oxygen, for example. 
     Subsequently, as shown in FIG. 14, by using the patterns  7  and  8  as a mask, the quartz substrate  1  is etched. Thereafter, the patterns  7  and  8  are removed and, then, a negative resist is applied to the whole surface and the exposure operation is performed to the substrate  1  from its bottom face side. As a development process is performed, the result is that, as shown in FIG. 15, a resist pattern  9  is formed only in a portion where the Cr film  2  is not present. Then, a photoresist is applied to the whole surface, and a pattern  10  is patterned through an exposure process and a development process. The Cr film  2  in a portion not covered by the pattern  9  or the pattern  10  is etched in accordance with the RIE method using a chlorine gas or a mixture gas of chloride gas and oxygen, for example, such as shown in FIG.  17 . 
     Then, as shown in FIG. 18, by using the patterns  9  and  10  as a mask, the quartz substrate  1  is etched. Finally, the patterns  9  and  10  as well as the Cr film  2  are removed. Here, in the etching process, a liquid mixture of cerium ammonium nitrate, perchloric acid and water, for example, may be used. In this manner, a six-level step-like diffractive optical element  1 ′ such as shown in FIG. 19 is completed. 
     Positions a and b in this step-like diffractive optical element  1 ′ (FIGS. 3 and 19) are determined in accordance with the first patterning, independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     Also, in this embodiment, the optical element can be manufactured at a step of one-third of the minimum resolvable line width of a drawing apparatus. Therefore, an optical element with a higher diffraction efficiency can be produced. 
     In this embodiment, the highest step a and the third step b therefrom are determined by the first mask. When an element with steps of a number 2 n  is to be produced, the highest step a and the (n)th step b therefrom are determined by the first mask. Also, two steps (without Cr film) may be formed in a first process while three steps (with Cr film) may be formed in the subsequent process. In that case, the highest step a and the third step b therefrom are determined by the first mask. 
     Therefore, generally, where steps of n are to be formed in a later process (with Cr film), the highest step a and the (n)th step b therefrom are determined by a first mask. 
     In a second embodiment of the present invention, as shown in FIG. 20, there is a quartz substrate  11  on which a Cr film  12  is formed by sputtering, with a thickness 1000 angstroms. Here, an anti-reflection film (not shown) of chromium oxide, for example, may be provided on the Cr film  2  as desired. 
     Then, a photoresist is applied to the quartz substrate  11  and, through an exposure process and a development process, a first-time resist pattern  13  is formed thereon, as shown in FIG.  21 . Subsequently, by using the resist pattern  13  as a mask, the Cr film  2  is etched. Here, the etching process may use a parallel plane plate type RIE (reactive ion etching) apparatus, for example, and an etching gas of a chlorine gas or a mixture gas of chlorine gas and oxygen, for example. 
     Then, as shown in FIG. 22, the resist pattern  13  is separated in accordance with an oxygen ashing method or by using a removing liquid. Additionally, as shown in FIG. 23, by using the pattern of Cr film  12  as a mask, the quartz substrate  11  is etched. Here, the etching process may use a RIE (reactive ion etching) apparatus as described above, for example, and an etching gas of a mixture gas of CF 4  and hydrogen, for example. The etching conditions may be, for example, as follows: CF 4  flow rate is 20 sccm, hydrogen flow rate is 3 sccm, pressure is 4 Pa, and PF power is 60 W. 
     Thereafter, a photoresist is applied to the whole surface and, through an exposure process and a development process, patterning of it is performed as shown in FIG.  24 . Then, by using the Cr film  12  and the resist pattern  14  as a mask, the quartz substrate  11  is etched by a RIE apparatus, such as shown in FIG.  25 . 
     Subsequently, the photoresist pattern  14  is separated and, thereafter, again a negative type resist  15  is applied to the whole surface (FIG.  26 ). Then, as shown in FIG. 27, the exposure process is performed to the substrate  11 , from its bottom face side. Additionally, as shown in FIG. 28, the exposure process is performed by using a photomask  16 , from its top face side. 
     As a development process is performed, the result is that, as shown in FIG. 29, a photoresist pattern  17  is formed only in a portion where the Cr film  12  is not present. Then, as shown in FIG. 30, the Cr film  12  in a portion not covered by the pattern  18  is etched in accordance with the RIE method using a chlorine gas or a mixture gas of chlorine gas and oxygen, for example. 
     Then, as shown in FIG. 31, by using the pattern  17  as a mask, the quartz substrate  11  is etched. Subsequently, as shown in FIG. 32, the resist pattern  17  is removed and, thereafter, the Cr film  12  is removed. As a result, a four-level step-like diffractive optical element  11 ′ such as shown in FIG. 33 is completed. 
     Positions a and b in this step-like diffractive optical element  11 ′ (FIG. 33) are determined in accordance with the first patterning of the Cr film  12 , independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     In a third embodiment of the present invention, as shown in FIG. 34, there is a quartz substrate  21  on which a Cr film  22  is formed by sputtering, with a thickness of 1000 angstroms. Here, an anti-reflection film (not shown) of chromium oxide, for example, may be provided on the Cr film  22  as desired. 
     Then, a photoresist is applied to the substrate  21  and, through an exposure process and a development process, a first-time resist pattern  23  is formed thereon, as shown in FIG.  35 . Subsequently, by using the resist pattern  23  as a mask, the Cr film  22  is etched. Here, in the etching process, a parallel plane plate type RIE (reactive ion etching) apparatus, for example, and an etching gas of a chlorine gas or a mixture gas of chlorine gas and oxygen, for example, may be used. Then, the resist pattern  23  is separated in accordance with an oxygen ashing method or by using a removing liquid. 
     Thereafter, a photoresist is applied to the whole surface and, through an exposure process and a development process, a resist pattern  24  such as shown in FIG. 36 is formed. Then, as shown in FIG. 37, by using the Cr film  22  and the resist pattern  24  as a mask, the quartz substrate  21  is etched. Then, the resist pattern  24  is removed (FIG.  38 ). 
     Subsequently, as shown in FIG. 39, by using the pattern of Cr film  22  as a mask, the quartz substrate  21  is etched by using a RIE (reactive ion etching) apparatus as described above, for example. The etching gas may be a mixture gas of CF 4  and hydrogen, for example. The etching conditions may be, for example, as follows: CF 4  flow rate is 20 sccm, hydrogen flow rate is 3 sccm, pressure is 4 Pa, and RF power is 60 W. 
     Thereafter, as shown in FIG. 40, a photoresist  25  is applied to the whole surface, and an exposure process is performed to the substrate  21  from its bottom face side. As a development process is performed, the result is such as shown in FIG.  41 . Then, as shown in FIG. 42, a photoresist is applied to the whole surface and, through an exposure process and a development process, a resist pattern  26  is patterned. Thereafter, as shown in FIG. 43, the Cr film  22  is etched by using a mixture liquid of cerium ammonium nitrate, perchloric acid and water, for example, while using the resist patterns  25  and  26  as a mask. 
     Subsequently, as shown in FIG. 44, by using the resist patterns  25  and  26  as a mask, the quartz substrate  21  is etched. Then, as shown in FIG. 45, the resist patterns  25  and  26  are removed, and the Cr film  22  is etched. Then, a four-level step-like diffractive optical element  21 ′ as shown in FIG. 46 is completed. 
     Positions a and b in this step-like diffractive optical element  21 ′ (FIG. 46) are determined in accordance with the first patterning of the Cr film  22 , independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     Also, in this embodiment, the optical element can be manufactured at a step of a half of the minimum resolvable line width of a drawing apparatus. Therefore, an optical element with a higher diffraction efficiency can be produced. 
     In a fourth embodiment of the present invention, as shown in FIG. 47, there is a quartz substrate  31  on which a Cr film  32  is formed by sputtering, with a thickness of 1000 angstroms. Here, an anti-reflection film of chromium oxide, for example, may be provided on the Cr film  32  as desired. 
     Then, a photoresist is applied to the quartz substrate  31  and, through an exposure process and a development process, a first-time resist pattern  33  is formed thereon, as shown in FIG.  48 . Subsequently, by using the resist pattern  33  as a mask, the Cr film  32  is etched. Here, the etching process may use a parallel plane plate type RIE (reactive ion etching) apparatus, for example, and an etching gas of a mixture gas of chlorine gas and oxygen, for example. 
     Then, as shown in FIG. 49, the resist pattern  33  is separated in accordance with an oxygen ashing method or by using a removing liquid. Subsequently, a photoresist is applied to the whole surface and, through an exposure process and a development process, a resist pattern  34  such as shown in FIG. 50 is formed. Then, as shown in FIG. 51, by using the Cr film  32  and the resist pattern  34  as a mask, the quartz substrate  31  is etched. 
     Thereafter, the resist pattern  34  is removed (FIG.  51 ). Then, as shown in FIG. 52, by using the pattern of Cr film  32  as a mask, the quartz substrate  31  is etched. Here, the etching process may be performed in accordance with a RIE (reactive ion etching) apparatus as described above, for example, and by use of an etching gas of a mixture gas of CF 4  and hydrogen, for example. The etching conditions may be, for example as follows: CF 4  flow rate is 20 sccm, hydrogen flow rate is 3 sccm, pressure is 4 Pa, and RF power is 60 W. 
     Thereafter, as shown in FIG. 54, the photoresist is removed and, then, a negative type resist pattern  35  is applied to the whole surface. Then, an exposure process is performed to the substrate  31  from its bottom face side (FIG.  55 ). Also, an exposure process is performed by using a photomask  36 , from the top face side of the substrate. As a development process is performed, a resist pattern  37  such as shown in FIG. 57 is produced. 
     Thereafter, as shown in FIG. 58, a portion of the Cr film  32  not covered by the pattern  37  is etched, by using a mixture liquid of cerium ammonium nitrate, perchloric acid and water, for example. Subsequently, as shown in FIG. 59, by using the pattern  37  as a mask, the quartz substrate  31  is etched. Then, as shown in FIG. 60, the resist pattern  37  is removed, and the Cr film  32  is removed by etching. Then, a four-level step-like diffractive optical element  31 ′ as shown in FIG. 61 is completed. 
     Positions a and b in this step-like diffractive optical element  31 ′ (FIG. 61) are determined in accordance with the first Cr film  32 , independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     Also, in this embodiment, the optical element can be manufactured at a step of a half of the minimum resolvable line width of a drawing apparatus. Therefore, an optical element of higher diffraction efficiency can be produced. 
     In a fifth embodiment of the present invention, a step-like diffractive optical element made of resin can be manufactured while using a step-like substrate, produced in accordance with any of the first to fourth embodiments, as a mold. 
     Initially, as shown in FIG. 62, a reaction setting resin, that is, ultraviolet radiation setting resin such as that of the acrylic series or epoxy series, or a thermo-setting resin, denoted at  43 , is applied by drops to a glass substrate  41  by a cylinder  42 . Subsequently, as shown in FIGS. 63 and 64, a step-like shape substrate  44  having been manufactured in accordance with any one of the first to fourth embodiments, is pressed against the resin  43  from above, whereby a replica layer  45  of the resin  43  is formed. 
     Here, before the substrate  44 , which functions as a mold, is pressed against the resin  43 , a mold releasing agent may be applied to the surface, as required. Subsequently, where an ultraviolet radiation setting resin is used, ultraviolet radiation is projected to the resin from the substrate (mold)  41  side, to solidify the resin. Where a thermo-setting resin is used, a heating treatment is performed to harden the resin. Subsequently, the substrate (mold)  44  is released, whereby a step-like diffractive optical element  46  as shown in FIG. 65 is completed. 
     Positions a and b in this step-like diffractive optical element  46  (FIG. 65) are determined in accordance with the first Cr film for the step-like substrate  44 , independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     Also, in this embodiment, the optical element can be manufactured at a step of a half to one-third of the minimum resolvable line width of a drawing apparatus. Therefore, an optical element of higher diffraction efficiency can be produced. 
     In a sixth embodiment, as shown in FIG. 66, a step-like shape substrate  51  produced in accordance with any one of the first to fourth embodiments may be provided with an aluminum film  52 , formed by sputtering and with a thickness of 1000 angstroms. A reflection type step-like diffractive optical element  53  can be completed in this manner. 
     Positions a and b in this step-like diffractive optical element  53  (FIG. 66) are determined in accordance with the first Cr pattern for the step-like substrate  51 , independently of the alignment. Therefore, the influence of an alignment error can be significantly reduced. 
     Also, in this embodiment, the optical element can be manufactured at a step of a half to one-third of the minimum resolvable line width of a drawing apparatus. Therefore, an optical element of a higher diffraction efficiency can be produced. 
     In a seventh embodiment, a diffractive optical element as manufactured in accordance with the first embodiment may be incorporated into a semiconductor exposure apparatus (stepper), as shown in FIG. 67, which uses ultraviolet radiation such as i-line or KrF, for example. 
     This exposure apparatus is arranged so that a reticle  62  is irradiated with light at a wavelength 248 nm from an illumination system  61 , and a pattern formed on the reticle  62  is transferred to a semiconductor substrate  65  placed on a stage  64 , by an imaging optical system  63 , at a reduction magnification of 1:5. The imaging optical system  63  is provided with a diffractive optical element  66  having been manufactured in accordance with the method of the first embodiment, this being for the purpose of reduction of chromatic aberration and the provision of aspherical effect. 
     This diffractive optical element  66  may have an appearance as illustrated in a perspective view of FIG.  68 . It may have a sectional shape such as shown in FIG.  69 . Optically, it functions as a convex lens. Although FIG. 69 shows an example of four-level structure, the following description will be made on an example with an eight-level structure. The surface level difference per single step is 610 angstroms, and the width of the outermost peripheral step is 0.35 micron. The diameter of the element  66  is 120 mm. 
     When light is incident on the diffractive optical element  66 , it may be transmitted therethrough while being separated mainly into a first order diffraction light, ninth order diffraction light and seventeenth order diffraction light. Of course, only the first order light contributes the imaging, and it occupies 90% or more of the incident light. The remaining few percent correspond to the ninth order light and the seventeenth order light. Since these diffraction orders are considerably different from the first order light that contributes to the imaging, these diffraction lights are directed out of the imaging optical system  63  and they do not have a large influence on the imaging. 
     This should be compared with the optical element of FIG. 70 described above. An intense diffraction light of the third order, for example, will be produced between the first and ninth orders of light when the optical element of FIG. 70, which is manufactured by using masks A, B and C, has three levels with a 610 angstrom level difference, a 0.35 micron width at the most peripheral step and a 120 mm diameter. Such unwanted light causes flare or the like upon the image plane resulting in a large deterioration of the image performance. 
     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.