Abstract:
A method of manufacturing a micro-structure having a plurality of step-like elements, includes the steps of etching a first region of a substrate, corresponding to a lowest step of the step-like element, with an appropriate depth, whereby a surface of the lowest step of the step-like element is defined, and etching, subsequently, a second region of the substrate, corresponding to another step of the step-like element, with an appropriate depth, whereby a surface of that step of the step-like element is defined.

Description:
FIELD OF THE INVENTION AND RELATED ART  
         [0001]    This invention relates to a method of manufacturing a diffractive optical element.  
           [0002]    A diffractive optical element is an optical element which uses a diffraction phenomenon of light, and it can be provided by a diffraction grating of various shapes and sizes. As an example, Japanese Laid-Open Patent Application, Laid-Open No. 258510/1994 shows a diffractive optical element with a diffraction grating having a step-like sectional shape. This diffractive optical element is formed by alternately accumulating, upon a substrate, layers of two materials having different reaction characteristics, with respect to an etching gas, while their film thicknesses are controlled precisely. After this, a patterning operation and an etching operation are performed to the alternate layers of the two materials on the basis of photolithography, by which a diffraction grating of step-like shape is produced.  
           [0003]    Where a diffraction grating having a step-like sectional shape is produced in accordance with the method disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 258510/1994 mentioned above, if the patterning is performed three times superposedly, an alignment error may occur between a step and a side wall facing it. If this occurs, the side wall may have a slope extending from the bottom of the step. That is, in the step-like structure, the side wall does not extend perpendicularly to the bottom.  
           [0004]    Further, in accordance with the method of forming a diffraction grating as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 258510/1994 mentioned above, a resist is applied to a surface having a surface step as defined by the etching process, and then the subsequent patterning operation is performed. This causes that the side wall of the patterned resist, particularly, the resist side wall of a lower step (smaller level difference) is easily affected by a wall on the opposite side, during the exposure operation, such that there occurs a tilt of the resist side wall relative to the bottom. As a result of this, the step-like shape to be defined by the subsequent etching process is formed with a similar tilt like the resist tile shape.  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of the present invention to provide a method of manufacturing a micro-structure having an accurate step-like sectional shape.  
           [0006]    It is another object of the present invention to provide a diffractive optical element with a diffraction grating having such a structure.  
           [0007]    In accordance with a first aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: etching a first region of a substrate, corresponding to a lowest step of the step-like element, with an appropriate depth, whereby a surface of the lowest step of the step-like element is defined; and etching, subsequently, a second region of the substrate, corresponding to another step of the step-like element, with an appropriate depth, whereby a surface of that step of the step-like element is defined.  
           [0008]    In accordance with a second aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: forming, on a substrate, a film having a thickness corresponding to the height of the step-like element: etching a first region of the film on the substrate, corresponding to a lowest step of the step-like element. With an appropriate depth, whereby a surface of the lowest step of the step-like element is defined; and etching, subsequently, a second region of the film on the substrate, corresponding to another step of the step-like element, with an appropriate depth, whereby a surface of that step of the step-like element is defined.  
           [0009]    In accordance with a third aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: forming, on a substrate, a film having a thickness corresponding to the height of the step-like element; etching a first region of the film on the substrate, corresponding to a lowest step of the step-like element, with an appropriate depth, whereby a surface of the lowest step of the step-like element is defined; and etching, subsequently, a second region of the film on the substrate, corresponding to another step of the step-like element, with an appropriate depth, whereby a surface of that step of the step-like element is defined, wherein the film includes a multilayered film comprising alternate layers of two materials having different etching rates, and wherein the or each layer of one of the two materials having a higher etching rate serves as a main portion of the step-like element while the or each layer of the other material having a lower etching rate serves as a stopper when the or each layer of the material of the higher etching rate is etched.  
           [0010]    In accordance with a fourth aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: forming, on a substrate, a film having a thickness corresponding to the height of the step-like element; etching a first region of the film on the substrate, corresponding to a lowest step of the step-like element, with an appropriate depth, whereby a surface of the lowest step of the step-like element is defined; and etching, subsequently, a second region of the film on the substrate, corresponding to another step of the step-like element, with an appropriate depth, whereby a surface of that step of the step-like element is defined; wherein the film includes a multilayered film comprising alternate layers of two materials having different etching rates; wherein the or each layer of one of the two materials having a higher etching rate serves as a main portion of the step-like element while the or each layer of the other material having a lower etching rate serves as a first stopper layer when the or each layer of the material of the higher etching rate is etched; and wherein the film further includes a second stopper layer of a material having an etching rate lower than those of the two materials and being provided on a side of the multilayered film facing the substrate, the second stopper layer serving as a stopper when the two materials are etched.  
           [0011]    In accordance with a fifth aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: etching a region of a substrate with an appropriate depth; and etching, subsequently, a surface defined by said etching and corresponding to a side wall of the step-like element.  
           [0012]    In accordance with a sixth aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: etching a region of a substrate to provide a surface of a step of the step-like element; and etching, subsequently, a surface defined by said etching and corresponding to a side wall of the step-like element, to correct an angle of the surface corresponding to the side wall.  
           [0013]    In accordance with a seventh aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: forming, on a substrate, a film having a thickness corresponding to the height of the step-like element; etching a region of the film of the substrate with an appropriate depth; and etching, subsequently, a surface defined by said etching and corresponding to a side wall of the step-like element, to correct an angle of the surface corresponding to the side wall; wherein the film includes a multilayered film comprising alternate layers of two materials having different etching rates; wherein the or each layer of one of the two materials having a higher etching rate serves as a main portion of the step-like element while the or each layer of the other material having a lower etching rate serves as a stopper when the or each layer of the material of the higher etching rate is etched; and wherein the or each layer of the one material having the higher etching rage is etched so that the or each layer of the other material having the lower etching rate is exposed, and thereafter the etching step for the correction of the angle of the surface corresponding to the side wall is performed while the or each layer of the material of the lower etching late is kept exposed.  
           [0014]    In accordance with an eighth aspect of the present invention, there is provided a method of manufacturing a micro-structure having a plurality of step-like elements, said method comprising: forming, on a substrate, a film having a thickness corresponding to the height of the step-like element; and etching a region of the film of the substrate with an appropriate depth; wherein the film includes a multilayered film comprising alternate layers of two materials having different etching rates; wherein the or each layer of one of the two materials having a higher etching rate serves as a main portion of the step-like element while the or each layer of the other material having a lower etching rate serves as a stopper when the or each layer of the material of the higher etching rate is etched; and wherein the or each layer of the one material having the higher etching rage is etched until the or each layer of the other material having the lower etching rate is exposed, and thereafter a resist pattern is formed on the substrate where the lower etching rate material layer is exposed, wherein the or each layer of the lower etching rate material is etched by use of the resist pattern and, thereafter, the or each layer of the higher etching rate material below it is etched.  
           [0015]    In a ninth aspect of the present invention, there is provided a method of manufacturing a diffractive optical element, including a process for forming a diffraction grating in accordance with the method corresponding to any one of the first to eights aspects of the present invention.  
           [0016]    In a tenth aspect of the present invention, there is provided a method of manufacturing a diffractive optical element, including a process for forming a mold for a diffraction grating in accordance with the method corresponding to any one of the first to eights aspects of the invention, and for forming the diffraction grating, for the diffractive optical element, by use of the mold.  
           [0017]    In an eleventh aspect of the present invention, there is provided a method of manufacturing an optical system having a diffractive optical element, including a process for providing the diffractive optical element in accordance with the method corresponding to any one of the first to eights aspects of the invention.  
           [0018]    In a twelfth aspect of the present invention, there is provided a method of manufacturing an optical system having a diffractive optical element, including a process for forming a mold for a diffraction grating in accordance with the method corresponding to any one of the first to eights aspects of the invention, and for providing the diffractive optical element by use of the mold.  
           [0019]    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  
       [0020]    [0020]FIG. 1 is a sectional view of a diffractive optical element according to a first embodiment of the present invention.  
         [0021]    [0021]FIGS. 2, 3,  4 ,  5 ,  6  and  7  are sectional views, respectively, for explaining manufacturing processes in the first embodiment of the present invention.  
         [0022]    [0022]FIG. 8 is a sectional view of a diffractive optical element according to a second embodiment of the present invention.  
         [0023]    [0023]FIGS. 9, 10,  11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 ,  29 ,  20 ,  21  and  22  are sectional views, respectively, for explaining manufacturing processes for a diffractive optical element according to a third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
         [0025]    [0025]FIG. 1 is a sectional view of a diffractive optical element having a four-level (four-stepped) step-like shape, according to a first embodiment of the present invention. Upon a substrate  1  made of an optical glass, a metal or a synthetic resin, there are sputtering films (etching films)  2  for defining steps through the etching operation, and etching stopper layers  3  being operable as etching stoppers for the respective levels (steps), which are alternately accumulated on the substrate  1 . Disposed between the substrate  1  and the etching stopper layer  3  at the bottom layer of the alternate accumulation film, is a second etching stopper layer  4 .  
         [0026]    In this embodiment, a silicon (Si) wafer is used for the substrate  1 , and a SiO 2  film is used for each layer  2  of the alternate accumulation film to be formed on the surface of the substrate  1 . Each of the layers  2  is formed with a thickness of 600 nm, by using a usual sputtering film forming apparatus. As regards the layers  3 , a Al 2 O 3  film having an etching rate different from that of the SiO 2  film is used. Each layer  3  is formed with a thickness of 10 nm, by using a similar sputtering film forming apparatus. The second etching stopper layer  4  comprises a metal film or a dielectric material film having a characteristic of very slow etching rate with respect to an etching gas, to be used for the etching of the sputtering layers  2  and the stopper layers  3 . As an example, a Cr 2 O 3  film may be used therefor. The Cr 2 O 3  film may be formed between the substrate  1  and the bottom layer of the etching stopper layer  3 , in accordance with a usual vacuum film forming method, for example.  
         [0027]    [0027]FIGS. 2 through 7 are sectional views, respectively, for explaining manufacturing processes.  
         [0028]    Initially, a substrate  1  having a diameter of 8 inches and having been cleaned sufficiently is place in a sample holder inside a casing, so that the surface thereof on which the film is to be formed faces a target of the sputtering apparatus. Then, the casing is vacuum evacuated sufficiently and, after that, an oxygen gas and an argon gas are introduced into the casing, with the inside pressure being set to 2 Pa. A high frequency wave of an output 400 W is applied from an RF voltage source to an electrode, by which a second etching stopper layer (Cr 2 O 3  film)  4  is uniformly formed on the surface, with a thickness 100 nm. After this film formation, the vacuum within the casing is once released, and the sample is placed into a heating high-temperature oven such that annealing is performed thereto in an atmospheric ambience, at 900 degrees and for 2 hours.  
         [0029]    After the sample is cooled sufficiently, it is loaded again on the sputtering apparatus in the same manner. The target is replaced by Al and, after the mixture ratio of the oxygen gas and the argon gas is adjusted to 1:1, a first etching stopper layer  3  is uniformly formed on the second etching stopper layer  4 , having been annealing processed, with a thickness 10 nm. Then, without releasing the vacuum, the target is replaced by SiO 2  and, after adjusting the mixture ratio of the oxygen gas and the argon gas to 1:10, a sputtering layer  2  is formed uniformly on the surface of the etching stopper layer  3 , with a thickness 600 nm. Subsequently, alternate layers of a layer  2  and a layer  3  are formed thereon twice, whereby an accumulation film comprising six layers are formed on the second etching stopper layer  4 .  
         [0030]    To a sample thus provided, a photoresist  5  being patterned is used, so as to produce a step-like shape with four levels (steps) through an etching operation. In FIG. 2, a photoresist  5  is formed on the alternate accumulation film by using a spinner, and a patterning of a width 1 micron corresponding to the fourth level (step) is exposed and then developed.  
         [0031]    Subsequently, this sample is loaded on an electrode of a usual RIE etching apparatus so that the surface with film can be etched. After sufficient vacuum evacuation, an etching gas of CF 4  and CHF 3  is introduced, and the pressure inside the casing is adjusted to 1 Pa. A voltage of output 110 W is applied to the electrode from a high frequency voltage source, and the etching operation is performed through a predetermined time by which one layer of the layers  2  can be etched from the sample surface. Then, while changing the etching gas by CCl 4 , the layer  3  is etched. These operations are repeated alternately, three times, and the alternate accumulation film is etched with a width 1 micron, so that, as shown in FIG. 3, the second etching stopper layer  4  is completely exposed or uncovered. By this procedure, as shown in  3 , the patterning and etching to the fourth level (step) from the top, which corresponds to the bottom step, is completed.  
         [0032]    The sample is then taken out of the etching apparatus, and the resist  5  is removed. As shown in FIG. 4, a photoresist  5  is formed again on the alternate accumulation film by using a spinner. Then, in this case, a step width of 2 microns, corresponding to the second level (step), plus a patterning alignment precision 0.3 micron, thus a total width 2.3 microns, is exposed and developed. Here, the 0.3 micron width is set at the etching stopper layer side as already been exposed by the etching.  
         [0033]    Like the preceding operation, the sample is loaded on the etching apparatus and, as shown in FIG. 5, the etching operation is performed through a predetermined time so that one of the layers  2  is etched. Similarly, while changing the etching gas, a layer  3  is etched. The sample is unloaded from the etching apparatus, and the resist  5  is removed. Subsequently, like the preceding operation, a photoresist  5  is formed on the alternate accumulation film by using a spinner, as shown in FIG. 6. Then, a step width of 1 micron, corresponding to the third level (step), plus a patterning alignment precision 0.3 micron, thus a total width 1.3 microns, is exposed and developed. Here, the 0.3 micron width is set again at the etching stopper layer side as already been exposed by the etching.  
         [0034]    Further, like the preceding operation, the sample is loaded on the etching apparatus, and, as shown in FIG. 7, the etching process is performed for a predetermined time so that one layer of the layers  2  is etched. Similarly, while changing the etching gas, the layer  3  is etched. Then, the sample is unloaded from the etching apparatus, and the resist  5  is removed. Subsequently, the sample having been etched is placed in a Cr 2 O 3  removing liquid and, with the application of ultrasonic waves, only the second etching stopper layer  4  is removed. The second etching stopper layer  4 , having been roughened by the etching operations to the second and third steps, is thus removed, whereby a smooth surface of the substrate  1  is exposed at the bottom of a groove. After this, it is sufficiently cleaned by washing by pure water, and then it is dried by using an N 2  blower, whereby a four-level diffractive optical element such as shown in FIG. 1 is completed.  
         [0035]    If an Al reflection surface is formed at this diffraction grating surface by using an ordinary vacuum deposition apparatus, a four-level reflection type diffractive optical element can be produced.  
         [0036]    In accordance with the method of this embodiment, a side wall facing a step or steps can be made perpendicular to the bottom face, as illustrated, and the etching bottom face is not roughened. Also, the produced shape is a rectangular shape with every steps having smooth surfaces and with regular level differences. Therefore, a diffractive optical element having a superior diffraction characteristic can be produced at low cost and stably. Here, it is to be noted that the sputtering film  2  or etching stopper film  3 , constituting the steps, are not limited to the combination of materials described in this embodiment. Other materials may be used with similar advantageous effects.  
         [0037]    [0037]FIG. 8 is a sectional view of a second embodiment. The Cr 2 O 3  film used in the first embodiment as the second etching stopper film  4  is replaced by a metal Cr film, and a sample is annealing processed by using a heating high-temperature oven, in a N 2  ambience, at 400 degrees and for two hours. Onto this second etching stopper layer  4 , a first etching stopper layer  3  of Al 2 O 3  film and a sputtering film (etching layer)  2  of SiO 2  film, constituting each steps, are alternately accumulated twice. Through corresponding photolithographic processes and etching processes, a diffractive optical element can be produced. The remaining portion of this embodiment is similar to the first embodiment.  
         [0038]    In this manner, like the first embodiment, a three-level reflection type diffractive optical element of rectangular shape, having a side wall perpendicular to the bottom face and having smooth and regular steps, without roughed faces, can be completed. By using the second etching stopper layer  4  of metal Cr film, a reflection factor similar to that of a substrate  1  of Si wafer to be used in an ordinary photolithographic process, can be obtained. Therefore, usual exposure conditions can be applied and thus the production can be switched smoothly. It is to be noted that similar advantageous effects are attainable with a diffractive optical element having levels other than that desired above.  
         [0039]    Further, in the first embodiment, the substrate  1  of Si wafer may be changed by a quartz wafer, and an anti-reflection film, not an Al reflection film, may be formed on the etched diffraction grating surface by using an ordinary vacuum deposition apparatus. It can produce a four-level transmission type diffractive optical element, having similar characteristics as of the first embodiment. Also, similar advantageous effects are attainable with use of a transparent optical material other than quartz used here.  
         [0040]    Further, in the first embodiment, an etching layer  2  may be directly formed on the second etching stopper layer  4 , without forming the etching stopper layer  3 , and, through subsequent alternate accumulation with the etching stopper layer  3  as in the first embodiment, a four-level diffractive optical element may be produced. In that occasion, an Al reflection film is not formed on the diffraction grating surface, and, as a mold, a ultraviolet-radiation setting resin is dropped thereon and an optical glass is adhered thereon. The the resin is set through projection of ultraviolet rays from the bottom face or top face of the diffraction grating, and thereafter, it is removed and replica-transferred, whereby a diffraction grating is produced. Alternatively, a thermo-plastic resin may be injected into a mold, and a diffraction grating can be produced by injection molding. Also, an Al reflection film may be formed on the grating surface of the thus produced diffraction grating, by which a reflection type diffractive optical element can be produced. In that occasion, a four-level reflection type diffractive optical element having similar characteristic as of the first embodiment can be manufactured with good reproducibility and at lower cost.  
         [0041]    When in the first embodiment the diffractive optical element is produced by alternately accumulating the etching stopper layers  3  and the etching layers  2  directly upon the substrate  1 , without provision of the second etching stopper layer  4 , a small groove may be formed at the groove portion corresponding to the fourth step from the above, that is, at the lowest step, or the bottom face may be roughened. However, the side wall facing the step or steps can be made perpendicular to the bottom.  
         [0042]    Further, in the first embodiment, the annealing process is performed after the formation of the second etching stopper layer  4 . This is to reduce or avoid the possibility that: a portion of the film may otherwise be separated during the formation of the alternate accumulation film or the film property may otherwise change locally in the substrate  1  surface, causing a difficulty in accomplishing the desired patterning; or film separation may otherwise occur during resist removing operation or a washing operation in the photolithographic process.  
         [0043]    When in the first embodiment an Al reflection film is formed on the diffraction grating surface to provide a reflection type diffractive optical element, after completion of the etching of the first etching stopper layer  3 , the second etching stopper layer  4  is removed by wet etching. This prevents roughening of the the surface of the lowest step, corresponding to the fourth step from the above.  
         [0044]    Referring now to FIGS.  9 - 22 , a third embodiment of the present invention will be descried in detail.  
         [0045]    FIGS.  9 - 22  are sectional views for explaining the manufacturing processes for a diffractive optical element having a step-like shape as in the first embodiment. FIG. 9 illustrates an accumulation film  11  prior to the etching process. A substrate  12  comprising a Si wafer of 8 inch diameter and having been cleaned, is placed on a sample holder so that the film forming surface thereof faces to a target of a sputtering apparatus, not shown. After the casing of the sputtering apparatus is vacuum evacuated sufficiently, the sample  12  is heated uniformly at 300° C., and an oxygen gas and an argon gas of the even quantity are supplied into the casing, with a set pressure 2 Pa. Then, a high frequency wave of an output 800 W is applied to the Al target, by using a RF voltage source, whereby an auxiliary film of Al 2 O 3  film  13   a  which functions as an etching taper adjusting layer is uniformly formed on the substrate  12 , with a thickness of about 15 nm.  
         [0046]    Subsequently, without releasing the vacuum in the casing, the Al target is replaced by a SiO 2  target while the the mixture ratio of the oxygen gas and the argon gas in the casing is adjusted to 1:10, and a SiO 2  film  14   a  which is a main film constituting the main portion of the step-like shape is formed on the Al 2 O 3  film  13   a , with a thickness of about 600 nm.  
         [0047]    The above-described process is repeated twice, by which Al 2 O 3  films  13   b  and  13   c  or thicknesses of about 45 and 36 microns and SiO 2  films  14   b  and  14   c  of thicknesses of about 570 and 579 microns are alternately accumulated in the substrate  12 , in this order from the substrate side. The thus produced accumulation film  1  is unloaded from the sputtering apparatus, after an atmosphere is introduced into the casing, following the cooling.  
         [0048]    To the accumulation film  1  unloaded out of the sputtering apparatus, an annealing process is performed inside a heating high-temperature oven at 450° C. and in an atmosphere ambience, for about two hours. Then, it is placed in a vacuum deposition apparatus so that it is opposed to a chromium evaporation source. After sufficiently vacuum evacuating the inside space of a casing of the vacuum deposition apparatus, the accumulation film  1  is uniformly heated to 300° C., and chromium is heated by using an electron gun. By this, a chromium film of a thickness of about 35 nm is deposited as a mask on the SiO 2  film  14   c  at the topmost step. After it is cooled sufficiently and an atmosphere is introduced into the casing, the accumulation film  11  in unloaded out of the vacuum deposition apparatus.  
         [0049]    While this embodiment uses a chromium film  15  as a mask having a different reactivity to an etching gas to be used for the etching of the Al 2 O 3  film and SiO 2  film  14 , any other metal thin film, dielectric material film or synthetic resin may be used.  
         [0050]    [0050]FIG. 10 shows a photoresist  16   a  formed on the chromium film  15 , as being patterned as desired. In this embodiment, a spinner is used to apply a photoresist  16   a  of 1 micron thickness onto the chromium film  15  surface, in the form of a line-and-space pattern of a 4-micron pitch and 3-micron opening. This patterning is formed by exposure and development processes to the resist according to known methods.  
         [0051]    Subsequently, as shown in FIG. 11, the exposed or uncovered portion of the chromium film  15  is placed in a chromium removing liquid, whereby it is removed by wet etching. Then, the accumulation film  11  is placed on an electrode of a RIE etching apparatus so that the film forming surface can be etched. After vacuum evaluation of the casing, an etching gas of CF 4  and CHF 3  is introduced into the casing, and the pressure is set to 1 Pa. Thereafter, an output of 100 W is applied to the electrode from a high-frequency voltage source. By the electric discharging, the exposed portion of the SiO 2  film  14   c  is etched. After checking the etching process through plasma measurement, the Al 2 O 3  film  13   c  which functions as a taper adjusting layer is etched for a predetermined time necessary for that the side wall of the SiO 2  film  14   c  becomes perpendicular to the substrate surface.  
         [0052]    Then, the atmosphere is introduced into the casing and the accumulation film  1  is unloaded, and the photoresist  16   a  is removed as shown in FIG. 12. Subsequently, as shown in FIG. 13, by using a spinner, a photoresist  16   b  of line-and-space with 4-micron pitch and 2-micron opening is patterned on the etched surface, and exposure and development processes are performed thereto. In order to cancel any deviation of alignment, the photoresist  16   b  on the chromium film  15  is patterned so that its edge is inside the chromium film  15 . However, in this embodiment, any tilt of the side wall portion or any surface waviness to be produced in the photoresist  16  is canceled.  
         [0053]    Subsequently, as shown in FIG. 14, the exposed portion of the Al 2 O 3  film  13   c  is removed by wet etching, so that it functions as an etching mask for the SiO 2  film  14   b.    
         [0054]    In FIGS.  15 - 20 , by using the above-described processes, the sample is set in a RIE etching apparatus and the etching, resist separation and patterning to the SiO 2  film  14  corresponding to the second step and below it of the step-like structure as well as the wet etching of the Al 2 O 3  film  13  are performed.  
         [0055]    With the provision of an alternate accumulation film having Al 2 O 3  films (auxiliary films)  13  having different thicknesses at different steps, even if a tilt portion of the photoresist  16  caused by the patterning is retracted by the etching, since the etching rate of the Al 2 O 3  film  13  below the photoresist  16  is considerably slow as compared with the SiO 2  film  14  and the photoresist  16 , it well functions as a mask. By making the film thickness of the Al 2 O 3  film  13  minimum, in consideration of the level difference of the steps, pattern size or the magnitude of tilt of patterning which varies with the material of the photoresist or the amount of exposure, for example, or the time necessary for etching the SiO 2  film  14  at each step to a desired depth, it is assured that the side wall of the SiO 2  film  14  is etched perpendicularly to the substrate surface.  
         [0056]    Even if any tilt is produced in the SiO 2  film  14  in the etching operation, since the etching rate of the Al 2 O 3  film  13  below is very slow as compared with the SiO 2  film  14  and the photoresist  16 , it well functions as a taper adjusting layer. To this end, the film thickness of the Al 2 O 3  film  13  is set minimum to meet the correction of the etching tilt amount of the SiO 2  film  14  which differs with steps, and by this, the side wall of the SiO 2  film  14  is etched perpendicularly to the substrate surface.  
         [0057]    After exposure and development of a desired patterning, the Al 2 O 3  film  13  is etched so that the main surface below it is uncovered and, subsequently, the SiO 2  film  14  is etched. The Al 2 O 3  film  13  having been thus patterned as desired is then surely used as a mask for the underlying SiO 2  film  14 . As a result, the side wall of the underlying SiO 2  film  14  can be etched perpendicularly to the bottom surface.  
         [0058]    Also, as shown in FIG. 21, for a Al 2 O 3  film  13  having been damaged by the etching, only its exposed portion may be removed by wet etching so that a smooth and flat SiO 2  film  14  is exposed. Subsequently, as shown in FIG. 22, a chromium film  15  damaged by the etching may be removed by wet etching, so that a smooth and flat SiO 2  film  14  is exposed. By doing so, a four-level diffractive optical element of rectangular shape, having uniform level difference and having a side wall facing the step extending perpendicularly to the bottom surface, can be produced.  
         [0059]    In this embodiment, while not shown in the drawings, an Al reflection film may be formed on the diffraction grating surface of a four-level diffractive optical element thus produced, by using a vacuum deposition apparatus. A reflection type diffractive optical element can be manufactured thereby.  
         [0060]    As a fourth embodiment, without forming the Al 2 O 3  film  13   a  which is an auxiliary film on the substrate  12  in the third embodiment, the SiO 2  film  14  (main film) may be directly formed on the substrate  12 . In that occasion, the number of accumulated layers for the SiO 2  film may be increased from three layers to seven layers, and the thicknesses of the SiO 2  films  4  may be set to 263 nm, 233 nm, 239 nm, 243 nm, 244 nm, 246 nm, and 246 nm, respectively, in an order from the substrate  12  side. Also, the thicknesses of Al 2 O 3  films  13  formed on the SiO 2  film  14  may be set to 30 nm, 24 nm, 20 nm, 19 nm, 17 nm, and 17 nm, respectively, in an order from the bottom SiO 2  layer.  
         [0061]    For the Al 2 O 3  film  13  at each layer, it is not wet etched but is dry etched in a RIE etching apparatus as the SiO 2  film  14 , while using a CCl 2  etching gas and while performing plasma measurement. The remaining processes may be similar to those of the third embodiment.  
         [0062]    Even if the SiO 2  film  14  on the substrate  12  (Si wafer) is formed directly, since Si has a different reactivity to the etching gas of the SiO 2  film  14 , it well functions as a taper adjusting layer.  
         [0063]    The diffractive optical element produced in the manner described above has uniform level differences among the steps like those of the third embodiment, and its step-like sectional shape, particularly, the side wall facing the step, is perpendicular to the bottom face. Thus, an eight-level reflection type diffractive optical element of rectangular shape can be accomplished.  
         [0064]    One layer of Al 2 O 3  film  13  may be omitted. This is effective to reduce the overall stress of the accumulation film  11 , such that an eight-level alternate accumulation film having a large contact force to the substrate  12  can be provided. Therefore, a diffractive optical element having a high diffraction efficiency can be produced.  
         [0065]    As regards the SiO 2  film  14 , the Al 2 O 3  film  13  and the chromium film  15 , similar advantageous results are attainable with a combination of materials other than those described above. For example, a SiO 2  film and a chromium film may be used as a main film and a mask, respectively, as in the present embodiment, while the same material of Si as the substrate  12  may be used as an auxiliary film. In that occasion, a CF 4  gas may be used for the etching of this auxiliary film to increase the etching selection ratio of SiO 2  to Si, to remove the same by etching. A similar diffractive optical element can be produced by this.  
         [0066]    As a fifth embodiment, the SiO 2  film  14  in the third embodiment may be replaced by an Al film, the Al 2 O 3  film  13  may be replaced by a Cu film, and the chromium film  5  may be replaced by a Mo film, respectively, and an accumulation film may be formed on the substrate  12  by using them. The Al film may be dry etched by using an etching gas, while the Cu film and Mo film may be removed by wet etching by using respective etching liquids.  
         [0067]    In the accumulation film  11  in that occasion, since all the materials constituting the film use metal films having high film forming speeds, the productivity is good.  
         [0068]    As a sixth embodiment, a quartz substrate may be used in place of the substrate  2  (Si wafer) used in the third embodiment. An anti-reflection film, in place of the Al reflection film, may be formed on the diffraction grating surface of the produced diffractive optical element by using a vacuum deposition apparatus. A transmission type diffractive optical element having a good diffraction characteristic can be produced thereby. Further, any transparent optical material other than quartz may be used.  
         [0069]    The Al 3 O 3  film  3  may be replaced by a Si 3 n 4  film, with thicknesses of 15 nm, 600 nm, 35 nm, 580 nm, 25 nm and 590 nm, in an order from the substrate  12  side, which may be formed into an alternate accumulation film of SiN 3  layers and SiO 2  layers. An oxygen gas may be added, as an additive gas, to a CF 3 Br etching gas, and the etching process may be performed while executing plasma measurement. A good diffractive optical element can be produced thereby.  
         [0070]    As a seventh embodiment, without forming the Al reflection film on the diffraction grating surface of the four-level diffractive optical element in the third embodiment, the produced diffractive optical element may be used as a mold. A ultraviolet-radiation setting resin may be dropped onto this diffractive optical element, and an optical glass may be adhered thereon. An ultraviolet radiation may be projected to the diffractive optical element, from its bottom face side or top face side, to set the resin. After this, the resin may be separated, whereby a diffractive optical element of replica transferred can be produced. An Al reflection film may be formed on the diffraction grating surface of the schematic view thus replica transferred, by which a reflection type diffractive optical element is produced.  
         [0071]    The reflection type diffractive optical element thus produced has uniform level difference among the steps, similarly to the first embodiment. Additionally, a reflection type diffractive optical elements can be produced with good reproducibility and with lower cost.  
         [0072]    As an eighth embodiment, a mold as produced in the seventh embodiment may be attached to an injection molding apparatus, and, while using a transparent thermo-plastic resin, a substrate  12  and a diffractive optical element may be formed as a unit, through the molding. Anti-reflection films may be formed by deposition on the surface of the diffractive optical element and the bottom face of the substrate. A transmission type diffractive optical element being integral with the substrate  12 , can be produced thereby.  
         [0073]    The diffractive optical element thus produced has uniform level differences at its steps. Also, a transmission type diffractive optical element can be produced with a good productivity and a good reproducibility, and with lower cost.  
         [0074]    Such diffractive optical element may be used as a color separation filter in an image reading apparatus or a liquid crystal projector, and it may be used as a diffraction lens in a photographing optical system or a projection optical system.  
         [0075]    If, in the diffractive optical element, the thicknesses of the Al 2 O 3  film layers  13   a ,  13   b  and  13   c  are not optimized but they are made all equal to a largest thickness 45 nm, the contact force to the substrate  12  is low, and surface deformation may occur due to local film floating. Also, since the Al 2 O 3  film layers  13   a ,  13   b  and  13   c  are removed three times by wet etching, lateral elongation may occur at the end portion of the topmost Al 2 O 3  film layer  13   c . Thus, a step-like diffractive optical element having a high-precision rectangular shape can not be accomplished. Further, the film forming time is very prolonged, and the productivity is low. Additionally, as regards the diffraction spectral characteristic of the produced diffractive optical element, the reflectivity is low and a desired spectral characteristic is not attainable.  
         [0076]    In the third embodiment described above, after the etching with SiO 2  film  14 , the patterning with the photoresist  16  is performed. This is because: If the patterning with the photoresist  16  is performed after the Al 2 O 3  film is removed, in the produced diffractive optical element, the edge of the photoresist  16  may be retracted during the etching process for the SiO 2  film  14 , such that a tilt may be produced at the side wall of the SiO 2  film  14 , like the photoresist  16 . In that occasion, a step-like diffractive optical element having a desired rectangular shape is not attainable.  
         [0077]    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.