Optical mask using phase shift and method of producing the same
This application is a continuation of application Ser. No. 07/757,324, filed Sep. 10, 1991, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to optical masks and methods of producing the same, and more particularly to an optical mask which uses a phase shift exposure to improve the resolution, and a method of producing such an optical mask. 
     There are demands to increase the operating speed of large scale integrated circuits (LSIs), such as a memory device having a large memory capacity, and to improve the integration density of the LSIs. For this reason, there are demands to realize a fine photolithography technique. 
     As one prominent means of satisfying the above demands, there is a phase shift exposure technique which employs an optical mask using a phase shift, and a coherent light source which emits an exposure light having a wavelength which is as short as possible. The optical mask using the phase shift will hereinafter be referred to as a phase shift optical mask. 
     FIG.1 shows in cross section an example of a conventional phase shift optical mask. The phase shift optical mask shown in FIG.1 includes a glass substrate 11, a chromium (Cr) layer 12 and a silicon dioxide (SiO 2 ) layer 13 which forms a phase shift part. 
     When producing this phase shift optical mask, the Cr light blocking layer 12 which blocks the exposure light is formed on the entire surface of the glass substrate 11 which is transparent with respect to the exposure light. Then, the SiO 2  layer 13 which is also transparent with respect to the exposure light is formed on the Cr light blocking layer 12 to a thickness such that the phase of the exposure light is shifted by 180°. 
     Thereafter, a resist layer (not shown) with an opening is formed on the SiO 2  layer 13 and the SiO 2  layer 13 is etched using a photolithography technique so as to form an opening 14. This opening 14 has the size and shape of a predetermined main light transmitting part. Next, the SiO 2  layer 13 is used as a mask and the Cr light blocking layer 12 which is within the opening 14 is subjected to an isotropic etching. As a result, the SiO 2  layer 13 overhangs above the Cr light blocking layer 12 to form the phase shift part. 
     However, when the phase shift optical mask is subjected to a cleaning or brushing during the production stage or in a stage before the phase shift optical mask is actually used, there are problems in that the SiO 2  layer 13 easily separates from the Cr light blocking layer 12 and that a part of the SiO 2  layer 13 may become damaged and come off the phase shift optical mask. 
     On the other hand, when producing the phase shift optical mask by the conventional technology, there is a limit to the precision with which the patterns may be formed by the photolithography technique. For this reason, there a problem in that it is extremely difficult to form, in the SiO 2  layer 13 an opening which has a desired size and shape. In addition, when subjecting the Cr light blocking layer 12 to a side etching, there is also a problem in that it is difficult to control the side etching quantity. 
     In other words, the light intensity distribution of the exposure light transmitted through the phase shift optical mask is dependent on a width L2 of the SiO 2  layer 13 overhanging the Cr light blocking layer 12, as will be described hereunder. FIG.2A shows the light intensity distribution of the exposure light transmitted through the phase shift optical mask shown in FIG. 1 when L1=0.50 μm and L2=0.15 μm, where L1 denotes the width of the Cr light blocking layer 12. Similarly, FIG. 2B shows the light intensity distribution for a case where L1=0.5 μm and L2=0.20 μm, and FIG. 2C shows the light intensity distribution for a case where L1=0.50 μm and L2=0.10 μm. In other words, FIGS. 2B and 2C respectively show the distributions for the cases where the width L2 of the SiO 2  layer (phase shift part) 13 is varied by ±0.05 μm on the wafer. 
     In FIGS. 2A through 2C, a solid line indicates the distribution at a defocus of 0.000 μm, a dashed line indicates the distribution at a defocus of 0.300 μm, a fine dotted line indicates the distribution at a defocus of 0.600 μm, a one-dot chain line indicate the distribution at a defocus of 0.900 μm, and a two-dot chain line indicates the distribution at a defocus of 1.200 μm. Further, the wavelength λ of the exposure light is 0.365 μm, the numerical aperture (NA) of an exposure lens is 0.54, and the coherency factor σ is 0.30. 
     In the case shown in FIG. 2B, the resist layer on the wafer is exposed by the peaks appearing on both sides of the distribution, and more of the resist layer is developed when compared to the case shown in FIG. 2A. The width of the opening in the resist layer after the developing step is approximately the distance in the distribution at the light intensity of 0.3. In the case shown in FIG. 2B, the width of the opening in the resist layer is 0.35 μm which is slightly smaller than the width of 0.36 μm which is obtained in the case shown in FIG. 2A. On the other hand, in the case shown in FIG. 2C, the width of the opening in the resist layer is 0.39 μm which is considerably greater than the 0.36 μm obtained in the case shown in FIG. 2A. Therefore, it can be seen that the deviation of ±0.05 μm of the width L2 exceeds the tolerable deviation range. 
     The side etching of the Cr layer 12 lacks stability, and the side etching rate is greatly dependent on the conditions of the preprocessing, which is carried out before the side etching, and also the etching pattern or etching area. Particularly, the side etching rate varies from 0.03 μm/min to 0.07 μm/min, and under this variation range, an error of 0.05 μm appears on the wafer at 3σ. Therefore, as described above, it is extremely difficult to control the width L2 of the SiO 2  layer 13 to a desired value which is within a tolerable range when the side etching of the Cr layer 12 is required. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful optical mask and method of producing the same, in which the problems described above are eliminated. 
     Another and more specific object of the present invention is to provide a phase shift optical mask used for exposing a pattern using an exposure light, comprising a substrate which is transparent with respect to the exposure light, a light blocking layer which is non-transparent with respect to the exposure light and is provided on the substrate, wherein the light blocking layer has an opening having a predetermined shape and size and being defined by a side wall of the light blocking layer, and a phase shift layer, which is transparent with respect to the exposure light and is provided on the light blocking layer and on the substrate surface which is exposed within the opening. The phase shift layer has a uniform thickness, and the light blocking layer has a predetermined thickness so that the phase of the exposure light transmitted through the phase shift layer, provided on the side wall of the light blocking layer, is shifted by approximately 180° relative to a phase of the exposure light which is transmitted through the phase shift layer provided on the substrate. According to the optical mask of the present invention, it is possible to improve the resolution and expose a fine pattern exceeding a limit of the conventional photolithography technique. In addition, no damage is made to the optical mask during the cleaning and brushing processes. 
     Still another object of the present invention is to provide a phase shift optical mask used for exposing a pattern on a wafer via an exposure lens using an exposure light, comprising a substrate which is transparent with respect to the exposure light, a light transmitting region provided on the substrate and having a width m(L+0.2 λ/NA), wherein m denotes a reducing projection magnification, L denotes a width of an opening actually developed on the wafer, λ denotes a wavelength of the exposure light and NA denotes a numerical aperture of an exposing lens, a first phase shift region which is transparent with respect to the exposure light and is provided on both sides of the light transmitting region, wherein the first phase shift region has a width (mλ/2 NA)[1.1-(NA/λ)(L+0.2 λ/NA)] on both sides of the light transmitting region, a first light blocking region which is non-transparent with respect to the exposure light and is provided on both outer sides of the first phase shift region, wherein the first light blocking region has a width 0.1 mλ/NA on both outer sides of the first phase shift region, a second phase shift region which is transparent with respect to the exposure light and is provided on both outer sides of the first light blocking region, wherein the second phase shift region has a width 0.1 mλ/NA on both outer sides of the first light blocking region, and a second light blocking region which is non-transparent with respect to the exposure light and is provided on both outer sides of the second phase shift region. According to the optical mask of the present invention, it is possible to expose a line pattern of 0.45×(λ/NA) μm or even less. 
     A further object of the present invention is to provide a method of producing a phase shift optical mask which is used for exposing a pattern using an exposure light, comprising the steps of forming an opening in a light blocking layer which is provided on a substrate, wherein the light blocking layer is non-transparent with respect to the exposure light, the substrate is transparent with respect to the exposure light and the opening has a predetermined shape and size defined by a side wall of the light blocking layer, and forming a phase shift layer which is transparent with respect to the exposure light on the light blocking layer and the substrate which is exposed within the opening, wherein the phase shift layer is formed to a uniform thickness. The light blocking layer has a predetermined thickness so that the phase of the exposure light transmitted through the phase shift layer provided on the side wall of the light blocking layer is shifted by approximately 180° relative to a phase of the exposure light transmitted through the phase shift layer provided on the substrate. According to the method of the present invention, it is possible to accurately form an opening using the conventional photolithography technique, and the size of the opening can be reduced to a degree or level, exceeding the limit of the conventional photolithography technique without deteriorating the accuracy. 
     Another object of the present invention is to provide a method of producing a phase shift optical mask used for exposing a pattern on a wafer via an exposure lens using an exposure light, comprising the steps of forming an opening, which has a width of 1.5 mλ/NA, in a first light blocking layer which is provided on a substrate, wherein the first light blocking layer is non-transparent with respect to the exposure light, the substrate is transparent with respect to the exposure light, m denotes a reducing projection magnification, L denotes a width of an opening actually developed on the wafer, λ denotes a wavelength of the exposure light and NA denotes a numerical aperture of an exposing lens, forming a first phase shift layer on the first light blocking layer and the surface of the substrate which is exposed within the opening, wherein the first phase shift layer is transparent with respect to the exposure light and has a stepped part in correspondence with the opening and the first phase shift layer has a thickness 0.1 mλ/NA on a side wall of the first light blocking layer defining the opening, forming a second light blocking layer on the first phase shift layer, wherein the second light blocking layer is non-transparent with respect to the exposure light and having a thickness 0.1 mλ/NA on the side wall of at the stepped part of the first phase shift layer, and forming a second phase shift layer on a side wall of the second light blocking layer on the first phase shift layer, wherein the second phase shift layer is transparent with respect to the exposure light and having a width (mλ/2 NA) [1.1-(NA/λ) (L+0.2 λ/NA) ]. According to the method of the present invention, it is possible to produce an optical mask which enables the exposure of a line pattern which is 0.45×(λ/NA) μm or even less. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing an essential part of an example of a conventional phase shift optical mask; 
     FIGS. 2A through 2C show different light intensity distributions of exposure light transmitted through the phase shift optical mask under respective different conditions; 
     FIG. 3A is a cross sectional view for explaining an operating principle of an optical mask according to the present invention; 
     FIG. 3B shows amplitude and phase characteristics of transmitted light in correspondence with FIG. 3A; 
     FIG. 3C shows amplitude and phase characteristics of synthesized transmitted light in correspondence with FIG. 3A; 
     FIG. 3D shows light intensity of transmitted light in correspondence with FIG. 3A; 
     FIG. 4 is a cross sectional view showing an essential part of a first embodiment of the optical mask according to the present invention; 
     FIG. 5 generally shows an exposure apparatus in which the first embodiment of the optical mask according to the present invention may be employed; 
     FIG. 6 is a plan view showing an actual pattern formed on a wafer by using the first embodiment of the optical mask; 
     FIG. 7A is a cross sectional view for explaining a first conventional phase shift optical mask; 
     FIG. 7B shows amplitude and a phase characteristic of light transmitted through a light transmitting region of the phase shift optical mask shown in FIG. 7A; 
     FIG. 7C shows amplitude and phase characteristics of light transmitted through a phase shift region of the phase shift optical mask shown in FIG. 7A; 
     FIG. 7D shows light intensity of transmitted light in correspondence with FIG. 7A; 
     FIG. 8A is a cross sectional view for explaining a second conventional phase shift optical mask; 
     FIG. 8B shows amplitude and phase characteristics of light transmitted through a light transmitting region of the phase shift optical mask shown in FIG. 8A; 
     FIG. 8C shows amplitude and phase characteristics of light transmitted through a phase shift region of the phase shift optical mask shown in FIG. 8A; 
     FIG. 8D shows light intensity of transmitted light in correspondence with FIG. 8A; 
     FIG.9 is a cross sectional view for explaining an operating principle of a second embodiment of the optical mask according to the present invention; 
     FIG. 10 is a cross sectional view showing an essential part of the second embodiment of the optical mask according to the present invention; 
     FIG. 11A shows amplitude and phase characteristics of light transmitted through a light transmitting region, a first phase shift region and a second phase shift region of the optical mask shown in FIG. 10; 
     FIG. 11B shows light intensity of transmitted light in correspondence with FIG. 11A; and 
     FIGS. 12A through 12C respectively are cross sectional views for explaining a second embodiment of the method of producing the optical mask according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, a description will be given of an operating principle of an optical mask according to the present invention, by referring to FIGS. 3A through 3D. 
     In FIG.3A, the phase shift optical mask includes a glass substrate 1, a light blocking layer 2, a first inorganic layer 3 and a second inorganic layer 4. A main light transmitting part A is surrounded by a phase shift part B. The first and second inorganic layers 3 and 4 are made of an inorganic material, and the second inorganic layer 4 has a thickness T. The total thickness PS, of the light blocking layer 2 and the first inorganic layer 3, corresponds to the required thickness of the material of layer 4 in each of the parts B, in which light propagating therethrough undergoes a phase shift of approximately 180° relative to light propagating through the transmitting part A. 
     Next, a description will be given of the resolution improving effect which is obtained by the edge emphasis type phase shift optical mask according to the present invention. 
     FIG. 3B shows the amplitude and the phase characteristic of the light which is transmitted through the main light transmitting part A and the phase shift part B of the phase shift optical mask. A curve I indicates the light transmitted through the main light transmitting part A, and each curve II indicates the light transmitted through the corresponding phase shift part B. As may be seen from FIG. 3B, the phase of the light transmitted through each phase shift part B is shifted by 180° with respect to the phase of the light transmitted through the main light transmitting part A. 
     FIG. 3C shows in curve III the amplitude and the phase characteristic of a synthesized light in which the light transmitted through the main light transmitting part A and the light transmitted through the phase shift part B are synthesized. The curve III of this synthesized light is sharper as compared to the curve I, because at the bottom portions of the curve III, the light transmitted through the main light transmitting part A is cancelled by the light which is transmitted through the phase shift part B and which has an inverted phase relative to that of the light transmitted through the main light transmitting part A. 
     In FIG. 3D, a curve IV indicates the intensity of the transmitted light. This curve IV corresponds to a square of the amplitude indicated by the curve III shown in FIG. 3C. The curve IV is very sharp because of the light transmitted through the phase shift part B. Hence, it may be seen from FIG. 3D that the resolution is improved when carrying out an exposure using the mask of the invention. 
     Next, a description will be given of a method of producing the phase shift optical mask shown in FIG. 3A. 
     First, the light blocking layer 2 is formed on the entire surface of the glass substrate 1, and the first inorganic layer 3 is formed on the entire surface of the light blocking layer 2. The total thickness PS of the light blocking layer 2 and the first inorganic layer 3 is set so that the exposure light is shifted by 180° by the second inorganic layer 4 which is later formed. 
     The thickness PS for realizing the 180° phase shift, by use of the second inorganic layer 4, is calculated as follows. That is, since the propagation velocity of the exposure light is inversely proportional to the refractive index of the medium through which the exposure light propagates, the thickness of the medium should be set to λ/2(n-1) for shifting the phase of the exposure light as propagated therethrough by 180°, as compared to the exposure light propagating through air, wherein n denotes the refractive index of the medium and λ denotes the wavelength of the exposure light. 
     In this case, it is unnecessary to provide the first inorganic layer 3 if the thickness PS can be provided using only the light blocking layer 2. However, when forming the light blocking layer 2 from a metal, problems are introduced due to the difference between the respective coefficients of thermal expansion of the layers if the metal light blocking layer 2 is made thick. Furthermore, when the metal light blocking layer 2 is thick, the etching of the metal light blocking layer 2 at the later stage becomes that much difficult. For this reason, it is desirable to form, on the light blocking layer 2, the first inorganic layer 3 which has approximately the same coefficient of thermal expansion as that of the glass substrate 1, and then to adjust the thickness PS in accordance with the total of the respective thickness of the light blocking layer 2 and the first inorganic layer 3. 
     Second, an opening 5 for exposure is formed in the first organic layer 3 and the light blocking layer 2 by a photolithography technique. The width of this opening 5 is A+2 T, wherein A denotes the width of the main light transmitting part A after completion and T denotes the thickness of the second inorganic layer 4 which is later formed. Hence, the width of the opening 5 is 2 T wider than the width of the opening 14 of the conventional phase shift optical mask shown in FIG. 1, thereby making it possible to accurately form the opening 5 with a satisfactory reproducibility using the normal photolithography technique. 
     Third, the second inorganic layer 4 is formed on the first inorganic layer 3 and the portion of the glass substrate 1 which is exposed within the opening 5, using a chemical vapor deposition (CVD) or the like. The thickness of the second inorganic layer 4 which is formed by the CVD is uniform, and the thickness T of the second inorganic layer 4 deposited on the first inorganic layer 3 can be made the same as the width T of the second inorganic layer 4 which is deposited on the inner wall defining the opening 5. 
     In this case, the depth of the stepped part, or depression extending from the surface of the second inorganic layer 4 is maintained the same as the total thickness PS (i.e., the combined respective thicknesses) of the light blocking layer 2 and the first inorganic layer 3. Thus, the depression functions as the main light transmitting part A, and a region which has the width T and is formed on the outer side of the depression on each side functions as the phase shift part B. 
     Therefore, the size of the completed main light transmitting part A can be controlled accurately using the width A+2 T of the opening 5 as a reference, which opening 5 is formed by the normal photolithography technique. 
     Next, a description will be given of a first embodiment of the optical mask according to the present invention, by referring to FIG. 4. 
     A phase shift optical mask 34 shown in FIG. 4 includes a Cr light blocking layer 7 which is formed on a quartz glass substrate 6, a SiO 2  first inorganic layer 8 which is formed on the Cr light blocking layer 7, an opening 10 which is formed in the SiO 2  first inorganic layer 8 and the Cr light blocking layer 7, and a SiO 2  second inorganic layer 9 which is formed on the SiO 2  inorganic layer 8 and the quartz glass substrate 6 which is exposed within the opening 10. The SiO 2  second inorganic layer 9 is transparent with respect to the exposure light. A main light transmitting part A is surrounded by the phase shift part B. 
     This first embodiment of the optical mask according to the present invention may be produced by a first embodiment of a method of producing the optical mask according to the present invention, as follows. 
     According to the first embodiment of the method, the Cr layer 7 is formed on the quartz glass substrate 6 and the SiO 2  layer 8 is formed on the Cr layer 7, so that a total thickness of the Cr layer 7 and the SiO 2  layer 8 becomes 4700 Å. 
     The total thickness of the Cr layer 7 and the SiO 2  layer 8 is calculated as follows. That is, since the propagation velocity of the exposure light is inversely proportional to the refractive index of the medium through which the exposure light propagates, the thickness of the medium should be set to λ/2(n-1) for shifting the phase of the exposure light by 180°, as compared to the exposure light propagating through air, where n denotes the refractive index of the medium and λ denotes the wavelength of the exposure light. 
     Hence, in the case where the light source is a mercury discharge lamp which emits as the exposure light a g-ray having a wavelength of 4358 Å and the refractive index n of the SiO 2  layer 9 is 1.46, the total thickness of the Cr layer 7 and the SiO 2  layer 8 can be calculated from λ/2(n-1) as 4737 Å or approximately 4700 Å. 
     Next, a resist layer (not shown) is formed on the entire surface of the SiO 2  layer 8, and a resist mask pattern having an opening is formed by a normal electron beam exposure technique. One side of this opening in the resist mask pattern has a length of 1.1×m μm when the reducing projection magnification of a reducing projection type exposure apparatus used therefor is m. Thereafter, the SiO 2  layer 8 and the Cr layer 8 are selectively removed via this opening in the resist mask pattern using a dry etching, process step thereby forming an opening 10. 
     Then, the SiO 2  layer 9 is deposited to a thickness of 1 μm on the entire surface of the SiO 2  layer 8 and the quartz glass substrate 6 which is exposed within the opening 10 using a CVD process step. This thickness of the SiO 2  corresponds to 0.2×m μm when M=5. 
     The phase shift optical mask 34 which is produced by the above described processes has a square main light transmitting part A having a length of each side of 0.7×m μm, and a border or frame shaped, phase shift part B having a width of 0.2×m μm. 
     When m=5 in this embodiment, for example, the square opening in the resist mask pattern which is formed on the SiO 2  layer 8 has a side of 5.5 μm, and when the phase shift optical mask 34 is completed, the square main light transmitting part A has a side of 3.5 μm and the frame shaped phase shift part B has a width of 1 μm. 
     When producing a semiconductor device using the first embodiment of the phase shift optical mask described above, a resist layer which is formed on a semiconductor surface is exposed by passing a g-ray having a wavelength of 4358 Å from a mercury discharge lamp through an optical pattern of the phase shift optical mask 34 and projecting the transmitted g-ray on the resist layer with a size reduction to 1/5th that of the original, for example. In this case, the NA of the exposure lens is 0.45. 
     FIG. 5 generally shows an exposure apparatus which is used for carrying out the first embodiment of the method. The g-ray emitted from a mercury discharge lamp 31 irradiates the phase shift optical mask (or reticle) 34 via a condenser lens 32 and an iris 33, and the g-ray transmitted through the phase shift optical mask 34 is converged and irradiated on a wafer 36 via a projection lens 35. The wafer 36 corresponds to the semiconductor surface referred above, and the resist layer which is exposed is formed on the wafer 36. 
     According to the first embodiment of the optical mask and the first embodiment of the method, the width T of the SiO 2  layer (phase shift part) 4 can be controlled accurately by controlling the deposition thickness of the SiO 2  layer 4. Using the existing techniques, the deposition thickness of the SiO 2  layer 4 can be controlled within a range of ±0.02 μm on the phase shift optical mask 34. However, on the wafer 36, this range is reduced to 1/5th that of the original due to the reduced production, and the error on the wafer 36 is on the order of ±0.004 μm at 3σ. This error of ±0.004 μm is considerably small compared to the error of ±0.05 μm which is introduced in the conventional case described above. 
     FIG. 6 shows a plan view of an actual pattern which is formed on the wafer 36 using the optical mask 34. For example, the optical mask 34 is particularly suited for use in forming a fine aperture 340 of the pattern shown in FIG. 6. 
     As described above in conjunction with the prior art, the phase shift optical mask was developed to improve the resolution of the photolithography technique. The phase shift optical mask includes a first region for transmitting the exposure light as it is and a second region for inverting the phase of the exposure light, and the second region is formed adjacent to, or in the vicinity of and surrounding (i.e., bordering) the first region. The widths of the first and second regions are determined by the wavelength of the exposure light and the NA of the exposure lens, and these first and second regions effectively utilize the phenomenon of light interference. 
     FIG. 7A shows a first conventional phase shift optical mask including a glass substrate 21, a Cr light blocking layer 22 and a SiO 2  phase shift layer 23. The amplitude of the exposure light which is transmitted through a light transmitting region 24 of this phase shift optical mask is shown in FIG. 7B, while the amplitude of the exposure light which is transmitted through the SiO 2  phase shift layer 23 is shown in FIG. 7C. In FIGS. 7B and 8C, a positive direction indicates a positive phase and a negative direction indicates an inverted phase. Hence, the light intensity of the exposure light transmitted through the phase shift optical mask, that is, the light intensity of the synthesized light made up of the exposure light transmitted through the light transmitting region 24 and the exposure light transmitted through the SiO 2  phase shift layer 23, becomes as shown in FIG. 7D. Therefore, it is possible to draw or expose an extremely thin line using this phase shift optical mask. 
     FIG. 8A shows a second conventional phase shift optical mask including a glass substrate 21, a Cr light blocking layer 25 and a SiO 2  phase shift layer 26. The amplitude of the exposure light which is transmitted through a light transmitting region 27 of this phase shift optical mask is shown in FIG. 8B, while the amplitude of the exposure light which is transmitted through the SiO 2  phase shift layer 26 is shown in FIG. 8C. In FIGS. 8B and 8C, a positive direction indicates a positive phase and a negative direction indicates an inverted phase. Hence, the light intensity of the exposure light transmitted through the phase shift optical mask, that is, the light intensity of the synthesized light made up of the exposure light transmitted through the light transmitting region 27 and of the exposure light transmitted through the SiO 2  phase shift layer 26, becomes as shown in FIG. 8D. Therefore, it is possible to draw or expose an extremely thin line using this phase shift optical mask. 
     Next, a description will be given of an operating principle of a second embodiment of the optical mask according to the present invention, in which the concepts of the first and second conventional phase shift optical masks shown in FIGS. 7A and 8A are combined so as to enable the drawing, or exposure, of an even thinner line. 
     Ideally, a phase shift optical mask which combines the concepts of the first and second conventional phase shift optical masks shown in FIGS. 7A and 8A has a structure as shown in FIG. 9. The phase shift optical mask shown in FIG. 9 includes a glass substrate 101, a Cr light blocking layer 102 which includes first and second light blocking regions 102A and 102B, a SiO 2  phase shift layer 103 which includes first and second phase shift regions 103A and 103B, and a light transmitting region 104. 
     But when the wavelength of the exposure light is 436 nm, the NA of the exposure lens is 0.45 and a line width of 0.4 μm is to be realized, for example, the width of the light transmitting region 104 must be set to m×0.6 μm, the width of the first phase shift region 103A must be set to m×0.25 μm, the width of the first light blocking region 102A must be set to m×0.1 μm, and the width of the second phase shift region 103B must be set to m×0.1 μm, where m denotes the reducing projection magnification which is normally 5 or less. 
     However, according to the existing techniques, it is extremely difficult to accurately align the first and second light blocking regions 102A and 102B and the first and second phase shift regions 103A and 103B which have the extremely small widths described above. Therefore, it is extremely difficult, if not impossible, to produce the phase shift optical mask shown in FIG. 9. 
     FIG. 10 shows an essential part of the second embodiment of the optical mask according to the present invention. The optical mask shown in FIG. 10 includes a light transmitting substrate 41, a light transmitting region 42, a first phase shift layer 43, a first light blocking layer 44, a second phase shift layer 45, and a second light blocking layer 46. For example, the light transmitting substrate 41 is made of quartz glass, the first and second phase shift layers 43 and 45 are made of SiO 2 , and the first and second light blocking layers 44 and 46 are made of Cr. 
     The first phase shift layer 43 is provided at the outer sides (i.e., the edges) of the light transmitting region 42 and thus defining same. The first light blocking layer 44 is provided at the outer edges of the first phase shift layer 43 and thus defining same. The second phase shift layer 45 is provided at the outer sides (i.e., edges) of the first light blocking layer 44 and thus defining same. Furthermore, the second light blocking layer 46 is provided at the outer sides of the second phase shift layer 45 and thus defining same. 
     The light transmitting region 42 has a width on the order of m(L+0.2 λ/NA), the first phase shift layer 43 has a width on the order of (mλ/2 NA) [1.1-(NA/λ) (L+0.2 λ/NA)], and the first light blocking layer 44 has a width on the order of 0.1 mλ/NA, where m denotes the reducing projection magnification, L denotes the width of the actual opening which is formed on the wafer, λ denotes the wavelength of the exposure light and NA denotes the numerical aperture of the exposure lens. 
     FIG. 11A shows the amplitude and phase characteristics of light transmitted through the light transmitting region 42, the first phase shift layer (region) 43 and the second phase shift layer (region) 45 of the optical mask shown in FIG. 10, and FIG. 11B shows the light intensity of the transmitted light in correspondence with FIG. 11A. In FIG. 11A, G denotes the amplitude of the light transmitted through the light transmitting region 42, H denotes the amplitude of the light transmitted through the first phase shift layer 43, and I denotes the amplitude of the light transmitted through the second phase shift layer 45. Hence, the intensity of the light transmitted through the optical mask shown in FIG. 10 as a whole is a synthesized light of G, H and I, as indicated by J in FIG. 11B. Hence, the effective exposure width can be set to 0.4 μm, for example, thereby enabling a considerable improvement of the resolution when compared to the conventional phase shift optical masks. 
     Of course, the light amplitudes and the light intensities of the light transmitted through the phase shift optical mask shown in FIG. 9 also become as shown in FIGS. 11A and 11B. 
     As will be described later in conjunction with FIG. 12C, it is also possible to provide a third phase shift layer 47 which covers the light transmitting region 42, the first phase shift layer 43, the first light blocking layer 44 and the second phase shift layer 45. 
     Next, a description will be given of a second embodiment of the method of producing the optical mask according to the present invention, by referring to FIGS. 12A through 12C. In this embodiment of the method, the second embodiment of the optical mask shown in FIG. 10 is produced. In FIGS. 12A through 12C, those parts which are the same as those corresponding parts in FIG. 10 are designated by the same reference numerals, and a description thereof will be omitted. 
     First, as shown in FIG. 12A, a Cr layer 46 is formed on the quartz glass substrate 41 to a thickness of 4700 Å which corresponds to λ/(2 n-2), where λ denotes the wavelength of the exposure light and n denotes the refractive index of SiO 2  which is used as the phase shift material of this optical mask. An opening having a width of 7.5 μm is formed in the Cr layer 46 using a photolithography technique. Then, a SiO 2  layer 45a is formed to a thickness of 5000 521 . This SiO 2  layer 45a covers the Cr layer 46 and the surface of the quartz glass substrate 42 which is exposed within the opening. Thereafter, a Cr layer 44a is formed on the SiO 2  layer 45a to a thickness of 5000 Å. As a result, each of the width of the side wall formed by the stepped part of the Cr layer 44a and the width of the side wall formed by the stepped part of the SiO 2  layer 45a is 0.5 μm. 
     Next, a reactive etching process step using a gas mixture of carbon tetrafluoride (CF 4 ) and oxygen (O 2 ) as the reactive gas is carried out to etch back the Cr layer 44a so as to leave a portion of the first light blocking layer 44 on the side wall of the stepped part of the SiO 2  layer 45a as shown in FIG. 12B. 
     A SiO 2  layer 43a is thereafter formed on the surface of the structure shown in FIG. 12B to a thickness of 1 μm, as shown in FIG. 12C. The width of the side wall formed by the stepped part of this SiO 2  layer 43a is 1 μm. 
     Then, when the SiO 2  layer 43a is etched back by a reactive etching process step using CF 4  as the reactive gas, the first phase shift layer 43 is left on the side wall at the stepped part of the first light blocking layer 44 as shown in FIG. 10. The etching of layer 43a is stopped at the common level of the blocking layer 44 and thus the part of the SiO 2  layer 45a which remains on the opposite side wall part of the first light blocking layer 44 and extends to a portion respectively aligned with the stepped part of the Cr layer (second light blocking layer) 46 forms the second phase shift layer 45. In FIG. 10, an opening is formed in the SiO 2  layer 43a, which defines the phase shift layer (43) on the inner edge of the light blocking layer 44 and which opening furthermore defines the light transmitting region 42. 
     As a modification of the second embodiment of the method, it is possible to omit the process of etching back the SiO 2  layer 43a and stop at the stage shown in FIG. 12C. In this case, corresponding portions of the SiO 2  layers 43a and 45a become the light transmitting region 47 as indicated in brackets in FIG. 12C. Furthermore, as indicated in brackets (i.e., parentheses) in FIG. 12C, the first phase shift layer (43) in this case has a triple-layer structure made up of the original first phase shift layer 43 and the underlying portions of the SiO 2  layers 43a and 45a, and the second phase shift layer 45 has a triple-layer structure made up of the original second phase shift layer 45 and the corresponding, overlying portions of the SiO 2  layer 45a and the SiO 2  layer 43a. 
     Since the second embodiment of the method carries out an etch back process after forming a phase shift layer and a light blocking layer on a stepped surface, it is possible to form a phase shift layer and a light blocking layer having an extremely narrow width with a high accuracy. Therefore, it is possible to expose line patterns having a width of 0.4 μm or even less. 
     As may be seen by comparing FIGS. 12C and 4, the second embodiment of the optical mask shown in FIG. 12C may be regarded as the first embodiment of the optical mask shown in FIG. 4 added with the phase shift layer 43 and the light blocking layer 44. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.