Mask, mask producing method and pattern forming method using mask

Abstract: A mask for use in exposure of a semiconductor wafer includes transparent portions (2, 31, 51) for transmitting light used in the exposure, formed by a transparent layer and corresponding to first areas of the wafer, these being areas which are to be exposed, opaque portions (14, 32, 53) for masking the light, formed by an opaque layer and corresponding to second areas of the wafer adjacent to the first areas, and phase shift portions (3a, 37A, 37B, 41A, 41B) for shifting the phase of the light so as to improve the contrast between the first and second areas, formed by a phase shift layer (3a) formed over the transparent layer and the opaque layer. According to the invention, in at least one part of the mask the phase shift portions are formed only along edges of the opaque portions (14, 32, 53) without extending between adjacent opaque portions. A method of producing such a mask, and a pattern forming method employing the mask, are also disclosed.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to masks, mask producing methods and pattern forming methods using masks, and more particularly to a mask which uses a phase shift of light, a mask producing method for producing such a mask and a pattern forming method which uses such a mask. When forming patterns of elements, circuits and the like on a semiconductor wafer in a production process of a semiconductor device, it is normal to employ a pattern transfer exposure which uses an ultraviolet light. The pattern which is to be transferred onto the wafer is formed depending on the existence of a metal thin film which is provided on a glass substrate, where the metal thin film is opaque with respect to the light and the glass substrate is transparent with respect to the light. The pattern on the glass substrate to be transferred onto the wafer is called a mask when this pattern is identical to a chip pattern which is actually transferred onto the wafer. On the other hand, the pattern on the glass substrate is called an enlarged mask or a reticle when this pattern is enlarged compared to the chip pattern which is actually transferred onto the wafer. When using the mask, the pattern is transferred onto a resist layer on the wafer and exposed using a parallel ray. On the other hand, when using the reticle, the pattern is transferred onto the resist layer on the wafer and exposed using a reduction lens system which projects a reduced pattern on the resist layer. In order to improve the resolution of the pattern which is transferred especially when the integration density of the integrated circuit is large and the pattern is fine, it is necessary to improve the contrast at an edge portion of the region which is exposed. FIG.1 shows an example of a conventional mask which is made up of an opaque layer having a predetermined pattern and a transparent substrate. FIG.2 shows an example of an optical system for forming a pattern on the wafer using the mask shown in FIG.1. In FIG.1, a mask 450 comprises a transparent substrate 452 and an opaque layer 451 which is made of a material such as chromium (Cr). The opaque layer 451 is formed to a predetermined pattern by lithography and etching processes. In FIG.2, a light C which is emitted by an exposure apparatus (not shown) illuminates the mask 450. The light C cannot be transmitted through the opaque layer 451 but is transmitted through a portion of the transparent substrate 452 not provided with the opaque layer 451. The transmitted light passes through an imaging lens system 453 and exposes a resist material 455 which is coated on a wafer 454. For example, a resist OFPR manufactured by Tokyo Ooka Kogyo K. K. of Japan may be used as the resist material 455. As a result, a pattern identical to the pattern of the mask 450 is formed on the wafer 454 by an etching process. When forming the pattern using an optical lens system, the exposure on the wafer 454 is made based solely on data related to the contrast which is determined by the existence of the opaque layer 451 on the transparent substrate 452. For this reason, there is a physical resolution limit to the pattern formation due to the wavelength of the light which is obtained via the optical lens system, and it is difficult to form a fine pattern depending on the wavelength of the light used for the exposure. Conventionally, the photolithography process uses a reticle which is made by forming an opaque layer on a transparent substrate and patterning the opaque layer. For example, the opaque layer is made of Cr and the transparent substrate is made of a transparent material such as glass and quartz. FIGS.3A through 3D are diagrams for explaining such a conventional pattern forming method. In FIG.3A, the optical system includes a light source 461, an illumination lens 462, and an imaging lens system 465. A reticle 464 is arranged between the illumination lens 462 and the imaging lens system 465. The light source 461 is made of a mercury lamp, an excimer laser or the like and is provided with a filter for emitting i-ray or g-ray. The light 463 from the light source 461 illuminates the reticle 464 via the illumination lens 462. For example, the partial coherency σ of the illumination lens 462 is 0.5. The reticle comprises a transparent substrate 468 and an opaque pattern 469 which is formed on the transparent substrate 468. For example, the transparent substrate 468 is made of glass and the opaque pattern 469 is made of Cr. The opaque pattern 469 on the reticle 464 is imaged on a photoresist layer 467 which is formed on a semiconductor substrate 466 via the imaging lens system 465. For example, the imaging lens system 465 has a numerical aperture (NA) of 0.50. According to this pattern forming method, the resolution can be described by K1·λ/NA, where K1 denotes a process coefficient which is normally 0.6 to 0.8 and λ denotes the wavelength of the light. The wavelength λ of the light 463 is approximately 365 nm in the case of the i-ray which is emitted using the mercury lamp and is 248 nm or 198 nm when the excimer laser is used. The numerical aperture NA differs depending on the imaging lens system 465 which is used but is approximately 0.5, for example. In order to improve the resolution, it is necessary to set K1 or λ to a small value and set NA to a large value. However, the values of K1 and NA cannot be set freely. In addition, the value of λ is restricted by the light source 461 and the optical system used. The resolution is determined when the wavelength λ of the light used for the exposure, the numerical aperture NA and the process coefficient K1 are determined, and it is impossible to image a pattern finer than the resolution. The light 463 emitted from the light source 461 illuminates the entire surface of the reticle 464, and the portion of the light 463 which illuminates the opaque pattern 469 is stopped by the opaque pattern 469. For this reason, only the portion of the light 463 which illuminates the portion of the reticle 464 not provided with the opaque pattern 469 is transmitted through the reticle 464 and is imaged on the photoresist layer 467 via the imaging lens system 465. FIG.3B shows an electrical vector E of the light transmitted through the reticle 464, and FIG.3C shows a light intensity P of the light transmitted through the reticle 464. A pattern having a light intensity distribution proportional to the square of the amplitude of the light illuminating the photoresist layer 467 is formed on the photoresist layer 467 and the photoresist layer 467 is selectively exposed. FIG.3D shows a portion of the reticle (opaque mask) 464 on an enlarged scale. A minimum width W of the pattern which can be exposed is restricted by the resolution which is determined by the imaging lens system 465. Next, a description will be given of the light intensity distribution for cases where the pattern which is exposed is a line pattern which is finer than the resolution, by referring to FIGS.4A through 4D. In FIGS.4A through 4D, the wavelength λ of the light used is 365 nm, the numerical aperture NA is 0.50 and the partial coherency σ of the light is approximately 0.5. FIG.4A shows the light intensity distribution for the case where the line pattern which is imaged has a width of 0.35 µm. The light intensity approaches zero approximately at a center position (0.0) and gradually rises on both sides of the center position (0.0). The line width is approximately 1.0 µm or greater at a position where the light intensity is a maximum. When the photoresist layer is developed using a position where the light intensity is approximately 0.2 as a developing threshold, it is possible to develop a pattern which has a designed width in the order of 0.35 µm. FIG.4B shows the light intensity distribution for the case where the line pattern which is imaged has a width of 0.30 µm. A minimum of the light intensity at the center position (0.0) is increased compared to the light intensity distribution shown in FIG.4A. The width of the light intensity distribution itself shown in FIG.4B is not much different from that of FIG.4A. FIGS.4C and 4D respectively show the light intensity distributions for the cases where the line pattern which are imaged have widths of 0.25 µm and 0.20 µm. In each of FIGS.4C and 4D, the minimum of the light intensity at the center position (0.0) is increased compared to the light intensity distribution shown in FIG.4A, similarly to the case shown in FIG.4B. The width of the light intensity distribution itself shown in each of FIGS.4C and 4D is not much different from that of FIG.4A. In other words, even when the pattern width is reduced exceeding the resolution, the pattern width of the light intensity distribution which is obtained does not decrease and the minimum of the light intensity at the center position (0.0) increases. In these cases, it is impossible to reduce the line width which is exposed, and the black level is exposed as a gray level. For these reasons, it is impossible to form an image which is finer than the resolution. On the other hand, a method of shifting the phase of the light which is transmitted through the mask and exposed on the wafer by 180° depending on the patterns of the mask is proposed in Marc D. Levenson, "Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol.ED-29, No.12, December 1982. According to this proposed method, the interference between the patterns is eliminated so as to improve the contrast on the wafer and improve the resolution of the exposure apparatus. However, it is difficult to apply this proposed method to masks and reticles having fine patterns, and there is a problem in that it is troublesome to generate pattern data peculiar to the phase shift pattern. For this reason, there is a demand to realize a phase shift pattern which is easily applicable to masks and reticles having fine patterns and does not substantially increase the number of processes such as the generation of the pattern data. The phase shift pattern of the phase-shifting mask which is used according to this proposed method is formed as follows. First, an auxiliary pattern is formed in a vicinity of a design pattern (white pattern) which is to be transferred onto the wafer, where the auxiliary pattern has a width smaller than that of the design pattern. Second, a phase shifter is formed on the auxiliary pattern. The phase shifter is an organic pattern made of a resist or the like which is formed by coating, exposure and developing processes, or an inorganic pattern which is formed by chemical vapor deposition and lithography processes. For example, a description will now be given of a method of forming a phase-shifting mask which uses a negative resist pattern as the phase shifter, by referring to FIGS.5A through 5D. In FIG.5A, an opaque layer 552 is formed on a glass substrate 551. An aperture pattern, that is, a design pattern 553 comprising a transmission region, and fine patterns 554A and 554B are formed in the opaque layer 552 by a lithography using an electron beam exposure. For example, the design pattern 553 has a width in the order of 1.5 µm, and the fine aperture patterns 554a and 554B have a width in the order of 0.5 µm. The fine aperture patterns 554A and 554B are formed as auxiliary patterns in a region neigboring the design pattern and separated by a distance in the order of 0.5 µm, for example. Next, as shown in FIG.5B, a transparent conductor layer 555 for preventing charge up at the time of the electron beam exposure is formed on the surface of the glass substrate 551 including the inner sides of the aperture patterns 553, 554A and 554B. Then, as shown in FIG.5C, a negative resist layer 656 is formed on the glass substrate 551 to a thickness such that the phase of the light which is transmitted through the negative resist layer 656 is shifted by 180°. A prebaking process is made if needed, and a phase shift pattern is formed on the fine patterns 554A and 554B by the electron beam exposure. A thickness D of the negative resist layer 656 can be obtained from the following formula (1), where λ denotes the wavelength of the light used for the exposure and n denotes a refractive index of the shifter material. D = λ/2(n - 1) When using the i-ray having the wavelength of 365 nm for the exposure, the refractive index n of the negative resist layer 656 is approximately 1.6 and the thickness D becomes approximately 304 µm. Next, as shown in FIG.5D, a developing process is made to selectively form phase shift patterns 556A and 556B on the fine patterns 554A and 554B, respectively. The-phase shift patterns 556A and 556B is made up of the negative resist layer 656 which has the thickness D. FIG.6 shows a phase profile corresponding to the pattern position of the i-ray transmitted through the phase-shifting mask shown in FIG.5D when the i-ray is used for the exposure. On the other hand, when a positive resist is used, a phase shift pattern 557 which is made up of the positive resist is selectively formed on the design pattern 535 as shown in FIG.7 by processes similar to those described in conjunction with FIGS.5A through 5D. In FIG.7, those parts which are essentially the same as those corresponding parts in FIGS.5A through 5D are designated by the same reference numerals, and a description thereof will be omitted. FIG.8 shows a phase profile corresponding to the pattern position of the i-ray transmitted through the phase-shifting mask shown in FIG.7 when the i-ray is used for the exposure. According to the phase-shifting mask shown in FIGS.5D and 7, the phase of the i-ray (i a and i c ) which is transmitted through the design pattern 553 and the phase of the i-ray (i b and i d ) which is transmitted through the fine patterns (auxiliary patterns) 554A and 554B differ by 180° as may be seen from FIGS.6 and 8. For this reason, the i-ray (i a and i c ) which is scattered in the horizontal direction from the region immediately below the design pattern is cancelled by the i-ray (i b and i d ) which is scattered in the horizontal direction from the region immediately below the auxiliary pattern, and the contrast at the ends of the exposure region is improved thereby improving the resolution. The aperture width of the auxiliary pattern is made narrow to such an extent that the light used for the standard exposure is insufficient to expose the resist layer to the bottom portion thereof. Hence, the auxiliary pattern will not be transferred onto the wafer when the exposure is made using the phase-shifting mask. For example, the methods described in conjunction with FIGS.5 through 8 are proposed in Japanese Laid-Open Patent Applications No.61-292643, No.62-67514 and No.62-18946. However, the conventional masks suffer from the following disadvantages. First, in the case of the mask which uses no phase shift, it is difficult to form a pattern which is narrower than the wavelength of the light due to the physical resolution limit of the optical system. When an attempt is made to realize a narrow line width, it is necessary to make a structural modification such as reducing the wavelength of the light and increasing the numerical aperture. Accordingly, it is impossible to form the fine patterns which are required in the future integrated circuits using the optical method. Second, in the case of the mask which uses the phase shift, the pattern formation can only be applied to a pattern such as the so-called line-and-space pattern which has regularity, and the pattern formation cannot be applied to the production of integrated circuits having various patterns. In addition, it is impossible to form a fine pattern because an opaque layer must always be provided. Third, in the case of the mask which uses the phase shift, the auxiliary pattern must be patterned by the exposure technique to a degree finer that the design pattern. For this reason, the fineness of the design pattern must be restricted in order that the resolution limit of the auxiliary pattern is not exceeded. Fourth, in the case of the mask which uses the phase shift, the number of steps required to generate the pattern data is large because it is necessary to generate in addition to the data related to the design pattern the pattern data related to the auxiliary pattern, the pattern data related to the phase shift and the like. Fifth, in the case of the mask which uses the phase shift, it is difficult to control the quality and thickness of the phase shifter which affects the refractive index when an organic material such as a resist is used for the phase shifter. For this reason, it is difficult to form a uniform phase shift pattern which has an accurate phase shift quantity. Sixth, in the case of the mask which uses the phase shift, the phase shifter is made of a material different from the glass substrate. Thus, a reflection occurs at the boundary of the phase shifter and the glass substrate thereby deteriorating the exposure efficiency. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful mask, a mask producing method and a pattern forming method in which the problems described above are eliminated. According to one aspect of the present invention, there is provided a mask for use in exposure of a semiconductor wafer, including: transparent portions for transmitting light used in the exposure, formed by a transparent layer and corresponding to first areas of said wafer, these being areas which are to be exposed; opaque portions for masking said light, formed by an opaque layer and corresponding to second areas of said wafer adjacent to said first areas; and phase shift portions for shifting the phase of said light so as to improve the contrast between the first and second areas, formed by a phase shift layer formed over the transparent layer and the opaque layer or by a phase shift region in the transparent layer; characterized in that in at least one part of the mask, said phase shift portions are formed only along edges of said opaque portions. According to a second aspect of the present invention, there is provided a method of producing the above mask, comprising the steps of: forming said opaque layer on said transparent layer; forming a resist pattern which is made of a resist material, on said opaque layer; forming the phase shift layer which transmits the light on said resist pattern layer, or forming the phase shift region in the transparent-layer, to a predetermined thickness to form the phase shift portions; and removing said resist pattern layer, whereby said phase shift portions remain only along an edge portion of said opaque layer in at least one part of the mask. According to a third aspect of the present invention, there is provided a method of producing the above mask, comprising the steps of: forming said opaque layer on said transparent layer; forming a stopper layer on said opaque layer; forming the phase shift layer on said opaque layer to a predetermined thickness; forming a resist pattern made of a resist material, on said phase shift layer; and etching said phase shift layer using said stopper layer as an etching stopper and said resist pattern as an etching mask, whereby said phase shift layer remains on said stopper layer only along an edge portion of said opaque layer in at least one part of the mask. According to a fourth aspect of the present invention, there is provided a pattern forming method for forming a pattern on a semiconductor wafer using the above mask, the method comprising illuminating the mask by light from a light source, and projecting the light transmitted through the mask onto a photoresist layer formed on the wafer by use of a lens system, whereby the photoresist layer is developed in said first areas of the wafer corresponding to the transparent portions of the mask. 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 a conventional mask provided with an opaque layer; FIG.2 generally shows an optical system which forms a pattern using the mask shown in FIG.1; FIGS.3A through 3D are diagrams for explaining a conventional pattern forming method; FIGS.4A through 4D show light intensity distributions of the prior art; FIGS.5A through 5D are cross sectional views for explaining a method of producing a conventional mask which has a phase shift layer and uses a negative resist; FIG.6 shows a phase profile of the light transmitted through the mask shown in FIG.5D; FIG.7 is a cross sectional view for explaining a method of producing a conventional mask which has a phase shift layer and uses a positive resist; FIG.8 shows a phase profile of the light transmitted through the mask shown in FIG.7; FIG.9 is a cross sectional view showing a mask useful for understanding the present invention; FIGS.10A through 10C are diagrams for explaining the light intensity at one edge portion of a phase shift layer; FIGS.11A through 11C are diagrams for explaining the light intensity at two edge portions of the phase shift layer; FIG.12 generally shows an optical system which is used for the exposure of the FIG. 9 mask; FIG.13 is a diagram for explaining a pattern forming method useful for understanding the present invention; FIGS.14A through 14E are cross sectional views for explaining a mask producing method useful for understanding the present invention; FIGS.15A through 15F are cross sectional views for explaining another mask producing method useful for understanding the present invention; FIG.16 is a plan view showing a mask pattern; FIGS.17A and 17B are diagrams for explaining the light intensity distribution in FIG.16; FIGS.18A through 18C are diagrams for explaining a first embodiment of a pattern forming method according to the present invention; FIG.19 is a cross sectional view showing a first embodiment of a mask according to the present invention; FIGS.20A and 20B are diagrams for explaining the light intensity at one edge portion of the phase shift layer; FIGS.21A and 21B are diagrams for explaining the light intensity at two edge portions of the phase shift layer; FIG.22 is a diagram for explaining a second embodiment of the pattern forming method according to the present invention; FIGS.23A and 23B are diagrams for explaining a pattern in FIG.19; FIGS.24A and 24B are diagrams for explaining the light intensity distribution in FIG.23; FIG.25 is a diagram for explaining the first embodiment of the mask applied to the line-and-space pattern; FIGS.26A through 26D are diagrams for explaining the light intensity distribution for different dimension conversion quantities and widths of the phase shift layer; FIG.27 is a diagram for explaining the conventional light intensity distribution in correspondence with FIG.25; FIGS.28A through 28F are diagrams for explaining the light intensity distribution for the case where a KrF excimer laser is used in FIG.25; FIGS.29A and 29B are diagrams showing a portion of FIG.19; FIGS.30A and 30B are diagrams for explaining the light intensity distribution in FIG.23; FIGS.31A and 31B are diagrams for explaining the formation of a black pattern on a white background; FIGS.32A and 32B are diagrams for explaining the light intensity distribution in FIGS.31A and 31B; FIGS.33A and 33B are diagrams for explaining the case where the first embodiment of the mask is applied to the formation of an integrated circuit pattern; FIGS.34A through 34C are diagrams showing a portion of the pattern shown in FIG.19; FIGS.35A and 35B are diagrams for explaining the light intensity distribution when a pattern width h = 0.3 µm; FIGS.36 through 41 respectively are diagrams for explaining the light intensity distributions when the pattern width h = 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0 µm; FIGS.42A and 42B are diagrams for explaining the case where the first embodiment of the mask is applied to the formation of a contact hole pattern; FIGS.43A and 43B are diagrams for explaining the light intensity distribution in FIGS.42A and 42B; FIGS.44A through 44F are cross sectional views for explaining a first embodiment of the mask producing method according to the present invention; FIGS.45A through 45E are cross sectional views for explaining a second embodiment of the mask producing method according to the present invention; FIGS.46A through 46E are cross sectional views for explaining a third embodiment of the mask producing method according to the present invention; FIG.47 shows a phase profile of the light transmitted through the mask shown in FIG.46E; FIGS.48A through 48C are cross sectional views for explaining a fourth embodiment of the mask producing method according to the present invention; FIG.49 shows a phase profile of the light transmitted through the mask shown in FIG.48C; FIGS.50A through 50H and 51A through 51G are diagrams for explaining mask producing methods not in accordance with the present invention but useful for understanding the same; FIGS.52A through 52F are diagrams for explaining a fifth embodiment of the mask producing method according to the present invention; FIGS.53A through 53F are diagrams for explaining a sixth embodiment of the mask producing method according to the present invention; PIGS.54A through 54E are diagrams for explaining a seventh embodiment of the mask producing method according to the present invention; FIGS.55A through 55C are diagrams for explaining the pattern forming method using the mask; and FIGS.56A and 56B show light intensity distributions for explaining the imaging using the edge portion of the phase shift layer. Before describing the preferred embodiments, a description will be given of a mask useful for understanding the present invention, but not in accordance with the same, by referring to FIG.9. In FIG.9, a mask 1 comprises a transparent substrate 2 which is transparent with respect to light L which is used for the exposure, and a mask pattern layer 5 which is formed on the transparent substrate 2. At least one portion of the mask pattern layer 5 is made up solely of a phase shift layer 3a. The light L can be transmitted through the phase shift layer 3a. The phase of the light which is transmitted through the phase shift layer 3a is shifted from the phase of the light which is transmitted through only the transparent substrate 2. Accordingly, an interference occurs and the light intensity decreases at the boundary of the light which is transmitted through only the transparent substrate 2 and the light which is transmitted through the phase shift layer 3a. For this reason, it is possible to form on a wafer (not shown) an interference pattern which is smaller than the wavelength of the light L which is used for the exposure. In addition, it is possible to adjust the phase shift quantity of the light and improve the resolution of the exposure pattern by adjusting the thickness of the mask pattern layer 5. FIGS.10A through 10C are diagrams for explaining the light intensity at one edge portion of the phase shift layer 3a of the mask 1. In FIG.10A, the phase of the light transmitted through the transparent substrate 2 and the phase shift layer 3a is shifted by approximately 180° with respect to the light transmitted through only the transparent substrate 2. Hence, an electrical vector E and a light intensity P of the light transmitted through the mask 1 respectively become as shown in FIGS.10B and 10C. As may be seen from FIG.10C, it is possible to expose a line pattern by using the light intensity change at the edge portion of the phase shift layer 3a. FIGS.11A through 11C are diagrams for explaining the light intensity at two edge portions of the phase shift layer 3a of the mask 1. In FIGS.11A through 11C, those parts which are the same as those corresponding parts in FIGS.10A through 10C are designated by the same reference numerals, and a description thereof will be omitted. In this case, the light intensity P of the light transmitted through the mask 1 becomes as shown in FIG.11C when a width W of the phase shift layer 3a is sufficiently small. Thus, it is possible to control the width of the line pattern by controlling the width W. FIG.12 generally shows an optical system which is used for the exposure using the mask 1. For example, a light source 6 is made up of a mercury lamp and the mercury lamp is provided with a filter (not shown) for passing only the i-ray which has the wavelength of 365 nm. The light from the light.source 6 passes through an illumination lens system 8 and reaches the mask 1 as the light L. The mask 1 is positioned at the focal distance of the lens system 8. The partial coherency σ of the light L is 0.5 in this example, but the partial coherency σ may be selected within a range such that 0.3 < σ < 0.7. The light transmitted through the mask 1 is imaged on a wafer 11 which is coated with a photoresist layer 10 via an imaging lens system 9. The imaging lens system 9 includes a 1/5 reduction lens and has a numerical aperture NA of 0.5. The wafer 11 is maintained flat by a chuck (not shown). Next, a description will be given of a pattern forming method which is not in accordance with the present invention but useful for understanding the same, by referring to FIG. 13 which shows the exposure state in FIG. 12. The light which passes through the illumination lens system 8 shown in FIG.12 is transmitted through the mask 1, and the light which is transmitted through the mask 1 can be divided into a light 7a which is transmitted through the mask pattern layer 5 of the mask and a light 7b which is transmitted through a portion of the mask 1 not provided with the mask pattern layer 5. The phase of the light 7a differs by 180° with respect to the phase of the light 7b. The lights (light beams) 7a and 7b are imaged on the photoresist layer 10 on the wafer 11 via the imaging lens system 9. A sharp light intensity change occurs at a portion which corresponds to the edge portion of the phase shift layer 3a due to the interference. Hence, it is possible to form on the wafer 11 a pattern which is smaller than the wavelength of the light used for the exposure. Next, a description will be given of a mask producing method which is not in accordance with the present invention but useful for understanding the same, by referring to FIGS. 14 and 15. In Figs 14 and 15, those parts which are essentially the same as those corresponding parts in FIG.9 are designated by the same reference numerals. In FIG.14A, the transparent substrate 2 is made of a material such as quartz and glass which transmits the i-ray. A resist material 3 is formed on the transparent substrate 2 as shown in FIG.14B. A resist such as an electron beam (EB) resist and a photoresist is used as the resist material 3. When the EB resist is used as the resist material 3, a resist pattern 4 shown in FIG.14C is formed by drawing by the electron beam and carrying out a developing process. Then, a silicon oxide which constitutes the phase shift layer 3a is sputtered on the surface of the resist pattern 4 as shown in FIG.14D to a thickness of 0.388 µm. Thereafter, the EB resist is removed by using a resist removal agent so that the mask pattern layer 5 of the phase shift layer (silicon oxide layer) 3a is formed on the transparent substrate 2 as shown in FIG.14E. On the other hand, in the arrangement shown in FIGS.15A through 15F, the transparent substrate 2 shown in FIG.15A is the same as that shown in FIG.14A but an aluminum oxide thin film 12 is formed on the transparent substrate 2 as shown in FIG.15B. As shown in FIG.15C, a silicon oxide which constitutes the phase shift layer 3a is sputtered on the thin film 12 to a thickness of 0.388 µm. Furthermore, the resist material 3 is formed on the phase shift layer 3a as shown in FIG.15D. A resist pattern 4 shown in FIG.15E is formed by drawing by the electron beam and carrying out a developing process. Thereafter, a plasma etching or a reactive ion etching (RIE) is carried out on the phase shift layer 3a by using a CF 4 gas. The resist material 3 is removed by the ashing of the oxygen plasma, and the mask pattern 5 of the phase shift layer (silicon oxide layer) 3a is formed as shown in FIG.15F. Because the thin film 12 is not etched by the CF 4 gas, the thin film 12 acts as an etching stopper. Thus, it is possible to obtain the mask pattern 5 which is made up of the phase shift layer 3a accurately having the thickness of 0.388 µm. The thickness of the phase shift layer 3a is set to 0.388 µm for the purpose of shifting the phase of the i-ray by 180°, that is, inverting the phase. The phase shift quantity and the thickness of the phase shift layer 3a can be described by the following general formula (2), where n denotes the refractive index of the phase shift layer 3a, λ denotes the wavelength of the light L used, S denotes the (phase shift quantity)/2π (or 1/2 when inverting the phase), and t denotes the thickness of the phase shift layer 3a. (n·t/λ) - (t/λ) = S In the methods shown in FIGS.14 and 15, the refractive index n of the phase shift layer 3a is 1.47, the wavelength λ of the light L used is 0.365 nm, and the phase shift quantity S is 1/2. Hence, the general formula (2) can be rewritten as the following formula (3). (1.47t/0.365) - (t//0.365) = 1/2 Therefore, the thickness t of the phase shift layer 3a is calculated as 0.388 µm from the formula (3). The phase shift quantity S is set to 1/2 (180°) in these methods because 180° is an optimum phase shift quantity for forming the pattern by the interference. FIG.16 shows a plan view of the mask pattern 5 of the mask 1 which is produced in the above described manner. When forming a line-and-space pattern with the resolution limit of 0.35 µm by use of a 1/5 recution lens system, a light intensity shown in FIG.17B can be obtained on the wafer 11 when a width b is set to 0.15 µm (reticle size of 0.75 µm) and a width a of a region 13 having no phase shift layer 3a is set to 0.55 µm (reticle size of 2.75 µm). A comparison will now be made of FIGS.17B and 17A. For the sake of convenience, it is assumed that the opaque layer 451 shown in FIG.1 is made of Cr and has a thickness in the range of 50 to 80 nm and a width of 0.35 µm (reticle size of 1.75 µm) for the conventional method. In addition, it is assumed that the interval of the opaque pattern is set to 0.35 µm (reticle size of 1.75 µm) for the conventional method. As shown in FIG.17A, the light intensity is approximately 50 % according to the conventional method, and it is evident that the contrast is poor. In this state, it is impossible to form a pattern on the photoresist layer 455 on the wafer 454 in FIG.2. But on the other hand, in the above described methods, the light intensity is approximately 80 % on the wafer 11 as shown in FIG.17B. Accordingly, the light intensity is greatly improved. In addition, the contrast (resolution) is greatly improved because there is no change at the dark portion. Next, a description will be given of a first embodiment of the pattern forming method according to the present invention, by referring to FIG.18. FIGS.18A and 18B respectively are a plan view and a cross sectional view showing the mask 1 which is for forming a pattern on the wafer, where large and fine patterns coexist in the pattern. An opaque layer 14 which is made of a Cr layer having a thickness of 50 to 80 nm is indicated by a hatched region. In addition, an exposed portion of the transparent substrate 2 is indicated by a white background. The exposed portion is divided into a large pattern region 15 and a fine pattern region 16. The phase shift layer 3a is formed within the fine pattern region 16. The large pattern region 15 is patterned by the conventional method, while the fine pattern region 16 is formed by use of the phase shift layer 3a. Large patterns and fine patterns usually coexist in the integrated circuit. Hence, this embodiment is especially suited for patterning the integrated circuit. FIG.18C shows a pattern which is imaged on the wafer 11 by using the mask shown in FIGS.18A and 18B. According to this embodiment, it is possible to form the pattern in a small number of steps even when the pattern includes fine patterns, and it is possible to sufficiently improve the resolution. Next, a description will be given of a first embodiment of the mask according to the present invention, by referring to FIG.19. A mask 1A shown in FIG.19 comprises the transparent substrate 2, an opaque layer 14 which is formed on the transparent substrate 2, and the phase shift layer 3a which is formed at the edge portion of the opaque layer 14. For example, the opaque layer 14 is made of Cr and is formed to a thickness of 50 to 80 nm. For the sake of convenience, it is assumed that a fine pattern is isolated on the left part of the transparent substrate 2 in FIG.19 and fine patterns are located adjacent to each other on the right part of the transparent substrate 2. In addition, a pattern 3b for forming a fine pattern is made up solely of the phase shift layer 3a. This pattern 3b is transferred onto the wafer 11 by the interference caused by the phase shifter, as will be described later in the specification. Distances a , b and c shown in FIG.19 will also be described later in the specification. FIGS.20A and 20B are diagrams for explaining the light intensity at the edge portion of the phase shift layer 3a of the mask 1A. In FIG.20A, the phase of the light transmitted through the transparent substrate 2 and the phase shift layer 3a is shifted by approximately 180° with respect to the light transmitted through only the transparent substrate 2. Hence, the light intensity P of the light transmitted through the mask 1A becomes as shown in FIG.20B. As may be seen from FIG.20B, it is possible to expose a line pattern by using the light intensity change at the edge portion of the phase shift layer 3a. FIGS.21A and 21B are diagrams for explaining the light intensity at two edge portions of the phase shift layer 3a of the mask 1A. In FIGS.21A and 21B, those parts which are the same as those corresponding parts in FIGS.20A and 20B are designated by the same reference numerals-, and a description thereof will be omitted. In this case, the light intensity P of the light transmitted through the mask 1A becomes as shown in FIG.21B. Thus, it is possible to control the width of the line pattern by controlling the width W. Next, a description will be given of a second embodiment of the pattern forming method according to the present invention, by referring to FIG.22 which generally shows an optical system which uses the mask 1A for the exposure. In FIG.22, those parts which are essentially the same as those corresponding parts in FIG.13 are designated by the same reference numerals, and a description thereof will be omitted. In FIG.22, the light L which originates from the light source 6 and reaches the mask 1A via the illumination lens system 8 cannot be transmitted through the opaque layer 14. The phase of the light 7a transmitted through the phase shift layer 3a is shifted and is inverted with respect to the phase of the light 7b which is transmitted through only the transparent substrate 2. The lights 7a and 7b are imaged on the photoresist layer 10 on the wafer 11 via the imaging lens system 9. In this case, the lights 7a and 7b mutually interfere and a sharp light intensity change occurs. Accordingly, it is possible to improve the contrast between the portion provided with no opaque layer 14 and the portion provided with the opaque layer 14. The conditions of the optical system shown in FIG.22 are the same as those of the optical system shown in FIG.13. Thus, the thickness of the phase shift layer 3a is set to 0.388 µm and the phase shift quantity is set to 180° based on the formula (3). Next, a description will be given of the effects obtained by forming the phase shift layer 3a at the edge portion of the opaque layer 14, by referring to FIGS.23 through 26. FIG.23A shows a case where no phase shift layer 3a is provided at the edge portion of the opaque layer 14, and FIG.23B shows a case where the phase shift layer 3a is provided at the edge portion of the opaque layer 3a with a width b of 0.15 µm. FIGS.24A and 24B respectively show the light intensity distributions for the cases shown in FIGS.23A and 23B. In the case shown in FIG.24B, it can be seen that the light intensity distribution changes sharply at the portion of the phase shift layer 3a in a vicinity of 0.0 µm on the x-axis. In other words, it can be seen from FIG.24B that the capability of forming the fine pattern is large in this case. In this case, the boundary of the phase shift layer 3a and the exposed portion of the transparent substrate 2 is 0.0 µm on the x-axis. Thus, when the boundary of the opaque layer 14 and the phase shift layer 3a is assumed to be -0.15 µm on the x-axis, the boundary between the black and white portions of the pattern becomes +0.1 µm from FIG.16B. Therefore, when the light used for the exposure is the i-ray having the wavelength of 365 nm and the numerical aperture NA of the lens system is 0.5, it is possible to form the resist pattern to the designed dimension when the exposed portion of the transparent substrate 2 is made 0.2 µm (0.1 µm on each side) larger than the designed dimension and the phase shift layer 3a is formed with a width of 0.15 µm on the periphery of the exposed portion of the transparent substrate 2. In the case where the mask 1A is a reticle which has the pattern with a dimension five times that of the resist pattern to be exposed on the wafer 11, it is possible to form the resist pattern to the designed dimension when the exposed portion of the transparent substrate 2 is made 1.0 µm (0.5 µm on each side) larger than the designed dimension and the phase shift layer 3a is formed with a width of 0.75 µm on the periphery of the exposed portion of the transparent substrate 2. In the description given hereunder, it is assumed for the sake of convenience that the reticle is made according to the same rule. The above rule must be changed when the wavelength λ of the light or the numerical aperture NA of the lens system changes. In other words, it is necessary to change the dimension conversion quantity of the exposed portion of the transparent substrate 2 and the width of the phase shift layer 3a formed in the periphery of the edge portion of the opaque layer 14. For example, when a KrF laser is used as the light source which emits a light having a wave length of 248 nm and the lens system used has a numerical aperture of 0.48, it is possible to obtain an optimum effect when the exposed portion of the transparent substrate 2 is made 0.13 µm (0.065 µm on each side) larger than the designed dimension and the phase shift layer 3a at the periphery of the edge portion of the opaque layer 14 is formed with a width of 0.06 µm. In the case where the mask 1A is a reticle which has the pattern with a dimension five times that of the resist pattern to be exposed on the wafer 11, it is possible to obtain the optimum effect when the exposed portion of the transparent substrate 2 is made 0.65 µm (0.325 µm on each side) larger than the designed dimension and the phase shift layer 3a is formed with a width of 0.30 µm on the periphery of the exposed portion of the transparent substrate 2. When the white patterns close upon each other, the region of the opaque layer 14 disappears between the white patterns and the white patterns are separated solely by the phase shift layer 3a. It is known from experience that the resolution is improved by 20 % when the phase shift method is employed. Hence, the dimension conversion quantity at the exposed portion of the transparent substrate 2 and the width of the phase shift layer 3a can be obtained from experience. That is, when the i-ray is used for the exposure and the numerical aperture NA of the lens system is 0.5, the resolution limit for the case where no phase shift is made can be described by 0.6 x (λ/NA) and the resolution limit for the case where the phase shift is made can be described by 0.6 x (λ/NA) x 0.8. Therefore, the width of the phase shift layer 3a can be calculated as 0.35 µm when numerical values are substituted. FIG.25 shows a case where this embodiment is applied to the so-called line-and-space pattern. In FIG.25, the phase shift layer 3a is formed at the edge portion of the opaque layer 14, and the phase shift layer 3a is also formed between the four spaces in which the transparent substrate 2 is exposed. FIGS.26A through 26D show the light intensity distributions of the line-and-space pattern with the resolution limit of 0.35 µm when the phase shift is made when the dimension conversion quantity a at the exposed portion of the transparent substrate 2 and the width b of the phase shift layer 3a are varied. FIG.26A shows the light intensity distribution for the case where a = 0.45 µm and b = 0.25 µm. FIG.26B shows the light intensity distribution for the case where a = 0.50 µm and b = 0.20 µm. FIG.26C shows the light intensity distribution for the case where a = 0.55 µm and b = 0.15 µm. FIG.26D shows the light intensity distribution for the case where a = 0.60 µm and b = 0.10 µm. It is necessary that a + b = 0.70 µm in order that the line-and-space pattern is formed. As may be seen from FIGS.26A through 26D, the peak of the light intensity decreases when the width b of the phase shift layer 3a is large. In addition, when the width b of the phase shift layer 3a is small, the light intensity at the space portion becomes large and the contrast decreases. An optimum contrast is obtained when b = 0.15 µm, and a = 0.55 µm in this case. The boundary between the phase shift layer 3a and the exposed portion of the transparent substrate 2 is shifted so as to enlarge the exposed portion of the transparent substrate 2 by 0.10 µm on each side from the designed dimension, and this case corresponds to the edge portion of the large pattern described above. The width b of the phase shift layer 3a must be set within 30 to 60 % of the resolution limit, that is, within the range of 0.144 to 0.228 µm. A high contrast can be obtained especially when the width b of the phase shift layer 3a is set within 40 to 50 % of the resolution limit. FIG.27 shows the light intensity distribution of the conventional mask shown in FIG.1 for the case where the width of the exposed portion of the transparent substrate 452 is 0.35 µm and the width of the opaque layer 451 is 0.35 µm. In the case where the mask shown in FIG.27 is a reticle which has the pattern with a dimension five times that of the resist pattern to be exposed on the wafer, FIG.27 corresponds to the light intensity distribution of the conventional reticle for the case where the width of the exposed portion of the transparent substrate 452 is 1.75 and the width of the opaque layer 451 is 1.75 µm. In FIG.27, the light intensity is approximately 55 % while the light intensity is approximately 80 % in FIG.26C. Hence, it is readily seen that the contrast is greatly improved in this embodiment. When the KrF excimer laser is used to emit a light having a wavelength of 248 nm and the lens system used has a numerical aperture NA of 0.48, the resolution limit is 0.25 µm when the phase shift is made. FIGS.28B through 28F show the light intensity distributions of for this case when the dimension conversion quantity a at the exposed portion of the transparent substrate 2 and the width b of the phase shift layer 3a are varied. FIG.28A shows for comparison purposes the light intensity distribution for the case where no phase shift is made. FIG.28B shows the light intensity distribution for the case where a = 0.35 µm and b = 0.15 µm. FIG.28C shows the light intensity distribution for the case where a = 0.36 µm and b = 0.14 µm. FIG.28D shows the light intensity distribution for the case where a = 0.38 µm and b = 0.12 µm. FIG.28E shows the light intensity distribution for the case where a = 0.40 µm and b = 0.10 µm. FIG.28F shows the light intensity distribution for the case where a = 0.42 µm and b = 0.08 µm. An optimum contrast is obtained when b = 0.12 µm as may be seen from FIG.28D. The width b of the phase shift layer 3a must be set within 30 to 60 % of the resolution limit. FIG.29A shows a portion of the mask 1A shown in FIG.19. In FIG.29A, the phase shift layer 3a is formed at both edge portions of the opaque layer 14, and the transparent substrate 2 is exposed between the phase shift layers 3a. On the other hand, FIG.29B shows a portion of the conventional mask 450 shown in FIG.1 for comparison purposes. In FIG.29B, the mask pattern made solely of the opaque layer 451 is formed on the transparent substrate 452. When comparing FIGS.29A and 29B, it is assumed that the width a of the exposed portion of the transparent substrate 2 shown in FIG.29A is 0.55 µm (reticle size of 2.75 µm), the width b of the phase shift layer 3a is 0.15 µm (reticle size of 1.75 µm), and the width d of the exposed portion of the transparent substrate 452 shown in FIG.29B is 0.35 µm (reticle size of 1.75 µm). FIGS.30A and 30B respectively show the light intensity distributions for the cases shown in FIGS.29A and 29B. As may be seen from FIG.30A, the light intensity is approximately 100 % in this embodiment while the light intensity is approximately 65 % in the conventional case, and it can be seen that the contrast is greatly improved according to this embodiment. Therefore, it is possible to obtain satisfactory results when the exposed portion of the transparent substrate 2 is formed larger than the designed pattern which is to be formed on the wafer and the phase shift layer 3a is formed on the peripheral portion of the exposed portion of the transparent substrate 2. Next, a description will be given of a case where a black pattern is formed on a white background, by referring to FIGS.31A and 31B. FIG.31A shows a mask for forming a black pattern of 0.35 µm using the phase shift. According to the rule described above, the mask pattern is made up solely of the phase shift layer 3a which has a width c of 0.15 µm (reticle size of 0.75 µm). In this case, the light intensity of the pattern imaged on the wafer becomes as shown in FIG.32A. On the other hand, FIG.31B shows a mask for forming the black pattern of 0.35 µm without the use of the phase shift. The mask shown in FIG.31B is made up of the opaque layer 14 having a width of 0.35 µm (reticle size of 1.75 µm). In this case, the light intensity of the pattern imaged on the wafer becomes as shown in FIG.32B. As may be seen by comparing FIGS.32A and 32B, there is a sharp decrease in the light intensity when the phase shift is used; and the resolution of the black pattern is improved. FIGS.33A and 33B show the case where this embodiment is applied to the formation of the integrated circuit pattern. FIG.33A is a plan view showing a portion of the mask 1A, and FIG.33B is a plan view showing a pattern which is imaged on the wafer 11. In FIG.33A, the phase shift layer 3a is formed at the edge portion of the opaque layer 14 and at the exposed portion of the transparent substrate 2 where the fine patterns are adjacent to each other. In other words, in FIG.33A there are provided an isolated region 15 and a pattern adjacent region 16. FIG.34A shows a pattern which is formed on the wafer 11 using the mask 1A, and a description will now be given of the interference caused by the mask 1A. In FIG.34A, it is assumed that a pattern width g is fixed to 0.35 µm and a pattern width h is varied to take the values 0.35, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0 µm. In this case, according to the rule described above, the mask pattern for the case where the patterns with the pattern width h of 0.35, 0.4 and 0.5 µm are adjacent becomes as shown in FIG.34B wherein the mask pattern is made up solely of the phase shift layer 3a and the white patterns are isolated. In addition, the mask pattern for the case where the patterns with the pattern width h of 0.6, 0.7, 0.8 and 1.0 µm are isolated from each other to a certain extent becomes as shown in FIG.34C wherein a region of the opaque layer 14 exists between the white patterns. For the sake of convenience, it is assumed that the pattern width h on the wafer 11 is 0.35 µm. FIG.35A shows the light intensity distribution for the mask shown in FIG.34B when the mask dimension a is 0.55 µm (reticle size of 2.75 µm), b is 0.15 µm (reticle size of 0.75 µm) and c is 0.15 µm (reticle size of 0.75µm). On the other hand, FIG.35B shows the light intensity distribution for the conventional mask shown in FIG.21B when the mask dimension e is 0.35 µm (reticle size of 1.75 µm) and f is 0.35 µm (reticle size of 1.75 µm). When FIGS.35A and 35B are compared, it can be seen that the light intensity is approximately 85 % according to this embodiment while the light intensity is approximately 55 % in the conventional case, and the contrast is improved according to this embodiment. FIG.36 shows the light intensity distribution when the pattern width h is 0.4 µm, FIG.37 shows the light intensity distribution when the pattern width h is 0.5 µm, FIG.38 shows the light intensity distribution when the pattern width h is 0.6 µm, FIG.39 shows the light intensity distribution when the pattern width h is 0.7 µm, FIG.40 shows the light intensity distribution when the pattern width h is 0.8 µm, and FIG.41 shows the light intensity distribution when the pattern width h is 1.0 µm. As is also clear from FIGS.36 through 41, the light intensity is large and the contrast is improved according to this embodiment. The following Table shows the pattern dimension of the isolated pattern 15 shown in FIG.33A for various values of the pattern width h. h (µm) Pattern Dimension (µm) 0.35 0.35 0.40 0.36 0.50 0.35 0.60 0.35 0.70 0.35 0.80 0.35 1.00 0.35 Isolated Pattern 0.35 As may be seen from the Table, the change in the pattern dimension of the isolated pattern 15 caused by the interference is within ±0.01 µm. Accordingly, it is possible to control the pattern dimension. But in order to more accurately control the pattern dimension, it is preferable to change the values of the pattern widths a , b, c and i in FIGS.34B and 34C. Next, a description will be given of the case where this embodiment is applied to the formation of a contact hole pattern. FIG.42A shows the mask 1A for forming a contact hole of 0.35 µm. According to the rule described above, the width a of the exposed portion of the transparent substrate 2 is 0.55 µm and the width b of the phase shift layer 3a is 0.15 µm. FIG.43A shows the light intensity distribution for this case using the mask 1A shown in FIG.42A. On the other hand, FIG.42B shows the conventional mask for forming the contact hole of 0.35 µm without the phase shift. In this case, the width ℓ of the exposed portion of the transparent substrate 2 is 0.35 µm. FIG.43B shows the light intensity distribution for this case using the mask shown in FIG.42A. As may be seen from FIGS.43A and 43B, the contrast of the contact hole can be greatly improved according to this embodiment. Next, a description will be given of first and second embodiments of the mask producing method according to the present invention, by referring to FIGS.44A through 44F and FIGS.45A through 45E. In FIGS.44A through 44F and FIGS.45A through 45E, those parts which are essentially the same as those corresponding parts in FIGS.14 and 15 are designated by the same reference numerals, and a description thereof will be omitted. The transparent substrate 2 shown in FIG.44A is made of quartz, for example. The opaque layer 14 is formed on the transparent substrate 2 as shown in FIG.44B. For example, the opaque layer 14 is made of Cr and is formed to a thickness of 50 to 80 nm. A resist material 3 such as an EB resist is formed on the opaque layer 14. The resist material is subjected to a usual mask production process (not shown) including the electron beam drawing, developing, etching and resist removal steps. Thereafter, a pattern 20 of the opaque layer 14 is formed as shown in FIG.44C. The resist material 3 which is an EB resist is formed on the pattern 20, and the resist pattern 4 shown in FIG.44D is formed by electron beam drawing and developing steps. Furthermore, the phase shift layer 3s is formed on the resist pattern 4 as shown in FIG.44E. For example, the phase shift layer 3a is made of a silicon oxide and has a thickness of 0.388 µm. Next, the resist material 3 is removed by use of a removal agent and the mask pattern layer 5 of the opaque layer 14 and the phase shift layer 3a is formed as shown in FIG.44F. In the embodiment shown in FIG.45, the pattern 20 of the opaque layer 14 is formed similarly to the steps shown in FIGS.44A through 44C. For example, the opaque layer 14 is made of Cr and has a thickness of 50 to 80 nm. A stopper layer 21 is formed on the pattern 20 as shown in FIG.45B. The stopper layer 21 acts as a stopper when making a plasma etching of an oxide layer on the pattern 20. For example, the stopper layer 21 is formed on the pattern 20 by sputtering a thin film of aluminum oxide which acts as a stopper for the plasma etching which uses CF 4 . The phase shift layer 3a is formed on the stopper layer 21 as shown in FIG.45C to a desired thickness. For example, the phase shift layer 3a is made of silicon oxide. The desired thickness of the phase shift layer 3a is set to 0.388 µm in this embodiment by use of the formula (3) so as to make a phase shift of 180°. The resist material 3 is formed on the phase shift layer 3a, and the resist pattern 4 is formed as shown in FIG.45D by electron beam drawing and developing steps. Then, the exposed phase shift layer 3a is etched by the CF 4 plasma and the resist material 3 is removed so as to form the mask pattern layer 5 which comprises the phase shift layer 3a as shown in FIG.45E. Next, a description will be given of a third embodiment of the mask producing method according to the present invention, by referring to FIGS.46A through 46E. This embodiment of the mask producing method produces a second embodiment of the mask according to the present invention shown in FIG.46E- The second embodiment of the mask has a designed pattern having a width of 1 µm, for example, and has a structure for shifting the phase of the light wherein a region corresponding to the phase shift pattern of the glass substrate is located at a position deeper than that of the designed pattern. In FIG.46A, a glass substrate 31 is made of quartz and has a thickness of 2 to 3 mm. An opaque layer 32 is formed on the glass substrate 31. For example, the opaque layer 32 is made of Cr and is formed on the glass substrate 31 by a sputtering CR to a thickness of 500 to 1000 Å. In addition, an aperture pattern 33 is formed in the opaque layer 32 by a normal lithography using the electron beam exposure. The aperture pattern 33 has a width of 0.85 µm in the case of the designed pattern having the width of 0.35 µm. Next, in FIG.46B, a conductor layer 34 is formed on the glass substrate 31 for preventing charge up at the time of the electron beam exposure. For example, the conductor layer 34 is made of MoSi 2 and is sputtered on the glass substrate 31 to a thickness of 200 to 300 Å. Thereafter, a negative EB resist layer 35 is formed on the glass substrate 31 and prebaked. In addition, the pattern having the width of 0.55 µm is subjected to an electron beam exposure approximately above the central portion of the aperture pattern 33 in the opaque layer 32. An exposed portion of the EB resist layer 35 is denoted by 35A. Then, as shown in FIG.46C, a normal developing is made to selectively leave the exposed resist layer 35A on the designed pattern forming region. In addition, an etching process is carried out using a gas mixture of CCl 4 and O 2 so as to selectively remove the exposed conductor layer 34 by using the exposed resist layer 35A as a mask. In FIG.46D, the exposed resist layer 35A and the opaque layer 32 are used as masks and an RIE which uses a gas mixture of CF 4 and O 2 is carried out on the top surface of the glass substrate 31 which is exposed in correspondence with phase shift patterns. Phase shift patterns 37A and 37B are formed on respective sides of a pattern 36 having a width of 0.55 µm on the glass substrate 31. The surface of the pattern 36 coincides with the top surface of the glass substrate 31. The phase shift patterns 37A and 37B have a width of 0.15 µm and has a top surface which is lower than the surface of the pattern 36. In order to shift the phase of the light which is transmitted through the phase shift .patterns 37A and 37B by 180° with respect to the light which is transmitted through the pattern 36, the phase shift patterns 37A and 37B are etched to a depth D. When the i-ray having the wavelength of 365 nm is used for the exposure and the refractive index of glass making up the glass substrate 31 is 1.54, the depth D in this case is approximately 0.36 µm from the formula (1). Next, in FIG.46E, the exposed resist layer 35A is removed by a normal ashing process. In addition, the conductor layer 34 is removed by a dry etching processing using a gas mixture of CCl 4 and O 2 . As a result, a mask 1C is completed. This mask 1C is the second embodiment of the mask according to the present invention. FIG.47 shows a phase profile of the light transmitted through the mask 1C in correspondence with the mask 1C shown in FIG.46E. In FIG.47, i a denotes the light transmitted through the central portion and i b denotes the light transmitted through the phase shift patterns 37A and 37B. According to the mask 1C, the phase shift pattern is made deep by using the negative resist at the time of patterning the phase shift pattern. However, when a positive resist is used the central portion is made deeper than the phase shift pattern. A third embodiment of the mask according to the present invention has such a structure and may be produced by steps similar to those shown in FIGS.46A through 46E. Next, a description will be given of a fourth embodiment of the mask producing method according to the present invention, by referring to FIGS.48A through 48C. In this embodiment, a mask 1D shown in FIG.48C is produced. In FIGS.48A through 48C, those parts which are essentially the same as those corresponding parts in FIGS.46A through 46E are designated by the same reference numerals, and a description thereof will be omitted. In FIG.48A, the opaque layer 32 is formed on the glass substrate 31, and the aperture pattern 33 is formed in the opaque layer 32. The aperture pattern 33 has a width of 0.85 µm. The conductor layer 34 for preventing charge up is formed on the glass substrate 31, and a positive EB resist layer 38 is thereafter formed on the glass substrate 31. Then, an electron beam exposure is made with respect to the designed pattern which is located on the resist layer 38 at the central portion of the aperture pattern 33 in the opaque layer 32. An exposed region is denoted by 38A. In FIG.48B, the exposed region 38A of the resist layer 38 is removed by a developing process. A dry etching using a gas mixture of CCl 4 and O 2 removes the conductor layer 34 exposed at an aperture 39 in the resist layer 38. An RIE using a gas mixture of CF 4 and O 2 etches the top surface of the glass substrate 31 which is exposed within the aperture 39 similarly to the above described embodiment. The glass substrate 31 is etched to the depth D which is approximately 0.36 µm. In FIG.48C, the resist layer 38 is removed by an ashing process, and the conductor layer 34 is removed by a dry etching using a gas mixture of CCl 4 and O 2 . Accordingly, the mask 1D is completed. FIG.49 shows a phase profile of the light transmitted through the mask 1D in correspondence with the mask 1D shown in FIG.48C. In FIG.49, i c denotes the light transmitted through the central poriton and i d denotes the light transmitted through the phase shift patterns 41A and 41B. Next, a description will be given of two mask producing methods not in accordance with the present invention but useful for understanding fifth to seventh embodiments of the mask producing method explained later. In FIG.50A, a transparent substrate 51 is made of glass, quartz or the like. A phase shift layer 52 is formed on the transparent substrate 51 as shown in FIG.50B. For example, the phase shift layer 52 is made of silicon oxide and is formed by chemical vapor deposition (CVD), sputtering, a spin-on-glass (SOG) technique or the like. In this method, the phase shift layer 52 has a thickness of 3900 Å so as to invert the phase of the light. In FIG.50C, an opaque layer 53 is formed on the phase shift layer 52. For example, the opaque layer is made of Cr and has a thickness of 700 to 1000 Å. Next, an EB resist is formed on the opaque layer 53. Electron beam drawing, developing and etching processes are carried out with respect to the opaque layer 53. As a result, a pattern 53a of the opaque layer 53 is formed as shown in FIG.50D. The pattern 53a of the opaque layer 53 is used as a mask when etching the phase shift layer 52 as shown in FIG.50E. In this method, the phase shift layer is an oxide layer and an RIE using a gas mixture of CF 4 and CHF 3 is used for the etching. Then, an EB resist layer 54 is formed as shown in FIG.50F, and an unwanted portion 53b of the opaque layer 53 is exposed by electron beam drawing and developing processes. Finally, the EB resist layer 54 is removed and the unwanted portion 53b of the opaque layer 53 is removed by an etching so as to complete a mask 1E. FIG.50H shows a plan view of the mask 1E in correspondence with FIG.50G. FIGS.51A through 51G show a variation of the above method. Corresponding parts to those in FIGS.50A through 50H are designated by the same reference numerals, and a description thereof will be omitted. Steps shown in FIGS.51A through 51C respectively correspond to the steps shown in FIGS.50A through-50C, but the phase shift layer 52 is omitted in this method. In PIG.51D, the pattern 53a of the opaque layer 53 is used as a mask when etching the transparent substrate 51. In this method, an RIE using a gas mixture of CF 4 and CHF 3 is used for the etching, and the transparent substrate 51 is etched to a predetermined depth. When inverting the phase of the light, the predetermined depth is set to 3900 Å. Then, an EB resist layer 54 is formed as shown in FIG.51E, and the unwanted portion 53b of the opaque layer 53 is exposed by electron beam drawing and developing processes. Finally, the EB resist layer 54 is removed the unwanted portion 53b of the opaque layer 53 is removed by an etching so as to complete the mask 1C shown in FIG.51F. FIG.51G shows a plan view of the mask 1C in correspondence with FIG.51F. In the above two mask producing methods, the opaque layer is used as a mask when etching the phase shift layer or the transparent substrate. Hence, it is possible to prevent a positioning error between the pattern of the opaque layer and the phase shift pattern by a self-alignment. Next, a description will be given of a fifth embodiment of the mask producing method according to present invention, by referring to FIGS.52A through 52F. In FIGS.52A through 52F, those parts which are essentially the same as those corresponding parts in FIGS.51A through 51G are designated by the same reference numerals, and a description thereof will be omitted. In FIG.52A, the opaque layer 53 is formed on the transparent substrate 51 and a pattern is formed by an electron beam drawing after forming an EB resist layer (not shown) on the opaque layer 53. This pattern is developed and the opaque layer 53 is patterned by an etching. For example, the opaque layer 53 is made of Cr and is sputtered on the transparent substrate 51. The EB resist layer is removed after the patterning. In FIG.52B, the pattern 53a of the opaque layer 53 is used as a mask when etching the transparent substrate 51 to a predetermined depth. The exposed transparent substrate 51 is etched by a parallel plate type RIE apparatus with a high-frequency excitation of 13.56 MHz. For example, the etching is carried out with a power of approximately 0.1 W/cm 2 at a pressure of 0.1 to 0.05 Torr using a gas mixture of CF 4 or CHF 3 and O 2 as the etchant gas. The etching depth is controlled to λ/2(n - 1) by detecting the depth using an end point detector. When A = 0.365 µm, λ/2(n - 1) = 0.39 µm. In FIG.52C, a positive resist layer 55 (for example, OFPR-800) is formed on the transparent substrate 51 and the pattern 53c to a thickness of approximately 1.0 µm after the etching of the transparent substrate 51 and prebaked at a temperature of 100°C. In FIG.52D, a light of the resist exposure region such as a monochromatic light of 40 to 60 mJ/cm 2 and a wavelength of 436 nm illuminates the transparent substrate 51 in its entirety from a back surface of the transparent substrate 51 so as to expose only the resist layer 55 on the transparent substrate 51. Thereafter, the transparent substrate 51 is submerged in a solution with 2.38 % of TMAH for 40 seconds, and a developing process is carried out so that only the resist layer 55 remains on the pattern 53c. Then, rinse and dry processes are carried out. After the resist pattern is formed, a side etching is carried out to remove a portion of the pattern 53c. The side etching quantity is controlled by the etching time so that approximately 0.4 to 0.8 µm of the opaque pattern 53 is removed on each side as shown in FIG.52E. Finally, the resist layer 55 remaining on the opaque layer 53 is removed by an O 2 ashing in FIG.52F, and a mask-which is substantially the same as the mask 1D shown in FIG.48C is completed. Next, a description will be given of a sixth embodiment of the mask producing method according to the present invention, by referring to FIGS.53A through 53F. In FIGS.53A through 53F, those parts which are essentially the same as those corresponding parts in FIGS.52A through 52F are designated by the same reference numerals, and a description thereof will be omitted. Steps shown in FIGS.53A through 53C respectively correspond to the steps shown in FIGS.52A through 52C. However, in FIG.53B, the transparent substrate 51 is etched λ/2(n - 1). When λ = 0.365 µm, λ/2(n - 1) = 0.39 µm. In FIG.53D, the light of the resist exposure region illuminates the transparent substrate 51 in its entirety from the back surface of the transparent substrate 51 so as to expose only the resist layer 55 on the transparent substrate 51. Thereafter, a developing process is carried out so that only the resist layer 55 remains on the pattern 53c. Then, in this embodiment, an overexposure is made, an overdeveloping is made, or an ashing is made for a short time after the developing and drying so as to partially expose the opaque layer 53 Next, as shown in FIG.53E, the exposed opaque layer 53 is etched using the resist layer 55 as an etching mask. In FIG.53F, the remaining resist layer 55 on the opaque layer 53 is removed by an O 2 ashing. As a result, a mask which is substantially the same as the mask 1D is completed. Next, a description will be given of an seventh embodiment of the mask producing method according to the present invention, by referring to FIGS.54A through 54E. In FIGS.54A through 54E, those parts which are essentially the same as those corresponding parts in FIGS.52A through 52F are designated by the same reference numerals, and a description thereof will be omitted. A step shown in FIG.54A may be the same as the step shown in FIG.52A. But in this embodiment, the resist layer 55 is formed on the transparent substrate 51 as shown in FIG.54B without etching the transparent substrate 51. In FIG.54C, only the resist layer 55 on the transparent substrate 51 is removed similarly to the step shown in FIG.52D. Thereafter, the resist layer 55 is used as an etching mask to etch the transparent substrate 51 λ/2(n - 1). When λ = 0.365 µm, λ/2(n-1) = 0.39 µm.. In FIG.54D, a portion of the pattern 53c of the opaque layer 53 is removed by a side etching. A step shown in FIG.54E may be the same as the step shown in FIG.52F. According to the embodiments described in conjunction with FIGS.52A through 54E, there is no need to match the positions of the pattern of the opaque layer and the phase shift pattern. Thus, it is possible to produce a mask having a pattern with a high accuracy and no positioning error. In addition, the adhesion between the phase shift layer and the substrate, the cleaning and the like may be affected depending on the material used for the phase shift layer when the phase shift layer and the substrate are independent. However, according to these embodiments, a portion of the transparent substrate itself is used as the phase shift layer or phase shift region, and the material of the phase shift layer does not become a problem. Accordingly, there is no need to take into consideration the stability and durability of the material used for the phase shift layer, and it is possible to easily form fine patterns exceeding the resolution limit of the exposure apparatus. Next, a detailed description will be given of the pattern forming method according to the present invention. FIGS.55A through 55C are diagrams for explaining the pattern forming method using the edge portion of the phase shift layer of the mask, and corresponds to FIGS.10A through 10C. In FIGS.55A through 55C, those parts which are essentially the same as those corresponding parts in FIG.12 are designated by the same reference numerals, and a description thereof will be omitted. The phase shift layer having a refractive index n and a thickness t is formed on the transparent substrate 2 so as to constitute the mask 1. The pattern of the mask 1 is imaged on the photoresist layer 10 on the semiconductor wafer 11 via the imaging lens system 9. The desired pattern is imaged by utilizing the edge portion of the phase shift layer 3a. The light incident to the mask 1 is transmitted through the phase shift layer 3a. Since the refractive index n of the phase shift layer 3a is different from that of air or vacuum, the phase of the transmitted light is shifted. The phase shift quantity S is described by S = (n - 1)t/λ or 2π(n - 1)t/λ in radians. For the sake of convenience, it is assumed hereunder that the phase shift quantity S is π so as to invert the phase of the transmitted light. As shown in FIG.55B, the phase of the electrical vector E of the light transmitted through the mask 1 is inverted in correspondence with the pattern of the phase shift layer 3a. When the light having the electrical vector distribution shown in FIG.55B illuminates the photoresist layer 10 and is absorbed thereby, the light intensity distribution on the photoresist layer 10 becomes proportional to E 2 . Hence, as shown in FIG.55C, a black pattern PB having a narrow width is formed at the position where the phase transition occurs. In other words, when the sign of the electrical vector E changes in FIG.55B, there is a point where the electrical vector E becomes zero. The light intensity distribution is proportional to E 2 , and a minimum value of the light intensity P is zero at the point where the electrical vector E is zero. Accordingly, it is possible to obtain a clear black pattern. Generally, when the phase shift layer 3a shifts the phase of the transmitted light, it is possible to obtain a pattern in which the phase shift is spatially distributed. When the phase shift is 180°, the light intensity P at the position where the 180° phase shift occurs is always zero, and a time integral thereof is also zero. Hence, the light intensity P at the edge portion of the phase shift layer 3a is zero. When the phase shift is other than 180°, the sign of the electrical vector E becomes the same or the opposite depending on the time, and the time integral of the light intensity P does not become zero. A description will now be given of an imaging using the edge portion of the phase shift layer, by referring to FIGS.56A and 56B. FIG.56A shows the light intensity distribution for a case where the phase shift layer makes a 180° phase shift, that is, inverts the phase of the transmitted light. In FIG.56A, a region on the left of the abscissa 0.0 corresponds to a region in which the phase shift quantity is 180°, and this region is adjacent to an aperture portion on the right with the phase shift quantity of zero at the position 0.0. FIG.56A shows the light intensity distribution at one edge portion of the phase shift layer. The light intensity decreases to approximately zero at the central portion of the pattern corresponding to the end of the phase shift layer. By utilizing this imaging principle, it is possible to obtain a line pattern exceeding the conventional resolution limit. FIG.56B shows the light intensity distribution for a case where the phase shift layer makes a 90° phase shift. When the phase shift quantity is 90°, the phase of the light transmitted through the phase shift layer and the phase of the light transmitted through the aperture portion may coincide or be inverted. Hence, the minimum value of the light intensity does not become zero when the time integral thereof is taken. In FIG.56B, the minimum value of the light intensity is approximately 0.5 or greater and is in the order of ½ the light intensity which is approximately 1.0 at the uniform portion. When the developing level is taken less than this light intensity, it is possible to carry out the developing in which the pattern shown is neglected. In addition, when the developing level is taken at an intermediate level between the minimum value and the uniform value of the light intensity, the pattern is developed at the central portion thereof. In the described embodiments, the partial coherency σ is set to 0.5. However, the partial coherency σ is not limited to 0.5 and may be set within the range of 0.3 ≤ σ ≤ 0.7. In addition, the light which is used for the exposure is not limited to the i-ray. Moreover, the light use for the exposure may illuminate the mask from the side of the transparent substrate provided within the phase shift layer or from the other side of the transparent substrate not provided with the phase shift layer. The materials used for the transparent substrate and the phase shift layer are of course-not limited to those of the described embodiments. For example, the transparent substrate may be made of any material as long as the transparent substrate is transparent with respect to the light which is used for the exposure. Furthermore, the phase shift layer may be made of a material selected from a group including SiO 2 , Al 2 O 3 and MgF 2 . The masks described in the embodiments include reticles. Accordingly, the pattern forming method according to the present invention is not limited to the application to the formation of patterns of semiconductor devices, and may of course be applied to the formation of patterns of masks and reticles.

Claims: A mask for use in exposure of a semiconductor wafer, including: transparent portions (2, 31, 51) for transmitting light used in the exposure, formed by a transparent layer and corresponding to first areas of said wafer, these being areas which are to be exposed; opaque portions (14, 32, 53) for masking said light, formed by an opaque layer and corresponding to second areas of said wafer adjacent to said first areas; and phase shift portions (3a, 37A, 37B, 41A, 41B) for shifting the phase of said light so as to improve the contrast between the first and second areas, formed by a phase shift layer (3a) formed over the transparent layer and the opaque layer or by a phase shift region in the transparent layer; characterized in that in at least one part of the mask, said phase shift portions are formed only along edges of said opaque portions (14, 32, 53). The mask as claimed in claim 1, wherein the phase shift portions (3a, 37A, 37B, 41A, 41B) produce a phase shift of said light within a range of 180° ± 30%. The mask as claimed in claim 1 or 2, wherein a said phase shift portion (3a, 37A, 37B, 41A, 41B) is provided at an edge portion of said opaque layer (14, 32, 53) for forming a line pattern when making the exposure. The mask as claimed in claim 1, 2, or 3, wherein a said phase shift portion (3a, 37A, 37B, 41A, 41B) is provided at a pair of edge portions of said opaque layer (14, 32, 53) for forming a pattern of arbitrary width when making the exposure. The mask as claimed in claim 4, wherein a boundary between said phase shift portions (3a, 37A, 37B, 41A, 41B) and said transparent portions (2, 31, 51) is located on an inner side of a designated black pattern. The mask as claimed in any of claims 1 to 5, wherein the phase shift portions (3a, 37A, 37B, 41A, 41B) include a first portion (3a1) and a second portion (3a2) which have mutually different thicknesses. The mask as claimed in any preceding claim, wherein the transparent layer (2, 31, 51) is made of quartz or glass. The mask as claimed in any preceding claim, wherein the phase shift layer (3a) is made of SiO 2 , Al 2 O 3 , or MgF 2 . The mask as claimed in any preceding claim, wherein the opaque layer (14, 32, 53) is made of Cr. A method of producing a mask as claimed in any preceding claim, comprising the steps of: forming said opaque layer (14, 20) on said transparent layer (2) ; forming a resist pattern (4) which is made of a resist material, on said opaque layer; forming the phase shift layer (3a) which transmits the light on said resist pattern layer, or forming the phase shift region in the transparent layer, to a predetermined thickness to form the phase shift portions; and removing said resist pattern layer, whereby said phase shift portions remain only along an edge portion of said opaque layer in at least one part of the mask. A method of producing a mask as claimed-in any one of claims 1 to 9, comprising the steps of: forming said opaque layer (14, 20) on said transparent layer (2); forming a stopper layer (21) on said opaque layer; forming the phase shift layer (3a) on said opaque layer to a predetermined thickness; forming a resist pattern (4) made of a resist material, on said phase shift layer; and etching said phase shift layer using said stopper layer as an etching stopper and said resist pattern as an etching mask, whereby said phase shift layer remains on said stopper layer only along an edge portion of said opaque layer in at least one part of the mask. The method as claimed in claim 11, wherein the stopper layer is made of Al 2 O 3 . A pattern forming method for forming a pattern on a semiconductor wafer (11) using a mask as claimed in any of claims 1 to 9, the method comprising illuminating the mask (1, 1A-1G) by light from a light source, and projecting the light transmitted through the mask onto a photoresist layer (10) formed on the wafer by use of a lens system (9), whereby the photoresist layer is developed in said first areas of the wafer corresponding to the transparent portions (2) of the mask. The pattern forming method as claimed in claim 13, wherein the light is projected at different focal positions on the photoresist layer (10) depending on a concavoconvex surface structure of said photoresist layer. The pattern forming method as claimed in claim 13 or 14, wherein a black pattern (PB) is projected on the photoresist layer (10) which has a light intensity lower than that at other portions of the photoresist layer, using interference generated in the vicinity of an edge portion of said opaque layer (14, 32, 53). The pattern forming method as claimed in claim 13 or 14, wherein a white pattern is projected on the photoresist layer (10) which has a light intensity higher than that at other portions of the photoresist layer using interference generated in the vicinity of an edge portion of the opaque layer (14, 32, 53). The pattern forming method as claimed in claim 14, 15, or 16, wherein said light is partially coherent with a coherency in the range 0.3 to 0.7. The pattern forming method as claimed in any of claims 13 to 17, wherein the mask has mutually adjacent phase shift portions with mutually different phase shift quantities, the difference in phase shift quantities being in the range 90° to 180° , and wherein a black pattern is generated on said resist by a boundary of the adjacent phase shift portions with a 180° phase shift quantity difference, and no pattern is generated on said resist by a boundary of the phase shift portions with a phase shift quantity difference of 90° or less. The pattern forming method as claimed in any of claims 13 to 17, wherein the mask has mutually adjacent phase shift portions with mutually different phase shift quantities, the difference in phase shift quantities being in the range 90° ± 30% to 180° ± -30%.

Inventors: KAWABATA TOSHIAKI, NAKAGAWA KENJI, SUMI KAZUHIKO, TAGUCHI MASAO, YAMAGUCHI SEIICHIRO, YANAGISHITA YUICHIRO, KAWABATA, TOSHIAKI, NAKAGAWA, KENJI, SUMI, KAZUHIKO, TAGUCHI, MASAO, YAMAGUCHI, SEIICHIRO, YANAGISHITA, YUICHIRO, KAWABATA, TOSHIAKI, C/O FUJITSU LTD., NAKAGAWA, KENJI, C/O FUJITSU LTD., SUMI, KAZUHIKO, C/O FUJITSU LTD., TAGUCHI, MASAO, C/O FUJITSU LTD., YAMAGUCHI, SEIICHIRO, C/O FUJITSU LTD., YANAGISHITA, YUICHIRO, C/O FUJITSU LTD.
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G03F1
IPC codes: G03F1