Mask, mask producing method and pattern forming method using mask

Abstract: A mask includes a transparent layer (2) which is transparent with respect to a light which is used for an exposure, and a mask pattern layer (5) which is formed on the transparent layer. At least a portion of the mask pattern layer (5) is made up solely of a phase shift layer (3a) for transmitting the light, so that a phase shift occurs between a phase of the light transmitted through the phase shift layer and a phase of the light transmitted through a portion of the mask having no phase shift layer.

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. EP-A-0 383 534 is relevant to the present application only under Article 54(3) EPC. It discloses a mask having transparent portions, light-shielding portions, and phase-shifting portions. The light-shielding portions are formed by a separate opaque layer. EP-A-0 090 924 discloses a mask according to the preamble of accompanying claim 1. Transparent portions are separated by light-shielding portions, with alternate transmitting portions being covered by phase-shifting material. Again, the light-shielding portions are formed by a separate opaque layer. EP-A-0 234 547 discloses a mask which does not employ phase-shifting. To ensure a smooth surface of the mask, a transparent layer is etched to form recesses, and light-shielding portions are formed in the recesses. According to a first aspect of the present invention, there is provided a mask for use in exposure of a semiconductor wafer, comprising a transparent layer provided with a mask pattern defining transparent portions for transmitting light used in the exposure, corresponding to first areas of said wafer, these being areas which are to be exposed, said mask pattern further defining mask portions for causing masking of said light in second areas of said wafer adjacent to said first areas; said mask further comprising phase shift portions for shifting the phase of said light within a range of 180° ± 30% so as to improve the contrast between the first and second areas; characterized in that at least some of the mask portions are formed by a phase shift layer used to form the phase shift portions. According to a second aspect of the present invention, there is provided a method of producing the above mask, comprising the steps of: providing a substrate made of a transparent layer; forming a resist layer on the substrate and pattering the resist layer to leave behind resist in areas where said transparent portions of the mask are to be formed; depositing the phase shift layer over the patterned resist layer; and removing the remaining resist, thereby removing the phase shift layer over the transparent portions, and leaving behind the phase shift layer over the phase shift portions and said at least some of the mask portions of the mask. Further aspects of the invention are methods of exposing a wafer as defined in accompanying claims 14 and 18. Reference is made, by way of example, to the accompanying drawings in which: 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 to 3D are diagrams for explaining a conventional pattern forming method; FIGS.4A to 4D show light intensity distributions of the prior art; FIGS.5A to 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 first embodiment of a mask according to the present invention; FIGS.10A to 10C are diagrams for explaining the light intensity at one edge portion of a phase shift layer; FIGS.11A to 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 first embodiment of the mask; FIG.13 is a diagram for explaining a first embodiment of a pattern forming method according to the present invention; FIGS.14A to 14E are cross sectional views for explaining a first embodiment of a mask producing method according to the present invention; FIGS.15A to 15F are cross sectional views for explaining a second embodiment of the mask producing method according to the present invention; FIG.16 is a plan view showing a mask pattern; FIG.17A and 17B are diagrams for explaining the light intensity distribution in FIG.16; FIGS.18A to 18C are diagrams for explaining a second embodiment of the pattern forming method according to the present invention; FIG.19 is a cross sectional view showing a second embodiment of the 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 third embodiment of the pattern forming method according to the present invention; FIG.23 is a diagram for explaining the second embodiment of the mask applied to the line-and-space pattern; FIGS.24A to 24D are diagrams for explaining the light intensity distribution for different dimension conversion quantities and widths of the phase shift layer; FIG.25 is a diagram for explaining the conventional light intensity distribution in correspondence with FIG.23; FIGS.26A to 26F are diagrams for explaining the light intensity distribution for the case where a KrF excimer laser is used in FIG.23; FIGS.27A and 27B are diagrams for explaining the formation of a black pattern on a white background; FIGS.28A and 28B are diagrams for explaining the light intensity distribution in FIGS.27A and 27B; FIGS.29A and 29B are diagrams for explaining the case where the second embodiment of the mask is applied to the formation of an integrated circuit pattern; FIGS.30A and 30B are diagrams showing a portion of the pattern shown in FIG.19; FIGS.31A to 31C are diagrams for explaining the pattern forming method using the mask, FIGS.32A and 32B show light intensity distributions for explaining the imaging using the edge portion of the phase shift layer; FIGS.33A and 33B are diagrams for explaining the formation of a loop shaped pattern in a fourth embodiment of the pattern forming method according to the present invention; FIGS.34A and 34B are diagrams for explaining the formation of an open shaped pattern in the same method; FIGS.35A and 35B are diagrams for explaining the formation of a point pattern in the same method; FIGS.36A to 36C are diagrams for explaining the formation of an intersecting line pattern in the same method; FIGS.37A and 37B are diagrams for explaining the formation of a T-shaped pattern in the same method; FIGS.38A to 38D are diagrams for explaining the formation of an interconnection pattern in the same method; FIGS.39A to 39C are diagrams for explaining the intersecting pattern; FIGS.40A to 40C are diagrams for explaining the interconnection pattern; FIGS.41A to 41C are diagrams for explaining the interconnection pattern; FIGS.42A to 42C are diagrams for explaining an oval pattern (point pattern); FIGS.43A and 43B are diagrams for explaining the line-and-space pattern; FIGS.44A and 44B are diagrams for explaining a mixed pattern of various sizes; FIG.45 is a cross sectional view showing another embodiment of the mask according to the present invention; and FIGS.46A to 46E, FIGS.47A to 47D and FIGS.48A to 48D are diagrams for explaining the light intensity distribution for phase shift layers having different thicknesses. A description will be given of a first embodiment of a mask according to the present invention, 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, 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 to 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 to 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 to 11C, those parts which are the same as those corresponding parts in FIGS.10A to 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 embodiment, 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 first embodiment of a pattern forming method according to the present invention, 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 ray 7a which is transmitted through the mask pattern layer 5 of the mask and a light ray 7b which is transmitted through a portion of the mask 1 not provided with the mask pattern layer 5. The phase of the light ray 7a differs by 180° with respect to the phase of the light ray 7b. The light rays 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 first and second embodiments of a mask producing method according to the present invention, 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 or 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 or 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, 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 embodiment shown in FIGS.15A to 15F, the transparent substrate 2 shown in FIG.15A is the same as that shown in FIG.14A but an aluminium oxide thin film 12 is formed on the transparent substrate 2 as shown in FIG.15B. As shown in FIG.15C, silicon oxide which constitutes the phase shift layer 3a is sputtered on the thin film 12 to the 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 embodiments 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 written 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 embodiments because 180° is an optimum phase shift quantity for forming a pattern by 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 reduction lens system, the 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 embodiments, 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 second 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 layers 3b, formed within the fine pattern region 16, are in accordance with the present invention. Other phase shift layers 3a, formed at the edges of the fine pattern region 16, are not in accordance with the invention. The large pattern region 15 is patterned by the conventional method, while the fine pattern region is formed by use of the phase shift layers 3a and 3b. 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 second embodiment of the mask, partly in accordance with 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 a phase shift layer 3a (not in accordance with the invention) 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, in accordance with the present invention. 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. By way of background explanation, 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. Also by way of background explanation, 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 the controlling the width W. Next, a description will be given of a third 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 ray 7a transmitted through the phase shift layer 3a is shifted and is inverted with respect to the phase of the light ray 7b which is transmitted through only the transparent substrate 2. The light rays 7a and 7b are imaged on the photoresist layer 10 on the wafer 11 via the imaging lens system 9. In this case, the light rays 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). FIG.23 shows the second embodiment of the mask applied to the so-called line-and-space pattern. In FIG.23, 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.24A to 24D 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.24A shows the light intensity distribution for the case where a = 0.45 µm and b = 0.25 µm. FIG.24B shows the light intensity distribution for the case where a = 0.50 µm and b = 0.20 µm. FIG.24C shows the light intensity distribution for the case where a = 0.55 µm and b = 0.15 µm. FIG.24D 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.24A to 24D, the peak of the light intensity decreases when the width b of the phase shift layer 3b 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 0.288 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.25 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.25 is a reticle which has a pattern five times larger than the resist pattern to be exposed on the wafer, FIG.25 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.25, the light intensity is approximately 55 % while the light intensity is approximately 80 % in FIG.24C. Hence, it is readily seen that the contrast is greatly improved in this embodiment. When the width b of the phase shift layer 3a is 30 to 60 % of the resolution limit, this means that the phase shift pattern should be smaller than the designed pattern by 20 to 35 % (one side) of the resolution limit. When the i-ray is used and the numerical aperture NA of the lens system is 0.5, the phase shift pattern is 0.070 to 0.123 µm (one side) smaller than the designed pattern. Furthermore it is known from experience that the width of the phase shift pattern for the case where the pattern of the phase shift layer is arranged between the opaque layer and the exposed portion of the transparent substrate is desirably 1.0 to 1.5 times the reduced width which is 20 to 35 % (one side) of the resolution limit. This width is 0.070 to 0.185 µm when the i-ray is used and the numerical aperture NA of the lens system is 0.5. 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.26A to 26F show the light intensity distributions 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.26A shows for comparison purposes the light intensity distribution for the case where no phase shift is made. FIG.26B shows the light intensity distribution for the case where a = 0.35 µm and b = 0.15 µm. FIG.26C shows the light intensity distribution for the case where a = 0.36 µm and b = 0.14 µm. FIG.26D shows the light intensity distribution for the case where a = 0.38 µm and b = 0.12 µm. FIG.26E shows the light intensity distribution for the case where a = 0.40 µm and b = 0.10 µm. FIG.26F 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.26D. The width b of the phase shift layer 3a must be set within 30 to 60 % of the resolution limit. Next, a description will be given of a case where a black pattern is formed on a white background, by referring to FIGS.27A and 27B. FIG. 27A shows a mask for forming a black pattern of 0.35 µm using the phase shift. According to the present invention, the mask pattern is made up solely of the phase shift layer 3a, and in this example 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.28A. On the other hand, FIG.27B shows a mask for forming the black pattern of 0.35 µm without the use of the phase shift. The mask shown in FIG.27B 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.28B. As may be seen by comparing FIGS.28A and 28B, 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.29A and 29B show the case where this embodiment is applied to the formation of the integrated circuit pattern. FIG.29A is a plan view showing a portion of the mask 1A, and FIG.29B is a plan view showing a pattern which is imaged on the wafer 11. In FIG.29A, the phase shift layer 3a is formed at the edge portion of the opaque layer 14 (not in accordance with the invention), but also at the exposed portion of the transparent substrate 2 where the fine patterns are adjacent to each other (i.e. in accordance with the invention). In other words, in FIG.29A there are provided an isolated region 15 and a pattern adjacent region 16. FIG.30A 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.30A, 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.30B wherein the mask pattern is made up solely of the phase shift layer 3a in accordance with the invention and the white patterns are isolated. The following Table shows the pattern dimension of the isolated pattern 15 shown in FIG.29A 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 and c in FIG.30B. Next, a detailed description will be given of a pattern forming method according to the present invention. FIGS.31A to 31C are diagrams for explaining the pattern forming method using the edge portion of the phase shift layer of the mask, and correspond to FIGS.10A to 10C. In FIGS.31A to 31C, 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 on 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.31B, 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.31B 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.31C, 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.31B, 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 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.32A and 32B. FIG.32A 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.32A, 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 adjacent to an aperture portion on the right with the phase shift quantity of zero at the position 0.0. FIG.32A 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. In addition, it is possible to obtain similar effects if the phase difference at the edge portion of the phase shift layer is 180° ± 30°. FIG.32B 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.32B, the minimum value of the light intensity is approximately 0.5 or greater and is in the order of 1/2 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 developing in which the pattern shown is neglected. If the phase difference at the edge portion of the phase shift layer is 90° ± 15°, the pattern is neglected and not formed under normal developing conditions (developing level of 35 %). 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. Next, a description will be given of a fourth embodiment of the pattern forming method according to the present invention, by referring to FIGS.33A and 33B. FIGS.33A and 33B are diagrams for explaining the formation of a loop shaped pattern according to this embodiment. FIG.33A shows a mask pattern. A mask 61 is formed on a aperture 60 in the transparent substrate. The mask 61 has a phase shift quantity corresponding to an optical path difference of a half-wave of the incoming light, and shifts the phase of the transmitted light by 180°. When such a mask pattern is imaged, the imaged pattern becomes as shown in FIG.33B. The portion where only the aperture 60 or the phase shift layer 61 is imaged has a constant light intensity and is white. A black pattern 62 is formed at a boundary portion of the aperture 60 and the phase shift layer 61. In other words, when the light transmitted through the aperture 60 and the light transmitted through the phase shift layer 61 mix due to interference, mutually opposite phases cancel each other and the light intensity becomes zero, thereby forming a black pattern. When the phase shift layer 61 is made of silicon oxide and the i-ray from a mercury lamp is the light which is used for the exposure, the refractive index of the phase shift layer 61 is approximately 1.47 which has a difference of approximately 0.47 with respect to the refractive index of air which is approximately 1.00. The thickness of the phase shift layer 61 is approximately 0.388 µm. In order to cancel the amplitude of the mixed light due to the interference, the inverted phase, that is, 180° phase shift, is most effective. However, the phase shift quantity is not limited to 180° and the light intensity can be effectively reduced within the range of 180° ± 30%. When the phase shift layer 61 having the phase shift quantity of 180° is formed above the aperture which makes no phase shift as in the case of the mask pattern shown in FIG.33A, a black pattern is formed at the portion corresponding to the edge portion of the phase shift layer 61. In this case, the black pattern has a closed loop shape. FIGS.34A and 34B respectively show a mask pattern for forming an open shape (line segment) and an imaged pattern thereof. In FIG.34A, the phase shift layer 61 is formed above the aperture 60, and one side of the phase shift layer 61 is used for forming a line segment. A phase shift layer 64 having a phase shift quantity of 90° is formed adjacent to the remaining (unwanted) sides of the phase shift layer 61. In other words, out of the four sides of the phase shift layer 61 having the phase shift quantity of 180°, one side 61a forms a boundary with a region in which the phase shift quantity is zero, and this side 61a forms a black pattern 65 in the imaged pattern as shown in FIG.34B. The other sides 61b, 61c and 61d are adjacent to the phase shift layer 64 which has the phase shift quantity of 90°. Hence, the phase shift when crossing these sides 61b, 61c and 61d is 90°, and the decrease in the light intensity is small at the edge portion of the phase shift layer 61 corresponding to these sides 61b, 61c and 61d. The peripheral sides of the phase shift layer 64 also have the phase shift quantity of 90° and the decrease in the light intensity is similarly small. It is possible to leave only the black pattern 65 by adjusting the developing level. In this case, a gray level is formed between the white and black levels, but it is of course possible to use two or more halftones. By using a plurality of kinds of phase shift layers having different phase shift quantities, it is possible to form a pattern of an open shape such as a line segment. FIGS.35A and 35B respectively show a mask pattern for forming a dot pattern and an imaged pattern thereof. In FIG.35A, the phase shift layer 61 having the phase shift quantity of 180° is formed above the aperture 60. The phase shift layer 64 which has the phase shift quantity of 90° surrounds the phase shift layer 61 excluding one vertex portion thereof. In other words, the phase shift layer 61 is adjacent to the aperture 60 at only the one vertex portion of the phase shift layer 61. When such a mask pattern is imaged, the imaged pattern becomes as shown in FIG.35B in which a black pattern 66 is formed only at the portion where the phase shift layer 61 and the aperture 60 connect. Because the phase shift quantity is 90° at the boundary of the phase shift layers 61 and 64, the decrease in the light intensity at this boundary is small compared to that at the portion corresponding to the black pattern 66. In addition, the outer peripheral portion of the phase shift layer 64 also has the phase shift quantity of 90°, and the decrease in the light intensity is similarly small. For this reason, it is possible to develop only the black pattern 66 as the pattern and treat the other patterns in the halftones as white patterns. A description was given above with respect to cases where the pattern to be formed is a line segment pattern. But in the following, a description will be given of the formation of a pattern which has an intersection. FIGS.36A and 36B show mask patterns for forming an intersecting pattern, and FIG.36C shows an imaged pattern which is formed by the mask patterns shown in FIGS.36A and 36B. FIG.36A shows a first mask pattern. The surface of the mask pattern is divided into four quadrants in correspondence with the regions which are divided by intersecting lines. The aperture 60 which makes no phase shift is provided in the first quadrant. The phase shift layer 61 which has the phase shift quantity of 180° is provided in the two quadrants which are adjacent to the aperture 60. A phase shift layer 67 which has a phase shift quantity of 360° is provided in the remaining quadrant. In other words, the boundary between two quadrants has a phase shift quantity of 180°. FIG.36B shows a second mask pattern. Similarly to the first mask pattern shown in FIG.36A, the surface of the mask pattern shown in FIG.36B is divided into four quadrants. The aperture 60 which makes no phase shift is provided in the first quadrant. The phase shift layer 61 which has the phase shift quantity of 180° is provided in the two quadrants adjacent to the aperture 60. An aperture 68 which makes no phase shift is provided in the remaining quadrant. In this case, the boundary between two quadrants also has a phase shift quantity of 180°. When the mask patterns shown in FIGS.36A and 36B are imaged, an imaged pattern shown in FIG.36C is obtained. That is, a portion having a uniform phase is imaged as a white pattern, and a portion having the 180° phase shift is imaged as a black pattern 69. The intersecting lines are not limited to straight lines and may be curved. FIGS.37A and 37B respectively show a mask pattern for forming a T-shaped pattern and an imaged pattern thereof. In FIG.37A, phase shift layers 61a and 61b having the phase shift quantity of 180° are formed adjacent to the aperture 60 which makes no phase shift. In addition, the aperture 68 which makes no phase shift is formed above the phase shift layer 61a so as to form a boundary having the phase shift quantity of 180°. As a result, a black pattern in FIG. 37B is imaged when this mask pattern is imaged. Furthermore, since a black pattern will be imaged if the phase shift layers 61b and 68 are directly adjacent to each other and form a boundary having the phase shift quantity of 180°, a phase shift layer 71 which has a phase shift quantity of 90° is formed between the phase shift layers 61b and 68. In other words, the decrease in the light intensity is relatively small at the boundary of the phase shift layer 71 because the phase shift quantity is only 90°. By appropriately adjusting the developing threshold value, such a decrease in the light intensity can be developed as a white pattern. As a result, a black pattern 72 shown in FIG.37B is imaged. In FIG.37A, there is a gap between the aperture 68 and the phase shift layer 61b. However, such a gap may be reduced if needed. In semiconductor devices, there is often a need to provide a wide region for making a contact at an intermediate part of an interconnection pattern. FIGS.38A to 38C show mask patterns for forming such an interconnection pattern, and FIG.38D shows an imaged pattern thereof. In FIG.38A, the aperture 60 which makes no phase shift and the phase shift layer 61 which has the phase shift quantity of 180° are adjacent to each other and form a linear boundary. A phase shift layer having a phase shift quantity of 180° is formed within the aperture 60 at the central portion of the boundary. An aperture which makes no phase shift is formed in the phase shift layer 61. The aperture 74 and the phase shift layer 75 both have a rectangular shape, and a boundary therebetween is aligned to the linear boundary between the aperture 60 and the phase shift layer 61. In FIG.38A, all the boundaries indicated by the solid line are accompanied by a 180° phase shift. In FIG.38B, the boundary which defines each region is formed similarly to FIG.38A. However, the aperture 74 shown in FIG.38A is replaced by a phase shift layer 76 which has a phase shift quantity of 360°. As in the case of the mask pattern shown in FIG.38A, there is a 180° phase shift between the upper and lower regions bounded by the straight horizontal boundary. In addition, a rectangular boundary having the phase shift quantity of 180° is formed at the central portion of the mask pattern. In FIG.38C, the mask pattern is generally divided into two regions by a straight horizontal boundary. In the upper region, the aperture 60 which makes no phase shift is formed on the right and the phase shift layer 61 which has the phase shift quantity of 180° is formed on the left. The aperture 60 and the phase shift layer 61 are adjacent to each other for a limited length. In addition, a phase shift layer 77 which has an intermediate phase shift quantity of 90° is formed between the aperture 60 and the phase shift layer 61. The lower region has a structure similar to that of the upper region. That is, in the lower region, the aperture 68 which makes no phase shift is formed on the left below the phase shift layer 61, and the phase shift layer 61 which has the phase shift quantity of 180° is formed on the right below the aperture 60. The aperture 68 and the phase shift layer 61 in the lower region are adjacent to each other for a limited length. In addition, the phase shift layer 77 which has the intermediate phase shift quantity of 90° is formed between the aperture 68 and the phase shift layer 61 in the lower region. As a whole, a boundary which accompanies a 180° phase shift is formed along the straight horizontal line and also at a vertical line segment portion at the central portion of the mask pattern. FIG.38D shows an imaged pattern which is obtained when the mask patterns shown in FIGS.38A to 38C are imaged. In this case, a black pattern 78 which has a wide region at the central portion is imaged. When making the length of the line finite in the mask patterns shown in FIGS.36A, 36B, 37A, 38A, 38B and 38C, it is sufficient to form in an unwanted portion a region having an intermediate phase shift quantity of 90° or the like. The mask patterns for forming various imaged patterns are described above, and now, a description will be given of the light intensity profiles which are obtained by numerical calculations for some of the described mask patterns. For these calculations, it is assumed for the sake of convenience that the light used for the exposure has a wavelength of 365 nm, the imaging lens system has a numerical aperture NA of 0.50, and the light from the illumination lens system has a partial coherency α of 0.50. Figs.39A to 39C are diagrams for explaining a pattern of intersecting lines. FIG.39A generally shows a pattern and a sampling region of the intersecting lines. The mask pattern shown in FIG.36B is employed, and the X, Y and Z coordinates shown on the right in FIG.39A are used. In addition, a region indicated by a phantom line is regarded as the sampling region, and the light intensity within the sampling region is calculated according to a mode. FIG.39B is a graph showing the light intensity profile within the sampling region shown in FIG.39A as a three dimensional model. It may be seen from FIG.39B that a deep valley is formed at the boundary portion accompanying the 90° phase shift. Although FIG.39B does not shows the light intensity profile for the lower half portion of the pattern shown in FIG.39A, it is apparent to those skilled in the art that the light intensity profile has a structure similar to that shown in FIG.39B for the lower half portion of the pattern shown in FIG.39A. FIG.39C shows a projection of the light intensity profile shown in FIG.39B on an XY plane when the light intensity profile is described by contour lines with respect to the light intensity. In other words, regions with the minimum light intensity are formed at a portion extending along the lower side in the direction X and a portion extending along the central portion in the direction Y. Portions where the light intensity gradually increases are formed adjacent to such regions with the minimum light intensity. FIGS.40A to 40C are diagrams for explaining the pattern of interconnections. FIG.40A shows the mask pattern for the interconnection pattern shown in FIG.38A, and a sampling region is indicated by a phantom line. The X,Y and Z coordinates shown on the right of FIG.40A are used. FIG.40B shows a light intensity profile within the sampling region shown in FIG.40A. A narrow valley is formed along the direction X, and the valley portion widens in a vicinity of a region in which X = 0. FIG.40C shows a projection of the light intensity profile shown in FIG.40B on the XY plane when the light intensity profile is described by contour lines with respect to the light intensity. It may be seen from FIG.40C that a valley portion is formed and the width of this valley portion widens at the central portion. FIGS.41A to 41C are diagrams for explaining the pattern of interconnections. FIG.41A shows the mask pattern for the interconnection pattern shown in FIG.38C, and a sampling region is indicated by a phantom line. On the horizontal axis at the central portion of FIG.41A there are formed the aperture 60 which makes no phase shift and the phase shift layer 61 which has the phase shift quantity of 180°. The aperture 60 and the phase shift layer 61 are adjacent to each other in part and the phase shift layer 77 which has the phase shift quantity of 90° is formed between the aperture 60 and the phase shift layer 61. In addition, the upper side of the aperture 60 makes an angle 1 to the horizontal direction. The two sides of the phase shift layer (intermediate region) 77 make an angle 2. Further, the upper side of the phase shift layer 61 makes an angle 3 to the horizontal direction. The phase shift layer 61 which has the phase shift quantity of 180° is formed adjacent to and symmetrically to the aperture 60 below the horizontal axis, and the aperture 68 which makes no phase shift is formed below the phase shift layer 61. In addition, another phase shift layer 77 is formed below the horizontal axis at a position symmetrical to the phase shift layer 77 above the horizontal axis. A narrow region at the central portion where the apertures 60 and 68 and the phase shift layer 61 meet has a width W1. In this sample, 1 = 2 = 3 = 60°, and W1 = 0.2 µm. FIGS.41B and 41C show the imaged pattern which is formed using such a mask pattern. FIG.41B shows a light intensity profile within the sampling region shown in FIG.41A. A deep valley is formed along the direction X, and the valley extends in the direction Y at the central portion. Moreover, a shallow branching valley is formed as shown. FIG.41C shows a projection of the light intensity profile shown in FIG.41B on the XY plane when the light intensity profile is described by contour lines with respect to the light intensity. FIGS.42A to 42C are diagrams for explaining a mask pattern having a structure similar to the mask pattern shown in FIG.35A. In FIG.42A, the aperture 60 which makes no phase shift is formed on the right and the phase shift layer 61 which has the phase shift quantity of 180° and a triangular shape is formed on the left. The aperture 60 and the phase shift layer 61 are adjacent to each other for width W2. In addition, the phase shift layer 64 which has the phase shift quantity of 90° is formed in each intermediate region between the aperture 60 and the phase shift layer 61. The light intensity distribution is calculated for a sampling region indicated by a phantom line. For example, a gap of 0.08 µm is set for the width W2, and two boundary lines which are formed by the phase shift layer 61 and the two phase shift layers 64 make an angle of 60°. FIG.42B shows the light intensity distribution within the sampling region shown in FIG.42A, and FIG.42C shows a projection of the light intensity profile shown in FIG.42B on the XY plane when the light intensity profile is described by contour lines with respect to the light intensity. As may be seen from FIG.42C, an oval region in which the light intensity is a minimum is formed in a vicinity of the position (0.0). FIGS.43A and 43B are diagrams for explaining a line-and-space pattern. FIG.43A shows a mask pattern for the line-and-space pattern. The line-and-space pattern is formed by an aperture region 80 and a phase shift region 81 which are alternately formed, where the aperture region 80 makes no phase shift and the phase shift region 81 has a phase shift quantity of 180°. For example, when the widths of the aperture region 80 and the phase shift region respectively are 0.5 µm and the light intensity profile of the imaged pattern is calculated, the light intensity profile shown in FIG.43B is obtained. A minimum of the light intensity is formed at a portion corresponding to the boundary between the aperture region 80 and the phase shift region 81. When making the calculation, it is assumed for the sake of convenience that the light used for the exposure has the wavelength of 365 nm, the numerical aperture NA of the lens system is 0.53, and the partial coherency σ of the light is 0.5. FIGS.44A and 44B are diagrams for forming a pattern in which various pattern sizes are mixed. FIG.44A shows a mask pattern. An opaque layer 85 for forming a large black pattern and a phase shift layer 86 are combined at the upper part. A phase shift layer 87 for forming the image of the phase shift layer itself is formed at the intermediate part. A phase shift layer 88 for forming an imaged pattern by the edge portion thereof is formed at the lower part. A phase shift layer 89 which has an intermediate phase shift quantity for preventing the imaging of the unwanted sides is formed in the periphery of the phase shift layer 88. For example, the phase shift layers 86, 87 and 88 have a phase shift quantity of 180°, and the phase shift layer 89 has a phase shift quantity of 90°. When the lens system used has a numerical aperture NA of 0.4 to 0.6 for the i-ray, a pattern of 0.5 µm or greater is formed by arranging the phase shift region 86 with the phase shift quantity of 90° around the opaque layer 85 as shown in the upper part of FIG.44A. When forming a black pattern of 0.3 to 0.5 µm, the mask is formed solely by the pattern of the phase shift layer 87 having the phase shift quantity of 180°, as shown in the intermediate part of FIG.44A. A pattern of 0.25 µm or less is formed by the boundary of the phase shift layer 88 which has the phase shift quantity of 180° and an aperture region 90 which makes no phase shift, as shown in the lower part of FIG.44A. In this case, the side of the unwanted phase shift region 88 is surrounded by the phase shift layer 89 which has the intermediate phase shift quantity so as not to be imaged by adjusting the developing level. FIG.44B shows the imaged pattern which is obtained by use of the mask pattern shown in FIG.44A. A black pattern 91 which corresponds to the mask which is made up of the opaque layer 85 and the phase shift region 86 is formed at the upper part. A black pattern 92 which corresponds to the phase shift layer 67 is formed at the intermediate part. A narrow black pattern 93 which corresponds to the boundary between the phase shift layer 88 and the aperture region 90 is formed at the lower part. When producing an integrated circuit by photolithography, the pattern of the reticle is imaged on the wafer using an imaging lens system. Accompanied by the increase in the integration density of the integrated circuit, the numerical aperture NA of the imaging lens system is increased so as to improve the resolution and cope with the fine integrated circuit patterns. However, when the numerical aperture NA is made large so as to improve the resolution, the focal point depth FD becomes small according to the following formula (4), where K 2 denotes a process coefficient. FD = K 2 (λ/NA 2 ) Accordingly, it is impossible to accurately form the pattern with respect to a concavoconvex surface. Next, a description will be given of an embodiment which can satisfactorily form a pattern even with respect to a concavoconvex surface. FIG.45 shows another embodiment of the mask according to the present invention. In FIG.45, those parts which are essentially the same as those corresponding parts in FIG.9 are designated by the same reference numerals, and a description thereof will be omitted. In this embodiment, the phase shift layer 3a comprises a shifter portion 3a1 having a thickness D1 and a shifter portion 3a2 having a thickness D1, where D2 > D1. By providing such shifter portions which have different thicknesses, it is possible to arbitrarily control the focal point of the image when making the exposure. Next, a description will be given of a fifth embodiment of the pattern forming method according to the present invention. In this embodiment, the thickness of the phase shift layer of the mask is made different locally where needed. For the sake of convenience, it is assumed that the mask 1A shown in FIG.19 and the optical system shown in FIG.22 are used to form the pattern, and a description will be given of the relationship of the thickness of the phase shift layer 3a and the deviation (defocus quantity) of the focal position. The light used for the exposure, the material used for the phase shift layer and the like are the same as those of the embodiment described above. When the thickness t of the phase shift layer 3a is calculated for obtaining each phase shift quantity S of 180°, 120° and 240°, respectively, t = 0.388, 0.259 and 0.517 µm. First, when t = 0.388 µm and S = 180°, the light intensity distribution becomes as shown in FIG.46C when the defocus quantity is zero. However, when the defocus quantities are set to +1.0 and +0.5 µm by taking as the positive (+) direction the direction towards the wafer 11 along the optical axis of the lens system 9, the light intensity distributions become as shown in FIGS.46D and 46E, respectively. As may be seen from FIGS.46A to 46E, the light intensity distribution changes in the same manner regardless of whether the focal point deviates in the positive direction or the negative direction. FIGS.47A, 47B, 47C and 47D respectively show the light intensity distributions which are obtained when t = 0.259 µm, S = 120° and the defocus quantities are +1.0, +0.5, 0 and -0.5 µm. As may be seen from FIGS.47A to 47D, the contrast is a maximum when the defocus quantity is +0.5 µm. FIGS.48A, 48B, 48C and 48D respectively show the light intensity distributions which are obtained when t = 0.517 µm, S = 240° and the defocus quantities are +0.5, 0, -0.5 and -1.0 µm. As may be seen from FIGS. 48A to 48D, the contrast is a maximum when the defocus quantity is -0.5 µm. According to this embodiment, the focal point can be controlled within the range of 0.5 µm by appropriately determining the thickness t of the phase shift layer. In the production process of the integrated circuit, a region of the integrated circuit may have a height greater than that at other regions. For example, in the dynamic random access memory (DRAM), the memory cell part is 0.5 to 1.0 µm higher than the other peripheral circuit parts when stacked capacitors are used. In such a case, when an attempt is made to form the pattern to the resolution limit by using a lens system having a large numerical aperture NA, the focal point depth FD becomes small and it is difficult to apply the pattern formation to the production of integrated circuits having concavoconvex surfaces. However, according to this embodiment, it is possible to apply the pattern formation to a DRAM using stacked capacitors. 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, comprising a transparent layer (2) provided with a mask pattern defining transparent portions for transmitting light used in the exposure, corresponding to first areas of said wafer, these being areas which are to be exposed, said mask pattern further defining mask portions (5, 3b, 14) for causing masking of said light in second areas of said wafer adjacent to said first areas; said mask further comprising phase shift portions (3a) for shifting the phase of said light within a range of 180° ± 30% so as to improve the contrast between the first and second areas; characterized in that at least some of the mask portions (5, 3b) are formed by a phase shift layer used to form the phase shift portions (3a). The mask as claimed in claim 1, wherein the phase shift portions (3a) include an edge portion for forming a line pattern when making the exposure. The mask as claimed in claim 1, wherein the phase shift portions (3a) include a pair of edge portions for forming a pattern of arbitrary width when making the exposure. The mask as claimed in claim 3, wherein the phase shift portions (3a) are patterned smaller than a designed black pattern which constitutes a said second area of said wafer. The mask as claimed in any of claims 1 to 4, wherein the phase shift portions (3a) include a first portion (3a1) and a second portion (3a2) having mutually different thicknesses. The mask as claimed in any of claims 1 to 5, wherein the phase shift layer is made of SiO 2 , Al 2 O 3 , or MgF 2 . The mask as claimed in any of claims 1 to 5, wherein the phase shift layer is constituted by said transparent layer, such that the phase shift portions (3a) are portions of said transparent layer having a different thickness from said transparent portions (2). The mask as claimed in any preceding claim, wherein said transparent layer (2) is made of quartz or glass. The mask according to any preceding claim, comprising further mask portions (14) which are formed by an opaque layer and which coexist with said mask portions (5, 3b) formed by the phase shift layer. The mask as claimed in claim 9, wherein the mask portions (3b) formed by the phase shift layer are mask portions for forming relatively fine patterns on the wafer, and the further mask portions (14) are mask portions for forming relatively large patterns on the wafer. A method of producing a mask as claimed in any of claims 1 to 6, comprising the steps of: providing a substrate (2) made of a transparent layer; forming a resist layer (3) on the substrate and patterning the resist layer to leave behind resist in areas where said transparent portions of the mask are to be formed; depositing the phase shift layer (3a) over the patterned resist layer; and removing the remaining resist, thereby removing the phase shift layer over the transparent portions, and leaving behind the phase shift layer over the phase shift portions and said at least some of the mask portions (5, 3b) of the mask. A method of producing a mask as claimed in any of claims 1 to 6, comprising the steps of: providing a substrate (2) made of a transparent layer; forming a stopper layer (12) on the substrate (2); forming the phase shift layer (3a) on the stopper 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 (12) as an etching stopper and said resist pattern (4) as an etching mask, thereby leaving behind the phase shift layer over the phase shift portions and said at least some of the mask portions (5, 3b) of the mask. The method as claimed in claim 12, wherein the stopper layer is made of Al 2 O 3 . A method of exposing a semiconductor wafer by illuminating a mask (1) with light from a light source, and projecting light transmitted through the mask onto a photoresist layer (10) on the wafer by use of a lens system (9), wherein the method comprises transmitting the light through transparent portions of the mask so as to expose first areas of said wafer, masking the light by mask portions (5b, 3b, 14) of the mask (1) so as to form second areas of said wafer adjacent to, and having a lower light intensity than, said first areas, and shifting the phase of said light within a range of 180° ± 30% by transmitting the light through phase shift portions of said mask (1) so as to improve the contrast between the first and second areas; characterized in that, in said step of masking the light, at least some of said second areas are formed by transmitting the light through mask portions (3b) made of the same phase-shift material as the phase-shift portions. The method as claimed in claim 14, wherein said step of shifting the phase of the light comprises transmitting the light through phase shift portions having different thicknesses depending on a concavoconvex surface structure of said photoresist layer, whereby the light is projected at different focal positions on the photoresist layer (10). The pattern forming method as claimed in claim 14 or 15, wherein said step of masking the light further comprises generating interference at an edge portion (3a) of the phase-shift material so as to form a black pattern (PB, 62, 65, 66, 69, 72, 78) as a said second area of said wafer. The pattern forming method as claimed in claim 14 or 15, wherein said step of shifting the phase of the light further comprises generating interference at an edge portion (3a) of the phase shift material so as to form a white pattern as a said first area of said wafer. A pattern forming method for forming a pattern on a semiconductor wafer (11) using a mask as claimed in any of claims 1 to 10, the method comprising illuminating the mask (1) 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, so that the photoresist layer is developed in said first areas of the wafer corresponding to the transparent portions of the mask; wherein said light is partially coherent with a coherency in the range 0.3 to 0.7.

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
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G03F1
IPC codes: G03F1, H01L21