Photomask and method of fabricating the same

Abstract: A photomask comprises light shielding areas with a light shielding layer formed on a mask substrate (10), and a light transmitting area defined on the mask substrate (10) by the light shielding areas, the light transmitting area being divided in a first area with a 90° phase shifter (16) formed on for shifting a phase of transmitted light by 90°, a second area with a 270° phase shifter (18) formed on for shifting a phase of transmitted light by 270°, and a third area with neither the 90° phase shifter (16) nor the 270° phase shifter (18) formed on, the first and the second areas being arranged without being overlapped on each other and with the light shielding areas or the third area located therebetween.

Description: BACKGROUND OF THE INVENTION The present invention relates to a photomask for use in optical lithography for the fabrication of semiconductor devices and so on, particularly to a photomask having phase shifters for shifting the phase of transmitted light, and to a method of fabricating the photomask. Accompanying demands for larger-scales and higher integration of semiconductor integrated circuits in recent years, micronized patterning is required. To improve resolution for optical lithography using photomasks there have been proposed phase shift masks using phase shifters for shifting phases of transmitted light. A phase shift mask of e.g., a line and space pattern of alternated light shielding and light transmitting areas is intended to improve resolutions of patterns by setting at 180° a phase difference between light transmitting areas with light shielding areas therebetween. But in the case a phase shift mask has a part in a light transmitting area, where a phase inversion of transmitted light takes place, i.e., an area (0° area) with no phase shifter formed thereon and an area (180° area) with a 180° phase shifter formed thereon are adjacent to each other, the intensity of the transmitted light is lowered at the part although the part is the light transmitting area. Consequently, by using such phase shift mask, resists adversely remain at parts where patterns are not to be formed. Attention has to be paid to arrangements of phase shifters so as not to form parts in light transmitting areas, where no phase inversions take place. To remove such limitation to the arrangement of phase shifters, a 90° shifter, for example, is arranged at a part where a phase is inverted, whereby a sharp shift of a phase in a light transmitting area has been prevented. Such conventional photomask is shown in FIG. 12. FIG. 12A is a plan view of the photomask, FIG. 12B is a sectional view along the line A-A' of FIG. 12A, and FIG. 12C is a sectional view along the line B-B' of FIG. 12A. Chrome layers 102 are formed in lines on a mask substrate 100. To shift a phase by 180° in every two ones of light transmitting areas sandwiched by the chrome layers 102, phase shifters 104, 106 each for shifting a phase by 90° are superposed on each other, and the lower phase shifter 104 alone is formed on borders of the light transmitting areas with no phase shifter formed thereon. In the borders of the light transmitting area without the phase shifters a phase is bluntly shifted by 90°, so that no sharp phase shifts takes place. Another conventional phase shift mask having phase shifters so arranged that the light transmitting area has no part where a phase is inverted in FIG. 13 (H. Ohtsuka et al., "Conjugate Twin-Shifter for the New Phase Shift Method to High Resolution Lithography", SPIE, Vol. 1463, pp. 112-123, Optical/Laser Microlithography IV (1991)). FIG. 13A is a plan view of the photomask, and FIG. 13B is a sectional view along the line A-A' of FIG. 13A. In this conventional phase shifter, chrome layers 102 are formed on a mask substrate 100, and 90° phase sifters 108 for shifting a phase by 90° and 270° phase shifters 110 for shifting a phase by 270° are alternately formed on each of light transmitting areas sandwiched by the chrome layers 102,. A phase is inverted in the boundary parts between the 90° phase shifter 108 and the 270° phase shifter 110. But in FIG. 13, because of the chrome layers 102 on the boundary parts, the light transmitting area has no parts where a phase inversion takes place. But in such conventional phase shift mask as shown FIG. 12, to shift a phase by 180° in the light transmitting areas, the shifters 104, 106 each for shifting a phase by 90° are superposed on each other. If a defect is found in a phase shifter of the photomask, it will be difficult to correct the shifter alone separately. The conventional shift mask of FIG. 13 has the problem that a light intensity is so lowered in the boundary parts of the phase shifters 108, 110 that a focus margin upon an exposure is lowered. SUMMARY OF THE INVENTION An object of the present invention is to provide a photomask and a method of fabricating the photomask, which can shift a phase in light transmitting areas without lowering the intensity of transmitted light, can easily correct defects of its phase shifters, and can secure a sufficient focus margin upon an exposure. The object of the present invention is achieved by a photomask comprising light shielding areas with a light shielding layer formed on a mask substrate, and a light transmitting area defined on the mask substrate by the light shielding areas, the light transmitting area being divided in a first area with a 90° phase shifter formed thereon for shifting the phase of transmitted light by 90° + m x 360° (m=0,1,2,...), a second area with a 270° phase shifter formed thereon for shifting the phase of transmitted light by 270° + n x 360° (n=0,1,2,...), and a third area for shifting the phase of transmitted light by 0°, characterized by the first and the second areas being arranged without being overlapped on each other and with the light shielding areas or the third area located therebetween, so that the first and second areas are at no part adjacent to each other. The object of the present invention is achieved by a method of fabricating a photomask according to claim 6, the method comprising the steps of forming a light shielding layer on parts of the surface to be light shielding areas, and forming an etching stopping layer on the entire area of the mask substrate, forming a first phase shifter on the entire surface for shifting the phase of transmitted light by 90° + m x 360° (m=0,1,2,...), forming a first resist layer on a first area of the light transmitting area not having the light shielding layer formed thereon, etching off a part of the first phase shifter with the first resist layer as a mask to leave the first phase shifter on the first area, forming a second resist layer with an opening corresponding to a second area of the light transmitting area, forming a second phase shifter on the entire surface for shifting the phase of the transmitted light by 270° + n x 360°(n=0,1,2,...), and removing the second resist layer to lift off the second phase shifter to leave the second phase shifter in the second area. Alternatively, the photomask may be fabricated using the method of claim 7. In the present invention, the first area with a 90° shifter formed on, and the second area with a 270° shifter formed on are arranged with a light shielding area or the third area with no phase shifter formed thereon located between the first and second areas. Consequently the photomask has no parts in the light transmitting area, where a phase of the transmitted light is sharply shifted, and consequently an intensity of the light is lowered. Furthermore, the 90° shifter and the 270° shifter are not overlapped on each other. Consequently even if a defect should take place in the 90° shifter or in the 270° shifter , the shifter could be easily corrected. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view of the photomask according to a first embodiment of the present invention; Fig. 2 (2A and 2B) represents views of the photomask according to the first embodiment of the present invention; Fig. 3 is a graph showing a relationship between focal points and minimum light intensity of the photomasks in Figs. 1 and 2, and Fig. 13; Fig. 4 (4A-4C) represents sectional views (1) illustrative of the steps of a first embodiment of the method of fabricating the photomask of the present invention; Fig. 5 (5A-5C) represents sectional views (2) illustrative of the steps of the first embodiment of the method of fabricating the photomask of the present invention; Fig. 6 (6A-6D) represents sectional views (1) illustrative of the steps of a second embodiment of the method of fabricating the photomask of the present invention; Fig. 7 (7A-7C represents sectional views (2) illustrative of the steps of the second embodiment of the method of fabricating the photomask of the present invention; Fig. 8 (8A-8E) represents sectional views of the steps of a method of correcting the photomask according to the embodiment of the present invention; Fig. 9 (9A-9D) represents sectional views of the steps of a method of correcting the photomask according to the embodiment of the present invention; Fig. 10 (10A-10E) represents sectional views of the steps of a method of correcting the photomask according to the embodiment of the present invention; Fig. 11 is a view of the photomask according to another embodiment of the present invention; Fig. 12 (12A-12C) represents views of the conventional photomask; and Fig. 13 (13A and 13B) represents views of the conventional photomask. DETAILED DESCRIPTION OF THE INVENTION The photomask according to one embodiment of the present invention will be explained with reference to Figs. 1 and 2. Fig. 1 is a plan view of the photomask, Fig. 2A is a sectional view along the line A-A' of Fig. 1, and Fig. 2B is a sectional view along the line B-B' of Fig. 1. Four chrome lines 12 of a light shielding layer are formed on a mask substrate 10 of quartz. An etching stopping layer 14 for blocking etching is formed on the entire mask substrate 10 including the chrome layers 12. The etching stopping layer 14 is formed of, e.g. Al 2 O 3 . Three light transmitting areas are defined between adjacent ones of the four chrome lines 12, and a 90° phase shifter 16 for shifting a phase by 90° is formed on the central transmitting area of the three transmitting areas. The 90° phase shifter 16 is e. g. an about 190 nm-thickness SiO 2 layer. This 90° phase shifter 16 is formed uninterruptedly up to the peripheral areas of the chrome lines 12. A 270° phase shifter 18 for shifting a phase by 270° is formed on each of the right and the left light transmitting areas of the three light transmitting areas sandwiched by each of the four chrome lines 12 and its adjacent one. The 270° phase shifters 18 are formed of e. g. an about 570 nm-thickness SiO 2 layer. The 270° phase shifters 18 are formed only on the light transmitting areas between the chrome lines 12. The 90° phase shifter 16 is separated from the shorter sides of each of the 270° phase shifters 18 by light transmitting areas without any phase shifter formed thereon. The 90° phase shifter 16 is also separated from the longer sides of each of the 270° phase shifters 18 by the light shielding areas of the chrome lines 12. In the photomask according to this embodiment, the 90° phase shifter 16 and the 270° phase shifter 18 are separated from each other by the light transmitting areas without any phase shifter formed them or by the light shielding areas of the chrome lines 12, and accordingly there is no part where a phase sharply shifts by an angle larger than 90°. Furthermore, the 90° phase shifter 16 and the 270° phase shifter 18 are not overlapped on each other, and consequently possible defects of the 90° phase shifter 16 and the 270° phase shifter can be corrected separately. According to the photomask according to this embodiment, a light intensity is not lowered in the outer boundary part of the phase shifters, and consequently a sufficient focus margin can be secured upon an exposure. A minimum light intensity obtained when a focal point is shifted was calculated on the photomask according to this embodiment of FIGs. 1 and 2, and the conventional photomask of FIG. 13. In the photomask according to this embodiment of FIGs. 1 and 2, a light intensity is most lowered in the areas near the boundaries of the phase shifters 18. In the conventional photomask of FIG. 13, a light intensity is most lowered in the areas, which are boundary parts of the phase shifters 108, 110. The calculated results are shown in FIG.3. Focal points are taken on the horizontal axis, and minimum light intensities obtained at the respective focal points are taken on the vertical axis. The lower limit of the practically allowable light intensities was set at 0.4. As shown in FIG. 3, in the conventional photomask, the minimum light intensity exceeds the limit value 0.4 between the focal points -0.5 µm and +0.5 µm. In the photomask according to this embodiment, the minimum light intensity exceeded the limit value 0.4 between the focal points - 0.5 µm and +1.0 µm. The focal margin of the conventional photomask was 1.0 µm, and the photomask according to this embodiment was 1.5 µm, which is 1.5 time that of the conventional photomask. Next, a first embodiment of the method of fabricating the photomask of the present invention will be explained with reference to FIGs. 4 and 5. First, a chrome layer 12 is evaporated on the entire surface of a mask substrate 10 of quartz and is patterned in lines. Subsequently on the entire surface an etching stopping layer 14 of Al 2 O 3 is evaporated. Then an about 190 nm-thickness SiO 2 layer 20 is evaporated on the entire surface (FIG. 4A). Next, a resist layer 22 which is so patterned that a region for the 90° phase shifter 16 to be formed thereon is masked is formed (FIG. 4B). Then the SiO 2 layer 20 is etched by, e.g., RIE (Reactive Ion Etching) with the resist layer 22 as the mask, and the 90° phase shifter 16 is formed (FIG. 4C). This etching of the SiO 2 layer 20 is stopped at the etching stopping layer 14 and the surface of the mask substrate 10 is never etched. Subsequently a resist layer 24 which is so patterned that a region for the 270° phase shifter 18 to be formed thereon is opened is formed (FIG. 5A). Then an about 570 nm-thickness SiO 2 layer 26 is evaporated on the entire surface (FIG 5B). The SiO 2 layer 26 is formed both on the etching stopping layer 14 and on the resist layer 24. Next, the resist layer 24 is removed to lift off the resist layer 24 on the SiO 2 layer 26, and the 270° phase shifter 18 is formed (FIG. 5C). In the method of fabricating the photomask illustrated in FIGs. 4 and 5, the 90° phase shifter 16 is formed by etching technique, and the 270° phase shifter 18 is formed by lift-off technique. Next, a second embodiment of the method of fabricating the photomask according of the present invention will be explained below with reference to FIGs. 6 and 7. First, a chrome layer 12 is evaporated on the entire surface of a mask substrate 10 of quartz, and is patterned in lines. Then an Al 2 O 3 etching stopping layer 14 is evaporated on the entire surface (FIG. 6A). Then a resist layer 28 which is so patterned that a region for the 90° phase shifter 16 to be formed on is opened is formed (FIG. 6B). Next, an about 190 nm-thickness SiO 2 layer 30 is evaporated on the entire surface (FIG. 6C). The SiO 2 layer 28 is formed both on the etching stopping layer 14 and on the resist layer 28. Subsequently the resist layer 28 is removed to lift off the SiO 2 layer 30 on the resist layer 28, and the 90° phase shifter 16 is formed (FIG. 6D). Then, similarly with the first embodiment, a resist layer 24 which is so patterned that a region for the 270° phase shifter to be formed is opened is formed (FIG. 7A), and an about 570 nm-thickness SiO 2 layer 26 is evaporated on the entire surface (FIG. 7B). The SiO 2 layer 26 on the resist layer 24 is lifted off, and the 270° phase shifter 18 is formed (FIG. 7C). In the method of fabricating the photomask illustrated in FIGs. 6 and 7, both the 90° phase shifter 16 and the 270° phase shifter 18 are formed by lift-off technique. Methods of correcting the photomask according to the above-described embodiment will be explained below with reference to FIGs. 8, 9 and 10. With reference to FIG. 8, a method of correcting the photomask will be explained by means of a case that the 90° phase shifter 16 or the 270° phase shifter has a chipped pattern 32 (FIG. 8A). A resist layer 34 which is patterned so as to be opened at a part corresponding to the chipped pattern 32 is formed (FIG. 8B). Then the chipped pattern is etched off with the resist layer 34 as the mask (FIG. 8C). Subsequently a SiO 2 layer 36 of a required thickness which is to be the phase shifter is evaporated on the entire surface (FIG. 8D). The SiO 2 layer 36 is formed both on the etching stopping layer 14 and on the resist layer 34. Then the resist layer 34 is removed to lift off the SiO 2 layer 36 on the resist layer 34, and a perfect phase shifter 38 without any defect is formed (FIG. 8E). With reference to FIG. 9 a method of correcting the photomask will be explained by means of a case that a residual pattern 40 is present at a part where neither a 90° phase shifter 16 nor the 270° phase shifter 18 should be formed (FIG. 9A). First a resist layer 42 which is patterned so as to be opened at a part corresponding to the residual pattern 40 (FIG. 9B). A normal pattern 44 is covered protectively with the resist layer 42. Then only the residual pattern 40 is etched off with the resist layer 44 as the mask (FIG. 9C). Subsequently the resist layer 44 is removed to leave only the normal pattern 44 (FIG. 9D). In the photomask according to the embodiments of the present invention, the 90° phase shifter 16 and the 270° phase shifters 18 are not overlapped on each other. Consequently corrections of possible defects of the 90° phase shifter 16 and of the 270° phase shifter 18 can be made separately. With reference to FIG. 10, a method of correcting the photomask will be explained by means of a case that the 90° phase shifter 16 or the 270° phase shifter has a chipped part 46 (FIG. 10A). A resist layer 48 which is patterned so as to be opened at a part including the chipped part 46 is formed (FIG. 10B). Then the part including the chipped part 46 is etched off with the resist layer 48 as the mask (FIG. 10C). Subsequently a SiO 2 layer 50 of a required thickness which is to be the phase shifter is evaporated on the entire surface (FIG. 10D). The SiO 2 layer 50 is formed both on the etching stopping layer 14 and on the resist layer 48. Then the resist layer 48 is removed to lift off the SiO 2 layer 50 on the resist layer 48, and a perfect phase shifter 52 without any defect is formed (FIG. 10E). Next, the photomask according to further another embodiment of the present invention will be explained below with reference to the plan view of FIG. 11. Three chrome lines 12 are formed of a light shielding layer on a mask substrate of quartz. An etching stopping layer (not shown) is formed on the entire surface of the mask substrate 10 including the chrome lines 12. A 270° phase shifter 18 for shifting a phase by 270° is formed on the left one of two light transmitting areas between each of the chrome lines 12 and its adjacent one, and on a light transmitting area which is right of the right one of the two light transmitting areas between each of the chrome lines 12 and its adjacent one. The 270° phase shifters 18 are formed of, e.g., an about 570 nm-thickness SiO 2 layer. A 90° phase shifter 16 for shifting a phase by 90° is formed on the right one of the two light transmitting areas sandwiched by each of the chrome lines 12 and its adjacent one. The 90° phase shifter is formed of, e,g,. an about 190 nm-thickness SIO 2 layer. The 90° phase shifter 16 is formed uninterrupted up to the peripheral areas of the chrome lines 12, but an allowance is provided for a light transmitting area on the right side of the right 270° phase shifter. In the photomask according to this embodiment, the 90° phase shifter 16 and each 270° phase shifter 18 are separated from each other by a light transmitting area without any phase shifter formed thereon or the light shielding areas of the chrome lines 12 formed thereon. Accordingly there is no part where a phase sharply shifts by an angle larger than 90°. The 90° phase shifter 16 and the 270° phase shifters 18 are not overlapped on each other. Accordingly as described above, corrections of possible defects of the 90° phase shifter 16 and the 270° phase shifters can be separately made. The present invention is not limited to the above-described embodiments and can cover other different variations and modifications. In the above-described embodiments, for example, the etching stopping layer is formed on the mask substrate and the chrome layers, but instead may be formed on the mask substrate and below the chrome layers. In the above-described embodiments, the formation of the 90° phase shifters are followed by the formation of the 270° phase shifters, but reversely the formation of the 270° phase shifters is followed by the formation of the 90° phase shifters. In the above-described embodiments, the phase shifters are formed of SiO 2 , but may be formed of other materials. Especially in a case that there is the selectivity of an etching process between a material of the phase shifters and a material of the mask substrate, the phase shifter may be formed only by lift-off technique. In a case that defect corrections of a phase shifter pattern are not necessary, the etching stopping layer may not be formed. As described above, according to the present invention, the first area where the 90° phase shifter is formed and the second area where the 270° phase shifter are formed sandwich therebetween the third area where no light shielding area and no phase shifter is formed. Accordingly there is no part where an intensity is lowered even when a phase sharply shifts even by 180° in the light transmitting area. Furthermore, the 90° shifter and the 270° phase shifters are not overlapped. Accordingly if the 90° shifters and the 270° phase shifters should have defects, the defects could be separately corrected with ease. Thus, the present invention much contributes to optical lithography for ultra-micronized patterns of semiconductor integrated circuits and so on.

Claims: A photomask comprising light shielding areas with a light shielding layer (12) formed on a mask substrate (10) and a light transmitting area defined on the mask substrate (10) by the light shielding areas, the light transmitting area being devided in a first area with a 90° phase shifter (16) formed thereon for shifting the phase of transmitted light by 90° + m x 360° (m=0,1,2,...), a second area with a 270° phase shifter (18) formed thereon for shifting the phase of transmitted light by 270° + n x 360° (n=0,1,2,...), and a third area for shifting the phase of transmitted light by 0°, characterized by the first and the second areas being arranged without being overlapped on each other and with the light shielding areas or the third area located therebetween, so that the first and second areas are at no part adjacent to each other. A photomask according to claim 1, wherein the light shielding layer (12) has a plurality of light shielding lines, which define a plurality of light transmitting spaces therebetween; the 90° phase shifter (16) is formed on a peripheral area of the light shielding lines and the light transmitting spaces; the 90° phase shifter (16) and the 270° phase shifter (18) are alternately formed on each of the light transmitting spaces; the 90° phase shifters (16) on the light transmitting spaces are formed uninterruptedly up to the 90° phase shifter (16) on the peripheral area; and light transmitting areas with neither the 90° phase shifter nor the 270° phase shifter are located between the 270° phase shifters (18) on the light transmitting spaces and the 90° phase shifter (16) on the peripheral area, so that the 270° phase shifter (18) is at no part adjacent to the 90° phase shifter (16). A photomask according to claim 2, wherein the 90° phase shifter (16) is formed on the light transmitting space adjacent to the light shielding line at the most outer side of the light shielding lines; the 270° phase shifter (18) is formed on an area adjacent to the light shielding line at the most outer side of the light shielding lines; and light transmitting areas with neither the 90° phase shifter nor the 270° phase shifter are located between the 270° phase shifters (18) on the area and the 90° phase shifter (16) on the peripheral area. A photomask according to any one of claims 1 to 3, wherein an etching stopping layer (14) for blocking etching of the mask substrate (10) is formed on the mask substrate (10). A photomask according to claim 4, wherein the etching stopping layer (14) is formed in the light transmitting area and on the light shielding layer (12) in the light shielding areas. A method of fabricating a photomask according to any one of claims 1 to 5, comprising the steps of: forming a light shielding layer (12) on parts of the surface of a mask substrate (10) to be light shielding areas, and forming an etching stopping layer (14) on the entire area of the mask substrate (10); forming a first phase shifter (16) on the entire surface for shifting the phase of transmitted light by 90° + m x 360° (m=0,1,2,...); forming a first resist layer (22) on a first area of the light transmitting area not having the light shielding layer (12) formed thereon; etching off a part of the first phase shifter (16) with the first resist layer (22) as a mask to leave the first phase shifter (16) on the first area; forming a second resist layer (24) with an opening corresponding to a second area of the light transmitting area, the first and the second areas being arranged without being overlapped on each other and with the light shielding areas or a third area for shifting the phase of transmitted light by 0° located therebetween, so that the first and second areas are at no part adjacent to each other; forming a second phase shifter (18) on the entire surface for shifting the phase of transmitted light by 270° + n x 360° (n=0,1,2,...); and removing the second resist layer (24) to lift off the second phase shifter to leave the second phase shifter (18) on the second area. A method of fabricating a photomask according to any one of claims 1 to 5, comprising the steps of: forming a light shielding layer (12) on parts of the surface of a mask substrate (10) to be light shielding areas, and forming an etching stopping layer (14) on the entire area of the mask substrate (10); forming a first resist layer (28) with an opening corresponding to a first area of a light transmitting area; forming a first phase shifter (16) on the entire surface for shifting the phase of transmitted light by 90° + m x 360° (m=0,1,2,...); removing the first resist layer (28) to lift off the first phase shifter and leaving the first phase shifter (16) on the first area; forming a second resist layer (24) with an opening corresponding to a second area of the light transmitting area, the first and the second areas being arranged without being overlapped on each other and with the light shielding areas or a third area for shifting the phase of transmitted light by 0° located therebetween, so that the first and second areas are at no part adjacent to each other; forming a second phase shifter (18) on the entire surface for shifting the phase of transmitted light by 270° + n x 360° (n=0,1,2,...); and removing the second resist layer (24) to lift off the second phase shifter and leaving the second phase shifter (18) on the second area.

Inventors: ASAI SATORU, HANYU ISAMU, ASAI, SATORU, HANYU, ISAMU, Asai, Satoru, c/o FUJITSU LIMITED, Hanyu, Isamu, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G03F1
IPC codes: H01L21, G03F1