Process for producing a phase shift photomask

Abstract: A photomask blank for preparing a reticle for use in the production of integrated circuits comprises a substrate 30, an electrically conductive layer 31, a light-shielding thin film 32 and an ionizing radiation resist 33 formed of a coated spin-on-glass layer. A pattern is directly formed on the layer 33 by electron beam exposure. After development with a solvent to give a resist pattern 35 (Fig 1c) exposed portions of the light-shielding layer 32 are etched by an etching glass plasma 36 (Fig 1d) to form a light-shielding pattern 37. The remaining resist 33 is incinerated by oxygen plasma 38 (Fig 1e) to form a photomask (Fig 1f). Next, a transparent film 39 is spin-coated over the pattern 37 (Fig 1g), and pattern-exposed to electron beam radiation 40 (Fig 1h), baked (Fig 1i) and developed to form a reticle bearing a phase shift pattern 41 (Fig 1j). The correction of defects in the phase shift layer is also disclosed.

Description: The present invention relates generally to photomasks used for producing high-density integrated circuits such as LSIs and VLSIs and more specifically to a process for producing phase shift layer-containing photomasks used for forming fine patterns with high accuracy. So far, semiconductor integrated circuits such as ICs, LSIs and VLSIs have been produced by repeating a so-called lithographic cycle wherein resists are applied on substrates to be processed, like Si wafers, and the substrates are then exposed to light through steppers to form the desired patterns, followed by development and etching. The photomasks used in such a lithographic step and called "reticles" are now increasingly required to have much higher accuracy as current semiconductor integrated circuits become higher in performance and integration than ever before. Referring to a DRAM that is a typical LSI as an example, a dimensional variation of a five-fold reticle for a 1 megabit DRAM, i.e., of a reticle five-fold greater in size than the pattern to be exposed to light is required to have an accuracy of 0.15 µm even where the mean value = ± 3 σ (σ is standard deviation). Likewise, five-fold reticles for 4- and 16-megabit DRAMs are required to have an accuracy of 0.1-0.15 µm and 0.05-0.1 µm, respectively. Furthermore, the line widths of device patterns formed with these reticles must be much finer, say, 1.2 µm for 1-megabit DRAMs and 0.8 µm for 16-megabit DRAMs, and various exposure processes are now being studied to meet such demand. With the next generation device patterns of, e.g. the 64 megabit DRAM class, however, stepper exposure systems using conventional reticles have been found to place some limitation on the resolution of resist patterns. Thus, a version of reticle based on a new idea, such as set forth in Japanese Provisional Patent Publication No. 58(1983)-173744, Japanese Patent Publication No. 62(1987)-59296, etc. and referred to as a phase shift mask, has been proposed in the art. Phase shift lithography using this reticle is a technique designed to improve the resolving power and contrast of projected images by operating the phase of light transmitted through the reticle. Photomasks such as those set forth in EP-A-90,924 also use a phase shift pattern to improve resolution and contrast, but the process for the production of such a photomask involves numerous production steps which leads to an increase in phase shift pattern or other deficiencies and a rise in production costs. EP-A-90,924, EP-A-0440467 and EP-A-0464492 all describe photomasks containing a phase shift layer and including a light shielding pattern and a phase shifter for effecting a phase difference between transparent regions isolated from each other by a light-shielding region and located adjacent to each other. Phase shift lithography will now be explained with reference to Figs. 1 and 2. Fig. 1 is a diagrammatical sketch showing the principal of the phase shift process and Fig. 2 is a diagrammatical sketch illustrating a conventional process. Figs. 1a and 2a are sectional views of reticles; Figs. 1b and 2b show the amplitude of light on the reticles; Figs. 1c and 2c depict the amplitude of light on wafers; and Figs. 1d and 2d illustrate the intensity of light on the wafers. In Figs. 1 or 2, reference numeral 1 stands for a substrate, 2 a light-shielding film, 3 a phase shifter and 4 incident light. In the conventional process, as illustrated in Fig. 2a, the substrate 1 formed of glass, etc. is provided thereon with the light-shielding film 2 formed of chromium, etc., only to form a given pattern of light-transmitting regions. In phase shift lithography, however, the phase shifter 3 to cause phase reversal (with a phase difference of 180°) is mounted on one pair of adjacent light-transmitting regions on a reticle, as sketched in Fig. 1a. According to the conventional process, therefore, the amplitude of light on the reticle is in the same phase, as illustrated in Fig. 2b, as is the amplitude of light on the wafer, as depicted in Fig. 2c, with the result that the patterns on the wafer cannot be separated from each other, as sketched in Fig. 2d. By contrast, the phase shift lithography enables the adjacent patterns to be distinctly separated from each other, as illustrated in Fig. 1d, because the light transmitting through the phase shifter is reversed in phase between the adjacent patterns, as depicted in Fig. 1b, so that the intensity of light on the pattern boundary can be reduced to zero. With the phase shift lithography, even patterns which cannot heretofore be separated from each other are thus made separable from each other, thereby achieving high resolution. Now, one example of conventional processes of producing phase shift reticles will be explained with reference to Fig. 6 which is a series of sectional views illustrating the steps of producing a typical phase shift reticle. As illustrated in Fig. 6a, a chromium film 12 is first formed on an optically polished substrate 11, and an ionizing radiation resist is uniformly coated and heated for drying thereon in conventional manner to form a resist layer 13. Then, a pattern is drawn on the resist layer 13 with ionizing radiation 14, as shown in Fig. 6b, followed by development and rinsing, thereby forming such a resist pattern 15 as sketched in Fig. 6c. If required, the edges, etc. of the resist pattern 15 are then cleared of such unnecessary matters as resist scums and whiskers by heating and descumming. Afterwards, portions exposed to open view between the resist pattern lines 15 and to be processed, i.e., the chromium layer 12 portions are dry-etched by an etching gas plasma 16, as shown in Fig. 6d, thereby to form a chromium pattern 17. After etching in this manner, the resist pattern 15, viz., the remaining resist is incinerated out by an oxygen plasma 18, as shown in Fig. 6e, thereby obtaining such a complete photomask as shown in Fig. 6f. Subsequently, this photomask is inspected to make a correction on the pattern, if required, followed by washing. After that, a transparent film 19 formed of, e.g. SiO 2 is formed on the chromium pattern 17, as depicted in Fig. 6g. Then, as depicted in Fig. 6h, an ionizing radiation resist 20 is formed on the transparent film 19 in similar manner as mentioned above, followed by alignment of the resist patterns 20, as shown in Fig. 6i. Subsequent drawing of a given pattern with ionizing radiations 21, development and rinsing give a resist pattern, as illustrated in Fig. 6j. Then, heating and descumming are effected, if required. After that, portions of the transparent film 19 exposed to open view between the resist pattern lines 22 are dry-etched by means of an etching gas plasma 23, as shown in Fig. 6k, to form a phase shifter pattern 24. Finally, the remainder of the resist is incinerated out by an oxygen plasma 25, as illustrated in Fig. 61. By the foregoing steps, such a phase shift photomask containing phase shifters 24 as shown in Fig. 6m is produced. According to the above-mentioned, conventional process for producing phase shift masks wherein the transparent film 19 serving as a phase shifter has been patterned on the chromium pattern 17, however, it is required to draw a predetermined pattern on the resist layer 20 with ionizing radiation, ionizing radiation exposure or other hardware gives out. It is further necessary to etch, either dry or wet, the resulting pattern after development to form the phase shifter pattern 24 and then remove the remainder of the resist. This results in an increase in the number of the production steps involved, making phase shifter pattern or other deficiencies likely to occur, and there is a rise in the production cost as well. If the number of the production steps is decreased, it will then be possible to prevent such deficiencies and achieve cost reductions. Having been accomplished with such situation in mind, this invention has for its object to provide a process of producing phase shift layer-containing photomask which uses a limited or reduced number of steps and so reduces cost, and makes deficiencies in the phase shifter pattern less likely to occur. According to the present invention there is provided a process for producing a phase shift layer-containing photomask including a light-shielding pattern and a phase shifter for effecting a phase difference between transparent regions isolated from each other by a light-shielding region and located adjacent to each other, characterized in that said phase shifter is formed by the steps of forming, using an ionizing radiation resist, an ionizing radiation resist pattern on a portion commensurate with said phase shifter pattern on the surface of a substrate on which at least a light-shielding pattern and a transparent film of uniform thickness are provided, coating a photoresist over the whole surface of the substrate with the ionizing radiation resist pattern formed thereon and full-exposing the photoresist to light from the rear side of the substrate, thereby to form a photoresist pattern on at least a portion commensurate with the light-shielding pattern, and dry-etching the transparent film of uniform thickness using the photoresist pattern or the photoresist pattern plus the ionizing radiation resist pattern as a mask to form a phase shifter pattern. There now follows a brief description of the drawings accompanying the specification; the Figures are not necessarily referred to in sequence in the text. Fig. 1 is a series of sketches illustrating a conventional phase shift process, Fig. 2 is a series of sketches illustrating the principles of a conventional process not involving phase shifting, Fig. 3 is diagrams illustrating examples of SOGs after formed by heating, Figs. 4 and 5 are a series of sectional sketches for illustrating the production processes of Examples 1 and 2, respectively, and Fig. 6 is a series of sectional views for illustrating a conventional process for producing phase shift masks. Before discussing the production process according to this invention, we will now give a brief explanation to spin-on-glass (hereinafter "SOG" for short) that is a film of an organic solvent solution of an organosilicon compound which has been converted into silicon oxide by coating, drying and heating. The starting materials used for SOG are a metal alkoxide such as tetraethoxysilane - Si(OC 2 H 5 ) 4 , bipolar solvents such as water and methanol and hydrochloric acid. To leave a methyl group (-CH 3 ) in SOG, triethoxymethylsilane - CH 3 Si(OC 2 H 5 ) 3 , diethoxydimethoxysilane - (CH 3 ) 2 Si(OC 2 H 5 ) 3 or trimethylethoxysilane - (CH 3 ) 3 Si(OC 2 H 5 ) may be added to tetraethoxysilane in an amount of a few % to a few tens %. One example of the mixing ratio of these starting materials is: Si(OC 2 H 5 ) 4 :CH 3 Si(OC 2 H 5 ) 3 :H 2 O:C 2 H 5 OH:HCl = 100:2:660:1050:6 Upon mixing together, the starting materials start hydrolysis and polycondensation. These reactions are schematically expressed as follows: wherein Et=C 2 H 5 Polycondensation:≡Si-OH + HO-Si≡→≡Si-O-Si≡+H 2 O Through such hydrolysis and polycondensation, an Si-O polymer (polysilicate) having a low molecular weight is obtained. To obtain a methyl group-containing SOG, a mixture of Si(OC 2 H 5 ) 4 with CH 3 Si(OCH 5 ) 3 is used as one of the starting materials. The reaction scheme is then expressed as follows: Polycondensation:≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H 2 O This low-molecular-weight SOG is spin-coated and softbaked (at 80-120°C) on a substrate, whereby the molecular weight of SOG is slightly increased. Heating [(CH 3 ) n/2 Si(OEt) a (OH) b O c ] n [(CH 3 ) n*/2 Si((OEt) a* (OH) b* O c* ] n* wherein a * <a, b * <b, c * >c, and n * >n Upon this substrate being further heated at 400-500°C, dehydration and dealcoholization proceed to cause a drastic increase in molecular weight, giving an intimate SOG film. [(CH 3 ) n*/2 Si(OEt) a* (OH) b* O c* ] n [(CH 3 ) n'/2 Si(OH) x O y ] n , wherein n', x and y approximate to infinity, zero and two, respectively. The methyl group-containing SOG allows the methyl group in the film to be left behind, even after heating at 400-500°C 3 One example of the SOG structure after heating at 400-500°C is illustrated in Fig. 3. When SOG is irradiated with electron beams, ion beams, radiation light such as X rays, γ rays and SOR or laser light - all hereinafter called the energy beams- - after the above-mentioned soft baking, polymerization takes place at the irradiated region, resulting in an increase in molecular weight. Due to a difference in molecular weight, this SOG has the property of leaving its irradiated region intact, when it is developed with a solvent like an alcohol after irradiation. By the way, when SOG which contains an acid generator such as halogenoacid onium salt and other compounds like triphenyl-sulfonium-trifrate, tetrabromobisphenol, 1,1-bis-parachlorophenyl-2,2,2-trichloroethane, etc. is irradiated with the energy beams after soft-baking, the oxygen generator gives out an acid which in turn acts on the hydroxyl (-OH) and ethoxy (-OC 2 H 5 ) groups remaining in SOG to allow dehydration or dealcoholization to proceed, resulting in an increase in molecular weight. When compared with the acid generator-free SOG, the acid generator-containing SOG has thus the property of being higher in molecular weight (sensitivity) even at a lower dosage of the energy beams. In the following Examples 1 and 2, reference will be made to the process for producing a phase shift photomask containing a phase shift pattern standing upright on the side, which can prevent deterioration of a light-shielding film at the time of etching the phase shift pattern. Example 1. Fig. 4 is a series of sectional sketches illustrating one embodiment of the process for producing this phase shift reticle. As shown in Fig. 4a, provided is a chromium reticle including a glass substrate 73 and a light-shielding pattern 75 thereon, which is built up of a double-structure light-shielding film consisting of chromium and low-reflecting chromium layers. It is noted that a layer 74 is a transparent, electrically conductive film for preventing "charging-up" and other phenomena. Then, an SOG film 76 is formed all over the surface of the chromium reticle to a thickness meeting the following relation: d =λ/2(n-1) Instead of this SOG film 76, use may be made of films formed of SiO by sputtering and SiO 2 and SiN by CVD (chemical vapor deposition) Then, an ionizing radiation resist layer containing a dye and composed mainly of novolak resin is formed on the chromium reticle including the SOG film 76, followed by conventional alignment. 'After that, a given pattern is drawn on the resist layer by ionizing radiation emanating from electron beam exposure hardware, etc., and is then developed and rinsed to form an ionizing radiation resist pattern 77, as sketched in Fig. 4c. As the ionizing radiation resist, use may be made of any other desired resist functioning to reduce light transmittance. Then, as illustrated in Fig. 4d, a photoresist 78 composed mainly of novolak resin is coated all over the surface of the chromium reticle on which the ionizing radiation resist pattern 77 has been formed. To this end, use may also be made of any other desired photoresist resistant to dry etching. Then, the chromium reticle is exposed to light all over the surface from the back side of the mask to form a photoresist pattern 78, as shown in Fig. 4e. When a dye-containing resist is used as the ionizing radiation resist 77 formed on the shifter layer 76 for forming the photoresist pattern 78, the ionizing radiation resist 77 absorbs light, so that the portion of the photoresist pattern 78 including that resist 77 drops in light transmittance. This assures that the photoresist 78 remaining on the ionizing radiation resist 77 comes to have a thickness sufficient for it to serve as an etching mask. Then, as sketched in Fig. 4f, the SOG layer 76 is dry-etched with a reactive plasma 80, while using the thus formed photoresist pattern 78 as a mask, to form a phase shifter mask 76. Finally, as depicted in Fig. 4g, the remaining resist is incinerated out by an oxygen plasma to obtain a complete phase shift photomask. In conjunction with the above-mentioned process, it is noted that when the resist used as the ionizing radiation resist pattern 77 is resistant to dry etching, it is not always required to incorporate a dye into that resist. When a dry-free, dry etching-resistant resist is used, the photoresist pattern 78 is formed only on the light-shielding pattern in the steps of Figs. 4d and 4e by the full-exposure 79 through the back side of the mask. In carrying out dry etching in the step of Fig. 4f, the photoresist pattern 78 on the light-shielding pattern 75 and the ionizing radiation resist pattern 77 on the position to be provided with the phase shifter then serve together as a dry etching mask to protect the light-shielding and phase shifter layers 75 and 76 against etching. Example 2. Example 1 is given to explain one example of the phase shift photomask in which the phase shifter layer 76 is provided on the substrate with the light-shielding pattern 75 formed on it. However, the above-mentioned production process is also applicable to a phase shift photomask in which a light-shielding pattern is provided on a substrate with a phase shifter layer formed on it. Referring now to Fig. 5 , provided is a quartz substrate 81 with a transparent, electrically conductive film 82 formed on it, as sketched in Fig. 5a. Then, as depicted in Fig. 5b, an SOG film 83 is formed on this conductive film to a thickness meeting the following relation: d = /2(n-1) and formed on this film 83 is a light-shielding film 84 consisting of chromium and low-reflecting chromium. As sketched in Fig. 5c, this substrate is coated thereon with an ionizing radiation resist 85, on which a light-shielding film pattern is drawn by ionizing radiation 86 emanating from electron beam exposure hardware, etc. Subsequent development and rising gives an ionizing radiation resist pattern 85. Using this resist pattern 85 as a mask, the light-shielding film 84 is etched to form such a light-shielding pattern 84 as shown in Fig. 5e. As illustrated in Fig. 5f, this substrate is coated thereon with a dye-containing ionizing radiation resist 87, followed by conventional alignment. After that, a pattern is drawn on a portion corresponding to the phase shifter by ionizing radiation 88 emanating from electron beam exposure equipment, etc. Subsequent development and rinsing gives such an ionizing radiation resist pattern 87 as depicted in Fig. 5g. Then, a photoresist 89 is coated all over the surface of this reticle, as shown in Fig. 5h, and the reticle is fully exposed to ultraviolet rays 90 from the back side of the mask. Subsequent development and rinsing gives a photoresist pattern 89 on portions corresponding to the phase shifter pattern and chromium pattern 84, as shown in Fig. 5i. Then, an SOG layer 83 is dry-etched using the formed photoresist pattern 89 as a mask, as shown in Fig. 5 j , thereby to form a phase shifter pattern 83, and the remaining resist is incinerated out by an oxygen plasma to obtain a complete phase shift mask, as shown in Fig. 5 k. According to the process of Examples 1 and 2 wherein the mask for dry etching is formed in a self-aligning manner, it is possible to prevent the light-shielding film from suffering damage at the time of dry etching. To be more precise, lowering of the optical density and surface reflectivity of the light-shielding film can be reduced and the side of the phase shifter pattern can be kept upright as well. It is thus possible to prevent the low-reflecting layer of the light-shielding film from suffering damage and avoid a lowering of the optical density of the light-shielding film, thereby to prevent deterioration of the pattern transferred on the wafer.

Claims: A process for producing a phase shift layer-containing photomask including a light-shielding pattern (75, 84) and a phase shifter (76, 83) for effecting a phase difference between transparent regions isolated from each other by a light-shielding region and located adjacent to each other, characterized in that said phase shifter (76, 83) is formed by the steps of forming, using an ionizing radiation resist (77, 87), an ionizing radiation resist pattern (77, 87) on a portion commensurate with said phase shifter pattern (76, 83) on the surface of a substrate (73, 81) on which at least a light-shielding pattern (75, 84) and a transparent film (76, 83) of uniform thickness are provided, coating a photoresist (78, 89) over the whole surface of the substrate with the ionizing radiation resist pattern (77, 87) formed thereon and full-exposing the photoresist (78, 89) to light (79, 90) from the rear side of the substrate (73, 81), thereby to form a photoresist pattern (78, 89) on at least a portion commensurate with the light-shielding pattern (75, 84), and dry-etching the transparent film (76, 83) of uniform thickness using the photoresist pattern (78, 89) or the photoresist pattern (78, 89) plus the ionizing radiation resist pattern (77, 87) as a mask to form a phase shifter pattern. A process according to Claim 1, wherein said transparent film (76, 83) is a spin on glass film.

Inventors: FUJITA HIROSHI, KURIHARA MASA-AKI, MIKAMI KOICHI, MIYASHITA HIROYUKI, TAKAHASHI YOICHI, FUJITA, HIROSHI, KURIHARA, MASA-AKI, MIKAMI, KOICHI, MIYASHITA, HIROYUKI, TAKAHASHI, YOICHI, Fujita, Hiroshi, c/o Dai Nippon Printing Co. Ltd., Kurihara, Masa-aki, Dai Nippon Printing Co. Ltd., Mikami, Koichi, c/o DAI NIPPON PRINTING CO., LTD., Miyashita, Hiroyuki, Dai Nippon Printing Co. Ltd., Takahashi, Yoichi, Dai Nippon Printing Co. Ltd.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON PRINTING CO., LTD.
Application year: 1991
Main classification: G03F1
IPC codes: G03F1