Photomask and manufacturing method therefor

Abstract: A photomask (13) for ultraviolet light with a contrast ratio 10 or more can be fabricated by generating color centers. These are obtained by irradiating a calcium fluoride crystal substrate with e.g. a KrF excimer laser (wavelength of 248 nm) at an energy of 10 J/cm² through an original photomask P2 formed by a chromium film on a quartz substrate, and forming a pattern constituted by color centers in regions corresponding to an original photomask pattern. In the same way, a similar photomask can be fabricated by irradiating a calcium fluoride crystal substrate with the X-ray having the wavelength of 0.5 to 1.0 nm through an original photomask formed by gold film on a silicon substrate.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photomask for ultraviolet (UV) to deep ultraviolet light exposure, and particularly to a photomask in which the mask pattern is formed by color centers. With improvements leading to higher and higher integration densities in semiconductor integrated circuits, submicron photolithography has now become indispensable. Accordingly, there is a corresponding need to use shorter wavelength light for the exposure operations. With these developments, the UV radiation known as the G-line or I-line classically obtained from a high pressure mercury lamp is now produced by an excimer laser. For example, dimer type excimer lasers consisting of a rare gas and a halogen gas - such as xenon chloride (XeCl), krypton fluoride (KrF) and argon fluoride (ArF) - emit laser radiation having wavelengths of 306 nm, 248 nm and 198 nm, respectively. Of these laser beams, the 306 nm wavelength produced by the XeCl excimer laser is not significantly different from the 365 nm wavelength obtained from the I-line of high pressure mercury lamps. Consequently, there is no real advantage to be gained from using such a laser source for the exposure of fine patterns, and only the light of 248 nm and 198 nm wavelengths (comprised in the so-called deep-UV spectrum) emitted by KrF excimer laser or ArF excimer laser is now thought to provide the kind of excellent results needed for the exposure to form a fine pattern. 2. Description of the Prior Art A UV or deep-UV photomask, corresponding to a light source for exposure to UV or deep-UV is necessary for photolithography. The prior art photomasks used for this application are typically fabricated by the following method. A thin, light-absorbing film such as chromium (Cr) or chromium oxide (CrO x ) is formed on a mask substrate that is transparent to the UV or deep UV-light. The surface is then coated with a resist film and patterned by electron beam lithography or UV light lithography. Using this resist pattern as the mask, the chromium, or chromium oxide, film is patterned and the desired photomask can thus be fabricated as described above. Not only is such a photomask fabrication method complicated, but the accuracy of the photomask formed is governed by both the accuracy of resist pattern and etching accuracy of chromium, or chromium oxide, film. Consequently, the resulting photomask proves to be unsuitable for the production of sub-micron devices. In the UV and deep-UV wavelength band, there is usually a significant amount of absorption of light, as well as ambient gas, by the optical system's components, such as the projection lens and the photomask substrate. Moreover, the irradiation at UV and deep-UV wavelengths generates color center type lattice defects in the optical materials, which has the effect of lowering their transmissivity. As a result, there arises the problem of the exposure intensity becoming unacceptably low. Accordingly, the selection of suitable materials for the photomask substrate is particularly important. The processes involved in the generation of color centers and their properties have been investigated from various aspects and described in detail, e.g. in the books by J. H. Schulman and W. D. Compton entitled: "Color Centers in Solids", The MacMillan Co. (1962) and by W. B. Fowler: "The Physics of Color Centers", (edited by W. B. Fowler) Academic Press (1968). A photomask obtained by electron beam irradiation of an ionic crystal to selectively create light shielding matter is disclosed in JP-A-57 212 447. The corresponding photomask is comprised of a NaCl sensitive film deposited on a glass substrate and overlaid with a conductive film for protection against electrical breakdown. In the field of photographic processing, e.g. for microfiches, it is known from US-A-3 512 969 to use a recording medium having a surface sensitized by the formation of color centers through selective exposure to penetrating radiation. The image is thereafter stabilized by means of an electric field. SUMMARY OF THE INVENTION It is an object of the present invention to simplify the fabrication of UV or deep-UV photomasks. It is another object of the present invention to improve the patterning accuracy of UV or deep-UV photomasks. These objects are achieved by a photomask of the present invention as defined in claim 1, to be used for photolithography utilizing light having a wavelength comprised in the ultraviolet to deep-ultraviolet wavelength region of the spectrum. Preferably, the color center is generated in a halogenide of an alkali metal such as crystalline lithium fluoride (LiF) or halogenide of an alkaline-earth metal such as crystalline calcium fluoride (CaF 2 ) to form the UV to deep-UV light absorbing material. CaF 2 is particularly well suited as a substrate material because it is highly transmissive to UV to deep-UV light. The color center may be formed by selectively irradiating the predetermined region of such a CaF 2 substrate with the UV or X-rays. The color center will normally absorb light in the visible light to deep-UV light range, and a photomask for the UV to deep-UV band may accordingly be fabricated by tailoring the color centers to the determined pattern. A pattern formed by color centers is essentially a pattern of light absorbing regions defined at the atomic level. For instance, such a pattern may be based on the displacement of lattice atoms or lattice ions in a CaF 2 crystal forming a substrate, or on the excited state of electrons generated by emission or trapping of electrons. Therefore, in the pattern consisting of color centers, the recessed and projected areas do not exist at the surface of substrate, unlike the pattern formed by the Cr thin film. Accordingly, a pattern consisting of color centers can be considered as a mask pattern having an extremely high resolving power. The present invention also relates to a method of manufacturing the aforementioned photomask (claim 3) and to its use for tranferring a pattern (claim 10). Moreover, the fabrication process for the inventive mask is simplified in comparison with conventional photomask fabrication methods which involve such processes as forming light absorbing material, e.g. thin film of Cr on the substrate, forming a resist thereon, patterning the resist, and selectively etching the light absorbing film. Another advantage of the invention is that since the color center is annealable by heat treatment, it is easy to revise (partially omit or add) pattern-work after fabrication of the photomask and to recover, or re-cycle the mask. As a result, the practical use for the photomask can be increased, and a cost reduction can also be obtained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a schematic sectional view indicating a layout of an original photomask for deep-UV processes having an original pattern and a photomask substrate for fabricating a pattern corresponding to the original pattern; Fig. 1(b) is a schematic sectional view indicating a step for fabricating a pattern corresponding to the original pattern on the photomask substrate by irradiation of deep-UV from the rear face of the original photomask; Fig. 1(c) is a schematic sectional view of a photomask fabricated by deep-UV irradiation; Fig. 2 is a schematic diagram indicating resist exposure of a wafer using a photomask fabricated by the method of the present invention; Fig. 3(a) is a schematic sectional view showing the formation of a positive type resist (OFPR-800) on an SiO 2 film formed on the surface of a silicon wafer; Fig. 3(b) is a schematic sectional view showing the formation of a negative resist (PVSS) on the positive type resist (OFPR-800); Fig. 3(c) is a schematic sectional view indicating a resist pattern formed by exposing the negative resist (PVSS) through the photomask obtained by the present invention and then developing the negative resist; Fig. 3(d) is a schematic sectional view indicating a resist pattern formed by removing the positive type resist (OFPR-800) using reactive ion etching by oxygen, with the resist pattern shown in Fig. 3(c) used as the mask; Fig. 4(a) is a schematic diagram of an original photomask for an X-ray process having an original pattern and a photomask substrate layout for fabricating a pattern corresponding to the original pattern; Fig. 4(b) is a schematic diagram indicating a step for fabricating a pattern corresponding to the original pattern on a photomask substrate by X-ray irradiation from the rear face of the original photomask; Fig. 4(c) is a schematic diagram of a photomask fabricated by X-ray irradiation; and Fig. 5 is a partial plan view of a photomask pattern having defects to be revised. DESCRIPTION OF THE PREFERRED EMBODIMENTS Three embodiments of the present invention will be explained hereunder with reference to the drawings from Figs. 1 to 5. First embodiment Figs. 1(a) to 1(c) are schematic sectional diagrams of the essential parts of the fabrication process of photomask by the present invention. In Fig. 1(a), an original photomask P1 forming an original pattern 2 is fabricated by electron beam lithography or UV-lithography by initially producing a light absorbing film, such as Cr film, on a substrate 1 such as a quartz (SiO 2 ) plate. A CaF 2 crystal substrate 3 that exhibits high transmissivity to UV to deep-UV radiation is located in a confronting position with respect to the photomask P1. The rear surface of the original photomask P1 is irradiated with the KrF excimer laser having the wavelength of 248 nm (symbolized by arrows 4 in Fig. 1(b)). Here, the energy of the laser beam 4 irradiating the substrate 3 through the photomask P1 should be at least 10 J/cm 2 . Since color centers are generated at parts of the substrate 3 irradiated by the laser beam 4, there will be produced on the substrate 3 a mask pattern 5 consisting of aggregations of color centers located at parts corresponding to the original pattern 2 shown in Fig.1(c). The original pattern 2 and mask pattern 5 are in a positive-negative relationship. The contrast of the thus obtained photomask - defined as the ratio of the light flux transmitted by the photomask without shielding from the mask pattern to the incident light flux which is shielded and attenuated by the mask pattern - is in excess 10. In the first embodiment, crystalline CaF 2 has been used as the substrate 3. But it is also possible to use quartz for the fabrication of a UV photomask, or to use a halogenide of alkali metal such as of crystalline lithium fluoride (LiF) or a halogenide of an alkaline-earth metal such as crystalline barium fluoride (BaF 2 ) for fabrication of photomask for deep-UV. There is a gradual decline in light transmissivity at the level of the substrate 1 of the original photomask P1 because color centers are also generated on the substrate 1 of original photomask P1 when forming a mask pattern 5 on the substrate 3. However, since the substrate 1 of original photomask P1 is uniformly irradiated with the deep-UV light, it will take longer for the contrast of the mask pattern 5 to fall to a given value, even when color centers are generated. Fig. 2 is a schematic diagram indicating the apparatus used to obtain the photomask shown in Fig. 1(c). The laser beam (shown in dotted lines) has a wavelength of 248 nm and is emitted from a KrF excimer laser 38. The beam is reflected by the mirrors 37, 36, 35 and irradiates the photomask 33 of the present invention through a lens 34. The light having passed the photomask 33 partially exposes a wafer 31 through a projection lens system 32 for patterning of resist. The photomask 33 and the wafer 31, which is placed on a stage 30 are aligned by mechanically translating and rotating the stage 30 until the pattern provided on the photomask 33 and the wafer 31 are aligned. Once the alignment has been achieved, the pattern on the photomask 33 is transcribed onto the resist film 31' overlying wafer 31. Although an KrF excimer laser 38 is described in the embodiment, an ArF excimer laser can also be used instead of the KrF excimer laser 38. It should be noted that Fig 2 does not show the control system for reasons of clarity. The resist patterning is carried out sequentially through the steps shown in Figs. 3(a) to 3(d). Figs. 3(a) and 3(b) show the sectional views of the various steps of the process for resist patterning. A suitable resist can for example be a double-layer type wherein the first layer is a Novolak system positive type photoresist OFPR-800 (a product of Tokyo Applied Chemical Industry Co.) and the second layer is a negative type photoresist PVSS (polyvinyl silsesquioxane). A positive type resist OFPR-800 shown in Fig. 3(a) is coated to the thickness of 1 to 1.5 microns as a first layer 62 on a silicon dioxide (SiO 2 ) film 61 formed on the surface of a silicon wafer 31. As shown in Fig. 3(b), a negative resist PVSS is coated in to a thickness of 0.2 microns as the second layer 63. The thus prepared silicon wafer 31 is loaded to the stage 30 (shown in Fig. 2) and exposed through the photomask 33. In this case, the irradiation energy by KrF excimer laser 38 is 300 to 400mJ/cm 2 . After the exposure, when the second layer 63 is developed, a resist pattern 63' as shown in Fig. 3(c) is obtained. Thereafter, when the first layer 62 is etched using the reactive ion etching by oxygen with the resist pattern 63' used as the mask, the resist pattern 62' as shown in Fig. 3(d) can be obtained, completing the resist patterning. Second embodiment In the first embodiment, the deep-UV is used as the light source for exposure; it may however, be replaced with soft X-rays, as will now be explained. Figs. 4(a) to 4(c) are schematic sectional views illustrating the various steps in the fabrication process of photomask, using soft X-rays. In Fig. 4(a), a substrate 11 of the original X-ray mask P2 is made of an X-ray transmissive material such as silicon (Si), silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ) or silicon carbide (SiC). The periphery of the substrate is fixed by a supporting frame. An original pattern 12 made of X-ray absorbing material such as gold (Au) or tantalum (Ta) is formed on the substrate 11. As in the first embodiment, the CaF 2 substrate 13 is located opposite the substrate 11 of original X-ray mask P2. The distance between the original pattern 12 and CaF 2 substrate 13 is set to 10 to 40 microns, and the original pattern 12 is therefore not in contact with the CaF 2 substrate 13. As shown in Fig. 4(b), the rear face of the substrate 11 of original X-ray mask P2 is irradiated with the soft X-rays. The substrate 13 is selectively irradiated with the soft X-rays through the original pattern 12. The result, shown in Fig. 4(c),is a mask pattern 15 consisting of color centers which is generated according to the original pattern 12 formed on the substrate 11. Although the second embodiment uses a CaF 2 crystal for the substrate 13, it is also possible to use quartz for the fabrication of a UV photomask, and a halogenide of alkali metal such as crystalline lithium fluoride (LiF) or a halogenide of an alkaline-earth metal, such as crystalline calcium fluoride (CaF 2 ) and crystalline barium fluoride (BaF 2 ) for the fabrication of deep-UV photomasks. The thickness of the Au or Ta film forming the original pattern 12 can be made so that a ratio: T(sub)/T(sub)T(m), where T(sub) and T(m) are the X-ray transmissivity of the substrate and the original pattern mask, respectively, i.e. an inverse number of transmissivity of original pattern 12, becomes greater than 10. The soft X-rays 14 may be obtained by synchrotron radiation in the wavelength of 0.5 to 1.0 nm. Third embodiment An embodiment of revision and recovery of photomask by the present invention will now be explained. Fig. 5 is a partial plan view of a mask pattern in the photomask. The mask pattern 55 is formed by color centers, and includes a projected defective part 51 and a recessed part 52. The size of these defective and recessed parts can be from several microns to several tens of microns. The projected defective part 51 is irradiated with a focused CO 2 laser beam or YAG laser beam so as to heat the portion of the substrate 53 to 200°C to 500°C. As a result, the color center forming the projected defective part 51 disappears, thereby eliminating the projecting defective part 51. When the recessed part 52 shown in Fig. 5 is selectively irradiated with the converted KrF excimer laser beam of 248 nm wavelength, or is irradiated through the photomask providing an aperture corresponding to the recessed part 52, a color center is generated at the recessed part 52, and thereby a complementary portion of the mask pattern is added to the recessed part 52. The mask pattern formed by color centers of the present invention is capable of not only revising a pattern, but also of rewriting the desired pattern at the predetermined position of substrate 3 by a similar method. Moreover, the substrate 3 or 13 may be re-used, by causing the entire mask pattern 5 or 15 formed on the substrate 3 or 13 to disappear. This can be achieved by eliminating color centers through a heat treatment throughout the substrate 3, 13, as shown in Figs. 1(c) and 4(c) at the temperature of 300°C to 500°C, under the inert gas environment. Here, it is apparent that the term "photomask" used in the embodiments of the present invention can also be applied to a reticule to be used for a reduced projection exposure method.

Claims: A photomask to be used for photolithography utilizing light having a wavelength comprised in the ultraviolet to deep-ultraviolet wavelength region of the spectrum, comprising : a first substrate (3; 13) made of a material which transmits the light ranging from ultraviolet to deep-ultraviolet wavelength region and in a predetermined region of which substrate is formed a mask pattern (5; 15) to be transcribed, said mask pattern being composed of color centers which absorb the light ranging from the visible to deep-ultraviolet wavelength region; characterized in that said first substrate (3; 13) is made uniquely of material in which color centers can be formed by irradiation with light in the ultraviolet to deep-ultraviolet wavelength region of the spectrum, or soft X-rays. A photomask according to claim 1, characterized in that said first substrate (13) is selected from the group comprising quartz, halogenides of alkali metals, and halogenides of alkaline earth metals. A method for fabricating a photomask to be used for photolithography, characterized in that said method comprises the steps of : providing a first substrate (3; 13) transmitting light ranging from ultraviolet light to deep-ultraviolet light and made uniquely of material in which color centers can be formed by irradiation with light in the ultraviolet to deep-ultraviolet wavelength region of the spectrum, or soft X-rays; providing an original photomask (P1; P2) having original pattern (2; 12) which is formed on an original substrate (1; 11) transmitting light ranging from ultraviolet light to deep-ultraviolet light or soft X-rays, and said pattern absorbing light which corresponds to the light transmitting through said original substrate; arranging said original photomask so that it confronts said first substrate (3; 13); and forming a pattern (5; 15) comprised of color centers corresponding to said original pattern (2; 12) on said first substrate (3; 13) by irradiating said first substrate with the light transmitting through said original photomask (P1; P2). A method for fabricating a photomask according to claim 3, characterized in that said first substrate (1; 11) forming said pattern formed by color centers is selected from the group of materials comprising quartz, halogenides of alkali metals and halogenides of alkaline earth metals. A method for fabricating a photomask according to claim 3, characterized in that said pattern (5) is formed by color centers generated through irradiation of a calcium fluoride crystal substrate (1) a krypton fluoride excimer laser (38) producing radiation at 248 nm wavelength with an energy in excess of 10 J/cm 2 . A method for fabricating a photomask according to claim 3, characterized in that said pattern (15) is formed by color centers generated through irradiation of a calcium fluoride crystal substrate (13) with X-rays of 0.5 to 1.0 nm wavelengths. A method for fabricating a photomask according to claim 3, characterized in that said method further comprises a step of eliminating color centers by selectively heating said color centers formed, corresponding to the original pattern, on the region of said first substrate (3; 13) which transmits the light ranging from ultraviolet light to deep ultraviolet light. A method for fabricating a photomask according to claim 3, characterized in that said method further comprises a step of eliminating a pattern formed by color centers by heating the entirety of said first substrate (3; 13) forming said pattern. A method for fabricating a photomask according to claim 7, characterized in that said color centers is eliminated by heating the part forming said color centers to a temperature in the range of 200°C to 500°C through irradiation of said color centers formed in calcium fluoride with a focused YAG (yttrium-aluminum-garnet) laser (38). Use of the photomask according to claim 1 or 2 for transcribing a mask pattern (5. 15) formed on said photomask ( 3; 13) onto a photosensitive medium provided on a second substrate by exposing the photomask, comprising the steps of: positioning said photomask, comprising the first substrate (3; 13) made of a material which transmits the light ranging from an ultraviolet to a deep-ultraviolet wavelength region and the mask pattern (5; 15) to be transcribed composed of color centers which absorb light ranging from the visible to deep-ultraviolet wavelength in predetermined regions of said first substrate (1; 11), to said second substrate to be irradiated; and transcribing said mask pattern onto said photosensitive medium by irradiation with light ranging from ultraviolet light to deep-ultraviolet light on said photosensitive medium through said photomask. Use of the photomask according to claim 1 or 2 for transcribing a mask pattern according to claim 10 characterized in that said irradiating light is produced by a krypton fluoride excimer laser emitting at a wavelength of 248 nm or an argon fluoride excimer laser emitting at a wavelength of 198 nm.

Inventors: BAN YASUTAKA, TOKITOMO KAZUO, BAN, YASUTAKA, TOKITOMO, KAZUO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: G03F1
IPC codes: G03F1