SILVER BASED PHOTOMASKS

Abstract: In the field of semiconductor photomask manufacture there is a need to produce photomasks more rapidly and with fewer flaws. There is also a need for photomasks capable of producing much finer resolution circuits than hitherto. There is disclosed a method of manufacturing a photomask (10) in which a substrate (11) is coated with an emulsion of gelatin (18) and a photosensitive silver halide (17). On exposure to an electron beam (15) or a laser beam, controlled to generate a desired pattern of movement, the silver halide reduces to create a photomask (10) after development and fixing.

Description: SILVER BASED PHOTOMASKS This invention relates to photomasks, ie. masks that are used mainly in the processing of semiconductor materials during production of integrated circuits (i.c.s) and similar devices. A photomask comprises a substrate (typically of quartz) usually having a flat upper surface to which is bonded a layer of Chromium in a pattern of lines and interconnections desired in an i.e. manufactured using the photomask. Radiation (typically in the ultra-violet wavelength) is projected via the photomask onto a semiconductor material. The radiation is blocked where the Chromium is present and passes through the mask at locations that lack Chromium. Under appropriate chemical and physical conditions the radiation that passes through the mask to the semiconductor is used to define the circuit pattern. In the semiconductor manufacturing industry production of the masks is laborious and limited in the ultimate resolution achievable. The most common process is described below in conjunction with Figure 1 of the appended drawings. The masks are manufactured by producing the quartz substrate 11 having a flat upper surface 12 and completely coating the surface 12 with a layer 13 of Chromium. Next a layer 14 of resist is bonded onto the top surface of the Chromium layer 13 to coat it completely. A resist is a material whose properties are altered by electron bombardment or laser irradiation. The electron beam resist most commonly used in photomasks is polybutyl sulphone. The desired circuit pattern is then formed in the resist layer, by, for example, projection of a beam 15 of electrons from an electron beam generator the output of which is usually computer controlled. This is shown at step (i) in Figure 1. The resist is then developed by means of an appropriate liquid, thereby producing openings in the resist, as shown at 14a (step (ii)). Thereafter a liquid etchant (typically eerie ammonium nitrate solution) is arranged to contact the partly completed mask. The etchant attacks the Chromium in the regions of the mask where the resist layer has been removed. The result is the partially completed mask 10b, shown at step (iii) in Figure 1 , having modified photoresist layer 14a and a similarly modified (ie. etched) Chromium layer 13a. Finally the photoresist layer is removed typically by means of an oxygen gas plasma treatment. This results in the completed photomask 10c having the modified Chromium layer 13a that is the pattern desired in the i.c.s manufactured using the photomask. This manufacturing process is time consuming primarily because it is a multi-stage process. The use of a liquid to remove the unwanted Chromium is disadvantageous. The etching solution dissolves the Chromium isotropically (ie. in all directions simultaneously) so that the lines etc. cut by the liquid etchant broaden as the dissolution takes place. This limits the resolution to which the lines may be defined. In other words, the narrowness of the lines etc, is limited by the liquid etchant. Usually the production of the i.c.s involves an optical reduction system that reduces the size of the pattern on the photomask by typically a factor of five. Using the photomask route described above, the narrowest lines attainable are approximately lμm wide. This limits the resolution of the circuitry that may be formed on a semiconductor. The use of a liquid etchant to create the circuit pattern in the photomask is undesirable for the further reason that the resulting liquid effluent is associated with safety hazards since it contains chromium. The known method for producing photomasks can lead to the production of faulty i.c.s. For example a small region of the photoresist or the Chromium may be inadvertently not removed in the photomask manufacturing process. Such flaws must be corrected before the photomask is supplied to the i.e. manufacturer. Thus there is a need to improve the speed of manufacture, and the resolution of photomasks by means of a process which does not produce undesirable chromium-containing effluent. According to a first aspect of the invention there is provided a method of manufacturing a photomask comprising the steps of: providing a substrate supporting an emulsion including a photosensitive silver halide; and directing photons and/or electrons at the emulsion to interact with the silver halide to define a desired pattern on the substrate, after development, of silver. The substrate preferably is quartz or glass. This method is highly advantageous because: (i) the amount of electron dose needed to create the pattern in the emulsion on the quartz substrate is significantly less than that needed for photoresist removal; (ii) the emulsion can readily include silver bromide particles of a size that permits production of extremely high resolution silver particles in the mask, thereby improving the resolutions achievable in the i.c.s.; (iii) the structure of a blank for forming the photomask is simpler, having only one layer instead of the two layers of the prior art; (iv) the method of the invention does not produce undesirable effluent; (v) the creation of the desired pattern on the photomask is a simpler process. This helps to speed the manufacture of photomasks. More importantly it reduces the chances of particles (of photoresist or Chromium in the prior art) remaining in parts of the mask from which they should be absent. Thus the manufacture of photomasks can be speeded up considerably (up to 100 times compared with the prior art, according to initial observations); resolutions down to dimensions of O.lμm or better are achievable in the i.c.s; and there are fewer chances that the mask will generate faulty i.c.s. Conveniently the emulsion is of or includes a photosensitive silver halide, especially silver bromide; and a gel, preferably gelatin. Thus the emulsion may be similar to, or may indeed be, a photographic emulsion. The method of the invention preferably includes the step of applying the emulsion onto the substrate by conventional methods used to manufacture photographic emulsion plates. Conveniently the emulsion is capable of defining a pattern of a resolution in the range 0.1 to 1 micron. In practice this is achieved through judicious choice of the silver bromide particle size. The invention also resides in a photomask manufactured by a method as defined herein. According to further aspects of the invention there is provided a blank or a photomask comprising a substrate supporting an emulsion including a photosensitive silver halide. The emulsion may be of or may include a photosensitive silver halide, especially silver bromide, and a gel, particularly gelatin. Preferably the emulsion is stabilised. Optionally the emulsion is capable of defining a pattern of a resolution in the range 0.1 to 1 micron. The substrate preferably is quartz or glass. The invention also extends to use of a blank or a photomask as defined herein in the manufacture of an electronic device, especially a semiconductor device. According to yet a further aspect of the invention there is provided an emulsion for use in a method and/or a blank and/or a photomask as defined herein, the emulsion including a photosensitive silver halide, especially silver bromide, and being capable of defining a pattern, on a substrate, having a resolution in the range 0.1 to 1 micron. Conveniently the emulsion is of or includes a photosensitive silver halide, especially silver bromide, and a gel, especially gelatin. There now follows a description of preferred embodiments of the invention, by way of example, with reference being made to the accompanying drawings in which: Figure 1 shows schematically a prior art method of manufacturing a photomask; and Figure 2 shows schematically a blank for forming a photomask; a method; and a photomask according to the invention. The prior art method steps shown in Figure 1 are described hereinabove. Referring to Figure 2 there is shown a blank 9 according to the invention and having a quartz or glass substrate 11 having a substantially flat upper surface 12 to which is secured an emulsion of preferably silver bromide particles 17 and gelatin 18 in a layer 16. Another photosensitive silver halide may alternatively be used, although silver bromide is presently preferred because of ease of availability. Gelatin is the preferred gel since it suspends the silver halide particles homogenously at spacings consistent with the desired line resolutions in the photomask. Also, of course, gelatin is transparent. The emulsion is initially coated, eg. by spinning, onto the upper surface of the quartz substrate. Thereafter an electron beam writer or a laser writer, such as used conventionally to manufacture photomasks as described hereinabove is used under computer control to apply a beam of electrons or photons to the stabilised emulsion. The controller for the electron beam generator causes the electron beam to describe a desired pattern of movement. The beam in turn causes the reaction: whereby the silver bromide on which the electron beam impinges reduces to solid silver and bromine. As indicated in Figure 2, as an alternative to electron beam energy a laser beam may equally well be used. Indeed, the method of the invention would work using virtually any source of photon energy, but electron and laser beams 15 are preferred because: (i) they are high energy beams that give rise to very short processing times; (ii) they are coherent beams whose intensity, resolution and direction can be controlled and varied using a computer controller. Since silver bromide is particularly photosensitive the electron beam dose required in the first step of the mask production process is much smaller than that required for a conventional electron beam resist, such as polybutyl sulphone. In fact we have found that the required electron beam dose is typically at least 50 times less any in some cases need only be 100 times less than that required for polybutyl sulphone. Hence, the manufacture of the photomask is advantageously quick. Use of the laser or electron beam as indicated in step (i) of Figure 2, followed by immersion in a developer results in the partly completed photomask lOd shown at step (ii). In the photomask lOd the layer 16 has been modified to include regions 20 of solid silver and regions of the unreacted AgBr/gelatin emulsion 17,18. In a subsequent step, the layer 16 is "fixed", eg. through use of sodium thiosulphate (IV) solution, which removes the excess, unreacted Ag Br as a soluble complex ion (Ag (S 2 O 3 ) 2 ) 3" so that the silver alone remains in situ in layer 16. The thus completed photomask lOe, whose layer 16 consists only of silver 20 and gelatin 18, is shown at step (iii) in Figure 2. Once manufactured the photomask of the invention may be used in the same way as prior art photomasks, ie. it may be inserted into a string of components including a focussing device and a reducing apparatus known per se whereby to project ultra-violet radiation onto a semiconductor substrate for the purposes of etching the semiconductor and/or producing conductors such as noble metal tracks. The preferred emulsion may be manufactured by precipitating AgBr particles and mixing them with gelatin to create an emulsion of the desired properties. If necessary the rate of precipitation of the particles may be controlled to a rate suitable to produce fine particles that permit the fine resolutions mentioned herein to be achieved. Typically the diameters of the nucleated AgBr particles are in the range 1.0 - 0. lμm.

Claims: CLAIMS 1. A method of manufacturing a photomask comprising the steps of: providing a substrate supporting an emulsion including a photosensitive silver halide; and directing photons and/or electrons at the emulsion to reduce the silver halide to define a desired pattern, on the substrate, of silver and unreduced silver halide. 2. A method according to Claim 1 wherein the emulsion is of or includes a photosensitive silver halide and a gel. 3. A method according to Claim 1 or Claim 2 wherein the silver halide is silver bromide. 4. A method according to Claim 2 or any claim depending therefrom wherein the gel is or includes gelatin. 5. A method according to any preceding claim including the step of applying the emulsion to the substrate. 6. A method according to any preceding claim including the step of stabilising the emulsion on the substrate. 7. A method according to any preceding claim wherein the emulsion is capable of defining a pattern of a resolution in the range 0.1 to 1 micron. 8. A method according to any preceding claim wherein the substrate is or includes quartz. 9. A method according to any preceding claim wherein the substrate is or includes glass. 10. A method according to any preceding claim including the sub-step of directing an electron beam at the emulsion to reduce the silver halide. 11. A method according to any preceding claim including the step of applying a developer to convert irradiated silver bromide particles to silver. 12. A method according to any preceding claim including the step of fixing the silver by removing unreduced silver halide. 13. A method according to Claim 12 including applying sodium thiosulphate (IV) solution to the emulsion to fix the silver. 14. A photomask manufactured by a method accordmg to any preceding claim. 15. A blank for a photomask comprising a substrate supporting an emulsion including a photosensitive silver halide. 16. A blank according to Claim 15 wherein the emulsion is of or includes a photosensitive silver halide and a gel. 17. A blank according to Claim 16 wherein the gel is or includes gelatin. 18. A blank according to any of Claims 15 to 17 wherein the emulsion is stabilised. 19. A blank according to any of Claims 15 to 18 wherein the emulsion is capable of defining a pattern of a resolution in the range 0.1 to 1 micron. 20. A blank according to any of Claims 15 to 19 wherein the substrate is or includes quartz. 21. A blank according to any of Claims 15 to 19 wherein the substrate is or includes glass. 22. A photomask comprising a substrate supporting silver and a transparent gel, the silver defining a desired pattern. 23. A photomask according to Claim 22 wherein the transparent gel is or includes gelatin. 24. A photomask according to Claim 22 or Claim 23 wherein the silver is fixed. 25. A photomask according to any of Claims 22 to 24 wherein the substrate is or includes quartz. 26. A photomask according to any of Claims 22 to 24 wherein the substrate is or includes glass. 27. A photomask according to any of Claims 22 to 26 including silver bromide converted to silver by a developer. 28. Use of a photomask according to Claim 14 or any of Claims 22 to 27 in the manufacture of a semiconductor device. 29. Use of a blank according to any of Claims 15 to 21 in the manufacture of a semiconductor device. 30. An emulsion for use in the method of any of Claims 1 to 13 or in a blank or a photomask according to any of Claims 14 to 29, the emulsion including a photosensitive silver halide and being capable of defining a pattern, on a substrate, having a resolution in the range 0.1 to 1 micron. 31. An emulsion according to Claim 30 of or including a photosensitive silver halide and a gel. 32. A method generally as herein described, with reference to or as illustrated in Figure 2 of the accompanying drawings. 33. A photomask generally as herein described, with reference to or as illustrated in Figure 2 of the accompanying drawings. 34. A blank generally as herein described, with reference to or as illustrated in Figure 2 of the accompanying drawings. 35. An emulsion generally as herein described, with reference to or as illustrated in Figure 2 of the accompanying drawings. 36. Use of a blank or a photomask generally as herein described, with reference to or as illustrated in Figure 2 of the accompanying drawings.

Inventors: CAIRNS JAMES ANTHONY, CAIRNS, JAMES, ANTHONY
Applicants: CAIRNS JAMES ANTHONY, UNIV DUNDEE, CAIRNS, JAMES, ANTHONY, UNIVERSITY OF DUNDEE
Application year: 1999
Main classification: G03F1
IPC codes: G03F1, H01L21