Patent Number: 041705122
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT A substrate 1 of small, thin glass, typically 0.211 glass, Corning Glass Corp., approximately 200 micrometers thick is thoroughly cleaned of all particulates and contamination by standard techniques strong enough to withstand handling and thin enough to be etched in a reasonable time (any glass thickness can be used). A film of polyamic acid 2, typically E. I. DuPont Corp. Product PI-2530 as described in U.S. Pat. No. 3,179,634 or other polyimide plastic precursors, is spun onto the surface of the substrate 1 by conventional spinning techniques known to those skilled in the art for spinning photoresists. Thicknesses of 0.05-5 micrometers can be obtained by spinning. Any of the well-known coating techniques such as dipping or spraying will also be satisfactory. These thicknesses can be obtained using various dilutions of polyamic acid in a solution of N-Methyl-2-pyrollidone and acetone or any of the solvent combinations suggested by the manufacturer of polyamic acid. One such solution is 4 parts of polyamic acid, 1 part N-Methyl-2-pyrollidone, and 1 part acetone which results in a polyimide thickness of about 1 micrometer when spun at 6000 rpm. Once a film of polyamic acid 2 on the glass 1 has been obtained, the polyamic acid 2 is converted to polyimide 2 by curing the film at 150.degree. C. for 15 minutes and then at 250.degree. C. for 60 minutes. Curing times and temperatures are not critical and other combinations are possible. After the film 2 has been cured, the absorber pattern of the mask 3 is fabricated on the surface 10 of the film 2. The polyimide film 2 is chemically inert and is not significantly affected by most of the chemical or processes used in state-of-the-art mask microfabrication processes and known to those skilled in the art. In addition, the substrate 1 can act as a heat sink for the thin polyimide film or membrane 2. The heat sink substrate 1 allows the use of mask fabrication techniques such as metal evaporation and ion beam etching which generate substantial heat on the surface of the membrane to be used to form the mask of metal, such as gold or platinum. After the absorber pattern mask 3 has been fabricated, a mask holder 4 is glued by adhesive 11 (using for example Tracom 2115 epoxy) to the polyimide 2 as shown in FIG. 3. Also, an etching holder 5 is glued to the polyimide 2 outside of the area encompassed by the mask holder 4. The adhesive 6 used to seal the etching holder 5 to the polyimide, and the etching holder itself must be resistant to the action of the etchant used to subsequently dissolve the substrate 1. Copper tubing is a convenient etching holder and an adhesive 6 resistant to concentrated HF which has been found to be satisfactory is Devcon.RTM. 5 min. epoxy. The etching holder 5 and substrate 1 are held in a solution of Hydrofluoric acid 8 in container 12 shown in FIG. 4. A convenient etching solution for Corning 0211 glass is 2 parts of HF to 1 part H.sub.2 O which etches at a rate of approximately 10 micrometers per minute. Other glasses and etchant solutions may be used. During etching a backing liquid 9 such as isopropyl alcohol can be used to provide a positive pressure against the polyimide surface 2 to prevent the entry of any etchant through pinholes in the polyimide film 2 if the metal mask 3 or mask holder 4 is of a material which is attacked by the etchant. Otherwise, the backing liquid 9 is not required. It is possible to easily ascertain when the glass 1 has been completely dissolved by the etchant 8. This is done by dipping the copper tube-substrate combination into water and observing the etched surface. If the glass has been removed the water will form droplets on the surface which easily run off the polyimide. If glass still remains attached to the polyimide film 2 the water will uniformly coat the glass surface. In this case, further etching is necessary. Once the glass has been removed, the etchant is rinsed from the copper tube 5 and polyimide film 2 by immersion in water and isopropyl alcohol. The backing liquid 9 is removed and the membrane is dried by evaporation or is blown dry using a jet of dry nitrogen. Finally, the superfluous polyimide membrane outside of the mask holder 4 is cut away to yield the completed mask as shown in FIG. 5. An alternative is to transfer the completed mask to a smaller mask holder. An adhesive 7 (Tracom 2115, for example) is applied to the surface of a mask holder 4' and the mask holder is placed on the polyimide membrane 2, as shown in FIG. 6. When the adhesive has hardened, the superfluous membrane is cut away to yield a completed mask as shown in FIG. 7. Although the etching holder 5 must be fabricated from a material which is resistant to the action of the substrate etchant 8, no such restriction applies to the mask holder 4 or 4'. Any rigid material can be used as a mask holder and any shape or thickness of holder is compatible with the process. As an example, the mask holder 4 can be a silicon ring with a flat face. The flatness of the holder 4 is not critical if the holder is mounted before etching of the substrate 1 because the adhesive 7 will conform to the polyimide membrane surface 2, whose flatness has been determined by the flatness of the substrate 1. It will be apparent to those skilled in the art that the glass substrate 1 can be masked and selectively etched by conventional techniques to leave a ring 1' of glass, as shown in FIG. 8, instead of using a separate mask holder 4. It will also be apparent that the supported membrane with a metallic coating, patterned or otherwise, as described in this invention, has other uses than as a mask and the invention is not to be so limited. Although the process has been described in detail using a specific plastic, polyimide in its liquid and hardened form, and a metal deposit thereon, it will be apparent that many other plastics may be used which may be applied in liquid form and subsequently hardened by heat evaporation of a solvent or chemical reaction. The materials deposited on the plastic may be any that are capable of being sputter deposited, vapor deposited or chemically deposited to form a bond to the plastic and need not be limited to a metal such as gold or platinum, which are commonly used for lithographic masks. It is evident that those skilled in the art, once given the benefit of the foregoing disclosure, may now make numerous other uses and modifications of, and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in, or possessed by, the apparatus and techniques herein disclosed and limited solely by the scope and spirit of the appended claims.