Supported membrane composite structure and its method of manufacture
The Government has rights in this invention pursuant to Contract No. F19628-76-C-0002 awarded by the U.S. Air Force and the Defense Advanced Research Projects Agency. 
    
    
     This is a division of Ser. No. 800,759, filed May 26, 1977, now U.S. Pat. No. 4,170,512. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the fabrication of metal patterns on thin polyimide membranes and more particularly to the production of masks for x-ray lithography. 
     Soft x-ray lithography (see Soft X-Ray Lithographic Apparatus and Process, Smith et al, U.S. Pat. No. 3,743,842, July 3, 1973) has been shown to be an effective and convenient means of replicating high resolution patterns. This capability has been demonstrated by the replication in an x-ray sensitive resist of a repetitive grating with linewidths less than 160 nanometers. In general, the masks used in x-ray lithography consist of a thin transmitter membrane which acts as a mechanical support for the absorber pattern. High attenuation in the mask membrane leads not only to long exposure time, but also to a reduction in the effective contrast of the mask. This is because the softer x-rays which are desirable for lithography are attenuated more than the harder x-rays. It is therefore desirable to minimize the attenuation of the soft x-rays in the mask transmitter membrane by utilizing a material with low absorptivity and by making the thickness of the transmitter membrane as small as possible. 
     Several materials have been used to fabricate x-ray transmitter membranes. These include silicon (See &#34;Soft X-Ray Mask Support Substrate&#34;, David L. Spears et al, U.S. Pat. No. 3,742,230, June 26, 1973), Al 2  O 3  (See P. A. Sullivan and J. H. McCoy, IEEE Trans. Electron. Devices, ED-22, 412, (1976)), Si 3  N 4  (See E. Spiller et al, Solid State Technol., 19 62, (1976)) and Mylar® and polyimide (J. S. Greeneich, IEEE Trans. Electron Devices, ED-22 434, (1975)). Silicon membranes thinner than 3 micrometers have not been fabricated. Silicon membranes are also opaque to visible radiation, a fact which makes the use of optical alignment techniques difficult. Al 2  O 3  membranes as thin as 0.2 micrometers have been produced. However, they are quite fragile and are limited to small areas. Si 3  N 4  and Si 3  N 4  -SiO 2  membranes of 0.1 and 0.2 micrometer thickness, respectively, have been fabricated, but they also are very fragile and have been limited to areas smaller than 3 mm by 3 mm. Mylar® membranes have been used extensively for x-ray masks. The minimum thickness of Mylar® films used for x-ray masks has been limited to the commercially available thickness of 3 micrometers. A major difficulty encountered in the use of Mylar® mask transmitter membranes has been the extreme roughness of the surface of commercially available Mylar®. In addition, it is necessary to provide a means of cooling the thin, heat sensitive polymer film when depositing metal on it or during ion beam etching of the x-ray absorber pattern. Commercially available polyimide films as thin as 6 micrometers have been used as x-ray transmitter membranes. The surface of commercial polyimide is considerably smoother than the surface of commercial Mylar®. However, the difficult problem of heat-sinking the membrane during metal deposition and etching of the absorber pattern remains. 
     It is therefore the object of this invention to provide a novel and useful mask and a process for fabricating an x-ray mask whose transmitter membrane has low x-ray attenuation, has a smooth surface, and is optically transparent. It is a feature of this invention that the thin mask membrane remains attached to a glass substrate during fabrication of the absorber pattern thus eliminating any problems of heating during absorber pattern formation. 
     SUMMARY OF THE INVENTION 
     X-ray masks with optically smooth polyimide transmitter membranes 0.05-50 micrometers thick which are supported on a rigid holder can be fabricated by the following procedure. 
     A glass substrate is coated with a film of polyamic acid (typically Du Pont product PI-2530, U.S. Pat. No. 3,179,634). The film is polymerized by heating, to yield polyimide. The desired absorber pattern is fabricated on the polyimide surface by any of the thin film lithographic techniques known to those skilled in the art. A metal tube is sealed to the polyimide surface using an adhesive, typically, an epoxy, and the glass substrate is dissolved by etching in hydrofluoric acid. The mask membrane can be transferred to a rigid holder smaller than the metal tube by glueing the holder to the top or bottom surface of the membrane and cutting away the excess membrane outside the ring. Distortion of the membrane during mounting of the holder can be avoided by glueing the holder to the polyimide prior to the etching of the substrate. 
     An attractive feature of this process is the fact that the membrane remains attached to the glass substrate during fabrication of the absorber pattern. The problem of heat-sinking a thin membrane is eliminated and handling of the membranes is simplified. 
     Other features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic cross section of the polyimide film on a glass substrate. 
     FIG. 2 is a diagrammatic cross section of the polyimide film with an absorber pattern. 
     FIG. 3 is a diagrammatic cross section of the polyimide film with an absorber pattern on a glass substrate with a metal tube and a rigid mask holder sealed to the polyimide. 
     FIG. 4 is a schematic cross-sectional representation of the apparatus used in the process of etching of the glass substrate. 
     FIG. 5 is a cross section of a completed mask prepared by bonding the rigid mask holder before etching of the substrate. 
     FIG. 6 is a cross-sectional view of the transfer of a polyimide membrane to a rigid holder. 
     FIG. 7 is a cross-sectional view of a completed mask where the membrane has been mounted after substrate etching. 
     FIG. 8 is a cross-sectional view of a mask fabrication step where the mask is to be supported by a ring of the etched substrate after the etching holder is removed. 
    
    
     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. Du Pont 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. Thickness 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 esults 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° C. for 15 minutes and then at 250° 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® 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 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&#39; 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&#39;. 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&#39; 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 such 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.