Method of making a cathode for RF sputtering amorphous semiconducting thin films
United States Patent 1 Chen et al.  
 [ 1 Apr. 15, 1975 METHOD OF MAKING A CATIIODE FOR RF SPUTTERING AMORPHOUS SEMICONDUCTING THIN FILMS [75] Inventors: Arthur C. M. Chen; Jish-Min Wang,  
 both of Schenectady, NY.  
 [73] Assignee: General Electric Company,  
 Schenectady, NY.  
 [22] Filed: Mar. 30, 1973 [21] App]. No.: 346,468  
 [52] US. Cl 65/18; 65/59 [51] Int. Cl C03c 27/02 [58] Field of Search 65/59, 18  
 [56] References Cited UNITED STATES PATENTS 3,389,215 6/1968 Rice et al 65/59 X Primary Examiner-Arthur D. Kellogg Attorney, Agent, or Firm.lerome C. Squillaro; Joseph T. Cohen [57] ABSTRACT An improved method for making cathodes useful in making amorphous semiconductor thin films by sputtering is disclosed. The cathode is formed by hot pressing powdered, bulk amorphous semiconductor glass onto a metallic substrate at approximately the softening temperature of&#39;the glass.  
 5 Claims, 3 Drawing Figures PMENTEDAFR 1 SL975 3,877, 918 v 7 sum 2 o 2 I I I I /l I METHOD OF MAKING A CATHODE FGR RF SPUTTERING AMORPHOUS SEMMQNDUQTENG THEN lFllLMS This invention relates to methods of making cathodes and, more specifically, to an improved method of making a cathode for RF sputter-depositing amorphous semiconductor thin films.  
  The preparation of amorphous semiconductor thin films. e.g. germanium-tellurium-arsenic compounds. is complicated by the high vapor pressure of some constituents at low temperature and the large differences in the vapor pressures of the materials; or, stated an&#39; other way, the temperature at which these materials have the same vapor pressure varies widely. in the Ge- Te-As system noted above. the temperatures at which these materials have the same appreciable vapor pressure, e.g. l Torr., are l400 375 300C, respectively.  
  Another difficulty is that some consitutents of the semiconductor. such as arsenic and tellurium, tend to form polyatomic species upon subliming. Further, the sticking coefficient for the constituents is frequently different.  
  For these reasons, known vacuum evaporation methods applied to the bulk material do not produce thin film compositions corresponding to that of the bulk starting material.  
  Of the several methods of making thin chalcogenide films, only RF sputtering is commercially feasible. Multiple source evaporation and flash evaporation can be used to produce limited quantity films for scientific investigation, but require careful control of deposition parameters. Even so, these latter two methods produce films at a low yield rate, i.e. a large percentage of the product is likely to have defects. such as pinholes. Further. there are uniformity and reproducibility problems.  
  The RF sputtering process is not without difficulty either. For example, the composition of the resulting thin film may differ from that of the bulk starting material. This is believed due to thermal evaporation from the source due to heating, causing material to be deposited at different rates.  
  In the prior art, chalcogenide glasses are mixed and cast into a wafer approximately one-fourth inch thick and then bonded, typically with epoxy, to a metallic substrate. The thickness is required due to the characteristic brittleness of chalcogenide glasses (i.e. glasses made with an element from period Vl-A of the periodic table). Chalcogenide glasses also characteristically have low thermal conductivity. This, combined with the thickness of the casting, provide a cathode likely to have hot spots, particularly at high sputtering rates. Due to the unequal vapor pressures at a given temperature of the material. thermal evaporation due to hot spots results in more of one constituent coming off the cathode than another, which produces a variable composition of the compound being formed.  
  in view of the foregoing, it is therefore an object of the present invention to provide an improved method of making a cathode source for sputter-depositing amorphous semiconducting thin films.  
  Another object of the present invention is to provide a sputtering cathode having reduced thermal evaporation during RF sputtering.  
  A further object ofthe present invention is to provide improved RF sputtering apparatus for producing reproducible composition amorphous semiconductor thin films.  
  The foregoing objects are achieved in accordance with one embodiment ofthe present invention wherein an improved cathode comprising a high thermal conductivity metallic substrate covered with a layer of powdered bulk amorphous semiconductor not greater than 0.100 inch thick is formed by grinding high purity materials such as germanium, tellurium and arsenic to a fine mesh. mixing the materials together and alloying them to provide a uniform composition. The alloyed composition is then ground to a fine particle size and under selected temperatures and pressures the fine particies are pressed into a desired cathode structure on a supporting substrate. Cathodes made by this method produces RF sputter-deposited films of substantially uniform composition with a high degree of reproducibility.  
  A more complete understanding of the present invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:  
  FIG. 1 is a flow diagram of the steps in the preparation of improved cathodes in accordance with the present invention.  
  FlG. 2 illustrates a suitable press for making sputtering cathodes in accordance with the present invention.  
  FIG. 3 illustrates a cathode in accordance with the present invention in place in a RF sputter deposition system.  
  Referring to FIG. 1, the cathodes of the present invention are made by mixing together high purity starting materials, alloying. quenching, crushing the bulk material and hot pressing.  
  Specifically, high purity (99.99-99.9997z) starting materials. such as germanium, tellurium. and arsenic. in the desired composition. are ground to a relatively small size, typically 100 mesh. These materials are then thoroughly mixed and put in a scalable quartz crucible for alloying.  
  For alloying, the crucible is evacuated, flushed once or twice with argon, or other inert gas, evacuated, down to approximately I u. and sealed. The crucible is then placed in a furnace, rotated continuously and held at approximately 1000C for about 24 hours to produce an alloy of uniform composition.  
  The crucible is then quenched in water to rapidly solidify the amorphous glass. After cooling, the crucible is cracked away from the glass, which is then crushed or ground to a particle size of mesh or smaller in preparation for pressing onto a metallic substrate in a hot press as illustrated in FIG. 2.  
  During the hot-press operation, the glass powder is raised to the softening temperature of the glass, typically l0-l5C below the melting point of the glass, while the press applies a pressure of between 500 and 1500 p.s.i. for lO-ZO minutes. The temperature and pressure may conveniently be increased together. The temperatures, pressure, and time may vary from glass to glass. but are readily determined empirically.  
  Of the three variables, temperature is the most critical. The temperature must be raised only to the softening temperature of the glass and not to the melting point. in the following table, the approximate softening temperature is given for various composition chalcogenide glasses.  
  In general. in the Ge-Te-As system. the softening temperature is between 250 and 500C. As a specific example of the values for pressure and time. for the composition Ge Te As a pressure of 1000 psi. for  
 IS minutes is suitable.  
  After the pressure is released. the now completed cathode is permitted to cool in place and is then removed.  
  As illustrated in FIG. 2. the hot-press comprises a lower ceramic spacer 10. a bottom graphite plunger 11 having an upper surface shaped to accept metallic plate 12, for example aluminum. which serves as the substrate for the cathode. Top graphite plunger 14 is mechanically coupled by way of graphite spacer l and ceramic spacer 16 to platen 17 of the hot-press. Walls 18 of the press are also formed of graphite and are surrounded by layers of insulating material comprising wall 19 of zirconia and an asbestos layer 20. Surrounding the hot-press is a suitable high temperature insulator such as Pyrex glass layer 21 which serves to electrically insulate induction coils 22 from the hot-press. A thermocouple 23 may also be provided for monitoring the temperature within the furnace. Volume 13. defined by aluminum plate 12. walls 18 and top plunger 14 contains the powder formed as described above.  
  In operation. the powder formed by regrinding the amorphous semiconductor is poured into chamber 13 to the desired depth (0.04 to 0.100 inches) and approximately leveled. Plunger 14 is then inserted and coupled to platen 17 as described above. The pressure applied is simultaneous with heating from induction coils 22 which serves to soften the amorphous semiconductor and enable the particles to bond to each other and to metal plate 12.  
  Metal plate 12 preferably comprises aluminum or other suitable high thermal conductivity metal. Since metal plate 12 is typically machined (turned) and relatively smooth. adhesion of the softened amorphous semiconductor to metal plate 12 can be aided by roughening the surface of metal plate 12 or by flame spray coating a thin (0.0050.010 inch) layer of the same metal upon the surface of metal plate 12. Flame spray coating is a process known in the art wherein small pieces of metal. for example. aluminum. are heated in a flame while being conveyed by a jet of air. The metal melts to form small liquid droplets which are then in effect spray painted upon a surface. The resultant surface gives the impression of painted sandpaper and is suitably rough for adhesion by the softened amorphous semiconductor. It should be understood that sanding, sandblasting or flame spray coating are but some of the ways that the surface of metal plate 12 may be suitably roughened.  
  6 Graphite lS utilized as the walls of the press since it has been found that the chalcogenide glasses in the softened state attack such materials as stainless steel and that these materials cannot therefore be used as the walls of the press. A suitable non-reactive material for the walls of the press has been found to be graphite.  
 Other. non-reactive materials may also be utilized.  
 5 FIG. 3 illustrates a conventional sputtering chamber in which the improved sputtering cathode of the present invention may be utilized. Specifically. the sputtering chamber comprises a bell jar overlying a metallic. grounded base 31 having passageways therethrough for evacuation or the admission of a suitable gas which has been purified by a gas purifier. Specifically, evacuation of the chamber is provided by pump 33 connected to the chamber by conduit 32 containing nitrogen trap 34. Any desired gas may be admitted to the chamber by way of conduit 35 and valve 36.  
  Within the chamber substrate holder 37 contains a plurality of cooling apertures 38 for a suitable coolant. Substrate holder 37 contains substrate 39 for receiving the amorphous semiconductor thin film 41 to be deposited. If desired, a shutter 42 is provided for controlling the sputtering duration. Shutter 42 is illustrated in the open position by a solid line and in the closed position by dash line 43. The sputtering cathode comprises metal substrate 12 having the powdered amorphous semiconductor layer 44 attached thereto. A cooling backing 45 may be provided for substrate 12 which is electrically connected by way of rod 46 to a source of RF energy.  
  As known. in operation. the energization of the cathode 44 causes the material to be sputter deposited as a thin film 41 upon substrate 39. Utilizing the cathode in accordance with the present invention. containing a 0.10 inch thick amorphous semiconductor powder, produces a thin film having a composition within 0.5% of the composition of the starting material.  
  There is thus provided by the present invention an improved method of making a cathode in which the source material bonds itself in a thin layer to a metallic substrate. The improved cathode enables reproducible thin films to be deposited and provides thin films having a composition corresponding to the composition of the starting material on the cathode.  
  In view of the foregoing it will be apparent to those of ordinary skill in the art that various modifications can be made within the spirit and scope of the present invention. For example, while described in conjunction with the more common chalcogenide. Ge-Te-As, other amorphous semiconductor glasses may be utilized such as Ge-Te-Sb, and Ge-Se-As.  
 Patent of the United States is:  
  1. The method of making an improved cathode for RF sputtering of chalcogenide glasses comprising the steps of:  
 grinding a bulk glass comprising germanium-tellurium-arsenic to a particle size of less than 80 mesh; preparing a metallic substrate with a roughened contact surface for the glass;  
 heating and pressing together the ground bulk and said metallic substrate, said heating being at the softening temperature of said glass and said pressing being at a pressure of between 500 and 1500 pounds per square inch.  
  2. The method as set forth in claim 1 wherein said heating is at a temperature of 250-500C and said pressure is approximately l000 pounds per square inch.  
 What we claim as new and desire to secure by Lettersgermanium. tellurium and arsenic:  
 heating the mixture to about l0O0C to alloy the material.  
 quenching the mixture in water to form a bulk amorphous glass:  
 crushing the bulk glass to a particle size of less than mesh to form a powder; and  
 hot pressing said powder onto said substrate at a temperature of approximately 400C with a pressure of about 1000 pounds per square inch.