Semiconductor substrate heater and reactor process and apparatus

Claim 1

1 An apparatus for heating semiconductor substrate. comprising: a heating container comprised of thermally conauc- tive material: support means for supporting at least one substrate against the external surface of said container at a substrate heating location is oreferred as the heat transfer medium. Bismuth or some eutectic mixture of indium and bismuth or indium and tin can also be used. Indium has very low viscosity in the molten state even at temoera- tures slightly above the melting point, similar to water. Low viscosity is crucial because conductive and convective heat transferring properties are inversely proportional to heat transfer uniformity. Hence, the lower the viscosity of the medium the less resistance to flow of the heat transfer medium and thus the heat, the higher the efficiency in heat transfer. Moreover. both indium and bismuth possess the requisite temperature range of melting points of less that 350°C and boiling points greater than 1000°C which is necessary for the reaction process. The melting point values of indium and bismuth are 156°C and 271°C, respectively; and the boiling point values of the same are 2087°C and 1420°C, respectively. The melting point of a 50 weight % indium - bismuth eutectic is approximately 89°C and the boiling point is over 1500°C. Indium does not exhibit the toxicity of bismuth or many other metals. Other material properties considered in selecting metals as the heat transferring medium and which are appropriate for insuring maximum system and product efficiencies include: a metal nonreactive to the container materials and the materials used in the reaction, a metal non-toxic and non- hazardous to the personnel handling such materials, a metal non-permeable through the container walls and in which the material of the container walls is not soluble and a thermally conductive metal with a low vapor pressure at temperatures up to and exceeding 1200°C. Furthermore, unless the metal has been contaminated by spilling or the like. indium and bismuth are not easily oxidized and therefore are indefinitely usable. While the preferred embodiment of the application has been described with respect to a thin film chemical vapor deposition process but is equally applicable to other semiconductor vapor fabrication processes such as physical vapor deposition. plasma enhanced chemical vapor deposition and thermal oxidation. The present invention produced an unusually uniform temperature across the substrate heating locations and areas. Fig. 5A illustrates different temperatures measured at the designated positions on the surface of the heating apparatus 15. Fig. 5B illustrates different temperatures measured on the exposed face of a four-inch diameter silicon wafer positioned on the heater 16. In Fig. 5A. the scnematic outline of the heating apparatus 15 is shown with the outline of a six-inch wafer also shown on the surface. Tests were made using a container 16. ten inches wide. eleven inches high and 1-3 4 inch tnick. made of 304 stainless steel whicn had been neat treated after welding and with the wall thickness on one wafer side of the container 16 1 4 inch thick and on the other side 12 inch thick The container included five 1.2 inch diameter rods of the general configuration illustrated in Fig. 3, two 8-inch rods. two 10- inch rods and one 12-inch rods. positioned witnin a heat transfer medium of pure indium that was about nine inches deep in the container 16. The container was positioned inside and spaced from a gold plated stainless steel heat shield. To each of the five rods, 1.5 kilowatts was applied. and in the particular tests, there was less than five kilowatts directed into the heating medium. The rods were high temperature cartridge heaters. models CM 1 2 inch insulated stainless steel sold by Fast Heat of Elmhurst. Illinois. Two different sets of temoerature measurements were taken at each of the five designated positions on the front wall of container 16. The first temperature measurements which are enclosed in rectangles were taken with energy applied to the heater rods and the temperature of the indium inside the container determined by a thermocoupie TC to be 798°C. The measurements at this temperature and in the indicated configuration snow a variation of no greater and ±38% on the wafer heating location of the face of the container 16. The second set of temperature readings which are shown circled were taken after the power had been turned off to the heaters and the temperature reading of the indium via the thermocouple had fallen to 336° and show a temperature variation of no greater than ±9%. Fig. 5B illustrates temperature readings taken using the apparatus described above with reference to Fig. 5A except that the heat shield was not used and a temperature measurements were made directly on the surface of a 4-inch diameter wafer mounted on the front face of the heating apparatus 15. Five cartridge heaters were used in this test all type CIR 3/8-inch diameter heaters made by Chromalox Industrial Heating Products of Pitts- burgh. Pennsylvania. There were two 8-inch rods type 2080. two 10-inch rods type 2101. anc one 12-inch rod type 2121..7 kw was applied to each of the 8-inch rods and 1 kw was applied to eacn of the other three rods. The readings shown in rectangles were taken with energy applied to the heater rods and with the temperature of the indium stabilized at 459°C as determmed through tne thermocouple TC- The temperature variation was nc more than ±12%. The set of circled readings was taken as the indium was being heated up anc was read to be 350°C The variation in temperature across the wafer was no more than 11 % Obviously. various modifications can be made a heat transfer means comprising a low melting point, high boiling point metal contained within said container in contact with the walls of said container at least to the level of the top of the substrate heating location for uniformly transferring heat to said container and said substrate heating location: and heating means for heating said heat transfer means.