Temperature gradient means in reactor tube of vapor deposition apparatus
This invention relates to the fabrication of semiconductor devices and more particularly to the fabrication of silicon-on-sapphire (SOS) semiconductor devices. 
     An SOS semiconductor device is an active device formed on a sapphire substrate. The present practice is to form islands of silicon, about 0.5-0.6 μm thick, on a smooth surface of a wafer of sapphire, spinel or monocrystalline beryllium oxide and then further processing the silicon islands by forming the traditional drain, source and gate members therein. SOS devices are a particularly attractive tool for the large scale integrated (LSI) circuit designer in that it has a high packing density due to the natural isolation from one island to another which results from use of the insulating sapphire substrate. In the fabrication of SOS devices, the individual circuits are first formed on a larger wafer which is then separated to form small chips which chips, inturn, are then mounted for use in a system. 
     However, as distinguished from the more traditional methods utilized in the formation of bulk silicon devices, it has been found that certain bulk silicon processing procedures, which heretofore have been used to fabricate SOS devices, cannot be utilized if one is to protect the integrity of an SOS device and to increase its yield. Specifically, sapphire wafers tend to be more sensitive to thermal shock than bulk silicon wafers and processing systems must be devised specifically for SOS wafers that can accommodate the thermal sensitivity of SOS wafers. 
     In accordance with the teachings of our invention a diffusion furnace is described having particular utility in the processing of SOS devices wherein a temperature gradient, per unit length of furnace tube, is introduced into the system that minimizes the thermal shock to which a sapphire wafer may be subject prior to the time the wafer is introduced into the furnace tube, at the commencement of processing. It is also been found that the temperature gradient utilized at the commencement of processing also serves to minimize any thermal shock to the wafer when the wafer is withdrawn from the furnace at the discontinuation of the processing step. 
    
    
     In the Drawings: 
     FIG. 1 is a schematic representation of a typical furnace system utilized to process semiconductor devices and indicating the placement of our novel temperature gradient region introduced therein; 
     FIG. 2a is a schematic representation of a typical furnace system illustrating the placement of thermocouple probes used for the measurement of the temperature gradient produced by our novel system; 
     FIG. 2b is graph comparing the temperature gradient in the apparatus of the invention with that of prior art systems. 
    
    
     Referring now to FIG. 1 there is shown a schematic representation of a typical furnace system having a load/unload station 10 adjacent a gas scavenger station 12. In the prior art furnace systems, the gas scavenger station is typically adjacent furnace zone 16 in order to remove the reactants used in the various processing steps. However, as will be described later, scavenger system 12 and furnace zone 16 in our novel system are separated by gradient zone 14. Adjacent the far end of furnace zone 16 is a gas cabinet 18 in which is stored the various gases and reactants utilized in the process. For example, nitrogen, hydrogen, oxygen, etc. may be introduced into the various furnace tubes 24 and 32 from tanks or sources (not shown) in cabinet 18. Traditionally this is done in controlled amounts at various times, as dictated by the process. Furnace tubes 24 and 32 are typically quartz tubes while liners 26 and 30 are of some refractory material. Both the tubes and the liners have to retain their physical shape and chemical identity in the presence of high temperatures and reactants. 
     In operation, wafers are mounted on boats 36, 38 at the load/unload station 10 for introduction into furance tubes 24 and 32 respectively. Since the gases and reactants are introduced, from cabinet 18, under pressure into furnace tubes 24 and 32 and since both furnace tubes 24 and 32 terminate in scavenger chamber 12 it will be seen that furnace tubes 24 and 32 are under positive pressure. To prevent these gases from exhausting into the room through scavenger chamber 12, the gases are exhausted by means of exhaust fan 20 to the outside atmosphere, as indicated by arrow 22. Thus, since scavenger chamber 12 is under a negative pressure it is also at a slightly elevated temperature with respect to the room temperature, and exhaust fan 20 removes any heat escaping from furnace tubes 24 and 32. Thus, the wafers are provided with additional cooling before being brought out into the still lower room temperature. In operation, boats 36, 38, containing sapphire wafers, are introduced into furnace tubes 24 and 32 at a temperature slightly above room temperature. Depending upon the speed at which boats 36 and 38 move into furnace tubes 24 and 32 will determine the initial temperature to which sapphire wafers are subjected. As soon as boats 36 and 38 enter furnace tubes 24 and 32 respectively, they are subjected to a slightly elevated temperature with respect to the room temperature but considerably lower than the temperature of furnace 16. Depending upon the particular temperatures being used and the speed at which the boats are introduced into the furnace 16 will determine the amount of insulation 28 and 34 to be utilized in order to provide the proper temperature gradient. As illustrated in FIG. 1, furnace liner 26 extends further into gradient zone 14 than liner 30. This difference in length is utilized to illustrate the differences in temperature that one may expect when utilizing differenet processing procedures. For example, one may be utilizing the temperature of about 900°-1000° in furnace tube 24 but only using about 700° in furnace tube 32. In such a situation, the temperature gradient, in the two tubes, may be tailored to provide a similar temperature gradient to boats 36 and 38. 
     By way of example when using a processing temperature of about 850°-900° C. and an introduction/removal boat speed of about 2 inches (50.8 mm)/min, we have found that if temperature gradient zone 14, between scavenger 12 and furnace 16, has a length of about 18-20 inches (457.2-508 mm), one would utilize a layer of insulation 28 of about 1 inch (25.4 mm) thick by 6 (152.4 mm) inches wide. One such insulation material 28 which has been found to provide satisfactory results is labelled &#34;FIBERFRAX DURABACK INSULATION&#34; and is made by Carborundum Co., Niagara Falls, N.Y. Under these circumstances, sapphire wafers are subjected to the minimal thermal shock and will thus produce the least amount of breakage. In the above-mentioned circumstances it is not necessary to extend the furnace liner 26 or 30 into the gradient zone 14. 
     Referring now to FIGS. 2a and 2b there is shown a typical furnace wystem using our invention (FIG. 2a) together with the temperature profile (FIG. 2b). In FIG. 2a a furnace tube 24 having a length of about 80.5 inches and a diameter of about 4 5/32 inches (105 mm) is provided with thermocouples 40, 42 and 44 positioned about 17 inches (431.8 mm) apart. Thermal couple 40 is positioned about 261/2 inches (673.1 mm) from the rear of furnace tube 24 while thermocouple 44 is positioned 20 inches (508 mm) from broken line 46 which line represents the end of a prior art furnace tube. An extension having the same inside dimension as the furnace tube is added to furnace tube 24 and constitutes gradient zone 14. The gradient zone 14 or extension furnace tube 24 is inserted between furnace zone 16 and scavenger zone 12 and is provided with insulation sleeve 28, as previously described. 
     The efficacy of our system to reduce breakage and thus increase yields was verified by inserting boat 36 (FIG. 1), having wafers thereon, into furnace tube 24 and into the area between thermocouples 40 and 44. Thereafter, the normal processing was performed on the wafers and at the conclusion of processing the boat was withdrawn, at the rate of 2 inches (50.8 mm)/minute. Various measurements were taken of temperature and boat position after the loaded boat passed thermocouple 44 but before it entered scavenger zone 12. These curves are the plotted curves in the graph of FIG. 2b. The curve labelled II is the plot of temperature versus distance from thermocouple 44 as the boat as being withdrawn from a furnace tube toward broken line 46 in a furnace system with no gradient zone. The curve labelled I is the plot of temperature versus distance from the thermocouple 44 as the boat was being withdrawn from furnace tube 24 that was provided with the gradient zone 14 of our invention. The following chart presents the actual measured values that were plotted as curves I and II respectively in FIG. 2b. The data presented in the column labelled &#34;DIST (in)&#34; is the distance, in inches, beyond thermocouple 44. 
     
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             TEMP. (°C.)                                           
DIST. (in)     I            II                                            
______________________________________                                    
 0             870          870                                           
 2             870          870                                           
 4             860          860                                           
 6             835          835                                           
 8             805          805                                           
10             760          760                                           
12             700          700                                           
14             615          615                                           
16             530          530                                           
18             460          400                                           
19             425          300                                           
20             359          200                                           
22             325          115                                           
24             260                                                        
26             210                                                        
28             165                                                        
30             130                                                        
32             115                                                        
34             100                                                        
36             85                                                         
38             70                                                         
40             55                                                         
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     Thus, we have provided a furnace system wherein the processed wafers are made to exit the furnace tubes at a lower temperature than heretofore possible. Since the wafers are now at a lower temperature when they enter the relatively cool scavenger zone, they are subject to less of a thermal stress resulting in fewer broken wafers thus increasing yields. 
     While we have described the process and shown the measured values involved with withdrawing a boat loaded with processed sapphire wafers, it should be obvious to those skilled in the art that similar data would be obtained (although slightly displaced) in a situation where a boat loaded with wafers is inserted into a system with no temperature gradient zone 14 as compared to inserting a boat loaded with sapphire wafers in a furnace system having a temperature gradient zone 14.