Abstract:
A method of semiconductor filming wherein a thin film is deposited on a wafer under an atmospheric pressure, which comprises the steps of simultaneously supplying a reactive gas and an inert gas to a reaction tube and maintaining a partial pressure of the reactive gas constant by adjusting the flow rates of those gases, whereby stability in film quality is improved.

Description:
This application is a division of application Ser. No. 08/571,538, filed Dec. 13, 1995, now U.S. Pat. No. 5,798,141. 
    
    
     TECHNICAL BACKGROUND 
     The present invention relates generally to a system for producing a semiconductor and in particular to a method and apparatus for semiconductor filming wherein a thin film, such as an oxidation film, is deposited on a surface of a silicon wafer. 
     Semiconductor manufacture involves a step wherein an oxidation film is generated as an insulation film on a wafer surface, which step is carried out by means of an oxidation system. A known oxidation system will be explained hereunder with reference to FIG. 3. 
     Inside a reaction tube 1, there is provided a boat 3 carrying a lot of horizontally oriented wafers 2 laid in a multi-storied fashion. The boat 3 is vertically disposed, via a boat stand 5, on a furnace opening cover 4 which airtightly covers a lower end of the reaction tube 1. In a ceiling of the reaction tube 1, there are provided a plurality of gas introduction holes 6 communicating with a gas introduction pipe 7. An exhaust pipe 8 communicates with the inside of the reaction tube 1 at a lower part thereof. The gas introduction pipe 7 is connected to a source of oxygen gas not shown and has a valve 9 and a flow rate adjuster 10 thereabove. 
     Referring to FIG. 4, oxidation operation in the known system will now be explained. 
     After the wafer 2 are loaded on the boat 3, the boat 3 is introduced into the reaction tube 1. The internal temperature of the reaction tube 1 is elevated at a rate of 5-10° C./min to a preset temperature (900° C. in FIG. 4) and maintained at that temperature. The exhaust pipe 8 is opened to feed oxygen from the gas introduction pipe 7 into the reaction tube 1 to thereby oxidize the wafer 2 surface. Upon completion of the oxidation process, the gas is exhausted from the exhaust pipe 8. 
     After the oxidation process is completed, the internal temperature is lowered at a rate of 2-5° C./min to a predetermined temperature, e.g., 800° C. and maintained at that temperature. After lapsing of a predetermined time period, the boat 3 is taken out of the reaction tube 1. 
     Since the exhaust gas is eventually exhausted to the atmosphere, the back pressure of the exhaust gas is greatly influenced by the atmosphere. Thus, internal pressure of the reaction tube 1 is liable to be influenced by the atmospheric pressure. Further, because the oxidation rate of the wafer depends on the partial pressure of oxygen, a change in the atmospheric pressure leads to a change in the partial pressure, and the oxidation rate changes as the atmospheric pressure changes. Consequently, there has been a problem that the quality of the oxidation process varies depending on the atmospheric pressure during such process and thus becomes unstable. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to maintain an absolute pressure of a reaction gas during a process such as an oxidation process to thereby achieve improved stability in product quality. 
     According to one aspect of the present invention, the above object is met by a method of semiconductor filming wherein a thin film is deposited on a wafer under an atmospheric pressure, the method comprising the steps of simultaneously supplying a reactive gas and an inert gas to a reaction tube and maintaining a partial pressure of the reactive gas constant. 
     In one particular form of the method, a flow rate of the inert gas is changed relative to a change in the atmospheric pressure and the partial pressure of the reactive gas is maintained constant. 
     In another particular form of the method, a flow rate of the reactive gas is changed relative to a change in the atmospheric pressure and the partial pressure of the reactive gas is maintained constant. 
     In a further particular form of the method, a flow rate of one of the reactive and inert gases is regularly changed relative to a change in the atmospheric pressure and a flow rate of the other one of the reactive and inert gases is changed relative to a change in the flow rate of the one of the reactive and inert gases and a change in the atmospheric pressure whilst the partial pressure of the reactive gas is maintained constant. 
     According to another aspect of the present invention, there is provided an apparatus for carrying out semiconductor filming, which comprises a reactive gas supply pipe for supplying a reactive gas to a reaction tube, an inert gas supply pipe for supplying an inert gas to the reaction tube, a flow rate adjuster disposed on each of the reactive gas supply pipe and the inert gas supply pipe, a barometer for measuring an atmospheric pressure, and a controller for controlling a flow rate of at least one of the flow rate adjusters based on the measurements of the barometer. 
     In a particular form of the apparatus, the controller includes an arithmetic means for operating the flow rates of the inert and reactive gases for maintaining a partial pressure of the reactive gas employing as variables a change in the atmospheric pressure, the flow rates of the inert and reactive gases, the partial pressure of the reactive gas and the atmospheric pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view illustrating an embodiment of the present invention. 
     FIG. 2 is a flow chart showing the operation of the embodiment. 
     FIG. 3 is a schematic perspective view illustrating a known apparatus. 
     FIG. 4 is a graph illustrating a manner of oxidation film deposition. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiment of the present invention will now be explained with reference to the accompanying drawings. 
     In FIG. 1, same reference numerals will be used for corresponding parts shown in FIG. 3. 
     Inside a reaction tube 1, there is provided a boat 3 carrying a plurality of horizontally oriented wafers 2 laid in a multi-storied fashion. The boat 3 is vertically disposed, via a boat stand 5, on a furnace opening cover 4 which airtightly covers a lower end of the reaction tube 1. In a ceiling of the reaction tube 1, there are provided a plurality of gas introduction holes 6 communicating with a gas introduction pipe 7. An exhaust pipe 8 communicates with the inside of the reaction tube 1 at a lower part thereof. The gas introduction pipe 7 is branched to provide a reactive gas supply pipe 11 connected to an oxygen gas source (not source) as a source of reactive gas and an inert gas supply pipe 12 connected to a source of inert gas (not shown). 
     The reactive gas supply pipe 11 has a flow rate adjuster 10. Upstream and downstream of the flow rate of adjuster 10, there are provided a valve 13 and a valve 9, respectively. Similarly, the inert gas supply pipe 12 has a flow rate adjuster 14, upstream and downstream of which there are provided a valve 16 and a valve 15, respectively. 
     On the inventive apparatus, there is also provided a controller 17 for controlling the flow rates of the flow rate adjusters 10 and 14, in which pressures detected by a barometer 18 are input. 
     Operation of the apparatus will be described hereinbelow with reference to FIG. 2. 
     When the valves 13 and 9 are opened, the flow rate of the flow rate adjuster 10 is set by means of the controller 17 and a reactive gas, e.g. oxygen, is supplied at a constant flow rate. Then, the valves 16 and 15 are opened to place the inert gas in condition to be supplied through the gas introduction pipe 7 to the inside of the reaction tube 1, and the flow rate of the flow rate adjuster 14 is set by means of the controller 17. The flow rate is controlled such that the absolute pressure of the oxygen gas at a reference atmospheric pressure corresponds to a set pressure. 
     A value indicative of the atmospheric pressure detected by the barometer 18 is input into the controller 17. The controller 17 computes a deviation between the detected atmospheric pressure and reference atmospheric pressure. It then computes the flow rate of the inert gas at which the partial pressure of the oxygen gas is maintained constant under such deviation and controls the flow rate adjuster 14 to allow flow of the relevant gas at the computed flow rate. 
     The partial pressure of oxygen may be held constant in several ways. For example, the flow rate of the inert gas may be maintained constant while the flow rate of the oxygen gas is varied in correspondence with a change in the atmospheric pressure. Alternatively, the flow rate of either the inert gas or oxygen gas may be changed regularly, that is, in accordance with a functional equation, and the flow rate of the other gas may be changed relative to that change and a change in the atmospheric pressure. 
     Relations between the inert gas flow rate, oxygen gas flow rate, atmospheric pressure and partial pressure of the oxygen gas are represented by Equation 1 and Equation 2, shown below, where the inert gas flow rate is R i  ; the oxygen gas flow rate is Ro 2  ; the partial pressure of the oxygen gas is Po 2  ; and the atmospheric pressure is P A . 
     
         R.sub.i =Ro.sub.2 (P.sub.A /Po.sub.2 -1)                   Equation 1 
    
     
         Ro.sub.2 =R.sub.2 (P.sub.A /Po.sub.2 -1)                   Equation 2 
    
     These operational equations 1 and 2 are input for presetting the controller 17. The controller 17 performs the required operations, and the flow rate adjusters 10 and 14 are controlled as explained hereunder. 
     When the flow rates of the oxygen gas and the partial pressure of the oxygen gas are maintained constant, an operation is carried out according to Equation 1. Based on the detected atmospheric pressure and the preset partial pressure of the oxygen gas, the desired flow rate of the inert gas is computed. A control signal is then output to the flow rate adjuster 14 to control same to allow the relevant gas to flow at the computed flow rate. When the flow rates of the inert gas and the partial pressure of the oxygen gas are maintained constant, the operation is carried out according to Equation 2. Based on the detected atmospheric pressure and preset partial pressure of the oxygen gas, the desired flow rate of the oxygen gas, is computed. A controlled signal is then output to the flow rate adjuster 10 to allow the oxygen gas to flow at the computed flow rate. When one of the flow rates of the inert and oxygen gases is to be changed regularly, an operation is performed according to either Equation 1 or Equation 2 to obtain a variable for maintaining the partial pressure of the oxygen gas constant. 
     Table 1 below shows an implementation where the flow rate of the oxygen gas is maintained constant and the partial pressure of oxygen is maintained constant at 900 hpa. The values shown in Table 1 are obtained by Equation 1 above. 
     
                                           TABLE 1__________________________________________________________________________ATMOSPHERIC OXYGEN  INERT GAS                       OXYGEN PARTIALPRESSURE    FLOW RATE               FLOW RATE                       PRESSURE[hpa]       [SLM]   [SLM]   [hpa]__________________________________________________________________________1  1,000    9       1       9002  980      9       0.8     9003  950      9       0.5     900__________________________________________________________________________ 
    
     As is now apparent, notwithstanding occurrence of a change in the atmospheric pressure, it is possible to maintain the partial pressure of the oxygen gas constant by changing the flow rate of the inert gas. Further, since the pressure inside the reaction tube is kept substantially equal to the atmospheric pressure, existing oxidation furnaces can also be used as they are. 
     Next, in a first way of controlling the inventive apparatus, the atmospheric pressure is first detected by the barometer 18 prior to treatment of the wafer 2 or during a predetermined time period of wafer insertion and completion of the temperature rise. Then, the flow rates of the inert and oxygen gases being processed for maintaining the partial pressure of the oxygen gas at the preset value are computed, the flow rates being maintained constant during the process. 
     In a second way of controlling the apparatus. additionally to the first way, a value detected by the barometer 18 may be input to adjust the flow rates at least once during the wafer treatment. In a third way of controlling the apparatus, the flow rates may be adjusted intermittently by inputting values detected by the barometer 18 at predetermined time intervals. Alternatively, the flow rates may be adjusted continuously by continuously inputting values detected by the barometer 18. Selection of these ways of controlling the apparatus depends on factory locations. For example, in districts experiencing a diversity of weather changes, the third way of controlling the apparatus may be desirable as the atmospheric pressure is likely to change. The first and second ways of controlling the apparatus may be suited to districts with little atmospheric pressure changes. 
     With the inventive system thus arranged, it is possible not only to avoid discrepancies between or errors in treatment conditions due to regional differences, e.g., altitudinal differences of factory locations, but also to avoid errors in batches of treatment, apparatuses and factories while achieving compatibility of treatment data. 
     As it may readily be appreciated by those of ordinary skill in the art, the present invention should not be limited to use on an oxidation apparatus but it may also be applied to a phosphorus diffusion apparatus, a PYRO oxidation (hydrogen gas burning oxidation) apparatus, a hydrogen chloride oxidation apparatus, etc., when an inert gas is additionally used and the partial pressure of an oxidation gas are controlled, thus achieving stability in product quality. 
     Because, as thus far explained, the partial pressure of a reactive gas can be maintained constant in the apparatus according to the present invention, it achieves deposition of thin films of uniform film thickness and quality and hence provides products of improved quality and improved reproductiveness. Further, because the internal pressure of a reaction chamber may be kept substantially equal to an atmospheric pressure, existing oxidation furnaces may be used as they now stand, without making any alterations thereto.