Abstract:
The present invention relates to a method for increasing the grain size of a polysilicon layer, which includes exposing a silicon oxide wafer in a deposition chamber to an amount, effective for the purpose, of nitrogen at a flow rate of at least about 240 standard liters per minute (slm).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to semiconductor device manufacturing, and, more particularly, to a method for controlling device speed by adjusting polysilicon grain size.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the manufacture of semiconductor devices, wafers, such as silicon wafers, are subjected to a number of processing steps. The processing steps include depositing or forming layers, patterning the layers, and removing portions of the layers to define features on the wafer. One such process used to form the layers is a procedure known as chemical vapor deposition (CVD), where reactive gases are introduced into a vessel containing the semiconductor wafers. The reactive gases facilitate a chemical reaction that causes a layer to form on the wafers.  
           [0003]    One deposition process involves the formation of polycrystalline silicon (polysilicon) layers on the wafer by decomposing SiH 4  molecules to Si atoms, which in turn combine to form Si grains, or “polysilicon.”. There are numerous factors that affect the deposition rate and deposition polysilicon film characteristics of a deposition tool. These factors include, e.g., the flow rate of reactive gases through the chamber and the temperature/pressure of the chamber.  
           [0004]    Polysilicon furnaces both within and without a nitrogen box (the furnace wafer loading area immediately below the furnace main body) may play an important role in controlling polysilicon grain size, thereby governing device speed. An important consideration is the actual N 2  (nitrogen) flow which continuously treats the silicon wafer surface in the N 2  box. A continual need exists for effective mechanisms for increasing the grain size of a silicon wafer.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention relates to a method for increasing the grain size of a polysilicon layer in a furnace, which includes exposing a silicon oxide wafer in a deposition chamber to an amount, effective for the purpose, of nitrogen at a flow rate of at least about 240 standard liters per minute (slm). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The present invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:  
         [0007]    [0007]FIG. 1 is a graph of the effect of increased nitrogen flow in a furnace; and  
         [0008]    [0008]FIG. 2 is a graph of polysilicon film volume fraction versus control wafer resistivity in a furnace. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0009]    According to the method of the present invention, a maximizing of polysilicon grain size results in an increase in device speed. In the method of the present invention, when an oxide wafer (i.e., a surface capped with a thin O 2  boundary layer) was placed in an N 2  box, the wafer was exposed to bulk N 2  flow, with absorbed O 2  being removed as a result. Upon treatment with the N 2  flow, absorbed oxygen bonded with oxide molecules, and the oxygen depletion site was filled with N 2  molecules. Nitrogen molecules “interfere” with silicon in forming a polysilicon seed; therefore fewer seeds, and a larger grain size was the result.  
         [0010]    A prototypical furnace includes four tube furnaces, which are used for curing polymers, sintering, and growing oxides and nitrides on silicon wafers. There are also four gas flow timers for the furnace. The orientation of the timers matches the orientation of the tubes with which they correspond; with a maximum time of twelve hours, the flow timers are used to control the time the gas flows in the furnaces. The minimum amount of time for the process to function correctly is about two hours. When the timer reaches zero, the gas discontinues to flow.  
         [0011]    There are three flow meters located just below the gas flow timers. Beneath each of the flow meters are two flow meter valves. The cut-off valves are the valves closest to the flow meters. The handles are at a 90° angle when the flow meters are closed. When the handles are parallel with the flow meter, they are open. The lower valves are the needle valves, which control the gas flow into the flow meters. To the right of the flow meters is the three-way valve. This valve controls which gas flows into the middle flow meter (tube  2 ). The two selectable gases are oxygen and nitrogen. The narrow, pointed end of the handle points to the gas that is being used. When tube  2  is not in use, the valve is turned to the off position. When furnace tube  2  is used, which performs oxidation, the oxygen cylinder in the chase behind the furnace is on. Note that in order to check that the gas pressure is sufficient, the cylinder gauge closest to the cylinder is read. The gauge to the left indicates the gas line pressure; it is set to approximately 25 psi.  
         [0012]    The furnace temperature is controlled by a programmable temperature control unit. There is one control for every furnace; there are programmable temperature controllers on each of the temperature control units. These controllers monitor each of the furnace zone heaters, and allow the programming of several process steps. Thus, in order to operate the furnaces, the following steps are performed: (1) set the appropriate atmosphere, (2) load the furnace, (3) set the temperature program, (4) run the process, (5) end the run.  
         [0013]    The first step in setting the atmosphere is to set the gas flow timer to the length of the particular run. This prevents the waste of gas in the system. The next step is to set the flow of gas using the flow meters.  
         [0014]    The first step in loading the furnace is to place a sample into a quartz boat, which is found in the nitrogen box next to the furnace. The cap on the end of the tube is removed, and the boat placed just inside the opening. A metal rod is used in order to carefully push the boat to the middle of the furnace tube. The desired temperature in degrees Celsius is set for the controllers, and the temperature cooled to about room temperature when the program is done. In order to remove the boat from the furnace, the loading procedure is reversed.  
         [0015]    In the testing of the present invention, an analysis of the effect of an increase in nitrogen flow, via nitrogen shower or other acceptable method was conducted in a furnace. As shown in FIG. 1, the V f  (the polysilicon film volume fraction, which is an index of polysilicon grain size) trends down (i.e., larger grain size, as nitrogen flow increased). In FIG. 2, note that R s  (control wafer resistivity) trends down (i.e., higher speed) as V f  trends down (i.e., a larger grain size). Therefore, it is apparent that device speed is significantly enhanced by the amount of the N 2  flow rate in a furnace.  
         [0016]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.