Abstract:
A process for purifying methylsilane is described. Methylsilane containing impurities such as carbon dioxide, chlorosilane and atmospheric gases is fed from a source container unit to an adsorption unit at −40° C. The adsorption unit is connected to a collection unit at −190° C. where the purified methylsilane is transported and can be stored.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processes for purifying methylsilane. More particularly, the present invention provides for purifying and condensing methylsilane gas through cryogenic adsorption techniques. 
     BACKGROUND OF THE INVENTION 
     Organosilicon gases, particularly methylsilane are employed in the semiconductor industry as coatings and films. A typical process for producing methylsilanes is called the “direct process.” This is the reaction of methyl chloride and silicon in the presence of a copper catalyst. The effluent from the reactor, however, is still a mixture of methylsilanes and high boiling materials such as disilanes, polysiloxanes, silylmethylenes and the like. This effluent must then be distilled to separate the methylsilane from the other silanes present. 
     Additionally, these synthesis methods also result in the generation of olefinic and chlorinated hydrocarbons. These individual species can cause separation problems with the above-mentioned distillation process, as well as problems relating to color and stability of the resulting product. 
     A typical sample of methylsilane gas may find various impurities in it that can interfere with the use of methylsilane in semiconductor fabrication and processing. These impurities include for example hydrogen, nitrogen, argon, oxygen, methane, ethane, carbon dioxide, silane, chlorosilane and dimethylsilane. 
     Applicants have discovered that the use of a cryo-adsorption process will remove impurities, particularly chlorosilanes and carbon dioxide, better than processes such as distillation. 
     SUMMARY OF THE INVENTION 
     The present invention provides for a process for the purification of methylsilane. The process provides for the steps of providing a source of methylsilane containing impurities, directing the methylsilane to an adsorption vessel for adsorption and degassing and directing the purified methylsilane to a collection vessel. 
     The adsorption vessel will contain an appropriate adsorbent such as magnesium silicate and will be kept at a temperature of about −40° C. 
     The impurities typically found in methylsilane gas are hydrogen, nitrogen, argon, oxygen, methane, ethane, carbon dioxide, silane, chlorosilane and dimethylsilane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic representation of the system used to purify methylsilane. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic representation of the overall purifying system  10 . Vessel  12  contains the methylsilane that is to be purified. Typically, it is a cylinder and valve arrangement which attaches to line  13  and connects via  13  to an isolation valve  14 . 
     Helium is supplied to the system from container  15 . The helium can be any grade that will effectively aid in purging the system, such as grade  6 . The container  15  is typically a cylinder and regulator which connects to an inlet valve  17  via line  16 . Lines  18  and  19  connect valves  17  and  14  respectively to valve  20  which is the inlet valve to the adsorption unit  22 . 
     The inlet valve  20  attaches to adsorption unit  22  via a line  21 . The line can prior to connecting to the unit. The adsorption unit  22  is typically fashioned from stainless steel and will contain an appropriate adsorbent. Representative examples include but are not limited to magnesium silicate which is available from Aldridge Chemicals as Florosil, alumina, silica gel, molecular sieves and zeolites. The absorption unit is kept at a temperature ranging from about −20° C. to about −40° C., with a temperature of −40° C. preferred, by a container  24 . The container is typically a stainless steel dewar capable of holding liquid nitrogen. 
     Line  28  connects the adsorption unit with an outlet isolation valve  26  to line  31  which is attached to a pressure gauge  30 . The purified methylsilane will travel through line  28  to line  31  to an isolation valve  32  which connects to the methylsilane receiver unit  35 . This unit is typically a cylinder which is capable of containing gas. The receiver unit is within a stainless steel container  33  capable of holding liquid nitrogen. The receiver unit is typically held at a temperature ranging from about −170° C. to about −190° C. with a temperature of about −190° C. preferred. 
     Line  28  further connects to line  39  and further connects to line  36  through valve  40 . Line  36  connects with a capacitance manometer vacuum gauge. Line  39  continues through a high vacuum isolation valve  37  to line  38  which attaches to a vacuum system  41 . The vacuum system is a turbo molecular vacuum pump/diaphragm backing pump system which is vented through line  42 . 
     Line  28  further connects to a vacuum venturi isolation valve  34  which leads to a vacuum venturi  54  which utilizes nitrogen gas which is delivered through line  52 . Line  50  connects a scrubber system to the venturi. 
     Typically, the components of this system are made of a material that can be manipulated and shaped and can withstand the somewhat large changes in temperature that the system undergoes. This material may be stainless steel, but could also be glass in portions. 
     In one embodiment of the present invention, the system as described above and in FIG. 1 is first checked for vacuum integrity. In step  1 , all valves  14 ,  17 ,  20 ,  26 ,  30 ,  32 ,  34 ,  40  and  37  are closed, the vacuum system is started and allowed to work up for 30 minutes. Valve  37  is opened in step  2  and the capacitance manometer meter  43  is turned on. The meter is allowed to warm up for 30 minutes in step  3 . 
     In step  4 , the source cylinder of methylsilane  12  is attached with its valve closed to the manifold valve  14 . The helium cylinder  15  with its valve closed is attached to valve  17  in step  5 . The receiver cylinder  35  is attached to manifold valve  32  and the cylinder is opened in step  6 . After 30 minutes, step  7  is performed by opening valves  40 ,  26 ,  20 ,  17  and  19  while keeping valve  34  closed. Step  8  is to allow the manifold system to evacuate to a stable pressure reading. In step  9 , valve  37  is closed and the manometer meter  43  checked for leaks. Lastly, in step  10 , the entire system  10  is purged 10 times with grade 6 helium to 100 psig, held at pressure for 30 minutes, followed by vacuum evacuation to a low-pressure stable reading. 
     Once the system has been satisfactorily purged and evacuated, the purification procedure may begin. First, the source cylinder  12  is cooled to −40° C. using a delimonene/dry ice bath, in a stainless steel dewar. Next, the adsorber vessel  22  is cooled to −40° C. with a delimonene/dry ice bath in a stainless steel dewar  24 . The receiver cylinder  35  is then cooled to −190° C. using liquid nitrogen in a stainless steel dewar  33 . All temperature measurements were made using a thermocouple and meter which are not illustrated in FIG.  1 . 
     Step  4  ensures the vacuum integrity of the manifold system by opening valves  14 ,  40  and  37  while keeping the source cylinder  12  and valves  17  and  34  closed. Next, valves  40  and  37  are closed and the source cylinder slowly opened up with its pressure measured at  30 . 
     Boiling of the liquid nitrogen is an indication that gas is flowing from the source cylinder through the adsorption vessel and collecting in the receiver cylinder. When the boiling has stopped, the cooling bath is removed from the source cylinder and the cylinder is allowed to warm to room temperature. The source cylinder is then warmed with, for example, a heat gun until there is no observable liquid nitrogen boiling in the receiver cylinder. 
     At this stage, the purification transfer is complete and shutdown is performed. Valve  32  is closed, the line to nitrogen dewar is removed, the source cylinder is closed and valve  34  is opened allowing the manifold to vent to the scrubber through line  50 . Once the receiver cylinder has warmed to room temperature, valve  32  is closed and the receiver cylinder removed and forwarded for analysis. The entire system can then be purged and evacuated as described above. 
     The above procedure was performed on a commercially available sample of methylsilane. The sample as received had the following impurities as determined through gas chromatography measurement as shown in Table I. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Component 
                 Amount (ppm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Hydrogen 
                 50 
               
               
                   
                 Nitrogen 
                 0.70 
               
               
                   
                 Ar and O 2   
                 &lt;0.5 
               
               
                   
                 Methane 
                 0.13 
               
               
                   
                 Ethane 
                 0.96 
               
               
                   
                 Carbon Dioxide 
                 1.92 
               
               
                   
                 Silane 
                 &lt;0.5 
               
               
                   
                 Chlorosilane 
                 6.5 
               
               
                   
                 Dimethylsilane 
                 — 
               
               
                   
                   
               
             
          
         
       
     
     After running the purification process once, the sample of methylsilane had the following levels of contaminants as shown in Table II. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Component 
                 Amount (ppm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Hydrogen 
                 8 
               
               
                   
                 Ar and O 2   
                 &lt;0.1 DL 
               
               
                   
                 Carbon Dioxide 
                 &lt;0.1 DL 
               
               
                   
                 Chlorosilane 
                 &lt;0.5 DL 
               
               
                   
                 Nitrogen 
                 &lt;0.1 DL 
               
               
                   
                 Methane 
                 &lt;0.1 DL 
               
               
                   
                 Ethane 
                 &lt;0.2 DL 
               
               
                   
                 Silane 
                 &lt;0.5 DL 
               
               
                   
                   
               
             
          
         
       
     
     As can be seen, only one residual contaminant was in higher than trace amounts after the purification process. 
     While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the 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.