Abstract:
A method for making a higher silane from a lower silane comprises heating a lower silane containing stream without exposing it to temperatures more than 20° C. more than the maximum temperature of a first reaction temperature range. The heated lower silane containing stream is introduced into a first reaction zone and allowed to react. The method further comprises mixing a first gaseous mixture from the first reaction zone with a higher silane containing stream and introducing the mixed streams into a second reaction zone operating within a second reaction temperature range. A second gaseous mixture exiting the second reaction zone is separated into various streams. One stream containing unreacted lower silanes is recycled to an earlier heating step and first reaction zone. The higher silane containing stream is mixed with the first gaseous mixture. Average residence time is low to prevent decomposition and formation of undesired silane byproducts.

Description:
TECHNICAL FIELD 
     The present invention relates to a system and a process for producing higher silanes useful in engineered silicon materials including semiconductors. 
     BACKGROUND OF THE INVENTION 
     Silicon semiconductor devices have become nearly ubiquitous in society. They can be found in portable devices such as MP3 players, watches, cell phones, etc. They can be found in most vehicles. They can be found in the workplace in computers, PDAs, telephone systems, elevators, and in numerous other products. They can also be found in homes in microwaves, TVs, radios, refrigerators, toys, just to name a few. The ever increasing presence of silicon chips makes it increasingly important to find new ways to manufacture silicon chips for less. 
     Currently monosilane is used in the manufacture of silicon chips and generally in making materials having films of polycrystalline silicon, epitaxial silicon or amorphous silicon. The monosilane is decomposed at very high temperatures. Because higher silanes including disilane and trisilane are more easily decomposed than monosilane and are low in loss by evaporation during film formation, it is possible to attain a decrease in the film forming temperature, an improvement in the film forming rate and an increase in the formed film yield by using higher silanes. Thus a need exists to manufacture higher silanes cheaply and in large amounts. 
     Higher silanes can be manufactured by pyrolysis. However at the high temperatures used for pyrolysis much of the monosilane is converted into elemental silicon, a useless byproduct. Hence, a need exists for a method of making higher silanes with less waste. 
     The manufacture of higher silanes by pyrolysis also creates undesirable other silanes. Silicon chip manufacturing requires a very high purity feed. Removing large quantities of impurities wastes reactants and desired products and requires expensive purification. This wastes starting material and requires additional purification. Thus, a need exists for a method creating less impurities. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, novel methods are provided for making disilane, trisilane, and other higher silanes. In accordance with one aspect of the invention, a method is provided for making a higher or higher than higher silane from a lower silane, including, but not limited to making disilane from monosilane, trisilane from disilane, and trisilane from monosilane. The method includes heating a first lower silane containing stream so that the stream is in a first reaction temperature range while avoiding exposing the stream to a temperature greater than about 20° C. more than the maximum temperature of the first reaction temperature range. Preferably the stream is not exposed to a temperature greater than about 10° C. more than the maximum temperature of the first reaction temperature range. Preferably the lower silane containing stream is at a pressure in excess of atmospheric pressure and contains less than 20% by volume non-reacting diluents such as hydrogen. Preferably when making trisilane from disilane, the first reaction temperature range is within from about 250° C. to about 450° C. Preferably when making disilane from monosilane, the first reaction temperature range is within from about 350° C. to about 550° C. 
     Next the heated first lower silane containing stream within the reaction temperature range is introduced into the first reaction zone where it is maintained within the reaction temperature range to form a higher silane reaction product. Preferably the first reaction zone has an average residence time of about 15 seconds to about 60 seconds. Preferably less than 20%, more preferably less than 10%, even more preferably less than 6%, and most preferably less than 3% of the lower silane is converted to the higher silane in each pass through the first reaction zone. The first reaction zone is a volume maintained within the reaction temperature range. It may include a catalyst. A first gaseous mixture containing the lower silane and the higher silane formed in the first reaction zone exits the first reaction zone. 
     In a first embodiment, the first gaseous mixture is purified to produce the higher silane. In a second embodiment, the first gaseous mixture is used to make a higher than higher silane. In the first embodiment, the first gaseous mixture is separated into a first higher silane containing stream having a relatively high concentration of the higher silane and a second lower silane containing stream having a relatively high concentration of the lower silane. Preferably, the higher silane containing stream is separated from any higher than higher silane impurities to purify the higher silane containing stream. 
     The second lower silane containing stream is heated so that the second stream is in a first reaction temperature range while avoiding exposing the second stream to a temperature greater than about 20° C. more than the maximum temperature of the first reaction temperature range and is introduced into the first reaction zone. Preferably, the second lower silane containing stream and the first lower silane containing stream are combined before they are heated to a temperature within the reaction temperature range. 
     In the second embodiment, the first gaseous mixture and a second higher silane containing stream having a relatively higher concentration of the higher silane than the first gaseous mixture are introduced into a second reaction zone to form a higher than higher silane. The first gaseous mixture and the higher silane stream can be mixed together before introduction into the second reaction zone. 
     A second gaseous mixture exits the second reaction zone. It contains the lower silane, the higher silane, and a higher than higher silane. It is separated into a third lower silane containing stream having a relatively high amount of the lower silane, into the higher silane containing stream, and into a higher than higher silane containing stream having a relatively high amount of the higher than higher silane. Preferably the higher than higher silane containing stream undergoes an additional separation to remove impurities. 
     The third lower silane containing stream is heated so that the third stream is in a first reaction temperature range while avoiding exposing the third stream to a temperature greater than about 20° C. more than the maximum temperature of the first reaction temperature range and is introduced into the first reaction zone. Preferably the third lower silane containing stream is not exposed to a reaction temperature greater than about 10° C. more than the maximum temperature of the first reaction temperature range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a process flow diagram for making disilane and higher silanes. 
         FIG. 2  is a process flow diagram for making trisilane and higher silanes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention may have 1, 2, or more reaction zones. In one embodiment, there is one reaction zone as shown in  FIG. 1 . A single reaction zone process is most useful for making a higher silane from a lower silane, for example, disilane from monosilane, trisilane from disilane, tetrasilane from trisilane, etc. A first lower silane containing stream  120  is introduced into a preheater  100 . Preheater  100  heats lower silane containing stream  120  to a temperature within the first reaction temperature range. 
     Preferably, preheater  100  heats the first lower silane containing stream  120  rapidly, more preferably in less than 20 seconds, even more preferably in less than 10 seconds, even more preferably in less than 6 seconds, and most preferably less than 1 second. Preheater  100  could heat this rapidly by exposing first lower silane containing stream  120  to hot wall temperatures. However, this would encourage the thermal decomposition of the silanes and the formation of undesirable higher than higher silanes. Consequently, the wall surface exposed to the lower silane containing stream should have a temperature not more than about 25° C. more than the maximum temperature of the first reaction temperature range, preferably not more than about 20° C., more preferably not more than about 15° C., and most preferably not more than about 10° C. 
     Preheater  100  has a conventional design. It consists of a metal pipe wrapped in electrical resistance heaters, and in insulation. First lower silane containing stream  120  flows inside the pipe. Temperature probes can be provided to modulate the power output of the heaters to ensure that lower silane stream  120  is not exposed to overly hot temperatures. In order to heat quickly, the pipe preferably has a relatively small diameter. 
     Preheater  100  heats first lower silane containing stream  120  to form a heated lower silane containing stream  122 , which is introduced into a first reactor or first reaction zone  102 . First reaction zone  102  is designed to maintain the temperature of heated lower silane containing stream  122  within the reaction temperature range. The lower limit of the first reaction temperature range is the minimum temperature below which the reaction for making the higher silane does not appreciably occur. The upper limit is the maximum temperature to which lower silane containing stream  120  and heated lower silane containing stream  122  are heated. Preferably, if the lower silane is monosilane and the higher silane is disilane, the first reaction temperature range is within from about 350° C. to about 550° C., more preferably from about 400° C. to about 500° C., even more preferably from about 425° C. to about 475° C., and most preferably from about 440° C. to about 460° C. Preferably, if the lower silane is disilane and the higher silane is trisilane, the first reaction temperature range is within from about 250° C. to about 450° C., more preferably from about 280° C. to about 400° C., even more preferably from about 305° C. to about 375° C., and most preferably from about 330° C. to about 350° C. 
     First reaction zone  102  can be a pipe wrapped in electrical resistance heaters and insulation like the preheater. However, because the heat transfer requirements for first reaction zone  102  is much less than preheater  100 , the electrical resistance heaters can have a lower power output and the diameter of first reaction zone  102  can be larger. For convenience, first reaction zone  102  and preheater  100  are separate. However, they can in fact be part of the same piece of equipment. 
     Preferably, the residence time within first reaction zone  102  is relatively short, preferably less than about 5 minutes, more preferably less than about 2 minutes, and most preferably, between about 15 seconds and about 60 seconds. The low residence time tends to reduce the conversion rate per pass, but boosts the overall output of the higher silane as it reduces the amount of higher silane that decomposes and the formation of undesirable higher than higher silanes. Preferably, the conversion rate per pass is less than 20%, more preferably less than 10%, even more preferably less than 6%, and most preferably less than 3%. 
     A first gaseous mixture  124  exits first reaction zone  102 . Gaseous mixture  124  contains predominantly lower silane, some higher silane, and smaller amounts of higher than higher silanes. It may also contain hydrogen and lower than lower silanes. Gaseous mixture  124  is introduced into a distillation tower  104 . Distillation tower  104  has a condenser  112  that uses liquid nitrogen to condense and separate gaseous mixture  124  into an overhead stream  130  containing relatively high amounts of hydrogen and/or lower than lower silanes, a higher silane containing stream  132  containing relatively high amounts of the higher silane, and a second lower silane containing stream  126  that is recycled back to preheater  100 . Overhead stream  130  exits the system. Higher silane containing stream  132  is collected in the pot  108  of distillation column  104  so that it can be further purified by distillation to remove undesirable higher than higher silanes after the reaction process has been shut down. 
     In a second embodiment, the process has two reaction zones as shown in  FIG. 2 . The process is particularly well suited for making a higher than higher silane from a lower silane, for example, trisilane from monosilane, tetrasilane from disilane, etc. The process has a preheater  100   a  and a first reaction zone  102   a . If the process is used to make trisilane from monosilane, preheater  100   a , first reaction zone  102   a , lower silane containing stream  120   a , and heated lower silane stream  122   a  can be the same as preheater  100 , first reaction zone  102 , lower silane containing stream  120 , and heated lower silane stream  122  for making disilane from monosilane in the first embodiment. The first gaseous mixture  124   a  exiting first reaction zone  102   a  is introduced into a second reaction zone  110  and mixed with a second higher silane containing stream  134 . Alternatively first gaseous mixture  124   a  can be mixed with second higher silane containing stream  134  before being introduced into second reaction zone  110 . 
     Preferably, second higher silane containing stream  134  is at a temperature and is of a flow rate so that first gaseous mixture  124   a  is cooled to a temperature within a second temperature range better suited for converting higher silane into a higher than higher silane. If the higher than higher silane is trisilane and the higher silane is disilane, preferably the second reaction temperature range is within from about 250° C. to about 450° C., more preferably from about 280° C. to about 400° C., even more preferably from about 305° C. to about 375° C., and most preferably from about 330° C. to about 350° C. 
     Because the temperatures conducive to creating a higher than higher silane are conducive to creating undesirable higher than higher than higher silanes, it is desirable to minimize the amount of time in the second reaction temperature range. In fact, it is believed that it might be desirable that second higher silane containing stream  134  is at a temperature and is of a flow rate so that first gaseous mixture  124   a  is cooled down to a temperature below 350° C. and preferably below 300° C. when streams  134  and  124   a  are completely mixed together. It is also believed that it may be preferable that the mixing occur rapidly. By designing the system so that streams  134  and  124   a  rapidly mix to achieve a temperature below 350° C. and preferably below 300° C., there is very little time for reactions consuming higher than higher silanes. 
     Second reaction zone  110  can be similar to first reaction zone  102   a  in design. A second gaseous mixture  136  exits second reaction zone  110  and is introduced into distillation column  104   a.    
     Distillation column  104   a  has four outputs. The first output is an overhead stream  130   a  containing predominantly hydrogen and a lower than lower silane. Overhead stream  130   a  exits the system. A higher than higher silane containing stream  132   a  having relatively high amounts of the higher than higher silane is another output. It is allowed to collect into the pot  108   a  of distillation column  104   a  for distillation to remove impurities such as the higher silane and any higher than higher than higher silanes after reaction zones  102   a  and  110  have been shut down. A third output is a third lower silane containing stream  140  having relatively high amounts of the lower silane. Third lower silane containing stream  140  is recycled back to preheater  100   a . A fourth output is a second higher silane containing stream  134  having relatively large amounts of the higher silane. It is introduced to second reaction zone  110  or mixed with first gaseous mixture  124   a  before introduction to second reaction zone  110 . Second higher silane containing stream  134  may be further cooled or heated prior to its introduction into second reaction zone  110  or mixture with the first gaseous mixture  124   a.    
     In general for both embodiments, the pressure of lower silane containing streams  120  and  120   a  and the pressure of first gaseous mixture  124  and  124   a  can be any pressure as long as the reactants are gaseous. Preferably, the pressure is more than atmospheric. Increasing the pressure above atmospheric allows for smaller equipment and it makes any distillations or condensations easier to perform. 
     Preferably reaction zones  102 ,  102   a , and  110 , and preheaters  100  and  100   a  are designed to operate in plug flow. More preferably, preheaters  100  and  100   a  are designed to operate in highly turbulent flow to provide high heat transfer rates. Such high heat transfer rates are not necessary for reaction zones  102 ,  102   a  and  110 . Flow rates and pipe diameters for reaction zones and preheaters should be sized accordingly. In addition, baffles and distribution plates may be used to prevent uneven flow distributions and channeling. Maldistribution may create hot spots thereby resulting in the decomposition of silane. 
     Hydrogen is believed to limit the decomposition of silane because it is a byproduct of that decomposition. However, in general for both embodiments, the concentration of non-reacting diluents, such as hydrogen, in lower silane containing streams  120  and  120   a  can be less than 20% by volume, and can be less than about 10%. This allows for smaller equipment sizes and increases the efficiency of separations. In particular, reducing the concentration of diluent greatly reduces the size of condensers  112  and  112   a . Because of the low residence times in first reaction zone  102  and  102   a  and the gentle heating of lower silane containing stream  120  and  120   a , having low concentrations of diluents does not result in excessive decomposition of silanes into silica and hydrogen. On the other hand, it may be desirable to have concentrations of diluents higher than 20% when condenser size is less important. 
     In accordance with the present invention, disilane, trisilane, and higher silanes can be produced cheaply and abundantly from a lower silane such as monosilane. The use of a preheater, not exposing the process streams to overly hot temperatures, and minimizing the residence time minimizes the creation of undesirable silane impurities and minimizes the decomposition of reactants and products into elemental silicon. In the two reactor zone process, the mixing of the first gaseous mixture  124   a  and the second higher silane containing stream  134  rapidly quenches the hot gaseous mixture achieving control over the residence time at higher temperatures minimizing waste and impurities. 
     Example 1 
     Approximately 16 kg/hr of disilane at 30 psia was fed to a pre-heater where it was heated to 350° C. The preheater was constructed of approximately 30 feet of ⅜″ diameter 316 stainless steel tubing. The first reactor has a volume of about 50 L and had a length to diameter ratio of approximately 5:1. The reactor was held at 350° C. Byproduct silane and any hydrogen was removed as an overhead stream. The outlet gas from the reactor was composed of 4.1% monosilane, 93.5% disilane, 2.17% trisilane, and 0.15% tetrasilane. No higher silanes or hydrogen were detected. 
     Example 2 
     The conditions were the same as example 1 except an 8 L reactor was used. The outlet gas from the reactor was composed of 2.124% monosilane, 96.67% disilane, 1.138% trisilane, and 0.07% tetrasilane. No higher silanes or hydrogen were detected. 
     Example 3 
     The equipment was the same as example 2. 2.5 kg/hr of monosilane was heated to 460° C. Silane was not purged in the overhead. The outlet gas from the reactor was composed of 98.33% monosilane, 1.414% disilane, 0.236% trisilane, and 0.019% tetrasilane. No higher silanes or hydrogen were detected. 
     Example 4 
     The conditions were the same as example 3 except 3.4 kg/hr of monosilane was heated to 440° C. The outlet gas from the reactor was composed of 99.63% monosilane, 0.341% disilane, and 0.027% trisilane. No higher silanes or hydrogen were detected.