BACKGROUND OF THE INVENTION

This invention relates in general to a method for the hydrogenation of silicon tetrachl oride and more specifically to a high pressure plasma (HPP) method for the hydrogenation of silicon tetrachl oride to produce trichlo osilane.

Trichlorosi 1 ane is the most widely used silicon source gas for the production of polycrystal 1 i ne silicon. SiHC13 is reduced with hydrogen at an elevated tempera¬ ture to deposit pure polycrystal 1 i ne silicon. By-products of this reaction are unreacted SiHC13 , Si Cl , HCl , other chl orosi 1 anes and polymeric chlorosi 1 anes . Less than about one third of the input Si HC13 is converted to silicon and about two thirds is converted to Si C14. The SiC14 cannot be used efficiently for polysilicon growth and thus is essentially a low-value waste product.

Attempts have been made to convert silicon tetrachl ori de to trichlorosilane; that is, to convert the waste material SiC14 to a useful starting material SiHC13. In one such attempt, S C14 is converted to Si HCl 3 in a hydrogen reaction at high temperatures (1000-1200°C) and at high reactor pressures ^* (typi cal ly 30-50 atmospheres). In this conversion, however, conversion efficiency and throughput are too low to be practical. Another problem encountered is that of silicon deposition in the hydro- genation reactor during the conversion.

Accordingly, a need existed for a method of converting silicon tetrachl ori de to trichlorosilane in order to reduce the cost of starting materials and thereby to reduce the cost of polycrystal i ne silicon. It is therefore an object of this invention to provide an efficient method for converting si 1.icon tetrachloride to trichlorosilane.

It is a further object of this invention to provide a high pressure plasma method for the hydrogenation of silicon tetrachl oride.

It is still another object of the invention to provide a method for the hydrogenation of silicon tetrachl ori de capable of high throughput and without silicon deposition during the conversion process.

SUMMARY OF THE INVENTION

The foregoing objects are achieved in the invention through the use of a high pressure plasma (HPP) for the hydrogenation of silicon tet achl ori de. Hydrogen and silicon tetrachl ori de are reacted in the presence of a high pressure RF plasma to form Si HCl 3 , Si H£ 12 ■> ^anc - -"-Cl by the reaction

HPP H ₂ + Si C14 --> SiHCl 3 + -SiH ₂ C12 + HCl

The process is optimized to enhance the production of Si HCl 3. Details of the invention will be further appreciated after a consideration of the following detailed description of the invention taken in connection with the drawi ngs .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus for practice of the i nvention ; FIG. 2 illustrates a high pressure plasma module for impedance matching and for introduction of reactant gases; and

FIG. 3 illustrates in cross-section a high pressure plasma nozzle assembly.

DETAILED DESCRIPTION OF THE INVENTION

A plasma can be defined as an approximately neutral cloud of charged particles. The plasma may be formed, for example, by an electric glow discharge in a strong electric field. The types and characteristics of plasmas can vary widely; two types commonly of interest are the low pressure and high pressure plasmas. The boundary line which distin¬ guishes between the two types of plasma is a pressure of about 13.3 KPa, but for practical purposes ^' the high pressure plasma (HPP) is typically produced at a pressure of about one atmosphere (101 KPa). An ^* important distinction between low pressure and high pressure plasma relates to temperature: in a low pressure plasma the electron temperature can be much greater than the gas temperature; in contrast, the conditions found in a high pressure plasma lead to thermal equilibrium in which the electron and gas temperatures are nearly identical . The gas temperature in the high pressure plasma can typically reach 3000-5000°K.

FIG. 1 illustrates an apparatus suitable for practice of the invention. The apparatus comprises an RF generator 10 operating at 13.56 MHz, an impedance matching module 12 and a dual flow nozzle 14 for sustaining a high pressure RF plasma beam 16. The power rating of the RF generator is selected for the particular application. While the exact frequency of the generator is not critical to the inven¬ tion, this particular frequency is chosen in accordance with FCC regulations. A co-axial cable 18 connects the RF generator to the impedance matching module.

The impedance-matching module is illustrated in FIG. 2. The module is a TT network and consists of a. tubular coil 20 and two variable capacitors 22 and 24 connected between the input and output of the coil , respectively, and ground. Coil 20 is made of two concentric tubes 26, 28 as shown in cross section in FIG. 2a and provides for

conveying separate inner and outer gas streams, respec¬ tively, through the RF circuit. The output of generator is connected to the input side of the ir network by coaxi cable 18. The concentric tubes of coil 20 can be made o any material that is a good electrical conductor and tha is unreactive with Si C14- Stainless steel coated with copper on its outer surface, for example, is suitable fo the coil material. The copper coating reduces ohmic los in the coil. When the π network is tuned for resonance, the voltage at the output 30 of the network reaches a maximum, a voltage sufficient to create and maintain a hi pressure plasma at the tip of the nozzle.

The dual-flow high pressure plasma nozzle is illus¬ trated in more detail in the cro.ss-sectional view of FIG. 3. Output 30 of coil 20 having concentric inner and out tubes 26, 28, conveying two different gas streams, is attached to nozzle 14. The nozzle is comprised of a met shell 32 made of stainless steel or other metal that is resistant to the chlorosilane ambient. An inner electrod 34 is formed of a refractory metal such as molybdenum or tungsten. An insulator sheath 36 forms the end of the nozzle. The sheath is formed of an insulator such as bo nitride which has high dielectric strength at the RF frequency and is resistant to the chlorosilane ambient. One of the reactant gases is conveyed through inner tube to inner electrode 34. A second gas is conveyed through outer tube 28 and then through a plurality of ports 38, o openings, which are bored through metal shell 32 and whic are arranged concentrically about opening 40 into which inner tube 26 and inner electrode 34 are positioned. The nozzle thus permits the isolation of the two gas streams until they exit at the tip of the nozzle.

FIG. 1 also illustrates a gas control system for controlling input amounts of the silicon tetrachl oride an hydrogen reactants. Sources of the silicon tetrachl ori de hydrogen, and an inert gas such as helium are shown at 42

44, 46, respectively. The gases are conveyed to a gas control system 48 which comprises appropriate valves and mass flow controllers for the safe and precise control of the reactant flows. A mole ratio of H ₂ to Si C14 of between 4 and 5 is preferred to optimize the Si Cl 4 to SiHCl 3 conversion efficiency. A mole ratio of 4.2, for example, results in a conversion efficiency of about 50%. Mole ratios higher than about 5 can be used to further increase the conversion efficiency, but the higher mole ratios tend to lead to some silicon production in addition to the trichlorosilane production. The gases are conveyed from the gas controller to the impedance matching module where they enter the inner and outer tubes of coil 20.

The high pressure plasma reaction takes place within a reactor 50. The reactor is simply a quartz tube 52, sealed at the ends by end seals 53 and 54. The end seals serve the purpose of sealing the quartz tube and controlling the ambient within the reactor. Dimensions of the quartz tube are not critical ; a diameter about four times the length of the plasma beam and a length of about ten times the diameter is satisfactory.

The exact pressure within the reactor depends on the flow rate of reactants, the RF power of the plasma beam (because of gas expansion by heating) and the resistance of the gas flow line exiting the reactor. Control of the pressure at any one specific value is not required. The reactor is not evacuated, however, and thus the pressure within the reactor is consequently above one atmosphere. Reaction products resulting from the high pressure plasma hydrogenation exit the reactor through end seal 53. The total output of the system comprises unreacted Si C14 and H ₂ and the reaction products SiHC13 _a SiH ₂ Cl ₂ , and ΗC1. This gas mixture is separated by conventional techniques and the unreacted Si C14 and H are recycled through the process. This separation can be accomplished, for example, by passing the output gases first through a

condensation apparatus 56 at -78°C. At this temperature all of the Si Cl 4, SiHCl _β , SiH ₂ Cl , and some of the HCl will be condensed. The condensate is distilled to separate the individual components. Most of the HCl and all of the H ₂ pass through the condensation apparatus and into a carbon adsorption bed 58. HCl is adsorbed in the bed while the H passes through the bed and is recycled. The HCl is subsequently boiled from the carbon bed and the bed regenerated. The SiH ₂ Cl is valuable elsewhere, e.g. for use in the expitaxial growth of silicon.

The following general example illustrates the practice of the invention. The reactor is purged with helium or other inert gas to remove all ai from the system. Hydrogen is introduced into both the inner and outer gas streams of the dual-walled coil at the impedance matching network. The RF generator is turned on and the power is increased to a level which is suitable for creating a plasma. The input and output capacitors of the TT network are tuned to resonance. The creation of a plasma beam at the dual-flow nozzle and a low reflected power measured at the RF generator are indications of resonance.

After the plasma is created, Si C14 is gradually introduced into the inner gas stream while gradually reducing the H ₂ flow in that stream. Changing the gas from hydrogen to silicon tetrachl oride affects the tuning of the network; it is therefore necessary to simultaneously retune the impedance matching network to sustain the plasma. When all of the hydrogen in the inner gas stream is replaced by silicon tetrachl oride the flow rates of the two gases are adjusted to obtain the desired flow rates and mole ratio of the reactants. Alternatively, the H ₂ can be introduced through the inner stream and Si Cl 4 through the outer.

The two reactants exit the high pressure plasma nozzle and react. The extremely high temperatures resulting from tne plasma favor the hydrogenat-i on reaction with little or

7 -

no catenation of molecules. Silicon tetrachl oride is thus hydrogenated to form trichlorosilane and dichl oros 1 ane with little formation of potentially detonatable chlorosilane polymers. In comparison, a hydrogenation reaction carried out in low pressure plasmas tends to produce these detonatable chlorosilane polymers in considerable quantities.

An in-line gas chro atograph is used to analyze the output gases from the high pressure plasma reactor so that the conversion efficiency can be instantaneously and continuously determined and monitored. Using the gas chromatograph data, the RF power is adjusted to optimize the conversion efficiency. As the RF power is increased the conversion efficiency is found to increase, reach a maximum, .and then decrease. The power level for optimum conversion efficiency depends, however, on the input reactant mole ratio and flow rate. For optimum performance, therefore, the RF power level is adjusted for the particular mole ratio and flow rate and those variables are then precisely maintained.

The following more detailed example further illus¬ trates the practice of the invention. Using a high pressure plasma apparatus as described above, the impedance matching module is adjusted and a plasma initiated. The outer gas stream is adjusted to 2 liters per minute of hydrogen. Silicon tetrachl oride is introduced in the inner gas stream by bubbling 4 liters per minute of hydrogen through silicon tetrachloride maintained at room temperature. The mole ratio of hydrogen to silicon tetrachl oride is about 4.2. The RF power is adjusted to about 1.7 KW. The hydrogenation is allowed to continue for about 6.5 minutes and the silicon-bearing reaction products are ^" analyzed to be, in volume percent, about 50.1% Si C14, 41.3% S HC13 and about 8.6% SiH ₂ Cl ₂ . Approximately 50% by volume of the input Si Cl 4 is converted to either SiHC13 or SiH ₂ Cl ₂ . Excellent mass balance of the

input Si Cl 4 and H ₂ , with the products Si Cl 4, SiHCl 3 , SiH ₂ Cl and HCl collected from the exhaust of the HPP reactor is observed (after correcting for unreac H ), indicating that the amount of polymeric material formed is negligible.

There has thus been provided, in accordance with th invention, a method for the hydrogenation without catenation of silicon tetrachl oride. This hydrogenation is achieved by the high pressure plasma reaction of hydrogen and silicon tetrachl oride. The high pressure plasma results in high conversion efficiency and high throughput because of the high temperature encountered i the plasma. An additional benefit of the high pressure plasma, in contrast to low pressure plasma, is the lesse equipment requirements accruing from working near atmospheric pressure as opposed to a vacuum environment.

While the invention has been described and illustra with regard to specific examples, it is not intended tha the invention be so limited. It will be appreciated, fo example, that the dual-flow coil and plasma nozzle can b constructed of materials and have configurations other t those illustrated. Further, the flow rates and powers u are optimized for the particular apparatus conf guration used and will, in general, be a function of the specific reactor designs.

Accordingly, it is intended that the invention embr all such variations and modif cations as fall within the scope of the appended claims.