WO1996006802A1

WO1996006802A1 - Method for the preparation of trisilane from monosilane

Info

Publication number: WO1996006802A1
Application number: PCT/FR1995/001093
Authority: WO
Inventors: Alain Villermet; Pierre Didier
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 1994-09-01
Filing date: 1995-08-17
Publication date: 1996-03-07
Also published as: FR2724162A1; FR2724162B1

Abstract

Method for the production of trisilane from monosilane, by which gaseous monosilane is passed into a reactive area where it is subjected to an electric discharge produced by a high frequency current. The method is characterized in that a) the monosilane is used in the form of a mixture with at least one inert gas chosen from the group consisting of helium and argon; b) the pressure of the gaseous mixture in the reactive area is from 0.1 to 3 105 Pa, and c) the gaseous mixture is placed in contact, within the reactive area under the electric discharge, with a wall cooled to a temperature which must be over -120 °C and at most -90 °C, preferably in the range of 115 °C - 100 °C. The invention is for use, in particular, in the electronic industry.

Description

PROCESS FOR THE PREPARATION OF TRISI LANE FROM MONOSI LANE

The invention relates to a process for preparing trisilane from monosilane by electrical discharge and cryogenic trapping.

The use of polysilanes in the microelectronics industry is of great interest. Thus, the use of disilane to produce semiconductor amorphous silicon deposits, in replacement of monosilane, has very important advantages: increased deposition rates and lowering of the deposition temperature.

Thus, by considering the example of polycrystalline silicon deposits on high resolution flat screen, it has been shown that the quality of the screen is notably linked to the size of the silicon grains: this level of quality is all the more high as the grain size is large. When monosilane is used as the precursor of silicon, a grain size of the order of 0.5 micron is obtained. Research aimed at improving the grain size has shown that by using disilane, it is possible to multiply the size of the polycrystalline silicon grains by approximately a factor of 10. Furthermore, it has been possible to demonstrate , that the deposition rate is, any other parameter equal, approximately 20 times greater in the case of disilane than in the case of monosilane.

These considerable advantages nevertheless run up against the very high price of disilane (approximately 10 times higher than that of monosilane). The Applicant recently proposed, in the French patent application published under the number FR-2, 702.467, integrated here for reference, a process for the production of disilane from monosilane, according to which gaseous monosilane is passed through a reaction zone where it is subjected to an electric discharge generated by a high frequency current, the monosilane being used in mixture with at least one inert gas chosen from helium and argon and the pressure of the gaseous mixture in the reaction zone being between 0 , 1 and 3 10 ^ Pa. The gas mixture is then brought into contact, in this reaction method, in the reaction zone under electrical discharge, with a wall cooled to a temperature sufficiently low that the saturated vapor pressure of the disilane is negligible. but not low enough for the monosilane to be condensable at working pressure. The temperature of the cold wall is preferably in the range [-120 ° C, -145 ° C]. Requirement above also describes a reactor suitable in particular for the implementation of this process.

Industrialists developing flat screen technology have recently shown a certain interest in being able to carry out comparative experiments between disilane and another higher silane which is trisilane, extrapolating the results obtained with monosilane and disilane leaving to consider even better grain size and deposition speed, in other words, better screen quality and increased productivity.

However, the experiments using trisilane prove in practice difficult to carry out. The main reason for this is that the supply of trisilane is currently very marginal, only very small quantities of trisilane being available on the market and the prices charged being generally dissuasive. These dissuasive prices are the result of great difficulties in implementing the processes for synthesizing trisilane (it should be remembered that trisilane is currently obtained by chemical reaction of chlorinated higher silanes on lithium aluminum hydride, a delicate reaction generating large quantities of waste).

It then appears that the development of the trisilane market, in conjunction with real needs expressed by microelectronics manufacturers, requires above all the development of an efficient and inexpensive process for the synthesis of this compound.

The process for producing disilane, described in the document already cited, additionally mainly generates disilane, by-products in very small quantities such as hydrogenated amorphous silicon but also trisilane. In this context, the Applicant has continued the work leading to the above-mentioned process, so as to propose a new process dedicated to the synthesis of trisilane.

The present invention therefore aims to provide a process for the synthesis of trisilane, allowing, using sufficient yields, to reach concentrations of trisilane of at least 10 to 20% in the collected mixture, which can then, depending on the concentration requirements. expressed by the user, be used as is or serve as the basis for a possible subsequent concentration by distillation.

The present invention therefore relates to a process for producing trisilane from monosilane, according to which gaseous monosilane is passed through a reaction zone where it is subjected to an electric discharge generated by a high frequency current, characterized in that: a) the monosilane is used in the form of a mixture with at least one inert gas chosen from the group formed by helium and argon; b) the pressure of the gas mixture in the reaction zone is between 0.1 and 3 10 ⁵ Pa, and c) the gas mixture is brought into contact, in the reaction zone under electrical discharge, with a wall cooled to a strictly temperature greater than -120 ° C and less than or equal to -90 ° C, preferably situated in the range [-115 ° C, -100 ° C].

The process of the invention is therefore based on the joint use of an electrical discharge which forms plasma and an adequate cryogenic trap in the reaction zone itself.

The electric discharge, which is of the so-called silent or dielectric barrier type (for example a corona discharge), acts on the molecules of monosilane to create ions, radicals, excited molecules, these species reacting with each other to form in the first place disilane. The latter, which taking into account the cold wall temperature conditions practiced is not trapped, then also undergoes electric shock to in turn form excited species which react, in particular with those of monosilane, to form trisilane. The trisilane formed is trapped by condensation on the cold walls of the reactor, it is therefore extracted from the active area and does not undergo discharge.

The phases involved during the discharge / trapping process are therefore in particular the following:

- a gas phase comprising disilane, inert gas, hydrogen, but also traces of unreacted monosilane,

a solid phase in the form of particles of hydrogenated amorphous silicon, and

- A liquid phase consisting essentially of trisilane in which it could nevertheless be dissolved in more or less small amount of the disilane. At the end of the reaction period, with a view to recovering the trisilane stored in liquid form, the feeding of the gaseous mixture is advantageously stopped, advantageously the reactor being purged using an inert gas such as helium and then placing the reactor under vacuum, in order to expel light gases (helium, hydrogen, monosilane, etc.), the trisilane is then collected in the form of a gaseous mixture with possibly traces of helium and disilane.

The absolute pressure of the gas mixture can range from 0.1 to 3 10 ^ Pa, preferably from 1 to 1.3 10 ^ Pa. The composition of the gaseous mixture obtained will very largely depend on the operating pressure. Indeed, it appears that the partial pressure of the starting monosilane can advantageously be between 0.01 and 0.110 ^ Pa, preferably between 0.04 and 0.08 10 ^ Pa, a partial pressure of less than 0 , 01 10 ^ Pa giving rise to a low productivity of trisilane and a partial pressure of monosilane of more than 0.110 ^ Pa requiring a high initiation voltage of the electric discharge which can cause damaging breakdowns in the material.

If the operation is carried out at a pressure close to the minimum pressure of 0.110 ^ Pa, it is advantageously possible to use a starting gaseous mixture which is predominantly a monosilane. If one operates at atmospheric pressure or slightly above, the starting gas mixture advantageously contains 1 to 10%, preferably 4 to 8% by volume of silane, and 90 to 99%, preferably 92 to 96% by volume. volume of inert gas. It should be noted that the gas mixture can include, in addition to the monosilane and the inert gas, a small amount of hydrogen (less than 10% by volume for example) without this being detrimental to the process.

The frequency of the electrical current adopted seems to influence the yield of trisilane. Thus, for example, it has been found that with the installation used for the examples described below, a frequency of 3 kHz appreciably improves the yield of trisilane compared to a frequency of 50 kHz. The frequency may therefore advantageously (in the current state of knowledge) be between 1 and 50 kHz, preferably between 1 and 10 kHz, and more preferably between 2 and 5 kHz. The residence time of the gas mixture in the reaction zone is advantageously short, for example less than 10 seconds, preferably less than 4 seconds, and better still less than 2 seconds.

The temperature of the cold wall is an important element of the process, it must be low enough for the saturated vapor pressure of the trisilane to be negligible but insufficiently low for the disilane to be trapped in large quantities. It has been found that a cold wall temperature strictly greater than -120 ° C and less than or equal to -90 ° C, and preferably situated in the range [-115 ° C, -100 ° C1 constitute satisfactory conditions meeting the objectives sought. Below -120 ° C, the conditions for obtaining the disilane would be favored, and above -90 ° C, the trisilane would be insufficiently trapped.

Using a reactor such as that described in the above-mentioned document and integrated for reference, several syntheses of mixtures have been carried out. comprising significant amounts of trisilane. The experimental conditions common to all these examples of implementation are as follows:

- total flow of the gas mixture feeding the reactor = 100 cm ^ / min. ; - concentration of silane in the incoming gas mixture = 5%;

- neutral carrier gas = helium;

- pressure in the reaction zone = 1, 1 10 ^ Pa.

For each of the four examples described below, the following information will be given and commented on: the cold wall temperature used, the yields obtained in disilane and in trisilane, as well as the composition of the recovered gas. The yields given correspond to the yields of conversion of the silane transformed into disilane or into trisilane. The given composition of the recovered gas corresponds to the gas composition which is obtained after the synthesis by discharge and evacuating the reactor after purging with helium to expel light gases (helium, hydrogen, monosilane, etc.).

Example j. (according to the invention):

Temperature = - 100 ° C; electrical power = 9.3 Watt.

Composition of the recovered gas: disilane = 70.9%, trisilane = 29, 1%.

Yield (disilane) = 6% Yield (trisilane) = 3.7%.

Example 2 (according to the invention):

Temperature = -110 ° C; electrical power: 8.3 Watt

Composition of the recovered gas: disilane = 85.4%, trisilane = 14.6%.

Yield (disilane) = 32.8% Yield (trisilane) = 8.4%.

Example 3 (comparative):

Temperature = -120 ° C; electrical power: 8.8 Watt

Composition of the recovered gas: disilane = 96.14%, trisilane = 3.86%.

Yield (disilane) = 56.9% Yield (trisilane) = 3.4%.

Example 4 (comparative):

Temperature = -135 ° C; electrical power: 8.6 Watt

Composition of the recovered gas: disilane = 96.64%, trisilane = 3.36%.

Yield (disilane) = 50.1% Yield (trisilane) = 2.6%.

The results obtained for the first two examples of implementation show that it is possible to obtain a gas comprising at least 10% of trisilane, which can be used as necessary or as a base for further concentration by distillation.

The conditions for the first example of implementation

(temperature located at the top of the temperature range), give rise, by the significant formation of powders, to a relatively low yield, but to a relatively high purity of trisilane due to the fact that relatively little disilane is trapped in the liquid phase.

For the second example of implementation where we are in the middle of the temperature range, we observe on the contrary a higher trapping of the disilane in the liquid phase, and therefore a lower purity of trisilane.

The two comparative examples which follow are used under low temperature conditions, for which we fall back on conditions favoring the obtaining of mixtures composed mainly of disilane.

It goes without saying that the embodiments described are only examples and that they could be modified, in particular by substitution of technical equivalents, without departing from the scope of the invention.

For example, one could alternately operate two reactors of the kind described, one of these supplying an application with trisilane produced in a previous synthesis operation, while the other is in the process of synthesis of trisilane, so that the user can have a continuous source of trisilane.

Claims

1. Method for producing trisilane from monosilane, according to which gaseous monosilane is passed through a reaction zone where it is subjected to an electric discharge generated by a high frequency current, characterized in that: a) the monosilane is used in the form of a mixture with at least one inert gas chosen from the group formed by helium and argon; b) the pressure of the gas mixture in the reaction zone is between 0, 1 and 3 10 ⁵ Pa, and c) the gas mixture is brought into contact, in the reaction zone under electrical discharge, with a wall cooled to a strictly temperature greater than -120 ° C and less than or equal to -90 ° C, preferably situated in the range [-115 ° C, -100 ° C].

2. Method according to claim 1, characterized in that the pressure is between 1 and 1.3 10 ^ Pa.

3. Method according to claim 1, characterized in that the gas mixture contains 1 to 10% by volume of monosilane and 90 to 99% by volume of inert gas.

4. Method according to claim 3, characterized in that the gas mixture contains 4 to 8% by volume of monosilane and 92% to 96% by volume of inert gas.

5. Method according to claim 1, characterized in that the frequency of the current generating the electric discharge is between 1 and 50 kHz, preferably between 1 and 10 kHz, and more preferably between 2 and 5 kHz.