DE4127819A1 - Discontinuous silicon@ prodn. by thermal decomposition - in which deposition occurs on inner wall of silicon@ tube and deposit is collected by periodically melting - Google Patents

Abstract

Polycrystalline Si is deposited on, pref. the inside of, a tube shaped element by thermal decompsn. of a gaseous cpd., followed by melting of the deposited layer. The feature is that the tube is heated by an electrical current, using pref. direct conduction through the tube which is made of Si. Melting is achieved by increasing the current through the tube. The molten Si is collected in a container. The process of depsn. and removal is repeated periodically. The process can be carried out in multiple reactors connected electrically in series. The Si cpd. used is pref. silane or a chlorosilane cpd. Reactor features claimed are the provision of a reaction gas inlet which directs the gas to the inside of the tube and the use of a central opening which can be closed hermetically and friction-less and is used to remove the molten Si. USE/ADVANTAGE - The method allows the use of lower depsn. temps. than some current processes which allows various Si-cpds. and increased concns. of the Silane cpd. selected to be used. to increase depsn. rates of Si. The use of Si elements reduces the contamination levels of the Si deposited. The molten Si can be taken directly to single crystal pullers or cast into containers for the mfr. of polycrystalline Si.

Description

The invention relates to a method for periodic Ab cut from high purity silicon on a tubular Carrier body due to thermal decomposition of a silicon containing connection and the periodic melting of the deposited silicon layer and a device for Execution of the procedure.

High purity silicon is mainly used in semiconductors industry and in solar cell technology. Nearly all today's manufacturing processes are based on quartz sand Silicon source. What can be obtained from it by reduction Raw silicon is still too contaminated and must to be further concentrated. Use most procedures as a cleaning strategy the conversion of the silicon into a distillable compound and subsequently the cleavage of the cleaned compound with recovery of the silicon. The thermal decomposition of trichlorosilane with participation supply of reducing hydrogen with subsequent Ab separation of high-purity silicon on thin silicon rods, the so-called souls, is called "Siemens Process" known. A modern device for implementation this method is for example in the patent DE 28 54 707 C2. The available in this way polycrystalline silicon rods with diameters up to 200 mm are due to their geometry for further processing  Crystals are very suitable due to crucible-free zone pulling. But only about 10% of the bars produced go to this one high energy consumption and therefore expensive Path. The majority of the bars are mechanically shredded, melted and either to polycrystalline solar cells material cast or in the crucible pulling process Czochralski pulled to single crystals. When crushing the semiconductor material superficially through the crusher witness contaminated and must then be cleaned in caustic baths will. This extra work and energy expenditure for the re-melting of the silicon very disadvantageous.

There is therefore no shortage of attempts to batch process of obtaining high purity Silicon by the thermal decomposition of a gaseous, silicon-containing compound semi-continuously or continuously avoiding the disadvantages mentioned shape.

In the published patent application DE 25 33 455 a separator method using seven hundred, little over the Described melting point of the silicon of heated support rods ben. The deposited silicon remains liquid, drips off the rods and is continuously discharged. Because Kontami nations largely avoided by the carrier material should be, the support rods are in a preferred Execution itself made of silicon. Every single one of the large number of separator rods must be by direct electricity passage and using coolant circuits at a surface temperature just above the melting point of silicon are held so that the rods themselves are not melt away and silicon does not grow on the bars. The power supply, contacting and current control extremely difficult and  make it particularly prone to failure.

Another method uses a tubular support Silicon deposition body. As per the illustration in patent application WO 84/00 156 the inner walls of the Carrier body at a temperature above the melting point of the Silicon held, the deposited silicon the Gravity flows down to the reactor floor and overall will melt. The heat is supplied indirectly via a Resistance heater leading the outer circumference of the carrier body. The carrier body consists of graphite, which is superficial with the deposited silicon to silicon carbide right acts. Especially given the high process temperatures is the silicon melt due to contamination with coal material or other foreign substances originating from graphite endangers.

A key disadvantage of all, silicon in liquid Form of depositing processes is that they essentially on the use of silane as a target silicon-containing compound. Will the silicon ever but through the reductive decomposition of chlorosilanes, into special of trichlorosilane, with hydrogen as Reducing agents are carried out at temperatures, in which the silicon is obtained in liquid form very high partial press on hydrogen necessary to match the reaction to keep weight on the product page. This leads to a Inefficiency of the entire procedure because the Bil formation rates of liquid silicon due to the high dilution the silicon-containing component with hydrogen are low.

In contrast, more freely in terms of the choice of the decomposable Silicon compounds are semi-continuous processes  separation processes, such as that in the patent described US 42 65 859. There is silicon through thermal decomposition of a silicon-containing compound the inner walls of a tubular, multi-walled reactor in solid form and subsequently by tempera Door increase melted on the reactor walls. The disgust that of solid silicon allows lower Process temperatures and in the case of the use of example as trichlorosilane lower hydrogen partial pressures for the reduction, so that chlorosilanes economically vice versa can be set. That in the mentioned patent However, the method described is based on the use of glassy quartz, silicon carbide or graphite as wall material rial, which increases the potential risk of contact mination of the silicon produced by the reactor foreign matter remains. Quartz is considered Wall material particularly unfavorable because it is in the separator temperature becomes soft and its dimensional stability ver liert. The process has other serious disadvantages, which are from the outside of the reactor, this derive ring-shaped surrounding heating elements. these can none uniform tempera over the entire separation area Ensure that the hotter wall areas near the Gas inlet point covered with silicon much faster than the colder, more distant Ab cutting surfaces. Regarding the operation of several derar The external heating proves to be more of a separation reactor also disadvantageous because each system has its own Power supply needed.

The object of the invention described below was therefore, a deposition process for high-purity silicon admit the disadvantages mentioned in the prior art nik avoids. Furthermore, it was the object of the invention  Separating device for performing the method ben.

The problem is solved by a procedure for Ab cutting polycrystalline silicon on a tube carrier body by thermal decomposition of a silicon-containing, gaseous compound and subsequent Melting of the grown silicon layer, marked net in that the carrier body by direct Continuity is heated.

The supply of heat comes in the Abschei invention process is of central importance. Because the carrier body everyone is heated by direct current passage Area cutout of the separation area the same tempera tur, so that the silicon per unit area with the same Ra deposited and during the melting phase is melted down. The carrier body is made of pure silicon manufactured, causing contamination of the deposited Silicon can be avoided entirely. Another important one A feature of the invention is that the silicon is only on the Precipitates inner surface of the support body. This is through a constant rinsing of the outer surface of the carrier body with a silicon-free purge gas and the in the range of In nenfläche of the support body directed guidance of the Reak tion gas flow reached. Finally, the procedure allows an electrical connection of several separation plants in a series connection with a single, central current care.

The invention is explained in more detail below with reference to the figures. Fig. 1 shows the cross section of a particularly preferred embodiment of the deposition system according to the invention. Of course, the device shown is exemplary and does not limit the inventive idea in any way. Fig. 2 shows a block diagram of a preferred electrical interconnection of several separators.

The core of the system is shown in FIG. 1, a tubular body 1 made of pure silicon with a wall thickness of preferably 10 to 30 mm, a height of 2 to 4 m and an outer diameter of 400 to 1000 mm. The carrier body is clamped between the base plate 2 of the reactor and the reactor cover 3 in an electrically conductive manner. In direct contact with the upper and lower tube limitation are two graphite rings, wherein the upper graphite ring 4 a a silicon cap 5 with central openings for the supply and discharge of process gas and on the lower graphite ring 4 b a conical funnel 6 of silicon with a central, circular opening are stored. About a hydraulically adjustable bellows contact ring 7 made of silver, which is mounted between the reactor cover 3 and the upper graphite ring 4 a, a constant pressure is exerted on the support body during operation of the separating system, so that breaks in the support body due to length and diameter fluctuations be prevented. The lateral limitation of the reactor is formed by a pipe section 8 which is screwed to the reactor cover 3 and the base plate 2 in an insulated manner, so that an electrical short circuit via the outer walls of the reactor is impossible even if the support body is not completely closed. Before geous part of the tube is provided on its inner surface with a coating of silver to keep energy losses due to heat radiation during operation of the system to a minimum. The reactor is divided into an outer 9 and an inner 10 reactor space by the support body. The electrical current to be supplied is evenly brought up to the carrier body with the aid of a copper ring 11 surrounding the base plate and is taken up again by a corresponding ring on the reactor cover of the carrier body. The base plate of the reactor has a central opening which is sealed in a gas-tight manner during the deposition of silicon from a collecting trough 12 made of silicon, since the reactor pressure prevailing during this process phase presses the trough against the bearing surface sealed with, for example, polytetra fluorethylene. The drip pan has a laterally out of the reactor and there resting on a bearing, coated with silver arm 13 , which allows a smooth lifting and retraction of the pan at the beginning of the melting phase, exposing the central opening in the base plate. The outer boundary of the separator forming con construction parts such as base plate, reactor cover and pipe section are preferably made of double-walled stainless steel and can be cooled with water, for example. In addition, cooling devices are also provided for the power supply and discharge as well as for the bottom of the sump.

Further details of the separation device become clear in the course of the following description of the procedure. The process begins with the deposition of silicon on the inner wall of the tubular carrier body 1 . This is first because it is made of silicon and at room temperature hardly conducts the electric current, ignited, that is, heated until its electrical resistance has decreased so much that its temperature can be maintained and increased by direct current passage. To ignite a usual union, not shown in Fig. 1 heater is lifted in the vertical direction in the inner reactor space 10 through the central opening of the base plate. From a temperature of approx. 400 ° C, sufficient current flows through the carrier body when it is connected to a voltage source to regulate the further heat supply via the direct current passage. Then the auxiliary heat source is removed, the sump 12 is positioned and lowered by means of the rotatably mounted arm 13 above the opening of the base plate of the reactor, so that it closes the opening properly. The current intensity is controlled so that the carrier body receives the decomposition temperature of the silicon compound.

The invention distinguishes a reaction gas stream, a purge gas stream and an exhaust gas stream. The reaction gas passes via the feed line 14 directed into the inner reactor space 10 and leaves it through the exhaust line 15th The purge gas stream of hydrogen, argon or helium reaches the outer reactor chamber 9 via the feed lines 16 , reaches the inner reactor chamber 10 via the channels 17 and the opening of the funnel 6 , and also leaves the system via the exhaust line 15 . The reusable components from the exhaust gas are advantageously separated and used again.

The reaction gas containing the silicon-containing compound preferably consists of silane or a chlorosilane derivative, the thermal decomposition of the latter, like him thinks, usually the presence of hydrogen as Re require production agents. The next one is exemplary Course of the deposition phase with trichlorosilane as silicon source described. To achieve the highest possible separation rates aim, the trichlorosilane is conveniently in liquid state together with part of the for reduction required hydrogen into the inner reactor space brings where it is due to the prevailing high temperatures evaporated instantly. The one still missing for reaction Hydrogen comes into the inner reaction via the purge gas room.  

During the introduction of the silicon-containing reaction gas separates on the inner surface of the tubular support body from a solid silicon layer of uniform thickness. The purge gas is slightly above that in the inside of the reactor prevailing pressure of usually 0.05 to 0.2 bar and therefore prevents silicon deposition outside the separation area. The through the carrier body flowing electrical current has a comparative effect on the Silicon deposition. Form due to local temperature fluctuations in thickness differences in the silicon layer, the temperature remains in a thicker place due to a reduced resistance behind that at a thinner point back, with the result that the growth rate on the thin position becomes higher and the differences in thickness change again compensate. To take advantage of this effect Silicon deposition operated in a temperature range, in the deposition rate increases with temperature. In the The use of trichlorosilane in the reaction gas becomes the Trä body advantageous during the deposition phase of the process heated to 1050 to 1200 ° C.

If the growing silicon layer the desired thickness the separation is achieved by blocking the Zu drove the reaction gas and lowering the reactor pressure completed. The choice of this time is of many influences sizes dependent and must be optimized by tests. To in particular, those with the deposition time must be taken into account decreasing Ab available for deposition cutting surface, the volume of liquid silicon that after the melting for further processing and the maximum power supply capacity. For example it is cheap to deposit just as much silicon as for a mold or a crucible filling is required, each after whether the material is poured into blocks or too  crystals should be pulled. When operating the fiction According to the system, the deposition of a 5 to 10 mm turns out thick silicon layer as particularly advantageous.

Before the current flow through the carrier body for melting the deposited silicon is increased, all residues of the reaction gas are carried out of the reactor by the purge gas. It is also advantageous to maintain the purge gas flow even during the melting. In order to discharge the molten silicon from the reactor, the drip pan 12 is first raised smoothly over the rotatably mounted arm 13 and laterally offset, so that the central opening in the base plate of the reactor is exposed. The drip pan suitably has the shape of a high-walled shell, so that it can be tapped and collected without contamination from the edge of the funnel 6 during the melting phase from the previous process cycle silicon formed. To remove the peg from the drip pan, it is regularly moved laterally out of the reactor.

By increasing the current passage through the Silicon layer covered carrier body is the temperature increased beyond the melting point of silicon. The Silicon then begins to melt, the reactor runs down the inner wall and drips into the intended receptacle direction. Similar to defrosting ice Silicon only superficially molten during the internal part of the support body remains dimensionally stable. The tubular construction of the carrier body ensures that there may be local temperature differences on the separation surface due to the radial heat radiation compensate quickly so that the silicon at equal rates melts off. The inner surface temperature of the carrier body be  is preferred during the melting phase of the process wise 1410 to 1450 ° C. The heat loss on the outer surface of the support body is at the same time due to the heat beam through the cooled reactor walls sufficiently large to to be the outer wall before a superficial melting true.

Depending on the further use of the molten silicon, a suitable receiving device, not shown in FIG. 1, is positioned under the central opening in the base plate of the reactor. This can be, for example, a feed line for recharging a crucible, a crucible or a mold. It is particularly advantageous and in the sense of the invention that the silicon reaches the destination of its further processing as molten as possible or at least at a temperature close to the melting point. If as much or nearly as much material has been melted as had previously grown up, the melting phase of the process is ended. For this purpose, the electrical current flow is throttled and the reactor opening in the base plate with the drip pan closed ver. Briefly dripping silicon falls into the drip pan or forms the cones mentioned. When the inner surface of the carrier body has reached the intended separation temperature, the next separation phase begins with the feeding of reaction gas. The deposition and melting of silicon are repeated periodically in this way.

The simultaneous operation of several, preferably 2 to 20, of the separators according to the invention is particularly advantageous. It has proven particularly favorable to connect the systems in series and to provide a common power supply. This means that the actuator losses that occur when switching the thyristors can be kept considerably lower than in systems with individual power supply. In Fig. 2 the circuit is shown in a block diagram. Before commissioning, the switch 18 with which the first separating system 19 and the switch 18 'with which the further separating systems 19 ' can be integrated into the circuit are closed. To start up the first separation system, this is first ignited and then connected to the controllable power supply 20 by opening the switch 18 . Finally, the current flow is adapted to the conditions required for silicon deposition. Each additional separation system is ignited in an analogous manner and divided into the circuit. By closing the switch belonging to the respective separator system 18 or 18 ', a failed separator system can be electrically short-circuited at any time without influencing the overall system.

In summary, the particular strengths of the invention are according to the procedure using the Ab cutting plant in the high automation potential in the Flexibility in choosing the gaseous silicon source and the safe exclusion of product contamination by Plant components. The maintenance effort for operating the system is especially due to the hydraulic, which increases the service life Support of the support body low, which is the favorable The economy of the process is still improved.

Claims (11)

1. A method for depositing polycrystalline silicon on a tubular support body by thermal decomposition of a silicon-containing, gaseous compound and subsequent melting of the grown silicon layer, characterized in that the support body is heated by direct current passage. 2. The method according to claim 1, characterized in that the silicon on the inner surface of the tubular Carrier body is deposited. 3. The method according to claim 1 or 2, characterized characterized in that the direct passage of current through a carrier body made of silicon. 4. The method according to one or more of claims 1 to 3, characterized in that for melting the grown silicon layer through the current passage the carrier body is increased. 5. The method according to one or more of claims 1 to 4, characterized in that the melted Silicon molten in a holding device is collected. 6. The method according to one or more of claims 1 to 5, characterized in that the phases of Deposition and melting of silicon periodically to repeat.   7. The method according to one or more of claims 1 to 6, characterized in that the passage of current through one or more separators connected in series locations. 8. The method according to one or more of claims 1 to 7, characterized in that as silicon-containing Compound silane or a chlorosilane derivative used becomes. 9. Device for depositing polycrystalline silicon on a tubular support body by thermal Decomposition of a silicon-containing, gaseous Connection and subsequent melting of the grown silicon layer, characterized by a heatable via direct current passage Silicon carrier body. 10. The device according to claim 9, characterized by a in the area of the inner surface of the carrier body directional supply of the silicon-containing compound. 11. The device according to claim 8 or 9, characterized by a gas-tight and friction-free lockable central Opening to discharge the molten silicon.