DE2618273B2 - Process for the deposition of polycrystalline silicon - Google Patents

Description

15th

The invention relates to a method for the deposition of polycrystalline silicon from the Gas phase on heated carbon carriers.

In high-temperature processes in semiconductor technology, Reaction chambers are particularly recommended for diffusion, oxidation and epitaxial processes Silicon because of its higher purity, higher mechanical stability and lower than quartz Gas permeability

Such molded silicon bodies can be sawed out of silicon blocks with diamond saws. This However, the process is extremely time consuming and costly. Efforts have therefore been made for a long time then, such shaped bodies by depositing silicon from the gas phase on carrier bodies for example to produce silicon, tantalum or carbon.

The deposition of silicon on carbon or graphite was first introduced in 1927 by R. HöIbIing Success carried out (Z. anorg. General Chem. 40, 655-659 / 1927). Rod-shaped solid bodies were produced according to this process

According to one described in DE-AS 11 09 142 Method, however, quartz tubes are used as a carrier body, which on the outside with a Layer of carbon are coated on which hollow silicon body is deposited from the gas phase will. In this process, however, the separation of the support body and the molded body is great Trouble. «

This separation of the support body and the shaped body deposited thereon succeeds better after one in the DE-OS 22 15 143 described method in which a support body made of carbon before Deposition with a layer of silicon dioxide and such is subsequently coated with amorphous silicon. But even with this process are re-use the carrier body set narrow limits, and the carrier body must be carefully ground down and be recoated.

In DE-OS 23 21 507 a process for the production of silicon moldings by precipitation is disclosed of silicon from the gas phase on a substrate body made of graphite. This substrate body from graphite, t> o are put together from individual graphite plates. This is again a matter of a conventional process for the production of silicon moldings, because here too there is Substrate body again made of rigid graphite parts.

Also in other processes known from the literature, according to which silicon moldings are carried out Deposition of silicon from the gas phase on carriers made of carbon or graphite are, the reuse of the support body succeeds in the rarest of cases and even then once or twice at most.

This caused by the different expansion coefficients of carbon and silicon Difficulty separating the support body and shaped body, promoted by the brittleness of the Silicon, which in addition to the destruction of the carrier body often also causes the deposited body to explode Silicon molding leads, as well as the time-consuming and expensive production of the carrier body itself, which have to be carved out of massive blocks of carbon or graphite are the decisive ones Disadvantages of all known processes, which result in the use of silicon moldings so far only a few species areas are restricted.

The invention is therefore based on the object of producing silicon bodies of the most varied of types and shapes easily available, cheap carrier bodies to be deposited.

This object is achieved in that the carrier body is made of 0.1 to 2 mm thick, flexible Flat structures made of carbon are assembled.

Such flexible flat structures made of carbon in the form of flexible foils, laminates or knitted fabrics are commercially available.

Such knitted fabrics consist of carbon thread, which again. by carbonization of synthetic fibers for example polyacrylonitrile or polyvinyl alcohol can be produced. Graphite foils are particularly suitable, which, according to the manufacturer, are made of pure, well-ordered graphite, whereby through chemical and thermal treatment, the distances between the layers in the crystal lattice of graphite Multiples of the normal value of 3.35 A. The resulting light bulk density from worm-shaped individual parts are then compacted on calenders or presses to form the end product, the layers of the graphite lattice and the individual particles of the bulk material solely through application be firmly connected to each other again by mechanical pressure.

Such graphite foils, in thicknesses of 0.1 to 2 mm, preferably 0.2 to 0.6 mm for the production of Carrier bodies of any conceivable shape can be used extremely easily to process. You can cut with simple household scissors, by bending them into the desired shape Shaped and glued with commercially available carbon adhesives. As a carbon glue are included preferably used adhesives in which at the high deposition temperatures only carbon as solid residue remains.

It is also advantageous to use graphite foils provided with an adhesive layer on one side, especially in Use tape shape, which is used, for example, in the production of round support bodies can be. Round or curved support bodies of this type can, for example, be made simple in this way produce that wrapped a corresponding, lost wax mold with such a graphite adhesive tape it must be ensured that the graphite cross-sectional area in the later direction of current flow is always largely the same area, that is, that parts of the mold with a smaller inner diameter accordingly must be pasted over in several layers. After the wax mold is completely taped shut

the wax is removed by simply melting it out

Support bodies suitable for the production of silicon tubes are obtained, for example, by that a corresponding right-angled Folk ns tuck is knocked in around s and glued to the longitudinal seam. One Another possibility is the constant diameter, especially when using graphite foils Thinner wall thicknesses results in tape-shaped graphite foil on appropriate, preferably Wind up metallic measuring rollers in a spiral shape and glue them together. The gluing takes place here either by the fact that the film overlaps the seam around the circumference of the pipe in a helical manner wrapped and the adhesive between this upper lap- is pung is introduced, or in that the tape film wound on buttocks and taped over the seam with a narrow and as thin as possible adhesive tape will.

Tubular support bodies with a circular cross section can be produced in large numbers, for example and any length produced by that on a roller made of, for example, steel, the diameter of which is the Corresponds to the inner diameter of the silicon tube to be deposited minus the thickness of the graphite foil, with 2 "> a second roller is a belt made of graphite foil, the former tube at a certain angle of inclination is pressed around. The seams can be glued in the usual way or with a corrugated one The roller is interlocked and then pressed together.

This process can be carried out continuously, such that the wound and glued Graphite support tube continuously pulled off one side of the measuring roller with a twisting motion and r> is glued. With appropriate, exchangeable measuring rollers, tubes can be used in this way Manufacture any inside diameter. The support tubes wound in this way are self-supporting and can be continuously drawn onto the measuring roller -to and cut to the desired length at the other end will.

Correspondingly, tubes can of course not be round but, for example, oval or elliptical Manufacture hollow diameter. Support tubes with an angular hollow diameter, for example in the form a polygon can be created by simply folding the graphite foil and then gluing the seams produce.

For the deposition, for example, two tubes of this type are placed on two electrodes and connected to one another in a U-shape via a bridge made of the same material. The bridge may in this case in einfachster manner also from a composite made of graphite foil tube are made, these two tubes are cut from the longitudinal v> page two circular recesses with a diameter corresponding to the outer diameters of the pipes to be contacted in the distance. With the two circular recesses, the bridge is simply plugged onto the two pipes.

However, the process is also well suited for the production of polycrystalline silicon as a starting product for casting processes or for the crucible drawing process according to Czochralski. While this hi Material has so far been produced by deposition on thin monocrystalline silicon rods, the in the amount of silicon deposited in the unit of time is initially small and only with increasing thickening of the The separator support and thus the increased separation area achieved useful values can be achieved according to the invention Process by depositing silicon, for example on the inner surface of hollow cylinders polycrystalline silicon can be produced from graphite foil with high deposition rates right from the start. is half of the inner diameter of the separating cylinder has grown, so the separation process is started expediently terminated, since the deposition rate then decreases noticeably

During the actual deposition, the support body arrangement composed of sheet-like structures made of graphite, preferably graphite foil, is generally heated by a suitable heating device, e.g. B. by direct current passage, brought to about 1050 to 125O ⁰ C. preferably 1120 to 1180 ⁰ C and deposited silicon from the decomposition of the deposition gas to the desired wall thickness.

In principle, for example, silicon hydrogen, monochlorosilane, dichlorosilane, Trichlorosilane or tetrachlorosilane, usually in a mixture with hydrogen, are used, with a Trichlorosilane-hydrogen mixture is preferred.

Four process variants are shown schematically in the figures

F i g. 1 shows the production of silicon tubes with a precisely defined hollow cross-section.

F i g. 2 shows the production of silicon tubes with more precisely defined over the entire tube length Cross sectional area.

3 shows the production of polycrystalline silicon as a starting material for subsequent casting or Crucible pulling process with current dissipation through the reactor base plate.

Fig.4 shows the production of polycrystalline Silicon as the starting material for subsequent casting or crucible drawing processes with current dissipation through the Reactor cover plate.

In a reactor 1 made of metal or quartz, one or more carrier tubes 2, preferably composed of graphite foil, are inserted into the graphite electrode, which is seated on electrode carrier 3 made of silver, for example! 4 and heated to the deposition temperature by direct passage of current of preferably 1120 to 1180 ^{° C.} From a trichlorosilane-hydrogen mixture, which is introduced into the reactor through the gas feed line 5, silicon is deposited on the outer surface of the support tube 2 composed of graphite foil to the desired thickness. The residual gases from the trichlorosilane decomposition exit the reactor 1 again through the gas discharge line 6. In the case of a metal reactor, the course of the deposition can be observed through a sight glass 7 made of quartz.

The surface of the silicon tube 8, which is precisely defined in terms of its dimensions, is the inner surface. Of the The hollow cross-section is predetermined by the cross-section of the support tube 2. With thin tubes of a few millimeters of wall thickness corresponds to the outer boundary line of the pipe cross-section in the The shape of the inner boundary line, which is determined by the shape of the support tube 2, is still largely the same and for example triangular 8, square 9, octagonal 10, circular 11 or oval 12. With larger ones Wall thicknesses of the silicon tubes 8 and correspondingly longer deposition times lead to an increasing Rounding of the corners, so that for example a

Silicon tube, which is deposited on a carrier tube according to FIG. 9, would have a largely circular outer boundary line of the cross-sectional area from a wall thickness of approximately 7-10 mm. In addition to the absolute deposition time, the deposition speed also plays an important role here. Is deposited faster, for example at speeds of over 2000 g Si / hour. m ² , the deposited silicon molded body retains the shape given by the carrier body rather than if slowly, for example with 200 g Si / hour. m ^{2 is} deposited.

After the silicon deposition is complete, the Silicon tubes H taken from the reactor 1 and the graphite foil by etching, burning off or quite simply removed again from the deposited silicon tube 8 by sandblasting

Although the anisotropy coefficient of the electrical resistivity is of the order of approx. 50 to 200 areas do not occur when power is supplied to support bodies made from this material unusual difficulties.

Are hollow silicon bodies in which it is not only depends on the shape of their hollow cross-section, desired, so those with a precisely defined inner and in particular outer contour, a method variant as shown in FIG. 2 is recommended is shown: In a metal or quartz reactor 13 with the gas supply and gas discharge nozzle 14 and

15, two tubes 16 and 17, preferably made of graphite foil, are placed one inside the other and above and held together at the bottom with annular graphite holders 18. The two support tubes 16 and 17 are by a heater, preferably by a high-frequency induction heating coil 19 movable in the vertical direction, for example a water-cooled one Silver coil, brought to a deposition temperature of preferably 1120 to 1180 ° C at the lower end. Through the Upper tube cover 18 is a gas lance 20 made of, for example, quartz, from which the deposition gas of, for example, trichlorosilane-hydrogen is introduced is blown through the two graphite foil tubes 16 and 17 limited space. the Induction heating coil 19 is at a certain speed from the bottom of the support tube assembly moved upwards, the speed being matched to the supply of the decomposition gas got to. The space enclosed by the two support tubes 16 and 17 thus becomes successive Filled with silicon from the ground up. As the deposition zone rises, the quartz lance 20 becomes withdrawn accordingly upwards. The residual gases exit through the nozzle 21.

During the deposition process, the The reactor 13 is charged with a protective gas, for example argon, via the gas supply nozzle 14 and the The air contained is displaced via the gas discharge nozzle 15. The choice of the protective gas is unproblematic because it does not come into contact with the actual reaction space rapid oxidation of the zones of the graphite foil heated to the deposition temperature by the heating device 19 to prevent. On the other hand, a pressure is set with the protective gas in the reactor 13 which is approximately the same as the pressure in the separation zone within the two support pipes

16 and 17 corresponds

After the deposition has ended, the graphite foil adhering inside and outside of the molded silicon body produced in this way is conventionally thus removed again, for example, by etching, burning or sandblasting

According to this process variant, hollow silicon bodies with a precisely defined cross-sectional area can be produced. for example round tubes 22, tubes that are round on the outside and square on the inside 23 or vice versa 24, outside and inside square hollow bodies 25, outside and inside triangular bodies 26, or also solid bodies with, for example, a cross-shaped cross-section 27. It

In this way, full bodies and Manufacture hollow bodies with a defined different cross-sectional area, for example funnels 28 and tapering tubes 29 as shown in Figure 2 in longitudinal section. Thus, silicon

i ") to produce hollow bodies, which are connected with ground joints, so they can be plugged into each other in a gastight manner. This allows laboratory equipment like them so far only made of glass or quartz, reconstruct accordingly in silicon if equipment for example, are to be exposed to high temperatures.

When it comes to the deposition, it is not the exact shape of the silicon body that is in the foreground, but rather of interest more the fast manufacture of polycrystalline We recommend silicon as the base material for subsequent refinement processes using silicon melt a variant of the method as shown in FIGS In a gas-tight, thermally insulated reactor 30 from

for example stainless steel, which is against the outside against direct Heat dissipation is covered with a suitable insulating layer, for example made of glass wool a hollow cylinder 31, closed on one side, made of graphite foil, which at its lower, open end

r> is attached in an electrically conductive manner to a carbon ring 32, which in turn rests with an annular groove milled out on its underside on a corresponding, cooled power supply electrode 33 made of metal, for example silver, between the hollow cylinder 31

• «ι and the inner wall of the reactor is also located A radiation protection 34 made of sheet molybdenum, for example, is also advantageous in order to prevent heat from being radiated contain. The ring-shaped power supply electrode 33 is hollow on the inside and is supplied and

4i outlet nozzle 35 from a suitable cooling medium, for example water, flows through the bottom of the hollow cylinder 31 made of graphite foil is conductive in the middle connected to the central conductor tube 36 made of, for example, silver. The central conductor tube 36, which as

V) current discharge functions, is continuously charged with a suitable cooling medium such as water via the cooling lance 37

Before the actual deposition is started, the reactor 30 is connected via the gas supply pipe zen 38 is charged with a protective gas, for example argon, and the air it contains is displaced via the gas outlet nozzle 39. The choice of protective gas is important This is unproblematic as it does not come into contact with the actual reaction space. The function

tf> of the protective gas consists in the rapid oxidation of the by direct passage of current to the deposition temperature of the hollow cylinder 31 made of graphite foil impede. On the other hand, the protective gas is used to set a pressure in the reactor 30 which is approximately the same as the pressure

b5 in the separation zone within the hollow cylinder 31 is equivalent to

During the actual separation, the Gas supply line 40 in the base plate 41 of the reactor 30

the deposition gas passed under a same pressure as possible to the pressure of the protective gas in the interior of the through direct passage of current to a temperature of about 1050 to 1250 ° C, preferably 1120-1180 ⁰ C heated hollow cylinder 31 made from graphite foil, thereby located on the inner surface of the Hollow cylinder 31 made of graphite foil a constantly growing layer of polycrystalline! Silicon is deposited, which increasingly takes over the power line. Uniform growth is ensured by the preferably water-cooled central power line 36, which cools areas that are growing too quickly compared to more distant competing areas. The protective gas pressure acting on the outside of the hollow cylinder 31 made of graphite foil, as well as the pressure of the deposition gas, is generally set so that it is about 0 to 5 bar, preferably 0.05 to 0.5 bar, above the external atmospheric pressure. The residual and reaction gases pass through the gas discharge

42 in the reactor bottom plate 41 again. The reactor bottom plate 41 made of, for example, silver-plated Steel is largely hollow and gets over the nozzle

A suitable cooling medium, for example water, flows through 43 and 44. Through a sight glass 45 made of quartz, the deposition process can be continuously observed.

In principle, for example, silicon hydrogen, monochlorosilane, dichlorosilane, Trichlorosilane or tetrachlorosilane, usually in a mixture with hydrogen, are used, with a Trichlorosilane-hydrogen mixture is preferred.

The best way to stop the deposition is after half of the initial inside diameter has been reached and the deposited polycrystalline silicon after removing the graphite shell by sandblasting for Example through the crucible pulling process according to C ζ ο c h r a I s k i converted into monocrystalline material.

In Fig. 4 is finally another of the ones shown in FIG. 3 shown very similar experimental set-up for the production of polycrystalline silicon is shown, which only differs in that the current is diverted via a, for example, silver-plated Steel existing central conductor tube 46 upwards over the upper cover plate 47 of the reactor 30 he follows

This embodiment is energetically even more favorable, since the heat dissipation, as it is in the in FIG. 3 The process variant shown takes place on the central guide tube 36, is avoided. The opposite of the Method variant described with reference to FIG reduced evenness of the silicon growth layers can generally be accepted because the deposited silicon is, for example, melted down and closed after the graphite layer has been separated off Solar cells can be cast in a thin layer.

According to the method described, it is possible for the first time to produce silicon bodies that are inexpensive and uncomplicated Way to manufacture in just about any shape.

example 1

In a reactor as shown in FIG. 1 is shown, Consists of a 130 cm high cylinder jacket made of quartz, which is covered by two silver plates at the top and bottom completed, each equipped with eight graphite electrodes, eight support tubes are made Graphite foil fitted between the graphite electrodes. The support tubes are made from a graphite foil strip 15 cm wide and 0.5 mm thick at a pitch angle of 45 "but wound and the helically surrounding the pipe seam with a 0.2 mm thick and 3 cm wide graphite tape with a Carbon glue glued and cut to a length of 100 cm.

A gas mixture of 7% by volume of trichlorosilane and 93% by volume of hydrogen is then fed into the reactor for 48 hours at a rate of approx. 100 m ³ /

to hours initiated.

After decomposition of the gas mixture on the heated to U 50 ° C by direct passage of current After the graphite foil has been separated by sandblasting, support tubes are made of silicon tubes with a Evenly thick wall thickness of 1 cm obtained.

Example 2

In a reactor as shown in FIG. 2 is shown, Consists of a 250 cm long, heat-insulated stainless steel cylinder of 60 cm diameter with a die The quartz slit running along the longitudinal front is sandwiched between two massive graphite plates nested graphite support tubes with a square cross-section in the graphite plates fitted into milled grooves.

The 200 cm long graphite support tubes, the inside of which has a square with a side length of 10 cm and the outside a square with a side length of 12 cm in cross-section, are made by simply folding a correspondingly cut graphite sheet with a thickness of 0.5 mm and pasting the Seam made with a 3 mm wide and 0.2 mm thick graphite foil strip
Before the deposition, the reactor is filled with argon as a protective gas.A water-cooled induction heating coil is slowly guided over the entire length of the support tubes from the bottom of the reactor over the course of 15 hours, while the decomposition gas of 15 vol.% Trichlorosilane and 85 vol .% Hydrogen is blown into the space between the two support tubes. The silicon is deposited in each case in the deposition zone heated by the induction heating coil, which traverses the full length of the support tubes.

The separation of the graphite foil from the silicon square tube produced in this way is carried out analogously to the example 1.

Example 3

In a reactor as shown in Fig. 3, consisting of a 120 cm long heat-insulated stainless steel cylinder with an inner diameter of 60 cm, the inner wall of which is provided with a molybdenum sheet to allow heat energy to be radiated prevent a 100 cm high hollow cylinder made of graphite foil with an inner diameter of 40 cm and a wall thickness of 0.5 mm and used on the lower circumference with that on the ring-shaped water-cooled, silver-made power supply electrode attached to the carbon ring. the The upper cover of the hollow cylinder, also made of graphite foil, becomes central with the water-cooled one and connected to the current conductor, which is also made of silver.

Subsequently, the air in the reactor is both outside and displaced within the hollow cylinder with argon and subsequently in the interior of the heated by direct current flow up to 1150 ⁰ C. hollow-

Linders introduced a separation gas made of 12% by volume trichlorosilane and 88% by volume hydrogen from graphite foil. Of the Argon pressure acting on the outside of the hollow cylinder, as well as the pressure of the deposition gas in the Inside the hollow cylinder are set to 0.1 bar above the external atmospheric pressure.

With an average inflow of 250 standard cubic meters of separation gas per hour, this increases over the course of 48 Hours a silicon layer of approx. 10 cm on the inner wall of the hollow cylinder made of graphite foil. Since the

10

The separation rate would now decrease rapidly due to the ever decreasing separation area, If the deposition is broken off, the hood of the reactor is pulled off upwards, the graphite hollow cylinder with the silicon body removed from the electrodes, the graphite foil from the deposited Silicon is removed by sandblasting and, after comminution, is used as the starting product for finishing, for example by pulling the crucible according to C ζ ο c h r a I -

to s k i, supplied.

For this purpose 4 sheets of drawings

Claims (2)

Patent claims: 1. Process for the deposition of poly crystalline silicon from the gas phase on heated Carrier bodies made of carbon, and subsequent removal of the carrier body, characterized in that that the support body 0.1 to 2 mm thick, flexible sheet-like structure made of carbon used 2. The method according to claim 1, characterized in that that graphite foil is used as the flat structure