DE2618273A1 - PROCESS FOR THE DEPOSITION OF POLYCRYSTALLINE SILICON - Google Patents

Description

WACKER-CHEMITRONIC * "Munich, April 5, 1976

Society for FE / Pat / Dr.F / sm 2R1 ^Q ? ^r 'l

Elektronik-Grundstoffe mbH

Wa-Ch 7601

Process for the deposition of polycrystalline! Silicon

The invention relates to a method for the deposition of polycrystalline! Heated silicon from the gas phase Carrier bodies made of carbon.

In high-temperature processes in semiconductor technology, in particular for diffusion, oxidation and epitaxial processes, reaction chambers made of silicon are recommended, due to their greater purity, greater mechanical stability and lower gas permeability than quartz.

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

The deposition of silicon on carbon or graphite was first successfully carried out by R.Hölbling in 1927 (Z. anorg. general Chem. 40, 655-659 / 1927). Rod-shaped solid bodies were produced by this method.

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According to a method described in DT-AS 1 109 142 on the other hand, quartz tubes are used as the carrier body, which on the outside with a layer of carbon are coated on which hollow silicon bodies are deposited from the gas phase. In this procedure prepares but the separation of the support body and shaped body presents great difficulties.

This separation of the carrier body and the molded body deposited thereon is more successful according to one in DT-OS 2 215 described method, in which a carrier body made of carbon before the deposition with a layer of Silicon dioxide and subsequently amorphous silicon is coated. But also with this procedure are the Reuse of the carrier body is subject to narrow limits, and the carrier body must be carefully in each case sanded and recoated. Also with other methods known from the literature, according to which silicon moldings by depositing silicon from the gas phase on carrier bodies made of carbon or If graphite are produced, the carrier bodies can only be reused in the rarest of cases even then, at most once or twice.

This due to the different expansion coefficients Difficulty caused by carbon and silicon in separating the support body and shaped body, promoted by the brittleness of silicon, which in addition to the destruction of the carrier body, often also to a detonation of the deposited silicon molding

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leads, as well as the time-consuming and costly production of the carrier body itself, which is made from solid blocks of carbon or graphite have to be worked out represent the decisive disadvantages of all known processes, whereby the use of silicon moldings is limited to only a few special fields so far.

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

This object is achieved in that the support bodies are made of flexible sheet-like structures 0.1 to 2 mm thick Carbon can be put together.

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

Such knitted fabrics consist of carbon threads, which in turn by carbonizing synthetic fibers from, for example, polyacrylonitrile or polyvinyl alcohol. Graphite foils, which, according to the manufacturer, are made of pure, well-ordered graphite, are particularly suitable are produced, the distances between the layers in the crystal lattice by chemical and thermal treatment of the graphite can be expanded to a multiple of the normal value of 3.35 A. The resulting light The bulk weight of worm-shaped individual parts is 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 by application be firmly connected to each other again by mechanical pressure.

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Such graphite foils, which in thicknesses from 0.1 to 2 now, preferably 0.2 to 0.6 mm can be used for the production of support bodies of any conceivable shape, can be processed extremely easily. You can cut through with simple household scissors Bend them into the desired shape and glue them with commercially available carbon adhesives. As a carbon glue Adhesives are preferred for which, at the high deposition temperatures, only Carbon remains as a solid residue.

It is also advantageous to use graphite foils provided with an adhesive layer on one side, especially in tape form, for example during manufacture can be used by round support bodies. Such round or curved support bodies can be for example, simply by making a corresponding, lost wax mold with such a Graphite tape is wrapped, care must be taken that in the later direction of current flow the graphite cross-sectional area is always largely the same area, that is, that parts of the mold with less Inner diameter must be glued over in several layers accordingly. After the wax mold is complete is stuck shut, the wax is removed by simply melting it out.

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Carrier bodies suitable for the production of silicon tubes are obtained, for example, in that a corresponding rectangular piece of film is wrapped around and is glued to the longitudinal seam. Another possibility that leads to greater diameter constancy, in particular when using graphite foils of thinner wall thicknesses, is to use ribbon-shaped graphite foil Wind up helically on appropriate, preferably metallic, measuring rollers and glue them to one another. The gluing takes place either in that the film on the helically encircling the pipe circumference Seam wound overlapping and the adhesive is introduced between this overlap, or by that the tape film is butt wound and the seam is covered with ■ a narrow and as thin as possible adhesive tape will.

Tubular support bodies with a circular cross-section can be produced, for example, in large numbers and Any length produced by placing on a roller made of, for example, steel, the diameter of which corresponds to the inner diameter of the silicon tube to be deposited minus the thickness corresponds to the thickness of the graphite foil, with a second roller, a band made of graphite foil, which rotates the first tube at a certain angle of inclination is pressed. The seams can be glued in the usual way or with a corrugated roller are interlocked and then pressed together.

This process can be carried out continuously, in such a way that the wound and glued graphite support tube under a twisting movement on one side of the measuring roller and pulled off the other Side is continuously rewound and glued. With appropriate, interchangeable measuring rollers can be opened

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produce this type of pipe with any inside diameter. The support tubes are wound in this way self-supporting and can be continuously drawn onto the measuring roller and at the other end in desired Length to be cut off.

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 of a polygon can be produced by simply folding the graphite foil and then gluing the seams together.

For the separation, for example, two such tubes are attached to two electrodes and over a bridge made of the same material connected to each other in a U-shape. The bridge can also be made in the simplest way consist of a tube composed of graphite foil, with two circular recesses on its long side with diameters corresponding to the outer diameters of the pipes to be contacted at a distance between these two pipes be cut out. With the two circular recesses, the bridge is easy on the two pipes attached.

The process is also well suited for the production of polycrystalline silicon as a starting product for Casting process or for the crucible pulling process according to Czochralski. While this material has so far been deposited by deposition is produced on monocrystalline silicon thin rods, the amount of silicon deposited in the unit of time is initially low and only with increasing thickening of the separator support and thus increased separation area Reached useful values can be achieved by the process according to the invention by the deposition of silicon

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for example on the inner surface of hollow cylinders Graphite foil can be produced with high deposition rates of polycrystalline silicon right from the start. Is in half the inner diameter of the separation cylinder has grown, the separation process is expediently aborted, since the separation rate then decreases noticeably.

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

In principle, for example, silicon hydrogen, Monochlorosilane, dichlorosilane, trichlorosilane or Tetrachlorosilane, usually mixed with hydrogen, are used, a trichlorosilane-hydrogen mixture is preferred.

Four process variants are shown schematically in the figures:

Figure 1 shows the production of silicon tubes with exactly defined hollow cross-section.

FIG. 2 shows the production of silicon tubes with a precisely defined cross-sectional area over the entire tube length.

FIG. 3 shows the production of polycrystalline silicon as a starting material for subsequent casting or crucible drawing processes with current discharge through the reactor base

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FIG. 4 shows the production of polycrystalline silicon as a 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 preferably composed of graphite foil are grounded Carrier tubes 2 are inserted into graphite electrodes 4, which are seated on electrode carriers 3 made of silver, for example 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 supply line 5, is silicon on the outer surface of the graphite foil composed Support tube 2 deposited to the desired thickness. The residual gases from the trichlorosilane decomposition pass through the Gas discharge line 6 from the reactor 1 again. In the case of a metal reactor, the course of the deposition can be seen through a sight glass 7 made of quartz can be observed.

The surface of the silicon tube 8, which is precisely defined in terms of its dimensions, is the inner surface. The hollow cross-section is predetermined by the cross section of the support tube 2. For thin pipes with a wall thickness of a few millimeters the outer boundary line of the pipe cross-section is still largely the inner boundary line, which is determined by the shape of the support tube 2 and, for example, triangular 8, square 9, octagonal 10, circular 11 or oval 12. With greater wall thicknesses of the silicon tubes 8 and a correspondingly longer deposition time, a increasing rounding of the corners, so that for example a silicon tube, which is deposited on a carrier tube according to FIG. 9, from a wall thickness of approx. 7-10 mm

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a largely circular outer boundary line the cross-sectional area would have. In addition to the absolute separation time, the separation speed also plays a role here an essential role. Is deposited faster, for example with

2 speeds over 200O g Si / hour. m so keeps the deposited silicon moldings tend to have the shape given by the carrier body than if slowly, that is

2
for example with 200 g Si / hour. m is deposited.

After the silicon deposition has been completed, the silicon tubes 8 and the graphite foil are removed from the reactor 1 by etching, burning or quite simply by sandblasting from the deposited silicon tube 8 removed again.

Although the anisotropy coefficient of the specific Electrical resistance in the order of magnitude of approx. 50 to 200 occur when the power is supplied of carrier bodies made of this material do not pose any unusual difficulties.

Are hollow silicon bodies, which are not limited to the Shape of their hollow cross-section matters, desired, so those with a precisely defined inner and in particular outer contour, a process variant as shown in Figure 2 is recommended: In a metal or quartz reactor 13 with the gas supply and gas discharge nozzle 14 and

15 are two tubes, preferably made of graphite foil

16 and 17 placed one inside the other and held together at the top and bottom with annular graphite holders 18. The two support tubes 16 and 17 are by a heater, preferably by a vertically movable high-frequency induction heating coil 19, for example a water-cooled silver coil, at the lower end to a deposition temperature of preferably 1120 to 1180 ° C brought. A gas lance made, for example, of quartz, is inserted through the upper pipe cover 18, from which the deposition gas

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from, for example, trichlorosilane hydrogen in the through the two graphite foil tubes 16 and 17 limited space is blown. The induction heating coil 19 is with a certain Speed from the bottom of the carrier tube assembly driven upwards, the speed must be adjusted to the supply of the decomposition gas. The one of the two The space enclosed in the carrier tubes 16 and 17 is thus gradually filled with silicon from the bottom up. With increasing The quartz lance 20 becomes the deposition zone accordingly withdrawn upwards. The residual gases exit through the nozzle 21.

During the deposition process, the reactor 13 is provided with a protective gas, for example Argon, charged and displaced the air contained via the gas discharge nozzle 15. The choice of protective gas is included unproblematic as it does not come into contact with the actual reaction space. The function of the protective gas consists in the rapid oxidation of the zones heated to the deposition temperature by the heating device 19 To prevent graphite foil. On the other hand, a pressure is set with the protective gas in the reactor 13, which is approximately the Pressure in the separation zone within the two support tubes 16 and 17 corresponds.

After the deposition has ended, the silicon molded body produced in this way adheres to the inside and outside Graphite foil in the usual way, for example by etching, burning off or sandblasting again removed.

According to this variant of the method, hollow silicon bodies can be produced with a precisely defined cross-sectional area, for example round tubes 22, tubes that are round on the outside

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and inside square are 23 or vice versa 24, outside and inside square hollow bodies 25, outside and inside triangular Body 26, or also solid body with, for example, a cruciform cross-section 27. It can also be in this way also produce solid bodies and hollow bodies with a defined different cross-sectional area, for example Funnel 28 and tapering tubes 29 as shown in Figure 2 in longitudinal section. It It is thus possible to produce hollow silicon bodies that are connected with ground joints, that is, plugged into one another in a gas-tight manner can. In this way, laboratory devices, as they were previously only made of glass or quartz, can be adjusted accordingly Recreate in silicon if equipment is to be exposed to high temperatures, for example.

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 rather the fast production of polycrystalline silicon as a base material for subsequent refinement processes via silicon melt is recommended a variant of the method as shown in FIGS is.

In a gas-tight, thermally insulated reactor 30 made of, 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 is electrically connected at its lower, open end is conductively attached to a carbon ring 32, which in turn has a milled out on its underside Ring groove on a corresponding, cooled power supply

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Guide electrode 33 made of metal, for example silver, is seated. Between the hollow cylinder 31 and the inner wall of the reactor there is also advantageously a radiation protection 34 made of, for example, molybdenum sheet contain the radiation of heat. The annular power supply electrode 33 is hollow inside and is over A suitable cooling medium, for example water, flows through the inlet and outlet nozzles 35. The bottom of the hollow cylinder 31 made of graphite foil is conductive in the middle with the central conductor tube 36 made of, for example, silver tied together. The central conductor tube 36, which functions as a current conductor, is connected to a cooling lance 37 with a A suitable cooling medium such as water is continuously charged.

Before the actual deposition is started, the reactor 30 is equipped with a gas feed nozzle 38 Protective gas, for example argon, is charged and the air contained is displaced via the gas outlet nozzle 39. the Choosing the protective gas is not a problem, since it does not come into contact with the actual reaction space. The function of the protective gas consists in the rapid oxidation of the by direct current passage to the deposition temperature To prevent heated hollow cylinder 31 made of graphite foil. On the other hand, it is with the protective gas in the reactor 30, a pressure is set which corresponds approximately to the pressure in the separation zone within the hollow cylinder 31.

During the actual separation, the separation gas is discharged through the gas supply line 40 in the base plate 41 of the reactor 30 under the same pressure as possible compared to the pressure of the protective gas into the interior of the direct Current passage to a temperature of approx. 1050 to 1250 ° C, preferably 1120 to 1180 ° C heated hollow cylinder

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made of graphite foil, whereby on the inner jacket of the liohlzy Linders 31 made of graphite foil is a constantly growing layer deposited from polycrystalline silicon, which increasingly takes over the power line. Even growth will be achieved ensured by the preferably water-cooled central power dissipation 36, the quasi-rapidly growing places cools down more strongly compared to more distant competing areas. The one on the outside of the hollow cylinder made of graphite foil 31 acting protective gas pressure and the pressure of the separation gas is generally set so that it is about 0 to 5 bar, is 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 off again. The reactor bottom plate 41 made of, for example, silver-plated steel is largely hollow and a suitable cooling medium, for example water, flows through the connection piece 43 and 44. The deposition process can be continuously observed through a sight glass 45 made of quartz.

In principle, for example, silicon hydrogen, Monochlorosilane, dichlorosilane, trichlorosilane or tetrachlorosilane, can usually be used in a mixture with hydrogen, a trichlorosilane-hydrogen mixture being preferred will.

The deposition is best aborted after half the initial internal diameter has been reached and the deposited polycrystalline silicon after removing the graphite shell by sandblasting, for example by the Czochralski crucible pulling process converted into monocrystalline material.

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Finally, in FIG. 4, another of the ones shown in FIG. 3 is shown very similar experimental set-up for the production of polycrystalline silicon shown, which is only The difference is that the current is conducted away via a steel that is made of, for example, silver-plated steel central conductor tube 46 takes place upwards over the upper cover plate 47 of the reactor 30.

This embodiment is energetically even more favorable, since the Heat dissipation, as occurs in the process variant shown in FIG. 3 on the central guide tube 36, is avoided will. The reduced evenness of the process variant compared to the process variant described with reference to FIG As a rule, silicon growth layers can be accepted, since the deposited silicon remains Separation of the graphite layer, for example, can be melted down and cast into solar cells in a thin layer.

According to the method described, silicon bodies are thus achieved for the first time in an inexpensive and uncomplicated manner Way to manufacture in just about any shape.

example 1

In a reactor as shown in Fig. 1, consisting of a 130 cm high cylinder jacket made of quartz, which at the top and is closed at the bottom by two silver plates, each equipped with eight graphite electrodes eight support tubes made of 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 an angle of inclination 45 ° wound on butt and the helical

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the pipe circumferential seam with a 0.2 mm thick and 3 cm wide graphite tape glued with a carbon glue and on trimmed 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 with a «
directs.

at a speed of approx. 100 m / hour. in-

After decomposition of the gas mixture to 1150 ° C through direct current flow through heated support pipes, after separation of the graphite foil by sandblasting, Silicon tubes with a uniform wall thickness of 1 cm received.

Example 2

In a reactor as shown in Fig. 2, consisting of a 250 cm long, thermally insulated stainless steel cylinder 60 cm in diameter with a quartz viewing slit running along the longitudinal front are placed between two massive Graphite plates two 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 inner one a square with a side length of 10 cm and the outer one has a square with a side length of 12 cm in cross-section, are cut to size by simply folding one accordingly Graphite foil with a thickness of 0.5 mm and pasting over the seam with a 3 mm wide and 0.2 mm made of thick graphite foil strips.

Before the deposition, the reactor is filled with argon as a protective gas. An induction heating coil cooled with water

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is slowly guided over the entire length of the support tubes over the course of 15 hours from the bottom of the reactor while The decomposition gas consisting of 15% by volume of trichlorosilane via the gas lance and 85% by volume of hydrogen is blown into the space between the two support tubes. The deposition the silicon takes place on the one with the induction heating coil that traverses the full length of the support tubes heated separation zone. The separation of the graphite foil from the silicon square tube produced in this way takes place analogous to example 1.

Example 3

In a reactor as shown in Fig. 3, consisting from a 120 cm long thermally insulated stainless steel cylinder with an inner diameter of 60 cm, whose Inside wall is provided with a molybdenum sheet to prevent the radiation of heat energy, 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 on the lower circumference with the one on the ring-shaped water-cooled, silver-made power supply electrode attached to the carbon ring. The upper, The cover of the hollow cylinder, which is also made of graphite foil, becomes central, with the water-cooled and likewise connected from silver existing current conductor.

Then the air in the reactor, both outside and inside the hollow cylinder, is displaced by argon and subsequently into the interior of the by direct current passage Hollow cylinder made of graphite foil heated to 1150 C a separation gas consisting of 12% by volume of trichlorosilane and 88% by volume of hydrogen was introduced. The one on the outside of the hollow cylinder acting argon pressure, as well as the pressure of the ab-

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separating gas inside the hollow cylinder, v / ground in the process adjusted to 0.1 bar above the external atmospheric pressure.

With an average inflow of 250 standard cubic meters of separation gas per hour, a silicon layer of approx. 10 cm grows over the course of 48 hours on the inner wall of the hollow cylinder made of graphite foil. Since the deposition rate would now decrease rapidly due to the ever decreasing deposition area, the deposition is broken off, the hood of the reactor is pulled upwards, the graphite hollow cylinder with the silicon body is removed from the electrodes, the graphite foil is removed from the deposited silicon by sandblasting and after comminution a ± & The starting product for refinement, for example by crucible pulling according to Czochralski.

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Claims (3)

Claims 1. Process for the deposition of polycrystalline Silicon from the gas phase on heated carbon carriers / characterized that the carrier body from 0.1 to 2 mm thick, flexible sheet-like structures be composed of carbon. 2. The method according to claim 1, characterized ge indicates that graphite foil is used as the flat structure made of carbon. 3. The method according to any one of claims 1 and 2, characterized in that the graphite foil serving as the carrier body is removed from the silicon body formed after the silicon has been deposited by sandblasting. 709 8U / 049P ORIGINAL! NSPECTD