WO2014118019A1 - Method for depositing polycrystalline silicon - Google Patents

Abstract

The invention relates to a method for depositing polycrystalline silicon. A silicon-containing gas is introduced into a reactor, and polycrystalline silicon is deposited on a carrier body, which is heated to a temperature of at least 550 °C by a direct passage of current, by reducing the silicon-containing gas. The invention is characterized in that at least one first metal, selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper, and zinc, is present during the reduction of the silicon-containing gas, said metal acting as a first catalyst, and additionally at least one second metal, which is different from the first metal and is selected from the group consisting of copper, silver, and gold, is present, said second metal acting as a second catalyst.

Description

 Method of depositing polycrystalline silicon

The invention relates to a method for the deposition of polycrystalline silicon.

High purity polycrystalline silicon (polysilicon) serves as a starting material for the production of single crystal silicon for Czochralski (CZ) or zone melting (FZ) semiconductor, for the production of single or multicrystalline silicon by various drawing and casting processes

Production of solar cells for the photovoltaic, as well as component in electrodes of lithium ion batteries. Polysilicon is usually produced by means of the Siemens process. In this case, a reaction gas comprising one or more silicon-containing components and optionally hydrogen in a reactor comprising by direct

Introduced current passage heated carrier body, which deposits on the carrier bodies silicon in solid form.

As the silicon-containing components are preferably silane (SiH4), monochlorosilane (SiH ₃ Cl), dichlorosilane (DCS, SiH ₂ Cl ₂ ), trichlorosilane (TCS, SiHCl ₃ ), tetrachlorosilane (SiCl ₄ ) or

Mixtures of these substances used.

The Siemens procedure is usually in one

In the most common embodiment, the reactor comprises a metallic base plate and a coolable bell which is placed on the base plate, so that a reaction space in the

Inside of the bell arises. The baseplate is provided with one or more gas inlet openings and one or more

Exhaust ports for the outgoing reaction gases and with

Mounts provided with the aid of the support body in

Reaction space kept and supplied with electrical power become. In EP 2 077 252 A2 the typical structure of one used in the production of polysilicon is used

Reactor type described. Each carrier body usually consists of two thin ones

 Filament rods and a bridge, which usually connects adjacent rods at their free ends. Most commonly, the filament rods are made of single or polycrystalline silicon

manufactured, rarely come metals or alloys or

Carbon used. The filament rods are mounted vertically in electrodes located at the bottom of the reactor, via which the connection to the power supply takes place. High-purity precipitates on the heated filament rods and the horizontal bridge

Polysilicon, whereby their diameter increases with time. After the desired diameter is reached, the process is terminated by stopping the supply of silicon-containing components.

The deposition process is usually controlled by the specification of rod temperature and reaction gas stream or composition. The measurement of the rod temperature is carried out with

Radiation pyrometers mostly at the reactor wall

facing surfaces of the bars. The rod temperature is controlled by controlling the electrical power

either fixed or depending on the bar diameter

specified. The amount and composition of the

Reaction gases are given as a function of time or bar diameter. The deposition with TCS or its mixture with DCS and / or STC is usually carried out at rod temperatures between 900 and 1100 ° C, a supply of silicon-containing component (s) (in total) of 0.5 to 10 kMol / h per 1 m ² the rod surface, wherein the mole fraction of this component (s) in the feed stream (in total) is between 10% and 50% (the remainder 90% to 50%

usually hydrogen). From US4481232 a method for producing high purity silicon is known, comprising forming a copper silicide alloy and positioning the alloy within a container, placing a filament over the alloy; Supplying a transport gas adapted to transport silicon, heating the filament and the alloy to temperatures at which the gas on the alloy surface with silicon

reacting and depositing silicon on the filament, which alloy is capable of trapping impurities at those temperatures. Copper is used to promote the reaction of silicon to a silane. From the resulting silane silicon is immediately deposited again.

US 4759830 and US 4773973 relate to processes for the production of thin layers of elemental silicon on an electrically conductive material suitable as an electrode by electrolytic deposition of silicon from a

Molten salt, which is a silicon halide, a

 Aluminum halide, an alkali metal or ammonium halide and a halide of a transition metal

wherein the electrolysis is carried out at temperatures of 100 to 350 ° C in an inert atmosphere and optionally under pressure. As halides of a transition metal are disclosed chromium (II) iodide (CrJ 2), manganese (II) iodide (MnJ 2),

Iron (II) iodide (FeJ2), copper (I) iodide (CuJ), hafnium (IV) iodide (HfJ4), nickel] odid (NiJ2), vanadium (II) iodide (VJ2) or

Mixtures of these iodides.

The electrolytic deposition of the silicon can be cathodic, wherein the cathode material such as copper, chromium, molybdenum, nickel, platinum, iron or stainless steels, preferably of aluminum, silicon or graphite and as

Anode material such as platinum, silicon or graphite used.

The electrolytic deposition of the silicon may be carried out anodically from a melt comprising a silicon halide

Aluminum halide, an alkali metal or ammonium halide containing, as the anode material such as aluminum and cathode material such as silicon or graphite

used. The electrolytic deposition of the silicon may e.g. from a melt containing silicon tetraiodide, aluminum triiodide and lithium iodide.

The halide of a transition metal represents the so-called catalyst, which controls the silicon deposition and the quality of the silicon layers on e.g. Copper, chromium, molybdenum, nickel, iron and chromium steel or inorganic glasses e.g. from tin dioxide or tin dioxide / indium oxide mixtures

improved and a silicon layer formation on one

Silicon carrier under the conditions of that method in the first place. Namely, no silicon deposition is observed without this catalyst.

The Müller-Rochow synthesis is a process for

large-scale production of methylchlorosilanes. When

Catalyst is copper, which elementary or z. B. is used in the form of copper oxide. Zinc, tin, phosphorus and other elements also act as promoters. The reaction takes place at about 300 ° C and 0.5-2 bar (ü).

EP2036117 AI discloses that according to conventional technique

Silicon nanowires using a catalytic

Metal layer to be grown on a substrate.

With metal, it may e.g. to trade gold on one

Substrate is deposited. This will become catalytic

Formed metal islands. Subsequently, silicon from a silicon source is used to effect growth of silicon nanowires at the metal islands.

EP2082419 A2 discloses a process for metal catalyzed nanowire growth in which as metal catalysts

Transition metals from the periodic table, u.a. Copper, silver, gold, nickel, palladium, platinum, cobalt, rhodium, iridium, indium, iron, ruthenium, tin, osmium, manganese, chromium,

Molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium and Gallium, including mixtures of one or more of these metals.

Current technologies for the use of silicon in semiconductor technology make the use of raw silicon with increasing demands on the purity of the material

required.

The object of the present invention was to provide a more economical method for the separation of

To provide polycrystalline silicon.

The object is achieved by a method for depositing polycrystalline silicon, wherein a silicon-containing gas is introduced into a reactor and by reduction of the silicon-containing gas polycrystalline silicon is deposited on a by direct current passage to a temperature of at least 550 ° C heated carrier body, characterized characterized in that during the reduction of the silicon-containing gas at least one first metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and zinc is present as a first

Catalyst acts and also at least one different from the first metal second metal selected from the group

consisting of copper, silver and gold, which acts as a second catalyst

The silicon-containing gas is preferably a halosilane, preferably trichlorosilane.

Preferably, trichlorosilane is used in the presence of hydrogen.

There are two partial reactions. Si-H and Si-Cl bonds have to be cleaved. As will be explained later, two types of catalysts are required to both

Accelerate partial reactions. The following combinations have proven to be particularly suitable:

Copper and titanium / iron / nickel;

Silver and chromium / manganese / iron / zinc;

 Gold and Titanium / Chrome / Manganese / Iron / Cobalt / Nickel / Zinc

Particularly preferred are the combinations of gold and titanium and gold and chromium.

Very particularly preferred is the use of copper and nickel. It is also particularly preferred to use copper and silver as well as copper and gold in combination.

The at least first and at least second metals shall be present in such quantities during the reduction of the silicon-containing gas that in the deposited polycrystalline silicon at least one element selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and Zinc at a concentration of 18 pptw - 40 ppbw titanium, 0.2 pptw - 5 ppbw chromium, 0.5 pptw - 15 ppbw manganese, 7 pptw - 50 ppbw iron, 0.012 pptw - 25 ppbw cobalt, 0.9 pptw - 8 ppbw nickel, 3 pptw - 12 ppbw copper or 0.6 pptw - 11 ppbw zinc;

and at least one second element other than the first element selected from the group consisting of copper, silver and gold in a concentration of 3 pptw - 12 ppbw copper, 0.15 pptw - 21 ppbw silver and 0.001 pptw - 0.3 ppbw gold is. The invention therefore also relates to polycrystalline silicon containing at least one first metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and zinc; and at least one second metal other than the first metal selected from the group consisting of copper, silver and gold;

wherein the at least two selected metals are present in polycrystalline silicon in the following amounts: 18 pptw - 40 ppbw titanium, 0.2 pptw - 5 ppbw chromium, 7 pptw - 50 ppbw iron, 0.012 pptw - 25 ppbw cobalt, 0.9 pptw - 8 ppbw nickel, 3 pptw - 12 ppbw copper, 0.6 pptw - 11 ppbw zinc, 0.15 pptw - 21 ppbw silver, 0.001 pptw - 0.3 ppbw gold, 0.5 pptw - 15 ppbw manganese.

Table 1 shows the minimum and maximum concentrations of metals in the deposited polycrystalline silicon.

Table 1

Preferably, the metal (s) are used in combination with carbon. It is particularly preferred if copper, nickel and

Carbon are present.

For this purpose, the concentration of methane in the silicon-containing gas should be 2-18 ppm. The process is associated with an immense economic advantage. In investigations of the deposition rates in reactors of the type shown in Fig. 1 has been shown that by the presence of the two catalyst metals compared

conventional methods (without metals) a thickness increase of polycrystalline silicon of at least 50% is achieved. The metals act as catalysts. This is associated with lower process times at lower process temperatures and thus a significant energy saving. Even at a temperature of 550 ° C, effective

Deposition rates.

In particular for applications in photovoltaics and in battery technology, the polysilicon produced is

excellent. For applications in microelectronics, the use of the polysilicon produced is few

preferred, since the requirements for the purity of

Polysilicon should be usually too high. However, it has been shown that even the smallest amounts of the existing metals, which have no negative impact on the quality of polycrystalline silicon, have a positive effect. It is essential that the catalytically active substances in solid, deposited polycrystalline silicon are effective. Disintegration of the silanes in the gas phase is undesirable. This would lead to gas phase nucleation and dusting, which is undesirable. The reaction mechanism must therefore be effective in the surface of the polycrystalline silicon. This is ensured by the fact that the metals are predominantly dissolved in solid silicon and do not remain free in the gas phase. The introduced metals can freely diffuse in already deposited polycrystalline silicon. As soon as they reach a surface and they are asymmetrically surrounded by silicon, stronger bonds form

Substrate having metal silicide properties. Due to their bonding structure, these metal silicides have an attractive effect on the surrounding hydrogen of the silanes and on the existing chlorine. The proton-withdrawing effect of

Pseudosilicide deprives the environment hydrogen, probably as a proton.

The silicon-containing gas, especially trichlorosilane, releases hydrogen. This is a first

Partial reaction. The so-destabilized silane decomposes, thereby depositing silicon. This is a second partial reaction.

The inventors assume - without wishing to be bound by this hypothesis - that both partial reactions are accelerated by catalysts, wherein the first partial reaction is accelerated by a first catalyst and the second partial reaction by a second catalyst, which can not be clarified the first and second partial reactions are each accelerated by the ferrous metals or by the noble metals. Copper is obviously suitable for both

Partial reactions act as a catalyst.

If trichlorosilane is reduced, chlorine is produced. This ensures that metals are removed from the reactor as volatile metal chlorides. At the same time it ensures a homogeneous distribution of the effective catalysts in the

Separation chamber.

Surprisingly, it has been found that combinations of the elements are more effective than the individual elements and especially carbon, which is ineffective in pure form, in combination with metals shows special properties, such as the surface quality of the polycrystalline silicon influenced. If it comes to dust formation, ie to a gas phase nucleation, the dust particles also take up metals. Also in the dust particles would then form metal silicides and it would come to silicon deposition. This is undesirable.

With a carbon addition, the Gasphasennukleation can effectively prevent.

As carbon source, methane or other carbon-containing gases are used, such as ethane, propane, butane, pentane, ethene, solvent vapors, the iso-forms of the

Hydrocarbons, methylsilane and others. Due to the low reactivity of methane at the temperatures mentioned methane is preferred. The other gases are more reactive and their methane equivalent can exceed the unused amount of

Carbon in the exhaust stream can be determined.

If copper is introduced as one of the catalysts, one obtains a otherwise identical process conditions

Polysilicon layer, which becomes 20 times thicker with maximum catalyst action. Here, polysilicon is deposited on the silicon core in the experimental reactor of FIG. 1. The same process conditions are temperature, pressure, gas flows and heating current. Only the metal addition by the catalyst source 9 in FIG. 1 is changed

 (Detailed view, see Fig. 2).

It can be observed here that the maximum effect can no longer be increased by increasing the amount of metal above a certain level. Only a slowing growth with too low supply was determined.

If gold is introduced as one of the catalysts, one obtains a otherwise identical process conditions

Polysilciumschicht on the growth zone, which is 60 times thicker with maximum catalyst action. It can be observed here that the maximum effect can no longer be increased by increasing the amount of gold above a certain content. The effect can no longer be increased up to the solubility limit of gold in silicon.

In the manufacture of a solar cell must be avoided that the introduced elements doping the silicon, as this complicates the creation of a p-n junction and the

Solar cell efficiency negatively affected.

A special role is played by aluminum. In the standard solar cell, the contacting is carried out by means of a highly doped with aluminum zone. This happens at the p-doped side of the p-n junction. That bothers you there

Aluminum as an impurity not. However, on the n-side, the doping is counteracted by aluminum.

Therefore, aluminum and elements are the 3rd main group

 (Boron group) unsuitable as catalysts.

Metals that occupy lattice sites are generally active as p- or n-doping. In addition, the effect

Locating lattice positions locally, i. the diffusion is obstructed.

Therefore, B, C, Mg, Ca, Zn, Pt are unsuitable while Li, O, Cr, Mn, Fe, Ni, Cu, Ag, Au are interstitial impurities.

In order to always have metal atoms available on the surface, they should diffuse to the surface, as only there can they interact with the atmosphere of the deposition.

At temperatures greater than 500 ° C this is usually the case. A high even at low temperatures

Diffusion coefficient has copper. In order to enable the function of the pn junction, the charge carriers must be separated. This is all the more easily possible the longer the charge carrier lifetime. Some of the metals act on and as recombination centers.

Particularly advantageous are copper and silver.

The release of the metals in the reactor is preferably carried out by arc in the cold gas phase, namely in the silanes, preferably in an inert gas. However, the metals can be incorporated in other ways in the reaction gases. For example, the metals may already be incorporated into the chlorosilane in an upstream process, e.g. Trichlorosilane, be introduced. It is also possible that metals are added to the hydrogen in a process upstream of the deposition.

Preferably, the metals are introduced by means of a gas-tight, temperature-resistant and temperature and pressure swing-resistant spark plug.

A high voltage pulse leads to an arc in the

Supply gas of the deposition process.

The spark plug comprises two electrodes, each one

Include catalyst metal.

Both electrodes can be completely made of one

Consist catalyst metal. Alternatively, a

Catalyst metal attached to the electrode tip, e.g. be soldered or welded, be.

The discharged amount of catalyst metal is through

Ignition voltage and charge (Joule) controlled. Fig. 1 shows an apparatus for carrying out the method. Fig. 2 shows a detail view of the spark plug. List of reference numbers used

11 Graphite power supply

 12 power connection

21 Feed silanes: SiH ₄ , DCS, TCS

22 Feed Auxiliary Gases: Hydrogen, HCl, CH ₄

 23 exhaust

 3 water cooling

 4 silicone gasket

 5 bellows

 6 Pressure-resistant screw connection

 71 IR radiation window

 72 IR heating lamp / pyrometer

 8 reaction vessel

 81 silicon soul (growth zone)

 9 spark plug / catalyst source

 91 metals

 92 Adjusting screw for adjusting the arc length

In Fig. 1, a device for carrying out the method is shown schematically.

The reaction vessel 8 is a

Pressure vessel in which a silicon core 81 is held for the deposition of polysilicon layers.

The holder consists of two graphite power supply 11, which are mounted so that they are suitable to supply heating current to the silicon core.

The electrical connection is made by means of power connections 12.

To these power connections and the components of

Silicone gasket 4 to protect against high process temperatures, the graphite power supply with a water cooling 3

cooled. The thermal expansion and the expansion of the pressure vessel at

Pressure change are compensated by the bellows 5.

For maintenance and cleaning purposes, the components of the process chamber with a pressure-resistant screw 6

screwed.

 Since the silicon core 81 is not electrically conductive at room temperature, it must be brought by means of an infrared heater 72 to a temperature at which the heating current from the graphite power supply lines 11 is effective.

For this heating, the reaction vessel 8 with a

Infrared radiation window 71 provided. After swinging out the HIR heating device, the IR radiation window 71 is used to observe the processes inside the reaction vessel, in particular by means of an IR pyrometer 72 to monitor the temperature. For the deposition of polysilicon are process gases

necessary.

The supply of silanes 21 takes place at the bottom of the chamber, as well as the supply of the auxiliary gases 22.

The discharge of the exhaust gas 23 takes place in the upper region of the process chamber.

The catalysts are in the flow of auxiliary gases 22

brought in .

The catalytically active metals are released by means of a slightly changed spark plug 9. The spark plug serves as a catalyst source. In Fig. 2, the spark plug 9 is shown in more detail.

The existing between the electrodes of the spark plug arc mobilized by the plasma discharge metals 91 from the electrodes.

The distance between the electrodes can be adjusted and readjusted with a set screw 92 for adjusting the arc length.

The amount and the proportion of the released metals are adjusted by the electrode spacing, the ignition voltage, the charge, and the tip angles of the electrodes. If discharges are carried out under the same conditions until the detection limit of the metals has been far exceeded, the amount of metal can be assigned to a single discharge and thus quantitative ratios below the detection limits can be set.

Claims

claims 1. A method for depositing polycrystalline silicon,  wherein a silicon-containing gas in a reactor  initiated and by reduction of the silicon-containing Gas polycrystalline silicon is deposited on a by direct current passage to a temperature of at least 550 ° C heated carrier body, characterized  characterized in that during the reduction of the silicon-containing gas there is present at least one first metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and zinc which acts as a first catalyst and also at least one of the first Metal is a different second metal selected from the group consisting of copper, silver and gold, which acts as a second catalyst. 2. The method of claim 1, wherein nickel and copper are present during the reduction of the silicon-containing gas. 3. The method according to any one of claims 1 to 2, wherein carbon is present during the reduction of the silicon-containing gas. 4. The method according to any one of claims 1 to 3, wherein as  Silicon-containing gas trichlorosilane is used in the presence of hydrogen. 5. The method according to any one of claims 1 to 4, wherein the first or second metal or both metals by an arc in the Silicon-containing gas or are released in an inert gas. 6. The method according to any one of claims 1 to 5, wherein the  at least first and the at least second metal in such Amounts during the reduction of the silicon-containing gas are present in the deposited polycrystalline silicon at least one element selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and zinc in a concentration of 18 pptw - 40 ppbw titanium, 0.2 pptw - 5 ppbw chromium, 0.5 pptw - 15 ppbw manganese, 7 pptw - 50 ppbw iron, 0.012 pptw - 25 ppbw cobalt, 0.9 pptw - 8 ppbw Nickel, 3 pptw - 12 ppbw copper or 0.6 pptw - 11 ppbw zinc;  and at least a second element other than the first element selected from the group consisting of copper, silver and gold in a concentration of 3 pptw - 12 ppbw Copper, 0.15 pptw - 21 ppbw silver and 0.001 pptw - 0.3 ppbw gold. 7. Polycrystalline silicon containing  at least one first metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper and zinc,  and at least one second metal other than the first metal selected from the group consisting of copper, silver and gold,  wherein the at least two selected metals are present in polycrystalline silicon in the following amounts: 18 pptw - 40 ppbw titanium, 0.2 pptw - 5 ppbw chromium, 7 pptw - 50 ppbw iron, 0.012 pptw - 25 ppbw cobalt, 0.9 pptw - 8 ppbw nickel, 3 pptw - 12 ppbw copper, 0.6 pptw - 11 ppbw zinc, 0.15 pptw - 21 ppbw silver, 0.001 pptw - 0.3 ppbw gold, 0.5 pptw - 15 ppbw manganese. A polycrystalline silicon according to claim 6, wherein 0.9 pptw - 8 ppbw of nickel and 3 pptw - 12 ppbw of copper are contained.