Process for producing polycrystalline silicon

Abstract

Polycrystalline silicon with low contamination by impurities, especially boron and phosphorus, is manufactured by the Siemens process or by the fluidized bed process, in which deposition of polycrystalline silicon takes place in a reactor maintained within a clean room of the 1 to 100,000 class, and air entering the facility enclosing the reactors is filtered by a multiple stage filtration system wherein coarse and fine filter elements contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic and aluminum. Following production of the polycrystalline silicon, the polycrystalline silicon may be further treated by steps such as comminution, classifying, wet-chemical treatment, and packing, all these further steps also preferably taking place within a clean room of the 1 to 100,000 class.

Claims

1. A process for producing polycrystalline silicon, comprising depositing polycrystalline silicon on at least one support body, in order to obtain at least one polycrystalline silicon rod, or depositing polycrystalline silicon on silicon particles, in order to obtain polycrystalline silicon granules, wherein deposition is effected in a reactor within a cleanroom of the 1 to 100,000 class, and conducting filtered air into the cleanroom, wherein the air is preliminarily filtered by first passing it through at least one coarse filter which separates out particles larger than or equal to 10 μm, the coarse filter being a filter of the G4 class constructed from polypropylene, and/or through a fine dust filter of the M6 class constructed from polyester, and then, following preliminary filtration, passing the preliminarily filtered air through an airborne particle filter which separates out particles smaller than 1 μm, wherein the coarse filter and fine dust filter contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic and aluminum.

2. The process of claim 1, wherein the airborne particle filter is an airborne particle filter with a PTFE membrane.

3. The process of claim 2, further comprising comminuting at least one polycrystalline silicon rod into chunks by a comminution system, wherein the comminuting of the polycrystalline silicon rods to polycrystalline silicon chunks takes place in a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing.

4. The process of claim 1, wherein fine and coarse dust filters, each contain less than 0.01 % by weight of sulfur and less than 0.1 % by weight of Sn.

5. The process of claim 1, wherein all adhesives and frames in which the filters are installed contain less than 0.1 % by weight of boron and phosphorus, less than 0.01 % by weight of arsenic and aluminum, and less than 0.1 % by weight of Sn.

6. The process of claim 1, further comprising comminuting at least one polycrystalline silicon rod into chunks by a comminution system, wherein the comminuting of the polycrystalline silicon rods to polycrystalline silicon chunks takes place in a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing.

7. The process of claim 6, further comprising subjecting the polycrystalline silicon chunks to a wet-chemical treatment in a cleaning apparatus and optionally subsequently drying in a dryer, wherein the cleaning apparatus and the dryer are located within a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing.

8. The process of claim 6, further comprising packing the polycrystalline silicon chunks into plastic bags, wherein the packing is performed within a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing, with the proviso that, in the case of a wet-chemical treatment and optionally of a subsequent drying operation of the polycrystalline silicon chunks, any transport line from cleaning system and/or drier to the packing system is likewise within a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing.

9. The process of claim 1, wherein polycrystalline silicon chunks derived from comminuting the polycrystalline silicon rod, or the polycrystalline silicon granules are classified, wherein the classification apparatus is within a cleanroom of the 1 to 100,000 class, wherein the same filters are used as in the depositing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a plot of surface boron contamination of polysilicon against time for one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) The deposition of polycrystalline silicon on a support body is typically effected by introducing reaction gas comprising a silicon-containing component and hydrogen into a reactor containing at least one heated support body on which polycrystalline silicon is deposited, which affords at least one polycrystalline silicon rod.

(3) Preferably, the silicon rod is subsequently comminuted into polycrystalline silicon chunks.

(4) The polycrystalline granules are typically produced in a fluidized bed reactor by fluidizing silicon particles by means of a gas flow in a fluidized bed, the latter being heated to high temperatures by means of a heating apparatus. Addition of a reaction gas comprising a silicon-containing component and optionally hydrogen results in a pyrolysis reaction at the hot particle surface. This deposits elemental silicon on the silicon particles, and the individual particles grow in diameter.

(5) The silicon-containing component is preferably a chlorosilane, more preferably trichlorosilane.

(6) The filter which separates out particles larger than or equal to 1 μm is preferably a fine dust filter for particles of size 1-10 μm, i.e. a filter of classes M5, M6, F7-F9 according to EN 779.

(7) Preferably, the air passes first through a coarse dust filter which separates out >10 μm particles, i.e. a filter of the G1-G4 classes according to EN 779. In this case, the preliminary filtration comprises a coarse dust filter and a fine dust filter.

(8) The airborne particle filter is preferably an airborne particle filter with a PTFE membrane of the E10-E12, H13-H14, U15-U18 classes according to DIN EN 1822. Preference is given to an airborne particle filter with a PTFE membrane of the U15 class (100 class).

(9) It is likewise preferable to use AMC (airborne molecular contamination) filters, for example composed of activated carbon filters or anion filters, in order to separate out any gaseous boron and phosphorus compounds in the air. The AMC filter is connected upstream of the airborne particle filter. If fine dust and coarse dust filters are used in the preliminary filtration, the AMC filter is preferably introduced between the coarse dust filter and airborne particle filter.

(10) In the preliminary filtration (fine and coarse dust filters), filters made from synthetic, low-dopant materials are used. These are preferably mats with a PTFE membrane, comprising polyester fibers or comprising a polypropylene fabric, which contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic, aluminum and sulfur and <0.1% by weight of Sn. All the adhesives and frames in which the filter mats are installed should likewise contain <0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic and aluminum and <0.1% by weight of tin.

(11) It has been found that the preliminarily filtered air does not contain any dopant-containing particles. Therefore, conditions which were achieved in the prior art only after an outgassing phase of several months exist in the cleanroom.

(12) The invention envisages a plurality of filter stages for deposition of particles of different size.

(13) The airborne particle filters should achieve a deposition level of more than 99% for particles of size less than 0.2 μm. It has been found that this can be accomplished by a two-stage preliminary filtration.

(14) Preliminary filter stage 1 provides a coarse dust filter of the G1 to G4 class for >10 μm particles. This consists of a synthetic material, preferably polypropylene or polyester.

(15) Preliminary filter stage 2 provides a fine dust filter of the M5 or M6 or F7 to F9 class for 1 to 10 μm particles. This likewise consists of a synthetic material, preferably polypropylene or polyester.

(16) Final filter stage 3 provides an airborne particle filter of the E10 to U17 class for <1 μm particles. The airborne particle filter also consists of a synthetic material, preferably polypropylene or polyester.

(17) Also possible in principle is a two-stage system composed of a fine dust filter of the M5 or M6 or F7 to F9 class for 1 to 10 μm particles and an airborne particle filter of the E10 to U17 class for <1 μm particles.

(18) Preference is given, however, to a two-stage preliminary filtration with coarse and fine dust filters. This is because it has been found that this extends the lifetime of the airborne particle filters by about 3 months compared to the one-stage preliminary filtration. In the three-stage system, the airborne particle filters last for about 1 to 1.5 years.

(19) Both the deposition of the polycrystalline silicon rods and the comminution of the silicon rods into chunks preferably take place in a cleanroom of the 1 to 100,000 class.

(20) For the deposition, this means that all the reactors in which polycrystalline silicon is deposited are within a cleanroom. This applies both to deposition by the Siemens process and to deposition by means of a fluidized bed process, in order to produce granules. This has the advantage that, even on deinstallation from the reactors, the silicon rods or the granules see clean low-particulate air from the start.

(21) For the comminution, this means that the comminution system is within a cleanroom of the 1 to 100,000 class.

(22) The polycrystalline silicon chunks or polycrystalline silicon granules are optionally classified (for example by chunk sizes). It is preferable that the systems for classification are within a cleanroom of the 1 to 100,000 class.

(23) The polycrystalline silicon chunks are optionally subjected to a wet-chemical treatment. It is preferable that the cleaning systems and driers are within a cleanroom of the 1 to 100,000 class, more preferably within a cleanroom of the 1 to 100 class.

(24) The polycrystalline silicon chunks are typically packed in plastic bags. It is preferable that the packing system is within a cleanroom of the 1 to 100,000 class, more preferably within a cleanroom of the 1 to 100 class. If the polycrystalline silicon chunks have been subjected to wet-chemical treatment and drying beforehand, it is preferable when the entire transport line from cleaning system/drier to the packing system is within a cleanroom of the 1 to 100,000 class, more preferably within a cleanroom of the 1 to 100 class.

EXAMPLES AND COMPARATIVE EXAMPLES

(25) From air handling systems with a different preliminary filtration setup, a cleanroom having airborne particle filters with a PTFE membrane of the U15 class (100 class) installed in the roof is supplied with air.

(26) The analysis of the surface contamination was effected by the process described in US 2013/0186325 A1.

Comparative Example 1

(27) In the air handling system, coarse dust filters of the G4 class composed of glass fibers are present at stage 1, and fine dust filters of the M6 class composed of glass fibers having boron content >10% by weight at stage 2.

(28) In the cleanroom, silicon rods (brother rods) of length 20 cm and diameter 1 cm were laid out for 6 hours.

(29) Subsequently, in accordance with the method described in US 2013/0186325 A1, the values for the surface contamination on the silicon rods reported in table 2 were determined.

(30) TABLE-US-00002 TABLE 2 B/ppta P/ppta Al/ppta As/ppta C/ppba Dopants/ppta 130 8 1 0.6 7.92 139.6

Comparative Example 2

(31) In the air handling system, coarse dust filters of the G4 class composed of glass fibers are present at stage 1, and fine dust filters of the M6 class composed of glass fibers having boron content >10% by weight at stage 2.

(32) Directly after the installation of the filters and every four weeks thereafter, brother rods were laid out in the cleanroom for 6 hours each.

(33) Table 3 shows the surface contamination with B, P, Al, As and C found and the sum total of the dopants (B, P, Al, As) directly after the installation of the filters (0 w), after 4 weeks (4 w), after 8 weeks (8 w), after 12 weeks (12 w), after 16 weeks (16 w) and after 20 weeks (20 w).

(34) TABLE-US-00003 TABLE 3 B/ P/ Al/ As/ C/ Dopants/ ppta ppta ppta ppta ppba ppta  0 w 130 8 1 0.6 7.92 139.6  4 w 50 6 0.5 0.3 7.92 56.8  8 w 30 4 0.5 0.3 7.92 39.8 12 w 15 3 0.3 0.2 7.92 18.5 16 w 7 2 0.15 0.1 7.92 9.25 20 w 2 0.5 0.1 0.1 7.92 2.7

(35) FIG. 1 shows the plot of contamination with boron against time.

Example 1

(36) In the air handling system, coarse dust filters of the G4 class made from synthetic polypropylene are present at the first stage, and fine dust filters of the M6 class made from synthetic polyester material at the second stage.

(37) According to analytical studies, the filter mats of the coarse dust filter and the fine dust filter contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic and aluminum.

(38) In the cleanroom, brother rods were laid out.

(39) After 6 hours, the values for the surface contamination reported in table 4 were measured.

(40) TABLE-US-00004 TABLE 4 B/ppta P/ppta Al/ppta As/ppta C/ppba Dopants/ppta 4 0.2 0.01 0.001 7.92 2.21

Example 2

(41) In the air handling system, filters of the M6 class made from synthetic polyester material are present.

(42) According to analytical studies, the filter mats contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic, 0.01% by weight of aluminum and 0.2% by weight of tin.

(43) In the cleanroom, brother rods were laid out.

(44) After 6 hours, the values for the surface contamination reported in table 5 were measured.

(45) TABLE-US-00005 TABLE 5 B/ppta P/ppta Al/ppta As/ppta C/ppba Dopants/ppta 3 0.1 0.005 0.002 6.85 3.107

(46) On a laid-out silicon chunk, after 2 hours, tin values of 100 pptw were measured.

(47) The chunk was analyzed as described in U.S. Pat. No. 6,309,467 B1.

(48) For this purpose, the chunk is sprayed with HF/HNO.sub.3. The etching acid is collected in a cup. Subsequently, the acid is evaporated off and the residue is introduced into water. The metal content of the aqueous solution is measured by means of ICP-AES (inductively coupled ion plasma atomic emission spectroscopy). The measured values are used to calculate the metal content of the poly surface.

Example 3

(49) In the air handling system, filters of the M6 class made from synthetic polyester material are present.

(50) According to analytical studies, the filter mats contain less than 0.1% by weight of boron and phosphorus and less than 0.01% by weight of arsenic, 0.01% by weight of aluminum and 0.02% by weight of tin.

(51) In the cleanroom, brother rods were laid out.

(52) After 6 hours, the values for the surface contamination reported in table 6 were measured.

(53) TABLE-US-00006 TABLE 6 B/ppta P/ppta Al/ppta As/ppta C/ppba Dopants/ppta 2.5 0.15 0.007 0.0015 5.92 2.6585

(54) On a laid-out silicon chunk, after 2 hours, tin values of 5 pptw were measured. The chunk was again analyzed as described in U.S. Pat. No. 6,309,467 B1.