Polycrystalline silicon fragments and process for comminuting polycrystalline silicon rods

Abstract

Comminuted polysilicon with reduced contamination is prepared using multi-step comminution employing comminution with comminution tools of differing tungsten carbide content and/or grain size.

Claims

1. A multiple step process for comminuting polycrystalline silicon rods or portions thereof with a plurality of comminuting tools having comminuting surface(s) comprising silicon carbide, the process comprising at least the following steps: a) comminuting with a first comminuting tool whose comminuting surface(s) contain coarse tungsten carbide particles having a median weight grain size of 1.3 m, in an amount of 95 wt. % dispersed in a metal matrix; and b) subsequent to step a) further comminuting with a second comminuting tool whose comminuting surface(s) contain fine tungsten carbide particles having a median weight grain size of 0.5 m, in an amount of 80 wt. % dispersed in a metal matrix.

2. The process of claim 1, wherein the process further comprises at least one further step of comminuting with a comminuting tool whose comminuting surface(s) contain coarse tungsten carbide particles having a median weight grain size of 1.3 m, in an amount of 95 wt. % dispersed in a metal matrix, prior to step b).

3. The process of claim 2, wherein the process comprises at least one further step of comminuting with a further comminuting tool whose comminuting surface(s) contain fine tungsten carbide particles having a median weight grain size of 0.5 m, in an amount of 80 wt. % dispersed in a metal matrix, after a previous step of comminuting with a second comminuting tool whose comminuting surface(s) contain fine tungsten carbide particles having a median weight grain size of 0.5 m, in an amount of 80 wt. % dispersed in a metal matrix.

4. The process of claim 1, wherein the process further comprises at least one further step of comminuting with a further comminuting tool whose comminuting surface(s) contain fine tungsten carbide particles having a median weight grain size of 0.5 m, in an amount of 80 wt. % dispersed in a metal matrix, after a previous step of comminuting with a second comminuting tool whose comminuting surface(s) contain fine tungsten carbide particles having a median weight grain size of 0.5 m, in an amount of 80 wt. % dispersed in a metal matrix.

5. The process of claim 1, wherein the median weight grain size of the second comminuting tool is 0.2 m.

6. The process of claim 1, wherein the first comminuting tool is a manual hammer, a hammer mill, or a machine impact tool.

7. The process of claim 6, wherein the second comminuting tool is a jaw crusher, a roll crusher having two rolls, or a ball mill.

8. The process of claim 1, wherein the second comminuting tool is a jaw crusher, a roll crusher having two rolls, or a ball mill.

9. The process of claim 1, wherein the metal matrix of the first comminuting tool or the secondary comminuting tool comprises cobalt.

10. The process of claim 1, wherein the grain size of the tungsten carbide particles in the second comminuting tool surface is less than or equal to 0.2 m and the tungsten carbide content is greater than 90%.

11. The process of claim 1, wherein the grain size of the tungsten carbide particles in the second comminuting tool surface is less than or equal to 0.2 m and the tungsten carbide content is greater than 95%.

12. The process of claim 1, wherein the metal matrix of all comminuting tool surface(s) comprises cobalt.

13. The process of claim 1, wherein the tungsten carbide content of the first comminuting tool surface is less than 90%.

14. The process of claim 1, wherein the tungsten carbide content of the first comminuting tool surface is less than 65%.

15. The process of claim 1, further comprising at a final comminution step which takes place after steps a) and b) being effected with a comminuting tool having surface(s) with a higher tungsten carbide content, with a lower grain size of the tungsten carbide particles than in surface(s) of any comminuting tool used in any preceding comminution steps, or with both a higher tungsten carbide content and a lower grain size as compared with the tungsten carbide content and grain size of any comminuting tool used in any prior comminuting step.

16. The process of claim 1, wherein following comminuting in step a), polycrystalline chunks obtained from step a) are heated to a temperature >500 C. and subsequently quenched in a cold medium prior to further comminuting in step b).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block flowsheet illustrating one embodiment of the claimed invention which is a three-step comminution of polysilicon.

(2) FIG. 2 illustrates one embodiment of the present invention where a jaw crusher having faces of coarse-grained silicon carbide is followed by a roll crusher having roll faces of fine-grained silicon carbide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The remainder to 100% in the material of the tool surface of the process of the invention is preferably cobalt binders, which may also include up to 2%, but preferably less than 1%, of other metals.

(4) Additional carbides are preferably present to an extent of less than 1%, of which Cr.sub.3C.sub.2 and VC are <0.4%.

(5) The sintering outcome of the invention is also affected by addition of carbon. It is further known that a balanced carbon level is important for achieving the optimal properties of the hard metal. Inferences in this respect can be drawn, for example, via the magnetic saturation, which may be in the range of 7-14 Tm{circumflex over ()}3/kg, or 75-110%. The carbon content based on WC is about 6%, and has a tendency to be somewhat higher.

(6) For comminution of polycrystalline silicon rods, manual hammers, hammer mills and machine impact tools are suitable, in which case preference is given to using the coarser grains having grain size greater than or equal to 0.8 m.

(7) Likewise envisaged is the use of jaw and roll crushers and ball mills, in which cases preference is given to using the finer grains smaller than or equal to 0.5 m.

(8) The finer grains preferably have a grain size of less than or equal to 0.2 m, in combination with a tungsten carbide content of greater than 80%, preferably greater than 90%, more preferably greater than 95%.

(9) The coarser grains preferably have a grain size of greater than or equal to 1.3 m, in combination with a tungsten carbide content of less than 95%, preferably less than 90%, more preferably 65-80%.

(10) Preferably, the process comprises at least two comminution steps, the final comminution step being effected with a comminuting tool having a higher tungsten carbide content or a lower grain size of the tungsten carbide particles than in the comminuting tool used in one of the preceding comminution steps.

(11) Preferably, the process comprises at least two comminution steps: at least one comminution step with a comminuting tool having a grain size of the tungsten carbide particles of greater than or equal to 0.8 m, preferably greater than or equal to 1.3 m, or at least one comminution step with a comminuting tool having a grain size of the tungsten carbide particles of less than or equal to 0.5 m, preferably less than or equal to 0.2 m.

(12) Preferably, the process comprises at least two comminution steps, wherein the at least two comminuting tools used therein have different WC grain sizes, selected from the group consisting of WC grain size less than 0.5 m, WC grain size 0.5-0.8 m, WC grain size greater than 0.8 m.

(13) It is especially preferable when the process comprises at least one comminution step with a comminuting tool having a grain size of the tungsten carbide particles of greater than or equal to 0.8 m, and a comminution step with a comminuting tool having a grain size of the tungsten carbide particles of less than or equal to 0.5 m.

(14) Preferably, the process comprises at least one comminution step by means of WC tools having a low WC content (<90%, preferably <85%) and/or larger grains >0.8 m and at least one further comminution step by means of WC tools having an increasingly higher WC content (>90, preferably >95%) and/or small grains <0.5 m.

(15) Preferably, the last comminution step, more preferably the last two comminution steps, is/are effected with WC tools having WC content>85%, preferably >90% and/or a grain size <0.5 m, more preferably <0.2 m.

(16) Preferably, the comminution of the rods, preferably the second comminution step, is followed by a thermal treatment of the chunks at a temperature of >500 C. with subsequent quenching in a colder medium, followed by further comminution steps.

(17) It has been found that the process according to the invention for breaking polycrystalline silicon rods results in polycrystalline silicon chunks having WC particles on the surface, the WC particles having a median size of less than 0.5 m or a median size of greater than 0.8 m.

(18) Preferably, the median size of the WC particles is less than 0.2 m.

(19) Preferably, the median size of the WC particles is greater than 1.3 m.

(20) It is likewise possible to obtain polycrystalline silicon chunks having WC particles on the surface thereof, the particle sizes of the WC particles on the surface being in bimodal or multimodal distribution, with at least one maximum in the distribution at less than 0.6 m and/or at least one maximum in the distribution at greater than 0.6 m.

(21) Preferably, at least one maximum in the distribution is at less than 0.5 m. More preferably, at least one maximum in the distribution is at less than 0.2 m. Preferably, at least one maximum in the distribution is at greater than 0.8 m. More preferably, at least one maximum in the distribution is at greater than 1.3 m.

(22) It has been found that, surprisingly, the tungsten carbide content, or the hardness, has a much smaller influence on the abrasion than the grain size of the WC particles of the comminuting tools, which has not been considered to date. For the same hardness, a tool having smaller grains and a smaller tungsten carbide content showed much lower abrasion than a tool having larger grains and a higher WC content.

(23) It was also surprising that tungsten contamination on the polysilicon, given several comminution steps, is determined predominantly by the last comminution step.

(24) This enables, in a process comprising several comminution steps, the use of less wear-resistant but tough hard metal tools in the initial comminution steps, for example in the initial breaking. This is advantageous. In the last comminution step, in contrast, it should be ensured that a tool having a particularly suitable WC type, namely having a relatively fine WC grain size and/or relatively high tungsten carbide content, is used. The inventive polycrystalline silicon chunks, which feature WC particles on the surface with defined particle sizes or particle size distributions, likewise have surprising advantages. These become visible when the polysilicon is melted and is processed further on the part of customers, for example by crystal pulling to give single crystals for solar or semiconductor applications.

(25) The influence of the WC grain size in comminuting tools on the melting characteristics or the pulling performance in the customer's hands was unforeseeable.

(26) In principle, WC particles on the surface of polysilicon (like other extraneous substances/metals too) can lead to dislocations in crystal pulling. For example, it is conceivable that very large WC particles are not melted because of the very high melting point of about 2800 C. and, as a result, lead to such dislocations. Smaller particles too, which are easier to melt, given the same total contamination, can lead to dislocations in single-crystal pulling because of the much higher number thereofup to a factor of more than 1000.

(27) However, the inventors were able to show that, with the use of WC having relatively large grains or else having relatively small grains, better results are achieved than with the grains according to the prior art, namely fine grains of 0.6 m; cf. US2003159647 A1.

(28) A grain size decreasing with each breaking step is especially preferable.

(29) The preference is especially given to initial breaking using a WC type with large grains (>0.8 m), and a WC type with small grains (<0.5 m) for the last breaking step(s).

(30) More particularly, however, it is possible to achieve better results through the combination of a plurality of comminution steps with tools of various grain sizes, namely lower contamination, higher service lives and better pulling performance. The dimensions of the processing tools can be increased, and hence processes can be run with higher throughput and lower costs.

(31) In addition, no complex reprocessing of the chunks is required, for example through a wet-chemical cleaning operation. Overall, the production process becomes much more economically viable.

(32) FIG. 1 shows one embodiment of the process described herein, in which a three stage comminution of polysilicon is effected. FIG. 1 also applies, for example, to a two-stage comminution where, for instance, either the first stage or the last stage shown in FIG. 1 is deleted.

(33) FIG. 2 illustrates one embodiment of the invention where coarse polysilicon chunks 1 are crushed between the jaws 3 of jaw crusher 2. The jaws 3 are faced with a facing material containing coarse grained silicon carbide 4. The smaller, comminuted chunks 5 fall as indicated by arrow 6 into roll crusher 7 in which the rolls 8 are faced with a facing material containing small grained silicon carbide 9 to comminute the polycrystalline silicon into smaller chunks 10.

EXAMPLES

(34) Comminution into chunks results in chunk sizes (CS), which can be assigned to the following size classes, each of which is defined as the longest distance between two points on the surface of a silicon chunk (=max. length):

(35) Chunk size 0 [mm] 1 to 5;

(36) Chunk size 1 [mm] 4 to 15;

(37) Chunk size 2 [mm] 10 to 40;

(38) Chunk size 3 [mm] 20 to 60;

(39) Chunk size 4 [mm] 45 to 120;

(40) Chunk size 5 [mm] 90 to 200

(41) Chunk size 6 [mm] 130 to 400

Example 1

(42) Manual breaking of polycrystalline silicon rods with a manual hammer (WC in Co matrix)

(43) a. (prior art) 88% WC, 12% Co and fine grains (0.5-0.8 m): small, visible WC splinters, i.e. high contamination

(44) b. 88% WC, 12% Co and coarse grains (2.5-6.0 m): no visible WC splinters, i.e. low contamination

(45) c. 80% WC, 20% Co and fine grains (0.5-0.8 m): no visible WC splinters

Example 2

(46) Initial breaking as in example 1 b. and further breaking with a roll crusher to target size CS4, classification and analysis of the surface contamination of sample pieces of a component fraction according to the prior art with ICPMS (ICP=inductively coupled plasma) to DIN 51086-2; hardness figures according to Vickers, test force 10 kp).

(47) a. (prior art) hardness HV10 1650: 90% WC+10% Co, very fine grains (0.5 m to 0.8 m): CS1 tungsten 2000 pptw

(48) b. hardness HV10 1630: 94% WC+6% Co, fine grains (0.8 m to 1.3 m): CS1 tungsten 4000 pptw

(49) c. hardness HV10 1590: 85% WC+15% Co; ultrafine grains (0.2-0.5 m): CS1 tungsten 1000 pptw

Example 3

(50) Manual initial breaking according to example 1 b., then further breaking to target size CS2 with large jaw crusher (88% WC & 12% Co and very fine grains (0.5-0.8 m)), then two breaking steps with a smaller jaw crusher (88% WC & 12% Co very fine grains (0.5 m to 0.8 m)) and a last breaking step

(51) a. with jaw crusher (88% WC & 12% Co very fine grains (0.5 m to 0.8 m): CS2 tungsten 500 pptw (prior art), or

(52) b. with jaw crusher (93.5% WC & 6.5% Co ultrafine grains (0.2 m to 0.5 m): CS2 tungsten 200 pptw

(53) (a. and b. each at about the same comminution ratio)

Example 4

(54) As example 3 b., but with thermal 800/1 h pretreatment and subsequent quenching in water at 20 and vacuum drying after the second breaking step.

(55) Result: CS2 tungsten 50 pptw

Example 5

(56) Poly-Si rods are broken in a controlled manner with several breaking steps and different WC types to CS2, such that the end product of the comparison groups each has about the same W contamination of about 500 pptw, but each group differs by the grain size on the product.

(57) Subsequently, the material was pulled to a single crystal by the CZ process and the dislocation-free length was measured.

(58) The mean dislocation-free length is determined from the ratio of a possible cylindrical crystal rod length (calculated from starting weight minus cone and residual melt losses) and actual length of several crystals.

(59) a. (prior art) manual initial breaking (88% WC/12% Co/very fine grains 0.5-0.8 m) to CS4, followed by two breaking steps with a jaw crusher (88% WC/12% Co/grains 0.5-0.8 m) to CS2:

(60) dislocation-free length 70%

(61) b. manual initial breaking (88% WC/12% Co/coarse grains 2.5-6.0 m) to CS4, three breaking steps with a jaw crusher (88% WC/12% Co/coarse grains 2.5-6.0 m) to CS2:

(62) dislocation-free length 95%

(63) c. manual initial breaking (88% WC/12% Co/ultrafine grains 0.2-0.5 m) to CS4, one breaking step with a jaw crusher (88% WC/12% Co/ultrafine grains 0.2-0.5 m) to CS2:

(64) dislocation-free length 93%