Production of silicon particles with reduced surface metal content

Abstract

The present invention relates to a method for producing silicon particles having reduced surface metal content, comprising the steps of crushing a silicon rod or block to form silicon particles, contacting the silicon particles with a preliminary etching bath containing 6.6-12 wt.% hydrofluoric acid and 40-65 wt.% nitric acid, and contacting the silicon particles with at least one main etching bath containing 5.3-6.5 wt.% hydrofluoric acid and 40-65 wt.% nitric acid.

Description

[Technical Field]

[0001] The present invention relates to a method for producing silicon chunks with reduced surface metal content, which comprises crushing a silicon ingot or block and contacting the resulting silicon chunks with a pre-etching bath and at least one main etching bath. [Background technology]

[0002] Polycrystalline silicon (polysilicon) is usually produced by the Siemens process (chemical vapor deposition process). Polysilicon is the starting material for the production of monocrystalline silicon, for example by the Czochralski process. In addition, polysilicon is required for the production of polycrystalline silicon, for example by the block casting process. For both processes, the polysilicon obtained in the form of ingots according to the Siemens process must be crushed into chunks.

[0003] In semiconductor applications, reducing surface contamination is a priority and is crucial to avoid adversely affecting minority charge carrier lifetimes (recombination and generation lifetimes) and to avoid minute metal precipitation. For chip applications, for example, the total content of metal impurities may only be within the pptw range.

[0004] The process of crushing polysilicon is a fundamental source of contamination, especially metal contamination. Crushing is typically carried out using roll or jaw crushers. While the use of particularly wear-resistant materials (e.g., tungsten carbide, silicon nitride, silicon carbide, and polycrystalline diamond) can reduce metal contamination, a certain level of wear during the crushing procedure can never be completely prevented. Therefore, cleaning of crushed polysilicon is essential, at least for its further use.

[0005] Cleaning is primarily accomplished using a wet chemical etching process, in which the crushed polysilicon is successively transferred to different acid and / or alkaline baths. The purpose of cleaning is to remove surface impurities such as metals and dopants, as well as grease and oil. Removal of elemental tungsten, in particular, presents a challenge, as the uncleaned material after crushing can have a tungsten content of 400-1000 pptw due to the common use of WC / Co-based crushing tools.

[0006] US6,309,467B1 is 6.66*10 -11 g / cm 2 They disclose silicon chunks with Fe and Cr contents less than 1000 ppm. This is obtained by a multi-step wet chemical process in which the material passes through different corrosive HF / HNO3 and purification (HF, HCl, and H2O2) baths. The material is guided through individual tanks by an elevating motion. However, it is not possible to efficiently remove tungsten from the Si surface.

[0007] US2010 / 0132746A1 describes a cleaning apparatus with multiple etching tanks, in which the HNO3 content increases continuously from the first etching bath to the last. The HF content in all baths is only 0.1% to 0.5%. The cleaned material still has a surface metal content of less than 0.01 ng / mL, likely due to the low HF content.

[0008] US2021 / 0114884A1 discloses an etching process, particularly for removing tungsten from silicon surfaces. The process involves passing the silicon through three etching baths, with the middle bath containing an aqueous alkaline solution containing H2O2 and tetramethylammonium hydroxide (TMAH), and the remaining two containing HF / HNO3 solutions. After treatment, the surface metal content is 15 pptw or less, and the tungsten content is 0.9 pptw or less.

[0009] The cleaning process according to US 2014 / 0037959 A1 also includes an alkaline etching step in addition to the HF / HNO3-based step. In contrast to US 2021 / 0114884 A1, this step is carried out by introduction. The disadvantage of alkaline cleaning steps is their generally slow reaction time, which prevents particularly rapid sequencing of the cleaning system. Furthermore, organic bases such as TMAH can lead to an increase in the carbon content at the surface. Furthermore, handling additional corrosive components results in additional equipment costs.

[0010] US 2013 / 0189176 A1 describes a two-stage cleaning process for polysilicon, passing it through a pickling mixture (HF / HCl / H2O2) and then through an etching bath (HF / HNO3). The cleaned material has a surface metal content between 10 and 100 pptw. Tungsten content is reported to be between 0.1 and 10 pptw. However, the polysilicon is not milled using milling tools containing tungsten carbide. The described cleaning conditions are not sufficient to efficiently remove tungsten impurities resulting from the use of tungsten-containing milling tools. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 6,309,467 [Patent Document 2] US Patent Application Publication No. 2010 / 0132746 [Patent Document 3] U.S. Patent Application Publication No. 2021 / 0114884 [Patent Document 4] US Patent Application Publication No. 2014 / 0037959 [Patent Document 5] US Patent Application Publication No. 2013 / 0189176 Summary of the Invention [Problem to be solved by the invention]

[0012] In view of the drawbacks described, it was an object of the present invention to provide an efficient method for removing surface metals in which as few different chemicals as possible are used, the focus here being on the efficient removal of tungsten. [Means for solving the problem]

[0013] The object is to provide a method for producing silicon chunks, preferably polysilicon chunks, with reduced surface metal content, comprising: a) crushing a silicon ingot or silicon block into silicon chunks; b) contacting the silicon chunks with at least one pre-etching bath containing 6.6% to 12% by weight of hydrofluoric acid (HF) and 40% to 65% by weight of nitric acid (HNO3); c) contacting the silicon chunks with at least one main etching bath containing 5.3 wt. % to 6.5 wt. % HF and 40 wt. % to 65 wt. % HNO3 This is achieved by a method comprising: DETAILED DESCRIPTION OF THE INVENTION

[0014] For information on silicon milling and equipment for carrying out this method, see US2006 / 008970A1 and US2014 / 0037959A1.

[0015] The silicon chunks are preferably brought into contact with the respective etching baths by a combination of downward and upward movements, with the chunks in a process pan and the etching solutions in respective reservoir tanks. Therefore, in other words, the contacting may involve a immersion process. The composition of the etching baths can be continuously monitored by titration.

[0016] Surprisingly, it has been found that treating silicon chunks in at least one upstream etching bath with an increased HF concentration compared to the main etching bath can achieve significantly better tungsten removal. Particularly advantageously, this improvement does not come at the expense of poorer removal of other metals from the intended total surface metal content (SFM). In addition to tungsten, the metals under consideration were as follows: Fe, Cr, Ni, Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Tl, U, V, Y, and Zr. The total SFM is the sum of the listed metals expressed in parts per tonne (pptw), hereafter referred to as SFM. The total SFM can be reduced by the method of the present invention to less than 13 pptw, preferably less than 12 pptw, and more preferably less than 11 pptw at the 99% total quantile.

[0017] Furthermore, the cleaning method of the present invention can advantageously be limited to the use of only one corrosive solution (HNO3 / HF).

[0018] Preferably, the silicon chunks are contacted with only one pre-etching bath.

[0019] Preferably, the pre-etching bath contains 6.6% to 12% by weight, more preferably 7% to 10% by weight, of HF.

[0020] The contact time of the silicon chunks in the pre-etching bath is preferably 1 to 30 seconds, more preferably 2 to 25 seconds, and more specifically 4 to 20 seconds, and therefore the etching wear caused thereby is typically only about 1 to 3 μm.

[0021] The temperature of the pre-etching bath is preferably 1 to 60°C, more preferably 5 to 50°C, and more particularly 8 to 40°C. The temperature in the etching circuit is detected via a temperature sensor (measurement principle: PT100) in the media circuit and can be controlled to a set value. Further monitoring can be carried out via an additional temperature sensor (measurement principle: PT100) directly in the etching bath.

[0022] According to a particularly preferred embodiment, the temperature of the pre-etching bath is set depending on the chunk size class of the silicon chunks to be cleaned: for chunk size 2 (CS2) the temperature is preferably 1-10°C, for CS3 preferably 8-60°C, and for CS4 preferably 1-15°C.

[0023] Chunk size classes 0-4 (CS0-CS4) are basically defined using the grain size of the chunk, which is defined as the longest distance between two points on the surface of the silicon chunk. The chunk size classes encompass fractions with the following grain size ranges: CS0: 1~6mm CS1: 3~15mm CS2: 4~45mm CS3: 10~65mm CS4: 20~150mm

[0024] CS4 chunks generally have the smallest specific surface area and minimal tungsten contamination from the crushing process because they only come into contact with the crushing tool (e.g., a two-roll crusher) once. Therefore, the contamination level is fairly low and can be removed by cleaning at low etching temperatures.

[0025] CS2 chunks generally have the highest specific surface area, thus increasing tungsten contamination from the spalling process. The large surface area causes high local temperatures in the chunks during the etching process. Therefore, the etching bath temperature is kept lower than usual to ensure reaction control.

[0026] CS3 chunks generally exhibit the most unfavorable relationship between specific surface area, tungsten contamination, and local temperature at the chunk surface during etching, so the etching bath temperature is usually increased to obtain more effective removal by cleaning.

[0027] Silicon chunks can be classified by a mesh sieve, and the edge length of the square mesh corresponds to the upper limit of CS. For example, US2016 / 0214141A1 describes a classification process using a vibrating sieve.

[0028] Preferably, the CS contains at least 90% by weight of silicon chunks within each size range.

[0029] A bed of (silicon) chunks, preferably with a narrow chunk size distribution, is typically measured and analyzed using a particle size analyzer. Preferably, for CS3 and CS4, the weight is first determined gravimetrically, and then the maximum chunk length and width of each chunk are determined optically by image processing. For chunk sizes smaller than CS3, the maximum chunk length and width are determined using light / laser scattering techniques. From the calculated average length-to-weight ratio or width-to-length ratio (aspect ratio), conclusions can be drawn about the cubicity or circularity of the chunk bed. For CS4, the maximum length-to-weight aspect ratio is preferably in the range of 0.2 to 1.0 mm / g. For CS3, the typical value range is between 1.2 and 5.0 mm / g. For CS2 and sizes smaller than 45 mm, the width-to-length ratio is determined when using light / laser scattering measurements. The aspect ratio here represents the more solid volume fraction of the bed and is less than the width-to-length ratio of 0.5. This value is preferably between 0.4 and 0.8.

[0030] The form of the chunks processed by the method of the present invention is essentially irrelevant to its performance / effectiveness.

[0031] The at least one main etching bath in step c) of the method preferably has a temperature of 1 to 15° C., more preferably 2 to 12° C., more particularly 4 to 10° C. If two or more main etching baths are used, they preferably do not differ or only slightly (variation range ±1° C.) in their temperature.

[0032] The silicon chunks are particularly preferably contacted with only one main etching bath, preferably where the main etching bath contains 53% to 65% by weight of HNO3.

[0033] The residence time (immersion time) of the silicon chunks in one main etching tank is preferably 35 to 180 seconds, more preferably 60 to 150 seconds.

[0034] Furthermore, it may be preferred to contact the silicon chunks with a first and a second main etching bath in step c), with the second main etching bath having a higher content of HNO. Preferably, the first main etching bath contains 40% to 65% by weight, more preferably 45% to 60% by weight, of HNO, and the second main etching bath contains 53% to 65% by weight of HNO.

[0035] When two main etching tanks are used, the residence time of the silicon chunks in the first main etching bath is preferably 35 to 180 seconds, more preferably 60 to 150 seconds, and in the second main etching bath is preferably 35 to 180 seconds, more preferably 60 to 150 seconds.

[0036] If more than two main etching baths are used, the HNO3 content preferably increases from the first to the last main etching bath.

[0037] The HF content preferably remains constant.

[0038] The acid mixture from the main etching bath can be fed to the pre-etching bath via a cascade through the tank overflow. Such a cascade is beneficial for economical use of acid. By specific metering, the required concentration in each etching tank is established and maintained.

[0039] After step c), the method preferably comprises a further step d), in which the silicon chunks are contacted with a hydrophilization bath containing an ozone-water mixture containing 5 to 30 ppm, preferably 7 to 15 ppm, of ozone.

[0040] Furthermore, it may be preferable to contact the silicon chunks with an ultrapure water bath after each of steps a), b), c) and optionally d). Contact with an ultrapure water bath may also be performed after only one or more of steps a) to d).

[0041] Preferably, a so-called inlet cleaning of the silicon chunks is carried out in an ultrapure water bath after the comminution in step a) and before the transfer into the pre-etching bath.

[0042] Furthermore, it is preferable to bring the silicon chunks after step d) into contact with an ultrapure water bath having a temperature of preferably 50 to 95°C, more preferably 60 to 90°C.

[0043] The preferred residence time of the silicon chunks in the ultrapure water bath (regardless of which method step follows) is between 15 and 180 seconds, more preferably between 30 and 150 seconds.

[0044] Preferably, no ultrapure water bath is placed between the pre-etching bath and the at least one main etching bath.

[0045] In certain circumstances, the silicon chunks can be further contacted with a pickling bath, which preferably contains 10% to 13% by weight of HCl, 4% to 6.5% by weight of HF, and 1.4% to 2% by weight of hydrogen peroxide. The pickling bath can be arranged in particular between steps a) and b).

[0046] The method may include a drying step in which the silicon chunks are dried by convection drying and / or vacuum drying.

[0047] The drying step can be followed by step c), step d), or an ultrapure water bath after one of the steps described.

[0048] Preferably, an ultrapure water bath (hot water washing) having a temperature of 80°C to 95°C is placed before the drying step.

[0049] Preferably, the drying step involves convection drying at 60-100°C, preferably 70-90°C, followed by vacuum drying at 2-8 kPa (temperature range 18-25°C), preferably 3-5 kPa. Typical residence times for the silicon chunks in convection drying are 800-3000 seconds (e.g., 1250 seconds at 80°C). In vacuum drying, residence times are typically 50-400 seconds (e.g., 100 seconds at room temperature and 3.5 kPa).

[0050] A further aspect of the invention relates to silicon chunks, more particularly produced according to the described method, having an SFM of less than 20 pptw, preferably less than 15 pptw, more preferably less than 11 pptw at the 99% quantile, and more particularly less than 8 pptw. In determining the surface metal content, the following metals are taken into consideration: Fe, Cr, Ni, Al, Ca, Ag, W, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Tl, U, V, W, Y, Zr.

[0051] The tungsten content at the 99% quantile is preferably less than or equal to 1 pptw, more preferably less than or equal to 0.6 pptw, and even more particularly less than or equal to 0.3 pptw.

[0052] The content of Fe, Cr, Ni and W is preferably 5 pptw or less, more preferably 4 pptw or less, and more specifically 3 pptw or less.

[0053] The surface metal content can be determined according to SEMI MF1724. The surface layer of the chunk is stripped (overetched) using a mixture of HF (40 wt%) / HNO3 (65 wt%) in a 1 / 4 v / v ratio (e.g., 250 mL of HF and 750 mL of HNO3), then fumigated until dry. The residue is redissolved in a beaker in a 1 / 1 v / v ratio of HF (40 wt%) / HNO3 (65 wt%) (e.g., 25 μL each) and HO (e.g., 1450 μL per sample) and then analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 8900-ICP QQQ). The initial mass of the chunk is weighed according to its size and adjusted to its specific surface area. The initial mass of the chunk is 15 to 180 g.

[0054] The detection limit (method detection limit, MDL) is calculated using the blank scattering. For this purpose, the following calculation according to the DIN 32486 direct method (blank value method) specified in the guidelines for method validation in the BLMP (German Federal / State Measurement Program: ISSN 0722-186X) is used:

[0055]

number

[0056] The annually averaged detection limits for various elements are 0.06 pptw for W, 1.03 pptw for Fe, 0.27 pptw for Cr, and 0.51 pptw for Ni. The detection limits for other elements from the Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Tl, U, V, W, Y, and Zr series are between 0.01 pptw and 2.01 pptw on an annual average. The elements Mo, Li, V, Mn, Zr, Pb, Sr, Y, Bi, Cd, Tl, and U are here below 0.03 pptw. Co, Ba, and As are below 0.07 pptw, and Na, Mg, Al, K, Ti, Cu, Zn, Ag, Sn, and Sb are below 0.78 pptw. Ca as an environmental element has the highest detection limit of 2.01 pptw, which gives an average detection limit of 0.3 pptw across all 30 elements. [Example]

[0057] <General information> Polycrystalline polysilicon is crushed with WC / Co-containing crushing tools (WC). Tungsten contamination poses a challenge. Due to the so-called "keep-it-clean" handling of the resulting polysilicon chunks between crushing and packaging, cross-contamination due to contact with foreign surfaces (e.g., gloves, metal surfaces, etc.) is prevented (the "keep-it-clean" concept). Cleaning of the chunks in the etching system in specific 5 kg process pans allows for direct, contactless packaging of the chunks in a cleanroom in product bags (the "contactless" principle). Therefore, "keep-it-clean" describes a concept that ensures that the crushed Si pieces come into contact with as few potentially contaminated surfaces as possible until the packaging process. In this way, etching wear can be minimized. The Si chunks have a particle size of 1200–3500 cm, depending on the chunk size and morphological characteristics. 2 / kg. A suitable measurement method is dynamic image analysis depending on the chunk size, for example using a Retsch Camsizer or a Haver Boecker (Geometric Product Specifications (GPS) - Dimensional Measuring Devices, Height Gauges - Design and Metrological Characteristics (ISO 13225:2012), German version EN ISO 13225:2012).

[0058] The morphological characteristics of the chunks can be detected by a camera system and represented by the morphological indices disclosed in WO2021 / 121558A1.

[0059] Five kg of silicon chunks are used in each example. The chunks originate from the same batch from the Siemens process. The chunks are placed in a process carrier for 5 kg product quantities, preferably made of plastic (e.g., PVDF), and are successively immersed in various cleaning baths. The etching baths are made of polyvinylidene fluoride. Typical bath filling volumes are between 300 and 700 L. The cleaning process proceeds automatically in the cleaning line, with immersion occurring by descending and ascending movements. The acid composition of the baths is continuously monitored by pH titration. Analysis of the washed products is performed in each case after the last process step by ICP-MS, with the sample amount depending on the chunk size (see SEMI MF1724 for a description of the method). For CS3, the sample amount is approximately 80 g.

[0060] [Example 1] A 5 kg CS3 silicon chunk goes through the following process steps: - Pre-etching bath: 45 wt% HNO3 and 7.0 wt% HF in ultrapure water, bath temperature: 50°C, dwell time: 4 seconds (generally, the dwell time can be achieved by either single or multiple immersions) - Main etching bath: 50 wt% HNO3 and 5.8 wt% HF in ultrapure water, bath temperature: 8°C, residence time: 130 seconds; - Ultrapure water bath: Bath temperature: 18℃, residence time: 120 seconds - Convection drying: Temperature 80℃, residence time: 1250 seconds - Vacuum drying: Pressure: 3.5 kPa, residence time: 100 seconds

[0061] [Example 2] 5 kg of CS2 silicon chunks are passed through the process steps described in Example 1. In contrast to Example 1, the temperature of the pre-etching bath is 4° C. and the residence time in the main etching tank (temperature 5° C.) is only 100 seconds.

[0062] [Example 3] 5 kg of CS4 silicon chunks are passed through the process steps described in Example 1. In contrast to Example 1, the temperature of the pre-etching bath is 4° C. and the residence time in the main etching tank (temperature 8° C.) is only 100 seconds.

[0063] [Example 4] 5 kg of CS2 silicon chunks with uncompressed morphology (chunks with growth structures such as dendrites / corals, cracks and holes) are passed through the process steps described in Example 1. In contrast to Example 1, the temperature of the pre-etching bath is 4°C and the residence time in the main etching tank is only 100 seconds at a temperature of 5°C.

[0064] [Comparative Example 1] A 5 kg CS3 silicon chunk goes through the following process steps: - Main etching bath: 50 wt% HNO3 and 5.8 wt% HF in ultrapure water, bath temperature: 8°C, residence time: 130 seconds - Ultrapure water bath: rinse tank after main etching tank, bath temperature: 18°C, residence time: 120 seconds - Convection drying: Temperature: 80°C, residence time: 1250 seconds - Vacuum drying: Pressure: 3.5 kPa, residence time: 100 seconds

[0065] Comparative Example 2 A 5 kg CS2 silicon chunk goes through the following process steps: - Main etching bath: 5.8 wt% HF, 50 wt% HNO3, bath temperature: 5°C, residence time: 100 seconds - Ultrapure water bath: Bath temperature: 18℃, residence time: 120 seconds - Convection drying: Temperature 80℃, residence time: 1250 seconds - Vacuum drying: Pressure: 3.5 kPa, residence time: 100 seconds

[0066] Comparative Example 3 5 kg of CS4 silicon chunks were subjected to the process steps of Comparative Example 2, with the difference that the temperature of the main etching bath was 8°C.

[0067] Comparative Example 4 5 kg of CS2 silicon chunks in uncompressed form are subjected to the process steps described in Comparative Example 2.

[0068] Comparative Example 5 A 5 kg CS3 silicon chunk goes through the following process steps: - Pickling bath: HCl content: 12% by weight, HF content: 5% by weight, H2O2 content: 2% by weight, bath temperature: 18℃, residence time: 415 seconds - Ultrapure water bath: Bath temperature: 18℃, residence time: 80 seconds - Main etching bath: HF content 5.8 wt.%, HNO3 content 50 wt.%, bath temperature: 8°C, residence time: 130 seconds - Ultrapure water bath: Bath temperature: 18℃, residence time: 150 seconds - Convection drying: Temperature: 80°C, residence time: 1250 seconds - Vacuum drying: Pressure: 3.5 kPa, residence time: 100 seconds

[0069] Table 1 summarizes invention examples 1 to 4.

[0070] [Table 1]

[0071] Table 2 summarizes Comparative Examples 1 to 5.

[0072]

Table 2

Claims

1. 1. A method for producing silicon chunks with reduced surface metal content, comprising: a) crushing a silicon ingot or silicon block into silicon chunks; b) contacting the silicon chunks with at least one pre-etching bath containing 6.6% to 12% by weight of hydrofluoric acid and 40% to 65% by weight of nitric acid; c) contacting the silicon chunks with at least one main etching bath containing 5.3% to 6.5% by weight of hydrofluoric acid and 40% to 65% by weight of nitric acid. A method comprising:

2. 2. The method according to claim 1, characterized in that the pre-etching bath contains from 6.6% to 12% by weight, preferably from 7% to 10% by weight, of hydrofluoric acid.

3. 3. The method according to claim 1 or 2, characterized in that the contact time in the pre-etching bath is from 1 to 30 seconds, preferably from 2 to 25 seconds, more preferably from 4 to 20 seconds.

4. A method according to any of claims 1 to 3, characterized in that the pre-etching bath has a temperature of from 1 to 60°C, preferably from 5 to 50°C, more preferably from 8 to 40°C.

5. 5. The method according to claim 1, wherein the temperature of the pre-etching bath is set depending on the chunk size of the silicon chunks, and the temperature is as follows: 1-10°C for chunk size 2, 8-60°C for chunk size 3, and 5-15°C for chunk size 4.

6. 6. The method according to any of claims 1 to 5, characterized in that the at least one main etching bath has a temperature of 1 to 15°C, preferably 2 to 12°C, more preferably 4 to 10°C.

7. 7. The method according to any of the preceding claims, characterized in that the silicon chunks are contacted in step c) with a main etching bath, said bath preferably containing 53% to 65% by weight of nitric acid.

8. 8. The method according to any of claims 1 to 7, characterized in that the silicon chunks are contacted in step c) with a first and a second main etching bath, the second main etching bath having a higher nitric acid content.

9. 9. The method of claim 8, wherein the first main etching bath contains 40% to 65% by weight of nitric acid and the second main etching bath contains 53% to 65% by weight of nitric acid.

10. 10. The method according to any of the preceding claims, comprising a further step after step c) of contacting the silicon chunks with a hydrophilization bath containing an ozone-water mixture comprising 5 to 30 ppm, preferably 7 to 15 ppm, of ozone.

11. 11. The method according to claim 1, wherein after each step or after each individual step a) to c) and / or after step d), the silicon chunks are brought into contact with a bath of ultrapure water.

12. 11. The method according to claim 10, characterized in that the silicon chunks after step d) are contacted with an ultrapure water bath having a temperature of 50-95°C, preferably 60-90°C.

13. The method according to any one of claims 1 to 12, comprising a drying step in which the silicon chunks are dried by convection drying and / or vacuum drying.

14. 14. The method according to claim 13, characterized in that the drying step comprises convection drying at 60-100°C, preferably 70-90°C, followed by vacuum drying at 2-8 kPa, preferably 3-5 kPa.

15. 15. Silicon chunks having a surface metal content (wherein the metals considered are Fe, Cr, Ni, Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Tl, U, V, W, Y, Zr) of less than 13 pptw, preferably less than 12 pptw, more preferably less than 11 pptw, more particularly silicon chunks produced by a method according to at least one of claims 1 to 14.

16. 16. Silicon chunks according to claim 15, characterized in that their tungsten content is less than or equal to 1 pptw, preferably less than or equal to 0.6 pptw.

17. 17. Silicon chunks according to claim 15 or 16, characterized in that the content of Fe, Cr, Ni and W therein is 5 pptw or less, preferably 4 pptw or less, more particularly 3 pptw or less.

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