Production of silicon particles having a reduced surface metal content

Abstract

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

Description

[0001] Production of silicon fragments with reduced surface metal content

[0002] The invention relates to a process for producing silicon fragments with a reduced surface metal content, in which a silicon rod or block is crushed and the resulting silicon fragments are brought into contact with a pre-etching bath and at least one main etching bath.

[0003] Polycrystalline silicon (polysilicon) is typically produced using the Siemens process (a chemical vapor deposition process). Polysilicon is the starting material in the production of single-crystal silicon, which is produced, for example, using the Czochralski process. Polysilicon is also required for the production of multicrystalline silicon, for example, using ingot casting. For both processes, the rod-shaped polysilicon obtained using the Siemens process must be crushed into fragments.

[0004] In semiconductor applications, reducing surface contamination is a priority and crucial to both avoid negatively impacting minority carrier lifetimes (recombination and generation lifetimes) and preventing micro-metal precipitates. For chip applications, for example, the total content of metallic impurities must be in the pptw range.

[0005] The comminution process of polysilicon fundamentally represents a source of contamination, especially for metals. Comminution is typically performed using roller or jaw crushers. While metal contamination can be reduced by using particularly abrasion-resistant materials (e.g., tungsten carbide, silicon nitride, silicon carbide, and polycrystalline diamonds), a certain amount of abrasion during the comminution process can never be completely prevented. Therefore, cleaning the crushed polysilicon is essential, at least for the aforementioned further use.

[0006] Wet-chemical etching processes are primarily used for cleaning, in which the crushed polysilicon is successively placed in various acid and / or alkali baths. The goal of cleaning is to remove surface contaminants, such as metals and dopants, as well as greases and oils. The removal of tungsten, in particular, presents a challenge because, due to the common use of WC / Co-based crushing tools, uncleaned material can have a tungsten content of 400-1000 pptw after crushing.

[0007] US 6,309,467 Bl discloses silicon fragments with a content of Fe and Cr of less than 6.66*10 -11 g / cm 2This is obtained through a multi-stage, wet-chemical process, in which the material passes through various caustic HF / HNO3 and cleaning (HF, HCl, and H2O2) baths. The material is transported through the individual tanks by lifting and lowering movements. However, tungsten cannot be efficiently removed from the Si surface.

[0008] US 2010 / 0132746 Al describes a cleaning apparatus with a plurality of etching baths. The HNO3 content increases continuously from the first to the last etching bath. The HF content is only 0.1–0.5% in all baths. The cleaned material still has a surface metal content of less than 0.01 ng / mL, which is likely due to the low HF content. US 2021 / 0114884 Al discloses an etching process, particularly for removing tungsten from the silicon surface. Three etching baths are run through, the middle one containing an aqueous alkali solution with H2O2 and tetramethylammonium hydroxide (TMAH), and the other two containing an HF / HNO3 solution. The surface metal content after treatment is 15 pptw or less, with the tungsten content being 0.9 pptw or less.

[0009] The cleaning process according to US 2014 / 0037959 Al also includes an alkaline etching step in addition to the HF / HNOß-based etching step. Unlike US 2021 / 0114884 Al, this takes place initially. The disadvantage of alkaline cleaning steps is the generally slow reaction times, which prevents a particularly fast cycle time of a cleaning system.

[0010] Furthermore, organic bases such as TMAH can lead to an increase in the carbon content on the surface. Furthermore, handling another corrosive component requires additional equipment.

[0011] US 2013 / 0189176 A1 describes a two-stage purification process for polysilicon, which involves a pickling bath (mixture of HF / HCl / H2O2) and then an etching bath (HF / HNO3). The purified material has a surface metal content of between 10 and 100 pptw. The tungsten content is stated to be 0.1 to 10 pptw. However, the comminution of the polysilicon is not carried out using crushing tools containing tungsten carbide. The described conditions would not be sufficient for the efficient removal of tungsten impurities resulting from the use of tungsten-containing crushing tools. Given the disadvantages mentioned, the object of the present invention was to provide an efficient process for removing surface metals using as few different chemicals as possible. The focus here is on the efficient removal of tungsten.

[0012] This object is achieved by a method for producing silicon fragments, preferably polysilicon fragments, with a reduced surface metal content, comprising the steps of a) comminuting a silicon rod or block into silicon fragments, b) bringing the silicon fragments into contact with at least one pre-etching bath containing 6.6 to 12 wt.% hydrofluoric acid (HF) and 40 to 65 wt.% nitric acid (HNO3), c) bringing the silicon fragments into contact with at least one main etching bath containing 5.3 to 6.5 wt.% HF and 40 to 65 wt.% HNO3.

[0013] Regarding the comminution of the silicon and the equipment required to carry out the process,

[0014] US 2006 / 008970 Al and US 2014 / 0037959 Al.

[0015] The silicon fragments are preferably brought into contact with the respective etching bath using a combination of lowering and lifting movements, with the fragments being located in a process tray and the etching solutions each being located in a storage basin. In other words, the contacting can be an immersion process. The composition of the etching baths can be continuously monitored via titration. Surprisingly, it has been shown that significantly better tungsten removal can be achieved by treating the silicon fragments in at least one upstream etching bath which has a higher HF concentration than the main etching bath. This improvement is particularly advantageous not at the expense of poorer removal of other metals from the total surface metal content (OFM) under consideration.In addition to tungsten, the following metals were considered: Fe, Gr, Ni, Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Ti, U, V, Y, Zr. The total OFM results from the sum of the mentioned metals in pptw and is referred to below as OFM. The total OFM could be reduced by the method according to the invention in the 99% quantile to less than 13 pptw, preferably less than 12 pptw, particularly preferably less than 11 pptw.

[0016] Furthermore, it is advantageous that the cleaning process according to the invention could be limited to the handling of only one corrosive solution (HNO3 / HF).

[0017] Preferably, the silicon fragments are brought into contact with only one pre-etching bath.

[0018] The pre-etching bath preferably contains 6.6 to 12 wt.%, preferably 7 to 10 wt.%, HF.

[0019] The contact time of the silicon fragments in the pre-etching bath is preferably 1 to 30 s, particularly preferably 2 to 25 s, especially 4 to 20 s. The resulting etching removal is typically only about 1 to 3 μm. The temperature of the pre-etching bath is preferably 1 to 60°C, particularly preferably 5 to 50°C, especially 8 to 40°C. The temperature in the etching circuit can be measured and regulated to a setpoint using a temperature sensor (measuring principle: PT100) in the media circuit. Further monitoring can be performed using another temperature sensor (measuring principle: PT100) directly in the etching bath.

[0020] According to a particularly preferred embodiment, the temperature of the pre-etching bath is adjusted depending on the size class of the silicon fragments to be cleaned. The temperature for a size class of 2 (BG2) is preferably 1 to 10°C, for a size class of 3 (BG3) it is preferably 8 to 60°C, and for a size class of 4 it is preferably 1 to 15°C.

[0021] The fraction size classes 0 to 4 (BG0 to BG4) are generally defined based on the grain size of the fragments, where the grain size is defined as the longest distance between two points on the surface of a silicon fragment. The fraction size classes summarize fractions with grain size ranges as follows.

[0022] BGO: 1 to 6 mm

[0023] BG1 : 3 to 15 mm

[0024] BG2 : 4 to 45 mm

[0025] BG3 : 10 to 65 mm

[0026] BG4 : 20 to 150 mm

[0027] Fragments of BG4 generally have the smallest specific

[0028] Surface and the lowest tungsten contamination from the crushing process. This is due to the only single contact with the crushing tool (e.g., two-roll crusher). Therefore, the contamination level is relatively low, allowing cleaning at reduced etching temperatures.

[0029] Fragments of BG2 generally exhibit the highest specific surface area and therefore increased tungsten contamination from the crushing process. This large surface area leads to high local temperatures on the fragments during the etching process. For reliable reaction control, the etching bath temperatures are therefore usually kept lower.

[0030] BG3 fragments generally exhibit the most unfavorable ratio of specific surface area, tungsten contamination, and local temperatures at the fragment surface during etching. To achieve more effective cleaning, the etching bath temperature is usually increased.

[0031] The silicon fragments can be classified using mesh sieves, with the edge length of the square mesh corresponding to the upper limit of a BG. For example, US 2016 / 0214141 A1 describes a classification process using vibrating sieves.

[0032] Preferably, a BG comprises at least 90 wt . % of silicon fragments within the respective size range.

[0033] Bulk materials of (silicon) fragments with preferably narrow size distributions are usually measured and analyzed using a grain size measuring device. For sizes BG3 and BG4, the weight is first determined gravimetrically and then the maximum fragment length and width of individual fragments are determined optically using image processing. For fragment sizes below BG3, the maximum fragment length and width are determined using the principle of light / laser scattering. Conclusions about the cubicity or roundness of the bulk materials can be drawn from the calculated average length to weight ratio or width to length ratio (aspect ratio). For size BG4, the aspect ratio of weight to maximum length is preferably in a value range between 0.2 and 1.0 mm / g, while for size BG3 the typical value range is between 1.2 and 5.0 mm / g.For BG2 and smaller than 45 mm when using light / laser scattering, the ratio is calculated from the width to the length. The aspect ratio represents the more cubic volume fraction of the fill that falls below the width-to-length ratio of 0.5. This value is preferably between 0.4 and 0.8.

[0034] The morphology of the fragments treated by the method according to the invention is fundamentally irrelevant for its implementation / effectiveness.

[0035] The at least one main etching bath in process step c) preferably has a temperature of 1 to 15°C, particularly preferably 2 to 12°C, in particular 4 to 10°C. If two or more main etching baths are used, it is preferred that they do not differ in temperature or differ only insignificantly (fluctuation range ± 1°C).

[0036] Particularly preferably, the silicon fragments are brought into contact with only one main etching bath. The main etching bath preferably contains 53 to 65 wt.% HNO3.

[0037] The residence time (immersion time) of the silicon fragments in only one main etching tank is preferably 35 to 180 s, particularly preferably 60 to 150 s. Furthermore, it may be preferable to bring the silicon fragments into contact with a first and a second main etching bath in step c), wherein the second main etching bath has a higher HNO3 content. Preferably, the first main etching bath contains 40 to 65 wt.%, particularly preferably 45 to 60 wt.% HNO3, and the second main etching bath contains 53 to 65 wt.% HNO3.

[0038] The residence time of the silicon fragments when using two main etching baths is preferably 35 to 180 s, particularly preferably 60 to 150 s, in the first and preferably 35 to 180 s, particularly preferably 60 to 150 s, in the second main etching bath.

[0039] If more than two main etching baths are used, the HNO3 content increases preferentially from the first to the last main etching bath.

[0040] The HF content preferably remains constant.

[0041] The acid mixture from the main etching bath can be fed into the pre-etching bath via a cascade through a tank overflow. Such a cascade allows for the economical use of the acid. The required concentrations of the respective etching tanks are adjusted and maintained by specific dosing.

[0042] Preferably, after step c), the process comprises a further step d), in which the silicon fragments are brought into contact with a hydrophilization bath containing an ozone-water mixture with 5 to 30 ppm, preferably 7 to 15 ppm, of ozone. Furthermore, it may be preferred that after each of steps a), b), c), and optionally d), the silicon fragments are brought into contact with an ultrapure water bath. Contacting with the ultrapure water bath can also take place after only one or more of steps a) to d).

[0043] Preferably, after comminution in step a) and before transfer to the pre-etching bath, a so-called initial rinsing of the silicon fragments takes place in an ultrapure water bath.

[0044] Furthermore, it is preferred that the silicon fragments are brought into contact with an ultrapure water bath after step d), which preferably has a temperature of 50 to 95°C, particularly preferably of 60 to 90°C.

[0045] The preferred residence time of the silicon fragments in the ultrapure water bath (regardless of which process step it is arranged after) is 15 to 180 s, particularly preferably 30 to 150 s.

[0046] Preferably, no ultrapure water bath is arranged between the pre-etching bath and the at least one main etching bath.

[0047] Under certain circumstances, the silicon fragments can also be brought into contact with a pickling bath. This preferably contains 10 to 13 wt.% HCl, 4 to 6.5 wt.% HF, and 1.4 to 2 wt.% hydrogen peroxide. The pickling bath can be arranged, in particular, between steps a) and b).

[0048] The process may include a drying step in which the silicon fragments are dried by convection drying and / or vacuum drying. The drying step may follow step c), step d), or an ultrapure water bath after one of the aforementioned steps.

[0049] Preferably, an ultrapure water bath with a temperature of 80°C to 95°C is arranged before the drying step (hot water rinsing).

[0050] The drying step is preferably a convection drying at 60 to 100°C, preferably at 70 to 90°C, immediately followed by a vacuum drying at 2 to 8 kPa (temperature range 18 to 25°C), preferably at 3 to 5 kPa. Typical residence times of the silicon fragments during convection drying are 800 to 3000 s (at 80°C e.g.

[0051] 1250 s). For vacuum drying, it is typically 50 to 400 s (at room temperature and 3.5 kPa, for example, 100 s).

[0052] A further aspect of the invention relates to silicon fragments, in particular produced by the described method, which have an OEM in the 99% quantile of less than 20 pptw, preferably less than 15 pptw, particularly preferably less than 11 pptw, in particular less than 8 pptw. When determining the surface metal content, the following metals are taken into account: Fe, Or, Ni, Al, Ca, Ag, W, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Ti, U, V, W, Y, Zr.

[0053] The tungsten content in the 99% quantile is preferably

[0054] < 1 pptw, particularly preferably < 0.6 pptw, especially

[0055] < 0.3 pptw .

[0056] The Fe, Cr, Ni, and W content is preferably < 5 pptw, particularly preferably < 4 pptw, and especially < 3 pptw. The surface metal content can be determined according to SEMI MF1724. A surface layer of the fragments is removed (overetching) using a mixture of HF (40 wt%) / HNO3 (65 wt%) in a ratio of 1 / 4 v / v (e.g., 250 mL HF and 750 mL HNO3) and then fumed until dry. The residue is redissolved in the beaker with HF (40 wt.%) / HNO3 (65 wt.%) in a ratio of 1 / 1 v / v (e.g., 25 pL each) and H2O (e.g., 1450 pL per sample) and then analyzed using inductively coupled plasma mass spectrometry (ICP-MS; Agilent 8900-ICP QQQ). The initial weight of the fragments depends on the fragment size and is adjusted to the specific surface area. It ranges from 15 to 180 g.

[0057] The detection limits (MDL) are calculated using the blank value scatter. The following calculation according to DIN 32486 direct method (blank value method), specified in the guideline for method validation in the BLMP (Federal / State Measurement Program; ISSN 0722-186X), is used: ng Resolving Volume [mL] (e.g., 3 mL) * 1000 [^] MDL fpptwl = 3 * Standard Deviation of the Blank Values ​​— - * - - — - - ; - — - —

[0058] LmLJ Sample weight [g]

[0059] The detection limits for the various elements are on average over the year 0.06 pptw for W; 1.03 pptw for Fe; 0.27 pptw for Cr; 0.51 pptw for Ni. The detection limits for the other elements from the series Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Ti, U, V, W, Y, Zr are on average over the year between 0.01 pptw and 2.01 pptw. The elements Mo, Li, V, Mn, Zr, Pb, Sr, Y, Bi, Cd, Ti and U have a maximum of 0.03 pptw. Co, Ba, and As at max. 0.07 pptw, and Na, Mg, Al, K, Ti, Cu, Zn, Ag, Sn, and Sb at max. 0.78 pptw. Ca, as an environmental element, has the highest detection limit of 2.01 pptw. Across all 30 elements, this results in an average detection limit of 0.3 pptw.

[0060] Examples

[0061] General Information:

[0062] Polycrystalline polysilicon is broken using a WC / Co-containing breaking tool (WC). Tungsten contamination poses a challenge. The so-called keep-it-clean handling of the resulting polysilicon fragments between breaking and packaging prevents cross-contamination from contact with foreign surfaces (e.g., gloves, metallic surfaces, etc.) (keep-it-clean concept). Cleaning the fragments in the etching system in special 5 kg process trays enables direct, contactless packaging of the fragments in the cleanroom in product bags (no-touch principle). Keep-it-clean thus describes a concept that ensures that the broken Si fragments come into contact with as few potentially contaminated surfaces as possible until the packaging step. In this way, etching removal can be reduced to a minimum.The Si fragments have a specific surface area of ​​1200 to 3500 cm, depending on the fragment size and morphological properties. 2 / kg. A suitable measurement method is dynamic image analysis, depending on the fragment size, e.g., with Camsizer, Retsch or Haver Böcker (Geometric Product Specification (GPS) - Length measuring instruments; Vertical length measuring instruments - Design and metrological characteristics (ISO 13225:2012); German version EN ISO 13225:2012). Morphological properties of the fragments can be recorded using camera systems and represented by a morphology index, as disclosed in WO 2021 / 121558 A1.

[0063] 5 kg of silicon fragments are used in each of the examples. The fragments originate from the same batch of a Siemens process. The fragments are placed in process carriers for 5 kg product quantities, preferably made of plastic (e.g. PVDF), and are immersed one after the other in the various cleaning baths. The etching baths are made of polyvinylidene fluoride. Typical fill volumes of the baths are between 300 and 700 l. The cleaning process is automated in a cleaning line, with immersion being accomplished by lowering and lifting movements. The acid composition of the baths is continuously monitored by pH titration. The analysis of the cleaned product was carried out after each last process step using a fragment-size-dependent sample quantity via ICP-MS (for a method description, see SEMI MF1724). For BG3, the sample quantity is approximately 80 g.

[0064] Example 1

[0065] 5 kg of silicon fragments of the BG3 go through the following process steps:

[0066] - Pre-etching bath: 45 wt% HNO3 and 7.0 wt% HF in ultrapure water;

[0067] Bath temperature: 50°C; residence time: 4 s (the residence time can generally be achieved by single or multiple immersion).

[0068] - Main etching bath: 50 wt.% HNO3 and 5.8 wt.% HF in ultrapure water; bath temperature: 8°C; residence time: 130 s

[0069] - Ultrapure water bath: Bath temperature: 18°C; residence time: 120 s;

[0070] - Convection drying: Temp. 80°C; Residence time: 1250 s - Vacuum drying: Pressure: 3.5 kPa; Residence time: 100 s

[0071] Example 2

[0072] 5 kg of silicon fragments from BG2 undergo 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 s.

[0073] Example 3

[0074] 5 kg of silicon fragments of the BG4 undergo 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 s.

[0075] Example 4

[0076] 5 kg of silicon fragments of BG2 with non-compact morphology (fragments exhibit growth structures such as dendrites / corals, cracks, and holes) undergo 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 s at a temperature of 5 °C.

[0077] Comparison example 1

[0078] 5 kg of silicon fragments of the BG3 go through the following process steps.

[0079] - Main etching bath: 50 wt.% HNO3 and 5.8 wt.% HF in ultrapure water; bath temperature: 8°C; residence time: 130 s

[0080] - Ultrapure water bath: Rinse tank after the main etching tank. Bath temperature: 18°C; residence time: 120 s. - Convection drying: Temperature: 80°C; residence time: 1250 s.

[0081] - Vacuum drying: Pressure: 3.5 kPa; Residence time: 100 s

[0082] Comparison example 2

[0083] 5 kg of silicon fragments from BG2 undergo the following process steps.

[0084] - Main etching bath: 5.8 wt% HF; 50 wt% HNO3; bath temperature: 5°C;

[0085] Dwell time: 100 s

[0086] - Ultrapure water bath: Bath temperature: 18°C; residence time: 120 s;

[0087] - Convection drying: Temp. 80°C, residence time: 1250 s

[0088] - Vacuum drying: Pressure: 3.5 kPa; Residence time: 100 s

[0089] Comparison example 3

[0090] 5 kg of silicon fragments of BG4 undergo the process steps from Comparative Example 2 with the difference that the temperature of the main etching bath is 8 °C.

[0091] Comparison example 4

[0092] 5 kg of silicon fragments of BG2 with non-compact morphology undergo the process steps described in Comparative Example 2.

[0093] Comparison example 5

[0094] 5 kg of silicon fragments of size BG3 go through the following process steps.

[0095] - Pickling bath: HCl content: 12 wt.%; HF content: 5 wt.%; H2O2 content: 2 wt.%; bath temperature: 18°C; residence time: 415 s

[0096] - Ultrapure water bath: Bath temperature: 18°C; residence time: 80 s

[0097] - Main etching bath: HF content 5.8 wt.%; HNO3 content 50 wt.%; bath temperature: 8°C; residence time: 130 s

[0098] - Ultrapure water bath: Bath temperature: 18°C; residence time: 150 s

[0099] - Convection drying: Temp.: 80°C; Residence time: 1250 s - Vacuum drying: Pressure: 3.5 kPa; Residence time: 100 s

[0100] Table 1 summarizes Examples 1 to 4 according to the invention. *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, TI, U, V, Y, Zr

[0101] Table 1

[0102] Table 2 summarizes comparative examples 1 to 5.

[0103] *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, TI, U, V, Y, Zr

[0104] Table 2

Claims

Patent claims 1. A process for producing silicon fragments with reduced surface metal content, comprising the steps of a) comminuting a silicon rod or block into silicon fragments, b) contacting the silicon fragments with at least one pre-etching bath containing 6.6 to 12 wt.% hydrofluoric acid and 40 to 65 wt.% nitric acid, c) contacting the silicon fragments with at least one main etching bath containing 5.3 to 6.5 wt.% hydrofluoric acid and 40 to 65 wt.% nitric acid.

2. Process according to claim 1, characterized in that the pre-etching bath contains 6.6 to 12 wt.%, preferably 7 to 10 wt.%, of hydrofluoric acid.

3. Process according to claim 1 or 2, characterized in that the contact time in the pre-etching bath is 1 to 30 s, preferably 2 to 25 s, particularly preferably 4 to 20 s.

4. Method according to one of the preceding claims, characterized in that the pre-etching bath has a temperature of 1 to 60°C, preferably 5 to 50°C, particularly preferably 8 to 40°C.

5. Method according to one of the preceding claims, characterized in that the temperature of the pre-etching bath is adjusted depending on the size of the silicon fragments, the temperature at Fraction size 2 is from 1 to 10°C, fraction size 3 is from 8 to 60°C, fraction size 4 is from 5 to 15°C. Method according to one of the preceding claims, characterized in that the at least one main etching bath has a temperature of 1 to 15°C, preferably of 2 to 12°C, particularly preferably of 4 to 10°C. Method according to one of the preceding claims, characterized in that in step c) the silicon fragments are brought into contact with a main etching bath, which preferably contains 53 to 65 wt.% nitric acid. Method according to one of the preceding claims, characterized in that in step c) the silicon fragments are brought into contact with a first and a second main etching bath, wherein the second main etching bath has a higher nitric acid content. Method according to claim 8, characterized in that the first main etching bath contains 40 to 65 wt.% nitric acid and the second main etching bath contains 53 to 65 wt.% nitric acid.A process according to any one of the preceding claims, comprising, after step c), a further step d) of contacting the silicon fragments with a hydrophilization bath containing an ozone-water mixture with 5 to 30 ppm, preferably 7 to 15 ppm, of ozone. A process according to any one of the preceding claims, characterized in that after each or some of steps a) to c) and / or after step d), the silicon fragments are contacted with an ultrapure water bath.

12. The method according to claim 10, characterized in that the silicon fragments are brought into contact after step d) with an ultrapure water bath which has a temperature of 50 to 95°C, preferably 60 to 90°C.

13. A method according to any one of the preceding claims, comprising a drying step in which the silicon fragments are dried by convection drying and / or vacuum drying.

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

15. Silicon fragment, in particular produced by a process according to at least one of claims 1 to 14, which has a surface metal content of < 13 pptw, preferably < 12 pptw, particularly preferably < 11 pptw, the following metals being taken into account: Fe, Cr, Ni, Al, Ca, Ag, Zn, As, Co, Cu, Na, K, Ti, Mg, Mo, Mn, Sn, Ba, Bi, Cd, Li, Pb, Sb, Sr, Ti, U, V, W, Y, Zr.

16. Silicon fragment according to claim 15, characterized in that its tungsten content is < 1 pptw, preferably < 0.6 pptw.

17. Silicon fragment according to claim 15 or 16, characterized in that its content of Fe, Cr, Ni and W is < 5 pptw, preferably < 4 pptw, especially < 3 pptw.

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