A method for determining surface metal impurities of small poly-silicon pieces

Abstract

The application provides a kind of determination method of small broken material surface metal impurity of polysilicon, and the method comprises the following steps: taking small broken material sample of polysilicon in a beaker, adding leaching solution, and digesting; taking the digested sample, cooling to room temperature, and transferring into a volumetric flask, then adding HNO3 for constant volume; shaking the volumetric flask, mixing the solid phase and liquid phase uniformly, and then centrifuging; after centrifugation, taking the supernatant to make a standard curve by standard addition method, the first addition amount of the standard curve should be close to the element content in the sample, and a blank is also prepared; the ICP-MS instrument is tuned to be optimal, the standard addition concentration is taken as the abscissa, the measured ion count CPS value is taken as the ordinate, the standard curve is drawn, and the sample is measured. The application avoids secondary pollution, and does not change the volume of acid leaching solution, and also does not have the problem of incomplete flushing of element impurities.

Description

Technical Field

[0001] This invention relates to the field of polycrystalline silicon technology, and in particular to a method for determining metallic impurities on the surface of small polycrystalline silicon fragments. Background Technology

[0002] In polycrystalline silicon production, the content of metallic impurities on the polycrystalline silicon surface is one of its important technical indicators. This content severely restricts the minority carrier lifetime, the photoelectric conversion efficiency of photovoltaic solar cells, and the performance of electronic devices, thus requiring strict control. With advancements in monocrystalline silicon manufacturing processes, small polycrystalline silicon fragments are also being used in monocrystalline silicon production. Simultaneously, the process has introduced requirements for the metallic impurities in these fragments, leading to new requirements for their detection. However, current national standards are not applicable to the detection of these small polycrystalline silicon fragments.

[0003] First, the current standard GB / T24582-2009, "Determination of Surface Metals in Polycrystalline Silicon by Acid Leaching-Inductively Coupled Plasma Mass Spectrometry," specifies polycrystalline silicon blocks with a sample size of approximately 3cm*3cm*3cm. This standard is not applicable to the determination of surface metals on small polycrystalline silicon fragments (1mm-3mm). Second, in GB / T24582-2009, the separation of the leaching solution from the polycrystalline silicon block after acid leaching is achieved by using PTFE tweezers to remove the leached block, followed by rinsing and collecting the rinsing solution in the leaching solution. For small fragments, due to their small size and large quantity, using PTFE tweezers is unsuitable, making it impossible to separate the solid polycrystalline silicon from the leaching solution after acid leaching. Consequently, the determination of surface metals on small polycrystalline silicon fragments cannot be performed. Third, GB / T24582-2009, "Determination of Metals on the Surface of Polycrystalline Silicon by Acid Leaching-Inductively Coupled Plasma Mass Spectrometry," does not specify the number of rinses when washing the leached material with deionized water, making it impossible to confirm whether the metal impurities on the surface of the leached polycrystalline silicon have been completely removed. Fourth, GB / T24582-2009 uses the external standard method. Due to the presence of silicon matrix and salts in the sample solution, the physical properties of the sample solution and the standard solution are mismatched, such as viscosity and surface tension. This leads to inconsistent atomization efficiencies between the two solutions in the ICPMS injection system. The silicon matrix can also affect the ionization of the sample solution in the ICPMS plasma, resulting in matrix suppression or matrix gain. Fifth, small fragments have a small volume and a large surface area, resulting in a large contact area with the leaching solution and a fast reaction rate. If the concentration of the leaching solution is too high, the polycrystalline silicon fragments will be leached to an excessive depth, dissolving some of the matrix metal in the leaching solution. If the concentration of the leaching solution is insufficient, the leaching depth of the polycrystalline silicon surface will be insufficient, and the metal impurities will not be completely dissolved. Both of these situations will affect the accuracy of the metal determination on the surface of the polycrystalline silicon fragments. Summary of the Invention

[0004] This invention provides a method for determining metallic impurities on the surface of small polycrystalline silicon fragments.

[0005] To achieve the objectives of this invention, the technical solution is as follows:

[0006] A method for determining metallic impurities on the surface of small polycrystalline silicon fragments, the method comprising the following steps:

[0007] (1) Determine the heating and digestion time after adding the silicon material surface stripping agent based on the particle size of the polycrystalline silicon fragments;

[0008] (2) Take a small polycrystalline silicon fragment sample into a beaker, add silicon surface stripping agent, cover and digest;

[0009] (3) Remove the digested sample, cool it to room temperature, transfer it into a centrifuge tube, and add HNO3 to bring the volume to the mark.

[0010] (4) Centrifugal separation to separate the solid and liquid phases;

[0011] (5) After centrifugation, the supernatant was collected in a digestion tube and evaporated to dryness in a graphite heater.

[0012] (6) Secondary acid removal of silicon matrix: After volatilization to dryness, add HNO3 and continue volatilization to dryness.

[0013] (7) After evaporating to dryness, add HNO3 for extraction;

[0014] (8) Optimize the performance of ICPMS instrument tuning;

[0015] (9) Draw the standard working curve;

[0016] (10) Repeat the above sample preparation process, use the above standard curve to measure the digestion solution, and at the same time prepare process blanks and three parallel samples.

[0017] Preferably, the beakers and PFA digestion tubes used in the method are made of PFA, and the centrifuge tubes are made of PTFE.

[0018] Preferably, in step (2), the digestion temperature is 60°C;

[0019] The temperature at which steps (5) and (6) are evaporated to dryness is 170°C.

[0020] Preferably, in steps (3) and (6), the concentration of HNO3 is 30%;

[0021] Step (7): The concentration of HNO3 is 1%.

[0022] Preferably, the method is used for the determination of polycrystalline silicon fragments with a particle size of 1 mm to 3 mm.

[0023] More preferably, the method includes the following steps:

[0024] (1) Determine the heating and digestion time after adding the silicon material surface stripping agent based on the particle size of the polycrystalline silicon fragments.

[0025] (2) Take 10g of polycrystalline silicon small fragments (accurate to ±0.1g) into a 50ml PFA covered beaker, add 30ml of silicon surface stripping agent, cover and digest on a hot plate at 60℃, and the digestion time is determined according to the particle size.

[0026] (3) Remove the digested sample, cool it to room temperature, transfer it to a 50ml PTFE centrifuge tube, and add 1% HNO3 to bring it to the mark.

[0027] (4) Centrifugal separation to separate the solid and liquid phases;

[0028] (5) After centrifugation, let stand for 10 minutes, take 5 ml of the supernatant into a 30 ml PFA digestion tube, and evaporate to dryness at 170°C in a graphite heater.

[0029] (6) Secondary acid removal of silicon matrix: After evaporation to dryness, add 2ml HNO3 (1+1) and continue evaporation at 170℃ to dryness.

[0030] (7) After evaporating to dryness, add 5 ml of 1% HNO3 for extraction.

[0031] (8) Optimize the performance of ICPMS instrument tuning.

[0032] (9) Draw the standard working curve:

[0033] (10) Repeat the above sample preparation process, use the above standard curve to measure the digestion solution, and at the same time prepare process blanks and three parallel samples.

[0034] Preferably, in step (1),

[0035] (1.1) Requirements for reagents and equipment:

[0036] Hydrofluoric acid: 48%, equivalent to grade 2 in SEMI C28.

[0037] Nitric acid: 65%, equivalent to grade 2 in SEMI C35.

[0038] Hydrogen peroxide: 30%, equivalent to grade 2 in SEMI C30.

[0039] Deionized water: All water should be of type E-1 as described in ASTM D5127 or other equivalent quality deionized water.

[0040] PFA beakers were soaked in 10% nitric acid solution for 48 hours, rinsed with plenty of deionized water, soaked in fresh 10% nitric acid solution for 48 hours, rinsed with deionized water, dried and stored in a clean environment.

[0041] Other equipment, such as PTFE test tubes and pipette tips, should be purified according to the method described in section "5" above.

[0042] Instruments and equipment:

[0043] ICP-MS: Inductively Coupled Plasma Mass Spectrometer with Dynamic Collision Reaction Cell;

[0044] Air environment:

[0045] The area used for sample collection, acid leaching, and ICP-MS analysis must be enclosed in a clean room, with the minimum clean room standard being Class 6 as defined in ISO 14644-1.

[0046] (1.2) The components of the silicon surface stripping agent are: HNO3, HF, H2O2, C6H 14 O6, H2O.

[0047] (1.3) Volume ratio of silicon surface stripping agent components: HNO3:HF:H2O2:C6H 14 O6:H2O = 1:1:1:0.2:40;

[0048] (1.4) Explanation of the reaction mechanism between silicon surface stripping agent and silicon: Nitric acid oxidizes the silicon surface layer, hydrofluoric acid dissolves silicon oxide, hydrogen peroxide enhances the oxidizing property of the stripping agent and accelerates the reaction rate, and mannitol forms a stable complex with the metal ions in the stripped material.

[0049] (1.5) The method of using the silicon surface stripping agent is as follows: under the heating condition of 60°C, the mass reduction of the polycrystalline silicon sample after treatment with the silicon surface stripping agent is controlled to be 0.5%-1% of the sample mass.

[0050] Specifically, it includes:

[0051] (1.5.1) Under heating conditions of 60℃, the amount of polycrystalline silicon samples with different particle sizes can be controlled by controlling the reaction time with the silicon surface stripping agent.

[0052] (1.5.2) Control of the amount of polycrystalline silicon sample surface stripping: The amount of mass reduction of the polycrystalline silicon sample after treatment with silicon material surface stripping agent is between 0.5% and 1% of the sample mass.

[0053] (1.5.3) Simultaneously weigh three 10g (±0.1g) polycrystalline silicon samples into 50ml PFA beakers, add 30ml of silicon surface stripping agent to each, and heat on a 60℃ hot plate for 10min, 20min, and 30min respectively. Remove and cool to room temperature, filter and dry the solution, and accurately weigh the sample after stripping with the silicon stripping agent. The heating time is sufficient for the sample surface stripping reaction time if the sample mass reduction is in the range of 0.5%-1.0%.

[0054] Preferably, in step (4), the specific steps of centrifugation are as follows:

[0055] (4.1) Remove the digested sample, cool it to room temperature, transfer it to a 50ml PTFE centrifuge tube, and add 1% HNO3 to bring it to the mark.

[0056] (4.2 Centrifuge parameters: speed greater than 1000 r / min, centrifugation time greater than 10 min.)

[0057] Preferably, in step (6), the specific steps for removing the silicon substrate by adding acid and then evaporating are as follows:

[0058] (6.1) After evaporating to dryness, add 2 ml of HNO3 (1+1) and continue evaporating to dryness at 170℃ to remove silicon matrix to less than 1000 ppm.

[0059] Preferably, in step (8), the detection conditions for ICP-MS using the ammonia collision reaction mode are as follows:

[0060] Atomizing gas flow rate: 0.93 L / min; Auxiliary gas flow rate: 12 L / min; Plasma gas flow rate: 16 L / min;

[0061] Inductively coupled plasma radio frequency 1600 watts; quadrupole ion deflector fixed voltage -12 volts; supercone park voltage 5 volts; omniring park voltage -215 volts; inner target lens voltage 6 volts; outer target lens voltage -9 volts; deflection outlet voltage -4 volts; differential aperture voltage -4 volts; cell bias voltage (CRO) -3.5 volts; axial field voltage (AFT) 175 volts; gas flow rate 0.6 ml / min; analog voltage -2050 volts; pulse voltage 1200 volts; mass calibration and resolution 0.7 amu; mass calibration and resolution accuracy ±0.03 amu; rectangular tube position correction In / 114.904 Maximum.

[0062] Preferably, the specific steps for matrix matching and standard working curve plotting in step (9) are as follows:

[0063] (9.1) Accurately weigh the extract, add a mixed standard solution containing Fe, Cr, Ni, Cu, Zn, K, Na and Al to prepare a concentration of 1 ppb, and measure it on the instrument. This is the first point of the standard curve.

[0064] (9.2) Accurately weigh the 1 ppb concentration solution obtained from the above determination, add a mixed standard solution containing Fe, Cr, Ni, Cu, Zn, K, Na and Al to prepare a 2 ppb concentration, and measure it on the instrument. This is the second point of the standard curve.

[0065] (9.3) Accurately weigh the 2ppb concentration solution obtained from the above determination, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a 5ppb concentration, and measure it on the instrument. This is the third point of the standard curve.

[0066] (9.4) Plot a standard curve with the standard concentration as the x-axis and the ion count as the y-axis based on the test results of the three sample digestion solutions containing different concentrations of mixed standards.

[0067] Compared with the prior art, the beneficial effects of the present invention are:

[0068] 1) This invention proposes a method for solid-liquid phase separation of small-particle-size polycrystalline silicon after removal by a silicon surface stripping agent. Current standards for detecting metallic impurities on the surface of polycrystalline silicon specify a size of 3cm*3cm*3cm. After acid leaching, the polycrystalline silicon block is removed with PTFE tweezers and rinsed with deionized water. The rinsing solution is collected in the acid leaching solution. This method is unsuitable for small polycrystalline silicon fragments with a particle size of 1mm-3mm, mainly because the solid-liquid phase cannot be separated after acid leaching of metallic impurities. Conventional filtration and washing introduce new impurities, and the volume of the leaching solution is uncontrollable. Firstly, thorough washing is not guaranteed; secondly, the volume of the washing solution dilutes the acid leaching solution, leading to measurement errors. This invention removes impurities by quantitatively adding a silicon surface stripping agent, then transferring the mixture to a 50ml centrifuge tube for centrifugation. The supernatant is then directly used for subsequent measurements, avoiding secondary contamination and eliminating the problem of incomplete rinsing of elemental impurities.

[0069] 2) This invention proposes a method for controlling the amount of polycrystalline silicon surface stripping agent to remove small polycrystalline silicon particles of different particle sizes. By controlling the reaction time between the polycrystalline silicon surface stripping agent and the polycrystalline silicon sample at 60°C on a hot plate, the amount of polycrystalline silicon surface stripping agent to be removed is controlled within the range of 0.5%-1.0% of the total mass of the sample, ensuring that surface metal impurities are completely removed while ensuring that no excessive silicon substrate is dissolved.

[0070] 3) This invention proposes to detect metallic impurities on the surface of polycrystalline silicon material by matrix matching. In the current standard, the external standard method cannot avoid signal suppression or signal gain caused by matrix interference during the measurement process. After adding nitric acid to remove part of the silicon matrix, the standard curve is plotted by adding the stripping solution of the silicon material to be tested, thus achieving matrix matching and improving the accuracy of detection. Detailed Implementation

[0071] The present invention will be further described below with reference to the embodiments, but the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0072] Example 1

[0073] A method for determining metallic impurities on the surface of small polycrystalline silicon fragments, the method comprising the following steps:

[0074] (1) Determine the heating and digestion time after adding the silicon material surface stripping agent based on the particle size of the polycrystalline silicon fragments.

[0075] (2) Take 10g of polycrystalline silicon small fragments (accurate to ±0.1g) into a 50ml PFA covered beaker, add 30ml of silicon surface stripping agent, cover and digest on a hot plate at 60℃, and the digestion time is determined according to the particle size.

[0076] (3) Remove the digested sample, cool it to room temperature, transfer it to a 50ml PTFE centrifuge tube, and add 1% HNO3 to bring it to the mark.

[0077] (4) Centrifugal separation to separate the solid and liquid phases;

[0078] (5) After centrifugation, let stand for 10 minutes, take 5 ml of the supernatant into a 30 ml PFA digestion tube, and evaporate to dryness at 170°C in a graphite heater.

[0079] (6) Secondary acid removal of silicon matrix: After evaporation to dryness, add 2ml HNO3 (1+1) and continue evaporation at 170℃ to dryness.

[0080] (7) After evaporating to dryness, add 5 ml of 1% HNO3 for extraction.

[0081] (8) Optimize the performance of ICPMS instrument tuning.

[0082] (9) Draw the standard working curve:

[0083] (10) Repeat the above sample preparation process, use the above standard curve to measure the digestion solution, and at the same time prepare process blanks and three parallel samples.

[0084] Preferably, the method is used for the determination of polycrystalline silicon fragments with a particle size of 1 mm to 3 mm.

[0085] Preferably, the description of the silicon surface stripping agent in step (1) is provided.

[0086] (1.1) Reagent requirements:

[0087] (1.2) The components of the silicon surface stripping agent are: HNO3, HF, H2O2, C6H 14 O6, H2O.

[0088] (1.3) The ratio of the surface stripping agent for silicon material is: HNO3:HF:H2O2:C6H 14 O6:H2O = 1:1:1:0.2:40

[0089] (1.4) Explanation of the reaction mechanism between silicon surface stripping agent and silicon: Nitric acid oxidizes the silicon surface layer, hydrofluoric acid dissolves silicon oxide, hydrogen peroxide enhances the oxidizing property of the stripping agent and accelerates the reaction rate, and mannitol forms a stable complex with the metal ions in the stripped material.

[0090] (1.5) Instructions for use of silicon surface stripping agent:

[0091] (1.5.1) Under heating conditions of 60℃, the amount of polycrystalline silicon samples with different particle sizes can be controlled by controlling the reaction time with the silicon surface stripping agent.

[0092] (1.5.2) Control of the amount of polycrystalline silicon sample surface stripping: The amount of mass reduction of the polycrystalline silicon sample after treatment with silicon material surface stripping agent is between 0.5% and 1% of the sample mass.

[0093] (1.5.3) Simultaneously weigh three 10g (±0.1g) polycrystalline silicon samples into 50ml PFA beakers, add 30ml of silicon surface stripping agent to each, and heat on a 60℃ hot plate for 10min, 20min, and 30min respectively. Remove and cool to room temperature, filter and dry the solution, and accurately weigh the sample after stripping with the silicon stripping agent. The heating time is sufficient for the sample surface stripping reaction time if the sample mass reduction is in the range of 0.5%-1.0%.

[0094] Preferably, in step (4), the specific steps of centrifugation are as follows:

[0095] (4.1) Remove the digested sample, cool it to room temperature, transfer it to a 50ml PTFE centrifuge tube, and add 1% HNO3 to bring it to the mark.

[0096] (4.2 Centrifuge parameters: speed greater than 1000 r / min, centrifugation time greater than 10 min.)

[0097] Preferably, in step (6), the specific steps for removing the silicon substrate by adding acid and then evaporating are as follows:

[0098] (6.1) After evaporating to dryness, add 2 ml of HNO3 (1+1) and continue evaporating to dryness at 170℃ to remove silicon matrix to less than 1000 ppm.

[0099] Preferably, in step (8), the detection conditions for ICP-MS using the ammonia collision reaction mode are as follows:

[0100] Atomizing gas flow rate: 0.93 L / min; Auxiliary gas flow rate: 12 L / min; Plasma gas flow rate: 16 L / min;

[0101] Inductively coupled plasma radio frequency 1600 watts; quadrupole ion deflector fixed voltage -12 volts; supercone park voltage 5 volts; omniring park voltage -215 volts; inner target lens voltage 6 volts; outer target lens voltage -9 volts; deflection outlet voltage -4 volts; differential aperture voltage -4 volts; cell bias voltage (CRO) -3.5 volts; axial field voltage (AFT) 175 volts; gas flow rate 0.6 ml / min; analog voltage -2050 volts; pulse voltage 1200 volts; mass calibration and resolution 0.7 amu; mass calibration and resolution accuracy ±0.03 amu; rectangular tube position correction In / 114.904 Maximum.

[0102] Preferably, the specific steps for matrix matching and standard working curve plotting in step (9) are as follows:

[0103] (9.1) Accurately weigh the extract, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a concentration of 1 ppb, and measure it on the instrument. This is the first point of the standard curve.

[0104] (9.2) Accurately weigh the 1 ppb concentration solution obtained from the above determination, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a 2 ppb concentration, and measure it on the instrument. This is the second point of the standard curve.

[0105] (9.3) Accurately weigh the 2ppb concentration solution obtained from the above determination, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a 5ppb concentration, and measure it on the instrument. This is the third point of the standard curve.

[0106] (9.4) Plot a standard curve with the standard concentration as the x-axis and the ion count as the y-axis based on the test results of the three sample digestion solutions containing different concentrations of mixed standards.

[0107] Example 2

[0108] 1. The main components of the silicon surface release agent are: HNO3, HF, H2O2, and C6H. 14 O6, H2O.

[0109] 1.1 Hydrofluoric acid is the only inorganic acid that can dissolve silicon. Elemental silicon is actually insoluble in hydrofluoric acid alone. To dissolve silicon, a mixture of concentrated nitric acid and hydrofluoric acid can be used. Hydrofluoric acid is a weak acid and does not have strong oxidizing properties, so it cannot oxidize elemental silicon. Although nitric acid has strong oxidizing properties, the tetravalent silicon produced does not have a suitable ligand to form a stable compound in solution. However, the fluoride ions provided by hydrofluoric acid are good ligands for tetravalent silicon. Therefore, using a mixture of nitric acid and hydrofluoric acid can achieve the purpose of dissolving the elemental silicon. Adding hydrogen peroxide enhances the oxidizing properties and accelerates the reaction rate. Adding mannitol forms a complex with the metal ion to be tested.

[0110] 2. Determination of sample peeling amount

[0111] 2.1 Determination of silicon content and total metal element content of the same sample with different stripping amounts:

[0112] Table 1

[0113] Serial Number Peeling amount (%) Silicon content (ppm) Total amount of metallic elements (ppb) 1 0.1 125 1.25 2 0.2 238 1.34 3 0.3 365 1.42 4 0.4 482 1.57 5 0.5 537 3.86 6 0.6 648 3.92 7 0.7 751 3.96 8 0.8 825 4.10 9 0.9 936 4.12 10 1.0 1025 4.21 11 1.1 1150 4.28 12 1.2 1361 4.36

[0114] From the data in Table 1 above, it can be seen that when the stripping amount reaches 0.5%, the total amount of metal elements increases significantly. Further increases in the stripping amount do not result in a significant increase in the total amount of metal elements, indicating that when the stripping amount is below 0.5%, the metal impurities on the silicon surface are not completely removed, leading to lower measured data. When the stripping amount is greater than 1.0%, the silicon content exceeds 1000 ppm. Subsequent acid removal of the silicon substrate will not be able to remove the silicon substrate to below 100 ppm. Therefore, it can be concluded that the surface metal stripping amount for polycrystalline silicon small fragment samples should be controlled between 0.5% and 1.0%.

[0115] 3. Determination of the concentration ratio of the silicon surface stripping agent

[0116] 3.1 Concentrated hydrofluoric acid reacts violently with nitric acid and silicon, exothermically, and the temperature rises sharply. The sample spurts out as a paste and cannot be separated. A certain amount of water must be added. However, if too much water is added, the reaction rate is too slow, and if too little water is added, the reaction is too violent.

[0117] 3.2 The following lists the peeling amount of different particle sizes of silicon surface stripping agents with different concentration ratios after 30 minutes on a 60°C hot plate.

[0118] Table 2

[0119]

[0120] The data in Table 2 above shows that when the concentration ratio of the silicon stripping agent is 1:1:1:0.2:40, it meets the requirement of 0.5%-1% stripping amount.

[0121] 4. Determination of stripping reaction time

[0122] 4.1 Three 10g (±0.1g) polycrystalline silicon samples of different particle sizes were simultaneously weighed into 50ml PFA beakers. 30ml of silicon surface stripping agent (1:1:1:0.2:40) was added to each beaker, and the samples were heated on a 60℃ hot plate for 10min, 20min, and 30min respectively. After cooling to room temperature, the solutions were filtered and dried. The mass of the samples after stripping with the silicon stripping agent was accurately weighed. The heating time required for a sample surface stripping reaction time to result in a mass reduction of 0.5%-1.0% met the requirements. A set of experimental data is given below.

[0123] Table 3

[0124]

[0125] The data in Table 3 above shows that the amount of peeling varies for samples with different particle sizes at the same temperature and different reaction times. Therefore, before the experiment, it is necessary to determine the reaction time based on the particle size of the sample to control the amount of peeling between 0.5% and 1.0%.

[0126] 5. The role of the complexing agent mannitol and the determination of its proportion

[0127] 5.1 The metal impurity elements of 10g of the same sample after reaction with surface stripping agents containing different proportions of mannitol were determined.

[0128] Table 4

[0129]

[0130] The data in Table 4 above show that the data for the surface stripper in serial number 1 without the addition of mannitol is lower. As the amount of mannitol added gradually increases, the measured value of the total amount of surface metal element impurities gradually increases. However, from the volume increase of 0.2 to 0.4, the measured value of surface metal impurities no longer increases, indicating that when the volume ratio of mannitol is 0.2, the surface metal impurities have been completely complexed.

[0131] Example 3

[0132] 1. Matrix effect, interference from silicon substrate

[0133] 1.1 When the silicon matrix content is greater than 100 ppm, matrix inhibition will occur during ICPMS measurement, resulting in signal attenuation. Standard mixed solutions (Fe, Cr, Ni, Cu, Zn, K, Na, Al) with silicon matrix contents of 90 ppm and 100 ppm were prepared, with each element concentration of 1.00 ppb. These solutions were then measured on the ICPMS. Ten consecutive measurements were performed using the same standard solution. The data are as follows:

[0134] Table 5

[0135]

[0136] As can be seen from the data in Table 5 above, with a standard solution of 90 ppm silicon matrix, the signal did not attenuate after ten consecutive measurements, and the recovery rate of each spiked element met the requirements of 90-105%. With a standard solution of 100 ppm silicon matrix, the signal continuously attenuated after ten consecutive measurements, matrix inhibition occurred, and the recovery rate dropped to 75-84%, which did not meet the measurement requirements.

[0137] 2. Add acid and evaporate twice to remove the silicon substrate.

[0138] 2.1 After the polycrystalline silicon sample reacts with the surface stripping agent, some silicon is released in the form of silicon tetrafluoride, and some exists in the stripping solution in the form of fluorosilicic acid. This is the main reason for the large silicon matrix. Nitric acid is added to remove the silicon matrix in the form of fluorosilicic acid by using the principle of strong acid to dissolve weak acid and high boiling point acid to remove low boiling point acid.

[0139] 2.2 The silicon content of the supernatant of the stripping solution after centrifugation was determined to be 936 ppm. At the same time, 5 ml of 10 portions of the supernatant after centrifugation were taken and evaporated to dryness at 170°C in a graphite furnace digestion tube. 2 ml of 30% HNO3 was added and evaporated to dryness again at 170°C. The silicon content was then extracted with 3 ml of 1% HNO3 and determined.

[0140] Table 6

[0141]

[0142] As can be seen from the data in Table 6 above, the silicon content of the stripping liquid after centrifugation was 936 ppm. After a second evaporation with acid, the silicon content was reduced to below 100 ppm, which meets the requirements of ICPMS determination, and the matrix inhibition was eliminated.

[0143] 3. Matrix matching and standard curve preparation.

[0144] 3.1 After acid addition and secondary evaporation, the silica matrix concentration was reduced to below 100 ppm, eliminating matrix inhibition. However, the silica matrix still existed. To improve detection accuracy, a matrix matching method was used to construct a standard curve. A 1% HNO3 extract after acid addition and secondary evaporation was taken. The extraction tubes were weighed to constant weight before extraction. The ion counts of each element in the extract were determined on an ICP-MS. Then, the remaining extract was accurately weighed, and a mixed standard solution containing the analyte was added to prepare a solution with an analyte concentration of 1 ppb. The ion counts of each element were measured again. This process was repeated until the third point, a 3 ppb solution, was tested for ion counts. A standard curve was then fitted. A background equivalent concentration < 1 ppt indicates that matrix interference was completely eliminated.

[0145] 4. Specific Implementation Data (ICP MS Model: PE 5000G)

[0146] 4.1 Select electronic-grade polycrystalline silicon small fragments (particle size approximately 1.0 cm) and test them according to the above steps. The elements to be tested are: Fe, Cr, Ni, Cu, Zn, Al, K, and Na.

[0147] 4.2 On-machine testing: Plot a standard curve with the elemental ion count (CPS) value as the ordinate and the standard concentration as the abscissa, and test the sample and digestion blank.

[0148] 4.3 The same sample was measured 10 times simultaneously, and the measurement data are as follows:

[0149] Table 7

[0150]

[0151]

[0152] As shown in Table 7 above, the RSD is better than the requirements of GB / T24582-2009 "Determination of Metals on Polycrystalline Silicon Surface by Acid Leaching-Inductively Coupled Plasma Mass Spectrometry".

[0153] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The embodiments and features described in these embodiments can be arbitrarily combined without conflict. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for determining metallic impurities on the surface of small polycrystalline silicon fragments, characterized in that: The method includes the following steps: (1) Determine the heating and digestion time after adding the silicon surface stripping agent based on the particle size of the polycrystalline silicon fragments; The silicon surface stripping agent has a volume ratio of HNO3:HF:H2O2:C6H 14 A mixture of O6:H2O = 1:1:1:0.2:40; the method of using the silicon surface stripping agent is as follows: under heating conditions of 60℃, control the mass reduction of the polycrystalline silicon sample after treatment with the silicon surface stripping agent to be 0.5%-1% of the sample mass; (2) Take a small polycrystalline silicon fragment sample into a beaker, add silicon surface stripping agent, cover and digest; (3) Remove the digested sample, cool it to room temperature, transfer it to a centrifuge tube, and add HNO3 to bring the volume to the mark; (4) Centrifugal separation to separate the solid and liquid phases; (5) After centrifugation, the supernatant was collected in a digestion tube and evaporated to dryness in a graphite heater. (6) Secondary acid removal of silicon matrix: After volatilization to dryness, add HNO3 and continue volatilization to dryness; (7) After evaporation to dryness, add HNO3 for extraction; (8) Optimize the performance tuning of the ICP-MS instrument; (9) Draw the standard working curve; (10) Repeat the above sample preparation process, use the above standard curve to measure the digestion solution, and at the same time prepare process blanks and three parallel samples; The specific steps for plotting a standard working curve are as follows: (9.1) Accurately weigh the extract, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a concentration of 1 ppb, and measure it on the instrument. This is the first point of the standard curve. (9.2) Accurately weigh the 1 ppb concentration solution after the above determination, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a 2 ppb concentration, and measure it on the instrument. This is the second point of the standard curve. (9.3) Accurately weigh the 2ppb concentration solution after the above determination, add a mixed standard solution containing Fe, Gr, Ni, Cu, Zn, K, Na and Al to prepare a 5ppb concentration, and measure it on the instrument. This is the third point of the standard curve. (9.4) Plot a standard curve with the standard concentration as the x-axis and the ion count as the y-axis based on the test results of the three sample digestion solutions containing different concentrations of mixed standards.

2. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 1, characterized in that: The beakers and PFA digestion tubes used in the method are made of PFA, and the centrifuge tubes are made of PTFE.

3. The method for determining metallic impurities on the surface of polycrystalline silicon fragments according to claim 1, characterized in that: Step (2): The digestion temperature is 55-65℃; The temperature at which steps (5) and (6) are evaporated to dryness is 160-180℃; In steps (3) and (6), the concentration of HNO3 is 25-35%; Step (7): The concentration of HNO3 is 0.5-1.5%.

4. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 3, characterized in that: Step (2): The digestion temperature is 60℃; The temperature at which the evaporation to dryness occurs in steps (5) and (6) is 170°C. In steps (3) and (6), the concentration of HNO3 is 30%; Step (7): The concentration of HNO3 is 1%.

5. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 1, characterized in that: The method is used to determine small polycrystalline silicon fragments with a particle size of 1 mm to 3 mm.

6. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 1, characterized in that: In step (1), the reagents and equipment requirements are as follows: Hydrofluoric acid: 48%; Nitric acid: 65%; Hydrogen peroxide: 30%; Purification method for beakers and test tubes: Soak in 10% nitric acid solution for 48 hours, rinse with deionized water, add fresh 10% nitric acid and soak for 48 hours, rinse with deionized water, dry and store in a clean environment; ICP-MS: Inductively Coupled Plasma Mass Spectrometer with Dynamic Collision Reaction Cell; Air environment: Areas used for sample collection, acid leaching, and ICP-MS analysis must be enclosed in a clean room, with a minimum clean room standard of Class 6 as defined in ISO 14644-1.

7. The method for determining metallic impurities on the surface of polycrystalline silicon fragments according to claim 1, characterized in that: In step (4), the specific steps of centrifugal separation are as follows: (4.1) Remove the digested sample, cool it to room temperature, transfer it to a 50ml PTFE centrifuge tube, and add 1% HNO3 to bring it to the mark; (4.2) Centrifuge parameters: speed greater than 1000 r / min, centrifugation time greater than 10 min.

8. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 1, characterized in that: In step (6), the specific steps for removing the silicon substrate by adding acid and then evaporating are as follows: (6.1) After evaporating to dryness, add 2 ml of HNO3 and continue evaporating at 170°C to dryness to remove silicon matrix to less than 100 ppm.

9. The method for determining metallic impurities on the surface of small polycrystalline silicon fragments according to claim 1, characterized in that: In step (8), the detection conditions for ICP-MS using the ammonia collision reaction mode are as follows: Atomizing gas flow rate: 0.93 L / min; Auxiliary gas flow rate: 12 L / min; Plasma gas flow rate: 16 L / min; Inductively coupled plasma radio frequency 1600 watts; quadrupole ion deflector fixed voltage -12 volts; Supercone Park voltage -5 Volts; Omniring park voltage -215 Volts; Inner target lens voltage -6 Volts; Outer target lens voltage -9 Volts; Deflection outlet voltage -4 Volts; Differential orifice voltage -4 Volts; Cell bias voltage (CRO) -3.5 Volts; Axial field voltage (AFT) 175 Volts; Gas flow rate 0.6 ml / min; Analog voltage -2050 Volts; Pulse voltage 1200 Volts; Mass calibration and resolution 0.7 amu; Mass calibration and resolution accuracy ±0.03 amu; Rectangular tube position correction In / 114.904 Maximum.

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