JP6472768B2 - Determination of impurities in silicon crystal by photoluminescence method and selection method of polycrystalline silicon - Google Patents

Description

  The present invention relates to a technique for quantifying impurities in polycrystalline silicon used as a raw material for producing a silicon single crystal by a simple method.

  Since polycrystalline silicon is used as a raw material for growing single crystal silicon used in the manufacture of semiconductor devices and the like, impurities as dopants contained in the polycrystalline silicon are required to have a very low concentration. Under such a background, Patent Document 1 (Japanese Patent Laid-Open No. 2013-151413) proposes an invention of a low-dopant polycrystalline silicon mass.

  In order to realize the provision of polycrystalline silicon with a low impurity concentration, a technique for quantifying it is also required. As a standardized evaluation method, the photoluminescence method is used, and ASTM-F1389-00 (single crystallized sample) is used. A method defined in JIS-H-0615 is known. The standard methods for these standards are based on the premise that a polycrystalline sample is single-crystallized. As a standardization method for single-crystallizing, a method based on the zone melting method according to ASTM-F1723-02 is known. ing.

Patent Document 2 (Japanese Patent Laid-Open No. 5-58789) discloses ultra-trace elements (phosphorus, arsenic, boron, aluminum, etc.) in a silicon single crystal manufactured by chemical vapor deposition, and a silicon single crystal. As a method of fractionating and quantifying ultra-trace elements (phosphorus, arsenic, boron, aluminum, etc.) in chlorosilanes, which are raw materials when producing, a silicon single crystal produced by chemical vapor deposition is used on the irradiated surface. Fractionation of ultratrace elements in a silicon single crystal characterized by irradiating a laser beam with an energy of 3100-3358 mW / cm ² and measuring the emitted spectrum photoelectrically to quantify the trace elements in the silicon single crystal The invention of “quantitative method” is disclosed.

JP 2013-151413 A JP-A-5-58789

  However, since all of the above methods require that the evaluation sample is single-crystallized, the step of single-crystallizing the impurity in the polycrystalline silicon by the photoluminescence method is indispensable. However, even in the single crystallization method based on the zone melting method defined in ASTM-F1723-02, contamination in the single crystallization process cannot be completely avoided. It is extremely difficult to quantify the concentration of the original impurities in the sample.

  The present invention has been made in view of such a problem, and an object of the present invention is to determine a single sample of an evaluation sample when quantifying impurities in polycrystalline silicon used as a raw material for producing a silicon single crystal by a photoluminescence method. An object of the present invention is to provide a technique capable of easily quantifying without requiring a crystallization step.

  In order to solve the above problems, a method for quantifying impurities in a silicon crystal according to the present invention is a method for quantifying impurities in a silicon crystal by a photoluminescence method, wherein a part of polycrystalline silicon is melted by high-frequency heating. An evaluation sample is collected from a part of the polycrystalline silicon sample that is maintained in a state where the crystallites are convected in the silicon melt for a predetermined time and then cooled and crystallized, and the evaluation sample is used. Impurity determination is performed.

  For example, the impurity is any one of phosphorus, arsenic, boron, and aluminum.

  Preferably, the crystallite convection state in the silicon melt is maintained for at least 5 minutes.

  For example, the collection area of the evaluation sample has a diameter of 3 mm or more and a thickness of 1 mm or more in terms of a disk.

  Further, the polycrystalline silicon according to the present invention has an impurity concentration evaluated by the above-described method, wherein the concentrations of phosphorus and arsenic are each 0.05 ppba or less, and the concentrations of boron and aluminum are 0.05 ppba or less, In the method for selecting polycrystalline silicon according to the present invention, the impurity concentration evaluated by the above method is such that the concentrations of phosphorus and arsenic are each 0.05 ppba or less, and the concentrations of boron and aluminum are each 0.05 ppba or less. Polycrystalline silicon is selected as polycrystalline silicon for semiconductor manufacturing.

  The method for quantifying impurities in a silicon crystal by the photoluminescence method according to the present invention does not require a single crystallization step as in the conventional method, so that rapid quantification is possible. In addition, since there is no fear of contamination due to single crystallization, the original impurity concentration can be quantified.

It is a PL spectrum obtained from four types of plate-like samples having different melting times. It is PL spectrum obtained from the plate-shaped sample (Example) obtained from the area | region which melt | dissolved a part of polycrystalline silicon by the high frequency heating. It is PL spectrum obtained from the plate-shaped sample (comparative example) obtained after making it single crystal by the zone melting method.

  An evaluation sample is collected from a polycrystalline silicon mass (for example, a polycrystalline silicon rod grown by the Siemens method) for quantifying the impurity concentration in the following procedure. For example, a cylindrical sample (core sample) is cut out with a 19 mm diameter core drill, and the tip is tapered. After that, for example, etching with a mixed solution (1: 5, volume ratio) of hydrofluoric acid (50 wt%) and nitric acid (70 wt%) is performed for 5 minutes to remove surface processing stains. Then, this is suspended in the vertical direction, and a tapered tip (for example, a conical region having a diameter of 5 mm and a height of 2 to 3 mm) is heated at high frequency to be melted. This state is maintained for a predetermined time while finely adjusting the current value of the high-frequency heating device so that the melt does not melt and fall.

  The core sample may be sampled from any direction of the polycrystalline silicon rod. In addition, the distance for causing the melting region and the high-frequency device to grip the measurement sample needs to be about 40 to 50 mm, and this distance prevents the sample core from being held at a high temperature.

  Maintaining the above-mentioned melting state means that the melted polycrystal starts convection in the melt, and eventually the crystallites are aligned in a certain direction or the crystallites grow greatly due to the influence of gravity or high frequency. Happens. This maintenance time is preferably longer than at least 5 minutes, for example, 10 minutes. At this time, the inside of the high-frequency heating apparatus is replaced with Ar gas (for example, a flow rate of 18 liter / min), and the internal pressure is set to 0.03 MPa or the like. Note that the height of the tip portion to be tapered is set to be within 5 mm.

  The maintenance time of the molten state is set longer than at least 5 minutes. If this time is short, the convection of crystallites in the silicon melt becomes insufficient, and a clear photoluminescence spectrum is obtained from this region. It is because it is not possible. In addition, melting is performed by high-frequency heating in order to avoid contamination during processing as much as possible. On the other hand, if melting is performed in a muffle furnace, a lamp heating furnace, or the like, contamination is likely to occur.

  After maintaining the molten state, it is naturally cooled and crystallized. Then, an evaluation sample is collected from this crystallization region. The sampling area of the evaluation sample at this time preferably has a diameter of 3 mm or more and a thickness of 1 mm or more in terms of a disk shape in consideration of ease of handling in a subsequent process. In addition, the laser light irradiation area | region at the time of PL measurement is a diameter of about 3 mm, and the depth involved in light emission is about 100 micrometers.

  In the appearance observation with a polarizing microscope after etching, no difference in appearance from that of the single crystal was observed. However, when X-ray diffraction analysis was performed, (111), (220), (311), (400 ) Diffraction peaks from each crystal plane were confirmed.

  One side of this plate sample is lapped, the surface is etched with a mixed aqueous solution of hydrofluoric acid and nitric acid, the surface is irradiated with light, photoluminescence (PL) measurement is performed, and the concentration of phosphorus, arsenic, boron, aluminum, etc. Perform quantification. In addition, fixed_quantity | quantitative_assay follows the method as described in ASTM-F1389-00 and JIS-H-0615.

  The resistivity measurement is based on the 4-probe method. In this method, the current that flows when voltage is applied to the two outer needles is measured with the two inner needles. Since the distance between the four probes is 1 mm (3 mm in total), the resistivity can be measured by the four-probe method if the plate-like sample has a diameter of 3 mm or more.

  As described above, the method for quantifying impurities in a silicon crystal by the photoluminescence method according to the present invention is a method for quantifying impurities in the silicon crystal by a photoluminescence method, wherein a part of polycrystalline silicon is melted by high-frequency heating. An evaluation sample is collected from a part of the polycrystalline silicon sample that is maintained in a state where the crystallites are convected in the silicon melt for a predetermined time and then cooled and crystallized, and the evaluation sample is used. It is characterized by quantifying impurities.

  Since this method does not require a single crystallization step as in the conventional method, rapid quantification is possible. In addition, since there is no fear of contamination due to single crystallization, the original impurity concentration can be quantified.

  Impurities quantified by the method according to the present invention are, for example, phosphorus, arsenic, boron, aluminum and the like.

  In practicing the present invention, it is preferable to maintain the convection state of the crystallites in the silicon melt for at least 5 minutes.

  Moreover, it is preferable that the collection area | region of the evaluation sample mentioned above is a diameter of 3 mm or more and thickness is 1 mm or more in disk conversion.

  Impurity concentrations evaluated by the method for quantifying impurities in silicon crystals by the photoluminescence method according to the present invention are such that the concentrations of phosphorus and arsenic are each 0.05 ppba or less, and the concentrations of boron and aluminum are each 0.05 ppba or less. If polycrystalline silicon is selected as polycrystalline silicon for semiconductor manufacturing, it is possible to select polycrystalline silicon for actual measurement (quality control).

  Four plate samples (melting time: 3 minutes, 5 minutes, 8 minutes, and 11 minutes) having different melting times were collected from the straight body portion of one polycrystalline silicon rod in accordance with the above-described procedure. FIG. 1 shows PL spectra obtained from these samples. Table 1 summarizes the concentrations and resistance values of P, As, B, and Al determined from the PL spectrum. The calculated resistance values in the table were calculated as resistance value (Ω-cm) = 93 / ((P concentration + As concentration) − (B concentration + Al concentration)).

  A clear peak is not observed in the PL spectrum from a sample having a melting time of 5 minutes or less, whereas a clear peak is observed from a sample having a melting time longer than 5 minutes.

  Next, a plate-like sample (Example) obtained from a region where a part of polycrystalline silicon was melted by high-frequency heating, and a plate-like sample (Comparative Example) obtained after single crystallization by zone melting method, The impurity concentration determined by PL measurement was compared. In order to confirm reproducibility, two plate-like samples were prepared for each sample. The PL spectra obtained from these samples are shown in FIG. 2 (Example) and FIG. 3 (Comparative example). Table 2 summarizes the concentrations and resistance values of P, As, B, and Al determined from the PL spectrum.

  Since these plate samples are collected from the same part of the same polycrystalline silicon rod, their impurity concentrations are essentially the same. However, according to the quantitative results, the plate-like sample (comparative example) obtained after single crystallization by the zone melting method is the plate-like sample (example) obtained from the region melted by high-frequency heating. It shows a higher impurity concentration. This fact indicates that a numerical value higher than the original impurity concentration is obtained due to contamination accompanying single crystallization.

  “CZ single crystal” in Table 2 is CZ single crystal silicon grown from polycrystalline silicon obtained by collecting the plate sample of the example, and the impurity concentration thereof is the plate sample of the example. It almost agrees with the numerical value obtained by quantification.

  Thus, the method according to the present invention is particularly effective for accurate impurity analysis of extremely low-concentration products such as P concentration of 0.05 ppba or less and B concentration of 0.05 ppba or less.

  The present invention provides a technique capable of easily quantifying an evaluation sample without requiring a single crystallization step when quantifying impurities in polycrystalline silicon used as a raw material for producing a silicon single crystal by a photoluminescence method.

Claims (4)

A method for quantifying impurities in a silicon crystal by a photoluminescence method,
The polycrystal obtained by melting a part of polycrystalline silicon by high-frequency heating, maintaining a crystallite convection state in a silicon melt for 8 minutes or more , and then cooling to crystallize without single crystallization. A method for quantifying impurities in a silicon crystal by a photoluminescence method, wherein a polycrystalline evaluation sample is collected from a part of a silicon sample, and impurities are quantified using the evaluation sample.   The method for quantifying impurities in a silicon crystal by a photoluminescence method according to claim 1, wherein the impurity is any one of phosphorus, arsenic, boron, and aluminum. The method for quantifying impurities in a silicon crystal by a photoluminescence method according to claim 1 or 2 , wherein the sampling region of the evaluation sample has a diameter of 3 mm or more and a thickness of 1 mm or more in terms of a disk. The impurity concentration evaluated by the method according to any one of claims 1 to 3 is such that the concentrations of phosphorus and arsenic are each 0.05 ppba or less, and the concentrations of boron and aluminum are 0.05 ppba or less, respectively. A method for selecting polycrystalline silicon, wherein crystalline silicon is selected as polycrystalline silicon for semiconductor manufacturing.