JP7626052B2 - Method for evaluating carbon concentration of silicon sample, method for evaluating silicon wafer manufacturing process, method for manufacturing silicon wafer, and method for manufacturing silicon single crystal ingot - Google Patents

Description

The present invention relates to a method for evaluating the carbon concentration of a silicon sample, a method for evaluating the silicon wafer manufacturing process, a method for manufacturing a silicon wafer, and a method for manufacturing a silicon single crystal ingot.

In recent years, there has been research into evaluating the carbon concentration of silicon samples (see, for example, Patent Documents 1 and 2).

JP 2017-191800 A JP 2021-15905 A

For silicon wafers used as semiconductor substrates, it is desirable to reduce impurity contamination that causes degradation of device characteristics. In recent years, carbon has attracted attention as an impurity contained in silicon wafers, and methods for reducing carbon contamination of silicon wafers have been studied.

To reduce carbon contamination, it is desirable to evaluate the carbon concentration of silicon samples and, based on the evaluation results, manage the silicon wafer manufacturing process and the manufacturing process of silicon single crystal ingots from which silicon wafers are cut so as to reduce carbon contamination during the manufacturing process. Finding a new method for evaluating the carbon concentration of silicon samples is useful for performing such process management.

One aspect of the present invention aims to provide a new method for evaluating the carbon concentration of a silicon sample.

One aspect of the present invention is
performing a hydrogen atom introduction process for introducing hydrogen atoms into the silicon sample to be evaluated;
subjecting the evaluation target silicon sample after the hydrogen atom introduction treatment to an evaluation method for evaluating a trap level in a band gap of silicon; and evaluating a carbon concentration of the evaluation target silicon sample based on an evaluation result regarding a density of a trap level at Ec (bottom energy of the conduction band)-0.15 eV obtained by the evaluation.
Including,
A method for evaluating a carbon concentration in a silicon sample (also referred to as a "carbon concentration evaluation method"), further comprising: performing a heat treatment of heating the silicon sample to a heating temperature of 800° C. or higher before the hydrogen atom introduction treatment; and the silicon sample to be evaluated is a nitrogen-containing silicon sample.
Regarding.

The above "heating temperature" refers to the temperature of the silicon sample surface heated by the heat treatment.

The present inventors have conducted extensive research in order to provide a new method for evaluating the carbon concentration of a silicon sample, and as a result have come to the following new findings.
The introduction of hydrogen atoms in the above evaluation method can form a trap level of Ec-0.15 eV in the band gap of silicon. The trap level of Ec-0.15 eV is a trap level caused by a complex formed by carbon atoms and hydrogen atoms (hereinafter, also referred to as a "carbon-hydrogen complex"). By forming such a trap level, it is possible to obtain an evaluation result regarding the density of the trap level. An example of such an evaluation result is the peak intensity (DLTS signal intensity) obtained by measurement using the DLTS method. This point will be described in detail later.
Regarding the trap level, the trap level of Ec-0.15 eV in the band gap of silicon after hydrogen atom introduction is a trap level related to a carbon-hydrogen complex, and the density of this trap level can be correlated with the carbon concentration of the silicon sample. Therefore, the evaluation result related to the density of the trap levels obtained by the evaluation performed after hydrogen atom introduction treatment, that is, the evaluation result correlated with the density of the trap levels, can be correlated with the carbon concentration of the silicon sample, so that the carbon concentration of the silicon sample to be evaluated can be evaluated based on the evaluation result.
However, this trap level of Ec-0.15 eV is located in the band gap of silicon close to the trap level (carrier trap) of nitrogen-related defects specific to nitrogen-containing silicon samples. Therefore, if hydrogen atoms are introduced into a nitrogen-containing silicon sample while the carrier trap is active, and the trap level of Ec-0.15 eV is evaluated, the carbon concentration obtained based on the evaluation result of this evaluation will also include the influence of the carrier trap. Therefore, in order to increase the sensitivity of the carbon concentration measurement, it is desirable to reduce or eliminate the influence of the carrier trap to obtain an evaluation result that is more correlated with the carbon concentration.
Regarding the above, the present inventors considered that if hydrogen atoms are introduced into a nitrogen-containing silicon sample after passivating the trap levels of nitrogen-related defects specific to the nitrogen-containing silicon sample, the influence of the carrier traps can be reduced or eliminated, and evaluation results that are more favorably correlated with the carbon concentration can be obtained. The present inventors then focused on the fact that the trap levels of nitrogen-related defects specific to nitrogen-containing silicon samples are deactivated by heat treatment at 800° C. or higher (see N. E. Grant et al. “Thermal activation and deactivation of grown-in defects limiting the lifetime of float-zone silicon”, Phys. Status Solidi RRL, 1-5 (2016)), and as a result of further intensive investigations, they have completed the carbon concentration evaluation method according to one aspect of the present invention.
In addition, the above Patent Document 1 (JP 2017-191800 A) does not describe the heat treatment before the hydrogen atom introduction treatment. In addition, the above Patent Document 2 (JP 2021-15905 A) discloses a heat treatment in a temperature range (600 ° C. or more and 650 ° C. or less: see claim 2, paragraphs 0023, 0024, etc. of JP 2021-15905 A) that is significantly lower than the heating temperature of the heat treatment performed before the hydrogen atom introduction treatment in the carbon concentration evaluation method. Therefore, none of the documents gives any suggestion for the carbon concentration evaluation method.

In one embodiment, the evaluation method can be the DLTS (Deep-Level Transient Spectroscopy) method.

In one embodiment, the hydrogen atom introduction process can include contacting the silicon sample to be evaluated with hydrofluoric nitric acid.

In one embodiment, the silicon sample to be evaluated can be an FZ silicon single crystal sample. In the present invention and this specification, "FZ silicon single crystal" refers to a silicon single crystal produced by the FZ (Floating Zone) method.

In one embodiment, the silicon sample to be evaluated can be a CZ silicon single crystal sample. In the present invention and this specification, "CZ silicon single crystal" refers to a silicon single crystal manufactured by the CZ method (Czochralski method).

One aspect of the present invention is
evaluating the carbon concentration of silicon wafers produced in a silicon wafer manufacturing process to be evaluated by the carbon concentration evaluation method; and evaluating the degree of carbon contamination in the silicon wafer manufacturing process to be evaluated based on the results of the evaluation.
A method for evaluating a silicon wafer manufacturing process (hereinafter also referred to as a "manufacturing process evaluation method"),
Regarding.

One aspect of the present invention is
evaluating a silicon wafer manufacturing process by the manufacturing process evaluation method; and manufacturing silicon wafers in the silicon wafer manufacturing process after performing a carbon contamination reduction treatment in the silicon wafer manufacturing process in which the degree of carbon contamination is determined to be at an acceptable level as a result of the evaluation, or in which the degree of carbon contamination is determined to exceed the acceptable level as a result of the evaluation.
A method for producing a silicon wafer, comprising:
Regarding.

One aspect of the present invention is
Growing a silicon single crystal ingot;
Evaluating the carbon concentration of a silicon sample cut out from the silicon single crystal ingot by the carbon concentration evaluation method.
Determining manufacturing conditions for a silicon single crystal ingot based on the results of the above evaluation; and
growing a silicon single crystal ingot under the determined production conditions;
A method for producing a silicon single crystal ingot, comprising:
Regarding.

According to one aspect of the present invention, a new method for evaluating the carbon concentration of a silicon sample can be provided. More specifically, according to one aspect of the present invention, a carbon concentration evaluation method can be provided that can accurately evaluate the carbon concentration of a nitrogen-containing silicon sample.

FIG. 1 is an explanatory diagram showing the configuration of a silicon single crystal manufacturing apparatus (FZ method). FIG. 1 is an explanatory diagram showing the configuration of a silicon single crystal pulling apparatus (CZ method). 1 shows the DLTS spectrum obtained in Example 1. 1 shows the DLTS spectrum obtained in Comparative Example 1. 1 shows the DLTS spectrum obtained in Comparative Example 2. 1 shows the DLTS spectrum obtained in Comparative Example 3. 1 shows DLTS spectra obtained in Example 2, Comparative Example 4, and Comparative Example 5.

[Method for evaluating carbon concentration of silicon sample]
One aspect of the present invention relates to a method for evaluating a carbon concentration of a silicon sample, the method including: performing a hydrogen atom introduction process for introducing hydrogen atoms into a silicon sample to be evaluated; subjecting the silicon sample to be evaluated after the hydrogen atom introduction process to an evaluation method for evaluating a trap level in a band gap of silicon; and evaluating a carbon concentration of the silicon sample to be evaluated based on an evaluation result regarding a density of a trap level at Ec-0.15 eV obtained by the evaluation; and further including performing a heat treatment for heating the silicon sample to be evaluated to a heating temperature of 800° C. or higher before the hydrogen atom introduction process; and the silicon sample to be evaluated is a nitrogen-containing silicon sample.
The carbon concentration evaluation method will be described in more detail below.

<Silicon sample to be evaluated>
The silicon sample to be evaluated in the carbon concentration evaluation method can be, for example, a silicon single crystal sample, and a specific example can be a silicon single crystal sample cut from a silicon single crystal ingot. For example, a sample cut into a wafer shape from a silicon single crystal ingot or a sample obtained by cutting a part from a sample cut into a wafer shape can be subjected to evaluation. The silicon sample to be evaluated can also be various silicon wafers (e.g., polished wafers, epitaxial wafers, etc.) used as semiconductor substrates or silicon samples cut from such silicon wafers. The silicon wafer can also be a silicon wafer that has been subjected to various processing treatments (e.g., polishing, etching, cleaning, etc.) that are usually performed on silicon wafers. The silicon sample can be n-type silicon in one form, and p-type silicon in another form. The resistivity of the silicon sample can be, for example, about 1 to 1000 Ω·cm, but is not particularly limited.

The silicon sample to be evaluated in the carbon concentration evaluation method is a nitrogen-containing silicon sample.
One example of the nitrogen-containing silicon sample is a FZ silicon single crystal sample. Since the FZ method often involves growing a crystal in a nitrogen gas-containing atmosphere, the FZ silicon single crystal usually contains nitrogen.
Another example of the nitrogen-containing silicon sample is a CZ silicon single crystal sample. In the CZ method, nitrogen doping may be performed at the stage of melting the raw material in order to control crystal defects. Therefore, an example of a nitrogen-containing CZ silicon single crystal sample is a nitrogen-doped CZ silicon single crystal sample.
For any form of nitrogen-containing silicon sample, the nitrogen concentration, as determined by FT-IR (Fourier Transform Infrared Spectroscopy), can be, for example, 1E+12 atoms/cm ³ or more, and can be, for example, 1E+16 atoms/cm ³ or less. However, the nitrogen concentration of the silicon sample to be evaluated by the carbon concentration evaluation method is not particularly limited. Note that, as is well known, "E+" indicates a power. For example, "E+12" indicates "×10 ¹² ".

<Heat Treatment>
In the carbon concentration evaluation method, the silicon sample to be measured is subjected to a heat treatment before hydrogen atoms are introduced. This heat treatment is performed at a heating temperature of 800° C. or more for the reasons described above. From the viewpoint of further reducing the influence of the trap level (carrier trap) of nitrogen-related defects specific to nitrogen-containing silicon samples, the heating temperature is preferably 850° C. or more, and more preferably 900° C. or more. In addition, since general heat treatment is usually performed at a temperature of 1200° C. or less, the heating temperature can be, for example, 1200° C. or less.
The heat treatment can be carried out using a known heating means such as a heating furnace.
The atmosphere in which the heat treatment is carried out (for example, the atmosphere inside a heating furnace) may be an air atmosphere, or may be an atmosphere other than air, for example, a single atmosphere of oxygen, nitrogen, argon, or the like.
The heat treatment time can be, for example, 10 minutes or more, preferably 30 minutes or more, and more preferably 60 minutes or more. Here, the heat treatment time refers to the time during which the surface temperature of the silicon sample is at the heating temperature. It is considered that the longer the heat treatment time, the more carrier traps can be inactivated, but the longer the heat treatment time, the longer the time required to determine the carbon concentration. From the viewpoint of shortening the time required to determine the carbon concentration, the heat treatment time is preferably 120 minutes or less, and more preferably 90 minutes or less.
In addition, the silicon sample to be evaluated during the period from the heat treatment to the hydrogen atom introduction treatment and during the period from the hydrogen atom introduction treatment to the evaluation can be placed in an air atmosphere at room temperature, unless otherwise specified. Here, room temperature can be, for example, a temperature in the range of 15° C. to 30° C.

<Hydrogen atom introduction treatment>
The silicon sample to be evaluated is subjected to a hydrogen atom introduction process after the above-mentioned heat treatment. By introducing hydrogen atoms, a trap level due to a complex (carbon-hydrogen complex) formed by carbon atoms and hydrogen atoms can be formed. The introduction of hydrogen atoms may be performed by a dry process (dry type) or a wet process (wet type, i.e., using a solution). For example, the introduction of hydrogen atoms by dry processing can be performed by ion implantation, hydrogen plasma, etc. Note that the introduction of hydrogen atoms in the present invention and this specification also includes a form in which hydrogen atoms are introduced in the state of ions or plasma.

The introduction of hydrogen atoms by wet processing can be performed by contacting (for example, immersing) the silicon sample in a solution. One example of the hydrogen atom introduction process by wet processing is a process in which the evaluation target silicon sample is contacted with hydrofluoric nitric acid (hydrofluoric nitric acid process). However, the solution to be contacted with the silicon sample is not limited to hydrofluoric nitric acid, and may be an acid solution or a base solution. Specific examples of the acid solution include solutions containing HF such as hydrofluoric acid (hydrofluoric acid aqueous solution), a mixed solution of hydrofluoric acid and nitric acid (hydrofluoric nitric acid), a mixed solution of sulfuric acid and hydrogen peroxide, a mixed solution of hydrochloric acid and hydrogen peroxide, and the like. In addition, examples of the base solution include a sodium hydroxide solution, a potassium hydroxide solution, a mixed solution of ammonia water and hydrogen peroxide, and the like. The above various solutions are preferably aqueous solutions (solutions containing water), and more preferably aqueous solutions. The acid concentration of the acid solution and the base concentration of the base solution are not particularly limited. As an example, hydrofluoric acid may be, for example, hydrofluoric acid (hydrofluoric acid aqueous solution) with an HF concentration of 1 to 25 mass %, and hydrofluoric nitric acid may be, for example, a mixed solution of nitric acid (nitric acid aqueous solution) with an _HNO3 concentration of 69 mass % and hydrofluoric acid (hydrofluoric acid aqueous solution) with an HF concentration of 50 mass %. Wet processing with each solution may be performed, for example, by contacting (for example, immersing) the evaluation target silicon sample in the solution for 1 to 10 minutes. Before and/or after contact with the solution, one or more processes such as water washing and drying may be performed as pre-processing and/or post-processing.

After the hydrogen atom introduction process, the silicon sample to be evaluated is subjected to an evaluation method that evaluates the trap levels in the silicon band gap.

<Evaluation of silicon sample after hydrogen atom introduction treatment and evaluation of carbon concentration>
In the carbon concentration evaluation method, a silicon sample to be evaluated after being subjected to a heat treatment at a heating temperature of 800° C. or more and a subsequent hydrogen atom introduction treatment is evaluated by an evaluation method for evaluating a trap level in the band gap of silicon, and the carbon concentration of the silicon sample to be evaluated is evaluated based on the evaluation result regarding the density of the trap level at Ec-0.15 eV obtained by such evaluation.

Examples of evaluation methods for evaluating the trap levels in the band gap of silicon include the DLTS method, the lifetime method, the ICTS method (Isothermal Capacitance Transient Spectroscopy), the low-temperature photoluminescence (PL) method, and the cathodoluminescence (CL) method. In addition, electron beam irradiation treatment was essential for evaluating the carbon concentration by the conventional PL method and the CL method (luminescence method). In contrast, according to the carbon concentration evaluation method, it is considered that the trap levels can be formed in an activated state by the hydrogen atom introduction treatment, so that the carbon concentration can be evaluated based on the density of the trap levels without performing the electron beam irradiation treatment. As for the measurement method using various evaluation methods, publicly known techniques can be applied. In the present invention and this specification, "no electron beam irradiation treatment" means that a treatment of actively irradiating an electron beam to a silicon sample is not performed, and electron beam irradiation that occurs unavoidably under sunlight, lighting, etc. is allowed. An electron beam is a flow of electrons obtained by applying an accelerating voltage to electrons. Electron beam irradiation processing has problems such as a long lead time, the need for large-scale equipment, increased costs, and an increase in the number of steps due to the need to create a protective oxide film in addition to the electron beam irradiation process. Therefore, it is preferable to be able to evaluate the carbon concentration of a silicon sample without performing electron beam irradiation processing. However, in one embodiment of the carbon concentration evaluation method, electron beam irradiation can also be performed by a known method.

For example, the DLTS method is a preferred evaluation method from the viewpoint of enabling more sensitive carbon quantification. When the DLTS method is used as an evaluation method, the DLTS spectrum of the trap level caused by the carbon-hydrogen complex can be separated by fitting the DLTS spectrum obtained as the sum of each peak obtained by the DLTS method using a known method. For example, in a DLTS measurement at a frequency of 250 Hz, the trap level density of Ec-0.15 eV can be measured based on the peak intensity (DLTS signal intensity) of the peak at temperatures of 90 to 110 K. The greater the peak intensity (DLTS signal intensity) of the DLTS spectrum of the trap level of Ec-0.15 eV separated by fitting processing, the higher the density of the trap level, and the higher the carbon concentration of the silicon sample to be evaluated can be determined.

In the DLTS method, a measurement sample is usually obtained by cutting out a part of a silicon sample to be evaluated, and a semiconductor junction (Schottky junction or pn junction) and an ohmic layer are formed on the measurement sample to produce a diode (sample element), and measurement (DLTS measurement) is performed. In general, it is preferable that the surface of the sample to be subjected to DLTS measurement is highly smooth. Therefore, etching, polishing, etc. can be optionally performed on the silicon sample to be evaluated before the measurement sample is cut out, or on the measurement sample cut out from the silicon sample to be evaluated, in order to improve the surface smoothness. Mirror etching is preferable for the etching. In addition, it is preferable for the polishing to include mirror polishing. For example, when the silicon sample to be evaluated is a silicon single crystal ingot or a part of an ingot, it is preferable to polish the measurement sample cut out from the silicon sample to be evaluated and then produce the sample element, and it is more preferable to polish the sample element after mirror polishing. As the polishing, a known polishing process applied to silicon wafers, such as mirror polishing, can be performed. On the other hand, silicon wafers are usually obtained through polishing processes such as mirror polishing. Therefore, when the silicon sample to be evaluated is a silicon wafer, the surface of the measurement sample cut from the silicon wafer usually has a high degree of smoothness even without polishing.

DLTS measurements are usually performed by the following method. A semiconductor junction (Schottky junction or pn junction) is formed on one surface of a silicon sample, and an ohmic layer is formed on the other surface to create a diode (sample element). The transient response of the capacitance of this sample element is measured by periodically applying a voltage while performing a temperature sweep. The voltage is usually applied by alternately and periodically applying a reverse voltage that forms a depletion layer and a forward voltage that fills the trap level in the depletion layer with carriers. The depth and width of the measurement region can be controlled by the magnitude of the reverse voltage. In one embodiment, the measurement region can be formed, for example, in a region about 1 μm to 60 μm deep from the surface of the silicon sample to be evaluated, with a width of about 1 to 50 μm, and preferably with a width of about 1 to 10 μm. The depth and width of the measurement region can be calculated by a known method from the magnitude of the reverse voltage, the magnitude of the forward voltage, and the resistivity of the silicon sample to be evaluated. On the other hand, the thickness of the silicon sample to be evaluated can be, for example, 20 μm or more and 1000 μm or less. However, it is not limited to this range. A DLTS spectrum can be obtained by plotting the DLTS signal against temperature. The DLTS spectrum obtained as the sum of each peak detected by the DLTS measurement can be fitted by a known method to separate the DLTS spectrum of each trap level and detect the peaks.

<Evaluation of carbon concentration>
In any case where any method is used as the evaluation method, the evaluation of the carbon concentration based on the evaluation result regarding the density of the trap level at Ec-0.15 eV can be performed using a calibration curve, or can be performed without using a calibration curve. When a calibration curve is not used, for example, the carbon concentration can be evaluated by a relative judgment criterion that judges that the carbon concentration is higher as the value obtained as the evaluation result is larger. For example, it can be judged that the carbon concentration is higher as the value of the peak intensity (DLTS signal intensity) of the DLTS spectrum is larger. In addition, when a calibration curve is used, it is preferable to create a calibration curve that shows the correlation between the trap level density and the known carbon concentration obtained from the evaluation result (for example, DLTS signal intensity) obtained for the silicon sample to be evaluated. The relational equation for obtaining the trap level density from various evaluation results is publicly known. In addition, the above-mentioned known carbon concentration can be obtained by measuring by a method other than the evaluation method used for the evaluation of the silicon sample to be evaluated. For example, when the evaluation target silicon sample is evaluated by the DLTS method, the above-mentioned known carbon concentration can be obtained, for example, by the SIMS method (Secondary Ion Mass Spectrometry) or the FT-IR method (Fourier Transform Infrared Spectroscopy). A relational expression for obtaining a carbon concentration from the evaluation results obtained by these methods is also known. The silicon sample (silicon sample for creating a calibration curve) that is subjected to evaluation by the same evaluation method as the evaluation target silicon sample in order to create a calibration curve and the silicon sample for obtaining a known carbon concentration are preferably silicon samples cut from the same silicon sample (for example, the same ingot, the same wafer, etc.) or silicon samples that have undergone the same manufacturing process. For the creation of the calibration curve, reference can also be made to paragraphs 0038 to 0040 of Patent Document 1 (JP Patent Publication No. 2017-191800). The silicon sample for preparing the calibration curve is preferably a silicon sample that has been subjected to various treatments such as heat treatment and hydrogen atom introduction treatment in the same manner as the silicon sample to be evaluated.

[Method for evaluating silicon wafer manufacturing process and method for manufacturing silicon wafer]
One aspect of the present invention relates to a method for evaluating a silicon wafer manufacturing process, the method including: evaluating a carbon concentration of a silicon wafer manufactured in a silicon wafer manufacturing process to be evaluated by the carbon concentration evaluation method; and evaluating a degree of carbon contamination in the silicon wafer manufacturing process to be evaluated based on a result of the evaluation.

Another aspect of the present invention relates to a method for manufacturing silicon wafers, including evaluating a silicon wafer manufacturing process using the above-mentioned method for evaluating a silicon wafer manufacturing process, and manufacturing silicon wafers in a silicon wafer manufacturing process after performing a carbon contamination reduction process in the silicon wafer manufacturing process in which the degree of carbon contamination is determined to be at an acceptable level as a result of the above evaluation, or in which the degree of carbon contamination is determined to exceed the acceptable level as a result of the above evaluation.

The silicon wafer manufacturing process to be evaluated in the above manufacturing process evaluation method can be some or all of the processes for manufacturing product silicon wafers. The manufacturing process for product silicon wafers generally includes cutting out wafers from silicon single crystal ingots (slicing), surface treatments such as polishing and etching, cleaning processes, and further post-processes (epitaxial layer formation, etc.) that are performed as necessary depending on the application of the wafer. All of these steps and treatments are publicly known.

In the manufacturing process of silicon wafers, carbon contamination may occur in the silicon wafers due to contact between the silicon wafers and the components used in the manufacturing process. By evaluating the carbon concentration of the silicon wafers manufactured in the manufacturing process to be evaluated and grasping the degree of carbon contamination, it is possible to grasp the tendency of carbon contamination to occur in the product silicon wafers due to the manufacturing process to be evaluated. That is, it can be determined that the higher the carbon concentration of the silicon wafers manufactured in the manufacturing process to be evaluated, the more likely carbon contamination is to occur in the manufacturing process to be evaluated. Therefore, for example, if an allowable level of carbon concentration is set in advance and the carbon concentration obtained for the silicon wafers manufactured in the manufacturing process to be evaluated exceeds the allowable level, it can be determined that the manufacturing process to be evaluated has a high tendency of carbon contamination occurrence and cannot be used as a manufacturing process for product silicon wafers. It is preferable that the manufacturing process to be evaluated that is determined in this way is used for manufacturing product silicon wafers after performing a carbon contamination reduction treatment. Details of this point will be described further below.

The carbon concentration of the silicon wafer produced in the silicon wafer manufacturing process to be evaluated is determined by the carbon concentration evaluation method according to one aspect of the present invention described above. The silicon wafer produced in the silicon wafer manufacturing process to be evaluated can be a silicon wafer into which nitrogen has been introduced at any stage. As described above, for FZ silicon single crystals, nitrogen is usually introduced into the silicon single crystal ingot during crystal growth. For CZ silicon single crystals, as described above, nitrogen may be introduced by performing a nitrogen doping process at the stage of melting the raw material. For the nitrogen doping process, known techniques can be applied. Details of the carbon concentration evaluation method are as described above. The silicon wafer to be subjected to the carbon concentration evaluation is at least one silicon wafer produced in the silicon wafer manufacturing process to be evaluated, and may be two or more silicon wafers. When the carbon concentrations of two or more silicon wafers are determined, for example, the average value, maximum value, etc. of the determined carbon concentrations can be used to evaluate the silicon wafer manufacturing process to be evaluated. In addition, the silicon wafer may be subjected to carbon concentration evaluation in the wafer form as it is, or a portion of the wafer may be cut out and subjected to carbon concentration evaluation. When two or more samples are cut out from one silicon wafer and subjected to carbon concentration evaluation, the average value, maximum value, etc. of the carbon concentration determined for the two or more samples can be determined as the carbon concentration of the silicon wafer.

In one embodiment of the silicon wafer manufacturing method, the silicon wafer manufacturing process is evaluated by the manufacturing process evaluation method, and a silicon wafer is manufactured in a silicon wafer manufacturing process in which the degree of carbon contamination is determined to be at an acceptable level as a result of the evaluation. This makes it possible to ship high-quality silicon wafers with a low carbon contamination level as product wafers. In another embodiment of the silicon wafer manufacturing method, the silicon wafer manufacturing process is evaluated by the manufacturing process evaluation method, and a carbon contamination reduction process is performed on a silicon wafer manufacturing process in which the degree of carbon contamination is determined to exceed the acceptable level as a result of the evaluation, and then a silicon wafer is manufactured in this silicon wafer manufacturing process. This makes it possible to reduce carbon contamination caused by the manufacturing process, and therefore makes it possible to ship high-quality silicon wafers with a low carbon contamination level as product wafers. The above-mentioned acceptable level can be set appropriately according to the quality required for the product wafer. In addition, examples of the carbon contamination reduction process include replacement and cleaning of members included in the silicon wafer manufacturing process. As an example, when a SiC susceptor is used as a susceptor, which is a member on which a silicon wafer is placed, during the manufacturing process of silicon wafers, the susceptor may deteriorate after repeated use, causing carbon contamination of the contact area with the susceptor. In such cases, the carbon contamination caused by the susceptor can be reduced by, for example, replacing the susceptor.

[Method for manufacturing silicon single crystal ingot]
One aspect of the present invention relates to a method for producing a silicon single crystal ingot, including growing a silicon single crystal ingot, evaluating the carbon concentration of a silicon sample cut from the silicon single crystal ingot by the carbon concentration evaluation method, determining production conditions for the silicon single crystal ingot based on results of the evaluation, and growing the silicon single crystal ingot under the determined production conditions.

The growth of silicon single crystal ingots can be performed by known methods such as the FZ method and the CZ method. For example, FZ silicon single crystal ingots produced by the FZ method may be contaminated with carbon due to carbon contamination in the silicon raw material, deterioration of carbon-containing material components constituting the single crystal production equipment, and carbon intake from the atmospheric gas. In addition, silicon single crystal ingots grown by the CZ method may be contaminated with carbon due to carbon contamination in the raw polysilicon and CO gas generated during growth. It is preferable to evaluate such contaminated carbon concentration and determine the production conditions based on the evaluation results in order to produce silicon single crystal ingots with reduced carbon contamination. For this purpose, the carbon concentration evaluation method according to one aspect of the present invention described above is suitable as a method for evaluating the contaminated carbon concentration.

For details of the shape of the silicon sample cut out from the silicon single crystal ingot, etc., the above description of the silicon sample to be evaluated in the carbon concentration evaluation method can be referred to. The number of silicon samples to be subjected to the carbon concentration evaluation is at least one, and may be two or more. When the carbon concentrations of two or more silicon samples are obtained, for example, the average value, maximum value, etc. of the obtained carbon concentrations can be used to determine the manufacturing conditions of the silicon single crystal ingot. For example, when the obtained carbon concentration is at a predetermined allowable level, a silicon single crystal ingot with less carbon contamination can be manufactured by growing the silicon single crystal ingot under the manufacturing conditions when the silicon single crystal ingot cut out from the silicon sample with the evaluated carbon concentration was grown. On the other hand, for example, when the obtained carbon concentration exceeds the allowable level, a silicon single crystal ingot with less carbon contamination can be manufactured by growing the silicon single crystal ingot under the manufacturing conditions determined by adopting a means for reducing the carbon concentration. As a means for reducing carbon contamination, for example, for the CZ method, one or more of the following means (1) to (3) can be adopted. For example, with respect to the FZ method, one or more of the following means (4) to (6) can be adopted.
(1) Use high-grade polysilicon with less carbon contamination as the raw material polysilicon.
(2) Properly adjusting the pulling speed and/or the argon (Ar) gas flow rate during crystal pulling in order to suppress dissolution of CO into the polysilicon melt.
(3) Making design changes or changing the mounting position of carbon components contained in the lifting device.
(4) Use high-grade silicon raw materials with less carbon contamination.
(5) Increasing the flow rate of gas introduced into the single crystal manufacturing apparatus suppresses the intake of carbon from the atmospheric gas.
(6) Replacing components made of carbon-containing materials contained in the single crystal manufacturing equipment, changing the design of the components, changing the installation position, etc.

Thus, according to one aspect of the present invention, it is possible to provide silicon single crystal ingots and silicon wafers with low carbon concentrations.

An example of growing a silicon single crystal ingot by the FZ method will be described below with reference to Fig. 1. However, the present invention is not limited to the example shown below.
The silicon single crystal manufacturing apparatus 10 shown in FIG. 1 is a silicon single crystal manufacturing apparatus using the FZ method.
The silicon single crystal manufacturing apparatus 10 shown in FIG. 1 has a furnace 13 that accommodates a silicon raw material rod 11 and a rod-shaped silicon single crystal 12 after growth.
Examples of the silicon raw material rod 11 include a silicon polycrystalline rod, a partially grown FZ silicon single crystal rod, a silicon single crystal rod produced by the CZ method, etc. In the furnace 13, a ring-shaped induction heating coil 14 having a slit serving as a heat source for forming a floating zone 12a between the silicon raw material rod 11 and the silicon single crystal 12 is provided. An inert gas is supplied into the furnace 13 from a gas supply port 16.
In addition, in the furnace 13, an upper holding jig 17 for holding the silicon raw material rod 11, a lower holding jig 19 for holding the seed crystal 18, an upper shaft 21 for vertically moving and rotating the silicon raw material rod 11, and a lower shaft 22 for vertically moving and rotating the grown rod-shaped silicon single crystal 12 are provided. Furthermore, a gas exhaust port 24 for discharging gas in the furnace is provided at the top of the furnace 13. In addition, in the silicon single crystal manufacturing apparatus 10 shown in FIG. 1, the upper shaft 21 which is the rotation center of the silicon raw material rod 11 and the lower shaft 22 which is the rotation center of the silicon single crystal 12 are provided on the same axis, but the upper shaft 21 and the lower shaft 22 can be shifted (i.e., eccentric) to grow a single crystal. By shifting the upper shaft 21 and the lower shaft 22, the molten part is stirred during single crystallization, so that the uniformity of the quality of the manufactured single crystal can be improved. The amount of eccentricity can be appropriately set according to the diameter of the single crystal.
A gas supply port 16 for supplying an inert gas is provided on the furnace wall near the induction heating coil 14, for example, on the furnace wall at approximately the same level as the induction heating coil 14. The inert gas introduced into the furnace from this gas supply port 16 is supplied not only to the slits but also to the red-hot silicon raw material rods 11 near the floating zone.
When manufacturing an FZ silicon single crystal ingot by the FZ method using the silicon single crystal manufacturing apparatus 10 shown in Fig. 1, first, the portion of the silicon feed rod 11 where melting starts is machined into a cone shape, and the surface is etched to remove processing distortion. Then, the silicon feed rod 11 is placed in a furnace 13, and the silicon feed rod 11 is fixed to an upper holding jig 17 of an upper shaft 21 installed in the furnace 13 with screws or the like. Meanwhile, a seed crystal 18 made of silicon single crystal and having a target crystal orientation is attached to a lower holding jig 19 of a lower shaft 22.
Next, while introducing an inert gas from the gas supply port 16 into the furnace 13, a high-frequency current is applied to the induction heating coil 14 by the high-frequency oscillator 25 to melt one end of the silicon feed rod 11, and the seed crystal 18 is fused to the melt. After that, the seed is drawn to form a narrowed portion 15 to eliminate dislocations, and the molten silicon is solidified to grow a single crystal. While rotating the upper shaft 21 and the lower shaft 22 in opposite directions, the silicon feed rod 11 is moved relative to the induction heating coil 14 in the axial direction, and the molten portion is gradually moved from the fused portion toward the other end of the silicon feed rod 11 to form a single crystal. Next, the silicon feed rod 11 and the silicon single crystal 12 thus produced are lowered to form a floating zone 12a between the silicon feed rod 11 and the rod-shaped silicon single crystal 12, and the floating zone 12a is moved to the other end of the silicon feed rod 11 at a constant speed to grow the silicon single crystal 12.

Next, an example of growing a silicon single crystal ingot by the CZ method will be described with reference to Fig. 2. However, the present invention is not limited to the following example.
FIG. 2 is an explanatory diagram showing the configuration of a silicon single crystal pulling apparatus using the CZ method.
The silicon single crystal pulling apparatus 30 shown in FIG. 2 includes a chamber 41, a support rotation shaft 42 provided vertically penetrating the center of the bottom of the chamber 41, a graphite susceptor 43 fixed to the upper end of the support rotation shaft 42, a quartz crucible 44 housed in the graphite susceptor 43, a heater 45 provided around the graphite susceptor 43, a support shaft drive mechanism 46 for raising and lowering and rotating the support rotation shaft 42, a seed chuck 47 for holding a seed crystal, a pulling wire 48 for suspending the seed chuck 47, a wire winding mechanism 49 for winding up the pulling wire 48, a heat shielding member 52 for preventing heating of the silicon single crystal ingot 50 due to radiant heat from the heater 45 and the quartz crucible 44 and for suppressing temperature fluctuations in the silicon melt 51, and a control device 53 for controlling each part.
A gas inlet 54 for introducing an inert gas into the chamber 41 is provided at the top of the chamber 41. The inert gas is introduced into the chamber 41 from the gas inlet 54 via a gas pipe 55, and the amount of the inert gas introduced is controlled by a conductance valve 56.
A gas exhaust port 57 is provided at the bottom of the chamber 41 for exhausting the inert gas within the chamber 41. The inert gas within the sealed chamber 41 is exhausted to the outside from the gas exhaust port 57 via an exhaust gas pipe 58. A conductance valve 59 and a vacuum pump 60 are provided midway along the exhaust gas pipe 58, and a reduced pressure state within the chamber 41 is maintained by sucking in the inert gas within the chamber 41 with the vacuum pump 60 while controlling the flow rate with the conductance valve 59.
Furthermore, a magnetic field supplying device 61 for applying a magnetic field to the silicon melt 51 is provided outside the chamber 41. The magnetic field supplied from the magnetic field supplying device 61 may be a horizontal magnetic field or a cusp magnetic field.

The present invention will be further described below with reference to examples. However, the present invention is not limited to the embodiments shown in the examples. The following treatments and operations were carried out in an atmospheric environment at room temperature unless otherwise specified.

[Preparation of FZ silicon single crystal samples]
A wafer-shaped sample was cut out from a silicon single crystal ingot (n-type silicon) grown in an atmosphere containing nitrogen gas by the FZ method, and processed into a silicon wafer (diameter 200 mm) by mirror polishing and other processing. The resistivity was 50 Ω cm. A plurality of silicon samples were cut out from this silicon wafer, one of which was used as a silicon sample for FT-IR measurement, and the remaining plurality of silicon samples were used as silicon samples for DLTS measurement in Example 1 and Comparative Examples 1 to 3, respectively.

[Measurement by FT-IR method]
The silicon sample for FT-IR measurement was subjected to FT-IR measurement to measure the carbon concentration, oxygen concentration and nitrogen concentration. The measurement results were as follows.
Carbon concentration: <2E+15atoms/ ^cm3
Oxygen concentration: <1E+16 atoms/ ^cm3
Nitrogen concentration: 4E+14atoms/ ^cm3

[Example 1]
The DLTS measurement silicon sample (FZ silicon single crystal sample) prepared above was subjected to the following treatments (A), (B), (C) and (D) in order. In the following (B), a hydrogen atom introduction treatment was performed on the silicon sample after the heat treatment in the following (A). In the following (C), a Schottky junction was formed on one surface of the silicon sample after the hydrogen atom introduction treatment, and in the following (D), an ohmic layer (Ga/In layer) was formed on the other surface to fabricate a diode.

(A) Heating in a heating furnace at a heating temperature of 900°C for 60 minutes; (B) Immersing the silicon surface of the sample in hydrofluoric nitric acid (a mixed solution of nitric acid (nitric acid aqueous solution) with a _HNO3 concentration of 69% by mass and hydrofluoric acid (hydrofluoric acid aqueous solution) with a HF concentration of 50% by mass) for 5 minutes; (C) Forming a Schottky electrode (Au electrode) on the silicon surface of the sample by vacuum deposition; (D) Forming a backside ohmic layer (Ga/In) by rubbing in gallium and indium.

A reverse voltage to form a depletion layer and a forward voltage to capture carriers in the depletion layer were applied alternately and periodically to the Schottky junction of the diode fabricated above. The reverse voltage was -2 V and the forward voltage was 0 V. This allows the DLTS measurement region to be a region 2.6 to 6.0 μm deep from the surface of the silicon sample. The transient response of the diode capacitance generated in response to the above voltage was measured.
The voltage application and capacitance measurement were performed while sweeping the sample temperature in a predetermined temperature range. The DLTS signal intensity ΔC was plotted against the temperature to obtain a DLTS spectrum. The measurement frequency was 250 Hz.
The obtained DLTS spectrum was subjected to fitting processing (True shape fitting processing) using a program manufactured by SEMILAB, and a DLTS spectrum having a peak at 90 to 110 K was separated. The DLTS spectrum thus obtained is shown in FIG.
In the DLTS spectrum shown in Fig. 3, a trap level caused by a carbon-hydrogen complex near 101K, i.e., a trap level peak at Ec-0.15 eV, can be clearly confirmed. The trap level density was calculated from the DLTS signal intensity at 101K using a known relational expression. Furthermore, the carbon concentration was calculated from the trap level density using a separately prepared calibration curve, and was found to be 2.9E+14 atoms/ ^cm3 .

[Comparative Example 1]
The DLTS measurement silicon sample (FZ silicon single crystal sample) prepared above was subjected to the same treatment and operation as described in Example 1, except that the heat treatment (A) above was not performed. FIG. 4 shows the DLTS spectrum obtained in Comparative Example 1.
In the DLTS spectrum shown in Fig. 4, a large peak of E+11 to E+12/ ^cm3, which is the level density of nitrogen-related defects, is observed near 98 K, and as a result, the peak of the trap level caused by the carbon-hydrogen complex near 101 K (peak of about 1E+10/cm3 as shown in Fig. ³ ) cannot be identified. In addition, there is a possibility that the peak caused by the nitrogen-related defects will be erroneously recognized as the peak of the trap level caused by the carbon-hydrogen complex.
The trap level density was calculated from the DLTS signal intensity at the peak position near 98K using a known relational expression, and the carbon concentration was calculated from the trap level density using a separately prepared calibration curve, and was found to be 2.0E+16 atoms/ ^cm3 .
From the above results, it was confirmed that the carbon concentration in Comparative Example 1 was overestimated.

[Comparative Example 2]
The DLTS measurement silicon sample (FZ silicon single crystal sample) prepared above was subjected to the same treatment and operation as described in Example 1, except that the heating temperature of the heat treatment (A) above was changed to 500° C. Figure 5 shows the DLTS spectrum obtained in Comparative Example 2.
In the DLTS spectrum shown in Fig. 5, a large peak of E+11 to E+12/cm3, which is the level density of nitrogen-related defects, is observed near 98 K, as in the DLTS spectrum shown in Fig. ⁴ , and as a result, the peak of the trap level caused by the carbon-hydrogen complex near 101 K (the peak of about 1E10/ ^cm3 as in Fig. 3) cannot be identified. Also in the DLTS spectrum shown in Fig. 5, as in the DLTS spectrum shown in Fig. 4, there is a possibility that the peak caused by the nitrogen-related defects may be erroneously recognized as the peak of the trap level caused by the carbon-hydrogen complex.

[Comparative Example 3]
The DLTS measurement silicon sample (FZ silicon single crystal sample) prepared above was subjected to the same treatment and operation as described in Example 1, except that the hydrogen atom introduction treatment (B) above was not performed. FIG. 6 shows the DLTS spectrum obtained in Comparative Example 3.
Comparative Example 3 is a comparative example in which hydrogen atom introduction treatment was not performed after the heat treatment. As shown in Fig. 6, no peak was observed in the DLTS spectrum. In Comparative Example 3, since hydrogen atom introduction treatment was not performed, a trap level due to a carbon-hydrogen complex was not formed, and therefore, it is considered that no peak appeared near 101K.

[Preparation of CZ silicon single crystal samples]
A wafer-shaped sample was cut out from a nitrogen-doped CZ silicon single crystal ingot (n-type silicon) grown by the CZ method, which had been subjected to nitrogen doping treatment at the raw material melting stage, and processed into a nitrogen-containing silicon wafer (diameter 300 mm) by mirror polishing and other processing. The resistivity was 50 Ω cm. A plurality of silicon samples were cut out from this silicon wafer, one of which was used as a silicon sample for FT-IR measurement, and the remaining plurality of silicon samples were used as silicon samples for DLTS measurement in Example 2, Comparative Example 4, and Comparative Example 5, respectively.

[Measurement by FT-IR method]
The silicon sample for FT-IR measurement was subjected to FT-IR measurement to measure the carbon concentration, oxygen concentration and nitrogen concentration. The measurement results were as follows.
Carbon concentration: <2E+15atoms/ ^cm3
Oxygen concentration: 3.91E+17atoms/ ^cm3
Nitrogen concentration: 1.19E+14 atoms/ ^cm3

[Example 2]
The silicon sample for DLTS measurement (CZ silicon single crystal sample) prepared above was subjected to the same treatment and operation as described in Example 1, except that the heating temperature of the heat treatment (A) above was changed to 1100°C.

[Comparative Example 4]
The silicon sample for DLTS measurement (CZ silicon single crystal sample) prepared above was subjected to the same treatments and operations as described in Example 1, except that the heat treatment (A) above was not performed.

[Comparative Example 5]
The DLTS measurement silicon sample (CZ silicon single crystal sample) prepared above was subjected to the same treatment and operation as described in Example 1, except that the heating temperature of the heat treatment (A) above was changed to 1100° C. and the hydrogen atom introduction treatment (B) above was not performed. That is, Comparative Example 5 is a comparative example in which the hydrogen atom introduction treatment was not performed after the heat treatment.

Fig. 7 shows the DLTS spectra obtained in Example 2, Comparative Example 4, and Comparative Example 5. In Fig. 7, "au" indicates an arbitrary unit.
From the results shown in FIG. 7, the following points can be confirmed.
The peak disappears once by heat treatment at a heating temperature of 1100°C (Comparative Example 5), and then hydrogen atom introduction treatment makes it possible to separate only the peak of the trap level caused by the carbon-hydrogen complex at Ec-0.15 eV (Example 2). In Comparative Example 4, the 98 K peak of the nitrogen-related defect level overlaps, and it is not possible to accurately detect only the peak of the trap level (near 101 K) caused by the carbon-hydrogen complex. The peak seen near 87 K is a level caused by the COH complex, and appears when oxygen is contained in the crystal (it does not appear in FZ silicon single crystals because the oxygen concentration in the crystal is usually very low).

The present invention is useful in the technical fields of silicon single crystal ingots and silicon wafers.

Claims (8)

performing a hydrogen atom introduction process for introducing hydrogen atoms into the silicon sample to be evaluated;
subjecting the evaluation target silicon sample after the hydrogen atom introduction treatment to an evaluation method for evaluating a trap level in a band gap of silicon, and evaluating a carbon concentration of the evaluation target silicon sample based on an evaluation result regarding a density of the trap level at Ec-0.15 eV obtained by the evaluation;
Including,
The method for evaluating a carbon concentration of a silicon sample further comprises performing a heat treatment of heating the silicon sample to a heating temperature of 800° C. or higher before the hydrogen atom introduction treatment, and the silicon sample to be evaluated is a nitrogen-containing silicon sample. The carbon concentration evaluation method for a silicon sample according to claim 1, wherein the evaluation method is the DLTS method. The method for evaluating the carbon concentration of a silicon sample according to claim 1 or 2, wherein the hydrogen atom introduction process includes contacting the silicon sample to be evaluated with hydrofluoric nitric acid. The method for evaluating the carbon concentration of a silicon sample according to any one of claims 1 to 3, wherein the silicon sample to be evaluated is an FZ silicon single crystal sample. The method for evaluating the carbon concentration of a silicon sample according to any one of claims 1 to 3, wherein the silicon sample to be evaluated is a CZ silicon single crystal sample. Evaluating the carbon concentration of a silicon wafer manufactured in a silicon wafer manufacturing process to be evaluated by the carbon concentration evaluation method according to any one of claims 1 to 5, and evaluating the degree of carbon contamination in the silicon wafer manufacturing process to be evaluated based on the results of the evaluation;
A method for evaluating a silicon wafer manufacturing process, comprising: Evaluating a silicon wafer manufacturing process by the evaluation method according to claim 6, and manufacturing silicon wafers in a silicon wafer manufacturing process in which the degree of carbon contamination is determined to be at an acceptable level as a result of the evaluation, or in a silicon wafer manufacturing process in which the degree of carbon contamination is determined to exceed the acceptable level as a result of the evaluation, after performing a carbon contamination reduction treatment on the silicon wafer manufacturing process;
A method for manufacturing a silicon wafer, comprising: Growing a silicon single crystal ingot;
Evaluating the carbon concentration of a silicon sample cut out from the silicon single crystal ingot by the carbon concentration evaluation method according to any one of claims 1 to 5;
Determining manufacturing conditions for a silicon single crystal ingot based on the results of the evaluation; and
growing a silicon single crystal ingot under the determined production conditions;
A method for producing a silicon single crystal ingot, comprising: