Method for evaluating quartz crucible

Abstract

To provide a method for evaluating a quartz crucible that, in evaluating a sample cut out from a quartz crucible to be evaluated by the Raman spectroscopy, can suppress fluorescent light generated from the sample and calculate the degree of crystallinity of the sample accurately and simply in a short time.SOLUTION: A method for evaluating a quartz crucible by the Raman spectroscopy includes: a sample cut-out step of cutting out a sample from a quartz crucible to be evaluated; a heat treatment step of performing heat treatment on the cut-out sample; a Raman mapping step of acquiring, by the Raman spectroscopy, Raman spectra at a plurality of points of the sample after the heat treatment; and a step of calculating the degree of crystallinity of the sample from the plurality of Raman spectra acquired in the Raman mapping step. A condition for the heat treatment in the heat treatment step is a condition for reducing fluorescent light generated from the sample in acquiring the Raman spectra in the Raman mapping step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating a quartz crucible by Raman spectroscopy. [Background technology]

[0002] In the production of silicon single crystals using the Czochralski method, silicon melt is generally placed in a quartz crucible whose inner surface is made of amorphous SiO2. During the production of silicon single crystals, the quartz crucible reacts with the silicon melt, forming a crystalline SiO2 cristobalite crystal layer on the inner surface of the quartz crucible that comes into contact with the silicon melt. Over time while the silicon single crystal is being pulled, this cristobalite crystal layer spreads on the inner surface of the quartz crucible and also grows inward, producing brown ring-shaped spots called brown rings.

[0003] The formed cristobalite crystal layer is prone to peeling off from the inner surface of the quartz crucible, and the cristobalite crystal layer that peels off during the pulling of the silicon single crystal may become liberated from the quartz crucible into the silicon melt and reach the silicon single crystal growth interface during the pulling process, causing dislocations in the silicon single crystal. Therefore, an evaluation technique that can calculate the content of the cristobalite crystal layer on the inner surface of the quartz crucible is important. This content indicates the degree of crystallization of the inner surface of the quartz crucible, and is hereinafter also referred to as the crystallinity.

[0004] Patent Document 1 discloses an evaluation technique for obtaining Raman spectra of a quartz crucible before and after pulling a single crystal, and determining whether the silica glass crucible is good or bad and capable of suppressing the occurrence of dislocations when pulling a silicon single crystal.

[0005] Furthermore, Patent Document 2 discloses a quartz crucible and a method for producing a silicon single crystal, which can pull up a silicon single crystal without generating dislocations by calculating a value obtained by dividing the sum of peak intensities derived from a crystalline structure in a Raman spectrum by the sum of peak intensities derived from an amorphous structure and using a quartz crucible containing a crystalline structure and an amorphous structure in a predetermined ratio. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-119619 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-142047 Summary of the Invention [Problem to be solved by the invention]

[0007] However, Patent Document 1 does not disclose a method for calculating the content (crystallinity) of the cristobalite crystal layer.

[0008] Furthermore, the method described in Patent Document 2 obtains an averaged Raman spectrum within the laser light irradiation area, so it is not possible to calculate the content (crystallinity) of the cristobalite crystal layer, nor is it possible to calculate the distribution of the crystallized area on the inner surface of the quartz crucible.

[0009] Here, since the cristobalite crystal layer (hereinafter sometimes simply referred to as cristobalite) grows not only in the planar direction but also in the thickness direction, in order to accurately grasp the crystallization state of the inner surface of the quartz crucible, a method is required that can visualize the crystallized region on the surface of the quartz crucible.

[0010] One such evaluation method is multi-point measurement using a Raman microscope (Raman mapping). Raman measurements are usually performed without pretreatment, but during Raman measurements, excitation by laser light can cause fluorescence from the evaluation sample, which can interfere with the Raman scattering of the target substance.

[0011] Even when measuring a quartz crucible, if the evaluation sample contains point defects that act as fluorescence centers, laser irradiation can cause fluorescence, obscuring the cristobalite peak and making it difficult to identify the crystal structure. When such fluorescence is detected, the measurement point can be irradiated with a laser for a certain period of time to induce fluorescence fading before performing Raman measurement. This reduces the amount of fluorescence during measurement and reveals the Raman scattered light of the target substance. However, Raman mapping has the problem of poor throughput due to the large number of measurement points, and prolonged irradiation can consume the life of the laser lamp.

[0012] Patent Documents 1 and 2 do not describe a method for solving such problems.

[0013] The present invention has been made to solve the above problems, and aims to provide a method for evaluating a quartz crucible that can suppress fluorescence emitted from a sample cut from the quartz crucible to be evaluated and calculate the crystallinity of the sample accurately, simply, and in a short time when the sample is evaluated by Raman spectroscopy. [Means for solving the problem]

[0014] In order to solve the above problems, the quartz crucible evaluation method of the present invention is a method for evaluating a quartz crucible using Raman spectroscopy, and includes a sample cutting step of cutting a sample from the quartz crucible to be evaluated, a heat treatment step of heat treating the cut sample, a Raman mapping step of obtaining Raman spectra at multiple locations on the heat-treated sample using Raman spectroscopy, and a step of calculating the crystallinity of the sample from the multiple Raman spectra obtained in the Raman mapping step, wherein the heat treatment conditions in the heat treatment step are set to conditions that reduce the fluorescence generated from the sample when obtaining the Raman spectrum in the Raman mapping step.

[0015] In this method of evaluating a quartz crucible, a sample cut from the quartz crucible to be evaluated is pre-heat-treated under conditions that reduce the fluorescence emitted from the sample when a Raman spectrum is acquired. This makes it possible to suppress the fluorescence when a Raman spectrum is acquired from the heat-treated sample, and it is possible to accurately acquire Raman spectra at multiple locations on the sample. As a result, the crystallinity of the sample can be accurately, simply, and quickly calculated from these accurate multiple Raman spectra.

[0016] Furthermore, it is preferable that the sample to be measured is cut out from a quartz crucible after the silicon single crystal has been produced.

[0017] If such a sample is used, the crystallization of the inner surface of the quartz crucible progresses during the silicon single crystal manufacturing process, and therefore the degree of crystallinity after manufacturing of the silicon single crystal can be calculated accurately, simply, and in a short time.

[0018] Furthermore, the heat treatment step preferably involves performing heat treatment at 300° C. to 1000° C. for 20 minutes or longer on a plurality of locations on the sample where the Raman spectrum is to be acquired.

[0019] By performing this type of heat treatment, the point defects that act as fluorescence centers can be eliminated without causing the sample to undergo a phase transition from an amorphous structure to cristobalite, thereby suppressing the fluorescence emitted from the sample when acquiring a Raman spectrum.

[0020] In addition, the step of calculating the degree of crystallinity preferably involves analyzing and imaging the multiple Raman spectra obtained in the Raman mapping step, and calculating the degree of crystallinity from the area ratio of cristobalite regions in the image obtained by the imaging.

[0021] With this calculation method, by analyzing and imaging the multiple Raman spectra acquired in the Raman mapping step, a Raman image can be obtained that visualizes the distribution of amorphous regions and cristobalite regions that have undergone a phase transition from the amorphous state, and the degree of crystallinity can be calculated accurately, simply, and in a short time from the area ratio of the cristobalite regions in the Raman image. [Effects of the Invention]

[0022] In the quartz crucible evaluation method of the present invention, a sample cut from the quartz crucible to be evaluated is pre-heat-treated under conditions that reduce the fluorescence emitted from the sample when a Raman spectrum is acquired. This makes it possible to suppress the fluorescence when acquiring a Raman spectrum from the heat-treated sample, and allows accurate acquisition of Raman spectra at multiple locations on the sample. As a result, the crystallinity of the sample can be accurately, simply, and quickly calculated from these accurate multiple Raman spectra. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a flowchart showing an example of a method for evaluating a quartz crucible according to the present invention. [Figure 2] FIG. 1 is a schematic diagram of a Raman microscope that can be used for the Raman spectroscopy used in the present invention. [Figure 3] 1 shows a Raman spectrum obtained in Example 1. [Figure 4] 1 is a Raman spectrum obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will be described in detail below, but the present invention is not limited thereto.

[0025] As mentioned above, when evaluating a sample cut from a quartz crucible to be evaluated using Raman spectroscopy, there was a need for a method of evaluating a quartz crucible that could suppress the fluorescence emitted from the sample and accurately, easily, and quickly calculate the crystallinity of the sample.

[0026] The inventors have conducted extensive research into the above-mentioned problems and have discovered a method for accurately, easily, and quickly calculating the crystallinity of a sample by heat-treating the cut-out quartz crucible sample and then evaluating it using Raman spectroscopy, thereby completing the present invention.

[0027] In other words, the quartz crucible evaluation method of the present invention is a quartz crucible evaluation method using Raman spectroscopy, and includes a sample cutting step of cutting a sample from the quartz crucible to be evaluated, a heat treatment step of heat treating the cut sample, a Raman mapping step of obtaining Raman spectra at multiple locations on the heat-treated sample using Raman spectroscopy, and a step of calculating the crystallinity of the sample from the multiple Raman spectra obtained in the Raman mapping step, wherein the heat treatment conditions in the heat treatment step are set to conditions that reduce the fluorescence generated from the sample when obtaining the Raman spectrum in the Raman mapping step.

[0028] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto.

[0029] Figure 1 is a flowchart showing an example of a method for evaluating a quartz crucible according to the present invention. The method includes a sample cutting step (Fig. 1(a)) in which a sample is cut from a quartz crucible to be evaluated, a heat treatment step (Fig. 1(b)) in which the cut sample is heat-treated, a Raman mapping step (Fig. 1(c)) in which Raman spectra are obtained at multiple locations on the heat-treated sample by Raman spectroscopy, and a step (Fig. 1(d)) in which the crystallinity of the sample is calculated from the multiple Raman spectra obtained in the Raman mapping step. The heat treatment step is performed under conditions that reduce the fluorescence emitted from the sample when a Raman spectrum is obtained in the Raman mapping step. These conditions will be explained in detail below when describing each step.

[0030] "Sample cutting process" This is the process of cutting out the sample to be measured from the quartz crucible.

[0031] Although not particularly limited, it is more preferable to use a material cut from a quartz crucible after the production of a silicon single crystal, as this allows for accurate, simple, and quick calculation of the degree of crystallinity of the quartz crucible that has been achieved through the production of the silicon single crystal.

[0032] Specifically, the quartz crucible used to pull the silicon single crystal can be cut out using a band saw at a position below the melt line, where no residual silicon melt is attached, and where the quartz crucible surface is exposed. The cut size is not particularly limited as long as it is large enough to fit into the sample chamber of the Raman microscope in the Raman mapping step, but when the sample is heated using a hot plate in the heat treatment step, it is preferable to cut out a piece with a thickness of 3 mm or less so that the temperature of the sample surface can be sufficiently increased.

[0033] "Heat treatment process" Next, the cut sample is subjected to a heat treatment.

[0034] Although not particularly limited, it is preferable to perform heat treatment at 300° C. to 1000° C. for 20 minutes or longer on a plurality of locations on the sample where Raman spectra are to be obtained.

[0035] Experiments have confirmed that by heating the sample under these conditions, fluorescence can be reduced and no phase transition occurs. The reason for the reduction in fluorescence is thought to be that the point defects that act as fluorescence centers disappear during heat treatment. On the other hand, while it is thought that amorphous SiO2 may undergo a phase transition from amorphous to cristobalite at high temperatures, no phase transition occurs under the above conditions.

[0036] The heating time is not particularly limited, but can be 20 minutes or more as described above, and since it is better not to make the time too long when productivity is taken into consideration, it can be, for example, 1 hour or less.

[0037] The device for performing the heat treatment is not particularly limited, but it is preferable to use a hot plate in consideration of ease of handling.

[0038] "Raman mapping process" Next, Raman spectra are obtained at multiple locations on the heat-treated sample by Raman spectroscopy.

[0039] First, Raman mapping is performed. Figure 2 is a schematic diagram of a Raman microscope 1. As shown in Figure 2, an optical microscope image of the surface of a cut-out quartz crucible sample 2 is acquired using an observation camera 3, and the size of the measurement area and pixel size are specified. The size of the measurement area and pixel size can be specified without any particular restrictions, as long as they are within the range measurable by the Raman microscope used for the measurement.

[0040] Although not particularly limited, in order to calculate the area ratio with high accuracy, it is preferable to determine the measurement region and pixel size so that the total number of measurement points is 10,000 or more. The stage height of the Raman microscope is adjusted so that the spot diameter of the laser irradiated from the laser light source 4 on each pixel is minimized, that is, so that the focus is on the surface of the sample 2.

[0041] Next, laser light is irradiated onto each pixel, and the resulting Raman scattered light is dispersed by a diffraction grating (spectroscope) 5 and detected by wavelength by a CCD detector 6. The signal obtained by the CCD detector is converted into a Raman spectrum by a PC 7, and the Raman spectrum of each pixel is obtained.

[0042] In Raman mapping, the means for irradiating the sample with excitation light can be any means that emits laser light that is commonly used in Raman spectroscopy, without any particular limitations.

[0043] In addition, the intensity and exposure time of the laser irradiating the sample in the Raman mapping are specified so that the maximum intensity of the acquired Raman spectrum is less than the display upper limit of the Raman microscope used for the measurement.

[0044] Generally, the Raman signal increases linearly with the laser light intensity, but the fluorescence intensity saturates. On the other hand, if the laser intensity is too high or the exposure time is too long, the maximum intensity of the spectrum will exceed the display limit of the Raman microscope used for measurement, making subsequent analysis impossible.

[0045] Therefore, in order to obtain a spectrum that can be analyzed while reducing the influence of fluorescence, it is preferable to set the laser intensity to the maximum value and the exposure time so that the maximum intensity of the spectrum is below the display upper limit of the Raman microscope used for measurement.

[0046] In a preferred embodiment, the Raman spectrum can be obtained using, for example, a commercially available Raman microscope (e.g., Raman Drive manufactured by Nanophoton Corp.) The obtained Raman spectrum of each pixel can be analyzed by linear multiple regression analysis using the least squares method (Classical Least Squares (CLS)) to generate an image.

[0047] "Step of calculating crystallinity" Next, the crystallinity of the sample is calculated from the multiple Raman spectra acquired in the Raman mapping step.

[0048] Although not particularly limited, the step of calculating the crystallinity preferably involves analyzing and imaging the multiple Raman spectra obtained in the Raman mapping step, and calculating the crystallinity from the area ratio of the cristobalite region in the image obtained by imaging.

[0049] The analysis and imaging of a plurality of Raman spectra can be carried out as follows using analysis software (for example, the aforementioned Raman Viewer manufactured by Nanophoton Corporation).

[0050] [analysis] The CLS analysis mentioned above is a mixture spectrum

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[0051] [Imaging] The visualization is performed by assigning a different color to each coefficient calculated by CLS, and plotting the coefficient with the largest value in the designated color (for example, by assigning α to red, β to blue, and γ to green, and then performing CLS analysis on the spectrum obtained from a pixel, and plotting red at that pixel if α is the largest of the calculated α, β, and γ).

[0052] For all spectra acquired from each pixel, the spectra of the amorphous SiO2 and cristobalite crystal layer alone are used as reference spectra. By performing the above CLS analysis and imaging, each pixel in the measurement area can be colored as a cristobalite crystal layer region or an amorphous SiO2 region, and the respective distributions can be visualized.

[0053] [Calculation of crystallinity] The crystallinity of the quartz crucible can be calculated by using known spectra of amorphous SiO and cristobalite crystal layers as reference component spectra and calculating the area ratio of the cristobalite crystal layer region imaged as described above as the crystallinity. The area ratio can be calculated as the ratio of the number of pixels colored as cristobalite in the imaging process to the total number of pixels.

[0054] The series of steps of obtaining, analyzing, imaging, and calculating the area ratio of the Raman spectrum in the process of calculating the crystallinity described above can be performed using, for example, a commercially available Raman microscope (e.g., Raman Drive manufactured by Nanophoton Inc.) and analysis software (e.g., Raman Viewer manufactured by Nanophoton Inc.). [Example]

[0055] The present invention will be described in more detail below with reference to examples.

[0056] [Example 1]

[0057] First, a quartz crucible with a diameter of 800 mm used for pulling silicon single crystals was cut into a piece measuring 10 mm x 10 mm x 3 mm at a position 125 mm from the top end of the quartz crucible.

[0058] The cut quartz crucible sample was heat-treated in air for 20 minutes with a hot plate set to a temperature of 500°C. When the temperature at the measurement point where the Raman spectrum was measured using Raman spectroscopy was measured with a thermal camera, it was displayed as 350°C to 500°C, and the sample was heated with a temperature distribution within this temperature range.

[0059] After that, to confirm whether the fluorescence was reduced by the heat treatment, a Raman spectrum was recorded at the same measurement point using a Raman microscope (Nanophoton Raman Drive) at a laser intensity of 9 × 10 6 W / cm 2 The exposure time was 0.2 seconds, and 10 cycles were acquired every 0.2 seconds. The acquired Raman spectrum is shown in Figure 3. This measurement shows the change in the Raman spectrum at the measurement point over time. Even in the first cycle, the intensity of the spectrum did not saturate, and peaks attributed to cristobalite were detected.

[0060] If the fluorescence is not reduced, a broad fluorescence peak will appear at 600 cm -1However, in this example, even in the spectrum of the first cycle, there is almost no rise in the baseline, and it is presumed that the fluorescence is sufficiently reduced by the heat treatment.

[0061] From this result, the laser intensity is 9 × 10 6 W / cm 2 It was determined that a spectrum free from the influence of fluorescence could be obtained under the conditions of a laser intensity of 9 × 10 and an exposure time of 0.2 seconds. 6 W / cm 2 The exposure time was determined to be 0.2 seconds.

[0062] Next, an optical microscope image of the measurement location was obtained using a Raman microscope, and Raman mapping was performed with a measurement area of 450 μm × 500 μm, a pixel size of 3 μm square, and a total of 25,000 measurement points. It was confirmed that Raman spectra were obtained at all measurement points that were free from the influence of fluorescence.

[0063] Next, CLS analysis and imaging were performed on the acquired Raman spectrum, using the spectra of amorphous SiO2 and the cristobalite crystal layer as reference spectra. The crystallinity was calculated to be 90.3% from the area ratio of the cristobalite region in the obtained Raman image. Microscopic observation of the areas evaluated as crystallized and those evaluated as not crystallized confirmed that the evaluation was correct.

[0064] [Comparative Example 1] First, the 800 mm diameter quartz crucible used for pulling silicon single crystals was cut into 10 mm x 10 mm x 3 mm pieces at a position 125 mm from the top of the quartz crucible. The cut quartz crucible was not heat-treated, and the Raman spectrum at the same measurement point was measured using a Raman microscope with a laser intensity of 9 x 10 6 W / cm 2The exposure time was 0.2 seconds, and 20 cycles were acquired every 0.2 seconds. The acquired Raman spectra are shown in Figure 4. As a result, the 600 cm -1 Although the fluorescence in the vicinity was reduced, the baseline rose due to the influence of the fluorescence even after 2 seconds or more (10 cycles or more) had passed since irradiation, and there were wavenumber bands where the spectral intensity was saturated. Therefore, in order to obtain a spectrum free from the influence of fluorescence, it was determined that irradiating each pixel with the laser for 2 seconds or more before measurement was necessary, and since the measurement time was expected to be long, Raman mapping could not be performed, and the degree of crystallinity could not be calculated.

[0065] As described above, according to the example of the present invention, a 10 mm x 10 mm x 3 mm sample was cut out from an 800 mm diameter quartz crucible to be evaluated, and the cut sample was heat-treated at 350 °C to 500 °C for 20 minutes. Raman spectra were obtained at 25,000 points on the heat-treated sample using Raman spectroscopy. From these Raman spectra, the crystallinity of the sample was calculated to be 90.3%, confirming that the evaluation was correct. On the other hand, for Comparative Example 1, which was not heat-treated, the crystallinity could not be calculated. Therefore, in Example 1, fluorescence could be suppressed by heat treatment, and as a result, the crystallinity of the sample could be calculated accurately, simply, and in a short time.

[0066] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and that provides similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0067] 1...Raman microscope, 2...sample, 3...observation camera, 4...laser light source, 5...diffraction grating (spectroscope), 6...CCD detector, 7...PC.

Claims

1. A method for evaluating a quartz crucible by Raman spectroscopy, comprising: a sample cutting step of cutting a sample from the quartz crucible to be evaluated; a heat treatment step of performing heat treatment on the cut-out sample; a Raman mapping step of acquiring Raman spectra at multiple locations on the heat-treated sample by Raman spectroscopy; calculating the crystallinity of the sample from the plurality of Raman spectra acquired in the Raman mapping step; Including, A method for evaluating a quartz crucible, characterized in that the heat treatment conditions in the heat treatment step are set to conditions that reduce the fluorescence emitted from the sample when a Raman spectrum is obtained in the Raman mapping step.

2. 2. The method for evaluating a quartz crucible according to claim 1, wherein the sample to be measured is cut out from the quartz crucible after the silicon single crystal has been produced.

3. The method for evaluating a quartz crucible according to claim 1 or 2, characterized in that the heat treatment process involves performing heat treatment at 300°C to 1000°C for 20 minutes or more at multiple locations on the sample where Raman spectra are obtained.

4. 3. The method for evaluating a quartz crucible according to claim 1, wherein the step of calculating the degree of crystallinity comprises analyzing and imaging the multiple Raman spectra obtained in the Raman mapping step, and calculating the degree of crystallinity from the area ratio of cristobalite regions in the image obtained by the imaging.

5. 4. The method for evaluating a quartz crucible according to claim 3, wherein the step of calculating the degree of crystallinity comprises analyzing and imaging the multiple Raman spectra obtained in the Raman mapping step, and calculating the degree of crystallinity from the area ratio of cristobalite regions in the image obtained by the imaging.

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