JP2000214105A - Method for measuring impurity in quartz - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an equipment neutron activation analysis method for accurately obtaining measurement result even at a micro region while excluding contamination factors originating, especially from, environment and an analysis instrument. SOLUTION: When an impurity in a quartz is to be measured by an equipment neutron activation analysis for enclosing a sample into a neutron irradiation container, by irradiating it with neutrons, performing cooling, and then measuring γ rays, a container made of quartz that is etched by a fluoric acid is used as a neutron irradiation container, the sample is transferred from the neutron irradiation container to another container, and then γ rays are measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for measuring trace impurities by instrumental neutron activation analysis (INAA). More specifically, the present invention relates to an analytical method capable of accurately analyzing the impurity concentration in a trace region including impurities that are hardly decomposed in the measurement of impurities in high-purity quartz by performing a series of operations before and after neutron irradiation. It is.

[0002]

2. Description of the Related Art Quartz materials are currently used in various fields such as the optical field. In particular, semiconductor materials are used as CVD reactors and cleaning tanks for producing Si wafers and LSIs, and as crucibles for producing Si single crystals. It is used. If there are many impurities in these quartz products, the impurities in the quartz products may be eluted and scattered during the production of Si wafers and LSIs.
It is known that the manufacturing yield decreases due to deterioration of electrical characteristics and the like. At the time of manufacturing these quartz products, impurities may be mixed in from a burner used for processing, a tool used for molding, or the like, and impurities contained in the raw material quartz powder cannot be ignored.

With the recent increase in the degree of integration of LSIs and the like, it has been desired to improve the yield, and it has been required that the impurity concentration of quartz powder used as a raw material for quartz products be as low as possible. Therefore, a highly sensitive and accurate impurity analysis method is required for process design / improvement and quality control / guarantee by grasping the impurity concentration for each process.

Conventionally, trace impurities in quartz powder have been analyzed by:
A wet decomposition method has been used as a pretreatment. In the wet decomposition method, quartz is thermally decomposed in a fluorocarbon resin container with hydrofluoric acid (hydrofluoric acid) or a mixed acid containing the same to form hexafluorosilicic acid, which is then dried or concentrated, and then heated or heated. An acid for recovery is added to the concentrated solution, and the volume is further increased with pure water. The diluted acidic solution thus obtained is introduced into an inductively coupled plasma emission spectrometer (ICP-AES), a coupled plasma mass spectrometer (ICP-MS) or a graphite furnace atomic absorption spectrometer (GFAAS) to determine the element concentration. The impurity concentration in the quartz powder is calculated from the calculated concentration. In such a wet decomposition method,
It is said that all steps from hydrolysis to instrumental analysis are preferably performed in a clean room or a clean bench installed in a clean booth from the viewpoint of preventing contamination of impurities. In addition, even for acids and pure water used for hydrolysis and dilution, those for semiconductors or special grades containing very few impurities are used.

[0005] In the wet decomposition method, as described above, strict conditions for eliminating impurities are required. However, even if the analysis is performed while satisfying these conditions, for example, SiC, WC, etc. are required in the production process of the sample quartz powder. It has been found that when hardly soluble ceramics, such as those described above, are mixed, these ceramics are difficult to dissolve and hinder accurate analysis. Therefore, in order to compensate for such a disadvantage of the wet decomposition method, it is necessary to analyze the quartz powder without dissolving impurities (non-destructive) and with high sensitivity. Instrumental neutron activation analysis (INAA) is a representative example of such an analysis technique. In this method, a sample is sealed in a neutron irradiation container such as polyethylene, polyimide or quartz, inserted into a transfer container, and transferred into the reactor. After irradiating the sample with neutrons generated inside the reactor and cooling it, the semiconductor detector measures the γ-rays generated when the radioactive elements generated by the (n, γ) reaction and the excited nuclei decay or return to the ground state. It is a technique to analyze. This method is said to be capable of performing analysis without using a special environment such as a clean room or using high-purity acids as compared with the above-mentioned wet decomposition method.

In the actual measurement, the quantification of elements immediately after neutron irradiation (immediately after activation) is often performed after a certain cooling time, so that when quantifying, a known amount of W together with the analysis sample is used.
A standard sample containing a target element of _st (g) is irradiated simultaneously and under the same conditions, and the radioactivity (A, _Ast ) of both samples is measured.
The amount W (g) of the target element is determined by equation (1) (references: Yoshikazu Hashimoto, Tsunehiko Ohtoshi, activation analysis / PIXE analysis, 1st edition, Kyoritsu Shuppan, Tokyo, 19)
86).

[0007]

W = W _st × A / A _st (1)

[0008] The nondestructive trace analysis represented by INAA has disadvantages such as a long time to obtain a result and an increase in analysis cost. In general, a wet decomposition method is applied to impurity analysis. However, nondestructive microanalysis has the advantage of being able to deal with the problem of hardly soluble impurities in the wet digestion method as described above, and has also been used as a cross-check means for wet digestion methods used routinely. Establishment is significant.

[0009]

However, when the trace impurities in quartz powder were actually analyzed by the above-mentioned INAA, compared with the instrumental analysis after the wet decomposition, especially the Fe and Na were analyzed.
It has been found that a value several times to several tens times larger is analyzed for such elements. Specifically, in the sample quartz powder in which the impurity concentration in the quartz powder is measured at a level of 10 to 100 ppb in the wet decomposition method, the impurity concentration of ppm level may be measured in INAA. The reason why such a large difference occurs is that there is no possibility that hardly soluble substances are mixed as impurities.
It is said that elements such as a are unlikely to become particularly hardly soluble impurities in wet decomposition, while these elements are contained in a large amount in the surrounding environment and materials of the irradiation container and the like, so that they are mixed. It seems to have caused an analysis error. As described above, even in the case of INAA, which was conventionally said to be able to analyze even a trace area without paying attention to the pretreatment, in the analysis of a trace area of 100 ppb level or less, a factor that may be an analysis error before and after the analysis occurs. I knew I could do it. However, it is not easy to completely prevent the elements such as Fe and Na from being mixed from the surrounding environment, the irradiation container and the like, and to establish a reliable measuring method.

In order to eliminate the errors in the trace analysis in the INAA analysis, it is necessary to prevent the contamination of impurities from the environment at the time of sampling, to reduce the impurity concentration of the irradiation container itself, and to prevent the contamination of impurities during the sealing of the irradiation container (complete dissolution). In the case of sealing), it is considered useful to prevent contamination from the transport container to the irradiation container due to recoil contamination or the like.

In particular, from the viewpoints of the above, C. Yonezawa and M. Hoshi, Bunseki Kagaku, 3
9, 1990, 25) uses a high-purity quartz container derived from high-purity silicon tetrachloride whose surface has been dissolved and washed with hydrofluoric acid as a neutron irradiation container in the literature. And sealing with a polyamide film. This polyamide film is for preventing metal from adhering from the transport container to the irradiation container during neutron irradiation. After neutron irradiation and cooling,
It states that by performing γ-ray measurement as it is, it is possible to perform analysis without analysis errors due to contamination and the like. However, according to the study of the present inventors, such a method is extremely effective especially in the above, but in terms of lowering the blank of the quartz container itself, even a high-purity synthetic quartz container derived from silicon tetrachloride is used. It has been found that even if is used, there is a possibility that it greatly varies depending on the environment in which the container is made or the jig, and that Na and Fe may be mixed in several tens of ppb or more. Further, it was found that even if the surface of the quartz container was dissolved and washed with hydrofluoric acid, the washing could not be completely performed, and as a result, there was a great possibility that the analytical value would be greatly affected. The fluctuation of the impurity concentration for each quartz container was large, and the reliability of analysis in a trace concentration region was poor.

In addition, the above document only considers the effect of hydrofluoric acid cleaning on the irradiation container and does not suggest any application to the measurement of impurities in quartz samples. In fact, Bovine Liver and Orchard Leaves are contained in the quartz sample container and analyzed, and have nothing to do with the quartz sample.

[0013]

Therefore, the present inventors have further studied diligently. As a result, regarding the analysis of trace impurities in quartz powder by INAA, the sample quartz was sealed in a high-purity neutron irradiation container washed by a specific method in a highly-clean environment, and then cooled by neutron irradiation and transferred to another container. If a series of operations of taking out and measuring γ-rays is performed, it becomes clear that analysis with extremely small analysis errors can be achieved.
The present invention has been reached.

That is, according to the present invention, when a sample is sealed in a neutron irradiation container, irradiated with neutrons, cooled, and then subjected to gamma-ray measurement, the neutron activation analysis is used to measure impurities in quartz powder. A method for measuring impurities in quartz powder, wherein a gamma ray measurement is performed after using an etched quartz container and transferring a sample from a neutron irradiation container to another container. In the present invention,
In particular, by taking out and measuring from the irradiation container to another container after neutron irradiation, reliable analysis of the trace impurity concentration in quartz powder became possible without being affected by the impurity concentration of the quartz irradiation container or its fluctuation.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (Target Sample) The quartz that is a sample to be measured in the present invention is not particularly limited. The shape of the sample is not limited, but the size is usually within 1 cm 3, preferably 0.5 cm 3.
cm cubic. In this range, the measurement error can be substantially ignored. The sample may be in the form of powder or granules. It is suitably used for measuring impurities in synthetic quartz powder, particularly synthetic quartz powder used for high-purity applications, and examples thereof include those obtained by a gas phase method and a sol-gel method. Quartz powder obtained by the sol-gel method can achieve a fairly high level of purity by refining the raw materials, but it is necessary to know the presence and degree of impurities from the equipment because it comes into contact with the vessel during the reaction. . In this respect, it is useful to apply the present invention to quartz powder obtained by the sol-gel method. Such a high-purity quartz powder is suitably used as a material of a high-purity quartz product used for applications such as semiconductors and optical fibers. When the measurement is performed, the measurement method of the present invention may be applied by pulverizing and collecting an ingot or the like in a powder state.

In addition to the quartz powder, the measuring method of the present invention can be used for high-purity ceramics such as SiC which has recently been used for jigs for making LSIs. Especially ICP-M
Analysis of trace impurities in high-purity hard-to-decompose ceramics, which is a material (hard to be wet-decomposed) that is difficult to apply S (p
(pb level) is also suitable for the measurement method of the present invention.

(Neutron Irradiation Vessel) In the measuring method of the present invention, first, the neutron irradiation vessel is washed. It is desirable that the subsequent operations including the washing be performed in a clean room or a clean bench installed in the clean room. Performing in such a clean environment is also important in preventing contamination from the environment during sampling. According to the inventor's experience, ppb
In the case of level analysis, the analysis values could fluctuate significantly at the time of sampling, possibly due to environmental causes. However, it is more preferable to perform the operation in a clean booth of at least class 10000, preferably in a clean room of class 1000 or less.

The neutron irradiation container is made of high-purity material.
Quartz is desirable. The size depends on the size of the irradiation hole to the reactor to be used, but in the examples described later, one having a diameter of 1 cm × length 2 cm × thickness of 1 mm was used, and a lid was also made to fit the same diameter. . High-purity natural quartz was used as the material. In addition, SiC and other high-purity ceramics can be used, but those made of quartz are most preferable in consideration of ease of surface cleaning and fabrication.

The above-mentioned C. Yonezawa and M. Hosh
As described in the literature of i, Bunseki Kagaku, 39, 1990, 25), it is preferable to use a synthetic material derived from silicon tetrachloride among quartz as the material for the material, but it is described earlier. Even if synthetic quartz is used as the material, the processing of the plate or tube or the molding process into the irradiation container should be performed in an environment where impurities are easily mixed, or if a tool that is prone to contamination is used, the impurities will remain in the irradiation container. Will be mixed in. In addition, synthetic quartz containers are generally expensive, and are difficult to use in INAA, in which the irradiation container is discarded after neutron irradiation. Again, as far as the cost permits, it is better to use one derived from synthetic quartz powder, but it is only necessary to use natural products and synthetic products depending on the concentration to be analyzed. In this case, what should be considered as impurities are a metal element, boron and phosphorus.

In addition to these, radiation-resistant polyimide is used as a container. However, neutron irradiation is performed in consideration of an analysis error due to shaving of an analysis sample against quartz powder, and a degree that metal adhered to the surface by pickling or the like can be reduced. For the container, ceramic-based quartz from which hard and high-purity materials can be obtained is preferred. The selection criterion for the natural product and the synthetic product of the quartz used for the container may be appropriately selected according to the analysis target concentration, but generally, when the analysis target concentration is (1) several tens ppb or more, natural quartz product,
(2) It is desirable to use a synthetic quartz product or a natural quartz product of high purity in the case of a few ppb or less, and (3) a synthetic quartz product in the case of a sub-ppb or less. However, elements (Al, T
If it is desired to analyze other than i, Fe, Ca, etc.), the surface of the quartz container to be used can be applied to high sensitivity analysis of several tens ppb or less even if the surface of the quartz container used is etched and cleaned with hydrofluoric acid. it is conceivable that.

(Cleaning of Neutron Irradiation Vessel) The neutron irradiation vessel made of quartz described above is washed with hydrofluoric acid. The cleaning is preferably performed by immersing the substrate in a 50% hydrofluoric acid for about 30 minutes in a clean bench installed in a clean room to clean the surface by etching. The concentration of hydrofluoric acid is 1% by weight or more. A mixed acid containing hydrofluoric acid such as a mixed acid of hydrofluoric acid and nitric acid may be used for cleaning. The container used for immersion is preferably Teflon (trademark) resin (fluororesin), particularly PFA (polyfloral alkyl resin), but is not limited thereto.
What has been washed by immersing this container in nitric acid in advance, may be taken out immediately before washing with pure water and then used. After the immersion, the hydrofluoric acid may be separated by decantation, washed with pure water, placed in a PFA container that has been washed in the same manner as the Teflon container, and dried on a hot plate. It is better to use only cleaned Teflon tweezers for handling. As the hydrofluoric acid used at this time, it is preferable to use a semiconductor grade having an impurity concentration of 1 ppb or less, which is generally used in the semiconductor industry and the like.

The etching time may be determined from a value calculated in advance by actual measurement or the like. According to the study of the present inventors, 1
It has been clarified from the etching time, the concentration of the etching solution, the weight of the quartz powder remaining after etching, and the concentration of the impurities that the concentration of impurities that tend to be unevenly distributed on the quartz surface, which seems to have been contaminated during processing, can be reduced by etching at 0 μm or more. Therefore, these etching conditions may be used. The concentration of the metal element and impurities such as boron and phosphorus in the hydrofluoric acid may be 100 ppb or less. In this case, it is desirable to carefully perform post-washing with pure water. The cleaning with hydrofluoric acid may be performed by etching using the above-described method, by using an ultrasonic wave, by spray cleaning, or in a vapor atmosphere of hydrofluoric acid or a mixed acid thereof.

(Neutron Irradiation) Thereafter, the sample is weighed and covered with a neutron irradiation container, covered with a lid, and sealed with a quartz burner to obtain a sample. But it is good. After that, it is put into a transport container, sent into the reactor, and irradiated with neutrons. The neutron irradiation may be performed by a known method, and the neutron irradiation time and intensity may be determined depending on the reactor to be used, the target analysis concentration / element, and the situation.

(Transfer to another container) As described above, in the case of the quartz irradiation container, trace impurities contained in the quartz irradiation container affect the analysis value in a trace concentration region due to the problem of contamination of the quartz irradiation container itself. After irradiation and cooling, it is important to transfer to another container and measure γ-rays. If you do this,
Contamination after neutron irradiation does not need to be considered in INAA itself, which has a great merit in measurement and investigation, and enables accurate measurement in a trace amount region without being affected by the purity or cleaning condition of the irradiation container. However, if there is a large amount of impurities in the inner wall or surface layer of the quartz irradiation container, even if the above operation is performed, the sample will be contaminated by recoil and the like, and the reliability of analysis in a trace analysis area will be poor. Therefore, minimal cleanliness is required. Therefore, as described above, it is preferable to use a high purity quartz powder among natural products.

(Γ-ray irradiation) Gamma-ray irradiation is performed by placing a sample irradiated with neutrons in a measurement room shielded by lead or steel. At this time, contamination is not a problem even if it is placed in a metal because it has been irradiated with neutrons. Γ-rays that can be an external background are almost blocked. A semiconductor detector (Ge single crystal) is used for the measurement. The actual analysis is performed by the following mechanism. That is, when the radioactive excited nuclei generated by neutron irradiation decay and return to the original element, γ rays having a specific wavelength (energy) are generated. The γ-rays vary greatly from element to element, and the semiconductor detector converts the energy into an electric signal for each energy and measures the intensity. Therefore, the output of the normal result usually shows energy on the horizontal axis and intensity on the vertical axis. In addition, since some elements have similar γ-ray energies, highly sensitive and accurate analysis can be performed by utilizing the difference in half-life of radioactive excited nuclei.

According to the present invention described above, an analysis error factor which can be eliminated from a container when used for neutron irradiation and γ-ray measurement, and which can eliminate environmental contamination by sampling in a clean environment. The law was established.

[0027]

Next, the present invention will be described more specifically with reference to examples. (Comparative Example 1) In a clean booth placed in a class 1000 clean room, 5 g of a quartz powder sample was weighed into a washed Teflon container, and after being heated and decomposed on a hot plate while adding hydrofluoric acid, nitric acid and a trace amount of sulfuric acid, Concentrated. Acids and pure water to be used were those for semiconductors with very few impurities and those of special grade. Thereafter, diluted nitric acid was added and dissolved, and the volume was increased in a separate container with pure water. After that, the diluted nitric acid solution was introduced into an inductively coupled plasma mass spectrometer (ICP-MS) or a graphite furnace atomic absorption spectrometer (GFAAS) and analyzed. The analysis results are shown in Table 1.

(Comparative Example 2) About 3 g of a quartz powder sample was weighed, washed with nitric acid, placed in a dry polyethylene bag, sealed, further wrapped in a plastic bag, and irradiated with neutrons. Irradiation conditions were as follows: reactor: Rikkyo reactor, irradiation hole: central test tube, neutron flux density:
3.7 × 10 ¹² (n · cm ⁻² s ⁻¹ ), irradiation time: 6 hours. After cooling, the external plastic bag was replaced, a new plastic bag was placed, and γ-rays were measured. The results are shown in Table 1.

(Comparative Example 3) About 30 g of a quartz powder sample was weighed.
Then, it was placed in a high-purity quartz container and irradiated with neutrons. Irradiation conditions are
Reactor: Missouri large reactor, neutron flux density: 1.94 × 10
¹³(N · cm^-2s ^-1), Irradiation time: 45 hr, cold
After rejection, the γ-ray was measured as it was. The results are shown in Table 1.

Example 1 The following operation was performed in a class 1000 clean room or a class 10 clean bench installed in a clean room. The container for neutron irradiation is 1cm in diameter x 2cm in length.
A lid made of high-purity natural quartz having a wall thickness of 1 mm was prepared to fit this diameter. These quartz containers and lids are washed with pure water, then immersed in nitric acid, and taken out of the PFA bottle that has been washed immediately before. Used for the operation.

The neutron irradiation container and the lid made of quartz were immersed in 50% hydrofluoric acid (for a semiconductor manufactured by Morita Chemical) for about 30 minutes in a clean bench installed in a clean room, and the surfaces were etched and cleaned. Handling was performed using washed Teflon tweezers. After the immersion, the neutron irradiation container was washed with pure water after decantation of hydrofluoric acid, placed in a PFA container that had been cleaned in the same manner as the Teflon container, and dried on a hot plate. Then, 3 g of the weighed quartz powder sample was put into a neutron irradiation container, covered with a lid, and wrapped while suppressing the lid with high-purity aluminum foil that had been washed and dried with high-purity ethanol for semiconductors.

After that, this was put into a transport container, sent into the reactor, and irradiated with neutrons. Irradiation conditions are reactor: JAERI J
RR-3M, neutron flux density: 1 × 10 ¹⁴ (n · cm ⁻² s
^-1 ), irradiation time: 6 hours. Thereafter, the sample was cooled, the quartz powder sample irradiated to the measurement container was taken out of the irradiation container, and γ rays were measured. The results are shown in Table 1.

[0033]

[Table 1]

As described above, in comparison with the value of Comparative Example 1, a polyethylene bag having a high impurity concentration is used as the neutron irradiation container as in Comparative Example 2, or the washed quartz container of Comparative Example 3 is used. Also, when the γ-ray was measured as it was after the irradiation, a concentration several to several tens of times was observed. However, the treatment before irradiation is performed in the clean room as in the first embodiment,
Neutron irradiation was performed in a quartz container washed with hydrofluoric acid, cooled, transferred to another container, and subjected to γ-ray measurement, thereby giving almost the same value as in Comparative Example 1. Thus, according to the present invention, neutron irradiation and γ
The analysis error factor derived from the container used for X-ray measurement can be eliminated,
By performing sampling in a clean environment, we have established an analytical method that can eliminate environmental contamination.

[0035]

According to the present invention, an analytical method capable of accurately analyzing the impurity concentration in a trace amount range, even for hardly decomposable substances, in the measurement of impurities in high-purity quartz has been established.

Claims (2)

[Claims] 1. A sample which is sealed in a neutron irradiation container, irradiated with neutrons, cooled and then subjected to gamma ray measurement. When measuring impurities in quartz by neutron activation analysis, quartz which is etched with hydrofluoric acid as a neutron irradiation container A method for measuring impurities in quartz, wherein a gamma ray measurement is performed after a sample is transferred from a neutron irradiation container to another container using a container made of neutrons. 2. The method for measuring impurities in quartz according to claim 1, wherein the quartz is a synthetic quartz powder obtained by a sol-gel method.