JPH0357422B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an apparatus and a decomposition method for decomposing various samples with acid, and more specifically,
The present invention relates to a sample decomposition device used for the purpose of preparing a sample solution for analyzing minute amounts of impurities in semiconductor crystals such as silicon crystals and gallium arsenide crystals, and a decomposition method thereof.

[Technical background of the invention and its problems] Silicon crystals and gallium arsenide crystals are used as semiconductor device substrates, but these crystals contain sodium (Na), potassium (K), iron (Fe), etc. The presence of such impurities, even if the amount thereof is extremely small, has a significant effect on the electrical characteristics of the device. Therefore, in order to improve the characteristics of the device, it is necessary to reduce the content of these impurities as much as possible.

As a prerequisite for taking such measures, it is essential to accurately analyze the concentration of impurities in the crystal.

Conventionally, flameless atomic absorption spectrometry has been widely applied as an analysis method. In this method, the sample solution to be applied to the analyzer is generally prepared as follows. That is, for example, in the case of a silicon crystal, a crystal sample is directly decomposed by dissolving it in a mixed acid solution of hydrofluoric acid, nitric acid, and sulfuric acid, and the resulting decomposed solution is evaporated to dryness to obtain a residue.
This method involves diluting this residue with pure water to a certain volume. In the case of gallium arsenide crystals, the only difference is that a mixed acid solution of hydrochloric acid and nitric acid is used.

However, in such a method of preparing a sample solution, contamination from each reagent used is extremely large.

For example, even if these reagents are purified by non-boiling distillation or ion exchange methods, they usually contain Na.
Contains 0.1 ppb or more of impurities such as K. Therefore, when such a reagent is used, it is extremely difficult to analyze impurities of 5 ppb or less in the obtained sample solution.

[Object of the Invention] An object of the present invention is to solve the above-mentioned problems and to provide an apparatus and method for preparing an analysis sample solution with almost no contamination from a sample decomposition reagent or the environment.

[Summary of the Invention] The sample decomposition device of the present invention includes: a sealed container; at least one acid solution storage container disposed within the sealed container; disposed within the sealed container, and containing a sample and pure water. is characterized by comprising a storage container in which the acid solution storage container is stored; a heating means for heating the acid solution storage container; and a magnetic field generation means that exerts a magnetic force on the acid solution storage container; The method is characterized in that an acid solution for sample decomposition is evaporated in a magnetic field, the acid vapor is absorbed into pure water in which the sample is immersed, and the sample is decomposed with the obtained high-purity acid solution.

The apparatus of the present invention will be explained based on an illustrated example. In the figure, 1 is a closed container in which the sample is decomposed. The material constituting the container 1 may be any material as long as it is not corroded by acid vapor, which will be described later, and has a certain degree of heat resistance.In particular, in the case of decomposing semiconductor crystals, hydrofluoric acid, etc. may be used. For example, Teflon is suitable.

Two types of containers are arranged in the closed container 1. One type is an acid solution storage container 2 that stores an acid solution for sample decomposition, and the other type is a storage container 3 that stores a sample to be decomposed and pure water for immersing it. Needless to say, the materials of these containers must have properties similar to those of the container 1.

The acid solution stored in the container 2 is arbitrarily selected depending on the type of sample 4. Furthermore, the number of containers 2 may be two or more, without any limitation. For example, if the sample 4 is a silicon wafer, the hydrofluoric acid and nitric acid are placed in separate containers 2.
It may be stored in a container, or it may be mixed acid and stored in one container.

A sample 4 and pure water 5 are stored in the container 3.
The pure water used is preferably purified water obtained by removing impurities, such as deionized water or distilled water. In order to smoothly decompose the sample 4, it is preferable to provide a convex portion 6 as shown in the figure on the bottom of the container 3, for example, to support the sample at a point.

In addition, although there is only one container 3 in the figure, in order to decompose many samples at the same time, for example, this container 3
You can pile up multiple containers, or partition one container into multiple rooms.

Furthermore, it is effective to cover the ventilation hole with a lid provided on the side wall of the container 3 without opening the top surface, since it is possible to prevent contamination from the acid solution dripping from the ceiling of the container 1 or the like.

Reference numeral 7 denotes heating means for the acid liquid storage container, which is provided to evaporate the acid liquid in the container 2 and generate acid vapor. The type of heating means, such as electric resistance heating, does not matter. These heating means are preferably disposed outside the container 1 from the viewpoint of preventing contamination. Further, if necessary, it may be provided to heat the container 3.

8 is a magnetic field generating means that applies magnetic force to the acid liquid storage container. The magnetic force referred to here is the force that attracts ferromagnetic substances such as Fe and Ni in the acid solution in the container to prevent them from evaporating, and a magnetic field generating means is provided to prevent evaporation. be done. The type may be either a permanent magnet or an electromagnet. These magnetic field generating means are preferably disposed outside the container 1 from the viewpoint of preventing contamination. However, oxidation resistance,
If the magnetic field generating means 8 is coated with a material having excellent chemical resistance and heat resistance, for example, Teflon, it may be placed inside the containers 1 and 2.

In this device, the acid liquid in the container 2 is heated by the heating means 7 and evaporated, turning into acid vapor and filling the container 1. During evaporation, impurities made of ferromagnetic substances such as Fe and Ni are attracted by the magnetic force of the magnetic field generating means and do not evaporate, resulting in higher purity of the acid vapor.

The acid vapor moves like the arrow and reaches the pure water 5 in the container 3.
The high purity is absorbed into the acid solution and restored. Thus,
The sample is decomposed by this high-purity acid solution, resulting in a sample solution that is totally free of external contamination.

Although the above explanation has focused on the case where the sample is a semiconductor crystal, this apparatus and method are not limited to this. It is very clear from its principle that it can be applied to any combination of a compound semiconductor (such as silicide, InSb, or InAs) and an acid solution capable of decomposing it.

[Examples of the Invention] Example 1 A phosphorus-doped silicon wafer having a specific resistance of 1.5 Ωcm and a thickness of 436 μm was prepared. This was stored in container 3 along with 50 ml of deionized water. 50 each as acid solution
% hydrofluoric acid and 400 ml of 60% nitric acid were selected and stored separately in container 2. The spatial volume of container 1 was approximately 8500 cm ³ . In addition, a Teflon-coated permanent magnet 8 (60 mm x 60 mm) is used as a magnetic field generating means.
×5 mm, 5 kGauss) was placed under container 2. Both containers were made of Teflon.

The heating means (heater plate) was activated to heat each acid solution at 210°C for 150 minutes. Wafer melting was confirmed.

The obtained sample solution was applied to a flameless atomic absorption spectrometer to analyze Na, K, Fe, and Cr. The measurement conditions are as follows.

Drying: 30 seconds at 120℃, Ashing: 600℃ for Na, K for
700℃, Fe and Cr at 1000℃ for 30 seconds each. Atomization: 2500℃ for Na, 2700℃ for K, 2800℃ for Fe and Cr
for 8 seconds each. Carrier gas: with argon
300ml/min, but no flow during atomization.
Measurement wavelength: Na: 589.0nm, K: 766.5nm, Fe:
248.3nm, Cr is 359.4nm. Light source for interference absorption correction:
For Na and K, use a tungsten halogen lamp.
For Fe and Cr, use a deuterium lamp.

As a result, Na: 3ppb, K: 1ppb, Fe:
7ppb, Cr: 0.5ppb.

Comparative Example 1-1 Analysis was conducted under the same conditions as in Example 1, except that the permanent magnet 8 was not used. Na is 3ppb, K is
1ppb and Cr was analyzed as 0.5ppb, but Fe could not be analyzed because the amount of Fe contamination at the time of sample decomposition was 11ppb.

Comparative Example 1-2 Example 1 was added to a mixed acid consisting of 10 ml of 50% hydrofluoric acid, 5 ml of 60% nitric acid, 0.1 ml of 96% sulfuric acid, and 5 ml of pure water.
Soak the silicon wafer used in
Digested for 20 minutes at °C. The resulting solution is approx.
After heating at 160°C for 120 minutes and evaporating to dryness, it was diluted to 5 ml with pure water.

When analyzed under the same measurement conditions as Example 1,
The amount of impurity contamination during sample decomposition is Na
14 ppb, K 9 ppb, Fe 26 ppb, and Cr 6 ppb, so none of the elements could be analyzed.

Example 2 Silicon wafer has a specific resistance of 6.9Ωcm and a thickness of 320μm
The amount of deionized water was 40 ml, and the heating time of the acid solution was 130 ml.
A sample solution was prepared in the same manner as in Example 1, except that the temperature was 30 minutes, and flameless atomic absorption spectrometry was performed under the same measurement conditions.

Na: 2ppb, K: 1ppb, Fe: 2ppb, Cr:
A result of 0.4ppb was obtained.

Comparative Example 2-1 Analysis was conducted under the same conditions as in Example 2, except that the permanent magnet 8 was not used. Na is 2ppb, K is
1ppb and Cr was analyzed as 0.4ppb, but Fe could not be analyzed because the amount of Fe contamination at the time of sample decomposition was 13ppb.

Comparative Example 2-2 Example 2 was added to a mixed acid consisting of 10 ml of 50% hydrofluoric acid, 5 ml of 60% nitric acid, 0.1 ml of 96% sulfuric acid, and 5 ml of pure water.
Soak the silicon wafer used in
Digested for 20 minutes at °C. The resulting solution is approx.
After heating at 160°C for 120 minutes and evaporating to dryness, it was diluted to 5 ml with pure water. The analysis was performed under the same conditions as in Example 2. The amount of impurity contamination during sample decomposition was Na 14ppb, K
9ppb, Fe 26ppb, and Cr 6ppb, so none of the elements could be analyzed.

Example 3 The sample was a gallium arsenide wafer with a thickness of 470 μm obtained by the LEC method, the acid solution was 400 ml of 35% hydrochloric acid,
400ml of 60% nitric acid was added, and the heating time of the acid solution was
An analytical sample solution was prepared in the same manner as in Example 1, except that the heating time was 140 minutes.

The obtained sample solution was applied to a flameless atomic absorption spectrometer to analyze Fe, Cr, and Ni. The measurement conditions are as follows.

Drying: 30 seconds at 120℃. Ashing: 1250℃ for Fe, Cr
1350℃ and 1200℃ for Ni for 60 seconds each. Atomization: 8 seconds each at 2800℃. Carrier gas: Argon at 300ml/min, but not during atomization. Measurement wavelength: 248.3nm for Fe, 248.3nm for Cr
359.4nm, Ni 232.0nm. Light source for interference absorption correction:
Uses a deuterium lamp.

Results were obtained for Fe: 8ppb, Cr: 2ppb, and Ni: 3ppb.

Comparative Example 3-1 Analysis was conducted under the same conditions as in Example 3, except that the permanent magnet 8 was not used. Cr was analyzed to be 2 ppb, but the amount of Fe and Ni contamination during sample decomposition was 13 ppb.
Since the concentration was 7 ppb, Fe and Ni could not be analyzed.

Comparative Example 3-2 Example 3 with a mixed acid of 10 ml of 35% hydrochloric acid and 5 ml of 60% nitric acid
After disassembling the wafer, it was diluted to 100ml with pure water. This solution was analyzed under the same conditions as in Example 3. The amount of impurity contamination during sample decomposition was Fe 24ppb, Cr
7ppb and Ni 14ppb, so neither element could be analyzed.

[Effect of the invention] As is clear from the above explanation and analysis results, the sample solution prepared using the apparatus of the present invention has the following effects:
Unlike conventional methods, contamination from reagents, the environment, etc. is suppressed, so the analysis results obtained are extremely sensitive; for example, in the case of silicon wafers, it is approximately 100 times more sensitive.
In the case of gallium arsenide, it is about five times as large, making it extremely useful.

[Brief explanation of drawings]

The figure is a schematic diagram showing an example of the device of the present invention. DESCRIPTION OF SYMBOLS 1... Airtight container, 2... Acid solution storage container, 3... Sample and pure water storage container, 4... Sample, 5... Pure water, 6... Convex part, 7... Heating means, 8... Magnetic field generation means.

Claims (1)

[Claims] 1. A sealed container; at least one acid solution storage container disposed within the sealed container; a storage container disposed within the sealed container and containing a sample and pure water; A sample decomposition device comprising: a container; heating means for heating the acid liquid storage container; and magnetic field generating means for exerting a magnetic force on the acid liquid storage container. 2. The sample decomposition device according to claim 1, wherein the sample is a semiconductor crystal wafer. 3. The sample decomposition device according to claim 1, wherein the storage container is a container with a lid. 4. Evaporate the acid solution for sample decomposition in a closed container in a magnetic field, absorb the acid vapor into the pure water in which the sample is immersed, and decompose the sample with the resulting high-purity acid solution. A sample decomposition method characterized by: 5. The method according to claim 4, wherein the sample is a semiconductor crystal wafer and the acid solution is nitric acid, hydrofluoric acid, or hydrochloric acid.