JPH098088A - Silicon wafer analyzing method - Google Patents

Abstract

PURPOSE: To clarify dissolved amounts of an oxide layer, a diffusion region layer, and each layer in a residual part, and enable high precision analysis by an atomic absorption method, by making an oxide film, a metal diffusion region layer, and the residual part of a silicon wafer solutions, and performing quantitative analysis by an atomic absorption method. CONSTITUTION: An oxide film, a metal diffusion region layer, and the residual part of a silicon wafer are dissolved and turned into solutions, and quantitative analysis of the respective dissolved solutions is performed by an atomic absorption method. A frameless atomic absorption method is an atomizing method wherein the atomizing part is constituted of a graphite furnace 3. Electrodes 2 make a current flow in the graphite furnace 3, which is heated by Joule's heat generated by the current. An irradiation light L is generated from a light source of a hollow cathode-ray tube 1. The atomized state in the graphite furnace 3 is irradiated with the light 1, which reaches a detector 6 through a spectroscope 5. Absorption of the light is measured with the detector 6, and outputted to a display part 8 through a signal processing part 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon wafer analysis method relating to elemental analysis related to characteristics of semiconductor elements such as impurities in silicon wafers in silicon semiconductor devices and trace heavy metal contamination in a wafer process.

[0002]

2. Description of the Related Art With the high integration of semiconductors, it is becoming more and more important to reduce impurity contamination on silicon wafers. This is particularly because metal contamination affects junction leakage, oxidation-induced defects, oxide film withstand voltage, and the like, and impurities in the manufacturing process are a major factor of yield reduction.

In addition, since power devices such as power semiconductor devices are basically vertical devices,
In addition to impurity contamination on the silicon wafer, the ppb level trace amount of impurities in the silicon material and its distribution have a great influence. This is because metal contamination in the oxidation / diffusion process itself acts as a charge generation / recombination center, causing device failure. In addition, it is synergistic with residual damage and strain in the device manufacturing process, which causes crystal defects. When crystal defects are introduced, defects such as increase of leak current and deterioration of reliability are caused. Moreover, since the pollution has an accumulation effect from the early stage of the process, it is affected by the pollution from the previous steps.

That is, there is a large amount of contamination from the wafer manufacturing stage.
However, the tendency to influence the manufacturing yield becomes more remarkable.
You. In order to control this on the production line, metal
Quantity 10 ^Tenatoms / cm² Silicon wafer with level as the lower limit of quantification
Ha trace metal analysis method is required. Also silicon
The wafer has hydrogen chloride on metallic silicon (purity 97-99%).
React and react with trichlorosilane (SiHCl_ThreeGet) this
After rectification, the hydrogen content is reduced by hydrogen reduction or thermal dissolution reaction.
Make pure polycrystalline silicon. Then re-melt and single crystal
Turn it into a got. In this case, 260% pure 100% silicon
It shows a high resistivity of 000 Ω · cm. Function as a semiconductor
In order to obtain the appropriate doping agent (B, P, As, Sb
To control the resistivity. Other than the doping agent
It is necessary to prevent the mixture of pure substances as much as possible.
Group 3 elements of the Periodic Table classified into 8 groups by quality are 0.3pp
b or less, Group 5 elements less than 1.5 ppb, heavy metals less than 0.1 ppb
Below, other elements other than carbon and oxygen must be 0.001 or less.
It is said that it must be done.

This ingot is formed into a wafer (thickness 500 to 75
0 μm) and polished to make a mirror surface wafer. A semiconductor element is manufactured using this wafer as a main material.

[0006]

In light of the above, there is a demand for a microanalysis method in which the ppt to ppb level is the lower limit of quantification for the amount of metal (concentration) in the silicon wafer surface oxide film and its silicon substrate. . Moreover, in order to estimate the contamination source during the wafer process, it is important to know the impurity distribution in the depth direction from the surface of the silicon wafer, that is, the hierarchical analysis and evaluation.

[0007] Generally, as a method for analyzing an extremely small amount of impurities in silicon of a semiconductor material, general methods such as emission analysis, solid mass spectrometry, activation analysis, infrared spectroscopy, fluorescent X-ray analysis and lifetime measurement (electrical method) are used. The method is described in "Analytical Chemistry Handbook", 4th edition (1991), edited by Japan Society for Analytical Chemistry. In addition, as a trace metal analysis method for analyzing the surface of a silicon wafer, a total reflection X-ray fluorescence analysis, an atomic absorption method combined with a gas phase dissolution method, a plasma mass spectrometry method and the like are generally performed.

[0008] However, the above-mentioned general trace analysis method for silicon is a metal analysis method for a silicon base material which requires a piece of silicon of 1 to 2 mm or about 1 to 10 g of silicon of a sample, and a silicon single analysis method. Oxygen in crystals
It is a carbon analysis method and a metal analysis method only for the silicon wafer surface, and is not suitable for hierarchical analysis and evaluation of the silicon wafer surface oxide film, diffusion region layer, and silicon substrate.

Furthermore, since there is no fixed optimum method for the analytical sample preparation method important for achieving this analytical purpose, it is not suitable for this analysis. The present invention has been made in view of the above points, and an object thereof is suitable for a highly sensitive and highly accurate trace metal analysis and evaluation method of heavy metal components such as iron, copper, chromium, and nickel, and silicon sampled for each layer from a silicon wafer. It is to provide a wafer analysis method.

[0010]

In order to achieve the above object, in a silicon wafer analysis method for quantitatively analyzing a metal element in a silicon wafer having an oxide film on the outermost surface and a diffusion region layer under the oxide film, The entire oxide film is dissolved into a solution, then the metal diffusion region layer is dissolved into a solution having a predetermined thickness, and then the remainder of the silicon wafer is dissolved into a solution having a predetermined thickness or a predetermined weight one or more times. Quantitative analysis will be performed by the atomic absorption method.

The dissolution solution is formed by dropping hydrofluoric acid onto the surface oxide film of the silicon wafer to form a dissolution solution. After removing the oxide film by dissolution, the diffusion region layer is hydrofluoric acid and nitric acid. It is assumed that the mixed solution is used as a solution, and after the diffusion region layer is dissolved and removed, the remaining portion of the silicon wafer is made into a solution using a mixed acid of hydrofluoric acid and nitric acid.

The mixed acid used for dissolving the diffusion region layer has a concentration of hydrofluoric acid of 18 to 22% and nitric acid of 3%.
It is good to be 0 to 40%. The concentration of the mixed acid used in the solution of the remainder of the silicon wafer is 20 to 25%, and the solution may be heated and dissolved in a closed container. It is preferable that the dissolving solution is formed by using a jig having a plurality of protrusions having a constant height on a flat plate and holding the dissolving solution only between the silicon wafer and the flat plate placed on the protrusions.

[0013]

According to the present invention, for a silicon wafer having an oxide film on the outermost surface and a diffusion region layer underneath it, the entire oxide film is dissolved into a solution and then the metal diffusion region layer is dissolved into a solution of a predetermined thickness. And then the rest of the silicon wafer to 1
Alternatively, since the solution was made into a solution having a predetermined thickness or a predetermined weight by multiple times and each of these solutions was quantitatively analyzed by the atomic absorption method, the amount of dissolution for each layer of the oxide layer, the diffusion region layer and the rest was clear. Yes, it can be analyzed with high accuracy by the atomic absorption method.

Further, since the surface oxide film is mainly composed of SiO _2, only the oxide film can be dissolved and made into a solution by hydrofluoric acid (HF). The reaction formula is represented by the following formula.

[0015]

Embedded image Further, since the diffusion region layer is mainly composed of Si and contains a trace amount of heavy metal, the sample preparation for making it into a solution is performed with HF.
And it is a lysis solution by mixed acid of nitric acid (HNO _3). The action of the mixed acid used here is represented by the following formula.

[0016]

Embedded image In any case, the target sample solution containing H ₂ SiF ₆ as a reaction product is iron, copper, nickel,
Metals such as chromium are melting at the same time. This preparation method is carried out by contacting the fluororesin (polytetrafluoroethylene) jig having protrusions with the jig made of the above mixed acid so that the surface to be treated faces downward so as to dissolve a thickness of about 50 μm from the surface. It will be possible. About 50μ from the surface
To dissolve the thickness of m, the concentration of the mixed acid and the contact time are adjusted so that it can be easily performed.

Further, the S of the rest of the silicon wafer itself
The solution of i as a solution is to dissolve about 1 g of sample for the analysis of trace metals. However, as in the above case, a mixed acid of HF and HNO ₃ is used and a closed container is used. The pollution of can be prevented.

[0018]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a flow chart showing an analytical sample preparation procedure according to an example of the present invention. This silicon wafer is GTO
(Gate Turn Off) Used in wafer processes such as thyristors, consisting of a surface oxide film, diffusion region layer, and silicon wafer base material. The flow charts that enable evaluation by the sample preparation method and the measurement method for the purpose of trace heavy metal analysis of each layer site are shown.

A procedure for preparing an analytical sample will be described. (1) For quantitative analysis of trace metals in the surface oxide film of a silicon wafer, since the surface film is SiO ₂ , only the oxide film can be dissolved and dissolved by HF. It can be analyzed by an absorption method. The procedure is, for example, H
Add 400 μl of F dropwise. HF is a high-purity reagent (concentration 38%)
100 μl of each of them was dropped from the center of the wafer horizontally, vertically, horizontally, and 5 times after the HF spread over the entire surface.
Leave it for a minute to completely dissolve the oxide film. To wash the surface of this liquid and then the wafer, rinse the entire surface with ultrapure water.
Collect in a container made of fluororesin and adjust the total volume to 3 ml. The chemical reaction of this method is represented by the following formula.

[0020]

Embedded image Only the oxide film of the wafer can be dissolved and the heavy metal in the film is also dissolved and recovered at the same time. In this way, the sample preparation for HF dropping dissolution solution can be easily performed. After that, a trace amount of heavy metals is measured through a liquid separation process. (2) To quantitatively analyze a trace amount of metal in the silicon wafer diffusion region layer, a silicon wafer obtained by dissolving and removing the oxide film is used. Since its surface is mainly composed of Si, the sample preparation for making it into a solution can be made into a solution by dissolving with a mixed acid of HF and HNO ₃ , and this treatment liquid can be analyzed by the flameless atomic absorption method.

As the procedure, for example, the product from which the surface oxide film has been removed is washed beforehand with ultrapure water. Next, it is dried, but it is naturally dried in a clean bench to prevent contamination at this time.
Since the diffusion region layer is melted to a thickness of about 50 μm from the surface, HF and H
A predetermined amount of NO ₃ mixed acid is put into contact with the surface to be treated facing downward. The reaction with the mixed acid used here is represented by the following formula.

[0022]

Embedded image The concentration of mixed acid is HF 20%, HNO ₃ 35%,
The contact time with mixed acid is 100 seconds. Transfer the dissolved sample solution to a platinum evaporation dish with a volume of 100 ml and concentrate. This liquid was collected in a container made of fluorine resin and the total amount was 3 m.
Adjust the volume to l.

By this method, the diffusion region layer of the wafer having a thickness of about 50 to 60 μm can be dissolved into a solution, and trace metals are similarly dissolved and recovered. After that, measurement of trace metals is performed. (3) To quantitatively analyze a trace amount of metal on the silicon wafer substrate, a silicon wafer obtained by dissolving and removing the diffusion region layer is used. The sample preparation in which the remainder of the silicon wafer itself is made into a solution can be made into a solution by dissolving with a mixed acid of HF and HNO ₃ , and this treatment liquid can be analyzed by the flameless atomic absorption method.

As the procedure, for example, the one from which the diffusion region layer is removed is washed with ultrapure water in advance. Next, it is dried, but it is naturally dried in a clean bench to prevent contamination at this time.
To dissolve the rest of silicon wafer (bulk), about 1 g of sample is solubilized for trace metal analysis. Grinding of the sample is operated in a clean bench, and crushing is about 2 mm in size.
The degree. This cleaning is performed by immersion cleaning with HF for about 1 minute, and then finish cleaning with ultrapure water.

Next, the same drying as described above is performed, and a predetermined amount is weighed to form a solution. The concentration of the mixed acid of HF and HNO ₃ used for dissolution is 20 to 25%, and it is dissolved by heating in a closed container. This method uses a microwave melting device / method to prevent contamination from the processing atmosphere. Then transfer the sample solution to a platinum evaporation dish with a volume of 100 ml and concentrate. The concentration operation is performed in a clean draft to prevent contamination. This liquid is collected in a container made of a fluororesin and adjusted to a constant volume of 3 ml. After that, measurement of trace metals is performed.

By this method, the remainder of the silicon wafer can be dissolved into a solution, and trace metals can be dissolved and collected at the same time. In this way, a method using a closed container was used to form a solution of about 1 g in an amount so that the contamination from the processing atmosphere could be prevented quickly. In the sample processing step, in order to prevent contamination from the processing atmosphere, a clean draft and a clean bench are used in the clean room to improve cleanliness.

(4) Next, the method for measuring the elements will be described. FIG. 2 is a diagram showing a configuration of a flameless atomic absorption spectrometer according to an embodiment of the present invention. This is for measuring each element of the treatment liquid prepared in advance for the purpose of trace element analysis of each layer portion of the silicon wafer of the sample.

The flameless atomic absorption method is currently the most useful atomization method in which the atomization part is composed of the graphite furnace 3 and which has the highest sensitivity. This feature is that high sensitivity can be obtained by injecting a very small amount of sample into a tubular graphite furnace and completely vaporizing it. In this case, a 5 to 50 μl level of the sample solution S is injected from the micropipette 4 into the small hole in the upper part of the graphite furnace 3. The electrode 2 is for heating with the Joule heat generated by passing an electric current through the graphite furnace 3. Hollow cathode tube 1
The irradiation light L emitted from the light source is irradiated to the atomized state in the graphite furnace 3, reaches the detector 6 through the spectroscope 5, and the light absorption is measured by the detector 6. Then, the absorbance is output to the display unit 8 via the signal processing unit 7.

For the heating cycle in the flameless atomic absorption method, a program that goes through the processes of drying, ashing, and atomization after injecting the sample solution is set in advance. The absorption signal appears immediately after atomization and its peak value or integrated value is recorded. This absorption signal intensity is proportional to the element concentration in the sample solution, so use this relationship as a calibration curve for the standard solution,
Quantitative analysis of elements is performed.

The advantages of this flameless atomic absorption / graphite furnace atomization method are as follows. The detection limit of many elements is superior to that of flame atomic absorption method, analysis with a small sample amount of μl level is possible, interference control method during analysis measurement, background correction method, automatic control function, high Suitable as a sensitive elemental analysis method. In this way, a calibration curve is created by the least-squares method from the relationship between the absorbance and the concentration of trace metals under the following measurement conditions.

Table 1 shows the measurement conditions for the target metallic element according to the present invention.

[0032]

[Table 1]

FIG. 3 is a diagram showing a calibration curve of Fe according to the present invention. FIG. 4 is a diagram showing a calibration curve of Cu according to the present invention. FIG. 5 is a diagram showing a Ni calibration curve according to the present invention. FIG. 6 is a diagram showing a Cr calibration curve according to the present invention. The calibration curve obtained at this time has good linearity. The calibration curve coefficients in FIGS. 3 to 6 are represented by the formulas for determining the absorbance, and the empirical formulas for determining the concentrations are as follows. The unit of the concentration X (element) in the liquid is ppb, RI, y is the absorbance.

[0034]

## EQU1 ## X (Fe) = 75.0075 y + 0.00 × 10 ^-3 , correlation coefficient = 0.9826

[0035]

[Equation 2] X (Cu) = 79.9361 y + 2.72 × 10 ⁻³ , correlation coefficient = 0.998

[0036]

[Formula 3] X (Ni) = 401.501 y + 5.95 × 10 ⁻³ , correlation coefficient = 0.9932

[0037]

## EQU4 ## X (Cr) = 31.0451 y-8.42 × 10 ^-4 , correlation coefficient = 0.9993 It is understood that the correlation coefficient between the absorbance and each element concentration in the liquid is 0.9826 or more. Table 2 shows the results of repeated examination of the analysis accuracy by using the sample prepared by preparing the standard solution and applying the calibration curve. It can be seen that the repetitive analysis accuracy is extremely good when the coefficient of variation is 1% or less at the concentration of 50 ppb or less in the liquid.

[0038]

[Table 2]

An actual sample was analyzed to confirm the effectiveness of the analysis method according to the present invention. Silicon wafer is GTO (Gate
Turn Off) Used in wafer processes such as thyristors, it is a 4-inch size silicon wafer consisting of a surface oxide film, a diffusion region layer, and a non-diffusion part. B (boron) was ion-implanted into two wafers in the same lot under the same conditions, and then a wafer (A) vacuum-diffused and a wafer (B) diffused in an oxygen atmosphere were subjected to the above calibration curve. It analyzed and compared.

Table 3 shows the analysis results of the wafer (A) subjected to vacuum diffusion and the wafer (B) subjected to diffusion in an oxygen atmosphere.

[0041]

[Table 3]

In the analysis of the liquid obtained by treating each layer portion of the 4-inch silicon wafer, trace metals could be quantified at 0.02 to 56.1 ppb. Furthermore, based on the above data, in the evaluation of the number of atoms per unit area and unit volume, in the surface layer of the silicon wafer, the diffusion region layer, the evaluation of 10 ⁹ to 10 ¹⁵ atoms / cm ² ,
Remaining silicon wafer (bulk), 10 ¹² to 10 ¹⁴ atoms / cm
Can be evaluated as ³ .

In the vacuum diffusion wafer (A), the lifetime of the diffusion region layer was as small as 6.5 μs, but the results of the above analysis showed that the diffusion region layer was contaminated with several tens of ppb of heavy metal. In the vacuum diffusion wafer (A), the lifetime of the diffusion region layer was as small as 6.5 μs. According to the present invention, since it is possible to analyze a trace amount of metal in each layer portion of a silicon wafer, it can be said that it is an extremely practical method applicable to heavy metal evaluation in a semiconductor wafer process.

[0044]

According to the present invention, in a silicon wafer analysis method for quantitatively analyzing a metal element in a silicon wafer having an oxide film on the outermost surface and a diffusion region layer below the oxide film, all of the oxide film is dissolved into a solution. Then, a sample preparation step in which the metal diffusion region layer is dissolved into a solution having a predetermined thickness and then the rest of the silicon wafer is dissolved into a solution having a predetermined thickness or a predetermined weight one or more times, and each of these dissolution solutions is subjected to atomic absorption. Since the method comprises the quantitative step of quantitative analysis by the method, the oxide film of the silicon wafer, the diffusion region layer, and the rest can be hierarchically sampled, and the trace metal can be quantitatively determined with high accuracy.

The above-mentioned dissolution into solution is performed by pouring the solution for dissolution to the height of the projections of a shallow container having a plurality of projections having a constant height on the bottom surface, and only the surface of the silicon wafer to be dissolved is in contact with the solution. This is preferably done by placing a silicon wafer. Since the analysis sample preparation step and the quantification step are included, and the analysis sample preparation step is hierarchically prepared for each layer portion of the silicon wafer material, it is possible to evaluate a trace amount of metal for each layer portion.

This preparation method is simple in that HF is dropped onto the oxide film and tilted so as to contact the entire film, and this solution is recovered. In the quantification step, the level of ppt to ppb can be quantified by optimizing the analysis conditions of the flameless atomic absorption method for the analysis of trace metals. Also,
Although the surface oxide film mainly composed of SiO _2, the diffusion region layer mainly composed of silicon, and the rest have different properties when dissolved into solution, a solution solution method corresponding to each of these layers has been established. it can.

[Brief description of drawings]

FIG. 1 is a flow chart showing a procedure for preparing an analytical sample and a procedure for measuring elements according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a flameless atomic absorption spectrophotometer device according to an embodiment of the present invention.

FIG. 3 is a diagram showing a calibration curve of Fe.

FIG. 4 is a diagram showing a calibration curve of Cu.

FIG. 5 is a diagram showing a calibration curve of Ni.

FIG. 6 is a diagram showing a calibration curve of Cr.

[Explanation of symbols]

 1 Hollow Cathode Tube 2 Electrode 3 Graphite Furnace 4 Micropipette 5 Spectrometer 6 Detector 7 Signal Processing Section 8 Display Section S Sample Liquid L Irradiation Light

Claims (5)

[Claims] 1. A silicon wafer analysis method for quantitatively analyzing a metal element in a silicon wafer having an oxide film on the outermost surface and a diffusion region layer below the oxide film. To a predetermined thickness, and then the remaining portion of the silicon wafer is made into a predetermined thickness or a predetermined weight dissolution solution one or more times, and each of these dissolution solutions is quantitatively analyzed by the atomic absorption method. Wafer analysis method. 2. The method for analyzing a silicon wafer according to claim 1, wherein the dissolution solution is formed by dissolving hydrofluoric acid on the surface oxide film of the silicon wafer to form a dissolution solution. After the removal, the diffusion region layer is made into a solution using a mixed acid of hydrofluoric acid and nitric acid. After the diffusion region layer is dissolved and removed, the remainder of the silicon wafer is mixed with hydrofluoric acid and nitric acid. A method for analyzing a silicon wafer is characterized in that it is dissolved into a solution. 3. The method for analyzing a silicon wafer according to claim 2, wherein the mixed acid used for dissolving the diffusion region layer into a solution has a hydrofluoric acid concentration of 18 to 20% and a nitric acid concentration of 30%.
The method for analyzing silicon wafers is characterized in that it is -40%. 4. The method for analyzing a silicon wafer according to claim 2, wherein the concentration of the mixed acid used in the solution for the rest of the silicon wafer is 20 to 25%, and the solution is heated and dissolved in a closed container. Silicon wafer analysis method. 5. The method for analyzing a silicon wafer according to claim 2, wherein the dissolution and solution is performed by using a jig having a plurality of protrusions having a constant height on a flat plate, and a silicon wafer placed on the protrusions. A method for analyzing a silicon wafer, characterized in that the dissolution solution is held only between the flat plate and the plate.