JP3331106B2 - Method for analyzing impurities in semiconductor thin film or semiconductor substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for analyzing impurities in a semiconductor thin film or a semiconductor substrate, and more particularly to a method for analyzing metal impurities present in a silicon semiconductor thin film or a silicon semiconductor substrate.

[0002]

2. Description of the Related Art Metal impurities such as Na and Fe present in a semiconductor thin film and a semiconductor substrate generally cause deterioration of oxide film breakdown voltage and crystal defects, and greatly affect semiconductor device characteristics. give.
Therefore, in the device manufacturing process, metal impurities are removed by washing, gettering, or the like. Due to the recent high integration, miniaturization and high performance of semiconductor devices, the allowable metal impurity concentration has been further lowered, and accordingly, higher sensitivity has also been required for impurity analysis methods to grasp and control contamination. Have been.

As a technique for analyzing impurities in a semiconductor thin film or a substrate, a chemical analysis technique as shown in Table 1 is mainly used. These techniques attempt to improve the analytical sensitivity by devising, for example, distilling the reagent to be used and concentrating the decomposition solution, but they are not necessarily sufficient yet. One of the limiting factors of sensitivity in current chemical analysis is insufficient sensitivity of a measuring device (such as an atomic absorption device). Therefore, to improve the sensitivity of impurity analysis, it is considered effective to increase the amount of decomposition of the semiconductor sample and increase the absolute amount of metal impurities.

[0004] However, in order to decompose a large amount of sample, more reagent is required, and contamination from the reagent occurs. On the other hand, if the amount of reagent used is reduced as much as possible, a large amount of silicon compound remains, and there is a problem that the amount of reagent for re-dissolving when performing measurement with a liquid sample increases, and it is not easy to improve the analysis sensitivity. difficult. For example, M. B. Shabani et al. Report the analysis of the concentration of metal impurities in a substrate by decomposing 1 g of a sample (Analytical Society of Japan Fall Meeting 2H09p405 (199)
5)) Only the theoretical (on-apparatus) detection limit values are shown, but no actual analysis results have been reported yet.
Table 1 summarizes such prior art.

[0005]

[Table 1] An object of the present invention is to provide a method capable of decomposing a large amount of a semiconductor thin film or a semiconductor substrate with a minimum necessary amount of reagents and analyzing the concentration of metal impurities with high sensitivity without generating a large amount of residues upon decomposition of the semiconductor. It is.

[0006]

Means for Solving the Problems The present inventors use an acidic solution containing hydrofluoric acid and nitric acid at a specific ratio when decomposing a semiconductor thin film or a semiconductor substrate, or use hydrofluoric acid. It has been found that the above problem can be solved by using an acidic solution containing the solution at the time of decomposition and then decomposing with nitric acid.

That is, according to the present invention, a semiconductor thin film or a semiconductor substrate comprises hydrofluoric acid and nitric acid, and the mixing ratio of hydrofluoric acid to nitric acid is 0.3% by weight.
Preparing an analysis sample from the residue obtained after the step of decomposing and liquefying the liquid obtained by the decomposition, heating and evaporating the liquid obtained by the decomposition, and the step of concentrating the liquid obtained by the decomposition. A method for analyzing impurities in a semiconductor thin film or a semiconductor substrate, comprising: a step of analyzing a sample to be analyzed;

In another aspect of the present invention, nitric acid may be added in the concentration step after decomposing the substrate or the thin film with an acidic solution containing hydrofluoric acid.

According to a preferred aspect of the present invention, the thin film or the substrate is immersed in the acidic solution,
The thin film or the substrate may be decomposed and liquefied,
The thin film or the substrate may be decomposed and liquefied by bringing the thin film or the substrate into contact with the vapor of the acidic solution.

Further, sulfuric acid may be added in the concentration step. The analysis of the analysis sample is preferably an atomic absorption spectrometer, a high frequency induction plasma mass spectrometer,
Alternatively, it is performed by a total reflection X-ray fluorescence spectrometer.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors prepared a silicon semiconductor thin film or a silicon semiconductor substrate (also simply referred to as a silicon substrate, a substrate or a wafer in this specification) using a nitric acid solution containing hydrofluoric acid and nitric acid. When the residue generated when decomposed by the reaction was examined, the main component was diammonium hexafluorosilicate (NH ₄ ).
₂ SiF ₆ , and also confirmed to contain trace amounts of metal impurities to be analyzed.

FIG. 1 shows an X-ray diffraction spectrum of the residue.
Table 2 shows the nuclear magnetic resonance spectrum of the residue. These all indicate the presence of diammonium hexafluorosilicate. By reducing the amount of this residue,
The concentration of metal impurities in a sample to be finally analyzed can be increased, and highly sensitive analysis of metal impurities can be performed.

[0013]

[Table 2] FIG. J. Mechanism of formation of diammonium hexafluorosilicate (NH ₄ ) ₂ SiF ₆ by the self-reduction reaction of nitrite ion proposed by Meyer et al. (Chemistry and Industry, Chemical Society of Japan, KAKTAF 48)
(4) (1995)). In general, it is presumed that this reaction occurs when a silicon semiconductor substrate or the like is decomposed by hydrofluoric acid and nitric acid. If the amount of nitric acid is small at the time of decomposition, the reaction shown in FIG. 2 causes a self-reduction reaction of nitrite ions to produce ammonia and subsequently ammonium ions, and finally (NH ₄ ) ₂ SiF ₆
Is generated. However, it can be seen that when the amount of nitric acid increases, the self-reduction reaction of nitrite ions is suppressed, ammonia is hardly generated, and the amount of (NH ₄ ) ₂ SiF ₆ also decreases.

FIG. 3 shows an example of a typical silicon semiconductor substrate disassembling procedure. As shown in FIG. 3A, the silicon semiconductor substrate 301 is immersed in a mixed solution 302 of hydrofluoric acid and nitric acid in a Teflon beaker 303 to be decomposed. Thereafter, as shown in FIG. 3B, the decomposition solution containing the dissolved metal impurities 304 is concentrated. When the concentration was further continued, FIG.
As shown in (c), the decomposition solution is dried and a residue 305 is generated. The residue 305 is redissolved in pure water 306 to prepare an analysis sample. A suitable mixing ratio of hydrofluoric acid and nitric acid in such a method was studied.

In the present specification, the mixing ratio of hydrofluoric acid to nitric acid is {(concentration of hydrofluoric acid used, weight%) × (amount of hydrofluoric acid used mL)} / {( Concentration of used nitric acid, weight%) x (amount of used nitric acid m
L)｝.

FIG. 4 shows the results, in which the mixing ratio of concentrated hydrofluoric acid (38% by weight) and concentrated nitric acid (68% by weight) was changed, and the residue generated after concentrating the decomposition solution of the silicon semiconductor substrate was removed. Silicon compound ((NH ₄ ) ₂ SiF
FIG. 6 is a correlation diagram showing the result of examining the amount of ( ₆ ) in detail.
Since it was difficult to directly measure the weight of the silicon compound in the residue, the evaluation was performed based on the minimum amount of pure water required to dissolve the residue. Judge as residue, 100μL
The case where it exceeded was evaluated as a large amount of residue. As shown in FIG. 4, when the mixing ratio of hydrofluoric acid to nitric acid (HF / HNO ₃ ) is larger than 0.6, a large amount of residue is generated regardless of whether the amount of Si is 1 g or 2 g. When it was less than 3, it did not completely decompose. Therefore, when the mixing ratio of hydrofluoric acid and nitric acid is in the range of 0.3 to 0.6, the silicon semiconductor substrate is decomposed and (NH ₄ ) ₂ S
It can be seen that the residue consisting iF ₆ itself is reduced.
In addition, even when the mixing ratio is less than 0.3 or greater than 0.6, a certain amount of hydrofluoric acid and nitric acid is contained, and if the amount of the acidic solution is particularly large, the silicon semiconductor substrate or the like may be used. Decomposed and (NH ₄ ) ₂ SiF
_{Although the} residue composed of ₆ itself may decrease, it is not preferable because contamination by the used acid increases.

As described above, if the silicon substrate is decomposed by the mixed solution of hydrofluoric acid and nitric acid having the HF / HNO ₃ mixing ratio in the range of 0.3 to 0.6, and the decomposed solution is heated and concentrated, Since the amount of the residue composed of NH ₄ ) ₂ SiF ₆ itself is reduced and a small amount of solvent is required for re-dissolving to prepare an analysis sample, highly sensitive analysis of metal impurities becomes possible.

Hydrofluoric acid and nitric acid that can be used in the present invention require the use of a high-purity reagent in order to prevent deterioration of analysis accuracy due to contamination from these acids. However, in some cases, reagents for the electronics industry can be used. The concentration of the acid to be used is not particularly limited and may be diluted, but it is preferable to use concentrated hydrofluoric acid and concentrated nitric acid in consideration of the decomposition time and the concentration time. As described above, nitric acid can be used in the decomposition solution, or can be added during decomposition, before the concentration step, during the concentration step, or after the concentration step, whereby (NH ₄ ) ₂ S
The amount of residue of iF ₆ itself can be reduced.

The decomposition temperature can be appropriately selected according to the necessity of the reaction rate, safety and the like if the acidic solution used for the decomposition is a liquid.

The decomposition is not limited to immersion in an acidic solution, but may be performed by steam. Decomposition by immersion is preferable when a large amount of a silicon semiconductor substrate is simply and quickly dissolved, and decomposition by steam is preferable when metal impurities caused by an acidic solution pose a problem in analysis accuracy. In the case of decomposition by steam, generation of steam can be performed, for example, by heating an acidic solution. The heating can be performed, for example, by heating a container containing the acidic solution to a temperature of from room temperature to 230 ° C., for example, to 200 ° C.

As shown in FIG. 5, when the decomposition is carried out by steam, the decomposition time is lengthened to make (NH ₄ )
It is possible to reduce the amount of ₂ SiF _{6. In} this example, the decomposition is completed by decomposition for 3 hours or more. In this case, there was no residue composed of (NH ₄ ) ₂ SiF ₆ .

When the residue consisting of (NH ₄ ) ₂ SiF ₆ itself is extremely small (to the extent that it is not observed visually, for example, 2 ng or less), it is preferable to add sulfuric acid and concentrate. This is because when the metal impurities are completely dried and the metal is converted into an oxide, the metal is hardly dissolved in pure water and the recovery rate is reduced. However, when sulfuric acid is added, the metal exists in a solution state (ion). This is considered to be due to the fact that it can be dissolved well, and a decrease in the recovery rate can be prevented. In this case, the concentration and the amount of addition are not particularly limited. For example, 10 μL of 33% by weight sulfuric acid is added. Also, (NH ₄ ) ₂
When there is a residue composed of SiF _{6, the} recovery rate does not decrease even without adding sulfuric acid.

The element to be analyzed by the method of the present invention is not particularly limited, but is preferably Na, Al, Ca, C
r, Mn, Fe, Cu, Ti, Mo, Au, Ag, W,
More preferably, Na, Al, Ca, Cr, Mn, F
e, Cu.

In the present invention, the metal impurity concentration can be measured using an atomic absorption spectrometer, a high-frequency inductively coupled plasma mass spectrometer (ICP-MS) or a total reflection X-ray fluorescence spectrometer (TREX). .

When analysis is performed by an atomic absorption spectrometer, the residue after concentration is diluted with about 100 μL of pure water to prepare an analysis sample. In this device, the sensitivity is sub-ppb
It is of the order, and has the advantage that it is highly sensitive to Na, Cu, and Zn, and is not easily affected by Si contained in the matrix. However, since the destructive measurement is performed for each element, the number of measurable elements and the number of measurements are limited.

When performing analysis by ICP-MS,
An analysis sample is prepared as in the case of the atomic absorption spectrometer.
It is preferable to use ETV (electrothermal vaporization) as the introduction system because a sample volume of about 100 μL is sufficient. Analysis with higher sensitivity (ppt to hundreds of ppt) is possible than an atomic absorption spectrometer, and heavy metals (Mo, Au, Ag, Ag,
There is an advantage that measurement of W) and measurement of multiple elements simultaneously are possible. Conventionally, the application was limited because the sample was easily affected by the matrix and it was difficult to measure a sample containing a residue at a high concentration.However, the present invention can reduce the residue and reduce the influence of the matrix. , Can be suitably used.

In the case of performing analysis by TREX, the decomposition solution is concentrated in advance to 1 mL or less, and the concentrated solution is completely dried by heating on a sufficiently clean Si wafer. Thereafter, fluorescent X-ray analysis is performed on this concentration point.
K, Ca, Ti, Cr, Mn, Fe, Co, Ni, C
Although there is an advantage that the simultaneous analysis of u and Zn can be performed with high sensitivity, measurement is difficult if there are many residues, and there has been a limitation on the application in the past. However, the present invention can reduce the residues. In the present invention, it can be suitably used.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. Example 1 Analysis of Metal Impurity Concentration in Silicon Substrate The silicon substrate was decomposed by the method shown in FIG.
The silicon substrate was subjected to a heat treatment 1 at 1190 ° C. for 24 hours in an O ₂ and N ₂ atmosphere, and a heat treatment 2 was carried out in an O ₂ atmosphere at 780 ° C. for 3 hours and then at 1000 ° C. for 16 hours. Heat treated for hours was analyzed.

[0029] the silicon substrate 2 g (301) Teflon beaker 303 placed, hydrofluoric acid (HF38 wt%, Tama Chemical Co., ultra-high purity analytical reagents, TAMAPURE-AA series) and concentrated nitric acid (HNO ₃ 68 Weight%, Tama Chemical Co., Ltd., ultra high purity analytical reagent, TAMAPURE-AA series)
Forty-five milliliters of 45 acidic solution of No. 45 was put in three times, and the silicon substrate was decomposed at room temperature for 0.5 hours. After the decomposition was completed, the decomposition solution was heated at 200 ° C. and concentrated. After concentration, the residue 305 was diluted with 100 μL (306) of pure water, and the concentration analysis of the metal impurities 304 was performed by an atomic absorption spectrometer or a high frequency inductively coupled plasma mass spectrometer.

Table 3 shows the results of analysis of the concentration of metal impurities in the silicon substrate subjected to the different heat treatments by an atomic absorption spectrometer. It can be seen that this method can measure that the contaminant form of the silicon substrate differs depending on the heat treatment conditions. As described above, according to the present invention, the amount of residue can be reduced even with a small amount of liquid, and the detection limit is 8 × 10
¹¹ cm ^-3 and sensitivity of step etching method 1 × 10 ¹³ cm ^-3
It is possible to grasp low-concentration contamination, which was unknown in the past, by two orders of magnitude.

[0031]

[Table 3] Table 4 shows the results obtained by applying this method to a silicon substrate to which a known amount of contamination has been added in advance and determining the recovery rate.
Elements other than Zn: 95% for Na, Cr, Fe and Cu
The above recovery rates were obtained, and it was clear that there was no problem in the reliability of this method.

[0032]

[Table 4] Example 2 Analysis of Metal Impurity Concentration in Nitride Film A silicon nitride film (semiconductor thin film) 602 on a silicon substrate 601 is decomposed by a hydrofluoric acid vapor 603 by a vapor phase decomposition method shown in FIG. Hydrofluoric acid vapor is generated by evaporating (at room temperature) hydrofluoric acid in a closed container, and the silicon nitride film is exposed to the vapor for 2 hours to decompose. Since a large amount of silicon is dissolved in the solution 604 generated by the decomposition, the solution 604 is concentrated as it is (NH ₄ ) ₂ S
iF ₆ remains as a white solid. Therefore, the decomposition solution 60
4 was added with 1 mL of concentrated nitric acid (605).
01 is heated, and the decomposition solution 604 is concentrated to dryness. After drying,
The residue (metal impurity) 606 at the enrichment site is analyzed by TREX to analyze the metal impurity concentration of the nitride film.

Table 5 shows the results of the analysis. Conventionally, the total reflection condition was not satisfied by (NH ₄ ) ₂ SiF ₆ existing after drying, and analysis by TREX was impossible. However, by adding nitric acid to the decomposition solution at the time of concentration, (NH _4)
2 ) The residual SiF ₆ was eliminated and analysis by TREX became possible.

[0034]

[Table 5]

[0035]

As described above in detail, according to the present invention, the production of diammonium hexafluorosilicate (NH ₄ ) ₂ SiF ₆ , which is a main component of residues generated when a semiconductor thin film or a semiconductor substrate is decomposed, is suppressed. Can be reduced. Therefore, it is possible to provide a highly sensitive method for analyzing the metal impurity concentration of a semiconductor thin film or a substrate with a minimum necessary amount of reagent and without causing excessive contamination or a large amount of residue. Further, diammonium hexafluorosilicate (NH ₄ ) ₂
By preventing the generation of SiF ₆ , metal impurity analysis by the TREX method, which has been impossible to measure, can be performed.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction spectrum of a residue obtained by concentrating and drying a solution obtained by decomposing a semiconductor.

FIG. 2 Diammonium hexafluorosilicate (NH ₄ ) ₂ Si by a self-reduction reaction of nitrite ions
FIG. 4 is an explanatory diagram of a generation mechanism of F ₆ .

FIG. 3 is a diagram illustrating an example of a method for analyzing a semiconductor substrate.

FIG. 4 is a correlation diagram between the amount of a silicon substrate to be decomposed, the mixing ratio of hydrofluoric acid and nitric acid, and the amount of a decomposed liquid and a residue.

FIG. 5 shows the decomposition time of hydrofluoric acid and nitric acid vapor and the residual (NH ₄ ) in the decomposition of a 2 g silicon substrate.
₂ is a correlation diagram between SiF ₆ content.

FIG. 6 is a diagram illustrating an example of a method for analyzing impurities of a semiconductor thin film or a semiconductor substrate according to the present invention.

[Explanation of symbols]

301 silicon substrate 302 mixed solution of hydrofluoric acid and nitric acid 303 beaker made of Teflon 304 metal impurity 305 (NH ₄ ) ₂ after the decomposition solution is concentrated to dryness
Residue containing SiF ₆ 306 Pure water 601 Silicon substrate 602 Silicon nitride film 603 Hydrofluoric acid vapor 604 Solution obtained by decomposing nitride film 605 Nitric acid 606 Residue after concentration and drying of decomposed solution

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01N 1/28 G01N 23/223 G01N 27/62 H01L 21/66 JICST file (JOIS)

Claims (6)

(57) [Claims] An acidic solution comprising a semiconductor thin film or a semiconductor substrate containing hydrofluoric acid and nitric acid and having a mixing ratio of hydrofluoric acid to nitric acid of 0.3 to 0.6 by weight concentration. Decomposing and liquefying, heating the liquid obtained by the decomposition, concentrating by volatilizing, preparing an analytical sample from a residue generated after the concentrating step, analyzing the analytical sample A method for analyzing impurities in a semiconductor thin film or a semiconductor substrate. 2. A step of decomposing a semiconductor thin film or a semiconductor substrate with an acidic solution containing hydrofluoric acid to liquefy the liquid, and heating and volatilizing the liquid obtained by the decomposition to concentrate the liquid. A step of adding nitric acid, a step of preparing an analytical sample from a residue generated after the concentration step, and a step of analyzing the analytical sample. . 3. The semiconductor thin film or the semiconductor substrate according to claim 1, wherein the thin film or the substrate is immersed in a solution of the acidic solution to decompose and liquefy the thin film or the substrate. Impurity analysis method. 4. An impurity in a semiconductor thin film or a semiconductor substrate according to claim 1, wherein the thin film or the substrate is brought into contact with vapor of the acidic solution to decompose and liquefy the thin film or the substrate. Analysis method. 5. The method for analyzing impurities in a semiconductor thin film or a semiconductor substrate according to claim 1, wherein sulfuric acid is added in said concentration step. 6. The semiconductor thin film according to claim 1, wherein the analysis of the analysis sample is performed by an atomic absorption spectrometer, a high frequency induction plasma mass spectrometer, or a total reflection X-ray fluorescence spectrometer. Or a method for analyzing impurities in a semiconductor substrate.