JPH06213805A - Etching method in ultramicroanalysis of metal on surface of silicon wafer - Google Patents

Abstract

PURPOSE:To prevent contamination from chemical and to decrease contamination from a container by performing decomposition with the mixed vapor of nitric acid and hydrofluoric acid in a specified ratio, and analyzing liquid of dew condensation. CONSTITUTION:Aqueous solution containing nitric acid of 14.3-14.9mol/l and hydrofluoric acid of 1.1-0.2mol/l is heated. The very minute amount of metal in the layer down to the depth of 1mum from the oxide film of the surface of a wafer and a silicon-bulk surface layer is dissolved with the generated vapor and the liquid of dew condensation generated by the cooling at the surface of a silicon wafer. The liquid of dew condensation is analyzed. In order to perform the ultramicroanalysis of the metal on the surface of the wafer 1, the weight of the wafer 1 is measured. The amount of the liquid of the dew condensation is obtained based on the difference between the weight and the weight before the processing and the specific gravity of the liquid of the dew condensation, which is obtained beforehand. Then, the liquid of dew condensation is recovered with a micropipette and measured with a frameless atomic absorbing analyzer as a sample. The weight of the silicon wafer after drying is measured as required, and the etching amount is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, as a pre-process for quantifying ultra trace elements contained in an oxide film and a silicon bulk layer on the surface of a silicon wafer, etches the oxide film and the silicon bulk layer of a silicon wafer to obtain an ultra trace metal element. It relates to an etching method for making an analysis.

[0002]

2. Description of the Related Art The following are examples of surface analysis methods in the method for quantifying ultratrace impurity metals on the surface of a silicon wafer. Place the wafer horizontally or vertically in an airtight container and vaporize hydrofluoric acid at room temperature or by heating.
It is said that the hydrogen fluoride gas reacts with the oxide film to decompose when left for about 2 hours. (JP-A-60
-69531, JP-A-1-98944). However, this method has a problem in that the surface is decomposed only with hydrofluoric acid gas, which requires a long period of time for decomposition, and that metals such as Cu having a smaller ionization tendency than silicon are not decomposed.

In addition to the hydrofluoric acid vapor, mixed acid (2
A method of decomposing the surface of a silicon wafer using a vapor of 0% hydrofluoric acid and 30% nitric acid to prepare a sample solution (JP-A-3-
No. 188642), but nitric acid for recovering the reaction product is added because a condensed liquid sufficient for quantification cannot be obtained by this method. This is inconsistent with the vapor decomposition method for preventing the contamination from the chemical liquid, and is a pollution factor even though the amount of the chemical liquid is small.

[0004]

In these conventional methods, the oxide film is decomposed by hydrofluoric acid, but the silicon bulk layer is not decomposed, so that the true cleanliness of the silicon wafer surface cannot be evaluated. Elements such as redeposit on the surface of the silicon wafer and cannot be measured,
The addition of nitric acid has a problem such as concern about contamination from the chemical solution. Therefore, an object of the present invention is to provide an improved etching method for ultra trace metal analysis of a silicon wafer surface, which solves the above problems.

[0005]

DISCLOSURE OF THE INVENTION The present invention is an acid obtained by mixing nitric acid and hydrofluoric acid.
14.9 mol / l and hydrogen fluoride 1.1-0.2 m
By the steam generated by heating the aqueous solution containing ol / l and the dew condensation liquid generated by cooling on the surface of the silicon wafer,
A method for etching ultra-trace amounts of metal on the surface of a silicon wafer, characterized in that an ultra-trace amount of metal in a layer from the silicon wafer surface oxide film and / or the silicon bulk surface layer to a depth of 1 μm is dissolved and the condensed liquid is analyzed. .

[0006]

As described above, according to the present invention, in order to avoid the contamination from the chemical solution, the surface of the silicon wafer is decomposed using the vapor of the chemical solution, nitric acid is added as an oxidant to the decomposition gas, and nitric acid and hydrofluoric acid are mixed. The vapor decomposes silicon, and the nitric acid contained in the dew condensation solution also dissolves elements such as copper. Dew condensation on the silicon wafer surface is sufficient (10-1000 μl) for the reaction decomposition solution. Since the condensation liquid is analyzed without adding chemicals, contamination from the environment such as devices, instruments, and chemical liquids is eliminated, and ultratrace impurity metals can be quantified accurately and quickly. .

In the present invention, the nitric acid and hydrogen fluoride contents in the mixed acid of nitric acid and hydrofluoric acid are limited to the above specific ranges, respectively, which will be described later when the specific ranges are exceeded. As described above, the amount of etching on the surface of the silicon wafer may not be appropriate, or the amount of the dew condensation liquid may be less than the amount required for the subsequent quantitative analysis.

Further, in the present specification, the term "mixed acid" hereinafter means an acid obtained by mixing nitric acid and hydrofluoric acid, which means that nitric acid 1
4.3-14.9 mol / l and hydrogen fluoride 1.1-
It means an aqueous solution containing 0.2 mol / l, and the "silicon bulk layer" means a depth of 1 μm from the surface layer of the silicon bulk.
Up to layers.

The method for etching the surface of a silicon wafer according to the present invention is, for example, (a) placing a mixed acid of nitric acid and hydrofluoric acid in a container capable of sufficiently heating the bottom and the bottom of the side surface and placing the silicon wafer on the top of the container. Fixed horizontally,
(B) The container is heated to generate a mixed vapor of nitric acid and hydrofluoric acid. (C) The mixed vapor is cooled on the surface of the silicon wafer to cause dew condensation. At this time, the ultra-trace amount of metal on the surface of the silicon wafer is in the dew condensation liquid. And (d) collecting the condensed liquid as an analytical sample without adding a chemical liquid.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a vertical sectional view of an example of an apparatus used for carrying out the present invention. To carry out ultratrace metal analysis on the surface of a silicon wafer according to the present invention using this apparatus, it is carried out as follows. First, the weight of the silicon wafer 1 is measured. Next, the heater 3 at the bottom of the decomposition liquid heating / evaporating vessel 2 and the bottom of the side surface are heated to keep the mixed acid 4 at 60 to 65 ° C. and generate a sufficient mixed vapor of nitric acid and hydrofluoric acid.
Vacuum tweezers 6 on the upper part of the container 2 where it is hard to heat
Then, the silicon wafer 1 is fixed horizontally and exposed to steam for a certain time. The vapor that has contacted the silicon wafer 1 is cooled on the surface and condenses. The condensation liquid on the surface of the silicon wafer is collected by spraying a gas (clean nitrogen or the like). The weight of the silicon wafer is measured, and the amount of the dew condensation liquid is obtained from the difference between the weight before the treatment and the previously determined specific gravity of the dew condensation liquid. The condensed liquid is collected with a micropipette, and this is used as a sample for measurement by a flameless atomic absorption spectrometry (ICP-MS) device. If necessary, the weight of the dried silicon wafer is measured to obtain the etching amount.

As a result of the above steps and, the ultra-trace amount metal of the native oxide film on the surface of the silicon wafer and the silicon bulk layer reacts with the decomposition gas and the dew condensation liquid to be decomposed and dissolved in the dew condensation liquid. Therefore, the impurities to be detected are
In (1), it remains in the dew condensation liquid on the surface of the silicon wafer until it is collected by the micropipette, and does not cause a positional change that causes mixing of other impurities. Further, since the chemical solution is not used for collecting impurities, it is not necessary to worry about the contamination from the chemical solution.

Table 1 shows that, as an etching solution, a mixed solution of nitric acid aqueous solution (68% by weight nitric acid) and hydrofluoric acid (38% by weight hydrogen fluoride) having various mixing ratios was used. Fig. 3 shows the results of observing the amount of dew condensation on the silicon wafer and the etching amount by vapor-depositing the silicon wafer to vapor-etch it.

[0013]

[Table 1]

As shown in Table 1, when the amount of hydrofluoric acid in the mixed solution was large, it reacted instantly and a large amount was etched, but the wafer surface was dried and no dew condensation liquid was obtained.
When hydrofluoric acid was decreased, the amount of dew condensation liquid gradually increased, and as shown in Table 1, when mixed at a volume ratio of nitric acid (68% by weight) 97: hydrofluoric acid (37% by weight) 3. The amount of dew condensation liquid was large, the amount of etching was appropriate, and the size and distribution of dew condensation particles were uniform and optimum. However, when the amount of hydrofluoric acid was extremely reduced, the etching amount became insufficient. From these results, the contents of nitric acid and hydrogen fluoride in the mixed acid used in the present invention are limited to 14.3 to 14.9 mol / l and 1.1 to 0.2 mol / l, respectively. Note that FIG.
FIG. 4 shows an example of the etching depth and the amount of dew condensation liquid with respect to the exposure time when etching is performed with such a mixed acid vapor of a specific ratio according to the present invention.

[0015]

As described above, according to the present invention, chemicals are not dropped directly on the surface of a silicon wafer for decomposition, but decomposed with a mixed vapor of nitric acid and hydrofluoric acid in a specific ratio to analyze the condensed liquid. Therefore, contamination from chemicals can be prevented, and further, from the reaction to the recovery, the contamination from the container can be reduced because it is not carried out on the surface of the silicon wafer to be measured and transferred to another container. Further, not only the natural oxide film but also the silicon bulk layer is decomposed by the mixed acid vapor to be a measurement target, so that the cleanliness control of the silicon wafer can be improved.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view of a surface decomposition apparatus used for carrying out the present invention.

FIG. 2 is a graph showing the relationship between the exposure time, the etching depth, and the amount of dew condensation liquid obtained in one example of the present invention.

[Explanation of symbols]

1 ... Silicon wafer, 2 ... Heating / evaporating container, 3 ... Heater, 4 ... Nitric acid + hydrofluoric acid, 5 ... Mixed acid vapor, 6 ... Vacuum tweezers.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 8, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

[Correction content]

[Fig. 2] ─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 26, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction content]

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a vertical sectional view of an example of an apparatus used for carrying out the present invention. To carry out ultratrace metal analysis on the surface of a silicon wafer according to the present invention using this apparatus, it is carried out as follows. First, the weight of the silicon wafer 1 is measured. Then, the heater 3 at the bottom of the decomposition liquid heating / evaporating vessel 2 and the bottom of the side surface are heated to keep the mixed acid 4 at 75 to 80 ° C. and generate a sufficient mixed vapor of nitric acid and hydrofluoric acid.
Vacuum tweezers 6 on the upper part of the container 2 where it is hard to heat
Then, the silicon wafer 1 is fixed horizontally and exposed to steam for a certain time. The vapor that has contacted the silicon wafer 1 is cooled on the surface and condenses. The condensation liquid on the surface of the silicon wafer is collected by spraying a gas (clean nitrogen or the like). The weight of the silicon wafer is measured, and the amount of the dew condensation liquid is obtained from the difference between the weight before the treatment and the previously determined specific gravity of the dew condensation liquid. The condensed liquid is collected with a micropipette, and this is used as a sample for measurement by a flameless atomic absorption spectrometry (ICP-MS) device. If necessary, the weight of the dried silicon wafer is measured to obtain the etching amount.

Claims (1)

[Claims] 1. A method for decomposing a silicon wafer surface for quantifying an ultratrace impurity metal on a surface of a silicon wafer, wherein the acid is a mixture of nitric acid and hydrofluoric acid.
4.3-14.9 mol / l and hydrogen fluoride 1.1-
Due to the vapor generated by heating an aqueous solution containing 0.2 mol / l, and the dew condensation liquid generated by cooling on the surface of the silicon wafer, the oxide film on the surface of the silicon wafer and / or the layer from the silicon bulk surface layer to a depth of 1 μm An etching method for ultra trace metal analysis of a silicon wafer surface, which comprises dissolving a trace amount of metal and analyzing the condensed liquid.