JPH0459700A - Washing of silicon wafer - Google Patents

Abstract

PURPOSE:To obtain a high-cleanness silicon wafer in which metal-based pollutants and fine particle pollutants are remarkably reduced by washing the silicon wafer using an aqueous solution containing respectively specified amount of hydrochloric acid, sulfuric acid, nitric acid and hydrogen fluoride. CONSTITUTION:In washing a silicon wafer, an aqueous solution containing >=1wt.% hydrochloric acid or >=1wt.% sulfuric acid, 1-30wt.% nitric acid and 0.01-10wt.% hydrogen fluoride is used as a washing liquid. If using the above- mentioned washing method for a silicon wafer, a high-cleanness silicon wafer can be obtained and lowering of quality of the silicon wafer such as occurrence of a defect inducing an oxide film or lowering of the life time can be avoided. In addition, deterioration of electric properties can also avoided be in case of using the silicon wafer for a highly integrated device such as IC or LSI.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a cleaning method for bringing the surface of a silicon wafer into a highly clean state.

BACKGROUND OF THE INVENTION In recent years, as devices have become more highly integrated, there has been a strong desire to further clean the surfaces of silicon wafers that serve as their substrates.

In general, silicon wafers are products with smooth surfaces that are cut from cylindrical single-crystal silicon into discs and then undergo lapping, etching, polishing, and cleaning processes to remove cutting distortion and contaminants. Become. If metal contaminants such as transition metals are present on the surface of the silicon wafer of this oldest product, it will cause quality problems of the silicon wafer such as the occurrence of oxidation-induced stacking faults and shortened lifetime. It also causes characteristic deterioration. Furthermore, the presence of particulate contaminants (also referred to as particles) such as silicon particles and dust causes disconnection or short circuits in device wiring. For this reason, cleaning is performed to remove these contaminants and cleanse.

The conventional cleaning method for silicon wafers is RCAc.
rRCA cleaning method invented by r+Kern et al.
], method of cleaning with a dilute aqueous hydrofluoric acid solution, (2) method of cleaning with a mixed aqueous solution of hydrochloric acid and hydrogen peroxide, (method of cleaning with a mixed aqueous solution of Machi VL acid and hydrogen peroxide, A method of cleaning with a mixed aqueous solution of hydrogen oxide, a method of cleaning with a mixed aqueous solution of hydrogen oxide, or a method of cleaning with a combination of these cleaning solutions is used.The cleaning solutions used in each of these cleaning methods include w1.
High purity EL with minimal particle contamination and metal contaminants
Chemicals called grades are used. Also, ultrapure water is used.

Among the conventional cleaning methods, the dilute hydrofluoric acid aqueous solution (f) has the ability to dissolve the natural oxide film that is usually about 10 times thick on the surface of a silicon wafer, so it removes metal contaminants in the oxide film. However, if the silicon wafer surface is contaminated with metal contaminants that have a smaller ionization tendency than Si, such as Cu, Cu cannot be completely removed, and depending on the concentration of the impurity Cu in the cleaning solution. The silicon wafer becomes contaminated again, and there is also the problem that it is difficult to reduce particulate contaminants.

・? This mixed aqueous solution of hydrochloric acid and hydrogen peroxide utilizes the high dissolving power of hydrochloric acid for metal-based contaminants, but it does not have the ability to dissolve oxide films. Metal contaminants that are particularly soluble in reducing acids (e.g. MCa, Cr, Fe, ML Zn
The effect of removing 'J) is high.

However, metal contaminants that are easily soluble in oxidizing acids but difficult to dissolve in reducing acids (e.g. CO1Cu, Mn
, Ni, etc.) has a low removal effect. Furthermore, it is less effective in removing metal contaminants present in the oxide film or at the interface between the oxide film and silicon.

・This mixed aqueous solution of sulfuric acid and hydrogen peroxide is generally used to remove organic contaminants and resist. However, sulfuric acid also has a high ability to dissolve metal contaminants (it has the same ability to remove metal contaminants as tusk cleaning solution).

The mixture of ammonia and hydrogen peroxide in L4 has the ability to dissolve silicon itself, so it is highly effective in removing metal contaminants present on it. However, the oxide film formed in the basic cleaning solution has the property of easily absorbing metal contaminants (eg, A1.Fe, Zn, etc.) in the cleaning solution, and there is a high possibility that the silicon wafer will be re-contaminated. However, since this method is highly effective in removing particulates, it is widely used by wafer manufacturers, device manufacturers, and chair manufacturers.

In addition, as a method to solve the problems of the above conventional method, 60
A slide etching method has been proposed in which a silicon wafer is immersed and cleaned in a mixed aqueous solution of nitric acid (wt%) and hydrogen fluoride (up to 1.1 wt%) (Ritsu).

Takizawa et al., “Extended Abstr.
acts of 5olid 5tateDevice
s and Materials”, 1988, p.
145).

However, even the highest purity nitric acid, called semiconductor grade, contains metal contaminants on the order of PPb or sub-PPb, such as AQ, Ca, Cr, Cu, Fe.
, Mg, Ni, Zn, etc., and the concentration of metal contaminants in the cleaning solution containing nitric acid at a high concentration of 60% is high. Metal impurities are eluted and their concentration increases.

Then, an oxide film is formed on the surface of the silicon wafer due to the strong oxidizing power of the highly concentrated nitric acid, and metal contaminants in the cleaning solution are easily incorporated into the oxide film. Therefore, it must be said that even the slide etching method has a limit in high cleaning of silicon wafers.

Furthermore, the silicon wafer is subjected to oxidation treatment in an oxidation furnace to form oxide H in which metal contaminants on the surface of the silicon wafer are incorporated into the oxide film, and then the silicon wafer is treated with hydrofluoric acid. A method called sacrificial oxidation is also used in which the oxide film is dissolved and the metal contaminants in the oxide film are dissolved in hydrofluoric acid and removed. However, some metal contaminants may diffuse into the silicon substrate (e.g. Cu),
Some substances (for example, Fe) form silicides that are insoluble in hydrofluoric acid. For this reason, it is not only impossible to remove metal contaminants, but it may also cause oxidation-induced stacking faults and a decrease in lifetime and dielectric breakdown of the oxide film. Therefore, it must be said that even the sacrificial oxidation method has a limit in high cleaning of silicon wafers.

Problems to be Solved by the Invention The present invention solves the following problems:
Determined by flameless atomic absorption spectrometry. In addition, by measuring the lifetime (hereinafter referred to as recombination lifetime) using the microwave reflection method, which is one of the methods for evaluating the cleanliness of silicon wafers, we have determined that the recombination lifetime of silicon wafers as a product and the metal The relationship between system pollutants was investigated.

As a result, the metal contaminants on the silicon wafer surface vary depending on the silicon wafer manufacturing process, but depending on the impurity metals originally contained in the chemicals used and the materials constituting the manufacturing equipment, there may be concentrations of M, CaCr, Cu, Fe, etc. , Mg
, Xi, Zn, etc. Also,
It has been found that when cleaning is performed with a mixed aqueous solution of ammonia and hydrogen peroxide, the silicon wafer becomes contaminated with Fe and the recombination lifetime is reduced.

The inventor has developed rough pink, etched silicon wafers,
Table 1 shows the results of measuring the amounts of contaminant metals, especially transition metals Cr, Cu, Fe, Ni, and Zn, after each step of polishing and cleaning. Wrapping has a pH of 10
Polishing was carried out using a polishing liquid containing alumina dispersed as abrasive grains in an alkaline aqueous solution with a pH of about 11, etching was carried out with a mixed acid of nitric acid and hydrofluoric acid, and polishing was carried out using an alkaline aqueous solution with a pH of about 10 with silica dispersed as abrasive particles. A polishing liquid was used for cleaning, and a mixed aqueous solution of ammonia and peroxide* was used for cleaning.

As can be seen from Table 1, silicon wafers are processed by Cr, Cu, F[!] during the lapping process. , Ni, and Zn.

In addition, ND in the table indicates that the value is below the lower limit of analytical quantification, and Cr is 1. OX 10”atoo+s/c
less than z2, Cu and Fe are 2. OX 10109at
o/cz less than 2, N1 is 2. QX 10” atoms
/c less than 2. In the etching process, metal contaminants from the previous process are considerably removed, but Cr, C:u
, Fe, and Zn. In the polishing process, Cr, Cu, FF! , N1, and Zn increase.

After the washing process, the concentration is often below the lower limit of quantification, but cr, Cu
There are occasional wafers that are contaminated with , Fe, Ni, and Zn. In addition, the silicon wafers in each process are contaminated with Kl, Ca, and Mg.

If the silicon wafer is contaminated in each process, it is thought to be due to the following reasons.

■The polishing liquid used in the lapping process is made by adding an organic amine to water as a medium and dispersing alumina in it, in order to have an etching effect on the silicon while grinding the silicon surface with abrasive grains. It has an alkaline level of 11. In addition, the polishing liquid contains impurities such as Ca,
It contains a large amount of Cr, Cu, Fe, Hg, Ni, Zn, etc. For example, Fe is contained in an amount of 500 to 1000 ppm. For this reason, contamination due to adsorption of impurity metals in the polishing agent to the silicon surface, which has become active due to the grinding and etching effects during polishing, or Si
Abrasive particles and impurity metals in the abrasive sink into the fine cracks in the strained layer that occur in the surface layer (depth: 15p).
m or more) (Monduma et al., “7th
"Semiconductor and Integrated Circuit Symposium", 1974, p.
103).

■ Even after the etching (etching amount: 20 gm/single side) process, detected Or, Fe, etc.
This is thought to be a contaminant that has penetrated into minute cracks in the strained layer that occurs on the Si surface layer.

■The polishing liquid used in the polishing process is made by adding potassium hydroxide or ammonia to water as a medium and dispersing silica in it, in order to have an etching effect on the silicon while grinding the silicon surface with abrasive grains. is alkaline with approximately 1O. In addition, the polishing liquid contains impurities such as AQ, Ca, Cr, Cu, Fe,
Mg, Ni, and Zn'Q are included in the four clays. For example, F
e also includes 1 to 3 ppll. Therefore, silicon wafers are considered to be contaminated during the polishing process for the same reason as during the rough pink process.

4. Contamination in the cleaning process, especially Fe contamination, is caused by a trace amount of impurity Fe contained in the cleaning solution, which is a mixed aqueous solution of ammonia and hydrogen peroxide. Even if the amount of impurity Fe in the cleaning solution is as small as 0.5 ppb, it will adsorb and contaminate the silicon wafer surface at a rate of about 8 x 10" atoms/cm2, resulting in a reduction in the recombination lifetime (
Otsuka et al., “Proceedings of the 34th Semiconductor and Integrated Circuit Technology Symposium,” 1988, p. 37).

As mentioned above, there are various situations in which silicon wafers become contaminated. Therefore, when cleaning silicon wafers, it is important to know the location of metal contaminants. That is, it is necessary to decide whether the cleaning liquid needs to have silicon etching properties or not, depending on whether the contamination is on the surface of the silicon wafer or if it is embedded in the silicon substrate. It is also important to know the form of metal contaminants. In other words, metal contaminants are selected from metal particles, oxides, salts, alloys such as silicides, and silicates by selecting the type and combination of acids that can efficiently dissolve and remove them. It is also important to determine the optimal cleaning conditions.

The present invention is designed to prevent contamination during the manufacturing process of silicon wafers and the final cleaning of products, such as Cr, Cu, Fe,
Metal contaminants such as Ni and Zn and particulate contaminants are extremely reduced, and there are no quality problems such as the occurrence of oxidation-induced stacking faults or a decrease in lifetime, and when it is made into a device, it is possible to reduce the dielectric breakdown of the oxide film and reduce the electrical The purpose is to obtain highly clean silicon wafers with no deterioration in characteristics.

Means and actions to solve the problem The gist of the present invention is as follows.

When cleaning silicon wafers, an aqueous solution containing 1% by weight or more of hydrochloric acid or sulfuric acid, 1 to 30% by weight of nitric acid, and 0.01 to 10% of hydrogen fluoride is used as a cleaning solution. A method for cleaning a silicon wafer, characterized by:

When cleaning silicon wafers, the method of the present invention utilizes the excellent ability of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid to dissolve metal contaminants, and also effectively combines these acids to improve cleaning performance. It provides an excellent cleaning solution.

That is, in the present invention, metal contaminants that dissolve well in reducing acids are dissolved and removed with hydrochloric acid, sulfuric acid, and hydrofluoric acid, and metal contaminants that dissolve well in oxidizing acids are dissolved and removed with nitric acid. Remove. At the same time, an oxide film is formed on the surface of the silicon wafer using the silicon oxidation effect of nitric acid.
Furthermore, by utilizing the oxide film dissolving power of hydrofluoric acid, the cleaning solution has the ability to etch silicon, removing not only metal contaminants on the oxide film of silicon wafers, but also inside the oxide film, the oxide film, and the silicon substrate. This method efficiently and thoroughly removes metal contaminants at the interface and in the silicon substrate.

In the present invention, the concentrations of hydrochloric acid, sulfuric acid, nitric acid, and hydrogen fluoride in the cleaning solution are HCu, H2SO, HNO3, respectively.
and weight percent as HF. Hydrochloric acid or sulfuric acid is 1
less than 1% by weight, less than 1% by weight of nitric acid, and 0% of hydrogen fluoride
．． If the amount is less than 0.01% by weight, the effect of removing metal contaminants will be insufficient. Even if the concentration of hydrochloric acid or sulfuric acid is increased, the removal effect of metal contaminants remains the same.
The upper limit of the concentration of these acids is not particularly limited and is set to 1% by weight or more.

If hydrogen fluoride is increased to more than 10% by weight, particulate contaminants tend to increase, so the concentration of hydrogen fluoride is reduced to 0.
01 to 10% by weight. Also, by increasing the concentration of nitric acid,
If it exceeds 0% by weight, the amount of silicon etched becomes excessive and the wafer surface becomes rough and cloudy, so the concentration of nitric acid is set to 1 to 30% by weight.

In the present invention, the cleaning temperature is not particularly limited, but in order to facilitate the dissolution of metal contaminants by hydrochloric acid or sulfuric acid and nitric acid, and to improve the resistance to oxidation of silicon by nitric acid, especially when the concentration of nitric acid is low. In order to accelerate the force, it is good to use high temperature force. However, if the temperature is higher than necessary, hydrochloric acid, f! There is a possibility that the composition of the facial cleansing liquid may deviate from the scope of the present invention due to accelerated formation of acid, hydrogen fluoride, or water. Moreover, it takes a long time to raise the temperature, which significantly impairs workability. Therefore, it is desirable that the cleaning temperature be in the range of room temperature to 80°C.

Furthermore, even if other ingredients than those mentioned above are added to the cleaning solution, such as salts such as ammonium fluoride, the cleaning ability will not be impaired.

Furthermore, it may be carried out in combination with conventionally known cleaning methods. For example, it is more effective to wash with a mixed aqueous solution of ammonia and hydrogen peroxide, further wash with a dilute hydrofluoric acid aqueous solution, and then wash with the method of the present invention.

Example: about 480 x 101010 ato/am2 of Cr, about 87
QX 1010atatas/c112 Fe, approx. +5
The results are shown in which a silicon wafer whose surface was contaminated with 1 cm2 of Ni was immersed in various cleaning solutions shown in Table 2 and cleaned for 10 minutes. Immediately rinse the silicon wafer with running water for at least 5 minutes in ultrapure water.
After drying with a spin dryer, the sample was analyzed for metal contamination on the surface.

The analysis was carried out on the surface layer 1p of silicon sea urchin/＼ using nitrofluoric acid solution.
m was dissolved by kneeling, and the metal element concentration in the solution was determined by flameless atomic absorption spectrometry.

Note that ND in the table indicates a value below the lower limit of analytical quantification, and Cr is 1. OX less than 10"atoms/cm2, Cu and Fe 2.0X10"atoms/C1
Less than 12, Ni is 2. OX 10”atoms/c+w
Less than 2.

The particles on the surface of the silicon wafer are measured using a surface particle measuring device.
More than 0 pieces/wafer is indicated by an X mark. In addition, the presence or absence of clouding was visually checked by shining light from a condensing lamp onto the wafer surface, and those with no clouding were marked with an O mark, and those with clouding were marked with an x mark.

Examples of the present invention, comparative examples thereof, and conventional examples are shown in Table 2.

In Table 2, Comparative Example No.1. No6 and No1
8 has less hydrochloric acid or sulfuric acid and nitric acid, so F
The remaining amounts of e and Cu are large, and the same <N.

Sample No. 17 contained too much hydrogen fluoride, so particulate contaminants remained.

The conventional N025 slice-etch method uses HNO3: 60
Although cleaning was performed with an aqueous solution of 1% by weight and 1% by weight of HF:O, the effect of removing Cu was high, but the effect of removing Fe was low.

The No2B ammonia hydrogen peroxide method uses an aqueous solution of NH3: 4.1% by weight and H2O2: 4.4% by weight, but among the various cleaning solutions, it has the lowest Fe removal effect and also the lowest Cr removal effect. .

No. 27 Hydrochloric acid hydrogen peroxide method uses HCl5.1% by weight H70
2: Washed with a 4.4% by weight aqueous solution. This solution is a cleaning solution that is an indicator of the effectiveness of removing metal contaminants, but compared to the method of the present invention, it does not remove metal contaminants, especially Fe and C.
r, Cu removal effect is low.

The No. 28 dilute hydrofluoric acid method involves cleaning with an aqueous solution of HF: 1% by weight, but the removal effect of Fe, Cr, Cu, and Ni is low.

In all of the examples of the present invention, particulate contaminants and metal contaminants on the surface of the silicon wafer are extremely reduced, and no clouding occurs on the surface after cleaning.

Effects of the Invention When silicon wafers are cleaned by the method of the present invention, highly clean silicon wafers with significantly reduced levels of both metallic contaminants and particulate contaminants can be obtained, and silicon Wafer quality deterioration is avoided and IC-? ) The possibility of deterioration of electrical characteristics when used in highly integrated devices such as LSIs is also avoided.

Claims (1)

[Claims] When cleaning silicon wafers, it contains 1% by weight or more of hydrochloric acid or 1% by weight or more of sulfuric acid, and 1 to 30% by weight of nitric acid, and further contains 0.01 to 10% by weight of hydrogen fluoride. A method for cleaning silicon wafers, characterized in that a cleaning solution is an aqueous solution containing water.