JPH0691061B2 - Silicon wafer cleaning method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a cleaning method for bringing the surface of a silicon wafer into a highly clean state.

2. Description of the Related Art In recent years, as devices have been highly integrated, it has been strongly desired to further clean the surface of a silicon wafer which is a substrate thereof.

Generally, a silicon wafer is cut into a disk shape from a cylindrical single crystal silicon, and then undergoes lapping, etching, polishing, and cleaning processes to become a product with a smooth surface from which processing strains and contaminants have been removed. . If metal-based contaminants such as transition metals are present on the surface of the silicon wafer of this final product, quality problems of the silicon wafer such as generation of oxidation-induced stacking faults and reduction of lifetime will occur, and further deterioration of the electrical characteristics of the device will occur. It can be a cause. Further, the presence of fine particle contaminants (also referred to as particles) such as silicon particles or dust causes disconnection or short circuit of device wiring. Therefore, cleaning is performed to remove these contaminants and make them clean.

Kern of RCA is the conventional cleaning method for silicon wafers.
Invented "RCA cleaning method", that is, a method of cleaning with a dilute aqueous solution of hydrofluoric acid, a method of cleaning with a mixed solution of hydrochloric acid and hydrogen peroxide, a method of cleaning with a mixed solution of sulfuric acid and hydrogen peroxide, a method of cleaning with ammonia and a peroxide. 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 performed. As the cleaning liquid used in each of these cleaning methods, chemicals called EL grade of high purity, in which fine particle contamination and metallic contaminants are reduced as much as possible, are used. Also, ultrapure water is used as water.

Among the conventional cleaning methods, the dilute hydrofluoric acid solution has the ability to dissolve the natural oxide film that normally exists on the silicon wafer surface with a thickness of about 10 Å, so the effect of removing metallic contaminants in the oxide film is not high. However, if the surface of the silicon wafer is contaminated with a metal-based contaminant having a smaller ionization tendency than Si, such as Cu, Cu cannot be completely removed, and the silicon wafer is contaminated again depending on the concentration of the impurity Cu in the cleaning liquid. To be done. In addition, there is a problem that it is difficult to reduce particulate contaminants.

The mixed aqueous solution of hydrochloric acid and hydrogen peroxide utilizes the high dissolving power of hydrochloric acid with respect to metallic contaminants, but does not have the ability to dissolve an oxide film. Therefore, the effect of removing the metallic contaminants on the oxide film, particularly the metallic contaminants (eg, Al, Ca, Cr, Fe, Mg, Zn, etc.) that are easily dissolved in the reducing acid is high. However, it is easy to dissolve in an oxidizing acid, but difficult to dissolve in a reducing acid (for example, Co, Cu,
The effect of removing Mn, Ni, etc.) is low. Further, the effect of removing the metallic contaminants existing in the oxide film or at the interface between the oxide film and silicon is also low.

The mixed aqueous solution of sulfuric acid and hydrogen peroxide is generally used for removing organic contaminants and resist. But,
Sulfuric acid also has a high ability to dissolve metal contaminants, and has the same ability to remove metal contaminants as a cleaning liquid.

Since the mixed solution of ammonia and hydrogen peroxide has a function of dissolving the silicon itself, it has a high effect of removing the metal-based contaminants existing thereon. However, the oxide film formed in the basic cleaning liquid has a property of easily taking in metal contaminants (for example, Al, Fe, Zn, etc.) in the cleaning liquid,
Silicon wafers are likely to be recontaminated. But,
Since this method has a high effect of removing fine particles, it is widely used by wafer makers and device makers.

As a method for solving the problems of the above conventional method, a light etching method has been proposed in which a silicon wafer is immersed in a mixed aqueous solution of 60 wt% nitric acid and 0.1 wt% or less of hydrogen fluoride for cleaning ( Ritsuo Takizawa et al., “Extended Ab
stracts of Solid State Devices and Materials ", 198
8 years, p145).

However, even the highest purity nitric acid called semiconductor grade contains ppb order or sub ppb order metal contaminants, for example, Al, Ca, Cr, Cu, Fe, Mg, Ni, Zn, etc. The concentration of metallic pollutants in the cleaning liquid containing nitric acid as high as 60% is high, and when a cleaning tank made of quartz glass is used, metallic impurities in the quartz glass are eluted,
Its concentration increases more and more.

Then, an oxide film is formed on the surface of the silicon wafer due to the strong oxidizing power of the high-concentration nitric acid, and the metal-based contaminants in the cleaning liquid are easily taken into the oxide film. Therefore, it must be said that there is a limit to the high cleanliness of the silicon wafer even with the slight etching method.

Furthermore, the silicon wafer is subjected to an oxidation treatment in an oxidation furnace to form an oxide film in which metallic 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 in which the oxide film is dissolved and the metallic contaminants in the oxide film are dissolved in hydrofluoric acid and removed is also used. However, some metal-based contaminants diffuse (eg, Cu) into a silicon substrate or form a silicide insoluble in hydrofluoric acid (eg, Fe). For this reason, not only the metal-based contaminants cannot be removed, but also the generation of oxidation-induced stacking faults, the lifetime, and the decrease in dielectric breakdown of the oxide film may be caused. Therefore, it must be said that the sacrificial oxidation method has a limit to the high cleaning of the silicon wafer.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention relates to the relationship between the manufacturing process of a silicon wafer, the type of metal-based contaminant on the surface of the silicon wafer, and the amount of contamination.
It was examined by flameless atomic absorption spectrometry. In addition, by measuring the lifetime (hereinafter referred to as recombination lifetime) by the microwave reflection method, which is one of the methods for evaluating the cleanliness of a silicon wafer, the recombination lifetime of the silicon wafer as a product and the metal The relationship of system pollutants was investigated.

As a result, the metallic contaminants on the surface of the silicon wafer differ depending on the manufacturing process of the silicon wafer, but depending on the impurity metals originally contained in the chemicals used and the materials constituting the manufacturing apparatus, Al, Ca, Cr, Cu. , Fe, Mg, Ni, Zn, etc. were found to be contaminated. It was also found that when cleaning was performed with a mixed aqueous solution of ammonia and hydrogen peroxide, the silicon wafer was contaminated with Fe and the recombination lifetime was reduced.

The inventor of the present invention wraps and etches silicon wafers,
Table 1 shows the results of measuring the amounts of contaminant metals, especially transition metals such as Cr, Cu, Fe, Ni, and Zn, after the polishing and cleaning steps. Lapping is performed with an alkaline aqueous solution having a pH of 10 to 11 using a polishing liquid in which alumina is dispersed as abrasive grains, etching is performed with a mixed acid of nitric acid and hydrofluoric acid, and polishing is performed with an alkaline aqueous solution having a pH of about 10 as abrasive grains. The polishing is performed with a polishing liquid in which silica is dispersed, and the cleaning is performed with a mixed aqueous solution of ammonia and hydrogen peroxide.

As can be seen from Table 1, silicon wafers are contaminated with Cr, Cu, Fe, Ni and Zn during the lapping process. In addition, ND in the table
Indicates that the value is below the analytical lower limit of quantification. Cr is 1.0 x 1
It is less than ⁰⁹ atoms / cm ² , Cu and Fe are less than 2.0 × 10 ⁹ atoms / cm ² , and Ni is less than 2.0 × 10 ¹⁰ atoms / cm ² . In the etching process, the metal-based pollutants in the previous process are considerably removed, but they are still contaminated with Cr, Cu, Fe, and Zn. Cr, Cu, Fe, Ni and Zn increase again in the polishing process. After the cleaning process, it is often below the lower limit of quantification, but Cr, Cu, Fe, Ni, Zn
Occasionally there are wafers that are contaminated with. In addition, the silicon wafer in each process is contaminated with Al, Ca, and Mg.

It is considered that the silicon wafer is contaminated in each process for the following reason.

The polishing liquid used in the lapping process is one in which an organic amine is added to water, which is a medium, and alumina is dispersed therein, in order to have an etching action on silicon while grinding the silicon surface with abrasive grains, and a pH of 10-11. It is alkaline. Further, the polishing liquid contains impurities such as Ca, Cr, Cu, Fe,
Contains a large amount of Mg, Ni, Zn, etc. For example, Fe also contains 500 to 1000 ppm. Therefore, during the polishing due to the adsorption of the impurity metal in the polishing agent on the silicon surface which has become the surface state due to the grinding and etching action in the polishing, or during polishing,
Abrasive particles and impurity metal in the abrasive penetrate into the minute cracks in the strained layer on the Si surface layer (depth: 15μ
(more than m) may cause pollution (Kadoma et al., “No. 7”)
"Semiconductor and Integrated Circuits Symposium", 1974, p.10
3).

Even after the etching (etching amount: 20 μm / one side) process,
The detected Cr, Fe, etc. are considered to be contaminants embedded in fine cracks in the strained layer generated in the Si surface layer during lapping.

The polishing liquid used in the polishing step is one in which potassium hydroxide or ammonia is added to water, which is a medium, and silica is dispersed therein in order to have an etching action on silicon while grinding the silicon surface with abrasive grains. About 10
It is alkaline. Further, the polishing liquid contains a large amount of Al, Ca, Cr, Cu, Fe, Mg, Ni, Zn and the like as impurities. For example, Fe also contains 1 to 3 ppm. Therefore, it is considered that the silicon wafer is also contaminated in the polishing process for the same reason as the contamination in the lapping process.

Contamination in the cleaning process, particularly Fe contamination, is caused by a very small amount of impurity Fe contained in the cleaning solution of the 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, the surface of the silicon wafer will be 8 × 10 ¹¹
Adsorption of about atoms / cm ² and contamination will reduce the recombination lifetime (Otsuka et al., “34th Semiconductor and Integrated Circuit Technology Symposium Proceedings”, 1988, p.37).

As described above, there are various situations where a silicon wafer is contaminated. For this reason, it is important to know the location of the metal-based contaminant when cleaning the silicon wafer for cleaning. That is, it is necessary to determine whether or not the cleaning liquid needs to have the etching property of silicon depending on the contamination of the surface of the silicon wafer or the contamination embedded in the silicon substrate. It is also important to know the form of metallic pollutants. That is, the metal-based pollutant is selected from a simple metal, an oxide, a salt, an alloy such as a silicide, and a silicate, and the kind and combination of acids for efficiently dissolving and removing them are selected, and It is important to determine the optimum cleaning conditions.

The present invention significantly reduces metal contaminants such as Cr, Cu, Fe, Ni, and Zn and particulate contaminants during the final cleaning of the product and contamination received during the manufacturing process of silicon wafers, and induces oxidation-induced stacking faults. It is an object of the present invention to obtain a silicon wafer of high cleanliness, which does not have quality problems such as occurrence of defects and deterioration of lifetime, and does not cause deterioration of dielectric breakdown of an oxide film or deterioration of electrical characteristics when used as a device.

Means and Actions for Solving the Problems The gist of the present invention is as follows.

When cleaning a silicon wafer, 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% by weight of hydrogen fluoride is used as a cleaning solution. A method for cleaning a silicon wafer, comprising:

According to the method of the present invention, when cleaning a silicon wafer, the ability to dissolve the excellent metallic contaminants of hydrochloric acid or sulfuric acid, nitric acid and hydrofluoric acid is utilized, and the effective combination of these acids improves the cleaning ability. It provides an excellent cleaning liquid.

That is, according to the present invention, metal-based contaminants that are soluble in reducing acids are dissolved and removed with hydrochloric acid or sulfuric acid and hydrofluoric acid, and metal-based contaminants that are soluble in oxidizing acids are dissolved with nitric acid. Remove. At the same time, an oxide film is formed on the surface of the silicon wafer by utilizing the silicon oxidation function of nitric acid,
Furthermore, by utilizing the ability of hydrofluoric acid to dissolve the oxide film, the cleaning liquid has the ability to etch silicon, and not only the metallic contaminants on the oxide film of the silicon wafer but also the oxide film and the interface between the silicon substrate and the silicon substrate. Also, it efficiently and thoroughly removes metallic contaminants in the silicon substrate.

In the present invention, the concentrations of hydrochloric acid, sulfuric acid, nitric acid, and hydrogen fluoride in the cleaning liquid are weight% as HCl, H ₂ SO ₄ , HNO ₃ and HF, respectively. Less than 1% by weight of hydrochloric acid or sulfuric acid,
When the nitric acid content is less than 1% by weight and the hydrogen fluoride content is less than 0.01% by weight, the effect of further removing the metallic contaminants is insufficient. Even if the concentration of hydrochloric acid or sulfuric acid is increased, the effect of removing metal contaminants does not change. Therefore, the upper limit of the concentration of these acids is not particularly limited and is set to 1% by weight or more.

If the concentration of hydrogen fluoride is increased to more than 10% by weight, the particulate contaminant tends to increase.
It was set to 10% by weight. If the concentration of nitric acid exceeds 30% by weight, the etching amount of silicon becomes excessive and the wafer surface becomes rough and cloudy.
-30% by weight.

In the present invention, the cleaning temperature is not particularly limited, but in order to facilitate the dissolution of metal-based contaminants by hydrochloric acid or sulfuric acid and nitric acid, and also the oxidation of silicon by nitric acid, especially the silicon oxidation ability when the nitric acid concentration is low. It is better to raise the temperature to accelerate it. However, if the temperature is raised to an unnecessarily high temperature, evaporation of hydrochloric acid, nitric acid, hydrogen fluoride or water is promoted, and the composition of the cleaning liquid may be out of the range of the present invention. In addition, it takes time to raise the temperature, and workability is significantly impaired. Therefore, the washing temperature is preferably in the range of normal temperature to 80 ° C.

In addition, the cleaning ability will not be impaired even if a component other than those mentioned above, for example, a salt such as ammonia fluoride is added as a component of the cleaning liquid.

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

Cr embodiment about ^{480 × 10 10 atoms / cm 2} , about ^{87 × 10 10 atoms / cm 2} C
u, about 3000 × 10 ¹⁰ atoms / cm ² Fe, about 150 × 10 ¹⁰ atoms / cm ²
The results obtained by immersing the silicon wafer whose surface was contaminated by Ni in the various cleaning solutions shown in Table 2 and cleaning it for 10 minutes are shown.
The washed silicon wafer was immediately washed twice in ultrapure water with running water for 5 minutes or more, dried with a spin dryer, and then subjected to analysis of contaminant metals on the surface.

The analysis was performed using a nitric hydrofluoric acid solution to obtain a surface layer of the silicon wafer of 1 μm.
Was etched and dissolved, and the metal element concentration in the solution was quantified by flame atomic absorption spectrometry. In addition, ND in the table indicates that the value is below the analytical lower limit of quantification, and Cr is
Less than 1.0 × 10 ⁹ atoms / cm ² , 2.0 × 10 ⁹ atoms / c for Cu and Fe
It is less than m ² and Ni is less than 2.0 × 10 ¹⁰ atoms / cm ² .

The fine particles on the surface of the silicon wafer were measured by a wafer surface fine particle measuring device, and 10 particles / wafer or less were indicated by a circle, and 10 particles / wafer or more were indicated by a cross. In addition, light from a condenser lamp was applied to the surface of the wafer to visually check whether or not there was fogging, and those with no fogging were indicated with a circle, and those with fogging were indicated with a cross.

Table 2 shows examples of the present invention, comparative examples thereof, and conventional examples.
In Table 2, the comparative examples N No. 1, No 6 and No 18 have a small amount of hydrochloric acid or sulfuric acid and nitric acid, so that they are particularly Fe and Cu.
The residual amount of No. 17 was large, and similarly, No. 17 had too much hydrogen fluoride, so fine particle contaminants remained.

The conventional No25 light etching method is HNO ₃ : 60% by weight, HF: 0.
Although washed with a 1 wt% aqueous solution, it has a high Cu removal effect but a low Fe removal effect.

No26 ammonia hydrogen peroxide method NH _3: 4.1 _{wt%, H 2 O 2: 4} .
Although it was washed with a 4% by weight aqueous solution, it has the lowest Fe removal effect and the lowest C removal effect among the various cleaning solutions.

The No. 27 hydrochloric acid-hydrogen peroxide method was washed with an aqueous solution of HCl: 5.1% by weight and H ₂ O ₂ : 4.4% by weight. Although this liquid is a cleaning liquid that serves as an index of the effect of removing metal contaminants, it has a lower effect of removing metal contaminants, especially Fe, Cr, and Cu, as compared with the method of the present invention.

The No. 28 diluted hydrofluoric acid method was washed with an aqueous solution of HF: 1% by weight, but the effect of removing Fe, Cr, Cu, Ni was low.

In each of the examples of the present invention, fine particle contaminants and metallic contaminants on the surface of the silicon wafer are extremely reduced, and no haze is generated on the surface after cleaning.

EFFECTS OF THE INVENTION When a silicon wafer is cleaned by the method of the present invention, a silicon wafer of high cleanliness in which both metallic contaminants and fine particle contaminants are extremely reduced is obtained, and silicon wafers such as generation of oxide film-induced defects and reduction in lifetime are obtained. The deterioration of the wafer quality is avoided, and the risk of deterioration of electrical characteristics when used in highly integrated devices such as ICs and LSIs is also avoided.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Sohei 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Co., Ltd. Inside the Hikari Steel Works (56) Reference JP-A-64-77130 (JP, A) 56-112747 (JP, A)

Claims (1)

[Claims] 1. When cleaning a silicon wafer, 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 0.01 to 10% by weight of fluorination. A method for cleaning a silicon wafer, which uses an aqueous solution containing hydrogen as a cleaning solution.