JPH07230975A - Method of cleaning semiconductor substrate - Google Patents

Abstract

(57) [Abstract] [Purpose] With regard to cleaning of semiconductor substrates, it is intended to put into practical use a cleaning method that does not adsorb impurities. [Structure] After a semiconductor substrate is chemically treated, the semiconductor substrate is washed to remove reaction products, and as cleaning water used to obtain a clean substrate surface, an acid component that decomposes or evaporates by heating. The method for cleaning a semiconductor substrate is characterized in that pure water whose pH is adjusted to 4 at most is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cleaning a semiconductor substrate. Semiconductors include single semiconductors typified by silicon (Si) and compound semiconductors typified by gallium arsenide (GaAs). Various devices such as semiconductor integrated circuits and lasers are manufactured, but these are cylindrical. Sliced single crystal ingot with thickness of about 500 μm and perform mechanical polishing and chemical polishing to make a substrate (wafer) with smooth surface. Thin film forming technology and photolithography technology (photolithography) on this wafer Various semiconductor devices are manufactured by applying impurity element injection technology.

Here, in semiconductor devices, particularly in semiconductor integrated circuits, the miniaturization of unit elements has increased the capacity, and the minimum width of conductor lines has reached sub-micron. Therefore, pattern formation is required. Device formation is carried out in a dust-free chamber because disconnection of the line easily occurs even if a small amount of dust adheres to the wafer being processed.

Further, in order to prevent deterioration or fluctuation of electric characteristics due to adsorption of impurity ions other than dust, cleaning with pure water is thoroughly performed in cleaning after chemical treatment such as development and selective etching performed in a photo-etching process. To prevent wafer contamination.

[0004]

2. Description of the Related Art In the formation of semiconductor devices, a wafer manufacturing department or a manufacturer supplies a wafer whose surface has been cleaned, but in order to thoroughly clean the wafer, the following surface treatment is performed step by step. After the wafer surface is cleaned and the wafer surface is cleaned, the device is formed. The cleaning process will be described below by taking Si as an example. Removal of adhered dust, removal of adhered organic substances and metal components, removal of sulfuric acid and metal components, that is, for example, ammonia (NH ₄ OH) and hydrogen peroxide (H ₂ O ₂ ) Immerse the wafer in a liquid consisting of water (H ₂ O),
After the dust adhering to the surface is floated from the substrate, it is washed with pure water to remove the dust.

Next, for example, as sulfuric acid (H ₂ SO ₄ ) and H ₂ O ₂
The wafer is immersed in a liquid consisting of water and water (H ₂ O) to oxidize and remove organic substances such as fat adhering to the surface, and to dissolve and remove metallic impurities adhering to the surface, Next, cleaning is performed using pure water.

[0006] Next, the to example nitric acid (HNO ₃₎ was immersed in an aqueous solution, a previously undissolved by H ₂ SO ₄ metallic impurities with溶Su, remove H ₂ SO ₄ components adhered to the surface Then, it is thoroughly washed with pure water.

As described above, the pure water cleaning is used as a finishing in the final step of the chemical work, but the pure water cleaning is an indispensable step not only in the wafer cleaning but also in the steps using the photo-etching technique. is there.

For example, in the process of manufacturing an integrated circuit, a process of forming a large number of integrated circuit forming regions separated by elements by a field oxide film on a wafer will be described as an example.
Approximately a silicon dioxide (SiO ₂ ) film is deposited on a Si wafer by thermal oxidation.
After forming the film to a thickness of 100 Å, a silicon nitride (Si ₃ N ₄ ) film is formed to a thickness of about 1000 Å by vapor phase epitaxy (CVD method), and element isolation is performed using photo-etching technology. After removing the Si ₃ N ₄ film at the position where
Wet oxidation is performed at a high temperature of 1000 ° C to form a thick field oxide film made of SiO ₂ and having a thickness of about 5000Å.
A large number of integrated circuit formation regions are formed by dissolving and removing the Si ₃ N ₄ film at the integrated circuit formation position using (H ₃ PO ₄ ), thoroughly washing with pure water and drying.

As described above, the deionized water cleaning process performed at the end of the wafer cleaning process and the chemical treatment process is an essential process, and is performed in order to eliminate the adhesion of impurity ions that cause deterioration of characteristics. ing. The pure water used to connect each stage is made by flowing running water, groundwater, etc. through an ion exchange resin, and converting the cations and anions contained in the water into hydrogen ions (H ⁺ ) and hydroxide ions (OH ^-) in are those obtained by substitution, therefore pH is in the 7.

[0010]

Although a chemical treatment such as photo-etching is frequently used for forming a semiconductor device, the electrical characteristics of the semiconductor device are sensitively affected by impurity ions. Are thoroughly performed to eliminate the adhesion of impurity ions.

However, the inventor removes dust, organic matter, metal ions, etc. by the above-mentioned cleaning method, and
As a result of measuring the recombination lifetime obtained from the microwave reflectance of a high-purity Si wafer washed with ultrapure water of MΩ · cm or more, it was found that the wafer was sufficiently washed with ultrapure water. Regardless, the lifetime is short, from now on,
There was concern that the wafer surface would be contaminated. Therefore, the problem is to eliminate the pollution and improve the characteristics of the semiconductor device.

[0012]

[Means for Solving the Problems] The above-mentioned problems are as cleaning water used to obtain a clean substrate surface by chemically cleaning the semiconductor substrate and then cleaning the semiconductor substrate to remove reaction products. The problem can be solved by configuring a method for cleaning a semiconductor substrate, which is characterized in that pure water whose pH is adjusted to at most 4 by adding an acid component that decomposes or evaporates by heating is used.

[0013]

Although the inventor has cleaned the wafer with ultrapure water, it has a relation with the pH (hydrogen ion concentration) of pure water as a reason for showing the phenomenon that impurity elements are attached. I thought it might be.

That is, most of the metal ions, non-metal ions, acid ions, etc. are removed from the pure water by ion exchange. However, so-called impurity ions that deteriorate the characteristics of the semiconductor device are considerably present in them. ing.

These ions are hydrolyzed because they have a pH of 7 and are neutral, while the surface of the wafer is subjected to a cleaning treatment using H ₂ SO ₄ , H ₂ O, HNO ₃ or the like. The activated impurity ions are hydrolyzed, and thus the hydrolyzed impurity ions are easily attached to the wafer surface. Therefore, in order to eliminate the adhesion of impurity ions, it is necessary to change the pH of pure water because it is necessary to prevent hydrolysis, and an experiment was conducted.

As a method, 1 ppb of calcium ion (Ca ⁺⁺ ) and magnesium ion (Mg ⁺⁺ ) in the form of nitrate are added to pure water, and the pH is adjusted with nitric acid (HNO ₃ ) and ammonia (NH ₄ OH). And immerse the Si wafer in this pure water,
Atomic absorption analysis was performed on the dried product, and the concentrations of Ca and Mg atoms adsorbed on the surface were measured.

The results are shown in FIG. 1 and FIG. 2. From this result, it was revealed that there is no adsorption when the pH is 4 or less, and the adsorption amount increases as the pH increases, and decreases when the pH exceeds 10.

It should be avoided that washing with a solution far out of neutrality causes a chemical reaction due to the acid component or alkaline component, but the present invention is because pH 4 is extremely weak as acidic. Cleaning is performed using pure water whose pH is adjusted to 4.

Next, it is necessary that the acid component for adjusting the pH to 4 is one that does not cause chemical contamination, can be easily removed by heating, and needs only a small amount of addition. From this viewpoint, HNO ₃ is suitable for forming Si semiconductor devices, while acetic acid (CH ₃ COOH) is suitable for cleaning materials that react with nitrate ions (NO ₃ ⁻ ).

[0020]

【Example】

Example 1: (Example in which HNO ₃ was used for pH adjustment) HNO ₃ was added to pure water which has been conventionally used to adjust the pH to 4, and this was used in the final cleaning step of the Si wafer.

That is, with respect to the obtained Si wafer, NH ₄ O
After immersing in a cleaning solution consisting of H, H ₂ O ₂ and H ₂ O to remove fine dust, it was washed with pure water of pH 4 and dried with hot air, then with HCl and H ₂ O ₂ After immersing in a cleaning solution of H ₂ O (commonly called SC2) to remove metal components, pH
It was thoroughly washed with pure water of 4 and then dried with hot air.

As a result of examining the surface impurity concentration of such Si wafer by atomic absorption spectrometry, the metal contamination level was 3 × 10 ¹⁰ cm ^-2 or less. Example 2: (Example of using CH ₃ COOH for pH adjustment) CH ₃ COOH was added to pure water that has been conventionally used to adjust the pH to 4, and this was used in the final cleaning step of the Si wafer. .

That is, regarding the obtained Si wafer, NH ₄ O was used.
After immersing in a cleaning solution consisting of H, H ₂ O ₂ and H ₂ O to remove fine dust, it is washed with pure water of pH 4 and dried with hot air, then H ₂ SO ₄ and H ₂ It is immersed in a cleaning solution consisting of O ₂ and H ₂ O to remove organic substances and metal ions, and then HNO ₃
Residual metal ions and H ₂ SO ₄ were removed by using an aqueous solution, and then thoroughly washed with pure water having a pH of 4, and then dried by hot air.

As a result of examining the impurity concentration on the surface of such Si wafer by atomic absorption spectrometry, the metal contamination level was 3 × 10 ¹⁰ cm ^-2 or less.

[0025]

The use of the present invention makes it possible to eliminate the adsorption of impurity ions generated during cleaning with pure water, thereby improving the characteristics of semiconductor devices.

[Brief description of drawings]

FIG. 1 is a relationship diagram between the pH of pure water and the concentration of attached Ca atoms.

FIG. 2 is a diagram showing the relationship between the pH of pure water and the concentration of attached Mg atoms.

Claims (3)

[Claims] 1. A semiconductor substrate is chemically treated, and then the semiconductor substrate is washed to remove reaction products and used as washing water to obtain a clean substrate surface, which is decomposed or evaporated by heating. A method of cleaning a semiconductor substrate, which comprises using pure water having an acid component added to adjust pH to at most 4. 2. The method for cleaning a semiconductor substrate according to claim 1, wherein the acid component decomposed by heating is nitric acid. 3. The method for cleaning a semiconductor substrate according to claim 1, wherein the acid component evaporated by heating is acetic acid.