JPH0689888A - Method of washing semiconductor wafer - Google Patents

Abstract

PURPOSE:To decrease the metal impurity amount left on the washed wafer surface down to a prescribed value or less by a method wherein impurity removal effects from a wafer are sufficiently secured and an adhesion of the metal impurity included in chemical liquid to the wafer surface also is prevented. CONSTITUTION:By using chemical liquid composed of NH4OH, H2O2, pure water, a semiconductor wafer is washed (1, 2), and after metal impurity concentration of the surface is set at 1X10<11> atoms/cm<2> or less each element, the semiconductor wafer is washed, and further by using chemical liquid in which NH4OH of 27 to 31wt%, H2O2 of 20 to 33wt%, and pure water are mixed at a ratio of 1:1:5 to 50, the wafer is washed under conditions of a washing temperature 20 to 50 deg.C and a washing time 1 to 3min. (3).

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 wafer, and more particularly to an improvement for reducing contamination by metal impurities in a chemical solution.

[0002]

2. Description of the Related Art A method of cleaning semiconductor wafers such as silicon is called RCA method in which the wafers are successively passed through a large number of cleaning tanks containing various chemicals to remove contaminants adhering to the surfaces thereof. The wet cleaning method is generally used.

In the RCA method, the final cleaning step is
Cleaning is carried out for the purpose of removing organic substances and particles (mostly crushed particles of silicon) on the surface of the silicon wafer.
Ammonia-based chemical liquid is used in which ammonia water, about 30 wt% hydrogen peroxide water, and pure water are mixed in the following volume ratios. Aqueous NH ₄ OH: H ₂ O _{2 water: H 2 O = 1: 1} : 5

[0004]

By the way, commercially available ammonia water and hydrogen peroxide water have a very small amount (several pp).
Although b), aluminum and heavy metals are inevitably contained, and it is inevitable that these metal impurities remain on the wafer surface after cleaning. Such metal impurities cause a decrease in device performance and a yield in a submicron LSI in which the minimum pattern width of the device is on the order of 0.1 μm.

Therefore, conventionally, hydrochloric acid, hydrogen peroxide solution, which has been originally used for removing heavy metal impurities after the final step of cleaning with the above-mentioned ammonia-based chemical solution,
In some cases, a cleaning method using a hydrochloric acid-based chemical solution prepared by mixing pure water or a hydrochloric acid-based chemical solution prepared by mixing hydrochloric acid and pure water is performed again as a final step. However, this method requires an additional facility for final cleaning with a hydrochloric acid-based chemical and also requires management, which inevitably increases wafer manufacturing costs.

When the wafer is cleaned with an ammonia chemical, the surface of the wafer is etched, and the particles embedded and fixed in the natural oxide film formed on the surface of the wafer are released and removed. It is known that a natural oxide film containing no impurities is formed to make the wafer surface hydrophilic. For this reason, it is considered that the ability to remove metal contamination has a positive correlation with the etching rate in the conventional wisdom, and the ability to remove metal impurities decreases when the etching rate decreases due to a decrease in temperature of the cleaning liquid or a reduction in cleaning time. It was supposed to be.

However, as a result of the detailed experiments conducted by the present inventors, the metal impurity level on the surface was set to 1 × 10 ¹⁰ for each element in accordance with the submicron LSI wafer standard.
When reducing to atoms / cm ² or less, if the cleaning temperature is raised to increase the etching rate, the metal impurities contained in the ammonia-based chemical solution adhere to the wafer surface,
A new fact has been found that the amount of contamination increases with time and the specified value cannot be satisfied. In other words, contrary to the conventional wisdom, a new finding was obtained that, when the cleaning conditions were low temperature and short time, metal contamination from the ammonia-based chemical solution could be reduced.

Further, the inventors of the present invention conducted further experiments to determine the level of metal impurities on the surface of the wafer before cleaning, which was introduced into the ammonia-based chemical, at 1 × 10 ¹¹ atoms / c for each element.
if in the m ² or less, even in the above washing conditions were found that can be easily satisfied the prescribed values.

[0009]

The present invention has been made based on the above findings. First, in the method of claim 1 of the present invention, 27 to 31 wt% NH ₄ OH and 20 to 33 w are used.
A semiconductor wafer is cleaned under the conditions of a cleaning temperature of 20 to 50 ° C. and a cleaning time of 1 to 3 minutes by using a chemical solution in which t% H ₂ O ₂ and pure water are mixed in a ratio of 1: 1: 5 to 50. It is characterized by that.

According to the method of claim 2 of the present invention, NH
_The semiconductor wafer is washed with a chemical solution consisting of ₄ OH, H ₂ O ₂ and pure water, and the concentration of metal impurities on the surface is set to 1 × 10 ¹¹ atoms / cm ² or less for each element. Wash with water, and then add 27-31 wt% NH _4.
OH, 27-31 wt% H ₂ O ₂ and pure water 1: 1:
Using a chemical solution mixed at a ratio of 5 to 50, a cleaning temperature of 20 to
It is characterized in that the semiconductor wafer is cleaned under the conditions of 50 ° C. and a cleaning time of 1 to 3 minutes.

If the cleaning temperature is lower than 20 ° C., the ability to remove contaminants adhering to the surface of the wafer before cleaning is lowered,
After cleaning, metal impurities above the specified value remain on the wafer surface. On the other hand, if the cleaning temperature exceeds 50 ° C., metal contamination from the cleaning liquid increases. Further, if the cleaning time is less than 1 minute, sufficient cleaning cannot be performed in the above temperature range, and if it is longer than 3 minutes, metal contamination from the cleaning liquid increases. It has been found that the elements that can be reduced in adhesion amount by the cleaning method of the present invention are boron, aluminum, iron and zinc.

If the concentration of NH ₄ OH or H ₂ O ₂ is less than the above range, the cleaning effect is insufficient, and if it is more than the above range, metal impurity contamination increases.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process chart showing a semiconductor wafer cleaning method according to an embodiment of the present invention. In the method of this embodiment, a silicon wafer is cleaned with an ammonia-based chemical solution at a relatively high temperature for a long time (pre-cleaning step 1), and then with pure water (rinse step 2), with an ammonia-based chemical solution. The wafer was washed at a low temperature for a short time (post-washing step 3), washed again with pure water (rinsing step 4), and the wafer was further dried (drying step 5). In this case, the post-cleaning step 3 corresponds to the method of claim 1 of the present invention, and steps 1-3 correspond to the method of claim 2 of the present invention.

In the pre-cleaning step 1, a cleaning liquid prepared by mixing ammonia water having a concentration of 28 wt%, hydrogen peroxide water having a concentration of 30 wt% and pure water in a volume ratio of 1: 1: 10 is used. The Si wafer was washed at a temperature of 80 ° C. for 10 minutes.

In the post-cleaning step 3, a chemical solution having the same composition as in the pre-cleaning step 1 was used, and the cleaning was carried out at a cleaning temperature of 20 to 50 ° C. and a cleaning time of 1 to 3 minutes. The wafer to be subjected to the post-cleaning step 3 was subjected to the pre-cleaning step 1 and the rinsing step 2 and the surface impurity concentration was 1 × 10 ¹¹ at each element.
It was set to be oms / cm ² or less.

FIG. 2 is a graph showing the relationship between the amount of metal impurities in the ammonia-based chemical solution and the concentration of metal impurities (Al, Fe) adsorbed on the cleaned wafer surface. As is clear from this graph, the amount of impurities in the chemical liquid defines the concentration of metal impurities on the wafer surface.

FIG. 3 shows the cleaning time in the post-cleaning step 3,
The relationship between the temperature and the Al and Fe concentrations after cleaning is shown. Al is SIMS (Secondary Ion Mass Spectrography), Fe
Is TXRF (Total Reflection X-Ray Fluorescency)
It was measured by. The unit is an arbitrary unit (au).
As is clear from this graph, as the cleaning time is longer and the cleaning temperature is higher, the amount of metal impurities adsorbed on the wafer surface increases.

FIG. 4 is a graph showing the relationship between the concentration of metal impurities on the wafer in the post-cleaning step 3 and the cleaning time when the cleaning temperature is 50 ° C. As is clear from this graph, the initial contamination of the wafer is 3 after the cleaning is started.
It is completely removed in 0 second to 1 minute, and thereafter, metal impurities adhere to the wafer from the cleaning liquid.

As shown in the above results, according to the method of the present invention, the impurity concentration is set to a prescribed level (10 ¹⁰ for each element) by keeping the cleaning conditions with the ammonia-based chemical solution at a low temperature for a short time, contrary to conventional wisdom. It was found that a wafer with a number of atoms / cm ² ) or less can be produced.

[0020]

As described above, according to the first aspect of the present invention.
27-31w in the method for cleaning semiconductor wafers
Using a chemical solution in which t% NH ₄ OH, 20 to 33 wt% H ₂ O ₂ and pure water are mixed in a ratio of 1: 1: 5 to 50,
Since the semiconductor wafer is cleaned under the conditions of the cleaning temperature of 20 to 50 ° C. and the cleaning time of 1 to 3 minutes, the metal impurities contained in the chemical solution may adhere to the wafer surface while sufficiently ensuring the effect of removing impurities from the wafer. prevent. As a result, the amount of metal impurities remaining on the surface of the wafer after cleaning can be reduced to a specified value or less, and the final cleaning step using the hydrochloric acid-based chemical solution for the purpose of the metal impurities can be omitted.

Further, in the semiconductor wafer cleaning method according to the second aspect of the present invention, the semiconductor wafer is cleaned by using a chemical solution consisting of NH ₄ OH, H ₂ O ₂ and pure water, and the metal impurity concentration on the surface is adjusted by each element. Is set to 1 × 10 ¹¹ atoms / cm ² or less, the semiconductor wafer is washed with water, and then the cleaning according to claim 1 is performed, so that the impurity concentration is below a specified level (10 ¹⁰ atoms / cm ² for each element). Wafers can be easily created.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a semiconductor wafer cleaning method of the present invention.

FIG. 2 is a graph showing the relationship between the amount of metal impurities in the chemical solution in the ammonia chemical cleaning step and the concentration of metal impurities (Al, Fe) adsorbed on the wafer surface after cleaning in the example.

FIG. 3 is a cleaning time in an ammonia chemical cleaning process,
6 is a graph showing the relationship between temperature and Al and Fe concentrations after cleaning.

FIG. 4 is a graph showing a relationship between a change in metal impurity concentration of a wafer due to a post-cleaning step and a cleaning time.

[Explanation of symbols]

 1 Pre-cleaning step 2 Rinse step 3 Post-cleaning step (corresponding to claim 1) 4 Rinse step 5 Drying step

Claims (2)

[Claims] 1. NH ₄ OH of 27 to 31 wt%, and 20
˜33 wt% H ₂ O ₂ and pure water are mixed at a ratio of 1: 1: 5 to 50, and a semiconductor wafer is cleaned under conditions of a cleaning temperature of 20 to 50 ° C. and a cleaning time of 1 to 3 minutes. A method for cleaning a semiconductor wafer, comprising: 2. A semiconductor wafer is cleaned with a chemical solution consisting of NH ₄ OH, H ₂ O ₂ and pure water, and the metal impurity concentration on the surface is set to 1 × 10 ¹¹ atoms / cm ² or less for each element. After that, the semiconductor wafer is washed with water and further 27-
And 31 wt% NH ₄ of OH, 20~33wt% of H ₂ O ₂
And a pure water in a ratio of 1: 1: 5 to 50 are used to clean the semiconductor wafer under the conditions of a cleaning temperature of 20 to 50 ° C. and a cleaning time of 1 to 3 minutes. Cleaning method.