JP4817887B2 - Semiconductor substrate cleaning method - Google Patents

Description

  The present invention relates to a method for cleaning a semiconductor substrate, particularly a silicon wafer.

  In a semiconductor manufacturing process, it is widely known that particles (dust) and metal adhering to the surface of a semiconductor substrate, particularly a silicon wafer, cause wiring defects and deterioration of electrical characteristics, respectively. For this reason, chemical cleaning for removing these contaminants has been frequently used. One typical chemical solution is an ammonia-hydrogen peroxide solution mixture (APM or SC1). Further, even in cleaning using an ammonia-hydrogen peroxide solution mixture, an organic compound such as a complexing agent or a surfactant is often used for the purpose of further improving the cleaning effect (Non-Patent Document 1, 2).

On the other hand, it is known that vacancies exist as one of so-called grown-in defects, particularly in silicon crystals obtained by the CZ method, and the presence thereof has an undesirable effect on, for example, the oxide film breakdown voltage characteristics of MOS diodes. It is also known to give. According to TEM observation or the like, it is shown that such holes are octahedral voids surrounded by one or two pairs of (111) planes. A structure in which these holes appear as pits on the surface by wafer processing is generally called COP. Since the size of the COP is increased by performing an etching process such as SC1 cleaning, when the etching process becomes longer, the smaller one that has no problem until then becomes larger, and as a result, the COP is detected. The number of detection increases. Further, the etching process such as SC1 cleaning not only grows the size of the COP that has already been exposed on the surface as described above, but also etches the silicon wafer itself, so that it is buried in a bulk within about 500 nm from the surface. As a result, the number of detected COPs is increased (Non-patent Document 3). As described above, when a silicon wafer having vacancies is etched with SC1 or the like, the number of COP detections of a certain size or more, which is a problem in wafer quality, increases, and the wafer quality is deteriorated.
K. Mori et al. , 1993 Semiconductor Pure Water and Chemicals Conference, Santa Clara, 1993 Chemical Processings, p. 122 S. Ojima et al. , IEICE Technical Report, Technical Report of IEICE, SDM95-86, p. 105, 1995 Wafer surface integrity creation and evaluation technology, published by Science Forum, Inc., 1998, pages 25-26, pages 36-40

  Provided is a silicon wafer cleaning method capable of effectively suppressing an increase in COP (number and density) on a wafer surface.

  As a result of diligent research to realize a silicon wafer cleaning method without the above problems, the present inventors have found that an oxide film on the silicon wafer surface by ozone has a specific etching resistance with respect to the SC1 cleaning solution, Based on this finding, the present invention has been completed.

That is , in the silicon wafer cleaning method according to the present invention , an initial oxide film is formed on the surface of the silicon wafer with a solution containing ozone , and then the initial oxide film formed by the ozone is converted into ammonia, hydroxylated. tetramethylammonium, characterized by etching with a mixed solution of any one or more chemicals and hydrogen peroxide of choline.

The etching amount by the mixed solution, it characterized der Rukoto less than the thickness of the initial oxide film formed me by the ozone.

Furthermore, the ozone concentration in the solution containing ozone is 10 to 200 ppm.

According to the present invention, a silicon wafer having an oxide film formed by ozone is etched with a mixture of one or more chemicals of ammonia, tetramethylammonium hydroxide, and choline and hydrogen peroxide , Contamination due to particles, heavy metals, etc. on the silicon wafer surface can be completely removed while effectively suppressing the appearance of a new COP.

  The best mode for carrying out the method of the present invention will be described in detail below.

(Silicon wafer)
There is no particular limitation on the silicon wafer having a surface that can be cleaned by the cleaning method of the present invention (hereinafter referred to as “the present cleaning method”). A silicon wafer obtained from a silicon single crystal produced by a generally known production method is applicable. There is no particular limitation on the size and shape of the silicon wafer. Specifically, a circular silicon wafer having a diameter of 100 to 300 mm can be mentioned.

  COP in this cleaning method means a surface defect observed as a pit when an octahedral void which is a kind of grown-in defect present in a silicon crystal is exposed on the wafer surface. The size and number of pits exposed on the surface can be measured by various measuring methods. For example, a known laser particle counter can be used. In general, since the laser particle counter has a limit on the minimum detectable particle size, in reality, it is desirable to measure a COP having a specific size or more that causes a problem in wafer quality. For example, COP of 0.08 μm or more is measured. More specifically, it is possible to measure using a laser particle counter including 0.08, 0.09, 0.10, 0.11 μm or more in size distribution.

(impurities)
Further, the impurities on the surface of the silicon wafer that are cleaned and removed by the cleaning method include not only impurities generated and deposited in the previous process but also impurities that may be generated and deposited during the cleaning process. Impurities that may be generated and adhered during the main cleaning step include complexing agents and surfactants used by adding to the SC1 cleaning liquid used in the present invention. Impurities include those having both organic and inorganic properties. Furthermore, solid deposits (particles) of various sizes and shapes, deposits of organic or inorganic ionic species, and adsorbates are also included. Furthermore, the impurities on the surface of the silicon wafer include not only those chemically adsorbed on the surface of the silicon wafer but also those physically adsorbed. Further, it also contains impurities that are diffused to a depth of approximately 100 nm or less from the surface.

(Oxide film by ozone)
The present invention is characterized in that an oxide film is formed in advance on the surface of a silicon wafer used for SC1 cleaning. Further, the oxide film is formed by ozone. The method for forming an oxide film on the silicon wafer surface by ozone is not particularly limited, and various known methods can be preferably used. Specific examples include a solution containing ozone and an inert gas containing ozone. Examples of the solution containing ozone include an aqueous ozone solution and an aqueous ozone solution containing other acids. Although there is no restriction | limiting in particular about the ozone concentration of ozone aqueous solution, The range of 10-200 ppm is preferable.

  In addition to the physical properties of film thickness and surface uniformity, the oxide film by ozone is also characteristic in the chemical properties for etching by SC1, as will be described below. The physical properties of film thickness and surface uniformity can be measured by ellipsometry or X-ray photoelectron spectroscopy. Although the thickness of the oxide film of the present invention is not particularly limited, it is known that the thickness of the oxide film is automatically about 1 nm because chemical oxidation of silicon is governed by diffusion of reactive species. .

(Cleaning method)
The SC1 cleaning liquid used in this cleaning method is a cleaning liquid that contains generally known ammonia and hydrogen peroxide, and includes all cleaning liquids that are also abbreviated as APM. Moreover, the liquid which uses organic alkalis, such as tetramethylammonium hydroxide and choline, instead of ammonia is also contained. Moreover, what is marketed as SC1 washing | cleaning liquid for silicon wafers can also be used preferably. The concentration of ammonia contained in the SC1 cleaning liquid used in this cleaning method is preferably in the range of 0.01 to 5%, and the concentration of hydrogen peroxide is preferably in the range of 0.01 to 5%. Further, cleaning agents containing various additives in addition to ammonia and hydrogen peroxide are also included.

  It is known that the SC1 cleaning solution etches the entire silicon wafer surface when cleaning a silicon wafer having a chemical oxide film on the surface in advance. However, it has been known that the etching amount is usually about 0.1 to 1 nm per minute, but no knowledge based on quantitative measurement has been made. In particular, there has been no knowledge that the etching amount with the SC1 cleaning solution depends on the oxidizing agent for forming the oxide film.

As a result of detailed examination of the etching behavior of the chemical oxide film on the silicon wafer surface by the SC1 cleaning liquid, the present inventors have determined that the amount of etching of the chemical oxide film on the silicon wafer surface by the SC1 cleaning liquid is the oxidation used to form the oxide film. It was found that it depends on the type of agent. That is, an SC1 cleaning solution having the same composition is applied to an oxide film having the same thickness formed by hydrogen peroxide (H ₂ O ₂ ), hydrogen fluoride-ozone (HF / O ₃ ), and ozone water (O ₃ ). The result of measuring the etching amount of the silicon wafer after immersion for a predetermined time is shown in FIG. From this result, the following can be understood. Here, the etching amount of the silicon wafer was determined by cleaning a wafer on which an etching resistant mask had been formed in advance, removing the mask after cleaning, and then measuring the level difference with AFM.
(1) The etching amount per hour is small in the order of H ₂ O ₂ , HF / O ₃ , and O ₃ .
(2) The etching amount of H ₂ O ₂ increases linearly with respect to the immersion time, but HF / O ₃ and O ₃ have an induction period in the initial stage of immersion.
(3) The higher the O ₃ concentration, the longer the induction period at the beginning of immersion, and the smaller the etching amount as a result.

In particular, when the treatment is performed at an O ₃ concentration of 46 ppm, it is understood that the etching does not substantially proceed when the immersion time is less than about 100 seconds.

Therefore, when processing at the same time SC1 cleaning solution, since O ₃ etching of oxide film formed by containing washing solution is extremely small, it is possible to substantially suppress an increase in COP occurring for etching by SC1 cleaning.

  The SC1 cleaning time of the silicon wafer is not particularly limited, and an appropriate cleaning time can be selected according to the balance between the type and amount of contamination to be removed and the desired COP increase suppression capability. If the cleaning time is too short, sufficient contamination cannot be removed, but if the cleaning time is too long, the etching of the oxide film proceeds excessively, resulting in an increase in COP. Specifically, a silicon wafer subjected to standard intentional contamination is subjected to ozone treatment for a predetermined time to form an oxide film, and the silicon wafer is used for SC1 cleaning for a predetermined time, and then the contamination is actually removed. Can be optimized to optimize the degree of ozone treatment and SC1 cleaning time. However, if the oxide film that existed before the SC1 cleaning is completely removed, etching proceeds regardless of whether the oxide film forming means is ozone treatment, and the initial purpose of suppressing the increase in COP cannot be achieved. It is self-explanatory. Therefore, it is necessary to set conditions so that the etching amount by SC1 cleaning is less than the initial thickness (usually 1 nm) of the oxide film existing before SC1 cleaning.

For example, in the case of a silicon wafer intentionally contaminated with Si ₃ N ₄ particles, a sufficient cleaning effect and a COP increase suppressing effect are exhibited by treating with a 25 ppm ozone water for 4.5 minutes and performing a subsequent SC1 cleaning for about 2 minutes.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, this invention is not limited to these Examples. For example, void defects and bit-like defects other than COP are expected to have the ability to suppress or increase the same as COP.

Example 1 Particle Contamination Removal An 8-inch silicon wafer was used as a sample. Si ₃ N ₄ particles (manufactured by Mitsuwa Chemicals Co., Ltd.) were prepared on the surface of the silicon wafer by a dip method so that the number of particles was about 200 to 800 particles of φ0.20 μm or more. Measurement of particles on the surface of the silicon wafer was performed using a laser scattering surface foreign object meter (SurfScan 6200, manufactured by KLA-Tencor, laser scattering method). As the ozone treatment conditions, an oxide film was formed by treatment with 25 ppm ozone water at room temperature for 4.5 minutes. For comparison, an oxide film was formed on a silicon wafer treated with 3% hydrogen peroxide solution at 60 ° C. for 4.5 minutes. These silicon wafers were cleaned by immersing them in an APM (1: 5: 50) cleaning solution at 60 ° C. for 2 minutes. Under this condition, the etching amount by SC1 of the wafer on which the oxide film was formed with 25 ppm ozone water was about 0.05 nm, which was sufficiently smaller than the initial oxide film thickness of 1 nm. Table 1 shows the particle removal rate on the surface of the obtained silicon wafer.

  From this result, it can be seen that the particles are removed to the same extent by washing for 2 minutes in both the oxide film using hydrogen peroxide and the oxide film using ozone.

  The increase in COP on the surface of the obtained silicon wafer is shown in FIG. From this result, it can be seen that the increase in COP is greatly suppressed in the case of an oxide film by ozone. This result is explained by the fact that the oxide film formation by ozone has the property that it is difficult to be etched at the initial stage by the SC1 cleaning. That is, contaminants are removed without etching the surface of the oxide film more than necessary, and etching of pits, which are COPs existing on the surface, is also suppressed.

(Example 2) Suppression of metal adsorption As a sample, an 8-inch silicon wafer was cleaned with dilute hydrofluoric acid (0.5%) to remove the natural oxide film on the surface, and the wafer was subjected to APM cleaning solution at 60 ° C or 10 ppm ozone. An oxide film was formed by treatment with water for 4.5 minutes each. These wafers were immersed in ultrapure water containing 1 ppb of Ca for 10 minutes, rinsed with clean ultrapure water for 10 minutes, and dried with a spin dryer. The amount of Ca deposited on the surface of the dried wafer was quantified by vapor phase decomposition (VPD) -atomic absorption analysis (AAS). The results are shown in FIG. It was found that the oxide film formed with ozone water has a small amount of Ca adsorption. This is considered to mean that there are few Ca adsorption sites in an oxide film, and it can be said that the oxide film formed with ozone water has the characteristic of suppressing subsequent Ca contamination.

  According to the method of the present invention, contaminants on the surface of the silicon wafer are sufficiently removed, and an increase in the number of COPs (density) on the surface is suppressed, which is an extremely useful silicon wafer cleaning method.

FIG. 1 shows the measurement results of etching with an SC1 cleaning solution of oxide films formed with various oxidizing agents. FIG. 2 shows the COP increase on the surface of the silicon wafer of Example 1. FIG. 3 shows the amount of Ca deposited on the silicon wafer surface of Example 2.

Claims (3)

An initial oxide film is formed on the surface of the silicon wafer with a solution containing ozone , and then the initial oxide film formed by ozone is treated with one or more chemicals selected from ammonia, tetramethylammonium hydroxide, and choline. Etching with a mixed solution with hydrogen oxide, a method for cleaning a silicon wafer. The etching amount due to the mixture, characterized in der thickness less than Rukoto initial oxide film formed by the ozone, the method of cleaning a silicon wafer according to claim 1, wherein. 2. The method for cleaning a silicon wafer according to claim 1, wherein an ozone concentration in the solution containing ozone is 10 to 200 ppm.

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