JP3192610B2 - Method for cleaning porous surface, method for cleaning semiconductor surface, and method for manufacturing semiconductor substrate - Google Patents

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 substrate having a porous structure exposed on its surface, and more particularly to a method for selectively etching a semiconductor, separating a dielectric material, or cleaning a surface thereof. The present invention relates to a method for cleaning a porous surface suitable for a method for cleaning a porous silicon semiconductor substrate which requires strictest control. The invention also relates to a method for cleaning a semiconductor surface. Further, the present invention provides a semiconductor substrate
And a method for producing the same.

[0002]

2. Description of the Related Art A method for forming a porous structure typified by porous silicon is described in US Pat. Uhlir was introduced in 1956 (Bell. Syst. Tech. J, 35, p.
p. 333).

[0003] Thereafter, applied technologies such as selective etching layers or those which are oxidized and used as isolation regions, and epitaxial growth on porous silicon have been developed. The present applicant uses SOI (Siliconon) by using a single-crystal silicon thin film epitaxially grown on porous silicon in Japanese Patent Application Laid-Open No. Hei 5-21338.
(Insulator) substrate.

In recent years, the photoluminescence phenomenon of porous silicon has been discovered, and attention has been paid to self-luminous materials utilizing not only the structure but also the characteristics of physical properties.

[0005] As a method for forming porous silicon, an anodizing method using a known electrochemical cell structure in a mixed solution of hydrofluoric acid / pure water / ethanol is generally used. Since a large number of foreign substances adhere to the porous silicon, it is better to remove the foreign substances by cleaning when epitaxially growing on the porous silicon. Conventional cleaning is only rinsing of the electrolytic solution inside the pores with pure water. Even at present, there is no example introduced about the active cleaning method of the surface.

As is well known, cleaning before and after a processing process is essential in a semiconductor process and cannot be avoided even in a porous silicon substrate. Conventionally, as a method for cleaning a bulk substrate (non-porous), W. Ker
RCA cleaning developed by others (RCA Revi
ew, 31, pp. 187-205, 1970), a chemical wet type combining chemical solutions such as sulfuric acid / hydrogen peroxide solution, ammonia / hydrogen peroxide solution, hydrochloric acid / hydrogen peroxide solution, hydrofluoric acid / pure water, and the like. Cleaning is used as a method that is effective in removing foreign substances on the surface.

Recently, Kojima et al. (IEICE Technical Report, SDM95-86, ICD)
95-95, pp. 105-112, July 1995
Mon), high-frequency (megasonic) ultrasonic waves with a frequency of about 1 MHz are irradiated on a bulk substrate in a mixed solution of hydrofluoric acid / hydrogen peroxide / pure water / surfactant or ozone-added pure water to remove foreign matter. A way to do that has been proposed.

The feature of this method is that the silicon substrate is oxidized and etched by hydrofluoric acid and hydrogen peroxide solution, foreign matter on the surface is lifted off from the substrate, and the surface potential of the foreign matter is neutralized by a surfactant to neutralize the foreign matter. Cleaning is to prevent re-adhesion. In addition, the combined use of megasonic is intended to remove organic substances attached to the substrate surface due to ion generation from pure water by megasonic, in addition to applying energy when lifting off foreign substances, Cleaning with chemicals is fundamental. The purpose of using ozone pure water is to enhance the organic substance removing effect.

In the ultrasonic cleaning, the conventional cleaning using a low frequency of about several tens kHz to about 400 kHz gives a severe shock wave to the substrate surface by liquid cavitation (expansion and compression action) due to liquid resonance action, so that a substrate of several tens μm is provided. This is "liquid resonance cleaning" for removing surface foreign matter. On the other hand, 800
High-frequency waves in the range of kHz to 1.6 MHz are "sonic wave scrub cleaning" that removes by applying kinetic energy to the foreign matter by the resonance phenomenon, and also removes foreign matter of sub-micron size without damaging the fine pattern. It is possible.

Due to these characteristics, low-frequency cleaning has a problem in that fine patterns are damaged by cavitation impact, and has not been used in a semiconductor process of 4 mega DRAM or later. On the other hand, high-frequency cleaning has attracted attention as a method capable of cleaning minute foreign matters without damaging the pattern.

[0011]

According to the experience of the present inventors, a substrate having a porous structure on its surface has a fine and dense structure and a structure having long pores. For this reason, when a chemical solution is used in conventional chemical wet cleaning, the chemical solution penetrates deep inside the pores, and it is difficult to completely remove the chemical solution even after long-time pure water rinsing, and epitaxial growth on a porous structure is difficult. Etc. adversely affect the process.

[0012] In addition, when a conventional low-frequency ultrasonic wave is superimposed on pure water to physically remove foreign matter, even in a relatively high frequency region of 200 kHz, since the porous structure is too brittle, There is a problem that the sound pressure of the shock wave of cavitation causes collapse of the porous material.

[0013] Such a problem is caused by the structure of the porous silicon, the experience of the present inventors is not unique, and the reason why active cleaning of the porous silicon surface has not been performed conventionally. Can be attributed to a similar problem.

Further, according to a study by the present inventors, after forming a porous structure by anodization, the surface of a porous silicon substrate rinsed with pure water was subjected to laser reflection intensity distribution as shown in FIG. It was found that hundreds of foreign substances having a diameter of 0.3 μm or more obtained from the wafers adhered to a wafer having a diameter of 5 inches. The classification of L1, L2, and L3 in the bar graph indicates the classification of the size of the foreign matter obtained from the laser reflection intensity from the foreign matter, and increases in the order of L1 <L2 <L3.

The number of foreign substances adhering during anodization is shown in FIG.
As shown in Fig. 8, while passing through the anodizing chemical conversion batch in single-wafer processing, foreign substances in the liquid are trapped on the substrate and gradually decrease, but are reduced to several or less on the RCA-cleaned bulk surface. This is an extraordinary number when compared with the semiconductor process.

The foreign substances adhering during the anodization are reduced to some extent by the circulation of the electrolytic solution and the collection of the foreign substances by the filter, but are not sufficient. The adhesion of foreign matter may be caused by anodizing equipment, foreign matter mixed in electrolyte, dust from workers during processing, and the porous silicon surface becomes hydrophobic due to anodization in high concentration hydrofluoric acid electrolyte. Since the silicon substrate is likely to be electrostatically charged and adsorb foreign matter, it is not easy to prevent the adhesion.

As a matter of course, such foreign matter causes abnormal growth and defects such as pinholes in subsequent processes, particularly in a film forming process, and has been an obstacle to the application of porous silicon. (Object of the present invention) Therefore, an object of the present invention is to remove foreign substances adhering to the surface of the porous silicon without using a chemical solution which may affect the subsequent processes and to prevent the collapse of the porous silicon surface. It is an object of the present invention to provide a new cleaning method that can be efficiently removed without causing the cleaning.

It is another object of the present invention to provide an economical cleaning method which can be easily introduced without significantly changing the conventional cleaning step, is efficient, and does not use a special chemical solution.

[0019]

According to a first method for cleaning a porous surface of the present invention, a method for cleaning a porous surface of a substrate having a porous structure on at least the surface thereof has a frequency of 600.
Pure water with high frequency in the range of kHz to 2 MHz
The cleaning is performed to remove foreign substances attached to the porous surface of the substrate.

The second method for cleaning a porous surface of the present invention comprises the steps of:
The surface of the substrate to be cleaned has a structure in which openings of a large number of pores are exposed, and the inner wall surface of the pores has a structure in which a porous structure material is exposed or covered with a different material. This is a method for cleaning the porous surface. The different material is a material different from the porous structure material, and may be a film deposited on the surface of the porous structure material or a film formed by treating the porous structure material by oxidation, nitridation, or the like. Materials are selected as needed.

In the method for cleaning a porous surface according to the present invention, the substrate can be cleaned by immersing the substrate in a pure water bath and superposing a high frequency.

In the method for cleaning a porous surface according to the present invention, the high frequency can be superposed and cleaned in parallel with the porous surface of the substrate immersed in the pure water bath.

In the method for cleaning a porous surface according to the present invention, the substrate during high-frequency cleaning can be intermittently pulled out of the liquid by immersing the substrate in the pure water bath.

In the method for cleaning a porous surface according to the present invention, the porous surface of the substrate can be cleaned by spraying a pure water shower in which high frequency is superimposed on pure water while rotating the substrate.

In the present application, the porous structure is defined as a porous structure composed of a large number of fine and communicating pores having a pore diameter and a wall thickness between pores of about several hundreds Å to several tens μm on the surface of the substrate. 2 shows a structure formed over a thickness of from several hundred μm.

[0026]

The third method for cleaning a porous surface of the present invention is a method for cleaning a porous surface of a substrate having at least a porous structure on the surface, wherein high frequency ultrasonic waves having a frequency in the range of 600 kHz to 2 MHz are applied. Deionized water that is superimposed and deaerated so that the concentration of the dissolved gas is 5 ppm or less,
The cleaning is performed to remove foreign substances attached to the porous surface of the substrate.

The fourth method for cleaning a porous surface according to the present invention comprises:
The surface of the substrate to be cleaned has a structure in which the openings of a large number of pores are exposed, and the inner wall surface of the pores has a structure in which a porous structural material is exposed or covered with a different material. This is a method for cleaning the porous surface. The different material is a material different from the porous structure material, and may be a film deposited on the surface of the porous structure material or a film formed by treating the porous structure material by oxidation, nitridation, or the like.

In the cleaning method of the present invention, the substrate is immersed in a pure water bath having degassed pure water so that the concentration of dissolved gas is 5 ppm or less, and the substrate is cleaned by superimposing the high-frequency ultrasonic waves. Can be.

In the method for cleaning a porous surface according to the present invention, while rotating the substrate, the porous surface of the substrate is degassed so that the concentration of dissolved gas becomes 5 ppm or less, and the high-frequency ultrasonic wave is superimposed. Cleaning can also be performed by spraying a pure water shower.

The fifth method for cleaning a porous surface of the present invention comprises the steps of:
In the method for cleaning a porous surface of a substrate having at least a porous structure on the surface, the porous surface of the substrate is treated to be hydrophilic, and the cleaning of the hydrophilic porous surface is performed at a frequency of 600 kHz to 2 MHz. Cleaning is performed with pure water on which high-frequency ultrasonic waves in a range are superimposed to remove foreign substances attached to the surface of the base.

The porous substrate to be cleaned in the present invention has a structure in which the openings of the pores are exposed on the surface thereof, and has a pore structure communicating with the surface openings.

In the present invention, the foreign matter to be removed is
It is preferably attached to the surface of the porous substrate and having a size larger than the diameter of the porous opening.

The sixth method for cleaning a porous surface according to the present invention comprises:
In the method for cleaning a porous surface of a substrate having at least a surface with a porous structure, the liquid for treating the porous surface of the substrate to be hydrophilic and cleaning the porous surface made hydrophilic has a frequency of 600 kHz. The cleaning is performed by superimposing a high-frequency ultrasonic wave in a range of 2 to 2 MHz to remove foreign substances attached to the surface of the base.

In the method for cleaning a porous surface according to the present invention, the hydrophilic treatment of the porous surface may be a treatment for forming an oxide film on the surface of the substrate and the inner wall of the porous hole.

In the method of cleaning a porous surface according to the present invention, the hydrophilic treatment of the porous surface may be a treatment of immersing the substrate in pure ozone water obtained by dissolving ozone in pure water.

In the method for cleaning a porous surface of the present invention, the hydrophilic treatment of the porous surface may be a treatment of immersing the substrate in an aqueous solution of hydrogen peroxide diluted with pure water.

In the method for cleaning a porous surface according to the present invention, the liquid may be pure ozone obtained by dissolving ozone in pure water.

In the method for cleaning a porous surface according to the present invention, the liquid may be an aqueous solution of hydrogen peroxide diluted with pure water.

In the method for cleaning a porous surface according to the present invention, the hydrophilic treatment of the porous surface is a treatment for forming an oxide film on the surface of the substrate and the inner wall of the porous hole. It is possible to remove at least the oxide film on the surface of the substrate.
it can. In the method for cleaning a semiconductor surface according to the present invention, cleaning is performed to remove foreign substances adhering to the surface of the semiconductor substrate with pure water degassed so that the concentration of dissolved nitrogen is 5 ppm or less and superimposed with ultrasonic waves. It is characterized by the following. Semiconductor of the present invention
The method of manufacturing a body substrate includes the steps of:
Preparation of a substrate having a porous structure on at least the surface
A washing step of washing the substrate, the porosity of the substrate
Forming a non-porous single crystal layer on the porous structure,
Before bonding the other substrate on the non-porous single crystal layer
Leaving the non-porous single crystal layer on another substrate,
Removing the exposed surface of the non-porous single crystal layer
Planarizing, wherein the cleaning step is performed according to the present invention.
Applying a method of cleaning the porous surface to the substrate.
And features.

The pure water is water from which impurities are removed as much as possible. In a semiconductor process, various levels of water are used as pure water for cleaning, depending on the degree of integration of a semiconductor device to be manufactured. In the present invention,
Water satisfying the following criteria was used as pure water.

Resistivity [MΩ · cm]> 17.5 Particles [particles / ml] <20 (for particles having a size exceeding 0.1 μm) Bacteria [particles / 100 ml] <50 Total silica [ppb] <5 TOC [ppb] <50 (Total organic carbon) DOC [ppb] <50 (Dissolved oxygen concentration) Mettalic ion [ppt] <500

[0043]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiments of the present invention are shown below, but any combination is within the scope of the present invention. (Embodiment 1) The method for cleaning a porous surface of the present invention has a frequency of 200 kHz to 8.4 MHz, preferably 600 kHz.
Ultrasonic waves in a high frequency band from kHz to 2 MHz, more preferably from 800 kHz to 1.6 MHz, are irradiated on the surface of the porous substrate while being superimposed on pure water.

In the method for cleaning a porous surface of the present invention, the material of the substrate is not particularly limited as long as the substrate has a porous structure on the surface as shown in FIG. For example, a semiconductor material such as Si or GaAs, a ceramic material, or the like can be used.
FIG. 2 shows a structure in which a semiconductor thin film of amorphous Si, polycrystalline Si, GaAs, or the like, or a layer made of metal deposition is deposited on the inner wall surface of the pores of the Si porous substrate using a chemical vapor deposition method or the like.

Hereinafter, cleaning of a porous silicon substrate will be described as an example of the method for cleaning a porous surface of the present invention.

The size of the foreign matter that can be removed by ultrasonic waves is determined by the frequency. For example, the size of a foreign matter that can be removed at a high frequency of 800 kHz or more is about 0.1 μm, and the molecular acceleration applied to the foreign matter at this time reaches about 250,000 times the gravitational acceleration of the earth's surface. It is said to be removed. In addition, the wavelength is as short as 0.8 mm in pure water, it is irregularly reflected on the liquid surface, and partially transmits to the atmosphere, so it does not generate standing waves such as low frequency ultrasonic waves in pure water. Less unevenness.

Since the wavelength is short and the directivity is high, it is possible to reduce the damage to the fine and fragile porous silicon surface by applying an ultrasonic wave in parallel to the substrate surface. In addition, the high frequency has a small amplitude and the number of times of rubbing the substrate surface is excellent in removing foreign substances, and the specific resistance is reduced by the generation of ions in pure water. Minimal reattachment of foreign matter.

However, there is no example in which ultrasonic cleaning is used for cleaning the porous silicon surface, and even less, there is no example in which foreign matter on the porous silicon surface is removed with pure water. The present inventor has found that, unlike a bulk substrate, when cleaning is performed with pure water to which a high frequency is applied, the high frequency has a certain range due to the characteristics of the porous substrate. This will be described with reference to FIG. In ultrasonic cleaning of a substrate, the frequency used depends on the size of foreign particles to be removed. For example, from FIG. 1, 80 to 90 kHz for removing foreign matter having a particle size of 1 μm from the substrate.
In order to remove an ultrasonic wave having a frequency of about 0.1 μm or a particle having a particle size of about 0.1 μm, an ultrasonic wave having a frequency of about 800 to 900 kHz may be applied.

However, in the porous Si substrate, according to the experiment of the present inventor, as shown in FIG. 1, the collapse of the porous material was observed below 200 kHz. Can be seen. This is because the use of ultrasonic waves for cleaning causes a problem not present in the bulk substrate in the porous substrate.

As described above, the porous structure is p ⁺
Type, p ^- type or n ^- type porous silicon has a fine structure of several hundred angstroms or less, and according to experiments performed by the present inventor, when ultrasonic waves having a frequency lower than 200 kHz are used, the porous structure is vulnerable to cavitation. The quality surface collapses.

According to the experiment of the present inventor, 8.4 M
When an ultrasonic wave having a frequency exceeding Hz is used, the fine porous structure itself resonates, and the porous material similarly collapses. The resonance frequency depends on the porous structure, and the lower limit frequency of the ultrasonic wave that can be used in a relatively large porous structure with a pore diameter and a silicon wall thickness of several hundred nm to several tens of μm such as n ⁺ type porous silicon Will be even higher.

Therefore, the ultrasonic cleaning of the porous silicon surface is set to a frequency band of 200 kHz to 8.4 MHz, preferably a high frequency of 600 kHz to 2 MHz. More preferably, by using high-frequency ultrasonic waves in a frequency band called megasonic cleaning in the range of 800 kHz to 1.6 MHz, the risk of collapse of the porous structure can be avoided.

As for the high frequency cleaning, Japanese Patent Laid-Open Publication No. Sho 51-2264 discloses that a semiconductor wafer is cleaned with high frequency ultrasonic waves in the range of 200 kHz to 5 MHz. And high-frequency ultrasonic waves to ammonia (chemical solution), and there is no disclosure of cleaning the porous substrate with pure water. Also, JP-A-6-2
Japanese Patent No. 75866 discloses that the porous semiconductor is immersed in pure water obtained by applying ultrasonic waves to the porous semiconductor. This is intended to improve the light emission characteristics by immersing the porous semiconductor in pure water, and is intended for cleaning. There is no disclosure of the ultrasonic frequency. In addition, IEICE technical report, SDM
95-86, ICD95-95, pp. 95-95. 105-11
2, July 1995, it is disclosed that the bulk substrate is cleaned with a chemical solution to which ultrasonic waves are applied and rinsed with pure water to which a high frequency is applied. There is no disclosure of cleaning.

On the other hand, another problem peculiar to ultrasonic cleaning of porous silicon is generation of bubbles during cleaning.

The gas taken into the porous silicon by the anodizing treatment or the subsequent drying is replaced with pure water outside the pores during the ultrasonic cleaning, and is discharged. Adhere as. These bubbles hinder the propagation of ultrasonic waves, reduce the effect of removing foreign substances, and promote the adsorption of foreign substances, causing reattachment to the substrate.

The bubbles are also generated by factors other than the substrate structure. Generally, bubbles are generated by cavitation in low-frequency ultrasonic waves, but are also generated by dissolved gases in pure water even in high-frequency ultrasonic waves. Micro bubbles adhering to the surface of the porous silicon cannot be removed by high-frequency cleaning without using a shock wave, but can be removed by intermittently lifting the substrate from pure water during ultrasonic cleaning.

On the other hand, although not limited to ultrasonic cleaning, a problem of batch cleaning in which a plurality of substrates are collectively stored in a carrier and immersed in a cleaning bath is a problem of Teflon generally used for supporting substrates during cleaning. The cleaning carrier is charged only by being immersed in pure water, and there is a problem that the substrate at the end of the carrier is inductively charged and adsorbs foreign matter.

At present, batch cleaning without a carrier is also possible. However, when a carrier is used, a dummy substrate used only for cleaning is arranged at the end of the carrier, so that foreign substances on the substrate at other carrier positions can be removed. Adhesion can be avoided. When the porous silicon layer is formed only on one side, foreign matter adheres to the porous surface by setting the porous silicon substrate on the carrier end in an inverted manner and setting the other substrates in the normal direction. Can be avoided.
This is a countermeasure in the case of batch type cleaning. This operation is unnecessary in the case of single wafer type spinner cleaning in which high frequency ultrasonic waves are superimposed on a pure water shower on the rotating substrate surface to perform surface cleaning.

In the cleaning with the pure water shower, there is a problem that foreign matter is attracted by charging of the substrate due to frictional charging between the nozzle and the flowing water. However, this problem is avoided by lowering the specific resistance of the pure water by superimposing a high frequency. be able to. However, when using cleaning by a shower, it is necessary to remove foreign substances only by high-frequency vibration and to reduce the water pressure so as to avoid collapse of the porous silicon.

As described above, according to the present embodiment, foreign matter on the surface can be removed by using only pure water and high-frequency ultrasonic waves to avoid collapse of the porous silicon.

Further, a foreign substance newly adhering to the surface of the porous silicon substrate having the porous inner wall thermally oxidized is re-adhered to the hydrophobic surface after etching the oxide film on the outermost surface layer with the diluted hydrofluoric acid water tank. Can be removed by high-frequency ultrasonic cleaning with pure water. (Embodiment 2) As described above, according to the study of the present inventors, a porous structure was formed by anodization and the surface of a porous silicon substrate rinsed with pure water was subjected to laser reflection intensity distribution. It was found that hundreds of foreign particles having a size of 0.3 μm or more adhered to a 5-inch diameter wafer (FIG. 2).
8). The adhesion of foreign matter may be caused by anodizing equipment, foreign matter mixed in electrolyte, dust from workers during processing, and the porous silicon surface becomes hydrophobic due to anodization in high concentration hydrofluoric acid electrolyte. Since the silicon substrate is likely to be electrostatically charged and adsorb foreign matter, it is not easy to prevent the adhesion.

Further, the porous structure has a fine and dense structure and a structure having long pores. Therefore, when a chemical solution is used in a chemical wet cleaning such as the conventional RCA cleaning, the inside of the pores becomes deep. It is difficult to completely remove the chemical solution even if the chemical solution infiltrates and rinses with pure water for a long time, which adversely affects the post-process such as epitaxial growth.

Such a problem is caused by the structure of the porous silicon, and the experience of the present inventors is not unique, and the reason why the active cleaning of the porous silicon surface has not been performed conventionally. Can be attributed to the same problem.

In the first embodiment, as a method of cleaning the surface of a fine and fragile porous structure without a preferable cleaning method in the past, pure water having a frequency in the range of 200 kHz to 8.4 MHz, preferably 600 kHz to 2.0 MHz is used. More preferably, it is a method of cleaning with pure water on which high-frequency ultrasonic waves in the range of 800 kHz to 1.6 MHz are superimposed.

However, when ultrasonic cleaning is performed using pure water, bubbles are generated from the pure water even at a high frequency,
In particular, a phenomenon in which bubbles adhere to the surface of the porous silicon having the hydrophobic surface from which the surface oxide film has been removed is observed.

The air bubbles adhering to the surface are hard to be eliminated especially when the substrate to be cleaned is washed by immersing it in a pure water bath on which ultrasonic waves are superimposed. That is, the fine air bubbles adhering to the substrate are not eliminated even by the flowing water during the cleaning, and while the air bubbles are small, they cannot be moved on the surface of the substrate only by their own buoyancy and are fixed. Bubbles not only impede the propagation of ultrasonic waves and reduce the cleaning effect, but also attract fine foreign matter in pure water to the gas-liquid interface, and contaminate the surface of the substrate to be cleaned with foreign matter.

Therefore, the first embodiment provides a method for avoiding the problem of reverse contamination of the substrate to be cleaned due to the generation of bubbles as described above.
In this method, the substrate to be cleaned is periodically pulled up from the pure water tank during cleaning to remove air bubbles, thereby avoiding reverse contamination by foreign substances and realizing a cleaning effect by high-frequency ultrasonic waves.

FIG. 3 shows the state of bubbles remaining on the wafer. FIG. 3A shows the position and the number of foreign substances of 0.2 μm or more on the surface of a 5-inch diameter bulk wafer when bubbles were not removed by ultrasonic cleaning in pure water. b) shows the position and the number of similar foreign substances when air bubbles are removed by lifting every 5 minutes. As is clear from the comparison between FIG. 3A and FIG. 3B, when the bubbles are not removed, the foreign substances are concentrated along the bubble rising direction,
This tendency is mitigated by air bubble removal.

In this cleaning method, porous silicon having a hydrophobic surface is placed in a room-temperature overflow pure water tank at 950 kHz.
When the ultrasonic wave of z is superimposed and the substrate is pulled out of the pure water tank for removing bubbles every 5 minutes and washed for 20 minutes, foreign substances on the surface of the porous substrate which could not be washed conventionally are reduced from 30% to 4%.
About 0% can be removed, and if the same cleaning is repeated for 20 minutes, 60% to 80% of foreign matter before cleaning can be removed.

However, on the hydrophilic porous silicon surface having an oxide film on the surface, such fixation of air bubbles does not occur, and the air bubbles quickly rise to the water surface and are eliminated. Thus, almost 90% of foreign matter could be removed by high-frequency ultrasonic cleaning without periodically lifting the substrate.

In the first embodiment, foreign substances are removed by cleaning with pure water in which ultrasonic waves in a high frequency range are superimposed on pure water, and during the cleaning, the substrate to be cleaned is periodically pulled up from the pure water tank. I have.

However, depending on the application, there is a case where it is desired to further remove foreign matter. However, in the above-described cleaning method, if the cleaning time in ultrasonic pure water is further increased in order to further remove foreign substances, there is a possibility that a natural oxide film may be formed by pure water, as well as the problem of work efficiency. This cleaning effect is not improved even when the temperature of pure water is increased.

The method of periodically pulling up the substrate from the pure water tank during cleaning is a very complicated operation when, for example, cleaning depends on the labor of an operator. Furthermore, it is difficult to completely avoid the generation of bubbles during cleaning and the attachment to the substrate, even if the bubble removal operation is performed by regular lifting.
Further improvements are desired from the viewpoint of reproducibility and stability of washing.

Therefore, in the second embodiment, as a result of further study, the cleaning was performed using pure water from which the dissolved gas was degassed, whereby bubbles were generated during ultrasonic cleaning and the pure water was adhered to the substrate. Prevents contamination of the substrate by foreign matter.

In cleaning a semiconductor substrate, it is known that O ₂ and CO ₂ are degassed in order to prevent oxidation on the semiconductor substrate. However, N ₂ which is a reducing gas is not particularly considered to be a problem. Conversely, the saturation concentration (17.8 at 25 ° C.)
ppm, and 6.7 ppm in hot pure water at 80 ° C. ) Were previously using N ₂ as takes in washing water. The present inventors, as shown in FIG. 4, the pure water incorporating N ₂ 25 ° C., while varying the residual oxygen concentration, 47 kHz, 950
When the number of generated bubbles was examined by ultrasonic waves at a frequency of kHz, the number of generated bubbles was larger at a frequency of 950 kHz than at 47 kHz, and at 5 ppm, more than 100 bubbles were generated in each case. It has been found that this causes foreign matter adhesion. It has also been found that it is difficult to avoid the generation of bubbles even when pure water is heated to 80 ° C.

Therefore, the present inventor performed degassing including N _2, and while changing the residual oxygen concentration in the same manner, at 47 kHz and 95%.
When the number of bubbles generated was examined using an ultrasonic wave having a frequency of 0 kHz, the frequency was about 6 ppm at 47 kHz, and 3 ppm at 950 kHz.
It was found that bubbles almost disappeared at ~ 5 ppm,
It has been found that by performing degassing (including N ₂ ), the generation of bubbles is suppressed regardless of the frequency of the ultrasonic wave, and foreign substances can be removed from the substrate.

The inventor of the present invention has developed orbisphere laboratories (orbisphere laboratories).
s) The dissolved gas concentration in pure water in the square overflow tank in the water supply state was measured in detail using a dissolved oxygen / dissolved nitrogen sensor manufactured by s). As a result, water was supplied from the tank bottom of the overflow tank, When megasonic ultrasonic waves having a frequency of 950 kHz are radiated at an output of 600 W, as shown in FIG. 5, the overflow operation depends on the water supply amount but the optimum water supply amount is 0.2 m ³ / hr to 0.4 m. ^In the range of ³ / hr, the concentration of dissolved nitrogen in pure water as feed water is 5 pp.
It has been found that no bubbles are generated in the entire tank in the case where the concentration is from m to 5.5 ppm and the dissolved oxygen concentration is in the range of 3.83 ppm to 4.3 ppm or less.

In general, in a semiconductor process, pure water in which nitrogen gas is dissolved after primary degassing is used at a use point.

The present inventor has also proposed that the dissolved oxygen concentration be 7.3.
8ppb, 14.57p when the dissolved nitrogen concentration is about the saturation concentration
Although pure water of pm was used, bubbles were generated by megasonic irradiation when dissolved nitrogen was present at 5 ppm or more even when only dissolved oxygen was degassed.

From this fact, megasonic ultrasonic waves were converted to pure water.
In order to suppress the generation of bubbles when irradiating, it is necessary to dissolve in pure water.
Controls the concentration of gas with high dissolved concentration
Need to be at least the main constituent gas of air
Nitrogen and oxygen, and CO _Two Control each gas concentration
(Dalton partial pressure method
Rule).

The main reason for paying attention to the main constituent gas of air is that the tank generally used for cleaning has a structure in which the liquid surface is open to the atmosphere. This is because it is not possible to ignore the amount of the gas constituting the redissolved in the liquid from the liquid surface.

The re-dissolution of air in contact with the liquid surface is remarkable in the case of a water storage tank. For example, even if deaerated water is supplied from the tank bottom and stored, the dissolved gas concentration in the liquid surface direction is lower than that of the tank bottom. As the concentration increases, a distribution is formed in the dissolved gas concentration in the tank, and the distribution in the tank is made uniform to a high concentration over time, and it becomes difficult to control the concentration of the dissolved gas (Henly's law).

On the other hand, in the case of an overflow tank, if degassed water is supplied from the bottom of the tank, pure water whose dissolved gas concentration has been adjusted is always supplied. It is considered that the concentration of dissolved gas in can be controlled to be constant.

However, actually, depending on the amount of water supplied, the degassed water that has reached the liquid level comes into contact with the atmosphere and re-dissolves the gas, and is circulated again in the tank without being partially drained. A phenomenon of thickening is seen. In order to solve such a problem, the most important points are the optimal design of the tank structure and the optimal setting of the water supply.

As an example, the width 28c used by the inventor
m, depth 23 cm, depth 25 cm in a square overflow tank, dissolved oxygen concentration (DO value) and dissolved nitrogen concentration (N
₂ values) 25cm deep (bottom of tank) and 12.5cm deep
6 (a), 6 (b), 7
(A) and FIG. 7 (b).

At the use point, pure water having an oxygen concentration of 1.88 ppb and a nitrogen concentration of 1.542 ppm, which was degassed at the use point, was supplied from the bottom of the tank (25 cm deep) at a water supply of 0.3 m ³ / hr, and overflowed. The concentration of dissolved oxygen and the concentration of dissolved nitrogen at each position in the tank were measured.

The arrows in the figure indicate the direction of water supply at the bottom of the tank.

The dissolved gas concentration at the water supply port is the lowest regardless of the gas type, and at the bottom of the tank, the concentration increases with distance from the water supply port along the flow of pure water, and the depth is 12.5 cm.
Although the concentration is higher than that at the bottom of the tank, it shows a substantially uniform concentration distribution, with an oxygen concentration of about 150 ppb and a nitrogen concentration of about 1.
It was 8 ppm.

Here, depending on the position and direction of the water supply port and the amount of water supply, turbulence occurs in the water flow and the concentration in the tank becomes high. Therefore, for example, the concentration in the tank is controlled by the dissolved gas concentration of the secondary degassed water. In such a case, it is necessary to make an optimum design for reducing the concentration in the tank as described above.

The dissolved gas concentration of the degassed water in the overflow tank performing the megasonic irradiation slightly increases with the irradiation time.

FIGS. 8 (a) and 8 (b) show the dissolved oxygen concentration distribution and FIG. 9 (a) showing the dissolved nitrogen concentration distribution in the overflow where megasonic irradiation was performed at an output of 600 W for 10 minutes from the bottom of the tank. And FIG. 9 (b). Although it is a slight change, it is necessary to set the dissolved gas concentration of the degassed water to be supplied in consideration of such concentration change under actual use conditions in order to accurately control the dissolved gas concentration in the tank .

That is, the surface cleaning method of the present invention is a method for removing foreign substances adhering to the surface of a substrate with pure water degassed so that the dissolved gas concentration becomes 5 ppm or less and superimposed with ultrasonic waves. It is for cleaning. Note that the concentration of the dissolved gas of 5 ppm or less means that the concentration of the dissolved gas is 5 ppm or less regardless of the type of the dissolved gas. ,nitrogen,
And the concentration of dissolved gas such as CO ₂ may be 5 ppm or less.

However, in the megasonic cleaning experiment of the present inventor, as shown in FIG. 10, the dissolved gas concentration was controlled to be below the boundary where no bubbles are generated (dissolved nitrogen concentration 5 ppm or less, dissolved oxygen concentration 3.8 ppm or less). When using water, the cleaning effect by megasonic irradiation is excellent, but degassed water in which dissolved gas is reduced by secondary degassing to the limit (dissolved oxygen concentration 150 ppb, dissolved nitrogen concentration When 1.8 ppm) was used, no air bubbles were generated, but it was also clear that the cleaning effect by megasonic irradiation was not obtained at all.

The reason for this is not clear, but the principle of megasonic cleaning is not only based on the so-called “sonic scrub cleaning” as conventionally known, but also by “liquid resonance cleaning” by cavitation such as low-frequency ultrasonic cleaning. The generation of bubbles by megasonic irradiation in the presence of a high concentration of dissolved gas is another proof of the liquid resonance phenomenon.

However, in megasonic cleaning, it is known that the sound pressure generated by cavitation impact is extremely low, and even if liquid resonance exists, the cavitation phenomenon is weak.

It is said that in ultrasonic vibration, the cavitation radius decreases with an increase in frequency. If sound pressure is generated by cavitation, it is assumed that high-frequency resonance occurs more densely than low-frequency resonance. That the cavitation impact is weak is not necessarily contradictory. The smaller the cavitation radius and the higher the density, the better the ability to remove smaller particle size particles.

Irrespective of the frequency, the cavitation phenomenon creates a kind of vacuum space in pure water by fracturing water molecules by ultrasonic vibration, expands, and finally shrinks this space rapidly. Thus, it can be considered that sound pressure is generated.

The generation of bubbles is such that the gas dissolved in water is degassed into this vacuum space, and then the space shrinks faster than the gas in the space re-dissolves in pure water. When the gas density is high, it can be considered that the gas having no place to go remains as bubbles.

Bubbles adhering to the hydrophobic substrate that are not removed by overflowing are removed for several minutes (2 to 3 minutes) by stopping ultrasonic irradiation and supplying deaerated water to the limit.
Will disappear later.

From this, it is considered that the gas degassed in the minute vacuum space due to cavitation is redissolved in pure water.

By controlling the concentration of the dissolved gas, the generation of bubbles at the time of ultrasonic irradiation can be suppressed because the amount of gas remaining as bubbles is reduced by lowering the density of gas degassed and released in this minute vacuum space. It is nothing but reducing.

However, it is not clear why megasonic cleaning effect does not appear when pure water degassed to the utmost limit is used, and conversely, an increase in particle adhesion is caused.

It is assumed that the contribution of the dissolved gas to the cavitation phenomenon exists, that is, the minute space is repeatedly evacuated and the dissolved gas is released into the space (degassing), and is repeatedly dissolved in pure water during contraction. It can be considered that it is formed by the process, or is caused by the electrochemical action caused by the change in the potential of the substrate caused by megasonic irradiation and the generation of ionic species from the dissolved gas in pure water, as conventionally known.

The surface cleaning method of the present invention has been found in the course of an experiment on a method of cleaning a porous surface. Instead, the present invention can be applied to, for example, cleaning of a silicon wafer and an SOI substrate. The present invention is preferably used for cleaning a substrate having a hydrophobic surface. However, even in a substrate having a hydrophilic surface, adhesion of foreign substances can be more reliably prevented if the generation of bubbles is suppressed.

The method for cleaning a porous surface of the present invention comprises
kHz to 8.4 MHz, preferably 600 kHz to 2 MHz, more preferably 800 kHz to 1.6 MHz.
Ultrasonic waves in the high frequency band in the range of z are superimposed on pure water and irradiated onto the surface of the porous silicon substrate, and the dissolved gas is cleaned using degassed pure water to remove bubbles during ultrasonic cleaning. The present invention prevents contamination of a substrate by foreign matter in pure water due to generation and adhesion to the substrate, and realizes stable and quick cleaning even on a porous silicon surface having a hydrophobic surface.

In the method for cleaning a porous surface of the present invention, the material of the substrate is not particularly limited as long as the substrate has a porous structure on the surface. For example, semiconductor materials such as Si and GaAs,
A ceramic material or the like can be used. Further, as shown in FIG. 2, a structure in which a layer composed of a semiconductor thin film such as amorphous Si, polycrystalline Si or GaAs or a metal thin film is deposited on the inner wall surface of the pores of the Si porous substrate by using a chemical vapor deposition method or the like. The porous surface cleaning method of the present invention can also be used to remove foreign substances adhering to the surface of the substrate having the above.

On the other hand, in the prior art related to the present invention, the following is performed. To realize the recent sub-micron or deep-micron VLSI, it is required to suppress the formation of a natural oxide film, and Morita et al. Use it for cleaning to form a natural oxide film in pure water. It is reported that dissolved oxygen in pure water is a critical factor, and it is reported that removal of dissolved oxygen to the utmost is the minimum condition for suppressing the formation of a natural oxide film (Ultra ·clean·
Technology, Vol. 1, No. 1, pp. 22-2
8, 1989).

At present, the dissolved oxygen concentration in water is 5 pp.
As a method for degassing to an extreme concentration region of b or less, a method in which a film is degassed in a physical degassing method and a method in which a catalyst is combined with a reduction method in a chemical degassing method is known. In particular, the membrane degassing method has recently been widely used because it has low contamination of pure water and can remove dissolved gases other than oxygen.

However, these techniques do not relate to the removal of foreign substances from the substrate, and do not suggest cleaning of a porous substrate at all.

Hereinafter, the method for cleaning a porous surface of the present invention will be further described. In ultrasonic cleaning, bubbles are generated during cleaning. As a source of bubbles, the gas taken in after drying the porous silicon is replaced by pure water outside the pores during ultrasonic cleaning and discharged, and oxygen dissolved in pure water used for cleaning is used. Dissolved gas such as nitrogen and nitrogen is converted into bubbles by ultrasonic cavitation.

The air bubbles adhered to the surface of the hydrophobic substrate not only impede the propagation of ultrasonic waves and decrease the effect of removing foreign substances, but also promote the adsorption of foreign substances to the substrate, causing contamination by foreign substances and a decrease in the cleaning effect. Become.

As described above, bubbles adhering to the surface of the porous silicon can be removed by intermittently pulling the substrate out of pure water during the ultrasonic cleaning. In the case of using pure water as a dissolved gas, the cleaning effect is naturally limited even if such measures are taken.

As described above, about 90% of foreign matter is removed by high-frequency ultrasonic cleaning with pure water of the surface of a porous silicon substrate which has been made hydrophilic by oxidation, and thus, the porous silicon substrate is taken into the inside of the porous body. It is considered that the degree of interference with washing due to deaeration of bubbles is slight as compared with the problem caused by the generation of bubbles from pure water.

In the case of a porous material having a hydrophobic surface, the amount of air bubbles deaerated from the inside of the porous material is considered to be much smaller than the amount of air bubbles generated from pure water.

In addition, when the porous inner wall is also hydrophobic, the bubbles themselves are less likely to be degassed, so that they are less likely to interfere with the cleaning. Therefore, when one of the causes of bubble generation is the dissolved gas of pure water used for cleaning, in order to further enhance the cleaning effect of the substrate, the dissolved gas in pure water is degassed as in the present invention. It is most effective to use it.

This is not limited to the case where the substrate is immersed in a pure water bath and the ultrasonic wave is superimposed in water for cleaning, and the effect is also obtained for the case where the ultrasonic wave is superimposed on the pure water shower and the substrate is sprayed and cleaned. I do.

The gas dissolved in water has a dissolved oxygen concentration of 8.8 when air at 25 ° C. and 1 atm is in contact with water.
It is considered that the concentration of dissolved nitrogen is 26 ppm and the concentration of dissolved nitrogen is 13.9 ppm.

Generally, pure water used in the semiconductor field is supplied by purging a pure water tank in a polishing system with nitrogen to maintain purity.

Therefore, it is considered that nitrogen is dissolved in pure water in a substantially saturated state. For example, the saturated dissolved concentration of nitrogen gas (purity: 99.999%) in pure water at 25 ° C. and 1 atm is as high as 17.8 ppm. Moreover, the dissolution concentration depends on the water temperature, and the dissolvable concentration decreases as the water temperature increases.

When this pure water is heated to 80 ° C., the dissolvable concentration becomes 6.7 ppm, and an excess nitrogen of 11.1 ppm of the difference is generated as bubbles.

If the dissolved gas in pure water is removed to a saturation concentration or less using a membrane deaerator, the generation of bubbles due to heating is prevented. Since bubbles can be generated even at a concentration of, it is more desirable to remove the dissolved gas to the limit concentration region.

Fortunately, a hydrophobic film is disposed at a portion corresponding to the interface, and the partial pressure is reduced by reducing the pressure on the secondary side with a vacuum pump to deaerate the pure water on the primary side. If the deaerator is connected to the outlet of the pure water production apparatus and used, it is possible to obtain pure water having a dissolved oxygen concentration in the extreme concentration region of 5 ppb or less with pure water of 60 ° C. or less even now.

By using pure water from which the dissolved gas in pure water has been removed in this manner, even if the pure water is heated, the generation of bubbles by high-frequency ultrasonic waves is eliminated, and the bubbles are deposited on the hydrophobic substrate surface. There is no sticking.

By preventing the generation of air bubbles, foreign substances can be further removed in the same cleaning time as in the prior art without performing the lifting operation of the hydrophobic porous silicon substrate, and pure water during cleaning is heated. This does not impair this effect.

As described above, according to the present invention, it is possible to avoid the collapse of porous silicon and remove foreign substances on the surface with high efficiency in a short time only by pure water from which dissolved gas has been removed and high-frequency ultrasonic waves. Became.

The function of the present embodiment can be effectively applied to a substrate other than silicon as long as it is a substrate to be cleaned having a similar fine and brittle porous structure.
The functions and effects of the present invention are not limited to silicon.

As a method for forming porous silicon, an anodization method in a mixed solution of hydrofluoric acid / pure water / ethanol is generally used. However, the silicon substrate during the anodization is formed by a chemical conversion apparatus or an operator. Many foreign substances adhere to the surface due to dust. (Embodiment 3) In Embodiment 3, the porous silicon surface is subjected to a hydrophilic treatment and subjected to ultrasonic cleaning. The ultrasonic cleaning of the porous silicon surface is set to a frequency band of 200 kHz to 8.4 MHz, preferably a high frequency of 600 kHz to 2 MHz. More preferably, from 800 kHz to 1.6.
If high-frequency ultrasonic waves in a frequency band called megasonic cleaning in the MHz range are used, the risk of collapse of the porous structure can be avoided.

However, in the above-described cleaning method by the present inventors, even when high-frequency ultrasonic waves are applied, a phenomenon is observed in which bubbles are generated from pure water, and particularly, bubbles adhere to the surface of the hydrophobic substrate.

The air bubbles not only impede the propagation of ultrasonic waves and lower the cleaning effect, but also attract fine foreign matters in pure water to the gas-liquid interface and contaminate the substrate surface with the foreign matters. Almost no air bubbles adhere to the substrate surface on the hydrophilic surface,
Bubbles adhere and adhere to the hydrophobic surface, and it is difficult to completely eliminate the adherence of foreign matters due to bubbles by flowing water or ultrasonic waves in the cleaning tank.

Hereinafter, this point will be described in more detail. FIG. 11 shows the state of attachment of bubbles and the state of movement of foreign matter on the hydrophobic substrate when a high frequency is superimposed on pure water. 3
1 is a quartz high-frequency cleaning tank, 32 is a high-frequency vibration plate, 33 is pure water from which dissolved gas has not been degassed, 34 is bubbles, 35 is foreign matter, 36 is a porous silicon substrate having a hydrophobic surface, 3
8 denotes a high-frequency traveling wave.

In general, in low-frequency ultrasonic cleaning, air bubbles are generated by cavitation in pure water, but in high-frequency cleaning, cavitation is reduced, but dissolved gas is vaporized due to local temperature rise due to high-frequency vibration in pure water. Occurs,
Bubbles are generated. The source of these bubbles is a dissolved gas such as oxygen or nitrogen dissolved in pure water.

The gas-liquid interface of the bubbles has a higher energy than that in the liquid and collects and traps fine foreign matter in pure water. Moreover,
Since the hydrophobic substrate surface is energetically stable with respect to gas, air bubbles are likely to adhere thereto. Further, since pure water cannot enter between the air bubbles and the substrate surface, the air bubbles are difficult to peel off from the substrate. For example, the diameter is 1 mm. As long as the bubbles are small, the buoyancy alone cannot move the substrate surface.

When a high frequency ultrasonic wave is applied to the substrate surface,
It receives kinetic energy in the direction of the traveling wave, but is insufficient to move the bubbles, and the bubbles adhere to the substrate surface.

The adhesion of air bubbles to the substrate surface promotes the collection of minute foreign matter in pure water. When the air bubbles adhere and move on the substrate surface by their own buoyancy, they are collected along the movement path. The contaminants adhere to the substrate surface and contaminate the substrate.

In addition, air bubbles adhering to the substrate surface impede the propagation of high frequency to the substrate surface and reduce the cleaning effect. That is, if bubbles are generated in the high-frequency cleaning of a substrate having a hydrophobic surface, the cleaning of the substrate surface by the high frequency and the contamination of the substrate by the foreign matter in the pure water due to the adhesion of the bubbles proceed simultaneously, resulting in a cleaning effect. Is reduced.

[0136] Even when washing is performed by superimposing ultrasonic waves on a pure water shower, bubbles are generated and adhered. However, if the shower pressure is increased to remove the bubbles, the fragile porous surface is broken. In some cases.

The present inventor has conducted research to improve the cleaning effect. As a result, the method of Embodiment 1 in which the substrate being cleaned is periodically pulled up from the pure water tank to eliminate air bubbles and used for cleaning is used. The method of the second embodiment has been found in which the dissolved gas of pure water is degassed in advance to prevent the generation of bubbles and to eliminate the operation of lifting the substrate from the pure water tank.

In the following, description will be made on the porous silicon surface of Embodiment 3 subjected to hydrophilic treatment and ultrasonic cleaning.

FIG. 13 shows an example of a flow chart of a manufacturing process up to epitaxial growth on porous silicon and a cleaning method. In the figure, SPM is H ₂ SO ₄ / H ₂ O
₂ mixed liquid, DIW is pure water, DHF is dilute HF liquid, APM is NH ₄ OH / H ₂ O ₂ / H ₂ O mixed liquid, and S / D is spinner drying.

The cleaning of the bulk silicon substrate before the anodization is the same as the conventional cleaning with a chemical solution, but a porous hole opening is exposed on the substrate surface from the formation of the porous silicon by the anodization to the epitaxial growth. .

As is clear from the flow chart, while the pore openings of the porous silicon are exposed on the surface, SPM or AP generally used for cleaning a bulk silicon substrate is used.
Chemicals such as M or HPM (HCl / H ₂ O ₂ / H ₂ O mixture) cannot be used, and are limited to DHF and pure water.

As described above, a large number of foreign substances adhere to the surface in the process of forming porous silicon by anodization.

When a single-crystal silicon film is epitaxially grown on the surface of a porous silicon substrate, an oxide film is formed on the porous inner wall surface in order to reduce structural changes in the porous structure during a heating process at a high temperature. (Formation of a natural oxide film or a low-temperature oxidation step in FIG. 16) is performed. Further, at least an oxide film on a porous (silicon substrate) surface is selectively removed immediately before the growth to perform epitaxial growth. In this case, the oxide film on the porous inner wall surface is left. Specifically, the oxide film on the porous silicon surface is removed by immersion in DHF for a short time, and before the DHF solution penetrates deep into the porous pores, the substrate is pulled up from the DHF tank and rinsed with pure water. I do.

For example, in the case where epitaxial growth is formed on porous silicon at a temperature of about 1000 ° C. by a thermal CVD method, the inner wall of the porous silicon hole is formed before the epitaxial growth.
Oxidation at a low temperature of 1 ° C. for 1 hour to form an oxide film.

However, after the thermal oxidation step, about 100 new foreign substances adhere to the surface of the substrate. It is considered that such foreign matter is generated by abrasion caused by rubbing between the quartz boat and the quartz furnace tube on which the substrates are aligned.

Since these oxides remove the oxide film on the surface of the porous silicon by etching with a dilute HF solution immediately before the epitaxial growth, it is often thought that the foreign matter is lifted off from the surface at the same time as the oxide film is removed.

However, actually, the number of foreign matters after etching with the diluted HF solution hardly changes or increases. This is presumably because the oxide film on the porous surface is removed to make the surface hydrophobic, so that the foreign matter floating in the etching tank is adsorbed to the substrate by flowing water charging of the substrate when the substrate is lifted and adheres again.

As shown in FIG. 13, since the anodization uses a high-concentration HF electrolyte, the surface of the porous silicon is hydrophobic after the anodization in the same manner as after the removal of the oxide film by DHF after the low-temperature oxidation. And sex.

As described above, when air bubbles are generated during the high frequency cleaning with pure water, the air bubbles adhere to the surface of the hydrophobic substrate and hinder the cleaning action, and also cause foreign substances in the pure water to collect on the substrate surface. Cause contamination. FIG. 12 shows the result of inspecting the adhesion state of foreign substances on the substrate surface using a foreign substance inspection apparatus when the hydrophobic porous silicon substrate is subjected to high frequency cleaning in pure water without lifting the substrate or performing deaeration of pure water. . FIG. 12 shows the floating direction of the bubbles. The high-frequency traveling wave was also in the same direction. Adhesion and contamination of foreign matter is recognized along the floating path of bubbles attached to the substrate surface.

Therefore, when using pure water which has not been degassed, the substrate is periodically lifted during high frequency cleaning, and high frequency cleaning is performed using pure water from which dissolved gas in pure water has been degassed. In FIG. 13, high-frequency cleaning (steps S ₁ , S ₂ , S ₂₎ is performed using degassed pure water.
That is why we do ₃ ).

However, in the cleaning of the substrate, the method of periodically lifting the substrate from the pure water tank during the cleaning is a very complicated operation, for example, when the cleaning depends on the labor of the operator.

Although the degassing of the dissolved gas in the pure water can compensate for the gas concentration at the outlet of the deaerator, even when the pure water is stored in a high-frequency cleaning tank, the degassing of the dissolved gas can be performed under flowing water. Since redissolution of oxygen and nitrogen from inside occurs in a short time,
In order to compensate for the concentration of dissolved gas in pure water in the cleaning tank, it is necessary to devise a structure of the cleaning tank and a gas seal.

Thus, the present inventors have further studied and as a result, have reached the present invention. That is, the method for cleaning a porous surface according to the third embodiment of the present invention includes the steps of: oxidizing the porous surface to a hydrophilic surface so that the cleaning effect is easily exhibited in high-frequency cleaning with pure water; Foreign matter on the surface of the substrate is removed by high frequency cleaning with pure water. If a clean surface oxide film of the porous silicon substrate from which such foreign matter has been removed is immediately removed by etching with a diluted HF solution and a single crystal silicon film is epitaxially grown on the porous silicon surface, a good single crystal silicon film can be obtained. Can be formed. (Embodiment 4) The method for cleaning a porous surface according to Embodiment 4 of the present invention comprises the steps of oxidizing a porous surface to a hydrophilic surface and making the porous surface hydrophilic so that the cleaning effect is easily exerted in high frequency cleaning. In this method, high-frequency ultrasonic waves are superimposed on a liquid for cleaning the surface of a substrate to remove foreign substances on the surface of the substrate.

In the method for cleaning a porous surface of the present embodiment, the material of the substrate is not particularly limited as long as the substrate has a porous structure on the surface. For example, semiconductor materials such as Si and GaAs,
A ceramic material or the like can be used. Further, as shown in FIG. 2, a structure in which a layer composed of a semiconductor thin film such as amorphous Si, polycrystalline Si or GaAs or a metal thin film is deposited on the inner wall surface of the pores of the Si porous substrate by using a chemical vapor deposition method or the like. The porous surface cleaning method of the present invention can also be used to remove foreign substances adhering to the surface of the substrate having the above. When a layer composed of a semiconductor thin film or a metal thin film is formed directly on the inner wall surface of the pores of the porous substrate as shown in FIG. 2, the substrate surface oxide film and the inner wall oxide film formed by the hydrophilic treatment are formed. After removing the film, a layer made of a semiconductor thin film or a metal thin film may be formed.

As described above, porous silicon immediately after anodization has a hydrophobic surface because a concentrated HF mixed solution is used as an electrolytic solution. When an oxide film is formed on this surface, the surface becomes hydrophilic. If it is a hydrophilic surface, even if bubbles are generated when high-frequency cleaning is performed with pure water that has not been degassed, there is no adhesion of bubbles to the substrate surface, and high-frequency cleaning with pure water has a high removal rate. it can.

That is, as shown in FIG. 14, the cleaning step S ₁₂
If the substrate is made hydrophilic by low-temperature oxidation before, even if bubbles are generated on the surface of the hydrophilic substrate as well, the substrate surface is always stable because it is stable to pure water and has good wettability. Covered with water to prevent air bubbles from sticking.

Therefore, the foreign matter collected by the air bubbles does not transfer to the substrate and the propagation of the high frequency is not hindered, so that the cleaning effect can be sufficiently exhibited.

As described above, in the pure water high frequency cleaning of the hydrophilic substrate, it is not necessary to take measures such as a periodic lifting operation of the substrate and deaeration treatment of the pure water. Even if bubbles are generated, a high cleaning effect can be expected only by immersing the substrate in a pure water high-frequency bath during cleaning.

It is well known that the surface of a silicon oxide film is hydrophilic. However, the present inventors have discovered that the low temperature oxidized surface of porous silicon is also hydrophilic.

As already described with reference to FIG. 13, this low-temperature oxidation treatment is performed as a treatment before epitaxial growth on porous silicon in order to reduce structural change of the porous structure during the heating process at a high temperature. And is not specifically introduced for the cleaning of the present invention.

The low-temperature oxide film is also formed by interstitial oxygen atoms between the crystal lattices on the substrate surface as in the case of thermal oxidation at a high temperature. If there is a foreign substance on the surface, it is also formed at the interface with the substrate. Is removed from the surface by etching of the oxide film together with foreign matter adhering to the surface.

However, if the surface of the oxide film before the etching is not clean, foreign matter is brought into the etching tank, and the foreign matter adheres again to the substrate after the etching.

For this reason, it is preferable that the cleaning after the oxidation step is performed before the oxide film is removed by etching.

Therefore, in FIG. 14, the porous silicon is made hydrophilic by oxidation or the like, and then subjected to high frequency cleaning with pure water to effectively remove foreign substances on the surface, thereby creating a clean oxide film surface.
By removing the surface oxide film with a dilute HF solution immediately before the epitaxial growth or the like and performing the epitaxial growth or the like, a clean porous silicon surface usable in a semiconductor process can be provided.

In addition, foreign substances on the surface of the oxide film which cause reattachment of foreign substances in the dilute HF etching step have already been removed and a clean oxide film surface is secured, so that foreign substances are brought into the dilute HF liquid tank. And the problem of recontamination is reduced.

As described above, the difference between the manufacturing methods shown in FIGS. 13 and 14 is that the hydrophobic surface after the oxide film removing step of the porous silicon surface by the dilute HF solution is performed between the thermal oxidation step and the epitaxial growth step. The high-frequency cleaning with water is changed from the one in which high-frequency cleaning with water is performed to the porous silicon having a hydrophilic surface between the oxidation step and the dilute HF etching step.

As described above, by simply changing the cleaning timing, it is not necessary to periodically take up the substrate during cleaning or take measures against air bubbles such as deaeration of pure water, and the foreign matter on the porous silicon surface can be effectively removed. Can be removed.

The hydrophilic treatment may use an oxidizing effect of ozone water or hydrogen peroxide, which will be described later, instead of thermal oxidation. In this case, the substrate may be simply immersed, but it is more preferable to apply high frequency ultrasonic waves. As the hydrophilic treatment, dry oxidation treatment includes atmospheric pressure oxidation in a high-concentration ozone gas atmosphere or a high-concentration oxidizing atmosphere, or plasma oxidation using the above gas as a base material under reduced pressure.

On the other hand, a large number of foreign substances adhere to the hydrophobic surface after anodization, and when these are put into a thermal oxidizing apparatus, depending on the type of foreign substances, the foreign substances may be firmly burned on the surface or cause contamination of the oxidizing apparatus. Become.

In the manufacturing flow shown in FIG. 14, cleaning is performed by combining the high-frequency cleaning of the hydrophobic surface with pure water and the degassing treatment of the dissolved gas in the pure water (step S ₁ ).

The cleaning step including such a deaeration treatment can be replaced by the cleaning method of the present invention as shown in FIG. Step S ₁₁ in FIG. 15 is due to the cleaning method of porous surface according to the second embodiment of the present invention, performs the high-frequency cleaning with pure water containing dissolved ozone.

As is well known, pure water in which ozone is dissolved has a strong oxidizing action. The effect of removing organic substances largely depends on the oxidizing action of organic substances. The strong oxidizing action of ozone pure water can be used as a hydrophilic treatment for a hydrophobic surface, and has the effect of oxidizing the porous silicon surface after anodization to create a hydrophilic surface. If high frequency cleaning is performed on the created hydrophilic surface, the hydrophilic treatment and high frequency cleaning are performed in the same step.

Furthermore, even if pure ozone water enters the pores of porous silicon similarly to pure water, it is easily discharged as steam or oxygen gas by subsequent heating, and does not adversely affect the subsequent steps.

The oxide film on the surface of the porous silicon and the inner wall of the hole formed by ozone water having an ozone concentration of about 10 to 13% is conventionally etched by DHF to remove a natural oxide film before the low-temperature oxidation step. To be removed.

Although high-frequency cleaning with ozone pure water exerts an effect on the hydrophobic porous silicon surface, there is no problem if it is applied to cleaning of oxidized porous silicon already having a hydrophilic surface. Therefore, it can be used for cleaning porous silicon both after anodization and after low-temperature oxidation.

Further, from the viewpoint of hydrophilic treatment of porous silicon by wet oxidation, a hydrogen peroxide aqueous solution (H ₂ O ₂ / H ₂ O) of about 2% or less diluted with pure water other than ozone water is used. It can be used for high frequency cleaning.

The cleaning using a chemical solution can be easily discharged even if it enters the pores of the porous silicon, and does not adversely affect the subsequent steps.

As shown in FIG. 16, low-temperature oxidation may be performed after high-frequency cleaning is performed using pure water in which ozone is dissolved in the manufacturing flow of FIG. An oxide film is formed on the surface of the porous silicon after the anodization with pure ozone water, and the surface is uniformly oxidized to a deep hole of the porous inner wall by a low-temperature thermal oxidation treatment. In addition, the rate of oxidation of silicon by oxygen in the atmosphere is low, and there is no need to remove the oxide film with pure ozone water, and the oxide film stripping step before low-temperature thermal oxidation is not required. Further, the number of steps can be reduced.

It is known that the surface of a hydrophobic substrate becomes hydrophilic after APM cleaning.
PM cannot be used for the hydrophilic treatment of hydrophobic porous silicon.

[0180]

Next, the present invention will be described in more detail by way of examples. Examples 1 and 2 are examples of the first embodiment. (Example 1) The porous silicon substrate used for cleaning was R
A porous silicon layer having a thickness of about 10 μm is formed on one side of a CA-cleaned p ⁺ -type silicon substrate having a diameter of 5 inches by anodization. After rinsing with pure water and spinner drying,
The number of foreign substances was measured using a surface foreign substance inspection device.

Next, as shown in FIG. 17, the porous silicon substrate 3 was set on the cleaning carrier 4 in the formation batch in the quartz tank 2 in which the pure water 1 overflowed, and the high-frequency ultrasonic tank 5 was passed through the quartz tank 2. Frequency of about 1 MH
Cleaning was performed by applying high frequency ultrasonic waves of z, power 150 W in parallel to the substrate 3.

At the carrier end, RC as a cleaning dummy was used.
The bulk silicon substrate 7 cleaned in A was placed, and the substrate together with the carrier 4 was pulled out of the pure water every 5 minutes to remove attached bubbles, and washed in pure water for 20 minutes.

In order to evaluate the effect of removing foreign matters, after cleaning, spinner drying was performed, foreign matters were measured, and the same washing was repeated again for 20 minutes, after which foreign matters were measured.

The measurement of foreign matter was obtained from a laser reflection intensity distribution within the 5-inch substrate surface, and evaluated in a mode for measuring foreign matter having a size of 0.3 μm or more.

FIG. 18 shows a change in the number of foreign particles due to this series of cleaning. In FIG. 18, data of the carrier end dummy 7 is not shown.

After the RCA cleaning as shown in FIG.
It can be seen that the number of foreign substances increases extremely after anodization (B in the figure) as compared with the number of foreign substances on the substrate surface in A) (the same data as FIG. 9 described above). Note that the classification of L1, L2, and L3 in the bar graph of FIG. 18 indicates the classification of the approximate magnitude of the laser reflection intensity from the foreign matter, and increases in the order of L1, L2, and L3 (L1 <L2 <L3).

Since the dummy substrate is arranged at the carrier end, the data of the first batch of formation is not contamination from the carrier but depends on the order of the batch of anodization.

As the conventional porous silicon which has been finished only with pure water rinsing, the one having a surface contaminated with such foreign substances has been used.

When this is cleaned by high frequency ultrasonic cleaning in pure water of the present invention for 20 minutes (C in the figure), foreign substances on the surface are removed from 13% to 51%, and similarly, the carrier 4 is removed every 5 minutes. The substrate was lifted up to remove air bubbles. 20
The cleaning (D in the figure) for 63 minutes removed 63% to 84% of the pure water rinsed after the chemical conversion (B in the figure). Also,
From the data of the foreign substance inspection device after cleaning, there was no change in unevenness due to collapse of the porous silicon surface.

When air bubbles are not removed, L
A phenomenon is seen in which minute foreign matters of one size collectively adhere to the substrate along the bubble rising direction (in the embodiment, coincide with the ultrasonic wave propagation direction) so as to cross the substrate. (Example 2) Next, an example of low-temperature oxidation and high-frequency ultrasonic cleaning after removing a surface oxide layer, which are indispensable pretreatments for epitaxial growth on a porous silicon surface, will be described.

In Example 1, a porous silicon substrate (D in the figure) which was subjected to high-frequency ultrasonic cleaning with pure water after anodization was used.
Oxidation at low temperature in an oxygen atmosphere at 00 ° C for 1 hour (in the figure,
E) When a foreign substance was measured, a new foreign substance was attached as shown in FIG.

This is adhered in the oxidation furnace and during the process, and is superimposed on the number of foreign substances immediately after chemical formation in the conventional porous silicon with pure water rinse only.

After setting this in a carrier in the same manner as described above and immersing it in diluted hydrofluoric acid, the washing apparatus of Example 1 (FIG. 1)
In step 7), high-frequency ultrasonic cleaning with pure water overflow was performed for 20 minutes while removing the air bubbles from the substrate together with the carrier 4 every 5 minutes (F in the figure). 54% to 80% of foreign matter was removed as compared to immediately after, and the number of porous silicon substrates was reduced to 65 or less.

This is considered to be a synergistic effect with the removal action in which the foreign matter is lifted off from the surface in the oxide film peeling step. However, the foreign substance once lifted off by the conventional diluted hydrofluoric acid and pure water rinsing alone cannot remove the foreign matter. Since the hydrophobic substrate is re-adhered by flowing water charging and about several hundred foreign substances are detected, it can be considered that the high-frequency ultrasonic cleaning effectively removes the foreign substances and effectively prevents the re-adhesion.

As in Example 1, no abnormality was detected on the porous silicon surface after cleaning.

The effect of removing attached air bubbles in the high-frequency cleaning in pure water performed in the first and second embodiments will be described.

Adsorption by air bubbles is remarkable for minute foreign matters, and the state of the aggregate adhered to the substrate surface is remarkable. However, as already explained, in the ultrasonic cleaning, even in the high frequency band, the generation of bubbles from the dissolved gas in pure water is observed, and when the porous silicon substrate is further dried, the inside of the pores is reduced. It is conceivable that air bubbles are generated because the gas (air) is replaced with pure water and discharged out of the holes. In order to clarify the origin of the bubbles adhering to the substrate surface, the evaluation was performed by using a foreign substance of 0.2 μm or more on the surface of the hydrophobic bulk substrate on which the porous structure was not formed.

FIG. 19 shows that the bulk substrate is subjected to RCA cleaning in advance to minimize the number of foreign substances on the surface, and then dipped in diluted hydrofluoric acid.
It is a figure which rinses with pure water for 5 minutes, spinner-drys, and shows the result of having measured the number of foreign substances. It is clear that the number of foreign particles on the substrate at the carrier end is larger than that at other positions. This is considered to be due to the conventionally known induction charging of the closest wafer due to carrier charging. The numbers indicating the slot positions in the figure represent the slot numbers of wafer carriers which are conventionally frequently used in the semiconductor industry, and are formed at equal intervals from 1 to 25 at a pitch of 5 mm from the end of the carrier.

FIG. 20 is a diagram showing the number of foreign substances when high-frequency cleaning with a frequency of 1 MHz and a power of 150 W is performed in the pure water rinsing step for 5 minutes. As shown in FIG. 20, the number of foreign substances on the substrate is reduced by the high-frequency cleaning. However, depending on the slot position of the wafer carrier, on the contrary, there is a substrate in which minute foreign substances (L1) increase sharply.

The increased foreign substances were densely packed in parallel to the bubble rising direction and the ultrasonic wave propagation direction based on the slot position in the carrier and the position distribution in the substrate.

From this, it can be seen that simply ultrasonic cleaning in pure water contaminates the substrate against foreign substances against the purpose.

FIG. 21 shows that only the foreign matter measurement surface of the substrate at the carrier end is set upside down with respect to the carrier side, the wafers in the other carriers are set up in a normal rotation, and the carrier is set up every 5 minutes. Lift the board from the high-frequency pure water bath,
It is a figure which shows the number of foreign substances when air bubbles are removed by repeating the operation of immersing again in pure water for 20 minutes.

Although there is an effect due to the cleaning time, the bubbles are reduced to the same level as the pure water rinse after the RCA cleaning at each position in the carrier (FIG. 19) as compared with FIG. 20, and the density in the substrate is eliminated. Was.

From the above results, it was clarified that the generation of air bubbles from pure water by high-frequency ultrasonic waves and the adhesion to the hydrophobic substrate surface were caused by the contamination of the substrate by the foreign substances.

For this reason, in the above example, the substrate was pulled up from pure water every 5 minutes. However, by repeatedly pulling up the substrate every shorter time within the same cleaning time, the substrate was pulled up within pure water. The cleaning effect can be enhanced by further reducing the number of air bubbles adhered to the substrate.

Foreign matter on the inverted substrate at the carrier edge is also reduced, and it will be clarified whether this is due to only high-frequency cleaning or a synergistic effect with the inverted set.

The bulk substrate at the end of the carrier was set in a normal rotation and a reverse rotation, and RCA cleaning having the highest cleaning ability was performed to count foreign substances of 0.2 μm or more.

FIG. 22 shows the number of foreign substances in the case where the bulk substrate is set in an inverted state and cleaned in the slot at the final end of the carrier where the influence of the induced charge from the carrier is greater, and FIG. .

In the normal rotation state, as in the case of immersion in diluted hydrofluoric acid, the number of foreign substances on the carrier end substrate is prominent, but the number is so small that no difference is recognized even when compared with other substrates by the reversal set. Few.

From this, it can be seen that the reduction in the number of foreign matters after the high frequency cleaning of the reversal set substrate in FIG. 21 of the third embodiment also includes the effect of avoiding adhesion by the setting method.

As described above, when performing high-frequency cleaning in a batch-type bath, attention should be paid to the method of setting the carrier end substrate and the prevention of air bubble adhesion.

In the above embodiment, a specific example was described using a batch-type cleaning apparatus. However, the cleaning effect of the present invention is not limited to the apparatus configuration.

Similarly, conditions such as frequency and high-frequency power, cleaning time, and liquid temperature are examples of proving the cleaning effect of the present invention, and the present invention relates to a frequency band (600 kHz to 2 kHz).
MHz), and other conditions can be set arbitrarily.

That is, in the embodiments of the present invention, the case where a high frequency is superimposed on pure water has been described. However, when a minute amount of a surfactant or ozone is added to pure water, the effect of removing organic substances and foreign substances is known. Therefore, a method combining these with the high-frequency cleaning of the porous silicon surface unique to the present invention can be easily assumed from the present invention.

Further, in the above embodiment, an example of a silicon semiconductor is given as a substrate material having a porous structure.
It is clear in the above description that the purpose of the present invention is to clean a substrate having a fine, dense, and fragile porous structure on its cleaning surface, and it is sufficient if the porous structure is the same. It is not limited to the substrate material. (Example 3) Examples 3 and 4 are examples of the second embodiment. As shown in FIG. 24, the membrane deaerator 1 used in the fifth embodiment is connected to a pure water supply pipe between a conventional pure water producing apparatus 12 and an ultrasonic cleaning apparatus 13 and used. It is desirable that the pure water produced from the membrane deaerator 11 be discharged from the bottom of the ultrasonic cleaning device 13 in order to prevent the intake of air.

As a result, pure water having a dissolved gas concentration of 5 ppm or less is supplied to the ultrasonic cleaning device 13.

[0217] Using degassed pure water,
A porous silicon substrate having a hydrophobic surface from which a natural oxide film is removed is set in a cleaning carrier in a flowed quartz tank, immersed for 20 minutes, and passed through a quartz tank through a vibrator of a high-frequency ultrasonic tank to generate high-frequency power at 150 W. Ultrasonic waves were applied in parallel to the substrate to perform ultrasonic cleaning (at a frequency of 950 kHz). Only the immersion was performed without lifting the substrate. FIG. 28 schematically shows the relationship between the porous silicon substrate and the high-frequency traveling wave. In this manner, the porous surface is arranged by arranging the cleaning surface of the porous silicon substrate and the traveling direction of the high-frequency traveling wave in parallel. Foreign matter adhering to the high quality silicon substrate can be more effectively removed.

In the degassed pure water, no bubbles were generated by the application of high-frequency ultrasonic waves, and no bubbles were fixed on the substrate surface.

In FIG. 25, pure water which has not been degassed is used.
The substrate is lifted every minute, and the same
For example, when the same cleaning is performed for 20 minutes (for a total of 40 minutes), and when the cleaning is performed under the above-described cleaning conditions of the present invention only by immersion in degassed pure water, the foreign matter removal rate of the porous surface is reduced. Each is shown in comparison.

The removal rate of the present invention by washing for 20 minutes was 9%.
Reaches 0%, not degassed (N ₂ saturated concentration) is superior in comparison with the washing 40 minutes in a high-frequency cleaning with pure water (60-80%). (Embodiment 4) The cleaning effect of the present invention is not limited to the immersion cleaning of a substrate in a pure water tank as in Embodiment 3 described above.

For example, when spinner cleaning is performed by spraying a shower of pure water in which high-frequency ultrasonic waves are superimposed on the surface of a spinning substrate as already proposed, degassed pure water is used as shown in FIG. By doing so, the generation of bubbles can be prevented, and the problem of interference with the propagation of ultrasonic waves due to the bubbles can be avoided. (Example 5) Examples 5 to 8 are examples of the third embodiment.
The cleaning flow up to the epitaxial growth of the porous silicon substrate of Example 5 is the same as the cleaning flow shown in FIG. However, here, the cleaning steps S ₂ and S ₃ are not performed. If there is a request to reduce the number of adhered foreign substances, the cleaning steps S ₂ and S ₃ may be performed as appropriate.

Immediately after the anodization, the surface is hydrophobic, so that during the high frequency cleaning with pure water, the substrate is periodically pulled up or deaerated to avoid bubbles from adhering to the substrate. (Here, deaeration treatment of pure water was performed).

For cleaning, one dummy substrate at the edge of the Teflon carrier for cleaning and 24 porous silicon substrates are collectively processed, and the substrate is left immersed at a frequency of 950 kHz, a high frequency output of 150 W, normal temperature, and overflow of running water. Performed for 20 minutes under flow conditions.

The pure water was used after degassing the dissolved gas concentration to 5 ppb or less (step S ₁ ). By this washing, about 90% of the foreign substances adhered in the anodizing step could be removed.

Although the cleaned porous silicon surface is hydrophobic, a natural oxide film is formed in the storage substrate. Therefore,
Immediately before the low-temperature oxidation treatment, high-frequency cleaning with pure water again and D
The natural oxide film is removed by HF and dried in dry oxygen at 40
A thermal oxide film is formed at a low temperature of 0 ° C. for one hour.

Although the surface after low-temperature oxidation becomes a hydrophilic surface,
During the oxidation process, about a hundred foreign substances adhere to the substrate surface. For this reason, after oxidation, high frequency cleaning with pure water is performed to remove foreign substances on the surface, and a clean oxide film surface is created and stored in a dedicated box.

Since the porous silicon surface after oxidation is hydrophilic, it is not necessary to take measures to prevent air bubbles from adhering to the substrate surface, and the substrate is immersed in a high-frequency cleaning tank with pure water that has not been degassed. Can be washed.

The substrate was washed in pure water without degassing, immersed in a 950 kHz frequency at a high frequency output of 150 W, at room temperature, and in a running water overflow tank for 20 minutes.

By this cleaning, 90% of the foreign substances adhered in the oxidation step could be removed. Since the storage after oxidation and washing is covered with an oxide film, there is no problem in application of porous silicon for a long period of time.

The stored porous silicon substrate with an oxide film is epitaxially grown by removing only the oxide film on the surface of the porous silicon with DHF immediately before the substrate is put into the epitaxial growth apparatus, by the number of pieces which can be processed by the epitaxial growth apparatus.

At this time, in the epitaxial growth, since a film is formed from the surface of porous silicon, DHF
The oxide film on the inner wall of the hole of the porous silicon is left in the etching step by the above method.

The pure water used for the high-frequency cleaning with pure water was either degassed or not depending on the surface condition of the substrate, and two types of pure water were used: hydrophobic (after anodization) and hydrophilic (after low-temperature oxidation). Pure water degassed together may be used, or the substrate may be lifted (without degassing). (Embodiment 6) Next, an example of another method for cleaning a porous silicon substrate up to epitaxial growth will be described.

FIG. 15 shows a cleaning flow of this embodiment. In the cleaning of the hydrophobic porous silicon surface after anodization in Example 5 described above, high-frequency cleaning was performed using degassed pure water. In this example, about 10 to 13% of ozone was dissolved. High frequency cleaning was performed under the same conditions using ozone pure water.

The ozone pure water was obtained by dissolving ozone gas obtained from a well-known wet-type ozone generator combining pure water electrolysis and a hollow fiber filter in pure water.

The oxide film formed on the surface of the porous silicon and the inner wall of the hole by the ozone pure water after the anodization is removed by DHF etching just before the low-temperature oxidation step as in the related art.

The use of pure ozone water requires the addition of a new device, but the concentration control in the cleaning tank is easier than in the deaeration of pure water.

In this embodiment, since the oxide film has already been formed and becomes hydrophilic, the high-frequency cleaning after low-temperature oxidation was performed using pure water that has not been degassed. It does not occur any problem even high-frequency cleaning using (it is possible to perform the step S ₁₂ of FIG. 15 under the same conditions as step S _11.). (Embodiment 7) Still another embodiment of the present invention will be described.

The cleaning flow is the same as in the sixth embodiment.
Although not shown, the surface of the hydrophobic porous silicon after anodization
High-frequency cleaning is performed using ozone pure water to make the surface hydrophilic.
Is diluted with pure water to a low concentration of less than 2%
Hydrogen hydride aqueous solution (H_Two O_Two / H _Two O) to make it hydrophilic
The method was changed to high-frequency cleaning.

As for the cleaning method, as in the case of using degassed pure water or ozone pure water, the cleaning effect is exhibited even when immersed.

However, in general, when a chemical solution is used, the chemical solution is stored in the cleaning tank for cleaning. However, in the cleaning method of the present invention, the foreign matter removed does not dissolve in the chemical solution but stays in the tank.

For this reason, in this embodiment, the pure water used for cleaning is drained out of the cleaning tank together with the foreign matter removed by overflowing the running water, and the drained diluted aqueous hydrogen peroxide solution contains 0.1% water. High-frequency cleaning was performed while circulating and supplying again to the cleaning tank through a filter for collecting particles of about 1 μm. The consumed hydrogen peroxide solution was periodically replenished. (Embodiment 8) In general, cleaning in a cleaning tank is suitable for processing a large number of substrates at one time. However, the recent demand for large-diameter substrates requires an increase in the cleaning tank volume, that is, the use of a chemical solution. Will increase.

In addition, in consideration of the amount of foreign matter adhered after the anodization, the use of a cleaning tank involves the risk of recontamination of the substrate by the foreign matter, so that the foreign matter is effectively discharged out of the tank. It is desired.

As a method for solving such a problem, it is conceivable to combine the known high-frequency cleaning with pure water of the present invention with a single-wafer cleaning apparatus that combines a chemical shower and a chemical discharge with a spinner.

The present invention is basically a cleaning using pure water, and has an advantage that the cleaning can be performed at a lower cost as compared with a case where a chemical solution is used.

In addition, the single-wafer shower cleaning is suitable for cleaning the porous silicon surface after anodization due to its processing ability.

In cleaning the hydrophobic surface, adhered foreign substances can be effectively removed by performing high-frequency cleaning while oxidizing before cleaning or making the surface hydrophilic with a shower made of ozone water or diluted hydrogen peroxide solution.

As a matter of course, high-frequency cleaning using a pure water shower can also be applied to cleaning of the porous silicon surface having a hydrophilic surface oxidized at a low temperature.

The semiconductor substrate having a porous surface, which has been washed by the method described above, is suitably used for producing a semiconductor device such as a MOS-FET. FIG.
(A) to (f) of FIG. 29 are schematic cross-sectional views illustrating an example of a process for manufacturing such a semiconductor device.

As shown in FIG. 29A, the silicon substrate 40 cleaned by the above-described method has a non-porous silicon single crystal region 42 (bulk silicon region) remaining without being made porous. , And a porous silicon single crystal layer 41. First, this silicon substrate 40 is placed in an oxygen atmosphere for 4 hours.
By heating at a temperature of 00 ° C. for one hour, an oxide film was formed on the inner walls of the pores of the porous silicon single crystal layer 41 and on the surface of the porous silicon single crystal layer 41. This oxide film is used, for example, in a process of forming an epitaxial layer later.
It is formed to prevent silicon atoms from migrating inside the hole due to a rise in temperature and blocking the hole.

Next, the surface of the porous silicon single crystal layer 41 was treated with hydrofluoric acid to remove the oxide film on the surface of the porous silicon single crystal layer 41 except for the oxide film on the inner wall of the hole. afterwards,
As shown in FIG. 29B, the porous silicon single crystal layer 4
A non-porous silicon single crystal layer (epitaxial layer) 43 having a thickness of 0.3 μm was epitaxially grown on 1 by a CVD method. For forming such a non-porous silicon single crystal layer 43, a method selected from a molecular beam epitaxy method, a plasma CVD method, a low pressure CVD method, a photo CVD method, a liquid phase growth method, and a sputtering method can be used. . The thickness of the non-porous silicon single crystal layer 43 is not limited to the above-mentioned thickness, and may be any thickness.
It is formed to a thickness of 0 nm to 2 μm.

Next, as shown in FIG. 29C, the surface of the non-porous silicon single crystal layer 43 is partially oxidized,
As the insulating layer 44, an oxide film having a thickness of 200 nm was formed. The thickness of the insulating layer 44 is not limited to the above-mentioned thickness, but may be any thickness, but is typically 50 nm to 2 μm.
Formed to a thickness of

Next, as shown in FIG. 29D, another silicon substrate 45 is overlaid on the insulating layer 44 and their surfaces are brought into close contact with each other, and then heat-treated at a temperature of 1180 ° C. for 5 minutes. And bonding was performed. As a result, the first
A silicon substrate 40 as a second substrate and a silicon substrate 45 as a second substrate are bonded together with an insulating layer 44 interposed therebetween, and as shown in FIG. A positioned multilayer structure 50 was formed.

Next, as shown in FIG. 29E, the non-porous silicon single crystal region 42
Was removed, and the porous silicon single crystal layer 41 was exposed.
First, the non-porous silicon single crystal region 42 is removed by polishing the non-porous silicon single crystal region 42 with a grinder while leaving a slight thickness adjacent to the porous silicon single crystal layer 41, and then removing the remaining non-porous silicon single crystal region. Silicon single crystal region 4
2 was removed by dry etching. In addition to such a polishing method, a method of separating the non-porous silicon single crystal region 42 from the multilayer structure 50 with the porous silicon single crystal layer 41 as a boundary can also be used. Since the porous silicon single crystal layer 41 has lower mechanical strength than the non-porous silicon single crystal region 42, the non-porous silicon single crystal layer 43, etc., the silicon substrate 45 and the non-porous silicon single crystal region 42 The non-porous silicon single crystal region 42 is separated without damaging the non-porous silicon single crystal layer 43 by applying a peeling force perpendicular to the surface of the substrate or a shearing force parallel to the surface of the substrate. It is possible to Other methods for separating the non-porous silicon single crystal region 42 include a method of inserting a wedge-shaped member into the porous silicon single crystal layer 41, a method of spraying a water jet on the porous silicon single crystal layer 41, and the like. Can be used. Further, a part of the porous silicon single crystal layer 41 is provided with a region where the porosity indicating the ratio of the volume of the hole to the volume of silicon is larger than that of the other part, and the separation surface (cut surface) of this region is provided. ), The non-porous silicon single crystal region 42 may be separated. Thus, the non-porous silicon single crystal region 4
In the case where the method of separating 2 is used, the porous silicon single crystal layer 41 can be exposed in a short time, and the semiconductor device can be manufactured efficiently. Further, the porous silicon single-crystal layer 41 remaining at the time of separation is removed from the separated non-porous silicon single-crystal region 42, flattened if necessary, and then partially porous. By doing so, the non-porous silicon single crystal region 42 is
It can be reused as the silicon substrate 40 as shown in FIG.

Next, as shown in FIG. 29 (f), the porous silicon single crystal layer 41 is removed by etching, and a thin nonporous silicon single crystal layer 41 is formed on a silicon substrate 45 via an insulating layer 44. A so-called SOI (silicon-on-insulator) substrate 51 having a crystal layer 43 was formed. The etching of the porous silicon single crystal layer 41 was performed by a chemical etching method using an aqueous solution containing hydrofluoric acid and aqueous hydrogen peroxide as an etchant. Depending on the conditions, the etching rate of the porous silicon is more than 10 ⁵ times faster than that of the non-porous silicon depending on the conditions.
Therefore, the porous silicon single crystal layer 41 could be selectively removed with good controllability, while leaving the non-porous silicon single crystal layer 43 having a uniform thickness and flatness.

Finally, the SOI substrate 5 shown in FIG.
1 was heat-treated at 1100 ° C. for 1 hour in a hydrogen atmosphere to further flatten the surface of the non-porous silicon single crystal layer 43. After this heat treatment, the non-porous silicon single crystal layer 4
When the surface of No. 3 was evaluated with an atomic force microscope,
The mean square roughness in the corner area was approximately 0.2 nm.

For the non-porous silicon single crystal layer 43 of the SOI substrate 51 formed as described above, a semiconductor device such as a MOS-FET, a DRAM, or a solar cell is manufactured by using a well-known semiconductor process. be able to.

In the above example, the silicon substrate is used as the second substrate, but a light-transmitting substrate such as a quartz substrate or a glass substrate can be used. When such a light-transmitting substrate is used, it can be suitably used for an optical sensor, a liquid crystal display, and the like. When the second substrate is made of an insulating material such as a quartz substrate or a glass substrate, or when the second substrate is
When a silicon substrate on which an insulating layer such as an O ₂ layer is formed is used, the insulating layer 44 in FIG. 29C is not necessarily required. However, in order to separate the non-porous silicon single crystal layer 43 where a semiconductor device is to be manufactured later from the bonding interface as much as possible and to protect from the influence of impurities and the like, an insulating layer is formed on the non-porous silicon single crystal layer It is desirable to be formed.

[0258]

As described above, the present invention removes foreign substances on the surface of a substrate having a porous structure on the surface by high-frequency ultrasonic waves, for which there has been no effective cleaning method because of the conventional porous structure. It can be effectively removed and washed without any collapse of the porous material.

Further, since the cleaning effect is exhibited only by pure water, there is no fear that a problem due to the chemical solution remaining inside the porous material may occur, and the introduction into the conventional process is easy.

In addition, since it can be achieved by a simple configuration in which a high-frequency cleaning step is added to the conventional anodizing treatment or the pure water rinsing step after the oxidation treatment, it is introduced from the viewpoint of workability, economy and stability. Is easy.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the frequency of ultrasonic cleaning and the size of particles to be removed, and the frequency range of the present invention.

FIG. 2 is a cross-sectional view illustrating an example of a porous silicon substrate.

FIG. 3 shows only immersion (a) and withdrawal every 5 minutes (b)
FIG. 9 is a diagram showing a state of adhesion of foreign matter on a wafer after cleaning in the case.

FIG. 4 is a graph showing the residual oxygen concentration and the number of generated bubbles in ultrasonic cleaning in pure water.

FIG. 5 is a graph showing a dissolved gas concentration at which no bubbles are generated in a megasonic irradiation region.

FIG. 6 is a graph showing a dissolved oxygen concentration distribution in a tank.

FIG. 7 is a graph showing a concentration distribution of dissolved nitrogen in a tank.

FIG. 8 is a graph showing a dissolved oxygen concentration distribution during megasonic irradiation.

FIG. 9 is a graph showing a dissolved nitrogen concentration distribution during megasonic irradiation.

FIG. 10 is a graph showing the dependence of dissolved gas concentration in pure water on megasonic cleaning.

FIG. 11 is a conceptual diagram illustrating the influence of bubbles due to pure water during high-frequency cleaning of a hydrophobic substrate.

FIG. 12 is a plan view illustrating adhered foreign matter after high-frequency cleaning when bubbles are attached to a hydrophobic substrate.

FIG. 13 is a diagram showing an example of a flow of cleaning a porous silicon substrate with pure water.

FIG. 14 is a drawing showing an example of a flow of cleaning a porous silicon substrate.

FIG. 15 is a view showing an example of a flow of cleaning a porous silicon substrate.

FIG. 16 is a view showing an example of a flow of cleaning a porous silicon substrate.

FIG. 17 is a sectional view showing an example of the high-frequency ultrasonic cleaning device of the present invention.

FIG. 18 is a graph showing the number of foreign substances on the porous silicon surface after the anodization and after the low-temperature oxidation, in which the high-frequency ultrasonic cleaning of the present invention was performed.

FIG. 19 is a graph illustrating the effect of removing bubbles in high-frequency cleaning according to the present invention.

FIG. 20 is a graph illustrating the effect of removing bubbles in high-frequency cleaning according to the present invention.

FIG. 21 is a graph illustrating the effect of removing bubbles in high-frequency cleaning according to the present invention.

FIG. 22 is a graph illustrating contamination of a foreign substance in a cleaning carrier.

FIG. 23 is a graph illustrating contamination of a foreign substance in a cleaning carrier.

FIG. 24 is a conceptual diagram showing an example of the configuration of a high-frequency ultrasonic cleaning device and a degassed pure water production device of the present invention.

FIG. 25 is a graph comparing the removal rate of foreign substances when the high frequency ultrasonic cleaning of the present invention is performed on a porous silicon surface having a hydrophobic surface with a cleaning method without degassing.

FIG. 26 is an apparatus conceptual diagram in the case where the high frequency ultrasonic cleaning of the present invention is applied to a shower type cleaning apparatus.

FIG. 27 is a diagram schematically showing a relationship between a porous silicon substrate and a high-frequency traveling wave.

FIG. 28 is a graph showing the number of foreign substances on the porous silicon surface in the order of the formation batch, which was completed only by pure water rinsing after the conventional anodization.

FIG. 29 is a schematic cross-sectional view showing one example of a process for manufacturing a semiconductor device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pure water 2 Quartz tank 3 Porous silicon substrate 4 Cleaning carrier 5 High frequency ultrasonic tank 6 Vibrator 7 Dummy substrate 11 Film deaerator 12 Pure water production apparatus 13 High frequency ultrasonic cleaning apparatus 31 Quartz high frequency cleaning tank (running water) 32 High frequency vibration plate 33 Pure water from which dissolved gas has not been degassed 34 Bubbles 35 Foreign substances 36 Porous silicon substrate having hydrophobic surface 38 High frequency traveling wave

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 21/304 B08B 3/12

Claims (30)

(57) [Claims] 1. A method for cleaning a porous surface of a substrate having at least a porous structure on a surface thereof, wherein foreign matter adhering to the porous surface of the substrate is washed with pure water having a frequency of 600 kHz to 2 MHz superimposed thereon. A method for cleaning a porous surface, comprising performing cleaning for removal. 2. The substrate surface to be cleaned has a structure in which openings of a large number of pores are exposed, and the inner wall surface of the pores has a structure in which a porous structural material is exposed or covered with a different material. The method for cleaning a porous surface according to claim 1. 3. The method for cleaning a porous surface according to claim 1, wherein the substrate is immersed in a pure water bath, and the high frequency is superimposed and washed. 4. The method for cleaning a porous surface according to claim 3, wherein the high frequency is superposed in parallel with the porous surface of the substrate immersed in the pure water bath to perform cleaning. 5. The method for cleaning a porous surface according to claim 3, wherein the substrate being immersed in the pure water bath is intermittently pulled out of the liquid during high frequency cleaning. 6. The porosity according to claim 1, wherein the porous surface of the substrate is washed by spraying a pure water shower in which high frequency is superimposed on pure water while rotating the substrate. How to clean quality surfaces. 7. The method for cleaning a porous surface according to claim 1, wherein the pure water is pure water degassed so that the concentration of a dissolved gas becomes 5 ppm or less. 8. The method according to claim 1, wherein the substrate has a dissolved gas concentration of 5 ppm.
The method for cleaning a porous surface according to claim 7, wherein the cleaning is performed by immersing in a pure water bath having deaerated pure water and superimposing the high-frequency ultrasonic waves as follows. 9. A cleaning method, wherein the porous surface of the substrate is degassed while rotating the substrate so that the concentration of the dissolved gas is 5 ppm or less and a pure water shower on which the high-frequency ultrasonic waves are superimposed is sprayed. The method for cleaning a porous surface according to claim 7, wherein the cleaning is performed. 10. The method for cleaning a porous surface according to claim 1, wherein the porous surface of the substrate is treated to be hydrophilic. 11. A method for cleaning a porous surface of a substrate having at least a porous structure on a surface thereof, wherein the liquid is a liquid for treating the porous surface of the substrate to be hydrophilic and cleaning the hydrophilic porous surface. And the frequency is 600
A method for cleaning a porous surface, comprising superimposing a high-frequency ultrasonic wave in the range of kHz to 2 MHz to perform cleaning for removing foreign substances adhering to the surface of the substrate. 12. The cleaning of the porous surface according to claim 10, wherein the hydrophilic treatment of the porous surface is a process of forming an oxide film on the surface of the substrate and the inner wall of the porous hole. Method. 13. The cleaning of the porous surface according to claim 10, wherein the hydrophilic treatment of the porous surface is a process of immersing the substrate in pure ozone water in which ozone is dissolved in pure water. Method. 14. The method for cleaning a porous surface according to claim 10, wherein the hydrophilic treatment of the porous surface is a process of immersing the substrate in an aqueous solution of hydrogen peroxide diluted with pure water. . 15. The method for cleaning a porous surface according to claim 11, wherein the liquid is ozone pure water obtained by dissolving ozone in pure water. 16. The method for cleaning a porous surface according to claim 11, wherein the liquid is an aqueous solution of hydrogen peroxide diluted with pure water. 17. The hydrophilic treatment of the porous surface is a treatment for forming an oxide film on the surface of the substrate and the inner wall of the porous hole. After the cleaning of the porous surface of the substrate, at least the oxide film on the surface of the substrate is removed. Claim 10 characterized by being removed
The method for cleaning a porous surface according to claim 16. 18. A method for cleaning a porous surface of a substrate having a porous structure on at least the surface, wherein the dissolved nitrogen concentration is 5 ppm or less and the frequency is 60
A method for cleaning a porous surface, comprising performing cleaning for removing foreign substances adhered to the porous surface of the base with pure water on which a high frequency in a range of 0 kHz to 2 MHz is superimposed. 19. The substrate according to claim 18, wherein the surface of the substrate to be cleaned has a structure in which the openings of a large number of pores are exposed, and the inner wall surface of the pores is coated with a different material. A method for cleaning porous surfaces. 20. The method for cleaning a porous surface according to claim 18, wherein the substrate is immersed in a pure water bath and the high frequency is superimposed and washed. 21. The method for cleaning a porous surface according to claim 20, wherein the cleaning is performed by superimposing the high frequency in parallel with the porous surface of the substrate immersed in the pure water bath. 22. The method for cleaning a porous surface according to claim 20, wherein the substrate being immersed in the pure water bath is intermittently pulled out of the liquid during high frequency cleaning. 23. The porous material according to claim 18, wherein the porous surface of the substrate is cleaned by spraying a pure water shower in which high frequency is superimposed on pure water while rotating the substrate. How to clean quality surfaces. 24. The method for cleaning a porous surface according to claim 18, wherein the porous surface of the substrate is treated to be hydrophilic. 25. The method for cleaning a porous surface according to claim 24, wherein the hydrophilic treatment of the porous surface is a treatment for forming an oxide film on the surface of the substrate and the inner wall of the porous hole. 26. The cleaning of the porous surface according to claim 24, wherein the hydrophilic treatment of the porous surface is a treatment of immersing the substrate in pure ozone water in which ozone is dissolved in pure water. Method. 27. The method for cleaning a porous surface according to claim 24, wherein the hydrophilic treatment of the porous surface is a process of immersing the substrate in an aqueous solution of hydrogen peroxide diluted with pure water. . 28. The hydrophilic treatment of the porous surface is a treatment for forming an oxide film on the surface of the substrate and the inner wall of the porous hole. After the cleaning of the porous surface of the substrate, at least the oxide film on the surface of the substrate is removed. The method for cleaning a porous surface according to claim 24, wherein the porous surface is removed. 29. A cleaning process for removing foreign substances adhering to the surface of a semiconductor substrate with pure water degassed so that the concentration of dissolved nitrogen is 5 ppm or less and superimposed with ultrasonic waves. A method for cleaning semiconductor surfaces. 30. A method of manufacturing a semiconductor substrate, comprising: preparing a substrate having a porous structure on at least a surface thereof.
Degree, the cleaning step of cleaning said substrate, form a non-porous monocrystalline layer on the porous structure of the substrate
Bonding a different substrate on the non-porous single crystal layer
Degree, the leaving the non-porous monocrystalline layer on the another board substrate
Removing the exposed surface of the non-porous single crystal layer.
Degree, wherein the said cleaning step, any one of the claims 1,11,18
Applying the method for cleaning a porous surface according to the above to the substrate.
A method for manufacturing a semiconductor substrate, comprising: