JP2010245405A - Method for roughening silicon surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for roughening a silicon surface so that unevenness at nano-order level. <P>SOLUTION: A fluorine-based material gas containing CF<SB>4</SB>, Ar and H<SB>2</SB>O is introduced into a plasma space 13 close to or below an atmospheric pressure to generate a fluorine-based reactive gas containing HF. The fluorine-based reactive gas, an acidic reactive gas containing O<SB>3</SB>and a diluting inert gas such as N<SB>2</SB>not passing through the plasma space 13 are mixed to obtain a treatment gas, and the treatment gas is allowed to contact a workpiece consisting of a silicon substrate 9. The flow rate of the fluorine-based reactive gas and the diluting inert gas is set at (fluorine-based reactive gas):(diluting inert gas)=2:3 to 2:12, and preferably 2:5 to 2:8. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for roughening the surface of silicon, and more particularly to a roughening method suitable for a silicon substrate for solar cells.

Solar cells made of silicon substrates are well known. Electric power is output by the incident light being absorbed by the silicon substrate. Therefore, in order to obtain high power generation efficiency, it is preferable that the reflectance of the silicon substrate with respect to incident light is low and the absorption rate is high.
Patent Document 1 describes a method of performing wet etching by immersing a silicon substrate in a mixed solution of hydrofluoric acid and hydrogen peroxide to which a silver salt is added. As a result, the surface layer of the silicon substrate is made porous and the reflectance is lowered.
Patent Document 2 describes that silver fine particles are deposited on the surface of a silicon substrate by electroless plating, and then the silicon substrate is immersed in a mixed solution of hydrofluoric acid and hydrogen peroxide to perform wet etching. As a result, fine irregularities are formed on the surface of the silicon substrate to reduce the reflectance.

JP 2005-183505 A JP 2007-194485 A

In the wet etching methods as described in Patent Documents 1 and 2, for example, if an electrode is formed on the surface of the silicon substrate, the electrode may be damaged. In addition, the smoothness of the back surface of the silicon substrate may be reduced. Furthermore, it becomes difficult to respond depending on the thickness of the silicon substrate.
In view of the above circumstances, an object of the present invention is to sufficiently roughen the surface of silicon by a method other than wet etching. In particular, an object of the present invention is to provide a roughening method capable of sufficiently reducing the reflectance when a silicon substrate is applied to a solar cell.

In order to solve the above problems, the present invention is a method for roughening the surface of an object to be processed made of silicon,
Fluorine source gas is introduced into the plasma space near atmospheric pressure to generate fluorine reaction gas,
A treatment gas containing the fluorine-based reaction gas, the oxidizing reaction gas, and the inert gas for dilution that does not pass through the plasma space is brought into contact with the object to be treated;
The flow rate ratio between the fluorine-based reactive gas and the inert gas for dilution is (fluorine-based reactive gas) :( inert gas for dilution) = 2: 3 to 2:12.
Thereby, nano-order irregularities can be formed on the surface of silicon, and the surface can be sufficiently roughened. When applied to a solar cell, the light reflectance can be reduced to 17% or less, and sufficient power generation efficiency can be obtained.

It is preferable that the flow rate ratio between the fluorine-based reaction gas and the inert gas for dilution is (fluorine-based reaction gas) :( inert gas for dilution) = 2: 5 to 2: 8.
Thereby, the surface of silicon can be more reliably roughened. When applied to a solar cell, the light reflectance can be reduced to 12% or less, and the power generation efficiency can be further increased.

The contact time of the processing gas at each position of the object to be processed is preferably 24 seconds to 48 seconds.
By setting the contact time to 24 seconds or longer, a necessary and sufficient surface roughening amount can be secured. By making the contact time 48 seconds or less, it is possible to avoid unnecessary processing, to suppress the consumption of the processing gas, and to prevent the processing time from being prolonged.

The fluorine-based source gas includes at least a fluorine-based source component, preferably further includes a carrier component, and more preferably further includes an additive component containing hydrogen.
Examples of the fluorine-based raw material component include perfluorocarbons (PFC) such as CF ₄ , C ₂ F ₆ , C ₃ F ₆ , C ₃ F ₈ , hydrofluorocarbons such as CHF ₃ , C ₂ H ₂ F ₂ , and CH ₃ F ( HFC), SF ₆ , NF ₃ , XeF _{2 and the} like.
Examples of the carrier component include an inert gas other than a rare gas such as Ar, He, and Ne, and a rare gas such as N ₂ .
Examples of the hydrogen-containing additive component include water (H ₂ O), hydrogen peroxide (H ₂ O ₂ ), and alcohol (OH group-containing compound).
Water is preferably used as the hydrogen-containing additive component. Thereby, a fluorine-based reaction component such as HF can be reliably generated.
The unit flow per 2g / m ³ ~9.6g / m ³ of water in the process gas after having added to the fluorine source gas, it is preferable to introduce the fluorine source gas into the plasma space. Water addition amount per the unit processing gas flow rate ^{(2g / m 3 ~9.6g / m} 3) is sufficiently smaller than a typical silicon etch (e.g., etching of the silicon substrate for a liquid crystal display).

The vicinity of atmospheric pressure refers to a range of 1.013 × 10 ^{4 to} 50.663 × 10 ⁴ Pa, and 1.333 × 10 ^{4 to} 10.664 considering the ease of pressure adjustment and the simplification of the apparatus configuration. × 10 ⁴ Pa is preferable, and 9.331 × 10 ^{4 to} 10.9797 × 10 ⁴ Pa is more preferable.
Examples of the fluorine-based reaction component of the fluorine-based reaction gas include HF, COF ₂ , and F ₂ .
The oxidizing reaction gas contains an oxidizing reaction component having an oxidizing action on silicon. Examples of the oxidizing reaction component include ozone (O ₃ ), oxygen radicals, oxygen (O ₂ ), NO, NO ₂ , and N ₂ O. The oxidizing reaction component is preferably ozone.
The inert gas for dilution is for diluting a processing gas containing a fluorine-based reactive gas and an oxidizing reactive gas. Examples of the inert gas for dilution include nitrogen (N ₂ ) and air.

  According to the present invention, nano-order irregularities can be formed on the surface of a silicon substrate, and sufficient surface roughening can be performed.

It is a front view which shows explanatoryally the surface roughening apparatus which concerns on 1st Embodiment of this invention. It is a graph of the reflectance of the light of wavelength 550nm with respect to the frequency | count of scanning of atmospheric pressure plasma etching, showing the result of Example 1. It is a graph of the reflectance with respect to the wavelength of incident light which shows the result of Example 1. It is the photograph which image | photographed the surface of the sample which made the scanning frequency | count of the atmospheric pressure plasma etching in Example 1 40 times with the electron microscope, (a) is before a process, (b) is after a process. It is a graph which shows the measurement result of the reflectance with respect to the flow volume of the inert gas for dilution in Example 2. FIG. It is a graph which shows the relationship between the flow volume of the inert gas for dilution in Example 2, and the moisture content in fluorine-type raw material gas.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The to-be-processed object of this embodiment is a silicon substrate for solar cells. The silicon substrate may be polycrystalline silicon, single crystal silicon, or amorphous silicon.

  As shown in FIG. 1, the surface roughening apparatus includes an atmospheric pressure plasma etching apparatus 1. The atmospheric pressure plasma etching apparatus 1 includes a stage 7 (processing object support means), three gas supply sources 4, 5 and 6, and a processing head 2. A silicon substrate 9 to be processed is placed on the stage 7. A moving means 8 is connected to the stage 7. The moving means 8 reciprocates the stage 7 left and right at a constant speed. The moving means 8 can be constituted by a linear motion mechanism, an air cylinder or the like.

The fluorine-based source gas supply source 4 generates a fluorine-based source gas. The fluorine-based material gas includes a fluorine-based material from the fluorine-based material supply unit 4a and a carrier gas from the carrier gas supply unit 4b. In this embodiment, CF ₄ is used as the fluorine-based material. Ar is used as a carrier gas. The flow rate of CF ₄ and Ar and the flow rate ratio thereof are adjusted by flow rate adjusting means (not shown) such as a mass flow controller and a flow rate control valve.

Further, the fluorine-based source gas supply source 4 is provided with a humidifier 4c. The humidifier 4c adds water (H ₂ O) as a hydrogen-containing additive component to a fluorine-based source gas composed of a mixed gas of CF ₄ and Ar. For example, part or all of the source gas (CF ₄ + Ar) from the supply units 4a and 4b is introduced into the space above the water surface of the humidifier 4c. Thereby, the saturated water vapor in the upper space of the humidifier 4c is mixed with the raw material gas. The source gas may be bubbled into liquid phase water in the humidifier 4c. The humidifier 4c may be heated to promote water evaporation. By adjusting the ratio of the gas that passes through the humidifier 4c and the gas that does not pass through the humidifier 4c among the source gases from the supply units 4a and 4b, the amount of water added can be adjusted.

Other PFCs such as C ₂ F ₆ , C ₃ F ₆ , and C ₃ F ₈ may be used as the fluorine-based raw material instead of CF ₄ , such as CHF ₃ , C ₂ H ₂ F ₂ , and CH ₃ F. HFC may be used, and SF ₆ , NF ₃ , XeF ₂ or the like may be used.
The carrier gas (Ar) has a role of generating stable plasma in the discharge space 13 to be described later in addition to the role of carrier of the fluorine-based raw material. As the carrier gas, other inert gases such as He and N ₂ may be used instead of Ar.
As a hydrogen-containing additive component, hydrogen peroxide water may be used instead of water, or an OH group-containing compound such as alcohol may be used.

The oxidizing reaction gas supply source 5 includes an oxygen supply unit 5a and an ozonizer 5b. The ozonizer 5b generates ozone (O ₃ ) using oxygen (O ₂ ) from the oxygen supply unit 5a as a raw material, and outputs an oxidizing reaction gas composed of a mixed gas of ozone and oxygen. The flow rate of the oxidizing reaction gas from the ozonizer 5b can be adjusted by adjusting the flow rate of the oxygen gas by flow rate adjusting means (not shown) such as a mass flow controller or a flow rate control valve attached to the oxygen supply unit 5a.

The dilution inert gas supply source 6 stores nitrogen (N ₂ ) as the dilution inert gas. The nitrogen gas flow rate is adjusted by flow rate adjusting means (not shown) such as a mass flow controller and a flow rate control valve attached to the supply source 6.

The processing head 2 is separated above the stage 7 and supported by a gantry (not shown). The processing head 2 has an upper plasma source 10 (fluorine-based reactive gas generator) and a lower nozzle 20. The plasma source 10 includes a pair of electrodes 11 and 11 that face each other. A solid dielectric layer (not shown) is formed on the opposing surface of the electrode 11. One electrode 11 is connected to the power source 3. The other electrode 11 is electrically grounded. The power source 3 applies, for example, a pulse wave voltage between the electrodes 11 and 11. The applied voltage is not limited to a pulse wave, and may be a continuous wave. By the voltage supply from the power supply 3, the space between the pair of electrodes 11, 11 becomes a plasma space 13 having a substantially atmospheric pressure. A gas supply path 4 d extending from the fluorine-based source gas supply source 4 is connected to the upstream end (upper end) of the plasma space 13. In the plasma space 13, the fluorine-based source gas is turned into plasma, and a fluorine-based reaction gas is generated. Fluorine-based reactive gas, including HF, a fluorine-based reactive components, such as COF _2.

  Three systems of gas outlets 21, 22, and 23 are formed inside the nozzle 20. The fluorine-based reactive gas blow-out path 21 is arranged at the center of the nozzle 20 with the gas flow direction directed up and down. The upper end portion of the fluorine-based reactive gas outlet path 21 is connected to the downstream end of the plasma space 13. The lower end portion of the fluorine-based reactive gas outlet passage 21 reaches the lower surface of the nozzle 20 and forms an outlet 24. The air outlet 24 has a slit shape extending in a direction orthogonal to the paper surface of FIG.

The upstream end of the oxidizing reaction gas blowing path 22 is connected to the oxidizing reaction gas supply source 5 via the oxidizing reaction gas supply path 5c.
The upstream end of the diluting inert gas blowing path 23 is connected to the diluting inert gas supply source 6 via the diluting inert gas supply path 6a.
The oxidizing reaction gas blowing path 22 and the diluting inert gas blowing path 23 merge with the fluorine-based reactive gas blowing path 21. In the junction part of these three paths 21, 22, and 23, the gas from each path 21, 22, and 23 is mixed together, and process gas is produced | generated. This processing gas is blown out from the blowout port 24.
Although not shown, a rectifying unit is provided inside the nozzle 20 to homogenize the gas passing through the paths 21, 22, and 23 in a direction perpendicular to the paper surface of FIG. The rectifying unit includes elements that increase and decrease gas conductance such as a chamber, a slit, and a row of a plurality of small holes.

  A pair of suction ports 25, 25 are formed on both the left and right sides of the air outlet 24 on the lower surface of the nozzle 20. Each suction port 25 has a slit shape extending in a direction perpendicular to the paper surface of FIG. An exhaust passage 26 extending from each suction port 25 is formed in the nozzle 20. The exhaust passage 26 is connected to exhaust means (not shown).

A method of roughening the surface of the silicon substrate 9 using the atmospheric pressure plasma etching apparatus 1 having the above-described configuration will be described.
A silicon substrate 9 is placed on the stage 7. The moving means 8 reciprocates (scans) the stage 7 and thus the silicon substrate 9 left and right. The left end of the reciprocating range is a position where the entire silicon substrate 9 is shifted to the left from the left suction port 25, and the right end of the reciprocating range is the suction port 25 on the right side of the nozzle 20 where the entire silicon substrate 9 is located. The position is shifted to the right. Thereby, each point on the surface of the silicon substrate 9 passes through the entire processing region between the left and right suction ports 25 and 25 of the nozzle 20 for each one-way movement.
The moving speed is preferably about 1 m / min to 6 m / min.
The distance between the lower surface of the nozzle 20 and the upper surface of the silicon substrate 9 is preferably about 0.5 mm to 5 mm.

In the fluorine source gas supply source 4, a fluorine source gas composed of a mixed gas of CF ₄ and Ar is generated. The flow rate ratio between CF ₄ and Ar is preferably about CF ₄ : Ar = 1: 59 to 1:19.
Water is added to the fluorine-based source gas (CF ₄ + Ar). The amount of water added is preferably set so that the dew point of the fluorine-based raw material gas after water addition is about 12 ° C to 24 ° C. Water amount for blowing flow rate of the processing gas from the air outlet 24, a unit flow rate per treatment ^gas, is preferably 2g / ^{m 3} ~9.6g / m 3 approximately.

The fluorine-based source gas (CF ₄ + Ar + H ₂ O) after addition of water is introduced into the interelectrode space 13 of the plasma source 10 through the fluorine-based source gas supply path 4d. In parallel, an electric field is applied between the electrodes 11 and 11 by voltage supply from the power source 3 to form an atmospheric pressure discharge. As a result, the raw material gas is turned into plasma in the discharge space 13 to generate a fluorine-based reaction gas containing a fluorine-based reaction component such as HF. This fluorine-based reaction gas is led out to the blowing passage 21 of the nozzle 20.

Further, oxygen from the oxygen supply unit 5a is ozonized by the ozonizer 5b to generate an oxidizing reaction gas (O ₂ + O ₃ ). The ozone concentration in the oxidizing reaction gas is, for example, about 8 vol%. This oxidizing reaction gas is led out to the oxidizing reaction gas blowing passage 22 through the supply passage 5c. The flow rate ratio between the fluorine-based reaction gas and the oxidizing reaction gas is preferably (fluorine-based reaction gas) :( oxidizing reaction gas) = 2: 1 to 2: 6, and (fluorine-based reaction gas) :( oxidation) Sexual reaction gas) = 2: 1.2 is more preferable.
Note that the flow rate of the fluorine-based reaction gas is substantially equal to the flow rate of the fluorine-based source gas (CF ₄ + Ar + H ₂ O), and further approximately equal to the flow rate of the fluorine-based source gas (CF ₄ + Ar) before water addition.

Further, nitrogen gas (N ₂ ) from the dilution inert gas supply source 6 is sent to the dilution inert gas blowing path 23 through the supply path 6a (without passing through the plasma space 13). The flow rate of nitrogen gas from the inert gas supply source 6 for dilution is set so that the flow rate ratio with the fluorine-based reaction gas is (fluorine-based reaction gas) :( nitrogen gas) = 2: 3 to 2:12. Preferably, (fluorine-based reaction gas) :( nitrogen gas) = 2: 5 to 2: 8.

  In the nozzle 20, the fluorine-based reaction gas in the fluorine-based reaction gas blowing path 21, the oxidizing reaction gas in the oxidizing reaction gas blowing path 22, and the nitrogen gas in the inert gas blowing path 23 for dilution are joined together. Mix to produce process gas. This processing gas is blown out from the blowout opening 24 and brought into contact with the silicon substrate 9. The supply flow rate of the processing gas is preferably about 6 slm to 24 slm per unit length (1 m) in the direction orthogonal to the paper surface of FIG.

Thereby, silicon on the surface of the substrate 9 reacts with ozone in the processing gas and is oxidized. This silicon oxide reacts with a fluorine-based reaction component such as HF to generate a volatile component such as SiF ₄ . Thus, the silicon on the surface of the substrate 9 is etched. Moreover, since the processing gas is sufficiently diluted with nitrogen gas (N ₂ ), it can be etched gently. Thereby, nano-order irregularities can be formed on the surface of the silicon substrate 9, and the surface of the silicon substrate 9 can be roughened. Therefore, the reflectance of the silicon substrate 9 can be lowered. For example, the reflectance of the silicon substrate 9 is set to 17% or less by setting the flow ratio of the fluorine-based reaction gas and nitrogen to (fluorine-based reaction gas) :( nitrogen gas) = 2: 3 to 2:12. Can do. By setting the flow ratio of the fluorine-based reaction gas and nitrogen to (fluorine-based reaction gas) :( nitrogen gas) = 2: 5 to 2: 8, the reflectance of the silicon substrate 9 can be reduced to 12% or less. (See Example 2).
As a result, the light absorptance of the solar cell can be sufficiently increased, and consequently the power generation efficiency can be increased.

In FIG. 1, the processing gas is divided into left and right hands on the silicon substrate 9 immediately below the blow-out port 24 and flows down to the lower sides of the left and right suction ports 25, as shown by a thick line g with an arrow. Sucked into. Accordingly, each point on the surface of the silicon substrate 9 is processed in contact with the processing gas during a period between the right under the left suction port 25 and the right under the right suction port 25. If the distance between the left and right suction ports 25 is 25, the moving speed of the silicon substrate 9 is V, and the number of scans of the silicon substrate 9 is n, the accumulated processing time of each point on the surface of the silicon substrate 9 ( The contact time (T) with the processing gas is
T = n × L / V (Formula 1)
become. Here, one scan refers to one-way movement of the silicon substrate 9. Therefore, n = 2 when the silicon substrate 9 reciprocates once.

  The number of scans n is set so that the processing time T is about T = 24 to 48 seconds for one silicon substrate 9. By setting the processing time T to 24 seconds or more, a necessary processing amount can be secured and the reflectance of the silicon substrate 9 can be sufficiently lowered. By setting the processing time T to 48 seconds or less, it is possible to avoid excessive processing, and it is possible to prevent wasteful consumption of processing gas and prolonged processing time.

According to the surface roughening method by atmospheric pressure plasma etching, the problems of the surface roughening method by wet etching can be eliminated or alleviated. That is, even if an electrode is formed on the surface of the silicon substrate 9, if the mask is used, damage to the electrode can be prevented. Further, it is difficult for the processing gas to hit the back surface of the silicon substrate 9, and the smoothness of the back surface can be easily prevented from being lowered. Furthermore, it can be reliably processed regardless of the thickness of the silicon substrate 9.

The present invention is not limited to the above embodiment, and various modes can be adopted within the scope of the gist.
For example, an inert gas such as nitrogen from the inert gas supply source 6 for dilution is blown out from the nozzle 20 without being mixed with the fluorine-based reactive gas and the oxidizing reactive gas in the nozzle 20, and the nozzle 20 and the silicon substrate. 9 may be mixed with the fluorine-based reaction gas and the oxidizing reaction gas in the space between the two.
The oxidizing reaction gas from the oxidizing reaction gas supply source 5 is blown out from the nozzle 20 without being mixed with the fluorine reaction gas in the nozzle 20, and the fluorine reaction in the space between the nozzle 20 and the silicon substrate 9. It may be mixed with gas.
The moving means 8 may be connected to the processing head 2 so that the processing head 2 is reciprocated. The surface treatment may be performed in a state where the relative position between the processing head 2 and the silicon substrate 9 is fixed.

Before the atmospheric pressure plasma etching, the silicon substrate 9 may be wet-etched with hydrofluoric acid, hydrogen peroxide solution or the like. By wet etching, irregularities on the order of microns can be formed on the surface of the silicon substrate 9. Thereafter, further nano-order irregularities can be formed on the surface of the micron-order irregularities by atmospheric pressure plasma etching. As a result, the reflectance of the silicon substrate 9 can be reliably lowered.
After the above atmospheric pressure plasma etching, the silicon substrate 9 may be wet etched with an alkaline aqueous solution such as NaOH. The alkali concentration may be about 1 wt%. The immersion time may be about 10 seconds. As a result, the surface of the silicon substrate 9 can be further roughened, and the reflectance can be further reliably reduced. The present invention is not limited to solar cells as long as the surface of the silicon substrate is roughened, and can also be applied to processing of silicon substrates for other uses.

Examples will be described. Needless to say, the present invention is not limited to these examples.
A plurality of polycrystalline silicon wafers sliced from a silicon ingot were prepared. As the surface roughening apparatus, an atmospheric pressure plasma etching apparatus having substantially the same structure as that of the apparatus 1 shown in FIG. 1 was used.
CF ₄ 0.15 slm and Ar 2.85 slm were mixed to obtain 3 slm fluorine-based source gas. Water (H ₂ O) was added to the fluorine-based source gas (CF ₄ + Ar) in such an amount that the dew point was 20 ° C. The fluorine-based source gas (CF ₄ + Ar + H ₂ O) after the addition of water was introduced into the atmospheric pressure discharge space 13 of the plasma source 10 to generate plasma, and a fluorine-based reactive gas 3 slm containing HF or the like was generated. The input power to the plasma source 10 was 3.5 kW.
An oxidizing reaction gas (O ₂ + O ₃ ) having an ozone concentration of 8 vol% was generated by the ozonizer 5b. This oxidizing reaction gas (1.8 slm) was supplied to the nozzle 20.
Further, 9 slm of nitrogen gas was supplied from the inert gas supply source 6 for dilution to the nozzle 20.
In the nozzle 20, a fluorine-based reaction gas of 3 slm, an oxidizing reaction gas of 1.8 slm, and a dilution nitrogen gas of 9 slm were mixed to obtain a processing gas of 13.8 slm. The flow ratio of the fluorine-based reaction gas and the dilution nitrogen gas (N ₂ ) in the processing gas was (fluorine-based reaction gas) :( nitrogen gas) = 1: 3. This processing gas was blown out from the blowout opening 24 and brought into contact with the silicon wafer 9 to perform atmospheric pressure plasma etching. The length of the outlet 24 in the direction orthogonal to the paper surface of FIG. 1 was 600 mm. The distance between the lower surface of the nozzle 20 and the silicon wafer 9 was 1 mm.
The treated gas was sucked from the pair of suction ports 25 and 25. The distance L between the pair of suction ports 25, 25 was L = 40 mm.
The silicon wafer 9 was reciprocated (scanned) simultaneously with the spraying of the processing gas. The moving speed was 2 m / min. Therefore, the time (processing time) that the processing gas contacts each point on the surface of the silicon wafer by one scanning (one-way movement) was 1.2 seconds (see Formula 1).
The number of scans was changed for each sample silicon wafer. The number of scans was five, 10 times, 20 times, 40 times, 60 times, and 80 times.

  The reflectance of each sample after processing was measured using a spectrophotometer (Hitachi U-4100). The results are shown in FIGS. FIG. 2 shows the measurement results of the reflectance when the wavelength of the incident light on each sample is λ = 550 nm. FIG. 3 shows the measurement results of the reflectance of each sample with respect to the wavelength λ of incident light. In FIG. 2, the data with the annotation “As slice → AP Etching” corresponds to each sample processed in the first embodiment. “As slice → AP Etching” indicates that direct atmospheric pressure plasma etching was performed after slicing from the silicon ingot. In addition, the reflectance of an untreated sample, that is, a silicon wafer sliced from an ingot was also measured.

As shown in FIG. 2, the unprocessed sample had a scan number n = 0, and its reflectance was about 30% with respect to the incident wavelength λ = 550 nm. It was confirmed that in the range of the number of scans from 0 to 20 times, the reflectivity rapidly decreases as the number of scans, that is, the processing time T increases. When the number of scans was 20, the reflectivity was about 12%. When the number of scans was 20 or more, the reflectance did not change much even if the number of scans was increased or decreased. Moreover, as shown in FIG. 3, when the number of scans is 20 or more, the reflectivity converges to a substantially constant range (about 10% to 17%) regardless of the wavelength λ of the incident light and regardless of the number of scans. It was way.
From this, it has been found that if the scan is performed at least 20 times, that is, if the processing time T is 24 seconds or more, the silicon surface can have a desired low reflectance. In consideration of processing variations, the number of scans is set to 20 to 40 times, that is, the processing time T is set to 24 to 48 seconds, so that a desired processing amount can be reliably obtained and useless processing can be prevented. I can say that.

  FIG. 4 is a photograph of the surface of a sample taken with an electron microscope with 40 scans, that is, with a processing time T of T = 48 seconds. FIG. 4A is before processing, and FIG. 4B is after processing. It was confirmed that nano-order irregularities were formed on the silicon surface by atmospheric pressure plasma etching.

In addition, the data with the annotation “As slice → Acid Etching → AP Etching” in FIG. 2 shows that the silicon wafer obtained by slicing from the silicon ingot is wet-etched with acid or alkali, and then is subjected to atmospheric pressure plasma etching. It is a measurement result of the reflectance in the case of. The etchant used for wet etching was HF, HNO ₃ , HAc or the like in the case of acid treatment, and NaOH-NaOCl or the like in the case of alkali treatment. In both cases, the cell conversion efficiency was about 17%. The atmospheric pressure plasma etching conditions and the apparatus configuration were the same as those in the case of direct atmospheric pressure plasma etching (As slice → AP Etching).
When the atmospheric pressure plasma etching was performed after the wet etching, the reflectance could be made lower than in the case of only the atmospheric pressure plasma etching.

Using the same apparatus as in Example 1, atmospheric pressure plasma etching was performed on each sample consisting of nine silicon wafers.
The fluorine-based raw material gas was a mixed gas of CF ₄ 0.1 slm and Ar 1.9 slm 2.0 slm, and water was added thereto. The dew points of the fluorine-based source gas after the addition of water were set to three types of 12 ° C., 18 ° C., and 24 ° C. according to the sample.
The oxidizing reaction gas was an ozone-containing gas (O ₂ + O ₃ ) with an ozone concentration of 8 vol%, and the flow rate was 1.2 slm.
The inert gas for dilution was nitrogen (N ₂ ), and the flow rates thereof were three types of 3 slm, 6 slm, and 12 slm depending on the sample. Therefore, the flow rate ratio between the fluorine-based reaction gas and the inert gas for dilution was three types: (fluorine-based reaction gas) :( inert gas for dilution) = 2: 3, 2: 6, 2:12.
The direct current of 160 V and 3.1 A was converted into AC power of 30 kHz and supplied to the plasma source 10.
The distance between the lower surface of the nozzle 20 and the silicon substrate 9 on the stage 7 was 1.5 mm.
The temperature of the silicon wafer was 25 ° C. (room temperature).
The scanning speed was 2 m / min.
The number of scans was 40.

The reflectance was measured for the treated sample using the same measuring instrument as in Example 1. The wavelength of incident light was 550 nm. The results are shown in FIG.
The reflectivity varied with the flow rate of the inert gas for dilution. When the fluorine source gas has a low dew point (12 ° C.), the lower the flow rate of the inert gas for dilution, the lower the reflectivity. However, when the dew points are 18 ° C. and 24 ° C., the inert gas for dilution Whether the flow rate is too small or too large, the reflectivity is high, and it has been confirmed that there is an inert gas flow for dilution that minimizes the reflectivity. If (fluorine-based reactive gas) :( inert gas for dilution) = 2: 3 to 2:12, the reflectance can be reduced to 17% or less by adjusting the amount of water added to the fluorine-based raw material gas. confirmed. (Fluorine-based reactive gas): (Inert gas for dilution) = 2: 5 to 2: 8 It was confirmed that the reflectance could be reduced to 12% or less.

FIG. 6 shows that in Example 2, the amount of water added to the fluorine-based raw material gas is divided by the flow rate of the processing gas blown from the blowout port 24 to be converted into the amount of water added per unit flow rate of the processing gas, This shows the relationship between the amount of water added and the flow rate of the inert gas for dilution (N ₂ ). (Fluorine-based reaction gas): (Inert gas for dilution) = 2: 3 to 2:12 The water addition amount corresponding to the dew point of 12 ° C. to 24 ° C. in the flow rate ratio range is 2 g / m per unit treatment gas flow rate. It is about ^{3 to} 9.6 g / m ³ . The amount of water added is sufficiently smaller than the amount of water added in the case of etching a silicon substrate for liquid crystal display, for example.

  The present invention is applicable to roughening a silicon substrate for solar cells, for example.

DESCRIPTION OF SYMBOLS 1 Atmospheric pressure plasma etching apparatus 2 Processing head 3 Power supply 4 Fluorine-type raw material gas supply source 4a Fluorine-type raw material supply part 4b Carrier gas supply part 4c Humidifier 4d Fluorine-type raw material gas supply path 5 Oxidation reaction gas supply source 5a Oxygen supply part 5b Ozonizer 5c Oxidizing reaction gas supply path 6 Diluting inert gas supply source 6a Diluting inert gas supply path 7 Stage (processing object support means)
8 Moving means 9 Silicon substrate (object to be processed)
10 Plasma source (fluorine-based reactive gas generator)
DESCRIPTION OF SYMBOLS 11 Electrode 13 Plasma space 20 Nozzle 21 Fluorine type reactive gas blowing path 22 Oxidizing reactive gas blowing path 23 Diluting inert gas blowing path 24 Blowing port 25 Suction port 26 Exhaust channel

Claims (4)

A method for roughening the surface of a workpiece made of silicon,
Fluorine source gas is introduced into the plasma space near atmospheric pressure to generate fluorine reaction gas,
A treatment gas containing the fluorine-based reaction gas, the oxidizing reaction gas, and the inert gas for dilution that does not pass through the plasma space is brought into contact with the object to be treated;
The surface roughness of silicon is characterized in that the flow rate ratio of the fluorine-based reaction gas and the inert gas for dilution is (fluorine-based reaction gas) :( inert gas for dilution) = 2: 3 to 2:12 Method.   2. The flow rate ratio of the fluorine-based reaction gas and the inert gas for dilution is set to (fluorine-based reaction gas) :( inert gas for dilution) = 2: 5 to 2: 8. The surface roughening method as described.   The surface roughening method according to claim 1 or 2, wherein a contact time of the processing gas at each position of the object to be processed is 24 seconds to 48 seconds. Claims, characterized in that the introduction of units of water flow per 2g / m ³ ~9.6g / m ³ of the process gases after having added to the fluorine source gas, the fluorine-based material gas into the plasma space Item 4. The surface roughening method according to any one of Items 1 to 3.