JP2000012517A - Method for surface treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase light absorption efficiency by forming texture on a Si or GaAs substrate or the like used for a solar cell panel to reduce reflectance of the panel surface. SOLUTION: In this surface treatment, surface of a semiconductor substrate 3 is plasma etched with a plasma etching equipment in which the semiconductor substrate 3 is grounded, wherein a gas mixture comprising SF6 gas containing fluorine atoms in the molecule and HCl gas containing a chlorine atom in the molecule is used. Because of the etching rate difference between SF6 gas and HCl gas for the same crystallographic plane, the etching becomes hard to proceed in isotropic basis, and texture is produced on the surface of the semiconductor substrate 3 even with plasma etching equipment of low impact energy. Therefore, because any apparatus for insulating the substrate side electrode against substrate carrying mechanism is not necessary, the equipment cost is reduced, and the cost of solar cell panel is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an improvement in a method of forming a surface texture on a substrate, a GaAs substrate, or the like to improve light receiving efficiency.

[0002]

2. Description of the Related Art Conventionally, Si substrates for solar cells, G
In order to reduce the surface reflectance of aAs substrates and the like, improve the light receiving efficiency and increase the conversion efficiency of solar cells, the surfaces of these Si and GaAs substrates are etched to make the surface uneven (surface texture). The surface treatment of forming is performed.

In FIG. 12A, reference numeral 101 denotes an RIE (Reactiv
e Ion Etching: Reactive ion etching.
The RIE apparatus 101 has a chamber 102, a gas introduction system 110, and a gas exhaust system 120.

[0004] A substrate-side electrode 104 is provided on the inner bottom surface of the chamber 102 via an insulator 105, and a mounting table 108 is provided on a side of the substrate-side electrode 104 facing the inner wall surface of the chamber 102. . The chamber 102 is grounded, and an RF power supply 107 is connected to the substrate-side electrode 104 via a matching box 106. When the RF power supply 107 is driven, high-frequency power can be supplied to the substrate-side electrode 104. I have.

[0005] Outside the chamber 102, a gas introduction system 110 and a gas exhaust system 120 are provided. The gas introduction system 110 and the gas exhaust system 120 have a gas introduction pipe 113 and an exhaust pipe 123 communicating with the inside of the chamber 102, respectively. The gas introduction system 110 and the gas exhaust system 120 have a gas cylinder 112 and an exhaust pump 122, respectively. ing.

The gas introduction pipe 113 and the exhaust pipe 123 are provided with valves 111 and 121, respectively.
Gas is introduced into the chamber 102 by opening the chamber 11, and the chamber 102 can be evacuated by activating the exhaust pump 122 and opening the valve 121.

In the RIE apparatus 101 having the above configuration, the vacuum valve 121 is opened in advance, and the exhaust pump 122 is driven to make the inside of the chamber 102 a high vacuum state. Is transported into the chamber 102 by a transfer device (not shown), and is placed on the mounting table 108.

In this state, the valve 111 is opened and an etching gas is introduced from the gas cylinder 112 to
07 is activated to supply high-frequency power to the substrate-side electrode 104. Then, the etching gas is turned into plasma by the high-frequency power, and the ions and excited active species generated in the plasma simultaneously act on the surface of the substrate 103, thereby etching the surface of the substrate 103.

In such RIE, when a plurality of gases having different etching characteristics in the same plane orientation are used as an etching gas, the RIE has a relatively high energy of ions colliding with the surface of the substrate 103. Is enlarged, the etching does not progress isotropically, and irregularities (texture) 130 are formed on the surface of the substrate 103 as shown in FIG.

In this manner, the texture 130 is formed on the surface.
When the substrate 103 on which is formed is used for a solar cell panel,
The reflectance of the surface of the substrate 103 is reduced as compared with the case where the surface of the substrate 103 is flat, the absorptance of light received from the surface of the substrate 103 is improved, and the light receiving efficiency of the solar cell is improved.

When the substrate 103 formed through the above steps is used for a solar cell panel, since the texture 130 is formed on the surface of the substrate 103, the reflectance of the surface of the substrate 103 is determined when the surface of the substrate 103 is flat. Is reduced as compared with For this reason, light incident on the surface of the substrate 103 is hardly reflected, and is easily absorbed inside the substrate 103.
The light receiving efficiency of the solar cell is improved.

However, the above-described RIE apparatus 101
Then, in order to actually process a plurality of substrates, it is necessary to carry in / out the substrate 103 by a carrier device (not shown). However, since high-frequency power is supplied to the substrate-side electrode 104, this high-frequency power is It is necessary to separately provide a facility for insulating the transfer device from the substrate-side electrode 104 so that the transfer device is not applied to the transfer device and causes inconvenience. For this reason, there has been a problem that the equipment cost of the device is increased, and as a result, the production cost of the solar cell panel is increased.

In the plasma etching apparatus, since the substrate side is grounded, it is not necessary to insulate the substrate transfer system from high-frequency power as in the above-described RIE apparatus 101, and the equipment cost can be reduced accordingly. Since the energy at which ions collide with the substrate is lower than in the RIE method and the etching reaction tends to proceed isotropically, the texture is large enough to improve the light receiving efficiency of the solar cell panel. Was difficult to form.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in order to solve such problems of the prior art.
An object of the present invention is to provide a technology capable of reducing the production cost of a solar cell panel.

[0015]

According to a first aspect of the present invention, there is provided a plasma processing apparatus comprising: a processing chamber; a substrate holder provided in the processing chamber and connected to a ground potential; and a plasma excitation source. A surface treatment method for forming irregularities on the surface of the semiconductor substrate by plasma-etching the surface of the semiconductor substrate where the semiconductor is exposed, using an etching apparatus, wherein the substrate is placed in a vacuum atmosphere in the treatment chamber. Holding the substrate in the processing chamber; introducing a reaction gas containing a mixed gas of a gas containing a fluorine atom in a molecule and a gas containing a chlorine atom in a molecule into the processing chamber; and Supply high frequency power from the source,
The method may further include a step of forming irregularities on the exposed surface of the semiconductor of the semiconductor substrate by converting the reaction gas into plasma and plasma-etching the exposed surface of the semiconductor.

According to a second aspect of the present invention, there is provided the surface treatment method according to the first aspect, wherein a volume ratio of the gas containing chlorine atoms in the molecule to the reaction gas is 40% or more. It is characterized by the following.

According to a third aspect of the present invention, in the surface treatment method according to any one of the first and second aspects, the plasma excitation source supplies a high-frequency power in a range of 50 kHz to 13.56 MHz. It is characterized by doing.

In the present invention, when forming the surface texture structure of the solar cell, the semiconductor substrate is held by the substrate holder using a plasma etching apparatus in which the substrate holder is connected to the ground potential, and the reaction gas is converted into molecules as a reactive gas. The substrate surface is plasma-etched using a mixed gas of a gas containing a fluorine atom and a gas containing a chlorine atom in a molecule.

When a silicon single crystal substrate is etched with a gas containing F (fluorine atom) in a molecule and a gas containing Cl (chlorine atom) in a molecule, the bonding distance of Si is 5 to 6 °. On the other hand, the ionic radius of F is about 2 to 3 °, which is smaller than the bonding distance of Si. Therefore, when etching Si with F, it is easy to enter between Si atoms from any plane orientation and to break bonds between atoms.

However, Cl has an ionic radius of 8 ° and is larger than the bonding distance of Si, so Cl enters from a plane direction in which Si atoms are separated from each other by more than the bonding distance of Si, and the bond is broken. You can only do it.

Therefore, at the time of etching, a difference occurs between the etch rate of the F gas and the etch rate of the Cl gas for the same plane orientation. This difference in etch rate makes it difficult for the etching to proceed isotropically, making it easier to form irregularities (texture) on the substrate surface.

Therefore, when a mixed gas of a gas containing a fluorine atom in a molecule and a gas containing a chlorine atom in a molecule is used, even if etching is performed using a plasma etching apparatus having low ion collision energy with the substrate, Texture can be formed.

In such a plasma etching apparatus, since the substrate-side electrode is grounded, it is necessary to insulate the substrate-side electrode from the transfer device as in the case of using an RIE apparatus in which high-frequency power is applied to the substrate-side electrode. Since there is no such equipment, the equipment cost can be reduced accordingly, and the production cost of the solar cell panel can be reduced.

In the present invention, the volume ratio of the gas containing chlorine in the molecule to the reaction gas may be 40% or more. With this configuration, the difference in etch rate between the gas containing a fluorine atom in the molecule and the gas containing a chlorine atom in the molecule in the same plane orientation is enlarged, so that a large texture can be formed. it can.

Further, in the plasma etching, since the energy at the time when the ions collide with the substrate surface is smaller than that of the RIE, a high frequency in the range of 50 kHz to 13.56 MHz from the plasma excitation source is provided. When power is supplied, the energy when ions collide with the substrate surface increases, and the texture formed on the substrate surface can be increased.

[0026]

Embodiments of the present invention will be described below. In FIG. 1A, reference numeral 1 denotes a plasma etching apparatus used in the embodiment of the present invention.

The plasma etching apparatus 1 has a chamber 2, a gas introduction system 10 and a gas exhaust system 20. A high-frequency electrode 8 is provided on the inner bottom surface of the chamber 2 via an insulator 9. A substrate holder 4 is provided on the ceiling side inside the chamber 2.

The gas introduction system 10 and the gas exhaust system 20 have a gas introduction pipe 13 and an exhaust pipe 23 communicating with the inside of the chamber 2, respectively.
2 respectively.

The gas introduction pipe 13 and the exhaust pipe 23 are provided with valves 11 and 21, respectively. When the valve 11 is opened, gas can be introduced into the chamber 2, and the exhaust pump 22 is started to activate the valve 21. When opened, the inside of the chamber 2 can be evacuated.

The chamber 2 and the substrate holder 4 are grounded. A high-frequency power source 7 is provided outside the chamber 2 via a matching box 6. When the high-frequency power source 7 is activated, high-frequency power can be supplied to the high-frequency electrode 8.

To form a texture on the surface of a Si single crystal substrate in order to form a solar cell panel with the plasma etching apparatus 1 having the above configuration, the vacuum valve 21 is opened in advance, and the exhaust pump 22 is started. The inside of the chamber 2 is evacuated to vacuum, and the vacuum state is maintained.
The i-single-crystal substrate 3 is carried into the chamber 2 by a transfer device (not shown), and is held by the substrate holder 4.

In this state, the valve 11 is opened and a mixed gas of SF ₆ and HCl is supplied from the gas cylinder 12 to the chamber 2.
Introduce within. At this time, HCl relative to the entire mixed gas
The volume ratio of the gas is set to 40%, and the total flow rate of the mixed gas is set to 500 SCCM. And the pressure 33.
Under the condition of 33 Pa (250 mTorr), 380 kHz,
A high frequency power of 0.8 W / cm ² is supplied to the high frequency electrode 8.

Then, the etching gas is turned into plasma by the high frequency power, and S
The surface of i-single-crystal substrate 3 is etched. At this time,
SF containing fluorine atom in molecule as etching gas
₆ and a gas, a mixed gas of HCl gas containing a chlorine atom in the molecule, the volume ratio of HCl gas to the mixed gas is set to be a 40%, SF ₆ gas, HC
A difference occurs in the etch rate in the same plane orientation between the 1 gases. Since the etching hardly progresses isotropically due to the difference in the etch rate, irregularities (texture) 30 as shown in FIG. 1B are formed on the surface of the Si single crystal substrate 3.

After forming the texture 30 on the surface of the Si single crystal substrate 3 in this manner, the supply of high frequency power and the introduction of the etching gas are terminated, and the Si single crystal substrate 3 is carried out of the chamber 2 by a transfer device (not shown). The etch rate of Si and the step between the irregularities (hereinafter referred to as surface roughness) were measured.

After that, other conditions are not changed from the above conditions,
The same measurement was performed a plurality of times while changing the volume ratio of HCl gas to the entire etching gas (hereinafter, simply referred to as the volume ratio of HCl gas) within the range of 0 to 40%. The measurement results thus obtained are shown in curves (A) and (C) of FIG.

In FIG. 2, the horizontal axis indicates the volume ratio of HCl gas, and the vertical axis indicates the Si etch rate and the surface roughness. Curve (A) shows the relationship between the etch rate of Si in plasma etching and the volume ratio of HCl gas, and curve (C) shows the relationship between the surface roughness and the volume ratio of HCl gas. I have.

According to FIG. 2, as the volume ratio of HCl gas increases, the Si etch rate gradually decreases as shown by the curve (A), and the surface roughness increases as shown by the curve (C). It can be seen that the surface roughness, which was 0 μm when the gas volume ratio was 0%, increased to 1.2 μm when the gas volume ratio was 40%.

The inventors of the present invention performed the same measurement as described above for the RIE method.

The curves (B) and (D) in FIG.
In the same manner as (A) and (C), the measurement results when the substrate surface is etched by RIE and subjected to surface treatment are shown.
Curve (B) shows the relationship between the etch rate of Si and the volume ratio of HCl gas during RIE, and curve (D) shows RIE.
In this case, the relationship between the surface roughness and the volume ratio of the HCl gas in each case is shown.

As the volume ratio of HCl gas increases,
As shown in curve (B), the Si etch rate decreased and the curve
As shown in (C), when the volume ratio of HCl gas was 0%, the surface roughness was 0 μm, but when the volume ratio was 40%, it was 0.5 μm.
m.

When comparing the curves (C) and (D), the curves
In any of (C) and (D), the introduction of HCl gas increases the surface roughness, but curve (C) has a larger surface roughness than curve (D) when the volume ratio of HCl gas is 40%. , About 2.4 times the curve (D). Thus, it has been found that the effect of increasing the surface roughness by introducing the HCl gas is more remarkable in plasma etching than in RIE.

Further, the inventors of the present invention mixed Cl ₂ gas with SF _{6 in place of} HCl gas, and set other conditions to the same conditions as the measurement explained in FIG.
The measurement similar to the measurement performed in was performed. FIG. 3 shows the measurement results.

In FIG. 3, curves (E) and (F) represent the etch rate of Si when the Si substrate is etched using the etching gas to which Cl ₂ gas is added, and the volume ratio of Cl ₂ gas to the entire etching gas. (Hereinafter simply referred to as a volume ratio of Cl ₂ gas). Curve (E) shows the result by plasma etching, and curve (F) shows the result by RIE.

Curves (G) and (H) show the relationship between the volume ratio of Cl ₂ gas and the surface roughness, respectively.
(G) shows the result by plasma etching, and the curve (H) shows the result by RIE.

The curves (E) and (F) in FIG. 3 show that as the volume ratio of HCl gas increases, the etch rate of Si gradually decreases in both RIE and plasma etching.

On the other hand, as shown by curves (G) and (H), the surface roughness can be reduced by RIE when the volume ratio of Cl ₂ gas is 30%.
The surface roughness increased from about 0.5 μm to about 1.5 μm in plasma etching, and the increase is larger in plasma etching than in RIE. As shown in FIGS. 2 and 3, the increase in the volume ratio of the chlorine-based gas is
It can be seen that the effect on the surface roughness is greater in plasma etching than in RIE.

In forming a solar cell panel, if the surface roughness is 1 μm or more, the reflectance can be reduced and the light receiving efficiency of the solar cell can be improved to a practically sufficient level. According to curves (C) and (G) of FIG.
If the volume ratio of the gas containing chlorine, such as gas or Cl ₂ gas, is at least 40% or more, the surface roughness can be made 1 μm or more, so that the light receiving efficiency of the solar cell panel can be improved to a sufficient extent. I understand.

As described above, in the present embodiment, using the plasma etching apparatus 1 shown in FIG. 1A, a size sufficient for forming a solar cell panel (a surface roughness of 1 μm or more) is used.
Can be formed on the surface of the Si single crystal substrate 3. In the plasma etching apparatus 1, since the Si single crystal substrate 3 is grounded as shown in FIG. 1, there is no need to insulate the transfer device of the Si single crystal substrate 3 from high frequency power. Therefore, the equipment cost for that purpose can be reduced, and the production cost of the solar cell panel can be reduced.

Further, the inventors of the present invention used a plasma etching apparatus 1 shown in FIG.
z, high frequency power of 0.8 W / cm ² and pressure 2
Under the condition of 6.66 Pa (200 mTorr), SF ₆ gas and H
Using a mixed gas with Cl gas, the volume ratio of HCl gas is 4
The texture was formed on the surface of the Si single crystal substrate by plasma etching at 0% and a total flow rate of the mixed gas of 100 SCCM.

FIG. 4 is an enlarged photograph showing the surface state of the Si single crystal substrate on which the texture has been formed as described above. FIG. 5 shows that the frequency of the high-frequency power is 380 kHz.
FIG. 4 is an enlarged photograph showing a surface state when a texture is formed on the surface under the same conditions as those in FIG.

The inventors of the present invention set the other conditions to be the same as those of the surface treatment performed in FIG. 4 and set only the SF ₆ gas as the etching gas, and subjected the surface treatment to the Si single crystal substrate by plasma etching. . FIG. 6 is an enlarged photograph showing the surface state of the Si single crystal substrate at that time, and FIG. 7 shows that the frequency of the high-frequency power was set to 380 kHz, and the other conditions were the same as those in FIG. 3 is an enlarged photograph showing a surface state when a is formed. Note that in the enlarged photographs of FIGS.
0000 times. 4 and 6, the texture shown in FIG. 4 is larger, and when comparing FIGS. 5 and 7, it can be seen that the texture shown in FIG. 5 is larger. In the surface treatment of plasma etching, HC
It was confirmed that by adding 40% of l, the texture formed on the surface could be increased.

Further, the inventors of the present invention have conducted RIE.
The texture was formed on the substrate surface using the apparatus, and the surface state was observed. First, 13.56 MHz, 0.8
A high-frequency power of W / cm ² is supplied to the electrode on the substrate side, and a mixed gas of SF ₆ gas and HCl gas such that the volume ratio of HCl gas becomes 40% under the condition of a pressure of 33.33 Pa (250 mTorr). And a total gas flow rate of 100 SCCM was used to form a texture on the surface of the Si single crystal substrate by the RIE method.

FIG. 8 is an enlarged photograph showing the surface state of the Si single crystal substrate when the texture is formed by such an RIE method, and FIG.
Hz, and other conditions are the same as those of the surface treatment of FIG.
5 is an enlarged photograph showing a surface state when a Si single crystal substrate is subjected to a surface treatment by the IE method.

Further, only SF ₆ was used as a reaction gas, and all other conditions were the same as those of the surface treatment shown in FIG.
The surface treatment of the i single crystal substrate was performed. FIG. 10 is an enlarged photograph showing the surface state at that time, and FIG. 11 shows the surface state when the substrate surface was processed by the RIE method under the same conditions as in FIG. 10 except that the frequency of the high-frequency power was 380 kHz. It is an enlarged photograph shown. The magnification in FIGS. 8 to 11 is equal to 25,000 times.

When comparing FIGS. 8 and 10, the texture shown in FIG. 8 is larger than the texture of FIG. 10, and when comparing FIGS. 9 and 11, the texture shown in FIG. It turns out that it is larger than the texture of FIG.

As described above, even when the RIE method is used, H
It was confirmed that a large texture could be formed on the substrate surface by adding 40% of Cl. However, comparing FIG. 5 showing the surface state by plasma etching with FIG. 9 showing the surface state by RIE, the texture shown in FIG. It can be seen that, under the same conditions, the surface roughness due to the introduction of the chlorine-based gas is larger in the plasma etching. Therefore, in order to increase the surface roughness by adding a gas containing a chlorine atom in a molecule to a gas containing a fluorine atom in a molecule in an amount of 40% or more, plasma etching must be performed by RI.
Preferred over E.

In this embodiment, the volume ratio of the gas containing chlorine to the etching gas is confirmed only up to 40%. However, the present invention is not limited to this, and if the volume ratio is 40% or more, A texture sufficient to constitute a solar cell panel can be formed on the surface.

In the present embodiment, the high-frequency power source 7 serving as a plasma excitation source supplies high-frequency power of 380 kHz, but the present invention is not limited to this, and the frequency is 50 kHz.
z to 13.56 MHz, preferably 150 kHz to 50
High frequency power in the range of 0 kHz may be supplied. By doing so, the energy when ions collide with the substrate surface can be increased, so that a larger texture can be formed. Further, in the present embodiment, SF ₆ is used as a gas containing a fluorine atom in the molecule, but the present invention is not limited to this, and a gas such as CF ₄ may be used.

[0059]

As described above, according to the present invention, an inexpensive solar cell panel can be manufactured.

[Brief description of the drawings]

FIG. 1A shows a configuration of a plasma etching apparatus used in the surface treatment method of the present invention. FIG. 1B is a cross-sectional view showing a state of the substrate after the surface treatment.

FIG. 2 is a graph showing the relationship between the volume ratio of HCl and the Si etch rate and surface roughness for both plasma etching and RIE.

FIG. 3 is a graph showing the relationship between the volume ratio of Cl ₂ and the Si etch rate and surface roughness for both plasma etching and RIE.

FIG. 4 is a first photograph showing a state of a fine pattern formed on a substrate after surface treatment.

FIG. 5 is a second photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 6 is a second photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 7 is a fourth photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 8 is a fifth photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 9 is a sixth photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 10 is a seventh photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

FIG. 11 is an eighth photograph showing a state of a fine pattern formed on the substrate after the surface treatment.

12A is a diagram showing a configuration of an RIE apparatus used in a conventional surface treatment method. FIG. 12B is a cross-sectional diagram showing a state of the substrate after the surface treatment.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 plasma etching apparatus 2 chamber (processing chamber) 3 single crystal silicon substrate (semiconductor substrate) 4 substrate holder 6 matching box 7 high-frequency power source (plasma excitation source) 8 high-frequency electrode 9 insulator 10 gas Introduction system 20 ... Gas exhaust system

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kafumi Ota 523 Yokota, Yamatake-cho, Yamatake-gun, Chiba F-term (reference) 5F004 AA03 BA04 BB11 BC02 BC03 DA00 DA01 DA04 DA18 DA29 DB01

Claims (3)

[Claims] 1. A semiconductor etching method comprising: a processing chamber; a substrate holder provided in the processing chamber and connected to a ground potential; and a plasma etching apparatus having a plasma excitation source. A surface treatment method for forming irregularities on the surface of the semiconductor substrate by plasma etching, wherein a step of holding the substrate on the substrate holder in a state where the processing chamber is in a vacuum atmosphere, A step of introducing a reaction gas containing a mixed gas of a gas containing a fluorine atom and a gas containing a chlorine atom in a molecule, supplying high-frequency power from the plasma excitation source, turning the reaction gas into plasma, Plasma etching the exposed surface of the semiconductor to form irregularities on the exposed surface of the semiconductor of the semiconductor substrate. Surface treatment method. 2. The surface treatment method according to claim 1, wherein a volume ratio of the gas containing a chlorine atom in the molecule to the reaction gas is 40% or more. 3. The plasma excitation source has a frequency of 50 kHz.
The surface treatment method according to claim 1, wherein a high-frequency power of not less than z and not more than 13.56 MHz is supplied.