JP2002329710A - Method for making surface of silicon substrate rough - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming projections and depressions uniformly and efficiently over the entire surface of a semiconductor substrate, particularly such as a silicon substrate used in a solar cell. SOLUTION: A method for making the surface of a silicon substrate rough using a dry etching method, in which the silicon substrate is dry-etched with silicon chips or other silicon substrates being arranged around it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for roughening a silicon substrate, and more particularly to a method for roughening a silicon substrate used for a solar cell or the like.

[0002]

2. Description of the Related Art A solar cell converts light energy such as sunlight incident on a surface into electric energy. Various attempts have hitherto been made to improve the conversion efficiency into electric energy. One of the techniques is a technique for reducing the reflection of light applied to the surface of the substrate. By reducing the reflection of light applied to the surface of the substrate, the conversion efficiency to electric energy can be increased.

[0003] Main solar cells are classified into crystalline, amorphous, and compound solar cells according to the type of materials used. Among them, most of them currently available in the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. The single-crystal silicon solar cell has the advantage that it is easy to increase the efficiency because the quality of the substrate is good, but has the disadvantage that the manufacturing cost of the substrate is large. On the other hand, polycrystalline silicon solar cells have the disadvantage that it is difficult to increase the efficiency due to poor substrate quality, but they have the merit that they can be manufactured at low cost. In recent years, a conversion efficiency of about 18% has been achieved at the research level due to the improvement in quality of polycrystalline silicon substrates and advances in cell technology.

On the other hand, polycrystalline silicon solar cells at the mass production level have been distributed on the market because of their low cost. In addition, higher conversion efficiency has been required.

When a solar cell element is formed using a silicon substrate, if the surface of the substrate is etched with an aqueous alkali solution such as sodium hydroxide, fine irregularities are formed on the surface of the substrate, and reflection of the surface of the substrate is reduced to some extent. Can be reduced.

When a single-crystal silicon substrate having a (100) plane orientation is used, a pyramid structure called a texture structure can be uniformly formed on the surface of the substrate by such a method. Is dependent on the plane orientation of the crystal. Therefore, when a solar cell element is formed on a polycrystalline silicon substrate, the pyramid structure cannot be formed uniformly, and therefore, there is a problem that the overall reflectance cannot be reduced effectively. .

In order to solve such a problem, when a solar cell element is formed on a polycrystalline silicon substrate, fine projections are formed on the surface of the substrate by reactive ion etching (Reactive ion etching).
It has been proposed to form by the Ion Etching method (for example, JP-B-60-27195, JP-A-5-7515).
No. 2, JP-A-9-102625). That is, fine projections are uniformly formed without being affected by the plane orientation of an irregular crystal in polycrystalline silicon, and even in a solar cell element using polycrystalline silicon, the reflectance is more effectively reduced. Is what you do.

However, the conditions for forming the irregularities are very delicate and vary depending on the structure of the device.
Consideration of conditions is often very difficult. If fine projections cannot be formed uniformly, the photovoltaic conversion efficiency of the solar cell will decrease and the value of each solar cell will be determined by its power generation efficiency. Must be improved.

A reactive ion etching apparatus used in the reactive ion etching method is generally of a parallel plate electrode type, in which an RF voltage is applied to an electrode on which a substrate is provided, and the other side is applied. And the inner side wall is connected to ground. The inside of the container is evacuated by a vacuum pump. After the evacuation is completed, an etching gas is introduced, and the substrate to be etched inside is etched while keeping the pressure constant. By taking such a procedure, the reactive ion etching apparatus has a long waiting time for evacuation and air leakage. In addition, the reactive ion etching apparatus is often used for precision small semiconductor elements such as LSIs, but when used for a solar cell, the area of the solar cell itself is large.
There is a problem that the number of processed sheets per operation is small and the cost is high. Therefore, when a reactive ion etching apparatus is used in a solar cell manufacturing process, it is also important how to perform the processing with a high tact.

For this purpose, an etching method using a residue formed on the surface by reactive ion etching as a mask has been studied (for example, Japanese Patent Application No. 2000-26).
No. 3023). This increases the unevenness forming speed,
There is a problem that no irregularities are formed around the substrate to be etched.

The present invention has been made in view of such problems of the prior art, and provides a method for efficiently and uniformly forming unevenness on the entire surface of a semiconductor substrate, particularly a silicon substrate used for a solar cell. The purpose is to.

[0012]

According to a first aspect of the present invention, there is provided a method for roughening a surface of a silicon substrate, wherein the surface of the silicon substrate is roughened by a dry etching method. In the method, a silicon piece or another silicon substrate is provided around the silicon substrate, and the silicon substrate is dry-etched.

In the method for roughening the surface of a silicon substrate, it is preferable that the dry etching method is a reactive ion etching method.

[0014] In the above method for roughening a silicon substrate,
It is preferable that the silicon piece or another silicon substrate is disposed around the silicon substrate at an interval of 5 mm or less, and the silicon substrate is dry-etched.

[0015]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing a solar cell formed using the method of the present invention. In FIG. 1, 1 is a silicon substrate, 1a is a surface uneven structure, 1b is a light-receiving surface side impurity diffusion layer, and 1c is a back surface side impurity diffusion layer (BS).
F), 1d denotes a front anti-reflection film, 1e denotes a front electrode, and 1f denotes a back electrode.

The silicon substrate 1 is a monocrystalline or polycrystalline silicon substrate. This substrate may be either p-type or n-type. In the case of single crystal silicon, it is formed by a pulling method or the like, and in the case of polycrystalline silicon, it is formed by a casting method or the like. Polycrystalline silicon can be mass-produced and is extremely advantageous over single crystal silicon in terms of manufacturing cost. An ingot formed by a pulling method or a casting method is sliced into a thickness of about 300 μm, and 10 cm × 1
The silicon substrate is cut into a size of about 0 cm or about 15 cm × 15 cm.

On the surface side of the silicon substrate 1, fine irregularities 1a are formed in order to effectively take in the incident light without reflecting it. This involves introducing a gas into a evacuated chamber, maintaining it at a constant pressure, and applying RF power to an electrode provided in the chamber to generate plasma and generate ions, which are active species generated. The surface of the substrate is etched by the action of radicals or the like.

This method, called the reactive ion etching (RIE) method, is shown in FIGS. 2 and 3, 2a is a mass flow controller, 2b is a silicon substrate, 2c is an RF electrode, 2d is a pressure regulator, 2e is a vacuum pump, and 2f is an RF power supply. Introducing an etching gas and a gas for generating an etching residue from the mass flow controller 2a into the apparatus, generating a plasma at the RF electrode 2c to excite and activate ions and radicals, and setting the silicon substrate on the RF electrode 2c. Etching is effected on the surface of 2b. In the apparatus shown in FIG. 2, the RF electrode 2c is installed in the apparatus and the surface of one silicon substrate 2b is etched. In the apparatus shown in FIG. Are simultaneously etched on the surface of the silicon substrate 2b.

Among the generated active species, a method that enhances the effect of ions acting on etching is generally called a reactive ion etching method. There is a similar method such as plasma etching, but the principle of plasma generation is basically the same, and the distribution of active species acting on the substrate is changed to a different distribution depending on the chamber structure, electrode structure, generation frequency, etc. I'm just there. Therefore, the present invention is effective not only in the reactive ion etching method but also widely in the plasma etching method in general.

In the present invention, for example, methane trifluoride (CH
F ₃ ) 20 sccm, chlorine (Cl ₂ ) 50 sccm,
Oxygen (O ₂ ) is 10 sccm, SF ₆ is 80 sccm, and H ₂ O is added thereto at a flow rate of 1 sccm.
Etch with W for about 3 minutes. Thereby, an uneven structure is formed on the surface of the silicon substrate. During the etching, the silicon is etched and basically vaporized, but a part thereof is not completely vaporized and the molecules are adsorbed and remain on the surface of the substrate as a residue.

If the conditions for forming the irregularities such as the gas conditions, the reaction pressure, and the RF power are set to such a condition that the silicon residue remains on the surface of the silicon substrate after the etching, the irregularities can be surely formed. However, the aspect ratio of the irregularities needs to be optimized depending on the conditions. Conversely, it is impossible to form irregularities under any conditions under the condition that no residue remains on the surface of the substrate.

Conventionally, when etching is performed by this method, there is a problem that unevenness is not sufficiently formed on the peripheral portion of the substrate to be etched (FIG. 4). Therefore, as a result of studying this problem, the present inventor has found that the cause is that the residue is not sufficiently adhered to this portion, and has accomplished the present invention.

In the present invention, 5 m around the substrate to be etched
This problem is solved by installing a silicon piece or another silicon substrate having substantially the same material as the substrate to be etched within m (FIG. 5). By doing so, it is possible to supplement the supply of the residue at the edge portion. In other words, conventionally, the silicon compound released when the substrate to be etched is etched is deposited again as a residue on the etching substrate, and this is used as a micromask to form irregularities. The edge of the etched substrate does not receive the residue sufficiently because there is no silicon substrate around,
As a result, the formation of unevenness was insufficient. Therefore, by disposing a silicon piece or a silicon substrate outside the edge portion, a residue deposited from a silicon compound generated by etching the silicon piece or the silicon substrate can be used. As a result, irregularities are sufficiently formed up to the edge of the substrate to be etched.

It is desirable that the distance between a silicon piece or a silicon substrate provided outside the edge portion and a target substrate to be etched be within 5 mm. When the distance is 5 mm or more, the supply of the residue becomes insufficient, and the formation of the unevenness becomes insufficient. Conversely, approaching is fine, ideally with no gaps at all.

The fine irregularities 1a have a conical shape or a continuous shape, and the size can be changed by controlling the gas concentration or the etching time by the RIE method. The width and height of the fine unevenness 1a are each formed to 2 μm or less. In order to form the fine unevenness 1a uniformly and accurately with controllability over the entire required portion of the silicon substrate 1, 1
μm or less is preferred. It is desirable that the aspect ratio (height / width of the unevenness 1a) of the fine unevenness 1a is 2 or less. When the aspect ratio is 2 or more, the fine unevenness 1a is damaged in the manufacturing process, and when a solar battery cell is formed, a leak current becomes large and good output characteristics cannot be obtained.

After forming the irregularities using a reactive ion etching apparatus or a similar plasma etching apparatus, the etching residue remaining on the surface of the silicon substrate is removed. Thereby, the characteristics of the solar cell to be manufactured can be improved. As a method for removing the etching residue, for example, ultrasonic waves are applied in a water tank after forming the unevenness by a reactive ion etching apparatus or a similar plasma etching apparatus and taking out the substrate. As a type of the ultrasonic wave applying device, the frequency of a main cleaning ultrasonic device which is usually commercially available is several tens kHz to several hundreds kHz, and the vibrator to be applied has various materials, shapes and outputs. There are types, but the type of this device can be selected according to the ease of surface residue removal. The ease of removing the residue varies depending on the shape and size of the irregularities, the remaining amount of the residue, the thickness of the substrate, and also varies depending on the frequency of the ultrasonic wave.
Even under conditions where it is relatively difficult to remove the residue, the residue can be removed by extending the application time.

On the surface side of the semiconductor substrate 1, a layer 1b in which a semiconductor impurity of the opposite conductivity type is diffused is formed. The layer 1b in which the opposite conductivity type semiconductor impurity is diffused is provided for forming a semiconductor junction in the silicon substrate 1. For example, when diffusing an n-type impurity, POCl _{3 is used.}
, A coating diffusion method using P ₂ O ₅ , and an ion implantation method for directly diffusing P ⁺ ions. The layer 1 containing the opposite conductivity type semiconductor impurity is formed at a depth of about 0.3 to 0.5 μm.

On the front side of the silicon substrate 1, an anti-reflection film 1d is formed. The anti-reflection film 1d is provided to prevent light from being reflected on the surface of the silicon substrate 1 and to effectively take light into the silicon substrate 1. The antireflection film is made of a material having a refractive index of about 2 in consideration of a refractive index difference from the silicon substrate 1 and the like, and has a thickness of 500 to
Silicon nitride film or silicon oxide (SiO
₂ ) It is composed of a film.

On the back surface of the silicon substrate 1, it is desirable to form a layer 1c in which one-conductivity-type semiconductor impurity is diffused at a high concentration. The layer 1c in which the one-conductivity-type semiconductor impurity is diffused at a high concentration forms an internal electric field on the back surface side of the silicon substrate 1 in order to prevent a decrease in efficiency due to carrier recombination near the back surface of the silicon substrate 1. is there.

That is, as a result of the carrier generated near the back surface of the silicon substrate 1 being accelerated by this electric field, power is effectively taken out, and in particular, the photosensitivity at long wavelengths is increased and the solar cell characteristics at high temperatures are reduced. Reduction can be reduced. As described above, the sheet resistance on the back side of the silicon substrate 1 on which the layer 1c in which the one-conductivity-type semiconductor impurity is diffused at a high concentration is about 15Ω / □.

On the front side and the back side of the silicon substrate 1, a front surface electrode 1e and a back surface electrode 1f are formed. The front electrode 1e and the back electrode 1f are mainly made of Ag powder,
An Ag paste composed of a binder, a frit, or the like is screen-printed and fired, and a solder layer is formed thereon. The surface electrode 1e has, for example, a plurality of finger electrodes (not shown) formed with a width of about 200 μm and a pitch of about 3 mm, and interconnects the plurality of finger electrodes.
It is composed of the busbar electrodes (1e). Back electrode 1f
Is composed of, for example, a large number of finger electrodes (not shown) having a width of about 300 μm and a pitch of about 5 mm, and two bus bar electrodes (1f) interconnecting the large number of finger electrodes.

[0032]

As described above, in the silicon substrate roughening method according to the present invention, a silicon piece or another silicon substrate is provided around the silicon substrate and the silicon substrate is dry-etched. When the surface is roughened by etching while attaching the etching residue to the surface of the substrate, it is possible to form unevenness uniformly in the plane.
Thereby, light absorption is improved up to the edge portion, and the characteristics of the solar cell are improved. In addition, since the in-plane uniformity is improved, a solar cell and a solar cell module with beautiful appearance can be manufactured.

[Brief description of the drawings]

FIG. 1 is a view showing an example in which a method for roughening a silicon substrate according to the present invention is applied to a method for manufacturing a solar cell.

FIG. 2 is a view showing an example of a reactive ion etching apparatus used for a silicon substrate roughening method according to the present invention.

FIG. 3 is a diagram showing another example of the reactive ion etching apparatus used for the silicon substrate roughening method according to the present invention.

FIG. 4 is a schematic diagram of a surface of a silicon substrate in a state where a portion having insufficient unevenness due to conventional unevenness formation exists.

FIG. 5 is a schematic view showing an installation state when a silicon piece for uniformly forming unevenness on a substrate to be etched for forming unevenness is installed.

DESCRIPTION OF SYMBOLS 1; silicon substrate, 1a; surface uneven structure, 1b; impurity diffusion layer, 1c; backside impurity diffusion layer, 1d;
1e; front electrode, 1f; back electrode, 2a; mass flow controller, 2b; silicon substrate, 2c;
2d; Pressure regulator, 2e; Vacuum pump, 2f; RF power supply, 4a ... part where etching is insufficient, 4b ... part where etching is uniformly formed, 5a ... .Substrate to be etched for forming irregularities, 5b..., Silicon pieces for uniformly forming irregularities on the substrate to be etched for forming irregularities

Claims (3)

[Claims] In a method of roughening a silicon substrate, wherein a surface of the silicon substrate is roughened by a dry etching method, a silicon piece or another silicon substrate is disposed around the silicon substrate to form the silicon substrate. A method for roughening a silicon substrate, characterized by dry etching. 2. The method of claim 1, wherein the dry etching method is a reactive ion etching method. 3. The silicon substrate according to claim 1, wherein the silicon piece or another silicon substrate is arranged around the silicon substrate at an interval of 5 mm or less, and the silicon substrate is dry-etched. Roughening method.