JP5901744B2 - Dry etching method - Google Patents

Description

  The present invention relates to a dry etching method for forming a texture structure on a silicon substrate surface, and more specifically, a texture that exhibits a high light scattering containment effect on the surface of a silicon substrate in a manufacturing process of a crystalline solar cell. It relates to what is used to form the structure.

  In a crystalline solar cell using a monocrystalline or polycrystalline silicon substrate, the silicon substrate surface is roughened by forming an uneven shape by dry etching (providing a texture structure), and is incident on the silicon substrate surface. 2. Description of the Related Art Conventionally, efforts have been made to improve photoelectric conversion efficiency by reducing light reflection. In addition, when a silicon substrate for a crystalline solar cell is obtained by slicing a silicon ingot, the process from removing the damaged layer of the silicon substrate that occurs at the time of slicing to the process of forming the texture structure is collectively processed by dry etching. This is known, for example, from Patent Document 1.

In the above-mentioned Patent Document 1, in order to remove the damaged layer on the surface of the silicon substrate generated at the time of slicing in the processing chamber of the dry etching apparatus, for example, oxygen gas and SF ₆ gas for removing the damaged layer are predetermined. Introduced at a flow rate, the stage holding the silicon substrate is powered from a high frequency power source, and the silicon substrate surface is dry etched to remove the damaged layer. Subsequently, for example, oxygen gas, texture forming Cl ₂ gas and NF ₃ gas are introduced into the processing chamber, power is supplied to the stage from a high-frequency power source, and the silicon substrate surface is dry etched to remove the damaged layer. A texture structure is formed on the surface of the silicon substrate.

  By the way, as described above, when a texture structure is formed on the surface of the silicon substrate by dry etching, reaction products generated during the etching are deposited on the surface of the etched silicon substrate. In addition, the silicon surface is repeatedly concave and convex, but has sharp edges with sharp peaks and valleys (that is, a saw blade shape when the cross-sectional shape is viewed). In such a case, when an antireflection film is formed on the silicon substrate surface in a later step using, for example, a vacuum film forming apparatus, the film is not efficiently formed on the top or valley, resulting in poor coverage, etc. Problems arise.

  For this reason, it is proposed to perform a round process in which the reaction product is removed and the top and valleys are rounded by wet etching using an etching solution such as a mixed solution of hydrofluoric acid and nitric acid. However, a separate wet etching facility is required for round processing, which not only results in poor productivity, but also necessitates treatment of waste liquid, resulting in high costs.

JP 2011-35262 A

  In view of the above points, the present invention effectively demonstrates the effect of confining the silicon substrate with a textured structure that effectively exhibits the light scattering containment effect and can be formed with good coverage even when a predetermined thin film is formed in a subsequent process. An object of the present invention is to provide a low-cost dry etching method that can be manufactured well.

  In order to solve the above problems, the present invention provides a dry etching method for forming a texture structure on a silicon substrate surface, wherein a fluorine-containing gas and a halogen-containing gas are placed in a film forming chamber under reduced pressure where the silicon substrate is disposed. A first step of introducing a first etching gas containing oxygen and oxygen gas, supplying electric power for discharge to etch the surface of the silicon substrate, and forming a film under reduced pressure in which the silicon substrate etched in the first step is disposed A second step of introducing a second etching gas containing a fluorine-containing gas into the chamber and supplying electric power for discharge to further etch the surface of the silicon substrate.

According to the present invention, a texture structure is formed on the silicon substrate surface in the first step. That is, for example, a fluorine-containing gas such as CF ₄ (flow rate ratio of 20 to 60%) and a halogen-containing gas such as a halogen gas such as Cl ₂ or a hydrogen halide gas such as HBr (flow rate ratio of 25 to 70%) Then, a first etching gas containing oxygen gas (flow rate ratio: 10 to 40%) is introduced into the processing chamber, and for example, high-frequency power is supplied to the substrate stage holding the silicon substrate in the processing chamber. As a result, plasma is formed in the processing chamber, and active species and ion species in the plasma enter the surface of the silicon substrate and etching proceeds. At this time, silicon oxide, hydrocarbon fluoride, or the like deposited on the substrate surface serves as a mask, so that the silicon surface is etched into an uneven shape and roughened to form a texture structure.

Next, for the texture structure formed on the silicon substrate surface in the second step, reaction products such as silicon oxide and hydrocarbon fluoride deposited on the silicon substrate surface during the etching in the first step are removed. The cleaning process is performed in a vacuum atmosphere. That is, for example, a second etching gas made of a fluorine-containing gas such as CF ₄ is introduced into the processing chamber, and high-frequency power is supplied to the substrate stage that holds the silicon substrate in the processing chamber. Thereby, the reaction product deposited on the silicon substrate surface by the active species and ion species in the plasma is removed. In this case, the wall surface (including the deposition prevention plate) of the vacuum chamber that defines the processing chamber is also cleaned. In this case, the etching gas may contain oxygen gas.

  As described above, in the present invention, the texture structure can be efficiently manufactured on the silicon substrate by performing dry etching to form the texture structure and to clean the surface of the silicon substrate after the first step. In addition, since wet etching is not used, high productivity can be achieved, and cost can be reduced.

In the present invention, in the film formation chamber under reduced pressure where the silicon substrate that has been etched in the second step is disposed, either a fluorine-containing gas or a halogen-containing gas is used as a main component, and oxygen gas is added thereto. It is preferable to further include a third step of further etching the silicon substrate surface by introducing 3 etching gas and supplying electric power for discharge. According to this, the round processing is continuously performed on the texture structure formed on the silicon substrate surface. In other words, for example, a third etching gas containing a fluorine-containing gas such as CF ₄ (flow rate ratio: 40 to 95%) and oxygen gas (flow rate ratio: 5 to 60%) is introduced into the processing chamber. High frequency power is applied to the substrate stage holding the silicon substrate. As a result, active species and ion species in the plasma are incident on the silicon substrate surface and etching proceeds, and the top and valley portions in the texture structure of the silicon substrate surface formed in the first step are rounded. In this case, the oxygen gas is added in order to obtain an optimum shape by controlling the etching rate by adjusting the flow rate of the oxygen gas. As a result, it is possible to perform dry processing up to the round processing on the texture structure, and it is possible to form a film with good coverage even when a predetermined thin film is formed in a subsequent process.

  In the present invention, the introduction of the halogen-containing gas and the oxygen gas in the first etching gas into the processing chamber is stopped without stopping the discharge power supply in the same processing chamber. It is preferable to perform the first step and the second step continuously by switching from the etching gas to the second etching gas. Also, in the same processing chamber, without stopping the supply of electric power for discharge, the introduction of oxygen gas is resumed, and the second etching gas is switched to the third etching gas to change the second and third steps. Is preferably carried out continuously. By performing batch dry etching in the same processing chamber, it is possible to further improve productivity and reduce costs.

  In the present invention, in the film forming chamber under reduced pressure on which the silicon substrate etched in the first step is disposed, either a fluorine-containing gas or a halogen-containing gas is used as a main component, and oxygen gas is added thereto. The etching gas is further introduced, and a third step of further etching the silicon substrate surface by supplying electric power for discharge is further included, and the silicon substrate surface etched in the third step is further etched in the second step. Also good. Also in this case, the texture structure can be efficiently manufactured on the silicon substrate by performing dry etching each of the formation of the texture structure, the round processing on the texture structure, and the cleaning of the surface of the silicon substrate after the third step. In addition, since wet etching is not used, high productivity can be achieved, and cost can be reduced.

  In the present invention, the introduction of either the fluorine-containing gas or the halogen-containing gas in the first etching gas into the processing chamber is stopped without stopping the discharge power supply in the same processing chamber. Then, it is preferable to perform the first step and the third step continuously by switching from the first etching gas to the third etching gas. In addition, it is preferable that the third process and the second process are continuously performed by switching from the third etching gas to the second etching gas without stopping the discharge power supply in the same processing chamber. . By performing batch dry etching in the same processing chamber, it is possible to further improve productivity and reduce costs.

  The introduction of oxygen gas in the first step and the third step is performed by a single gas introduction system having a flow rate control means, the oxygen flow rate ratio in the first step is in the range of 10 to 40%, and in the third step It is preferable to control the flow rate control means so that the oxygen flow rate ratio is in the range of 5 to 60%. Here, when the oxygen flow rate ratio in the first step (the flow rate ratio of the oxygen gas in the first etching gas in the total flow rate of the first etching gas introduced into the processing chamber) is outside the above range, a texture structure is formed. On the other hand, if the oxygen flow rate ratio in the third step is outside the above range, the etching rate is too fast or too slow and the etching shape cannot be controlled.

The schematic diagram which shows the structure of the dry etching apparatus which enforces the dry etching method of embodiment of this invention. (A) And (b) is the SEM photograph of the board | substrate obtained in Example 1 of this invention. (A)-(c) is the SEM photograph of the board | substrate obtained in Example 2 of this invention.

  Hereinafter, referring to the drawings, the object to be processed is a single crystal or polycrystalline silicon substrate (hereinafter simply referred to as a substrate W) used for a crystalline solar cell, and a texture structure is formed on the surface thereof. A dry etching method of the embodiment will be described. In addition, since the structure of a crystalline solar cell is well-known, detailed description is abbreviate | omitted here.

  FIG. 1 shows a dry etching apparatus EM that can perform the dry etching method of the present embodiment. In the following description, a direction from a shower plate, which will be described later, to the substrate W will be described as a lower side, and a direction from the substrate W to the shower plate will be described as an upper side.

  The dry etching apparatus EM includes a vacuum chamber 1 that can be held under reduced pressure to a predetermined degree of vacuum via a vacuum exhaust unit 11 that includes a rotary pump, a turbo molecular pump, and the like, and defines a film forming chamber 12. A substrate stage 2 is provided in the lower space of the film forming chamber 12. An output 31 from the high frequency power source 3 is connected to the substrate stage 2. A shower plate 4 is provided above the film forming chamber 12 so as to face the substrate stage 2. The shower plate 4 is held at the lower end of an annular support wall 13 protruding from the inner wall surface of the vacuum chamber 1, and a gas introduction for introducing an etching gas into a space 41 defined by the support wall 13 and the shower plate 4. A system 5 is provided.

The gas introduction system 5 includes a merging gas pipe 51 that communicates with the space 41. The merging gas pipe 51 includes gas pipes 53a, 53b in which flow control means 52a, 52b, 52c having a closing function such as a mass flow controller are interposed. 53c are respectively connected to the first to third gas sources 54a, 54b, 54c. Thus, the flow rate can be controlled for each gas type and introduced into the processing chamber 12. In the present embodiment, the gas of the first gas source 54a contains fluorine such as CF ₄ , NF ₃ , SF ₆ , and CxHyFz so that the first step, the second step, and the third step can be continuously performed. The gas of the second gas source 54b is made of a halogen-containing gas such as a halogen gas such as Cl ₂ or a hydrogen halide gas such as HBr, and the gas of the second gas source 54b is made of oxygen gas. Hereinafter, the etching method of the first embodiment using the dry etching apparatus EM will be specifically described.

First, in a state where the processing chamber 12 reaches a predetermined degree of vacuum (for example, 10 ⁻⁵ Pa), the substrate W is transported by a vacuum robot (not shown) and is held on the substrate stage 2. Next, as a first step, the first etching gas is supplied from the first to third gas sources 54 a, 54 b, 54 c through the flow rate control valves 52 a to 52 c of the gas introduction system 5 from the space 41 to the shower plate 4. And introduced into the processing chamber 12. The first etching gas is composed of CF ₄ as a fluorine-containing gas, Cl ₂ as a halogen-containing gas, and oxygen gas. The ratio is in the range of 20 to 60%, the flow rate ratio of the halogen-containing gas is in the range of 25 to 70%, and the flow rate ratio of the oxygen gas is in the range of 10 to 40% (in this case, the pressure in the processing chamber 12 under reduced pressure) 30 to 250 Pa). The flow ratio of each gas can be appropriately set within the above range according to the size of the processing chamber 12 and other process conditions (the same applies to the third step). At the same time, discharge power is supplied to the substrate stage 2 via the high frequency power source 3. The input power in this case is appropriately set so that the power density is 0.5 to 1.8 W / cm ² . As a result, a texture structure is formed on the surface of the substrate W in the first step. That is, plasma is formed in the processing chamber 12, and active species and ion species in the plasma are incident on the surface of the substrate W and etching proceeds. At this time, oxygen deposited on the surface of the substrate serves as a mask, so that the silicon surface is etched into a concavo-convex shape and roughened to form a texture structure.

After performing the etching in the first step for a predetermined time, a cleaning process is continuously performed as a second step. That is, without stopping the supply of electric power for discharge from the high frequency power source 3, the flow rate control means 52b is closed to stop the introduction of Cl ₂ as a halogen-containing gas into the processing chamber 12, and the flow rate control means. The valve 52c is closed to stop the introduction of the oxygen gas into the processing chamber 12, thereby switching from the first etching gas to the second etching gas. By this second step, reaction products (also referred to as “etching residues”) such as silicon oxide and hydrocarbon fluoride deposited on the substrate surface during the etching in the first step are removed. At the same time, the inner wall surface (including the deposition prevention plate) of the vacuum chamber 1 that defines the processing chamber 12 is also cleaned.

  After performing the etching in the second step for a predetermined time, the third step is continuously performed. That is, without stopping the supply of electric power for discharge from the high-frequency power source 3, the flow rate control means 52c is opened to restart the introduction of the oxygen gas into the processing chamber 12, and from the second etching gas to the third etching gas. Switch to etching gas. At this time, the flow rate ratio of the fluorine-containing gas to the total flow rate of the gas introduced into the processing chamber 12 is in the range of 40 to 95%, and the flow rate ratio of the oxygen gas is in the range of 5 to 60% (in this case, under reduced pressure). The pressure in the processing chamber 12 is 20 to 150 Pa).

  By this third step, round processing is performed on the texture structure formed on the surface of the substrate W. That is, the top and valleys in the texture structure of the silicon substrate surface formed in the first step are rounded by the active species and ion species in the plasma incident on the surface of the substrate W. In this case, the oxygen gas is added in order to obtain an optimum shape by controlling the etching rate by adjusting the flow rate of the oxygen gas. When the texture structure is covered with the reaction product in the third step, the second step may be performed again.

  By controlling the flow rate control means so that the oxygen flow rate ratio in the first step is in the range of 10 to 40% and the oxygen flow rate ratio in the third step is in the range of 5 to 60%, the texture structure in the first step Formation and the round process in a 3rd process can be performed efficiently. If the oxygen flow rate ratio in the first step (the flow rate ratio of oxygen gas in the total flow rate of the first etching gas introduced into the processing chamber) is outside the above range, there is a problem that a texture structure cannot be formed. When the oxygen flow rate ratio in the third step (the flow rate ratio of oxygen gas in the total flow rate of the third etching gas introduced into the processing chamber 12) is outside the above range, the etching rate is too fast or too slow, and etching is performed. There is a problem that the shape cannot be controlled.

  Next, an etching method according to the second embodiment using the dry etching apparatus EM will be described. The etching method of the second embodiment is the same as the etching method of the first embodiment except that the order of the second step and the third step is reversed. Here, the first process, the third process, and the second process are successively performed in this order in the same processing chamber by switching the etching gas without stopping the supply of the discharge power from the high-frequency power source 3. It is preferable. Since the conditions such as the gas flow rate in each step are the same as those in the first embodiment, detailed description thereof is omitted here. According to this embodiment, since the cleaning is performed last, it is possible to reliably prevent the round textured structure from being covered with the reaction product. On the other hand, since the reaction product formed on the substrate surface in the first step contains silicon, this reaction product is also etched during the round processing in the third step, so that a desired surface state is obtained in the third step. The processing time may be longer than that of the first embodiment. Note that either one of the first embodiment and the second embodiment may be appropriately selected in consideration of processes before and after the dry etching in the solar cell production line, various conditions of the dry etching, and the like. Good.

  According to the above, by performing the formation of the texture structure and the round processing on the texture structure with a single dry etching apparatus EM, the light scattering confinement effect is exhibited and a predetermined thin film is formed in the subsequent process. Even in this case, it is possible to efficiently manufacture a texture structure on the silicon substrate so that the film can be formed with good coverage. In addition, since wet etching is not used, high productivity can be achieved, and cost can be reduced. When slicing the silicon ingot to obtain the substrate W, the step of removing the damaged layer of the substrate W generated during slicing can be performed in the same processing chamber 12 prior to the first step. In this case, since the dry etching conditions described above can be used, detailed description thereof is omitted here.

Next, an example carried out using the dry etching apparatus EM shown in FIG. 1 will be described. In the first embodiment, a polycrystalline silicon substrate obtained by a known method is used as the substrate, and the first etching gas is CF ₄ , Cl _2, and oxygen gas as the conditions of the first step, and its CF ₄ : Cl ₂ The flow rate of oxygen gas is 300: 1000: 200 sccm (the flow rate ratio at this time is 20: 67: 13%), the pressure of the processing chamber 12 during etching is 60 Pa, the input power from the high-frequency power source 3 is 2 kW, The treatment was performed for 90 seconds. As conditions for the second step, the second etching gas is CF ₄ , the flow rate of CF ₄ is 300 sccm, the pressure in the processing chamber 12 during etching is 60 Pa, and the input power from the high-frequency power source 3 is 1.5 kW. The cleaning process was performed for 10 seconds. As conditions for the third step, the third etching gas is CF ₄ and oxygen gas, and the flow rate of CF ₄ : oxygen gas is 300: 50 sccm (the flow ratio at this time is 86: 14%). The chamber pressure was 60 Pa, the input power from the high-frequency power source 3 was 1.0 kW, and the treatment was performed for 10 seconds. FIG. 2A shows an SEM photograph of the substrate immediately after the second step is performed, and FIG. 2B shows an SEM photograph of the substrate immediately after the third step. This shows that the texture structure can be efficiently manufactured on the silicon substrate by dry etching.

In the second example, a polycrystalline silicon substrate obtained by a known method was used as the substrate, and the first step, the third step, and the second step were sequentially performed in this order. At this time, as conditions for the first step, the first etching gas is CF ₄ , Cl _2, and oxygen gas, and the flow rate ratio of CF ₄ : Cl ₂ : oxygen gas is 60: 30: 10%. The pressure in the processing chamber 12 was set in the range of 20 to 150 Pa, the input power from the high-frequency power source 3 was 29.5 kW, and the processing time was 75 seconds. As conditions for the third step, the third etching gas is CF ₄ and oxygen gas, the flow rate ratio of CF ₄ : oxygen gas is 90: 10%, and the pressure of the processing chamber during etching is in the range of 20 to 150 Pa. The input power from the high frequency power source 3 was 15 kW, and the processing time was 15 seconds. As conditions for the second step, the second etching gas is CF ₄ , the CF ₄ is introduced at a flow rate equivalent to that in the third step, and the pressure in the processing chamber 12 during etching is within the range of 20 to 150 Pa. The input power from the high-frequency power source 3 was 15 kW, and the processing time was 10 seconds. 3A shows the substrate immediately after the first step, FIG. 3B shows the substrate immediately after the third step, and FIG. 3C shows the substrate immediately after the second step. SEM photographs are shown respectively. According to this, the substrate surface is entirely covered with the reaction product after the first step, and the top and valleys of the texture structure are rounded after the second step, and the reaction product (lighted white in the photograph) is formed on the top. However, after the third step, the top reaction product was completely removed, and it was found that the texture structure can be efficiently produced on the silicon substrate by dry etching.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to said thing. In the above embodiment, the dry etching apparatus EM capable of performing the dry etching method of the present invention has been described by taking the case where power is supplied to the substrate stage 2 as an example. The dry etching method of the present invention, such as the dry etching apparatus used, can be widely applied to other types of dry etching apparatuses.

  In the above embodiment, the first step, the second step, and the third step are performed in a single dry etching processing apparatus EM. However, this can be performed separately, and further, the halogen-containing gas is added in the second step. Although the example of stopping is described as an example, the fluorine-containing gas may be stopped. In the above embodiment, the fluorine-containing gas, the halogen-containing gas, and the oxygen gas are used as separate gas sources. However, the mixed gas of the fluorine-containing gas and the oxygen gas, and the mixed gas of the halogen-containing gas and the oxygen gas are used. It is good also as a gas source.

  In the above embodiment, the case where the oxygen gas is stopped in the second process has been described as an example. However, even if the meteor control means 52c is controlled to reduce the introduction amount of the oxygen gas in the second process rather than the first process. Good. At this time, the flow rate ratio of the halogen-containing gas incorporated in the total flow rate of the gas introduced into the processing chamber 12 is set in the range of 60 to 90%, and the flow rate ratio of the oxygen gas is set in the range of 10 to 40% (in this case, the pressure is reduced). The pressure in the lower processing chamber 12 is 40 to 150 Pa).

  EM ... dry etching apparatus, 12 ... processing chamber, 2 ... substrate stage, 3 ... high frequency power source, 4 ... shower plate, 5 ... gas introduction system, 52a-52c ... mass flow controller (flow rate control means), 53a-53c ... gas pipe 54a to 54c (respectively for fluorine-containing gas, halogen-containing gas and oxygen gas), gas source, W ... substrate (silicon substrate).

Claims (4)

A dry etching method for forming a texture structure on a silicon substrate surface,
A first etching gas containing a fluorine-containing gas, a halogen-containing gas containing a halogen other than fluorine, and an oxygen gas is introduced into a film forming chamber under reduced pressure where a silicon substrate is disposed, and electric power for discharge is supplied to form silicon. A first step of etching the substrate surface into an irregular shape to form a textured structure ;
A deposition chamber under reduced pressure of arranging the etched silicon substrate in about the first factory, the main component either one of fluorine-containing gas and a halogen-containing gas, to which a second etching gas obtained by adding oxygen gas A second step of rounding the top and valleys in the texture structure formed in the first step by introducing a discharge power and further etching the silicon substrate surface;
A deposition chamber under reduced pressure of arranging the etched silicon substrate in the second step, by introducing a third etching gas containing fluorine-containing gas, further etching the silicon substrate surface discharge power by introducing And a third step of removing the reaction product deposited on the surface of the silicon substrate in the first step . In the same processing chamber, the introduction of either the fluorine-containing gas or the halogen-containing gas in the first etching gas into the processing chamber is stopped without stopping the charging power supply. the dry etching method according to claim 1 wherein the etching gas and the first step is switched to the second etching gas and a second step, characterized in that the continuously performed. In the same processing chamber, the second process and the third process are continuously performed by switching from the second etching gas to the third etching gas without stopping the charging power supply. The dry etching method according to claim 1 or 2 . The introduction of oxygen gas in the first step and the second step is performed by a single gas introduction system having a flow rate control means, the oxygen flow rate ratio in the first step is in the range of 10 to 40%, and the oxygen flow rate in the second step the dry etching method according to any one of claims 1 to 3, characterized by controlling the flow rate control means so that a ratio in the range of 5% to 60%.

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