JP2009088098A - Method of patterning dielectric film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of patterning a dielectric film which allows patterning and reduces or fully removes a residue after etching paste thermal treatment, by etching the dielectric film having such a thickness not capable of being etched in single etching paste processing through simple processing using the etching paste to form a patterned film. <P>SOLUTION: The method of patterning a dielectric film includes a step of forming a first dielectric film on a silicon substrate, a step of forming a second dielectric film on the first dielectric film, a step of partially exposing the first dielectric film by partially etching the second dielectric film with use of an etching paste, and a step of etching the exposed first dielectric film using an etching solution. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for patterning a dielectric film, and more particularly, to a method for easily patterning a dielectric film grown on a silicon substrate. The dielectric film patterning method of the present invention can be suitably applied to the semiconductor field, in particular, to the field related to solar cells.

  Dielectric films such as a silicon oxide film and a silicon nitride film are formed on a silicon substrate by various methods such as CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), spin coating, and thermal oxidation. Excellent function. Examples include an “anti-diffusion film” that prevents impurities from thermally diffusing into the silicon substrate, an “anti-reflection film” that prevents surface reflection using the interference of light, and the silicon substrate surface is electrically Examples thereof include a “passivation film” that is activated to prevent surface recombination of carriers. Such a function is a very important function for forming semiconductor devices, particularly solar cells.

  For example, in order to form a PN junction essential for a solar cell, it is necessary to diffuse an impurity having a conductivity type opposite to that of a single crystal or polycrystalline silicon substrate into a part of the surface of the silicon substrate. . Further, in order to form a BSF (Back Surface Field) in order to improve the performance of the solar cell, it is necessary to diffuse an impurity having the same conductivity type as that of the silicon substrate into a part of the surface of the silicon substrate. . In order to partially diffuse impurities in this way, it is necessary to form a “diffusion prevention film” that covers a part of the surface of the silicon substrate to prevent the diffusion of impurities into the silicon substrate.

  Moreover, the solar cell is characterized by the function of converting light energy into electrical energy. Therefore, in order to increase the conversion efficiency of the solar cell, it is necessary to have a function of taking as much incident light as possible into the silicon substrate and conversely not emitting the light taken into the silicon substrate to the outside of the silicon substrate. For example, on the surface of a silicon substrate having a flat surface shape, incident light having a wavelength of 600 nm is reflected by 30% or more, but only by forming a silicon oxide film having a thickness of about 110 nm on the “antireflection film”. And the reflectance can be suppressed to 10% or less. As a result, the conversion efficiency of the solar cell can be improved by nearly 30%.

  Further, in a solar cell, electric energy is obtained by absorbing incident light, generating electron and hole carrier pairs, moving each carrier to an N-type layer and a P-type layer, respectively, and taking them out from an external electrode. However, if the carrier recombines with the carrier of the opposite conductivity type before reaching the electrode, the carrier disappears and cannot be taken out as electric energy. The carrier recombination phenomenon may occur inside the silicon substrate, but most of the carrier recombination occurs on the silicon substrate surface if appropriate surface treatment is not performed. Therefore, it is important to form a “passivation film” that electrically inactivates the silicon substrate surface and prevents carrier recombination on the substrate surface. For example, by forming a silicon nitride film on a polycrystalline silicon substrate by plasma CVD, the hydrogen in the silicon nitride film terminates dangling bonds on the substrate surface and inside the substrate, thereby increasing the life of the carrier and cell characteristics. It is known that can be improved.

  Conventionally, when these dielectric films as anti-diffusion films, anti-reflection films, and passivation films are used for mass-produced terrestrial solar cells, precise patterning of the dielectric films is not always necessary. This is because, in a conventional mass-produced terrestrial solar cell, the impurity is diffused only on the entire surface on one side of the substrate, so that the dielectric film can be deposited on one side by plasma CVD before the impurity can be diffused. Because it is good. Further, since the antireflection film and the passivation film formed of a dielectric film are insulator films, they cannot be in electrical contact. However, an electrode paste containing glass frit and metal particles is screen-printed on the surface of the silicon substrate on which the dielectric film is formed, and then fired at a high temperature, so that the electrode material penetrates the insulator film and makes electrical contact. The obtained phenomenon, so-called fire-through, becomes possible. For this reason, in order to obtain contact with the electrode, a step of partially opening the dielectric film to expose the silicon substrate surface is not necessarily required.

  However, in recent years, in order to realize high efficiency of solar cells, there is an increasing need for impurity diffusion not on the entire surface of the substrate but on a part of the substrate surface. For example, by performing high-concentration impurity diffusion in the region immediately below the electrode on the light-receiving surface side of the solar cell, the electrical contact between the impurity diffusion layer on the silicon substrate surface and the electrode material is improved, and in other regions low purity There is a “selective emitter structure” that aims to improve the sensitivity of short wavelength light. In addition, there is a “comb-shaped back contact structure” in which P-type and N-type impurity diffusion regions are alternately provided only on the back side of the solar cell. Furthermore, when the fire-through to the dielectric film cannot be used due to the electrode material and the process temperature, it is necessary to partially pattern the dielectric film which is a diffusion prevention film, an antireflection film or a passivation film.

  Here, as a method of partially patterning the dielectric film, an etching method using a photolithography technique may be mentioned. However, when patterning of the anti-diffusion film, patterning of the antireflection film, and patterning of the passivation film are all performed by the photolithography technique, there is a problem that the manufacturing time per device is remarkably increased and the manufacturing cost is increased.

Therefore, an etching paste as disclosed in Patent Document 1 has been developed as a printable paste-like etching medium for etching silicon oxide and silicon nitride. The etching paste contains an etching component, a solvent, a thickener, and other additives, and after printing on the surface of the silicon substrate on which the dielectric film is formed using a printing technique such as screen printing or pad printing, By applying a heat treatment as necessary, the dielectric film can be etched to form a pattern. By using this etching paste, it becomes possible to pattern a silicon oxide film or a silicon nitride film in a short time and at a low cost.
Special table 2003-531807 gazette

  However, when patterning a dielectric film formed on the surface of a silicon substrate using an etching paste, the amount of etching components contained in the printed etching paste is finite. There is a limit to the thickness of the dielectric film that can be etched. For example, when a silicon oxide film formed by APCVD (Atmosphere Pressure Vapor Deposition) is patterned with an etching paste containing phosphoric acid as an etching component, it can be etched only to a thickness of about 250 nm by a single process. In order to etch a silicon oxide film having a thickness larger than that, it is necessary to print an etching paste again in the region once etched. In this case, a precise alignment technique is required, and the production process is increased, which causes a problem that the production cost increases.

  Moreover, the residue of the etching paste after etching also becomes a problem. As described above, since the etching paste contains a thickener and other additives as components, even if a heat treatment of about 300 ° C. is applied after printing, it does not burn and remains as a residue, and adheres to the silicon substrate surface. . Since this residue is difficult to remove by running water washing, a more complicated process is required to remove the residue, leading to an increase in production cost.

  The present invention is intended to solve the above-described problems, and an object of the present invention is to etch a dielectric film having a thickness that cannot be etched by a single etching paste process by a simple process using the etching paste. It is another object of the present invention to provide a dielectric film patterning method capable of forming a pattern and reducing or completely removing residues generated after heat treatment of an etching paste.

  The present invention includes a step of forming a first dielectric film on a silicon substrate, a step of forming a second dielectric film on the first dielectric film, and an etching paste. A step of partially exposing the first dielectric film by partially etching the second dielectric film, and a step of etching the exposed first dielectric film using an etchant. A method for patterning a dielectric film is provided.

  Here, it is preferable that the first dielectric film is mainly made of silicon oxide, and the second dielectric film is mainly made of silicon nitride. In this case, the etching solution is preferably an etching solution having an etching rate for silicon oxide higher than that for silicon nitride. As such an etchant, an etchant containing hydrofluoric acid can be given.

  It is also preferable that the first dielectric film is mainly made of silicon nitride, and the second dielectric film is mainly made of silicon oxide. In this case, the etching solution is preferably an etching solution having an etching rate for silicon nitride higher than that for silicon oxide. As such an etchant, an etchant containing phosphoric acid can be used.

  According to the present invention, even when a dielectric film that is too thick to be etched by a single etching paste process is patterned, etching and patterning are performed by a simple method without printing the etching paste in the same region again. be able to. In addition, after etching the second dielectric film with the etching paste, a residue of the etching paste may remain on the first dielectric film. However, in the step of etching the exposed first dielectric film, the residue Since etching can be performed at the same time, residues derived from the etching paste can be reduced or completely removed.

  Hereinafter, the present invention will be described in detail with reference to FIG. FIG. 1 is a schematic process diagram showing a preferred example of a method for patterning a dielectric film according to the present invention. In the present invention, first, the first dielectric film 102 is formed on the surface of the silicon substrate 101 (FIG. 1A). The first dielectric film 102 is preferably a silicon oxide film mainly made of silicon oxide or a silicon nitride film mainly made of silicon nitride. The silicon oxide film and the silicon nitride film preferably contain most of silicon oxide and silicon nitride, respectively, but may contain impurities. The silicon oxide film is preferably a silicon oxide film formed by CVD, PVD, spin coating, thermal oxidation, or the like, and the silicon nitride film is preferably a silicon nitride film formed by CVD, PVD, or the like.

  Here, the thickness of the first dielectric film 102 to be patterned is not particularly limited. According to the method of the present invention, the thickness exceeds 250 nm and cannot be etched by a single etching paste. Even if it exists, it can etch by a simple method.

  Next, as shown in FIG. 1B, a second dielectric film 103 is formed on the surface of the first dielectric film 102. When the first dielectric film 102 is a silicon oxide film, the second dielectric film 103 is a silicon nitride film, and when the first dielectric film 102 is a silicon nitride film, the second dielectric film The film 103 is a silicon oxide film. Similarly to the above, the silicon oxide film and the silicon nitride film preferably contain most of silicon oxide and silicon nitride, respectively, but may contain impurities. The silicon oxide film is preferably a silicon oxide film formed by CVD, PVD, spin coating, thermal oxidation, or the like, and the silicon nitride film is preferably a silicon nitride film formed by CVD, PVD, or the like.

  The thickness of the second dielectric film is preferably such that it can be etched by a single etching paste. Specifically, it is preferably 500 nm or less, and more preferably 250 nm or less.

  Next, as shown in FIG. 1C, an etching paste 104 is printed on the second dielectric film 103 by, for example, a screen printing method in accordance with a desired patterning shape. Here, as the etching paste 104, a conventionally known one can be adopted, but it is preferable to use a material containing phosphoric acid as an etching component and water, an organic solvent and a thickener as components other than the etching component. Can do. As the organic solvent, for example, at least one kind of alcohol such as ethylene glycol, ether such as ethylene glycol monobutyl ether, ester such as propylene carbonate, or ketone such as N-methyl-2-pyrrolidone can be used. Although organic solvents other than those described above can be used, it is preferable to select a solvent having a boiling point of about 200 ° C. and a viscosity change of the etching paste 104 that hardly occurs during printing. As the thickener, for example, at least one of cellulose, ethyl cellulose, cellulose derivatives, polyamide resin such as nylon 6 or polymer polymerized with vinyl group such as polyvinyl pyrrolidone can be used.

  Next, by heating the silicon substrate 101 on which the etching paste 104 is printed, the portion of the second dielectric film 103 on which the etching paste 104 is printed is etched as shown in FIG. The first dielectric film 102 is partially exposed to be removed. At this time, as shown in FIG. 1D, the residue 105 derived from the etching paste 104 is usually left on the exposed surface of the first dielectric film 102.

  Here, the heating temperature in the heat treatment is preferably 400 ° C. or less. If the heating temperature exceeds 400 ° C., the etching paste 104 may be burned and cannot be completely removed. In the case where the second dielectric film 103 is a silicon oxide film, the heating temperature in the heat treatment is preferably 300 ° C. or higher. When the heating temperature is less than 300 ° C., etching tends to be insufficient. When the second dielectric film 103 is a silicon nitride film, the heating temperature is preferably 200 ° C. or higher. When the heating temperature is less than 200 ° C., the etching tends to be insufficient.

  The treatment time in the heat treatment is preferably 30 seconds or more and 180 seconds or less. When the treatment time in the heat treatment is less than 30 seconds, there may be a portion that cannot be etched sufficiently even when the heating temperature is high. In addition, if the heating is performed for a long time, the etching paste 104 is denatured and it is difficult to remove after the heating. Therefore, the processing time in the heat treatment is preferably within a range not exceeding 180 seconds.

  Note that the etching rate when the etching paste 104 is printed on the silicon oxide film formed by atmospheric pressure CVD and then heat-treated at 300 ° C. is about 150 nm / min, and the silicon nitride film formed by plasma CVD. The etching rate when the etching paste 3 was printed and then heated at 300 ° C. was about 240 nm / min. When an etching paste whose etching component is phosphoric acid is used as the etching paste 104, it is difficult to react at room temperature, and phosphoric acid is difficult to vaporize even during heating. Fine etching with a high aspect ratio close to etching performed using a lithography process becomes possible.

  The method for the heat treatment is not particularly limited, and for example, it can be performed by heating using a hot plate, a belt furnace or an oven. As described above, when an etching paste whose etching component is phosphoric acid is used as the etching paste 104, phosphoric acid is difficult to vaporize, so that there is little fear of corroding the apparatus, and a belt furnace or oven can be used. In particular, heating by a belt furnace or an oven is preferable in that a temperature difference hardly occurs between the peripheral portion and the central portion of the silicon substrate 101 and variation in etching can be suppressed.

  After the heat treatment, the residue 105 derived from the etching paste 104 after the heat treatment may be removed by immersing the silicon substrate 101 in water, performing ultrasonic cleaning, and cleaning with running water. In addition to cleaning with running water, RCA cleaning, cleaning with a mixed solution of sulfuric acid and hydrogen peroxide, and cleaning with a diluted hydrogen fluoride aqueous solution or a cleaning solution containing a surfactant may be performed. Although most of the residue can be removed by such washing, some residue may remain.

  In this step, the second dielectric film 103 patterned using an etching paste functions as a mask in etching the first dielectric film 102 in the next step. The mask can be easily manufactured using an etching paste as compared with a patterning method using, for example, a photoresist. Therefore, according to the method of the present invention, in manufacturing a device in which a dielectric film is partially formed on a silicon substrate, the manufacturing process and the manufacturing time can be shortened.

  Next, the silicon substrate 101 is immersed in an etching solution to remove the exposed first dielectric film 102, that is, the first dielectric film 102 immediately below the region where the second dielectric film 103 is etched ( FIG. 1 (e)). Here, the etching solution is preferably an etching solution having a property that the etching rate with respect to the first dielectric film 102 is larger than the etching rate with respect to the second dielectric film 103, and the difference in the etching rate is more Larger is preferable. Specifically, in the case where the first dielectric film 102 is a silicon oxide film and the second dielectric film 103 is a silicon nitride film, the etching solution preferably contains hydrofluoric acid. In the case where the first dielectric film 102 is a silicon nitride film and the second dielectric film 103 is a silicon oxide film, the etching solution preferably contains phosphoric acid, and more preferably contains hot phosphoric acid. desirable.

  By this step, the residue 105 derived from the etching paste 104 remaining on the first dielectric film 102 is reduced or completely removed together with the etching of the first dielectric film 102. According to the method of the present invention, since the residue 105 of the etching paste 104 remaining on the first dielectric film 102 is removed by etching the first dielectric film 102, cleaning / cleaning such as ultrasonic cleaning is performed. The residue can be easily reduced or completely removed without performing a removal operation.

  Finally, the second dielectric film 103 is removed (see FIG. 1F). The removal of the second dielectric film 103 can be performed, for example, by immersing the silicon substrate 101 in an etching solution and etching it. The etchant for etching the second dielectric film 103 is an etchant having a property that the etching rate for the second dielectric film 103 is higher than the etching rate for the first dielectric film 102. It is preferable that the difference in the etching rate is larger. Specifically, when the first dielectric film 102 is a silicon oxide film and the second dielectric film 103 is a silicon nitride film, the etching solution preferably contains phosphoric acid, and particularly contains hot phosphoric acid. It is more desirable. In the case where the first dielectric film 102 is a silicon nitride film and the second dielectric film 103 is a silicon oxide film, the etchant preferably contains hydrofluoric acid. Since the silicon substrate surface is exposed in a desired region at the stage shown in FIG. 1E, the step of removing the second dielectric film 103 may be omitted.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these.

<Example 1>
First, N-methyl-2-pyrrolidone, nylon 6 and a phosphoric acid aqueous solution (the concentration of phosphoric acid is 85% by mass), N-methyl-2-pyrrolidone: nylon 6: phosphoric acid aqueous solution = 4: 3: 3. An etching paste containing phosphoric acid as an etching component was prepared by mixing at a mass ratio of 2 and sufficiently stirring. The viscosity of this etching paste was 25 Pa · s. Next, a p-type silicon polycrystalline substrate having a width of 125 mm, a length of 125 mm, and a thickness of 200 μm was prepared, and a silicon oxide film having a thickness of 500 nm was formed on one surface of the silicon substrate. Thereafter, a silicon nitride film with a thickness of 80 nm was formed on the silicon oxide film. And the etching paste produced above was printed on a part of the silicon nitride film on the silicon substrate by the screen printing method.

  Subsequently, the silicon substrate on which the above-mentioned etching paste is printed is heated by heating in a belt furnace at 300 ° C. for 60 seconds, and the etching paste is printed out of the silicon nitride film on the silicon substrate. The part was etched. After the heat treatment, the silicon substrate is immersed in water, ultrasonically applied and subjected to ultrasonic cleaning for 5 minutes, and then the surface of the silicon substrate is washed with running water for 5 minutes to perform running water etching, thereby etching after the heat treatment. Most of the paste residue was removed to expose a portion of the silicon oxide film on the silicon substrate.

  Subsequently, the silicon substrate was immersed in hydrofluoric acid having a concentration of 10% by mass for 2 minutes to remove the exposed silicon oxide film and expose a part of the silicon substrate. Finally, the silicon nitride film was removed by immersing the silicon substrate in 85% by mass of hot phosphoric acid (temperature 170 ° C.) for 20 minutes.

<Example 2>
First, N-methyl-2-pyrrolidone, nylon 6 and a phosphoric acid aqueous solution (the concentration of phosphoric acid is 85% by mass), N-methyl-2-pyrrolidone: nylon 6: phosphoric acid aqueous solution = 4: 3: 3. An etching paste containing phosphoric acid as an etching component was prepared by mixing at a mass ratio of 2 and sufficiently stirring. The viscosity of this etching paste was 25 Pa · s. Next, a p-type silicon polycrystalline substrate having a width of 125 mm, a length of 125 mm, and a thickness of 200 μm was prepared, and a silicon nitride film having a thickness of 250 nm was formed on one surface of the silicon substrate. Thereafter, a silicon oxide film having a thickness of 200 nm was formed over the silicon nitride film. Then, the etching paste prepared above was printed on a part of the silicon oxide film on the silicon substrate by a screen printing method.

  Subsequently, the silicon substrate on which the above-mentioned etching paste is printed is heated in a belt furnace by heating at 300 ° C. for 60 seconds, and a portion of the silicon oxide film on the silicon substrate where the etching paste is printed Was etched. After the heat treatment, the silicon substrate is immersed in water, ultrasonically applied and subjected to ultrasonic cleaning for 5 minutes, and then the surface of the silicon substrate is washed with running water for 5 minutes to perform running water etching, thereby etching after the heat treatment. Most of the paste residue was removed to expose a portion of the silicon nitride film on the silicon substrate.

  Subsequently, the silicon substrate was immersed in hot phosphoric acid having a concentration of 85% by mass (temperature: 170 ° C.) for 60 minutes, thereby removing the silicon nitride film at the exposed portion and exposing a part of the silicon substrate. Finally, the silicon oxide film was removed by immersing the silicon substrate in hydrofluoric acid having a concentration of 10% by weight for 1 minute.

<Comparative Example 1>
The conventional patterning of the dielectric film is, for example, as follows with reference to FIG. In this comparative example, it is assumed that the dielectric film is thick enough that it cannot be etched by a single etching paste treatment. First, as shown in FIG. 2A, a dielectric film 202 such as a silicon oxide film or a silicon nitride film is formed on the surface of the silicon substrate 201. Next, as shown in FIG. 2B, the first etching paste 203 is printed on the dielectric film 202 by, for example, a screen printing method. Next, the silicon substrate 201 after the first etching paste 203 is printed is subjected to a heat treatment to etch a part of the dielectric film 202 on which the etching paste 203 is printed, as shown in FIG. Remove. At this time, the residue 204 derived from the etching paste 203 remains on the etching surface.

  Next, as shown in FIG. 2D, the second etching paste 205 is printed on the part of the dielectric film 202 that has been partially etched. Next, after performing the heat treatment of the second etching paste 205, the silicon substrate 201 is cleaned and the second etching paste 205 is removed (see FIG. 2E). Through the above steps, the remaining portion of the dielectric film 202 can be removed, and an opening 206 that partially exposes the surface of the silicon substrate 201 can be provided. However, since the residue 207 after baking the second etching paste 205 is difficult to remove, there is a possibility that it remains in the opening 206 of the silicon substrate 201 even after cleaning.

<Comparative example 2>
The dielectric film was patterned in the same manner as in Comparative Example 1 except that the silicon oxide film as the dielectric film had a thickness of 100 nm and was etched with a single etching paste. The etching paste having the same composition as in Example 1 was used, and the heating time and heating temperature were the same as in Example 2.

  FIG. 3 is an electron micrograph showing the results of patterning in Example 1 and Comparative Example 2. FIG. 3 (a) is that of Example 1, and FIG. 3 (b) is that of Comparative Example 2. From these photographs, it can be seen that in Example 1, the thickness of the oxide film (silicon oxide film) is five times that in Comparative Example 2, but the patterning shapes of both are the same.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  According to the present invention, it is possible to provide a method for manufacturing a selective emitter structure solar cell and a comb-shaped back contact structure solar cell exhibiting high conversion efficiency at a low cost by simplifying a patterning method of a thick dielectric film. .

It is a schematic process drawing which shows a preferable example of the patterning method of the dielectric film which concerns on this invention. It is a schematic process drawing which shows an example of the patterning method of the conventional dielectric film. It is an optical microscope photograph which shows the result of the patterning of Example 1 and Comparative Example 2.

Explanation of symbols

  101 silicon substrate, 102 first dielectric film, 103 second dielectric film, 104 etching paste, 105 residue.

Claims (7)

Forming a first dielectric film on the silicon substrate;
Forming a second dielectric film on the first dielectric film;
Partially exposing the first dielectric film by partially etching the second dielectric film using an etching paste; and
Etching the exposed first dielectric film using an etchant;
A method for patterning a dielectric film comprising:   2. The dielectric film patterning method according to claim 1, wherein the first dielectric film is mainly made of silicon oxide, and the second dielectric film is mainly made of silicon nitride.   The dielectric film patterning method according to claim 2, wherein the etchant is an etchant having an etching rate for silicon oxide higher than an etching rate for silicon nitride.   The dielectric film patterning method according to claim 3, wherein the etchant contains hydrofluoric acid.   2. The dielectric film patterning method according to claim 1, wherein the first dielectric film is mainly made of silicon nitride, and the second dielectric film is mainly made of silicon oxide.   6. The method of patterning a dielectric film according to claim 5, wherein the etchant is an etchant having an etching rate for silicon nitride higher than that for silicon oxide.   The dielectric film patterning method according to claim 6, wherein the etching solution contains phosphoric acid.