JP2006332509A - Method for roughening surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for roughening the surface of a base body efficiently forming irregularities on the whole surface of the base body or a base body group, particularly a silicon board used for a solar cell uniformly. <P>SOLUTION: In the method for roughening the surface, the surface of the base body or the base body group is roughened by a dry etching method. The method for roughening the surface contains a first process in which a member composed of the same component as the base body is fitted around the base body or the base body group, and the upper end of the member is made higher than the surface of the base body or the base body group. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a surface roughening method, and more particularly to a surface roughening method in which a positional deviation of a substrate is controlled.

  A solar cell converts light energy such as sunlight incident on a surface into electric energy. Various attempts have been made to improve the conversion efficiency into electric energy. One of them is a technique for reducing the reflection of the light irradiated on the surface of the substrate, and the conversion efficiency to electric energy can be increased by reducing the reflection of the light irradiated on the surface of the substrate.

  Major solar cells are classified into crystalline, amorphous, and compound types depending on the type of materials used. Of these, most of the crystalline silicon solar cells currently on the market are in the market. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single crystal silicon solar cells have the advantage that the substrate quality is good, and thus it is easy to increase the efficiency, but the substrate manufacturing cost is high. On the other hand, polycrystalline silicon solar cells have the merit that they can be manufactured at a low cost, although the substrate quality is inferior and it is difficult to increase the efficiency. In recent years, conversion efficiency of about 18% has been achieved at the research level due to the improvement of the quality of the polycrystalline silicon substrate and the advancement of cell technology.

  On the other hand, mass-produced polycrystalline silicon solar cells have been distributed to the market because of their low cost, but in recent years, demand has increased further as environmental issues have been addressed. Higher conversion efficiency has been demanded.

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

  For example, 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. In the case of forming a solar cell element, since etching with an alkaline aqueous solution depends on the crystal plane orientation, the pyramid structure cannot be formed uniformly, and the overall reflectance cannot be effectively reduced.

  In order to solve such problems, it has been proposed that when a solar cell element is formed of a polycrystalline silicon substrate, a fine protrusion is formed on the surface of the substrate by a reactive ion etching (Reactive Ion Etching) method. (For example, refer to Patent Document 1). In other words, the texture structure, which is a fine protrusion, is uniformly formed on the surface of the polycrystalline silicon, regardless of the plane orientation of the irregular crystal of polycrystalline silicon. It is intended to reduce the rate more effectively.

  However, the conditions for forming the irregularities are very delicate, and change depending on the structure of the apparatus, so it is often very difficult to examine the conditions. When fine projections cannot be formed uniformly on the silicon surface, the photoelectric conversion efficiency of the solar cell is lowered. That is, in order to improve the characteristics of the solar cell element, it is desirable to uniformly form fine protrusions on the silicon surface to improve the conversion efficiency of the solar cell.

  Here, the reactive ion etching apparatus used in the reactive ion etching method is generally a parallel plate electrode type, and an RF voltage is applied to the side of the electrode on which the substrate is placed, and the other side and the inside The side wall is connected to ground. The inside of the container is evacuated by a vacuum pump, and after the evacuation is completed, an etching gas is introduced, and the substrate to be etched is etched while keeping the pressure constant. Since such a procedure is followed, the reactive ion etching apparatus has a long waiting time for evacuation and air leakage. Reactive ion etching apparatuses are often used for precision small semiconductor elements such as LSIs, but when used for solar cells, the area of the solar cell itself is large, so the number of treatments per process is small and the cost is low. There was a problem that became high. Therefore, when a reactive ion etching apparatus is used in a solar cell manufacturing process, it is an important point how high-tact processing is performed.

  Until now, an etching method using a residue formed on the surface by reactive ion etching as a mask has been studied (for example, see Patent Document 2). This increases the unevenness formation speed, but there is a problem that unevenness is not formed around the substrate to be etched.

  FIG. 7 shows a top view of a silicon surface that has undergone a conventional etching process. In FIG. 7, 101 is a silicon substrate, 104a is a portion where etching is insufficient, and 104b is a portion where etching is uniformly formed. In the insufficiently etched portion 104a, a sufficient silicon compound (residue) is not generated during the etching process, resulting in insufficient etching as compared with the center of the silicon substrate.

Therefore, by arranging dummy silicon pieces made of the same material as the substrate around the silicon substrate, irregularities are also formed around the silicon substrate. (For example, see Patent Document 3)
JP-A-9-102625 JP 2002-76404 A JP 2002-329710 A

  However, in this method, the silicon substrate to be etched needs to be disposed close to the dummy silicon piece to 5 mm or less. In this way, it is relatively easy to arrange the substrate to be etched close to 5 mm or less when the tray or the apparatus is small, but it becomes difficult when the tray becomes large, so positioning accuracy is high. Importantly, when the distance between the silicon substrate and the silicon piece is long, there is a region where etching of the silicon substrate is insufficient, and unevenness cannot be formed uniformly on the substrate surface.

  The present invention has been made in view of such problems of the prior art, and provides a method of roughening a substrate or substrate group in which irregularities are efficiently and uniformly formed on the entire surface of the substrate or substrate group. With the goal.

  In order to achieve the above object, in the roughening method for a substrate or group of substrates according to claim 1, in the roughening method for roughening the surface of the substrate or group of substrates by dry etching, the substrate or substrate A member made of the same component as the base is provided around the group, and the first step is made such that the upper end of the member is higher than the surface of the base or the base group.

  A roughening method for a substrate or a group of substrates according to claim 2 is a roughening method for a substrate or a group of substrates according to claim 1, wherein the substrate or the substrate group is placed in the first step. The plate has a groove around the plate, and the member is fixed to the groove.

  Further, a roughening method for a substrate or a group of substrates according to claim 3 is a roughening method for a substrate or a group of substrates according to claim 1, wherein the substrate or the substrate group is placed in the first step. The formed plate is provided with a recess in which the base body or the base body group is juxtaposed.

  As described above, the roughening method for a substrate according to the present invention is a roughening method in which the surface of a substrate or a group of substrates is roughened by a dry etching method. And a first step in which the upper end portion of the member is positioned higher than the surface of the base body or the base group. As a result, by providing the upper end portion of the member at a position higher than the surface of the substrate, the member plays a role of positioning, so that the substrate can be easily placed near the member, and the member has the same component as the substrate. Therefore, it becomes possible to adhere a uniform etching residue to the surface of the substrate. In addition, it is possible to suppress the positional deviation of the substrate. As a result, the surface of the substrate or the substrate group can be uniformly roughened.

  Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the text, a silicon substrate will be described as an example of a substrate for convenience.

  FIG. 1 is a view showing a solar cell element formed by using the method of the present invention. In FIG. 1, 1 is a silicon substrate, 1a is an uneven surface structure, 1b is a front side impurity diffusion layer, 1c is a back side impurity diffusion layer (BSF), 1d is an antireflection film, 1e is a front electrode, and 1f is a back electrode. ing.

  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 monocrystalline 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 cut into a size of about 10 cm × 10 cm or 15 cm × 15 cm and sliced to a thickness of about 300 μm to form the silicon substrate 1.

  On the surface side of the silicon substrate 1, a fine surface uneven structure 1a is formed in order to effectively capture incident light without reflecting it. This is because a gas is introduced into a vacuumed chamber, held at a constant pressure, and plasma is generated by applying RF power to an electrode provided in the chamber. The surface of the substrate is etched by the action of radicals or the like.

  2 and 3 show the structure of a reactive ion etching (RIE) method, which is one of the dry etching methods according to the present invention. 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 source. An etching gas and etching residue generation gas are introduced into the apparatus from the mass flow controller 2a, and plasma is generated by the RF electrode 2c to excite and activate ions and radicals, and a silicon substrate installed on the RF electrode 2c. Etching by acting 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. However, like the apparatus shown in FIG. 3, the RF electrode 2c is installed on the outer wall of the apparatus. You may etch the surface of the several silicon substrate 2b simultaneously.

  Of the generated active species, a method that increases the effect of ions on etching is generally called a reactive ion etching method. Plasma etching is a similar method, but the principle of plasma generation is basically the same, and the distribution of the active species acting on the substrate is changed to a different distribution depending on the chamber structure, electrode structure, generation frequency, etc. There is only. Therefore, the present invention is effective not only for the reactive ion etching method but also for a wide range of plasma processing methods such as plasma CVD.

In the present invention, for example, methane trifluoride (CHF ₃₎ 20 sccm, chlorine (Cl ₂₎ of 50 sccm, oxygen _{(O 2)} of 10 sccm, 80 sccm of SF _6, while passing further 1sccm of H ₂ O in addition to these, Etching is performed at a reaction pressure of 7 Pa and RF power of 500 W for generating plasma for about 3 minutes. Thereby, an uneven structure is formed on the surface of the silicon substrate. During etching, silicon is etched and basically vaporized, but some molecules are not vaporized and molecules are adsorbed and remain on the surface of the substrate as a residue.

  Further, when the unevenness forming conditions such as gas conditions, reaction pressure, and RF power are set such that a silicon residue remains on the surface of the silicon substrate after etching, the unevenness can be reliably formed. However, the aspect ratio of the irregularities needs to be optimized depending on conditions. On the contrary, it is impossible to form irregularities under any conditions under which no residue remains on the surface of the substrate.

  In the present invention, in a roughening method in which the surface of a substrate or a group of substrates is roughened by a dry etching method, a member made of the same component as the substrate is provided around the substrate or the group of substrates, and The first step was made such that the upper end of the member was higher than the surface.

  In this way, by arranging the upper end portion of the member made of the same component as the base at a position higher than the surface of the base or the base group, the member plays a role of positioning, and the base can be easily placed near the member. In addition, since the member has the same component as the base, the etching residue can be attached to the outer peripheral portion of the base. Furthermore, since the upper end portion of the member is located higher than the surface of the base, the member serves as a guide, and it is possible to suppress the positional deviation of the base, and there are other substrates and members in the vicinity of the base. It will be. As a result, the surface of the substrate or the substrate group can be uniformly roughened. Further, if the distance between the upper end of the member and the surface of the base is 5 mm or less, the surface of the base or the base group can be more uniformly roughened.

  FIG. 4 shows an example of the roughening method according to the present invention. In FIG. 4, 5a indicates a substrate and 5b indicates a member. As shown in FIG. 4B, by making the thickness of the member 5b thicker than that of the base 5a, the upper end of the member is positioned higher than the surface of the base 5a or the base group, so that the member 5b Can play a role of positioning. Further, by using such a member, the weight of the member 5b is increased as compared with the prior art, so that the member 5b is prevented from moving due to the influence of vibration or the like generated during conveyance. Furthermore, since the upper end portion of the member is at a position higher than the surface of the base body 5a, the position shift of the base body 5a can be prevented by the member 5b. As a result, the surface of the base body 5a or the base group can be uniformly roughened. it can. Further, the member 5b need not be particularly fixed with an adhesive or the like.

  The substrate 5a is not limited to a silicon substrate for solar cells, and can be appropriately applied to, for example, a glass substrate.

  FIG. 5 shows another example of the roughening method according to the present invention. In FIG. 5, 5a is a base, 5b is a member, 6 is a plate, and 7 is a groove. In the first step described above, the plate 6 is disposed on the RF electrode 2c, and the groove 5 is provided around the base 6a or the plate 6 on which the base group is placed, and the member 5b is fixed to the groove 7. preferable.

  By providing the groove 5 with the member 5b, the member 5b can be reliably fixed, so that the positioning effect by the member 5b is high, and a great effect is also obtained with respect to the positional deviation of the base body 5a.

  The plate 6 is made of a glass-based material or aluminum. The processing method of the plate 6 is not particularly limited, but when the plate 6 is made of metal, the groove portion 7 may be formed by pressing, or when the plate 6 is made of other materials, the groove portion 7 is formed by cutting. It doesn't matter. Furthermore, another member provided with the groove 7 may be produced and the other member may be fixed to the plate 6.

  FIG. 6 shows another example of the roughening method according to the present invention. In FIG. 6, 5a is a base, 5b is a member, 6 is a plate, and 8 is a recess. As shown in FIG. 6, it is preferable to provide the plate 6 with a recess 8 in which the base body 5a or the base group is arranged. By arranging the base body 5a or the base group in parallel in the recess 8, the upper end of the member is at a position higher than the surface of the base body 5a or the base group, so that the member 5b can play a positioning role. A great effect can be obtained even with respect to the displacement of 5a.

  When the base 5a or the base group is arranged in the recess 8 as shown in FIG. 6, the member 5b arranged around the base 5a may be a silicon piece or a silicon block thicker than the thickness of the silicon substrate. . Furthermore, by performing thermal spraying of silicon, silicon adheres to the plate 6, so that the member 5b can be securely fixed, and the positioning effect is enhanced.

  Fine irregularities 1a are formed on the surface of the substrate 5a or the substrate group after the first step. The fine irregularities 1a have a conical shape or a continuous shape, and the size can be changed by controlling the gas concentration or etching time by the RIE method. The width and height of the fine irregularities 1a are each set to 2 μm or less. In order to form the fine irregularities 1a uniformly and accurately over the entire necessary portion of the silicon substrate 1, the thickness is preferably 1 μm or less. The aspect ratio of the fine irregularities 1a (height / width of the irregularities 1a) is desirably 2 or less. When this aspect ratio is 2 or more, fine irregularities 1a are damaged in the manufacturing process, and when a solar battery cell is formed, a leak current increases and good output characteristics cannot be obtained.

  Then, after forming irregularities on the surface of the substrate 5a or a group of substrates using a reactive ion etching device or a similar plasma etching device, the etching residue remaining on the surface of the substrate 5a is removed. Furthermore, the characteristics of the solar cell can be improved by having the concavo-convex structure from which the formed residue is removed. As a method for removing the etching residue, for example, an unevenness is formed by a reactive ion etching apparatus or a similar plasma etching apparatus and the substrate 5a is taken out, and then ultrasonic waves are applied in the water tank. As the types of apparatuses for applying ultrasonic waves, the frequencies of main commercially available ultrasonic cleaning apparatuses are several tens of kHz to several hundreds of kHz, and the vibrators to be applied have various materials, shapes, outputs, etc. Although there are types, the type of this device can be selected depending on the ease of removing the residue on the surface. The ease of residue removal varies depending on the shape and size of the unevenness, the remaining amount of residue, the thickness of the substrate 5a, and the like, and also varies depending on the frequency of the ultrasonic wave. However, the residue removal is a relatively difficult condition. However, the residue can be removed by lengthening the application time.

Further, on the surface side of the silicon substrate 1, a surface side impurity diffusion layer 1b in which a reverse conductivity type semiconductor impurity is diffused is formed. The surface-side impurity diffusion layer 1b is provided to form a semiconductor junction in the silicon substrate 1. For example, when n-type impurities are diffused, a vapor phase diffusion method using POCl ₃ , P ₂ O ₅ and the ion implantation method for directly diffusing p ⁺ ions. The surface side impurity diffusion layer 1b is formed to a depth of about 0.3 to 0.5 μm.

An antireflection film 1 d is formed on the surface side of the silicon substrate 1. The antireflection film 1 d is provided to prevent light from being reflected from the surface of the silicon substrate 1 and to effectively take light into the silicon substrate 1. The antireflection film 1d is made of a material having a refractive index of about 2 in consideration of the refractive index difference with the silicon substrate 1, and is a silicon nitride film or a silicon oxide (SiO ₂ ) film having a thickness of about 500 to 2000 mm. Consists of.

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

  A front surface electrode 1 e and a back surface electrode 1 f are formed on the front surface side and the back surface side of the silicon substrate 1. The front electrode 1e and the back electrode 1f are formed by screen-printing and baking an Ag paste mainly composed of Ag powder, binder, frit and the like.

  The embodiments of the present invention are not limited to the above-described examples, and various modifications can be made without departing from the scope of the present invention. FIG. 8 is a structural view showing a cross section of another embodiment according to the present invention. In FIG. 8, 5a is a base, 5b is a member, 6 is a plate, and 9 is a pad. In the above-described example, the base 5a is placed on the flat plate 6. However, as shown in FIG. 8, a base-fixing pad 9 may be placed under the base 5a. By doing so, it is possible to prevent the base body 5a from being displaced due to the influence of vibration or the like generated during conveyance, and the surface of the base body 5a can be uniformly roughened.

  9 and 10 are views in which the base according to the present invention is placed obliquely. In FIG. 9, 5a is a base, 5b is a member, 6 is a plate, and 7 is a groove. In FIG. 10, 5a is a base, 5b is a member, 6 is a plate, 7 is a groove, and 9 is a pad.

  Thus, as shown in FIGS. 9 and 10, the base 5a may be inclined and placed. By placing the base body 5a with a height difference of about 500 μm, the base bodies 5a overlap each other even if the base body 5a is displaced, and the distance between the base bodies 5a is also different because the base body 5a is displaced in one direction. The problem of opening can be prevented, and the surface of the substrate 5a can be uniformly roughened. The surface of the plate 6 itself may be inclined as shown in FIG. 9, or the pad 9 may be placed with a height difference as shown in FIG. Further, a material that is easy to slide may be provided on the base 5a mounting surface, and the base 5a may be intentionally slid in one direction. For example, when the base 5a is a solar cell element silicon substrate, the thickness of the silicon substrate is 100 to 200 μm from the viewpoint of cost reduction. Since the base 5a and the like having such a thickness are light in weight and easily displaced, the effect of preventing the displacement of the base 5a is increased by performing the above method. Further, the above method is not limited to the etching apparatus, and can be used in combination with a substrate processing apparatus provided with a transfer mechanism for transferring a plurality of substrates 5a.

  Furthermore, FIG. 11 shows another embodiment according to the present invention. In FIG. 11, 5a is a base, 5b is a member, 6 is a plate, and 7 is a groove. As shown in FIG. 11, the member 5b may be processed into an L shape, and the base 5a may be placed on the member 5b. By mounting in this manner, the surface of the substrate 5a can be more uniformly roughened.

  Further, in FIGS. 3 to 6, the bases 5a or the bases 5a and the members 5b are arranged with a distance, but this distance is preferably 5 mm or less, and more preferably no such distance. .

It is a figure which shows the example which applied the roughening method of the base | substrate and base group based on this invention to the manufacturing method of a photovoltaic cell. It is a figure which shows an example of the reactive ion etching apparatus used for the roughening method of the base | substrate which concerns on this invention, and a base | substrate group. It is a figure which shows the other example of the reactive ion etching apparatus used for the roughening method of the base | substrate and base group based on this invention. (A) It is an upper view which shows one Embodiment based on this invention.

(B) It is a structure figure showing the section which shows one embodiment concerning the present invention.
It is a structural diagram showing the cross section of other embodiment which concerns on this invention. It is a structural diagram showing the cross section of other embodiment which concerns on this invention. It is a schematic diagram of the surface of the silicon substrate in a state where the unevenness is insufficient due to conventional unevenness formation. It is a structural diagram showing the cross section of other embodiment which concerns on this invention. It is a structural diagram showing the cross section of other embodiment which concerns on this invention. It is a structural diagram showing the cross section of other embodiment which concerns on this invention. It is a structural diagram showing the cross section of other embodiment which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1; Silicon substrate 1a; Surface uneven | corrugated structure 1b; Surface impurity diffusion layer 1c; Back surface impurity diffusion layer 1d; Antireflection film 1e; Front surface electrode 1f: Back surface electrode 2a; Mass flow controller 2b; Vacuum pump 2f; RF power source 5a; base 5b; member 6; plate 7; groove 8; recess 9; pad 101; silicon substrate 104a;

Claims (3)

In the roughening method for roughening the surface of the substrate or group of substrates by dry etching,
A roughening method comprising a first step comprising a member comprising the same component as the substrate around the substrate or substrate group, wherein the upper end of the member is positioned higher than the surface of the substrate or substrate group. . 2. The surface roughening method according to claim 1, wherein the first step has a groove around the plate on which the substrate or the substrate group is placed, and the member is fixed to the groove. The roughening method according to claim 1, wherein the first step includes a recess in which the base body or the base group is arranged in parallel on a plate on which the base body or the base group is placed.

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