JP2015041713A - Paste composition and solar battery element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paste composition which is to be used for forming an electrode on the backside of a silicon semiconductor substrate, and which makes possible to increase an open-circuit voltage, and to suppress the reduction in current density; and a solar battery element including a backside electrode which is formed by use of such a composition.SOLUTION: A paste composition is to be used for forming an electrode on the backside of a silicon semiconductor substrate which a crystalline silicon solar battery includes. The paste composition comprises: aluminum powder; Al-B alloy powder; glass powder; and an organic vehicle. In the paste composition, the concentration of the boron is 0.005-0.05 mass%.

Description

  The present invention generally relates to a paste composition and a solar cell element, and specifically, a paste composition used when forming an electrode on the back surface of a silicon semiconductor substrate constituting a crystalline silicon solar cell, and It is related with the solar cell element in which the back surface electrode was formed using it.

  As an electronic component having an electrode formed on the back surface of a silicon semiconductor substrate, a solar cell as disclosed in Japanese Patent Application Laid-Open No. 2003-69056 (Patent Document 1) and Japanese Translation of PCT International Publication No. 2009-530845 (Patent Document 2). Devices are known.

  FIG. 1 is a diagram schematically showing a general cross-sectional structure of a solar cell element.

As shown in FIG. 1, the solar cell element is configured using a p-type silicon semiconductor substrate 1 having a thickness of about 200 μm. On the light receiving surface side of the p-type silicon semiconductor substrate 1, an n ⁺ layer 2 is formed as an n-type impurity layer having a thickness of 0.3 to 0.6 μm, and an antireflection film 3 and a grid electrode 4 are formed thereon. .

An aluminum electrode layer 5 is formed on the back side of the p-type silicon semiconductor substrate 1. The aluminum electrode layer 5 is formed by applying a paste composition made of aluminum powder, glass frit, and organic vehicle by screen printing or the like, drying, and baking for a short time at a temperature of 660 ° C. (melting point of aluminum) or higher. Has been. During the firing, aluminum diffuses into the p-type silicon semiconductor substrate 1, thereby forming an Al—Si alloy layer 6 between the aluminum electrode layer 5 and the p-type silicon semiconductor substrate 1. A p ⁺ layer 7 is formed as an impurity layer by atomic diffusion. Due to the presence of the p ⁺ layer 7, a BSF (Back Surface Field) effect that prevents recombination of electrons and improves the collection efficiency of generated carriers can be obtained.

The mechanism by which the BSF layer as the p ⁺ layer 7 is formed can be roughly described as follows. First, when the p-type silicon semiconductor substrate 1 to which the paste composition is applied is heat-treated at a high temperature (generally 700 to 900 ° C.) higher than the temperature of the solidus line of the Al—Si alloy (577 ° C.) in the phase diagram. In addition, the Al-Si alloy melt is formed by melting Al contained in the paste and cell-derived Si. Thereafter, the Al-Si melt is solidified again by rapidly cooling the p-type silicon semiconductor substrate 1 on which the Al-Si alloy melt is formed to near room temperature. At this time, since the diffusion of aluminum atoms into silicon is slower than the diffusion of silicon atoms into aluminum, a part of the aluminum atoms in the Al-Si alloy melt is left behind in the silicon. At the same time when the Si alloy layer 6 is formed, the p ⁺ layer 7 (that is, the BSF layer) is formed as a silicon layer containing a high concentration of aluminum.

  On the other hand, for the purpose of further improving the conversion efficiency of solar cells, a method for forming a uniform BSF layer, a method for increasing the concentration of impurities contained in the BSF layer, and the like have been conventionally studied. For example, in the solar cell described in Japanese Patent Application Laid-Open No. 2003-69056 (Patent Document 1), at least one boron-containing material selected from the group consisting of boron powder, inorganic boron compounds and organic boron compounds is used as a paste composition. The BSF effect is improved by adding to the above.

  In the solar cell described in JP-T-2009-530845 (Patent Document 2), a paste composition containing an Al—B alloy is used in order to increase the concentration of impurities in the BSF layer. As described above, a method for increasing the concentration of impurities in the BSF layer by adding a boron-containing material having the same trivalent valence as aluminum to the paste composition has been conventionally studied.

JP 2003-69056 A Special table 2009-530845

  However, according to the method described in Japanese Patent Application Laid-Open No. 2003-69056 (Patent Document 1), since the boron-containing material is distributed alone in the paste composition, considering the mechanism in which the BSF layer is formed, It is believed that boron atoms diffuse unevenly into the BSF layer. Then, the non-uniform diffusion of boron in the BSF layer becomes an obstacle to increasing the open circuit voltage.

Regarding the Al-0.2 mass% B alloy containing 0.2 mass% of boron used in the example of the solar cell described in JP-T-2009-530845 (Patent Document 2), The position where the liquidus is shown is around 1000 ° C., and at the temperature at which the solar cell element is actually fired (that is, generally 700 to 900 ° C.), the intermetallic compound AlB ₂ remains as boron. Since an aluminum liquid phase coexists, boron atoms cannot be diffused into the BSF layer.

  Furthermore, when the paste composition is applied to the back surface of the solar cell element, if the amount of boron in the paste composition is large, an excessive concentration of impurities contained in the BSF layer may cause a decrease in light reflectance on the back surface. There is. The decrease in the light reflectance on the back surface causes a decrease in current density.

  Accordingly, an object of the present invention is to solve the above-described problem, and is a paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate, which increases an open-circuit voltage and has a current density. It is providing the solar cell element provided with the paste composition which can suppress a reduction | decrease, and the back surface electrode formed using the composition.

  The inventors of the present invention have conducted extensive research to solve the problems of the prior art, and as a result, in order to form an electrode on the back surface of the silicon semiconductor substrate, a mixture of aluminum powder and Al—B alloy powder is used. It has been found that the above object can be achieved by using a paste composition containing a powder and containing a specific amount of boron. Based on this finding, the paste composition according to the present invention has the following characteristics.

  A paste composition according to the present invention is a paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate constituting a crystalline silicon solar cell, and comprises an aluminum powder, an Al—B alloy powder, glass Contains powder and organic vehicle. The concentration of boron in the paste composition is 0.005% by mass or more and 0.05% by mass or less.

  Preferably, the Al—B alloy powder contains 0.01 mass% or more and 0.07 mass% or less of boron.

  The solar cell element according to the present invention includes an electrode formed by applying a paste composition having any one of the above-described features on the back surface of a silicon semiconductor substrate and then baking the paste composition.

  As described above, according to the present invention, in order to form an electrode on the back surface of the silicon semiconductor substrate, a mixed powder of aluminum powder and Al—B alloy powder is included, and a specific amount of boron is included. A paste composition capable of increasing an open-circuit voltage and suppressing a decrease in current density by using a paste composition to be used, and a solar cell element including a back electrode formed using the composition Can be provided.

It is a figure which shows typically the general cross-section of the solar cell element to which this invention is applied as one embodiment.

  As a result of intensive studies to solve the problems of the prior art, the present inventors have developed an aluminum powder and an Al-B alloy into a paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate. It has been found that the above object can be achieved by including a powder mixed with the powder and including a specific amount of boron. Based on this finding, the paste composition according to the present invention has the following characteristics.

  A paste composition according to the present invention is a paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate constituting a crystalline silicon solar cell, and comprises an aluminum powder, an Al—B alloy powder, glass Contains powder and organic vehicle. The density | concentration of the boron in the paste composition of this invention is 0.005 mass% or more and 0.05 mass% or less.

  The Al—B alloy powder used in the paste composition of the present invention preferably contains 0.01% by mass or more and 0.07% by mass or less of boron.

According to the paste composition according to the present invention containing an aluminum powder and an Al-B alloy powder and containing a specific amount of boron as a paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate For example, it is possible to increase the open circuit voltage by making the diffusion of boron into the BSF layer uniform, and it is possible to suppress the decrease in current density due to the deterioration of the reflectance of light on the back surface of the solar cell element.
<Solar cell element>
As shown in FIG. 1, a p-type solar cell element as one embodiment of a solar cell element according to the present invention is configured using, for example, a p-type silicon semiconductor substrate 1 having a thickness of 180 to 250 μm. Is done. On the surface of the silicon semiconductor substrate 1 on the light-receiving surface side, an n ⁺ layer 2 as an n-type impurity layer having a thickness of 0.3 to 0.6 μm, and an antireflection film (passivation) made of, for example, a silicon nitride film thereon. Film) 3 and grid electrode 4 are formed. The grid electrode 4 as the surface electrode is formed, for example, by screen-printing a silver paste and firing it.

An aluminum electrode layer 5 is formed on the back surface of the silicon semiconductor substrate 1 opposite to the light receiving surface. The aluminum electrode layer 5 is formed by applying the paste composition of the present invention by screen printing or the like, drying it, and baking it at a temperature exceeding 660 ° C. (melting point of aluminum) for a short time (fire-through method). ing. The paste composition of the present invention includes aluminum powder, Al—B alloy powder, glass powder, and an organic vehicle. During the firing, aluminum diffuses into the silicon semiconductor substrate 1, whereby an Al—Si alloy layer 6 is formed between the aluminum electrode layer 5 and the silicon semiconductor substrate 1, and at the same time, due to the diffusion of aluminum atoms. A p ⁺ layer (BSF layer) 7 as an impurity layer is formed. Due to the presence of the p ⁺ layer 7, a BSF (Back Surface Field) effect that prevents recombination of electrons and improves the collection efficiency of generated carriers can be obtained. In this way, the back electrode 8 composed of the aluminum electrode layer 5 and the Al—Si alloy layer 6 is formed on the back surface side of the silicon semiconductor substrate 1, and the silicon semiconductor substrate 1 facing the aluminum electrode layer 5 is further formed. A BSF layer 7 is formed in the region.

<Paste composition>
The paste composition of the present invention is a paste composition that is applied to the back surface opposite to the light receiving surface of the silicon semiconductor substrate 1 in order to form the aluminum electrode layer 5 described above. An alloy powder is contained, and a glass powder and an organic vehicle are contained as a binder. The concentration of boron in the paste composition is 0.005 mass% or more and 0.05 mass% or less. Furthermore, Preferably, the Al-B alloy powder used for a paste composition contains 0.01 mass% or more and 0.07 mass% or less of boron.

<Aluminum powder>
The aluminum powder contained in the paste composition exhibits an effect as an electrode due to its conductivity. Also, the aluminum powder forms the Al—Si alloy layer 6 and the p ⁺ layer (BSF layer) 7 between the aluminum powder and the silicon semiconductor substrate 1 when the paste composition is baked. ⁺ Layer is obtained.

  The shape of the aluminum powder is not particularly limited. The shape of the aluminum powder is preferably spherical. The ratio of the major axis to the minor axis of the aluminum particles constituting the aluminum powder is preferably 1 or more and 1.5 or less. By using the powder containing aluminum particles having such a shape, the filling property of the aluminum particles in the aluminum electrode layer 5 can be increased, and the electrical resistance as an electrode can be effectively reduced. Further, the number of contacts between the silicon semiconductor substrate 1 and the aluminum particles is increased, and a good Al—Si alloy layer 6 can be formed.

  The average particle size of the aluminum particles constituting the aluminum powder is preferably 1 μm or more and 10 μm or less. If the average particle diameter of the aluminum particles is in the range of 1 μm or more and 10 μm or less, good dispersibility can be obtained. There exists a possibility that aluminum particles may aggregate that an average particle diameter is less than 1 micrometer. If the average particle diameter exceeds 10 μm, the dispersibility of the aluminum particles may be deteriorated.

  The aluminum purity in the aluminum powder is preferably 99.7% or more. Even if impurities are mixed in the aluminum powder, mixing of impurities is allowed if the total amount of Fe and Si in the aluminum powder is less than 0.1%.

<Al-B alloy powder>
By including the Al—B alloy powder in the paste composition of the present invention, the concentration of impurities in the BSF layer can be increased by boron. Moreover, if the content of boron in the Al—B alloy powder in the paste composition is 0.01% by mass or more and 0.07% by mass or less, a liquid phase is sufficiently formed when the solar cell element is fired. Therefore, a good BSF layer can be formed.

  The shape of the Al—B alloy powder is not particularly limited. The shape of the Al—B alloy powder is preferably spherical. When the sphericity is 0.5 or more as the shape of the Al—B alloy powder, the filling performance in the aluminum electrode layer 5 can be increased, so that a decrease in electrical resistance can be suppressed.

  The sphericity herein can be obtained, for example, by observing spherical Al—B alloy powder particles with a scanning electron microscope (SEM). In a plurality of arbitrarily selected particles (for example, 20 particles), the minimum diameter of each particle (this diameter is determined based on the observation field in the microscope or the photograph in which the range of the field is photographed) The shortest distance between the two line segments obtained by sandwiching the two line segments, and the maximum diameter (this diameter is on the field of observation in the microscope or the field of view) Is the longest distance among the distances between two line segments obtained by sandwiching each particle between two parallel line segments). . The average of the minimum diameter and the maximum diameter is calculated for each particle. Furthermore, the average value of the minimum and maximum diameters of a plurality of arbitrarily selected particles is calculated (that is, the average of the shortest and maximum diameters of a plurality of arbitrarily selected particles is added). Divide the value by the number of particles used).

  The average particle size of the Al—B alloy powder in the paste composition of the present invention is preferably greater than 1 μm and less than 10 μm. When the average particle size is 1 μm or less, the dispersibility in the paste is deteriorated, and when it is 10 μm or more, the reactivity is lowered.

<Glass powder>
The glass powder is said to have an effect of assisting the reaction between the aluminum powder and the silicon semiconductor substrate and the sintering of the aluminum powder itself. The glass powder is selected from the group consisting of lead (Pb), bismuth (Bi), vanadium (V), boron (B), silicon (Si), tin (Sn), phosphorus (P), and zinc (Zn). You may contain 1 type, or 2 or more types.

  Further, as the glass powder, lead-containing glass powder or lead-free glass powder such as bismuth, vanadium, tin-phosphorus, zinc borosilicate, alkali borosilicate, or the like can be used. In particular, in view of the influence on the human body or environmental resistance, it is desirable to use lead-free glass powder.

  Moreover, it is preferable that the softening point of glass powder is 750 degrees C or less. If the glass powder has a softening point exceeding 750 ° C., the performance of the passivation film may be significantly impaired when a specific passivation film is used.

  Furthermore, it is preferable that the average particle diameter of the glass particle which comprises glass powder is 1 micrometer or more and 3 micrometers or less.

  In addition, content of the glass powder contained in the paste composition of this invention is although it does not specifically limit, It is preferable that they are 0.1 weight part or more and 15 weight part or less with respect to 100 weight part of aluminum powder. There exists a possibility that adhesiveness with the silicon semiconductor substrate 1 may fall that content of glass powder is less than 0.1 weight part. There exists a possibility that the electrical resistance of the aluminum electrode layer 5 formed as content of glass powder is 15 weight part or more may increase.

<Organic vehicle>
As the organic vehicle, a solvent in which various additives and a resin are dissolved as necessary is used. As the solvent, known solvents can be used, and specific examples include diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, dipropylene glycol monomethyl ether and the like. As various additives, for example, an antioxidant, a corrosion inhibitor, an antifoaming agent, a thickener, a coupling agent, an electrostatic imparting agent, a polymerization inhibitor, a thixotropic agent, an anti-settling agent and the like can be used. . Specifically, for example, polyethylene glycol ester compound, polyethylene glycol ether compound, polyoxyethylene sorbitan ester compound, sorbitan alkyl ester compound, aliphatic polycarboxylic acid compound, phosphate ester compound, amide amine salt of polyester acid, polyethylene oxide Series compounds, fatty acid amide waxes and the like can be used. Known resins can be used, such as ethyl cellulose, nitrocellulose, polyvinyl butyral, phenol resin, melanin resin, urea resin, xylene resin, alkyd resin, unsaturated polyester resin, acrylic resin, polyimide resin, furan resin, Thermosetting resin such as urethane resin, isocyanate compound, cyanate compound, polyethylene, polypropylene, polystyrene, ABS resin, polymethyl methacrylate, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyvinyl alcohol, polyacetal, polycarbonate, polyethylene terephthalate, Polybutylene terephthalate, polyphenylene oxide, polysulfone, polyimide, polyethersulfone, polyarylate, polyetheretherke Emissions, polytetrafluoroethylene, can be used in combination of two or more kinds of such as silicon resin. As the organic vehicle included in the paste composition of the present invention, a resin may be used without being dissolved in a solvent.

  In addition, content of the organic vehicle contained in the paste composition of this invention is although it does not specifically limit, It is preferable that they are 30 to 100 weight part with respect to 100 weight part of aluminum powder. If the content of the organic vehicle is less than 30 parts by weight or exceeds 100 parts by weight, the printability of the paste composition may be lowered.

  Examples of the present invention and comparative examples will be described below.

(Preparation of paste composition)
The paste compositions of Examples 1 to 4 and Comparative Examples 1 to 4 were prepared as follows.

Example 1
An aluminum powder having a spherical shape with a particle size of about 3 to 6 μm and an Al-0.011 mass% B alloy powder are prepared. A total of 100 parts by weight of aluminum powder and Al-0.011 mass% B alloy powder are mixed with 1.5 parts by weight of glass powder and 40 parts by weight of an organic vehicle using a known mixer, and paste An aluminum paste having a boron content of 0.005% by mass in the composition was obtained.

(Example 2)
An aluminum powder having a spherical shape with a particle size of about 3 to 6 μm and an Al-0.032 mass% B alloy powder were prepared, respectively, and a total of 100 parts by weight of aluminum powder and Al- An aluminum paste having a boron content of 0.02 mass% in a paste composition, in which 1.5 parts by weight of glass powder and 40 parts by weight of an organic vehicle are mixed with 0.032 mass% B alloy powder. Got.

Example 3
An aluminum powder having a spherical shape with a particle size of about 3 to 6 μm and an Al-0.051 mass% B alloy powder were prepared, respectively, and a total of 100 parts by weight of aluminum powder and 1.5 parts by weight of glass powder and 40 parts by weight of organic vehicle are mixed with Al-0.051% by mass B alloy powder, and the boron content in the paste composition is 0.034% by mass. An aluminum paste was obtained.

Example 4
A spherical aluminum powder having a particle diameter of about 3 to 6 μm and an Al-0.070 mass% B alloy powder were prepared, respectively, and a total of 100 parts by weight of aluminum was prepared in the same manner as in Examples 1, 2, and 3. 1.5 parts by weight of glass powder and 40 parts by weight of organic vehicle are mixed with the powder and Al-0.070 mass% B alloy powder, and the boron content in the paste composition is 0.05 mass. % Aluminum paste was obtained.

(Comparative Example 1)
An aluminum powder having a spherical shape with a particle size of about 3 to 6 μm is prepared. An aluminum paste was obtained by mixing 1.5 parts by weight of glass powder and 40 parts by weight of an organic vehicle with 100 parts by weight of aluminum powder using a known mixer.

(Comparative Example 2)
A spherical aluminum powder having a particle size of about 3 to 6 μm and an Al-0.006 mass% B alloy powder are prepared. For a total of 100 parts by weight of aluminum powder and Al-0.006 mass% B alloy powder, 1.5 parts by weight of glass powder and 40 parts by weight of organic vehicle are mixed in a known mixer, An aluminum paste having a boron content of 0.002% by mass in the paste composition was obtained.

(Comparative Example 3)
An aluminum powder having a spherical shape with a particle size of about 3 to 6 μm and an Al-0.083 mass% B alloy powder were prepared, respectively, and a total of 100 parts by weight of aluminum powder and Al- An aluminum paste having a boron content of 0.06 mass% in a paste composition, in which 1.5 parts by weight of glass powder and 40 parts by weight of an organic vehicle are mixed with 0.083 mass% B alloy powder. Got.

(Comparative Example 4)
A spherical aluminum powder having a particle size of about 5 to 8 μm and an Al-0.240 mass% B alloy powder were prepared, respectively, in the same manner as in Comparative Examples 2 and 3, and a total of 100 parts by weight of aluminum powder and 1.5 parts by weight of glass powder and 40 parts by weight of organic vehicle are mixed with Al-0.240% by mass B alloy powder, and the boron content in the paste composition is 0.2% by mass. An aluminum paste was obtained.

(Formation of aluminum electrode layer)
In a 5-inch single crystal cell, the implementation obtained above using a screen printer on the entire back surface of the silicon semiconductor substrate 1 on which the silver paste was previously printed on the surface (light receiving surface) of the silicon semiconductor substrate 1 The paste compositions of Examples 1 to 4 and Comparative Examples 1 to 4 were applied at a thickness of 40 μm. 250 Mesh was used for the screen mesh. Then, after each cell coated with each paste composition was dried at a temperature of 100 ° C. for 10 minutes, it was fired in an air atmosphere in an infrared firing furnace. In firing, the temperature of the firing zone of the infrared firing furnace was set to 750 to 800 ° C. By this firing, an aluminum electrode layer 5 was formed on the silicon semiconductor substrate 1 of each cell as shown in FIG. In this way, eight types of single crystal cells having different p ⁺ layers (BSF layers) 7 were obtained.

(Evaluation of conversion efficiency)
The IV characteristics of each cell obtained as described above were measured using a solar simulator manufactured by Wacom Denso Corporation. The solar simulator was set to AM1.5 conditions. About the IV characteristic of each cell, what was displayed on the said solar simulator was used. The conversion efficiency Eff (%) is calculated by the following formula.

Conversion efficiency Eff (%) = (current density I × open voltage V) × fill factor value F.I. F.

  Table 1 shows the evaluation results of Examples 1 to 4 and Comparative Examples 1 to 4.

  When considering improvement in the conversion efficiency Eff, attention should be paid to the power value W (= I · V), which is the product of the current density and the open circuit voltage, so that the paste composition does not contain boron in Comparative Example 1. In comparison, by using Al—B alloy powder or a mixture of aluminum powder and Al—B powder as in the paste compositions of Examples 1 to 4 and Comparative Examples 2 and 3, the reduction and release of current density are reduced. It can be seen from Table 1 that it is possible to achieve both increase in voltage (that is, increase in power value I · V).

  Furthermore, it can be seen from Table 1 that according to the paste compositions of Examples 1 to 4 above, it is possible to suppress the decrease in current density and increase the open circuit voltage as the boron content in the paste composition increases. On the other hand, when the boron content in the paste composition is less than 0.005% by mass, it can be seen that the increase in the open circuit voltage does not appear remarkably (see Comparative Example 2). It can also be seen that there is almost no difference between the conversion efficiency Eff of Comparative Example 1 and the conversion efficiency Eff of Comparative Example 2. On the other hand, when the boron content in the paste composition is more than 0.05% by mass (see Comparative Example 3), the current density significantly decreased as the boron content increased. Moreover, in the paste composition of the comparative example 3, compared with the paste composition of the comparative example 1 which does not contain boron, it was confirmed that conversion efficiency Eff is equivalent or less. Furthermore, in the paste composition of Comparative Example 4 containing excessive boron, the open-circuit voltage was remarkably reduced and the conversion efficiency Eff was also significantly reduced as compared with the paste composition of Comparative Example 1 not containing boron.

  As described above, from the results shown in Table 1, by using the silicon semiconductor substrate 1 coated with the paste composition of the present invention (Examples 1 to 4), the conversion efficiency of the solar cell element can be further improved. Recognize.

  It should be considered that the embodiments and examples disclosed above are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments and examples but by the scope of claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the scope of claims. .

1: p-type silicon semiconductor substrate, 2: n-type impurity layer, 3: antireflection film, 4: grid electrode, 5: aluminum electrode layer, 6: Al—Si alloy layer, 7: p ⁺ layer, 8: back electrode .

Claims (3)

A paste composition used for forming an electrode on the back surface of a silicon semiconductor substrate constituting a crystalline silicon solar cell,
Containing aluminum powder, Al-B alloy powder, glass powder, and organic vehicle,
The paste composition whose density | concentration of the boron in the said paste composition is 0.005 mass% or more and 0.05 mass% or less.   The paste composition according to claim 1, wherein the Al-B alloy powder contains 0.01 mass% or more and 0.07 mass% or less of boron. The solar cell element provided with the electrode formed by apply | coating the paste composition of Claim 1 or Claim 2 on the back surface of a silicon semiconductor substrate, and baking.

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