JP2011176097A - Laminated soi wafer, and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an SOI wafer capable of reducing warping of the SOI wafer caused by the difference between thicknesses of a BOX (Buried Oxide) layer and a rear face oxide film without carrying out bonding heat treatment for a long time. <P>SOLUTION: This manufacturing method includes a process for forming an oxide film 21a on a supporting wafer 20, a process for superimposing an activating wafer 30 on the supporting wafer 20 so as to be abutted to the oxide film 21a, and a process for bonding them by heat treatment. When the thickness of the oxide film 21a is A and the thickness of the oxide film on the surface of the activating wafer 30 is B, the following relationship is satisfied; A-B>2 μm. Accordingly, the thickness of the BOX layer 40 can be made more than 2 μm, and the difference between the thickness of the BOX layer 40 and the thickness of the rear face oxide film 22a is reduced, and the high-withstand voltage SOI wafer can be manufactured without carrying out the bonding heat treatment for a long time. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a bonded SOI (Silicon On Insulator) wafer and a method for manufacturing the same, and more particularly to a bonded SOI wafer having excellent gettering ability and reduced warpage and a method for manufacturing the same.

  An SOI wafer is known as a kind of silicon wafer. The SOI wafer is a type of silicon wafer in which a BOX layer (Buried Oxide layer: buried oxide film) is formed immediately below an active layer on which a semiconductor element such as a transistor is formed. Compared with a normal silicon wafer that isolates elements by applying a reverse bias to the pn junction, SOI wafers have a smaller stray capacitance, which enables not only high-speed switching operation but also power consumption. It can also be reduced.

  As a method for manufacturing an SOI wafer, a bonding method and a SIMOX (Separation by Implantation of Oxygen) method are known. Among these methods, the bonding method prepares a support wafer and an active wafer, forms an oxide film on one or both surfaces of the wafers, and then superimposes them and adheres them by heat treatment. This is a method of manufacturing an SOI wafer by reducing the thickness (see Patent Documents 1 and 2).

  Patent Document 1 discloses that the difference between the thickness of the oxide film formed on the supporting wafer and the thickness of the oxide film formed on the active wafer is 2 μm or less, thereby preventing the occurrence of slip dislocation during the adhesion heat treatment. It describes what you can do.

JP 2001-93788 A JP 2002-184960 A Japanese Patent Laid-Open No. 9-293845 JP 2002-134721 A

  However, in order to provide an SOI wafer capable of forming a high-breakdown-voltage element, there are cases where the thickness of the BOX layer must be designed to be thicker than 2 μm. In this case, in the method described in Patent Document 1, the difference between the film thickness of the BOX layer and the film thickness of the oxide film formed on the back surface of the supporting wafer becomes large, so that a large warp occurs in the SOI wafer. There was a problem that. Specifically, when the film thickness of the oxide film formed on the supporting wafer is A and the film thickness of the oxide film formed on the active wafer is B, the thickness of the BOX layer is A + B by bonding. On the other hand, since the film thickness of the oxide film (back surface oxide film) formed on the back surface of the supporting wafer remains A, a film thickness difference of B occurs between the two. As an example, when the thickness of the BOX layer (= A + B) is designed to be 4 μm, in order to make the difference between A and B 2 μm or less as in the invention described in Patent Document 1, the value of B is at least 1 μm (in this case, the value of A is 3 μm), which is the film thickness difference between the BOX layer and the back oxide film.

  In order to reduce the warpage of the wafer due to the film thickness difference between the BOX layer and the back surface oxide film, it is necessary to increase the film thickness of the back surface oxide film by performing an adhesion heat treatment for a long time. , Productivity will decrease. This problem becomes more prominent as the thickness of the BOX layer increases. Patent Document 2 describes that the thickness (A) of the oxide film formed on the supporting wafer is made thinner than the thickness (B) of the oxide film formed on the active wafer. In this case, since the film thickness difference between the BOX layer and the back surface oxide film becomes larger, the problem of warpage becomes more serious.

  As described above, in the conventional method for manufacturing an SOI wafer, it is necessary to perform an adhesive heat treatment for a long time in order to reduce warpage of the SOI wafer due to a film thickness difference between the BOX layer and the back surface oxide film. There was a problem of low nature.

  Therefore, an object of the present invention is to provide a method for manufacturing an SOI wafer that can reduce warpage of the SOI wafer due to the film thickness difference between the BOX layer and the back surface oxide film without performing an adhesive heat treatment for a long time. That is.

  On the other hand, Patent Documents 3 and 4 describe SOI wafers having gettering capability. However, in any case, it is necessary to perform damage processing before bonding, and there is a problem in that the number of steps increases.

  Therefore, another object of the present invention is to provide an SOI wafer having a high withstand voltage that can be manufactured without increasing the number of steps and has an excellent gettering capability.

  The present inventor has earnestly studied how the relationship between the thickness A of the oxide film formed on the supporting wafer and the thickness B of the oxide film formed on the active wafer is related to the occurrence of slip dislocation. Went. As a result, if the difference between the film thickness A and the film thickness B exceeds 2 μm, slip dislocation may occur in the wafer with the thicker oxide film, while slip dislocation does not occur in the wafer with the thinner oxide film. It became clear. The present invention has been made based on such technical knowledge.

  The SOI wafer according to the present invention is a bonded SOI wafer in which an active layer is provided on a supporting wafer via a buried oxide film, and the buried oxide film has a thickness of more than 2 μm. Is characterized in that slip dislocations are present and no slip dislocations are present in the active layer.

  According to the present invention, since slip dislocations exist in the supporting wafer, this functions as a gettering site for capturing heavy metals and the like. In addition, the occurrence of slip dislocation can be controlled by the thickness of the oxide film formed on the supporting wafer and the active wafer before bonding, so that it is manufactured without adding a process such as damage processing. Is possible. Furthermore, since the buried oxide film has a thickness of more than 2 μm, it is possible to form a high breakdown voltage element.

  In this invention, it is preferable that the film thickness of the back surface oxide film formed on the back surface of the supporting wafer is more than 2 μm. According to this, since the film thickness difference between the buried oxide film and the back surface oxide film becomes small, it becomes possible to suppress the warpage of the wafer. In addition, since the thickness of the oxide film on the active wafer can be reduced, manufacturing can be performed without performing the adhesive heat treatment for a long time.

  In the present invention, the difference between the thickness of the buried oxide film and the thickness of the back oxide film is preferably 0.2 μm or less. According to this, it becomes possible to suppress the curvature of a wafer to 20 micrometers or less.

The SOI wafer manufacturing method according to the present invention includes an oxide film forming step of forming a first oxide film on at least one surface of a support wafer, and an active wafer so as to be in contact with the first oxide film. A method for producing a bonded SOI wafer, comprising: a superimposing step of superimposing on the supporting wafer; and a bonding heat treatment step of adhering the superposed supporting wafer and the active wafer by heat treatment, When the thickness of the first oxide film is A and the thickness of the second oxide film on the surface of the active wafer on the side in contact with the first oxide film is B,
A-B> 2 μm
It is characterized by satisfying.

  According to the present invention, the thickness of the BOX layer can be more than 2 μm, and the difference between the thickness of the BOX layer and the thickness of the back oxide film is reduced. Therefore, it is possible to manufacture a high breakdown voltage SOI wafer. In addition, since the film thickness A is larger than the film thickness B by more than 2 μm, it becomes possible to generate slip dislocations in the supporting wafer without generating slip dislocations in the active wafer, and gettering is performed on the SOI wafer. It is also possible to give ability.

  In the present invention, the thickness B of the second oxide film is preferably 0.2 μm or less. According to this, it is possible to reduce the warpage generated in the SOI wafer without performing the adhesive heat treatment for a long time.

  In the present invention, it is preferable that the film thickness B of the second oxide film is substantially zero. According to this, the difference between the thickness of the BOX layer and the thickness of the back oxide film becomes substantially zero when the supporting wafer and the active wafer are overlapped. Here, “substantially zero” indicates a state in which the second oxide film is not actively formed on the active wafer by thermal oxidation or the like. Therefore, it is ideally completely zero (that is, there is no second oxide film), but includes a case where a very thin natural oxide film is formed on the surface of the active wafer. Since the natural oxide film is significantly thinner than the thickness of the first oxide film (greater than 2 μm), it can be considered substantially zero in the present invention.

  In the oxide film forming step, a back surface oxide film having a film thickness A is also formed on the other surface of the supporting wafer, and the thickness of the back surface oxide film is increased by performing the bonding heat treatment step in an oxygen atmosphere. It is preferable to increase within a range not exceeding A + B. According to this, since the difference between the thickness of the BOX layer and the thickness of the back oxide film is reduced by the adhesive heat treatment, it is possible to reduce the warpage. More preferably, if the thickness B of the oxide film formed on the active wafer is matched with the amount of increase in the thickness of the backside oxide film that occurs in the time required for the adhesive heat treatment, it is possible to eliminate unnecessary time and processes. It becomes possible to manufacture an SOI wafer without warping.

  As described above, according to the present invention, it is possible to provide an SOI wafer with high withstand voltage that can be manufactured without increasing the number of steps and has excellent gettering capability.

  In addition, according to the present invention, there is provided an SOI wafer manufacturing method capable of reducing the warpage of the wafer due to the film thickness difference between the BOX layer and the back oxide film without performing the adhesive heat treatment for a long time. Is possible.

It is a schematic sectional drawing which shows the structure of the bonding SOI wafer by preferable embodiment of this invention. It is a schematic diagram for demonstrating the manufacturing method of the bonding SOI wafer by preferable embodiment of this invention. 6 is a graph showing measurement results of Example 2.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view showing the structure of a bonded SOI wafer according to a preferred embodiment of the present invention.

  As shown in FIG. 1, the bonded SOI wafer 10 according to the present embodiment includes a supporting wafer 20, a buried oxide film (BOX layer) 40 provided on one surface 21 of the supporting wafer 20, and a BOX layer 40. And an active layer (SOI layer) 30 a provided on the supporting wafer 20.

  The supporting wafer 20 plays a role of ensuring the mechanical strength of the bonded SOI wafer 10 and the thickness thereof is not particularly limited, but may be about 725 μm. The supporting wafer 20 includes a large number of slip dislocations S, which function as gettering sites for capturing heavy metals and the like. Although details will be described later, most or all of the slip dislocations S included in the supporting wafer 20 are not present before the bonding, but are generated after the bonding heat treatment after the bonding. is there. Further, a back surface oxide film 22 a is formed on the back surface 22 of the supporting wafer 20. The thickness of the back oxide film 22a is A + a. Although details will be described later, the film thickness A is a film thickness before the adhesive heat treatment and is more than 2 μm. Further, the film thickness a is an increase in the film thickness due to the adhesive heat treatment, and the specific thickness depends on the temperature and time for performing the adhesive heat treatment.

  The active layer 30a is a region for forming a semiconductor element such as a transistor, and the thickness thereof is not particularly limited, but may be 0.3 to 250 μm, for example, about 15 μm. There are no slip dislocations in the active layer 30a. The active layer 30a is a portion made of an activation wafer 30 described later.

  The BOX layer 40 serves to insulate and isolate the semiconductor elements formed in the active layer 30a and has a film thickness of A + B. The value of A + B is more than 2 μm. Thus, since the film thickness of the BOX layer 40 is more than 2 μm, it is possible to form a high breakdown voltage semiconductor element in the active layer 30a. Here, the film thickness A is the thickness of the oxide film on the surface 21 of the supporting wafer 20 before the adhesive heat treatment, and the film thickness B is the thickness of the oxide film on the surface of the active wafer before the adhesive heat treatment. . Therefore, if the film thickness B and the film thickness a are matched, the film thicknesses of the insulating films (BOX layer 40 and back oxide film 22a) formed on both surfaces of the supporting wafer 20 are matched. It is possible to prevent the warpage of the resulting wafer. Although it is not essential that the film thicknesses of the BOX layer 40 and the back surface oxide film 22a coincide completely, it is preferable that the difference in film thicknesses is 0.2 μm or less. According to this, it is possible to sufficiently reduce the warpage of the wafer due to the film thickness difference.

  FIG. 2 is a schematic diagram for explaining a method for manufacturing a bonded SOI wafer 10 according to a preferred embodiment of the present invention.

  First, a support wafer 20 and an activation wafer 30 that do not include slip dislocations are prepared. In the present invention, it is not essential that the supporting wafer 20 does not contain slip dislocations. However, since it is difficult to obtain good adhesion characteristics for wafers having slip dislocations, the supporting wafer 20 also slips before bonding. It is preferable not to include dislocations. On the other hand, since the active wafer 30 is finally used as the active layer 30a, it should not include slip dislocations that affect at least device characteristics. Note that the thicknesses of the supporting wafer 20 and the active wafer 30 do not have to be the same.

  Next, the front surface 21 and the rear surface 22 of the supporting wafer 20 are cleaned by performing cleaning, pure rinsing, and hydrofluoric acid organic cleaning in this order on the supporting wafer 20 and then thermally oxidizing them. Then, oxide films 21a and 22a each having a film thickness A are formed (oxide film forming step). The film thickness A is more than 2 μm. In the present invention, it is not essential to form the oxide film 22a on the back surface 22 of the support wafer 20. However, in order to sufficiently reduce the warpage of the wafer, the back surface 22 of the support wafer 20 is also the same as the oxide film 21a. It is preferable to form an oxide film 22a having a thickness.

  On the other hand, the active wafer 30 does not need to be thermally oxidized, and is preferably in a state where no thermal oxide film is formed on both surfaces thereof. It is preferable to clean the surface of the active wafer 30 by performing pure rinse and hydrofluoric acid organic cleaning in this order before bonding. Ideally, it is desirable that the surface of the active wafer 30 immediately before bonding is in a state where no oxide film is formed, but when exposed to an atmosphere containing oxygen, a natural oxide film is inevitable on the surface. To occur. However, since the natural oxide film is very thin, it is not essential to remove it.

However, in the present invention, a thermal oxide film should not be formed on the active wafer 30, and when the thickness of the oxide film on the surface of the active wafer 30 is B,
A-B> 2 μm
A thermal oxide film may be formed on the active wafer 30 as long as it satisfies the above requirement. Even in this case, the film thickness B is preferably as thin as possible, and more preferably 0.2 μm or less. This is because, as will be described later, the film thickness B is the film thickness difference between the BOX layer 40 and the back oxide film 22a immediately after bonding, and the warpage increases as the film thickness difference increases. . Therefore, if the film thickness B is substantially zero, the difference in film thickness between the BOX layer 40 and the back surface oxide film 22a immediately after bonding is also substantially zero, and no warpage occurs. Here, the reason that the thermal oxide film is allowed to be formed on the active wafer 30 is that the thickness of the back oxide film 22a slightly increases if the bonding heat treatment after bonding is performed in an oxygen-containing atmosphere. . However, in this embodiment, the thickness of the back surface oxide film 22a is more than 2 μm, and the rate of increase of the film thickness by the adhesive heat treatment is small. Therefore, if the film thickness B is too large, the film of the BOX layer 40 and the back surface oxide film 22a. In order to eliminate the thickness difference, it is necessary to perform an adhesive heat treatment for a long time. Considering this point, the film thickness B is preferably as thin as possible, and more preferably 0.2 μm or less.

  Next, the wafers 20 and 30 are cleaned, purely rinsed, and hydrofluoric acid organic cleaned in this order, and then terminated with hydrogen. Then, the supporting wafer 20 and the activating wafer 30 are overlapped so that the oxide film 21a is in contact with the activating wafer 30, and both are pressed with a predetermined pressure (overlapping step). Thereby, a BOX layer 40 having a film thickness A + B is formed between the supporting wafer 20 and the activating wafer 30. At this time, the film thickness of the back surface oxide film 22 a formed on the back surface 22 of the supporting wafer 20 remains A.

  Next, the superposed support wafer 20 and the active wafer 30 are heat-treated at a temperature of 1100 ° C. or higher to bond them together (heat treatment step). Thereby, the supporting wafer 20 and the activation wafer 30 are firmly bonded via the BOX layer 40. Further, if the adhesion heat treatment is performed in an oxygen-containing atmosphere, the thickness of the back oxide film 22a slightly increases. However, when the thickness of the oxide film already formed is increased by thermal oxidation, the increase rate of the film thickness decreases as the thickness of the oxide film already formed increases. In the present embodiment, since the thickness of the back surface oxide film 22a is very large, exceeding 2 μm, the thickness of the back surface oxide film 22a hardly increases when the adhesive heat treatment is performed in a short time. In order to increase the film thickness of the back surface oxide film 22a to some extent, it is necessary to spend a long time for the adhesive heat treatment. However, considering productivity, it is actually possible to obtain a film thickness increase amount exceeding 0.2 μm. Not right. Therefore, in order to eliminate the film thickness difference between the BOX layer 40 and the back oxide film 22a by the adhesive heat treatment, as described above, the film thickness B is preferably as thin as possible, and more preferably 0.2 μm or less.

  On the contrary, when the film thickness B is zero or very thin, the back oxide film 22a becomes thicker than the BOX layer 40 if the adhesive heat treatment in the oxygen-containing atmosphere is performed for a longer time than necessary. Therefore, the adhesion heat treatment is preferably performed in a range where the thickness of the back oxide film 22a does not exceed A + B.

  In this bonding heat treatment, the film thickness A of the oxide film 21a formed on the support wafer 20 side is larger than the film thickness B of the oxide film formed on the active wafer 30 side by more than 2 μm. A strong stress is generated on the wafer 20 side, and many slip dislocations are generated. Such slip dislocations may occur only on the wafer having the thicker oxide film when the difference in film thickness of the oxide film formed on the two bonded wafers exceeds 2 μm. This has been confirmed by the inventors' experiments. On the other hand, slip dislocation does not occur in the wafer having the thinner oxide film. For this reason, when the adhesive heat treatment is performed in the present invention, slip dislocation occurs on the supporting wafer 20 side, while no slip dislocation occurs on the active wafer 30 side.

  Next, the surface of the active wafer 30 is ground to reduce the thickness of the active wafer 30 to obtain an active layer 30a having a thickness of about 0.3 to 250 μm. Then, the bonded SOI wafer 10 according to the present embodiment is completed by performing terrace polishing on the peripheral portion of the active layer 30a.

  In the bonded SOI wafer 10 thus completed, the thickness of the BOX layer 40 is more than 2 μm, the supporting wafer 20 has slip dislocations, and the active layer 30a has slip dislocations. Absent. For this reason, the characteristics of the device formed in the active layer 30a are not deteriorated by slip dislocations, and a large number of slip dislocations existing in the supporting wafer 20 function as gettering sites for capturing heavy metals and the like. In addition, since slip dislocations included in the supporting wafer 20 are naturally generated by the adhesive heat treatment, it is not necessary to add a special process for forming a gettering site. In addition, since the supporting wafer 20 having no slip dislocation is used before bonding, unlike the case of bonding a wafer including slip dislocation from the beginning, it is possible to obtain good adhesion characteristics.

  Furthermore, in this embodiment, since the film thickness B of the oxide film on the surface of the active wafer 30 is thin (preferably 0.2 μm or less), the film thickness of the BOX layer 40 and the back oxide film 22a immediately after the bonding is performed. The difference is also reduced. For this reason, not only the warpage generated in the SOI wafer due to such a film thickness difference is reduced, but also the film thickness difference between the BOX layer 40 and the back surface oxide film 22a is sufficiently reduced by, for example, 0.2 μm or less by a relatively short adhesive heat treatment. It becomes possible to reduce it.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, it is included in the range.

  Hereinafter, although the Example of this invention is described, this invention is not limited to this Example at all.

  A plurality of silicon wafers were cut out from the same silicon single crystal ingot grown by the Czochralski method, half of which was wafer X and the other half was wafer Y. These wafers X and Y have a diameter of 200 mm and a thickness of 725 μm. None of the wafers contains slip dislocations.

  Next, after forming oxide films of various thicknesses on the wafers X and Y by thermal oxidation, the wafers X and Y were superposed in various combinations. Then, both were bonded by performing adhesive heat treatment at 1200 ° C. The adhesive heat treatment was performed for 1 hour.

  Then, a razor blade was inserted into the bonding surface between the wafer X and the wafer Y to separate them, and whether or not slip dislocation occurred in each wafer was observed by an X-ray topograph (Lang method). The evaluation results of the wafer X are shown in Table 1, and the evaluation results of the wafer Y are shown in Table 2.

  As shown in Tables 1 and 2, no slip dislocation occurred in any of the wafers when the difference in oxide film thickness was 2 μm or less. On the other hand, when the film thickness difference between the oxide films is more than 2 μm (3 μm or more in this embodiment), slip dislocation occurred on the wafer having the thick oxide film. On the other hand, slip dislocation did not occur in the wafer having the thinner oxide film. As a result, when the adhesive heat treatment is performed after two wafers having a difference in film thickness of the oxide film formed on the surface exceeding 2 μm are overlapped, slip dislocations are observed on the wafer having the thick oxide film. On the other hand, it was confirmed that slip dislocation did not occur in the wafer having the thinner oxide film.

  Next, among the plurality of silicon wafers prepared in Example 1, wafer X having an oxide film thickness of 1 μm and wafer Y having an oxide film thickness of 5 μm were used and bonded at room temperature. After being combined, both were bonded by performing an adhesive heat treatment at 1200 ° C. The adhesive heat treatment was performed for 1 hour. As a result, slip dislocation occurred on the wafer Y side, but no slip dislocation occurred on the wafer X. Next, the wafer X was ground and terrace polished to form an active layer having a thickness of 5 μm. Thus, a bonded SOI wafer for Sample 1 was completed.

  On the other hand, among the plurality of silicon wafers prepared in Example 1, a wafer X and a wafer Y, each having an oxide film thickness of 3 μm, were used to produce a bonded SOI wafer of Sample 2 in the same manner as described above. . In the bonded SOI wafer of sample 2, slip dislocation did not occur in either wafer X or Y.

Next, Cu was intentionally contaminated at a concentration of 10 ¹³ atoms / cm ² in the active layer of the bonded SOI wafers of Samples 1 and 2. Then, after heat treatment at 1100 ° C. for 2 hours, the Cu concentration on the surface of the active layer was measured. The measurement results are shown in FIG.

  As shown in FIG. 3, the bonding of sample 1 in which the supporting wafer (wafer Y) includes slip dislocations as compared to the bonded SOI wafer in sample 2 in which the supporting wafer (wafer Y) does not include slip dislocations. In the SOI wafer, the Cu concentration on the surface was low. Thereby, it was confirmed that the slip dislocation generated in the supporting wafer functions as a Cu gettering site.

DESCRIPTION OF SYMBOLS 10 SOI wafer 20 Support wafer 21 Front surface 22 of support wafer Back surface 21a of support wafer Oxide film 22a Back surface oxide film 30 Active wafer 30a Active layer 40 BOX layer S Slip dislocation

Claims (7)

A bonded SOI wafer in which an active layer is provided on a supporting wafer via a buried oxide film,
The buried oxide film has a thickness of more than 2 μm;
Slip dislocation exists in the supporting wafer,
A bonded SOI wafer characterized in that slip dislocations do not exist in the active layer.   2. The bonded SOI wafer according to claim 1, wherein the thickness of the back oxide film formed on the back surface of the supporting wafer is more than 2 μm.   The bonded SOI wafer according to claim 2, wherein a difference between the film thickness of the buried oxide film and the film thickness of the back surface oxide film is 0.2 μm or less. An oxide film forming step of forming a first oxide film on at least one surface of the supporting wafer;
An overlaying step of overlaying an active wafer on the supporting wafer so as to be in contact with the first oxide film;
A bonded SOI wafer manufacturing method comprising: an adhesion heat treatment step of adhering the superposed support wafer and the active wafer by heat treatment,
When the film thickness of the first oxide film is A and the film thickness of the second oxide film on the surface of the active wafer on the side in contact with the first oxide film is B,
A-B> 2 μm
A method for producing a bonded SOI wafer, wherein:   5. The method for manufacturing a bonded SOI wafer according to claim 4, wherein a film thickness B of the second oxide film is 0.2 μm or less.   6. The method for manufacturing a bonded SOI wafer according to claim 5, wherein the film thickness B of the second oxide film is substantially zero. In the oxide film forming step, a back surface oxide film having a film thickness A is also formed on the other surface of the supporting wafer,
The bonded SOI wafer according to any one of claims 4 to 6, wherein the adhesion heat treatment step is performed in an oxygen atmosphere to increase the thickness of the back oxide film within a range not exceeding A + B. Manufacturing method.

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