JP2015013352A - Method of making fine mechanical structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable shape change and characteristic control in MEMS using single crystal SiC.SOLUTION: A deformation part 105 is formed in, for example, a rectangular parallelepiped shape, the deformation part having a first face 105a and a second face 105b parallel to each other. For example, the first face 105a is a (0001) face, and the second face 105b is a (000-1) face. The deformation part 105 formed in this way is subjected to thermal oxidation at the same, so that a silicon oxide layer 151 is formed on the first face 105a and a silicon oxide layer 152 is formed on the second face 105b.

Description

  The present invention relates to a method for producing a fine mechanical structure for producing a fine structure made of SiC.

  Microelectromechanical systems (MEMS) that exhibit various functions in mechanical operation are used in various applications such as sensors. Currently, MEMS are mainly manufactured using Si. On the other hand, silicon carbide (SiC) having characteristics such as a high Young's modulus, a high yield strength at a high temperature, and a high chemical stability as compared with Si has attracted attention as a material for MEMS. The research and development of MEMS using SiC (SiC-MEMS) has centered on 3C-SiC deposited on Si, polycrystalline SiC, and amorphous SiC. However, 3C-SiC has many crystal defects, and polycrystalline SiC and amorphous SiC have problems that the above-described characteristics are inferior to single-crystal SiC.

  On the other hand, some studies have been conducted to make MEMS using high-quality SiC single crystal wafers. When a high quality SiC single crystal wafer is used, fine processing is difficult, but there are few crystal defects, and the original characteristics of SiC can be obtained. The inventors laminated a p-layer and an n-layer of SiC on a single crystal SiC wafer, and performed selective etching of the p-layer or n-layer using photoelectrochemical etching or electrochemical etching, thereby forming a thin film (membrane). Has proposed a technique for fabricating a basic structure of MEMS such as a bridge, a cantilever, and the like (see Non-Patent Document 1).

  A cantilever resonator made of single crystal SiC has excellent resonance characteristics with a very high Q value of 230000. This is about 10 times the Q value of the cantilever resonator formed by the 3C-SiC thin film formed on the Si substrate, and corresponds to 20 times the Si cantilever resonator (see Non-Patent Document 1). This result is a very useful characteristic for MEMS applications, such as a high sensitivity sensor.

Watanabe Adachi, Naoki Watanabe, Hajime Okamoto, Koji Yamaguchi, Tsuyoshi Kimoto, Satoshi Suda, “High-Q Single Crystal 4H-SiC Microcantilever”, Society of Applied Physics, SiC and Related Wide Gap Semiconductor Research Group 21 Proceedings of the annual lecture, P-102, 2012. Y. Song et al., "Modified Deal Grove model for the thermal oxidation of silicon carbide", JOURNAL OF APPLIED PHYSICS, vol.95, no.9, pp.4953-4957, 2004.

  However, single crystal SiC-MEMS has a big problem. In the technique of single crystal SiC-MEMS, a fine MEMS is configured using an SiC film homoepitaxially grown on a single crystal SiC substrate. In such a MEMS, both the substrate and the MEMS structure are made of SiC, and there is no internal stress in the MEMS structure. In general MEMS, the internal stress is controlled by the deposition conditions of the thin film of the layer constituting the MEMS, whereby the shape change by stress relaxation and the characteristic control by the internal stress are performed. This is not possible with single crystal SiC-MEMS.

In GaAs-based MEMS, internal stress can be controlled by depositing mixed crystal semiconductors having different lattice constants such as Al _x Ga _1-x As and In _x Ga _1-x As. However, in the case of SiC, Si _x C _1-x does not exist in the vicinity of x = 0.5, and the above-described method using a laminated structure having different composition ratios cannot be used.

  The present invention has been made to solve the above problems, and an object of the present invention is to enable shape change and characteristic control in a MEMS using single crystal SiC.

  A method for producing a micromechanical structure according to the present invention includes a deformable portion having a first surface and a second surface that are parallel to each other, and a support portion that supports a portion of the deformable portion, which are made of single-crystal SiC on a substrate. And a thermal oxidation step of forming a silicon oxide layer on each of the first and second surfaces by thermally oxidizing the surface of the microstructure after forming the microstructure, The first surface and the second surface are crystal surfaces having different growth rates of the silicon oxide layer formed by thermal oxidation.

  In the method for manufacturing a fine mechanical structure, the first surface may be a crystal plane within a range of 10 degrees from the (0001) plane.

  In the method for manufacturing a fine mechanical structure, the thickness of the silicon oxide layer formed on the first surface may be 10 to 200 nm. In addition, the thickness of the silicon oxide layer formed on the first surface is preferably 5 times or more the thickness of the silicon oxide layer formed on the second surface.

  As described above, according to the present invention, it is possible to obtain an excellent effect that shape change and characteristic control can be performed in a MEMS using single crystal SiC.

FIG. 1 is an explanatory diagram for explaining a method of manufacturing a fine mechanical structure in an embodiment of the present invention. FIG. 2 is a cross-sectional view showing a partial configuration example of a MEMS including a micro mechanical structure manufactured by the method for manufacturing a micro mechanical structure in the embodiment of the present invention. FIG. 3 is a cross-sectional view showing a partial configuration example of a MEMS including a micro mechanical structure manufactured by the method for manufacturing a micro mechanical structure in the embodiment of the present invention. FIG. 4 is a cross-sectional view showing a configuration of a fine mechanical structure produced by the fine mechanical structure producing method according to the embodiment of the present invention. FIG. 5 is a cross-sectional view showing a configuration of a fine mechanical structure produced by the fine mechanical structure producing method according to the embodiment of the present invention. FIG. 6 is a characteristic diagram showing a result of measuring a deformed portion before thermal oxidation produced by the method for producing a fine mechanical structure in the embodiment of the present invention with an optical profiler. FIG. 7 is a characteristic diagram showing the result of measuring the deformed portion after thermal oxidation produced by the method for producing a micromechanical structure in the embodiment of the present invention with an optical profiler. FIG. 8 is a characteristic diagram showing the resonance characteristics (in vacuum) of the deformed portion before thermal oxidation produced by the method for producing a micromechanical structure in the embodiment of the present invention. FIG. 9 is a characteristic diagram showing the resonance characteristics (in vacuum) of the deformed portion after thermal oxidation produced by the method for producing a fine mechanical structure in the embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram for explaining a method of manufacturing a fine mechanical structure in an embodiment of the present invention. In FIG. 1, the cross section in the process in the middle of preparation is typically shown.

  First, as shown in FIG. 1A, a substrate 101 made of single crystal SiC is prepared. For example, the main surface of the substrate 101 may be a (0001) plane. The substrate 101 only needs to be made of p-type single crystal SiC. Next, an n-type layer 102 made of n-type SiC is formed on the surface of the substrate 101. For example, n-type single crystal SiC may be epitaxially grown on the substrate 101. For example, the n-type layer 102 may have a thickness of about 1 μm.

  Next, as shown in FIG. 1B, an opening region 103 penetrating the n-type layer 102 is formed. For example, the opening region 103 may be formed by selectively etching the n-type layer 102 by reactive ion etching (RIE) using a Ni mask formed by a well-known photolithography technique and etching technique. . Note that the Ni mask used to form the opening region 103 is removed.

  Next, the fine structure 104 is formed as shown in FIG. 1C by selectively etching the substrate 101 through the opening region 103 by electrochemical etching. In this example, the fine structure 104 is, for example, a plate-like cantilever including a support portion 106 constituted by a part of the substrate 101 and a deformable portion 105 supported at one end by the support portion 106. . In this case, the deformable portion 105 is composed of the n-type layer 102. The length of the deformable portion 105 from the portion fixed by the support portion 106 to the front end portion on the left side of FIG. 1 is, for example, 120 μm.

  In the microstructure 104 formed as described above, the deformed portion 105 formed in a plate shape or a rectangular parallelepiped shape has a first surface 105a and a second surface 105b that are parallel to each other. The front surface and the back surface of the deformable portion 105 are a first surface 105a and a second surface 105b. As described above, the substrate 101 has a (0001) plane as the main surface, and the deformed portion 105 is formed on the epitaxially grown n-type layer 102 thereon. Accordingly, the first surface 105a is a (0001) surface, and the second surface 105b is a (000-1) surface. This is a state where the first surface 105a and the second surface 105b are crystal planes with different growth rates of the silicon oxide layer formed by thermal oxidation. In addition, according to the formation of the fine structure 104 described above, the second surface 105b is in a state of facing the substrate 101 side.

  After forming the microstructure 104 as described above, the first surface 105a and the second surface 105b are thermally oxidized at the same time. As shown in FIG. 1D, the silicon oxide layer 151 is formed on the first surface 105a. The silicon oxide layer 152 is formed on the second surface 105b. The substrate 101 on which the microstructure 104 is formed may be heated to about 900 to 1000 ° C. in an oxygen gas atmosphere in a well-known thermal oxidation furnace.

  As described above, the first surface 105a and the second surface 105b are in the state of crystal planes with different growth rates of the silicon oxide layer formed by thermal oxidation. In this example, the second surface 105b is The growth rate of the silicon oxide layer is fast. Accordingly, the silicon oxide layer 152 is formed thicker than the silicon oxide layer 151. Note that the silicon oxide layer formed by thermal oxidation is formed not only on the first surface 105a and the second surface 105b but on all surfaces of SiC exposed in the thermal oxidation process.

  After the thermal oxidation as described above, the internal temperature of the thermal oxidation furnace is cooled to room temperature (about 20 to 25 ° C.) and the substrate 101 (fine structure 104) is carried out, as shown in FIG. In addition, a part of the deformation portion 105 is deformed in a direction away from the substrate 101. As described above, since the silicon oxide layer 152 is formed thicker during thermal oxidation, the deformed portion 105 is warped toward the thinner silicon oxide layer 151 when cooled.

  As described above, in the present invention, the crystal orientation of the single crystal SiC that forms the microstructure is appropriately selected, and thermal oxidation is performed under the selected conditions, so that the microstructure can be obtained without performing lithography or special processing. For example, by forming silicon oxide layers with different thicknesses on two parallel surfaces of a plate-shaped deformed part, for example, it is possible to control the internal stress of a microstructure formed from single crystal SiC. There are features.

  The thermal oxidation of single crystal SiC has been investigated in detail in the course of research and development of MOSFETs using SiC. By thermal oxidation, SiC of SiC becomes a silicon oxide layer, and C is mainly discharged as carbon dioxide gas. It is known that the thermal oxidation rate greatly depends on the SiC crystal plane orientation. The surface with the slowest oxidation rate is in the vicinity of the (0001) plane, and the fastest surface is in the vicinity of the (000-1) plane (see Non-Patent Document 2).

  Therefore, if one surface parallel to each other of the deformed portion constituting the MEMS is set to the vicinity of the (0001) plane and the other surface is set to the vicinity of the (000-1) plane, one surface is obtained by simply performing thermal oxidation. In addition, silicon oxide layers having different thicknesses can be formed on the other surface. Considering the process for producing the SiC microstructure, the plane that can be favorably homoepitaxially grown is not a complete (0001) plane and (000-1) plane, but 0.5 to 10 degrees from the plane. It is preferable to make the surface inclined to some extent.

  As described above, when the shape of the deformed portion is formed by electrochemical or photoelectrochemical etching, layers of different conductivity types are formed. For this reason, epitaxial growth and ion implantation are performed. When these are performed on the (0001) plane and the (000-1) plane, an off angle of several degrees or more is generally added. However, in the present invention, the difference in the growth rate of the thermal oxide film formed on the (0001) plane and the (000-1) plane is used. This difference decreases as the off-angle is increased, and the above-described difference disappears in the plane where the off-angle is 90 degrees.

  In order to obtain a layer thickness difference of more than twice between the silicon oxide layer formed on the first surface and the silicon oxide layer formed on the second surface by thermal oxidation, the off angle should be suppressed to about 10 degrees. desirable. From the viewpoint of obtaining this large layer thickness, it is best to set the off angle to 0. On the other hand, from the viewpoint of ease of manufacture, the off angle is preferably set to 0.5 degrees or more, but about 10 degrees. If there is a misorientation, the difference in thermal oxidation rate between the first surface and the second surface is sufficient and there is no problem.

  For example, if the deformed portion has a cantilever structure, the cantilever can be warped by performing the above-described thermal oxidation process. A cantilever with a warped shape is a very useful structure in a MEMS that handles fluid.

  For example, as shown in FIG. 2, a fine structure including a deforming portion 205 and a supporting portion 206 formed on a single crystal SiC substrate 201 having a main surface of (0001) plane is arranged in a flow path 211. As described above, the silicon oxide layer 251 and the silicon oxide layer 252 are formed in the deformed portion 205 by thermal oxidation, and the deformable portion 205 is along the direction away from the substrate 201. The channel 211 is configured by a channel groove formed in a channel substrate 201 a fixed on the substrate 201.

  According to the MEMS having this fine mechanical structure, if the change of the displacement of the deforming portion 205 due to the flow of the fluid in the flow path 211 is detected, a sensor for detecting the state of the flowing fluid can be obtained. The deforming portion 205 is warped in a state where the tip portion approaches the inner wall on the flow path substrate 201a side, and if a state in which a fluid flows from the right side to the left side in FIG. For example, the tip of the deformable portion 205 is displaced toward the substrate 201. If this displacement state is detected, the state of the fluid flowing in the flow path 211 can be detected.

  Further, for example, as shown in FIG. 3, if the tip of the deformable portion 205 is placed close to the inner wall of the flow channel 211 on the flow channel substrate 201a side, it can be used as a check valve. In the direction 301, the fluid flows when the deforming portion 205 is deformed toward the substrate 201, and in the direction 302, the deforming portion 205 deforms and contacts the inner wall side on the flow path substrate 201a side to stop the fluid flow.

  Depending on the MEMS device to be manufactured, as shown in FIGS. 4 and 5, there may be a case where a microstructure 404 including a deformable portion 405 that bends in the in-plane direction of the plane of the substrate 401 on which the support portion 402 is formed may be necessary. . In FIG. 4, (a) shows a cross section taken along the line aa 'in (b). FIG. 5 also shows a cross section similar to FIG. Considering the rectangular parallelepiped deformable portion 405, the above-described state can be realized by making the surface 401a of the substrate 401 a crystal plane orthogonal to the (0001) plane. FIG. 5 shows a state in which two support portions 402 each having a plurality of deformation portions 405 are arranged facing each other on the surface where the deformation portions 405 are formed.

  For example, if the front surface 401a is a (11-20) plane or a (1-100) plane, the back surface of the substrate 401 becomes a (−1-120) plane or a (−1100) plane equivalent to this crystal plane, and thermal oxidation is performed. The speed is the same, and silicon oxide layers with the same thickness are formed on the front and back surfaces. On the other hand, if the longitudinal direction of the rectangular parallelepiped deformable portion 405 is appropriately set, the first surface 405a and the second surface 405b which are side surfaces orthogonal to the front surface 401a (back surface) are the (0001) surface and (000-1). It becomes a surface. As a result, the silicon oxide layer 451 formed on the first surface 405a and the silicon oxide layer 452 formed on the second surface 405b have different layer thicknesses, and the deformed portion 405 has a planar direction (in-plane) of the surface 401a. Direction).

  Further, the amount of warpage can be controlled even on the same wafer by changing the longitudinal state (length, etc.) of the rectangular parallelepiped deformable portion 405. If cantilevers (deformed portions) facing various directions are produced on one chip, different warpage amounts can be added to each.

  Even in the case described above, the surface 401a of the substrate 401 is not completely orthogonal to the (0001) plane, and there is no problem even if there is a deviation of about 20 degrees from the orthogonal plane. By the way, in the case of tilting from the (11-20) plane to the (0001) plane or the (000-1) plane direction, the off-angle should be about 10 degrees as described above. Further, when tilting from the (11-20) plane to the (1-100) plane direction, the side surfaces are the (0001) and (000-1) planes, and the off angle may exceed 10 degrees. The angle difference between the (11-20) plane and the (1-100) plane is 30 degrees, and the range of 15 degrees from each plane includes all planes that are crystallographically perpendicular to the (0001) plane. It comes to be.

  The amount of warpage of the fine structure (deformed portion) includes the shape, such as the width, length, and thickness of the deformed portion, the difference in thickness between the silicon oxide layers on the first surface and the second surface parallel to each other, and the formation of the silicon oxide layer. It can be controlled arbitrarily according to the atmosphere and forming temperature.

  As described above, it is possible to introduce internal stress and warpage into the deformed portion constituting the fine structure made of single crystal SiC.

  By the way, the extremely high Q value of the fine structure (micro mechanical structure) of single crystal SiC used as MEMS is derived from SiC. When a silicon oxide layer is formed, the Q value is lowered and the advantage of single crystal SiC is obtained. May be reduced. As a result of the experiment, if the thickness (thickness) of the silicon oxide layer is up to about 50 nm, the Q value will be about half, but it was confirmed that the Q value far exceeding the MEMS formed from Si is maintained. There is no problem.

  Next, the actually produced fine mechanical structure will be described. Below, the result of actually producing the fine mechanical structure (the length of the deformed portion is 120 μm) in the embodiment described with reference to FIG. 1 will be described. The result of measuring the shape of the produced deformed portion with an optical profiler is shown in FIG. FIG. 6 shows the measurement result of the deformed portion before the thermal oxidation.

  In the graph of FIG. 6, the horizontal portion on the right side is the support portion, and the portion that once falls toward the left side of FIG. 6 from here is undercut to start the portion that becomes a hollow structure. The part which shows a part and is gradually raised on the left side is a deformation | transformation part (length 120 micrometers). Moreover, the part which has fallen by 0.01 mm of a graph becomes a front-end | tip part of a deformation | transformation part (cantilever).

  In FIG. 6, the vertical axis is enlarged, but in the case of the deformed portion, it can be said that it is substantially horizontal only by warping upward by about 0.3 μm with respect to the length of 120 μm. This is because there is almost no internal stress because it is made of single crystal SiC. Although unwanted (not required) warpage is a problem in MEMS, the above-described results show that such a problem hardly occurs in a fine structure composed of single crystal SiC.

  Next, a state in which warpage is given by performing thermal oxidation on the above-described deformed portion will be described. Thermal oxidation was performed in an oxygen gas atmosphere at 1000 ° C. for 2 hours. Under this condition, the silicon oxide layer on the first surface which is the (0001) plane has a thickness of about 5 nm, and the silicon oxide layer on the second surface which is the (000-1) plane has a thickness of about 40-50 nm. FIG. 7 shows the result of measuring the shape of the microstructure (deformed part) after thermal oxidation thus produced with an optical profiler. As shown in FIG. 7, it is very clearly warped upward (in the direction of the first surface away from the substrate side). The amount of warpage has reached 2 μm, which is much larger than 0.3 μm before thermal oxidation.

The main mechanisms for warping are as follows. There is a difference in thermal expansion coefficient between SiC and SiO ₂ . Considering that the strain is relaxed at the temperature during thermal oxidation (thermal oxidation temperature), SiC contracts more than SiO ₂ in the process of lowering the temperature from the thermal oxidation temperature to room temperature. Tensile stress acts on.

  In the example described above, since the silicon oxide layer on the second surface is thicker, the relationship between the silicon oxide layer on the second surface side and SiC is dominant in the generation of stress due to the difference in shrinkage. At this time, since SiC contracts more significantly than the silicon oxide layer on the second surface, the tip of the deformed portion (cantilever) made of SiC is warped upward away from the substrate side.

  If you want to warp downward (on the substrate side), use a single crystal SiC substrate with the main surface of the (000-1) plane opposite to the above-described configuration, and the substrate side is the first surface Then, a deformed portion whose second surface parallel to this is in the vicinity of the (000-1) plane is formed, and then thermal oxidation is performed.

  As adjustment (control) of the thickness of the silicon oxide layer, for example, the silicon oxide layer formed after thermal oxidation may be etched to some extent. By this etching, the thicknesses of the silicon oxide layers formed on the first surface and the second surface may be controlled.

  By the way, in order to give a significant warp to the deformed portion, it is important that the thickness of the silicon oxide layer on the side to be formed thicker (second surface) is 10 nm or more. On the other hand, as the thickness of the silicon oxide layer increases, the resonance characteristics of the deformed portion deteriorate. When utilizing the resonance characteristics, the thickness of the silicon oxide layer formed in the deformed portion is preferably 200 nm or less. When the resonance characteristic is not used, the thickness of the silicon oxide layer formed in the deformed portion may exceed 200 nm.

  In addition, the silicon oxide layer formed on the second surface may be five times thicker than the silicon oxide layer formed on the first surface. As described above, the thicker the silicon oxide layer, the easier it is to give a significant warp to the deformed portion. However, by providing a difference of 5 times or more in the thickness of the two silicon oxide layers described above, the silicon oxide layer is more Even in a thin state, a large warp can be given to the deformed portion.

  By the way, even if the shape can be controlled as described above, if the high Q value, which is a characteristic of MEMS using single crystal SiC, is lost, the attractiveness is reduced by half. 8 and 9 show the resonance characteristics (in vacuum) of the deformed portion (cantilever) before and after thermal oxidation produced as described above. FIG. 8 is a characteristic diagram showing the resonance characteristics (in vacuum) of the deformed part before thermal oxidation. FIG. 9 is a characteristic diagram showing the resonance characteristics (in vacuum) of the deformed portion after the above-described thermal oxidation.

  As shown in FIG. 8, it shows a very high value of Q = 17,000 before thermal oxidation. Further, as shown in FIG. 9, after thermal oxidation, although it is slightly lowered, a high value of Q = 90000 is maintained. It is important to set the shape and dimensions of the deformed portion, the thickness of the silicon oxide layer by thermal oxidation, and the conditions within a range in which the required Q value can be maintained.

  As described above, according to the present invention, the first surface and the second surface that are parallel to each other are used by using the point that the silicon oxide layer formed by thermal oxidation in single-crystal SiC has different crystal planes. Since the silicon oxide layers having different thicknesses are formed on the first surface and the second surface by thermally oxidizing the deformed portion, it is possible to change the shape and control the characteristics in the MEMS using single crystal SiC. It becomes like this.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in the above description, a cantilever has been described as an example of the deformable portion, but the present invention is not limited to this. The deformable portion may have a dual-supported beam (bridge) structure or a membrane structure. Regardless of the structure, for example, the MEMS described with reference to FIG. 2 can be realized.

  The membrane structure may be formed as follows, for example. First, a through hole is formed in an n-type single crystal SiC layer formed on a p-type single crystal SiC substrate having a (0001) plane as a main surface, and a plane is formed through the formed through hole by electrochemical etching. A single crystal SiC substrate is etched uniformly in the direction to form a cavity. Thereby, the membrane structure provided with the membrane (deformed part) centering on the through hole can be formed. The back surface on the cavity side of the membrane structure is the (000-1) plane. Thereafter, if thermal oxidation is performed, a silicon oxide layer is also formed on the inner wall surface of the cavity through the through hole, and the membrane surface on the (0001) plane outside the cavity and the (000-1) inside the cavity (000-1). The thickness of the silicon oxide layer is different from that of the membrane back surface. As a result, a fine mechanical structure is obtained in which the membrane deforms convexly on the side opposite to the cavity.

  DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... N-type layer, 103 ... Opening region, 104 ... Fine structure, 105 ... Deformation part, 105a ... First surface, 105b ... Second surface, 106 ... Supporting part, 151, 152 ... Silicon oxide layer.

Claims (4)

A shape forming step of forming a microstructure comprising a deformable portion having a first surface and a second surface that are parallel to each other, and a support portion that supports a portion of the deformable portion, formed of single-crystal SiC on a substrate. When,
A thermal oxidation step of forming a silicon oxide layer on each of the first surface and the second surface by thermally oxidizing the surface of the microstructure after forming the microstructure; and
The method for manufacturing a fine mechanical structure, wherein the first surface and the second surface are crystal surfaces having different growth rates of a silicon oxide layer formed by thermal oxidation. In the manufacturing method of the micro mechanical structure of Claim 1,
The method for manufacturing a micromechanical structure, wherein the first surface is a crystal plane within a range of 10 degrees from a (0001) plane. In the manufacturing method of the micro mechanical structure of Claim 1 or 2,
The method for manufacturing a micromechanical structure, wherein the silicon oxide layer formed on the first surface has a thickness of 10 to 200 nm. In the manufacturing method of the micro mechanical structure of any one of Claims 1-3,
The method for manufacturing a micromechanical structure, wherein the thickness of the silicon oxide layer formed on the first surface is not less than five times the thickness of the silicon oxide layer formed on the second surface.

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