Patent Publication Number: US-11643324-B2

Title: MEMS sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application corresponds to Japanese Patent Application No. 2019-146424 filed in the Japan Patent Office on Aug. 8, 2019, and the entire disclosure of the application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a MEMS sensor. 
     BACKGROUND ART 
     Patent Document 1 (Japanese Patent Application Publication No. 2016-17747) discloses a pressure sensor manufactured by a manufacturing method that has a step of preparing a SOI substrate in which a first silicon substrate and a second silicon substrate are stacked together with an oxide layer between the first and second silicon substrates and removing both the second silicon substrate and the oxide layer so that the first silicon substrate is exposed, a step of forming a diaphragm by forming a cavity in the first silicon substrate that has been exposed, and a step of sealing the cavity by joining a surface in which the cavity of the first silicon substrate is formed and a base substrate configured to have silicon together without interposing the oxide layer between the surface and the base substrate. 
     SUMMARY OF INVENTION 
     The present inventor has found that characteristics of a piezoresistance formed at a silicon diaphragm change depending on the plane orientation of silicon. 
     Therefore, an object of the present invention is to provide a MEMS sensor that is capable of raising characteristics of a piezoresistance to a higher grade than in the past. 
     A MEMS sensor according to an aspect of the present invention includes a silicon substrate that has a first surface and a second surface on a side opposite to the first surface and that has a cavity in the first surface, a silicon diaphragm that has a first surface and a second surface on aside opposite to the first surface and in which the second surface is joined directly to the first surface of the silicon substrate, and a piezoresistance formed at the first surface of the silicon diaphragm, and, in the MEMS sensor, a plane orientation of the first surface of the silicon substrate and a plane orientation of the first surface of the silicon diaphragm differ from each other. 
     EFFECTS OF INVENTION 
     According to the MEMS sensor according to one aspect of the present invention, the plane orientation of the first surface of the silicon substrate and the plane orientation of the first surface of the silicon diaphragm differ from each other. This makes it possible to employ a plane orientation in which characteristics of piezoresistances become optimal as the plane orientation of the first surface of the silicon diaphragm without being restricted by the plane orientation of the first surface of the silicon substrate. 
     Additionally, the cavity is formed not at the silicon diaphragm but at the silicon substrate that is thicker than the silicon diaphragm. Therefore, it is possible to form the cavity highly accurately and easily. Additionally, the MEMS sensor is formed by Si—Si direct junction between the silicon substrate and the silicon diaphragm. In other words, the substrate and the diaphragm are equal to each other in linear expansion coefficient, and therefore it is possible to restrain the occurrence of stress in the junction interface that results from a temperature change. Consequently, it is possible to stabilize characteristics of piezoresistances. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a plan view of a MEMS sensor according to a first preferred embodiment of the present invention. 
         FIG.  2    is a cross-sectional view of the MEMS sensor according to the first preferred embodiment of the present invention, showing a cross section along line II-II of  FIG.  1   . 
         FIG.  3 A  to  FIG.  3 F  are views showing part of a process of manufacturing the MEMS sensor according to the first preferred embodiment of the present invention. 
         FIG.  4    is a view showing a modification of the step of  FIG.  3 B . 
         FIG.  5 A  is a view to describe the piezoresistance coefficient of a piezoresistance formed at a (100) surface. 
         FIG.  5 B  is a view to describe the piezoresistance coefficient of a piezoresistance formed at a (110) surface. 
         FIG.  6    is a plan view of a MEMS sensor according to a second preferred embodiment of the present invention. 
         FIG.  7    is a cross-sectional view of the MEMS sensor according to the second preferred embodiment of the present invention. 
         FIG.  8 A  to  FIG.  8 G  are views showing part of a process of manufacturing the MEMS sensor according to the second preferred embodiment of the present invention. 
         FIG.  9    is a plan view of a MEMS sensor according to a third preferred embodiment of the present invention. 
         FIG.  10    is across-sectional view of the MEMS sensor according to the third preferred embodiment of the present invention, showing a cross section along line X-X of  FIG.  9   . 
         FIG.  11 A  to  FIG.  11 G  are views showing part of a process of manufacturing the MEMS sensor according to the third preferred embodiment of the present invention. 
         FIG.  12 A  to  FIG.  12 C  are views, respectively, showing structures  1  to  3  of MEMS sensors used in examples. 
         FIG.  13    is a view showing a relationship between He processing time and the amount of change in atmospheric pressure value of the structures  1  to  3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First, an itemized description will be given of preferred embodiments of the present invention. 
     A MEMS sensor according to one preferred embodiment of the present invention includes a silicon substrate that has a first surface and a second surface on a side opposite to the first surface and that has a cavity in the first surface, a silicon diaphragm that has a first surface and a second surface on aside opposite to the first surface and in which the second surface is joined directly to the first surface of the silicon substrate, and a piezoresistance formed at the first surface of the silicon diaphragm, and, in the MEMS sensor, a plane orientation of the first surface of the silicon substrate and a plane orientation of the first surface of the silicon diaphragm differ from each other. 
     According to this arrangement, the plane orientation of the first surface of the silicon substrate and the plane orientation of the first surface of the silicon diaphragm differ from each other. This makes it possible to employ a plane orientation in which characteristics of the piezoresistance become optimal as the plane orientation of the first surface of the silicon diaphragm without being restricted by the plane orientation of the first surface of the silicon substrate. 
     Additionally, the cavity is formed not at the silicon diaphragm but at the silicon substrate that is thicker than the silicon diaphragm. Therefore, it is possible to form the cavity highly accurately and easily. Additionally, the MEMS sensor is formed by Si—Si direct junction between the silicon substrate and the silicon diaphragm. In other words, the substrate and the diaphragm are equal to each other in linear expansion coefficient, and therefore it is possible to restrain the occurrence of stress in the junction interface that results from a temperature change. Consequently, it is possible to stabilize characteristics of the piezoresistance. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the plane orientation of the first surface of the silicon substrate may be a (100) surface, and the plane orientation of the first surface of the silicon diaphragm is a (110) surface. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the piezoresistance may be formed in a longitudinal shape that extends in a direction of a &lt;111&gt; axis. 
     According to this arrangement, the plane orientation of the first surface of the silicon diaphragm is a (100) surface, and the piezoresistance is formed in a longitudinal shape that extends in a direction of a &lt;111&gt; axis. Hence, it is possible to enlarge the longitudinal piezoresistance coefficient of the piezoresistance, and therefore it is possible to improve the sensibility of the piezoresistance. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the cavity may be formed in a quadrangular shape in a plan view, and the piezoresistance may include a plurality of piezoresistances disposed at each side of the cavity in a plan view, and the plurality of piezoresistances may be each formed in a longitudinal shape that extends in the direction of the &lt;111&gt; axis. 
     According to this arrangement, the plane orientation of the first surface of the silicon substrate is a (100) surface, and each of the piezoresistances is formed in a longitudinal shape that extends in the direction of the &lt;111&gt; axis. Hence, it is possible to enlarge the longitudinal piezoresistance coefficient of the piezoresistance, and therefore it is possible to improve the sensibility of the piezoresistance. 
     Additionally, the piezoresistance is disposed at each side of the cavity. Hence, when the silicon diaphragm vibrates, the piezoresistances disposed at a pair of first and third sides facing each other of the cavity are pulled in the longitudinal direction, and resistance increases. On the other hand, the piezoresistances disposed at a pair of second and fourth sides facing each other of the cavity are pulled in the width direction, and resistance decreases. As a result, it is possible to make a large difference in the resistance value between the piezoresistances of the first and third sides and the piezoresistances of the second and fourth sides. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the plurality of piezoresistances may include a first piezoresistance, a second piezoresistance, a third piezoresistance, and a fourth piezoresistance, and the MEMS sensor may include a wiring that is electrically connected to the first to fourth piezoresistances and that forms a bridge circuit including the first to fourth piezoresistances. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the silicon diaphragm may have a predetermined thickness. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the silicon diaphragm may have a thickness of 3 μm to 30 μm. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the cavity may be sealed with the silicon diaphragm. 
     According to this arrangement, the MEMS sensor is formed by Si—Si direct junction between the silicon substrate and the silicon diaphragm, and therefore an oxide film is not formed therebetween. As a result, it is possible to prevent a gas from entering the cavity through the oxide film even when the MEMS sensor is used in a gaseous environment in which the radius of an atom, such as helium (He) or hydrogen (H 2 ), is small. As a result, it is possible to hold the degree of vacuum of the inside of the cavity, and therefore it is possible to prevent the property fluctuation of the MEMS sensor. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the silicon diaphragm may have a through-hole that leads to the cavity. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the cavity may be formed in a quadrangular shape in a plan view, and the silicon diaphragm may have a plurality of through-holes that lead to the cavity. Each of the plurality of through-holes may be formed at each corner portion of the cavity in a plan view. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the silicon diaphragm may have a second cavity that faces the cavity. The second cavity may be formed at the second surface of the silicon diaphragm. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the second cavity may be formed annularly along a circumferential edge of the cavity in a plan view. 
     In the MEMS sensor according to one preferred embodiment of the present invention, the cavity may be sealed with the silicon diaphragm. 
     According to this arrangement, the MEMS sensor is formed by Si—Si direct junction between the silicon substrate and the silicon diaphragm, and therefore an oxide film is not formed therebetween. As a result, it is possible to prevent a gas from entering the cavity through the oxide film even when the MEMS sensor is used in a gaseous environment in which the radius of an atom, such as helium (He) or hydrogen (H 2 ), is small. As a result, it is possible to hold the degree of vacuum of the inside of the cavity, and therefore it is possible to prevent the property fluctuation of the MEMS sensor. 
     In the MEMS sensor according to one preferred embodiment of the present invention, a width of a junction interface between the silicon substrate and the silicon diaphragm that is defined by a distance from the circumferential edge of the cavity to an end surface of the silicon substrate in a plan view may be 50 μm to 500 μm. 
     Detailed Description of Preferred Embodiments of the Present Invention 
     Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG.  1    is a plan view of a MEMS sensor  1  according to a first preferred embodiment of the present invention.  FIG.  2    is a cross-sectional view of the MEMS sensor  1  according to the first preferred embodiment of the present invention, showing a cross section along line II-II of  FIG.  1   . 
     The MEMS sensor  1  is applicable to various sensors, such as an atmospheric pressure sensor or a pressure sensor. The MEMS sensor  1  includes a silicon substrate  2  and a silicon diaphragm  3 . 
     The silicon substrate  2  has a first surface  4  and a second surface  5  on the side opposite to the first surface  4 . The first surface  4  and the second surface  5  of the silicon substrate  2  may be referred to as a front surface and a rear surface of the silicon substrate  2 , respectively. The silicon substrate  2  additionally has an end surface  6 . In the present preferred embodiment, the silicon substrate  2  is formed in a quadrangular shape in a plan view, and the end surface  6  includes four end surfaces  6  that form four sides of the silicon substrate  2  in a plan view. The end surface  6  of the silicon substrate  2  may be referred to as a lateral surface of the silicon substrate  2  or as a third surface. 
     In the present preferred embodiment, the first surface  4  of the silicon substrate  2  is a (100) surface, and the second surface  5  is a (100) surface, and the end surface  6  is a (110) surface. 
     The thickness of the silicon substrate  2  is, for example, 100 μm to 775 μm. The silicon substrate  2  has a cavity  7  formed in the first surface  4 . The depth of the cavity  7  is, for example, 5 μm to 20 μm. The cavity  7  is formed in a substantially quadrangular shape in a plan view as shown in  FIG.  1   . More specifically, the cavity  7  is formed in a quadrangular shape that has four corner portions a corner of each of which has been rounded off in a plan view, i.e., the cavity  7  is formed in a quadrangular shape that has a first corner portion  46 A, a second corner portion  46 B, a third corner portion  46 C, and a fourth corner portion  46 D. Therefore, the cavity  7  has a first side  7 A, a second side  7 B, a third side  7 C, and a fourth side  7 D in a plan view. 
     The silicon diaphragm  3  is made of a silicon material thinner than the silicon substrate  2 , and has a first surface  8  and a second surface  9  on the side opposite to the first surface  8 . The first surface  8  and the second surface  9  of the silicon diaphragm  3  may be referred to as a front surface and a rear surface of the silicon diaphragm  3 , respectively. In the present preferred embodiment, the first surface  8  of the silicon diaphragm  3  is a (110) surface, and the second surface  9  is a (110) surface. The silicon diaphragm  3  has a predetermined thickness. The thickness of the silicon diaphragm  3  is, for example, 3 μm to 30 μm. Additionally, the silicon diaphragm  3  is membranous, and hence may be referred to as a silicon membrane. 
     The silicon diaphragm  3  is joined directly to the silicon substrate  2 . More specifically, the second surface  9  of the silicon diaphragm  3  is joined directly to the first surface  4  of the silicon substrate  2 . Hence, a junction interface  10  of Si—Si junction is formed between the silicon substrate  2  and the silicon diaphragm  3 . 
     The junction interface  10  is formed in a closed-annular shape that surrounds the cavity  7  in the present preferred embodiment, and hence the cavity  7  is sealed by the silicon diaphragm  3 . The width W 1  of the junction interface  10  is, for example, 50 μm to 500 μm in a plan view. The width W 1  of the junction interface  10  may be defined by a distance from a circumferential edge  11  (first to fourth sides  7 A to  7 D) of the cavity  7  to the end surface  6  of the silicon substrate  2 . 
     The MEMS sensor  1  additionally includes piezoresistances R 1  to R 4  serving as strain gauges, metallic terminals  12  to  16 , and metallic wirings  17  to  20 . 
     The piezoresistances R 1  to R 4  are diffusion resistances formed on the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3 , and may be referred to as “gauges,” respectively. In the present preferred embodiment, the silicon diaphragm  3  includes a movable portion  21  that moves in response to pressure fluctuations of the cavity  7  while facing the cavity  7  and a fixed portion  22  joined to the silicon substrate  2 . 
     The piezoresistances R 1  to R 4  are disposed with substantially equal intervals between the piezoresistances R 1  to R 4  along a circumferential direction of the movable portion  21  formed in a substantially quadrangular shape in a plan view. More specifically, the first piezoresistance R 1  is disposed at the first side  7 A of the cavity  7 , and the second piezoresistance R 2  is disposed at the second side  7 B of the cavity  7 , and the third piezoresistance R 3  is disposed at the third side  7 C of the cavity  7 , and the fourth piezoresistance R 4  is disposed at the fourth side  7 D of the cavity  7  in a plan view. 
     The first piezoresistance R 1  and the third piezoresistance R 3  that face each other with the center of the movable portion  21  between the first and third piezoresistances R 1  and R 3  are each formed in a longitudinal shape extending in a mutually-facing direction of the first and third piezoresistances R 1  and R 3  (i.e., direction that crosses the first side  7 A and the third side  7 C of the cavity  7 ). The first piezoresistance R 1  and the third piezoresistance R 3  straddle between the movable portion  21  and the fixed portion  22  in a plan view. In other words, the first piezoresistance R 1  and the third piezoresistance R 3  straddle between the inside and the outside of the cavity  7  in a plan view. 
     The number of first piezoresistances R 1  and the number of third piezoresistances R 3  formed in the present preferred embodiment are four and four, respectively. The four first piezoresistances R 1  are arranged with intervals between the first piezoresistances R 1  along a boundary portion between the movable portion  21  and the fixed portion  22  (i.e., along the circumferential edge  11  of the cavity  7 ). A first relay wiring  23  is formed between mutually-adjoining first piezoresistances R 1 . 
     The first relay wiring  23  connects the four first piezoresistances R 1  together in series. The first relay wiring  23  is connected to one end portion in a longitudinal direction of each of the first piezoresistances R 1 , and some of the first relay wirings  23  (in the present preferred embodiment, two) are formed at the movable portion  21  of the silicon diaphragm  3 , and the remaining first relay wirings  23  are formed at the fixed portion  22  of the silicon diaphragm  3 . The first relay wiring  23  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     Likewise, the four third piezoresistances R 3  are arranged with intervals between the third piezoresistances R 3  along the boundary portion between the movable portion  21  and the fixed portion  22  (i.e., along the circumferential edge  11  of the cavity  7 ). A third relay wiring  25  is formed between mutually-adjoining third piezoresistances R 3 . The third relay wiring  25  connects the four third piezoresistances R 3  together in series. 
     The third relay wiring  25  is connected to one end portion in a longitudinal direction of each of the third piezoresistances R 3 , and some of the third relay wirings  25  (in the present preferred embodiment, two) are formed at the movable portion  21  of the silicon diaphragm  3 , and the remaining third relay wirings  25  are formed at the fixed portion  22  of the silicon diaphragm  3 . The third relay wiring  25  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     On the other hand, the second piezoresistance R 2  and the fourth piezoresistance R 4  that face each other with the center of the movable portion  21  between the second and fourth piezoresistances R 2  and R 4  are each formed in a longitudinal shape extending in a direction perpendicular to a mutually-facing direction of the second and fourth piezoresistances R 2  and R 4  (i.e., direction along the second side  7 B and the fourth side  7 D of the cavity  7 ). The second piezoresistance R 2  and the fourth piezoresistance R 4  are housed inside the movable portion  21  in a plan view. 
     The number of second piezoresistances R 2  and the number of fourth piezoresistances R 4  formed in the present preferred embodiment are four and four, respectively. The four second piezoresistances R 2  are arranged with intervals between the second piezoresistances R 2  in a 2×2 matrix manner. A second relay wiring  24  is formed between mutually-adjoining second piezoresistances R 2 . 
     The second relay wiring  24  connects the four second piezoresistances R 2  together in series. The second relay wiring  24  is connected to one end portion in a longitudinal direction of each of the second piezoresistances R 2 , and all of the second relay wirings  24  (in the present preferred embodiment, three) are formed at the movable portion  21  of the silicon diaphragm  3 . The second relay wiring  24  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     Likewise, the four fourth piezoresistances R 4  are arranged with intervals between the fourth piezoresistances R 4  in a 2×2 matrix manner. A fourth relay wiring  26  is formed between mutually-adjoining fourth piezoresistances R 4 . The fourth relay wiring  26  connects the four fourth piezoresistances R 4  together in series. 
     The fourth relay wiring  26  is connected to one end portion in a longitudinal direction of each of the fourth piezoresistances R 4 , and all of the fourth relay wirings  26  (in the present preferred embodiment, three) are formed at the movable portion  21  of the silicon diaphragm  3 . The fourth relay wiring  26  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     A first contact wiring  27  is connected to each end portion in the longitudinal direction of a pair of first piezoresistances R 1 , which are outer ones, of the four first piezoresistances R 1 . A pair of first contact wirings  27  are connected to the first piezoresistance R 1  in the fixed portion  22 , and extend in mutually-opposite directions along the boundary portion between the movable portion  21  and the fixed portion  22  (i.e., along the circumferential edge  11  of the cavity  7 ). The first contact wiring  27  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     Likewise, a third contact wiring  29  is connected to each end portion in the longitudinal direction of a pair of third piezoresistances R 3 , which are outer ones, of the four third piezoresistances R 3 . A pair of third contact wirings  29  are connected to the third piezoresistance R 3  in the fixed portion  22 , and extend in mutually-opposite directions along the boundary portion between the movable portion  21  and the fixed portion  22  (i.e., along the circumferential edge  11  of the cavity  7 ). The third contact wiring  29  is a diffusion wiring (p+ type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     A second contact wiring  28  is connected to each end portion in the longitudinal direction of a pair of second piezoresistances R 2 , which are closer to the boundary portion (the circumferential edge  11  of the cavity  7 ) between the movable portion  21  and the fixed portion  22 , of the four second piezoresistances R 2 . The pair of second contact wirings  28  are connected to the second piezoresistance R 2  in the movable portion  21 , and extend in the same direction while crossing the boundary portion (the circumferential edge  11  of the cavity  7 ) between the movable portion  21  and the fixed portion  22 . The second contact wiring  28  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     A fourth contact wiring  30  is connected to each end portion in the longitudinal direction of a pair of fourth piezoresistances R 4 , which are closer to the boundary portion (the circumferential edge  11  of the cavity  7 ) between the movable portion  21  and the fixed portion  22 , of the four fourth piezoresistances R 4 . The pair of fourth contact wirings  30  are connected to the fourth piezoresistance R 4  in the movable portion  21 , and extend in the same direction while crossing the boundary portion (the circumferential edge  11  of the cavity  7 ) between the movable portion  21  and the fixed portion  22 . The fourth contact wiring  30  is a diffusion wiring (p +  type region) formed at the first surface  8  of the silicon diaphragm  3  by introducing impurities, such as boron (B), into the silicon diaphragm  3  with a high concentration. 
     Referring to  FIG.  2   , an insulating layer  31  is formed on the first surface  8  of the silicon diaphragm  3 . The insulating layer  31  may be made of, for example, silicon oxide (SiO 2 ) or silicon nitride (SiN). The insulating layer  31  covers the movable portion  21  and the fixed portion  22  of the silicon diaphragm  3 . The thickness of the insulating layer  31  may be, for example, 0.1 μm to 2.0 μm. 
     The metallic terminals  12  to  16  include a first metallic terminal  12 , a second metallic terminal  13 , a third metallic terminal  14 , a fourth metallic terminal  15 , and a fifth metallic terminal  16 . The first to fifth metallic terminals  12  to  16  are formed on the insulating layer  31 . The first to fifth metallic terminals  12  to  16  are arranged with intervals between the first to fifth metallic terminals  12  to  16  along one end surface  6  of the silicon substrate  2  in a plan view. The first to fifth metallic terminals  12  to  16  are each made of aluminum (Al) in the present preferred embodiment. The first to fourth metallic terminals  12  to  15  may be referred to as an earth terminal (GND), an negative-side voltage output terminal (Vout−), a voltage application terminal (Vdd), and a positive-side voltage output terminal (Vout+) in accordance with objects to which these metallic terminals  12  to  15  are respectively connected. The fifth metallic terminal  16  is a terminal of the substrate, and is set to become an electric potential greater than the voltage application terminal (Vdd). 
     The first to fourth metallic wirings  17  to  20  are wirings to form a bridge circuit (Wheatstone bridge) by means of a bridge connection of the piezoresistances R 1  to R 4 . 
     Specifically, the first metallic wiring  17  connects the first piezoresistance R 1  and the second piezoresistance R 2  together at the fixed portion  22 , and is connected to the first metallic terminal  12 . The second metallic wiring  18  connects the second piezoresistance R 2  and the third piezoresistance R 3  together at the fixed portion  22 , and is connected to the second metallic terminal  13 . The third metallic wiring  19  connects the third piezoresistance R 3  and the fourth piezoresistance R 4  together at the fixed portion  22 , and is connected to the third metallic terminal  14 . The fourth metallic wiring  20  connects the fourth piezoresistance R 4  and the first piezoresistance R 1  together at the fixed portion  22 , and is connected to the fourth metallic terminal  15 . 
     The first to fourth metallic wirings  17  to  20  are each made of aluminum (Al) in the present preferred embodiment, and are formed on the insulating layer  31 . The first metallic wiring  17  is connected to the first and second contact wirings  27  and  28  through the insulating layer  31 , and the second metallic wiring  18  is connected to the second and third contact wirings  28  and  29  through the insulating layer  31  (see  FIG.  2   ). The third metallic wiring  19  is connected to the third and fourth contact wirings  29  and  30  through the insulating layer  31  (see  FIG.  2   ), and the fourth metallic wiring  20  is connected to the fourth and first contact wirings  30  and  27  through the insulating layer  31 . 
     A fifth wiring  32  is connected to the fifth metallic terminal  16 . The fifth wiring  32  includes a metallic wiring  33  formed on the insulating layer  31  and a diffusion wiring  34  formed on the first surface  8  of the silicon diaphragm  3 . Referring to  FIG.  2   , the diffusion wiring  34  is formed in an annular shape surrounding the cavity  7  in a plan view. The diffusion wiring  34  may pass through a space below the first to fifth metallic terminals  12  to  16 . In other words, the diffusion wiring  34  may overlap with at least one of the first to fifth metallic terminals  12  to  16  in a plan view. 
     A passivation film (not shown) with which the first to fourth metallic wirings  17  to  20  and the first to fifth metallic terminals  12  to  16  are covered may be formed on the insulating layer  31 . 
       FIG.  3 A  to  FIG.  3 F  are views showing part of a process of manufacturing the MEMS sensor  1  according to the first preferred embodiment of the present invention.  FIG.  4    is a view showing a modification of a step of  FIG.  3 B . 
     In order to manufacture the MEMS sensor  1 , the silicon substrate  2  is prepared, and the cavity  7  is formed in the silicon substrate  2 , for example, as shown in  FIG.  3 A . The cavity  7  may be formed by selectively applying dry etching to the first surface  4  of the silicon substrate  2 . 
     Thereafter, a SOI substrate  35  is prepared as shown in  FIG.  3 B . The SOI substrate  35  includes a support substrate  36  made of silicon, a BOX (Buried Oxide) layer  37  on the support substrate  36 , and an active layer  38  made of silicon on the BOX layer  37 . For example, the thickness of the support substrate  36  may be 625 μm to 775 μm, and the thickness of the BOX layer  37  may be 0.5 μm to 2.0 μm, and the thickness of the active layer  38  may be 3 μm to 30 μm. 
     The support substrate  36  has a first surface  39  contiguous to the BOX layer  37  and a second surface  40  on the side opposite to the first surface  39 . In the present preferred embodiment, the first surface  39  of the support substrate  36  is a (100) surface, and the second surface  40  is a (100) surface. The active layer  38  has a first surface  41  contiguous to the BOX layer  37  and a second surface  42  on the side opposite to the first surface  41 . In the present preferred embodiment, the first surface  41  of the active layer  38  is a (110) surface, and the second surface  42  is a (110) surface. 
     Thereafter, the SOI substrate  35  is reversed up and down, and the SOI substrate  35  is bonded onto the silicon substrate  2  so that the second surface  42  of the active layer  38  comes into contact with the first surface  4  of the silicon substrate  2 . Thereafter, annealing treatment is performed, for example, at 1000° C. to 1200° C. for 30 minutes to 90 minutes. Hence, the SOI substrate  35  is joined to the silicon substrate  2 . 
     The substrate to be joined to the silicon substrate  2  may be the second silicon substrate  43  as shown in  FIG.  4   . For example, the thickness of the second silicon substrate  43  may be 625 μm to 775 μm. The second silicon substrate  43  has a first surface  44  and a second surface  45  on the side opposite to the first surface  44 . In the present preferred embodiment, the first surface  44  of the second silicon substrate  43  is a (110) surface, and the second surface  45  is a (110) surface. In this case, the following way is recommended, i.e., the second silicon substrate  43  is reversed up and down, and the second silicon substrate  43  is bonded onto the silicon substrate  2  so that the first surface  44  of the second silicon substrate  43  comes into contact with the first surface  4  of the silicon substrate  2 , and the second silicon substrate  43  and the silicon substrate  2  are joined together. 
     In other words, it is recommended to bond the SOI substrate  35  or the second silicon substrate  43  to the silicon substrate  2  so that the (110) surface of the active layer  38  or of the second silicon substrate  43  is directed upwardly both in  FIG.  3 B  and in  FIG.  4   . 
     Thereafter, the support substrate  36  and the BOX layer  37  of the SOI substrate  35  are removed as shown in  FIG.  3 C . The support substrate  36  and the BOX layer  37  can be removed by, for example, grinding, etching, or the like. Thereafter, the active layer  38  is processed until the active layer  38  attains a desired thickness (thickness of the silicon diaphragm  3 ). The active layer  38  may be thinned by, for example, grinding, etching, polishing, or the like. If the active layer  38  is pre-formed with a desired thickness at the stage of the SOI substrate  35 , the step of thinning the active layer  38  can be omitted after the BOX layer  37  is removed. Hence, the silicon diaphragm  3  made of the active layer  38  is formed. 
     On the other hand, if the second silicon substrate  43  is used instead of the SOI substrate  35 , it is recommended to thin the second silicon substrate  43  up to a desired thickness by grinding, etching, polishing, or the like. 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  3 D , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  3 E , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . The first to fourth relay wirings  23  to  26 , the first to fourth contact wirings  27  to  30 , and the diffusion wiring  34  of the fifth wiring  32  are formed. 
     Thereafter, the insulating layer  31  is formed on the silicon diaphragm  3  by, for example, a CVD method as shown in  FIG.  3 F . Thereafter, the first to fourth metallic wirings  17  to  20  and the first to fifth metallic terminals  12  to  16  are formed on the insulating layer  31  by, for example, a sputtering method and by patterning. The MEMS sensor  1  is obtained through these steps. 
     In the MEMS sensor  1 , when the movable portion  21  of the silicon diaphragm  3  receives pressure (for example, gas pressure) from the first surface  8  side, differential pressure is generated between the inside and the outside of the cavity  7 , and, as a result, the movable portion  21  displaces in the thickness direction of the silicon diaphragm  3 . Because of this displacement, a silicon crystal forming the piezoresistances R 1  to R 4  is distorted, and resistance values of the piezoresistances R 1  to R 4  change. 
     For example, when a constant electrode bias is being applied to the voltage application terminal (third metallic terminal  14 ), a voltage between the output terminals (the second and fourth metallic terminals  13  and  15 ) changes in accordance with a change in each resistance value of the piezoresistances R 1  to R 4 . Therefore, it is possible to detect the magnitude of the pressure generated in the MEMS sensor  1  on the basis of its voltage change. 
     The MEMS sensor  1  is formed by Si—Si direct junction between the silicon substrate  2  and the silicon diaphragm  3 , and therefore an oxide film is not formed therebetween. As a result, it is possible to prevent a gas from entering the cavity  7  through the oxide film even when the MEMS sensor  1  is used in a gaseous environment in which the radius of an atom, such as helium (He) or hydrogen (H 2 ), is small. As a result, it is possible to hold the degree of vacuum of the inside of the cavity  7 , and therefore it is possible to prevent the property fluctuation of the MEMS sensor  1 . 
     Additionally, the plane orientation of the first surface  4  of the silicon substrate  2  and the plane orientation of the first surface  4  of the silicon diaphragm  3  differ from each other. This makes it possible to employ a plane orientation in which characteristics of the piezoresistances R 1  to R 4  become optimal as the plane orientation of the first surface  4  of the silicon diaphragm  3  without being restricted by the plane orientation of the first surface  4  of the silicon substrate  2 . More specifically, the plane orientation of the first surface  4  of the silicon substrate  2  is a (100) surface, and the plane orientation of the first surface  4  of the silicon diaphragm  3  is a (110) surface. The fact that the plane orientation of the formation surface of each of the piezoresistances R 1  to R 4  is preferably the (110) surface rather than the (100) surface can be described by a comparison between  FIG.  5 A  and  FIG.  5 B . 
       FIG.  5 A  is a view to describe the piezoresistance coefficient of a piezoresistance formed at a (100) surface.  FIG.  5 B  is a view to describe the piezoresistance coefficient of a piezoresistance formed at a (110) surface. 
     When a comparison is made between  FIG.  5 A  and  FIG.  5 B , the lateral piezoresistance coefficient of  FIG.  5 B  is somewhat smaller than the lateral piezoresistance coefficient of  FIG.  5 A . Therefore, regarding the lateral piezoresistance coefficient, it is preferable to form the piezoresistances R 1  to R 4  at the (100) surface rather than the (110) surface. However, when a comparison is made between  FIG.  5 A  and  FIG.  5 B  regarding the longitudinal piezoresistance coefficient, the longitudinal piezoresistance coefficient of  FIG.  5 B  is far greater than the longitudinal piezoresistance coefficient of  FIG.  5 A . In other words, in consideration of the amount of increase of the longitudinal piezoresistance coefficient, it is preferable, from a comprehensive perspective, to form the piezoresistances R 1  to R 4  at the (110) surface rather than the (100) surface although the lateral piezoresistance coefficient is somewhat inferior. 
     Additionally, the longitudinal piezoresistance coefficient and the lateral piezoresistance coefficient become large at a &lt;111&gt; axis in the (110) surface as shown in  FIG.  5 B , and therefore it is preferable to dispose the piezoresistances R 1  to R 4  on the &lt;111&gt; axis. In other words, in  FIG.  1   , it is preferable for each of the piezoresistances R 1  to R 4  to be formed in a longitudinal shape that extends in the direction of the &lt;111&gt; axis. Hence, it is possible to enlarge the longitudinal piezoresistance coefficient and the lateral piezoresistance coefficient of each of the piezoresistances R 1  to R 4 , and therefore it is possible to improve the sensibility of the piezoresistances R 1  to R 4 . 
     Additionally, the piezoresistances R 1  to R 4  are disposed at the sides  7 A to  7 D of the cavity  7 , respectively. Hence, when the silicon diaphragm  3  vibrates, the piezoresistances R 1  and R 3  disposed at the pair of first and third sides  7 A and  7 C facing each other of the cavity  7  are pulled in the longitudinal direction, and resistance increases. On the other hand, the piezoresistances R 2  and R 4  disposed at the pair of second and fourth sides  7 B and  7 D facing each other of the cavity  7  are pulled in the width direction, and resistance decreases. As a result, it is possible to make a large difference in the resistance value between the piezoresistances R 1  and R 3  of the first and third sides  7 A and  7 C and the piezoresistances R 2  and R 4  of the second and fourth sides  7 B and  7 D. 
     Additionally, the cavity  7  is formed not at the silicon diaphragm  3  but at the silicon substrate  2  that is thicker than the silicon diaphragm  3 . Therefore, it is possible to form the cavity  7  highly accurately and easily. Additionally, the MEMS sensor  1  is formed by Si—Si direct junction between the silicon substrate  2  and the silicon diaphragm  3 . In other words, the substrate and the diaphragm are equal to each other in linear expansion coefficient, and therefore it is possible to restrain the occurrence of stress in the junction interface  10  that results from a temperature change. Consequently, it is possible to stabilize characteristics of the piezoresistances R 1  to R 4 . 
       FIG.  6    is a plan view of a MEMS sensor  51  according to a second preferred embodiment of the present invention.  FIG.  7    is a cross-sectional view of the MEMS sensor  51  according to the second preferred embodiment of the present invention.  FIG.  7    chiefly shows a characterized part of the MEMS sensor  51  for convenience, and does not show a cross section at a specific location of  FIG.  6   . Additionally, in the second preferred embodiment, only chief differences distinguished from the first preferred embodiment are described, and the same reference sign is given to a component that is equivalent to each component described above, and a description thereof is omitted. 
     In the MEMS sensor  51 , a plurality of through-holes  52 A,  52 B,  52 C, and  52 D are formed in the silicon diaphragm  3 . Referring to  FIG.  6   , the plurality of through-holes  52 A,  52 B,  52 C, and  52 D are formed at the first corner portion  46 A, the second corner portion  46 B, the third corner portion  46 C, and the fourth corner portion  46 D of the cavity  7 , respectively, in a plan view. Hence, the inside and the outside of the cavity  7  lead to each other through the through-holes  52 A,  52 B,  52 C, and  52 D. Therefore, in the MEMS sensor  51 , the cavity  7  is not sealed by the silicon diaphragm  3  unlike the MEMS sensor  1 . 
     The shape of each of the through-holes  52 A,  52 B,  52 C, and  52 D may be, for example, a quadrangular shape in a plan view as shown in  FIG.  6   , or may be a circular shape in a plan view or a triangular shape in a plan view. 
     Likewise, in this MEMS sensor  51 , the plane orientation of the first surface  4  of the silicon substrate  2  and the plane orientation of the first surface  4  of the silicon diaphragm  3  differ from each other. This makes it possible to employ a plane orientation in which characteristics of the piezoresistances R 1  to R 4  become optimal as the plane orientation of the first surface  4  of the silicon diaphragm  3  without being restricted by the plane orientation of the first surface  4  of the silicon substrate  2 . Consequently, it is possible to stabilize characteristics of the piezoresistances R 1  to R 4 . 
     Additionally, the through-holes  52 A to  52 D are formed, and therefore it is possible to exclude the influence of outside pressure. 
       FIG.  8 A  to  FIG.  8 G  are views showing part of a process of manufacturing the MEMS sensor  51  according to the second preferred embodiment of the present invention. 
     In order to manufacture the MEMS sensor  51 , the silicon substrate  2  is prepared, and the cavity  7  is formed in the silicon substrate  2 , for example, as shown in  FIG.  8 A . The cavity  7  may be formed by selectively applying dry etching to the first surface  4  of the silicon substrate  2 . 
     On the other hand, the SOI substrate  35  is prepared, and a plurality of through-holes  52 A to  52 D are formed in the active layer  38  of the SOI substrate  35  as shown in  FIG.  8 B . The plurality of through-holes  52 A to  52 D may be formed by allowing the active layer  38  to be selectively subjected to dry etching, for example, until the through-holes  52 A to  52 D reach the BOX layer  37  from the second surface  42  of the active layer  38 . This manner in which the plurality of through-holes  52 A to  52 D are formed by using the SOI substrate  35  makes it possible to easily adjust the depth of the plurality of the through-holes  52 A to  52 D on the basis of the thickness of the active layer  38 . 
     Thereafter, the SOI substrate  35  is reversed up and down, and the SOI substrate  35  is bonded onto the silicon substrate  2  so that the second surface  42  of the active layer  38  comes into contact with the first surface  4  of the silicon substrate  2  as shown in  FIG.  8 C . Thereafter, annealing treatment is performed, for example, at 1000° C. to 1200° C. for 30 minutes to 90 minutes. Hence, the SOI substrate  35  is joined to the silicon substrate  2 . 
     The substrate to be joined to the silicon substrate  2  may be the second silicon substrate  43  as shown in  FIG.  4   . In this case, it is recommended to form a concave portion, which corresponds to each of the plurality of the through-holes  52 A to  52 D, at the second silicon substrate  43 . This concave portion becomes each of the plurality of the through-holes  52 A to  52 D by allowing the second surface  45  of the second silicon substrate  43  to reach the bottom portion of the concave portion when the second silicon substrate  43  is thinned. 
     Thereafter, the support substrate  36  and the BOX layer  37  of the SOI substrate  35  are removed as shown in  FIG.  8 D . The support substrate  36  and the BOX layer  37  can be removed by, for example, grinding, etching, or the like. Thereafter, the active layer  38  is processed until the active layer  38  attains a desired thickness (thickness of the silicon diaphragm  3 ). The active layer  38  may be thinned by, for example, grinding, etching, polishing, or the like. If the active layer  38  is pre-formed with a desired thickness at the stage of the SOI substrate  35 , the step of thinning the active layer  38  can be omitted after the BOX layer  37  is removed. Hence, the silicon diaphragm  3  made of the active layer  38  is formed. 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  8 E , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  8 F , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . The first to fourth relay wirings  23  to  26 , the first to fourth contact wirings  27  to  30 , and the diffusion wiring  34  of the fifth wiring  32  are formed. 
     Thereafter, the insulating layer  31  is formed on the silicon diaphragm  3  by, for example, the CVD method as shown in  FIG.  8 G . Thereafter, the first to fourth metallic wirings  17  to  20  and the first to fifth metallic terminals  12  to  16  are formed on the insulating layer  31  by, for example, the sputtering method and by patterning. The MEMS sensor  51  is obtained through these steps. 
       FIG.  9    is a plan view of a MEMS sensor  61  according to a third preferred embodiment of the present invention.  FIG.  10    is a cross-sectional view of the MEMS sensor  61  according to the third preferred embodiment of the present invention, showing a cross section along line X-X of  FIG.  9   . In the third preferred embodiment, only chief differences distinguished from the first preferred embodiment are described, and the same reference sign is given to a component that is equivalent to each component described above, and a description thereof is omitted. 
     In the MEMS sensor  61 , a second cavity  62  is formed at the silicon diaphragm  3 . The second cavity  62  is formed at the second surface  9  of the silicon diaphragm  3 , and faces the cavity  7 . 
     In the present preferred embodiment, the second cavity  62  is formed annularly along the circumferential edge  11  of the cavity  7  in a plan view. Hence, a convex portion  63  surrounded by the second cavity  62  is formed at the second surface  9  of the silicon diaphragm  3 . The second cavity  62  is not necessarily required to be annular, and may be configured so that, for example, the entirety of the movable portion  21  of the silicon diaphragm  3  is concaved. The depth of the second cavity  62  is, for example, 0.1 μm to 25 μm. 
     In the MEMS sensor  61 , a cavity  64 , which is formed by combining the cavity  7  and the second cavity  62  together, is formed by the second cavity  62 . 
     The cavity  64  integrally includes a first portion  65  having a first size T 1  and a second portion  66  having a second size T 2  greater than the first size T 1  in the thickness direction of the silicon substrate  2 . The first portion  65  is a portion sandwiched between a bottom surface of the cavity  7  and the top of the convex portion  63 . The second portion  66  is a portion sandwiched between the bottom surface of the cavity  7  and a bottom surface of the second cavity  62 . 
     Although a lateral surface of the cavity  7  and a lateral surface of the second cavity  62  are flush with each other as shown in  FIG.  10    in the present preferred embodiment, these lateral surfaces may deviate from each other. For example, the lateral surface of the second cavity  62  may be placed at a more outward position than the lateral surface of the cavity  7 . 
     Likewise, in this MEMS sensor  61 , the plane orientation of the first surface  4  of the silicon substrate  2  and the plane orientation of the first surface  4  of the silicon diaphragm  3  differ from each other. This makes it possible to employ a plane orientation in which characteristics of the piezoresistances R 1  to R 4  become optimal as the plane orientation of the first surface  4  of the silicon diaphragm  3  without being restricted by the plane orientation of the first surface  4  of the silicon substrate  2 . Consequently, it is possible to stabilize characteristics of the piezoresistances R 1  to R 4 . 
     Additionally, the formation of the second cavity  62  makes it possible to raise the sensibility, and it is possible to prevent a breakdown by setting the size of T 1  at such a size as to come into contact with the bottom portion of the cavity  7  until the silicon diaphragm  3  is dented and broken because of a raised external pressure. 
       FIG.  11 A  to  FIG.  11 G  are views showing part of a process of manufacturing the MEMS sensor  61  according to the third preferred embodiment of the present invention. 
     In order to manufacture the MEMS sensor  61 , the silicon substrate  2  is prepared, and the cavity  7  is formed in the silicon substrate  2 , for example, as shown in  FIG.  11 A . The cavity  7  may be formed by selectively applying dry etching to the first surface  4  of the silicon substrate  2 . 
     On the other hand, the SOI substrate  35  is prepared, and the second cavity  62  is formed in the active layer  38  of the SOI substrate  35  as shown in  FIG.  11 B . The second cavity  62  may be formed by allowing the active layer  38  to be selectively subjected to dry etching, for example, from the second surface  42  of the active layer  38  up to a halfway position in the thickness direction of the active layer  38 . 
     Thereafter, the SOI substrate  35  is reversed up and down, and the SOI substrate  35  is bonded onto the silicon substrate  2  so that the second surface  42  of the active layer  38  comes into contact with the first surface  4  of the silicon substrate  2  as shown in  FIG.  11 C . Thereafter, annealing treatment is performed, for example, at 1000° C. to 1200° C. for 30 minutes to 90 minutes. Hence, the SOI substrate  35  is joined to the silicon substrate  2 . 
     The substrate to be joined to the silicon substrate  2  may be the second silicon substrate  43  as shown in  FIG.  4   . In this case, it is recommended to form the second cavity  62  at the first surface  44  of the second silicon substrate  43 . 
     Thereafter, the support substrate  36  and the BOX layer  37  of the SOI substrate  35  are removed as shown in FIG.  11 D. The support substrate  36  and the BOX layer  37  can be removed by, for example, grinding, etching, or the like. Thereafter, the active layer  38  is processed until the active layer  38  attains a desired thickness (thickness of the silicon diaphragm  3 ). The active layer  38  may be thinned by, for example, grinding, etching, polishing, or the like. If the active layer  38  is pre-formed with a desired thickness at the stage of the SOI substrate  35 , the step of thinning the active layer  38  can be omitted after the BOX layer  37  is removed. Hence, the silicon diaphragm  3  made of the active layer  38  is formed. 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  11 E , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . 
     Thereafter, impurity ions (in the present preferred embodiment, boron (B)) are selectively implanted into the first surface  8  of the silicon diaphragm  3  as shown in  FIG.  11 F , and annealing treatment is performed. Hence, the piezoresistances R 1  to R 4  are formed at the silicon diaphragm  3 . The first to fourth relay wirings  23  to  26 , the first to fourth contact wirings  27  to  30 , and the diffusion wiring  34  of the fifth wiring  32  are formed. 
     Thereafter, the insulating layer  31  is formed on the silicon diaphragm  3  by, for example, the CVD method as shown in  FIG.  11 G . Thereafter, the first to fourth metallic wirings  17  to  20  and the first to fifth metallic terminals  12  to  16  are formed on the insulating layer  31  by, for example, the sputtering method and by patterning. The MEMS sensor  61  is obtained through these steps. 
     Although the preferred embodiments of the present invention have been described above, the present invention can be carried out in other preferred embodiments. 
     For example, various design changes may be made within the scope of the matters described in the claims. 
     EXAMPLES 
     Next, the present invention will be described on the basis of examples, and yet the present invention is not limited by the following examples. 
     In the examples, it was verified how much a gas having a small atomic radius can be prevented from entering the inside of the cavity  7  by means of Si—Si direct junction between the silicon substrate  2  and the silicon diaphragm  3 . Specifically, that was verified by use of MEMS sensors having Structure  1  to Structure  3  shown in  FIG.  12 A  to  FIG.  12 C , respectively. 
     Structure  1  includes a silicon substrate  71  having a through-hole  72 , a glass substrate  73 , and a silicon membrane  74 . The glass substrate  73  is joined to a rear surface of the silicon substrate  71 . The silicon membrane  74  is joined to a front surface of the silicon substrate  71  through a silicon oxide (SiO 2 ) film  75 . Hence, the through-hole  72  of the silicon substrate  71  is sealed with the glass substrate  73  and the silicon membrane  74 . 
     Structure  2  includes a silicon substrate  76  having a cavity  77  and a silicon membrane  78 . The cavity  77  is formed on the front surface side of the silicon substrate  76 , and has a bottom portion at a halfway position in the thickness direction of the silicon substrate  76 . The silicon membrane  78  is joined to a front surface of the silicon substrate  76  through a silicon oxide (SiO 2 ) film  79 . Hence, the cavity  77  of the silicon substrate  76  is sealed with the silicon membrane  78 . 
     Structure  3  is a structure formed in imitation of the first preferred embodiment described above. That is, Structure  3  includes a silicon substrate  80  having a cavity  81  and a silicon membrane  82 . The cavity  81  is formed on the front surface side of the silicon substrate  80 , and has a bottom portion at a halfway position in the thickness direction of the silicon substrate  80 . The silicon membrane  82  is joined directly to a front surface of the silicon substrate  80 . Therefore, a junction interface of Si—Si junction is formed between the silicon substrate  80  and the silicon membrane  82 . Additionally, the front surface of the silicon substrate  80  (joint surface with the silicon membrane  82 ) is a (100) surface, and the front surface of the silicon membrane  82  (surface on the side opposite to a joint surface with the silicon substrate  80 ) is a (110) surface. 
     In any of Structures  1  to  3 , the inside of each of the cavities  77  and  81  (through-hole  72 ) of the MEMS sensors is kept in a vacuum state in an initial state. 
       FIG.  13    shows results obtained by measuring the amount of change in pressure of the inside of each of the cavities  77  and  81  (through-hole  72 ) of the MEMS sensors when the MEMS sensors of Structures  1  to  3  are stored in a He atmosphere of about 2.5 atmospheric pressure and when 0 h (initial value), 24 h, and 72 h elapse. 
     In Structures  1  and  2 , the silicon oxide films  75  and  79  are interposed between the silicon substrates  71  and and the silicon membranes  74  and  78 , respectively. Therefore, He (helium) penetrates the silicon oxide films  75  and  79 , and enters the inside of the cavity  77  (through-hole  72 ), and pressure of the inside of the cavity  77  (through-hole  72 ) varies in proportion to the length during which the MEMS sensor is stored in the He atmosphere as shown in  FIG.  13   . As a result, the degree of vacuum of the cavity  77  (through-hole  72 ) is reduced, and the dented amount of each of the silicon membranes  74  and  78  changes, and hence there is a concern that the property fluctuation of the MEMS sensor will be caused. 
     On the other hand, in Structure  3 , the junction between the silicon substrate  80  and the silicon membrane  82  is Si—Si direct junction. Therefore, pressure fluctuations hardly occurred after the MEMS sensor was stored in the He atmosphere for a long time as shown in  FIG.  13   . Therefore, it has been found that it is possible to prevent a gas from entering the cavity through the oxide film even when the MEMS sensor of Structure  1  is used under a gaseous environment having a small atomic radius, such as helium (He) or hydrogen (H 2 ). Additionally, based on Structure  3 , it has been shown that it is possible to achieve the above-described effect even when the plane orientation of the front surface (the first surface  4  described above) of the silicon substrate  80  and the plane orientation of the front surface (the first surface  8  described above) of the silicon membrane  82  differ from each other.