Patent Publication Number: US-2023146603-A1

Title: Pressure sensor chip, pressure sensor, and manufacturing method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2020-131925 filed on Aug. 3, 2020 and is a Continuation application of PCT Application No. PCT/JP2021/025839 filed on Jul. 8, 2021. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressure sensor chip to measure pressure from outside, to a pressure sensor including the pressure sensor chip, and to a method of manufacturing the pressure sensor chip and the pressure sensor. 
     2. Description of the Related Art 
     A differential pressure sensor configured to measure differential pressure between two pressures is known as a type of a pressure sensor. For example, the differential pressure sensor is applied to a fluid pipe having a constricted section built in and measures the differential pressure between pressures upstream and downstream of the constricted section. The differential pressure measured can be converted to a flow rate using the relational expression between flow rate and differential pressure, which is described in JIS 28762 of the Japan Industrial Standards. 
     To perform the above conversion, it is necessary to obtain a static pressure, such as a gauge pressure relative to the atmospheric pressure or an absolute pressure relative to the vacuum, in addition to the differential pressure. 
     Japanese Unexamined Patent Application Publication No. 2003-42878 discloses a pressure sensor equipped with a differential-pressure sensor chip configured to measure differential pressure and a static-pressure sensor chip configured to measure static pressure. 
     SUMMARY OF THE INVENTION 
     In the pressure sensor disclosed in Japanese Unexamined Patent Application Publication No. 2003-42878, however, the differential-pressure sensor chip and the static-pressure sensor chip are provided as separate chips, which leads to an increase in the size of the pressure sensor. 
     In order to reduce the size of the pressure sensor, the differential pressure sensor and the static pressure sensor may be integrated in one chip. The integration, however, faces the following challenges as described in Japanese Unexamined Patent Application Publication No. 2003-42878. 
     The pressure sensor measures pressure in accordance with the amount of bending of a diaphragm. The diaphragm provided for the static pressure sensor needs to withstand a large pressure and accordingly have a greater thickness compared with the diaphragm provided for the differential pressure sensor. If the differential pressure sensor and the static pressure sensor were formed integrally in the same chip in the pressure sensor disclosed in Japanese Unexamined Patent Application Publication No. 2003-42878, the diaphragm for the differential pressure sensor and the diaphragm for the static pressure sensor would need to have different thicknesses in the same semiconductor substrate. However, forming diaphragms having different thicknesses in the same semiconductor substrate complicates the manufacturing process. 
     Accordingly, the size reduction of the pressure sensor is achieved in Japanese Unexamined Patent Application Publication No. 2003-42878 by disposing the differential-pressure sensor chip and the static-pressure sensor chip closely on the same base. However, the differential-pressure sensor chip and the static-pressure sensor chip are separate chips and are not integrated in the same chip in the pressure sensor of Japanese Unexamined Patent Application Publication No. 2003-42878, which limits the degree of size reduction of the pressure sensor. 
     Preferred embodiments of the present invention provide pressure sensor chips in each of which two diaphragms can be integrated in the same chip without complicating the manufacturing process. 
     According to an aspect of a preferred embodiment of the present invention, a pressure sensor chip including a first diaphragm and a second diaphragm to measure pressure includes a base, a first layer, a second layer, a third layer, and a fourth layer. The first layer includes a first cavity and is joined to the base. The second layer is joined to a surface of the first layer opposite to the base. The third layer includes a second cavity and is joined to a surface of the second layer opposite to the first layer. The fourth layer includes a third cavity and is joined to a surface of the third layer opposite to the second layer. In the pressure sensor chip, the second layer includes the first diaphragm between the first cavity and the second cavity, and the fourth layer includes the second diaphragm between the second cavity and a space in communication with outside. In addition, a first end of the third cavity is in communication with outside, and a second end of the third cavity is in communication with the second cavity. The first cavity is sealed, and a pressure in the first cavity is lower than a pressure in the second cavity. 
     According to preferred embodiments of the present invention, two diaphragms can be integrated in the same chip without complicating the manufacturing process. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a longitudinal sectional view illustrating a pressure sensor according to a first preferred embodiment of the present invention. 
         FIG.  2    is a plan view illustrating a pressure sensor chip included in the pressure sensor of  FIG.  1   . 
         FIG.  3    is a cross-sectional view taken along line A-A in  FIG.  1   . 
         FIG.  4    is a cross-sectional view taken along line B-B in  FIG.  1   . 
         FIG.  5    is a cross-sectional view taken along line C-C in  FIG.  1   . 
         FIG.  6    is a view illustrating an equivalent circuit of the pressure sensor chip of  FIG.  1     
         FIG.  7    is a longitudinal sectional view illustrating a pressure sensor in which a fourth layer is thinner than a second layer. 
         FIG.  8    is a longitudinal sectional view illustrating a pressure sensor according to a second preferred embodiment of the present invention. 
         FIG.  9    is a plan view illustrating a pressure sensor chip included in the pressure sensor of  FIG.  8   . 
         FIG.  10    is a cross-sectional view taken along line D-D in  FIG.  8   . 
         FIG.  11    is a cross-sectional view taken along line E-E in  FIG.  8   . 
         FIG.  12    is a cross-sectional view taken along line F-F in  FIG.  8   . 
         FIG.  13    is a longitudinal sectional view illustrating a pressure sensor according to a third preferred embodiment of the present invention. 
         FIG.  14    is a longitudinal sectional view illustrating a pressure sensor according to a fourth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to an aspect of a preferred embodiment of the present invention, a pressure sensor chip including a first diaphragm and a second diaphragm to measure pressure includes a base, a first layer, a second layer, a third layer, and a fourth layer. The first layer includes a first cavity and is joined to the base. The second layer is joined to a surface of the first layer opposite to the base. The third layer includes a second cavity and is joined to a surface of the second layer opposite to the first layer. The fourth layer includes a third cavity and is joined to a surface of the third layer opposite to the second layer. In the pressure sensor chip, the second layer includes the first diaphragm between the first cavity and the second cavity, and the fourth layer includes the second diaphragm between the second cavity and a space in communication with outside. In addition, a first end of the third cavity is in communication with outside, and a second end of the third cavity is in communication with the second cavity. The first cavity is sealed, and a pressure in the first cavity is lower than a pressure in the second cavity. 
     According to this configuration, the pressure in the first cavity and the pressure in the second cavity act on the first diaphragm. As a result, the first diaphragm can measure pressure relative to the pressure in the first cavity that is sealed and airtight. The pressure in the second cavity and the pressure from the outside act on the second diaphragm. As a result, the second diaphragm can measure the differential pressure between these two pressures. In other words, according to this configuration, a single pressure sensor chip can measure differential pressure between two pressures as well as absolute pressure relative to the vacuum. Moreover, two diaphragms are integrated in a single pressure sensor chip. This enables size reduction of the pressure sensor chip. 
     According to this configuration, the first diaphragm and the second diaphragm are located in different layers. As a result, the thickness of the second layer included in the first diaphragm and the thickness of the fourth layer included in the second diaphragm can be made different without complicating the manufacturing process. 
     In the pressure sensor chip, the first cavity may be under vacuum or substantially under vacuum. This enables the first diaphragm to function as a diaphragm to measure an absolute pressure. 
     In the pressure sensor chip, the fourth layer may have a thickness smaller than a thickness of the second layer. According to this configuration, the second diaphragm is thinner than the first diaphragm, which enables a small pressure difference to bend the second diaphragm. If the pressure in the first cavity is low, in other words, if the reference pressure for the first diaphragm is low, the amount of bending of the first diaphragm increases. This may lead to the breakage of the first diaphragm. According to this configuration, however, the first diaphragm is thicker than the second diaphragm, which can reduce the likelihood of the first diaphragm breaking. 
     In the pressure sensor chip, the base, the second layer, and the fourth layer may be electrical conductors, and the first layer and the third layer may be electrical insulators. According to this configuration, the pressure sensor chip can operate as a capacitance sensor. 
     In the pressure sensor chip, the first diaphragm does not need to overlap the second diaphragm as viewed in plan. This can reduce the likelihood of the pressure in the first cavity affecting the second diaphragm via the second layer and the second cavity. This enables the second diaphragm to measure differential pressure accurately. 
     In the pressure sensor chip, the first diaphragm may overlap the second diaphragm as viewed in plan. As a result, the first diaphragm becomes large, which can increase the sensitivity of the first diaphragm. 
     According to another aspect of a preferred embodiment of the present invention, a pressure sensor includes the above-described pressure sensor chip and a covering portion that covers the pressure sensor chip. The covering portion includes a fourth cavity through which the third cavity is in communication with the outside and also includes a fifth cavity through which the surface of the fourth layer positioned opposite to the third layer is exposed to outside. In addition, the second diaphragm is positioned between the second cavity and the fifth cavity. 
     According to this configuration, the covering portion can protect the pressure sensor chip. 
     In the pressure sensor, the covering portion may include a tubular-shaped first cap protruding from a periphery of the fourth cavity in a direction away from the pressure sensor chip and a tubular-shaped second cap protruding from a periphery of the fifth cavity in a direction away from the pressure sensor chip. According to this configuration, the pressure sensor can be connected easily to outside devices through the first and second caps. 
     According to another aspect of a preferred embodiment of the present invention, a method of manufacturing a pressure sensor chip includes joining a first layer to a base, the first layer including a first cavity patterned therein, joining a second layer to a surface of the first layer opposite to the base, joining a third layer to a surface of the second layer opposite to the first layer, the third layer including a second cavity patterned therein, the third layer being joined in such a manner that a first diaphragm is formed in the second layer between the first cavity and the second cavity, and joining a fourth layer to a surface of the third layer opposite to the second layer, the fourth layer including a third cavity patterned therein, the fourth layer being joined in such a manner that a first end of the third cavity is in communication with outside, that a second end of the third cavity is in communication with the second cavity, and that a second diaphragm is formed in the fourth layer between the second cavity and a space in communication with the outside. 
     According to another aspect of a preferred embodiment of the present invention, a method of manufacturing a pressure sensor includes joining a first layer to a base, the first layer including a first cavity patterned therein, joining a second layer to a surface of the first layer opposite to the base, joining a third layer to a surface of the second layer opposite to the first layer, the third layer including a second cavity patterned therein, the third layer being joined in such a manner that a first diaphragm is formed in the second layer between the first cavity and the second cavity, joining a fourth layer to a surface of the third layer opposite to the second layer, the fourth layer including a third cavity patterned therein, the fourth layer being joined in such a manner that the third cavity is in communication with the second cavity, and forming a covering portion so as to cover the base, the first layer, the second layer, the third layer, and the first layer, the covering portion including a fourth cavity and a fifth cavity, the covering portion being formed in such a manner that the fourth cavity exposes the third cavity to outside, that the fifth cavity exposes a surface of the fourth layer to outside opposite to the third layer, and that a second diaphragm is formed in the fourth layer between the second cavity and the fifth cavity. 
     According to the above manufacturing methods, pressure sensor chips and pressure sensors can be manufactured without using a complicated step, for example, a step of changing the thickness of the same layer. 
     First Preferred Embodiment 
       FIG.  1    is a longitudinal sectional view illustrating a pressure sensor according to a first preferred embodiment of the present invention. 
     A pressure sensor  1  is capable of detecting a small pressure variation. As illustrated in  FIG.  1   , the pressure sensor  1  includes two diaphragms, in other words, a first diaphragm  26  and a second diaphragm  27 , which will be described in detail later. The pressure sensor  1  detects pressure due to the first diaphragm  26  and the second diaphragm  27  bending. The pressure sensor  1  is able to measure two separate pressures. 
     As illustrated in  FIG.  1   , the pressure sensor  1  includes a substrate  10 , a pressure sensor chip  20 , an application specific integrated circuit (ASIC)  30 , a first covering portion  40 , and a second covering portion  50 . The application specific integrated circuit  30  is hereinafter referred to as the “ASIC  30 ”. 
     The substrate  10  is a tabular member. The substrate  10  is made of a material, such as a resin (for example, epoxy resin or phenol resin), a ceramic material, or aluminum. The substrate  10  has wiring traces, pads, and through-holes or the like, which are made of a copper or the like and are formed on surfaces of the substrate  10 . The wiring traces, pads, and through-holes are electrically connected to one another. 
       FIG.  2    is a plan view illustrating the pressure sensor chip included in the pressure sensor of  FIG.  1   . 
     As illustrated in  FIG.  1   , the pressure sensor chip  20  is mounted on the substrate  10 . Known methods can be used to mount the pressure sensor chip  20 . In the first preferred embodiment, the pressure sensor chip  20  includes a base  25  (to be described later), and the base  25  is adhered to an upper surface  10 A of the substrate  10  using an adhesive. 
     As illustrated in  FIGS.  1  and  2   , the exterior of the pressure sensor chip  20  of the first preferred embodiment is shaped like a cuboid. Note that the shape of the pressure sensor chip  20  is not necessarily the cuboid but may be, for example, a round column. The pressure sensor chip  20  is a MEMS (micro-electro-mechanical systems) device. The pressure sensor chip  20  has a structure in which multiple layers are laminated. The structure of the pressure sensor chip  20  will be described in detail later. 
     As illustrated in  FIG.  1   , the ASIC  30  is mounted on the upper surface  10 A of the substrate  10 . Known methods can be used to mount the pressure sensor chip  20 . In the first preferred embodiment, the ASIC  30  is adhered to the upper surface  10 A using an adhesive. 
     The ASIC  30  is connected to the pressure sensor chip  20  using an electrically conductive wire made of aluminum or copper (for example, a wire  31  illustrated in  FIG.  1   ). The ASIC  30  is also connected to a pad  33  formed on a surface of the substrate  10  using an electrically conductive wire  32  made of aluminum or copper. 
     The ASIC  30  processes signals received from the pressure sensor chip  20  via the wire  31  or the like and outputs processed signals to an external device via a wire  32  and the like. The signal processing of the ASIC  30  includes at least one of the following functions or features. 
     For example, the signal processing includes conversion processing in which analog signals received from the pressure sensor chip  20  are converted to digital signals. In the first preferred embodiment, the signals received from the pressure sensor chip  20  are current values that correspond to the amounts of bending of the first diaphragm  26  and the second diaphragm  27 , which will be described later. For example, the signal processing includes filtering processing in which low-frequency signals are obtained by filtering out high-frequency noise components of the digital signals obtained after the conversion processing. For example, the signal processing includes correction processing in which the signals obtained after the filtering processing are corrected through arithmetic operation using data from an external temperature sensor and preset correction factors. The temperature sensor is disposed, for example, on the substrate  10  near the pressure sensor chip  20 . The correction factors are stored, for example, in an internal memory included in the ASIC  30 . 
     The first covering portion  40  and the second covering portion  50  are made of a resin, such as epoxy resin. The first covering portion  40  and the second covering portion  50  are examples of the covering portion. 
     The first covering portion  40  covers the pressure sensor chip  20 . In the first preferred embodiment, the first covering portion  40  covers the surfaces of the pressure sensor chip  20  except for the surface being in contact with the substrate  10 . The first covering portion  40  includes two cavities  41  and  42  that pierce the first covering portion  40 . The cavities  41  and  42  expose a portion of the pressure sensor chip  20  to outside. The cavity  41  is an example of the fourth cavity. The cavity  42  is an example of the fifth cavity. 
     The second covering portion  50  is joined to the first covering portion  40 . The second covering portion  50  is joined to the surface of the first covering portion  40  that is opposite to the surface being in contact with the pressure sensor chip  20 . The second covering portion  50  includes two tubular caps  51  and  52 . In the first preferred embodiment, the caps  51  and  52  are shaped like tubes. The caps  51  and  52  protrude outward in a direction away from the first covering portion  40  and the pressure sensor chip  20 . An internal space  53  of the cap  51  is in communication with the cavity  41 . An internal space  54  of the cap  52  is in communication with the cavity  42 . The cap  51  is an example of the first cap. The cap  52  is an example of the second cap. 
     The following describes the structure of the pressure sensor chip  20  in detail. Note that in the following description, the pressure sensor chip  20  is shaped like the cuboid of which the sides extend in three directions, which are defined as a longitudinal direction  2 , a transverse direction  3 , and a height direction  4 . In  FIG.  1   , the transverse direction  3  corresponds to the depth direction of the illustration. The height direction  4  is defined such that the substrate  10  is located at a lower position and the second covering portion  50  is located at an upper position in  FIG.  1   . 
     As illustrated in  FIG.  1   , the pressure sensor chip  20  includes a first layer  21 , a second layer  22 , a third layer  23 , a fourth layer  24 , and a base  25 . 
     The first layer  21  and the third layer  23  are electrical insulators. In the first preferred embodiment, the first layer  21  and the third layer  23  are made of silicon dioxide. The second layer  22 , the fourth layer  24 , and the base  25  are electric conductors. In the first preferred embodiment, the second layer  22 , the fourth layer  24 , and the base  25  are made of silicon. 
     The base  25  is adhered to the upper surface  10 A of the substrate  10  by an adhesive or the like. The first layer  21  is joined to an upper surface  25 A of the base  25 . The second layer  22  is joined to an upper surface  21 A of the first layer  21 , which is the surface opposite to the surface facing the base  25 . The third layer  23  is joined to an upper surface  22 A of the second layer  22 , which is the surface opposite to the surface facing the first layer  21 . The fourth layer  24  is joined to an upper surface  23 A of the third layer  23 , which is the surface opposite to the surface facing the second layer  22 . Accordingly, the pressure sensor chip  20  is formed by laminating the base  25 , the first layer  21 , the second layer  22 , the third layer  23 , and the fourth layer  24  in this order from the bottom. 
     In the first preferred embodiment, the first layer  21 , the second layer  22 , the third layer  23 , and the fourth layer  24  have thicknesses (i.e., lengths in the height direction  4 ) in an approximate range of about 2 μm to about 5 μm, for example. 
     The pressure sensor chip  20  is covered by the first covering portion  40  except for the lower surface of the base  25  that is joined to the substrate  10 . In other words, the first covering portion  40  covers the upper surface  24 A of the fourth layer  24  and the side surfaces of the base  25 , the first layer  21 , the second layer  22 , the third layer  23 , and the fourth layer  24 . 
       FIG.  3    is section A-A of the pressure sensor chip  20  illustrated in  FIG.  1   . As illustrated in  FIGS.  1  and  3   , a cavity  21 B is formed in the first layer  21 . The cavity  21 B pierces the first layer  21  in the height direction  4 . The cavity  21 B is an example of the first cavity. 
       FIG.  4    is section B-B of the pressure sensor chip  20  illustrated in  FIG.  1   . As illustrated in  FIGS.  1  and  4   , no cavity is formed in the second layer  22 . As illustrated in  FIG.  1   , the cavity  21 B of the first layer  21  is interposed between the base  25  and the second layer  22 . 
     The top end of the cavity  21 B is sealed by the second layer  22 , while the bottom end of the cavity  21 B is sealed by the base  25 . Accordingly, the cavity  21 B is made airtight. In the first preferred embodiment, the cavity  21 B is under vacuum. 
     The cavity  21 B is not necessarily under true vacuum but may be substantially under vacuum. When the pressure in the cavity  21 B is less than about 3000 Pa and greater than 0 Pa, the cavity  21 B is substantially under vacuum. When the pressure in the cavity  21 B is 0 Pa, the cavity  21 B is under vacuum. The cavity  21 B is not necessarily under vacuum nor substantially under vacuum. Whatever the inside condition of the cavity  21 B may be (under vacuum, substantially under vacuum, or under any other conditions), the pressure in the cavity  21 B is lower than the pressure in a cavity  23 B (to be described later). 
       FIG.  5    is section C-C of the pressure sensor chip  20  illustrated in  FIG.  1   . As illustrated in  FIGS.  1  and  5   , a cavity  23 B is formed in the third layer  23 . The cavity  23 B pierces the third layer  23  in the height direction  4 . The cavity  23 B is an example of the second cavity. 
     As illustrated in  FIG.  5   , the cavity  23 B has a first space  23 Ba, a second space  23 Bb, and a third space  23 Bc. The first space  23 Ba is in communication with the third space  23 Bc. The third space  23 Bc is in communication with the second space  23 Bb. 
     The first space  23 Ba overlaps the cavity  21 B of the first layer  21  as the pressure sensor chip  20  is viewed in the height direction  4 , in other words, as viewed in plan. 
     A portion of the second layer  22  that overlaps the first space  23 Ba and the cavity  21 B as viewed in plan (see  FIG.  5   ) defines and functions as the first diaphragm  26  (see  FIG.  4   ). In other words, a portion of the first layer  21  that is interposed between the first space  23 Ba and the cavity  21 B (see  FIG.  1   ) defines and functions as the first diaphragm  26 . The first diaphragm  26  can bend in the height direction  4  because spaces are provided at the upper and lower sides of the first diaphragm  26  as illustrated in  FIG.  1   . In the first preferred embodiment, the first diaphragm  26  is shaped like a rectangle as viewed in plan, and the lengths of the sides of the rectangle are in the range of about 200 μm to about 500 μm, for example. 
     As illustrated in  FIG.  5   , the second space  23 Bb does not overlap the cavity  21 B of the first layer  21  as viewed in plan. In the first preferred embodiment, the size and the shape of the second space  23 Bb are the same as the size and the shape of the first space  23 Ba as viewed in plan. However, at least one of the size and the shape of the second space  23 Bb can be different from that of the first space  23 Ba. 
     In the first preferred embodiment, in the transverse direction  3 , the length of the third space  23 Bc is smaller than the length of the first space  23 Ba and the length of the second space Bb. In other words, the width of the third space  23 Bc is smaller than the width of the first space  23 Ba and the width of the second space Bb. In the first preferred embodiment, the third space  23 Bc does not overlap the cavity  21 B of the first layer  21  as viewed in plan. 
     As illustrated in  FIGS.  1  and  2   , a cavity  24 B is formed in the fourth layer  24 . The cavity  24 B pierces the fourth layer  24  in the height direction  4 . The cavity  24 B is an example of the third cavity. 
     As illustrated in  FIG.  1   , the bottom end of the cavity  24 B is in communication with the first space  23 Ba of the cavity  23 B of the third layer  23 . In other words, the cavity  24 B overlaps the first space  23 Ba as viewed in plan. The top end of the cavity  24 B is in communication with the outside of the pressure sensor chip  20 . The bottom end of the cavity  24 B is an example of the second end of the third cavity. The top end of the cavity  24 B is an example of the first end of the third cavity. 
     As described above, the upper surface  24 A of the fourth layer  24 , which is opposite to the surface facing the third layer  23 , is covered by the first covering portion  40  as illustrated in  FIG.  1   . The first covering portion  40  is in contact with the upper surface  24 A. 
     The cavity  41  of the first covering portion  40  is positioned directly above the cavity  24 B. Accordingly, the cavity  41  exposes the cavity  24 B to outside of the pressure sensor chip  20 . In addition, the cavity  23 B and the cavity  24 B are in communication with the outside of the pressure sensor  1  through the cavity  41  and also through the internal space  53  of the cap  51  of the second covering portion  50 . 
     The cavity  42  of the first covering portion  40  is positioned directly above the fourth layer  24 . The cavity  42 , however, does not overlap the cavity  24 B as viewed in plan. Accordingly, the cavity  42  exposes the upper surface  24 A of the fourth layer  24  to outside of the pressure sensor chip  20 . In addition, the upper surface  24 A is in communication with the outside of the pressure sensor  1  through the cavity  42  and also through the internal space  54  of the cap  52  of the second covering portion  50 . 
     As viewed in plan, the cavity  42  overlaps the second space  23 Bb of the cavity  23 B of the third layer  23 . As illustrated in  FIGS.  1  and  2   , a portion of the fourth layer  24  that overlaps the cavity  42  and the second space  23 Bb as viewed in plan defines and functions as the second diaphragm  27 . In other words, a portion of the fourth layer  24  that is interposed between the cavity  42  and the second space  23 Bb defines and functions as the second diaphragm  27 . The second diaphragm  27  can bend in the height direction  4  because spaces are provided at the upper and lower sides of the second diaphragm  27 . In the first preferred embodiment, the second diaphragm  27  is shaped like a rectangle as viewed in plan, and the size of the second diaphragm  27  is substantially the same as that of the first diaphragm  26 . The size of the second diaphragm  27  may be different from the size of the first diaphragm  26 . 
     The cavity  42  is a space outside the pressure sensor chip  20 . In other words, the second diaphragm  27  is interposed between the second space  23 Bb and the space in communication with the outside of the pressure sensor chip  20 . The cavity  42  is in communication with the outside of the pressure sensor  1  through the internal space  54  of the cap  52 . Accordingly, the second diaphragm  27  is interposed between the second space  23 Bb and the space in communication with the outside of the pressure sensor  1 . 
     In the first preferred embodiment, as illustrated in  FIG.  2   , the second diaphragm  27  does not overlap the first diaphragm  26  as viewed in plan. 
     The first diaphragm  26  and the second diaphragm  27  are operable to measure pressure, which will be described in detail below. 
     As illustrated in  FIG.  2   , the fourth layer  24  includes a projection  24 D at one end thereof in the longitudinal direction  2 . The projection  24 D is formed at the one end of the fourth layer  24  in the longitudinal direction  2  by removing opposite corner portions of the fourth layer  24  positioned in the transverse direction  3 . A pad  24 C is formed on the upper surface of the projection  24 D. As illustrated in  FIG.  1   , the wire  31  is connected to the pad  24 C. This electrically connects the fourth layer  24  having the second diaphragm  27  to the ASIC  30 . 
     As illustrated in  FIG.  5   , the opposite corner portions of the third layer  23  positioned in the transverse direction  3  are removed similarly to the fourth layer  24 . In the second layer  22 , as illustrated in  FIG.  4   , only one of the opposite corner portions positioned in the transverse direction  3  is removed at one end of the second layer  22  in the longitudinal direction  2  in the manner similar to the fourth layer  24  and the third layer  23 . As illustrated in  FIG.  3   , one of the opposite corner portions of the first layer  21  positioned in the transverse direction  3  is removed similarly to the second layer  22 . 
     Accordingly, as illustrated in  FIG.  2   , the upper surface  22 A of the second layer  22  and the upper surface  25 A of the base  25  are exposed to outside of the pressure sensor chip  20  at respective sides of the projection  24 D in the transverse direction  3 . A pad  22 B is formed on the exposed upper surface  22 A, and a pad  25 B is formed on the exposed upper surface  25 A. These pads  22 B and  25 B are electrically connected to the ASIC  30  by wires (not illustrated), as is the case for the pad  24 C. Accordingly, the second layer  22  including the first diaphragm  26  and the base  25  are electrically connected to the ASIC  30 . 
       FIG.  6    is an equivalent circuit of the pressure sensor chip of  FIG.  1   . 
     As illustrated in  FIG.  1   , the first diaphragm  26  of the second layer  22  opposes the base  25  with the cavity  21 B being interposed therebetween. In addition, the second layer  22  and the base  25  are both electric conductors as described above. Accordingly, the first diaphragm  26  and the base  25  define a capacitor C 1  illustrated in  FIG.  6   . 
     As illustrated in  FIG.  1   , the second diaphragm  27  of the fourth layer  24  opposes the second layer  22  with the second space  23 Bb of the cavity  23 B being interposed therebetween. In addition, the second layer  22  and the fourth layer  24  are both electric conductors as described above. Accordingly, the second diaphragm  27  and the second layer  22  form a capacitor C 2  illustrated in  FIG.  6   . 
     Thus, the equivalent circuit illustrated in  FIG.  6    is formed in the pressure sensor chip  20 . 
     The lower surface of the first diaphragm  26  faces the cavity  21 B. The upper surface of the first diaphragm  26  faces the cavity  23 B. As described above, the pressure inside the cavity  21 B is lower than the pressure inside the cavity  23 B. Accordingly, the first diaphragm  26  bends toward the cavity  21 B. The amount of bending of the first diaphragm  26  changes in accordance with the pressure in the cavity  23 B. In other words, the first diaphragm  26  is operable to measure pressure relative to the pressure in the sealed cavity  21 B. In the first preferred embodiment, the cavity  21 B is under vacuum, and accordingly, the pressure in the cavity  21 B is measured as an absolute pressure. 
     The lower surface of the second diaphragm  27  faces the cavity  23 B. The upper surface of the second diaphragm  27  faces the cavity  42 . Accordingly, the amount of bending of the second diaphragm  27  increases as the differential pressure between the cavity  23 B and the cavity  42  increases, and the amount of bending of the second diaphragm  27  decreases as the differential pressure decreases. When the pressure in the cavity  23 B is greater than the pressure in the cavity  42 , the second diaphragm  27  bends toward the cavity  42 , in other words, bends upward. When the pressure in the cavity  42  is greater than the pressure in the cavity  23 B, the second diaphragm  27  bends toward the cavity  23 B, in other words, bends downward. 
     Note that in the state illustrated in  FIG.  1   , the cavity  23 B and the cavity  42  are both open to the atmosphere via the caps  51  and  52  and accordingly the differential pressure is zero. When the caps  51  and  52  are connected to pipes or the like, the caps  51  and  52  can receive fluids from different locations or fluids of different types. In this case, the above differential pressure can be other than zero. 
     The greater the amount of bending of the first diaphragm  26 , the smaller the distance between the first diaphragm  26  and the base  25 . As a result, the capacitance of the capacitor C 1  increases. On the other hand, the smaller the amount of bending of the first diaphragm  26 , the greater the distance between the first diaphragm  26  and the base  25 . As a result, the capacitance of the capacitor C 1  decreases. 
     The greater the amount of downward bending of the second diaphragm  27 , the smaller the distance between the second diaphragm  27  and the second layer  22 . As a result, the capacitance of the capacitor C 2  increases. On the other hand, the smaller the amount of downward bending of the second diaphragm  27  or the greater the amount of upward bending of the second diaphragm  27 , the greater the distance between the second diaphragm  27  and the second layer  22 . As a result, the capacitance of the capacitor C 2  decreases. 
     Signals that reflect the conditions of the first diaphragm  26  and the second diaphragm  27  are sent to the ASIC  30  via the pads  22 B,  24 C,  25 B. When the ASIC  30  receives the signals, the ASIC  30  performs the above-described processing (such as the conversion processing, the filtering processing, and the correction processing) and outputs processed signals. The signals processed by the ASIC  30  that correspond to the state of the first diaphragm  26  are the signals indicating the absolute pressure of the fluid taken into the cap  51 . The signals processed by the ASIC  30  that correspond to the state of the second diaphragm  27  are the signals indicating the differential pressure between the fluids taken into respective caps  51  and  52 . 
     A method of manufacturing the above-described pressure sensor chip  20  is described below. The process of manufacturing the pressure sensor chip  20  includes four steps, in other words, the first to fourth steps described below. 
     First of all, the first layer  21  is joined to the upper surface  25 A of the base  25 . The first layer  21  is made of silicon dioxide, and the cavity  21 B is patterned therein. The base  25  is made of silicon. The step of joining the first layer  21  to the base  25  is an example of the first step. The patterning in this step and in the following steps is carried out using a known method, such as etching. The joining is carried out using a known method, such as pressure bonding under high temperature environment. 
     Next, the second layer  22  made of silicon is joined to the upper surface  21 A of the first layer  21  in such a manner that the second layer  22  covers the cavity  21 B. The cavity  21 B is thereby sealed by the base  25  and the second layer  22 . The step of joining the second layer  22  to the first layer  21  is an example of the second step. 
     In the present preferred embodiment, at least the second step of the first to fourth steps is carried out under vacuum. Accordingly, the sealed cavity  21 B remains under vacuum. Note that the second step can be carried out in conditions other than under vacuum. In such a case, the pressure in the cavity  21 B is set to be lower than the pressure in the cavity  23 B of the third layer  23 , which will be joined to the second layer  22  in the third step. 
     Next, the third layer  23  is joined to the upper surface  22 A of the second layer  22 . Third layer  23  is made of silicon dioxide, and the cavity  23 B is patterned therein. The third layer  23  is joined to the second layer  22  in such a manner that the first space  23 Ba of the cavity  23 B overlaps the cavity  21 B as viewed in plan. In other words, the third layer  23  is joined to the second layer  22  in such a manner that the first diaphragm  26  is formed in the second layer  22  at the position between the cavity  21 B and the cavity  23 B. In addition, the third layer  23  is joined to the second layer  22  in such a manner that the second space  23 Bb of the cavity  23 B does not overlap the cavity  21 B as viewed in plan. The step of joining the third layer  23  to the second layer  22  is an example of the third step. 
     Next, the fourth layer  24  is joined to the upper surface  23 A of the third layer  23 . The fourth layer  24  is made of silicon, and the cavity  24 B is patterned therein. In the state of the fourth layer  24  being joined to the third layer  23 , the upper surface  24 A of the fourth layer  24  is exposed to outside. Accordingly, the top end of the cavity  24 B is in communication with the outside. 
     The fourth layer  24  is joined to the third layer  23  in such a manner that the cavity  24 B overlaps the cavity  23 B as viewed in plan. In other words, the fourth layer  24  is joined to the third layer  23  in such a manner that the bottom end of the cavity  24 B is in communication with the cavity  23 B. 
     In addition, the fourth layer  24  is joined to the third layer  23  in such a manner that a portion of the fourth layer  24  other than the cavity  24 B overlaps the second space  23 Bb of the cavity  23 B as viewed in plan. In other words, the fourth layer  24  is joined to the third layer  23  in such a manner that the second diaphragm  27  is formed in the fourth layer  24  at the position between the cavity  23 B and the space that the upper surface  24 A faces (i.e., the outside space). 
     The step of joining the fourth layer  24  to the third layer  23  is an example of the fourth step. 
     The pressure sensor chip  20  is manufactured by carrying out the first to fourth steps. 
     Next, the pressure sensor chip  20  manufactured through the above steps is mounted on the substrate  10 . Components, such as the ASIC  30  and resistors, are also mounted on the substrate  10  as needed. The pressure sensor chip  20  and the above components are mounted on the substrate  10  using a known method, such as a surface mounting technology or a through-hole mounting technology. In the first preferred embodiment, the pressure sensor chip  20  and the ASIC  30  are adhered to the substrate  10  using an adhesive (not illustrated) that is applied to the bottom surfaces of the pressure sensor chip  20  and the ASIC  30 . 
     Next, wires, such as the wires  31  and  32 , are wired using a known method. In the first preferred embodiment, the wires  31  and  32  are wired using wire bonding. One end of the wire  31  is connected to the pad  24 C formed on the fourth layer  24 , and the other end is connected to the ASIC  30 . Other wires that connect the pressure sensor chip  20  to the ASIC  30  are wired in a manner similar to the wire  31 . Both ends of the wire  32  are connected respectively to the pad  33  and the ASIC  30  that are disposed on the upper surface of the substrate  10 . 
     Next, the first covering portion  40  made of a resin is formed so as to cover the upper surface  10 A of the substrate  10  and also cover the pressure sensor chip  20  and the ASIC  30  that are mounted on the upper surface  10 A. The first covering portion  40  covers the surfaces of the pressure sensor chip  20  except for the lower surface of the base  25 . In other words, the first covering portion  40  covers and the upper surface  24 A of the fourth layer  24  and the side surfaces of the first layer  21 , the second layer  22 , the third layer  23 , and the fourth layer  24 . 
     The first covering portion  40  is formed so as to cover the upper surface  10 A of the substrate  10  using a known method, such as injection molding. In the first preferred embodiment, the first covering portion  40  in a softened state is injected toward the upper surface  10 A of the substrate  10 . In this step, a die is used to form the cavities  41  and  42  that pierce the first covering portion  40  in the height direction  4 . The cavities  41  and  42  are formed directly above the upper surface  24 A of the fourth layer  24 . The cavity  41  is formed so as to overlap the cavity  24 B as viewed in plan. As a result, the cavity  41  exposes the cavity  24 B to outside. The cavity  42  is formed so as to overlap the second space  23 Bb of the cavity  23 B as viewed in plan. As a result, the cavity  42  exposes a portion of the upper surface  24 A of the fourth layer  24  positioned directly above the second space  23 Bb to outside. Thus, the second diaphragm  27  is formed in the fourth layer  24  at the position between the cavity  42  and the second space  23 Bb. 
     Next, the second covering portion  50  made of a resin is formed so as to cover an upper surface  40 A of the first covering portion  40 . As is the case for the first covering portion  40 , the second covering portion  50  covers the upper surface of the first covering portion  40  using a known method. In this step, a die is used to form the caps  51  and  52  in the second covering portion  50 . In the first preferred embodiment, the caps  51  and  52  are shaped like tubes projecting upward. The cap  51  is formed directly above the cavity  41 , which enables the cavity  41  to communicate with the outside through the internal space  53  of the cap  51 . The cap  52  is formed directly above the cavity  42 , which enables the cavity  42  to communicate with the outside through the internal space  54  of the cap  52 . 
     The step of forming the first covering portion  40  and the second covering portion  50  so as to cover the pressure sensor chip  20  is an example of the fifth step. 
     According to the first preferred embodiment, the pressure in the cavity  21 B and the pressure in the cavity  23 B act on the first diaphragm  26 . This enables the first diaphragm  26  to measure pressure relative to the pressure in the sealed cavity  21 B. The pressure in the cavity  23 B and the pressure from the outside act on the second diaphragm  27 . The pressure from the outside acts on the upper surface  24 A of the fourth layer  24  through the internal space  54  of the cap  52  and through the cavity  42 . This enables the second diaphragm  27  to measure the differential pressure between the above two pressures. In other words, according to the first preferred embodiment, a single pressure sensor chip  20  can measure the differential pressure between two pressures as well as the pressure relative to the pressure in the cavity  21 B. Moreover, two diaphragms (the first diaphragm  26  and the second diaphragm  27 ) are integrated in the single pressure sensor chip  20 . As a result, the size of the pressure sensor chip  20  can be reduced. 
     According to the first preferred embodiment, the first diaphragm  26  and the second diaphragm  27  are formed in different layers. As a result, the thickness of the second layer  22  in which the first diaphragm  26  is formed and the thickness of the fourth layer  24  in which the second diaphragm  27  is formed can be made differently without complicating the manufacturing process. 
     In the first preferred embodiment, the cavity  21 B is under vacuum. This enables the first diaphragm  26  to function as a diaphragm to measure absolute pressure. 
     According to the first preferred embodiment, the pressure sensor chip  20  can operate as a capacitance sensor. 
     According to the first preferred embodiment, the first diaphragm  26  does not overlap the second diaphragm  27  as viewed in plan. This reduces the likelihood of the pressure in the cavity  21 B affecting the second diaphragm  27  via the second layer  22  and the cavity  23 B. This enables the second diaphragm  27  to measure the differential pressure accurately. 
     According to the first preferred embodiment, the first covering portion  40  can protect the pressure sensor chip  20 . 
     According to the first preferred embodiment, the pressure sensor  1  can be connected easily to outside devices through the caps  51  and  52 . 
     According to the method of manufacturing the pressure sensor chip  20  and the pressure sensor  1  of the first preferred embodiment, the pressure sensor chip  20  and the pressure sensor  1  can be manufactured without using a complicated step, for example, a step of changing the thickness of the same layer. 
     The shapes of the cavities  21 B,  23 B, and  24 B of the pressure sensor chip  20 , the shapes of the cavities  41  and  42  of the first covering portion  40 , and the shapes of the caps  51  and  52  of the second covering portion  50  are not limited to those described in the first preferred embodiment. 
     For example, the shape of the cavity  21 B, the shapes of the first space  23 Ba and the second space  23 Bb of the cavity  23 B, and the shape of the cavity  24 B are described as rectangles as viewed in plan in the first preferred embodiment. These shapes, however, may be others, such as circles. 
     The positions and sizes of the cavities  21 B,  23 B,  24 B of the pressure sensor chip  20 , the positions and sizes of the cavities  41  and  42  of the first covering portion  40 , and the positions and sizes of the caps  51  and  52  of the second covering portion  50  are not limited to those described in the first preferred embodiment. The positions and sizes, however, preferably satisfy the following four conditions. 
     The first condition is that at least a portion of the cavity  21 B overlaps at least a portion of the cavity  23 B as viewed in plan. The first diaphragm  26  is a portion of the second layer  22  that overlaps the cavity  21 B and the cavity  23 B as viewed in plan. The second condition is that at least a portion of the cavity  24 B overlaps at least a portion of the cavity  23 B as viewed in plan. This enables the cavity  23 B to communicate with the outside through the cavity  24 B. Third condition is that at least a portion of the cavity  24 B overlaps at least a portion of the cavity  41  as viewed in plan. The fourth condition is that the cavity  24 B does not overlap the cavity  42  as viewed in plan. 
     In the first preferred embodiment, for example, the cavity  24 B overlaps only the first space  23 Ba of the cavity  23 B as viewed in plan. The cavity  24 B, however, can overlap the third space  23 Bc instead of, or in addition to, the first space  23 Ba. 
     In the first preferred embodiment, as illustrated in  FIG.  5   , the third space  23 Bc of the cavity  23 B is narrower than the first space  23 Ba and the second space  23 Bb. The position, size, and shape of the third space  23 Bc, however, are not limited to those illustrated in  FIG.  5   . For example, the width of the third space  23 Bc (i.e., the length in the transverse direction  3 ) can be longer or shorter than the width illustrated in  FIG.  5   . Moreover, the third space  23 Bc can extend in the longitudinal direction  2  while inclining toward one side or the other side in the transverse direction  3 . Note that the third space  23 Bc preferably extend straight and does not curve or bend. 
     In the first preferred embodiment, the thicknesses (the lengths in the height direction  4 ) of the first layer  21 , the second layer  22 , the third layer  23 , and the fourth layer  24  are described as being the same as illustrated in  FIG.  1   . The thicknesses, however, can be different from each other. 
     For example, as illustrated in  FIG.  7   , the thickness of the fourth layer  24  can be smaller than the thickness of the second layer  22 . Due to the second diaphragm  27  being thinner than the first diaphragm  26 , the second diaphragm  27  can be bent by a small pressure difference. In addition, because the cavity  21 B is under vacuum or has a lower pressure, the first diaphragm  26  may bend largely and break. The first diaphragm  26 , however, is thicker than the second diaphragm  27 , which can reduce or prevent the likelihood of the first diaphragm  26  breaking. 
     The first layer  21  and the third layer  23  can include multiple layers. In this case, the cavity  21 B pierces the multiple layers included in the first layer  21  in the height direction  4 , and the cavity  23 B pierces the multiple layers included in the third layer  23  in the height direction  4 . 
     In the first preferred embodiment, the capacitor C 1  is between the first diaphragm  26  and the base  25  in the pressure sensor chip  20 , and the capacitor C 2  is between the second diaphragm  27  and the second layer  22  (see  FIG.  6   ). Accordingly, the pressure sensor chip  20  operates as the capacitance sensor. The pressure sensor chip  20 , however, is not limited to the capacitance sensor. For example, strain gauges may be provided on the first diaphragm  26  and the second diaphragm  27 , and the pressure sensor chip  20  may operate as a piezoelectric sensor. 
     Second Preferred Embodiment 
       FIG.  8    is a longitudinal sectional view illustrating a pressure sensor according to a second preferred embodiment of the present invention.  FIG.  9    is a plan view illustrating a pressure sensor chip included in the pressure sensor of  FIG.  8   .  FIG.  10    is section D-D of the pressure sensor chip illustrated in  FIG.  8   .  FIG.  11    is section E-E of the pressure sensor chip illustrated in  FIG.  8   .  FIG.  12    is section F-F of the pressure sensor chip illustrated in  FIG.  8   . 
     The pressure sensor of the second preferred embodiment is different from the pressure sensor of the first preferred embodiment in that as viewed in plan, the first diaphragm overlaps the second diaphragm and the cavity in the third layer has a rectangular shape. 
     The cavity  21 B of the second preferred embodiment (see  FIG.  10   ) is larger than the cavity  21 B of the first preferred embodiment (see  FIG.  3   ). As illustrated in  FIG.  8   , the cavity  21 B of the first layer  21  extends from a position under the cavity  41  to a position under the cavity  42 . Accordingly, the first diaphragm  26  of the second preferred embodiment (see  FIG.  11   ) is larger than the first diaphragm  26  of the first preferred embodiment (see  FIG.  4   ). As a result, the first diaphragm  26  overlaps the second diaphragm  27  as viewed in plan, as illustrated in  FIG.  9   . 
     As illustrated in  FIG.  12   , the width of the third space  23 Bc of the cavity  23 B, in the other words, the length of the third space  23 Bc in the transverse direction  3  is equal to the length of the first space  23 Ba and the length of the second space  23 Bb in the transverse direction  3 . Accordingly, the cavity  23 B is shaped like a rectangle. In  FIG.  12   , the border lines between the third space  23 Bc and the first space  23 Ba and between the third space  23 Bc and the second space  23 Bb are indicated by dotted lines. 
     According to the second preferred embodiment, the first diaphragm  26  is larger than that of the first preferred embodiment, which can increase the sensitivity of the first diaphragm  26  compared with that of the first preferred embodiment. 
     Third Preferred Embodiment 
       FIG.  13    is a longitudinal sectional view illustrating a pressure sensor according to a third preferred embodiment of the present invention. 
     The pressure sensor of the third preferred embodiment is different from the pressure sensor of the first preferred embodiment in that the pressure sensor chip is disposed on the ASIC. 
     The ASIC  30  of the third preferred embodiment (see  FIG.  13   ) is larger than the ASIC  30  of the first preferred embodiment (see  FIG.  1   ). As illustrated in  FIG.  13   , the pressure sensor chip  20  is mounted on the upper surface of the ASIC  30 . The pressure sensor chip  20  is fixed to the upper surface of the ASIC using a known method, such as adhesion using an adhesive. 
     Fourth Preferred Embodiment 
       FIG.  14    is a longitudinal sectional view illustrating a pressure sensor according to a fourth preferred embodiment of the present invention. 
     The pressure sensor of the fourth preferred embodiment is different from the pressure sensor of the first preferred embodiment in that the first covering portion and the second covering portion are formed integrally and the internal spaces of the caps are shaped differently. 
     In the fourth preferred embodiment, as illustrated in  FIG.  14   , the first covering portion  40  and the second covering portion  50  of the first preferred embodiment are integrated with each other. In the fourth preferred embodiment, a covering portion in which the first covering portion  40  and the second covering portion  50  are integrated is referred to as a “covering portion  60 ”. 
     The covering portion  60  includes caps  61  and  62 . The structure of the cap  61  is substantially the same as that of the cap  51  of the first preferred embodiment. The structure of the cap  62  is substantially the same as that of the cap  52  of the first preferred embodiment. In the fourth preferred embodiment, the caps  61  and  62  are shaped like tubes. However, internal spaces  63  and  64  in respective caps  61  and  62  are different from the internal spaces  53  and  54  of the caps  51  and  52  of the first preferred embodiment. More specifically, the smaller the inside diameter of each of the internal spaces  63  and  64 , the lower the position (in other words, as it goes closer to the pressure sensor chip  20 ). The internal space  63  of the cap  61  is in communication with the cavity  24 B of the fourth layer  24 . The internal space  64  of the cap  62  is positioned above a portion of the fourth layer  24  where the cavity  24 B is not provided. 
     In the manufacturing process of the pressure sensor chip  20 , the covering portion  60  is formed so as to cover the upper surface  10 A of the substrate  10  using a known method, such as injection molding, as is the case for the first covering portion  40  of the first preferred embodiment. In this step, a die is used to form the caps  61  and  62  in the covering portion  60 . The die can be used to form the internal spaces  63  and  64  of the caps  61  and  62  because the diameter of each internal space decreases with the position being lower. Note that the shapes of the internal spaces  63  and  64  and the external shapes of the caps  61  and  62  are not limited to those illustrated in  FIG.  14   . 
     Configurations of different preferred embodiments can be combined appropriately with one another, and resulted combinations are able to provide advantageous effects similar to those obtained in the original preferred embodiments. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.