Abstract:
A pressure switch with improved sealing of an airtight chamber, and improved electrical characteristics reducing chattering, increasing response rate, and minimizing the pressure necessary for activation. The pressure switch includes an upper substrate with a diaphragm readily deformed by an applied stress, a lower substrate overlapped with the upper substrate to form the airtight chamber, a contact electrically switched in response to the deformation of the diaphragm, and a sealing member continuously surrounding the airtight chamber, disposed between the first and second substrate, and hermetically sealing the airtight chamber.

Description:
BACKGROUND OF THE INVENTION 
     1) Technical Field of the Invention 
     This invention relates to a pressure switch with an airtight chamber partially defined by a diaphragm for electrically switching thereof in response to the stress applied to the diaphragm. 
     2) Description of Related Art 
     Some types of pressure switches have been so far proposed for a use of automobiles and industrial machines, in which a diaphragm of the pressure switch formed by partially thinning the semiconductor substrate is applied. Referring to FIGS. 16 through 18, the details of the conventional pressure switch disclosed in JPA06-267381, as an example, will be described hereinafter. 
     The conventional pressure switch  100  as shown in FIG. 16 basically comprises a silicon substrate  110  made of p-type single crystal and a glass substrate  130 . The silicon substrate  110  includes, in its middle portion, a depression  111  formed on one surface (top surface), a recess  112  formed on the other surface (bottom surface) opposing to the depression  111 , and a diaphragm  113  defined by and between the depression  111  and a recess  112  (with a thickness of several ten micrometers). The silicon substrate  110  further comprises a pair of p-type diffusion layers  114 ,  115 , which are formed on the top surface, and spaced apart (electrically isolated) from each other through the depression  111 . A pair of terminal electrode pads  116 ,  117  made of aluminum is also deposited on the top surface of the silicon substrate  110  for electrically connecting the pressure switch to the peripheral devices. A first wire layer  118  made of material such as aluminum is deposited on and extends along the diffusion layer  114  (left side), a side-wall, and a bottom of the depression  111 . 
     On the other hand, the glass substrate  130  is joined on the top surface of the silicon substrate  110  so that an airtight chamber (reference pressure chamber) is defined between the depression  111  and the glass substrate  130 . A second wire layer  131  also made of material such as aluminum is formed on a part of a bottom surface of the glass substrate  130  opposing the diffusion layer  115  (right side). The first and second wire layers  118 ,  131  are opposing each other within the airtight chamber  119 , and each includes a contacting tip  120 ,  132  made of titanium, respectively. The diaphragm  113 , when stressed, is deformed close to the glass substrate  130  so that the contacting tips  120 ,  132  contact each other so as to electrically connect the terminal electrode pads  116 ,  117  through the p-type diffusion layer  114 ,  115  and the wire layers  118 ,  131 . Thus, the pressure switch can be switched in accordance with deformation (incurvature) of the diaphragm. 
     The silicon substrate  110  is designed to include a pair of offset paths  121 ,  133  pre-formed on the p-type diffusion layers  114 ,  115  for offsetting the thickness of the wire layers  118 ,  131  thereby to smoothen the joint surface where the silicon substrate  110  and the glass substrate  130  are joined together. In general, in order to achieve the high reliable pressure switch, the silicon substrate  110  and the glass substrate  130  should be hermetically sealed to define the airtight chamber  119 , thereby maintaining its airtightness for a long time period. 
     SUMMARY OF THE INVENTION 
     Nevertheless, according to the above conventional pressure switch, the pre-formation of the offset path  122 ,  133  requires a precise control of the manufacturing process for the wire layers  118 ,  131  as well as the offset path  122 ,  133 , so that both layers and paths have the same thickness. In fact, such control is too difficult to be achieved, and a high productivity can hardly expected especially in a mass production line. 
     Referring to the silicon substrate  110  as shown in FIG. 17, the diffusion layers  114 ,  115  are formed apart from each other via a region  122 , in which the diffusion layer is not deposited. In general, a surface of the diffusion layer, when grown, is swelled by approximately one micrometer than the original surface so that a micro-step is formed between regions in which the diffusion layer is deposited and not. Therefore, the micro-step caused by the thickness of the diffusion layers  114 ,  115  as well as the thickness of the wire layers  118 ,  131  should be taken into consideration in order to smoothen the joint surface between the silicon substrate  110  and the glass substrate  130 . Indeed, the glass substrate  130  is gapped apart from the silicon substrate  110  at the region  122  in which the diffusion layer is not deposited so that the pressure switch  100  is as shown in FIG.  18 . This causes a problem deteriorating the airtightness of the airtight chamber  119  thereby to reduce the reliability of the pressure switch  100 . 
     In addition to that, the formation of the contacting tips  120 ,  132  made of material such as titanium causes each a step-like boss at the overlapping portions of the contacting tips  120 ,  132  on the wire layers  118 ,  131 , as clearly shown in FIG.  18 . In general, the contacting tips  120 ,  132  should have the contacting surface as wide as possible in order to improve the electrical switching characteristics of the pressure switch  100 , for instance, to reduce a resistance between the wire layers  118 ,  131 , to minimize the chattering that is a noise vibration, and to optimize the deviation of pressure among pressure switches that is necessary for activating thereof. This would require that the step-like bosses of the contacting tips  120 ,  132  have complementary configurations each other, which is almost impossible to control to produce. 
     Further, as described above, the first wire layer  118  (left side) is formed on and extending along the diffusion layer  114  (left side), a side-wall and a bottom of the depression  111 . The first wire layer  118  is bent at the portion between the top surface of the silicon substrate  110  and the side-wall of the depression  111 , and at the portion between the side-wall and the bottom of the depression  111 , thus the first wire layer  118  is easily broken at those bending portions. 
     Therefore, the present invention addresses the difficulties and problems as mentioned above. The first object of the present invention is to provide a pressure switch with an airtight chamber of which airtightness can be maintained for a long time period. 
     The further object of the present invention is to provide a pressure switch, which switches with less chattering at a higher response speed, and requires the minimized stress necessary for activating the pressure switches. 
     The pressure switch according to the first aspect of the present invention, comprises: a first substrate having a first opposing surface and a diaphragm capable of being readily deformed by a stress applied thereto; a second substrate having a second opposing surface overlapped with the first opposing surface of the first substrate to form an airtight chamber between the first and second substrate; a contact mechanism including, a first and second contact deposited within the airtight chamber and on the first opposing surface of the first substrate, a third contact deposited within the airtight chamber and on the second opposing surface of the second substrate, capable of being electrically connected with the first and second contact in response to the deformation of the diaphragm; and a sealing member continuously surrounding the airtight chamber, the sealing member disposed between the first and second opposing surface, thereby hermetically sealing the airtight chamber off the atmosphere. 
     The pressure switch according to the present invention, further comprises; a first and second conductive layer deposited on the first opposing surface of the first substrate, the first conductive layer being continuously surrounded by and spaced apart from the second conductive layer; and wherein the sealing member is the second conductive layer. 
     In the pressure switch according to the present invention, the first substrate is made of semiconductor material and the second substrate is made of glass. 
     The pressure switch according to the second aspect of the present invention, comprises: a first substrate having a first opposing surface and a diaphragm capable of being readily deformed by a stress applied thereto; a second substrate having a second opposing surface overlapped with the first opposing surface of the first substrate to form an airtight chamber between the first and second substrate; a contact mechanism including, a first contact deposited within the airtight chamber and on the first opposing surface of the first substrate, a second and third contact deposited within the airtight chamber and on the second opposing surface of the second substrate, capable of being electrically connected with the first contact in response to the deformation of the diaphragm; and a sealing member continuously surrounding the airtight chamber, the sealing member disposed between the first and second opposing surface, thereby hermetically sealing the airtight chamber off the atmosphere. 
     The pressure switch according to the present invention, further comprising: a first and second conductive layer deposited on the second opposing surface of the second substrate; and wherein the second and third contact deposited on the first and second conductive layer. 
     In the pressure switch according to the present invention, the first and second substrate are made of semiconductor material. 
     In the pressure switch according to the present invention, the sealing member includes a layer made of alkali glass. 
     In the pressure switch according to the present invention, the first substrate is made of semiconductor material, and the second substrate is made of glass. 
     The pressure switch according to the third aspect of the present invention, comprises: a first substrate having a first opposing surface and a diaphragm capable of being readily deformed by a stress applied thereto; a second substrate having a second and third opposing surface, the second opposing surface overlapped with the first opposing surface of the first substrate to form an airtight chamber between the first and second substrate; a contact mechanism including, a first contact on the first opposing surface of the first substrate, a second and third contact on the second opposing surface of the second substrate, capable of being electrically connected with the first contact in response to the deformation of the diaphragm; a third substrate having a fourth opposing surface overlapped with the third opposing surface of the second substrate; a sealing member disposed between the first and fourth opposing surface, continuously surrounding the airtight chamber, thereby hermetically sealing the airtight chamber off the atmosphere. 
     In the pressure switch according to the present invention, the second substrate includes a first wire member and a second wire member, and the first and second wire member is spaced away from each other, and wherein the second and third contact is disposed on the first and second wire member, respectively. 
     In the pressure switch according to the present invention, the first and second substrate is made of semiconductor and the third substrate is made of glass. 
     The pressure switch according to the present invention, further comprises a stiffening means disposed on the the diaphragm for stiffening a portion of the diaphragm adjacent to the first, second and third contact. 
     In the pressure switch according to the present invention, the diaphragm has a circular configuration. 
     In the pressure switch according to the present invention, the airtight chamber is filled with inert gas. 
     The pressure switch according to the present invention, further comprises a pair of terminal means electrically connected to the first and second conductive layer, respectively. 
     The pressure switch according to the present invention, further comprises a pair of terminal means electrically connected to the first and second wire member, respectively. 
     In the pressure switch according to the present invention, each of the first, second and third contact is substantially flat. 
     In the pressure switch according to the present invention, each of the first, second and third contact is made of gold. 
     In the pressure switch according to the present invention, the first substrate is high resistive. 
     The pressure switch according to the present invention, further comprises an insulating layer disposed on the first opposing surface. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: 
     FIG. 1 is a cross sectional view of the pressure switch according to Embodiment 1 of the present invention; 
     FIG. 2 is a cross sectional view taken along lines II—II in FIG. 1; 
     FIG. 3 is the similar cross sectional view to that of FIG. 1 while the pressure switch is activated on; 
     FIG. 4 is a cross sectional view of the pressure switch according to Embodiment 2 of the present invention; 
     FIG. 5 is a cross sectional view taken along lines V—V in FIG. 4; 
     FIG. 6 is the similar cross sectional view of the pressure switch further including an insulating layer; 
     FIG. 7 is a cross sectional view of the pressure switch according to Embodiment 3 of the present invention; 
     FIG. 8 is a cross sectional view of the pressure switch according to Embodiment 4 of the present invention; 
     FIG. 9 is a cross sectional view taken along lines IX—IX in FIG. 8; 
     FIG. 10 is microscopic view of contacts of the pressure switches according to Embodiment 1 and Modification 1 thereof, at the moment the movable contacts is connecting with the fixed contact; 
     FIG. 11 is a graph of the contacting resistance versus the time of the pressure switches according to Embodiment 1 and Modification 1 thereof, while the switches are activated on; 
     FIG. 12 is a cross sectional view of the pressure switch according to Modification 1 of Embodiment 1; 
     FIG. 13 is a cross sectional view of an another pressure switch according to Modification 1 of Embodiment 1; 
     FIG. 14 is a cross sectional view of a further another pressure switch according to Modification 1 of Embodiment 1; 
     FIG. 15 is a cross sectional view of the pressure switch according to Modification 2 of Embodiment 1; 
     FIG. 16 is a cross sectional view of the conventional pressure switch; 
     FIG. 17 shows a top surface of the silicon substrate taken along lines XVII—XVII in FIG. 16; and 
     FIG. 18 is a cross sectional view of the conventional pressure switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the attached drawings, the details of embodiments according to the present invention will be described hereinafter. In those descriptions, although the terminology indicating the directions (for example, “upper”, “lower”, “right”, and “left”) are conveniently used just for clear understandings, it should not be interpreted that those terminology limit the scope of the present invention. 
     (Embodiment 1) 
     A pressure switch according to Embodiment 1 is described in FIGS. 1 and 2. As clearly shown in FIG. 1, the pressure switch  1  basically comprises an upper substrate  10  and a lower substrate  20  disposed beneath the upper substrate  10 . The upper substrate  10  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Also, the lower substrate  20  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Preferably, the upper substrate  10  and the lower substrate  20  are made of silicon, and glass, respectively. However, the present invention should not be limited to those materials. 
     A middle portion of the upper surface of the upper substrate  10  is processed to form a recess  11  thereby having a thinned bottom portion, which defines a diaphragm  12 . Although not specifically limited thereto, if the upper substrate  10  is made of silicon, any suitable etching processes may be used for forming the recess  11 . The upper substrate  10  may be thinned before the etching process, if desired. The diaphragm  12  should have a thickness such that the diaphragm  12  is, when stressed and unstressed, easily deformed to the direction of the thickness (vertical direction in the drawing). The diaphragm  12  preferably has a thickness, for example, of several ten micrometers. 
     As particularly shown in FIG. 2, the first and second conductive layer  13 ,  14  are deposited on the lower surface of the upper substrate  10 , so that the second conductive layers  14  is continuously surrounded by and spaced away from the first conductive layers  13 . On the lower surface of the upper substrate  10  beneath the diaphragm  12  (as shown with a dotted line in FIG.  2 ), the first conductive layers  13  is extending from the left side and protruding to right side in the drawing, and the second conductive layers  14  is extending from the right side and protruding to left side in the drawing. Both protruding portions of the first and second conductive layers  13 ,  14  oppose each other approximately in the middle of the diaphragm  12  with some predetermined interval. 
     Although a various processes may be used for depositing the first and second conductive layer  13 ,  14  on the lower surface of the upper substrate  10 , if the upper substrate  10  is made of n-type silicon, for example, those conductive layers  13 ,  14  may be advantageously formed by implanting or diffusing impurity such as boron into the silicon substrate thereby to grow the p-type diffusion layer (high impurity-doped layer). Each of those conductive layers  13 ,  14  has a portion within the diaphragm  12 , on which low resistive and relatively soft metal (for example, gold) is laminated, so that a pair of movable contacts  15 ,  16  is formed. The movable contacts  15 ,  16  preferably have surfaces as wide as possible to contact over the wide surfaces with a fixed contact which will be described later, thereby reducing the resistance between the movable contacts  15 ,  16  through the fixed contact. 
     Each of those conductive layers  13 ,  14  has an another portion outside the diaphragm  12 , on which low resistive and relatively soft metal (for example, gold) is laminated, so that a first and second terminal electrodes  17 ,  18  are formed, respectively. 
     Referring back to FIG. 1, the upper surface of the lower substrate  20  is processed by a known etching technology to form a depression  21  with a predetermined depth (for example, approximately 5 through 10 μm) in a region opposing to the diaphragm  12 . The depression  21  has a bottom surface on which conductive metal (for example, gold) is laminated by a known thin-film laminating technology to form a fixed contact  22 . The lower substrate  20  has a pair of holes  23 ,  24  bored in regions corresponding to the first and second terminal electrodes  17 ,  18  of the upper substrate  10 . 
     The upper substrate  10  and the lower substrate  20  formed as described above, are bonded together by an appropriate bonding technology (for example, an anode-bonding technology) so that the depression  21  opposes to the diaphragm  12 , and the first and second terminal electrodes  17 ,  18  are exposed by the pair of holes  23 ,  24 , respectively. Thus, the depression  21  and the lower surface of the diaphragm  12  define an airtight chamber  25 . Within the airtight chamber  25 , the movable contacts  15 ,  16  are opposing to and spaced away from the fixed contact  22  with a predetermined gap. A switching contact mechanism is comprised of those contacts  15 ,  16 , and  22 . 
     As shown in FIG. 1, the conductive layers  13 ,  14  have their surfaces swelling with a certain thickness greater than the original silicon surface while formed by diffusing impurity into the silicon substrate. Therefore, when the lower surface of the upper substrate  10  is bonded to the lower surface  20 , there will be a gap equivalent to the swelling thickness between the upper substrate  10  and the lower substrate  20 . However, according to the present invention, the first conductive layer  13  continuously surrounds the second conductive layer  14 , as described above (See FIG.  2 ). Therefore, the first conductive layer  13  continuously contacts with the upper surface of the lower substrate  20  thereby to hermetically seal the airtight chamber  25  off the atmosphere. Such hermetically sealing causes the airtight chamber  25  completely sealed off the atmosphere thereby to maintain its airtightness perfectly. 
     The first and second terminal electrode  17 ,  18  of the pressure switch  1  formed as described above are connected to a circuit to be switched. In this implementation, a stress applied to the diaphragm  12  (for example, mechanical stress or hydrodynamic pressure) deforms the diaphragm  12  to the direction of the lower substrate  20 , resulting in contacting the movable contacts  25 ,  16  with the fixed contact  22 , so that the first and second terminal electrode  17 ,  18  are electrically connected through the first and second conductive layers  13 ,  14 , and the movable and fixed contacts  15 ,  16 ,  22 . When the stress or pressure is released, the diaphragm  12  returns in a position as shown in FIG. 1 by its own elasticity, so that the movable contacts  13 ,  14  disconnect from the fixed contact  22 . 
     (Embodiment 2) 
     FIGS. 4 and 5 show a pressure switch  2  according to Embodiment 2. As clearly shown in FIG. 2, the pressure switch  2  basically comprises an upper substrate  30  and a lower substrate  40  disposed beneath the upper substrate  30 . The upper substrate  30  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Also, the lower substrate  40  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Preferably, the upper substrate  30  and the lower substrate  40  are made of silicon. However, the present invention should not be limited to the material. 
     The upper substrate  30  is processed to form a recess  31  on the upper surface and a depression  33  on the lower surface, defining a thinned diaphragm  32  between the recess  31  and the depression  33 . Although not specifically limited thereto, if the upper substrate  30  is made of single crystal silicon, any suitable etching processes may be used for forming the recess  31  and the depression  33 . The upper substrate  30  may be thinned before the etching process, if desired. The diaphragm  32  should have a thickness such that the diaphragm  32  is, when stressed and unstressed, easily deformed to the direction of the thickness (vertical direction, in the drawing). The diaphragm  32  preferably has a thickness, for example, of several tens of micrometers. The depression  33  has a bottom surface on which conductive metal (for example, gold) is laminated by a known thin-film laminating technology to form a movable contact  34 . 
     As particularly shown in FIG. 5, the lower substrate  40  has a region opposing to the recess  33 , on which a first and second conductive layer  41 ,  42  are deposited. Those conductive layers  41 ,  42  are spaced away from each other. And those conductive layers  41 ,  42  may be formed by a similar process to that disclosed in Embodiment 1. Also, covered on those conductive layers  41 ,  42 , is a pair of fixed contact  43 ,  44  made of conductive metal (for example, gold) opposing to the movable contact  34 . The movable contact  34  and the fixed contacts  43 ,  44  together constitute the switching mechanism, and preferably have surfaces as wide as possible to minimize the resistance between the fixed contacts  43 ,  44  through the movable contact  34 . 
     The lower surface of the lower substrate  40  is processed by a known etching process to form a pair of apertures  36 ,  37  exposing a portion of the first and second conductive layer  43 ,  44 . Laminated on the exposed portions of the conductive layers  41 ,  42  are a first and second terminal electrodes  45 ,  46  made of metal such as gold. 
     The upper substrate  40  and the lower substrate  50  formed as described above are bonded by an appropriate bonding technique (for example, an nickel-silicide bonding technology) so that the fixed contacts  45 ,  46  of the lower substrate  40  oppose to the movable contact  34  of the upper substrate  30 . The nickel-silicide bonding is performed, for example, by forming a Ti (titanium) layer as a base layer on a peripheral region of the lower surface of the upper substrate  30  made of silicon and an Ni (nickel) layer on the base layer, aligning the upper substrate  30  to the lower substrate  40 , and then annealing the upper substrate  30  and the lower substrate  40  at approximately 400° C. Elements of Ni from the upper substrate  30  and Si from the lower substrate  40  form a bonding layer (an eutectic alloy) thereby to bond the upper substrate  30  and the lower substrate  40 . 
     Thus, the recess  33  of the upper substrate  30  defines an airtight chamber  47  in conjunction with the upper surface of the lower substrate  40  opposing to the recess  33 . Within the airtight chamber  25 , the movable contact  34  is opposing to and spaced apart from the fixed contacts  43 ,  44  with a predetermined gap. Those contacts  34 ,  43 , and  44  together constitute a switching contact mechanism. The first and second terminal electrodes  45 ,  46  are exposed through the apertures  36 ,  37 , respectively. 
     Although each of the conductive layers  41 ,  42  and each of the fixed contacts  43 ,  44  covered thereon has a thickness, each of them is completely included within the airtight chamber  47  and none of them is interposed in a bonding surfaces of the upper substrate  30  and the lower substrate  40 . Thus, the bonding surfaces are maintained even without such micro-steps. Also, in the bonding surfaces of the upper substrate  30  and the upper substrate  40 , a bonding layer  48  continuously surrounds the airtight chamber  47 . Therefore, the airtight chamber  47  can be completely sealed off the atmosphere. 
     The first and second terminal electrodes  45 ,  46  of the pressure switch  2  formed as described above are connected to a circuit to be switched. In this implementation, a stress applied to the diaphragm  32  (for example, mechanical stress or hydrodynamic pressure) deforms the diaphragm  32  to the direction of the lower substrate  40 , resulting in contacting the movable contact  34  with the fixed contacts  43 ,  44 , so that the first and second terminal electrode  45 ,  46  are electrically connected through the first and second conductive layers  41 ,  42 , and the movable and fixed contacts  34 ,  43 , and  44 . When the stress or pressure is released, the diaphragm  32  returns in a position as shown in FIG. 4 by its own elasticity, so that the movable contact  34  disconnects from the fixed contacts  43 ,  44 . 
     The upper substrate  30  may be alternatively made of low resistive silicon. However, in this application, an insulating layer  35  should be formed on the lower surface of the upper substrate  30  as shown in FIG. 6, preventing the fixed contacts  43 ,  44  from electrically connecting through the upper substrate  30 . 
     (Embodiment 3) 
     FIG. 7 shows a pressure switch  3  according to Embodiment 3. As clearly shown in FIG. 7, the pressure switch  3  basically comprises an upper substrate  50  and a lower substrate  60  disposed beneath the upper substrate  50 . The upper substrate  50  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Also, the lower substrate  60  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Preferably, the upper substrate  50  and the lower substrate  60  are made of silicon. However, the present invention should not be limited to the material. 
     The upper substrate  50  is processed to form a recess  51  on the upper surface and a depression  53  on the lower surface, defining a thinned diaphragm  52  between the recess  51  and the depression  53 . Although not specifically limited thereto, if the upper substrate  30  is made of single crystal silicon, any suitable etching processes may be used for forming the recess  51  and the depression  53 . The upper substrate  50  may be thinned before the etching process, if desired. The diaphragm  52  should have a thickness such that the diaphragm  52  is, when stressed and unstressed, easily deformed to the direction of the thickness (vertical direction in the drawing). The diaphragm  52  preferably has a thickness, for example, of several tens of micrometers. The depression  53  has a bottom surface on which conductive metal (for example, gold) is laminated by a known thin-film laminating technology to form a fixed contact  54 . 
     As clearly shown in FIG. 7, a first and second fixed contacts  61 ,  62  are formed on the upper surface of the lower substrate  60 , extending from the middle to the left edge and right edge of the lower substrate  60 , respectively. Those fixed contacts  61 ,  62  are opposing to each other with a predetermined distance. The movable contact  54  on the upper substrate  50  is disposed on the lower substrate  60  such that the movable contact  54  opposes to the fixed contacts  61 ,  62 . Those contacts  54 ,  61 , and  62  together constitute a switching contact mechanism, and preferably have surfaces as wide as possible to contact over the wide surfaces thereby to reduce the resistance between the movable contacts  61 ,  62  through the fixed contact  54 . 
     Also, the lower substrate  60  is processed by a known etching process to form a pair of holes for partially exposing the fixed contact  61 ,  62 . 
     In addition, a bonding layer  65  is formed on the upper surface of the lower substrate  60  so that the bonding layer  65  continuously surrounds the fixed contact  61 ,  62 . The bonding layer  65  may be made of, for example, alkali glass containing potassium ion and sodium ion and may be laminated, for example, by an electron beam evaporating, a sputtering, or a spin-on-glass technology with a use of a Pylex® glass. The bonding layer  65  has a thickness thicker at least than that of the fixed contacts  61 ,  62 . 
     The bonding layer  65  as formed described above is then bonded to the lower surface of the upper substrate  50  so that an airtight chamber  66  is defined by the depression  53  of the upper substrate  50 , the upper surface of the lower substrate  60  and the continuously surrounding bonding layer  65 . Within the airtight chamber  66 , the movable contact  54  is opposing to and spaced apart from the fixed contacts  61 ,  62  with a predetermined gap. Those contacts  54 ,  61 , and  62  constitute a switching contact mechanism. 
     Although each of the fixed contacts  61 ,  62  has a thickness, since the bonding layer  65  with a thickness thicker than those of the fixed contacts  61 ,  62  continuously surrounds the fixed contacts  61 ,  62 , the airtight chamber  66  can be completely sealed off the atmosphere with the perfect airtightness. 
     The first and second terminal electrode  61 ,  62  of the pressure switch  3  formed as described above are connected to a circuit to be switched. In this implementation, a stress applied to the diaphragm  52  (for example, mechanical stress or hydrodynamic pressure) deforms the diaphragm  52  to the direction of the lower substrate  60 , resulting in contacting the movable contact  54  with the fixed contacts  61 ,  62 . When the stress or pressure is released, the diaphragm  52  returns in a position by its own elasticity, so that the movable contact  54  disconnects from the fixed contacts  61 ,  62 . 
     The upper substrate  50  may be alternatively made of low resistive silicon. However, in this application, an insulating layer  55  should be formed on the lower surface of the upper substrate  50  as shown in FIG. 7, preventing the fixed contacts  61 ,  62  from electrically connecting through the upper substrate  50 . 
     (Embodiment 4) 
     FIGS. 8 and 9 shows a pressure switch  4  according to Embodiment 4. As clearly shown in FIG. 8, the pressure switch  4  basically comprises an upper substrate  70 , a middle substrate  80 , and a lower substrate  90 , in which the middle substrate  80  is interposed between the upper substrate  70  and the lower substrate  90 . The upper substrate  70  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. The middle substrate  80  is made of low resistive semiconductor material with a predetermined thickness (for example, 250 through 400 μm). Also, the lower substrate  90  is a thin board with a predetermined thickness (for example, 250 through 400 μm) that is made of insulating material or high resistive semiconductor material. Preferably, the upper substrate  70  and the middle substrate  80  are made of silicon, and the lower substrate  90  is made of glass. However, the present invention should not be limited to the material. 
     As clearly shown in FIG. 8, the upper substrate  70  is processed to form a depression  72  (with a thickness of approximately 5 through 10 μm) on the lower surface, defining a diaphragm  71  in a thinned portion corresponding to the depression  72 . Although not specifically limited thereto, if the upper substrate  70  is made of single crystal silicon, any suitable etching processes may be used for forming the depression  72 . The diaphragm  71  should have a thickness such that the diaphragm  71  is, when stressed and unstressed, easily deformed to the direction of the thickness (vertical direction in the drawing). The diaphragm  32  preferably has a thickness, for example, of several ten micrometers. The depression  72  has a bottom surface on which conductive metal (for example, gold) is laminated by a known thin-film laminating technology to form a movable contact  73 . 
     The middle substrate  80  has an upper surface on which a first and second fixed contacts  81 ,  82  made of conductive metal (for example, gold) are laminated opposing to the movable contact  73 . The movable contact  73  and the fixed contacts  81 ,  82  together constitute the switching mechanism, and preferably have surfaces as wide as possible to minimize the resistance between the movable contacts  81 ,  82  through the fixed contact  73 . 
     As clearly shown in FIG. 9, the middle substrate  80  is divided into three portions, that is, a first wire member  80   a  on which the first fixed contacts  81  is laminated, a second wire member  80   b  on which the second fixed contact  82  is laminated, and the peripheral sealing member  80   c  which continuously surrounds and is spaced apart from the first and second portion  80   a ,  80   b . Thus, the first and second portion  80   a ,  80   b  and the peripheral sealing member  80   c  are divided to have a space  83  therebetween, so that those portions are electrically isolated one another. The middle substrate  80 , when made of silicon, can be divided into three members  80   a ,  80   b  and  80   c  by, for example, etching the middle substrate  80  using the deep-dry etching technique after the middle substrate  80  is bonded on the upper substrate  70 . This results in that the middle substrate  80  are divided into those members  80   a ,  80   b ,  80   c  with the dividing space  83 . Further, a first and second terminal electrodes  84 ,  85  made of conductive metal (for example, gold) are deposited on the lower surface of the first and second members  80   a ,  80   b , respectively. 
     Referring again to FIG. 8, the lower substrate  90  has a pair of holes  91 ,  92  which is opposing to and exposing to the first and second terminal electrode  84 ,  85 , respectively. 
     The upper substrate  70 , the middle substrate  80 , and the lower substrate  90  are bonded together by an appropriate bonding technique (for example, anode-bonding technology) so that the movable contact  73  opposes to the fixed contacts  81 ,  82  with a predetermined distance, and the pair of apertures  91 ,  92  oppose to the first and second terminal electrodes  84 ,  85 . Thus, an airtight chamber  74  is defined beneath the diaphragm  73  in accordance with the depression  73 . Within the airtight chamber  74 , the movable contact  73  is opposing to the fixed contacts  81 ,  82 , and those contacts  73 ,  81 , and  82  together constitute a switching contact mechanism. The first and second terminal electrodes  84 ,  85  are exposed through the holes  91 ,  92 , respectively. 
     In this embodiment, although the airtight chamber  74  is connected to the dividing space  83 , the dividing space  83  is completely surrounded by the lower surface of the upper substrate  70 , the upper surface of the lower substrate  90 , and the peripheral member  80   c . Therefore, the airtight chamber  74  can be completely sealed off the atmosphere, thereby maintaining the airtightness perfectly. 
     The first and second terminal electrodes  84 ,  85  of the pressure switch  4  formed as described above are connected to a circuit to be switched. In this implementation, a stress applied to the diaphragm  71  (for example, mechanical stress or hydrodynamic pressure) deforms the diaphragm  71  in the direction of the middle substrate  80 , causing the movable contact  73  in contact with the fixed contacts  81 ,  82 , so that the first and second terminal electrode  84 ,  85  are electrically connected through the low resistive first and second wire members  80   a ,  80   b , and the movable and fixed contacts  81 ,  82 , and  73 . When the stress or pressure is released, the diaphragm  71  returns in a position as shown in FIG. 8 by its own elastic nature, so that the movable contact  73  disconnects from the fixed contacts  81 ,  82 . 
     The upper substrate  70  may be alternatively made of low resistive silicon. However, in this application, an insulating layer  75  should be formed on the lower surface of the upper substrate  70  as shown in FIG. 8, preventing the fixed contacts  81 ,  82  from electrically connecting through the upper substrate  70 . 
     Each one of the airtight chambers as described above, is preferably filled with inert gas such as nitrogen and helium. Alternatively, the airtight chamber may be vacuated. Thus, contacts made of conductive material such as gold can be prevented from deteriorating and discharging with another contacts in accompanying with switching the pressure switch of the present invention. 
     (Modification 1) 
     A first modification according to Embodiments 1 through 4 of the present invention, in which the diaphragm is improved, will be described hereinafter with reference to FIGS. 10 through 14. Although FIGS. 10 through 14 are illustrated based upon Embodiment 1, it will be readily understood that such modification can be applied to other embodiments. 
     As described above, the pressure switch  1  according to Embodiment 1 of the present invention is switched on, when the diaphragm  12  is deformed by the stress or pressure to connect the movable contacts  15 ,  16  contact with the fixed contact  22 . The diaphragm  12  is most greatly deformed on which the pressure is applied. And the stress is generally applied on the middle portion of the diaphragm  12 . FIG. 10A shows a microscopic view of the diaphragm  12  at the moment the switch  1  is being switched on. Thus, contacting surfaces of the movable contacts  15 ,  16  contacting with the fixed contact  22  are very small at the beginning, and gradually expanded as the diaphragm is getting flat. When the movable contacts  15 ,  16  fail to contact entirely with the fixed contact  22  (i.e. when the contacting surfaces are small), the chattering, that is, a noise vibration between the movable contacts  15 ,  16  and the fixed contact  22  is easily caused by an unstable stress. Also, even where the stress is constantly applied to the diaphragm  12 , as clearly shown by a dotted line in FIG. 11, it takes a certain time from a moment when the movable contacts  15 ,  16  first touch to the fixed contact  22  (at t=T 0 ) and a moment when the movable contacts  15 ,  16  contact thoroughly with the fixed contact  22  (at t=T 1 ). In other words, a certain time period from T 0  through T 1  is required to achieve the full-contact resistance of the pressure switch  1 . As the pressure switch needs longer time period between T 0  through T 1  to have a full contact, the response of the pressure switch is slower, which should be improved. 
     To address this problem, the middle portion of the diaphragm  12  is made less deformed by providing a ridge  19  around the middle portion of the diaphragm  12  thereby to stiffen the diaphragm  12  adjacent to the ridge  19 . Referring to FIG. 10B also showing a microscopic view of the diaphragm  12  with the ridge  19  at the moment the switch  1  is being switched on, the movable contacts  15 ,  16  are readily maintained flat, and entirely contacted with the fixed contact  22 . Furthermore, as shown by a real line in FIG. 11, the resistance of the pressure switch can be instantly reduced to the full-contact resistance. Thus, the pressure switch  1  having less chattering and high-speed response can be obtained. 
     FIG. 12 shows the ridge  19  as having a pyramid configuration or a conical configuration, the ridge  19  may have any configuration such as a cylinder or a cube as shown in FIG. 13, for stiffening the diaphragm  12 . 
     Further, although FIG. 12 shows the ridge  19  as being formed on the upper surface of the diaphragm  12  of Embodiment 1, the ridge may be formed on the lower surface of the diaphragm  32  and within the airtight chamber  47  to stiffen the diaphragm  32  as well, as shown in FIG. 14 for an another ridge according to Embodiment 2. 
     (Modification 2) 
     A second modification according to Embodiment 1 to 4 of the present invention, in which the diaphragm is improved, will be described hereinafter with reference to FIG.  15 . Although FIG. 15 is illustrated based upon Embodiment 1, it will be readily understood that such modification can be applied to other embodiments. 
     As described above, the pressure switch  1  according to Embodiment 1 of the present invention is switched on, when the diaphragm  12  is deformed by the stress or pressure so that the movable contacts  15 ,  16  contact with the fixed contact  22 . In case where the diaphragm  12  has a top-view with a square configuration as shown in FIG. 1, the distance from the center to the edge of the diaphragm  12  varies depending upon the direction to the edge. Therefore, the tension also depends upon the position of the diaphragm  12 , so that the diaphragm  12  is, in position, unevenly loaded, which is not favorable for the long-term reliability. 
     To solve this problem, the diaphragm  12  is designed to have a top-view with a circular configuration instead of the square configuration, so that the unevenness of the tensility (load) to the diaphragm  12  can be normalized thereby to achieve the robust and reliable pressure switch. In addition, the use of the circular diaphragm advantageously minimizes the stress for activating the pressure switch, in comparison with the stress for activating the pressure switch with the diaphragm having different configurations but the same dimension. In case where the most sensitive pressure switches capable of being activated with the minimized stress is required, such a pressure switch with the circular diaphragm is useful.