Abstract:
A pressure sensor assembly comprising: three stacked silicon wafers which form a support, a sensor and a cover wherein the sensor includes a cavity extending from the bottom of the sensor up towards the top of the sensor to form a cavity bottom and a diaphragm; a dielectric layer covering the bottom of the sensor and the cavity and wherein the support is coupled to the dielectric layer along the bottom of the sensor; a plurality of ports located on a top of the support within an area defined by the cavity, the plurality of ports extending through the support to its bottom and wherein the cover is coupled to the top of the sensor covering the diaphragm; and, a second cavity cut into a bottom of the cover wherein the second cavity is sized and positioned to surround the diaphragm.

Description:
TECHNICAL FIELD 
     The present patent document relates to pressure sensors and methods of making the same. More particularly, the present patent document relates to pressure sensors made from silicon wafers. 
     BACKGROUND 
     Current designs of pressure sensors for high pressure backside applications are inadequate for many applications. Typical pressure sensors use a diaphragm as the pressure detecting element of the pressure sensor. The diaphragm is exposed to a positive or negative pressure on only one side creating a pressure differential across the diaphragm. This pressure differential causes the diaphragm to flex and the amount of flex may cause a change in, e.g., resistance or capacitance, which may be measured with an electrical circuit. 
     One problem with current designs is that they do not limit the flow of pressure to the diaphragm. Accordingly, when the pressure sensor is exposed to a drastic change, the diaphragm experiences a very rapid pressure change. This rapid pressure change could cause the diaphragm to flex past its yield point and permanently damage the diaphragm and ultimately the pressure sensor. 
     Another issue with the current design is that the silicon diaphragm is directly exposed to the medium of which the pressure is to be measured. A conductive fluid at high electric potential impinging on the silicon surface will cause a large electric current to flow through the sensor and result in damage to the sensor and/or the circuitry attached to it. 
     Yet another issue with the current design is that there is no means of detection when the diaphragm has reached its displacement limit. Such a means of detection could be used to mitigate the displacement limit condition. 
     SUMMARY OF THE EMBODIMENTS 
     In view of the foregoing, an object according to one aspect of the present patent document is to provide a pressure sensor assembly. Preferably the methods and apparatuses address, or at least ameliorate one or more of the problems described above. To this end, a pressure sensor assembly is provided. In one embodiment, the pressure sensor assembly comprises: a first silicon wafer with a top and a bottom; a cavity formed in the silicon wafer, the cavity extending from the bottom of the first silicon wafer up towards the top of the first silicon wafer to form a cavity bottom such that a diaphragm is formed between the cavity bottom and the top of the first silicon wafer; a dielectric layer covering the bottom of the first silicon wafer and the cavity; a conductive layer, e.g., a metal such as aluminum, or highly doped polysilicon, covering the top of the diaphragm in the first silicon wafer; a second silicon wafer coupled to the dielectric layer along the bottom of the first silicon wafer and covering the cavity; a plurality of ports located on a top of the second silicon wafer within an area defined by the cavity, the plurality of ports extending through the second silicon wafer to a bottom of the second silicon wafer; a third silicon wafer coupled to the top of the first silicon wafer such that the third silicon wafer covers the diaphragm; and, a second cavity cut into a bottom of the third silicon wafer wherein the second cavity is sized and positioned such that the second cavity surrounds the diaphragm. 
     In preferred embodiments, the depth of the second cavity is designed to mechanically limit motion of the diaphragm to less than 3 times a full-scale pressure displacement. In an even more preferred embodiment the depth of the second cavity is less than or equal to 0.9 μm. 
     In yet other embodiments, the second cavity may also include a means to electronically detect that the diaphragm has reached its displacement limit. In such embodiments, a conductive layer may cover, or be appropriately placed on, the cavity bottom. A corresponding conductive layer may cover, or be appropriately placed on, the top of the diaphragm. Accordingly, when the diaphragm reaches its maximum displacement, the conductive layer on top of the diaphragm comes in contact with the conductive layer on the bottom of the second cavity. The layers may be appropriated designed, sized and shaped such that contact between the layers completes an electrical circuit, thus electronically detecting the maximum displacement of the diaphragm has occurred. In some embodiments, the conductive layer in the bottom of the second cavity may be in electrical communication with a contact located on another area of the sensor assembly. In a preferred embodiment, this contact may be located on top of the third silicon wafer. The conductive layers may be made from a metal such as aluminum, or highly doped polysilicon. 
     In some embodiments, the cavity bottom that forms the diaphragm is at least 80% of the way to the top of the first silicon wafer. In other embodiments other depths may be used. 
     In preferred embodiments, the dielectric layer insulates the electric circuit on the sensor from the medium impinging on the diaphragm. In preferred embodiments, the dielectric layer insulates the electric circuit on the sensor from the support. In preferred embodiments, the dielectric layer is made of silicon dioxide or silicon nitride or a combination of silicon dioxide and silicon nitride. In some embodiments, no dielectric layer is used and the first wafer is coupled directly to the second silicon wafer. 
     In some embodiments, the pressure sensor assembly may be further coupled to a substrate. In such embodiments, the substrate may be coupled to the bottom of the second silicon wafer. The substrate may include a channel that runs from a top of the substrate to a bottom of the substrate. The channel opening at the top of the substrate may cover the plurality of port openings on the bottom of the second silicon wafer. In some embodiments, the substrate may be a pedestal. 
     Other embodiments of a pressure sensor assembly are also described. In one embodiment, the pressure sensor assembly comprises: a first silicon wafer having a top surface and a bottom surface and a mid-plane defined half-way between the top surface and the bottom surface wherein the bottom surface includes a cavity that extends up towards the top surface past the mid-plane to a cavity bottom; a diaphragm formed between the cavity bottom and the top surface; a second silicon wafer coupled to the bottom surface of the first silicon wafer, the second silicon wafer covering the cavity to form a chamber; a plurality of channels that extend through the second silicon wafer and into the chamber; a third silicon wafer coupled to the top surface of the first silicon wafer, the third silicon wafer including a second cavity, the second cavity sized and positioned to cover the diaphragm and wherein a bottom of the second cavity forms a mechanical stop for the diaphragm; and wherein the first silicon wafer and the second silicon wafer are electrically isolated from each other by a dielectric layer deposited between them. 
     In preferred embodiments, the bottom of the second cavity may be covered with a conductive layer positioned to close an electrical circuit if the pressure applied to the diaphragm causes a conductive layer on top of the diaphragm to come in contact with the conductive layer on the bottom of the second cavity. 
     In another aspect of the present patent document, a method for manufacturing a pressure sensor assembly is provided. In a preferred embodiment, the method comprises: forming a sensor from a first silicon wafer by creating a cavity that extends up into the first silicon wafer from a bottom; forming a conductive layer over the top of the first silicon wafer; forming a dielectric layer over the bottom and cavity of the first silicon wafer; forming a support from a second silicon wafer by creating a plurality of channels that pass through the second silicon wafer from a top surface to a bottom surface; bonding the bottom of the first silicon wafer to the top surface of the second silicon wafer such that a chamber is formed by the cavity and the plurality of channels open into the chamber; forming a cover from a third silicon wafer by creating a second cavity that extends up into the third silicon wafer from a bottom; bonding the bottom of the third silicon wafer to a top of the first silicon wafer such that the second cavity surrounds the diaphragm. 
     In some embodiments, the depth of the second cavity is cut to mechanically limit motion of the diaphragm to less than 3 times a full-scale pressure displacement. In some methods, the depth of the second cavity is less than or equal 0.9 μm, 1.1 μm, 1.3 μm or 1.5 μm. 
     In other embodiments, a conductive layer may also be formed in the bottom of the cavity of the third silicon layer. The conductive layer may be in electrical communication with a contact located on the top of the third silicon wafer. In addition, a conductive layer may also be formed on top of the diaphragm of the first silicon layer. The conductive layer may be positioned such that when the diaphragm reaches a maximum displacement condition, the conductive layer on top of the diaphragm comes in electrical communication with the conductive layer on the bottom of the cavity in the third silicon wafer and completes a circuit, which may be used to detect the maximum displacement condition. 
     In some embodiments of the method, the dielectric layer is made of silicon dioxide or silicon nitride or a combination of silicon dioxide and silicon nitride. And, in some embodiments of the method, the pressure sensor assembly is coupled to a substrate at the bottom of the support. 
     As described more fully below, the embodiments of a pressure sensor described herein ameliorate or alleviate some of the problems described above. Further aspects, objects, desirable features, and advantages of the apparatus and methods disclosed herein will be better understood from the detailed description and drawings that follow in which various embodiments are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an isometric view of one embodiment of a pressure sensor assembly mounted on a substrate. 
         FIG. 2  illustrates a cross sectional view of the pressure sensor assembly of  FIG. 1 . 
         FIG. 3  illustrates a close up view of the cross section of  FIG. 2 . 
         FIG. 4  illustrates a plurality of cross sections of a silicon wafer during the process of forming one embodiment of a sensor. 
         FIG. 5  illustrates a plurality of cross section of a silicon wafer during the process of forming one embodiment of a support. 
         FIG. 6  illustrates a cross section of the combination of the sensor from  FIG. 4  with the support from  FIG. 5 . 
         FIG. 6A  illustrates the cross section of  FIG. 6  with a conductive layer added to the top surface of the pressure sensor. 
         FIG. 7  illustrates a plurality of cross sections of a silicon wafer during the process of forming one embodiment of a cover. 
         FIG. 7A  illustrates a plurality of cross sections of a silicon wafer during the process of forming an embodiment of a cover configured to electronically detect a maximum displacement condition of the diaphragm. 
         FIG. 8  illustrates a cross section of the combination of the sensor from  FIG. 4 , the support from  FIG. 5 , and the cover from  FIG. 7  to form one embodiment of a sensor assembly. 
         FIG. 9  illustrates the sensor assembly of  FIG. 8  with the sides of the cover diced off. 
         FIG. 10  illustrates a cross section of one embodiment of a sensor assembly designed to electronically detect the maximum displacement of the diaphragm. 
         FIG. 11  illustrates a cross section of one embodiment of a sensor assembly with the dielectric layer configured differently from the embodiment of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The words “top” and “bottom” are used in the description that follows to help provide an orientation to the reader. The words are not meant to limit the scope of the embodiments to those oriented a particular direction with respect to gravity but rather simply to provide a relative orientation and/or direction with respect to other parts or portions of a particular embodiments. For example, if the direction “top” is established, the “bottom” will be the opposite side of the embodiment from the top and vice versa. Accordingly, “up” is in a direction from the “bottom” towards the “top” and down is in a direction from the “top” towards the “bottom.” 
       FIG. 1  illustrates an isometric view of one embodiment of a pressure sensor assembly mounted on a substrate. In the embodiment shown in  FIG. 1 , the entire assembly  100  is comprised of the pressure sensor assembly  50  and a substrate  10 . The pressure sensor assembly  50  may be coupled to the substrate  10  using a number of different methods. In a preferred embodiment, the pressure sensor assembly  50  is bonded to the substrate  10  by a eutectic bonding technique, a glass-frit bonding technique, or an adhesive bonding technique. The substrate  10  shown in  FIG. 1  is exemplary and many other sizes or shapes of substrate  10  may be used. 
     The pressure sensor assembly  50  is comprised of a cover  52 , a sensor  54  and a support  56 . In embodiments that include a substrate  10 , the support  56  is coupled to the top of the substrate  10 . The sensor  54  is coupled to the top of the support  56  and the cover  52  is coupled to the top of the sensor  54 . 
     In a preferred embodiment, the cover  52 , sensor  54  and support  56  are all made from silicon (Si). In an even more preferred embodiment, the cover  52 , sensor  54  and support  56  are all made from a silicon wafer. Accordingly, the sensor assembly may be comprised by three stacked silicon wafers. In other embodiments, other materials may be used. Making the components of the sensor assembly  50  from silicon is advantageous because they may be manufactured using advance manufacturing techniques such as those used in Microelectromechanical Systems (MEMS) devices. In general, the sensor assembly  50  may be a MEMS device. 
     Typical silicon wafers for use with the preferred embodiments may be between 300 μm and 400 μm in thickness. In a preferred embodiment, the silicon wafer has a thickness of 350 μm. In other embodiments, other thicknesses may be used for the silicon wafers of the cover  52 , sensor  54  and support  56 . In different embodiments, different thicknesses may be used for each component. In a preferred embodiment, the crystalline orientation of the silicon wafer is 100 on the Miller index. However, other crystalline orientations may be used in other embodiments. 
     In a preferred embodiment, the cover  52 , sensor  54  and support  56  are all made of silicon wafers and are bonded together with a silicon bond. However, other bonding techniques may be used and in particular, other bonding techniques may be used if a material other than a silicon wafer is used. 
       FIG. 2  illustrates a cross sectional view of the embodiment of a pressure sensor assembly  50  including a substrate  10  of  FIG. 1 . The pressure sensor assembly  50  is coupled to the substrate  10  at the bottom of the support  56 . In the embodiment shown in  FIG. 2 , the substrate includes a channel  12  that runs from a top  11  of the substrate  10  to a bottom  13  of the substrate  10 . In other embodiments, other types of channels may be used. The channel in the substrate  10  may be all the same diameter or may include more than two different diameters or may be square or rectangular or polygonal. The channel in the substrate  10  may be of various different designs but should provide communication from the bottom of the pedestal  10  up to the bottom of the support  56 . 
     As may be seen in  FIG. 2 , the support  56  includes a plurality of ports  60  and the sensor  56  includes a cavity  58 . The bonding of the support  56  to the sensor  54  causes the cavity  58  to become a chamber  58 . The plurality of ports  60  extend through the thickness of the support  56 . In preferred embodiments, the openings of the ports are aligned such that they open in the chamber  58 . The plurality of ports  60  provide a path for the pressure to pass from the bottom of the support  56  up into the cavity  58 . 
       FIG. 3  illustrates a close up view of the cross section of  FIG. 2 . As may be seen in  FIG. 3 , a plurality of ports  60  extend through the support  56 . The ports  60  may also be referred to as channels or an array of channels. Using a plurality of smaller ports  60  is advantages over a single larger port because the array of smaller ports  60  dampen the pressure spike of the inrushing medium to protect the sensor  54 . In particular, the plurality of ports  60  dampen the pressure spike to the diaphragm  59  of the pressure sensor  54 . In preferred embodiments, the plurality of ports  60  are constructed with a significantly reduced diameter from a typical single larger port. In preferred embodiments, the ports  60  are formed by drilling holes into the support  56  using a Deep Reactive-Ion Etch (DRIE) tool or by ultrasonic drilling or by mechanical drilling. The shape of the ports  60  can be round or square or rectangular or polygonal. The flow velocity V f  in each channel is a function of the pressure difference Δp between the pressure P out  outside the chamber  58  and the pressure P in  inside the chamber  58 , the density of the fluid σ, the ratio of the diameter D of the channel to the length L of the channel, and the friction coefficient f D  inside the channel. This relationship is described by the equation V f =√(2/σ Δp/f D  D/L). To this end, the diameter and length of the ports  60  may be selected such that the flow of the medium into the chamber is reduced compared to a single large port. In preferred embodiments, for the fluid and the maximum pressure difference for which the sensor will be used, the ratio D/L is selected such that the flow is reduced to 1/10 th  of the flow when a single large port is employed. In other embodiments, other reduction ratios may be designed for. 
     In the embodiment shown in  FIG. 3 , five ports extend from the bottom of the support  56  up into the chamber  58 . However, in other embodiments other number of ports may be used. In a preferred embodiment, between 2 and 10 ports are used. In a more preferred embodiment, between 5 and 10 ports are used. In yet other embodiments between 5 and 50 ports may be used. In still yet other more complex embodiments, 50 or more ports may be used. 
     In embodiments where the ports are confined to the thickness of the support  56 , the length of the ports  60  is defined by the thickness of the silicon wafer. Accordingly, only a diameter needs to be selected. In a preferred embodiment, the diameter of a port is 1/100 or less the maximum diameter of the chamber  58 . In yet other embodiments, the diameter of a port is 1/25 to 1/15 the maximum diameter of the chamber  58 . In still yet other embodiments, the diameter of a port is 1/15 to 1/10 of the maximum diameter of the chamber  58 . In still yet other embodiments, the diameter of a port is 1/10 to ⅕ the maximum diameter of the chamber  58 . 
     The plurality of ports  60  may be aligned with the channel opening  15  at the top of the substrate  10  such that the channel opening  15  covers the plurality of port openings  60  on the bottom of the support  56 . In other embodiments, some portion of or more of the port openings  60  may be outside the channel opening  15 . 
     As may be seen in  FIG. 3 , the sensor  54  includes a cavity  58 . The cavity extends from the bottom surface of the sensor  54  down to a cavity bottom such that a diaphragm is formed between the cavity bottom and the top of the sensor  54 . In a preferred embodiment, the cavity bottom extends at least 80% of the way towards the top surface of the sensor  54 . In an even more preferred embodiment, the cavity bottom extends between 85% and 95% of the way to the top surface of the sensor  54 . The depth of the cavity  58  determines the diaphragm  59  thickness which in turn determines the full-scale pressure range of the pressure sensor  54 . The thickness of the sensor  54  is the distance between the bottom surface and the top surface. The sensor  54  has a mid-plane half-way between the top surface and the bottom surface. In preferred embodiments, the cavity  58  extends up towards the top surface past the mid-plane to a cavity bottom. 
     The minimum diaphragm thickness  59  (or maximum cavity  58  depth) is dictated by the output target and limited by the manufacturing process. There is no maximum diaphragm  59  thickness (or minimum cavity  58  depth). In a preferred embodiment, the diaphragm  59  thickness is typically from 10 μm to 95% of the sensor wafer thickness. 
     The sensor  54  includes an electric circuit disposed on the top side of the sensor  54 . The electrical circuit is used to measure the deflection of the diaphragm and thus, the pressure. The electric circuit may be formed by a conductive layer  51 . The conductive layer may be made from any metal. The conductive layer  51  is used to form circuitry embedded in the silicon of the sensor. In preferred embodiments, the conductive layer  51  is a different conductive layer from conductive layer  57  (shown in  FIG. 6A ). In some embodiments, they may be made from different materials and/or have different thicknesses. In other embodiments, the conductive layer  57  may be formed as part of the electrical circuit  51  or may be attached thereto. 
     In order to electrically isolate the electrical circuit from a potential short circuit, a dielectric layer may be used between the sensor  54  and the support  56 . In preferred embodiments, the dielectric layer covers both the cavity  58  and the bottom side of the sensor  54 . The dielectric layer may be made from any insulating material. In a preferred embodiment, the dielectric layer is made of silicon dioxide or silicon nitride. 
     The primary source of a potential short to the electric circuit is created when a conductive fluid at high electric potential enters the sensor assembly and impinges on the silicon surface. Contact between the fluid at high electrical potential and the silicon may cause a large electric current to flow through the sensor and result in damage to the sensor and/or the circuitry attached to it. Accordingly, the electric circuit needs to be electrically isolated from the cavity and channels which harbor the incoming fluid. This may be accomplished as seen in the cross section of a sensor assembly  50  in  FIG. 11  where a dielectric layer  30  covers the entire bottom surface of the sensor. In a preferred embodiment, this includes the surface of the cavity  58  and the surface between the sensor  54  and the support  56 . In another embodiment, the same electrical isolation may be accomplished by applying a dielectric layer  30  over all the surfaces exposed to the incoming fluid as may be seen in the cross section of a sensor assembly  50  shown in  FIG. 10 . In a preferred embodiment, this may include the entire chamber  58  and the surfaces of the ports  60 . 
     Returning to  FIG. 3 , the cover  52  is coupled to the top of the sensor  54 . In a preferred embodiment, the cover  52  also includes a cavity  62 . The cavity  62  is cut into the cover  52  on the side coupled to the sensor  54 . The cavity  62  extends up into the cover to a cavity bottom. Thus, when the cover  52  is assembled to the sensor  54 , the cavity  62  forms a gap above the diaphragm  59 . In a preferred embodiment, the cavity  62  in the cover  52  is sized and positioned such that the cavity  62  surrounds the bottom of the diaphragm  59 . Also in a preferred embodiment, the depth of the cavity  62  is designed such that the bottom of the cavity  62  limits the motion of the diaphragm  59  towards the cover  52 . Thus, the cavity is sized and positioned such that when the cover is coupled to the sensor  54 , the bottom of the cavity  62  acts as a mechanical stop to excessive displacement of the diaphragm  59 . 
     In a preferred embodiment, the diaphragm  59  is typically displaced about 0.3-0.5 μm at full-scale pressure. To be effective as a stop, the gap formed by the cavity  62  needs to be greater than the full-scale displacement but less than the distance where the diaphragm will yield or burst if no stop is present. A diaphragm yield or rupture typically occurs at 3 to 5 times the full-scale pressure displacement. Accordingly, for a diaphragm with a typical full scale displacement of 0.3 μm, the cavity  62  may have a depth greater than 0.3 μm but less than 0.9 μm. In yet another embodiment with a diaphragm with a typical full scale displacement of 0.3 μm, the cavity  62  may have a depth greater than 0.3 μm but less than 1.5 μm. In other embodiments, other cavity depths may be used depending on the design of the diaphragm. 
     A series of figures will now be described to illustrate the process of constructing one embodiment of a pressure sensor assembly  50 . The pressure sensor assembly  50  in this embodiment will be constructed from 3 separate silicon wafers bonded together. The pressure sensor assembly  50  will include a plurality of ports  60 , a cover  52  with a mechanical stop and a dielectric layer to electrically isolate the sensor  54  from the support  56 . In other embodiments, other combination or configuration may be constructed. 
       FIG. 4  illustrates a plurality of cross sections of a silicon wafer during the process of forming one embodiment of a sensor  54 . In step  1 , the process preferably begins with an n-type &lt;100&gt; Si wafer, 10Ω cm +/−20% with a 350 μm thickness. In step  2 , an oxidation layer is added along with SiN deposition. In a preferred embodiment a photoresist spin is performed. The cavity mask is created by an appropriately placed and sized SiN etch and oxide etch followed by a photoresist strip. In step  3 , a KOH etch is used to create the cavity  58  of the sensor  54  and accordingly, the diaphragm  59 . The KOH etch is preferably to a depth of about 90% of the wafer thickness. Corner rounding is preferably also performed in this step. In step  4 , the SiN and oxide layers are stripped. 
       FIG. 5  illustrates a plurality of cross section of a silicon wafer during the process of forming one embodiment of a support  56 . In step  1 , the process preferably begins with a &lt;100&gt; Si wafer with a 350 μm thickness. In step  2 , the mask for the plurality of ports  60  is created by applying an oxidation layer, performing a photoresist spin and then exposing the port mask with an oxide etch. In step  3 , the plurality of ports  60  are formed with deep reactive ion-etching (DRIE) followed by a photoresist strip. Finally in step  4 , the oxide layer is stripped off. 
       FIG. 6  illustrates a cross section of the combination of the sensor  54  from  FIG. 4  with the support  56  from  FIG. 5 . In a preferred embodiment, the two silicon wafers are silicon-to-silicon bonded. The assembly is then oxidized to prepare to form the electrical circuit that detects the diaphragm displacement on the top surface of the sensor  54 . A series of steps is performed to form the electrical circuit as is well known to those skilled in the art. Included in this series of steps is the deposition and patterning of a metal, for example, aluminum. The patterned metal may not only form the electrical circuit, it may also serve as part of the electrical contact that forms the maximum displacement detection switch on the top side of the diaphragm  59 . 
       FIG. 6A  illustrates the cross section of  FIG. 6  with a conductive layer  57  added to the top surface of the pressure sensor  54 . As may be seen in  FIG. 6A , a portion of the conductive layer may be formed over the diaphragm  59  to provide a contact for the maximum displacement switch. 
       FIG. 7  illustrates a plurality of cross sections of a silicon wafer during the process of forming one embodiment of a cover  52 . In step  1 , the process preferably begins with a &lt;100&gt; Si wafer, 0.1Ω cm or less with a 350 μm thickness. In step  2 , a photoresist spin is performed followed by a recess mask. The cavity  62  is formed by DRIE. Then a photoresist strip is performed. 
       FIG. 7A  illustrates a plurality of cross sections of a silicon wafer during the process of forming an embodiment of a cover configured to electronically detect a maximum displacement condition of the diaphragm. In preferred embodiments, the cover  52  may not only act as a mechanical stop, but may also be configured to act as an electrical detection switch. In such embodiments, the cavity  62  is covered with a conductive layer  63  which is electrically connected to the cover  52 . The conductive layer may be made from any metal. Thus, when the diaphragm  59  reaches the bottom of cavity  62 , the conductive layer  57  on the diaphragm  59  will come into contact with the conductive layer  63  in the cavity of the cover  52 . Preferably, the contact forms a closed circuit. This closed circuit may be detected with a detection circuit which may disable the source of the high pressure or provide relief by opening another valve. 
     As may be seen in  FIG. 7A , in step  3 , a conductive layer, such as aluminum, is deposited on the bottom side of the cover  52 . Then a photoresist spin is performed followed by a masking and etching step. In step  4 , a conductive layer, such as aluminum, is deposited on the top side of the cover  52 . The conductive layer on the top side of the cover  52  may be used as a contact to the conductive layer  63 . Then, a photoresist spin is performed followed by a masking and etching step. 
       FIG. 8  illustrates a cross section of the combination of the sensor  54 , support  56  and cover  52  to form a sensor assembly  50 . In a preferred embodiment, the cover  52  is silicon-to-silicon bonded to the top of the sensor  54 . The stack of three silicon wafers bonded together may then be Ti/Pt/Au deposition on the bottom side of the wafer stack.  FIG. 9  illustrates the sensor assembly of  FIG. 8  with the sides of the cover  52  diced off. 
       FIG. 10  illustrates a cross section of one embodiment of a sensor assembly designed to electronically detect the maximum displacement of the diaphragm. As may be seen in  FIG. 10 , a conductive layer  63  is deposited on the bottom of the cavity  62  in the cover  52 . In addition, a conductive layer  57  is deposited on the top of the diaphragm  59 . When the diaphragm flexes to its maximum displacement, the two conductive layers  57  and  63  come in contact and complete a circuit. The completion of the circuit is detected and the system knows the diaphragm has reached its maximum displacement. In preferred embodiments, detection of the maximum displacement may cause an action to reduce or remove the over pressure condition. 
     As may also be seen in  FIG. 10 , the dielectric layer covers the inside of the cavity  58  and the inside of the ports/channels  60 . Accordingly, the dielectric layer  30  electrically isolates the cavity  58  and the ports/channels  60  from the sensor  54 . As may be seen in this embodiment, the sensor  54  and the support  56  are coupled directly together. 
       FIG. 11  illustrates a cross section of one embodiment of a sensor assembly with the dielectric layer configured differently from the embodiment of  FIG. 10 . As may be seen in  FIG. 11 , rather than covering all of the cavity  58  and the ports  60 , the dielectric layer  30  covers the entire bottom surface of the sensor  54 . Although the dielectric layer is configured different in the embodiment of  FIG. 11 , it serves the same function of electrically isolating the sensor  54  from the support  56 . As may be seen in  FIG. 11 , in such an embodiment the sensor  54  is not coupled directly to the support  56  because the dielectric layer  30  is placed between them. 
     Although the embodiments have been described with reference to preferred configurations and specific examples, it will readily be appreciated by those skilled in the art that many modifications and adaptations to the pressure sensors and process of making the same are possible without departure from the spirit and scope of the embodiments as claimed hereinafter. Thus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the embodiments as claimed below.