Abstract:
A strain-sensing device comprises a metal, glass, ceramic, or plastic cell that has formed within it a diaphragm characterized by a thin layer of material bounded by a thick layer of material. A silicon strain gauge, either junction isolated or dielectric isolated, is attached directly to the diaphragm. The strain gauge has at least one sensing element that is aligned such that applied pressure to the diaphragm induces a strain in the sensing element. The silicon strain gauge has a triangular shape that is optimizes the performance and reliability of the sensor with the added benefit of making it more affordable as well.

Description:
FIELD OF THE INVENTION 
     The present invention relates to media compatible pressure sensing devices and methods for their fabrication. More specifically, this invention relates the design of a media compatible pressure-sensing device capable of high sensitivity to pressure with improved reliability. 
     BACKGROUND OF THE INVENTION 
     Measuring pressure in ultra clean environments or environments containing harsh media has always been a challenge. There has been a continuous effort to produce affordable, reliable, media compatible pressure sensors. 
     Originally metal strain gauges were prevalent. These strain gauges comprised four resistors arranged in a Wheatstone bridge configuration such that two opposite resistors would increase in resistance with applied pressure and the other two would decrease with applied pressure. The resistance change was due to the dimensional changes in the metal resistors. 
     Other circuit configurations have also been used, but the four-resistor Wheatstone bridge configuration is still prevalent. 
     The more sensitive silicon micro electromechanical system (“MEMS”) based devices replaced many of the metal strain gauges. In these devices, resistors are formed within a silicon diaphragm by ion implantation or diffusion. These resistors exhibit a piezoresistive effect such that two opposite resistors increase in resistance and two decrease in such a way that each output changes in opposite ways. Metal (such as aluminum) is applied to the diaphragm for interconnects and pads for wire bonding. MEMS devices often require protection to make them media compatible. 
     Metal, glass, ceramic, plastic, or other chemically compatible diaphragms are used to protect silicon pressure sensors from harsh media. When using such diaphragms, a fluid (such as oil) is used to transfer the pressure from the chemically compatible diaphragm to the silicon diaphragm. 
     Some applications cannot tolerate the chance of an oil leak if there were to be some sort of diaphragm rupture. In this case, silicon strain gauges are used. Silicon strain gauges are often relatively long and thin. They are fragile, difficult to match and difficult to handle. Their use requires them to be attached directly to a metal, glass, ceramic, or plastic diaphragm. 
     A silicon chip can also be attached directly to a metal, glass, ceramic, or plastic diaphragm. In this case, all four resistors can be placed on one chip. These chips are less fragile, easier to position, and intrinsically matched. However, all four resistors must necessarily be very close together. Optimizing performance by judiciously locating resistors on the diaphragm cannot be done without adding additional Wheatstone bridge circuits that can make temperature compensations more difficult. 
     All of the above technologies are commercially available today. 
     SUMMARY OF THE INVENTION 
     A pressure transducer in accordance with the present invention comprises a novel pressure sensing structure. This structure includes a body of a first material within which a diaphragm is constructed. The diaphragm, in one embodiment contains a relatively thick boss centrally located. This diaphragm provides media isolation from the sensor. One or more pressure sensing elements is attached to the diaphragm with a second material. Each pressuresensing element comprises a triangular chip with one or more strain-sensing elements on it. In one embodiment, the triangular chip is a semiconductor material such as silicon. Typically, each strain-sensing element is a resistor that is electrically isolated by a dielectric layer in a silicon-on-insulator structure. Dielectric isolation enables performance at higher temperatures. Alternatively, junction isolated resistors can be used. The pressure sensing chips are located close to areas of maximum absolute stress. The triangular shape improves the reliability. 
     The resistors are typically piezoresistive. In lieu of resistive strain sensing elements, in other embodiments, capacitive strain sensing elements are used. In such an embodiment, stress changes the capacitance exhibited by the capacitive strain sensing elements. 
     In another embodiment, piezoelectric strain sensing elements are used. Stress changes the voltage across the piezoelectric strain sensing elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section side view of a thin diaphragm with a silicon strain gauge bonded to the surface near an edge of the diaphragm. 
     FIG. 2 is a cross section side view similar to FIG. 1 except that the diaphragm contains a thick boss region in the center of the diaphragm. The silicon strain gauge is mounted near the edge of the boss on the thin diaphragm. 
     FIG. 3 is a cross section view of the silicon sensor chip. In this embodiment the chip has a silicon-on-insulator structure. 
     FIG. 4 is a cross section view of the silicon sensor chip with a junction isolated resistor. 
     FIGS. 5A,  5 B, and  5 C depict various embodiments of single resistor formed within a triangular chip. 
     FIGS. 6A,  6 B, and  6 C depict various embodiments of triangular chips comprising two resistors each. The resistors are typically used to form half of a Wheatstone Bridge. 
     FIG. 7 is a schematic diagram of a full Wheatstone bridge circuit composed of two half bridges. Six wire bonds are required. 
     FIGS. 8A,  8 B, and  8 C depict various embodiments of two-resistor placement in a parallel configuration on a triangular chip. These two resistors do not typically form half bridges. 
     FIG. 9 is a schematic diagram of a full Wheatstone bridge circuit composed of 4 resistors of which the two opposite resistors reside on the same chip. Eight wire bonds are required. 
     FIGS. 10A and 10B are top views of the structure shown in FIG.  1 . FIG. 10A displays the proper position and orientation of any of the chip configurations shown in FIGS. 5A,  6 A, and  8 A. FIG. 10B displays the proper position and orientation of any of the chip configurations shown in FIGS. 5B,  5 C,  6 B,  6 C,  8 B, and  8 C. 
     FIGS. 11A and 11B are top views of the structure with the boss as shown in FIG.  2 . FIG. 11A displays the proper position and orientation of any of the chip configurations shown in FIGS. 5A,  6 A, and  8 A. FIG. 11B displays the proper position and orientation of any of the chip configurations shown in FIGS. 5B,  5 C,  6 B,  6 C,  8 B, and  8 C. 
     FIG. 12A,  12 B,  12 C,  12 D are top views of structures with two triangular chips positioned along the diaphragm in order to compensate for body mounting stress. FIGS. 12A and 12B apply to diaphragms with a central boss. 
     FIG. 13 is a top view of a chip with an alternate strain-sensing element comprising a meandering resistor. 
     FIG. 14 is an alternative embodiment of a pressure sensor comprising a rectangular chip. 
    
    
     DETAILED DESCRIPTION 
     In reference to the drawings, like numerals represent like materials through the various figures. 
     In FIG. 1, a body  1  is composed of a material resistant to or impervious to the media for which it is intended. This material can be a metal such as any grade of steel, galvanized steels, and any stainless steel alloy as well as molybdenum. This material also can also be a ceramic, glass, plastic and other polymer materials such as Teflon (PTFE), Ultem and nylon. Within this body a relatively thin diaphragm  4  is formed such that its dimensions are sized according to known scientific principles so that the proper amount of strain is generated at the locations of the pressure sensing elements to achieve the desired performance for the pressure range specified. Diaphragm  4  can be formed by any method including stamping, etching, welding, and machining to achieve the desired dimensions. The non-diaphragm part of body  1  must be thick enough to be considered rigid within the desired pressure range. 
     In another embodiment, in lieu of diaphragm  4 , a diaphragm  6  is provided that includes a central boss structure  5  composed of the same material as diaphragm  6  (FIG.  2 ). The thickness of boss structure  5  is such that it is rigid for all intents and purposes compared to a diaphragm  6 . Diaphragm  6  is annular. 
     To this diaphragm  4  or  6  at least one pressure-sensitive element  3  is attached. The material used to attach element  3  to the diaphragm can be any appropriate material, e.g. a eutectic material, solder, glass, epoxy or other polymer material. If solder is used, the backside of the pressure-sensing element must be wetable by the solder or be covered with a metal layer that is wetable by the solder. Bonding can be performed using any other appropriate method as well. 
     Description of the Pressure-sensing Element 
     Pressure-sensing element  3  may be of any material exhibiting the piezoresistive effect. This may include silicon, silicon composites, gallium arsenide, and the like. Most commonly, monocrystaline silicon would be used because of its relative affordability. 
     Referring to FIG. 3, a first embodiment of element  3  consists of a silicon-on-insulator structure. Element  3  comprises a substrate  7  that can be monocrystaline silicon. Substrate  7  is typically less than 100 μm thick. A buried layer of dielectric material  8  is formed in substrate  7 . Buried layer  8  is preferably silicon dioxide, but any dielectric compatible with the adjoining materials would do. A top layer  10  of preferably monocrystaline silicon is formed on buried layer  8  and doped with boron to form p-type silicon material. Top layer  10  serves as a resistor, and has piezoresistive properties. The doping of top layer  10  can be done with ion implantation, diffusion, epitaxial growth or any combination thereof. A cap layer  11  of dielectric (preferably silicon dioxide) covers the resistor areas. The field (i.e. portions of the structure away from the resistor) is etched using dry etching techniques or wet etching techniques to remove the silicon from around the resistor areas down to dielectric layer  8 . This leaves the patterned resistor areas electrically isolated from any other structures on the pressure-sensing element. (Although only one resistor is shown in FIG. 3, as explained in more detail below, more than one resistor can be formed on substrate  7 .) 
     Buried layer  8  can be formed in a number of ways, e.g. using a BESOI or SIMOX process. See, for example, Auberton-Herve et al., “SOI Materials for ULSI Applications”, published in Semiconductor International in October 1995, incorporated herein by reference. See also “New Bonding Technology for SOI: Unibond” published by SOITEC USA, Inc. of Peabody, Mass., also incorporated herein by reference. 
     Openings are made in cap layer  11  for contacts to the resistor(s). Metal  9  is then deposited, patterned and etched, leaving pads for wire bonding and connections to the individual resistor(s). The metal can be any metalization scheme suitable for wire bonding. This metal may be aluminum, TiW/Au, Cr/Au, Cr/Ni/Au or any other wire bondable structure. The resistors are aligned in the &lt;110&gt; crystal direction in order to achieve a maximum change in resistance with applied strain. Because the structure of FIG. 3 uses a dielectric layer (layer  8 ) to isolate the resistor (layer  10 ), it is advantageous if any part of the required operating temperature range exceeds 125° C. In addition, dielectric resistor isolation is thought to contribute to long term sensor stability. This structure is superior to a similar structure based on polysilicon resistors since monocrystaline silicon resistors have a significantly greater sensitivity to strain. However, the present invention can also be practiced using polysilicon resistors. 
     Another embodiment of sensing element  3  is shown in FIG.  4  and comprises a monocrystaline n-doped silicon substrate  7  within which a layer of boron doped, p-type silicon  10 ′ is formed. The doping can be done with ion implantation and/or diffusion. A dielectric layer  8 ′, preferably silicon dioxide, covers the resistor areas and the field. This leaves the patterned resistor area junction isolated. Openings are made in dielectric layer  8 ′ for electrical contacts to the resistor(s). Metal  9  is deposited, patterned and etched, leaving pads for wire bonding and connections to the individual resistor(s). The metal can be any metalization scheme suitable for wire bonding. This metal may be aluminum, TiW/Au, Cr/Au, Cr/Ni/Au or any other wire bondable structure. The resistors are aligned in the &lt;110&gt; direction in order to achieve maximum change in resistance with applied strain. This embodiment is sufficient for operating temperatures up to 125° C. and is significantly more affordable. (Above 125° C., an undesirable amount of leakage current may flow between p-type region  10 ′ and substrate  7 .) 
     Description of the Triangular Shaped Pressure-sensing Element 
     For the embodiments of FIGS. 3 and 4, a triangular shape of the sensing element offers an improvement over the existing state of the art. The magnitude of the strain is highest at edge  14  of diaphragm  4  (for the embodiment of FIG. 1) and at both the inside and outside edges  14 , 18  of diaphragm  6  with the central boss  5  (for the embodiment of FIG.  2 ). This is also the location for greatest change in the strain. It is desirable to position a sensing element at the point of greatest strain in order to maximize the sensitivity to pressure If sensing element  3 , being of finite dimensions, straddles this location, element  3  will experience a wide variation of strain across it. All of this can lead to mechanical fatigue and premature failure. Placing element  3  completely on the diaphragm side of the highest stress point creates a more uniform strain across it. Using a triangular shaped sensing element with one point touching the highest stress point minimizes strain effects that can lead to failure, yet maximizes the average strain across the strain-sensing e lement. In contrast, a rectangular sensing element can be placed in the same position with an edge parallel to the diaphragm edge but the number of pressure cycles it could withstand would be significantly less. Temperature cycling exposes the sensing element to the same type of failure mechanism. 
     FIGS. 5A,  5 B and  5 C illustrate in plan view three embodiments of a triangular chip comprising a single piezoresistive strain sensing element  13 . The strain sensing element is shown as a simple rectangular region. The required metal pad and contacts are not shown in FIGS. 5 for sake of clarity. Preferably, the longitudinal axes of strain sensing elements  13  are parallel to the &lt;110&gt; crystal direction (or a member of the &lt;110&gt; family of axes) for maximum pressure sensitivity in the finished device. The advantage of a single strain-sensing element per triangular chip is that each chip can be placed at different places around the diaphragm in order to compensate for body mounting stresses. 
     FIGS. 10A and 10B show two embodiments of triangular sensing element  16  with one strain-sensing element orientation and sensing element  17  with another strain-sensing element orientation. FIGS. 10A and 10B are top views where the point of greatest strain is represented by a dashed line  14 . In both cases the sensing element is aligned such that the &lt;110&gt; direction (indicated by line  15 ) is perpendicular to a tangent to line of greatest strain  14 . In the special case of a circular diaphragm the &lt;110&gt; direction is parallel to the radius of the diaphragm. 
     In addition, the sensing element touches the line of greatest stress at a point with the corner of the triangle. 
     FIGS. 11A and 11B are other embodiments where the diaphragm contains a central boss. The region of greatest stress is represented by an inside dashed line  18  around the boss and an outer dashed line  14 . The chip locations shown in FIGS. 10A and 10B function here as well, but the preferred embodiment is to place the chips touching inner line  18 . In both cases the chip  19  (and  20 ) is aligned such that the &lt;110&gt; direction  15  is perpendicular to a tangent to the line of greatest strain  18 . 
     More than one chip can be placed on the diaphragm. FIGS. 12A, B, C, and D show other embodiments with two chips placed on the diaphragm in order to compensate for body mounting stress. (Body mounting stress is the stress caused by mounting the sensor on a structure where pressure is to be measured.) The optimum choices depend upon the stresses transferred to the diaphragm and are therefore dependent upon the application and configuration of the body. FIGS. 12 show four ways of orienting the sensors. The scope of this invention is not limited to these configurations. 
     Description of Strain-sensing Element Orientation on a Triangular Chip 
     As mentioned above, FIGS. 5A to  5 C illustrate in plan view three embodiments of a single rectangular sensing element  13  formed in a triangular chip. In other embodiments, non-rectangular sensing elements can be used. An example of such an embodiment is shown in FIG. 13, which illustrates a meandering strain-sensing region. Again, the required metal pad and contacts are not specifically illustrated in FIG. 13 for sake of clarity. 
     FIGS. 6A,  6 B, and  6 C represent three options for aligning double strain-sensing elements  13  on the triangular chip  12 . In all cases the strain-sensing elements  13  are parallel to the &lt;110&gt; family of orthogonal axes for maximum pressure sensitivity in the finished device. In this embodiment the two strain-sensing elements  13  are positioned normal to each other. In one embodiment, elements  13  are electrically connected such that each chip forms a half bridge circuit structure. Two chips can then be used to create a full Wheatstone bridge with each chip located at a different place around the diaphragm in order to compensate for body mounting stresses as shown in FIG.  7 . 
     FIG. 7 schematically illustrates how the sensing elements in a first chip  12   a  (shown as resistors  31 ) and the sensing elements of another chip  12   b  (shown as resistors  32 ) can be coupled together to form a Wheatstone bridge. Chip  1   2   a  in FIG. 7 also includes metalization  35  and electrical contacts  33   a ,  33   b  and  33   c.  Chip  12   b  includes metalization  37  and electrical contacts  34   a ,  34   b  and  34   c.  During use, leads  33   a  and  34   a  are typically connected to a first voltage source terminal, leads  33   c  and  34   c  are coupled to a second voltage source terminal, and the voltage across leads  33   b ,  34   b  is sensed to determine the stress applied to the diaphragm. 
     In one embodiment, 5V DC is applied across leads  33   a ,  33   c.  In another embodiment, 12V DC is applied. In other embodiments, non-DC voltages are applied. The circuitry coupled to the Wheatstone bridge can be as described in “Solid State Pressure Sensors Handbook”, Vol. 16, published by Sensym, Inc. of Milpitas, Calif. in 1998, incorporated herein by reference. See, for example, pp. 8-70 to 8-73 and 8-92 to 8-93. 
     Four embodiments of how the dual-sensor chips of FIG. 6 can be applied to a diaphragm are shown in FIG.  12 . Other possible orientations for other triangular options will be readily apparent to one of ordinary skill in light of this specification. 
     FIGS. 8A,  8 B, and  8 C represent another three options for aligning double strainsensing elements  13  on the triangular chip  12 . In all cases the strain-sensing elements  12  are preferably parallel to a member of the &lt;110&gt; family of axes for maximum pressure sensitivity in the finished device. In the embodiments of FIG. 8, the two strain-sensing elements  13  are positioned parallel to each other. The resistors in this embodiment are used as the opposite resistors in a full bridge. Thus, in FIG. 8, each chip  12  does not constitute a half bridge circuit structure. However, two chips can still be used to create a full bridge with each chip located at a different place around the diaphragm in order to compensate for body mounting stresses as shown in FIG.  9 . Strain-sensing elements  31  are located on a first chip. Strain-sensing elements  32  are located on a second chip. Leads  40  and  41  are typically coupled together and to a first power source. Leads  42  and  43  are typically connected together and to a second power source. Leads  44  and  45  are typically connected together and form one output terminal of the Wheatstone bridge, whereas leads  46  and  47  are typically connected together and form the other output of the Wheatsone bridge. The interconnect wiring from chip to chip is more complex, requiring more wire bonds than the embodiment of FIG. 6, but one method is not preferred over the other for reasons other than affordability. 
     While the invention has been described with respect to specific embodiments, those skilled in the art will appreciate that changes can be made without departing from the spirit and scope of the invention. For example, in lieu of using triangular chips, other shapes can be used. These other shapes typically include a corner pointed toward one of the areas or lines of greatest stress in the diaphragm. For example FIG. 14 illustrates a quadrilateral shaped (e.g. rectangular) chip  50  in which one corner  50   a  of chip  50  is pointed toward or touching line  14  of greatest stress. The rectangle edges are not parallel to the edge of the diaphragm. In other words, for the case of a circular diaphragm, a line tangent to the line  14  of greatest stress at a point touching or closest to corner  50   a  of chip  50  is not parallel to the sides of chip  50  closest line  14 . 
     In lieu of resistors using boron-doped p-type silicon, other dopants can be used. Also, n-type silicon can be used, but the optimum sensitivity to stress in n-type silicon is along other crystal directions. See, for example, S. M. Sze, “Semiconductor Sensors” published by John Wiley and Sons, Inc. in 1994, p. 160-181, incorporated herein by reference. 
     Chips containing sensing elements can be made from materials other than silicon, e.g. as described above. Different techniques can be used to attach the chips to the diaphragm. Different materials can be used to form the sensor. Accordingly, all such changes come within the invention.