Abstract:
A piezoresistive strain concentrator that converts mechanical movement into electrical output and a process for fabricating the concentrator are provided. The device includes a strain sensing structure composed of a piezoresistive strain sensitive element that spans a gap in a substrate. The strain sensing structure is supported on a strain concentrating structure also spanning the gap that has vertical walls extending to a cross-section at the base of the gap, both structures being etched from the substrate. The structure of the strain-concentrating support for the strain sensitive element is fabricated by deep reactive ion etch (DRIE). The strain sensing structure has an increased sensitivity, a low gage factor and an increased resistance to buckling and fracture compared to previous strain gage structures. Several of the strain sensing structures can be connected in a sequence in a bridge circuit.

Description:
BACKGROUND OF THE INVENTION 
   In pressure and acceleration sensors, it is desired to produce a relatively large signal power from a relatively small amount of energy absorbed from the medium. The goal is to minimize the mechanical energy necessary to produce a desired output signal. In pressure sensors, energy is absorbed from the medium as pressure deflects a diaphragm. Generally, a bar deeply notched at the center and its ends is placed across a diaphragm. Gages are placed on the plane surface opposite the notched bottoms. The strain of the bending bar is concentrated at the bottom of the notches. In acceleration sensors, energy is absorbed from the acceleration field as the seismic mass deflects relative to its reference frame. For example, a structure that is used features gages that are etched free from the substrate over an elastic hinge, a so-called “freed-gage.” With the hinge carrying the transverse load and the gages much further from the neutral axis of bending than the outer surfaces of the hinge, the gages become the most highly strained material. In both the acceleration and pressure sensor, efficiency permits high sensitivity via a small physical size. 
   A common approach taken by manufacturers of transducers has been to create a large field of strained surface and to place onto the more strained areas strain gages of a convenient size. Alternatively, structural means have been used to concentrate strain in piezoresistors. In piezoresistive sensors, signal is produced by changing the resistance of one or more strain-sensitive resistors excited by an electric current. Hence, in a simple plane diaphragm pressure sensor with embedded gages, much of the periphery and a broad area of the center are brought to the state of strain needed to provide signal in the gages. Although gages are placed in areas of highest strain, much of the strain energy is expended in the periphery and center areas which lack strain gages. 
   In a freed-gage structure only the piezoresistive material sees the full level of strain; the hinge and force-gathering structures are much less strained. Though the freed strain gage was an improvement over previous strain gages, it is still not the optimal structure to detect strain. Manufacturing tolerances impose a minimum cross section on the freed-gage; hence, for the required signal power, some minimum amount of material must be strained. The manufacturing process also imposes an upper limit on the resistivity in the freed gage, which limits the gage factor and thus, the sensitivity of the gage. In addition, heat dissipation limits the length of a device, such that the gages must be stitched back and forth across a gap over a hinge until there is enough total length to give the needed resistance. Thus, there is still a need for a stress concentrating structure that overcomes the short-comings of the freed-gage structure. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a strain sensitive element for use in a device that senses mechanical movement of at least two relatively movable parts and converts that movement into electrical output. The device comprises a substrate composed of silicon crystal material. The substrate includes a gap extending across a portion thereof and a cross-section that extends across the gap, defining the relative movable parts. At least one strain sensitive element is provided on the silicon substrate, having two end portions and a neck portion that extends across the gap. The neck portion of the strain sensitive element is supported on a strain concentrating structure that also extends across the gap. The strain concentrating structure has vertical walls extending to the cross-section at the base of the gap and is derived from the same material as the substrate. Electrode means, electrically connected to the end portions are provided to detect changes in electrical resistance between the end portions, when the neck portion is subjected to stress in the direction of a current through the strain sensitive element that results from the relative movement of the substrate parts. In one embodiment, the silicon substrate is oriented in the (110) plane and comprises an n-type impurity with the strain sensitive element aligned in the [111] direction and comprising a p-type impurity. In another embodiment, the silicon substrate is oriented in the (100) plane and comprised of a p-type impurity with the strain sensitive element aligned in the [001] direction and comprised of an n-type impurity. 
   In another aspect of the invention, at least two strain sensitive elements connected in a series are provided on the silicon substrate and are supported by corresponding strain concentrating structures of a device that senses mechanical movement of at least two relatively movable parts and converts that movement into electrical output. In a further embodiment, six strain sensitive elements connected in a series are provided on a silicon substrate and supported by corresponding strain concentrating structures. The strain sensitive elements are heavily doped and have a small cross-sectional width. 
   In another embodiment, the device of the present invention is made from a silicon substrate derived from n-type semi-conductive material oriented in the (110) plane. One surface of the substrate includes a triple-bossed diaphragm. The other surface of the substrate includes four gaps extending across a portion of the substrate defining a flexible cross-section and the relative moveable parts. Four strain sensitive element pairs are provided on the silicon substrate around the gaps, each strain sensitive element having two end portions interconnected by an intermediate neck portion that extends across the gap. Each neck portion is supported on a corresponding strain concentrating structure that extends across the gap that has vertical walls extending to the cross-section at the base of the gap and is derived from the same material as the substrate. The four strain sensitive element pairs derived from p-type semi-conductive material and oriented in the [111] direction are connected as a bridge circuit. In a preferred embodiment, the bridge circuit is a Wheatstone bridge circuit. Electrode means, electrically connected to the end portions are provided to detect changes in electrical resistance between the end portions, when the neck portions are subjected to stress in the direction of a current through the strain sensitive elements that results from the relative movement of the substrate parts. An insulated crossover on the substrate at the level of the strain sensitive element connects the strain sensitive elements in a bridge circuit sequence such that adjacent legs of the bridge have opposite senses of strain. 
   In another embodiment, the device includes a reference cavity to capture a reference pressure and is adapted for deposit on a catheter to measure fluid pressure. 
   The invention also relates to a method of fabricating a device for sensing mechanical input and converting mechanical movement of at least two relatively movable parts into electrical output. The method comprises: fabricating a sensor wafer from which the diaphragm and strain sensitive elements are fabricated and a support wafer for mechanical rigidity of the sensor wafer; aligning and bonding the sensor and support wafers; and deep reactive ion etching (DRIE) the diaphragm and strain sensitive elements areas. In one embodiment of the invention, the sensor wafer is made from a single silicon crystal of an n-type impurity, having a main face with a (110) orientation, with the [111] direction identified and two flat polished sides having thermally grown oxide on both surfaces. 
   A method of fabricating the sensor wafer comprises: heavily diffusing p-type boron into the conduction areas on the sensor surface; photopatterning the surface and opening links to strain sensitive elements; lightly diffusing p-type boron onto the strain sensitive elements through areas opened on the surface; and deep reactive ion etching the photopatterned areas to form the diaphragms and structures supporting the strain sensitive elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1A  is a view in perspective of a piezoresistive device in accordance with the present invention in which a single strain sensitive element and a strain-concentrating support structure are depicted on the substrate. 
       FIG. 1B  is a view in perspective of the embodiment in  FIG. 1A  having both electrical connections on the fixed side of the device. 
       FIG. 2  is a view in perspective of a second embodiment of the invention in which two strain sensitive elements in a series and their corresponding strain-concentrating support structures are depicted on the substrate. 
       FIG. 3A  is a view in perspective of a third embodiment of the invention in which four pair of strain sensitive elements and their corresponding strain-concentrating support structures extend respectively across four gaps that span the substrate and are connected in a bridge circuit. 
       FIG. 3B  is a side cross-sectional view of the embodiment in  FIG. 3A  along the  3 B line. 
       FIG. 3C  is a side cross-sectional view of the embodiment in  FIG. 3A  along the  3 C line. 
       FIGS. 4A–4F  are views in perspective illustrating a sequential process of fabricating a sensor wafer. 
       FIG. 5A  is a cross-sectional view of a fourth embodiment of the invention of a catheter featuring a pressure sensor to measure fluid pressure. 
       FIG. 5B  is the view of the back end of the embodiment in  FIG. 5A . 
       FIG. 6  is a side cross-sectional view of an embodiment of the invention in which six strain sensitive elements in a series and their corresponding strain-concentrating support structures are depicted on the substrate. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A description of the preferred embodiments of the invention follows. Referring to  FIG. 1A , there is shown a piezoresistive device  10 , illustrating the invention, with a silicon substrate  1 , having a gap  2  separating relatively movable substrate ends  12  and  13 . Defined within gap  2  is a flexible cross-section  3  that spans gap  2 . As can be seen in  FIG. 1A , a neck portion  4  of strain sensitive element  14  extends over gap  2 , supported on a strain concentrating structure  5 . Strain sensitive element  14  is isolated from the bulk of the substrate by a p-n junction. The strain sensitive element  14  can be lightly doped e.g., with boron to a suitable depth such as about 2 microns. Neck portion  4  of strain sensitive element  14 , is connected to electrical pads  6  and  9  at each end thereof. Alternatively, in  FIG. 1B  both electrical connections are on a fixed end of the device, substrate end  12 . Ohmic contacts  7  and  11  are near the end of substrate ends  12  and  13 , respectively, having between them the conductivity of the substrate. On a movable end of the substrate, substrate end  13 , piezoresistor  14  is connected to adjacent ohmic contact  8 . Functional connections to piezoresistor  14  are then ohmic contacts  11  and  8  on substrate end  13 . 
   As will be appreciated, flexible cross-section  3  defines an elastic hinge  15  that is parallel to substrate  1  and determines the direction of flexibility of movable substrate ends  12  and  13 . Force applied planar to substrate  1  will cause substrate ends  12  and  13  to tilt relative to each other around hinge  15  of cross-section  3 , creating strain on strain concentrating structure  5  that is transmitted to neck portion  4  of strain sensitive element  14  and sensed electrically via electrical pads  6  and  9 . In particular, changes are detected in electrical resistance when the neck portion is subjected to stress in the direction of current through the strain sensitive element that results in the relative movement of the parts. 
   Referring to  FIG. 2 , piezoresistive device  16  is shown with two strain sensitive elements  23  arranged in a manner similar to that described above. Substrate  17  having a gap  20 , spanned by strain sensitive elements  23  are supported by strain concentrating structures  24  that are perpendicular to flexible cross-section  21  which defines elastic hinge  25 . Both strain sensitive elements  23  can be lightly doped to a depth of about 2 microns. The dual strain sensitive elements  23  have individual electrical pads  26  on substrate end  18  and electrical pad  29  on substrate end  19 . Electrical pads  26  have electrical contact terminals  27  positioned thereon, while electrical pad  29  contains electrical contact terminal  28 . Electrical contact terminals  28  and  27  may be composed of metal. 
     FIG. 6  shows a cross-section of piezoresistive device  73 , which is depicted with six strain sensitive elements  75 . Each strain sensitive element is supported on a corresponding strain concentrating structure  74 . In this embodiment, strain sensitive elements  75  are preferably heavily doped with boron to a depth of approximately 0.3 microns, allowing the strain sensitive element to have a very small cross-sectional width C2 of about 4 microns, for instance. 
   In piezoresistive devices  10 ,  16  and  73  the silicon substrate can be oriented in the (110) plane and comprised of an n-type impurity with the strain sensitive element aligned in the [111] direction and comprised of a p-type impurity. Alternatively, the silicon substrate of the devices can be oriented in the (100) plane and comprised of a p-type impurity with the strain sensitive element aligned in the [001] direction and comprised of an n-type impurity. 
   Referring now to  FIGS. 3A–3C , a piezoresistive stress concentrator  30  is shown, illustrated by a device made from silicon substrate  31 . Functionally, stress concentrator  30  is composed of four stress concentrators of the type shown in  FIG. 2  that are incorporated into a pressure sensor. As can be seen in  FIG. 3B , sculpted on one side of substrate  31  is triple-bossed diaphragm  50 , composed of central boss  33  and two outer bosses  34  and  35 . Four gaps extend across a portion of the substrate  31 , such that outer gap  36  is outside of outer boss  35 , inner gap  37  is between outer boss  35  and central boss  33 , inner gap  38  is between central boss  33  and outer boss  34  and outer gap  39  is outside of outer boss  34 . Torsion bar conduction paths  48  run along outer bosses  34  and  35 , to terminals  47  on rim  32  that extend across diaphragm  50 . In response to pressure applied to one surface, diaphragm  50  will deflect, with central boss  33  moving plane parallel to the rim  32  and outer bosses  34  and  35  tilting relative to rim  32 . Turning to  FIG. 3A , defined within each gap are flexible cross-sections  40 . Four strain sensitive element pairs  41 ,  42 ,  43  and  44  are provided on the surface of substrate  31  around gaps  36 ,  37 ,  38  and  39  respectively with each strain sensitive element within the pair supported on stress concentrating structures  45 . Each strain sensitive element can be doped with boron, preferably to a level of approximately 3×10 8  per cubic centimeter. The strain sensitive element pairs are connected in an electronic bridge circuit sequence, such as a Wheatstone bridge, the inner strain sensitive elements  42  and  43  wired to terminals  47  at the corners of substrate  31  via torsion bar conduction paths  48  such that adjacent legs of the bridge have opposite senses of strain and change in resistance. Insulated crossover  46  accommodates the physical geometry of the device as it relates to formation of a bridge circuit. As seen in  FIG. 3A , trenches  49  are cut on either side of the conduction path to a depth leaving the thickness T 1  shown in  FIG. 3C . The stress concentrator may be cemented to a support and wired to a circuit, or the support and connecting functions may be provided by a single complex structure applied to the terminal surface. 
   With pressure applied to the planar surface, deflection of the bosses of the diaphragm occurs as described above, causing outer strain sensitive elements  41  and  44  to become compressed and the inner strain sensitive elements  42  and  43  to become stressed, mechanical movement that the piezoresistive strain sensitive elements detect as a change in resistance. In a balanced bridge circuit, the change in resistance due to deflection unbalances the bridge to provide an electrical output signal. 
   Referring now to  FIGS. 4A–4F , a sequence of steps is shown for fabricating the sensor wafer portion of piezoresistive stress concentrator  30 . As illustrated in  FIG. 4A , a single crystal n-type silicon substrate  51  has a main face with a (110) orientation, two flat polished sides  54  in the [111] direction and has formed a thermal oxidized layer  52  on the top surface and an oxidized layer  53  on the bottom surface. In an embodiment, the total thickness variation of the sensor wafer is about 2 microns. As can be seen in  FIG. 4B , a preliminary heavy diffusion of p-type boron is made into areas  53  which will serve as conductors on the sensor surface.  FIG. 4C  shows the photopatterning of the surface and opening of the oxide for a light diffusion of boron onto strain sensitive elements  56 . In an embodiment, strain sensitive elements  56  can be lightly diffused with at least 3×10  18  boron, to a depth of about 1.1 microns and 265 ohms per square. In  FIG. 4D , the surface of substrate  51  is masked against deep reactive ion etching (DRIE) with sputtered aluminum  60 , and the DRIE pattern opened in the aluminum to form the diaphragm, stress concentrating structures  57  to support strain sensitive elements  56 , flexible cross-sections  58  and trenches  59  on either side of conduction path  55 . 
   Substrate  51  is etched with DRIE to a final depth that is about 28 percent of the original wafer thickness. As shown in  FIG. 4E , sputtered aluminum  60  from  FIG. 4D  is removed and a thin oxide  61  is grown on the surface. Conduction path  62  is photopatterned and contact holes opened through the oxide. In  FIG. 4F , aluminum  63  is deposited on the surface of substrate  51  to a depth of about 0.7 microns and the surface photopatterned to define electrical traces  65  and thermocompression bonding sites  64 . 
   A support wafer necessary for mechanical rigidity and electrical connection is also fabricated. The support wafer is composed of a single silicon crystal of an n-type impurity. To form the complete piezoresistive device, the sensor wafer is aligned with the support wafer and the two wafers thermocompression bonded. 
   Referring to  FIGS. 5A and 5B , a catheter  66  employing a piezoresistive pressure sensor  72  illustrates an embodiment of the invention. A reference pressure tube  71  is epoxy-sealed to pressure sensor  72 . Silicone elastomer  69  also seals pressure sensor  72  to reference tube  71 . Silver-plated copper wires  70  connect to terminals of pressure sensor  72 . A face  68  of pressure sensor  72  may be coated with a thin film of tantalum. A support wafer can be bonded to pressure sensor  72  which provides both electrical vias for the terminals and a plumbing via for reference tube  71 . The catheter can be used to measure fluid pressure. 
   As will be appreciated from the discussion of the above, the invention provides a device and a process for fabricating a device that senses mechanical movement and converts that movement into electrical output through a strain concentrating structure that supports a strain sensitive element. The invention improves over prior gage structures in several ways. For instance, having the strain sensitive element supported on a strain-concentrating support structure eliminates the process constraint on resistivity, allowing the resistivity to only be constrained by acceptable thermal behavior of the strain sensitive element. The supported strain sensitive element can be doped with boron at one-tenth or less of the levels of that of the freed-gage structure, resulting in higher sensitivity to strain. Unlike in the freed-gage structure, the material on the supported strain sensitive element does not need to resist etching, and thus can be chosen for more desired properties like a high gage factor and matched temperature coefficients of resistance and gage factor. Hence, the supported strain sensitive element is heat-sunk by its support; the resistive heat generated in the element is carried away not only along the length of the element, but also downward into the support. Finally, the strain-concentrating structure of the strain sensitive element is much more resistant to buckling under compressive load than the freed-gage structure, making it much less brittle. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.