Patent Publication Number: US-6338275-B1

Title: Schottky-barrier semiconductor device

Description:
This application is a divisional and claims the benefit of application Ser. No. 08/760,782, filed Dec. 5, 1996, now U.S. Pat. No. 6,034,404. 
    
    
     U.S. Government may have certain rights in this invention pursuant to ONR grant No. N00014-95-C-0355. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to Schottky-barrier semiconductor devices, and uses therefor. More particularly, the invention is directed to Schottky-barrier semiconductor devices for use in measuring strain, temperature and the like. 
     Methods are known in the art for measuring mechanical strain, which are based on measurements of resistance, the piezoelectric effect, or the acousto-optical effect. Conventional strain gauges vary in speed, sensitivity, bandwidth, system complexity and cost. 
     Conventional resistive strain gauges have a conductive element, such as a metal ribbon, epoxied to a flexible member. The gauge is included as a component in a balanced Wheatstone bridge resistive network. The strain produced by vibrations transfers through the flexible member to the metal ribbon and alters its resistivity. This fractional change in the resistivity unbalances the bridge producing a current proportional to the strain. The resistive strain gauge is a narrow band device, responding only in a limited bandwidth in the vibrational spectrum. Dynamic calibration of the bridge is a tedious process and integration in sliding contacts—in which it is desirable to measure strain—requires careful packaging. 
     Some of the disadvantages of the resistive strain gauge are overcome by using a voltage or capacitive method to measure strain. Certain crystals such as quartz and PZT are piezoelectric in nature. Under stress or strain they become polarized, and the polarization appears as a voltage across the crystal, or alternatively as a capacitive charge. The piezoelectric transducer is packaged in a metal fixture that is mated to the vibrating assembly. The voltage produced by the transducer is amplified electronically. However, sensitive piezoelectric crystals are expensive to fabricate, package, and calibrate. Furthermore, each crystal requires its own calibration. 
     Strain coupled to an acousto-optic material such as quartz, LiNbO 3 , or water produces changes in the refractive index of the material. Laser light incident on the material can undergo Bragg diffraction or deflection. The angle of deflection is proportional to the vibrating stimulus, and the deflection is measured on a position-sensitive photodetector array. This is a sensitive method of detecting high frequency vibrations. However, high quality crystals for acoustic sensing are expensive, and the experimental apparatus required for a simple measurement is fairly complex, and unsuitable for measuring strain at multiple locations in a commercial engine, hydraulic system, transmission or the like. 
     Methods are known in the art for measuring temperature. A common technique used for electrical temperature sensing is to measure the motion of charges in a sensor element. Temperature sensors are commonly constructed as resistors, bi-metal junctions, or semiconductor devices, and require a variety of measurement and calibration techniques. 
     In resistive temperature sensors, the mean-free path of the electrons between scatter events is related to the temperature. As the temperature increases, the vibratory motion of ions in the resistive material increases, and electrons traveling through the resistive material have a higher scattering probability. The electron mean-free path is reduced and the sensor becomes more resistive. When the temperature declines, the inverse process is true. Resistive sensors often have carbon-glass or metal-film resistors. For example, if aluminum is used as the metal film sensor material, its resistivity changes from 3.55 μΩ-cm at 300° K. to 2.45 μΩ-cm at 273° K. and 0.3 μΩ-cm at 77° K.. 
     Bi-metal junctions are formed when two dissimilar metals make physical contact. A potential barrier is formed at the junctions. Electrons are thermally excited over the barrier, and the current across the barrier is temperature sensitive. Bi-metal thermocouples are most useful at high temperatures. 
     Semiconductor temperature sensors are usually constructed as p-n junction diodes, bipolar junction or field-effect transistors. The motion of electrons through a crystal is temperature dependent. The crystalline lattice is in a state of vibratory motion in well-defined phonon modes. The phonons scatter electrons randomly. If the temperature of the material is comparable to its Debye temperature, the mode density, governed by Bose-Einstein statistics, is fairly high. The probability of an electron-phonon scatter event is proportionally high, and the electron&#39;s mean-free path is relatively short. As the sensor temperature decreases the motion of the crystalline lattice “freezes.” Fewer phonon modes are excited in the lattice, and the electron motion is relatively unhindered by any interaction with the lattice. Electrons, may however, undergo Coulomb scattering by a sparse population of impurity ions until the carrier motion itself freezes. This occurs at very low temperatures approaching a few degrees Kelvin. 
     An alternative technique for temperature sensing in semiconductors is to simply monitor the thermionic emission across the potential barrier in a p-n or a metal-semiconductor junction. The emission probability is proportional to the classical Maxwell-Boltzmann distribution. Sensors using this alternative technique are particularly effective at high temperatures where thermionic emission increases exponentially. 
     Any of the above-described temperature sensors may be used in a conventional temperature-sensing system as a part of a balanced resistive network powered by a constant-current source. The temperature is measured by monitoring the voltage across a reference resistor held at a fixed temperature. The temperature dependent current through the sensor is deduced from this voltage. 
     It is known in the art of semiconductor technology that when a semiconductor is brought into contact with a metal, a barrier layer is formed in the semiconductor from which charge carriers are severely depleted. This barrier is known as a Schottky barrier. a Schottky diode is formed in the region where a metal contacts a lightly doped semiconductor. Schottky diodes have a faster response time and lower operating voltage than doped silicon junction diodes. Metal in contact with a highly doped semiconductor (5×10 17  atoms per cubic centimeter), however, forms a regular ohmic contact. The Schottky barrier forms because the work function of the metal is greater than the work function of the doped semiconductor, and the metal depletes the semiconductor in the region around the contact of charge carriers, typically electrons, leaving in the semiconductor a depletion layer of positively charged donor ions that is practically stripped of electrons. 
     Semiconductor devices using the Schottky barrier are known in the art. Scares, S. F., “Photoconductive Gain in a Schottky Barrier Photodiode”, Jpn. J. Appl. Phys., Vol. 31, pp. 210-216 (1992), discloses a pair of metal contacts on a lightly-doped n-type semiconductor for use in photodetection. At each interface between metal and semiconductor, a Schottky barrier forms. In “Heterodyne Ultraviolet Photodetection” (Dissertation of S. F. Scares, University of New Mexico, Albuquerque, submitted December, 1989), metal contacts are deposited about 3 microns apart on a lightly-doped silicon substrate, and each has an area in contact with the substrate of about 50 square microns to about 250 square microns. The semiconductor dopant density is selected so that the depletion regions of the two contacts almost extend to each other. It is further known that application of a bias across the pair of Schottky barriers enhances the detection of photo events. 
     There is a need for a simple, inexpensive, fast, robust strain gauge for inclusion in gears and bearings in aircraft engines and transmissions, for example, and engines and transmissions generally. Resistive gauges are difficult to package and time consuming to calibrate. Piezoelectric crystals are expensive and require complex electronic circuits to detect minute stress-induced voltages. Acousto-optic techniques are based on expensive laser techniques. 
     There is a further need for a temperature gauge for these same engine environments, especially a gauge that can cooperate with a strain gauge, where strain measurements may be thrown off by changes in temperature due to friction or combustion heat from operation. 
     SUMMARY OF THE INVENTION 
     The invention is embodied in a novel and useful device for measuring physical variables such as mechanical strain and ambient temperature by directly or indirectly measuring changes which these physical variables impart to Schottky-type electrical barriers in the device. 
     The device comprises a flexible, lightly-doped semiconductor substrate in the form of a leaf, with at least one metal contact formed on the upper surface thereof, forming a Schottky barrier at the interface of the metal contact and the substrate. More specifically, the device has a pair of metal contacts formed on the upper surface of the substrate for connection to current or voltage measuring equipment, each metal contact forming a Schottky-barrier diode. The Schottky-barrier electrical potential at a metal contact is altered by strain in the substrate leaf, and this electrical potential change is registered in this invention as an indication of that strain. In this manner, the device is a strain gauge. 
     The electrical potential change can be registered by measuring the current flowing through the Schottky barrier at a metal contact under an external electrical bias. Alternatively, the electrical potential change can be registered by measuring the voltage across the Schottky barrier at a metal contact. 
     The substrate is preferably formed as a leaf, e.g., sufficiently thin from its upper surface to its lower surface to be flexible. Furthermore, at least one metal contact must interface with the substrate over a sufficiently large area of its upper surface to provide a measurable current or voltage signal indicative of the strain, or of some other physical variable change such as a change of temperature. 
     The inventive gauge is small, simple, and fast-to-respond. It does not require extensive calibration. It does not require expensive equipment for measuring a response to strain. The range of the device is superior to devices in the art. 
     The device can be epoxied at its lower surface to an engine part, transmission gear, or any other component, the strain in which is to be measured. Alternatively, the device can be formed by vapor deposition directly on the underlying engine part during fabrication of the part. 
     Under strain transmitted from the strained engine part, through the epoxy and into the semiconductor substrate, the dopant atoms are redistributed in the semiconductor lattice. The dielectric tensor of the semiconductor substrate is also altered by the strain, producing a bulk polarization across the semiconductor lattice. The Schottky-barrier electrical potential varies as a function of the distribution of the dopant atoms and as a function of the dielectric tensor, so that the changes of these two factors caused by strain likewise causes a change in the Schottky-barrier electrical potential. 
     The change in electrical potential can be measured directly with voltage measuring equipment connected to the metal contact or contacts, as an indication of the strain. As a preferable alternative, an electrical bias is applied across the Schottky barrier at a metal contact, and the current flow across the barrier is monitored. As the Schottky-barrier electrical potential raises or lowers with strain, the current flow across the barrier is reduced or increased, respectively, as an indication of the strain. 
     Importantly, the device must be thin enough that strain is adequately imparted from the underlying engine part, for example, through the substrate lattice to the region at the upper surface of the substrate where the Schottky barriers are found. Thinness is also critical because the thinner a device is, the more freely it flexes with the underlying member, without exceeding its elastic limit and fracturing. 
     The device is formed by a sequence of known semiconductor fabrication techniques. A lightly-doped semiconductor substrate may be directly deposited on an underlying engine part by means of vapor deposition, or a lightly-doped semiconductor substrate may be provided as a prefabricated wafer. At least one metal layer is deposited on a doped semiconductor substrate over an area of sufficient size to provide a measurable change in the Schottky barrier with strain in the substrate. The metal layer is about 30 nanometers thick, and preferably about 5,000 to about 10,000 square microns in area. Preferably, two such metal contacts are formed on the upper surface of the substrate at distinct locations about 1-100 microns apart across the semiconductor substrate surface. For measuring strain, the substrate is thinned down, or deposited, to a thickness of about 10 microns. Additional metal layers may be deposited on the initial metal contact or contacts to provide electrical connectivity to other equipment. Many devices may be created on a single prefabricated wafer, and subsequently cut apart into individual devices. 
     The overall size of the gauge is about 100 to about 200 microns on a side, making it a very small device, capable of being attached to or embedded in a variety of surfaces in engine, hydraulic or motor components. It can be attached to a surface with an appropriate epoxy. Each of the metal contacts is connected to electrically-conductive leads which permit direct or indirect measurement of the Schottky-barrier potential by measuring equipment, such as ammeters or voltmeters. 
     Orientation of the gauge with respect to the desirably-measured strain is important in two respects. First, the orientation of the crystal lattice of the semiconductor substrate with respect to the direction of strain is important. Greater sensitivity to a given stress can be obtained in the device if the device is produced and used in a manner such that the anticipated stress occurs in the direction of greatest packing density in the semiconductor lattice of the leaf. Second, the orientation of the two contacts with respect to the crystal orientation is important. It is preferred that a line drawn between the metal contacts is parallel to the direction of greatest packing density in the semiconductor lattice of the leaf. 
     The device of the present invention can also be used to measure temperature. When a device is subjected to an ambient temperature increase or decrease, the distribution of charge carriers which have sufficient energy to cross the barrier increases or decreases, respectively. With an electrical bias provided across the metal contacts, a change in measured current serves as an indication of temperature change. 
     A pair of devices can be used in a neighborhood to provide both strain and temperature information about that neighborhood of the underlying engine part. Temperature information can be used to adjust the strain gauge information to compensate for temperature effects within the strain gauge, so that the strain gauge indication is a true indication of underlying strain. In certain engine parts, strain is cyclical, and the strain signal can be deembedded from the combined strain-temperature effects on current flowing across the Schottky barrier. 
     A pair of devices may also be placed together on the same stressed surface, arranged orthogonally to one another in a Poisson arrangement. Both devices are subject to the same temperature, and therefore produce the same temperature indication, e.g., the same thermionic emission current. However, due to the orthogonal arrangement of the devices, stress in the underlying engine part imparted to the semiconductor leaves of each of the devices results in different currents in each device attributable to the stress. This provides for deembedding the stress signal from the background temperature signal. The stress signal appears primarily in just one device, while the temperature signal appears in both devices. Processing of the signal can yield a corrected value indicative of the stress alone. 
     Furthermore, a single device can comprise three contacts, arranged in a modified Poisson arrangement in the form of a right-triangular configuration, where the center contact provides a current sink or source. The temperature signal between the center contact and each leg contact is the same. The stress signal is found primarily between just one leg and the contact. This single-chip, modified Poisson arrangement is biased by a Wheatstone bridge circuit. Stress produces current imbalance in the bridge. 
     The invention provides means for accurately and rapidly measuring strain in engine parts at virtually any location at which these small devices are emplaced. The devices are simple and cheap to construct, robust, and accurate over a broad range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a Schottky-barrier strain gauge embodiment of the present invention; 
     FIG. 2 is a diagrammatic view of the strain gauge shown in FIG. 1 mounted on a surface for measuring strain therein; 
     FIG. 3 is a graph of current versus time for the strain gauge shown in FIG. 2, undergoing strain; 
     FIG. 4 is a diagrammatic view of a Schottky-barrier temperature sensor of the present invention; 
     FIG. 5 is a perspective view of an alternative embodiment of the temperature-sensing Schottky device; 
     FIG. 6 is a graph of current versus temperature of the Schottky device; 
     FIG. 7 is a diagrammatic view of another Schottky-barrier strain gauge of the present invention; 
     FIG. 8 is a diagrammatic view of a use of embodiments of the device of the present invention in an application; 
     FIG. 9 is a sectional view of the strain gauge shown in FIG. 2; 
     FIG. 10A is a top view of another Schottky-barrier strain gauge of the present invention; 
     FIG. 10B is a partially sectional view of the Schottky-barrier strain gauge of FIG. 10A; 
     FIG. 11A is a top view of another Schottky-barrier strain gauge of the present invention; 
     FIG. 11B is a partially sectional view of the Schottky-barrier strain gauge of FIG. 10B; and 
     FIG. 12 is a schematic diagram of a Wheatstone bridge circuit incorporating the Schottky-barrier strain gauge of FIG. 10 or FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A semiconductor sensor embodying the present invention is shown in FIG. 1, and is generally identified by reference numeral  100 . The semiconductor sensor  100  has a lightly-doped semiconductor substrate leaf  105 , and a pair of conductive contacts  110  and  115  located thereon. The contacts  110 ,  115  each comprise a layer of metal  120  at an interface  125  with the semiconductor substrate leaf  105 . Additional layers  130  of other suitable metals may rest on top of the initial metal layer  120 . The initial metal layer  120  should be chosen to provide a suitable Schottky barrier at the interface  125  to the semiconductor substrate leaf  105  at each contact  110  and  115 . The additional metal layers  130  may be chosen to improve conductivity out from the sensor  100  to conductive leads to measuring equipment or the like. 
     The metal contacts  110 ,  115  can have a thickness in the range from about 30 nm to about 300 nm, depending on the application of the sensor  100 , and whether additional layers of metal  130  are added on top of the initial metal layer  120 . Preferably, the metal layer  120  is about 30 nm thick. The lateral separation between the pads  110  and  115  should be in the range of about 1 micron to about 100 microns, and more preferably about 10 microns. Each contact  110  and  115  has an area of about 5,000 to 10,000 square microns, and is approximately 75 to 100 microns on a side, to provide sufficient area of contact with the substrate leaf to obtain a measurable alteration of the Schottky barrier with strain, or with other physical variables. 
     An important aspect of the sensor in a strain gauge embodiment is that the thickness of the substrate leaf does not dilute the sensitivity of the sensor. For use as a strain gauge, the semiconductor substrate leaf  105  should be thinned to a thickness of about 10 microns, or, if the device is being fabricated directly on the underlying engine part, the substrate leaf  105  should be deposited to a thickness of about 10 microns. 
     Dilution of sensitivity occurs in two respects. First, strain imparted at the lower surface of substrate through the epoxy from the engine part becomes diluted at greater distances away from the lower surface by relaxation in the crystal lattice of the substrate. The metal contacts and the Schottky barriers formed thereby are at the upper surface of the substrate, and if the substrate is thick, the strain appearing at the upper surface may be a negligible fraction of the strain imparted through the epoxy. Second, the substrate exerts its own resistance to stress upon the underlying strained engine part to which it is epoxied. A thicker substrate exerts a greater resistance to tension or compression, and a greater amount of strain may be lost in the epoxy layer, thereby diluting sensitivity in proportion to the thickness of the substrate. 
     It is preferable that the substrate leaf  105  be sufficiently thin so that compression or tension communicated through an epoxy or other binder from an underlying engine part to the lower portions of the substrate—thereby causing proportional strain in a bottommost section of the substrate—can be mechanically communicated through a minimal amount of intervening substrate to a upper portions of the substrate, where are found the Schottky-type depletion regions adjacent the metal contacts. Furthermore, a thin leaf-type gauge is preferable because it moves more freely with the tension and compression of the underlying engine part, exerting less of its own resistance to the stress. 
     If the device is fabricated on a prefabricated semiconductor wafer, and subsequently thinned, it is furthermore preferable to polish the underside of the Schottky strain gauge to a sub-micron smoothness to improve the fracture strength of the substrate. 
     The substrate leaf  105  may comprise, for example, doped silicon with a dopant concentration on the order of about 10 5  dopant atoms per cubic centimeter. The metal layer  120 , forming the Schottky barrier with the silicon substrate, may be nickel, deposited as a layer of 30-nm thickness. Additional layers  130  of chromium and gold may be deposited on top of the nickel, with respective thicknesses of 30 nm and 200 nm. The Schottky-barrier potential of the nickel-silicon interface  125  is about 0.61 eV. The region of carrier depletion in the silicon extends about 1 micron into the silicon substrate  105  from each metal contact  110 ,  115 . 
     The metal-semiconductor-metal sensor  100  can be used in a variety of applications, including sensing tensile or compressive strain, and measuring temperature. 
     As shown in FIG. 2, a metal-semiconductor-metal Schottky-barrier sensor  200  can be employed as a strain gauge. The sensor  200  has a 10-micron-thick lightly-doped silicon substrate  202  in a leaf-like form, which is epoxied with an appropriate epoxy to the surface  204  of a strain-bearing component  206 , such as a bearing, gear, cam and crank shaft, hydraulically-pressurized surface, or the like. One suitable epoxy for metal surfaces is, by way of example, Ablestik 789-3 adhesive, available from Ablestik Electronic Materials &amp; Adhesives, in Rancho Dominguez, Calif. Electrically conductive leads  208  and  210  are attached to a pair of contacts  212  and  213  on the substrate  202  using any of a wide variety of well-known conducting arrangements in the art, suitable for the environment in which the sensor  200  is employed. The electrical leads  208  and  210  connect to voltage measuring equipment  214 , or in the alternative to a voltage bias source  216  and current measuring equipment  218 . Alternatively, one of the electrical leads  208  may terminate at a local ground  220 , while the other lead  210  serves as a signal lead. However, improved, low-noise operation is achieved by connecting both leads to shielded cable  222  leading to the signal processing equipment. To protect the connection of the external electrical leads  208  and  210  to the contacts  212  and  213 , and to reduce electrical interference or contact with other electrical sources, the gauge  200  can be embedded in an epoxy bead  224 , or otherwise packaged according to packaging techniques known in the art. 
     Metal contacts  212  and  213  may be 30-nm-thick layers of nickel, each covering an area of about 5,000 to about 10,000 square microns of substrate  202 . At the interface between each metal contact  212 ,  213  and the substrate  202 , there forms in the substrate a Schottky-type electrical barrier, due to the propensity of the nickel to attract electrons from the semiconductor lattice in the vicinity of the interface. 
     The strain gauge  200  has several modes of operation. When the substrate is subjected to stress due to the transmission of mechanical stress through the epoxy from the underlying component  206 , the ions in the semiconductor lattice are redistributed, producing a two-fold change in the semiconductor substrate  202 . The distribution of dopant ions is proportional to the degree of lattice compression or tension. In addition, the dielectric tensor of the semiconductor is reordered in a proportional fashion to the applied stress. A bulk polarization is produced across the stressed crystal of the semiconductor substrate  202 . A shift in the potential at each barrier occurs, which varies as a power of the quantity N d /∈ s , (dopant-ion distribution/dielectric constant). The electric field arising from the polarized lattice is distributed across the gauge  200 , and appears as a voltage across each contact  212  and  213 , which can be measured as an indication of strain. The voltage offset produced is on the order of microvolts and is very susceptible to interference from stray electrical noise sources. The voltage measuring equipment  214  must be very sensitive, and careful electrical measuring techniques known in the art should be employed, since the gauge is not biased. 
     The electric field also affects the barrier potential, and an alternate mode of operation is to measure the emission current across the metal-semiconductor Schottky-barrier junction at a metal contact under a constant external bias. The external bias  216  can be as simple as a battery, which yields sensitive signals measurable with a pico-ammeter  218  in series with the gauge  200 . This mode of operation is preferred as being easier to measure and more stable under an external bias than measuring voltage. A quiescent current level at an applied bias of 5 volts, for example, was approximately 0.8 microamps. 
     An exemplary chart of measured current  302 , referenced to such a quiescent level  304 , is shown in FIG.  3 . Therein is depicted the dynamic sensor response, measured as a change in current with respect to the quiescent current, as a function of time, through which strain was applied. It can be seen that a negative trough  306  in sensor response occurs when tensile stress is applied. A positive peak  308  in sensor response occurs when compressive stress is applied. Thus, as is expected, when a component to which the inventive strain gauge is attached is subjected to stress such that the gauge undergoes tensile strain, the gauge current decreases. When the Schottky strain gauge undergoes compressive stress, the current increases. 
     It may be appreciated that the device of the present invention must comprise at least one metal contact forming a Schottky barrier in the semiconductor substrate. While it is preferable to provide two metal contacts, as described above, the invention requires only one metal contact which forms a Schottky barrier with the substrate. However, current-measuring and voltage-measuring equipment typically require two conductive inputs, and therefore, whereas one conductive input may be connected to a singular metal contact on a substrate, another conductive lead must be connected to the substrate at another location, essentially forming yet another metal-semiconductor interface. In the embodiment of FIG. 2, having two metal contacts, it may be understood that the application of an electrical bias to the metal contacts  212 ,  213  effectively forward-biases one of the Schottky-barrier diodes and reverse-biases the other Schottky-barrier diode. The current measurement made thus is indicative of the strain experienced by the reverse-biased Schottky-barrier diode. The device shown in FIG. 2 advantageously provides for an indication of strain in the substrate regardless of which lead  208 ,  210  is connected to which metal contact  212 ,  213 , since both metal contacts are the same size. However, the invention includes a device having just one metal contact of the requisite size and forming the requisite Schottky barrier with the substrate. 
     Turning to FIG. 9, the strain gauge  900  is shown in a sectional view. The dopant density and the dielectric constant are fixed quantities in a semiconductor material of the substrate  902  in mechanical equilibrium, but are modified by strain. The strain redistributes the donor ions within the depletion region  904  of the Schottky-barrier diode at the contact  906 . The depletion region  904  at each Schottky-barrier diode exists in a delicate state of charge neutrality between the donor ions in the lattice and the excess electrons that have diffused to the metal contact  906 . Under the influence of compressive or tensile stress, the lattice deforms and becomes polarized locally. The polarization is neutralized by free electrons attracted from the metal  906  to the interface just within the depletion region  904  at each Schottky barrier. The Schottky-barrier potential at the interface  908  between nickel and silicon is thus lowered by compressive stress, resulting in larger emission current flow under external bias. Tensile stress tends to reduce the bulk polarization in the depletion region  904 , and electrons from the semiconductor substrate  902  are relinquished by diffusion to the metal  906 . As a result, tensile stress reduces emission current flow under external bias. The Schottky-barrier potential is extremely sensitive to local charge distributions, and rises accordingly, reducing the emission current flow across the barrier under external bias. 
     The semiconductor sensor of the present invention may also be used as a temperature sensor according to the following way, as depicted in FIG. 4. A Schottky-barrier temperature sensor  402  has a lightly-doped silicon substrate  404  and two metal contacts  406  and  407  thereon, and can be epoxied to a surface  408  in a vicinity in which it is desired to measure temperature. The metal contacts  406  and  407  can be nickel, by way of example, or any other suitable metal which forms Schottky barriers when deposited onto the doped silicon substrate  404 . The contacts  406  and  407  preferably have a thickness of about  30  nanometers, and cover an area of about 5,000 to 10,000 square microns. Electrically conductive leads  410  and  411  are connected from the pads  406  and  407 , and connect to a biasing circuit  412  and a low-noise pico-ammeter  414  in series. To improve the low-noise quality of the sensor, shielded cable  416  can be used for the leads  410 ,  411  from the contacts. To protect the Schottky temperature sensor  402 , it may be embedded in an epoxy bead  418 , or otherwise packaged according to packaging techniques known in the art. The silicon substrate material can be directly bonded to a variety of conductive and nonconductive mechanical parts without the need for a special insulating backing. However, if desired, insulating material  420  may be added to the underside of the semiconductor substrate  404  where appropriate in attaching the sensor to a surface  408 . 
     In an alternative embodiment, as shown in FIG. 5, a Schottky temperature sensor  504  is used to measure the temperature of an integrated circuit die  502  having an electronic circuit  503  formed thereon. The Schottky sensor  504  is fabricated on the die  502  upon which the circuit  503  is being constructed, by fabrication techniques known in the art and fabrication techniques described elsewhere in this specification. The metal contacts  508  and  510  are deposited with the dimensions herein specified, namely a thickness of about 30 nm and an area of about 5,000 to about 10,000 square microns. Measurements of current are taken via conductive leads  512  and  514  which lead to a simple bias circuit  516  and ammeter  518  in series. In this embodiment, the circuit die  502  is a doped semiconductor, and serves as the substrate for contacts  508  and  510 , which form the requisite Schottky barriers in the die  502 , as described above. 
     As another alternative, a Schottky temperature sensor of the present invention can be embedded within a component, to obtain its bulk temperature. 
     When the symmetric metal-semiconductor-metal Schottky-barrier diode arrangement is in thermal equilibrium, electrons in a continuum of states in the semiconductor and metals have energies described by the Maxwell-Boltzmann distribution. Thermally emitted electrons (thermionic emission current) near the interface between the metal contacts and the substrate have energies distributed over a range. Some electron energies are large enough to overcome the Schottky-barrier potential at each metal contact, whereas some electrons do not have sufficient energy. As the temperature at the Schottky-barrier diode junction increases, larger numbers of electrons have sufficient energy to cross the Schottky barrier at a metal contact. When the temperature drops, proportionately fewer electrons have the necessary energy to cross the barrier. Electrons at both Schottky barriers in the device, and on either side of each barrier can overcome the barrier with sufficient energy. 
     Currents arising from thermionic emission move in opposite directions across the barriers. The net resultant current is zero if the sensor is unbiased. Thermionic electrons must move in one direction to constitute a current which is measurable and which may vary as a function of temperature. Unidirectional current may be accomplished by applying an external bias voltage from the biasing circuit  412  or  516 , such that the external bias voltage is greater than the “flat-band” voltage of the diodes. The “flat-band” voltage is the bias point threshold at which the applied electric field forces charges to move unidirectionally across the metal-semiconductor-metal diode pair. 
     The temperature-dependent current established by the biasing can be measured by a series pico-ammeter, such as  414  and  518 . The current j(T) varies with temperature T according to: 
     
       
         j(T)˜exp[−eφ SB /kT], 
       
     
     where φ SB  is the potential energy of the Schottky barrier, k is the Boltzmann constant, and T is the temperature. 
     By way of example, FIG. 6 shows a response  605  of the thermionic emission current to changes in temperature, for a nickel-silicon-nickel device (the preferred embodiment of the temperature sensor) having a 20-micron separation between contacts, each contact being 75 microns wide and 100 microns long, under a bias of 2 volts, and where the quiescent current at room temperature was about 0.5 microamps. The response is measured as emission current as a function of temperature. 
     The Schottky temperature sensor performs well for temperatures below 500° C., and is particularly suitable for applications where the temperature is between 0° C. and 100° C. Use of the Schottky temperature sensor at temperatures above approximately 500° C. may result in annealing of the silicon and nickel layers to form highly resistive nickel silicide, which impairs electron emission. The small physical size of the Schottky sensor makes it particularly appealing for sensing of localized temperature in a sliding bearing surface, and provides very rapid response to thermal fluctuations due to the low thermal mass and relatively large surface area. 
     The semiconductor temperature and strain sensors of the present invention may be fabricated according to well-known methods in the art. A doped silicon wafer of 0.5 mm thickness and 2-inch diameter, which is widely available in the semiconductor fabrication marketplace, having a dopant concentration of about 10 15  dopant atoms per cubic centimeter, may be used as a substrate for the sensor. The silicon wafer is first cleaned in a solution of H 2 O 2  and H 2 SO 4  in proportions of 1:3, respectively, for five minutes. It is then rinsed in de-ionized water and dried in a nitrogen atmosphere. The wafer is then spin-coated with a 3-micron layer of photoresist and subjected to a 100° C. prebake. It is then exposed to 350-nm ultraviolet light through a chrome-emulsion mask having the desired device pattern. The mask for a 2-inch silicon wafer may have a multitude of device patterns, each having two contacts spaced from 1 to 100 microns apart, where each contact is about 75 microns wide along the dimension perpendicular to a line dividing the contacts, and about 100 microns long along the dimension parallel to the line dividing the contacts. The exposed wafer is then soaked in chlorobenzene to assist in the process of lift-off. The wafer is then developed and subjected to a 100° C. postbake. The wafer is coated with a 30-nm layer of nickel, a 30-nm layer of chromium, and a 200-nm layer of gold in an electron beam evaporator. Thereafter, soaking in acetone causes removal of the unexposed photoresist and removal of the unused areas of metal. The wafer may then be diced into individual dies each having a pair of Ni—Cr—Au contacts. At all times process temperatures should be kept under 200° C. to assure that stable Schottky contacts are formed at the interface between silicon substrate and nickel. 
     Finally, to enable the device to be used for the measurement of strain, the devices may be thinned down to a thickness of about 10 microns, and polished on the underside of the silicon substrate. It is preferable that the underside be polished to a sub-micron smoothness to improve fracture strength. 
     As mentioned above, it is contemplated to use the semiconductor device of the present invention for measurement of strain or temperature. In the measurement of strain, it has been noted in the above description that the strain gauge is also subject to variations in emission current from temperature fluctuations as well as from the strain sought to be measured, and in order to compensate for the temperature fluctuations, a temperature sensor can be placed in.the vicinity of the strain gauge, and its output used to compensate for thermal fluctuations in the current response of the strain gauge. In the measurement of large strain, the temperature signal is negligible compared to the strain signal. It is contemplated that the temperature sensor of the present invention can be used in combination with the strain gauge of the present invention to provide a measure of strain and temperature together, where desirable. Alternatively, the strain gauge of the present invention can be used with any temperature sensor known in the art, to provide a temperature-corrected measure of strain. 
     For example, as shown in FIG. 7, a Schottky strain gauge  702  according to the present invention, comprising a substrate  704  and a pair of metal contacts  706  and  708  may be epoxied to a surface  710  of a component  711  for strain measurements, while a temperature sensor  712 , comprising a semiconductor substrate  714  and a pair of metal contacts  716  and  718  according to the present invention can be embedded marginally deeper in a non-flexing region of component  711 , so as to be shielded from surface stress, even while supplying temperature information for de-embedding the strain signal from the strain gauge. Leads  720  and  722  conductively connect from the metal contacts  706  and  708  of the strain gauge  702  to an ammeter  724  and biasing circuit  726  in series. Leads  728  and  730  conductively connect from metal contacts  716  and  718  of temperature sensor  712  to another ammeter  732  and biasing circuit  734  in series to provide measurement of temperature. Information on measurement of current indicative of strain combined with some temperature effects can be provided to a processor  736  from ammeter  724  and biasing circuit  726 . Information on measurement of current indicative of mostly temperature alone can be provided to the processor  736  from ammeter  732  and biasing circuit  734 . The processor can be hardwired or programmed according to known methods in the art to provide an output indicative of the strain alone based on the combined inputs of the strain gauge and temperature sensor. 
     It is further contemplated that where a strain gauge is used on an underlying engine part which experiences cyclical strain, the processor  736  can be used with strain gauge  702  alone. The processor  736  can be programmed or hardwired to deembed the temperature-corrected strain from the measured current from ammeter  724  (which indicates both strain and temperature effects) by filtering out the constant temperature effect from the time-varying strain effect. 
     As an alternative to embedding a temperature sensor within the component, as shown in FIG. 7, a pair of Schottky-barrier gauges  1010 ,  1012  can be arranged on the surface  1015  of a component in the orthogonal arrangement shown in FIGS. 10A and 10B. This arrangement also provides for temperature correction of strain measurements. 
     Gauge  1010  has metal contacts  1017  and  1018  on a semiconductor leaf  1019 . Gauge  1012  has metal contacts  1020  and  1022  on a semiconductor leaf  1024 . Metal contacts  1017 ,  1018 ,  1020  and  1022  may comprise one or more layers of different metals, as described hereinabove with respect to the device of FIG.  1 . Each semiconductor substrate leaf  1019 ,  1024  preferably has a thickness of about 10 microns. Each metal contact  1017 ,  1018 ,  1020  and  1022  preferably has an area in the range of about 5,000 to 10,000 square microns, and is approximately 75 to 100 microns on a side. Metal contacts  1017  and  1018  are preferably about 1-100 microns apart across the surface of the substrate leaf  1019 , and metal contacts  1020  and  1022  are similarly spaced apart across the surface of substrate leaf  1024 . 
     Schottky barriers are formed at the interfaces  1030 ,  1033 ,  1035  and  1037  between metal contact  1017  and leaf  1019 , metal contact  1018  and leaf  1019 , metal contact  1020  and leaf  1024 , and metal contact  1022  and leaf  1024 , respectively. Metal contacts  1017  and  1018  are connected to an electrical bias, such that electrical current flows between them. Similarly, metal contacts  1020  and  1022  are connected to an electrical bias, causing current to flow therebetween as well. 
     Gauges  1010  and  1012  preferably have substantially similar dimensions, and thus yield the same temperature-dependent thermionic emission signal. However, gauges  1010  and  1012  may be of dissimilar size, in which case they may yield differing thermionic emission signals at the same temperature. The size-dependent temperature response of the gauges may be correlated to one another, such that if a temperature change occurs, resulting in a change in thermionic emission current in, by way of example, gauge  1012 , an estimate may be made of exactly how much the thermionic emission current in gauge  1010  must change, other parameters being constant. 
     Gauges  1010  and  1012  are preferably fabricated such that the orientation of the lattice of the semiconductor leaf  1019  with respect to a hypothetical line connecting metal contacts  1017  and  1018 , is the same as the orientation of the lattice of the semiconductor leaf  1024  with respect to a hypothetical line connecting metal contacts  1020  and  1022 . Therefore, with the gauges  1010  and  1012  arranged as shown in FIG. 10A, the orientation of the lattices of semiconductor leaves  1019  and  1024  are orthogonal. A directed stress applied to the underlying surface  1015  and transmitted to the semiconductor leaves  1019  and  1024 , will result in different redistributions of dopant ions in the leaves, as well as a different reordering of the dielectric tensor in each. 
     With the application of directional compression or tension, a stress-generated signal appears in one of the gauges, gauge  1012  by way of example, which is substantially absent in the other gauge, gauge  1010 . Since the temperature signal is the same across both gauges, the strain signal can be easily deembedded from the overall current signal coming from gauge  1012 , utilizing the temperature signal coming from gauge  1010 . 
     Turning now to FIGS. 11A and 11B, another embodiment of the device  1111  is shown to have three contacts  1113 ,  1116  and  1119  on a substrate leaf  1125 , which is mounted on the surface  1127  of a strained component. The contacts are arranged in a right-triangular geometry such that contact  1116 , located at the right angle of the triangle, is a common current sink or source to contacts  1113  and  1119 , which are located at the legs of the triangle. Schottky barriers are formed at the interfaces  1130 ,  1133  and  1136  between metal contacts  1113 ,  1116  and  1119 , and the substrate  1125 , respectively. Thermionic current indicative of temperature flows approximately equally between each leg contact  1113 ,  1119  and the common contact  1116 , respectively, because the temperature throughout the device  1111  is uniform. 
     Semiconductor substrate leaf  1125  preferably has a thickness of about  10  microns. Each metal contact  1113 ,  1116 , and  1119  preferably has an area in the range of about 5,000 to 10,000 square microns, and is approximately 75 to 100 microns on a side, to provide sufficient area of contact with the substrate leaf to obtain a measurable alteration of the Schottky barrier with strain. Metal contacts  1113  and  1116  are preferably about 1-100 microns apart across the surface of the substrate leaf  1125 , and metal contact  1119  is similarly spaced from metal contact  1116 . Metal contacts  1113 ,  1116  and  1119  may each comprise one or more layers of metal, as described hereinabove with respect to the device shown in FIG.  1 . 
     The device in FIGS. 11A and 11B is biased by a Wheatstone bridge circuit  1210 , shown in FIG. 12. A Wheatstone bridge generally is a device well known to those skilled in the art. A lead  1212  is connected to metal contact  1116 . Schottky diodes  1215  and  1217  comprise the current pathways of metal contact  1116  with metal contact  1113  and  1119 , respectively. A lead  1218  is connected to metal contact  1113 . A lead  1219  is connected to metal contact  1119 . A detector  1220  monitors current or voltage in the bridge. A variable resistor  1225  is used to balance the bridge when the Schottky-barrier strain gauge  1111  of FIGS. 11A and 11B is in thermal equilibrium but not subject to stress, to compensate for any minor differences in thermionic emission current flowing in each of the respective Schottky diodes  1215  and  1217 . Resistors  1228  and  1230  complete the Wheatstone bridge. 
     Stress applied to the component is transmitted throughout the surface  1127  to the device  1111 . The dopant ion distribution is changed by strain in the lattice, resulting in both a polarization in the lattice and a migration of electrons to or from the metal contact. The Schottky barrier at each metal contact  1113 ,  1116  and  1119  is altered by the redistribution of dopant ions. The Schottky barrier at metal contact  1116  is forward biased, and therefore the effects on current flow at the Schottky barriers at metal contacts  1113  and  1119  predominate. The polarization caused by the strain gives rise to an electric field, the effect of which on the current flowing between metal contacts  1113  and  1116  is non-negligibly different from the effect it has on the current flowing between metal contacts  1119  and  1116 . This difference is due to the directionality of the polarization in the lattice with respect to the direction of current flow in each Schottky diode  1215  and  1217 . As a result, the Wheatstone bridge, which is balanced for thermal currents, becomes unbalanced due to the strain. Strain is indicated at the detector  1220 , with temperature effects having already been removed by the arrangement of the Wheatstone bridge circuit. 
     Engines and transmissions in automobiles, helicopters, aircraft and machinery in general have lubricated sliding contacts in gears and bearings, in which it is desirable to monitor adverse temperatures and pressures. Such conditions may arise as lubricants break down during operation, and friction in sliding contacts increases beyond control. All of these systems can benefit tremendously from the incorporation of embedded sensors based on the device of the present invention. The signals from these sensors can be electronically processed, digitized and analyzed in a computer to provide a measure of system worthiness during field operation. 
     By way of example, FIG. 8 depicts the use of sensors of the present invention in an opposed-pad conical bearing  802 , along with the hydraulic system required for bearing operation. The hydraulic fluid is pumped by a pump  804 , driven by a motor  806 , from a main reservoir  808  through an intake line  810  where a constant pressure is maintained by a relief pressure valve  812 . Excess fluid from the line is drained into a second reservoir  814 . The fluid arrives through restrictors  816 ,  818 , to the pads in the bearing. The spent fluid passes through drainage grooves  820  in the bearing into the second reservoir  814  where it is filtered and recirculated to the main reservoir. Sensors for pressure  822 , temperature  824 , and stress  826  are shown, respectively placed at locations such as the pump  804 , inlet line  828 , valve  812 , restrictors  816 ,  818 , pads  830 , sliding surfaces  832  and outlets  834 . The information is supplied to a computer that evaluates the operation and the condition of the bearing. 
     Temperature sensors according to the present invention may be included in the fabrication process for integrated circuits at an early stage of the fabrication, and serve as temperature sensors for the remainder of the fabrication process, by depositing two nickel or other suitable metal contacts on a suitably-doped region of the silicon wafer upon which fabrication is taking place. The contacts are then connected as described above to a voltage bias and a current meter. Local sensing of temperature during fabrication can improve the quality and yield of the process. 
     It may be seen that the semiconductor device of the present invention allows for improved strain and temperature measurement systems. The device has dimensions which make it ideal for embedding or attaching to multiple locations in an engine or other machinery, and furthermore the small dimensions of the device permit almost instantaneous stabilization of response. The devices are cost-effective to manufacturer, requiring only a few simple semiconductor fabrication steps. 
     While the invention has been described above in specific embodiments, the scope of the invention is not intended to be limited thereto, and it should be understood that other embodiments of the device and methods for its use lie within the scope of the invention, as defined by the claims.