1. Field of the Invention
This invention relates to temperature sensitive circuits and structures employing SiC, AlN and/or AlxGa1xe2x88x92xN (x greater than 0.69) as a temperature sensitive device.
2. Description of the Related Art
SiC, AlN and AlxGa1xe2x88x92xN (x greater than 0.69) are temperature sensitive materials whose resistance changes with temperature and are useful for hostile environment sensor and electronic applications. However, temperature sensors employing these materials have been limited in their usable temperature range because of mechanical deterioration of the sensor structure caused by higher temperature levels and thermal shock. The general state of the art is summarized in xe2x80x9cMaterials for High Temperature Semiconductor Devicesxe2x80x9d: Committee on Materials for High Temperature Semiconductor Devices, National Materials Advisory Board, Commission on Engineering and Technical Systems, National Research Council, National Academy Press, Washington, D.C. 1995, pages 68-70, and in O. Nennewitz, L. Spiess and V. Breternitz, xe2x80x9cOhmic Contacts to 6Hxe2x80x94SiCxe2x80x9d, Applied Surface Science, Vol. 91, 1995, pages 347-351. Whereas the goal temperature in these references is only 600xc2x0 C., a significantly higher operating range would be desirable.
Specific structures for SiC temperature sensors are also known. A method for electrical isolation of the back-side of a SiC wafer or device chip is described in Q. Y. Tong, U. Gosele, C. Yuan, A. J. Steckl and M. Reiche, J. Electrochem. Soc., Vol. 142, No. 1, 1995, pages 232-236. A method of bonding SiC slabs for heat sink purposes is described in P. K. Bhattacharya, J. Electronics, Vol. 73, No. 1, 1992, pages 71-83. In J. B. Casady et al., xe2x80x9cA Hybrid 6Hxe2x80x94SiC Temperature Sensor Operational from 25C to 500Cxe2x80x9d, IEEE Transactions on Components, Packaging and Manufacturing Technologyxe2x80x94Part A, Vol. 19, No. 3, September 1996, a SiC JFET structure was integrated with an operational amplifier for temperature sensing up to 500xc2x0 C.
AlN dies have also been used for high temperature applications. In R. Holanda, xe2x80x9cThin-Film Thermocouples on Ceramicsxe2x80x9d, NASA Technical Briefs, March 1997, page 62, Pt vs PtRh metal thin films were deposited on AlN dies for use as thin-film thermocouples; the drift of the thermocouple junction vs temperature (to 1500xc2x0 C.) is discussed. In Y. H. Chaio, A. K. Knudsen and I. F. Hu, xe2x80x9cInterfacial Bonding in Brazed and Cofired Aluminum Nitridexe2x80x9d, ISHM ""91 Proceedings, 1991, pages 460-468, the reactions for joining interfaces between AlN and several metals is discussed. A multilayer AlN/W structure is shown in which the interface joining is due to interlocking grain-boundaries. In Savrun et al., mentioned above, the thermal stability of WSi2, NdSi2 and TiSi2 films, deposited on AlN dies for the purpose of developing SiC hybrid circuits, was investigated. All silicides were found to change composition upon heating (up to 1000xc2x0 C.). The films were said to be promising for hybrid circuits with a maximum operating temperature of 600xc2x0 C.
Various SiC, AlN and AlxGa1xe2x88x92xN temperature sensors are also described in the following references:
G. Busch, Helvetica Physica Acta, Vol. 19, No. 3, 1946, pages 167-188.
J. A. Lely and F. A. Kroeger, xe2x80x9cIn Semiconductors and Phosphorsxe2x80x9d, Proceedings of Intl. Colloquium-Partenkirchen, Ed. M. Schoen and H. Welker, N.Y., Interscience Pub., Inc., 1958, pages 525-533.
M. I. Iglitsyn et al., Soviet Physicsxe2x80x94Solid State, Vol. 6, No. 9, March 1995, pages 2129-2135.
O. A. Golikova et al., Soviet Physicsxe2x80x94Semiconductors, Vol. 5, No. 5, September 1971, pages 366-369.
Westinghouse Astronuclear Laboratory, xe2x80x9cSilicon Carbide Junction Thermistorxe2x80x9d, 1965.
T. Nagai and M. Etoh, xe2x80x9cSiC Thin-Film Thermistorsxe2x80x9d, IEEE Transactions on Industry Applications, Vol. 26, No. 6, November/December 1990, pages 1139-1143.
SiC is generally considered to have a temperature coefficient of resistance (TCR) that varies exponentially with temperature. This thermistor-like TCR, together with circuit stability limitations, have precluded its use for applications that require temperature to be monitored over large ranges, in which a scaling of electronic controls and readouts requires a sensor with an approximately linear TCR, e.g., resistance temperature detectors and thermocouples.
This invention seeks to provide circuit structures, and systems employing such structures, that maintain their physical integrity at highly elevated temperatures, up to 1300xc2x0 C. or more. The invention further seeks to provide a SiC temperature sensing mechanism with a substantially linear TCR.
An improved high temperature structure is achieved through the use of an AlN ceramic die to which a circuit device which comprises SiC, AlN and/or AlxGa1xe2x88x92xN (x greater than 0.69) is adhered by an electrically conductive mounting layer. The mounting layer has a thermal coefficient of expansion (TCE) within 1.0xc2x10.06 that of the die and circuit device, and is preferably formed from W, WC and/or W2C. It can be discontinuous, with a plurality of mutually separated mounting elements that are connected to different portions of the circuit device through respective electrodes. The die""s surface is roughened to establish an adhesion to the mounting layer.
In one embodiment the mounting layer includes a W, WC and/or W2C adhesive layer which is adhered to the die, plus an optional metallization layer that is adhered to the adhesive layer and bonded to electrodes on the circuit device. When used, the metallization layer has a TCE which is not greater than about 3.5 times that of the adhesive layer over a temperature range of interest.
The structure can also include a plurality of electrode pads lateral to the circuit device that have the same composition as the mounting layer, are electrically and mechanically connected to the die, and electrically connected to the mounting layer. When a metallization layer is used and the electrode pads comprise lateral extension of the mounting layer, the metallization layer preferably has a greater thickness at the electrode pads than at the device electrodes.
Lead wires can be connected to the device through the electrode pads and mounting layer, and an encapsulation formed from a reacted borosilicate mixture (RBM) formed over the device, mounting layer, electrode pads and a portion of the lead wires on the die. The encapsulation preferably includes an oxide interface between the RBM and the encapsulated elements. It forms an environmental barrier having a TCE that closely matches that of the device and die, or a viscosity which is less than its Littleton softening point (xcx9c107 poise). An alternate encapsulation technique employs a cover, of the same material as the die which extends over the device and is bonded to the die by an encapsulation formed from the RBM, or reaction bonded to the die by the RBM.
The new high temperature structure can be used as a contact/immersion temperature sensor for applications that are now performed by integrated circuits, pyrometers, resistance temperature detectors, thermistors and thermocouples, and electromechanical and volume devices such as metal coils and strips, volumetric tubes and bulb thermometers. Other sensor applications include radiation detectors, precision flow rate monitoring and control of gases, tank fluid level monitors, humidity sensors, chemical reaction temperature sensors and electronic circuits that employ a resistance which varies with temperature. The invention can additionally be used in pressure sensors, chemical sensors and high temperature electronic circuits.
The invention also exploits a previously unrecognized property of SiC, which is that it can be doped to have a substantially linear TCR. With n-type doping, a linear TCR is achieved within the temperature range of about 22xc2x0 C.-1300xc2x0 C. With p-type doping, the TCR exponentially decreases with increasing temperature until a temperature in the range of about 100xc2x0 C.-600xc2x0 C. (depending upon the concentration of p-type and n-type dopant atoms), above which an approximately linear positive TCR is achieved up to about 1300xc2x0 C.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of particular embodiments, taken together with the accompanying drawings.