Oriented zinc-oxide layers have been combined with NMOS planar technology and silicon micromachining to produce integrated sensors empolying the pyroelectric effect in thin deposited films. Several fully integrated sensors have been designed and fabricated to detect differential temperatures as small as 18uK (and up to 2K) that arise in response to physical or chemical variables being sensed. The following fully-integrated pyroelectric sensors have been designed, fabricated, and tested: infrared detector array, gas-flow sensor, and chemical-reaction sensor. These sensors have been fabricated along with a charge-coupled device imager, tactile-sensor array, cantilever-beam accelerometer, and surface-acoustic-wave vapor sensor on the same silicon chip demonstrating the possibility of monolithic multi-sensing.
The infrared detector utilizes chromium-coated zinc oxide supported on a small silicon therma-mass with thickness of approximately 25um to sense the absorbed heat of chapped incident radiation. The induced pyroelectric charge is proportional to the intensity of the incoming infrared radiation and is amplified by on-chip MOSFET circuits. The voltage responisivity of a single detector element of area measuing 70x70um^2 is 4.3*10^4 VW^-1, measured at a chopping frequency of 24 Hz. The measured broad-band detectivity D is 3.1*10^7cmHzW^-1 and the response time is 0.76s being limited by the theremal properties of the support membrane. A 64-element detector array has been fabricated for real-time imagine application. The uniformity of voltrage responsivity across an eight-element row is approximately 5%.
in the gas-flow sensor (or anemometer) a thin-film on-chip polycrystalline-silicon resistor serves as a heater, and two symmetrically places zinc oxide strips are cooled differentially, depending on the velocity of the stream of gas flowing over the sensor's surface. The pyroelectrically sensed temperature difference is a measure of the flow velocity. The induced pyroelectric charge is coupled to gates of MOS transistor amplifiers arranged in a differential-pair configuration. The output voltage of the differential pair amplifer varies linearly with flow velocity. The measured output voltage for a nitrogen flow velocity of 2 m/s is 184mV when the chip is heated to a temperature of 36C.
The chemical-reaction sensor detects the heat liberated in the selective chemisorption of gas molecules on the surface of specific metals.
The fabricated sensor ultilzed a thin platinum 61m overlaying a zinc-oxide film on a thinned-silicon membrane as a heat absorber. The chemisorption of carbon-monooxide gas molecules on the platinum surface releases a heat of reaction that induces a pyroelectric charge. Sensitivity to smaller than 10^-12 moles of carbon monoxide has been demonstated, with a corresponding minimum detectable change in temperature of 18uK.
The pyroelectric coefficient in zinc-oxide thin films was studied for the first time. The value measured (1.4*10^-8cm^-2K^-1) is in good agreement with published data for bulk single crystals. Pyroelectric charge decay times of 2.8*10^6s were also found giving the resulting sensor structures a nearly deresponse.
The ability to detect simultaneously several different physical and chemical variables has also been demonstarted in this study through the fabrication of a multi-function sensor chip. The chip contains conventional MOS devices for signal conditioning, array accessing, and output buffering along with the following sensors (all based either upon the pyroelectric or piezoelectric effect in zinc-oxide thin-films): infrared detector array, gas-flow sensor, chemical-reaction sensor, cantilever-beam accelerometer, tactile-sensor array, surface-acoustic-wave (SAW) vapor sensor, and infrared charge-coupled device imager. The chip measures 8*9 mm^2 and is compatibly fabricated in conjunction with a converntional 3um silicon planar process. Fabrication is carried out in an eleven-mask process with 3um minimum feature sizes. Two layers of chemical vapor deposited silicon dioxide are used to encapsulate the zinc-oxide thin-film dielectrically. In the multi-sensor chip, anisotropic etching of silicon is used to form thin-membrane structures. Polycrystalline silicon is used to form the back-side electrode of the zinc-oxide sensors. Compatible fabrication technology, sensor design, and resulting sensor performance properties are described.