Kenneth G. Kreider

Fast temperature programmed sensing for micro-hotplate gas sensors

We describe an operating mode of a gas sensor that greatly enhances the capability of the device to determine the composition of a sensed gas. The device consists of a micromachined hotplate with integrated heater, heat distribution plate, electrical contact pads, and sensing film. The temperature programmed sensing (TPS) technique uses millisecond timescale temperature changes to modify the rates for adsorption, desorption, and reaction of gases on the sensing surface during sensor operation. A repetitive train of temperature pulses produces a patterned conductance response that depends on the gas composition, as well as the temperature pulse width, amplitude, and specific sequence of pulses. Results are shown for the vapors of water, ethanol, methanol, formaldehyde, and acetone.

Sputtered thin-film pH electrodes of platinum, palladium, ruthenium, and iridium oxides

Abstract Thin-film metal oxides have been investigated for use as pH electrodes. These materials may have potential for measuring pH under conditions that are not favorable for glass electrodes. Reactive sputtering of platinum, palladium, ruthenium, and iridium metal targets in argon-oxygen atmospheres is used to produce 1 μm thick electrodes on alumina and silicon substrates. The structure of the deposits is determined and compared to the fully crystalline structure developed after annealing at 420 °C. The pH response is measured for pH 2-11 versus a glass electrode. Exposures of up to 24 h at pH 2–11 are used to study the stability under harsh conditions. Palladium and platinum oxides are found to be less stable than ruthenium oxide. X-ray photoelectron spectroscopy studies are used to determine the surface chemical state and verify the stoichiometry of the sensing surface. The stability of thin-film RuO 2 indicates the usefulness of further testing at high temperatures and wider pH ranges.

Optimized temperature-pulse sequences for the enhancement of chemically specific response patterns from micro-hotplate gas sensors

Abstract Microfabricated solid-state gas sensors have been of continuing interest because of the potential for arrays of devices with low power consumption. Devices based on a micromachined ‘hotplate’ offer the additional advantage of a wide operating temperature range with a rapid thermal time constant of order 1 ms. By operating the device in a temperature-programmed mode, reaction kinetics on the sensing film surface are altered, producing a time-varying response signature that is characteristic of the gas being sensed. Approaches to optimizing such temperature programs to maximize the differences in response signatures for gases of interest or to enhance the sensitivity of the device are discussed.

Transparent thin film thermocouple

A transparent thin film thermocouple and a method of manufacturing comprig a positive element of indium tin oxide (ITO) and a negative element of indium oxide (In2 O3) formed on a surface by reactive sputtering with the elements being electrically joined to form a hot junction for conversion of heat into electricity.

High temperature materials for thin-film thermocouples on silicon wafers

Abstract We have developed an instrumented calibration wafer for radiometric temperature measurements in rapid thermal processing (RTP) tools for semiconductor processing. The instrumented wafers have sputter deposited thin-film thermocouples to minimize the thermal disturbance of the wafer by the sensors. The National Institute of Standards and Technology (NIST) calibration wafer also employs platinum–palladium wire thermocouples to achieve a combined standard uncertainty of 0.4°C in the temperature measurement of the thin-film thermocouple junction at 900°C. The high temperatures of the wafer has required the development of new thin-film material systems. We have reported the results of our testing and characterization of sputtered platinum, palladium, rhodium, and iridium thin films using titanium bond coats on thermally oxidized silicon wafers. Depth profiling with secondary ion mass spectrometry was used to determine the diffusion profiles from the metal film to the silicon after heat treatments as high as 1000°C. Electron microscopy and optical microscopy were used to follow the reactions and the deterioration of the thermoelectric films. In addition, performance tests up to 1000°C in the NIST RTP test bed were used to determine the stability of the material systems. Failure mechanisms and limitations of the thin-film thermocouple materials have been discussed with data on hysteresis and drift in thermometry performance. The results of our evaluations indicated that Rh/Ir thin-film thermocouples have the best properties for wafer temperatures above 900°C.

Platinum/palladium thin-film thermocouples for temperature measurements on silicon wafers

Abstract A platinum versus palladium thin-film thermocouple system has been established for measuring temperatures on silicon wafers in a rapid thermal processing (RTP) tool. The application includes a silicon wafer with an array of thin-film thermocouples welded to wire thermocouples, used to calibrate radiometric temperature measurements of the RTP tool. The thin-film thermocouples have advantages over present technology using wire thermocouples because the films cause less disturbance of the heat transfer to and from the silicon wafer during periods of high heat flux. High-purity platinum and palladium films have been chosen because of their stability, their resistance to oxidation at high temperatures (to 900 °C), and their compatibility with Pt/Pd wire thermocouples. The research effort has concentrated on developing solutions to the adhesion problems for platinum and palladium on silicon; measuring the change in resistance of the films after exposure in air to high temperatures (up to 1050 °C); quantifying the drift and hysteresis of the thin-film thermocouples in calibration tests; and determining the effects of composition, stress, and morphology on the Seebeck coefficient. The results of the testing indicate that the Pt/Pd thin-film thermocouples on silicon can perform satisfactorily up to 850 °C. We present a discussion of the factors that affect the stability of the films; these include diffusion, chemical reactions, and thermal-mechanical stress, as well as the limitations of this material system.

Growth of SnO2 films on micromachined hotplates

Arrays of micromachined hotplates have been used for materials processing on a microscopic scale. The temperature of individual elements ‘‘micro‐hotplates’’ of an array is controlled by addressing a given element with a specified current and measuring the temperature from a resistance change. This unique temperature control capability has been exploited to deposit SnO2 overlayers onto micro‐hotplates with individually controlled temperatures using reactive sputter deposition and organometallic chemical vapor deposition. Post‐deposition heating in vacuum was used to alter the stoichiometry of films. The result is an array of separately, but simultaneously, processed films. The micro‐hotplates have excellent thermal isolation from other devices (transistors, logic elements) on the chip. Electrical contact pads allow for in situ electrical characterization of the films. The use of micro‐hotplates allows high‐temperature growth to occur on portions of a silicon substrate, while other portions remain at room t...

Mechanistic and response studies of iridium oxide pH sensors

Abstract Results are presented on the pH-potential response of iridium oxide films prepared by d.c. magnetron reactive sputtering. Freshly deposited films exhibit a nearly Nernstian response to pH and little hysteresis. The redox sensitivity of films prepared by sputtering in water-saturated oxygen and annealed in an oxygen atmosphere has been examined as well. In addition, methods are described for preparing model iridium oxide sensor surfaces for ultrahigh vacuum surface analytical studies. Stoichiometric IrO 2 -like surfaces are shown to be relatively inert to gas phase water. Hydroxylation of IrO 2 -like surfaces can be induced, however, by r.f. water plasma treatment.

Transient thermal response of plasma-sprayed zirconia measured with thin-film thermocouples

Abstract A pulsed laser heat source method for measuring the transient thermal response of thin-film thermocouples (TFTCs) is shown to obtain results for the temperature jump and temporal response which are consistent with those predicted employing a simple model. Some deviation from this model is observed for the thinnest films on alumina, which has a very high thermal conductivity, and on Min-K 2000, which has a very low thermal conductivity. Poor adherence of the thin film to the substrates, the interface thermal conductance, is suggested to be the origin of this effect. The technique is determined to be sensitive to the thermal properties of the substrate on dimensions of the thermal diffusion lengths. This experimental method is preferable to calculations of the thermal response of the TFTCs based on bulk thermal data. It should prove useful as a method for characterizing in situ TFTCs, which may vary from batch to batch.

Thin film thermocouples for internal combustion engines

The feasibility of fabricating thin film thermocouples on internal combustion engine hardware was investigated. The goal was to find a procedure that would be useful for the measurement of the surface metal temperature of valves, valve seats, combustion chamber surfaces, cylinder walls, and piston heads during engine operation. The approach pursued was to coat the engine hardware material with an aluminum‐containing, oxidation‐resistant ferrous alloy (FeCrAlY) which forms a thermal oxide layer with good electrical resistance. This thermal oxide was coated with a thin layer of reactively sputtered aluminum oxide and sputtered thin film type S thermocouple legs of platinum and platinum plus rhodium. This project was used to investigate the materials problems related to obtaining good adhesion in the metal/metal–oxide/oxide/metal laminate and the electrical insulating properties of the oxide. Thermal oxidation, reactive sputtering of Al2O3, and platinum alloy sputtering were investigated using optical micros...