Objective: Demonstrate feasibility of silicon micromachined, thin-film sensor array technology for multi-analyte, real-time detection and concentration measurement of gases.
Problem: Increasing global competition has placed new demands on the chemical process industry for more efficient use of materials, better process reproducibility, and environmental safety. Similar measurement concerns are encountered in the automotive field where engine performance and reduced emissions are issues. Meeting these demands requires a low-cost technology for the measurement of gas species, which can provide immediate, on-site analysis for the detection of reaction products, exhaust gases, leaks, etc.
Approach: Advances in microfabrication technology now make possible miniaturization of conventional low-cost metal oxide sensors into a planar array form. At NIST, a sensor array platform has been developed which uses a microhotplate as the generic device structure. The microhotplate has three functional layers: a heater, a thermometer/heat distribution plate, and sensing film electrical contacts. Devices are fabricated using CMOS processing combined with post-CMOS process silicon micromachining, and sensor film deposition. NIST holds three patents on this technology. To make this technology viable, fundamental advances are required in three areas: materials, measurement techniques, and transducing mechanisms.
The most important challenge is to develop methods for fabricating and lithographically defining sensing films with the desired characteristics of sensitivity, selectivity, and stability. Catalyst-doped metal oxide materials, which have shown good characteristics on the conventional sensors, must now be produced in a thin film form. Reactive sputter deposition and chemical vapor deposition (CVD) methods are adapted to take advantage of the self-heating of the micro-hotplates for thermally processing the sensing film. With CVD, varying the precursor composition and selecting which microhotplates in an array are activated, array elements with different materials can be produced.
Because of their 50 to 250-µm size the elements can be heated and cooled rapidly with time constants of 1-2 milliseconds over a large operating temperature range (>800 °C). This capability supports a novel sensing approach, temperature programmed sensing (TPS). The effects which produce a response signal are based on thermally-activated processes, such as adsorption, reaction, and desorption. By varying the temperature in a defined, repeated pattern using millisecond scale temperature changes, the sensor generates response signatures that are characteristic of adsorbed species/sensing material combinations. Neural network and chemometric-based approaches to this problem are being used to optimize the generation of patterns and for signal analysis during sensing.
The third area of research is to discover the mechanisms behind the signal
transduction process. Depending on whether the thin film structure is
polycrystalline, oriented polycrystalline, or epitaxial, the dominant mechanisms
may be related to grain-boundaries or the properties of the oriented-surface. The
rapid temperature variations used to generate signals require a better
understanding of reaction kinetics on these materials. Surface analytical
techniques combined with electrical measurements are used to address these
issues.