Kinetically Controlled Chemical Sensing Using Micromachined Structures

Microdevices produced by machining silicon can offer a variety of control functionality on a miniature scale,1,2 with general advantages connected to not only reduced size (less invasive), but also lower power requirements, lower cost, and reproducibility of manufacturing. Micromachining has already made impacts in a number of technical areas, and it promises to be an increasingly important and broadly applicable technology. In the general field of sensors, micromachining has had its biggest impact, to date, in physical sensing. Commercialized devices include accelerometers and pressure sensors. This paper, however, focuses on the capabilities micromachined structures offer in the area of chemical sensing. It demonstrates how the inherently complex phenomena encountered in chemical sensing can be more completely probed using micromachined devices and arrays. The specific array devices described here consist of replicated low-mass suspensions we call “microhotplates”, where each element is covered with a sensing film. Two concepts are discussed. The first concept is the use of rapid heating of these low-mass devices to introduce “kinetic selectivity”. The second concept is the use of compositionally different surfaces and their varied interactions with gas phase molecules to introduce “materials selectivity”. While temperature control and the use of differing active materials (be they partially or highly selective) have been applied to chemical sensing for many years, what we wish to emphasize here is the enabling powers of these miniaturized structures and arrays in employing these concepts in entirely new ways. The approach provides a capability for “tuning”, through selection of film suites and operating programs, and thereby can address a wide range of gas and vapor sensing applications (in areas such as automated process control, environmental monitoring, and personal safety). The same generic base configuration can be used for varied applications, and fabrication aspects of the technology make it quite adaptable for manufacturing. While other transduction modes may be considered for micromachined chemical sensors, for example, capacitance changes or calorimetric effects, we will focus on conductometric sensing. The approach is based on the technology that underlies Taguchi sensors developed decades ago. In our case, however, the semiconducting oxide materials such as SnO2 and ZnO are applied as planar films, which are modified to enhance their sensing characteristics by surface-dispersing very low coverages of catalytic metals, for example, Pd and Pt. The paper is structured as follows. First we indicate the basis for thermally controlled sensing by describing the importance of temperature-dependent processes in solid-state transduction. We next indicate the approach employed to incorporate materials and kinetic selectivity to the microhotplates, and then describe the fabrication procedures as well as the characteristics of these micromachined devices. Self-lithographic chemical vapor deposition (CVD) is then presented as an efficient method for depositing active sensing films which lends itself readily to manufacturing. The next sections illustrate response characteristics (including temperature programmed analyte “signatures”) and an example of how training and predictive modeling are used to optimize performance. The closing sections then summarize benefits of the technology and indicate levels of adaptibility and research needs.