Configurable electrodes for capacitive-type sensors and chemical sensors

This paper describes a novel design for capacitive sensors or chemical sensors, which features configurable interdigitated electrodes: The electrode spacing can be varied by means of switches on the CMOS chip. This new design allows for performing two capacitive measurements with one single-sensor capacitor so that the number of sensors required to acquire a certain amount of information can be significantly reduced. The use of the same sensor and the same polymer layer for two measurements at a different electrode periodicity provides a better signal quality for the difference signal since detrimental influences, such as humidity and sensor drift, are similar for both electrode configurations and are strongly correlated. Such high signal quality is required for, e.g., the successful recognition of n-octane in the presence of tenfold larger background signals of humidity or, in general, for the determination of low analyte concentrations in humid air. The baseline drift in the concentration predictions based on the differential signal from the two electrode configurations was an order of magnitude lower than that for uncorrelated signals produced by two separate interdigitated capacitors on the same chip. Since the number of required sensors is reduced and, owing to the differential readout of two electrode configurations, reference capacitors are no longer necessary, the overall chip size and/or the number of sensor chips and, consequently, costs can be considerably reduced.

I. Introduction

Gas sensors are increasingly used for applications such as process control, quality control, and environmental monitoring. Capacitive chemical sensors, which rely on changes in the dielectric properties of a sensing material upon analyte exposure, are promising devices for chemical sensing due to the low power consumption, which favors their usage in hand-held devices. For potential commercialization, low-cost production is essential and can be achieved, e.g., by miniaturization and integration—leading to the field of microsensors [1] [2]–[3] [4] [5] [6]. Integrating capacitive sensors in CMOS technology is especially interesting as no micromechanical parts such as membranes or cantilevers are needed, rendering the chip more rugged and reducing the number of post-processing steps and, hence, costs.