CMOS Single‐chip Gas Detection Systems: Part II

Abstract

Sensor arrays based on industrial CMOS‐technology combined with post‐CMOS micromachining (CMOS MEMS) are a promising approach to low‐cost sensors. In the first part of this article [1], the state of research on CMOS‐based gas sensor systems was reviewed, and a platform technology for monolithic integration of three different transducers on a single chip was described. In this second part, the transduction principles of three polymer‐based gas sensors are detailed and the read‐out circuitry is portrayed. The first transducer is a micromachined resonant cantilever. The absorption of analyte in the chemically sensitive polymer causes shifts in resonance frequency as a consequence of changes in the oscillating mass. The cantilever acts as the frequency‐determining element in an oscillator circuit, and the resulting frequency change is read out by an on‐chip counter. The second transducer is a planar capacitor with polymer‐coated interdigitated electrodes. This transducer monitors changes in the dielectric constant upon absorption of the analyte into the polymer matrix. The sensor response is read out as a differential signal between the coated sensing capacitor and a passivated reference capacitor, both of which are incorporated into the input stage of a switched capacitor second‐order ΣΔ‐modulator. The third transducer is a thermoelectric calorimeter, which detects enthalpy changes upon ab‐/desorption of analyte molecules into a polymer film located on a thermally insulated membrane. The enthalpy changes in the polymer film cause transient temperature variations, which are detected via polysilicon/aluminum thermocouples (Seebeck effect). The small signals in the μV‐range are first amplified with a low‐noise chopper amplifier, then converted to a digital signal using a ΣΔ‐A/D‐converter and finally decimated and filtered with a digital decimation filter.