CMOS Single Chip Gas Detection Systems — Part I

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.