CMOS Monolithic Metal–Oxide Gas Sensor Microsystems

This paper presents two mixed-signal monolithic gas sensor microsystems fabricated in standard 0.8-
$muhbox m$
CMOS technology combined with post-CMOS micromachining to form the microhotplates. The on-chip microhotplates provide very high temperatures (between 200
$^circ$
C and 400
$^circ$
C), which are necessary for the normal operation of metal–oxide sensing layers. The first microsystem has a single-ended architecture comprising a microhotplate (diameter of 300
$muhbox m$
) and a digital proportional-integral-derivative (PID) microhotplate temperature controller. The second microsystem has a fully-differential architecture comprising an array of three microhotplates (diameter of 100
$muhbox m$
) and three digital PID microhotplate temperature controllers (one controller per microhotplate). The on-chip digital PID temperature controllers can accurately adjust the microhotplate temperatures up to 400
$^circ$
C with a resolution of 2
$^circ$
C. Further, both microsystems feature on-chip logarithmic converters for the readout of the metal–oxide resistors (which cover a measurement range between 1
$hbox kOmega$
and 10
$hbox MOmega$
), 10-bit A/D converters, anti-aliasing filters, 10-bit D/A converters,
$I ^2 C$
serial interfaces, and bulk-chip temperature sensors. Carbon monoxide (CO) concentrations in the sub-parts-per-million (ppm) range are detectable, and a resolution of 0.2 ppm CO has been achieved.