CMOS Monolithic Metal–Oxide Gas Sensor Microsystems

Abstract

This paper presents two mixed-signal monolithic gas sensor microsystems fabricated in standard 0.8- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$muhbox m$</tex> CMOS technology combined with post-CMOS micromachining to form the microhotplates. The on-chip microhotplates provide very high temperatures (between 200 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C and 400 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$muhbox m$</tex> ) 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$muhbox m$</tex> ) 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C with a resolution of 2 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C. Further, both microsystems feature on-chip logarithmic converters for the readout of the metal–oxide resistors (which cover a measurement range between 1 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$hbox kOmega$</tex> and 10 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$hbox MOmega$</tex> ), 10-bit A/D converters, anti-aliasing filters, 10-bit D/A converters, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$I ^2 C$</tex> 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.