Monolithic CMOS MEMS Oscillator Circuit for Sensing in the Attogram Range

Abstract

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter presents the design, fabrication, and demonstration of a CMOS/microelectromechanical system (MEMS) electrostatically self-excited resonator based on a submicrometer-scale cantilever with <formula formulatype="inline"> <tex>$\sim$</tex></formula>1 ag/Hz mass sensitivity. The mechanical resonator is the frequency-determining element of an oscillator circuit monolithically integrated and implemented in a commercial 0.35-<formula formulatype="inline"> <tex>$\mu$</tex></formula>m CMOS process. The oscillator is based on a Pierce topology adapted for the MEMS resonator that presents a mechanical resonance frequency of <formula formulatype="inline"><tex>$\sim$</tex></formula>6 MHz, a relative low quality factor of 100, and a large motional resistance of <formula formulatype="inline"><tex>$\sim$</tex> </formula>25 M<formula formulatype="inline"><tex>$\Omega$</tex></formula>. The MEMS oscillator has a frequency stability of <formula formulatype="inline"><tex>$\sim$</tex></formula>1.6 Hz resulting in a mass resolution of <formula formulatype="inline"><tex>$\sim$</tex></formula>1 ag (1 ag <formula formulatype="inline"><tex>$= \hbox{10}^{ - 18}$</tex></formula> g) in air conditions. </para>