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LINEAR MEMS DIFFERENTIAL CAPACITANCE SENSING SCHEME

IP.com Disclosure Number: IPCOM000009871D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2002-Sep-25
Document File: 6 page(s) / 495K

Publishing Venue

Motorola

Related People

Gary O'Brien: AUTHOR [+3]

Abstract

MEMS (micro electro-mechanical system) devices have been used previously to measure acceleration parallel to Cartesian coordinate axes (x, y, z). Similarly, rotational rates have been measured with respect to x, y, and z axes. MEMS acceleration sensors are typically comprised of differential capacitance parallel plates in which the dielectric gap is modulated as a function of applied acceleration. The sensor differential capacitance is signal conditioned using a CMOS control circuit. The CMOS control circuit translates the acceleration mechanical input signal from the sensor into an electrical output such as an output voltage (mV/g) or frequency (Hzlg).

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MOTOROLA

LINEAR MEMS DIFFERENTIAL CAPACITANCE SENSING SCHEME

by Gary O'Brien, David J. Monk and Bishnu Gogoi

INTRODUCTION

MEMS (micro electro-mechanical system) devices have been used previously to measure acceleration parallel to Cartesian coordinate axes (x, y, z). Similarly, rotational rates have been measured with respect to x, y, and z axes. MEMS acceleration sensors are typically comprised of differential capacitance parallel plates in which the dielectric gap is modulated as a function of applied acceleration. The sensor differential capacitance is signal conditioned using a CMOS control circuit. The CMOS control circuit translates the acceleration mechanical input signal from the sensor into an electrical output such as an output voltage (mV/g) or frequency (Hzlg).

DEFINITION OF PROBLEM

Parallel plate (capacitive) MEMS accelerometer sensing schemes typically experience an undesired electrostatic attraction force in the axis of proof mass travel. This scenario is labeled as "Case 1 " as shown in Figure I.

The undesired electrostatic force is inversely proportional to the square of the dielectric gap described by Equation 3.0 as shown in Figure I. The capacitance measurement for Case 1 acceleration sensing is inversely proportional to the dielectric gap given by c= !!.2!! as shown in Figure 1.

Z

The undesired electrostatic spring constant results in non-linear output error. This significantly limits the linear operating range of the sensor and generally requires higher circuit amplification to realize desired transducer sensitivity. As a consequence, the system signal-to-noise ratio is decreased adversely. Also, electrostatic latch-up can occur due to the non-linear forces inversely proportional to the instantaneous dielectric gap.

Motorola. Inc. 2000

The parallel plate capacitance scheme for Case 2, as shown in Figure 1, is modulated via Y axis movement. Although Case 2 does not directly suf fer from a parallel electrostatic force in the axis of desired proof mass motion, as Y becomes significantly smaller than Yo, the parallel plates will experience a vertical electrostatic force modulated by Y. The undesired vertical electrostatic force will modulate the Z axis reference dielectric gap.

As a result, a non-linear component will be coupled into the capacitance measurement versus Y axis displacement. Minimizing the non-linear effect, as described in Case 2, requires that the Y axis spring constant be significantly stiffer than Z the axis component. Significant differences in the Y axis versus Z axis spring constants are typically difficult to implement and involve a trade-off of sensitivity versus cross action rejection. However, if large proof mass travel with linear capacitance output is desired, Case 2 is a clear choice.

The differential MEMS capacitor is fabricated using multiple polysilicon mechanical layers as shown in Figure 2. The configuration shown in Figure 2 is for a rotational rate sensor. Note that the two plates in Figure...