A Novel Approach to Design Dual Axis MEMS Capacitive Accelerometer

Authors:

Prashant Singh, Pooja Srivastava

Student, Dept. of Microelectronics

Indian Institute of Information Technology-Allahabad

Presented by:

Prashant Singh

Outline• Definition

• MEMS• Accelerometer

• Accelerometer classification• Capacitive Accelerometer• Accelerometer design• Proposed Accelerometer Model• Simulation Results

• Proof Mass Support Modeling• Accelerometer Modeling

• Results and Conclusion• References

Definition

• MEMS• Micro-Electro-mechanical-system• Integration of mechanical unit, electrical unit, sensor and

actuator on a single substrate.

• Accelerometer• Inertial sensor

• Newton’s 1st law (Mass of inertia)

• Used to measure: (i) Acceleration,

(ii) Displacement,

(iii) Force

(iv) Inclination angle

Accelerometer Classification

• Fabrication Technique: • Surface Micromachining- additive process• Bulk Micromachining- subtractive process

• Sensing Technique• Read out principle

• Displacement based: Capacitive

Tunneling

Optical

Hall effect

Thermal

Magnetic

• Stress based: Piezoelectric

Piezoresistive

Capacitive Accelerometer• Based on Change in capacitance between Comb fingers.

• {Capacitance change} α {Force applied on Proof Mass}• Comb structure Large capacitance value• Advantages

• High resolution• Good DC response• Linear output• low power dissipation• Easy incorporation with CMOS

Accelerometer Design• 2nd order Spring-mass-damper model

• Reference frame excited ‘Y’• Proof mass ‘X’ (due to inertia)• Basic system equation

Proposed Accelerometer Model• Modeling on COMSOL Multiphysics• Dual axis accelerometer

• Cantilever: Out-of-Plane• Spring: In-plane

Proposed accelerometer model

Simulation Results (proof mass support modeling)

• Proof mass support • Proof mass parameters

• Suspension Mass= 0.055896 µg• Spring Stiffness,

643.94kN/m

Parameters Beam (um) Spring (um)

Length 20 100

Width 10 5

Height 10 10

Resonance Frequency for Spring support• Resonance frequency depends only on Spring parameters.• Beam has no effect on resonance frequency.

• Resonance frequency=1.75 MHz • Bandwidth= 1 MHz

Spring support Modeling• Effect of Spring flexure height on free

point displacement• Effect of Spring flexure height on

Resonance frequency

Resonance Frequency for Beam support• Resonance frequency depends on beam as well as

Spring parameters.

• Resonance frequency=1.1 MHz • Bandwidth= 1 MHz

Spring support Modeling• Effect of Beam flexure height on

free point displacement• Effect of Beam flexure height on

Resonance frequency

Accelerometer Modeling• Accelerometer Parameters

Parameters Comb Proof mass

Length(µm) - 200

Width(µm) - 100

Finger length(µm) 50 -

Finger width(µm) 5 -

Finger overlap(µm) 2.5 -

Finger gap(µm) 40 -

Capacitor pair 52 -

Height 10 10

Accelerometer Modeling Contd..• Total Mass= Proof mass+ Comb finger mass+ Support mass

=0.892µg• Capacitance per finger pair (At rest),

1.41667nF• Total no. of capacitors = 52• Total capacitance value = 73.6668nF

Accelerometer Modeling Contd..• Frequency domain analysis

• To determine accelerometer resonance frequency.

• In-Plane resonance frequency- Transverse motion.• Spring support in use.

• Fr=0.3 MHz• BW= 0.4 MHz

Accelerometer Modeling Contd..• Out-of-Plane Resonance frequency.• Beam support in use.

• Fr= 0.32 MHz• BW= 0.25 MHz

Results and Conclusion• Dual Axis accelerometer is designed.• In-Plane operation

• Spring support in use.• Fr= 0.3 MHz• BW= 0.4 Mhz

• Out-of-Plane operation• Cantilever beam support in use• Fr= 0.32 MHz• BW= 0.25 MHz

• Application• Moderate frequency operation• Military

References • G.M. Rebeiz, J.B. Muldavin, "RF MEMS switches and switch

circuits“, Microwave Magazine, IEEE , vol.2, no.4, pp.59-71, 2001.• S. Pacheco, et al., Microwave and Optoelectronics Conference, pp.

770-777, 2007.• http://www.memsnet.org/mems/what-is.html.• B.V. Amini, F. Ayazi, “A 2.5-V 14-bit CMOS SOI capacitive

accelerometer”, IEEE Solid-State Circuits 39(12):2467–2476, 2004.• G. Kovacs, Micromachined Transducers Sourcebook; New York:

McGraw Hill, 1998.• Chi Yuan Lee, Guan Wei Wu, Wei Jung Hsieh, “Fabrication of

micro sensors on a flexible substrate”, Sensors and Actuators,(2008).

Thank You