A Novel MEMS Capacitive Microphone with Semiconstrained Diaphragm Supported with Center and Peripheral Backplate Protrusions

Abstract

Audio applications such as mobile phones, hearing aids, true wireless stereo earphones, and Internet of Things devices demand small size, high performance, and reduced cost. Microelectromechanical system (MEMS) capacitive microphones fulfill these requirements with improved reliability and specifications related to sensitivity, signal-to-noise ratio (SNR), distortion, and dynamic range when compared to their electret condenser microphone counterparts. We present the design and modeling of a semiconstrained polysilicon diaphragm with flexible springs that are simply supported under bias voltage with a center and eight peripheral protrusions extending from the backplate. The flexible springs attached to the diaphragm reduce the residual film stress effect more effectively compared to constrained diaphragms. The center and peripheral protrusions from the backplate further increase the effective area, linearity, and sensitivity of the diaphragm when the diaphragm engages with these protrusions under an applied bias voltage. Finite element modeling approaches have been implemented to estimate deflection, compliance, and resonance. We report an 85% increase in the effective area of the diaphragm in this configuration with respect to a constrained diaphragm and a 48% increase with respect to a simply supported diaphragm without the center protrusion. Under the applied bias, the effective area further increases by an additional 15% as compared to the unbiased diaphragm effective area. A lumped element model has been also developed to predict the mechanical and electrical behavior of the microphone. With an applied bias, the microphone has a sensitivity of -38 dB (ref. 1 V/Pa at 1 kHz) and an SNR of 67 dBA measured in a 3.25 mm × 1.9 mm × 0.9 mm package including an analog ASIC.

Keywords: MEMS; capacitive microphone; center protrusion; effective area; finite element modeling; peripheral protrusion; reduced order modeling; serpentine spring.

Figure 1

(a) A schematic cross-sectional view of a generic Knowles MEMS microphone package. (b) A top–down optical image of the MEMS die collected using a microscope. The movable diaphragm is suspended with eight compliant springs along the perimeter. The center and eight peripheral posts on the backplate prevent the movable diaphragm from electrostatic collapse onto the backplate.

Figure 2

SEM images of critical features of the proposed design. (a) Top view of the MEMS microphone die, (b) serpentine spring design, (c) perforated backplate with antistiction bumps, and (d) backplate anchor design to substrate.

Figure 3

Proposed design fabrication sequence: (a) 1st sacrificial layer deposition and polysilicon deposition for diaphragm with dimples. (b) Sensing gap formation with sacrificial layer deposition and patterning. (c) Si₃N₄ deposition for formation of backplate with polysilicon electrode with a center and peripheral protrusions. (d) Backplate patterning, formation of the back cavity, and gold metal pads with sacrificial layer release.

Figure 4

A reduced order model for a single-motor MEMS microphone.

Figure 5

Lumped element representation of a distributed system.

Figure 6

Defined boundary conditions and z−displacement field for: (a) constrained diaphragm and (b) spring−supported diaphragm with peripheral and center post under bias.

Figure 7

Lumped element representation of a system with distributed mass.

Figure 8

Experimental setup for the diaphragm compliance measurement using a scanning LDV.

Figure 9

Measurement and simulation of the diaphragm acoustic compliance vs. bias voltage for the semiconstrained with peripheral post and center post boundary condition. The polynomial fitting curve is for the simulation.

Figure 10

Hexagonal symmetry of the perforated backplate.

Figure 11

(a) Defined boundary condition for the 1/6th unit cell model with Si₃N₄ backplate and underlying polysilicon electrode. (b) Electric potential distribution with ground and terminal voltage. (c) Electric field distribution with fringing field effect.

Figure 12

(a) Electrostatic force as a function of the gap between the electrodes for solid and perforated plates. (b) Capacitance as a function of the gap for solid and perforated plates.

Figure 13

(a) Developed fine mesh for the full FEA model. (b) Developed mesh for the spring. (c) A 45-degree symmetry deflection model of the diaphragm under bias. (d) Full diaphragm deflection model showing a donut deflected shape under bias. (e) Capacitance as a function of bias voltage with collapse 50 V, parasitic capacitance not included. (f) Diaphragm deflection along the radius cross-section at different bias voltages.

Figure 14

The first four vibration modes of the FEA model of the microphone diaphragm: (a) $f_1=54.5$ kHz, (b) $f_{2 }=61.2$ kHz, (c) $f_3=132.4$ kHz, and (d) $f_4=204.5$ kHz. The red and blue surface profiles indicate regions of maximum and minimum deflections, respectively.

Figure 15

Simulation result of diaphragm deflection under the bias voltage as a function of sound pressure.

Figure 16

Simplified electroacoustic lumped model representing noise sources.

Figure 17

(a) Total damping obtained using port damping and radiation damping as function of frequency, and (b) total mass obtained using port mass and radiation mass as a function of frequency.

Figure 18

Cross-section of the velocity field in the FEM domain at 1 kHz. Red is associated with high velocity, and blue is associated with low velocity.

Figure 19

Measured frequency response along with the simulation.

Figure 20

Measured and simulated noise spectral density. The component-wise noise spectra are plotted together to show their contributions to the simulated noise spectrum in total.

Figure 21

Pie charts for the total acoustic noise contributors from (a) MEMS only and (b) a package including MEMS and ASIC.

Figure 22

Noise spectral density in air and vacuum with respect to frequency.

Figure 23

Measured THD curve.