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. 2018 Oct 19;18(10):3545.
doi: 10.3390/s18103545.

Design and Modeling of a MEMS Dual-Backplate Capacitive Microphone with Spring-Supported Diaphragm for Mobile Device Applications

Néstor N Peña-García  1 , Luz A Aguilera-Cortés  2 , Max A González-Palacios  3 , Jean-Pierre Raskin  4 , Agustín L Herrera-May  5   6
Affiliations

Design and Modeling of a MEMS Dual-Backplate Capacitive Microphone with Spring-Supported Diaphragm for Mobile Device Applications

Néstor N Peña-García et al. Sensors (Basel). .

Abstract

New mobile devices need microphones with a small size, low noise level, reduced cost and high stability respect to variations of temperature and humidity. These characteristics can be obtained using Microelectromechanical Systems (MEMS) microphones, which are substituting for conventional electret condenser microphones (ECM). We present the design and modeling of a capacitive dual-backplate MEMS microphone with a novel circular diaphragm (600 µm diameter and 2.25 µm thickness) supported by fifteen polysilicon springs (2.25 µm thickness). These springs increase the effective area (86.85% of the total area), the linearity and sensitivity of the diaphragm. This design is based on the SUMMiT V fabrication process from Sandia National Laboratories. A lumped element model is obtained to predict the electrical and mechanical behavior of the microphone as a function of the diaphragm dimensions. In addition, models of the finite element method (FEM) are implemented to estimate the resonance frequencies, deflections, and stresses of the diaphragm. The results of the analytical models agree well with those of the FEM models. Applying a bias voltage of 3 V, the designed microphone has a bandwidth from 31 Hz to 27 kHz with 3 dB sensitivity variation, a sensitivity of 34.4 mV/Pa, a pull-in voltage of 6.17 V and a signal to noise ratio of 62 dBA. The results of the proposed microphone performance are suitable for mobile device applications.

Keywords: FEM model; Sandia Ultra-Planar Multi-level MEMS Technology V (SUMMiT V) fabrication process; capacitive microphone; dual backplate; electret condenser microphones; spring-supported diaphragm.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Schematic view of a MEMS microphone with the sound port in the substrate.
Figure 2
Figure 2
3D view of the MEMS microphone formed by (a) circular diaphragm and two backplates with holes and (b) springs array located on the diaphragm edge; (c) Detail of the cross-section view of the differential capacitive MEMS microphone.
Figure 3
Figure 3
Schematic view of the microphone design based on the SUMMiT V surface-micromachining process. (a) Anchor structure of the diaphragm and backplates, and electrical connection of the (b) diaphragm and (c) bottom backplate of the microphone.
Figure 4
Figure 4
Deflection of the microphone diaphragm.
Figure 5
Figure 5
(a) Geometrical parameters of the polysilicon spring and (b) schematic representation of a partial section of the out surface of the microphone diaphragm.
Figure 6
Figure 6
Schematic view of the deflections of the (a) spring and (b) diaphragm used in the model of the MEMS microphone.
Figure 7
Figure 7
Spring and piston model of the diaphragm.
Figure 8
Figure 8
Model of the microphone capacitor.
Figure 9
Figure 9
Model of the deformed capacitor.
Figure 10
Figure 10
(a) Cross-section view of a blackplate hole model with squeeze-film damping and damping through hole; (b) 3D view of the blackplate hole model and (c) its dimensions.
Figure 11
Figure 11
(a) Acoustic resistance and (b) equivalent hydraulic circuit of the MEMS microphone.
Figure 12
Figure 12
Elements of the electroacoustic lumped model of the MEMS microphone.
Figure 13
Figure 13
Electroacoustic lumped model of the MEMS microphone.
Figure 14
Figure 14
Sensitivity of the MEMS microphone diaphragm.
Figure 15
Figure 15
Thermomechanical noise model of the MEMS microphone.
Figure 16
Figure 16
Electrical noise model of the charge amplifier.
Figure 17
Figure 17
Theoretical frequency response of the MEMS microphone. The first (31 Hz) and third (27 kHz) red dots represent the bandwidth, and the second (15.8 kHz) red dot indicates the resonant frequency of the microphone.
Figure 18
Figure 18
Mesh of the FEM model of the microphone diaphragm obtained through ANSYS Workbench software.
Figure 19
Figure 19
The first four vibration modes of the FEM model of the microphone diaphragm: (a) f1 = 21.657 kHz; (b) f2 = 32.891 kHz; (c) f3 = 68.232 kHz and (d) f4 = 94.674 kHz. The red and blue surfaces represent the maximum and minimum displacements, respectively.
Figure 20
Figure 20
Displacements of the FEM model of the microphone diaphragm caused by a sound pressure of 30 Pa.
Figure 21
Figure 21
Maximum principal stress on the support springs and microphone diaphragm caused by a sound pressure of 30 Pa.
Figure 22
Figure 22
Mesh of a module formed by a hole and a section of the backplate, which is obtained through ANSYS APDL software.
Figure 23
Figure 23
Electrical potential distribution of the proposed module: (a) 3D view and (b) cross-section view.
Figure 24
Figure 24
Deflections of the microphone diaphragm caused by a sound pressure of 30 Pa. These deflections are determined using analytical (AM) and FEM models.
Figure 25
Figure 25
Deflections (w0) of the microphone diaphragm center as a function of the sound pressure. These deflections are calculated with a bias voltage of 3 V and using the lumped element (LEM) and FEM models.
Figure 26
Figure 26
Relative error of the output voltage with a bias voltage of 3 V.
Figure 27
Figure 27
PSD of acoustic noise sources in the microphone.
Figure 28
Figure 28
PSD of the output voltage noise in the microphone and amplifier.
See this image and copyright information in PMC

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