AbstractIn this paper, a CMOS condenser microphone with high sensitivity and low polarization voltage was designed, simulated and fabricated. Due to CMOS process temperature variant and lattice defects, the poly-membrane would be invoked normal stress and gradient stress. These two residual stresses would deform the membrane and increase the membrane's rigidity. For these concerns, an interlace slots design is utilized to reduce the normal stress up to 90%, and the annealing process is applied to decrease the gradient stress. The acoustical sensitivity was increased considerably to -45dBV at 2.7V bias voltage, and, the noise level is -85dBV at 1KHz

Chen, Jien-Ming

Kuo, Chin-Fu

Wu, Chia-Yu

Elsevier - Publisher Connector 

Proceedings of the Eurosensors XXIII conference 
Low Polarization Voltage and High Sensitivity CMOS Condenser 
Microphone Using Stress Relaxation Design 
Chia-Yu Wu, Jien-Ming Chen, Chin-Fu Kuo  
South Branch/Industrial Technology Research Institute Taiwan, R.O.C. 
Abstract 
In this paper, a CMOS condenser microphone with high sensitivity and low polarization voltage was designed, simulated and 
fabricated. Due to CMOS process temperature variant and lattice defects, the poly-membrane would be invoked normal stress 
and gradient stress. These two residual stresses would deform the membrane and increase the membrane’s rigidity. For these 
concerns, an interlace slots design is utilized to reduce the normal stress up to 90%, and the annealing process is applied to 
decrease the gradient stress. The acoustical sensitivity was increased considerably to -45dBV at 2.7V bias voltage, and, the noise 
level is -85dBV at 1KHz.  
 Keywords: CMOS, microphone, gradient stress, stress release 
1. INTRODUCTION  
Over the last few decades, there has been a dramatic increase in the number of publication on MEMS 
Microphone research. The study of microphone fabrication has apparently moved from order MEMS process to 
standard CMOS process. Silicon micromachining technique had been successfully applied to the fabrication of 
miniature microphones on silicon wafer. The Knowles SiSonicTM microphone has been successfully 
commercialized for using in cell phones, cameras and other high volume consumer electronics. One major 
advantage that MEMS microphones over traditional microphones is the capability of handling the heat produced 
during the reflow soldering processes on a typical surface mount production line. However,  the MEMS microphone 
is wire bond connected to a ASIC chip that will induce additional parasites, The motivation of monolithic 
integration of sensors with electronic part comes form four requirements, lowing the manufacturing cost and 
simplifying total process, solving interconnect bottleneck, miniaturizing total size and reducing the parasitic 
capacitance. One example is the accelerometer made by Analog Device [1]. A piezoelectric microphone with on-
chip CMOS amplifier [2] had been demonstrated. The square diaphragm with the size 2mm on a side are fabricated 
by combining a  silicon- nitride diaphragm with ZnO thin film layers. The measured sensitivity is 80uV/Pa at 3KHz 
and the frequency range is form 800Hz to 30KHz. The capacitive sensing is more widely used to detected 
membrane vibration Condenser microphone has benefits of high sensitivity, flat frequency response, and low noise 
level [3]. A FET condenser microphone with sensitivity up to 38mV/Pa had been reported in [4]. Basically, the 
sensitivity of microphone can be boosted by increasing membrane size and bias voltage, reducing the air gap, and 
relieving the residual stress. However, it is difficult to eliminate the thin film residual stresses, especially those of 
CMOS process. It is straightforward to release the residual stress through the boundary of structures [5] 
 
 
Procedia 
Chemistry  
www.elsevier.com/locate/procedia
1876-6196/09 © 2009 Published by Elsevier B.V. 
Procedia Chemistry 1 (2009) 859–862 
doi:10.1016/j.proche.2009.07.214
Open access under CC BY-NC-ND license.
  
variant 、higher polarization voltage and large diaphragm area. But when we consider the THD and stability, the 
gap and polarization voltage would become tradeoff terms. The gap in a MEMS microphone is much smaller so the 
electric field is on the order of 2-4 V/um [7]. This means that to build a similar sensitivity the diaphragm compliance 
of the MEMS microphone must larger than that of ECM diaphragm.  
 
 
ps
sb
m
o CC
C
d
V
k
AS += σ         (1) 
3. MICROPHONE DESIGN AND RESULT 
Condenser microphone has the benefits of high sensitivity, flat frequency response, and low noise level. The 
sensitivity of microphone could be boosted by increasing membrane size and bias voltage, reducing the air gap and 
relieving the residual stress. However, the residual stress is difficult to eliminate especially in CMOS process. The 
best way to liberate the stress is to free the boundary. The novel design of condenser microphone with interlaced 
slots for stress relieving was presented. The diagram of our interlace slots design is shown in Fig.1. The radius and 
angle of the slots are two important parameters that determine the effect of the stress releasing. So the normal stress 
induced by the foundry process can be effectively relieved. The design parameters are listed in table 1. According to 
finite element simulation results, the novel interlaced slots structures have the capability of relieving the normal 
stress up to 90% [8]. This technique can not only enhance performance of microphone but also minimize the effect 
of process variation. Gradient stress would become an uncertain effect when the overlap slots were released. 
Because the membrane with slots edge can be treated as free boundary, the gradient stress of membrane would make 
the membrane convex or concave. In order to have flat plate, we use the monitor key [9] to evaluate the gradient 
stress, and using Coventorware to evaluate the membrane deformation in Fig.2. Hence we can use this FEM model 
to define the acceptable resonated frequency and stress range by modifying the annealing temperature of process in 
Fig3. The novel condenser microphone was implemented by using MXIC 2P2M 0.5μm CMOS process and post 
process. As shown in Fig4, the diaphragm and back-plate of microphone are realized by Poly2 and Metal layers, and 
their thicknesses are 0.3μm and 0.9μm, respectively. The gap between those two layers is 1.4μm. Then the acoustic 
back chamber was formed by back side ICP etching. Finally the sacrificial oxide layer was removed by Silox Vapox 
III solution and thus the microphone structure was released. The membrane profile was measured by interferometer 
is shown in Fig.5. Combining the normal stress and gradient stress which we measured into simulation tool, we can 
minimize the inaccuracy between simulation and experiment below 10%. To measurement the performance of the 
fabricated microphone, the microphone and interface ASIC were integrated by probing and characterized in the 
wafer level acoustic test system. With the polarization voltage increasing form 2.5V to 2.7V, the microphone 
sensitivity at 1KHz tone rises form-56dBV to -45dBV. The background noise duo to open space is about -85dB, 
according to the experience, the noise should be smaller than -95dB at the anechoic chamber. And die level anechoic 
chamber testing is ongoing. Fig.6(a) and Fig.6(b) show the measured results.  The results indicate the way to design 
and fabricate of high sensitivity and low polarization CMOS condenser microphones. 
 
 
Table 1: Design parameters. 
 Diameter Slot width
Slot 
gap
Slot 
Plurality Thickness
1st 
Frequency
Ring 
type 350um 1um 5um 24 0.3um 40KHz 
 
2. CONDENSER MICROPHONE OVERVIEW 
 
The condenser consists of a high compliant diaphragm, a perforated but rigid backplate, and a back chamber, The 
acoustic holes in backplate allow the air between the diaphragm and the blackplate freely moving. The capacitance 
was polarized by a voltage source to a high impendence amplifier. If the distance between diaphragm and the 
backplate is d and the sound pressure altered sensing capacitance is Cs and the parasitic capacitance is Cp , the area 
of diaphragm is A, σis membrane intrinsic stress, Km is equivalent spring constants of diaphragm, and Vb is 
polarization voltage. Form this governing equation[6] we can conclude that high sensitivity can be designed by low 
intrinsic stress、low diaphragm spring constants、small gap、low parasitic capacitance and large capacitance 
J. Courbat et al. / Procedia Chemistry 1 (2009) 859–862 860
   
(a)
<1MPa 100MPa
                                             (b)  
Fig.1 (a) 1/12 simply Ring type FEM stress relaxing design (b) half model cross section  
  
Fig. 2 Coventorware simulation result show the center deflection is 0.63um 
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100
50MPa_normal_-
25MPa_gradient 
-50MPa_gradient
50MPa_normal
-50MPa_normal_-
50MPa_gradient
KHz
nm Membrane compliance & 1st resonance frequency
 
Fig. 3 Coventorware simulation results show the center compliance with membrane resonated frequency in difference stress condition. 
(a)
-
Vsignal
Cmic
Cpara
Vbias
poly
metal
              (b)  
Fig. 4: (a)The diaphragm of Microphone cross section (b) SEM picture of fabricated microphone 
Center deflection is 
0.63um 
J. Courbat et al. / Procedia Chemistry 1 (2009) 859–862 861
  
0.6um
  
Fig. 5 White light interferometer measurement data, the center deflection of membrane is 0.6um 
 
(a)         (b)   
Fig. 6(a): The diagram indicates signal tone response of Ring-type design is -56dBV at 2.5V polarization voltage. Green line: 2.5V bias 1KHz 
single tone excited. Orange line: 2.5V bias . Red line: 0V bias 1KHz signal tone excited  (b): The diagram indicates signal tone response of Ring-
type design is -45dBV at 2.7V polarization voltage, we can see some nonlinearity tone was excited. Green line: 2.7V bias 1KHz single tone 
excited. Orange line: 2.7V bias. Red line: 0V bias 1KHz signal tone excited 
Acknowledgements 
The authors would like to thank the technical staff at MSTC for processing and measurement the microphone  
References 
1. Analog Device, Inc., Norwood,MA, www.analog.com 
2. E. S. Kim, R.S.Muller, and P.R.Gary, L. Midjord, and T. S. Larsen, Journal of Microelectromechanical Systems, Vol. 12,NO. 6, Dec. 
2003 
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Systems, Vol. 12,NO. 6, Dec. 2003 
4. E.Graf, W.Kronast, S. Duhring, B. Muller, and A. Stoffel, Sensors and Actuators A, Vol.37 -38, pp.708-711, 1993 
5. US5490220 : Feb.06,1996, Inventors – Peter V. Loeppert, Hoffman Estates. 
6. A. Mizoguchi, J.Acoust., J. Acoust. Soc. Jpn., vol. 31, no. 10, pp. 593–601, 1975, pp. 593-601 
7. Peter V. Loeppert, and S.B. Lee, Solid-State Sensors and Actuators and Microsystems Workshop Hilton Head Island, South 
Carolina,June4-8, 2006 
8. C.Y.Wu, J.M.Chen and J.Y.Chen,Eurosensors 2008 proceeding, M2-04.15,pp.81,2008  
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-45dBV -56dBV 
862 J. Courbat et al. / Procedia Chemistry 1 (2009) 859–862 


Low Polarization Voltage and High Sensitivity CMOS Condenser Microphone Using Stress Relaxation Design 

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Low Polarization Voltage and High Sensitivity CMOS Condenser Microphone Using Stress Relaxation Design

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