This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 41.229.253.196This content was downloaded on 02/05/2015 at 09:22

Please note that terms and conditions apply.

Fabrication of a dual-planar-coil dynamic microphone by MEMS techniques

View the table of contents for this issue, or go to the journal homepage for more

2010 J. Micromech. Microeng. 20 065004

(http://iopscience.iop.org/0960-1317/20/6/065004)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 20 (2010) 065004 (7pp) doi:10.1088/0960-1317/20/6/065004

Fabrication of a dual-planar-coil dynamicmicrophone by MEMS techniquesRay-Hua Horng1,5, Kuo-Feng Chen2, Yao-Cheng Tsai3, Cheng-You Suen3and Chao-Chih Chang4

1 Department of Electro-Optical Engineering, National Cheng Kung University, Tainan, Taiwan,Republic of China2 Institute of Photonics and Communications, National Kaohsiung University of Applied Sciences,Kaohsing, Taiwan, Republic of China3 Institute of Precision Engineering, National Chung Hsing University, Taichung, Taiwan, Republic ofChina4 Taiwan Carol Electronics Co. Ltd, No. 202, Tung Kuang Road, Taichung, Taiwan, Republic of China

E-mail: rhhorng@mail.ncku.edu.tw

Received 19 February 2010, in final form 6 April 2010Published 11 May 2010Online at stacks.iop.org/JMM/20/065004

AbstractA dual-planar-coil miniature dynamic microphone, one of the electro-acoustic transducersworking with the principle of the electromagnetic induction, has been realized bysemiconductor micro-processing and micro-electro-mechanical system (MEMS) techniques.This MEMS microphone mainly consists of a 1 m thick diaphragm sandwiched by two spiralcoils and vibrating in the region with the highest magnetic flux density generated by a doublemagnetic system. In comparison with the traditional dynamic microphone, besides theminiaturized dimension, the MEMS microphone also provides 325 times the vibration velocityof the diaphragm faster than the traditional microphone. Measured by an audio analyzer, thefrequency response of the MEMS microphone is only 4.5 dBV Pa1 lower than that of thetraditional microphone in the range between 50 Hz and 20 kHz. The responsivity of54.8 dB Pa1 (at 1 kHz) of the MEMS device is competitive to that of a traditionalcommercial dynamic microphone which typically ranges from 50 to 60 dBV Pa1(at 1 kHz).(Some figures in this article are in colour only in the electronic version)

1. Introduction

A microphone is a type of transducer which can convertacoustic waves into electrical signals. One of the earliesttypes of microphones was introduced with the telephonesystem by Siemens in the 1870s [1]. Nowadays, microphoneshave been extensively employed in many applications suchas broadcasting, hearing aids, telecommunications and taperecording. In general, microphones can be classifiedinto three major types which are piezoelectric, capacitive,electromagnetic-induced microphones with different physicalmechanisms to convert sound vibrations into electricalsignals. Piezoelectric microphones produce electrical signalsby applying mechanical stress on piezoelectric materials. But5 Author to whom any correspondence should be addressed.

besides encountering the issues of the high output impedanceand the high self-noise level, piezoelectric microphones[2, 3] need the external preamplifier, which does require apower source, to reduce the equivalent noise level. Theforegoing drawbacks make piezoelectric microphones lesscompetitive than other well-developed ones. A condensermicrophone uses a variable parallel-plate capacitor to sensethe acoustic vibration changing the distance between twoplates and causing output voltage variation. Although, withhigh sensitivity and good frequency response, condensermicrophones become one of the mainstream choices; however,they are merely preferred in the applications of high-qualitystudio recording and the laboratory due to high self-noise,high internal impedance and the requirement of an externalpower supply to maintain a nearly constant electric chargeon two plates. Moreover, in the last decade, miniature

0960-1317/10/065004+07$30.00 1 2010 IOP Publishing Ltd Printed in the UK & the USA

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

output voltage

case

diaphragm

N SS

acoustic pressure

permanent magnet

coil

Figure 1. Schematic structure of the traditional dynamicmicrophone.

condenser microphones based on silicon micromachiningtechniques were the subject of research and development[4, 5]. Silicon-based condenser microphones are widelyrecognized as the next-generation product to replace theconventional electret condenser microphone (ECM). Theyhave potential advantages in terms of miniaturization, highsensitivity, low noise level, environmental endurance, massproduction capability and flat frequency responses in a widebandwidth [6]. However, miniature condenser microphonesstill have many disadvantages: they require a battery orexternal power supply to bias the plates and have highinternal impedance, among other things [7]. There is aloss of signal strength if a microphone is connected to aninput with lower impedance. Dynamic microphones, one ofelectromagnetic-induced microphones, work via the principleof electromagnetic induction using a moving coil to sensethe change of the magnetic field and produce the electricity.Dynamic microphones [8, 9] instead offer low self-noise andlow internal impedance and, particularly, have no preamplifier;thus, batteries or external power supplies are not required.However, the traditional dynamic microphone still suffers frominsensitivity due to the slow vibration velocity as a resultof the heavy diaphragm (16 m thick and 25 mm indiameter) and the non-integrated spiral moving coil verticallyattached to the diaphragm, which makes the whole devicequite unwieldy, as shown in figure 1. The letters N and Sin the figure show the two magnetic poles for generating themagnetic field as having moving coils immersed and sensingthe change of the magnetic field. In recent decades, with thedevelopment of semiconductor micro-processing and micro-electro-mechanical system (MEMS) technology, miniaturizedmicrophones [10, 11] have been the subject of research anddevelopment for the purposes of moderating the physicaldimensions, lowering the noise and increasing the vibrationvelocity.

In this paper, it is shown that the unwieldy disadvantagesof traditional dynamic microphones can be overcome by usingMEMS techniques to fabricate a thin vibrating diaphragm(only about 1 m thick) integrated with double planar spiralcoils. Obtaining a high electromotive force (emf) output is themost important index for the performance of a miniaturizeddynamic microphone. Therefore, according to Lenzs law ofinduction

emf = Blv, (1)

where B is the magnetic flux density, l is the total lengthof the coil and v is the vibration velocity of the coil; themaximized performance of the device can be obtained if thecoil with the highest vibration velocity moves up and downin the region with the highest magnetic flux density. Besidesproposing the MEMS fabrication of the diaphragm integratedwith the coil, the design of the magnetic circuits includingthe single magnetic circuit (SMC) and the double magneticcircuit (DMC), and the arrangement of the major componentassembly are also studied. Here, the commercial finite-element software COMSOL is used to simulate and analyzethe distribution of the magnetic field for finding the positionwith the highest B where the moving coil will be locatedafterward.

2. Fabrication of the MEMS diaphragm integratedwith dual moving coils

The moving-coil structure was fabricated on a p-type, double-side polished (1 0 0)-Si substrate 300 m thick. First, eachdevice is designed as a round shape with a diameter of 16 mm.The silicon surface is cleaned with the RCA cleaningprocedure, which is followed by the deposition of 600 nmsilicon nitride acting as an etching stop layer by inductivelycoupled plasma-enhanced chemical vapor deposition, asshown in figure 2(a). The first metal layer is patternedwith Ti/Al (1 m thick) by photolithography and thermalevaporation, which is immediately followed by a lift-offprocess, as shown in figure 2(b). The first metal layer consistsof the first spiral coil, the first semi-circular arc pad (SCAP)and the second SCAP. The first spiral coil with 22 windings hasan equal line and space (50 m/50 m). The two SCAPs act asthe connection pads. The first spiral coil is directly connectedwith the first SCAP, which is, however, disconnected with thesecond SCAP on purpose, as the first metal layer patterned.Subsequently, a photosensitive polyimide layer with 1 mthick was spun on the first spiral coil to create a diaphragm,and the two connection holes are exposed, as shown infigure 2(c). The second spiral coil (1 m thick and 50 m/50 m line/space) with the same fabrication process as thefirst metal layer is patterned on the top of the polyimide. Thetwo ends of the second spiral coil are connected to the firstspiral coil and the second SCAP via the connection holes, asshown in figure 2(d). Next, a thin seed layer is depositedon the circular edge of the diaphragm with 2 mm width, andelectroplated to the 90 m thick Cu as a supporting frame, asshown in figure 2(e). Consequently, the completed diaphragmstructure with spiral coils is released from the Si substrate bywet etching, as shown in figure 2(f ).

The schematic layouts and the alignment relationship ofthe three major components (the first/second coil layers andthe diaphragm) are shown in figure 3(a), which can be fullyunderstood at a glance for the MEMS miniaturized device.The outer of the first coil is directly connected to the firstSCAP at the same metal layer, and the inner of the first coil isconnected to the inner of the second coil via the near centralconnection hole of 300 300 m2 on the diaphragm. Theouter of the second coil is connected to the second SCAP via

2

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

SiNx

Silicon (100)

1st Spiral Coil1st SCAP 2nd SCAP

Connection HolesPolyimide

Connection Pad2nd Spiral Coil

Circular Supporting Frame

Signal Output

(a)

(b)

(c)

(d )

(e)

( f )

Figure 2. Schematic fabrication processes of the MEMS dynamicmicrophone: (a) silicon nitride deposition, (b) patterning of the firstmetal layer, (c) formation of the diaphragm, (d) patterning of thesecond metal layer, (e) formation of the supporting frame and(f ) removal of the substrate.

the edge connection pad of 600 1200 m2. The dashed linesdescribe the alignment relationship for the three major layers.The possible misalignment is, thus, negligible for thecapability of the contact aligner due to the huge areas ofthe connection pads. Eventually, the two SCAPs will actas the signal outputs. Figure 3(b) schematically shows the

integration of the three layers. The semi-transparent SCAPsunderneath the diaphragm can be observed. The first coilis visually blocked by the second coil if it is viewed fromthe top. The photograph of the portion of the MEMSminiaturized dynamic microphone is shown in figure 3(c),which is corresponding to figures 3(b) and 2(d) prior to theformation of the supporting frame. The portions of the brightsecond coil and the SCAPs can be seen and consistent withfigure 3(b). There is the bright arc line between the SCAPsand the coil as a result of the edge reflection of the developedphotoresist for patterning the supporting circular frame.Figure 3(d) shows the photograph of the MEMS microphonewith the supporting frame, which is corresponding tofigure 2(e). The SCAPs are visually blocked by the 2 mmwide supporting frame.

3. Design of magnetic circuits

In order to immerse the coil in the region with the highestmagnetic flux density to maximize the device performance,the distribution of the magnetic field is also simulated with theCOSMOL software which is designed to obey the boundaryconditions of Maxwells equations for magnetism. Maxwellsmagnetostatic equations can be written as

H = J (2) B = 0, (3)

where H is the magnetic field intensity, J is the current densityand B is the magnetic flux density. Maxwells equationsbecome definite when constitutive relations between the fieldquantities are specified. For an isotropic medium in theabsence of permanent magnetization, the relation can beexpressed by

B = 0rH, (4)where 0 is the permeability of the vacuum and r is therelative permeability of the material. For any vector A, ( A) = 0, equation (3) can be satisfied by

H = B. (5)Therefore, these equations yield the second-order differentialequation:

((0r)1( H)) = J. (6)Assume that current is parallel to the z-axis. The two-dimensional magnetostatic field problems are governed withthe two-dimensional Poisson equation:

x

(1

0r

H

x

)+

y

(1

0r

H

y

)= J. (7)

Therefore, the magnetic potential can be solved by using thefinite element method, while the magnetic flux density can becomputed from equation (5) and becomes

B = (Bx, By) =(

H

y,H

x

). (8)

In this study, the magnetostatic simulation models were builtin COMSOL R in two dimensions. In order to implement abetter description of the problem, the following assumptionsare made:

3

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

(a) (b)

(c) (d )Figure 3. (a) Layout of the diaphragm and coil. (b) The schematic diaphragm integrated with coils. The photograph of the fabricated device(c) without and (d) with the supporting frame.

(1) permanent flux and relative permeability in permanentmagnets is 5 kGauss and 1, respectively;

(2) materials in the soft magnet have a nonlinear relationbetween the magnetic flux and the magnetic field; thus,the so-called BH curve is introduced as an interpolationfunction;

(3) magnetic insulation is set at the exterior boundary of theair domain.The designs of the SMC and DMC are proposed in this

paper. First, the photographs of the face-up (left) and theface-down (right) SMCs are shown in figure 4(a). An SMCconsists of a hollow cylindrical magnet (made of NdFeB)which is mounted into a cylindrical concave yoke aligned wellwith the same cylindrical axis. The right-hand side image infigure 4(a) is the face-down SMC with three inlets openedfor incoming sound waves. The schematic cross-sectionaldiagram of SMC with dimensions is shown in figure 4(b). Thecylindrical magnet has an outer diameter of 6 mm and height of3 mm. The concave yoke has an outer diameter of 12 mm andheight of 4 mm. The gap between the magnet and the yoke is1 mm, so the diameter of the magnet is hollow. Instead, a DMCconsists of two similar SMCs with face-to-face arrangement,typically separated by a gap of 12 mm. Figures 5(a) and (b)show the simulated spatial distribution of the magnetic field ofthe SMC and DMC, respectively. The x-axis (red line) for theSMC is placed 0.5 mm above the top edge of the SMC. Thex-axis for the DMC is placed at the middle of the two SMCs.The diaphragm will be placed along the x-axis for component

(b)

Magnet Yoke Inlet for incoming sound waves

(a)

Figure 4. (a) Photograph of a face-up SMC (left) and a face-downSMC (right). (b) Schematic cross-sectional diagram for the SMCstructure.

assembly. The two same face-to-face SMCs in the DMC areseparated by 1 mm from each other. The origin of the x-axes

4

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

(a)

(b)

Figure 5. Simulated spatial distributions of the magnetic field for the (a) SMC and (b) DMC.

starts from the left edge of both SMC and DMC. The curvelines in the figures present the magnetic flux which can beanalogous to the electric current for electricity. Then, the moremagnetic flux lines flowing through a certain cross-sectionalarea will produce higher magnetic flux density. Therefore,the region with the densest distribution of the magnetic fluxon a certain area is preferred to be selected as the location ofthe moving coil. Approximately, the highest magnetic fluxdensity from figure 5 occurs at 3 and 9 mm on the x-axis.Figure 6 quantitatively shows the distributions of the magneticflux density of the SMC and DMC as a function of the x-axis.Both curves are symmetrical to the center of the cylindricaldevice at x = 6 mm. The maximal magnetic flux densitiesfor both SMC and DMC, occurring nearly at x = 3 and 9 mmas known in figure 5, are 0.2035 and 0.3747 T, respectively,and also located in the range of the moving coil (x = 1.23.375 mm and x = 8.62510.8 mm). The two small side lobes(0.075 T) of DMC near the cylindrical center are producedby the repellence from the same magnetic poles, which providequite a weak influence on the moving coils due to the improperposition. The full widths at half maxima (FWHM) for the SMCand DMC are approximately 4.2 and 2 mm, respectively. Inspite of the wider FWHM for the SMC, the lower magneticflux density makes the SMC undesirable for the choice ofthe magnetic system. Continuously, the preferable air gapbetween the two face-to-face SMCs of the DMC system is alsosimulated. The x-axis is placed at the middle of two SMCs.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 2 4 6 8 10 12x-axis (mm)

Ma

gnet

ic flu

x d

ensi

ty (T

)

SMCDMC

Figure 6. Distributions of the magnetic flux density of the (a) SMCand (b) DMC along the x-axis in a 2D plane model.

Figure 7 shows the distributions of the magnetic flux densitiesas a function of x with different air gaps of 1, 1.5 and 2 mm.The curves in figure 7 have the same properties as those infigure 6(b). The maximal magnetic flux densities with airgaps of 1, 1.5 and 2 mm are 0.3747, 0.3033 and 0.2523 T,respectively. The maximal value is decreased with theincreasing distance of the air gap. Adding the considerationof the assembly, the air gap of 1 mm is, therefore, chosenas the preferable distance for the DMC system in thispaper.

5

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

DMC System

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 2 4 6 8 10 12x-axis (mm)

Ma

gne

tic flu

x de

ns

ity (T

) Air gap=2.0 mmAir gap=1.5 mm

Air gap=1.0 mm

Figure 7. Simulated distributions of the magnetic flux density withvarious distances (1, 1.5 and 2 mm) of an air gap for the DMCsystem.

00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Mag

netic

flu

x d

ensi

ty (T

)

X axis (mm)

MDC System Air gap= 1mm

Gauss meter

Probe

12108642

Figure 8. Measured distribution of the magnetic flux density for theDMC system with a 1 mm air gap. The inset shows the setup formagnetic flux density measurement.

4. Measurement results and discussion

Figure 8 shows the typical measured magnetic flux as afunction of x with the air gaps of 1 mm for the DMC system.The measured system is shown in the inset of figure 8. Herethe probe of the Gauss meter was controlled by the stage withthe resolution of 0.01 mm. From the obtained result, themaximal magnetic flux is 0.419 T located 3.5 and 8.7 mm atx-position. The corresponding simulation data is 0.3747 T asshown in figure 7. The difference of the maximal magnetic fluxbetween simulation and measurement data could be due to thesimulated magnetic material parameters being different realmaterial parameters. However, the tendency of the magneticflux distribution is almost the same with the simulation.

The most important factor determining the performanceof the dynamic microphones is the frequency responsivity,a measure of inputoutput gain of a sensor or transducersystem. A B&K audio analyzer generating 1 Pa air pressureis used to measure the frequency response of the traditionaland MEMS dynamic microphones, as shown in figure 9.

100 1000 10000-80

-75

-70

-65

-60

-55

-50

-45

-40Frequency Response

Traditional dynamic microphone -52.6 dBV/Pa Miniature dynamic microphone -54.8 dBV/Pa

Res

pons

itivy

(dVB

/Pa)

Frequency (Hz)

LFT=90 Hz HFT=15 KHz

LFM=110 Hz

Figure 9. Frequency response of the MEMS dynamic microphonecompared with a traditional dynamic microphone.

The responsivities of the two microphones are comparedbetween 50 Hz and 20 kHz. The responsivity of thetraditional dynamic microphone is on average 4.5 dBV Pa1higher than that of the MEMS dynamic microphone. Theresponsivities of 52.6 and 54.8 dBV Pa1 are obtained atthe frequency of 1 kHz for the traditional and MEMS devices,respectively. The corresponding sensitivity is 2.3 mV Pa1and 1.8 mV Pa1 for the traditional and MEMS devices. Onthe other hand, the bandwidth (defined the responsivity lower10 dVB Pa1 as compared with the responsivity at 1 K)for the traditional device is from about low frequency (LFT)90 Hz to high frequency (HFT)15 kHz. It is worth noting thatthe MEMS device presents better high frequency responsivityas HF > 10 kHz. From this frequency response, although theMEMS device is quite competitive to the traditional device,there still exist many factors which can be analyzed and furtherimproved.

Table 1 shows the comparison of a variety of parametersbetween the traditional and MEMS devices. As mentionedin the very beginning, B, l and v are three major factors thatinfluence the device performance. The sizes of the coil anddiaphragm in table 1 directly affect the vibration velocity lof the coil, which is also measured under 1 Pa air pressureby a laser Doppler vibration system. The vibration velocitiesof the traditional device and MEMS device are 0.032 and10.4 m s1, respectively. The vibration velocity of theMEMS device is 325 times faster than that of the traditionaldevice, which is attributed to the much slighter coil anddiaphragm by MEMS techniques. The coil length of thetraditional device is about 22 times longer than that of theMEMS device as a result of the space limitation of the planarcoil. To solve the issue of the limited length, the multiplepolyimide layers can be adopted for the next generationdevelopment. The resistance per unit of the MEMS deviceis 7.7 times larger than that of the traditional device. It revealsthat the 1 m thick coil for the MEMS structure is not thickenough to provide competitively low resistance per unit. Themaximal magnetic flux density of the traditional device is twicethat of the MEMS device. In fact, the highest magnetic flux

6

J. Micromech. Microeng. 20 (2010) 065004 R-H Horng et al

Table 1. Comparison of the MEMS device and a traditional device in a variety of parameters.

Type of microphone Traditional MEMS

Diaphragm Diameter (mm) 25 16Thickness (m) 16 1Vibration velocity (m s1) 0.032 10.4

Coil Material Al Ti/AlLength (cm) 816.4 36.5Resistance () 563 193

Magnet Material NdFeB NdFeBSize: diameter (mm) thickness (mm) 13 2 6 3Maximal magnetic flux density (T) 0.8 0.4

Frequency response Responsivity (dBV Pa1) at 1 kHz 52.6 54.8

density in figure 5(b) is located at the top edge of SMC withx = 3 mm. Thus, for future work, the magnet might need tobe redesigned to have a shorter thickness than the yoke.

5. Conclusion

In summary, the major components of a miniature dynamicmicrophone including a diaphragm, a dual planar coil and asupporting frame were fabricated and integrated by MEMStechniques. The 1 m thick MEMS diaphragm with theintegrated coil provides the vibration velocity of 10.4 m s1,which is 325 times faster than that of the traditional structure.A DMC system is simulated to find out the maximal magneticflux density (0.3747 T) and the corresponding location forthe moving coil. Consequently, the frequency response ofthis newly designed MEMS miniature microphone exhibitsa responsivity of 54.8 dBV Pa1 (at 1 kHz), which iscompetitive to the compared traditional dynamic microphone,one of the commercialized microphones.

Acknowledgments

This work was supported by the National Science Counciland the Ministry of Education (Taiwan, Republic of China)under contract no 95-2622-E-005-014-CC3 and the ATU plan,respectively.

References

[1] Coile R C 1939 The development of modern microphonesJ. Acoust. Soc. Am. 11 163

[2] Royer M, Holmen J O, Wurm M A, Aadland O S and Glenn M1983 ZnO on Si integrated acoustic sensor SensorsActuators A 4 357

[3] Schellin R and Hess G 1992 A silicon subminiaturemicrophone based on piezoresistive polysilicon straingauges Sensors Actuators A 32 555

[4] Kronast W, Muller B, Siedel W and Stoffel A 2001 Single-chipcondenser microphone using Porous silicon as sacrificiallayer for the air gap Sensors Actuators A 87 18893

[5] Zou Q, Li Z and Liu L 1996 Design and fabrication of siliconcondenser microphone using corrugated diaphragmtechnique Microelecromech. Syst. 5 197204

[6] Goto M et al 2007 High-performance condenser microphonewith single-crystalline silicon diaphragm and backplateIEEE Sensors J. 7 410

[7] Pederson M, Olthuis W and Bergveld P 1997 A siliconcondenser microphone with polyimide diaphragm andbackplate Sensors Actuators A 63 97104

[8] Yoshida S 1984 Dynamic microphone US Patent 4427845[9] Uzawa S and Kikuti Yoshio 1992 Diaphragm of dynamic

microphone US Patent 5148492[10] Miao J, Lin R, Chen L, Zou Q, Lim S Y and Seah S H 2002

Design considerations in micromachined siliconmicrophones Microelectron. J. 33 2128

[11] Tanijma T, Nishiguchi T, Chiba S, Morita A, Abe Mand Tanioka K 2003 High-performance ultra-small singlecrystalline silicon microphone of an integrated structureMicroelectron. Eng. 6768 50819

7

1. Introduction2. Fabrication of the MEMS diaphragm integrated with dual moving coils3. Design of magnetic circuits4. Measurement results and discussion5. ConclusionAcknowledgmentsReferences