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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 5, OCTOBER 2007 1173

A Highly Reliable Lateral MEMS Switch UtilizingUndoped Polysilicon as Isolation Material

Wendian Shi, Norman C. Tien, Member, IEEE, and Zhihong Li, Member, IEEE

AbstractThe lateral actuated switch requires an isolationstructure to provide mechanical coupling and electrical isolationbetween the actuator and the contacts. This isolation structureusually imposes extra difficulty on the fabrication process. In pre-vious reports, we demonstrated a thermal actuated lateral switch,where the nitride isolation structure was a weak point, leadingto reliability problems. In this paper, we developed a modifiedswitch utilizing undoped polysilicon as the isolation material. Theundoped-polysilicon isolation structure requires only one extrastep of sheltered implantation, and it provides robust mechanicalconnection. A 20-m-long undoped-polysilicon isolation structurehas a current leakage of less than 2 nA under a 15-V operation

voltage. The proposed switch works under a 12-V driving voltagewith 60-mW input power. The time response is measured to be130s, and a maximum operation frequency of 4.5 kHzis reached.An ON-state insertion loss of0.41 dB at 20 GHz and an OFF-stateisolation of20 dB at 20 GHz have been achieved on the normallow-resistivity silicon substrate. The undoped-polysilicon isola-tion method can be used in other surface-micromachined lateralswitches as well. [2007-0022]

Index TermsElectrical isolation, isolation structure, mechani-cal coupling, microrelay, radio frequency (RF) switch.

I. INTRODUCTION

THE LATERALLY actuated microelectromechanical sys-

tems (MEMS) switch has drawn more attention recentlydue to its in-plane design flexibility. The lateral switches can

be fabricated with the surface-micromachined polysilicon

process [1][3], the bulk-micromachined silicon process [4],

[5], the thick metal plating process [6], or other nonstandard

processes [7]. Various actuation approaches are also investi-

gated, including the electrothermal actuation [2][4], the elec-

trostatic actuation [8], the piezoelectric actuation [9], and the

electromagnetic actuation [10]. Among all these methods, the

electrostatic actuation and electrothermal actuation are most at-

tractive. The electrostatic actuation has the merits of low power

dissipation and high driving frequency, but a relatively high

actuation voltage is usually needed. In contrary, the thermalactuation offers the advantages of low driving voltage, high

driving force, and therefore low contact resistance [11], but the

Manuscript received November 24, 2006; revised April 2, 2007. This workwas supported by the National Nature Science Foundation of China underGrant 60528009. Subject Editor H. Zappe.

W. Shi and Z. Li are with the Department of Microelectronics, PekingUniversity, Beijing 100871, China (e-mail: [email protected]).

N. C. Tien is with the Department of Electrical Engineering and Com-puter Science, Case School of Engineering, Case Western Reserve University,Cleveland, OH 44106-7071 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2007.901121

high power consumption is a major drawback compared with

the electrostatic actuation [12], [13]. Therefore, the actuation

method should be selected according to the application require-

ments of the switches.

One major benefit of the lateral switch is the ability to

cofabricate the actuator, the contacts, the conductor paths, and

the supporting structures all in one single lithographic step [14].

However, an extra isolation structure is necessary for the me-

chanical coupling and electrical isolation between the actuator

and the contacts. It usually requires extra process steps or

special materials for this isolation structure.Reference [15] reported an isolation method of creating

insulative regions in conductive materials for the silicon-on-

insulator instruments. An inverse approach of creating con-

ductive regions in dielectric materials was developed for the

molded structures [16]. Both methods required combining

process steps including etching trenches, refilling dielectric/

conductive materials, and etching back to form the isolation

structure. To simplify the process, a maskless anisotropic etch

was used to form a dielectric sidewall for the isolation in the

thermal actuator [17], but this method could not avoid to form

dielectric sidewalls between the contact head and the signal

lines in lateral switches. Reference [10] reported a lateral switch

where a SiO2 layer was used as the isolation structure. Inthis case, the switch with the SiO2 layer was released with

an unusual epoxy sacrificial layer and oxygen plasma etch. It

is not suitable in other cases such as surfaced-micromachined

switches because the epoxy layer significantly limits the ther-

mal budget of the following processes. Photoresist was also

employed as the isolation material in a thermal actuated lateral

switch [1], but no detailed performance of the photoresist

isolation structure was shown.

In previous reports, we demonstrated a thermal actuated lat-

eral switch that has the advantages of low driving voltage, high

RF performance, and simple fabrication process [2], [3], [11].

This switch adopted a piece of low-stress silicon nitride film asthe isolation structure between the polysilicon actuator and the

contact head. The nitride isolation structure was a weak point,

which might cause reliability and yield problems, because

the nitride-polysilicon adhesion was not very strong. During

the operations of the switch, it was found that the nitride-

polysilicon adhesion was easy to break, particularly, when

contact force was increased to achieve low contact resistance.

Besides, the nitride-polysilicon interface might be attacked by

the hydrofluoric (HF) acid etching during structure release. The

SU8 photoresist has also been tried as the isolation material, but

the deformation that is caused by the stress mismatch was too

large to be tolerated.

1057-7157/$25.00 2007 IEEE


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1174 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 5, OCTOBER 2007

Fig. 1. Schematic overview of (a) the proposed switch and (b) the previously reported switch, and the cross-sectional comparison of (c) the undoped-polysiliconconnection and (d) the nitride connection.

Fig. 2. Schematic illustration of the loaded stress during the ON-state of the switch in the (a) push-type design and (b) pull-type design.

This paper proposes a modified thermal actuated lateral

switch that utilizes the undoped polysilicon as the isolation

material. The undoped-polysilicon isolation structure has the

advantages of a simple process, robust mechanical strength, and

high reliability. Due to this robust structure, a stable pull-type

actuation design and an optimized actuator can be employed

to reduce the switching time and power consumption. The

performances and lifetime of the switch are systematically

investigated.

II. DESIGN

A. Isolation Principle

Fig. 1 compares the schematic overview of the proposed mi-

croswitch [Fig. 1(a)] and the previously reported one [Fig. 1(b)]

[3]. The basic structures of these two devices are similar.

An electrothermal V-shaped actuator, which is made of dopedpolysilicon, is employed to provide the in-plane motion of the

switch. The doped-polysilicon contact head, signal lines, and

their sidewalls are coated with a gold film. Depending on the

design of actuation direction, as shown in Fig. 2, the V-shaped

actuator pushes or pulls the contact head and connects the RF

signal lines via sidewall contacts to turn on the switch. The

main improvement of the proposed microswitch is the isolation

structure between the actuator and contact head, as compared

in Fig. 1(c) and (d).

As shown in Fig. 1(d), the previously reported switch used a

nitride connection as the isolation structure. In our experiments,

it was found that the nitride connection was easy to break

in switches with a pull-type actuation design. Alternatively,

a push-type actuation design was employed in that work.

However, the nitride connection still caused some mechanical

failures and decreased the switchs yield and reliability in

long-term operation. The problem was mainly due to the poor

nitride-polysilicon adhesion. Section II-B simulates the stress

profile on the nitride connection. In the pull-type switch, thenitride-polysilicon adhesion sustains tensile stress, as shown in


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SHI et al.: LATERAL MEMS SWITCH UTILIZING UNDOPED POLYSILICON AS ISOLATION MATERIAL 1175

Fig. 3. Simulated stress profile on the nitride connection: (a) pull-type actuation design and (b) push-type actuation design.

Fig. 3(a), which causes the peeling-off of the nitride film. In the

push-type switch, the adhesion sustains a compressive stress, as

shown in Fig. 3(b), but the stress concentration on the corner ofthe adhesion interface contributes to the failures of the nitride

connection.

To handle this problem, the undoped-polysilicon connection

is developed in the proposed switch, as shown in Fig. 1(c).

The undoped-polysilicon connection is deposited with the

doped-polysilicon actuator and the contact head in the same

step of the low-pressure chemical vapor deposition (LPCVD)

process. Therefore, the homogeneous connection structure has

robust mechanical strength. It could sustain a large contact

force, which is beneficial for reducing the contact resistance.

Besides, the undoped-polysilicon connection provides a planar

surface, which is important for some other applications wheresucceeding lithography steps are necessary. The electrical isola-

tion performance of the undoped-polysilicon connection is also

sufficient as investigated in our experiments.

Due to the improvement of the isolation structure, the pull-

type actuation design can be employed in the proposed mi-

croswitch. Comparing with the push-type design, the pull-type

design has the advantage of good mechanical stability, as shown

in Fig. 2. The arrows in the beams indicate the direction of the

sustained stress during turning on the switch. In the push-type

design, the cross joint sustains compressive stresses in both the

x-axis direction and y-axis direction, so it is easy to buckleout of plane when the compressive stress is increased [18].

Contrarily, the cross joint of the pull-type design sustains tensilestresses in the y-axis direction, which makes the structuremore stable. Therefore, a larger contact force could be reached

without buckling.

B. Isolation Design and Simulation

As the actuator of the proposed switch is driven by the

Joule heating, the isolation structure also has to serve as the

thermal isolation between the actuator and the contact head.

The undoped polysilicon has a low thermal conductivity of

about 13.8 W/m K due to its phonon scattering effect at grain

boundaries [19], [20], which is comparable with the siliconnitride (1530 W/m K).

Fig. 4. Mesh model used in both the temperature profile simulation and the

contact force simulation.

TABLE IPARAMETERS USED IN THE COVENTORWAR E SIMULATION ANALYSIS

TABLE IITHERMAL BOUNDARY CONDITION USED IN THE SIMULATIONS

As the microswitch is suspended only a few micrometers

above the substrate, the heat conduction toward the substrate

contributes to an important factor of determining the heat

loss on the connection structure [21]. Therefore, the undoped-

polysilicon structures with different isolation patterns are em-

ployed to enhance the thermal isolation. Their thermal isolation

performances are simulated in CoventorWare 2005.

The mesh model of the simulation is shown in Fig. 4.The meshing element is the 27-node hexahedral. The element


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Fig. 5. Isolation structure designs of the undoped-polysilicon connection: (a) type I, (b) type II, and (c) type III.

size is 1 m in the planar directions (x-axis and y-axis) and0.2 m in the extruding direction (z-axis). The physical con-stants that are used in the analysis are listed in Table I, and the

following analysis does not consider the dependence of material

properties on the temperature. Table II summarizes the thermal

boundary conditions in the simulation. As the vertical spacing

between the switch and the substrate z is only 2 m, theconduction heat transfer coefficient U to the substrate can beapproximated by [21]

U = kair/z (1)

where kair is the air thermal conductivity. Using a kair of0.03 W m1 K1, U is calculated to be 15000 W m2 K1

[21]. The actuation voltage of the switch is applied between the

left side and the right side of the actuator beams.

Three different types of isolation structure designs are con-

sidered, as shown in Fig. 5. During the ON-state of the switch,

the simulated temperature profile on the actuator, the undoped-

polysilicon connection, and the contact head are shown in

Fig. 6, where a scaling factor of 5.0 is used to magnify the

deformation of the V-shaped beams. The results show that

a sufficient temperature drop along the undoped-polysilicon

connection can be achieved in all the three designs. As marked

in Fig. 6, the V-shaped beams temperatures in the type-IIdesign (735 K) and the type-III design (733 K) are slightly

lower than the beams temperature in the type-I design (763 K).

The type-III design provides the best thermal isolation perfor-

mance with a temperature of about 352 K in the contact head.

These results are obtained from static thermal analysis.

Fig. 7(a) shows the temperature profile of a switch with the

nitride connection. The thickness of the nitride film is 0.6 m,and its planar dimensions (50-m length and 8-m width) arethe same as the undoped-polysilicon connection in the type-I

design. Comparing Fig. 7(a) with Fig. 6(a), it shows that the ni-

tride connection has better thermal isolation performance than

the undoped-polysilicon connection. However, a 50-m-length

nitride film would greatly weaken the mechanical strength ofthe switch. In fact, the nitride connection that is used in our

previous switch has a length of only 6 m, and the accordingtemperature profile is shown in Fig. 7(b), which shows that thethermal isolation performance is not as good as the proposed

three types of undoped-polysilicon connection. In summary,

the robust undoped-polysilicon connection enables longer and

more complex isolation structures than the nitride connection,

and both factors contribute to the improved thermal isolation

performance.

C. Actuator Design Consideration

There have been reports on the static performances of the

V-shaped actuator such as the maximum motion, the contact

force, and the lifetime [17], [22], [23]. There are also reportson the dynamic performances of the actuator, such as studying

the electrothermal responses with the line-shape microstructure

[24] and simulating the time responses of the thermal beams

[21]. The design of the V-shaped actuator is a complex tradeoff

among the displacement, the contact force, and the mechanical

stability [17], [25], [26]. Due to the stable pull-type actuation

design, the optimized actuator with longer beam length and

fewer beam number can be employed in the proposed mi-

croswitch without leading to the out-of-plane buckling.

The schematic view of the V-shaped actuator in our switch is

shown in Fig. 8. For the V-shaped beams of different lengths,

the longer beam requires a lower heating temperature to providethe same displacement [17], which means that a lower operation

temperature is sufficient to turn on the switch. So, a 400-m-long actuator is employed here instead of the 200-m-longactuator in the previous report. The power consumption is

also an important consideration in the thermal actuated switch.

Therefore, the three-beam design is employed here instead of

the previous six-beam design to reduce the power consumption.

The final actuator design has dimensions of 400-m length,4-m width, 2-m thickness, and 7-m offset at the center. Thespacing between the V-shaped beams is 9 m. The gap heightbetween the actuator and the substrate is 2 m, and there is aninitial 3-m gap between the contact head and the signal lines.

Furthermore, the contact forces that are provided by theactuator under different driving voltages are simulated in


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Fig. 6. Simulated temperature profile on the microswitch with different connection structure designs, i.e., (a) type I, (b) type II, and (c) type III, when thesubstrate temperature is set to 300 K.

CoventorWare with the mesh model and boundary condition

of Section II-B. As shown in Fig. 9, the contact head moves

across the 3-m gap and reaches the signal lines at a drivingvoltage between 8 and 9 V, which matches with our experi-

ment well. Under a 12-V driving voltage, the actuator could

provide a contact force of about 310 N, which is sufficientto achieve a stable contact with the resistance in the range of

100300 m [27].The time for heating up or cooling down the V-shaped actu-

ator can be esteemed by the first mode thermal time constant

[24], [26], [28]. In our design, the actuators thermal time

constant is calculated to be 67 s, which indicates a maxi-mum cutoff frequency of 14.9 kHz. Meanwhile, the mechan-

ical resonant frequency of the microswitch is simulated to be

376 kHz in the y-axis direction, which is sufficiently higherthan the thermal cutoff frequency of 14.9 kHz.

The microswitch that is proposed in this report occupied an

area of approximately 500 m 100 m. The RF signal linesare separated by 40 m, and the contact area is designed to be

10 m 2 m or 14 m 2 m. The closing gap between thecontact head and the signal lines is 4 m. After sputtering agold layer with 0.5-m thickness, the gap distance is reducedto be about 3 m.

III. FABRICATION

The microswitch is fabricated with the polysilicon surface-

micromachined process, and a silicon wafer ( = 24 cm)with a 0.3-m-thick thermal oxide layer is used as the substrate.Fig. 10 shows the cross-sectional schematic view of the process.

First, a 0.2-m-thick Si3N4 film is deposited on the substrateby the LPCVD process. The Si3N4 film and the thermal oxide

film together form an insulation layer to reduce the substrate

parasitics at high frequencies, which is due to the lossy nature

of the silicon substrate. Then, a sacrificial layer of 2-m-thickLPCVD SiO2 film is deposited and patterned to form the anchor

position, as shown in Fig. 10(b). The sacrificial SiO2 layerremains undoped, which is different from the usual doped SiO2


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Fig. 7. Temperature profile on the switches with different nitride connection structures: (a) 50- and (b) 6-m-long nitride connections.

Fig. 8. Schematic view of the three-beam V-shaped actuator in the proposedswitch.

Fig. 9. Simulated contact force versus the input power. The correspondingactuation voltages are indicated at selected points.

sacrificial layer, such as borosilicate glass or phosphosilicate

glass, to prevent the dopant diffusing from the sacrificial layer

into the polysilicon structures.

Second, a 2-m-thick undoped LPCVD polysilicon film isdeposited at 610 C, and a 2-m-thick photoresist (Shipley6818) film is coated, patterned, and baked as the shelter layer

for the following implantation. As shown in Fig. 10(d), the

polysilicon film is implanted with the P+ at a dosage of

1 1016 cm2 and an energy of 80 keV, while the patternedphotoresist layer keeps the isolation area undoped. The

undoped area forms the undoped-polysilicon isolation structure

in the followed sequences. After a 0.5-m-thick LPCVD oxidelayer is deposited to avoid the self-diffusion effect, the thermal

annealing (1050 C, 1 h) is carried out to drive and activate the

dopant. The annealing also contributes to reduce the residual

stress in the polysilicon film. Then, the top oxide layer is

removed by the buffered HF acid (BHF) etching, and the

polysilicon film is patterned with inductively coupled plasma

etch process, as shown in Fig. 10(e). In this step, the undoped

area forms the isolation structure, while the doped area forms

the actuator, the contact head, and the signal lines.

Third, the method of partial release combined with liftoff

process is employed to form the sidewall metal contacts and

the metal on signal lines [3]. As shown in Fig. 10(f), a partial

release step is performed by dipping the wafer into 6 : 1 BHF

while exposing only the small region between the contact head

and the RF signal lines. Approximately 1.0 m of the sacrificialoxide in the gap area is removed to ensure the separation of

sputtered gold on the contact sidewalls between the contacthead and the signal lines. Then, the liftoff process was carried


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Fig. 10. Cross-sectional illustration of the fabrication process sequence.(a) Deposit the Si3N4 insulation layer. (b) Deposit and pattern the undopedSiO2 sacrificial layer. (c) Deposit the undoped polysilicon and form thephotoresist shelter. (d) Implant P+ into the polysilicon with the photoresistshelter. (e) Pattern the polysilicon structure after thermal annealing. (f) Partiallyrelease the sacrificial layer between the contacts. (g) Sputter and lift off the goldfilm to form the sidewall contacts. (h) Release the whole microswitch.

Fig. 11. SEM pictures of (a) the fabricated microswitch with transmission lineand (b) a close-up view of the switch.

out. A 0.5-m-thick gold film is sputtered and lifted off to leavegold only on the contact sidewalls and signal routing lines.

Finally, the device is fully released in the concentrated HF

acid for 15 min, as shown in Fig. 10(h). The sublimation dryingafter the HF release is employed to reduce the surface stiction of

Fig. 12. (a) Schematic view and (b) SEM picture of the on-chip testing

structure for measuring the electrical isolation performance of the undoped-polysilicon connection.

the thin actuator beams. Fig. 11(a) shows the scanning electron

microscope (SEM) picture of the whole fabricated microswitch

with the transmission line, and Fig. 11(b) shows a close-up

view. The whole fabrication sequence is completed by standard

MEMS processes with only four masks including the liftoff.

The undoped-polysilicon connection is realized by one step of

sheltered implantation without extra process.

IV. TEST AND DISCUSSION

A. Undoped-Polysilicon Isolation Structure

The undoped-polysilicon connection has good mechanical

strength and could sustain a large contact force, either in the

pull-type actuation design or in the push-type actuation design.

In our experiments, the undoped-polysilicon connection would

not break even when the V-shaped beams were burnt by Joule

heating under an 18-V driving voltage, which corresponds to a

contact force of higher than 500 N, as shown in Fig. 9.The electrical isolation performance of the undoped-

polysilicon connection has been investigated with the on-chip

testing structures, as shown in Fig. 12. The testing structures

are 8-m-wide 2-m-thick 200-m-long doped-polysilicon

bridges with the undoped-polysilicon connection at the center.The structures IV curves are measured with an HP series4156B parameter analyzer. Fig. 13 shows that the undoped-

polysilicon connection provides good electrical isolation in the

switchs operating range of15 V. The 10-, 20-, and 40-m-long connections provide the isolation with a current leakage

of no more than 3 nA under 15-V voltage, which is sufficient

for a thermal actuator. The 20- and 40-m-long connectionscurrent leakage remains no more than 5 nA when the voltage

rises to 40 V. The 5-m-long connection, as shown in Fig. 14,is insufficient for the isolation because the thermal annealing

process causes the lateral diffusion of the implanted P+ dopant.

Fig. 15 illustrates the IV curves of the 40-m-long connection

under different heating temperatures. The results show that thecurrent leakage of the connection increases with the rise of


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Fig. 13. IV curves of the undoped-polysilicon connections with threedifferent lengths: 10, 20, and 40 m.

Fig. 14. Comparing the IV curves of the on-chip testing structure with andwithout 5-m-long undoped-polysilicon connection.

the temperature. Therefore, a switch design with low actuation

temperature is preferred when using the undoped-polysilicon

connection. At 390-K temperature, the current leakage of

30 nA under 15 V voltage is still sufficient and the electrical

isolation performance could be further improved by employing

the optimized isolation structure design.

It is well known that the residual stress plays important roles

in MEMS structures. If the dopant causes any stress mismatch

between the doped and undoped polysilicon, an out-of-planedeformation of the released structure may occur. Therefore,

the released doped-polysilicon cantilevers, with and without

the undoped-polysilicon connection, have been employed to

investigate the possible stress mismatch. Compared to the can-

tilevers without undoped-polysilicon connection, as shown in

Fig. 16, the cantilevers with undoped-polysilicon connection

show no observable out-of-plane deformation. It seems that the

stress mismatch can be ignored, and this is also verified in the

fabricated microswitch.

Besides, the residual stress of the doped polysilicon causes

an in-plane displacement of the contact head after releasing.

If the stress is too large, the contact head might connect the

signal lines even when no voltage applies on the microswitch.The freestanding rotating indicator structure [29] is adopted to

Fig. 15. IV curves of the undoped-polysilicon connections under differenttemperatures.

diagnose the residual stress. As shown in Fig. 17, the deflec-

tion of the indicator is small. This indicates that the residual

stress of the doped polysilicon is also ignorable, and no initial

displacement of the contact head is observed in the released

microswitch.

B. DC Testing

In the dc testing of the microswitch, a driving voltage of

about 8.09.0 V is required to achieve a 3-m in-plane displace-ment. The switches start to be turned on under a driving voltage

of about 11.0 V. In our experiments, a driving voltage of 12 V

is used to obtain a stable metal contact with low contact resis-

tance. The driving voltage of 12 V is higher than the previouslyreported 3 V. It is mainly due to the low doping concentration

(7 1018 cm3) of the doped-polysilicon actuator, and a loweroperation voltage can be achieved with a higher doping level.

The microswitch provides a small contact resistance of

0.42 at a driving input of 12 V/5 mA, corresponding to thepower consumption of 60 mW. A control group using a six-

beam actuator requires 115-mW power consumption. The input

power is reduced effectively by reducing the number of actuator

beams. The RF signal line has a current handling capacity of

50 mA. It is measured by increasing the carrying current

through the signal path while monitoring the frequency re-

sponse of the switch. The switch fails to deliver signals properlyafter the carrying current was increased from 50 to 55 mA.

The failure is due to the damage of the metal contact, which

is caused by the gold welding [3].

A 100-Hz 50%-duty square-wave signal with a peak voltage

of 12 V is used to measure the frequency responses of the

switch. While the square-wave signal is applied in the actuator

as the excitation source, an oscilloscope is monitoring the

voltage output at one terminal of the signal path, while the

other terminal is supplied with a 1-V dc voltage. Fig. 18 plots

the rising-edge response time [Fig. 18(a)] and the falling-edge

response time [Fig. 18(b)]. The microswitch needs, on average,

114 s to switch on and less than 13 s to switch off. The

rising edge is much longer than the falling edge because theactuator needs to travel a distance of about 3 m before contact


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Fig. 16. (a) Schematic view of cantilever used for testing the stress mismatch and (b) the SEM image of the cantilevers tips.

Fig. 17. Deflection of the free-standing rotating indicator structure that is

made by doped polysilicon.

with the signal lines, while opening the microswitch is only

separating the contact head and the signal lines by removing

the input power [3]. The maximum operating frequency of the

microswitch is 4.5 kHz. It is measured by monitoring the output

signal when increasing the exciting signal frequency until the

microswitch fails to deliver signal properly.

Comparing with the previous report, the switching time is

reduced much, from about 290 to 130 s, while the maxi-mum operating frequency is increased from 2.5 to 4.5 kHz.

The improvement is mainly due to utilization of the 400-m-long beam actuator instead of the 200-m-long design. At a

temperature of 300 K, the calculated thermal time constant of43 s for the 200-m-long design is smaller than the 67 s

for the 400-m-long design. However, two important factorsmust also be considered. First, the electrothermal properties of

the polysilicon depend on the temperature, particularly, when

considering the specific heat, which affects the thermal timeresponse much [20]. To provide the same contact force of

300 N, as simulated by CoventorWare, the 400-m-longdesign needs an average temperature of 600 K on the V-shaped

beams, while the 200-m-long design needs a much highertemperature of 1000 K. Second, the temperature distribution

along the V-shaped beam is not uniform [21]. The parts of

the V-shaped beam that are located at two sides near the

anchors might response faster than the part at the center, and

the 400-m-long design provides longer parts at two sides fordeformation. Above all, the experiment results show that the

switching time is obviously reduced by utilizing the 400-m-long actuator.

C. RF Performance

For RF applications, the lateral switch is more favorable

in microstrip circuits. For measurement convenience, however,

the coplanar waveguide (CPW) transmission line is utilized to

characterize the RF performances of the switch in this paper.

Due to the lateral dimension of the switch, it is difficult to

employ a standard 50- CPW line. Therefore, a nonstandardCPW transmission line is utilized for the switch circuit. The

transmission line layout of the proposed switch is shown in

Fig. 11(b), where the RF performances are not optimized.

The insertion loss that is induced by the transmission line iseliminated by utilizing a de-embeded structure [3].


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Fig. 18. Measured frequency responses of the microswitch as (a) the rising-edge response time and (b) the falling-edge response time. CH1 is the inputsignal of the excitation source. CH2 is the output signal of the switch.

The on-wafer RF performance of the microswitch was mea-

sured with an HP series network analyzer (Model 8510C)

and cascade groundsignalground microprobes with 150-mpitch. A standard shortopenloadthrough calibration kit was

used, and the two-port S-parameters were measured from

100 MHz up to 20.1 GHz.During the on-state of the microswitch, the insertion loss is

extracted by subtracting the measured throughline loss (with-

out the microswitch) from the two-port S-parameter measure-

ment of the microswitch. As shown in Fig. 19, the microswitch

has a low insertion loss of0.41 dB up to 20 GHz. Insertion

loss of the same microswitch at different driving voltages have

also been investigated. The higher driving voltage provides a

smaller RF insertion loss because of the corresponding higher

contact force, as shown in Fig. 9. No reliability problem occurs

due to increasing the contact force, which caused the nitride-

polysilicon adhesion failure in the previous work.

Fig. 20 shows the OFF-state isolation of the microswitch, and

an isolation of about 20 dB at 20 GHz can be achieved. Thesubstrate leakage of the microswitch is slightly high mainly due

Fig. 19. ON-state insertion loss of the microswitch at different drivingvoltages.

Fig. 20. OFF-state isolation of the microswitch.

to the normal low-resistivity silicon wafer. Besides, the thermal

oxide insulation layer that is used in the proposed microswitch

is only 0.3 m thick, whereas the oxide insulation layer that isused in the previous work was 1 m thick. Both the OFF-stateisolation and ON-state insertion loss at high frequency can

be further improved by employing the high-resistivity silicon

wafer.

D. Reliability

Two dominant switch failure mechanisms have been ob-

served in our experiments, including the contact degrada-

tion and the mechanical failure that is caused by the nitride

connection.

In our previously reported switch, the nitride-polysilicon

adhesion could not sustain the tensile stress, which leads to

the failure of nearly all the pull-type switches. In the push-type

switches, the nitride connection could sustain a limited com-

pressive stress, but its mechanical failures still contribute to an

important factor of low yield and reliability problems in long-

term operations. With the undoped-polysilicon connection, no

mechanical failure of the proposed switch was observed amongover 20 devices for reliability testing.


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Fig. 21. Change of the contact resistance versus switching cycles.

TABLE IIISUMMARY OF THE MICROSWITCH PERFORMANCE

In the proposed switch, the metal contact degradation plays

the main role of the failure mechanism. A 1000-Hz 50%-dutysquare-wave signal with a peak voltage of 12 V is used as the

actuation input in reliability tests. The results show that the

device can operate over 1 109 cycles under cold switching.Fig. 21 depicts the change of contact resistance versus the

number of operating cycles. No significant degradation of

the contact resistance was observed. The undoped-polysilicon

connection could sustain a larger contact force, which leads to

a more stable metal contact, contributing to the improvement

of the switchs lifetime. However, the average lifetime of the

devices is only about 0.6 106 in hot switching tests with acarrying current of about 10 mA on the signal path. The gold

welding, process variation of sputtering, and surface roughness,

which are in charge of the contact failure and affect the mi-

croswitch lifetime, have been investigated before [3], [30].

During the long-term operation, the undoped-polysilicon

connection sustains a long-term thermal annealing due to the

Joule heating in the actuator beams, as shown in Fig. 6. It

might cause slow self-diffusion of the dopant from the doped-

polysilicon actuator into the undoped-polysilicon connection

and affect the electrical isolation performance. The 20-m-long undoped-polysilicon connection is sufficient to avoid this

effect as verified in our experiments. The connection structure

shows no observable degradation of the electrical isolation

performance after one billion cycles operation.

The overall performances of the proposed microswitch aresummarized in Table III.

V. CONCLUSION

This paper has reported a modified thermal actuated lat-

eral switch utilizing the undoped-polysilicon connection as

the mechanical coupling and electrical isolation structure.

The undoped-polysilicon isolation structure has the advantage

of high reliability as its robust mechanical strength could

sustain the large contact force necessary for a stable metalcontact. Meanwhile, it provides promising electrical isolation

performance.

The proposed microswitch requires a driving voltage of 12 V

and an input power of 60 mW. The switching time is measured

to be 130 s, and a maximum operating frequency of 4.5 kHzis reached, which is nearly double of our previous report. An

ON-state insertion loss of0.41 dB and an OFF-state isolation

of 20 dB at 20 GHz have been achieved on normal low-

resistivity silicon substrate. Improved RF performances can be

obtained by employing the high-resistivity silicon substrate.

The proposed microswitch operates over one billion cycles

without significant degradation of the contact resistance, andthe electrical isolation performance of the undoped-polysilicon

connection shows no observable degradation.

The simplicity of this four-mask fabrication process and

the easy realization of the insulative mechanical coupling en-

able the further possibility of cascading different actuation

approaches together into one device. The undoped isolation

structure can also be used in other surface-micromachined

polysilicon lateral switches.

ACKNOWLEDGMENT

The authors would like to thank the staff at the NationalKey Laboratory of Nano/Micro Fabrication Technology, Peking

University, for their help in the fabrication process. They would

also like to thank Y. Tan, X. Wang, and X. Ji for their help in

the simulation and for fruitful discussions.

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Wendian Shi was born in Zhejiang Province, China,in 1983. He received the B.S. degree from PekingUniversity, Beijing, China, in 2004. He is currentlyworking toward the M.S. degree at the Departmentof Microelectronics, Peking University.

His research interests include design and fabrica-tion of microelectromechanical systems (MEMS), inparticular, RF MEMS and Bio MEMS.

Norman C. Tien (S87M89) received the B.S.degree from the University of California, Berkeley,the M.S. degree from the University of Illinois,Urbana Champaign, and the Ph.D. degree from theUniversity of California, San Diego.

He was a Professor and the Chair of the De-partment of Electrical and Computer Engineering,University of California, Davis, and held a jointappointment as Professor of electrical engineeringand computer science at the University of California,Berkeley. He also served as a Codirector of the

Berkeley Sensor and Actuator Center. In 2006, he joined the Department ofElectrical Engineering and Computer Science, Case School of Engineering,Case Western Reserve University, Cleveland, OH, where he is currently theNord Professor of Engineering and the Dean of the Case School of Engineering.His research interests are micro- and nanotechnology, in particular, appli-cations in wireless communications, biomedical systems, and environmental

monitoring.Dr. Tien is the Ohio Eminent Scholar in Condensed Matter Physics. He was

the recipient of a National Science Foundation CARRER Award.

Zhihong Li (M02) received the B.S. degree andthe Ph.D. degree, majoring in VLSI technology andreliability, from Peking University, Beijing, China, in1992 and 1997, respectively.

He then joined the MEMS Group, Departmentof Microelectronics, Peking University. From 2000to 2004, he was a Postdoctoral Researcher atCornell University, Ithaca, NY, and the University ofCalifornia, Davis. He is currently a Professor andthe Director of the MEMS Center, Department ofMicroelectronics, Peking University. His research

interests include design and fabrication of microelectromechanical systems(MEMS), in particular, RF MEMS and Bio MEMS.

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