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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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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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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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SHI et al.: LATERAL MEMS SWITCH UTILIZING UNDOPED POLYSILICON AS ISOLATION MATERIAL 1183
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.
REFERENCES
[1] E. J. J. Kruglick and K. S. J. Pister, Lateral MEMS microcontactconsiderations, J. Microelectromech. Syst., vol. 8, no. 3, pp. 24271,Sep. 1999.
[2] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, Low-voltage lateral-contact microrelays for RF applications, in Proc. 15th IEEE Int. Conf.
Micro-Electro-Mechanical Syst., Jan. 2002, pp. 645648.
[3] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, A low-voltage lateralMEMS switch with high RF performance, J. Microelectromech. Syst.,vol. 13, no. 6, pp. 902911, Dec. 2004.
[4] R. W. Moseley, E. M. Yeatman, A. S. Holmes, R. R. A. Syms,A. P. Finlay, and P. Boniface, Laterally actuated, low voltage, 3-port RFMEMS switch, in Proc. 19th IEEE Int. Conf. Micro-Electro-Mech. Syst.,Jan. 2006, pp. 878881.
[5] Z. Li, D. Zhang, T. Li, W. Wang, and G. Wu, Bulk micromachinedrelay with lateral contact, J. Micromech. Microeng., vol. 10, no. 3,pp. 329333, 2000.
[6] R. Wood, R. Mahadevan, V. Dudley, A. Cowen, E. Hill, andK. Markus, MEMS microrelays, Mechatronics, vol. 8, no. 5, pp. 535547, Aug. 1998.
[7] J. Simon, S. Saffer, F. Sherman, and C.-J. Kim, Lateral polysilicon mi-crorelays with a mercury microdrop contact, IEEE Trans. Ind. Electron.,vol. 45, no. 6, pp. 854860, Dec. 1998.
[8] A. Q. Liu, M. Tang, A. Agarwal, and A. Alphones, Low-loss lateralmicromachined switches for high frequency applications, J. Micromech.Microeng., vol. 15, no. 1, pp. 157167, Jan. 2005.
-
8/4/2019 Lateral Switch2
12/12
1184 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 5, OCTOBER 2007
[9] Y. Shi and S. G. Kim, A lateral, self-cleaning, direct contactMEMS switch, in Proc. 18th IEEE Int. Conf. Micro-Electro-Mech. Syst.,Jan. 2005, pp. 195198.
[10] R. L. Borwick, III, P. A. Stupar, and J. DeNatale, A hybrid approach tolow-voltage MEMS switches, in Proc. 12th IEEE Int. Conf. Solid-StateSensors, Actuators Microsyst. (Transducers), Jun. 2003, pp. 859862.
[11] Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, A micromachinedRF microrelay with electrothermal actuation, Sens. Actuators A, Phys.,
vol. 103, no. 1, pp. 231236, Jan. 2003.[12] R. Chan, R. Lesnick, D. Becher, and M. Feng, Low-actuation voltage RFMEMS shunt switch with cold switching lifetime of seven billion cycles,
J. Microelectromech. Syst., vol. 12, no. 5, pp. 713719, Oct. 2003.[13] S. D. Lee, B. C. Jun, S. D. Kim, H. C. Park, J. K. Rhee, and K. Mizuno,
An RFMEMS switch with low-actuation voltage and high reliability,J. Microelectromech. Syst., vol. 15, no. 6, pp. 16051611, Dec. 2006.
[14] G. M. Rebeiz, RF MEMS: Theory, Design and Technology. Hoboken,NJ: Wiley, 2003, pp. 148149.
[15] T. J. Brosnihan, J. M. Bustillo, A. P. Pisano, and R. T. Howe, Embeddedinterconnect and electrical isolation for high-aspect ratio, SOI inertialinstruments, in Proc. 9th IEEE Int. Conf. Solid-State Sensors Actuators(Transducers), Jun. 1999, pp. 10021005.
[16] L. Muller, J. M. Heck, R. T. Howe, and A. P. Pisano, Electrical isola-tion process for molded, high-aspect-ratio polysilicon microstructures,in Proc. 13th IEEE Int. Conf. Micro-Electro-Mech. Syst., Jan. 2000,pp. 590595.
[17] L. Que, J.-S. Park, and Y. B. Gianchandani, Bent-beam electrothermalactuatorsPart I: Single beam and cascaded devices, J. Microelectro-mech. Syst., vol. 10, no. 2, pp. 247254, Jun. 2001.
[18] M. Chiao and L. Lin, Self-buckling of micromachined beams underresistive heating, J. Microelectromech. Syst., vol. 9, no. 1, pp. 146151,Mar. 2000.
[19] A. D. McConnell, S. Uma, and K. E. Goodson, Thermal conductivityof doped polysilicon layers, J. Microelectromech. Syst., vol. 10, no. 3,pp. 360369, Sep. 2001.
[20] S. Uma, A. D. McConnell, M. Asheghi, K. Kurabayashi, andK. E. Goodson, Temperature dependent thermal conductivity of undopedpolycrystalline silicon, Int. J. Thermophys., vol. 22, pp. 605616, 2001.
[21] R. Hickey, D. Sameoto, T. Hubbard, and M. Kujath, Time and frequencyresponse of two-arm micromachined thermal actuators, J. Micromech.
Microeng., vol. 13, no. 1, pp. 4046, 2003.[22] Y. B. Gianchandani and K. Najafi, Bent-beam strain sensors, J. Micro-
electromech. Syst., vol. 5, no. 1, pp. 5258, Mar. 1996.[23] L. L. Chu, L. Que, A. D. Oliver, and Y. B. Gianchandani, Lifetimestudies of electrothermal bent-beam actuators in single-crystal siliconand polysilicon, J. Microelectromech. Syst., vol. 15, no. 3, pp. 498506,Jun. 2006.
[24] L. Lin and M. Chiao, Electrothermal response of lineshape microstruc-tures, Sens. Actuators A, Phys., vol. 55, no. 1, pp. 3541, Jul. 1996.
[25] E. T. Enikov, S. S. Kedar, and K. V. Lazarov, Analytical model for analy-sis and design of v-shaped thermal microactuators, J. Microelectromech.Syst., vol. 14, no. 4, pp. 788798, Aug. 2005.
[26] J. M. Maloney, D. S. Schreiber, and D. L. DoVoe, Large-force elec-trothermal linear micromotors, J. Micromech. Microeng., vol. 14, no. 2,pp. 226234, Feb. 2004.
[27] D. Hyman and M. Mehregany, Contact physics of gold microcontacts forMEMS switches, IEEE Trans. Co mpon. Packag. Technol., vol. 22, no. 3,pp. 357364, Sep. 1999.
[28] D. S. Schreiber, W. J. Cheng, J. M. Maloney, and D. L. DoVoe, Surface
micromachined electrothermal V-beam micromotors, in Proc. ASME Int.Mech. Eng. Congr. Expo., 2001, pp. 17.
[29] B. P. V. Dieenhuizen, J. F. L. Goosen, P. J. French, and R. F. Wolffenbuttel,Comparison of techniques for measuring both compressive and tensilestress in thin film, Sens. Actuators A, Phys., vol. 37/38, pp. 756765,1993.
[30] N. E. McGruer, G. G. Adams, L. Chen, Z. J. Guo, andY. Du,Mechanical,thermal, and material influences on ohmic-contact-type MEMS switchoperation, in Proc. 19th IEEE Int. Conf. Micro-Electro-Mech. Syst. , Jan.2006, pp. 230233.
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.