ABSTRACT This paper reports the design and fabrication of a lowvoltage lateral-contact microrelay for RF applications. The silicon surface micromachined relay utilizes electrothermal actuators and low-stress silicon nitride as a structural connection as well as electrical and thermal isolation. The sidewall contact is sputtered gold. The driving voltage is measured to be as low as 8V. RF testing shows that the microrelay has an off-state isolation of20dB at 12GHz. The simplicity of this four-mask fabrication process enables the possible integration with other RF MEMS components

Actuator Center

Berkeley Sensor

Daniel T Mccormick

Norman C Tien

Ye Wang

Zhihong Li

CiteSeerX

LOW-VOLTAGE LATERAL-CONTACT MICRORELAYS  
FOR RF APPLICATIONS 
Ye Wang1, Zhihong Li2, Daniel T. McCormick1 and Norman C. Tien
Berkeley Sensor and Actuator Center, Department of Electrical and Computer Engineering,  
University of California at Davis, Davis, CA 95616 
1also at Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 
2also at Institute of Microelectronics, Peking University, Beijing, China 
Email: yw51@cornell.edu
ABSTRACT 
This paper reports the design and fabrication of a low-
voltage lateral-contact microrelay for RF applications.  
The silicon surface micromachined relay utilizes electro-
thermal actuators and low-stress silicon nitride as a 
structural connection as well as electrical and thermal 
isolation.  The sidewall contact is sputtered gold. The 
driving voltage is measured to be as low as 8V.  RF testing 
shows that the microrelay has an off-state isolation of -
20dB at 12GHz. The simplicity of this four-mask 
fabrication process enables the possible integration with 
other RF MEMS components. 
INTRODUCTION 
With the recent progress of MicroElectroMechanical 
System (MEMS) technology, more and more attention has 
been focused on the development of MEMS devices for 
RF applications.  MEMS switches are one of the most 
promising future micromachined products that have 
attracted numerous research efforts in recent years. They 
have many potential applications including signal routing 
in RF system front-ends, impedance matching networks, 
filter-path selection circuits and other high-frequency 
reconfigurable circuit applications.  Compared to their 
conventional electromechanical or solid-state counterparts, 
micromachined relays offer many advantages in terms of 
low insertion loss, high off-state isolation and linearity, 
high breakdown voltage and integration capability. The 
majority of the reported MEMS switches employs 
electrostatic actuation [1-4] and operates with large 
actuation voltage.  Few are lateral relays [5-6] and those 
often require non-standard post process and they are only 
suitable for DC applications.  Some of the conventional 
parallel-plate MEMS switches have limited RF 
performance due to loss caused by capacitive coupling 
between the contacting bar and the substrate at high 
frequency.  An adequate closing gap to achieve high 
isolation usually results in a large actuation voltage. In 
addition, the signal line and the contacting bar have to be 
realized by two separate process steps. 
We present a lateral-contact microrelay fabricated on a 
silicon substrate using surface micromachining 
technology.  This relay is electro-thermally actuated and 
exhibits high RF performance due to its lateral contact 
mechanism.  The microrelay consists of polysilicon and 
low-stress silicon nitride as structural materials and 
sputtered gold as the contact metal.  Metal contact is 
realized by in-plane motion and sidewall connection. 
Figure 1. Schematic illustration of the microrelay.
DESIGN 
Figure 1 illustrates the operation principle of the 
microrelay.  It utilizes electro-thermal actuators, for which 
extensive research has been done both theoretically and 
experimentally [7-8].  The thermal expansion caused by 
current flowing through a V-shaped beam actuates the 
contact head through silicon nitride connection.  The 
resulting in-plane motion of the contact head allows it to 
move forward and connect the RF signal lines via sidewall 
contact. 
V-shaped actuator beams were chosen for their rectilinear 
displacements caused by resistive heating and their large 
output force in the range of millinewtons [7]. This will 
provide a stable contact with low contact resistance in the 
range of milliohms [9].  Actuator beams of the same 
dimension are cascaded to produce larger displacement, as 
depicted in Figure 2.  A current flow between terminal A 
and B will cause all three V-beams to expand due to 
resistive heating.  The thermal expansion of the two side 
beams increases the vertical displacement of the central 
beam.  This generates a larger deflection at the tip 
compared to a single actuator beam. 
Signal 
Lines 
Contact 
Head
SiN 
Isolation 
Electro-thermal 
Actuator 
0-7803-7185-2/02/$10.00 ©2002 IEEE 645
Figure 2. Schematics of the cascaded thermal actuator 
beams (top view).
The nominal dimension of a single actuator beam has a 
length of 200µm, width and thickness of 2µm and center 
offset of 10µm.  The actuator beam with this design value 
is calculated to have a high shock resistance > 5000g (g = 
9.8m/s2) and self-resonance frequency of 31.9kHz, which 
indicates good mechanical robustness. The microrelay 
itself occupies an area of 200×220µm 2 if not considering 
the RF testing pad.  Test structures of cascaded actuator 
beams with various dimensions were designed and 
simulated. Table 1 shows the design values and ANSYS 
simulation of their displacements under a given thermal 
load. Type 1 design gives the nominal dimensions.  
Table 1. Various actuator test structures and their 
simulated displacements. 
Length Width Offset Disp. 
Type1 200µm 2µm 10µm 5.4µm
Type2 240µm 2µm 10µm 7.2µm
Type3 240µm 1µm 15µm 6.2µm
Type4 260µm 1.5µm 10µm 7.9µm
Type5 280µm 2µm 10µm 8.6µm
Type6 300µm 2µm 15µm 9.2µm
Gold is chosen as contact metal because of its low 
resistivity, good stability and efficiency in RF signal 
propagation. It has a skin depth of 0.71µm at 12GHz and 
0.45µm at 30GHz. Thickness of 0.5µm is used due to 
sputtering limit. Sputtered gold is known to have higher 
hardness which gives less surface damage for metallic 
microcontacts [9]. 
Different contact head shapes including round, square and 
angled have been designed to explore their reliabilities.  
Contact head areas between 700µm 2 to 1200µm 2 are 
designed after considering the tradeoff between contact 
resistance and the load the contact head puts on the 
extension beams.  The closing gap between the contact 
head and the signal lines is between 3-5µm to ensure good 
sidewall coverage of the sputtered gold in the trench.  
FABRICATION 
Figure 3. Schematic of the process flow: (a) deposition of 
the low-stress SiN as isolation; (b) deposition and 
patterning of sacrificial SiO2 and low-stress SiN 
connection; (c) deposition and patterning of poly-Si and 
partial release; (d) Sputtering and lift-off of Au and HF 
release.  
The fabrication process is depicted in Figure 3.  First, 
0.6µm of Low-Pressure Chemical-Vapor-Deposited 
(LPCVD) low-stress silicon nitride is deposited at 
temperature of 850°C. It is used as an isolation layer to 
reduce substrate loss.  Then 2µm of sacrificial oxide is 
deposited and anchors are patterned.  Afterwards another 
0.6µm of LPCVD low-stress silicon nitride is deposited 
and patterned.  It serves as the structural connection as 
well as the electrical and thermal isolation between the 
driving structure and the contact structure.  Figure 4 (a) is 
the SEM micrograph of a piece of silicon nitride 
connecting two polysilicon structures; no stress mismatch 
between the two materials was observed.  A few different 
structural connections are designed to explore the 
possibilities, as shown in Figure 4 (b).  Next 2µm of in situ
(a) 
SiN 
(d) 
Au 
SiO2
(b) 
SiN 
(c)
Poly-Si 
A B
Length 
Width 
Offset 
0-7803-7185-2/02/$10.00 ©2002 IEEE 646
doped n-type polysilicon film is deposited at 620°C.  It is 
patterned using 0.4µm oxide as hard mask.   A partial 
release step is then performed at the closing gap region to 
ensure the separation of sputtered gold on the contact head 
sidewall and the signal lines, as well as the removal of 
unwanted gold in the area between them. 
In the next step, a thin layer of gold (0.3-0.5µm) is 
sputtered and lifted off, leaving gold only on the contact 
sidewalls and signal routing lines.  Figure 5 shows a 
uniform coverage of gold on the contact sidewalls. The 
warpage on the edge of the signal line is due to lift-off.  
Finally, the device is released in hydrofluoric acid and the 
polysilicon and silicon nitride structures are suspended 
above the substrate.  A supercritical CO2 drying after HF 
release is helpful to reduce the surface stiction of the thin 
actuator beams. 
(a) 
(b) 
Figure 4. (a) SEM image of the SiN structure connecting 
two polysilicon structures; (b) SEM images of different 
structural connections. 
Figure 5. SEM image of the contact sidewalls and signal 
line coated with gold. 
The fabrication is completed using standard MEMS 
processes with only four masks, including lift-off, and no 
post-processing is required.  Contact metal is realized 
using one-step gold sputtering.  The simplicity of this 
process allows design flexibility, and possible integration 
of this microrelay with other passive RF MEMS 
components.  
RESULTS AND DISCUSSIONS 
Electrical testing was done with an HP 4145B Parameter 
Analyzer at room temperature in an air ambient 
environment.  It shows the microrelay can be operated at 
voltages of 8-15V with contact resistance less than 1Ω.   
The operating voltage can be further reduced by 
decreasing the closing gap between the contact head and 
the signal lines or using highly doped polysilicon.  Contact 
heads with round and square shape showed better 
reliability than the angled-shape contact heads. 
During the microrelay on-state, the power consumption of 
the thermal actuator is in the range of 20-40mW.  The 
break down voltage of the signal path is measured to be as 
high as 500V. 
2
3
4
5
6
7
1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0
S im
M e a
D
is
pl
ac
em
en
t [
um
]
In p u t P o w e r [m W ]
Figure 6. Measured actuator displacement vs. simulated 
values at various input power. 
The measured actuation displacements at different input 
power for the actuator beam arrays with nominal 
dimension were plotted in Figure 6.  Simulation 
parameters were adjusted to account for their changes due 
to thermal heating.  This gives good agreement between 
the measured displacement and simulation, which shows a 
linear proportion to the input power.  Test structures listed 
in Table 1 were tested and their actuation distances were 
compared with ANSYS simulations. Theoretical 
calculation [7-8] is also used to verify the results. 
RF testing was performed using an HP8510C network 
analyzer and coplanar ground-signal-ground (GSG) probes 
with 150µm pitch.  Two-port S-parameters were measured 
and parasitics of the testing pads were de-embedded.  
Figure 7 shows the measured off-state isolation is -20dB at 
12GHz.  The measurements also indicate the microrelay 
has an insertion loss of less than -1dB at 12GHz with most 
resistive loss occurs on the signal lines.   
Contact Head 
Au 
Signal Line 
SiN 
Poly-Si 
0-7803-7185-2/02/$10.00 ©2002 IEEE 647
A comparison between some of the most recent 
microrelays and this design is tabulated in Table 2. It 
shows that the microrelay exhibits high isolation in the 
GHz frequency range comparable to those fabricated on 
GaAs substrate with similar actuation voltage. 
Frequency [GHz]
Is
ol
at
io
n 
[d
B
]
2 4 6 8 10
-80
-60
-40
-20
0
Relay OFF-state Isolation
Figure 7. Measured relay off-state isolation.
Table 2. Comparison of selected recent MEMS relays and 
this design. 
Ref. Motion Substrate Voltage Isolation 
 [1] Vertical GaAs 5V -30dB@4GHz 
 [2] Vertical GaAs 8V -42dB@5GHz 
 [3] Vertical Si 10V -8dB@6GHz 
[4] Vertical GaAs 70V -30dB@40GHz 
This 
Design 
Lateral Si 8V -20dB@12GHz 
Reliability study of the microrelay shows that metal 
contact welding causes the most failures. Surface 
roughness on the sidewall may also result in bad contacts.  
The high adherence force of gold plays a role in the 
contact degradation as well.  Gold and Nickel alloy can be 
considered as an alternative choice of contact metal due to 
its small adherence force and relatively low resistivity. 
CONCLUSION 
An electro-thermally actuated lateral-contact microrelay 
for RF applications has been designed and fabricated on 
silicon substrate using surface micromachining 
technology.  Low-stress silicon nitride as structural 
connection shows promising results with good electrical 
isolation and negligible stress mismatch.  The lowest 
actuation voltage of the microrelay is 8V.  The microrelay 
has an off-state isolation of -20dB at 12GHz and low 
insertion loss of less than -1dB.  The lateral contact 
mechanism of the microrelay provides a high RF 
performance. The measured off-state isolation in the GHz 
frequency range is comparable with some recently reported 
RF MEMS switches fabricated on semi-insulating 
substrate.  The simplicity of the fabrication process enables 
the possible integration of the microrelay with other 
passive RF MEMS components. 
ACKNOWLEDGEMNETS 
The authors would like to thank Prof. Edwin Kan for 
providing the testing equipment and the staff of Cornell 
Nanofabrication Facility.  The authors would also like to 
thank A. Yeh, H. Jiang, K. Yoo and Z. Liu for their fruitful 
discussions. 
REFERENCES 
[1] D. Hah, E. Yoon and S. Hong,  “A Low-Voltage 
Actuated Micromachined Microwave Switch Using 
Torsion Springs and Leverage”, IEEE Trans. 
Microwave Theory and Techniques, Vol. 48, No. 12, 
pp. 2540-2545, Dec. 2000 
[2] J. Y. Park, G. H. Kim, K. W. Chung and J. U. Bu, 
“Electroplated RF MEMS Capacitive Switches”, Proc. 
IEEE/ASME Int. Conference on Micro  
Electro Mechanical Systems (MEMS 2000), Interlaken, 
Switzerland, pp. 639-644, Jan. 2000 
[3] F. Plotz, S. Michaelis, G. Fattinger, R. Aigner and R. 
Noe, “Performance and Dynamics of a RF MEMS 
Switch”, Proc. IEEE Int. Conference on Solid-State 
Sensors and Actuators (Transducers ’01), Munich, 
Germany, pp. 1560-1563, June 2001 
[4] R. E. Mihailovich, M. Kim, J. B. Hacker, E. A. Sovero, 
J. Studer, J. A. Higgins and J. F. deNatale, “MEM 
Relay for Reconfigurable RF Circuits, IEEE 
Microwave and Wireless Components Letters, Vol. 11, 
No. 2, pp. 53-55, Feb. 2001 
 [5] E. J. J. Kruglick and K. S. J. Pister, “Lateral MEMS 
Microcontact Considerations”, IEEE J. 
Microelectromechanical Systems, Vol. 8, No. 3, pp. 
264-271, Sept. 1999 
[6] J. Simon, S. Saffer, F. Sherman and C.-J. Kim, “Lateral 
Polysilicon Microrelays with a Mercury Microdrop 
Contact”, IEEE Trans. Industrial Electronics, Vol. 45, 
No. 6, pp. 854-860, Dec. 1998 
[7] L. Que, J.-S. Park and Y. B. Gianchandani, “Bent-
Beam Electrothermal Actuators-Part I; Single Beam 
and Cascaded Devices, IEEE J. 
Microelectromechanical Systems, Vol. 10, No. 2, pp. 
247-262, June 2001 
[8] J. M. Maloney, D. L. DeVoe and D. S. Schreiber, 
“Analysis and Design of Electrothermal Actuators 
Fabricated From Single Crystal Silicon”, Proc. ASME 
Int. Mechanical Engineering Congress and Exposition, 
pp. 233-240, May 2000 
[9] D. Hyman and M. Mehregany, “Contact Physics of 
Gold Microcontacts for MEMS Switches”, IEEE 
Trans. Components and Packaging Technology, Vol. 
22, No. 3, pp. 357-364, Sept. 1999 
Reference [1] = -30dB 
Reference [2] = -42dB 
Reference [3] = -8dB 
0-7803-7185-2/02/$10.00 ©2002 IEEE 648


Low-voltage lateral-contact microrelays for RF applications,” IEEE,

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=39BD4D99D827DC14BF2EBACB461D3908?doi=10.1.1.1074.686&rep=rep1&type=pdf

Low-voltage lateral-contact microrelays for RF applications,” IEEE,

Abstract

Similar works

Full text

Available Versions

CiteSeerX