Radio frequency (RF) microelectromechanical (MEMS) switches offer superior performance when compared to the traditional semiconductor devices such as PIN diodes or GaAs transistors. MEMS switches have a return loss (RL) better than -25 dB, negligible insertion loss (IL), isolation better than -30 dB, and near zero power consumption. However, RF MEMS switches have several drawbacks the most serious being long-term reliability. The ability for the switch to operate for millions or even billions of cycles is a major concern and must be addressed. MEMS switches are basically grouped in two categories, capacitive and metal-to-metal contact. The capacitive type switch consists of a movable metal bridge spanning a fixed electrode and separated by a narrow air gap and thin insulating material. The metal-to-metal contact type utilizes the same basic design but without the insulating material. After prolonged operation the metal bridges, in most of these switches, begin to sag and eventually fail to actuate. For the metal-to-metal type, the two metal layers may actually fuse together. Also if the switches are not packaged properly or protected from the environment moisture may build up and cause stiction between the top and bottom electrodes rendering them useless. Many MEMS switch designs have been developed and most illustrate fairly good RF characteristics. Nevertheless very few have demonstrated both great RF performance and ability to perform millions/billions of switching cycles. Of these, nearly all are of metal-to-metal type so as the frequency increases RF performance decreases

Oldham, Daniel R.

Scardelletti, Maximilian C.

Zorman, Christian A.

NASA Technical Reports Server

Maximilian C. Scardelletti
Glenn Research Center, Cleveland, Ohio
Christian A. Zorman
Case Western Reserve University, Cleveland, Ohio
Daniel R. Oldham
Glenn Research Center, Cleveland, Ohio
RF MEMS Switches With SiC Microbridges for 
Improved Reliability
NASA/TM—2008-215201
May 2008
IEEE Paper 2961
https://ntrs.nasa.gov/search.jsp?R=20080023307 2019-08-30T04:43:59+00:00Z
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Maximilian C. Scardelletti
Glenn Research Center, Cleveland, Ohio
Christian A. Zorman
Case Western Reserve University, Cleveland, Ohio
Daniel R. Oldham
Glenn Research Center, Cleveland, Ohio
RF MEMS Switches With SiC Microbridges for 
Improved Reliability
NASA/TM—2008-215201
May 2008
IEEE Paper 2961
National Aeronautics and
Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Prepared for the
IEEE International Symposium on Antennas and Propagation
sponsored by IEEE, URSI, and APS
San Diego, California, July 5–12, 2007
Acknowledgments
The authors would like to thank Elizabeth McQuaid and Nick Varaljay for their CAD and fabrication efforts. This work was 
supported by the Wireless Communications group of the Integrated Vehicle Health Management (IVHM) project at NASA.
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NASA/TM—2008-215201 1 
RF MEMS Switches With SiC Microbridges for  
Improved Reliability  
 
Maximilian C. Scardelletti 
National Aeronautics and Space Administration 
Glenn Research Center 
Cleveland, Ohio 44135  
 
Christian A. Zorman 
Case Western Reserve University 
Cleveland, Ohio 44106  
 
Daniel R. Oldham 
National Aeronautics and Space Administration 
Glenn Research Center 
Cleveland, Ohio 44135  
Introduction 
Radio frequency (RF) microelectromechanical (MEMS) switches offer superior performance when 
compared to the traditional semiconductor devices such as PIN diodes or GaAs transistors (ref. 1). MEMS 
switches have a return loss (RL) better than –25 dB, negligible insertion loss (IL), isolation better than  
–30 dB, and near zero power consumption. However, RF MEMS switches have several drawbacks the 
most serious being long-term reliability. The ability for the switch to operate for millions or even billions 
of cycles is a major concern and must be addressed. MEMS switches are basically grouped in two 
categories, capacitive and metal-to-metal contact. The capacitive type switch consists of a movable metal 
bridge spanning a fixed electrode and separated by a narrow air gap and thin insulating material. The 
metal-to-metal contact type utilizes the same basic design but without the insulating material. After 
prolonged operation the metal bridges, in most of these switches, begin to sag and eventually fail to 
actuate. For the metal-to-metal type, the two metal layers may actually fuse together. Also if the switches 
are not packaged properly or protected from the environment moisture may build up and cause stiction 
between the top and bottom electrodes rendering them useless. Many MEMS switch designs have been 
developed and most illustrate fairly good RF characteristics. Nevertheless very few have demonstrated 
both great RF performance and ability to perform millions/billions of switching cycles (refs. 2 and 3). Of 
these, nearly all are of metal-to-metal type so as the frequency increases RF performance decreases. 
Silicon carbide (SiC) is widely known for its potential as a structural material for MEMS devices 
designed to operate in harsh environments (i.e., high temperature, radiation, wear, etc.). It has robust 
mechanical properties that make SiC very attractive for RF MEMS applications. When used as the 
structural material in micromachined bridges, the inherent stiffness and tensile stresses of SiC results in 
beams that are extremely resistant to sagging. Moreover, its chemical inertness makes SiC highly resistant 
to stiction. These properties makes SiC an ideal alternative to metals in surface micromachined bridge-
based RF MEMS switches for the reasons described above. Incorporation of insulating, amorphous SiC  
as the main mechanical structure in bridge-based RF switches eliminates the need to use a stiction-
preventing insulating film between the cantilever and transmission because the SiC itself is highly 
resistant to stiction, due to its chemical inertness coupled with its resistance to oxidation. 
In this paper we present RF MEMS switches which utilize 500- and 300-nm-thick amorphous  
SiC films for structural support in order to improve reliability. The MEMS switch incorporating the  
500-nm-thick microbridge actuates readily but suffers from poor RF performance while the switch  
with the 300-nm-thick SiC microbridge exhibits great RF performance but does not actuate reliably.  
 
NASA/TM—2008-215201 2 
MEMS Switch Design and Fabrication 
A schematic of the RF MEMS switch with a SiC microbridge is shown in figure 1(a). The MEMS 
switch is constructed out of coplanar waveguide (CPW) transmission line media. The SiC microbridge 
spans the width of the CPW completely thus making the effect of the bridge on the CPW negligible while 
in the up-position (off-state) and renders the MEMS switch in shunt configuration. Furthermore the by 
making the bridge extend completely over the CPW with no contact the MEMS switch can be biased 
directly and the CPW left at ground potential to bias other electronic components.  
The MEMS switch is fabricated on a sapphire wafer with a dielectric constant of 9.4 and a thickness 
of 500 µm. The CPW has a center conductor width (S), a slot width (W), and a ground width (G) of 130, 
60, and 300 µm, respectively, which equates to a characteristic impedance of 50Ω. Typical integrated 
circuit (IC) fabrication techniques where used to develop the switches. The CPW transmission lines 
consist of chrome (Cr) and gold (Au) with a thickness of 25 and 1200 nm, respectively. The SiC 
microbridge consisted of 300 nm or 500-nm-thick amorphous SiC films deposited by plasma enhanced 
chemical vapor deposition using a process detailed elsewhere (4). Since the SiC film is insulating, a thin 
layer of Cr/Au (25/120 nm) was deposited on top of the bridge to ensure a potential would be created 
between the bridge and the CPW to actuate the switch. An SEM micrograph of a released MEMS switch 
is shown in figure 1(b).  
  
 
(a)    
 
 
(b)  
 
Figure 1.—(a) Schematic diagram of the SiC-based RF MEMS switch, and (b) SEM micrograph 
of a SiC micrombridge-based switch. 
NASA/TM—2008-215201 3 
MEMS Switch Characterization 
The SiC RF MEMS shunt switch was characterized with the HP 8510C Vector Network Analyzer 
(VNA), a RF/Microwave Cascade Microtech probe station and 150 micron pitch ground-signal-ground 
probes from GGB Industries. MultiCal Software by NIST and a thru-reflect-line (TRL) calibration was 
performed to calibrate the system with a set of on-wafer TRL calibration standards. The RL and IL for the 
MEMS switch with the 500-nm-thick SiC microbridge while in the up position (off-state) is shown in 
figure 2(a). The RF performance of the switch is fairly good but when the switch is turned on when an 
actuation voltage of 52 V is applied to the DC bias pads and the bridge is set to the down position, as seen 
in figure 2(b), the RF performance is poor. This is principally due to the thickness of the SiC film which 
creates a small on-state capacitance. However, figure 3 shows that the switch actuates readily and reliably 
when repeatedly turned off and on. Furthermore this switch has been subjected to over 500,000 switching 
cycles to date with no signs of performance degradation. The noisiness illustrated in the figure is strictly 
from the actuation source. A much more precise and sensitive power source is needed to ensure accurate 
characterization. The RL and IL of the MEMS switch fabricated using the 300-nm-thick SiC microbridge 
is shown in figure 4. The RF performance in the down position (on-state) is outstanding. The S11 is less 
then 0.3 dB from 0 to 45 GHz and the isolation (S21) is better than –20 dB from 10 to 45 GHz. 
Unfortunately this switch does not actuate as readily as the 500 nm SiC switch. It is believed that this is 
due to the inherent stresses in the 300-nm-thick microbridge as compared with the 500-nm-thick 
microbridge and/or the influence of the metal conductor on the mechanical properties of this bridge. 
 
 
(a) Frequency (GHz)
0 10 20 30 40
M
ag
ni
tu
de
 (d
B)
-40
-30
-20
-10
0
S11 Up-position 
S21 Up-position 
    (b) Frequency (GHz)
0 10 20 30 40
M
ag
ni
tu
de
 (d
B
)
-40
-30
-20
-10
0
S11 Down-position 
S21 Down-position 
 
Figure 2.—SiC MEMS switch (a) in the up position (off-state) and (b) in the down position (on-state). 
 
Frequency (GHz)
0 10 20 30 40
M
ag
ni
tu
de
 (d
B)
-40
-30
-20
-10
0
S11 Actuation 
S21 Actuation 
    Frequency (GHz)
0 10 20 30 40
M
ag
ni
tu
de
 (d
B)
-40
-30
-20
-10
0
S11 Down-position 
S21 Down-position 
 
 Figure 3.—500 nm SiC MEMS actuation. Figure 4.—The 300 nm SiC switch in down position. 
 
NASA/TM—2008-215201 4 
Conclusion 
RF MEMS switches which utilize an insulating SiC microbridge as a structural layer have been 
developed. The MEMS switch with the 500 nm SiC microbridge exhibits good off-state performance but 
poor on-state characteristics due to the small on-state capacitance of the structure. However the 
mechanical reliability of the switch is exceptional. The MEMS switch with the 300 nm SiC thick 
microbridge has outstanding down position (on-state) RF characteristics which is do to the film being 
40 percent thinner than the 500 nm microbridge thus creating a much larger on-state capacitance. 
Unfortunately the 300-nm-thick microbridge does not actuate readily. It is believed that this is due to the 
stresses within the bridge.  
References 
1. Maximilian C. Scardelletti, George E. Ponchak, Afroz J. Zaman, and Richard Q. Lee, “RF MEMS 
Phase Shifters and Their Application in Phased Array Antennas,” IEEE Wireless and Microwave 
Technology Conference, Clearwater, Florida, April 7–8, 2005. 
2. McKillop, J., Fowler, T., Goins, D., and Nelson, R., “Design, Performance and Qualification of a 
Commercially Available MEMS Switch,” 36th European Microwave Conference, pp. 1399–1401, 
Sept. 2006. 
3. RadantMEMS, Stow, MA 01775.  
4. J.B. Summers, M.C. Scardelletti, R. Parro, and C.A. Zorman, “Development of Amorphous SiC for 
MEMS-based Microbridges, in Proceedings of SPIE—MEMS/MOEMS Components and Their 
Applications IV, San Jose CA, January 22–23, 2007, vol. 6464, pp. 64640H–1 to H–12. 
 
 
 
 
 
  
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RF MEMS Switches With SiC Microbridges for Improved Reliability 
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14. ABSTRACT 
Radio frequency (RF) microelectromechanical (MEMS) switches offer superior performance when compared to the traditional 
semiconductor devices such as PIN diodes or GaAs transistors. MEMS switches have a return loss (RL) better than -25 dB, negligible 
insertion loss (IL), isolation better than -30 dB, and near zero power consumption. However, RF MEMS switches have several drawbacks 
the most serious being long-term reliability. The ability for the switch to operate for millions or even billions of cycles is a major concern 
and must be addressed. MEMS switches are basically grouped in two categories, capacitive and metal-to-metal contact. The capacitive type 
switch consists of a movable metal bridge spanning a fixed electrode and separated by a narrow air gap and thin insulating material. The 
metal-to-metal contact type utilizes the same basic design but without the insulating material. After prolonged operation the metal bridges, 
in most of these switches, begin to sag and eventually fail to actuate. For the metal-to-metal type, the two metal layers may actually fuse 
together. Also if the switches are not packaged properly or protected from the environment moisture may build up and cause stiction 
between the top and bottom electrodes rendering them useless. Many MEMS switch designs have been developed and most illustrate fairly 
good RF characteristics. Nevertheless very few have demonstrated both great RF performance and ability to perform millions/billions of 
switching cycles. Of these, nearly all are of metal-to-metal type so as the frequency increases RF performance decreases. 
15. SUBJECT TERMS 
MEMS; CPW; SiC 
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