The feasibility of integrating the RF MEMS switches in space and wireless communication systems has generated tremendous interest in related design, fabrication and characterization methodologies. The space applications make long term reliability of the devices a very pertinent issue and involves both the process and device characterization. In this paper we describe the experimental setup and measurement results on RF MEMS switches fabricated for DC to 30GHz applications. The on-wafer experimental setup, based on standard manual microprobe station provides dual pulse actuation voltage waveforms with programmable period and amplitude. The usefulness of the dual-pulse testing is demonstrated by the minimal charge generation in the dielectric layer and capacitance measurements with negligible variations over long measurement periods

Collini, Cristian

Giacomozzi, Flavio

Lorenzelli, Leandro

Margesin, Benno

Pianegiani, Fernando

Rangra, Kamal J.

English

Unitn-eprints Research

       UNIVERSITY OF TRENTO  DEPARTMENT OF INFORMATION AND COMMUNICATION TECHNOLOGY   38050 Povo – Trento (Italy), Via Sommarive 14 http://www.dit.unitn.it              Electro-Mechanical Performance Analysis of RF MEMS Switches        Leandro Lorenzelli, Kamal J. Rangra, Cristian Collini,  Flavio Giacomozzi, Benno Margesin e Fernando Pianegiani        May 2004  Technical Report # DIT-03-058                                                         .   Electro-Mechanical Performance Analysis of RF MEMS Switches   Leandro Lorenzelli**, Kamal J Rangra1 ,Cristian Collini, Flavio Giacomozzi, Benno Margesin Fernando Pianegiani1 ITC-irst  Microsystems Division,  Via Sommarive,18 38050 Povo-Trento - Italy  1Department of Information Technology, University of Trento,  Via Sommarive,14 38050, Povo-Trento - Italy                                                 *Corresponding author, e-mail address: lorenzel@itc.it  ABSTRACT  The feasibility of integrating the RF MEMS switches in space and wireless communication systems has generated tremendous interest in related design, fabrication and characterization methodologies. The space applications make long term reliability of the devices a very pertinent issue and involves both the process and device characterization.  In this paper we describe the experimental setup and measurement results on RF MEMS switches fabricated for DC to 30GHz applications. The on-wafer experimental setup, based on standard manual microprobe station provides dual pulse actuation voltage waveforms with programmable period and amplitude, ranging from 10-5 to 1sec and 0-200 volts respectively. The usefulness of the dual-pulse testing is demonstrated by the minimal charge generation in the dielectric layer and capacitance measurements with negligible variations over long measurement periods.  1. INTRODUCTION  RF-MEMS devices and applications thereof are relatively new, among the prevalent MEMS technologies. The superior performance, volume, weight and cost reduction and the world wide market for communication systems with ever increasing emphasis on mentioned attributes make it a technology being followed by large number of research groups in industry and universities.  RF-MEMS switch, most promising of the RF devices, is being explored as “near ideal” switching device in a number of existing communication systems and subsystems with frequency range up to 80GHz [1,2].  The electrostatically actuated metal beam based switches out perform the other solid state switches such as FETs, HEMTs and PIN diodes [3], currently being in use. Although , RF  MEMS switches have the speed limitation as compared to the semiconductor counterparts, they exhibit very low insertion loss due the low resitivity  of the metal and  high isolation due to 1-3  microns of physical separation between switching elements.  The inevitable I-V non-linearities   associated with semiconductor junctions in PIN and FET devices are also non-existent in such  devices, except for the minor hysterises  in   C-V in shunt switches. This improves the distortion characteristics and power handling capabilities in the devices which becomes  important consideration in  systems having large number of switches and limited power resources. The RF MEMS switches exhibit no measurable harmonics or intermodulations and require negligible quiescent current consumption; typical switching energy is less than 10nJ [4]. The monolithic batch fabrication of switches when used as components in an integrated RF system, improves the overall performance and makes it more cost effective as compared to the design approach with solid state switches.   The electro-mechanical response of the RF MEMS switch can be approximated by a lumped one dimensional model [5]. The model considers the capacitive switch as a single rigid parallel plate capacitor, with top plate suspended over the fixed bottom plate by an ideal linear spring. The capacitor has effective area A and is separated by distance go. The upper plate, with material spring constant k  also referred to as membrane is movable under electrostatic actuation when a suitable bias is applied between the plates, bringing the gap to a minimum depending upon the dielectric layer thickness on the bottom plate. The same configuration can be used as a series switch with minor modification on the signal carrying conductor. The membrane consists of a thin metal layer (Au, Al or Cu, 2-3 mm thick) suspended across dielectric or polymer posts. The bottom plate is a 1.5 - 2 mm L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  On-wafer  electro-mechanical characterization of silicon MEMS switches thick metal layer with a 200-300 nm of dielectric layer on it. In the ‘OFF’ state, large air gap between the top and bottom metal plates constitutes a high impedance. The thin metal membrane deflects towards the bottom electrode under the effect of a bias voltage and  is capacitibly coupled to the bottom electrode  when the applied bias exceeds the pull-in voltage  Vpi, thus lowering  the switch impedance. The ‘ON’ to ‘OFF’ capacitance ratio determines the isolation and insertion loss of the device. The pull-in bias for this simple lumped model is given by [5],  Vpi = Akg030278e  where eo  is the dielectric constant of the dielectric material.  The spring constant for a switch depends on the geometrical dimensions of the beam, Young’s modulus and Poisson’s ratio. The process related residual stress (of the beam) dependence of  Vpi  coupled with environmental effects such humidity in the measurement setup and charge injection into the dielectric insulation layer,  lead to drifts in Vpi   over time and shorter life span of the devices. In view of the long term reliability issues for applications such as space communication variations in Vpi, switching speed and the life time are of crucial importance.  In the following sections we discuss the variation in actuation voltage, bridge capacitance, signal rise and fall  time for the fabricated RF MEMS test structures using an experimental setup developed around a standard micro probe station.                   2. FABRICATION  A series of test structures implemented for the technology characterization have been fabricated using a SPST/SPDT RF-MEMS switch fabrication process on 5kW cm, <100> p-type Si wafers. To minimize the dielectric losses devices and test structures are implemented  in CPW configuration with standard 50W line impedance. The fabrication steps for a representative RF MEMS switch are outlined in Figure 1. In order to integrate the 50kW resistors and DC blocking capacitors, devices are fabricated using a seven mask fabrication process. As shown in Fig.1a the biasing  resistors are patterned  in 300nm LPCVD polysilicon layer deposited on 1000 nm thick thermal oxide. The contact holes are patterned in 200 nm TEOS SiO2 layer, deposited on boron implanted poly silicon with a dopant activation cycle at 925o C for 60 minutes. This is followed by sputter deposition of 60 nm of Ti layer and 200 nm of TiN, by reactive sputter deposition technique to define underpass lines and diffusion barriers on the polysilicon contacts (Fig.1b). The  underpass and actuation pad areas are also fabricated in polysilicon in a separate process run with thicker oxide insulation layer to achieve better isolation.  Via holes are defined in low temperature SiO2 or PECVD Si3 N4  in step (Fig.1c). Si3 N4   covered actuation pads are used for analyzing the drift in Vpi due to charge injection compared to SiO2 insulation.  L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  On-wafer  electro-mechanical characterization of silicon MEMS switches Figure 1. Schematic outline of the fabrication process for RF-MEMS switch.  To pattern the movable membrane a 3 micron thick   photo resist layer, hard backed at  200o C to ensure good step coverage, is used as sacrificial layer (Fig.1d).  As a seed layer for electrochemical deposition of Au , a 20/100/10 nm thick Cr/Au/Cr layer is deposited by PVD followed by  a 3 µm thick photoresist layer to define the interconnects and the movable membrane in step (e). After a wet Cr etch, 1.5 micron Au layer is selectively grown in gold sulfite bath. The selected electroplating parameters result in required tensile stress. The next photolithography (Fig.1e) defines the CPW lines and anchor posts for the microbridge.  It is further followed by the selective deposition of 4 mm thick gold layer. After the removal of last masking and seed layers, the membrane is released using a modified plasma ashing process to avoid stiction problems.  3. TEST STRUCTURES DESIGN  The mechanical properties of the fabricated switches are characterized using composite electromechanical test structures specially designed and fabricated for easy access and shorter measurement cycles.   BridgesW = 150 umd=88um1 23 45 a.)   b.)  Figure 2. a.) Photograph of a test structure with contemporaneously actuated bridge;  b.) Photograph of an other test  structure with separately bridge contact.   In order to study the geometrical contributes in the elctromechanical actuation, test structures having bridge with different length and width have been described. Each structure consists of five microbridges with length (850 to 450 mm), width (150mm), thickness (1mm) and gap (3mm) corresponding to the SPST and SPDT devices under consideration. Different configuration of  microbridges have been implemented to be actuated  individually or contemporaneously. Figure 2 shows the photograph of a basic test structures utilized for the electromechanical tests.  4. EXPERIMENTAL RESULTS  The experimental setups used for studying the mechanical properties and experimental verification of the parameter such as actuation voltage, capacitance and ON – OFF timings are shown by the block diagram in Figure 3.                               a.)                b.)         Figure 3. Experimental setups for a.) quasi-static C-V measurements, b) life time measurements.    The quasi-static CV measurements on the structures are performed using the setup shown in Figure 3a consisting of an HP 4192A impedance analyzer, HP 4145B HP4145B Semiconductor Parameter AnalyzerPCHP4192A LF Impedance Analyzer DUTKarl Suss PM8Manual ProbeGPIBHP8112APulse Generator PCLeCroy LC574A OscilloscopeDUTKarl Suss PM8Manual ProbeGPIBKeithley 2410Source Meter BIAS Amplifier TTL SignalActuation VoltageRF Signal GeneratorL. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  On-wafer  electro-mechanical characterization of silicon MEMS switches semiconductor parameter analyzer and Karl Suss PM8 probe station. Measurements  with applied actuation voltage in the  range of 0 - 35 volts show minor increase in the obtained capacitance values due the lowering of the air bridges during the subsequent measurements. Increasing the applied voltage range to 1.5 to 2 times the pull-in voltage provides  direct measurements of the pull-in voltage  and capacitance of each bridge.  18 20 22 24 26 28 30 32 34 360.01.0x10-122.0x10-123.0x10-124.0x10-125.0x10-126.0x10-127.0x10-128.0x10-12Capacitance [F]Actuation voltage [V] Figure 4.  Set of three successive C-V measurement of  four (1-4) microbridges .     Figure 4 shows the CV plots for three structures having  the same geometrical characteristics on a wafer with SiO 2 as dielectric material, clearly identifying the individual actuation steps corresponding to respective ON – capacitance values.  The variations in the pull-in voltage and associated capacitance for 4 composite test structures are shown by Figure 5a and 5b . The comparatively large variations in pull-in voltage and corresponding capacitance in the case of  a few microbridges, may be attributed to the process non-uniformities and deformation of the membranes, changing the effective spring constant. CV measurements repeated over a time period, in case of devices with silicon nitride on actuation pads show a significant drift in  pull-in voltage values as compared to the devices with oxide insulation. Changes in capacitance values for certain devices may be due to the initial beam deformation and subsequent non-uniform bending of the beam under actuation force, as  shown in Figure 6.  1 2 3 424262830323436Actuation Voltage [V]Bridge number a.) 1 2 3 41.0x10 -122.0x10 -123.0x10 -124.0x10 -125.0x10 -126.0x10 -127.0x10 -128.0x10 -12Capacitance [F]Bridge number b.)  Figure 5.  a.) Actuation voltage and b.) capacitance vs bridge plots.    10 11 12 13 144.0x10-136.0x10-138.0x10-131.0x10-121.2x10-121.4x10-121.6x10-121.8x10-122.0x10-12Capacitance [F]Actuation voltage [V]•2. Initial stress•3. Switch on•1. Switch offCapacitance [F] L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  On-wafer  electro-mechanical characterization of silicon MEMS switches  Figure 6. Different capacitance values ascribed to different microbridge bending effect .  The block diagram of the set up for life time measurements and quasi static CV measurements using custom built actuation signal source is shown in Figure 3b. The actuation signal amplitude can be varied fro m 0-200 volts while 50% duty period ranges from  10 -5 sec to 1sec.  The reduction in actuation signal  amplitude after the device has switched  on results in reduction of  electric field across the dielectric layer by  more than 1 MV/cm. The field is enough to keep the device in on state while reducing the charging effects considerably.   10 20 30 40 50 60 70 80 901.5x10-132.0x10-132.5x10-133.0x10-133.5x10-134.0x10-134.5x10-135.0x10-13Capacitance [F]Actuation voltage [V] Figure 7.  C-V measurements after cycle of  stress procedures.   The drift in pull-in voltage is considerably reduced for repeated measurements on the device as shown in Figure 7. The field reduction also averts the stiction problems and premature failure of the switch [6].     5 CONCLUSIONS  In this preliminary work, an experimental setup and test structures for on-wafer characterization of the electromechanical performances of the suspended bridge have been illustrated in depth. The bridge capacitance and actuation voltage variations due to charging effects during quasi-static C-V measurements have been  investigated. In the experimental measurements the large variations in pull-in voltage and corresponding capacitance, have been observed in silicon nitride based devices with respect the silicon oxide based one. The different behavior may be attributed both to the charge stored in the silicon nitride during the fabrication and in the minor charge mobility in the silicon oxide. Future work will be devoted to examine carefully the minimal charge generation in the dielectric layer and capacitance measurements over long measurement periods.  ACKNOLEDGMENTS  This work is supported by ESA/ESTEC contract nr. 14628/NL/CK “Microwave electrostatic micro-machined devices for on-board applications”    REFERENCES  [1] G Gabreil M. Rebiez and Jeremy B. Muldabin,“RF MEMS Switches and switch circuits”, Technical Digest IEEE Microwave magazine, 2001, 59-71.    [2] Harrie A. Tilmans “MEMS components for wireless communications”, EuroSensors XVI, 16th  European Conference on Solid State transducers, 2002, 1-34, Prague, Czech Republic.   [3] J. Rizk,G-L Tan, J. Muldabin, G. Rebeiz “High Isolation W-band MEMS switches”, IEEE Microwave and Wireless components Letters, 2001, 11(1), 10-12.  [4 Z. J. Yao , S.Chen, S. Eshelman,  D. Denniston and C. Goldsmith, “Micromachined low loss microwave switches” IEEE J. MEMS 8, 1999, 129-34.  [5] C. Goldsmith, J. Randall, S. Eshelman, T. H. Lin, D. Denniston, S. Chen and B. Norvell, “Characteristics of micromachined switches at Microwave Frequencies”, IEEE MIT -S Digest 1996, 1141-1144.  [6] C. Goldsmith, J. Ehmke, A. Malczewski, B. Pillans, A. Yao, J. Brank and M. Eberly, “Lifetime characterization  of capacitive RF MEMS switches”, IEEE MIT-S Digest  2001, 227-230.  

Electro-Mechanical Performance Analysis of RF MEMS Switches

University of Trento : UNITN-eprints

  
 
 
 
 
 
UNIVERSITY 
OF TRENTO 
 DEPARTMENT OF INFORMATION AND COMMUNICATION TECHNOLOGY 
  
38050 Povo – Trento (Italy), Via Sommarive 14 
http://www.dit.unitn.it 
 
 
 
 
 
 
 
 
 
 
 
 
 
Electro-Mechanical Performance Analysis of RF MEMS Switches
 
 
 
 
 
 
 
 
Leandro Lorenzelli, Kamal J. Rangra, Cristian Collini,  
Flavio Giacomozzi, Benno Margesin e Fernando Pianegiani 
 
  
 
 
 
 
May 2004 
 
Technical Report # DIT-03-058 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
. 
  
Electro-Mechanical Performance Analysis of RF MEMS Switches 
 
 
Leandro Lorenzelli**, Kamal J Rangra1 ,Cristian Collini, Flavio Giacomozzi, Benno Margesin 
Fernando Pianegiani1 
ITC-irst  Microsystems Division,  Via Sommarive,18 38050 Povo-Trento - Italy  
1Department of Information Technology, University of Trento,  
Via Sommarive,14 38050, Povo-Trento - Italy 
 
 
                                              
*Corresponding author, e-mail address: lorenzel@itc.it 
 
ABSTRACT 
 
The feasibility of integrating the RF MEMS switches in 
space and wireless communication systems has generated 
tremendous interest in related design, fabrication and 
characterization methodologies. The space applications 
make long term reliability of the devices a very pertinent 
issue and involves both the process and device 
characterization.  In this paper we describe the 
experimental setup and measurement results on RF MEMS 
switches fabricated for DC to 30GHz applications. The on-
wafer experimental setup, based on standard manual 
microprobe station provides dual pulse actuation voltage 
waveforms with programmable period and amplitude, 
ranging from 10-5 to 1sec and 0-200 volts respectively. The 
usefulness of the dual-pulse testing is demonstrated by 
the minimal charge generation in the dielectric layer and 
capacitance measurements with negligible variations over 
long measurement periods. 
 
1. INTRODUCTION 
 
RF-MEMS devices and applications thereof are relatively 
new, among the prevalent MEMS technologies. The 
superior performance, volume, weight and cost reduction 
and the world wide market for communication systems 
with ever increasing emphasis on mentioned attributes 
make it a technology being followed by large number of 
research groups in industry and universities.  RF-MEMS 
switch, most promising of the RF devices, is being 
explored as “near ideal” switching device in a number of 
existing communication systems and subsystems with 
frequency range up to 80GHz [1,2].  The electrostatically 
actuated metal beam based switches out perform the other 
solid state switches such as FETs, HEMTs and PIN 
diodes [3], currently being in use. Although , RF  MEMS 
switches have the speed limitation as compared to the 
semiconductor counterparts, they exhibit very low 
insertion loss due the low resitivity  of the metal and  high 
isolation due to 1-3  microns of physical separation 
between switching elements.  The inevitable I-V non-
linearities   associated with semiconductor junctions in 
PIN and FET devices are also non-existent in such  
devices, except for the minor hysterises  in   C-V in shunt 
switches. This improves the distortion characteristics and 
power handling capabilities in the devices which becomes  
important consideration in  systems having large number 
of switches and limited power resources. The RF MEMS 
switches exhibit no measurable harmonics or 
intermodulations and require negligible quiescent current 
consumption; typical switching energy is less than 10nJ 
[4]. The monolithic batch fabrication of switches when 
used as components in an integrated RF system, improves 
the overall performance and makes it more cost effective 
as compared to the design approach with solid state 
switches.  
 The electro-mechanical response of the RF 
MEMS switch can be approximated by a lumped one 
dimensional model [5]. The model considers the capacitive 
switch as a single rigid parallel plate capacitor, with top 
plate suspended over the fixed bottom plate by an ideal 
linear spring. The capacitor has effective area A and is 
separated by distance go. The upper plate, with material 
spring constant k  also referred to as membrane is movable 
under electrostatic actuation when a suitable bias is 
applied between the plates, bringing the gap to a minimum 
depending upon the dielectric layer thickness on the 
bottom plate. The same configuration can be used as a 
series switch with minor modification on the signal 
carrying conductor. The membrane consists of a thin metal 
layer (Au, Al or Cu, 2-3 mm thick) suspended across 
dielectric or polymer posts. The bottom plate is a 1.5 - 2 mm 
L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  
On-wafer  electro-mechanical characterization of silicon MEMS switches 
thick metal layer with a 200-300 nm of dielectric layer on it. 
In the ‘OFF’ state, large air gap between the top and 
bottom metal plates constitutes a high impedance. The 
thin metal membrane deflects towards the bottom 
electrode under the effect of a bias voltage and  is 
capacitibly coupled to the bottom electrode  when the 
applied bias exceeds the pull-in voltage  Vpi, thus 
lowering  the switch impedance. The ‘ON’ to ‘OFF’ 
capacitance ratio determines the isolation and insertion 
loss of the device. The pull-in bias for this simple lumped 
model is given by [5], 
 
Vpi = 
A
kg
0
3
0
27
8
e
 
 
where eo  is the dielectric constant of the dielectric material.  
The spring constant for a switch depends on the 
geometrical dimensions of the beam, Young’s modulus 
and Poisson’s ratio. The process related residual stress (of 
the beam) dependence of  Vpi  coupled with 
environmental effects such humidity in the measurement 
setup and charge injection into the dielectric insulation 
layer,  lead to drifts in Vpi   over time and shorter life span 
of the devices. In view of the long term reliability issues 
for applications such as space communication variations 
in Vpi, switching speed and the life time are of crucial 
importance.  In the following sections we discuss the 
variation in actuation voltage, bridge capacitance, signal 
rise and fall  time for the fabricated RF MEMS test 
structures using an experimental setup developed around 
a standard micro probe station. 
                  
2. FABRICATION 
 
A series of test structures implemented for the 
technology characterization have been fabricated using a 
SPST/SPDT RF-MEMS switch fabrication process on 
5kW cm, <100> p-type Si wafers. To minimize the dielectric 
losses devices and test structures are implemented  in 
CPW configuration with standard 50W line impedance. 
The fabrication steps for a representative RF MEMS 
switch are outlined in Figure 1. In order to integrate the 
50kW resistors and DC blocking capacitors, devices are 
fabricated using a seven mask fabrication process. As 
shown in Fig.1a the biasing  resistors are patterned  in 
300nm LPCVD polysilicon layer deposited on 1000 nm 
thick thermal oxide. The contact holes are patterned in 200 
nm TEOS SiO2 layer, deposited on boron implanted poly 
silicon with a dopant activation cycle at 925o C for 60 
minutes. This is followed by sputter deposition of 60 nm 
of Ti layer and 200 nm of TiN, by reactive sputter 
deposition technique to define underpass lines and 
diffusion barriers on the polysilicon contacts (Fig.1b). The  
underpass and actuation pad areas are also fabricated in 
polysilicon in a separate process run with thicker oxide 
insulation layer to achieve better isolation.  Via holes are 
defined in low temperature SiO2 or PECVD Si3 N4  in step 
(Fig.1c). Si3 N4   covered actuation pads are used for 
analyzing the drift in Vpi due to charge injection compared 
to SiO2 insulation. 
 
L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  
On-wafer  electro-mechanical characterization of silicon MEMS switches 
Figure 1. Schematic outline of the fabrication process for 
RF-MEMS switch. 
 
To pattern the movable membrane a 3 micron thick   photo 
resist layer, hard backed at  200o C to ensure good step 
coverage, is used as sacrificial layer (Fig.1d).  As a seed 
layer for electrochemical deposition of Au , a 20/100/10 nm 
thick Cr/Au/Cr layer is deposited by PVD followed by  a 3 
µm thick photoresist layer to define the interconnects and 
the movable membrane in step (e). After a wet Cr etch, 1.5 
micron Au layer is selectively grown in gold sulfite bath. 
The selected electroplating parameters result in required 
tensile stress. The next photolithography (Fig.1e) defines 
the CPW lines and anchor posts for the microbridge.  It is 
further followed by the selective deposition of 4 mm thick 
gold layer. After the removal of last masking and seed 
layers, the membrane is released using a modified plasma 
ashing process to avoid stiction problems. 
 
3. TEST STRUCTURES DESIGN 
 
The mechanical properties of the fabricated switches are 
characterized using composite electromechanical test 
structures specially designed and fabricated for easy 
access and shorter measurement cycles.  
 
Bridges
W = 150 um
d=88um
1 2
3 4
5
 
a.) 
 
 
b.)  
Figure 2. a.) Photograph of a test structure with 
contemporaneously actuated bridge;  b.) Photograph of 
an other test  structure with separately bridge contact.  
 
In order to study the geometrical contributes in the 
elctromechanical actuation, test structures having bridge 
with different length and width have been described. Each 
structure consists of five microbridges with length (850 to 
450 mm), width (150mm), thickness (1mm) and gap (3mm) 
corresponding to the SPST and SPDT devices under 
consideration. Different configuration of  microbridges 
have been implemented to be actuated  individually or 
contemporaneously. Figure 2 shows the photograph of a 
basic test structures utilized for the electromechanical 
tests. 
 
4. EXPERIMENTAL RESULTS 
 
The experimental setups used for studying the mechanical 
properties and experimental verification of the parameter 
such as actuation voltage, capacitance and ON – OFF 
timings are shown by the block diagram in Figure 3.   
 
         
  
 
 
 
             a.) 
  
  
 
 
 
 
 
 
 
    b.) 
 
 
 
 
 
 
 
 
Figure 3. Experimental setups for a.) quasi-static C-V 
measurements, b) life time measurements.  
 
 The quasi-static CV measurements on the structures are 
performed using the setup shown in Figure 3a consisting 
of an HP 4192A impedance analyzer, HP 4145B 
HP4145B 
Semiconductor 
Parameter Analyzer
PC
HP4192A 
LF Impedance Analyzer 
DUT
Karl Suss PM8
Manual Probe
GPIB
HP8112A
Pulse Generator 
PC
LeCroy LC574A 
Oscilloscope
DUT
Karl Suss PM8
Manual Probe
GPIB
Keithley 2410
Source Meter 
BIAS Amplifier 
TTL Signal
Actuation Voltage
RF Signal Generator
L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  
On-wafer  electro-mechanical characterization of silicon MEMS switches 
semiconductor parameter analyzer and Karl Suss PM8 
probe station. Measurements  with applied actuation 
voltage in the  range of 0 - 35 volts show minor increase in 
the obtained capacitance values due the lowering of the 
air bridges during the subsequent measurements. 
Increasing the applied voltage range to 1.5 to 2 times the 
pull-in voltage provides  direct measurements of the pull-
in voltage  and capacitance of each bridge.  
18 20 22 24 26 28 30 32 34 36
0.0
1.0x10-12
2.0x10-12
3.0x10-12
4.0x10-12
5.0x10-12
6.0x10-12
7.0x10-12
8.0x10-12
C
ap
ac
ita
n
ce
 [
F
]
Actuation voltage [V]
 
Figure 4.  Set of three successive C-V measurement of  
four (1-4) microbridges .    
 
Figure 4 shows the CV plots for three structures having  
the same geometrical characteristics on a wafer with SiO 2 
as dielectric material, clearly identifying the individual 
actuation steps corresponding to respective ON – 
capacitance values.  
The variations in the pull-in voltage and associated 
capacitance for 4 composite test structures are shown by 
Figure 5a and 5b . The comparatively large variations in 
pull-in voltage and corresponding capacitance in the case 
of  a few microbridges, may be attributed to the process 
non-uniformities and deformation of the membranes, 
changing the effective spring constant. CV measurements 
repeated over a time period, in case of devices with silicon 
nitride on actuation pads show a significant drift in  pull-in 
voltage values as compared to the devices with oxide 
insulation. Changes in capacitance values for certain 
devices may be due to the initial beam deformation and 
subsequent non-uniform bending of the beam under 
actuation force, as  shown in Figure 6. 
 
1 2 3 4
24
26
28
30
32
34
36
A
ct
u
at
io
n
 V
o
lta
g
e 
[V
]
Bridge number
 
a.) 
1 2 3 4
1.0x10 -12
2.0x10 -12
3.0x10 -12
4.0x10 -12
5.0x10 -12
6.0x10 -12
7.0x10 -12
8.0x10 -12
C
ap
ac
it
an
ce
 [F
]
Bridge number
 
b.) 
 
Figure 5.  a.) Actuation voltage and b.) capacitance vs 
bridge plots.  
  
10 11 12 13 14
4.0x10-13
6.0x10-13
8.0x10-13
1.0x10-12
1.2x10-12
1.4x10-12
1.6x10-12
1.8x10-12
2.0x10-12
C
ap
ac
ita
nc
e 
[F
]
Actuation voltage [V]
•2. Initial stress
•3. Switch on
•1. Switch offC
ap
ac
ita
nc
e 
[F
]
 
L. Lorenzelli, K. Rangra, C. Collini, F. Giacomozzi , B.  Margesin, F. Pianegiani  
On-wafer  electro-mechanical characterization of silicon MEMS switches 
 
Figure 6. Different capacitance values ascribed to 
different microbridge bending effect . 
 
The block diagram of the set up for life time measurements 
and quasi static CV measurements using custom built 
actuation signal source is shown in Figure 3b. The 
actuation signal amplitude can be varied fro m 0-200 volts 
while 50% duty period ranges from  10 -5 sec to 1sec.  The 
reduction in actuation signal  amplitude after the device 
has switched  on results in reduction of  electric field 
across the dielectric layer by  more than 1 MV/cm. The 
field is enough to keep the device in on state while 
reducing the charging effects considerably.   
10 20 30 40 50 60 70 80 90
1.5x10-13
2.0x10-13
2.5x10-13
3.0x10-13
3.5x10-13
4.0x10-13
4.5x10-13
5.0x10-13
C
ap
ac
ita
nc
e 
[F
]
Actuation voltage [V]
 
Figure 7.  C-V measurements after cycle of  stress 
procedures.  
 
The drift in pull-in voltage is considerably reduced for 
repeated measurements on the device as shown in Figure 
7. The field reduction also averts the stiction problems and 
premature failure of the switch [6].   
 
 
5 CONCLUSIONS 
 
In this preliminary work, an experimental setup and test 
structures for on-wafer characterization of the 
electromechanical performances of the suspended bridge 
have been illustrated in depth. The bridge capacitance and 
actuation voltage variations due to charging effects 
during quasi-static C-V measurements have been  
investigated. In the experimental measurements the large 
variations in pull-in voltage and corresponding 
capacitance, have been observed in silicon nitride based 
devices with respect the silicon oxide based one. The 
different behavior may be attributed both to the charge 
stored in the silicon nitride during the fabrication and in 
the minor charge mobility in the silicon oxide. 
Future work will be devoted to examine carefully the 
minimal charge generation in the dielectric layer and 
capacitance measurements over long measurement 
periods. 
 
ACKNOLEDGMENTS  
This work is supported by ESA/ESTEC contract nr. 
14628/NL/CK “Microwave electrostatic micro-machined 
devices for on-board applications” 
 
 
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[2] Harrie A. Tilmans “MEMS components for wireless 
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[3] J. Rizk,G-L Tan, J. Muldabin, G. Rebeiz “High Isolation W-
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Kamal J. Rangra

Fernando Pianegiani

Archivio della ricerca - Fondazione Bruno Kessler

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Electro-Mechanical Performance Analysis of RF MEMS Switches

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