RF MEMS Simulation–High Isolation CPW Shunt Switches
Authored by:Desmond TanJames Chow
Ansoft Corporation
Ansoft 2003 / Global Seminars: Delivering Performance
Presentation #4
What’s MEMSw Micro-Electro-Mechanical Systems (MEMS) is the integration of
mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology
w The electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes),
w The micromechanical components are fabricated using compatible “micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
MEMS
Electronics
Mechanical
MEMS Revolution
Silicon-Based Microelectronics
Micromachining Technology
Integrated CircuitsSystem-on-chipsDecision Making
SensorsGather Information From
MechanicalBiologicalThermalChemicalOptical
Magnetic
Processing
ActuatorsMoving, Filtering, Rotating, Positioning, Pumping
ControlSignals
Technological Advantagesw Lighter, Smaller, Lower Power, More Reliable, Lower
Cost
Application - OpticsMiniaturized combinations of Optics, Electronics, and Mechanics form technology fields of
MicroOptoElectroMechanicalSystems (MOEMS)
Mechanical
Optical Electrical
MOEMS
Opto
Electronics
MEMSOpto
Mechanics
Use of microelectronics batch-processing techniques makes possible the design and construction of microsystems
Scanning Mirrors
Rotating Actuators
Membrane Switches
Application – Automotive Operation and safety
Force SensorsBrakes, Throttle Pedals
Pressure and inertia sensorsFor braking control
Silicon Nozzles forFuel injection
Air ConditioningCompressor
sensor
Manifold Air Pressure sensor
Inertial Navigation SensorsAcceleration, Yaw Rate
MicromachinedAccelerometer
For Airbag
Tire Pressure Sensors
Fuel SensorsLevel and vapour
pressure
Exhaust temperaturesensor
Application – Medical Diagnostics and Treatment
Blood Pressure Sensor
Cortical Probe
Brain Surgery
Muscle Stimulator
“MICRO” characteristics of MEMS makes it
suitable for medical applications
Typical Actuators
Thermal Actuators• Single Layer lateral-motion actuator
• Motion caused by uneven ohmicheating
• Returns to rest position after cooling
• Advantages : Low potentials, Large Forces, Large Deflections
• Disadvantages : Low Frequencies, Moderate Power, Large Arrays
Electrostatic Actuators• Opposing flexure-supported electrodes
• Continuous force balance operation – spring force counteracts electrode force
• Basic structure used in many forms : comb drives, micromirrors, membranes
• Advantages : Low Power, High Frequency, Simple
• Disadvantages : High Potentials, Low Force, Nonlinearity
Simulation of Shunt Capacitive CPW Switches
• An electromagnetic model for membrane microelectromechanical systems (MEMS) shunt
switches for microwave/millimeter-wave applications is generated.
• These shunt capacitive CPW switches are modeled on up and down capacitance using quasi static 3D
solvers and the transmission and isolation characteristics from full wave FEM solvers.
Shunt Capacitive Switch over a CPW- Up State Position
Cross Section
Top View
MEMS bridge
g W
Silicon Substrate
Silicon Nitride layer
LB
CPW
g
W
LB
w(membrane width)
Shunt Capacitive Switch over a CPW- Down State Position
Cross Section
Top View
CPW
g
W
LB
w(membrane width)
MEMS bridge
g W
Silicon Substrate
Silicon Nitride layer
MEMS Process Information
The Mems switch is fabricated on high-resistivity silicon substrate (εr = 11.9).
Gold 2umSilicon Nitride 0.2umGold 0.8umSilicon Dioxide 1um
Silicon 400um
MEMS Modeling Flow Chart
The MEMs switch is simulated on both up and down state of the MEMs bridge
SIQ3DHFSS
EM3DFS
Insertion &Return Loss
Low Frequency Capacitance
UpwardForce
ANSOFT OPTIMETRICS
CPW Switch
3D Model of the Up-State Switch
Plan View
Full geometry with substrate block Cross Section View showing up state
3D Model of the Down-State Switch
Full geometry with substrate block
Plan View
Cross Section View showing down state
Parametric and Optimetric Capability -Height
Membrane HeightParameterization
Parametric and Optimetric Capability -Width
Signal Trace WidthParameterization
Circuit Model for MEMs Shunt Switch
Cup/Cdown, L and R in series
Lumped elements of the bridge with capacitance up/down state
Rline/2 Rline/2
The switch shunt impedance :
Ztotal = R + jwL + 1/jwC
Where C = Cup or Cdown depend on the switch position.
The LC series resonant frequency of the shunt switch :
fo = 1/(2π√LC)
Therefore impedance of the shunt switch can be approximated by
1/jwC, for f << fo
Ztotal = R, for f = fo
jwL, for f >> fo
ANSOFT HFSS is used to extract the Return and
Insertion LossFor 110um membrane width (w)
S-Parameter for Membrane Width 110umat Up State Position
S-Parameter for Membrane Width 110umat Down State Position
At Up State Position:The Cutoff Frequency fc is expected to be above 800 GHz
At Down State Position:The Resonant Frequency fO is 50.38 GHz with Frequency sweep of 0.1 to 80 GHz
ANSOFT Optimetricsis used to extract the Return and
Insertion Losswith varying membrane width (w)
from 40 to 100um
S11 for Membrane Width of 40 to 70um in Up State Position
S21 for Membrane Width of 40 to 70um in Up State Position
S11 for Membrane Width of 40 to 100um in Down State Position
S21 for Membrane Width of 40 to 100um in Down State Position
ANSOFT SI (Q3D) with Optimetricsis used to extract the Capacitance value at Quasi Static Frequency
with varying membrane width (w)from 40 to 100um at Down State Position
The Capacitance is extracted by Ansoft SIwhen the membrane is at Down State Position,
this was formed between the membrane and the transmission line of the CPW structure.
These capacitance values dependent both on the dielectric constant of the oxide and the
conductor area.
Capacitance values with varying membrane width at Down State Position
Results obtained were consistent with the fundamental capacitance equation.
ANSOFT Electromagnetic 3D Field Simulator (EM3DFS) with Optimetricsis used to extract the downward force
produced by the membrane bridgewith varying height
Down State Force with varying membrane Z position with different static actuation voltage
Summaryw An RF MEMS switch model was created in Ansoft 3D
modeler which could be used in all Ansoft EDA toolsw S-parameter extraction using High Frequency Structure
Simulator (HFSS), identification of resonant and cutoff frequencies
w Capacitance value of MEMS bridge extracted using SI Q3D
w Downward force extracted using Maxwell EM3Dw Dimensional variation performed using Ansoft
Optimetrics allow various “what if” analysis on designw This illustration of Ansoft tools for RF MEMS design
shows the potential for simulation of other MEMS devices
References
w [1]. Elliot R. Brown, “RF-MEMS Switches for Reconfigurable Integrated Circuits”, IEEE Trans. on Microwave Theory and Techniques, Vol. 46, no.11, November 1998
w [2]. Jeremy B. Muldavin, Gabriel M. Rebeiz, " High-Isolation CPW MEMS Shunt Switches ", IEEE Trans. on Microwave Theory and Techniques, Vol. 48, no.6, June 2000.