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.