RF MEMS Simulation High Isolation CPW Shunt Switches Authored by: Desmond Tan James Chow Ansoft Corporation Ansoft 2003 / Global Seminars: Delivering Performance Presentation #4
What s MEMS Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology The electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), 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 Micromachining Technology Silicon-Based Microelectronics Integrated Circuits System-on-chips Decision Making Control Signals Processing Sensors Gather Information From Mechanical Biological Thermal Chemical Optical Magnetic Actuators Moving, Filtering, Rotating, Positioning, Pumping
Technological Advantages Lighter, Smaller, Lower Power, More Reliable, Lower Cost
Application - Optics Rotating Actuators Mechanical Opto MEMS Mechanics MOEMS Miniaturized combinations of Optics, Electronics, and Mechanics form technology fields of MicroOptoElectroMechanical Systems (MOEMS) Scanning Mirrors Optical Opto Electronics Electrical Membrane Switches Use of microelectronics batch-processing techniques makes possible the design and construction of microsystems
Application Automotive Operation and safety Inertial Navigation Sensors Acceleration, Yaw Rate Micromachined Accelerometer For Airbag Silicon Nozzles for Fuel injection Air Conditioning Compressor sensor Manifold Air Pressure sensor Fuel Sensors Level and vapour pressure Exhaust temperature sensor Tire Pressure Sensors Pressure and inertia sensors For braking control Force Sensors Brakes, Throttle Pedals
Application Medical Diagnostics and Treatment Blood Pressure Sensor Muscle Stimulator Cortical Probe Brain Surgery MICRO characteristics of MEMS makes it suitable for medical applications
Typical Actuators Thermal Actuators Single Layer lateral-motion actuator Motion caused by uneven ohmic heating 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 L B MEMS bridge g W Silicon Substrate Silicon Nitride layer Cross Section CPW g L B W Top View w(membrane width)
Shunt Capacitive Switch over a CPW - Down State Position MEMS bridge g W Silicon Substrate Silicon Nitride layer Cross Section CPW g L B W Top View w(membrane width)
MEMS Process Information The Mems switch is fabricated on high-resistivity silicon substrate (ε r = 11.9). Gold 2um Silicon Nitride 0.2um Gold 0.8um Silicon Dioxide 1um Silicon 400um
MEMS Modeling Flow Chart CPW Switch HFSS SI Q3D EM 3DFS ANSOFT OPTIMETRICS Insertion & Return Loss Low Frequency Capacitance Upward Force The MEMs switch is simulated on both up and down state of the MEMs bridge
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 Plan View Full geometry with substrate block Cross Section View showing down state
Parametric and Optimetric Capability - Height Membrane Height Parameterization
Parametric and Optimetric Capability - Width Signal Trace Width Parameterization
Circuit Model for MEMs Shunt Switch Rline/2 C up /C down, L and R in series Lumped elements of the bridge with capacitance up/down state Rline/2
The switch shunt impedance : Z total = R + jwl + 1/jwC Where C = C up or C down depend on the switch position. The LC series resonant frequency of the shunt switch : f o = 1/(2π LC) Therefore impedance of the shunt switch can be approximated by 1/jwC, for f << f o Z total = R, for f = f o jwl, for f >> f o
ANSOFT HFSS is used to extract the Return and Insertion Loss For 110um membrane width (w)
S-Parameter for Membrane Width 110um at Up State Position
S-Parameter for Membrane Width 110um at Down State Position
At Up State Position: The Cutoff Frequency f c is expected to be above 800 GHz At Down State Position: The Resonant Frequency f O is 50.38 GHz with Frequency sweep of 0.1 to 80 GHz
ANSOFT Optimetrics is used to extract the Return and Insertion Loss with 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 Optimetrics is 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 SI when 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 Optimetrics is used to extract the downward force produced by the membrane bridge with varying height
Down State Force with varying membrane Z position with different static actuation voltage
Summary An RF MEMS switch model was created in Ansoft 3D modeler which could be used in all Ansoft EDA tools S-parameter extraction using High Frequency Structure Simulator (HFSS), identification of resonant and cutoff frequencies Capacitance value of MEMS bridge extracted using SI Q3D Downward force extracted using Maxwell EM3D Dimensional variation performed using Ansoft Optimetrics allow various what if analysis on design This illustration of Ansoft tools for RF MEMS design shows the potential for simulation of other MEMS devices
References [1]. Elliot R. Brown, RF-MEMS Switches for Reconfigurable Integrated Circuits, IEEE Trans. on Microwave Theory and Techniques, Vol. 46, no.11, November 1998 [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.