RF MEMS TECHNOLOGY PLATFORM FOR CELL PHONES RF MEMS TECHNOLOGY PLATFORM FOR CELL PHONES Transient simulation of the closing Transient simulation of the closing of a MEMS switch with air gap of a MEMS switch with air gap modeled by FLUID136 elements modeled by FLUID136 elements Nicolas LORPHELIN Salim TOUATI UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 1
Outline Outline Presentation of RF MEMS switches Application Electrostatic actuation Ohmic / capacitive switches Delfmems switch functioning modeling process flow Modeling gap closing of FLUID136 elements Death of fluidic elements Rough membrane Remaining thin film Transient simulation of a beam supported on pillars Transient simulation of delfmems switch UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 2 2
Presentation of MEMS Presentation of MEMS (MicroElectroMechanical Systems) (MicroElectroMechanical Systems) UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 3 3
RF MEMS switches: RF MEMS switches: applications applications UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 4 4
RF MEMS switches: RF MEMS switches: Electrostatic actuation Electrostatic actuation Scalar model of a MEMS switch: spring-capacitor system k g 0 − g g t d d 2 − 1 2 = 0 2 0 S V Equilibrium equation: Pull-in stable unstable domain domain Pull-out d V pi = 27 0 S g 0 t d d V po = t d 2 k g 0 3 8k Pull-out voltage: Pull-in voltage: 0 S adapted from G.M. Rebeiz, RF MEMS theory, design and technology , Wiley-Interscience 2003 UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 5 5
RF MEMS switches: RF MEMS switches: Ohmic switches Ohmic switches 2 contacts 1 contact Example of series ohmic switch Example of series ohmic relay with metal contact isolated from actuation membrane The contact resistance depends on: - Contact force - Contact materials - Effective surface area in contact (due to roughness) - Power dissipated through the contact Hyouk Kwon et al, Investigation of the electrical contact behaviors in Au-to-Au thin-film contacts for RF MEMS switches , J. Micromech. Microeng. 18 (2008) UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 6 6
RF MEMS switches: RF MEMS switches: Capacitive switch Capacitive switch Capacitive switch with air gap stoppers g 0 C on = g 0 g 1 C off g 1 off state on state Capacitive switch with dielectric g 0 C on = 1 g 0 d C off t d off state on state UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 7 7
Delfmems switch Delfmems switch Ohmic switch on an interrupted line Anchorless membrane simply supported by two pillars Two pairs of electrodes (internal, external) which ensure two forced states Mechanical stoppers which allow the membrane to move but maintain it in position Internal electrodes Mechanical stop units (MSU) Contact location Pillar External electrodes UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 8 8
Delfmems switch Delfmems switch Two forced states OFF state ON state dielectric layer Delfmems membrane at ON state Delfmems membrane at OFF state UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 9 9
Delfmems switch: Delfmems switch: modeling modeling Modeling RF line by Modeling pillars with Modeling of a quarter of contact/target elements contact/target elements membrane due to symmetry contact contact target target Modeling electrostatic actuation Contact force on bumps by TRANS126 elements 350 300 contact force [µN] 250 200 150 100 50 0 0 5 10 15 20 25 30 applied voltage [V] UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 10 10
Delfmems switch: Delfmems switch: Process flow Process flow Burried electrodes and dielectric layer Silicon wafer Silicon Nitride Titanium-Tungsten Gold Silicon dioxide Chromium UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 11 11
Delfmems switch: Delfmems switch: Process flow Process flow First sacrificial layer, electroplating mold and electroplating of pillars and RF line Silicon wafer Silicon Nitride Titanium-Tungsten Gold Silicon dioxide Chromium UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 12 12
Delfmems switch: Delfmems switch: Process flow Process flow Second sacrificial layer, contact, dielectric and membrane Silicon wafer Silicon Nitride Titanium-Tungsten Gold Silicon dioxide Chromium UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 13 13
Delfmems switch: Delfmems switch: Process flow Process flow Third sacrificial layer and stopppers patterning Silicon wafer Silicon Nitride Titanium-Tungsten Gold Silicon dioxide Chromium UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 14 14
Delfmems switch: Delfmems switch: Process flow Process flow Realeasing, rinsing and drying Silicon wafer Silicon Nitride Titanium-Tungsten Gold ● Only 5 principal materials (excluding sticking layers) Silicon dioxide ● 3 sacrificial layers ● No photoresist for sacificial layers : better stability of Chromium process flow UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 15 15
Delfmems switch: Delfmems switch: performance performance Low actuation voltage: low gap Low contact resistance: High contact force High restoring force: membrane stiffness + external actuation Low switching time : needs to be simulated simulation of impact on contact simulation of closing of air gap with FLUID136 elements: feasible since ANSYS release 12 challenging due to: low gap high pressure high electrostatic force vanishing of air between the electrode and the membrane UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 16 16
Modelling squeeze film: Modelling squeeze film: FLUID136 elements FLUID136 elements FLUID136 elements are surfacic elements based on Reynolds equation which are adapted to model thin films with high lateral dimensions. Since release 12 it is possible to perform coupled transient fluid/structure simulations with air gap going near zero: Since ANSYS release 12 ANSYS release 11 Pressure, UX, UY, UZ are available DOFs Pressure is the only DOF (KEYOPT(3)=1 or 2) Large pressure changes can be modeled with The pressure change must be small compressible nonlinear Reynolds equation compared to ambient pressure. (KEYOPT(4)=1) Large pressure changes can be modeled with Displacement amplitudes must be small compressible nonlinear or incompressible compared to the film thickness. linearized Reynolds equation (KEYOPT(4)=1 or 2) Managing of the closing of air gap If gap goes below a defined fluid_mingap: reset it to fluid_mingap The element is considered “dead” for a fluid standpoint If gap goes below a defined mech_mingap: The element is considered “dead” for a mechanical standpoint Apply contact pressure UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 17 17
Simulation of gap closing: Simulation of gap closing: Death of fluid elements Death of fluid elements Modeling of fluid mingap Test structure: clamped-clamped beam zero pressure on the edge quarter of membrane anchoring X and Y symmetry Options for FLUID136 elements: KEYOPT(1)=3 High Knudsen number and accomodation factor KEYOPT(2)=0 Four node element KEYOPT(3)=2 DOF PRES, UX, UY, UZ Implicit treatment of cross-coupling terms. Adapted for gaps near zero KEYOPT(4)=1 Compressible nonlinear Reynolds equation KEYOPT(5)=2 If gap becomes lower than fluid_mingap, the element will be considered as “dead” KEYOPT(6)=0 If the gap becomes lower than mech_mingap no contact pressure will be applied UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 18 18
Simulation of gap closing: Simulation of gap closing: Death of fluid elements Death of fluid elements Displacement at the centre of the beam Pressure at the centre of the beam Length of the beam in contact with the electrode UK users conference, November 9 th th 2011, Gaydon, Warwickshire UK users conference, November 9 2011, Gaydon, Warwickshire 19 19
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