Modeling and Characterization of Dielectric-Charging Effects in RF MEMS Capacitive Switches

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1 Modeling and Characterization of Dielectric-Charging Effects in RF MEMS Capacitive Switches Xiaobin Yuan, Zhen Peng, and ames C. M. Hwang Lehigh University, Bethlehem, PA 1815 David Forehand, and Charles L. Goldsmith MEMtronics Corporation, Plano, TX 7575 Partially supported by the Air Force Research Laboratory under Contract No. F C-73 Funded by DARPA/MTO Harsh Environment, Robust Micromachined Technology (HERMIT) program

2 Outline Introduction Experimental setups Transient current measurement Charging model construction Accelerated life tests and model verification Temperature dependence Equivalent-circuit model Bipolar vs. unipolar charging Conclusion

3 Introduction Many microsystems will include electronics, optics and MEMS Main challenges for MEMS are reliability, packaging, and integration Wiggling is OK; touching is touchy; grinding is certain death Problems are worse for NEMS - even a low voltage in tight space cause charging problems MEMS are more reproducible than NEMS - study of MEMS reliability will help understand NEMS reliability problems

4 Motivation RF MEMS switches have low loss, low power, and high linearity RF MEMS switches for electronically steered antenna represent the first significant insertion opportunity of MEMS technology into aerospace/defense systems Lifetime of RF MEMS contact switches limited by stiction Lifetime of RF MEMS capacitive switches limited by dielectric charging No quantitative model exists to predict lifetime due to charging Accelerated life test is required because MEMS are slow Only after the failure mechanisms are understood and acceleration factors are quantified can life test be properly accelerated A dielectric-charging model can be used to design controlvoltage waveforms either to accelerate failure or to prolong lifetime

5 RF Device Modeling/Characterization Lab - One of the Best in Academia Pulsed harmonic load-pull power and waveform measurement Pulsed I-V and S parameter measurement ±1V, 2A, 5GHz, 65-2 C Femtoampere transient current measurement, 65-2 C

Experimental Setup DC Test Setup Microchamber with temperature and humidity control Triaxial probes/cables fa Precision semiconductor parameter analyzer Transient charging/discharging current

6 Experimental Setup DC Test Setup Microchamber with temperature and humidity control Triaxial probes/cables fa Precision semiconductor parameter analyzer Transient charging/discharging current directly measured RF Source VNA DC Source Bias-T MEMS Function Generator VNA Bias-T RF Detector Oscilloscope RF Test Setup 5 GHz VNA for transient pulsed S parameters ±1 V DC Source Arbitrary waveform generator

7 RF MEMS Capacitive Switches 12µ x 8µ MIM capacitor 25V pull down voltage 8V release voltage.6db insertion 35GHz 15dB 35GHz 1µs switching time

8 Real vs. False Switches Real Switch Al membrane (GND) False Switch Al membrane (GND) Cr signal line Oxide Cr signal line Oxide Sputtered silicon dioxide.3µm thick Linear relationship between actuation-voltage shift and accumulated charge in dielectric V = qhq/e e r Transient current measurements taken on false switch and used to construct charging model with charge location as an adjustment factor h (1/2) dielectric thickness Charging model fits actuation-voltage shift of real switch

9 Top vs. Bottom Charging V Al Membrane 4.5 ev Cr Vacuum Level.9 ev Eg ~ 9 ev 4.3 ev Al Top charging at higher voltage due to surface contamination Top charging very fast; top discharging very slow Metal/dielectric combination chosen to avoid top charging

10 Bipolar vs. Unipolar Charging ( ) before, (- -) after Positive charge causes actuation/release voltages to shift left Negative charge causes actuation/release voltages to shift right

11 Transient Current Measurements 1 3 V on 6 CURRENT (pa) V off 4 2 VOLTAGE (V) TIME (s) ΔQ = = 1,2 ΔQ exp( V / V )[1 exp( t ON / τ C )]exp( t OFF / τ D )

12 STEADY STATE CHARGE (q/cm 2 ) 1.E+13 1.E+12 1.E+11 1.E+1 Steady-State Charge Density Trap 1 Trap 2 Q = Q exp(v/v ) CONTROL VOLTAGE (V)

13 Charging/Discharging Time Constants TIME CONSTANT (s) Trap 1 Charging + Trap 1 Discharging Δ Trap 2 Charging х Trap 2 Discharging CONTROL VOLTAGE (V)

14 Model Parameters Positive Voltage τ C (s) τ D (s) ΔQ ( cm -2 ) V (V) Negative Voltage τ C (s) τ D (s) ΔQ ( cm -2 ) V (V) ΔQ = = 1,2 ΔQ exp( V / V )[1 exp( t ON / τ C )]exp( t OFF / τ D )

15 Modeled vs. Measured Transient Currents 1E-11 1E-11 CURRENT (A) 1E-12 1E-13 1E-14 4 V 3 V 2 V Charging Discharging CURRENT (A) 1E-12 1E-13 1E-14-4 V -3 V -2 V Charging Discharging 1E-15 1E TIME (S) TIME (S) Model constructed for charging under both positive and negative actuation voltages Good fit between modeled and measured transient currents

16 Charging under Square-Wave Control CHARGE DENSITY CHARGING B E A t ON S C t OFF DISCHARGING D On Time A B E Off Time C D TIME Net charge accumulation per switching cycle depends on ratchet action of charging/discharging Injected charge will saturate when the charging/discharging processes are balanced

17 ACTUATION VOLTAGE SHIFT (V) Duty Factor Acceleration DUTY FACTOR = 75 % 5 % 25 % PEAK VOLTAGE = - 3V, f = 1 Hz NUMBER OF CYCLES

18 ACTUATION VOLTAGE SHIFT (V) Voltage Acceleration PEAK VOLTAGE = - 35 V -3 V -25 V DUTY FACTOR = 5%, f = 1 Hz NUMBER OF CYCLES

19 ACTUATION VOLTAGE SHIFT (V) Frequency Independence DUTY FACTOR = 75 % 5 % 25 % PEAK VOLTAGE = - 3V, 16 s FREQUENCY (Hz)

20 Temperature Dependence Q = Q exp( Ea / kt )[1 exp( t ON / τ C )]exp( t OFF / τ D ) LOG CHARGE DENSITY (q/cm 2 ) Trap 1 Modeled Trap 1 Extracted --- Trap 2 Modeled ΔΔ Trap 2 Extracted INVERSE TEMPERATURE (1/K) TIME CONSTANTS (s) ΔΔ Trap 2 Charging xx Trap 2 Discharging Trap 1 Charging ++ Trap 1 Discharging Averaged Values TEMPERATURE (K) Steady-state charge density exhibits Arrhenius temperature dependence Time constants independent of temperature

Temperature Acceleration Q = Q exp( Ea / kt )[1 exp( t / τ )]exp( t Model agrees with measured increase in actuation-voltage shift as a function of temperature Switch more prone to stiction at higher

21 Temperature Acceleration Q = Q exp( Ea / kt )[1 exp( t / τ )]exp( t Model agrees with measured increase in actuation-voltage shift as a function of temperature Switch more prone to stiction at higher temperature due to both increased charging of dielectric and decreased stiffness of membrane electrode / τ ) Steady-state leakage current through dielectric increases with temperature, but did not help bleed away trapped charge ON C OFF D LOG CURRENT DENSITY (A/cm 2 ) ACTUATION-VOLTAGE SHIFT (V) (curve) modeled (symbol) measured 5 C 25 C C Steady-State Current Density under -3 V INVERSE TEMPERATURE (1/K) Actuation-Voltage Shift under -3 V STRESS TIME (s)

22 V S1 = Q 1 exp(v/v 1 ) t t VtPulse SRC6 V S2 = Q 2 exp(v/v 2 ) VtPulse SRC8 Diode DIODE1 Diode DIODE5 Diode DIODE7 Diode DIODE6 Equivalent-Circuit Model R Rc4 R=6.5 Ohm R C1 = τ C1 R Rd4 R=7 Ohm R D1 = τ D1 R Rc5 R=52.5 Ohm R C2 = τ C2 R Rd5 R=74.7 Ohm R D2 = τ D2 C C3 C=1. F C = 1 F C C5 C=1. F C = 1 F Compact model to simulate circuits of multiple MEMS and electronic devices under complex control waveforms Equivalent-circuit model an approximation of equation-based model Transient SPICE model implemented in Agilent s ADS circuit simulator

23 Complex Control Waveforms CONTROL VOLTAGE (V) t ON = 5 ms t P = 25 ms t P = 5 ms t OFF = 5 ms ACTUATION-VOLTAGE SHIFT (V) TIME (ms) TIME (s) Equivalent-circuit simulation correctly predict reduced charging under dual-pulse control wave Envelope simulation more efficient similar to that for wireless communication under complex modulation such as CDMA ACTUATION-VOLTAGE SHIFT (V) t P t ON t OFF TIME (s)

24 Bipolar vs. Unipolar Charging V Al Membrane 4.5 ev Cr Vacuum Level.9 ev Eg ~ 9 ev 4.3 ev Al Top charging at higher voltage due to surface contamination Top charging very fast; top discharging very slow Avoid bipolar charging!

25 Bipolar Charging under Positive Voltage 4 ACTUATION VOLTAGE SHIFT(V) V 4V 3V Stress Added 3V STRESS 1 4V STRESS 1 5V STRESS 1 5V STRESS 2 Stress released, waiting for recover TIME (s) Unipolar charging at 3V; bipolar charging at 5 V Unipolar: actuation voltage recovers to original value Bipolar: long-term actuation voltage drift after bottom charge dissipates but top charge remains

26 Bipolar Charging under Negative Voltage ACTUATION VOLTAGE SHIFT(V) V -3V STRESS 1-4V STRESS 1-6V -5V STRESS 1-4V -5V STRESS 2-6V STRESS 1-5V -6V STRESS 2 Stress Added -4 Stress released, waiting for recover TIME (s) Threshold for bipolar charging higher than that under positive voltage Barrier from Al to SiO 2 higher for holes than for electrons

27 Conclusion Model extracted from charging/discharging currents Model validated under accelerated life test conditions Model can be used to design control-voltage waveforms either to accelerate failure or to prolong lifetime Model can be used for quick evaluation of dielectrics Model provides deeper insight into the dielectric charging problem and allow more robust MEMS switches to be designed Envelope simulation by using equivalent circuit model provides quick approximation under complex control waveforms Temperature accelerates charging and softens membrane Avoid bipolar charging at all cost!

28 References 1. X. Yuan,. C. M. Hwang, D. Forehand, and C. L. Goldsmith, Modeling and characterization of dielectric-charging effects in RF MEMS capacitive switches, in IEEE MTT-S Int. Microwave Symp. Dig., une X. Yuan,. C. M. Hwang, D. Forehand, and C. L. Goldsmith, A transient charging model to predict actuation voltage shift in RF MEMS capacitive switches, in Proc. Soc. Optical Engineers, vol. 6111, an. 26, pp. 6111G1-6111G8. 3. C. Goldsmith, D. Forehand, X. Yuan and. Hwang, Tailoring capacitive switch technology for reliable operation, in Dig. Government Microelectronics Applications Conf., Mar X. Yuan,. C. M. Hwang, D. Forehand, and C. L. Goldsmith, Temperature acceleration of dielectric-charging effects in RF MEMS capacitive switches, to appear in IEEE MTT-S Int. Microwave Symp. Dig., une 26.

IN THE past decade, RF microelectromechanical system

IN THE past decade, RF microelectromechanical system 2640 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 10, OCTOBER 2006 A Transient SPICE Model for Dielectric-Charging Effects in RF MEMS Capacitive Switches Xiaobin Yuan, Member, IEEE, Zhen Peng, Student