Introduction to Microeletromechanical Systems (MEMS) Lecture 2 Topics MEMS for Wireless Communication Components for Wireless Communication Mechanical/Electrical Systems Mechanical Resonators o Quality Factor Oscillators Voltage-Tunable Capacitors Micromachined Inductors Filters Switches Antennas MEMS Overview Introduction & Background History & Market Methodology Devices & Structures Processes & Foundries Micromachining: lithography, deposition, etching,
MEMS for Wireless Communication Why microfabrication? Process integration Mass fabrication Beyond cell phones: combine sensing, actuation, computation, and communication intelligent sensor/actuator network ( smart dust ) Why mechanical components? High-frequency oscillators (GHz range and more) Stability w.r.t. temperature, aging Low loss oscillators (high quality factor) Tunable oscillators (voltage controlled) Components for Wireless MEMS Antennas Amplifiers Switches Resistively coupled Capacitively coupled Resonators Filters Oscillators It is difficult to build integrated high quality electronic resonators for high-frequency applications (»MHz) Use discrete components, or Use corresponding micromechanical devices instead
Mechanical / Electrical Systems x b L R Input :external force Output :displacement mx &&( + bx& + Kx( F m mass, b damping, K stiffness Transfer function : H ( s) x F K s 2 + m b m m F s + x K m F V i Input : voltage Output : voltagev Lq&& + Rq& + V H ( s) V o i s V i C o LC R s L C q( V LC i V o L induct., R resist., C capacit. Transfer function : 2 + + Mechanical / Electrical Systems Alternative circuit: L V i R C V o Input : voltage Lq&& + q& + L RC V H ( s) V o i s V Output : voltage i V o C q( V L inductance, R resistance, C capacitanc e Transfer function : 2 + LC RC s + LC i
Resonators Analogy between mechanical and electrical system: Mass m - inductivity L Spring K - capacitance C Damping b - resistance R (depending where R is placed in circui Solution to 2nd order differential equation: 2 ω H ( s) 2 ω 2 s + s + ω ω ω Q 2πf K m Q mechanical system, quality factor natural frequency ω LC electrical system Mechanical Resonator Frequency and phase shift under damping: x( Ae τ m b ω ω t 2τ - 2 2 4ω τ ϕ phase shift cos( ω t + ϕ ) damping time ω 2 b - 4Km Energy dissipation: E( E e t τ
Quality Factor How fast does energy dissipate? What is the maximum amplitude for a given frequency? Definition: Quality factor (Q factor) Ratio of stored energy E τ Q 2 π 2π ωτ and lost energy: E T Mechanical system: m Km Q ω b b Similar for electric systems: (a) L Q ω R R (b) Q ω RC R L C C L Quality Factor How fast does energy dissipate? Q m τ τ (mechanical) ω b What is the maximum amplitude for a given frequency? At resonance, amplitude is Q times the DC response Gain (db) ω ω (/s)
Summary: Mechanical/Electrical Resonator Mechanical resonator: Torsional resonator: Electrical resonator: mx && + bx& + Kx( m mass, b damping, K stiffness natural frequency ω K m (for small b) I && θ + b & θ + kθ I moment of inertia, b damping, k stiffness natural frequency ω k I Lq&& + Rq& + q( C L inductivit y, R resistance, C capacitanc e natural frequency ω LC Wireless Communication Frequency Spectrum RF IR UV X-rays VLF LF MF HF VHF UHF SHF EHF visible 5 5 2 n Hz wavelength cm µm
Wireless Communication Components [Nguyen et al., 998] Oscillators for Wireless Communication Current Technology: Quartz crystal Surface acoustic wave (SAW) Discrete elements (variable capacitors and inductors) Advantages: High quality factor: >, Extremely high stability against thermal variations and aging Extremely selective filtering (small channel bandwidth) Problem: requires incompatible materials (e.g., GaAs) assembly necessary
Oscillators for Wireless Communication Goals: High Q oscillators Integrated VCO s (voltage controlled oscillators) Compatibility with micromachining processes Low cost Challenges: Q is proportional to L (inductivity) or m (mass) Tunable devices MEMS devices with sufficiently high C and L Noise from temperature changes, vibrations, aging Inductor losses: eddy currents in substrate Oscillator Stability [Nguyen et al., 998]
Voltage-Tunable Capacitors 2µm [Young and Boser 996] Voltage-Tunable Capacitors 4 tunable parallel capacitors 2.4pF 2.35pF 3V tuning voltage, Q62 at GHz [Young and Boser 996]
Micromachined Inductors NiFe core under planar metal spiral 2.7 µh inductance Q6.6 at 4MHz Spiral inductor on substrateisolating platform/membrane.2 nh inductance Q6-8 at 4GHz [Najafi et al., 997] Micromachined Inductors Electroplated 3D coil inductors with 4nH, Q3 at GHz [Young and Boser 997]
Thin-Film Bulk Acoustic Resonators Thin-film bulk-acoustic mode piezoelectric resonator (FBAR) Q > f.5-7.5 GHz Thin-film resonator on substrate Acoustic isolation by strategic selection of separating layers [Lakin, Kline and McCarron 995] Comb-Drive Resonator V p V d I DC bias drive signal at ω d output current V p dc/dt feedback via transimpedance amplifier V c carrier signal at ω c output signal: [Figure: Maluf 2] frequency spectrum includes ω d, ω c, but also ω c ±ω d basis for frequency transfer (heterodyning)
Bandpass Filters Intuition: Two resonance modes Loosely coupled resonators: resonance modes are close and effectively form bandpass Additional coupled resonators widen frequency bandpass [Figure: Maluf, 2] Resonators and Filters [Nguyen et al., 998]
MEMS Medium Frequency Filters [Nguyen et al., 998] Cantilever Beam Switches [de los Santos 999] Issues: Maxium current when ON, max. voltage when OFF Impedance for DC to GHz frequency range Speed, stiction, lifespan, contact deterioration
Mercury Switches Side-driven mercury switch [Saffer et al. 996] Curved electrodes achieve small gap size (large el.stat. force) over larger actuation range Bumpers prevent electrode contact / short Mercury drop (selectively deposited only on Au patches) Originally fabricated in MUMPs [S. Saffer et al., 996] Capacitive Switches Metal membrane Electrostatic actuation Avoid stiction with dielectric layer Capacitive, not resistive coupling No DC component, which is ok for microwave frequencies [Goldsmith et al., 996]
Antennas [Gauthier, Courtay and Rebeiz 997]