CSE 495/595: Intro to Micro- and Nano- Embedded Systems

CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Applications: RF MEMS CSE 495/595: Intro to Micro- and Nano- Embedded Systems Prof. Darrin Hanna

Radio Basics Receiving signals and an oscillator Hz

Radio Basics Start simple

Radio Basics IF=455 kilohertz (AM) or 10.7 megahertz (FM/VHF)
Super heterodyning It reduces the signal from very high frequency sources where ordinary components wouldn't work (like in a radar receiver). It allows many components to operate at a fixed frequency (IF section) and therefore they can be optimized or made more inexpensively. It can be used to improve signal isolation by arithmetic selectivity

Radio Basics f1 + f2 and f1 – f2 asin(2πf1t) + asin(2πf2t)

RF MEMS Radio-frequency (RF) MEMS high volume production mass cost performance Potential markets cellular phones cordless phones for home use wireless computer networking radios global positioning system (GPS) receivers satellites missile guidance, military radar

RF MEMS Requirements transmit desired signals with low loss do not permit external signals or noise to join the transmitted signal filter out or not generate undesired signals higher-frequency harmonics At high frequencies, difficult to achieve

RF MEMS Transmission lines wavelength on order of system size Conductor between ground planes dielectric system impedance (such as RF) must match transmission line impedance

Signal Loss Losses conductor loss due to the resistance of materials used – heating modeled as resistance in series with the signal path low-resistive metals are used gold, copper, and aluminum contacts and between layers of different materials also add resistance dielectric loss due to atomic-scale dipoles excited in the dielectric heating modeled as an equivalent parallel conductance

RF MEMS Losses (cont’d) eddy currents induced in substrate modeled as a parallel conductance insulating or semi-insulating substrates minimize the loss due to eddy currents high-resistivity (>5,000 Ω∙cm) silicon substrates ok but semi-insulating gallium arsenide (GaAs) is better Gallium arsenide common microwave Insulators such as glass and alumina good process compatibility, cost, and thermal coefficient of expansion mismatches

Capacitors and Inductors
Capacitors and inductors have parasitics limits performance Two parameters describe their performance and enable comparisons between devices quality factor Q and the self-resonance frequency fSR Q is measure of loss in a linear-circuit element maximum energy stored during a cycle divided by the energy lost per cycle capacitor C with series resistance Rs, Q= 1/(2πfCRS)

Inductor quality inductor L with series resistance Rs, Q = 2πfL/RS

Q = 2πfL/RS and Q= 1/(2πfCRS) greater resistance gives a smaller Q as frequency goes up, current flows along an increasingly thin layer at the surface of conductors (the skin effect), increasing the resistance and lowering the Q capacitors dielectric loss, a function of frequency also contributes to a lower Q micromachined capacitors, the dielectric is usually a gas or vacuum, leaving series resistance as dominant loss loss can also occur in the substrate

inductors often have higher series resistance than capacitors far lower quality factors substrate loss as well

lines connecting a capacitor to a circuit and even plates have a small parasitic inductance circuit model has a capacitance C in series with an inductance Lpara low frequencies, the impedance of the inductor is small and the capacitor functions as normal. higher frequencies, capacitor impedance falls while the impedance of the inductor rises

lines connecting a capacitor to a circuit and even plates have a small parasitic inductance circuit model has a capacitance C in series with an inductance Lpara low frequencies, the impedance of the inductor is small and the capacitor functions as normal. higher frequencies, capacitor impedance falls while the impedance of the inductor rises limits useful frequency range Above fSR, inductance dominates and capacitor looks to a circuit like an inductor!

Inductors often implemented as coils of a conductor have parasitic capacitance between them circuit model with each turn of the inductor is represented by an incremental inductor and its parasitic capacitance

Inductor (cont’d) simplified model inductor L in parallel with a parasitic capacitor Cpara low frequencies, most current flows through the inductor above fSR the capacitance dominates and inductor ceases to function as one this often occurs at lower frequency for inductors than for capacitors

Inductor (cont’d) quality factor rises with frequency but parallel capacitance reduces the effective inductance at higher frequencies, Q eventually reaches a maximum before falling.

Micro-machined capacitors
constant capacitance fabricated side by side with transistors standard semiconductor integrated-circuit process sandwiching dielectric between two conductive layers on-chip capacitors reduces parts on a circuit board lowers cost noise reduction lowering both parasitic capacitance and resistance capacitance per unit area is relatively small

variable capacitors voltage-controlled oscillators (VCOs) / tuning circuits require variable capacitors varying dc voltage applied varies capacitance capacitance tuning ranges from 2:1 to over 10:1 with an applied voltage of 0–5V are available commercially Q at most on the order of 50 in the gigahertz frequency range. example requirement is Q > 50 for a 2-pF capacitor in the range of 1 GHz (cell phone)

surface micro-machined variable capacitors bottom plate residing on an insulated substrate, an air gap, and a flat top plate suspended by a spring structure

applying dc control voltage V creates electrostatic force Fe = є0AV2/(2g2) є0 is the permittivity of free space, A is the area of plate overlap, and g is the gap force pulls the top plate downward, increasing capacitance restoring spring force Fs = kg, k is spring constant and g is plate displacement spring force increases linearly with plate motion electrostatic force rises faster than linearly with the plate gap change results in both plate motion and capacitance change slowly at first, then rising rapidly

when displacement reaches 1/3 initial gap electrostatic force rises more rapidly than the spring force top plate snaps down toward the bottom plate limits controllable increase in capacitance for this type of variable capacitor to 50% parasitic capacitance lowers the possible tuning range.

PART II

Bulk Micro-machined capacitors
Bulk micro-machined variable capacitors comb-drive – most successful Capacitance scales linearly with the number of fingers and finger thickness and inversely to size of the gap

DRIE for high aspect ratio A:d typically resistance in the spring dominates Q

uses a single mask to DRIE though the top layer of Si on SOI stop on a 2-µm-thick buried layer of SiO2 etch with HF to remove buried oxide moving parts supercritical drying

heavy doping (>1020 cm-3) of the silicon springs and fingers does not yield sufficiently low resistance resistance is lowered by sputtering on a thin layer of Al coats the tops and sidewalls of the silicon top of the underlying silicon substrate is also coated gaps formed from undercut of the oxide, avoiding shorting

still substrate acts as the lossy plate of a capacitor low Q and a large parasitic capacitance changing Si substrate to highly-resistive material improves the Q and lowers the parasitic capacitance by factor of ten better Q is achieved with a glass substrate requires major changes in process flow

These are fabricated by bonding thin-silicon side of SOI wafer to the glass wafer with epoxy . Young’s moduli of epoxies are normally orders of magnitude lower than those of silicon and glass provides some stress isolation between two substrates thick side of SOI wafer removed by grinding and polishing off most of the silicon final silicon removal by etching in TMAH etch buried oxide to reveal thin layer of silicon

a 2-µm layer of Al is deposited for low series resistance and patterned using plasma etching photoresist and Al act as a mask in a DRIE step to etch straight down through Si to buried epoxy epoxy is etched in oxygen plasma undercuts the silicon to free the moving parts additional 0.25 µm Al sputtered to coat sidewalls for an even lower series resistance.

Fabricated capacitors fingers 2 µm wide, 20 µm thick, a gap of 2 µm an initial overlap of a few micrometers nominal capacitance of 2 pF requires 1,200 sets of interdigitated fingers! measured value for Q is 61 at 1 GHz self-resonance frequency is 5 GHz tuning range 4.55:1 at 5.2V from finger motion of 23 µm

Other concerns unintentional movements when device is subject to an external acceleration, such as vibration noise unintentional displacement depends on the acceleration, the mass, and the spring constant in the direction of motion finger capacitors motion of the movable fingers sideways, perpendicular to the intended direction of travel gap on one side is reduced, electrostatic force increases and eventually pulls the fingers together electrical short

Micro-machined inductors
low-cost discrete inductors many have inductances in the range of a few to tens of nanohenries when used with an IC, suffer from parasitic capacitance in traces and bond pads on the chip, in the bond wires connecting the chip to the circuit board, and in the inductor packaging limits the self-resonance frequency and therefore maximum operating frequency consume board space in portable electronics on-chip inductors are desirable

Example inductor parameters for on-chip VCOs in cellular phones in the 1–2 GHz range L = 5 nH and Q >30 Inductors are readily fabricated on ICs using CMOS spiral in one layer of metal and a connection to the center of the spiral in another layer of metal Losses from the resistance of the metal and eddy currents in the substrate limit the Q to less than 10 at 2 GHz

improving both quality factor and self-resonance frequency reduce parasitic capacitance and substrate conductive loss by changing to an insulating substrate not possible if circuitry is integrated on same chip raise the inductor above the substrate using an air gap or form cavity underneath to reduce parasitic cap. to substrate As an example, 24-nH inductors were made using a 12.5-turn spiral with an outer diameter of 137 µm on the substrate have a self-resonance frequency of 1.8 GHz raised 250 µm above the substrate have an fSR of 6.6 GHz

small solenoids on top of the substrate closely spaced ribbons of copper-plated metal make coils bottoms of the coils attached to substrate thick copper shield placed under the coils prevents eddy currents low loss an example inductor with three turns, each 200 µm wide and 535 µm in diameter, has an inductance of 4 nH, a high Q of 65 at 1 GHz, and a self-resonance frequency of 4.2 GHz using thin aluminum shield from CMOS process instead of thick copper (MEMS) lowers the Q by about 25%

MEMS Resonators mechanical system of a spring with spring constant k and a mass m has a resonant frequency In electronics, this is analogous to a series or parallel combination of capacitor and inductor, with a small series resistance (damper).

MEMS Resonators Illustration of Q (review from before)

MEMS Resonators quartz crystals IC oscillators have not been able to achieve large Q typical quartz crystal has a Q that reaches 10,000 or more quality factors above 1,000 are considered high for many electronic and RF applications frequencies of interest cover the range between 800 MHz and 2.5 GHz for front-end wireless reception and intermediate frequencies at 455 kHz and above

Comb Drive Resonator folded springs supporting a shuttle plate plate oscillates back and forth in the plane of the wafer surface applied voltage, either positive or negative, generates an electrostatic force between the left anchor comb and shuttle comb that pulls the shuttle plate to the left

Comb Drive Resonator

Comb Drive Resonator A comb-drive resonator made of polycrystalline silicon standard surface-micromachining beams with a thickness 2 µm, widths of 2 µm, and lengths of 185 µm system spring constant of 0.65 N/m moveable mass equal to 5.7 × kg resonates at 17 kHz same beam thickness and width but reducing the length to 33 µm resonates at 300 kHz Q can be over 50,000 in vacuum but rapidly decreases to below 50 at atmospheric pressure damping in air

Comb Drive Resonator higher resonant frequency total spring constant increased or the motional mass decreased increase the beam width and decreasing its length – increase spring k reducing motional mass is hard electron-beam lithography to write submicron linewidths, single-crystal silicon beams with lengths of 10 µm and widths of 0.2 µm reached a resonant frequency of 14 MHz same resonator dimensions use material with a larger ratio of Young’s modulus, E, to density than silicon silicon carbide and polycrystalline diamond

Beam Resonator MEMS with higher resonant frequency beam resonators University of Michigan, Ann Arbor reference frequency oscillators to replace quartz crystals in cell phones much smaller size ability to build several different frequency references on a single chip higher resonant frequencies ability to integrate circuitry, either on the same chip or on a circuit chip bonded to the MEMS all at a lower cost than the traditional technology

Beam Resonator simplest beam resonator clamped on both ends and driven by an electrode

Beam Resonator dc voltage applied between the beam and drive electrode center of the beam deflects downward removal allows it to travel back upward ac drive signal causes the beam to flex up and down large dc bias VD similar to comb-drive resonator beam oscillates at same frequency as the drive signal at resonance deflection amplitude is at its greatest

Beam Resonator An example polysilicon beam 41 µm long, 8 µm wide, 1.9 µm thick gap of 130 nm Applying voltages VD = 10V and ía = 3 mV measured resonant frequency is 8.5 MHz Q is 8,000 at a pressure of 9 Pa deflection amplitude is only 4.9 nm at center of beam at atmospheric pressure Q drops to 1,000 to maintain high Q, vacuum sealed cap in packaging

Beam Resonator dc bias adds downward electrostatic force force varies with distance opposes mechanical restoring force of the beam makes effective mechanical spring constant smaller resonant frequency falls by a factor proportional to C is the initial capacitance, k is the mechanical spring constant, and g is the gap without a dc bias resonant frequency can be electrically tuned

Beam Resonator Variance with temperature is a problem rate at which the resonant frequency falls polysilicon clamped-clamped beams –17 ×10–6/K (ppm/K) quartz crystals –1 ×10–6/K range (much better) compensation

Beam Resonators (dual)
place an electrode with a dc bias VC over a beam an effective second electrical spring is added to system reduces overall system spring constant

mounting ends of this top electrode on top of metal supports with faster thermal expansion rate gap increases with temperature reduces the electrical spring constant opposing mechanical spring mechanical spring constant falls results in combination varying much less with temperature than single beam +0.6 × 10–6/K polysilicon clamped-clamped beams –17 ×10–6/K (ppm/K) quartz crystals –1 ×10–6/K range (much better)

entire top electrode is made of metal process compatibility expands faster laterally than the underlying Si substrate it is clamped at the ends undesirably bows upward suspending the ends of beam off the substrate and putting slits near its ends, bowing is greatly reduced

with bias reduces the frequency shift with temperature –0.24 × 10–6/K comparable to the best quartz crystals specifications for this prototype beam length of 40 µm, width of 8 µm, thickness of 2 µm gap below resonant beam during operation of 50 nm gap above beam of ~ 250 nm with dc bias VD of 8V (lower) and VC also of 8V (upper) resonant frequency is 9.9 MHz Q of 4,100

bottom electrode and resonant beam fabricated from polysilicon sacrificial silicon dioxide layer in between sacrificial oxide layer also on top of the resonant beam sacrificial nickel spacer on sides of beam gold electroplated through a photoresist mask for top metal electrode nickel and silicon dioxide etched away to leave freestanding beams

Coupled Resonators as Bandpass Filters
MEMS resonators have very narrow bandpass characteristic suitable for setting the frequency in an oscillator circuit but not for a more general bandpass filter bandpass filters pass a range of frequencies, with steep roll-off on both sides two or more micro-resonators can be linked together by weak springs or flexures to create bandpass filters Q = fR / BW So for Q = 4,100 and fR=9.9 MHz BW = 2.4 kHz Highly selective

in phase no relative displacement between two masses oscillation frequency equal to natural frequency of a single resonator out of phase displacements in opposite directions higher oscillation frequency

physical coupling of the two masses effectively split the two overlapping resonant frequencies into two distinct frequencies how far apart these frequencies are depends on the stiffness of the coupling flexure for compliant coupling spring, the two split frequencies are sufficiently close to each other that they effectively form a narrow passband.

increasing number of oscillators in a linear chain widens the passband also increases the number of ripples in general, the total number of oscillation modes is equal to the number of coupled oscillators in the chain

MEMS Switches key desirable parameters in RF switches are low insertion loss and return loss (reflection) in closed state high isolation in the open state high linearity high power-handling capability during switching low operating voltage (for portables) high reliability (particularly a large number of cycles before failure) small size and low cost MEMS switches to be designed into new products, must surpass the performance of, or offer some other advantage over existing

MEMS Switches membrane shunt switch University of Michigan 2-µm-thick layer of gold suspended 2 µm above a 0.8-µm-thick gold signal line coated with ~0.15 µm of insulating SiN membranes span 300 µm and lengths of 20 to 140 µm 15-V dc voltage to signal line (in addition to the ac signal) pulls gold membrane down to the nitride

MEMS Switches membrane shunt switch (cont’d) use of insulator prevents switch from working at dc and low frequency but is expected to be more reliable than metal-to-metal contacts sides of gold membrane are supported by wide strips of same gold film in closed state, the connection is made by capacitive coupling, which is only useful at high frequency insertion loss in the closed state is less than 0.6 dB over the range of 10–40 GHz,

MEMS Switches cantilever series switch when closed, measured insertion loss of the switch is less than 0.2 dB from dc to 2 GHz

CSE 495/595: Intro to Micro- and Nano- Embedded Systems

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