Applications: RF MEMS

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

2. Radio Basics • Receiving signals and an oscillator Hz

3. Radio Basics • Start simple

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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 ismeasure 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)

12. Capacitors and Inductors • Inductor quality • inductor L with series resistance Rs, Q = 2πfL/RS

13. Capacitors and Inductors • Q = 2πfL/RSand 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

14. Capacitors and Inductors • inductors often have higher series resistance than capacitors • far lower quality factors • substrate loss as well

15. Capacitors and Inductors • 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

16. Capacitors and Inductors • 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!

17. Capacitors and Inductors • 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

18. Capacitors and Inductors • 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

19. Capacitors and Inductors • 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.

20. 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

21. Micro-machined capacitors • 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)

22. Micro-machined capacitors • 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

23. Micro-machined capacitors • 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

24. Micro-machined capacitors • 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.

25. Micro-machined capacitors • PART II

26. 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

27. Bulk Micro-machined capacitors • DRIE for high aspect ratio A:d • typically resistance in the spring dominates Q

28. Bulk Micro-machined capacitors • 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

29. Bulk Micro-machined capacitors • 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

30. Bulk Micro-machined capacitors • 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

31. Bulk Micro-machined capacitors • 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

32. Bulk Micro-machined capacitors • 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.

33. Bulk Micro-machined capacitors • 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

34. Bulk Micro-machined capacitors • 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

35. 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

36. Micro-machined inductors • 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

37. Micro-machined inductors • 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 fSRof 6.6 GHz

38. Micro-machined inductors • 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%

39. Micro-machined inductors

40. 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).

41. MEMS Resonators • Illustration of Q (review from before)

42. 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

43. 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

44. Comb Drive Resonator

45. 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 × 10-11 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

46. 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

47. 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

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

49. 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

50. 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