High-performance and Low-cost Capacitive Switches for RF Applications

Transcription

1 High-performance and Low-cost Capacitive Switches for RF Applications Bruce Liu University of California at Santa Barbara Toyon Research Corporation Toyon Research Corporation Fame

2 Outline Motivation for microelectromechanical (MEM) switches MEMS Switch performances and fabrication issues MEMS RF applications BST interdigital capacitors (IDCs) Low-loss BST phase shifters Future work and summary

3 Overview Z Z V Series Switch Z + V L - V Shunt Switch Z + V L - Ideal switching circuits Z Z V Z d Z + V L - V Z d Z + V L - Actual switching circuits V IL = 2log V L 2 log1+ Z = 2 log1+ Z / 2Z / 2Z d d,, series switch shunt switch

4 Switching Technologies At present, most commonly used switching devices are PIN diodes, GaAs FETs, and conventional mechanical switches (relays). V b I + - V I On state slope = 1/R on PIN diode = Z d Off R s R on C j O ff state V th V On GaAs FET + V gs - On state (V gs =) I d I d slope 1/R on V gs = I dss + V gs =-.5 V ds - V gs =-1 V min V gs =-V po source gate drain source gate drain r g R c r g r g C g C g r g r ds substrate C ds On state (V g =) Off state (V g <-V po ) R off V max Off state (V gs <-V po ) slope 1/R off V ds = Z d Off On R on C off

5 Key Switch Properties Important figures of merit for switches are: -isolation - insertion loss - power consumption - power handling capability - switching speed - cutoff frequency - cost of fabrication

6 MEMS (Micro ElectroMechanical Systems) Outgrowth of Micromachining - Creation of unique physical structures through the use of sacrificial layers resulted in miniature mechanical structures one a substrate MEMS Switch MEMS bridge t dielectric L air g G W substrate G substrate Switch up Switch down V p = 3 8Kg s 27ε WL Off C off Switch up MEMS Switch = Z d On C on Switch down

7 MEMS Switches in RF Applications MEMS Switch in a CPW configuration V bias RF choke MEMS bridge V bias + RFout DC Block DC Block coplanar waveguide C on /C off RF in substrate ε C ε r C Ww ε off Ww on g h - acts as RF switch or capacitor (1:1 ratios) - loss dominated by conductor loss - controlled by static DC voltage (1 nj switching energy) - low cost processing (~4 mask layers) - high cutoff frequency - minimum intermodulation distortion

8 Why RF MEMS? Comparative table Switch Type Isolation Insertion loss Power Power Switching Cost handling consumption speed PIN diodes Good Good Good Poor Good Good GaAs FETs Good Good Poor Good Excell Poor MEMS switches Escell Excell Excell Exell Poor Good Advantages: Very good isolation and insertion loss. Virtually no control circuit power dissipation in either ON or OFF state With proper design, can be capable of broad band and high power switching. Switching speed is more than sufficient for RF control circuit applications Relatively low cost (designed and fabricated by standard processing techniques). Essentially on every substrate. MEMS technology offers superior performance at lower costs and is expected to have tremendous impact on microwave systems.

9 Key Factors of MEMS Capacitive Switches MEMS switch typical structure SiN MEMS bridge substrate Critical parameters to be carefully considered : -the height of the membrane over the central conductor ( C OFF, V PD ) -the thickness and composition of the top membrane ( V PD, mechanical properties) -the size and the geometry of the membrane ( RF and DC performances) -the thickness and type of dielectric coating the central conductor ( C ON, breakdown) -type of substrate ( leakage, parasitic effects)

10 Typical Fabrication Process Ti/Au Substrate CPW Pattern Substrate SiN layer photoresist (PMGI) Ti/Al Substrate Sacrificial layer Substrate Membrane deposition Substrate Sacrifical layer removal

11 Membrane Release PMGI Stiction Ti/Au Critical Point Drying System Substrate Water or solvent Substrate solution Stiction Substrate High pressure chamber Stiction occurred in an air-dried sample 9 o angle view of the released suspended structure after critical point drier

12 Substrates & Metals Silicon GaAs Glass Si - Si 3 N 4 Si - SiO 2 High leakage current Low breakdown voltage Lowest leakage current High breakdown voltage Membrane: gold aluminum titanium platinum nickel Strong metal stress (Pt,Ni,Ti) Low metal stress (Al,Au)

13 Yield and Reliability Important Factors: compositions of MEMS top membrane low stress deposition reflow temperature and surface roughness of sacrificial layer Conclusions: risky design if span length longer than 3µm reflow temperature need to be adjusted for different types of MEMS switch design sputtered aluminum and electroplated low-stress nickel are good candidates as MEMS top membrane

14 DC Measurements Typical DC measurements DOWN state capacitance UP state capacitance Capacitive ratio Pull-down voltage 2-7pF.1-.5pF V

15 RF Simulations and Measurements S21 measured S21 fitted S21 HFSS S21 measured S21 fitted S21 HFSS -5 S21 MAG (db) S11 measured S11 fitted S11 HFSS S11 MAG (db) S21 MAG (db) S11 measured S11 fitted S11 MAG (db) S11 HFSS [GHz] [GHz] Switch UP CPW pad CPW pad R =.5Ω L = 2 ph Switch DOWN CPW pad CPW pad R =.5 Ω L = 2 ph C =.75 pf C = 2.7 pf Good agreements between measurements and HFSS simulations Instead of 3-D EM simulation, lump-element model can also fit measurements well, which gives us a simplified way to describe MEMS switch performance

16 More Topics on MEMS Switches How do we improve the switch performance? h A Switch down C down = ε ε r A h Increasing switching ratio (i.e. increasing C DOWN for given C UP )

17 Switches with Metal Caps Metal cap Switch up Switch down UP DOWN Pro: Much higher isolation in DOWN state than previous design Con:Direct metal-to-metal contact lower switching speed [Hz]

18 MEMS RF Applications Advantages of RF MEMS -High performance, low bias power consumption -Potential low cost manufacturing into a variety of substrates Limitations of RF MEMS -Slower switching speed -Potential lifetime limitations APPLICATIONS Phase Shifters Filters Reconfigurable Antenna

19 Programmable Delay Lines Variable Phase Velocity Design Analog or Digital MEMS Switches Switched-Capacitor Variable-Capacitor L l L l L l L l L l L l L l L l L l L l C l C s C l C s C l C s C l C s C l C s C l C s C l C s C l C s RF in 4-bit Digital Phase Shifter -22.5º -45º -9º -18º RF out V 1 V 2 V 3 V 4

20 Distributed Circuit Concepts Transmission-line Z, β Equivalent circuit Z v = = 1/ L / C ω < LC 2 / LC L C L C L C L C L C L L l L l L l L l L l Loaded Transmission-line C l C s C l C s C l C s C l C s Varactor provides variable phase delay: ( C ) φ = ω L C + l l s

21 MEMS Low-loss Distributed Phase Shifters Transmission line sections Z line, v line Z line, v line Z line, v line C MEMS C MEMS C MEMS Equivalent Circuit: L t C t C MEMS L sect : Length of transmission line per section C t : Transmission line capacitance per section L t : Transmission line capacitance per section Periodic loading of transmission lines with MEMS capacitive switches creates a structure with a variable phase velocity

22 Optimization MEMS Phase Shifters Loading Factor: x = Min/Max ratio: C max var C Return Loss: S11 S11,max.4 / sect l L 1 S 11,max a = 1 + S11,max min max 1 a ymin = Cvar / Cvar a x 2 Max phase shift/section: ( 1+ x xy ) Lsec δφ = 2π f 1 vi 1 fs = max 2π r C Loss = n 5 s var t + f 2 max sec t π Cvar Z L + fs n sec t L sec t α ( Z ) i ymin S 11,max =-2dB S 11,max =-17dB S 11,max =-14dB Insertion Loss [db] Loading Factor x Loading Factor x

23 Alternative MEMS Switch Configurations Challenge: Not overload the transmission line with an excessively large MEMS switch capacitance in the DOWN state Solution: the series configuration design C fixed C var Substrate C var C TOT = C var C fixed 2 C + 2C if C var >> C fixed CTOT 2C C << C C C var fixed var fixed TOT var fixed C fixed C fixed

24 One-bit MEMS Phase Shifters 35 Ground Ground MEMS capacitors Differential phase shift [degrees] Simulated Measured [GHz] UP state S-parameters [db] DOWN state S-parameters [db] S21 S21-1 S S11 MAG[dB] -2-3 S21 MAG[dB] S11 MAG [db] -2 S11-3 S21 MAG [db] [GHz] [GHz] The state of the art insertion loss performance, 154 /db at 25 GHz and 16 /db at 35 GHz, demonstrates the potential for the implementation of a very low loss multi-bit digital MEMS phase shifter.

25 Three-bit MEMS Phase Shifters DC Block Designed for 25GHz operation 3dB max. insertion loss Phase error less than 8.5 for all switching states 6 1 Differential Phase Shift (Degree) Insertion Loss [db] Return Loss [db] (GHz) [GHz]

26 Topology of 3-Pole Tunable Filters Coupling Capacitors C 12 C 23 C n n+1 5 Ω Z 1, L 1 Z 1, L 1 5 Ω Input Line Output Line Resonator 1 (a) Resonator n CPW Ground Coupling Capacitors Loading MEMS Capacitors Signal C1 C 12 C23 C 34 CPW Ground Loaded line Resonator (b) (a) Topology of capacitively coupled tunable filter based on DMTL resonators. (b) Layout of 3-section tunable filter in coplanar-waveguide form. - Capacitively loaded line sets up a slow wave structure - Resonator electrical length determined by capacitive loading of the line - Tuning range of the filter determined by the effective capacitance range of varactor - MEMS varactors have been demonstrated with low loss making them an excellent candidate for filter applications

27 Simulated Results of Tunable Filters Return Loss Insertion Loss 5 S11 (db) Cvar=12 ff Cvar=15 ff Cvar=18 ff S21 (db) Cvar=12 ff Cvar=15 ff Cvar=18 ff (GHz) (GHz)

28 Measured Tunable Filter Performance RF IN RF OUT Bias Pad 12% bandwidth at 2GHz with 3.6dB insertion loss in passband Loss is contributed by conductive loss, substrate leakage and radiation. micromachining 3.8% tuning range, which is limited by the tuning factor of MEMS varactors (~1.3:1) 1 SEM microphotograph -1 S11 [db] V=V V=5V V=6V S21 [db] V=V V=5V V=6V [GHz] [GHz]

29 Reconfigurable Microstrip Grid Antenna --- Toyon Project Reconfigurable antenna systems: The switch can be used to turn on or off various conductive paths. In this way various virtual antenna states can be realized. A matrix of RF switches in a metallic grid to control the RF current distribution and hence radiation and impedance characteristics of the antenna structure. Multiple frequency band operation Possible beam and null steering Lends itself to multiple feed locations Polarization diversity Microstrip structure with switches represents over 1 17 possible states.

30 4-Element Microstrip Grid Antenna SQUARE RECONFIGURABLE GRID ANTENNA INCORPORATING MEMS CONTROL SWITCHES.1 cm 1.5 cm.312 cm Aluminum housing 4 Gold traces 3.4 cm SMA connector (CDI p/n 538-2CC) 1 MEMS switches 4-places 2 Glass substrate (Er = 5.75) 2.5 cm.71 cm 3/24/ M. Gilbert Toyon Research

31 Preliminary Work on MEMS-based Grid Antenna FULL HFSS MODEL -- ALL MEMS DOWN SQUARE RECONFIGURABLE ANTENNA INCORPORATING MEMS CONTROL SWITCHES (switches at 3/4 of each leg length, diagonal trace from feed point) Radiation Pattern [Electric Field] S11 [db] simulated measured [GHz]

32 BST Thin Film Technology Motivation Phased Array Antennas The Phase-Shifter: A critical component Thin-Film BST Varactors Parallel Plate vs. Interdigital Capacitors Device Characterizations Phase-Shifters using Thin-Film BST

33 Wideband Phased Array Coarse Delay Control Sub-Array True Time Delay unit Fine Delay Control TTD TTD Antenna Array TTD Feed True Time Delay unit True Time Delay unit True Time Delay unit True Time Delay unit TTD TTD TTD desired phasefront True Time Delay unit True Time Delay unit True Time Delay unit TTD 12-bit Beam Control True Time Delay unit TTD TTD

34 Barium Strontium Titanate (BST) Key BST Properties Large field dependent permittivity --- Compact tunable circuits Intrinsically fast field response --- Fast switching speeds High breakdown fields, >3x1 8 V/cm --- High power handling capability Low drive currents (dielectric leakage) --- Low prime power requirement Simple fabrication --- Low cost - BST thin film properties differ markedly from those of bulk and much work is focused on low-cost deposition of BST thin film for microwave applications.

35 Influence of Loss and Tunability on Phase Shifter Performance Tunability: how much we can vary capacitance of film C max /C min Loss (db) for 36 degrees at 2 GHz Distributed Circuit Phase Shifter BST on Silicon, Tunability: Total Loss CPW conductor Loss.2.1 Effective Device Loss Tangent at 2 GHz Loss (db) for 36 degrees at 2 GHz 8 Silicon Substrate 7 BST Loss Tangent= BST Loss Tangent= BST Loss Tangent= BST film tunability Device loss and tunability are important

36 BST Device Considerations Desirable Features of a Viable Thin-Film Varactor Technology Reproducibility Inexpensive substrates Standard growth/processing steps Low loss tangent (tan δ <.1) High tunability (>2:1) Compatible with low-cost packaging - Integrated monolithic capacitors using sputtered/mocvd material on low-cost substrates - Sapphire is a cost effective wide area substrate that has excellent microwave and rf properties

37 Monolithic BST Device Structures Parallel Plate vs. Interdigitated Capacitor (IDC) w w w l w 3w Substrate SiN BST Pt Vertical polarization High tunability, efficient use of BST Low breakdown voltage Complex fabrication Horizontal polarization Low tunability, inefficient use of BST High breakdown voltage Easier fabrication

38 DC Measurements Tunability Interdigitated Structure 7 Capacitance (pf) Width=2um, Spacing=1um Width=2um, Spacing=2um -1Å film - 7-8% of material tunability --1 V control -Device tunability limited by finger-to-finger spacings - Ti/Au metalization - 3-mask process DC Bias (V) Good choice for low-cost circuits

39 RF Parameters Extraction Method Layout for Parameters Extraction: Open Short Load Equivalent Circuit Expression: Y S Y S Y S Y P Y P Y P Y D Y op Y D Y sh (Y L -Y op )(Y sh -Y op ) Y sh -Y L Q= ωc/g Y L G+iωC

40 Quality Factor Limits Capacitance [ff] w=2µm, s=.5µm w=2µm, s=1µm Total device loss consists of conductor loss and BST material loss: = + Qdevice Qcond Qbst Q bst, inherent to deposited BST film Q cond, determined by R metal and C device [GHz] Quality Factor w=2µm, s=.5µm w=2µm, s=1µm Devices with different finger to finger spacings but same other dimension parameters are fabricated on the same wafer Q device stays the same while Q cond varies Q bst dominates the total device Q device [GHz]

41 Quality Factor of BST Interdigital Capacitors 6 5 1nm (Ba,Sr)TiO 3 thin films deposited on sapphire substrate Quality Factor Ba/Sr=3/7 Ba/Sr=5/5 Quality factor rolls off as frequency increases Stoichiometry of deposited thin film determines its loss tangent Q>35 in X-band for (Ba 3,Sr 7 )TiO 3 thin film [GHz] T Surface = 7 C: RF power = 2* 15 W ( 3.3 W/cm 2 ): 9/1 Ar/O 2 (sccm): 1 nm

42 Tuning Factors of BST Interdigital Capacitors 8 7 Finger spacing = 1µm 6 Capacitance [ff] nm (Ba 3,Sr 7 )TiO 3 thin film on sapphire substrate 2 1 V 4V 1V ~2:1 tuning range -1V DC control [GHz] T Surface = 7 C: RF power = 2* 15 W ( 3.3 W/cm 2 ): 9/1 Ar/O 2 (sccm): 1 nm

43 X-band BST Phase Shifters 15mm mm Return Loss [db] V Insertion Loss [db] 4V -25 1V [GHz] Photograph of fabricated X-band BST phase shifter S-parameter measurements of distributed BST phase shifter

44 Measured Differential Phase Shift 6 8 Differential Phase Shift [Degree] V 4V 1V Phase Shift per db Loss [Degree/dB] [GHz] Differential phase shift at V, 4V and 1V bias voltage [GHz] Phase shift per db insertion loss at 1V bias voltage º-36º phase 8.2GHz with 5dB insertion loss Return loss is better than 1dB from DC to 1GHz

45 State of Progress What have been done so far? Developed a novel design of MEMS devices MEMS-based RF circuits (i.e. phase shifters, filters) BST interdigital capacitors and phase shifters What the plan for future work? BST-MEMS switches MEMS-based reconfigurable microstrip grid antenna MEMS single-controlled single-pole-double-throw (SPDT) switches

46 BST-MEMS Switches Barium Strontium Titanate (BST) Switch up Switch down High dielectric constant of BST (>25) >15dB more isolation than conventional MEMS switch in DOWN state BST What we will do? BST/Pt/Au/Ti/Sapphire Reduce pull-down voltage, avoid breakdown in BST thin film

47 Future Work on Reconfigurable Antenna --- Toyon Project Demonstrate the 4-element microstrip grid antenna using standard MEMS fabrication techniques RF measurements and simulations accounting for the parasitic effects in the model Finally expand this design to a 16-element switching reconfigurable antenna array

48 MEMS Single-Controlled SPDT Switches INPUT Control 1 Control 2 OUTPUT 1 OUTPUT 2 Coplanar Waveguide MEMS Switch 1 λ/4 MEMS Switch 2 Signal OUT 2 Signal IN λ/4 Signal OUT 3 Coplanar Waveguide Port 2 Port 2 Isolated Port 1 CDOWN Port 1 λ/4 λ/4 Port 3 Isolated λ/4 Port 3 (a) Switch Down (b) Switch Up

49 Summary MEMS based switches promise superior performance relative to conventional devices. Through its superior performance characteristics, the MEMS switches are developed in a number of existing circuits, including switches, phase shifters and filters. BST thin films have been characterized and used to demonstrate a º- 36º X-band phase shifter with only 5dB insertion loss. Replace Si 3 N 4 with high dielectric constant BST thin film in MEMS switch is expected to further improve the switch performance. MEMS technology offers the potential for building a new generation of low loss high-linearity reconfigurable antenna arrays for radar and communications applications.