Surface Acoustic Wave Devices - Challenges for Technology Development

Transcription

1 Surface Acoustic Wave Devices - Challenges for Technology Development Richard Gruenwald Vectron International April 2 th 215 Vectron SAW Products, Apr 215 1

2 Agenda SAW Functionality and Design Principles Challenges and Solutions: Miniaturization RF coexistence situations Power Handling Vectron SAW Products, Apr 215 2

3 Motivation Why Micro-Acoustic Filters (convert EM acoustic EM)? ( ) ( ) Size advantage over EM-filters (e.g. Microstrip, Cavity,...) Smaller size results in high power densities Established high-volume filter solution for 3 3 MHz 1.1mm 17mm Vectron SAW Products, Apr 215 4

4 SAW Applications BLUE: average +3 degc RED: average +9 degc magnitude in db magnitude in db Insertion magnitude (S21) in db frequency in MHz frequency in MHz Frequency in MHz Rx BB IF Rx IS LNA Rx Frontend VSWR IF Tx BB Key Applications: A. Transceivers: DPX PD Tx IS PA Filtering ( isolation ) of receive signal in RF (frontend and interstage) and IF Suppression of spurious signals in transmit path Separation of receive and transmit signal at antenna port - duplexer B. Oscillators: Signal generation (VCSO) 1- or 2-port SAW resonator Output (clean-up) filtering Osc Out PD Conductance / S magnitude in db Frequency / MHz frequency in MHz Vectron SAW Products, Apr 215 5

5 How do SAW/BAW devices work? Piezo-Electric Effect: electrical energy mechanical (acoustic) energy Surface Acoustic Waves: Filters Resonators Bulk Acoustic Waves: Crystals Monolithic Crystal Filters Achievable bandwidth / coupling coefficient k² Quartz (SiO 2 ) LiTaO 3 LiNbO 3 La 3 Ga 5 SiO high temperature stability Temperature sensitivity Manufacturing sensitivity Vectron SAW Products, Apr 215 7

6 How Does a SAW Filter Work? IN OUT h(t) h(t) * -T T t t jωt = h( t e dt H( ω ) ) = Electro-acoustic transfer function Acousto-electric transfer function Vectron SAW Products, Apr 215 9

7 Transversal SAW Filter FIR Filter CH1 S21 LOG 7 db/ REF db TFS15AL sample 12/29 CH1 PASS 4 Dec 29 8:36:33 1 REF :. db MHz db 1 2 IN OUT Cor BW: cent : 1_ loss Performance FIR Filter approach Lowest Shape factor: BW stopband / BW passband 1.3 Typically matching network required Insertion attenuation: IA 12dB 35dB Strong correlation between shape factor and size (>1³ wavelengths) Hld CENTER 15. MHz SPAN 1. MHz Frequency in MHz Vectron SAW Products, Apr 215 1

8 Pulse Compression Filter Magnitude (db) o SAW Modules (with Integrated Matching) o Matched Pairs of a) Expansion Filter b) Compression Filter o Analogue pulse compression (typ. radar application) hcorr ( t) = ha( τ) hb( t τ) dτ τ o Limitations apply for o τ B > 1 o τ > 1 µs (Amplitude response) ILoss = db x Magnitude (db) (Amplitude response) ILoss = db Magnitude (db) ILoss = db -1. db bw =.145 us -3. db bw =.2496 us -6. db bw =.3476 us -2. db bw =.583 us -3. db bw =.6582 us -4. db bw = u -5-6 Expander H A (f) Frequency (MHz) -5-6 Compressor H B (f) Frequency (MHz) Time (us) Vectron SAW Products, Apr Convolution h corr (t)

9 Loss Reduction Power-sensitive devices require low-loss components Constructive use of SAW reflections to trap acoustic energy in SAW resonator Electrical and/or acoustic coupling between resonant circuits forms filter passband Low-loss performance: IL 1 5 db Relative bandwidth constrained by material properties (electro-acoustic coupling ): BW rel.5 8% λ λ x= ϕ 4 refl. = π λ λ x= ϕrefl.= 2π magnitude in db frequency in MHz Vectron SAW Products, Apr

10 Resonator SAW Filter Impedance Element Filter: Y s1 Y s2 Y s.. Y sn Y p1 Y p.. Y pn L s C p C s R s Bandpass Filter: Resonance: f ( Z ) res = f = r 2π 1 L C s s Anti-resonance: Pole-zero distance: f ( Y ) res PZD= = f a f a f = r 2π = C S L + C s 1 2π C s p C C C p p p + C C s L s s 1 C s L s = f r 1+ C C s p 1 k 2 Bandstop / Notch SAW Filter: Inversion of series and shunt admittances Smaller bandwidth than bandpass filter Vectron SAW Products, Apr

11 Merging Transduction and Reflection IIR Filter Excitation Unidirectional performance: Loss reduction! Additional degrees of freedom for signal shaping weighting of excitation and reflection λ λ Acoustic reflections Vectron SAW Products, Apr

12 Wideband SAW Structures Slanted Finger IDT (SFIT) Filter: Parallel Connection of multiple narrow-band channels Insertion attenuation dB Fractional bandwidth 2%... 6% Challenging 2.5-D design procedure Vectron SAW Products, Apr

13 Package Technology RF Filters Same size 3x3mm² 2.5x2mm²: 2x1.6mm²: -44% -4% BGA - WLP available prototype S21 in db Migration to FlipChip Technology: Reduced space for package/chip inter-connects Reduced electro-magnetic influence of interconnects Increased chip space for SAW performance Performance advantages: Reduced insertion attenuation Improved skirt steepness Reduced fly back in GHz frequency range Hermetic package solution High reliability stud-bump solution S21 in db Frequency in MHz Wire-bonded 3x3mm² Flip-chip 2.5x2mm² Frequency in MHz Vectron SAW Products, Apr

14 Motivation for Technology Development e.g. LTE Frequency Bands 45 LTE Band S 21 / db f typ 2 15 f frequency / MHz Requirements towards Frequency a) Reduced transition band: f b) Increased fractional bandwidth: ² Vectron SAW Products, Apr

15 New Technologies Temperature Compensation S 21 / db f Guaranteed spec Temp. + prod. margin LiTaO 3 / LiNbO 3 + Al / Cu Frequency LiTaO 3 / LiNbO 3 + Al / Cu +.2λ SiO 2 S 21 / db Frequency High-performance filter / duplexer applications require minimum transition bandwidth f Temperature compensation required to ensure temperature-stable performance Achieved by deposition of optimized SiO 2 - overcoat (opposite expansion to piezo substrate) Mandatory for challenging bands and coexistence RF environments Vectron SAW Products, Apr

16 New Technologies Low-Tolerance Wafer Process S 21 / db f Guaranteed spec Temp. + prod. margin Frequency S 21 / db Frequency High-performance filter / duplexer applications require reduced / minimum transition bandwidth f Wafer-level area-selective ion-beam trimming to achieve % reduced process variations over one product Vectron SAW Products, Apr

17 Why High Power? System level: Design of receiver chain with low-noise amplifiers (LNA) and filters Improved signal-to-noise ratio Improved receiver sensitivity Reduced bit-error-rate High power critical for SAW devices: Low phase velocity ( ) ( ) Small structures High power densities power / dbm frequency / MHz Vectron SAW Products, Apr

18 SAW Propagation Effects SAW Device Functionality: Conversion between electric and mechanical energy Propagation and reflection of SAW within structure Amplitude of SAW ( m range) depends on applied RF power level and causes stress phenomena Resonant i.e. low-loss SAW filters face more challenging stress patterns than transversal (FIR) design techniques Narrow-band devices are more critical than wider bandwidth solutions Ref = dB IN OUT IN magnitude in db OUT frequency in MHz Vectron SAW Products, Apr

19 High-Power Damage Modes Damage progress: Acousto-migration causes extrusions and voids Damage of metallization system causes (typ.) increased insertion attenuation and reduced bandwidth Ultimately: Contact between extrusions catastrophic short-circuit magnitude [db] frequency [MHz] Vectron SAW Products, Apr

20 Power Handling Capabilities - SAW High power exposure micro-acoustic stress material deformation Extrusions and voids in fingers deteriorate performance (ultimately total failure) Lifetime and acceptable power level depends on multiple factors: Power density (abs. power, f c, size of acoustic elements) Duty cycle / modulation scheme (e.g. CW exposure vs. LTE-spectrum) In-band (electric acoustic) / out of band exposure ( IDT = capacitance ) magnitude in db up-link down-link frequency in MHz Vectron SAW Products, Apr

21 SAW Filter Lifetime Model Accelerated Lifetime Testing: Repeated power exposure and measurement /evaluation of device at maximum operating temperature worst-case frequency position Pre-defined performance degradation defines time-to-failure (e.g. /. %) Test is repeated for multiple devices and power levels (optionally temperatures) magnitude [db] frequency [MHz] IL > -.5dB: N = 716; h time to failure / h B -15dB / % TFS82A-std TFS82A-std corr. TFS82A-std lifetime TFS82A-std lifetime corr. TFS82A-std: P(15yr, T~P -3 ) = 15.5dBm power / dbm time / h Vectron SAW Products, Apr

22 SAW Filter Lifetime Model II TTF N = C P in e W kϑ Eyring lifetime model: C - constant (design, material, ) P in - power at filter input N - acceleration exponent (typically N = 3) W - activation energy k - Boltzmann s constant ϑ - ambient temperature Rules of thumb (N = 3): ϑ = +1K -5% TTF ϑ = +1K P in - 1dB P = +1dB -5% TTF Power durability specifications consider (default): Exposure (continuous wave) over 15 years Specified power applied at maximum operating temperature worst-case frequency position Frequency-dependence of power durability: Constant max. power density (W/mm²) P P ~ f1 max A IDT = Pf f1 f E.g. comparable design: P max,1ghz = P max,2ghz + 6dB 2 time to failure / h TFS82A-std TFS82A-std corr. TFS82A-std lifetime corr. -3 C C 3 C 6 C 12 C power / dbm Vectron SAW Products, Apr

23 High-power material system 4 35 Analysis of overstress risk power durability / dbm Conductance / S Frequency / MHz magnitude in db magnitude in db frequency in MHz IL dB -5-1 Impedance Element Filter LCRF Quartz resonator / -filter magnitude in db frequency in MHz Loss-reduced IF filter frequency in MHz -2 1 center frequency / MHz 1 Vectron SAW Products, Apr

24 SAW Filter Lifetime - Limitations TTF N = C P in e W kϑ Limits of accelerated test (e.g. 82MHz filter): + Test at 35dBm (3.2W) >1³ h test time required Test at 36dBm (4.W) Breakdown in <<1s overstress Test at worst-case frequencies (electric and acoustic fields) time to failure / h TFS82A-std: P(15yr, T~P -3 ) = 15.3dBm TFS82-HP: P(15yr, T~P -3 ) = 32.3dBm 1-2 TFS82A-std TFS82A-std corr. TFS82A-std lifetime TFS82A-std lifetime corr. TFS82-HP TFS82-HP corr. TFS82-HP lifetime TFS82-HP lifetime corr power / dbm Vectron SAW Products, Apr 215 3

25 Improved Power Durability - Technology Technological Solution: Improved power density handling though optimized metallization system Significantly improved stability against grain boundary diffusion effects! Standard system: Doped Aluminium aluminum layer layer 3dBm (~3W/mm²) optimized system: Adhesion layer 34dBm (~8W/mm²) Substrate Standard technology: 27dBm high power technology: 35dBm Vectron SAW Products, Apr

26 High Power Process Summary: Typ db power handling improvement for all SAW designs Power density capabilities close to breakdown limit Identical performance between standard and high-power system Technology fully qualified and in production Employed for New high-power designs High-power versions of existing products TFS82A-std.TiAl TFS82A-HP time to failure / h TFS82A-std: P(15yr, T~P -3 ) = 15.5dBm TFS82-HP: P(15yr, T~P -3 ) = 31.9dBm m a g n itu d e [d B ] TFS82A-std TFS82A-std corr. TFS82A-std lifetime TFS82A-std lifetime corr. TFS82-HP TFS82-HP corr. TFS82-HP lifetime TFS82-HP lifetime corr power / dbm frequency [MHz] Vectron SAW Products, Apr

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