Thin-Film Lithium Niobate Contour-Mode Resonators Renyuan Wang1, Sunil A. Bhave1, and Kushal Bhattacharjee2 1OxideMEMS lab, Cornell University, Ithaca, NY, USA 2RF Micro Devices, Inc., Greensboro, NC, USA Motivation • Motivation • Device Design • Fabrication • Results • Summary Motivation – Demand for Multi-frequency Band-select Filters Antenna Band-pass IF Filter Filter Array Mixer Mixer PLL/Oscillator MEMS Resonators or SAW resonators Integrated Multifrequency Filters Motivation – kt2 x Q Electromechanical coupling factor vs. quality factor Roll-off R. Ruby, AlN FBAR K. Hashimoto SAW F. Ayazi, sidewall AlN resonator BW 55k Nguyen et al, diamond disk G. Piazza, AlN Contour Mode Resonator Motivation – Lithium Niobate Thin-Film Contour-Mode Resonators Frequency Defined by Photolithography (Resonator Width, and Period of IDT) f m VAcoustic W Aluminum Nitride Contour Mode Resonators Lithium Niobate Contour Mode Resonators • Frequency is defined by photolithography, suitable for Multi-frequency filters integration • High coupling factor; leveraging the wafer polishing technique, device can be fabricated on a particular cut that is optimized for high coupling factor • High quality factor • kt2 < 3%, because z-axis of deposited AlN film is norm to wafer surface, the vertical to lateral transduction scheme is much less efficient than that of FBAR transduction Device Design • Motivation • Device Design • Fabrication • Results • Summary Device Design – Contour-Mode Resonators on LN Thin-Film Coupling Matrix Strain-Charge From 0 d 22 d31 0 0 0 d15 d 22 0 d15 0 d31 d33 0 0 2d 22 0 0 W L d 22 211012 C/N d31 11012 C/N d33 6 1012 C/N d15 68 1012 C/N • Key Design Parameters: Duty Cycle = Electrode Width/Period of Electrodes • • • • Targeted mode of vibration (S0 mode) Orientation of IDT Electrode duty cycle affects coupling factor and quality factor For a particular mode of vibration, both acoustic wave velocity and electro-mechanical coupling factor depend on Thickness/λ • Wafer cut: Black Y136 Cut Lithium Niobate • The following parameters are chosen based on COMSOL simulation: • W = 78.4um, 79.2um for 460MHz and 750MHz resonant frequencies • Electrode duty cycle = 50% for optimum coupling factor Device Design – 750MHz Contour-Mode Resonators on LN Thin-Film Contour plot of total displacement S0 Mode S G S G S • Key Design Parameters: G • • • • • Targeted S0 mode IDT parallel to x-axis Electrode duty cycle =50% 11 finger electrodes Thickness /λ = 0.126 COMSOL Simulation Admittance of the 750MHz Contour Mode Resonator Fabrication • Motivation • Device Design • Fabrication • Results • Summary Fabrication – Lithium Niobate Thin-Film Wafer Bonding and Grounding LN Device Wafer LN Device Layer Ion-Slice LN Carrier Wafer LN Carrier Wafer • Freedom of choosing wafer cut • Excellent film quality • Good thermal stability as device layer and carrier are both LN. Chemical Vapor Deposition • • • Poor quality factor and electro-mechanical coupling factor due to poor film quality [1] Thermal instability caused by the thermal mismatch between the layer stacks Possible damage of the thin-film from implantation, and peel-off process [1]. Michio Kadota, et. al., “FBAR using LN thin flim deposited by CVD”, IUS 2010 Fabrication – Anisotropic Etching of Lithium Niobate Ion Mill Reactive Ion Etching • • • Generally require metal hard mask, which increase the complexity of the process, and is hard to remove after etching. Require very high etching RF power Very hard to achieve clean side wall and good side wall verticality, due to the re-deposition of Li compound. Proton exchange can help with the etching, however it changes the property of the thin film. 1um wide slot on Lithium Niobate 1um • • • • RIE etching of Lithium Niobate [1] Using photoresist mask, easy to work with Good side wall verticality Clean side wall Can open less than less than 1um slots on Lithium Niobate [1]. Sarah Benchabane, et. al., “Highly selevtive electroplated nickel mask for Lithium Niobate dry etching” , J. App. Phys., 2009. Fabrication – Process Flow Glue - Thermally Matched to LN LN Device Wafer Buffer Layer 1 um Thick LN Device Layer 1um SiO2 1um SiO2 LN Carrier Wafer LN Carrier Wafer (b) Bonding of device wafer to carrier wafer (c) Grinding down the device layer to desired thickness 1um SiO2 Lithium Niobate (LN) Device Wafer (a) Deposition of sacrificial layer and buffer layer on device wafer Ion Mill Photoresist 1 um Thick LN Device Layer LN Device Layer LN Device Layer SiO2 SiO2 SiO2 LN Carrier Wafer LN Carrier Wafer LN Carrier Wafer (d) Ion mill defining device geometry (e) Lift-off top electrode (f) Release device in BOE Fabrication – Contour Mode Resonators • Etched Trench for Electrical Isolation and Stress relief Ground • Resonant frequency is defined by photolithography Resonators with different resonant frequencies are fabricated on the same chip Signal 460MHz • • Ion-Milled Lithium Niobate thin-film contour mode resonator with clean side wall and good side wall verticality Etched trenches to isolate the signal pad to reduce parasitic feed-through capacitance and help release the built-in stress, SEM shows no buckling or bowing 750MHz Experimental Result • Motivation • Device Design • Fabrication • Results • Summary Experimental Result – 750MHz Resonator Wide Span Sweep Designed Frequency, Corresponds to W = 79.2 um Fitted mBVD Model 22 Ohm Measurement fs = 747MHz Rs = 27 Ohm Fitted Curve 14.9 fF 158 fF 5 Ohm 3 uH fp = 781.5MHz 2 f kt2 1 s 8.6% f p Resonator Parameters from Fitted Model Qs = 538 Qp = 2800 kt2 = 8.6% kt2 x Qs = 51 Experimental Result – 750MHz Resonator Measurement COMSOL Simulation Targeted S0 Mode Experimental Result – 750MHz Resonator Zoom-in Measurement fs = 747MHz Rs = 27 Ohm Fitted Curve 2 f k 1 s 8.6% f p 2 t System impedance is adjusted to center the Q circle around the center fp = 781.5MHz Phase of Admittance Clear Transition from +90 ° to -90° kt2 x Qs = 53 Qs = 612 Qp = 3000 Experimental Result – 463MHz Resonator Designed Frequency, Corresponds to W = 78.4 um Measurement fs = 463MHz Rs = 51 Ohm kt2 = 7% Qp data is very noise, because the impedance at parallel peak is very large, and analyzer noise masked the measurement fp = 480MHz kt2 x Qs = 105 Qs = 1500 Experimental Result – kt2 x Q Electromechanical coupling factor vs. quality factor Roll-off (c) 750MHz 463MHz (d) BW 750MHz 460MHz (b) (a) (e) For RF filters, the kt2 determines the maximum possible BW, while the Q determines the filter roll-off. a) b) c) d) e) ) F. Ayazi, sidewall AlN resonator G. Piazza, AlN Contour Mode Resonator K. Hashimoto, LiTaO3 SAW R. Ruby, AlN FBAR Nguyen et al, diamond disk This work 55k Summary • With the flexibility lithography-defined resonator geometry, demonstrated multi-frequency resonators on a single chip we • Two resonators at two different frequencies are demonstrated • 463MHz resonator: kt2*Qs = 105, Qs = 1500, kt2 = 7% • 750MHz resonator: kt2*Qs = 52, Qs = 612, kt2 = 8.6% • Very large Qp was observed, which implies very good dielectric property from the LN thin film • For RF filters, the kt2 determines the maximum possible BW, while the Q determines the steepness of the filter roll-off. Lithium Niobate thin-film contour-mode resonators can potentially achieve both high Q and high kt2, and more importantly provide the flexibility to integrate multifrequency resonators and filters on a single chip. Acknowledgements • Sponsors • DARPA-MTO: ART Program Outline • Motivation • Demand for Integrated Multi-Frequency Filters • Device Design • High-Order Contour Mode Resonators on Lithium Niobate Thin-Film • Fabrication • LN Thin-Film: Epitaxial Deposition, Ion-Slice and Wafer Polishing • Anisotropic Etching of Lithium Niobate • Process Flow • Experimental Result • Multi-Frequency Resonators • Summary