Microelectromechanical systems (MEMS) is among the most revolutionary technologies of 21st century, with the applications ranging from industrial systems to consumer electronics. Using MEMS in battery-powered wireless devices has long been seen as the evolutionary step in achieving a highly miniaturized and efficient RF frontend (RFFE) for the next generation of communication systems. Piezoelectric MEMS resonators are the essential component of every RFFE as they do a phenomenal job of frequency generation and filtering while meeting the stringent specifications regarding power budget and form factor.
This thesis focuses on the design, fabrication and characterization of Lamb wave resonators on thin film LN for RF filters and oscillators. The goal of the project is to improve the electromechanical coupling (k_t^2) and quality factor (Q) of the resonators by first selecting an appropriate LN cut and in-plane device orientation for the intended mode and secondly, by optimizing different geometrical parameters such as electrode coverage (duty factor), gap, bus and anchor dimensions.
With the goal to improve k_t^2 of the resonator, we fabricate devices with varying electrode coverages from 20% to 70% for Al and Pt electrodes. Our results show that low electrode coverages (20% to 40%) do not have significant effect on k_t^2, but for higher electrode coverages (> 40%), k_t^2 reduces by ~35% due to the non-linear increase in static capacitance (C_o). The electrode material does not show any significant dependence on k_t^2 but devices with Al electrodes show larger Q (in our case x2) than devices with Pt electrodes due to higher resistive losses. We demonstrate devices with the highest reported k_t^2 of 31% and 40% for S0 and SH0 mode resonators with Q of 720 and 590, operating at around 500 MHz and 300 MHz, respectively, in X-cut LN.
In our next effort to further enhance Q, we fabricate devices by varying the geometrical dimensions of different inactive region elements such as anchor, bus and gap of the resonator. Each element plays a very important role in containing energy within the resonator cavity resulting in Q improvement. Our measurements suggest that a square anchor (L_a=W_a), a wider bus (W_b>0.3λ) and a longer gap (g=2λ) helps to improve Q of the resonator. The highest Q of 1900 and k_t^2 of 41% is achieved for the design parameters of g=2λ, W_b= 3λ/4, W_a= 3λ/4 and L_a= 3λ/4, resulting in the highest-ever achieved Figure-of-Merit (FoM) of 780 for SH0 mode resonators in X-cut LN.
We also demonstrated the fabrication of high performance A1 mode resonator operating in 3-6 GHz range in Z-cut LN. Our fabricated devices exhibit k_t^2 and Q up to 28% and 300, respectively. This performance is in agreement with the FEM simulations and is suitable for the 5G RF-filter technology.
Lastly, we use 2-port SH0 mode resonators to build oscillators and measure phase noise (PN) as it directly relates to its frequency stability. We investigate the dependence of PN on carrier power, Q and k_t^2 by measuring the PN for 60 oscillators using different SH0 mode resonators. Our results show that a higher carrier power and Q significantly reduces PN and improves frequency stability of the oscillator, whereas k_t^2 does not show significant influence on PN. We achieve a low phase noise of -96 dBc/Hz and -129 dBc/Hz, at the offset frequency of 1 kHz and 10 kHz, respectively, for the resonator with Q of 2500.