AlN and AlScN contour mode resonators for MEMS-based RF front ends

Miniaturization is a simple yet very powerful concept. Applied to devices, it opens to plethora of new applications. Microelectromechanical systems (MEMS) are the result of scaling down the dimensions of devices to the micron range or the nano range (NEMS). This approach, leads to objects that not only are smaller, but often perform better than their bigger counterparts. It is not then a surprise that MEMS have been object of intense research in several different fields. In particular, the MEMS communications market is consolidating as we observe an increasing trend that points at hyper-connectivity of people and objects. RF MEMS-based front-end have been proposed in order to create communication systems that have low power consumption, easy integration with CMOS circuits and small form factor. Piezoelectric MEMS resonators are the core elements of the RF front-end. Among other technologies, contour mode resonators (CMRs) allow to chose the operating frequency via design. In this thesis, techniques to improve the performance of CMRs are studied. Two resonator parameters, the quality factor (Q) and the electromechanical coupling (kt^2), are markers to assess the resonator performance and to compare to state-of-the-art. As a matter of fact, a simultaneous enhancement of both has a great impact on the performance of the filters and oscillators built using them. Since Q is design-depended whereas kt^2 is mostly material dependent, a single approach often fails to enhance both. The thesis is based on two milestones. The first is to combine design optimization techniques with a materials with enhanced piezoelectric response compared to state of the art (i.e. aluminum nitride, AlN) to foster a simultaneous Q-kt^2 enhancement. The second is to test if a Q-kt^2 trade-off exists and how much designers can circumvent it combining selected techniques for each of these two parameters. The first part of the work focuses on Q. A modified fabrication technique is developed to: (i) take care of the Q instability with respect to the release area dimension and simultaneously (ii) create an acoustic reflector. This resulted in a 5X improvement in Q stability and up to 70 % Q recovery. The second part focuses on improving kt^2 using aluminum scandium nitride (AlScN), with 17 % Sc concentration. Our results show that anchor losses are still a dominant dissipation mechanism and that using AlScN we find an improvement in CMRs kt^2 higher than 2X. The highest FoM=70 (Q = 1658, kt^2 = 4.1 %) is obtained for a resonator operating at 390MHz, showing a motional resistance of 64 Ohm. The extracted relevant piezoelectric coefficient for AlScN CMRs, d31, is -3,89 pm/V (2.25X improvement compared to AlN). Finally, 2-port CMRs, are used to build oscillators. A new measurement technique is developed: the 2-port CMRs are directly contacted via RF probes, allowing the measurement of the phase noise (PN) for a large number of resonators. The best PN for the AlScN-based oscillator, measured at the highest Q =1413, is -100 dBc/Hz at 1 kHz offset frequency. Interestingly, when compared to an AlN-based oscillator with nearly identical Q, the AlScN PN performance is better than the AlN PN performance by 10 dBc/Hz at 1 kHz offset frequency. Since the only thing changing in the close loop setup is the resonator, this finding indicates that, regardless of the resonator Q, there may be a difference in the intrinsic resonator flicker noise due to the piezoelectric layer used.

Evidence of Smaller 1/F Noise in AlScN-Based Oscillators Compared to AlN-Based Oscillators

Marco Liffredo, Andrea Lozzi, Luis Guillermo Villanueva Torrijo

In this paper we investigate the direct effect of resonator quality factor (Q) on the oscillator phase noise. We use 2-port contour mode resonators (CMRs), fabricated both with aluminum nitride (AlN) and aluminum scandium nitride (AlScN), as the frequency-determining element in the oscillator circuit. Over 70 oscillator configurations are tested using resonators with different Q and with different piezoelectric layer. The testing of so many devices is possible because, in our setup, interfacing the circuit to the resonator is streamlined using RF probes. Our results show that higher resonator Q yields better frequency stability of the oscillator, for both AlN and AlScN CMRs. Interestingly, the comparison between AlN-based oscillators and AlScN-based oscillators with equal Q shows that AlScN-based oscillators' Phase Noise is up 10 dBc/Hz better than the AlN oscillator at 1 kHz offset frequency, suggesting different intrinsic resonator flicker noise of the two piezoelectric layers. [2019-0200]

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2020

Development of Lamb wave resonators in thin film X-cut lithium niobate

Muhammad Faizan

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>0.3Î») and a longer gap (g=2Î») 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=2Î», W_b= 3Î»/4, W_a= 3Î»/4 and L_a= 3Î»/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.

EPFL2021

Al0.83Sc0.17N Contour-Mode Resonators With Electromechanical Coupling in Excess of 4.5%

Andrea Lozzi, Paul Muralt, Luis Guillermo Villanueva Torrijo

In this paper, we demonstrate the fabrication of contour-mode resonators (CMRs) with Al0.83Sc0.17N as a piezoelectric layer. Moreover, we assess the electromechanical coupling and the maximum achieved quality factor from 150 to 500 MHz. In comparison to pure aluminum nitride (AlN) CMRs, our results show electromechanical coupling coefficients of more than a 2x factor higher at around 200 MHz. The highest quality factor is measured on a CMR operating at 388 MHz and is in excess of 1600. From the characterization of devices operating at different frequencies, material parameters of the Al0.83Sc0.17N are extracted such as the stiffness constant, the relative permittivity, and the piezoelectric constant. In particular, the reported d(31) piezoelectric constant is equal to -3.9 pm/V. This represents a 2.25x improvement when compared to pure AlN. Finally, we report the first temperature compensation experimental results for Al0.83Sc0.17N CMRs. Our results show that about 1.5 mu m of sputtered oxide, deposited on top of a released resonator, allows near zero temperature coefficient of frequency variation for CMRs operating up to 500 MHz.

2019

Development of Lamb wave resonators in thin film X-cut lithium niobate

Muhammad Faizan

EPFL2021