Acoustic Engineering of High Quality Factor Piezoelectric AlN-on-Si Micromechanical Resonators


Student thesis: Doctoral Thesis

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Award date2 Jan 2020


Very high frequency (VHF) band microelectromechanical systems (MEMS) resonators have been emerged as a plausible alternative to quartz crystal resonators for frequency-selective elements and timing modules of wireless communication systems. Their small size, low cost due to batch fabrication, low operating power and fabrication process compatibility with mainstream complementary metal oxide semiconductor (CMOS) processing, make them highly attractive for these applications. Strong electromechanical coupling and high quality factor (Q) are two vital features of micromechanical resonators which are extensively researched for improving the performance of resonators. As high Q and low motional impedance are desired for low phase noise oscillators, their obtainable Q is limited by several energy loss mechanisms. These include gas damping, akhiezer damping, interface damping, anchor loss, thermoelastic damping (TED) etc. These damping sources greatly influence the performance of MEMS resonators. Several techniques have been introduced to precisely model these energy loss mechanisms and thereby predict resonator’s performance. This ultimately reduce time consumption as well as lowers the fabrication cost by minimizing the number of trial and errors.

This research focuses on designing, finite element (FE) modeling, fabrication and characterization of piezoelectric Aluminum Nitride (AlN) on Silicon (Si) Lamb mode resonators, a promising resonator technology among several other existing acoustic resonator technologies. The Q of AlN-on-Si resonators is typically limited by anchor loss, which can be described as the outflow of acoustic waves through tethers that suspend the resonant cavity in free space. Two streams of acoustic engineering have been proposed for reducing anchor losses and boosting Q in AlN-on-Si resonators. With both of these approaches, we have experimentally demonstrated sufficient reduction in anchor loss to achieve Qs around 10000 for Lamb wave mode piezoelectric resonators. Besides, we have shown that these approaches are able to reduce anchor loss to a point whereby electrode related losses become observable.

First approach investigates the effectiveness of the biconvex design in enhancing the quality factor Q and effective coupling coefficient (k2eff) of AlN-on-Si Lamb mode resonators. These include modifying the resonator geometry when extending the mode order to reduce the motional series resistance. Results for applying various curvatures are described to illustrate the effect of resonator length in providing the possibility to reliably enhance quality factor through an optimal range of curvatures. The drawbacks of over-curving in biconvex resonators are also elucidated. Besides the effect of varying electrode geometry and electrode coverage on the performance of biconvex resonators have been presented. As the Qs of these resonators are associated with numerous dissipation sources, which are often difficult to separate from each other in experiments. For better understanding, FE models are applied to estimate anchor losses and TED to a large set of AlN-on-Si resonators specifically designed to have significantly different Qs. The computed values of Q based on the proposed FE models agree well with the experimentally obtained Qs from a large set of resonators.

The second approach is to investigate the ability of phononic crystals (PnCs) to shape the behavior of acoustic waves in the PnC medium due to the generation of acoustic bandgap (ABG) that arises from the periodic alternation of acoustic properties (velocity and density) in the PnCs. A unique wide ABG solid disk shaped PnCs are hybridized with AlN-on-Si MEMS resonators to reduce anchor loss and thereby enhance quality factor. The effect of geometrical variations to the proposed PnC unit cell on their corresponding ABGs are described through simulations and validated by transmission measurements of fabricated delay lines that incorporate these solid disk shaped PnCs.