Substitution of Al by Sc has been predicted and demonstrated to improve the piezoelectric response in AlN for commercial market applications in radio frequency filter technologies. Although cosputtering with multiple targets have achieved Sc incorporation in excess of 40%, industrial processes requiring stable single target sputtering are currently limited. A major concern with sputter deposition of ScAl is the control over the presence of non-c-axis oriented crystal growth, referred to as inclusions here, while simultaneously controlling film stress for suspended microelectromechanical systems (MEMS) structures. This work describes 12.5% ScAl single target reactive sputter deposition process and establishes a direct relationship between the inclusion occurrences and compressive film stress allowing for the suppression of the c-axis instability on silicon (100) and Ti/TiN/AlCu seeding layers. An initial high film stress, for suppressing inclusions, is then balanced with a lower film stress deposition to control total film stress to prevent Euler buckling of suspended MEMS devices. Contour mode resonators fabricated using these films demonstrate effective coupling coefficients up to 2.7% with figures of merit of 42. This work provides a method to establish inclusion free films in ScAlN piezoelectric films for good quality factor devices.

Aluminum scandium nitride (Al1−xScxN/AlScN) (x = 0.32) Lamb wave resonators (LWR) have been fabricated and tested to demonstrate electromechanical coupling coefficients (kt2) in excess of 10%. The resonators exhibited an average kt2 and unloaded quality factor (Qu) of 10.28% and 711, respectively, when calculated from the measured data. Applying the Butterworth Van-Dyke (BVD) model to the measured data enabled the extraction of the resonator's lumped element parameters to calculate the motional quality factor (Qm), which neglects the contributions of the electrical traces. For the best measured resonator response, results from the BVD model showed a Qm of 1184 and a resulting figure-of-merit (FOM = K2 · Qm) of 100. Comparing the response of similar AlScN and AlN resonators shows that the AlScN LWR has a significantly lower motional resistance (Rm), suggesting that AlScN has a strong potential for use in piezoelectric microelectromechanical oscillators.

Piezoelectric acoustic resonators are widely utilized for RF devices but most can only utilize a few different resonant frequencies on the same substrate. Contour mode resonators (CMR) have resonant frequencies defined lithographically offering the advantage of an extensive frequency range on the same film, wafer, or die. This work will discuss our efforts in developing ScAlN for CMRs to achieve RF performance suitable for use in RF filters. First we will discuss our RF sputter deposition process for creating high-performance Sc 0.12 Al 0.88 N piezoelectric films and compare it with an established sputtered piezoelectric AlN. Fabrication and integration techniques for fabricating CMRs will be then detailed. Finally, electrical design and resonator performance is then discussed. These early investigations into ScAlN films suggest that performance metrics such as k 2 eff can substantially improve while simultaneously ensuring good resonator Q factors over a wide frequency range of devices.

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Aluminum nitride (AlN) radio frequency (RF) MEMS filters utilize piezoelectric coupling for high-performance electrical filters with frequency diversity in a small form factor. Furthermore, the compatibility of AlN with CMOS fabrication makes AlN extremely attractive from a commercial standpoint. A technological hurdle has been the ability to package these suspended resonator devices at a wafer level with high yield. In this work, we describe waferlevel packaging (WLP) of AlN MEMS RF filters in an all silicon package with solder balls on nickel vanadium / gold (NiV/Au) bond pads that are subsequently ready for flip chip bonding. For this integration scheme, we utilize a 150 mm device wafer, fabricated in a CMOS foundry, and bond at the wafer level to a cavity silicon wafer, which hermetically encapsulates each device. The cavity wafer is then uniformly plasma etched back using a deep reactive ion etcher resulting in a 100 µm thick hermetic silicon lid encapsulating each die, balled with 250 µm 90/10 Pb/Sn solder balls and finally diced into individually packaged dies. Each die can be frequencytrimmed to an exact frequency by rapid temperature annealing the stress of the metallization layers of each resonator. The resulting technology yields a completely packaged wafer of 900 encapsulated die (14 mm 2 by 800 µm thick) with multiple resonators and filters at various frequencies in each package.