Abhay Kochhar

Investigation of 20% scandium-doped aluminum nitride films for MEMS laterally vibrating resonators

This paper reports on the investigation of 1 μm thick films of 20% Scandium-doped Aluminum Nitride (ScAlN) for the making of piezoelectric MEMS laterally vibrating resonators (LVRs). The ScAlN films, which can be sputter-deposited such as undoped Aluminum Nitride (AlN) films, were used to demonstrate high performance resonators. These devices showed quality factor (Qs) in excess of 1000 in air centered around 250 and 500 MHz and enhanced electromechanical coupling (kt2) in the range of 3.2–4.5%. This kt2 is double the value of what has been achieved on similar resonators made out of AlN films. A 3-dB Qs of 1300 has been recorded both for 1-port and 2-port resonators at 250 and 500 MHz, while a maximum Qs of 1500 has been recorded for a 1-port resonator at 500 MHz. Along with experimental results from actual devices, this work also reports the etching characteristics of the piezoelectric material under Cl2/BCl3 chemistry to attain high selectivity and straight sidewall with a SiO2 hard mask. More broadly, enhancement of resonators design and fabrication process, suppression of spurious modes and increase in the concentration of Sc (theoretically up to 40%) will lead to significant performance improvements for many classes of piezoelectric MEMS, especially tunable filters.

Low loss delay lines based on suspended thin film of X-cut lithium niobate

Analog signal processing techniques based on surface acoustic wave (SAW) devices have been commercialized in the past 50 years. Delay lines [1] and resonators [2] have been implemented for a wide range of applications [3]. The use of small scale resonators has boomed because of the continuous growth of the cell phone business, whereas components such as delay lines have been relegated to smaller markets such as base stations. Emerging applications related to the Internet of Things have brought up the need for extremely low power and high speed signal processors. The maximum electromechanical coupling (kt2) of surface acoustics waves in lithium niobate (LiNbO3, dubbed LN) is 5.6 % [4]. Lamb waves resonators fabricated on suspended thin films of LN have demonstrated a kt2 around 30 % [5]. Taking advantage of such high kt2 to build delay lines is extremely beneficiary. However, due to the nature of Lamb waves, which require suspended plates, achieving long delays poses an important fabrication challenge. We overcome this technological challenge in this work.