UC Berkeley

The explosive development of wireless and mobile communication systems has continuously driven rapid technology innovation in component performance and system integration. In order to obtain faster signal processing and reduce integration complexity, the miniaturized and Complementary metal–oxide–semiconductor (CMOS) compatible micro-electromechanical system (MEMS) resonators are likely to be the driving core of a new generation of devices such as radio frequency (RF) filters and oscillators which are the main building blocks in the RF front-end. Thus, a high-performance MEMS resonator technology is highly in demand as the fundamental components in the RF front-end for an advanced wireless communication system.

Among various microelectromechanical resonator technologies, aluminum nitride (AlN) Lamb wave resonators (LWRs) have attracted great attention since it combines the advantages of surface acoustic wave (SAW) and bulk acoustic wave (BAW): the ability of high resonance frequency (fs) and multi-frequency on a single chip. The AlN-based structure provides for CMOS compatibility, and the lowest order symmetric (S0) Lamb wave mode exhibits high phase velocity up to 10000 m/s, weak phase velocity dispersion, small temperature coefficient of frequency (TCF), high quality factor (Q), and moderate electromechanical coupling coefficient (k2). However, the performance parameters needs to be further improved to enable the low-loss filters and stable oscillators. This dissertation addresses a number of issues and demonstrated the high-performance (high-fs., large-k2, high-Q, low-TCF, and low-resonance impedance (Zmin)) piezoelectric AlN LWRs to fulfill the technical requests for the RF front-end.

The k2 and fs optimization of the AlN LWR using the S0 Lamb wave mode is achieved by choosing electrode materials and thicknesses for specific electrode configurations. This study adopts the finite element analysis (FEA) to investigate the influence of electrodes on the dispersive characteristics of the S0 mode propagating in the multi-membranes. From the theoretical study, it is found that the phase velocity is directly related to the density (ρ) and equivalent phase velocity in the metal, while the coupling coefficients depend on ρ and acoustic impedance (Z) of the metal. Large-Z material is preferred for the IDT and light material is favored for the bottom electrode to optimize the k2. Thicker metal for the IDT loads the k2 more when AlN is thin and enhance k2 more when AlN is relatively thick.

In order to boost the Q, a new class of the AlN LWRs using butterfly-shaped plates is introduced in this dissertation for the first time. The butterfly-shape plates can effectively suppress the vibration in the tether location and reduce anchor loss. The 59˚ butterfly-shaped resonator enables an unloaded Q up to 4,758, showing a 1.42× enhancement over the conventional resonator. The experimental results are in good agreement with the simulated predictions by the 3D perfectly matched layer (PML)-based FEA model, confirming that the butterfly-shaped AlN thin plate can efficiently eliminate the anchor dissipation. What’s more, by employing the butterfly-shape AlN plates with rounded tether-to-plate transition which has smaller tether-to-plate angle, the suppression in the anchor loss and enhancement in the Q is even more effective, which is also demonstrate by the simulations. The fabricated butterfly-shaped resonator with 45˚ beveled tether-to-plate transition yields a Q of 1,979 which upwards 30% over a conventional rectangular resonator; while another AlN LWR on the butterfly-shaped plate with rounded tether-to-plate transition yields a Q of 2,531, representing a 67% improvement. In addition, the butterfly-shaped plate didn’t introduce spurious modes on a wide spectrum, compared to other Q increase techniques.

The TCF reduction along with the k2 and fs optimization is demonstrated by introducing the symmetrical SiO2/AlN/SiO2 sandwiched structure to replace the conventional temperature compensation structure AlN/SiO2. The symmetrical SiO2/AlN/SiO2 sandwiched plate experiences the less temperature-induced bending deformation than the conventional AlN/SiO2 bilayer plate when the operation temperature arises from room temperature to 600°C. The symmetrical SiO2/AlN/SiO2 sandwiched plate also enables the pure S0 mode which shows the higher vp and larger k2 than the QS0 mode in the AlN/SiO2 bilayer plate. In the SiO2/AlN/SiO2 sandwiched membrane, the single IDT with the bottom electrode configuration offers a simple process flow and a large k2 at the relatively thin hAlN region (hAlN = 0.1λ). The double-sided IDTs configuration provides a large k2 up to 4% at the relatively thick hAlN region (hAlN/λ = 0.4λ) even when the thick SiO2 layers are employed for thermal compensation at high temperatures. Based on the correct choices of the AlN and SiO2 thicknesses in the symmetrical SiO2/AlN/SiO2 sandwiched membrane, the LWRs can be thermally stable and retain a large k2 at high temperatures.

The low-Zmin and high-fs AlN LWR is studied by utilizing the first order symmetric (S1) mode propagating in a specific thickness of AlN. In order to achieve the larger electromechanical coupling coefficient and high phase velocity as well as avoid the negative group velocity in the S1 Lamb wave mode, the 4-μm-thick AlN layer and 3-μm-wide finger electrodes are employed in the Lamb wave resonator design. The experimental results show the S1 mode presents a Zmin of 94 Ω at 1.34 GHz, lower than the Zmin equal to 224 Ω of the S0 mode at 878.3 MHz.