Li-Peng Wang

Frequency tuning of film bulk acoustic resonators (FBAR)

This paper suggests a design for a Film Bulk Acoustic Resonator (FBAR) which utilizes a secondary piezoelectric layer for purposes of tuning the FBARs resonant frequency. Currently, many ceramic resonators have difficulties in on-chip integration, power handling and electrode fabrication. FBARs are not only simple to fabricate and capable of full integration with CMOS/RF IC circuitry, but are also compact and can achieve high frequencies (GHz) with high quality factors. It is widely accepted that piezoelectric actuators encounter a significant change in mechanical stiffness between their open-circuit and closed-circuit states. In addition, it has been previously shown that the resonant frequency of a multi-layer FBAR is a function of the acoustic impedances and, correspondingly, the acoustic velocities, of its respective layers. Since the effective modulus term of the acoustic velocity of an FBAR layer is dependent on both the mechanical properties and electromechanical coupling of its piezoelectric element, and since electromechanical coupling can be altered by means a previously investigated shunt capacitor tuning concept, the stiffness of the piezoelectric tuning layer can be adjusted to vary the resonant frequency of the FBAR. Since difficulties have existed in matching FBAR resonant frequencies to specified values or making the frequencies stable during temperature variations, an active tuning capability for FBARs could offer many possible improvements. This work describes the application of the shunt capacitor tuning to a FBAR resonator and looks at the effects that varying different FBAR parameters have on the frequency range and degree of tunability of the device.

Sub-μg Ultra-Low-Noise MEMS Accelerometers Based on CMOS-Compatible Piezoelectric AlN Thin Films

Piezoelectric accelerometers based on aluminum nitride thin films offer a number of advantages in microelectromechanical systems such as high signal-to-noise ratio, low dielectric loss, low power requirements and CMOS process compatibility. This paper reports the state-of-the-art piezoelectric accelerometer design with an emphasis on maximizing the sensitivity per area. The accelerometers have a novel sensing structure to increase the sensitivity without increasing the sensing area using the approach of stress concentrations. Furthermore, the sensing structure is designed to have low off-axis sensitivity and to be reliable with symmetric balanced bars between sensing beams. Experimental results confirm the significantly improved sensitivity of the accelerometers obtained with the new sensing structures. The tested charge sensitivity is 5.2pC/g and the measured total noise floor of sensor plus interface electronics is as low as 670ng/√Hz.

Method of Fabricating Multiple-Frequency Film Bulk Acoustic Resonators in a Single Chip

We report experimental results of a novel approach to integrate multiple-frequency film bulk acoustic resonators (FBAR) in a single chip. An additional tuning layer was deposited and patterned on a conventional Metal/AlN/Metal FBAR film stack. By controlling in-plane dimensions of the periodic tuning patterns, resonance frequencies are modulated according to the corresponding loading percentages. To obtain a desirable frequency response of the modulated resonance peaks (a pure frequency shift), the pitch of the tuning patterns needs to be smaller than the membrane thickness. Three thicknesses of the tuning layers are fabricated to demonstrate different tuning ranges and sensitivities. This approach provides a potential solution to integrate multiple-frequency FBAR filters of adjacent bands by only one additional lithographic patterning step

Sputtered A1N Thin Films for Piezoelectric MEMS Devices

Piezoelectric films have been demonstrated to be attractive for micromechanical systems (MEMS) devices. Among the piezoelectric films used, AlN film has been less explored. In this study, AlN resonators and accelerometers, utilizing longitudinal and transverse piezoelectric effects respectively, were demonstrated. Resonators with Q of 1000 and electromechanical coupling (kt 2) of 6.5% were achieved at ~2 GHz. Accelerometers were fabricated and tested with charge sensitivities ranging between 0.06 to 0.45 pC/g depending on device designs. Important material properties of sputtered AlN films - C33 of 435 GPa, e33 of 1.55 C/m2, and e31 of -0.58 C/m2 - were extracted by fitting finite element analysis (FEA) simulated values to measured results.

Characterization of polycrystalline AlN films using variable-angle spectroscopic ellipsometry

Polycrystalline aluminum nitride (AlN) films, prepared by reactive sputtering, were characterized by variable-angle spectroscopic ellipsometry (VASE) and x-ray diffraction (XRD) rocking curve. A VASE model, which included cylindrical symmetry, effective-medium approximation, and surface roughness, was used to extract the optical constants of AlN films: refractive index (n) and extinction coefficient (k). Microstructure of the sputtered polycrystalline AlN films was well reflected in the VASE model and was verified by scanning electron microscopy cross-sectional micrographs. In addition, a correlation between optical constants and full width at half maximum (FWHM) of XRD rocking curve was established. Films with smaller FWHM of the XRD rocking curve, an indication of fewer defects and better crystallinity, had a higher refractive index and smaller extinction coefficient. This study leads to a faster and simpler optical method for characterizing sputtered AlN films compared to the currently predominately us...

CH003: Stability of nanodots in ferroelectric thin films

Retention properties of written nanodots for Pb(Zr0.2Ti0.8)O3 have been investigated. The imaging voltage plays an important role when investigating small unstable domains. Even with an imaging voltage far below the coercive voltage the boundaries of the domains are switched during imaging by what the measured nanodomain stability is strongly influenced