High Frequency Bulk Acoustic Wave Research Articles

High-order mode solid mounted resonators with polarity inverted multilayered GeAlN/AlN films

High frequency bulk acoustic wave (BAW) resonators for beyond 5 G communications have low Q values and electromechanical coupling because of their ultra-thin piezoelectric monolayer films. Polarity inverted multilayered film BAW resonators operating in high-order mode resonance can have thicker piezoelectric layers than monolayer BAW resonators. In this paper, we fabricated and evaluated two- to four-layered polarity inverted GeAlN/AlN film solid mounted resonators (SMRs). They resonated in high-order mode. Their total film thicknesses were approximately two- to four-times thicker than that of a monolayer AlN film SMR. The polarity inverted GeAlN/AlN film SMRs had higher Q values than the monolayer SMR.

3.4 GHz strip-type thickness shear mode solidly-mounted bulk acoustic wave resonator using X-cut LiTaO3

This paper describes a high-frequency bulk acoustic wave resonator (BAWR) with a solidly-mounted (SM) structure using single crystal LiTaO3 (LT) thin plates. A Bragg reflector solidly supports the LT thin plate, which is fragile if self-suspended. The two kinds of BAWRs use a strip-type thickness shear mode in 0.56 μm thick X37°Y LT and X127°Y LT. The Bragg reflector is made of 5 pairs of Al and Ta films, i.e. 10 layers in total. The X37°Y LT SM structure BAWRs exhibited a resonance frequency (f r) of 3.250 GHz, an anti-resonance frequency (f a) of 3.463 GHz, a bandwidth (BW) of 6.6%, and an impedance (Z) ratio of 48 dB. The X127°Y LT BAWR has a similar characteristic with f r of 3.153 GHz, f a of 3.367 GHz, a BW of 6.8%, and a Z ratio of 46 dB. The X37°Y LT BAWR showed an advantage of 2 dB compared with the X127°Y LT one. The Al film acts as a low acoustic impedance film, but an acoustic impedance layer combination of Al and Ta films is not suitable and that of SiO2 and W films suitable among the four combinations, Al/Ta. SiO2/Ta, Al/W, and SiO2/W films. Although the acoustic impedance layer combination, the acoustic film quality, the structure, electrode design, and fabrication process of the fabricated BAWRs were not optimized yet, this result suggests the high potential of this device.

Study of Adsorption of Globular Proteins on Hydrophobic Surfaces

In this paper, we use micromachined, high-frequency, quartz bulk acoustic wave resonators to systematically study the physical and viscoelastic properties of spontaneously adsorbed globular protein films with molecular weights (MWs) spanning three decades on hydrophobic surfaces. Specifically, changes in the frequency and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor of the micromachined resonator array were studied as a function of concentration for five proteins, namely human serum albumin (HSA), immunoglobulin G (IgG), human fibrinogen (Fib), alpha-2-macroglobulin (AMG), and immunoglobulin M (IgM) at the fundamental and third resonance modes. The results obtained were interpreted using equivalent electrical impedance models for the multilayer stack on the quartz crystal microbalances surface. Discrete changes in the protein adsorption and the viscoelastic behavior with solution concentration were observed for all the five protein films. The spherical core-shell protein model is used to provide a simple explanation of the results. The work presents the first systematic and quantitative evaluation of the density, thickness, viscosity, and elastic modulus of adsorbed globular protein films and demonstrates the advantages of using micromachined high-frequency bulk acoustic wave resonators for obtaining these types of data.

A new type of solution-concentration sensor using the effects of generation and detection of bulk acoustic waves with interdigital transducers

A new type of solution-concentration sensor using a layered planar structure consisting of piezoelectric substrate–liquid solution layer–piezoelectric substrate is presented. Two interdigital transducers (IDTs) are deposited, respectively, at the two parallel interfaces of the layered structure. Under given conditions, the first IDT deposited at one interface and driven by a tone burst can generate a high frequency bulk acoustic wave (BAW) pulse into the solution layer only. This BAW pulse propagates through the solution layer and impinges on the second IDT deposited at the other interface. The second IDT then produces an electrical pulse output, containing information of the solution concentration of the liquid layer via the carrier phase delay of the BAW pulse propagation. Based on the theoretical consideration, a high sensitivity solution-concentration sensor using BAW generation and detection with the IDTs is designed and fabricated. Its feasibility and performance are also experimentally examined.

Systematic studies on globular proteins using micromachined high frequency bulk acoustic wave resonators

In this paper we use micromachined, high-frequency, quartz bulk acoustic wave resonator arrays to evaluate the physical and viscoelastic properties of adsorbed globular protein films with molecular weights spanning two orders of magnitude. Specifically, changes in the frequency and the Q-factor of the micromachined resonator array were studied as a function of concentration for three proteins, namely Human Serum Albumin (HSA), Immunoglobulin G (IgG) and Human Fibrinogen (Fib) at the fundamental and third resonance modes. The results obtained are interpreted using continuum mechanics approach for multilayer stack on the QCM surface. Discrete changes in the protein adsorption rate constant and the viscoelastic behavior was observed for all the three protein films. The thickness, density, elastic modulus and viscosity of protein layer were successfully obtained through this analysis.

A lateral-field-excited LiTaO3high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO(3). Prototype LFE LiTaO(3) sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

Aluminum nitride thin films for high frequency applications

A piezoelectric thin film sandwiched between two conductors is the basic structure for high frequency bulk acoustic wave devices. For that purpose, AIN thin films deposited by DC reactive sputtering on various electrode materials were investigated. Highly c-axis oriented thin films as thick as 2.5 μm were grown on various substrates and analyzed. Intrinsic stress values ranging from strong compression to high tension were observed and piezoelectric d33, f coefficients up to 3.9 pm/V were measured. The first fabricated acoustic resonators showed a pronounced resonance peak centered at 2 GHz with a very high Q factor.