Kushal Bhattacharjee

MEMS vibrating structure using a single-crystal piezoelectric thin film layer

The present invention relates to a micro-electro-mechanical systems (MEMS) vibrating structure having dominant lateral vibrations supported by a MEMS anchor system, and includes a single-crystal piezoelectric thin-film layer that has been grown with a specific crystal orientation. Since the MEMS vibrating structure has dominant lateral vibrations, its resonant frequency may be controlled by its size and shape, rather than layer thickness, which provides high accuracy and enables multiple resonators having different resonant frequencies on a single substrate.

Packageless SAW Devices with Isolated Layer Acoustic Waves (ILAW) and Waveguiding Layer Acoustic Waves (WLAW)

We report on two novel layered structure concepts and their acoustic wave properties for efficient, mechanically isolated, temperature compensated, and technologically attractive packageless SAW device applications as RF filters and duplexers. The piezoelectric substrate is covered with a layer of SiO2 or Pyrex that is in turn covered by a material of higher acoustical impedance creating an isolated layer acoustic wave (ILAW). Otherwise the wave inside a relatively low acoustic velocity waveguiding layer is confined by the higher velocity topmost layer made with high SAW velocity material thus ensuring the creation of waveguiding layer acoustic waves (WLAW). Modeling of acoustical properties of these waves (ILAW and WLAW) is confirmed by experimental results while useful figures of coupling, reflection and temperature stability are obtained. Extensive experiments with in-situ monitored depositions of multiple layers were performed and very good acoustical isolation of the waves was achieved.

Design and Fabrication of S 0 Lamb-Wave Thin-Film Lithium Niobate Micromechanical Resonators

Commercial markets desire integrated multifrequency band-select duplexer and diplexer filters with a wide fractional bandwidth and steep roll-off to satisfy the ever-increasing demand for spectrum. In this paper, we discuss the fabrication and design of lithium niobate (LN) thin-film S0 Lamb-wave resonators on a piezoelectric-on-piezoelectric platform. Filters using these resonators have the potential to fulfill all the above requirements. In particular, we demonstrated one-port high-order S0 Lamb-wave resonators with resonant frequencies from ~400 MHz to ~1 GHz on a black rotated y-136 cut LN thin film. The effective electromechanical coupling factor (k2eff ) ranges from 7% to 12%, while the measured quality factor ranges from 600 to 3300. The highest k2eff × Q achieved on this chip is 194, significantly surpassing contour mode resonators manufactured in other technologies.

High k t 2 ×Q, multi-frequency lithium niobate resonators

This paper presents design and vacuum measurements of lithium niobate (LN) contour-mode resonators (CMR). By carefully positioning the interdigital transducer (IDT), we achieved CMRs with k_t²×Q of 7%*2150=148 (IDT @ node) or resonators with very high k_t² of 12.3% and spur-attenuated response (IDT @ anti-node). In addition, we demonstrated resonators with frequencies ranging from 400MHz to 800MHz on a single chip.

3D finite element modeling of real size SAW devices and experimental validation

We report on several successful examples of using finite elements based software package for modeling SAW structures with dimensions above 100 wavelengths in the propagation direction including all the electrodes and spaces. The approach easily incorporates additional layers and elements with arbitrary 3D shapes that are common in SAW devices. Temperature behavior of the frequency response, SAW beam shape, bulk wave related data and other relevant information is stored in the solution and this helps determining the sources of response peculiarities. The modeling results for infinite periodic structures are validated against the proven FEMSDA software. Personal workstation is used for modeling. This work demonstrates the capabilities of finite elements based software running on contemporary personal computers to solve practical problems of SAW device design.

Thin-film Lithium Niobate contour-mode resonators

This paper presents Lithium Niobate (LN) thin-film contour mode resonators (CMR) on a piezoelectric-onpiezoelectric platform. Using this platform, we demonstrate, on a black Y136 cut Lithium Niobate thin-film, one-port high-order width extensional contour mode resonators at 463MHz and 750MHz. The electro-mechanical coupling factor (k_t²) and quality factor (Q) of the 750 MHz resonator is 8.6% and 612, resulting in a k_t²*Q of 53. The 463MHz resonator exhibits a k_t²*Q of 105, with a 7% k_t² and 1500 Q. With this technology, we can potentially achieve multi-frequency band-pass filters with both wide bandwidth and steep roll-off.

Temperature compensation of Longitudinal Leaky SAW with silicon dioxide overlay

Longitudinal leaky surface acoustic wave (LLSAW) is attracting considerable attention for its high velocity and reasonable coupling coefficient. The intrinsic temperature coefficient of frequency (TCF) for these waves for different longitudinal cuts with metal gratings is in the range of -110 to -90 ppm/degC. However, for certain filter applications the TCF of the LLSAW waves must be quite low in the range of ~ 20 to -20 ppm/degC. The introduction of a positive TCF overlay material (generally SiO2) alters the LLSAW wave characteristics, and also changes the coupling coefficient. The modified LLSAW mode retains the same high velocity characteristics with an estimated improved TCF. Thus, a systematic study has been attempted by us to evaluate the effect of SiO2 on some interesting longitudinal cuts, such as, YZ and 128deg lithium niobate. In this paper, we show the possibility to realize a good TCF value for LLSAW mode using a three dimensional (3-D) periodic finite element (FE) model. The predicted results show an improvement in the TCF value to -20 ppm/degC for the modified LLSAW mode which is in excellent agreement with the measurement results.

Temperature Compensation in SAW Filters by Tri-Layer Wafer Engineering

Bonded wafer concept is modified by introduction of a third layer that is used for compensation (or even overcompensation) of wafer warping thus increasing the amount of stress at the upper surface of the piezoelectric LiTaO3 layer resulting in significant improvement of temperature coefficient of frequency (TCF), that may become zero or even positive. We have successfully demonstrated variants of structure where a third layer with relatively higher coefficient of thermal expansion (CTE) is deposited or bonded on the back surface thus compensating or introducing opposite warping of the combined tri-layer structure. The experimental results and the modeling show that with appropriate choice of supporting substrate materials the unwanted warping can be eliminated and the TCF closer to zero is routinely obtained in LiTaO3/Si/Cu tri-layer structures with thick Al electrodes.

P3I-2 Cavityless Wafer Level Packaging of SAW Devices

Original implementations of the solution to cavityless WLP by means of isolation of waves are discussed. Variants of waveguiding layer acoustic wave (WLAW) (similar to boundary and interface waves) and isolated layer acoustic wave (ILAW) together with Bragg mirror-like additional acoustical isolation are compared in modeling and experimentally. The structures include metal electrode patterns and subsequent layers. The first among these layers is a dielectric layer, usually SiO2 (or Pyrex) that possesses temperature compensating properties, while the outer layers are formed with either metals or dielectrics. In order for the wave to be confined into the SiO2 layer the stack of the outer layers may be formed in different ways. For implementation of the WLAW concept, the main feature of subsequent layers is the increased acoustical velocity in comparison to the SiO2 layer. Thus the wave attenuates exponentially in the structure on both sides of the SiO2 waveguiding core. The ILAW concept is based on the application of high acoustical impedance materials providing abrupt change in boundary conditions between the layers. Further improvement of acoustical isolation in this approach is effectuated by means of alternating several layers with low and high acoustical impedance.

Approach to on-wafer controllable trimming of SAW filters

We have developed the concept and the processing details of SAW filter trimming by thin film deposition having in mind the applicability of such a process to controllable on-wafer trimming. Fast and repeatable deposition is the key element for individual processing of multiple wafer regions. We have discussed and experimentally compared the following options: deposition of platinum (or other metals) instead of gold, and deposition of selected dielectrics, notably alumina and ytterbium oxide. We show that the latter is a promising candidate for trimming due to high density and low stiffness while the former does not prove to be useful for leaky waves in LiTaO/sub 3/. We have obtained trimming ranges of about 0.1% for ladder structures, 1% for DMS filters and 4% for resonators on lithium tantalate with negligible loss increase and little distortion in response.