Paul B. Kirby

Thin-film bulk acoustic resonators and filters using ZnO and lead-zirconium-titanate thin films

This paper presents the findings of a design, modeling, and fabrication study of ZnO and PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ thin-film bulk acoustic resonators and filters. Measurements of the high-frequency responses of ZnO resonators having different area are used to develop an acoustic model that accurately represents resonator impedance data. The models are also used to interpret S-parameter measurements on thin-film PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/-based resonators and a value for the effective coupling coefficient deduced. ZnO and PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ ladder filters were designed based on measured impedance data from single resonators. Ladder filters based on PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ have been fabricated for the first time. It is shown that the high coupling coefficient in PbZr/sub 0.3/Ti/sub 0.7/O/sub 3/ leads to bandwidths in the range 100/spl sim/120 MHz at a center frequency of 1.6 GHz, larger than the bandwidths of ZnO-based filters.

Wafer-Level Transfer Technologies for PZT-Based RF MEMS Switches

We report on wafer-level transfer technologies to integrate PZT-based radio frequency (RF) microelectromechanical-systems switches on CMOS. Such heterogeneous integration can overcome the incompatibility of PZT material with back-end-of-the-line (BEOL) CMOS technology. The PZT stack and the transfer process have been optimized to avoid degradation of the PZT actuators during the transfer. In particular, we have optimized the seed layer for the growth of highly oriented PZT on a patterned TiO2-Pt layer, optimized the electrodes structure, and developed an Al2O3 capping layer to prevent degradation of PZT during the transfer process. A full wafer-level transfer process and a selective transfer technology allowing the distribution of RF switches from one source wafer to many receiving wafers has been demonstrated. The latest transfer process demonstrated exhibits great potential for cost optimization of wafer-level transfer of microdevices. In a separate experiment, we have demonstrated the BEOL CMOS compatibility of our integration technique. Switch characterization showed insertion loss of less than 0.5 dB and an isolation better than 30 dB for the 0.4- to 6-GHz frequency range with 15-V actuation voltage.

Thin film piezoelectric property considerations for surface acoustic wave and thin film bulk acoustic resonators

Abstract We report on the optimisation of thin film piezoelectric ZnO for production of resonant acoustic MEMS devices (SAW and FBAR/BAW). The ZnO was deposited by RF sputtering, and conditions were optimised to promote uniform polycrystalline orientation, high resistivity and smooth surface morphology. Both surface acoustic wave and thin film bulk acoustic resonators exhibited high Q values with low insertion loss (3–5 dB).

Development of micromachined RF switches with piezofilm actuation

Current developments in RF systems require high-performance switches for applications including signal routing, impedance matching and adjustable gain amplifiers. The use of micro-switches to replace traditional semiconductor components is increasingly common, because of their advantages in terms of electrical isolation and power loss. This paper reports on a research program relating to the development of a silicon micro-machined RF micro-switch that uses thin-film piezoelectric material for actuation. Piezoelectric actuation has potential advantages over electrostatic actuation in terms of achievable forces and simplicity of structural design. This paper gives an overview of the design and analysis of a prototype switch. The design concept, based on a cantilevered silicon beam or plate, is described. A low order mathematical model, incorporating the mechanical and electrical characteristics of the switch and the interaction between the silicon structure and the piezo-drive is summarized. This allows the basic behavior of the switch to be quantified, and provides a useful tool for design and optimization purposes. The outline design and manufacture/processing of a prototype switch is discussed.

In-plane excitation of thin silicon cantilevers using piezoelectric thin films

This paper deals with the actuation of in-plane and out-of-plane motions of silicon cantilevers, using a single thin film of lead zirconate titanate with a divided electrode configuration. In-plane actuation is demonstrated practically, and excellent agreement is obtained between theoretically predicted and experimentally measured resonant amplitudes, for the fundamental out-of-plane and in-plane modes of vibration of the fabricated test cantilevers.

Low-temperature CMOS-compatible 3D-integration of monocrystalline-silicon based PZT RF MEMS switch actuators on rf substrates

This paper presents a low temperature (200°C) CMOS-compatible fabrication process for integrating high-temperature deposited lead zirconate titanate (PZT) on thin film monocrystalline-silicon piezoelectric actuators, onto an RF substrate, and successful demonstration of this process for fabrication of metal-contact RF-MEMS switches. The patterned PZT/silicon multi-layer stack is transfer-bonded from a silicon-on-insulator (SOI) donor wafer to an AF-45 glass RF substrate using adhesive wafer transfer bonding. Furthermore, several strategies have been investigated to drastically reduce the post bonding misalignment created by the shear forces between the bonding chucks during wafer bonding.

Growth and high frequency characterization of Mn doped sol-gel PbxSr1-xTiO3 for frequency agile applications

In pursuit of thin film ferroelectric materials for frequency agile applications that are both easily adapted to large area deposition and also high performance, an investigation has been carried out into sol-gel deposition of 3% Mn doped (Pb0.4Sr0.6)TiO3. Large area capability has been demonstrated by growth of films with good crystallinity and grain structure on 4 in. Si wafers. Metal-insulator-metal capacitors have also been fabricated and development of an improved de-embedding technique that takes parasitic impedances fully into account has enabled accurate extraction of the high frequency dielectric properties of the PbxSr1−xTiO3 films. Practically useful values of e∼1000, tan δ∼0.03, and tunability ∼50% have been obtained in the low gigahertz range (1–5 GHz). Peaks in the dielectric loss due to acoustic resonance have been modeled and tentatively identified as due to an electrostrictive effect with an electromechanical coupling coefficient of ∼0.04 at an electric field of 240 kV/cm which is potenti...

Lagrange's formalism for modeling of a triaxial microaccelerometer with piezoelectric thin-film sensing

Lagranges equation has been used to construct a dynamic model of a triaxial microaccelerometer with piezoelectric thin-film sensing. A practical and representative structure is used throughout instead of the more usual spring-mass type simplified model. The elastic properties of both the silicon substrate and the PZT thin film are included by use of the laminated plate theory. The three out-of-plane bending motions of the accelerometer, symmetric, antisymmetric, and torsional, are analyzed. The accuracy of the dynamic model is confirmed by finite element analysis. The dependence of structural parameters on the characteristics of the accelerometer for two-end supported structures is discussed. The results show that the model gives close insight into the structural design of the triaxial microaccelerometer and will be a useful tool for the design, analysis, optimization, and characterization of a range of microaccelerometer devices, especially with regard to parameter optimization, and a tradeoff between sensitivity and resonant frequency.

Ferroelectric parallel-plate capacitors with copper electrodes for high-frequency applications

Tunable capacitors with a Cu∕PbxSr1−xTiO3∕Cu parallel-plate structure have been fabricated using a layer transfer method. The use of a Cu bottom electrode results in a giant electrode Q-factor×capacitor area product of QelecA=3.79×105μm2 at 1GHz. The dielectric constant at room temperature is 420 and the tunability amounts to 73% near a breakdown voltage of 35V. The major advantages of the layer transfer method include low electrode losses, the freedom to select an auxiliary substrate and seed layer for ferroelectric film growth irrespective of their high-frequency properties, and the possibility to utilize a large variety of device substrates as they no longer act as template for film growth.

Dynamic Operational Stress Measurement of MEMS Using Time-Resolved Raman Spectroscopy

A dynamic-stress analysis method, based on time-resolved micro Raman spectroscopy, has been developed for reliability studies of microelectromechanical systems. This novel technique is illustrated by measuring temporally and spatially resolved stress maps of a piezoelectrically actuated silicon microcantilever when driven at its first- (6.094 kHz) and second-order (37.89 kHz) resonant frequencies. Stress amplitudes of up to 180 plusmn 10 MPa were measured at the maximum stress locations. The time-resolved Raman stress measurements are compared to the results of finite-element analysis and laser Doppler vibrometry.[2008-0003].