Piezoelectric materials convert mechanical to electrical energy under stretching and bending conditions. Optimizing the coupling conversion is imperative to the electromechanical behavior of a micromachined membrane's performance. This paper discusses analytical calculations that were devised to determine the microscale structure that minimizes residual stress and outlines the implementation of fabrication technique variations including three different electrode configurations, trenching around the membrane, and reducing the total composite residual stress of the support structure using compressive silicon oxide. Lead zirconacte titanate (PZT) films between 1 and 3 μm thick with a ratio of Zr to Ti of 40:60 were deposited onto 3 mm square silicon membranes. The total tensile stress in the composite structure reaches 100 MPa during standard fabrication processing. Utilizing analytical calculations, a structure was determined that lowered the residual stress of the composite to 11 MPa and increased the electromechanical coupling 35 times. Changing the geometry of the electrode coverage decreased the residual stress of the composite by 40%. Trenching around the membrane provided a membrane with boundary conditions that approached simply supported and decreased the composite residual stress by another 16%. A comparison of the electromechanical behavior for these structures will be discussed, showing a route towards increasing electromechanical coupling in PZT MEMS.

Flexing piezoelectric membranes can be used to convert mechanical energy to electrical energy. The overall deflection of individual membranes is impacted by the residual stress in the system. Membranes comprised of silicon dioxide, Ti/Pt, lead- zirconate- titanate (PZT), and TiW/Au layers deposited on a micromachined boron doped silicon wafer were examined for both morphology and residual stress. By characterizing the membrane residual stress induced during processing with x-ray diffraction, wafer curvature, and bulge testing and identifying methods to reduce stress, the membrane performance and reliability can be optimized. For Zr:Ti ratios of 52:48, the residual stress in the PZT was 350 MPa tensile, with an overall effective stress in the composite membrane of 150 MPa. A reduction of stress was accomplished by changing the PZT chemistry to 40:60 Zr:Ti in the PZT to obtain a stress in the PZT of 160 MPa tensile and an overall effective membrane stress of 100 MPa. The crystallization of the 52:48 PZT film at 700 °C causes a 28% reduction in the thickness of the film.

PZT films between 1 and 3 μm thick were grown using solution deposition techniques to study the effects of crystal structure, orientation, chemistry and PZT/PZT crystallization interfaces on the piezoelectric output of these films. By varying the chemistry of the film from Zr-rich to Ti-rich the film orientation increased towards {h00}. PZT with 60 wt% Ti exhibited tetragonality and produced greater electrical output at a given strain than the rhombohedral films with concentrations less than 50 wt% Ti. Multiple steps of solution deposition left identifiable PZT/PZT interfaces within the film. TEM, FESEM, and Auger spectroscopy were used to characterize these interfaces, which form upon crystallization of the amorphous PZT film. Internal PZT interfaces are associated with both structural defects (voids) as well as chemical variations such as Pb deficiencies.

Thin films of lead zirconate titanate (PZT) are currently being used in a novel MEMS device to generate power. A piezoelectric stack consisting of platinum/PZT/gold is deposited by sputtering, spin coating, and subsequent heat treatments onto a thin silicon membrane, which is cyclically polarized by a flexing motion. The membrane must withstand strains between 0.1% and 0.5% for several billion cycles to provide a useful device. This study has examined the processing-structure-property relationships in developing the PZT film for use in this device. In the sol-gel deposition of PZT, pyrolysis and crystallization temperatures have been shown to alter both microstructure and properties of the piezoelectric film. The chemistry of the PZT film has also been tailored to increase piezoelectric output for this device. Ferroelectric properties are compared to the piezoelectric outputs, and fatigue behavior is measured on bulk silicon and on membranes.

The P3 Micro Heat Engine relies on a thin film PZT based transducer to convert mechanical energy into usable electrical power. In an effort to increase process yield for these were used on sputtered Ti/Pt bottom electrodes to compare roughness, grain size, and diffusion for annealing temperatures between 550 and 700 °C. For an optimized bottom electrode, process yield for various sized top electrodes were then studied for PZT thickness between 0.54 and 1.62 for reducing stress concentrations. Two PZT etching geometries on 2.3 μm thick Si/SiO2 membranes, with 1.5-3.5 mm side-lengths, were examined and one was used to increase the strain at failure by at least 40%. Integrating improvements in process yield and strain at failure, single PZT based MEMS devices capable of generating power of up to 1 mW and in excess of 2 volts have been demonstrated operating at frequencies between 300 and 1,100 Hz.

Piezoelectric oxide films, such as lead zirconate titanate (PZT), are now being integrated into MEMS applications. Many PZT derived systems are deposited using a sol-gel process, which can be used in a microelectronics processing route using spin coating as the deposition method. An application of interest for PZT films is in power generation, where a flexing membrane is used to transform mechanical to electrical energy. The current study was undertaken to identify the relationships between the processing, microstructure, and mechanical reliability of these films. Films were deposited onto both monolithic and bulk micromachined platinized silicon wafers using standard sol-gel chemistries, with roughness and grain size tracked using electron and scanning probe microscopy. Mechanical properties were evaluated in a dynamic bulge testing apparatus. Grain size variations in the Pt film between 35 and 125 nm are shown to have little effect on grain size of the subsequent PZT film and the adhesion of the PZT to the Pt film. Only the Pt film with 125 nm grains was shown to undergo any significant interfacial fracture. Fatigue tests suggest film lifetime is primarily limited by the number of pre- existing flaws in the film from processing. Reducing the microcrack density has been shown to produce films and devices that fail at strains of 1.4% and have mechanical fatigue lifetimes in excess of 100 million cycles at strains simulating the operating conditions.

A piezoelectric thin film MEMS device for generating power from a novel heat engine which approaches a Carnot cycle has been developed. The structure of the underlying electrode and PZT thin film generator has been optimized for increased adhesion. Atomic force microscopy was used to track electrode grain size and roughness; generating grain sizes of approximately 100 and 200 nm in diameter and a roughness of about 14-20 nm provide substantial improvements in film adhesion over systems with smaller grains and smoother surfaces. This has led to the ability to operate the engine at frequencies between 10 and 1500 Hz. The system of interest (a fluid filled cavity sealed by a micromachined silicon membrane and the PZT film) shows increased deflections for a given pressure applied to the membrane at frequencies where the system resonates. By operating the system dynamically, it is possible to generate more than 2 V from a single generator structure.

Piezoelectric films for a MEMS microengine have been deposited using solution deposition routines onto platinized silicon wafers. These films are used as membranes above a bulk micromachined cavity. A dynamic bulge tester and interferometer were used to characterize the deformation of the films when pressurized. The mechanical strain at failure, as well as the fatigue behavior, have been characterized. Membranes between 300 and 500 nm thick have been shown to sustain mechanical fatigue damage over approximately 10 million cycles at strains of 30% of the monotonic failure strain. Defects in the films due to growth and thermal stresses during processing, and their role in membrane failure, are identified. Crack growth is demonstrated in these films by compliance measurements during fatigue testing, and interfacial failure is identified between the PZT and Pt layers.

The results of a multihospital study involving a total of 588 twin pairs born in Chicago in 1970–1975 are reported, with special respect to differences in mortality between first and second twins by time as well as by cause of death. Mortality was higher in second than in first twins and most commonly occurred after delivery and was the result of immaturity and of respiratory distress syndrome.