The release of microstructures from a Si substrate depends directly on the underetching (isotropic) characteristics of the etchant used. For the purpose of this study, XeF2 gas was selected as the etchant medium. Etching by XeF2 is primarily a function of pressure, which determines the rate of interaction between the Si surface and the etchant gas. However, other factors play a large role in XeF2 etching characteristics. Testing was conducted to determine the etch rate and profile of XeF2 etching when various parameters of the structure design are changed (Si exposure area, size and dimension of structures, spacing of structures). The gas was introduced at a pressure cycle of approximately 4.0 Torr for 180 s. Uniformity of the etched surface is improved by increasing the number of etch cycles, or by increasing the gas pressure of the surrounding XeF2.
The development of lead zirconate titanate (PZT) films can be a fairly troublesome process when trying to obtain a viable thick crack-free film. Traditional methods for film fabrication via a pure sol-gel solution provide the best results, but still can have many problems. This paper maps out the specific spin-coating and annealing steps used in order to achieve a PZT film with minimal-to-no cracking and/or delamination. A seed layer of PbTiO3 (PT) was used in order to create nucleation sites for the subsequent PZT layers, virtually eliminating any delamination. All layers, including the PT base layer, were spin-coated at 3000 rpm onto a 100-mm silicon wafer (previously sputtered with Ti and Pt for adhesive and conductive purposes, respectively) and soft-baked at 150 °C for 10 min. Initial annealing procedures produced severe cracking, a result of the relatively high cooling rates through the Curie temperature (∼350 °C). The annealing process was refined, for individual layers, to 550°C for 120 s, with a cooling rate of 0.042 °/s between 400 and 300 °C. Final annealing was conducted at 600 °C for 30 min, with a cooling rate of 0.028 °/s between 400 and 300 °C. The resulting PZT layer was virtually crack-free. Platinum was sputtered again subsequent to PZT deposition in order to pole the piezoelectric material. A PZT/nanoparticle powder mixture was also investigated as the piezoelectric layer. PZT nanoparticles were suspended in the sol-gel precursor solution and then spin-coated also at 3000 rpm onto a 100-mm wafer and soft-baked at 150 °C for 10 min. The relatively slow cooling rate was extended between 500 and 100 °C in order to prevent any cracking that might occur along grain boundaries between the individual PZT nanoparticles. The resulting film was crack-free, however displaying areas of agglomerated nanoparticles.
Microscale actuators have been a subject of high interest in the past few years, especially for active structural control on the micro and nano scales. This includes manipulation of microparticles, two dimensional RF switching, on-chip electrical probing, and strain sensors. Previous developments for micro-actuators exhibit deflection in the vertical direction (both positive and negative). This paper presents a novel design for a cantilever piezoelectric microactuator. The design proposed is capable of actuation in the vertical as well as the horizontal direction by having a trapezoidal cross-section. This cross-section allows for the deposition of electrodes (platinum) onto sloped sidewalls, resulting in added degrees of freedom. The actuators are fabricated from link sections of traditional piezoelectric materials such as lead titanate zirconate (PZT). Currently, only single link actuators have been investigated. For the double link actuator, two sections will be connected by a joint consisting of silicon nitride, allowing for the independent actuation in the X, Y, and Z axes of each section for each specific cantilever beam. Through the utilization of voltage as the drive for the actuation, it is possible to counteract lingering oscillations in the beam resulting from initial motion. Control for this microactuator is comparable to rigid robotic structures, taking into account the modeled behavior of piezoelectric materials as well as the bending model of a trapezoidal cantilever beam. The research and development for this paper will include design, as well as dynamic modeling and simulation of a piezo-MEMS robot arm. Fabrication consists of a series of spin-casting and annealing steps to create the piezoelectric films, the polarity of which is determined by a poling process subsequent to fabrication. Conducting materials are deposited both before the PZT (bottom electrode) and after (top and sidewall electrodes), while their thicknesses will be minimalized in order to not adversely affect the beam stiffness of the piezoelectric links.
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