The present study reports the sol-gel deposition of PZT thick films on Pt/Ti/SOI substrate and its application to the micro cantilevers and 2D micro optical scanning mirrors. Crack-free PZT thick films (2.7 µm) have fabricated on Pt/Ti/SOI substrate. Thin SiO<sub>2</sub> layer on the top of the SOI substrate was found to play important role as a burrier layer to avoid breaking the Pt/Ti bottom electrode layer. The micro cantilevers and 2D micro scanning mirrors fabricated by MEMS technologies are flat suggesting the advantage of using SOI substrate instead of Si substrate. The deflection of the tip of the 800 µm-long x 250 µm-width micro cantilever was measured to be 5.9 µm at 5 V. The absolute value of the transverse piezoelectric constant |<i>d</i><sub>31</sub>| of the PZT thick film calculated from the deflection is as high as 84 pC/N. The scan angle of the cantilever via resonant actuation at 2387 Hz is as high as 40 degree with only 6 V (peak-to-peak). The response time of micro cantilevers were measured to be within 0.3-1 ms. These data indicate the potential application of the present 2D micro scanning mirrors to wavelength division multiplexing (WDM) systems driven at several voltage.
Resonant-typed microscanners based on a silicon diaphragm and actuated by PZT was designed and fabricated on purpose to improve the deformed microstructure while resonating at high frequency. In order to yield large actuating force, hybrid PZT deposition process: sol-gel method and laser ablation was developed to manufacture thick PZT films with well-crystallized perovskite phase for the applications of microscanners. In our previous study, a sol-gel derived PZT was used due to the high film quality, large deposition area and easy composition control. However, to make a thick and crack-free PZT film, several times of coating and thermal treatment is not only time consumption, but increases the risk of contamination and leads the complicated problem of thermal residual stress. In this paper, the hybrid-derived PZT film with thickness of 3 μm was prepared with simplified steps and reduced processing time. Regarding to the performance of microscanners, 1D scan motion with straight patterns and scan angle of 8±1° has been demonstrated, while resonating with 7 Vp at resonance frequency (2325 Hz). The 2D scan pattern with area of (8±1°)×(5±1°) and less deformed edged was also obtained successfully due to the improvement of the silicon-based flat mirror surface.
A varactor with a wide tuning range is essential to adjust the desired frequency band among a wide Gigahertz range and compensate for process deviation as well as effects of temperature. Nowadays, conventional varactors with high quality cannot be available in standard silicon processes; furthermore, they cannot avoid high losses at high frequencies due to the nature of semiconductors. A novel micromachined varactor with a wide tuning range is presented. It can provide a digital selection of capacitance. The electroplating process was singled out to fabricate such as device, with its feature of high aspect ratio. The design has been verified through a finite-element analysis to show a tuning range of 350%.
An investigation on the influence of etchant concentration, dissolving silicon content and additives during silicon anisotropic etching in TMAH has been carried out. Based on the Taguchi method, the etch rates of Si, Al, and SiO<SUB>2</SUB> were measured via under-etch experiments using the wagon- wheel mask pattern. The improvement on the surface quality was observed by agitating solution under ultrasonic vibration in TMAH solutions with additives. Furthermore, a new approach is developed to reduce wet etching time and to control etched gap depth between the released micro membrane and the silicon substrate. This method employs a polysilicon or an amorphous silicon thin layer embedded between the micro membrane and silicon substrate as a sacrificial layer, then this layer would be fast iso tropically etched away by TMAH solution.