PhD_Projects_Portfolio
- 1. Mandar Deshpande UIC PHD PROJECTS E-mail: deshmand@yahoo.com DESIGN AND DEVELOPMENT OF AN OPTICALLY POWERED MICROACTUATOR DESCRIPTION Optically powered microactuators were required as components in a novel retinal implant application for light powered microfluidic dispensing. The existing microactuators did not satisfy the requirements; particularly the available light input, which is 2-3 orders of magnitude smaller than sunlight, and the desired actuation frequency on order of 50 Hz. Additional requirements included size and compatibility with standard microfabrication techniques. DESIGN The various means for opto-mechancial transduction were analyzed and a new method was conceived that comprised of integration of a micro-solar cell (photodiode) with piezoelectric thin film microactuator on a silicon chip. A solar cell would transduce light to electric energy that will be used for driving a piezoelectric microactuator. An optimal design was derived using analytical and FEA modeling techniques implemented in ANSYS, MATLAB and SIMULINK. A detailed layout was prepared to facilitate microfabrication and testing of the device. FEA model of the microactuator showing deflection profile along a radial cross-section. 3D model and 2D schematic of the designed microactuator FABRICATION Fabrication posed a significant challenge as the integration of a silicon solar cell with a thin film piezoelectric microactuator involved several process conflicts because of high temperature steps and strong chemical etches. To address these challenges, the processes were developed first at component level and then combined. The processes were developed in- house in the university microfabrication facility. SEM of the microactuator showing the solar cell grid and the piezoelectric patch in the center. SEM of cross-section of the microactuator. TESTING A test setup was developed for mechanical, electrical, and optical characterization of the microactuator prototypes. Various instruments were used in the setup including microscope scanning laser vibrometer, picoammeter, signal sources, oscilloscope, optical illuminators and others, which were controlled via a PC using LabVIEW. The working of the optically powered microactuator was successfully demonstrated. A detailed characterization was conducted that included the time response, the piezoelectric actuator displacements and the solar cell output parameters. Experimental setup and schematic. Measured displacement profile for the microactuator. CONTRIBUTIONS This work contributed to development of optically powered microactuators that can work under very low light illuminations and at relatively high frequencies on the order of 1 kHz. A patent application is being prepared on this work in view of its broader applications in BioMEMS and optical communications devices.
- 2. Mandar Deshpande UIC PHD PROJECTS E-mail: deshmand@yahoo.com MICROFABRICATION OF AN OPTICALLY POWERED MICROACTUATOR DESCRIPTION An optically powered microactuator was proposed that integrated a micro-solar cell with a piezoelectric microactuator on a single silicon chip. The integration of the two devices through microfabrication posed significant challenges in form of deposition conditions for good quality PZT (Lead Zirconate Titanate) thin film, DRIE process development for diaphragm etching, and process conflicts because of high temperatures processes and strong chemical etches. PROCESS DEVELOPMENT To systematically address the challenges the process development was undertaken at the component level first, where the micro-solar cell and the piezoelectric microactuator were fabricated and tested separately. After successful component development, the fabrication processes were appropriately combined to fabricate the integrated actuator. Process Description (a) Alignment mark (Photolithography and RIE) (b) Oxide dopant mask (oxidation and photolithography) (c)-(e) Doping of phosphorus into silicon (Spin on dopant, diffusion and dry oxidation) (f) Bottom electrodes for PZT (Photolithography, e-beam deposition, Lift- off) (g) PZT thin film via sol gel method (CSD, photolithography, wet-etching, RTP) (h) Oxide etching for ohmic contact (photolithography, BOE etch) (i) Top electrode forming the solar cell grid (Photolithography, e-beam deposition, Lift-off) (j)-(l) Diaphragm by backside etching using DRIE (Photolithography, Backside alignment, Thick photoresist, Bosch Process) Key Processes Developed 1. Deposition parameters for Pt/Ti film to lower stress buildup 2. PZT thin film sol-gel process – Pyrolisis temperature and RTP profile to eliminate peeling/cracking of PZT film. 3. PZT thin film wet etch – New etch recipe that reduced undercut and showed high selectivity over oxide. 4. Thick photoresist processing (SPR-220-7.0) – improved process recipe that eliminated cracking problem for a 20 µm thick resist film. 5. DRIE parameters for etching large area silicon – ICP parameters for straight side wall profile for exposed area in the millimeter scale. Images of a device during fabrication steps. Alignment Oxide mask Doped Si Pt/Ta electrode PZT Thin film Oxide etch Au/Ti electrode DRIE RESULTS Working prototypes were successfully fabricated and tests showed satisfactory operation with reference to the desired requirements. This work contributed to developing a method for incorporating a solar cell with a conventional MEMS device to create optically powered MEMS. Processed wafer before dicing. SEM of a microactuator device. SEM images of cross-section showing constituent layers and the etched silicon diaphragm
- 3. Mandar Deshpande UIC PHD PROJECTS E-mail: deshmand@yahoo.com MEASUREMENT OF PROPERTIES OF PIEZOELECTRIC THIN FILMS ON MICROACTUATORS DESCRIPTION Accurate values of piezoelectric properties are useful for evaluating piezoelectric microactuators and tailoring their performance for various applications. There are several challenges with regards to measuring these properties. They cannot be measured directly and show significant variations with compositions, type of substrates, processing conditions and the driving voltage. Many new indirect measurement methods are being developed, but most require fabrication of specialized structures such as cantilevers, or provide properties measured under sensor mode, which does no reflect its behavior under actuation mode. In this work a method was developed that allowed for non- destructive and accurate measurement of the piezoelectric properties under practical actuation conditions. APPROACH The method involved developing a finite element (FE) model of the microactuator and structurally validating it by corroborating the resonance frequencies and modes with measurements. Following this, the piezoelectric properties in the FE model were adjusted to match the measured deflections, thus providing an accurate estimate of the properties as a function of the applied voltage. To validate the method, square piezoelectric microactuators were fabricated using KOH etched silicon diaphragms and PZT thin films. Next, first 8 resonance modes and frequencies were measured using a microscope laser scanning vibrometer and were used for validating a FE model built in ANSYS. The piezoelectric properties were then computed by comparing the FE results with the measured deflections. Microactuator fabrication process Oxidation - LPVCD Nitride - Photolith - RIE patterning - KOH - Photolith - e- beam Pt/Ti - Liftoff - PZT sol gel - Photolith - Wet-etch - RTP- Photolith - e-beam Ag/Cr - Liftoff. Fabricated microactuator. Experimental setup for measuring mode shapes and deflections. Cross-section showing KOH etched silicon diaphragm. FE Model of the microactuator. Experimental and FEM mode shapes and frequencies showing excellent corroboration. The transverse piezoelectric properties as a function of the applied voltage. RESULTS AND CONTRIBUTIONS The piezoelectric properties were shown to be a function of the applied voltage, with the values at higher voltages corroborating with the results published in other studies. These results showed that at low voltages the effective properties are significantly lower than that published in most studies.
- 4. Mandar Deshpande UIC PHD PROJECTS E-mail: deshmand@yahoo.com OTHER MEMS PROJECTS FABRICATION OF A COMPLIANT MEMS MANIPULATOR A project required microfabrication of a silicon compliant mechanism for manipulation of microscale objects. The design called for a millimeter scale frame structure with thickness of 20 µm. DRIE process on an SOI wafer was selected as the best approach for achieving the desired structure. A COMB-DRIVE BASED CORIOLIS FLOW RATE SENSOR In order to address the lack of MEMS flow rate sensors that can be integrated on a chip with microfluidic circuitry, a novel comb drive based flow rate sensor operating using the coriolis principle was designed. In this sensor the flow was directed through the supporting beams of the comb-drive, which would bend during motion because of coriolis force in proportion to the mass flow rate. A fabrication process was devised that allowed for on-chip fabrication of the comb drive sensor and the microfluidic connecting channels. The sensor showed high sensitivity with a theoretical resolution of less than 1 µL/s for water. SEM of millimeter scale complaint mechanism fabricated on a SOI substrate CAD drawing of the designed device AN ANALYTICAL MODEL FOR CIRCULAR PIEZOELECTRIC MICROACTUATORS DESCRIPTION Circular bending type piezoelectric actuators are widely used in MEMS for applications such as ultrasonic devices, pumps and others. The deflections generated by these actuators are sensitive to the relative dimensions of the constitutive structural components such as the substrate and the piezoelectric layer. An analytical model leading to a closed form equations has been long desired for convenient design optimization of the actuators, but has been difficult to derive because of the multilayered and non-homogenous structure of the actuator. METHOD An analytical model was derived using Classical Laminated Plate Theory and closed form equations were found for the deflections of circular piezoelectric actuators under voltage and pressure loading conditions. The equations were validated via corroboration with experiments and FE model, and a high accuracy within 96.5 % of the experimental results was obtained. Analytical framework for CLPT derivation. Computed deflections in FE. Experimental setup and the test actuator.