Design and Implementation of Silicon-Based MEMS Resonators for Application in Ultra Stable High Frequency Oscillators

The focus of this work is to design and implement resonators for ultra-stable high-frequency ( \u3e 100MHz) silicon-based MEMS oscillators. Specifically, two novel types of resonators are introduced that push the performance of silicon-based MEMS resonators to new limits. Thin film Piezoelectric-on-Silicon (TPoS) resonators have been shown to be suitable for oscillator applications due to their combined high quality factor, coupling efficiency, power handling and doping-dependent temperature-frequency behavior. This thesis is an attempt to utilize the TPoS platform and optimize it for extremely stable high-frequency oscillator applications. To achieve the said objective, two main research venues are explored. Firstly, quality factor is systematically studied and anisotropy of single crystalline silicon (SCS) is exploited to enable high-quality factor side-supported radial-mode (aka breathing mode) TPoS disc resonators through minimization of anchor-loss. It is then experimentally demonstrated that in TPoS disc resonators with tethers aligned to [100], unloaded quality factor improves from ~450 for the second harmonic mode at 43 MHz to ~11,500 for the eighth harmonic mode at 196 MHz. Secondly, thickness quasi-Lamé modes are studied and demonstrated in TPoS resonators for the first time. It is shown that thickness quasi-Lamé modes (TQLM) could be efficiently excited in silicon with very high quality factor (Q). A quality factor of 23.2 k is measured in vacuum at 185 MHz for a fundamental TQLM-TPoS resonators designed within a circular acoustic isolation frame. Quality factor of 12.6 k and 6 k are also measured for the second- and third- harmonic TQLM TPoS resonators at 366 MHz and 555 MHz respectively. Turn-over temperatures between 40 °C to 125 °C are also designed and measured for TQLM TPoS resonators fabricated on degenerately N-doped silicon substrates. The reported extremely high quality factor, very low motional resistance, and tunable turn-over temperatures \u3e 80 °C make these resonators a great candidate for ultra-stable oven-controlled high-frequency MEMS oscillators

Strategies and techniques for fabricating MEMS bistable thermal actuators.

Bistable elements are beginning to appear in the field of MEMS as they allow engineers to design sensors and actuators which require no electrical power and possess mechanical memory. This research focuses on the development of novel strategies and techniques for fabricating MEMS bistable structures to serve as no electrical power thermal actuators. Two parallel strategies were explored for the design and fabrication of the critical bistable element. Both strategies involved an extensive material study on candidate thin film materials to determine their temperature coefficient of expansion and as-deposited internal stress properties. Materials investigated included titanium tungsten, Invar, silicon nitride and amorphous silicon deposited using either sputtering or PECVD. Deposition parameters were experimentally determined to produce tensile, compressive and stress-free films. A full set of graphs are presented. To address the 3D MEMS topology challenge required for bistability, this research explored two different strategies for fabricating 3D non-planar hemispherical dome structures using minimal processing steps. The first approach used the thermal/chemical reflow of resist, along with traditional binary lithography with a single photomask. Specific thermal/chemical reflow conditions were experimentally developed to produce hemispherical dome over a wide range. The second approach introduced a novel maskless procedure for fabricating the dome using grayscale lithography. After evaluating the above results, it was decided to use engineered compressive stress in released thin film sandwiches to form the 3D dome structures required for bistable actuation. Three different types of released multi-layer diaphragms were studied: 1) oxide-polyimide diaphragms, 2) oxide-aluminum diaphragms, and 3) oxide-aluminum-polyimide diaphragms

Performance Comparison of Phase Change Materials and Metal-Insulator Transition Materials for Direct Current and Radio Frequency Switching Applications

Advanced understanding of the physics makes phase change materials (PCM) and metal-insulator transition (MIT) materials great candidates for direct current (DC) and radio frequency (RF) switching applications. In the literature, germanium telluride (GeTe), a PCM, and vanadium dioxide (VO2), an MIT material have been widely investigated for DC and RF switching applications due to their remarkable contrast in their OFF/ON state resistivity values. In this review, innovations in design, fabrication, and characterization associated with these PCM and MIT material-based RF switches, have been highlighted and critically reviewed from the early stage to the most recent works. We initially report on the growth of PCM and MIT materials and then discuss their DC characteristics. Afterwards, novel design approaches and notable fabrication processes; utilized to improve switching performance; are discussed and reviewed. Finally, a brief vis-á-vis comparison of resistivity, insertion loss, isolation loss, power consumption, RF power handling capability, switching speed, and reliability is provided to compare their performance to radio frequency microelectromechanical systems (RF MEMS) switches; which helps to demonstrate the current state-of-the-art, as well as insight into their potential in future applications

Localized annealing of polysilicon microstructures by inductively heated ferromagnetic films

The monolithic integration of dissimilar microsystems is often limited by conflicts in thermal budget. One of the most prevalent examples is the fabrication of active micro-electromechanical systems (MEMS), as structural films utilized for surface micromachining such as polysilicon typically require processing at temperatures unsuitable for microelectronic circuitry. A localized annealing process could provide for the post-deposition heat treatment of integrated structures without compromising active devices. This dissertation presents a new microfabrication technology based on the inductive heating of ferromagnetic films patterned to define regions for heat treatment. Support is provided through theory, finite-element modeling, and experimentation, concluding with the demonstration of inductive annealing on polysilicon inertial sensing structures. Though still in its infancy, the results confirm the technology to be a viable option for integrated MEMS as well as any microsystem fabrication process requiring a thermal gradient

On-a-chip microdischarge thruster arrays inspired by photonic device technology for plasma television

This study shows that the practical scaling of a hollow cathode thruster device to MEMS level should be possible albeit with significant divergence from traditional design. The main divergence is the need to operate at discharge pressures between 1-3bar to maintain emitter diameter pressure products of similar values to conventional hollow cathode devices. Without operating at these pressures emitter cavity dimensions become prohibitively large for maintenance of the hollow cathode effect and without which discharge voltage would be in the hundreds of volts as with conventional microdischarge devices. In addition this requires sufficiently constrictive orifice diameters in the 10µm – 50µm range for single cathodes or <5µm larger arrays. Operation at this pressure results in very small Debye lengths (4 -5.2pm) and leads to large reductions in effective work function (0.3 – 0.43eV) via the Schottky effect. Consequently, simple work function lowering compounds such as lanthanum hexaboride (LaB6) can be used to reduce operating temperature without the significant manufacturing complexity of producing porous impregnated thermionic emitters as with macro scale hollow cathodes, while still operating <1200°C at the emitter surface. The literature shows that LaB6 can be deposited using a variety of standard microfabrication techniques

Aluminum nitride deposition/characterization & pMEMs/SAW device simulation/fabrication

Aluminum Nitride (AlN) is a promising material for piezoelectric MicroElectroMechanical Systems (pMEMS) and Surface Acoustic Wave (SAW) devices. AlN is a direct bandgap semiconductor possessing moderate piezoelectric coefficients, a high Curie temperature, and a high acoustic velocity. Potential applications of AlN thin film devices include high temperature pMEMS microvalves for use in Solid Oxide Fuel Cell (SOFC) flow control systems and high frequency/sensitivity SAW platforms for use in biosensors.;Since AlN is a robust material capable of operating at high temperatures and harsh environments, it can be used in settings where other widely used piezoelectrics such as Lead Zirconate Titanate (PZT) and Zinc Oxide (ZnO) fail. Piezoelectric beams are commonly used in MEMS and have many possible applications in smart sensor and actuator systems. In this work, the results of 3-dimensional Finite Element Analysis (FEA) of AlN homogeneous bimorphs (d31 mode) are shown. The coupled-field FEA simulations were performed using the commercially available software tool ANSYSRTM Academic Research, v.11.0. The effect of altering the contact geometry and position on the displacement, electric field, stress, and strain distributions for the static case is reported.;Surface acoustic wave devices have drawn increasing interest for use as highly sensitive sensors. Specifically, SAW platforms are being explored for chemical and biological sensor applications. Because AlN has one of the highest acoustic velocities of all the piezoelectric materials, high frequency (and thus highly sensitive) sensors are feasible. In this work, AlN SAW Rayleigh wave platforms were designed, fabricated, and tested. The insertion loss of the SAW platforms for two InterDigitated Transducers (IDTs) separation distances is also presented

Investigation of the Effect of Process Parameters by Taguchi Method on Structural and Electrical Properties of RF Magnetron Sputtered SiO2 & pSi on Si Substrate

In this work, Taguchi Signal-to-noise (S/N) analysis was applied to investigate the effect of varying three process parameters, namely- sputtering power, working pressure and Ar gas flow rate on the surface, morphological and electrical properties of the RF sputtered SiO2 and Boron doped pSi over Si substrate. The contribution of a particular process parameter on these properties was also inspected by applying Analysis of Variance (ANOVA). SiO2 and pSi thin films were fabricated over Si substrate using RF magnetron sputtering system. Three sets of inputs for the three mentioned process parameters were chosen for sputtering SiO2 and pSi thin films. To deposit SiO2, 150W, 200W and 250W power levels were chosen, for pSi deposition- power levels were 100W, 150W and 200W; 5mTorr, 10mTorr and 15mTorr were chosen for pressure and three Ar gas flow rate levels at 5, 10 and 15 sccm were selected. By performing Taguchi L9 orthogonal array, nine combinations of sputtering parameters were prepared for depositing SiO2/Si and pSi/Si thin films. The surface morphological and electrical properties (resistivity per unit area and capacitance per unit area) of the sputtered samples were therefore inspected by analyzing the Taguchi design of experiment. Signal-to-noise (S/R) analysis presents how the properties were affected by the variation of each process parameter. ANOVA analysis showed that sputtering power and working pressure are the two dominant process parameters contributing more to surface morphological and electrical properties. A regression model for surface roughness of the SiO2/Si and pSi/Si thin film samples was also derived. The electrical properties of the SiO2/Si and pSi/Si thin films, however, didn’t show linearity and that is why it was not possible to derive a regression model for the electrical properties of SiO2/Si and pSi/Si sputtered thin films

Characterization of the RF Magnetron Sputtered p-Si & n-Si on Si Substrate and Effect of Process Parameters on their Structural & Electrical Properties

In our work, we applied Taguchi Signal-to-noise (SNR) analysis to investigate the effect of varying three process parameters, namely- sputtering power, working pressure, and Ar gas flow rate on the surface and morphological properties of the RF sputtered p-Si and n-Si on Si substrate. We also inspected the contribution of process parameters on those properties by applying Analysis of Variance (ANOVA). Characteristics of thin films fabricated by RF magnetron sputtering vary with the recipes. Process parameters such as power, pressure, process gas flow rate, substrate temperature, and target to substrate distance determine the quality of the thin films. Some of these parameters contribute more significantly towards a specific property of the thin film than other parameters. For RF sputtered p-Si on Si Substrate, we tried to determine which parameters contribute most to surface properties like grain size, micro-stress, and surface roughness and the effect of variation of these parameters. We applied the same procedure for n-Si thin films. Using 2 diameter targets of thickness 0.125 each, thin films of two kinds were fabricated on Si substrate using RF magnetron sputtering system. For each material (p-Si / n-Si), two sets of inputs for the three mentioned process parameters were chosen; for power, we chose 100W, 150W, and 200W; 5mTorr, 10mTorr, and 15mTorr were chosen for pressure, and we varied Ar gas flow rate at 5, 10 and 15 sccm (standard cubic centimeter per minute). We applied the Taguchi Design of experiments method to find out the optimized process parameter combination. By performing Taguchi L9 orthogonal array, nine combinations of the recipe were prepared for sputtering p-Si and n-Si. The surface and morphological properties of those nine samples were therefore inspected. We studied surface roughness (form AFM & Profilometer), grain size and shape ( from XRD diffractometer) and micro-stress (from W-H plot). Signal-to-noise (SNR) analysis presents how the properties like roughness and grain size change with the process parameters\u27 variation. We found that among the three process parameters, the contribution of sputtering power towards surface roughness and Ar pressure towards grain size are the greatest. We also observed that more sputtering power resulted in smoother surfaces and larger grain sizes. No significant effect was found for Ar gas flow rate from ANOVA test

Nanotube film-enhanced 3-D photoanode for application in microsystems technology

Surface area plays an important factor in the energy conversion performance of solar cells. It has also emerged as a critical factor in the evolution of high-performance micro-electro-mechanical systems (MEMS) and multifunctional microstructures most of which will benefit from integrated on-chip solar power. Presented here is the hierarchical fabrication and characterization of TiO2 nanotubes on non-planar 3-dimensional microstructures for enhanced performance of the photoanode in dye-sensitized solar cells (DSSCs). The objective is to increase photoanode performance within a 1 cm2 lateral footprint area by increasing the vertical surface area through the formation of TiO2 nanotubes on 3-D microstructures. In the interest of the seamless integration of DSSCs into MEMS applications, bulk micromachining using wet-etching was employed to fabricate 3-D microstructures in silicon. Anodization was used to form titania nanotubes within sputtered titanium thin films. Film quality, adhesion, and the formation of the nanotubes are discussed. Nanotubes with approximate outer diameter dimensions of 180 nm, inner diameter of 75 nm, and heights of 340 nm on 15 um-sq x 15 um-deep micro-wells were fabricated resulting in more than 5 times the increase in surface area over planar surfaces. Grazing incidence diffraction measurements were used to negate the substrate contribution while providing a detailed in-depth profile analysis to validate the preferred polycrystalline rutile and anatase orientation on the 3D surface-texture photoanode. The increase in surface area resulted in an equal increase in dye adsorption capacity and a 78% reduction in spectral reflectance. The optical enhancement of this hierarchically-structured nanotube film-enhanced (NFE) 3D photoanode correlated well to a high current density increase 10 times that of its flat counterpart. Fabrication of a DSSC utilizing the NFE 3D photoanode was also performed and tested for its photocurrent performance under solar simulation. Results suggest that although the surface-textured anode increases the performance of the photoanode, efficiency of the overall cell significantly depends on the architecture. A conceptual implementation of the NFE 3-D photoanode into microsystems is also discussed along with conclusions and suggestions for future work