Aron Michael

Design criteria for bi-stable behavior in a buckled multi-layered MEMS bridge

In this paper, the bi-stability of a buckled multi-layered micro-bridge with elastically constrained boundary conditions is studied theoretically and experimentally. The residual moment due to non-symmetric distribution of residual stress in the layers of the micro-bridge is taken into consideration. The buckled shape model, which characterizes the initial buckled deflection, is employed in this study. A systematic method of designing bi-stable buckled micro-bridges has been developed and applied to multi-layered structures. The method is tested against ANSYS simulation and shown to be in excellent agreement. Two multi-layer micro-bridges have been fabricated: (i) 2.5 µm thick low stress SiO2/1 µm thick high compressive stress SiO2/2 µm thick SCS Si; (ii) 1 µm thick high compressive stress SiO2/2 µm thick SCS Si. The fabricated bridges are tested for bi-stability by thermal actuation and the results agree well with the analysis. The intrinsic bi-stable nature of a buckled micro-bridge can only be guaranteed as long as the residual moment is within a certain threshold value.

Investigation of E-Beam Evaporated Silicon Film Properties for MEMS Applications

This paper investigates the crystallinity, microstructure, surface morphology, stress characteristics, and Youngs Modulus of ultrahigh vacuum (UHV) electron-beam (E-beam) evaporated silicon films with a low thermal budget. The films are evaporated at various substrate temperatures ranging from 200 °C-625 °C, deposition rates ranging from 50 to 400 nm/min, and annealed at 600 °C for various durations. Some of the preliminary results were reported by Michael and Kwok. The results indicate that the film characteristics of the evaporated silicon films are significantly different from and better suited for microelectromechanical systems (MEMS) applications than low pressure chemical vapor deposition silicon films, commonly used for MEMS devices. The very attractive properties of UHV E-beam deposited silicon films are remarkably low residual stress (both average and gradient), very smooth surface morphology, and thick layers at a low thermal budget with reasonably large deposition rates. Two different mechanisms have been identified as responsible for initiating the formation of crystal grains in these films: 1) kinetic energy of evaporated silicon atoms; and 2) thermal energy from the substrate heating. The first mechanism leads to fine columnar grains responsible for the smooth surface morphology and easily controllable low stress characteristics. The second mechanism results in coarse grain formation with a relatively higher proportion of (111) oriented grains. Cantilever beams of 30-μm thickness have been fabricated from such films with rms roughness of c5 nm, residual stress of 20 MPa, and stress gradient of c1.1 MPa/μm with Youngs Modulus of 169 GPa.

Wet anisotropic etching by TMAH with NCW-1002 surfactant on crystalline silicon surface

Paper reports the use of a new surfactant NCW-1002 as an addictive in TMAH wet anisotropic etching to improve the etching characteristic on three silicon principle planes (i.e. (100), (110), (111) planes). Concentrations of TMAH from 2.5% to 10% with addition of various concentration of NCW-1002 are studied to find an optimal combination for a improved smoothness and etch selectivity between (100) and (110) planes, which is necessary for the formation of 45°mirror plane (110) in (100) silicon surface. Etch rate and roughness of silicon planes were measured by Dektak II and AFM respectively. Besides, this paper will explain the formation of 45°slope. By improving the selectivity or extending the etching depth, we are able to enlarge the 45°portion on the mirror surface.

Theoretical analysis of initially buckled thermally actuated snapping bimorph microbridge

A theoretical analysis of initially buckled, and thermally actuated bimorph micro bridge is presented in this paper. The micro bridge is to be buckled by compressive residual stress developed in the beam during fabrication. An analytical model that characterizes the buckling shape is proposed, and used in the analysis. This model considers symmetrical rotational stiffness, and infinite axial stiffness at both ends of the bridge. Deflections versus temperature characteristics for a bimorph micro-bridge of length, 1000µm, thickness, 4µm, with various initial deflections ranging from 5µm to 20µm are obtained, and plotted. The results show that a pin-pin micro bridge (rotational stiffness of zero) exhibits bi-stability at lower snapping temperatures when negative thermal expansion material is used as one of the layers. A snapping temperature of less than 100°C is possible. It is also shown that clamped-clamped (rotational stiffness of infinite) micro bridge does not snap at all, and there is maximum allowable rotational stiffness below which snapping is possible.

Evaporated Thick Polysilicon Film With Low Stress and Low Thermal Budget

In this letter, we report on the use of e-beam evaporation under ultrahigh vacuum conditions to deposit smooth, low stress, and thick polysilicon films for low thermal budget applications. The film stress, surface morphology, and crystallographic characteristics have been investigated at various deposition temperatures, deposition rates, and annealing temperatures. Cantilever beams formed from such films have been released and characterized for the stress gradient and Youngs modulus. The results show that crystallized grains start forming in the evaporated films below 400°C substrate temperatures and the stress levels in the film can be easily controlled and minimized. Residual stress of 20 MPa and a stress gradient of only 1.6 MPa/μm have been obtained for a 10 μm film deposited at 500°C and annealed at 600°C.

Fabrication of smooth 45° micromirror using TMAH low concentration solution with NCW-601A surfactant on silicon

Paper reports improved results in the fabrication of 45° micromirrors using low concentration of TMAH with NCW-601A surfactant. 45° micro mirror is an essential component for obtaining 90° out-of-plane reflection of the optical beam. TMAH anisotropic wet etching on (100) silicon wafer with features aligned to the flat so that 45° slope is formed on (110) plane. This requires the etch rate of <110> planes to be lower than <100> planes. Etching rate selectivity depends on: temperature, concentration, and additives. Substantial undercutting of mask needs to be taken into consideration during the design. TMAH concentrations ranging from 2.5% to 10% with different concentrations of surfactant have been studied to achieve improved smoothness of the micromirror surface and better selectivity. SEM/AFM measurement show the roughness of mirror is less than 1nm. Results also show that the surface roughness varies along the 45° slope with the roughest portion at the top of the mirror. This paper will describe the techniques to reduce the size of the rough portion of the mirror.

High aspect ratio sharp nanotip for nanocantilever integration at CMOS compatible temperature

In this paper, we demonstrate a novel low temperature nanofabrication approach that enables the formation of ultra-sharp high aspect ratio (HAR) and high density nanotip structures and their integration onto nanoscale cantilever beams. The nanotip structure consists of a nanoscale thermally evaporated Cr Spindt tip on top of an amorphous silicon rod. An apex radius of the tip, as small as 2.5 nm, has been achieved, and is significantly smaller than any other Spindt tips reported so far. 100 nm wide tips with aspect ratio of more than 50 and tip density of more than 5 × 109 tips cm-2 have been fabricated. The HAR tips have been integrated onto an array of 460 nm wide cantilever beams with high precision and yield. In comparison with other approaches, this approach allows the integration of HAR sharp nanotips with nano-mechanical structures in a parallel and CMOS compatible fashion for the first time to our knowledge. Potential applications include on-chip high-speed atomic force microscopy and field emission devices.

E-beam Evaporated Polysilicon for Lead Zirconate Titanate-Based Micro-Actuators

This letter reports the application of ultra-high vacuum E-beam evaporated polysilicon (UHVEEPoly) film for lead zirconate titanate (PZT)-based piezoelectric micro-electromechanical systems (MEMSs) for the first time. The UHVEEPoly film is employed as a passive structural layer in unimorph piezoelectric cantilever micro-actuators to demonstrate its applicability in PZT-based MEMS. Two sets of PZT micro-cantilever actuators have been fabricated and characterized in this letter. The first set has 4-μm-thick UHVEEPoly as a structural layer while the second set is made of 4 μm thick single crystalline silicon structural layer obtained from an active layer of a siliconon-insulator wafer. The static and dynamic behaviors of the two sets of actuators including their initial deflection profiles have been measured and compared. Results confirmed that the two sets of actuators exhibit similar characteristics, indicating the unique suitability of the UHVEEPoly films for realizing PZT-based MEMS that enables a cost effective process and formation of versatile MEMS structures.

A novel bistable thermally actuated snap-through actuator for out-of-plane deflection

In this paper , the importance of boundary conditions for two- way bistable thermal snapping action is analysed by Ansys simulations. Several designs are developed and modified into a novel bistable actuator for out of plane deflection. It consists of 4 long epi-silicon single layered legs , a 1200um x 40um x 3um microbeam and 4 spring-supports in which both are polysilicon/oxide/episilicon layered. Buckling of the released structure is achieved by the compressive residual stress remaining in the oxide layer as a result of the processing. The structure switches between two stable equilibrium states by heating the legs and then heating one of the polysilicon layers to produce the thermal moment needed for snapping to occur, hence achieving two-way bistable operation.

Fabrication of nano-scale high aspect ratio ultra sharp tips for nano-mechanical structures at low thermal budget

In this paper, we demonstrate a novel approach to form ultra-sharp high-aspect-ratio and high density nanotips at low temperature suitable for integration with nano-electro-mechanical structures in CMOS compatible manner. For the first time, Cr thermally evaporated Spindt tips have been studied and achieved a tip radius as small as 3nm. An array of 250nm wide and 4μm tall nanotips have been fabricated with a high aspect ratio of 16 and high density of 1×109 tips/cm2. Furthermore, 250nm wide, 1.2μm tall tips have been integrated onto an array of 500nm wide silicon nanowires with high precision and yield. Potential application of the approach includes the development of high-speed parallel AFM on single chip.