We present a micromechanical device designed to be used as a non-volatile mechanical memory. The structure is composed of a suspended slender nanowire (width: 100 nm, thickness: 430 nm, length: 8 to 30 ?m) clamped at both ends. Electrodes are placed on each side of the nanowire to (1) actuate the structure during the data writing and erasing mode and (2) determine its position by measuring the capacitive bridge in the reading mode. The structure is patterned by electron beam lithography on a pre-stressed thermally grown silicon dioxide layer. When later released by plasma etching, the stressed material relaxes and the beam buckles by itself to a position of lower energy. These symmetric bistable Euler beams exhibit two stable deformed. This paper presents the microfabrication process and analysis of the static buckling of nanowires. Snapping of these nanowires from one stable position to another by mechanical or electrical means will also be discussed.

https://iopscience.iop.org/article/10.1088/0960-1317/18/4/045005/pdf

Bistable nanowire for micromechanical memory

The properties of electroplated Ni thin films have been systematically investigated as a function of plating temperature and current density. The resistivity and its temperature coefficient remain unchanged on varying the process conditions, though the values of these properties are approximately three times and one-half of those of bulk Ni material, respectively. Optimal conditions of J = 2 mA/cm(2) and 60 degrees C were found for stress-free Ni thin films. The modulus of elasticity of the Ni films is as high as that of bulk Ni (210 GPa) when plated at high temperature and low current density, and then decreases linearly with increasing plating current density, down to 85 GPa at a plating current density of 30 mA/cm(2). It is believed that higher plating rates produced fine-grained structures of low density, leading to a high tensile stress and low modulus of elasticity, while lower plating rates produced a dense material with a modulus of elasticity close to that of bulk Ni and a compressive residual stress. A clear correlation between modulus of elasticity and the stress exists, which reveals that a material under high tensile stress may posses a low modulus of elasticity, and is not suitable for fabrication of microelectromechanical systems devices.

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Effects of Process Conditions on Properties of Electroplated Ni Thin Films for Microsystem Applications

This paper describes the effects of specimen size and temperature on mechanical properties of nano-scale silicon dioxide (SiO2) wires at elevated temperatures. Mechanical characterization of the nano-scale fixed–fixed SiO2 wires was carried out by a quasi-static bending test under an atomic force microscope at temperatures ranging from 295 to 573 K in high vacuum. The SiO2 wires had upper widths of 230–800 nm and a thickness of 410 nm, and were prepared by thermal oxidation of nano-scale single crystal silicon (SCS) wires. All of the SiO2 wires showed a linear relationship between bending force and displacement until failure, and they fractured in a brittle manner. Elastic moduli of the wires depended only on the test temperature. However, fracture stress was influenced by not only temperature but also specimen size. As compared with nano-scale SCS wires of the same specimen size, the SiO2 wires exhibited much higher fracture strain. AFM observations also revealed that the maximum peak-to-valley distance of the top surface of the SiO2 wires dominated the fracture stress.

Quasi-static bending test of nano-scale SiO2 wire at intermediate temperatures using AFM-based technique

SiC is a candidate material for microelectromechanical and nanoelectromechanical systems, but the high residual stress created during the film grow limits the development of the material for these applications. To understand the stress relaxation mechanism in hetero-epitaxial 3C-SiC films, different micromachined free-standing structures have been realized. In this paper, assisted by finite-element method (FEM), a micromachined planar rotating probe was developed for residual stress analysis to split the stress into the following two components: 1) the gradient residual stress (σ1) related to the film defects and 2) the uniform stress (σ0) related to the substrate. Transmission electron microscopy characterization studies about the defect formation and the defect evolution as a function of thickness on 3C-SiC on the Si substrate revealed the problems due to the incorrect linear stress approximation in a heteroepitaxial thin film. With FEM, an exponential approximation of the stress relationship was studied, yielding a better fit with the experimental data. This paper shows that the new approximation of the total residual stress function reduces the actual disagreement between experimental and simulation data.

Advanced Residual Stress Analysis and FEM Simulation on Heteroepitaxial 3C–SiC for MEMS Application

We report on elastic instability of nanomechanical SiO2 beams with widths 20 nm<d<110 nm and lengths 5 μm<L<10 μm. The beams are fabricated from a silicon substrate with a 500 nm thermal oxide layer. After release from the silicon substrate by reactive ion etching the beams buckle due to the residual Si/SiO2 strain. The measured buckling displacements of the beams are compared with the predictions of nonlinear continuum elasticity theory. We observe a continuous buckling transition, qualitatively different than the critical transition predicted by Euler buckling theory, which we attribute to system asymmetry. Finally, we determine the effective potential energy of the fundamental buckling mode.

Elastic instability of nanomechanical beams

Thin film materials are normally under residual stresses as a result of fabrication processes. Unlike microelectronics devices, a micromechanical structure is no longer constrained by its underlying silicon substrate after anisotropic etch undercutting; therefore, residual stresses may result in the bending and buckling of a micromechanical structure. The buckling behavior has been exploited to measure the residual stresses of thin films. This characteristic can also be applied to fabricate out-of-plane three-dimensional micromechanical structures if their deflections are controllable. The buckling of a microbridge is difficult to predict since it is strongly dominated by its fabrication processes and boundary conditions. Currently the information regarding the buckling of micromachined structures is still not complete. The application of the buckling behavior is therefore limited. In this research, the effects of boundary conditions and gradient residual stresses on the buckling behavior of microbridges were studied using analytical and experimental approaches. The variations of the buckling amplitude orientations with the thickness and length of the microbridges were obtained; therefore, the buckling behavior can be predicted and then exploited to fabricate useful micromechanical structures. The potential application of this research lies in preventing the leakage of the microvalves.

https://iopscience.iop.org/article/10.1088/0960-1317/9/3/304/pdf

Hsin-Hua Hu

Papers

On the buckling behavior of micromachined beams