Amorphous Silicon Self‐Rolling Micro Electromechanical Systems: From Residual Stress Control to Complex 3D Structures

Abstract

In surface micromachining, the control of the residual stress of the films is crucial, as excessive stress can cause buckling, cracking, and peeling of the materials and fracture of the devices. However, the gradients of stress across the thickness of individual films or the stress mismatch in composite structural layers can be exploited to obtain highly complex 3D structures via self‐assembly, self‐bending, self‐positioning, or self‐rolling processes. Herein, the thin‐film residual stresses of amorphous silicon, TiW, AlSiCu, Cr, Au, and SiO2 are assessed using polymer substrates and tuned. Then, a stress‐mismatched bi‐layer of amorphous silicon (tensile) and TiW (compressive) is used in the fabrication of complex 3D self‐rolling micro electromechanical systems (MEMS) structures. The freestanding regions of the fabricated MEMS have a characteristic radius of curvature of ≈100 μm. Current‐induced thermal actuation of a suspended plate is demonstrated. The microfabrication process and materials used are compatible with standard cleanroom processing and with a variety of substrates. In the future, amorphous silicon photodiodes, photoconductors, or other semiconductor devices can be incorporated on complex MEMS fabricated by self‐rolling/self‐bending processes and applied to localized optical and electrical sensing.

Derivation of analytical expressions for the stress/strain distributions, bending plane and curvature radius in multilayer thin-film composites

“…An alternative approach is to use these analytical models to estimate certain mechanical properties of the thin-film, instead. This has been done in previous works (see [4,40] and [41]), where, for example, the curvature radius of a composite thin-film/compliant substrate is used to determine the residual stress of the thin-films, depending on the deposition conditions in a chemical vapor deposition system.…”

“…where the terms α ijkl are constants called elastic moduli. There are 3 4 = 81 distinct possible values of α ijkl , but due to the symmetries of the stress and strain tensors discussed above this number reduces to 36, and the previous expression reduces to [35][36][37]…”

“…This approach is intended to overcome the limitations of the traditional devices, constrained to rigid and planar geometries, and should therefore allow the fabrication of bio-compatible, lightweight, miniaturized and costefficient devices, capable of accommodating the mechanical deformation when integrated in curvilinear or complex threedimensional (3D) shaped surfaces [1]. Sophisticated 3D complex structures can be obtained by self-assembly, using strain engineering and the presence of residual stresses on bilayers or trilayers of different materials [2][3][4] or even surface-chemical reactions that induce the expansion/contraction of layers [5]. These strategies allow, for example, to devise monolithic integrated multiaxial magnetic sensors sensitive along the three orthogonal positions for 3D magnetic angular encoders [6].…”

A Comprehensive Review of Micro/Nano Precision Glass Molding Molds and Their Fabrication Methods

“…Although widely used for the fabrication of various micro/nanostructures, conventional lithography and etching processes are limited to producing two-dimensional (2D) structures [ 37 ]. To produce accurate three-dimensional (3D) structures, intricate planar approaches such as bulk micromachining and surface micromachining are required [ 38 , 39 , 40 ]. For this reason, another subtractive technique, ultraprecision tuning, has been employed, in which the material is removed by keeping the cutting tool stationary and rotating the Si workpiece.…”

Nanorolls Decorated with Nanotubes as a Novel Type of Nanostructures: Fast Anodic Oxidation of Amorphous Fe–Cr–B Alloy in Hydrophobic Ionic Liquid

“…5 Examples of selfrolling include the fabrication of nanobelts of SiGe/Si and SiGe/Si/Cr, self-rolling of tensile Si films on Ge sacrificial layers, rolled up tubes made from pre-stressed inorganic nanomembranes deposited on photoresist (polymer) sacrificial layers, self-assembled microstructures prepared using Cr/Cu bilayers, multilayer C/Si/C microtube anodes for lithium-ion batteries, and microtube optical cavities of SiOx/SiO 2 , silicon nitride/silicon oxide. 6 There are different models of self-rolling phenomenon: the strain gradient, 7 the selective etching of the underlying sacrificial layer, 8 the surface stress originated from the self-bending mechanism, 9 the Timoshenko bilayer model, 10 model based on "clamped ion" approximation, 11 and others. Despite the variety of literature on self-assembling devices, the application of Fe and Cr amorphous oxides attracted not enough attention.…”