Fabrication and characterisation of suspended microstructures of tantalum

Abstract

An investigation of the influence of deposition and post-fabrication processes on the final mechanical structure of tantalum beams is reported in the present study. The final deflection profiles of doubly supported beams made from compressive and tensile-stressed films have been studied experimentally. An optimum finite element model has been developed to predict the buckling behaviour of the doubly supported beams by considering the boundary conditions in the form of a compressive stress and an applied load. No matter which etch release method has been used, the initial stress state of the as-deposited films has been found to have a significant impact on the final deflection profile of the fabricated device. The compressive-stressed films have presented larger deflection in the final released beams than the tensile-stressed films. Taking into account the type of etch release methods, the beams that have been released in the dry etch release processes have been found to deform more vertically than those released in the wet-etch release method. Moreover, it has been found that the amplitude of vertical deflection increases with the increase of the beam length and thickness. The results indicate that optimum profiles of tantalum suspended structures can be obtained from the tensile-stressed films that have been released by the wet etching method with an aspect ratio of 1:48.

Compression and decompression of structural tantalum films exposed to buffered hydrofluoric acid

“…After release, fixed-fixed beams made from Ta films originally under compressive stress buckle, as expected because the compressive stress is beyond σ cr . However, similar to Al-Masha'al et al [11], it is found that beams made from tensile-stressed films also buckle. Figure 2 shows buckled beams that were deposited under a moderate tensile stress of 21 MPa.…”

“…Of particular interest in this work, a large shift, not precisely quantified but on the order of hundreds of MPa, in Ta fixed-fixed beams towards more compressive stress was measured by Al-masha'al et al after two different release processes [6,11]. In the first process, the sacrificial layer was a photoresist, and the release process was an oxygen plasma ash.…”

Electrostatically driven long‐microbeams for low‐frequency applications

“…Then, a DC magnetron sputtering system has been used to deposit four layers of tantalum using sputtering power of 500 W and sputtering pressure of 20, 10, 20, and 10 mTorr consecutively to form one layer of tantalum of 1 µm thick. As confirmed by curvature measurements, the corresponding residual stress of the deposited tantalum layers will be compressive, tensile, compressive and tensile [8, 9]. In this case, the final residual stress in the whole deposited layer can be compensated by the influence of the compressive and tensile stress component of the different layers.…”

“…In order to realise straight and long suspended beams, the process‐induced stress should be controlled, and the aspect ratio of static vertical deflection to beam length needs to be minimised. In our previous study, the influence of stress in tantalum films as a result of deposition and fabrication processes has been optimised [8, 9]. Tantalum has physical and chemical properties such as high melting point, high fracture toughness, and low ratio of Young's modulus to mass density that make it ideally suited for low‐frequency applications devices.…”

Delamination of polyimide in hydrofluoric acid

“…The importance of polyimide films is due to their outstanding thermal, mechanical, and chemical properties. For example, the low modulus, high mechanical strength and high thermal stability have enabled the polyimide films to be used as sacrificial layers in suspended MEMS-based structures [4,5]. Another important application of polyimide-based layers is their use as active structural components in MEMS sensors and resonators [6][7][8].…”