Electrochemical micromachining of microdevices from NiTi shape memory alloys

Abstract

This thesis aimed to develop a reproducible process for batch-fabricating microdevices required for MEMS and medical applications, such as micro actuators and stents, from heat-sensitive NiTi shape memory materials. Electrochemical micromachining was chosen to carry out this work. This is a non-traditional machining process involving photoresist processing and electrolytic etching which has received much attention recently for the processing of thin films. The electrolyte used was a non-aqueous solution of 5% sulphuric acid in methanol. The optimum parameters for the photoresist processing were obtained by evaluation of the thickness and exposure time of the KTFR photoresist coating. A quantitative investigation of the electrolytic etching of NiTi was carried out to study the influence of applied voltage, etch time and line width of the test pattern on the etching behaviour, e.g. etch rate, undercut, depth of etch and etch factor. The anodic polarisation behaviour of NiTi in 5% sulphuric acid in methanol was investigated under a potentiostatic control system to establish the optimum etching parameters. The materials used for the fabrication of micro actuators (required by Forschungszentrum Karlsruhe, Germany to make a prototype microvalve) were NiTi alloy thin film materials (sputtered or cold-rolled) with thicknesses ranging from 5 to 46J...lm displaying a one-way or two-way shape m:emory effect. A variety of optimised designs of micro actuator were successfully etched electrolytically at 8V. The etch rate was found to depend directly on the anodic current density. The addition of a third alloying element such as Pd or eu reduced the anodic current density and maintained a similar etch rate. However it resulted in the breaking of the films during etching due to the reduction in the ductility of the material. The materials for the micro fabrication of stents were 100J...lm thick NiTi sheets. The problem of non-uniform metal dissolution was observed. However, by adding a sacrificial etch band as a current 'robber', periodic rotation of the anode and properly adjusting the electrochemical and geometric parameters, the stents were etched successfully with improved yield and dimensional accuracy.