Microfabrication processing of titanium for biomedical devices with reduced impact on the environment

Abstract

This thesis presents research on a novel method of microfabrication of titanium (Ti) biomedical devices. The aim of the work was to develop a commercial process to fabricate Ti in a more environmentally friendly manner than current chemical etching techniques. The emphasis was placed on electrolytic etching, which enables the replacement of hazardous hydrofluoric acid-based etchants that are used by necessity when using Photochemical Machining (PCM) to produce intricate features in sheet Ti on a mass scale. Titanium is inherently difficult to etch (it is designed for its corrosion-resistant attributes) and as a result, Hydrofluoric acid (HF) is used in combination with a strong and durable mask to achieve selective etching. The use of HF introduces serious health and safety implications for those working with the process. The new technique introduces the use of a “sandwich structure”, comprising anode/insulator/cathode, directly in contact with each other and placed in an electrolytic etching cell. In this technique the same photolithography process is utilised to achieve selective etching on a metal substrate as in the PCM process. However, for the electrolytic etching stage, the inter- electrode gap (IEG) is reduced significantly from a few centimetres, as usually applied in electrochemical processes, to 4 μm. The intention behind this was to improve the current distribution experienced at the anode (Ti) during subsequent electrolytic etching. The sandwich structure was developed by deposition of a photoresist S1818 and Copper (Cu) on top of Ti. Firstly, a manual sanding of the substrate was applied in order to eliminate the oxide layers which could strongly affect a final electrolytic etching. The soft- and hard-bake stages involved in the processing of the S1818 resist were optimised to produce a stress-free Ti/S1818/Cu/S1818 structure. Ultimately a pattern would be imparted onto the S1818/Cu/S1818 that would ultimately be imparted through to the Ti layer during the last stage, electrolytic etching. In order to achieve this, a Cu electroless deposition was developed as a technique to obtain a conductive film which would act as a cathode during the electrolytic etching of the target, Ti layer. The results of the electrolytic etching of the Ti sandwich structure revealed flat-base profiles of half-etched (“half-etch” is the term used to signify an etch that does not penetrate completely through the thickness of the metal sheet) micro-holes in the Ti layer. The problem of delamination of the electroless Cu, in 10 % w/v HCl electrolyte used as an etchant, was solved by electroplating a 12 μm layer of Cu on top of the 60 nm Cu electroless deposited film. Using this technique, micro-features were achieved in Ti. The half-etched micro-holes were characterised to have an overall spherical shape corresponding to the imaged pattern and a preferred flat-base profiles (typically a raised land of material arises in conventional electrolytic etching). A series of parameters were tested in order to control the process of electrolytic etching through the Ti sandwich structure by measuring etch rate, surface roughness of the etched pattern and the etch factor. The applied current densities (CD) of 10, 15, 20, and 25 A/cm2 showed proportional dissolution to the applied current. Electrolytic etching with a CD of 20 A/cm2 demonstrated a high etch rate of 40 μm/min. and a relatively low Ra of 2.8 μm, therefore, it was utilised in further experimental work. The highest etch rate of 50 μm/min. and an improved distribution of half-etched micro-holes was achieved by the introduction of 4 crocodile connectors (2 per electrode) and mechanically stirring of the electrolyte (800 rpm) while performing the electrolytic etching. The maximum etch depth of 143.9 μm was produced in Ti when the electrolytic etching was performed at the same conditions for 3 minutes. The incorporation of ultrasonic agitation to the electrolytic etching and an electrolyte temperature of 130 C resulted in a decrease of the surface roughness of the etched micro-holes to 0.5 μm. The results of the Ti sandwich structure electrolytic etching proved the concept of minimising the IEG in order to obtain a uniform Ti dissolution on a feature scale, improved control of the electrolytic dissolution over the whole area of the sample with utilisation of the lower hazard etchant at the same time.