Advanced dry etching studies for micro- and nano-systems

Dry etching is a collective term used for controlled material removal by means of plasma generated ions. Dry etching includes several techniques, with reactive ion etching as one of the most used of its many derivatives. In this work inductively coupled plasma reactive ion etching has been applied for structuring of sapphire and many polymers.

Metals and metal alloys have been structured by physical sputtering with argon ions in an ion beam etching system. The materials for which etch characteristics have been investigated are commonly used in device fabrication at DTU-Danchip. Ion beam etching was first used for structuring of a magnetic device containing four different materials in nine layers.

The materials, tantalum, tantalum oxide, iridiummanganese, and permalloy, can all be etched by reactive ion etching, however the thin layers and the need to etch all layers in one process makes ion beam etching an ideal choice. The physical nature of ion beam etching ensures etch rates of the different layers to only differ slightly, however, the etch rate is low.

One problem with this technique can be redeposition of etched material at the sidewalls and at resist walls. This can introduce short circuits and even contaminate the surface with metal flakes after resist removal. Ion beam etching has also been used for etching of steel without any problems with redeposition.

For steel the etch rate was low which reduced the selectivity to the photo resist. Sapphire, a crystal of aluminum oxide, has a very low sputter rate limiting the applicability of ion beam etching. Structuring of sapphire is however interesting for fabrication of prepatterned substrates for gallium nitride epitaxial growth, among others.

Such a substrate needs a certain structure height which can be obtained by introducing reactive ion beam etching in a boron trichloride plasma. The etch rates of sapphire in such a plasma can be up to a hundred times faster than rates in ion beam etching. The anisotropy of the etch can be controlled by changing the plasma conditions and fabrication of sloped sidewalls can be achieved.

Reactive ion etching of polymers can be used for several purposes, such as polymer removal, surface properties alternation, or polymer structuring. For material removal any polymer can be etched in an oxygen plasma, including all the polymers used in this project, which include, SU-8, TOPAS®, PLLA, PCL, and PMMA.

However, just generating an oxygen plasma does not result in a controllable etch and may give rise to a poor surface for later use. It may be necessary to introduce other gases such as SF6 to reduce surface roughness. Roughness can also be introduced by the mask in the form of redeposition of material on the surface.

Since photo resist is a polymer the selectivity is inherently low and a hard mask below is a solution to increase selectivity. Nevertheless great controllability with many shared properties can be achieved for polymer etching in reactive ion etching.