Silicon etch process promises improved micromachines

A micromachining process for silicon chips developed in Britain is being used by the US Army in a project to develop a microscopic turbine generator to power battlefield telecoms equipment. The aim is to replace the 14kg battery pack that soldiers currently have to carry.

Using the Advanced Silicon Etch (ASE) process developed by Surface Technology Systems of Newport, the US army and Massachusetts Institute of Technology are collaborating on a project to build a power pack the size of a Zippo lighter. It will contain a tiny gas turbine etched in silicon and enough fuel to produce electricity to operate personal telecoms equipment.

The ASE process etches patterns into the surface of silicon wafers using highly reactive gas plasmas. The main advantage of the process over wet chemical etching is its ability to rapidly produce well-defined microscopic trenches in silicon, ideal for creating so-called micro-electromechanical systems.

Surface Technology Systems has refined the original plasma etch process developed by automotive component company Robert Bosch in Germany. In a two-stage process, a plasma is used to deposit a thin layer of a Teflon-like polymer `resist’ on the surface of the silicon wafer. A different plasma is then used to remove the resist from the areas to be etched. The exposed channels of silicon are then rapidly etched away by fluorine gas generated in the second plasma. By continually repeating the two stages, microscopic trenches can be created.

Fast etch rates and the ability to produce narrow, deep trenches make ASE economically viable for a wide range of commercial applications. Bosch is already using ASE to make car airbag sensors just 1mm in diameter.

It also uses the process to manufacture the roll sensor installed on the Mercedes A-Class to stop it overturning on sharp turns. The sensor acts as a gyroscope that detects sideways movement.

Outside the automotive industry, Surface Technology Systems has supplied ASE equipment to telecoms companies Lucent and Motorola for their joint Scalpel programme to develop a non-optical lithographic method of laying down resist patterns on silicon.

And medical equipment suppliers are looking to use the process to create `lab-on-a-chip’ throw-away automated blood analysers. A drop of blood placed on the chip will be taken through micromachined channels to various analysis cells which will provide an instant read-out.

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