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MacRobert Award
2002 Finalist
Surface Technology Systems plc (STS)
 Surface Technology Systems plc (STS) have cornered the world market in plasma process silicon-etching equipment that is used to create minute machines with features smaller than the width of a human hair.
The company's etching process is based on a concept invented in Germany by Robert Bosch GmbH and licensed to STS plc for development in 1995. This process relies on the anisotropic etching of silicon; which means eroding the silicon wafer at different rates in different directions, so there is more control of the shape produced.
Innovation
Since that time STS
have made many innovative improvements to
the original idea. Huma Ashraf, Dr Janet
Hopkins, Dr Leslie Lea and Andrew Chambers
are the team members responsible for this
development. The resulting product, the
Advanced Silicon etch process (ASEŽ), was
released in 1995 and is now the standard for
deep silicon etching. This process allows a
greater performance and degree of control
than the original "Bosch Process". STS were
awarded the Queen's Award for Technological
Achievement in 1998 for the ASEŽ process.
STS have acquired around 80 per cent of the
world market for the deep silicon etching
applications used in micro-electromechanical
systems (MEMS) and micro-optoelectrical
mechanical systems (MOEMS).
The etching process that STS plc has developed uses alternating etching and deposition steps to create deep patterns and trenches in silicon wafers. It is possible to control the etching process by repeating these steps and controlling the etch time.
 Step 1 - A fluorine based plasma is used to deposit a layer of PTFE like polymer on all exposed surfaces.
Step 2 - Plasma chemistry and conditions are altered so that the surfaces parallel to the surface of the wafer, i.e. the bottom of the trench, has the PTFE coating removed at a much higher rate than the rest of the trench.
Step 3 - The plasma chemistry and conditions are changed again so that the exposed silicon surface at the base of the trench is then preferentially etched, while the vertical walls are protected.
This process can be modified to produce the type of etching needed for specific applications, by altering the plasma chemistry and the etching conditions. STS specialise in customising this to the needs of the specific customer.
The beauty of the ASEŽ
process is that it creates trenches with
virtually vertical walls, while other
alternative technologies leave the trenches
with shallower, angled walls, far less
useful for MEMS applications. The technology
uses non-toxic, fluorine-based chemistry,
operates at room temperature, and has been
shown to be capable of producing MEMS
devices at a high volume.
There is a growing
need in the industry for having an increased
etch-rate without sacrificing the precision
and control of the ASEŽ process. Thus the
latest version, ASEHRM (high rate) launched
in 2001, allows faster etching while
maintaining accuracy and degree of control
of the ASEŽ process.
Commercial Applications
The ASEŽ and ASEHRM are used to make a wide variety of MEMS, such as tiny silicon turbines, gyroscopes, sensors and switches measuring only a few micrometres across. These MEMS machines are used as components in many industries. Current commercial MEMS-based products manufactured using ASEŽ include opto-electronic switches, gyroscopes for safety-critical applications such as airbag sensors and radio-frequency MEMS for mobile phones.
Future applications for ASEŽ technology include Lab on a Chip or power generation on a micro scale, for example MIT have investigated using ASEŽ to etch both the vanes of a microturbine assembly and to release the turbine assembly via a high aspect ratio through-wafer etch. Ultimately this microturbine would form part of a small generator unit to replace battery packs in portable electrical devices.
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