Franz Lärmer, Andrea Urban

A method for anisotropically etching silicon

Technical field
Robert Bosch
When German inventors Dr. Franz Lärmer and Andrea Urban created the Bosch Process by adapting micro-process techniques, they opened up a new world of affordable car safety devices.

European Inventor of the Year 2007 in the category "Industry"

Occasionally something small can make a huge difference. Little did Robert Bosch employees Dr Franz Lärmer and Andrea Urban (born née Schilp) realise that their process to help manufacture tiny devices from a simple piece of silicon would revolutionise the field of microelectromechanical Systems (MEMS).

Their 1994 patent - a method for anisotropically etching silicon - enabled the design of more sophisticated, compact devices that were cheaper to manufacture and are now widely used in today's automobile safety features.

Lärmer describes himself as a physicist with a "strong affinity towards chemistry." Born in the Bavarian town of Waldsassen, he studied physics and mathematics at the Technical University of Munich, before furthering his studies in physics at the Swiss Federal Institute of Technology in Zurich. Urban, born in Waiblingen, in the southwestern German state of Baden-Wuerttemberg, studied surface engineering technology at Aalen Polytechnic.

Technology used in many applications

The technology developed from their patent enjoyed immediate success upon introduction, marketed first by Bosch in the form of an airbag in 1997. Although using silicon accelerometers for airbag crash sensors wasn't a brand new idea, the major barrier to their widespread introduction had always been cost. The Bosch Process suddenly made it much cheaper to manufacture these safety devices, rendering them affordable for all cars, not just high-end models.

Surface Technology Systems, a UK-based etch tool manufacturer, became the first to take up a license after working with Bosch to commercialise the process. In 1998 and 2000, Bosch used the process to introduce new generations of gyroscopes, mainly for automobile anti-skidding systems and rollover protection.

Bosch remains the leading manufacturer of automotive MEMS sensors. The company says it uses Lärmer and Urban's technology to produce 50 million sensors per year, covering all types of acceleration sensors and gyroscopes. No larger than a fingernail, these chips can pinpoint the moment a vehicle starts to skid.

The sensors are also used in other applications such as mobile phones or laptops. Indeed, the technology has affected practically all MEMS fields, from health applications (including DNA chips and disposable blood pressure sensors) to high-resolution inkjet printing heads.

According to NEXUS Market Analysis, the MEMS market will grow by 11 percent per year from its current level of $33 billion, reaching a value of $57 billion by 2009.

Although not all devices use the Bosch process, it is clear that without this invention, the manufacture of affordable acceleration sensors and miniature gyroscopes - which lie at the heart of essential car safety systems such as anti-lock braking and airbag devices - would have been unthinkable.

How it works

Lärmer and Urban developed a method for manufacturing high-precision silicon sensors using plasma technology while working in sensor and process development at Robert Bosch Corporate Research. The purpose of their invention was to etch deep microstructures with vertical sidewalls into silicon wafers at high speed and with great accuracy.

The invention concerns a method for the anisotropic etching of features defined by an etching mask (preferably recesses with precisely defined sides in silicon) produced using a plasma etching technique.

The etching process has been around for some 30 years in the world of MEMS. A common problem with etching deep into silicon was that the process also etched sideways, eating into the wall of the structure being created and resulting in distorted walls.

The inventors' method overcame the problem by using a fluorocarbon-based plasma to deposit an etch-resistant layer before taking a subsequent etch step. The technique, nowadays simply referred to as the Bosch Process, allows deep etching in a well-defined manner.

The technology revolutionised MEMS at the time and is currently used worldwide to manufacture silicon MEMS.



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