Award Winning Microsystems
While many people just talk about revolution, Franz Lärmer can actually look back on his very own: It was 1990 when the Bosch Research physicist hit on the idea of how silicon could be structured freely and with high precision. He and his colleagues worked on this idea for two years until the breakthrough came in 1992 with a technical solution which got entire teams of design engineers involved: Bosch Research and the Automotive Electronics Division in Reutlingen vigorously drove forward so-called Surface Micromechanics (OMM) until the first standard product came off the production line in 1996 in the form of an acceleration sensor as the triggering unit for airbags.
Researchers use process systems from established semiconductor technology in the so-called "Bosch process". First they seal the silicon surface with photosensitive resist. The areas to be worked are then cleared of photosensitive resist by photographic exposure and development. These areas are then exposed to a plasma in cyclic alternation between etching and passivating gases: Silicon is removed from the surface by the etching gas sulfur hexafluoride with the assistance of ions from the plasma. A cavity is created. The passivating gas octafluorocyclobutane lines this cavity with a plastic layer to protect the sidewalls. The sequence of lower etching and wall lining is then repeated. This creates a trough with extremely vertical walls and a freely specifiable geometry.
"For me, the revolution lies in the fact that this new technology of plasma etching allows us to produce any silicon structure we want, and thereby to completely replace the classical wet etching of micromechanics" explained Lärmer. "Today, throughout the world, silicon is structured practically only by using the Bosch process".
Sacrificial layers release the filigree structures
Above all, vertical structures can be etched with great precision and efficiency into silicon surfaces with the Bosch process. When the material under the structures, the so-called sacrificial layer, is then removed, floating elements are obtained, for example a bar, that bends as soon as the entire component is accelerated or braked. This electrically measurable bending turns the micromechanical component into an acceleration sensor.
To undercut the structures and thereby make them floating, Lärmer's colleague Michael Offenberg introduced sacrificial-layer etching in vaporous hydrofluoric-acid. First the sacrificial layer of silicon dioxide (SiO2) is applied in a sandwich structure with silicon as the base. A further silicon layer is deposited on top by a process called epitaxy. The researchers can structure this upper layer by using the Bosch process until they hit the SiO2 underneath. The SiO2 then dissolves in an atmosphere of hydrofluoric-acid vapor and releases the vibrating bar and spring elements, in the inertial sensors for instance, without conglutinating them.
Bosch utilizes a specific form of sacrificial-layer technology in the OMM pressure sensors that went into series production in 2007: A small area of a silicon substrate is treated electrochemically with an acid to create a porous, boat-form silicon modification. The researchers deposit a further layer of silicon on top. Under temperature treatment of approximately 1000 °C, the porous silicon disintegrates like a bubble bath and leaves behind a cavity with an internal pressure of just one millibar. For purposes of comparison, normal air pressure at sea level is approximately 1000 millibars. The silicon layer above the cavity bends at the reference air pressure as a function of the external air pressure level. Integral electromechanical transducer elements register this bending and deliver electrical measured values for conversion of the membrane deflection into the ambient pressure. In this development, Bosch researchers from Gerlingen and the Automotive Electronics Division in Reutlingen, above all under the overall control of Stefan Finkbeiner, have also cooperated very closely to provide the new manufacturing technologies and systems, and to develop the sensor products so that they are ready for production.
