Bosch bets big on tiny MEMS

EE Times: Was that when you invented your proprietary DRIE [deep-reactive ion-etching] technique?

Münzel: Yes, this was the beginning of DRIE. Other MEMS suppliers at that time used very thin poly layers--so thin that they were too stress-sensitive. And the procedures for releasing the mechanical structure [from the sacrificial materials surrounding them] was just not mass producible.

So our first invention was our epi poly [epitaxial polysilicon] and the DRIE process, which enabled us to create structures perpendicular to the surface with variable geometries. Then we invented a release process using a dry HF [hydrofluoric acid] process, which was another breakthrough. This release process was much simpler than what our competitors did at that time.

Finally, we invented a wafer-scale encapsulation technology that was so stable, we could assemble and package our MEMS chips in inexpensive plastic packages.

EE Times: What was your first MEMS part to use DRIE?

Münzel: It was the world's first accelerometer to be housed in a plastic package, which provided a cost advantage compared with the very expensive ceramic and metal housings offered at the time. And because of the stability of our thick, deep MEMS structures, our surface-micromachined MEMS chips also offered a reliability advantage.

This technology has endured to today. In 2005, we crossed the 100 million mark for our MEMS sensors in automotive applications.

EE Times: So do you use the DRIE process for all your sensors?

Münzel: We use epi poly with dry etching and wafer encapsulation with all our inertial sensors--accelerometers and gyros. For pressure sensors, we have to form a membrane for this process. We used bulk micromachining in the past, but for the next generation we are using surface micromachining based on a porous-silicon process.

EE Times: Why was Bosch Sensortec established?

Münzel: Because we see emerging demand for nonautomotive MEMS sensors. For consumer products, the way you approach customers, the timing of delivery and the sensors themselves have different requirements, so we founded Bosch Sensortec to serve these new markets. Our automotive sensors remain within the Bosch Automotive Electronics Division--the division that pioneered MEMS at Bosch.

Frank Melzer: Bosch Sensortec was founded 2005. It works closely with the Automotive Electronics Division. We rely on Bosch core expertise in product development and mass production. But the development times and product life cycle in our markets are shorter. To adapt our organization [to the requirements of new markets] in the fastest possible way, we set up Bosch Sensortec as an independent entity.