New Facility Enables Microsystem Technology

A new method of producing miniaturized thermal imaging sensors promises to permit large-scale manufacturing of automotive safety devices. The technique, developed at the Fraunhofer Institute for Microelectronic Circuits and Systems (Fraunhofer IMS), takes advantage of precise etching.

Infrared cameras see more than the naked eye and could make traffic safer. Indeed, thermal imaging cameras are already used in certain applications — in the construction industry and the military, for example. Such infrared cameras, however, are barely available in mobile applications; for example, in automotive safety systems. The reason is that long-range infrared microsensors are difficult to produce commercially.

On June 22, Fraunhofer IMS opened a new facility in which the production of such microsystem technology (MST) is possible. MST involves minute sensors, valves or other mechanical components that are integrated into semiconductor chips. For instance, in airbags they serve as motion sensors, and they are no thicker than a human hair. If MST is to be applied on semiconductors and integrated, one has to master the art of etching — which is where the researchers at Fraunhofer IMS come in.

A researcher operates the production machine at Fraunhofer IMS’s new microsystem technologies laboratory and cleanroom facility. (Photo: Fraunhofer IMS)

To apply MST to a semiconductor, one starts with a silicon wafer substrate, which is topped with a sacrificial layer that serves as a spacer, and then by a functional layer. The sacrificial layer is later etched away, leaving only the desired sensor structure behind. The problem is that traditional methods allow vertical etching into the layers, said Marco Russ, project manager at Fraunhofer IMS.

Unsupported structures are decisive for the mechanical functions of many items of MST, Russ added. In other words, the etching must work not only vertically but evenly in all directions. Experts call this process “isotropic etching.” This ensures that the etching substance not only eats vertically to the substrate but also digs itself under the function layer, like a tunnel. What remains is an unsupported structure of the function layer that is only 100 nm thin and connected to the substrate only at certain suspension points.

“A conventional technique is etching with liquids,” Russ said. However, capillary forces can occur when the etching fluid dries. The result is that the filigree membranes are glued to the substrate or even destroyed. In addition, most etching liquids do not permit the choice of just any combination of materials for the function and sacrificial layers.

“We bypass these problems with our new facility," Russ said. “We can use two different gases in the processing chambers of the machine instead of fluids." The gases are highly selective: hydrogen fluoride has strong etching properties on silicon dioxide but does not affect silicon. The exact reverse is the case with xenon difluoride gas.

“This way, we can select which material is better suited to be the function layer," Russ said.

For more information, visit: www.ims.fraunhofer.de