New facility delivers microsystem technology

Compiled by Photonics Spectra staff

A new method that employs precise etching to produce miniaturized thermal imaging sensors promises to enable large-scale manufacturing of automotive safety devices.

Because they see more than the naked eye, infrared cameras could improve traffic safety. They already are being used in the construction industry and the military; however, infrared cameras are barely available in mobile applications such as automotive safety systems because long-range infrared microsensors are difficult to produce commercially in the quantities necessary.

Now, the Fraunhofer Institute for Microelectronic Circuits and Systems (Fraunhofer IMS) has opened a facility to produce microsystem technology (MST). MST involves minute sensors, valves or other mechanical components that are integrated into semiconductor chips. In air bags, they serve as motion sensors. To implement MST on semiconductors, the art of etching must be mastered, which is where the scientists at Fraunhofer IMS come in.

A researcher operates the production machine at Fraunhofer IMS’s new microsystem technologies laboratory and cleanroom facility. Courtesy of Fraunhofer IMS.

To apply the technology to a semiconductor, the scientists assembled three layers. The lowest is a silicon wafer substrate, which is topped with a sacrificial layer serving as a spacer and followed 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 only 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, he added, explaining that etching must work not only vertically but also evenly in all directions, a process called isotropic etching.

This procedure ensures that the etching substance not only eats vertically to the substrate, but also digs itself under the function layer. What remains is an unsupported structure of the function layer that is only 100 nm thick and connected to the substrate only at certain suspension points.

The conventional technique is to etch with liquids, which results in the filigree membranes becoming glued to the substrate or even destroyed. Additionally, most etching liquids do not permit the choice of just any combination of materials for the function and sacrificial layers. The scientists said the new facility would bypass these problems by using 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. The researchers can select which material is better suited for the function layer.