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Precious metals boost performance of LEDs, chips

12 Dec 2012

Copper, gold and tin-based deposition processes aid manufacture, functionality of integrated electronics devices.

By the use of metals such as gold, copper or tin and associated special galvanizing processes, researchers from the Fraunhofer Institute for Microelectronic Circuits and Systems IMS, Duisburg, Germany, are improving the function of semiconductors and simplifying the manufacture of microelectronic systems.

Research project leader Prof. Holger Vogt says that the LED industry in particular could benefit from his team’s novel approaches to device production. “LEDs have already conquered the automotive industry; for example, it is already possible today to identify the make of a car by the design of its LED headlights. LED lighting has also found many other applications in new cars: as well as brake lights, parking lights or fog lights, there is also interior illumination, displays, and the infotainment system.”

But unlike traditional halogen or xenon lights, light emitting diodes require drivers. Their most important task is to continuously supply the LEDs with power. They also perform complex tasks and may control several LEDs in series, or switch individual diodes on in multiple stages if, for example, interior lighting needs to be dimmed.

Thus the requirements relating to the drivers are enormous: they need to be immune to the high temperature and voltage differences in a car and in certain cases be resistant to aggressive chemicals. In order to guarantee reliable luminosity, a higher voltage must flow through the circuits of the LED drivers.

Copper increases current

Vogt’s team has developed a process to manufacture the chips that meets these complex needs: it is based on galvanization, a process in the semiconductor industry, in which particular metals are deposited in critical locations on the semiconductors.

Vogt’s department at the IMS, is focusing on copper. “This means we can enable a higher current flow through the chips”, explains Vogt. “That is important, because for many applications, new designs mean that the chips must become smaller and smaller while the current flowing through them remains the same.

However, integrating new materials, such as a layer of copper, is not always without problems, since there are limits to the regular processes for manufacturing chips. It is for this reason that the scientists at the IMS specially constructed a manufacturing line for “post processing” – called the MST Lab & Fab – to be able to subsequently improve the chips on the substrate wafers, depending on the requirements of the application.

In addition to copper, the engineers are also able to deposit other metals or compounds such as copper-tin or gold-tin onto the chips. “These layers can be soldered”, explains Vogt. That enables a substantial advantage: a cover can be soldered onto the chip, right there on the wafer. “The result is the smallest housing for a chip that can be achieved.”

This technique can be used to surround and protect sensitive sensors without adversely affecting functionality. One example is in bolometers, sensors that are used to measure temperature. Because the housings for bolometers must additionally also be put into a vacuum environment to provide accurate measurements, their manufacture is conventionally complex and thus expensive. However, with the help of the MST Lab & Fab, housings that are more cost-effective and therefore suitable for mass production can be manufactured.

Opto-chip soldering

In addition, the researchers in the MST Lab & Fab can construct complex components within a single housing. For example, they are able to solder together two chips, such as an opto-chip with highly sensitive photo sensors with a CMOS chip, which can measure individual photons, to each other, using the copper galvanization process. Such microelectronic components are suitable for night-vision devices or for low-light microscope applications.

About the Author

Matthew Peach is a contributing editor to optics.org

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