Ultrafast 3D circuit enables higher switching capacity

The Berlin-based Ferdinand Braun Institute for Ultra-High Frequencies (FBH) has launched an IC process that enables transistors switching at frequencies of well beyond 200GHz. The process has another capability, which could be of interest to the semiconductor industry. This helps chip designers to create 3D integrated circuits.

The transfer substrate process developed by the FBH researchers uses InGaAs transistors as active elements. At the initial step, very thin indium phosphide and InGaAs layers are applied to a substrate wafer. "These layers are partly less than 10nm thick," said Wolfgang Heinrich, head of microwave technology department, FBH. In the second step, the layers are structured by means of normal etching and metalizing processes. Then the structured front side of the wafer is bonded to a ceramic carrier and the substrate wafer then is removed using a standard thinning process.

"By removing the substrate wafer, the rear side of the active layers is laid open, and further layers can be bonded on top," he added. In the produced circuits, the wafer has no more negative impact on the dielectric coefficient of the transistors, since it has been removed. "No more silicon-induced dirt effects," Heinrich said. "Thus, the cut-off frequency is well beyond 200GHz", he stressed. According to the specifications published by the institute, f_T is 410GHz; f_max is up to 480GHz.

Aside from its high-working frequencies, the circuits produced using the process feature another interesting attribute. On top of the active layers, further layers can be applied, creating the possibility to manufacture 3D circuits with significantly higher integration levels compared to today's 2D chips.

According to Heinrich, the institute has not yet decided if and how it will commercialize the technology, and has no idea when it will be used in industrial production. However, he said the institute is in negotiation with at least one interested party.

- Christoph Hammerschmidt
EE Times Europe