On-chip RF transformers could shrink thanks to rolled-up membrane design

Researchers at the University of Illinois say they have devised more efficient RF transformers by depositing conductors on a membrane that, once complete, rolls up to create a high-turns-ratio transformer.

The transformers consist of two sets of rolled-up coils deposited on some kind of SiN_x material. Researchers say they’ve used the technique to realize up to about 1.1 nH inductances. They realized turns ratios ranging from 1.5:1 to 2.5:1 and self-resonant
frequencies from 11.5 GHz to over 20 GHz, with device footprints of 0.003 or 0.008 mm².

The construction technique basically consists of depositing layers of conductors on stretched membranes. The membrane rolls up into a tube once deposition is complete and the membranes are released from a substrate by etching away a sacrificial layer.

Fabrication of a transformer starts with a silicon-oxide substrate onto which a germanium sacrificial layer is evaporated. Then comes an oppositely strained silicon-nitride bilayer. Reactive ion etching is used to etch down to the SiO₂ to form a mesa. Then comes a metal layer of copper, nickel, aluminum, or gold, followed by photolithographic patterning to form primary and secondary coils. On top of this goes an Al₂O₃ thin film serving as a cover. Finally, an etch window is removed to give access to the germanium sacrificial layer. Once the germanium is gone, the remaining material rolls up into a tube.

The thickness of the various deposited layers determines the inner diameter of the tube. The horizontal layout of the conductors determines the number turns in the primary and secondary coils and the spacing between them.

Researchers note that the material rolls up with no air gap between the turns, important for realizing close tolerances in performance specs. But the rolled-up configuration introduces a number of parasitic elements not found in conventional chip transformers. Researchers put an appreciable amount of work into devising simulation models that correctly predict these parasitic factors.

Of course, there is a lot to do before rolled-up transformers become commercial. Researchers say the fabrication yields are pretty low so far, particularly for complex designs. And they would like to use conductors cheaper than the noble metals found on transformers fabbed to date. This study used gold wire, but the team has successfully demonstrated the fabrication of their rolled devices using industry-standard copper. The next step will be to use thinner and more-conductive metal such as graphene, making possible smaller and more flexible transformers. Researchers speculate such an advance may make it possible for the devices to be woven into the fabrics of high-tech wearables.

Finally, devices fabbed so far don’t change much even when subjected to high temperatures, but researchers say they’ve yet to characterize the thermal and mechanical stability of designs to the degree necessary for commercial devices.

The researchers described their work in a paper appearing in Nature Electronics, “Three-dimensional radio frequency transformers based on a self-rolled-up membrane platform.”