Electroplating on Unconventional Ultra-Compliant Substrates for Travelling Wave Tube Amplifiers

Abstract

We report gold electroplating on ultra-compliant substrates comprising helical slow wave structures (SWSs) for traveling wave tube amplifiers (TWTAs) [1]. The novel ultra-compliant substrates are composed of edge-tethered tri-layer metal ribbons with a helical geometry of microscale diameter. After electroplating with gold, we obtain overall thicknesses of a few um. We discuss different controllable electroplating conditions that influence thickness, uniformity, roughness, and related properties of deposited gold films on helical ribbons. In addition to increasing conductance of the electroplated helical ribbons, electroplating stabilizes the helix at its equilibrium diameter, with the pitch predicted by Prakash et al.[1].Our method of fabrication of ultra-compliant helical ribbons starts with defining strips of width 5 µm to 10 µm by optical lithography, metal evaporation, and lift-off, deposited on Si substrates coated with a sacrificial layer of Ge or GeOx,. We use Cr/Au/Cr tri-layers to create an inherent stress gradient that causes the strip to self-assemble into a helix, after etching with XeF2 for selective removal of a sacrificial layer. Diameter and pitch of the released helices are controlled by varying the thickness, the elastic modulus, the residual stress, and the in-plane geometry of the deposited tri-layer metal strips. The gold electroplating process uses a sulphate-based gold solution in a two-electrode electrochemical setup with the helix as the cathode and a platinized mesh as the anode [2]. We deposit gold to a thickness of a few um, using a pulsed current source with variable parameters. Direct current can also be used with smaller deposition times. Our results demonstrate the application of electroplating to unconventional ultra-compliant helix of nanoscale dimensions. Reference [1] Divya J. Prakash,., Matthew M. Dwyer, Marcos Martinez Argudo, Mengistie L. Debasu, Hassan Dibaji, Max G. Lagally, Daniel W. van der Weide, and Francesca Cavallo. 2020. “Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.” ACS Nano, 2021, 15, 1229-1239. doi:10.1021/acsnano.0c08296.[2] Max G. Lagally, Anjali Chaudhary, Daniel van der Weide, Divya J. Prakash, and Francesca Cavallo, Improved Self-assembly of Helices via Electrodeposition on Freestanding Nanoribbons for TWT Application, provisional US patent application (P220249US01)Work supported by U.S. AFOSR-Award No FA9550-22-1-0086.