Superconducting Nb & Cu Resonator/CPW Fabrication

Microfabrication of superconducting Nb resonators/ CPWs for cryogenic measurements and Cu resonators/ CPWs for room-temperature testing.

Overview

For multiple projects I have done the fabrication and characterization of:

  • Nb resonators and CPWs for low-temperature, cryogenic experiments
  • Cu resonators and CPWs for room-temperature S-parameter testing

The key components of this work:

  • Electroplating Indium (In) bump structures on resonator & coplanar waveguide (CPW) chips for spin pumping and magnetic control interfaces. Details in Electroplating.
  • DC lines enabled current-controlled magnon–magnon and magnon–photon coupling in hybrid quantum systems.
  • Established a repeatable fabrication workflow and validate resonator performance across temperatures and magnetic fields.
Left – Nb resonators for cryogenic testing; Right – Cu resonators for room-temperature characterization. Both have photoresist spun for dicing.

Fabrication Process

Devices were fabricated using optical lithography and thin-film deposition:

  • Substrates: High-resistivity double sided polished Si wafers
  • Deposition: DC sputtering of Nb (200 nm), and Cu (150 nm)
  • Patterning: UV lithography, lift-off for Cu and Nb
  • Geometry: Quarter-wave CPW resonators (50 Ω feedline, λ/4 open stub)
  • Indium Bumps: Electroplated onto patterned trenches to create low-resistance current injection contacts for tunable coupling experiments
  • Packaging: Wire-bonded to custom PCBs for VNA measurements

Measurements

Room-Temperature (Cu): S-parameter and impedance characterization using a VNA.

Low-Temperature (Nb): Cryogenic measurements of S21 and Q-factor at 1.7 K, demonstrating high-quality resonator performance and compatibility with hybrid magnonic architectures.

Nb resonator performing under magnetic fields measured at 1.7 K: S21 measurements (a) and corresponding Q-factors (b)

Electroplating Indium (for CPW chips)

For CPW wafers: (a) CPW layout used for current-controlled magnon coupling, (b) Profilometry of In bump pattern trenches, , (c) 3D view of indium bumps, and (d) Electroplated bumps after resist stripping.

Key Outcomes

  • Established a reproducible fabrication workflow for Nb and Cu resonators/CPWs
  • Developed indium bump integration for current-driven magnon–magnon and magnon–photon coupling
  • Verified resonator performance trends across magnetic fields
  • Provided a baseline for cryogenic hybrid quantum experiments
  • Integrated Microwave Simulation → Mask/ PCB Design → fabrication → measurement pipeline

Tools & Methods

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