Optimizing Tin Electroplating for MEMS Meissner-Effect Transition-Edge Sensors

Abstract

Electroforming metal features into patterned polymer molds targeting MEMS microfabrication allows for the enhancement of electrical, mechanical, magnetic, and thermal properties1. Uniform and repeatable electroforming processes are important to achieve the final desired metal geometry to optimize performance. MEMS fabrication processes typically rely on average plating rates based on historical depositions to determine future plating rates and offer the user options to vary electroplating time, among other options, to meet the desired device height(s). Process repeatability is often difficult to achieve in these scenarios and can be challenging in R&D applications with limited throughput. Inherent changes from one sample to the next can occur in many forms including a change in active area of lithographically patterned photoresist molds, resistance of substrates/seed layers below electroplated layers, geometry of the layout, chemistry make-up, electroplating setup, and fast deposition rates can all affect plating rates from sample to sample. Some changes can be controlled through careful inspection and tracking, then accounted for. However, some changes cannot be observed, and deposition rates can change from sample to sample leading to ever-changing plating rates.These challenges were encountered during electroplating of tin disks coupled with planar superconducting coils of niobium targeting the microfabrication of Meissner-Effect Transition-Edge Sensor devices (ME-TES)2. We investigated two and three electrode setups using a modified Dow Chemical Solderon make-up chemistry targeting pure tin depositions as opposed to tin and silver alloy depositions. Those changes were coupled with chronopotentiometry, chronoamperometry, and pulse plating experiments to optimize tin electrodeposition. Once found, the optimum tin deposition process was built into a graphical user interface (GUI)3 that predicts electroplating rates and was then successfully applied to microfabricate ME-TES’s. This process methodology could benefit other metal MEMS-based electrodeposition processes such as nickel, copper, gold, magnetic alloys, etc. to optimize electroplating.(1) Buchheit, T. E.; Christenson, T. R.; Schmale, D. T.; Lavan, D. A. Understanding and tailoring the mechanical properties of LIGA fabricated materials. Materials Science of Microelectromechanical Systems (Mems) Devices 1999, 546, 121-126. DOI: Doi 10.1557/Proc-546-121.(2) Finnegan, P. Packaging & Solid-state Materials and Fabrication Processes. In Packaging & Solid-state Materials and Fabrication Processes, June 2023.(3) St. John, C. In-Situ Analysis of Chronopotentiometry Data to Predict Electrodeposition Rates. In 243rd ECS Meeting with SOFC-XVIII, Boston, MA, 2023.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525 SAND2022-10288A