Low Temperature CVD Process Development for Forming Niobium Layers on Copper

Technical Report · Sun Mar 26 00:00:00 EDT 2017

OSTI ID:1832479

McNeal, Shawn R.; Arrieta, Victor M.

Innovative process development is needed to reliably deposit superconducting niobium films possessing performance capabilities near those of bulk niobium on less costly copper accelerator component structures such as cavities, complex couplers, and nosecones as an alternative to solid bulk niobium. Unlike bulk or thick-film niobium, most thin-film coatings do not afford practical post-coating surface conditioning options. Thin-film niobium coatings on copper are prone to copper diffusion through the entire ~5-μm coating thickness and poor thermal contact (i.e., not metallurgically/diffusion bonded), limiting their use to low-to medium-gradient applications. Niobium-coated copper fabrication technologies for cost-effective high-Q, high-field superconducting radio frequency (SRF) components are needed for the economic viability of future accelerator facilities. Ultramet, in collaboration with Cornell University’s SRF Group, is demonstrating thick-film (~150 µm) chemical vapor deposited (CVD) niobium on OFHC copper that is compatible with state-of-the-art cavity surface conditioning methods and has great potential to achieve bulk-like performance capabilities for use in high-gradient applications, but at lower cost by minimizing or eliminating many costly, time-consuming, and often performance-limiting fabrication steps as well as affording an estimated 90-95% reduction in the amount of niobium required. In this project, the feasibility of depositing thick-film niobium layers on OFHC copper using lower temperature CVD (850-900ºC) was demonstrated. The 30- to 150-µm CVD niobium coatings were shown to be diffusion-bonded to the copper, and interdiffusion analysis showed that the bulk of the coating thickness remained pure niobium even after elevated temperature exposure for extended periods. Coating adherence was excellent, and the coated surface looked similar to bulk niobium. The BCS portion of surface resistance was good. Residual surface resistance was elevated, but this was not related to RRR and was likely due to surface inclusions that can be reduced or eliminated by conditioning in future development. Process variables deemed critical for material optimization, process scaling, and accelerator cavity and component fabrication and conditioning efforts were identified. Ultramet’s thick-film niobium CVD process is uniquely well-suited for coating complex SRF accelerator component geometries because the virtually 100% dense coatings are formed on the substrate at the molecular level and purity levels in excess of 99.99% are achievable. The CVD coating process exhibits the greatest throwing power, or ability to uniformly deposit materials onto/into intricately shaped or textured substrates. This research is a necessary step toward the commercial and scientific application of advanced accelerator component-forming technologies that will represent a significant technical milestone in developing reliable fabrication techniques for reproducible niobium-coated copper accelerator components offering high-gradient operation and substantial cost reductions for SRF programs worldwide.

• Research Organization: Ultramet (Pacoima, CA)
• Sponsoring Organization: USDOE Office of Science (SC), High Energy Physics (HEP)
• DOE Contract Number: SC0015727
• OSTI ID: 1832479
• Type / Phase: SBIR (Phase I)
• Report Number(s): DOE-ULTRAMET-0015727-1; ULTRA-TR-17-16415
• Country of Publication: United States
• Language: English

Related Subjects

43 PARTICLE ACCELERATORS
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
36 MATERIALS SCIENCE
42 ENGINEERING
superconducting radio frequency (SRF) cavity
niobium
copper
chemical vapor deposition (CVD)
diffusion bond
high-energy physics
nuclear physics
accelerator