Reliability of ultra-thin metal films integrated onto embossed and bonded liquid-crystal-polymer (LCP) sheets for neural-interface applications

Liquid crystal polymer (LCP) is increasingly used in flexible implantable bioelectronic devices due to its low moisture uptake, chemical stability, and ability to form robust thermoplastic bonds. However, integrating fine-pitch thin-film metallization into bonded embossed LCP structures presents challenges related to pattern fidelity, bond integrity, alignment accuracy, and long-term electrical reliability, particularly when the metal thickness is small relative to the surface roughness. In this work, we present and characterize a fabrication process for integrating a 500-nm-thick sputtered Cr/Au thin-film metallization onto a 25-μm-thick embossed high-temperature LCP (HT-LCP) substrate, patterned into long (20 cm) and narrow (8 μm) traces using lift-off. Bond integrity between the metallized HT-LCP and a low-temperature LCP (LT-LCP) layer was evaluated using peel testing, while structural and electrical integrity were assessed using NanoCT imaging and resistance measurements. Long-term reliability was evaluated using reactive accelerated aging (RAA) at 87 °C in physiological saline with 10 mM hydrogen peroxide. The results show that the thin metal layer does not degrade bond strength and that embedded traces maintain structural and electrical integrity through bonding and aging. After 12 days of RAA testing, no measurable changes in electrical performance were observed. Electrochemical impedance spectroscopy demonstrated that electrodes coated with a 100-nm sputtered Pt layer exhibited approximately 2 × lower impedance than flat Pt electrodes, attributed to increased surface roughness. Additionally, the bonded LCP structure was thinned from 50 μm to 10 μm using CF4/O2 reactive ion etching with >90% uniformity. These results demonstrate that thin-film metallization integrated into bonded embossed LCP systems can achieve high interconnect density without compromising mechanical or electrical reliability. This work provides practical guidelines for the design of thin, flexible, and durable LCP-based implantable bioelectronic devices.