Thin Film Metallization Stacks Serve as Reliable Conductors on Ceramic-Based Substrates for Active Implants

Abstract

Substrates and packages of active implantable medical devices are often fabricated from ceramics, such as alumina. Screen-printed PtAu paste is the state-of-the-art metallization for functional structures. Due to solid-state and liquid diffusion of Au during thermal exposure, solder times are limited. Otherwise, metal structures tend to delaminate. Moreover, it was shown that PtAu with solder fails after 37.4 years. We established a thin film metallization on the alumina process to overcome these disadvantages. We used sputtered platinum with an underlying adhesion layer made of tungsten–titanium to increase the adhesion strength of the alumina substrate. We avoided using gold in this work due to its high diffusion tendency. All used materials provided relatively low diffusion properties, which increases independence from joining techniques and mechanical longevity during use. Utilizing the Design of Experiment (DoE) methodology, we derived an optimal Pt thickness of 500 nm with 43 nm of WTi as an adhesion-promoting layer. After accelerated aging at 150 °C, corresponding to 125 years at body temperature (37 °C), the contact pad adhesion strength was 32.75 ± 7.08 MPa. This exceeded the safety limit of 17 MPa by far, set as a recommendation for robust screen-printing metallization processes. Soldering times of up to 120 s did not influence the adhesive strength. The new process reduced the minimum track distance to 50% of screen-printing values and is capable to be transferred into rapid prototyping techniques. It helps to make the assembly process independent of the manufacturing person in order to increase the yield of device fabrication and—most important in implantable device manufacturing—to make it more robust and thereby safer for the patient.