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

Abstract

Hermetic and non-hermetic packages of active implantable medical devices are often fabricated of ceramics like alumina. Screen printed PtAu paste is the state of the art metallization for functional structures. Due to solid state and liquid diffusion of Au at 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 alumina process to overcome these disadvantages. The metallization consists of sputtered platinum with an underlying adhesion layer made of tungsten-titanium to increase the adhesion strength to the alumina substrate. We avoided using gold in this work due to its high diffusion tendency. Instead, the materials in use provide relatively low diffusion properties, which potentially increases the long term mechanical performance and usability during assembly and packaging.Utilizing the Design of Experiment (DoE) methodology, we derived an optimal Pt thickness of 500 nm with 43 nm of WTi as adhesion promoting layer. After accelerated aging at 150 °C, corresponding to 125 years at body temperature (37 °C), the contact pad adhesion strength was with 32.75 MPa ± 7.08 MPa still significantly higher than the safety limit of 17 MPa, following the recommendations for a robust screen-printing metallization process. Moreover, 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 of rapid prototyping. It helps to make the assembly process independent of the manufacturing person, to increase the yield of device fabrication and -most important in implantable device manufacturing-to make it more robust and thereby more safe for the patient.

3D-Printed Hermetic Alumina Housings

“…The fracture is partly cohesive and partly adhesive in nature and shows extended scattering from 2 MPa up to the detection limit of 88 MPa. Unlike the previous studies [ 14 , 19 ], partial adhesive failure of the metal-ceramic interface was observed here, which may explain the lower adhesion and large variance. One difference was the larger surface roughness, which is decisive for the wettability during sputter deposition as well as the soldering process [ 36 ].…”

“…The mean ultimate tensile strength needed to de-attach the wires was 31.1 MPa ( n = 47). Also, the median strength (20.1 MPa, n = 47) was higher than the conservative threshold of 17 MPa, set by a comparable study showing also a 32 MPa mean adhesion strength on commercial alumina [ 19 ]. No clear difference in maximum pull force between the copper rods soldered on the two different pad geometries was found.…”

“…Challenges due to coefficient of thermal expansion mismatches or pressure distributions during high-temperature firing on the shape of the ceramic to be metalized are minimized. Likewise, liquidus diffusion should be negligible, which occurs for medical implants with platinum-gold metallization embedded in a glass matrix [ 19 ]. In contrast, the developed sputter-deposited WTi-Pt metallization showed excellent long-term stability on smooth Al 2 O 3 at elevated temperatures in air and saline [ 19 ].…”

“…Likewise, liquidus diffusion should be negligible, which occurs for medical implants with platinum-gold metallization embedded in a glass matrix [ 19 ]. In contrast, the developed sputter-deposited WTi-Pt metallization showed excellent long-term stability on smooth Al 2 O 3 at elevated temperatures in air and saline [ 19 ]. The adhesion promoter WTi10 increases the adhesion strength by a factor of three [ 14 ], presumably forming a covalent bond with the oxygen atom of the ceramic substrate [ 35 ].…”

“…For this purpose, the samples were coated with a metal layer stack of tungsten/titanium (WTi10) and platinum (50 nm and 500 nm respectively) by sputter deposition (Leybold Univex 500, Leybold Vacuum GmbH, Cologne, Germany). WTi10 acts as an adhesion promoter to ensure a higher adhesion of the sputter-deposited platinum [ 19 ] which is used as the conductor in active implants. The structuring of the metals was realized by masking the uncoated parts with polyimide adhesive tape before deposition and by laser ablation with an infrared Nd:YAG nanosecond laser (ACL Laser GmbH, Nohra, Germany) at a wavelength of 1064 nm after deposition and peeling off of the coarse adhesive tape mask.…”