Corrosion Protection of Silicon Micro Systems with Ultra-Thin Barrier Films for Miniaturized Medical Devices

Miniaturized medical devices are becoming increasingly adopted by doctors and patients because they enable new treatment and monitoring capabilities, minimally invasive surgery, improved portability and ease of use. Recent examples include micro pacemakers, micro cochlear implants and ex-situ micro glucose sensors. However, implantable micro devices employing packaging technologies other than metallic enclosures are yet to be seen. Such metallic containers are expensive and can make up to 80% of the volume of the final medical micro device. Physiological monitors such as in-situ pressure sensors and BioMEMS could profit significantly from advances in thin barrier films for corrosion protection of silicon micro devices. Coating films that stop the diffusion and permeation of harmful substances are necessary to protect both the patient and the micro device. Ceramic films deposited by chemical vapor deposition techniques are good candidates for this task due to their low permeability to gases, low chemical reactivity and high conformality. However, few studies are available about the corrosion protection offered by biocompatible coatings to microelectronic devices in representative biological environments. Nine materials previously found non-cytotoxic in-vitro were studied: Al₂O₃, BN, DLC, HfO₂, SiC, SiN, SiO₂, SiOC, and TiO₂. Ultra-thin films of these materials (5-100 nm) were deposited by plasma enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) on substrates commonly found in electronic micro devices: crystalline silicon, copper, tungsten nitride and polyimide. Characterization of films was performed by x-ray photo electron spectroscopic (XPS), x-ray diffraction (XRD), x-ray reflectometry (XRR), atomic force microscopy (AFM), variable angle spectroscopic ellipsometry (VASE) and helium gas permeability measurements. Aging tests of silicon, copper and tungsten nitride substrates coated with these films were performed for several weeks at different temperatures in phosphate buffer saline (PBS), a saline solution that simulates the chemical environment of human blood plasma. Changes in thickness and chemical composition were monitored by VASE, XPS and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). It was found that SiO₂ and SiN films (generally used for protection in the microelectronics industry) are not stable in PBS at 37°C, even though they act as good hermetic barriers against gases. In contrast, films of Al₂O₃, DLC, SiOC and TiO₂ showed very low chemical reactivity in PBS, and one of them was effective as hermetic and diffusion barrier for the corrosion protection of metallic substrates of interest for silicon micro systems. A discussion on the advantages and disadvantages of deposited films and recommendations for corrosion protection of miniaturized medical devices are also included.