AVS2012 Session TF-WeM: Thin Films for Encapsulation, Packaging, and Biomedical Devices

Encapsulation of 3-D electronic biomedical implants with complex geometries and tight gaps between components is one of the greatest challenges to achieve long-term functionality and stability. We have investigated a new encapsulation scheme that combines atomic layer deposited (ALD) Al₂O₃ and Parylene C for biomedical implantable system and will present results to quantify the performance of this system. Our approach combines the highly effective moisture barrier properties of ALD alumina, and Parylene as a barrier to many ions and for preventing contact of alumina with liquid water. 52 nm of Al₂O₃ was deposited by plasma-assisted (PA) ALD on interdigitated electrodes (IDEs). AFM micrographs show that as-deposited Al₂O₃ films on fused silica substrate have RMS surface roughness of 0.48 nm. XPS spectra determined that PA-ALD films had nearly stoichiometric O/Al ratio of 1.4. A 6-µm thick Parylene-C layer was deposited by CVD using Gorman process on top of Al₂O₃ and used A-174 (Momentive Performance Materials), an organosilane, as adhesion promoter. The IDEs coated with Al₂O₃-Parylene C were soaked in phosphate buffered saline (PBS) solution for a period of about 9 months at both body temperature (37 °C) and elevated temperatures (57 to 80 °C) for accelerated lifetime testing. Electrochemical impedance spectroscopy (EIS) and chronoamperometry were used to evaluate the integrity and insulation performance of the soft encapsulation. The leakage current was ~ 20 pA by applying 5 V DC bias and impedance was ~ 3.5 MΩ at 1 kHz with phase of close to -87° by using EIS for samples under 67°C about 9 months (approximately equivalent to 72 months at 37°C), indicating no significant degradation. The encapsulation performances of combining alumina and Parylene C, Parylene C only and alumina only coatings were compared and the bi-layer coating shows its superiority of at least 5 times longer lifetime than the rest two coating approaches. The continuous 5 V bias voltage has no obvious effect on alumina and Parylene coated samples while it shortened the lifetime of Parylene coating by at least a factor of 4. Complex topography can shorten the lifetime of coating dramatically comparing with planar structures, especially with the existence of micromotion inside the body. The lifetime of alumina and Parylene coated devices with hand-wound coils and SMD capacitors was only about 50% or less of that of planar test structures. The long-term (more than 6 years of equivalent lifetime) insulation performance of the novel double-layer encapsulation shows its potential usefulness for chronic implantable electronic microsystems.

Encapsulant barrier coatings, which prevent the permeation of water through flexible plastic substrates, are an enabling technology for the commercialization of OPV devices. Such protective coatings are made of multilayer stacks where multiple dense, inorganic layers are alternated with soft, organic ones. The inorganic layer contains inevitably some pinholes and defects. The roles of the organic layer are creating a tortuous and longer path among the defects of two successive inorganic layers, filling the pores of the inorganic underlayer limiting the propagation of defects from one inorganic layer to the other and smoothening the substrate surface roughness.

We obtained good barrier properties (WVTR= 10^-2 g/cm²/day at 25°C, RH=85%) with a bilayer obtained by coupling initiated-PECVD (iPECVD) and plasma enhanced CVD (PECVD) at very low thickness of inorganic layer (25 nm).

SiO_x layers were deposited through PECVD in MW plasma at high power and high oxygen dilution. The silanol and organic groups were not detectable by IR spectroscopy, resulting in dense film with high flexibility and high critical tensile strain. High critical tensile strain implies that the coating can be bent and stretched to a relatively big extent before cracking. Inorganic films obtained by other technologies (i.e. Al₂O₃ ALD coatings) showed smaller critical strain values.

Organic coatings were deposited through a new process named iPECVD with enhanced monomer structure retention compared to a conventional plasma deposition and faster deposition rate if compared to conventional iCVD processes from organosilicon monomer. The deposition conditions were tuned to obtained good planarizing properties. The deposition of planarizing organic layers was demonstrated by depositing the coating on the top of some microspheres (1 µm in diameter) which served as model defects on the surface. Increasing the thickness of the coating, the degree of planarization (DP), both local (DLP) and global (DGP), increases. The DLP increases much faster than the DGP: when the coating is 1µm-thick the DLP is already 99%, for the global planarization instead a 1.8µm-thick-coating is needed to reach DGP= 99%.

The great advantage of a similar approach is that we deposit the multilayer in a large-area reactor, maintaining the same organosilicon precursor and the same reactor configuration for both deposition of silica-like and organosilicon layers. A detailed investigation of the barrier and mechanical properties changing the number of layers in the stack and the measurements conditions will be presented in order to demonstrate the robustness of the following approach to create flexible ultra-high barrier layer.