Improved Parylene-Based Barrier Layers Using Low Temperature CVD Technologies

Implantable devices, demanding efficient protection against body fluids as well as maximal miniaturization, require a high degree of biological safety. An ultra-tight encapsulation solution which enables safe long-term implantation close to inflammatory tissue and which has a superior temperature stability for multiple sterilization cycles is highly desirable. In this work, it is shown how these needs can be addressed by an innovative thin film encapsulation technology made of alternating organic and inorganic layers.<br/><br/>Parylene, as organic thin film, is a well-established polymer material, exhibiting excellent electrical insulation and dielectric properties, being often the material of choice for biomedical applications. However, parylene barrier properties are not sufficient to ensure a protection for long-term implantation in the human body of several years. The inorganic materials are dense and hence offer extremely good barrier properties. However, inorganics alone suffer from pinhole and micro-crack defects, which induce a poor long-term hermeticity. On the other hand, the combination of parylene with inorganic thin films provides excellent barrier properties and improves the scratch resistance, limiting the water permeation through potential defects.<br/><br/>This study investigates the barrier performances of inorganic monolayers and parylene multilayers in combination with inorganic films deposited by low temperature chemical vapor deposition (CVD) processes. Parylene VT4 film, deposited by the standard Gorham process (low pressure CVD), was used for its excellent thermal stability and its superior capability to enter into small voids or channels of substrates compared to other parylene types. Silicon-based layers, deposited by plasma-enhanced CVD (PECVD) were examined as single layer, bilayers and combined with parylene film as multilayers. Three precursors and two plasma generation methods (inductively and capacitively coupled plasma) were tested and evaluated for inorganic PECVD layers. Similarly, atomic layer deposition (ALD), including thermal and plasma oxidation methods, was used to compare three metal-oxide layers. Water vapor transmission rate (WVTR) measurements were performed to quantify the barrier performances using electrolytic and diode laser spectroscopy detection methods. The most efficient single layers show a WVTR in the range of 10⁻³ g m⁻²d⁻¹, at 38°C and 90% relative humidity conditions, while the results of WVTR measurements for parylene-inorganic (1dyad) combinations are in the range of 10⁻⁴ g m⁻²d⁻¹at 38°C and 90% R.H. Multilayer stack (more than 1 dyad) will be assessed and WVTR value will be measured for different organic/inorganic layer combinations. Finally, thin film encapsulations will be evaluated on functional active device, (e.g., electronic circuit) immersed in saline solution in order to determine the barrier performances close to real implantable applications.