Deposition Parameters Determining Insulation Resistance and Crystallinity of Parylene C in Neural Implant Encapsulation

摘要

Miniaturized neural implants often use Parylene C as an FDA approved material for system encapsulation. The quality of the layer with respect to water absorption and salt intrusion limits the lifetime of these implants. Within this study we measured the resistance of Parylene C (poly-chloro-para-xylylene) thin films in saline solution as a function of film thickness, pressure while deposition and substrate metal. The measurement of the resistance was done with an electrochemical setup. In addition, the crystallinity of the deposited films was investigated with an x-ray diffractometer (XRD) and film thickness was measured with a white-light interferometer. A low resistance indicates that there are defects (pinholes) in the layer. Samples with a thickness larger than 5 urn seemed pinhole free. Furthermore, higher pressure while deposition resulted in films with a higher resistance although the layers were thinner. The investigations showed that the film's insulation quality does not only depend on thickness, substrate metal and pressure while deposition but also on the preprocessing of the substrate. Preliminary results conclude that detailed process control beyond the standard Gorham process parameters significantly increase the encapsulation quality of Parylene C for micro-machined neural implants.