Diffusion-Limited Deposition of Parylene C

Diffusion-limited deposition (DLD) allows the deposition of parylene C layers of two different thicknesses in one step with smooth transitions in between. Needlelike microelectrode structures for implantation in the brain were successfully coated with a taper along the needles' shafts. Diffusion limitation was obtained in the parylene coater by containers with perforated cover lids through which the needles were placed. The polymer layer condensing on the shafts is remarkably thinner than that on the regions outside the container. In this paper, we characterized the parameters influencing the DLD of parylene C. Systematic depositions were made using nine screens with different aperture sizes and screen thicknesses. The parylene thickness declined from approximately 9 to 1 mu m and less due to the DLD. The geometric parameters affected the transitions' lengths (180 to 630 mu m), shapes, slopes, and symmetries. The process was established to obtain uncovered electrode sites, a mandatory property to assure a direct metal-tissue contact to record bioelectric signals.