Thin-Film Microfabrication
Thin-film microfabrication is the process of building an electrode array by depositing and patterning thin conductive and insulating layers on a wafer, using the photolithography, deposition and etching steps of the semiconductor and MEMS industries.
NeuroNexus silicon probes and polymer grids are made using this thin-film technology. An array design is drawn with computer-aided design tools and converted into a multilayer photomask set, the masks transfer the probe features onto the wafer photolithographically, and sequential lithography, deposition and etching build the layered device. After fabrication the individual devices are released from the wafer and go to assembly, testing and final quality control.
The device is built as a stack of thin films. Fabrication begins with a substrate that provides structural support. A thin-film dielectric is deposited over it for electrical insulation and additional mechanical support. Conductive material is then deposited and patterned into the individual interconnect traces that route each site's signal along the shank. A second dielectric is deposited over the traces, and vias are opened through it to reach the conductors beneath. Finally, metal is deposited and patterned to form the electrode sites and the bond pads. Materials are selected for biocompatibility, electrical performance, mechanical properties and suitability for implantation. The fine-feature process places traces and other features at the sub-micron scale, which is what lets many sites share a shank a few tens of micrometers wide.
Two substrate materials give the two platforms.
Thin-film silicon is rigid enough to penetrate tissue — brain, spinal cord, peripheral nerve and heart — at a typical thickness of 15 or 50 µm. Notably, silicon at this thickness is flexible in plane, allowing it to bend without breaking, but still provides sufficient rigidity for insertion.
Thin-film polyimide is flexible in all planes such that it conforms to a complex, moving tissue surface. It is used for ECoG, EEG, nerve-cuff and cardiac interfaces at a typical thickness of 12 µm. Both thicknesses can be customized. Because fabrication runs in batches, several designs share one wafer, and both platforms support customization of site geometry, site spacing, shank dimensions and overall array layout.
Electrode quality follows from the design and the microfabrication process. See What Makes a Thin-Film Electrode Good and Silicon Probes and Thin-Film Grids: An Introduction for additional information.
Typical values
| Silicon substrate thickness, typical | 15 or 50 µm |
|---|---|
| Polyimide substrate thickness, typical | 12 µm |
| Functional layers | substrate, dielectric, traces, dielectric with vias, sites and bond pads |
| Trace feature size | sub-micron |