Encapsulation and Packaging of a Semiconductor Multielectrode Array for Cortical Implantation

摘要

A 16 x 16 multielectrode semiconductor array, known as the AFIT brain chip, must be encapsulated prior to implantation into the cerebral cortex of a rhesus monkey. The encapsulant, which must protect the chip from the saline-like solution of the cerebro-spinal fluid, was a polyimide: Dupont PI 2555. The polyimide was spun onto the chip, and the electrode openings etched, using negative photolithography and wet chemical etching. An eight-micron thick layer of polyimide was obtained by repeating this process several times. Although actual implantation was not performed, an implantable package was designed and fabricated which will allow for chronic use with only one surgical operation. A surgical technique for implanting the package is suggested. The fabricated package was not fully functional, however, because an epoxy used to protect bond wires from the stress of the polyimide could not withstand the high temperature cure. The functioning part of the chip was tested in vitro and worked continuously for three weeks. There was no damage to the polyimide encapsulant, but some of the aluminum electrodes showed signs of deterioration.