Biomedical Engineering Reference
In-Depth Information
integrated circuits for signal amplification and spike detection may be packaged on the flexible sub-
strate using flip-chip bonding techniques in future designs.
7.1.2 Process Flow
All processing for building the electrode array is performed on the surface of a 100-mm-diameter
silicon wafer covered with Kapton tape (which provides adequate adhesion for the subsequent poly-
imide layers). The polyimide bottom insulation layer (PI 2611; HD Microsystems, Cupertino, CA)
is spin deposited and cured to a final thickness of 20 µm. Sputtered nickel, 100 Å, is patterned to
define the probe, wiring, and bond pad dimensions. Then, Ni is electrodeposited on the Ni seed
to an average thickness of 20 µm via a 10-mA direct current for 4 hours in a nickel sulfamate bath
(Nickel S; Technic Inc.). Adhesion promoter (VM9611, HD Microsystems) is next applied fol-
lowed by three spin coatings of the PI 2611 to achieve the final 20-µm top layer of insulation. Al
(1000 Å) is patterned as a hard mask for the subsequent oxygen plasma etch. The etching process
includes an O
2
reactive ion etch that removes the polyimide from the top of the bond pads and the
probe tips. The remaining polyimide under the probe tips is isotropically etched in a plasma barrel
asher. Then the probe-cable assembly is removed from the substrate wafer and primed for parylene-
C deposition with a silane adhesion promoter (Acros Organcis, Geel, Belgium). The parylene-C
vapor deposition step insulates the shank of the metal probes to a thickness of 2-4 µm. Then the
probe ends are manually cut with a blade to expose bare metal for the electrode sites. Finally, the
probes are immersed in an electroless gold plating solution (TechniIMGold AT, 600, Technic Inc.,
Cranston, RI) that covers the electrode sites as well as the bond pad sites with 0.1 µm of gold. An
Omnetics connector is then fixed to the bond pads with silver epoxy.
7.1.3 In Vivo Testing
Adult male 250-g Sprague-Dawley rats were used to test the recording performance of the flexible
electrode arrays. All procedures have been approved by the University of Florida Institutional Ani-
mal Care and Use Committee Board and were performed in the University of Florida McKnight
Brain Institute. Before surgery, the rats were anesthetized and the surgical site was thoroughly ster-
ilized. The top of the skull was then exposed by a midsaggital incision from between the eyes, and
the landmarks bregma and lambda are located on the skull [
45
]. The microwire array was implanted
to a depth of 1.66 mm into the forelimb region of the primary motor cortex. The electrodes are
stereotaxically moved to the appropriate site and lowered to the appropriate depth using a micro-
positioner (1 mm/h) to minimize distress to the brain tissue (FHC, Bowdowinham, ME) as shown
in Figure
7.4
. The array was then grounded using a 1/16-in.-diameter stainless steel screw. A low-
profile Omnetics connector was used to attach the recording wire.
