Fabrication. We fabricated the arrays using the following protocol:
First, 200 nm of SiO2 was thermally grown on a silicon wafer to insulate the substrate from the other layers. This process was done in a high-temperature atmospheric furnace to grow silicon dioxide (SiO2) on silicon wafers (¡«3 h at 1,100 ¡ãC).
In subsequent steps, the bottom conductive electrodes were patterned and deposited through optical photolithography, and metal deposition processes, followed by lift-off steps.
To do that, first, standardsilicon wafers were prebaked on a hot plate at 200 ¡ãC for ¡«2 h. Then the manual resist spinning step was performed by applying 10 drops of hexamethyldisilizane to the wafers as an adhesive layer.
This was followed by the coating of the wafers with MaP 1215 photoresists. The wafers were then transferred to a contact aligner system to perform precision mask-to-wafer alignment followed by near-UV photoresist exposure (¡«3 s). Exposed photoresists were developed, and the patterned wafers were transferred to an evaporation system to deposit the bottom metallic electrodes.
In the evaporation system, first, 3 nm of Cr was deposited as an adhesive layer, which was followed by the deposit of 100 nm of gold to create the sensors¡¯ bottom conductive electrodes.
The wafers then went through a metal lift-off process (¡«30 min in Acetone) to remove the remaining photoresists and form the final configuration of the bottom electrodes. The next step was deposition of 30 nm of silicon dioxide (sensing region) using plasma-assisted atomic layer deposition technique. This is a high-quality, conformal, uniform, pinhole and particle-free oxide film to minimize the electrical shorting effect and maximize the sensors¡¯ yield.
This was followed by the fabrication of the top conductive electrodes. A similar procedure as for the bottom electrodes was followed to fabricate the top electrodes.
These conductive electrodes were then coated with a protective oxide layer (SiO2). These protective layers were deposited using a plasma-enhanced chemical vapor deposition system. Several etching steps followed by a lithography step were performed to form channels underneath the sensors. Then, the oxide from electrical measuring pads, called bonding pads, was removed. To achieve this, the patterned wafers went through a wet etching process (6:1 Buffered Oxide Etch) to expose these bonding pads.
The final step was further cleaning of the sensing tips and forming them with sharp edges in the channel. This step was achieved using the focused ion beam etching process.
(this idiot).