Environmental Engineering Reference
In-Depth Information
Next, the glass probes were gradually pulled out using a computer-controlled
(via LabView v.7) motorized linear translation stage (Newport Corp., Irvine, CA)
to taper them down to 20
m at the tip. In the final etch step, approximately
1 mm length of the tapered probe was immersed into the same etchant for further
sharpening by meniscus etching, yielding ~200 nm tips [55, 62,]. This final etch step
was self-terminating, which permitted consistent and reliable fabrication of micro-
electrode sensor tips. Following this final etching step, the sharpened glass probes
were cleaned in sulfuric peroxide solution (H
2
SO
4
and H
2
O
2
in a 7:3 (v/v) ratio),
followed by acetone, methanol, and DI water for 5 min each. The total etch time
was approximately 40 min for one glass wafer.
Metallization.
The tapered glass probes with sharpened tips were metalized on
all sides by thermal evaporation (Fig. 6.3d). A 200 nm thick layer of gold (Au)
was deposited as a conductive layer on top of a 20 nm thick layer of titanium (Ti)
adhesion layer [57]. A glass cover slip was used to mask the microelectrode array
base on both sides to prevent metal deposition and electrically isolate individual
microelectrodes.
Packaging
. Following the batch fabrication steps, the metalized glass wafer
was cross-cut to separate individual MEAs. For easier handling and establish-
ing electrical connections with individual sensors, MEAs were packaged with
copper-clad laminate glass-epoxy (also known as printed circuit board or PCB)
carriers. Using PCB is also beneficial for future system integration with IC cir-
cuitry. The laminate was 790
×
20
μ
μ
m thick (D&L Products, Inc.) with a 35
μ
m
thick layer of copper and a 33
m thick layer of dry film negative photoresist.
The copper layer was photolithographically patterned and etched in ferric chlo-
ride to define electrical traces on the carrier surface. Following photolithography,
carriers were cut to the exact size from the patterned board by circuit milling
(Quick Circuit 5000, T-Tech). Individual microelectrodes were fixed to carriers
using UV-cured epoxy (3301, Loctite, Rocky Hill, CT). Conductive silver epoxy
(Ablebond 8700E, Emerson & Cuming, Billerica, MA) was used to establish the
electrical connections to individual microelectrodes with copper strip lines on PCB
(Fig. 6.3f).
To insulate individual microelectrodes, a 1.5
μ
μ
m thick layer of Parylene C
was coated over the entire substrate (PDS 2010 Parylene Labcoter, Specialty
Coating Systems) (Fig. 6.3g). Parylene C is a well known biocompatible poly-
mer material [63, 64]. It is inert and is optically transparent. Using simple vapor
deposition, Parylene C is deposited easily and uniformly on any substrate. The sur-
face roughness is very low, similar to that of Teflon, and the surface is pinhole
free.
Sensor tip formation.
Microelectrode tips were first beveled (BV-10 Beveler,
Sutter Instrument Co.) at 45
◦
above horizontal (for better penetration) for 30 min
on a rotating plate under visual control through a microscope to remove Parylene
C and Ti/Au layers. The resulting structure with exposed Au, schematically shown
in Fig. 6.4(b), formed the solid-state ORP sensor. Gold gives more reliable mea-
surement of ORP than platinum for this application, as platinum may catalyze some
additional reactions at its surface [45, 65].
