Environmental Engineering Reference
In-Depth Information
Fig. 6.4
Fabrication of sharp MEA tips using meniscus etching. Meniscus height falls dynamically
to form a taper with geometry dependant on the initial meniscus height [55, 57]
to 400ºC, followed by cooling to ambient temperature. After annealing, the diced
wafer was cleaned with acetone, methanol, and DI water for 5 min each.
Etching.
The glass probes were sharpened into needle-type microelectrodes using
the chemical sharpening process, termed
meniscus etching
, which uses surface ten-
sion force at the glass-etchant interface. The process is schematically illustrated in
Fig. 6.4. Glass probes are immersed into HF-based etchant with an organic layer
on top, typically paraffin oil or vegetable oil, to modify contact angle at the glass-
etchant interface. The etchant wets the surface of the probes and gradually reduces
their dimensions. The surface tension force at the glass-etchant interface reduces
with the diminishing dimensions, forcing the height of the meniscus to decrease
with time until the etching front reaches the center of the probe, and forming a
sharp tip. The process is self-terminating. The balance of two opposing forces, the
surface tension and the weight of the etchant, determines the final tip geometry. This
process has been used previously in fabrication of optic probes [61] as well as redox
potential sensors [55, 56] and dissolved oxygen sensors [57, 58].
The probes were first etched in the microelectrode etchant solution for 20 min
with agitation to smooth the diced surface and reduce the probe dimensions to
~95
m in width. The microelectrode etchant solution was prepared by mixing HF,
HNO
3
, and H
2
O in a 10:7:33 (v/v/v) ratio. The lateral and transverse etching rates
were ~2.5 and ~2.3
μ
m/min, respectively. The ~10% increase in lateral etch rate was
due to the greater roughness of the unpolished vertical sidewalls of the glass beams
formed by dicing, offering a greater surface area for the etching process [61]. At the
end of this etch step, the cross-section of individual probes was 85
μ
×
90
μ
m due to
the starting probe dimensions of 175
μ
m in thickness and 190
μ
m in width [55].
