Fabrication of Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 127

mechanism for this type occurs in the optical iber and energy is not
allowed to escape the iber to be recaptured. Therefore, this optical
iber not only acts as a signal transmission line, but also as a sensing
component. To improve the sensitivity of optical iber sensors for
use in iber optic biosensors, the iber must be processed a second
time by stripping a portion of cladding so the light-sensitive core
layer can be in contact with the sample solution. The area where
the cladding is removed is called an optical iber window, as shown
in Fig. 6.16. The iber has a window gap structure and the depth
of this gap can be controlled. A shallow depth would only require
the removal of the jacket layer, while a deeper depth allows the
iber core to be exposed. This section describes three methods to
fabricate optical iber windows: etching, polishing, and high energy
ultrashort pulse laser processing.
Figure 6.16 Schematic diagram of optical iber window.
6.4.1 EtchingMethod
The core of glass iber is fused silica. Fused silica is a noncrystalline
(glass) form of silicon dioxide (quartz, sand). To generate good
lat etching results, the BOE (Buffered Oxide Etch), rather than HF
(Hydroluoric Acid) is used to etch the optical iber [39] because the
BOE has lower etching rate. Besides, BOE etching is only effective
for SiO 2 , and the plastic coating covering on the D-shaped window
optical iber cannot be removed. Therefore, the etching length can be
determined by controlling the length of the bare iber, and the etching
depth can be determined by controlling the etching time. However,
the etching rate is dependent on the temperature and concentration
of the BOE. Thus, the etching temperature must be controlled if
accurate etching depth is required. For single mode optical ibers,
an etching depth of at least 50 μm must be obtained to let the
evanescent wave through. Before etching, part of plastic coating
must be stripped, followed by BOE etching for a properly controlled
etching time. The maximum etching time can be measured as the