8 Femtosecond Laser Direct Writing for 3D Microfluidic Biochip …
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8.2.3 Additive Processing
Additive processing as discussed herein primarily refers to two-photon polymerization (TPP) (Fig. 8.1c) [44–47], which is currently one of the primary approaches
to laser-based 3D manufacturing with nanometer-scale precision. Generally, TPP is
based on two-photon absorption in a photocurable resin in response to irradiation
by a fs laser, which only occurs in the central region of the focal volume at which
the laser intensity exceeds the TPP threshold. The width of the solidified region is
usually less than the focal spot size and so a fabrication resolution on the order of
100 nm can be achieved when focusing the laser beam with an objective lens having
a numerical aperture of approximately 1.4. This technique has been widely applied
to the manufacture of 3D functional microdevices for biochip applications, such as
micromixers [48, 49], microfilters [49–51], remotely controlled micromachines [52]
and cell counters [27]. In addition to TPP, the selective metallization of microchannels [53–56] and fs laser induced photoreduction [57–60] can be also considered as
additive processing methods. As an example, using the 3D capabilities of fs laser
induced modification, the deposition of thin metal films inside glass channels can be
performed in a spatially selective manner [56].
8.3 Fabrication of 3D Microfluidic Devices
Direct writing with a fs laser can prepare microfluidic devices with 3D configurations in many different transparent materials, such as glass [15–18] and polymers
[61]. Subtractive processes such as FLAE and LAFLD are typically employed for
the fabrication of glass-based devices and, to date, photosensitive Foturan glass
and fused silica have been used with FLAE with the greatest degree of success.
Foturan is a lithium-aluminosilicate glass doped with trace amounts of silver and
cerium ions, and has a relatively low glass transition temperature that allows the
fabrication of microfluidic structures with smooth surfaces, in addition to permitting the use of low laser power and higher etching rate. In contrast, fused silica
exhibits a wide range optical transmission down to UV ranges, as well as minimal
autofluorescence. FLAE processing of Foturan glass by laser-induced photochemical reactions can be described as follows. During fs laser irradiation, silver ions
in the focal volume are reduced due to the generation of free electrons by multiphoton absorption, thus precipitating silver atoms. As a result of successive thermal
treatment, these precipitated silver atoms fuse together to form silver nanoclusters
that act as nuclei for the growth of crystalline lithium metasilicate in the lasermodified regions. The crystallized regions exhibit an etching rate in HF up to 50
times that in the unirradiated areas [34, 62]. Thus, 3D hollow microstructures can be
created via the selective chemical etching of the laser-exposed regions using a diluted
HF solution. An additional thermal treatment after the etching smooths the etched
251
8.2.3 Additive Processing
Additive processing as discussed herein primarily refers to two-photon polymerization (TPP) (Fig. 8.1c) [44–47], which is currently one of the primary approaches
to laser-based 3D manufacturing with nanometer-scale precision. Generally, TPP is
based on two-photon absorption in a photocurable resin in response to irradiation
by a fs laser, which only occurs in the central region of the focal volume at which
the laser intensity exceeds the TPP threshold. The width of the solidified region is
usually less than the focal spot size and so a fabrication resolution on the order of
100 nm can be achieved when focusing the laser beam with an objective lens having
a numerical aperture of approximately 1.4. This technique has been widely applied
to the manufacture of 3D functional microdevices for biochip applications, such as
micromixers [48, 49], microfilters [49–51], remotely controlled micromachines [52]
and cell counters [27]. In addition to TPP, the selective metallization of microchannels [53–56] and fs laser induced photoreduction [57–60] can be also considered as
additive processing methods. As an example, using the 3D capabilities of fs laser
induced modification, the deposition of thin metal films inside glass channels can be
performed in a spatially selective manner [56].
8.3 Fabrication of 3D Microfluidic Devices
Direct writing with a fs laser can prepare microfluidic devices with 3D configurations in many different transparent materials, such as glass [15–18] and polymers
[61]. Subtractive processes such as FLAE and LAFLD are typically employed for
the fabrication of glass-based devices and, to date, photosensitive Foturan glass
and fused silica have been used with FLAE with the greatest degree of success.
Foturan is a lithium-aluminosilicate glass doped with trace amounts of silver and
cerium ions, and has a relatively low glass transition temperature that allows the
fabrication of microfluidic structures with smooth surfaces, in addition to permitting the use of low laser power and higher etching rate. In contrast, fused silica
exhibits a wide range optical transmission down to UV ranges, as well as minimal
autofluorescence. FLAE processing of Foturan glass by laser-induced photochemical reactions can be described as follows. During fs laser irradiation, silver ions
in the focal volume are reduced due to the generation of free electrons by multiphoton absorption, thus precipitating silver atoms. As a result of successive thermal
treatment, these precipitated silver atoms fuse together to form silver nanoclusters
that act as nuclei for the growth of crystalline lithium metasilicate in the lasermodified regions. The crystallized regions exhibit an etching rate in HF up to 50
times that in the unirradiated areas [34, 62]. Thus, 3D hollow microstructures can be
created via the selective chemical etching of the laser-exposed regions using a diluted
HF solution. An additional thermal treatment after the etching smooths the etched