250
J. Xu et al.
8.2.2 Subtractive Processing
The volumetric removal of laser-exposed regions from transparent materials to form
hollow microstructures within bulk glass or polymer specimens via fs laser direct
writing is defined as subtractive processing (Fig. 8.1b). Similar to the WG fabrication process, subtractive processing takes advantage of the benefits of laser direct
writing to eliminate the need for multiple stacking and bonding during 3D fabrication of microfluidic devices. Currently, two general types of subtractive methods are
employed for the creation of 3D microfluidic devices. One is referred to as fs laserassisted chemical etching (FLAE) or fs laser irradiation followed by chemical etching
(FLICE) [32–35]. This method is actually a two-step process (Fig. 8.1b). The initial
step involves using focused fs laser irradiation to induce localized modification only
over the focal volume within a glass substrate. During this step, direct writing with
the focused laser beam can generate modified regions extending from the interior of
the sample to the surface in a spatially selective manner. The second step is selective removal of the laser-modified regions by successive chemical etching in diluted
hydrofluoric acid (HF) solutions, thus forming hollow 3D structures within the glass.
Both photosensitive Foturan glass and fused silica are typically used in conjunction
with this approach. The other subtractive method is known as liquid-assisted fs laser
3D drilling (LAFLD), in which a glass substrate immersed in a liquid, such as water, is
ablated from the rear surface to the interior by a focused fs laser beam to create hollow
3D microstructures. In this process, the liquid plays an important role by efficiently
removing debris from the laser-ablated regions during drilling, potentially producing
long channels with complicated 3D structures [36–40]. In contrast to FLAE, this
technique can be applied to any material that is transparent to the laser beam. This
method can also be used to produce very narrow channels in glass substrates. As an
example, channels with diameters of only approximately 700 nm and with arbitrary
geometries have been fabricated in fused silica by carefully adjusting the laser pulse
energy to near the ablation threshold [40]. However, as the drilling length approaches
several hundred micrometers, the debris generated by the ablation process tends to
clog the channel, and so in practice, the channel length is limited to approximately
1 cm. The fs laser direct writing ablation of porous glass immersed in water followed
by a post-annealing step has been demonstrated as a means of overcoming this limitation [41–43]. In this technique, fs direct writing ablation of porous glass in distilled
water is initially performed. During this process, nanopores in the glass form a 3D
connective network that allows liquid to flow inside the glass, efficiently removing
debris from the ablated regions. Following laser ablation, the sample is annealed to
consolidate it into a compact glass. This step completely eliminates the nanopores
while retaining the 3D channels inside the glass.
J. Xu et al.
8.2.2 Subtractive Processing
The volumetric removal of laser-exposed regions from transparent materials to form
hollow microstructures within bulk glass or polymer specimens via fs laser direct
writing is defined as subtractive processing (Fig. 8.1b). Similar to the WG fabrication process, subtractive processing takes advantage of the benefits of laser direct
writing to eliminate the need for multiple stacking and bonding during 3D fabrication of microfluidic devices. Currently, two general types of subtractive methods are
employed for the creation of 3D microfluidic devices. One is referred to as fs laserassisted chemical etching (FLAE) or fs laser irradiation followed by chemical etching
(FLICE) [32–35]. This method is actually a two-step process (Fig. 8.1b). The initial
step involves using focused fs laser irradiation to induce localized modification only
over the focal volume within a glass substrate. During this step, direct writing with
the focused laser beam can generate modified regions extending from the interior of
the sample to the surface in a spatially selective manner. The second step is selective removal of the laser-modified regions by successive chemical etching in diluted
hydrofluoric acid (HF) solutions, thus forming hollow 3D structures within the glass.
Both photosensitive Foturan glass and fused silica are typically used in conjunction
with this approach. The other subtractive method is known as liquid-assisted fs laser
3D drilling (LAFLD), in which a glass substrate immersed in a liquid, such as water, is
ablated from the rear surface to the interior by a focused fs laser beam to create hollow
3D microstructures. In this process, the liquid plays an important role by efficiently
removing debris from the laser-ablated regions during drilling, potentially producing
long channels with complicated 3D structures [36–40]. In contrast to FLAE, this
technique can be applied to any material that is transparent to the laser beam. This
method can also be used to produce very narrow channels in glass substrates. As an
example, channels with diameters of only approximately 700 nm and with arbitrary
geometries have been fabricated in fused silica by carefully adjusting the laser pulse
energy to near the ablation threshold [40]. However, as the drilling length approaches
several hundred micrometers, the debris generated by the ablation process tends to
clog the channel, and so in practice, the channel length is limited to approximately
1 cm. The fs laser direct writing ablation of porous glass immersed in water followed
by a post-annealing step has been demonstrated as a means of overcoming this limitation [41–43]. In this technique, fs direct writing ablation of porous glass in distilled
water is initially performed. During this process, nanopores in the glass form a 3D
connective network that allows liquid to flow inside the glass, efficiently removing
debris from the ablated regions. Following laser ablation, the sample is annealed to
consolidate it into a compact glass. This step completely eliminates the nanopores
while retaining the 3D channels inside the glass.