02-02-2013, 11:44 AM
Three-dimensional micromachining of glass using femtosecond laser for lab-on-a-chip device manufacture
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ABSTRACT
Three-dimensional (3D) micromachining of photosensitive
glass is demonstrated by photochemical reaction using
femtosecond (fs) laser for lab-on-a-chip application. True 3D
hollow microstructures embedded in the glass are fabricated by
fs laser direct writing followed by heat treatment and successive
wet etching. The modification mechanism of the photosensitive
glass by the fs laser and advantage of this process are discussed.
Various microcomponents for the lab-on-a-chip devices
such as microfluidics, microvalves, microoptics, microlasers,
etc. are fabricated by using this technique and their performance
is examined.
Introduction
Conception of lab-on-a-chip revolutionized various
fields, in particular, chemical and biological fields for
human gene and protein analysis, new drug development,
medical inspection, synthesis of new materials, environmental
monitoring, and so on. A micro-total analysis system
(μ-TAS) that is one of the lab-on-a-chip devices realized fielddeployable
chemical analysis with high efficiency, high accuracy,
and high performance [1]. The μ-TAS is composed of
microfluidic components, such as microcells, and microchannels,
for the infusion of reagents and the storage of reactants,
micromechanical components, such as microvalves,
micromixers, and micropumps, for the control of reagent
flow and reaction, and microoptical components, such as micromirrors,
microlenses, microgratings, waveguides and microoptical
sensors, for the in situ analysis of reactants. These
microcomponents are integrated in a single small chip with
a size of the order of mm2 ∼ tens cm2, and then the infusion,
mixture, and reaction of reagents and the analysis of reactants
are successively conducted.
Experimental
The photosensitive glass used in this work was
commercially available Foturan glass from Schott Glass Corporation.
The experiments were carried out using a commercial
fs lasermicromachining workstation at HOYACANDEO
OPTRONICS. Figure 1 shows a schematic illustration of the
workstation. The laser wavelength, pulse width and repetition
rate were 775 nm, 140±5 fs and 1 kHz, respectively. To
guarantee a high beam quality, a 6mm diameter of the output
laser beam was reduced to 3mm by an aperture in front
of the focusing system. The focusing system was a 20× microscope
objective with a numerical aperture (NA) of 0.46.
Samples under fabrication were translated by a PC controlled
x-y-z stage with a resolution of 0.5 μm. The fabrication process
was displayed on the PC monitor by a charge-coupled
device (CCD). Three-dimensional latent images were written
inside the photosensitive glass by the fs laser direct writing.
Determination of process parameters
The first step of this work is to check a critical
dose of the photoreaction, because the high precision is only
achievable at the condition just above this critical dose. The
critical dose Dc is defined as the lowest dose necessary for
achieving selective etching of the photosensitive glass at the
exposed region. The model proposed by Fuqua et al. [9] assumes
that to make the photosensitive glass selectively soluble
in a HF solution, the density of nuclei should reach a critical
range to form an interconnected network of crystallites,
which is defined as the critical density c. One can expect that
the density of nucleation site should be proportional to the
dose.
Advantages of the present technique
Such a higher order of the six-photon process for
the present technique is beneficial to the internal fabrication
with high spatial resolution, because the absorption and reaction
region can be confined to a region inside the glass much
smaller than the laser spot size if the pulse energy is controlled
very close to the critical dose,which is one of themost important
parameters for precision microfabrication.
Another important feature of this process is pure photochemical
reaction based on reducing reaction using free
electrons generated by the inter-band excitation. Threedimensional
hollow microchannels can be fabricated even
inside fused silica by fs laser irradiation followed by post
chemical etching with a HF acid solution [4]. However, we
revealed that when an objective lens with NA of 0.46 was
used this technique needed laser power of∼ 10mWand scanning
speed of∼10 μm/s at1 kHzrepetition rate of laser pulse
maybe due to photophysical or photothermal reaction. On
Fabrication of micromechanical components
In μ-TAS,micromechanical components like a microvalve,
a micromixer, and a micropump must be integrated
into the glass chip with the microfluidic components to control
flow and reaction of chemical reagents. The micromechanical
components should be able to move in the microfluidics
freely. The freely movable microcomponents can also
be fabricated by the present technique. Figure 9 shows the
experimental scheme for fabricating the freely movable microplate
inside the photosensitive glass. First, dark color regions
inside the glass are exposed with a scanning focused
fs laser beam, and then baked to form the modified regions
(Fig. 9a). To fabricate this structure, large volumes of the glass
must be exposed with the fs laser. To shorten the writing time,
the laser fluence was increased to 170 mJ/cm2, so that the
scanning speed could be increased to2mm/s. Afterwet etching
in a HF solution, the dark color regions in Fig. 9a can be
completely removed and a glass microplate is left in a hollowchamber
embedded in the glass. This glassmicroplate can
move inside the hollow chamber freely (Fig. 9b).
Conclusions
We have demonstrated the feasibility of true 3D
micromachining in the photosensitive glass by use of a fs
laser operated at near-IR wavelength for lab-on-a-chip applications.
Photoreaction mechanism study revealed that free electrons were generated in the glass for precipitation of Ag
atoms by inter-band excitation through defect levels with
a six-photon process. The photochemical reaction using the
six-photon process would perform high throughput processing
with high spatial resolution. Based on the examined process
parameters and photoreaction mechanism, 3D microfluidic
structures were fabricated inside the glass.