a) b) c) d)
18 Industrial Laser Solutions MAY/JUNE 2017
Experimental ablation rates
Ablation rates of a DUV laser beam and a green laser beam (515nm)
with various different materials were made to compare and evaluate their characteristics (FIGURE 1). A single-crystal silicon carbide
(SiC) wafer, borosilicate glass, and stainless steel were prepared
as workpieces. These materials were individually irradiated with
a focused DUV or green laser beam by one pulse only (FIGURE 2).
Subsequently, the maximum depth of the hole made by irradiation was measured. Ablation rates for each material were obtained
by conducting irradiation and measurement with different levels of
pulse energy. The vertical axis of the chart in FIGURE 1 indicates the
fabrication depth, while the horizontal axis represents the energy
intensity at the central part of the irradiated laser beam.
When ablation rates were compared between the DUV and green
laser beams, those of the DUV laser beam were higher in every
material. The DUV laser beam also achieved a higher ablation rate
for the SiC material, which has a large bandgap.
Generally, when inducing ablation by a short-pulse
laser, the photon energy of the laser beam must be
higher than the bandgap of the workpiece material.
Since photon energy is in inverse proportion to
laser wavelength, the photon energy of the DUV
laser beam is about two times higher when compared to the green laser, and exceeds the bandgap
of the SiC material. Therefore, the ablation phenomena on the SiC material can be fully induced by the
DUV laser beam. With the DUV laser, a high ablation rate for the borosilicate glass was obtained.
Accordingly, it is assumed that the photon energy
of the DUV laser beam contributed largely to this
high ablation rate.
The above results verified the superiority of the
DUV laser to the green laser beam, and also made
it possible to estimate the optimum pulse number
needed to obtain the fabrication depth required for
individual materials. Thus, higher drilling quality and
optimized drilling rates are obtained.
Helical drilling features
Percussion drilling, widely used for micro-drilling, is
a method to drill holes by irradiating a pulsed laser
beam in a fixed place without allowing the beam to move or rotate.
Therefore, since the laser spot profile is directly transcribed on
workpieces, it is difficult to drill holes with high accuracy.
Alternately, a helical drilling method has been used in the
hole-drilling test described previously. Helical drilling is carried
out while precisely rotating the path of the laser beam at a high
speed, which enables drilling with high roundness and quality. In
addition, an optical head we developed allows the control of the
incidence angle as well as the rotation diameter of the laser beam,
which can achieve any hole diameter and cross-sectional shape.
Using the drilling characteristics obtained from the one pulse irradiation test described above, laser irradiation conditions were determined, based on which hole-drilling was performed on a stainless
steel (SUS304, thickness: 0.2mm) and single-crystal SiC wafer
FIGURE 3. Examples of hole-drilling and micro-grooving include a straight hole (a), tapered hole (b), reverse tapered hole (c), and
perpendicular incidence hole (d).
FIGURE 2. A single-crystal SiC wafer (a), borosilicate glasses (b and c), and
stainless steel (d) were individually irradiated with a focused DUV or green laser
beam by one pulse only.