silicon carbide wafer
Borosilicate glass Stainless steel
DUV laser Green laser
www.industrial-lasers.com MAY/JUNE 2017 Industrial Laser Solutions 17
10µM-DIAMETER DRILLED HOLES
YUJI IMAMIYA, SATORU AKAMA,
YOSHIHITO FUJITA, and HARUHIKO NIITANI
In recent years, a microfabrication method utilizing a short-pulse, short-wavelength laser has attracted atten- tion in connection with the growing demand for further miniaturization and increased quality of ultraprecision machining. Using a picosecond-pulse laser enables ablation with no thermal effects, achieving high-preci- sion fabrication with superior quality. Additionally, using
a short-wavelength laser can make the spot diameter of a laser
beam smaller, enabling finer fabrication.
Currently, popular short-pulse lasers for microfabrication include
green lasers (532nm), followed by ultraviolet (UV) lasers (355nm).
On the other hand, deep-ultraviolet (DUV) lasers at 266nm have
not been widely used because of difficulties in handling.
At Mitsubishi Heavy Industries
Machine Tool, we have devel-
oped a helical drilling system that
employs a DUV laser. With it, we
conducted hole-drilling tests in
which we achieved 10µm-diam-
eter hole drilling with an aspect
ratio of 10. Moreover, because
of the high photon energy of the
DUV laser, high-quality hole drilling was accomplished on a workpiece material on which precision
machining and fabrication based
on chemical actions are extremely
difficult. This occurs because of its
high degree of hardness and chemical stability.
DUV laser focusing system
In laser microfabrication, a laser beam is focused and irradi-
ated on the surface of the target workpiece, and the irradiated
part of the workpiece is removed by the photon energy. When
the focused laser spot diameter is smaller, the area removed by
laser irradiation also gets smaller, increasing the microfabrication
quality. There are multiple means to achieve a smaller focused
laser spot diameter, such as utilizing a short-wavelength laser,
enlarging the beam diameter before focusing, and shortening
the focal length.
However, when the wavelength of the laser light reaches the UV
range, the laser’s absorbance through glass materials increases.
Therefore, it is necessary to ease the strain on optical element
materials such as the lens. In other methods, the convergence angle
is large, which is likely to cause a gap in size of the hole between its
entrance and exit and to reduce the focal depth, limiting the shape
of the hole created by drilling.
A newly developed optical laser focusing system was adopted
for the DUV laser. The durability of the lens was ensured by testing
the durability against the DUV laser beam of the glass used for the
optical element beforehand, while selecting the optimal glass material and setting appropriate laser irradiation intensity. Furthermore,
by optimizing the focal length and lens form, the convergence angle
is minimized and a long focal depth is achieved, while the focused
laser spot diameter remains very small.
FIGURE 1. Fabrication depth of individual materials corresponding to the energy intensity of DUV
and green laser are shown, where normalized values are utilized for the vertical and horizontal
axes; all charts utilize a unified scale.