MNS Synova vFinal DR 27/6/08 10:28 Page 15
MICRONANOSYSTEMS
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pumped IR lasers with pulse widths of 0.4 to several further benefits compared to conventional,
200µs and repetition rates up to 2 kHz, to ‘dry’ laser cutting, such as extended working
multimode Q-switched diode pumped lasers distance and perfectly parallel kerfs walls, as there
operating at 1064, 532, or 355 nm, with pulse is no divergence of the beam. The heat affected
widths of 90 to 400 ns, also with repetition rates zone (HAZ) problem, which is inherent with
up to 50 kHz. The only constraint on the laser conventional lasers, is also non existent thanks to
wavelength is that it must be compatible with the the water jet, which cools the material between
water transmission spectrum. Power levels up to laser pulses. Contamination is eliminated as well, Fig. 2. Comparison of
200W are available using dual laser systems. as the water jet expels the molten material more diamond blade saw
The laser beam is coupled to the optical head, efficiently from the kerf than the assist gas used (left) and LMJ dicing
where lenses focus the light through a quartz with conventional laser cutting. In addition, the of hexagonal dies
window into a chamber filled with water under low thin film of water maintained on the surface of the (right)
pressure, into the water jet as it exits the nozzle. work piece during cutting prevents any deposition
From this point on, the laser beam is guided along of particles on the work piece material surfaces.
the cylindrical jet by means of the total internal
reflection at the at the air/water interface, due to The benefits
their differences in refractive index. When it An example of the boost in yield that can be
reaches the work piece, the laser beam ablates the achieved is the dicing of hexagonal dies, as shown
material by melting and vaporisation. in Fig. 2. On the left hand side the limitations of
The diamond or sapphire nozzles have aperture using a conventional diamond blade saw for dicing
diameters varying between 25 and 100µm. New,
smaller nozzles are under development, allowing
kerf widths of only 18 µm. Depending on the
nozzle diameter, the pressure of the pure de-ionised
water, supplied from an external pump, ranges
using a water jet offers several further
from 50 to 500 bar. However, the mechanical
benefits compared to conventional,
forces applied by the water jet are negligible (less
than 0.1 N).
‘dry’ laser cutting, such as extended
In comparison, the assist gas jet used in
working distance and perfectly
conventional laser cutting applies a force of around
1N. Water consumption is very low, averaging only
parallel kerfs walls, as there is no
about 1.5 l/hour, making the process very
environmentally friendly.
divergence of the beam
Like the other laser based technologies, the
water jet guided laser features omni directional
cutting. However, the process speed is higher with
thin materials. For example, a cutting speed of up are readily evident, in which large areas of
to 300 mm/s can be achieved on 50 µm thick material are wasted. In comparison, the LMJ due
silicon. Furthermore, using a water jet offers to its omni directional capabilities is able to dice
leaving minimal wastage. The result is an
astounding 33% increase in die yield from each
wafer. Added to this is the benefit of improved
fracture strength, resulting from not having to halt
the cutting at each corner, but cutting a small
radius, which allows the laser beam to continue
cutting uninterrupted, albeit momentarily at a
slightly slower speed.
Fig. 3. shows a 350 µm thick wafer after dicing
has been carried out with a 50 µm nozzle, which
clearly shows that the die yield has been maximised
from the available surface area and wastage of
material has been reduced to the absolute
minimum. The actual kerf widths are ~45 µm.
Being able to cut narrow kerfs in the material
also means providing more real estate area on the
Fig. 3. View of wafer after hexagonal chip wafer, increasing yield and resulting in less
dicing with the LMJ wastage. The LMJ is able to make extremely small
July 2008 www.micronanosystems.info
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