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Laser Micromachining and Laser
Ablative Cleaning
Laser Micro- and Meso-Machinig
Laser micro- and meso-machining (meso refers
to a few hundred microns to a few millimeters) are of interest especially
for high-strength materials where mechanical methods are not strong enough
while etching-based methods are too slow. When the aspect ratio (depth vs.
width) of the features to be machined is high, laser also provides an edge.
Most lasers used for micro- and meso-machining are Q-switched YAG lasers
with nanosecond pulse width, especially its third harmonic, that is, 355 nm
wavelength (UV), is in widespread use. Ultra-short pulsed lasers with pico-
and femto-second pulse have shown great promise in material processing but
are not in widespread use.
UV laser micro- and meso-machining of metallic
materials has been used in microelectronic, aircraft engine, and other
industries. Using the Q-Switch techniques, UV lasers (Excimer lasers and
especially frequency-tripled Nd:YAG lasers) offer short pulse duration and
high peak power, which limits heat affected zone and makes the material
removal process dominated by ablation. While material removal mechanism for
polymers by such UV lasers have been well understood, more detailed modeling
work is needed for metals to predict optimal process conditions for ablation
dominance (in metals, melting cannot be entirely avoided but can be
minimized and thus better finished) and to limit heat affected zone. Models
may use enthalpy method to track the solid/liquid interface, Stefan and
kinetic boundary conditions be applied at the liquid-vapor interface,
property discontinuity across the Knudsen layer be considered, and relevant
experimental work be carried out to validate the modeling effort and assist
optimization. |
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Laser Ablative Cleaning for Microelectronics
Industry and Art Restoration
One of the mechanisms for laser cleaning is
through ablation, which is similar to ablative laser machining. Laser
cleaning has potential applications ranging from oxide removal from copper
substrate and nano-particle removal from silicon wafers in microelectronics
industry to crust removal from marble or limestone in art restoration. A
number of fundamental issues are being addressed, including effect of laser
pulsing characteristics on oxide layer removal, effect of laser fluence on
the discoloration of marble, and mechanism and prediction of laser wet
cleaning of marble encrustation. |
