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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.
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 (see publications).
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