Microscale Testing and Characterization Techniques for Benchmarking Crystal. . .
107
nique is so promising is that the precision at which samples can be manufactured
is close to that of what is possible with an FIB, but laser machining is orders
of magnitude faster. The use of a femtosecond laser provides a much cleaner cut
than picosecond or nanosecond lasers, because vaporization is more complete and
redeposition much lower. The ablation of material through the use of femtosecond
laser is difficult to image experimentally, but molecular dynamics (MD) simulations
suggest that during femtosecond machining, material is vaporized more rapidly and
less heat is stored in the sample, where the former reduces particle redeposition and
the latter minimizes the heat-affected zone [79]. Moreover, unlike FIB and EDM,
femtosecond laser machining is material agnostic, can be applied to a broad range
of materials, and can be used in laboratory air [28].
However, the use of femtosecond laser machining is still under development, and
there are drawbacks. One of the most difficult things to control is the redeposition
of material that accompanies ablation. A plume of material is released from the
sample and tends to redeposit back on the sample or on surrounding surfaces. This
can result in redeposited material collecting on other samples or on laser optics. The
redeposition can be managed during machining by blowing air over the sample, but
this may not be possible for fragile materials or sample geometries. Another option
is to use more passes of the sample geometry at a higher speed, which can generate
less redeposition, as less material is ejected from the sample and therefore can more
easily be removed via vacuum during each pass. In an open-air laboratory setting,
environmental effects can also have a nontrivial effect on machining. Fluctuations
in temperature and humidity can affect the positioning of optics both internal and
external to the laser, which may require recalibration. For this reason, the laser
must be maintained within a stable climate to guarantee consistent performance.
Additionally, imaging during femtosecond laser machining can be difficult because
of the offset between the focal planes of the camera and laser. A second camera and
objective could be implemented to observe machining of the sample from the side
or at an angle, but it would not provide the best view of the machining process.
Finally, as with the FIB, there is a limit to the size of the sample that can be
machined using the femtosecond laser. Although ablation rates are significantly
higher than with the FIB, there is still a limit at which laser milling becomes
inefficient. Making trenches past a certain depth becomes impractical because there
is less potential for redeposited material to escape and because of the depth of
field of the objective lens. Femtosecond laser machining shows a lot of promise
for samples with thickness between 10–300 μm, with great speed and precision at a
length scale that is unreachable by other machining techniques, and works for many
classes of materials.
3.4 Comparison of Machining Techniques
FIB machining is viable for microtensile samples on the order of tens of microns,
laser machining for samples on the order of tens to a few hundreds of microns,
107
nique is so promising is that the precision at which samples can be manufactured
is close to that of what is possible with an FIB, but laser machining is orders
of magnitude faster. The use of a femtosecond laser provides a much cleaner cut
than picosecond or nanosecond lasers, because vaporization is more complete and
redeposition much lower. The ablation of material through the use of femtosecond
laser is difficult to image experimentally, but molecular dynamics (MD) simulations
suggest that during femtosecond machining, material is vaporized more rapidly and
less heat is stored in the sample, where the former reduces particle redeposition and
the latter minimizes the heat-affected zone [79]. Moreover, unlike FIB and EDM,
femtosecond laser machining is material agnostic, can be applied to a broad range
of materials, and can be used in laboratory air [28].
However, the use of femtosecond laser machining is still under development, and
there are drawbacks. One of the most difficult things to control is the redeposition
of material that accompanies ablation. A plume of material is released from the
sample and tends to redeposit back on the sample or on surrounding surfaces. This
can result in redeposited material collecting on other samples or on laser optics. The
redeposition can be managed during machining by blowing air over the sample, but
this may not be possible for fragile materials or sample geometries. Another option
is to use more passes of the sample geometry at a higher speed, which can generate
less redeposition, as less material is ejected from the sample and therefore can more
easily be removed via vacuum during each pass. In an open-air laboratory setting,
environmental effects can also have a nontrivial effect on machining. Fluctuations
in temperature and humidity can affect the positioning of optics both internal and
external to the laser, which may require recalibration. For this reason, the laser
must be maintained within a stable climate to guarantee consistent performance.
Additionally, imaging during femtosecond laser machining can be difficult because
of the offset between the focal planes of the camera and laser. A second camera and
objective could be implemented to observe machining of the sample from the side
or at an angle, but it would not provide the best view of the machining process.
Finally, as with the FIB, there is a limit to the size of the sample that can be
machined using the femtosecond laser. Although ablation rates are significantly
higher than with the FIB, there is still a limit at which laser milling becomes
inefficient. Making trenches past a certain depth becomes impractical because there
is less potential for redeposited material to escape and because of the depth of
field of the objective lens. Femtosecond laser machining shows a lot of promise
for samples with thickness between 10–300 μm, with great speed and precision at a
length scale that is unreachable by other machining techniques, and works for many
classes of materials.
3.4 Comparison of Machining Techniques
FIB machining is viable for microtensile samples on the order of tens of microns,
laser machining for samples on the order of tens to a few hundreds of microns,