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Introduction
Chapter
1: Introduction to Machining with Lasers
Chapter
2: Time Scales
Chapter
3: Machining with Long Pulses
Chapter
4: Nanosecond Machined Samples
Chapter
5: Machining with Ultrafast Laser Pulses
Chapter
6: Femtosecond Machined Samples
Chapter
7: Contamination, Debris, Etc.
Chapter
8: Heat Affected Zone (HAZ)
Chapter
9: Machining Accuracy
Chapter
10: Sub-micron Features
Chapter
11: Machining Inside Bulk Materials
Chapter
12: Introduction to Waveguides
Chapter
13: Active Waveguides
Chapter
14: Shortcomings of Femtosecond Lasers
Chapter
15: Materials We've Machined
Chapter
16: Conclusion
Appendices:
References
and Glossary |
Sub-Micron
Features
"...the
unique combination of multiphoton absorption and saturated
avalanche ionization provided by ultrafast laser pulses that
makes it possible to machine materials on dimensions much
smaller than 1 micron."
Let's
now use this high-reproducibility concept to create sub-micron
features in materials.
If your
objective is to create the smallest feature you can possibly
make by machining with light, you must focus that light on
the smallest spot you can possibly make. The size of the spot
is determined by several factors, but for our purposes we
limit this discussion to making the statement that the smallest
spot that you can get is about the same as the wavelength
of the light you are using to make it. Thus, if the wavelength
of light is about 0.5 microns, then the smallest spot you
can create is about 0.5 microns. However, as noted above,
while both ultrafast pulse lasers and long pulse lasers can
both operate at wavelengths of 0.5 microns, the long pulse
laser is not capable of creating a machined feature that is
much less than about 10 microns because of heat diffusion
into the surrounding material.
Referring
to Figure 10-1, below, we can see how an ultrafast laser pulse
can create features substantially below that of the central
wavelength of the laser pulse itself. First, we focus the
ultrafast laser on a spot with a profile which has a peak
intensity in the center of the beam and smoothly decreases
radially outward from the center (a "Gaussian" spot).
Remember we said earlier that machining with ultrafast laser
pulses is a threshold process? If we adjust the intensity
of the laser spot on the surface of the material (which is
a very easy thing to do) so that just the peak of the beam
is above threshold, then we will remove material only in that
very limited area! That very limited area can be as little
as one-tenth the size of the spot itself.
| Figure
10-1: Creating Sub-Micron Features |
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| Click
on the above image to view a series of three animations
of long-pulse avalanche ionization and a side-by-side
comparison of the results.
NOTE:
You must have the Macromedia™ Flash Player to
view the animations. You
may "right-click" on the animation to display
a pop-up menu for controlling the animation or use the
Pause and Play buttons in the animation itself.
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Now
imagine that we generate ultrafast laser pulses that have
a central wavelength of 0.2 microns. Using ultrafast pulses
of light we should be able to create features as small as
0.02 microns, or 20 nanometers.
It
is important to note that it is the unique combination of
multiphoton absorption and saturated avalanche ionization
provided by ultrafast laser pulses that makes it possible
to machine materials on dimensions much smaller than 1 micron.
Without this unique characteristic of machining with ultrafast
laser pulses and the highly deterministic nature of the
process, it would not be possible to achieve these results
without a heat-affected zone. More specifically, it is not
possible to get comparable highly repeatable sub-micron
machining with long pulse laser systems.
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