PLD Intelligent Window
An Intelligent Window
The PVD Products Intelligent Window minimizes build-up on the optical port of a
laser. This build-up decreases not only the laser efficiency, but also the life
of the optical window. Our Intelligent Windows counteract these negative effects,
and are customized to fit your research applications.
Fluence
It is well known that one of the most important parameters in the PLD process is
the amount of laser energy per unit area, or fluence, that impinges on the ablation
target. The fluence strongly affects such physical properties as the removal rate
of target material as well as the internal and kinetic energy of the ablated species.
Changes in the amount of material removed per pulse clearly will affect the deposition
rate of the growing thin film. Furthermore, changes in the energetics of the ablation
plume will directly affect the crystallinity of the deposited film, the intrinsic
stress and electronic or optical properties of the deposited film. Thus, accurately
setting the fluence and keeping the fluence constant for the entire deposition process,
and from run to run, is key to consistent material properties and film growth rates.
The problem: ablated material build-up
Material scattering
While the ablation plume in the PLD process is forward directed, it is an obvious
fact that some of the ablated material lands on the optical port where the laser
beam enters the deposition chamber. A small fraction of the ablated material lands
on this window with every laser pulse.
Laser inefficiency
Depending on the chamber design, background gas pressure, the laser beam's angle
of incidence with respect to the ablation target, the film build-up on the window
can become a significant problem. As material builds up on the surface of the laser
optic, it starts to absorb a fraction of the incident laser beam's energy. With
more build-up, the amount of energy absorbed can become significant enough to reduce
the On-Target Laser Fluence (OTLF).
Window Ablation
Morever, this absorbed energy may be directed towards ablating material off of the
window surface. The removal of material from the window surface in this fashion
can be even more deleterious to the deposition process as it degrades the optical
quality of the window, and thus the shape of the focused spot on the target.
Example
A damaged window optic
This photograph is an example of film built up on a fused silica window optic obtained
from one of our customers. You can clearly see where the rectangular shaped laser
beam was impinging on the window.
The pulsed excimer laser beam was in fact constantly removing the film that was
freshly deposited by the previous laser pulse. However, a small fraction of the
fused silica material comprising the window was also being removed by the deposited
film with each laser pulse. This, in turn, damaged the optic significantly by degrading
its surface quality.
The solution: Intelligent Windows
PVD Products has developed a solution to this problem that we call the Intelligent
Window (IW). This product has two different, but equally important features:
- Keeps the optical beam path clean for an extended time period
- Allows the user to monitor and properly adjust the laser energy that has traversed
the complete optical train. Measuring the energy that has gone through the complete
optical train just before it hits the target is the best way to set up the deposition
process.
Intelligent Window design
The Intelligent Window includes a large, internal UV grade fused-silica disc housed
inside a pair of vacuum flanges. This disc is connected to a rotary feedthrough
and can be easily rotated when desired. Behind the disc is a high quality, AR coated
optic that makes the atmosphere/vacuum seal using a VitonTM
O-ring. Ablated target material is deposited onto only a small section of the internal
disc that is defined by an aperture located just in front of the disc.
The IW comes in a variety of sizes as can be seen mounted in the photographs of
our PLD 3000 and PLD 5000
systems.
Longevity
The longevity of the internal disc provides from 20 to 50 times longer life than
a standard optical window. Process gas is also bled into the Window area to keep
a steady over pressure of gas near the fused silica disc to further increase its
longevity.
When the section of the silica disc has become coated or degraded, the disc can
simply be rotated to expose a new, fresh optical surface. The time between rotations
will strongly depend on the user chamber design as well as process parameters.
Beam splitter
Furthermore, located inside the vacuum system the IW has a reflecting beam splitter
that is mounted to a linear actuator. When inserted into the beam path, excimer
radiation that hits the beam splitter is reflected out of the chamber through a
side port and can be measured directly by a flange mounted pyroelectric joule meter
probe. The beam splitter can be inserted into the optical beam path whenever desired.
For instance, the splitter could be inserted into the beam path during preparation
for the deposition run to accurately determine the energy, or fluence, which will
hit the ablation target. The beam splitter can also be inserted into the beam path
in the middle of a run to see if it has become necessary to rotate the internal
window and thus keep the variations in laser fluence to a minimum. When the beam
splitter is extracted from the beam path, a shield protects it so that it does not
become coated by ablated material.
Customization
The windows can be designed and fabricated with custom flanges to mount to your
existing chamber's optical port, with or without the beam splitter/Joule meter option.
The IW comes standard with a high quality optic coated for either 248-nm (KrF),
or 193-nm (ArF) with alternate wavelengths available on request.
Windows can also be designed to be UHV compatible and operate well below 10-8
Torr if desired. They also include either a ¼" SwagelockTM
or a ¼" Male VCRTM fitting for process gas introduction
to the chamber close to the window surface.
Other options include motorized disc rotation, electropneumatic linear actuators,
as well as software to monitor the laser beam energy, rotate the disc position,
and monitor and control the excimer laser beam output energy.
A spare fused silica "sacrificial" disc is also provided. When the perimeter of
the first disc is fully coated, it can be easily replaced with the spare. The first
disc can then be cleaned with polishing compound.
For more details or to obtain a quote about our Intelligent Windows please
contact us.