Transcript Sono-Tek Corporaton

ISCST 2012
Advantages of TCO via Ultrasonic Spray Under
Atmospheric Conditions
Authors:
Robb W Engle, Director of Technical Services, Sono-Tek Corporation, Milton, NY
Morgan Dart, Engineering Assistant, Materials Science, Sono-Tek Corporation
Advantages
of TCOat
viathe
Ultrasonic
Spray Under Atmospheric
Conditions
Presented
16th International
Coating Science
&
Technology Symposium, September 11, 2012, Atlanta, GA
1
Define TCO
• TCO, or Transparent Conductive Oxide layers are
typically metal oxides, often doped, that are used on the
optical surfaces of PV panels, glass, and interactive
consumer devices.
• TCO is measured by how
well it conducts current versus
how little it blocks light (photon)
transmittance.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
2
History of TCO Deposition
• Spray Pyrolysis was first
used in development of
TCO layer technology.
This was difficult at the
time, to grow into
manufacturing processes.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
3
Sputtering
• In sputtering, a “target” containing the
metallic particles is bombarded from
the back with ions. Collisions with the
target of the correct potential cause atoms
of the metal to be freed from the opposite
side. These “freed” atoms, under vacuum,
coat the surfaces available to them,
including the substrate.
• Capitol equipment and operating expenditures are quite high
in that:
– the process typically runs in the fractions of millitorrs of vacuum,
– the electricity used in the process is quite high, including serious
cooling challenges,
– and finally the transfer efficiency of the process is often low,
typically below 25%.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
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Spray Pyrolysis
• From the Greek for “fire” and “separating”, this
processes uses high heat to catalyze a reaction.
• Many recognize four different styles of spray
pyrolysis:
–
–
–
–
carrier removal
contact change
plume manipulation
plume and contact change (in order from coolest to hottest).
• Non uniform droplet sizes wreak havoc on the
consistency and reliability of spray pyrolysis.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
5
Ultrasonic Nozzle Design
• Ultrasonic nozzles create high frequency mechanical vibrations
by stimulating piezoelectric transducers with high frequency
oscillating electrical energy. When liquid is fed through the
nozzle, it is atomized at the nozzle tip and emerges as a fine
mist of mathematically defined micron-sized droplets.
Atomized liquid is generated
As a soft, low-velocity spray
Resulting in minimal overspray
Active Electrode
Titanium Rear Horn
Ground Electrode
Rear Housing
Front Housing
Ground Lug
O-Ring Seal
Liquid Feed Channel
O-Ring Seal
Atomizing Surface
Titanium Amplifying
Section (Front Horn)
Piezoelectric
Transducers
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
O-Ring Seal
Electrical Connector
to the Broadband
Ultrasonic Generator
6
Define Ultrasonic Atomization
• When liquid is added to a resonating nozzle, waves are formed
on the atomizing surface perpendicular to the axis of vibration
underneath.
• Increasing the power of electrical energy causes the wave
peaks to get so high that droplets fall off the tips of the wave.
These droplets have a mathematically definable size.
• Nozzle tip vibration, wave formation and ultrasonic atomization:
Droplets forming on nozzle tip
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
7
Uniform Drop Sizes
• Drop size is governed by the
frequency at which the nozzle
vibrates, and by the surface
tension and density of the
liquid being atomized.
Frequency is the predominant
factor. Ultrasonic nozzles have
a tight drop distribution curve
compared to pressure nozzles,
contributing to a much higher
uniformity of spray.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
Drop distribution comparison
= 60 kHz Sono-Tek Ultrasonic Nozzle
= Pressure Nozzle
8
Process Challenges
• High Temperature
• Chemical Compatibility
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
9
High Temperature
• The temperatures required for methods 2,3 and 4
of spray pyrolysis are quite hot, often ranging from
150 to 1000 degrees C on the substrate.
• Temperatures this hot require considerable
engineering to ensure capitol equipment isn’t
damaged. Ultrasonic atomization equipment must
be cooled to surface this environment, as must
many other components such as motors and
solenoid coils.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
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Chemical Compatibility
• Much more challenging than the temperature
aspect is the chemistry used to dissolve metals.
• At a minimum, acids such as Nitric or Hydrochloric
are commonly used.
• More common are processes using Fluorine
doped oxide layers.
• HF represents one of the
most difficult materials
in engineering, even more
so when it is heated.
Corrosive resistant ultrasonic nozzle
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
11
Advantages of Ultrasonic Process
• Various tests have been documented
and published demonstrating the
feasibility of atmospheric TCO layers
(not under vacuum).
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
12
Uniform Droplet Size
• The first key advantage to the
ultrasonic atomization process
is the uniform size of the droplets
produced. As they are produced
from a mathematically defined
waveform (fig.1), the droplets
are significantly uniform in size.
• These similarly sized droplets all
require the same amount of energy
to achieve the next target state,
Fig 1 – Drop size calculation equations
and all reach the target state at the
same time in the process.
• This eliminates the challenge of too small droplets having
their chemistry “overcooked” and too large droplets reaching
the substrate too wet or “undercooked.”
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
13
Plume Velocity versus Pyrolysis State
• Since the process engineers have
direct control over the speed of the
plume, they can allow the plumes to
fall in a more predictable, controllable rate.
• This gives key control to the pyrolysis reaction,
not otherwise available in high velocity plumes,
such as those from dual fluid atomization
equipment.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
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Summary
• There is a profound and impending market for
efficiently produced TCO in advanced energy and
consumer products.
• The costs of starting up a production facility and
operating a sputtering line are prohibitive.
• Ultrasonic atomization has been proven to
distinguish the process through two key
advantages.
• Recent improvements in the technology
are now overcoming the two main
process challenges.
Advantages of TCO via Ultrasonic Spray Under Atmospheric Conditions
15