OLED & LIGHTING
No 19 | July 2017 |
structuring and singula-
tion over other methods.
This article reviews the
mechanism and benefits
of laser ablation and
provides an overview of
the tradeoffs involved
in choosing a USP laser
for a particular task.
There are two basic
means by which a laser
can ablate or process
a material. Continuous output lasers and
pulsed lasers with pulsewidths in the tens
of nanoseconds range remove material
through a photothermal interaction. Here,
the focused laser beam acts as a spatially
confined, intense heat source which
vaporises (essentially boils away) material.
This allows relatively high material removal
rates; however, for the most demanding
tasks, peripheral heat affected zone (HAZ)
damage can be a significant problem. This
undesirable damage can include delamina-
tion of surface coatings, microcracking,
changes in the bulk material properties, and/
or the presence of some recast material.
The second mechanism for laser material
removal is based on photoablation, typically
accomplished using USP lasers whose short
pulsewidths lead to very high peak powers.
This power is sufficient to directly break the
molecular or atomic bonds which hold the
material together, rather than simply heating
it, resulting in “cold” material removal. Plus,
the material is exposed to the laser light
for such a short time that the energy is not
carried beyond the area of impact. This mini-
mises the heat affected zone. Additionally,
this is a very clean process, leaving no recast
material that could require post-processing.
For OPE applications, a major advan-
tage of ultrafast processing is that it
works on virtually any material, even
ones that are normally transparent at the
laser wavelength. This, coupled with the
ability to very precisely control ablation
depth, makes it ideal for either cutting
partially or completely through substrates
comprising layers of disparate materials.
The typical tradeoff of USP lasers is a
higher capital cost than longer pulse length
lasers. As a result, ultrafast processing
provides the best rate of return for applica-
tions that demand the greatest possible
precision, quality and smallest HAZ.
USP laser process
Commercial USP lasers are currently
available with output in either the infrared,
visible or ultraviolet, and over a range of
pulse repetition rates and output powers.
Choosing the optimum source for a specific
application typically represents a compromise
between quality, processing speed and cost.
For example, process quality (specifically
kerf width) improves in virtually all materials
as wavelength decreases. But, shorter wave-
length lasers have lower material removal
rates than longer wavelength sources (all
other factors being equal), so processing
speed also decreases with wavelength.
Similarly, process speed can often be
increased by using higher laser power or
pulse repetition rates. However, laser cost
is usually tied directly to these parameters,
so increasing them raises cost. Determin-
ing the optimum laser characteristics for
a given process typically requires some
practical testing, and is often best done
in cooperation with an experienced laser
supplier that offers applications development
services utilising a wide range of products.
In conclusion, many laser processes
already established in display and semicon-
ductor fabrication are directly applicable to
roll-to-roll OPE production. USP lasers are
useful to cut inhomogeneous layers, either
partially or completely, in a single process.
They produce a minimal HAZ and kerf, and
deliver better finish quality than virtually any
other technique. Furthermore, for many
applications, USP laser processing delivers
a highly desirable combination of superior
finish quality with and higher process utilisa-
tion, resulting in a lower cost-of-operation.
A 220µm thick simulated display layer on glass scribed with a Coherent
HyperRapid NX laser with output in the infrared, green and ultraviolet.
This clearly demonstrates the improvement in cut quality with
“Dark Yellow,” 30µm thick polyimide sheets scribed with a nanosecond
and USP laser. The nanosecond laser processed at 66mm/sec, while the
USP laser cut at 193mm/sec. The nanosecond laser produced a
substantial HAZ – seen as a darkening in this image – while the USP laser
produced none, and also delivered a substantially smaller cut width
Ultra-short pulse laser from Coherent
Founded in 1966, Coherent,
Inc. is one of the world’s leading
providers of lasers and laser-based
technology for scientific, commercial
and industrial customers. Its com-
mon stock is listed on the Nasdaq
Global Select Market and is part
of the Russell 2000 and Standard
& Poor’s MidCap 400 Index.