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OLED & LIGHTING

8

No 19 | July 2017 |

OPE

journal

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.

Laser Ablation

Mechanics

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

parameters

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

decreasing wavelength

“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

Information

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.