ANALYSIS: DISPLAY MANUFACTURING
Precision laser cutting of OLED polymer stacks for bendable display technologies
Jim Bovatsek, of MKS Spectra Physics, shares how UV picosecond lasers can effectively cut layered stacks of organic polymers in flexible OLED display production
Organic polymers are employed as substrates and other components in many microelectronic and display devices. In microelectronics, for example, polyimides (PIs) are used as passivation layers, insulators and alpha-particle barriers. Flexible electronics devices use PIs for both substrates and encapsulation layers, while liquid crystal displays (LCDs) use polyimides as alignment layers. New PIs that are highly transparent at visible wavelengths (clear PI) are used as substrates and encapsulants in flexible organic LCD (OLCD) and organic light emitting diode (OLED) displays. Acrylic resins with hard, inorganic particulate
additives are used for scratch- resistant, anti-fingerprint hard coat layers (HCs) and polyethylene terephthalate (PET) for transparent substrates and protective layers in flexible OLED manufacturing processes. Recently, novel thin film stacks
for flexible, and touch-sensitive, OLEDs have been developed that facilitate emerging designs for foldable smartphones and rollable displays1
. These
stacks typically employ a flexible PET layer with a thin layer of pressure sensitive adhesive (PSA) as a removable protective layer. Optically clear PI is used as a substrate and encapsulant, while engineered polymer HCs or ultrathin glass (UTG) are employed for anti- scratch, anti-fingerprint layers. Polyvinyl alcohol (PVA) films are used for polariser layers.
Achieving a small bending radius of curvature in all the display’s component layers while simultaneously retaining functionality over a long lifetime of bending cycles is challenging for OLED device manufacturing. UTG, which is brittle and difficult to manufacture and handle, is especially problematic and is therefore replaced with clear PI/HC layers in many OLED devices. Laser cutting for device singulation and feature generation is the only method with the precision required for OLED manufacturing. UTG films can be laser-cut using Bessel beam processing and an IR picosecond laser, but this method does not work for OLED polymer stacks. IR laser sources cut materials photothermally, producing a localised, intensely
heated cutting zone that often generates a significant peripheral heat affected zone (HAZ). While UTG is tolerant of relatively high temperatures and therefore resistant to HAZ formation, organic polymers are easily thermally damaged and IR laser sources often produce unacceptable HAZ.
The right tool High-power, ultrashort pulse (picosecond, ps, or femtosecond, fs) UV laser sources are therefore used for precision cutting of many polymer films. UV wavelengths facilitate bond-breaking over photothermal vaporisation during the cutting process, and the cut is produced via a photoablative mechanism. OLED cutting using ps pulsed UV lasers requires polymer- specific laser source parameters for each film since organic polymers can exhibit a wide range in photoablation threshold values. In addition, the polymers used in OLED stacks exhibit very dissimilar HAZ characteristics. Laser cutting of OLED stacks thus requires that the laser source parameters be adjusted to previously optimised values for each layer within the stack
Figure 1a: Experimentally determined ablation thresholds for clear PI, PET and HC
Figure 1b: Optical microscope images showing dissimilar ablation characteristics for clear PI, PET and HC materials (single pulse energy ≅ 5× material ablation threshold)
Figure 1c: Test results for HC cut optimisation
14 LASER SYSTEMS EUROPE SUMMER 2022
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MKS Instruments
MKS Instruments
MKS Instruments
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