Precision laser cutting of OLED polymer stacks for bendable displays

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Jim Bovatsek, applications engineering manager for 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.

The right tool for the job

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 (ps) laser, however, 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. Therefore, high-power, ultrashort pulse (picosecond, ps, or femtosecond, fs) UV laser sources are 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 if best overall quality and throughput are to be achieved.

MKS has evaluated the use of a high-power, picosecond UV hybrid fibre laser (Spectra-Physics IceFyre UV picosecond Laser) for photoablative cutting of a multi-layer polymer stack like those used in foldable OLED applications2. The IceFyre UV50 laser outputs >50W of UV power with pulse energy >40µJ (100’s µJ in burst mode), repetition rates from single shot to 10MHz, and typical pulse widths of 10ps. The system also allows pulse-tailoring using TimeShift ps technology to produce programmable burst shapes with constant pulse energy, adjustable pulse repetition frequency (PRF), and variable burst-pulse separations as low as 10ns. 

A polymer test stack, developed by a supplier to the OLED display industry, was used to determine single-pulse ablation thresholds and cutting results. It consisted of a 50μm thick clear PI film, a 12μm thick engineered composite HC layer on one surface (an organic polymerised film with embedded inorganic particles such as high-hardness glass/ceramic nanoparticles), and a 50μm thick protective PET film adhered to the HC layer with a 4μm thick PSA coating. The challenge addressed in these tests was to adjust the laser source parameters to produce high quality cuts at high cutting speeds for the complete stack.

Each material in the stack was evaluated for single-pulse ablation threshold using the Liu method3. The results, shown in Figure 1a, exhibited considerable range.

Figure 1a: Experimentally determined ablation thresholds for clear PI, PET, and HC

Clear PI had a relatively low threshold of ~0.25J/cm2, similar to that of conventional PI and consistent with strong linear absorption of the UV energy. The PET material had a threshold value more than twice that of clear PI, at ~0.56J/cm2, consistent with higher transparency at 355nm. The engineered composite HC layer displayed the highest threshold, at 2.4J/cm2, approximately 10x that of clear PI. This result is expected for glassy materials and ultrashort UV pulse lasers. The single-pulse tests also highlighted the very dissimilar ablation characteristics amongst the materials. Whereas clear PI exhibited clean photoablation, PET and HC materials showed signs of thermal melt and brittle fractures, respectively (Figure 1b).

Figure 1b: Optical microscope images showing dissimilar ablation characteristics for clear PI, PET, and HC materials (single pulse energy  5× material ablation threshold)

Following the determination of ablation threshold values and characteristics for each of the stack materials, optimised laser source parameters were determined for cutting individual layers. Figure 1c shows a representative series of results, in this case for the HC layer. The successful cutting process for the HC layer required low laser source energy, high PRF, and high pulse overlap. Similar tests were conducted for cutting the clear PI and PET layers.

Figure 1c: Test results for HC cut optimisation

Once the optimised laser source parameters were obtained for cutting each material, stack cutting tests determined the optimised OLED stack cutting process. The threshold studies had shown that stack cutting presented a significant challenge, since laser source parameters for optimised throughput and quality differed significantly for each layer. Here, the ability of the IceFyre laser source to change parameters on-the-fly proved invaluable. Once the clear PI layer had been cut using a large focus spot size and moderate laser fluence, a single focus and PRF adjustment optimised the laser source for cutting the HC layer. After cutting the HC layer, the final, more thermally sensitive PET layer required only a laser source adjustment to a lower PRF value. The IceFyre laser automatically outputs the maximum pulse energy at each of the triggered PRFs, or it can be programmatically adjusted to an appropriate setting, if desired. The final, optimised stack cutting process yields excellent results, as shown in Figure 2.

Figure 2a: Stack cutting results showing optimized cuts for PI, HC, and PET layers

Figure 2b: A cross-section of the laser cut film stack showing no evidence of delamination or debris smearing across the cut surfaces

The OLED stack cutting process had an overall cutting speed of >400mm/s which is similar to or slightly better than that which can be achieved with other polymer stacks of similar thickness. The cuts showed edge heating HAZ values of <10μm for the clear PI and PET layers and <5 μm for the critical HC layer. The cross-sectional inspection showed no evidence for delamination, adhesive smearing, or HAZ across the cut surfaces. 

Conclusion

Layered stacks of organic polymers (i.e., polyimides, PET, polyvinyl alcohol, etc.) constitute critical components in new microelectronics and display devices such as foldable phones and roll-up monitors and TVs. New, advanced laser technologies are needed for the precise cutting processes used in device singulation and feature generation when manufacturing devices containing these polymer stacks. Lasers that employ ultrashort pulse (USP) technology combined with UV wavelengths have been proven out in this application, successfully providing the precision cutting capability for a diverse range of polymer layer stacks. The data provided in this article demonstrates how the Spectra-Physics IceFyre picosecond hybrid fibre UV laser with TimeShift ps technology can be used to provide a flexible platform for the development of manufacturing processes that yield both high quality device features and high throughput. 

References

  1. Z. Gao, L. Yu, Z. Li, W. Shi, C. LI, L. ge Yuan, X. Sun and D. Fu, "31 inch Rollable OLED Display Fabricated by Inkjet Printing Technology," International Converence on Display Technology (ICDT 2020), vol. 52, no. S1, pp. 312-314, 2021.
  2. J. Bovatsek, "Ultrashort pulse laser cutting of clear polyimide and hard coat film stacks for flexible OLED displays," in Lasers in Manufacturing Conference 2021, Munich DE, 2021.
  3. J. M. Liu, "Simple Technique for Measurement of Pulsed Gaussian-Beam Spot Sizes," Optics Letters, vol. 7, no. 5, p. 196, 1981.

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