ANALYSIS: DISPLAY MANUFACTURING


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


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


exhibited clean photoablation, PET and HC materials showed signs of thermal melt and brittle fractures, respectively (figure 1b). 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. Once the optimised laser


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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 (100s 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. 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 among the materials. Whereas clear PI


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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 2a and figure 2b. 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 <5μm for the critical HC layer. The cross- sectional inspection showed


“UV USP lasers provide the precision cutting capability for a diverse range of polymer layer stacks”


no evidence for delamination, adhesive smearing, or HAZ across the cut surfaces.


Conclusion Layered stacks of organic polymers (that is, polyimides, PET, polyvinyl alcohol, and so on) 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 technology combined with UV wavelengths have been proven 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. l


Jim Bovatsek is a manager of applications engineering at MKS Spectra Physics


References [1]


[2]


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 Conference on Display Technology (ICDT 2020), vol. 52, no. S1, pp. 312-314, 2021.


[3]


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.


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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