Patterning electrode for cholesteric liquid crystal display by pulsed laser ablation

doi:10.1016/j.optlaseng.2010.05.004

Optics and Lasers in Engineering

Volume 48, Issue 10, October 2010, Pages 1008-1011

https://doi.org/10.1016/j.optlaseng.2010.05.004 Get rights and content

Abstract

An ultraviolet wavelength laser is used to manufacture the metal electrode of a cholesteric liquid crystal display (Ch-LCD) without a shadow mask. The proposed method effectively ablates the metal layer without damaging the liquid crystal layer and underlying indium tin oxide electrode. The width and morphology of the laser ablated sections were investigated as a function of laser power as well as scan speed. The minimum ablated line is around 43 μm and the laser system operates at 1.5 W at a scan speed of 200 mm/s. The characteristics of a Ch-LCD prepared using laser patterning under the optimum ablation process conditions are similar to those of a Ch-LCD prepared using the screen printing method.

Introduction

Flexible reflective cholesteric liquid crystal displays (Ch-LCDs) have been recently developed. The conducting electrodes are used on stacking structures to fabricate the devices [1]. Flexible bistable-display media are becoming as practical as displays for some niche products. In general, the two dimensional electrodes of flexible displays are usually manufactured by traditional wet etching process. Conventional lithography is a multi-step wet process, which includes resist coating, resist exposure, resist etching, indium tin oxide (ITO) etching, and resist stripping [2]. However, the liquid crystal layer or dark layer cannot be exposed to organic solvents, so the metal electrode cannot be patterned by the conventional wet etching process. Thus, electrodes patterned using the screen printing technique are currently applied to form the metal row conductor [3]. The metal conductor must be completely ablated to avoid the two metal lines from linking together. Furthermore, the patterning process cannot damage the liquid crystal layer and the underlying ITO electrode. With the development of laser micromachining techniques, diode pumped Q-switched neodymium-doped yttrium lithium fluoride (Nd:YFL) laser has been applied to ablate ITO films on glass substrate. The characteristics of ITO thin films patterned at different laser wavelengths have been investigated. Improved pattern morphology at higher processing speeds can be obtained using UV irradiation or a short pulse laser [4], [5], [6], [7], [8]. Lasers provide high resolution and do not require a mask, which makes it possible to ablate a conductor layer without damaging the liquid crystal layer and underlying ITO electrode.

In the present study, the properties of the metal conductor of a cholesteric liquid crystal display produced using laser ablation are investigated. The effects of the laser power and scan speed on the quality of patterning are evaluated. The characteristics of a Ch-LCD prepared using laser patterning are compared to those of a Ch-LCD prepared using the screen printing method.

Section snippets

Experimental

Fig. 1 shows a schematic diagram of the workstation used for electrode patterning with laser ablation. The system for laser patterning consisted of a Q-switched DPSS 355 nm laser (Coherent), which had a pulse duration of 30 ns, a repetition rate of 40 kHz a stability of less than 5%, a scanner (Scanlab) at the focus of a 250 mm focal length lens, and an XYZ stage. The metal conductor patterning was carried out at a laser power of 1–6 W; the scan rate was varied from 200 to 500 mm/s.

In order to

Results and discussion

Due to the high power of the laser, not only the silver layer but also the underlying liquid crystal layer and ITO film can be etched away. If the ITO film is etched away, electrical current cannot flow. Material removal occurs only if the pulse energy is above an ablation threshold, which strongly depends on material characteristics. For multilayer structures, the ablation threshold of each layer is the most important factor for the selective removal of an interested layer. Therefore, the

Conclusion

A patterning process for the metal electrode layer on Ch-LCDs using laser technology was proposed. Good quality trenches were obtained by adjusting the laser power and scan speed. Furthermore, electro-optical characteristics of the Ch-LCD prepared using the laser ablation method are the same as those of a Ch-LCD prepared using the screen printing method. Therefore, laser patterning has potential as a method for manufacturing the electrode of Ch-LCDs.

Acknowledgements

The authors acknowledge the support from the Ministry of the Economic Affairs (MOEA), Taiwan ROC., for conducting this project.

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Patterning electrode for cholesteric liquid crystal display by pulsed laser ablation

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Appl Suf Sci

Opt Lasers Eng

Opt Laser Technol

Recent progress in color flexible reflective cholesteric displays

SID Symp Dig Tech Pap