Flexible Thin Film Display Based on Optical Waveguide for Signage
Authors: Park Bong Je , Park Sun Tak , Kyung Ki Uk , Yun Sung Ryul , Nam Se Kwang 
Published: February 2020

Flexible thin film displays based on optical waveguide have been demonstrated. Flexible thin film displays consist of light source, optical waveguide, and scatters. Here, scatters have a role to change the direction of light source from parallel to perpendicular to optical waveguide so that one can easily see patterns. The total thickness and transparency of thin film displays are less than 100 um and over 90%, respectively.
I. Introduction
Flexible displays are one of main issues in a display field. Most flexible display technologies have mainly focused on the active ones such as OLED and LCD. However, only a few efforts to passive signage displays have been made using LED sources or fluorescent lamps with a hard acrylic panel. Especially, flexible displays using optical waveguide are almost rare. Only two optical waveguide type displays have been considered using an optical fiber and polymer waveguide.[1] First one is constructed by guiding laser light through an optical fiber arranged in a spiral manner. The light leaks out via the grooves fabricated on the optical fiber. Second one is a polymer waveguide with successive branches which distribute the optical power from a laser to two-dimensional emission points on a plane. Therefore, it is necessary to develop thin, transparent, and flexible displays for signage.
II. Experiment
The overall fabrication process is the following. First, the under clad and core polymers are spin-coated on a Si wafer and cured by UV, respectively. Second, optical waveguide and scatters are formed by photolithography and reactive ion etching (RIE) process. Third, the upper clad polymer is spin-coated on the optical waveguide and cured. Finally, the polymer film is detached from the Si wafer.
III. Results and discussions
The principle of flexible signage display is the following. Light source is coupled into the optical waveguide, transferred to the scatters, and emitted to air as shown in Fig.1. If the thin film including the waveguide and scatters is an elastic polymer, the signage display could have a good flexibility.
We used Bisphenol A ethoxylate diacrylate (Aldrich) and Tetra diacrylate (Aldrich) as core and clad, respectively. The refractive indices of BAED and TEGDA are 1.5647 and 1.5031 at 632nm, respectively. Under clad layer was spin-coated on Si wafer at 500 rpm for 20 seconds and uv-cured with 2 kW power for 5 minutes. The thickness of under clad layer was about 27 um. Core layer was spin-coated at 3700 rpm for 30 seconds and uv-cured with 2 kW power for 5 minutes. The thickness of core layer was about 15 um. After then, photolithography process has been done to make optical waveguide pattern on core layer with SU-8 photoresist (PR). SU-8 PR coated Si wafer was placed into RIE system to etch the core area without SU-8 PR. Since our Pdos20 process etched about 8um thickness of core layer, the etching process was done twice to etch 15um thickness of core layer. Core optical waveguide is finally made. Before making upper clad layer by spin coater, scatters have to be made in the core layer by using photolithography and RIE process. The scatters have a role to change the direction of light source from parallel to perpendicular to optical waveguide so that one can easily see pattern. If there are no scatters, light source is strictly guided within core layer and it is hard to see pattern that we want. Second photolithography process was done on core optical waveguide layer with SU-8 PR to make scatter pattern. SU-8 PR coated Si wafer was again placed into RIE system to etch small dot shaped scatter pattern. After developing SU-8 PR layer, upper clad layer was spin coated at 500 rpm for 20 seconds and uv-cured for 5 minutes. The thickness of upper clad layer was about 26um. The final thin film was detached on Si wafer. The total thickness of the film was about 70 um and the transparency was 91%. The thin film display was extremely bendable as shown in Fig.2. There was no light extraction loss even when the film was bended.
In order to couple light source into our optical waveguide type display, we used a 4 ch single mode fiber block. Light source was 650 nm red LD with 50 mW. A 4 ch single mode fiber block and our display were placed on Auto-aligner system and attached with epoxy resin because 20um diameter core of single mode fiber has to be exactly placed to 15um core layer of our display to transfer light source into the display.
References
[1] Y. Okuda and I. Fujieda, “Polymer waveguide technology for flexible display applications,” Proc. SPIE 8280, 82800W (2012).