Large-area and thin light guide plates fabricated using UV-based imprinting

As the demand of larger and thinner flat panel display increasing, conventional methods such as injection molding and hot embossing to fabricate light guide plates (LGPs) become difficult and unsuitable. This study reports a low-cost and high-throughput method to fabricate large-size (320 mm × 240 mm, 15” in diagonal) LGPs by using UV-based imprinting process. With the UV-based imprinting process, a large-size LGP with thickness down to 0.8 mm has been successfully fabricated. The optical property of fabricated LGP has been verified. This study has demonstrated the fabrication of large-size and thin LGPs by using UV-based imprinting process, and the possibility of UV-based imprinting process for fabricating other large thin optical elements. ©2008 Optical Society of America OCIS codes: (220.0220) Optical design and fabrication; (220.4000) Microstructure fabrication; (230.0230) Optical devices; (230.3720) Liquid-crystal devices References and links 1. Y. K. Shen, W. Y. Wu, S. Y. Yang, H. M. Jian, and C. C. Chen, “Study on Numerical Simulation and Experiment of Light guide plate in injection molding,” Journal of Reinforced Plastics and Composites 23, 1187-1206 (2003) 2. J. H. Lee, H. S. Lee, B. K. Lee, W. S. Choi, and H. Y. Choi, “Simple liquid crystal display backlight unit comprising only a single-sheet micropatterned polydimethylsiloxane (PDMS) light-guide plate,” Opt. Lett. 32, 2665-2667 (2007). 3. S. R. Park, O. J. Kwon, D. Shin, S. H. Song, H. S. Lee, and H. Y. Choi, “Grating micro-dot patterned light guide plates for LED backlights,” Opt. Express 15, 2888-2899 (2006). 4. C. H. Wu and C. H. Lu, “Fabrication of an LCD light guide plate using closed-die hot embossing,” J. Micromech. Microeng. 18, 035006 (2008). 5. P. H. Huang, T. C. Huang, Y. T. Sun, and S. Y. Yang, “Fabrication of large area resin microlens array using gas-assisted ultraviolet embossing,” Opt. Express 16, 3041-3048 (2008).


Introduction
Liquid crystal displays (LCDs) have been widely used in many applications, such as computer and notebook monitors, digital camera, and personal digital assistants (PDAs) due to their high brightness, low energy consumption, and low weight.Because the liquid crystals can not emit light, a back light unit (BLU) is required to provide the light, usually through converting the linear light source into surface light source.Among the optical elements of BLU, the light guide plate (LGP) is regarded as a key component.LGP is a thin transparent plate with microstructures such as dots array or V-cut grooves arranged in a designed pattern on its bottom for the light uniformly over the whole area.Conventionally, the LGP with microstructures is fabricated by injection molding process [1].Recently, methods such as backside 3-D diffuser lithography followed by PDMS casting [2], dot-matrix holographic interference followed by UV embossing [3], and hot embossing process have been also employed to fabricate LGPs [4].In recent years, the LCD monitors has been required to be larger, thinner, and cheaper.Among the forementioned methods, backside 3-D diffuser lithography and dot-matrix holographic interference methods are unsuitable for fabricating large-size LGPs due to their complexity.Injection molding large thin LGPs has its difficulty due to the large flow resistance during filling and packing.Flow-induced molecular orientation and residual stresses results in warpage of the LGPs.Besides, the cost of injection mold and machine is very high.Although hot embossing is relatively inexpensive, it has the bottlenecks of limited embossing area and time consumed for heating-up and cooling-down.In addition, the heating and cooling may also release the residual stress and cause the warpage of the LGPs.Therefore, it is important to develop a fast process to fabricate large-size thin LGPs at room temperature and low pressure.
Recently, we have proposed a process which integrates the gas-assisted pressing and UVcuring mechanisms to fabricate large-area microlens array at room temperature and low pressure [5], which has shown its potential and possibility of being employed to fabricate large-area microstructures.In this study, based on gas-assisted pressing and UV-curing imprinting principles, the apparatus and process for fabricating large-size and thin LGPs has been designed, implemented and tested.Under the gas pressuring and UV irradiating, a largesize LGP has been fabricated.The optical property of the LGP is verified by using a laboratory-constructed back light module (BLU)

Experiment setup
As shown in Fig. 1, the apparatus based on the mechanisms of gas-assisted pressing and UVcuring consists of a upper chamber equipped with a gap-retained holder, a seal film, a lower chamber, and a UV-LED lamp.For the ease of operation, the upper and lower chambers are mounted in a commercial vertical injection machine which has a maximum clamping force of 250 tons, enabling the semi-automatic chamber open and close operations.The nitrogen gas can be introduced into the upper chamber to exert a pressing pressure during imprinting.In addition, a gap-retained holder is designed, used, and equipped in the upper chamber.The gap-retained holder, which holds the substrate with a maximum size of 320 mm × 240 mm, is aimed to retain a gap between the stamper and substrate for effective vacuuming during vacuuming stage.The lower chamber is employed to place the stamper and is vacuumed to reduce air bubble during imrpinting.Since the traditional UV-lamp is a linear light source and is difficult to provide a uniform irradiation over large-area, a UV-LED lamp is designed and implemented for completely covering, irradiating, and curing a large-area.The UV-LED lamp has the wavelength of 375-395nm and a power of 10 mW/cm 2 .A PET film with a thickness of 180μm is used as the seal film.The seal film is needed to separate the nitrogen gas in the upper and lower chambers and to pressurize gap-retained holder.The 0.8mm-thick PMMA plates (320mm × 240mm) are used as the substrates in the experiment.An ultraviolet-curable resin with a refractive index of 1.56 is employed.
As shown in Fig. 2, the large-area stainless-steel stamper (320mm × 240mm) with dots patterns of LGP, fabricated by photolithography and wet etching, is designed and supplied by Industrial Technology Research Institute of Taiwan.The stamper is 0.8 mm in thickness.The dots patterns of LGP are 80 μm in diameter and 20 μm in depth, while the pitch of dots patterns are ranging from 140 μm to 700 μm which varies from the long edge to center of the stamper.In this experiment, moreover, the surface of stamper is treated with anti-sticking of fluoride because of the sticking problem is crucial during UV-based imprinting process, especially in large imprinting area.After the anti-sticking treatment, the water contact angle of stamper surface is measured to be 128.84ºusing an automatic contact angle meter (FAT-125), indicating that the stamper surface has the lower surface energy to facilitate the de-molding between stamper and substrate.
The UV imprinting process for fabricating a large-size (15" in diagonal) light guide plate is described as following.
(a) The PMMA substrate is attached to the gap-retained holder, and the stainless-steel stamper with cavity of dots patterns is coated with the UV resin and fixed on the holder of lower chamber.(b) The upper and lower chambers are closed.At the same time, the PMMA substrate held by the gap-retained substrate holder is not in contact with the stamper and retains a distance of approximately 5 mm with the stamper, facilitating the ease of air evacuating.Subsequently, the lower chamber is evacuated to remove the air and prevent the air bubble entrapment.(c) After evacuating, the nitrogen gas is introduced into upper chamber to produce a pressing pressure which is applied to the gap-retained holder that the gap-retained can bring the substrate downward and in contact with the UV resin coated on the stamper surface.After pressing the stack of substrate and stamper for a specific while, the UV irradiating is conducted at room temperature to cure the UV resin of dots pattern which is transferred from stamper to PMMA substrate.(d) After curing, the nitrogen gas is introduced into lower chamber to separate the stamper and the substrate.After curing, the nitrogen gas is expelled out from the upper chamber.Without the pressing of embossing pressure, the gap-retained holder moves upward.At the same time, the substrate can be detached automatically from the stamper due to the stamper is treated with anti-sticking.Finally, the upper and lower chambers are opened, and the large-size light guide plate of PMMA can be obtained.

Results and discussions
As shown in Fig. 3(a), the dos patterns of LGP have been successfully transferred from the stamper to the whole 320 mm × 240 mm PMMA substrate with negligible air bubble defects by UV-based imprinting process with gas-assisted pressing capacity.The result demonstrates the capacity of this proposed process for the fabrication of large-size LGP with thin thickness down to 0.8 mm.Results also demonstrate that using a gap-retained holder is helpful to avoid the air bubble defects.Figures 3(b) and 3(c) shows the optical microscope (OM) images of the replicated dots patterns on the PMMA substrate under the processing conditions of 1 kgf/cm 2 of pressing pressure, 30 seconds of pressing duration (including UV irradiating time), and 200 mJ/cm 2 of UV-curing dose.Results show that the profile is acceptable.
In order to verify the optical property of the fabricated LGP, a laboratory-constructed back light module is employed as shown in Fig. 4. The laboratory-constructed back light module is composed of white LED light source, a DC power supply, two prism sheets, a diffuser, a reflection sheet, and the fabricated LGP. Figure 5(a) shows the photography of lights emanating from the fabricated 320 mm × 240 mm × 0.8 mm LGP when the edge LEDs are on.This indicates that the fabricated LGP using proposed process has high potential for use in a conventional LCD monitor.Figure 5(b) shows the measured corresponding light intensity distribution over the whole LGP.Although the light intensity distribution is not uniform, it is acceptable.The reason why the light intensity distribution is not uniform is the design of micro-dot patterns.In fact, the distribution of micro-dot patterns on stamper is designed for the LGP used in edge-light backlight module with cold cathode fluorescent lamps (CCFLs).In this study, however, the light source of laboratory-constructed back light module employed to inspect and verify the optical property of fabricated LGP used white LEDs instead of CCFLs because the required size of CCFLs were not available, meaning the optical property (light intensity distribution) may be different.The uniformity of light intensity distribution could be upgraded by designing the distribution of micro-dot patterns on stamper for using in the back light module with white LEDs light source.To sum up, the proposed process with gas-assisted pressing and UV-curing mechanisms can be employed to fabricate large and thin LGP, and the optical property has been verified to be acceptable.In addition, this process also shown its potential for fabricating an ultra-thin light guide plate even other optical elements (brightness enhanced films, reflection films, diffusers) for the application in flexible display.

Conclusions
This study presents an effective process to fabricate large-area and thin light guide plates by the mechanisms of gas-assisted pressing and UV imprinting.This process takes advantage of the uniform pressing pressure of gas-assisted embossing and the low pressure and fast curing of the UV imprinting.Under the processing conditions of 1 kgf/cm 2 of pressing pressure, 30 seconds of pressing duration (including UV irradiating time), and 200 mJ/cm 2 of UV-curing dose, a large and thin (320 mm × 240 mm × 0.8 mm) light guide plate has been successfully fabricated with the micro-dot patterns.In addition, the fabricated LGP is free of air bubble defect by using a gap-retained substrate holder, demonstrating gap-retained substrate holder is useful to facilitate the evacuating efficiency.The optical property of fabricated LGP has been proven and is of acceptable.This process has proven its potential for the fabrication of largesize and thin LGPs in conventional LCD display even the flexible display by using thin plastic films as the substrates.

Fig. 1 .
Fig. 1.Schematic diagram showing the apparatus and process of the UV-based imprinting process with gas-assisted pressing mechanism.

Fig. 2 .
Fig. 2. Photograph and OM images of the stainless steel stamper of LGP.

Fig. 4 .Fig. 5 .
Fig. 4. The laboratory-constructed back light module for inspecting the optical property of fabricated LGP.