纳米压印光刻技术-诱导自对准液晶取向层新型工具-指导essay

纳米压印光刻技术-诱导自对准液晶取向层新型工具-指导essay  Nanoimprinting Lithography as Novel Tool Inducing Self-Aligned Liquid Crystals For Alignment Layer

Huang-Ming Philip Chen*

Department of Photonics and Display Institute, National Chiao Tung University

Rm. 506, MIRC Building, 1001 Ta Hsueh Road, Hsin-Chu 30010, Taiwan, R.O.C

Chih-Ho Chiu, Hui-Lung Kuo, Pin-Cheng Chen, Chun-Hsiang Wen and Yi-Chun Liu

Materials Research Labs. of Industrial Technology Research Institute

B77, 195-5 Chung-Hsin Rd. sec. 4, Chutung, Hsin-Chu 310, Taiwan, ROC.

Abstract

Polymerizable nematic liquid crystal (LC) was applied as resist for our newly developed nanoimprinting lithography process.The LC resist was self-aligned during imprinting process at very low impriting pressure, 1.5 bar and the patterned area was able to achieve 4x4 cm2. The optical anisotropic was verified through polarized optical microscopy (POM). The various thickness of LC resist was evaluated and compared alignment ability with polyimide, and isotropic resists. 微磨碎LC薄膜

准备通过这种新颖的方法提供了新的应用液体LC细胞内晶体对准层以及制备的光学各向异性的技术。The micro-grated LC films prepared by this novel method offer new applications for liquid crystal as alignment layer within LC cells as well as preparation of optical anisotropic films.  http://www.ukassignment.org/dxessay/

1. Introduction

液晶（LC）对准层是形成高性能的LC细胞关键因素之一。Liquid crystal (LC) alignment layer is one of the key elements in forming high performance LC cells. LC alignment layers can be prepared by contact, i.e. mechanical buffing,[1] and non-contact,such as photoinduced surface alignment techniques.[2] Up-todate,mechanical buffing still dominates the mainstream LC cell preparation due to polyimide (PI) layer yields robust and uniform alignment ability under mechanical buffing.[3-5] Studies have shown that Nanoimprint lithography (NIL) applications are able to

create surface relief patterns.[6-9] In particular, certain directional order of light emitting molecules has been demonstrated on the surface of submicron patterned polymer

matrix.[10] All patterned polymer matrix mentioned above were mainly made from amorphous materials with relatively high imprinting pressure.[11] Here we 我们提出一种新颖的液晶印迹压力。对准技术，在此我们利用液晶的印迹抵抗，可以自对准没有预涂对齐层1.5印记下在大面积上的4×4平方厘米的压力。report a novel liquid crystal alignment technique, in which we utilize liquid crystal as imprinting resist that can be self-aligned under 1.5 bar imprinting pressure on a large area of 4x4 cm2 without pre-coated alignment layer. Hence, the novel alignment technique has great potential for future liquid crystal cell fabrication and optical anisotropic film preparation.

2. Experiments

The new NIL process creating alignment surface is depicted in Scheme 1.不同音高的模具压印通过电子束光刻或光刻制造。 Molds with different pitches for the imprinting step were fabricated by E-beam lithography or photolithography. The pitches were from 1.8 to 0.35μ m with the line/space ratios from 0.4 to 2. A toluene solution, comprising SLM-90519 (reactive liquid crystal oligomer, Wacker Chemical) and Irgacure 369 (photo-initiator, Ciba), 准备为LC抵制和LC聚合物薄膜。标准压印工艺条件是1.5巴的印迹压力120℃，20分钟LC抗蚀剂，然后通过紫外线照射。was prepared for both LC resist and LC polymer films. The standard imprinting process condition is 1.5 bar of imprinting pressure at 120 oC for 20 minutes on LC resist, followed by UV irradiation. To ease the peel-off process from the mold, N-dodecyltrichlorosilane was applied as a

releasing agent (purchased from TCI). Molds were immersed in the mixture of n-trichlorosilane and toluene under nitrogen for one hour. After releasing, these molds were rinsed with toluene and dried by a stream of nitrogen. The post-baked procedure for releasing film from molds was done at 150 oC for 3 mins after imprinting process.

Scheme 1. NIL fabrication for surface microstructures using LC as resist.

3. Results and Discussion

The direct evidence in Figure 1 suggests that the novel NIL process is capable of aligning LC resist. A 4 x 4 cm2 patterned area which prepared by the mold of 1.8μ m/period and 200nm in depth showed optical anisotropy under polarizing optical microscope (POM). Light transmission was fully dependent on the azimuthal angle of patterned area under crossed polarizers.

The results in Figure 1 clearly showed light blockage (dark state) in the patterned area, when grating lines were paralleled to one of the polarizers’ axes. The maximum light transmission (bright state) occurred when grating lines of the patterned area are placed at either 45o or 135o. In all cases above, un-patterned area appeared in all azimuthal angles showed the Grandjean texture only. The driving force for self-aligned LC resist can be attributed to the laminar flow and capillary effect during the hot pressing inside the mold. This mechanical pressing induced various shear rate distribution which yielded the preferred orientation along the flow direction. [12]

Figure 1. Pictures of the liquid crystal grating film under POM.(a) grating line direction is parallel (0o) to the axis of polarizer, and (b) at 45o.

The order parameter is also strongly affected by thickness of LC resist and depth of mold. Various thickness of resist was examined by mold with 1.8 μm/period and 200nm in depth. As shown in Figure 2, the concentration of resist needs to be greater than 5.0 weight percent solution (240 nm in film thickness) to show good molecular order. This result implies that the thickness of resist needs to be greater than mold’s depth to obtain best result.

 

Order Parameter

Figure 2. Order parameter on different thickness of LC resist. Polymerizable nematic liquid crystal (SLM-90519), the same material utilized for LC grating film, was doped with S-428 dichroic dye (Mitsubishi Chemical) as an order indicator for evaluation. The dye-doped LC layer was prepared into 3.6±0.3 μm thick films by spin coating. The order parameter, S, is determined by the absorption of A⊥ and A‖ at 500 nm of the wavelength using the equation ( ) ( 2 ) C C S = A − A A + A ⊥ ⊥ .[13]

The high order parameter, S = 0.62, can be achieved on 1.7 μm LC grating film, which was comparable to the PI buffed surface (S = 0.67), but much better than the one using isotropic grating film (S = 0.33), as shown in Figure 3. This result suggests that the LC grating film possesses a similar alignment capability, comparing with the PI buffed surface. The pretilt angle of LC on polyimide, LC grating film, and isotropic grating film were measured based on crystal rotation method and found to be 1.7,

 

Wavelength, nm

Figure 3 Linear dichroism of LC with dichroic dye (S-428) on LC grating film, isotropic grating film and PI treated alignment layer. Both Desolite 4D5-57 and SLM-90519 grating films showed the same relief pattern under NIL treatment. In Figure 4, the LC molecules on LC grating film, however, has better extinction ratio than isotropic grating film at all annealing and curing temperatures.液晶分子的上各向同性光栅膜透露有限的择优取向，由于它们的分子约束微槽表面上。 LC molecules on isotropic grating film revealed limited preferred orientation, due to their molecular confinement on the micro-groove surfaces. In addition to the confinement effect, the order of LC molecules on LC grating film was enhanced by molecular arrangement. The self-aligned sample of

SLM-90519, however, possesses a greater degree of molecular alignment ability, which is absent in an isotropic sample of

Desolite 4D5-57. This is the reason why LC on SLM-90519 has much higher order parameter than the LC on Desolite 4D5-57. These results suggest that micro-groove surfaces can induce the alignment ability of liquid crystalline materials. Molecular level’s alignment, however, is the key to achieve well ordered liquid crystals.

 

4. Conclusion

The novel NIL technique for LC alignment was successfully achieved in this study. 没有任何预取向层，这简单的方法，可以有效地调整LC。Without any pre-alignment layer, this simple approach can align LC effectively. Specifically, 施加低压印压力液晶抵抗1.5bar ，产生以自对准的光栅膜，是非常容易在实际中操作。applying low imprinting pressure at 1.5 bar on liquid crystal resist, yielding a self-aligned grating film, is much easier to operate in practical sense. Furthermore, LC resist possesses aligning capability can also be utilized as an alignment layer itself. The results indicate that a good liquid crystal alignment can not solely depend on the

grooved surface, but rather rely on the molecular level of surface alignment ability.#p#分页标题#e#

5. Acknowledgements

The funding of this work was provided by the MOEA of the Republic of China under the frontier research program and was partially supported by MOEA Technology Development for Academic Project # 94-EC-17-A-07-S1-046.

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