Electrowetting Display Pixels Fabricated By Nanoimprint Lithography

Display devices plays an increasingly important role in the information society. Liquid crystal displays still occupy the mainstream position in the current display marketplace. However, due to limitations of its own principles, there are still some shortcomings, such as low brightness and contrast in outdoors environment, small view angle range, high power consumption, low utilization of light. New display technology and equipment has been a hot research topic. Among variety of potential display technologies, electrowetting display (EWD) technology has obvious advantages. EWD is an innovative solution for the display applications space because of its unique combination of attractive merits:low brightness, exceptional view ability (all angles) even in bright sunlight, fast video speed response, color conversion speed, real world color saturation, low energy consumption. EWD can meet various demands of displaying, so it is a very promising display technology.EWD pixels are usually fabricated by conventional photolithography process, while the process is very cumbersome. In this paper, we use nanoimprint lithography (NIL) technology to replace conventional photolithography to fabricate EWD pixels, successfully simplify the process, expand the idea of reducing the cost of fabricating EWD and help to promote the EWD toward practical use. The main achievements are summarized as follows:1.Study of the processes that the typical EWD fabricated by NIL:Cytop and SU-8 photoresist are used as the hydrophobic dielectric and the hydrophilic grid materials, respectively. It is impossible to deposit SU-8 uniformly on Cytop because Cytop surface is hydrophobic while SU-8 is hydrophilic. A SiO2 layer was deposited on the Cytop layer using plasma enhanced chemical vapor deposition (PECVD) at 30℃, SU-8 can then be uniformly spin-coated on the SiO2 layer with good adhesion since the SiO2 surface is hydrophilic. An anti-sticking thin functional layer is applied on the mold to reduce the surface energy before imprint. However, an anti-sticking layer on the Si mold is not enough to guarantee a successful imprinting of SU-8. The imprint procedures and parameters should be optimized. According to cross-linking proceeds of SU8, UV pre-exposure and thermal NIL were combined to imprint SU8, and achieved a good release effect by optimizing the parameters. We found that at the situation of exposing for 1.5min under 15W UV lamp, maintaining the pressure of 0.3MPa and the temperature of 110℃ for 5 min, the release effect is the best. Rectangle pixels,100 μm in width,300 μm in length and 13 μm in depth are successfully fabricated according to this situation. Since NIL is essentially a mechanical molding process, a residual layer always exists in the resist after imprinting. This residual layer should be removed by additional oxygen plasma etching process to complete the pattern transfer. Although measures have been taken to attain uniform distribution of pressure over the entire mold surface, variations in the residual layer thickness still remain. This makes it difficult to remove the residual layer completely by RIE, because the Cytop layer underneath will be inevitably etched in areas where the residual SU-8 is thinner. In this way, the aforementioned SiO2 layer acts as a barrier to protect the Cytop layer. The cured SU-8 was etched in an O2/SF6 plasma, in which the small amount of SF6 was added to enhance the etching speed. The insertion of a SiO2 layer between the Cytop layer and the resist is an important modification in the NIL process. It not only ensures the successful spin-coating of SU-8 resist, but also protects the Cytop layer from being etched. The SiO2 layer left on top of Cytop can be removed easily by hydrofluoric acid. Tested after filled the oil into the fabricated SU-8 grid and sealed under distilled water. Test results show that controllable contraction and recovery of oil film in single EWD pixels was realized, that is to say the display works based on electrowetting dispalys principle. Although the test results still need improve, for example the operating voltage is too large and slow recovery speed, but it illustrates that it is feasible to fabricate EWD pixels by NIL.2. Study of the processes that the bistable EWD fabricated by NIL:Firstly, Maskless lithography technique and the hydrochloric acid solution etch process were combined to fabricate ITO double bottom electrode structure, then determined the best methods and parameters to imprint SU8 3010 to fabricate EWD pixels, that is to heat after apply pressure and maintain the pressure of 0.3MPa and the temperature of 110℃ for 2 min. Because residual SU8 layer can be used as a dielectric layer in the bistable EWD, there is no need to consider the issue of removing residual layer. Alignment effect is not very good due to the use of opaque silicon mold. This type of EWD avoids occurring the contact angle saturation and hysteresis phenomenon, thus reduce the error analysis. Besides, for bistable EWD, the power consumption is lower because the power is only needed to switch the pixel but not required to maintain the pixel state once switched.