Development and Study of a novel precision pattern-coating technology：“Air-Bubble Wet Coating”

Various applications for micro-patterning have been proposed. In recent years, micro-patterning technology has been applied in many new fields, such as semiconductor, flat panel display, biomedical, and so on. In response to the global environmental issue, varieties of pattern coating technology are being developed and applied. The methods combine fluid control and mechanical motion with a roll-to-roll process to define micro pattern directly, and avoids the drawbacks of lithography method, such as time and power consuming, low material utilization, and high pollution in the traditional batch type process.
Pattern coating technology can be classified into two groups as template-needed methods and template-less methods. The latter has higher pattern flexibility because it does not need to fabricate new templates when pattern changes. The template-less methods include ink-jet printing, pen printing, stopper coating and air-bubble coating, in which the last one is the subject being investigated in this study. Amount the templat-less methods, ink-jet printing has the best patterning flexibility, but is limited by low viscosity fluids and has the drawback of poor film uniformity. The pen printing method has also the advantage of pattern flexibility, but the coating speed is restricted by the pattern accuracy. The stopper coating method has the benefit of traditional coating methods; however, the operation frequency is restricted by the machine inertia and the maintenance of the complex electro-mechanical systems can be a challenge. Therefore, in this study, low inertia micro two-phase flow is introduced as the discontinuous working fluid to improve the precision and extend the workable frequency range.
The purpose of this study is to develop the conceptual roll-to-roll precision pattern coating technology, coined Air-Bubble Coating, and to build up the prototype equipment for it. This method can be considered as an advanced-die-coating method, which combines the well-know die coating with the microfluidic technology. Micro two-phase flow is utilized to be the upstream discontinuous coating source and micro-patches can be generated in the coating process. Therefore, the method has the advantages of wide range of fluid viscosity and high coating speed, as with die coating methods, and high precision and response, as with microfluidic system.
The dissertation work is divided into three parts to develop the new concept technology and build up the corresponding theories, step by step:
(1)Using capillary tube as coating head to investigate the coating mechanism:
The process of micro two-phase segments feeding in capillary tube is used to investigate the effects of operation parameters on the two-phase segment size and uniformity. From the experiment, it has been found that the installed air chamber and the lubrication of leading liquid segment affect the segment size uniformity significantly. Theoretical models are proposed to explain the phenomenon.
The feasibility of Air-Bubble coating method with capillary tube is conceptually proven. Moreover, the U(fluid speed)- V(coating speed) coating windows with different coating gaps, length of liquid segments and bubbles have also been discussed.
(2)To coat micro patches sequentially with single T-junction channel
The purpose is to find out the most stable micro two-phase flow generation method and the workable range for Air-Bubble coating. In the study, it has been found that micro two-phase flow has much better stability by using a dual constant pressure source than a commonly used dual constant flow rate source, which is usually a dual syringe pump system. With a dual pressure driven source, the pressures Pc and Pd are chosen as the operation variables, and the concept and physical explanation of effect pressure (Pe) are proposed. The experimental and theoretical methods are further used to investigate the effects of channel geometry (length ratio of upstream and downstream channel, G*), fluid properties (viscosity and surface tension) and pressure ratio (Pe/Pc) on the characteristics (bubble length, frequency and velocity) and flow rate ratio(