This work dealt with the large area micro electro mechanical (MEMS) made by a roll-to-roll printing system. A Fabry-Perot color interferometer device was introduced into this MEMS as a demonstrator. The roll-to-roll process system was firstly set up to support the manufacturing of the designed MEMS on a large flexible substrate. Optical, electrical, and mechanical characterization were applied on this demonstrator to evaluate the MEMS design as well as the roll-to-roll printing process capabilities later on. The successful demonstration proved that both the large area flexible MEMS was realizable and the printing process was applicable. This work then built significant impacts in the MEMS design and application together in the low-cost mass production fields.

1. Introduction

Electronic paper display devices realized by eletrophoretic, eletrochromic, electrowetting, and electromechanic have been published or even commercialized. However, all these devices are with limited display areas and were fabricated by conventional high-cost, low-throughput, and small-batch photolithography process. Furthermore, all these devices suffer from monochrome or impure colors. Thus, to design and develop a suitable manufacturing system for low-cost, high-throughput, and large-area production is highly expected for electronic devices. On the other hand, even though printed electronic products are only with single or double layers, they show promising capabilities especially for flexible substrates.

Flexible Display System: Some academic and commercial ideas were reviewed, evaluated, and compared. All these systems are using liquid material to achieve both flexibility and color variety thus all of these systems suffer reliability concerns.

MEMS Controlled Display System: Micro electro mechanical system (MEMS) display is a well developed technology with commercialization. Four different control types were introduced to set up the basic of this study. The main drawback of MEMS lies on the rigidity.

Printing System: Conventional printing techniques were evaluated to replace current photolithography not only for efficiency but also to support flexible substrates. Good candidates were defined in this section.

Study Goal and Target Application: The main target application is large area flexible decoration with attraction. Four main goals were set up as evaluation references. An example of targeted market was also given here.

Thesis Structure: The over all structure of this thesis was not only graphically illustrated but also explained in detail.

By taking advantages of photolithography processed displays and printing system processed electronics, this study focuses both on the development of novel printing techniques and large area full color flexible display. The goal of this study is to setup a multiple printing-step roll-to-roll manufacturing system and to realize prototype of large scale flexible display by this system.

2. System Design and Simulation

In chapter 1, some flexible display ideas and control mechanism have been introduced. Within them, the Fabry-Perot was evaluated as the most promising system to be controlled by MEMS. This study then chose the MEMS as the flexible display's control system with Fabry-Perot color interference concept.

In this chapter, a multilayer structure will be firstly introduced to explain the Fabry-Perot interferometer system and its application on color filtering. Some equations and calculations will also be given to verify its characteristics followed by simulations. In the same time, a special discussion on materials' behavior will also be given which strongly influence the color filtering effects. Further simulations with different structure designs which include air channels and thickness of different layers will also be carried out to optimize the whole system.

Model: In order to explain and predict its operation, a MEMS model composed of a cantilever and a flat plate was set up and simulated for movement under applied voltage. This is the first time to introduce this superposition model which expects the MEMS device performance and implies improvement solutions.

Structure: Special design concerns were placed on the spacer (barrier) layer. Not only novel air channel shows promising advantages for operation, but also spacer layer itself implies air evacuation path solutions. Great amount of simulations were done to support the MEMS model with predicted best settings.

Color filtering: Transmissive Fabry-Perot color interference is the basic idea for color filtering. By modifying the interferometer (cavity), designed wavelength (color) can be picked out. Detailed material-oriented designs were focused in this part. The display device's dimensional structure was decided according to color designs. A novel quantitative method to judge the color purity by color purity deviation (CPD) was proposed and used. By quantifying the purity value on CIE chromaticity diagram, use understands how the purity was improved from a target and how far the experimental data is from the target.

3. Fabrication

With the successful structure set up and model simulation, this chapter will deal with fabrication details. As introduced in chapter 1, a novel process should be used for the special requirement on not only the structure's flexibility but also on the dot spacer layer design. Several promising solutions were examined in chapter 1 and with the structure set up in chapter 2; material, process, and concerns will be discussed layer by layer in this chapter. The main idea is to reproduce a three dimensional circuit structure. New printing processes which show great possibility to replace conventional photolithography process were discussed in this section. A combination of printing processes in series which becomes a roll-to-roll system was developed with high production speed. During the process, both discrete and continuous roll-to-roll system were used and from the operation point of view, both auto and semi-auto system were used.

Material: Candidate material's characteristics of each layer were discussed in this part. Substrate material was chosen to screen out hazardous ultraviolet (UV) light; the electrode material was chosen for the best color filtering effects; the isolation material was chosen with roll-to-roll printing capability; the spacer material was chosen with both printing and lamination capability.

Electrode Patterning: A lift-off process is modified from photolithography was introduced and developed in this part. Each part of this technique showed high-speed, low-cost, and large-area process capabilities.

Thick Layer Printing: Gravure printing was developed for different layers. Rheology characteristic of gravure printing was evaluated for process optimization. A uniform layer is expected for isolation layer, which is the critical part in color modulation. Good relationship between cylinder cell design and printed structure shapes was also setup.

Lamination: Lamination of two layers is not possible in photolithography process; however, printing process provides capability by using adhesive ink. According to the study done on gravure printing, unmerged ink patterns in spacer layer provide similar function as air channel design. Relative contrast (CR) in spacer area was defined. With extra doping in spacer ink, high relative contrast was achieved to improve the process efficiency.

Roll-to-Roll System: This part shows the integrity and flexibility of previously mentioned printing processes. Detail process parameters such as speed, resolution, temperature, and pressure of each step were discussed. Gravure cylinder parameters of mesh, width, density, depth, screen angle, and contact angle were analyzed to support the rheology designs.

Characteristics: Electrical (electrostatic movement) and optical results was presented in this subsection.

4. Characterization

An active-matrix driven transmissive MEMS-controlled flexible display array designed in chapter 2 was manufactured by roll-to-roll printing processes mentioned in chapter 3. According to the study made for gravure and flexography printing techniques, lift-off (flexography) was chosen for lower electrode patterning while gravure was chosen for isolation and spacer layer. The whole structure was made partly by continuous and partly by discrete roll-to-roll system described above. An area size of 64×118mm (21×39 array) was achieved and a test area of 3×3 array was cut out from the substrate for characterization.

Optical Performance: With the well design of electrode material, device's transmittance achieved 50% in average. The transmittance peaks of three primary colors also showed balanced intensity. The decision of choosing PEN as substrate helped on cutting unwanted UV lights for this transmissive device.

Structural Performance: Newly designed air channel successfully evacuated air trapped inside a display pixel thus helped the on the operation voltage from over 100V to less than 20V. An optimized result with simulation support indicated the best spacer coverage design of 90%.

The color variations according to the view angles satisfied the purpose and goal of decoration application. The variable colors also enhanced this decoration device's attraction.

Electrical Performance: Not only the operation voltage was reduced, but also a dynamic active matrix array was realized. Both successful individual and multiple pixel control of the demonstrator showed its varieties on pattern programming.

Mechanical Performance: A series test proved that this flexible display device can be operated under bending conditions with radii larger than 54mm.

Yield Performance: A 97% and a 100% lift-off process yield for electrode layer was achieved on the sheet to sheet and within sheet, respectively.

5. Discussion

After review the MEMS display device's electrical, mechanical, and optical behaviors in chapter 4, this chapter will deal with some special considerations. These considerations came with the original design and sometimes worsened along the long term operation or the mass production. However, these considerations do not necessarily happen on all samples under all process or operation conditions. Thus the discussions on these considerations help on verifying some root causes of issues and also help on improving the device into a more complete design. The discussions in this chapter will cover the fundamental, process, and operation topics. Potential issues found during this study such as alignment accuracy, color degradation, surface condition, and true color were raised in this chapter with suggested solution.

Alignment Accuracy: The demonstrator was proved to be rotary shifted by semi-auto lamination process. This apparent misalignment can be minimized by process with long substrate with the same method.

Color Degradation: Demonstration colors first enlarged to a larger extent of the display pixel along with activation time then changed colors with elevated operation voltage before finally burned out. The unevenness of isolation layer was the root cause. A solution for this degradation of using two isolation layers was raised. Another roll-to-roll isolation layer process done with sputter is also believed to be flat and solid to persist under pressure.

Surface Condition: Even though the macro view of printed then cured isolation layer done by gravure printing was uneven, the micro view of gravure printed surface covered the surface roughness variation and degradation generated from each process step in the roll-to-roll system. This behavior relieved many potential reliability and color purity concerns published before.

True Color: This study used single electrode material to simplify the process complexity with averaged acceptable primary colors. A solution for true color simulation suggested different electrode materials for different colors was performed for future refinement.

6. Conclusion

In order to manufacture the large area MEMS controlled flexible display device, several novel printing processes have been developed before the realization of this device. The final chapter of this thesis will conclude the achievements from both process and device sides. Here relist the study goals mentioned at the end of chapter 1: support large scales, support flexibility, support vivid colors, and setup a process line to support this device's manufacturing.

The conclusion will qualify the importance and the achievement of this study and will quantify the improvement of each proposal and a prospection section will propose solutions to perfect this idea in the future.

Process: The most important part in the process is the inauguration of a roll-to-roll system. Within this system, novel ideas of gravure printing, flexography printing, and lift-off technique were realized.

Device: An overall solution for the MEMS flexible display device from design, modeling, simulation, and evaluation were performed in this thesis. Based on the printing process developed before, a demonstrator was successfully evaluated for comprehensive optical, electrical, and mechanical properties.

Prospection: A complete structure composed by three primary colors with different layer materials and layer stack was evaluated. This prospection can serve as a reference for the future studies.

本論文は "Large Area MEMS Flexible Color Pixel Sheet by Roll-to-Roll Printing Technology" (邦訳:ロール・ツー・ロール印刷技術による大面積MEMSフレキシブルカラーピクセルシート)と題し、長尺のプラスチック・フィルム材料表面上にロール・ツー・ロール印刷技術による連続加工を施し、大面積にわたって静電駆動型のMEMS(Micro Electro Mechanical Systems)アクチュエータ構造を集積加工する技術について、その設計方法、製作方法、応用試作例、その評価方法についてまとめたものであり、英文による全6章で構成されている。

第1章は "Introduction" (序論)であり、本研究の背景技術について述べている。従来のMEMS技術では半導体製膜技術やフォトリソグラフィ技術を用いて小型・高精細なデバイスを集積化する手法が主流であったのに対して、本章ではグラビア印刷、フレキソ印刷、ラミネート加工などのロール・ツー・ロール印刷技術を用いることで大面積のデバイス加工が可能であることを述べるとともに、特にその応用先としてフレキシブル画像ディスプレィや電子ペーパーを取り上げ、デバイスの原理と印刷による製造方法について解説し、本論文の目的と研究の意義、論文の構成について説明している。

第2章は "System Design and Simulation" (システム設計および解析)であり、ロール・ツー・ロール印刷技術を用いた大面積MEMSとして、その特性がもっとも発揮できる応用として静電駆動型のファブリ・ペロ干渉計型のカラーピクセルのデバイス原理を提案し、白色の透過光から赤、緑、青の三原色を色純度良く取り出すための光学設計と、静電駆動力による光干渉長の制御方法について説明している。

第3章は "Fabrication" (製作方法)であり、ロール・ツー・ロール印刷型のフレキソ印刷、リフトオフ加工、グラビア印刷、ラミネート加工等を組み合わせることによって、半導体プロセスのフォトリソグラフィと製膜加工に相当するプロセスを大面積にわたって実施する方法について説明している。特に、フレキソ印刷によって所望の形成パターンのネガ型インクを先に塗布しておき、その上から所望の材料を連続蒸着した後に、最初のインクを有機溶剤で選択的に除去することで、リフトオフ型のパタニングが可能であることを実験的に示している。また、第2章で提案した静電駆動型のファブリ・ペロ干渉計型のカラーピクセルをロール・ツー・ロール印刷技術によって連続加工するための具体的方法について述べ、実際に印刷技術によってデバイスを試作した結果について述べている。

第4章は "Characterization" (評価技術)であり、第3章で製作したデバイスの光学特性(透過率、色純度)、色味の観察角度依存性、コントラスト、静電駆動特性、色純度の改善方法、マトリクス型のピクセルの個別駆動実験結果、および、印刷による製造歩留まりについて述べている。

第5章は "Discussion" (考察)であり、ロール・ツー・ロール印刷技術を用いたMEMS構造製作の際の、層間アラインメント(位置決め)精度、ラミネート貼り合わせ加工時の角度アラインメント精度について述べるとともに、製作した静電駆動型カラーピクセルに電圧を印加し続けた際に生じるデバイスの劣化と色味の変化に関する実験結果について報告している。

第6章は "Conclusion" (結論)であり、本論文で示した成果を総括している。

以上これを要するに、本論文は、従来の半導体微細加工技術におけるフォトリソグラフィ工程、製膜工程、エッチング工程、ウエハ貼り合わせ工程などを、それぞれロール・ツー・ロール型のフレキソ印刷、グラビア印刷、リフトオフ洗浄、および、ラミネート加工に置き換えることで、従来にない大面積・長尺にわたるMEMSデバイスの加工を高速加工で実現する新たな製造手法を考案するとともに、実際に各種ロール・ツー・ロール印刷技術を用いて静電駆動ファブリ・ペロ干渉計型のフレキシブルカラーピクセルシートを製作してその動作を実証し、印刷による製膜厚みや位置合わせ等の加工精度について実験的に検証したものであり、電気工学に貢献するところが少なくない。

よって本論文は博士(工学)の学位請求論文として合格と認められる。