**1.3.2 Flexography printing**

376 Microelectromechanical Systems and Devices

Manufacturing technique for flexible electronic devices, especially for display devices, is the basic but also a crucial factor. As described and summarized previously, conventional CMOS photolithography techniques are not 100% applicable on polymer flexible substrates owing to heat, UV exposure, chemical treatment, and plasma bombardment. Thus, new process which is suitable for polymer substrate should be firstly developed to support the MEMS display system design. With polymer material's natural paper-like characteristic – stocks in a roll, conventional paper printing process seems workable for patterning the system's circuits as well as its structure. Here, some newly developed printing process will be introduced and evaluated to see how it can be modified and applied on a flexible

After reviewing some trench patterning techniques, it is necessary to consider flat, continuous, and uniform layer stack. Within printing techniques, gravure printing is one of the most famous systems for ink printing on materials such as paper, plastic, and clothes. Its advantages are low cost, addable multiple inks, and gray scale. Its characteristic of low cost comes from the continuous mass production; its addable multiple colors comes from the combination of individual colors prepared by different cylinder to form a color mixture; its gray scale comes from the different designs of cell depth, cell density, cell angle, cell size, and cell shape. The cells are made by laser engraving and are recessed from the cylinder surface. A schematic plot of gravure printing in working is illustrate in Figure 8 and is usually used for continuous process. As shown in this figure, the ink cell on the cylinder represents how dense, how large, how high the printed patterns will be. Thus, gravure

Fig. 6. A three-layer (two-interface) optical interferometer.

Fig. 7. A reflective Fabry-Perot interferometer iMOD.

**1.3 Novel printing process system** 

polymer substrate based display system.

**1.3.1 Gravure printing** 

Gravure printing is not possible to support thin layer deposition because in order to let printed inks merge, dense, deep, large ink cells are expected. Thus the transferred ink layer are usually ranging from 5-10m. Flexography, as illustrated in Figure 9, uses a pattern plate to introduce ink from the anilox roller to the substrate. Since the patterns on the plate are

Fig. 9. An example of flexography printing system.

Possibilities for Flexible MEMS:Take Display Systems as Examples 379

interface even though both the reflective light and the transmissive light decay in the

Color interference takes place when there is optical path length difference (Γ) between two or more light components traveling together. In this figure, the reflective Light 1 and the

Here, *n1* and *n2* is the index of refraction of Intermediate 1 and Intermediate 2, respectively. Since the angle of incidence equals to the angle of reflectance, which is *θ2* in

> cos 2 *d*

AB BC

AD ACsin 1

 

2EBsin 1

*d <sup>n</sup>* 

Replace AB , BC , and AD by Equation 4 and Equation 5 into Equation 3,

2(AEtan )sin 2 1 2 tan sin 2 1 *d*

AD 2 tan ( sin ) 2

*n*

> 

2 2 1

(AB BC) AD 2 1 *n n* (3)

(4)

(5)

(6)

(7)

intermediates.

Figure 10, thus,

According to Snell's law:

The distance AD becomes

Fig. 10. A basic Fabry-Perot interferometer.

reflective Light 2 have optical path length difference Γ:

sin sin 1 12 2 *n n*

raised from the plate surface, the transferred ink amount depends only on the substrate's surface condition and ink's characteristic. However, flexography is suitable for thin layer (<2m) deposition for good uniformity. There's also no ink rheology requirement generated from gravure printing's ink merge process. For pattern isolation, both hot embossing and laser ablation introduce fast and simple solutions but also bring some uniformity concerns. For layer deposition, even though gravure is used for thick layer and flexography is used for thin layer, how precise the thicknesses are seriously influence the optical design in a display device. But apparently, to perform integration process on a flexible substrate, the printing techniques described in this section are compulsory. Other printing techniques such as screen, inkjet, and offset are not suitable for this study but are widely discussed for flexible electronic devices' applications.

### **1.4 Target application**

Section 1.1 already detail described why flexible display is necessary in the future and how those promising technologies are being realized and commercialized nowadays. The technologies introduced in section 1.1 have different target applications and markets and thus are with different concepts. It is obvious that no single technology can satisfy all requirements with all advantages such as low power consumption, high brightness, and fast response time. As a result, this chapter wants to cover and target at the large scale flexible display area for signage, advertisement, and decoration purpose. This means that this chapter is not necessarily pursuing a fine resolution, vivid true color, and fast response which are fundamental factors for TVs and monitors. Nevertheless, this study still looks for and tries to realize these good characteristics as reasonable as possible under some natural limitation such as availabilities of materials and configuration of apparatus. One of the most interesting examples of its applications for this study is to replace the mosaic windows which are usually decorated in churches as a motive. When the above targets are realized, the large scale flexible display sheet will be very distinguishable from previously mentioned flexible display systems and also those MEMS devices listed in section 1.2. Finally, this device will not only support uneven surfaces but will also be programmable to change the mosaic patterns without artificial backlight.
