**5.6 Processes**

410 Microelectromechanical Systems and Devices

Even though the combinational full color display is not a must of the goal for decoration applications, this study implied a full color possibility by combining all three primary colors. However, the original design, which used a unified spacer thickness of 600nm, showed different combinational layer thickness. When combine the structures in Figure 11, a total thickness difference will appear since the isolation (Intermediate 5) thicknesses are different. This means that when using a unified lower substrate, the upper substrate will not be flat as shown in Figure 42. The uneven layer will not only cause process difficulties but will also result in reliability concerns. To overcome this, a structure with unified upper layer which requires different spacer heights and different isolation thickness for different colors should be designed. Some candidate solutions suggested in Table 9 provide uniform OFF state colors (same CIE coordinate). The process detail will be complicated but by doing this, the process issue disappears and the spacer heights can be controlled by gravure printing's

Fig. 41. Simulated best design with Au electrodes.

cylinder and ink engineering discussed in section 3.4.

Fig. 42. Unified spacer height will leave an uneven upper layer.

**5.5 Structures** 

Recent research and development in the printed electronics field indicated usable Ag nano particle ink, which can be a good candidate for printed electrode. But designer also has to consider the influence of high resistivity of this ink because even though the resistivity does not influence the electrostatic behavior, the device requires a high electrical yield for connection lines. After these discussion focused on specific issues from process, material, and operation point of view with possible solutions, one can further expect a perfect final display device which is controlled by MEMS and manufactured by printing processes with flexibility.
