**5.3 Surface condition**

408 Microelectromechanical Systems and Devices

because the electric power was removed suddenly thus the display area did not 100% return to its original state. A study done with a periodical electric power supply which continuously switched between two polarities and thus the charges could be removed and the upper layer could return back to the original state. The electrodes with accumulated charges were further supported with more and more charges under the cumulative stress test, thus the display area expanded larger and larger. Some solutions for this reliability

In order to obtain high and solid insulating material in a roll-to-roll process system, a sputter process is necessary and the sputter still matches the mass production requirement as the metal sputter process does. To continuously include the isolation layer sputter in the same apparatus of metal target can further reduce the process time (vacuum pumping) and

Since the highest working temperature of plastic material is normally lower than 200°C, the over 350°C curing temperature for SiO2 curing is not acceptable. This issue can be solved by replacing the plastic substrate by glass. Even though the glass substrate is fragile for shear stress, it can be made thin and thus flexible. Section 3.1 gave one example of how thin, how

Either the sputtered SiO2 or the completely cured SiO2 ink will not solve the profound impact of electrostatic adhesion between upper electrode layer and the isolation layer. As a result, the sputtered SiO2 or the completely cured SiO2 ink will still degrade owing to the detachment of electrode material from the upper substrate. To prevent the electrode material from detaching from its substrate, a good solution is to cover the electrode with the isolation material. When this device is ON, the combinational thickness matches the color design value in the second section and the output color is expected to be the same. However, as summarized in Table 8, the OFF state non interfered color needs to be specially designed. The thickness in this table was easily divided the isolation layers into equivalent two parts from its originally designed value. Compared to the data for single layer, the data for double layer will form a larger triangle which is a drawback explained

Red=160 (0.30, 0.24)

Blue=245 (0.22, 0.26)

CIE coordinate when OFF

Green=325 (0.25, 0.26) ---

Red=80×2 (0.32, 0.24) acceptable Green=162×2 (0.29, 0.21) acceptable Blue=122×2 (0.23, 0.44) large

Difference

issue are thus proposed in three ways:

**B. Complete drying and curing process** 

large, and how flexible can the thin glass be.

Structure Isolation layer

thickness (nm)

Table 8. OFF color difference from single and double layer designs.

the equipment space (add one target instead of add a machine).

**A. Solid isolation material** 

**C. Doubled isolation layers** 

in section 4.1.

Single layer

Double layer

Previously study concluded that the surface unevenness was the root cause for the poor color purity. The reference also concluded that the same issue caused the isolation breakdown. Furthermore, the surface unevenness structures distributed on the substrate surface mentioned in the reference were dense and uniformly spread on the substrate, the color interference of the defects then should not be as uniform as the data disclosed in the same reference. But even though the hypothesis was not correct, this surface defect should be eliminated. Figure 40 is the surface profile analyzed by atomic force microscope (AFM, SII NanoTechnology Inc., Nanopics). Here *Rq* is the root mean square value of measured data. The surface condition was monitored (a) before metal electrode sputtering, (b) after metal electrode sputtering, (c) after pattern lift-off, and (d) after isolation layer printing. There was no significant difference for the first three steps which suggested the substrate's profile was always inherited and followed from layer to layer until the SiO2 was capped. Compared to the sputter and lift-off process, the gravure SiO2 printing and the following drying and curing processes covered all these surface defects. The SiO2 droplets transferred out from the gravure cylinder reflowed, spread, and finally merged before drying process provided a more uniform surface than the original substrate did. Thus the printing rheology is a key point for study.

Fig. 40. Surface profile after different processes.
