**5. Summary and discussion**

After review the micro MEMS display device's electrical, mechanical, and optical behaviors in previous section, this section 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. 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.

Fig. 38. The schematic plot for manual alignment and semi-auto lamination.

#### **5.1 Alignment accuracy**

406 Microelectromechanical Systems and Devices

the line width were relatively wider than the lines in the test pattern set, these 2mm lines were thus with less ink wetting induced variations from gravure printing. Even though the study goal is a large area MEMS controlled flexible display device, to develop a process which can support the requirement of the display system emerged parasitically. Thus this characterization section reviewed not only the device itself, but also the yield of the

Fig. 37. Sheet resistance yield plots of electrode layer test patterns on the same sheet.

Fig. 38. The schematic plot for manual alignment and semi-auto lamination.

After review the micro MEMS display device's electrical, mechanical, and optical behaviors in previous section, this section 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. Thus the discussions on these considerations help on verifying some root causes of issues and also help on improving the device into a more

production line.

**5. Summary and discussion** 

complete design.

This MEMS flexible display device was made partially by automatic continuous roll-to-roll system and partially by semi-auto discrete processes. Since the alignment apparatus was not yet installed in the roll-to-roll system shown in Figure 23, the alignment process during lamination of the two layers was performed manually. As shown in Figure 38, the lamination was performed with the test printer by the following procedures:


The solution for misalignment is to install the lamination process into the continuous production line and control the same misalignment amount over a long distance. Figure 39 is the schematic plot for this idea. Let *L1*>>*L2* and *a* is the smallest misalignment done by semi-auto system with manually alignment. Since the *a* value is fixed no matter how long the substrate is, when the process was aligned with a long substrate (*L1*) and cut into smaller sheets (*L2*) the misalignment amount *b* will be smaller than *a*. This kind of comparison was made base on the concept of unit length (here, the *L2*). For example, the misalignment amount roughly reduced to 10% on the small area when the process distance was 10 times longer; the misalignment amount roughly reduced to 1% on the small area when the process distance was 100 times longer. When good alignment is expected on small areas, alignment mark can be added on both layers and registered and adjusted optically. Moreover, a feedback system which is capable to adjust the cylinder's location will also be helpful to adjust the alignment.

Fig. 39. The longer substrate helps on reducing the manual misalignment per unit length.

#### **5.2 Color degradation**

During the experiments and evaluations, a color degradation issue was found. The display color degraded from the original color after long term, high stress (voltage) operation. Note that the reliability test was cumulatively stressed from the low voltage → short term → long term → high voltage → short term → long term. Previous study attributed similar behavior to the reliability of thin electrode layer (12nm aluminum) and the strong electrostatic adhesion between upper electrode layer and the isolation layer. When they are in contact under stress, the upper electrode pealed off from the upper substrate and became incapable for color interference anymore. There was a light trend of display area with test sequence. This was because the isolation layer thickness difference and the charges started to accumulate from the thinnest areas. However, the charges were not smoothly removed

Possibilities for Flexible MEMS:Take Display Systems as Examples 409

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

The purer red provided by Ag appeared owing to Ag's suitable optical parameters. However, the simulated and experimental data were not close to target red identified in Figure 13. Other metals such Au and Cu were examined to see how they provide interfered colors. Figure 41 is the simulations done with Au and Cu electrodes. As indicated by its color purity deviation (*CPD*), the red color by Au and green color by Cu provided smallest values. However, these values were not small enough to be true colors. Thus true colors of red and green require either new electrode materials or a multiple layer which serves as a single electrode layer to perform color interference. Other popular metals such as chrome (Cr) and cobalt (Co) were exempt from the simulation database because of their poor color

**5.3 Surface condition** 

is a key point for study.

**5.4 True colors** 

Fig. 40. Surface profile after different processes.

and transmittance performance during simulation.

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 issue are thus proposed in three ways:
