**4.3.2 Combinational performance**

Figure 34 is the three primary color pixels of the 3×3 MEMS flexible display demonstrator made partially by continuous and partially by discrete roll-to-roll printing processes operated under <20V. According to simulations, the contact area should be 93%(15V), 92%(20V), and 94%(20V) for red, green, and blue, respectively. These data typically match the expectations and the trend in Table 7. The big difference between these data and previous study is the Newton's ring's size. In the previous study the Newton's ring can be found under both small and large display apertures but the Newton's ring can merely be found in the green and blue pixel of this study because of the air channel design. This kind of improvements can be attributed to:


The first merit was fully explained in section 4.2 and the second merit can be explained with Figure 35. Since the spacer structures were printed with pyramid shapes as shown in Figure 27(b), their oblique surfaces provide supports for the upper layer when ON. Also because of the black pigment doping in the spacer ink, the Newton's ring's color was blocked by the spacer structure.

Fig. 34. The 3×3 test sample under ON state. A single pixel (square) is designed as 2000μm.

Fig. 35. A schematic plot to explain how Newton's ring was suppressed.

#### **4.4 Yield performance**

Section 4.1 to section 4.4 reviewed the MEMS flexible display device's characteristics. This section will review the roll-to-roll process's integrity from the mass production point of view.

#### **4.4.1 Sheet to sheet uniformity**

404 Microelectromechanical Systems and Devices

Figure 34 is the three primary color pixels of the 3×3 MEMS flexible display demonstrator made partially by continuous and partially by discrete roll-to-roll printing processes operated under <20V. According to simulations, the contact area should be 93%(15V), 92%(20V), and 94%(20V) for red, green, and blue, respectively. These data typically match the expectations and the trend in Table 7. The big difference between these data and previous study is the Newton's ring's size. In the previous study the Newton's ring can be found under both small and large display apertures but the Newton's ring can merely be found in the green and blue pixel of this study because of the air channel design. This kind

1. Low operation voltage – By using the air channel design to evacuate air pressure when

2. Small Newton's ring – By replacing the steep spacer made by photolithography by the

The first merit was fully explained in section 4.2 and the second merit can be explained with Figure 35. Since the spacer structures were printed with pyramid shapes as shown in Figure 27(b), their oblique surfaces provide supports for the upper layer when ON. Also because of the black pigment doping in the spacer ink, the Newton's ring's color was blocked by the

Fig. 34. The 3×3 test sample under ON state. A single pixel (square) is designed as 2000μm.

Section 4.1 to section 4.4 reviewed the MEMS flexible display device's characteristics. This section will review the roll-to-roll process's integrity from the mass production point of

Fig. 35. A schematic plot to explain how Newton's ring was suppressed.

**4.3.2 Combinational performance** 

of improvements can be attributed to:

oblique ink spacer made by gravure printing.

ON;

spacer structure.

**4.4 Yield performance** 

view.

Figure 36 is the cumulative plot for electrode's sheet resistance (*Rs*). Since *Rs* excludes the influence by thickness, it represents a normalized impedance to its area with the following equation:

$$R\_S = \frac{R \times w}{l} \tag{27}$$

where *R* is the sheet resitance. Note that the real line length (*l*) and real line width (*w*) will differ from the designed value, only the real value should be used to correlate with area size. Even though section 3.1 suggested a better flexography printing resolution along MD direction, electrical test revealed that the finest resolution was about 40m for both transverse direction (TD) and MD. The data in this figure came from continuous 10m substrate with repeated 18 patterns for 8 times (sheets). The failure rate was 2.78% (4 out of 144) which is very compatible with current commercial semiconductor process lines. The whole patterning process done by the continuous roll-to-roll system including sacrificial ink printing, metal sputtering, and ultrasonic assisted lift-off was successfully developed and proved. From the figures we also understood that narrower lines were with larger standard deviations which implied poorer resolution controls. From the results, the smaller standard variation value of vertical patterns (0.54ohm/sq) also suggested better printing integrity along the MD direction. The TD patterns (whose standard variation is 0.85ohm/sq) showed finer lines but was by chance. Thus when one wants to try to obtain fine lines, it is suggested to design patterns normal (90°) to the printing direction but when one wants to obtain stable performance, it is suggested to design patterns along the printing direction. With these data, the developed lift-off process is suggested for the wider than 55m line width applications.

Fig. 36. Sheet resistance yield plots of electrode layer with (a) TD and (b) MD pattern.

#### **4.4.2 Within sheet uniformity**

Another test line set which occupies the whole sheet was used to check the within sheet uniformity. These test lines were designed only along the MD direction. Figure 37 is the cumulative plot for a 2mm long line which was used to fabricate passive matrix samples. Compared to Figure 36, these data were perfectly distributed with 100% as a sharp line since

Possibilities for Flexible MEMS:Take Display Systems as Examples 407

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

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

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

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

lamination was performed with the test printer by the following procedures:

2. Align the real upper layer with the duplicated layer and put them on the roller,

1. Prepare a duplicated lower layer pattern on a thin substrate,

5. Remove the dummy layer to obtain the laminated device.

4. Activate the roller to laminate the two real layers,

**5.1 Alignment accuracy** 

adjust the alignment.

**5.2 Color degradation** 

3. Put the real lower layer on the plate,

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 production line.

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