**4.3.1 Factors influencing electrical performance**

The main target on the electrical performance of this MEMS display device is to reduce its operation voltage as described in the design part in section 2.3. However, to reduce the upper layer thickness together incorporate with the handling issue in which the electrostatic force is too strong on the <20m PEN. The thin PEN will be easily attracted to the rubber pad and other equipment parts during printing and lamination processes by electrostatic force. The ultra thin upper layer will also induce special concerns on reliability. If extra layer should be added unto the whole structure, the transmittance and the optical performance should also be re-designed. Thus to reduce the thickness of the upper layer is not adequate.


Table 6. *CPD* and its CIE coordinate under different spacer height settings.

To reduce the spacer height is not also a proper solution because the spacer height in OFF state also influences the output color as described in section 4.1. Table 6 is the list of simulated *CPD* under OFF state with white target of (0.31, 0.31) on CIE 1931 chromaticity diagram. The design goal not only fell on the small *CPD* but also required a small *CPD* difference between different colors. Both 400nm and 600nm spacer height designs are with smallest *CPD* differences

( 0.10 0.07 0.12 0.09 0.03 ) but from the gravure printing characteristic point of view in

Figure 27(c), the original 600nm design falls on the center part of the linear region thus provides more confidence on process control. Since the isolation layer thickness is the key for color interference, to reduce its thickness while keeping the same color design is then inaccessible. The final possibility fell on the pixel size and since this study aims on a large area display device for decoration, a 2000m pixel size was set in section 2.3. Note that even though a 15V operation voltage was simulated in the same section, actual driving voltage was far higher than expectation in previous publication as listed in Table 7.


Table 7. Operation voltage difference between simulation and real device [35].

Possibilities for Flexible MEMS:Take Display Systems as Examples 405

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

*R w Rs <sup>l</sup>*

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.

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

(27)

**4.4.1 Sheet to sheet uniformity** 

**4.4.2 Within sheet uniformity** 

equation:
