**3.5 Lamination and finishing**

396 Microelectromechanical Systems and Devices

Figure 27(a) is a schematic plot for a single cell on the cylinder, the pyramid structure was laser engraved from the stainless steel cylinder. Figure 27(b) is the three dimensional printed structures measured optically and Figure 27(c) is an experimental plot for the printed pattern's size and its height. The interesting experiment result falls on that the printed pattern's size is linearly positively related to its designed dimension but the printed pattern's height is only in small range positively related to its designed dimension and finally trends to a saturation behavior. To explain this, the pyramid model in Figure 27(a) is used: The ink transfer process is a balance of force competition between the interface between the substrate (*Fis*) and the ink and the interface between the ink and the cell wall (*Fic*). When *Fis* is larger than *Fic*, the ink will tend to adhere to the substrate based on a premise that the ink quality is uniform within the whole droplet. Inversely, the ink will tend to stay inside the cell. With the relationship between the cell width (*V*) and the cell depth

> <sup>1</sup> <sup>2</sup> tan ( ) *<sup>D</sup> V*

(21)

(22)

*V D* 2 3 (23)

(24)

(25)

(26)

4 4 *A A c t*

4

*A A c t*

 

*A V s*

 

<sup>2</sup> 8 3

2

<sup>2</sup> <sup>12</sup>

<sup>1</sup> <sup>2</sup> tan ( )

*D*

<sup>1</sup> <sup>1</sup> tan ( ) <sup>2</sup>

26.6

*<sup>c</sup> <sup>V</sup>* 

A comparison of *Ac* and *As* indicates that the difference of adhesion force becomes larger and larger when increasing the cell volume. As a result, the transfer of ink from the cell to the substrate becomes more and more difficult and final reaches its limitation. This special behavior will be alleviated if design the stylus angle to a larger value and the linear region can be extended. In order to keep the spacer dot height (*H*) – cylinder cell depth (*D*) curve linear, a critical angle *γc* , from Equation 21, can be calculated based on *Ac*=*As* or *V*=4*D*:

*D*

*D*

*VD*

Since the angle *γ* is 30°, the relationship between *V* and *D* is:

It is also obvious that the area of the opening area of the cell (*As*) is:

When replace *V* in Equation 22 with Equation 23:

The stylus angle is approximately 120° and the total area (*Ac*) of four cell walls (*At*) is:

(*D*), angle *γ* can be calculated:

After the process for both substrates in Figure 23 separately but before their lamination, one more study was performed to improve the MEMS flexible display device's contrast. Previous study only targeted on the demonstration of a single display pixel and did not address much on the overall performance of combined performance. A final compromise was made for the adhesive ink: a 35wt% glue solid content with 30wt% tiny black pigment (Sun Chemical, 049-72784) to block the transmission light from spacer areas. The lamination process was performed manually right after the spacer printing process. A final drying process was also applied right after the lamination step under 120°C for 1h to remove the extra solvent from the inks to reduce the reliability risks. The continuous substrate processed by roll-to-roll facility will be rewound back into a roll for stock and transfer before cut out into pieces for applications but the small area demonstrators fabricated by discrete tools discussed in these sections will be used for test in the next part.
