**2.4 The horizontal structure design**

#### **2.4.1 Air pressure consideration**

386 Microelectromechanical Systems and Devices

percentage under certain applied voltage. Similarly, one can also expect the operation voltage for specific contact area. The following simulations were performed under the

0.3m, *E* = 6.1GPa, and *g* = 0.6m. Figure 17 is the simulation result with different pixel size (*L*). From these figures we understand that under the same applied voltage, a larger pixel will result in a larger contact area. From these figures we also understand that when one wants to achieve, for example, 90% contact area, a great operation voltage difference (55V for 1000m pixel and 15V for 2000m pixel) appears in Figure 17(a)-(b). Figure 17(c) is the simulation results with different spacer thickness (*g*). We understand that the operation voltage can be further reduced from 15V to 10V when change the spacer thickness from 600nm to 300nm. An examination in Figure 17(d) also indicated that when change the upper layer's thickness (*h*) from 16m to 8m, the operation voltage can be further reduced from 10V to 5V. Thus, a combination of these improvement designs, one can expect and design a low operation voltage device with this MEMS model. Other parameters concerning material

*<sup>r</sup>* and *E*, can also help on the operation voltage lowering but will not

*<sup>r</sup>*= 3, *h* = 16m, *L* = 2000m, *t* =

*0* = 8.85×10-12 A2s4kg-1m-3,

Fig. 17. Simulation results with different parameters from the MEMS model.

following parameter settings:

characteristics such as

be considered here.

Until now, the device structure is designed and discussed vertically in detail thus its horizontal dimension and structure should also be considered. Previous report indicates high operation voltage with an enclosed Intermediate 4 in Figure 11 which is shown here in Figure 18(a) from its top. The author suggested some solutions to lower down the operation voltage such as to use thinner upper layer, to use thinner Intermediate 4, and to design a larger system. However, if the Intermediate 4 is trapped inside the system when ON, it will become a movement barrier or cause reliability issue unexpectedly. Since atmospheric air is designed for Intermediate 4, it is very possible to reduce the air pressure trapped inside the enclosed spacer area to alleviate the operation voltage. Figure 18(b) is the top view of a newly designed structure. Compared to Figure 18(a), the new design has some openings (air channel) on specific locations. These air channels serve as air evacuation paths when the device is ON. Figure 19 to Figure 21 are the simulations done with commercial software MEMSOne to explain how flexible can the air channel be designed and how the corresponding structure moves. Note that since there are design limitations on the structure by this software, the color legend means the areas in moving instead of its absolute displacement value. The value of opening ratio means the air evacuation efficiency while different designs imply different display shape because the air channel area can also be turned on if the opening ratio is large. Compared to the baseline design, we understand that increasing the opening ratio helps on enlarging the MEMS movement area. In the design of Figure 19, which represents the basic design in Figure 18 where the air channel was put in the center part of one spacer side, the displacement area increases when the opening ratio increases. The extreme model (Figure 19(c)) indicates that the displacement switched to the air channel area rather than the pixel area.

Fig. 18. Renewed spacer layer design's top views.

Similar effects also appeared on the Figure 20 designs, in which the air channel was divided into two sub-channels and were put at the both ends of one spacer side. One can find that up to 40% opening ratio, the displacement area follows the Figure 19 designs but the opening ratio of 60% (not shown here) and 80% (Figure 20(c)) ones helped the displacement continued to expand inside the pixel area. With this design improvement, we can positively change the unexpected displacement area caused by air channel to a reasonable and expectable area within the pixel. Based on Figure 20, the sub-channels were moved to the two ends of one spacer side and the spacer corners were also removed. The opening ratio of

Possibilities for Flexible MEMS:Take Display Systems as Examples 389

Fig. 20. Simulation results of opening ratio of (a) 0%, (b) 40%, (c) 80% for design 2.

one spacer side was thus still the same. The simulation result in Figure 21 did not change when the opening ratio is smaller than 20%, but interesting displacement took place at the spacer corners for opening ratio larger than 40%. This kind of motivation came from Figure 19(c), in which the displacement expanded to the direction with spacer's opening ratio large enough. Similarly, when the opening ratio in Figure 21(c) was large enough, the displacement area expanded to the corners when keeping its shape the same of a square. The structure design and simulation showed very promising results to realize the design in Figure 11.

Fig. 19. Simulation results of opening ratio of (a) 0%, (b) 40%, (c) 80% for design 1.

one spacer side was thus still the same. The simulation result in Figure 21 did not change when the opening ratio is smaller than 20%, but interesting displacement took place at the spacer corners for opening ratio larger than 40%. This kind of motivation came from Figure 19(c), in which the displacement expanded to the direction with spacer's opening ratio large enough. Similarly, when the opening ratio in Figure 21(c) was large enough, the displacement area expanded to the corners when keeping its shape the same of a square. The structure design and simulation showed very promising results to realize the design in

Fig. 19. Simulation results of opening ratio of (a) 0%, (b) 40%, (c) 80% for design 1.

Figure 11.

Possibilities for Flexible MEMS:Take Display Systems as Examples 391

promising options for curved or flexible applications with large curvature, its fragility still limits its realization on flexible electronic devices especially for portable products. The potential safety and reliability concerns also put a barrier between its benefit and realization. In contrary to the fragile glass, elastic polymer material (plastic) is a very good option for the substrate. Because the plastic substrate will be used for the flexible display system, some

are primary material selection principles. Within polymer materials, one can screen out polyvinyl chloride (PVC), polycarbonate (PC), polyethylene (PE), and polyimide (PI) from stability, reliability, deformation, and transparency point of view, respectively. With these concerns, polyethylene teraphthalate (PET) and polyethylene naphthalate (PEN) are relatively suitable for this flexible MEMS design. PET is also famous for its low cost and high transmittance in visible region while PEN is famous for its high temperature stability and sharp cutoff performance for UV light. According to these characteristics, PEN was

Besides embossing and laser ablation, which are patterning techniques for isolation instead of layer stack, the other printing methods are all printing process related ideas. However, within the printing process ideas, the screen printing and ink jet printing are batch processes which do not provide any help on improving the low throughput in photolithography. A compromise between resolution and throughput results in the flexography and gravure printing. Their working concepts have been explained in section 1.3 and the detail process

In printing process, ink plays a very important role. Refer to Figure 11, four layers should be processed besides plastic substrates. Within these four layers, two electrodes are Ag; and the isolation is SiO2. Since the two substrates have to be laminated after process, the spacer should also cover the lamination job. A commercial standard spin-on SiO2 (TOK, OCD T7- 12000-T) was chosen for isolation. This material is composed of RnSi(OH)4-n and additives (diffusion dopants, glass matter forming agent, and organic binder) dissolved in organic solvents (ester, ketone, and mainly consisting of alcohol) in liquid form and thus is suitable for printing process. Its SiO2 solid content is 12wt% and its thickness can be controlled by curing temperature, time, and spin speed if prepared by spin-on process. Because the rollto-roll (reel-to-reel, R2R) system uses gravure printing, whose printing thickness can be adjusted by cylinder cell design, only the curing temperature and time were studied for the thickness control. Figure 22 is the thickness change after thermal and UV treatment which are two optional steps in the process system. Curing temperature was controlled between 100-150°C for less than 30min in this study. After the thermal treatment a 2min 12.5mW/cm2 UV exposure was applied. The thickness change was mainly because the evaporation of solvent and the thickness is basically inversely related to temperature and

parameters and system specifications will be discussed in the following sections.

2. Transparency – High transmittance in visible region (400-700nm) is necessary, 3. Cutoff – Unexpected wavelength (<400nm and >700nm) should be screened out,

5. Reliability – Have to be highly moisture, gas, and chemical resistive,

special requirements including:

chosen as the substrate material.

**3.1.3 Ink** 

1. Flexibility – Low Young's modulus (*E*) is highly expected,

4. Stability – Should be thermally and electrically stable,

**3.1.2 Process environment for plastic substrates** 

Fig. 21. Simulation results of opening ratio of (a) 0%, (b) 40%, (c) 80% for design 3.
