**3.1.2 Process environment for plastic substrates**

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

#### **3.1.3 Ink**

390 Microelectromechanical Systems and Devices

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

As introduced before, a novel process should be used for the special requirement on not only the structure's flexibility but also on the dot spacer layer design. Several promising solutions were examined in section 1.3 and with the structure set up before; material, process, and

According to Figure 11, the whole structure will be made and the device will be operated in atmospheric ambient. Besides the Intermediate 1 in Figure 11, the multilayer structure contains six layers. The substrate material plays a crucial role for flexibility since it is the thickest part of the structure. When its thickness is below 500m, it is with sufficient flexibility for display applications. However, when a large curvature is expected, only thinner substrates can satisfy this requirement. The latest glass manufacturing techniques support 30m thick commercial products for large scale (300×400mm). Even though thin glass substrates provide very

**3. Fabrication based on printing techniques** 

concerns will be discussed layer by layer here.

**3.1 Material selection and system setup** 

**3.1.1 Substrate** 

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

Possibilities for Flexible MEMS:Take Display Systems as Examples 393

process steps contain not only previously mentioned flexography and gravure printing but also conventional sputtering, cleaning, and drying/heating units. In the same time another or more substrate rolls can also be processed with similar sequences to form required patterns and layer stacks. These processed substrates might be finally laminated together and rewound back into a roll to complete this "roll-to-roll" process concept. Of course the final products can also be cut into sheets as a "roll-to-sheet" system. Note that even there is no multilayer combination and lamination process, the single substrate process is still a rollto-roll system with roll-to-sheet capability. Before the whole continuous system is set up, individual parts in Figure 23 will be evaluated and single process step will be developed

The concept of flexography printing was described in section 1.3. As mentioned previously, the flexography plate contains patterns raised from the surface and introduces very little ink from an anilox instead of the ink tank. Besides the adhesion force between flexography plate and ink, the plate merely plays a crucial role for ink transferring. Unfortunately, no commercial Ag ink can satisfy the specs and thus the direct pattern printing by flexography became inapplicable. A drawback of flexography printing laid on the uneven printed surface. The printed surface uniformity depends on a compromise between pattern integrity (contrast) and flatness: high pattern integrity requires thick (high solid content) ink, which in turn leaves peaks and valleys on the printed surface. Another drawback is that the printing process (not only limited to flexography) is highly direction oriented system: the pattern integrity is better along the printing direction (machine direction, mechanical direction, MD). Figure 24 is a printed example that along the printing direction more complete test lines are available. One can make modifications on ink quality and printing speed but can relatively induce worse surface uniformity and lower throughput unexpectedly. This printing behavior implies that one should design finer resolution along the printing direction to avoid broken lines. The process speed of Figure 24 was set to be 5m/min and its technique of lift-off will be explained in the next section. However, the drawback of surface uniformity is also just the merit of PR in photolithography: the rough surface provides good chemical resolving path during PR removal. With this advantage, a

photolithography-like lift-off process was developed based on flexography printing.

Fig. 24. Printing direction is with better resolution thus horizontal lines are thinner.

firstly with discrete units in the following sections.

**3.2 Flexography printing** 

time. Since the R2R system is designed for high volume production and the plastic substrate is with low operation temperature limit, the data point of 140°C for 1min was set as the process variation boundary. On the other hand, since the spacer has to be sticky but does not have to be precise for physical and optical requirements, commercial adhesive glue (Herberts, EPS71) mixed with 35wt% hardener (Herberts, KN75) was chosen as the spacer material.

Fig. 22. Spin-on SiO2 thickness variation after thermal treatment.

#### **3.1.4 The roll-to-roll system**

The final production system setup is illustrated in Figure 23. The flexible substrate is unwound from a roll and is then transferred into several different process steps. These

Fig. 23. Setup of the roll-to-roll process system.

process steps contain not only previously mentioned flexography and gravure printing but also conventional sputtering, cleaning, and drying/heating units. In the same time another or more substrate rolls can also be processed with similar sequences to form required patterns and layer stacks. These processed substrates might be finally laminated together and rewound back into a roll to complete this "roll-to-roll" process concept. Of course the final products can also be cut into sheets as a "roll-to-sheet" system. Note that even there is no multilayer combination and lamination process, the single substrate process is still a rollto-roll system with roll-to-sheet capability. Before the whole continuous system is set up, individual parts in Figure 23 will be evaluated and single process step will be developed firstly with discrete units in the following sections.

#### **3.2 Flexography printing**

392 Microelectromechanical Systems and Devices

time. Since the R2R system is designed for high volume production and the plastic substrate is with low operation temperature limit, the data point of 140°C for 1min was set as the process variation boundary. On the other hand, since the spacer has to be sticky but does not have to be precise for physical and optical requirements, commercial adhesive glue (Herberts, EPS71) mixed with 35wt% hardener (Herberts, KN75) was chosen as the spacer

The final production system setup is illustrated in Figure 23. The flexible substrate is unwound from a roll and is then transferred into several different process steps. These

Fig. 22. Spin-on SiO2 thickness variation after thermal treatment.

**3.1.4 The roll-to-roll system** 

Fig. 23. Setup of the roll-to-roll process system.

material.

The concept of flexography printing was described in section 1.3. As mentioned previously, the flexography plate contains patterns raised from the surface and introduces very little ink from an anilox instead of the ink tank. Besides the adhesion force between flexography plate and ink, the plate merely plays a crucial role for ink transferring. Unfortunately, no commercial Ag ink can satisfy the specs and thus the direct pattern printing by flexography became inapplicable. A drawback of flexography printing laid on the uneven printed surface. The printed surface uniformity depends on a compromise between pattern integrity (contrast) and flatness: high pattern integrity requires thick (high solid content) ink, which in turn leaves peaks and valleys on the printed surface. Another drawback is that the printing process (not only limited to flexography) is highly direction oriented system: the pattern integrity is better along the printing direction (machine direction, mechanical direction, MD). Figure 24 is a printed example that along the printing direction more complete test lines are available. One can make modifications on ink quality and printing speed but can relatively induce worse surface uniformity and lower throughput unexpectedly. This printing behavior implies that one should design finer resolution along the printing direction to avoid broken lines. The process speed of Figure 24 was set to be 5m/min and its technique of lift-off will be explained in the next section. However, the drawback of surface uniformity is also just the merit of PR in photolithography: the rough surface provides good chemical resolving path during PR removal. With this advantage, a photolithography-like lift-off process was developed based on flexography printing.

Fig. 24. Printing direction is with better resolution thus horizontal lines are thinner.

Possibilities for Flexible MEMS:Take Display Systems as Examples 395

screened out in section 3.1 for its low process speed and batch production characteristic. From the ink engineering point of view, ink jet printing and gravure printing provide similar ink droplet behavior on the substrate but the control variety of ink jet printing is less yet it is also a kind of batch process. When thoroughly review the cell design of a gravure cylinder in Figure 26, one can easily find its control varieties such as: cell width, wall width, channel width, channel depth, cell density, screen angle, depth, and stylus angle. All these factors' combination results in a final parameter of volume. Table 1 is a list of designed patterns for different printed thickness as well as wetting performance. The adjustable parameters are the printing speed, the pressure force, and the contact angle between the

Fig. 26. A cell design set on the gravure cylinder and parameter definitions.

**Design I II III IV V VI VII VIII**  Mesh (line/cm) 60.8 53.8 41 47.8 116.4 104.3 89.9 80.9 Cell width (μm) 152 167.5 232.1 197.6 78.8 91.1 103.4 114.8 Wall width (μm) 12.6 18.3 11.7 11.7 7.1 4.8 7.8 8.8 Channel width (μm) 24 41 47 35 9 19 24 28 Channel depth (μm) 3.5 7.4 8.3 9.5 2 0 4.5 4.8 Cell density (%) 85.3 81.3 90.6 89.1 84.2 90.2 86.5 86.3 Cell depth (μm) 53.1 51.7 42.5 53 52.6 53.6 52.8 53.1 Screen angle(°) 56.9 60.8 58.6 59.4 31 34.4 39.1 43.5 Stylus angle (°) 121.5 119.8 129.8 130.1 119.1 118 118.7 118.3

Fig. 27. (a) Cell model, (b) printed structures, and (c) relationship between (a) and (b) of

doctor blade and the cylinder.

Table 1. Detail list of cell designs.

gravure printing.

#### **3.3 Lift-off**

PR is recognized to be a patterning mask which influences the final structure on the substrates. With the uneven property of flexography printed surface, the ink was also treated as a sacrificial layer which provides protections on the substrate for layer stacking in the same figure. Here we modified Figure 23 from destructive process to constructive process for the same comparison basis. One can easily find that a process step of PR (ink) coating was omitted – the sacrificial pattern was directly printed from flexography plate while the photolithography exposed UV light through a photo mask. Similar to photolithography, this lift-off process is also a multilayer capable step as long as the sacrificial layer is thicker than the total thickness of multilayer. Figure 25 is a step-by-step example of lift-off process substrate. The thickness of sacrificial layer was ranging from 1- 2m which was higher and sufficient for multilayer stack with total thickness less than 1m. The Ag sputtering was controlled with 20nm according to the color interference design in section 2.2. The third and the last step for the lift-off process is the sacrificial layer removal. In corresponding to the ink composition, basic chemicals such as acetone and ethyl acetate are suitable to dissolve it. It was also obvious that the substrate was flexible and was capable for large curvature process at this step. With ultra sonic vibration's help, the sacrificial ink dissolved in acetone in only seconds. Another important factor of heat influence on the removal efficiency was also carried out because of the drying unit after the sacrificial ink flexography printing. Here the sputter process influence was omitted because it was controlled under 60°C.

Fig. 25. Printing process is simpler than photolithography on step numbers: (a) sacrificial ink printing, (b) metal layer coating, and (c) sacrificial layer removal. The dimension of the square pattern in (a) and (b) is designed as 2000μm.

#### **3.4 Gravure printing**

The main difference between flexography printing and gravure printing lies on the controllability of printed layer. As shown in Figure 8 to Figure 9 and experimental results in section 3.2, it is apparent that the printed layer thickness and surface uniformity by flexography is merely controllable. The only solution for thick layer patterning is to choose from screen printing, ink jet printing, and gravure printing. The screen printing was

PR is recognized to be a patterning mask which influences the final structure on the substrates. With the uneven property of flexography printed surface, the ink was also treated as a sacrificial layer which provides protections on the substrate for layer stacking in the same figure. Here we modified Figure 23 from destructive process to constructive process for the same comparison basis. One can easily find that a process step of PR (ink) coating was omitted – the sacrificial pattern was directly printed from flexography plate while the photolithography exposed UV light through a photo mask. Similar to photolithography, this lift-off process is also a multilayer capable step as long as the sacrificial layer is thicker than the total thickness of multilayer. Figure 25 is a step-by-step example of lift-off process substrate. The thickness of sacrificial layer was ranging from 1- 2m which was higher and sufficient for multilayer stack with total thickness less than 1m. The Ag sputtering was controlled with 20nm according to the color interference design in section 2.2. The third and the last step for the lift-off process is the sacrificial layer removal. In corresponding to the ink composition, basic chemicals such as acetone and ethyl acetate are suitable to dissolve it. It was also obvious that the substrate was flexible and was capable for large curvature process at this step. With ultra sonic vibration's help, the sacrificial ink dissolved in acetone in only seconds. Another important factor of heat influence on the removal efficiency was also carried out because of the drying unit after the sacrificial ink flexography printing. Here the sputter process influence was omitted because it was

Fig. 25. Printing process is simpler than photolithography on step numbers: (a) sacrificial ink printing, (b) metal layer coating, and (c) sacrificial layer removal. The dimension of the

The main difference between flexography printing and gravure printing lies on the controllability of printed layer. As shown in Figure 8 to Figure 9 and experimental results in section 3.2, it is apparent that the printed layer thickness and surface uniformity by flexography is merely controllable. The only solution for thick layer patterning is to choose from screen printing, ink jet printing, and gravure printing. The screen printing was

square pattern in (a) and (b) is designed as 2000μm.

**3.3 Lift-off** 

controlled under 60°C.

**3.4 Gravure printing** 

screened out in section 3.1 for its low process speed and batch production characteristic. From the ink engineering point of view, ink jet printing and gravure printing provide similar ink droplet behavior on the substrate but the control variety of ink jet printing is less yet it is also a kind of batch process. When thoroughly review the cell design of a gravure cylinder in Figure 26, one can easily find its control varieties such as: cell width, wall width, channel width, channel depth, cell density, screen angle, depth, and stylus angle. All these factors' combination results in a final parameter of volume. Table 1 is a list of designed patterns for different printed thickness as well as wetting performance. The adjustable parameters are the printing speed, the pressure force, and the contact angle between the doctor blade and the cylinder.

Fig. 26. A cell design set on the gravure cylinder and parameter definitions.


Table 1. Detail list of cell designs.

Fig. 27. (a) Cell model, (b) printed structures, and (c) relationship between (a) and (b) of gravure printing.

Possibilities for Flexible MEMS:Take Display Systems as Examples 397

When the angle γ is controlled smaller than *γc*, the adhesion force *Fis* and *Fic* are at least balanced thus the transferred ink amount will be in linear relationship with the cell volume. For the structure design, the desired thicknesses fall in the linear region in Figure 27(c), thus the stylus angle of 120° was used. Experimented results shows that bigger and denser cell designs helped the printed ink to spread and merge (wetting). Further study and optimization should be done after this work for flatter surface. The final gravure printing process parameters for the isolation layer is summarized in Table 2. The best resolution of

> Speed (m/min)

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

The discrete roll-to-roll printing system was used to check the printing capability, to characterize ink properties, and to study the system design. From system's point of view the continuous roll-to-roll printing processes handled the large area substrate before it was cut into small pieces for discrete roll-to-roll printing processes. From automation's point of view both the continuous and discrete roll-to-roll printing processes were automatically performed but the lamination process was semi-auto for alignment. The final production result of this study was not influenced by any factor of the system or the automation settings. Figure 28 is the pictures for the final demonstrators of (a) 3×3 active matrix array and (b) 21×39 passive matrix array. The 3×3 active matrix array device was examined and

Contact angle between doctor blade and cylinder (°)

Pressure (N)

550 32 60 Green

gravure printing was 200m.

Target Cylinder design

(160nm) <sup>V</sup>

(325nm) VII

(245nm) VI

**3.5 Lamination and finishing** 

**4. Characterization and analysis** 

evaluated for various characteristics.

Red

Blue

part.

(Table 1)

Table 2. The process parameters for different SiO2 targets.

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 (*D*), angle *γ* can be calculated:

$$\gamma = \tan^{-1}(\frac{2D}{V}) \tag{21}$$

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

$$\begin{aligned} A\_{\mathcal{C}} &= 4A\_{\mathcal{f}} \\ &= 4VD \end{aligned} \tag{22}$$

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

$$V = 2\sqrt{3}D\tag{23}$$

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

$$\begin{split} A\_{\mathcal{C}} &= 4A\_{\mathcal{f}} \\ &= 8\sqrt{3}D^2 \end{split} \tag{24}$$

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

$$\begin{aligned} A\_S &= V^2\\ &= 12D^2 \end{aligned} \tag{25}$$

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*:

$$\begin{aligned} \gamma\_{\mathcal{C}} &= \tan^{-1}(\frac{2D}{V}) \\ &= \tan^{-1}(\frac{1}{2}) \\ &= 26.6^{\circ} \end{aligned} \tag{26}$$

When the angle γ is controlled smaller than *γc*, the adhesion force *Fis* and *Fic* are at least balanced thus the transferred ink amount will be in linear relationship with the cell volume. For the structure design, the desired thicknesses fall in the linear region in Figure 27(c), thus the stylus angle of 120° was used. Experimented results shows that bigger and denser cell designs helped the printed ink to spread and merge (wetting). Further study and optimization should be done after this work for flatter surface. The final gravure printing process parameters for the isolation layer is summarized in Table 2. The best resolution of gravure printing was 200m.


Table 2. The process parameters for different SiO2 targets.
