**1.2.1 Reflection**

The most famous commercial reflective MEMS display system is Texas Instruments (TI) digital micromirror device (DMD). This device is fabricated on silicon (Si) substrate with complicated mechanical movement design. The key module – micromirror – is mounted on the center part of a torsion beam supported platform and the tilt angle of the platform can be controlled by electrostatic force to ±12°. With this setting, the reflection light from light source can be directed to display location (ON state, with color) or a shutter (OFF state, black). This is the realization of basic optical MEMS mirror design and people can generate three primary colors by implementing three DMD modules together or by using a color filter on a single DMD module.

#### **1.2.2 Diffraction**

When optical slits' sizes are well designed, optical diffraction takes place when light goes through the slits. Take white light as an example, different wavelength components diffract into different direction. Thus when viewing angle is fixed, different diffraction grating designs generate different primary colors. Sony's (originally developed by Silicon Light Machines) grating light valve (GLV) is one of the applications. Its MEMS part lies on the control and movement of its thin periodical metal ribbons. The ribbons reflect incident light under OFF state and specific wavelength is diffracted into designed direction when electrostatic force is applied. The individual control off each ribbon makes the system with different diffraction spatial frequencies for different colors. This device is usually made with complementary metal oxide semiconductor (CMOS) process.

#### **1.2.3 Switching**

374 Microelectromechanical Systems and Devices

system is considered for flexible application. In which, electrostatic force controlled micro scale system with mechanical movement for color filtering is set to solve the reliability problem. With these settings, the structure becomes a micro electro mechanical system

MEMS device is usually fabricated on solid substrate with batch photolithography process. In this section, some commercially realized MEMS display system will be discussed and

The most famous commercial reflective MEMS display system is Texas Instruments (TI) digital micromirror device (DMD). This device is fabricated on silicon (Si) substrate with complicated mechanical movement design. The key module – micromirror – is mounted on the center part of a torsion beam supported platform and the tilt angle of the platform can be controlled by electrostatic force to ±12°. With this setting, the reflection light from light source can be directed to display location (ON state, with color) or a shutter (OFF state, black). This is the realization of basic optical MEMS mirror design and people can generate three primary colors by implementing three DMD modules together or by using a color

When optical slits' sizes are well designed, optical diffraction takes place when light goes through the slits. Take white light as an example, different wavelength components diffract into different direction. Thus when viewing angle is fixed, different diffraction grating

Fig. 5. The basic composition of an electrochromic system.

**1.2 MEMS controlled display system** 

filter on a single DMD module.

**1.2.1 Reflection** 

**1.2.2 Diffraction** 

reviewed by its color modification classifications.

(MEMS).

Shutter is one of the applications in MEMS field and most uses of shutters are on optical or display categories. Pixtronix's digital micro shutter (DMS) is the representative device. DMS is fabricated by photolithography process on solid substrate with a suspension beam on opposite sides. The mechanical movement of the shutter layer opens and closes the output light from below generated by white light source. Its full color presentation comes from ultra fast switch rate which allows >1000 colors per second and avoids video fragments and color breakups.

#### **1.2.4 Interference**

Color interference takes place when a light beam is interfered by itself. To achieve this, one can put a dielectric material in another intermediate as shown in Figure 6. A key point to generate self interference is to have both reflection and transmission light at each interface. For example, light goes through Interface 1 or Interface 2 will generate two waves: one transmission light and one reflection light. Interference happens when transmission light from Interface 1 (t1) encounters reflection light from Interface 2 (r2). As a result, both constructive and destructive interference lights are formed as output. The Fabry-Perot interference condition (Equation 2) describes the constructive (visible) light under certain criteria. This Equation implies that when the incident angle (*θ*) and index of refraction (*n*) are understood, the output interference wavelength (*λ*) can be determined by dielectric material's thickness (*d*). The index *m* in the equation means any positive integer. A multiple layer stack for Fabry-Perot interference is normally the basic design concept of wavelength filters for visible colors and invisible transmission applications . Qualcomm's interferometric modulator (iMOD) took this advantage and commercialized small scale, low power consumption, high contrast device for display application. As shown in Figure 7, the reflection light's wavelength is determined by the gap distance between the solid substrate and a deformable metal membrane. Its OFF states reflect three primary colors and its ON states interfere the output lights to invisible region to compose a full color display.

$$2nd = m\lambda \cos \theta\_1 \tag{2}$$

Although most of these MEMS ideas require solid substrate and CMOS photolithography process, some of them already showed flexible system with soft substrate when explaining MEMS with generalized terms: A mechanical movement system controlled by electrostatic force in micrometer scale. The rest issues lie on how to process or manufacture such flexible system.

Possibilities for Flexible MEMS:Take Display Systems as Examples 377

printing is usually used for thick layer transfer. When ink's solid content is low (less sticky), printed high pattern density ink will spread and then merge together. This behavior

Gravure printing is not possible to support thin layer deposition because in order to let printed inks merge, dense, deep, large ink cells are expected. Thus the transferred ink layer are usually ranging from 5-10m. Flexography, as illustrated in Figure 9, uses a pattern plate to introduce ink from the anilox roller to the substrate. Since the patterns on the plate are

provides a solution for continuous thick layer preparation.

Fig. 8. An example of gravure printing system.

Fig. 9. An example of flexography printing system.

**1.3.2 Flexography printing** 

Fig. 6. A three-layer (two-interface) optical interferometer.

Fig. 7. A reflective Fabry-Perot interferometer iMOD.

#### **1.3 Novel printing process system**

Manufacturing technique for flexible electronic devices, especially for display devices, is the basic but also a crucial factor. As described and summarized previously, conventional CMOS photolithography techniques are not 100% applicable on polymer flexible substrates owing to heat, UV exposure, chemical treatment, and plasma bombardment. Thus, new process which is suitable for polymer substrate should be firstly developed to support the MEMS display system design. With polymer material's natural paper-like characteristic – stocks in a roll, conventional paper printing process seems workable for patterning the system's circuits as well as its structure. Here, some newly developed printing process will be introduced and evaluated to see how it can be modified and applied on a flexible polymer substrate based display system.

#### **1.3.1 Gravure printing**

After reviewing some trench patterning techniques, it is necessary to consider flat, continuous, and uniform layer stack. Within printing techniques, gravure printing is one of the most famous systems for ink printing on materials such as paper, plastic, and clothes. Its advantages are low cost, addable multiple inks, and gray scale. Its characteristic of low cost comes from the continuous mass production; its addable multiple colors comes from the combination of individual colors prepared by different cylinder to form a color mixture; its gray scale comes from the different designs of cell depth, cell density, cell angle, cell size, and cell shape. The cells are made by laser engraving and are recessed from the cylinder surface. A schematic plot of gravure printing in working is illustrate in Figure 8 and is usually used for continuous process. As shown in this figure, the ink cell on the cylinder represents how dense, how large, how high the printed patterns will be. Thus, gravure printing is usually used for thick layer transfer. When ink's solid content is low (less sticky), printed high pattern density ink will spread and then merge together. This behavior provides a solution for continuous thick layer preparation.

Fig. 8. An example of gravure printing system.
