**1.1.4 Electrochromic display**

372 Microelectromechanical Systems and Devices

Compared to particle based Gyricon and electrophoretic display systems, electrowetting system uses all liquid material for color modification. This kind of concept takes advantage of Young-Lippmann's equation (Equation 1) to modify different surface energies between the droplet and the substrate underneath, which in turn changes the contact angle between the droplet and the substrate. Here, *γLG* is the surface tension between liquid and gas, *γSG* is the surface tension between solid and gas, *γSL* is the surface tension between solid and liquid, *θ* is the contact angle, *V* is the applied voltage, and *C* is the electric capacitance per

2

(1)

2 *CV*

unit area in the region of contact between a metal surface and the electrolyte drop.

*LG SG SL*

 

According to Equation 1 and Figure 3, the applied voltage will change the droplet's contact angle. A smaller contact angle represents a larger droplet diameter while a larger contact angle represents a smaller droplet diameter. By following this concept, an electrowetting display system was designed: The intermediate liquid was dyed for different colors while the water was kept transparent. Under normal (OFF) condition, the intermediate color liquid is laying under transparent water thus the reflective light shows intermediate liquid's color. Under operation (ON) condition, the intermediate color liquid is pressed into a specific corner of a pixel, leaving the rest area only with transparent water. Thus the reflection light with background color appears. By switching this system ON and OFF, two different colors can be switched for display purpose as shown in Figure 4. Since electrowetting system is using all liquid material for color modification, it is supposed to be flexible for display application. Some main advantages and disadvantages of electrowetting

cos

1. Gray scale is ideally controllable by applied voltage (contact angle),

 

**1.1.3 Electrowetting display** 

system are listed below:

3. Good color purity (not by mixing colors),

2. Low stability (gravity influence on liquid),

4. Can be designed for transmission or reflection type.

1. Low resolution (limited by liquid suppressed at corners),

3. High power consumption (continuous power supply when ON).

Fig. 3. Contact angle change by applied voltage is the basic of electrowetting.

Advantage

2. Low cost,

Disadvantage

Gyricon and electrophoretic systems are using physical control by electric field outside on colored particles and electrowetting system is also using physical control but the main material contains only liquid. Here, the electrochromic system is using chemical concept to change material's charging condition in dielectric electrolyte in order to change its light absorption band as shown in Figure 5. The electrochromic material can be either dissolved in the electrolyte or coated on the electrode substrate. Typical materials in electrochromic system are WO3 and TiO2. WO3 had been reported with capability to change its colors between transparent under oxidation state and blue under reduction state; TiO2 had been reported with capability to change its colors between transparent under reduction state and white under oxidation state. Thus, using only one electrochromic material can realize a twocolor system and a combination of two electrochromic materials can realize a multiple color system. When the electrochromic material is thinly coated on the electrode, the whole structure will be bendable; when the electrochromic material is dissolved in electrolyte, the whole structure will also be bendable. Thus the electrochromic system can be used as a flexible color filtering device. Some main advantages and disadvantages of electrowetting system are listed below:

Advantage


Disadvantage


All previous described technologies are supposed to be applicable on flexible substrate since all Gyricon, electrophoretic, electrowetting, and electrochromic (if material dissolves in electrolyte) are using liquid as intermediate or main material. But this also implies a reliability concern under critical operation conditions such as: high/low temperature, vibration or shock, and gravity influence, not mention to the jeopardy when the whole system is breaking and the chemical or electrolyte is leaking.

After reviewing these technologies from their basic operations, a summary can be made: Both physical and chemical concepts make the system slim and simple, thus the whole system can be fabricated on a thin substrate for flexible applications while reliability is a special concern. In contrary, stable, predictable, reliable, and reproducible mechanical

Possibilities for Flexible MEMS:Take Display Systems as Examples 375

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

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 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

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

(2)

complementary metal oxide semiconductor (CMOS) process.

**1.2.3 Switching** 

color breakups.

**1.2.4 Interference** 

a full color display.

system.

2 cos <sup>1</sup> *nd m*

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).

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

#### **1.2 MEMS controlled display 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 reviewed by its color modification classifications.
