Backgrounds

Chapter 1 Introduction

1.1 Backgrounds

With the rapid development of the optoelectric and biomedical industries, the number of applications for transparent conductive oxide (TCO) thin films, aspheric glass lenses, and hydrophobic and hydrophilic templates has increased tremendously, especially in touch screen and disease testing task applications. To reduce heavy investment in processing equipment, minimize chemical impact on the environment, and increase processing speeds, laser dry etching has become an important manufacturing method, and is widely used to remove superfluous materials. Moreover, laser-machining technologies include milling, drilling, cutting, marking, scribing, annealing, and texturing.

Common high-power laser sources for industry include the CO₂ laser, with 10.6 μm and 9.3 μm wavelengths; the Nd:YAG laser, with 1064 nm, 532 nm (doubled), 355 nm (tripled), and 266 nm (4^th harmonic) wavelengths; the fiber laser, with 1070 nm; and the excimer laser, with wavelengths from 157 nm to 351 nm (figure 1.1). Figure 1.2 to 1.3 show schematic diagrams of the machined results for different laser pulse widths [1,2]. The pulse width for Nd:YAG laser (λ: 1064 nm) is short than for the CO2 laser (λ: 10.6 μm), thus, the Nd:YAG machined quality, including heat affected zones (HAZs), micro cracks, surface debris, and shock waves, is much better than that produced by CO2 laser machining. Moreover, the cost of Nd:YAG and CO₂ laser sources are less than others of similar power output. Therefore, manufacturers adopt the nanosecond pulsed Nd:YAG laser for the manufacture of indium tin oxide (ITO) thin films for touch panels, surface texturing of silicon substrates for hydrophilic surface applications, backside writing of glass substrates for hydrophobic surface applications, and surface treatment of hard coatings for glass-molding applications. In this dissertation, a pulsed Nd:YAG laser was used to investigate the mechanical, optoelectric, and surface characteristics of machined specimens.

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Figure 1.1 Common high-power laser sources for industry.

Figure 1.2 Long-pulse laser matter interaction [1].

Figure 1.3 Short-pulse laser matter interaction [2].

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1.1.1 Electrode forming of TCO thin films

In recent years, transparency conductive materials were extensively used in 3C market products (i.e. computer, communication, and consumer electronics) to meet the rapid development of the electro-optical and semiconductor industry. Common transparent conductive oxide thin film materials, such as TiO2, SnO2, In2O3, and ZnO are used. The thin films of ZnO composition doped with aluminum, gallium, and tin elements are named aluminum zinc oxide (Al:ZnO), the gallium zinc oxide (Ga:ZnO), and the zinc tin oxide (ZTO) [3] films, respectively. Particularly, among them, indium tin oxide (ITO) material is popularly used in the flat panel display industry. Due to the high optical transparency and better electrical conductivity, ITO films have attracted great interests for various electrode or conductor applications in solar cells [4,5], flat panel displays [6], liquid crystal displays (LCDs) [7], and in organic light emitting diodes [8]. Figure 1.4 shows the touch panel applications including an Elonex e-Book reader and an Apple iPhone [9,10].

Figure 1.4 Products of touch panel application. (a) Elonex e-Book reader [9] and (b) Apple iPhone [10].

In order to fulfill light, thin, short, small, and flexible requirements in the electronic gadgets, plastic substrates are developed and convinced to be better candidates to replace the glass substrate in portable electronic products. The most common used substrate materials include polycarbonate (PC), polyestersulfone (PES), polyethylene terephtalate (PET),

(a) (b)

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polyimide (PI), polyarylate (PAR), and polyolefin. Figure 1.5 shows the schematic diagram of fundamental structure for resistive touch panel [11]. In the electrode manufacturing process, the transparent conductive material films were coated on these substrates first using the various deposition methods, and then etching the deposited film to become the electrode of the pre-determined pixel sizes.

Figure 1.5 Schematic diagram of fundamental structure for resistive touch panel [11].

Traditional electrode patterning techniques used the photolithography and chemical wet etching to form the patterns on the thin deposited films. The film electrodes manufacturing process includes sequentially (a) photoresist coating, (b) soft bake, (c) exposure, (d) lithography, (e) hard bake, (f) etch, and (g) photoresist stripping [12]. Figure 1.6 shows the semiconductor processes for patterning electrodes of transparent conductive oxide. Because of these processes increasing the heavy investment of semiconductor lithography process equipment and the chemical harm to the environment, a novel technique of direct laser writing is applied on the film surface to obtain the designed electrode.

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Figure 1.6 Semiconductor processes for patterning electrodes of transparent conductive oxide.

1.1.2 Wettability of material surfaces

Wettability is an important characteristic of solid surfaces. Due to continual development in the field of nanotechnology in recent years, a deeper understanding of the relationship between the microstructure of solid surfaces and wettability has been obtained.

Wettability can usually be shown through the contact angle between water and a solid surface.

Figure 1.7 shows the schematic diagram of a wettable surface [13]. When the contact angle is smaller than 90°, the surface is referred to as a hydrophilic surface, and when the contact angle is greater than 90°, it is referred to as a hydrophobic surface. A solid surface with a contact angle greater than 150° is referred to as a superhydrophobic surface.

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Figure 1.7 Schematic diagram of a wettable surface [13]. (a) Superhydrophobic surface, (b) hydrophobic surface, and (c) hydrophilic surface.

The wettability of a solid surface is decided by the chemical composition and microscopic geometric structure. Under the effects of the solid surface chemical composition, the higher than free energy of the solid surface, the higher the wettability is. The opposite is also true. However, when adjusting the surface free energy using chemical methods, the contact angle cannot usually be increased beyond 120°. In order to achieve a larger contact angle, the surface material must be planned on a microscopic level. The microstructure of the surface must be smaller than the micro-size of the liquid droplet. This micro structure can effectively increase the hydrophilic or hydrophobic qualities of the material. Hydrophobic materials are widely used to prevent pollution [14], prevent corrosion [15], reduce fluid resistance [16-18] and also include self-cleaning mechanisms [19-21]. Hydrophilic materials are widely used to disperse water evenly, dry faster, dissipate static electricity, and resist the dirt and spots. Figure 1.8 shows a digital microfluidic biochip and a multifunctional biochemical detection biochip system for the surface wettability application [22].

Figure 1.8 Products of surface wettability application [22]. (a) Digital microfluidic biochip and (b) multifunctional biochemical detection biochip system.

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1.1.3 Hard coatings for glass-molding dies

Glass materials have excellent optical properties, such as a high refractive index and low optical energy absorption in visible light spectrum, and high chemical and heat resistance, and are one of important materials used in the variety of key components and devices. Glass materials are much more suitable than plastic materials applied for high temperature, humid or harsh environments. For those reasons, glass lenses are dominantly used in optoelectronic, chemical, and biomedical devices. Low cost and high performance glass lenses are mostly produced by high precision molding technology. Figure 1.9 shows the photo-pictures of a glass-molding machine and a glass-molding die [23]. Consequently, the protective films are necessary for the glass molding dies to resist elevated temperature and pressure. The protective films of different chemical compositions and processes are developed to satisfy the different glasses operated conditions and to improve and extend the molding die lifetime.

In the glass-molding processes, several steps were needed, including (a) set glass material onto molds, (b) vacuuming inside of the chamber, (c) purging exposure inside of the chamber by nitrogen gas, (d) heat mold and glass materials to specified temperature by infrared lamps, (e) vacuuming inside of the chamber, (f) clamping molds and pressing glass materials, (g) cooling down molds and molded lenses with nitrogen gas, and (h) un-loading molded lenses from molds after cooling to the specified temperature as shown in the Fig. 1.10 [24]. Figure 1.11 shows the application of glass-molding process for fabricating various lenses such as bi-concave lens, bi-convex lens, meniscus lens, insertion lens, prism, f-θ lens, micro lens array, fiber array, etc [24].

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Figure 1.9 Pictures of a glass-molding system [23]. (a) Glass-molding machine of Toshiba-GMP207HV and (b) glass-molding die.

Figure 1.10 Flow diagram for glass-molding processes [24].

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Die

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Figure 1.11 Application of glass-molding process for fabricating various lenses [24].