Rr rr rr rr tr

Pumping light

Jl-ClU

Jl-ClU

Mirror trtr tr tr tr tr trtr tr tr tr tr

Mirror

&&Jin Jin

&&Jin Jin

Fig. 3.4. Schematic diagram for light amplification through stimulated emission

Partially reflecting mirror tr tr tr tr tr tr

Fig. 3.4. Schematic diagram for light amplification through stimulated emission

3.3.1 Monochromacity

A monochromatic light means that all the photons have exactly one frequency. Therefore, in terms of color, a monochromatic light can be said a perfectly pure color. However, no light source is truly monochromatic but has finite band. Laser light is said to be quasi-monochromatic, meaning its linewidth is as narrow as possible. This narrow linewidth or purity of color is very useful in many application for instrumentation and micromanufacturing.

3.3.2 Directionality

Laser light propagates straight with very little divergence. This exceptional directionality of a laser beam originates from the design of optical cavity shown in Fig. 3.4. The divergence of a modern laser beam is less than 1 mrad. This small divergence benefits in many aspects for applications. For example, the laser energy is not lost during propagation and easy to focus onto a small area with a simple lens. By contrast conventional light diverges very rapidly and collection of it is very difficult.

3.3.3 Brightness

Brightness is defined as the power emitted per unit area per unit solid angle (the SI unit of brightness is W/m2 sr). Because the divergence of a laser beam is much smaller than conventional sources, the small solid angle into which the laser beam is emitted ensures an extremely high brightness. It can be shown that a 3 mW He-Ne laser with a divergence angle of 1 mrad and beam diameter of 1 mm is 240 times brighter than the sun (Wilson and Hawkes 1987).

3.3.4 Coherence

Light is said to be coherent when the relative phase of all photons is the same. The time duration for which different photons maintain the same relative phase is defined as coherence time. Because the relative phase among photons can change due to path or material difference they travel through, coherence is an essential property for applications to instrumentation like interferometry. It is known that the coherence length, obtained by multiplying the coherence time by the speed of light, of a laser beam is as long as 3x105 m while that of white light is only about 3 |im (Das 1991)

3.3.5 Spatial Profile

Spatial profile defines how laser energy is distributed over the cross section of the beam. The most important and thoroughly analysed profile is the Gaussian (or TEM00) mode beam. A Gaussian beam is axisymmetric and characterised by the following equation

Fig. 3.5. Spatial energy distribution of a Gaussian laser beam

Where, I is irradiance (W/cm2), I0 is the irradiance at the center of the beam, r is radial coordinate, w0 is the radius of the beam where I drops to I0/e2 (Fig.3.5). A

full discussion of Gaussian beam can be found in references (Verdeyen 1995; Milonni and Eberly 1988)

3.3.6 Temporal Profile

Laser beam can be emitted from the laser continuously or intermittently. If the laser beam is continuously emitted, it is called continuous wave (CW) laser. When the laser beam is emitted with finite interval or in a single shot manner, it is called a pulse laser. The time duration for which pulse laser beam is being emitted is the pulse width or pulse duration. The term repetition rate represents how many pulses are emitted per second.

Fig. 3.6. Temporal mode of lasers: (a) CW laser, (b) pulse laser

3.4 Practical Lasers

Lasers are typically classified by active medium although classification by pulse width or other design characteristics is also possible. Nowadays, various types of lasers are commercially available and Table-3.1 shows the characteristics of most widely used lasers in industry and research. CO2 laser is the most widely used laser in industry. Because of its unparalleled high output power (as high as 45,000 W (Ready 1997)), it has been the workhorse for conventional manufacturing such as cutting, welding, marking and drilling. Excimer laser is important for semiconductor processing. Because of the short wavelength, excimer lasers allow the fabrication of fine patterns required for high density circuit integration. Recent research reported that fabrication of a device with 50 nm feature size is viable by optical lithography with F2 excimer lasers (Pong and Wong 2002). Semiconductor lasers (also called as diode lasers) are the most used lasers in quantity and shares the largest global market. Because of their small size, low price, lightweight, low power consumption, and the possibility of mass production, semiconductor lasers are widely used for instrumentation, telecommunications, compact disc players and data storage applications. Nd-YAG laser is another type of solid state laser with relatively high output power and wavelength (e.g., 1064 nm). Nd-YAG lasers are used as widely as CO2 lasers in industry for the benefit of easy operation and low maintenance cost. Nd-YAG lasers produce output power as high as 2,400 W

for continuous operation or up to 2 J for Q-switched pulse. Because Nd-YAG laser beam can be transmitted through an optical fiber, remote processing is possible with this laser, which is a great advantage for industry application such as automotive body welding. Using a technique known as harmonic generation, the Nd-YAG laser beam can be converted to a shorter wavelength of 532, 355, or 266 nm (Kuhn 1998).

Table 3.1. Properties of typical lasers (Ready 1997; Kuhn 1998; Carter et al. 2004)

Type

Laser

Active medium

Characteristics

Applications3

Gas

He-Ne

Ne

Power: 0.5-35 mW TEMoo mode, CW Low divergence Wavelength: 632.8 nm1

Interferometry Holography Alignment Velocimetry

Ar

Ar+

Power: upto 25 W TEM00 mode2, CW Low divergence Wavelength: 514.5, 488 nm1

Measurements Microfabrication Entertainment Lithography

CO2

CO2

Power: 100-10,000 W Multimode2, CW & Pulse Divergence increases with output power, X: 10.6

Metal/Paper/Plastic Cutting, Drilling, Welding, Marking

Excimer

Noble gas +Halogen gas

Energy4: 0.01-2 J Multimode, Pulse Medium divergence X: 351, 308, 248, 193, 157 nm

Photolithography Micromachining Eye surgery

Solid State

Nd-YAG

Nd

Power: up to 2400 W TEM00 & Multimode, CW & pulse, Low divergence X: 1064, 532, 355, 266

Welding, Marking,

Micromachining,

Drilling

Saphire

Ti-Saphire

Energy: up to 2 mJ TEM00 mode

Ultrashort pulse: 10-1000 fs 800 nm

Ultraprecision micromachining Nonlinear processing

Semicon ductor

Semicond ucting compound

Power: 0.3-2.5 W Very small size: ~10 Large divergence: up to 30° 2X: 780-880, 1150-1650

CD & DVD player Barcode scanner Data storage Telecommunication

Fiber

Rare earth material

Power: up to 100,000 W TEM00 mode

High efficiency; X: 350-2100

Telecommunication Welding, Marking Micromachining

Liquid

Dye

Organic dye

Energy4: up to 150 mJ Low divergence, X: 370-900, wide range

Research Diagnostics

'Most common frequency, Typical configuration, Typical examples, Pulse energy

'Most common frequency, Typical configuration, Typical examples, Pulse energy

Two other types of solid state lasers, which are relatively new but very important in emerging or future laser technologies are ultrashort pulse laser and fiber laser. Ultrashort pulse laser is a laser that outputs a pulse shorter than a picosecond (1 picosecond=10-12 second). Ti:Saphire laser is the most representative one in ultrashort pulse lasers and the pulsewidth ranges typically from 10's to 100's of femtoseconds (1 femtosecond=10-15 second). With this short pulse, extremely fine micromachining that cannot be realised with other method becomes possible (Ostendorf 2002). In a fiber laser, the optical fiber itself plays the role of an active medium and produces high quality laser beam. Fiber lasers have several advantages over other lasers in compactness, high amplification (or gain) efficiency, robustness, high thermal stability and simplicity of operation. (Schreiber et al. 2004). It is expected that fiber lasers may replace existing CO2 or Nd-YAG lasers in many industrial applications.

3.5 Laser Technology in Micromanufacturing

Laser technology is widely applied to semiconductor processing, electronics packing, optical communication, medicine, and so on. Using lasers fabrication of microstructures on the order of micrometer (1 |im=10-6 m) or micromachining of hard materials like ceramic, glass, or stainless steel those are difficult to mechanically process can be easily achieved.

3.5.1 Background

The possibility of micromachining using laser beam is based on the fact that laser beam can be focused into a very small size. With high magnification objective lens, a spot diameter of 1-2 |im size is possible. Laser also plays a crucial role as an illumination source for photolithography in semiconductor manufacturing. Early 1990s, lasers have replaced halogen lamps in photolithography to reduce the critical dimension of the circuit patterns produced on silicon wafer by imaging a mask. The critical dimension in current photolithography is about 0.18 nm on the manufacturing line but the minimum pattern size of 70 nm is already achieved in research (Yim et al. 2003). All laser applications in micromanufacturing are in principle based on the absorption of the laser light by the workpiece, which leads to thermal or chemical changes in the workpiece. The development of a laser microprocessing technique and its proper application require understanding of thermal, physical, and chemical phenomena during laser beam interaction with materials and their variation with respect to the key laser parameters such as wavelength, pulse width, and energy.

3.5.2 Absorption and Reflection of Laser Light

Fig. 3.7. shows schematically the absorption and reflection of incident laser light by a solid. When the irradiance of the incident laser beam is I0 and the reflectance of the solid surface is R, an amount of RI0 is reflected at the surface while the rest is absorbed. The irradiance of the transmitted light decreases exponentially with distance in accordance with the following equation,

Where, a is the absorption coefficient having a dimension of m-1. If we take the inverse of the absorption coefficient, 1/a, it represents how far the light propagates before complete absorption, known as the optical penetration depth. The absorption coefficient is expressed in terms of the wavelength of the incident laser beam as,

Where, k is the imaginary part of the index of refraction of an absorbing medium and termed as the extinction coefficient. From Eq.3.5 it is seen that the shorter the wavelength the shallow the penetration depth is. The reflectance and absorption coefficient of a medium varies significantly with wavelength and Fig. 3.8 shows theoretically determined properties of crystalline silicon, gold, and aluminum (von Allmen and Blatter 1995). In general, metals are opaque for laser light and thus light is completely absorbed within about 10 nm. The reflectance of typical metals like gold, aluminum, copper, etc. is very high for infrared light, close to 1, and thus direct processing of these metals with Nd-YAG or CO2 laser is inefficient in energy point of view. In the case of silicon, which is the central material in semiconductor industry, infrared beam transmits over several hundreds of micrometers but ultraviolet beam is completely absorbed within about 10 nm thickness. Therefore, it is important to select the proper wavelength in accordance with the intended process.

Fig. 3.7. Schematic diagram for reflection and absorption of laser energy by a solid

Fig. 3.7. Schematic diagram for reflection and absorption of laser energy by a solid

3.5.3 Application Technology Fundamentals

The spot diameter of a focused laser beam can be reduced to a few micrometers depending on lens' focal length, laser wavelength, and the diameter of unfocused beam. For Gaussian beam in air, the spot size d can be estimated using the following equation, rf = ^ (3.6)

Where, X is the wavelength, f is focal length of the lens, and D is diameter of the beam before focusing. For example, if a Nd-YAG laser beam (X=1064 nm) with initial diameter of 2 mm is focused with a f=50 mm lens, the spot diameter will be about 34 |im.

Suppose that the laser output is from a pulse laser with pulsewidth of 5 ns and moderate pulse energy of 100 mJ. Then, the irradiance at the laser spot becomes 22x1015 W/m2, which is overwhelmingly high value almost unachievable with other sources. Upon irradiation by a laser beam with this high energy density, almost all materials are immediately melted or vaporised.

This is how laser can effectively machine hard materials like diamond, glass, and ceramic. Laser irradiation of a medium may cause varying thermal effects from simple heating to melting, evaporation, or ionisation of the material. For high power laser processing, all of these phenomena occur almost simultaneous within the pulse duration, which makes the process very complicated and difficult to control. For applications like heat treatment of metal surface, annealing of polysilicon plates for liquid crystal display, texturing of computer hard disk, laser irradiation induces only heating and melting of the material with no evaporation. The substrate may undergo changes in mechanical and physical properties or surface morphology as a result of irradiation. For example, microbumps of 10-20 |im diameter and 40 nm height (Fig.3.9) can be fabricated on hard disk surface using laser melting to solve stiction problem between the hard disk surface and header (Baumgart et al. 1995).

For applications like laser trimming for fine adjustment of resistance of electronic components, drilling of printed circuit board, marking of electronic or medical components, marking of silicon wafer, fabrication of ink jet printer nozzle, and so on, laser beam removes material from the workpiece in the form of vapor, droplets or particles, or even in solid flakes. When evaporation occurs during laser microprocessing, temperature of the substrate can reach several thousand degree and the vapor pressure can be tens of atmosphere. For higher energy conditions, the vapor can absorb incident laser beam and be ionised causing significant secondary effects such as shielding of the sample surface from the laser beam. The high temperature and pressure during laser processing, and possible ionization of the vapor can lead to significant variation in the process and thus defining optimal conditions and precise control of them are crucial for real application.

Fig. 3.8. Reflectance and absorption coefficient as a function of wavelength for different materials (von Allmen and Blatter 1995)
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Fig. 3.9. Laser-textured microbumps on super-smooth NiP/Aluminum disk surface. The diameter and height of each bump are approximately 15 and 45 nm, respectively. (source: www.almaden.ibm.com)

The high vapor pressure developed during laser irradiation can be used for cleaning of contaminants on electronic components. Because no mechanical or chemical treatment is necessary, laser cleaning is free from possible mechanical damage and is environment friendly. Conventionally, laser beam is fired directly on the surface to be cleaned but in this case thermal damage of the sample may occur. Recent technique adopts a method to focus the laser beam in air above the surface and generate plasma in which cleaning is done by the high pressure of the expanding plasma. An example of plasma cleaning of intentionally contaminated solid surface is shown in Fig. 3.10.

Fig. 3.10. Laser cleaning of contaminated surface. (Lee 2005, Courtesy: IMT Co. LTd.)

Fig. 3.10. Laser cleaning of contaminated surface. (Lee 2005, Courtesy: IMT Co. LTd.)

The heating and phase change of material during laser processing is basically thermal process. However, the precise micromachining technologies up to date is based on photochemical processes by excimer lasers whose output wavelength lies in the ultraviolet regime between 157-351 nm. The short wavelength of excimer laser beam has great importance in application point of view, especially for the photolithography in semiconductor manufacturing. In photolithography, the desired circuit pattern is first produced on a mask and this mask pattern is imaged onto a silicon wafer. Laser light is the illumination source during imaging and the minimum feature size R of thereby produced patterns is limited by diffraction by the following equation (Gower 1993),

Where, NA is the numerical aperture of the imaging optics. As it can be seen from Eq. (7), R decreases linearly with wavelength, which implies that the circuit density increases in proportion to 1/X2. Actually, adopting a shorter wavelength excimer laser is the primary reason for the ever-increasing computer memory capacity in the semiconductor industry.

The short wavelength of excimer lasers is also a valuable property in direct micromachining. As explained earlier photon energy is inversely proportional to the wavelength as Ep=hc/X. Photon energy of excimer lasers is much greater than that of CO2 laser or Nd-YAG laser. When organic material like polymer is irradiated with excimer laser beam, the chemical bonding between atoms and molecules brake by incident photons and material removal in atomic or molecular form can take place. In this case, almost no heat affected zone is formed in the workpiece and micromachining with very sharp edge profile is possible (Fig. 3.11) (Gower 1999). Photon energy of various types of lasers and the bonding energies of representative chemical bonds are shown in Fig. 3.12 (Gower 1993).

Fig. 3.11. Ink-jet printer nozzles drilled by an excimer laser. The diameter of the nozzles is about 30 and the substrate is polyimide (Gower 1999)

Fig. 3.11. Ink-jet printer nozzles drilled by an excimer laser. The diameter of the nozzles is about 30 and the substrate is polyimide (Gower 1999)

Fig. 3.12. Strengths of some common molecular chemical bonds compared with excimer laser photon energies (Gower 1993)

Since the late 1990's, there has been tremendous research and development in ultrafast laser technologies, especially for femtosecond lasers. Pulsewidth of femtosecond lasers ranges typically 10's to 100-200 fs although shorter or longer pulses are also available. To see how short this time is, let us calculate the distance light propagates within 100 fs as L=cxAt=(3x1014 |im/s) x(100x10"15 s)=30 |im.

This result shows that light travels a distance equivalent only about half of a hair thickness during 100 fs. This short pulse of femtosecond lasers has significant impacts on material processing in the following aspects. First, ultrashort laser pulse easily leads to extremely high irradiance like 1015-1020 W/m2. At this high energy density, many nonlinear optical phenomena occur. Second, because of the ultrashort pulse, material removal occurs in thermally nonequilibrium conditions. The result of this nonequilibrium processing is a sharp machining edge with almost no heat-affected zone on the workpiece, even for metallic samples. An excellent explanation of ultrafast phenomena can be found in Craig (Craig 1998). Fig. 3.13(a) shows a striking example of femtosecond laser micro fabrication in which a realistic image of a bull is manufactured in the size of a red blood cell using two photon absorption principle (Kawata et al. 2001). Other results of femtosecond laser micromachining superior to long pulse laser fabrication or nonlaser methods have been demonstrated for various materials including metals (Tonshoff et al. 2000) and glasses (Minoshima et al. 2002).

Fig.13.(a) A micrograph of the micro-bull. The scale bar is 2^m long (Kawata et al. 2001); (b) Example of circuit repair by laser-induced chemical vapor deposition on LCD glass c ;'x

Fig.13.(a) A micrograph of the micro-bull. The scale bar is 2^m long (Kawata et al. 2001); (b) Example of circuit repair by laser-induced chemical vapor deposition on LCD glass

The high focusability of a laser beam can also be an advantage when micromachining is required only at a local area of a large workpiece. For example, flat panel displays such as plasma display panel (PDP) or liquid crystal display (LCD) are core products in electronics industry and their global market expands rapidly. Due to the large size of the PDP or LCD glasses, currently as large as 1.2x1.8 m2, it is crucial to manufacture the circuits on these glasses free of defects because a discard of the large glass will result in a significant loss of productivity. Should there be a local defect on these glasses, it is best to fix the damage instead of discarding the entire glass. Laser can be effectively used for this type of local repair of electronic circuits. Fig. 3.13 (b) shows an example of this type of repair of an open circuit on LCD glass using laser-induce chemical vapor deposition technique (Han and Jeong 2004). A metal interconnect with a width about 10 |im is locally deposited without influencing the surrounding circuit elements. Besides deposition, laser-induced chemical etching is also possible for drilling and cutting (Shin and Jeong 2003) or fabrication of three-dimensional microstructures (Bauerle 2000)

3.6 References

Bauerle D (2000) Laser Processing and Chemistry. 3rd edn. Springer Carter A, Tankala K, Samson B, Machewirth D, Khitrov V, Manyam U (2004) Continued advances in the designs of double clad fibers for use in high output power fiber lasers and amplifiers. Int. Symp, Laser Precision Microfabrication, Japan, SPIE Proc. 131 Craig B (1998) Ultrafast pulses promise better processing of fine structures, Laser Focus World 9:79

Das P (1991) Lasers and Optical Engineering, Springer

Gower MC (1993) Excimer lasers: current and future applications in industry and medicine in laser processing in manufacturing. In: Crafer RC, Oakley PJ (eds.), Chapman & Hall

Gower MC (1999) Excimers tackle micromachining, Industrial Laser Solutions for Manufacturing 4:14

Han SI, Jeong SH (2004) Laser-assisted chemical vapor deposition to directly write three-

dimensional microstructures, J of Laser Applications 16:154-160 Hecht E (1998) Optics, 3rd edn. Addison-Wesley

Kawata S, Sun HB, Tanaka T, Takada K (2001) Finer features for functional microdevices,

Nature 412, 697 Kuhn KJ (1998) Laser Engineering, Prentice

Lee JM (2004) Personal communication. Technical Report, IMT Co. LTd. Milonni PW, Eberly JH (1988) Lasers, Wiley

Minoshima K, Kowalevicz AM, Ippen EP, Fujimoto JG (2002) Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing, Optics Express 10:645-649 Ostendorf A (2002) Precise sturucturing using femtosecond lasers, Review of Laser Engineering 30:221

Pong WT, Wong A (2002) Feasibility of 50-nm device manufacture by 157-nm optical lithography: An initial assessment. Proceedings of IEEE conference, Electron devices meeting, Hong Kong, p 31 Ready JF (1997) Industrial Applications of Lasers, 2nd edn. Academic Press Schreiber T, Limpert J, Liem A, Roser F, Nolte S, Zellmer H, Tunnermann A (2004) High power photonic crystal fiber laser systems. Proc., Int. Conf. on Transparent Optical Networks 1, Wroclaw, pp 131-135 Shin YS, Jeong SH (2003) Laser-assisted etching of titanium foil in phosphoric acid for direct fabrication of microstructures, J of Laser Applications 15:240-244 Tonshoff HK, Momma C, Ostendorf A, Nolte S, Kamlage G (2000) Microdrilling of metals with ultrashort laser pulses, J of Laser Applications 12:23-30 Verdeyen JT (1995) Laser Electronics. 3rd edn. Prentice

Allmen vM, Blatter A (1995) Laser-beam interactions with materials, Physical principles and applications. 2nd edn Springer Wilson J, Hawkes JFB (1987) Lasers principles and applications. Prentice Yim YS, Shin KS, Hur SH, Lee JD, Baik IG, Kim HS, Chai SJ, Choi EY, Park MC, Eun DS, Lee SB, Lim HJ, Youn SP, Lee SH, Kim TJ, Kim HS, Park KC, Kim KN (2003) 70 nm NAND flash technology with 0.025^m2 cell size for 4 Gb flash memory, Technical Digest of IEEE International Electron Devices Meeting, IEDM, USA, vol. 34, pp 1-5

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