**1. Introduction**

260 Recent Advances in Nanofabrication Techniques and Applications

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Fabrication technologies on the micro and nanometer scale are becoming more and more important from the viewpoint of industrial applications, for example: high-resolution lithography for manufacturing high-density recording media, high-resolution displays, and high sensitivity biomolecule sensor arrays. Electron-beam lithography, interferometric lithography, electron-beam evaporation of constituent materials and lift-off procedures were used to fabricate negative refractive-index metamaterials at the near-infrared (NIR) wavelengths (Zhang et al., 2005; Dolling et al., 2006; Dolling et al., 2007). Due to its capability for large-area fabrication, conventional photo-lithography is a wide-spread fabrication technology. Because the minimum feature size is limited by optical diffraction, sub-micrometer structures can be created by deep ultra-violet (DUV) at hundred-nanometer wavelengths or extreme ultra-violet (EUV) at ten-nanometer wavelengths lithography techniques (Gwyn et al., 1998). Because these technologies require complex vacuum optics, their cost remains prohibitive.

Ultrashort pulsed lasers, particularly femtosecond lasers, could offer an alternative to currently used micro and nanostructuring methods (Nishiyama et al., 2008; Qi et al., 2009; Allsop et al., 2010; Sugioka et al., 2010). High peak power can be reached for femtosecond pulses at relatively low energy per pulse. Due to the high radiation intensity, nonlinear effects dominate the interaction of tightly focused femtosecond laser beams with materials.

Pulsed laser micromachining involves the removal of material through the ablation process which consists in some consecutive physical processes: laser energy absorption, material heating, material expelling, and material cooling (Liu et al., 1997; Stuart et al., 1996).

The first step in laser ablation is the absorption of laser energy by the target material. The absorption mechanism depends on laser intensity (laser fluence and pulsewidth) and can be accomplished by linear and nonlinear processes. For opaque materials at laser radiation wavelength, linear absorption is the main mechanism at long pulsewidths with low intensity, whereas the nonlinear absorption becomes dominant at ultrashort pulsewidths with high intensity. For transparent materials, absorption comes from nonlinear processes through laser-induced optical breakdown. It is a process where a normally transparent material is first transformed in absorbing plasma by avalanche ionization and multiphoton

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 263

 resulting melt layer thickness is much smaller than in case of long pulses. After the laser pulse irradiation is finished, there is a rapid cooling due to the steep temperature gradient. Because little liquid is involved, the ablation process becomes highly precise in comparison to the long pulse case. By precision micromachining with femtosecond laser pulses, feature

Micro- and nanostructures can be created also through some nonlinear optical processes that occur when the interaction of high intensity femtosecond laser radiation with materials is performed under the ablation threshold. By tightly focusing the femtosecond laser pulses into the bulk of transparent materials, 3D micro-structures can be produced by a permanent refractive-index modification inside a small focal volume (Will et al., 2002; Osellame et al., 2003; Vega et al., 2005). Laser energy is deposited in this volume by multi-photon absorption and avalanche ionization. The photogenerated hot electron plasma rapidly transfers its energy to the lattice giving rise to high temperatures and pressures. The nonelastic thermomechanical stress created in the focal region produces a local densification with an increase of refractive index over a micrometer-sized volume of the material. This index gradient allows one to fabricate complicated photonic structures in many transparent materials like fused silica (Will et al., 2002; Shah et al., 2005), silicate and phosphate glasses (Homoelle et al., 1999; Ams et al., 2005), chalcogenide thin films (Zoubir et al., 2004), sapphire (Wortmann et al., 2008), poly(methyl methacrylate) (Sowa et al., 2006). Various microstructures contributing to developments in integrated optics, optical communications, and optical data storage have been obtained by refractive-index patterning: optical waveguides (Will et al., 2002; Osellame et al., 2003; Zoubir et al., 2004; Ams et al., 2005; Sowa et al., 2006; Allsop et al., 2010), beam splitters (Homoelle et al., 1999), micro-channels (Wortmann et al., 2008), directional couplers (Streltsov & Borelli, 2001), three-dimensional data storage (Glezer et al., 1996), diffraction gratings (Takeshima et al., 2004), photonic crystals (Takeshima et al., 2005). In case of irradiation of UV-photopolymerizable materials with high-intensity tightly focused NIR femtosecond laser pulses, a nonlinear optical process of multi-photon absorption can take place. In such photopolymers, the two-photon absorption of NIR femtosecond laser pulses induces photochemical reactions and then photopolymerization, just like in the case of a single UV photon absorption (Maruo et al., 1997; Witzgall et al., 1998). Unlike the UV-radiation photo-polymerization by single photon absorption, a highprobability of two-photon absorption occurs only in a very tiny volume of material near the center of the focused laser spot, leaving behind photo-polymerized patterns with micrometer or sub-micrometer dimensions. Direct laser writing by multiphoton polymerization of photoresists has emerged as a technique for the rapid, cheap and flexible fabrication of 3D structures with a resolution beyond the diffraction limit, leading to advanced applications in telecommunication, photonics, metamaterials and biomedicine (Deubel et al., 2004; Rill et al., 2008; Ovsianikov et al., 2009; Farsari & Chichkov, 2009). In this chapter, we present some experimental results in the field of femtosecond laser micromachining and micro/nanostructuring of both opaque and transparent materials. Three-dimensional (3D) micro-structuring of negative photo-resists by two-photon photopolymerization (TPP) using low energy high repetition rate NIR femtosecond lasers is described in the Section 2 of the paper. Fabrication of GHz range composite right/left handed (CRLH) devices by a technology that combines classical photo-lithography and femtosecond laser processing is described in the Section 3. Near-field laser lithography reaching a resolution beyond the diffraction limit through the interaction of ultra-short

sizes with submicrometer resolution can be obtained (Chimmalgi et al., 2003).

ionization. At relatively low laser intensity, the avalanche ionization process in a transparent dielectric material is seeded by free electrons coming from material impurities, thermal or linear optical ionization of shallow energy levels of inclusions. Free electrons can absorb laser energy through the inverse Bremsstrahlung process, when they collide with the bound electrons and the material lattice. Seed electrons can be accelerated at energy exceeding the ionization potential of the bound electrons. The collisions of seed electrons with bound electrons give rise to an avalanche ionization process growing exponentially from the initial very low seed electrons density. When plasma with a critical density is created, the transparent material is broken down and becomes absorbing at laser radiation wavelength. If the laser field strength is very high, as in the case of femtosecond-pulse laser-matter interaction, bound electrons can be directly ionized through multiphoton absorption. For longer laser pulses (microsecond-nanosecond pulsewidths) where the field strength is lower, the multiphoton ionization contribution is negligible and laser-induced breakdown is dominated by avalanche ionization. For ultrashort pulses, multiphoton ionization determines the breakdown threshold behavior: only when the laser intensity exceeds a certain threshold the plasma density grows to the critical value where irreversible breakdown takes place. The avalanche ionization fluence breakdown threshold exhibits large fluctuations due to statistical variations of the number of seed electrons already present in the material. Multiphoton ionization can directly generate free electrons, it is not related to the impurity seed electrons. Consequently, the laser induced breakdown threshold at ultrashort pulsewidths becomes more precise.

The material is heated up by the electrons energy transfer to the ions and the lattice. The amount of laser energy transfer during the laser pulse depends on the laser pulse duration and the energy coupling coefficient. Due to the thermal gradient, the absorbed energy can leave the laser focal volume by heat conduction and a larger volume is heated. For absorbing materials, such as metals and semiconductors, the laser pulse energy is absorbed in a surface layer whose thickness is given by the skin penetration depth, <sup>1</sup> *ls* , where is the absorption coefficient. The heat penetration depth due to the thermal conduction is given by the diffusion length, *<sup>d</sup> D <sup>l</sup> l* , where *D* is the heat diffusion coefficient and *<sup>l</sup>* is the laser pulsewidth. For microsecond and nanosecond laser pulses, *<sup>d</sup> <sup>s</sup> l l* , and the volume of the heated material, hence the temperature, is determined by the heat diffusion length. Therefore, for long pulses, the fluence breakdown threshold varies with the laser pulsewidth as *Fth <sup>l</sup>* . If the laser pulsewidth decreases, the diffusion length decreases too. For laser pulses in the femtoseconds range, *<sup>d</sup> <sup>s</sup> l l* , the skin penetration depth determines the heated volume during the laser pulse. As a result, the breakdown threshold becomes independent of the pulsewidth.

For long pulses, a larger volume of material is heated and melted, but only a small layer of material reaches the vaporization temperature. Material removal is accomplished through melt expulsion driven by the vapor pressure and the recoil of the light pressure. This physical process involving fluid dynamics and vapor conditions is an instable one. For micro-processing applications, the resolidification of the melt after the ablation can lead to very irregular shapes of the holes or cuts. On the other hand, for ultrashort pulses, the laser energy is deposited in a thin layer with a thickness limited by the skin penetration depth. The localized energy heats the material very quickly at high temperatures to the vapor phase with high kinetic energy. The material removal takes place mainly by direct vaporization. Because most of the heated material reaches the vaporization temperature, the

ionization. At relatively low laser intensity, the avalanche ionization process in a transparent dielectric material is seeded by free electrons coming from material impurities, thermal or linear optical ionization of shallow energy levels of inclusions. Free electrons can absorb laser energy through the inverse Bremsstrahlung process, when they collide with the bound electrons and the material lattice. Seed electrons can be accelerated at energy exceeding the ionization potential of the bound electrons. The collisions of seed electrons with bound electrons give rise to an avalanche ionization process growing exponentially from the initial very low seed electrons density. When plasma with a critical density is created, the transparent material is broken down and becomes absorbing at laser radiation wavelength. If the laser field strength is very high, as in the case of femtosecond-pulse laser-matter interaction, bound electrons can be directly ionized through multiphoton absorption. For longer laser pulses (microsecond-nanosecond pulsewidths) where the field strength is lower, the multiphoton ionization contribution is negligible and laser-induced breakdown is dominated by avalanche ionization. For ultrashort pulses, multiphoton ionization determines the breakdown threshold behavior: only when the laser intensity exceeds a certain threshold the plasma density grows to the critical value where irreversible breakdown takes place. The avalanche ionization fluence breakdown threshold exhibits large fluctuations due to statistical variations of the number of seed electrons already present in the material. Multiphoton ionization can directly generate free electrons, it is not related to the impurity seed electrons. Consequently, the laser induced breakdown

The material is heated up by the electrons energy transfer to the ions and the lattice. The amount of laser energy transfer during the laser pulse depends on the laser pulse duration and the energy coupling coefficient. Due to the thermal gradient, the absorbed energy can leave the laser focal volume by heat conduction and a larger volume is heated. For absorbing materials, such as metals and semiconductors, the laser pulse energy is absorbed

is the absorption coefficient. The heat penetration depth due to the thermal conduction is

the laser pulsewidth. For microsecond and nanosecond laser pulses, *<sup>d</sup> <sup>s</sup> l l* , and the volume of the heated material, hence the temperature, is determined by the heat diffusion length. Therefore, for long pulses, the fluence breakdown threshold varies with the laser

too. For laser pulses in the femtoseconds range, *<sup>d</sup> <sup>s</sup> l l* , the skin penetration depth determines the heated volume during the laser pulse. As a result, the breakdown threshold

For long pulses, a larger volume of material is heated and melted, but only a small layer of material reaches the vaporization temperature. Material removal is accomplished through melt expulsion driven by the vapor pressure and the recoil of the light pressure. This physical process involving fluid dynamics and vapor conditions is an instable one. For micro-processing applications, the resolidification of the melt after the ablation can lead to very irregular shapes of the holes or cuts. On the other hand, for ultrashort pulses, the laser energy is deposited in a thin layer with a thickness limited by the skin penetration depth. The localized energy heats the material very quickly at high temperatures to the vapor phase with high kinetic energy. The material removal takes place mainly by direct vaporization. Because most of the heated material reaches the vaporization temperature, the

, where *D* is the heat diffusion coefficient and *<sup>l</sup>*

. If the laser pulsewidth decreases, the diffusion length decreases

, where

is

in a surface layer whose thickness is given by the skin penetration depth, <sup>1</sup> *ls*

threshold at ultrashort pulsewidths becomes more precise.

given by the diffusion length, *<sup>d</sup> D <sup>l</sup> l*

becomes independent of the pulsewidth.

pulsewidth as *Fth <sup>l</sup>*

 resulting melt layer thickness is much smaller than in case of long pulses. After the laser pulse irradiation is finished, there is a rapid cooling due to the steep temperature gradient. Because little liquid is involved, the ablation process becomes highly precise in comparison to the long pulse case. By precision micromachining with femtosecond laser pulses, feature sizes with submicrometer resolution can be obtained (Chimmalgi et al., 2003).

Micro- and nanostructures can be created also through some nonlinear optical processes that occur when the interaction of high intensity femtosecond laser radiation with materials is performed under the ablation threshold. By tightly focusing the femtosecond laser pulses into the bulk of transparent materials, 3D micro-structures can be produced by a permanent refractive-index modification inside a small focal volume (Will et al., 2002; Osellame et al., 2003; Vega et al., 2005). Laser energy is deposited in this volume by multi-photon absorption and avalanche ionization. The photogenerated hot electron plasma rapidly transfers its energy to the lattice giving rise to high temperatures and pressures. The nonelastic thermomechanical stress created in the focal region produces a local densification with an increase of refractive index over a micrometer-sized volume of the material. This index gradient allows one to fabricate complicated photonic structures in many transparent materials like fused silica (Will et al., 2002; Shah et al., 2005), silicate and phosphate glasses (Homoelle et al., 1999; Ams et al., 2005), chalcogenide thin films (Zoubir et al., 2004), sapphire (Wortmann et al., 2008), poly(methyl methacrylate) (Sowa et al., 2006). Various microstructures contributing to developments in integrated optics, optical communications, and optical data storage have been obtained by refractive-index patterning: optical waveguides (Will et al., 2002; Osellame et al., 2003; Zoubir et al., 2004; Ams et al., 2005; Sowa et al., 2006; Allsop et al., 2010), beam splitters (Homoelle et al., 1999), micro-channels (Wortmann et al., 2008), directional couplers (Streltsov & Borelli, 2001), three-dimensional data storage (Glezer et al., 1996), diffraction gratings (Takeshima et al., 2004), photonic crystals (Takeshima et al., 2005). In case of irradiation of UV-photopolymerizable materials with high-intensity tightly focused NIR femtosecond laser pulses, a nonlinear optical process of multi-photon absorption can take place. In such photopolymers, the two-photon absorption of NIR femtosecond laser pulses induces photochemical reactions and then photopolymerization, just like in the case of a single UV photon absorption (Maruo et al., 1997; Witzgall et al., 1998). Unlike the UV-radiation photo-polymerization by single photon absorption, a highprobability of two-photon absorption occurs only in a very tiny volume of material near the center of the focused laser spot, leaving behind photo-polymerized patterns with micrometer or sub-micrometer dimensions. Direct laser writing by multiphoton polymerization of photoresists has emerged as a technique for the rapid, cheap and flexible fabrication of 3D structures with a resolution beyond the diffraction limit, leading to advanced applications in telecommunication, photonics, metamaterials and biomedicine (Deubel et al., 2004; Rill et al., 2008; Ovsianikov et al., 2009; Farsari & Chichkov, 2009).

In this chapter, we present some experimental results in the field of femtosecond laser micromachining and micro/nanostructuring of both opaque and transparent materials. Three-dimensional (3D) micro-structuring of negative photo-resists by two-photon photopolymerization (TPP) using low energy high repetition rate NIR femtosecond lasers is described in the Section 2 of the paper. Fabrication of GHz range composite right/left handed (CRLH) devices by a technology that combines classical photo-lithography and femtosecond laser processing is described in the Section 3. Near-field laser lithography reaching a resolution beyond the diffraction limit through the interaction of ultra-short

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 265

A large series of negative photoresists, among them SU-8, organically modified ceramics (Ormocer), or organically modified silicate (Ormosil), have the maximum of the absorption band in the UV-blue spectral range. In such photopolymers, the two-photon absorption of NIR Ti:sapphire femtosecond laser radiation induces photochemical reactions and then photo-polymerization, just like in the case of a single UV photon absorption. In contrast with the single photon processing, the two-photon absorption occurs in a very tiny volume of material, near the centre of the focused spot, allowing creation of features with resolution

The laser workstation for material processing consists in the following main parts: the femtosecond laser, the beam delivery and focusing optics, and the scanning mechanics. In our experiments we use a set-up compatible with different type of laser structuring methods, by laser ablation as well as by TPP. The set-up consists in a modular microscope for laser writing built to be coupled with laser beams at different radiation wavelengths, depending on the experimental requirements. For applications such as laser ablation, requiring high pulse energy from tens nano-Joules up to micro-Joules, a laser amplifier CPA2101 system (Clark-MXR) was employed. This laser emits femtosecond pulses of as much as 0.6 mJ energy, 200 fs pulse duration at 775 nm wavelength and 2 KHz repetition rate. In experiments requiring high repetition rate, such as TPP, a 75 MHz repetition rate femtosecond laser oscillator Synergy Pro (Femtolasers) was coupled with the processing microscope. The oscillator delivers laser pulses of 5 nJ pulse energy, 10 fs duration at 790 nm central wavelength, with spectral bandwidth of 100 nm. The beam delivery optics is interchangeable and can be easily replaced with optics adapted for other working

**CCD**

**A** 

Fig. 1. Experimental set-up for laser processing. A – Attenuator with half waveplate and Glan Polarizer; DC – Dichroic mirror; L1 – Focusing lens; S – Sample; *XYZ* –Translation

wavelengths, such as harmonics of fundamental wavelength.

**S** 

**DC**

**XYZ** 

**L1** 

below 1 micrometer.

**2.1 Experimental set-up** 

stage; CCD – Video camera.

pulsed laser beams with small size particles is presented in Section 4. In the Section 5, a micro-printing method based on femtosecond laser induced forward transfer (LIFT) of various materials deposited in thin films is demonstrated.
