(a) The coherence of laser radiation is the result of the stimulated emission process in the atomic or molecular system. It relates to the narrow spectral bandwidth into which all available energy is channelled, thus achieving a very high spectral density. In addition to the tremendous properties of interference and diffraction, this property allows a wide range of applications including spectral pumping and tuning, efficient nonlinear propagation, selective absorption and activation and others.

(b) The directionality of the radiation by the formation of Gaussian beams is the result of the optical resonance necessary to achieve and sustain laser oscillation. Laser beams have increased spatial coherence and exhibit minimal deterioration due to diffraction, thus enabling efficient energy transmission and delivery on the target under processing.

(c) Both the above aspects are responsible for the celebrated high intensity of the laser beam, which together with the spatial and temporal coherence, as well as the polarization control, are crucial in materials processing operations.

(i) Wavelength of radiation, propagation in free space and absorption by the workpiece under processing. Selection of specific wavelength is important in many cases. It determines the beam delivery technology and the nature of operations.

(ii) Emission at CW and/or pulsed operation. It determines the nature and range of operations, as well as the technical and financial aspects of the deployed technology.

(iii) Efficiency of operation in technical and economic terms and power efficiency in reference to electrical-to-optical power conversion. It affects the operational costs and the market value of the final products.

(iv) Reliability of operation, technology complexity and automation. They are very important in the industrial floor.

(v) Acquisition and operational costs, production efficiency and final product price. They affect primarily the industrial production and also research operations.

(a) Carbon dioxide laser: CO₂ laser emitting at 10.6 μm from the molecular transitions of carbon dioxide. Highly efficient (~ 30%) system offering both pulsed and CW operations from mW to kW CW output power levels and pulses of mJ to kJ energy. Systems widely deployed in industrial production, from heavy industry to microelectronics.

(b) Excimer lasers produce UV radiation by the metastable excited molecular dimers ArF (193 nm), KrF (248 nm), XeCl (308 nm) and the more recently developed F₂(157 nm). They only emit pulsed operation and are pumped by high current discharges or electron beams. Their efficiency varies depending on their type. They deliver mJ to Joule pulses, of a few nanoseconds’ duration. Special hybrid dye-excimer systems offer sub-picosecond radiation. Excimer lasers offer highly energetic photons and enable unique operations, even though their beam coherence and spatial quality are reduced and limited by the electric discharge effects.

(c) Nd:YAG, Nd:Glass and related rare earth solid sate laser systems. Nd-based solid state sources have been and are widely used owing to their high efficiency operation at the NIR and the potential of visible and UV emission by use of frequency doubling and optical parametric oscillator systems. By these means they cover the range from the UV to the mid-IR, although at varying laser efficiencies. In addition to the fundamental frequency ω: 1064 nm, significant wavelengths deployed are 2ω: 532 nm, 3ω: 355 nm and 4ω: 266 nm. The use of various laser hosts and ions give new possibilities with prime candidates the Er⁺ (1.5–1.6 μm) and the Ho⁺ (2.1 μm) laser systems. Solid state technology has become widespread and enables highly efficient operations by the use of spectrally selective pumping by diode lasers. Systems deployed in CW at power levels of less than 1 mW to kW and pulsed modes emitting from the quasi CW to picoseconds pulses of nJ to tens of J energy are well established in industry. The wide range of free space systems is now enriched by the production of all-fibre lasers which deliver picosecond and femtosecond pulse trains in the NIR.

(d) Semiconductor diode lasers are deployed owing to the achievement of considerably high power levels delivered by laser diodes arrays. They operate in both pulsed and CW modes, achieving power levels approaching the 1 kW in the NIR range from ~ 800 nm to ~1600 nm. They are small, compact and reliable sources of very robust construction and in most cases offer fibre delivery alleviating the hazards of invisible radiation.

(e) Ti-Sapphire, Ti:Al₂O₃ laser system emitting broadband radiation at ~ 800 nm. This is one of the latest solid state laser developments which operate in the CW and pulsed modes. The broad gain bandwidth allows efficient mode locked operation producing 150 fs pulse trains at ~ 100 MHz repetition rates and above. By use of pulse compression and amplification techniques, these lasers offer currently sub-50 fs pulses of mJ energy, yielding extremely high peak power levels and several TW/cm² intensities on target. These systems offer an extended wavelength selection by use of optical parametric oscillators (UV to NIR). Their use defines the current trends in several fields.

(a) Irrespective of the nature of the material, the workpiece is described by a 3D interphase profile function of the form n_c(x, y, z), which can be analysed in terms of spatial frequencies.

(b) The incident field is diffracted by this interphase in both semi-spaces breaking up into components determined by the particular geometrical and physical characteristics of the problem. The presence of suitable spatial frequencies in the sub-wavelength regime may reduce severely the reflectivity and can effectively produce opacity by presenting an artificial refractive index to the incident wave. Radiation may thus be coupled into the sample and become absorbed, irrespective of the prime natural reflection properties of the material. A totally different absorption response is therefore attained by the otherwise assumed ‘planar’ but textured interphase.

(c) The existence of localized imperfections produces scattering which leads to local modifications of the field. The ideal planar wave incident on the workpiece produce large local field intensities which can affect the material significantly. Multi-pulse irradiation produces localized damage in the material even at relatively low average intensity. Subsequent pulses may thus find totally different surface properties, which in nominal terms respond quite unexpectedly.

(i) Electronic excitation and de-excitation upon radiation absorption: Transitions between bands by use of a single photon or many photons (multiphoton), associated with radiation emission (fluorescence, luminescence) and also non-radiative energy transfer via coupling to the lattice which may cause significant structural modifications.

(ii) Photo-ionization: Removal of electrons to the vacuum state usually followed by avalanche ionization, which leads to materials breakdown, triggering ablation and plasma formation.

(iii) Molecular photo-dissociation: Absorption and excitation to repulsive states leads to the dissociation of molecular bonds and disintegration of the material.

(iv) Photochemical reactions: Activation of chemical reactions, synthesis of new molecules or manipulation effects.

(v) Laser-plasma effects: formation, plasma post-ionization, resonant effects and superheating.

(vi) Other atomic and molecular interactions: radiation-induced forces, optical trapping and atomic cooling.

(a) Thermal effects are of prime interest and their participation cannot be excluded in any case. They range from the initial phase transformation to melting and re-solidification, evaporation and pyrolysis, processes evolving under thermodynamic equilibrium.

(b) Photomechanical effects relate to the direct or indirect application of mechanical forces. At low light intensities they concern radiation pressure and gradient field forces. In addition, thermomechanical stress effects, acoustic waves and shockwaves can be induced at high intensities, especially upon photo-ablation.

(c) Photochemical effects, including photo-dissociation and photolysis, photo-catalysis, photochemical reactions, photosynthesis and photopolymerization.

(d) Photo-desorption and photo-ejection from surfaces include molecular, atomic, ionic and electronic ejection by absorption of radiation. This is termed non-explosive materials ablation as it is effectively removal of materials in the atomic/molecular level.

(e) Photo-ionization of atoms and ions, plasma formation and driving of atomic beams.

(f) Explosive photo-ablation is a composite explosive effect embracing most of the above phenomena, developing under non-equilibrium thermodynamic conditions. It is associated with violent ejection of plasma accompanied by neutral species such as molecules, clusters and particulates.

(i) In the UV spectral range (1–2) strong absorption is caused by interband transitions from the valence to conduction bands. The fundamental absorption edge, 2, may be in the visible or near-IR for semiconductors. The absorption coefficient is typically above α ~ 10⁵ cm⁻¹. For larger photon energy in the X-ray region and beyond, this absorption decreases rapidly. The energy and momentum conservation, respectively, ΔE = E_f – E_i = ħω(β) and k_f – k_i = β are valid. Towards the longer wavelengths, the edge 2 defines the limit of the band gap, E_g = hv, and yields a sharp decrease of absorption. Usually at low temperatures for bulk materials, this region comprises excitonic states, 3. Rayleigh and Mie scattering are always present and result in increased coupling effects.

(ii) Further to the exciton states, 3, absorption is due to impurity centres in the gap, 4, which become ionized. Intraband absorption concerns also the case of free electrons in the conduction band or holes in the valence band. High intensities may yield reflectivity increase due to free electrons. In contrast to the metallic behaviour, this type of absorption depends on the electron density and is limited. Electron density in insulators is negligible. For photon energies below the energy gap, hv < E_g, absorption decreases with ω², defining the free electron absorption edge, 6, that extends to the lowest limit of the energy gap. Impurity absorption changes considerably the behaviour of the materials and has dramatic effects in the overall response. In the band gap where the material can in practice be fully transparent, ion centres or other imperfections lead to strong resonance absorption which depending on the matrix environment can present a quite broadened response. A linear absorption spectrum is thus observed. Characteristic examples here are the hydroxyl (O-H) bonds in silica, of special interest in optical fibres, which limit the NIR transmission, and the C-H bonds in polymers, with several chromophores producing dramatic absorption in the otherwise transparent materials.

(iii) In the far-IR region (> 20 μm) the interactions between photons and phonons are direct, in region 5. Photons interact with the optical modes due to dipole moment of the lattice and the comparable energy content (0.05–0.02 eV). At high frequencies the probability for the participation of multiphoton processes increases and absorption constants can approach 10⁵ cm⁻¹ in polar crystals. In covalent crystals, however, smaller values of 10–100 cm⁻¹ are observed. Lattice imperfections again play a crucial role and lead to nonlinear coupling. Very low frequency effects, 7–8, such as cyclotron and spin resonances are not important in our case.

(iv) High laser intensities reveal a range of nonlinear processes in solids, owing to non-harmonic driving of the materials polarization as P ~ χE + χ⁽²⁾ E² + χ⁽³⁾ E³ + …. Further to the frequency generation processes, nonlinear Raman or Brilluin scattering induce strong radiation coupling to the solid via stimulated processes. Of great significance here is the nonlinear absorption process occurring at high laser intensities, in which the simultaneous presence of multiple photons results in excitation and ionization of atoms and molecules, even by use of low photon energies. The response is analogous to the nth-power of the intensity as, Φ ~ Iⁿ, where n is the process order defined as the number of participating photons.

Photophysical effects

While laser radiation is an important tool for metalworking and the microelectronics packaging industries, laser-based processing in photonics technology meets niche applications (Basting and Marowsky, 2005). Laser annealing has been widely applied for the phase transformation of semiconductor interfaces such as in thin film transistor (TFT) production, as well as for selective materials doping. Thus, laser offers selective processing in the material bulk (Gamaly et al., 2006) leading to structure modification, crystallization, optical poling glass and polymers, and the induction of nonlinear optical properties Along these lines, photorefractive processes represent a large class of refractive index modification of materials by the application of usually intense optical fields. The refractive index of a material can be tailored by structured light even at milli-Watt power levels. Such changes may relate to the excitation of specific impurity centres or vacancies resulting in localized refractive modifications. In several cases these effects are also associated with strong photochromism owing to the excitation of relatively deep levels which affect the absorption properties, especially by use of UV radiation. Characteristic examples concern silica and other glass bulk and optical fibres with significant impact for fibre grating technology and other widespread photonic applications.

Structural modifications can be induced even by low intensity laser beams in amorphous and glass oxides (Mailis et al., 1998), and also non-oxide chalcogenide glass materials (Gill et al., 1995). The strength and the permanency of these effects depend on materials, laser wavelength and radiation intensity in the interaction region. Dynamic processes may be based on relatively long-lived effects that relate to impurity state excitation. They are followed by electronic or ionic relaxation phenomena which provide a degree of reversibility to the original materials state after removal of the irradiation. Intense irradiation, especially highly energetic deep-UV photons, may yield permanent structural changes usually associated with structural deformations. These effects result in significant variation of the refractive index and provide the means for the fabrication of micro-optics, optical waveguides and other photonic devices. In this case the high-intensity electric field rather than thermal effects has effectively modified the internal structure locally in the bulk in the area of beam focus, inducing large optical nonlinearities. Such effects take advantage of multiphoton processes induced by ultra-short high-intensity pulses, while refractive filaments in silica bulk offer new possibilities for photonic structure fabrication in the future (Haglund, 2006).

Quite different processing operations are possible by the deployment of laser radiation forces (Sigel et al., 2002). The application of structured optical fields leads to the production of reversible structures. They are formed by compressive forces in semi-dilute entangled soft matter causing rapid osmotic extraction of the solvent and subsequent solidification, forming three-dimensional micro-objects, discussed in a later chapter.

Photochemical effects

Some of the most well-known photochemical processes are those met in the reduction of silver in photographic emulsions. The dissociation of silver halide produces silver nanoclusters which darken the emulsion and develop the image, via the reaction AgX + hν → Ag + X. This process can also be activated by a focused or an interfering laser beam and can produce optical gratings in the bulk or the surface, a toolbox for the advent of holography and its numerous applications. The reduction of metallic compounds by using light is today a source for nanocluster production and it is applied as an alternative to chemical reduction methods.

The exposure of photoresists in lithographic processes use routinely high power UV lasers in the industrial floor. The relatively narrow spectrum of excimer lasers emitting at 248 nm (KrF) and 193 nm (ArF) and the high-intensity radiation allow tuning of the resist properties. In addition pulsed exposure allows fast processing speeds and high industrial yield. Deep-UV exposure has been advantageous in reducing the feature size of lithographic processing to the 100 nm size for contact methods. Resist technology has thus been developed in recent years to cope with lithography towards the extreme UV range. A favourable candidate appeared with the development of high power F₂ excimer laser emitting at 157 nm. Even though this radiation promised to provide reduction of feature size well below the state of the art, two main problems hindered the further deployment of the technology. First, the delivery of beams at 157 nm requires complex vacuum or nitrogen or noble gas purged transmission lines, which increases the complexity of the technology. Second, the highly energetic photon of 7.9 eV produces molecular dissociation of the photoresist and large amounts of debris which is deposited on masks or near field optics thus decreasing the efficiency and the accuracy of the lithographic process.

Current industrial developments in the micro-electronics and photonics industries incorporate high power UV laser lithography for large-scale production and multiphoton lithographic processing currently provides significant feature size reduction. This is caused by the use of the high-intensity section of the focused beam, which can be tuned to reach processing regions sized below the optical diffraction limits. In this context scanning laser lithography by femtosecond lasers leads to 3D structure development, as an evolution of conventional stereolithography. Complementary approaches relating to metallized structures have also been presented.

Further to the photo-polymerization processes, intense UV beams have been used for the deposition of metals and the growth of semiconductors, which can be realized in gas, liquid and solid environments. For example, the use of organometallic compounds has provided selective deposition of metals by photo-dissociative reactions such as M (–C_x H_y–) + hv → M↓ + C_xH_y↑. Metal hydrides have been similarly used with significant examples of amorphous silicon growth through silane decomposition in a series of photoreactions with deep-UV radiation to yield hydrogenated amorphous silicon through: SiH₄ + hν → {SiH₂ + 2H} + hν + Si → α-Si:H. Further applications involve oxidation and reduction processes with important examples being the production of reactive gas such as oxygen and nitrogen roots for oxide and nitride materials growth. The above processes may be realized as part of more complex processes such as physical vapour, molecular beam deposition, or laser ablative processes.

Photoreactions may also be used to achieve selective corrosive processing, while in most cases a suitable chemical equilibrium may be established depending on the application sought.

As a general rule the surface or bulk photochemical process embraces:

The localization or not of the targeted process depends exactly on the laser interaction region and the extent and nature of the process achieved via localized or delocalized primary and/or secondary reactions. Effects may take an explosive character under intense illumination, thus denoting a rather ‘grey borderline’ between subtle and ablative processes.

In its relevant chapters this book provides an exhaustive account of state-of-the-art photophysical and photochemical processing, expanding in detail on the emerging research and future technologies.

Main ablation schemes

Of extreme fundamental and technological interest are the ablative processes realized upon interaction of solid or liquid materials with high-intensity laser pulses. Figure 1.5 depicts the typical interaction geometries implemented to date. In Fig. 1.5a, an intense laser pulse 1 is focused on surface of a solid target 2. The material absorbs the pulse energy and the material is explosively photo-ablated and forms a deposit, 3, on suitable oppositely positioned substrate, 4. In this case a pulsed laser deposition (PLD) scheme is realized, in which a highly energetic plasma plume 5 is formed. Figure 1.5b emphasizes the direct laser ablative processing scheme, where the target is etched with high precision by a highly focused beam or a complex pattern imaged on the surface, 2, at high resolution (Vainos et al., 1996). Both operations require laser intensity on target exceeding the characteristic threshold value. This threshold value is materials, surface and wavelength dependent. Below this value photo-desorption effects are possible which may be assisted by the substrate and/or the matrix.

The forward ablation scheme (Bohandy et al., 1986) and the resulting micro-printing are illustrated in Fig. 1.5c. A transparent target substrate embodying a thin or thick film 5 is used. The irradiating pulsed beam is transmitted through the substrate 2 and is absorbed by the film 5 which is thus ablated at its interphase. The film material is expelled by the vaporized layer and forms a deposit 3 on the receiving substrate 4. The accuracy of this micro-printing operation performed by laser-induced forward transfer (LIFT) can reach the submicron levels (Zergioti et al., 1998). The remaining film is etched with high precision as shown in Fig. 1.5d, forming patterns, 3, of complementary design to those being micro-printed.

As we will discuss later in this chapter, both backward and forward ablation modes have achieved quite remarkable results. A wide range of materials, from glass and amorphous to high crystallinity epitaxial films have been achieved, while high precision direct etching has been proved a viable scheme capable for micro-fabrication.

Ablation mechanisms

Depending on the nature of the material, exposure of a local region of the materials interphase a (usually) focused high-intensity beam produces a rapid, abrupt vaporization of the surface material under thermodynamic non-equilibrium conditions. A superheated volume builds up close to the surface, the Knudsen layer, establishing internal temperature much higher than the vaporization temperature and a very high pressure. The density of this layer is proportional to the laser pulse intensity and the very small mean free path aids to equalize instantaneously the internal temperature of the layer.

This superheated layer is expanded from the near surface region expelling the possibly present ambient gas. Depending on pulse intensity this expansion may be subsonic (speed < Mach 1), sonic (speed = Mach 1) or supersonic (speed > Mach 1). In case of Mach 1, the external ambient pressure determines the vaporization. Mach 1 is a limiting case from which mass continuity is satisfied by the production of new vapour that adds on, finally creating a shock wave which propagates in the neighbouring materials.

This is exactly the case which onsets a series of laser interactions relating to the production of plasma as shown in Fig. 1.5. The intense light pulse produces fast ionization and the formation of a highly energetic directional plasma plume ejected as shown in Fig. 1.5a. Depending on the process parameters this plasma can be produced either by radiation heating of the ejected vapour, or by photo-ionization and direct photo-dissociation of the material or their combination. Plasma can be established under local thermodynamic equilibrium (LTE), governed by the Saha equation:

[1.12]

where respectively: n_g, n_e, n_i are the number densities of atoms, electrons and ions, Z⁺ and Z⁰ the partial functions of ions and neutral atoms, m_e the electron mass, T temperature, k_B and h are Boltzmann’s and Planck’s constants respectively, and E_i is the ionization energy.

Plasma extends and expands above the interaction region with a visible plume formed with temperatures reaching above 10000°K and pressures of the order of 10⁴–10⁷ Pa. Depending on intensity, particles are ejected at supersonic speeds, above 10⁶ cm/s. Depending on the number density and radiation wavelength, plasma may absorb or reflect radiation, with such effects naturally becoming more profound by using IR radiation and long laser pulses.

Bearing in mind the above, it is important to underline that there is not an integrated theory available to date capable of describing well these complex phenomena. The thermal, photophysical, photomechanical and photochemical mechanisms are coupled together and constitute this explosive photo-ablation effect. The participating strength of each mechanism depends on the specific experimental parameters, which determine the evolution of events, and no absolute borders can be defined in all cases.

There are two types of ablation processes demonstrated to date. The backward ablation scheme described above (Fig. 1.5a) leads to a highly directional plasma plume which is characterized by multiple constituent components. The ejected matter comprises electrons and ionic species, neutral atoms, complexes, clusters and micro-particles, all ejected at different speeds from the ablation volume and following different spatiotemporal distributions. This latter effect is of extreme importance and offers significant advantages in applications requiring filtering of specific species. On the one hand the eroded material exhibits a minimal distillation effect, and the remaining part after ablation preserves the original composition of the target, and on the other hand, the ejected material also preserves its overall composition and can be deposited on a substrate achieving the original stoichiometry and in many cases the crystallinity of the target material. While there are exceptions, it has been observed that the less thermal the nature of the ablation process is, the more stoichiometric the expected deposits would be.

Influence of the actual experimental parameters

There are a number of important parameters which influence the ablation process, its nature and individual characteristics. These parameters are interdependent and affect not only the coupling of energy to the target but also the nature of the process itself as follows:

The nature of the target material determines the overall process and it is in full dependence on the laser parameters. The optical and physical properties are decisive in the development and evolution of these effects. Wavelength and surface properties are the main factors that determine the degree of energy coupling into the target, through the absorption property and the diffractive coupling effects induced by the surface texture.

According to our previous discussion, radiation incident on the planar surface is absorbed in the penetration depth, δ = α⁻¹, where α is the absorption coefficient at the particular wavelength. The surface texture of spatial frequency comparable to the wavelength is acting to increase coupling of radiation, while sub-wavelength components are enhancing the most this nominal apparent absorption. The absorption of most materials in the deep-UV region is usually very high and the penetration depth is minimal, with the exception of highly transparent fluorides and related compounds. The penetration depth of UV radiation in metals is a few nanometres, except in some alkali. Infrared radiation can be more penetrating and thus energy is deposited in a larger interaction region than the UV.

The physical properties of the material undertake the next task of transferring this energy to the bulk. The nature of the material then determines the evolution of effects. In a case of solid having finite thermal conductivity, it is the combination of penetration depth δ and thermal diffusion length, L_th:

[1.13]

where, D_th is the thermal diffusion coefficient and τ is the laser pulse duration, which determines the interaction. In effect, the energy is deposited in a depth δ = α⁻¹ and is converted to heat. During the laser pulse this heat is diffusing in depth L_th. A maximum energy accumulation effect is thus expected when during the irradiation time, τ, all energy deposited in δ cannot diffuse away, but remains in the interaction region with δ ~ L_th. Provided the pulse energy is high enough, this fact will lead to efficient ablation.

The intensity of the laser beam thus becomes the most crucial parameter. It relates to the energy deposition rate per unit area. While the pulse energy fluence is an important factor, it is effectively the pulse duration, τ, what governs the overall process. Figure 1.6 presents a range of effects evolving at different pulse length values and the respective intensities. In Fig. 1.6a CW radiation or a relatively long pulse 1, up to the nanosecond regime, irradiates the sample 2. Radiation may be absorbed within a specific volume, 3, by electrons in the valence band or impurity states, 4. This energy is transferred to the lattice via collisions and heats the material bulk. The more than few nanoseconds’ duration of interaction greatly exceeds the energy relaxation time (~ picosecond), meaning that interaction extends during the laser pulse irradiation. Region 3 is thermally transformed and melts under laser exposure. Non-destructive chemical effects may also take place.

The use of high intensity, relatively long nanosecond laser pulses produces an electron avalanche inducing strong collisions with the lattice and finally causes materials breakdown by avalanche ionization yielding the photo-ablation effect. In molecular solids such as polymeric materials high energy photons may yield excitation to higher molecular states and enable the reach of dissociative states and molecular breakdown with minimal thermal effects. Even though in the case of polymer ablation thermal action cannot be excluded, the low thermal conductivity may yield a rapid passage to the evaporation state with minimal heat diffusion effects in the non-irradiated volume (Urech and Lippert, 2010).

In the nanosecond regime photo-ablation yields a visible ablated crater, 7, accompanied by a heat affected zone, 5, and re-solidification zone, 6.

Quite different effects are met by using high-intensity ultra-short pulses (Pronko et al., 1995). In Fig. 1.6b a sub-picosecond duration pulse, 8, irradiates the sample, 9. Irrespective of the laser wavelength a relatively high energy is confined in a small laser spot, thus leading to a smaller diameter interaction region, 10. This is caused by two effects: (a) the short interaction time becomes comparable to the energy relaxation time in the material and consequently it confines in space the whole event; (b) electrons are excited and become ionized by multiphoton absorption from the valence or impurity bands, or from a molecular state, as shown in 11 of Fig. 1.6b. The high-intensity pulse induces extreme acceleration and direct photo-ionization of electrons, leading to direct ablation. High laser intensities thus produce ablation with a smaller crater formation, 12, and minimal heat affected and re-solidification zones, 13. It is now proved experimentally that multiphoton effects enable high-resolution processing.

Laser ablation is characterized by intensity (and energy density) threshold effects and specific erosion (etching) rate behaviours. These are strong functions of the material, the wavelength of radiation and the pulse duration, through the processes outlined above. Multiphoton processing has a smaller ablation threshold and attains linearity of the etching rate. Radiation is absorbed and interacts in a shallow region near the surface, which yields high quality ablative processing. On the other hand, the energy density (fluence) value represents the total energy deposited per unit target volume and determines the extent of the overall etching process. In the long pulse regime, even in the nanosecond scale, the longer duration of interaction leads to larger ablation volumes, but with inferior etching quality effects. The ablation threshold is higher and the etching rate as a function of energy density shows a linear behaviour followed by a nonlinear saturation region. Figure 1.7 presents typical experimental data for threshold and etching (erosion) for polymer and metal substrates as a function of energy density. It is noted that the beam intensity is about 50 000 times higher in the ultra-fast case, though the energy deposited per unit area remains a quite important parameter. The material dependence is in any case distinguishable with the metal alloy showing clear improvement of the erosion rate and lowering of ablation threshold. It is worth noting here the difference between single and multi-pulse experiments. In the latter case the gradual deterioration of the structure results in lowering the apparent threshold and increasing the overall etching rates. Characteristic also is the difference between sub-picosecond and 20 ns pulses where the great difference in peak intensity leads to severe reduction of the etching threshold (Mailis et al., 1999).

Considering the ablation process the intensity affects the plasma distribution leading a narrow cone of plume and matter ejection. Energy density also affects significantly the products of ablation. While femtosecond laser pulses were initially thought to yield highest quality ablation products and a smaller particulate content this has not been verified experimentally and nanosecond pulses are still seen to produce superior results in PLD experiments.

Appraisal of methods and apparatus

The growth of photonic materials imposes stringent requirements on the nature of the ablation process, the materials used and the conditions applied. Photonics require extreme quality with respect to optical properties such as transparency, bulk homogeneity and surface quality, in addition to specific individual properties, such as composition, doping or other physical properties.

To comply with these requirements, extreme control of the process is required and a huge effort has been made in this respect. These initial efforts have been directed to ferroelectric and paraelectric oxides, such as respectively barium titanate (BaTiO₃) and bismuth silicate (Bi₁₂SiO₂₀) as holographic photorefractive materials (Youden et al., 1991). This progress has been followed by further materials including garnets and sapphire, which led to the achievement of the first laser actions by PLD grown in Ti-Sapphire, Nd:YAG and Nd:GGG optical waveguides (Anderson et al., 1997a, 1997b). More recently, nanostructured thin film oxide materials for photonic sensor applications have been developed (Mazingue et al., 2005). The overall methodology is developed and presented in reference to the typical PLD experimental configuration presented in Fig. 1.8a.

A high or ultra-high vacuum (HV or UHV) reactor vessel is especially designed and constructed to incorporate optical windows and feed-through ports. The ambience of the vessel can be modified by gas injection and monitored by, for example, a quadrupole mass spectrometer (not shown). A pulsed laser beam 1 is directed through a transparent window and is focused on the target material 2. The target from which material is to be ablated may be in a solid crystalline, glass or sintered ceramic form. The target is usually attached on a rotating or translating holder platform, to ensure refreshing of the material undergoing ablation. This material is laser sputtered and grown as film, 3, on an oppositely placed substrate, which is usually a glass or crystalline plate, or wafer. A train of laser pulses, usually delivered at a repetition rate of a few Hz to some tens of Hz, ablate the target material producing a plasma plume, 4, which is directional and flushes the substrate. The ejected material is thus deposited on the substrate and grows a film, 3, on the substrate, while measures are taken to ensure uniform coverage in film growth. In Fig. 1.8b and 1.8c two distinct examples of photonic structures are shown: a highly compacted waveguide optical amplifier (Vainos et al., 1999) and indium oxide nanocomposite comprising nanocrystals and metal clusters in the amorphous matrix (Grivas et al., 2002).

While the overall laser sputtering process appears to be simple in its principle and provides a large flexibility in its operation, there are several interdependent parameters which need to be tuned for the successful growth of a specific structural composition. Even though there are various commercial systems available which operate reliably for specific relatively simple coating processes, the specific individuality and the interdependency of parameters make PLD a not yet industrially standardized process.

Interdependency of PLD processing parameters and methodologies

Several important issues determine the PLD methodology, with specific aspects of the scheme concerning:

In conclusion, PLD has proved to be a quite versatile growth tool which can be adapted to the application needs. While the system is quite flexible, the very nature of the process has not yet allowed industrial standardization and reliability for large-scale production. The limited area and uniformity of deposition, together with the difficult control of parameters in large-scale and long-run operations are some present impediments. The latter yields an overall incompatibility with the existing micro-electronics and photonics foundry, thus limiting wide exploitation of the scheme.

Advanced schemes for photonics are discussed in a later chapter.

Laser micro-etching operations

The use of laser beams finely focused in the micron scale produces materials micro-ablation. This operation is limited in the micron or submicron size laser spot area and requires relatively low pulse energy in the range of a few milli-joules. In this regime the strong focusing in micron size spots of nanosecond pulses leads to extremely high intensities in the GW/cm² range corresponding to energy density values of several J/cm². These values are well above the ablation threshold of most materials including the highly transparent glass and crystalline compounds. The micro-ablation process thus leads to a series of processing operations allowing direct materials treatment spatially. These are now considered viable as alternatives to well-established lithographic methods, offering specific advantages and unique features as analysed here (Mailis et al., 1999).

Figure 1.9 provides an outline of a direct laser micro-fabrication station. The beam, 1, is delivered by a laser source (not shown) and is transmitted through a beam-shaping optics system, 2. The system performs beam expansion, filtering and possibly homogenization. The processed laser beam is directed through a folding mirror, 5, into a focusing objective, 3, which delivers the radiation onto the workpiece, 4, for processing. This workpiece is positioned on a platform allowing controlled x-y-z motion to enable focusing in the z-direction and x-y surface processing. It is noted here that depending on the specific configuration folding mirror 5 may be replaced by a raster scanning system, especially to be used with long focal length focusing. The overall processing area is monitored by ocular system, 6, which may include video imaging. We note here that such a system may be used in projection lithography in which case the laser intensity and power throughput needs may be reduced in favour of resolution.

The operation of the system is assisted by the controlled motion of the platform especially by use of high precision nanomotion-motorized stages controlled by position encoders currently available with sub-100 nm accuracy. This allows precise processing of the workpiece in raster scanning and/or vector scanning modes. In addition, the beam-shaping system may comprise a suitable imaging mask or diaphragm which is projected onto the surface of the workpiece for direct etching of complex patterns such as holographic patterns and images, in conjunction with beam homogenizers.

The above imaging operation mode is considered to be most appropriate as it allows precise intensity control to be applied in the system through calibration procedures. It allows high precision ablative processing via high-resolution imaging, yielding controlled intensity profile and pattern transfer onto the workpiece. This scheme, combined with absolute motion control, enables high precision direct processing. It is worth underlining that the use of a focused beam is not always appropriate due to the Gaussian or other highly peaked beam profile at the waist which may lead to inhomogeneous processing. Nevertheless, in some cases this allows a type of super-resolved processing in the sub-wavelength regime (previously shown in Fig. 1.6b), utilizing only the high-intensity part of the beam, while the beam shoulders have intensity below the required ablation threshold.

Figure 1.10 depicts the first developed examples of micro-fabrication of computer-generated holographic optics etched directly on the surface of acrylic and polished stainless steel plate with their optical reconstructions having appreciable finesse though non-optimal configurations have been applied. Fig. 1.10c shows a Fresnel micro-lens of about 500 μm diameter. Fig. 1.10d shows a polymer optical fibre processed for sensor applications with long light coupling slot. This unique operation proves the generic processing capability of the scheme. Such operations benefit from the nature of ablation which is able to cope with different materials of composite nature without concern for differential erosion or preferential etching effects, provided the intensity is well above the ablation threshold. The quality and the speed of processing depend clearly on the laser parameters and the physical properties of the material. All issues discussed previously concerning the surface quality and the ablation mode established for processing also apply here. Bearing in mind the sensitive and the precise nature of the operation, the laser parameters must be tuned exactly in relation to the physical properties of the workpiece and the application sought in order to achieve high quality results. It is clear that excessive intensity may cause severe damage to the sample, while reduced intensity may lead to gradual deterioration of surface quality, making the required process impossible.

Specific examples of photonics-related processing include the production of surface relief patterns, the production of micro-lens arrays, the processing of optical fibres and the tailoring of waveguide structures, as well as photonics packaging applications. The main advantages of the direct laser processing operation are drawn from its materials generic nature and the flexibility of application on surfaces of arbitrary geometry, practically on every solid surface. The scheme has been recently explored successfully in the fabrication of authenticity micro-holograms for security applications in the frame of the EU project initiative ‘HOLAUTHENTIC’, which resulted in a laser-based authentication system.

Laser micro-printing approaches

A complementary operation has been developed for photonics applications by use of the typical micro-fabrication station (Fig. 1.9). The material to be transferred is deposited on a transparent ‘carrier’ substrate as previously presented in Fig. 1.5c. An ultra-short laser pulse irradiates the film after transmission in the carrier plate and ablates the material at the substrate–film interphase. The net result is the ejection of the material in free space, propelled by the ablation gas products, and its deposition in the nearby positioned substrate. Figure 1.11 presents the dynamics of this forward ablation process by Schlieren imaging of the ejected material in free space (Zergioti et al., 2002). The velocity of the ejected material is a function of intensity of the laser pulse and determines the kinetic energy which is in turn deposited on the substrate. It is worth noting here that this operation can be performed either in vacuum or in gas atmosphere, while velocities in the range of 1 Mach have been recorded. It is noted that in this particular case the produced ablates are expanding in flight, while in other configurations the solid or liquid ejects are propelled and deposited on the substrate. This is the case with either metal targets or inorganic deposition by use of dynamic release layers, which rapidly photo-dissociate and detonate upon absorption of laser radiation.

Figure 1.12 presents the first laser micro-printing operation in the submicron regime by using a single 0.5 ps laser pulse at 248 nm. Series of 700 nm diameter metallic Cr dots deposited on glass, silica and silicon substrates show excellent adherence, a fact which verifies the energetic nature of the process. By using a modified deposition cell micro-printing on optical fibres has been possible, as shown in the examples in Fig. 1.12b for dot series on D-fibre and high curvature etched cylindrical fibre. Such operations may lead the way to photonic sensor applications, enabling enhanced optical coupling and advanced interrogation.

While the first application of laser-induced forward transfer concerned the repair of photomasks, the reduction of printed patterns and the high quality of deposits led the original developments in the field of photonics. Micro-printing operations thus first addressed the fabrication of photonic micro-structures in the form of computer-generated holograms of metals and metal-oxides.

Since these original developments several techniques have been developed for photonics applications, with most recent advances relating to the fabrication of waveguide devices by depositing metallic Ti stripes on lithium niobate for subsequent in-diffusion for waveguide fabrication. Further developments involve the use of polymeric dynamic release layers (Banks et al., 2008), which upon photo-dissociation produce a considerable amount of gas that enables efficient propulsion of the transfer layer, used especially in cases of transferring inorganic oxides, such as GGG and YAG laser garnets. It is worth mentioning that the method has so far allowed the deposition of submicron metal nanodroplets, achieved currently in the range of 500 nm and below to 300 nm diameter, by using amplified femtosecond Ti-Sapphire laser pulses in forward and backward transfer modes. Metallic films show a most suitable behaviour for decreased feature size, because the film melts locally releasing nanodroplets due to the unstable hydrodynamic conditions induced that enable propelling of matter to the oppositely placed substrates.

A case of global laser-based micro-fabrication

In Fig. 1.13 an example of a global laser-based device fabrication approach is presented (Koundourakis et al., 2001). The process commences with the PLD of metal oxide. In this specific case a non-stoichiometric mesoscopic indium oxide (InO_x) nanocomposite is grown by reactive PLD using metallic indium target as shown in Fig. 1.13a. The material is deposited by use of nanosecond laser pulses at 248 nm on fused silica substrate. Its structure comprises several defects in the form of oxygen vacancies and metal nanoclusters, which enable charge transfer processes to evolve leading to variations of its electrical and optical properties. The latter effects lead to holographic recording in the material due to localized photorefractive effects induced by interfering UV laser beams.

This thin-film-on-carrier-plate structure is used in the laser processing station to micro-print a diffraction grating on a transparent glass plate by 0.5 ps 248 nm pulses, as presented in Fig. 1.13b. This grating is fabricated by repetitive transfer of grating lines sized in the micron range. We note that the build-up of the grating is achieved by precise relative positioning of the carrier plate and the receiver substrate, with the distance between film carrier and substrate kept in the few microns range.

The functionality of the deposited structure has been investigated by diffraction experiments under UV illumination at different temperatures. Fig. 1.13c depicts the typical experimental configuration used for functionality testing. The inset Fig. 1.13d shows an atomic force microscope image of the final diffractive structure. A UV laser beam is turned on and off and the diffraction efficiency of the micro-printed grating is altered within a certain response time determined by the participating charge transfer processes. In a manner identical to the parent InO_x nanocomposite material, the refractive index of the grating is altered by the illuminating UV beam. This produces changes in the observed diffraction efficiency as shown in Fig. 1.13e in a similar time scale to that observed in the original ‘parent’ material.

It is worth mentioning that the alternative approach would be the lithographic processing of the planar film in order to produce the desired diffraction grating with expected similar results. Further to the complexity of this multi-step processing operation, a unique advantage of the laser-based scheme is drawn from its potential to produce micro-printed structures of arbitrary design on practically every given surface of arbitrary morphology, even of high curvature or discontinuous geometry, with no restriction. This is the significant advantage of the scheme in addition to the generic and inert processing nature of laser ablation. Indeed, as discussed above, lithographic approaches involve steps of wet deposition of masks, possibly followed by chemical or reactive etching which affects the nature of sensitive materials.