**2. Pulsed laser deposition (PLD)**

Pulsed laser deposition (PLD) is a well-established method used to grow thin films from a wide range of materials, enabling a stoichiometric transfer of these. Although PLD was introduced in 1965, it was applied intensively in the late 1990s [41, 42]. PLD is a physical vapor deposition technique where an external high-power laser (typically an UV laser source) ablates a target based on a single or a combination of compounds depending on the desired composition of the film [43]. In comparison with other deposition methods such as sputtering, molecular beam epitaxy, chemical vapor deposition, or thermal evaporation, PLD has the following advantages: (i) any type of substrate can be used for depositing thin films; (ii) by using UV laser sources, a wide range of materials can be ablated; (iii) the pressure during the deposition process can be choose from 10<sup>−</sup><sup>7</sup> mbar up to 1 mbar; (iv) due to progressive growth with each laser pulse, a rigorous control of the thickness is possible; (v) the stoichiometry can be preserved or changed in a controlled manner during the deposition; (vi) the kinetic energy of the evaporated species can be moderated in order to control the film growth properties; (vii) a background gas can be used in order to obtain the adequate reactive atmosphere; (viii) multilayered thin films can be obtained by switching different target materials in the deposition cycle; and (ix) assure the purity of the initial composition because the ablation source is the light [42–45]. As any deposition technique, the PLD process has also some drawbacks: (i) limited deposition area for standard setups; (ii) the uniformity of the deposition is influenced by energy profile and inhomogeneity of the laser pulse; (iii) macroscopic and microscopic droplets are sometimes ejected from the target [45, 46].

PLD is a versatile method that proved its potential in different research areas considering that a wide class of the materials can be ablated using excimer lasers and deposited as thin films [42, 44, 47–53]. Thus, metal films, semiconductor films, superconductors, ceramic layers, oxides, insulators can be easily obtained by this laser technique [54, 55]. Moreover, nanostructures with different morphologies such as nanowires, nanoflowers, nanorods, nanotubes, and even quantum dots based on ZnO, ITO, graphene, molybdenum disulfide (MoS2), tungsten disulfide (WS2), cadmium selenide (CdSe) can be deposited by PLD [45, 47, 56–58]. The thin films or nanostructures fabricated by PLD were integrated in various devices: photovoltaics, environmental sensors, actuators, light emitters, ferroelectrics, photocatalysis, biomaterials, medical implants, etc. [45, 47, 59].

A common PLD deposition setup is depicted in **Figure 1**. Hence, the growth of the thin film is the result of the interaction between the laser beam and the target. When the laser fluence (the energy delivered per unit area at given pulse duration) reaches the ablation threshold, the vaporization of the material from the target surface takes place, process followed by the generation and expansion of the plasma plume. Further, the plasma species (free electrons, ions, neutral atoms, molecules) with appropriate energy nucleates on the deposition support [45, 59, 60]. In PLD, the film growth and the film quality depend generally on various experimental parameters:

**Figure 1.** *Schematic representation of PLD deposition chamber.*

laser fluence, laser wavelength, pulse duration, repetition rate, target-substrate distance, background gas and its pressure, quality of the target substrate temperature, etc. Because the influence of each deposition parameter on the properties of films deposited by PLD, from specific materials, was extensively discussed and analyzed in literature, in the following we briefly resumed their importance [42, 44, 47–53].

The laser fluence is one of the principal parameters because it impacts the kinetic energy of the species presented in the plasma plume and their movement toward the deposition substrate [52]. As was discussed by Schou, the chosen laser fluence must be high enough to induce target ablation but not so high to avoid the re-sputtering and possible implantation of some species in the film [53].

The laser wavelength is connected with the energy absorbed by the target material [61], thinner films being obtained when the target material is transparent to the laser wavelength used during the deposition. Lower threshold fluences and also low ablation rates are obtained when short laser wavelengths are used [48]. Thus, the laser wavelength must be selected depending on the material type intended to be deposited.

The pulse duration parameter can be controlled to prepare films with expected performances. In general, nanosecond pulse lasers are implied in the PLD deposition [48]. When long laser pulses are implied, the absorbed laser energy firstly heats the target surface to the melting point, and afterward at the vaporization temperature,

#### *Pulsed Laser Deposition of Transparent Conductive Oxides on UV-NIL Patterned Substrates… DOI: http://dx.doi.org/10.5772/intechopen.105798*

the thermal wave penetrates the target and produces the melting of the material, evaporation appearing from the liquid phase. In the case of the femtosecond-pulse lasers, the vapor and plasma phases appear quickly, therefore the heat conduction is negligible, and as a consequence, the liquid phase is absent [62].

The pulse repetition rate influences the deposition rate, this being related to the duration necessary to get a specific thickness of the film [63]. The number of the particles, which are found as islands, grown firstly on the deposition substrate, subsequently tend to diffuse and aggregate depending on the pulse repetition rate, a higher density of islands being favored by the increase of this parameter. Moreover, it was emphasized that using higher pulse frequencies, a high density of small-size islands can be obtained facilitating the diffusion of some adatoms from islands top to the substrate, in this way films characterized by a smooth surface being obtained. At lower pulse frequencies, a low density of islands is formed resulting in rougher surfaces [64].

Although some PLD films can be fabricated just in ultrahigh vacuum, most of them required a background gas; this parameter affects the plume dynamics and furthers the growth and properties of the films [52, 65]. The background gas decreases the kinetic energy of the species presented in the plasma plume, a high pressure of this can decrease the sputtering of the film, but at the same time can lead to the preferential diffusion of some species to the deposition support [53, 66]. Argon, helium, or nitrogen is frequently used in the PLD deposition, but the most studied gas is still oxygen, due to the possibility of producing films with controlled oxygen content [50].

The target-substrate distance influences the mass ratio of the species that reach the substrate, thus influencing the thickness of the obtained film. A higher distance is equivalent with a reduction of the deposited material while a lower distance has as effect a rebound of the species due to their high kinetic energies [67]. Thus, it is essential to choose an optimal target-substrate distance. Some studies show that TCO layers on flexible substrate characterized by cracks or peeling off are obtained when the deposition is performed at lower target-substrate distance (4 cm) while cracksfree, smoother films are obtained at higher target-substrate distances (6 or 8 cm) [26].

The substrate temperature can influence the film growth and its surface morphology [67]. Even if the deposition can be carried on at room temperature leading usually to amorphous films, it was highlighted that at higher substrate temperatures, the adatom mobility increased resulting in crystalline films [52, 67]. When the temperature of the deposition substrate is increased, even the low kinetic energy species can be capable of constituting uniform layers [47].

Accordingly, the optimal PLD deposition conditions for developing high-quality complex films from a large number of materials can be found by tuning the experimental parameters involved in this laser process [50, 67].

### **3. Ultraviolet nanoimprint lithography (UV-NIL)**

Nowadays, the transition from millimeter to micro and further to nano dimensions, the tendency to pass from rigid to flexible electronics, and also the continuous need of device enhanced efficiencies based on surface patterning using the principles of the plasmonic and photonic theories have forced the industry to search nanopatterning techniques that can be used in volume manufacturing [68]. In order to gain the industrial attention, these patterning techniques need to fulfill at least some key attributes such as: (i) high resolution; (ii) ability to

simultaneously pattern different types of structures; (iii) high throughput and low defectivity; and (iv) reduced costs [69].

Under the name "NIL" can be found the classical thee imprint techniques: microcontact printing (μ-CP), hot-embossing (also known as thermal NIL), and UV-NIL, but also the newly added roll imprint process, laser-assisted direct imprint, reverse imprint lithography, substrate conformal imprint lithography, ultrasonic NIL [32]. As a general definition, the nanoimprint lithography can be understood as a physical pressing process to replicate the master patterns into a polymer negative resist by thermal or ultraviolet curing [38]. Master is the name of the so called "mother"

**Figure 2.** *Schematic representation of UV-NIL process.*

#### *Pulsed Laser Deposition of Transparent Conductive Oxides on UV-NIL Patterned Substrates… DOI: http://dx.doi.org/10.5772/intechopen.105798*

template that is usually fabricated using electron beam lithography on silicon substrates. From this master, in the case of UV-NIL, rigid or soft stamps (negative copies of the master pattern designs) based on elastomeric materials can be manufactured. Thus, common materials based on silicone polymers (usually modified formulas of polydimethylsiloxane), polyimides, or polyurethanes are applied as free-standing membranes or attached to a flexible or rigid backplane [33, 37, 38, 70]. Actually, these cheaper manufactured stamps are used in the lithography process reducing the production costs and thus prolonging the lifetime of the master, this being fabricated by more time-consuming and expensive methods.

The steps involved usually in the UV-NIL process are presented in **Figure 2**. Relatively simple, they can be described as follows: (i) spin-coating deposition of both primer and photoresist on the desired substrate, each followed by a heat treatment; (ii) alignment of the stamp with the coated substrate; (iii) adding them in contact, pressing and irradiating them with UV radiation; and (iv) detaching the mask after UV curing.

The advantages of using NIL in comparison to other photolithography techniques are arising from the fact that using a direct contact between the stamp and the coated substrate, the resolution is given by the resolution of the patterns existing on the surface of stamp, which can be beyond the diffraction limits or beam scattering. However, exactly this advantage can easily become the disadvantage of the technique due to the resist filling rheology behavior and demolding capabilities [32, 33]. Therefore, one of the common defect mechanisms that appear in the NIL processes is connected with the detachment of the stamp after resist curing, when the polymer may stick on the stamp surface due to the interfacial forces (adhesion and friction forces) that appear between the resist and the stamp material. Interfacial forces are strongly linked to the quality of the stamp (design, roughness, antisticking layer, and material type), to the resist material and to the residual stress that appears during the UV irradiation due to the shrinkage of the resist that makes the stamp to adhere more to the resist surface. Taking into account all these aspects, a special attention must be paid to the selection of the materials and the process parameters that must be optimized in function of the stamp characteristics and pattern design [71, 72].
