**6.1 Mechanical characterization**

The mechanical characterization comprises the determination of residual stress commonly created during deposition, and the physical properties such as Young's modulus and hardness.

The well-known Stoney Equation [36] is widely used to characterize the stress generated in a film deposited onto a thicker substrate. The principle behind this equation is the linear correlation between the stress crated within the film, and the amount of bending produced in the substrate as a result of the constraint condition among them. The application of this technique requires the knowledge of the radius of curvature of the substrate before and after the film deposition. The radius of curvature is often measure with an optical profilometer or drawing on any high resolution microscopy method that allows to reproduce the sample profile. The accuracy of this method is subjected to the compliance of certain conditions, i.e. the film thickness is much thinner than the substrate, very small strain and rotations, both film and substrate should be homogeneous and isotropic, equi-biaxial stress in the plane of the film, spherical deformation of the system film/substrate, and spatial invariability of stress and curvature in the whole surface. Even though some of these conditions are not fully in compliance, this technique has still been used for stress determination in a number of investigations including semiconductor-based thin films in the solar sector, thin film transistor industry among others [37, 38].

Physical properties such as hardness and Young's modulus are usually characterized drawing on the nano-indentation technique [29, 39]. This method consists in moving a sharp indenter towards the surface of the film until making an effective contact, then the applied load and the displacement are repeatedly recorded. In this way a correlation of the applied load through the indenter and the depth of indentation is established. As the indenter penetrates into the film, the slope of the loading curve progressively increases due to stronger contact between them; this correlation allows for the calculation of the hardness at any point of the curve by dividing the load to the contact area in that point. The Young's modulus of the film instead can be determined from the unloading curve once the contact area is defined considering that the Young's modulus and Poisson's ratio of the indenter are known. This is possible thanks to the direct correlation that is established between the backward motion of the indenter and the elastic properties of the film during the unloading.

### **6.2 Chemical composition**

A number of techniques are available to obtain the elemental composition of thin films, which can be classified in two groups, ion scattering and spectroscopicbased techniques. In the first group both Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) are included. They both are based on the elastic scattering of energetic ions produced when they strike the atoms in the film surface; in this process they transfer an amount of energy to the target species atoms via collisions generating backscattered, forward scattered, and recoils particles. While in RBS the backscattered yield and its energy distribution are measured by a detector, in ERDA the corresponding quantities for the coiled particles are recorded allowing the formation of an energy spectrum from which the compositional depth profile can be extracted. These both techniques can be used as complement of each other as the RBS is suitable for detection of heavy elements while ERDA provides a higher accuracy for light elements. The main disadvantage of RBS is a shallow depth into the film that can be probed while in ERDS complications to distinguish elements with similar masses arise.

In the spectroscopic-based techniques, the X-ray photoelectron spectroscopy (XPS) is one of the most widely employed for elemental composition characterization. The XPS is based on photoelectric effect through which electrons localized either in the core or valence band are emitted when a beam of X-rays, with an energy higher than the binding energy of the electrons, is irradiated onto the film surface. Then, these ejected electrons are driven first to an analyzer to measure their kinetic energy, and subsequently they arrive to a detector where the number of electrons is counted considering their kinetic energies. The information of this kinetic energy along with the known photon energy allow to compute the electron binding energy. A spectra is then formed that correlates the electron count vs. the calculated binding energy. The binding energy constitutes a signature for the identification of each constituent element of the film since each element possesses a unique value. On the other side, the intensity of the spectra instead reflects the concentration of the element. This technique is suitable for probing the sample in a depth of a few nanometers as it is limited by the interactions of emitted electrons with the atoms present in the film.

#### **6.3 Microstructure and morphology**

A number of methods are available for the microstructure characterization in terms of crystalline volume fraction, crystallite size, and crystallographic orientation.

The distinctive microstructure and material phase of thin films as a result of deposition conditions can be extracted from Raman microscopy, and X-ray diffraction (XRD). The Raman technique [40] is a non-destructive method based on the scattering of incident photons, coming generally from a laser beam, caused by the vibrational modes of molecules or atoms present in the film. The scattering of photons can be either elastic, or inelastic; within the latter, photons can be scattered with a frequency lower than incident photons (stokes) or higher than incident photons (anti-stokes). A Raman spectrum is formed considering the intensity and the frequency of the scattered light (inelastic) corresponding to the specific vibrational mode of the molecule in question. The material phase fraction in volumetric terms can be extracted from the Raman line-shape. The different peaks observed in the Raman spectrum designates different material phases, which can be extracted by isolating the integrated intensity of the required peak. This isolation is usually carried out by a deconvolution of the Raman spectra using specialized software that allows to perform a peak fitting analysis. The average crystallite size can also be extracted from the Raman spectrum by means of a correlation length model which is suitable for sizes larger than 5 nm. However, certain considerations are necessary to avoid the superposition of simultaneous effects such as stress and local heating due to a high laser power.

The X-ray diffraction technique [41] is based on the scattering principle where a monochromatic beam of X-rays is directed onto a specimen which contains a set of lattice planes. The incident X-rays are scattered in different angles by the lattice planes according to the Bragg's law, giving rise to constructive interference which build up the peaks of the diffraction pattern. The peak shape defines the material phase for the substance under study providing a broad peak for amorphous regions, and sharp peaks for crystalline ones. In this way, the fraction of crystallinity can be determined once the integrated intensities of each characteristic peak is obtained. The peak width also provides information about the crystallite size, which shows a broadening for small crystallites and shrinking for larger ones. The crystallite size is computed drawing on the well-known Scherrer's formula. In the case of crystalline orientation it can be inferred from the relative change of the peak height.

#### *Thin Films/Properties and Applications DOI: http://dx.doi.org/10.5772/intechopen.95527*

The morphology of the film can be investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM is one of the most widely used tools to characterize the morphology of thin films. This technique is based on the interplay of a beam of electrons with the sample surface. This interaction produces the emission of electrons from the sample with different energies due to either elastic, inelastic scattering or photons, which are collected by a detector to produce a distribution map based on the intensities of the signal. Each of the three emitted scattered electrons allows the reproduction of images which provide different information about the sample, i.e. images from secondary electrons (inelastic scattering) are suitable for the study of topographical features. From a top view this technique permits the visualization of the crystalline columns emerging at the sample surface, and the different material phases, either crystalline and/or amorphous, due to the contrast created by the secondary electrons, while the film thickness can be evaluated form a cross-sectional SEM analysis.

TEM consists in the emission of a beam of electrons which are directed onto a very thin sample (a few hundred nm) to allow the transmission of such electrons. The intensity with which these transmitted electrons exit the sample depends on the density and thickness of the sample; thus, these structure-dependent intensities give rise to the formation of a contrast that is projected as an image on a fluorescent screen. In essence, a compact structure produces a higher scattering of electrons projecting a darker image while a porous structure projects a brighter image in a bright field image dominated exclusively by transmitted electrons. TEM constitutes a powerful technique with a spatial resolution higher than SEM for the characterization of microstructure, crystallite size, crystalline orientation, and the film thickness can be obtained via a cross-sectional TEM image.

In some characterization cases two or more techniques are complementarily employed to extract thorough information from thin films. For example, due to the in-depth probing the Raman technique can provide the size of the small crystallites that form a columnar structure in the film while the SEM can be used to determine the diameter of the crystalline columns emerging at the sample surface. Furthermore, the selection of the appropriate characterization technique strongly depends on the characteristics of the film such as the thickness, whether it is a conductor, semiconductor or insulator, whether it is possible to achieve a high vacuum, among others.

#### **6.4 Electrical properties**

Perhaps the most widely used technique to measure the electrical resistivity is the four-point probe method. It consists of four metal tips linearly arranged keeping the same separation from each one where an electric current is applied in the outer two probes while the potential difference is measured in the inner two probes. From these measurements the sheet resistance (*R*sh) of the film can be calculated by means of the following equation. The Sheet resistance is defined as the ratio of the resistivity (*ρ*) to the film thickness (*t*).

$$R\_{sh-\frac{\rho}{t}} \tag{1}$$

An extra issue related to the measurement of carrier concentration and carrier mobility appears in semiconductor characterization. The Van der Paw technique [42] provides a solution to obtain these parameters. This method is based on two independent measurement, i.e. resistivity and Hall coefficient. The characteristic resistances are first obtained by the application of a current in two adjacent contacts while the potential difference is measured in the other two remaining contacts, all

of which are located in the periphery of a sample with an arbitrary shape. Then these results are incorporated into an expression developed by Van der pauw where *d* is the film thickness, *f* is a correction factor that depends on the ration of the characteristic resistances *R*A and *R*B.

$$
\rho = \frac{\pi d}{\ln 2} \left( \frac{R\_A + R\_B}{2} \right) f \tag{2}
$$

The Hall effect consists in the creation of a voltage when an electric current (*I*) is applied between opposite contacts of a semiconductor that is under the effect of a magnetic field perpendicular to the plane of the sample (*B*). This potential difference is created by the migration of charge carriers, either electrons or holes, to the edge of the sample induced by the magnetic force. The sign of this voltage is determined by the type of major carriers dominating the electronic transport. This Hall voltage (*V*H) is used to obtain the Hall coefficient (*R*HS) and the carrier concentration (*N*S) through the following equation.

$$R\_{\rm HS} = \frac{V\_H}{IB} = \frac{r}{qN\_S} \tag{3}$$

where *r* is the scattering factor, and *q* is the elementary charge.

The carrier Hall mobility (*μH*) is obtained by combining the Hall coefficient with the resistivity previously calculated

$$
\mu\_H = \frac{R\_{HS}}{\rho} \tag{4}
$$

### **7. Applications and challenges of thin films**

Thin films technology has historically been used in a wide range applications going from decorative purposes in its early stage, evolving for optical purposes latter on, and an almost endless range of applications with the appearance of advanced deposition techniques, supported by the rapid development of vacuum technology and electrical power. Overall, thin films are used to enhance the properties of bulk materials by depositing a layer with the desired physical and chemical characteristics to improve their functionality. In the following section a brief description of the most technological relevant fields of application of thin films is presented.

*Advanced electronics-optoelectronic devices* have become an important field for the application of a number of thin film types. In particular, MOSFET and CMOS absorb a great amount of the technological development in semiconductor thin films. The fabrication of MOSFETs requires the use of dielectric thin films, i.e. silicon dioxide (SiO2) [43, 44] to insulate the conducting channel from the gate. This thin film has been used due to its ease of fabrication, high impedance due to a large band gap, resistance to high temperatures and chemicals. Also, metallic films are required for the fabrication of multiple microelectronic devices, opto-electronics and optical devices [13, 45]. Al thin films are usually deposited in the channel between the source and drain of MOSFETs to allow the voltage for its operation. Instead, Cu thin films are commonly used in CMOS as gate metallization due to its high electrical conductivity and higher resistance to electromigration. Thin films have also played an important role in data storage devices due to their good magnetic properties in an attempt to replace the traditional flash memory devices for non-volatile memory devices [46]. Thin films such as BiFeO3, lead-zirconium titanate films, amorphous Si, organic compounds among others are being explored as candidates as based-material for this application [47–50].

#### *Thin Films/Properties and Applications DOI: http://dx.doi.org/10.5772/intechopen.95527*

The use of *thin films in the photovoltaic sector (PV)* is conceived as a potential solution to reduce the cost per watt in the generation of electricity. This sector has been experiencing a rapid market penetration due to the accelerated achievement of higher efficiencies and the development of thin film structures with better stability. In fact, record efficiencies about 23.3% and 22.1% have been reached using copper indium gallium selenide (CIGS) and CdTe thin films as based materials, respectively [51]. Overall, the advantages of thin films in the PV sector is related to the high absorption coefficient of the absorber layer, which permits to reduce considerably the material thickness, contributing to the reduction of material cost; also thin film technology allows the deposition of multiple-juntion devices to capture most of the solar spectrum to increase the conversion efficiency [52]. Additionally, thin films can be deposited into flexible substrate for roll-to-roll manufacturing of PV modules [53]. The state-of-the art of thin films for PV application was initially dominated by amorphous silicon, but evolved into the more efficient CdTe and CIGS, and lately organic and perovskite-based PV cells are under investigation due to its reduced processing cost and feasibility to deposit at low temperatures in flexible substrates [54, 55].

*Thins films and coating applications* are involved in a large number of fields including optics, and in sectors where the improvement of the mechanical and chemical properties of bulk materials provide a better functionality or larger lifespan. Optical thin films are widely employed in eyeglasses to improve the vision through the use of a polymer-based optical element that is coated to the spectacles. In addition, the undesired transmission of ultraviolet light and undesired reflection are prevented by the use coatings materials able to absorb wavelengths lower than 400 nm, and the use of antireflective coatings usually made of dielectric materials [56]. Architectural glazing has drawn on thin film coatings to enhance the energy efficiency in office buildings [57, 58]. The heat transfer can be managed from outside and inside the buildings by a suitable filtering of the spectral regions of light. Only transmission of visible light from the outside, and reflection of infrared radiation from inside can be set by making the windows to become a multifunctional device-like with thin films with different spectral response, saving energy from air conditioning and heating for the former and latter cases, respectively. Coatings as a means to increase the wear resistance and reduction of friction in cutting tools can be obtained by multilayer deposition of ceramic coatings, i.e. TiN, TiC [13]. Coatings for corrosion resistance are widely spread in numerous sectors including pipes coated with SiC, stainless steel components coated with oxides, i.e. SiO2, Al2O3, engine parts coated with high temperature corrosion protection such as MoSi2 among others [13] .

*Organic thin films* have attracted a great attention owing to certain unique properties, in particular, flexibility and low cost material processing which are essential to expand the scope of application of many technologies. In photovoltaics for example, although still low, the efficiency has been improved considerably from 0.04% up to about 8.3% [54] in organic-polymer based modules, but its evolution remains fuelled by the low cost of material processing, i.e. printing, spraying, and the possibility to fabricate flexible modules. Likewise, the intrinsic complex fabrication process and rigidity of Si-based field-effect transistors can be somehow overcome by organic thin films field-effect transistors. The use of organic thin film have already been proven in various applications such as memory devices, sensors, electronic papers, and smart cards [59, 60].

The use of thin films has gained a considerable space in *biomedical applications* due to their ability to provide biocompatible and functional properties, for example, invasive devices, tissue engineering substrates, drug delivery, and antimicrobial coatings, to name a few. The surface of implants have to comply special chemical and mechanical properties, and Ti6Al4V thin films appear to provide appropriate conditions for femur implants [61]. This structure apart from offering a good adhesion and harness, promotes the formation of a calcium layer through a chemical interaction with the biological fluids, improving the osseointegration. Polymer-based thin films have demonstrated to have a good resistance to protein adsorption, which is essential to provide a biocompatible behavior to implants. In this respect, poly (ethylene glycol) PEG, PEGylated thin films are suitable for bone, dental implants and for tissue engineering purposes [62, 63]. Composite thin films have also been used to provide the appropriate mechanical and biological properties to implants in neuronal applications. For example, silicon-based implants have been coated with a nanostructure formed by amorphous silica with fillers of aluminum, silicon dioxide or silver in order to provide microbial protection. Inorganic thin films with piezoelectric properties deposited on flexible substrates are also being investigated for the fabrication of nano-generators and nano-sensors for biomedical applications [64]. These piezoelectric devices have the capacity to convert mechanical energy provided by the movement of internal organs into electrical energy to power for example pacemakers or nano-sensors. Due to the high sensitivity to mechanical movement these devices can also be used to monitor the cell deformation at nanoscale. Higher performance piezoelectric devices have been fabricated using perovskite such as BaTiO3, PZNT, and PMN-PT [64, 65].

The broad scope of thin film applications require of tailored physical, mechanical and chemical properties which are linked to the resulting structure and morphology, and they in turn depend on the deposition techniques and deposition parameters adopted. Accordingly, a number of challenges remain to be tackled for a complete understanding of the connection among the different phases involved in the fabrication of thin films. Overall, various versions of CVD and PVD deposition techniques present a still expensive final product, lack of reproducibility, inappropriate attachment of the film to the substrate, high deposition temperatures which prevent the use of cheaper substrates, and limited control over the final properties. Thus, deposition technology needs to evolve with a higher precision to control the microstructure, and with a higher deposition rate suitable for large area deposition to reduce the cost. Moreover, although nanostructured thin films are promising for cutting edge applications such as microelectronics, optics, photovoltaics, and biomedicine, some of them need to be transferred to specific substrates for an appropriate operation. Consequently, the now poor transferring technique has to progress to take fully advantage of thin film technology. Even though a number of characterization techniques are available for tracking almost every feature of thin films, many of them struggle when the film thickness approaches a few nanometers. For example Raman or X-ray diffraction characterization might degrade due to the inevitable contribution from the substrate to the acquired spectra. Sophisticated models to analyze the data are therefore needed to isolate the relevant information. Of primary importance for biomedical applications is the knowledge of protein adsorption in substrates for the appropriate selection of materials; however, the characterization tools for this purpose are still at their infant stage, and they are based on complicated models for data analysis. Therefore, more advanced and specific in vitro models can pave the way for a rapid identification of suitable thin films. In spite of the significant progress in deposition and characterization techniques, the prediction of film properties as a function on the microstructure is still very difficult. This occurs due to the complex transport properties derived from the multiple defects, grain boundaries, material phases, quantum confinement effects in very thin films, interface scattering, among others. Thus, advanced models that incorporate all of these structural variants are necessary to establish the appropriate connection between microstructure and film properties to progress in the thin film technology.
