**2. Deposition methods**

Thin-films are in general developed to provide special properties, i.e. electrical, optical, mechanical, chemical, that satisfy the needs for specific applications. The desired properties are determined by the resulting film structure, which strongly dependents on the selected deposition method, film material, and substrate. In line with the wide range of applications of thin films, a number of deposition methods have been developed/improved to optimize the film properties, of which, the most commonly employed are described in this section. Broadly speaking, thin-film production can be realized based on two technological groups, namely physical and chemical deposition methods.

#### **2.1 Physical deposition methods (PVD)**

Physical deposition methods are usually referred as to physical vapor deposition methods (PVD) because the process entails the generation of vapor. PVD essentially consists in removing growth species from a source or target material via evaporation, then this vapor is transported to the substrate surface, and eventually it solidifies in the surface, forming the film. The evaporation is generally carried out under a reduced pressure chamber to avoid impurities in the film formation which are produced due to collisions between vapor particles and residual gas particles in their displacement from the source to the substrate surface. PVD techniques are known to offer a number of advantages, including the deposition of almost any material, high reproducibility of film properties, the use of a large range of substrate materials, the possibility of tailoring the film properties through modification of deposition parameters in single element deposition, and obtaining films with high purity. On the other side, among the main disadvantages are the use of sophisticated and costly monitoring systems for the control of the deposition rate and film thickness, and the mismatch between the composition of the deposited film and the composition of the evaporant in the case of alloys and compounds.

PVD techniques can be classified according to the method employed in the generation of vapor. The most common PVD techniques are vacuum-based evaporation and its heating versions, sputtering, laser ablation, cluster beam and ion pattering, of which, only the most important will be described in detail in the present section. For further detail on the versions of PVD techniques, the reader is referred to [1–3].

### *2.1.1 Vacuum evaporation*

It is among the most popular PVD techniques due to its simplicity in operation and high deposition rate. This technique uses heating sources to evaporate the deposition material onto the substrate surface where it condenses forming a thin film, all within a vacuum chamber. This technique is suitable for deposition of elements or compounds at temperatures below 2000 K [4]. According to the method used to evaporate the target material, this technique can be subdivided into resistanceheated evaporation, and electron-beam evaporation.

In *resistance-heated evaporation* the target material is deposited in one of the multiple configurations of the evaporation source including coil, boats, and baskets. Due to the high temperature required, they are commonly fabricated from refractory metals with high melting points, such as tungsten, molybdenum, and tantalum although stainless steel can also be used in cases where the target material has a low evaporation temperature. The essential condition for the proper selection of an evaporation source is that its evaporation point does not have to be reached at the operation temperature. Once selected an appropriate evaporation source the vacuum chamber needs to reach pressures lower than 10−5 mbar to optimize the sample coverage during deposition, and provide a high purity of the film. The evaporation of the target material is carried out by the heating of the source through which a high electric current is forced to pass. The deposition rate is controlled with the source temperature due to its direct relationship with the vapor pressure of the target material. Among the main disadvantages of this technique are the limited upper temperature (2000 K) which constrains the use of materials suitable for evaporation, a limited film thickness, and the possibility of contamination related to the heater filament.

In *electron-beam evaporation* the target material is deposited in a crucible which is design to match the heating and power density of the electron-beam. To avoid contamination of the target material in the evaporation stage, the crucible should not be prone to evaporation or erosion at high temperatures. Accordingly, the most widely used crucibles are the water-cooled copper, and ceramic hearth. With these crucibles, which have high melting points (3000–4000) K, it is possible to reach higher temperatures than those achieved for resistance-heated evaporation, and thus, it allows the evaporation of refractory material, reactive materials like titanium and aluminum, dielectrics (SiO2), boron, carbon, silicon among others. The evaporation process takes place under vacuum through the incidence of an intensive beam of electrons, emitted by a thermionic filament, which is accelerated towards the target material by an electric field. These energetic electrons collide in the surface of the target material generating a local melting after having being deflected by a magnetic field usually with an angle of 180° or 270°. The deposition rate achieved is high, in the order of 1000 nm/min, relying on the target material, and the distance between the source and substrate. One of the main disadvantages of this technique is the generation of ionized radiation (X-rays) which can penetrate the film producing damage.

*Molecular-beam epitaxy (MBE)* is the most advance solution for the deposition of compounds and alloys due to its simultaneous control over the evaporation rates of the different constituents, with high precision. This technique enables the epitaxial grow of thin films under ultra-high vacuum (UHV) conditions onto a hot single-crystalline substrate. While the UHV prevents contamination of the film from impurities present in the growth environment producing a high-purity film with an improved morphology, the substrate temperature promotes the diffusion of adsorbed species on its surface to achieve an epitaxial growth. The growing species are provided by the atomic or molecular beams generated by the heating

of a convenient source material which can be either elements or compounds, and that are normally contained in crucibles of high purity. The proper selection of crucibles is fundamental to assure the purity of the molecular beam; the most widely employed crucibles are fabricated based on graphite, pyrotilic boron nitride for high temperatures, and also quartz and stainless steel are for low melting point materials.

One of the most important features of this technique is its ability to accurately control the composition and doping profile in the growth direction. This can be realized thanks to the UHV that enables the film growth to occur in the molecular regime. In this regime the atoms and molecules do not collide in their way to the substrate because the mean free path is larger than the distance from the crucible to the substrate. This feature allows to modify as desired the composition of the feeding phase, blocking abruptly one or more streams of atoms being either a constituent atom or a dopant element with the help of mechanical shutters. Regarding that this blocking process can be carried out in very short times, and the low deposition rate from MBE, it is possible to achieve extremely thin thicknesses between the layers of different composition and/or doping. Advanced nanostructures, such as quantum-wells, superlattices and quantum-dots have been successfully fabricated with this deposition technique [5–7].

#### *2.1.2 Sputtering*

Sputtering essentially consists in the bombardment of the target material with energetic particles to dislodge atoms from its surface which travel through the plasma to eventually condensate onto the substrate. Three sputtering techniques are the most employed for thin film growth, including DC diode, RF-diode, and magnetron diode.

*In the DC diode sputtering*, the bottom electrode, called cathode, contains the target material to be deposited while the top electrode holds the substrate. An inert gas, i.e. Ar, Ne, Kr, or Xe [8] is fed into the sputtering reactor at a reduced pressure. A plasma is then formed by the application of a voltage between the cathode and anode with the inert gas inside. The electrons emitted from the cathode are accelerated towards the anode and in their way ionize the gas molecules producing positively charged ions which accelerate towards the cathode, establishing a discharge. The glow discharge can be made self-sustained if appropriate conditions of gas pressure, voltage and distance between electrodes are adopted. If that occurs, a continuous bombardment of positive energetic ions against the cathode is established promoting its surface sputtering, and the subsequent condensation of a thin film onto the substrate. It should be noted that the target material in this system is necessarily a metal as the glow discharge can be maintained only between metals. Thus, deposition of insulators might not be possible using this technique. The main disadvantages of this technique are the low deposition rates, and the absence of self-sustained glow discharge at very low pressures.

*Radio-frequency (RF) sputtering* offers a solution for the deposition of insulating materials by preventing the accumulation of positive ions in the front side of the insulator to maintain the discharge. A continuous sputtering process of the insulator target is set by the application of an alternating signal to the cathode with a frequency corresponding to radio-frequency (13.56 MHz) [9]. In this way, a larger number of electrons arrive at the surface target during the positive half-cycle which overcompensate the number of ions that accumulate during the negative half-cycle, giving the target a net negative charge. This self-biased charge process is possible due to the higher mobility of electrons with respect to the positive ions in the plasma. Apart from insulators, this technique has successfully been adopted for

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

the deposition of metals and semiconductors, however, the sputtering rate remains lower than that of the vacuum deposition described above.

The *magnetron sputtering* technique permits to improve the sputtering rate with respect to the previous versions through the optimisation of the ionization of the sputtering gas molecules [4]. In this system, a magnetic field is applied between the cathode (target material) and anode (substrate) in addition to the electrical field where they both make the electrons to change their paths within the discharge from linear to spiral. This longer path followed by electrons in their travel from the target to the anode increases the time they are in contact with the gas molecules, raising the probability of ionization. Also, the fact that electrons remain a longer time in the plasma produces an increase of the current density at the cathode side, which in turn, raises the sputtering rate of the target material. This higher effective gas ionization allows to reduce the sputtering gas pressure to maintain the discharge, which significantly reduces the collisions of sputtered atoms within the plasma in their travel towards the substrate, increasing the deposition rate. A columnar microstructure is usually obtained with this technique with defects concentrated in between the columns. Silicon nitride films are often produced with this technique [10], but in combination with RF sputtering, it is possible to produce films based on Ti, Cr, Fe, Mo, Ag, Cu, among others [11].

#### *2.1.3 Laser ablation*

Laser ablation is based on a similar configuration as the previous techniques, i.e. usually an evacuated chamber with a target material to be evaporated, a substrate placed parallel to the target where the film condensates, and in this case, an additional high-power pulsed laser placed outside the deposition chamber which emits the energy for inducing the ablation of the target material [12]. This technique is broadly employed in the deposition of alloys, compounds, polymers, semiconductors, and multilayers due to its excellent stoichiometry transfer from the target to the film. Oxide thin films can also be deposited if oxygen is introduced in the chamber as a background gas. The irradiation from the laser power, i.e. KrF (248 nm), is focused on the target producing a rapid local heating until reaching the melting point, and eventually producing evaporation that will be deposited in the substrate. The laser ablation process and the quality of the sample are affected by a number of parameters, including the characteristics of the target material, deposition conditions, laser parameters, substrate temperature. Although this technique is widely recognized for its diverse and fast applicability, the actual ablation process is yet to be fully understood because the material ejection is not produced solely by a thermal process but also a photochemical is likely present.

#### **2.2 Chemical deposition methods (CVD)**

CVD is a deposition method where a volatile compound of a pre-established substance is introduced into a reactor, usually along with an inert gas, to induce a chemical reaction which produces a solid thin film onto a substrate at an elevated temperature. In this technique, unlike PVD, the reaction does not have to be produced under vacuum conditions. Due to its versatility to work with a broad range of reactants and precursors, this technique enables the deposition of a variety of structures, including metal alloys and compound semiconductors with an excellent control of purity and doping (stoichiometric film) [13]. Compared to PVD, this technique offers higher deposition rates, better conformance in rough substrates, easy deposition onto complex surfaces, and high throughput. However, some disadvantages, such as the use of high substrate temperatures, and the toxicity

and flammability of the reactive gases have prevented it from being used in lowscaled developments, but is well justified in applications where high-throughput is required, i.e. semiconductor industry. The CVD processes can be classified based on the type of source employed to initiate the chemical reaction, the range of pressure under which the deposition is carried out, and the type of reactant used. The most established CVD method are described below.

*The thermal activated CVD* is the most conventional method where the thermal energy produced inside the reactor triggers the chemical reactions. Two variants are noticeable within this process in relation to the pressure range under which the deposition materializes, namely atmospheric pressure CVD (APCVD), and low pressure CVD (LPCVD) [14]. Both variants essentially comprise the same chemical reactions which overall consist in the creation of vapor from reactants; then these vapor species are directed into the reactor, where depending on the deposition parameters, homogeneous chemical reactions take place in the gas phase while heterogeneous reactions occur near the substrate surface. Finally, the crystallization of a solid film occurs in the substrate surface. An alteration of the rate-limiting process occurs when the pressure is reduced in LPCVD about 1000-fold with respect to the atmospheric pressure CVD [13]. This reduced pressure substantially increases the rate of mass transport of reactants overpassing the rate of reactions in the substrate surface. This makes the kinetics in the substrate surface to be the rate-limiting step in the deposition process. In the APCVD instead, the mass transport of reactants is lower than the rate of reactions in the substrate surface, thus making the process limited by the diffusion of reactants. The introduction of unwanted impurities is reduced in LPCVD with respect to the APCVD. Moreover, high uniform films with a higher throughput can be produced for commercial applications using LPCVD, however, a disadvantage in this variant is the still high temperature needed for deposition. Polysilicon and silicon nitride films are among the films produced with this technique [15].

*Plasma-enhanced* CVD (PECVD) is one of the most widely used variations of CVD because it provides an alternative for deposition at lower temperatures using organic, inorganic and inert precursors [4]. In this method a plasma energy, in addition to the thermal energy, is incorporated to improve the dissociation of the reactive gases. The plasma is created when an energy, usually in the form of an electric field, is introduced into the reactor which contains the reacting gases in the space between two electrodes. Then, complex chemical reactions occur in the plasma under reduced pressures which produce energetic ions and radicals that travel towards the substrate bombarding its surface and promoting reactions that give rise to a solid film. Unlike APCDV or LPCDV, this variant allows deposition of conformal films at lower temperatures (200–400 K) [13]. This deposition method has been enhanced through the excitation of plasma by radio frequency field (RF = 13.56 MHz), and microwave frequency field (2.45 GHz) which draws on the effect of electron cyclotron resonance (ECR) to reduce the pressure. The higher frequency of the latter allows for a higher-energy and a higher concentration of electrons in the plasma leading to a boost in the degree of ionization up to ~1000 times than that realized with RF field [16]. PECVD is an established deposition method for research and industrial production, including antireflective coatings, microelectronics, photovoltaics, and transistors.

*Metalorganic* CVD (MOCVD) basically follows the same process as the CVD where volatile metalorganic compounds are used as precursors instead of inorganic ones. This technique allows the deposition of a broad range of materials with an amorphous, polycrystalline and epitaxial microstructure. The metalorganic compounds are decomposed through pyrolysis reactions at low temperatures which permits to carry out the film deposition at lower temperatures than in thermal CVD.

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

Both deposition temperature and pressure play an essential role in determining the rate-limiting process. Deposition is usually performed in the range 550–1150 K, noticing a kinetic reaction limited behavior for temperatures below 500 K, and a diffusion-rate limited for temperatures above 800 K [13]. At low pressures (<1 kPa) the reactions are kinetic limited while above that threshold the reactions become diffusion-rate limited. This technique is attractive because the gas flow rate and the partial pressure of the precursors can be controlled allowing the fabrication of films with the right stoichiometry at high deposition rates. Among the disadvantages are the relative high cost of metalorganic compounds along with the difficulty to obtain highly purity version with minimum oxygen content to fabricate high quality semiconductors. Both semiconductor and superconductor thin films have been deposited through this technique [16]. A list of the most used metalorganic precursors for the deposition of semiconductor, metallic, and dielectric films is presented in Ref. [13].
