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Advance Deposition Techniques for Thin Film and Coating Advance Deposition Techniques for Thin Film and Coating

Thin films have a great impact on the modern era of technology. Thin films are considered as backbone for advanced applications in the various fields such as optical devices, environmental applications, telecommunications devices, energy storage devices, and so on . The crucial issue for all applications of thin films depends on their morphology and the stability. The morphology of the thin films strongly hinges on deposition techniques. Thin films can be deposited by the physical and chemical routes. In this chapter, we discuss some advance techniques and principles of thin-film depositions. The vacuum thermal evaporation technique, electron beam evaporation, pulsed-layer deposition, direct current/radio frequency magnetron sputtering, and chemical route deposition systems will be discussed in detail.


Introduction
Nowadays, most of the technologies are used for minimizing the materials into nano-size as well as nano-thickness leading to the emergence of new and unique behaviors of such materials in optical, electrical, optoelectronic, dielectric applications, and so on. Hence, a new branch of science/materials science is called thin films or coatings. Thin film can be defined as a thin layer of material, where the thickness is varied from several nanometers to few micrometers. Like all materials, the structure of thin films is divided into amorphous and polycrystalline structure depending on the preparation conditions as well as the material nature. Thin films comprise two parts: the layer and the substrate where the films are deposited on it. Also, thin films can be composed of different layers such as thin-film solar cells, electrochromic cells, and so on.
In order to obtain thin films with good quality, there are two common deposition techniques: physical and chemical depositions. It can be summarized as shown in Table 1 This chapter describes some common deposition techniques for thin films in detail to give some confidential and important points of view for readers on how thin films can be formed.

Evaporation techniques
Evaporation methods are considered as the common deposition of materials in the form of thin-layer films. The general mechanism of these methods is obtained by changing the phase of the material from solid phase to vapor phase and converting again to solid phase on the specific substrate. It takes place under vacuum or controlled atmospheric condition.
The technique of thermal evaporation is strongly dependent on two parameters: thermally vaporized material and applying a potential difference to the substrate under medium-or higher-vacuum level ranging from 10 −5 to 10 −9 mbar. The schematic diagram for thermal evaporation is shown in Figure 1 taken from elsewhere [9].

Electron beam evaporation
This type of evaporation is another method of physical deposition where the intensive beam of electrons is generated from a filament and steered through both electric and magnetic fields to hit the target and vaporize it under vacuum environment as shown in Figure 2. Thin films prepared by electron beam evaporation are of good quality and purity [10].  Large categories of materials can be prepared by electron beam evaporation technique [11] such as amorphous and crystalline semiconductors [12], metals [13], oxides [14], and molecular materials [15].

Laser beam evaporation (pulsed-laser deposition)
Pulsed-laser deposition (PLD) is another physical deposition technique to deposit the thinfilm-coating system [16]. During the thin-film deposition process, the laser beam is used to ablate the material for depositing the thin films inside a vacuum chamber as shown in Figure 3. Different kinds of laser sources are being used to ablate the target. The most common sources are Nd-YAG laser, KrF (248 nm), and XeCl (308 nm). When the laser beam strikes the target material, it produces the plume which could deposit on the various substrates. The created plume may contain neural-and ground-state atoms and ionized species. In the case of metal oxide thin films, oxygen is used to deposit the oxides of metals [18]. The thin-film quality from the PLD depends on the various parameters such as wavelength of the laser, energy, ambient gas pressure, pulsed duration, and the distance of the target to the substrate [19]. The ablation process during the deposition may control and monitor by using laser-induced fluorescence [20], laser ablation molecular isotopic spectroscopy [21], and optical emission spectroscopy [22]. The morphology of the deposited thin films is also affected by the substrate temperature. The coating of thin films through PLD follows three modes: Frank--van der Merwe, Stranski--Krastanov, and Volmer--Weber [23,24]. PLD has some advantages over other physical deposition systems because of its fast deposition time and its compatibility to oxygen and other inert gases.

Sputtering technique
Sputtering technique is mostly used for depositing metal and oxide films by controlling the crystalline structure and surface roughness [11,25]. The simple form of the sputtering system consists of an evacuated chamber containing metallic anode and cathode [25] in order to obtain a glow discharge in the residual gas in the chamber. Also, an applied voltage in the order of several KeV with pressure more than 0.01 mbar is sufficient for film deposition. The sputtering process depends on the bombardment of the ions released from the discharge to the molecules in the cathode leading to the liberation of the molecules from the cathode with higher kinetic energy. The atomic weight of the bombarding ions should be nearly to that of the target material in order to maximize the momentum transfer. These molecules move in straight lines and strike on the anode or on the substrate to form a dense thin film [25]. The diagram of the sputtering system is shown in Figure 4. The process of sputtering has several advantages. High-melting point materials can be easily formed by sputtering. The deposited films have composition similar to the composition of the starting materials. Sputtering technique is available to use for ultrahigh vacuum applications. The sputtering sources are compatible with reactive gases such as oxygen. Contrarily,' thick coatings cannot be obtained and there is a difficulty to deposit uniformly on complex shapes.
There are two common types of sputtering process: direct current (DC) and radio frequency (RF) sputtering. The first one depends on DC power, which is generally used with electrically conductive target materials. It is easy to control with low-cost option. The RF sputtering uses RF power for most dielectric materials. A common example for sputtered films is aluminum nitride films. These films were prepared by both DC-and RF-sputtering technique, and their structure and optical properties were compared [26,27].

Chemical deposition techniques
Although the production of thin films via physical methods as previously described gives good quality and functionalizes properties, it is highly expensive and perhaps requires a large amount of material target. Since the need to produce good-quality thin films with low economical cost is necessary, chemical deposition techniques are widely used globally. These techniques are cheap producing good-quality films. Most of them do not require expensive equipment. The chemical deposition is strongly dependent on the chemistry of solutions, pH value, viscosity, and so on. The most common chemical deposition has been obtained via solgel route, chemical bath deposition, electrodeposition, chemical vapor deposition (CVD), and spray pyrolysis technique. This section is concerned only on sol-gel and chemical bath deposition techniques because they can form good film quality with low equipment requirement.

Sol-gel technique
The sol-gel technique is broadly used for the synthesis of oxide materials [28]. Sol-gel process is one of the famous wet-chemical methods. It works under lower-temperature processing and gives better homogeneity for multicomponent materials. The word "'sol"' means the formation of a colloidal suspension and 'gel' means the conversion of 'sol' to viscous gels or solid materials. Two routes are used to prepare transition metal oxides (TMOs) as follows: a. Preparing of inorganic precursors via inorganic salts in aqueous solution.
b. Preparing of metal alkoxide precursors via metal alkoxides in nonaqueous solvents.
In this section, we are concerned on the famous route "the metal alkoxide precursor solution by an alcoholic solution."

Alkoxide precursors in organic solvents
The sol-gel technique is based on the polycondensation of metal alkoxides M (OR) z in which R represents an alkyl group (R = CH 3 , C 2 H 5 , …) and z is the oxidation state of the metal atom M z+ [29]. It can be synthesized via the reaction of metal salt (chloride, acetate, nitrate, etc.) with alcohol as follows: After this process, two important steps should be involved:

2.
Condensation: condensation is the second step after hydrolysis leading to the departure of a water molecule. The process of condensation can be either olation process or oxolation process.
• Olation: a hydroxyl bridge ("ol" bridge) is formed between two metal centers as shown in Figure 5.
• Oxolation: oxolation is a reaction in which an oxo bridge (-O-) is created between two metal centers. When the metal is coordinately unsaturated, oxolation with rapid kinetics leads to edge-or face-shared polyhedral as shown in Figure 6.  Hence, olation process occurs mainly for lower oxidation states of cations (z < 4), whereas oxolation is mainly observed with cations of high oxidation state (z > 4) [29,31].
The previous description provides the preparation of the precursor solution. In order to make thin film from the precursor solution, there are two processes for the production of the films, that is, dip-coating and spin-coating techniques.

Dip-coating technique
Dip-coating technique is almost used to fabricate transparent layers of oxides on a transparent substrate with a high degree of planarity and surface quality [32]. Other substrates are also  Scriven [33] described the dip-coating process in five stages: immersion, start-up, deposition, drainage, and evaporation. Hence, the evaporation normally accompanies the start-up, deposition, and drainage steps as shown in Figure 7.

Spin-coating technique
Another technique is also available for usage after the precursor solution is prepared known as spin coating or spinning. The solution is dripped onto a spinning substrate and spreads evenly. The spinning process is most suitable for the coating of small disks or lenses but is not very economical. The process of spinning film can be described as shown in Figure 8.

Chemical bath deposition technique
Chemical bath deposition method is also known as solution growth technique or controlled precipitations [34]. It is the oldest method to deposit films on a substrate. Solution growth technique is mostly used to prepare chalcogenide films as well as metal oxide films. Also, the deposition can be performed at lower temperatures. In the solution growth method, the precursor solution of metal ions must be complexed by ligands. The complex solution is almost obtained with ammonia solution, triethanol amine, ethylene-diamine-tetraacetic acid (EDTA), citric acid, and so on. When the complexation is completed, the addition of the anions should take place. These anions come from the thiourea, thioacetamide, thiosulfate, and sodium Sulfide solutions [34] as sources of sulfur anions or selenourea and sodium selenosalfate for selenium anions to deposit the chalcogenides. Substrates are put in vertical, horizontal, or specific position inside the solution and left until the desired film thickness is obtained. The deposition of oxide films is quite different than chalcogenides. After making the complexation by controlling the pH value, the substrate is immersed in the solution under the desired temperature varied in the range 60°-100 °C to deposit in most cases the metal hydroxide films. The hydroxide film can then be transferred to oxide by the annealing process. Figure 9 represents the simple chemical bath deposition method taken from Ref. [34]. Indeed, much  reviews and literature, which describes the chemical bath deposition for both chalcogenide and oxide films, are found elsewhere [34][35][36].