INTRODUCTION:

The development of today’s modern society purely depends on the advancement of science and technology, which is possible due to contribution of scientists and technologists since ages. This development is not possible with technological progress in the field of thin film science. Thin film studies have directly or indirectly advanced many new areas of research in solid-state physics and chemistry, which are based on the phenomena uniquely characteristic of the thickness, geometry, and structure of the film [1]. Thin film technology has revolutionized the field of optics, electronics and magnetic etc., considering the new and improved high performance optics, electronics, magnetic and photovoltaic devices. The main advantage of thin film devices are low material consumption and possible use of flexible substrates in various fields [2].

Chemical Vapour Deposition (CVD) is commonly used to deposit conformal films and augment substrate surfaces in ways that more traditional surface modification techniques are not capable of. It is extremely useful in the process of atomic layer deposition which deposits extremely thin layers of material. Micro fabrication is the term that describes the processes of fabrication of micrometer and nanometer size structures. Micro fabrication processes widely use CVD to deposit materials. It is often used in the semiconductor industry to produce thin films. A variety of applications for such films exist. The semiconductor industry is the aggregate collection of companies engaged in the design and fabrication of semiconductor devices [3].

The solution growth (sol-gel) method is based on the phase transformation of a sol obtained from metallic alkoxides or organometallic precursors. This sol which is a solution containing particles in suspension is polymerized at low temperature, in order to form a wet gel. The solvent is removed by drying the gel and the next step is a proper heat treatment. Some of the advantages of the sol-gel method are its versatility and the possibility to obtain high purity materials, the provision of an easy way for the introduction of trace elements, allowance of the synthesis of special materials and energy savings by using low processing temperature [4]. Sol-gel method is a technique used in ceramics industry since the 1960’s. Dishch prepared the block glass in 1971 [5]. Since then, studies in this filed have developed very fast. Nowadays, the sol-gel method has been extensively used to obtain various kinds of functional oxide films due to its simplicity and low cost [6-9].

Spray pyrolysis is a simple and low cost technique for the synthesis of semiconductor films. This technique involves spraying of a solution containing soluble salts of the desired compound onto preheated substrates, where the constituents react to form a chemical compound. The chemical reactants used for making solution are selected such that the products other than the desired compound are volatile at the temperature of deposition [10]. Spray pyrolysis is a processing technique being considered in research to synthesis thin and thick films of any composition, ceramic coatings, and powders. It does not require high quality substrates or chemicals.

Various reviews concerning spray pyrolysis techniques have been published. Mooney and Radding have reviewed the spray pyrolysis method, properties of the deposited films in relation to the conditions, specific films, and device application [11]. Tomar and Garcia have discussed the preparation and the properties of sprayed films as well as their application in solar cells, anti-reflection coatings, and gas sensors [12]. Albin and Risbud presented a review of the equipment, processing parameters, and optoelectronic materials deposited by the spray pyrolysis technique [13]. Pamplin has published a review of spraying solar cell materials as well as a bibliography of references on the spray pyrolysis technique [14]. Recently, thin metal oxide and chalcogenide films deposited by spray pyrolysis and different atomization techniques were reviewed by Patil [15]. Bohac and Gauckler have discussed the mechanism of chemical spray deposition and presented some examples of sprayed YSZ films [16].

The art of electroplating metals and metallic alloys has been in practice for nearly a century. Further, the viability of using the electrodeposition technique as a tool of material technology is attracting attention as a means of obtaining films of a wide variety of materials including semiconductors, superconductors, polymer films, materials for bio stimulation, specific electronic device application materials etc. Some of the key advantages of the electrodeposition techniques are it is possible to grow films over large areas as well as irregularly shaped surfaces. Compositionally modulated structures or non-equilibrium alloys can be electroplated.

Electrodeposition was originally used for the preparation of metallic mirrors and corrosion resistant surfaces among other things. In its simplest form electrodeposition consists of an electrolyte containing metal ions, an electrode or substrate on which the deposition is desired, and a counter electrode. When a current flows through the electrolyte, the cations and anions move towards the cathode and anode, respectively, and may deposit on the electrode after undergoing a charge transfer reaction. The discovery of electrodeposition can be traced back to Michael Faraday and his famous laws of electrolysis. The first law states that the total amount of chemical change produced by an electric current is proportional to the total charge passing through the electrolyte. The second law states that the masses of the different substances liberated in the electrolysis are proportional to their chemical equivalent weights [17].

The anodization process can be done either at constant voltage (potentiostatic) or constant current (galvanostatic). If the applied voltage exceeds the dielectric breakdown limit of the oxide, the oxide will no longer be resistive to prevent further current flow and oxide growth, which will lead to more gas revolution and sparking. This technique is known as Anodic Spark Deposition (ASD) or Micro-Arc Oxidation (MAO). For example, it has been reported that the breakdown potentials for H₃PO₄ and H₂SO₄ were around 80 and 100 V, respectively. Below the breakdown limit, the anodic oxide film was relatively thin and usually non-porous using non-fluorine electrolytes. A constant temperature during the anodization process is usually required to maintain a homogeneous field enhanced dissolution over the entire area[18].

Sputtered films typically have a better adhesion on the substrate than evaporated films. Sputtering sources contain no hot parts and are compatible with reactive gases such as oxygen. Sputtering can be performed top down while evaporation must be performed bottom up. Advanced processes such as epitaxial growth are possible. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film [19].

The thin solid films were probably first obtained by electrolysis in 1838. Bunsen and Grove obtained metal films in 1852 by means of chemical reaction. Faraday obtained metal films in 1857 by thermal evaporation of metallic elements. Thin films are two dimensional solids. In these solids the third dimension is negligibly smaller than the two dimensions. Thin films can be obtained from various deposition techniques. An improper selection of deposition technique causes varied and irreproducible results on the films. For this reasons the understanding of thin films has made tremendous advantages in past decade [20-22].Thin films have been prepared using various techniques including thermal evaporation [23], spray pyrolysis [24], chemical vapor deposition [25], electrochemical deposition [26], sol-gel [27, 28], sputtering [29-31], chemical solution deposition [32-36], etc. Among these, chemical solution deposition, also called as a chemical bath deposition, is an advantageous technique due to its low cost, low-temperature operating condition and freedom to deposit materials on a variety of substances. In the present comparative study, the important chemical deposition methods of thin films were studied.

DISCUSSION:

Classification of Deposition methods:

Commonly, the thin film deposition methods are classified as physical and chemical deposition methods. Physical deposition methods are classified into thermal evaporation, electron beam evaporation, activated reactive evaporation, molecular beam epitaxy evaporation, ion plating and pulsed laser deposition. Chemical deposition methods are classified into chemical vapor deposition, solution growth, spray pyrolysis, electrodeposition, anodization and sputtering.

Classification of Chemical Deposition methods:

1. Chemical Vapour Deposition (CVD) Method:

The most popular chemical deposition technique to deposit thin films of materials on various substrates is chemical vapour deposition (CVD) method. In this technique, source gases are introduced into the reaction chamber and energy is applied through heat, high frequency, high voltage, or other techniques that result in the decomposition of the source gas and reaction of the chemicals to form a film. This process is used to produce high purity, high performance and high efficiency solid materials. The technique is frequently used in the semiconductor industry to produce desired thin films. Deposited thin film is a layer of depositing material ranging from fractions of a nanometer to several micrometers in thickness. Frequently, volatile by products are also produced by this technique, which are removed by gas flow through the reaction chamber. A wafer is a thin slice of semiconductor material, for example, a silicon crystal used in the manufacture of IC’s and other micro devices. Fig. 1 shows the experimental set up of chemical vapour deposition method [37].

The broad classifications of CVD:

A number of forms of CVD techniques/ processes are in wide use. These processes differ by their activation process and process conditions.

1.1 Classified by operating pressure:

Atmospheric Pressure CVD (APCVD): These CVD processes held at atmospheric pressure.

Low-Pressure CVD (LPCVD):

These CVD processes held at sub-atmospheric pressure. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer. Most modern CVD processes are either LPCVD or UHVCVD [38].

Ultra-High Vacuum CVD (UHVCVD):

These CVD processes held at a very low pressure, typically below 10-6 Pascal.

1.2Classified by physical characteristics of vapour:

Aerosol Assisted CVD (AACVD):

In this CVD process the precursors are carried to the substrate by means of a liquid or gas aerosol, which can be generated ultrasonically. This technique is suitable for use with non-volatile precursors.

Direct Liquid Injection CVD (DLICVD):

In this DLICVD process the precursors are in liquid form or solid dissolved in a convenient solvent. Liquid solutions are injected in a vaporization chamber towards injectors. Then the precursor vapors are transported to the substrate as in classical CVD process. This technique is suitable for use with liquid or solid precursors. High growth rates can be reached by using this technique.

1.3 Plasma methods:

Plasma-Enhanced CVD (PECVD):

This CVD process utilizes plasma to boost chemical reaction rates of the precursors. The PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductor materials [39].

Remote Plasma-Enhanced CVD (RPECVD):

The RPECVD process is similar to PECVD except that the wafer substrate is not directly in the plasma discharge region. By removing the wafer from the plasma region and allows processing temperature down to room temperature [40].

Atomic Layer CVD (ALCVD):

This ALCVD process deposits successive layers of different substances to produce layered crystalline films.

Combustion Chemical Vapor Deposition (CCVD):

This CCVD process is a chemical process by which thin film coatings are deposited onto substrates in the open atmosphere [41].

Hot Wire CVD (HWCVD):

This HFCVD uses a hot filament to chemically decompose the source gases[42, 43].

Frequently, materials used in CVD process are silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, silicon dioxide, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and diamond. The CVD process is also used to produce synthetic diamonds. Gallium Arsenide (GaAs) is used in some ICs and photovoltaic devices. Amorphous polysilicon is used in the manufacturing of photovoltaic devices [44, 45].

2. Solution Growth method (Sol-Gel):

In this technique a material to be grown is dissolved in an appropriate solvent of desired concentration and by controlling the various process parameters such as pH, concentration of gel solution, setting time of the gel solution, concentration of reactance, temperature and period of crystal growth etc. then precipitate the crystals from the supersaturated solution. Large crystals can be grown by adjusting the rate of crystallization. Different approaches such as isothermal evaporation of the solvent or a gradual cooling of the saturated solution can be used to create the supersaturated solution. The most important feature of this technique is that it allows the formation of materials with a great variety of structures and compositions.

The Fig. 2 shows the experimental set up of solution growth by single and U tube method [46]. There are two types of growth which can be takes place in the chemical reaction. One in which the growth takes place by the reaction of one component with the other. Whereas in the other with the reaction of one component impregnated in the gel medium. In this technique the crystals grow inside the gel. The process is a highly controlled one because the reactants combine due to the diffusion of ions through fine pores.

Spray pyrolysis involves many processes occurring either simultaneously or sequentially. The most important of these are aerosol generation and transport, solvent evaporation, droplet impact with consecutive spreading, and precursor decomposition. The deposition temperature is involved in all mentioned processes, except in the aerosol generation. Consequently, the substrate surface temperature is the main parameter that determines the morphology and properties of the film. By increasing the temperature, the morphology of the film can be changed from a cracked to a porous microstructure. In many studies the deposition temperature was reported indeed as the most important spray pyrolysis parameter. Understanding these processes will help to improve film quality.

Thin film deposition using spray pyrolysis can be divided into three main steps, atomization of the precursor solution, transportation of the resultant aerosol, and decomposition of the precursor on the substrate. Atomization of liquids has been investigated for many years. The key is to understand the basic atomization process of the atomization device in use. In particular, it is important to know which type of atomizer is best suited for which application and how the performance of the atomizer is affected by variations in liquid properties and operating conditions. Air blast, ultrasonic, and electrostatic atomizers are normally used in spray pyrolysis techniques [53].

Spray pyrolysis method has been employed for the deposition of dense films, porous films, and for powder production. Even multi-layered films can be easily synthesized by using this versatile technique. Spray pyrolysis has been used for several decades in the glass industry [54] and in solar cell production [55]. Numerous reports were published on the mechanism of liquid atomization. Rizkalla and Lefebvre examined the influence of liquid properties on air blast atomizer spray characteristics [56]. Lampkin presented results concerning the application of the air blast atomizer in a spray pyrolysis setup [57]. Recently, a theory of ultrasonic atomization was published [58]. Ganan-Calvo et al. have studied the electrostatic atomization of liquids and derived scaling laws for droplet size from a theoretical model of charge transport [59, 60]. Recently considerable attention has been given to the preparation of thin metal oxide thin films for photovoltaic and photo catalytic applications by various techniques. Various metal oxide semiconductors such as TiO₂, WO₃, ZnO, SrTiO₃ etc. have been prepared by the spray pyrolysis technique. Under thin film deposition, procedures right from the substrate cleaning, preparation of solution and formation of different thin films such as fluorine doped tin oxide (FTO), titanium dioxide (TiO₂), tungsten oxide (WO₃) and multi-layered oxide (TiO₂-WO₃) were illustrated in the literature [61].

4. Electrodeposition Methods:

4.1 Chemical synthesis method:

This is one of the methods for the synthesis of conducting polymers in which the reaction takes place by adding the monomer and different oxidants in an aqueous medium. The chemical reaction is carried out with time and different temperature environment. However, control over polymer morphology is extremely limited, purification can be problematic and processing is virtually impossible. There is no control over the polymerization especially on the thickness, which certainly affects different process parameters.

4.2 Electrochemical synthesis method:

Electrochemical polymerization of conducting polymers is generally carried out by constant current or galvanostatic method, constant potential or potentiostatic method and potential scanning or cycling or sweeping method. Standard electrochemical technique, which employs a divided cell containing a working electrode, a counter electrode and a reference electrode generally, produces the best quality films [62-68].

4.2.1 Potentiostatic deposition method

In the potentiostatic deposition method, a predetermined optimum voltage is applied during synthesis with no control over the resulting current in the system. The potentiostatic deposition is generally carried out using a potentiostat to control the potential of the working electrode (WE). However, consistency is only achieved if the natures of the solutions and electrodes are rigorously controlled [69]. The reference electrode (RE) has to be kept closer to the working electrode so that it will minimize the ohmic drop within the electrolyte and reliable measurement of working electrode potential can be done.

4.2.2 Galvanostatic deposition method

In the galvanostatic method, a fixed oxidation current is supplied with no control over the resulting potential of the system. Galvanostatic method is more widely used because it provides more control over film thickness, and it is reproducible too. The galvanostatic method enables a more uniform polymer film to be produced by applying constant current. The magnitude of the current density during polymerization depends upon the intended application because this can affect the morphology and conductivity of the polymer film as well as the potential of the system during synthesis. At high potentials, the integrity of reagents to be incorporated in the film may be affected. Fig. 4 shows the experimental set up of electrochemical deposition method [70].

4.2.3 Cyclic voltammetry (CV) method:

In CV, the voltage applied to the working electrode (WE) is scanned linearly from an initial value to a predetermined limit of the monomer oxidation known as the switching potential where the direction of the scan is reversed. On reaching initial value again the scan may be halted or reversed alternately or continued to further value. The current response can be plotted against the applied potential. CV growth of polymer film is achieved by consecutive potential sweeps in a solution containing monomer and supporting electrolyte. However, the positive potential limit is critical since the initiation of polymerization on the electrode surface commences only above a certain potential. If the potential is too positive, the polymer can be over-oxidized, and if it is too low, the film formation can be impeded [71]. Oxidation and reduction peaks are appeared during forward and reverse scan respectively. By observing the appearance and disappearance of redox peaks and variation in sweep rates, it is possible to determine the correlation between various electrochemical processes and redox peaks. However, CV polymerization is difficult to control, and the generation of reproducible polymer films is difficult to accomplish.

The advantages of electrochemical synthesis method over the chemical method are it is simple and less expensive technique. Therefore, electrodeposition of conducting polymer on oxidizable conducting glass is extremely economical. Unlike chemical method, there is no need of catalyst and therefore, the electrodeposited polymers and co-polymers are essentially pure and homogeneous. Doping of the polymer with desired ion can be considered simultaneously by changing the nature of counter ions in the solution. The conducting polymers can be obtained directly in thin film form as coatings on electrodes and the properties of these coatings can be controlled effectively by proper choice of the electrochemical process parameters. Reduction in the possible pollution by adopting a suitable system for electropolymerization can be achieved using modern sophisticated instrument. The most salient feature of this method is that polymerization, doping and processing takes place simultaneously. Opportunity to carry out various in-situ spectroscopic studies, conductivity and even determination of conducting levels at various potentials is possible by this method.

5. Anodization Method:

In this method chemical baths are used to prepare the surface of aluminum to receive an electrical charge which will increase the thickness of the oxide layer and make it harder, less likely to corrode and more durable. Typical anodization procedures include alkaline cleaning, acid activation, and electrolyte anodizing. Acid activation is performed in a mixture of nitric acid and hydrofluoric acid (HF) to remove the natural titanium oxide layer and surface contaminants. The electrolyte anodization is carried out in an electrochemical cell, which usually has a three electrode configuration (titanium anode, platinum cathode and Ag/AgCl reference electrode). When a constant voltage or current is applied between the anode and cathode, electrode reactions (oxidation and reduction) in combination of field driven ion diffusion lead to the formation of an oxide layer on the anode surface. Fig. 5 shows the experimental set up of anodization method [72].

It is a surface modification method. It can lead to topography changes and could be used with other treatments together. First, anodization provides a controlled way to create nano-roughness or even nano-features. Generally, there are two mechanisms that are responsible for Osseo integration of bone: biomechanical interlocking and biological interactions. For biomechanical interlocking, it depends on the roughness, and surface irregularity. Current femoral stems made of titanium alloys are usually macro-textured to provide such surface features for bone to mechanically interlock. For biological interactions, it involves complex systems. Considering roughness in different scales, it is reported that increased micro or submicron roughness could enhance bone cell function, such as ALP activity [73, 74], while some other studies have revealed the enhanced cell implant interactions on nanoporous or nano phase materials. Ideally, the future titanium implant should possess roughness in all three scales: macro, micro, and nano. One possible approach to accomplish this is by subjecting implants to techniques like polishing and mechanical grinding that promote micro-roughness, and then to induce nano tubular structures by a quick anodization process. Second, micro or nano HA films produced using anodized titanium shows some advantages over conventional ones. Although plasma spray is still widely used for HA coatings on titanium, anodization has a strong role to play to incorporate Ca and P into Ti coatings [75-77].

Fig. 5 shows the experimental set up of anodization method.

Anodization has the ability to form uniform and thin HA or calcium phosphate layers on titanium implants of various shapes. Moreover, HA deposited onto the anodized titanium could be nano-scale in dimension. One problem that still needs to be more fully investigated is the bonding strength between apatite crystals and the anodic oxide. Again, anodization can be used to incorporate drug delivery into titanium-based implants to enhance new bone formation. Porous ASD surfaces could be used as matrices for drug storage and release [78]. Similarly, the nano-tubular structures could serve as reservoirs of chemical mediators, such as bone morphogenetic protein-2 (BMP-2) and osteogenic protein-1 (OP-1, BMP-7) [79]. In the future, studies should concentrate on embedding these growth factors into the unique porosity that can be well controlled on titanium for orthopedic applications.

6. Sputtering Method:

This technique is used to deposit thin films of a material onto a substrate. First creating gaseous plasma and then accelerating the ions from this plasma into some source material, which is eroded by the arriving ions via energy transfer and is ejected in the form of neutral particles either individual atoms, clusters of atoms or molecules. These ejected neutral particles will travel in a straight line unless they come into contact with nearby surface or some other particles. For this purpose energy source (RF, DC, and MW) is required to feed and maintain the plasma state while the plasma is losing energy into its surroundings. Fig. 6 shows the experimental set up of sputtering method [80].

By using electromagnetic methods the charged atomic particles can be easily controlled. A weakly charged gas particle which exhibit collective behavior is called plasma, the source material is called the target and the emitted molecules or atoms are said to be sputtered off. Sputtered molecules or molecules or atoms from the target make their way onto the substrate through diffusion. Ions and neutralized gas molecules or atoms may also embed on the substrate as impurities. The ions incident on the substrate may also re-sputter the surface. Chemical reactions may occur. Deposition rate is proportional to the sputtered yield. An optimum pressure exists for high deposition rates. Higher pressure means more collisions and ions. Lower pressure means less scattering. In sputtering, the target material and the substrate is placed in a vacuum chamber. A voltage is applied between them so that the target is the cathode and the substrate is attached to the anode. Plasma is created by ionizing a sputtered gas (chemically inert, heavy gas like Argon). The sputtered gas bombards the target and sputters off the material we like to deposit[81]. Ion beam sputtering, Reactive sputtering, Ion assisted deposition, High target utilization sputtering, High power impulse magnetron sputtering (HIPIMS) and Gas flow sputtering etc are the various types of sputtering deposition.

Fig. 6 shows the experimental set up of sputtering method.

Following are the advantages of sputtering technique; it can be used as diffusive spreading for coating. It can coat around corners. It can process alloys and compounds. High temperatures are not needed during the sputtering. Even organic compounds have been sputtered. It can coat large areas more uniformly. Large target sources mean less maintenance. Even materials with very high melting points are easily sputtered while evaporation of these materials in a resistance evaporator or Knudsen cell is problematic or impossible. Sputter deposited films have a composition close to that of the source material. The difference is due to different elements spreading differently because of their different mass but this difference is constant. Disadvantages of the sputtering process are that the process is more difficult to combine with a lift off for structuring the film. This is because the diffuse transport, characteristic of sputtering, makes a full shadow impossible. One cannot fully restrict where the molecules or atoms go, which can lead to contamination problems. Active control for layer by layer growth is difficult and inert sputtering gases are built into the growing film as impurities [82].

The various industrial applications of this technique are: it is used extensively in the semiconductor industry to deposit thin films of different materials in various IC’s processing. Thin antireflection coatings on glass for optical applications are also deposited by using sputtering technique. The reason is that the low substrate temperatures sputtering technique is an ideal method to deposit contact metals for thin film transistors. Perhaps the most familiar products of sputtering are low emissivity coatings on glass, used in doublepane window assemblies. The coating is a multi-layer containing silver and metal oxides such as zinc oxide, tin oxide, and titanium oxide. A large industry has developed around tool bit coating using sputtered nitrides, creating the familiar gold colored hard coat [83]. Sputtering is also used as the process to deposit the metal layer during the fabrication of CDs and DVDs. Hard disk surfaces use sputtered CrOx and other sputtered materials. Sputtering is one of the main processes of manufacturing optical waveguides and for making efficient photovoltaic solar cells [84].

CONCLUSIONS:

In order to optimize the desired film thickness and characteristics, good understanding and knowledge of the various chemical deposition processes is necessary. The important chemical deposition methods of thin film deposition are chemical vapor deposition, solution growth, spray pyrolysis, electrodeposition, anodization and sputtering. These chemical deposition methods can be found in the fabrication and processing technology industries. In this study, the processes of chemical deposition methods and their significance in the whole process of making a substrate deposition are studied. These methods are mostly used for creating thin or thick films or metalized substrates. They have their own unique way of depositing materials on the substrates or wafers and thus having their own advantages, disadvantages and limitations in their industrial applications. One can select the appropriate deposition method as per the requirement of the desired physical and chemical properties of the film or coating for desired application purpose.

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Received on 01.11.2017 Modified on 29.11.2017

Asian J. Research Chem. 2018; 11(1):195-205.

DOI:10.5958/0974-4150.2018.00039.1