Surface coating, a broad division of surface engineering, is used to amend the physical and chemical properties and morphology of a surface. A Surface coating either consists of one homogeneous composition, crystalline phase composition and microstructure, or has an inhomogeneous multilayer or composite structure. The structure of the multilayer and nanocomposite can be periodic or entirely random.
Surface coatings are now being used in virtually every area of technology, including automotive, aerospace, missile, power, electronic, biomedical, textile, petroleum, petrochemical, chemical, steel, power, cement, machine tools, and construction industries and have been adapted to fulfill a wide variety range of desired functional properties, including physical, chemical, electrical, electronic, magnetic, mechanical, wear-resistant, and corrosion-resistant properties at the required substrate surfaces. Almost all types of materials, including metals, ceramics, polymers, and composites, can be deposited onto similar or dissimilar materials. It is also possible to form coatings of advanced materials (e.g. metglas, polymers, superlattices, photocatalysts), graded deposits, metamaterials, multicomponent deposits, etc.) [1]
In general, surface coatings are necessary, for a variety of reasons including economics, materials conservation, unique properties, or the engineering and design flexibility which can be obtained by separating the surface properties from the bulk properties. This near-surface region is produced by depositing a coating onto it (i.e., surface coating) by processes such as physical or chemical vapour deposition, electrodeposition, and thermal spraying, or by altering the surface material by the in-diffusion of materials or by ion implantation of new material so that the surface layer now consists of both the parent and added materials
SURFACE COATING TECHNIQUES:
Virtually every property of the surface coating depends on and can be modified by the deposition process. So microstructure, surface morphology, tribological, electrical, and optical properties of the surface coating are all controlled by the deposition process. A single material can be used in several different applications and technologies, and the optimum properties for each application may depend on the deposition process used. Since not all deposit technologies yield the same properties or microstructures [1], the deposition process must be chosen to fit the required properties and application. Several publications have presented a detailed review of thin-film deposition processes; [1-4] thus only brief descriptions of the thin-film deposition processes are presented in this chapter.
It is informative to list the steps in the formation of a deposit. The three basic steps are [1]:
1. Synthesis or creation of the depositing species
2. Transport from source to substrate
3. Deposition onto the substrate and subsequent film growth.
Because of the overlap in process mechanisms and the formation of hybrid deposition processes, no definite scheme can accurately define and classify all coating processes. There are many different ways of depositing a thin film; the most common ones are based on deposition from a vapor of some sort. These techniques may further be subdivided into Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). In PVD the vapor is composed of atoms and molecules that simply condense on the substrate, whereas in CVD the vapor undergoes a chemical reaction on the substrate, the product of which forms the film.
PHYSICAL VAPOR DEPOSITION TECHNIQUES
PVD covers a number of deposition technologies in which materials are physically created in the vapor phase by energetic bombardment of a source (e.g. sputtering target) and subsequent ejection of material. The basic PVD processes are evaporation, sputtering and ion plating. A number of specialized PVD processes have been derived from these processes and extensively used, including reactive ion plating, reactive sputtering, unbalanced magnetron sputtering, High-Power Pulsed Magnetron Sputtering (HPPMS), and filtered cathodic arc deposition.
Evaporation is a process in which the boiling is carried out in a vacuum having almost no surrounding gas; the escaping vapor atom will travel in a straight line for a specified distance before it collides with structures in the vacuum chamber or residual gas atoms. Boiling is caused by thermal or electron beam heating of a source material.
Molecular Beam Epitaxy is an evaporation process performed in an ultra-high vacuum for the deposition of compounds of extreme regularity of layer thickness and composition from well-controlled deposition rates.
Reactive Evaporation is a process in which small traces of a reactive gas are added to the vacuum chamber; the evaporating material reacts chemically with the gas so that the compound is deposited onto the substrate.
Sputter Deposition
A vacuum process which uses a different physical phenomenon to produce the microscopic spray effect is called sputter deposition. When a fast ion strikes the surface of a material (target), atoms of that material are ejected by a momentum transfer process. As with evaporation, the ejected atoms or molecules can be condensed on a substrate to form a surface coating. DC Sputtering, Radio Frequency (RF) Sputtering, Magnetron sputtering are examples of sputtering. Another type of sputtering is reactive Sputter Deposition which involves a partial pressure of a reactive gas which reacts with the sputtered material to form a compound surface coating.
Ion Plating is an atomistic vacuum coating process in which the depositing film is continuously or periodically bombarded by energetic atomic-sized inert or reactive particles that can affect the growth and properties of the film. The source of depositing atoms can be from vacuum evaporation, sputtering, arc vaporization, or a chemical vapor precursor. Reactive Ion Plating is ion plating technique that involves a partial pressure of a reactive gas which reacts with the sputtered material to form a compound surface coating. Chemical Ion Plating is similar to reactive ion plating but uses stable gaseous reactants instead of a mixture of evaporated atoms and reactive gases. In most cases, the reactants are activated before they enter the plasma zone.
Ion Beam Deposition is a process in which a beam of ions generated from an ion beam source, impinge and deposit on the substrate.
Ion Beam Assisted Deposition (IAD) - two versions are possible. In dual ion beam assisted deposition an ion beam is used to sputter a target and a second ion beam is used to bombard the growing film to change microstructure and properties. In this case, conventional evaporation or sputtering techniques are used to generate a flux of the depositing species.
Ion Implantation is very similar to ion plating, except that in it all of the depositing material is ionized, and accelerating energies are significantly higher. The depositing ions are thus able to penetrate the surface barrier of the substrate and become implanted in the substrate rather than on it.
CHEMICAL VAPOR DEPOSITION TECHNIQUES:
Chemical vapor deposition (CVD) of thin films occurs when the reacting gas species come in contact with a heated substrate that catalyzes the reaction to produce the solid films. CVD can be classified to thermal CVD, laser CVD, photo CVD by the method used to apply the energy necessary to activate the CVD reaction, i.e., temperature, photon, or plasma respectively.
Plasma-Enhanced Chemical Vapor Deposition (PECVD) is a form of CVD that involves creation of plasma of the reacting gases and subsequent deposition onto a substrate. The plasma is generally created by an RF, DC or microwave discharge between two electrodes located in the space into which the reactive gases are introduced.
Plasma-Assisted Chemical Vapor Deposition (PACVD) is a process similar to PECVD where the reaction between the precursors is stimulated or activated by creating plasma in the vapor phase using techniques such as RF, microwave or cyclotron resonance excitation.
Hybrid Deposition Systems:
Hybrid deposition systems benefit from the possibility of combining both the PECVD and PVD approaches in one chamber [5]. This allows one to fabricate different coating architectures including multilayers or graded layers, or doped or nanostructured (nanocomposite) coatings with specific optical, mechanical, and other characteristics [6-7].
One of the major issues in most modern deposition technologies is the unintentional heating of substrate materials. This may cause degradation of the mechanical properties of such materials, especially upon exposure to temperatures that are much higher than the tempering temperatures [8]. In the case of CVD processes, because the temperatures are typically much higher, CVD may not be a good choice for heat-sensitive substrates. If a low temperature CVD is used, a post deposition hardening heat treatment is often a must, but depending on the coating type, major problems may occur on the hard coating itself. Chief among them is severe oxidation or partial delamination of coatings from the substrate surface due to chemical reactions and thermal distortions. High-speed steels, certain intermetallics, and cemented carbides can safely be exposed to deposition temperatures as high as 450 â-¦C without major structural, chemical, or mechanical degradations. In contrast, for the deposition of nanocomposite coatings on titanium, aluminum, and magnesium-based engineering materials, one has to use relatively low temperatures; otherwise, major degradations or distortions may take place in such substrates. Because of their light weights, these materials are much desired for structural components in all kinds of transportation applications. There is also interest in using these materials in tribological and mechanical applications for aerospace and biomedical needs. However, mainly because of their high sensitivity to heat, extra precautions must be taken to avoid degradation in mechanical properties or loss of structural integrity of light alloy substrates. For the production of nanostructured or composite coatings, it may be necessary to combine two or more of the deposition methods mentioned earlier. Furthermore, the use of hybrid deposition system is important for producing coatings with strong bonding, dense structure, and superior mechanical properties. For example, in such systems, one can do sputtering and PECVD, sequentially, or both together.
1.4 Characterization of Thin Film
The key to success in material science is to characterize and understand a material's formation process, structure, and properties. A change in any of these three (process-structure-properties) will affect the other two accordingly. For any material, the manufacturing process conditions should bam mapped as minute as possible. Subsequently, the structure and its properties need to be characterized and coupled to the manufacturing process. The micro structural characterization can be especially troublesome in thin film related areas, where substrate influence has to be identified. Today, high resolution transmission electron microscopy (HRTEM) and various characterization techniques can provide atomic scale structural result and information of growth dynamics. Subsequently, the mechanical properties, e.g. hardness, can be investigated through nano indentation experiments. A deep understanding of how different process parameters affect the structure and properties of a particular film, help in the efforts of designing and optimizing film.
In this thesis, the "Ti-Si-N" system has been examined through deposition of titanium nitride layer on silicon wafer using dense plasma focus. The TiN films on Silicon (100) wafers are deposited using a compact 2.3 kJ Mather-type plasma focus. The charging voltage was 12 kV. The operating pressure was 2.5 mbar. These samples were placed at different angular positions (0o, 10o and 20o) for different (05, 15, 25, and 45) number of focus shots, at a distance of Z = 9cm from the anode tip. The deposited thin layer of "TiN" was characterized by using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Raman Spectroscopy techniques.
1.5 SCOPE OF THE RESEARCH
The investigations of particles and radiation emission from the focus are useful for basic studies on plasma physics, as well as for technological and industrial applications.
In order to extend the usefulness of the plasma focus for applications, it is important to better understand the fundamental processes in hot plasmas, as well as to optimize a certain device for a specific application.
The main objective of this research was to deposit and characterize TiN/SiNx thin films using "2.3 KJ" plasma focus and through this seek answer to how deposition of TiN on "Si" wafers evolves.
The research could be further pursued in the next years, for both academic results and applications, on the same device or on other dense plasma focus machines, both in NIE / NTU and in other laboratories.
Thesis Objectives:
At present, surface coating processes are being rapidly advancing and are extensively applied for medical, energy and industrial technologies.
In many cases no single process can achieve films with required properties for multi layer and Nanocomposite thin films. Therefore, Hybrid coating processes are also becoming important. Plasma-enhanced chemical vapor deposition (PECVD) is combined with sputtering, sputtering is combined with electron beam evaporation, and electron beam evaporation is combined with physical vapor deposition (PVD). Plasma focus is one of the potential candidates which are used as hybrid deposition device for thin film deposition.
Plasma Focus is a compact powerful pulsed source of multiple radiations: x-rays, neutrons, fast electrons, ions and plasma stream [35]. It generates high energy (~1-2 keV), high density (~1025- 1026 m-3), and short duration plasma column (~10-7 s) [35]. Plasma Focus device (PF) has been successfully used as a pulsed ionizing radiation source for many applications including pulsed neutron activation analysis [36-37], as a high flux X-ray source for lithography and radiography [38-42]. It has also been used as highly energetic radiation source for processing of materials in the form of surface modification, thin films coatings [43-52]. All the radiations are produced at the high-current discharge in a vacuum chamber filled with different gases. The sort of radiation depends on the type of gases inside the reactor, material of electrodes, geometry and other parameters [42]. For this reason, Plasma focus is very well fitted to a number of applications, in spite of the fact that a quite complicated picture of physical processes ruling a generation of radiation still is not understood completely.
Plasma Focus - A Potential Candidate for Surface coatings
When compared with other plasma based thin film deposition systems, plasma processing with dense plasma focus has several attractive features.
Dense plasma focus (DPF) deposition process is a hybrid deposition process that combines key features of three existing vacuum deposition methods: the PVD sputtering process, Electron Beam evaporation and plasma-enhanced chemical vapour deposition (PECVD).
DPF provides a high deposition rate process for solid films. So, comparatively thick films can be deposited in less deposition time.
It is cost effective system compared to other deposition systems.
A simple capacitor discharge is sufficient to power the plasma focus.
DPF is a source of intense radiation burst; Ions radiating from DPF device are more energetic (40 keV to 2 MeV).
The gas consumption is considerably small as compared to other deposition techniques.
Radiation pulse of very short duration is used to deposit thin films.
Additional substrate heating is not required during film deposition because the substrates are heated during ion beam treatment.
Plasma focus device is operated under pressure conditions which can easily be maintained.
Good adhesion between the deposited films and substrate is achieved.
The recent upsurge in demonstration of plasma focus device as a potentially suitable candidate for surface engineering motivated us to employ it in surface engineering scheme. The idea was to take advantage of the high-energy radiation from dense plasma focus to deposit and characterize
Nanocomposite coatings consisting of a nanocrystalline titanium nitride (TiN) imbedded in amorphous Silicon nitride matrix by plasma Enhanced chemical vapour deposition process.
Nanocomposite coatings consisting of a nanocrystalline titanium-aluminum-nitride by plasma Enhanced chemical vapour deposition process.
Nanocomposite coatings consisting of a nanocrystalline titanium-aluminum-nitride (TiAlN) imbedded in amorphous Silicon nitride matrix by plasma Enhanced chemical vapour deposition process.
Deposition of Silicon nitride onto Titanium by plasma Enhanced chemical vapour deposition process
Deposition of Aluminum nitride onto Titanium by plasma Enhanced chemical vapor deposition process
Composite Materials:
A composite material is used where the properties of the surface are different from those of the core. Thus, materials with surface coatings are used in the applications ranging from microelectronics, display devices, chemical corrosion, tribology including cutting tools, high temperature oxidation/ corrosion, solar cells, thermal insulation and decorative coatings (including toys, automobile components, watch cases, etc.).
A large variety of materials is used to produce these coatings. They are metals, alloys, refractory compounds (e.g., oxides, nitrides, and carbides), intermetallic compounds (e.g., GaAg) and polymers in single or multiple layers [1]. The thickness of the coatings ranges from a few atom layers to millions of atom layers. The microstructure and hence the properties of the coatings can be varied widely and at will, thus permitting one to design new material systems with unique properties. (A material system is defined as the combination of the substrate and coating.) Historically, coating technology evolved and developed in the last 30 years in several industries, i.e., decorative coatings, microelectronics and metallurgical coatings. They used similar techniques but only with the passage of time have the various approaches reached a common frontier resulting in much useful cross-fertilization.
That very vital process isMaterials selection plays a crucial role in the operation and capabilities of a fabricated device. Among the material properties that must be considered are: the morphology and crystal structure of each material; thermal and electrical compatibility between materials; and ability to withstand processing conditions without unacceptable degradation. Even after a material has been selected, there is the possibility that a new deposition method or material processing step could result in better device performance. This section examines the material properties of the materials synthesis in this research.
Engineered materials are the future of thin film technology. Transition metal (TM) nitrides have been the most studied and investigated compounds since the beginning of the use of hard coatings to improve the performance of mechanical components.
Aluminum Nitride (AlN) coatings:
AlN has physical properties desirable to the larger research community. As an III-V semiconductor, many of its material properties are a result of its close-packed crystal structure. Under ambient conditions, the thermodynamically-stable structure of AlN is hexagonal wurtzite, although at very high pressure it can take the cubic rock salt form [9]. The c/a ratio is 1.60 and its close-packed covalent bonds result in its very high hardness and excellent thermal conductivity, [10-11]. AlN ceramics are found in industrial power supplies and inverters to efficiently dissipate waste heat from these devices [12]. Because of its hardness and corrosion resistance, it is often used as a wear- resistant and anti-corrosive coating, both in industrial and consumer applications [13].AlN is a very chemically-resistant material. Electrically, AlN is a wide band gap semiconductor with a conduction band edge 6.2 eV above the valence band. This property has led to the use of AlN in a number of photonic applications [14] its unique combination of properties, including excellent thermal conductivity, good electrical resistance, low dielectric loss, and high piezoelectric response, making the material suitable for a variety of applications in optoelectronic devices [15]
Silicon Nitride (Si3N4) Coatings:
The Si3N4 tool ceramics belong to materials that have a real possibility to replace steel and sintered carbides in future. Employment of these materials makes high speed machining possible with high feed rates both by turning and by milling [16].The Si3N4 nitride ceramics demonstrates nearly the ideal properties in various applications in a wide range of temperatures. Its high strength, hardness and oxidation resistance, good thermal conductivity and resistance to thermal shocks feature its advantages.
Nanocomposite-TiN/amorphous-Si3N4 coatings:
In recent years, there has been great interest in the synthesis of thin films based on the refinement of the grain size with two or more phases present at the nano-scale. There are many types of nanostructured films, such as superlattices, nanograded, nano-scale multilayers and nanocomposite coatings [17]. The type of coating that has attracted the greatest amount of interest from researchers is the nanocomposite type. Nanocomposite thin films are a new class of materials that exhibit enhanced properties such as high hardness, high toughness and thermal stability. Nanocomposite coatings are composed of nanocrystalline grains of transition-metal nitride or carbide surrounded by an amorphous hard tissue. A substantial number of nanocomposite coatings have been studied [18-20]. Among them, nanocomposite coatings of TiN/Si3N4 have been studied extensively due to their high hardness and elastic modulus, improved wear resistance and high oxidation resistance [21-23]
Titanium Aluminum Nitride (TiAlN) coatings:
Thin films of transition metal nitrides have been widely used in many engineering applications especially due to their high hardness, chemical inertness and excellent wear resistance. Among them, the properties and the applications of TiN coatings have been studied extensively. The main disadvantage of TiN is its limited oxidation resistance (approximately 450-500 oC). The addition of other elements such as Al, Cr, Si, etc. increases the oxidation resistance of TiN [24-26]. TiAlN coatings have been developed for the engineering applications as an alternative to TiN since 1986 [27]. It has been reported that, the addition of aluminum to TiN, thus forming TiAlN, improves the oxidation behavior and the thermal stability of the coating, by forming a stable oxide layer on the surface of the film during oxidation [28-29].
Nanocomposite -TiAlN/amorphous-Si3N4 coatings:
The prominent hardness of the nanocomposite TiN/Si3N4 coatings and improved thermal stability of TiAlN coatings led to the exploration of TiAlN/ Si3N4 nanocomposite coatings. Cutting tool materials with (Ti, Al) N coating had been shown to be superior to other conventional binary coatings and could be used at significantly higher cutting speed [30] since the oxidation resistance was improved [31-33]. Silicon nitride films are useful for structural applications due to their attractive properties of hardness and chemical inertness. It was reported that an amorphous matrix, such as Si3N4, could provide a higher stability against oxidation than that of crystalline metallic nitride [34]. Therefore, nanolaminate materials consisting of nanocrystal-(Ti1-xAlx)Ny and amorphous- Si3N4 [designated as nc-(Ti1-xAlx)Ny and a- Si3N4] can be seen as the future novel cutting tool coating materials.
