{"id":243,"date":"2018-12-07T04:49:38","date_gmt":"2018-12-07T04:49:38","guid":{"rendered":"http:\/\/msp08.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=243"},"modified":"2018-12-07T05:12:05","modified_gmt":"2018-12-07T05:12:05","slug":"thin-films-deposition-technique","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/chapter\/thin-films-deposition-technique\/","title":{"rendered":"Thin films deposition technique"},"content":{"raw":"
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Thin films of various materials have been the focus of much research owing to their vast applications in electronic and optoelectronic devices. These applications stem from the ability to deposit stable thin films of controlled morphology and thickness. The traditional procedures to produce thin films involving casting and spin coating do not meet the requirements of advanced device technologies. With advanced industrial requirements, uniform and stable nanometer thick films are needed. Additionally, many functional materials are not soluble in the common solvents. In such cases, vapor deposition techniques can be promising candidates to fabricate advanced functional devices. In this module, the participants will learn:<\/p>\r\n \r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Vacuum deposition technique\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Physical vapor deposition technique\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Chemical vapor deposition technique\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Advantages and limitations of these techniques\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Applications of these techniques\r\n\r\n \r\n\r\n1.\u00a0\u00a0 Vacuum Deposition<\/strong>\r\n\r\n \r\n

Vacuum deposition is a group of various deposition techniques employed to deposit thin films or layers of a material onto a substrate by atom-by-atom or molecule-by-molecule manner. The processing is carried out at pressures lower than the ambient pressure (i.e., vacuum). The thickness of the deposited films varies from atomically thin to a few millimeters. This technique can also be used to produce free-standing films of a material. Consequently, alternate layers of different materials can also be deposited using this technique, as an example, this technique can be used to produce optical coatings where layers of different materials are present on top of each other.<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

Figure 1 Operating principle of vacuum deposition [2].<\/strong><\/p>\r\n \r\n\r\nThe purposes for depositing films under vacuum are:\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The particle density is greatly reduced; therefore, the mean free path during collisions is long.\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Less contamination<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Low pressure plasma conditions<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/span>Composition of the gas and vapor can be easily controlled<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/span>Flow of the vapors can also be controlled<\/span>\r\n\r\n \r\n\r\nCondensing or depositing vapors can be produced by:<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Thermal evaporation<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Sputtering<\/span>\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Arc vaporization, etc.<\/span>\r\n\r\n \r\n

In reactive deposition (where certain chemical reactions occur during <\/span>deposition process), the following reactions can take place:<\/span><\/p>\r\n

-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 the depositing species may react with a component of the gases present in the reactor (e.g., Ti + N\u00a0<\/span>\u2192 TiN)<\/span><\/p>\r\n

-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 the depositing species may react with a co-depositing species (Ti + C \u2192 TiC).<\/span><\/p>\r\n \r\n

The plasma environment activates the gases (N2 \u2192 2N) and decomposes the precursor vapors (SiH4 \u2192 Si\u00a0<\/span>+\u00a0 4H). The other uses of plasma include:<\/span><\/p>\r\n

-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Precursors can be vaporized by sputtering<\/span><\/p>\r\n

-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The substrate can be cleaned by ion sputtering<\/span><\/p>\r\n

-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 To densify the structure and control properties (ion plating).<\/span><\/p>\r\n\r\n<\/div>\r\n

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These processes can be classified on the basis of the type of vapor source employed in film deposition in the following two categories:<\/p>\r\n1.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Physical vapor deposition: this technique uses a solid or liquid vapor source\r\n\r\n2.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Chemical vapor deposition: chemical vapor\r\n\r\n \r\n

Vapor deposition processing includes techniques which deposit materials in a vapor state by condensation process, chemical reactions, or certain types of conversion processes. The deposition process is known physical vapor deposition (or PVD) if a liquid or solid source is used to create the vapor phase. However, if vapors are produced by a chemical reaction, the process is chemical vapor deposition (CVD). Generally, a combination of both these techniques is used.<\/p>\r\n \r\n\r\n1.1 Applications<\/strong>\r\n\r\n \r\n\r\nVacuum deposition techniques find diverse range of applications, such as:\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Electrical, semiconducting as well as insulating coatings.\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Optical coatings\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Reflective coatings\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Film lubricants\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Low emissivity glass coating, smart film coatings\r\n\r\n-\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Diffusion barrier coatings, etc.\r\n\r\n \r\n\r\n2.\u00a0\u00a0 Physical Vapour Deposition Method<\/strong>\r\n\r\n \r\n

Physical vapor deposition (PVD) includes a group of vacuum deposition techniques employed to synthesize thin films as well as coatings. In PVD, the material goes from a condensed phase (as precursor) to a vapor phase and then back to the condensed phase (deposited as thin films). Commonly used PVD techniques include sputtering, laser surface alloying, ion plating and ion implantation. It is widely employed to produce thin films for mechanical, optical, chemical or electronic functions, such as semiconductor devices including thin film solar panels, aluminized PET film for food packaging and\u00a0balloons, and coated cutting tools for <\/span>metal working, etc. The most frequent coatings developed by this technique are titanium nitride, zirconium nitride, chromium nitride, titanium aluminum nitride.<\/span><\/p>\r\n \r\n

In PVD, the film is deposited over the entire exposed area of the object. It is basically a vaporization coating method involving atomic scale transport of the material to be coated. The gas phase precursor condenses onto the substrate, thereby creating the required layer. No chemical reactions occur during the deposition process. The process is performed under vacuum and comprises the following steps (<\/span>Figure<\/strong> 2(a)<\/strong>):<\/span><\/p>\r\n \r\n

Evaporation<\/em><\/strong><\/p>\r\n

The target (material to be coated\/deposited) is incident with <\/span>high energy source like an electron\/ion beam.\u00a0<\/span>The atoms from <\/span>target surface are removed, thereby vaporizing them.<\/span><\/p>\r\n \r\n

Transport<\/em><\/strong><\/p>\r\n

The atomic <\/span>vapours are carried from <\/span>target surface to the surface of the substrate requiring coating.<\/span><\/p>\r\n \r\n

Reaction<\/em><\/strong><\/p>\r\n

This step is introduced if the deposition is to be of compounds of target metal atoms such as metal oxides, nitrides, carbides <\/span>and the like materials. When the metal atoms of the target vaporize, they react with the gas (intentionally introduced to react with target metal) during <\/span>transport phase, thereby depositing products of these metal atoms.<\/span><\/p>\r\n \r\n

Deposition<\/em><\/strong><\/p>\r\n

It involves coating build-up on <\/span>surface of the substrate. Based on <\/span>specific method, certain reactions may take place between the target material and reactive gases on the surface of <\/span>substrate, concurrently along with the deposition process.<\/span><\/p>\r\n\r\n<\/div>\r\n

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Figure 2 (a) Flowchart of the PVD technique, and (b) schematics of the setup used for PVD.<\/strong><\/p>\r\n \r\n

Figure 2(b) <\/strong>shows the experimental setup used for PVD technique. The experiment is performed in a quartz or alumina (ceramic) tube. Depending on the application, the tube may be fitted either horizontally or vertically. Before performing experiment, reaction chamber is evacuated at the pressure in 10-4 to 10-7 Torr range. Then heating element is turned on and with a constant flow rate, carrier gas is introduced into the chamber. Introduction of carrier gases increases pressure inside the chamber and it becomes ~200-500 Torr. Flow rate of carrier gas depends on structure of required nanomaterial because morphology of produced nanostructure greatly depends on pressure of chamber and flow rate of carrier gas. After\u00a0achieving necessary conditions inside the chamber, gas flow and temperature of <\/span>chamber are kept constant for deposition time. Precursor materials are vaporized at high temperature, <\/span>low pressure conditions. These <\/span>vapours are then transferred by inert gases to lower temperature zone, where they progressively supersaturate. When they reach the substrate surface, nucleation and subsequent growth of desired nanostructures <\/span>takes place. The growth can be terminated by turning off the furnace. The reaction setup is cooled to the surroundings by flowing inert gas through it.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\n2.1 Ion Plating (via plasma)<\/strong>\r\n\r\n \r\n

Metals including titanium, aluminum, gold, copper, and palladium are deposited on the surface of a feature via plasma based ion plating. The thicknesses of the deposited layers vary between 0.008 to 0.025 nm. The advantages of this technique are good adhesion, surface finish, in-situ substrate cleansing before coating and good control over the film geometry. However, its disadvantages include tight control of process parameters, plasma contamination, and possible contamination of substrate and deposited layer by the bombarded gas species. It is widely used in X-ray tubes, piping threads, turbine blades in aircraft engines, steel drilling bits, etc.<\/p>\r\n \r\n\r\n2.2 Ion implantation<\/strong>\r\n\r\n \r\n

It does not create an overall new layer, but forms alloys with substrate surface, thereby altering the chemical composition of the surafce. For instance, nitrogen is employed to enhance the wear resistance in metals. Substrate cleaning prior to the deposition process is highly critical in this technique. As it works on the species present on the surface of the substrate, therefore, it is highly prone to contamination led problems. The process is carried out in room temperature and the time required for deposition depends on substrate\u2019s temperature resistance and the desired coating material.<\/p>\r\n \r\n

Ion implantation chemistry is only limited by the number of elements which can be vaporized and ionized within a vacuum chamber. Advantages are reproducibility, no posttreatment and low waste production. If the coating is exposed to elevated temperatures, it cannot produce good finish. Limitations are complications in quality control, insufficient know-how and equipments.<\/p>\r\n \r\n

Typical applications include anti-wear coatings for high value components in biomedical devices, tools, and gears and balls in aerospace industry. It is also used in semiconductor industry to deposit gold, ceramics, etc. onto a variety of substrates (e.g., plastic, ceramic, silicon, gallium arsenide, etc.).<\/p>\r\n \r\n\r\n2.3 Sputtering<\/strong>\r\n\r\n \r\n

Sputtering alters the physical properties of any surface by etching mechanism. A gas plasma is created between two electrodes, that is, the cathode (comprising the material to be deposited) and an anode (which acts as the substrate on which deposition is required). Typical depositions are thin films with thickness varying from 0.00005 \u2013 0.01 mm. Typical depositions are of chromium, titanium, aluminum, copper, molybdenum, tungsten, gold, and silver.<\/p>\r\n \r\n\r\nApplications include decorative coatings like watchbands, eyeglasses, and jewelry.\r\n\r\n \r\n

In comparison to other deposition techniques, sputtering is an economic and cost effective process and thus, it is extensively used in numerous industries. It is widely used in electronics industry for producing heavily sputtered coatings and films. Such coatings include depositing thin film wires on chips, recording heads, magnetic and magneto-optic recording media, etc. Automotive industry uses sputtering to prepare decorative films for plastic. In buildings, it is used to create reflective films for large pieces of architectural glass. The food packaging industry uses sputtering to produce thin plastic films for packaging.<\/p>\r\n \r\n\r\n2.4 Surface alloying<\/strong>\r\n\r\n \r\n

Surface alloying (modification) by using lasers facilitates alloy formation by introducing the selected material into the melt pool. This process produces surfaces exhibiting good performance at elevated temperatures, improved wear and corrosion resistance, enhanced mechanical behaviors, and good appearance.<\/p>\r\n \r\n

Laser cladding is the most common process in surface alloying. Laser cladding is used for selective deposition over a specific area. In this technique, a thin metal layer is mixed with some base metal by combined application of pressure and heat. Elaborate sample preparation step is not required, yet the surface might require to be made rough before coating. Grinding and\/or polishing are usually done after deposition.<\/span><\/p>\r\n \r\n

However, readily oxidized materials require inert gas streams for deposition. Laser power, its frequency <\/span>and speed affect the deposition rates. The thickness of the deposited layer varies between <\/span>few hundred micrometers to millimeters. If the deposited layer is highly dense, film cracking may result in delamination. Furthermore, this technique only deposits in the line of sight.<\/span><\/p>\r\n \r\n

3.\u00a0\u00a0 Chemical Vapour Deposition Method (CVD)<\/strong><\/p>\r\n \r\n

CVD is the most extensively utilized technique to prepare good quality, extraordinary-performance solid materials. Its\u2019 most popular applications include producing thin-film coatings on surfaces, but it can also be used to synthesize <\/span>high quality powders and bulk materials. Composite materials can also be fabricated by the infiltration technique. CVD is also employed to deposit almost all the elements of the periodic table, sometimes in their pristine forms, but more often combined compounds.<\/span><\/p>\r\n \r\n

Conventionally, a precursor gas (or gases) is introduced inside a chamber enclosing the pre-heated substrates needs to be coated with. Chemical reactions take place on and near these heated substrates, leading to the deposition of thin films of <\/span>desired material on <\/span>surface of the substrate. The deposition is accompanied by <\/span>production of byproducts which needs to be removed from the reaction chamber along with the non-utilized precursor gases. <\/span>Figure 3<\/strong> includes a conventional CVD experimental setup along with the schematics of the process.<\/span><\/p>\r\n \r\n

The steps involved can be described as follows:<\/span><\/p>\r\n\r\n