Module-2_Unit-3_NSNT 
                       
                      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: 
                           -    Vacuum deposition technique 
                           -    Physical vapor deposition technique 
                           -    Chemical vapor deposition technique 
                           -    Advantages and limitations of these techniques 
                           -    Applications of these techniques 
                       
                           1.  Vacuum Deposition 
                       
                      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. 
                       
                                                                                                                                  
                                                     Figure 1 Operating principle of vacuum deposition [2]. 
                       
                      The purposes for depositing films under vacuum are: 
                           -    The particle density is greatly reduced; therefore, the mean free path during collisions is long. 
                       
                       
                           -    Less contamination 
                           -    Low pressure plasma conditions 
                           -    Composition of the gas and vapor can be easily controlled 
                           -    Flow of the vapors can also be controlled 
                       
                      Condensing or depositing vapors can be produced by: 
                           -    Thermal evaporation 
                           -    Sputtering 
                           -    Arc vaporization, etc. 
                       
                      In reactive deposition (where certain chemical reactions occur during deposition process), the following 
                      reactions can take place: 
                           -    the depositing species may react with a component of the gases present in the reactor (e.g., Ti + N 
                                → TiN) 
                           -    the depositing species may react with a co-depositing species (Ti + C → TiC). 
                      The plasma environment activates the gases (N → 2N) and decomposes the precursor vapors (SiH  → Si 
                                                                                    2                                                                   4
                      + 4H). The other uses of plasma include: 
                           -    Precursors can be vaporized by sputtering 
                           -    The substrate can be cleaned by ion sputtering 
                           -    To densify the structure and control properties (ion plating). 
                       
                      These processes can be classified on the basis of the type of vapor source employed in film deposition in 
                      the following two categories: 
                           1.  Physical vapor deposition: this technique uses a solid or liquid vapor source 
                           2.  Chemical vapor deposition: chemical vapor 
                      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. 
                       
                      1.1 Applications 
                      Vacuum deposition techniques find diverse range of applications, such as: 
                           -    Electrical, semiconducting as well as insulating coatings. 
                           -    Optical coatings 
                           -    Reflective coatings 
                           -    Film lubricants 
                           -    Low emissivity glass coating, smart film coatings 
                           -    Diffusion barrier coatings, etc. 
                       
                       
                           2.  Physical Vapour Deposition Method 
                      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 
                       
        
       balloons, and coated cutting tools for metal working, etc. The most frequent coatings developed by this 
       technique are titanium nitride, zirconium nitride, chromium nitride, titanium aluminum nitride. 
        
       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 (Figure 
       2(a)): 
        
       Evaporation 
       The target (material to be coated/deposited) is incident with high energy source like an electron/ion beam. 
       The atoms from target surface are removed, thereby vaporizing them. 
       Transport 
       The atomic vapours are carried from target surface to the surface of the substrate requiring coating. 
       Reaction 
       This step is introduced if the deposition is to be of compounds of target metal atoms such as metal oxides, 
       nitrides, carbides 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  transport  phase,  thereby  depositing 
       products of these metal atoms. 
       Deposition 
       It involves coating build-up on surface of the substrate. Based on specific method, certain reactions may 
       take place between the target material and reactive gases on the surface of substrate, concurrently along 
       with the deposition process. 
        
                                                   
         Figure 2 (a) Flowchart of the PVD technique, and (b) schematics of the setup used for PVD. 
        
        
       Figure 2(b) 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 
        
        
       achieving  necessary  conditions  inside  the  chamber,  gas  flow  and  temperature  of  chamber  are  kept 
       constant  for  deposition  time.  Precursor  materials  are  vaporized  at  high  temperature,  low  pressure 
       conditions. These 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 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. 
        
         2.1 Ion Plating (via plasma) 
       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. 
        
         2.2 Ion implantation 
       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’s temperature resistance and the desired coating material. 
       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. 
       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.). 
        
         2.3 Sputtering 
       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 – 0.01 mm. Typical depositions are of chromium, titanium, aluminum, 
       copper, molybdenum, tungsten, gold, and silver. 
       Applications include decorative coatings like watchbands, eyeglasses, and jewelry. 
       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. 
        
         2.4 Surface alloying 
       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.