2. THIN FILM PREPARATION AND CHARACTERIZATION TECHNIQUES

Transcription

1 2. THIN FILM PREPARATION AND CHARACTERIZATION TECHNIQUES 2.1. Introduction Various methods of deposition of thin films including RF magnetron sputtering technique in which the current samples were prepared for investigations are elaborately discussed in this chapter. Also the procedures followed for obtaining the samples for present study are neatly explained. The configurations of the sputtering unit and the various instruments used for current deposition and characterization are also briefly explained Types of deposition techniques Thin film deposition is an act of preparing a thin film onto a surface called substrate or onto previously deposited layers. Here the deposition techniques control the thin film thickness. Thin films vary in thickness from a few hundred angstroms to tens of microns. Depending on the process involved in deposition techniques, the two major categories arise as chemical and physical deposition techniques Chemical deposition techniques In this deposition technique, a solid layer is left out on a solid surface which is the result of a fluid precursor undergoing chemical change on it. Thin films with conformal deposition is possible in this technique as deposition occurs on all parts of the solid surface due to the surrounding fluids. The two major categories of chemical deposition are chemical solution deposition (CSD) and chemical vapor deposition (CVD). In CSD, the unit species of material is in liquid or solution form during deposition and there will be no chemical reaction happening with the substrate and the substrate provides only a physical support for deposition. Also the deposition process occurs at lower temperatures and at atmospheric pressure. In contrast to these features, the unit species of materials are applied in vapor form during CVD deposition and also chemical reactions occur on the surface of the substrate. High temperature and low or high pressure is also needed for the deposition to undertake Chemical vapour deposition(cvd) Thin films with good uniformity, compatibility, high rate of deposition and conformal growth can be prepared by Chemical Vapor deposition technique[1]. Control over deposition rate, chemical and phase composition and microstructure of the deposited thin films are possible in this versatile deposition technique. Various materials are deposited as thin films using this technique and it involves the reaction of a molecule with the surface of the substrate which is induced by heat[2]. Organic/ inorganic molecules containing the desired atoms in the form of vapor are allowed to pass through a heated semiconductor wafer. The molecules break down into the desired atoms by the heated 35

2 surface and deposit on it layer by layer with qualities controlled by the composition of the gas. All the exposed surfaces of the substrate are deposited by the molecules which is the characteristic feature of this technique. The unwanted products generated during the breaking of molecules into the desired atomic species, are carried away by the hot gas supplied. The main steps that are followed during this type of deposition technique are: Introducing the reactants in gaseous phase nearer to the substrate. Initiating the chemical reactions by heat, photons, electrons, ions or a combination of them. Deposition of the molecules over the substrate. Removal of the gas-phase undesired radicals. In this way, high quality thin films with perfect alignment with substrate surface are obtained. A schematic representation of the important steps that undertake during CVD process is presented in Figure 2.1. Area selective deposition in case of substrates exposing different phase to the vapor is also possible in this type of technique in which the deposition is localized to the desired phase of the substrate leaving the other phase regions of it without deposition. This type of deposition is employed in the fields of microelectronics and optical communication industries. Recently uniform nanocrystalline V 2 O 5 thin films, free from carbon were deposited by N. K. Nandakumar et.al. on silicon using CVD technique[3]. Metal Organic Chemical Vapor Deposition (MOCVD) is a special case of CVD in which one or more of the precursors are applied as an organic metal. Due to the similar arrangement in the structures of V 2 O 5 and V 6 O 13, the dependence of the orientation, crystallinity and phase formation of these thin films with respect to the deposition parameters will throw light on understanding more about the potential of these two Vanadium oxides and MOCVD is the suitable one for the same[4] Spray pyrolysis A simple and inexpensive method of preparing thin and thick films, ceramic coatings, metal and metal oxide powders in large scale is spray pyrolysis[5,6]. Any element can be easily doped in the required ratio via solution medium in this method. It is a simple and cost-effective processing method in which different types of materials can be deposited in to thin film form[7]. Spray pyrolysis is a thermally stimulated reaction. The main components of a spray pyrolysis equipment are an atomizer, precursor solution, substrate heater and a temperature controller. The atomizer may be an air blast in which liquid is exposed to a stream of air or ultrasonic frequencies which produce short wavelengths or electrostatic 36

3 in which liquid is exposed to a high electric field[8]. In spray pyrolysis, the aqueous solution containing soluble salts of the ingredient atoms of the needed compound is sprayed on to the heated surface of the substrate in which endothermic decomposition takes place and results in single crystallite or a cluster of crystallites of the necessary compound is formed[9]. Initially, the necessary chemical reactants are selected carefully so that when they, in solution form are sprayed on the heated surface, the other undesired chemical products and the excess solvent should be volatile at the temperature of deposition[10]. The thermal energy needed for the thermal decomposition and also the recombination of the species taking part in the combination after sintering and recrystallization of the crystal clusters, is supplied by the heated substrate. A schematic block diagram of a spray pyrolysis arrangement is shown in Figure 2.2. In short, spray pyrolysis needs the following steps: Transforming liquid precursor or solution of precursor in to micro sized droplets Making solvent to evaporate Allowing solute to condense Making the solute to decompose and react Sintering the solid particles V 2 O 5 thin films were synthesized by using this method of deposition which is a low cast and large area technique[6,11] Electrodeposition Another name for electrodeposition is electroplating and this technique is an inexpensive one to prepare thin layers of high quality metals over the inexpensive and easily available base materials[12]. It is used to coat electrically conductive textile materials with a metal layer by applying electrical current which ends in forming a thick, stiff and heavy metal coat on textile. Some of the metals that can be coated on fabric surfaces using this technique are cadmium, chromium, copper, gold, iron, nickel, silver and zinc. The electrolytic cell in which this electrodeposition process occurs, contains an electrolyte and two electrodes. The chemical species containing the required metal are dissolved in to the electrolyte which is an ionic conductor, as suitable solvent or they may be converted in to the liquid state for making a molten salt[13]. In the electrolytic cell, the coating metal is taken as the anode and the layer to be coated is taken as cathode so that when a low voltage current is supplied, ions residing in the electrolyte reach cathode and deposit on it. Most of the time, the metallic deposit is in crystalline phase and so this method of deposition is also called as electrocrystallization. However, the structure of the initial layers of the thin films deposited on a polycrystalline substrate is highly influenced 37

4 by that of the substrate and when the thickness increases, the bath condition dominates the influence of substrate in determining the crystal structure of the film deposited. This can also be achieved by reducing the crystal size of the substrate as it will increase the crystalline boundary regions and thereby reducing the influence of substrate[14]. High response time (2-20 seconds) and repeating cycle ( 8 x 10 4 ) in EC reaction was obtained for V 2 O 5 thin films deposited on ITO glass plates in electrodeposition process[15]. High surface area mesoporous V 2 O 5 thin films which show high capacity of lithium ions were fabricated by using this method of deposition[16] Anodization Anodization is an unique electrochemical process to prepare oxide thin films of certain metals like aluminium, niobium, tantalum, titanium, tungsten and zirconium in which aluminium, niobium and tantalum oxide thin films play vital role as capacitor dielectrics in commercial and technological field. The thickness and properties of the thin films deposited in this method depend on the metal. Thick oxide coatings of aluminium obtained by anodizing aluminium alloys in particular acidic electrolytes show high density of microscopic pores which make them applicable as corrosion preventives in automobiles and aerospace structures and also as electrical insulation. The anodizing cell contains the metal workpiece as anode by connecting it to the positive terminal of a dc power supply and any electronic conductor which is inert in the anodizing bath as cathode by connecting to the negative terminal of the supply. When the dc power supply is switched on, electrons are thrown out of the metal surface which acts as a positive terminal leaving behind ions and these ions on the metal surface interact with water and form an oxide layer upon the metal. Hydrogen gas is formed when the electrons react with the hydrogen ions in the cathode during their return path. The electrolytes in the anodizing cell should be carefully selected so that the oxide layer will be insoluable in it and also the bath composition should be carefully studied as the formation of a barrier or a porous thin film depends mainly on it. 4-5 μm long nanotubes of mixed V 2 O 5 TiO 2 arrays were deposited on Ti-V alloy plates and Ti foils by anodization[17,18] Solution growth Ternary thin films of sulphides, selenides and halides which show high applications in photovoltaic, opto-electronic and other device applications can be prepared by solution growth method, a modified process of chemical precipitation[19,20]. The advantages of using this technique for preparing thin films are cheap and simple[21]. In this process, the dry, well cleaned glass plates were immersed vertically in reaction baths having the desired mixture with distilled water for deposition. The solution is allowed to deposit on the glass substrates for some time and then the glass plates are removed and are rinsed with distilled water and finally dried in air. It was the first and widely used method for 38

5 producing artificial crystals which have wide range of applications such as mass crystallization, pharmaceuticals production and growing small crystals for studying their crystallographic and physical properties. Isothermal, aqueous solution method is adopted recently for growing epitaxial ZnO thin films and also the same low temperature method can be used to grow polycrystalline, epitaxial ZnO films Screen printing One of the thick film formation technologies is screen printing and this method is used for getting thin films of conductors, dielectrics and resistances. In this method of deposition, the substrate is placed in a carriage and it is fitted below a screen which contains the relevant configuration to be printed in the form of open mesh areas created by the process called photolithography[9]. The substrate is placed in an air free recess which is in the platform of the substrate holder and the substrate holder can be moved repeatedly between the mounting position to the printing position with the help of the moving carriage. During printing process, the substrate should be kept closer to the screen and the gap between the two is called as breakaway or snap-off distance. Micrometer screws in the screen mounting are used to rotate or move the screen in X and Y directions with respect to the substrate for keeping the screen parallel to the substrate and also for adjusting the snap-off distance. The precision of the printed pattern is determined by the amount of paste supplied on the surface of the substrate and the main parameter that determines this is the snap-off distance. Over the surface of the screen, a small amount of paste is poured in and it is forced to go down to the substrate through the open mesh areas by a flexible wiper called squeegee which moves across the screen surface, deflecting it vertically. Thus a printed pattern of paste is left behind on the surface of the substrate after removing the squeegee and the screen restores back to its original position by its natural tension. Now the substrate can be removed from the carriage and it has the desired pattern printed on its surface Physical vapor deposition techniques In physical vapor deposition, deposition of required material occurs on the surface of substrate by releasing them from the source and this process uses mainly the sputtering and evaporation techniques for the release of material from the source. The advantage of this technique of deposition over CVD is lower process risk in case of metal deposition. Its disadvantages are inferior quality of thin films, thin films with less step coverage, high resistivity thin film preparation in case of metals and thin films with more defects and traps in case of insulators Thermal evaporation The general concept in evaporation technique involves heating a metal and thereby creating its vapor in high vacuum ( below 10-5 torr) so as to control the scattering by 39

6 molecules in gaseous state and then allowing metal vapor to diffuse and recondense on the other surfaces as solid form. The advantages of thermal evaporation technique are cost effective volume production and reproducibility and this method is a straight forward one for metal deposition. Also the influence of sputtering gas inside the chamber during deposition over the prepared thin films in sputtering techniques is absent in this method of deposition. Resistive heating created by dc current or eddy currents induced by magnetic field is used in the simplest thermal evaporation process for heating the metal charge. The schematic representation of a conventional thermal evaporation system with main parts is shown in Figure 2.3. The metal charge placed inside a metal boat normally made of tungsten or inside wound coils, is conductively heated by passing high current to the boat or coils. In this method, metals with low melting point such as Ag, Al and Au can also be easily deposited. In the case of high melting point metals such as Ta, W, Mo and Ti, the former method is not applicable as they require very high temperature to attain a favorable vapor pressure and deposition rates. In this case, the metals can be placed inside a ceramic crucible which is surrounded by a coil and by the rf excitation of the coil, eddy currents are induced in the metal which results in inductive heating[22]. Surface contamination should be removed by subjecting the boat to preliminary heating. The distance between the source and the target is carefully chosen in such a way that it is smaller than the mean free path of the evaporating atoms so that the collision of these atoms with the atoms of the residual gas inside the chamber will be negligible[14]. The velocity of colliding atoms which depends on the temperature of the evaporation source, the intensity of beam settling on the surface and the contamination caused by evaporated material are some more parameters which affect the nature and properties of the evaporated film. Electroanalytical characterization of thermally evaporated, homogeneous V 2 O 5 thin films are better [23] compared with thin films prepared in other methods Electron beam evaporation This is another method of evaporation in which an electron beam directed towards the target metal is used to heat. In this method of deposition, a wide range of materials can be evaporated which includes refractory metals such as tungsten, metals with low vapor pressure such as platinum and alloys. This method is also useful for producing alloys of materials whose melting points are different or whose components go directly from solid to vapor with no liquid phase. The schematic diagram of an electron beam evaporation system is shown in Figure 2.4. High vacuum of 10-5 or less is needed for the process to begin. The source of the electron beam, normally a tungsten filament in an electron beam gun which is placed under the metal charge is used to produce an electron beam. To avoid contamination by the evaporant, the gun assembly is placed outside the evaporation area. Strong electric and magnetic fields are used to accelerate the electron beam coming from the electron gun and also to bend the path of this electron beam to a 270 C circular 40

7 arc and thereby focssing it to hit on the target. When the electrons with high kinetic energy hit the surface of the target, it is transformed in to the corresponding thermal energy. Several million watts per square inch is attained in this process and the surface of the evaporant thus gains a large amount of heat and vaporization begins and the vapor finally condenses over the surface of the substrate[24]. The evaporant holder should be protected with a coolant or else it will melt due to the high heat generated by the electron beam. The main advantage of this method of deposition is that the central portion of the charge which is hit by the electron beam alone is heated up whereas the outer area of the metal and also the parts of crucible are at lower temperature. Also the high rate of deposition of this technique makes it a favorable method for industrial purposes Molecular beam epitaxy(mbe) Molecular beam epitaxy is a vapor deposition technique which needs ultra high vacuum in the order of 10-9 Torr[25]. A sketch of Molecular Beam Epitaxy system with its main components is shown in Figure 2.5. In this Ultra High Vacuum (UHV) deposition technique, interaction of one or more molecular or atomic beams on the surface of a crystalline substrate which is already been heated leads to diffusion and thereby an epitaxial growth of thin films. The beams of atoms will not react with each other or with the vacuum chamber gases due to the long mean free path of the atoms and the interaction occurs only when they reach the substrate surface. The beam contains angular distribution of atoms or molecules from the heated solid source materials which are kept in vacuum cells. Kundsen cell or electron beam is selected as vapor sources in MBE of metal films. The metal source is indirectly heated by the K-cell through a tungsten filament. In an electron beam source, the electrons are accelerated towards the target in vacuum. In K-cell, the highly divergent and directional beam propagates through a small aperture behind which the heatable crucible is placed. The flow of gas can be rapidly stopped and allowed by using a shutter in front of the aperture which controls the molecular beam emitted and thereby controlling the amount of deposited material and even a single layer of atoms is possible to obtain. Thus the layer structure can be controlled by adjusting the shutter speed and also the amount of dopants that should be added to the layers can be carefully adjusted. The electron beam sources are more common than the K-cell sources in MBE of metal films as the vapor pressures of many metals are well below 1500 C. Normally, the substrate is chosen as a single crystal of a semiconductor such as GaAs or other III-V compound. In some cases, Si or CdTe are also used as substrate material[26]. As mobility of surface plays a vital role in MBE, the substrate is usually heated to a high temperature but it shows a drawback of inter diffusion of deposited atoms in to the substrate. The advantages of MBE are high quality epitaxial growth of structures with monolayer control as the deposition rate very slow, preparation of thin films both at the research and the industrial production level, clean growth environment, accuracy in control of the beam fluxes and growth condition, easy 41

8 implementation of in situ diagnostic instruments and compatibility with other thin film deposition methods using high vacuum condition. The diagnostic instruments for MBE are Auger Electron Spectroscopy (AES), Scanning Electron Microscopy (SEM), Ellipsometry, Laser interferometric method, Reflection High-Energy Electron Diffraction (RHEED) and Surface Acoustic Wave Devices Ion plating In ion plating deposition technique, atomic size energetic particles are used to bombard the substrate and depositing atoms continuously or periodically. For cleaning the deposition surface, prier sputtering is essential. For obtaining good adhesion, high density of material deposition, assistance in chemical reactions, residual stress modification and also for modification in structural and morphological properties of the depositing film, deposition surface is bombarded during deposition. Atomically clean interface can be maintained by continuously bombarding the surface between the cleaning and the deposition periods of the process. Other names for this physical vapor deposition technique are ion assisted deposition (IAD) and ion vapor deposition (IVD)[27]. There are two major categories of ion plating and they are plasma-based ion plating and vacuum-based ion plating. In the first category, the negatively biased substrate attracts the positively charged ions in the surrounding plasma and thus the highly energetic ions hit the surface of the substrate and for this purpose the substrate can be the cathode electrode for generating plasma inside the chamber. The substrate can also be placed in the plasma-generation region or in a region far away from the active plasma generation region. In vacuum based ion plating, the ion beam from a source is allowed to bombard and deposit the film material in vacuum. Here the source of vaporization and the source of high energy bombarding ions are separate and normally this type of deposition is called as ion beam assisted deposition (IBAD). Thin films of TiO 2, Ta 2 O 5, ZrO 2, Al 2 O 3 and SiO 2 with high packing densities and high refractive indices are deposited onto substrates kept at room temperature using this technique of deposition[28] Activated reactive evaporation In reactive evaporation process, films of compound materials are formed by depositing atoms under partial pressure atmosphere of the reactive gas. Activated reactive evaporation (ARE) is the deposition process in which the oxide films are deposited by evaporating the film material under a low pressure plasma containing oxygen. The major difference between ARE and normal reactive process lies in activating the reactive gas and thus making it more chemically reactive so that the process can be achieved even at low gas pressure[27]. In ARE, the metal vapor and the reactive gas are ionized in the space between the metal vapor source and the substrate[29]. Plasma plays an important role in deposition of compound materials by enhancing the reactions that activate it. Also it modifies the growth kinetics and thereby changes the structural and morphological 42

9 characteristics of the deposited materials[30]. During the deposition process, the surface gains negative potential when it comes in contact with the plasma and the gas phasenucleated particles also gain negative potential when in contact with the plasma which prevents these ultrafine particles to deposit on the substrate surface. Usually the substrate is given a negative bias and in this case, the evaporation technique is called as bias active reactive evaporation (BARE) Sputtering technique Ejection of material from a source called target and depositing it on a substrate kept in a vacuum chamber is called as sputtering. Required deposition pressure is maintained inside the chamber during the sputtering process. Materials with high melting points are easily sputtered and their thin films are prepared by using sputter deposition technique while it is impossible with resistance evaporator or Knudsen cell to evaporate these materials. Also the composition of the sputtered thin films is similar to that of the source material and also the adhesion of the thin films of the material over the substrate is better than that in the evaporated thin films. Other advantages in this method of deposition are the usage of sputtering source without any hot parts and the compatibility of the whole set-up with reactive gas such as oxygen. The sputtering gas is normally an inert gas such as argon. Initially negative charge is applied to the target which causes plasma or glow charge and sputtering starts. The high speed positive charged gas ions, ie the ionized A + ions generated in the plasma region are attracted towards the negatively biased target material and collide with it making a momentum transfer[31]. These collisions eject atomic size particles from the target and allow them to deposit as a thin film on the surface of the substrate which is connected with the positive electrode. When the ionized A + ions hit the surface of the target material, secondary electrons are also produced which help further ionization process in the chamber. When the sputtering gas pressure is increased, the ionization probability increases and hence the number of ions and the conductivity of the gas also increase which reduces the breakthrough voltage. This generates stable plasma and thus sufficient amount of ions are available during the deposition period for sputtering the target material. The parameters of sputtering technique which affect the properties of the resulting films are listed below: The sputter current It determines the rate of deposition process The applied voltage It determines the maximum energy with which the sputtered particles ejected from the target surface and the sputter yield ie the ratio of the number of sputtered particles and the sputtering ions[32]. Sputtering gas pressure & Target- substrate distance They control the mean free path of the sputtered particles and thereby the porosity, crystallinity and texture of the deposited thin films. 43

10 Reactive gas mixture It controls the stoichiometry of the thin films prepared. Substrate temperature It controls the density of the thin films and also the behavior of growth with respect to crystallinity. Bias-voltage to substrate It determines the growth of the layer, either by accelerating the electrons towards the substrate or keeping them away from the same. Some of the advantages of using magnetron sputtering are lower voltage needed for striking plasma, controlling uniformity, reduction in substrate heating especially in the case of Si wafer by electron bombardment and high deposition rate. When compared with thermal evaporation technique, this method has some advantages as presence of high energy atoms, low vacuum path, more collision, smaller grain size and better adhesion. Thin films of various materials are deposited in manufacturing integrated circuits in the semiconductor industry by using this technique. Anti reflection thin film coatings deposited on glass substrates by sputtering have various optical applications. In thin film transistor fabrication, contact metals are deposited effectively by this process as in this method of deposition, the substrates can be maintained at low temperatures. Other applications of thin films prepared in this technique are sensors, photovoltaic cells ( solar cells), metal cantilevers and interconnects. DC and RF modes are available in magnetron sputtering and DC sputtering deals with conducting materials. In the case of non conducting material, sputtering will be stopped in DC mode due to the constant building up of positive charge over the surface of the target material. Both conducting and non conducting targets are sputtered in RF mode Thin film preparation by RF Magnetron Sputtering In this method of sputtering, the percentage of electrons taking part in ionization process is increased by keeping powerful magnets which in turn increases the probability of electrons hitting the argon atoms, the length of the electron path and hence the ionization efficiency. Sputtering effectively at lower pressures is made possible by generating magnetic field inside the chamber[14]. The schematic representation of RF Magnetron sputtering unit with the main components is shown in Figure 2.6. The operating parameters of RF magnetron sputtering are target composition, sputtering gas pressure and composition, sputter RF power, substrate temperature, target to substrate distance and deposition rate[33] Sputtering unit To produce highly insulating oxide thin films, RF magnetron sputtering method is adopted which requires RF power supplies and impedance matching networks. Also this is the convenient method to prepare thin, homogeneous, uniform and pure films of 44

11 various materials. Here a ring shaped magnet is mounted below the target to increase the ionization rate by raising the number of secondary electrons ejected from the surface of the target[34]. The magnetic field thus developed traps the electrons in cycloids and keeps them circulating over the target surface which increases the dwell time of electrons in the gas and thereby raises the ionization probability. This forms the plasma ignition at very low pressures compared with the conventional sputtering methods. When a nonconducting target is bombarded with positive ions, a charging surface is developed closer to the surface which shields the applied electrical field and consequently the ion current will die off. This makes the restriction of using dc sputtering for depositing conducting materials only like metals or doped semiconductors. For depositing dielectric films, RF sputtering or reactive sputtering are preferred. In RF sputtering, the applied ac voltage accelerates the positively charged ions towards the target surface at one phase and neutrality is achieved in the other phase. In reactive sputtering, reactive gases such as oxygen or nitrogen are fed in to the chamber in addition to the sputtering inert gas such as argon. In the present work, the RF magnetron sputtering unit used for deposition of thin films is a HINDIVAC Planar magnetron RF/DC sputtering unit, Model-12 MSPT, Bangalore with MHz as the working frequency, supported by a vacuum system which consists of an oil diffusion pump in conjunction with an oil sealed rotary pump and the photograph of the RF magnetron sputtering unit used for depositing thin films in this study is shown in Figure 2.7. The vacuum chamber is made up of non magnetic quality stainless steel with internal diameter of 290 mm and cylindrical length of 400 mm. The target can be placed in a target holder which is attached with the fixed flat plate inside the chamber. A 0 ring is sealed between the chamber and the flat plate to achieve vacuum and a cooling water pipeline is used to cool the outer wall including the chamber window by circulating cool water.the chamber has a view port with glasses in the front for viewing the sputtering process. The HINDIVAC diffusion pump used in the unit is backed by a 300 liters per minute, double stage, direct driven rotary vane pump with an overload protection. A hand operated high vacuum valve fixed at the base plate helps to isolate the chamber from the pumping system and thus the chamber can be brought to atmospheric pressure without switching off the pumping system. For preventing the foreign bodies falling into the high vacuum valve, the base plate is fixed with the stainless steel mesh over it. In addition with this, a combination single lever plate type valve is used for roughing and backing operations. The diffusion pump is isolated from the rest of the system by choosing the backing position when roughing is in progress. As these two valves are interlocked and a single lever is available for their operation, they can not be selected simultaneously. The chamber pipeline is fixed with an air admittance valve which helps to release the vacuum from the chamber after the deposition process is over. To measure the low pressure developed inside the stainless steel chamber, both pirani and penning gauges are used. The analog pirani gauge operates in conjunction with 45

12 two pirani gauge heads, one fixed over the rotary vane pump pipeline and the other on the sputtering chamber. The pirani gauge is used to measure both the roughing and the backing pressure and is in the range of 0.5 mbar to 1 x 10-3 mbar. The analog penning gauge measures the fine pressure inside the chamber which is in the order of 1 x 10-3 mbar to 1 x 10-6 mbar. The substrate heater is sandwiched with two stainless steel hot plates each with 5 diameter and feed Vacuum pumps & Gauges While depositing thin films, vacuum is essential inside the sputtering chamber in order to increase the mean free path of the sputtered atoms and also to reduce the surface and bulk contaminations. Gases behave differently in two regimes of pressure which should be carefully studied while working with vacuum units. The first one is the viscous flow regime of pressure in which the gases flow as a fluid and the mean free path of the gas molecules is very small compared to the size of the vacuum chamber. The second one is the high vacuum molecular flow regime in which the mean free path of the gas molecules is greater than the dimensions of the sputtering chamber. Vacuum is needed during deposition for protecting the vapor source from oxidation and corrosion. Also in vacuum deposition techniques, the structure and properties of the thin films depend on the vacuum developed inside the chamber, the residual gases and their partial pressure. The different levels in vacuum are listed below: Low vacuum : Torr Medium vacuum : Torr High vacuum : Torr Very high vacuum : Torr Ultra high vacuum : Below 10-9 Torr The pumps used to create these levels of vacuum and the gauges used to measure them are different and in this section these are elaborately discussed. (i) Oil Sealed Rotary Vane Pump Another name for rotary mechanical pump is roughing pump as it can t create vacuum below 100 mtorr. To generate the necessary fore-vacuum for reaching high vacuum, oil sealed rotary pumps are required. The schematic diagram of a rotary pump is shown in Figure 2.8. Rotary vane pumps are mostly used to generate vacuum in medium-sized vacuum systems than rotary piston pump as the former can displace gases in the rate of m 3 /h[35]. The working principle of the sliding vane rotary pumps is explained as below. An eccentric rotor (A) rotated by a keyed shaft is used for gas transport. After one 46

13 cycle of rotation, the gas is isolated from the inlet and during the next cycle, it is compressed and exhausted. The rotor is mounted eccentrically in a stator (B) and it contains two spring-loaded diametrically sliding vanes which are used to press it against the inner surface of the stator. The pumping speed S of the rotary pump is given as S = 2Vn where V is the volume between the two sliding vanes and n is the number of rotations per unit time[36]. The speed of rotation is in the range of rpm for the rotary pumps that use speed reduction pulley whereas it is in the range of rpm for those with direct drive pump. An oil film with low vapor pressure serves to seal the vanes and the area between the rotor and housing and also the small gap at the seat in order to reduce friction, wear and tear. The temperature of this oil is higher at around 80 C in the case of direct drive pumps whereas it is in the range of 60 C for low speed pumps. Smaller size rotary pumps are air cooled and larger ones are water cooled. Care must be taken not to pump a large amount of water, acetone or any other vapors which can be condensed during pumping operation. Otherwise these condensing vapors will condense during the compression process and thereby will contaminate the fluid before the exhaust valve opens. And if it is permitted for a long run, formation of gum will happen on the moving parts of the pump which will eventually cause the pump to seize. (ii) Diffusion pump To achieve and maintain higher vacuum in sample chambers than is possible by use of positive displacement pumps, diffusion pumps are used. They are called so because vacuum is developed in these pumps by the diffusion of air molecules in to the active zone of the pump where trapping and removal of them occur. A forevacuum of about 5 to 10 Pa is essential for an oil diffusion pump to operate and in such type of pumps[35], oil of low vapor pressure is used for pumping gas molecules. The schematic of oil diffusion pump with its main components is shown in Figure 2.9. A gate valve isolates the oil diffusion pump from the chamber and normally a mechanical pump such as rotary pump is used to generate the initial necessary vacuum. In an oil diffusion pump, oil is boiled at the base and thus hot oil vapor is produced and it is allowed to rise through a funnelshaped baffle set and the supersonic jet of vapor is directed towards the sides of the pump which are kept cool by circulating cold water in tubes surrounding the upper portion of the pump[36]. When air molecules diffuse into this portion of the pump, collision of them with the vapor molecules occur in which trapping of air molecules results. By the circulating cold water, the oil vapor is cooled down and it condenses and moves down towards the bottom portion of the pump. The heater placed at the bottom of the pump body heats the condensed oil and thus reboiling of oil happens. In this process, air 47

14 molecules are released through the diffusion pump outlet and they are compressed to ambient pressure by the mechanical forepump and exhausted. Oil diffusion pumps can generate a working pressure of about 1 x 10-5 Torr inside the chamber. When modern fluids and accessories are properly used, a pressure approaching mbar can also be achieved in such type of pumps. In this oil diffusion pumps, the speed of pumping for all types of gases is high and also the cost per unit pumping is low for achieving high and ultra high vacuum when compared with other types of pumps. Also there are no moving parts in this pumps which makes them more durable and reliable than the turbomolecular and cryopumps. The trend of backstreaming of oil into the vacuum chamber and thereby contaminating the surfaces inside the chamber is the main drawbak of this pumping system. Carbonaceous or siliceous deposits may also result if hot filaments inside the chamber make contact with this backstream of oil. (ii) Pirani Gauge Pirani gauge is used to measure the pressure obtained by a rotary pump. It is used to measure both fore vacuum and roughing vacuum with its two gauge heads. Its working rule is based on the variation of thermal conductivity of the gas which depends on the number of gas molecules present in the vacuum system and this, in turn depends on the pressure of the system. The diagram of a pirani gauge is shown in Figure The system contains a constant voltage electrically heated filament whose heat loss varies with change in pressure as explained above. As the temperature coefficient of resistance of the filament in the Pirani gauge head is very high, even a slight change in the pressure of the vacuum system will result in a big change in resistance and as this filament forms one arm of a Wheatstone bridge, the out of balance current can be read as pressure on a meter[36]. When the filament is filled with contaminants, the gauge head must be washed with acetone and then it should be dried. Also, to remove the filament deposits, 10V dc is applied across it. These measures taken on the filaments help the gauge to behave consistently. (iv) Penning Gauge Vacuum in the range of 1 Pa to 10-7 Pa inside the chamber can be measured using Penning gauge and this low pressure can be measured in two different ranges with the help of an instant range changing toggle switch. The Penning gauge contains a cold cathode ionization gauge with two electrodes as anode and cathode and its diagram is shown in Figure A current limiting resistor is used to apply a potential difference of 2.3 kv between the anode and the cathode in the gauge head. An 800 gauss permanent magnet is placed in such a way that the magnetic field introduced by it should be at right angle to the plane containing the electrodes in the gauge and this introduction is to raise the ionization current. Due to the presence of this magnetic field, the electrons emitted from the cathode of the gauge head take helical path before reaching the anode and thus 48

15 the chance of collision with the gas molecules even at low pressure is improves a lot. Due to ionization, secondary electrons are produced and the ionization rate also increases rapidly. These electrons are captured by the anode and when the number of electrons produced per second is equal to the sum of positive ion current to the cathode and the electron current to the anode, equilibrium is reached and is used to measure the low pressure of the gas. When the electrode surface of the Penning gauge is contaminated, the discharge current reduces and this causes the gauge to indicate a low pressure. These gauges are long live and rugged when compared with other types of gauges. Also the electrodes are very strong and they don t posses filaments and thus they are free from breakage and burn out of filaments. The damages caused by the sudden overwhelming inward flow of air and vibrations are limited in these gauges. The gentle rubbing away by means of friction is sufficient to clean the electrodes without damaging them[37] Target preparation Preparation of V 2 O 5 target In this study, 30 gm of V 2 O 5 powder of 99.9% purity was grinded to prepare a fine powder of V 2 O mg of polyvinyl alcohol was dissolved in 100 ml of distilled water and the solution was used as a binder by adding 5 drops of it for every half an hour in to the powder V 2 O 5 during the grinding process. The powder was then subjected to a 60 ton hydraulic pressure to get a circular shape target of 6mm in thickness and 50mm in diameter with the help of a circular steel piston and a ring. The target was then removed from the holder and it was sintered to 300 C for about 2 hrs to tighten the target material ie to create a solid disc from the compressed V 2 O 5 powder. The target disc was allowed to cool for a day and was then mounted over the steel platform inside the chamber and was fixed in place with the help of screws. Another V 2 O 5 target disc similar to the above one was also prepared for depositing thin films under different sputtering parameters Preparation of mixed V 2 O 5 and CeO 2 targets Powder mixture of V 2 O 5 and CeO 2 in a molar ratio of 1:1 was taken by adding gm of V 2 O 5 and gm of CeO 2 and the mixture was grinded for three days. In this preparation also polyvinyl alcohol solution was used as the binder. The powder mixture was then subjected to sintering at 600 C for 5 hours. The XRD pattern of the powder mixture revealed that the mixture possessed 76% of CeVO 4 and 23% of V 2 O 5 and there was no evidence of CeO 2 peaks. To remove the V 2 O 5 phase from the mixture, it was again subjected to sintering at 600 C for 5 hours and now the mixture possessed 80% of CeVO 4 and 20% of V 2 O 5. The powder was sintered again and again for removing the V 2 O 5 phase from the mixture and after 5 trials, a powder mixture with CeVO 4 W (Wakefieldite) phase was obtained. A target of 6mm in thickness and 50mm diameter was obtained by subjecting it to a 60 ton hydraulic pressure with the help of a circular 49

16 steel piston and a ring. It was then sintered to 300 C for about 2 hrs. V 2 O 5 and CeO 2 powder mixture in a molar ratio of 2:1 was obtained by adding gm of V 2 O 5 and gm of CeO 2 and grinding of this mixture was undertaken for two days and was then sintered to 600 C for 5 hours. The mixture possessed 49% of CeVO 4, 18% of CeVO 3 and 33% of V 2 O 5 which was revealed by studying the peaks formed in the XRD patterns. By subjecting it to 5 times sintering at 600 C for 5 hours, a mixture with pure CeVO 4 phase was obtained. The pressed powder mixture with 6mm thickness and 50mm diameter was the fourth target which was then sintered to 300 C for about 2 hrs. To make the fifth target which should be from the powder mixture of V 2 O 5 and CeO 2 in a molar ratio of 1:2, gm of V 2 O 5 and gm of CeO 2 powders were taken. The powder mixture was then grinded for three days and was sintered to 600 C for 5 hours. The mixture possessed Wakefieldite phase of CeVO 4. The final target of 6mm in thickness and 50mm diameter was obtained by subjecting it to a 60 ton hydraulic pressure and then it was sintered to 300 C for about 2 hrs Preparation of various substrates The substrates used in this work to deposit thin films are the Corning glass 7059 with dimension 8 cm x 6 cm. The following methods were adopted for cleaning the glass substrates. (i) Chemical treatment: This technique involves cleaning the substrate by subjecting it to chemical treatment and in this process, the surface is cleaned from the dust and other particles such as organic and inorganic contaminants. Contaminants such as grease and other oxide materials can also be removed effectively from the surface by using this method. (ii) Ultrasonic cleaning: Ultrasound ( khz) is used by the ultrasonic cleaner to clean delicate item such as glass plates and usually the cleaning solvent will be the distilled water. The high frequency ultrasonic sound waves are generated by an ultrasound generating transducer which is lowered into the fluid stored in a stainless steel tank inside which the substrates are piled side by side. The generated ultrasonic sound waves are used to agitate the fluid which induces cavitation bubbles. When these millions of microscopic bubbles act on the contaminants such as dust, dirt, oil, pigments, grease, polishing compounds, flux agents, fingerprints, soot wax and mold release agents adhering to the substrates, these are removed thoroughly from the surface of the substrate due to the intense disturbance caused by them. In our present study, the substrates were initially washed in soap solution by gently scrubbing the surface of the substrate by the cotton pieces soaked in soap solution. Then 50

17 the substrates were rinsed in running water and then in deionized water thoroughly so that the traces of soap solution could not be found over the surface. The substrates were then immersed in a chromic acid bath and were heated for half an hour. This is for dissolving the fine silica layer formed over the surface of the substrate. Then the substrates were rinsed well with the deionized water and then dried with acetone. Finally the fine dusts on the surface were removed by subjecting the substrates to ultrasonic cleaning. The other substrates used for deposition of V 2 O 5 and Ce-V mixed oxide thin films are SnO 2 :F coated glasses, ITO coated glasses and p-type Si wafers with (100) orientation and 50 Ωcm resistivity. The SnO 2 :F coated glass, ITO coated glass substrates were submitted to the ultrasonic bath and the Si wafers were stored in a clean room and washed with ethanol before deposition Thin Film deposition Thin films were deposited in our work by using a HINDIVAC Planar magnetron RF/DC sputtering unit, Model-12 MSPT, Bangalore with MHz as working frequency. In the chamber, the diameter of the top electrode is 10 cm and that of the bottom electrode on which the target is mounted, is 7 cm and the distance between the two is 6 cm. The V 2 O 5 target with 6mm in thickness and 50mm in diameter is mounted on the stainless steel backing plate and the different substrate plates are fixed on the substrate table which is placed vertically above the target holder. A shutter is available in the space between the target and the substrate for controlling the thickness of the thin film prepared during deposition and also for preventing the target from contamination while loading or unloading the substrate. Initially, the cool water is allowed to flow through the walls of the chamber and the high vacuum valve should be in closed position. The rotary pump is switched on and the combination valve is turned to backing position so that a low pressure of 0.05 mbar should be obtained on the pipelines which can be studied by the pirani gauge selected in GH-1 position. Now the roughing position is selected and a pressure of 0.05 mbar is developed in the chamber which can be monitored by the GH-2 position of the pirani gauge. In backing position, the diffusion pump is turned on and the high vacuum valve is open after 45 minutes which is the time needed for the silicon oil of the pump to evaporate. When the pressure inside the chamber is less than 10-6 mbar, high vacuum is developed inside the chamber and the deposition process begins with applying the sputtering argon gas inside the chamber. The sputtering gas pressure is maintained as 1.33 x 10-2 mbar and it is a non reactive sputtering. The RF power is maintained as 100 W and the sputtering process is admitted for 30 minutes. Similar method is followed for preparing V 2 O 5 thin films in room temperature by changing the RF power as 125 W, 150 W, 175 W and 200 W. Another set of V 2 O 5 thin films were prepared by varying the substrate temperature from 100 C to 300 C in steps of 50 C, by keeping the RF power at 150W. In order to study the annealing effects on the as deposited thin films, all samples were heat treated at 400 C for 5 hrs. The above mentioned procedure is adopted 51