MAGNETRON SPUTTERED METAL NITRIDE THIN FILMS FOR DEVICE APPLICATIONS

Abstract

Metal nitride materials show enhanced performance in terms of physical and chemical properties as compared to their metal counterparts. In last decade, the thin films based on chromium nitride (CrN), titanium nitride (TiN), tungsten nitride (WN) and aluminium nitride (AlN) material have been studied extensively due to their wide range of potential applications such as protective coating, gas sensing, charge storage, diffusion barrier and memory storage. Moreover, the main reason for rising importance of metal nitride thin films is tailoring the properties by optimizing the thickness and composition. However, some binary metal nitrides are suggested as protective coating for cutting tools but those are not sufficient to fulfil the future demands. Therefore, ternary metal nitride nanocomposite thin films fabricated by addition of other metal into binary nitride matrix have attracted a great attention as refractory materials due to their wide spectrum of mechanical and electronic properties. In last couple of years, several advanced memory storage concepts have been evolved, but due to possessing tuneable electrical properties the resistive switching of metal nitride thin films is promising approach in terms of data storage. Recently, hierarchical nanostructures such as nanoball and nanowire of metal nitride thin films have been utilized for gas sensing applications due to providing high surface area. The main objective of the thesis is to synthesize the various metal nitride thin films such as aluminium nitride (AlN), tungsten nitride (WN) and chromium tungsten nitride (CrWN) using DC magnetron sputtering technique. Thereafter, the resistive switching properties of AlN and WN thin films were investigated for non-volatile memory application in detail. Subsequently, the room temperature hydrogen gas sensing properties of palladium (Pd) capped WN thin film fabricated on porous silicon (PSi) substrate were also examined. The structural, corrosion and mechanical properties of CrWN nanocomposite thin films were investigated for industrial application such as protective coating for cutting tools. This thesis is organized into six chapters. The summary of each chapter is discussed below: Chapter 1 begins with historical overview and very incisive literature survey on the synthesis and properties of selected metal nitride thin films such as aluminium nitride (AlN), tungsten nitride (WN), chromium nitride and chromium tungsten nitride (CrWN). The chapter also provides essential information to understand the non-volatile memory storage, hydrogen gas sensor and protective coatings on cutting tools. ii Chapter 2 present the details of the experimental techniques which have been employed for the synthesis and characterization of several metal nitride thin films. All thin film samples were fabricated using reactive DC magnetron sputtering technique. Various characterization techniques, such as X-rays diffraction (XRD), Raman Spectroscopy, X-Ray Photoelectronic Spectra (XPS), Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) have been discussed in detail. Electrical properties of the aluminium nitride and tungsten nitride thin films were studied using Keithley 4200 semiconductor characterization system (SCS). Hydrogen gas sensing properties of nanoporous W2N thin films were characterized using a custom made sensing setup. The Nanoindentation was used to study the mechanical properties such as hardness (H), elastic modulus (E), plasticity index (H/E) and resistance to plastic deformation (H3/E2) of CrWN nanocomposite thin films. In addition, a three electrode potentiostat was used to measure the corrosion rate of CrWN nanocomposite thin films. Chapter 3 is divided into two sections. Section 3.1 describes highly stable and bipolar resistive switching (RS) behaviour of aluminium nitride (AlN) thin film sandwiched between Cu (top) and Pt (bottom) electrodes. Resistive switching properties in Cu/AlN/Pt structure are induced by the formation/disruption of Cu conducting filaments in AlN thin film. Excellent non-volatile resistive switching characteristics have been observed at the voltage of + 2.6 V and - 1.7 V. Trap controlled space charge limited current (SCLC) and ohmic behaviour are the dominant conduction mechanisms at high resistance state (HRS) and low resistance state (LRS) respectively. The resistance ratio between HRS and LRS is found to be of the order of ~ 104. Moreover, the Cu/AlN/Pt structure also exhibited endurance upto > 104 cycles and a non-volatile retention time for > 104 sec. Section 3.2 illustrates the influence of top electrode (TE) material on resistive switching properties of DC magnetron sputtered tungsten nitride (WN) thin film in TE (Ti, Al and Cu)/WN/Pt stack configuration. The Ti/WN/Pt and Al/WN/Pt structure exhibit two resistance states i.e. LRS and HRS, which were caused by formation and rupture of nitrogen vacancy related ionic filaments. Formation of additional Cu filaments in Cu/WN/Pt configuration is responsible for three resistance states (or say multilevel) switching. This study suggests that the electrode engineering of tungsten nitride thin film have potential for non-volatile and multilevel resistive memory application. Chapter 4 depicts the room temperature hydrogen (H2) gas sensing performance of the palladium modified tungsten nitride (Pd/W2N) nanoballs (NBs) grown on the porous silicon substrate using reactive DC magnetron sputtering. Porous silicon substrate has attracted iii enormous amount of consideration for gas sensing application due to its high reactive surface morphology. Reversible change in resistance was observed during hydrogenation and dehydrogenation process at room temperature. The H2 gas sensing performances together with sensing mechanism of the Pd/W2N NBs structure under low sensing range (5-500 ppm) were discussed in detail. Moreover, the prime requirements for sensing performance including stability, reproducibility and selectivity measurement are also studied at room temperature. Chapter 5 describes the growth of chromium tungsten nitride (Cr1-xWxN) nanocomposite thin films on silicon (100) substrate using reactive magnetron co-sputtering. X-ray diffraction pattern of Cr1-xWxN thin films reveal the presence of (111) and (200) orientation for different tungsten concentrations (0 < x < 0.61). In Cr1-xWxN thin films, the atomic concentration (x) of tungsten (W) was controlled by varying the DC power on the W target. It is observed that, the addition of small amount of W atoms led to significant changes in the structural, electrochemical and mechanical properties of the Cr1-xWxN films. The crystallite size of Cr1-xWxN (0 < x < 0.61) thin films varies from 31.1 nm to 15.2 nm due to variation in nucleation rate and self-shadowing effect during deposition process. Electrochemical properties of these thin films were studied by Tafel polarization curves. The results show that, the addition of a certain amount of W atoms enhanced the corrosion rate which may be due to higher ratio between real surface area and projected area. Hardness of the Cr1-xWxN thin films tends to increase with the decrease in grain size in accordance with the Hall-Petch relation. For Cr0.48W0.43N thin film, the highest hardness of 43.18 GPa and elastic modulus of 341.02 GPa were achieved at the grain size of 15.2 nm. Chapter 6 describes major conclusions drawn after thorough discussion and in-depth analysis that presented in individual chapters. A brief report on the scope for future work is also included.