Deposition and Characterization of Magnetron Sputtered Beta-Tungsten Thin Films Jiaxing Liu Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences COLUMBIA UNIVERSITY 2016 © 2016 Jiaxing Liu All Rights Reserved ABSTRACT Deposition and Characterization of Magnetron Sputtered Beta-Tungsten Thin Films Jiaxing Liu β-W is an A15 structured phase commonly found in tungsten thin films together with the bcc structured W, and it has been found that β-W has the strongest spin Hall effect among all metal thin films. Therefore, it is promising for application in spintronics as the source of spin- polarized current that can be easily manipulated by electric field. However, the deposition conditions and the formation mechanism of β-W in thin films are not fully understood. The existing deposition conditions for β-W make use of low deposition rate, high inert gas pressure, substrate bias, or oxygen impurity to stabilize the β-W over α-W, and these parameters are unfavorable for producing β-W films with high quality at reasonable yield. In order to optimize the deposition process and gain insight into the formation mechanism of β-W, a novel technique using nitrogen impurity in the pressure range of 10-5 to 10-6 torr in the deposition chamber is introduced. This techniques allows the deposition of pure β-W thin films with only incorporation of 0.4 at% nitrogen and 3.2 at% oxygen, and β-W films as thick as 1μm have been obtained. The dependence of the volume fraction of β-W on the deposition parameters, including nitrogen pressure, substrate temperature, and deposition rate, has been investigated. The relationship can be modeled by the Langmuir-Freundlich isotherm, which indicates that the formation of β-W requires the adsorption of strongly interacting nitrogen molecules on the substrate. The dependence of β-W formation on the choice of underlayer materials has also been investigated. The β-W phase can only be obtained on the underlayer materials containing non- metallic elements. The dependence is explained by the existence of strong covalent bonds in β-W compared with that in α-W. The nickel and permalloy underlayers are the only exception to the above rule, and β-W has been successfully deposited on permalloy underlayer using very low deposition rate for spin-diffusion length measurement of β-W. The permalloy thin films usually take the (111) texture, since its (111) planes have the lowest surface energy. However, permalloy thin films deposited on β-W underlayer can achieve (002) texture using amorphous glass substrates. Therefore, the permalloy/β-W bilayer system can work as a seed layer for the formation of (002) textured films with fcc or bcc structure. The mechanism of the (002) texture formation cannot be explained by the existing models. The β-W to α-W phase transition was characterized by differential scanning calorimetry. The enthalpy of transformation is measured to be 8.3±0.4 kJ/mol, consistent with the value calculated using density functional theory. The activation energy for the β-W to α-W phase transformation kinetics is 2.2 eV, which is extremely low compared with that of lattice and grain boundary diffusion in tungsten. The low activation energy might be attributed to a diffusionless shuffle transformation process. Table of Contents List of Figures ............................................................................................................................... iv List of Tables .............................................................................................................................. viii Acknowledgements ...................................................................................................................... ix 1 - Introduction ............................................................................................................................. 1 1. 1 Introduction ................................................................................................................... 1 1.2 Organization of the Dissertation .................................................................................... 2 2 – Background and Motivation .................................................................................................. 4 2.1 Tungsten Thin Films in Electronics Industry ................................................................ 4 2.2 β-Tungsten Thin Films in Spintronics ........................................................................... 5 2.2.1 Properties of β-Tungsten Thin Films .................................................................. 5 2.2.2 Spin Hall Effect in β-Tungsten Thin Films ........................................................ 6 2.3 Deposition of Tungsten Thin Films ............................................................................. 10 2.3.1 Chemical Vapor Deposition .............................................................................. 10 2.3.2 Sputter Deposition ............................................................................................ 12 3 – Experimental Techniques ..................................................................................................... 15 3.1 Sputter Deposition ....................................................................................................... 15 3.1.1 Sputter Deposition Equipment .......................................................................... 15 3.1.2 Gas Impurity Monitoring .................................................................................. 17 3.1.3 Substrate Temperature Control ......................................................................... 18 3.2 Characterization Techniques ....................................................................................... 19 3.2.1 X-Ray Diffraction ............................................................................................. 19 3.2.2 X-Ray Reflectivity ............................................................................................ 21 3.2.3 X-Ray Photoelectron Spectroscopy .................................................................. 23 3.2.4 Transmission Electron Microscopy .................................................................. 25 3.2.5 Electrical Resistivity Measurement .................................................................. 31 3.2.6 Differential Scanning Calorimetry .................................................................... 33 i 4 – Deposition Conditions of β-W Thin Films .......................................................................... 36 4.1 Experimental Details ................................................................................................... 36 4.1.1 Sputter Deposition of Tungsten Thin Films...................................................... 36 4.1.2 Characterization of Tungsten Thin Films ......................................................... 37 4.2 Results ......................................................................................................................... 39 4.1.2 Film Composition Measurement....................................................................... 39 4.1.2 Relationship between β-W Formation and Deposition Conditions by XRD and Resistivity Measurement ................................................................................................ 45 4.1.3 Microstructure of β-W Thin Films by TEM ......................................................... 52 4.3 Discussion ................................................................................................................... 56 4.3.1 Advantages of β-W Deposition with Nitrogen Impurity .................................. 56 4.3.2 Modeling the Effect of Deposition Parameters on β-W Formation .................. 57 4.4 Summary ..................................................................................................................... 62 5 – Deposition of β-W on Other Underlayers and Substrates................................................. 64 5.1 Motivation ................................................................................................................... 64 5.2 Experimental Details ................................................................................................... 65 5.3 Results ......................................................................................................................... 67 5.3.1 Relationship Between Underlayer Materials and β-W Formation .................... 67 5.3.2 Deposition of β-W on Permalloy Underlayer ................................................... 69 5.4 Discussion ................................................................................................................... 72 5.4.1 Effect of Underlayer Crystalline Structure ....................................................... 72 5.4.2 Effect of Underlayer Chemistry ........................................................................ 73 5.5 Summary ..................................................................................................................... 76 6 – Formation of (002) Textured Permalloy on β-W ............................................................... 77 6.1 Motivation ................................................................................................................... 77 6.2 Experimental Details ................................................................................................... 78