CARBON NITRIDE DEPOSITION BY MAGNETRON SPUTTERING: STRUCTURAL, MECHANICAL, ELECTRICAL AND OPTICAL PROPERTIES Presented by MIGUEL ALBERTO MONCLUS PALAZON as the fulfilment of the requirement for the degree of DOCTOR OF PHILOSOPHY at DUBLIN CITY UNIVERSITY SCHOOL OF ELECTRONIC ENGINEERING Research supervisor Dr. David Cameron February 2000 Caminante no hay canino, se hace amino al andar. Antonio Machado DECLARATION I declare that all the unreferred work described in this thesis is entirely my own, and no portion of the work contained in this thesis has been taken from the work of others save and to the extent that such work has been cited and acknowledged within the text of my work. SIGNED: Date 2-2 ~ 2' < 9 0 I.D. Number S' ^ O ' M ACKNOWLEDGEMENTS Firstly, I’d like to thank my supervisor Dr. David Cameron. I am extremely grateful for all the advice and suggestions given towards the problems solved. His guidance, especially during the hard times I went through kept me going until the end. To Shaestagir Chowdhury, with whom I worked most of the time, and as a result our team work produced very valuable results. Also thanks to Dr. Robert Barkley and Michael Collins from Trinity College for their help with some measurements. To the technicians in the electronic engineering department: Conor Maguire, Liam Meany, Robert Clare, Stephen Neville and Paul Wogan for providing me with the required supplies and for their kindness. My sincere thanks are expressed to the technicians in the mechanical engineering department and the mechanical workshop. I’m particularly grateful to Liam Domican for his technical support. I’d also like to thank Dr. Pat McNally, for his continuous advice and support. Thanks to old and present fellow research students in D.C.U.- Michael Murphy, Emmet Caulfield, John Curley, Colm O’Leary, Ivonne Kavanagh, Jahangir Alam, David Neary, Donnacha Lowney, Jarujit Kanatharana and countless others; I appreciate their help and support. A Suzanne Carey, por sus animos de guays. Also to Denis Dowling and Kevin Donelly in Enterprise Ireland, for their valuable advice and for trusting me in the use of the Nanoindenter. Finally, my most sincere gratitude is extended to my parents, who have given me their utmost support to all that I have tried to do. I can’t ever repay them enough. Mama y papa: muchisimas gracias por todo. Also to my brother and sisters, for always supporting me along the way. Thank you. Miguel A. Monclus Palazon, February 2000. II CARBON NITRIDE DEPOSITION BY MAGNETRON SPUTTERING: STRUCTURAL, MECHANICAL, ELECTRICAL AND OPTICAL PROPERTIES. ABSTRACT Carbon nitride films were deposited using a magnetron sputtering technique based on the Penning type geometry. For deposition, graphite targets were used in a nitrogen/argon gas mixture. The technique employed consists of two opposing cathodes, with two very strong magnets placed behind them. The magnetic field created with this configuration, in conjunction with the electric field, provides a high ion flux at the substrate, which results in high deposition rates (up to 3fim/h) and increased nitrogen incorporation in the films (up to 45 at.% N). The effect of deposition parameters, such as nitrogen partial pressure (Npp), substrate bias, and deposition temperature on the film properties has been investigated. The effect of nitrogen addition on the structural properties of carbon nitride films has been characterised in terms of its composition, infrared and Raman spectra and X-ray photoelectron spectroscopy (XPS). Nitrogen content in the films was measured by Rutherford Backscattering (RBS) and film thickness by a surface profilometer. Infrared spectra revealed that the films have an amorphous structure, in which nitrogen is mainly incorporated in C=N sp2-type bonding configurations, with some proportion of O N sp-type bonding, which increases with nitrogen incorporation. Above 20 at.% N, a structural rearrangement occurs in which N preferentially bonds to itself. Core level XPS peaks due to carbon and nitrogen Is electrons were assigned to different types of bonds in accordance with Raman and IR spectra and by comparison with other assignments found in literature. A Nanoindenter instrument was employed to investigate the mechanical properties of the films. Nanoindentation load-displacement curves provide a 'mechanical fingerprint' of the material’s response to contact deformation. Hardness and elastic modulus values obtained for carbon nitride films using the Oliver and Pharr technique ranged from 8-12 GPa and 90-120 GPa respectively. Hardness was shown to decrease with Npp but was not dependent on total nitrogen content. Hardness was better correlated to the O N bond concentration; the increased amount of O N bonds causes a weakened structure as they serve as network terminators in the carbon backbone structure. The hardness of the films correlates with the presence of a graphite-like structure dominated by sp2 bonding structure. The effect of substrate bias, annealing and deposition III temperature on the film’s hardness and structure is also described. Higher coefficients of friction were found for harder films deposited at different substrate bias. The stress is shown to be concentrated at the film-substrate interface whereas the bulk of the film is stress-free and increases at higher substrate bias as ion bombardment increases. The XPS and ultraviolet photoelectron spectroscopy (UPS) valence band (VB) spectra have been reported. The evolution of bands corresponding to jt and a bonding with Npp reveals that there is a greater degree of sp2 bonding as the nitrogen partial pressure (Npp) is initially increased compared to the pure carbon samples. However, with further increase in Npp (reaching 100%), the bonding becomes more sp3-like. The films’ electronic states were further investigated as a function of Npp using techniques such as electron energy loss spectroscopy (EELS) and electron spin resonance spectroscopy (ESR). The observations reported point to the reasons why crystalline (5-C3N4 material has been found only with high Npp despite the fact that nitrogen content is not significantly enhanced in this situation. Various electrical analysis techniques were employed to study the electrical properties of carbon nitride films. The four-point probe and van der Pauw methods were used for resistivity measurements. From these analyses, the resistivity was found to increase with Npp, and to be controlled by the amount of C=N bonding, which causes a decrease in the number density of electrons available for conduction. N is incorporated in the films in a non-doping configuration. From temperature-dependent resistivity measurements, it was proposed a transition from metallic to semiconducting behaviour as nitrogen is incorporated in the films. A conduction mechanism typical of low mobility amorphous semiconductors was suggested. An increase in both negative substrate bias and deposition temperatue produce films with lower resistivity. The optical gap was estimated from absorption coefficient measurements in the ultraviolet-visible region. Refractive index and extinction coefficient measurements are also reported. An electron emission configuration revealed that it is possible to obtain moderate emission current from a number of carbon nitride films. Despite the fact that the analysis techniques considered here give results consistent with an amorphous carbon nitride solid, there is some evidence of crystalline P-C3N4 areas co-existing with the mainly amorphous structure for films deposited at 100% Npp. There is room for further optimisation of the quality of carbon nitride films deposited by reactive Penning-type magnetron sputtering with respect to composition, structure and application-relevant properties. IV CONTENTS DECLARATION I ACKNOWLEDGEMENTS II ABSTRACT III CHAPTER 1 - INTRODUCTION 1 1.1 General overview 1 1.2 Carbon nitride compounds - possible structures and properties 4 1.3 General review of carbon nitride deposition processes 10 1.3.1 PVD vs. CVD deposition techniques 10 1.3.2 Deposition techniques used for carbon nitride 11 1.4 Amorphous carbon nitride semiconductors 18 1.5 Organisation of the thesis 21 References 22 CHAPTER 2 - CARBON NITRIDE DEPOSITION: SPUTTERING TECHNIQUE 27 2.1 Principles of sputtering 27 2.2 Applications of sputtering 31 2.2.1 Sputter etching 31 2.2.2 Sputter deposition 32 2.3 Typical deposition system 32 2.3.1 RF sputtering 34 2.3.2 Reactive sputtering 34 2.3.3 Magnetically enhanced sputtering 35 2.3.4 Bias sputtering 35 2.4 Magnetron sputtering systems 36 2.4.1 Types of magnetrons 37 2.4.2 Summary on magnetron sputtering 39 2.5 Thin film growth 39 2.6 Film microstructure 42 2.6.1 Deposition Parameters affecting film microstructure 42 2.6.2. Structure Zone Model 44 2.6.3 Stress 46 2.6.4 Substrate effects 47 2.6.5 Adhesion 47 2.7 Conclusions on the sputtering technique 48 2.8 The use of the sputtering technique for carbon nitride deposition 49 References 53 CHAPTER 3 - PENNING TYPE MAGNETRON SPUTTERING SOURCE AND ITS USE FOR CARBON NITRIDE DEPOSITION 55 3.1 Introduction 55 3.2 Penning type sputtering source 56 3.2.1 Physical Description 57 3.2.2 Plasma Characteristics 59 3.2.3 Operating Characteristics 62 3.3 High vacuum system description 63 3.4 Carbon nitride deposition 69 3.4.1 Experimental details 69 3.4.1.1 Substrate types and cleaning 69 3.4.1.2 Carbon nitride films series 70 3.4.2 Substrate heater 72 3.4.2.1 Substrate heating configuration 72 3.4.2.2 Substrate temperature 74 3.5 Summary 75 References 76 CHAPTER 4 - STRUCTURAL