The development of magnetic tunnel junction fabrication techniques Clifford Alastair Elwell Darwin College, Cambridge. A dissertation submitted for the degree of Doctor of Philosophy at the University of Cambridge July 2002 The development of magnetic tunnel junction fabrication techniques The discovery of large, room temperature magnetoresistance (MR) in magnetic tunnel junctions in 1995 sparked great interest in these devices. Their potential applications include hard disk read head sensors and magnetic random access memory (MRAM). However, the fabrication of repeatable, high quality magnetic tunnel junctions is still problematic. This thesis investigates methods to improve and quantify the quality of tunnel junction fabrication. Superconductor-insulator-superconductor (SIS) and superconductor-insulator- ferromagnet (SIF) tunnel junctions were used to develop the fabrication route, due to the ease of identifying their faults. The effect on SIF device quality of interchanging the top and bottom electrodes was monitored. The relationship between the superconducting and normal state characteristics of SIS junctions was investigated. Criteria were formulated to identify devices in which tunneling is not the principal conduction mechanism in normal metal-insulator-normal metal junctions. Magnetic tunnel junctions (MTJs) were produced on the basis of the fabrication route developed with SIS and SIF devices. MTJs in which tunneling is the principal conduction mechanism do not necessarily demonstrate high MR, due to effects such as magnetic coupling between the electrodes and spin scattering. Transmission electron microscope images were used to study magnetic tunnel junction structure, revealing an amorphous barrier and crystalline electrodes. The decoration of pinholes and weak-links by copper electrodeposition was investigated. A new technique is presented to identify the number of copper deposits present in a thin insulating film. The effect of roughness, aluminium thickness and voltage on the number of pinholes and weak-links per unit area was studied. High frequency testing of read heads at wafer level was performed with a network analyser. Design implications for read head geometry were investigated, independent of magnetic performance. This technique has great potential to aid the rapid development of read and write heads whilst improving understanding of the system. ii Preface This dissertation, which is submitted for the degree of Doctor of Philosophy at the University of Cambridge, describes work carried out from October 1998 to July 2002 in the Department of Materials Science and Metallurgy, University of Cambridge. Except where specific reference is made, this is entirely the result of my own work and includes nothing that is the outcome of work done in collaboration. No part of this work has been or is being submitted for any other qualification at this or any other university. Clifford Alastair Elwell Device Materials Group, Dept. Materials Science, University of Cambridge July, 2002. Acknowledgements This work would not have been possible without the support of a number of people. I would firstly like to thank my supervisor, Dr Mark Blamire for all his help, support, generosity and ideas over the years. I would also like to thank the head of the research group, Prof. Jan Evetts for use of the research facilities and some useful discussion. Dr Zoe Barber has provided practical help and discussion with sputtering. Dr Gavin Burnell deserves particular mention, due to the generous time he has spent with me over the last four years. His programming skills, practical help and proof reading have been invaluable. I would like to thank Seagate Technology for providing the CASE award and the EPSRC for my studentship. Thanks to Dr Alan Johnston for a great deal of help and support during the 3 months I spent in Londonderry. I’d also like to thank Paul Scullion, Robert Lamberton, William O’Kane, and Adrian Marsh, for their help. Thanks to everybody in Springtown for the craic, in addition to the above, Peter, Niru, Declan, Colin and Dennis. iii I was also grateful to receive wafers for testing, courtesy of Seagate and, in particular, Patrick Doherty. All my proof readers have been great: Frances Gibson, Karen Yates, Mike Gibson, Angus Bryant, Angela MacKay, Gemma France, Mike Elwell and John Durrell. In the words of Socrates: “The unexamined life is not worth living.” I’d like to thank everyone in the lab for their help and company over the last four years. In particular, thanks to Richard Moseley and Wilfred Booij for a great deal of help in the cleanroom when I arrived in the lab. Thanks also to Noel Rutter, Phil McBrien, Neil Todd, Robert Kinsey, Jose Prieto, Nadia Stelmashenko and John Durrell for general assistance. Thanks to Chris Bell for the FIB work and Stephen Lloyd for TEM images. My office-mates for three years deserve mention for their stimulating discussion… Mike Hogg, Ashish Garg, Yee Cheng and Moon-Ho Jo. Needless to say, my friends and family have provided immeasurable support over the last 4 years. Sincere thanks to all of you – this PhD would not have been possible without you. Particular thanks to Frances who has experienced all the highs and lows of my PhD and given unstinting support. Thanks to my parents for everything – they could not have been better. iv To Frances and to my parents. v Table of Contents Chapter 1:.......................................................................................................1 Introduction....................................................................................................1 1.1 Motivation for MTJ development ....................................................................2 1.1.1 Read head principles and technology.......................................................3 1.1.2 Motivation for this thesis...........................................................................6 1.2 The form of this thesis .......................................................................................6 Chapter 2:.......................................................................................................8 Theoretical introduction...............................................................................8 2.1 Tunneling............................................................................................................9 2.1.1 One dimensional rectangular barrier ......................................................9 2.1.2 Normal metal - insulator - normal metal junctions..............................14 2.1.3 Superconductivity – a brief review.........................................................16 2.1.4 Superconductor-insulator-ferromagnet.................................................19 2.1.5 Superconductor-insulator-superconductor...........................................26 2.2 Magnetism ........................................................................................................27 2.2.1 Basic magnetism reviewed ......................................................................27 2.2.2 Ferromagnetism.......................................................................................28 2.2.3 Magnetic tunnel junctions.......................................................................36 2.3 Summary...........................................................................................................39 Chapter 3:.....................................................................................................40 Experimental methods.................................................................................40 3.1 Substrate choice and preparation...................................................................41 3.2 Deposition of thin films....................................................................................42 3.2.1 Deposition system.....................................................................................42 3.2.2 Evacuation of the system.........................................................................44 3.2.3 Gas pressure .............................................................................................45 3.2.4 Deposition rate calibration......................................................................46 3.3 Processing .........................................................................................................46 3.3.1 Photoresist ................................................................................................48 3.3.2 Optical lithography..................................................................................48 3.3.3 Ion milling.................................................................................................51 3.3.4 Silica deposition........................................................................................52 3.3.5 Contact pad deposition............................................................................53 3.4 Testing...............................................................................................................54 3.4.1 Film characterisation...............................................................................55 3.4.2 Device characterisation ...........................................................................57 3.5 Nomenclature ...................................................................................................62 3.6 Summary...........................................................................................................62