Ultra-High-Resolution Optical Imaging for Silicon Integrated-Circuit Inspection Keith Andrew Serrels, MPhys (Hons) Submitted for the degree of Doctor of Philosophy Heriot-Watt University Ultrafast Optics Group School of Engineering and Physical Sciences (EPS) David Brewster Building Riccarton Campus EH14 4AS May 2009 The copyright in this thesis is owned by the author. Any quotation from the thesis or use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information. ABSTRACT This thesis concerns the development of novel resolution-enhancing optical techniques for the purposes of non-destructive sub-surface semiconductor integrated-circuit (IC) inspection. This was achieved by utilising solid immersion lens (SIL) technology, polarisation-dependent imaging, pupil-function engineering and optical coherence tomography (OCT). A SIL-enhanced two-photon optical beam induced current (TOBIC) microscope was constructed for the acquisition of ultra-high-resolution two- and three-dimensional images of a silicon flip-chip using a 1.55µm modelocked Er:fibre laser. This technology provided diffraction-limited lateral and axial resolutions of 166nm and 100nm, respectively - an order of magnitude improvement over previous TOBIC imaging work. The ultra-high numerical aperture (NA) provided by SIL-imaging in silicon (NA=3.5) was used to show, for the first time, the presence of polarisation-dependent vectorial- field effects in an image. These effects were modelled using vector diffraction theory to confirm the increasing ellipticity of the focal-plane energy density distribution as the NA of the system approaches unity. An unprecedented resolution performance ranging from 240nm to ~100nm was obtained, depending of the state of polarisation used. The resolution-enhancing effects of pupil-function engineering were investigated and implemented into a nonlinear polarisation-dependent SIL-enhanced laser microscope to demonstrate a minimum resolution performance of 70nm in a silicon flip-chip. The performance of the annular apertures used in this work was modelled using vectorial diffraction theory to interpret the experimentally-obtained images. The development of an ultra-high-resolution high-dynamic-range OCT system is reported which utilised a broadband supercontinuum source and a balanced-detection scheme in a time-domain Michelson interferometer to achieve an axial resolution of 2.5µm (in air). The examination of silicon ICs demonstrated both a unique substrate profiling and novel inspection technology for circuit navigation and characterisation. In addition, the application of OCT to the investigation of artwork samples and contemporary banknotes is demonstrated for the purposes of art conservation and counterfeit prevention. ii To my Family… For reminding me everyday that I can. iii ACKNOWLEDGEMENTS First of all, I would like to say a huge thank you to my academic supervisor Prof. Derryck Reid. His belief and confidence in me to demonstrate my potential within his group will not be forgotten. Thank you for your guidance, support, ideas and encouragement over the past 3 and a half years. You are an exceptional supervisor and mentor. Enormous gratitude must be given to Dr Euan Ramsay. His knowledge, advice and patience have been immensely valuable, not only in the lab, but also during our generally random coffee-break conversations. I would like to take this opportunity to wish both him and his family every success in the future. To all the members of the Ultrafast Group (past and present) for their support and helpful conversations – Euan Ramsay (experimental work and MATLAB simulations), Karl Tillman, Stuart Campbell, Reda El-Agmy, Łukasz Kornaszewski, Jinghua Sun, Barry Gale, Tobias Lamour and Teresa Ferrerio, as well as final year MPhys students Bernd Stark and Michael Renner (OCT and MATLAB development)…… it’s been a pleasure! To my family. Mum, Dad, Craig, Shaun, Nana Wilma, Scott, Nana Elsie, Grandad Harry, Christine, Davy, Conor and, of course, Tristin. Thank you all for your encouragement, support and love. I would not be where I am today without you. I would also like to thank all the academic and technical staff within the physics department who I have had the opportunity to work with – especially Martin Thomson for generating ASAP simulations and Henry Bookey for providing the PM-DS-HNLF. Their valuable contributions towards this work are greatly appreciated. Finally, to all the friends I have made within the department and beyond who have made me laugh and kept me sane. The old office and Friday lunchtime crowd - Ian Thomson, Ryan Warburton, Spyros Brown and Richard Moag. Apologies to anyone else I have forgotten to mention (you know who you are!) A special thanks to Marcel Leutenegger for the advice on vectorial-field modelling. iv ACADEMIC REGISTRY Research Thesis Submission Name : Keith Andrew Serrels School/PGI: Engineering and Physical Sciences Version: (i.e. First, Final Degree Sought: PhD / Physics Resubmission, Final) Declaration In accordance with the appropriate regulations I hereby submit my thesis and I declare that: 1) the thesis embodies the results of my own work and has been composed by myself 2) where appropriate, I have made acknowledgement of the work of others and have made reference to work carried out in collaboration with other persons 3) the thesis is the correct version of the thesis for submission and is the same version as any electronic versions submitted*. 4) my thesis for the award referred to, deposited in the Heriot-Watt University Library, should be made available for loan or photocopying and be available via the Institutional Repository, subject to such conditions as the Librarian may require 5) I understand that as a student of the University I am required to abide by the Regulations of the University and to conform to its discipline. * Please note that it is the responsibility of the candidate to ensure that the correct version of the thesis is submitted. Signature of Date: Candidate : Submission Submitted By (name in capitals): Signature of Individual Submitting: Date Submitted: For Completion in Academic Registry Received in the Academic Registry by (name in capitals): Method of Submission (Handed in to Academic Registry; posted through internal/external mail): E-thesis Submitted Signature: Date: v ABBREVIATIONS FA Failure Analysis IC Integrated-Circuit CMOS Complimentary Metal Oxide Semiconductor FIB Focused Ion Beam IRM Infrared Microscope SEM Scanning Electron Microscope AFM Atomic Force Microscope SNOM Scanning Near-Field Optical Microscope SPM Scanning Probe Microscopy EBIC Electron Beam Induced Current OBIC Optical Beam Induced Current TOBIC Two-Photon Optical Beam Induced Current OBIRCH Optical Beam Induced Resistance Change LIVA Laser Induced Voltage Alteration LADA Laser Assisted Device Alteration TIVA Thermally Induced Voltage Alteration SHG Second Harmonic Generation EFISHG Electric-Field Induced Second Harmonic Generation THG Third Harmonic Generation TPA Two-Photon Absorption TPF Two-Photon Fluorescence SIL Solid Immersion Lens h-SIL Hemispherical Solid Immersion Lens s-SIL Super Solid Immersion Lens FOSSIL Forming the substrate into a SIL ESA Excited State Absorption EDFA Erbium-Doped Fibre Amplifier NALM Nonlinear Amplifying Loop Mirror APM Additive Pulse Mode-locking NPR Nonlinear Polarisation Rotation FWHM Full-Width Half-Maximum PSF Point-Spread Function OCT Optical Coherence Tomography vi OCM Optical Coherence Microscopy S/N Signal-to-Noise A2D Analogue-to-Digital SC Supercontinuum CCD Charge-Coupled Device NEP Noise Equivalent Power vii TABLE OF CONTENTS Chapter 1: Introduction 1.1 Background and Motivation 3 1.2 Review of Conventional and Advanced Imaging Modalities in Semiconductor Failure Analysis 6 1.2.1 The Optical Microscope 6 1.2.2 The Infrared Microscope (IRM) 8 1.2.3 The Confocal Microscope 11 1.2.4 The Scanning Electron Microscope (SEM) 13 1.2.5 Electron Beam Induced Current (EBIC) Microscope 15 1.2.6 Laser Signal Injection Microscopy (OBIC, TOBIC, OBIRCH, LIVA, TIVA) 15 1.2.7 The Thermal Imaging Microscope 18 1.2.8 Atomic Force Microscope (AFM) 18 1.3 Nonlinear Effects and Techniques 20 1.3.1 Second Harmonic Generation (SHG) 21 1.3.2 Third Harmonic Generation (THG) 23 1.3.3 Two-Photon Absorption (TPA) 24 1.3.4 TPA in Semiconductor Integrated-Circuits 26 1.3.5 Two-Photon Fluorescence (TPF) 30 1.4 Solid Immersion Techniques for Nanoscale Semiconductor Inspection and Probing 33 1.4.1 Optical Resolution in the Far-Field 34 1.4.1.1 Surface Microscopy 34 1.4.1.2 Sub-Surface Microscopy 35 1.4.2 The Solid Immersion Lens 36 1.4.2.1 Hemispherical Solid Immersion Lens (h-SIL) 37 1.4.2.2 Super Solid Immersion Lens (s-SIL) 38 1.4.2.3 Magnification Effects in Solid Immersion Lenses 40 1.4.3 Non-Integrated-Circuit Applications of SILs 42 1.4.3.1 Optical Data Storage 42 1.4.3.2 Photoresponse Mapping 43 1.4.3.3 Spectroscopic Studies of Semiconductor Nanostructures 44 viii 1.4.4 Applications of SILs in Semiconductor Integrated-Circuit Failure Analysis 48 1.4.4.1 SIL-Enhanced Confocal Imaging 48 1.4.4.2 SIL-Enhanced Angular Spectrum Tailoring for IC Characterisation 49 1.4.4.3 SIL-Enhanced Widefield Microscopy of ICs 50 1.4.4.4 Substrates for Enhanced IC Failure Analysis 50 1.4.4.5 Other Potential Applications of Solid Immersion Lenses in IC Characterisation 51 1.4.5 SIL Modifications 52 1.4.6 Microfabricated SILs 53 1.5 Conclusions 53 1.6 Thesis Structure 54 References 55 Chapter 2: Modelocked Er:Fibre Laser and Erbium-Doped