Laser-based Packaging of Micro-devices Norbert Lorenz A dissertation submitted for the degree of Doctor of Philosophy Heriot-Watt University School of Engineering and Physical Sciences October 2011 This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that the copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without the prior written consent of the author or of the University (as may be appropriate). Abstract In this PhD thesis the development of laser-based processes for packaging applications in microsystems technologies is investigated. Packaging is one of the major challenges in the fabrication of micro-electro-mechanical systems (MEMS) and other micro- devices. A range of bonding processes have become established in industry, however, in many or even most cases heating of the entire package to the bonding temperature is required to effect efficient and reliable bonding. The high process temperatures of up to 1100°C involved severely limit the application areas of these techniques for packaging of temperature sensitive materials. As an alternative production method, two localised heating processes using a laser were developed where also the heat is restricted to the joining area only by active cooling. Silicon to glass joining with a Benzocyclobutene adhesive layer was demonstrated which is a typical MEMS application. In this laser-based process the temperature in the centre of the device was kept at least 120°C lower than in the bonding area. For chip- level packaging shear forces as high as 290 N were achieved which is comparable and some cases even higher than results obtained using conventional bonding techniques. Furthermore, a considerable reduction of the bonding time from typically 20 minutes down to 8 s was achieved. A further development of this process to wafer-level packaging was demonstrated. For a simplified pattern of 5 samples the same quality of the seal could be achieved as for chip-level packaging. Packaging of a more densely packed pattern of 9 was also investigated. Successful sealing of all nine samples on the same wafer was demonstrated proving the feasibility of wafer-level packaging using this localised heating bonding process. The development of full hermetic glass frit packaging processes of Leadless Chip Carrier (LCC) devices in both air and vacuum is presented. In these laser-based processes the temperature in the centre of the device was kept at least 230°C below the temperature in the joining region (375°C to 440°C). Testing according to MIL-STD- 883G showed that hermetic seals were achieved in high yield processes (>90%) and the packages did withstand shear forces in excess of 1 kN. Residual gas analysis has shown that a moderate vacuum of around 5 mbar was achieved inside the vacuum packaged LCC devices. A localised heating glass frit packaging process was developed without any negative effect of the thermal management on the quality of the seal. ii Dedication This thesis is dedicated to Matthias Baunack (1953 – 2006) iii Acknowledgements Firstly, I would like to express my gratitude to Professor Duncan Hand for giving me the opportunity to work within the Applied Optics and Photonics Group and for guiding and supervising me throughout the last 3½ years; without him this work would not have been possible. I would like to thank my colleagues of the AOP group for their support during my PhD; and Robert for proofreading the thesis. Special thanks must go to Suzanne Millar for the good collaboration and the leak testing of the packaged devices. Furthermore, I would like to thank the technical staff at Heriot-Watt University for their support; in particular Mark Stewart for engineering my bonding setup and Neil Ross for dicing sheer endless amounts of wafers. I feel honoured that my research was so highly recognised by the Association of Laser Users (AILU). I would like to express my sincerest thanks to the committee of AILU for awarding me the AILU‟s Young UK Laser Engineer's Prize 2011. I am glad that during my time at Heriot-Watt University I made such great friends with so many people, especially Brian, Dave, Jack, Jens, Stefan and Yves – I will miss you all! Finally, I must thank my family and Teresa for their continuous moral support and their endless belief in me. I couldn‟t have done it without you and I could always rely on you! iv ACADEMIC REGISTRY Research Thesis Submission Name: NORBERT LORENZ School/PGI: School of Engineering and Physical Sciences Version: (i.e. First, Degree Sought Resubmission, Final) Final (Award and Doctor of Philosophy in Physics Subject area) 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 the Student Service Centre (SSC) Received in the SSC by (name in capitals): Method of Submission (Handed in to SSC; posted through internal/external mail): E-thesis Submitted (mandatory for final theses) Signature: Date: v Table of Contents Abstract ............................................................................................................................. ii Dedication ........................................................................................................................ iii Acknowledgements .......................................................................................................... iv List of Tables.................................................................................................................... ix List of Figures ................................................................................................................... x List of Symbols and Abbreviations ................................................................................ xvi List of Publications and Awards .................................................................................. xviii 1 Introduction ................................................................................................................ 1 1.1 Rationale ............................................................................................................. 1 1.2 Aims and Objectives .......................................................................................... 2 1.3 Summary of Chapters ......................................................................................... 2 2 Literature Review and Background ........................................................................... 4 2.1 Motivation – Challenges and Issues in Packaging of Micro-devices ................. 4 2.2 Traditional Direct Bonding Techniques ............................................................. 6 2.2.1 Silicon Fusion Bonding ............................................................................... 6 2.2.2 Anodic Bonding .......................................................................................... 8 2.3 Intermediate Layer Bonding ............................................................................. 12 2.3.1 Eutectic Bonding ....................................................................................... 12 2.3.2 Glass Frit Bonding .................................................................................... 16 2.3.3 Adhesive Layer Bonding........................................................................... 20 2.4 Localised Bonding Techniques ........................................................................ 26 2.4.1 Microwave Heating ................................................................................... 27 2.4.2 Induction Heating ...................................................................................... 28 2.4.3 Resistive Heating ...................................................................................... 30 2.4.4 Seam Sealing ............................................................................................. 32 2.4.5 Laser Joining ............................................................................................. 33 2.4.6 Summary of Localised Bonding Techniques ............................................ 38 vi 2.5 Previous Work at Heriot-Watt University ........................................................ 40 2.5.1 Laser Bonding of Silicon to Glass with BCB Adhesive Layer ................. 40 2.5.2 Laser-based Glass Frit Packaging of Miniature Devices .......................... 43 2.6 Quality Assessment / Military Standard Testing .............................................