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Copper Thermocompression for Mems Encapsulation Henri Ailas COPPER THERMOCOMPRESSION FOR MEMS ENCAPSULATION Master’s Programme in Chemical, Biochemical and Materials Engineering Major in Chemistry Master’s thesis for the degree of Master of Science in Technology submitted for inspection, Espoo, 5th of April, 2019 Supervisor Prof. Kari Laasonen Advisors D.Sc. Jaakko Saarilahti D.Sc. Jyrki Kiihamäki Aalto University, P.O. BOX 11000, 00076 AALTO www.aalto.fi Abstract of the master’s thesis Author Henri Ailas Title Copper thermocompression for MEMS encapsulation Degree programme Chemical, Biochemical and Materials Engineering Major Chemistry Code of major CHEM3023 Thesis supervisor Prof. Kari Laasonen Thesis advisors D.Sc. Jaakko Saarilahti, D.Sc. Jyrki Kiihamäki Date 5.4.2019 Number of pages 92+5 Language English Abstract Copper thermocompression is a promising wafer-level packaging technique, as it al- lows the bonding of electric contacts simultaneously to hermetic encapsulation. In thermocompression bonding the bond is formed by diffusion of atoms from one bond interface to another. The diffusion is inhibited by barrier forming surface oxide, high surface roughness and low temperature. Aim of this study was to establish a wafer-level packaging process for MEMS (Micro- ElectroMechanical System) mirror and MEMS gyroscope. The cap wafer of the MEMS mirror has an antireflective coating that limits the thermal budget of the bonding process to 250°C. This temperature is below the eutectic temperature of most com- mon eutectic bonding materials, such as Au-Sn (278°C), Au-Ge (361°C) and Au-Si (370°C). Thus a thermocompression bonding method needed to be developed. Cop- per was used as a bonding material due to its low cost, high self-diffusivity and re- sistance to oxidation in ambient air. The bond structures were fabricated using three different methods and the bonding was further enhanced by annealing. The bonded structures were characterized with scanning acoustic microscopy, scanning electron microscope and the bond strength was determined by shear testing. Exposing the bond structures to etchant during Cu seed layer removal was found to drastically increase the surface roughness of bond structures. This increase proved detrimental to bond strength and dicing yield and thus covering the bond surface during wet etching is recommended. The native oxidation on copper surfaces was completely removed with combination of ex situ acetic acid wet etch and in situ form- ing gas anneal. Successful thermocompression bonding process using sputtered cop- per films was established at a low temperature of 200°C, well below the thermal lim- itation set by the antireflective coating. The established wafer bonding process had high yield of 97% after dicing. The bond strength was evaluated by maximum shear strength and recorded at 75 MPa, which is well above the MIL-STD-883E standard (METHOD 2019.5) rejection limit of 6.08 MPa. Keywords Wafer-level packaging, thermocompression bonding, wafer bonding, MEMS, Cu ii Aalto-yliopisto, P.O. BOX 11000, 00076 AALTO www.aalto.fi Diplomityön tiivistelmä Tekijä Henri Ailas Työn nimi MEMS suljenta kuparin lämpöpuristusliitännällä Koulutusohjelma Kemian-, Biokemian-, ja Materiaalitekniikan koulutusohjelma Pääaine Kemia Pääaineen koodi CHEM3023 Vastuuopettaja Prof. Kari Laasonen Työn ohjaaja TkT Jaakko Saarilahti, TkT Jyrki Kiihamäki Päivämäärä 5.4.2019 Sivumäärä 92+5 Kieli Englanti Tiivistelmä Kuparin lämpöpuristusliitäntä on lupaava kiekkotason pakkausmenetelmä, sillä se mahdollistaa sekä sähköisten liitäntöjen, että hermeettisen suljennan toteuttami- sen samanaikaisesti. Lämpöpuristusliitännässä sidos muodostuu atomien diffuusi- osta liitospinnalta toiselle. Diffuusiota rajoittavat estokerroksen muodostava pinta oksidi, korkea pinnan karheus ja matala lämpötila. Diplomityön tavoitteena oli luoda kiekkotason pakkausmenetelmä mikroelektrome- kaaniselle (MEMS, MicroElectroMechanical System) peilille ja MEMS gyroskoopille. Peilin lasisen kansikiekon pinnalla oleva antiheijastava kalvo rajoitti liitännässä käy- tettävän lämpötilan korkeintaan 250°C:een, mikä on alempi lämpötila kuin useim- pien kiekkoliitännässä käytettyjen materiaaliparien eutektinen piste. Esimerkkinä mainittakoon mm. Au-Sn (278°C), Au-Ge (361°C) ja Au-Si (370°C). Kuparin alhainen hinta, korkea ominaisdiffuusio ja hidas hapettuminen ilmakehässä puoltavat sen va- lintaa liitäntämateriaaliksi. Liitäntärakenteet valmistettiin kolmella menetelmällä ja liitännän vahvuutta parannettiin lämpökäsittelyllä. Liitetyt rakenteet karakterisoitiin pyyhkäisy elektronimikroskoopin, akustisen mikroskoopin ja liitoslujuus-mittauksen avulla. Liitospintojen altistamisen hapolle havaittiin lisäävän pinnankarkeutta ja olevan si- ten haitallista liitokselle ja laskevan saantoa. Liitospintojen suojaaminen siemenker- roksen syövytyksen aikana on suotavaa. Pintaoksidi pystytään poistamaan täysin suorittamalla oksidin märkäetsaus jääetikalla sekä lämpökäsittely N2/H2 atmosfää- rissä. Sputteroidut kuparikalvot pystyttiin liittämään onnistuneesti yhteen 200°C lämpötilassa, mikä on alle anti-heijastavan pinnan asettaman lämpötilarajan. Tällä liitäntä menetelmällä saavutettiin kiekkoliitoksella yhteen liitettyjen sirujen sahauk- sessa korkea 97% saanto. Liitoslujuus määritettiin maksimi-leikkausvoiman avulla ja sen suuruudeksi mitattiin 75 MPa. Lujuus oli yli kymmenkertainen MIL-STD-883E standardin (METHOD 2019.5) asettamaan hylkäysrajaan 6.08 MPa nähden. Avainsanat Kiekkojen pakkaus, lämpöpuristusliitäntä, piikiekkoliitäntä, MEMS, Cu iii Acknowledgements This work was conducted in the MEMS team at VTT Technical Research Center of Finland. It has received financial support from the Horizon 2020 programme of the European Union under ’Programmable Systems for Intelligence in Automobiles’ (PRYSTINE-783190) and ‘Advancing fail-aware, fail-safe, and fail-operational elec- tronic components, systems, and architectures for fully automated driving to make future mobility safer, affordable and end-user acceptable’ (AutoDrive-737469). Both projects were partially funded by BusinessFinland in the framework of ECSEL, also. Part of the research was performed at the OtaNano - Micronova Nanofabrication Centre of VTT. Firstly, I would like to thank my thesis supervisor Prof. Kari Laasonen and my thesis advisors D.Sc. Jaakko Saarilahti and D.Sc. Jyrki Kiihamäki for enabling me to write this thesis. During this time, they have supported me with constructive suggestions and feedback. Secondly, I would like to thank VTT’s operators, especially Jaana Marles, who sacrificed considerable amount of her time teaching me how to use various la- boratory tools. I would also like to thank D.Sc. Tuomas Pensala whose superior knowledge of spelling helped me with the final touches on the outlook of the thesis. Lastly, I would like to appreciate my fellow thesis workers and dear friends Jonne Vähänissi and Henry Malm for much needed distraction from the thesis work and fruitful relaxation. 5.4.2019 Espoo, Finland Henri Ailas iv Table of Contents Abstract…………………………………………………………………………………………………………….…….ii Abstract (in Finnish)…………………………………………………………………………………………….….iii Acknowledgements ..................................................................................................... iv List of abbreviations .................................................................................................... vii 1 Introduction .......................................................................................................... 1 2 Wafer bonding for MEMS encapsulation ............................................................. 3 2.1 Micro-Electro-Mechanical Systems............................................................... 3 2.2 Diffusion bonding materials and typical bonding parameters ..................... 7 2.3 Grain formation in the bond interface ........................................................ 12 2.4 In situ oxide removal methods .................................................................... 17 2.5 Ex situ surface cleaning and oxide removal methods ................................. 20 2.6 Copper microstructure fabrication methods .............................................. 25 2.7 Bond area geometry.................................................................................... 27 2.8 Wafer alignment ......................................................................................... 28 2.9 Characterization methods for bonded wafers ............................................ 29 3 Experimental ....................................................................................................... 35 3.1 Structure of samples ................................................................................... 35 3.2 Measurement series ................................................................................... 35 3.3 Sample fabrication ...................................................................................... 39 3.4 Sputtering and electrodeposition ............................................................... 40 3.5 Lithography ................................................................................................. 41 3.6 Plasma stripping and resist removal ........................................................... 44 3.7 Etching ......................................................................................................... 44 3.8 Characterization methods ........................................................................... 46 3.9 Wafer bonding ...........................................................................................
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