(Anodisches) Niedertemperatur-Waferbonden für mikrosystemtechnische Anwendungen Mario Baum Fraunhofer-Institut für Elektronische Nanosysteme ENAS Jörg Frömel, Chenping Jia, Marco Haubold, Dirk Wünsch, Jörg Bräuer, Karla Hiller, Maik Wiemer Page 1 Fraunhofer ENAS Überblick Vorstellung Fraunhofer ENAS Anodisches Bonden – Einsatz Prozesstemperatur als Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Feldunterstütztes Direktbonden Niedertemperatur-Direktbonden Reaktives Bonden von Glas bei Raumtemperatur Zusammenfassung Page 2 Fraunhofer ENAS Fraunhofer Institute for Electronic Nano Systems Systems integration by using micro and nano technologies MEMS/NEMS design Development of MEMS/NEMS MEMS/NEMS test System packaging/wafer bonding International Offices: Since 2001 / 2005 Tokyo/Sendai-Japan Back-end of Line technologies for micro and nano electronics Process and equipment simulation Since 2002 Shanghai-China Micro and nano reliability Since 2007 Manaus-Brazil Printed functionalities Advanced system engineering Page 3 Fraunhofer ENAS Smart Systems Campus Chemnitz Lightweight Structures Engineering 3D-Micromac AG Start-up-building Fraunhofer ENAS Institute of Physics and Center for Microtechnologies at the CUT Page 4 Fraunhofer ENAS Fraunhofer ENAS / ZfM: A historical perspective and trends Applications Wafer Bonding / MEMS Packaging Interposer Integration MOEMS MEMS GEMAC Inclination Sensor Fabry-Perot Filter LITEF High Prec. Accelerometer X-Fab Embedded Interconnects * IZM MDI * ZfM Anodic Wafer Bonding Silicon Fusion Wafer Bonding 1994 Glass Frit Wafer Bonding 1996 Page 5 Fraunhofer ENAS 1998 2000 Adhesive Wafer Bonding BDRIE Technology 2002 * ENAS 2D AIM low g sensor E&H Pressure Sensor Eutectic Wafer Bonding Smart System Integration 2004 3D Integration Thin film Encapsulation Nano Structures & Effects Functional Substrates bond frame Nano Effects for Wafer Bonding Laser Assisted Wafer Bonding Thermo Compression Wafer Bonding Technologies 2006 2008 2010 Fraunhofer ENAS / ZfM: Wafer bonding portfolio Page 6 Fraunhofer ENAS Anodisches Bonden - Einsatz Joining of glass (borosilicate) and silicon substrates without intermediate layers or silicon materials with sputtered thin glass layers. Inclination sensor Technology steps: Gyroscope sensor Joining of starting materials Heating of the substrates Applying of an electrostatic field Cooling of the substrates Materials: Silicon Borosilicate glasses e.g. borofloat, pyrex, SD2 Intermediate layers like SiO2, Si3N4, Al Typical parameters: Temperature between 250°C and 510°C Voltage between 50 and 2000 V Page 7 Fraunhofer ENAS Micro mirror Aktiv Sensor GmbH Pressure sensor Anodisches Bonden - Einsatz The BDRIE technology flow for inertial sensors Cross section of the glass – Si – glass variant Active structure glass Si glass Contact hole with metallisation Example: Vibration sensor Page 8 Fraunhofer ENAS Anodisches Bonden - Einsatz The BDRIE technology flow for inertial sensors 1 Glass (400 µm thickness) with sandblasted holes Si 300 µm, with etched cavities on the bottom side Anodic bonding Page 9 Fraunhofer ENAS Anodisches Bonden - Einsatz The BDRIE technology flow for inertial sensors 2 Thinning of the Si wafer (grinding and polishing) down to 50 µm thickness of the active layer Dry etching of Si Page 10 Fraunhofer ENAS Anodisches Bonden - Einsatz The BDRIE technology flow for inertial sensors 3 Glass (500 µm thickness) Wet etching of a cavity Anodic bonding with active wafer Metal deposition for through contacts Issue: edges of the holes must be smooth and free of defects for successful thinlayer metallization Page 11 Fraunhofer ENAS Examples of holes and fabricated MEMS structures Sand blasted holes, view on bond side SEM of hole in glass-Si compound Page 12 Fraunhofer ENAS Example of inertial MEMS with metallized through holes Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Gehäuseausführung als Dreifachstapel Glas – Glas – Silizium 1. Bondinterface Glas – Glas Direktbonden (oberhalb 210°C) 2. Bondinterface Glas – Silizium Anodisches Bonden (bei 210°C) Elektrische Kontaktierung z.B. mittels vertikalen TSVs Metallisiertes TSV zur Kontaktierung Strukturiertes Siliziumsubstrat MEMS Glasverbund mit Kavität Schematische Darstellung der umhausten Struktur Page 13 Fraunhofer ENAS Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Fertigungstechnologie: 1. Sandstrahlprozess zur Perforation 2. Direktbonden nach Vorbehandlung 3. Temperung bei 400°C Gefertigte Perforation (links) / direkt gebondete Wafer (rechts) Page 14 Fraunhofer ENAS Prozessablauf Deckelfertigung Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Technologie: Nutzung der vorhandenen Aluminium Leiterbahnen als Bondschicht Interface Al-Glas erlaubt das Absenken der Prozesstemperatur während dem anodischen Bonden Ausreichende Bondfestigkeit bleibt erhalten Fakten: SUSS MicroTec SB8e Substratbonder Prozesstemperatur 210°C Grenzwert 200°C Bondspannung 800V Max. Strom 1,1 mA Kontaktierungsfenster im Oxid zur Anschluss des Al an das Substrat Page 15 Fraunhofer ENAS Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Testchips für die elektrische Charakterisierung sowie Hermetizitätsuntersuchungen Page 16 Fraunhofer ENAS Umhauste HF Schalter auf Waferlevel Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Test des Al-Glas Interface nach Bond @ 210°C mittels Mikro-Chevrontest Bruch des Glaschips als überwiegender Fehlermechanismus Max. Zugkraft Mittelwert 12,3 N Standardabweichung 2,2 N Anzahl getesteter Chips Aluminium = Bondfläche Bruch im Glaschip Resultat nach Chevrontest Bruch des Glaschips Page 17 Fraunhofer ENAS 12 Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Weitere Verringerung der Bondtemperatur untersucht Mithilfe von Zwischenschichten (Al) eine signifikante Minimierung erreicht Bondfestigkeit zeigt hohe Werte bei 150 °C Bondtemperatur Page 18 Fraunhofer ENAS Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Dependence of fracture toughness from bond temperature Bonding of borosilicate glass to silicon: • Fracture toughness heavily depend on bonding temperature • Main mechanism of bonding is oxidation at interface Page 19 Fraunhofer ENAS Forschungsschwerpunkt Anodisches Niedertemperatur-Waferbonden Dependence of fracture toughness from bond temperature Adding of thin metal layer in between the interface: • Oxidation is much easier than silicon • Fracture toughness dependence on bonding temperature very low Hata,S.; Froemel,J.; Gessner,T.; The improvement of the bonding strength of low temperature anodic bonding; Japan Institute of Electronics Packaging; The 19th JIEP Annual Meeting; (2005) Page 20 Fraunhofer ENAS Forschungsschwerpunkt Field Assisted Direct Bonding Anodic bonding : 300 V and 300…400°C Back grinding of glass wafer to ca. 20 µm thickness Surface and layer quality is better than after sputtering 2nd bonding: field assisted: 150°C and 100 V Page 21 Fraunhofer ENAS Field Assisted Direct Bonding Process parameters Substrate 1: 4-inch Si, p-(100), 200µm thickness Substrate 2: 4-inch glass, Borofloat, 500µm Temperature: start from120°C, step up to 150°C Bias voltage: start from 200V, shift gradually to 2000V at a pace of 200 ~ 300V Tool force: constant 300N All experiments performed under vacuum Page 22 Fraunhofer ENAS Field Assisted Direct Bonding Bonding quality inspection - IR imaging 150°C, 120V Page 23 Fraunhofer ENAS 150°C, 100V Result Summary A wafer bond was reached at 100 V and 150 °C The shear strength of the joined wafer pair is greater than the flexural strength of 200µm thick Si plate – break out Migration current is significantly lower than normal AB process (60…250µA max. vs. 10mA) Temperature is the major factor for this bonding, voltage the second Further steps: characterization of the strength and transfer to patterned wafers Page 24 Fraunhofer ENAS Forschungsschwerpunkt Oberflächenaktiviertes Direktbonden Bonding partners: Borosilicate glass – 4”, thickness 500 µm, CTE 3.25 ppm/K Silicon – 4”, thickness 525 µm, CTE 2.3 ppm/K Results: Tensile force of 5 N - Activated specimen is more than two times higher than the strength of the reference Bond strength of this wafer stack is as high as a conventionally anodic bonded Si/glass wafer stack Bond strength of the bonded interface is comparable to the bulk material Process Parameter: Pre-Treatment: CMP and RCA-procedure Optimal process gas: O2dH2O (humid oxygen) Annealing: 250°C, 6h, air Page 25 Fraunhofer ENAS Forschungsschwerpunkt Reaktives Raumtemperaturbonden Page 26 Fraunhofer ENAS Forschungsschwerpunkt Reaktives Raumtemperaturbonden Adv antages : Localized heating Stress-free bonding Bonding of heterogeneous materials (Si-metal, Si-ceramic...) Integration of new materials (Optical layers, bio-sensitive layers...) Very fast large area bonding / wafer bonding within several ms Room-temperature metallic bonding No special surface preparation needed (“just” wetting layer) Bonding at different atmospheres possible Different bonding geometries possible Page 27 Fraunhofer ENAS Conclusion Our motivation is decreasing the process temperature for wafer bonding techniques, optimizing the overall parameters and minimizing the thermal influence to the devices. As an independent research organization we are able to offer individual services for companies doing R&D. We see future trends in an increasing functionality in one system with decreasing size. Smart System Integration covers all aspects of research: Materials Technologies Packaging Equipment Page 28 Fraunhofer ENAS Source: Fraunhofer ENAS Thank you for your kind attention! Fraunhofer ENAS Dept. System Packaging Mario Baum Technologie Campus 3 D-09126 Chemnitz Phone: +49 - 371 – 45 00 12 61 Fax: +49 - 371 – 45 00 13 61 E-mail: [email protected] http://www.enas.fraunhofer.de Page 29 Fraunhofer ENAS
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