Precise bulk silicon wet etching – a key process for future wafer thinning technologies LEVITRONIX ULTRAPURE USER CONFERENCE 29th CMP USERS MEETING April, 11th to 12th Zurich, Switzerland Knut Gottfried, Manuela Zacher, Ines Hartwig, Ina Schubert, Sven Zimmermann, Ronny Martinka Fraunhofer Institute for Electronic Nanosystems Technology Campus 3 Chemnitz, Germany Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 1 Fraunhofer ENAS – a part of the Fraunhofer Society Berlin Paderborn Chemnitz Fraunhofer-Gesellschaft 60 Institutes More than 20,000 employees Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 2 Research at Fraunhofer ENAS addresses … … system integration using micro and nano technologies MEMS/NEMS design Development of MEMS/NEMS MEMS/NEMS test System packaging / Waferbonding Back-end of Line technologies for micro and nano electronics International Offices of Fraunhofer ENAS: Process and equipment simulation Since 2001/2005 Micro and nano reliability Printed functionalities Advanced system engineering (Since 2012 Project-Center in Sendai) Since 2002 Shanghai, China Since 2007 Manaus, Brazil Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 3 Tokyo/Sendai, Japan Outline • Background • Spin etching • Process / tool requirements • Tool concept and technical details • Experimental and Results • Summary Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 4 Wafer thinning in IC and MEMS fabrication Wafer thinning (back side grinding) is used in IC and MEMS fabrication in order to: • Achieve a desired device thickness (ICs, MEMS) • Ensure a specific thickness based on device functionality (MEMS) • Reduce substrate series resistance in vertical devices (Power devices) Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 5 Standard wafer thinning technology: grinding • Simple mechanical removal (grinding wheel & DI water) • Well developed / established (tools, wheels) • 2 stages: coarse grinding and (ultra) fine grinding • Serious substrate damage (up to several microns) result in high mechanical stress Depending on the specific application, post treatments required, such as: • Plasma etching • Spin etching • CMP D Knut Gottfried Dr. M Manuela Zacher IInes Hartwig IIna Schubert Dr. Sven Zimmermann D Ronny Martinka 6 Vertical system integration - 3D-integration 3D-integration implies any stacking of integrated devices (ICs, MEMS) and their vertical mechanical and electrical connection. (Dec 2011) In order to: • Raise integration density • Increase performance • Enhance functionality • Reduce power consumption • Minimize volume and weight Picture source: CEA Leti “3D Activities and Roadmap”, presented at EMC-3D European Technical Symposium Minatec June 29th, 2007 Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 7 Wafer thinning within 3D-integration Applications: Achieve minimal device / system thickness • Ensure producible through silicon via (TSV) aspect ratio • Blind TSV reveal 72 µm 15 µm • AR: 20 : 1 1,2 µm Requirements: © 2013 ENAS / ZFM • Low total thickness variation; (TTV < 1… 2 µm) • Good … excellent surface quality; (wafer bonding Ra < 1nm for a 10 µm x10 µm area) • Stress free substrates / ultra thin substrates / thin wafer handling © 2013 PennWell Corporation / IMEC Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 8 Wafer thinning within 3D-integration - conclusion • Wafer thinning is one of the main process modules within 3D-integration (besides TSV-fabrication and stacking / joining) • Grinding remains the technology of choice; however, due to the specific requirements post grinding treatments mandatory for many applications • Possible candidates are: Plasma etch, Wet etch, and CMP Wet etch, executed as spin etch, is a promising candidate, because • Comparatively simple process setup • Reasonable effort (tool costs, consumable costs) • High and tunable etch rate (much faster than CMP) • Wet or dry in and dry out process (direct processing of grinded wafers, no additional cleaning) Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 9 Outline • Background • Spin etching • Process / tool requirements • Tool concept and technical details • Experimental and Results • Summary Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 10 Spin etch process / tool: basic requirements • Process applicable to 100mm, 150mm, and 200mm wafers with minimum conversion time (less than 15 minutes) • Chemicals • KOH • HNO3/HF/CH3COOH (HNA) • Wafer rotation • Continuously rotation • Puddle mode • Dispense positions and modes • Fix position • Oscillating movement over a specific distance (wafer diameter) • Spray dispense • Flush dispense D Knut Gottfried Dr. M Manuela Zacher Ines Hartwig In Ina Schubert Dr. Sven Zimmermann Ronny Martinka 11 SPS Europe B.V. Polos Spin Process Station Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 12 SPS Europe B.V. Polos Spin Process Station - overview Tool combines a Polos spinner and a chemical delivery system in one unit Polos spinner Chemical delivery system • 100 mm to 200 mm wafers using type casted chucks • 4 day tanks, 2 with automatic fill, 2 with manual fill • 2 dispense arms • Tank recirculation • PLC controlled (alphanumeric) • One heated tank for KOH • Easy configuration changes by software (valve assignment, …) • Recirculation for KOH-processes • Special drain for HF-chemistry • Waste tank for special applications • PLC controlled (GUI) Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 13 Polos Spin Process Station - operating area Funnels for HNO3 / CH3COOH supply Process reactor (Spinner) KOH & HF supply Waste chemicals Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 14 Service / storage Polos Spin Process Station - chemicals dispense separate dispense arms for KOH and HNO3/HF/CH3COOH Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 15 • ¼ inch flush dispense line • ǩLQFKIOXVKGLVSHQVHOLQH • spray nozzle Polos Spin Process Station – chemicals supply CH3COOH HNO3 HF KOH up to 95°C Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 16 Polos Spin Process Station – chemicals supply details Levitronix BPS200 pump systems and Leviflow™ flow meters * • Continuous recirculation • Process supply • DI water dilution • Perfect in-situ mixing Needle valves to adjust • Recirculation flow • Chemical supply load balance * IR1600 flow meter (TOKYO KEISO) for CH3COOH) because pure acetic acid cannot be measured using an ultrasonic principle Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 17 Polos Spin Process Station – HNO3/HF/CH3COOH mixing Serial and parallel operation of 4 (5 if DI-water dilution) BPS200 pumps HF : HNO3 : CH3COOH ratio may vary, typical ratios • 1:5:1 • 1 : 5 : 0.3 Flow range up to 1liter for each chemical Dispense arm Mixing pump CH3COOH Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 18 HF HNO3 DI water • 20 ml : 100 ml : 20 ml • 200 ml : 1liter : 200 ml • 20 ml : 100 ml : 7 ml • 200 ml : 1 liter : 67 ml Outline • Background • Spin etching • Process / tool requirements • Tool concept and technical details • Experimental and Results • Summary Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 19 Experimental and results – KOH process Process parameters: Applications • KOH concentration 30% • TSV reveal (no Cu attack) • KOH temperature 80°C • MEMS fabrication • KOH recirculation mode • Low rate etching for stress relief • Flush dispense using ¼ inch line 680 • Oscillating dispense 675 Typical results: (10 µm Si removal) • Etch rate: 1,2 µm/min 670 665 660 • ıremoval < ± 5% 655 • TTV: < 2 µm 650 Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 20 Thickness after spin etch • Continuous rotation 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Experimental and results – HNO3/HF/CH3COOH process Process parameters: Applications • Continuous rotation • High rate etching for stress relief • Oscillating dispense mode • MEMS fabrication • TSV reveal approach (attacks Cu) • Flush dispense using ¼ inch line • Chemicals concentration: • HNO3 69 % • HF: 40 % 100 % Typical results: (10 µm Si removal) 465 460 455 450 • Etch rate: 10 µm/min 445 • ıremoval < ± 5% 440 • TTV: < 2 µm Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 21 Thickness after spin etch • CH3COOH: 470 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Process stability – HNO3/HF/CH3COOH process Acid flow monitoring Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 22 Process stability – HNO3/HF/CH3COOH process Wafer 1 Wafer 2 Wafer 3 Wafer 4 HNO3 HF Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 23 Experimental and results – damage removal / stress relief • Preparation of 4 different wafers as given below • Bow measurement using Dektak profiler • Laser induced SAW measurements to determine damage depths Coarse grind Fine grind Spin etch Wafer 1 100 µm * 30 µm *** 10 µm Wafer 2 200 µm * 30 µm *** 10 µm Wafer 3 300 µm * 30 µm *** 10 µm Wafer 4 300 µm ** --- 10 µm * feed rate 4 µm/s ** feed rate 6 µm/s *** feed rate 0.6 µm/s Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 24 Results of wafer bow measurements after … initial state coarse grind fine grind spin etch Si removed Wafer 1 4.2 µm 227 µm 36.1 µm 4.7 µm 140 µm Wafer 2 4.5 µm 299 µm 52.4 µm 8.8 µm 240 µm Wafer 3 4.3 µm 376 µm 84.1 µm 13.7 µm 340 µm Wafer 4 3.7 µm 370 µm --- 11.1 µm 310 µm • Wafer bow clearly correlates to the surface treatment • For wafer 2 to 4 the bow after spin etch remains higher – thickness! • No difference in final bow with /without fine grind • Wafer 4 rougher than wafer 1 to 3 Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 25 Coarse grind Fine grind Spin etch Wafer 1 100 µm 30 µm 10 µm Wafer 2 200 µm 30 µm 10 µm Wafer 3 300 µm 30 µm 10 µm Wafer 4 300 µm --- 10 µm Acoustic surface waves (SAW) measurements … to determine mechanical properties of films on substrates Penetration depth ~O higher frequency o increasing film impact Phase velocity depends on frequency p Dispersion = ( ,, ; , , , ;) c E N … … … … Phase velocity Elastic modulus Density Poisson's ratio Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 26 f … frequency S … substrate F … film 50 … 250 MHz “LAwave” – measurement system (by Fraunhofer IWS) Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 27 LSAW results wafer 1 Wafer 1 Coarse grind Fine grind Spin etch 100 µm 30 µm 10 µm Calculated damage depth ࢉ 1.2 µm Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 28 LSAW results wafer 4 Wafer 4 Coarse grind Fine grind 300 µm --- Calculated damage depth ࢉ 1.2 µm Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 29 Spin etch 10 µm Coarse grind Fine grind Spin etch Wafer 1 100 µm 30 µm 10 µm Wafer 2 200 µm 30 µm 10 µm Wafer 3 300 µm 30 µm 10 µm Wafer 4 300 µm LSAW results - comparison wafer 1 and 4 Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 30 --- 10 µm Experimental and results – conclusion • Bow and laser induced SAW measurements are suitable candidates to analyze thinning process related substrate damages • LSAW measurements allow a more precise qualitative analysis as well as a rough quantitative predication • A removal of 10 µm silicon using a HF/HNO3/CH3COOH spin etch process is suitable to remove grinding induced substrate damages almost completely ¾ However, a fine grinding step seems to be mandatory to achieve that Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 31 Outline • Background • Spin etching • Process / tool requirements • Tool concept and technical details • Experimental and Results • Summary Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 32 Summary • 3D-integration is a challenging playground for wafer thinning technologies • Specific requirements to TTV, surface quality, and allowable stress • Customized / application specific technologies (process setups) • Spin etching could be a well suitable process to meet the 3D specifics • Simple, fast, flexible, and tunable • Reasonable effort • SPS Polos Spin Process Station @ Fraunhofer ENAS offers unique capabilities for research and process development • Fraunhofer ENAS is able to drive this technology into your application Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 33 Acknowledgement I would like to thank my colleagues Manuela, Ina, and Ronny for their very active contribution to work. Especially, I would like to thank Ines for LSAW measurements and Sven for wafer bow and roughness measurements. I would like to thank SPS Europe B.V. (Rein de Groot, Alessandro Barilaro, Raymond de Munnik, and many others) for a great collaboration, starting from the first ideas for such a tool till its final installation and setup. I would like to thank Levitronix (Wolfgang Dornfeld, Marco Brunner, Simon Stöckli) for their outstanding support in setting up pump systems and flow control systems. Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 34 Thank you for your kind attention! Dr. Knut Gottfried Manuela Zacher Ines Hartwig Ina Schubert Dr. Sven Zimmermann Ronny Martinka 35
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