Trends in Device Encapsulation and Wafer Bonding

Transcription

1 Trends in Device Encapsulation and Wafer Bonding Roland Weinhäupl, Sales Manager, EV Group

2 Outline Introduction Vacuum Encapsulation Metal Bonding Overview Conclusion

3 Quick Introduction to EV Group st Our philosophy Our mission in serving next generation application in semiconductor technology Equipment supplier for the semiconductor and MEMS industry equipment installations Privately held company founded in 1980 Headquartered in Austria - subsidiaries in USA, JP, KR, CN and TW Worldwide Sales and Customer Support Network Internal process development 20% of revenue is invested into R&D annually

4 MEMS, IoT & Wearables Sensor (R)evolution Novel Sensors: Development of new measurement units (IMU, environmental, health, ) Smart Software: Combo sensors as integration of multiple measurement units and data processing Size and Power Reduction: Steady volume reduction (footprint and thickness) as well as power reduction

5 Outline Introduction Vacuum Encapsulation Metal Bonding Overview Conclusion

6 Vacuum Sealing in MEMS Why? Vacuum encapsulation in MEMS is based on three primary drivers: 1. Reduce the power consumption caused by parasitic drag on a resonator: - Gyroscopes 2. Reduce convection heat transfer: - Microbolometers - Temperature-controlled devices 3. Prevent corrosion or other types of interaction with O 2 or H 2 O: - Parts with exposed Al or AlN Source: ST 6

7 Vacuum MEMS Cavity Pressure Different Applications demand different pressure levels MEMS Device Working Pressure Accelerometer mbar Absolute pressure sensor mbar Gyroscope mbar Rotation Acceleration Sensor mbar Resonant Magnetometer mbar Microbolometer < 10-4 mbar RF switch < 10-4 mbar Oscillators < 10-4 mbar Sources: A. Bonucci, A. Conte, M. Moraja, G. Longoni and M. Amiotti, Chapter 40: Outgassing and Gettering, in Handbook of Silicon-based MEMS: materials and technologies, pp. 585 V. Lindroos, M. Tilli, A. Lehto and T. Motooka (ed.), Elsevier, 2010 (ISBN: ) 7

8 Bonding Requirements for Vacuum Sealing CMOS compatibility Temperature limits of MEMS sensor and CMOS processing (Al-Ge or Al-Al) Contamination limitation Material compatibility Bonding Requirements

9 CMOS Compatibility Various CMOS-MEMS are using in production low temperature fusion bonding, eutectic, TLP bonding. Fusion bonding not compatible with high vacuum levels, even with getters. Only one eutectic system was proven to be compatible with CMOS: Al-Ge (volume production). Major disadvantage: process conditions too close to the limits of low temperature range (>400 C)! Al-Al thermo-compression bonding foreseen as a candidate. Major disadvantage: hard-to-handle, chemically-robust native oxide. 9

10 Bonding Requirements for Vacuum Sealing CMOS compatibility Temperature limits of MEMS sensor and CMOS processing (Al-Ge or Al-Al) Contamination limitation Material compatibility Bonding Requirements Stress management Bow and Warp management are gaining importance in vacuum sealing High temperature processing

11 Stress Management The use of MEMS specific patterning processes may result in high bow of incoming substrates, which subsequently can be responsible of inducing stress during bonding process. The specific MEMS requirements in terms of vacuum may impose substrates heating under vacuum for significant time, which may result in thermally-induced stress (high temperature difference between the two substrates) or extremely long process times. The use of high compression forces required by some bonding processes may induce stress in resonator structures. 11

12 Bonding Requirements for Vacuum Sealing CMOS compatibility Temperature limits of MEMS sensor and CMOS processing (Al-Ge or Al-Al) Contamination limitation Material compatibility Bonding Requirements Stress management Bow and Warp management are gaining importance in vacuum sealing High temperature processing Long term stability of vacuum inside devices Getters and high vacuum bake outs are competing New equipment concepts are underway for high vacuum sealing

13 Long Term Vacuum Stability The use of MEMS getters is becoming common in some vacuum MEMS. Disadvantages: hard to match thermally the various wafer bonding processes in use this results in decreasing the getter efficiency require additional patterning steps for getter deposition consume space inside device (High) vacuum baking prior bonding: a process which may avoid the use of getters but with most wafer bonder designs this results in unacceptable increase of process time. New equipment concepts address this issue. 13

14 Bonding Requirements for Vacuum Sealing CMOS compatibility Temperature limits of MEMS sensor and CMOS processing (Al-Ge or Al-Al) Contamination limitation Material compatibility Bonding Requirements Stress management Bow and Warp management are gaining importance in vacuum sealing High temperature processing Long term stability of vacuum inside devices Getters and high vacuum bake outs are competing New equipment concepts are underway for high vacuum sealing Direct vacuum measurement Testing schemes need to be optimized for high vacuum MEMS devices Testing integration into process flow Wafer level testing will be required

15 Vacuum Evaluation Inside Micropackages Test Method Sensitivity Limit/ Cavity Volume Membrane resonance 1 x 10-9 Pa m 3 /s / V > 0.5 mm 3 Optical deformation 5 x 10-9 Pa m 3 /s / V > 0.5 mm 3 Characteristics Requires thin (~20 µm) membranes, may affect sensor structure Inspection of metal housings; sensitivity affected by cap geometry Helium leak test 5 x Pa m 3 /s / V > 1 mm 3 Kr 85 radioactive tracer 1 x Pa m 3 /s / V > 0.5 mm 3 Limited to sealed volumes > 5 mm 3 Handling of radioactive test gas Q-factor monitoring 1 x Pa m 3 /s / Requires resonator in vacuum 2 mm 3 > V > mm 3 package Internal pressure 1 x Pa m 3 /s / 2 mm 3 > V > 0.01 mm 3 Integrated µ-pirani pressure sensor IR transmission 5 x Pa m 3 /s / Transmission of oxidized metal layers. 2 mm 3 > V > mm 3 No getter use possible. Residual gas analysis (RGA) Gas volume: >10-12 Pa l Destructive test, laborious for small packages 15

16 Outline Introduction Vacuum Encapsulation Metal Bonding Overview Conclusion

17 Overview Wafer Bonding Processes

18 Vacuum Basics for Wafer Bonding Wafer Bonding Specific Outgassing

19 Wafer Bonding Processes: Main Features Bonding temperature Re-melting temperature Bond cycle time Anodic Glas Frit TLP Eutectic Metal TC 350 C 450 C na 350 C 450 C Same as bonding 180 C 300 C Higher than bonding 300 C 450 C Same as bonding 5 20 min min min min Line width >20 µm 200 µm 500 µm Tolerance to topography Vacuum range Getters compatibility 100 C 400 C na min >30µm >30 µm >30 µm µm 1 µm 1 µm 0 Low - Medium Low - Medium Medium - High Medium- High Medium- High Yes Yes Yes Yes Yes Leak rate Low Low Very low Very low Low 19

20 Overview Wafer Bonding Processes

21 Eutectic Wafer Bonding Melting temperature Material A Temperature Time Material B Melting Point of the alloy is lower than of the individual materials Thickness of the material B and diffusion coefficient define process time

22 Eutectic Wafer Bonding: Step 1 Contacting Melting temperature Temperature x (µm) Time Wafers are aligned and brought into contact 1.00 c (a.u.) 0.00 Due to surface roughness no direct joint of the materials

23 Eutectic Wafer Bonding: Step 2 Heating Melting temperature Temperature x (µm) Time - Interdiffusion Diffusion start already in solid state Slow process but fast heating ramps with high uniformity improve process 1.00 c (a.u.) 0.00 Sill interface between the wafers as the gaps are not closed in the solid state

24 Eutectic Wafer Bonding: Step 3 Isothermal Stage Melting temperature Temperature Liquid x (µm) Time Temperature is ramped up above the eutectic temperature c (a.u.) 0.00 Process temperature is kept stable till the whole interface is liquid Interface gaps are closed and the wafers are joined

25 Eutectic Wafer Bonding: Step 4 Cooling Melting temperature Temperature x (µm) Time 1.00 c (a.u.) 0.00 Finally the wafers are cooled down to solidify the interface The remelt temperature is the same as the process temperature

26 Overview Wafer Bonding Processes * Also referred as: SLID solid liquid inter-diffusion

27 Transient Liquid Phase Wafer Bonding High melting material Low melting material Temperature Melting temperature Low melting material Time High melting material Melting Point of the low meting material defines process temperature Thickness of the low melting material and diffusion coefficient define process time

28 TLP Wafer Bonding: Step 1 Contacting Temperature Melting temperature Time Wafers are aligned and brought into contact Due to surface roughness no direct joint of the materials

29 TLP Wafer Bonding: Step 2 Heating Temperature Melting temperature Time - Interdiffusion Diffusion start already in solid state Slow process but fast heating ramps with high uniformity improve process Sill interface between the wafers as the gaps are not closed in the solid state

30 TLP Wafer Bonding: Step 3 Reach Process Temperature Temperature Melting temperature Liquid Time Liquefaction of the low melting material Closing of the interface gaps and joint of the wafers -Liquid Interdiffusion Fast process due to the higher mobility of the atoms

31 TLP Wafer Bonding: Step 4 Isothermal Process Melting temperature Temperature Liquid Time Material composition changes due to diffusion The alloy with higher melting point solidifies

32 TLP Wafer Bonding: Step 5 Cooling Melting temperature Temperature Time Finally the whole Interface is solid and the wafers are cooled down The remelt temperature is now higher than the process temperature

33 Metal Based Wafer Bonding Processes TLP (Transient Liquid Phase) Bonding Exemplary work done at EV Group regarding Cu-Sn TLP Bonding A B C D E 1 2 3

34 Outline Introduction Vacuum Encapsulation Metal Bonding Overview Conclusion

35 Overview Metal Wafer Bonding Techniques Solder Bonding Eutectic Bonding Metal TC Bonding Liquid Phase Yes Yes, local No Roughness Req. Low Medium High Feature Size Medium/Large Medium Small GaAs Au:Sn layer InP Au:Sn eutectic Bonding Ni-Sn TLP Bonding Cu-Cu Bonding

36 Conclusion MEMS devices gained a significant role in various applications areas as consumer products, medical devices, automotive Sensor fusion is the next trend in MEMS manufacturing, driven by IoT, embedded sensing & wearables Integrated combo sensors with different vacuum requirements are challenging wafer bonding Combining the right bonding process with the right integration scheme is key Material and temperature management are difficult (e.g. getter vs. CMOS integration) Low-T thermo-compression bonding will play an increasing role for future, high performance MEMS devices

37 Thank you for your attention! Please visit us at Booth #1324 Roland Weinhäupl, Data, design and specifications may not simultaneously apply; or depend on individual equipment configuration, process conditions and materials and may vary accordingly. EVG reserves the right to change data, design and specifications without prior notice. All trademarks, logos, website addresses or equipment names that contain the letters or words "EVG" or "EV Group" or any combination thereof, as well as the following names and acronyms are registered trademarks and/or the property of EV Group: ComBond, CoverSpin TM, EZB, EZ Bond, EZD, EZ Debond, EZR, EZ Release, GEMINI, HERCULES, HyperIntegration, IQ Aligner, LowTemp TM, NanoAlign, NanoFill TM, NanoSpray TM, NIL-COM, NILPhotonics TM, OmniSpray, SmartEdge, SmartView, The Triple "i" Company Invent- Innovate-Implement, Triple i. Other product and company names may be registered trademarks of their respective owners.