Current and Future Directions in Hybridization for Pixelated Particle Detectors

1. Current and Future Directions in Hybridization for Pixelated Particle Detectors Alan Huffman Center for Materials and Electronic Technologies huffman@rti.org

2. Outline • Who is RTI? • Solder Bump Technology • Bumping process • Post bump processes • Wafers thinning • Dicing control • Hybridization • Current Programs and Results • CMS • MEDIPIX • Future Technologies for Hybridization • 3D integration technology • Alternative bump materials • Alternatives to sawing

3. A Crisis of Identity…Who is RTI? • RTI acquired the research groups formerly known as MCNC Research & Development Center in March 2005 • RTI/MCNC has over 15 years experience in the development and implementation of flip chip technology, including the spin off of Unitive Electronics in 1998 (Amkor) • Fine pitch flip chip (<100 µm) has been ongoing since 1997

4. Important Points of Pixel Devices for Bumping • I/O pitch typically less than 100 µm • High interconnect counts, from a few thousand to over 65,000 • Large readout and sensor chip size (~ 1 cm2 and larger) • Multi-chip modules (MCM) typically needed to create large area sensor arrays • Materials used must withstand high radiation environment • Flux-free assembly processes are a necessity

5. Fine Pitch Solder Bumping • Formation of fine pitch solder bumps uses essentially the same processes as ‘standard’ pitch flip chip • Tighter control must be maintained over the processes than for typical wafer level packaging (WLP) applications due to smaller geometries • Additional post-wafer bumping processes are sometimes needed (i.e. wafer thinning) which can easily damage small solder bumps

6. Plate Solder or Wettable Metal Incoming Wafer With I/O Pads Repassivation With BCB Strip Resist Template UBM Deposition Reflow Apply and Define Plating Template Etch Field UBM RTI Fine Pitch Bumping Process Flow

7. Solder Bumped ROC and Sensor (US-CMS) 25 µm bump base diameter and 25 µm bump height Ni/Au bump bonding pads

8. Solder Bumped ROC and Sensor (MEDIPIX) 50 µm pitch readout chip with eutectic Sn/Pb bumps 50 µm pitch sensor chip with Ni/Au bump bond pads

9. Post-Bumping Wafer Thinning • Wafer thinning is done after bumping to prevent excessive handling and processing of thin wafers • A protective layer is applied to the wafer to protect the bumps during the taping, thinning, and de-taping processes • Wafer thinning process consists of two steps • Grind: to quickly remove Si from the wafer backside • Stress relief: to remove the damaged Si layer and alleviate the stress created in the silicon during the grind • Protective layer is removed prior to dicing

10. Dicing Considerations • Thinned ROC wafers are more susceptible to damage during dicing and require different blades and parameters • Dicing kerf must be very close to the active area (50 µm or less) on ROCs to allow close placement in multi-chip module assembly • Thin, high resistivity silicon sensors are susceptible to chipping and microcracking during dicing, which increases the leakage current

11. Poorly Diced Sensor Wafers

12. Cleanly Diced Sensor

13. Assembly Processes • Flip chip assembly of fine pitch bumped devices requires high placement accuracy bonder • Assembly of multi-chip module detectors have ROCs in very close proximity (~150 µm); process must not disturb previously placed die • Use of flux for reflow is undesirable due to difficulty removing flux residue under large chips

14. Standard Vs. Fine-Pitch Assembly 50um Pitch 250um Pitch • Chip-to-substrate gap reduces from 65µm to 22µm for 25µm diameter bumps

15. Replaces flux in assembly process Solder-bearing parts treated prior to assembly Short (10-15 min) treatment time Leaves no residues on chip or substrate Plasma Assisted Dry Soldering (PADS) • Proven applications in SMT, MEMS, photonics, and standard flip chip packaging and assembly processes

16. Current Programs

17. CMS Detector Modules • Readout chips are fabricated on full thickness 8-inch silicon wafers and are thinned to 200 µm prior to assembly, 4160 bumps per chip • Sensor wafers are fabricated on thin, high resistivity wafers • Bump size is 25 micron base diameter with a minimum I/O pitch of 50 microns • 6 different module sizes: 1x1, 1x2, 1x5, 2x3, 2x4, 2x5 • Full detector will require over 800 total modules with about 5000 individual readout chips • Total number of bumped connections is over 20,000,000

18. Pixilated Detector Module Assemblies 2x4 detector module in test fixture Courtesy: US-CMS FPix Collaboration

19. Yield Data • Recent evaluation of CMS detector modules (1x1, 1x2, 1x5, 2x3, 2x4, 2x5 arrays, 76 total modules) • 1134 bad bump connections out of about 2,000,000 • Bump bonding yield of 99.94% • Leakage current measurements previously completed on 61 modules • 60 of 61 modules meet leakage current specifications at 250V • 59 of 61 modules meet leakage current specifications at 600V • Power consumption on all modules within spec Courtesy: US-CMS FPix Collaboration

20. Yield Data Sensor Wafer 029 Courtesy: US-CMS FPix Collaboration

21. Yield Data Courtesy: US-CMS FPix Collaboration

22. MEDIPIX Consortium - CERN • X-ray/gamma ray detector devices working in single photon counting mode • 55 µm pitch, uniform in both directions • Detector modules of 1x1 (~1 in2) and 2x2 (~4 in2) • MEDIPIX ASIC is used in conjunction with different sensor devices for a number of applications • X-ray imaging • Biological radiography • Neutron detection

23. Pixilated Detector Module Assemblies MEDIPIX 2x2 detector array

24. MEDIPIX2 Images Courtesy: MEDIPIX Collaboration

25. MEDIPIX2 Images Courtesy: MEDIPIX Collaboration

26. Future Hybridization Technologies • 3D Integration • Alternative Bump Materials • Alternative Singulation Processes

27. 3D Integration • Through via interconnects (TVI) are formed through bulk silicon in active devices • Allows multiple device layers to be interconnected front-to-back • TVIs can be formed before or after devices are physically joined together • Significant process differences between vias first process and vias last process • Process used dictated by device design and process compatibility • Allows array sizes not limited to 1xN or 2xN modules: true area array ROC placement

28. Benefits of 3D Integration: Pixelated Devices • 3-D Integration allows massively parallel signal processing • Dramatically increased electronic functionality in each pixel Detector/Sensor Arrays Actuator Arrays Spatial light modulators w/digital control of optical wave front phases Mirror MEMS Actuator 3-D Interconnects 3-D Interconnects DARPA Coherent Communications, Imaging & Targeting (CCIT) program 3-D ROIC • 3-D Sensor Arrays • Large formats with high resolution • On-chip signal processing • Reduction of size, weight & power • 3-D Actuator Arrays • Large formats with high resolution • Low switching energy & latency • Reduction of size, weight & power

29. Test Structure Operability Test 65,536 interconnects in ~1 cm2 Si IC 25 mm 256x256 ROIC 20 mm Operability Map Insulator Copper 14 Defective pixels Si IC Nonfunctional cell Demonstrated 99.98% operability in 256x256 arrays with 4 mm vias on 30 mm pitch

30. Imaging Demonstration FPA cross section Thermal image Demonstrated image from 256x256 MWIR FPA built on 2-layer stack with 4 mmdiameter3-D interconnects (one per cell)

31. Alternative Bump Materials • Non-collapsible bump materials may be useful for extremely small bump interconnections (~5 µm dia.) Sn-capped Cu bumps

32. Alternatives to Saw Dicing • Silicon etching using Bosch process allows damage-free singulation of ROCs and sensor devices • Dicing streets must be free of metal Deposit and pattern photoresist Bosch etching complete Photoresist removal Bosch etching

33. Conclusion • RTI has developed a number of technologies to enable the successful bumping and hybridization of pixel devices • Currently applying these technologies to CMS and MEDIPIX projects for detector manufacture • New technologies under development will someday enable smaller pixel sizes in larger arrays with more functionality

34. FinAlan Huffmanhuffman@rti.org