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

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

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

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

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

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

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

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

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

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

Poorly Diced Sensor Wafers

Cleanly Diced Sensor

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

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

Plasma Assisted Dry Soldering (PADS) 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 Proven applications in SMT, MEMS, photonics, and standard flip chip packaging and assembly processes

Current Programs

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

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

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

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

Yield Data Courtesy: US-CMS FPix Collaboration

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

Pixilated Detector Module Assemblies MEDIPIX 2x2 detector array

MEDIPIX2 Images Courtesy: MEDIPIX Collaboration

MEDIPIX2 Images Courtesy: MEDIPIX Collaboration

Future Hybridization Technologies 3D Integration Alternative Bump Materials Alternative Singulation Processes

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

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

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

Imaging Demonstration FPA cross section Thermal image Demonstrated image from 256x256 MWIR FPA built on 2-layer stack with 4 mm diameter 3-D interconnects (one per cell)

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

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 Bosch etching Photoresist removal

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

Fin Alan Huffman huffman@rti.org