• Nuclear decay product - Helium nucleus with +2 charge - High kinetic energy (2-8 MeV) - 10-30 m range in silicon • Packaging Technology transition - Wafer Level / 3D Packaging • Alpha sources closer in proximity to sensitive nodes • Low alpha materials required for increasing number of applications
10
Particle Energy (MeV)
Understand alpha behavior in Pb-free materials 2012 IEEE SER Workshop
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Paradigm Shift • Current Industry Understanding - Secular equilibrium is an inclusive explanation of time dependent alpha emission levels
• Secular Equilibrium Explanation Insufficient - Experimental data departs from theoretical prediction models • This is a concern… - Alpha emission from the packaging materials potentially higher than projected
Sn Alpha Emissivity vs Time 14
B
Counts/khr·cm2
12
C Sec. Equil. Model Sec. Equil. Model
10
8
6
4
2
0 0
100
200
300
400
500
Time (Days)
600
700
800
2012 IEEE SER Workshop
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Experimental Design • Hypothesis: Microsegregation transports emitters towards the center of the mass during solidification Elements partition between liquid and solid phases Solute distribution is heterogeneous
• Select Sn materials with range of surface emissions - Measure alpha flux in sequential layers of the volume - Dissolve >15 μm from the surface with each iteration Alpha Flux Measurements
Initial Surface
Layers removed by acid dissolution 700 µm
Final Surface
2012 IEEE SER Workshop
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Alpha Emissivity vs. Depth
Alpha Emission vs Depth: 100 μm 80
Linear Concentration
Counts∙khr-1·cm-2
70
Gradients
y = 0.86x + 12 R² = 0.98
60
A B
50
C
40
y = 0.22x + 8.4 R² = 0.99
30 20
y = 0.019x + 0.78 R² = 0.35
10 0 0
20
40
60
80
100
120
Depth from Surface (μm)
Alpha surface emissivity A>B>C 2012 IEEE SER Workshop
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Microsegregation Discussion • Distribution curve slope is determined by emitter concentration in the material • Trace impurities transported into the center of mass - Alpha particles detected from only top ~ 20 μm - Initial alpha measurements may be erroneously low • Heterogeneous emitter distribution does not explain the time dependant alpha data - Surface alpha emitter concentration increasing
Additional time dependent mechanism at work 2012 IEEE SER Workshop
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Polonium Diffusion Experiment • Hypothesis: - Alpha emission increasing over time from 210Po diffusion • Reproduce experimentally to confirm mechanism - Doped low alpha Sn with 210Po - Sample heated & compared against room temp control • Measure/Heat/Measure cycle repeated Alpha emissivity vs [210Po]
Alpha Spectrum ‐ Sn Heat Experiment Significant increase in 210Po
3
Pre heat
Alpha‐hr‐1
2.5
Heat Cycle 1 Control
2 1.5 1 0.5 0
1
2
3
4
Energy (MeV)
5
6
1.0E+03
2 π Emission cts ∙khr‐1∙cm‐2
3.5
1.0E+02 1.0E+01 1.0E+00 5.8E‐01
5.8E+00
5.8E+01
5.8E+02
5.8E+03
1.0E‐01 1.0E‐02
210Po equivalent concentration (atoms/g Sn)
2012 IEEE SER Workshop
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Emissivity vs. Time: 210Po doped Sn
Diffusion accelerated by temperature 2012 IEEE SER Workshop
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Dynamic Alpha Emissivity • Heat conditioning drives majority of polonium to the surface - Heating on consecutive days did not change Po activity - Diffusion is accelerated ~240x at 200 °C vs. 23 °C • Surface alpha emission determined by Po concentration - Po concentration > 10-4 parts per trillion unacceptable (>2 α·khr1·cm-2)
Analyzed volume
Time elapse
210Po
diffusion to surface
-1·cm-2 Transport Alpha Emitters Confirmed Material of state after packaging >2 α·khr 2012 IEEE SER Workshop
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Packaging implications • Microsegregation effects in packaging processes - Phase transition processes susceptible - Process temperature accelerated diffusion • Polonium mobility considerations - Static position of emitters oversimplified - Negate to some extent shielding assumptions made in design • Diffusion transports emitters in volume to surface Surface Po Activity vs Sphere Diameter 1.E+04 1.E+03
α∙khr‐1∙cm‐2
1.E+02 1.E+01 1.E+00 1.E‐01
1
50
100
150
1.E‐02 1.E‐03 1.E‐04
Po atoms/g 1E+04 1E+03
1.E‐05 1.E‐06
Sphere Diameter (μm) 2012 IEEE SER Workshop
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Conclusions • Microsegregation and diffusion effects can result in increased alpha emissions • Both mechanisms should be considered when evaluating packaging processes in high reliability applications • Most direct solution is to selectively eliminated Po from feedstock materials
2012 IEEE SER Workshop
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Acknowledgements • Michael Fields: Analytical data • Michael Paladin: Sample Preparation • Thomas Ploegman: Sample Preparation • Jack Long: Sample Preparation
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