Analysis of Mechanical Spalling Method for Cost Effective Production
of III-V Solar Cells
Anna Braun, Dustin Crouse
1, Dong Wu
1, Corinne Packard
1,2Colorado school of Mines
1, National Renewable Energy Lab
2Intent
III-V solar cells achieve significantly
higher efficiency than commercially available
silicon solar cells; however, the cost of
germanium substrates on which cells are grown makes them economically impractical. Substrate reuse is a proven method to reduce the cost. By electroplating stressed nickel onto the cell, it can be delaminated from the substrate allowing multiple substrate reuses. Understanding how current density affects the residual stress can help achieve the maximum number of reuses.
Objectives
• Repeatedly achieve a <10μm spall depth
• Determine the affect of bath chemistry and current density on nickel stress
• Redesign the spalling jig to control more variables that affect spall depth
Methods
• Stressed nickel layer is plated on germanium • A strain mismatch is created and causes a
fracture to propagate through the substrate
Stress Analysis
• The Stoney formula is used to calculate the nickel stress
𝜎𝜎𝑓𝑓 = 6ℎ𝐸𝐸𝑠𝑠ℎ𝑠𝑠2𝜅𝜅
𝑓𝑓(1 − 𝑣𝑣𝑠𝑠)
• XRD measurements were used to find the
curvature of samples plated at different current densities.
𝜅𝜅 = tan(Δ𝜃𝜃)Δ𝑑𝑑
• ESEM pictures of the XRD samples were
analyzed to find the thickness of the wafer and the film.
Conclusions
• For t = 3.90 min and J = 60 mA/cm² • Spall depth < 10 μm
• >90% repeatability achieved
• Keyence microscopy provides spatial map of topological features
• The new spalling jig allows fine speed control following a linear trend line
• The nickel modulus was not affected by the current densities or bath chemistries tested • The stress in the nickel film generally
increases with current density
Future Work
Stress Analysis
• Complete the stress analysis for the Watts Nickel and Ni-P bath
Spalling Jig
• Recalibrate for used of new spalling jig
• Test the effect of speed and force on the resulting spall depth
Electroplating
• Test bath stability
• Implement a float switch in the electroplating bath
Acknowledgments
National Science Foundation award
DMR-1461275, REU Site: Research Experiences for
Undergraduates in Renewable Energy
This project is supported by the Department of Energy’s Office of Renewable Energy and
Efficiency under contract number 0990-1594.
Special acknowledgement to our collaborators at the National Renewable Energy Laboratory: Aaron Ptak, John Simon, and David Young. Cassi Sweet, Chloe Castaneda, and Will Major have each made significant contributions to the project.
Force
Ge Substrate Ni Stressor
Film Spalling Depth
• No control over the force of the roller on the sample
• Variations in positioning of the roller and wafer • No precise way to control the speed
Old Jig:
New Jig:
Speed Calibration:
Redesigning the Spalling Jig
y = 635.38x - 32.775 R² = 0.9876 0 100 200 300 400 500 600 700 0% 20% 40% 60% 80% 100% 120% S p e d (mm/s ) Percent Speed
Max Speed Near Center of Slider
• Various current densities and plating times were tested
• Mechanical profilometry was used to determine the spall depth
Spall Depth Repeatability
Macro image: 200x 3D map: 500x High Current Density
1.50 min N/A
Low Current Density 9.90 min
~17 μm Mid Current Density
4.05 min N/A
Mid Current Density 3.90 min ~7 μm Spalled Germanium Nickel Film (Germanium side)
Nickel Modulus Testing
Watts Nickel 0.57 M H3BO3 1.26 M NiCl2 • 6H2O Ni-P Bath 10.0 mM H3PO3 0.6 M NiCl2 • 6H2O Reduced Phosphorous 5.0 mM H3PO3 0.6 M NiCl2 • 6H2O • 25 μm of nickel was plated on copper squares • In each bath chemistry, 2-3
samples were plated for 3 current densities • Nanoindentation was
used to find the elastic modulus Nickel Copper Avg: 227.7 GPa Avg: 208.4 GPa Avg: 228.6 GPa 0 500 1000 1500 2000 2500 30 40 50 60 70 80 90 S tre s s (MP a )
Current Density (mA/cm²)
