Metamaterial AR Coatings for Alumina
Joey Golec4/1/20
Method and Design
• Precision dicing of sub-wavelength structures creates a metamaterial that simulates dielectric layers which act as an AR coating
• This method has been used for all of the metamaterial AR coatings for the silicon optics
• Since alumina is much harder than silicon, constraints get placed on blade thickness
Alumina
Vacuum
At mm-wavelengths
Alumina
Vacuum
n1_eff
n2_eff
Coating Designs
• Only the MF coating has been fabricated and only on a 6 inch diameter wafer
• The UHF and LF designs are very preliminary and are subject to blade constraints, etc.
*Not to scale
Layer Kerf(K) Depth (D)1 0.687 mm 1.45 mm2 0.182 mm 0.946 mm
K1
K2
D1
D2
Pitch (P) = 1.554 mmLF (20/40 GHz)
P
PK1
K2
D1
D2
Layer Kerf(K) Depth (D)1 0.220 mm 0.425 mm2 0.07 mm 0.289 mm
Pitch (P) = 0.522 mm
P
KD
MF (90/150 GHz)
Layer Kerf(K) Depth (D)
1 0.110 mm 0.162 mm
Pitch (P) = 0.300 mmUHF (220/280 GHz)
Prototype Fabrication• We fabricated the AR coating
on a 6-inch diameter wafer
• Blade wear was the main complication of the fabrication process and resulted in the thick cut not being full depth
• If we implement automatic compensation for blade wear the fabrication procedure is simplified drastically but blade changes are inevitable
• I estimate it would take ~16 days to fabricate a 40 cm diameter optic
1.044 mm
Prototype Performance Forecast
• The 6-inch diameter prototype is at UChicago right now and measurements of the reflection performance are going to be made as soon as the labs open back up
• I modeled what the “imperfections” of the coating we fabricated
• The bottom plot to the right is the simulated performance of the fabricated coating
• The model with the rounded edges can be optimized to perform as well as the flat bottom cut model
Future Steps and Conclusions
• Measure the performance of the 6-inch diameter MF prototype and confirm it matches simulations
• AR coat the other side of the 6-inch prototype and measure to confirm percent level reflection
• In parallel, write code infrastructure to compensate for blade wear
• Investigate the viability of the coatings for other frequencies
• Barring significant speedups, it takes a couple weeks to completely fabricate a 40 cm diameter optic