anti-reflection coated silica windows for electra stuart searles, john sethian naval research...
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Anti-Reflection Coated Silica Windows for Electra
Stuart Searles, John Sethian Naval Research Laboratory
Washington, D.C.
Russell SmilgysScience Applications International Corporation
McLean, Virginia
November 13+14, 2001
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Immediate Objective
Acquire 30 cm Electra window with AR coating able to survive 5 hours of laser operation.
Window Exposure:
0.7 J/cm2 @ 5 Hz (100 ns pulse width) Radiation excited by 450 kV e-beam into laser gas Inside cavity: 1 atm Ar, Kr + 0.3% fluorine, trace HF Outside cavity: Air
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Electra Window Specifications
• Transmission at 248 nm (> 99.5%)• Damage threshold 1.4 J/cm2 in 100 ns @ 5 Hz
• Surface quality 20-10 scratch-dig Maximum inclusion size 0.1 mm
• Survive highly corrosive environment
- Long term exposure to fluorine
- Exposure in combination with UV
- Exposure in combination with radiation
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AR Coated Window Availability
• Few vendors able and willing to coat large windows.
• Current vendor supplies AR coated fused silica windows 20 cm × 20 cm for NIKE amplifier. Survives hundreds of single laser pulses @ 0.25 J/cm2 (100 nsec pulse duration).
• Current vendor unproven to meet Electra’s specifications.
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Electra Window Design
Fused silica windows (bare windows on order)
- Corning code 7980, KrF grade fused silica
Absorption @ 248 nm = 0.001136/cm
AR (antireflection) coating on both surfaces
- Conventional AR coating on atmospheric side
Dielectric stack: e.g. SiO2/MgF2
- Fluorine resistant AR coating on laser cavity side Dielectric stack: e.g. Al2O3/AlF3
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Technical Approach
• Acquire 1” diameter coated windows- Coating system at SAIC in McLean, Virginia- Commercial vendors
• Test
- Gas cell: HF and F2 exposure with and without UV
- Lumonics laser: laser damage threshold
• Evaluate- Spectrophotometry: transmission, optical constants- Microscopy: optical, and SEM- Interferometry: ZYGO
- Electra laser cavity (planned): electrons, x-rays, UV
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Coating System at SAIC in McLean,VAEquipped for electron beam evaporation with ion beam assist (IBAD)
- Vacuum chamber 8’ by 12’ cylinder- Base pressure 10-7 Torr- Up to four different coating materials in a run- Accommodates sheets up to 30”×60”
- Fixture mounts six 1” substrates- Heats to 300°C- Provision for cover sheet to block dust during pump down
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Laser Damage Testing of Windows
Use Lumonics KrF laser to simulate Electra
Laser specification:
• 30 ns pulse duration @ 10 Hz
• Spot size 2 × 3 mm
• Achieve up to 4 J/cm2 with focusing optics
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Exposure to Fluorine and HF
• Diatomic fluorine is highly reactive.
• Atomic fluorine is more reactive than diatomic fluorine.
- Atomic fluorine created by UV photodisssociation.
• HF formed by reaction of fluorine with water vapor.
- HF highly corrosive to optics.
- HF can be eliminated from Electra cavity by water vapor control.
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One Month Exposure to 5% F2
• Uncoated silica window
- Strongly degraded surfaces
• Vendor ‘C’ with AR coated surfaces
- Low density of deep pits (up to 3 microns)
- General surface unaffected (6 nm rms)
• Next test: need to monitor and control HF levels
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ZYGO Oblique Plot, Vendor ‘C’• One month exposure
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Combination of UV, Fluorine
• 4 J/cm2 600 pulses and 5% F2
• Vendor ‘C’ window with AR coated surfaces
- General surface unaffected
- Three pits
• Vendor ‘A’ window with AR coated surfaces
- General surface unaffected
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ZYGO Oblique Plot, Vendor ‘C’• Exposure to UV, F2
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ZYGO Solid Plot, Vendor ‘C’• Exposure to UV, F2
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Plans• Extend tests to larger areas and develop test procedures.
• Measure induced absorption in fused silica rods to evaluate possible limitation in the regime of millions of pulses.
• Proceed with upgrade of coating system at SAIC to coat windows up to 60 cm in size for Electra and NIKE.