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

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

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

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.

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

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

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

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

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.

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

ZYGO Oblique Plot, Vendor ‘C’• One month exposure

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

ZYGO Oblique Plot, Vendor ‘C’• Exposure to UV, F2

ZYGO Solid Plot, Vendor ‘C’• Exposure to UV, F2

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.