IFE Final Optics Using SiC substrates and Al coatings

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IFE Final Optics Using SiC substrates and Al
coatings

• M. S. Tillack, J. Pulsifer, K. Sequoia
• UC San Diego
• E. Hsieh, T. Walsh
• Schafer Corporation
• W. Kowbel
• MER Corporation

High Average Power Laser Program Project
Meeting SNLA Albuquerque, NM April 9-10, 2003
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Review of progress to date 1. Design concept
and key issues
Key Issues Shallow angle instability
Damage resistance/lifetime Goal 5 J/cm2, 108
shots Contamination resistance Optical
quality Fabrication Radiation resistance
The reference mirror concept consists of a
radiation-resistant substrate with a thin
metallic coating optimized for high reflectivity
(Al for UV, S-pol, shallow q)
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Review of progress to date 2. Accomplishments
and findings

• No signs of a shallow angle instability under
  any conditions
• Testing of diamond-turned surfaces showed high
  damage threshold at 532 nm, much lower at 248 nm
• UV damage observed in air, requiring testing in
  vacuum

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Review of progress to date2. Accomplishments
and findings, contd.

• All evidence suggests that surface contaminants
  are tolerable due to strong cleaning by laser
• Thin coatings have been difficult to produce with
  sufficient quality, and may have a lower damage
  resistance than Al plates
• We began to study perturbations to transmitted
  light in order to establish limits on
  microscopic damage

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Outline of the talk

• Coating options and issues
• Damage testing results for Al on SiC
• Test of carbon dust on a diamond-turned mirror
• Test of polished Al
• Studies of perturbations to transmitted light

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Coated optics are now being evaluated

• Substrate types
• superpolished CVD-SiC (RH)
• functionally graded SiC foam (MER)
• SiC/SiC composite (MER)
• Si on SiC on SiC/SiC (MER)

• Coating types
• MER
• evaporation coating
• PVD sputter coating
• Schafer
• Sputter coating
• e-beam evaporation
• Sputter plus e-beam

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All testing was performed with our KrF laser
400 mJ, 25 ns, 248 nm
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Interface thermal stress can be very high with
thin coatings of Al on SiC

• Plane stress analysis
• Stress at free surface 0
• Heat load resulting from 10 J/cm2 laser
• Peak stress at interface
• 40 MPa _at_30 ns
• Yield stress 10 MPa

• gt2 mm is desired
• gt1 mm is difficult

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Conventional wisdom coating quality
deteriorates with thickness beyond 300 nm
1 mm coating of Al on SiC
300 nm coating of Al on SiC
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7 mirrors were coated at Schafer Corp.

• 50 nm sputtered 0.5 mm e-beam evaporation
• 50 nm sputtered 1 mm e-beam
• 100 nm sputtered 2 mm e-beam
• 250 nm e-beam
• 1.5 mm e-beam
• 100 nm sputtered
• 200 nm sputtered

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Exposure causes pinpoint defects to grow
mirror 41, s/n 10157-024

• 50 nm sputtered 1.0 mm e-beam
• exposed to 500 shots above 5 J/cm2

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Nevertheless, the mirrors survive exposure

• Same mirror 50 nm sputtered 1.0 mm e-beam
• Surface exposed to 3.5 J/cm2 in vacuum for 5000
  shots
• Then 2000 shots at 5 J/cm2
• Local darkening appeared as well as pinpoint
  defect growth

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Damage accelerates beyond 5 J/cm2, perhaps
related to coating thickness?

• 50 nm sputtered 0.5 mm e-beam
• Surface exposed to 7 J/cm2 in vacuum for 1000
  shots
• No damage observed, so surface was exposed 1000
  more shots

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Damage morphology for 2 mm coating

• 100 nm sputtered 2.0 mm e-beam
• Surface exposed to 3.5 J/cm2 in vacuum for 1000
  shots, then 5.0 J/cm2 for 1000 shots

• No pinpoint defects, hence no defect-driven
  damage. Some darkening.

Is the surface cleaner due to thicker sputter
layer or thicker e-beam?
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Damage to 200 nm sputter coating

• 200 nm sputter coating only
• Surface exposed to 3.5 J/cm2 in vacuum for 100
  shots _at_1 Hz, then 3.5 J/cm2 for 5000 shots _at_5 Hz,
  5.0 J/cm2 for 2000 shots at 5 Hz

• No pinpoint defects on initial surface
• No defect-driven damage
• Some darkening
• Is oxide layer changing?

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Yielding at interface is not observed. Why?

• Strength of Cu increases with strain rate above
  104/s
• In other words, plastic deformation is rate
  limited
• Similar behavior is expected in Al

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Several new mirrors were fabricated at MER

• All mirrors have 300 nm Al by PVD
• Substrates include

• Sectioned Rohm Haas wafer of CVD SiC
• CVD SiC on graphite
• 40 mm CVD SiC on SiC/SiC composite
• 40 mm CVD Si on SiC/SiC composite

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RH substrate coated with 300 nm Al

• Surface exposed to 4-8 J/cm2 in air for several
  shots
• Immediate damage occurred due to poor substrate
  condition

after
before
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CVD Si on SiC/SiC has a generally smooth surface,
but many isolated defects
Mirror surface before exposure
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These defects light up at low doses

• Surfaces exposed to 3.5 J/cm2 in vacuum for 100
  shots

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Summary Good defects Bad defects

• Pits even deep ones have little effect on
  damage threshold
• Weakly attached impurities larger than 1 mm are
  removed by the laser without seeding damage
• Weakly attached impurities smaller than 1 mm may
  remain
• Embedded protrusions smaller than 1 mm may be
  etched away in time
• Embedded impurities or protrusions larger than 1
  mm are disastrous
• just one of these can ruin a mirror

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Experiments with carbon dust

• Surface exposed to 2-5 J/cm2 in air for 1000
  shots
• Larger contaminants removed smaller ones remain
  without apparent damage to substrate

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First experiments with polished Al

• 1-mm 99.999 pure Al, bonded with CA to 3-mm
  thick Al alloy
• SiC sanding wheels used to prepare surface for
  polishing
• Polishing with 5, 1, and 0.04 mm alumina (Al2O3)
  suspension.

25 nm avg. roughness
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Damage morphology to polished Al
2-5 J/cm2, lt100 shots, vacuum
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More damage morphology to polished Al
100 X
500 X
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The transmitted wave is an important diagnostic
for surface damage
The requirement on damage is 2 change in
spatial profile and not the appearance of visible
damage
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Surface map of mirror scan
An old, damaged diamond-turned surface was used
to highlight various changes to the transmitted
beam
Surface map
Measurements were made using an 8-bit camera
with 640x480 resolution We plan to acquire a
12-bit XGA camera for future studies
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Summary Conclusions

• Defects on thin-film mirrors continue to plague
  us.
• Schafer Al coatings on superpolished SiC show
  promise.
• Some of these surfaces can operate over long
  periods of time after surface changes occur.
  Future damage studies will examine the reflected
  wavefront as well as visible damage.
• Overcoating the Al to eliminate oxide effects
  should be tried (PVD Products).
• Monolithic Al mirrors provided good resistance
  previously. More testing of polished and
  diamond-turned Al, as well as Al-coated Al and
  novel Al microstructures (nanocomposited Al?)
  should be considered.