IFE Final Optics Using SiC substrates and Al coatings - PowerPoint PPT Presentation

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IFE Final Optics Using SiC substrates and Al coatings

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Title: IFE Final Optics Using SiC substrates and Al coatings


1
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
2
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)
3
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

4
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

5
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

6
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

7
All testing was performed with our KrF laser
400 mJ, 25 ns, 248 nm
8
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

9
Conventional wisdom coating quality
deteriorates with thickness beyond 300 nm
1 mm coating of Al on SiC
300 nm coating of Al on SiC
10
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

11
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

12
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

13
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

14
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?
15
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?

16
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

17
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

18
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
19
CVD Si on SiC/SiC has a generally smooth surface,
but many isolated defects
Mirror surface before exposure
20
These defects light up at low doses
  • Surfaces exposed to 3.5 J/cm2 in vacuum for 100
    shots

21
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

22
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

23
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
24
Damage morphology to polished Al
2-5 J/cm2, lt100 shots, vacuum
25
More damage morphology to polished Al
100 X
500 X
26
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
27
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
28
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.
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