Al COATED SiC MIRRORS FOR

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Al COATED SiC MIRRORS FOR FINAL OPTICS
Edmund Hsieh, Brian Motta, Kelly Youngblood and
Tom Walsh Schafer Laboratories, 303 Lindbergh
Ave., Livermore, Calif. 94551 Mark
Tillack University of California at San Diego, La
Jolla, CA 92093 Presented to HAPL Program
Workshop, September 24-25, 2003 Madison, Wisconsin
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ABSTRACT

• We are supporting Dr. Tillack of UCSD in
  making the SiC mirrors for the Final Optics.
  Aluminum was chosen as the reflective surface and
  the requirements for the Al coatings on the SiC
  substrates are still evolving. One of the
  critical requirements is that the Al coating has
  to be extremely smooth. For that reason, physical
  vapor deposition is the most logical process for
  coating the mirrors. We did both sputtering and
• e-beam depositions of various thicknesses on
  the SiC mirrors and some of the mirrors were
  tested and evaluated. Results and a plan for the
  future work will be presented.

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SUMMARY PLAN

• The pin-point defects observed on some of the
  coated mirrors are due to defective SiC
  substrates.
• We have established a procedure to strip the Al
  from SiC substrates and concluded that indeed the
  substrates can be reused.
• Extremely smooth Al coatings with thicknesses of
  up to 2.5 microns were deposited by e-beam
  deposition. The surface smoothness is in the
  range of less than a few nanometers. We have
  repeated the depositions of microns thick Al
  coatings with extremely smooth surface.
• Rohm Hass, our supplier, can produce SiC
  substrates with no pin-point defects and with
  very low defect densities. We have two of such
  substrates on hand and we can set up acceptance
  specifications for future substrates.
• In examining the damaged site on substrate -034,
  the catastrophic coating failure seems to start
  from a single defect on the mirror. We notice
  also that there were one or two defects under the
  laser footprint which did not lead to
  catastrophic failures. We can only say that not
  all defects lead to failure.
• The damaged sites on substrates -034(2u thick)
  and -026(0.25u thick) look quite different.
  Coating thickness may be an important factor in
  mirror durability.
• Our plan is to work closely with Dr. Tillack to
  come up with a durable mirror for Final Optics
  and be responsible for the scaled-up (0.5 meter
  sized mirrors) to produce the prototype mirrors
  for the test reactor.

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Defective SiC substrates cause Pin-Point defects
in mirrors

• We observed Pin-Point defects, a more or less
  uniform distribution of sub-micron sized pits, on
  uncoated SiC substrates.
• After striping the Al coating off from a mirror
  with Pin-Point defects, the striped surface shows
  similar Pin-Point defects.
• We re-coated the striped substrate and the
  resultant mirror also showed Pin-Point defects.
• Lastly, we coated a substrate with no Pin-Point
  defects and obtained a mirror with no Pin-Point
  defects. In combination, we showed that the
  coating process did not contribute to the
  Pin-Point defects.

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Procedure to strip Al coated SiC mirrors for
re-use

• Heat three beakers with 200 ml of DI H2O on a hot
  plate to 50C. Dissolve 5 grams of KOH in the
  first beaker, 1 gram in the second and none in
  the third. Fill two more beakers with 200 ml of
  DI H2O and do not heat.
• Grab the coated mirror by the side with tweezers.
  Place in each beaker for one minute, gently
  agitating, starting with the strong solution to
  the fresh rinse. Keep exposure to air at a
  minimum to avoid drying which causes water spots.
  Rinse with Methanol, Isopropyl and Acetone, in
  that order, over Petri dish. Place mirror on
  folded lab cloth so it sits at an angle, and blow
  off with copious filtered dry nitrogen starting
  from the high side to the low side.
• Inspect under high magnification and return for
  re-use.

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Damaged area on mirror 026 shows no melting but
has multiple catastrophic defects
5x
10x
? 50u?
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Damaged area on mirror 034 shows melting and
the failure was initiated from a single defect
5x
10x
? 50u?
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Damaged Area on mirror 034 (high mag)
? 2u ?
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Plan for the proto-type 0.5 meter SiC mirrors

• Rohm Hass indicated that they are interested to
  supply the SiC substrates.
• We can handle the intermediate (up to 0.15 meter)
  mirrors at our present facility.
• We have a working relationship with Surface
  Optics Corporation which has facility to coat
  meter sized substrates.
• Surface Optics and Schafer jointly worked on a
  SBRI phase I grant to deposit uniform multi-layer
  coatings on over meter sized mirrors and we can
  work with Surface Optics to coat our proto-type
  0.5 meter mirrors.