SURFACE CHARACTERISTICS OF 3 TO 12 MICRON

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SURFACE CHARACTERISTICS OF 3 TO 12 MICRON Al
COATINGS ON SiC SUBSTRATES
Ed Hsieh, Brian Motta, Tom Walsh, Kelly
Youngblood Schafer Laboratories, Livermore,
CA Mark Tillack University of California at San
Diego, La Jolla, CA
Presented to HAPL Program Workshop, February 5
6, 2004 Georgia Institute of Technology, Atlanta,
GA
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EXTENDED ABSTRACT

• The work presented here is in support of
  Final Optics. We showed previously that a few
  nanometer surface smoothness can be obtained for
  a 2u thick Al coating e-beam deposited on
  ultra-smooth SiC substrates. The Grazing Angle
  Incidence laser test on the mirror showed
  promising results. It survived for slightly over
  50,000 shots _at_ 4 J/cm2. Based on stress analysis,
  a thicker Al coating may improve the
  survivability of the mirror.
• There is no problem in depositing thicker Al
  coatings. However, the surface smoothness
  degrades with deposition thickness. We are to
  determine the upper limit of coating thickness
  which will not cause enough degradation to
  distort the reflected laser beam at a grazing
  angle incidence.

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EXTENDED ABSTRACT (CONT.)

• We e-beam deposited 3, 5, 8, and 12µm of Al
  coatings on SiC substrates and examined them
  under optical microscope and WYKO interferometer.
  The WYKO scans are presented here. The scans for
  a bare and a coated (2µm of Al) SiC substrates
  from the previous experiments are included here
  as a reference. We are surprised to find that the
  Al coatings did not degrade the averaged
  roughness, Ra, or the RMS roughness, Rq,
  significantly from the bare SiC substrate.
  However, the average peak to valley separation,
  Rz, seems to increase with increasing thickness.
  For optical application, Rz may be more
  indicative of the surface roughness. Optically,
  at 1,000 magnification, we begin to see specks
  on the surface of coatings with thickness of 3µm.
  The density and size of the specks increase
  with thickness. At 8µm thickness, the surface
  appears slightly frosty, typical of thick Al
  coatings, and at 12µm the frostiness is
  prominent. At this point, we believe that the
  upper thickness limit may be around 5µm. All
  coated SiC substrates will be tested and results
  reported at a later date.

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(Si)
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(3µm Al)
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(5µm Al)
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(5µm Al)
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(8µm Al)
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(12µm Al)
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