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SURFACE CHARACTERISTICS OF 3 TO 12 MICRONAl COATINGS ON SiC SUBSTRATES

Ed Hsieh, Brian Motta, Tom Walsh, Kelly YoungbloodSchafer Laboratories, Livermore, CA

Mark TillackUniversity of California at San Diego, La Jolla, CA

Presented to HAPL Program Workshop, February 5 & 6, 2004Georgia 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 @ 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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0

2

4

6

8

10

12

14

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18

20

Roughness (nm)

SiWafer

10157-034

10157-024

10144-072

10157-010

10157-015

Sample

Surface Roughness

R-a

R-z