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Al COATED SiC MIRRORS FOR “FINAL OPTICS”*

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

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Al COATED SiC MIRRORS FOR “FINAL OPTICS”*

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  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. 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. Procedure to strip Al coated SiC mirrors for re-use

  6. Damaged area on mirror #026 shows no melting but has multiple catastrophic defects 5x 10x | 50u|

  7. Damaged area on mirror #034 shows melting and the failure was initiated from a single defect 5x 10x | 50u|

  8. Damaged Area on mirror # 034 (high mag) | 2u  |

  9. 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.

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