TY - JOUR
T1 - Progress of coating stress compensation of silicon mirrors for Lynx x-ray telescope mission concept using thermal oxide patterning method
AU - Yao, Youwei
AU - Chalifoux, Brandon D.
AU - Heilmann, Ralf K.
AU - Chan, Kai Wing
AU - Mori, Hideyuki
AU - Okajima, Takashi
AU - Zhang, William W.
AU - Schattenburg, Mark L.
N1 - Funding Information:
This work was supported by NASA Grant Nos. NNX14AE76G and NNX17AE47G
Funding Information:
This work was supported by NASA Grant Nos. NNX14AE76G and NNX17AE47G.
Publisher Copyright:
© The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.
PY - 2019/4/1
Y1 - 2019/4/1
N2 - A thermal oxide patterning method has proven to be effective for correcting coating-stress-induced distortion on flat silicon wafers. We report progress on developing this method for correcting curved silicon mirrors distorted by front-side iridium coatings. Owing to the difference in geometry, a finite element model has been established to calculate the appropriate duty cycle maps in thermal oxide hexagon patterns used for compensation. In addition, a photolithographic process, along with three-dimensional printed equipment, has been developed for creating patterns precisely on the back side of curved mirrors. The developed method has been used to recover the original surface shape of two silicon mirrors which are 100-mm long, 0.5-mm thick, having 312-mm radius of curvature, and 30 deg in azimuthal span (Wolter-I geometry). These mirrors' front sides are sputter-coated by 20-nm iridium layers with ∼-70 N / m integrated stress. Measurement results show that the developed method can mitigate coating-induced distortion by a factor of ∼5 in RMS height and ∼4 in RMS slope error, corresponding to ∼0.5 arc sec RMS slope error. Residual errors after correction are dominated by mid-frequency ripples created during the annealing process, which will be resolved in the future. The presented method is precise and inexpensive and a potential candidate for resolving the coating stress issue for Lynx optics in the future.
AB - A thermal oxide patterning method has proven to be effective for correcting coating-stress-induced distortion on flat silicon wafers. We report progress on developing this method for correcting curved silicon mirrors distorted by front-side iridium coatings. Owing to the difference in geometry, a finite element model has been established to calculate the appropriate duty cycle maps in thermal oxide hexagon patterns used for compensation. In addition, a photolithographic process, along with three-dimensional printed equipment, has been developed for creating patterns precisely on the back side of curved mirrors. The developed method has been used to recover the original surface shape of two silicon mirrors which are 100-mm long, 0.5-mm thick, having 312-mm radius of curvature, and 30 deg in azimuthal span (Wolter-I geometry). These mirrors' front sides are sputter-coated by 20-nm iridium layers with ∼-70 N / m integrated stress. Measurement results show that the developed method can mitigate coating-induced distortion by a factor of ∼5 in RMS height and ∼4 in RMS slope error, corresponding to ∼0.5 arc sec RMS slope error. Residual errors after correction are dominated by mid-frequency ripples created during the annealing process, which will be resolved in the future. The presented method is precise and inexpensive and a potential candidate for resolving the coating stress issue for Lynx optics in the future.
KW - Coating stress
KW - Figure correction
KW - Silicon mirror
KW - Stability
KW - Thermal oxide pattern
KW - X-ray
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U2 - 10.1117/1.JATIS.5.2.021011
DO - 10.1117/1.JATIS.5.2.021011
M3 - Article
AN - SCOPUS:85065488522
SN - 2329-4124
VL - 5
JO - Journal of Astronomical Telescopes, Instruments, and Systems
JF - Journal of Astronomical Telescopes, Instruments, and Systems
IS - 2
M1 - 021011
ER -