Optical Analysis of IBF Polished Silicon Wafers for Purpose in Exoplanet Direct Imaging

Abstract

NASA’s direct imaging of exoplanets missions and projects such as WFIRSTand LUVOIR require fabricated coronagraph masks to control scatteringand diffraction of light. The High-contrast Imager for Complex ApertureTelescope (HiCAT) testbed at Space Telescope Institute (STScI) advancesthe coronagraphy technology for these missions. The lithographypatterned mask is fabricated on ion beam figuring (IBF) polished waferwith cryogenically etched black-silicon. This study shows IBF polishedwafers could improve the surface figure on the wafers to only a fewnanometers RMS and black silicon Bidirectional Reflectance DistributionFunction (BRDF) measurement shows very low specular reflectance.

Results

Kaseylin Yoke, Oregon State UniversityRon Shiri, Optics Branch, Code 551

Background

Interferometers:• Uses interference patterns to “scan” the surface of the wafers and

create a surface mapIon beam figuring (IBF):• Uses an ion beam in a vacuum to polish optical surfaces• The accelerated ions remove a pre-set amount of material based on

the surface map of the waferExoplanet direct imaging:• In order to directly image an exoplanet, the central light in its solar

system must be blocked• The light from the star must be reduced by ten orders of magnitude

Methodology

Conclusion/Further Studies

For corresponding pucks, both the B7 andB34 interferometers yield roughly thesame results for both RMS and PV, and theIBF polishing of the pucks reduces the RMSsignificantly. It has been discussed that IBFcould lower the RMS even further if it ranover individual pucks in sections, ratherthan as a whole. This could be an area offuture study. The next step in this projectwould be to coat the wafers with blacksilicon and perform tests to determinecertain light-absorption properties of thecoated wafers.

References/Acknowledgements

[1] Soummer, Remi, Brady, R., G., Brooks, Keira, . . . MarshallD. (2019, March 13). High-contrast imager for complexaperture telescopes (HiCAT): 5. first results with segmented-aperture coronagraph and wavefront control. Retrieved fromhttps://arxiv.org/abs/1903.05706

[2] Fuerschbach, K., Rolland, J. P., & Thompson, K. P. (2014).Theory of aberration fields for general optical systems withfreeform surfaces. Optics Express, 22(22), 26585.https://doi.org/10.1364/oe.22.026585

[3] Zernike expansion schemes. (n.d.). TeliscopeOptics.Net.Retrieved July 28, 2020, from https://www.telescope-optics.net/zernike_expansion_schemes.htm

Thank you to Ron Shiri, Brooke Fujishima, Michael Biskach,Christine Jhabvala, and Joseph Mcmann, Ray Ohl, EdWollack, and Aruna Ramanayaka for their expertise andsupport on this project. Special thanks to the Oregon SpaceGrant Consortium for providing the funding for this project.

Introduction

NASA’s direct imaging of exoplanets missions and projects such as WFIRSTand LUVOIR require fabricated coronagraph masks to control scatteringand diffraction of light. In this study, we intend to enhance the pathfindermasks for High-contrast Imager for Complex Aperture Telescope (HiCAT) atSpace Telescope Institute (STScI) while assessing their opticalperformance independently. The fabrication of pupil mask involves siliconwafers to be sliced and polished at 𝜆/20 using recently procured ion beamfiguring (IBF) system where the absorbent region will be black-siliconetched using Oxford Instrument Etching system currently undercommission at the GSFC Detector Characterization Lab. A comparisonbetween pre- and post-IBF polished silicon wafers done in Summer 2019showed that IBF polishing greatly decreases the surface figure error onthe silicon wafers. In this study, the results from Summer 2019 will beverified and corrected. Additionally, a Zernike polynomial decompositionwill be performed on each wafer to determine the exact surfaceaberrations present.

Zernike polynomial decomposition:To determine the exact aberrations onthe wafer surfaces, a Zernikepolynomial decomposition wasperformed on each wafer. Figures weregenerated with the firth 15 FringeZernike polynomials with theircorresponding RMS values for eachwafer. The code also printed out thecoefficients for each polynomial. TheRMS and coefficients were rankedfrom highest to lowest to show whichpolynomial had the greatest surfaceaberration. The Zernike decompositionfor ESM2 are shown in the third part ofthe Results section. To the right are thefirst 15 Fringe Zernike polynomials andtheir names. Note that the number isnot a ranking, merely the number ofeach polynomial in the series.

For this project, a total of eight silicon wafers were analyzed. Each wafer was 2 inches in diameter. Six ofthe wafers (Pucks 1, 2, 16, 17, 18, and 19) were 5 mm thick, and two (ESM1, ESM2) were 500 µm thick.To conserve space, only the analysis for Puck 16 and ESM2 are shown in the results section.

Obtaining data:The data for this project was obtained during Summer 2019. Each puck was mounted and aligned withthe interferometers in B7 and B34. After being run through both the B7 and B34 interferometers, thewafers were put through the IBF machine in B34. The wafers were polished using the interferometrydata from B34. Interferometry data was taking both before IBF polishing and then again after thepolishing, and all data was then run through a Matlab script for analysis. .

Pre/post-IBF analysis:The raw interferometer data from both B7 and B34, pre- and post-IBF, was run through a Matlab script(provided by Ron Shiri) to produce surface maps of each wafer. Minor alterations were made to thescript to perform actions such as masking out noise in the data and changing the aperture percentage.The code was also altered to perform a Zernike polynomial decomposition on each wafer. As there wereerrors in the surface maps generated during the Summer 2019 internship, the surface maps were re-generated with corrections and verified with results from Timo Saha on the same data. Apertures of100%, 95%, 90%, and 80% were generated (Mr. Saha’s results did not include the 80% aperture surfacemaps). The comparison with Mr. Saha is shown in the first part of the results section, and is for Puck 16.The re-generated surface maps for ESM2 are shown in the second section.

1. Comparison with Timo Saha

2. New surface maps

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3. Zernike decomposition

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post-IBF pre-IBF

• Example Zernike decomposition: ESM2 B7, pre- vs. post-IBF

• Coefficients show the first 3 (piston, tip, tilt) are removed

• Highest aberration:• Pre-IBF: 4th• Post-IBF: 9th

• Lowest aberration:• Pre-IBF: 14th• Post-IBF: 12th