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<p>In this study, a simple two-surface interferometer was expanded to a three-surface interferometer to separate the upper and lower geometric thickness irregularities. The rigorous optical path difference (OPD) for each interference frequency was derived, and an important phase relationship was determined from the obtained rigorous OPD. A separation method for the two types of geometric thickness irregularities was proposed based on the rigorous OPD and Sellmeier equation for the measurement of the blank mask. Finally, the proposed method was confirmed by the simultaneous measurement of the three main phases using the 2<i>N</i>—1 phase-calculation algorithm, a wavelength-modulation Fizeau interferometer, and the phase relationship between the three main phases. The accuracy of the proposed method was less than 16&#xa0;nm in PV and 3&#xa0;nm in RMSE, which are extremely low considering the scale of the measured geometric properties and the size of the blank mask.</p>

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Precise Three-Surface Interferometry for Thickness Separation in Blank Masks

  • 期刊论文

  • 发表日期：

    2025年5月5日

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  • Hwan Kim,
  • Yangjin Kim,
  • Yusuke Ito,
  • Naohiko Sugita

摘要

In this study, a simple two-surface interferometer was expanded to a three-surface interferometer to separate the upper and lower geometric thickness irregularities. The rigorous optical path difference (OPD) for each interference frequency was derived, and an important phase relationship was determined from the obtained rigorous OPD. A separation method for the two types of geometric thickness irregularities was proposed based on the rigorous OPD and Sellmeier equation for the measurement of the blank mask. Finally, the proposed method was confirmed by the simultaneous measurement of the three main phases using the 2N—1 phase-calculation algorithm, a wavelength-modulation Fizeau interferometer, and the phase relationship between the three main phases. The accuracy of the proposed method was less than 16 nm in PV and 3 nm in RMSE, which are extremely low considering the scale of the measured geometric properties and the size of the blank mask.