利用雙波長相移干涉術量測步階深度

Abstract

光學表面微輪廓儀引入雙波長相移干涉術進行測量計算可擴展步階深度量測的限制。為了要提昇量測成功機率與重複性，增加縱向深度與坡度量測彈性，我們嘗試各種相位重建技術並且比較其還原結果。 量測系統採用的架構為斐佐式干涉儀，所使用的光源為紅光氦氖雷射﹙632.8 nm﹚和綠光氦氖雷射﹙543.5 nm﹚。雙波長的計算程序如下：分別使用兩種光源量測後，將兩種相位數據相減，可得到一組新的相位，對該數據進行相位重建，再轉換成深度分布即可。採用雙波長相移干涉術求得的表面輪廓，就如同使用紅外光源﹙3.85 mm﹚進行量測的效果。 和單一波長測量的結果相比，使用雙波長相移干涉術計算的訊噪比較差。這是因為深度與相位分布、量測波長成正比，如果存在等量的相位雜訊，波長越長換算成深度變化的雜訊也越大。因此雙波長量測的結果會有較大的雜訊。如果雙波長的雜訊小於二分之一的單一波長，將雙波長量測結果用來校正單一波長相位分佈，可以得到較理想的相位分布。如此可以增加單一波長量測的動態範圍﹙深度/平整度﹚和量測訊噪比。

Two-wavelength phase-shifting interferometry is applied to an optical profiler extending the unambiguous step height measurement range. In order to improve feasibility and repeatability of the measurement and to increase the ability of measuring the depth and steep, we try different kinds of phase unwrapping techniques and compare the results with each other. The measurement system consists of a Fizeau interferometer and an optical microscope. The measurement wavelengths used in this work are 632.8 and 543.5 nm. After measuring with the both wavelengths independently, the step height can be obtained by subtracting the two measurement results, unwrapping the newly derived phase, and then converting to surface profile. The profile is determined as if the surface were tested at a synthesized equivalent wavelength mm. The signal-to-noise ratio (SNR) at the equivalent wavelength is lower than those at the single wavelength measurements due to an error magnification proportional to the ratio of two wavelengths. The equivalent wavelength data can be used to correct visible wavelength measurements, as long as the noise in the equivalent wavelength phase is less than one-half of that in single-wavelength. This correction extends the dynamic range of smaller wavelengths and increase the SNR of the measurements.