Reduced-phase dual-illumination interferometer for measuring large stepped objects.

Area of Science:

• Optical Metrology
• Quantitative Phase Imaging
• Interferometry

Background:

• Traditional interferometry struggles with samples exhibiting large step height variations.
• Phase wrapping ambiguity complicates accurate surface topography reconstruction in quantitative phase imaging.
• Accurate measurement of micro-scale and macro-scale step heights is crucial in various scientific and industrial applications.

Purpose of the Study:

• To introduce and demonstrate a novel reduced-phase dual-illumination interferometer (RPDII) for topography measurement.
• To overcome the limitations of phase wrapping in quantitative phase imaging for samples with large step height variations.
• To achieve single-shot, unambiguous, and accurate measurement of sample thicknesses.

Main Methods:

• Utilizing a 4f optical system with a transmission grating to generate two collimated beams incident at different angles.
• Employing a reduced-phase dual-illumination interferometer (RPDII) for simultaneous recording of two phases and their difference.
• Controlling the relative phase difference to remain below 2π, thus eliminating phase wrapping ambiguity.

Main Results:

• Experimental demonstration of the RPDII's principle and feasibility.
• Successful measurement of metal layer thicknesses of 150 μm and 660 μm.
• Accurate topography reconstruction despite step height variations exceeding 1000 times the laser wavelength, without unwrapping algorithms.

Conclusions:

• The RPDII is a viable technique for single-shot quantitative phase imaging of samples with large step height variations.
• The method effectively eliminates phase wrapping ambiguity, leading to precise and reliable topography measurements.
• RPDII offers a robust solution for measuring significant step heights in metrology and surface characterization.