Nanoscale surface topography imaging using phase-resolved spectral domain optical coherence tomography

Abstract

Microscopic surface topography plays an important role in studying the functions and properties of materials. Microscopic surface topography measurement has been widely used in many areas, such as machine manufacturing, electronic industry and biotechnology. Optical interferometry is a popular technique for surface topography measurement with an axial resolution up to nanoscale. However, the application of this technique is hampered by phase wrapping, which results in a limited measurement range for this technique. Various digital algorithms for phase unwrapping have been proposed based on the phase continuity between two adjacent points. However, several significant challenges still exist in recovering correct phase with this technique. Optical coherence tomography (OCT) is a non-contact three-dimensional imaging modality with high spatial resolution, and it has been widely used for imaging the biological tissues. In this paper, we demonstrate a method for nanoscale imaging of surface topography by using common-path phase-resolved spectral domain OCT to reduce the influence of phase wrapping. The system includes a superluminescent diode with a central wavelength of 1310 nm and a spectral bandwidth of 62 nm, an optical fiber circulator, a home-made spectrometer, and a reference arm and a sample arm in common-path arrangement. The reference mirror and the sample under investigation are positioned on a same stage in order to further reduce the influence of ambient vibration. The phase difference between two adjacent points is calculated by performing Fourier transform on the measured interferometric spectrum. The phase difference distribution of the surface is obtained first. And then, the surface topography of the sample is constructed by integrating the phase difference distribution. In the traditional methods, phase wrapping occurs if the absolute value of the measured phase is greater than . However, in the present method, phase wrapping occurs if the absolute value of the phase difference between two adjacent points is greater than . The maximal detectable absolute value of the phase difference between two adjacent points increases from for the traditional methods to 2 for the present method. The experimental results indicate that the present system has a high stability and the maximum fluctuation is less than 0.3 nm without averaging. The accuracy of the system is tested with a piezo stage, and the mean absolute deviation of the measured results is 0.62 nm. The performance of the present system is also demonstrated by the surface topography imaging of an optical resolution test target and a roughness comparison specimen. The experimental result shows that the present system is a potential powerful tool for surface topography imaging with an axial resolution better than 1 nm.