Abstract
Optical coherence tomography (OCT) is a non-invasive technique for cross-sectional imaging. It is particularly advantageous for applications where conventional microscopy is not able to image deeper layers of samples in a reasonable time, e.g. in fast moving, deeper lying structures. However, at infrared and optical wavelengths, which are commonly used, the axial resolution of OCT is limited to about 1 μm, even if the bandwidth of the light covers a wide spectral range. Here, we present extreme ultraviolet coherence tomography (XCT) and thus introduce a new technique for non-invasive cross-sectional imaging of nanometer structures. XCT exploits the nanometerscale coherence lengths corresponding to the spectral transmission windows of, e.g., silicon samples. The axial resolution of coherence tomography is thus improved from micrometers to a few nanometers. Tomographic imaging with an axial resolution better than 18 nm is demonstrated for layer-type nanostructures buried in a silicon substrate. Using wavelengths in the water transmission window, nanometer-scale layers of platinum are retrieved with a resolution better than 8 nm. XCT as a nondestructive method for sub-surface tomographic imaging holds promise for several applications in semiconductor metrology and imaging in the water window.
Highlights
A self-evident way to improve the resolution further is the reduction of the central wavelength
XCT is limited in its applications by transmission windows of the host material of the sample owing to the strong photo-absorption of XUV radiation
We already successfully build and tested an XUV interferometer for monochromatic light[17] and we will adapt it to fit the needs of XCT in the future. Another drawback of the current setup is the lateral resolution of a few hundert micrometers only
Summary
A self-evident way to improve the resolution further is the reduction of the central wavelength. The development of extremely broadband XUV and soft-X-ray sources in the framework of attosecond pulse generation[9,10] and other physics disciplines in the last years suggests a realization of OCT in the XUV regime such that coherence lengths of a few nanometers are in reach. This variant of OCT is referred to as XCT11,12. Our XCT setup is a variant of common-path OCT where the sample arm and the reference arm share the same path[16] This has been realized by a highly reflective capping gold or platinum layer on top of the sample, which provides the reference beam. We completely avoided the use of a freestanding broadband beam splitter as it is typically used in OCT because it is technically extremely demanding (see Supplementary Notes online)
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