Characterization of semiconductor materials using synchrotron radiation
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This work covers the use of scattering-type near-field optical microscopy with the Metrology Light Source (MLS) as a broadband synchrotron radiation source to characterize different semiconductor materials. They carried out scans across nanoscale Si-based FT surface structure using nano-high spectroscopy.
Figure 1. Schematic diagram of the experimental s-SNOM setup using broadband synchrotron radiation in the IR regime from the electron storage ring MLS. The IR radiation is coupled out at a bending magnet and guided by several mirrors (not shown in the image) to the experimental setup. The focused IR beam has a diameter of about 80 μm (inset with an optical microscopy image).
The following tests were carried out on a commercially available scattering-type scanning near-field optical microscope (Neaspec GmbH, Germany) that consists of an asymmetric Michelson interferometer and an AFM. The MLS is a low-energy storage ring that can produce spectral f -IR and THz radiation (629 MeV electron energy). A planar water-cooled mirror couples the infrared radiation out of the ring at the bending magnet, and other planar and cylindrical mirrors direct it to the experimental equipment. The sample with rectangular SiO2 patterns on a Si substrate was used in the near-field imaging tests (Fig. 1).
Figure 2. Topography ((a) and (c)) and corresponding optical images (O2-, O3- and O4-signal) ((b), (d), (e), and (f)) obtained from a Si -sample with rectangular SiO 2 structures. The 20 nm high SiO 2 patterns with an edge length of 1 μm x 1.5 μm on (100) Si substrate appear bright in the AFM image and dark in the corresponding near-field image due to the stronger absorption of SiO 2 compared to Si in the mid-IR range. The acquisition time for the scan is about 20 minutes (Hermann et al., 2014) .
A topographic image and the optical image (2nd harmonic optical signal, hereafter referred to as O2-signal) from the semiconductor sample are displayed in Fig. 2(a). With a pixel integration time of 39 ms and an image size of 180 x 180, the acquisition process took almost 20 minutes. In the topographic image, the four SiO 2 structures are visible as bright rectangles on the Si substrate. The SiO 2 structures appear optically black in the simultaneously captured optical near-field image (Fig. 2 (b)). The average refractive index within the wavelength range of the incident IR light, which is lower for SiO 2 than for Si substrate, can be used to explain this contrast in the near-field image. The results demonstrated that it is feasible to use synchrotron radiation to obtain an optical image from the surface of a 6 m × 6 m area of a semiconductor sample in a reasonable amount of time with a suitable signal-to-noise ratio up to the 4th harmonic.
References
[1] Hermann, P., Hoehl, A., Ulrich, G., Fleischmann, C., Hermelink, A., Kästner, B., Patoka, P., Hornemann, A., Beckhoff, B., Rühl, E., & Ulm, G. (2014). Characterization of semiconductor materials using synchrotron radiation-based near-field infrared microscopy and nano-FTIR spectroscopy. Optics Express , 22 (15), 17948. https://doi.org/ 10.1364/oe.22.017948