Tip-enhanced Near-field Raman Spectroscopy Using an Scanning Tunneling Microscope with Side Illumination Optics

摘要

Raman spectroscopy (RS) is a key tool to characterize residual stress in silicon devices because the vibrational frequencies of a silicon substrate change with its stress. However, due to the intrinsic optical diffraction limit, conventional micro-Raman spectroscopy can only have a probe resolution of around 1 μm~2, which is not sufficient for nanotechnology-oriented electronic industry. Low sensitivity is another problem to be solved to maximize the potential of this technique. In this study, a novel Raman spectrometer, which can overcome the optical diffraction limit, was built with the attempt to improve the resolution as well as the detection sensitivity. This approach instrument, which is based upon tip-enhanced near-field effects, has a nanoscale resolution by deploying a silver-coated tungsten tip mounted on a scanning tunneling microscope (STM) with side illumination optics. It features fast and reliable optical alignment, versatile sample adaptability and effective far-field signal suppression. The performance was evaluated by observing the enhancement effects on silicon substrates and single-walled carbon nanotubes (SWCNTs). It was found that apparent enhancement as high as 120% on silicon substrates could be achieved using the depolarization technique. It is believed that this technique is promising for future diagnosis of semiconductor materials and devices at nanoscales, especially for stress mapping of semiconductor devices.