Introduction: near-field optical microscopy
Scanning Near-field Optical Microscopy (SNOM) overcomes the resolution limit of conventional optical microscopes. In addition to the topography, the optical properties of micro- or even nanostructures can be revealed simultaneously by this technique.
Due to diffraction, the resolution of conventional optical microscopy is restricted to approximately half of the wavelength used. This limit can, however, be circumvented by a novel type of microscopy: Scanning Near-field Optical Microscopy (SNOM or NSOM). It belongs to the scanning probe methods – serial image acquiring techniques that rely on the local interaction between a probe and the sample – and is therefore similar to e. g. Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM or SFM).
These techniques are characterized by a specially designed small probe that allows to detect the sample surface properties with a high lateral and vertical resolution. This benefit is to be attributed to the extremely short range of the particular interaction utilized to image the sample, e. g. the fast decrease of the quantum-mechanical electron tunnelling probability with distance (STM) or the short-range potential of interatomic or intermolecular forces (SFM). In the case of SNOM, it is the exponential decay of the optical near-field – the so-called evanescent modes – that yields the high resolution. These components of the radiation field can otherwise not be made use of in conventional (i. e. far-field) light microscopy.
The basic principle of SNOM is to scan a submicroscopic radiation source (or, alternatively, a submicroscopic radiation detector or scatterer) as so-called near-field probe across the sample surface at a scan-height of only some few nanometers. Unlike in conventional microscopy, the resolution does not depend on the wavelength of the radiation but mainly on the geometry of the probe, i. e. particularly the aperture diameter and shape as in the case of the probes presented here.
Probes with a well-defined geometry and a high transmission efficiency (or a high detection efficiency in the case of active detecting probes) are therefore crucial if SNOM is expected to yield sensitive and reproducible results.
Near-field infrared optical microscopy is a promising technique for applications in many fields of science and technology. It allows, for instance, to investigate the influence of near-field effects on microstructure measurements. Furthermore, in combination with spectroscopic techniques it makes up a potentially powerful tool for the analysis of the local chemical composition of submicroscopic or even nanoscopic structures – a crucial challenging task in semiconductor technology as well as in modern biology and medicine.
Fig.: Distance dependence of near-field optical resolution: series of scanning transmission images recorded at 20 nm high gold/palladium test structures on a silicon wafer. The scan height is subsequently reduced from approx. 300 nm (left image) to approx. 10 nm (right image), thus reaching a lateral resolution of about 80 nm. Please note that a wavelength of 1064 nm is used here; therefore these 80 nm correspond to a lateral resolution of l/13, i. e. about one order of magnitude better than in conventional optical microscopy.
English: Introduction: Scanning near-field optical microscopy