Introduction: optical near field microscopy
Optical near-field microscopy allows the imaging of much smaller structures than is possible with conventional microscopes. Simultaneous to object topography, optical properties of micro- and nanostructures can now be acquired.

The resolution in conventional optical microscopy is limited by diffraction effects to about half of its radiation wavelength. This limit is overcome by a new type of microscopy: optical near-field microscopy (SNOM). It is related to scanning tunneling (STM) and atomic force microscopy (AFM).

These methods are characterized by the fact that an appropriately designed or functionalized small probe tip can detect the surface properties of interest with high lateral and vertical resolution. The methods are all rasterized, i.e. images are obtained by combining many individual measured values in points and lines (-> raster image).

The high resolution of all these scanning probe microscopic methods is based on the exploitation of extremely short-range interactions between probe and sample as imaging mechanisms: e.g. with STM on the rapid decrease of the quantum mechanical electron tunnel probability with the distance, with AFM on the short-range potential of interatomic or interfering electron tunnels. molecular forces, and in SNOM on the exponentially decaying optical near field (so-called „evanescent modes“), which cannot be used for imaging in conventional microscopy.

Comparison: conventional <<>> SNOM

The principle of optical scanning near-field microscopy in particular consists in moving a submicroscopic radiation source (alternatively also: radiation detector or radiation scattering body) in the form of a so-called near-field probe over the sample surface at a distance of only a few nanometers – and thus within the range of the near-field – in the form of a grid. The resolution is essentially given by the geometry of the probe (i.e. usually by the aperture diameter) and not by the radiation wavelength. For sensitive and reproducible measurements it is necessary to provide probes with well defined probe geometry with high radiation transmission (or radiation detectivity for actively detecting probes, luminescence intensity for actively emitting probes and high scattering efficiency for scattering body probes).

Interesting fields of application for optical near-field microscopy open up, for example, with the investigation of the influence of optical near-field effects in microstructure measurement technology, or with the investigation of the material composition of submicroscopic structures through the use of spectroscopic techniques. The latter is of great importance for many questions in semiconductor technology as well as biology and medicine.

SNOM distance

Figure: Distance dependence of the near-field optical resolution: Series of transmission raster images on a silicon wafer with 20 nm high gold/palladium test structures. In the left image, the near-field probe was guided over the sample at a height of approx. 300 nm. This distance is reduced to approx. 10 nm in the right image, whereby a lateral optical resolution of approx. 80 nm is achieved.

The wavelength of the radiation used here is 1064 nm – i.e. the lateral resolution is many times higher than conventional microscopy allows (see scale in the images).