Alternative Applications of Scanning Probe Microscopy

A lot of text follows! If you are just interested in some examples go to the sections of  Optical Nearfield Microscopy as well as Lithography.

Basically, a scanning probe microscope is an instrument for positioning a sharp tip at a defined distance above the surface of some sample. In contradistinction to other positioning instruments, the scanning probe microscope works on the nanometer scale (10E-9 m). In standard modes, forces are measured in order to scan a surface topography og record forces (lateral forces, Force spectroscopy). Besides the mechanical interaction, however, a measuring tip can interact in many other ways with a surface. By using a conducting tip it is possible to perform measurements of e.g. potential or current. Several different measuring methods exist, designated by three or four letter abbreviations such as SSRM (resistance), TUNA (current), SCM (capacitance), KFM (Potential); these can all be realized through the use of different amplifiers or measurement equipment. In many cases, a special scanner model is used: As an example, for measurements of small currents it is recommended to use a tunnel current amplifier built into the scanner to minimize external disturbances. For Kelvin Probe Measurements (KFM) as well as for Capacitance measurements the small dimensions of the cantilever can be used to convert electrical measurements into measurements of forces. Furthermore, it is possible by means of special cantilevers to measure other properties such as e.g. magnetic forces and temperature effects.

One of the most versatile and at the same time also very complex SPM measurement technique is the optical nearfield microscopy (Scanning Nearfield Optical Microscope, SNOM or NSOM). This class of instruments are used for investigation of electromagnetic interactions (light) with a resolution below the wavelength of the used electromagnetic field. Also here are a lot of possibilities: With an apertureless SNOM you use a normal metal or semiconductor tip and use the electrogmagnetic interaction between the sample surface and the tip to amplify optical signals. If you illuminate the sample surface at the position of the tip, the interaction between the sample and light causes a light amplification and it is possible to obtain electromagnetic information in the order of magnitude of the tip geometry. This is used especially for amplification of Raman signals (Tip Enhanced Raman or Tip Enhanced Raman Spectroscopy: TERS).

Anonther possibility is to use a fine fiber glass tip to concentrate the light. This way, the fiber glass can be used for lighting as well as for collection of the detected light (Illumination Collection Mode SNOM), and it is possible to carry out photoluminescense experiments with resolutions below the light wavelength. This is especially interesting when a combination of CCD camera and monochromator is used as detector and measures a complete optical spectrum in each point of the surface (Optical Nearfeld Spectroscopy). For a lot of investigations, this will be even more interesting, when these measurements can be carried out at very low temperatures. E.g. in optical semiconductors, many quantum mechanical effects can only be seen at very low temperatures.

That this actually works can be seen from the following examples. They were recorded with such an instrument (Optical Low Temperature Nearfield Spectroscope). The scanner is a DME DualScope DS45-40 SNOM. The measurements originate from "Institut für Angewandte Physik, Technische Universität Braunschweig". (If you are interested, you can see more videos on another page.)

Emission intensity dependent on wavelength	Dynamics spectrum of the optical emission
InGaN LED photoluminescense at app. 50 K Emission intensity at various  wavelengths (cf. image right) Distance between images: 3 nm, all images from one single measurement Image size: 15 square micron

The measurements prove the existence of high energy barriers in high efficiency blue InGaN LEDs (see Suppression of Nonradiative Recombination by V-Shaped Pits in GaInN/GaN Quantum Wells Produces a Large Increase in the Light Emission Efficiency, A. Hangleiter et al., Phys. Rev. Lett. 95, p. 127402 (2005)). The barrier emission shows a line width corresponding to the resolution capability of the optical spectrometer. Therfore, you only see a short flashes at various positions on the surface.

High resoution 5 micron scan
at room temperature

All images originate from one single measurement. As the recording of the spectrum from each point contains a three-dimensional information (X/Y coordinates, wavelength), the data must be processed in some manner to display as an image.

As comparison, the individual image to the left shows the optical emission intemsity of an InGaN LED at room temperature. The structures in this also give a good illustration of the resolution capability of the fiber glass tip which in this measurement is somewhat better than 100 nm.

One of the greatest challenges for such measurements is the manufacture of the fiber glass tips. In this case was used an etched tip with aluminum coating. To manufacture such fiber glass tips you need a coating instrument with a rotating holder, access to suitable chemical facilities for etching the fiber tip, and an electron microscope to check the results of the etching and coating as well as a photoluminiscence sample for checking the resolution capabilities of the manufactured fiber tip.

The optical information can also be represented in other ways. As an example, you can make an image of the width of the emission peaks or the displacement of the emission maximum, as shown below.

Analogous to an ordinary Atomic Force Microscope, a Scanning Nearfield Optical Microscope also provides topography information. Because of the high light intensity at the fiber tip, the topography information is often disturbed, however, but in many cases as the example above, it can be used. In the topography image you see a surface with a lot of cracks. The image to the right was made from the wavelength displacement and shows a clear displacement towards red at the cracked locations. The displacement towards red is a result of the mechanical relaxation of the layer at the cracks.

If these examples do not suffice, here is a page with more videos of  optical  Nearfield spectroscopy...

With a scanning probe microscope you can not only record information, but you can also transfer information onto a sample. The DME scan software provides the possibility to duringing scaning modulate any channel with the information from a bitmap. It is possible to modulate e.g. various electrical pulse heights and widths as well as forces and positions.

The image to the right shows a writing impressed into a piece of ordinary adhesive tape. Here, the resolution corresponds to the tip geometry. The used tip has half cone angle of app. 25°. the size of the writing is app. (width x height) 4.85 x 1.75 micron.