Scanning Near-Field Optical Microscopy (SNOM or NSOM) - Different Methods of Operation

Topics Covered

Background

How to Conduct an Experiment Using Scanning Near-Field Optical Microscopy (SNOM or SNOM)

Feedback Mechanisms: Shear Force Feedback and Normal Force Feedback

Normal Force Feedback for Experiments Using Atomic Force Microscopes (AFMs)

Different Operating Methods for Scanning Near-Field Optical Microscopy (SNOM)

Photon Scanning Tunneling Microscope (PSTM)

Plasmon Near-Field Microscope

Using Scanning Near-Field Optical Microscopy (SNOM) in Lithography

Background

SNOM is the acronym for Scanning Near-Field Optical Microscopy, an alternative name for NSOM (Near-Field Scanning Optical Microscopy). The basic principle of such a kind of technique is quite simple: light passes through a sub-wavelength diameter aperture and illuminates a sample that is placed within its near field, at a distance much less than the wavelength of the light. The resolution achieved is far better than that which conventional optical microscopes can attain.

How to Conduct an Experiment Using Scanning Near-Field Optical Microscopy (SNOM or NSOM)

In order to make an NSOM/SNOM experiment, a point light source must be brought near the surface that will be imaged (within nanometers). The point light source must then be scanned over the surface, without touching it, and the optical signal from the surface must be collected and detected. There are a few different ways to obtain a point light source: One can use pulled or etched optical fibers (tapered optical fibers) that are coated with a metal except for at an aperture at the fiber's tip. The light is coupled into the fiber and is then emitted at the sub-wavelength (50 nm or larger) aperture of the fiber. Or, one can use a standard AFM cantilever with a hole in the center of the pyramidal tip. A laser is focused onto this hole, which is of sub-wavelength dimensions.

AZoNano, Nanotechnology - This image shows the scheme of a pulled optical fiber with an aluminium coating (the most diffused probe for SNOM systems).

Figure 1. Scheme of a pulled optical fiber with an aluminium coating (the most diffused probe for SNOM systems).

AZoNano, Nanotechnology - This picture shows an optical fiber mounted on a tuning fork (read the description below).

Figure 2. This picture shows an optical fiber mounted on a tuning fork (see description below).

AZoNano, Nanotechnology - This image shows an electron microscopy image of the end of the cantilever with a hollow aluminium tip, viewed from the bottom.

Figure 3. Electron microscopy image of the end of the cantilever with hollow aluminium tip, viewed from the bottom.

AZoNano, Nanotechnology - This image shows an electron microscopy image of the hollow aluminium tip. The standard aperture size is 100 nm.

Figure 4. Electron microscopy image of the hollow aluminium tip. The standard aperture size is 100 nm.

Feedback Mechanisms: Shear Force Feedback and Normal Force Feedback

The resolution of an NSOM/SNOM measurement is defined by the size of the point light source used (typically 50-100 nm). The distance between the point light source and the sample surface is usually controlled through a feedback mechanism that is unrelated to the NSOM/SNOM signal. Currently, most instruments use one of the following two types of feedback: shear force or normal force. In the shear force feedback, or tuning fork feedback, the straight tip is mounted to a tuning fork, which is then oscillated at its resonance frequency. The amplitude of this oscillation is strongly dependent on the tip-surface distance, and it can be effectively used as a feedback signal.

Normal Force Feedback for Experiments Using Atomic Force Microscopes (AFMs)

In the normal force feedback (the standard feedback mode used in AFM), it is possible to perform experiments in contact and in intermittent contact mode. This feedback mechanism is only possible with cantilevered, tapered optical fibers and with AFM cantilevers with holes.

Different Operating Methods for Scanning Near-Field Optical Microscopy (SNOM)

Scanning near-field optical microscopy can be performed in many different ways of operation (see figures 5 and 6). Most common today is the use of aperture probes for transmission microscopy, either in illumination (a) or in collection (b). However, many samples or substrates are opaque, so that working in reflection is necessary (c). The reflected light can be collected by optics close to the tip, or by the fiber probe itself, in which case often uncoated fiber tips are used.

AZoNano, Nanotechnology - This diagram shows different methods of operation for Scanning Near-Field Optical Microscopy (SNOM).

Figure 5. Diagram showing different methods of operation for Scanning Near-Field Optical Microscopy (SNOM).

Photon Scanning Tunneling Microscope (PSTM)

A different approach is taken by the Photon Scanning Tunneling Microscope (PSTM), where evanescent waves are created at the sample surface by oblique far-field illumination (d). The probe tip acts as a scatterer of the evanescent field, leading to homogeneous waves which can be easily detected. Easy to operate, this mode suffers somewhat from difficulties in data interpretation. Of high interest is this arrangement with inverted light path, the i-PSTM or Tunnel Near-Field Optical Microscope (TNOM) or forbidden light near-field optical microscope.

Plasmon Near-Field Microscope

Similarly to the PSTM, light can be scattered from the evanescent field by other probe tips, such as a force microscope tip on a cantilever (e). In the Plasmon Near-Field Microscope, surface plasmons are generated at the surface of a sample on a thin film metallic substrate, and scattered by a probe tip (f). The SNOM system at the Materials and Microsystems Laboratory is an AlphaSNOM by Witec, where the optical probe is a silicon cantilever with a hole in the center of a metallic tip.

AZoNano, Nanotechnology - This picture show the image obtained with the optical probe in reflection configuration.

Figure 6. This is a topographic picture showing a Scanning Near-Field Optical Microscopy (SNOM) image of a sub-micrometric triangular pattern of holes drilled on polymethyl methacrylate (PMMA) by electron beam lithography and wet etching, performed in the Materials and Microsystems Laboratory.

AZoNano, Nanotechnology - This is a topographic picture showing a Scanning Near-Field Optical Microscopy (SNOM) image of a sub-micrometric triangular pattern of holes drilled on polymethyl methacrylate (PMMA) by electron beam lithography and wet etching, performed in the Materials and Microsystems Laboratory.

Figure 7.  This picture show the image obtained with the optical probe in reflection configuration.

Using Scanning Near-Field Optical Microscopy (SNOM) in Lithography

SNOM can be fruitfully used for direct writing optical nanolithography. In fact, topography and physical properties of photosensitive surfaces such as photoresists may be changed by laser pulses emitted from the SNOM probe, offering resolutions unachievable by conventional optical and laser systems (going well beyond the diffraction limit). The first results obtained in several labs dealt with nanometric strips on standard positive photoresist and on self-assembled monolayers of alkanethiols (with transfer to metallic substrates) by using laser sources in the visible-UV range.

Primary author: Fabrizio Giorgis.

Source: Materials and Microsystems Laboratory, the Polytechnic of Turin and the National Institute for Physics of Matter (INFM).

 

For more information on this source please visit Materials and Microsystems Laboratory.

 

Date Added: Apr 21, 2005 | Updated: Jun 11, 2013
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