Scanning Near-Field Microscopy (SNOM) - Principles and Modes of Operation by NT-MDT

:: AZoNanotechnology Article

Topics Covered

Background
Introduction
Shear Force Microscopy
Key Element of SNOM
Transmission Mode of SNOM
Reflection Mode of SNOM
Luminescence Mode of SNOM

Background

NT-MDT Co. was established in 1991 with the purpose to apply all accumulated experience and knowledge in the field of nanotechnology to supply researchers with the instruments suitable to solve any possible task laying in nanometer scale dimensions. The company NT-MDT was founded in Zelenograd - the center of Russian Microelectronics. The products development are based on the combination of the MEMS technology, power of modern software, use of high-end microelectronic components and precision mechanical parts. As a commercial enterprise NT-MDT Co. exists from 1993.

Introduction

The resolving power of classical optical microscopes is restricted by Abbe's diffraction limit to about to one-half of the optical wavelength.Howevwr, it is possible to overcome this limit.

If a subwavelength hole in a metal sheet is scanned close to an object, a super-resolved image can be built up from the detected light that passes through the hole. Scanning near-field microscopy based on this principle was first proposed by Synge and demonstrated at microwave frequencies by Ash and Nicholls with a resolution of l/60. At visible wavelengths this principle (optical stethoscopy, near-field optical-scanning microscopy, SNOM) was demonstrated by Pohl et al. In Betzig et al have demonstrated using fiber probes to image a variety of samples with a number of different contrast mechanisms.

To make the system easier to use and to extend its applicability to samples of orbitrary topography, it would be advantageous to have a distance regulation mechanism capable of automating the initial approach and maintaining the aperture at a fixed distance from the sample over the entire course of a scan. Several mechanisms have been proposed previously to SNOM and related evanescent field techniques, including electron tunneling, capacitance, photon tunneling, near-field reflection.

At present the most-used method of probe-sample distance regulation relies on the detection of shear forces between the end of near-field probe and the sample. Shear Force based system allows Shear Force Microscopy alone, or simultaneous Shear Force and Near-Field imaging, including Transmission mode for transparent samples, Reflection mode for opaque samples and Luminescence mode for additional characterisation of samples.

Figure 1. Schematic of a combined shear force and near-field scanning optical microscope.

Shear Force Microscopy

At present the most-used method of probe-sample distance regulation relies on the detection of shear forces between the end of near-field probe and the sample. Shear force based system allows Shear Force Microscopy alone, or simultaneous Shear Force and Near-Filed imaging, including Transmission mode for transparent samples, Reflection mode for opaque samples and Luminescence mode for additional characterisation of samples.

To hold the optical probe near surface nonoptical scheme with quartz tuning fork as sensor is used. It allows to increase ratio of useful signal to noise in comparison with optical holding schemes. It is very important at operations with limiting resolution. Also photoinduced carriers does not appear. It is necessary requirement when some properties of semiconductor are investigated.

At the heart of nonoptical method for obtaining of information about surface lies idea to use response of quartz tuning fork attached to optical fiber on interaction with surface. System fiber-quartz is excited in transverse vibrations with help of external feed element on quartz resonance frequency. Further piezoeffect is used: in the presence of mechanical oscillations electrical outputs of quartz have voltage response, which is used as information signal about amplitude of fiber oscillation.

Shear Force Microscopy is realized in the following way. Piezodriver via quartz tuning fork excite oscillations of the fiber probe with some initial amplitude. Suitable output value of quartz is Ao. After approaching sample surface the amplitude of fiber probe oscillations reaches some set-point value and quartz output reaches value A. After that scanning of the sample surface is conducted with maintaining this value by the feedback system.

Key Element of SNOM

The key element of the Near-Field Scanning Microscope (SNOM) is a tiny aperture (end of laser illuminated fiber probe in our case) scanned along the sample in very close proximity, typically less than 10 nm.

At present the most-used method of probe-sample distance regulation relies on the detection of shear forces between the end of near-field probe and the sample. Shear Force based system allows simultaneous Shear Force and Near-Filed imaging, including Transmission mode for transparent samples, Reflection mode for opaque samples and Luminescence mode for obtaining additional characterisation of samples.

At the heart of nonoptical method for obtaining of information about surface lies idea to use response of quartz tuning fork attached to optical fiber on interaction with surface. System fiber-quartz is excited in transverse vibrations with help of external feed element on quartz resonance frequency. Further piezoeffect is used: in the presence of mechanical oscillations electrical outputs of quartz have voltage response, which is used as information signal about amplitude of fiber oscillation.

Transmission Mode of SNOM

Transmission mode of SNOM is realized simultaneously with Shear Force Microscopy, which in turn is realized in the following way. Piezodriver via quartz tuning fork excite oscillations of the fiber probe with some initial amplitude. Suitable output value of quartz is A₀. After approaching sample surface the amplitude of fiber probe oscillations reaches some set-point value and quartz output reaches value A. After that scanning of the sample surface is conducted with maintaining this value by the feedback system.

Under the scanning the sample is illuminated by the fiber probe and the passed through sample light via objective is directed on the photomultiplier tube.

Figure 2. Transmission mode.

Reflection Mode of SNOM

Reflection mode of SNOM is realized simultaneously with Shear Force Microscopy, which in turn is realized in the following way. Piezodriver via quartz tuning fork excite oscillations of the fiber probe with some initial amplitude. Suitable output value of quartz is A₀. After approaching sample surface the amplitude of fiber probe oscillations reaches some set-point value and quartz output reaches value A. After that scanning of the sample surface is conducted with maintaining this value by the feedback system.

Under the scanning the sample is illuminated by the fiber probe and the scattered light is directed by the mirror via objective on the photomultiplier tube.

Figure 3. Reflection mode.

Luminescence Mode of SNOM

Luminescence mode of SNOM is realized simultaneously with Shear Force Microscopy, which in turn is realized in the following way. Piezodriver via quartz tuning fork excite oscillations of the fiber probe with some initial amplitude. Suitable output value of quartz is A₀. After approaching sample surface the amplitude of fiber probe oscillations reaches some set-point value and quartz output reaches value A. After that scanning of the sample surface is conducted with maintaining this value by the feedback system.

Under the scanning the sample is illuminated by the fiber probe and the passed through sample light via objectives and notch filter is directed on the photomultiplier tube.

Figure 4. Luminescence mode.

Source:NT-MDT Co.

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