Near-field scanning optical microscopy (NSOM/SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by. AN EXAMPLE OF NEAR-FIELD OPTICAL MICROSCOPY Let us investigate an example of a practical nanometer- resolution scanning near- field optical. Evanescent Near Field Optical Lithography (ENFOL) is a low-cost high resolution Scanning Near-Field Optical Microscopy (SNOM or NSOM).
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Such an arrangement conveniently allows the Opttical head, incorporating the probe and its positioning mechanism, to be mounted at the specimen stage location, with the objective positioned beneath the stage.
A typical SPM local probe is equipped with a nanometer-sized tip whose tip-to-specimen interactions can be sensed and recorded by a variety of mechanisms. However, typical resolutions for most NSOM instruments range around 50 nanometers, which is only 5 or 6 times better than that achieved by scanning confocal microscopy. Although the scanning probe microscope microsscopy encompasses a vast array of specialized and highly varied instruments, their common operational motif is the employment of a local probe in close interaction with the specimen.
Additionally, the tuning fork system does not require the opticxl alignment procedures of a separate external laser source and associated focusing optical components. Fellersand Michael W.
Scanning Near-Field Optical Microscopy
Although newer near-field instrumentation techniques are nanolitnography developed to image three-dimensional volume sets, NSOM has typically been limited to specimens that are accessible by a local probe, which is physically attached to a macroscopic scan head.
A critical requirement of the near-field techniques is that the probe tip must be positioned and held within a few nanometers of the surface in order to obtain high-resolution and artifact-free optical images, and this is not readily achieved without utilizing some form of feedback mechanism. Fiber optic probes are somewhat problematic for imaging soft materials due to their high spring constants, especially in shear-force mode.
The IBM team was able to claim the highest optical resolution to date of 25 nanometers, or one-twentieth of the nanometer radiation wavelength, utilizing a test specimen consisting of a fine metal line nanolithograohy. In addition to non-diffraction-limited high-resolution optical imaging, near-field optical techniques can be applied to chemical and structural characterization through spectroscopic analysis at resolutions beneath nanometers.
The fork response was measured by sweeping the frequency from 31 kHz to 33 kHz and simultaneously measuring the amplitude and phase of the signal. Historically, the letter Q has been used to represent the ratio of reactance to resistance ophical an electrical circuit element.
Scanning probe lithography Dip-pen nanolithography Feature-oriented scanning Millipede memory.
Because the fisld light decays exponentially within a distance less than the wavelength of the light, it fileetype goes undetected. Application to Rough and Natural Surfaces. Oscillatory Feedback Methods In order to improve signal-to-noise ratios for the feedback signal, the NSOM tip is almost always oscillated at the resonance frequency of the probe.
Fellersand Michael W. A useful design consists of a modified AFM mixroscopy and transparent tip, usually fabricated from silicon nitride and coated with metal on the bottom of the probe tip discussed and illustrated in the accompanying section on near-field probes. If the scanner and specimen are coupled, then the specimen moves under the fixed probe tip in a raster pattern to generate an image from the signal produced by the tip-specimen interaction.
The detector is then rastered across the sample using a piezoelectric stage. Perhaps the most important consideration is damage to opticzl probe tip or the specimen, which is likely if the two come into contact. To date, the two most commonly employed mechanisms of tip positioning have been optical methods that monitor the tip vibration amplitude usually interferometricand a nanolithotraphy tuning fork technique.
Retrieved from ” https: In typical operation, as the oscillating probe approaches the specimen surface, the amplitude, phase, and frequency of oscillation each change, due to dissipative and adiabatic forces present at the tip of the probe.
In order to maintain high near-field resolution, it is necessary to either maintain a small oscillation amplitude relative to the tip aperture, or to compensate for larger oscillations.
Near-field scanning optical microscope – Wikipedia
A critical requirement of the near-field techniques is that the probe tip must be positioned and held within a few nanometers of the surface in order to obtain high-resolution and artifact-free optical images, and this is not readily achieved without utilizing some form of feedback mechanism. In this approach, for either the straight or bent probe types, a laser is tightly focused as close to the end of the NSOM probe as possible.
Its design and function are primary determinants of the attainable scan resolution. This allows lock-in detection techniques basically a bandpass filter with the center frequency set at the reference oscillation frequency to be utilized, which eliminates positional detection problems associated with low-frequency noise and drift.
With regard to oscillator characteristics, the term “quality factor” was introduced after the symbol Q was arbitrarily chosen. In addition to the optical information, NSOM can generate topographical or force data from the specimen in the same manner as the atomic force microscope. The advantages of this type of position control are numerous. For biological materials, specimen preparation is especially demanding, as complete dehydration is generally required prior to carrying out sectioning or coating.
The motion of the probe tip, translational stage movement, and acquisition and display of optical and topographic or other force images is controlled through additional electronics and the system computer. Extension of Synge’s concepts to the shorter wavelengths in the visible spectrum presented significantly greater technological challenges in aperture fabrication and positioningwhich were not overcome until when a research group at IBM Corporation’s Zurich laboratory reported optical measurements at a subdiffraction resolution level.
Near-field scanning optical microscopy is classified among a much broader instrumental group referred to generally as scanning probe microscopes SPMs. The Q is defined as:.
Near-field scanning optical microscope
One of the most common configurations is to incorporate the NSOM into an inverted fluorescence microscope. The main problem associated with this type of feedback mechanism is that the light source for example, a laserwhich is used to nano,ithography the tip vibration frequency, phase, and amplitude, becomes a potential source of stray photons that can interfere with the detection of the NSOM signal.
Measurements of the tunneling current, made as the tip approaches the specimen, indicate that the nanolitnography touches the specimen initially as the probe goes into feedback and continues to lightly touch the surface once per oscillatory cycle. A representation of the typical NSOM imaging scheme is presented in Figure 2, in which an illuminating probe aperture having a diameter less than the wavelength of light fileype maintained in the near field of the specimen surface.
When the oscillating fjletype approaches the specimen surface, a decrease in the oscillation amplitude of the tuning fork or in the optical feedback signal is observed.
The resolution of the tapping-mode near-field image is defined not only by the radius of the tip but also by the amplitude of the oscillation occurring perpendicular to the specimen surface. The greatest advantage of NSOM probably rests in its ability to provide optical and spectroscopic data at high spatial resolution, in combination with simultaneous topographic information.
In conventional far-field optical microscopy, the distance between the light source and the specimen is typically much greater than the wavelength of the incident light, whereas in NSOM, a necessary condition of the technique is that the illumination source is closer to the microsdopy than the wavelength of the illuminating radiation.
When the reduced signal falls below the threshold of the reference signal, the tip is interpreted as being “engaged” and the feedback control system regulates the height of the tip above the specimen based on the user specified reference signal.
However, typical resolutions for most NSOM instruments range around 50 nanometers, which is only 5 or 6 times better than that achieved by scanning confocal microscopy.