Scanning probe microscopy
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|Scanning probe microscopy|
Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded with the invention of the scanning tunneling microscope in 1981.
Many scanning probe microscopes can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode.
The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution. They owe this largely to the ability of piezoelectric actuators to execute motions with a precision and accuracy at the atomic level or better on electronic command. One could rightly call this family of techniques "piezoelectric techniques". The other common denominator is that the data are typically obtained as a two-dimensional grid of data points, visualized in false color as a computer image.
Established types of scanning probe microscopy 
- AFM, atomic force microscopy 
- Contact AFM
- Non-contact AFM
- Dynamic contact AFM
- Tapping AFM
- BEEM, ballistic electron emission microscopy
- CFM, chemical force microscopy
- C-AFM, conductive atomic force microscopy
- ECSTM electrochemical scanning tunneling microscope
- EFM, electrostatic force microscopy
- FluidFM, fluidic force microscope
- FMM, force modulation microscopy
- FOSPM, feature-oriented scanning probe microscopy
- KPFM, kelvin probe force microscopy
- MFM, magnetic force microscopy
- MRFM, magnetic resonance force microscopy
- NSOM, near-field scanning optical microscopy (or SNOM, scanning near-field optical microscopy)
- PFM, Piezoresponse Force Microscopy
- PSTM, photon scanning tunneling microscopy
- PTMS, photothermal microspectroscopy/microscopy
- SCM, scanning capacitance microscopy
- SECM, scanning electrochemical microscopy
- SGM, scanning gate microscopy
- SHPM, scanning Hall probe microscopy
- SICM, scanning ion-conductance microscopy
- SPSM spin polarized scanning tunneling microscopy
- SSRM, scanning spreading resistance microscopy
- SThM, scanning thermal microscopy
- STM, scanning tunneling microscopy
- STP, scanning tunneling potentiometry
- SVM, scanning voltage microscopy
- SXSTM, synchrotron x-ray scanning tunneling microscopy
Of these techniques AFM and STM are the most commonly used for roughness measurements.
Probe tips 
The nature of an SPM probe depends entirely on the type of SPM being used. However, certain characteristics are common to all, or at least most, SPMs.
Most importantly the probe must have a very sharp apex. The apex of the probe defines the resolution of the microscope, the sharper the probe the better the resolution. For atomic resolution imaging the probe must be terminated by a single atom.
For many cantilever based SPMs (e.g. AFM and MFM) the entire cantilever and integrated probe fabricated by acid [etching], usually from silicon nitride. Conducting probes, needed for STM and SCM among others, are usually constructed from platinum/iridium wire for ambient operations, or tungsten for UHV operation. Other materials such as gold are sometimes used either for sample specific reasons or if the SPM is to be combined with other experiments such as TERS. Platinum/iridium (and other ambient) probes are normally cut using sharp wire cutters, the optimal method is to cut most of the way through the wire and then pull to snap the last of the wire, increasing the likelihood of a single atom termination. Tungsten wires are usually electrochemically etched, following this the oxide layer normally needs to be removed once the tip is in UHV conditions.
It is not uncommon for SPM probes (both purchased and "home-made") to not image with the desired resolution. This could be a tip which is too blunt or the probe may have than one peak, resulting in a doubled or ghost image. For some probes in situ modification of the tip apex is possible, this is usually done by either crashing the tip into the surface or by applying a large electric field. The latter is achieved by by applying a bias voltage (of order 10V) between the tip and the sample, as this distance is usually 1-3 Angstroms, a very large field is generated.
Advantages of scanning probe microscopy 
- The resolution of the microscopes is not limited by diffraction, but only by the size of the probe-sample interaction volume (i.e., point spread function), which can be as small as a few picometres. Hence the ability to measure small local differences in object height (like that of 135 picometre steps on <100> silicon) is unparalleled. Laterally the probe-sample interaction extends only across the tip atom or atoms involved in the interaction.
- The interaction can be used to modify the sample to create small structures (nanolithography).
- Unlike electron microscope methods, specimens do not require a partial vacuum but can be observed in air at standard temperature and pressure or while submerged in a liquid reaction vessel.
Disadvantages of scanning probe microscopy 
- The detailed shape of the scanning tip is sometimes difficult to determine. Its effect on the resulting data is particularly noticeable if the specimen varies greatly in height over lateral distances of 10 nm or less.
- The scanning techniques are generally slower in acquiring images, due to the scanning process. As a result, efforts are being made to greatly improve the scanning rate. Like all scanning techniques, the embedding of spatial information into a time sequence opens the door to uncertainties in metrology, say of lateral spacings and angles, which arise due to time-domain effects like specimen drift, feedback loop oscillation, and mechanical vibration.
- The maximum image size is generally smaller.
- Scanning probe microscopy is often not useful for examining buried solid-solid or liquid-liquid interfaces.
Visualization and analysis software 
In all instances and contrary to optical microscopes, rendering software is necessary to produce images. Such software is produced and embedded by instrument manufacturers but also available as an accessory from specialized work groups or companies. The main packages used are
- Freeware : Gwyddion, WSxM (developed by Nanotec).
- Commercial : FemtoScan Online  (developed by Advanced Technologies Center), SPIP (developed by ImageMet), MountainsMap SPM (developed by Digital Surf).
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