Принципы нанометрологии (1027506), страница 45
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As a tip is driveninto a soft sample the contact area increases as the sample deforms around thetip. Hence, increasing the contact force between the tip and sample increasesthe contact area, which in turn increases the number of interactions betweenthe tip and sample.
Therefore, increasing contact force results in an increasedadhesive force between the tip and sample. To compare measured adhesionvalues, the contact force should be selected such that it does not vary fromexperiment to experiment. Additionally, slow scan rates should be used toallow the tip and sample to separate during retraction.7.4 Scanning probe microscopy of nanoparticlesAccurate measurement of nanoparticles using AFM requires intermittent ornon-contact mode imaging. This reduces the lateral forces allowing imagingof the particle. For contact mode imaging, the high lateral force will displacethe weakly attached particles except under certain conditions. A closed-loopxy scanning system is also recommended, to minimise the drift of theElectron microscopypiezoelectric scanner in the x and y directions.
For very small particles it isalso important to have enough resolution for the z scanner, i.e. the dynamicrange of the z scanner should be reduced as much as possible, usually byusing a low-voltage mode of operation.SPM measurement of nanoparticles differs from that of electronmicroscopy in that it produces images in three dimensions, via the deflectionof a sharp probe and, unlike electron microscopy, simple lateral measurement is not practicable due to large tip–nanoparticle distortion. The onlypractical solution to this is to measure the heights of the nanoparticle ratherthan the lateral dimensions.7.5 Electron microscopy7.5.1 Scanning electron microscopyThe scanning electron microscope (SEM) uses a very fine beam of electrons,which is made to scan the specimen under test as a raster of parallelcontiguous lines (see [54,55] for thorough descriptions of electron microscopy). Upon hitting the specimen electrons will be reflected (backscatteredelectrons) or generated by interaction of the primary electrons with thesample (secondary electrons).
The specimen is usually a solid object and thenumber of secondary electrons emitted by the surface will depend upon itstopography or nature. These are collected, amplified and analysed beforemodulating the beam of a cathode ray tube scanned in sympathy with thescanning beam. The image resembles that seen through an optical lens but ata much higher resolution.The dimensions of the probe beam determine the ultimate resolvingpower of the instrument. This is controlled in turn by the diffraction at thefinal aperture. The ultimate probe size in an SEM is limited by diffraction,chromatic aberration and the size of the source.Typical SEMs can achieve image magnifications of 400 000 and havea resolution of around 1 nm with a field emission system and an in-lensdetector. The magnification of the system is determined by the relative sizesof the scan on the recording camera and of the probe on the specimen surface.The magnification is, therefore, dependent upon the excitation of the scancoils, as modified by any residual magnetic or stray fields.
It also dependssensitively on the working distance between the lens and the specimen. It isnot easy to measure the working distance physically but it can be reproducedwith sufficient accuracy by measuring the current required to focus the probeon the specimen surface.199200C H A P T ER 7 : Scanning probe and particle beam microscopyThe camera itself may not have a completely linear scan, so distortions ofthe magnification can occur. In considering the fidelity of the image, it isassumed that the specimen itself does not influence the linear response of thebeam; in other words that charging effects on the specimen surface arenegligible.
If calibration measurements of any accuracy are to be made, anymetal coating employed to make the surface conducting should be very thincompared to the structure to be measured, and is best avoided altogether ifpossible.Since charging is much more serious for low-energy secondary electronsthan for the higher-energy backscattered electrons, it is preferable to use thebackscattered signal for any calibration work, if the instrument is equipped tooperate in this mode.
For similar reasons, if the specimen is prone tocharging, the use of a low-voltage primary beam rather than an appliedconductive coating is much to be preferred, but the resolution is lost again.The indicated magnification shown on the instrument is a useful guide butshould not be relied upon for accuracy better than 10 %.In all forms of microscopy, image degradation can occur from a number offactors. These include poor sample preparation, flare, astigmatism, aberrations, type and intensity of illumination and the numerical apertures of thecondenser and objective lens [56].Electron backscattered diffraction (EBSD) provides crystallographicorientation information about the point where the electron beam strikesthe surface [57].
It has a spatial resolution down to 10 nm to 20 nmdepending on the electron beam conditions that are used. Because of theunique identification of crystal orientation with grain structure, EBSD canbe used to measure the size of grains in polycrystalline materials, and canalso be used to measure the size of crystalline nanoparticles when these aresectioned.
As EBSD relies on the regularity of the crystal structure, it canalso be used to estimate the degree of deformation in the surface layers ofa material.7.5.1.1 Choice of calibration specimen for scanning electronmicroscopySince there are various potential sources of image distortion in an SEM, itwould be convenient to have a calibration artefact that yields measurementsover the whole extent of the screen and in two orthogonal directions. Thusa cross-ruled diffraction grating or a square mesh of etched or electron beamwritten lines on a silicon substrate is an ideal specimen. The wide range ofmagnification covered by an SEM requires that meshes of different dimension are available to cover the full magnification range. There are manygratings and meshes that are commercially available.Electron microscopyAt progressively higher magnifications, copper foil grids, cross-ruledsilicon substrates and metal replica diffraction gratings are available [58].
Allthe artefacts should be mounted flat on a specimen stub suitable for the SEMin use, and the stage tilt should be set at zero [59]. The zero tilt condition canbe checked by traversing the artefact in x and y directions to check that thereis no change in beam focus and, therefore, no residual tilt.
The beam tiltcontrol should be set at zero.It is important that the working distance is not changed during theexamination of a specimen or when changing to a calibration specimen. Theindications of working distance given on the instrument are not sensitiveenough to detect changes which could affect measurement accuracy inquantitative work. It is better to reset the exchange specimen stub againsta physical reference surface which has already been matched to the stubcarrying the specimen [59].The ideal case is to be able to have a magnification standard on the samespecimen stub as the sample to be measured, since there is then no ambiguity in the operating conditions (working distance, accelerating voltage, etc.)[60].
For nanoparticles, this can be ensured by using a grid, as suggestedabove, or even more integrally by dispersing a preparation of polystyrene latexspheres on the specimen so that each field of view contains some of thecalibration spheres. It has to be emphasised that, although the various‘uniform’ latex suspensions do indeed have a well-defined mean size, thedeviation from the mean allows a significant number of particles of differentsize to be present.
It is essential, therefore, to include a statistically significant number of latex spheres in the measurement if the calibration is to bevalid.7.5.2 Transmission electron microscopyThe transmission electron microscope (TEM) operates on the same basicprinciple as a light microscope but uses electrons instead of light. The activecomponents that compose the TEM are arranged in a column, withina vacuum chamber. An electron gun at the top of the microscope emitselectrons that travel down through the vacuum towards the specimen stage.Electromagnetic electron lenses focus the electrons into a narrow beam anddirect it onto the test specimen. The majority of the electrons in the beamtravel through the specimen.
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