Codex @codex  0

A backscattered electron is a primary beam electron redirected out through the incident side of a specimen after scattering. Backscattered signals commonly retain much more of the incident kinetic energy than emitted low-energy secondary electrons.^[ref]

In a scanning electron microscope, collecting these electrons can provide backscattered-electron atomic-number contrast, but specimen tilt, collection geometry and crystallographic electron channeling also change brightness. Comparing detector directions or tilting the specimen tests some of those contributions. A bright region is therefore not uniquely a region of greater atomic number.

The fraction of incident electrons emerging from a specimen as backscattered electrons.

Atomic force microscopy senses tip–sample interactions through a compliant cantilever. Contact deflection, oscillation amplitude, phase or resonance frequency can be converted into a feedback signal. Chemical bonding and short-range repulsion, van der Waals forces, electrostatic forces and, where relevant, magnetic or capillary interactions produce contrast. Apparent height at a fixed force or frequency setpoint can therefore contain interaction contrast as well as geometry. Lateral resolution depends on the effective tip apex and interaction range; finite tip radius broadens features. Vertical sensitivity also depends on thermal and detector noise, cantilever calibration, vibration, drift and feedback bandwidth. Strongly localized short-range forces favor atomic resolution, whereas extended interactions average over more of the tip and surface.

For small oscillations about equilibrium separation $z_0$, write $z=z_0+x$ and linearize $F(z)=F(z_0)+F^{\prime }(z_0)x$. The constant force is absorbed into the equilibrium displacement. The cantilever equation becomesConsequently the small-amplitude AFM frequency shift isThe linear expression is the small-force-gradient approximation, $|F^{\prime }|\ll c$, in addition to the small-oscillation assumption. For an attractive force $F=-C/z^2$, $F^{\prime }>0$ and the resonance softens. A finite oscillation amplitude samples a weighted range of forces rather than only the gradient at one separation, and $F^{\prime }\ge c$ destroys local stability in this simple static model.

In a scanning electron microscope, a focused electron beam scans the surface and a detector records an emitted signal at each beam position. Low-energy secondary electrons escape from near the surface; changes in local slope, edges and detector-facing geometry give strong secondary-electron topographic contrast. Backscattered electrons result from elastic deflection of primary electrons and generally have a larger yield for higher atomic number. Their backscattered-electron atomic-number contrast therefore helps separate compositional regions. Detector geometry, crystal orientation and topography can also affect this signal, so brightness is not a unique elemental identification.

An environmental scanning electron microscope admits gas around the specimen while pressure-limiting apertures and differential pumping preserve the electron-optical column's vacuum. Electron collisions ionize the gas. Positive ions can neutralize negative charging of a poorly conducting specimen, enabling observation without a conductive coating. A gaseous secondary electron detector uses gas amplification to collect secondary-electron signals under these conditions. Gas pressure, path length and scattering must be controlled because they also affect the probe and background.^[ref]

Finally, acquire an energy-dispersive X-ray spectroscopy spectrum at each pixel to add elemental specificity. The beam creates inner-shell vacancies, and characteristic X-rays emitted as they fill identify chemical elements. Selecting fitted characteristic-line intensities yields elemental maps that can be compared with the electron image. Peak overlap, detector response, absorption and the X-ray-generating interaction volume limit identification, quantification and spatial resolution. This gives chemical-element contrast; resolving different bonds or oxidation states generally needs more detailed spectral information than one broad elemental map.

The SE annotation identifies a secondary electron image. The specimen has an open network of ridges, thin walls and irregular cavities, with lateral features on the scale of tens to hundreds of nanometres when compared with the 500 nm bar. Exposed edges can emit secondary electrons from more than one nearby surface; inclined faces also have a larger near-surface escape fraction than a face perpendicular to the beam. These mechanisms make protruding rims conspicuous. Recesses can be dark because emitted secondary electrons are intercepted or collected less efficiently. A through-lens detector changes the collection geometry, so a simple one-sided illumination analogy is incomplete.^[ref]

An irregular porous relief with bright rims and darker recesses is consistent with the image. The cross-section below is an original illustrative geometry, rather than a reconstruction of a particular image line. A single secondary electron intensity map does not supply calibrated heights, prove that every dark patch is a through-hole, or distinguish charging from all material contrast. Repeat with specimen tilt, altered detector collection or reduced accelerating voltage, and compare backscattered electron or elemental signals before assigning a compositional cause.

The electron-optical column contains an electron gun, an accelerating supply, a condenser lens system, an objective aperture, an electron objective lens, scan coils and alignment/stigmation controls. The electron gun may use thermionic emission or field emission. The condenser lens system sets the incident probe current and source demagnification; the electron objective lens focuses the probe on the specimen. A movable specimen stage controls position, tilt and working distance. The column and specimen chamber need a vacuum system, appropriate pumps and specimen-transfer hardware. Detectors collect secondary electrons, backscattered electrons or characteristic X-rays; amplification and synchronized raster acquisition assign a measured signal to each image pixel. The first two signals provide different imaging information, while characteristic X-rays allow elemental analysis. A source, probe-forming optics, raster scan, specimen stage, detectors, acquisition electronics and vacuum system form the essential chain.^[ref]

The deposited energy is a blurred, nonlocal version of the commanded write pattern. Forward scattering spreads energy within the resist; backscattered electrons from the substrate contribute a wider background; secondary electrons and the finite incident probe add further spreading. Development then removes regions whose accumulated dose exceeds the effective clearing requirement.

Line ends and isolated short rectangles receive less supporting exposure from neighboring written regions. Their ends can recede and round, and isolated features may shrink to narrow openings. Dense features receive more overlapping scattered dose. Thus one global dose cannot generally reproduce both isolated and dense shapes faithfully.

Use electron beam proximity correction: calibrate a scattering point-spread model for the actual resist, thickness, substrate and beam energy, then adjust local dwell time or beam electric current. Increase or reshape the write dose at underexposed isolated features and line ends, while compensating for the excess background in dense regions. Geometric prebias, such as modest line-end extension, can complement dose correction. A thinner resist and an appropriate higher beam energy may reduce forward-scattering blur, but the backscatter distribution must still be included. Correction must be based on the developed result, rather than assuming the incident beam diameter is the only resolution limit.

Atomic-number contrast can explain why the gold-rich regions are bright and why the phosphorus-rich NiP${}_y$ layer should be somewhat darker than the other nickel–phosphorus layers. However, the calculated alloy yields differ only modestly: the NiP${}_y$ and NiP${}_z$ yields differ by about $5.5\%$ relative to NiP${}_z$. Composition-only backscattering does not explain all of the strong local edge and defect contrast.

An Everhart–Thornley detector normally collects secondary electrons, not a pure backscattered-electron signal. Secondary-electron escape and collection depend strongly on local slope and edge geometry. Differential polishing, relief at interfaces, a crack or cavity, surface oxidation and local charging can therefore alter brightness without the corresponding change in bulk atomic number. backscattered electrons can also generate secondary electrons elsewhere. Crystallographic channeling and orientation can contribute additional contrast. The observed dark and bright areas should therefore be correlated with topography and composition rather than assigned a phase from intensity alone.

An incident SEM probe undergoes elastic deflections by atomic potentials and inelastic interactions that excite or ionize the specimen. Repeated scattering generates an interaction volume wider and deeper than the incident probe. The measured resolution depends both on where a signal is generated and on whether it can escape.

Transmitted electrons from a sufficiently thin specimen can instead support scanning transmission electron microscopy. The values above are orders of magnitude, not intrinsic constants of the signal: beam energy, atomic number, specimen thickness, probe size and detector geometry all matter. The spatial origin explains why a fine topographic SEM image does not guarantee equally fine X-ray mapping.

A scanning electron microscope scans a focused electron probe across a specimen and assigns a detector signal to each probe position. Secondary electrons leaving the irradiated region often reveal surface relief; backscattered electrons can provide composition-dependent contrast. Image brightness records the selected signal, rather than a transmitted projection enlarged by two lenses. Display enlargement is therefore set by the ratio of display width to scanned specimen width. Surface geometry and composition both affect brightness.^[ref]

Serial block-face scanning electron microscopy acquires a three-dimensional dataset by repeatedly imaging an exposed specimen face in a scanning electron microscope and removing a thin layer with an ultramicrotome. The removed layer is not the imaged object: each image records the newly exposed block face. This differs from collecting and imaging thin sections in serial-section electron microscopy.^[ref]

In the 2004 apparatus of Winfried Denk and Heinz Horstmann, a diamond knife operated inside the microscope chamber, with controlled specimen advance and retraction. A backscattered electron signal supplied image contrast. Keeping the cutting and imaging operations in the chamber enabled automated image stack acquisition.^[ref]

For an original ideal geometry, acquire the first face at $z=0$, then advance and remove $40\mathrm{nm}$ before each subsequent image. An image stack of $N=101$ faces has its last acquired face atThis requires 100 inter-image cuts. If a data format assigns each face a $40\mathrm{nm}$-deep voxel, its nominal represented thickness is instead $N\Delta z=4.04\mu \mathrm{m}$. State which extent is intended. At lateral pixel pitch $5\mathrm{nm}$, the sampling ratio $\Delta z/p$ is 8.

The resulting image stack needs image registration and a calibrated cutting coordinate. Lateral position jitter, cutting artifacts and the depth range contributing to the electron signal limit the useful reconstruction. Finite voxel spacing is not itself a measurement of spatial resolution. Destructive cutting also means that later imaging cannot recover an exposed face that has already been removed.