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Secondary electron contrast in low-vacuum∕environmental scanning electron microscopy of dielectrics
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View: Figures


Image of FIG. 1.
FIG. 1.

Schematized vacuum system of an environmental SEM. Pressure-limiting apertures separate regions of the column that are differentially pumped (; ).

Image of FIG. 2.
FIG. 2.

Idealized cross section of the region surrounding the specimen for one possible configuration. The cascade amplification field is determined by the potential difference between the anode and specimen surface, divided by the gap distance . The anode can be a deliberately biased detector, such as the gaseous SE detector (GSED). In other configurations it is simply the grounded pole piece, in which case, the signal can be collected from the stub (, typically in the range of ; ).

Image of FIG. 3.
FIG. 3.

(a) Inverse mean-free-path (i.e., scattering events per millimeter) as function of electron energy for various gases at . Helium–dashed line; water vapor–solid line; nitrogen–dotted line. The information is presented in this manner because the inelastic mean-free-path (IMFP) scales linearly with pressure. For example, doubling the pressure halves the mean-free-path. (b) Mean number of scattering events experienced by a primary electron enroute to the specimen as a function of the pressure-distance product in water vapor, computed for various electron energies. Assuming Poisson-scattering statistics, an average number of scattering events equivalent to unity correspond to of the electrons reaching the specimen unscattered. As a rule of thumb, therefore, the shaded region represents the parameter space in which high-quality images can be obtained.

Image of FIG. 4.
FIG. 4.

A gold-on-carbon resolution standard imaged at high vacuum and with and of water vapor. A conventional Everhart–Thornley SE detector was used for the high-vacuum image, while a gaseous SE detector was used in low-vacuum mode. In all cases, the , with . Resolutions measured using a cross correlation procedure were 1.54, 2.35, and , respectively (Ref. 20). Signal-to-noise ratios were 7.57, 3.9, and 1.83, respectively. Mechanical vibrations are present in the images. However, these have a minimal effect on the measured changes in resolution with gas pressure. of water vapor is the minimum pressure necessary to stabilize hydrated specimens. Images courtesy of Daniel Phifer, FEI Company.

Image of FIG. 5.
FIG. 5.

Primary electron energy deposition profiles calculated for using the Monte Carlo program CASINO (Ref. 31). The simulations were executed using initial electron energies of 1, 5, 10, and , as shown in the figure. The curves are approximately proportional to SE generation profiles. The shaded area represents the size of the SE escape region typically encountered in the case of dielectrics in a SEM ( maximum primary electron penetration range).

Image of FIG. 6.
FIG. 6.

(a) Schematic of the charge distribution encountered in the case of an electron-beam-irradiated insulator in a high-vacuum environment. It consists of an electron-depleted near-surface region and an underlayer of implanted electrons . A free electron (represented by the particle in the middle) drifts towards the surface under the influence of the field. (b) Simplified electronic structure diagram corresponding to the situation in Fig. 6(a). Negative charge implanted in the bulk (not shown) creates an electric field represented by the slope of the energy bands. Charge traps are indicated by dashed lines inside the forbidden energy gap. Trapped electrons can be excited into the conduction band (1) after which they are free to drift towards the surface, losing energy through phonon scattering (2). Electrons can either be retrapped (3) or eventually reach the surface (4), where they can recombine with excess holes within the SE escape region, or gaseous ions at the surface (in the case of low-vacuum SEM). Only electrons with energy greater than can be emitted as secondary electrons ( top of valence band; bottom of conduction band; ; ).

Image of FIG. 7.
FIG. 7.

Axial potential function for an electron-irradiated dielectric in a low-vacuum environment sketched at three length scales: (a) anode-gas-sample-stub, (b) sample surface-maximum primary beam penetration range , and (c) sample surface-maximum SE escape range. The potential function is sketched for the case where electrons are implanted within the interaction volume, but the surface potential is positive due to the presence of the ionic space charge at and above the sample surface, and excess holes below the surface ( at the sample surface; between the sample and the anode; ; ).

Image of FIG. 8.
FIG. 8.

Schematic illustration of the SE signal intensity , plotted as a function of distance , above the sample surface . Adapted from Ref. 72. [ emission yield; mean SE-ion recombination probability (determined by the lateral SE and ion distributions within the SE-ion recombination volume); maximum height at which SE-ion recombination can occur; minimum height at which SE gas amplification can occur.]

Image of FIG. 9.
FIG. 9.

Gas gain , and recombination-induced SE signal damping coefficient , measured as a function of gas pressure . Data from Ref. 72 (; ; ; ; ).

Image of FIG. 10.
FIG. 10.

Effect of ions and external fields on surface electronic structure and secondary electron emission from a dielectric. (a) A simplified energy diagram of a surface infinitely far from an ion, in the absence of applied electric fields. Only the electrons that have a sufficient component of their velocity normal to the surface can surmount the surface barrier. (b) An energy diagram for an ion near the surface, in the presence of the electric field generated by the biased anode, specimen charging, and the ionic space charge. The surface-potential barrier is lowered, and electrons in the conduction band that could not normally escape can be emitted or captured by the ion. The additional emitted electrons are indicated by the shaded part of . Adapted from Ref. 32. [ ; ; SE energy distribution just below the specimen surface; SE energy distribution just above the surface.]

Image of FIG. 11.
FIG. 11.

Alternating positive (bright) and negative ferroelectric domains in . Image adapted from Ref. 82 (; ).

Image of FIG. 12.
FIG. 12.

An image pair of the junction in a cross-sectioned diode. In the left-hand image, the -doped side is floating while the -doped side is grounded. Conditions are reversed for the right-hand image.

Image of FIG. 13.
FIG. 13.

A polished grain of gibbsite imaged with water vapor. The bottom image was taken at a fast scan rate (eight frames integrated to reduce noise). Increasing the electron flux by slowing the scan rate (top image) causes many of the SE contrast features to invert.

Image of FIG. 14.
FIG. 14.

High-resolution image of sapphire showing surface detail only visible when the sample was allowed to charge, obtained using an accelerating voltage of and a gas pressure of .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Secondary electron contrast in low-vacuum∕environmental scanning electron microscopy of dielectrics