Specimen Current (The Specimen As Detector)
Consider the interactions of the beam electrons with the specimen to produce
backscattered and secondary electrons. For a 20 keV beam on copper, about 30 out
of 100 beam electrons are backscattered ( = 0.3). The remaining 70 beam
electrons lose all their energy in the solid, are reduced to thermal energies,
and are captured by the solid. About 10 units of charge are ejected as
secondary electrons ( = 0.1). This leaves 60 excess charges in the target.
What is the fate of these electrons? To understand this, an alternative view is
to consider the
currents, defined as charge per unit time, which flow in and
out of the specimen. Viewed in this fashion, the specimen can be treated as an
electrical junction, as illustrated schematically in Figure 4sc.1, and is
subject to the fundamental rules which govern junctions in circuits. By
Thevinin's junction theorem, the currents flowing in and out of the junction
must exactly balance, or else there will be net accumulation or loss of
electrical charge, and the specimen will charge on a macroscopic scale. If the
specimen is a conductor or semiconductor and if there is a path to ground from
the specimen, then electrical neutrality will be maintained by the flow of a
current, designated the "specimen current" (also known as "target current" or
"absorbed current"), either to or from ground, depending on the exact conditions
of beam energy and specimen composition. What is the magnitude of the specimen
current?
Considering the specimen as a junction, the current flowing into the junction is
the beam current, iB, and the currents flowing out of the junction are the
backscattered electron current, iBSE, and the secondary electron current, iSE.
For charge balance to occur, the specimen current, iSC, is given by:
iSC = iB - iBSE - iSE
(4sc.1)
For the copper target, the backscatter current will be iBSE =  iB = 0.3 iB and
the secondary electron current will be iSE =  iB = 0.1 iB.
Substituting
these values in equation (4sc.1) gives the result that the specimen current will
be iSC = 0.6 iB, double the largest of the conventional imaging currents, which
is the backscattered electron signal. If a path to ground (literally a wire
from the specimen to a grounding point on the SEM stage) is not provided so that
the specimen current can flow, the specimen will rapidly charge.
Note that in formulating equation (4sc.1) no consideration is given to the large
difference in energy carried by the backscattered and secondary electrons.
Since current is the passage of charge per unit time, the ejection of a 1 eV
secondary electron from the specimen carries the same weight as a 10 keV
backscattered electron.
The specimen obviously serves as its own collector for the specimen current. As
such, the specimen current signal is readily available just by insulating the
specimen from ground and attaching a wire to ground to collect the specimen
current. Does the specimen current signal actually convey useful information?
As described below under contrast formation, the specimen current signal
contains exactly the same information as that carried by the backscattered and
secondary electron currents. Since our detectors measure a convolution of
backscattered and/or secondary current with other characteristics such as energy
and/or directionality, the specimen current signal can give a unique view of the
specimen (Newbury, 1976).
To make use of the specimen current signal, the current must be routed through
an amplifier on its way to ground. The difficulty is that we must be able to
work with a current similar in magnitude to the beam current, without any high
gain physical amplification process such as electron-hole pair production in a
solid state detector or the electron cascade in an electron multiplier. To
achieve acceptable bandwidth at the high gains necessary, most current
amplifiers take the form of a low input impedance operational amplifier (Fiori
et al., 1974). Such amplifiers can operate with currents as low as 10 pA and
still provide adequate bandwidth to view acceptable images at slow visual scan
rates (1 500-line frame/s).
Compositional Contrast with Specimen Current
To understand the appearance of the specimen in an image prepared with the
specimen current signal, consider how the signals change between any two
different locations. Using equation (4sc.1) as a starting point, the difference
in signals between any two locations can be calculated. To simplify the
argument, consider that the backscattered and secondary electron signals are
combined into an "emissive" signal:
iE = iBSE+ iSE
(4sc.2)
With this substitution, equation (4sc.1) becomes:
iB = iE + iSC
(4sc.3)
The difference in the signals between any two pixels is found by taking
differences for each term:
iB = iE + iSC
(4sc.4)
Because the electron optical column is carefully constructed to maintain a
constant beam current, the difference in the beam current between any two points
in the image is zero, except for statistical fluctuations. Equation (4sc.4) can
thus be rearranged to give the relationship between the emissive and the
specimen current signals:
iB = 0
iSC = -iE
(4sc.5)
The sense of the contrast is thus opposite in the specimen current image
compared to the image recorded with a detector of emissive mode signals.
Images of Composition in the Specimen Current Signal
The contrast reversal predicted by equation 4sc.5 can be seen in Figure 4sc.2,
which is an image of a flat specimen with regions of differing composition.
Where certain regions appear dark in the emissive mode image of Figure 4sc.2(a),
these same images appear bright in the specimen current image of Figure
4sc.2(b).
While this contrast reversal may seem to be a trivial change, a more subtle
difference exists between the specimen current and emissive mode signals.
Specimen current is sensitive only to the numbers of electron electrons leaving
the specimen and is completely insensitive to their trajectories. As long as a
secondary or backscattered electron leaves the specimen, it contributes
information to the specimen current signal, regardless of its fate to be
collected or to be lost. Of all possible detectors, specimen current is the
only detector which is sensitive to number effects only. Trajectory and energy
effects are completely eliminated. If the specimen is biased to suppress
secondary emission, the specimen current signal can be rendered sensitive to
backscattered electron effects only (Heinrich 1966).
Because the direct specimen current image gives the reversed sense of atomic
number contrast, it is common practice to apply a signal processing operation to
artificially reverse the sense of the signal (reversed specimen current image)
so that a brighter area corresponds to higher atomic number.
Images of Topography in the Specimen Current Signal
To understand the appearance of the topography specimen current image, two
properties of specimen current must be recalled. First, from equation (4sc.5),
the sense of topographic contrast is expected to be reversed in the specimen
current image as compared to the emissive mode image. Second, specimen current
is only sensitive to number effects, and is completely insensitive to trajectory
effects, which contribute strongly to emissive mode images prepared with the E-T
detector, and effects of the energy distribution of backscattered electrons, to
which solid state detectors are sensitive.
The direct specimen current image of a rough, fractured surface of high purity
iron is shown in Figure 4sc.3(a). The sense of the topography appears reversed
relative to the emissive mode image, Figure 4sc.3(b), as expected from equation
(4sc.5). The fine scale dimples on the grain surfaces appear prominently in the
specimen current image. An unexpected effect is the uniform appearance of
facets at approximately equal tilt to the beam. The facets of the grain
boundary triple junction in the lower left of the image appear uniform in the
specimen current image, but these facets have different brightness in the
positively-biased E-T image and in the dedicated backscattered electron image
because of trajectory effects, which are completely absent in the specimen
current image. Since both backscattering and secondary electron emission
increase monotonically with tilt, the magnitude of the specimen current can be
used to quantitatively assess the local tilt angle.
The distraction of the reversal of topography encountered in the direct specimen
current image can be eliminated by using contrast reversal in the signal
processing chain to produce the image shown in figure 4sc.3(c), which displays
the proper sense of the topography, as expected from the point of view of the
emissive mode detector. Unless this contrast reversal is applied, the sense of
topography obtained from a direct specimen current image will be incorrect.
Signal processing for specimen current
Situations sometimes arise in which it is desirable to reverse the contrast
which naturally appears in an image. An example is the direct specimen current
image, which has the opposite sense of contrast to the corresponding emissive
mode image. Since we intuitively expect to interpret images according to the
characteristics of the emissive mode, direct specimen current images are often
confusing, both for topographic contrast, where the sense of topography appears
reversed, and for atomic number contrast, where light elements are unexpectedly
bright compared to heavy elements. It is therefore useful to artificially
reverse the contrast during signal processing. This reversal is accomplished by
the following signal transformation:
Sout = Smax - Sin (4sc.6)
An example of this contrast reversal transformation applied to an image of a
rough surface is shown in Figure 4sc.3(c).
Specimen Current for Separation of Contrast Components
The specimen current image is invaluable as a reference image for separation of
contrast components through comparison of emissive (backscattered and secondary
electron) images with specimen current images (Newbury, 1976). The specimen
current signal is totally dominated by number effects and is completely
independent of trajectory effects, while an asymmetric backscattered electron
detector such as the negatively-biased E-T detector is extremely sensitive to
trajectory effects. Figure 4sc.4 shows a comparison of images of a two phase
lead-tin eutectic alloy with surface topography. Figure 4sc.4(a)shows a
negatively-biased E-T detector and Figure 4sc.4(b) the corresponding specimen
current image, with the contrast reversed so that the phase with the higher
average atomic number appears bright. In Figure 4sc.4(a), the ridges of the
topography dominate the image because the negatively-biased E-T detector is
located at a low take-off angle above the surface, which increases the effect of
the apparent oblique illumination, and increases the sensitivity of the image to
the trajectory effects inherent in the topographic contrast. In Figure
4sc.4(b), the atomic number contrast dominates the image obtained with the
specimen current signal, and the topographic contrast is greatly diminished.
Because the specimen current image is only sensitive to the numbers of SE and
BSE leaving the specimen and not to their trajectories, all of the shadowing
effects encountered in conventional emissive mode images (BSE only or SE+BSE)are
eliminated from specimen current image.
References
Fiori, C. E., Yakowitz, H., and Newbury, D. E. (1974).
Institute, Chicago, Illinois, p. 167.
SEM/1974, IIT Research
Heinrich, K. F. J. (1966). In Proc. 4th Intl. Conf. on X-ray Optics and
Microanalysis, eds. R. Castaing, P. Deschamps, and J. Philibert, Hermann, Paris,
159.
Newbury, D. E. (1976).
SEM/1976/I, IIT Research Inst., Chicago, Illinois, 111.
Figure Captions
4sc.1 Illustration of currents which flow in and out of specimen: iB , beam
current; iBS, backscattered electron current; iSE, secondary electron current;
iSC, specimen current. The junction equivalent of the specimen is also shown.
4sc.2 Atomic number (compositional contrast) observed in a composite specimen
consisting of a copper grid, silicon chip (square, and silver dag on an aluminum
stub. (a) Backscattered electron image derived from a negatively-biased
Everhart - Thornley detector. (b) Direct specimen current image of the same
region; note the contrast reversal compared to 4sc.2(a)and the complete lack of
the strong shadowing seen in the highly directional BSE image.
4sc.3(a) Fracture surface of polycrystalline iron viewed with various detectors:
(a)Negatively-biased Everhart-Thornley, BSE only; (b) Positively-biased
Everhart-Thornley, SE+BSE; (c) Dedicated backscattered electron detector having
a large solid angle (sum mode)and placed at a high take-off angle, showing
compositonal differences. (d) Dedicated backscattered electron detector having a
large solid angle (difference mode)and placed at a high take-off angle, showing
topography; (e) Direct specimen current signal showing contrast reversal from
(a)(b), and (c); (f) Reversed contrast specimen current image showing same
general sense of contrast as (a). E0 = 15 keV.
4sc.4 Separation of contrast components (composition and topography) in a leadtin eutectic alloy with (a) highly directional, low take-off angle, negativelybiased Everhart - Thornley detector collecting only BSEs; weak atomic number
contrast is observed, but the strongest features are surface irregularities
(b)specimen current (contrast reversed, where the atomic number contrast
dominates and the topography is almost completely eliminated.