Kelli Gagnon
Lab 3 – Electron Beam Parameters
9.5/10
Introduction
A stable probe current is essential for SEM imaging, as information from the sample
is collected by a point-to-point scan over a duration of time, rather than all at once.
Obtaining proper filament saturation ensures that minor fluctuations in the filament
current will not affect the beam current (that is, the portion of the filament current
that leaves the gun through the hole in the anode; this current becomes further
condensed by lenses and the objective aperture as it passes through the column, the
portion that reaches the sample is known as “probe current”). GOOD At saturation,
an increase in the filament current (DO YOU MEAN FILAMENT HEATING CURRENT?
– THIS SHOULD HAVE ALMOST NO EFFECT ON BEAM CURRENT WHEN AT
SATURATION) results in an increase in electron emission; as the emission current
flows through the bias resistor, the negative bias voltage on the Wehnelt cap also
increases, opposing the increase in emission (Fig 1). GOOD DESCRIPTION OF
FEEDBACK AT SATURATION Furthermore, ensuring that the filament is set at or
below saturation will increase its overall lifespan.
Figure 1. Cross section of electron gun showing bias resister and Wehnelt cap
I THINK YOU SHOULD SPECIFICALLY REFERENCE THESE IMAGES USED IN FIGURES
UNLESS YOU MAKE THEM YOURSELF (MAYBE YOU DID MAKE THE FARADAY CUP
IMAGE?)
The objective of this lab was to observe how the strength of the probe current
changes as the filament is being saturated and as different lens settings are applied
to the SEM. We used a Faraday cup (Fig 2) to trap the electrons released by the
beam and obtained accurate probe current measurements with a pico-ammeter. By
obtaining an accurate probe current value, we could then observe how it fluctuates
when emission current and various lens settings were changed. The probe diameter
was calculated at two different probe currents by scanning a silicon wafer with an
electron-opaque edge. The convergence angle of the beam was calculated by
measuring the beam at two different working distances; based on these values we
also estimated functional aperture. Using a drill bit, we discerned the depth of field
from two images and observed the effect of working distance on depth of field.
Figure 2.Cross-section of a Faraday cup measuring probe current. Probe current enters
the cup through the aperture, all electrons are absorbed and measured by the
picoammeter.
Methods
A mounted silicon chip and Faraday cup were placed in the sample chamber; a wire
from the Faraday cup was connected to the pico-ammeter via a port inside the
chamber. The chamber was evacuated and the filament was turned on and warmed
up using the knob below the HT button. The filament was set to just below
saturation. To achieve max saturation the current had to be aligned over the anode
hole in the Wenhelt cap. The emission current increased as the filament was heated
and was 83uA at the point of saturation (found on the L.C. readout of the panel).
To find the probe current, the aperture of the Faraday cup (Figure3) was located
and centered on screen. To direct the current into the hole, the hole was zoomed in
until it completely engulfed the screen (alternately, a spot beam could be directed
into the hole).
Figure 3. Aperture of the faraday cup imaged at 350x.
The spot size was adjusted to achieve probe currents of 10nA and 0.1nA; the
Condenser settings and Emission currents were recorded (Table 1, Results). The
spot size was then adjusted to 8, 12 and 18; the objective lens was kept constant and
the probe currents for each spot size were recorded (Table 2, Results).
Finally, the probe diameter was measured by observing the rise/fall in signal
(Figure 4) as the beam was swept over the sharp edge of the silicon chip (Figure 5).
THIS METHOD WOULD NOT BE CLEAR TO SOMEONE NEW TO THE LAB – IT IS
TRICKY BUT MAY INVOLVE ONE MORE FIGURE SHOWING THE BEAM SWEEP
ACROSS THE EDGE OR JUST A BIT MORE TEXT this was done using probe currents
of 0.1nA and 1.0nA and recorded along with condenser settings (TABLE 4).
Figure 4. Silicon wafer edge, which was used to measure probe diameter.
Figure 5. Method of measuring probe diameter from a tracing of a signal waveform
(MORE LIKE A SIGNAL INTENSITY VS DISTANCE PLOT) at spot sizes 8 and 12,
diameter was measured and corrected for 50,000x magnification at each setting.
The beam convergence angle (Figure 6) was calculated by measuring the diameter
of the beam at two different working distances (the difference between them
defined as DELTA “z”) with focus held constant to obtain “di” and “df” (initial and
final beam diameters). With these values, the convergence angle (a) was
determined using the following equation: tana=(1/2 df-di)/z. With this angle, the
functional aperture was able to be estimated as well (FIGURE 8 AND TABLE 5).
Figure 6.Schematic diagram of the depth-of-field in an SEM image, showing location of
convergence angle and how depth-of-field may be calculated.
Depth of field was observed using a drill bit flute. Using the equation
D=0.2mm/Mag*a, depth of field was calculated at magnifications 50x and 5000x.
Depth of field was also experimentally measured from an image of the drill bit by
measuring the amount of the image that appeared in focus in ImageJ.
Results
Objective Aperture
Condenser setting
Probe current
Emission current
(spot size/I.C.L.)
(L.C.)
50 um, virtual pos.
18/ 95mA
10 nA
83 uA
50 um, virtual pos.
8/ 168mA
0.1 nA
83 uA
Table 1. Effect of condenser settings on probe current and emission current.
Changing the spot size affected the probe current (probe current increased with
spot size), but the emission current remained unchanged. This is as to be expected,
as the condenser lens is located below the anode where emission current is
measured. The probe current is much smaller than the emission current, as the
anodes are taking up much of the electrons The objective aperture is also holding
back electrons as the aperture is smaller than the beam; these electronstravel
through the metal sides of the column to ground.
Spot size
Condenser lens current
Probe Current
8
168 mA
0.1 nA
12
117 mA
1.0 nA
18
95 mA
10 nA
Table 2. Effects of condenser lens settings on probe current; objective lens held
constant at 400 mA.
wd (estimated)
Objective lens current
Probe current
25 mm
435 mA
0.8 nA
30 mm
406 mA
1.1 nA
35 mm
390 mA
1.1 nA
Table 3. Effects of objective lens current on probe current, condenser lens held
constant at 117 mA.
The condenser lens current is adjusted by lowering or raising spot size; the affects
the probe current by cropping the beam as the spot size decreases. This can be seen
in that the probe current increased by a magnitude of 10 as the spot size is
increased by a value of 6. The objective lens current is controlled by focus or
working distance; as the beam has already passed through the condenser lenses,
changing the objective lens current will not affect it as much. These results were
plotted in Figure 7.
Figure 7. Objective lens and condenser lens effect on probe current. Objective lens is in
red, condenser lens is in blue. Note that the condenser lens has a much greater effect
on probe current than objective lens. GREAT PLOT
Probe diameter was measured using the procedure in Fig. 5 while the spot size was
adjusted and all other parameters were held constant. It was observed that probe
diameter increases with increasing spot size (Table 4); this is important as an image
obtained with a smaller probe diameter will have a better resolution with more fine
detail.
Aperture
Probe Current
Probe Diameter
50 um, virtual pos.
Condenser Setting
(spot size/I.C.L.)
8 / 168 mA
0.1 nA
20 nm
50 um, virtual pos.
12/ 117 mA
1.0 nA
40 nm
Table 4. Effects of condenser settings on probe diameter.
Convergence angle and the functional aperture were calculated using the method
illustrated in Figure 8. Probe diameter values (di, df and Δ d) were calculated using
the method shown in Figure 5. The values for these calculations are listed in Table 5.
Figure 8. Calculations for beam convergence angle and estimating functional aperture
using the values obtained in Table 5. NICE DIAGRAM
di (nm)
df (nm)
Δ d (nm)
zi (mm)
zf (mm)
Δ z (mm)
a (rad)
fa (um)
80
1160
1080
14
13.5
.5
0.001
30.24
Table 5: Measured and calculated values for beam convergence, using a spot size of 12
with the aperture held constant and in the 50um virtual position.
Depth of field was calculated at working distances of 11 mm and 31 mm, and at
magnifications of 50x and 5000x using the formula D=0.2mm/Ma. The 0.2 mm factor
accounts for the limitations on the human eye when viewing an SEM screen. It was
observed that depth of field is greater with larger working distances when the
convergence angle is narrower, which is expected. These values are listed in Table 6.
Finally, images of the drill bit flute at wd 11 mm and 30 mm were captured at 50x
and the depth-of-field was experimentally measured in ImageJ (Figure 9). Figure 10
illustrates how these measurements represent the depth of field for the image.
wd
Convergence
D at 50x
D at 5000x
11 mm
0.0028 rad
1.429 mm
0.014 mm
31 mm
0.00098 rad
4.082 mm
0.041 mm
Table 6.The effect of working distance on depth of field.Aperture diameter 50um,
functional aperture diameter 30.24 um. The greater working distance for 50x has a
depth of field 2.8 times larger than the smaller wd.
Figure 9. Depth of field measured experimentally in ImageJ. Left is working distance
11, right is wd 31. The greater working distance has a depth of field 2.3 times larger
Than the smaller wd.
IT ISNT CLEAR THAT THE WD
SHOWN HERE ARE NOT THE
ACTUALLY MEASUREMENTS
FOR DEPHT OF FIELD – I DO
THINK THAT YOU
UNDERSTAND
Figure 10.Illustration of drill bit flute showing depth of field at 11mm and 31mm
working distance.
References
1) Goldstein, J.; Newbury, D.; Joy, D.; Lyman, C.; Echlin, P.; Lifshin, E.; Sawyer, L.;
Michael, J., Scanning Electron Microscopy and X-Ray Microanalysis. 3 ed.; Kluwer
Academic/Plenum Publishers: New York, 2003.
2) Lyman, C.E.; Goldstein, J. et al., Scanning electron microscopy, X-ray
microanalysis, and analytical electron microscopy: a laboratory workbook.
Plenum Press: New York, 1990.
3) Neff, D and Norton, M. SEM Laboratory 2: Beam Parameters. Marshall
University, 2010.
4) http://www.marshall.edu/mbic/instrumentation/SEM/sem.html
5) http://www.science.marshall.edu/dneff/course2013/semLAB2/drilldepth_2013.pptx
YOU FORGOT THESE QUESTIONS BUT I THINK YOU GOT MOST OF THE BIG
IDEAS
How does the beam current at the specimen compare with the emission
current at the gun? Where did the rest go? Note that the emission current did
not change for the high-beam-current setting because no change was made to the
gun parameters. Name three ways to change the emission current. Why
would you want to change the emission current?
Can saturation be determined by measuring the secondary electron signal
with the E-T detector instead? The emission current (readout L.C. on panel)?
Compare the ratio of probe diameters to the ratio of probe currents
(low/high) to the ratio of probe diameters (low/high) for the two operating
conditions
Why is there such a difference in the influence of these two lenses on the probe current?