Direct measurement of the electron beam of a scanning electron microscope

Apparatus for measuring the electron beam diameter of a scanning electron croscope includes: a transducer which supports a heated wire acting as a knife edge; an electron collector; and a display. The electron beam is scanned across the knife edge to obtain a change in current density which is received by the electron collector and shown as a trace on the display. This trace is a relative measurement of electron beam diameter. The electron beam is scanned a second time with the transducer moving the heated wire abruptly during the second scan to cause a shift in the current density trace on the display. The amount of shift between the traces of the initial and second scans is a reference distance against which the relative measurement of electron beam diameter may be measured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to apparatus for measuring the diameter of the 
electron beam of a scanning electron microscope (SEM). More particularly, 
the invention relates to the use of a heated wire acting as a knife edge 
for measuring the electron beam diameter and a transducer assembly for 
providing a known reference distance against which the measured electron 
beam diameter is compared. 
2. Description of the Prior Art 
The method currently used to measure the resolution of an SEM is to make a 
micrograph at high magnification of an object which contains a wealth of 
high contrast and fine detail. A measurement is then made of the shortest 
distance which is resolved on the micrograph and converted via the known 
magnification into a distance resolved on the specimen. Since the smallest 
resolvable distance is approximately equal to the beam diameter, 
measurements of the resolution have been used to determine beam diameters. 
The difficulty with this approach comes from the fact that the result 
depends upon the interaction between the specimen and the electron beam. 
The technique is also very slow. There is no way it could be used to 
determine the optimum settings for a combination of the variables that 
determine the beam diameter. 
Another method has been proposed in an article "SEM Parameters and Their 
Measurement," D. C. Joy, SCANNING ELECTRON MICROSCOPE, 1974 (Part I) 
Proceedings of the Seventh Annual Scanning Electron Microscope Symposium, 
ITT Research Institute. The method uses a thick, rectangularly-shaped 
knife edge positioned at the narrowest part (i.e., focal point) of the 
electron beam. An aperture and electron detector below the knife edge are 
used to collect the current from the electron beam. The electron beam is 
scanned across the knife edge and the current density received by the 
electron detector is plotted on an x-y graph to give an indication of 
electron-beam diameter. 
SUMMARY OF THE INVENTION 
The present inventor recognized several heretofore unknown problems with 
the prior art measurement system described by D. C. Joy and provides a 
solution to those problems. One major problem involves the buildup of 
carbonaceous material on the knife edge. This causes inaccuracies for two 
reasons: first, the carbonaceous material has a low density and results in 
a knife edge which is highly transparent to electrons when it is 
uncharged; and second, as the contamination layer acquires a charge, it 
deflects the electron beam by increasingly larger amounts. Another problem 
involves the use of a fracture edge of a silicon wafer as the knife edge. 
Since silicon is of low density, a thin piece will allow electrons to pass 
through and a thick section must be used in order to have a relatively 
opaque knife edge. Since the electron beam has a finite divergence at its 
focal point, the top and bottom edges of the thick, straight, vertical 
knife edge will strike portions of the electron beam above and below the 
focal point which are wider than the focal point. This will result in an 
apparently larger diameter than one would measure with a mathematically 
ideal knife edge which would be of zero thickness and mathematically 
opaque to electrons. A third problem involves the limited accuracy with 
which the beam diameter could be measured even if a non-contaminating, 
thin opaque knife edge were available. The accuracy of such a 
determination is limited to the accuracy with which the magnification of 
the microscope is known, 25% at best. 
To solve the above-stated problems, the present invention utilizes a heated 
wire which acts as a knife edge. A current source provides a current 
through the wire to heat it. The SEM scans the electron beam across the 
heated wire and an electron collector receives the changing current of the 
electron beam which will provide a relative measurement of electron-beam 
diameter on, for example, an x-y trace. This measurement must be compared 
with a known reference distance to determine what the measurement is in 
units of distance. To accomplish this, the SEM scans the electron beam 
across the knife edge a second time. During the scan, a transducer 
assembly causes the heated wire to move abruptly a known distance. 
Movement of the heated wire will cause the x-y trace of the electron beam 
current to be shifted. The magnitude of the shift is related to the known 
distance the heated wire moves and is therefor a known reference distance 
against which the relative measurement of electron beam diameter from the 
first scan may be measured. 
A novel feature of the invention is the use of heat to prevent the buildup 
of carbonaceous materials on the knife edge. 
A second novel feature of the invention is the use of a cylindrical knife 
edge (i.e., wire) which causes measurement of the electron-beam diameter 
to be made at the narrowest possible point of the beam. 
A third novel feature of the invention is the use of a high-density 
conductor, preferably tungsten, as the knife edge. This minimizes the 
number of electrons which travel through the knife edge after striking it 
and subsequently are received by the electron detector. 
A fourth novel feature is the use of a transducer assembly to produce a 
known reference distance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Apparatus for measuring the diameter of an electron beam in a scanning 
electron microscope is outlined in FIG. 1. The apparatus is generally 
composed of: a transducer assembly 10, a heated wire 12 supported by the 
transducer assembly, a current source 14 providing current to the heated 
wire, and a frame member 16 which attaches the transducer assembly to the 
stage of the scanning electron microscope. 
Transducer assembly 10 includes: a support 18, an elastic compliance member 
20 (partially shown), a pair of transducer discs 22, 24 (or plates), a 
voltage source 25 and mechanical biasing element 26. 
Support 18 contains a massive aluminum conducting member 28 for heat 
dissipation, ground lead 29, insulator 30, spacer 32, beryllium-copper 
strip 34, and anchor 36 having a locking screw for heated wire 12. 
Insulator 30 is integrally attached to aluminum conducting member 28 and 
supports spacer 32. Beryllium-copper strip 34 is attached to spacer 32 and 
acts as a conductor and a spring. Power lead 38 extends from current 
source 14 to beryllium-copper strip 34. 
Heated wire 12 is stretched under tension between strip 34 and anchor 36. 
The stretched length of the heated wire 12 is about 1 mm. The tension on 
the wire and its short length make the wire stiff and prevent movement of 
it when an electron beam is scanned across it. Heated wire 12 is a 
high-density conductor such as tungsten and may have a diameter of 0.001 
inches. With such a diameter, it passes 0.45 amps to have the proper 
current density for preventing contamination on the wire. Correspondingly 
higher current at the same current density could be passed through larger 
diameter wires. 
Elastic compliance member 20 is an elastic material capable of being 
compressed and returning to its original shape after compression. In the 
preferred embodiment, it is machined out of a single piece of brass so 
that it is essentially a thin walled cylinder with heavy integral end 
faces. 
Transducer discs 22, 24 are washer-shaped circular discs which may be made 
of a material designated by Military Standard 1376 revised form -- Navy 
type III -- very hard lead zirconate - titanate with a Curie point larger 
than or equal to 290.degree. C. An example of this is "PZT-8" produced by 
the Clevite Corporation which is composed of lead zirconate -- lead 
titanate ceramic transducer material with chemical additives including 
Fe.sub.2 O.sub.3. They are placed in contact with each other and oriented 
so that their polarization vectors are oppositely directed. Different 
numbers of transducer discs could be used depending on their respective 
thicknesses. PZT-8 was chosen as the transducer material because of its 
reproducibility. Other materials having reproducible characteristics of 
the same order could also be used. Transducer discs 22, 24 are in contact 
with each other through a thin copper ring 23 placed therebetween. They 
are oriented so that their polarization vectors are oppositely directed. 
The outside surfaces are kept at ground potential while the driving 
voltage from voltage source 25 is applied through lead 39 to the copper 
ring 23 between the central common surfaces. The transducer discs are 
covered with a metal foil at ground potential to shield the interior of 
the microscope from the voltages that are applied to the transducer. 
Otherwise the trajectories of the electrons would be affected when a 
voltage is applied to the transducer. 
Mechanical biasing element 26 is a stainless steel bolt which is much more 
than 10 times stiffer than elastic compliance member 20. It extends 
through transducer discs 22, 24 (and is electrically insulated therefrom), 
support 18, elastic compliance member 20, and frame member 16. The nut on 
the end of mechanical biasing element 26 is hidden by frame member 16. The 
function of mechanical biasing member 26 is to maintain a mechanical bias 
on transducer discs 22, 24. This mechanical bias is necessary to make the 
transducer displacement-voltage behavior reproducible and virtually 
hysteresis-free. 
FIG. 2 is a side view of the apparatus of FIG. 1. Mechanical biasing member 
26 extends through transducer discs 22, 24, support 18, elastic compliance 
member 20, and frame member 16. Attached to support 18 is beryllium-copper 
strip 34 which has power lead 38 attached. An electron collector 40 is 
shown to be directly beneath beryllium-copper strip 34 and heated wire 12 
(not visible). Power source lead 39 is attached to the copper ring between 
the inner surfaces of transducer discs 22, 24. 
FIG. 3 is a vertical sectional view of electron collector 40 which in the 
preferred embodiment is a Faraday cell. An insulator 42 is the outer 
surface of the electron collector 40. In the top of insulator 42 is a 
tungsten ring 43 which has a 200-micron aperture. On top of tungsten ring 
43 is a grounded copper shield 44, and enclosing the bottom of the Faraday 
cell is an electronic charge-collecting copper element 46. Connected to 
copper element 46 is shielded lead 48 which transfers the electron charges 
collected to a display 49 made up of an electrometer and x-y recorder. 
FIG. 4 shows diagrammatically the measurement of the diameter of an 
electron beam of an electron scanning microscope utilizing the apparatus 
of the prior art. Scanning electron microscope 50 produces an electron 
beam which scans in the direction indicated. A portion of the electron 
beam is shown as excluded and the narrowest portion of the beam, which is 
the focusing point at which a specimen would normally be placed, is blown 
up. A knife edge 52, which may be a copper mesh (10-micron repeat, of the 
type used in vidicon tubes) or the fractured edge of a silicon slice, is 
shown at the point of focus of the beam. A charge collector is made up of 
a shield 54, with a 100-micron aperture therein, and an electron detector 
56. 
In the prior art the focused probe is scanned normal to the knife edge 52 
(in the direction indicated) and the electron charges are collected by the 
detector 56. The intensity of the collected charge is graphically plotted 
to provide an indication of electron beam diameter. 
Several major problems occur with the knife edge of the prior art which 
decrease accuracy of the electron beam measurement. Briefly, these 
problems involve the buildup of carbonaceous materials on the knife edge, 
the use of a thick rectangular knife edge, the use of a low-density 
conductor (silicon or copper) for the knife edge, and the lack of an 
accurate knowledge of the distances involved. These problems will be 
considered in detail subsequently. 
FIG. 5 shows diagrammatically the measurement of the diameter of the 
electron beam of the present invention. Scanning electron microscope 50 
generates the same scanning beam shown in FIG. 4. Heated wire 12 having a 
diameter of 0.001 in. is used as the knife edge at the focal point of the 
beam. Electron collector 40 shown in detail in FIG. 3 is used to collect 
the electron charges and transfer them to an electrometer and x-y recorder 
which will graphically plot the diameter of the electron beam. 
FIG. 6 shows an x-y plot with the y direction representing the electron 
collector current and the x direction representing the scan voltage. Trace 
58 on the graph represents electron beam current collected as the electron 
beam is scanned past the knife edge. Trace 60, a broken line, represents 
electron beam current collected as the electron beam is scanned partially 
over the knife edge and the transducer is activated which shifts the 
heated wire a known distance, causing the trace to be shifted. The amount 
by which the trace is shifted is related to the known magnitude of the 
knife edge movement. The distance of the trace shift, shown by arrow 62, 
is used as a known reference distance from which the diameter of the beam 
may be calculated. The beam diameter is measured by determining the 25 and 
75% points (25 and 75% of the maximum beam current) on the x-y plot of the 
current received by the Faraday cell. Use of these two points makes it 
possible to avoid the error caused by using the flat portions of the 
current curve (horizontal portion of the trace in FIG. 6). Scattered 
electrons from the knife edge have a much larger effect on the horizontal 
portion of the curve than on the steeper portion. A theoretical analysis 
of the effect is given in "The Direct Measurement of SEM Beam Diameters," 
William H. Vaughan, Scanning Electron Microscopy/1976, Part I, Proc. 9th 
Annual SEM Symposium of IITRI (Illinois Institute of Technology Research 
Institute), Chicago, Ill., 60616, Apr. 1976 pps 745-755, which is hereby 
incorporated by reference. 
In operation, two steps are generally required to obtain a measurement of 
the electron beam diameter. The first is to obtain a measurement of the 
electron beam diameter which is relative and has no units of distance. The 
second step is to establish a reference distance against which the 
relative electron beam measurement may be compared and then the distance 
can be determined in distance units. 
To obtain a relative measurement of electron beam diameter, the scanning 
electron microscope 54 is adjusted such that the narrowest point of its 
beam (i.e., focal point) is focused on the edge of heated wire 12. 
Scanning electron microscope 54 then scans the electron beam along a line 
perpendicular to heated wire 12 which is stationary and above electron 
collector 40. As the electron beam scans across the knife edge, the 
resulting variation in the current of the electron beam is received by 
electron collector 40 and sent to the electrometer whose output drives an 
x-y recorder. The x motion of the recorder is driven by the same signal 
that drives the electron beam across the sample. The output of the 
electrometer is fed to the y input of the recorder. The resulting plot on 
the x-y recorder is that shown in FIG. 6. 
The second step in the measurement of the electron beam diameter, 
establishment of a reference distance, is accomplished as follows. 
Scanning electron microscope 54 causes the electron beam to scan across 
the knife edge again. While the scan is taking place, a D.C. voltage is 
set up across the transducer causing the knife edge to be shifted a known 
distance. This causes a shift in the beam current plot on the x-y 
recorder, as shown by trace 60 on FIG. 6. The distance between the initial 
and shifted scans of the electron beam, shown by arrow 62 of FIG. 6, is 
measured on the x-y plot (i.e., the difference in scan voltage is 
determined for the same beam current on the two curves). It corresponds to 
a distance in the specimen in the microscope equal to the calibrated 
displacement of the transducer for the applied voltage. In this way 
displacement along the x axis of the x-y plot is calibrated with an 
accuracy that is determined by a National Bureau of Standards calibration. 
Looking more closely at how the transducer moves the heated wire, a voltage 
from a D.C. voltage source 25 is applied through lead 39 to the central 
common surface of transducer discs 22, 24. This increases the thickness of 
the transducer discs. The mechanical biasing member 26 provides a 
mechanical bias to transducer discs and to elastic compliance member 20. 
An increase in thickness of the transducer discs causes the elastic 
compliance member to compress. Support 18 which is positioned between 
transducer discs 22, 24 and elastic compliance member 20 will be displaced 
by the amount that the elastic compliance member 20 compresses. Since 
heated wire 12 is supported on support 18 the heated wire will move 
through the electron beam the same distance as the support. As the heated 
wire moves through the beam, the electron collector 40 will sense the 
change in current in the electron beam and an x-y trace such as that shown 
as trace 60 of FIG. 6 will result. 
It should be noted that although optimum accuracy of measurement is 
achieved by comparing the relative measurement of the electron beam 
diameter with the reference established by the use of the transducer, the 
relative measurement of electron beam diameter could also be compared with 
a reference established by using the known magnification of the scanning 
electron microscope as previously discussed with respect to prior art. The 
accuracy of this method is not optimum however. If this method were used 
the transducer would not be needed and only a support for the heated wire 
would be needed. An example of such a support could be support 18, shown 
in FIG. 1, attached to an appropriate frame. 
One improvement of the invention is the use of a heated wire on the knife 
edge. Heating the knife edge is necessary to get valid results because a 
single passage of the electron beam across the knife edge can cause a 
detectable amount of contamination buildup. The contamination buildup is 
caused by the hydrocarbons that are present in all similar vacuum systems 
due to the diffusion pump fluid and the elastromers that are used as 
seals. This forms a surface film on everything within the microscope. When 
this film of hydrocarbons is struck by an energetic electron beam, the 
high volatility products go off as gases leaving a low-conductivity 
carbonaceous layer behind. The low conductivity of this layer causes it to 
acquire a charge when exposed to the full electron beam. The resulting 
electrostatic deflection of the beam causes the edge trace to become 
diffused and makes an accurate measurement of the beam diameter 
impossible. Heating the wire causes hydrocarbon molecules striking the 
wire to statistically spend less time on the surface of the wire and thus 
a significant buildup of a carbonaceous material does not form. In 
experiments conducted using a wire which was not heated, the current 
density traces such as that shown in FIG. 6 leveled out after several 
scans until the trace became irregular and almost linear. When a heated 
wire was used, accurate, reproducible, current density traces were 
consistently achieved. As mentioned previously, 0.45 amps passed through a 
0.001 in. wire was sufficient to prevent all detected effects due to 
contamination. Correspondingly higher currents at the same current density 
are sufficient for larger diameter wires. 
Another important aspect of this invention is the use of a high-density 
cylindrical (i.e., tungsten wire) knife edge rather than the thick 
rectangular knife edge disclosed in the prior art. The cylindrical knife 
edge is desired since the electron beam has a finite divergence angle. The 
electron beam is best cut off using a cylindrical knife edge since it 
begins measurement of the electron beam at the narrowest portion of the 
electron beam. This is clearly shown by comparison of the knife edge shown 
in FIGS. 4 and 5. A 0.001 in. diameter tungsten wire for a knife edge is 
considered near optimum for 20 KV electrons at a semi-convergence angle of 
0.01 radians for several reasons. If the diameter is less than 0.001 in., 
more electrons will pass through the wire and enter the electron 
collector. If the diameter is substantially greater than 0.001 inches, the 
wire does not cause the beam to be measured in its narrowest portion. This 
is the same problem as with the prior art knife edge. Furthermore, it is 
desirable to use tungsten since it is a high-density conductor through 
which electrons will not easily pass, and it is also strong which is 
necessary for a thin wire of 0.001 in, diameter. 
Another important aspect of this invention is the use of two points on the 
steep portion of the current trace from which to make measurements which 
characterize the beam diameter. In this way the effect due to electrons 
that have been scattered from the top of the knife edge and subsequently 
picked up by the Faraday cell is minimized. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.