U.S. patent number 3,646,344 [Application Number 04/889,762] was granted by the patent office on 1972-02-29 for scanning electron beam apparatus for viewing potential distribution on specimen surfaces.
Invention is credited to William Charles Nixon, Graham Plows.
United States Patent |
3,646,344 |
Plows , et al. |
February 29, 1972 |
SCANNING ELECTRON BEAM APPARATUS FOR VIEWING POTENTIAL DISTRIBUTION
ON SPECIMEN SURFACES
Abstract
In electron beam apparatus, in which a finely focused probe is
caused to impinge on the surface of a specimen and the resulting
secondary electrons are collected and used to show potential
distribution on the specimen surface the detector includes, between
the collector and the specimen, a positive grid and a further grid,
this further grid being of variable low or negative potential to
form effectively a potential barrier of adjustable height to
discriminate between electrons coming from regions of different
potential.
Inventors: |
Plows; Graham (Hertfordshire,
EN), Nixon; William Charles (Cambridge,
EN) |
Family
ID: |
9701950 |
Appl.
No.: |
04/889,762 |
Filed: |
December 31, 1969 |
Foreign Application Priority Data
Current U.S.
Class: |
250/310;
324/754.22 |
Current CPC
Class: |
H01J
37/268 (20130101); H01J 37/244 (20130101); H01J
2237/24507 (20130101); H01J 2237/2443 (20130101); H01J
2237/2448 (20130101); H01J 2237/2449 (20130101); H01J
2237/2445 (20130101) |
Current International
Class: |
H01J
37/244 (20060101); H01J 37/26 (20060101); H01j
037/28 () |
Field of
Search: |
;250/49.5A,49.5PE,49.5AE |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Electronics; Vol. 37, No. 16; May, 1964; pp. 119. .
Tharp et al; Journal of Applied Physics; Vol. 38, No. 8; July,
1967; pp. 3320-3330. .
Lander et al.; Review of Scientific Instr; Vol. 33, No. 7; July
1962, 782, 783..
|
Primary Examiner: Birch; Anthony L.
Claims
We claim:
1. Electron beam apparatus comprising electron gun means for
forming a high-energy probe of electrons having an energy of the
order of thousands of electron volts, lens means focusing said
probe, a specimen having a surface thereof in the path of said
probe for impact thereon of said probe, a detector adapted to
receive secondary electrons emerging from said specimen surface as
a consequence of the impact thereon of said probe said detector
subtending a restricted solid angle at said specimen surface, image
display means connected to said detector to receive signals
therefrom generated by said secondary electrons, deflection means
acting on said probe to deflect said probe laterally, said image
display means being synchronized with said deflection means such as
to produce an image of a region of said specimen surface in terms
of said secondary electrons, a first grid between said specimen
surface and said detector, said first grid being at a substantial
positive electric potential with respect to said specimen, such as
to create a voltage gradient between said first grid and specimen
surface which is not less than 5,000 volts per meter, and said
potential being not less than 25 volts, a second grid between said
first grid and said detector, said second grid being at a potential
which is negative and low but variable with respect to the
specimen, the potential of said second grid being of the order of
between zero and -5 volts.
2. Electron beam apparatus as set forth in claim 1 including a
third grid between said second grid and said collector.
3. Electron beam apparatus as set forth in claim 2 wherein said
third grid is electrically connected to said first grid.
4. Electron beam apparatus as set forth in claim 1 wherein each of
said grid and said collector are planar and are mutually
parallel.
5. Electron beam apparatus as set forth in claim 4 wherein said
grids and collector lie perpendicular to a normal to the specimen
surface.
6. Electron beam apparatus as set forth in claim 1 wherein said
voltage gradient is not less than 10.sup.5 volts per meter.
Description
This invention relates to electron beam apparatus of the kind in
which a beam of electrons, known as an electron probe, is caused to
impinge on a specimen under examination and information about the
voltage distribution at the specimen surface is obtained by using a
detector to pick up electrons emanating from the specimen. Normally
the beam will be focused to a fine spot and will be caused to scan
the surface of the specimen and the signal is used to control the
brightness of a cathode-ray tube display synchronized with the
scanning of the beam, so that an image of the specimen surface is
formed.
Voltage contrast in the image may itself be of significance, for
example where the specimen is a solid-state electronic component or
a portion of an integrated circuit, or the voltage contrast may be
indicative of some other physical phenomenon which it is desired to
examine.
The detector is normally placed on the same side of the specimen as
the beam but to one side of the beam and is intended to pick up the
secondary electrons emerging from the specimen. The detector
normally subtends a relatively small solid angle at the specimen
surface. As the beam scans over an area of uniform potential the
mean number of secondary electrons picked up by the detector
remains the same, but when a region of different potential is
reached the absolute energy distribution of secondaries is upset
and there is a change in the number of secondaries picked up,
resulting in image contrast.
However, such an arrangement is not predictable. For a given
voltage, difference dark may correspond to positive and light to
negative or vice versa.
Further, it is not normally possible to arrange that, for a given
electron probe current, the collected current changes by a certain
amount for a given change in specimen surface voltage. Hence, the
magnitude and sign of the specimen surface voltage difference
cannot be deduced from the change in collected current.
A further difficulty is that the secondary electrons, as they move
away from the point of impact and are attracted in curved paths and
at relatively low velocities towards the detector may also be
influenced by fields produced by potential gradients or potential
differences in other parts of the specimen surface. Moreover the
detector picks up not only true secondary electrons of low energy
but also high-energy reflected primaries which are substantially
unaffected by potential contrast. Finally it also picks up other
low-energy secondary electrons generated in the surrounding casing
and specimen mount by impingement of the reflected primaries, and
these likewise are not affected by the potential contrast in the
specimen, and so swamp still further the wanted signal. They may
form as much as 30 percent of the low-energy electrons reaching the
detector.
The main aim of the present invention is to provide a configuration
of electrodes, including an electron detector, such that the sense
of the change in collected current for a given change in specimen
surface voltage is always the same, irrespective of the absolute
value of either voltage, that is to say the collected current is a
monotonic function of surface voltage. A further aim is to attempt
to ensure that this configuration of electrodes can be made to give
the same change in collected current for the same change in
specimen surface voltage, whether the specimen surface voltage
change be in space or in time, and wherever the electron probe
might be situated or might move upon the specimen surface. A
further aim is to provide that the collected electron current has
as far as possible a linear relationship with the potential at the
specimen surface. A further aim is to provide that the collected
electron current is so far as possible independent of all influence
other than the number of electrons emitted from the small area of
specimen surface where the electron probe is instantaneously
incident, and the voltage of that same point.
According to the invention we propose to provide a detector
subtending a substantial solid angle at the point of impact of the
probe and to provide grids between the detector and the point of
impact, the potential on at least one of the grids being such as to
allow the passage only of the electrons of above a certain level of
energy, the "threshold energy," which may be set to any chosen
value, and the potential on at least one other grid being at a
substantial positive value to attract a high proportion of the
available secondary electrons.
Preferably it is the first grid encountered by the electrons that
is at a substantial positive potential. The presence of this grid
also produces a field that masks any disturbances of the field
above the specimen induced by potential differences on the specimen
surface in the neighborhood of the point of impact. The grids and
the detector may be of part-spherical form, with their common
center of curvature at the point of impact, or they could be planar
(this is necessitated in many cases by the nature of the detector)
or cylindrical, frustoconical, or of any other convenient form.
The invention will now be further described by way of example with
reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of the apparatus according to
the invention;
FIG. 2 is a graph showing the energy distribution of secondary
electrons; and
FIG. 3 is a graph showing the relationship between collector
current and potential difference.
Referring first to FIG. 1, the probe-forming system is orthodox,
comprising an electron gun G, an anode A, a first electron-optical
lens L1 and a final electron lens L2. The beam or probe of
electrons formed by this system impinges on a specimen S and is
caused to scan a small area of the specimen, for example a square
measuring a fraction of a millimeter each way, by scanning coils C
fed from a time base T which also controls the deflection plates of
a cathode-ray tube CR. The brightness of the spot on the screen of
the tube is controlled by the signal from a detector, indicated
generally at D, so that there is reproduced on the screen an image
of the scanned area of the specimen, the contrast in the image
being determined by the change in signal at the detector D as the
probe moves over the surface of the specimen.
Where the detector is simply picking up the secondary electrons
from the specimen and the contrast to be displayed is potential
contrast at the specimen surface, the known detectors suffer from
the defects mentioned above. The changes that are to be detected
are masked by backscattered high-energy primary electrons, by
secondary electrons generated by impact of backscattered primary
electrons on parts of the instrument around the specimen, and the
by fact that the very potential contrast which one is seeking to
detect produce fields above the surface of the specimen which
themselves influence the paths of the low-energy secondary
electrons and which thus mask unpredictably the contrast one is
seeking to observe.
Normally the electron gun will be at a large negative potential,
(for example -20 kv.), and the specimen at earth potential, so that
the electrons of the probe will have an energy (in the example
quoted) of 20 kev. Any of these which are backscattered from the
specimen will have energies of a similar order, i.e., of various
values up to 20 kev. Consequently they will be fast and not easily
deflected by small electric or magnetic fields. The true secondary
electrons, on the other hand have low energies of varying values,
mostly under 5 electron volts, with a peak around about 21/2
electron volts. This is shown diagrammatically in FIG. 2.
Those secondary electrons leaving a point in the specimen surface
which is at earth potential have, on arriving at given grid or
other surface of a certain potential an amount of energy
represented by their intrinsic energy around 21/2 to 4 electron
volts, to which is added (or from which is subtracted) that energy
gained or lost as a result of the potential difference between the
earthed specimen and the grid or other surface in question. Where
the specimen surface is not at earth potential the energy gained or
lost will be different.
In the detector illustrated we provide a grid G1 at a relatively
high-positive potential, a control grid G2, of adjustable low
potential, and a third grid G3 for a purpose to be explained below,
between the specimen surface and an electron collector in the form
of a scintillator SC, the light from which is taken by a light pipe
to a photomultiplier P to produce the electrical signal that is
amplified and used to modulate the beam in the cathode-ray tube
CR.
The grid G1 serves simply to attract the secondary electrons from
the point of impact of the beam on the specimen surface towards the
detector and has the additional function of producing an electrical
field above the specimen surface that masks any field distortions
caused in this region by the presence of potential differences in
the specimen. In a typical example the grid G1 may be at a positive
potential of 500 volts but the actual voltage will depend on the
distance from the specimen; it is desirable that the potential
gradient should be not less than 5,000 volts per meter. This would
give a value of 25 volts where the grid is 5 millimeters from the
specimen surface. Preferably the gradient is at least 10.sup.5
volts per meter. However, regardless of distance the potential on
G1 should be at least 25 volts to have a worthwhile effect.
The heart of the invention lies in the control grid G2, which forms
a potential barrier of variable height to discriminate between
secondary electrons emanating from regions of the specimen surface
of different potential. The grid G2, like the grids G1 and G3 can
be in the form of a mesh of wires and its effective potential is
not that on the wires themselves but that of the field in the gaps
between the wires, which is largely determined by the potential on
the wires but is also influenced to some extent by potentials of
surrounding electrodes. If this effective potential is zero,
secondary electrons coming from a point on the specimen surface
which is at zero potential will get through the grid G2 as they
will have a potential energy of about 21/2 electron volts or more.
Electrons coming from a region of the specimen surface which is at,
say, -5 volts will get through it easily as they will have the
added energy of 5 electron volts derived from their acceleration in
the field from -5 volts up to zero. Electrons coming from a region
at +5 volts, on the other hand, are retarded and only those which
started with an intrinsic energy of greater then 5 electron volts
will get through the grid. As will be seen from FIG. 2, this will
only be a small proportion. Thus the signal picked up by the
detector will vary as the probe scans over a region of the specimen
surface that varies in potential, those portions which are more
negative producing a larger signal in the detector than those which
are more positive.
In practice the absolute potentials are influenced by the work
function potential of the elements of which the specimen is made
but as this is the same all over the specimen it will not affect
the reasoning above.
The detector is thus able to discriminate between regions of the
specimen surface of different potential and a potential contrast
image is formed on the screen of the cathode-ray tube CR. By
adjusting the potential of the grid G2 it is possible to vary the
mean potential level at which the scanned region just produces no
signal in the detector, and so one can discriminate at will between
regions of positive and negative potential or between regions
having different levels of potential of the same sign. Effectively
the grid G2 forms a barrier B as indicated in FIG. 2, all those
electrons which lie to the right of the barrier being able to pass
through it. For maximum discrimination the barrier will normally be
set approximately to the peak of the curve, or to the point of the
curve of steepest slope, or the point of inflexion, to the right of
the peak, or somewhere between these two points. Linearity of
response is also desirable, i.e., a uniform change in signal at the
detector for a uniform change in potential at the specimen surface,
and this is best achieved by working at the point of inflexion.
FIG. 3 shows approximately how the collector current Ic varies with
the value of the difference between the effective potential Vg on
the grid G2 and the potential Vs on the specimen surface.
Effectively this represents the integral of the curve of FIG. 2,
but with the zero of the horizontal axis shifted by approximately 5
volts to allow for the fact that the potential difference in
question has to be up to 5 volts negative to overcome the intrinsic
energy of the secondary electrons. It is desirable to work over the
nearly linear part of the curve around -21/2 volts.
A scintillator is preferably used to pick up the electrons, as it
has a very low-noise factor. Its potential is unimportant to the
above reasoning and is of any typical value used for scintillators
for example 10 kv. The grid G3 has the function of a suppressor
grid to avoid the signal being upset by secondary emission from the
collector itself, resulting from impact of high-energy
backscattered primary electrons. For convenience we connect this
third grid to the first grid G1, so that it has the same potential
of 500 volts in the present example.
In order to improve linearity of response we may vary the potential
on the control grid G2 cyclically in a sinusoidal or other manner
about its mean value. As the absolute value of sensitivity varies
to some extent with the potential on the control grid, i.e., away
the straight part of the curve of FIG. 3, the cyclic variation does
itself introduce some distortion and so its amplitude should be not
more than necessary.
In the example illustrated the grids and the scintillator are
arranged in line on an axis which is normal to the surface of the
specimen, and the electron probe is brought in at an angle to clear
them but it would be possible for the probe to be normal to the
surface, the detector then being to one side. The grids need not be
flat but could, for example be part-spherical or even cylindrical,
surrounding the normal to the specimen, and in that case the
collector may be other than a scintillator, since a scintillator is
generally necessarily flat.
In the example illustrated we provide a window W defining the area
of scintillator exposed. The purpose of this window is to increase
the proportion, in the collected electron current, of electrons
emitted with small energies (and thus contributing to the voltage
contrast) to the proportion of electrons emitted with high
energies. The positive first grid G1 will tend to pull in towards
the normal to the specimen the preferred low-energy secondaries,
more than it does high-energy electrons. Again, the high-kinetic
energy of these preferred electrons at the first grid G1 means they
are less bent by the retarding field between the grids G1 and G2
than they would be without the additional kinetic energy given by
the first grid. Finally, the positive third grid G3 will again pull
in towards the specimen normal those electrons penetrating the grid
G2 with a low energy. At the other extreme, an electron emitted
with high energy will be much less deflected from its course by the
various grid potentials. The window W allows through to the
collector only those electrons which are close to the normal to the
specimen. Hence the net effect is that electrons from a larger
solid angle of emission are collected for low-emission energies
than for high-emission energies.
The detector arrangement in accordance with the invention may be
used in conjunction with AC sorting methods, or in conjunction with
the stroboscopic scanning methods disclosed in our copending U.S.
Pat. application Ser. No. 813,176 filed on Apr. 3, 1969.
* * * * *