Method of displaying an image of phase contrast in a scanning transmission electron microscope

A method of and apparatus for displaying an image of phase contrast in a scanning transmission electron microscope, in which an electron beam flux transmitted through a specimen is deflected with a high frequency and repeatedly moved on an aperture provided in a region where a transmitted electron beam and a scattered electron beam interfere, and among signal components detected by a detector through the aperture, only a signal synchronous with the high frequency is sampled and detection-rectified, whereby the difference of the intensities of the region where both the electron beams interfere is detected so as to display the image of phase contrast of the specimen.

A method of and apparatus for displaying an image of phase contrast in a 
scanning transmission electron microscope. 
BACKGROUND OF THE INVENTION 
This invention relates to improvements in a method of and apparatus for 
displaying an image of phase contrast in a scanning transmission electron 
microscope (STEM). 
In general, an image of phase contrast in a scanning transmission electron 
microscope is obtained with a method in which a small aperture is provided 
between a specimen and a detector so as to narrow a detecting angle. 
FIG. 1 is a diagram showing the outline of the method. In case of 
illuminating a specimen 2 (hereinafter, a phase object having a 
grating-like structure shall be considered as an ideal specimen) with a 
primary electron beam 1, electron beams transmitted through the specimen 2 
can be divided into scattered electron beams 3 generated by scattering 
within the specimen 2 and a transmitted electron beam 4 transmitted 
through the specimen 2 without interacting therewith. Each scattered 
electron beam 3 involves a phase shift relative to the transmitted 
electron beam 4 on account of the scattering within the specimen. The 
phase contrast causes the scattered electron beam 3 and the transmitted 
electron beam 4 to interfere when they overlap each other. It is a part 5 
that is subjected to the interference. Since information on the phase 
contrast is obtained through the interference phenomenon, the variation of 
the intensity of the electron beam in the interfering region 5 needs to be 
detected in order to form an image of phase contrast. To this end, there 
has been adopted an expedient in which an aperture plate 6 is inserted 
into the interfering region 5 to confine a narrow detecting angle and to 
enhance the coherence, whereupon a signal is obtained. This expedient has 
realized optically the same condition as that in the case of forming an 
image of phase contrast in a transmission electron microscope (TEM). The 
intensity of the electron beam having passed through an aperture 6' of the 
aperture plate 6 is detected by a detector 7. The detected signal is 
amplified by an amplifier 8, and is supplied for the intensity modulation 
of a CRT (cathode-ray tube) 9. On the other hand, the scanning on the 
screen of the CRT 9 is synchronous with the scanning of the primary 
electron beam which is effected by a power supply for scanning 10 and 
deflection coils 11 (in the figure, the arrow 12 indicates the direction 
of the scanning on the specimen 2). Therefore, an image including the 
information of the phase contrast is formed on the screen of the CRT 9. 
This is the so-called image of phase contrast. 
However, the image of the phase contrast acquired with this method is of 
the superposition between the contrast based on the phase shift (phase 
contrast) and the contrast (bright field image) ascribable to the decay of 
the electron-beam intensity occurring when the primary electron beam is 
transmitted through the specimen. It is not an image which consists only 
of the information of the phase shift. 
As an expedient for improving the drawback, there has been proposed a 
method in which only the phase contrast is emphasized to display an image 
(refer to Optik, vol. 41, p. 452-456, 1974). This method is characterized 
in that two detectors are used to construct a new detector system without 
disposing the aperture plate 6. 
FIG. 2 is a top plan view of the detector system in this method. Detectors 
(I) and (II) have their detecting faces arranged so as to lie in contact 
with each other through a straight line l within an identical plane. When 
they are installed at the position of the detector 7 in FIG. 1, the spots 
of the electron beams transmitted through the specimen become as shown by 
three circles in FIG. 2. In the figure, numeral 14 designates the spot 
formed by the transmitted electron beam, numerals 13 and 13' indicate the 
spots formed by the scattered electron beams, and hatched parts A and B 
are the interfering regions. With the detector system thus constructed, 
the difference between the electron-beam intensities detected by the 
respective detectors (I) and (II) is taken. Then, since the bright field 
images are isotropic, they are canceled, and only the difference (A-B) of 
the electron-beam intensities in the interfering regions A and B can be 
derived as a signal. Accordingly, an image of phase contrast formed by the 
use of this signal consists only of the information of the phase shift. 
With such detector system, however, the two detectors are required, which 
gives rise to such disadvantages (1) that the coadjustments between the 
detectors are necessary, (2) that the mechanical butt of the detecting 
faces of the two detectors needs to be precise, and (3) that since the 
symmetry with respect to an optical axis is necessary, the axial alignment 
is necessitated, resulting in complicated operations. 
SUMMARY OF THE INVENTION 
This invention has been made with note taken of the above-mentioned 
drawbacks, and has for its object to provide a novel method of displaying 
an image of phase contrast in a scanning transmission electron microscope, 
the method making it possible to readily observe the image of phase 
contrast. 
In order to accomplish the object, according to this invention, a method of 
and apparatus for displaying an image of phase contrast in a scanning 
transmission electron microscope wherein the difference between 
electron-beam intensities in regions in which an electron beam resulting 
from the transmission-through-a-specimen of a primary electron beam 
illuminating the specimen and electron beams caused by scatterings within 
the specimen interfere is detected, thereby to display the image of phase 
contrast of the specimen, is so constructed that an electron beam flux 
transmitted through the specimen is deflected with a high frequency and 
repeatedly moved on an aperture plate disposed in the interfering region, 
and that among signal components detected by a detector through the 
aperture plate, only a signal synchronous with the high frequency is 
sampled and detection-rectified, thereby to detect the difference of the 
intensities of both the electron beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereunder, this invention will be described in detail with reference to 
preferred embodiment. 
FIG. 3 is a view of an exemplary embodiment of this invention, while FIGS. 
4(a)-4(f) are waveform diagrams showing signals in various parts of the 
embodiment. In FIG. 3, a primary electron beam 21, which is focused to a 
beam, fine is scanned in two dimensions on a specimen 22 by deflection 
coils 31. FIG. 4(a) shows an example (saw-tooth wave) of a signal which is 
supplied from a scanning power supply 30 to the deflection coil 31 at that 
time. FIG. 4(b) shows that part of the specimen which is scanned by the 
primary electron beam according to FIG. 4(a) within the corresponding 
period of time; for example, at a time T in FIG. 4(a), the primary 
electron beam lies at a position X on the specimen, as seen in FIG. 4(b). 
A deflector 33 for deflecting the electron beam flux 23 transmitted 
through the specimen is disposed between the specimen 22 and an aperture 
plate 26. The deflector is supplied with a radio frequency of, for 
example, approximately 10 KHz, as shown in FIG. 4(c), to deflect the 
electron beam flux 23 after the transmission through the specimen in one 
direction or in two dimensions. 
Here, the distance which the primary electron beam is scanned (moved) on 
the specimen within at least one cycle of the radio frequency needs to be 
less than the resolution of the apparatus (STEM). That is, letting d 
denote the resolution of the apparatus, v denote the scanning velocity of 
the primary electron beam on the specimen, and f denote the frequency of 
the radio-frequency signal applied to the deflector 33, the inequality of 
v/f&lt;d needs to be met. Accordingly, by way of example, in the case where 
the resolution d of the apparatus is 3 angstroms (A), where the cycle of 
the saw-tooth wave (FIG. 4(a)) to scan the primary electron beam is 
approximately 100 msec. for the display of an image of phase contrast and 
where the scanning velocity v of the primary electron beam on the specimen 
at this time is approximately 10.sup.-4 cm/sec., the frequency f may be 
greater than 10/3 kHz. Therefore, supposing by way of example that the 
deflector 33 is supplied with 10 kHz as stated above, this is effective to 
deflect the electron beam flux 23 after the transmission through the 
specimen, for three cycles at every point of the specimen 22. 
The electron beam flux 23, which is deflected by the radio frequency in a 
manner to be repeated, unidirectionally crosses an aperture 26' of an 
aperture plate 26 continuously and iteratively, and it executes one 
reciprocation on the surface of the aperture plate 26 with respect to one 
cycle of the radio frequency supplied to the deflector 33. Only electrons 
which have passed through the aperture plate 26 are detected by a detector 
27 (for example, a photomultiplier tube). Accordingly, the signal which is 
detected by the detector 27 through the aperture plate 26 is attended with 
time fluctuations in correspondence with the intensity distribution of the 
electron beam flux 23 as illustrated in FIG. 4(d). In this regard, when 
the beam scans a portion of the specimen which changes in thickness, the 
detected signals produce a phase shift. Thus, in the sample as shown in 
FIG. 4b, the signals detected when the beam scans one portion of the 
specimen in which the gradient of thickness changes from positive to zero 
will be opposite in phase from signals detected when the beam scans the 
other portion of the specimen in which the gradient of thickness changes 
from zero to negative. Also, when the beam scans a portion of the specimen 
having a constant thickness, as shown in the center of FIG. 4b, the 
detected signals do not produce a phase shift, but become constant, as 
shown by the flat portion of the signals in FIGS. 4d and 4e. 
When, among the components of the signal, the component (FIG. 4(e)) 
synchronized with the radio frequency supplied to the deflector 33 is 
noticed, it is none other than a result obtained in such a way that, in 
the foregoing method employing the two detectors, the electron beam flux 
23 is divided into two symmetrical parts, whereupon the electron-beam 
intensities of the respective parts simultaneously detected are derived 
alternately in time (in time division). 
Accordingly, the magnitude from the top to the bottom of the signal 
waveform shown in FIG. 4(e) and synchronized with the radio frequency 
supplied to the deflector 33 corresponds to the difference between the 
electron-beam intensities of the two symmetrical parts. Therefore, a 
square wave signal (FIG. 4(f)) obtained by detection-rectifying the 
waveform of FIG. 4(e) has the same nature as that of the difference signal 
detected by the use of the two detectors as stated before. 
The signal thus obtained is supplied for the intensity modulation of a CRT 
29. On the other hand, the scanning on the screen of the CRT 29 is 
synchronous with the scanning of the primary electron beam executed by the 
scanning power supply 30 and the deflection coils 31. Therefore, an image 
of phase contrast identical in nature to that obtained by the foregoing 
system of dividing the detecting surface into the two parts is formed on 
the screen of the CRT 29. 
The above-stated functions of supplying the radio frequency to the 
deflector 33, sampling the component synchronous with the radio frequency 
and carrying out the detection-rectification may well be realized with 
means separate from one another, but they can be provided as well by a 
lock-in amplifier 32, which is a well-known form of synchronous detector 
using a balanced amplifier. Thus, when an output signal from the lock-in 
amplifier 32 is used for the intensity modulation of the CRT 29, the image 
of phase contrast identical in nature to that obtained by the foregoing 
method employing the two detectors is formed on the screen of the CRT 29. 
In FIG. 3, numeral 28 indicates an amplifier for amplifying the signal 
detected by the detector 27. 
In the above embodiment, the case of employing the ideal specimen has been 
referred to in order to facilitate understanding of this invention. 
Needless to say, however, the basic principle holds as to general 
specimens. The aperture of the aperture plate 26 in the embodiment can be 
optionally selected as long as it has a geometry which can be received 
within the region A or B illustrated in FIG. 2. In addition, when the 
aperture plate 26 is provided as a movable aperture plate having a 
plurality of apertures and the optimum aperture is selected depending upon 
the condition of the specimen, etc., an appropriate condition for 
detecting the signal is established. Further, although the embodiment has 
referred to the case where the electron beam flux after the transmission 
through the specimen is deflected at the radio frequency unidirectionally, 
the invention is also applicable, in principle, to the case where it is 
deflected at the radio frequency in two dimensions. 
As set forth above, in comparison with the foregoing prior-art system of 
dividing the detecting surface into two parts, this invention has various 
advantages in that (1) the single detector suffices, so the structure is 
simplified, (2) any strict axial alignment with the optical axis of the 
detector system is not required, so that the operability of the system is 
enhanced, and (3) the handling of the signals can be fully effected by the 
lock-in amplifier, so surplus adjustments are dispensed with. The 
invention is greatly effective when put into practical use.