US3714425A

US3714425A - Reflecting mirror type electron microscope

Info

Publication number: US3714425A
Application number: US00112878A
Authority: US
Inventors: T Someya; H Sakural
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-02-07
Filing date: 1971-02-05
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: NL7100609A; GB1351513A; DE2105455A1

Abstract

A reflecting mirror type electron microscope in which a specimen maintained at a high negative potential is scanned by an electron beam focused in the vicinity of the specimen surface. A bright specimen surface image is produced by the electrons reflected by the electric field formed in front of the specimen without reducing the field of view.

Description

United States Patent n 1 Someya et al.

[ 51 Jan. 30, 1973 [541 REFLECTING MIRROR TYPE ELECTRON MICROSCOPE [76] Inventors: Teruo Someya, 3414 Haijima-machi, Akishima-shi, Tokyo; Higekata Sakural, No. 31 2-chome-17 Kichijoji, Higashi-machi, Musashino-shi, Tokyo, both of Japan [22] Filed: Feb. 5, 1971 [21] Appl. No.: 112,878

[30] Foreign Application Priority Data Feb. 7, 1970 Japan ..45/l0813 March 30, 1970 Japan ..45/26777 [52] U.S. Cl. ..250/49.5 A, 250/495 PE [51] Int. Cl .1101] 37/26, G0!n 23/22 [58] Field of Search .....2S()/49.5 R, 49.5 A, 49.5 TE, 250/495 PE [56] 9 References Cited UNlTED STATES PATENTS 3,614,311 10/1971 Fujiyasu et a1 ..250/49.5 X

1 6 1 1w 4 :fi

5 y ,JMPur/fas l a, 3

Fe [3 l a s Primary Examiner-William F. Lindquist AtIomeyWebb, Burden, Robinson & Webb [57] ABSTRACT A reflecting mirror type electron microscope in which a specimen maintained at a high negative potential is scanned by an electron beam focused in the vicinity of the specimen surface. A bright specimen surface image is produced by the electrons reflected by the electric field formed in front of the specimen without reducing the field of view.

9 Claims, 14 Drawing Figures PATENIEDJAN30 I975 3.714.425

SHEET 5 [IF 7 REFLEC'IING MIRROR TYPE ELECTRON MICROSCOPE The invention relates to a reflecting mirror type electron microscope and more particularly to a scanning mirror type electron microscope.

A mirror type electron microscope is disclosed in U.S. Pat. No. 2,901,627, entitled, Method of and Apparatus for the Electronic Magnification of Objects in which a parallel electron beam is directed at a specimen maintained at a negative potential. The electrons are reflected and then projected onto a fluorescent screen. In this arrangement, a dispersed electron beam illuminates the sample. Hence, the intensity of the electron image projected onto the screen is very poor. Moreover, in the existing type of mirror electron microscope, the screen image is a shadow image, making it difficult to obtain a simple explanation for the image.

It is an advantage according to this invention that a bright focused image is insured without reducing the field of view. Another advantage of this technique is the capability of observing a dark field image by means of the reflected electrons. A still further advantage of the present invention is to provide a reflecting mirror type electron microscope by which the potential distribution on the specimen surface can be observed quantitatively.

Briefly, according to this invention, there is provided a reflecting mirror type electron microscope in which the specimen is maintained at a high negative potential. An electron gun integral with the microscope directs an electron beam at the sample. The beam is focused by one or more electron lenses at a point in the vicinity of the specimen. Deflecting means are provided which cause the electron beam to scan across the specimen surface. The beam is reflected by the decelerating electric field in front of the specimen and accelerated generally backwardly away from the specimen according to the nature of the specimen surface. Means are provided for detecting the reflected electrons and thereby providing a magnified image of the specimen surface.

Other objects and advantages pertaining to this invention will become apparent from the following detailed description read in conjunction with the appended drawings in which:

FIG. 1 shows an optical view of a reflecting mirror type electron microscope in which the specimen under study is scanned by an incident electron beam.

FIGS. 2, 3, and 4 show modified embodiments of the reflecting mirror type electron microscope shown in FIG. 1.

FIG. 5 shows another embodiment of the present invention.

FIGS. 6A and 6B show a cross sectional and end view of a semiconductor type aperture used in the embodiment shown in FIG.5.

FIG. 7 shows the output voltage distribution curve of the output voltages detected by the semiconductor type aperture referred to in FIG. 6.

FIG. 8 shows a block circuit diagram for analyzing the specimen surface image.

FIG. 9 shows a modified embodiment of FIG. 5 capable of producing an obliquely illuminated shadow image of the specimen on the display means.

FIGS. 10A and 108 show a cross sectional and end view of the specimen.

FIGS. II and 12 are illustrative drawings showing the images obtained in the embodiment according to FIG. 9.

Referring to FIG. 1, an incident or illuminating electron beam generated by an electron gun 1 passes through an opening 20 provided in a fluorescent screen 2 and is focused by a projector lens 3 (shown as the analogous optical lens) so as to form a reduction image S of the crossover. The beam then passes through the. center of an aperture 5 located at the back focal plane of an objective lens 4 and is then focused by the objective lens 4 so as to form a reduction image S of S.

Two pairs of deflecting coils 7a and 7b, to which vertical and horizontal scanning signals are applied by a scanning signal generating circuit 8 by an amplifier 9, are arranged between the projector lens 3 and the aperture 5. Thus, the beam can be deflected by controlling the intensity of the scanning signals and can thereby be made to pass through the aperture 5 in any desired direction and deflecting angle. The electrons constituting the said beam, which run more or less in parallel with respect to each other are focused at a point r0 in the vicinity of the specimen 6. By varying the intensity of the scanning signals, the focus point m can be made to shift across or scan the specimen.

Further, a high negative voltage is applied to the specimen by a power source 20 to produce a beam decelerating electric field in the vicinity of said specimen, said field serving to reflect before the illuminating electrons constituting the beam reach the specimen surface and to accelerate the electrons in the opposite direction. The electrons passed through the aperture 5 are removed from the beam path by the deflecting coils 7a and 7b and focused on the same plane as S to form an electron spot S". The spot S" is then magnified and projected onto the fluorescent screen 2 by the projector lens 3.

The configuration of the electric field produced in front of the specimen is determined by the potential distribution and the irregularity of the specimen surface. As a result, the reflected electrons are deviated from the normal trajectory and accelerated by the said field and intercepted by the aperture 5, thereby reducing the number of reflected electrons passing through the said aperture. Consequently, a scanning reflected electron image corresponding to the configuration of the electric field in front of the specimen is observed on the fluorescent screen 2. The magnification M of the said image is expressed as follows:

where f is the focal length of the objective lens 4, l, is the distance between the aperture 5 and the second deflecting coils 7b, 1 is the distance between the first and second deflecting coils 7a and 7b, 1 is the distance between the first deflecting coils 7a and the projector lens 3, Mp is the magnification of the projector lens 3.

At the same time, secondary electrons produced by the reflected electrons striking the aperture 5 are multiplied and detected by a secondary electron multiplier 10. The detected signal isthen amplified by an amplifier 11 and fed into the grid of a display means 12. Furthermore, the scanning signal generated by the scanning signal generating circuit 8 is fed into a pair of deflecting coils 13 forming part of the display means 12, via an amplifier 14. As a result, a dark field image,

whose brightness portion is opposite to that of the image appearing on the fluorescent screen 2, is observed on the display means 12.

In the aforedescribed embodiment, since the electron image projected onto the fluorescent screen 2 is focused, the observed image is sufficiently bright to make analysis easy. Furthermore, because the beam is scanned, the specimen can be observed over .a wide field of view. Again, the dark field image, formed in accordance with the variation in the number of electrons impinging on the aperture 5, is observed on the display means at the same time as the light field image is observed on the fluorescent screen.

Referring to FIG. 2, an embodiment of this invention is shown wherein scanning is effected by only one pair of deflecting coils, an intermediate lens being provided between the deflecting coils 7 and the aperture 5 in order to deflect the beam. In this arrangement, the magnification coefficient M of the electron image appearing on the fluorescent screen 2 is expressed as follows:

Referring to FIG. 3, there is shown an embodiment wherein the reflected electrons are deflected and removed from the incident electron beam path by a magnetic field 16 (shown as a prism, the analogous optical device). Furthermore, the electrons dispersed by the electric field in front of the specimen impinge'on the aperture 5 and are directly fed into the amplifier l 1.

Referring to FIG. 4, yet another embodiment is shown wherein a detecting aperture 21, similar to aperture 5, replaces the fluorescent screen and is positioned anywhere between the deflecting coils and the electron gun. In addition, a switch 22 has been provided so that the bright field image resulting from the electrons detected by the aperture 21 via the amplifier 23 and the dark field image resulting from the electrons detected by the aperture 5 via the amplifier 1 1 can alternately be observed on the display means 12 as required.

Referring to FIG. 5, still another embodiment of this invention is shown wherein an aperture 25 is provided with a number of detecting elements for supplying quantitative information on the potential distribution and/or topographical irregularity of the specimen surface. The illuminating electron beam EB focused by the objective lens 4 is reflected at an angle y, as shown in the figure, according to the configuration of the electric field formed in front of the specimen which is determined by the potential distribution and/or topographical irregularity of the specimen surface. The reflected electrons then pass through the objective lens 4 and impinge on the aperture 25 which is made up of an N-type semiconductor plate and a diffused layer of P-type semiconductors, 27a, 27b, 27c. 2711, as shown in FIG. 6 (A and B), in order to provide several P-N junction elements.

The thickness of the semiconductor layer is such that the impinging electrons reach the base of the said layer. Moreover, since the P-type semiconductors forming the semiconductor layer are separated from each other, signals corresponding to the quantity of electrons impinging on each respective P-N junction detecting element can be fed out from terminals 28a, 28b, 28c

28:1 connected to each element. For example, if the reflected electrons EB impinge on the aperture 25, output voltages V V V Vn (see FIG. 7) are detected by the P-N junction detecting elements. And since the sum total voltage V of the output voltages detected by the detecting elements corresponds to the reflecting angle 7 V 7 x 'y y of the incident electron beam, the magnitude of the reflecting angle -y can readily be ascertained by measuring the total detected voltage.

The azimuth 0 of the maximum value V, of the output voltages detected by the detecting elements is known by the curve approximation of the output voltage distribution as shown in FIG. 7 and is expressed as follows:

Further, since 'ycorresponds to V. ya: and yy can be respectively expressed as follows:

yxaVcos 0 y y aVsin6 The output signals from the detecting elements 27a, 27b 27n are fed into the circuit shown in FIG. 8 comprising a comparator 29 and an integrator 30. The comparator 29 compares the intensities of the said output signals and selects the output signal having the maximum intensity in order to determine the azimuth 6. The integrator 30, on the other hand, integrates the output voltages detected by the detecting element, the integrated single corresponding to the sum total V. The output signals of the comparator 29 and the integrator 30 are fed into an operating circuit 31 in which Vcos6, Vsin6 viz., y): and 7 are calculated. Thus calculated, 7:: and 7y are then memorized by a memory device 32 and the information signals (ya: and 'yy) with respect to each point on the specimen are fed into a computer 33 where the specimen surface image is quantitatively analyzed in accordance with the specimen scanning.

In the embodiment shown in FIG. 9, the electric signals detected by the detecting elements 27a to h are converted into two sets of signals by means of a switch 40 comprising terminals 41a to h in circuit with the element respectively, two

electroconductive plates

42 and 43 and a rotatable insulation plate 44 the end of which are fixed to the said electroconductive plates respectively. When the illuminating beam is directed to a point A on a specimen,.for example, as shown in FIGS. 10 (A) and (B), signals la and lb are produced by

plates

42 and 43 respectively. la which corresponds to the quantity of reflected electrons striking the elements 27b, c, d and e is larger than Ib which corresponds to the quantity of reflected electrons striking the elements 27a, h, g andf. The said signals are then fed into a differential circuit 45 which subtracts lb from la, the difference being fed into the display means 12 as a plus signal.

Again, when the incident beam is directed to a point B on the specimen, the intensity of the signal la is equal to the intensity of the signal Ib, with the result that the output signal of the difierential circuit 45 is zero.

Moreover, when the incident beam is directed to a point C on the specimen, the intensity of la is smaller than the intensity of Ib, the difference, in this case, being fed into the display means 12 as a minus signal.

As a consequence of the aforegoing, a shadow image corresponding to a certain illumination condition, as shown in FIG. 11, depending on the irregularity of the specimen surface, is observed on the display means when the specimen is scanned.

If the insulation plate 44 is rotated by 90, the signals detected by the detecting elements 27d, e,fand g and 27h, a, b and c are grouped, differentiated and displayed, resulting in a shadow image as shown in FIG. 12.

In the abovedescribed embodiments, a P-N junction semiconductor is utilized for detecting the reflected electrons. This means, however, may be replaced by a scintillation counter or any other detecting means.

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

We claim 1. A reflecting mirror type scanning electron microscope comprising:

A. an electron gun for creating an electron beam;

B. means for maintaining a specimen at a high negative potential so that a decelerating electric field is produced in front of the specimen;

C. a lens for focusing the said beam in the vicinity of the specimen;

D. an apertured means located at the back focal plane ofthe said lens;

E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means such that the relative portions of the reflected beam passed and intercepted by the apertured means are indicative of the degree of dispersion of the electron beam; and

F. display means for displaying the specimen surface image according to the manner in which the electrons constituting said beam are reflected by said electric field.

2. A reflecting mirror type electron microscope according to claim 1 wherein the said display means is a fluorescent screen arranged in the path of the electron beam passed back through said apertured means.

3. A reflecting mirror type electron microscope according to claim 1 wherein the said display means comprises a means for detecting the reflected electrons and a display unit connected to the said detecting unit.

4. A reflecting mirror type electron microscope according to claim 1 wherein a magnetic field is provided to remove the reflected electrons from the beam path.

5. A reflecting mirror type scanning electron microscope comprising:

A. an electron gun for creating an electron beam;

C. a lens for focusing the said beam in the vicinity of the specimen;

D. an apertured means located at the back focal plane of the said lens;

E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means such that the relative portions of the reflected beam passed and intercepted by the apertured means are indicative of the degree of dispersion of the electron beam;

F. means for detecting the electrons intercepted by the apertured means;

G. means for displaying the specimen surface image according to the intercepted electrons reflected by said electric field;

H. means for detecting the electrons passed through said aperture; and i 1. means for displaying the specimen surface image according to said passed electrons reflected by said field.

6. A reflecting mirror type electron microscope according to claim 5 wherein the said display means includes a means for detecting the reflected electrons intercepted by the apertured means, a display unit connected to the said detecting means, and a fluorescent screen located between the electron gun and the apertured means in the path of the incident electron beam for displaying the specimen surface image according to the reflected electrons passed through the apertured means.

7. A reflecting mirror type electron microscope according to claim 5 wherein the said display means includes two detecting units for detecting the reflected electrons passed through and intercepted by the said apertured means respectively, a switch connected to the said detecting units and a display unit connected to the switch.

8. A reflecting mirror type scanning electron microscope comprising:

A. an electron gun for creating an electron beam;

C. a lens for focusing the said beam in the vicinity of the specimen;

D. an apertured means located in the back focal plane of the said lens, said means having a plurality of means for detecting incident electrons radially spaced about the aperture in said apertured means, said detecting means providing an output signal indicative of the intensity of incident electrons;

E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means causing the detecting means to produce output signals;

F. means for integrating the output signals of said detecting means in order to determine the total intensity of the reflected electrons collected by said detecting means said total intensity being indicative of the reflecting angle (y); and

G. means for comparing the output signal intensities of the detecting means in order to determine the azimuth (8) of the reflected electrons.

9. A reflecting mirror type scanning electron microscope comprising:

A. an electron gun for creating an electron beam;

C. a lens for focusing the said beam in the vicinity of the specimen;

D. an apertured means located at the back focal plane of the said lens, said means having a plurality of means for detecting incident electrons radially spaced about the aperture in said apertured means, said detecting means providing an output signal indicative of the intensity of the incident electrons;

E. means for deflecting said beam to cause the beam F. a switch means in circuit with said detecting means for connecting detecting means on opposite semi-circles of said apertured means, the two output signals of said connected detecting means being converted into two signals indicative of the difference in output of the detecting means in different selected semi-circles on the apertured means by said switch means;

G. means for obtaining the differential signal of said two signals; and

H. means for displaying the specimen surface image according to said differential signal.

Claims

1. A reflecting mirror type scanning electron microscope comprising: A. an electron gun for creating an electron beam; B. means for maintaining a specimen at a high Negative potential so that a decelerating electric field is produced in front of the specimen; C. a lens for focusing the said beam in the vicinity of the specimen; D. an apertured means located at the back focal plane of the said lens; E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means such that the relative portions of the reflected beam passed and intercepted by the apertured means are indicative of the degree of dispersion of the electron beam; and F. display means for displaying the specimen surface image according to the manner in which the electrons constituting said beam are reflected by said electric field.

5. A reflecting mirror type scanning electron microscope comprising: A. an electron gun for creating an electron beam; B. means for maintaining a specimen at a high negative potential so that a decelerating electric field is produced in front of the specimen; C. a lens for focusing the said beam in the vicinity of the specimen; D. an apertured means located at the back focal plane of the said lens; E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means such that the relative portions of the reflected beam passed and intercepted by the apertured means are indicative of the degree of dispersion of the electron beam; F. means for detecting the electrons intercepted by the apertured means; G. means for displaying the specimen surface image according to the intercepted electrons reflected by said electric field; H. means for detecting the electrons passed through said aperture; and I. means for displaying the specimen surface image according to said passed electrons reflected by said field.

8. A reflecting mirror type scanning electron microscope comprising: A. an electron gun for creating an electron beam; B. means for maintaining a specimen at a high negative potential so that a decelerating electric field is produced in front of the specimen; C. a lens for focusing the said beam in the vicinity of the specimen; D. an apertured means located in the back focal plane of the said lens, said means having a plurality of means for detecting incident electrons radially spaced about the aperture in said apertured means, said detecting means providing an output signal indicative of the intensity of incident electrons; E. means for deflecting said beam to cause the beam to pass through the apertured means in any desired direction whereby the lens causes the beam to scan across the specimen and approach the specimen substantially perpendicular thereto whereby said beam is reflected and dispersed according to the topology of the sample surface and whereby any portion of the beam reflected back substantially perpendicular to the specimen surface is again passed through the apertured means and the remaining portion is intercepted by the apertured means causing the detecting means to produce output signals; F. means for integrating the output signals of said detecting means in order to determine the total intensity of the reflected electrons collected by said detecting means said total intensity being indicative of the reflecting angle ( gamma ); and G. means for comparing the output signal intensities of the detecting means in order to determine the azimuth ( theta ) of the reflected electrons.