US 3558880 A
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United: States-Patent [mentors App].
Filed Patented Assignce Richard N. Kniseley Ames. Iowa;
Dean Van Zuuk. Sunnyvale. Calif.
Aug. 8, I968 Jan. 26, 1971 The United States of America as represented by the United States Atomic Energy Commission Primary ExaminerWilliam F. Lindquist Attorney-Roland A. Anderson ABSTRACT: In a scanning electron microprobe whereinan electron beam is scanned over the surface of a specimen in a E B E T partial vacuum to generate an image electron signal 3 C rawmg Flgs' therefrom, the generated image electron signal is filtered to US. Cl 250/495 accept only high-frequency components thereof. A cathode- Int. Cl ..II01 j 37/00, ray tube whose X and Y axis sweeps are synchronized with the GOln 23/00 scan of the electron beam over' the specimen has its display in- Field of Search 250/495 tensity modulated by the high-frequency components of the 1 49.5( 8 image electron signal.
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BACKGROUND OF THE INVENTION qualitative and quantitative elemental distribution studies a 'over a specimen surface to highly localized analysis on a submicron scale. In the electron microprobe, a microvolume at a particular point on the surface 'of aspecimen in a partial vacuum is excited by a focused beam of electrons. The beam,
less than I micron in diameter at the surface of the specimen, is accelerated by a voltage of I to 50 kv. The electron beam incident on the surface of the specimen produces back-scattered electrons from the specimen and absorbed electrons within the specimen (specimen current). These back-scattered electrons or absorbed electrons may be used to provide a picture which represents both the topographic and atomic number differences in the specimen. To obtain this picture, the electron beam is scanned in a raster pattern over a selected area of the surface of the specimen. The back-scattered electron signal or absorbed electron signal is applied to a cathode-ray tube to modulate the intensity of thecathode-ray tube display. With the X and Y sweeps of the cathode-ray tubesynchronized with the scan of the electron beam over the surface of the specimen, a picture display results from the cathode-ray tube representing both the topographic and atomic number differences in the specimen.
Presently, two basic types of electron beam scanning are utilized in the electron microprobe. In one, the scanning rate is relatively slow (approximately l hertz per 5 seconds). In this case,'a cathode-ray tube having a long persistence phosphor is used to retain the picture during theslow sweep. Since a long persistence phosphor is used, the "picture is retained for several seconds after the sweep is terminated and, when the specimen is moved and a new area is covered by the sweep, a delay of several seconds is necessary for the old picture to clear and a new one to build up on the cathode-ray tube screen. This is often inconvenient when attempting to study a specimen surface where the specimen position must be i changed frequently. Further, the delay occasioned by the long persistence phosphor is inconvenient when attempting to focus the electron beam to provide the best resolution at the specimen surface. In the other typeof scanning, the electron beam is scanned at a much higher rate (approximately 45 hertz per second). This scanning rate is fast enough to provide a livepicture whereby a short persistence phosphor can be used on the cathode-ray tube. This live" picture changes immediately when the specimen is moved or the beam focus is changed. However, the high ratescan system presents a major diffic'ulty in that the pictures obtained on the cathoderay tube display are generally of poor quality unless relatively high electron beam currents (greater than 0.1 microampere) are used. High electron beam currents incident on the specimen are undesirable, since they can cause excessive specimen heating. Further, even at high electron beam currents, the eathode-ray tube display is often smeared and lacks detail. This difficulty is found whether the back-scattered electron signal is used to modulate the intensity of the cathode-ray tube display or whether the specimen current signal is used.
7 Accordingly, it is one object of the present invention to provide a method and means for improved display of image electron signals in a scanning electron microprobe.
It is another object of the present invention to provide a method and means for displaying the physical properties of a specimen in a scanning electron microprobe wherein electron beam currents having a value less than 0.()()l microampcrc are incident on the specimen.
It is another object of the present invention to provide a method and means for displaying the physical properties of a specimen responsive to high-frequency components contained in the image electron signal of a scanning electron microprobe.
It is yet another object of the present invention to provide a method and means for displaying with high resolution the image electron signal of a scanning electron microprobe.
BRIEF DESCRIPTION OF THE DRAWINGS SUMMARY OF THE INVENTION In a scanning electron microprobe wherein an electron beam is scanned over the surface. of a specimen in a partial vacuum to generate an image electron signal therefrom, the generated image electron signal is filtered to accept only highfrequency components thereof. A cathode-ray tube whose X and Y axis sweeps are synchronized with the scan of the electron beam over the specimen has its display intensity modulated by the high-frequency components of the image electron signal. j v
In FIG. 1 a conventional electron scanning microprobe-is schematieallyshown modified according to the present invention. The conventional electron scanning microprobe comprises an electron beam sourcelO, which generates a narrow beam of electrons at energies from'l to 50 kv. In the illustratcd embodiment, a field-emission type electron sourcc is shown. However, it is to be understood that a heated-cathode electron sourcemay be substituted therefor. A magnetic lens I2 focuses the electron beam from electron beam source I0 onto the surface of a specimen 14 held in a mount 15. The electron beam is focused'into a spot a few microns in-diametcr on the surface of the specimen 14. It is to be noted that electrostatic focusing may be employed rather than the elec tromagnetic lens illustrated. A pair of deflecting electrodes I6 mounted on opposing sides of the electron beam and a pair of deflecting electrodes 18 mounted normal to electrodes 16 deflect the electron beam in the X-Y plane over the surface of the specimen 14. A sweep generator 20 provides two sawtooth 'aforedescribed electron beam generation and scan over the specimen surface is effected in a partial vacuum.
The electron beam impinging upon the surface of the specimen 14 generates back-scattered electrons from the surface of the specimen l4 and absorbed electrons within the specimen 14. These electrons, either the back-scattered or the absorbed, provide an image electron signal which may be used to generate a pictorial display of the physical properties of the specimen 14 being scanned. To detect the back-scattered electrons, an electrode (not shown) is spatially mounted above the surface of the specimen 14 upon which the electron beam is incident so as 'to capture back-scattered. electrons radiating from the scanned surface of the specimen I4. To detcct absorbed electrons in the specimen I4, an electrical conncction is made to the specimen I4, as shown in FIG. I. and a resistor 22 is connected between electrical ground and this electrical connection with the absorbed electron signal output being taken across the resistor 22.
For the practice of the present invention, either the backscattcred electron signal or the absorbed electron signal may be used as the image electron signal. For purposes of illustration, the absorbed electron signal from the specimen I4 is described.
The absorbed electron signal from the specimen [4 is detccted across resistor 22. A cathode-ray tube 26 having its X and Y axis sweeps driven by sweep generator 20 so that they are synchronized with the scan of the electron beam across the surface of specimen 14 has its intensity modulated by the absorbed electron signal as detected across resistor 22 to provide a pictorial display of the physical properties of the specimen 14. For the present invention, it has been discovered that the signal information generated across resistor 22 from the ab sorbed electron signal comprises two frequency ranges. One range comprises low frequencies and contains mostly information on gross structural features of the specimen 14. The second range comprises high frequencies and contains most of the detailed information concerning the specimen surface. The low-frequency components of the absorbed electron signal are generally higher in amplitude than the higherfrequency components and thus mask the detailed informa? tion concerning the specimen surface. The value of these frequencies-depends upon the scanning rate of the electron beam over the surface of the specimen 14. For example, where the beam is scanned over a specimen surface area of from 250 to 1000 square nanometers in a vertical direction at a rate of 45 hertz per second and a horizontal direction at a rate of 4.5 kilohertz per second, micron-sized details of the specimen will be contained in frequencies between- 50 kilohertz and 2 megahertz. If the scan rate over the surface of the specimen 14 is increased, then the frequencies containing the information on micron-sized detail will go higher and, conversely, if the scan rate of the electron beam over the surface of the specimen 14 is lowered, then the frequencies containing the micron-sized specimen detail will go down.
Thus, for the practice of the present invention, the output signal across resistor 22 (absorbed electron signal) is filtered to pass only the high-frequency components thereof, since these frequencies contain the information describing micronsized detail variations in the specimen physical properties. These high-frequency components of the electron image signal are applied as described to modulate the intensity of the cathode-raytube 26, wherefrom a display superior in resolution to that heretofore possible is achieved.
To effect the aforedescribed filtering, the output from the resistor 22 is fed by an isolation amplifier 28 to a filter network 30. The output from the filter network 30 is fed via a conventional pulse amplifier 32 to the intensity modulation input of the cathode-ray tube 26. The isolation amplifier 28 and filter network 30 are illustrated in schematic detail in FIGS. 2 and 3.
The isolation amplifier 28, illustrated in FIG. 2, acts as an impedance match between the specimen resistor and the filter network 30. The field-effect transistor 34 in the amplifier input maintains a high impedance and the transformer 36 connected in the emitter-follower output of the amplifier 28 eliminates 60 hertz pickup within the electron microprobe and also provides a limit on the frequency response ofthe system.
The filter network 30, illustrated in FIG. 3. comprises two capacitors 38 and 40 and a resistor 42, connected as shown. Adjustment of the value of capacitor 38 affects the lowcr limiting frequency of the bandwidth of the filter30. Adjustment of the value of capacitor 44 affects the upper limiting frequency ofthe bandwidth ofthe filter 30.
With the apparatus as heretofore described. a bandwidth at the output of the pulsed amplifier 32 is obtained which is typically represented by the lot shown in- FIG. 4. 'l'h eillustratcd bandwi th is used for t e aforedescribed condition where scanning of a specimen surface area of 250 to 1000 square nanometers is effected at a vertical rate of 45 hertz per second and a horizontal rate of 4.5 kilohertz per second, wherefrom the information characterizing micron-sized specimen features is contained in the frequencies between 50 hertz and 2 megahertz. As shown, the bandwidth is 3 decibels downat 50 kilohertz and 2 megahertz about a central frequency of 300 kilohertz. Using this bandwidth. the resolution of the display of cathode-ray tube 26 improved so that extremely good pictures of the specimen 14 were obtained with electron beam current values less than .OOI microampere incident upon the surface of the specimen l4.
Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments far different from the embodiments illustrated. Accordingly, the scope of the protection afforded the invention should not be limited to the particular embodiment illustrated in the drawings and described above, but should be determined only in accordance with the appended claims.
I. In a scanning electron microprobe comprising electronbeam-generating means, specimen-mounting means, means for focusing said electron beam onto a spot on the surface of said specimen, means for scanning said focused electron beam spot over the surface of said specimen to generate an image electron signal therefrom with the image electron signal having low-frequency components representative of gross specimen structure and highfrequency components representative of micron size specimen structure, and means for detecting said image electron signal, the combination with said image-electron-signal-detecting means and electronbeamscanning means of a cathode-ray tube for forming a picture representative of the structure of the portion of said specimen scanned by said electron beam, means for synchronizing the X and Y axis sweeps of said cathode-ray tube with the scan of said electron beam over the surface of said specimen, filtering means accepting said detected image electron signal to pass only said high-frequency components thereof determinative of said micron-sized specimen structure, and means for modulating the display intensity of said cathode-ray tube responsive to the amplitude of said high-frequency component image electron signal.
2. The apparatus according to claim 1 wherein said filtering means has a bandwidth about a central frequency responsive to the scan rate of said electron beam over the surface of said specimen.
3. The apparatus according to claim 1 wherein said electron beam has a Y axis scan rate of 45 hertz per second and an X axis scan rate of 4.5 kilohertz per second over a specimen area of approximately 1000 square nanometers and said filter means has a bandwidth 3 decibels down at 50 kilohertz and 2 megacycles about a central frequency of 300 kilohertz.