Scanning Electron Microscope With Color Display Means

Abstract

A scanning electron microscope and system wherein small surface potential differences are rendered visible as color shifts on a color CRT (cathode-ray tube) display device. A plurality of different surface potentials are detected respectively by scintillation detectors which are coupled to photomultipliers and then amplifiers. The amplifier output signals representative of several different means surface potentials at scanned points of the specimen are coupled respectively to the three signal control grids of a color CRT in the case of a three scintillating detector-three channel system and the color combinations of the primary colors displayed are indicative of surface potentials in between the mean potentials thus making small differences of the specimen surface potential readily discernible to the eye as small color shifts.

Description

This invention relates to improvements in scanning electron microscopes and more particularly to a scanning electron microscope which is capable of providing a color display derived from variations in surface potential.

The conventional scanning electron microscope system utilizes a collector electrode having a slit therein through which pass secondary electrons emitted from the bombarded point on the specimen having a certain constant energy. These secondary electrons are then accelerated to hit a scintillation detector, and a signal whose amplitude is proportional to the number of electrons having passed through the slit is detected by a photomultiplier. This signal is then supplied through an amplifier to a grid of a cathode-ray tube for effecting modulation of the brightness of the CRT. Meanwhile, the output of a scanning power source is simultaneously supplied to the deflecting plates of the scanning electron microscope, and to the deflecting plates of the cathode ray tube. By this simultaneous supply of scanning power, a secondary electron image of the specimen can be observed on the fluorescent screen of the cathode-ray tube.

Presently, technicians utilizing the above conventional scanning electron microscope observe the surface characteristics of the specimen as conveyed by display of surface potential scanning of the specimen in the form of a black and white picture or display as seen on the face of the CRT. The single collector of the above-described conventional SEM (scanning electron microscope) apparatus is moved to a second position for receiving secondary electrons having a different constant energy and the greys in the first and second black and white pictures obtained at the first and second collector locations are compared.

The above state of the art method for surface potential studies of the surfaces of specimens makes small differences in surface potential very difficult to detect by comparisons of greys in two or more black and white pictures.

It is therefore a primary object of the present invention to provide a scanning electron microscope system having increased sensitivity to surface potential and providing easier visualization thereof.

It is another object of the present invention to provide a scanning electron microscope apparatus in which means are provided for developing a multicolor display on a color cathode-ray tube in which small differences of surface potential are displayed as small color shifts in the display.

It is yet another object of the present invention to provide a scanning electron microscope having a plurality of detection systems in which the information derived is coupled to a single display device for the processing thereof.

Other objects, features, and, advantages of the present invention will become apparent as the detailed description proceeds.

According to one embodiment of the present invention the electron microscope system comprises means for emitting a primary electron beam and means for focusing the electrons into a spot on a specimen. A common scanning power source is provided for simultaneous scanning of the primary electron beam in the electron microscope and the plural electron beams in a color cathode-ray tube. A plurality of slit-shaped collector electrodes are positioned to receive deflected secondary electrons through the slits thereof having a corresponding plurality of different constant energy levels. A scintillation detector is positioned behind each of the slits and coupled to a photomultiplier which provides a signal which is then amplified and supplied to the signal control grids of a color CRT. In accordance with a preferred embodiment, three channels are provided each including a collector electrode having a slit, a scintillation detector, a photomultiplier and amplifier means, the first collector electrode and slit being positioned to receive secondary electrons having a first constant energy level and the output signal of the corresponding amplifier in the first channel is coupled to the signal control grid of the red gun in the color CRT. The second collector electrode and slit of the second channel is positioned to receive secondary electrons having a second constant energy level higher than said first constant energy level, the output signal from the amplifier in the second channel being coupled to the signal control grid of the green electron gun in the color CRT, while the slit of the third collector electrode in the third channel is positioned to receive secondary electrons having a third constant energy level higher than said second constant energy level and the output signal from the amplifier of the third channel corresponds to the blue color signal of a color CRT and is coupled to drive the signal control grid of the blue electron gun of the color CRT.

More complete understanding of the invention will best be obtained from consideration of the accompanying description and drawings in which:

FIG. 1 is a diagrammatic view showing the structure of a prior form of scanning electron microscope;

FIG. 2 is a diagrammatic view of a scanning electron microscope structure and apparatus for displaying surface potentials of a specimen on a color cathode-ray tube in accordance with an embodiment of the invention;

FIG. 3 is a diagrammatic view of a scanning electron microscope structure and system for recording and displaying in color simultaneously surface potentials of a scanned specimen in accordance with a further embodiment of the invention.

FIG. 1 is illustrative of a prior art scanning electron microscope and display arrangement as described in FIG. 1 of U.S. Pat. No. 3,474,245 to Kimura et al., which may be referred to for further detail and explanation thereof.

Briefly, the scanning electron microscope of FIG. 1 includes electron beam emission means consisting of a cathode 1 in the form of a hairpin-shaped tungsten filament, a grid 2 generally called the Wehnelt cylinder, and an anode 3. A voltage of 20 to 30 kilovolts is applied across the anode 3 and the cathode 1, and an electron beam 4 emitted from the heated tungsten filament is focused on a specimen 7 to be observed by means of two condenser lenses 5 and 6. The specimen 7 is placed on a specimen stage 8 and the speciment position is adjustable in both horizontal and axial directions. The electron beam 4 is deflected by two pairs of deflecting plates 9 so as to scan the specimen surface longitudinally and laterally thereof. When the specimen surface is bombarded by the primary electron beam 4, a secondary electron beam 10 is emitted from the bombarded point and the amount of the secondary electrons is variable depending on the material of that particular bombarded point and the incident angle of the primary electron beam 4. This secondary electron beam 10 generally has an energy of less than 50 electron volts. A mesh 11 having a control aperture for the passage therethrough of the incident beam is disposed opposite the specimen 7 and is kept at a negative potential of from several to several tens of volts so that the secondary electron beam 10 is deflected in a manner as shown and only those secondary electrons having a certain constant energy are passed through the slit 12 in collector electrode 12a. These secondary electrons are then accelerated to hit against a scintillator 13, and a signal whose amplitude is proportional to the number of electrons having passed through the slit 12 is detected by the photomultiplier 14. This signal is then supplied through an amplifier 15 to grid 17 of the cathode-ray tube 16 for effecting modulation of the brightness of the tube 16. Meanwhile, the output of the scanning power source is simultaneously supplied to the deflecting plates 9 of the scanning electron microscope column, and to deflecting plates 19 of the cathode-ray tube 16. By this simultaneous supply of scanning power, a secondary electron image of the specimen 7 can be observed on the fluorescent screen of the cathode-ray tube 16.

Turning now to FIG. 2 wherein the same numerals represent parts corresponding to those of the SEM of FIG. 1 it will be observed that the output display device of the system is a color cathode-ray tube 16a rather than a black and white picture tube 16 as shown in the prior art system of FIG. 1. An electromagnetic deflection coil 19a is shown for purposes of simultaneously deflecting the plurality of electron beams of the tube 16a and causing them to scan a raster on the faceplate of tube 16a. In FIG. 2, there are three collector electrodes 23a, 24a, and 25a each having a corresponding slit 23, 24, and 25. Slit 23 in collector electrode 23a comprises the input to the secondary electron detection system of the first channel. Secondary electrons in secondary electron beam 33 having the lowest certain constant energy are deflected by the field formed in the region of the beam along a curved path in the manner shown and received by scintillator 13a after passage through slit 23. Coupled to scintillator 13a is photomultiplier 14a which provides an output signal whose amplitude is proportional to the number of electrons having passed through the slit 23. This signal is then supplied through an amplifier 15a to feed the red signal control grid 17a. Secondary electrons in secondary electron beam 34 which have a certain constant energy higher than the secondary electrons of beam 33 are deflected further in the field formed in the beam deflection region along a curved path and pass through slit 24 in collector electrode 24a and are then detected by scintillator 13b coupled to photomultiplier 14b, the output of scintillation detection 13b and photomultiplier 14b then being fed to amplifier 15b and the amplified signal being the green color signal supplied to drive green signal control grid 17b at the output of the second channel between the SEM and color cathode-ray tube 16a. The third channel input is provided by collector electrode 25a which receives through slit 25 therein secondary electrons in secondary electron beam which have a constant energy level higher than the secondary electrons of beam 34 and which are consequently deflected further through the deflection field between the specimen 7 and collector electrode 25a. These secondary electrons which reach and pass through slit 25 strike scintillator 13c which is coupled to photomultiplier 14c. The output of scintillation detector 13c and photomultiplier 14c is then amplified in amplifier 15c and the amplifier output in the third channel is coupled to signal control grid 17c. While the several collector electrodes 23a, 24a, and 25a are shown spaced successively further away from the specimen and parallel to mesh 11 which is kept at a negative potential of several to several tens of volts to provide the deflection of secondary electron beams 33, 34, and 35 as shown, it should be recognized that other deflecting fields and electrode geometry, as for example as is shown in the embodiment of FIG. 3, may be utilized to provide detection of secondary electron beams at selected energy levels to furnish the three inputs to the three channels shown. While a three-color CRT coupled to the three channel outputs is shown, it should be recognized that a two-color display device at the output of two channels using two collectors at the input of the channels may be utilized to provide a two-color display of surface potentials of a scanned specimen. A penetration-type color CRT of known type may be utilized as the output display device in a two-channel arrangement of the system.

A further embodiment of the invention is shown now in FIG. 3. The collector structure of this embodiment differs from that of FIG. 2 in that instead of separate collectors 23a, 24a, and 25a insulatively separated from each other there is included in FIG. 3 a single collector plate 26 having corresponding slits 23, 24, and 25 therein. Thus the individual collectors of the three channels are seen to be maintained at the same potential because of the integral collector plate 26 common to the three channel inputs. Light pipes 43, 44, and 45 are coupled between scintillators 13a, 13b, and 13c and photomultipliers 14a, 14b, and 14c, respectively. This manner of light transmission from scintillators to photomultipliers in each channel permits close positioning of scintillators behind closely spaced slits 23, 24, and 25 in collector plate 26. Photomultipliers 14a, 14b, and 14c are thus not required to be mounted directly behind and in the limited space available to the rear of scintillator buttons 13a, 13b, and 13c. It should be noted that collector plate 26 having a plurality of slits 23, 24, and 25 is positioned at right angles to mesh 11 and also parallel to the primary electron beam 4 for collecting deflected secondary electron beams 23, 24, and 25. Recording means 75 is shown coupled to record the third channel output signal obtained through switching means 76 shown in the third channel position making connection to terminal 79 for sensing the output of amplifier 15c. In the same manner, the output signals representing scanned surface potentials derived in the second and first channels may be recorded when switch 76 is connected respectively to terminals 78 and 77. Alternatively, plural channel recording means may be coupled to output terminals 77, 78, and 79 when it is desired to analyze and compare the three channel output voltages available at output terminals 77, 78 and 79 to obtain more exact surface potential variations than can be observed visually on the screen of color CRT 16a. Small differences of surface potential are displayed as small color shifts. The measurement of potentials in between those primarily measured by each collector and the ability to see small potential differences as color shifts are possible because of the Gaussian distribution of electron trajectories for any one surface potential. Other color combinations than those in the embodiments shown wherein the highest surface potentials show up as blue and the lowest potentials as red may be utilized to provide the display.

Claims

What I claim is:

1. In a scanning electron microscope:

beam generating means for emitting a primary electron beam,

support means for supporting a specimen,

at least one focusing electron lens of the magnetic type positioned between said beam generating means and said support means for focusing the primary electron beam on the specimen,

first deflecting means positioned between said beam generating means and said focusing means for deflecting the primary electron beam to scan the specimen,

first detector means for detecting secondary electrons having a first constant energy which are emitted from the portion of the specimen struck by the primary electron beam,

second detector means for detecting secondary electrons having a second constant energy greater than said first constant energy which are emitted from the portion of the specimen struck by the primary electron beam,

a color cathode-ray tube having first and second color control grids for modulating the intensities of first and second scanning electron beams in response to the output signals of said first and second detector means respectively, and

second deflecting means for simultaneously deflecting the first and second scanning electron beams in the color cathode-ray tube in synchronous relation with the scanning operation of the primary electron beam.

2. A scanning electron microscope comprising electron beam generating means for generating and directing a beam of primary electrons along a beam path towards a specimen, means for mounting said specimen, means for focusing said electron beam into a spot on said specimen, means for raster scanning said focused electron spot over the surface of said specimen, means for detecting secondary electrons at different discrete energy levels thereof, a color cathode-ray tube, and means for coupling the output of said means for detecting secondary electrons to said color cathode-ray tube whereby the display intensities of the primary colors thereof are responsive respectively to the outputs of said detecting means at said different discrete energy levels thereof.

3. The apparatus of claim 2 wherein said means for detecting secondary electrons at discrete energy levels comprises a plurality of photomultiplier-coupled scintillation detectors mounted adjacent said specimen in position to detect secondary electrons at said different discrete energy levels.