Patent GB-1000560-A (GB)
Title: Electron probe system
IPC: H01J37/256
Filed: 1961-09-29
Abstract: 1,000,560. Radiation measurements. ELION INSTRUMENTS Inc. Sept. 29, 1961 [March 2, 1961], No. 35345/61. Heading G1A. [Also in Divisions H3 and H4] General: A specimen 22, Fig. 1, is scanned by a high voltage (e.g. 50 kv) beam of electrons to stimulate emissions or reflections which are detected at 32, 60, 52 to produce corresponding electric signals which are applied to modulate cathode-ray tubes 40, 42 which are scanned synchronously with the scanning of the specimen, the arrangement being characterized by the provision of (1) means for adjusting the location of the reg region of the scanning on the specimen, (2) means for varying the magnitude of the scan effected over the specimen relative to the scans in the cathode-ray tubes so as to permit in effect a variable magnification of the image of the specimen presented by the cathode-ray tubes, and (3) switch means for supplying scanning signals as well as additional deflection signals alternately our succession time periods. Detector 32 is a phosphor element responsive to electrons reflected from the specimen, the resulting light being directed to a photo-multiplier 34. Detectors 50, 52 are scintillation or Geiger-Muller counters responsive to X-rays from the specimen which are reflected by spectrometer crystals 46, 48. The outputs of detectors 50, 52 are supplied to scalers or analysers 54 (such as pulse height analysers) and rate meters 56. Provision is also made by way of switch 36 to derive a signal from the specimen current. The derived signals are applied to a control unit 38 whereby selected signals may be applied to modulate the cathode-ray tubes. A bias source 44 may be connected to the specimen. The specimen stage 24 may be moved by motors 26, 28 to permit initial positioning of the specimen with respect to the electron beam. The scan generator 30 which supplies the signals for scanning the specimen and cathode-ray tubes comprises a vertical and a horizontal saw tooth signal generator together with means for varying the speed, amplitude and position of the resulting scans. After initial adjustment, the position and size of scans of the cathode-ray tubes are not varied, whilst the position of the scan or the specimen is varied to permit any desired area of interest to be selected and the size of the scan is then adjusted to obtain a desired degree of image magnification. The horizontal and vertical sawtooth generator have their speed controls coupled so as to maintain a constant number of lines in each raster during adjustment. The amplitude controls are also coupled so as to maintain a constant aspect'  ratio. The scan generator may be operated with both generators functioning so as to effect a television-type raster scan, or only one generator may function to produce a line scan. Operation: In general the apparatus is first operated in a search mode with a raster scan to permit a general examination of the specimen and is then operated in a line scanning mode to permit detailed examination by line scan across a selected part of the specimen. During the search mode, the cathode-ray display is caused to present both the modulated raster representing the specimen image and a line 524 or 526, Fig. 4, whose length indicates the magnitude of one. of the detector signals. This is achieved by causing the slower speed (vertical) scan generator to produce a waveform such that following each scan there is an interval before scanning restarts. During this interval the electron beams for both the specimen and cathoderay tubes are stationary at the centre of the area of scan. The resulting detector signal is converted to an alternating signal of a magnitude proportional to the detector signal and is applied to deflect the beam in the cathode-ray tubes so as to produce a line 524, 526. By appropriate selection of the horizontal or vertical deflection plates and the polarity of the signal the line may be caused to extend either up, down, to the left or to the right. Due to the phosphor decay characteristic, both the raster and line are visible simultaneously, the line extending from the point in the raster where the measurement is made. Reference is made to the simultaneous or time sequential display of more than one detector signal. Normally the search mode operation repeats cyclically, but provisions is made for the raster scan to take place only once very slowly and for the resulting cathode-ray tube image to be photographed. For this purpose one of the cathode-ray tubes has a short persistence phosphor. During the line scanning mode scanning takes place under the control of switch means alternately with a raster scan and a line scan so that the cathode-ray tube image comprises a raster together with a modulated line such as 135, Fig. 4. It is important that the line scan should be at a certain angle with respect to a certain feature of the specimen (e.g. at right angles to a fault 137) and for this purpose the line is produced by the vertical sawtooth signal which is applied to the horizontal and vertical deflection plates in controllable proportions to permit the angle of scan to be adjusted. Furthermore, it is important that the scan should proceed in a predetermined direction over the fault and to this end provision is also made to control the polarity of the scanning signals applied to the deflection plates so that the beam is moved in the desired predeterminated direction. For recording purposes the line scan may be made to take place very slowly and the resulting cathode-ray tube display photographed or the resulting signals recorded magnetically. Provision is made for a high speed raster scan for use during initial adjustment and focusing. The cathode-ray tubes may be used for monitoring signals throughout the apparatus. Cathode-ray tube operating circuits; valve sawtooth generators: Vertical sawtooth generator, Fig. 5. Capacitor 162 is charged through adjustable resistor chain 146,148 from a constant-current source comprising series connected triodes 142, 144, the resulting sawtooth variation being applied through cathode-followers 182,184 to produce an output at line 202 and also being applied via a further cathode-follower 186 and an earthedgrid amplifier 194 to the anode of triode 142 to provide linearizing feedback. The output on line 202 is also applied to the normally non-conducting left-hand triode 204 of differential amplifier 204, 206, the grid of the right-hand triode of which is set by potentiometer 208 at a potential corresponding to the desired maximum amplitude of the sawtooth. When this amplitude is reached, triode 204 conducts, triode 206 ceases to conduct, and a relay 154 is released. At contact 154-1 capacitor 162 is discharged to terminate the saw-tooth, the discharge taking place to a starting potential set at the cathode of cathode-follower 198 by grid potentiometer 200. Reoperation of relay 154 is delayed adjustably by CR circuit 220, 222 so that each sawtooth 226 and flyback stroke 228 is followed by an interval 230. By operation of switch 158 the generator may be set up to operate in a singlestroke mode under the control of start and stop swtiches 708, 210. The instantaneous amplitude of the sawtooth is indicated by cathode-follower voltmeter 217, 219 connected between the grids of differential amplifier 204, 206. A circuit including a potentiometer for varying the magnitude of the sawtooth wave before it is applied to cathode-ray deflecting plates and for adding an adjustable amount of D.C. voltage for shift purposes is described with reference to Fig. 6 (not shown). The circuit also includes a relay switch controlled by a flip-flop circuit triggered by the sawtooth wave for controlling the alternate line-scan and rasterscan operation in the lino-scan mode. Horizontal sawtooth generator, Fig. 7. Capacitor 290 or 292 as determined by switch 158-5 is charged rapidly to the supply voltage by an impulse 298 which renders cathode-follower 296 conductive and thereafter discharges through a constant-current path comprises of series connected triodes 300, 302 and adjustable resistor chain 304, 306 to generate the sawtooth. The output is derived through cathode-followers 350, 360 an adjustable potentiometer 364 and potentiometer means 374 for adding an adjustable D.C. shift voltage. Pulses 298 for triggering cathode-follower 296 are obtained from one or the other of two multivibrators 308, 326. Switches 150 for adjusting the resistor chains in the constant-current path of both thehorizontal and vertical generators together with time-constant networks of the multivibrators are coupled mechanically so as to maintain a constant relationship between the frequencies of the two generators. Cathode-ray blanking signals are obtained via cathode-follower 346 on lead 348. Deflection amplifiers. In the circuit for controlling the scanning of the specimen, Fig. 8, the circuit comprises D.C. coupled push-pull stages 386, 388 and 390, 392 with the input signal applied from the scanning generator over lead 258 to a differential amplifier 380, 382 which receives a feed-back signal from the output of stage 390, 392 via lead 384. 86 indicates the vertical amplifier. 108 comprises a generally similar circuit for the horizontal amplifier. The amplifier outputs are applied to the deflection plates 18, 20 via relay controlled change-over switches 400, 416 whereby the scan direction may be controlled. This feature is utilised during the line-scanning mode (see above) where the vertical scan signal is applied in adjustable proportions to both the horizontal and vertical deflection plates so as to create a line scan at a predetermined angle. In the circuit for controlling the cathode-ray display tubes, Fig. 9, a similar arrangement of D.C. coupled push pull stages is employed with feed-back to the input, but the output stage differs in detail in that each arm of the pair comprises two series-conn