| 摘要 |
959,430. Discharge apparatus. CARL ZEISS-STIFTUNG, [trading as C. ZEISS [Firm of]]. Nov. 12, 1962 [Nov. 18, 1961], No. 42582/62. Heading H1D. [Also in Division H4] In electron/ion beam apparatus for working material, correct focusing is obtained by periodically varying the current flowing through an electromagnetic lens 13, Fig. 1, measuring the amplitude of charged-particle emission from the material 15 being worked, and setting the focusing lens current at the value corresponding to measurement of an extreme value in the charged-particle emission amplitude. As shown, the charged-particle emission is collected by interceptor 30, which may be positively biased. It is found with materials having a high electron emission and high-melting point that a gradually improving beam focus initially produces an increasing electron collection at interceptor 30 until positive ions are produced and result in a sharp minimum of the negative current collected at interceptor 30 at correct focusing. However, with materials of low electron emission and low melting point a maximum of the negative current collected at interceptor 30 indicates correct focusing. The workpiece 15 may be connected to ground through a resistance, the voltage drop in which is fed to an oscillograph, or the current collected at the workpiece may be measured to indicate chargedparticle emission therefrom. If a continuous, rather than a pulsed, electron beam is used, no clear extreme value of charged particle emission may occur, and in this case the current through lens 13 is set at the point of symmetry, or a predetermined point, between the flanks of current rise and fall at interceptor 30. If the beam focus is to be set above or below the surface of workpiece 15, then the charged particle emission current is subjected to a time delay before actuating the switch 36 which sets the lens current. In an alternative arrangement, two interceptors 45, 44, Fig. 10, are employed, one collecting electrons deflected by magnetic field 43 and the other collecting ions, the currents at 45, 44 being compared at 46 to determine the required extreme value. The current through lens 13 may oscillate about a mean value determined by the height of the surface of workpiece 15. This height is sensed by a feeler 31 associated with a potentiometer, the signal from which is subjected to a time delay in accordance with the speed at which workpiece 15 is moved across the beam path, and then rotates a potentiometer winding in lens current generator 34 to set the mean value. The sliding arm of the potentiometer is oscillated periodically by motor 35 until the latter is stopped by switch 36. Automatic focusing may be carried out only intermittently, the interceptor 30 being swivelled away from the beam path in the intervening period to avoid soiling. |
