Patent GB-1105115-A (GB)
Title: Improvements in and relating to the observation of the impact spot of a charged particle beam for machining of a workpiece
IPC: H01J37/315
Filed: 1965-07-20
Abstract: 1,105,115. Electron beam machining apparatus. STEIGERWALD STRAHLTECHNIK G.m.b.H. 20 July, 1965 [24 July, 1964], No. 30773/65. Heading H1D. [Also in Divisions B3 and G1] In electron beam apparatus for machining a workpiece the size and/or shape of the beam impact spot is checked by utilizing electrons and/or X-rays emanating from the impact spot to form an image on a fluorescent screen or to generate a signal at an output electrode enabling remote reconstitution of the image. The image on a fluorescent screen may be viewed directly or an associated photoelectric detector may be used to derive a signal for reconstitution of the image. The viewing system may include an image converter tube. Electrons 16 possibly reflected from the workpiece 13 may be imaged by magnetic lenses 17, 18 on to the fluorescent screen 19 after deflection at 21, a limiting diaphragm being provided at 50<SP>1</SP>. In alternative embodiments the reflected electrons are reflected at right angles to the axis of the main beam 12 before passing through a lens on to the fluorescent screen, Fig. 3 (not shown); the reflected electrons are, after deflection, accelerated on to the fluorescent screen by a system of electrodes, Fig. 5 (not shown); the main lens 10 is used also as the first lens of the imaging system for the reflected electrons, both the gun 2, 3, 4 and the fluorescent screen 19 being at an angle to the axis of the lens 10 and a deflecting field being provided for separating the main and reflected electron beams either above or below the lens, Fig. 6 (not shown); the main lens 10 is used also as the first lens of the imaging system, but the reflected electrons are deflected out of the path of the main beam after returning through the lens 10, Fig. 7 (not shown); the reflected electrons are used to project on to the fluorescent screen the shadow image of a knife edge or a circular diaphragm, Figs. 9 and 10 (not shown), or of two longitudinally separable diaphragms, one serving as a central stop to the other, Fig. 11 (notshown). In another arrangement the electrons and/or X-rays emanating from the impact spot are imaged on a fluorescent screen through a pin hole diaphragm, Fig. 8 (not shown). In an arrangement in which the reflected electrons are imaged on a target electrode the electrons are scanned by an electrostatic deflection plate system over the electrode which is smaller than the image of the beam spot on the workpiece and the detected signal is fed to an oscilloscope with a synchronized deflection system, Fig. 4 (not shown). The surroundings of the beam spot on the workpiece may also be imaged by providing an additional defocused beam or the beam may be pulsed and defocused or scanned rapidly over the area to be imaged in the intervals between the pulses. Alternatively the central part of the beam may be focused so that the central portion is used for machining while an outer annular zone is used for imaging the area. An electric deflection field may be provided between the main lens 10 and the workpiece 13 to maintain the image spot stationary on the fluorescent screen when the beam is deflected in particular machining operations over the workpiece, Fig. 6 (not shown). In a system for automatically maintaining the beam in focus a photo-electric detector arranged in front of the fluorescent screen is connected to a circuit controlling the current of the main focusing lens, Fig. 12 (not shown). The current through the lens is periodically varied and a switching pulse cuts off the current varying unit when the focus is optimal to keep the lens current at a uniform level.