| 摘要 |
PURPOSE:To determin the position of a positioning mark highly accurately, by performing electron-beam scanning along the paths crossing at a mark from the two different points, and sensing the scanning speed of the beam by using the difference in time lengths from the beginning of the scanning and to sensing of the mark. CONSTITUTION:An electron beam is scanned along a path 301 from a point X0- X1, and the beam is turned on and off during the scanning. Scattering of electrons, when the beam crosses a target 302, is sensed; and the time, when the sensed signal crosses a threshold level 303, is sensed. The period from the beginning of the beam scanning to the time at which the threshold crossing is sensed is counted N1-N4 by a counter which is connected to an oscillator with a specified frequency. Then, scanning from a point X0' which is well separated from the point X0 is counted at the same speed to obtain the values N1'-N4'. The scanning speed C is obtained by dividing the distance (X0-X0') by the difference in the arithmetic means of the counted values. The position of the mark can be obtained by X0+C(N1+... +N4)/4 very accurately, and is not affected by the scanning speed of the beam. |
