EBPG hardware tour Page 6

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Another laser in the system measures the sample height. The height is measured on each exposure field, after which the EBPG corrects the focus and field size. This measurement and correction takes just a few milliseconds. You might wonder how the height measurement is converted to a focus correction. Well I’m going to tell you anyway. Think of the plot of focus versus height-signal: it’s more or less a straight line, and the slope never changes. The y-intercept of that straight line is determined while calibrating the system before an exposure. During calibration, the EBPG automatically focuses on a reference mark, and then measures the height with the laser sensor. This sets the offset; that is, the y-intercept of focus versus height.

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