in the measurement of photolithographic instruments, the Z-axis position at which, for evaluative lines in the image, the optical image has the highest contrast, and at which the evaluative line pattern will consequently appear with the correct width and pitch. Line focus may vary across the image field and is properly given as a Z-axis value for a specified image site in the image field . It also varies with the line angle, and the line focus must therefore include a specification of the angle, such as sagittal, tangential, or some other angle. [Adapted from SEMI P25-94]
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