Measuring electron-beam landing angle in real time

With the ever increasing need for higher resolution and placement accuracy in mask making, the requirement for eliminating position shifts due to targ et height variations is becoming a critical factor in the overall image pla cement error budget. In theory this problem has been eliminated with the IB M patented variable axis immersion lens which can achieve perpendicular lan ding in all parts of the major magnetic field as well as in the electrostat ically deflected subfield. The initial setup and periodic checks, however, require very high accuracy measurements. In the past we have relied on a te dious method which involves exposure and detection of registration marks at various heights, in conjunction with a very accurately calibrated laser st age, followed by position measurements with a state-of-the-art metrology to ol. In this article we describe a method which makes this measurement with high accuracy, in real time, using a standard oscilloscope display. (C) 199 9 American Vacuum Society. [S0734-211X(99)17906-X].