The introduction of new techniques such as double patterning will reduce overlay process tolerance much faster than the
rate at which critical feature dimensions are shrinking. In order to control such processes measurements with
uncertainties under 0.4nm are desirable today and will become essential within the next few years. This very small error
budget leads to questions about the capability of the imaging technology used in overlay tools today and to evaluation of
potential replacement techniques. In this paper we will show that while imaging technology is in principle capable of
meeting this requirement, the real uncertainty in overlay within devices falls well short of the levels needed. A proper
comparison between techniques needs to focus on all of the possible sources of error, and especially those that cannot be
simply reduced by calibration or by repeating measurements. On that basis there are more significant problems than the
relative capability of different measurement techniques. We will discuss a method by which overlay within the device
area can be controlled to the required tolerance.
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