Intra-field etch induced overlay penalties

The etch induced on-product overlay performance across wafer has received quite some attention recently. Global wafer overlay penalties have been observed by realizing that the etch direction is not always perpendicular to the wafer surface and may vary slightly as a function of the wafer radius due to the geometry and plasma parameter settings of the etch tool. In particular close to the wafer edge, for radii in between 130-mm and 150-mm, the etch direction may change even more strongly and is not constant over time. This is due to a consumable part inside the etch tool, the so-called focus ring. Control solutions based on optical overlay metrology have been developed and have found their way into tunable focus rings. The general concept is to keep the etch direction perpendicular to the wafer surface throughout the life-time of the focus ring. The general belief is that these global etch induced overlay penalties can be mitigated by applying these newly developed hardware control solutions. In this experimental work, we go one level deeper and consider the more local etch induced overlay penalties. This time etch effects on length scales on the order of exposure field and/or die level are addressed. The intra-field etch induced overlay penalties are characterized by considering the overlay measurement after resist development (ADI) and after etch (AEI). Surprisingly, the observed penalties are on the order of similar to 1-nm within each individual exposure field despite the fact that away from the wafer edge the etch direction is considered to be close to perpendicular to the wafer surface. In this experimental work, etch tool parameters like low frequency (LF) power and pressure have been varied to reveal the nature of these die-level overlay penalties. Based on the experimental results, we present a hypothesis of the underlying mechanism that explains the etch induced intra-field overlay penalties and provide solution directions to mitigate these kinds of overlay penalties.