Study on Abnormal Intra-Field CD Uniformity in Exposure Scan Direction

Abstract

Critical dimension uniformity (CDU) of hole layer is becoming more and more crucial alongside with the technology node being driven into 28 nm and beyond, since the critical dimension (CD) variation of 2-dimensional (2D) hole pattern is inherently larger than that of 1D pattern (line/space). As the process window becomes more marginal with the more advanced technology node, EFESE tilt (focus drilling method) is a simple but useful way to enhance depth of focus (DOF) (1-4). Despite the compromises in image contrast and exposure latitude (EL), it’s still worth the trade-off; especially in BEOL photo process where incoming wafer topography eats significantly into focus control window. We report an abnormal up to 6 nm ADI CD trend-down in Y-direction (exposure scan direction) in the strictly repeated via-hole patterns within an about 9 mm x 6 mm chip in base line (BL) photo condition wafer (short as BL wafer). No CD trend-down or trend up in X-direction. This hole layer BL photo condition uses EFESE tilt to improve DOF. This CD trend-down phenomenon is investigated and a model of “effective EFESE tilt” is proposed and verified. Based on the model, we made a further step into the impact of applying EFESE tilt on not only intra- but also inter-field CDU. Through all this analysis, we give an insight of the safety zone for applying EFESE tilt for future reference.