Since the device integration and relating lithography process progressively increasing in complexity, more integrated approach to process control has been very necessary. There is more function in scanners that can be used to improve process window, CDU and OVL etc. When semiconductor technology node achieves to 28 nm, some function of scanner comes out contribution on process window increase. Focus drilling is the scanner implementation to increase the depth of focus (DoF) for contact-hole or via fabrication processes [1][2][3][4][5][6][7]. The basic principle is to smear out the maximum image contrast at best focus (BF) over a specific defocus range. This function applies a wafer stage R x -tilt (X-axis tilt) when scanning along the Y-direction (scanning direction). This allows the wafer stage to scan along a plane parallel to its tilt. This paper relies on Focus drilling to improve DoF of 28nm Via process, meanwhile also to analyze its negative impact on EL and CDU etc. And then try to find a balance focus range (Rx tilt) for 28 nm product. But the experiment result showed focus marginal gain is about 15nm, meanwhile it is hard to increase DoF through Rx tilt expanded.
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.
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