Impact of mask CDU and local CD variation on intra-field CDU

Abstract

The control of critical dimension uniformity (CDU), especially intra-field CDU, is an important aspect for advanced lithography, and this property must be controlled very tightly since it affects all of the exposure fields. It is well known that the influence of the mask CDU on the wafer intra-field CDU is becoming dominant because the mask error enhancement factor (MEEF) is quite high for low-k1 lithography. Additionally, the abovementioned factors impact the CDU through global (field-level) and local (grating-level) variations. In this paper, we analyze in detail CDU budgets by clarifying the impact of local CD variation. The 50-nm staggered hole features using Att-PSM showed a mask global CDU of 1.64 nm (3sigma at the mask level) and a wafer intra-field CDU of 2.30 nm, indicating that the mask global CDU was a major part of the intra-field CDU. By compensating for the contribution of the mask CD, the wafer intra-field CDU can be reduced to 0.986 nm. We analyzed the budgets of wafer intra-field CDU, which is caused by local CD variation (mask and process) and measurement noise. We determined that a primary cause of the wafer intra-field CDU after applying a mask CD correction was these local CD variations, which might disturb the proper use of dose correction for the mask CD. We demonstrated that the impact of mask local CD variation on the correction flow can be greatly reduced by averaging multiple point measurements within a small area, and therefore discuss the optimum conditions allowing for an accurate intra-field CDU determination. We also consider optimization of the CD sampling scheme in order to apply a dose correction on an exposure system to compensate for the mask CDU.