Analysis of wafer heating in 14nm DUV layers

Abstract

To further shrink the contact and trench dimensions, Negative Tone Development (NTD) has become the de facto process at these layers. The NTD process uses a positive tone resist and an organic solvent-based negative tone developer which leads to improved image contrast, larger process window and smaller Mask Error Enhancement Factor (MEEF)[1]. The NTD masks have high transmission values leading to lens heating and as observed here wafer heating as well. Both lens and wafer heating will contribute to overlay error, however the effects of lens heating can be mitigated by applying lens heating corrections while no such corrections exist for wafer heating yet. Although the magnitude of overlay error due to wafer heating is low relative to lens heating; ever tightening overlay requirements imply that the distortions due to wafer heating will quickly become a significant part of the overlay budget. In this work the effects, analysis and observations of wafer heating on contact and metal layers of the 14nm node are presented. On product wafers it manifests as a difference in the scan up and scan down signatures between layers. An experiment to further understand wafer heating is performed with a test reticle that is used to monitor scanner performance.