Track process monitoring via laser scattering imaging

The growing complexity of photolithography processes necessitates stringent defect monitoring to meet the increasingly rigorous yield requirements. Mitigation of defectivity at the track level involves both product wafer scans, as well as blanket wafer inspections for process monitoring and tool qualification. Some of the common defect types that are characteristic of the track process are comets, striations, water marks, chuck marks, edge effect etc [1]. In this work, we review two case studies that highlight the benefit of using laser-based scattering imaging for detecting coating-induced defects in blanket wafers [2]. First, we used a whole wafer, laser-based scattering image to detect, as well as quantify, a buildup of material near the wafer center. Grid analysis of the whole wafer image enabled us to bin out the defective regions of the wafer surface, and as a result automatically flag defective monitor wafers. Achievement of process improvement is reflected in the post-change process scan images. This methodology of process signature monitoring led to the optimization of the spin coating process which was implemented by the track team. In the second case, whole wafer laser scattering imaging was again successfully used to visually capture the spin coating classical striation signature in a film from a trilayer stack. This was further quantified by using a spectroscopic ellipsometer, which clearly highlighted the thickness variation of the resist film as a function of radius across the wafer with maximum thickness variation was observed near the center of the wafer surface. Further studies for this layer will focus on a quantification strategy for verification of process anomaly. This work was performed by IBM Research and Development at various facilities.