Process variation impacts on optical overlay accuracy signature

On Product Overlay (OPO) control is a critical factor in advanced semiconductor manufacturing. As feature sizes become smaller, OPO budgets become tighter, leaving less room for overlay (OVL) measurement inaccuracy. Over the last few years, overlay metrology’s focus has shifted inwards, towards accurate measurement conditions, as we aim to capture ever-smaller process and scanner variations. One method used to break down the OPO error budget is combining one or more accuracy flags and correlating them to various process impacts. Analyzing the overlay accuracy signature generated by accuracy flags can be useful for data validation, inspection and correlation to different processes and metrologies. In this paper, an extensive OVL accuracy experiment demonstrates the use of this new method. First, the method is applied to several wafers designed with intentional process variation, including variations in etch duration, Chemical Mechanical Polishing (CMP) duration, amorphous silicon (a-Si) thickness and titanium nitride (TiN) thickness. OVL results from the experimental wafers are compared with results from the reference (nominal) wafer.