Reticle process monitoring and qualification based on reticle CDU and wafer CDU correlation

Abstract

Reticle process of record (POR) sometimes needs fine tuning for some reasons such as multiple layer process, better critical dimension uniformity (CDU) or new etch chamber. The sidewall angle and corner rounding will be varied due to the reticle processing tuned comparing to previous POR. However, because the reticle critical dimension (CD) measurement is based on middle side lobe measurement or other algorithm, the reticle CD cannot reflect the changes of reticle sidewall angle and corner rounding variation which are critical for 65nm node and below. Each of the scanner, wafer process, reticle and metrology tool contributes to the intra-field wafer CD. Normally, the reticle contribution to the wafer CDU should be as small as possible, that is less than 33%. By averaging all wafer CD of individual features to obtain a wafer CD reference independent of feature location and wafer die, the correlation of wafer measurement to target (MTT) and reticle MTT can be obtained. The correlation can accurately qualify and monitor the tuning processing of reticle. We have manufactured two masks for active layer of 65nm tech node by different reticle process. One used the original POR process of active layer, while another used multi-layer-reticle (MLR) process. The correlations between wafer CDU and reticle CDU of these reticles are demonstrated for both isolated and dense features in vertical and horizontal direction, respectively. Similar experiments were implemented and the correlations for both dense and isolated structures are demonstrated as well, for two different POR process for first metal layer of 40nm tech node. Referring to the wafer and reticle MTT correlation, the contribution of reticle CDU to wafer CDU can be used as an evaluation methodology for reticle processing. The wafer and reticle CDU correlations for 45nm node poly and contact layers POR process are also demonstrated.