Phase behavior through pitch and duty cycle and its impact on process window

Abstract

Immersion lithography has moved into 45nm node and will soon go into 32nm node. Alternating Phase Shifting Masks (alt. PSM's) are one of the most effective methods to enhance resolution and process window. . However there are two major challenges: intensity balancing and quartz dry etch process. The dry etch process requires not only an uniform quartz etch but also a good linearity over a wide range of feature sizes to ensure a 180° phase shift through pitch and duty cycle. Phase errors lead to an image placement error during printing becoming even worse through focus. As feature sizes shrink imaging effects and 3D mask effects impact the phase shift and accurate phase shift measurement becomes extremely important. In this paper we report on phase shift measurements through pitch and duty cycle on alt. PSM taken on the newly developed phase metrology system Phame® and compare them to rigorous 3D simulations. Furthermore we correlate the phase shift measurements to process window data such as maximum exposure latitude. Through pitch investigations on alt. PSM show that for print pitches below 200nm (wafer level) the phase shift drops significantly below 180° which will lead to an image placement error during printing and a shrinking process window. Furthermore a strong correlation between phase shift and maximum exposure latitude is shown. Largest maximum exposure latitude is achieved for phase shift close to 180°. Phame® enables optical phase shift measurement in critical production features down to 120nm half pitch providing the opportunity to optimize the quartz dry etch process in terms of signature and linearity. This will help to optimize the phase shift of critical features on alt. PSM for largest process window and hence increase end of line yields for reducing overall chip manufacturing costs.