Alternating aperture phase shift masks (AAPSM) continue to offer high contrast imaging for 65nm half-pitch using conventional 193nm illumination. The transition to high NA lithography systems including immersion lithography, and the ever-decreasing feature sizes have made the topography of the photomask a significant issue in the final resist image. Therefore, the influence of the alternating phase shift depth, the trench profile, and the critical dimension control through variable feature width must be considered and understood for optimized wafer imaging.
This paper will examine the impact on imaging based on three photomasks, each employing different quartz etch chemistries. The three methods used to define the well structures include two all dry and a partial wet etch approach. As the photomask features continue to decrease, slight changes in the quartz etched trench profile and depth can severely affect the wafer prints, as the effective 180 degree phase shift for imaging is not achieved. In this work we correlate the imaging performance through pitch to a systematic evaluation of the photomask topography using complementary photomask metrology techniques.
The actual depth and profile of the structures is obtained on a FEI Stylus nano-profilometer (SNP-XT) and from destructive cross sections. The CD linearity is measured on a top-down reticle CD SEM (KLA 8100XR). Based on photomask metrology data, rigorous electro-magnetic field (EMF) simulations of the various topographic profiles are performed. As a first printing performance estimate the photomasks are evaluated on a Zeiss AIMSfab193. Comparisons between the different evaluations will be made against wafer prints, obtained on an ASML PAS5500/1100 ArF scanner working with a 0.75NA projection lens.
This study will lead to an understanding of the impact of possible limitations of the current quartz etching processes on the imaging performance of alternating phase-shift masks for 65nm half-pitch.