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Phase-shift mask (PSM) technology in combination with 193nm illumination remains a viable option for high
contrast imaging towards 45nm half-pitch applications. The advent of hyper NA (immersion) lithography increases the
imaging sensitivity towards the photomask properties, such as mask-induced polarization. In addition, the use of PSM
technology implies taking into account the inherent photomask topography effects for a balanced through pitch imaging. A
good quartz etch depth control of +/-1o through pitch is required for optimized wafer imaging [1]. Therefore, a new PSM
material stack was proposed based on a transparent etch stop layer (TESL) in order to meet the stringent phase depth
requirements beyond 65nm half-pitch [2]. This extra layer allows over-etching of the quartz, resulting in a good etch depth
linearity and uniformity.
This study examines the manufacturability and printability of TESL-based masks. We examine the effect of an
improved quartz etch depth linearity on the through-pitch process windows for a TESL-based alternating aperture (AA)PSM.
Moreover, due to the different stack of photomask material compared to a classical photomask blank, the impact on
printability is investigated by simulations, AIMS and wafer imaging. The image imbalance compensation by trench biasing
needs to be optimized for through-pitch process windows.
The actual depth and line width of the structures is systematically probed within the photomask field. Based
on photomask metrology data, rigorous electro-magnetic field simulations are compared to wafer prints, obtained on an
ASML XT1250Di ArF immersion scanner working with a 0.85NA projection lens and to AIMS results from Zeiss
AIMS fab 193i.
Furthermore, feature sizes on the order of the lithography wavelength induce photomask polarization effects in the
imaging path [3]. The degree of polarization is compared to the polarization behavior of a conventional PSM.
In summary, this study assesses the capability of TESL PSM towards the 65nm node through-pitch imaging.
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