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As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever.
To ensure an accurate printing both attenuated and alternating PSMs (Phase Shift Masks) need precise control of phase
as a function of both pitch and target sizes. However critical target CDs fall much below conventional phase metrology
tools capabilities. Interferometer-based phase shift measurements are limited to large CD targets and require custom
designed features in order to function properly, which limits phase measurement. AFM (Atomic Force Microscopy)
methods are able to capture small feature sizes but do not consider any diffraction effects which are caused by the
topography of the features itself when getting close to the used wavelength.
Imaging simulations, both, in a rigorous and a Kirchhoff regime, show the dependency of the phase in the image plane
of a microlithography exposure tool on numerical aperture and pitch due to the loss of phase information in the imaging
pupil. Additionally, for small features the phase is strongly impacted by polarization and 3D mask effects. For these
feature sizes, the image phase does not coincide with the etch depth equivalent phase calculated from the nominal depth
and optical constants of the shifter material. Deviations up to 20° have been observed leading to strong variations in the
imaging quality and process window variations during scanner printing. Considerations of CD variation between 0 and
pi features by simulation show lowest 0/pi CD variation and therefore largest process window if the scanner relevant
phase is at 180°. The simulation results illustrate the importance to measure the scanner relevant phase, effective in the
image plane of the scanner.
Consequently Zeiss, in collaboration with Intel, has developed a laterally resolving Phase Metrology Tool - Phame® -
for in-die phase measurements. The optical metrology tool is able to perform in-die phase measurement on alternating
PSM, attenuated PSM and CPL masks down to 120nm half pitch at mask. On-axis measurement results have already
been published.
In this paper we elaborate on off-axis phase measurement theory and procedure. Furthermore we present first off-axis
measurement results over varying features sizes using different illumination conditions.
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