Mask metrology has long been separated into critical dimension (CD) vs. pattern placement (Registration) in terms of
both the parametric definitions as well as measurement techniques applied. The combined effect of measured CD and
placement errors on mask-to-mask overlay (OL) is hard to model let alone calculate in definitive terms. As device size
continues to shrink, novel lithography solutions being considered for 45nm technology node and beyond such as double
exposure and patterning techniques are projected to tighten the overlay requirement much faster than originally
anticipated.
Electron optics is generally the preferred solution for small feature size in-die sampling by virtue of its high image
resolution, measurement precision, low cross-field distortion and absence of tool induced shift. In this paper we propose
to examine and identify the key elements of a new approach in applying electron optics to a mask metrology system that
combines CD and pattern placement. We will then present the results from our experiments with a prototype wide field
scanning electron microscope (WFSEM) using reticle with optical proximity correction (OPC) features.
As lithography mask process moves toward 45nm and 32nm node, phase control is becoming more important than ever.
Both attenuated and alternating PSMs (Phase Shift Masks) need precise control of phase as a function of both pitch and
target sizes. However conventional interferometer-based phase shift measurements are limited to large CD targets and
requires custom designed target in order to function properly, which limits phase measurement.
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, polarization, and on the so-called balancing of the mask for
features close to the size of the used wavelength. 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.
Additionally, for PSMs generating phase jumps deviating from 180°, the resulting phase in the image plane of a
microlithography exposure tool depends on the transmitted diffraction orders through the aperture of the imaging
system.
Consequently Zeiss, in collaboration with Intel, has started the development of a laterally resolving Phase Metrology
Tool (Phame) for in-die phase measurements.
In this paper we present this optical metrology tool capable of phase measurement on individual line/spaces down to
120nm half pitch. Alternating PSM, Attenuated PSM, Cr-less masks were measured on various target sizes and
simulations were performed to further demonstrate the capability and implication of this new method to measure the
scanner relevant phase in-die, taking into account NA, polarization, and rigorous effects.
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