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Precise and accurate feature positioning in SEMs is becoming more critical. Moving the stage to a predetermined location
must be done with accuracy and precision that put the feature ofinterest in the field ofview at a magnification high enough
to detect orrecognize the same feature. Ifthis is notdone, some sort ofsearch, either automatic ormanual must be performed.
This may not only be bothersome, but detrimental to inspection or measurement throughput performance. Ultra precise
stages - for example, those using laser interferometers or linear encoders - are capable of positioning precisions, if not
accuracies, to 0. 1 micron. In both optical and SEM systems where inspection is normal to the plane ofthe waler(cailed zero
tilt), precise locating of features is possible without serious attention being paid to the bow or warp of a wafer. From the
SEMI Standards Manuals, it is seen that a 200 mm wafer may have up to 65 microns of bow. In optical lithography tools
and optical inspection or measurement systems, a vacuum chuck may alter or reduce the bow. However, in the vacuum
chamber of the SEM this technique does not work. The bow or warp remains. The problem occurs in going to a particular
numerical address whenthe waferis tilted, ifthat numerical address was determined at some different tilt -themost probable,
of course, being zero iilt. Tilting of the wafer will cause the initially observed feature to move through an arc of "unknown"
extent (unknown because it is a function of the bow and the bow is not known at that point). A 60 degree tilt of awafer
with 40 microns of bow can cause about 35 microns oflaten.l displacement of a feature from where it would be expected
for a wafer with no bow. The effect of this displacement on detectability is discussed. Actual displacement measurements
on a 125 mm wafer ait plotted. These plots are compared with those derived from measurements made by optical and SEM
systems specially set up to measure bow magnitudes. Bow-magnitude data obtained from a separate bow-measuring
insirument or from data taken in-situ in the SEM itself can be used to correct the positioning error that would occur with
that particular wafer. Bow related effects may be a practical limitation on the open-loop positional precision capabily of the
SEM at non-zero tilts.
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