A new small-spot, film-thickness measuring system operating near Brewster's angle, shows 0.1 Angstrom or 0.1% precision on SiO2 films in the 10-400 Angstrom range. The illuminating beam is focussed to a e-2 intensity diameter of 8 microns. The elliptical spot on the surface is easily placed in the center of test areas, such as a 50 micron square pad. The measurement technique takes into account the non Brewster's angle rays present within the focussed beam and the Gaussian intensity distribution of these rays. The resulting theoretical and experimental reflectance-vs.-film-thickness curves are shown to agree. Precision tests for various film-thicknesses are shown. Measurements are made in 2-3 seconds. Proper focussing and placement of the illuminating beam is achieved by autofocussing of a separate microscope viewing system.
Today, precision of a few nanometers is requiid to measure critical dimensions (CD's). Measurement tools must and are being designed to be more stable over both short and long periods. In addition, the high spatial resolution of a scanning electron microscope (SEM) is making it possible to determine moi about the semiconductor feature itself. This paper reports on new data collection and analysis techniques that yield more meaningful and reliable values for SEM-measured CD's including information about (1) CD variation along the feature, (2) individual edge roughness and (3) variations due to the measuring instniment. At TV rates an electron beam is raster-scanned over a small area of interest containing a pitch or line. A CD value is computed from the video signal associated with each of the raster's horizontal scans across the sample. The average of these separately computed CD values is taken to be the CD of record. This method preserves information about "apparent" edge roughness and orientation. However, the contribution of "real" edge roughness is determined only through further analysis. A correlation program was created to compare edge and CD data sets. Plots of various correlations showed that contributions to the standard deviation of edge and CD data sets were quantifiable. For instance, a correlation of a CD data set with itself, but with data acquired at a different time, generated a number that could be associated with the contribution of random video noise. A correlation of a CD data set with itself, but with its position shifted in the data collection window, differentiated contributions of the real feature roughness and other data set variations not assodated with the sample. The correlation piots also revealed information about the frequency of these various contributions.
KEYWORDS: Semiconducting wafers, Scanning electron microscopy, Inspection, Electron beams, Integrated circuits, Metrology, Process control, Wafer-level optics, Electron microscopes, Optical testing
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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