|
Physical vapor deposition (PVD) aluminum films present unique challenges when detecting particulate
defects with a Surface Scanning Inspection System (SSIS). Aluminum (Al) films 4500Å thick were
deposited on 300mm particle grade bare Si wafers at two temperatures using a Novellus Systems INOVA®
NExT,.. Film surface roughness and morphology measurements were performed using a Veeco Vx310®
atomic force microscope (AFM). AFM characterization found the high deposition temperature (TD) Al
roughness (Root Mean Square 16.5 nm) to be five-times rougher than the low-TD Al roughness (rms 3.7
nm). High-TD Al had grooves at the grain boundaries that were measured to be 20 to 80 nm deep.
Scanning electron microscopy (SEM) examination, with a Hitachi RS6000 defect review SEM, confirmed
the presence of pronounced grain grooves. SEM images established that the low-TD filmed wafers have
fine grains (0.1 to 0.3 um diameter) and the high-TD film wafers have fifty-times larger equiaxed plateletshape
grains (5 to 15 um diameter).
Calibrated Poly-Styrene Latex (PSL) spheres ranging in size from 90 nm to 1 μm were deposited in circular
patterns on the wafers using an aerosol deposition chamber. PSL sphere depositions at each spot were
controlled to yield 2000 to 5000 counts. A Hitachi LS9100® dark field full wafer SSIS was used to
experimentally determine the relationship of the PSL sphere scattered light intensity with S-polarized light,
a measure of scattering cross-section, with respect to the calibrated PSL sphere diameter. Comparison of
the SSIS scattered light versus PSL spot size calibration curves shows two distinct differences. Scattering
cross-section (intensity) of the PSL spheres increased on the low-TD Al film with smooth surface roughness
and the low-TD Al film defect detection sensitivity was 126 nm compared to 200 nm for the rougher high-
TD Al film. This can be explained by the higher signal to noise attributed to the smooth low-TD Al.
Dark field defect detection on surface scanning inspection systems is used to rapidly measure defectivity
data. The user generates a calibration curve on the SSIS to plot the intensity of the light scattering derived
at each National Institute of Standards and Technology (NIST) certified PSL deposition spot that was
deposited. It is not uncommon for the end user to embark upon the time consuming process of attempting
to "push" the maximal SSIS film specific sensitivity curve beyond the optical performance capability of the
SSIS.
Bidirectional reflectance distribution function (BRDF) light scattering modeling was utilized as a means of
determining the most appropriate polarity prior to the SSIS recipe creation process. The modeling utilized
the Al refractive index (n) and extinction coefficient (k) and the SSIS detector angles and laser wavelength.
The modeling results allowed predetermination of the maximal sensitivity for each different Al thickness
and eliminate unnecessary recipe modification trial-and-error in search of the SSIS maximal sensitivity.
The modeling accurately forecasted the optimal polarization and maximal sensitivity of the SSIS recipe,
which, by avoiding a trial and error approach, can result in a substantial savings in time and resources.
|
