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SSL Seminar Series 2002 No.6
Title: Near Surface and Interfacial B-profiling: An (Un)solvable
Problem in SIMS.
Speaker: Prof Wilfried Vandervorst (Materials and
Components Analysis Group, IMEC)
Date: August 28, 2002
Time: 3:00-4:00pm
Venue: Physics Conference Room (Blk S13 M01-15)
Abstract:
Despite the general (successful) use of SIMS in the semiconductor
industry for dopant profiling, several problems continuously plague
the applicability of SIMS in deep submicron technology. When studying
for instance shallow junction formation, one routinely is faced
with the need to quantify the dopant profile in the first nm quite
accurately. The interpretation of the near-surface profiles (first
few nm) is complex as sputter yield transients and ionization yield
changes impact the reliability of the SIMS-profiles. Moreover routinely
these layers are capped with (thin) oxides and, after annealing,
quite often interfacial segregation of the dopants is observed/suspected.
Quantitative analysis of these kind of structures then needs not
only a control of the matrix effects but at the same time a very
high depth resolution. The issue of near surface quantification
relates to the problem of knowing the amount of oxygen at the surface
as this sets the instantaneous sputter yield and ionization degree.
Based on extensive studies using XPS and LEIS, we have established
a detailed correlation between the surface oxygen concentration
and ionization probability. At the same time we have developed a
MISR-methodology, based on ion signal ratios, that allows to determine
in-situ the ionization degree. Hence using the MISR as function
of fluence and the correlation to oxygen concentration and ionization,
it is possible to correct the matrix signal for the changes in ionization
yield. When applied to a B-profile, the same MISR-technique can
be used to correct the B-intensity as well. The correction for the
sputter yield variation establishes a correct thickness of the layer
as can be deduced from the comparison with the profile from a capped
structure. All yield variations have been removed as well with a
small (spurious) peak remaining at the outermost surface. Obviously
a strongly enhanced emission is present initially. The latter leads
to a double peak structure in the near B-profile with the second
peak related to backward segregation. Complementary high resolution
(<0.2 nm depth resolution) ERD-measurements confirm that the surface
B-spike is solely a SIMS-artifact and not present in the sample.
Extensive studies with various energies, incidence angle always
show a similar SIMS profile i.e. B peak at the surface of the oxide
layer. Only in case the oxide is capped with an additional Si-layer
an interfacial peak can be seen. The surface peak must be related
with a rapid transport of the B through the oxide within the collision
cascade. The latter has been quantified by using samples with different
oxide thickness (up to 5 nm). In all cases this transport can be
seen, suggesting an extreme mobility of the B under ion irradiation.
About the Speaker:
After obtaining his Ph.D. in applied science in 1983 from the Katholieke
Universiteit Leuven (University of Leuven), Wilfried Vandervorst
has spent his entire career doing fundamental research on the use
and improvement of material characterization methods for advanced
semiconductor processing.He has contributed significantly in the
fields of dopant profiling,with the prime emphasis on the various
fundamental aspects of Secondary Ion Mass Spectrometry (SIMS),and
on carrier profiling using spreading-resistance probing and derived
techniques. He also pioneered device characterization based on scanning-probe
microscopy using the patented concepts of scanning spreading resistance
microscopy and nanopotentiometry. Wilfried Vandervorst is recognized
worldwide as a leading authority in various aspects of ULSI characterization.He
has more than 400 publications in internationally reviewed journals
and conference proceedings,from which 45 were invited papers. He
also holds 12 patents. In 1995, Wilfried Vandervorst was awarded
the IMEC prize for outstanding achievements for building up a materials
characterization laboratory within IMEC.Through his work, IMEC has
become a center of excellence in materials and process characterization.
Currently, Wilfried Vandervorst is the Leader of the Materials and
Components Analyis group at IMEC and the visiting professor at the
Katholieke Universiteit Leuven. In 2002, he was appointed as IMEC
Fellow.
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