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)
August 28, 2002
Time: 3:00-4:00pm
Physics Conference Room (Blk S13 M01-15)

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.