Barrier Metal Ions Drift into Porous Low k Dielectrics under Bias-Temperature stress
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1249-F05-09
Barrier Metal Ions Drift into Porous Low k Dielectrics under Bias-Temperature stress Ming He,a Ya Ou,a Pei-I Wang,a Lakshmanan.H.Vanamurthy,b Hassaram Bakhru,b and TohMing Lua a Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180, U.S.A. b College of Nanoscale Science and Engineering, University at Albany, Albany, NY 12203, U.S.A. ABSTRACT Ta family has been used as barrier to prevent Cu diffusion into interlayer dielectric in IC applications. Recent experiments demonstrated a more severe flatband voltage shift (∆VFB) occurred for Ta/porous low k dielectrics/Si capacitors compared to that of Cu/porous low k dielectrics/Si capacitors after a moderate bias temperature stress (BTS). The flatband voltage shift under BTS was interpreted as the penetration of Ta ions into porous low k dielectrics. However, this interpretation has been under debate. In this paper, by using Secondary Ion Mass Spectrometry (SIMS) backside sputter depth profile technique, we report a direct evidence of Ta ions inside porous methyl silsesquioxane (MSQ) in a Ta/MSQ/Si structure after BTS. INTRODUCTION Tantalum and tantalum based metal have been successfully used in the integrated circuits (IC) as diffusion barrier to block Cu penetration into interlayer dielectric (ILD) in the Cu/SiO2 integration scheme.1,2,3 When this back end of line (BEOL) structure moves to Cu/porous low k dielectric, the barrier layer is even more critical4 and Ta family is still widely perceived as a good candidate with stable interface on porous low k dielectrics.5 But this assumption was challenged by recent experiments on the Capacitance-Voltage (C-V) measurement on Ta/low k dielectrics/Si MIS capacitor under BTS.6-9 Typically a large negative flatband voltage shift (∆VFB) was observed after moderate BTS, implying positive ions penetration into dielectrics. We have suspected these positive charges to be Ta ions.6,7 However, this interpretation has been a matter of controversy, and alternative explanations such as protons drift (under the influence of moisture) and dielectric polarization have both been proposed.8,9,10 Also, it has been shown that the drifted ions would act as electron traps, which can enhance the leakage current and cause reliability problems.11 Therefore, it is important to identify the chemical nature of the ions by a direct elemental measurement. In this paper, a secondary ion mass spectrometry (SIMS) backside sputter depth profile technique was chosen to determine the possibility of Ta ions drift inside the dielectric of the MIS capacitors driven by BTS. Depth profiling from backside of the Si substrate eliminates the need and the complexity of selectively etching away Ta on the porous low k dielectrics. The depth profiling from backside can also reduce the ion beam mixing effect during sputtering,12 which would otherwise generate misinterpretation.10 EXPERIMENT AND RESULTS The porous low k dielectric is methyl silsesquixane (MSQ) provided by Freescale. 300 nm thick porous MSQ with dielectric const
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