Ultrashallow SIMS Study of Implanted Dopants in NiSi/Si(100)
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Ultrashallow SIMS Study of Implanted Dopants in NiSi/Si(100) Nikolai L. Yakovlev, Andrew S.W. Wong, Doreen M.Y. Lai and Dongzhi Chi Institute of Materials Research & Engineering, 3 Research Link, 117602 Singapore. ABSTRACT Ultrashallow doping of silicon wafers with boron was done by implantation of B+ or BF2+. Nickel silicide on them was formed by annealing of thin Ni layer. As it was revealed by secondary ion mass spectrometry, after the growth of NiSi, boron remains in Si just under the silicide layer, but fluorine accumulates in the NiSi layer. This accumulation suppresses agglomeration of the silicide. INTRODUCTION Nickel monosilicide (NiSi) is an attractive material for local contacts in silicon based electronic devices due to its low resistivity which is maintained even for line width down to 100 nm [1] and due to lower consumption of silicon and smoother silicide/Si interface with respect to TiSi2 and CoSi2. However in contrast to those, NiSi has lower thermal stability [2]. Above 700°C, it agglomerates, i.e. the silicide layer is not continuous. At higher temperatures, it transforms into higher resistivity disilicide (NiSi2) [3]. In this work, we have studied nickel silicide layers made by annealing of metal Ni layers on boron implanted substrates. Implantation of dopants gives steep decay of their concentration at the depth of a dozen of nanometres. An important point is behaviour of the dopants during the annealing and consumption of Si by the growing silicide. Distributions of elements through the depth of the films were measured in this work using time-of-flight secondary ion mass spectrometry (ToF SIMS), lateral structure of the layers was observed using scanning electron microscopy (SEM). EXPERIMENTAL The Si(100) substrates were n-type doped with phosphorus at the concentration 2⋅1015/cm3. Then they were submitted to implantation of either B+ ions, or BF2+ ions with the energy 20 keV at the dose 5⋅1015 ions/cm2. The substrates were covered with 30 nm Ni layer by magnetron sputtering in argon. In order to obtain nickel silicide, these structures were annealed at different temperatures in the range of 400°C to 800°C, in nitrogen atmosphere during 60 seconds. Integrity or agglomeration of NiSi films was observed using 7 keV electrons in a field emission gun SEM built by Jeol. The mode of detection was back-scattered electrons, so the bright areas correspond to heavier material NiSi, darker areas are Si, fig 1. Depth profiles of distribution of components were measured using ToF SIMS instrument built by ION-TOF. The mass resolution of the ToF detector was 8000 which allows distinguishing different species within one integer mass, e.g. 60Ni and 30Si2. The SIMS was operated in dual beam mode: 3 keV Cs+ ion gun was used for sputtering and 25 keV Ga+ ion gun was used for analysis. Negative secondary ions were detected; depth
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profiles of F-, 30Si- and 60Ni- are presented in the figures. Among boron containing ions, SiB- clusters had the highest intensity; hence their depth profiles are plotted
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