In-situ real time studies of nickel silicide formation
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It is the objective of this research to follow the formation of NiSi in real time, in order to gain a basic understanding of the phase transformations, and to develop analytical procedures to follow these processes in real time using non-destructive, in-situ techniques. Several nickel silicide phases (Ni 2Si, NiSi, NiSi 2), with different properties, form in various temperature ranges below 1000 0 C. In order to study the NiSi formation process, we need to be able to form and detect the pure NiSi phase. To this end, we use in-situ real time ellipsometry (RTE) in a rapid thermal processing environment. Since previous studies of the optical properties of nickel silicides6' 7 are limited, we measured the optical properties of the three nickel silicide phases and used these results as a database. We use RTE and spectroscopic ellipsometry (SE), in conjunction with RBS, to identify the phases, and the extent of phase formation for anneals at various temperatures, and times. We have observed that long time anneals can lead to agglomeration of the silicide phases even at low temperatures. EXPERIMENTAL Samples of 25nm nickel films were electron beam deposited on single crystal Si'. These samples were annealed for various times (2, 5, 10, 15 and 20min) in our rapid thermal processing chamber at several temperatures (550, 650 and 750'C). The chamber used for the anneals is a 121 Mat. Res. Soc. Symp. Proc. Vol. 569 ©1999 Materials Research Society
stainless steel, water cooled, turbo pumped system with a base pressure of 10-8 Torr, and is equipped with an in-situ ellipsometer. Sample heating is done with quartz iodine lamps heating the chamber through a fused silica window. The system is not capable of performing stable temperature anneals for less than 30sec due to the time it takes for the temperature to stabilize in the chamber. Therefore, samples annealed for 30sec at temperatures, ranging from 175 - 1000°C, were done in a Heat Pulse 8108 rapid thermal processing system, at STEAG RTP Systems Inc.' Spectroscopic ellipsometry data was taken in the energy range of 1.8 - 4.1 eV using a custom built ex-situ ellipsometer. Real time optical data was taken at 2.06eV during the anneals. AFM was used to image the surface of the samples, before and after the anneals. The samples were analyzed using RBS, equipped with a tandem accelerator, using a 2.3MeV He' ion beam at a 600 tilt angle. Depth profiling and elemental ratios of Ni:Si at varying depths were obtained with RBS. Ellipsometry is a sensitive optical technique that can be used to measure the dielectric function (-) for films and surfaces. The two measurables of ellipsometry are ellipsometric angles
T and A, which are related to E. c is complex where the real part, E,, is related to the refractive index, n, and the imaginary part, E2, is proportional to the absorption coefficient, k. The Bruggeman effective medium approximation (BEMA) was used for ellipsometric modeling which assumes that for a mixed composition film, each component can be represented by its bulk dielectric f
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