Activation Energy for the C49-TO-C54 Phase Transition of Polycrystalline TiSi 2 Films with under 30nm Thickness
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ACTIVATION ENERGY FOR THE C49-TO-C54 PHASE TRANSITION OF POLYCRYSTALLINE TiSi2 FILMS WITH UNDER 30nm THICKNESS
Y.Matsubara, K.Noguchi and K.Okumura ULSI Device Development Laboratories, NEC Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa 229, Japan
ABSTRACT The C49-to-C54 transition in TiSi2 was investigated by resistance measurement and x-ray diffraction technique. The resistance measurement showed that the C49-to-C54 transition has an activation energy strongly dependent on the titanium thickness. The energy increased with the thinning of the TiSi 2, from 4.6±0.3 eV for TiSi2 formed with 50nm titanium, to 10.5±0.3 eV formed with 20nm titanium. Furthermore, x-ray diffraction result showed that (004)-oriented phase in C54 TiSi2 is responsible for the increase in activation energy. This orientation dependence of the activation energy probably originates from anisotropy in C54 crystal growth during the transition.
INTRODUCTION For very large scale integrated circuit, it is highly desirable to use refractory metal silicides such as TiSi 2, CoSi 2 and MoSi 2 in order to reduce parasitic resistance in the circuit. TiSi2 has the lowest sheet resistance of these, and is therefore most attractive material for sub-half micron devices fabricated with a salicide process[l]. However, there are several restricting factors in the TiSi 2 formation process such as ones related to diffusion area width[2], titanium sputter thickness[3,4,5] and impurity concentration[6,7,8]. These factors change the silicide growth rate and the phase transition temperature. As the device size shrinks, TiSi 2 thickness should also be scaled-down to realize shallower source/drain junctions. Thus, to keep a feasible process window, further understanding is required on the thickness dependence of the TiSi 2 formation. H. J. W. van Houtum et al. recently reported[4,5] that the transition temperature from the metastable C49 phase to the C54 phase increases with decreasing film thickness. However, kinetics observation concerning the thickness effects for the C49-to-C54 phase transition has not been reported yet[9,10,11,12]. In this work, we report the activation energy for the C49-to-C54 phase transition using both resistance measurement and x-ray diffraction(XRD) technique.
EXPERIMENT (100) oriented p-type Si wafers with a resistivity of 130*cm were used. Titanium film was sputtered with 20-50nm thickness in 10nm increments. To form TiSi2, a two step rapid thermal annealing (RTA) process[3] was employed as described below. The rapid thermal annealing was performed in an AG Heat Pulse equipment. The ramp rate was Mat. Res. Soc. Symp. Proc. Vol. 311. 01993 Materials Research Society
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100°C/s on heating. The first annealing was at a temperature of 700'C for 30 seconds. The crystal structure of TiSi 2 examined by XRD, was C49 phase with no observable C54 phase when the titanium thickness was below 50nm. The thickness of C49 structure TiSi2 measured by cross-sectional TEM was 30nm for Ti thickness of 20nm, and 70nm for Ti thickness of 50nm. Unreac
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