Thermal Stability Study on Cu/Tin/TiSi 2 /Si
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ABSTRACT We have investigated TiN/TiSi 2 films as diffusion barriers between copper and silicon. The TiN/TiSi 2 films were formed by annealing Ti/Si via rapid thermal process at a temperature of 850'C for 30 seconds in N2 ambient. The contact resistance and leakage current of the Cu/TiN/TiSi 2/n+-p Si contact system were measured after sintering at 350-600°C for 30 minutes in an N2 ambient. We found an abrupt increase of contact resistance occurred at a sintering temperature of 6000C. From the leakage current of n'-p diode, we observed that the thermally stable temperature was about 475°C. Meanwhile, XTEM photographs showed small Cu 3Si crystallites with size of about 0.25 uim precipitated in the n'Si substrate at a sintering temperature 5000C. The formation of Cu 3Si increases the occupied volume, generates the gap between TiSi 2 and n'Si, and gradually increases the specific contact resistance. SIMS profiles also showed that a certain amount of Cu atoms diffused into the n÷-p Si junction and that Si atoms existed in the Cu film. The Cu 3Si formation across the n+-p junction can explain the abrupt increase in the leakage current measurement at 500'C. So the failure mechanisms for contact resistance and leakage current are different.
INTRODUCTION With the continuing trend towards reduction of lateral dimensions in integrated circuit technology the pressure to replace Al as the element for metallization has increased. Cu, with its low electrical resistivity (1.7 jfl-cm for Cu versus 2.5 jtfQ-cm for Al) and superior resistance to void formation and electromigration, is an obvious candidate in this regard. However, one of the major setbacks for Cu is its high diffusivity and the introduction of deep levels in silicon, resulting in deterioration of devices even at low temperatures [1]. Hence a diffusion barrier is necessary between the Cu metallization and Si substrate. The reaction between Cu and Si or silicide with TiN barrier layers has been reported by several workers [2-5]. Reactively sputtered TiN as a diffusion barrier between Cu and Si has been studied by Wang et al. [2]. The leakage current densities of the Cu/TiNx/n+-p Si contact system increased to the order of uA/cm 2 up to 650-850'C for 30 seconds in N 2 ambient. A 30 nm layer of TiNt simultaneously grown with TiSi 2 by rapid thermal annealing was proven to be an effective diffusion barrier between Cu and TiSi2 up to 5000 C from Rutherford backscattering spectrometry [3]. Such reports have not contained details on the degradation study from the electrical requirement of VLSI. In this paper, the results of contact resistance and leakage 391 Mat. Res. Soc. Symp. Proc. Vol. 320. `ý,1994 Materials Research Society
current measurements of the Cu/TiN/TiSi 2/n+-p Si contact system are reported as well as scanning electron microscopy (SEM), cross section transmission electron microscopy (XTEM) and secondary-ion mass spectrometry (SIMS) measurements. The failure mechanisms on contact resistance and leakage current are also studied. EXPERIMENT (100) p-type Si waf
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