Interactions of Cu with PtSi and TiSi 2 with and Without Como Barrier Layers
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Al for interconnects in VLSI applications, as it offers advantages of low electrical resistivity and high resistance to electromigration. Metallurgically Cu is less reactive than Al, while the reactions of Cu with a number of silicides have been reported in the literature [4-6]. An understanding of the reaction is of technological importance as well as scientific interest because the reactions in most of the cases are not thermodynamically favored. We present here some different results from the previous findings on thermal stabilities of Cu on PtSi and TiSi 2 layers. The effectiveness of an amorphous CoMo film as a barrier layer has also been evaluated, and the results are compared to that reported in Al-based metallization. EXPERIMENTAL PROCEDURE Si (100) wafers were degreased with organic solvents and dipped in dilute HF before being loaded into an evaporator at a base pressure of 5 x 10-8 Torr. The evaporator is equipped with dual electron guns and a resistance heater. Thin films of 80-nm thick Pt or 90-nm thick Ti was deposited on Si, and then heated at 450 *C or 700 *C in the evaporation chamber for 30 min to form PtSi or TiSi2 , respectively. After the samples were cooled to room temperature, a 45-nm thick Cu overlayer was deposited on the silicides without breaking the vacuum. This ensures a relatively clean interface between Cu and silicides. For diffusion barrier studies a 50-nm thick CoMo film was interposed between Cu and Pt by co-evaporation. In all the cases the samples were encapsulated with a Si0 2 layer and stored in vacuum to protect Cu against oxidation. The as-prepared samples were then annealed in a quartz-tube vacuum furnace at a pressure of 1 x 10-7 Torr. Rutherford backscattering spectrometry (RBS) and Auger electron spectrometry 385 Mat. Res. Soc. Symp. Proc. Vol. 320. @1994 Materials Research Society
(AES) were employed to monitor the interdiffusion and reaction. Phase formation was identified by glancing angle x-ray diffraction (XRD). RESULTS
cu/PtSi/Si The as-prepared sample consists of a PtSi layer with a thickness of 160 nm between Cu and Si substrate. Figure 1 shows the backscattering spectra of Cu/PtSi/Si samples before and after annealing at 300 "C for various time. Upon annealing the Cu overlayer decreases in width, and some Cu migrates across the PtSi layer to form Cu silicides at the PtSi/Si interface. Although the Pt signal progressively moves towards the sample surface, the PtSi layer retains its stoichiometric composition and original thickness. The reaction proceeds in a layer-by-layer fashion until the entire Cu layer has been consumed in silicide formation. The phase formation was investigated with XRD as a function of annealing time. The asdeposited sample consisting of PtSi and Cu yields a x-ray diffraction pattern shown in Fig. 2(a). Samples annealed at 300 "C for 30 min exhibit a similar diffraction pattern in Fig. 2(b) with only one additional line which can be identified as the Cu3 Si (320) reflection. After annealing for 45 miin the Cu signals disappear, while more
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