Refractory Metal Nitride Encapsulation for Copper Wiring
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temperatures below 450°C. Cu can diffuse through CoSi2 to form Cu3Si at the CoSi2/ Si interface while the integrity of the CoSi2 layer is maintained. In the Cu/CrSi2/Si
Cu Interactions
T°C Cu
Si
\c ZrO 2 /Zr 5 Si 4
900°C
Hf
Ta
Nb
Mo
NbO/Nb 5 Si 3
W
After heating the Cu-Nb bilayer structure, the Nb atoms diffuse to the surface of the Cu at 500°C. The interfacial reaction temperatures for the Cu-Mo and Cu-Ta bilayer structures are higher than 750°C during annealing in vacuum. Annealing Cu-Ti and Cu-Zr bilayers in vacuum results in interfacial reaction between Cu and Ti at 350°C. The first phase identified by TEM is CuTi. At 400°C, the Cu 3 Ti compound is observed to coexist with Ti and CuTi. In the Cu-Zr system, interfacial reaction between Cu and Zr occurs at 400°C. In addition to CuZr compound formation, amorphization occurs during the reaction. Cu does not react with W until 6()0°C according to the RBS. Cross-sectional TEM shows a nonuniform interface and the local reacted region has been identified as the Cu-W solid solution. The interfacial reaction temperature is greatly influenced by the purity of the most refractory metals. In general, refractory metals have a strong affinity to oxygen. A high temperature is generally needed to cause the reaction of Cu/refractory metal containing oxygen. The reactions for various Cu/refractory metal bilayer systems are summarized in Table I, where the reaction temperatures and observed first phase in these systems are indicated. Refractory metals in Group IV A tend to form compounds with copper at relatively low temperatures. Refractory metals in Group V A and VI A do not form compounds with copper and have very low solubility in copper. To avoid the residual refractory metal in copper lines in the structures during heat treatment, refractory metals in Group V A or VI A are favorable as choices for refractory metal nitride-encapsulated copper structures. It is worthwhile to point out that most pure metals in Group V A and VIA are not good diffusion barriers when they are in contact with copper and silicon. For instance, compared with the reaction temperature (700°C) in the Cu/Cr bilayer structure, the failure temperature in the Cu/Cr/Si structure is around 400°C.1 The low failure temperature is due mainly to copper diffusing through chromium to react with the silicon, which can act as a strong sink to the formation of Cu3Si. On the other hand, the thickness of the chromium layer and the oxygen impurity level inside chromium are two factors also important to the effectiveness of the barrier properties.
Refractory Metal/SiO2 Reactions Wang et al. systematically studied the interfacial reaction kinetics in the refractory metal/SiO2 systems.2" These refractory metals are divided into two groups, in terms of phase formation and reaction tempera-
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Refractory Metal Nitride Encapsulation for Copper Wiring
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Cu/Refractory Metal/SiO2 Reactions in an NH 3 Ambient
with SiO2 nor form compounds with copper during annealing, the formation of a nitride layer is determine
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