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The benefits of an interconnect technology depend on the properties of the metal and the ILD, as we outlined in Chapter 2. However, the practical feasibility of a certain metal/ILD combination ultimately rests to a large extent on the characteristics of the interface between the metal and ILD. Unless this interface is strong and remains stable over time, the reliability of the circuit, and thus its usefulness, may be compromised. As an aside, let us remark that interfaces are not only critical for the interconnect structure but also play a key role in and on the active devices, for example in the first-level metal contact areas to source and drain, in the gate contacts, etc. Consideration of those kinds of interface issues is outside the scope of the present book. The interested reader may find further information in Refs. [7,73,170]. The use of Cu as the metal, either with SiO2 or a low-k dielectric as the ILD, presents a unique challenge primarily because the Cu/ILD interface is in general much less strong than the corresponding Al/ILD interface. In fact, a major reason for the success of Si technology to date has been the stability of the interface in the workhorse metallization scheme of Al and SiO2.
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