Multilevel metallization is becoming an increasingly interesting area of research as circuit fabrication technologies are scaled down to deep submicron dimensions. The mainstream of today's interconnection technology is AlSiCu metallization. Al-based solutions, however, seem to be limited in resistivity as well as in electromigration performance. Because of its low electrical resistivity and its resistance to electromigration, copper is considered to be a promising new solution for on-chip interconnections. The performance of copper would allow its use in wiring with very small linewidths, as required for ULSI circuits. Before considering copper metallization in ULSI processing, however, major problems still have to be overcome.

Taking into account these considerations, a European project is now devoted to the evaluation of copper-based metallization for ULSI applications. The project, Copper Interconnection (COIN), associates the efforts of seven European partners.

The objectives of COIN are to evaluate and develop a set of optimized solutions which demonstrate to our industrial partners the interest of using on-chip copper metallization. High deposition rates, low temperature processing, thermal stability, reliability, and low cost are the main goals.

The COIN project is devoted to an exploratory study concerning the major issues to be addressed in order to reach the above targets. The choice is to limit, for as long as possible, the modifications of existing multilevel metallization (MLM) structures to a simple replacement of the aluminum by copper interconnection levels. The refractory metal layers in MLM structures (TiSi2, Ti, TiN, W) will be maintained in the copper metallization scheme as will the use of SiO2 as the dielectric material. Another consideration in our approach is the possibility of achieving this substitution with minimum modification to existing fabrication lines. Thus, processes for copper deposition and copper patterning will be developed using existing machines and machine concepts for as long as possible.