The investigation of copper for use as an interconnection metal in the ultra large-scale integration (ULSI) era of silicon integrated circuits has accelerated in the past several years. The obvious advantages for using copper to replace currently used Al are related to its lower resistivity (1.7 pR-cm vs. 2.7 @-cm for Al) and its higher electromigration resistance (several orders of magnitude higher compared with Al). The goal of this review is to examine the properties of copper and its applicability as the interconnection metal. A comparison of electromigration behavior of various possible interconnection metal in standard "bulk" state is made. This is followed by a review of the calculations made comparing (a) the RC (resistance x capacitance) time constants of various material systems and (b) the joule heating of the interconnection materials. A comparative study of various metal systems for the application as the interconnect metal is then made. These discussions will clearly establish the superiority of copper over other metals despite certain limitations of copper. We then review the properties, both physical and chemical, and materials science of copper. The concept of using alloys of copper with a minimal sacrifice on resistivity to gain reliability is also discussed. This is followed by the review of the deposition, pattern definition and etching. passivation, need of the diffusion barrier (DB) and adhesion promoter (AP), planarization and dual damascene process using chemical mechanical planarization, and reliability. This review shows that copper will satisfy the needs of the future integrated circuits and provide high performance and reliability as long as we provide an appropriate barrier to diffusion in the underlying devices and the dielectric.

The properties of thin films of Cu with 1 at. % Al are explored. As-deposited films of Cu(1 at. % Al) oxidize orders of magnitude more slowly than do those of pure Cu. After Cu(1 at. % Al) films are annealed in Ar at 400 °C for 30 min, very thin protective layers of aluminum oxide form on the surface. These thin oxide layers stop further oxidation of the copper. Cu(1 at. % Al) films also adhere better to SiO2 than do films of pure copper. Unlike pure Cu, films of Cu(1 at. % Al) remain microscopically smooth after anneals at temperatures up to 700 °C. In addition, Cu(1 at. % Al) films show no diffusion of Cu (as measured by Rutherford backscattering spectroscopy) into SiO2 at temperatures up to 700 °C. The addition of Al to Cu does increase its resistivity by about 2 μΩ cm per 1 at. % Al, but a possible procedure to avoid this problem is proposed.

No abstract

The properties of thin films of Cu-doped with different percentages of Mg were investigated. It was found that as-deposited films of Cu (2 at. % Mg) oxidize orders of magnitude more slowly than do those of pure Cu. More importantly, when Cu(2 at. % Mg) films are annealed in Ar at 400 °C for 30 min, a thin protective layer of magnesium oxide forms on the surface and completely stops further oxidation. This annealing step also reduces the resistivity of films to the value essentially the same as that of pure sputtered copper films. Films of Cu (2 at. % Mg) also adhere to SiO2 much better than do films of pure copper. Furthermore, annealing studies show that this material remains microscopically smooth and shows no diffusion into SiO2 at temperatures up to 700 °C.

The formation of copper silicide by reaction of silane with sputtered copper films has been observed at temperatures as low as 300 °C. The growth kinetics have been monitored by both sheet resistance and x-ray diffraction techniques. Cu5Si is the first phase to form followed next by Cu3Si, coincident with the loss of the original copper layer. The silicide layer provides significant oxidation protection for the underlying copper up to 550 °C in air.