Effect of Hydrogen on Thin Cu/Ti and Cu Films
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EFFECT OF HYDROGEN ON THIN Cu/Ti AND Cu FILMS CHRISTOPHER A. APBLETT AND PETER J. FICALORA Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180 ABSTRACT Thin films of Cu and Cu on Ti were prepared by sputtering films onto SiO 2 substrates. The films were annealed at temperatures between 350 and 4000C in vacuum and hydrogen ainlbients. The stress was, measured in-situ during the anneals. The stress in vacuum initially was compressive, then became tensile and remained so. The stress in hydrogen started in compression and remained so throughout the anneal. The origin of these stresses is explained as a result of compound formation. In vacuum anneals, TiCu forms during the compressive stage with an activation energy of 1.7 eV, then TiCu3 forms during the tensile stage with an activation energy of 2.5 eV. In hydrogen, a single compound, TiH 2, forms under the Cu with an activation energy of 0.93 eV. Studies on the Cu films indicate that hydrogen reduces the incremental stress formed in the filns during annealing. A possible explanation is presented for the stress change in that. hydrogen has been shown to assist in annealing dislocations. Experimental evidence of an instantaneous change in the stress state upon the introduction of a new ambient is also presented. INTRODUCTION In an attempt to increase the speed of integrated circuitry, the microelectronics industry is attempting to lower the time needed for signal switching by lowering the RC time constant of the interconnect metallization. These attempts are two fold: first, to shrink the interconnect distance that a metallization line must travel, and second, to lower the resistivity of the metal involved. One of the most attractive ways of lowering the interconnect distance is to expand the metallization into the third dimension; the process of so-called multilevel metallization. For the second criterion, the use of copper (bulk resistivity 1.67 jAl-cm) is being considered as a replacement for aluminum (bulk resistivity 2.8 jAQ-cm), which is presently used. [1] Copper suffers from some problems in its use as a multilevel interconnect. [2] It has relatively poor adhesion to Si0 2, and diffuses rather rapidly into Si. Both of these effects may be controlled through the use of a properly selected diffusion barrier. Titanium is an excellent choice for this application, as it adheres very well to both Si0 2 and Cu, and it prevents Cu from reaching the Si. A new problem arises, however, because Ti and Cu will interact. [3] The Cu-Ti phase diagram shows several intermetallic formations at temperatures likely to be encountered during metallization anneals. The stresses generated by these interactions may be prohibitively high, and may raise the resistivity, defeating the use of copper in the first place. Hydrogen has been found to limit the stress generation [4] and the compound formation [5] when present in the annealing ambient. No quantitative analysis of the stress generation in these films, or the effect that hydrogen has on the stress gen
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