Copper metallization for ULSL and beyond

Abstract

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.

Increasing the mean grain size in copper films and features

“…As shown, for FCC materials like Cu the strain energy density has a minimum value for grains with {100} texture and a maximum for grains with {111} texture. Moreover, the figure also shows that the strain energy density variation is low as the orientation is varied from (111) to (110), but that it decreases sharply as the orientation is varied from (111) to (100). Therefore, in biaxially strained films, the strain energy differences of grains with different orientations may contribute to the driving force for secondary grain growth, thereby favouring grains with orientations that minimize the strain energy density.…”

“…Suggested driving forces for the grain growth include the stress, which is accumulated during the plating process, dislocation loops, and interface and grain boundary [95]. The activation energy of secondary grain growth in electroplated copper films has been found to be in the range of 0.8-1.1eV [52,90,[98][99], which is quite close to grain boundary diffusion energy (~0.9eV) and seems to be independent of thickness [100]. This suggest an average atomic diffusion behaviour that is similar to grain boundary self-diffusion.…”

“…When strain energy density minimization dominates over surface and interface energy minimization, grain growth can be a strain relief mechanism. The use of these texture maps to explain the textural evolution of the studied copper films in this work is justified, because the two main observed orientations during textural evolution were (111) and (100). Dependent on the film thickness h, a threshold strain ε T can be calculated from equation 2.19, using ∆E ε, (111) (100) orientation always has the minimum anisotropic energy.…”

High Conductivity and Adhesion of Cu-Cr-Zr Alloy for TFT Gate Electrode

“…As the large-size and high-resolution thin-film transistor liquid crystal display (TFT-LCD) with high operating frequency becomes more widely used, the copper gate electrode is a promising candidate to substitute for aluminum, owing to its high conductivity to reduce the resistance-capacitance (RC) propagation delay as well as its higher stress-migration and electromigration resistance [1]. However, before it can be used for this purpose, copper has a serious drawback to be addressed, as it is well-known to exhibit poor adhesion to substrates such as SiO 2 or glass [2][3][4].…”

X-ray characterization of oriented β-tantalum films

“…However, an underlying layer is necessary for the adhesion of Cu to dielectrics, and more importantly, as a diffusion barrier against the migration of copper ions through the dielectric layer [1]. Tantalum (Ta) metal and Ta-N intermetallics are being used for Cu adhesion and as diffusion barriers.…”