A critical assessment is given of the phase equilibria of the AI-Cu system. In the first section, the stable equilibrium diagram is constructed. In the second section, metastable and constrained equilibria are considered. These include metastable solvus curves of the AI-rich side of the diagram; on the Cu-rich side, transformations involving the metastable bcc phase include the order-disorder transition, metastable invariant reactions, and martensitic and massive transformation. In the third section, crystal structure and lattice-parameter data are reviewed. Finally, new calculations of selected phase equilibria are presented which are based on computer optimization of phase-diagram and thermochemical data.

The aluminium-copper system

We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm2 (tensile) to −2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10 °C/min up to 850 °C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta‐to‐alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta‐to‐alpha phase transition occurred varied with deposition pressure and substrate biasing. Incomplete compressive stress relaxation at high temperatures was observed if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800 °C due to substrate biasing during the deposition. We conclude that the main stress relief mechanism for tantalum films with intrinsic compressive stresses to completely relax their stress is the beta‐to‐alpha phase transition, while for intrinsically tensile films, this transformation has a much smaller effect on the stress.

The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

Mechanical stress in interconnection is a problem of growing importance in VLSI devices. Open circuits due to metal cracking and voiding and short circuits due to hillocks are stress-related phenomena. The origins of this stress are discussed including intrinsic stresses from the synthesis of the films and thermally induced stresses. A measurement technique based on the determination of wafer curvature with a laser scanning device is utilized to directly measure the film stress in situ as a function of temperature during thermal cycling. The changes in stress observed during thermal cycles are interpreted quantitatively and mechanisms that lead to plastic deformation and their relationship to hillocks are discussed. In the stress vs. temperature measurements, several regions have been identified including elastic and plastic behavior both under compression and tension, the yield strength, recrystallization, gain growth, hardening, and solid-state reactions. The effects of deposition conditions on these regions are also examined. >

Mechanical stress as a function of temperature in aluminum films

N.M. Mayer

Papers

Structural changes of evaporated tantalum during film growth

Crystallization of amorphous Al-Cu films

The growth of unusually large grains in thin Al-Cu films