Quantitative analysis of strengthening mechanisms in thin Cu films: Effects of film thickness, grain size, and passivation

Abstract

Thermal stresses in thin Cu films on silicon substrates were examined as a function of film thickness and presence of a silicon nitride passivation layer. At room temperature, tensile stresses increased with decreasing film thickness in qualitative agreement with a dislocation constraint model. However, in order to predict the stress levels, grain-size strengthening, which is shown to follow a Hall–Petch relation, must be superimposed. An alternative explanation is strain-hardening due to the increase in dislocation density, which was measured by x-ray diffraction. At 600 °C, the passivation increases the stress by an order of magnitude; this leads to a substantially different shape of the stress-temperature curves, which now resemble those of aluminum with only a native oxide layer. The effect of passivation is shown to be very sensitive to the deposition and test conditions.

Dislocation Plasticity in Thin Metal Films

“…In this case, stress can be imposed on the film by changing the temperature of the system. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The constraint of the substrate implies that the generated thermal strain is essentially offset by the generation of some combination of elastic and plastic strain in the film. If is the equi-biaxial stress in the film, and p is the equi-biaxial plastic strain, then rates of change are related by /M ϩ p ϩ (␣ film Ϫ ␣ sub )Ṫ 0,…”

“…8,18 Therefore, the rate of temperature change has an influence on the stress-temperature dependence observed, and the time-dependence of the plastic deformation needs also to be taken into account for a complete description. Furthermore, it has been discussed that the stress-temperature evolution is also influenced by strain-hardening or a Bauschinger effect 14,20,21 as the film is plastically strained, despite the fact that the imposed plastic strains of about 0.5% are rather small.…”

X-Ray Diffraction Studies of Forward and Reverse Plastic Flow in Nanoscale Layers During Thermal Cycling

“…An Anton Paar DHS 900 heated stage regulated the sample temperature under vacuum, thereby minimizing oxidation and the associated effects on stress-temperature behavior. [23] No evidence of oxidation was observed in post-heating diffraction patterns.…”

Probing thickness-dependent dislocation storage in freestanding Cu films using residual electrical resistivity

“…The structural miniaturization, driven primarily by applications involving microelectronics and microelectromechanical systems, has led many experimental [1][2][3][4][5][6][7] and computational 8,9 studies to focus on the physical mechanisms responsible for size effects in metallic thin films. Two contrasting size-dependent responses have been reported in the literature for the tensile behavior of freestanding films.…”