A review of the sputtered film stress literature shows that the intrinsic stress can be tensile or compressive depending on the energetics of the deposition process. Modeling studies of film growth and extensive experimental evidence show a direct link between the energetics of the deposition process and film microstructure, which in turn determines the nature and magnitude of the stress. The fundamental quantities are the particle flux and energy striking the condensing film, which are a function of many process parameters such as pressure (discharge voltage), target/sputtering gas mass ratio, cathode shape, bias voltage, and substrate orientation. Tensile stress is generally observed in zone 1-type, porous films and is explained in terms of the grain boundary relaxation model, whereas compressive stress, observed in zone T-type, dense films, is interpreted in terms of the atomic peening mechanism. Modeling of the atomic peening mechanism and experimental data indicate that the normalized moment...

Intrinsic stress in sputter-deposited thin films

The spatial distribution of sputter-etch effects is analysed theoretically under the assumption of random slowing-down of the bombarding ions. When a surface is bombarded at locally oblique incidence the most pronounced sputtering effect is likely to be observed not at the very point of impact, but further “downstream”. This effect may cause a significant reduction of the local sputtering yield on top of a spike or a ridge, and an increase on the bottom of a groove or a crater. As a consequence, small irregularities on a relatively smooth surface may be enhanced during bombardment. It is concluded that a microscopically flat surface is unstable under high-dose ion bombardment unless atom migration acts as a dominating smoothing effect. Conversely, sharp cones appear to be surprisingly stable under bombardment. Even on a clean surface, cones erode more slowly than a plane surface, provided that their dimensions are of the order of, or less than, the penetration depth of the ions.

A mechanism of surface micro-roughening by ion bombardment

Sputtering by ion bombardment theoretical concepts

Surface science aspects of etching reactions

The intrinsic stresses of Al, Ti, Fe, Ta, Mo, W, Ge, Si, AlN, TiN, and Si3N4 films prepared by ion beam sputtering were investigated at low Td/Tm values. The intrinsic stress is compressive and its origin is explained in terms of the ion peening model. Knock‐on linear cascade theory of forward sputtering is applied to derive a simple scaling law with the film’s physical properties. The results show that the stress is directly proportional to the elastic energy/mole, given by the quantity Q=EM/(1−ν)D, where E is Young’s modulus, M the atomic mass, D the density, and ν Poisson’s ratio. Stress data taken from the literature for a variety of materials deposited by low‐pressure magnetron sputtering, and rf and ion beam sputtering also fit the correlation with Q. Furthermore, the model predicts a square‐root dependence on the incident ion energy, suggesting that the stress is momentum rather than energy driven.

An intrinsic stress scaling law for polycrystalline thin films prepared by ion beam sputtering

Measurements of the reflected fractions of sodium, potassium, and krypton ions impinging on polycrystalline silver and gold and amorphous germanium targets have been made at medium keV energies. The reflected fraction depends strongly on mass ratio and weakly on energy. Previous calculations of ion reflection (based on random slowing-down by elastic collisions in an infinite medium) have been extended. Corrections to the calculated values due to electronic stopping and surface effects have been estimated. Agreement between measurements and calculations is satisfactory.

J. B⊘Sttiger

Papers

On the reflection coefficient of keV heavy-ion beams from solid targets