J. M. E. Harper

Theory of ripple topography induced by ion bombardment

When an amorphous solid is etched by an off‐normal incidence ion beam, a ripple topography often results. A theory explaining the origin of these waves is presented. For incidence angles close to the normal, we find that the ripple wave vector is parallel to the surface component of the beam direction, provided that longitudinal straggling of the beam is not too large. The ripple orientation is rotated by 90° when the beam is close to grazing incidence. The wavelength given by the theory varies as λ∼( f T)−1/2 exp(−ΔE/2kBT) for high temperatures T and low fluxes f, where ΔE is the activation energy for surface self‐diffusion. The predicted magnitude of the wavelength is in reasonable accord with experiments in this regime.

Tantalum as a diffusion barrier between copper and silicon: Failure mechanism and effect of nitrogen additions

The interaction of Cu with Si separated by thin (50 nm) layers of tantalum, Ta2N, and a nitrogen alloy of Ta has been investigated to determine the factors that affect the success of these materials as diffusion barriers to copper. Intermixing in these films was followed as a function of annealing temperature by in situ resistance measurements, Rutherford backscattering spectra, scanning electron microscopy, and cross‐section transmission electron microscopy. Ta prevents Cu‐silicon interaction up to 550 °C for 30 min in flowing purified He. At higher temperatures, copper penetration results in the formation of η‘‐Cu3Si precipitates at the Ta‐Si interface. Local defect sites appear on the surface of the sample in the early stages of this reaction. The Ta subsequently reacts with the substrate at 650 °C to form a planar hexagonal‐TaSi2 layer. Ta silicide formation, which does not occur until 700 °C in a Ta‐Si binary reaction couple, is accelerated by the presence of Cu. Nitrogen‐alloyed Ta is a very similar...

Mechanisms for microstructure evolution in electroplated copper thin films near room temperature

We present a model which accounts for the dramatic evolution in the microstructure of electroplated copper thin films near room temperature. Microstructure evolution occurs during a transient period of hours following deposition, and includes an increase in grain size, changes in preferred crystallographic texture, and decreases in resistivity, hardness, and compressive stress. The model is based on grain boundary energy in the fine-grained as-deposited films providing the underlying energy density which drives abnormal grain growth. As the grain size increases from the as-deposited value of 0.05–0.1 μm up to several microns, the model predicts a decreasing grain boundary contribution to electron scattering which allows the resistivity to decrease by tens of a percent to near-bulk values, as is observed. Concurrently, as the volume of the dilute grain boundary regions decreases, the stress is shown to change in the tensile direction by tens of a mega pascal, consistent with the measured values. The small ...

Theory of thin‐film orientation by ion bombardment during deposition

We study the development of orientational order in thin films grown with off‐normal incidence ion bombardment during deposition. The overall orientational order in our model results from the dependence of the sputtering yield on grain orientation. We demonstrate that the degree of orientational order at the surface of a thick film grows slowly with increasing ion flux until, at a critical value of the flux, it begins to rise more steeply and then saturates at its maximum value. The time needed to approach the thick‐film limit displays a peak as the ion flux is varied. We compare our work with the experimental results of Yu et al. [Appl. Phys. Lett. 47, 932 (1985)] and use our results to show how the deposition technique can be optimized.

Note on the origin of intrinsic stresses in films deposited via evaporation and sputtering

An attempt is made to provide a broad and brief review (with a sufficient number of references) of the question of intrinsic stresses in films deposited via evaporation and sputtering. Films deposited via evaporation are usually found initially in a disordered state, at the limit in an amorphous condition. Very broad thermodynamic principles imply that disorder is usually accompanied by an increase in volume (with few exceptions, e.g. water). Any relaxation from a disordered state to a more ordered state will, therefore, be accompanied by a decrease in volume and the formation of tensile stresses. The situation is modified in films deposited in the presence of impurities. Then one often finds the formation of compressive stresses. Several mechanisms may account for this result: (a) direct diffusion of interstitial impurities in the bulk of the underlying film: (b) surface and grain boundary diffusion of the impurities leading to compound formation (and swelling) in the grain boundaries, and at low temperatures in intergranular voids; (c) some attention is paid to a third model where compressive stresses could result from impurity adsorption not at the top surface of the growing films, but one monolayer below. The presence of compressive stresses in films deposited via sputtering is briefly reviewed in terms of the atomic peening mechanism. An experiment exploring the interrelationship between the purification effect of ion bombardment, thus causing tensile stresses, and the more normal formation of compressive stresses is discussed.

Technology and applications of broad-beam ion sources used in sputtering. Part II. Applications

The developments in broad‐beam ion source technology described in the companion paper (Part I) have stimulated a rapid expansion in applications to materials processing. These applications are reviewed here, beginning with a summary of sputtering mechanisms. Next, etching applications are described, including microfabrication and reactive ion beam etching. The developing area of surface layer applications is summarized, and related to the existing fields of oxidation and implantation. Next, deposition applications are reviewed, including ion‐beam sputter deposition and the emerging technique of ion‐assisted vapor deposition. Many of these applications have been stimulated by the development of high current ion sources operating in the energy range of tens of hundreds of eV. It is in this energy range that ion‐activated chemical etching is efficient, self‐limiting compound layers can be grown, and the physical properties of vapor‐deposited films can be modified. In each of these areas, broad ion beam techn...

Technology and applications of broad‐beam ion sources used in sputtering. Part I. Ion source technology

The technology of broad‐beam ion sources used in sputtering applications is reviewed. The most frequently used discharge chambers are described, together with procedures for predicting performance. A new, compact ion source is described. Ion acceleration is reviewed, with particular emphasis on recent low‐energy techniques. Some of these techniques include three‐grid, small‐hole two‐grid, and one‐grid ion optics. A new material for fabrication of high‐precision ion optics is silicon. Because no stresses are introduced with the etching techniques used, the finished grid can be held to very close tolerances. A recent innovation for sputtering applications is the use of Hall‐current acceleration. This technique uses a magnetic field interacting with an electron current to provide the accelerating electric field, thereby avoiding the usual space‐charge limit on ion current density that is associated with gridded optics. Electron emission is also reviewed, with new hollow cathodes promising improved lifetimes....

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

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. Incom...

Control of thin film orientation by glancing angle ion bombardment during growth

Glancing angle ion bombardment during thin film deposition is shown to have a pronounced alignment effect on crystallographic orientation. Restricted fiber texture is achieved in Nb films deposited at room temperature onto amorphous silica substrates by Ar ion beam sputtering, with simultaneous bombardment by 200 eV Ar+ ions at 20° from glancing angle. The alignment direction corresponds to a channeling direction for the incident ions between (110) planes, for which a low sputtering yield is expected. The degree of alignment is measured as a function of ion/atom arrival rate ratio up to 1.3 Ar+ ions per Nb film atom, and is shown to increase monotonically with the fraction resputtered. Also, the fiber axis tilts slightly towards the incident ion beam direction, favoring the channeling direction.

Reduction of the C54–TiSi2 phase transformation temperature using refractory metal ion implantation

We report that the ion implantation of a small dose of Mo into a silicon substrate before the deposition of a thin film of Ti lowers the temperature required to form the commercially important low resistivity C54–TiSi2 phase by 100–150 °C. A lesser improvement is obtained with W implantation. In addition, a sharp reduction in the dependence of C54 formation on the geometrical size of the silicided structure is observed. The enhancement in C54 formation observed with the ion implantation of Mo is not explained by ion mixing of the Ti/Si interface or implant‐induced damage. Rather, it is attributed to an enhanced nucleation of C54–TiSi2 out of the precursor high resistance C49–TiSi2 phase.