Eric Chason

Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering

When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.

Roughening instability and ion‐induced viscous relaxation of SiO2 surfaces

We characterize the development of nanometer scale topography (roughness) on SiO2 surfaces as a result of low energy, off‐normal ion bombardment, using in situ energy dispersive x‐ray reflectivity and atomic force microscopy. Surfaces roughen during sputtering by heavy ions (Xe), with roughness increasing approximately linearly with ion fluence up to 1017 cm−2. A highly coherent ripple structure with wavelength of 30 nm and oriented with the wave vector parallel to the direction of incidence is observed after Xe sputtering at 1 keV. Lower frequency, random texture is also observed. Subsequent light ion (H, He) bombardment smoothens preroughened surfaces. The smoothing kinetics are first order with ion fluence and strongly dependent on ion energy in the range 0.2–1 eV. We present a linear model to account for the experimental observations which includes roughening both by random stochastic processes and by development of a periodic surface instability due to sputter yield variations with surface curvature ...

The Dynamic Competition Between Stress Generation and Relaxation Mechanisms During Coalescence of Volmer-Weber Thin Films

Real-time measurements of stress evolution during the deposition of Volmer–Weber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystalline Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior occurs even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the film/substrate interfacial strength. For Ag films, interfacial shear dominated the early relaxation behavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the Al/SiO2 interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures...

Ion beams in silicon processing and characterization

General trends in integrated circuit technology toward smaller device dimensions, lower thermal budgets, and simplified processing steps present severe physical and engineering challenges to ion implantation. These challenges, together with the need for physically based models at exceedingly small dimensions, are leading to a new level of understanding of fundamental defect science in Si. In this article, we review the current status and future trends in ion implantation of Si at low and high energies with particular emphasis on areas where recent advances have been made and where further understanding is needed. Particularly interesting are the emerging approaches to defect and dopant distribution modeling, transient enhanced diffusion, high energy implantation and defect accumulation, and metal impurity gettering. Developments in the use of ion beams for analysis indicate much progress has been made in one-dimensional analysis, but that severe challenges for two-dimensional characterization remain. The ...

Extensions of the Stoney Formula for Substrate Curvature to Configurations with Thin Substrates or Large Deformations

Two main assumptions which underlie the Stoney formula relating substrate curvature to mis-match strain in a bonded thin film are that the film is very thin compared to the substrate, and the deformations are infinitesimally small. Expressions for the curvature-strain relastionship are derived for cases in which thses assumptions are relaxed, thereby providing a biasis for interpretation of experimental observations for a broader class of film-substrate configurations.

Stress evolution during metalorganic chemical vapor deposition of GaN

The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.

Model for stress generated upon contact of neighboring islands on the surface of a substrate

In the early stage of growth of a metal film on a substrate by the Volmer–Weber mechanism, a tensile stress in the film is observed to arise at about the point in the process when islands of deposited material begin to coalesce. The mechanism commonly proposed as the origin of this tensile stress is that the coalescing islands deform in order to form a relatively low energy grain boundary, at the expense of some surface energy by surface area reduction, and that this proceeds until a stress is generated that has magnitude sufficient to prevent further area reduction. Several models have been proposed for this process, but the inferred tensile stress estimates have been much larger than observed stress magnitudes in many cases. The purpose here is to introduce a model for the process based on the theory of contact of elastic solids with cohesion. A description of the process is developed on this premise for one-dimensional, two-dimensional, and three-dimensional states of deformation of coalescing islands....

Stress and Defect Control in GaN Using Low Temperature Interlayers

In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.

Brittle-ductile relaxation kinetics of strained AlGaN/GaN heterostructures

The authors have directly measured the stress evolution during metal organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to directly determine a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained Al{sub x}Ga{sub 1{minus}x}N on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.

Whisker formation in Sn and Pb-Sn coatings: Role of intermetallic growth, stress evolution, and plastic deformation processes

We have simultaneously measured the evolution of intermetallic volume, stress, and whisker density in Sn and Pb–Sn alloy layers on Cu to study the fundamental mechanisms controlling whisker formation. For pure Sn, the stress becomes increasingly compressive and then saturates, corresponding to a plastically deformed region spreading away from the growing intermetallic particles. Whisker nucleation begins after the stress saturates. Pb–Sn layers have similar intermetallic growth kinetics but the resulting stress and whisker density are much less. Measurements after sputtering demonstrate the important role of the surface oxide in inhibiting stress relaxation.