J. C. Bilello

Growth anisotropy and self-shadowing: A model for the development of in-plane texture during polycrystalline thin-film growth

The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e., oblique incidence deposition of anisotropic materials), an in-plane texture will develop. Grains that have “fast” crystallographic growth directions aligned with the “fast” geometric growth direction overgrow grains that do not exhibit this alignment. Furthermore, the rate of texturing increases with the degree of each anisotropy. This model was used to simulate in-plane texturing during thin-film deposition. The simulation results are in excellent quantitative agreement with recent experimental results concerning the development of in-plane texture in sputter deposited Mo films.The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e.,...

Analysis of thin film stress measurement techniques

Abstract Residual stresses in several magnetron sputtered Mo thin films, with thicknesses from 100 nm to 1.60 μm, were determined using double-crystal diffraction topography (DCDT), sin 2 ψ , and the high-resolution X-ray diffraction technique (HRXRD). The Mo films had a range of microstructures that included random and polycrystalline, textured out-of-plane, and textured in-plane. When the average biaxial stresses over the entire film thickness were determined for the films using the aforementioned techniques, the results were comparable in magnitude. However, the stresses determined with the substrate curvature technique, DCDT, were consistently smaller than those obtained with the sin 2 ψ and HRXRD techniques. The difference may arise for several reasons. For example, the HRXRD and sin 2 ψ measurements of a textured film may not be indicative of the mean film stress, and thus may differ from the curvature measurement. Also, substrate curvature techniques measure extrinsic stresses, or stresses that arise solely from the presence of the substrate. The techniques which analyze the film directly, such as sin 2 ψ and HRXRD, measure the extrinsic stresses and the intrinsic stresses that arise from defects or morphology changes within the film. The additional information that can be obtained from the depth-sensitive HRXRD technique concerning stress variations within thin films was also highlighted.

Microstructure and residual stress of very thin Mo films

Abstract Very thin sputtered Mo films ( thicknesses A ) have been investigated in order to gain a better understanding of the relationship between the development of microstructure, morphology and residual stress. Mo films were grown by planar magnetron sputtering (3 mTorr Ar) onto Si substrates with native oxide. Examination of a large number of Mo films using double crystal diffraction topography showed that a large compressive stress (~ 1 GPa) was accumulated in the thinnest films studied ( ~33 A ). The film stress relaxed with increasing thickness and eventually crossed over to the tensile regime. The microstructure of these films have been studied using two techniques, transmission electron microscopy and grazing incidence X-ray scattering. The evolution of microstructure in Mo films has been examined by monitoring the in-plane grain size as a function of film thickness, beginning with 33 A thick films. Both techniques showed that the grain width increased rapidly at the earliest stages of growth, but exhibited a slower increase in growth by thicknesses ~ 800 A .

Combined transmission electron microscopy and x‐ray study of the microstructure and texture in sputtered Mo films

The microstructure and texture of thin Mo films sputtered onto the native oxide of Si(100) wafers were investigated with both conventional reflection x‐ray pole figures, and transmission electron microscopy and diffraction. Films were grown at two deposition rates (powers), 34 nm/min (1.5 kW) and 67 nm/min (3.9 kW), onto both moving and stationary substrates, under otherwise identical experimental conditions. The microstructure of the Mo films evolved into a zone 2 microstructure within the first 2 μm of growth. The development of both out‐of‐plane and in‐plane textures was found to be influenced by deposition rate and geometry. Films grown at the lower deposition rate exhibited predominantly {110} textures, while films grown at the higher rate exhibited predominantly {110} textures up to a film thickness of ∼0.5 μm and {111} textures above a film thickness of ∼1 μm. Films with the {110} textures developed grains with elongated footprints and faceted surfaces, while films with the {111} textures developed...

Evolution of anisotropic microstructure and residual stress in sputtered Cr films

A series of Cr films with varying thicknesses have been prepared using a multiple moving substrate deposition geometry. These films have been investigated with several experimental techniques, including synchrotron x-ray scattering, pole figures, electron microscope, and double crystal diffraction topography. It was found that the in-plane stresses are highly anisotropic in these Cr films. The anisotropic stresses, characterized by two principal stresses in two characteristic directions defined by the deposition geometry, are quantified based on a methodology given in the Appendix. The plan view transmission electron microscopy observations reveal that the Cr films develop well-organized microstructures. The grains, which are elongated along the radial direction, are crystallographically aligned as well. The development of crystallographic texture in the Cr films, further revealed by pole figures and azimuthal (φ) x-ray scans, depends on both the deposition geometry and the film thickness. The preferentia...

Depth dependence of residual strains in polycrystalline Mo thin films using high-resolution x-ray diffraction

The magnitude of the stress in a thin film can be obtained by measuring the curvature of the film–substrate couple. Crystal curvature techniques yield the average stress throughout the film thickness. On a microscopic level, the details of the strain distribution, as a function of depth through the thickness of the film, can have important consequences in governing film quality and ultimate morphology. A new method, using high‐resolution x‐ray diffraction to determine the depth dependence of strain in polycrystalline thin films, is described. The technique requires an analysis of the diffraction peak shifts of at least six independent {hkl} scattering vectors, at a variety of penetration depths from the free surface of the film. The data are then used to determine the magnitude and directions of the strain eigenvalues in a laboratory reference frame for each penetration depth from the free surface of the film. A linear elastic model was used to determine the strains in successive slabs of the film. Result...

Controlling strength and toughness of multilayer films : a new multiscalar approach

Multiscalar films are produced in order to combine both toughness and strength into a multilayer film. These structures incorporate both a strengthening phase and a toughening phase in a compositionally modulated microcomposite. The mechanical properties and microstructure for thick (∼50 μm) Mo/W multiscalar films have been characterized. A detailed microstructural analysis (including transmission electron microscopy, scanning electron microscopy, and x‐ray techniques) of Mo/W multiscalar films has shown that large single‐crystal columns of Mo interspersed with epitaxial layers of W extend for the entire film thickness. The microstructure is a zone‐II‐type microstructure, yet the temperatures during deposition are well below the lower limit (0.3 T/Tm) previously reported for such microstructures. Hardness and tensile tests have shown that a multiscalar approach is capable of tailoring a desired strength and toughness into a multilayered film.

SURFACE ROUGHNESS AND IN-PLANE TEXTURING IN SPUTTERED THIN FILMS

Real surfaces are not flat on an atomic scale. Studying the effects of roughness on microstructural evolution is of relevance because films are sputtered onto nonideal surfaces in many applications. To this end, amorphous rough substrates of two different morphologies, either elongated mounds or facets, were fabricated. The microstructural development of films deposited onto these surfaces was examined. In particular, the development of a preferred crystallographic orientation in the plane of growth in 400 nm thick Mo films grown on the rough substrates was studied using scanning electron microscopy, transmission electron diffraction, and high resolution x-ray diffraction (using φ scans in the symmetric grazing incidence x-ray scattering geometry with a synchrotron light source). It was found that the degree of texturing was dependent upon the type of roughness and its orientation during deposition. By limiting the average oblique angle of incident adatom flux, rough surfaces slowed the development of in-...

STRAIN GRADIENTS AND NORMAL STRESSES IN TEXTURED MO THIN FILMS

The high-resolution x-ray-diffraction technique was used to explore strain variations in sputtered Mo films with thicknesses of 170, 260, and 800 nm that possess a (110) out-of-plane texture. The strains in crystallographic planes perpendicular to the surface of each film were found to be nominally constant and compressive at all x-ray penetration depths. Near the surface of each film, the inclined-plane strains were compressive, and then relaxed as the penetration depths approached each entire film thickness. The strain tensor in a laboratory reference frame for each film, as a function of penetration depth, revealed that the normal strain ezz was tensile near the surface of each film, and then relaxed to a nominally constant value as the penetration depths approached the entire film thickness. The penetration depth over which the normal strain decayed to a nominally constant value increased as the total film thickness increased. A consequence of the large normal strains near the free surface of each fil...

Mechanical properties of tough multiscalar microlaminates

Strength and toughness were produced concurrently and optimized in two Mo/W multilayers by using a multi-scale approach which intersperses a tough phase, thick ({approx}1 or {approx}5 {mu}m) layers of one component (Mo), with a strong phase, 29 layer stacks of Mo/W (layer thickness = 4.0 nm). Specimens were sputtered onto 76 mm dia Si wafers, and had overall thicknesses of {approx}51 and {approx}31 {mu}m, respectively. Microstructure and fracture were examined using SEM. Uniaxial tensile and hardness testing were performed to measure strength and toughness; both increased with tough phase thickness. Several models were used to estimate K{sub IC} and fracture strength. Upper bound models showed {approx}2- and {approx}6-fold increases in K{sub IC} and fracture strength, respectively, when tough phase thickness was increased from 1 to 5 {mu}m, while hardness decreased from {approx}430 to {approx}380 kg/mm{sup 2}. The results are discussed in terms of multiscalar design concepts.