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 preferential orientation of film growth is [110] for thinner films (<1.6 μm), and then becomes [111] for thicker films. Correspondingly, the in-plane texture varies in a conformal manner. In the former case, [100] and [110] directions of grains preferentially align along the radial direction and the direction of platen rotation, respectively. In the latter case, the preferential orientation of grains in the radial direction becomes [112], while that in the direction of rotation remains to be [110]. The occurrence of the anisotropic stresses and their dependence on film thickness is related to the evolution of the anisotropic structure and in-plane texture. The correlation is discussed in terms of the modulus effect associated with in-plane texture, the stress relief at intercolumnar voids, and the texture transition. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69911/2/JAPIAU-92-12-7183-1.pd

Evolution of in-plane texture in reactively sputtered CrN films

The microstructure and texture of chromium nitride films reactively sputtered on silicon substrates were investigated using x-ray scattering, pole figures, transmission electron microscopy, and atomic force microscopy. Under the given deposition geometry, the CrN films were shown to develop a in-plane texture. The three preferred crystallographic orientations of the CrN films approximately coincided with the characteristic directions associated with the deposition geometry. There appear to be two regimes that govern the microstructural evolution and texture development for reactively sputtered chromium films. The first one involves the deposition conditions that lead to the formation of a single, stable phase such as stöichiometric CrN (above certain level of nitrogen partial pressure). In this regime, the film growth appears to be controlled by local epitaxy in individual columns, competitive grain growth, and kinetic roughening. The film characteristics resulted from this regime include the development of the in-plane texture, well-organized microstructures with relatively coarse grains, increased surface roughness, and large tensile stress. The second regime involves the transitional region prior to formation of the stable phase CrN in which significant microstructural refinements take place. This transitional region is associated with the thermodynamically metastable phase CrNxCrNx or the presence of multiple phases. The continuous renucleations during film growth disrupt the local epitaxy and impede kinetic roughening. This leads to film characteristics manifested by weakened or no texture, ultrafine microstructure (e.g., nanocrystalline structures), reduced surface roughness, and a tendency for residual stress to transit from tensile to compressive.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87655/2/023525_1.pd

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-plane texture. Comparison between experimental data and theoretical predictions showed that a recent analytical model is able to reasonably predict the degree of texturing in films grown onto these surfaces. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70129/2/JAPIAU-84-3-1346-1.pd

Growth textures of thick sputtered films and multilayers assessed via synchrotron transmission Laue

The growth textures of thick sputtered Mo metallizations and Mo/W multilayers, were characterized via a synchrotron white‐beam (WB) x‐ray transmission Laue technique. Transmission x‐ray diffraction studies of Mo specimens up to 61 μm thick were performed with WB synchrotron radiation; while the practical thickness limit for similar observations using a conventional laboratory Cu K(α) x‐ray source is ten times smaller. This unique approach used polychromatic x rays to simultaneously produce diffraction from a wide spread of orientations of many crystallographic planes for all the grains within a relatively large specimen volume (≊60×106 μm3). These patterns were obtained for polycrystalline 31‐ and 61‐μm‐thick Mo/W multilayer specimens, and a 35‐μm‐thick‐monolithic Mo foil specimen. In all three cases the alignment of specimen grains was similar to what would be expected for single‐crystal transmission patterns, except that the recorded intensity distributed was less localized. The WB transmission images were indexed using a reciprocal space construction for the Laue case. In the multilayers, the grains were oriented out‐of‐plane such that 〈110〉 crystallographic planes were aligned in the direction of sputter growth, while in the monolithic Mo specimen 〈111〉 crystallographic planes were so aligned, i.e., perpendicular to the deposition substrate. A spread in orientation of ∼5° was measured in the multilayer specimens, while the monolithic Mo specimen showed a spread of ∼30° when compared to a perfect single‐crystal orientation. Preferred orientation was also observed within the plane of growth to varying degrees for all three samples. © 1995 American Institute of Physics.  Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71326/2/JAPIAU-78-6-3812-1.pd

The evolution of texture in thin films and multilayers via synchrotron transmission Laue and grazing-incidence X-ray scattering

Sputter-deposited films and multilayers are used for a wide variety of applications including protective coatings on turbine engine blades, magnetic recording heads, optical elements, electronic packaging, X-ray filters and monochromator components to name a few examples. This wide range of interest requires growth thicknesses from a few nanometres to tens of micrometres depending on the product. In many applications, specific film textures in the growth direction as well as in the plane of growth are required. The control and manipulation of these textures can be accomplished by using advanced characterization techniques to select particular processing conditions. A variety of X-ray methods including grazing-incidence X-ray scattering, conventional pole figure studies and synchrotron white-beam transmission Laue scattering were used to study texture evolution for the thinnest films up to the thickest multilayer coatings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48901/2/jd950d58.pd

Measuring Socioeconomic Inequalities in Relation to Malaria Risk: A Comparison of Metrics in Rural Uganda.

Socioeconomic position (SEP) is an important risk factor for malaria, but there is no consensus on how to measure SEP in malaria studies. We evaluated the relative strength of four indicators of SEP in predicting malaria risk in Nagongera, Uganda. A total of 318 children resident in 100 households were followed for 36 months to measure parasite prevalence routinely every 3 months and malaria incidence by passive case detection. Household SEP was determined using: 1) two wealth indices, 2) income, 3) occupation, and 4) education. Wealth Index I (reference) included only asset ownership variables. Wealth Index II additionally included food security and house construction variables, which may directly affect malaria. In multivariate analysis, only Wealth Index II and income were associated with the human biting rate, only Wealth Indices I and II were associated with parasite prevalence, and only caregiver's education was associated with malaria incidence. This is the first evaluation of metrics beyond wealth and consumption indices for measuring the association between SEP and malaria. The wealth index still predicted malaria risk after excluding variables directly associated with malaria, but the strength of association was lower. In this setting, wealth indices, income, and education were stronger predictors of socioeconomic differences in malaria risk than occupation

Susceptibility of Anopheles gambiae to Natural Plasmodium falciparum Infection: A Comparison between the Well-Established Anopheles gambiae s.s Line and a Newly Established Ugandan Anopheles gambiae s.s. Line

Much of our understanding of malaria transmission comes from mosquito feeding assays using Anopheles mosquitoes from colonies that are well adapted to membrane feeding. This raises the question whether results from colony mosquitoes lead to overestimates of outcomes in wild Anopheles mosquitoes. We successfully established an Anopheles colony using progeny of wild Anopheles gambiae s.s. mosquitoes (Busia mosquitoes) and directly compared their susceptibility to infection with Plasmodium falciparum with the widely used An. gambiae s.s. mosquitoes (Kisumu mosquitoes) using gametocyte-infected Ugandan donor blood. The proportion of infectious feeds did not differ between Busia (71.8%, 23/32) and Kisumu (68.8%, 22/32, P = 1.00) mosquitoes. When correcting for random effects of donor blood, we observed a 23% higher proportion of infected Busia mosquitoes than infected Kisumu mosquitoes (RR, 1.23; 95% CI, 1.10–1.38, P &lt; 0.001). This study suggests that feeding assays with Kisumu mosquitoes do not overestimate outcomes in wild An. gambiae s.s. mosquitoes, the mosquito species most relevant to malaria transmission in Uganda

The Impact of Multiple Rounds of Indoor Residual Spraying on Malaria Incidence and Hemoglobin Levels in a High-Transmission Setting.

BACKGROUND: Indoor residual spraying (IRS) is widely used as a vector control measure, although there are conflicting findings of its effectiveness in reducing malaria incidence. The objective of this study was to estimate the effect of multiple IRS rounds on malaria incidence and hemoglobin levels in a cohort of children in rural southeastern Uganda. METHODS: The study was based upon a dynamic cohort of children aged 0.5-10 years enrolled from August 2011 to June 2017 in Nagongera Subcounty. Confirmed malaria infections and hemoglobin levels were recorded over time for each participant. After each of 4 rounds of IRS, malaria incidence, hemoglobin levels, and parasite density were evaluated and compared with pre-IRS levels. Analyses were carried out at the participant level while accounting for repeated measures and clustering by household. RESULTS: Incidence rate ratios comparing post-IRS to pre-IRS incidence rates for age groups 0-3, 3-5, and 5-11 were 0.108 (95% confidence interval [CI], .078-.149), 0.173 (95% CI, .136-.222), and 0.226 (95% CI, .187-.274), respectively. The mean hemoglobin levels significantly increased from 11.01 (pre-IRS) to 12.18 g/dL (post-IRS). CONCLUSIONS: Our study supports the policy recommendation of IRS usage in a stable and perennial transmission area to rapidly reduce malaria transmission

Sources of persistent malaria transmission in a setting with effective malaria control in eastern Uganda: a longitudinal, observational cohort study.

BACKGROUND: Symptomatic malaria cases reflect only a small proportion of all Plasmodium spp infections. Many infected individuals are asymptomatic, and persistent asymptomatic Plasmodium falciparum infections are common in endemic settings. We aimed to quantify the contribution of symptomatic and asymptomatic infections to P falciparum transmission in Tororo, Uganda. METHODS: We did a longitudinal, observational cohort study in Tororo district, Uganda. We recruited participants of all ages from randomly selected households within this district. Participants were eligible if the selected household had no more than nine permanent residents and at least two members younger than 10 years, and the household was their primary residence, and they agreed to come to the study clinic for any fever episode and avoid antimalarial medications outside the study. Participants were followed-up by continuous passive surveillance for the incidence of symptomatic infections; routine assessments (ie, standardised clinical evaluation and blood samples) were done at baseline and at routine visits every 4 weeks for 2 years. P falciparum parasite density, gametocyte density, and genetic composition were determined molecularly using quantitative PCR (qPCR), quantitative reverse transcriptase PCR (qRT-PCR), and amplicon deep sequencing, respectively. Membrane feeding assays were also done to assess infectivity to mosquitoes. The contribution of different populations to the infectious reservoir was estimated for symptomatic infections, asymptomatic but microscopically detected infections, and asymptomatic but qPCR-detected infections; and for age groups younger than 5 years, 5-15 years, and 16 years or older. FINDINGS: Between Oct 4, 2017, and Oct 31, 2019, 531 individuals were enrolled from 80 randomly selected households and were followed-up for 2 years. At baseline, P falciparum was detected in 28 (5·3%) of 531 participants by microscopy and an additional 64 (12·1%) by qPCR and declined thereafter. In 538 mosquito feeding experiments on 107 individuals, 446 (1·2%) of 37 404 mosquitoes became infected, with mosquito infection rates being strongly associated with gametocyte densities (β=2·11, 95% CI 1·62-2·67; p<0·0001). Considering both transmissibility of infections and their relative frequency, the estimated human infectious reservoir consisted primarily of asymptomatic microscopy-detected infections (83·8%), followed by asymptomatic submicroscopic infections (15·6%), and symptomatic infections (0·6%). Children aged 5-15 years accounted for more than half of the infectious reservoir (58·7%); individuals younger than 5 years (25·8%) and those 16 years or older (15·6%) contributed less. Samples from four children contribued to 279 (62·6%) of 446 infected mosquitoes after multiple mosquito-feeding assays. INTERPRETATION: Individuals with asymptomatic infections were important drivers of malaria transmission. School-aged children contributed to more than half of all mosquito infections, with a small minority of asymptomatic children being highly infectious. Demographically targeted interventions, aimed at school-aged children, could further reduce transmission in areas under effective vector control. FUNDING: US National Institutes of Health, Bill & Melinda Gates Foundation, and the European Research Council