Repeated antigen testing of 12 severe acute respiratory coronavirus virus 2 (SARS-CoV-2)–positive nursing home residents using Abbott BinaxNOW identified 9 of 9 (100%) culture-positive specimens up to 6 days after initial positive test. Antigen positivity lasted 2–24 days. Antigen positivity might last beyond the infectious period, but it was reliable in residents with evidence of early infection.

This study aimed to identify an appropriate simple mathematical model to fit the number of coronavirus disease 2019 (COVID-19) cases at the national level for the early portion of the pandemic, before significant public health interventions could be enacted. The total number of cases for the COVID-19 epidemic over time in 28 countries was analysed and fit to several simple rate models. The resulting model parameters were used to extrapolate projections for more recent data. While the Gompertz growth model (mean R2 = 0.998) best fit the current data, uncertainties in the eventual case limit introduced significant model errors. However, the quadratic rate model (mean R2 = 0.992) fit the current data best for 25 (89%) countries as determined by R2 values of the remaining models. Projection to the future using the simple quadratic model accurately forecast the number of future total number of cases 50% of the time up to 10 days in advance. Extrapolation to the future with the simple exponential model significantly overpredicted the total number of future cases. These results demonstrate that accurate future predictions of the case load in a given country can be made using this very simple model.

A multidisciplinary, high-resolution paleoecological study (Lepidoptera and plant remains, macroscopic charcoal, pollen) was conducted on a 4000-yr peat monolith extracted from the margin of an ombrotrophic peatland on Anticosti Island (Gulf of St. Lawrence, eastern Canada) to reconstruct the long-term natural disturbances (insect outbreaks, forest fires) of a balsam fir/spruce forest. We hypothesized that an activity of insect defoliators (spruce budworm, hemlock looper) was the main disturbance factor of conifer forests during the Late Holocene. The earliest remains of spruce budworm and hemlock looper were found ca. 3220 and 2350 cal yr BP, respectively. Peaks of insect head capsules occurred from ca. 1640 to ca. 625 cal yr BP. Low balsam fir pollen concentrations during this period suggest a lengthy episode (∼ 1000 yr) of high insect activity, resulting in extensive fir dieback and mortality. The long-term dynamics of the pristine balsam fir/spruce forests were mainly governed by the activity of insect defoliators. The limited extent and possibly the low occurrence of forest fires in the maritime environment of Anticosti Island allowed the development of mature coniferous stands propitious for insect infestations. Insect head capsules appeared to be a useful and effective tool for establishing insect presence and activity during the Holocene.

Metal-catalyzed graphitization from vapor phase sources of carbon is now an established technique for producing few-layer graphene, a candidate material of interest for post-silicon electronics. Here we describe two alternative metal-catalyzed graphene formation processes utilizing solid phase sources of carbon. In the first, carbon is introduced as part of a cosputtered Ni-C alloy; in the second, carbon is introduced as one of the layers in an amorphous carbon (a-C)/Ni bilayer stack. We examine the quality and characteristics of the resulting graphene as a function of starting film thicknesses, Ni-C alloy composition or a-C deposition method (physical or chemical vapor deposition), and annealing conditions. We then discuss some of the competing processes playing a role in graphitic carbon formation and review recent evidence showing that the graphitic carbon in the a-C/Ni system initially forms by a metal-induced crystallization mechanism (analogous to what is seen with Al-induced crystallization of amorphous Si) rather than by the dissolution-upon-heating/precipitation-upon-cooling mechanism seen when graphene is grown by metal-catalyzed chemical vapor deposition methods.

We have studied the kinetics of NiSi agglomeration and NiSi2 phase formation during heating of NiSi on Si, using simultaneous in situ measurements of resistance, light scattering and x-ray diffraction. NiSi is a desirable contact to Si because of its low resistivity, limited Si consumption and low formation temperature. However, the formation of the higher resistivity phase NiSi2 must be avoided for device applications. Ni thin films 5 to 30 nm thick were deposited on substrates of poly-Si and silicon-on-insulator (SOI) and were studied using heating rates from 0.3 to 27 °C/s. At low heating rates and for the thinnest films studied, NiSi agglomeration precedes NiSi2 nucleation by as much as 350°C. The agglomeration temperature decreases with decreasing film thickness and linewidth. Once the film is agglomerated, the formation of NiSi2 is delayed to higher temperature by its low nucleation site density and decreased contact area. We conclude that agglomeration is the primary failure mechanism limiting the morphological stability of NiSi as a contact material to Si devices.

We examine how the substrate temperature during Ti film sputter deposition influences the subsequent texture formation in TiSi2 thin films. Titanium films of 32 nm thickness were sputtered onto Si(001) at elevated substrate temperatures varying between 100 °C and 900 °C. After the depositions, in situ x-ray diffraction (XRD) measurements were performed to study the thin film reactions in real time, as the samples were annealed. The XRD results show that the substrate temperature significantly influences the texture of the initial Ti film as well as the texture of the resulting C54-phase TiSi2. The preferred Ti orientation gradually changes from (002) to (101) fiber texture as the deposition temperature increases up to 500 °C. Films deposited at 600 °C transformed into the C49 phase during deposition while films deposited at 700 °C and higher temperatures transformed into the C54 phase during deposition. The series of deposited films was annealed up to 1000 °C in He to complete the C54 phase formation while monitoring the texture evolution in situ using a position sensitive x-ray detector. The XRD results show that the final C54 phase texture changes from a dominant (311) orientation normal to the substrate to a (010) orientation for substrate temperatures between 600 °C and 700 °C. The C49-C54 phase transformation temperature is also lowered for these deposition temperatures. Ex situ pole figure analysis of the film deposited at 700 °C confirms the dominant C54 (010) texture and shows an in-plane orientation with C54 [001] ∥ Si [110]. For substrate temperatures between 800 °C and 900 °C, the C54 texture changes dramatically. In this case, θ - 2 θ scans do not show a preferred C54 orientation, but pole figure analysis indicates weak inplane orientations.

We have studied the formation of titanium silicides in the presence of an ultra-thin layer of Ta, interposed between Ti and Si. In-situ x-ray diffraction (XRD), resistance measurements and elastic light scattering were used to study the thin film reactions in real time during ramp anneals to 1000°C. On poly-Si substrates the Ta thickness was varied from 0 to 1.5 nm while the Ti thickness was held constant at ∼27 nm. The time-resolved XRD shows that the volume fraction of C40 and metal-rich silicide phases grows with increasing Ta layer thickness. Increased Ta layer thicknesses also delay the growth of the C49 disilicide phase to higher temperatures. Among the Ta thicknesses we examined, 0.3 nm is the most effective in lowering the C49-C54 transformation temperature. Films with Ta layers thicker than 0.5 nm do not completely transform into the C54 phase. The texture of the C54 phase is also sensitive to the Ta thickness. The C54 disilicide film is predominantly (010) textured for the Ti / 0.3 nm Ta sample. The final C54 texture is significantly different for Ta layers thinner or thicker than the optimal 0.3 nm. This suggests that the most effective thickness for lowering the C54 formation temperature is related to the development of a strong (010) texture. The possibility of a template effect by the C40 or metal-rich Ti5Si3 phases is also discussed on the basis of texture considerations.

Ta films were grown by plasma-enhanced atomic layer deposition (PE-ALD) at temperatures from room temperature up to 300 °C using TaCl5 as source gas and RF plasma-produced atomic H as the reducing agent. Post-deposition ex situ chemical analyses showed that the main impurity is oxygen, incorporated during the air exposure prior to analysis with typically low Cl concentration below 1 at %. The X-ray diffraction indicates that ALD Ta films are amorphous or composed of nano-grains. The typical resistivity of ALD Ta films was 150-180 μΩ cm, which corresponds to that of β-Ta phase, at a wide range of growth parameters. The conformality of the film is 100 % up to an aspect ratio of 15:1 and 40 % for aspect ratio of 40:1. The thickness per cycle, corresponding to the growth rate, was measured by Rutherford back scattering as a function of various key growth parameters, including TaCl5 and H exposure time and growth temperature. The maximum thickness per cycle values were below 0.1 ML, probably due to the steric hindrance for TaCl5 adsorption. Bilayer structures consisting of Cu films deposited by sputtering and ALD Ta films with various thicknesses were prepared and the diffusion barrier properties of ALD Ta films were investigated by various analysis techniques consisting of X-ray diffraction, elastic light scattering, and resistance analysis. The results were compared with Ta thin films deposited by sputtering with comparable thicknesses. Also, the growth of TaN films by PE-ALD using consecutive exposures of atomic H and activated N2 is presented.

We propose a modified self-aligned silicide (salicide) process that uses Ge implantation and a silicon cap to reduce the silicon substrate consumption by 75% as compared with a conventional salicide process. We have used Ge implants to increase the cobalt disilicide formation temperature. This forces the cobalt to react primarily with a deposited silicon cap, thus minimizing consumption from the silicon substrate. We expect this process to be useful for making silicide on shallow junctions and thin SOI films, where silicon consumption is constrained.

We discuss a modified self-aligned silicide (salicide) process that uses a silicon cap to reduce the substrate silicon consumption by 50% as compared with a conventional salicide process. We have used a metal-silicon mixture to form the metal-rich phase reliably in the first anneal. After etching the unreacted mixture we deposit a silicon cap. This forces the metal to react with the silicon cap as well as with the substrate during the second anneal, thus minimizing silicon consumption from the substrate. The unreacted portion of the silicon cap is selectively etched, leaving a structure with a raised source and drain. We expect this process to be useful for forming silicide on shallow junctions and thin SOI films, where silicon consumption is constrained.

The biaxial stress in Co thin-films has been investigated in situ by measuring changes in substrate curvature that occurred during deposition and annealing.Films of Co, 35 to 500 nm in thickness, were deposited by UHV magnetron sputtering at room temperature on Si (100) and poly-Si substrates.Results show that during Co deposition the bending force increased linearly with film thickness; a signature of constant stress.In addition, the stress evolution during silicide formation was measured under constant heating rate conditions from room temperature up to 700°C. The stress-temperature curve was correlated with Co2Si, CoSi, and CoSi2 phase formation using in situ synchrotron X-ray diffraction measurements.The room temperature stress for the CoSi2 phase was found to be ∼0.8 GPa (tensile) in the films deposited on Si (100) and ∼1 GPa (tensile) on the films deposited on poly-Si.The higher tensile stress in the poly-Si sample could be a result of Si grain growth during annealing.

The mechanisms are studied for enhanced formation of C54–TiSi2 at about 700 °C when rapid thermal annealing at 3 °C/s in N2 is performed on 32-nm-thick codeposited Ti–5.9 at.% Ta on Si(100) single-crystal substrates. The enhancement is related to an increased C54–TiSi2 nucleation rate due to the development of a multilayered microstructure. The multilayer microstructure forms at temperatures below 600 °C with the formation of an amorphous disilicide adjacent to the Si substrate and a M5Si3 (M = Ti, Ta) capping layer. This amorphous disilicide crystallizes at higher temperatures to C49–TiSi2. The multilayer microstructure introduces an additional interface that increases the area available for the heterogeneous nucleation of C54. The capping layer is identified as hexagonal Ti 5Si3 or its isomorphous compound (Ti1–xTax)5Si3. Crystal simulations demonstrate that C54(040) has a lattice mismatch of 6–7% relative to Ti5Si3(300) suggesting that a pseudomorphic epitaxial relationship may lower the interfacial energy between these two phases and reduce the energy barrier for C54 nucleation. A C40 disilicide phase was also observed at temperatures above that required to form C54–TiSi2 suggesting that, in the present experiments, the C40 phase does not play a major role in catalyzing C54 formation.

This paper examines the challenges for silicide contacts in current and future CMOS technologies and assesses the capability of TiSi2 and related materials, as well as CoSi2, to meet the technology requirements. Specific issues such as maintaining low resistance in narrow lines, source/drain silicon consumption, and silicide thermal budget, are discussed. In this regard we evaluate the salient features of the basic titanium salicide process, and variations such as alloying, implantation, selective CVD, and laser silicidation. Data are presented from recent experiments to assess progress in each approach towards meeting the necessary goals. Finally, projections of the future of silicide processing are given.