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During the past quarter-century, the field of high pressure research has undergone a quiet revolution. This has resulted from the creation and development of the diamond-anvil cell (DAC). Because diamonds are the hardest known materials, the highest static pressures can be attained and indefinitely held in these devices, recent pressure records have surpassed 400 GPa.
Photospheric parameters and abundances of 13 chemical species are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (—0.41 ≲ [Fe/H] ≲ +0.11). A significant overabundance of several elements (e.g., Na, Mg, Al, Ca) is observed. The temperatures derived from the (V — R) and (V — I) colours are found to be significantly affected by activity processes.
Post-traumatic stress disorder (PTSD) in response to the World Trade Center (WTC) disaster of 11 September 2001 (9/11) is one of the most prevalent and persistent health conditions among both professional (e.g. police) and non-traditional (e.g. construction worker) WTC responders, even several years after 9/11. However, little is known about the dimensionality and natural course of WTC-related PTSD symptomatology in these populations.
Data were analysed from 10 835 WTC responders, including 4035 police and 6800 non-traditional responders who were evaluated as part of the WTC Health Program, a clinic network in the New York area established by the National Institute for Occupational Safety and Health. Confirmatory factor analyses (CFAs) were used to evaluate structural models of PTSD symptom dimensionality; and autoregressive cross-lagged (ARCL) panel regressions were used to examine the prospective interrelationships among PTSD symptom clusters at 3, 6 and 8 years after 9/11.
CFAs suggested that five stable symptom clusters best represent PTSD symptom dimensionality in both police and non-traditional WTC responders. This five-factor model was also invariant over time with respect to factor loadings and structural parameters, thereby demonstrating its longitudinal stability. ARCL panel regression analyses revealed that hyperarousal symptoms had a prominent role in predicting other symptom clusters of PTSD, with anxious arousal symptoms primarily driving re-experiencing symptoms, and dysphoric arousal symptoms primarily driving emotional numbing symptoms over time.
Results of this study suggest that disaster-related PTSD symptomatology in WTC responders is best represented by five symptom dimensions. Anxious arousal symptoms, which are characterized by hypervigilance and exaggerated startle, may primarily drive re-experiencing symptoms, while dysphoric arousal symptoms, which are characterized by sleep disturbance, irritability/anger and concentration difficulties, may primarily drive emotional numbing symptoms over time. These results underscore the importance of assessment, monitoring and early intervention of hyperarousal symptoms in WTC and other disaster responders.
Longitudinal symptoms of post-traumatic stress disorder (PTSD) are often characterized by heterogeneous trajectories, which may have unique pre-, peri- and post-trauma risk and protective factors. To date, however, no study has evaluated the nature and determinants of predominant trajectories of PTSD symptoms in World Trade Center (WTC) responders.
A total of 10835 WTC responders, including 4035 professional police responders and 6800 non-traditional responders (e.g. construction workers) who participated in the WTC Health Program (WTC-HP), were evaluated an average of 3, 6 and 8 years after the WTC attacks.
Among police responders, longitudinal PTSD symptoms were best characterized by four classes, with the majority (77.8%) in a resistant/resilient trajectory and the remainder exhibiting chronic (5.3%), recovering (8.4%) or delayed-onset (8.5%) symptom trajectories. Among non-traditional responders, a six-class solution was optimal, with fewer responders in a resistant/resilient trajectory (58.0%) and the remainder exhibiting recovering (12.3%), severe chronic (9.5%), subsyndromal increasing (7.3%), delayed-onset (6.7%) and moderate chronic (6.2%) trajectories. Prior psychiatric history, Hispanic ethnicity, severity of WTC exposure and WTC-related medical conditions were most strongly associated with symptomatic trajectories of PTSD symptoms in both groups of responders, whereas greater education and family and work support while working at the WTC site were protective against several of these trajectories.
Trajectories of PTSD symptoms in WTC responders are heterogeneous and associated uniquely with pre-, peri- and post-trauma risk and protective factors. Police responders were more likely than non-traditional responders to exhibit a resistant/resilient trajectory. These results underscore the importance of prevention, screening and treatment efforts that target high-risk disaster responders, particularly those with prior psychiatric history, high levels of trauma exposure and work-related medical morbidities.
To examine the prevalence of the C677T polymorphism of the methylene tetrahydrofolate reductase (MTHFR) gene and the A2756G polymorphism of methionine synthase (MS), and their impact on antidepressant response.
We screened 224 subjects (52% female, mean age 39 ± 11 years) with SCID-diagnosed major depressive disorder (MDD), and obtained 194 genetic samples. 49 subjects (49% female, mean age 36 ± 11 years) participated in a 12-week open clinical trial of fluoxetine 20–60 mg/day. Association between clinical response and C677T and A2756G polymorphisms, folate, B12, and homocysteine was examined.
Prevalence of the C677T and A2756G polymorphisms was consistent with previous reports (C/C = 41%, C/T = 47%, T/T = 11%, A/A = 66%, A/G = 29%, G/G = 4%). In the fluoxetine-treated subsample (n = 49), intent-to-treat (ITT) response rates were 47% for C/C subjects and 46% for pooled C/T and T/T subjects (nonsignificant). ITT response rates were 38% for A/A subjects and 60% for A/G subjects (nonsignificant), with no subjects exhibiting the G/G homozygote. Mean baseline plasma B12 was significantly lower in A/G subjects compared to A/A, but folate and homocysteine levels were not affected by genetic status. Plasma folate was negatively associated with treatment response.
The C677T and A2756G polymorphisms did not significantly affect antidepressant response. These preliminary findings require replication in larger samples.
The formation of a 12nm thick, continuous and thermally stable COSi2 layer was described in our previous work [MRS Proc. 238, 587 (1992)]. Interdiffusion in the Co/Ti-Si multilayer system has been further studied and the initial Ti(O) thickness is shown to be a critical parameter in controlling its effectiveness as a diffusion barrier, and in modulating the Co-Si and Ti-Si compctctive reactions. Three Ti(O) and three Co layers with thickness from ∼5nm 20nm were deposited sequentially, with Ti(O) as the first layer, on Si-(100) substrates by dual source thermal evaporation. The morphology of the CoSix/Si interface was strongly influenced by Ti(O) thickness from ∼5nm to ∼10nm, and a 12nm thick uniform CoSi2 layer with ∼28μΩ-cm resistivity was produced as decribed previously. When the initial Ti(O) thickness was increased to ∼20nm and the Co thickness set at -10nm, Co diffusion was suppressed and Ti reacted with Si yielding an ∼10nm amorphous TiSix layer at 550°C. This amorphous layer transformed to a 15nm thick uniform C-54 TiSi2 layer after selective removal of upper layers and a 750°C plus 800°C annealing. A flat silicide/Si interface and a ∼58μΩ-cm resistivity were obtained. The significance of both thermodynamic and kinetic factors in the compctetive reactions is discussed.
A prototype RTP system has been developed which allows for in-situ emissivity and temperature measurements. The wafer emissivity is measured by using an optical detector at a wavelength of 2.4 μm and by modulation of the lamp power. This method permits accurate temperature determination in the range from 400 to 1200°C, independent of wafer backside roughness, backside layers, and transmit tance. The feasibility of the temperature measurement technique is demonstrated by using wafers with built-in thermocouples and highly As-doped wafers with different backside roughnesses or layers. The emissivity variations during processing can also be used to study thin film reactions in-situ. This is demonstrated for Co silicidation using probing wavelengths varying from 0.6 to 3.2 μm.
The evolution of modern integrated circuit technology to sub micron dimensions has brought about a number of challenges, specially in the field of metallization. Decreasing junction depths have imposed stringent demands on the materials used for the electrical contacts. This is due to the potential interactions between the contact metal and silicon (or suicide in the case of salicide processes) causing junction leakage and/or shorting. The solution most commonly applied to this problem is that in which a barrier material is interposed between the metallization and the contact. The material most often selected for this purpose has been TiN. TiN can be deposited via the reactive sputtering of Ti in a N2 atmosphere or it can be also obtained by sputtering Ti and then reacting it with either N2 or NH3. Shrinking VLSI dimensions have brought about the need for improved planarization for the purposes of metal definition. It has also prevented the tapering of contacts for space saving reasons. Both of these issues resulted in deep, straight wall contacts with aspect ratios greater than 1 that cannot be metallized appropriately with conventional sputtering techniques. These requirements have driven the development of a conformai CVD TiN barrier process. This paper describes the evolution of the TiN metallization barrier from the requirements of 1.2μ to 0.35μ technologies.
The formation of NiSi and NiSi2 upon annealing of an ion-amorphized Ni/Si structure has been studied by various surface analytical techniques to characterize the morphology, stoichiometry and interface sharpness of the NiSi2 layer. In comparison with reactions of nickel on crystalline silicon (c-Si), sharpening 0ofthe NiSi2/C-Si interface is obtained for appropriate amorphization depths. Moreover, the surface roughness of the NiSi2 films is significantly reduced by implantation. The NiSi2 formation temperature is, however, not reduced as observed for structures with nickel deposited on amorphous silicon prepared by evaporation. This dissimilarity can be explained by an unexpected low crystallization temperature or the ion-amorphized structure, where Ni-ennanced solid phase epitaxy occurs at a temperature as low as 425°C.
The effects of TEOS (Tetra-ethyl-ortho-silicate) concentration on the physical, chemical and electrical characteristics of undoped SiO2 films deposited by plasma (PECVD) and thermal CVD (ThCVD) processes, are described. It is shown that the (TEOS/O3) or (TEOS/O) ratio strongly influences the various film properties. The uniformity of film thickness is better at higher (TEOS/O3) or (TEOS/O2) ratios in both processes. In ThCVD the SiO2 films are denser (low etch rate) when deposited at low (TEOS/O3 ) ratio. This study has shown that the TEOS concentration can be used to tailor film properties.
Copper ball bonding is a new technology which is expected to replace the traditional gold ball bonding and paid attention in woled recently, the technology is very important to reduce cost and improve reliability of microelectronic components. In this paper, the copper wire ball bonding processes have been studied by means of the MW-EFO metal wire ball forming device and the JWYH-2 thermosonic ball bonder. The bond strength of the ball bond under varied ultrasonic power and bonding time have been tested. The tested results show that the maximal strength of copper ball bond can be over 20g, it has advantage over the gold wire ball bonding. In this paper, the deformation process of the copper ball bond has been analyzed and viewed by SME.
Electron-beam (e-beam) evaporated low Au content NiGe(Au)W ohmic contacts with a contact resistance (Rc) as low as 0.15 Ω-mra have been reported. Due to the high melting point of W it is desirable to deposit this layer by means other than e-beam evaporation. However, the use of nearly oxygen-free sputtered W, yields contact resistances in excess of 0.7 Ω-mm. By replacing the sputtered W by a reactively sputtered metallic W oxide, containing ∼25 at. % oxygen, the low contact resistance (Rc < 0.15 Ω-mm) is restored. Contacts employing a reactively sputtered W nitride in place of W oxide also yield high Rc's (∼0.75 Ω-mm). Auger depth profiles of the reacted contacts show a significant outdiffusion of Ga from the GaAs substrate in the presence the oxygenated W but not in the low oxygen and the W nitride contacts. These results and the fact that our previously reported e-beam evaporated W contacts also contain ∼25 at. % oxygen, suggest an oxygen assisted ohmic contact formation mechanism.
The current/voltage characteristics of ion beam synthesised CoSi2/Si (n - type) Schottky barrier diodes implanted with phosphorus to doses between 5 × 1012 and 2 × 1013 ions cm-2are examined after annealing at temperatures in the range 400° - 1000°C. For each dose of implanted phosphorus, the effective barrier height of the CoSi2/Si interface is successively reduced as the anneal temperature increases. The results of Secondary Ion Mass Spectroscopy (SIMS) analysis indicate that these changes are due to an increase in the space charge density at the interface. For lower annealing temperatures the increase in space charge density is attributed to activation of the phosphorus in the tail of the dopant distribution which extends across the CoSi2/Si interface. For higher annealing temperatures larger increases in the space charge density are attributed to a modified dopant distribution resulting from phosphorus diffusion and activation at the interface. For doses of 1 × 1014 P* cm-2and 2×1015P*cm2, ohrnie characteristics are seen after annealing at temperatures of 1000°C and 500°C respectively.
X-ray diffraction studies and current-voltage measurements have been performed on a (100) oriented single crystal thin film of CoxGa1-x (x = 0.42) grown epitaxially on n-GaAs, from 300°C to 900°C. At this composition, CoxGa1-x, which has a broad range of homogeneity and a variable lattice parameter, is lattice matched to GaAs better than 0.5%. A Schottky barrier height of 0.68eV and an ideality factor of 1.07 have been measured up to 500°C, with significant barrier degradation at 600°C. At 700°C formation of the CoGa3 phase and a shift in CoxGa1-x stoichiometry to its bulk thermodynamically most stable composition of Co.45Ga.55 was observed with x-ray diffraction. At 800°C Co2AS formed, and at 900°C only CoGa3 and Co2As phases remained in contact with GaAs.
We introduce a new quantitative description for electronic noise at Schottky contacts. The model combines spatially inhomogeneous current transport across the interface with the modulation of the local barrier height due to trapping dynamics of charged states located at or close to the interface. The experimentally observed increase of noise power with decreasing temperature is explained by the inhomogeneity of the interface. Our model fits experimental data obtained from different silicide/silicon Schottky contacts and the detailed analysis of measured noise spectra yields information about the interfacial potential fluctuations.
The influence of metallization and processing on Schottky barrier formation provides the basis for one of several fruitful approaches for controlling junction electronic properties. Interface cathodo-and photoluminescence measurements reveal that electrically-active deep levels form on GaAs(100) surfaces and metal interfaces which depend on thermally-driven surface stoichiometry and reconstruction, chemical interaction, as well as surface misorientation and bulk crystal quality. These interface states are discrete and occur at multiple gap energies which can account for observed band bending. Characteristic trends in such deep level emission with interface processing provide guides for optimizing interface electronic behavior. Correspondingly, photoemission and internal photoemission spectroscopy measurements indicate self-consistent changes in barrier heights which may be heterogeneous and attributable to interface chemical reactions observed on a monolayer scale. These results highlight the multiple roles of atomic-scale structure in forming macroscopic electronic properties of compound semiconductor-metal junctions.
Silicon device performance has improved dramatically over the past two decades. During that time, the portion of total circuit delay attributable to interconnect wiring has increased to nearly half. Changes in the use of aluminum wiring and silicon oxide insulators must be considered to further improve performance.
The generation of ULSI circuitry now under development in the industry depends heavily on innovations in materials and processes to achieve significant improvements in wiring density and circuit performance. The subsequent generation places even greater demands on materials improvements for lower resistivity wiring and a lower dielectric constant insulator. Integrating materials and methods compatible with semiconductor processing while ensuring device reliability will create a significant financial challenge in addition to the technical challenge.
This presentation enumerates the interconnection challenges facing the materials scientist and the circuit designer. The ultimate limits in materials improvements are predictable and finite. Further improvements will be derived from design innovations such as treating interconnects as active rather than passive devices. Accurate physical modelling of MLM structures will complement even more complex electrical modelling. Manufacturing sciences will grow in sophistication if implementation is to succeed. The financial burden of these technical challenges will foster collaborations among semiconductor manufacturers, equipment vendors, and research universities.