Two dimensional computer simulation of plasma immersion ion implantation

The biggest advantage of plasma immersion ion implantation (PIII) is the capability of treating objects with irregular geometry without complex manipulation of the target holder. The effectiveness of this approach relies on the uniformity of the incident ion dose. Unfortunately, perfect dose uniformity is usually difficult to achieve when treating samples of complex shape. The problems arise from the non-uniform plasma density and expansion of plasma sheath. A particle-in-cell computer simulation is used to study the time-dependent evolution of the plasma sheath surrounding two-dimensional objects during process of plasma immersion ion implantation. Before starting the implantation phase, steady-state nitrogen plasma is established inside the simulation volume by using ionization of gas precursor with primary electrons. The plasma self-consistently evolves to a non-uniform density distribution, which is used as initial density distribution for the implantation phase. As a result, we can obtain a more realistic description of the plasma sheath expansion and dynamics. Ion current density on the target, average impact energy, and trajectories of the implanted ions were calculated for three geometrical shapes. Large deviations from the uniform dose distribution have been observed for targets with irregular shapes. In addition, effect of secondary electron emission has been included in our simulation and no qualitative modifications to the sheath dynamics have been noticed. However, the energetic secondary electrons change drastically the plasma net balance and also pose significant X-ray hazard. Finally, an axial magnetic field has been added to the calculations and the possibility for magnetic insulation of secondary electrons has been proven.

The megalopa stage of Portunus spinimanus Latreille, 1819 and Portunus gibbesii (Stimpson, 1859) (Decapoda, Brachyura, Portunidae) from the southeastern Atlantic coast of the United States

The identification of megalopae from plankton samples is difficult, because this larval stage is the least well known among crab larvae, unknown in some species and poorly described in others. Wild megalopa specimens of some swimming crabs (family Portunidae Rafinesque, 1815) were captured alive from neuston samples obtained during summer surveys near the coast of Charleston, South Carolina (U.S.A). For identification purposes, larvae were reared to the 8th juvenile instar. After reaching the 5th juvenile instar, the juvenile crabs exhibited morphological features suitable for identification to the species level. The specimens belonged to two species of Portunidae, Portunus spinimanus Latreille, 1819 and P. gibbesii (Stimpson, 1859). Their megalopae were described in detail and compared to other portunid megalopae known from the southeastern Atlantic coast of the U.S.A. Species-specific characters of portunid megalopae are the number of carpal spines on the chelipeds, the relative size of the sternal spines (7th sternite), the number of antennal flagellum segments, and the setation of mouthparts. Copyright © 2007 Magnolia Press.

Economic relations of Brazil and the United States and the Enterprise for the Americas Initiative

Includes bibliography

The social roots of United States protectionism

Includes bibliography

A collection of documents on economic relations between the United States and Central America, 1906-1956

Includes bibliography

The United States and Latin America and the Caribbean: Highlights of economics and trade

Foreword by Alicia Bárcena

Some considerations on the possible effects of the failure of the United States Congress to renew the ATPDEA

Includes bibliography

Hispanics in the United States

Includes bibliography

Social Security in the United States: Overview and Outlook

Includes bibliography

Development of eco-efficient and sustainable urban infrastructure in Asia and Latin America: eco-efficiency and sustainable infrastructure in the United States and Canada

Incluye bibliografía

Trade in services negotiations: a review of the experience of the United States and the European Union in Latin America

Includes bibliography

Trends in United States trade with Latin America and the Caribbean and trade policy towards the region

Includes Bibliography

Estimating the effects of United States food safety and agricultural health standards on agro-food exports from Latin America and the Caribbean

Includes bibliography

Observatory of customs controls for United States imports: technical standards

Includes bibliography

A comparative study of health care policy in the United States and Canada: what policymakers in Latin America might and might not learn from their neighbors to the North

Includes bibliography