Ulf Helmersson

Bio: Ulf Helmersson is an academic researcher from Linköping University. The author has contributed to research in topics: Sputter deposition & Thin film. The author has an hindex of 54, co-authored 237 publications receiving 11954 citations. Previous affiliations of Ulf Helmersson include Chinese Academy of Sciences.

A novel pulsed magnetron sputter technique utilizing very high target power densities

Abstract: Using a novel pulsed power supply in combination with a standard circular flat magnetron source, operated with a Cu target, a peak power density of 2800 W cm -2 was achieved. This results in a very intense plasma with peak ion current densities of up to 3.4 A cm −2 at the substrate situated 10 cm from the target. The ionized fraction of the deposited Cu flux was estimated to be approximately 70% from deposition rate measurements. The potential for high-aspect-ratio trench filling applications by high power pulsed magnetron sputtering is demonstrated by deposition in via-structures. The high power pulsed technique also results in a higher degree of target utilization and an improved thickness uniformity of the deposited films compared with conventional d.c. magnetron sputtering.

Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness

Abstract: Single‐crystal TiN/VN strained‐layer superlattices (SLS’s) with layer thicknesses lTiN =lVN =λ/2 (where λ is the period of the superlattice) ranging from 0.75 to 16 nm have been grown on MgO(100 ) substrates by reactive magnetron sputtering. Cross‐sectional transmission electron microscopy (TEM) and x‐ray diffraction examinations showed that the films were single crystals exhibiting coherent interfaces and several orders of superlattice reflections. There was no evidence in either plan‐view or cross‐sectional TEM analyses of misfit interfacial dislocation arrays. The primary defects observed were dislocation loops with a diameter of 8–10 nm extending through several layers and small defects with a diameter of 1–2 nm that were confined within single layers. Microindentation hardness values H, measured as a function of λ in films with a total thickness of 2.5 μm, increased from 2035±280 kg mm−2 for Ti0.5V0.5N alloys (i.e., λ=0) to reach a maximum of 5560±1000 kg mm−2 at λ=5.2 nm and then decreased rapidly t...

High power impulse magnetron sputtering discharge

Abstract: The high power impulse magnetron sputtering (HiPIMS) discharge is a recent addition to plasma based sputtering technology. In HiPIMS, high power is applied to the magnetron target in unipolar pulse ...

On the film density using high power impulse magnetron sputtering

Abstract: The influence on thin film density using high power impulse magnetron sputtering (HIPIMS) has been investigated for eight different target materials (Al, Ti, Cr. Cu, Zr, Ag, Ta, and Pt). The densit ...

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Perspective on the Development of Lead-free Piezoceramics

Abstract: A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Microstructural evolution during film growth

Abstract: Atomic-scale control and manipulation of the microstructure of polycrystalline thin films during kinetically limited low-temperature deposition, crucial for a broad range of industrial applications, has been a leading goal of materials science during the past decades. Here, we review the present understanding of film growth processes—nucleation, coalescence, competitive grain growth, and recrystallization—and their role in microstructural evolution as a function of deposition variables including temperature, the presence of reactive species, and the use of low-energy ion irradiation during growth.