This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.

Quantification of Grain Boundary Segregation in Nanocrystalline Material

In the world of tomographic imaging, atom probe tomography (APT) occupies the high-spatial-resolution end of the spectrum. It is highly complementary to electron tomography and is applicable to a wide range of materials. The current state of APT is reviewed. Emphasis is placed on applications and data analysis as they apply to many fields of research and development including metals, semiconductors, ceramics, and organic materials. We also provide a brief review of the history and the instrumentation associated with APT and an assessment of the existing challenges in the field.

Atom Probe Tomography 2012

Hydrogen embrittlement is a complex phenomenon, involving several length- and timescales, that affects a large class of metals. It can significantly reduce the ductility and load-bearing capacity and cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Despite a large research effort in attempting to understand the mechanisms of failure and in developing potential mitigating solutions, hydrogen embrittlement mechanisms are still not completely understood. There are controversial opinions in the literature regarding the underlying mechanisms and related experimental evidence supporting each of these theories. The aim of this paper is to provide a detailed review up to the current state of the art on the effect of hydrogen on the degradation of metals, with a particular focus on steels. Here, we describe the effect of hydrogen in steels from the atomistic to the continuum scale by reporting theoretical evidence supported by quantum calculation and modern experimental characterisation methods, macroscopic effects that influence the mechanical properties of steels and established damaging mechanisms for the embrittlement of steels. Furthermore, we give an insight into current approaches and new mitigation strategies used to design new steels resistant to hydrogen embrittlement.

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Understanding and mitigating hydrogen embrittlement of steels: a review of experimental, modelling and design progress from atomistic to continuum.

The design of atomic-scale microstructural traps to limit the diffusion of hydrogen is one key strategy in the development of hydrogen-embrittlement–resistant materials. In the case of bearing steels, an effective trapping mechanism may be the incorporation of finely dispersed V-Mo-Nb carbides in a ferrite matrix. First, we charged a ferritic steel with deuterium by means of electrolytic loading to achieve a high hydrogen concentration. We then immobilized it in the microstructure with a cryogenic transfer protocol before atom probe tomography (APT) analysis. Using APT, we show trapping of hydrogen within the core of these carbides with quantitative composition profiles. Furthermore, with this method the experiment can be feasibly replicated in any APT-equipped laboratory by using a simple cold chain.

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Direct observation of individual hydrogen atoms at trapping sites in a ferritic steel

Dopant distributions in n-MOSFET structure observed by atom probe tomography.

Using 1M DMA-TEG, the analyses of 5sigma Vth fluctuation in 65 nm-MOSFETs were carried out. Physical and electrical analyses confirmed that random dopant fluctuation is dominant though NMOSFET has larger fluctuation as compared with PMOSFET. To explain this phenomenon, a B clustering model is proposed. In the case of clustering with 5 to 6 B atoms in the channel, Vth fluctuation of NMOSFET can be explained.

Analyses of 5σ V th fluctuation in 65nm-MOSFETs using takeuchi plot

Three dimensional dopant distributions in polycrystalline Si gate of n-type (n-) and p-type (p-) metal-oxide-semiconductor field effect transistor (MOSFET) structure were investigated by laser-assisted three dimensional atom probe. The remarkable difference in dopant distribution between n-MOSFET and p-MOSFET was clearly observed. In n-MOSFET gate, As and P atoms were segregated at grain boundaries and the interface between gate and gate oxide. No diffusion of As and P atoms into the gate oxide was observed. On the other hand, in p-MOSFET, no segregations of B atoms at grain boundaries or the interface were observed, and diffusion of B atoms into the gate oxide was directly observed.

Dopant distribution in gate electrode of n- and p-type metal-oxide-semiconductor field effect transistor by laser-assisted atom probe

The fabrication of future nanoscale semiconductor devices calls for precise placement of dopant atoms into their crystal lattice. Monolayer doping combined with a conventional spike annealing method provides a bottom-up approach potentially viable for large scale production. While the diffusion of the dopant was demonstrated at the start of the method, more sophisticated techniques are required in order to understand the diffusion, at the near surface, of P and contaminants such as C and O carried by the precursor, not readily accessible to direct time-of-flight secondary ion mass spectrometry measurements. By employing atom probe tomography, we report on the behavior of dopant and contaminants introduced by the molecular monolayer doping method into the first nanometers. The unwanted diffusion of C and O-related molecules is revealed and it is shown that for C and O it is limited to the first monolayers, where Si–C bonding formation is also observed, irrespective of the spike annealing temperature. From the perspective of large scale employment, our results suggest the benefits of adding a further process to the monolayer doping combined with spike annealing method, which consists of removing a sacrificial Si layer to eliminate contaminants.

Behavior of phosphorous and contaminants from molecular doping combined with a conventional spike annealing method

The PZT polarization hysteresis characteristics in a Pt/PZT/Pt ferroelectric capacitor are degraded by annealing in a hydrogen-containing atmosphere due to the catalytic effect of the top Pt electrode. This can be avoided by using an IrO2/PZT/Pt capacitor structure as we proposed. During hydrogen annealing of the as-grown capacitor, the top IrO2 electrode is deoxidized, degrading the capacitor characteristics. However, the polarization hysteresis characteristics are preserved after hydrogen annealing at 300° C if the IrO2/PZT/Pt capacitor is pre-annealed in oxygen at 600° C.

Fumiko Yano

Papers

Dopant distributions in n-MOSFET structure observed by atom probe tomography.

Analyses of 5σ V th fluctuation in 65nm-MOSFETs using takeuchi plot

Dopant distribution in gate electrode of n- and p-type metal-oxide-semiconductor field effect transistor by laser-assisted atom probe

Behavior of phosphorous and contaminants from molecular doping combined with a conventional spike annealing method

IrO2/Pb(ZrxTi1-x)O3(PZT)/Pt Ferroelectric Thin-Film Capacitors Resistant to Hydrogen-Annealing Damage