Muriel Braccini

Rheological behavior of the mushy zone at small strains

The modeling of the hot-tearing phenomena at the end of solidification requires the development of specific constitutive equations for the mushy zone. The presence of liquid films in the solid-liquid mixture has important consequences on the rheological behavior of the mushy zone. This aspect is particularly treated in order to propose a constitutive equation for the mushy zone in the low strain region. Starting from the behavior of the fully solid material, the mush is modeled as a porous medium saturated with liquid with partial cohesion. The cohesion of the mush is treated as an internal variable for which an evolution equation is proposed. Experimental data on a Sn-Pb model alloy both in the fully solid state and in the mushy state are used in order to study the advantages and the limitations of the proposed constitutive equation.

Study of stress in tensile nitrogen-plasma-treated multilayer silicon nitride films

The authors conducted a physico-chemical analysis of tensile sequential-nitrogen-plasma-treated silicon nitride films, which function as stressor liners in complementary metal oxide semiconductor (CMOS) technologies. These films are made of stacked nanometer-thick, plasma-enhanced, chemical vapor-deposited layers which were individually treated with N2-plasma, to increase stress. This study allowed us to monitor the evolution of the films’ chemical composition and stress as a function of process parameters such as deposition and post-N2-plasma duration. Consistent with secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM) and other physico-chemical analysis results, it was shown that the elementary component of the films can be modeled with a bi-layer consisting of an untreated slice at the bottom that is covered by a more tensile post-treated film. In addition, we observed that longer plasma treatments increase residual stress, SiN bond concentration and layer density, while redu...

Determination of the Rheological Behavior of Partially Solidified Alloys for the Prediction of Casting Defects

The aim of this paper is to detail the various experiments wich have been set-up to study the rheological behavior of partially solidified alloys for the purpose of modelling hot tear deformation. Before that, however, the general requirements for conducting these experiments will be presented. Some results concerning Al and Mg alloys will illustrate the approach

Characterisation of silicon nitride thin films used as stressor liners on CMOS FETs

Performance enhancement thanks to process induced strained technology is mandatory in recent CMOS technology (90 nm and beyond) to meet the device specifications [1]. In fact the strain induced in the Si channel results in band structure distortion and mobility increase [2]. Silicon Nitride (SiN) Contact Etch Stop Layers (CESL) are widely used for this purpose and bring for example up to 70 % saturation current increase in PMOS transistors. Typical physical and chemical characterisations are usually carried out but mechanical measurements are not common on such high stressed thin films. The purpose of this paper is the measurement of Youngs modulus and hardness by nanoindentation and picosecond acoustic. In this paper, we demonstrate that contrary to what the nanoindentation theory expect, one can mechanically characterise our 200 nm thickness high stressed SiN thin film. Moreover, picosecond acoustic appears as the best method for thinner films mechanical characterisation.

Methodology to Determine the Toughness of a Brittle Thin Film by Nanoindentation

Nanoindentation is the most convenient local technique for measuring elastic modulus, hardness, and fracture toughness of dielectric thin films. This approach is applied to bulk silicon and dielectric thin films (porous and non-porous) on silicon substrate. Reproducible stable cracks are generated from the edges of a cube corner indentor. The validity of theoretical model of use to estimate the toughness from cracks length has been checked on these reference cases. To calculate the toughness of thin film on Si substrate, we first established the loading range in which the cracks only affect the thin film without substrate damage. Several corrective terms have been introduced to the classical toughness estimation formula to take into account the proximity of the film/substrate interface and the residual stress pre-existing in the film. This approach is discussed by comparing experimental results obtained including these improvements to literature results.