Y. Wouters

Characterization of Chromia Scales Grown on Pure Chromium in Different Oxidizing Atmospheres

Abstract Scanning electron microscopy, X-ray diffraction, photoelectrochemistry and microphotoelectrochemistry have been used to characterize thin chromia scales, in the micrometer range, grown on pure chromium at 900°C in oxygen and in water vapour. The duplex structure formed, more easily observable in water vapour grows by the opposite transport of chromium and of oxide/hydroxide ions. The external chromia subscale exhibits the usually reported 3.5 eV bandgap whereas the internal subscale presents a reduced gap possibly due to impurity contribution. Imaging the photocurrent generated in this subscale allows the observation of good metal-oxide interface properties of samples grown in H2O, whereas samples grown in O2, (liable to cracking during cooling), exhibit partially disrupted zones.

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)2O3 initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.

Possible connection between nodule development and presence of niobium and/or titanium during short time thermal oxidation of AISI 441 stainless steel in wet atmosphere

Abstract AISI 441 ferritic stainless steel is a good candidate for metallic interconnects of solid oxide fuel cells (SOFCs). In this alloy, the minor elements Ti and Nb are used to stabilise the ferritic structure but their influence on steel durability is not well understood. This study focuses on the early stages of oxidation (24 h) at 800°C of AISI 441 under 5%H2O in O2 following the cathodic SOFCs conditions. The typical duplex oxide scale, composed of a (Mn,Cr)3O4 spinel top layer and a Cr2O3 rich sublayer is observed, with oxide nodules growing in places. These objects, in the micrometre range in size, are studied by FIB tomography. The analyses reveal a complex structure and a development strongly linked to the presence of niobium and/or titanium compound(s) in the subjacent substrate.

Electromigration multistress pattern technique for copper drift velocity and Black’s parameters extraction

Package electromigration tests were performed on dual damascene copper interconnects issued from the 90, 65 and 45 nm-node technologies. By varying the stress conditions during the steady state regime of the resistance increase, we study drift velocity evolution as a function of the current density j. Thus we show some large deviation of the Blacks law, which assumes a linear dependence of the lifetime with j. Finally we have determined the Blacks exponent n in a broad range of current density j (2.5-50 mA/mum2). For 250 mum long lines, a n=1 is obtained between 10 and 20 mA/mum2, it increases up to 2 for j=2.5 mA/mum2.

Effect of reservoir on electromigration of short interconnects

As the interconnect cross-sections are ever scaled down, a particular care must be taken on the tradeoff between increase of current density in the back end of line and reliability to prevent electromigration (EM). Some lever exists as the well-known Blech effect [1]. One can take advantage of the EM induced backflow flux that counters the EM flux. As a consequence, the total net flux in the line is reduced and additional current density in designs can be allowed in short lines. However, the immortality condition is most of the time addressed with a standard test structures ended by two vias [2]–[3]. Designs present complex configurations far from this typical case and the Blech product (jL)c can be deteriorated or enhanced [4]. In the present paper, we present our study of EM performances of short lines ended by an inactive end of line (EOL) at one end of the test structure. Significant differences on the median time to failure (MTF) are observed with respect to the current direction, from a quasi deletion of failure to a significant reduction of the Blech effect. Based on the resistance saturation, a method is proposed to determine effective lengths of inactive EOL configurations corresponding to the standard case.

Use of bidirectional current stress for in depth analysis of electromigration mechanism

Electromigration under bidirectional current is studied on dual damascene copper interconnects for the 65 nm node. Physical analyses confirm void location a both ends of the line and copper transport over long distance. Resistance evolution was studied and correlated to void healing/growth kinetics. Finally, we show the interest of bidirectional tests to study multimodal failure mode.

Grain boundary as relevant microstructure feature for electromigration in advanced technology studied by Electron BackScattered Diffraction

The work presented in this paper shows the links between electromigration (EM) in copper interconnects and microstructure of copper. Metal lines of 70 nm width corresponding to minimum width of 45–40 nm technology node are aged by electromigration (EM) test. Electron Backscattered Diffraction (EBSD) technique is then used to characterize microstructure and orientation of copper grains around void(s) resulting from EM. Advanced TEM and STEM characterization have also been used to assess reliability of EBSD technique (void localization, texture determination) applied to very small dimensions. Finally, the influence of cobalt (Co) as capping or sidewall liner and Aluminum-Copper alloy (CuAl) seed layer were investigated by EM tests and physical characterization. Our results confirm that critical microstructure parameter for electromigration phenomena in copper is grain boundary and in particular high angle misoriented grain boundaries.

Haematite and chromia dissolution in the zirconia matrix during thermal oxidation of Laves-phases γ–Zr(Fe,Cr)2 on Zircaloy-4

Abstract The life time of the Zircaloy-4 alloy is linked to the role that intermetallic particles play during the growth of the zirconia scale. In the present work, a photoelectrochemical technique (PEC), has been used to study the phenomena of oxidation of intermetallic particles but also of dissolution of thermal oxidised particles. Both phenomena have been addressed in the literature and studied in particular with TEM; in this work, they were clearly imaged by photoelectrochemical microscopy at a mesoscopic view. Our results do contribute to a better understanding of the major impact that intermetallic precipitates could have on the oxidation rate.

Microstructure local effect for electromigration reliability improvement and Cu damascene lines design rules relaxation

Implication of microstructure during electromigration void formation process is studied in this paper. Dual damascene Cu lines with cathode end width variations are characterized. The idea of these variations being to force different microstructure profiles within the same test structure. The narrow-to-wide (NTW) width transition structure shows better electromigration performances than usual structure with a constant width (CW) along the line length. Bamboo Cu grains in the NTW structure wider segment are evidenced to slow down Cu atoms migration, inducing a local Blechs-like effect. This results in delayed void nucleation time and reduced void growth rate leading to increased lifetime. By this study, we show how great advantage can be taken on lifetime with this new design.

Improved statistical analysis at low failure rates in Cu electromigration using an innovative multilink test structure

An innovative electromigration test structure is described in this paper. This new structure consisting of serial connected links is designed to address very early percentiles of lognormal electromigration failure time distribution and highlight extrinsic failures. The simplicity of implementation, data treatment and the correlation with elemental dual damascene test lines make this structure a pretty good candidate for the future of interconnects reliability.