Tom Van der Donck

Correlation between Cu microstructure and TSV Cu pumping

Cu pumping is the irreversible extrusion of Cu from Cu-filled through-silicon vias (TSVs) exposed to high temperatures during back-end of line (BEOL) processing. The distribution of Cu pumping values over the TSVs of a single wafer has a large intrinsic spread. As potential BEOL reliability issues due to Cu pumping will first occur at the highest pumping TSVs, they can be mitigated if the fundamental cause for this large intrinsic spread is known and under control. This paper describes a clear correlation between Cu pumping and TSV Cu microstructure based on the grain size at the top of 5×50 μm TSV, disregarding twin boundaries. For the mitigation of TSV Cu pumping the ideal microstructure was shown to consist of a single grain spanning the whole TSV cross section, bringing down the highest measured Cu pumping value from 248 nm to 73 nm. This effect was attributed to the absence of rapid diffusion paths and grain boundary sliding ability.

Effect of deposition parameters on the stress gradient of CVD and PECVD poly-SiGe for MEMS applications

The effect of the deposition parameters and Ge content on the stress gradient in poly-SiGe films was investigated. The films, ranging in thickness from 1.2 to 2.3 μm, were deposited by chemical vapor deposition (CVD) at 450 °C and plasma enhanced chemical vapor deposition (PECVD) at 520 °C. The Ge content was varied between 45 and 64 at%. Xray diffraction revealed that both PECVD and CVD films were polycrystalline. The stress gradient was determined by measuring the deflection of 1 mm long released cantilevers. The stress gradient was found to decrease with increasing Ge content. A CVD film with 55 at% Ge was thinned using a very low power SF6/O2 plasma. The stress gradient was measured as a function of film thickness. The stress profile was calculated by matching the bending moment of the calculated profile to the bending moment obtained from the measured stress gradient. The largest change in stress occurs right at the thin film/substrate interface. PECVD films were found to possess a lower stress gradient compared to CVD films with similar thickness. This was explained by differences in TEM microstructure: CVD films have more Vshaped grains, while PECVD films have more columnar grains.

Surface acoustic wave depth profiling of a functionally graded material

The potential and limitations of Rayleigh wave spectroscopy to characterize the elastic depth profile of heterogeneous functional gradient materials are investigated by comparing simulations of the surface acoustic wave dispersion curves of different profile-spectrum pairs. This inverse problem is shown to be quite ill posed. The method is then applied to extract information on the depth structure of a glass-ceramic (alumina) functionally graded material from experimental data. The surface acoustic wave analysis suggests the presence of a uniform coating region consisting of a mixture of Al2O3 and glass, with a sharp transition between the coating and the substrate. This is confirmed by scanning electron microscope with energy dispersive x-ray analysis.

Stacked Boron Doped Poly-Crystalline Silicon-Germanium Layers: an Excellent MEMS Structural Material

In this work stacked boron doped poly-crystalline Silicon-Germanium (poly-SiGe) layers, which can be applied as structural MEMS layers, were studied. A standard 1 µm base layer, deposited at 480 oC chuck temperature, is stacked until the required thickness (e.g. 10 x for a 10 µm thick layer). This 1 µm base layer consists of a PECVD seed layer (+/− 75 nm), a CVD crystallization layer (+/− 135 nm) and a PECVD layer to achieve the required thickness with a high growth-rate. The top part of this PECVD layer can optionally be used for optimizing the stress gradient by a stress compensation layer. This approach resulted in 4 µm thick poly-SiGe MEMS structural layers with low tensile stress (50 MPa), low resistivity (2 mΩcm) and a low strain gradient ( −5 /µm).

Texture in steel plates revealed by laser ultrasonic surface acoustic waves velocity dispersion analysis

HIGHLIGHTSA laser‐ultrasonic approach for texture measurement was proposed.A clear SAW angular dependence which was strongly connected with texture was found.SAW velocity angular dispersion within single grains was numerically determined.SAW velocities and variation in {1 1 1} texture were smaller than in {1 1 0} texture. ABSTRACT A photoacoustic, laser ultrasonics based approach in an Impulsive Stimulated Scattering (ISS) implementation was used to investigate the texture in polycrystalline metal plates. The angular dependence of the ‘polycrystalline’ surface acoustic wave (SAW) velocity measured along regions containing many grains was experimentally determined and compared with simulated results that were based on the angular dependence of the ‘single grain’ SAW velocity within single grains and the grain orientation distribution. The polycrystalline SAW velocities turn out to vary with texture. The SAW velocities and their angular variations for {1 1 0} texture were found to be larger than that the ones for {1 1 1} texture or the strong &ggr; fiber texture. The SAW velocities for {0 0 1} texture were larger than for {1 1 1} texture, but with almost the same angular dependence. The results infer the feasibility to apply angular SAW angular dispersion measurements by laser ultrasonics for on‐line texture monitoring.

Mechanisms of fine extinction band development in vein quartz: new insights from correlative light and electron microscopy

Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz.

Statistical Distribution of Through-Silicon via Cu Pumping

Cu pumping is defined as the irreversible extrusion of Cu from Cu-filled through-silicon vias (TSVs) exposed to high temperatures. The distribution of Cu pumping values over the TSVs of a single wafer has a large intrinsic spread. In previous publications both a lognormal distribution and a distribution of the maximum of two normal variables were used to fit experimental data. In this paper, these two types of statistical distribution are compared, showing that the maximum of two normal distributions provides a better fit, in particular at the right tail which is more significant for the potential reliability impact of Cu pumping. Also, it is shown how Cu pumping is determined by the network of random high angle grain boundaries in the Cu region near the TSV top, as an extension of a previous analysis which occurred at the TSV top surface only. This relation between Cu pumping and Cu microstructure provides a physical interpretation of the maximum of two normal distributions, based on the deformation mechanisms underlying Cu pumping.