Rafael Estevez

Chip to wafer copper direct bonding electrical characterization and thermal cycling

Copper direct bonding technology is considered to be one of the most promising approach for matching the miniaturization needs of future 3D integrated high performance circuits (3D-IC). In this study, we discuss the recent achievements in copper direct bonding technology with oxide/copper mixed surface and present the latest electrical and physical characterizations of chip to wafer bonding structures after annealing at 400°C and thermal cycling tests. In addition, electrical performance of chip to wafer bonding on 300mm wafers is also presented. Finally, thermo-mechanical finite element simulations showing the impact of the annealing conditions on the closure of the interface are shown.

Relationship Between Microstructure, Strength, and Fracture in an Al-Zn-Mg Electron Beam Weld: Part II: Mechanical Characterization and Modeling

This paper presents an experimental and modeling study of the mechanical behavior of an electron beam welded EN-AW 7020 aluminum alloy. The heterogeneous distribution of mechanical properties is characterized by micro-tensile tests and by strain field measurements using digital image correlation technic. These results are related to the microstructural observation presented in the companion paper. The mechanical behavior of the weld is simulated by a finite element model including a Gurson-type damage evolution model for void evolution. The model is shown to be capable of describing accurately experimental situations where the sample geometry is varied, resulting in stress triaxiality ratios ranging from 0.45 to 1.3.

Structural optimization under overhang constraints imposed by additive manufacturing technologies

Abstract This article addresses one of the major constraints imposed by additive manufacturing processes on shape optimization problems – that of overhangs, i.e. large regions hanging over void without sufficient support from the lower structure. After revisiting the ‘classical’ geometric criteria used in the literature, based on the angle between the structural boundary and the build direction, we propose a new mechanical constraint functional, which mimics the layer by layer construction process featured by additive manufacturing technologies, and thereby appeals to the physical origin of the difficulties caused by overhangs. This constraint, as well as some variants, is precisely defined; their shape derivatives are computed in the sense of Hadamards method, and numerical strategies are extensively discussed, in two and three space dimensions, to efficiently deal with the appearance of overhang features in the course of shape optimization processes.

A Possible Mechanism for Protrusions Formation at the Metal/Oxide Interface During Short Time Oxidation of Ferritic Stainless Steel

High temperature oxidation of ferritic stainless steel for short durations leads to the formation of an original morphology at the metal/oxide interface. This interface is composed of metallic protrusions localized in a chromium-rich oxide layer through a discontinuous silica film. In this paper we propose a mechanism based on preferential diffusion paths for the oxygen through the oxide that are governed by the distribution of the hydrostatic pressure in this layer. We point out that the mechanical contrast between the oxide and the metal subjected to creep can be critical for the hydrostatic pressure gradient magnitude inside the oxide layer. This observation is likely to promote the formation of protrusions for specific conditions of temperature and time of exposure to oxidation.

Mechanics of cracking failure in a silver layer deposited by inkjet printing on a flexible substrate

This work intends to investigate strain fields around cracks in thin metallic film deposited on flexible substrate through an experimental approach. The studied sample consists of a pre-cracked silver nanoparticles inkjet printed layer with a thickness of 2 μm deposited on 125 μm thick polyimide substrates. The mechanical induced failure experiments are performed on a tensile testing device installed under a scanning electron microscope (SEM) allowing in situ cracking observations through digital images correlation (DIC). The samples are deformed up to 2 %. In order to follow the deformation during the test though DIC, speckle patterns of 100 nm wide and 50 nm depth are milled on the film surface with focused ions beam (FIB). The displacement field is obtained experimentally with DIC, the strain field being too noisy to be exploited. These experimental results are compared with finite elements models (FEM). Three-dimensional models are developed using the ABAQUS code and allow assessing the strain distribution in a cracked film around the crack tip. These FEM results agree with the experimental strain in front of the crack tip. The crack disturbs the imposed homogeneous strain at a distance of 200 μm laterally and 70 μm ahead.

DESIGN OF ISOTROPIC MICROSTRUCTURES VIA A TWO-SCALE APPROACH

Architectured materials are promising to reach extreme properties and ultimately address issues related to lightweight or non conventional properties for bulk materials (eg. high specific rigidity, extremal conductivity or auxetism (negative Poisson’s ratio)) [1]. A very efficient way to obtain optimal forms is via inverse homogenization, i.e. using shape and topology optimization techniques in order to achieve target material properties [2]. A great number of publications has been devoted to the design of isotropic materials with extreme properties. Isotropy is usually prescribed via a combination of symmetric planes and penalization techniques, which are quite delicate to handle in an optimization framework. In this work, we present an approach for the design of isotropic multi-materials with extremal conductivity via laminate geometries, consisting in anisotropic phases [3]. More specifically, we design composites with extremal conductivity using rank-1 laminates, composed by two orthotropic phases along parallel layers. The second phase is obtained by a 90-degree rotation of the first one, while their volume fractions are explicitly chosen so that the laminate is isotropic. The orthotropic phases are considered to have their own periodic micro-structure, composed by multiple phases. By optimally distributing the different phases in a periodic cell, we can achieve the Hashin-Shtrikman bounds for the isotropic laminate. We present examples in two dimensions using the level-set method for shape and topology optimization [4]. Alexis Faure, Georgios Michailidis, Rafael Estevez, Guillaume Parry and Grégoire Allaire

Low temperature metal bonding for 3D and power device packaging

Low temperature copper-copper direct bonding at ambient air on plain and patterned surfaces was developed at CEA LETI. In this paper we will review this bonding in terms of simulation models, process technology, electrical characterization and reliability. Wafer to wafer, die to wafer and self-assembly will be analyzed.