Luca Del Carro

All-Copper Flip Chip Interconnects by Pressureless and Low Temperature Nanoparticle Sintering

Flip chip interconnects purely made out of Cu, so-called all-Cu interconnects, have the potential to overcome the present current capacity limit of state-of-the-art solder based interconnects, while meeting the demand for ever decreasing interconnect pitches. Parasitic effects in solder based interconnects, caused by interdiffusion of various metals, are mitigated in all-Cu interconnects. In this work, all-Cu interconnects were formed by the use of low temperature and pressureless sintering of Cu nanoparticles. Thereby, a Cu paste material was applied between the Cu pillars of a silicon chip and the Cu pads on a silicon substrate by a dip transfer method. The electrical and mechanical properties of sintered Cu were characterized on films of the same Cu pastes. The porous films resulted in 4.4 times higher electrical resistivity and one order of magnitude reduced mechanical stiffness and tensile strength compared to bulk Cu. All-Cu interconnects with a diameter of 30 μm and a pitch of 100 μm were formed with an optimized Cu particle distribution and sintering procedure. Resistances down to 1.7 ± 0.5 mO were measured for these all-Cu interconnects which is comparable to solder based benchmark interconnects. However, the porosity of the sintered Cu interconnect results in lower shear strength compared to the solder benchmark.

On the Evaporation of Colloidal Suspensions in Confined Pillar Arrays

The thermal and electrical transport capabilities of materials in electronic packaging are key to supporting high-performance microelectronic systems. In composite and hybrid materials, both of these transport capabilities are limited by contact resistances. We propose a directed nanoparticle assembly method to reduce contact resistances by transforming point contacts between micrometer-sized objects into quasi-areal contacts. The nanoparticle assembly is directed by the formation of liquid bridges in contact points during the evaporation of a colloidal suspension. In this work, we experimentally study the evaporation of colloidal suspensions in confined porous media to yield uniform nanoparticle assembly, as required for electronic packaging. The evaporation pattern of liquids in confined pillar arrays is either branched or straight, depending on the surface tension of the liquid and on the pore size defined by the pillar size and spacing. Stable evaporation fronts result in uniform nanoparticle deposition above a bond number threshold of 10

Morphology of Low-Temperature All-Copper Interconnects Formed by Dip Transfer

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Review on Percolating and Neck-Based Underfills for Three-Dimensional Chip Stacks

-3. However, at reduced evaporation dynamics, liquid pinning results from colloidal particle accumulations at the liquid–vapor interface, ultimately leading to undesired colloidal bridging between pillars.

Laser Sintering of Dip-Based All-Copper Interconnects

A high thermal conductivity of the underfill material is key for efficient heat removal from 3D chip stacks. Recently, the excellent thermal properties of underfills in which nanoparticle self-assembly formed necks between the filler particles were demonstrated. To industrially apply neck-based thermal underfills, the robustness of neck formation must be improved. Accordingly, it is crucial to understand the mechanisms of suspension drying in confined spaces. In this work, we present a study of particle self-assembly during colloidal suspension evaporation. The processes were directly observed by fluorescence imaging. In contrast to earlier work, particle self-assembly was studied not only within a model pillar array, but also inside a more representative filler-particle bed. First, microparticle assembly was investigated in porous cavities containing silicon micropillar arrays with different inter-pillar spacings. The 1-μm polystyrene particles used assembled by capillary bridging between the pillars. Pinning of the evaporation front at the outermost pillar row and particle transport to the cavity edges occurred. Accordingly, a large gradient in particle deposition was observed. The pinning could be mitigated by high evaporation temperatures, which led to fast propagation of the drying front. This resulted in directional bridging and a more uniform particle deposition. Moreover, a defect-free particle assembly on the millimeter scale was achieved by guiding the front with spiral pillar arrangements. Second, neck formation was studied in cavities filled with 250-μm filler spheres. In these particle beds, two evaporation stages were observed: vapor invasion along the larger pores and subsequent drying of the capillary bridges linking the filler particles. Homogeneous neck formation was obtained by evaporating a 1 wt% polystyrene suspension at 30 °C. Higher temperatures and concentrations both resulted in enhanced particle deposition at the cavity edges.

Erratum: “Review on Percolating and Neck-Based Underfills for Three-Dimensional Chip Stacks” [ASME J. Electron. Packag., 2016, 138(4), p. 041009; DOI: 10.1115/1.4034927]

Flip-chip interconnects made entirely from copper are needed to overcome the intrinsic limits of solder-based interconnects and match the demand for increased current densities. To this end, dip-based all-copper interconnects are a promising approach to form electrical interconnects by sintering copper nanoparticles between the copper pillar and pad. However, the remnant porosity of the copper joint formed between the pillar and the pad limits the performance of this technology. Moreover, the applicability of this technology in the printed circuit board (PCB) industry is endangered by thermo-mechanical stresses that arise during the sintering and by the unknown compatibility with standard finishing layers used to prevent the oxidation of the copper. This work reports three main advances in dip-based all-copper interconnect technology. First, a reduction in the porosity level of the copper joint is obtained by application of pressure during the bonding. Second, a decrease of the bonding temperature to 160 °C is achieved. Third, the compatibility of this technology with standard finishing layers is demonstrated.