Jean Berthier

On the influence of surfactants in electrowetting systems

Buffer liquids currently used in biotechnology contain surfactants. These surfactants modify the wetting properties of the liquid and affect the behavior of droplets in electrowetting on dielectric (EWOD) microsystems. An analysis of the effect of surfactants in EWOD-based microsystems, at equilibrium and in a non-equilibrium state, is given here. First, a reference for the value of the surface tension and its time variation is obtained by the use of the pendant drop method, showing that the time variation of the surface tension follows a decreasing exponential law. Next, equilibrium experiments have been performed on a specially designed EWOD experimental microsystem. It has been observed that the Lippmann–Young theoretical equation for electrowetting is satisfied at equilibrium, even in the presence of surfactants. However, at the CMC, equilibrium surface tension values are different from those obtained by the pendant drop method. This difference is explained by an internal convective motion that remixes the surfactants. This internal motion is observed by following the motion of fluorophores. Consequently, surfactant concentrations about ten times the CMC are required to obtain the same values of the surface tension in the pendant drop method and by EWOD measurements. In a second section, it is shown that the behavior of surfactant-loaded droplets in EWOD-based microsystems can be explained by considering that the surface tension relaxes with time according to the exponential law found in the first section.

Self-alignment of silicon chips on wafers: A capillary approach

As the limits of Moore’s law are approached, three-dimensional integration appears as the key to advanced microelectronic systems. Die-to-wafer assembly appears to be an unavoidable step to reach full integration. While robotic methods experience difficulties to accommodate fabrication speed and alignment accuracy, self-assembly methods are promising due to their parallel aspect, which overcomes the main difficulties of current techniques. The aim of this work is the understanding of the mechanisms of self-alignment with an evaporating droplet technique. Stable and unstable modes are examined. Causes for misalignments of chips on wafers and their evolution are investigated with the help of the SURFACE EVOLVER numerical software. Precautions for suitable alignment are proposed.

Impact of containment and deposition method on sub-micron chip-to-wafer self-assembly yield

3D technologies need a high speed high alignment accuracy chip-to-wafer hybridation technique. This paper will focus on chip-to-wafer self assembly processes coupled with direct bonding hybridation. Submicronic alignment accuracy and a 90 per cent self-assembly process yield are obtained. The self-assembly process yield is analyzed in term of alignment accuracy and direct bonding quality. The impact of the chips surface state (hydrophilic, hydrophobic, and mixed) on fluid containment efficiency and self-assembly process yield will be discussed. Topological containment (canthotaxis effect) is also evaluated with regards to structures height. Finally, the alignment yield as a function of deposition parameters will be described.

High density chip-to-wafer integration using self-assembly: On the performances of directly interconnected structures made by direct copper/oxyde bonding

We present here our latest results on chips-to-wafer 3D structures obtained with the self-assembly technology adapted on copper/oxide patterned surfaces. Technological integration, bonding quality and alignment accuracy are presented and the electrical contact of the interconnection is evaluated. High speed high alignment accuracy chip-to-wafer hybridation technique is mandatory for 3D technology. Chip-to-wafer self-assembly processes coupled to direct bonding hybridization is on the merge to breakthrough this issue. In a previous work [1], we demonstrated submicronic alignment accuracy and a 90% self-assembly process yield with this technique. In this paper, we discuss on interconnect electrical characterization of self-assembled chips compared to chips assembled with conventional Pick and Place method. Interface resistance is evaluated on daisy chain and Kelvin structures. The quantification of the alignment is measured thanks to vernier and is in the range of a few hundred nanometers. The liquid drop impact on assembled structure, considering the different aspects (bonding quality, Cu-chemical oxidation, mechanical chip level bow and electrical resistance) is discussed.

Magnetic confinement of paramagnetic micro and nano-particles away from solid walls

In many biological processes, like medical diagnostic, using paramagnetic micro or nanosized particles, adherence of these particles to the walls is a major drawback to the efficiency of the process. Usually, one tries to use electrostatic repulsion and/or special wall chemical treatment to reduce the number of particles sticking to the walls. Another way of keeping the particles away from the walls, based upon magnetic repulsion, is presented here. The advantage is that it does not require changing the chemical composition of the fluid/walls, it acts only externally on the magnetic colloid. The method is first described by numerical simulation using a finite element approach. Then an experimental confirmation of the principle is shown.

Scaling rules for flow focusing devices: From standard to large FFDs for cell encapsulation in alginates

Flow focusing devices (FFDs), with dimensions smaller than 100µm have recently been used for cell encapsulation. However, encapsulation of larger clusters of cells, like islets of Langerhans, is of considerable interest. Producing larger capsules requires larger FFDs. We focus here on the geometrical scaling-up between a small FFD (50µm) and a larger one (100µm, 200µm, 1mm). We determine the driving pressure conditions to obtain the droplet regime at any size of the FFD. A similar behavior of the device is experimentally obtained by a scaling of the dimensions by a ratio λ and a scaling of the driving pressures by the ratio 1/λ.