Wafer Scale Insulation of High Aspect Ratio Through-Silicon Vias by iCVD.

In microelectronics, one of the main 3D integration strategies consists of vertically stacking and electrically connecting various functional chips using through-silicon vias (TSVs). For the fabrication of the TSVs, one of the challenges is to conformally deposit a low dielectric constant insulator thin film at the surface of the silicon. To date, there is no universal technique that can address all types of TSV integration schemes, especially in the case requiring a low deposition temperature. In this work, an organosilicate polymer deposited by initiated chemical vapor deposition (iCVD) was developed and integrated as an insulating layer for TSVs. Process studies have shown that poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (P(V3D3)) can present good conformality on high aspect ratio features by increasing the substrate temperature up to 100 °C. The trade-off is a moderate deposition rate. The thermal stability of the polymer has been investigated, and we show that a thermal annealing at 400 °C (with or without ultraviolet exposure) allows the stabilization of the dielectric films by removing residual oligomers. Then, P(V3D3) was integrated in high aspect ratio TSV (10 × 100 µm) on 300 mm silicon wafers using a standard integration flow for TSV metallization. Functional devices were successfully fabricated (including daisy chains of 754 TSVs) and electrically characterized. Our work shows that the metallization barrier should be carefully selected to eliminate the appearance of voids at the top corner of the TSV after the Cu annealing step. Moreover, an appropriate integration process should be used to avoid the appearance of cohesive cracks in the liner. This work constitutes a first proof of concept of the use of an iCVD polymer in a quasi-industrial microelectronic environment. It also highlights the benefit of iCVD as a promising technique to deposit conformal dielectric thin films in a microelectronic pilot line environment.

Elucidating the Mechanisms of Two Unique Phenomena Governed by Particle-Particle Interaction under DEP: Tumbling Motion of Pearl Chains and Alignment of Ellipsoidal Particles.

Particle-particle interaction plays a crucial role in determining the movement and alignment of particles under dielectrophoresis (DEP). Previous research efforts focus on studying the mechanism governing the alignment of spherical particles with similar sizes in a static condition. Different approaches have been developed to simulate the alignment process of a given number of particles from several up to thousands depending on the applicability of the approaches. However, restricted by the simplification of electric field distribution and use of identical spherical particles, not much new understanding has been gained apart from the most common phenomenon of pearl chain formation. To enhance the understanding of particle-particle interaction, the movement of pearl chains under DEP in a flow condition was studied and a new type of tumbling motion with unknown mechanism was observed. For interactions among non-spherical particles, some preceding works have been done to simulate the alignment of ellipsoidal particles. Yet the modeling results do not match experimental observations. In this paper, the authors applied the newly developed volumetric polarization and integration (VPI) method to elucidate the underlying mechanism for the newly observed movement of pearl chains under DEP in a flow condition and explain the alignment patterns of ellipsoidal particles. The modeling results show satisfactory agreement with experimental observations, which proves the strength of the VPI method in explaining complicated DEP phenomena.

Elucidating the mechanism governing cell rotation under DEP using the volumetric polarization and integration method.

Cell rotation can be achieved by utilizing rotating electric fields through which torques are generated due to phase difference between the dipole moment of cells and the external electric field. While reports of cell rotation under non-rotating electrical fields, such as dielectrophoresis (DEP), are abound, the underlying mechanism is not fully understood. Because of this, contradicting arguments remain regarding if a single cell can rotate under conventional DEP. What's more, the current prevailing DEP theory is not adequate for identifying the cause for such disagreements. In this work we applied our recently developed Volumetric Polarization and Integration (VPI) method to investigate the possible causes for cell rotation under conventional DEP. Three-dimensional (3D) computer models dealing with a cell in a DEP environment were developed to quantify the force and torque imparted on the cell by the external DEP field using COMSOL Multiphysics software. Modeling results suggest that eccentric inclusions with low conductivity inside the cell will generate torques (either in clockwise or counter-clockwise directions) sufficient to cause cell rotation under DEP. For validation of modeling predictions, experiments with rat adipose stem cells containing large lipid droplets were conducted. Good agreement between our modeling and experimental results suggests that the VPI method is powerful in elucidating the underlying mechanisms governing the complicated DEP phenomena.

Elucidating the DEP phenomena using a volumetric polarization approach with consideration of the electric double layer.

Dielectrophoretic (DEP) phenomena have been explored to great success for various applications like particle sorting and separation. To elucidate the underlying mechanism and quantify the DEP force experienced by particles, the point-dipole and Maxwell Stress Tensor (MST) methods are commonly used. However, both methods exhibit their own limitations. For example, the point-dipole method is unable to fully capture the essence of particle-particle interactions and the MST method is not suitable for particles of non-homogeneous property. Moreover, both methods fare poorly when it comes to explaining DEP phenomena such as the dependence of crossover frequency on medium conductivity. To address these limitations, the authors have developed a new method, termed volumetric-integration method, with the aid of computational implementation, to reexamine the DEP phenomena, elucidate the governing mechanism, and quantify the DEP force. The effect of an electric double layer (EDL) on particles' crossover behavior is dealt with through consideration of the EDL structure along with surface ionic/molecular adsorption, unlike in other methods, where the EDL is accounted for through simply assigning a surface conductance value to the particles. For validation, by comparing with literature experimental data, the authors show that the new method can quantify the DEP force on not only homogeneous particles but also non-homogeneous ones, and predict particle-particle interactions fairly accurately. Moreover, the authors also show that the predicted dependence of crossover frequency on medium conductivity and particle size agrees very well with experimental measurements.