Superfilling of Through-Silicon-Vias by Cu Electrodeposition with Iodide-based Levelers and Improvement of Filling Efficiency by Adding Thiourea

Abstract

The scaling of Cu interconnection for the fabrication of densified chips is reaching physical and technological limitation. Intensive researches have been focused on the 3D packaging by using through-silicon-vias (TSVs) to solve the scaling limitation. In the present work, iodide-based levelers were introduced to fill the TSVs without voids, and filling mechanism of TSV and methods to improve the filling efficiency are introduced. Based on the previously reported molecular structures of levelers, a new leveler containing cationic quaternary ammonium and anionic iodide ion was suggested. The suggested leveler exhibited convection-dependent adsorption characteristic, that is, strong inhibition under forced convection. Electrochemical analyses demonstrated that the leveler enhanced the adsorption strength of suppressor while Cu deposition was promoted by the displacement of accelerator in the stagnant condition. TSV-scaled trenches were filled by adding the accelerator, suppressor, and leveler. Based on the electrochemical analysis and filling results, the filling mechanism was examined: strong inhibition by suppressor and leveler on the top of the trenches, while accumulation of SPS on the bottom that developed the growing surface. Step current was applied to reduce the filling time in the three-additive system. Step current is composed of (1) low current for the bottom-up growth, and (2) consecutively applied high current for the decrease in the filling time. The growing surface is developed by the accumulation of accelerator during the first step current. Negligible Cu deposition occurred on the side walls near the top where the surface coverage of suppressor and leveler was high enough under the strong convection. Cu was selectively deposited only on the growing surface and the bottom-up growth continued. The filling time was reduced by about 47% with step current deposition compared to that of galvanostatic filling. To improve the filling performance by modifying the molecular structures of levelers, various kinds of organic levelers were tested. However, the levelers did not affect Cu electrodeposition although the molecular structures were modified by changing the methyl to allyl, hydroxyl to ester group, or elongating the carbon chain. It is obvious that quaternary ammonium ion or iodide ion determines the behavior of the levelers since all the levelers contain it in common. The characteristic as the leveler was more clearly observed in the order of leveler containing Cl- < Br- < I- when levelers containing various halide ions were electrochemically analyzed. Electrochemical analysis was conducted with the addition of NaI to investigate the effect of iodide ion in detail. I- ionized from NaI exhibited the similar electrochemical behavior, of convection-dependent adsorption and synergetic inhibition with suppressor, to the organic levelers. Moreover, cyclic voltammetry demonstrated that I- retarded the displacement adsorption of SPS. The addition of NaI, instead of organic levelers, also enabled the void-free filling of trenches. Void-free filling was achieved with the addition of KI and NH4I, as well, through the I- ionized from the inorganic compounds, which supported that I- functioned as the leveler. TSV was filled without voids at an applied current density of 1 mA/cm2, in the three-additive system containing I-. However, filling failed at higher current density of 2 mA/cm2. Thiourea was added to the three-additive composition in order to improve the filling performance at 2 mA/cm2. Low concentrations of TU increased the adsorption strength of suppressor, while the suppressing layer was disrupted when high concentrations of TU were added. Void-free filling of TSV was enabled at 2 mA/cm2 by adding low concentrations of TU which enhanced suppressing power and retarded the displacement adsorption of accelerator on the TSV side walls. High concentrations of TU, of course, deteriorated the filling performance by detaching suppressor on the side walls. The filling time was reduced in half by optimizing the 4-additive composition.