High aspect ratio (∼15) and ultrafine pitch (∼35μm) through-wafer copper interconnection columns were fabricated by aspect-ratio-dependent electroplating. By controlling the process parameters, ultrafine copper grains with nanoscale twins (twin lamellar width ∼20nm) were obtained in the copper columns. Transmission electron microscope reveals that the density of these nanotwins depends on the location along the length of the columns. The highest twin density was achieved at the bottom of the column where the electroplating starts. The presence of higher density of the nanotwins yields significant higher hardness (∼2.4GPa) than that with lower twin density (∼1.8GPa). The electrical conductivity of the electroplated copper (2.2μΩcm) is retained comparable to the pure copper.

In this paper, we report the fabrication of high aspect ratio, highly dense, very fine pitch on-chip copper-pillar-based interconnects for advanced packaging applications. Photoresist molds up to a thickness of 80 µm and having feature sizes as small as 5 µm were fabricated using multi-step coating of the positive tone AZ9260 photoresist. Spin coating and lithography parameters were optimized to achieve smooth and vertical sidewalls. Copper interconnects having an aspect ratio up to 6 and a pitch size of 25 µm were electroplated in the fabricated resist mold. Due to a very small pitch size, the total number of interconnects per cm2 chip area is 160 000, which is much larger than the conventional solder-based interconnects. The electrical resistance of the electroplated copper interconnects was measured by 4-probe kelvin measurement configuration and was found to be in the range of 8–10 mΩ and the corresponding electrical resistivity was calculated as 2.4 µΩ cm. Such low resistive interconnects can carry much larger electrical current without significant electrical loss, which is ideally suitable for next generation packaging applications. X-ray diffraction has shown the presence of the (2 2 0) texture along the length of electroplated copper pillars. Transmission electron microscope reveals the presence of nanoscale copper twins along the length of copper interconnects.

Interactions between the cathodic and anodic processes during electroless Cu deposition using glyoxylic acid reducing agent are studied. It is shown that application of the mixed potential theory by decoupling the anodic and cathodic processes leads to erroneous predictions of the electroless deposition rate and the surface mixed potential observed in this electroless system. Chronoamperometry under various conditions of bath composition, electrode surface preparation and applied potential provides evidence for an autocatalytic mechanism in the glyoxylic acid based electroless Cu system. Specifically, it is observed that the electroless Cu surface accelerates the glyoxylic acid oxidation and the glyoxylic acid in turn enhances the cupric-ion reduction leading to a positive feedback loop. The autocatalysis explains shortcomings in the simplistic application of the mixed potential theory to the decoupled partial reactions, and suggests a more complex reaction mechanism in which the partial reactions are coupled.

In this paper we present the mechanical and microstructural characterization results of through-wafer electroplated copper interconnects. Copper was deposited in very high aspect ratio (∼15), narrow (15 µm) through-vias in silicon, which were earlier created by deep reactive ion etching. The two critical mechanical properties, i.e. hardness and modulus of elasticity, and the microstructure of the electroplated copper interconnect were determined by nanoindentation, atomic force microscope and x-ray diffraction techniques. A location-dependent hardness characteristic was shown along the length of electroplated copper interconnects. The modulus and the hardness of copper interconnects at the bottom segment (124 GPa and 1.8 GPa) were found to be higher than those at the top segment (116 GPa and 1.1 GPa). The reason behind these variable hardness values in the copper interconnect was due to the different grain sizes and the microstructure in the electroplated copper. These varying grain sizes were caused by the incremental current densities used during electrodeposition. The thermal strain, generated due to the coefficient of thermal expansion mismatch, was measured by the digital image speckle correlation technique. From the results, the thermal strain in the Y-direction was found to be more dominant than that in the X-direction. The grain sizes and the preferred texture orientation in the electroplated copper were characterized by the x-ray diffraction technique.

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