Electrical, Morphological, and Compositional Characterization of Screen-Printed Al Contacts Annealed in Horizontal and Vertical Configurations

The electrical, morphological, and compositional characteristics of screen-printed Al paste contacts on p -doped Si wafers have been investigated in horizontal and vertical thermal annealing configurations over a wide temperature range. The horizontal configuration refers to an industrial six-zone conveyor belt rapid thermal annealing furnace. The vertical configuration refers to a modified three-zone quartz tube furnace with vertically stacked wafers. The contact resistivity was measured by using the transmission line method. In the horizontal configuration, the resistivity exhibited a pronounced minimum at temperature of ∼ 870°C, while higher temperatures resulted in a rapid increase in the contact resistivity. In contrast, the resistivity variation in the vertical configuration was linear. The lowest contact resistivities measured were 136 mΩ cm 2 in the horizontal and 103 mΩ cm 2 in the vertical configuration, demonstrating a 24% reduction with the latter approach. The surface morphology and composition of the Al/Si contact interface were determined by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy. The measured elemental concentrations were curve-fit to accurately measure the width of the interface regions. The Al/Si contact region was observed to consist of five parts: (a) a top sintered paste layer of Al/Si spheres, (b) voids between the Al/Si spheres, (c) an Al/Si eutectic region, (d) an epitaxially grown Al-doped Si layer, and (e) the lightly Al-diffused Si substrate. Sintered Al/Si spheres were observed to consist of a solid core of Al embedded in a thin shell of Al, Al 2 O 3 , SiO 2 , and Si. The rapid rise in resistivity at high temperatures is attributed to enhanced oxidation of Al and Si islands, resulting in thicker Al 2 O 3 /SiO 2 films between metallic Al spheres. The lower resistivity observed in the vertical configuration was attributed to larger, more uniform Al–Si eutectic regions, higher density of Al/Si films within the paste region, and transformation of sintered Al spheres into larger pseudosquare islands. The proposed Al/Si interface model was further supported by the higher resistance measured for the pulsed laser-based Al/Si contact with high Si concentrations in the Al/Si eutectic region. An approximately linear reduction in resistivity as a function of time over a broad range varying from microseconds to seconds reinforced the proposed model and suggests that longer, steady-state annealing is the preferred approach to achieve the lowest contact resistivity.