Additive manufacturing (AM) is the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. This technological advancement that was made possible by the transition from analog to digital processing, enables the creation of lighter, stronger systems and parts. AM is highly preferred credit to the many advantages it holds over traditional fabrication approaches. At present, AM of metal interconnects has been actively pursued with varying degrees of success for electronics and other microsystems.
Alternatively, sputtering – a manufacturing method used in state-of the-art electronics to produce thin, conformal coatings of nearly any material- can deliver ideal film microstructure surfaces. To this end, numerous studies have been conducted where various threshold conditions have been established. For instance, it has been documented that sputter atoms with insufficient energy will form tall, closely packed, and narrow columnar grains, with crystallographic orientation determined by the angle of arrival. As such, various alternative methods for ensuring that the arriving atoms have enough kinetic energy have been presented. In fact, past research by the authors of this work reported a novel ion-drag-focused, atmospheric pressure microplasma sputterer designed to directly print high-resolution, conductive features without the need for post-processing, e.g. annealing.
Regardless, the effects of deposition parameters have not yet been exhaustively studied. In this view, researchers from the Massachusetts Institute of Technology: Y. Kornbluth, R. Mathews, Dr. Lalitha Parameswaran, Dr. Livia Racz and led by Dr. Luis Velásquez-García explored the effects of deposition parameters on the microstructure and electrical resistivity of sub-100 nm thick gold films using an improved microplasma reactor. Their aim was to optimize the printing process and gain further insights into the parameters that govern the creation of void-free, highly conductive films. Their work is currently published in the research journal, Nanotechnology.
In their work, the scholars used a combination of a directed gas flow and electrostatic attraction to ensure that the sputtered material reached the substrate with a high normal velocity, while minimizing substrate heating. Altogether, they investigated the effects of deposition parameters, namely: gas flow rates, bias voltages, and nozzle-to-substrate separation, on the microstructure of <100 nm thick gold deposits made with a room-temperature, atmospheric-pressure, ion-drag microsputterer.
The research team reported that it was possible to produce continuous (96.5% coverage), highly electrically conductive deposits (45 μΩ cm), without heating the substrate or using a vacuum, by harnessing electric fields. All in all, they identified the main parameters that affect microsputtered film coverage and presented a simple piecewise model that described the majority of the experimental results obtained.
In summary, MIT scientists presented an experimental investigation of the effects of deposition parameters on the microstructure of room-temperature, atmospheric pressure, ion-drag-focused sputter deposition, using a DOE. Remarkably, through the application of statistical analysis, the researchers were able to develop a simple model that provides insight into the dynamics of such a printing method; where, based on their model, they identified electrostatic effects as the most important factor that influence the deposition process. In a statement to Advances in Engineering, Dr. Luis Velásquez-García, the lead author emphasized that their work was not an overdraw but in fact represented the right impetus to steer further research aimed at expounding on some of the effects that were not yet well captured by the model they presented.
Y S Kornbluth, R H Mathews, L Parameswaran, L M Racz, L F Velásquez-García. Room-temperature, atmospheric-pressure microsputtering of dense, electrically conductive, sub-100 nm gold films. Nanotechnology, Volume 30, Number 28.