Electrochemical Migration and Electro-Migration Reliability of Ag Interconnect Fabricated By Reverse-Offset Printing
Authors: Hwang Jae-Seon 
Published: February 2020

Wearable devices have drawn a great attention with rising of a new era of innovation. For wearable devices, metal interconnects on flexible substrate are essentially required. In this point of view, printing technology is suitable for fabricating wearable devices. This method can also significantly support the trend of high integrity of electronics. To meet this requirements, reverse-offset printing provides a high resolution and thin film thickness below 20 µm, 1 µm, respectively in various printing methods. However, offset printed interconnects to apply for wearable devices have reliability issues. Wearable devices contact to skin that has sweat contains sodium chloride (NaCl). Water with NaCl as an electrolyte is a much better conductor of electricity, therefore, electrochemical migration (ECM) can be a critical issue. As the dimension of interconnect width and pitch was decreased, electro-migration (EM) is also an important reliability problem.
In this study, we investigated ECM and EM characteristics induced by electric current in Ag interconnect installed on a polyimide (PI) substrate fabricated by reverse-offset printing. Water drop test (WDT) was selected to evaluate the time-to-failure (TTF) in a short time and spacing between interconnects are 100 µm, 200 µm and 300 µm. In-situ corrosion behavior in D.I. water and 0.1 wt.% NaCl solution was observed by using optical scope. DC current densities of range from 104 to 106 A/cm2 at 50 oC were applied to a dog born shaped organic Ag samples during 96 hours. The dog born shaped EM sample size is approximately 20 μm x 1 mm x 0.5 μm. Samples annealed at 250°C for 30 min in air condition. The surface morphology was investigated by atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) and the composition of filament was investigated by energy dispersive spectroscopy (EDS).
At low voltage ranges between 0.1 ~ 1 V for WDT, the time to failure (TTF) of non-coating Ag interconnects were measured in 100 seconds and at high voltage ranges between 3 ~ 12 V, they failed immediately in 20 seconds in D. I. water and even more susceptible to NaCl contained-solution conditions. According to previous research, TTF is increased by metallic coating process. Thus, we newly introduced organic coating in this study. Polyacrylamide (PA) is a flexible polymer which is non-toxic and inert forming a soft gel when hydrated, used in manufacturing soft contact lenses. PA was coated around of Ag interconnects by spin-coating. As expected, TTF of PA coating in NaCl condition was increased because PA coating played a great shield to prevent chloride ion penetration from anode to cathode in Ag interconnects.
Reverse-offset printed Ag interconnects shows gradual failure mode interestingly. Resistance change (%) of Ag interconnects were 5, 10, 30 and 50 % at 105 A/cm2current stressing for 9, 15, 23 and 24 hours, respectively. Agglomeration of Ag nanoparticles was accelerated by current induced test and Ag deficient regions were locally formed. It is considered that failure of Ag interconnect occurred due to joule-heating induced vaporization of Ag nanoparticles because of current-crowding effects, assisted by agglomeration of Ag. They cause electrical insulation in Ag interconnect, resulting in gradual changes of resistance.
We investigated the voltage dependence of ECM failure time under variable spacing of interconnects and failure by gradual resistance increase under current stressing. Lifetime model to predict the voltage and spacing dependence of ECM and effect of microstructure changes depending on various post annealing conditions in Ag interconnects will be discussed.