Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate
© Kim et al; licensee Springer. 2012
Received: 1 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The microstructural evolution and the adhesion of an Ag nanopaste screen-printed on a silicon substrate were investigated as a function of sintering temperature. Through the two thermal analysis methods, such as differential scanning calorimeter and thermo-gravimetric analysis, the sintering conditions were defined where the temperature was raised from 150°C to 300°C, all with a fixed sintering time of 30 min. The microstructure and the volume of the printed Ag nanopaste were observed using a field emission scanning electron microscope and a 3-D surface profiler, respectively. The apparent density of the printed Ag nanopaste was calculated depending on the sintering conditions, and the adhesion was evaluated by a scratch test. As the sintering temperature increased from 150°C to 300°C, the apparent density and the adhesion increased by 22.7% and 43%, respectively. It is confirmed that the printed Ag nanopaste sintered at higher temperatures showed higher apparent density in the microstructural evolution and void aggregation, resulting in the lower electrical resistivity and various scratched fractures.
Keywordssilver nanopaste screen printing sintering density adhesion.
Micro and nanofabrication are essential for the modern electronic devices . Recently, printed electronics has been highlighted by many researchers in academia and industry as emerging manufacturing technologies to fabricate portable and display devices [2–6]. The fabrication methods of printed electronics reported so far include direct printing techniques such as inkjet, gravure, and screen printing [7–9]. These techniques have been put forward as alternative methods for patterning conducting circuits due to the short manufacturing time, low cost, large-area patternability, and environmental friendliness compared to conventional photolithography . Printed electronics is based on an additive manufacturing technology and thereby requires heat treatment after the printing process. In addition, the features of the patterns directly printed on a substrate also depend on the heat treatment. Therefore, it is essential to understand the behaviors of nanoparticles in a sintering process in order to provide an insight into the printing techniques.
Part of an ongoing research project in our laboratory is to produce printed thin films with sufficient adhesion, which is directly related to the lifetime of the electronic devices. However, it is difficult to measure the adhesion of a printed film that has a weak and thin layer, and hence, this has been one of the key issues in this project. A scratch test is the most practical method for assessing the adhesion of the thin film to the substrate . This is because the critical load determined by the scratch test is widely regarded as the representative of film adhesion .
Based on these requirements, we investigated the effects of heat treatment on the microstructural evolution and electrical property of the screen-printed Ag nanopaste. The influence of sintering temperature on the adhesion was also characterized by the scratch test.
Results and discussion
The characteristics of thin printed patterns are dominated by the heat treatment applied. The influence of sintering temperature on the adhesion of the screen-printed Ag nanopaste was investigated. The scratch test, which is to measure the critical friction force of the film, was suggested to be a suitable method to evaluate the adhesion of printed patterns. Overall, the critical friction force increased by 43% as the sintering temperature increased from 150°C to 300°C. To rationalize these experimental results, the microstructural evolution and variation of density were investigated as a function of sintering temperature. The Ag nanopaste sintered at higher temperatures showed the accelerated condition. The calculated apparent density of the Ag nanopaste increased from 6.08 g/cm3 at 150°C to 7.46 g/cm3 at 300°C. It was concluded that the printed Ag films sintered at higher temperatures became more densely packed, which resulted in the lower electrical resistivity and the stronger adhesion of the printed Ag nanopaste.
differential scanning calorimeter
field emission scanning electron microscope
transmission electron microscope
This work was supported by the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R32-2009-000-10124-0).
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