Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate

  • Kwang-Seok Kim1,

    Affiliated with

    • Yongil Kim2 and

      Affiliated with

      • Seung-Boo Jung2Email author

        Affiliated with

        Nanoscale Research Letters20127:49

        DOI: 10.1186/1556-276X-7-49

        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.


        silver nanopaste screen printing sintering density adhesion.


        Micro and nanofabrication are essential for the modern electronic devices [1]. Recently, printed electronics has been highlighted by many researchers in academia and industry as emerging manufacturing technologies to fabricate portable and display devices [26]. The fabrication methods of printed electronics reported so far include direct printing techniques such as inkjet, gravure, and screen printing [79]. 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 [10]. 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 [11]. This is because the critical load determined by the scratch test is widely regarded as the representative of film adhesion [12].

        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.


        The Ag nanopaste (Silver nanopaste DGP, Advanced Nano Materials Inc., Kumho-ri Cheongwon-gun, South Korea) was composed of Ag nanoparticles with a mean size of 24 nm, which were dispersed in α-terpineol matrix at a solid loading of 73% by weight. The shape of the Ag nanoparticles was examined by a JEOL JEM-1200EX transmission electron microscope [TEM] (JEOL Ltd., Akishima, Tokyo, Japan). Two types of thermal analysis were performed on the Ag nanopaste: differential scanning calorimeter [DSC] and thermo-gravimetric analysis [TGA]. A screen printing machine (MT-550TV, Micro-Tec, Chiba City, Chiba, Japan) was used to duplicate the conductive patterns. The Ag nanopaste was printed using a 400-mesh screen mask onto a silicon [Si] substrate passivated with SiO2. All printed patterns were dried on a hot plate at 70°C for 10 min, and then sintered in a box-type, muffle furnace (RTA-BRT100, BLS Korea Inc., Seoul, South Korea) for 30 min under a sintering temperature ranging from 150°C to 300°C. A four-point probe method was adopted to measure the electrical resistivity. The apparent density of the screen-printed Ag nanopaste depending on the sintering temperatures was calculated from the precise volume and mass measured using a commercial precision scale (JL-180, Chyo Balance Corp., Minami-ku, Kyoto, Japan) and a 3-D surface profiler (Nanoview2000, Nanosystem Inc., Daejon City, South Korea), respectively. Figure 1 shows a schematic diagram of scratch testing, and the table in Additional file 1 lists the detailed scratch parameters. The scratch test was carried out using a commercial scratch tester (MSTX, CSM Instruments, Needham, MA, USA) equipped with a diamond indenter having a tip radius of 10 μm. The microstructural evolution was investigated using a field emission scanning electron microscope [FE-SEM] (JSM-7401F, JEOL Ltd., Akishima, Tokyo, Japan), and an optical microscope [OM] was used to observe the fracture surfaces of the screen-printed Ag nanopaste after the scratch test.
        Figure 1

        A Schematic diagram of scratch testing.

        Results and discussion

        Figure 2a, b shows a TEM image and the measured size distribution of the Ag nanopaste, respectively. In the TEM image, most of the Ag nanoparticles have a diameter of approximately 25 nm. It was also confirmed that the diameters of the Ag nanoparticles were within 10 and 30 nm with a narrow distribution, as shown in Figure 2b. This distribution of particle size was known to be affected by three major factors, i.e., the generation rate of the Ag embryos, the growth rate of the Ag nanoparticles, and the extent of surfactant absorbing or encapsulating if no agglomeration occurred between the nanoparticles [13].
        Figure 2

        A TEM image (a) and size distribution (b) of Ag nanoparticles.

        The thermal properties of the Ag nanopaste are shown in Figure 3. The DSC curve had a slightly exothermic slope from 30°C to 200°C at the rate of 10°C/min and one sharp endothermic peak at 230°C. It means that the necking reaction in the Ag nanopaste occurred at the point of the endothermic peak, resulting in the coarsened nanoparticles. Compared with the TGA curve, any moisture and solvent in the Ag nanopaste were dried or decomposed thermally from 100°C to 300°C at the rate of 10°C/min. In this temperature range, a maximum rate of weight loss was shown at 225°C, and the total weight loss was approximately 27 wt.%. Based on these results, the sintering temperature range was determined from 150°C to 300°C to investigate the influence of heat treatment on the microstructural evolution and property variation.
        Figure 3

        DSC (a) and TGA (b) curves of the Ag nanopaste.

        The microstructural evolution as a function of sintering temperatures was shown in Figure 4. The surface of the Ag nanopaste sintered at 150°C for 30 min exhibited a similar particle shape and size compared to the as-dried one. However, when the printed Ag nanopaste was sintered at 200°C for 30 min, the clusters were built via interconnections, resulting from interparticle necking that occurred after the drying of the dispersing agent and decomposition of the organic solvent. Therefore, this microstructural evolution matched well with the thermal analysis results, as shown in Figure 3. Above a sintering temperature of 200°C, the surface of the sintered Ag nanopaste drastically changed from discrete and spherical Ag nanoparticles to continuous and consolidated ones.
        Figure 4

        FE-SEM micrographs of the screen-printed Ag nanopaste sintered at various temperatures. (a, e) 150°C, (b, f) 200°C, (c, g) 250°C, and (d, h) 300°C.

        Figure 5 plots the variation in the apparent density of the Ag nanopaste as a function of sintering temperature. Except at 300°C, the average apparent density did not change much which was approximately 6.32 g/cm3 within the standard deviation of 0.22. At 300°C, the film of the Ag nanopaste as the result of volume shrinkage appeared after the interparticle neck growth, as shown in Figure 4d [14]. Because the apparent density was calculated from the values of the measured mass and volume, any change in mass or volume would consequently affect the density. Although some pores were observed inside the film of the Ag nanopaste, the main parameter was the volume shrinkage of the screen-printed Ag film compared to the mass decrement caused by the solvent evaporation. In this case, although the mass of the Ag nanopaste was reduced by solvent evaporation that resulted in observable pores inside the film, the dominating factor was still the volume shrinkage of the screen-printed Ag film. Therefore, the volume shrinkage led to the increase of apparent density.
        Figure 5

        The apparent density of the Ag nanopaste.

        Figure 6 reveals the influence of the sintering temperature on the electrical resistivity of the Ag nanopaste. The electrical resistivity dramatically decreased with the increasing sintering temperature, implying a conduction pathway between the Ag nanoparticles due to the interparticle neck formation and growth. This is because the mechanism of electrical conduction in a metal nanopaste features the point-to-point contact between the conductive nanoparticles [15]. In other words, when the conductive nanoparticles have been necked to a sufficient extent, the film of the printed nanopaste becomes relatively well conductive despite of its porosity. The Ag nanopaste sintered at 300°C showed the lowest electrical resistivity of 1.89 μΩ·cm.
        Figure 6

        The electrical resistivity of the Ag nanopaste.

        Figure 7 exhibits the adhesion strength of the Ag nanopaste printed on the Si substrate as a function of sintering temperature. A critical friction force is the force needed to pull out a film on a substrate. Overall, the critical friction force increased linearly with an increasing sintering temperature. As previously seen in Figure 4b, the nanoparticles on the surface of the Ag nanopaste formed clusters with a diameter of 130 to approximately 180 nm due to interparticle necking at a sintering temperature of 200°C. The clusters grew larger with the three-dimensional interconnections as the sintering temperature increased, which increased from around 300 nm at 250°C (Figure 4c) to 600 nm at 300°C (Figure 4d). Therefore, as the sintering temperature increased, the clusters were connected more strongly, and hence, the surface area between the Ag nanopaste and the Si substrate increased, resulting in the higher friction force.
        Figure 7

        The critical friction force of the Ag nanopaste.

        In order to investigate the scratched surface of the Ag nanopaste, the scratches were observed using the OM. The panorama images illustrate the direction of scratch testing, and the morphologies of the entire scratches are shown in Figure 8a, b, c, d. As the sintering temperature increased, larger parts of the Ag nanopaste were pulled out due to the stronger connections in the Ag clusters. Figure 8e, f, g, h indicates the exact starting points to pull the film out from the substrate. The different fracture modes were identified depending on the sintering temperature. The printed Ag nanopaste sintered at 300°C exhibited a fracture behavior like a bulk Ag film.
        Figure 8

        Optical micrographs of the scratched Ag nanopaste surface sintered at various temperatures. (a, e) 150°C, (b, f) 200°C, (c, g) 250°C, and (d, h) 300°C.


        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


        optical microscope


        transmission electron microscope


        thermo-gravimetric analysis



        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).

        Authors’ Affiliations

        SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University
        School of Advanced Materials Science and Engineering, Sungkyunkwan University


        1. Yoo H, Shin H, Lee M: Direct patterning of double-layered metal thin films by a pulsed Nd:YAG laser beam. Thin Solid Films 2010, 518: 2775–2778.View Article
        2. Kim KS, Lee YC, Ahn JH, Jung SB: The effect of heat treatment on flexibility of silver nanopaste circuit screen-printed on polyimide. In IEEE NMDC2010: Topics in Nanostructure Characterization. IEEE Nanotechnology Materials and Devices Conference (NMDC)2010; October 2010. Monterey: IEEE; 2010:306.View Article
        3. Ko SH, Pan H, Grigoropoulos CP, Luscombe CK, Frechet JMJ, Poulikakos D: All-inkjet-printed flexible electronics fabrication on a polymer substrate by low-temperature high-resolution selective laser sintering of metal nanoparticles. Nanotechnology 2007, 18: 345202–345209.View Article
        4. Zaumseil J, Someya T, Bao Z, Loo YL, Cirelli R, Rogers JA: Nanoscale organic transistors that use source/drain electrodes supported by high resolution rubber stamps. Appl Phys Lett 2003, 82: 793–795.View Article
        5. Kim A, Lee H, Ryu C, Cho SM, Chae H: Nanoscale thickness and roughness control of gravure printed MEH-PPV layer by solvent printing for organic light emitting diode. J Nanosci Nanotechnol 2010, 10: 3326–3330.View Article
        6. Kim KS, Koo JM, Joung JW, Kim BS, Jung SB: Electrical characteristics of copper circuit using inkjet printing. J Microelectron Packag Soc 2010, 17: 43–49.
        7. Kim KS, Lee YC, Ahn JH, Jung SB: Evaluation of the flexibility of silver circuits screen-printed on polyimide with an environmental reliability test. J Nanosci Nanotechnol 2011, 11: 5806–5811.View Article
        8. Kim JW, Hong SJ, Kim YS, Kim YS, Lee JN, Kang NK: Recent advances in eco-friendly nano-ink technology for display and semiconductor application. J Microelectron Packag Soc 2010, 17: 33–39.
        9. Park SC, Cho SH, Jung HC, Joung JW, Park YB: Effect of temperature/humidity treatment conditions on the interfacial adhesion energy of inkjet printed Ag film on polyimide. J Kor Inst Met & Mater 2007, 45: 520–526.
        10. Noh BI, Yoon JW, Kim KS, Lee YC, Jung SB: Microstructure, electrical properties, and electrochemical migration of a directly printed Ag pattern. J Electron Mater 2011, 40: 35–41.View Article
        11. Jaworski R, Pawlowski L, Roudet F, Kozerski S, Petit F: Characterization of mechanical properties of suspension plasma sprayed TiO2 coatings using scratch test. Surf Coat Technol 2008, 202: 2644–2653.View Article
        12. Steinmann PA, Tardy Y, Hintermann HE: Adhesion testing by the scratch test method: the influence of intrinsic and extrinsic parameters on the critical load. Thin Solid Films 1987, 154: 333–349.View Article
        13. Liu J, Li X, Zeng X: Silver nanoparticles prepared by chemical reaction-protection method, and their application in electrically conductive silver nanopaste. J Alloy Compd 2010, 494: 84–87.View Article
        14. Kim KS, Lee YC, Kim JW, Jung SB: Flexibility of silver conductive circuits screen-printed on a polyimide substrate. J Nanosci Nanotechnol 2011, 11: 1493–1498.View Article
        15. Jang S, Seo Y, Choi J, Kim T, Cho J, Kim S, Kim D: Sintering of inkjet printed copper nanoparticles for flexible electronics. Scripta Mater 2010, 62: 258–261.View Article


        © Kim et al; licensee Springer. 2012

        This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.