Influence of nano silver filler content on properties of ink-jet printed structures for microelectronics Tomasz Falat 1* , Jan Felba 1 , Andrzej Moscicki 2 , Anita Smolarek 2 , Karlheinz Bock 3 and Detlef Bonfert 3 1 Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics, Wroclaw, Poland, 2 Amepox Microelectronics, Lodz, Poland, 3 Fraunhofer IZM, München, Germany *Tomasz.Falat@pwr.wroc.pl, www.lipec.info Abstract The influence of nano silver filler content on properties of ink-jet printed structures for microelectronics were investigated. Samples were prepared by using ink with different nano silver filler content: 44 %, 41 % and 38 %. The electrical measurements were performed during and after heating process. It was shown, that filler content does not have a strong influence on sintering time, but it changes the final resistivity of printed and sintered structures. The enhancement of sintering process by UV exposure was also investigated. Preliminary results are promising and the further study will be conducted. Introduction Electrically conductive microstructures and contacts with dimensions in the range of tens of micrometers nowadays plays very important role in microelectronic packaging. Performing the structures like lines, different patterns or dots arrays is possible under the condition that both special technologies and materials are applied. Since the late eighties of the last century, digital ink-jet technology has been widely explored by the electronic industry in developing new manufacturing processes [1]. Ink-jet technology needs a special liquid, usually called ink, which should satisfy at least the following three requirements: has very low viscosity, can be treated as a “true solvent” without component separation during high acceleration, and is able to make electrically conductive structures. If a suspension is used as the fluid for printing, the “conductivity” condition requires applying electrically conductive particles, and the “true solvent” demands particles with dimensions as small as possible, not higher than tens of nanometers. Ink for printing conductive microstructures is typically based on noble metals of nano-sized dimensions because of the chemical inertness in ambient atmosphere and good electrical conductivity – mainly Ag. We are able to produce the silver particles with very narrow size distribution. The smallest controllable size is 4 nm, and size distribution window of few nanometers (e.g. particle size: 4-10 nm) [2]. The silver nanoparticles can be produced with controlled size up to 60 nm. On the basis of this nano sized filler, the ink for ink-jet printing technology is formulated [3] and microstructures can be printed with the use of a professional drop-on-demand printer. The structures are noncondcutive just after the printing. To obtain good electrical conductivity, they need an additional energy, mainly supplied during a heating process. In our previous work [4] the properties of the structures made by ink-jet printing with the use of the ink containing the nano sized silver particles were investigated. It was shown, that thermal process influences strongly the electrical resistance, and finally resistivity of printed structures was only about three times higher than the value of the bulk material. Initial tests with the 4-10 nm silver particles showed that the resistivity of structures is about 4.5 µΩ·cm, while the bulk resistivity of silver is about 1.55 µΩ·cm. Also different electrical tests proved similarity between a bulk silver and the structures made by ink-jet printing with the use of nano sized silver particles. Such results suggest that during the heating process almost the whole binder is removed from an ink, and in the final layer only sintered silver particles remain. If so, the following question arises: whether decreasing the amount of filler in the ink influence the printed structure properties such as an electrical resistivity or not? The question is very important, as spare even small amount of very costly nano silver may bring significant economical effect in the mass production. It was investigated by electrical measurement of three inks with different content of nano silver filler. Inks and samples preparation The conductive inks were formulated basing on the Ag powder with particles in the atomic size range (4- 10 nm in diameter). The base properties of used inks have been shown in Table 1. Table 1. The base properties of inks for ink-jet printing Number of components One Consistency Very low viscous fluid Color Dark brown to black with metallic shine Viscosity *) 3.6 ÷ 18 mPa·s Thixotrophy index ~ 1.0 Surface tension value 28.5÷32.5 mN/m Specific gravity 1.3 ÷ 1.6 g/cm³ *) Brookfield LVDVII + CP; 100 rpm; 20 °C There were prepared three inks (named as #1, #2 and #3) witch differ in the filler content: 44 %, 41 % and 38 % respectively. The samples were printed on on microscopic glass slides by using the drop-on-demand printer (Dimatix DMP-2831) equipped with cartridge having a 12 nozzles of 22.5 µm diameter. The drops of volume about 10 pl were jetted with the frequency of 1 kHz. The drop spacing were 20 µm (1270 dpi). The structures were printed with the shape presented in Fig. 1. In the first step