Silver 나노입자를 첨가한 저온 소성용 페이스트의 제조

Abstract

Thick film technologies are used in several fields. Numerous types of electronic, mechanical, chemical and energy supply device are currently fabricated using thick film deposition processes. Thick film system of metal-glass-organic composite has been used to produce printed circuits because of their speed and repeatability in the electronic industry. Silver powders are used in electronics due to unique properties of high electrical and thermal conductivity. Silver paste containing PbO-based glass has been commonly used in microelectronic packages because silver has high electrical and thermal performance. Silver thick films with PbO-based glass are used extensively for making electrical contacts in silicon solar cells, hybrid circuits and other devices owing to their excellent electrical properties. The PbO-based glass plays a role in achieving densification of silver paste at low sintering temperature. In recent years, the PbO-based glass has been restricted because it is hazardous to health and environment. Thus, there have been intensive works on preparation of silver paste with Pb-free glass compositions such as Bi2O3-based, ZnO-based, and P2O5-based, respectively. However, the paste with Pb-free glasses requires high sintering temperature to obtain the high density and compatible conductivity, compared with the PbO-based paste. In this study, Pb-free silver paste for low sintering temperature was prepared in order to overcome poor densification of Pb-free silver paste using nanoparticles as a sintering aid. The nanoparticles having high surface energy can assist the densification at low sintering temperature. For the preparation of silver paste, the amount of nanoparticles and Pb-free frit was changed to investigate the effects of nanoparticles and Pb-free frit on microstructure and sheet resistivity of the films. Pb-free silver paste was prepared from two commercial silver powders (0.8 ㎛ and 1.6 ㎛ in size) and lab-synthesized nanoparticles (30-50 nm in size) by a chemical reduction method using surfactant. Silver particles with 5 wt%, 10 wt% or 20 wt% of the nanoparticles and its corresponding commercial silver powder were prepared. Then, 3 wt%, 6 wt% or 9 wt% of Pb-free frit (Bi2O3-based) was added to the mixed powders. Thick films of silver were prepared by a screen printing on an alumina substrate and the films were sintered at 400°C to 550°C. The effects of nanoparticles, Pb-free frit, and sintering temperature on microstructure and conductivity of silver paste were investigated. The degree of densification was important to improve the conductivity of silver thick film. The nanoparticles presented at the neck of micron-sized grains led to better interconnection and less amount of residual pores, i.e., the nanoparticles assist good conductive network at low temperature. However, excess amount of nanoparticles (>10 wt%) formed agglomerates at neck and free surface of micron-sized grains. As a result, in case of N5 Paste with 5 wt% of nanoparticles, the films had better microstructure and lower sheet resistivity than those of pastes with 10 wt% of nanoparticles or 20 wt% of nanoparticles. The glass frit also enhanced densification of the films by liquid phase sintering. However, the optimum amount of glass frit, 6 wt% in this study, should be used for dense microstructure since excessive content (9 wt%) provoked low conductivity due to formation of glass phase separated from particle network. Improvement of microstructural network by densification resulted in excellent conductivity of the sintered thick films. When films were prepared by N5G6 Paste with 5 wt% nanoparticles and 6 wt% glass frit, a minimum sheet resistivity of 2.5 μΩcm was obtained at sintering temperature of 550oC. It can be concluded that the silver paste obtained in this study can be a good substance for electronic devices, which could be extensively applied in microelectronic.