鈦/鎳/銀/金 金屬化鍍層於熱循環測試後界面反應之研究

Abstract

Light-emitting diodes (LEDs) are becoming a promising device for lighting. However, heat dissipation becomes a technological challenge which may reduce luminance as well as product life of the device. In order to solve this difficult problem, Si substrate was among the alternatives to replace traditional sapphire substrate. Si substrate offers many advantages such as good thermal conductivity, large wafer size and lower cost of the Si fabrication process. The Ti/Ni/Ag/Au multilayers combination, intended for LED metallization was deposited on Si substrate for interfacial interaction study. The interfacial interactions and electrical characteristics of the Ti/Ni/Ag/Au multilayers was investigated under various periods of thermal cycling between -40℃ and 125℃. The electrical properties of the multilayer thin films were measured by two-probe nano-electronics measurement. The depth profile was performed by ESCA (Electron Spectroscopy for Chemical Analysis). The interfacial reaction behavior was investigated with FIB (Focused Ion beam) and SEM. The electrical property of Ti/Ni/(Ag/Au) and Ni/(Ag/Au) multilayer show that the Ni/(Ag/Au) predominates the overall electrical performance of the multilayer combination. It is obvious that the resistance of Ni/(Ag/Au) interface is much higher than Ti/Ni interface. The results of ESCA (Electron Spectroscopy for Chemical Analysis) depth profile analysis indicate that Ag atoms diffuse passing through the pure Au layer to the surface after thermal cycling. The atomic concentration of Ag at the surface increases up to 68% after thermal cycling of 627 cycles. The outward diffusion of Ag atoms towards Au and Ni cause voids formation in Ag layer and thus higher electrical resistance. Four types of void morphology were observed (a) void formed between Au and Ag layer, (b) a small void formed in the Ag layer closing to the Ni layer. (c) several small size voids aggregating at the Ag layer. (d) larger arcuate void forming in Ag layer. The disparity of diffusion coefficient among Au, Ag and Ni elements give rise to large voids formation in the Ag layer. The formation of void is responsible for the poor electrical performance of the Ni/(Ag/Au).