Improvement of Adhesion and Microwave Transmission Characteristics of Indium Bump by Silver Coating for Low Temperature
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Improvement of Adhesion and Microwave Transmission Characteristics of Indium Bump by Silver Coating for Low Temperature Flip-Chip Applications Kun-Mo Chu1, Jung-Hwan Choi2, Jung-Sub Lee, Han Seo Cho3, Seong-Ook Park2, Hyo-Hoon Park3, and Duk Young Jeon 1 Dept. of Materials Sci. & Eng., Korea Advanced Institute of Science and Technology, 373-1, Kuseong-dong, Yuseong, Daejeon, 305-701, Korea 2 Information and Communications University, Optical Interconnection and Switching Lab., 103-6 Munji-dong, Yuseong, Daejeon, 305-714, Korea 3 Information and Communications University, Microwave and Antenna Lab., 103-6 Munji-dong, Yuseong, Daejeon, 305-714, Korea ABSTRACT We have conducted low-temperature flip-chip bonding for both optical interconnect and microwave applications. Flip-chip bonding of vertical-cavity surface-emitting laser (VCSEL) arrays was performed on a fused silica substrate that provides propagation paths of laser beams and also supports a polymeric waveguide. To avoid thermal damage of the polymeric waveguide during the flip-chip bonding, indium bumps were used and the bonding condition of the flip-chip was determined as a heating temperature of 150 °C and a pressure of 500 gf. Experimentally, a thin silver (Ag) layer coated on the indium bump was very effective to enhance the adhesion strength between the indium bump and the VCSEL chip pads. In addition, the microwave characteristic of coplanar waveguide (CPW) package was slightly improved by the Ag coating. INTRODUCTION Flip-chip interconnection is becoming popular for packaging of optical and microwave devices. The small dimensions of laser diodes require accurate positioning and assembly techniques for high coupling efficiency [1]. Flip-chip technology with bumps is generally used to reduce the interconnection length and inductance for very high frequency or wide-band systems because the bumps directly connect each terminal. The excellent electrical behavior of flip-chip contacts up to millimeter wave frequency range is well known to the microwave community. Compared to other contacting methods, flip-chip contacts are shorter and hence, possess smaller inductance and resistance [2]. Alloys containing indium are common in applications where relatively low processing temperature is required. For photonics and fiber optics devices and packaging, indium solder has become ever more popular due to its ductility [3]. In the soldering
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operation, indium oxidized easily to form oxides that have very high melting point. The solid oxide film adheres onto the molten solder and prevents the solder solution from having contact with the parts to be joined [4]. Consequently, quality joints cannot be achieved. In the present study, to investigate the effect of Ag layer coating on the indium bump, two kinds of test samples, optical interconnection plate and CPW package, were prepared. This paper will report the adhesion strength between VCSEL chip pad and solder bump, and the microwave characteristics of flip-chip assembled CPW packages using two kinds of
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