Fabrication and assessment of copper pillar micro-bumps with printed polymer cores cured by in-situ UV-LED

Abstract

Cu pillar micro-bump technology constitutes a new generation of interconnection technology for high-density fine-pitch flip-chip die stacking and 3D IC integration, which inherits the advantages from solder bump technology while overcoming the drawbacks. However, due to the significant mismatch of coefficient of thermal expansion (CTE) between a silicon chip and an organic substrate, the Cu pillar micro-bumps may suffer from high thermomechanical stress during device packaging and service, which may shorten lifespan and limit commercialization. Cu pillar micro-bumps with polymer cores have been demonstrated to effectively reduce thermomechanical stress and improve joint reliability. Unfortunately, current fabrication techniques of polymer core use traditional semiconductor fabrication processes and suffer from issues related to the chip size, pillar height, substrate topology and grass effect. Thus, it is important to investigate an improved method for polymer core fabrication in order to broaden the application. Therefore, the aim of this thesis is to 1) develop a new polymer core fabrication method to overcome the current limitations, 2) establish a polymer core surface metallization method to form the designed joint structure, 3) introduce bonding technology to form joints which are made of Cu pillar micro-bumps with printed polymer cores, and 4) investigate the reliability performance of the joints. An additive manufacturing process by in-situ UV LED curing during aerosol printing was developed for polymer core fabrication. To fulfill the dimension requirement of the Cu pillar joint, a high-intensity, focused emission UV LED package was designed, fabricated and optimized for in-situ UV LED curing. Micro-scale cylindrical polymer cores with a diameter of 20μm and a height of 30μm were achieved by synchronized UV curable acrylic resin printing and in-situ UV LED curing using an aerosol jet printer. From the printing results, it can be concluded that the printed polymer cores are uniform, and that the fabrication process is stable and reliable. Cu pillar micro-bumps with printed polymer cores were successfully achieved by surface metallization. Seed layer sputtering and electroplating on printed acrylic polymer were found to be the key processes for polymer core surface metallization. The diameter of the structure was 35μm while the height was 35μm. For comparison, conventional Cu pillars with similar dimensions were prepared using the similar processes. Shear test results showed that the shear strength of Cu pillar micro-bumps with printed polymer cores was 20% higher than that of conventional Cu pillars, this is because adhesion between UV-cured polymer and Al pads is superior to the adhesion between the TiW and Cu. The joints made of Cu pillar micro-bumps with printed polymer cores were achieved by flip-chip bonding technology. Silicon chips with Cu pillar micro-bumps with printed polymer cores were bonded to BT substrates to form the joints. Stress analysis by finite element simulation demonstrated that the Cu pillar micro-bumps with printed polymer cores exhibited reduced stresses in various test conditions, which indicated a better reliability performance than conventional copper pillars. The great improvement in reliability performance of the joints of Cu pillar micro-bumps with printed polymer cores was reported. The reliability of joints made of Cu pillar micro-bumps with printed polymer cores and joints made of conventional Cu pillars was investigated under temperature cycling condition and drop condition. Printed polymer cores increased the characteristic life by 32% in temperature cycling test (0℃-100℃), while the drop test showed that printed polymer cores increased the characteristic life by 4 times. It can be concluded that Cu pillar micro-bumps with printed polymer cores reduce stress and improve joint reliability, based on the results from mechanical and reliability tests.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology