Cu Pillar Bump Development for 7-nm Chip Package Interaction (CPI) Technology

Power, performance, and area gains are important metrics driving the complementary metal–oxide–semiconductor (CMOS) technology from older nodes to newer ones. Over past several decades, a steady downscaling of feature sizes of CMOS technology has been a leading force enabling continual improvement in circuit speeds and cost per functionality. Increase in functionality drives larger number of inputs/outputs (I/Os), and the scaling-driven small intellectual property (IP) block sizes force these larger number of I/Os to be accommodated by reduction of I/O pitches. The result is an unrelenting pressure to reduce bump pitches from one generation of CMOS to another. In contrast to 14-nm/16-nm nodes which used 150-um bump pitch coming out of a die, for 7-nm node, the industry is targeting 130-um bump pitch for high performance devices. With this pitch reduction, conventional tin/silver (SnAg) solder bumps face limitations in terms of bridging. Cu pillar bumps are the best candidate for smaller bump pitches. However, for large die sizes prevalent in high-performance computing (HPC), the Cu pillar bumps will induce higher stress on the silicon resulting in higher risks of extremely low K (ELK) cracking. If copper pillar bumps are not properly developed, then there is a risk of marginal reliability in terms of chip package interaction. The situation becomes even more dire in large die sizes, where coefficient of thermal expansion mismatch between silicon and laminate substrate magnifies the stress. The present article discusses successful development of Cu pillar bumps for 7-nm technology. The development program included a 2-step development path. In the first step, extensive thermomechanical modeling was carried out to find optimal design of copper pillar bump for robustness of interactions with 7-nm back end of line ELK layers. In the second step, a 460-mm2 7-nm Silicon test vehicle was fabricated, and its assembly process was optimized to characterize the copper pillar bumps and prove their extended reliability on 7-nm silicon. As a result of this development, copper pillar technology has been qualified on Advanced Micro Devices (AMD) products. Today, copper pillar is a fully integral part of AMD's ever-growing 7-nm product offering in HPC.