Cu pillar bump development for 7nm Chip package interaction (CPI) technology

Power, performance, and area gains are important metrics driving the 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 I/Os, and the scaling driven small 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 14nm/16nm nodes which used 150um bump pitch coming out of a die, for 7nm node the industry is targeting 130um bump pitch for high performance devices. With this pitch reduction, conventional 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, the Cu pillar bumps will induce higher stress on the silicon resulting in higher risks of 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 CTE mismatch between silicon and laminate substrate magnifies the stress. The present paper discusses successful development of Cu pillar bumps for 7nm technology. The development program included a 2-step development path. In the first step, extensive thermo-mechanical modeling was done to find optimal design of copper pillar bump for robustness of interactions with 7nm BEOL ELK layers. In the second step, a 460 mm2 7nm Silicon test Vehicle was fabricated and its assembly process was optimized to characterize the copper pillar bumps and prove their extended reliability on 7nm silicon. As a result of this development, copper pillar technology has been qualified on AMD products. Today, copper pillar is a fully integral part of AMD's ever-growing 7nm product offering in High Performance Computing.