Today we can cram more than one billion transistors onto a single microprocessor die thanks to 50 years of miniaturization of integrated circuits. However, the scaling is becoming a headache for engineers as transistor dimensions of only a few atoms are being approached.
One alternative concept is 3D integration, the stacking of individual integrated circuits on top of each other. This improves integration density, but also energy efficiency and compute performance thanks to proximity and improved wiring of electronic circuits, similarly to the neural network inside our brain.
But challenges arise as you stack the chips, namely the demand of increased current densities and communication bandwidth to the chip stack. Currently, microprocessors are connected to printed-circuit-boards (PCB) by several 10,000s of solder balls, which are arranged in a regular grid at the bottom side of the chip. The pain is that as much as 80% of the electrical interconnects are occupied for provisioning of up to 300 Amps of current and only 20% are left for the main purpose of signaling.
The solder ball interconnect technology was invented by IBM engineers in the 1960s resulting in the still widely used flip chip technology, also known as the controlled collapse chip connection (C4). A chip with solder balls is placed onto a PCB and both components are transferred into an oven. The solder melts above 230°C and instantly wets the pads of the PCB. During cool-down, the solder solidifies to form the electrical contact.
|Cross-section of All-Copper Interconnects formed by low temperature sintering of copper nano-particles|
A challenge is that voids form in the solder interconnects at high current densities and system failures occur. The voids are a consequence of the passing “electron wind”, causing material transport in the direction of the current flow. An alternative interconnect material to solder is copper, a material already used for electrical wires for most chips and PCBs. Copper is about ten-fold more resistant against the “electron wind”, resulting in what is termed electromigration.
IBM researchers invented together with colleagues from Intrinsiq Materials, SINTEF and the Technical University of Chemnitz a process where solder is replaced with copper to create All-Copper Interconnects. The physical limitation of the high melting temperature of copper (1085°C) was overcome. These interconnects can be formed via the self-assembly of copper nanoparticles between a copper pillar and a pad, followed by an annealing step at temperatures as low as 150°C. The resulting interconnect allows to increase the current density and to reduce the interconnect pitch. IBM scientists from Zurich are presenting their latest developments using copper today in San Diego at the IEEE Electronic Components and Technology Conference (ECTC).
To perform All-Copper Interconnects, the scientists are working with a paste containing copper micro and nano-particles, which is smeared out to yield a uniform Cu-paste film with so-called “doctor blading”. Cu-paste is transferred to copper pillars of a microprocessor chip simply by dipping. The chip is aligned and placed onto pads of a PCB. The final joint is achieved by annealing at a temperature well below the melting temperature of copper. This is an example of the beauty of nanotechnology; nano-particles have a very large surface compared to their volume and hence diffusion of copper atoms is promoted to form metallic contacts. Formic acid is applied during the annealing step to reduce the copper oxide from the nano-particles.
|Formation of All-Copper Interconnects: a) Copper film formation by doctor blading, b) dipping of
microprocessor into paste film, c) alignment and d) placement of chip to substrate, including annealing
The novel All-Copper Interconnects have the potential of overcoming the current density and interconnect pitch limitations of current solder joints and thereby support the continuation of the system integration, in particular 3D stacking for the next decades to come.
While multiple hours of lab work remains, Dr. Thomas Brunschwiler at IBM Research is optimistic. He adds, “If we can demonstrate the All-Copper Interconnects in high-volume production, it will have a significant impact on devices ranging from laptops to data centers and our vision of a supercomputer the size of a shoebox will become reality.”
This research is part of HyperConnect, a European funded activity with 10 partners from industry, research institutes and universities.