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IBM Researchers Use Sandcastle Mechanics to Improve Heat Dissipation in Stacked Chips

Q. How will 3-D stacked chips help with these issues?
Think of a suburb, where all of the homes are 10 blocks apart and only one story high. To get from house to house or block to block, you have to expend a lot of energy. This is what a computer board is like now. What we’d like to do is have the houses three, five, or even 10 stories high so you don’t have to use as much time and energy to get from house to house. That’s essentially the notion behind chip stacking, where the latency time and energy required for the chips to talk to one another is much lower. Eliminating this distance will help enormously in reducing data center energy consumption.

We think that 3-D chip stacks are one way to tackle this. If you look at one of the most efficient computational systems in nature, the human brain, you realize that it’s organized with 3-D communication paths. It has 10^11 neurons with 10^14 communication channels interconnected in three dimensions in a very compact manner so it fits into a volume of half a gallon. We absolutely have to explore this to get IT near the efficiency of brainlike computing.

Q. How does capillary bridging play into this?
One of the key aspects of the future of computing is going into the third dimension, but then you have to deal with issues such as heat removal. Currently, we’re able to transport heat away from conventional flat circuits, but if we stack a couple of chips on top of each other, that heat has to be transported through the stack in a vertical direction.

Currently, thermal underfill materials are polymers with some filler particles improving the thermal transport in it. Usually, these interspace filling materials are preformulated and inserted between 3-D interconnects with capillary force. Because they’re preformulated, there’s a limit to how high of a fill fraction of fillers we can put in and how much heat we can transport up the stack. What we’re trying to do with capillary bridging is create chip-stack materials in a different way so we can increase the amount of filler material that connects in the vertical direction while improving their point contact to effectively dissipate heat.

Q. How does capillary bridging work in this situation?
We’re using nanoparticles injected into a particle bed formed by micron-sized fillers to create improved thermal paths from chip to chip through the resulting particle network. These connections are created by the evaporation of the carrier fluid of the nanoparticle suspension—a fluid that contains these very tiny nanoparticles—within the tiny pores of the filler particle bed formed between the dies of the chip stack. The liquid forms capillary bridges due to the surface tension of the fluid during the evaporation step and the concentration of the nanoparticles increases as they stay in the fluid. Finally, so-called “necks” around the point contacts of the filler particles result by the assembly of the nanoparticles as all of the liquid is evaporated.

This effect is a similar to when people—who are actually taking advantage of the mechanical benefits of capillary bridging—build immense structures from sand mixed with water. To make shaping easier, people wet the sand during the building phase. After the building phase is finished, the water evaporates so that it eventually remains only in the very small contact between the sand particles, making the overall structure mechanically most stable. When the water completely evaporates, however, the structure will collapse with even the slightest outside influence.

With the research we’re conducting, the necks are formed by adding a mix of liquid and nanoparticles, followed by evaporation of the liquid, which pulls the nanoparticles into the contact zones between the large particles. Depending on which kind of nanoparticles are used and which kind of bigger particles or structures are used, you can create thermally conductive or electrically conductive neck paths, conducting heat or electric current.

In our case, the function of the necks is to ease the heat transport. This is crucial once we have a stack of more than two silicon chips. This is like having several concerts on different floors of a skyscraper that are over at the same time. When everyone wants to leave, the elevators can’t handle this and additional paths like stairs are needed. Those stairs must have the capacity to bring the concert attendants down in a reasonable amount of time so you don’t have heavy traffic—or heat, in a chip stack—around the elevator entrances.

Jim Utsler, IBM Systems Magazine senior writer, has been covering the technology field for more than a decade. Jim can be reached at jjutsler@provide.net.

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