Aiming for a cool market
Among the companies looking to meet the microelectronics industry’s need for heat dissipation is ALD Nanosolutions (ALDNanoSolutions.com), which uses atomic layer deposition to produce nanometer amorphous coating on a variety of surfaces-including boron nitride (BN) particles.
BN is an excellent thermal conductor and also a good electrical insulator, so it is a common additive into a polymer matrix to produce a thermally conductive bonding material. But it has difficult chemistry: a surface so inert that it mixes and distributes poorly in the viscous polymer. By coating BN particles with an alumina layer of just a few nanometers, the mixing behavior is improved and the thermal and electrical properties are retained, making the coated materials ideal for thermal bonding applications.
One issue with thermal bonding is that, during the assembly of a microelectronic chip to its heatsink, the thermally conductive filler materials tend to arrange themselves in a “magic cross” on diagonals from corner to corner of the bonded area. This reduces the thermal conductivity in the areas that the conductive filler has migrated from, and it increases the thickness along the lines to which the filler has migrated. Both those effects decrease the thermal conductivity of the bond between chip and heatsink.
Researchers at IBM’s Zurich Research Laboratory (www.zurich.ibm.com/news/07/cooling.html) investigated the factors driving the fluid flow and have developed a microchannel plate that evenly distributes the thermal filler during the normal assembly process. The copper plate with a network of 220- and 150μm channels serves to minimize the bond thickness and maximize the dispersal of the conductive filler, both of which serve to increase the thermal conductivity of the bond.
The image shows the paste after being applied using the new technology. The pattern arises from the hierarchical channel design of the interface that controls and optimizes the spread of the paste. (Image courtesy of IBM)
Celsia Technologies (celsiatechnologies.com) also uses a copper plate with engineered microchannels for its heat transfer solution, but for a different purpose. The microchannels are embedded in a sealed copper bar-about 1mm thick. When one surface of the copper plate is put in contact with a hot surface, distilled water within the microchannels vaporizes. Because of the small size of the microchannels, capillary motion and vapor pressure drive the vapor to the opposite side of the plate, in contact with a radiative heatsink. There the vapor recondenses and circulates back to the hot side, repeating the process to efficiently transfer heat away from the microelectronic circuit.
CoolChips (coolchips.com) also uses micro- and nano-structuring for its thermionic cooling technique. The physical principles of this cooling method have been understood for several decades: High-energy electrons can be excited off the surface of a hot material and transmitted across a gap. As long as the gap does not provide a thermal path back to the hot side, the cooling can be very efficient.
Despite knowledge of the principles, no technology incorporating them has been available-partly because the gap between the hot and cold electrodes must be both very small and very well controlled. In the 1950s, that was a showstopper, but it’s relatively straightforward with today’s manufacturing methods.
The second problem is the high voltage required to release the electrons from the hot electrode, the “work function.” CoolChips solves this with so-called “Avto Metals,” metals engineered with nanometer-scale features that modify the available energy levels for the electrons at the surface. By eliminating lower-energy electron levels, the highest-energy electrons become much easier to drive off the surface. Even with conservative design assumptions, the system efficiency can be as high as 50% to 55%.
It is likely that future thermal management solutions will incorporate several micro- and nanotechnological elements to maintain the rapid improvements in computing performance.