13-06-2018 | | By Rob Coppinger
A honeycomb style wafer of silicon placed over a mobile phone’s or other computing devices’ hot spots could cool 400% better than existing heat sinks.
Made of silicon with holes produced by an etching process, the heat is not transported away from the electronics through the holes. Instead, the holes act as guides for heat carrying sub-atomic particles called phonons. Phonons are the heat equivalent of photons. While photons are described as sub-atomic packets of light, phonons are packets of heat. Acting as barriers, the holes guide the phonons to travel from the silicon’s back to front while limiting their movement towards the wafer’s sides.
“In the in-plane direction we are reducing the thermal conductivity [with the holes]. In the other, cross-plane, direction, we are letting them [phonons] go as freely as possible so they can maintain high thermal conductivity,” said University of California (UCI) mechanical and aerospace engineering assistant professor, Jaeho Lee. He oversaw the research, conducted by UCI Nano Thermal Energy Research Group graduate student researcher, Zongqing Ren. ““This innovation could potentially be ideal for keeping electronic devices such as smartphones cool during operation,” said Ren. Lee was also the corresponding author for the work.
A heat sink for electronics is typically a solid slab of a class of materials known as chalcogenides, which include Bismuth telluride and other tellurides. However, the holey silicon is not solid and does not have to cover the whole of a device’s electronics. Instead, very small and thin wafers can deliver the cooling needed to hot spots. Lee explained that the wafer could as small as one millimetre across or as large as one centimetre to cover a hotspot. In thickness the wafer could be 50 to 500 microns deep.
This side-view taken using a scanning electron microscope shows the smooth boundaries of holey silicon after the deep reactive ion etching process that makes the holes. By Jaeho Lee Link.
The holes are produced using a process called reactive ion etching. This uses a plasma to remove material from the surface to create the hole. The radio waves that turn the chemical vapour into a plasma provide the energy to strip away the surface. There is also a non-plasma version of this process.
To transport the heat to the outward facing surface area, the holey silicon design aims to maximise, in one plane, one direction, the thermal conductivity, which equates to the ease with which a phonon can travel. Phonons travel very fast through a solid, at the speed of 8,000 metres per second. A metre is a million microns and a wafer can be up to 500 microns thick. The holes, of which there would be thousands, guide the phonons in the cross-plane direction, and limit their travel in the in-plane direction. This is called the ballistic transfer regime, where phonons encounter holes before they encounter other phonons.
This image shows the sandwich of hotspot, holey silicon wafer on top and on top of that, the micro-cooler heat sink and heat exchanger. The red indicates the travel of heat. By Jaeho Lee/Zongqing Ren Link.
Normally a phonon can expect to encounter another phonon and the time and distance it takes for this to happen is called the mean free path. The combination of the holes’ cross-plane guidance and a wafer as thin as 50 microns can ensure no phonons are encountered.
As well as mobile computing devices, another application is cooling for microchips that are embedded in larger machines that can have a hot environment. One example is actuators in electric cars and another is sensors that operate as part of the Internet of Things. Being able to draw out the heat will help electronics’ reliability and longevity.