When the island of Manhattan became too crowded, architects responded by building skyscrapers. The increasing density of components on “flat” computer chips is encouraging similar ideas, building upwards to create three-dimensional chips. But moving from flat interconnecting wires to 3D ones to link up different “storeys” has proved a tricky business – until now.
Instead of soldering prefabricated wires in place, as is traditionally done to connect two parts of a chip, Min-Feng Yu and Jie Hu at the University of Illinois at Urbana-Champaign have developed a technique to grow tiny 3D wires in situ which are tailor-made for their location.
Yu and Hu’s technique is a modified form of electroplating, in which an electric current is used to coat a conductive surface with a thin layer of metal, deposited from a liquid electrolyte. Such a technique theoretically offers a way to directly “write” metal wires onto a surface.
But using such electrodeposition to build tiny wires, rather than indiscriminately plating an entire surface, is challenging. One option is to hold the electrolyte in a pipette that’s then brought near to the conductive surface. Then, when an electrical potential is applied between the two, a thin “liquid bridge” is established. Copper or platinum particles in the electrolyte are attracted to the surface, creating a small metallic blob.
Yu and Hu realised that because the metallic blob continues to grow away from the surface and along the liquid bridge as long as the electrical potential is applied, the method offers a way to grow 3D wires that extend above the surface instead of just “two-dimensional” ones that run along it.
The trick is to carefully synchronise the speed with which the micropipette is withdrawn from the surface so that it matches the growth rate of the metal blob. When done correctly, the micropipette leaves a solid, 3D metallic structure in its wake. By moving the pipette parallel to the surface, instead of just vertically away from it, it is possible to create intricate 3D structures (see image). Such metallic bridges could link different parts of a conductive surface, or link two conductive surfaces or components at different heights on a 3D chip.
With a small enough pipette, the researchers have grown freestanding interconnects more than 80 micrometres long but just 100 nanometres wide. “That’s one order of magnitude smaller in diameter than is possible in current practice,” says Yu.
Take the heat
These interconnects can withstand current densities a million times higher than those that will burn out a traditionally soldered interconnect, because the electrodeposited metal forms a much stronger bond with the surface than a soldered joint.
The work provides an “elegant solution” to the problem of growing intricately shaped 3D wires in chip manufacture, says Peter Searson of Johns Hopkins University in Baltimore, Maryland. “It is likely to be an important new tool for the fabrication of three dimensional architectures at the micron scale.”
Yu and Hu now hope to shrink the diameter of their wires by yet another order of magnitude, while also scaling up the technique to create a quick, industrially compatible process. “Instead of fabricating wires one by one, we will try to find a way to fabricate them in an array fashion,” says Yu.
Journal reference: Science, DOI: 10.1126/science.1190496