Engineers at the NanoStructure Laboratory at Princeton University have developed a lithographic printing technique to produce 3D structures smaller than 40nm. The method involves the creation of a mould on a silicon wafer using e-beam lithography and etching.
Publishing in Applied Physics Letters, Professor Stephen Chou, Mintao Li and Lei Chen define the mould structure using a 35keV e-beam in a delicate process which takes weeks to complete.
"You have to use a very low current so that the beam spot size is very small," said Chou. "Once you have created the mould, you use it to deform the shape of a polymer and then transfer the pattern in the film on to a substrate."
Chou says that method is much easier than using conventional lithography to create 3D structures — a process that requires at least two lithographic steps.
The mould is formed using a bilayer of electron-sensitive polymer resists.
After each layer is coated on to the wafer, it is baked at 165°C. The electron beam defines the pattern as an imprint within the polymer layers.
Layers of chromium and nickel are then evaporated on to the surface and oxidised using an ion etching process. The nickel layer is dissolved and, after a second etching step, the chromium mould structure is complete. The mould can then imprint the pattern on to a 520nm polymer layer on a silicon substrate.
Chou and his team have evaporated titanium and gold on to the imprinted polymer and then lifted off the polymer to produce structures as small as 40nm. Titanium encourages adhesion between the gold and the silicon substrate.
While the creation of the mould is a slow and delicate process, the printing process which follows has the potential for low cost and high throughput.
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