Univ. of Maryland researchers have made a
breakthrough in the use of visible light for making tiny integrated circuits.
Though their advance is probably at least a decade from commercial use, they
say it could one day make it possible for companies like Intel to continue
their decades long tread of making ever smaller, faster, and cheaper computer
For some 50 years, the integrated circuits, or chips, that are at the heart
of computers, smart phones, and other high-tech devices have been created
through a technique known as photolithography, in which each computer chip is
built up in layers.
In photolithography, each layer of a conductive material is deposited on a
chip and coated with a chemical that hardens when exposed to light. Light
shining through a kind of stencil, known as a mask, projects a detailed pattern
onto the photoresist, which hardens where it’s exposed. Then, the unhardened
areas of photoresist and underlying metal are etched away with a chemical. Finally,
the remaining photoresist is etched away using a different chemical treatment,
leaving an underlying layer of metal with the same shape as the mask.
However, fitting more and more circuits on each chip has meant making
smaller and smaller circuits. In fact, features of circuits in today’s computer
chips are significantly smaller than the wavelength of visible light. As a
result, manufacturers have gone to using shorter and shorter wavelengths of
light (radiation), or even charged particles, to enable them to make these
Univ. of Maryland chemistry Professor John Fourkas and his research group
recently introduced a technique called RAPID lithography that makes it possible
to use visible light to attain lithographic resolution comparable to (and
potentially even better than) that obtained with shorter wave length radiation.
“Our RAPID technique could offer substantial savings in cost and ease
of production,” Fourkas said. “Visible light is far less expensive to
generate, propagate and manipulate than shorter wavelength forms of
electromagnetic radiation, such as vacuum ultraviolet or X-rays. And using
visible light would not require the use of the high vacuum conditions needed
for current short wavelength technologies.”
The key to RAPID is the use of a special “photoinitiator” that can
be excited, or turned on, by one laser beam and deactivated by another. In new
work just published online by Nature
Chemistry, Fourkas and his group report three broad classes of common dye
molecules that can be used for RAPID lithography.
In earlier work, Fourkas and his team used a beam of ultrafast pulses for
the excitation step and a continuous laser for deactivation. However, they say
that in some of their newly reported materials deactivation is so efficient
that the ultrafast pulses of the excitation beam also deactivate molecules.
This phenomenon leads to the surprising result that higher exposures can lead
to smaller features, leading to what the researchers call a proportional
velocity (PROVE) dependence.
“PROVE behavior is a simple way to identify photoinitiators that can be
deactivated efficiently,” says Fourkas, “which is an important step
towards being able to use RAPID in an industrial setting.”
By combining a PROVE photoinitiator with a photoinitiator that has a
conventional exposure dependence, Fourkas and co-workers were also able to
demonstrate a photoresist for which the resolution was independent of the
exposure over a broad range of exposure times.
“Imagine a photographic film that always gives the right exposure no
matter what shutter speed is used,” says Fourkas. “You could take
perfect pictures every time. By the same token, these new photoresists are
extremely fault-tolerant, allowing us to create the exact lithographic pattern
we want time after time.”
According to Fourkas, he and his team have more research to do before
thinking about trying to commercialize their new RAPID technology. “Right
now we’re using the technique for point-by-point lithography. We need to get it
to the stage where we can operate on an entire silicon wafer, which will
require more advances in chemistry, materials and optics. If we can make these
advances—and we’re working hard on it—then we will think about
Another factor in time to application, he explained, is that his team’s
approach is not a R&D direction that chip manufacturers had been looking at
before now. As a result, commercial use of the RAPID approach is probably at
least ten years down the road, he said.