Modern computer chips can build nanoscale structures. Until now, such tiny structures could only be formed on top of silicon wafers, but now a new technology can create nanoscale structures in a layer just beneath the surface.The method's inventors say it has broad application prospects in both photonics and electronics, and that one day it will be possible to create 3D structures on entire silicon wafers.
The technology relies on the fact that silicon is transparent to certain wavelengths of light.This means that a suitable laser can pass through the wafer surface and interact with the underlying silicon.But designing a laser that can pass through a surface without causing damage, while also enabling precise nanoscale fabrication underneath, is no simple matter.
Researchers at Bilkent University in Ankara, Turkey, achieved this by using spatial light modulation to create needle-shaped laser beams that better control where the beam's energy is distributed.By exploiting the physical interaction between lasers and silicon, they were able to create wires and planes with different optical properties that could be combined to create nanophotonic components beneath the surface.
Using lasers to fabricate inside silicon wafers is nothing new. But Onur Tokel, an assistant professor of physics at Bilkent University who led the research, explains that so far only micron-scale structures have been created. Scaling this approach to the nanoscale could unlock new capabilities, he said, because it can create features that are roughly the size of the wavelength of incoming light. When this happens, these structures exhibit a range of novel optical behaviors that, among other things, make it possible to create metamaterials and metasurfaces.
"Silicon is the cornerstone of electronics, photonics and photovoltaics," Tokel said."If we can introduce additional functionality inside the nanoscale wafer that complements these existing capabilities, it will bring about a completely different paradigm. Now you can imagine doing things within the volume, and maybe even eventually doing things in three dimensions. We believe this will open up exciting new directions."
Previous technologies were unable to fabricate at the nanoscale because laser light scatters once it enters the silicon, making it difficult to precisely deposit the energy. In a paper published in the journal Nature Communications, Tokel's team showed that they could solve this problem by using a special type of laser called a Bessel beam, which does not diffract. This means the laser can fight light scattering effects and remain narrowly focused inside the silicon, allowing it to deposit energy precisely.
When a laser shines onto a wafer, it creates tiny holes, called voids, in the area where the beam is focused.Tokel says this has occurred with previous methods, but the smaller gaps created by focusing the more tightly focused beams exhibit a "field enhancement" effect, causing the laser intensity to increase around them. This changes the silicon structure around the voids, further increasing the enhancement effect and creating a self-sustaining feedback loop. The team also found that they could change the direction of the field enhancement by changing the polarization of the laser light.
The end result is the creation of two-dimensional planar or linear structures as small as 100 nanometers in silicon wafers. These structures have a different refractive index than the rest of the wafer, but Tokel said it's not entirely clear what these structures are made of. Based on previous research, he believes the underlying crystal structure of the silicon wafer may have been modified. He added that electron microscopy studies should be able to clarify this in the future, but ultimately it is not necessary to know the exact underlying properties of these structures to create useful nanophotonic components.
To demonstrate this, the researchers created a nanoscale photonic device called a Bragg grating that can be used as an optical filter.According to the team, this is the first functional nanoscale optical element completely buried in silicon.
Maxime Chambonneau, a researcher at the University of Jena in Germany, said it was remarkable that the researchers were able to achieve nanoscale features because the relatively long laser pulses used by Tokel's team usually create large heat-affected zones, which lead to microscale changes. (Bilkent's team worked with pulses measured in nanoseconds, while other direct laser writing work has traditionally involved picosecond or femtosecond lasers.) Being able to create features smaller than light waves could open up a variety of possibilities, including improving the energy-harvesting capabilities of solar cells, Chambonneau said.
Because the manufacturing technique does not cause any changes to the wafer surface, Tokel said that in the future the technology could be used to create multifunctional devices with electronic components on the surface and photonic components buried underneath. The team is also investigating whether the method can be used to carve microfluidic channels beneath the surface of a chip. Pumping fluid through these channels improves heat dissipation, which helps cool electronic devices and make them run faster, Torkel said.
The biggest limitation of this approach, Tokel says, is that researchers can't precisely control where holes appear in specific areas. Currently, a small number of voids are unevenly distributed in the area where the laser beam is focused. Toker said,If they could position these cavities more precisely, they could nanofabricate in three dimensions, rather than simply producing lines or planes.
"If you could control these things individually and distribute them like a chain, that would be very exciting in the future," he added. "Because then you'll have more control, which will enable richer elements or systems."