The latest progress in laser chip manufacturing

Modern computer chips can construct nanoscale structures. So far, only these tiny structures can be formed on top of silicon chips, but now a new technology can create nanoscale structures in a layer beneath the surface. The inventor of this method stated that it has broad application prospects in the fields of photonics and electronics, and one day, people can manufacture 3D structures on the entire silicon wafer.

This technology relies on the fact that silicon is transparent to certain wavelengths of light. This means that a suitable laser can pass through the surface of the wafer and interact with the underlying silicon. However, designing a laser that can penetrate the surface without causing damage and can also perform precise nanoscale manufacturing underneath is not simple.

Researchers at Birkent University in Ankara, Türkiye, achieved this goal by using spatial light modulation to create needle shaped laser beams, so as to better control the distribution position of beam energy. By utilizing the physical interaction between laser and silicon, they are able to manufacture lines and planes with different optical properties, which can be combined to create nanophotonic elements beneath the surface.
The use of lasers for manufacturing inside silicon wafers is not a new phenomenon. But Onur Tokel, assistant professor of physics at the University of Kent who led the research, explained that so far, only micrometer scale structures have been produced. He said that extending this method to the nanoscale can unleash new capabilities, as it can create features that are comparable in size to the wavelength of the incident light. When this happens, these structures exhibit a range of novel optical behaviors, which makes it possible to manufacture metamaterials and metasurfaces, among other things.

Silicon is the cornerstone of electronics, photonics, and photovoltaic technology, "Tokel said. If we can introduce additional functionalities inside the nanoscale wafer to supplement these existing functionalities, it will bring a completely different paradigm. Now you can imagine doing things in volume, and even potentially in three-dimensional space. We believe this will open up exciting new directions.

Previous technologies were unable to manufacture at the nanoscale because once the laser enters the silicon, it scatters and it is difficult to deposit energy accurately. In a paper published in the journal Nature Communications, Tokel's team demonstrated that they can solve this problem by using a special laser called Bessel beam, which does not undergo diffraction. This means that lasers can counteract light scattering effects and maintain narrow focusing inside silicon, allowing for precise energy deposition.

When the laser is irradiated onto the wafer, tiny holes or gaps are generated in the area where the beam is focused. Tokel said that this situation has also occurred with previous methods, but the smaller gaps generated by the more tightly focused beam exhibit a "field enhancement" effect, resulting in an increase in laser intensity around them. This will change the silicon structure around the gap, further enhancing the enhancement effect and forming a self-sustaining feedback loop. The team also found that they can change the direction of field enhancement by altering the polarization of the laser.

The final result is to create a two-dimensional planar or linear structure with a minimum of 100 nanometers in the silicon wafer. The refractive index of these structures is different from the rest of the wafer, but Tokel stated that the composition of these structures is not yet fully understood. Based on previous research, he believes that the underlying crystal structure of silicon wafers may have been modified. He added that electron microscopy research should be able to clarify this in the future, but ultimately there is no need to understand the exact underlying properties of these structures to create useful nanophotonic components.

To demonstrate this, researchers have developed a nanoscale photonic device called a Bragg grating, which can be used as an optical filter. According to the team, this is the first functional nanoscale optical component completely buried in silicon.

Maxime Chambonneau, a researcher at the University of Jena in Germany, said that it is remarkable that researchers were able to achieve nanoscale features, as the relatively long laser pulses used by the Tokel team typically create large heat affected zones, leading to microscale variations. The Bilkent team uses pulses in nanoseconds, while other direct laser writing works traditionally involve picosecond or femtosecond lasers. Chambonneau suggests that creating features smaller than light waves could bring various possibilities, including improving the energy harvesting capability of solar cells.

Due to the fact that this manufacturing technology does not cause any changes to the wafer surface, Tokel stated that in the future, this technology can be used to manufacture multifunctional devices, with electronic components located on the surface and photonic components buried underneath. The team is still investigating whether this method can be used to carve microfluidic channels beneath the surface of chips. Tokel stated that pumping fluid through these channels can improve heat dissipation, thereby helping to cool electronic devices and make them run faster.

Tokel stated that the biggest limitation of this method is that researchers cannot precisely control the location of voids in specific areas. Currently, a small portion of voids are unevenly distributed in the area where the laser beam is focused. Tokel stated that if they could more accurately locate these voids, they could perform nanomachining in three-dimensional space, rather than simply producing lines or planes.
If you can individually control these things and distribute them like chains, then this will be very exciting in the future, "he added. Because in this way, you will have more control, which will make richer elements or systems possible.

Source: Semiconductor Industry Observation