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Ultrafast lasers increase functionality of semiconductor chips

Researchers have developed a direct laser writing technique that can achieve local material processing within the 3D space of semiconductor chips.

The method could exploit the sub-wafer surface space of chips to achieve higher integration densities and deliver extra functionality.

Semiconductors remain the backbone material of electronics integrated with modern devices such as smartphones, cars, robots and many other intelligent devices. Driven by the continuous need for miniaturised and powerful chips, current semiconductor manufacturing technologies are facing increasing pressure.

According to the researchers, from the LP3 Laboratory in France, the dominating manufacturing technology, lithography, has strong limitations to fully address these challenges. For this reason, they believe that being able to fabricate structures under the wafer surfaces would be highly desirable so that the full space inside the materials can be exploited.

In the International Journal of Extreme Manufacturing, the researchers have demonstrated such capability via the newly developed direct laser writing technique, which enables them to fabricate embedded structures inside various semiconductor materials. 

They demonstrated the successful sub-wafer modification of Si and GaAs, two important materials for the microelectronics industry which cannot be 3D processed using conventional ultrafast laser pulses. This is because the intense light causes highly efficient nonlinear ionisation inside narrow-gap materials, which creates free electrons that rapidly transform any semiconductor into metal-like materials – thus making it impossible for the light to propagate deep inside the matter. This transition deteriorates the focusing process and prevents the occurrence of sub-wafer semiconductor material modification using ultrafast lasers.

To solve this problem, the team used non-conventional ultrafast bursts of short-wave infrared (SWIR) pulses to circumvent the metallisation transition.  

“Previous research used too strong light pulses that were exciting the electrons too easily,” explained Dr Andong Wang, one of the main investigators of this paper. “Here, instead of using strong light pulses, we split the pulse energy into a large number of weaker pulses with an extremely fast repetition rate. These pulse trains, also named bursts, will avoid strong pulse excitation before the light is focused. Additionally, the pulses will repeat very fast so that the delivered laser energy can accumulate efficiently to cross the modification.”

According to the researchers, their work offers the first “very practical” solution for ultrafast laser writing inside semiconductor materials. 

Their next step will be to concentrate on the type of modifications that can be achieved inside these materials.

“Refractive index engineering is surely an important target given the growing importance of silicon photonics,” remarked Dr David Grojo, the team leader behind the work. “Laser writing would offer the possibility of direct digital fabrication of 3D architecture materials inaccessible by current manufacturing technologies. In the future, these new laser modalities may dramatically change the way advanced micro-chips are fabricated today.”

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