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3D Silicon Micromachining with Infrared ultrafast LasErs

Periodic Reporting for period 1 - SMILE (3D Silicon Micromachining with Infrared ultrafast LasErs)

Reporting period: 2015-09-30 to 2017-09-29

"The famous physicist Richard Feynman said, ""There is plenty of room at the bottom."", and is often quoted to highlight the successes of modern fabrication techniques. The continual progress in these enables new technologies we enjoy daily. In this respect, silicon, the bedrock of modern computers, mobile communications, and Si-photonics, has proven to be extremely capable. The material is incorporating ever more speed, usually described with Moore's law. Precisely due to these amazing successes, one may be surprised to hear that virtually none of these are taking advantage of the vast space available, below the surface, inside silicon wafers. In this sense, electronics and Si-photonics continue to live in Flatland. What more could be achieved, if the bulk of Si was opened to usage?

A team of scientists now found a way to pack various laser-written structures deep inside silicon chips (Tokel, et. al. Nature Photonics, 11, 639, 2017). In this novel approach, they use a focused infrared laser which exploits the inherent optical response of Si to create 1-μm-resolution building blocks in a sliver of silicon. For the first time, the researchers demonstrate arbitrary 3D fabrication in silicon.

But this was the first hurdle the researchers had to overcome. Next, they convert these 3D architectures into functional optical devices, such as lenses, waveguides, holograms. ""We achieve this by exploiting dynamics arising from nonlinear interactions, leading to controllable building blocks,"" says Dr. Onur Tokel of the Department of Physics at Bilkent. ""In any 3D fabrication method, there is a trade-off between speed, resolution, and complexity. With our approach, we are hitting the sweet spot. The critical realization is noticing that most practical components can be made out of needle-like building blocks. Our method enables creating such blocks, while also preserving a width of 1 µm for each block. Better yet, the rods can be combined to create a 2D layer, or even 3D shapes.""

A further outcome is related to 3D printing or sculpting. They found that by exposing modified areas to a specific chemical, it is possible to realize 3D sculpturing. They demonstrated various micro-components, such as microchannels, thru-Si vias, and micropillars. ""I should note that this is a direct-laser writing approach, inexpensive compared to conventional lithography,"" notes Dr. Serim Ilday, of the Department of Physics, one of the coauthors of the paper.

Inspired by the successes of ""on-chip"" devices, the team coined the term ""in-chip"", as a descriptor for this new class of components based on direct 3D fabrication. ""The possibilities are endless. It is likely that the method will enable entirely new in-chip devices, such as Si-photonics components, or microfluidic channels that may be used to efficiently cool electronic chips"", observed Prof. Ömer Ilday, another co-author of the paper from Bilkent.
(Adapted from dissemination:"
"Our primary goal is to develop a novel 3D laser-writing method deep inside silicon, towards novel applications. We are motivated by the myriad of applications based on 3D micromachining of glass that peaked in the early 2000's. These successes had been achieved using lasers at wavelengths for which glass is transparent. Here, similarly, we demonstrated the necessary laser technology, for discovering a interaction regime in Si, which eventually led to a truly 3D laser-structuring capability deep inside Si. We also demonstrated the first functional “in-chip” optical elements.

Our efforts led to a unique 3D laser-writing toolbox for silicon (Tokel, et. al. Nature Photonics, 11, 639, 2017; Optics Letters, 42,15, 3028, 2017). We used home-made lasers operating at 1.5 μm, and demonstrated that by exploiting the nonlinear mechanisms, it is possible to demonstrate i) the first fully 3D subsurface modification of Si without altering the surface, ii) the first functional elements buried deep inside silicon, and iii) realise a novel 3D fabrication method akin to 3D-printing to create a plethora of microstructures with large-volume coverage.

Our first objective was to employ the appropriate laser technology. It had recently been shown that simply increasing the pulse energies would not work, since plasma shielding diffracts light, precluding modification (J. Appl. Phys., 117, 153105, 2015). There was no method to realize subsurface modification with fs pulses prior to ours. Our extensive efforts enabled (i) the first fs laser modification deep inside Si, and (ii) the first functional waveguides buried in Si ( Tokel, et. al., “Femtosecond laser written waveguides deep inside silicon”, Optics Letters, 42,15, 3028, 2017).

Our approach next is based on exploiting the inherent optical response of Si. The laser-material system could be considered as an evolving entity, where once a modification of Si takes place, it can reconfigure the laser, which in turn can continue modifying Si. In ""Tokel, Nature Photonics, 11, 639, 2017"", we take such a novel approach, and show functional elements and 3D architectures with 1-μm resolution, without damaging chip surface:

• holograms for wavefront control,
• lenses and gratings for beam steering,
• waveguides for optical interconnects,
• multilevel, dense data storage,
• microfluidic channels for cooling,
• through-Si vias for electrical interconnects,
• microstructures for MEMS,
• slicing of a wafer for photovoltaics.

We introduced a plethora of subsurface—that is, ‘in-chip’— elements, which constitute an entirely new capability and a disruptive technology. This was possible with close collaboration of Dr. O. Tokel, Dr. S. Ilday and Dr. F. Ö. Ilday, from Bilkent.

For decades, the information technology has literally been written on the surface of Si chips, where lithography realised finer features leading to speed increases, summarised with Moore’s law. But what about the possibilities for photonic and other functional elements inside Si wafers?

Inspired by the successes of “on-chip” devices on glass, we coined the term “in-chip”, in order to differentiate our subsurface Si components from their conventional 2D analogues. This novel method uses a combination of direct laser writing and optional chemical etching to pack 3-D microstructures and photonic devices deep inside the bulk of Si, while leaving the chip surface unaltered. The idea similar to direct fabrication of 3D objects, referred to as 3D printing, capturing the public imagination on a scale rapidly approaching that of consumer electronics revolutions. Until recently, there was no method for 3D fabrication deep inside Si. We developed such a method, and created a variety of 3D photonic and microstructures, enabling various microelements and photonic devices. For instance, we demonstrated holograms for 3D image projections, aimed for 3D displays.

This new laser micro-fabrication technology and in-chip elements can be used in various fields, such as microfluidics, Si-photonics and MEMS devices. We also imagine hybrid-systems, with in-chip photonics integrated to electronics. These can incorporate waveguides for data transfer, holograms for diffractive optics, and advanced sensors. Notably, all of these advances have been in Si, the crown jewel of the microelectronics and Si-photonics. Thus, our demonstrations may find use in other applications which we may not even have foreseen.
Graphical abstract of in-chip devices