Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams

Abstract

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm−3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm−3). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a “ring” with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 1021 cm−3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon.

Ultrafast Laser Welding of Silicon

“…In contrast, ultrashort pulses (<10 ps) lead to limited heataffected zone. However, permanent modifications are extremely challenging to produce in the bulk and the exit surface of silicon with ultrashort laser pulses, [12] unless complex approaches are implemented for optimizing the spatial, [26][27][28] spectral, [29,30] or temporal [31][32][33] properties of the irradiation. These limitations originate from the high nonlinear refractive index of silicon (on the order of 10 À14 cm 2 W À1 [34,35] ), which causes microfilamentation at modest peak powers-and thus, an intensity clamping similar to that described for transparent media.…”

Silicon–metal and silicon–silicon ultrafast laser welding with domesticated filaments

“…In contrast, ultrashort pulses (< 10 ps) lead to limited heat-affected zone. However, permanent modifications are extremely challenging to produce in the bulk and the exit surface of silicon with ultrashort laser pulses, 12 unless complex approaches are implemented for optimizing the spatial, 19,35,36 spectral, 37,38 or temporal [39][40][41] properties of the irradiation.…”

Hybrid photoexcitation in ultralow-nitrogen diamond by mid-IR femtosecond laser pulses

“…Mid-infrared-range (MIR, 2-12 m) femtosecond (fs) laser pulses emerged as hardly technically accessible, but very promising toolkit in IR photonics of gases [1.2], liquids [3], solids [4,5] and even biological samples [6,7]. This spectral range, typically free of electronic transitions, corresponds to the transparency window of the most solid dielectrics and even semiconductors, while covers the most important ("fingerprint") molecular and optical-phonon (sub-lattice) vibrations in any media.…”

“…Moreover, MIR fs-laser pulses were recently demonstrated to be very promising in their "invasive" interactions with either UV-MIR transparent solid dielectrics, or UV-near-IR opaque semiconductors (both on surfaces and in bulk), when the underlying non-linear photo-processes become strongly-driven at high I 2 factors [1][2][8][9][10]. Specifically, recently structural modification in bulk silicon [4] and even diamond [11] was achieved, utilizing tightly focused MIR fs-laser pulses. Direct selective linear (single-photon) MIR fs-laser excitation of optical phonons in polar dielectrics (h-BN, 7.8 m) [12] and multi-photon MIR fs-laser excitation of optical bi-phonons (quasi-particle made of phonons with the opposite wave-vectors [13]) in non-polar dielectrics (diamond, 4.0 and 4.7 m) [11] were demonstrated to drive quantum bosonic lattice dynamics.…”