Single-Shot Multi-Stage Damage and Ablation of Silicon by Femtosecond Mid-infrared Laser Pulses

Abstract

Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MIR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ2 scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MIR pulses (λ = 2.7–4.2 μm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and μ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.

Single Femtosecond Laser-Pulse-Induced Superficial Amorphization and Re-Crystallization of Silicon

“…Upon irradiation of semiconductors with ultrashort (fs-ps) laser pulses, amorphous surface layers with a thickness of some tens of nanometers were reported, as determined by analytical techniques such as transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) analyses, micro-Raman spectroscopy (µ-RS), confocal scanning laser microscopy (CSLM), real-time reflectivity measurements (RTR), and fs-time-resolved microscopy (fs-TRM) [ 33 , 34 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. The specific interest of such a contact-less surface modification technology is based on the remarkably different structural, electrical and optical properties of the amorphous and the crystalline silicon phases that enable applications in electronics and photonics and manifest through alterations of the local chemical etching rate, electric conductivity, or the high refractive indices at telecom wavelengths [ 48 , 49 ].…”

Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR

“…In the mid-IR spectral region, promising materials for on-chip DLA technology are dielectric materials, which have a higher material damage threshold compared to silicon 14 , 15 . Well-developed applications such as ultrafast laser materials processing 16 and direct laser writing 17 may also have advantages in switching from visible and near-IR to mid-IR. A promising way for controlling the energy deposition inside transparent dielectric materials is to use a two-color ultrafast excitation 18 – 24 .…”

Extreme Sub-Wavelength Structure Formation from Mid-IR Femtosecond Laser Interaction with Silicon

“…Using our previously-described laser-induced damage (LID) setup [ 1 ], high purity (>1000 Ω-cm) FZ mono-crystalline silicon (111) and (100) wafers were exposed to a maximum of 10,000 pulses per site with a maximum average peak fluence of 1.6 J/cm , a focal diameter of 24 µm FWHM, at an angle of incidence (AOI) of 45 degrees ( p -polarization). Details of the focal spot characterization method and positioning of the sample surface with respect to the focal plane are described in detail in reference [ 1 ]. The (111) wafer is oriented such that the laser pulse electric field polarization was parallel to the (100) direction.…”

“…Strong field interaction with solids in the mid-infrared (MIR) regime of light provides an exciting platform in contrast with that of visible (VIS) and near-infrared (NIR) regimes [ 1 , 2 ]: first, the reduction of photon energy allows the interaction to be more field dominated than photon dominated, where the material valence band to conduction band electronic transition leaning away from multiphoton processes towards tunneling process; second, traditional semiconductors and other ’small bandgap’ materials, which are highly absorptive in the VIS-NIR regime, become transparent in the MIR, potentially changing ultrafast absorption dynamics; third, cycle averaged energy (a.k.a. ponderomotive energy, , where is the laser wavelength, is the peak laser electric field strength, and is the effective mass in the conduction band) of the free carriers increases significantly with longer wavelength, changing free carrier absorption and collisional ionization; fourthly, with decreasing plasma critical density as wavelength increases, semiconductors may become ideal plasmonic platforms, creating exotic metamaterials and surfaces [ 3 ].…”