Helium as a Protective Gas in Femtosecond Laser Processing of Crystalline Silicon: Tracking He Incorporation with High‑Sensitivity SIMS

Introduction

Femtosecond laser processing is used to create precise micro and nanostructures in silicon while also modifying material properties. In many cases, the process is carried out in a gas atmosphere rather than in vacuum. This helps reduce contamination of the optical window by ablated silicon nanoparticles, but it also introduces the possibility that the gas itself may enter the silicon during irradiation. The study focused on this problem and asked whether helium could serve as a protective gas without producing the unintended doping effects associated with other atmospheres. The authors investigated structural, optical, doping, and electrical changes in crystalline silicon treated with femtosecond laser pulses in helium.

Methods

The experiments were carried out on double-polished N-type silicon wafers placed in a chamber filled with helium at a pressure of 0.2 MPa. A pulsed laser was emitted from a titanium (Ti): sapphire fs laser system, with a pulse duration of 100 fs, a center wavelength of 800 nm, and a repetition rate of 1 kHz. A second set of samples was implanted with helium ions before laser treatment, and one implanted sample was later annealed. To characterize the resulting material, the researchers used scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, Raman spectroscopy, optical spectroscopy, Hall-effect measurements, and SIMS. Helium depth profiles were measured using dynamic magnetic‑sector SIMS (CAMECA IMS 7f‑Auto), operated in a depth‑profiling mode optimized for light elements.

Surface and Optical Results

The silicon surfaces processed in helium showed clear changes with increasing laser fluence. At low fluence, the surface displayed laser-induced periodic surface structures. At moderate fluence, worm-like protrusions appeared, and at higher fluence, blunt cone-like structures formed. Unlike the typical black silicon reported under other processing conditions, the helium-processed samples appeared gray, and the low-fluence sample displayed structural color at certain viewing angles. The surfaces were also relatively smooth and only sparsely covered with redeposited nanoparticles. The authors attributed this to the small atomic radius and low mass of helium, which allowed ablated silicon nanoparticles to travel farther and reduced redeposition on the silicon surface.

Optical measurements further reflected these surface differences. Laser treatment reduced reflectance in the UV-Vis range and increased absorptance from the ultraviolet into the near infrared. At the same time, samples processed at moderate and high fluence showed increased reflectance in parts of the near infrared. The authors proposed that this behavior was linked to the modified surface layer and the geometry of the primary microstructures. Because the helium atmosphere led to less nanoparticle redeposition, the optical behavior of those primary structures became easier to observe.

Structural Analysis

To examine crystal quality, the researchers analyzed the sample processed at 0.53 J/cm² in more detail. Transmission electron microscopy revealed a recrystallized surface layer above a single-crystal core region. High-resolution TEM images and diffraction patterns showed that the matrix largely preserved the monocrystalline nature of silicon, although local regions of disturbed atomic arrangement were present. Raman spectra did not show polycrystalline or amorphous peaks, but they did show peak broadening and redshift, indicating disturbance and tensile stress in the monocrystalline silicon. Based on these results, the authors concluded that helium-assisted laser processing modified the silicon surface without forming new phases and preserved a good monocrystalline phase more effectively than the other gas environments discussed in the study.

While surface‑sensitive techniques provide valuable chemical snapshots, understanding helium behavior in laser‑processed silicon fundamentally requires true depth‑resolved analysis.

SIMS was essential to the study because it provided depth-resolved information on helium incorporation, which is particularly challenging to characterize due to helium’s high ionization energy and the resulting high detection limit in SIMS measurements.. The maximum helium concentration reached only 6.3 × 10¹⁸ cm⁻³ at high fluence, close to the SIMS detection limit, and far lower than concentrations reported for other protective gases under comparable processing conditions. At such concentration levels, reliable helium detection requires sustained sputtering conditions and high secondary‑ion transmission, which are essential for quantitative depth profiling of light species. Among all characterization techniques used, SIMS uniquely enabled direct verification that helium incorporation remained minimal and that implanted or incorporated helium could diffuse out during laser processing or subsequent annealing.

The SIMS results also helped explain what happened in the helium-implanted samples after laser treatment. As laser fluence increased, helium concentration decreased and became more broadly distributed, indicating significant diffusion during laser-induced heating. The annealed implanted sample also showed reduced helium concentration. Together, these findings supported the mechanism proposed by the authors: helium enters the molten silicon surface during irradiation, interacts with vacancies, and then diffuses out during recrystallization.

"Among all characterization techniques used, SIMS uniquely enabled direct verification that helium incorporation remained minimal and that implanted or incorporated helium could diffuse out during laser processing or subsequent annealing." — Ji-Hong Zhao, Ph.D, Professor, Jilin University, Changchun, China.

Electrical Properties

Hall-effect measurements showed that the processed silicon remained N-type. Sheet resistance increased slightly and carrier concentration decreased somewhat, while electron mobility remained high. Overall, the electrical properties stayed within the same order of magnitude. The authors concluded that femtosecond laser processing in helium did not substantially alter the electrical characteristics of the silicon substrate. They also found that thermal annealing could remove residual helium from implanted silicon.

Conclusion

This study showed that helium can serve as an effective protective gas during femtosecond laser processing of crystalline silicon. Under the reported conditions, helium enabled smooth micro/nano-structured surfaces, supported preservation of a good monocrystalline phase, and resulted in low helium concentration with minimal impact on electrical behavior. SIMS was central to these conclusions because it provided the depth-resolved helium measurements needed to confirm limited incorporation and subsequent diffusion. Beyond detection, depth‑profiling SIMS using CAMECA IMS 7f-Auto was essential to validate helium diffusion and effusion mechanisms across processing, implantation, and annealing conditions.