Thierry Sarnet

Possible surface plasmon polariton excitation under femtosecond laser irradiation of silicon

The mechanisms of ripple formation on silicon surface by femtosecond laser pulses are investigated. We demonstrate the transient evolution of the density of the excited free-carriers. As a result, the experimental conditions required for the excitation of surface plasmon polaritons are revealed. The periods of the resulting structures are then investigated as a function of laser parameters, such as the angle of incidence, laser fluence, and polarization. The obtained dependencies provide a way of better control over the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material.

Formation of femtosecond laser induced surface structures on silicon: Insights from numerical modeling and single pulse experiments

Abstract Laser induced periodic surface structures (LIPSS) are formed by multiple irradiation of femtosecond laser on a silicon target. In this paper, we focus and discuss the surface plasmon polariton mechanism by an analysis of transient phase-matching conditions in Si on the basis of a single pulse experiment and numerical simulations. Two regimes of ripple formation mechanisms at low number of shots are identified and detailed. Correlation of numerical and experimental results is good.

Laser–ablative nanostructuring of surfaces

We present an overview of laser methods for nanostructuring of surfaces that are in the focus of ongoing research activities of our Institute (LP3). The methods imply the removal of material by laser radiation to provide either spontaneous nanostructuring of laser–illuminated surface or its controlled nano–modification. Here, the desired relief or architecture is achieved through a proper selection of radiation characteristics (pulse duration and wavelength) and parameters of environment (pressure of ambient gas, etc.). Examples of formed nano–architectures include penguin–like structures of black Si with enhanced light absorption in the visible, semiconductor (Si, Ge, ZnO, etc.) quantum dot nanostructures with UV/visible fluorescence, as well as periodic plasmonic nanoarrays. Exhibiting a series of unique properties, these structures are of importance for photovoltaics, optoelectronics and biological sensing applications.

Study on laser-induced periodic structures and photovoltaic application

We have irradiated silicon with a series of femtosecond laser pulses to improve light absorption at the silicon surface. The laser treated surface namely black silicon shows excellent optical properties on mono and multicrystalline silicon wafers with a reflectivity reduction down to 3%, without crystal orientation dependence. After the laser process, the front side of samples has been boron‐implanted by Plasma Immersion Ion Implantation to create the 3D p+ junction. Improved electrical performances have also been demonstrated with a 57% increase in the photocurrent, compared to non‐texturized surface.

Laser Textured Black Silicon Solar Cells with Improved Efficiencies

Femtosecond laser irradiation of silicon has been used for improving light absorption at its surface. In this work we demonstrate the successful implementation of femtosecond laser texturisation to enhance light absorption at Si solar cell surface. In order to adapt this technology into solar industry, the texturisation process is carried out in air ambient. The microstructure similar to what has been produced in vacuum can be made in air by using appropriate laser conditions. The texturised surface shows excellent optical properties with a reflectivity down to 7% without crystalline orientation dependence. Junction formation and metallisation proceeded after texturisation. Suns-Voc measurements are performed to evaluate the cell performance and decent electrical characteristics have been achieved.

Laser doping for microelectronics and microtechnology

The future CMOS generations for microelectronics will require advanced doping techniques capable to realize ultra-shallow, highly-doped junctions with abrupt profiles. Recent experiments have shown the potential capabilities of laser processing of Ultra Shallow Junctions (USJ). According to the International Technology Roadmap for Semiconductors, two laser processes are able to reach ultimate predictions: laser thermal processing or annealing (LTP or LTA) and Gas Immersion Laser Doping (GILD). Both processes are based on rapid melting/solidification of the substrate. During solidification, the liquid silicon, which contains the dopants, is formed epitaxially from the underlying crystalline silicon. In the case of laser thermal annealing dopants are implanted before laser processing. GILD skips the ion-implantation step: in this case dopants are chemisorbed on the Si surface before the laser shot. The dopants are then incorporated and activated during the laser process. Activation is limited to the liquid layer and this chemisorption/laser shot cycle can be repeated until the desired concentration is reached. In this paper, we investigate the possibilities and limitations of the GILD technique for two different substrates: silicon bulk and SOI. We also show some laser doping applications for the fabrication of micro and nanoresonators, widely used in the MEMS Industry.

Laser interaction with materials: introduction.

Laser-materials interaction is the fascinating nexus where laser physics, optical physics, and materials science intersect. Applications include microdeposition via laser-induced forward transfer of thin films, clean materials processing with femtosecond beams, creating color filters with nanoparticles, generating very high density storage sites on subpicosecond time scales, structuring solar cell surfaces for higher efficiency, making nanostructures that would be impossible by other means, and creating in-volume waveguiding structures using femtosecond laser filaments.

Edge Isolation Using Ultra-Short Pulse Laser Materials with a Top-Hat Beam Profile

Laser grooving is an existing industrial solution for solar cell junction isolation. However there is still plenty room to improve this process. Potential approach includes choosing proper laser wavelength, tuning laser pulse width and laser focus beam profile, etc. We have recently investigated laser edge isolation of crystalline silicon solar cells using an ultra-short pulse laser. In this study we carried out isolation test using the same laser with a top-hat beam profile. A comparative study between isolation using top-hat and Gaussian is launched. The geometry of laser scribed grooves and the electrical performance of the cells are characterised. More homogenous ablation and material removal are achieved using top-hat hence the dopants from the isolation groove area are eliminated efficiently. The results from I-V characterisation confirm that more efficient isolation process and better isolation quality can be achieved using top-hat.

Optimization of picosecond laser processing for microscopy sample preparation prior to Ion milling polishing

Microchips are more and more complex and designed in thick 3 dimensional packages. In order to access and characterize by electron microscopy (SEM and TEM) any detected defect responsible for malfunction of the device, a large quantity of matter needs to be removed without damaging the surrounded area. The available techniques, such as plasma Focused Ion Beam (FIB), allow achieving high quality surfaces but are limited by their low matter removal rate (∼104 μm3/s). In order to accelerate the process, other techniques such as laser micromachining of the sample prior to FIB polishing are envisioned [1, 2]. In this work, picosecond laser micromachining has been investigated at 3 wavelengths (343, 515 and 1030 nm) in silicon and tested in integrated circuits. In order to minimize the FIB polishing time, the laser induced damaged zone should be the least extended and the micromachined sidewalls should be as vertical and smooth as possible. It is shown that by using sufficiently high fluences and number of pulses, almost vertical and smooth sidewalls can be obtained (see Fig. la) [3]. Moreover, according to the TEM images, sidewalls are only covered by a few hundred nm thick debris layer with limited heat affected zones. These results, coupled with the high matter removal rate (∼106 μm3/s) demonstrate that picosecond laser machining fulfills the requirements for sample preparation. The processing method was successfully tested on microchips as shown by SEM imaging which reveals clean exposed interfaces of underlayers (see Fig. 1b). In addition, using a simple model allowing a better understanding of laser absorption in silicon for picosecond pulses, the conditions (wavelength and fluence) to achieve optimal matter removal rate and ablation efficiency were identified and validated by experimental results.

Laser interaction with materials: introduction

Laser-materials interaction is the fascinating nexus where laser physics, optical physics, and materials science intersect. Applications include microdeposition via laser-induced forward transfer of thin films, clean materials processing with femtosecond beams, creating color filters with nanoparticles, generating very high density storage sites on subpicosecond time scales, structuring solar cell surfaces for higher efficiency, making nanostructures that would be impossible by other means, and creating in-volume waveguiding structures using femtosecond laser filaments.