Laser Ablation Of Dielectrics Research Articles

Electron dynamics in fused silica after strong field laser excitation detected by spectroscopic imaging pump-probe ellipsometry

Laser ablation of dielectrics by ultrashort pulsed laser radiation is induced by nonlinear photoexcitation, such as multiphoton and tunneling processes, and the subsequent impact excitation, due to electron-electron collisions. Previous theoretical and experimental studies demonstrate that these two phenomena strongly depend on the transient optical properties of the dielectric and the electron-electron collision times. To estimate these transient optical properties and the collision time, the excited electrons are usually considered as a nearly free-electron gas, being described by the Drude theory. All previous experimental studies support using the Drude theory by time-resolved measurements of the transient reflectance during and after irradiation. However, a comprehensive characterization of the transient complex refractive index, and, thus, a more complex view of the optical properties upon irradiation above the ablation threshold fluence was still missing. Therefore, this paper presents the time-, spectral-, and fluence-resolved complex refractive index of fused silica during the interaction with ultrashort pulsed laser radiation (800 nm, 40 fs, and 4.4 J/${\mathrm{cm}}^{2}$) measured by spectroscopic imaging pump-probe ellipsometry. Comparing the state-of-the-art Drude model to the experimental data reveals that, although the Drude model replicates the transient reflectance of the material, it deviates significantly from the transient refractive index in the considered temporal range up to 200 fs after irradiation. Thus, the Drude model cannot correctly describe the transient optical properties, which leads to the assumption that the electrons cannot be considered as a nearly free-electron gas at this time. Alternatively, a Lorentz model, considering the excited electrons as bound, can replicate the measured data much better, which indicates that the electrons might be excited into localized states.

New Approach for the Ablation of Dielectrics from Silicon Using Long Wavelength Lasers

Laser ablation of dielectrics from silicon substrates represents a useful technique for e.g. the creation of local contacts. However, these dielectrics are transparent at the laser wavelengths normally employed for silicon solar cell processing, i.e. the first, second and third harmonics of solid state lasers (1064, 532 and 355nm). As a result of this, the ablation is indirect, and follows from energy deposition in the silicon rather than in the dielectric. This mechanism introduces defects in the silicon substrate, an effect which is detrimental to solar cell performance. Attempts have been made to limit the extent of the laser damage, by going to shorter wavelengths and shorter pulse durations.In this work, we suggest an alternative route to low-damage ablation of dielectrics by application of long wavelength laser pulses from e.g. CO2-lasers. At wavelengths above approx. 8μm, we find absorption bands in many of the dielectrics applied in solar cells. Simulations show that it may be possible to keep the silicon temperature below melting temperature, while reaching vaporization temperature in the dielectric. Experiments using laser pulses at 9.3μm with a duration of approx. 100ns show, however, that the silicon substrate experiences melting. We conclude that even shorter pulses must be applied for the method to be successful.

Laser ablation of SiO2 for locally contacted Si solar cells with ultra‐short pulses

AbstractWe apply ultra‐short pulse laser ablation to create local contact openings in thermally grown passivating SiO2 layers. This technique can be used for locally contacting oxide passivated Si solar cells. We use an industrially feasible laser with a pulse duration of τpulse ∼ 10 ps. The specific contact resistance that we reach with evaporated aluminium on a 100 Ω/sq and P‐diffused emitter is in the range of 0·3–1 mΩ cm2. Ultra‐short pulse laser ablation is sufficiently damage free to abandon wet chemical etching after ablation. We measure an emitter saturation current density of J0e = (6·2 ± 1·6) × 10−13 A/cm2 on the laser‐treated areas after a selective emitter diffusion with Rsheet ∼ 20 Ω/sq into the ablated area; a value that is as low as that of reference samples that have the SiO2 layer removed by HF‐etching. Thus, laser ablation of dielectrics with pulse durations of about 10 ps is well suited to fabricate high‐efficiency Si solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Laser ablation of dielectrics with temporally shaped femtosecond pulses

A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.

Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps

Laser-induced ablation has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. Barium aluminum borosilicate glass with an extremely high glass transformation temperature (∼600 °C) served as target material. The most significant observation was a substantial decrease of the ablation threshold fluence at pulse durations below 100 fs. All results indicate a dominant role of multiphoton absorption in addition to collisional ionization in this time domain.