Temperature dependent ablation threshold in silicon using ultrashort laser pulses

Abstract

We have experimentally investigated the ablation threshold in silicon as a function of temperature when applying ultrashort laser pulses at three wavelengths. By varying the temperature of a silicon substrate from room temperature to 320 C, we observe that the ablation threshold for a 3 ps pulse using a wavelength of 1030 nm drops from 0.43 J/cm 2 to 0.24 J/cm 2 , a reduction of 43%. For a wavelength of 515 nm, the ablation threshold drops from 0.22 J/cm 2 to 0.15 J/cm 2 , a reduction of 35%. The observed ablation threshold for pulses at 343 nm remains constant with temperature, at 0.10 J/cm 2 . These results indicate that substrate heating is a useful technique for lowering the ablation threshold in industrial silicon processing using ultrashort laser pulses in the IR or visible wavelength range. In order to investigate and explain the observed trends, we apply the two-temperature model, a thermodynamic model for investigation of the interaction between silicon and ultrashort laser pulses. Applying the two-temperature model implies thermal equilibrium between optical and acoustic phonons. On the time scales encountered herein, this need not be the case. However, as discussed in the article, the two-temperature model provides valuable insight into the physical processes governing the interaction between the laser light and the silicon. The simulations indicate that ablation occurs when the number density of excited electrons reaches the critical electron density, while the lattice remains well below vaporization temperature. The simulated laser fluence required to reach critical electron density is also found to be temperature dependent. The dominant contributor to increased electron density is, in the majority of the investigated cases, the linear absorption coefficient. Two-photon absorption and impact ionization also generate carriers, but to a lesser extent. As the linear absorption coefficient is temperature dependent, we find that the simulated reduction in ablation threshold with increased substrate temperature is linked to the temperature dependence of the linear absorption coefficient. Another factor influencing the ablation threshold is the wavelength dependence of the interaction with the excited electron plasma. This wavelength dependence can explain that we observe experimentally similar ablation thresholds for a wavelength of 1030 nm at 320 C and for 515 nm at room temperature, even though the linear absorption coefficient in the latter case is much higher.

Nanoscale patterning of graphene through femtosecond laser ablation

“…Hence, the damage threshold of Si at 343 nm is lower and is reported to be around 100 mJ/cm 2 . 18 Therefore, on samples such as ours, one might expect damage on Si taking place before (or at least together with) graphene. Note that since the SiO 2 layer is more transparent than Si, we can disregard its effect for these arguments.…”

Initiation time of near-infrared laser-induced slip on the surface of silicon wafers

“…Due to this thermal effect, laserinduced surface damage, such as slipping, cracking, melting, and ablation, occur. [6][7][8] Previous investigations of laser-induced damage by laser weapons have focused primarily on rockets, mortars, missiles, and aircraft made of metallic materials. These are all potential targets of laser weapons.…”

“…[12][13][14][15][16] The most common type of fatal, laser-induced thermal damage to silicon is slip damage since it is the first state of permanent damage and can easily result in fracture due to brittleness. 17,18 Even though many different types of laser-induced damages to silicon have been reported, 4,6,19,20 most studies have been related to ablation and fracture damages. Moreover, in the field of laser weapons, damage initiation time is very important, though this aspect has not been thoroughly investigated.…”

Thermal damages on the surface of a silicon wafer induced by a near-infrared laser

“…Even though prestigious achievements in laser-induced thermal damages in silicon crystals have been reported in the field of laser processing technology, most studies focused on the ablation effect induced by ultraviolet (UV) and visible (VIS) wavelength lasers, [13][14][15][16] and a few researchers have explained cracking and melting damage induced by an NIR laser. 17,18 In this paper, the thermal damages in silicon wafers, such as cracking and melting, induced by an NIR CW laser beam with a 1070-nm wavelength were investigated under various irradiances.…”