Laser removal of oxides and particles from copper surfaces for microelectronic fabrication

Bi-stability in femtosecond laser ablation by MHz bursts

In this work, a bi-stable behavior of laser ablation efficiency and quality was controlled by fluence and burst length. The plasma shielding of incoming laser radiation caused sudden jumps with a significant decrease in ablation efficiency for every even number of pulses in the burst. The attenuation of incoming laser radiation by plasma created by the previous pulse was incorporated into the toy model of burst ablation efficiency. The mathematical recurrence relation has been derived for the first time, binding ablation efficiency for the next pulse with the efficiency of the previous pulse, which predicts bi-stability, as well as sudden jumps occurring in ablation efficiency depending on the number of pulses in burst with the response to changes of the control parameter of peak laser fluence in the pulse. The modeling results using new recurrence relation showed stable and bi-stable ablation efficiency depending on burst fluence and the number of pulses, which agreed well with experimental data. The extremely efficient laser ablation has been achieved by optimizing the shielding effect using three pulses in the burst.

Laser cleaning and Raman analysis of the contamination on the optical window of a rubidium vapor cell

In this work, we present the laser cleaning of a Rubidium vapor cell and the Raman analysis of the contaminant material to be removed. The optical window of the vapor cell had gradually lost transparency due to the development of an opaque layer of unknown composition at the inner side during the normal operation of the cell. Laser cleaning was successfully performed by a frequency-doubled Nd:YAG laser focusing the beam inside the cell, avoiding any possible damage to the window. A single laser pulse was enough to clear away the black discoloration at the focal spot and locally restore the transparency of the window. The Raman spectra of the deposit showed peaks not yet described in the literature. Comparison with known Rubidium germanate spectra and simulation results strongly suggested that the unknown material was Rubidium silicate.

Portable wedge prism scanner for laser surface cleaning of corroded 304L stainless steel

A compact wedge prism scanner for laser surface cleaning is proposed, wherein the concept of system was studied based on geometric analysis. The final equations for the design express the transposition of the laser beam focal point and scanning radius. The results were verified through comparisons to both Zemax simulation and experiments. There was satisfactory agreement between the equations and Zemax simulation, but slight disagreement with the experiments. Additionally, two main factors of wedge prism scanner in commercial laser removal applications (circumferential overlap and spot overlap) was also discussed and the completely corrosion removal experiment indicated the potential use of our scanner.

Non-back-reflecting polygon scanner with applications in surface cleaning

Polygon mirror scanners are attracting considerable interest owing to their rapid speed and large scanning area. Here, we focused on the back-reflection effect of the polygon scanner. A new polygon scanner system was designed based on a geometric analysis. The final equations for the design express the position of the laser beam source having the largest scanning length without the reflected beam traveling back to the fiber. The proposed system performed a raster scan on an area. Additionally, a paint stripping experiment was conducted to demonstrate the potential use of our scanner in commercial laser cleaning applications.

Blind-zone formation in laser shockwave nano-cleaning

Laser shockwave cleaning (LSC) has attracted growing attention due to its advantages in non-contact, site-selective nanoparticle removal on microelectronic/optical devices. However, an uncleaned blind-zone formed directly under the laser-induced plasma kernel severely affects the cleaning effect. Laser shockwave cleaning of 300 nm polystyrene latex nanoparticles on silicon wafers is fully explored to understand the blind-zone formation mechanism. The size of the uncleaned blind-zone quickly increases from 0.84 to 19.50 mm² associated with a growing fraction of the uncleaned blind-zone area within the whole cleaned area from 0.05 to 0.93 as the plasma-substrate gap distance is increased from 0.5 to 2 mm and the laser fluence is increased from 75 to 150 J/cm². Besides, the variation of the blind-zone size is more strongly dependent on the plasma-substrate gap distance than the laser fluence. A time-resolved analysis of the laser-induced plasma evolution shows an inseparable relationship between the blind-zone and the geometric location of the plasma kernel. Theoretical analysis of the removal force in LSC based on the rolling mode reveals that the lack of dragging force acting on the nanoparticles in the region right under the plasma kernel impedes their removal and causes the uncleaned blind-zone formation.

Influence of parameters of a laser cleaning soil rust layer on the surface of ceramic artifacts

The laser cleaning effect can be affected by many parameters, including laser power, scanning speed, laser repetition rate, and pulse duration, and the cleaning effect with these parameters is investigated. It is found that the cleaning effect is negligible for an infrared high-repetition nanosecond-pulsed laser with repetition rate from 100 to 400 kHz and pulse duration from 58 to 240 ns and is significantly related to the laser power and scanning speed. Moreover, the cleaning thresholds related to laser power and scanning speed for a soil rust layer on ceramic artifacts are measured under the step distance of 281.3 μm. The heat conduction equation and thermal stress equation are used to simulate the cleaning effect with the influence of these parameters, which is in good agreement with the experimental data.

Enhancement of pulsed laser ablation in environmentally friendly liquid

Enhancement of pulsed laser ablation can be achieved in acetic acid as an environmentally friendly liquid. This paper evaluates microholes and textured features induced by a nanosecond pulsed laser under different processing circumstances. The microholes are fabricated by laser drilling in acetic acid and found to be 100% deeper than in air. The textured features achieved in the liquid demonstrate a higher content of Copper and a lower content of Oxygen. The improvement of laser ablation efficiency in the liquid is attributed to the strong confinement of plasma plume accompanying with shockwave and cavitation bubbles. Meanwhile, the laser enhanced chemical etching by the weak acid plays a critical role.