Jong H. Yoo

Explosive change in crater properties during high power nanosecond laser ablation of silicon

Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreemen...

Laser–plasma interactions in fused silica cavities

The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a cavity with aspect ratio of 4 increased faster with irradiance after the laser irradiance reached a threshold of 5 GW/cm2. The threshold irradiance of particulate ejection was lower for laser ablation in a cavity compared with on a flat surface; the greater the cavity aspect ratio, the lower the threshold irradiance. The ionization of silicon becomes saturated and the crater depths were increased approximately by 1 order of magnitude after the irradiance reached the threshold. Phase explosion was discussed to explain the large change of both plasma characteristics and mass removal when irradiance increased beyond a threshold value. Self-focusing of the laser be...

Laser-induced breakdown spectroscopy in industrial and security applications

Laser-induced breakdown spectroscopy (LIBS) offers rapid, localized chemical analysis of solid or liquid materials with high spatial resolution in lateral and depth profiling, without the need for sample preparation. Principal component analysis and partial least squares algorithms were applied to identify a variety of complex organic and inorganic samples. This work illustrates how LIBS analyzers can answer a multitude of real-world needs for rapid analysis, such as determination of lead in paint and childrens toys, analysis of electronic and solder materials, quality control of fiberglass panels, discrimination of coffee beans from different vendors, and identification of generic versus brand-name drugs. Lateral and depth profiling was performed on childrens toys and paint layers. Traditional one-element calibration or multivariate chemometric procedures were applied for elemental quantification, from single laser shot determination of metal traces at ~10 μg/g to determination of halogens at 90 μg/g using 50-shot spectral accumulation. The effectiveness of LIBS for security applications was demonstrated in the field by testing the 50-m standoff LIBS rasterizing detector.

Analysis of the absorption layer of CIGS solar cell by laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) was applied for the elemental analysis of the thin copper indium gallium diselenide (CuIn(1-x)Ga(x)Se(2) [CIGS]) absorption layer deposited on Mo-coated soda-lime glass by the co-evaporation technique. The optimal laser and detection parameters for LIBS measurement of the CIGS absorption layer (1.23 μm) were investigated. The calibration results of Ga/In ratio with respect to the concentration ratios measured by x-ray fluorescence and inductively coupled plasma optical emission spectroscopy showed good linearity.

Fluorine analysis using Laser Induced Breakdown Spectroscopy (LIBS)

Laser induced breakdown spectroscopy (LIBS) is evaluated for detecting fluorine using a commercially available J200 Tandem LA/LIBS system from Applied Spectra, Inc. The 685.6 nm fluorine atomic emission line was used to detect and determine the best calibration method for quantifying the amount of fluorine in a set of prepared NIST SRM 120c phosphate rock standards. The multivariate calibration model, based on partial least squares regression (PLS), provided the best accuracy and precision for the sample set analyzed in this study. The detection limit of fluorine using the J200 LIBS system with a 213 nm laser and an ICCD was determined to be 135 ppm from a phosphate rock standard. A rare earth element (REE)-rich mineral high in fluorine content was used to access the potential applications of mapping the fluorine content over a 16 mm2 surface area of the sample.

Thermionic electron emission from narrow band-gap semiconductors under picosecond laser excitation

A model is presented to relate picosecond laser induced thermionic electron emission to the carrier and phonon dynamics of semiconductors. Silicon is chosen as a model material since its optical and thermal properties are well characterized. The temporal profiles of thermionic emission current and total electron yield are obtained as a function of incident laser fluence for different surface conditions, below the silicon surface melting threshold. Two distinct regimes have been found for the dependence of the electron yield on the laser fluence due to thermionic emission and this behavior is related to Auger recombination.

Laser ablation plasmas for diagnostics of structured electronic and optical materials during or after laser processing

Laser induced plasma can be used for rapid optical diagnostics of electronic, optical, electro-optical, electromechanical and other structures. Plasma monitoring and diagnostics can be realized during laser processing in real time by means of measuring optical emission that originates from the pulsed laser-material interaction. In post-process applications, e.g., quality assurance and quality control, surface raster scanning and depth profiling can be realized with high spatial resolution (~10 nm in depth and ~3 μm lateral). Commercial instruments based on laser induced breakdown spectrometry (LIBS) are available for these purposes. Since only a laser beam comes in direct contact with the sample, such diagnostics are sterile and non-disruptive, and can be performed at a distance, e.g. through a window. The technique enables rapid micro-localized chemical analysis without a need for sample preparation, dissolution or evacuation of samples, thus it is particularly beneficial in fabrication of thin films and structures, such as electronic, photovoltaic and electro-optical devices or circuits of devices. Spectrum acquisition from a single laser shot provides detection limits for metal traces of ~10 μg/g, which can be further improved by accumulating signal from multiple laser pulses. LIBS detection limit for Br in polyethylene is 90 μg/g using 50-shot spectral accumulation (halogen detection is a requirement for semiconductor package materials). Three to four orders of magnitude lower detection limits can be obtained with a femtosecond laser ablation - inductively coupled plasma mass spectrometer (LA-ICP-MS), which is also provided on commercial basis. Laser repetition rate is currently up to 20 Hz in LIBS instruments and up to 100 kHz in LA-ICP-MS.