Ablation Processes

Pulsed laser deposition (PLD) is a laser-based technique used to grow high-quality thin films of complex materials on substrates like Silicon wafers. The material to be deposited (target) is vaporized by short and intense laser pulses and forms a plasma plume. Then, the vaporized target material from the plasma bombards the substrate and – under the right conditions – creates a thin homogenous layer on this substrate. For each laser shot, a layer of only a few nanometers of material is ablated to form the plasma plume in a process that typically last a few tens of picoseconds. To enable this process, nanosecond pulses with energies of tens to hundreds of mJ are necessary and UV wavelengths are usually preferred. These requirements match very well the performances of excimer lasers.

The first laser deposition experiments took place in the mid to late 1960s, but PLD gained tremendous interest after T. Venketesan in 1987 first applied this method to create high temperature superconductive (HTSC) films. Since then, many hundreds of lasers have been sold to drive research, process development and small-scale production of thin film devices, such as superconductive magnetic sensors (SQUIDs), thin film ferroelectrics and “high k” gate resistors, semiconductor alloys, carbon nano-tubes, and more.

High pulse energy lasers provide several benefits. First, they extend the range of usable target materials. Second, they enable to ablate a larger area of the sample with the desired fluence. This increases the deposition rate and reduces the plume angle, resulting in higher deposition efficiency. Third, higher pulse energy lasers provide a larger process window, allowing a more consistent process.

Coherent’s COMPex and LEAP laser series are designed for demanding high-pulse energy applications and are highly effective tools for pulsed laser deposition. They deliver the beam stability and energy stability performance required to achieve superior results in sophisticated thin film deposition experiments.

Laser Ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted into a plasma. The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material’s optical properties and the laser wavelength.​

Laser ablation is used typically for three main applications: laser pulse deposition, material processing and ablation of material for analytical purposes.

The first two applications are discussed elsewhere while the third one is discussed here, namely Laser Ablation inductively-coupled plasma mass spectrometry or LA-ICP-MS. This technique is used to analyze samples to determine their elemental composition and finds applications in geological and nuclear physics research, quality control of materials and pharmaceutical samples. Specific application examples include analysis of fluid inclusions in minerals, age determination of samples by isotope ratio analysis and analysis of high purity semiconductor materials. ​

In brief, the sample is placed in a vacuum chamber and ablated preferably with a nanosecond UV pulse of light. The ablated material is fragmented in elemental components that are fed into a mass spectrometer. In general, energetic UV pulses at 193 nm can ablate efficiently most materials and minimize the so-called fractionation or inhomogeneity of the individual particles fed into the spectrometer, leading to more precise and accurate measurements.​

Coherent 193 nm solid-sampling-system GeoLasHD is a self-contained laser ablation system for sample introduction in high-resolution LA-ICP MS (laser ablation inductively-coupled plasma mass spectrometry). GeoLasHD integrates the COMPex 193 nm ablation laser including beam homogenizing and shaping optics, a sample chamber, and unmatched microscopic sample observation capability. The sampling speed can be varied in a wide range from 1 Hz up to 100 Hz.​

​The use of IR and UV femtosecond pulses pushes LA-IC-MS to its ultimate frontier as the short pulses compared with nanosecond pulses further reduce the level of fractionation of the ablated material resulting in even more accurate measurements. Coherent Astrella and Legend Elite – possibly with their harmonic generators - are integrated into research and commercial ablation systems

​Laser ablation and its applications have been progressing since the initial appearance of pulsed lasers in the 1960s. While the initial effort was towards optimize processes with increasing pulse energies, as laser technology improved, research on material processing started focusing on shorter wavelengths and successively shorter pulses and tailored succession of pulses, either temporally or in energy. With adequately short and energetic pulses, so-called “cold” ablation, where only a minute part of the incident laser energy is waster as heat deposited in the target, became feasible.

There are several areas of development in ultrafast laser processing research. The first and most widely pursued topic involves the inherent precision of the ablation process enabled by sub-picosecond pulses. Such short pulses result in a lower ablation threshold than for longer pulses. Second and more importantly there is a drastic reduction in the energy flowing as heat towards the surrounding material. This happens because the short pulses have a duration that is comparable with the dynamics of the electrons in the target material; therefore, the atoms composing the material undergo a direct phase transition from solid to plasma or vapor, which is rapidly removed without a direct negative effect (HAZ or heat affected zone) on the adjacent material. The thickness of the ablated layer is typically only a few tens or hundreds of nanometers, allowing excellent depth control through the metered delivery of a pulse sequence. This fine control of ablation depth and minimal HAZ makes it straightforward to generate features such as precisely shaped holes in a wide range of engineering materials such as virtually any metal or ceramic and many polymers.​

The second area of unique application for femtosecond pulses concerns the multiple-photon interaction with the material, i.e. the simultaneous interaction of two or more photons with the same atom or molecule. Like in multiphoton excitation microscopy, the multiphoton effect makes the material “experience” the laser pulses as if they had a much shorter wavelength than their actual one. Since most material that are transparent to IR or visible light absorbs UV radiation, transparent materials like glass or sapphire can be easily ablated with femtosecond pulses. Since these materials tends also to be brittle, femtosecond pulses - with their minimal conductive heat transfer - are the ideal tool.​

​Finally, femtosecond pulses can produce direct surface modification or “micro-texturing”. For example, by controlling the laser polarization state and orientation with respect to the target, it is possible to generate laser-induced periodic structures (LIPS). These topologies show promise for controlling the hydrophobic/hydrophilic response of these laser-modified surfaces when in contact with liquids, enabling the possibility of “self-cleaning” or “super-wetting” surfaces. In addition, ultrashort lasers can be used to form oxide layers of controlled thickness, altering the color of the surface and enabling high-quality durable marking of the surface.

​The Coherent Monaco family is ideal for studies on femtosecond ablation because in addition to the availability of IR (1035 nm), green ((517nm) or UV (345 nm) wavelengths for optimum processing for different materials, it offers ultimate flexibility in repetition rates, pulse sequences, energy and pulse duration control. In addition, compact size and HALT/HASS protocols result in easy integration and 24/7 industrial reliability.