Laser ablation

The term laser ablation is widely used in the scientific literature in areas such as the production of thin films, laser sampling and processing of materials. In the physical literature, the term "ablation" (from the Latin. " Ablatio " - withdrawal) refers to a combination of complex physical and chemical processes, the result of which is the removal of a substance from the interface. According to the meaning of the Latin root, this term can be used to describe any removal of a substance. In this regard, the term "laser ablation" in a broad sense refers to the process of removal of a substance under the action of laser radiation, including both the removal of vaporized material and the volatile products of chemical etching.

One can also find in the literature an overly narrow interpretation of the term, when ablation is understood as the process of removal of a substance caused by the destruction of chemical bonds and the formation of free molecules, atoms and ions under the influence of light. It should be noted that the term is interdisciplinary, and appeared in the literature long before the advent of lasers. So it was used to describe the process of removing a substance when a metal sample is exposed to an electric discharge, a stream of hot gas, and plasma. The term "ablation protection" in astronautics and aviation is understood as a way to effectively reduce the overheating of the fuselage elements, due to the selection of heat for melting and evaporation of a layer of special protective material. In addition, it should be noted that this term is used in geology and glaciology to mean a decrease in the mass of a glacier or snow due to melting and evaporation.

Most researchers use the term laser ablation to mean the process of interaction of laser radiation with a substance, in which the process of melting, evaporation, or immediately sublimation occurs with the formation of vapors and low-temperature plasma, usually these processes are also accompanied by the expansion of particles and droplets of the initial substance.

The main features of laser ablation are as follows:

1. laser ablation is associated with direct absorption of laser pulse energy in matter;
2. laser ablation results in the formation of a plasma cloud;
3. laser ablation occurs at the interface between condensed and gaseous (or vacuum) or liquid phases;
4. laser ablation has a threshold character.

Laser ablation is used in various fields:

• substance analysis sample (LIBS, LA ISP OES, LA ICP MS)
• part processing (micromachining)
• thin films, including new materials (PLD)

Laser vapor deposition (PLA or PLD - pulsed laser deposition) is the process of rapid melting and evaporation of a target material as a result of exposure to high-energy laser radiation, followed by transfer of the sprayed material from the target to the substrate in vacuum and its deposition.

The advantages of the method include:

• high deposition rate (> 10 ¹⁵ atom · cm ⁻² · s ⁻¹ );
• rapid heating and cooling of the deposited material (up to 10 ¹⁰ K s ^–1 ), ensuring the formation of metastable phases;
• direct connection of the energy parameters of radiation with the kinetics of layer growth;
• the possibility of congruent evaporation of multicomponent targets;
• strict dosage of the material supply, including multi-component with a high evaporation temperature;
• aggregation into clusters of different sizes, charge, and kinetic energy (10 - 500 eV), which allows selection using an electric field to obtain a certain structure of the deposited film.

A detailed description of the LA mechanism is very complex, the mechanism itself includes the ablation of the target material with laser irradiation, the development of a plasma torch containing high-energy ions and electrons, as well as the crystal growth of the coating itself on the substrate. The aircraft process as a whole can be divided into four stages:

1. the interaction of laser radiation with the target - ablation of the target material and the creation of plasma;
2. plasma dynamics - its expansion;
3. applying material to a substrate;
4. film growth on the surface of the substrate.

Each of these stages is crucial for the physicomechanical and chemical parameters of the coating, and, consequently, the biomedical performance characteristics.

The removal of atoms from the bulk of the material is carried out by evaporation of the mass of the substance to the surface. The initial emission of electrons and coating ions occurs, the evaporation process by its nature is most often thermal. The penetration depth of laser radiation at this moment depends on the wavelength of the laser radiation and the refractive index of the target material, as well as the porosity and morphology of the target.

The first studies of laser ablation have been carried out since the advent of lasers in 1962 in ^[1] . Most of the works in the 1960s used laser pulses of microsecond duration. A thermal model was created for this type, which described the observed phenomena with high accuracy ^[2] . The development of laser technology has led to the fact that, in the early 80s, most of the work on laser ablation was performed using laser pulses of the nanosecond range. In the next decade, studies of picosecond laser ablation became increasingly important. In the past 20 years, the use of lasers with a femtosecond pulse duration has been widely developed ^[3]

At the second stage, the plasma of the material expands parallel to the normal to the target surface to the substrate due to Coulomb repulsion. The spatial distribution of the plasma torch depends on the pressure inside the chamber. The dependence of the shape of the torch on time can be described in two stages:

1. The plasma jet is narrow and directed forward from the normal to the surface (the duration of the process is several tens of picoseconds), scattering practically does not occur, stoichiometry is not violated.
2. Expansion of the plasma torch (the duration of the process is several tens of nanoseconds). The stoichiometry of the film may depend on the further distribution of the ablation material in the plasma torch.

The plume density can be described as the dependence cosn (x) close to the Gaussian curve. In addition to the sharply directed peak distribution, a second distribution is observed, which is described by the dependence cosΘ [43, 46]. These angular distributions clearly indicate that material entrainment is a combination of various mechanisms. The plasma expansion angle does not depend directly on the power density and is characterized mainly by the average ion charge in the plasma stream. An increase in the laser flow gives a higher degree of plasma ionization, a sharper plasma flow with a smaller expansion angle. For a plasma with charge ions Z = 1 - 2, the expansion angle is Θ = 24 ÷ 29 °. Neutral atoms are mainly deposited at the edge of the film spot, while ions with high kinetic energy are deposited in the center. In order to obtain uniform films, the edge of the plasma stream must be shielded. In addition to the angular dependence of the deposition rate, certain variations are observed in the stoichiometric composition of the vaporized material depending on the angle Θ during the deposition of multicomponent films. The oversized peak distribution preserves the stoichiometry of the target, while the wide distribution is non-stoichiometric. As a result, during laser deposition of multicomponent films, stoichiometric and non-stoichiometric components always exist in the plasma flow, depending on the angle of deposition.

Also, the dynamics of plasma expansion depends on the density of the target and its porosity.

For targets of the same material, but of different density and porosity, the time intervals for plasma expansion are different.

It is shown that the ablation rate along the propagation of laser radiation in a porous substance is (1.5-2) times higher than the theoretical and experimental results for the ablation rate in a solid, describe the mode and material.

We can distinguish the main important technological parameters of the aircraft that affect the growth and physicomechanical and chemical properties of the films when applying the material to the substrate:

• laser parameters - factors on which the energy density (J / cm2) mainly depends. The energy and speed of ablation particles depends on the laser energy density. The degree of ionization of the ablation material and the stoichiometry of the film, as well as the rate of deposition and growth of the film, in turn, depend on this.
• surface temperature - the surface temperature has a great influence on the density of nucleation (the first stage of the phase transition, the formation of the main number of steadily growing particles of a new, stable phase). As a rule, the nucleation density decreases with increasing substrate temperature. The roughness of the coating may also depend on the temperature of the substrate.
• condition of the surface of the substrate — the nucleation and growth of the coating depends on the state of the surface: pre-treatment (chemical treatment, the presence or absence of an oxide film, etc.), the morphology and surface roughness, and the presence of defects.
• pressure - the density of nucleation depends on the working pressure in the chamber of the spraying system, and as a result, the morphology and roughness of the coating, as well as pressure parameters, affect the stoichiometry of the surface. It is also possible to redistribute the material from the substrate back into the chamber at certain laser and pressure parameters.

At present, three mechanisms of film growth are described that are suitable for ion-plasma vacuum methods:

• The germinal mechanism of Volmer-Weber growth : realized on atomically smooth faces of a perfect crystal, which are the faces with small Miller indices. In this case, the growth of films occurs through the initial formation of two-dimensional or three-dimensional nuclei, which subsequently grow into a continuous film on the surface of the substrate.
• Layer-by-layer Frank – Van der Merwe growth mechanism : realized when steps are present on the substrate surface, the source of which is, in particular, the natural roughness of faces with large Miller indices. These faces are represented as a set of atomic steps formed by areas of close-packed porosities with small Miller indices.
• Stransky-Krastanov mechanism : is an intermediate growth mechanism. It consists in the fact that at first growth occurs on the surface by a layer-by-layer mechanism, then after the formation of a wetting layer (one or more monoatomic layers thick), a transition to the island growth mechanism occurs. A condition for the implementation of such a mechanism is a significant (several percent) mismatch of the lattice constants of the deposited material and the substrate material.

The laser ablation method has certain difficulties associated with obtaining films of substances that are weakly absorbing (oxides of various substances) or reflecting (a number of metals) laser radiation in the visible and near infrared spectral range. A significant drawback of the method is the low utilization of the target material, since its intensive evaporation occurs from a narrow erosion zone, determined by the size of the focal spot (~ 10-2 cm2), and, as a result, a small deposition area (~ 10 cm2). The value of the coefficient of beneficial use of the target material during laser sputtering is 1 - 2% or less. The formation of a crater in the erosion zone and its deepening changes the spatial angle of expansion of the substance, as a result of which the uniformity of the films both in thickness and composition deteriorates, and also destroys the target, which is especially typical for high-frequency sputtering (pulse repetition rate of about 10 kHz) . Increasing the uniformity of the films and increasing the life of the target requires the use of a high-speed system (~ 1 m / s) of plane-parallel scanning of the target, which avoids overlapping adjacent focal spots and, as a result, local overheating of the target and the formation of deep craters on it, which, however, significantly complicates the design intracameral device and the spraying process itself.

• Spectroscopy
• Spectroscopic methods
• Quantitative analysis
• Emission spectrum

• Laser ablation and its application - an exhibition prepared by the Department of SPSL SB RAS
• The role of laser ablation in the physics of selective evaporation of components
• Laser ablation, spark exposure
• Nanostructure generation during laser ablation of metals in liquids: new results (inaccessible link)