17 Laser Ablation Technique

S.S. islam

epgp books

 

 

 

Laser Ablation Technique

 

Laser ablation is an established and versatile tool in modern manufacturing. At the same time, the basics and applications of laser ablation are the subject of numerous current research studies. In laser ablation, material is removed from a solid (or a liquid) surface by bombarding laser beam onto it. At low values of laser flux, the material gets heated which causes its evaporation or sublimation. However, high laser flux converts the material into a plasma. Generally, laser ablation involves the removal of materials by using a pulse wave laser; but continuous wave laser beams can also be employed if its intensity is high enough to ablate the material. In this module, the participants will learn the laser ablation technique towards the synthesis of thin films.

 

Since its discovery, lasers are extensively utilized in numerous applications, e.g., in laser ablation technique. Although the earliest experimental studies of laser ablation were demonstrated around 1963, the technique itself could not be used in gas sensing till the mid 1990s.

 

In laser ablation, material is removed from the surface of a sample by laser irradiation. The technique is termed as ‘laser ablation’ in order to highlight the nonequilibrium vapor or plasma conditions developed by the intense laser pulses at the sample surface. It is different from laser evaporation wherein the material is heated and evaporated while maintaining thermal equilibrium. Figure 1 shows the general schematic of the setup used in laser ablation experiments. A laser ablation device typically consists of two parts: (a) pulsed laser, and (b) ablation chamber. The laser can be either a CO2 laser, Nd-YAG laser, ArF excimer laser, or XeCl excimer laser. The irradiated laser beam is absorbed on the sample surface, thereby rapidly increasing its temperature. This results in vaporizing the target material into the laser plume. This vaporized material may condense to form particles or clusters. Noticeably, there is no chemical reaction involved in formation of these clusters. Additionally, sometimes, the vaporized material may also react with introduced reactants (e.g., reactive gases), to produce novel materials. The condensed particles can be deposited onto a substrate or collected using a filtering mechanism. These collected particles can be coated over a substrate via drop-casting, spin coating, etc.

 

It is a similar to gas phase condensation technique with the exception that more confined plume of material is vaporized instead of the entire sample to produce vapours. Usually, a high-power laser is incident on a small spot of the material. The laser can be exposed either continuously or in short intense pulses. The laser power is high enough to rapidly raise the temperature of a small spot of material such that the material is vaporized. Due to small amount of the material used to prepare the vapours, this technique can only be used to synthesize small amounts of nanomaterials. This technique has been excessively employed to synthesize metal-oxide nanoparticles.

 

 

Figure 1 Schematics of a typical setup used for laser ablation technique [3].

 

The absorption depth of the laser energy and the resulting removal of material depend upon the optical properties of the material being ablated. Other parameters affecting this process include the duration of pulse irradiation and the wavelength of the laser used. Total mass removed from the material by one laser pulse is termed as ablation rate. Ablation process is also affected by the velocity of the scanning laser beam and covering of scanning lines.

 

The width of laser pulse can vary in a broad range of duration, e.g., from milliseconds to femtoseconds. Besides, the flux of the laser pulse can also be precisely controlled. Thus, laser ablation can be widely used in research as well as industrial applications.

 

Laser ablation is also a very old technique to prepare carbon nanomaterials like fullerenes and carbon nanotubes. To produce carbon nanomaterials, carbon is vaporized and deposited at different places. A high intensity laser is utilized to vaporize the carbon. Some of this carbon is transmitted into plasma and carbon nanotubes (CNTs) are synthesized in the plasma plume along with other allotropes of carbon. This plasma plume is deposited on chamber walls. Both types of CNTs – single-walled as well as multi-walled – can be produced using this technique. When pure carbon source is used, multi-walled CNTs are formed. And a mixture of multi- and single-walled CNTs is obtained when metal catalyst is used along with the carbon source.

 

Advantages

 

In this technique, a selected area of a solid material is focused by a laser beam for removing a single or multiple layers from the material. The recent advancements in fiber laser technology have facilitated the application of laser ablation with a very high level of accuracy, precision and efficiency, resulting in numerous advantages for both the process and the industries that it is applied to. We explore below these benefits of laser ablation.

 

Since this technique does not involve the use of solvents, it is considered to be environmentally benign and does not expose users to harmful chemicals (upon vaporization). It can be automated easily. The running cost is relatively lower than other conventional techniques, but instrumentation and installation are expensive. It is comparatively gentle than abrasive processes, for instance, carbon fibers present inside the composite material do not get damaged. The target material is not excessively heated. The following sections discuss the advantages of laser ablation in detail:

 

a.       Laser ablation can be performed using both a pulsed wave and continuous wave laser. Usually a high intensity is required in order to remove the material, thus pulsed wave laser is the most popular method

 

b.       Low heat transfer: Despite this high level of intensity, very little heat energy is transferred to the area surrounding the spot at which laser ablation is being performed. This has benefit of avoiding damage to the rest of the material. Since the laser is focused on a small spot, the amount of removed material can be easily controlled.

 

c.       Cost effective: It is an economic technique to remove the desired material. For instance, to remove thin films, conventional approaches involve multi-step processing. A pattern of the spot to be removed is created. Then the selected area is removed by using chemicals to etch it out of the material. Whereas, laser ablation is a fast, efficient, and economic technique. Thus, it is the most preferred technique in most cases for thin film removal.

 

d.       Environmentally benign: As well as its cost-effective and low energy waste benefits, laser ablation is a safer and more environmentally-friendly approach as it uses no solvents, it is relatively easy to automate with robots and is much gentler than using more abrasive techniques such as dry-ice blasting. This results in it being the ideal choice for a wide range of materials, including metals, ceramics, and plastics.

 

Applications

 

Laser ablation is primarily used for controlled removal of materials from a solid surface. For example, laser machining and laser drilling; where pulsed lasers are employed to drill very small, deep holes in hard materials. Short laser pulses ablate the exposed material so quickly that very little heat is absorbed by the surrounding material. Therefore, delicate and heat-sensitive materials (such as tooth enamel) can be treated with laser drilling.

 

Additionally, metallic coatings selectively absorb laser energy. Therefore, pulsed lasers are often utilized for cleaning surfaces, removing paints or coatings, or preparing surface for painting without causing damage to the underlying surface. A large area can be cleaned by a single pulse of high power laser, whereas low power laser exploit multiple small pulses to be scanned across the selected area.

 

Desired patterns can be created by selectively removing the coasting from dichroic filters via laser ablation. These patterns are employed in stage lighting for creating high dimensional projections. This technique can be used to remove rusting from iron objects as well.

 

Additionally, the materials removed by laser ablation, can be used to produce novel materials, which may not be possible via other techniques. One such example is synthesizing carbon nanotubes (CNTs) from the ablated soot. Guo and coworkers were the earliest to use laser for ablating pure graphite. This graphite was then mixed with catalysts (e.g., cobalt, platinum, copper, or their combinations, etc.). This mixture was then kept in an oven and a laser was focused on it. An inert gas (i.e. Ar) was pumped inside the chamber in the direction of laser beam. The temperature of the oven was maintained at ~1200°C. CNTs were formed from the ablated material from this block and were collected at the copper collector by the gas flow. Another variant of this application is using laser ablation for creating coatings by ablating the coating material from a source and depositing it onto the substrate surface. This is a type of physical vapor deposition and is described as pulsed laser deposition (PLD), and is used to produce coatings from materials that are not readily evaporated by other means. This technique is often employed for manufacturing elevated temperature superconductors and laser crystals.

 

Remote laser spectroscopy estimates the surface composition by analyzing the composition of ablated material from the surface. It is done by examining the wavelength of the light emitted by the plasma.

 

Applications requiring momentum transfer also use laser ablation. When the ablated materials expand, they apply a high pressure pulse on the underlying surface. The impact is analogous to striking the surface with hammer. This forms the basis of the damage mechanism used in laser weapons. This is also used to harden metal surfaces in industries. Furthermore, it also provides pulsed laser propulsion used in spacecrafts.

 

Inductively coupled plasma mass spectrometry uses laser ablation to collect samples for forensic and biological purposes. Applications in biology include destroying the nerves and other tissues to investigate their functions, e.g., sensory neurons of pond snail (Helisoma trivolvis) can be ablated off while the snail is still in embryonic stage in order to prevent use of these nerves. Likewise, it can also be employed to destroy individual cells during embryogenesis of an organism to examine the effects of missing cells on the development of the organism.

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    References

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    Review Questions

  1. List the potential uses of laser ablation in biology.
  2. How can laser ablation be used for liquids?
  3. What are the application areas of laser ablation technique?
  4. What are the differences between pulsed laser beam and continuous laser beam synthesis techniques for nanoparticle synthesis?