Femtosecond Pulse Laser Induced Ablation in Thin Metal Films

The effect of laser light on matter is tightly related both to the laser parameters and to the sample optical properties. If the laser light intensity is sufficiently high, above a certain threshold that corresponds to the irradiated material, can take place.

 In recent decades laser ablation became a common method for many applications, such as cutting, patterning, marking and more.

  Although the response of many materials to optical radiation is a classical topic treated in textbooks, in ablation, processes are escorted by many complicated transfer mechanisms of heat and matter, which are not fully understood...

 In order to better understand these transfer mechanisms, we are using chirped pulse amplified Ti:Sapphire laser pulses with varied time durations (70 fs - 8 ns) and energies (10 µJ - 300 µJ) on different samples, especially metals (Cu, Ni, Co and more).

 The laser pulse is focused on the target to a small area (diameter of 20 µm – 40 µm). Once ablation is achieved, a plasma plume is expelled out of the focused area on the sample.

 Among other species, the plasma contains excited atoms and ions of the surface material, which emit isotropic light as they return to the energy ground state. This light carries the irradiated material spectral fingerprint. By "sending" the emitted light into a spectrophotometer, we can tell which material is on the surface.

 This method is called Laser Induced Breakdown Spectroscopy, or LIBS in short. the output of a LIBS measurement is basically the intensity of the collected emitted light vs. its wavelength. In the following cartoon, a copper sample is ablated and the LIBS signal is collected by a lens into the spectrophotometer. The data is recorded by the computer and presented on screen...

 In the figure below, the 800 nm laser light irradiates a copper sample, which generates strong green light resulting from 3 main spectral peaks: 511, 515 and 522 nm.

 For a simplified illustration of the interaction between a laser pulse and a sample, click here.

After the sample was ablated, it is taken to characterization by Atomic Force Microscope (AFM) and/or Scanning Electron Microscope (SEM).

  Below, a SEM image of an ablation pattern with 20 μm in diameter, is shown. The sample in this example is a Si wafer coated by 600 nm of copper. The dark area in the center is the Si layer and the brighter area is off course, the copper. Other than the evaporated material (not shown), some of the surface material was melted and resolidified (see the beautiful ring). The resulted pattern is quite similar to an instant image of a stone thrown into a pool...