Laser-Material Interaction

    Understanding laser-material interaction is the key in deploying laser in all practical applications where deposition, ablation, drilling, evaporation etc. is the goal. Materials processing by laser depends on absorption of a given laser by the material to be processed. 

Matching the wavelength:

Many lasers can produce holes and marks in most materials, but the highest quality micromachining is achieved at the lowest cost by matching the laser wavelength to the job. Commonly used lasers in the micromachining industry are:

    Excimer: UV wavelengths (193, 248, 308, 351 nm) 
    Nd:YAG or Nd:YLF: UV, visible and  Near infrared wavelengths (266, 355, 533, 1067 nm) 
    CO₂ lasers: Deep infrared wavelength (10.6 µm) 
    Optimum laser wavelength is determined by the minimum feature size and the optical properties of the material.  Since material ablation requires that the laser energy be absorbed in a thin surface layer of the substrate, the laser wavelength must be matched to the optical absorption and reflection characteristics of the substrate. 

 The width and depth of focus of laser beam at a given wavelength is determined from the following formulas. Width of beam at focus, w₀, is given by:

w₀ = 2pf/l

where, f is the f-number (the ratio of the focal length to the diameter of the focusing lens) and l is the laser wavelength. A parameter known as the "Rayleigh range" is defined as the length along the beam path where the radius increases by a factor of Ö2. The depth of focus, d, is then defined as twice the Rayleigh range: 

d = 2pw₀²/l.

The value of d can also be calculated from the f-number of a lens:

d = 8lf²/p.

For instance, a 250 nm wavelength used with an f/4 optical system will have a depth of focus 0f 10.2 micron and a beam diameter at focus of 1.3 micron. Table 1 shows the ablation depth of a few common materials.

    Polymers often exhibit strong absorption in the ultraviolet and deep infrared, with weak absorption at visible wavelengths. Excimer and CO₂ lasers are good choices for micromachining of polymers. Transmission characteristics of different materials are shown below.

Fig. 3. Transmission characteristics of polymers as a function of incident wavelength.

Fig. 4. Transmission characteristics of metals as a function of incident wavelength.

Fig. 5. Transmission characteristics of glassy materials as a function of incident wavelength.

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© 1999 Anis Rahman