Page 62 - High Power Laser Handbook

34 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Excimer Lasers 35

The optical resolution that is achievable in laser material process-
ing is proportional to the laser’s wavelength. The short wavelengths
of excimer lasers enable them to be among the most precise optical
processing tools on the market. Depending on laser wavelength,
material, and optical system, excimer laser–based material process-
ing tools achieve feature sizes of 1 mm and smaller. The advantage of
micromachining with a short UV wavelength is shown in the follow-
ing equation for minimum feature size (MFS):
λ
MFS ≈ 1 ∗ k NA

where k is processing factor, λ is wavelength, and NA is numerical
1
aperture. With a practical numerical aperture of 0.12 and a process
factor of 0.5, the 248-nm excimer laser yields a resolution of 1 mm.
Moreover, whereas the short wavelength translates into smaller
lateral structures, it is the strong material absorption of the corre-
spondingly high photon energy (e.g., 5.0 eV for 248 nm or 6.4 eV for
193 nm) that translates into very limited vertical material impact. In
fact, the depth resolution of excimer laser thin-layer material process-
ing is in the submicron range and can be as small as 50 nm/laser
pulse depending on the material sample and the wavelength.
The short wavelength is directly absorbed by “transparent” mate-
rials, such as glass, quartz, Teflon, or transmissive conductor oxide
(TCO) films, allowing UV laser radiation to directly interact with the
small defined absorption volume and to minimize bulk heating
effects. This advantage has led to a set of successful applications in
which material processing with minimum heat effect is demanded.
Figure 2.15 shows an example of the excimer ablation in glass. The
crater’s flatness indicates the high homogeneity of the excimer laser
beam over the entire illumination area.

Figure 2.15 Excimer laser ablation pit in NIST (National Institute of
Standards and Technology, Gaithersburg Maryland, USA); glass obtained at
193 nm after 50 laser pulses.