The Excimer Laser

In this type of Laser, the active medium consists of an inert gas (X) or of a mixture of an inert gas and a halide gas (X + Y). The term excimer stands for excited dimmer which refers to a diatomic molecule of two inert gas atoms (XX) or a molecule of an inert gas atom and a halide gas atom (XY).  

The chemical mechanisms required to achieve the upper laser state are quite complex, involving in some cases up to 82 different reactions. The promotion of molecules to the upper laser state requires reactions, which include ionization, dissociation, and 

On the other hand, the energy released by the laser transition is associated with a dissociation process of the dimmer, which consequently takes place in the UV part of the spectrum. 

Excimer lasers operate at typical powers of 50-200 mJ in pulses of around 20 ns. Table 2.1 shows the emission wavelengths for different excimer lasers. 

Excimer lasers are of great importance for UV and vacuum UV (VUV) spectroscopy and photochemistry. They are also found in a wide range of applications. For example, they are used in micromachine medical devices, including refractive surgery, in photo-lithography for the microelectronics industry, for material processing, as optical pump sources for other type of lasers (dyes), and so on. More details about excimer lasers can be found in Rodhes (1979). 

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Figure B2.3.9. Schematic diagram of an apparatus for laser fluorescence detection of reaction products. The dye laser is syncln-onized to fire a short delay after the excimer laser pulse, which is used to generate one of the reagents photolytically.

The excimer laser radiation is pulsed with a typical maximum rate of about 200 FIz. Peak power of up to 5 MW is high compared with that of a nitrogen laser. 

Fig. 40. Schematic of an euv exposure tool. Key features are the excimer laser-driven x-ray source and the redective optical elements (including the mask) in...

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

This work was supported by a Faculty Research Grant from the University of California, Irvine and the Petroleum Research Fund administered by the American Chemical Society. The excimer laser... 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

Fig. 8. Portion of the infrared spectrum shown following photolysis of 30 mtorr of Fe(CO)5 in 100 torr of CO with a KrF excimer laser pulse. Adjacent traces are taken at 3-/nsecond intervals. The first trace is the one with the largest excursion from the abscissa. Nine traces are shown with the first being 3. seconds after the excimer laser pulse photolyzes the sample. Bands are labelled V, Fe(CO)5 IV, Fe(CO)4. [Reproduced from Ouderkirk et al. (68) with permission copyright 1983, American Chemical Society.]...

Similar results have been reported for DMA/PEMA (14). Figure 11 shows data for DSAE/PPSQ. Although the same effect is present, it is quantitatively different. The reduction in T is about a factor of 3 for a film initially bleached to T=32%, while for DPA about the same reduction is seen with T(initial) = 8% thus it is apparently less severe for DSAE. The most interesting result is that DMA/PEMA, when irradiated under N2 at 260 nm (+/-8nm FWHM bandwidth) by an Hg-Xe lamp, shows absolutely no change in transmittance with a dose of 100 J/cm2 (15). Thus the antibleaching is an intensity dependent effect that is absent at low intensities and occurs only with the excimer laser (typically -0.1-1 mJ/cm2 in -35 nsec). 

The irradiation of a polymer surface with the high intensity, pulsed, fer-UV radiation of the excimer laser causes spontaneous vaporization of the excited volume. This phenomenon was first described by Srinivasan (1) and called ablative photodecomposition. The attention of many researchers was drawn to the exceptional capabilities of photoablation (2). Etching is confined to the irradiated volume, which can be microscopic or even of submicron dimensions, on heat-sensitive substrates like polymers. In most experimental conditions, there is no macroscopic evidence of thermal damage, even when small volumes are excited with pulses of... 

A similar approach was described by Hartmann et al. (114) for methylperoxy radicals in laboratory photodissociation experiments. The excimer laser pho-... 

Hydrophilicity of polymeric channels can also be increased by photoablation. For instance, polymeric channels (37 pm deep) were photoablated through a copper foil mask. Relative to the original polymer, the photoablated surface is rougher and has increased hydrophilicity. The EOF increases in the following order PC < PS < cellulose acetate < PET [194]. The excimer laser ablation has... 

The initial investment cost of the Nd YAG is less than half that of the excimer laser... 

Until quite recently, direct measurements of o(>d2)(X) were limited by the very real experimental difficulties associated with the highly efficient deactivation of O ( D2) by O3, as well as the need to provide a sensitive probe for atomic oxygen atoms in the ground Pj state as well as in the electronically excited D2 state. The development of resonance spectroscopic techniques for time-resolved detection of O ( Pi) has permitted monitoring of this state at densities of ca. 10 cm with an instrumental bandwidth in excess of 10 MHz. When combined with the use of high intensity photolysis sources such as the excimer lasers and frequency quadrupled Nd/YAG, it has proved possible to measure directly the yield of 0( D2) and O( Pj) at several discrete wavelengths in the middle ultraviolet. 

Figure 3.59). They also showed that standing wave effects, though not totally absent, were barely noticeable despite the quasimonochromaticity of the excimer laser radiation. 

Also, the rotational populations were such that it was impossible to determine a single temperature, possibly because different mechanisms of CN formation resulted in this multi-temperature distribution. The vibrational and rotational temperatures were higher for the excimer laser (308 nm induced plasmas) than for the Nd YAG laser (1064 nm plasmas), viz. 18 000 K versus 10 000 K. 

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