KrF exciplex lasers

Finally, electrons have been generated with nanosecond pulses from the KrF exciplex laser at 248 nm via photodetachment of OH and Cl in aqueous and methanol solutions, and could be via one-photon photoionization in liquids with the ArF laser at 193 nm, and via one and two-photon processes in aqueous naphthol and naphtholate with nanosecond pulsed nitrogen lasers at 337 nm. With the improved fluxes and short pulses now available from gain modulation techniques applied to the TEA exciplex lasers, semiconductor lasers and F-center lasers, they may prove to be convenient sources for future studies of electron relaxation and transfer processes from the ultraviolet to infrared region. 

Reactions [13.1]-[13.11] illustrate the specific photochemical reactions involved in the lasing actions of KrF exciplex lasers. (Similar reactions are also involved in ArF exciplex lasers.)... 

Figure 13.38 Resolution capability of AZ EXP 2787 DUV 248-nm photoresist used in printing line/space features with a 0.65-NA KrF exciplex laser tool. (Courtesy of R. Dammel. )...

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance... 

Excimer laser A source of pulsed coherent radiation obtained from an exciplex. The proper name should be exciplex laser. Typical lasing species are noble gas hahdes (XeCl, KrF, etc.) emitting in the UV domain. 

The 1980s and 1990s saw the development of lithographic exposure sources such as KrF and ArF exciplex lasers. Today, the majority of advanced IC circuits are fabricated with these laser-based exposure tools. 

Examples of the application of exciplexes in hthography include KrF and ArF exciplex laser light sources for 248 nm and 193 nm hthographies, respectively. 

Figure 13.5 Schematic of potential energy curves for a rare-gas monohalide exciplex laser based on KrF. KrF is formed via two reaction channels. It decays to the ground state via dissociation into Kr and F while emitting a photon at 248 nm. (Adapted with permission from Francis Taylor Group LLC. " ) The diatomic halogen excimer lasers based on F2 also have similar potential energy curves.

Infrared spectra were measured using a Nicolet Magna-IR FTIR/550 spectrometer. The resist solutions were spin coated at 2500 RPM to produce -O.Tpm films on 8 inch double polished silicon waiers and heated at 150°C for 60s (unless otherwise stated) in hard contact with the bake plate. Following exposure to known doses ranging from 0-50 mJ/cm on a GCA XLS KrF (248 nm) or an ISI MicroStep (193 nm) exciplex laser stepper, the wafer was baked with minimal delay (<5 min) for 60s at 150 C. This experiment was repeated for post-exposure bake temperatures of 140,130 and 120°C. 

The key operational parameters of exciplex and excimer lasers used in optical lithographic applications include exposure-dose-related parameters comprising average power, pulse energy, repetition rate, and pulse width temporal coherence spatial coherence including beam dimensions, beam divergence, and beam uniformity and maintenance and reliability. Table 13.2 lists some of the key operational parameters of KrF, ArF, and F2 laser systems used in optical lithography. 

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