Experimentation and modeling of organic photocontamination on lithographic optics

Abstract

Photodeposition of organic films on transparent substrates irradiated in the presence of trace levels (ppb to ppm) of hydrocarbons has been experimentally investigated and a model is presented that describes the film growth behavior. The efficacy of a given organic precursor at forming a deposit is proportional to the product of its surface coverage (as governed by its partial pressure relative to its saturation partial pressure) and by its photon absorption cross section. These measurements are important in predicting the transmission characteristics of lithographic optics operating at 1 57-, 193-, and 248-nm wavelength. For example, a lens element irradiated continuously for one year (1 kHz, 0.1 mJ/cm2 per pulse) in the presence of 1 part per billion of t-butyl benzene would exhibit a transmission of 87% at 193 nm. The effects of oxygencontaining ambients are also documented, and methods for elimination andlor prevention of organic contamination are suggested.

Femtosecond pulse damage thresholds of dielectric coatings in vacuum

“…Nitrogen and toluene do not affect the breakdown threshold F(∞). The outcome with toluene is very different from the observation with ns pulses, where organic compounds formed graphitic deposits that accelerated failure [5,7]. Figure 3 shows that F(∞) is sensitive to both the pressure of water vapor and gaseous oxygen.…”

“…Previous studies with nanosecond lasers under vacuum and sealed atmospheric conditions have shown that the useful lifetime of the optics and multiple pulse damage fluence reduced significantly for dielectric films [5][6][7][8] and bulk materials [9,10] compared to atmospheric conditions. Two explanations have been given for these observations.…”

“…Two explanations have been given for these observations. The first involves the deposition of absorbing graphite layers from photolytic cracking of background organic contaminants that result from out-gassing of epoxies [7]. These deposits are reduced by the presence of oxygen in the atmosphere, but grow unchecked in sealed systems or under vacuum.…”

Controlled contamination of optics under 157-nm laser irradiation

“…The transmission then remains relatively constant after a total drop of approximately 1 .5%. Although spectroscopy has not been performed on the deposit, the film is believed to be highly conjugated, akin to the state of hydrocarbon films under prolonged irradiation [1,2]. The absorption coefficient at 157 urn for these films is approximately 7Iim (base 10) and therefore the measured 1 .5% transmission drop is associated with the accumulation of 1 nm of deposit, or approximately 2-3 monolayers.…”

“…Uncoated CaF2 has been used as the surface exposed to the contaminant, in order to provide a solid reference against which studies of optical coatings can be compared. Toluene is expected to be one of the more efficient species for producing residues due to its comparatively high absorption cross-section and ability to undergo photo-induced ring-opening polymerization at the surface [1] . As will be shown below, the addition of controlled concentrations of oxygen can have a significant impact on surface contamination: it can reduce it, prevent it from forming in the first place, or remove it after it has formed.…”