Analysis of adsorbed contaminants of CaF2 surfaces by infrared laser induced desorption

Abstract

157 nm photolithography technologies are currently under development and have been accepted as the leading candidate for fabrication of the next generation semiconductor devices after 193nm. At this and shorter wavelengths, molecular contamination of surfaces becomes a serious problem as almost all molecules absorb at 157nm and below. The light transmitted by a photolithographic tool can be significantly decreased by the presence of a few monolayers adsorbed on its many optical surfaces. We have developed a laser induced desorption, electron impact ionization, time-of-flight mass spectrometer (LIDTOFMS) to study contaminants on 157nm and other ultraviolet optics, e.g., polished CaF2. The LID TOFMS of CaF2(100) samples showed water ions, hydrocarbon ions, oxygen-containing hydrocarbon ions, as well as alkali metal ions (Na+,K+). For multiple irradiations of one site at fixed laser fluence, the ion intensities decreased as the number of pulses increased, suggesting that surface contaminants were being removed. A degenerate threshold model that assumes preferential adsorption at surface defects was employed to quantitatively analyze the LID data. Desorption thresholds for water and hydrocarbons were obtained from this model.

The influence of surface contamination on the ion emission from nanosecond-pulsed laser ablation of Al and Cu

“…As already mentioned, the intensity of the target ions is also sensitive to the surface contamination [1,19]. At a vacuum pressure of the order of 10 −6 mbar, the surfaces are known to be covered with a thin layer of adsorbates [28,29].…”

“…Previously, laser treatment of the metal surfaces has been performed, to remove these surface contaminations that are responsible for the fast ion structure [14][15][16]. It is also noted that the intensity of the ions that belong to the target material decreases with the removal of surface contamination [1,19], but the mechanism of the underlying phenomenon is not discussed. In addition, no exclusive study has been reported to investigate this target ion's intensity dependence on the amount of surface con tamination.…”

Light induced adsorption of Si nano-composites in LiF crystals at 157nm

“…The efficacy of cleaning contaminated surfaces depends not only on the type of the contaminant (e.g., C, Si) but also on the exposure wavelength. Considering the dependence on the type of contaminant, it was found that hydrocarbon ions, water ions, alkali metal ions can be removed after multiple irradiation of optical surface at 157 nm with trace levels of oxygen in the purge gas [33][34][35]. In certain cases, the films of molecular contaminants formed on optical surfaces can be removed by cleaning them with appropriate solvents or by applying several cleaning techniques such as CO 2 snow cleaning, ion or laser cleaning, on-orbit exposure to ambient atomic oxygen, synchrotron radiation heating and VUV irradiation [36].…”