Diagnosis of the flux emanating from the intermediate focus of an extreme ultraviolet light lithography source.

摘要

The implementation of extreme ultraviolet (EUV) light lithography as the solution for next generation lithography needs. In this paper, the effects of chamber pressure, buffer gas mass, and pinch gas mass on debris transport will be explored using the XTREME XTS 13-35 EUV light source. Utilizing the Sn Intermediate Focus Flux Emission Detector (SNIFFED), three triple Langmuir probes, as well as a set of Si witness plates placed along the mock-up collector optic and at the intermediate focus, it will be shown that the interaction between high energy electron and photons, energetic ions, and energetic neutrals with the buffer gas has a considerable impact on the creation and transport of non-EUV photon debris to the intermediate focus. The creation of an EUV light emitting plasma results in the propagation of three separate observable plasmas: one initiated by the high energy electrons decoupled from the plasma core, one caused by the energy retarded fast electrons coupled with the expansion of the high energy ions, as well as the expansion of the lower energy core of the EUV emitting plasma into the surrounding buffer gas. The generated plasmas are typically in the range of 3-6 eV with densities on the order of 1013 cm-3. It will be shown that electron temperatures and densities generally peak at 12 mTorr using Ar buffer gas and a N 2 fueled pinch. While electron temperatures greatly increase up to 11+/-2 eV with He buffer gas, and drop down to 6+/-1 eV for Ar buffer gas, the larger species with more electrons, and less ionization potential, have the highest density. In general there is very little effect observed in changing the pinch species used, except to change the arrival time of the second and third plasmas. With increased energy, and the consequent ionization, these buffer gas species sputter the chamber walls and introduce any contaminant there into the chamber atmosphere. If the pressure is not high enough, these species (oxygen and carbon) readily reach the intermediate focus and deposit on any surface after it. Furthermore, the presence of these expanding plasmas can contribute to a negative charge flux of ~-0.25+/-0.1x1011 e-cm-2 impending upon the intermediate focus facing surface, though the chamber pressure largely determines the amount of ions and electrons reaching the surface. The interaction between the intermediate focus facing components and the charged flux can lead to sputtering, or further deposition as the ions are accelerated through the built up sheath into the surface (depending on the suppression of the energetic ions and neutrals ejected from the EUV emitting plasma). The excitation of the buffer gas species also results in the transport of neutral atoms over 100 eV to the intermediate focus. This is largely affected by the chamber pressure (peak flux was observed at 6 mTorr with an arrival time of ~700 mus), buffer gas mass (40 AMU had the highest measured flux with an arrival time of ~800 mus), and pinch gas species (40 AMU pinch gas mass had the highest energy deposition into 40 AMU buffer gas, though arrival time was the same for all species. Furthermore, deposition rates at the intermediate focus were shown to peak at 2 mTorr with a rate of 1.5+/-0.3x10-4 nm/pulse and a total film concentration of oxygen and carbon totaling greater than 90%. Increasing pressure reduces deposition rate because of increased buffer gas suppression of depositing metals from the electrode, as well as increased etching by the higher density generated plasmas. Increasing buffer gas mass species were theoretically shown to decrease the deposition rate at the intermediate focus, though Sn and Cu particulates increased with increasing buffer gas mass due to arcing between the electrodes and resulting sputtering. Ultimately the understanding of the importance in choosing buffer gas mass, pinch gas mass, and chamber pressure are emphasized in regards to the transport of debris from the EUV emitting plasma to the intermediate focus.