Debris characterization and mitigation of droplet laser plasma sources for EUV lithography.

摘要

Extreme ultraviolet lithography (EUVL) is a next generation lithographic techniques under development for fabricating semiconductor devices with feature sizes smaller than 32 nm. The optics to be used in the EUVL steppers is reflective optics with multilayer mirror coatings on each surface. The wavelength of choice is 13.5 nm determined by the optimum reflectivity of the mirror coatings. The light source required for this wavelength is derived from a hot-dense plasma produced by either a gas discharge or a laser. This study concentrate only on the laser produced plasma source because of its advantages of scalability to higher repetition rates.;The first part of this study investigates debris emissions from tin-doped droplet targets, in terms of aerosols and ions. Numerous tin aerosols can be created during a single laser-target interaction. The effects these interactions are observed and the depositions are investigated using SEM, AFM, AES, XPS, and RBS techniques. The generation of aerosols is found to be the result of incomplete ionization of the target material, corresponding to non-optimal laser coupling to the target for maximum CE. In order to determine the threats of the ion emission to the collector mirror coatings from an optimal, fully ionized target, the ion flux is measured at the mirror distance using various techniques. The ion kinetic energy distributions obtained for individual ion species are quantitatively analyzed. Incorporating these distributions with Monte-Carlo simulations provide lifetime estimation of the collector mirror under the effect of ion sputtering. The current estimated lifetime the tin-doped droplet plasma source is only a factor of 500 less than the stepper manufacturer requirements, without the use of any mitigation schemes to stop these ions interacting with the mirror.;The second part of this investigation explores debris mitigation schemes. Two mitigation schemes are applied to tin-doped droplet laser plasmas; electrostatic field mitigation, and a combination of a foil trap with a magnetic field. Both mitigation schemes demonstrate their effectiveness in suppressing aerosols and ion flux. A very small number of high-energy ions still pass through the combination of the two mitigation schemes but the sputtering caused by these ions is too small to offer a threat to mirror lifetime. It is estimated that the lifetime of the collector mirror, and hence the source lifetime, will be sufficient when tin-doped targets are used in combination with these mitigation schemes. (Abstract shortened by UMI.)