| AVS 56th International Symposium & Exhibition | |
| Plasma Science and Technology | Thursday Sessions |
| Session PS1-ThA |
| Session: | Fundamentals of Plasma-Surface Interactions II |
| Presenter: | T.A.R. Hansen, Eindhoven University of Technology, The Netherlands |
| Authors: | T.A.R. Hansen, Eindhoven University of Technology, The Netherlands J.W. Weber, Eindhoven University of Technology, The Netherlands M.C.M. van de Sanden, Eindhoven University of Technology, The Netherlands R. Engeln, Eindhoven University of Technology, The Netherlands |
| Correspondent: | Click to Email |
Structures in the chip industry are approaching the 32 nm half pitch, which requires radiation in the VUV and EUV range. Cracking of hydrocarbon impurities in the vacuum by the radiation causes C growth on the VUV and EUV optics. Each nm of deposited carbon reduces the reflectivity of the optics by 1%. Fast removal of these contamination layers without damage to the underlying optics is essential for the next generation of lithography devices.
Etching with a low energetic plasma jet can be used to selectively remove coatings such as hydrogenated amorphous carbon (a-C:H) without damage to the underlying structure. Real time, in situ spectroscopic ellipsometry measurements indicate that the highest etch rates are obtained for an Ar/H2 plasma, rather than for a pure Ar or H2 plasma.
Even though the etch rate of a-C:H thin films is dependent on both temperature and roughness, the highest roughness in absolute values is attained by the plasma with the lowest etch rate. At low temperatures, the etch rate deviates from an Arrhenius relation, while the activation energy is similar for both the H2 and Ar/H2 plasma at higher temperatures.
The two orders of magnitude higher etch rate for the Ar/H2 plasma is due to chemical sputtering, which is a synergistic effect between atomic H and Ar+ ions with an ion energy below the threshold of 58 eV for physical sputtering. Chemical sputtering has been observed by Hopf et al. for energies above 20 eV and an H to Ar+ flux ratio over 100 [1]. In our plasma, however, the Ar+ ion energy is only a few eV’s and the estimated H to Ar+ ratio is lower than 5.
The etch products, released from the surface, consist mainly of CH4 and C2Hy, as shown by residual gas analysis. Time resolved optical emission spectra of the Ar/H2 plasma, from a few mm’s in front of an a-C:H sample, indicates also the presence of C2 and CH radicals. The CH radical is formed in the plasma phase through charge transfer between Ar+ ions and these larger hydrocarbons, and dissociative recombination. Similar plasma chemical processes occur during the remote plasma deposition of a-C:H films. However, in contrast with deposition, the CH rotational temperature shows an overpopulation in the higher excited states, indicating that the (internal state of the) parent molecule is different for an etch plasma than for a deposition plasma.
Spatially resolved optical emission measurements are Abel inverted, by means of the numerical Barr method. While there is some CH production throughout the entire plasma jet, the highest CH production occurs in front of the a-C:H sample.