Metrology and Optical Characterization of Plasma Enhanced Chemical Vapor Deposition, (PECVD), low temperature deposited Amorphous Carbon films

Amorphous Carbon films deposited by PECVD (R-F) have recently been introduced as a new material for semiconductor processing, e.g. in 193nm ARC lithography [1] and in the DRAM production [2]. A large amount of literature has already been published on with regard to the applications of this class of material [ 3]. Hence, it has been reported that Amorphous Carbon films undergo a hydrogen chemical desorption when deposited above 500 degrees C, together with an amorphous to graphite phase transition. Unfortunately, the intrinsic nature of the amorphous carbons depends strongly upon deposition techniques. Film morphology can be completely different from one case to another since there are so many deposition techniques. Optical characterization of these films has also undergone development for several decades. In Spectroscopic Ellipsometry (SE), several models have been proposed from simple Lorentz oscillators' absorption, toward the Tauc Lorentz or Forhoui Bloomer equations. Nevertheless, none of these models sufficiently quantitatively explain the expermienstal data. In some cases, a simple 'Effective Medium Approximation',(EMA), is able to determine the sp(2)/sp(3) bound present in the film. However, the validity limitation of this approach remains questionable when considering films in a wide range of film thicknesses. In line, metrology for semiconductor requires robust models, which account for parameters such as temperature deposition, stress and film resistivity as well. Different solutions are investigated from our optical measurements, including a biaxial anisotropy hypothesis, which has been proposed by J. Leng et al [3] from BPR (Beam Reflectometry Profile) and with SE measurements [4]. Our results are considered, together with other surface analysis techniques (XRD, IR and Raman) and confronted to TEM observations.