Paper PS-TuM11
Characterization of Patterned Porous Low-k Dielectrics after Plasma Patterning and Subsequent Wet Processing/Cleaning

Tuesday, October 20, 2015, 11:20 am, Room 210B

Porous dielectrics have been commonly used in micro- and nanotechnologies since the past decade. Their chemical composition and porous properties make them more susceptible to physical and chemical damage. In particular, pattern etching and subsequent processes for removal of resist layers and/or post-etch residues are critical steps that potentially modify the dielectric properties [1].

This study first focuses on the modification of the porous dielectric material (pore structure, surface sealing, wettability) using a patterned line/trench test structure of 45 nm ½ pitch (Fig. 1). Several methods were applied for characterization of the patterned structure used in this study, including ellipsometric porosimetry (EP), X-ray photoelectron spectroscopy (XPS), and Fourier transform Infrared spectroscopy (FTIR). Fig. 2 shows the variation of the polarization state, expressed here by Delta angle, as a function of the relative pressure of toluene (used as a probe for porosity change). For the 45 nm patterned structure, the rapid change in Delta angle reflects the adsorption of toluene into the porous network. This clearly indicates that the patterned low-k surface remained unsealed after the C₄F₈/CF₄-based etch plasma process. In contrast, the surface of the blanket low-k film was almost sealed, evidenced by a very slow and irreversible toluene adsorption. Note that the sealing layer is only concerned the surface. The approach and characterization methods utilized for determination of the sealing thickness and the internal hydrophobic/hydrophilic properties [2] of the patterned porous low-k structures will be discussed.

Another aspect of this study concerns the surface composition of the patterned feature, type of the residues generated during the plasma etch, and the effect of a subsequent wet clean step. The latter is usually required before the deposition of the barrier layer. Substantial amount of fluorinated etch residues were detected on both the TiN surface and the dielectric sidewall and bottom. As shown in Fig. 3, the XPS F 1s core-level spectrum for the surface after the OSG etch consisted of two main components: the peak centered at ~684.6 eV corresponds to F-Ti bonds and the peak at 688.4 eV can be assigned to C-F bonds. The efficiency of the removal/dissolution of CFx and TiFx by the wet chemistries can be clearly demonstrated using this 45 nm test structure. For instance, dilute HF and TMAH:H₂O₂ mixture are efficient for TiFx removal but only show very limited dissolution of CFx polymer. In the presentation, the change in the low-k dielectric and TiN properties due to the plasma etch and subsequent wet cleans will also be presented and discussed.