Characterization of Patterned Porous Low-kDielectrics: Surface Sealing and Residue Removal by Wet Processing/Cleaning

Abstract

The results described in this paper first demonstrate key differences between a plasma-exposed blanket porous dielectric sample and a patterned structure in terms of surface sealing induced by the patterning plasma using ellipsometric porosimetry characterization. While the blanket CVD organosilicate glass (nominal k-value = 2.4, ∼20% of porosity) surface was sealed by the C 4 F 8 /CF 4 -based plasma, the surface of the analogous patterned structure of 45 nm 1/2 pitch was found to be completely open after the same patterning plasma process. Second, 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 are presented. The experimental results demonstrate that the use of a relevant patterned test structure and its characterization represent an appropriate approach for the optimization of the etch and cleaning processes. The removal efficiency of various wet clean solutions, including dilute HF, citric acid, tetramethylammonium hydroxide:H 2 O 2 :H 2 O mixture, and a slightly alkaline formulated mixture, with regard to polymer residues, CFx, and metal-containing residues, TiFx, can be clearly distinguished using the patterned porous low-k stack. In back-end of line (BEOL) processing of advanced microelectronic CMOS integrated circuits, conductor lines are made using a damascene approach. For this, dielectric layers are locally etched through a patterned photoresist or metal hardmask layer using plasmas and followed by electroplating of Cu inside the etched patterns. In order to ensure accurate profile control, etching anisotropy, and minimize degradation of the exposed porous dielectric during the plasma etch, polymerizing plasma chemistries are employed to deposit a thin layer of polymer on the exposed surface.1-3 A good balance between the polymerization and the dielectric etch rate allows the polymer film to passivate the sidewalls without affecting the vertical etch rate, resulting in a vertical etch profile. The polymer formed during the dry etch process ("post-etch polymer residues" or PER), mostly fluorocarbon polymers for fluorocarbon-containing plasmas, must be removed prior to deposition of subsequent layers (metal) in the etched features to achieve high adhesion and good coverage. Removal of these polymers is usually performed by a combination of a short plasma treatment and wet clean using chemical solutions, or by wet cleans alone. It is known that this type of fluorocarbon polymer is chemically inert to many existing wet clean solutions, including aqueous solutions such as fluoride ion-containing or highly alkaline solutions, and solvent mixtures.4 A short plasma treatment carried out prior to the wet clean step enhances the polymer removal efficiency but also has a drawback of irreversibly damaging the porous dielectrics by material densification, carbon depletion, and increase of hydrophilicity. 1,2,5,6 An alternative approach for improving the residue removal efficiency involves structural modification o...

Formation and Wet Removal of Organic and Titanium-Containing Residues on Patterned TiN/Porous Low-k Structure

“…Typically, in the Ti 2p region, the high intensity peak at lower BE side (459.1 eV) can be assigned to the Ti 2p 3/2 component, while the high BE peak at 464.8 eV is due to the Ti 2p 1/2 component. 23,24 As already discussed in our previous publication, 18 the binding energy of these peaks gives a clear evidence that the surface of the TiN layer has been oxidized to TiO 2 , most likely during the opening of the TiN layer as well as during the patterning of the OSG layer. The thickness of the TiO 2 surface layer is estimated to be significantly thinner than 5 nm, as an additional but weak signal from TiN was also detected at a lower BE, ∼455.2 eV.…”

“…This behavior/characteristic was also observed for similar types of residues generated on a stack with OSG 2.4 as the dielectric. 18 The results shown in both Tables also confirmed that the TiN HM still remained at the surface. In general, the oxygen concentrations for all samples were significantly higher for the case with PET.…”

“…The patterned structure initially consisted of a Si substrate/OSG/SiO 2 HM/TiN HM (25 nm)/BARC/193-nm photoresist (PR) layer. As was described in a previous paper, 18 the TiN layer was first patterned through the PR mask using HBr/Cl 2 plasma, followed by the BARC and PR strip. The oxide HM and OSG etch was then performed using C 4 F 8 /CF 4 -based plasma in a dielectric etch chamber.…”

Characterization of Etch Residues Generated on Damascene Structures

“…In contrast to the F1s spectra, very similar Ti2p spectra were recorded regardless of the TOA's within the range of 22-78 deg., which in turn confirms the previous statement. The binding energies and the shape of the Ti2p spectra suggest that several types of Ti compounds are present at the surface, including TiO 2 , TiN, and TiFx [3,4]. The presence of the latter is strongly supported by the observed F1s spectrum as discussed above.…”