Abstract
The purpose of this research is to investigate electroless Cu deposition on various diffusion barriers such as TiN, Ta, TaN, and Ta2N, for the application of interconnects in ULSI devices. Since these diffusion barriers are not catalytic to electroless Cu deposition, an activation step should be carried out prior to the deposition of electroless Cu. Therefore, activation of barriers by different methods is a key point of this research. The research is mainly divided into five sections. In the first section, Cu was electrolessly deposited on Ta and TaN with sputtered Cu as a seed layer, and on TiN which was activated by a two-step activation using SnCl2/HCl and PdCl2/HCl solutions, respectively. Continuous Cu films were electrolessly deposited on these substrates. The Cu film deposited on Cu seeded barrier showed a low Ra surface roughness of 11 nm and a resistivity of 1.8 □□ cm, with a film thickness of 1 □m. A strong (111) texture was obtained for the Cu film deposited on Cu seeded TaN because of the strong (111) texture of the seed layer. Besides, complete fill of Cu into 0.2 □m vias with an aspect ratio of 5 was obtained. In the second section, a two-step activation was used to catalyze electroless Cu deposition on TaN barrier. It was found that a continuous Cu film formed on the catalyzed TaN through these steps: nucleation on catalytic sites, coalescence of Cu nuclei into dense islands, and merging of the islands to form a continuous film. With proper control of the catalyzation solutions, Cu films were electrolessly deposited with a high purity, low surface roughness, and low resistivity. Besides, gap fill was achieved for vias with a size lower than 0.2 □m and aspect ratio up to 5. In the third section, a novel PdCl2/BOE/HNO3 solution was used to activate Ta barrier for deposition of electroless Cu film by displacement deposition of Pd. Composition of the activation solutions was modified to see the change in surface morphology of Pd catalytic sites. Experimental results showed that the optimized solution had a BOE/HNO3 ratio of 3:2, and electroless Cu deposited on Ta barrier activated by this solution had a low resistivity and continuous morphology. The surface roughness of electroless Cu film on Ta barrier was high due to the insufficient Pd catalytic sites. In the fourth section, PdCl2/BOE/HNO3 solution was used to activate TaN and Ta2N barriers for electroless Cu deposition. It was found that with a pre-etching in a diluted HF solution, the activation of both TaN and Ta2N barriers by Pd displacement deposition was improved. The different phenomena between activation of TaN and Ta2N barriers were discussed. In the final section, we propose a Cu crystallization process during electroless deposition: self-induced repeated nucleation. This mechanism explains the agglomerations of Cu nano-crystallites in electroless Cu deposition on Pd activated TaN barrier. With this mechanism, low-angle boundaries are formed between the Cu nano-crystallites. The in-situ recrystallization of nano-crystallites into large grains was observed as exposing the sample to the electron beam of SEM, and it can be explained by the elimination of boundary dislocations by rotation of the nano-crystallites.