Post-CMP Cleaning Enhanced on Poly-Si Film

Post-CMP cleaning requirements become more stringent as device dimensions scale down. Cleaning solutions augmented with tetraalkylammonium hydroxides (TAAHs) that have various chain lengths of hydrocarbon substituents were developed for post-poly-Si chemical mechanical polishing (CMP) cleaning. Researchers from National Chiao Tung University (Hsinchu, Taiwan), National Nano Device Laboratories (Hsinchu), and Merck-Kanto, Advanced Chemicals Ltd. (Taoyuan, Taiwan) recently evaluated cleaning performance with respect to particle, organic, and metal removal, and surface roughness. Results of the study were reported in the June Journal of the Electrochemical Society .

The researchers also examined the influences of the added surfactants on the hydrophilicity and etching rate of the poly-Si surface, as well as the possible interaction mechanism between TAAH cleaning solution and the poly-Si surface. The surfactants included tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH). The results demonstrated that the cleaning solutions enhanced with TAAH solutions and a chelating agent — ethylenediaminetetraacetic acid (EDTA) — achieved significantly better removal efficiencies of particle and metal impurities than the 3% NH₄OH control solution.

1. Results of residual particle numbers after cleaning with each solution exhibit improved removal when using the TAAH solutions. (Source: National Chiao Tung University)

The TAAH-containing solutions showed improved particle removal compared with the solution without TAAH. Figure 1 displays the results of residual particle numbers after cleaning with each type of solution. The researchers found that the TAAH solutions removed hydrophobic particles by penetrating the particle-surface interfacial region. The tetraalkylammonium ions were adsorbed onto the poly-Si surface as well as the particle surface to facilitate particle removal. In fact, the peak intensities of residual C₂H₆, C₃H₈ and C₄ H₁₀ after cleaning with TAAH solutions were one- to two-fold higher than those cleaned with NH₄OH. However, the differences in these peak intensities became insignificant when RCA cleaning followed the post-CMP cleaning with various TAAH solutions. Furthermore, no deterioration in the electrical properties was observed.

2. Results indicated that the degree of roughness was dictated by the etching rate. (Source: National Chiao Tung University)

Electrical characterization of the post-CMP capacitors cleaned with TAAH-containing solutions further confirmed their improved cleaning performance. The results indicated that the solution containing TPAH and EDTA had the best overall cleaning efficiency and electrical properties.

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