The W-film chemical-mechanical-polishing (CMP) has been utilized for the formation of via-hole and metal wiring as an essential semiconductor process. In the W-film CMP, key performance parameters are high polishing rate, low surface roughness of W-film, and high selectivity between SiO₂ and W-film CMP. Recently, the remaining nanoscale abrasives in sub-micron W lines has been seriously issued as the design rule has scaled down below 20 nm in memory devices, as shown in Fig.1. In addition, the surface roughness after W-film CMP has been important since it influences directly the metal contact resistance. Thus, in our study, we designed a novel W-film CMP slurry using the sulfonate functional group (i.e., polystyrene sulfonate (PSS)) and 10-nm-diameter ZrO₂ abrasives to reduce remaining abrasive in the sub- micron W-lines and surface roughness after W CMP. Principally, a W CMP slurry was fabricated with nanoscale ZrO₂, Fe based catalyst, corrosion inhibitor, pH stabilizer, and anionic surfactant (i.e., PSS).

First, we investigated the dependency of the W-film polishing rate on the PSS concentration. The W-film polishing rate decreased from 126 to 53 Å/min when the concentration increased from 0 to 1.0 wt%, and then it increased from 53 to 100 Å/min when the concentration increased from 1.0 to 2.0 wt%, as shown in Fig. 2(a). This result indicates that the W-film polishing rate minimized at a specific concentration; i.e., 0.5 wt% in which mechanism will be presented in the conference. Second, the W-film surface roughness [root-mean-square (RMS) at 2 x 2-um scanning area] after CMP increased from 1.302 to 1.662 nm when the concentration increased from 0 to 0.5 wt%, and then it decreased from 1.662 to 1.289 nm when the concentration increased from 0.5 to 2.0 wt%, as shown in Fig.2(b). This result means that the surface roughness peaked at a specific concentration; i.e., 1.0 wt%. Finally, the area ratio of ZrO₂ abrasives remained at sub-micron W lines after W-film CMP decreased from 95.7 to 2.4 % when the concentration from 0 to 0.5 wt%, and then it increased from 2.4 to 97.6 % when the concentration from 0.5 to 2.0 wt%, as shown in Fig. 3. This result imply that the area ratio of ZrO₂ abrasives remained at sub-micron W lines after W-film CMP minimized at a specific concentration; i.e., 0.5 wt%. In our presentation, we will review the mechanism of polystyrene-sulfonate surfactant effect on tungsten-film chemical-mechanical-polishing properties improvement such as remaining abrasive in sub-micron W lines and surface roughness.

Figure 1. SEM images of the remaining nanoscale abrasives in sub-micron W lines.

Figure 2. Dependency of polishing rate (a) and surface roughness (b) as a function of PSS concentration

Figure 2. Dependency of the area ratio of ZrO₂ abrasives remained at sub-micron W lines after W-film CMP as a function of PSS concentration

Acknowledgement

This research was supported by the MOTIE(Ministry of Trade, Industry & Energy (10085643) and KSRC(Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. This investigation was financially supported by the Brain-Korea 21 PLUS Program in 2017 and Semiconductor Industry Collaborative Project between Hanyang University and SK Hynix