Reduction of Metal Contaminants Level on the Silicon Wafer Surface Using Chemical Additive during Chemical Mechanical Polishing

As the design rule of memory devices has scaled down toward 1x nm, metal contaminants on the silicon wafer surface has became the one of critical issues. In particular, Ni and Cu contaminants in the silicon wafer induce easy breakdown of gate oxide and the leakage current of devices.[1-2] Among various metal contaminant sources on the silicon wafer surface, metal contaminants in the polishing slurry for silicon wafer are one of the important metal contaminant sources.

In this investigation, we observed the electrical potential state of Ni, Cu contaminants and the colloidal silica abrasive in the polishing slurry and then, we found the chemical additives to reduce each metal contaminants during the silicon wafer polishing.

The polishing slurry for silicon wafer was basically composed of an colloidal silica abrasive and pH titrant. And we added the standard metal contaminants (1000ppm, Kanto Kagaku), and various chemical additives to reduce metal contaminant on the silicon wafer into the slurry. We used stock polished silicon wafer to observe the reduction efficiency of metal contaminants after the silicon wafer polishing using a polishing machine (poli-300, G&P Tech. inc., Korea). A High-resolution Transmission Electron Microsopy (HR-TEM) (JEM-2000, JEOL, Japan) are used to observe the image of colloidal silica abrasive in the slurry. The abrasive characteristics are measured by light scattering machine (ELS Z-2+, Otsuka Electronics, Japan).

 We used the nano-size colloidal silica abrasive as shown in the Fig. 1(a). The zeta potential of colloidal silica abrasive decreased with the increased of pH owing to the dissociation of Si-OH to Si-O^- on the colloidal silica surface as shown in the Fig. 2(b). In this study, the surface of colloidal silica abrasive has negative zeta potential because  the pH of our slurry is 10.5.

Fig. 2(a) and (b) show the pourbaix diagrams of Ni, Cu contaminants in the aqueous solution. In akaline region, Ni can exist in the ion state of HNiO_2­^- and Cu can exist as CuO ion. With these electrical potential difference between colloidal silica abrasive and metal contaminants in the slurry, we could expect that the behavior of metal contaminants in the slurry would be different and the prevention efficiency of metal contaminants onto the silicon wafer surface would be depend on the functional group of chemical additives in the slurry.

Fig.3 shows the prevention efficiency of metal contaminants on the silicon wafer surface during the silicon wafer polishing using the various chemical candidates, such as carboxyl acid, amine, hydroxly funtional group. We found that Ni contaminant was effectively reduced with the chemical additives having carboxyl acid functional group, and Cu contaminants are drastically decreased with the addition of chemical having hydroxyl functional gorup into the slurry.

In this study, we investigated prevention efficincy of Ni and Cu contaminants in the polishing slurry onto the silicon wafer during silicon wafer polishing using various chemical additives. We found that Ni and Cu contaminants on the silicon wafer reduced with the addition of checmial additives with carboxyl acid and hydroxyl functional groups, respectivly, after the silicon wafer polishing.

 Acknowledgement

  This study was financially supported by Semiconductor Industry Collaborative Project between Hanyang University and LG siltron., and the Post Brain Korea 21 Project in 2013.

Reference

[1] M. Miyazaki, M. Sano, S. Sumita, and N. Fujino, Jpn. J. Appl. Phys. 30, 2B, 295-297 (1991)

[2] T. Fukami, T. Takaku, US patent 6,060,396 (2000)