Research of New Concept Dishing-Less Tungsten CMP Slurry Using Using 25 Nm Crystal Zirconium Dioxide Abrasive

As design rule of semiconductor device shrinks, aspect ratio is becoming larger at wiring formation process [1], and the importance of chemical mechanical planarization (CMP) process is gradually increasing [2]. However, dishing and erosion of wiring generated from CMP process induced the fatal problems such as the increase of resistance and heat generation [3]. The solution for fatal defects such as dishing and erosion has been investigated for a long time, but the progress is still slight. Erosion can be controlled by enhancing polishing selectivity using surfactant to rapidly decrease polishing rate of SiO₂ film. However, dishing cannot be controlled in the case of the chemical-dominant CMP slurry, which mainly polishes the oxidized W surface. Therefore, CMP process of the adequately mixed mechanical and chemical-concept is necessary to dramatically improve dishing. This research suggests the possibility that the dishing was dramatically improved using 25 nm crystal zirconium dioxide (ZrO₂) abrasive.

All slurry was composed of an abrasive, a dispersant (except colloidal silica), a pH titrant, an oxidizer (H₂O₂), mixing with iron(III) nitrate (Fe(NO₃)₃) as catalyst, and deionized water (DIW). All of slurry was titrated to pH 2.3 with the addition of nitric acid. We used 6 x 6 cm W and oxide wafers to observe polishing performance using a polishing machine (poli-300, G&P Tech. inc., Korea). And we used the line pattern wafer. It has 10 nm width and about 250 nm thickness above SiO₂fillm. We deposited about 300 nm thick W film by Chemical vapor deposition method. Before and after the pattern polishing, the cross-sectional analysis of the patterned wafer was fabricated by a Forcused Ion Beam (Quanta 3D 200i, FEI Corp., USA). A High-resolution Transmission Electron Microscopy (HR-TEM) (JEM-2000, JEOL, Japan) image of line pattern was obtained with an accelerating voltage of 200 kV.

Figure 1 shows the HR-TEM of the colloidal silica and ZrO₂ particle used in the study. We used commercially available nano-sized colloidal silica (PL-7, Fuso, Japan), and ZrO₂(99.9%, SS-Nano, USA) nano-particles as the abrasive consisted of crystal forms of monoclinic.

Figure 2 shows the polishing rate of W and oxide films in colloidal silica and ZrO₂ slurries as a function of the Fe(NO₃)₃ concentration as catalyst. The polishing rate of W film was slightly increased from 56 to 1100 A/min in comparison with ZrO₂ slurry, while the polishing rate of oxide film was not changed to about 650 A/min according to Fe(NO₃)₃ concentration in the colloidal silica slurry. However, the polishing rate of W film was continually increased from 29 to 2390 A/min, while the polishing rate of oxide film was decreased from 372 to 85 A/min according to Fe(NO₃)₃ concentration in the ZrO₂slurry.

Figure 3 shows the TEM images of the line-pattern before and after polishing in the colloidal silica and ZrO₂ slurries. The average dishing and erosion was 6.8 and 10.7 nm in the colloidal silica slurry, respectively. In the ZrO₂slurry, however, the average dishing and erosion reduced in comparison with colloidal silica, which was 0.9 and 5.8 nm, respectively.

The ZrO₂ slurry in comparison with colloidal silica slurry indicated good performances such as high polishing rate, selectivity, low dishing and erosion, etc. However, we need more investigation to understand the CMP mechanism of the ZrO₂ slurry. We just guess that the good performances in comparison with colloidal silica are due to a small particles size and a crystal structure of the ZrO₂ abrasive in the slurry.