NH4OH Slurries

F ure 4.38 The distributions of beginning and ending potentials measured in each slurry. The diamond indicates the mean, the wide bar indicates one standard deviation, and the error bars indicate the range of the distribution. The distributions include 24 wafers for the NH4OH slurry, 8 wafers for the NH4NO3 slurry, and 10 wafers for the NH4CI slurry. 

In contrast to the 1 vol% NH4OH slurry, in a slurry of NH4OH plus ferricyanide ions a surface film readily forms on the copper. XPS analysis indicates that a surface film of copper ferricyanide (Cu3Fe(CN)6) is present on the copper surface after CMP in slurries of 1 vol% NH4OH plus 0.2-5.0 wt% potassium ferricyanide (K3Fe(CN)6). Figure 7.4 shows the XPS spectra for copper polished in 1 vol% NH4OH plus 0.5 wt% KjFeCCN). Fe,... 

Figure 7.3 XPS spectra for copper film polished in 1 vol% NH4OH slurry and rinsed immediatdy afto polish.

Polish Rate and Etch Rate in NH4OH Slurries... 

The key to the high solubility of the copper ions in the NH4OH slurry is the complexing action of the dissolved NH, gas that is prevalent in aqueous solutions of NH4OH. In pure water, copper ions are surrounded by polar water molecules which form coordination bonds with the ion. The solubility of copper in pure water is determined by the ability of the water molecules to shield the charge on a given copper ion firom other copper ions in solution. Complexing of copper occurs with NHj because the polar... 

In the NH4OH slurry, most of the Cu ions are complexed by the NH,. Thus, as the total concentration of copper species in the slurry increases, the Cu ion concentration does not increase substantially. Therefore, the additional Cu ions that build up when not flowing slurry do not interfere with the dissolution of copper to Cu (as long as the Cu ion concentration does not exceed one half the NH3 concentration). In the NH4NO3 slurry, most of the copper ions remain uncomplexed and Cu ions accumulate when there is no slurry flow. As the concentration of Cu ions increases, the driving force for reaction (7.2) decreases consequently, the dissolution rate of copper decreases and the efficiency of the mechanical abrasion decreases, yielding a lower polish rate. 

N03 ions may be introduced into the slurry by the addition of nitrate salts. The addition of NH4NO3 supplies not only the N03 ion but also ammonia. Figure 7.14 shows the polish rate of copper vs. the concentration of NH4NO3 in the 1 vol% NH4OH slurry. Only a nominal increase in the polish rate is observed in Figure... 

Figure 7.14 Copper polish rate vs. NH4NO3 concentration in the 1 vol% NH4OH slurry. (12S mm wafers polished on Strasbaugh 6CU polisher with 2.S wt% alumina abrasive, IS kPa, and 130 cm/sec.)...

Figure 7.15 also demonstrates the importance of the NH3 complexing agent in the Cu(N03)2 slurry. Recall from Section 4.6.5 that the copper polish rate decreases when Cu(N03)2 is added to the H2O only slurry. In the H20-only slurry, the increase in Cu ions inhibits dissolution of the abraded material. However, when the Cu(N03)2 is added to the 1 vol% NH4OH slurry, NH3 complexes the Cu " ions from the Cu(N03)2 salt so that the Cu ions do not inhibit copper dissolution. In addition, by complexing the Cu ions, the NH3 drives reaction (7.10) further to the right, increasing the supply of N03 . 

During the polishing of the thermally grown CujO the underlying copper is exposed only at the completion of the polishing. Therefore, the increase in polish rate of CujO with ferricyanide addition to the NH4OH slurry must be a result of an increase in the dissolution rate of the abraded CujO, which dissolves because of the increase in surface area. As the ferricyan-... 

Table lA Polish Rale vs. Abrasive Size in 1 vol% NH4OH Slurry... 

---