Abraded Material

Never allow slurry to dry on wafers. Strong bonds are formed when slurry dries, making the removal of the slurry nearly impossible. The bonds are formed between the abrasive, the abraded pad, the abraded material, and the wafer. The bond strength is such that even supplementary cleaning steps are insufficient to remove the slurry. 

Finally, assuming that none of the mechanically abraded material is redeposited, the material removal rate is... 

Dissolution of the abraded material is governed by electrochemical reactions such as ... 

Note that in the absence of the Ti/Cu couple. Figure 4.44a predicts the titanium dissolution current density to be 89 pA/cm This dissolution current density translates to a dissolution rate of only 3 nm/min, which is lower than the observed polish rate of 62 nm/min. It is possible that in the absence of copper ions, most of the mechanically abraded titanium is swept away from the wafer surface without dissolving The abraded material either falls into the pad as undissolved titanium or Ti02 or is adsorbed onto the abrasive particles. If this is the case, the titanium dissolution rate will be lower than the polish rate, since not all of the abraded material dissolves. 

Removal of the abraded material from the vicinity of the copper surface. 

The term abraded material refers to copper and/or copper oxides (or other compounds that form the surface film) that are mechanically dislodged by the abrasive action. Removal of the material from the vicinity of the copper surface may be achieved by one or more of the following mechanisms ... 

Adsorption of the abraded material onto the abrasive article, or... 

Mechanical removal of the abraded material, i.e., the abraded material is swept away from the surface by the turbulent motion of the slurry. 

In a third removal mechanism, the abraded material is swept away from the copper surface by the turbulent motion of the slurry. Assurances must be made that the abraded material does not return to the copper surface at a later point in the process. If the pad has long fibers, the abraded material may fall into the pad where it is held away from the surface. In an HjO only slurry, the presence of precipitates is indicated by the black color of the slurry during CMP. When polishing with a napped cloth comprised of long... 

The final method of removing the abraded material is to dissolve the material into the slurry as it is abraded firom the surface. This technique requires increasing the solubility and the dissolution rate of the abraded material into the slurry. Solubility must be high to prevent precipitate formation. The dissolution rate must be high to ensure that all of the abraded material is dissolved and the abrasion efficiency is maximized. 

In the copper slurry formulations described below, we have attempted to maximize the solubility and dissolution rate of the copper in the slurry, and therefore dissolution of the abraded material is expected to be the dominant removal mechanism. However, the removal of the abraded material may occur as a combination of several of the above mechanisms. For example, the abraded material may initially fall into the pad undissolved, where it then dissolves. Such a scenario was described in Section 4.6.2 where it was observed that when the concentration of polish by products in the pad is high, the slurry initially turns black, indicating incomplete dissolution and the formation of copper oxide precipitates. With time, however, the slurry turns blue, indicating dissolution and the formation of the Cu(NH3)2 complex. Thus, the optimum polishing conditions may provide for a combination of removal mechanisms. 

For NH4OH whether the slurry is flowing or not, the difference in polish rate is negligible because the NHj complexes the abraded copper entering the slurry. In both the 1 vol% NH4OH and 1.4 wt% NH4NO3 slurries, we hypothesize that the abraded copper is first dissolved as Cu and then complexed to Cu(NHj)2 Thus, dissolution of the abraded material occurs by the following sequence ... 

Oxidizing agents such as NOj and Fe(CN)/ may be added to the NH40H-based slurries to increase the copper polish rate by increasing the dissolution rate of the abraded material. In order to form the complex ion, the NH3 complexing agent first requires the oxidation of copper to Cu (reaction (7.2)). Reaction (7.2) requires an associated reduction reaction to sink the electrons. Even if the NH3 were to complex the copper metal directly in one step ... 

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 . 

At low etchant concentrations, the polish rate is limited by step 2. In this dissolution rate limited region, the abrasion rate is higher than the dissolution rate. The abraded material that is not dissolved quickly redeposits onto the surface, lowering the net rate of removal. Therefore, the polish rate is approximately equal to the dissolution rate. In the dissolution rate limited region, the slurry cannot dissolve more material, and therefore increasing the mechanical abrasion rate, by increasing the pressure, has no effect on the polish rate. However, increasing the etchant concentration increases the dissolution rate of abraded material and thus increases the polish rate. 

The model contends that material is removed from the surface primarily by mechanical abrasion. The abraded material is either dissolved into the slurry, swept away from near the surface as undissolved copper or copper oxides by the fluid motion of the slurry, or redeposited onto the surface. The polish rate is then the abrasion rate minus the redeposition rate. Material is removed from the surface by chemical etching secondarily only, if at all. 

Why does the etchant dissolve the abraded material but not the surface ... 

From the above discussions one concludes that the Preston equation may be applied to plots of polish rate vs. pressure or velocity and the resulting value compared to the theoretical value. The Preston equation predicts the abrasion rate of the surface. In all cases examined, the observed is lower than theory predicts because the efficiency of mechanical abrasion is lowered by incomplete removal of the abraded material from the vicinity of the surface. The unremoved abraded mataial redeposits onto the surface, lowering the net polish rate. 

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