Chemical Mechanical Polishing

Jeffrey Rockwell and Yuzhuo Li

Department of Chemistry

Clarkson University

Potsdam, NY 13699

(315) 268-3806

What is Chemical Polishing?

Chemical Mechanical Polishing (CMP) is a process that is used for the Planarization of semiconductor wafers. CMP takes advantages of the synergetic effect of both physical and chemical forces for polishing of wafers. This is done by applying a load force to the back of a wafer while it rests on a pad. Both the pad and wafer are then counter rotated while a slurry containing both abrasives and reactive chemicals is passed underneath.

What is Planarization and Why is it needed?

The goal of CMP is to obtain uniform Planarization globally across the wafer. The wafers consist of many small dies and patterns, which take the form of interconnected lines of copper and silica. These lines are called interconnects. Planarization occurs when the interconnects are polished to the point where both the copper and the silica lines are at the same level. When polishing, four different things can be achieved; No Planarization, Smoothing, Local, and Global, these different forms of polishing can be seen in Fig. 1.

Fig. Various forms of planarizations

In the case were No Planarization the different lines polish at different rates so that one type of line is higher than the other. In smoothing the polishing is more uniform but one type of line is still polish slower than the other. In local Planarization parts of the wafer are polished uniformly but other parts have an uneven polish. This is common with etch-back polishing. Global Planarization is where a uniform polish is achieved over the entire wafer causing it to be completely leveled. Global Planarization is essential for the construction of a computer chip with multiplayer structure.

What kind of problems can occur?

The most significant problems that arise when polishing are dishing, erosion, and corrosion. Dishing and erosion are forms of local planarization where areas of the wafer polish faster than others. In dishing the copper is “dished” out of the lines where as in erosion whole sections of the die are polished faster than others. Below is a picture were both dishing and erosion has occurred (Fig. 2).

Fig. 2 Illustration of dishing and erosion.

In corrosion within the copper lines consists of pits, fractures and depressions that occur due to attack from the chemistry of the slurry. Highly reactive slurries cause great deals of corrosion (Figs. 3 and 4).

Fig. 3 A polished wafer containing corroded copper lines.

Fig. 4 A wafer polished with an improved slurry. No visible corrosion found.

Current Research

We have minimized the erosion, dishing, and corrosion that occur during polishing. To do this we are using new a type of abrasive particles and a new kind of corrosion inhibitor as well as a set of optimized polishing parameters.

Acknowledgments

JR wishes to thank Earl Trye, Jason Keleher, John Westbrook for their contributions to this work.