As interconnect dimensions continue to decrease, the extendibility of conventional copper damascene processing becomes increasingly problematic. Small feature openings can lead to agglomeration of the thin seed that is required to avoid pinch-off, and this can then lead to poor nucleation during the electrodeposition process. Electromigration performance also suffers from the decrease in line dimensions. In addition, the increase in line resistance with shrinking dimensions means that the barrier is consuming valuable space within the feature. To address these extendibility issues, alternative metals, such as cobalt, are being actively explored.

Conventional damascene electroplating utilizes a combination of organic additives, namely, a suppressor, an accelerator, and a leveler, to achieve superconformal fill of interconnects. An alternative mechanism for superconformal cobalt fill is presented here. This mechanism, the Differential Current Efficiency Fill (DCEF) mechanism, utilizes sacrificial oxidants to achieve void-free superconformal fill. In the DCEF mechanism, the presence of a sacrificial oxidant promotes a difference in metal deposition rate between the field and the feature bottom. Addition of a single suppressor-type additive may assist in driving the plating rate differential. By appropriately selecting the sacrificial oxidant, the waveform, and the rotation rate, void-free superconformal fill can be achieved.

Figure 1 shows an example of the superconformal cobalt fill that can be achieved. Here, the rate and degree of fill are modulated by changing the rotation rate during the electrodeposition. Electrochemical polarization and current efficiency experiments are useful in determining appropriate process conditions, and these data will also be discussed.

The DCEF mechanism presents a new approach to achieving superconformal fill and will be important in extending electroplating to increasingly smaller interconnect dimensions. While the DCEF mechanism has been shown to work for superconformal cobalt fill, the concept may be applicable to other metals as well.

Figure 1: Superconformal cobalt fill: 1mA/cm² 240s, varied rotation rate