Electrodeposition of Cobalt for Advanced Interconnect Applications
Copper (Cu) damascene processes have been used to produce back end of line (BEOL) interconnect structures in integrated circuits (IC). As the critical dimension of BEOL structures approaches the electron mean free path of Cu or below, the Cu resistivity exponentially increases, posing significant challenges on further scaling. Metals with shorter electron mean free path, for example cobalt (Co), have been explored as the alternative material to replace Cu in the finest metal levels.The Co electrodeposition process for interconnect applications must produce Co films that can reproducibly fill deep vias or trenches without any defects. It is can be achieved by adding small amounts of organic additives to the plating bath, which lead to the Co electrodeposition preferentially at the bottom of trench, known as bottom-up filling, or super conformal filling, or simply “super-filling”. As the size of interconnect continues to shrink, additives are becoming the key to the successful application of Co electrodeposition in IC manufacture. In this dissertation, a new class of organic additives, dioximes (dimethylglyoxime, cyclohexane dioxime, and furil dioxime), have been investigated for their effects on the electrochemical deposition process of cobalt. In Chapter 2, the nucleation and growth behavior of Co deposition with the addition of dimethylglyoxime and cyclohexane dioxime are studied. Double-peak nucleation curves are observed during Co deposition for the first time. In Chapter 3, a descriptive model is established for the Co nucleation process using furil dioxime, where the suppression effect on Co deposition and the catalytic effect on hydrogen evolution are both more pronounced among the dioxime molecules. Mercaptopropanesulfonate, or MPS, a well-known accelerator used in Cu damascene process, is investigated during the Co deposition in Chapter 4. A potential oscillation is observed during galvanostatic deposition for the first time and a kinetically controlled mechanism is proposed. In Chapter 5, Co films are electrodeposited with different additives including dimethylglyoxime, sodium chloride, and 3-mercapto-1-propanesulfonate. It is found that the addition of 3-mercapto-1-propanesulfonate into the electrolyte significantly increases the S incorporation level and decreases the grain size, both contributing to a higher sheet resistance of film.