Superpolishing for Planarizing Copper Damascene Interconnects

Abstract

We demonstrate a superpolishing electrolyte, which consists of acid additives in conventional Cu polishing electrolytes (H 3 PO 4 ), for efficiently planarizing Cu damascene features. The significant additive concentration gradient in features, resulting in a selective Cu dissolution rate within features, is explored as a major mechanism that yields such electrolytes with high planarization efficiency. Moreover, another additive, polyethylene glycol as a suppressor, is also employed to reduce oxygen bubbling on polished films. Consequently, a smooth surface with a complete step height elimination is obtained in a 70 m trench after electropolishing.Chemical mechanical polishing ͑CMP͒ of Cu and barrier metals is widely recognized as the most promising technology for planarizing Cu damascene interconnects. 1-4 However, conventional Cu-CMP slurries contain mechanically hard abrasives that scratch and damage polished Cu surfaces. Recently, Cu electropolishing has been explored as a replacement of Cu-CMP due to its higher polishing rate, lower waste stream, no scratching, and no mechanical stress on substrates. TSMC company announced CMP-free and CMP-less approaches by integrating Cu electropolishing. 5 Sony company announced a 25 m wide line formed by soft polishing technology. 6 They used a complexing agent to block Cu surface and introduced a soft pad to remove Cu complex outside of pattern, thereby producing selective electrochemical dissolution.Cu electroplating generally produces a step on/between damascene features after deposition. Moreover, planarization efficiency ͑PE͒ of Cu electropolishing is a strong function of the feature size ͑also for Cu CMP͒. 7,8 Those issues necessitate additional modifications to conventional polishing techniques. Because Cu dissolution rate depends on the pH value of polishing electrolytes and increases with solution conductivity, an addition of acid additives into polishing electrolytes can enhance Cu removal rate. Based on this mechanism and additive distributions controlled in diffusion limit, a presence of concentration gradient of additives in damascene features forms a selective electrochemical dissolution, thus enhancing step height reduction and improving PE of Cu electropolishing. In this article, we demonstrate a superpolishing electrolyte with high PE by adding acid additives into the polishing electrolyte (H 3 PO 4 ). Additionally, oxygen formation during Cu electropolishing often causes severe etched pits on polished surfaces. With polyethylene glycol ͑PEG͒ as a suppressor in the electrolyte, the overpotential of oxygen formation is significantly elevated, thus leading to less damage on polished surfaces. A polishing electrolyte with those additives can planarize damascene interconnects even with feature sizes in several tens of micrometers. ExperimentalThe patterned wafer used in PE measurements was composed of a 30 nm thick ionized metal plasma ͑IMP͒-TaN layer as the diffusion barrier, a 200 nm thick IMP-Cu film as the seed layer, and a 1.7 m electroplated Cu as the ...

Water diffusion coefficients during copper electropolishing

“…For these reasons, rather than using Equation (4) to determine effective diffusion coefficients, this will be employed to approximate the extent to which the effective diffusion coefficient varies during each experiment. Figure 4 shows the current transients from Figure 2 fit to Equation (4). Such plots consistently show little curvature as t increases during the experiment.…”

“…Lastly, data fits to Equation (4) were not found to be very accurate at long times (Rfi1) due both to the functional form of f(R) and to the strong sensitivity to the exact choice of the limiting current density. For these reasons, rather than using Equation (4) to determine effective diffusion coefficients, this will be employed to approximate the extent to which the effective diffusion coefficient varies during each experiment.…”

“…Cu electropolishing has recently been proposed for surface planarization following Cu electroplating during damascene processing of Si-based semiconductor devices [1][2][3][4][5]. While local planarization occurs through the action of the electropolishing electrolyte, global planarization is more difficult.…”

“…This approach provides rapid Cu removal, but suffers from the drawback that surface features of varying size, such as those created by Cu electrodeposition from superfilling additives, may be difficult to planarize simultaneously. Additive-containing baths have been proposed to address this problem [4], but their performance has not yet been widely validated. Another approach involves use of a scanning cathode or segmented cathode configuration [6].…”

“…Other challenges to commercialization include island formation due to a loss of contact when the remaining Cu film becomes quite thin. Advantages relative to chemical mechanical planarization (CMP) include simplicity, cost-of-ownership, easier endpoint detection, particle contamination is not introduced, and better compatibility with porous low-k dielectrics, which are mechanically fragile and may be damaged during CMP [4].…”