In Situ Electrochemical Investigation of Tungsten Electrochemical Behavior during Chemical Mechanical Polishing

Abstract

The electrochemical behavior of tungsten during chemical mechanical polishing (CMP) was observed in order to investigate a proposed blanket passivation and abrasion mechanism for tungsten removal. The experiments were performed in a cell that allowed electrochemical measurements to be made during polish. Polish rates were determined from the same samples used in the cell. Alumina-based polish slurries containing potassium iodate, ferric nitrate, or ammonium persulfate were used. DC polarization experiments show no evidence of passive film formation on the tungsten duiing polish. Tungsten oxidation rates measured during polish account for removal rates that are ito 2 orders of magnitude below the measured polish rate. Values of the charge-transfer resistance (measured by ac impedance spectroscopy) during polish are ito 2 orders of magnitude higher than expected from the polish rate, thus corroborating the dc-based data. Polish rates under potentiostatic conditions were also measured. The current required to maintain the metal anodic of the open-circuit potential is well below the current expected from measured polish rates, assuming complete oxidation of the tungsten. The polish rate during cathodic potentiostatic conditions (-0.5 V with regard to the open-circuit potential) was similar to the polish rate at open circuit. We conclude that the formation of a blanket passive layer does not significantly contribute to tungsten removal during CMP. InfroductionChemical mechanical polishing (CMP) is the most effective and now the predominant method for the removal of excess tungsten (W) deposited by nonselective chemical vapor deposition (CVD) for the formation of contacts and vias used in integrated circuit (IC) multilevel interconnects. Figure la depicts the CVD tungsten film and patterned oxide prior to polish. Figure ib depicts the same surface after CMP. The majority of W CMP research to date has focused on empirical cause and effect relationships in which process variables, such as slurry composition, pad type, applied pressures, and platen and carrier speeds, are empirically modeled. These empirical models allow for adequate manufacturing process control; however, they provide little information on the fundamental * Electrochemical Society Active Member.Oxide Fig. 1. The result of a blanket tungsten deposition is shown in the top sketch. The tungsten has been deposited in the vias opened in the inter-level dielectric, but is also present as a blanket film on the surface. The excess tungsten has been polished back to the interlevel dielectric in the bottom sketch.tungsten removal mechanisms that occur during polish. Clearer understanding of the removal mechanism(s) will benefit next-generation designs of slurries and pads and will improve W CMP manufacturing processes.In this work we investigate the role of tungsten oxidation and passive film formation in the mechanism of tungsten removal during CMP, by comparing measurements of the electrochemical behavior of the CVD tungsten film with tungsten removal rates obta...

Dissolution kinetics of WO3 in acidic solutions

“…Among the most widespread applications of CMP is the planarization of tungsten (W) and interlayer dielectric (IDL). In the case of W, it has been proposed that the CMP process proceeds via successive steps of surface oxide (passivating layer) formation and oxide removal [2], although the detailed mechanisms remain unresolved [2][3][4][5][6][7][8][9]. As device features continue to shrink, the need to control the metal removal process at the nanometer scale becomes critical.…”

“…In the hydrometallurgical extraction literature there are some reports on the chemical dissolution behavior of WO 3 (s) in alkaline solution [18,19]. In contrast, no comparable information is available for acidic solution, although the stability of this oxide in low pH environments is important for CMP as well as other technologically important processes [1][2][3][4][5][6][7][8][9]20].…”

Selection of an oxidant for copper chemical mechanical polishing: Copper-iodate system

“…These models, however, have been mainly concerned with the mechanical aspects of the process. The chemical effects, especially for metal CMP, also play an important role [2,3,[9][10][11][12][13][14][15]. During the metal CMP process, the metal oxidation is driven by the reduction of an oxidant (cathodic reaction) on the metal surface; at steady-state, anodic current ¼ cathodic current.…”

Key Chemical Components in Metal CMP Slurries

“…Alumina and periodic acid based slurries for W CMP have been studied in detail by Hetherington and coworkers [35][36][37]. The impact of process parameters, slurry pH, abrasive and oxidizer concentrations, and also the impact on the process temperature were investigated.…”