Copper metallization in supercritical carbon dioxide: Applications and kinetics
Copper is the material of choice for advanced interconnects due to its low electrical resistance and superior electromigration resistance. However, as feature sizes are reduced in successive generations, the difficulty of filling high aspect ratio feature increases rapidly. Supercritical Fluid Deposition (SFD) is a promising hybrid technique for single step conformal metal deposition that is extendable to sub-45 nm device structures. The approach involves the reduction of organometallic compounds in supercritical media to yield high purity conformal films. High precursor concentration and low viscosity in supercritical fluids prevent mass transport limitations and promote excellent step coverage of very narrow features. The absence of surface tension in the supercritical fluids enables the complete wetting of complex surfaces. Moreover, since the effluent of the SFD often contains only CO2, hydrocarbons, and H2, this technique offers considerable environmental advantages.
In this work, a recirculating equilibrium system is presented for measuring organometallic compounds solubilities in supercritical carbon dioxide. Solubility of nickelocene, bis(2,2,6,6-tetramethyl-3,5-heptanedionate)copper, [Cu(tmhd) 2], and bis(2,2,7-trimethyloctane-3,5-dionato)copper, [Cu(tmod) 2], were obtained at different temperatures and pressures. The solubility results confirm high precursor concentrations, a distinct advantage in SFD process over vapor phase metallization techniques.
Furthermore, the versatility and effectiveness of the SFD process is demonstrated by employing novel precursors for Cu deposition. Different type of reaction chemistries were tested and compared to demonstrate the SFD process for Cu metallization.
Moreover, a revolutionary interfacial adhesion promotion method was developed and tested for Cu metallization. A thin poly(acrylic acid) layer (3.5 nm) was found to dramatically enhance the interfacial adhesion of Cu films to substrates with various barrier layers, including TaN, TiN and Ta.
While the utility of SFD for Cu metallization has been well demonstrated, the kinetic pathways and the mechanism of the process have not previously been studied in detail. In this dissertation, the first kinetic study of Cu metallization by hydrogen reduction of Cu(tmod)2 in supercritical carbon dioxide is reported. Film deposition rates in the temperature range of 220°C to 270°C, as a function of the relevant experimental parameters including precursor and H2 concentration, by-product concentration, and system pressure, were investigated. The implication of these results for mechanistic pathways for Cu deposition in CO2 is discussed. A rate expression for film growth rate in terms of system control parameters is proposed.