Water diffusion coefficients during copper electropolishing

Abstract

Cu electropolishing was studied using a rotating disc electrode in a variety of phosphoric acid-based electrolytes, including several with ethanol and other species added as diluents. Diluents allow a wider range of water concentrations and electrolyte viscosities to be accessed and also reduce the removal rate during Cu electropolishing, simplifying possible application to damascene processing. Transient and steady state currents in the mass transfer limited regime are shown to depend on both the number of water acceptor molecules associated with each dissolving Cu ion and on the effective diffusion coefficient of water. Transient analysis samples the bulk transport properties, whereas steady state analysis integrates them through the diffusion layer. Assuming that the effective diffusion coefficients appropriate to transient and steady state behavior are the same, about one water molecule is associated with each dissolving Cu ion. This analysis yields effective diffusion coefficients for water on the order of 10 )9 cm 2 s )1 . However, the data is also consistent with an assumption that six water molecules are associated with each dissolving Cu ion, but the effective diffusion coefficient appropriate for a Levich analysis is somewhat lower than that in the bulk electrolyte. This analysis yields effective diffusion coefficients for water on the order of 10 )8 -10 )7 cm 2 s )1 . The latter interpretation, that six water molecules are associated with each dissolving Cu ion, appears more likely since it provides almost exact agreement with the effective diffusion coefficient reported previously by Vidal and West. In combination with previously published impedance results ruling out a salt film mechanism, the good agreement between the transient and steady state analyses confirm that water is the acceptor species that complexes dissolving Cu ions in phosphoric acid-based electropolishing baths.

Effects of viscosity-dependent diffusion in the analysis of rotating disk electrode data

“…The linear fit has an R 2 value of 0.94. Using the slope of the linear regression, the effective diffusion coefficient (assuming s A = 6) is 5.3 · 10 -8 cm 2 s -1 which is similar to previously reported values [6,8,9]. However, over the range of data collected, a curvature results when a regression analysis is performed suggesting additional factors exist which are not captured by the Levich model.…”

“…As proposed elsewhere [6,8,9], water is assumed to be the limiting acceptor species in the removal mechanism. With this assumption the water concentration should approach zero near the surface.…”

Uso de eletrodos de cobre e eletrodos modificados como sensores eletroquímicos

“…O comportamento eletroquímico do cobre em soluções aquosas tem sido investigado, sendo dispensado interesse especial à elucidação do mecanismo de formação de depósitos superficiais de óxidos de cobre para estudos da proteção anódica, que tem como objetivo evitar fenômenos de corrosão [84][85][86]112]. O estudo da formação do filme sobre a superfície do eletrodo está usualmente relacionado com a estrutura morfológica do filme protetor, da cinética de formação e da sua dissolução [105].…”

“…O estudo do comportamento eletroquímico do cobre em soluções contendo íons fosfato foi direcionado fundamentalmente ao processo de eletropolimento em soluções concentradas de ácido fosfórico [112,[114][115][116][117]. Apesar da utilização do ácido fosfórico em vários processos em meio aquoso, ainda há controvérsias quanto à identidade das espécies formadas e mecanismos de reação [94,112,[115][116] Estes resultados podem ser explicados em função da formação de espécies solúveis de cobre e da passivação causada pelo crescimento do filme anódico [37,94,105,[136][137][138].…”

“…Apesar da utilização do ácido fosfórico em vários processos em meio aquoso, ainda há controvérsias quanto à identidade das espécies formadas e mecanismos de reação [94,112,[115][116] Estes resultados podem ser explicados em função da formação de espécies solúveis de cobre e da passivação causada pelo crescimento do filme anódico [37,94,105,[136][137][138]. …”