Planarization of Copper Thin Films by Electropolishing in Phosphoric Acid for ULSI Applications

Padhi, D.; Yahalom, J.; Gandikota, Srinivas; Dixit, G.

doi:10.1149/1.1523415

Cited by 62 publications

(54 citation statements)

References 14 publications

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“…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.…”

Section: Introductionmentioning

confidence: 99%

“…While local planarization occurs through the action of the electropolishing electrolyte, global planarization is more difficult. One common approach to global planarization is the use of a rotating wafer/ electrode configuration (RDE) for simultaneous Cu removal across the wafer surface [1,2,5]. 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.…”

Section: Introductionmentioning

confidence: 99%

“…Despite intensive investigation, the identity of the solution phase species involved in the rate-determining step remains controversial, with some groups proposing phosphate-containing species and others water [5,7,10,12]. Such knowledge may be important for commercial application to damascene processing of Si wafers, since this will determine the removal rate that is obtained.…”

Section: Introductionmentioning

confidence: 99%

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Water diffusion coefficients during copper electropolishing

Suni

2004

Journal of Applied Electrochemistry

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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.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Water diffusion coefficients during copper electropolishing

Suni

2004

Journal of Applied Electrochemistry

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“…In certain systems, the chemical complexation of metal ions with the electrolyte and water molecules will support phase separation and the formation of a viscous liquid phase (viscous layer) adjacent to the anode. [2][3][4][5][6][7][8] The layer of viscous liquid serves as a barrier to the diffusion of ions from the surface of the anode and vice-versa, thereby reducing the efficiency of the electropolishing process. The density of the viscous liquid is assumed to be similar to the density of the electrolyte solution.…”

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confidence: 99%

Editors' Choice—The Enhancement of Ion Transport in an Electrochemical Cell Using High Frequency Vibration for the Electropolishing of Copper

Dubrovski

Tietze

Zigelman

et al. 2018

J. Electrochem. Soc.

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A previous experiment showed that the rate of the electropolishing of a copper anode may be increased by twofold when generating a 60 KHz to 1.7 MHz frequency vibration in the anode. In this work we use theory to elucidate the mechanisms by which the vibration may enhance the transport of ions in the electrolyte solution and support the formation of dents in the anode, which was observed in experiment. We find that in the limit of weak ion convection the transport of ions mainly supports the formation of dents in the anode. However, in the limit of prominent ion convection we find an appreciable contribution of the vibration to the efficiency of the electropolishing process, in accordance with the previous experimental findings. The contribution of the vibration to ion transport is given by 2 √ PeDkC s /π √ π, in which the Peclét number, Pe, quantifies the ratio between the convective and diffusive fluxes of ions, and D, k, and C s are the diffusion coefficient of the ions, the wavenumber of the vibration, and the solubility limit of the ions in the electrolyte. Electropolishing is a well-known technique used to polish metal surfaces by employing electrochemistry.1 The most basic electropolishing system is a classic electrochemical cell, comprising an anode and a cathode in an electrolyte solution, as depicted in Fig. 1. The application of an electrical potential difference between the electrodes will decompose the surface of the anode -the metallic object to be polished -to metal ions that dissolve in the electrolyte solution. Sharp corners on the metal anode dissolve to ions faster than flat areas. Hence, the electropolishing process renders the surface of the anode smooth. In certain systems, the chemical complexation of metal ions with the electrolyte and water molecules will support phase separation and the formation of a viscous liquid phase (viscous layer) adjacent to the anode.2-8 The layer of viscous liquid serves as a barrier to the diffusion of ions from the surface of the anode and vice-versa, thereby reducing the efficiency of the electropolishing process. The density of the viscous liquid is assumed to be similar to the density of the electrolyte solution.It was previously found that exciting a high frequency (60-1700 KHz) mechanical vibration in the anode may enhance the efficiency of the electropolishing process by two-fold. 9,10 However, it appears that the vibration further supports the formation of dents on the surface of the anode. The experimental system investigated previously is depicted in Fig. 1. It is an electrochemical cell, which is comprised of copper electrodes, a power supply, and an electrolyte solution containing water, concentrated phosphoric acid, and isopropyl alcohol. A piezoelectric transducer generates vibration in the anode. Upon the application of an electrical potential difference between the electrodes, which initiates the electropolishing process, a viscous layer of a thickness of about 1-2 mm forms adjacent to the electrode, as depicted in Fig. 2. The viscous layer is...

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“…[3][4][5][6] Our group also demonstrated a one-additive ͑organic acid species͒ EP electrolyte, a so-called superpolishing electrolyte, which can improve the planarization-efficiency ͑PE͒ of Cu-EP. 7 This one-additive EP electrolyte provides the functionality of a selective Cu dissolution rate within damascene features.…”

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confidence: 99%

Two-Additive Electrolytes for Superplanarizing Damascene Cu Metals

Liu¹,

Shieh²,

Chih³

et al. 2005

Electrochem. Solid-State Lett.

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Two-additive electropolishing ͑EP͒ electrolytes exhibit an extremely high planarization-efficiency in Cu damascene schemes, independent of pattern sizes ͑1-50 m͒. This electrolyte is demonstrated by adding alcohols and organic acids to the H 3 PO 4 electrolyte. The high wetting ability of alcohols allows such additives to easily access the damascene bottom. This mechanism, assisted by the reduced polishing rate associated with the high surface viscosity caused by alcohol additives, greatly passivates the damascene bottom from elecropolishing. Accordingly, the superpolishing functionality reported by Chang et al. ͓Electrochem. Solid-State Lett., 6, G72 ͑2003͔͒ in the two-additive electrolyte outperforms in the one-additive electrolyte.Cu damascene schemes continue to dominate the next generation of interconnect preparation in the integrated circuit ͑IC͒ industry as feature sizes are scaled down below severaltens of nm. Chemical mechanical polishing ͑CMP͒ of copper and barrier metals is the current method for defining a planarized interconnection area. Recently, due to the introduction of fragile low-k films ͑such as porous dielectrics͒ as intermetal dielectrics ͑IMD͒, minimization of the mechanical components from the Cu-CMP becomes more important. 1,2 Copper electropolishing ͑Cu-EP͒ has recently been introduced to replace Cu-CMP because of its stress-free nature ͑no mechanical force component͒, high polishing rate, low volume of waste stream, no requirement of abrasives and no scratching.Many studies have reported the feasibility of Cu-EP for planarizing Cu damascenes. [3][4][5][6] Our group also demonstrated a one-additive ͑organic acid species͒ EP electrolyte, a so-called superpolishing electrolyte, which can improve the planarization-efficiency ͑PE͒ of Cu-EP. 7 This one-additive EP electrolyte provides the functionality of a selective Cu dissolution rate within damascene features. However, the PE of superpolished Cu still depends on the feature size; the best value is ϳ34% for 50 m patterns. With reference to two ͑or three͒-additive electroplating electrolytes, 8,9 the introduction of different EP functionalities from the various additives is a possible solution. Wetting additives for Cu electroplating, like polyethylene glycols ͑PEG͒, alcohols were found to access easily the reacting surfaces in EP. 10,11 Moreover, the solution viscosity for alcoholcontaining electrolytes is higher than for alcohol-free electrolytes. 12 Because Cu dissolution is suppressed by increased solution viscosity near polished interfaces, using alcohol-containing EP electrolytes for planarizing damascene features leads to numerous alcohol additives at the damascene bottom, thus passivating that area from EP. This proposed inhibitor, assisted by the superpolishing functionality generated by the second additive of organic acids, 7 hence constitutes the formula of two-additive EP electrolytes.This paper demonstrates an extremely efficient two-additive EP electrolyte, which consists of organic acid and alcohols in the conventional Cu EP e...

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Planarization of Copper Thin Films by Electropolishing in Phosphoric Acid for ULSI Applications

Cited by 62 publications

References 14 publications

Water diffusion coefficients during copper electropolishing

Water diffusion coefficients during copper electropolishing

Editors' Choice—The Enhancement of Ion Transport in an Electrochemical Cell Using High Frequency Vibration for the Electropolishing of Copper

Two-Additive Electrolytes for Superplanarizing Damascene Cu Metals

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