Potential Oscillations in Galvanostatic Cu Electrodeposition

Article pubs.acs.org/JPCC

Potential Oscillations in Galvanostatic Cu Electrodeposition: Antagonistic and Synergistic Effects among SPS, Chloride, and Suppressor Additives Nguyen T. M. Hai,† Jan Odermatt,† Valentine Grimaudo,† Karl W. Kram ̈ er,† Alexander Fluegel,‡ Marco Arnold,‡ Dieter Mayer,‡ and Peter Broekmann*,†,‡ †

Department of Chemistry and Biochemistry, University of Bern, Freiestr. 3, 3012 Bern, Switzerland BASF SE, Global Business Unit Electronic Materials, 67056 Ludwigshafen, Germany

‡

S Supporting Information *

ABSTRACT: Polymerizates of imidazole and epichlorohydrin (Imep) serve as one of the benchmarks for today's chemistry development of leveler additives in context of the industrial copper Damascene process. We therefore studied the synergistic and antagonistic interplay of the Imep polymer with other additives, commonly present in copper plating baths used for the state-ofthe-art IC manufacturing. Characteristic oscillations in the applied electrode potential appear in galvanostatic copper electrodeposition when Imep is used in combination with SPS (bis(sodium sulfopropyl) disulfide). We identified the reversible Cu(I) coordination chemistry of the Imep polymer as a second prospective driving force beyond interfacial anion/cation pairing toward the formation of such suppressor/leveler ensembles at the interface. OH groups of the pristine Imep polymer coordinate with H2O-Cu(I)-MPS units (primary effect) that appear as side products of the copper electrodeposition in the presence of SPS. The latter transforms during copper deposition into monomeric MPS (mercaptopropanesulfonic acid/sulfonate) as result of the adsorptive SPS dissociation on the copper surface. Electrostatic coupling between the anionic sulfonate of the MPS and the cationic imidazolium group in the formed linear, bidentate Imep-Cu(I)-MPS complex results into a neutral, hydrophobic species that finally precipitates (secondary effect). The presence of diamagnetic Cu(I) species in those precipitates is proven by elementary analysis in combination with magnetic SQUID measurements. The observed potential oscillations under galvanostatic conditions are discussed in terms of an alternating precipitation and dissolution of the Imep-Cu(I)-MPS suppressor ensemble at the copper/electrolyte interface. Linear sweep experiments prove a partially hidden, N-shaped negative differential resistance (HN-NDR) as physical origin for the observed instabilities under galvanostatic conditions. SIMS (secondary ion mass spectroscopy) depth profiling of copper films deposited under such oscillatory conditions reveals periodic modulations in the contamination level parallel to the surface normal. Cross-sectional FIB analysis of the grown copper deposit reveals periodically repeating lines of grain boundaries in the copper deposit.

1. INTRODUCTION Copper electroplating is of vital importance for today′s mass fabrication of logic and memory devices1,2 and their future three-dimensional integration.3 The bottom-up fill of vias and trenches is typically achieved by the use of particular plating additives4 which regulate the accelerated copper deposition at the feature bottom with respect to the suppressed copper deposition on the wafer surface. State-of-the-art bottom-up fill of Damascene features relies on two-component additive packages consisting of so-called type I suppressors (polarizers) and their specific antagonists (accelerators, depolarizers).5 Their nonuniform surface coverage inside and outside the features is considered as physical origin for the differential copper deposition © 2012 American Chemical Society

rate inside and outside the feature as a crucial prerequisite for such a superfill phenomenon.6−9 Most common type I suppressor additives for the Damascene processing are poly(alkylene glycol)s4,10−12 (PAGs) which require halides as essential coadditives for their suppressing action.12−15 Cu(I) appearing as intermediate species of the copper reduction is discussed as further essential constituent of these suppressor ensembles.14 SPS (bis(sodium sulfopropyl) disulfide) is added to those plating formulations as specific antagonist Received: October 5, 2011 Revised: February 28, 2012 Published: March 13, 2012 6913

dx.doi.org/10.1021/jp2096086 | J. Phys. Chem. C 2012, 116, 6913−6924

The Journal of Physical Chemistry C

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action to the Cu(I) coordination mode depending on the particular chemical environment at the interface. We will demonstrate that it is actually the reversible Cu(I) coordination chemistry of this Imep polymer in combination with the SPS surface chemistry that introduces an oscillatory instability into the interfacial suppressor ensembles under galvanostatic conditions. Such nonlinear phenomena are wellknown for numerous metal dissolution and deposition processes under electrochemical conditions.29 A lot of progress has been made within the past two decades in developing mathematical models describing spatiotemporal pattern formation that are typically associated with this kind of temporal instability.30−32 Diagnostic tools and classification schemes are available in the literature helping to categorize the new electrochemical oscillator system under investigation.29,32 A prime example for such an oscillatory behavior in context of copper electrodeposition is the formation of nanoscale multilayers of Cu2O and mixed Cu/Cu2O composites from an alkaline Cu(II) lactate solution.33 The observed potential modulations in the deposition experiment correlate with oscillatory changes in the resulting film composition and morphology along the growth direction. A characteristic feature of this kind of electrochemical oscillators is the massive inclusion of those passive films into the growing deposit as a crucial prerequisite for the formation of lamellar nanoscale Cu/ Cu2O systems.33 Nakanishi et al. report similar potential oscillations for galvanostatic copper deposition processes from o-phenanthroline-containing Cu(II) solutions as a prime example for acidic experimental conditions.34 Cross-sectional SEM inspection of the grown copper film also revealed a periodic change of the film morphology parallel to the surface normal in accordance with the potential oscillation seen in the respective potential transient curve.34 In the present study we discuss an electrochemical oscillator system which is similar to the one reported by Nakanishi et al. insofar as our suppressor chemistry also relies on a Cu(I) coordination chemistry. It will be demonstrated, however, that the Imep suppressor chemistry shows a considerably less pronounced tendency toward consumptive incorporation into the copper deposit upon growth which facilitates a fast postdeposition recrystallization. A molecular-level understanding of such oscillatory electrodeposition processes is still lacking for many of the systems described in the literature. This is because there is no direct experimental access to the active suppressor ensemble that often appears under reactive conditions only. Intermediates of those deposition or dissolution processes often serve as coadditives of the resulting suppressor ensemble at the interface. Their transient appearance under reactive conditions makes it often difficult to isolate and to characterize them under ex-situ conditions. In this current contribution we report an experimental approach toward synthesis and isolation of an active suppressor ensemble. These beaker-scale experiments are the prerequisite for a detailed compositional and (future) structural analysis of the Imep-Cu(I)-MPS coordination polymer as the active suppressor component. Our paper is structured as follows: The first part is dedicated to the electrochemical characterization of those Cu(I)− suppressor adducts particularly focusing on the nonlinear phenomena that appear during galvanostatic copper electrodeposition. This section is followed by a combined SIMS and

(depolarizer) of the PAG-Cu(I)-Cl suppressor ensemble. However, the actual antagonist (depolarizer) is the chemisorbed or the free MPS (mercaptopropanesulfonic acid/sulfonate) that forms at the surface in the course of the adsorptive SPS dissociation upon copper deposition.5 SERS studies on the accelerator adsorption on copper by Schultz et al. are lacking any S−S specific feature of the SPS, suggesting a dissociative adsorption of SPS on Cu.16 An undesired side effect of the successful superfill is the sustained copper deposition after the feature is filled (momentum plating; overplating). Bumps appear over isolated features after the successful feature fill.9,17,18 Their coalescence over dense arrays of Damascene features (mounding) is known to cause tremendous problems in the CMP (chemical mechanical polishing).19 A third component is therefore commonly added to those plating formulations that is expected to level out these undesired height modulations on the wafer surface.19 From an electrochemical point of view these leveling agents are constituents of another type of suppressor ensemble that cannot be deactivated by the MPS. This kind of additive has been denoted in ref 5 as type II suppressor. Such suppressors with leveling capabilities typically behave indifferent to the chemical composition and structure of the anionmodified copper surface. Interfacial anion/cation pairing between either chemisorbed chloride or the sulfonic head groups of the coadsorbed MPS and the covering (poly)cationic leveler additives is discussed as one the physical origins of such nonselective interaction with the copper surface besides simple hydrophobic effects.5,20 Classical concepts of leveling typically rely on mass transfer limitations of those additives in combination with their consumptive inclusion into the growing deposit.21−23 Kim et al. discuss an alternative leveling mechanism in context of the Damascene electroplating that relies on a specific deactivation of the accelerating SPS by the leveler additive.20,24 Most recent leveling concepts that are already introduced into the commercial practice of today's Damascene electroplating19 make particular use of additives which get more selectively active over Damascene features following the bottom-up fill. Such selectivity is indicative for a more specific interaction of those leveler additives either with the chemically modified copper surface or with intermediates of the copper deposition whose concentrations are locally increased after the superfill. As already pointed out by Reid et al.,19 our knowledge of the underlying leveler mode of action and the synergistic interplay with other coadditives is still rather poor. In this current contribution we will discuss Cu(I) coordination chemistry as a promising alternative concept beyond such interfacial anion/cation pairing. This approach might also explain the observed chemical selectivity. Prototypical model systems for such a Cu(I)-based coordination chemistry are polymerizates of imidazole and epichlorohydrin,25−28 referred hereafter as Imep (Figure 1).

Figure 1. Reaction scheme indicating the synthesis of Imep polymers.

What makes this chemistry unique is the fact that the Imep is capable to switch from a pure anion/cation pairing mode of 6914



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Cross-sectional analysis of the grown copper deposit was performed after FIB (focused ion beam) treatment using a dual beam FIB instrument (FEI, type 875 at IFOS, Kaiserslautern, Germany). The FIB cut was done perpendicular to the wafer coupon surface with a 30 keV Ga ion beam. Cross-sectional FIB micrographs were taken under an angle of 52° with respect to the surface normal. SQUID (superconducting quantum interference device) techniques were used to discriminate between diamagnetic Cu(I) and paramagnetic Cu(II) in the synthesized Cu(I) suppressor adducts. The magnetic susceptibility was measured on a Quantum Designs MPMS SQUID-XL between 10 and 300 K in fields of 0.05 and 0.5 T for paramagnetic and diamagnetic samples, respectively. Samples were prepared in gelatin capsules. Measurements were done in dc mode for a scan length of 5 cm and 3 scans for each temperature. The chemical composition of the Cu(I) precipitates was analyzed at BASF SE by means of atom absorption spectroscopy (AAS) for the Cu content in combination with a standard incineration train coupled with heat conductivity as well as infrared and luminescence detection for the determination of the C, S, O, N, and H content (basic elementary analysis). The chloride content was determined after the incineration treatment as HCl via standard titration with AgNO3. In order to identify Cu(I) and Cu(II) species in solution, UV−vis spectroscopy was applied using a Lambda 900 spectrometer (Supporting Information).

cross-sectional FIB analysis of the grown copper films relating the electrochemical characteristics to the resulting film properties. The third part of the paper is an attempt to rationalize the oscillatory behavior of the Imep chemistry in terms of a reversible formation and degradation of Imep-Cu(I)MPS suppressor ensembles. To demonstrate this polymeric Imep-Cu(I)-MPS coordination, adducts are synthesized on a beaker scale followed by their isolation and characterization by elementary analysis and SQUID techniques (superconducting quantum interference device).

2. EXPERIMENTAL SECTION Chronopotentiometric (potential transient) and voltammetric measurements were performed using an Autolab potentiostat/ galvanostat (PGSTAT 128) that was connected to a standard rotating disk electrode (RDE) setup (Pine). Pt disks served as working electrodes (5 mm in diameter). The counter electrode (Pt wire) was housed in a glass compartment separated from the main electrochemical cell by a ceramic frit. All potentials given in the text refer to an Ag|AgCl|KClsat. reference electrode. Prior to each galvanostatic copper deposition experiment, the Pt working electrode was preplated with copper for t = 20 s at a nominal current density of J = −20 mA/cm2 from plating bath 2 (see below). By this we obtained a thin and shiny copper film serving as substrate for the subsequent plating experiments. Blanket wafers were purchased from Hionix. Our wafer specimens comprise a 100 nm top layer of Cu followed by a Ta layer of 25 nm thickness and a 500 nm TOX/SiO2 film on a 50 mm thick Si substrate. Wafer-coupon plating experiments were carried out in a 500 mL beaker glass. Convective transport of reactants and additives in the beaker glass was achieved by magnetic agitation. The following plating baths were used: • Bath 1: 10 g/L H2SO4 (10.42 g/L H2SO4 96%, Merck, suprapur) and 40 g/L Cu2+ (157.2 g/L CuSO4·5H2O, Sigma-Aldrich). Bath 1 represents an additive-free plating solution. • Bath 2: 10 g/L H2SO4 (10.42 g/L H2SO4 96%, Merck, suprapur); 50 ppm chloride (186.8 mg/L HCl 30%, Merck, suprapur) and 40 g/L Cu2+ (157.2 g/L CuSO4·5H2O, Sigma-Aldrich). Bath 2 represents a chloride-containing plating solution free of other additives. SPS (bis(sodium sulfopropyl) disulfide, Raschig, Ludwigshafen, Germany) was purified by recrystallization before use. MPS (mercaptopropanesulfonic acid sodium salt) was used for the beaker-scale precipitation experiments as provided by Raschig without further purification. The Imep suppressor additive (polymerizate of imidazole and epichlorohydrin) was provided by BASF SE Electronic Materials (Ludwigshafen, Germany). The contamination level in the copper deposit was characterized by means of SIMS (secondary ion mass spectroscopy) depth profiling. Cs+ cations with a kinetic energy of Ekin = 14.5 keV served as primary ions. The sputter current density was determined to be J = 0.38 mA/cm2. The analyzed area on the wafer surface was 60 μm in diameter. The angle of incident was 24° off the surface normal. For the depth profile analysis of the copper deposits the following negative secondary ions were detected: Cu−, CN−, O−, Cl−, S−, and C−. The quantification of the SIMS data was achieved on the basis of copper standards provided by the IBM T.J. Watson Research Center (Yorktown Heights, NY).

3. RESULTS 3.1. Copper Deposition under Oscillatory Conditions. A first set of chronopotentiometric measurements was conducted to unravel synergistic and antagonistic effects among SPS, chloride, and Imep during copper electroplating. Addition of Imep (final Imep concentration: cimep = 109 mg/L) to the SPS-free but chloride containing plating solution at a nominal current density of J = −5 mA/cm2 causes an increase of the overpotential by about ΔE = −200 mV, thus pointing to a strong suppressing action of the Imep polymer with respect to the copper deposition (curve 1 in Figure 2a). These polycationic polymers typically form diffusional barriers at the interface, thereby physically limiting the transport of cupric ions to the electrode surface. Dosing 2 ppm SPS to the solution causes a reproducible, transient increase of the overpotential (curves 2 and 3 in Figure 2a; 3300 s > t > 3000 s), pointing to a synergistic interaction of the Imep with SPS, at least in the initial phase of the SPS/Imep interaction. Higher dosages of SPS (+ 20 ppm) initiate at t > 4000 s regular, strictly periodic oscillations in the deposition potential. This temporal instability involves most likely an alternating dissolution and precipitation of the Imep suppressor ensemble at the interface (see Discussion section). SPS and Imep are both identified as being essential to induce such an oscillatory behavior besides the Cu(II) that serves as charge carrier for the faradaic process. By varying the Imep/SPS ratio at constant current density, we can fine-tune the frequency of the potential oscillation. Low-frequency oscillations typically appear at high ratios of the Imep/SPS content (curve 2 in Figure 2a). In turn, high-frequency oscillations are observed at lower ratios of the Imep/SPS content (curve 3 in Figure 2a). Passing a critical threshold in the Imep/SPS ratio lets the potential oscillation completely disappear (Figure 2b). In the context of the Damascene copper plating the dimeric SPS is often discussed as the precursor of the monomeric MPS 6915



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(mercaptopropanesulfonic acid/sulfonate) as the actual antisuppressor additive (depolarizer) for the PAG. A first experimental hint that points to free MPS acting as a depolarizer (antisuppressor) of the Imep suppressor ensemble comes from the potential transient experiment presented in Figure 2c. Increasing amounts of free MPS were dosed to the Imep containing plating bath. Comparison of Figures 2b and 2c clearly evidences the monomeric MPS as the additive that impacts the oscillatory behavior much more heavily than the dimeric SPS (MPS precursor). Equivalent amounts of MPS and SPS lead to potential oscillations that are obviously higher in frequency in case of the MPS. Similar to SPS also the MPS causes a transition from an oscillatory behavior into a stable steady-state when a critical threshold in the MPS concentration is exceeded (total dosage >70 ppm; Figure 2c). Quite in contrast to the SPS, however, we observe a potential oscillation which spontaneously starts again after a certain plating time without any change of the external parameters (highlighted in Figure 2c). As already described by Healy et al.,35 the free MPS is not stable in Cu(II) containing solutions. MPS transforms back into the dimeric SPS leaving Cu(I) species behind.35 It is this slow dimerization reaction which causes a drop-down of the actual MPS concentration with time. Falling below such a critical threshold in the MPS concentration for a given Imep content lets the oscillation start again. This effect is highlighted in Figure 2c. A further additive impacting the oscillatory behavior is the chloride. While Imep and SPS are both considered as essential, the chloride is not capable to initiate such a temporal instability in combination with Imep (curve 1 in Figure 2a). We therefore consider the chloride as a nonessential species whose presence, however, impacts the interplay between SPS and Imep. Increasing amounts of chloride let the frequency of the potential oscillation stepwise raise going along with a decline in amplitude (Figure 2d). This behavior suggests a chloride-induced destabilization of the Imep suppressor ensemble at the interface (see Discussion section) similar to the observed action of the SPS and MPS. Exceeding a certain chloride concentration (cCl = 327 mg/L) for a given SPS content (cSPS = 50.5 g/L) causes a similar break-down of the oscillatory behavior as observed before for the SPS and the MPS, resulting in a steady-state potential that maintains by ΔE = −100 mV below the starting potential of the additive-free plating solution (Figure 2d). For a given set of reactant and additive concentrations regular, periodic potential oscillations of high amplitude occur within a narrow current density regime between J = −4 mA/cm2 and J = −7.5 mA/cm2 (Figure 3). These low current densities are well below the transport-limited regime of the copper deposition. The amplitudes of those periodic potential oscillations range from ΔE = 0.14 V at J = −4 mA/cm2 to ΔE = 0.17 V at higher current densities of J = −7.5 mA/cm2. More affected by the current density is the frequency of the potential oscillation that progressively drops down by increasing J until the oscillation completely disappears at J = −8 mA/cm2, at least on the time scale of 10 000 s (Figure 3a). Periods of the potential oscillation range from λ = 330 s at J = −4 mA/cm2 to λ = 2900 s at J = −7.5 mA/cm2. It should be noted, however, that the exact correlation between the applied current density J and the resulting oscillation periods λ also depends on the experimental starting conditions. The exact values of λ for a given current density J might scatter slightly while the overall trends described above are correct. Important for the current density dependence of the potential oscillation is the particular copper content in solution

Figure 2. (a) Chronopotentiometric measurement (J = −5 mA/cm2) identifying the SPS and the Imep polymer as essential additives for the oscillatory behavior. Curve 1: bath 2 + 109 mg/L Imep (t = 250 s). Curve 2: bath 2 + 109 mg/L Imep (t = 250 s) + 2.18 mg/L SPS (t = 3000 s) + 21.8 mg/L SPS (t = 4000 s). Curve 3: bath 2 + 10.9 mg/L Imep (t = 250 s) + 2.18 mg/L SPS (t = 3000 s) + 21.8 mg/L SPS (t = 4000 s). (b) Bath 1 + 109 mg/L Imep (t = 300 s) + SPS (1.09 mg/L at 2000 s; 5.45 mg/L at 4000 s; 21.8 mg/L at 7700 s; 54.5 mg/L at 10 200 s). (c) Bath 1 + 109 mg/L Imep (t = 300 s) + MPS (1.09 mg/L at 2100 s; 5.45 mg/L at 4700 s; 21.8 mg/L at 7000 s; 54.5 mg/L at 11 000 s). (d) Chronopotentiometric measurement (J = −5 mA/cm2) identifying chloride as a nonessential additive for the initiation of the oscillatory behavior which, however, impacts its frequency: bath 1 + 21.8 mg/L SPS (t = 250 s) + 109 mg/L Imep (t = 300 s) + Cl (1.09 mg/L at 2600 s; 5.45 mg/L at 3900 s; 54.5 mg/L at 10 000 s; 109 mg/L at 13 570 s; 218 mg/L at 16 000 s; 327 mg/L at 18 704 s). 6916



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Figure 4. Linear sweep experiments starting from the OCP of the copper film preplated on Pt-RDE. Curve 1: bath 2 (reference); curve 2: bath 2 + 21.8 mg/L SPS; curve 3: bath 2 + 109 mg/L Imep; curves 4 and 5: bath 2 + 21.8 mg/L SPS + 109 mg/L Imep. All curves were measured at sweep rates of dE/dt = 5 mV/s, except for curve 5 which was measured at dE/dt = 1 mV/s.

in terms of the intrinsic accelerating capability of the monomeric MPS that forms from the dimeric SPS precursor upon copper deposition by dissociative adsorption on the electrode surface.5,37,38 Such catalytic (depolarizing) effect of the formed MPS intermediate typically requires the presence of chloride in the plating bath while the SPS and MPS by their own act as mild suppressors.5,39,40 Inner-sphere electron transfer via chloride or sulfur bridges has been discussed as physical origin for the observed mild catalytic activity of the chloride or the chloride/MPS mixture.40,41 As demonstrated by curve 3 in Figure 4, Imep polymers strongly suppress copper deposition most likely due to their pure interfacial anion/cation-pairing mode of action. Such anion/cation-pairing of polycationic additives typically occurs through electrostatic coupling to the chemisorbed chloride anions that are omnipresent on the copper surface under the given experimental conditions.20,42−49 It is, however, the particular interplay between Imep and SPS which introduces an N-shaped characteristic into the J/Ework response (curves 4 and 5 in Figure 4) as a crucial mechanistic prerequisite for the occurrence of those temporal instabilities observed in our chronopotentiometric measurements (Figure 2).29,32 The potential regimes assigned to the NDRs are highlighted in Figure 4. We associate such characteristic altering of the overall shape of the linear sweep (curve 3 → curve 4) to a change from the pure interfacial anion/cation mode of suppression to the one that is governed by the Imep/Cu(I) coordination chemistry which interferes with the sophisticated SPS surface chemistry (see Discussion section).5,37,42,43 Sweep-rate-dependent measurements indicate an NDR that is less apparent under almost stationary experimental conditions at sweep rates of dE/dt = 1 mV/s (curve 5 in Figure 4) and below while the NDR is operating well on a faster time scale, e.g. at dE/dt = 5 mV/s (curve 4 in Figure 4) and above. This finding is clearly indicative for a negative differential resistance that is at least partially hidden. Origin of such a hidden NDR in the J/Ework characteristics is typically a further chemical reaction that takes

Figure 3. (a) Overview of the current density dependence of the potential oscillations: bath 2 + 21.8 mg/L SPS (t = 250 s) + 109 g/L Imep (t = 300 s). (b) Transition from a periodic into a damped potential oscillation at lower current densities: bath 2 + 21.8 mg/L SPS (t = 250 s) + 109 mg/L Imep (t = 300 s). Note: for the sake of clarity, the potential transients are shifted on the potential scale; the respective starting potentials are indicated.

(see Supporting Information). For higher copper concentrations (cCu = 60 g/L) we observed a much more extended range of current densities where potential oscillations occur than for lower copper concentrations. At cCu = 5 g/L only the high-frequency and damped oscillations were detected within a narrow range of low current densities. The physical origin of all these temporal instabilities is a socalled negative differential resistance (NDR)29,32 as evidenced in the corresponding J/Ework characteristic. Curve 1 in Figure 4 represents the linear sweep reference experiment in the absence of Imep and SPS. A slight increase of the reactivity is commonly observed when SPS is present in the electrolyte (curve 2 in Figure 4).9,12,36 This mild catalytic effect is typically rationalized 6917



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place at the interface parallel to the main faradic process.29 Such a hidden N-NDR is commonly denoted in the literature as HNNDR.29,32 Further support for our assumption of an HN-NDR comes from the experimental observation of an oscillatory behavior that remains restricted to the galvanostatic case with Ework as a free parameter. Plating experiments under strictly potentiostatic conditions, in contrast, do not show current oscillations at any ratio of the Imep, SPS, and chloride concentrations unless an extra ohmic resistance is inserted into the external circuit in series to the RDE working electrode (see Supporting Information).32 Note that the intrinsic ohmic resistance of a wafer coupon with a 100 nm Cu seed layer might be sufficiently high enough in order to initiate such a temporal instability even under strictly potentiostatic conditions (see Supporting Information). 3.2. Characterization of the Copper Deposits by SIMS and Cross-Sectional FIB. One preliminary hypothesis that might rationalize at least qualitatively the observed temporal instabilities is based on an oscillatory breakdown of the suppressor ensemble at the interface. From a mechanistic point of view such a degradation of the interfacial suppressor ensemble could rely either on a potential-dependent, massive incorporation into the deposit upon growth (consumptive degradation), or alternatively on its coordinative dissolution by complexing ligands that might form as intermediate species in a side reaction to the main faradaic process. The latter degradation pathway is based on the constraint of suppressor ensembles whose structures are indeed based on coordination chemistry and not solely on interfacial anion−cation pairing effects. A consumption of plating additives under oscillatory conditions might leave deposits behind with modulations in both their chemical composition and their film morphology as exemplified for the Cu/Cu2O33 and the Cu/o-phenanthroline34 systems. Figure 5 indeed demonstrates a correlation between the oscillatory behavior of the potential in the galvanostatic wafercoupon plating and an oscillation in the contamination level perpendicular to the growth direction. SIMS depth profiling shows an almost in-phase modulation of the N, C, S, Cl, and O contents (Figure 5b) with progressive erosion depth. Slight upward shifts of the Cl peak-centroid positions are most likely related to the sequential data detection in combination with different counting rates of the respective CN, C, S, Cl, and O signals. What is not exactly reproduced by our SIMS data, however, is the absolute number of maxima and minima in the respective chronocoulometric curve. Because of a strong attenuation of amplitudes in the SIMS oscillation with increasing erosion depths, only 10 oscillation periods are resolved (Figure 5b) while the corresponding potential transient reveals 16 periods in total (Figure 5a). At least for the first 10 oscillation periods we observe a period length that remains almost constant with d = 450 ± 30 nm. Such attenuation in the SIMS intensities is a well-known phenomenon, described for instance for the depth profiling of subnm thick boron delta layers periodically implanted into Si specimens which commonly serve as standards for the evaluation of the SIMS depth resolution.50 This effect is typically ascribed to a sputter-induced surface roughening and layer intermixing upon erosion that typically goes along with an increase of the FWHMs (full widths at half-maximum) of SIMS peaks with increasing crater depths.50,51 Such an artificial

Figure 5. (a) Copper electrodeposition on wafer coupon (A = 4.9 cm2) at J = −3 mA/cm2. The plating experiment was performed with bath 2 + 21.8 mg/L SPS + 109 mg/L Imep. (b) SIMS depth profile indicating the oscillatory incorporation of contaminants into the copper film upon growth.

smoothing of the actual concentration gradients are known to scale with the kinetic energy of the primary ions and the angle of incidence upon collision with the surface.50 We therefore assume that the actual concentration gradients in the copper deposits are most accurately described by the first peaks in the depth profile while the amplitudes at higher crater depths are significantly damped by the mentioned SIMS-specific artifacts. In particular, the S, Cl, and N concentrations around first peak in the concentration profile change tremendously by up to 2 orders of magnitude. We consider these changes in the contamination level as clear experimental evidence for the periodic breakdown of the suppressor ensembles upon galvanostatic Cu electroplating. It should be stressed, however, that the overall contamination level in copper films plated with the Imep chemistry is considerably low compared to other leveler additives that commonly are used in industrial copper electroplating. Janus Green B (JGB),52−55 6918



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micrograph in Figure 6c corresponds to the 25.6 μm thick copper film plated under low-frequency conditions at J = −6 mA/cm2. Well reproduced in all three experiments in Figure 6a−c is the slight increase of the modulation period by going from the wafer coupon surface to the dielectric. Dimensions of individual crystallites seen in the FIB micrograph are clearly in the micrometer range, pointing to an effective recrystallization even at room temperature. It should be noted, however, that the appearance of clear delta layers in conjunction with a wellrecrystallized copper film is most apparent in copper films plated under low-frequency conditions. Copper films plated under high-frequency conditions (curve 1 in Figure 6a,b), by contrast, appear more homogeneous in the FIB micrographs (see Supporting Information). Both frequency of the particular potential oscillation and the potential range covered by the potential oscillation seem to have an impact in the resulting film morphology. We interpret the observed sharp deltas in Figure 6c as spatially confined copper layers, revealing a comparably high contamination level which prohibits the further coalescence of neighbored copper grains. We further associate these deltas to the maxima in the contamination level seen in the SIMS depth profiling (Figure 6b) and to the maxima of the overpotential observed in the chronopotentiometry (Figure 6a). An unambiguous correlation between the contamination level and the applied potential can be deduced from plating experiments where the deposition process was stopped at vertex potentials Emax and Emin of the oscillation as the two most prominent data points in the potential transient curves (Figure 7). The respective SIMS experiments show a characteristic shift in the contamination level which is exemplarily demonstrated in Figure 7c for the N contamination. Identical shifts are detected for the C, S, O, and Cl contaminations.

a prototypical monomeric leveler of low molecular weight, produces copper films with a contamination level that is significantly higher than the one reported in the present study (see Supporting Information). The leveling concept of JGB makes particular use of such a consumptive inclusion into the deposit in combination with mass transfer effects as crucial prerequisite for its mode of action, e.g., in context of the 3D-TSV copper plating.19,53,56 Electroplated copper films typically undergo a postdeposition self-annealing even at room temperature with an actual self-annealing rate that was discussed to depend on the overall contamination level in the copper deposit.19 In order to correlate the oscillatory behavior seen in the chronopotentiometry and the SIMS with the resulting film morphology, we performed complementary FIB experiments. Figure 6a shows two oscillatory deposition experiments conducted under high- and low-frequency conditions. Both films reveal different absolute thicknesses of 4.2 μm (1: highfrequency deposition; J = −4 mA/cm2) and 25.6 μm (2: lowfrequency; J = −6 mA/cm2). The total number of periods in both chronopotentiometric experiments was, however, chosen to be equal with n = 15. Our SIMS data clearly reproduce both the high- and low-frequency oscillations in the respective chronopotentiometric experiments. For our discussion we restrict ourselves to the C contamination (Figure 6b). While the number of SIMS peaks (Figure 6b) corresponds well to the respective number of periods in the low-frequency potential oscillation (Figure 6a), not all peaks are well resolved in the high-frequency case. In particular, at higher crater depths individual peaks merge into a single broad one (1 in Figure 6b). Careful inspection of the corresponding cross-sectional FIB micrograph reveals indeed an almost periodic array of horizontal grain boundaries separating well-recrystallized domains in the deposited copper film. The presented FIB

Figure 6. (a) Copper electrodeposition on wafer coupon (A = 4.9 cm2) at J = −4 mA/cm2 (1; high-frequency oscillation) and J = −6 mA/cm2 (2; low-frequency oscillation); the gray bar indicates the time window relevant for the 3D-TSV plating (see Discussion section). (b) Corresponding SIMS depth profile. (c, d) SEM micrograph of one of the copper films deposited under oscillatory conditions described in (a). 6919



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change from the blue Cu(II) hexaquo complex with 3d9 configuration (Figure 8a) to the yellow H2O-Cu(I)-MPS with 3d 10 electron configuration (Figure 8b). We used a combination of UV−vis and linear sweep experiments to prove the disappearance of Cu(II) in accordance with the appearance of Cu(I) (see Supporting Information). Addition of Imep to the H2O-Cu(I)-MPS containing solution leads to a spontaneous precipitation reaction (Figure 8c). No precipitates are formed, however, when this polymerizate is added to the Cu(II) solution, thus pointing to a clear preference of the Imep for Cu(I). Not only is the Cu(I) essential for such a precipitation but also the MPS ligand itself. This can be deduced from additional experiments where the Imep polymer was added to solutions where the Cu(I) was stabilized solely by chloride. No precipitates were obtained when these solutions were brought into contact with Imep. After complete removal of the precipitate by filtration we did not detect any Cu(I) in the remaining filtrate solution by means of UV−vis and linear sweep experiments (see Supporting Information). The dissolution of the precipitate is achieved only by addition of an excess of complexing ligands to the solution. An excess of chloride leaves an almost colorless solution behind most likely due to the formation of bidentate Cu(I) dichloro complexes or the respective mixed Cl-Cu(I)-MPS species (Figure 8d). Note this solution is sensitive to oxidation in air. Cu(I) transforms into bluish Cu(II) within a couple of hours. A yellow or reddish solution is obtained by exposing the precipitate to an MPS containing solution most likely due to the formation of the bidentate MPS-Cu(I)-MPS complex (Figure 8e). Even when exposed to air this reddish solution remains stable for weeks, pointing to an MPS-Cu(I)-MPS complex that is less sensitive to oxidation compared to the respective chloride analogues. Our beaker-scale experiments clearly prove the strong concentration dependence in the (reversible) appearance and disappearance of the polymeric Imep-Cu(I)-MPS suppressor adducts. An oxidative decomposition of the Cu(I) precipitate is observed when it is brought into contact with an H2O2 solution. The resulting solution has a blue appearance pointing again to Cu(II) in solution. For the further chemical and physical analytics the precipitate needs to be dried under vacuum conditions at p = 10−2 mbar for 12 h after filtration. An excess of residual water in the precipitate leads to its oxidative decomposition in air within a couple of hours (bluish/brownish appearance due to the presence of Cu(II)). In its dried form the precipitate remains stable in (dry) air even for weeks. The inset in Figure 8c shows the isolated and dried precipitate. The result of the elementary analysis is summarized in Table 1. Our data clearly identify both Cu and S as constituents of the precipitate. Their molar ratio of Cu/S = 1/1.68 almost matches the ideal one of 1/2 expected for the incorporation of structurally intact Cu(I) thiolate into the polymer. The high oxygen content exceeding the one expected for the presence of one sulfonate and one OH group in the repeating polymer unit points to some residual water that is still present in the precipitate even after the vacuum treatment. For the ultimate proof of Cu(I) species in the precipitate we combined such chemical analytics with magnetic measurements to discriminate between diamagnetic Cu(I) of 3d10 and paramagnetic Cu(II) species of 3d9 electron configuration.

Figure 7. Copper deposition on wafer coupon at J = −3 mA/cm2 stopped at the maximum of suppression (a) and at the minimum of suppression (b). (c) Corresponding SIMS depth profiles indicating a phase shift in the N contamination.

These SIMS data suggest a higher incorporation rate at higher overpotentials for a given current density. 3.3. Synthesis and Characterization of Imep-Cu(I)MPS Adducts. The most important mode of action of those polycationic Imep additives besides their interfacial anion/ cation pairing relies on the formation of polymeric Cu(I) adducts. Rather stable Cu(I) species are formed as intermediates in the course of the copper deposition in particular when strongly complexing ligands are present in the plating bath such as chloride or organic additives carrying thiol functionalities.36 It is known that monomeric MPS (mercaptopropanesulfonic acid/sulfonate) originates upon plating from the respective dimeric SPS precursor.5,16 The intermediates of the copper deposition might act as crucial coadditives for the action of suppressor additives. Our beaker-scale experiments are intended to mimic the interaction of Imep polymers with those Cu(I) intermediates that appear during the copper deposition in the presence of SPS at the copper/electrolyte interface. For this purpose we produced Cu(I) thiolate complexes via the homogeneous redox reaction of Cu(II) and MPS according to Healy et al.:35 2[Cu(II)(H2O)6 ]2 + + 4HS−(CH2)3 −SO3H → 2[Cu(I)(−S−(CH2)3 −SO3H)(H2O)] + 4H+ + HO3S−(CH2)3 −S−S−(CH2)3 −SO3H + 10H2O (1)

The resulting Cu(I) species are most likely linear H2OCu(I)-MPS complexes. It is well documented in the literature that Cu(I) ions favor linear, bidentate coordination.57 The redox reaction can be followed by the characteristic color 6920



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Figure 8. (a−c) Series of photographs indicating the transition of Cu(II) to Cu(I) followed by the precipitation of the polymeric Imep-Cu(I)-MPS complex: (a) solution of 10 g/L H2SO4 + 51.42 mg/L HCl + 8.04 g/L CuSO4; (b) solution of 10 g/L H2SO4 + 51.42 mg/L HCl + 8.04 g/L CuSO4 + 24 g/L MPS; (c) solution of 10 g/L H2SO4 + 51.42 mg/L HCl + 8.04 g/L CuSO4 + 24 g/L MPS + 1 g/L Imep. (d) Dissolution of the filtered precipitate with an excess of KCl. (e) Dissolution of the filtered precipitate with an excess of MPS.

Table 1. Elementary Analysis of the Precipitate in Figure 8ca

a

element

weight [g/100 g]

mol %

C H N O S Cu Cl

23.2 (±1%) 4.2 (±5%) 5.5 (±5%) 25.6 (±1%) 18.2 (±6%) 21.7 (±2%) 0.8 (±10%)

21 46 4 18 6 4 1

The error margins of the respective analysis are given in parentheses.

Results of temperature-dependent SQUID measurements are presented in Figure 9. We used CuSO4·5H2O as reference for Cu(II). Its paramagnetism gives rise to a Curie law hyperbola of the magnetic susceptibility χ with a steep increase of χ toward lower temperatures (curve 1 in Figure 9). The susceptibility curve of CuSO4·5H2O is clearly above zero while the one of the precipitate is well below zero within the entire temperature range (curve 2 in Figure 9). The constant negative magnetic susceptibility of curve 2 is due to purely diamagnetic contributions from the precipitate. This is in agreement with diamagnetic Cu(I) and proves the absence of any paramagnetic ions like Cu(II) in the precipitate. As further reference we added the susceptibility curve for MPS sodium salt (curve 3 in Figure 9). Again, it shows a purely diamagnetic behavior.

Figure 9. SQUID measurements proving the presence of diamagnetic Cu(I) in the precipitate. Curve 1: paramagnetic Cu(II) reference (m = 61.94 mg CuSO4·5H2O, H = 0.05 T). Curve 2: precipitate containing the diamagnetic Cu(I) (m = 17.61 mg, H = 0.5 T). Curve 3: MPS reference (m = 20.24 mg, H = 0.5 T). The inset shows an enlarged section of curves 2 and 3 in the temperature range between 250 and 300 K. Both curves are below zero, according to diamagnetic contributions only.

4. DISCUSSION The Imep polymer can be considered as a prototypical suppressor additive that makes particular use of the Cu(I) thiolate production during copper deposition. According to our recently introduced classification scheme of suppressor additives,5 we consider the Imep-Cu(I)-MPS ensemble as a hybrid of the so-called type I and type II suppressors. While type I suppressor ensembles typically show an antagonistic interaction with respect to SPS (or better MPS), the type II suppressors are completely resistant against deactivation via the SPS (MPS).5 The antagonistic type I

suppressor/SPS interaction leads under competitive conditions therefore to a low steady-state potential while the overpotential remains on a high level for the type II suppressor/SPS interaction.5 A number of type II suppressors even show a synergistic interaction with the SPS, leading to a slight increase of the overpotential.5 Prototypical type I suppressors are poly(ethylene glycol)s (PEGs). Poly(ethylene imine)s (PEIs) are considered as prototypical type II suppressors. In contrast to these both extremes, we observe in the present case a bistable situation when the Imep suppressor interacts with the SPS (MPS). The potential oscillations during the 6921



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Figure 10. Reaction scheme demonstrating the SPS surface electrochemistry coupled with the Imep precipitation reaction.

the near-surface electrolyte (step 3, Figure 10) as consequence of the same anion desorption/readsorption dynamics during copper growth which allows the preceding SPS adsorption. This scenario was recently evidenced by a combination of in-situ STM studies (Supporting Information).5 With a certain probability free MPS readsorbs on the surface. The thiol-functionalized monomer (MPS) was proven to displace chloride from the surface much more effectively than the dimeric disulfide (SPS).5,38,43 According to Healy et al.,35 Cu(II) and free MPS react to MPS-stabilized Cu(I) and SPS (step 4 in Figure 10, eq 1). The latter goes back into the SPS reaction cycle (step 1 in Figure 10). Two intermediates of the SPS reaction cycle are crucial to understand the oscillatory behavior of the copper deposition in presence of Imep/SPS. H2O-Cu(I)-MPS complexes are supposed to be essential coadditives for the resulting Imep suppressor ensemble at the interface. This might explain the (transient) synergistic interplay between SPS and the Imep as observed in our electrochemical experiments (Figure 2a). Aggregation of the H2OCu(I)-MPS unit with the Imep polymer is followed by the surfaceconfined precipitation of the Imep-Cu(I)-MPS suppressor ensemble at the copper/electrolyte interface (step 5 in Figure 10). Our beaker glass experiments are well suited to mimic the

galvanostatic copper deposition involve periodically repeating cycles of suppressor film formation and its subsequent degradation without reaching a final steady state, at least within a narrow interval of external parameters such as concentrations of the involved species and the current density. This bistability is clearly associated in the present case to the reversible Cu(I) coordination chemistry involved. Crucial for the mechanistic understanding of the Imep suppressor/leveler is the life cycle of the SPS at the copper/electrolyte interface under reactive conditions that is superimposed on the faradaic process (Figure 10). The process of SPS dissociation starts with its physisorption on the chloride layer38,42,43 (step 1; Figure 10) followed by its dissociative adsorption on the metallic copper at defect sites within the chloride matrix (step 2; Figure 10), leaving monomeric MPS behind anchored to the copper through the thiol functionality.5 Defect sites in the chloride matrix as structural prerequisite for the dissociative SPS adsorption naturally appear at lower potentials through partial chloride desorption.38,43,48 More relevant for the industrial copper electroplating is the appearance of defect sites in the chloride adsorption layer at higher potentials due to anion desorption/readsorption processes that are accelerated by the copper electrodeposition/dissolution processes. Chemisorbed MPS can be released from the electrode surface into 6922



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contamination level, however, lets us conclude that the potential-dependent incorporation is not the dominating mechanism for the deactivation of the Imep-Cu(I)-MPS suppressor ensemble. It is the combination of these surface and electrolyte volume reactions that introduces certain chemical and electric feedback loops into the system (Figure 10), giving rise to the observed nonlinear behavior during galvanostatic copper deposition (Figures 2 and 3). To the best of our knowledge, this Imep suppressor chemistry is the first example of an electrochemical oscillator that might be relevant for the state-of-the-art copper Damascene plating. At this point the question naturally arises whether the oscillatory instability of the Imep-Cu(I)-MPS suppressor ensemble as observed in our galvanostatic RDE experiments might negatively impact its desired leveling effect in context of the Damascene process. The fill of smaller Damascene features involving also the subsequent plating of the so-called overburden is typically accomplished on time scales that are certainly not relevant for the observed oscillatory instabilities. 32 nm technology Damascene features are filled within a couple of seconds while shortest periodicities of the regular periodic oscillation described in this paper are in the range of 233s (see Supporting Information). More problematic seems the use of Imep or related chemistries showing these temporal instabilities during galvanostatic plating for the fill process of extremely large, micrometer-sized TSV features. These plating processes are accomplished on the time scale of minutes. It has been demonstrated that 5 μm × 50 μm TSV test structures can be filled void-free within 25 min.59 The respective time window for the plating TSV is indicated in Figure 6a.

situation when the Imep polymers get into contact with an excess of H2O-Cu(I)-MPS species (Figure 8). For the precipitation itself we propose a two-step mechanism. The primary step is the coordination of the Cu(I)-MPS adduct by the OH group of the Imep polymer (highlighted red in Figure 10). Recent DFT studies combined with MD simulations suggest a linear, bidentate complex of Cu(I) coordinated by the thiolate group of the MPS ligand and the structurally intact OH group of the Imep.58 Such Cu(I) coordination itself is, however, not the driving force for the observed precipitation. It has to be considered as a primary effect only which introduces deprotonated and therefore negatively charged sulfonate groups into the resulting suppressor ensemble via the MPS ligand. Subsequent intraand interchain anion/cation pairing effects (highlighted green in Figure 10) transform the pristine positively charged Imep polymer into a neutral and therefore more hydrophobic, highly branched polymer that finally precipitates. Interchain linking in the resulting precipitate occurs via electrostatic coupling between the anionic sulfonate group and the cationic imidazolium of neighbored polymer chains. Our DFT study58 confirms the significant contribution of the electrostatic attraction between the sulfonate group and the imidazolium unit to the overall binding energy while the coordination of the aromatic imidazolium to the Cu(I) has to be considered as negligible. In this scenario the imidazolium group is only the carrier of the positive charge that gets counterbalanced in the course of the precipitation reaction by the negatively charged sulfonate group of the MPS ligand.58 This mode of leveler action therefore relies on a combination of Cu(I) coordination chemistry combined with an intramolecular anion−cation pairing effect. It is this concept of surface precipitation that appears advantageous for the industrial Damascene process. This mechanism introduces a certain selectivity into the action of this additive that becomes active in particular in those regions of the wafer surface where the leveling action is indeed needed. The local concentration of H2O-Cu(I)-MPS is expected to be highest over filled Damascene features. The proposed mechanism of action is also in agreement with patents describing the Imep and related chemistries.25−28 Most of the patents dealing with those leveling agents indeed stress the particular interplay of those additives with so-called brighteners as crucial coadditives for the desired leveling effect. This clearly points to some sort of synergistic interplay between these two types of plating additives. SPS is one of the brighteners mentioned in those patents.25−28 Note that the terms brighteners, accelerators, and antisuppressors are commonly interchangeable. The particular diversity in the naming of those sulfur-containing additives simply points to the diverse roles they have to play in the course of the copper deposition. The antagonistic interplay between SPS and Imep results from the free MPS as the second important intermediate of the SPS cycle (Figure 10). By our beaker-scale experiments, we could indeed demonstrate a dissolution of the Imep-Cu(I)MPS precipitate when an excess of free MPS added (Figure 8). Such coordinative dissolution most likely involves the cleavage of the O−Cu bond in the precipitate (step 6, Figure 10). This coordinative degradation of the Imep-Cu(I)-MPS suppressor ensemble is assumed to be the decisive one. Incorporation of the Imep-Cu(I)-MPS suppressor ensemble into the copper film could be detected by our SIMS data. The overall low

5. CONCLUSIONS The Imep polymer was studied with respect to its specific interplay with reaction intermediates that appear during the additive-assisted copper electrodeposition. Beaker-scale experiments could clearly prove the pronounced Cu(I) coordination chemistry of this type of suppressor additive as an alternative action mode for leveling beyond the interfacial anion/cation pairing concept. Cu(I) species could be identified in the formed precipitates by combined elementary analytics and SQUID magnetic measurements. It is such Cu(I) coordination chemistry, however, that introduces particular nonlinear instabilities into the system represented by an oscillatory behavior of the deposition potential under galvanostatic conditions. Essential variables for the appearance of these potential oscillations are the concentrations of the Imep polymer, of the Cu(I)-MPS unit, of the free MPS, and of the Cu(II) species, the latter being the charge carriers of the faradaic process. The potential in the galvanostatic deposition mode can be considered as a further essential pseudoparameter. As demonstrated by SIMS measurements, these periodic instabilities of the suppressor ensemble lead to an oscillatory modulation of the contamination level in the deposited copper film.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S7. This material is available free of charge via the Internet at http://pubs.acs.org. 6923



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AUTHOR INFORMATION

Corresponding Author

*Phone: +41 31 631 4317; e-mail: peter.broekmann@ iac.unibe.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P. Broekmann acknowledges the financial support by the Swiss National Foundation (SNF). The authors acknowledge the discussions with Dr. Bock and Dr. Reuscher from the IFOS institute in Kaiserslautern (Germany).

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Potential Oscillations in Galvanostatic Cu Electrodeposition

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