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Seedless copper electroplating on Ta from a “single” electrolytic bath
Electrochimica Acta 55 (2010) 1656–1663
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Seedless copper electroplating on Ta from a “single” electrolytic bath David Starosvetsky, Nina Sezin, Yair Ein-Eli ∗ Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
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Article history: Received 12 May 2009 Received in revised form 11 October 2009 Accepted 18 October 2009 Available online 25 October 2009 Keywords: Electroplating Seedless copper Ta barrier Single bath Oxide removal
a b s t r a c t An alternative approach for copper electroplating on Ta surface from a “single” injected bath is being described in this work. Copper electrodeposition over a thin TaN/Ta barrier was performed in a two-step process: (1) activation conducted by electrochemically reduction of Ta oxide from the TaN/Ta barrier at a negative potential of −2 V for a short period (“removal” step) and (2) copper electroplating performed in the invariable electrochemical bath by injecting a solution containing Cu-ions. Supplementary Cu plating is continued by shifting the applied potential to −1.2 V in the same electrolytic bath. It was also established that addition of low content (up to 10 ppm) dimercaptothiadiazole (DMcT) improves Cu nucleation and growth on Ta surface and allows a conformal features fillings. Copper layer deposited is characterized with an excellent adhesion to the Ta surface. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The progress in miniaturization ULSI design, achieving size features below 32 nm, requires new approaches in copper interconnect (IC) metallization. Traditional metallization process, which includes intermediate PVD formation of Cu seed layer on Ta barrier film prior to feature filling by Cu electroplating, cannot be effective once feature dimensions are comparable with seed layer thickness. One option for metallization of such narrow features (<32 nm), being thoroughly studied in recent years, is to eliminate PVD Cu seed layer deposition step, along with a reduction in Ta barrier film thickness, down to a several nanometers [1–5]. Thus, copper electroplating should be performed eventually directly over the thin Ta barrier film. A major setback of direct Cu plating on Ta based materials is associated with the passivation oxide film of tantalum pentoxide, T2 O5 , developed on the Ta electrode surface while exposing it to aqueous solutions [6–8]. Tantalum oxide surface is characterized by both poor wetting and adhesion of the electrodeposited metals. Therefore, efficient copper plating could be performed on a Ta surface only with a removal of the oxide film. It is absolutely clear that a complete oxide removal process from a thin tantalum film should be conducted without an additional thinning of the Ta barrier film. Current approaches to Ta oxides removal are based on either chemical etching in fluoride-ions containing acidic solutions [3–5], or anodic polarization in saturated alkaline solutions [7]. Chemical etching in fluoride containing acidic solutions is usually charac-
terized with a relatively high etching rate [6,8] that can lead to an excessive thinning of the Ta barrier film [9,10]. The perforation of the Ta film at certain surface sites cannot be excluded completely in fluoride acid solution’s etching, taking into account the inconsistency of wafer surface having trenches and vias with different dimensions and aspect ration, as well as certain irregularities presented in the Ta film itself. In addition, fluoride acid solutions’ etching does not exclude subsequent re-oxidation of the Ta surface via contact with air and/or washing in aqueous media. On the other hand, consideration of Ta oxide removal by anodic bias application seems most peculiar and undesired. It is well known that metal’s anodic polarization is usually associated with the opposite effect; oxide film formation and growth occurs under this condition, and Ta is not excluded [6–10]. In particular, anodic step process (anodizing) is used to develop Ta oxide film on tantalum, having a predetermined thickness in capacitors fabrication [11]. Additional problems associated with Ta surface oxidation are related to the subsequent Cu plating process. Since Ta surface rapidly re-oxidized by immersion and exposure to an aqueous solution, copper electrodeposition should be conducted (at least during the initial deposition steps), under specific conditions which would prevent oxide reformation and growth. Thus, the aim of the present work was to determine conditions for seedless copper electrodeposition over a thin Ta barrier film while removing the surface oxide, and avoiding any thinning in Ta barrier thickness and its re-oxidation during the initial steps of Cu deposition process. 2. Experimental
∗ Corresponding author. Tel.: +972 4 829 4588; fax: +972 4 829 5677. E-mail address: [email protected] (Y. Ein-Eli). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.10.044
Electrochemical measurements were conducted with a penciltype tantalum electrode, produced by mounting a pure tantalum
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Fig. 1. Potentiodynamic characteristics obtained from Ta electrode polarized in KOH solutions at scan rate of 5 mV/s: (a) 5, 10, 25 wt.% KOH at 25 ◦ C (inset: ECORR transient obtained from Ta electrode at OCP in KOH solutions); (b) 10 wt.% KOH, at T of 25, 40 and 60 ◦ C (inset: ECORR transient obtained from Ta exposure at OCP to different temperatures).
rod (99.99 wt.%, 3.5 mm diameter) in an epoxy resin. The electrode was freshly wet-abraded to a 1200 grit finish prior to each experiment. Commercial patterned Si wafer having 20–30 nm TaN/Ta barrier film prepared by PVD deposition were used in Cu electroplating experiments (Imec, Belgium). Patterned Si wafer electrodes (2.5 cm × 2.5 cm) were positioned in a Teflon holder (with a working area of 1 cm2 ) equipped with an O-ring and with an Ohmic front contact of InGa eutectic alloy. Electrochemical studies were conducted in 500-ml electrochemical cell equipped with a saturated calomel reference electrode (SCE, Luggin capillary) and platinum plate counter electrode. All potentials quoted in this study are vs. SCE potential. Copper deposition electrolytes were prepared from potassium pyrophosphate (K4 P2 O7 , Carlo Erba Reagents) and copper pyrophosphate (Cu2 P2 O7 , Alfa Aeasar) dissolved in de-ionized (DI) water (18 M, Millipore). Base solution was 0.3 M (100 g) K4 P2 O7 . Both low and high copper ions content in the pyrophosphate electrolytes were used: 0.03 M (2 g/l) Cu2+ , 0.3 M (100 g/l) K4 P2 O7 (pH 9.3) and 0.2 M (12 g/l) Cu2+ , 0.53 M (175 g/l) K4 P2 O7 (pH 8.5). Subsequent to the addition of 0.03 M Cu2+ as Cu2 P2 O7 to 0.3 M K4 P2 O7 the solution’s pH reduces from 10.1 to 9.3, as a result of copper pyrophosphate hydrolysis. The pH of the final copper plating solution (0.53 M K4 P2 O7 + 0.2 M Cu2 P2 O7 ) was 8.5. Electroplat-
ing was conducted in pyrophosphate solutions additive-free and with the addition of 2,5-dimercapto-1,3,4-thiadiazole, 98% (DMcT, AcrosOrganics). DMcT is one of the components in PY61-H brightener composition, developed for copper plating baths [12,13]. Some experiments were conducted also in potassium hydroxide (KOH) solution and potassium pyrophosphate solution. Potentiostat (PAR 2273) was used in the electrochemical studies and the electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were conducted with a Ta pencil-type electrode at 5 mV amplitude sinusoidal signals in the frequencies range between 0.1 Hz and 100 KHz. Scanning electron microscope (SEM, LEO-982 Geminate FEG-HRSEM) top view and a dual-beam focused ion beam (FIB, FEI Strata 400-S) cross-sectional images were used in order to monitor copper nucleation and deposition processes. The qualitative evaluation of adhesive characteristics of the deposited copper film to the surface of a Ta foil (5 mm × 30 mm × 1 mm) was conducted with the use of a bending, a heat-quench and peel-off tests, while copper adhesion characteristics to wafer samples was evaluated by the latter two tests. Procedures of these tests are described by Magagnin et al. [14] and in ASTM [15]. Ta foil (having a deposited copper film) bending test was performed as followed: bending to 180◦ followed by sample straitening to the initial state. Bent surface zone was examined by SEM prior and subsequent to the bending test. Peel-off test was conducted with adhesive scotch tape with an angle of about 90◦ . Heat-quench was conducted by thermally heating the samples (deposited Ta foil or a wafer) in a tube furnace at 300 ◦ C for 2 h under hydrogen atmosphere.
3. Results and discussion 3.1. Ta oxide “removal”
Fig. 2. Impedance spectra obtained from Ta electrode immersed in 10 wt.% KOH solution at different temperatures (in frequency range between 104 and 10−1 Hz) subsequent to 30 s exposure at OCP.
“Removal” studies of Ta oxide film from Ta surface were conducted with a Ta pencil-type electrode under a cathodic polarization in both KOH and K4 P2 O7 solutions (the latter will serve as the supporting electrolyte for further copper electroplating). Fig. 1a presents potentiodynamic characteristics (10 mV/s scan rate) obtained from tantalum polarized in 5, 10 and 25 wt.% KOH solutions at 25 ◦ C in a wide potential range (between −2 and +0.4 V). Corrosion potential transients obtained from Ta electrode exposure at OCP in KOH solutions are presented, as well (inset). The onsets of anodic current in KOH solutions are located in a potential range between −1.1 and −1.2 V (vs. SCE/V). Above this potential values (up to 0.4 V) Ta electrode remains passive. One can also observe
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that the anodic current density in the passivity region slightly increases with KOH concentration, indicating a minor reduction in Ta passivity. Fig. 1b presents the effect of temperature on the potentiodynamic characteristic of Ta electrode in 10 wt.% KOH solution having a pH value of 10.2. The ECORR transient obtained from Ta during OCP exposure is shown, as well (inset). The anodic current density in the passivity region markedly increases (approximately, in one order of magnitude) once the alkaline electrolyte temperature is increased from 25 to 60 ◦ C, indicating a significant decrease in Ta passivity. The effect of various parameters, such as temperature, solution composition and applied potential, on the state of Ta electrode interface in alkaline solutions was studied by EIS. EIS measurements were performed in accordance with Sapra et al. [9], who studied oxide removal from Ta surface in HF solution. Here, analysis of the charge-transfer resistance value (diameter of semicircle) was used for only a qualitative evaluation of Ta oxide removal from the electrode surface. Fig. 2 presents Niquist plots obtained from Ta electrode immersed in 10 wt.% KOH at temperatures of 25, 40 and 60 ◦ C subsequent to OCP exposure for 30 s. As can be seen, the charge-transfer resistance of the Ta electrode in 10% KOH at OCP significantly decreased with temperature. However, even at 60 ◦ C it remains quite high (∼2.5 K cm2 ), indicating the presence of Ta oxide film on the electrode surface. Electrochemical knowledge and a well-established practice point that cathodic reduction is a proper method for electrode surface cleaning from its oxides. This approach, for example, is effectively used for cathodic reduction of oxide film from surface of ruthenium barrier layers [16]. In this work, we also studied the efficiency of cathodic reduction of oxide film covered Ta electrode surface with the use of EIS. Fig. 3 presents Niquist plots obtained from Ta electrode exposed to a solution containing 10% KOH subsequent to 30 s exposure at different applied potentials, all of them below OCP. One can see that negative shift of the applied potential leads to a decrease in the charge-transfer resistance, indicating a reduction of Ta oxide film. Especially, the most significant reduction of Ta oxide occurred at potentials below −1.5 V. The charge-transfer resistance of tantalum electrode was decreased by approximately four orders of magnitudes by shifting the applied potential from −1.5 to −2.1 V. The value of charge-transfer resistance obtained at a potential of −2.1 V was substantially reduced. Thus, by application of an appropriate cathodic polarization one can achieve an effective “removal” of Ta oxide from the Ta electrode surface, by simply reducing the oxide film into metallic Ta layer.
Fig. 3. Impedance Niquist spectra obtained from Ta electrode immersed in a solution of 10 wt.% KOH (25 ◦ C) subsequent to 30 s potentiostatic exposure at different applied potentials: OCP, −1.3, −1.5, −1.7 V. Inset: EIS of Ta at potential of −1.9 and −2.1 V in the same solution.
Fig. 4. Impedance spectra obtained from Ta electrode immersed in a solution containing 0.3 M K4 P2 O7 (25 ◦ C) subsequent to 30 s potentiostatic exposure at different applied potentials: OCP, −1.3, −1.5. Inset: EIS of Ta in 0.3 M K4 P2 O7 at potentials of −1.7 and −1.9 V.
Fig. 5. FIB cross-sectional view of Si/TaN/Ta interface: (a) initial state: as received wafer prior to potential application; (b) subsequent to 2 h of exposure at a potential of −2.0 V.
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Fig. 4 presents data obtained from similar EIS measurements conducted once Ta electrode was immersed in a solution containing 0.3 M potassium pyrophosphate (K4 P2 O7 ) having a pH of 10.1. Similarly to the results obtained in KOH solution, potentiostatic exposure of Ta electrode in 0.3 M K4 P2 O7 solution at potentials below −1.5 V also leads to a remarkable decrease in the chargetransfer resistance. This indicates that a “removal” by a cathodic reduction of Ta oxide surface film in this potential range occurs, as well. However, interruption or suspension in the cathodic polarization process results in a rapid development and growth of a fresh new oxide layer. To ensure that the cathodic pretreatment does not reduce the thickness of the Ta barrier film, wafer samples having a barrier film were exposed for a deliberately extended time of 2 h, at an applied potential of −2.0 V. The thicknesses of the barrier film prior and subsequent to the long cathodic pretreatment were measured by FIB cross-sectional view of Si/TaN/Ta interface, as shown in Fig. 5. It can be seen that no thinning of the barrier film was detected subsequent to the extended cathodic pretreatment. 3.2. Copper electroplating Copper electroplating over a Ta electrode surface was conducted in Cu2+ containing alkaline pyrophosphate electrolytes prepared from K4 P2 O7 and Cu2 P2 O7 . Copper ion in alkaline pyrophosphate solutions is being presented as a complex ion [Cu(P2 O7 )2 ]6− . This specie undergoes a reduction process under cathodic conditions [17]: [Cu(P2 O7 )2 ]6− + 2e− → Cu + 2[P2 O7 ]4− Following our earlier results on Ta oxide “removal”, a copper electrodeposition over a Ta electrode surface was conducted immediately after Ta oxides cathodic reduction. This was performed by 30 s exposure of the Ta electrode in 0.3 M K4 P2 O7 solution (this will be served from now on as the copper bath supporting electrolyte) at a potential of −2 V. Thus, copper electroplating is being performed in a two-step process: a removal of oxide film and a subsequent copper electrodeposition. In the next series of studies, copper deposition (first electroplating step) was performed by shifting the applied potential to much higher potential values (≥−1.4 V) immediately after the cathodic pretreatment (without interruption of the polarization). Small portion (50 ml) of pyrophosphate solution was taken from the 1 L pyrophosphate solution and poured into a separate container, dissolving 5 g Cu2 P2 O7 . Thus, the content of Cu2+ ions in the bath was 0.03 M. Cathodic pretreatment was conducted in the remaining supporting electrolyte solution (950 ml), which was agitated by a magnetic stirrer. The solution containing 5 g Cu2 P2 O7 (in 50 ml solution) was poured back to the electrolyte at the end of the cathodic pretreatment. Copper deposition was initiated by simultaneously applying a potential and adding the small portion (50 ml) solution, containing dissolved copper in a pyrophosphate based solution (0.015 M Cu2 P2 O7 + 0.3 M K4 P2 O7 ) to the bath of the supporting electrolyte (950 ml). Cathodic behavior of Ta electrode in copper pyrophosphate solutions is illustrated in Fig. 6. Cathodic polarization characteristics of Ta electrode were measured in both electroplating solutions containing 0.03 and 0.2 M Cu2+ . This was performed subsequent to a cathodic pretreatment – potentiostatic exposure at −2 V for 30 s. For comparison, the inset graph presents the cathodic curve obtained from Ta electrode polarized in the supporting pyrophosphate electrolyte (0.3 M K4 P2 O7 ) subsequent to cathodic pretreatment. One can see that copper electrodeposition in electrolyte containing lower Cu2+ concentration (0.03 M) is initiated at more negative potentials (−1.1 vs. −0.8 V detected for the
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Fig. 6. Cathodic polarization characteristics of Ta electrode subsequent to oxide “removal” in two copper electroplating solutions: 0.03 M Cu2+ (2 g/L) + 0.3 M K4 P2 O7 and 0.2 M Cu2+ (12 g/L) + 0.53 M K4 P2 O7 . Inset: Polarization characteristic of Ta electrode in the absence of cooper ion (0.3 M K4 P2 O7 solution).
high copper ion solution) and is characterized with a lower current density values compared to copper deposition in electrolyte containing higher Cu2+ content (0.2 M). In a solution containing 0.03 M Cu2+ the increase in cathodic current density was observed by negative shift of the applied potential down to −1.25 V. Cathodic current density remained practically unaffected (∼8 mA/cm2 ) in a wide potential range below −1.25 V (between −1.25 and −1.5 V). Maximum cathodic current density value obtained in electrolyte containing 0.2 M Cu2+ , was 5 times higher (40 mA/cm2 ) than the value recorded in electrolyte containing 0.03 M Cu2+ . Increase in the cathodic current density displayed in both curves at potentials below −1.5 V is associated with acceleration of hydrogen evolution. Cathodic current density values measured potentiodynamically in 0.3 M K4 P2 O7 solution (Fig. 6, inset) were significantly smaller compared with values obtained in Cu2+ containing electrolytes. Features of copper deposition on Ta surface (foil 0.25 mm thickness, pre-polished to 1200 grid finish) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 electrolyte under the application of different potentials are shown in Figs. 7 and 8. The required potential was applied subsequent to 30 s of pre-exposure at a potential of −2 V. Current–time pro-
Fig. 7. Current–time transients obtained from Ta electrode polarized in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution (pH 9.3) under different applied potentials.
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Fig. 8. SEM images obtained from Ta surface presenting copper nucleus electrodeposited at −1.1 V (a) and −1.2 V (b) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3). Total charge accumulated was 100 mC/cm2 .
files presented in Fig. 7 evaluate nucleation and growth of copper on Ta surface under applied potentials of −1.0, −1.1 and −1.2 V. As can be seen in Fig. 7, the cathodic current density gradually increased during polarization in all applied potentials, indicating increase in copper deposition rate. Copper deposition rate is pronouncedly increased by a negative shift in the applied potential to −1.2 V, as the cathodic current density is significantly increased at this potential. These results are in agreement with SEM observation obtained from Ta surface subsequent to copper deposition at −1.1 and −1.2 V (Fig. 8). Copper deposition under each of these potentials was terminated once a total charge of 100 mC was accumulated. Size inconsistent, separation and irregular shape of copper crystallites distributed over Ta surface is observed in the −1.1 V sample. The number of nucleated crystallites increases by negatively shifting the applied potential to −1.2 V, in agreement with the electrochemical studies, presented in Fig. 7. In order to achieve a conformal copper deposition over TaN/Ta barrier surface the following studies were conducted with copper pyrophosphate electrolytes having dimercaptothiadiazole (DMcT) as an additive. It is known from copper electroplating onto Pt electrode [13] that DMcT/copper pyrophosphate electrolyte system involves two additive species, which are in dynamic equilibrium: DMcT monomer and dimmer. Monomer species form a complex compound with copper ions [13]. In such case, copper electrodeposition is accelerated, presumably, due to assistance in nucleation
of nodule copper crystallites randomly distributed over the surface. Unlike DMcT monomers, dimmer species hinder copper deposition rate by blocking nucleation surface sites [13]. This dual decelerating/accelerating behavior of DMcT on Pt results eventually in enhanced leveling of copper deposition from pyrophosphate electrolytic bath. Here, we describe the use of DMcT in copper pyrophosphate electrolytic bath while Ta serves as the electroplated electrode. Fig. 9 illustrates the effect of DMcT addition (1–10 ppm) on the cathodic potentiodynamic characteristics obtained from Ta electrode in K4 P2 O7 solution containing 0.03 M Cu2+ . As can be seen, the maximum value of cathodic current density, associated with copper cathodic reduction rate, is significantly decreased with increase in DMcT concentration. Further studies of copper electroplating solutions containing only 3–5 ppm DMcT were performed, since at this intermediate concentration the acceleration–inhibition effect of DMcT is well established. The effect of potential on copper nucleation and growth in a solution containing 3 ppm DMcT and copper ions–pyrophosphate system (0.03 M Cu2+ + 0.3 M K4 P2 O7 ) is illustrated in the current transient curves measured at different applied potentials (Fig. 10) and SEM images obtained from copper deposition on a cathodically (−2 V) pre-treated Ta surface subsequent to a short potentiostatic exposure (passed charge 100 mC) at potentials of −1, −1.1 and −1.2 V (Fig. 11). As can be seen, the current transient shape obtained from a potentiostatic exposure in the presence of
Fig. 9. Cathodic potentiodynamic (5 mV/s) profiles obtained from polarizing Ta electrode in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3) at different DMcT content.
Fig. 10. Current–time transients obtained from Ta electrode polarized in 0.03 M Cu2+ + 0.3 M K4 P2 O7 containing 3 ppm DMcT (pH 9.3) at different applied potentials.
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Fig. 11. SEM images obtained from Ta surface showing copper crystallites electrodeposited (100 mC charge was recorded) at potentials of −1.1 V (a) and −1.2 V (b) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 containing 3 ppm DMcT (pH 9.3).
3 ppm DMcT is completely different from the one obtained in an additive-free solution (Fig. 7). Cathodic current density, related to copper deposition rate, rapidly reached its maximum immediately after potential application and gradually decreased during further exposure. As the applied potential was more negative the current peak was higher and appeared earlier. It is reasonable to suggest that DMcT addition markedly increases copper nucleus formation rate, saturating all possible nucleation sites at the Ta electrode surface during the initial stages of copper deposition. Fig. 11 illustrates nucleation and growth of copper at the initial stages of copper deposition (accumulated charge of 100 mC was recorded) on the surface of cathodically pre-treated (−2 V) Ta at a potential of −1.1 and −1.2 V (Fig. 11a and b, respectively) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution containing 3 ppm of DMcT, as an additive (pH 9.3). The presence of DMcT has a remarkable influence: copper crystallites density nucleated and developed under both applied potential of −1.1 and −1.2 V is markedly higher, compared with copper depositions obtained at the same potentials in DMcTfree solution (see Fig. 8). However, we should point out that despite this improvement, copper nucleation initiated by the addition of copper containing pyrophosphate solution subsequent to cathodic pretreatment at −2.0 V and positive shift of the applied potential cannot be considered as the most suitable one for a conformal filling of sub-micron trenches and vias: dimensions of nucleated copper
crystals are too large (>1 m) and density of nucleus remained quite small (the gap between them is higher than a few microns). In the following experiments, copper nucleation was initiated by adding a portion of a copper pyrophosphate (Cu2 P2 O7 ) solution into the supporting electrolyte (0.3 M K4 P2 O7 ) at the end of the cathodic pretreatment (without interrupting potentiostatic exposure at −2 V) and further exposure for 3–5 s under −2 V in Cu2+ containing electrolyte. Fig. 12 illustrates nucleation and growth of copper on the surface of Ta foil after 3 s exposure at −2.0 V in copper pyrophosphate solution (0.03 M Cu2+ + 0.3 M K4 P2 O7 ) without and with 3 ppm DMcT. One can see that even after 3 s exposure at −2.0 V in the copper pyrophosphate electrolyte with DMcT the surface of Ta foil was completely covered with fine Cu crystals. In the absence of DMcT the density of Cu nucleus is markedly lower while the size of the crystals is higher. Further study of copper deposition on Ta surface was conducted with commercial patterned Si wafers having 20–30 nm TaN/Ta barrier film. Identical copper pyrophosphate solution (0.03 M Cu2+ + 0.3 M K4 P2 O7 and 3 ppm DMcT) and an exact procedure of copper deposition was applied: 1. Activation stage: a cathodic reduction of Ta oxide via exposure to a supporting electrolyte, consisting of 0.3 M K4 P2 O7 solution at potential of −2 V for 30 s;
Fig. 12. SEM images obtained from of Ta foil surface presenting copper crystallites electrodeposited after 3 s exposure under applied potential of −2.0 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3) without additive (a) and with 3 ppm DMcT (b).
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Fig. 13. Front view SEM images of coupon wafer surface covered with continuous copper layer electrodeposited on TaN/Ta barrier film for 500 s at −1.2 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution (first electroplating step) without DMcT (a) and with 3 ppm of DMcT (b).
2. Seeding stage: injection of 0.03 M Cu2+ (from a solution of Cu2 P2 O7 ) into the supporting electrolyte (0.3 M K4 P2 O7 ) without polarization interruption and further exposure for 3–5 s at a potential of −2 V constitutes the first plating stage; 3. Second plating stage: potential shift to values above −1.4 V and exposing the seeded electrode surface at this potential for a time length capable of obtaining a continuous copper film over the wafer surface; 4. Final plating stage: third electroplating stage is conducted in electrolyte bath containing 0.2 M Cu2+ and 0.53 M K4 P2 O7 . This is done in order to accelerate copper deposition.
Fig. 13 presents front view of a coupon wafer surface covered with continuous copper layer deposited over a TaN/Ta barrier film in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution without DMcT (Fig. 13a) and with 3 ppm of DMcT (Fig. 13b). Electroplating was conducted according to the above-described procedure with a 500 s expo-
sure time at a potential of −1.2 V. As can be seen in the SEM images, copper deposition over a coupon wafer surface, obtained in pyrophosphate solution in the absence of DMcT, is not uniform and is characterized with the formation of large crystals (Fig. 13a). In the presence of 3 ppm of DMcT a uniform copper deposition was obtained over the whole wafer coupon surface including centered and peripheral zones. FIB cross-sectional view of Si/TaN/Ta patterned wafer surface having a copper layer deposited in DMcT containing Cu pyrophosphate solution is shown in Fig. 14. Fig. 14a and b presents cross-section of wafers having a copper film (∼100 nm) deposited for 500 s at a potential of −1.2 V (matching deposition stage, as shown in Fig. 13). The filling of small features in the pyrophosphate solution can be described as a conformal process. Fig. 14c and d illustrates Si/TaN/Ta cross-section of copper filled features obtained at the final plating stage. The final plating stage was conducted at a potential of −1 V for 100 s in a fresh pyrophosphate electroplating bath with higher Cu2+ content (0.2 M Cu2+ (as Cu2 P2 O7 ), 0.53 M K4 P2 O7 and 5 ppm DMcT, pH 8.5, 25 ◦ C).
Fig. 14. FIB cross-sections of Si/TaN/Ta patterned wafer surface covered with copper film, electrodeposited on TaN/Ta barrier film in copper pyrophosphate electrolytes: (a, b) after first electroplating stage conducted for 500 s at −1.2 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 and 3 ppm DMcT (pH 9.3) solution at 25 ◦ C; (c, d) after a second (final) electroplating stage conducted for 100 s at −1.0 V in 0.2 M Cu2+ (as Cu2 P2 O7 ) + 0.53 M K4 P2 O7 solution containing 5 ppm DMcT, pH 8.5, 25 ◦ C.
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The thickness of the deposited copper layer after the final electroplating stage was 450 nm. Our studies indicate that the best adhesion of Cu to the Ta electrode surface was obtained when copper nucleation (first plating step) was conducted for 3–5 s under the applied potential of −2.0 V in pyrophosphate electrolyte containing 0.03 M Cu2+ (0.3 M K4 P2 O7 + 0.015 M Cu2 P2 O7 , pH 9.3) and 3 ppm DMcT. Copper film deposited on the surface of a patterned wafer was characterized with a very good adhesion to the thin TaN/Ta barrier film. As was noted above, the adhesion of the deposited copper film to Ta surfaces (foil and wafer) was qualitatively evaluated by bending (only with Ta foil), heat-quench and peel-off tests. No exfoliations of copper film from the tantalum surface were observed subsequent to the application of the test methods, indicating a good adhesion between the deposited copper film and the Ta surfaces. 4. Conclusions We present in this extensive work an alternative approach for seedless copper electroplating on Ta surface, being initially covered in pristine passive oxide layer. The method described in this study involves the introduction of a new electroplating bath along with a removal of the oxide passivating film. The method is a generic one and may also be applied to other passivated metals and alloys (Ru and Ru/Ta mixed metals), being currently considered as barrier films for future seedless integrated systems. The following conclusions can be drawn from this pioneering study: • Copper electrodeposition over a thin TaN/Ta barrier can be performed in a two-step process: activation of TaN/Ta barrier by a cathodic reducing of Ta oxide (“removal” step) and subsequent copper electroplating performed in the invariable electrochemical bath. • Ta oxide reduction (“removal”) is being performed in 0.3 M K4 P2 O7 solution under the application of a potential of −2 V for 30 s. At this potential, Ta oxide is being reduced to metallic Ta. • Copper plating is initiated at a potential of −2 V by injecting low copper content Cu2 P2 O7 solution, (0.03 M Cu2+ ) and 3 ppm DMcT, into the supporting K4 P2 O7 electrolyte, followed by 3–5 s exposure in this solution. It was established in this work that DMcT additive improves Cu nucleation and growth on Ta surface, providing a conformal features filling. • Supplementary Cu plating is continued by shifting the applied potential to −1.2 V in the same electrolytic bath, while the final plating process can be performed in high copper ion content pyrophosphate solution. • Copper layer deposited is characterized with an excellent adhesion to the Ta surface.
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It should be pointed out that copper filling with the use of Cu pyrophosphate chemistry is totally different from the acid-sulfate ones, being used in current IC metallization. While the wafer features in this study are well-filled with Cu from the pyrophosphate chemistry, it is yet to be demonstrated that the chemistry is capable to fill features that are much narrower (below 50 nm width). The organic additives, particular developed to the acid-based Cu plating chemistries, enable a rapid bottom-up fill, allowing a defect-free filling of narrow features. Thus, it is anticipated that pyrophosphate chemistry’s additives, to be developed in future research, may lead to enhanced nucleation densities and similar or even better narrow features fillings than current acid bath technology. Acknowledgements This research was financially supported by the Technion Research Foundation, a joint grant from the Center for Absorption in Science of the Ministry of Immigrant Absorption, and by the Committee for Planning and Budgeting of the Council for Higher Education under the framework of the KAMEA Program. The authors would like to thank Imec, Belgium, for patterned wafer supply. References [1] S. Kim, D. Duquette, J. Electrochem. Soc. 154 (2007) D195. [2] M. Zheng, J.J. Kelly, H. Deligianni, J. Electrochem. Soc. 154 (2007) D400. [3] H. Hui Hsu, C. Chan Hsieh, M. Hsian Chen, S. Jein Lin, J. Wei Yeh, J. Electrochem. Soc. 148 (2001) C590. [4] Z. Wang, H. Li, H. Shodiev, I. Ivar Suni, Electrochem. Solid-State Lett. 8 (2005) G283. [5] A. Radisic, Y. Cao, P. Taephaisitphongse, A. West, P.C. Searson, J. Electrochem. Soc. 148 (2001) C41; A. Radisic, Y. Cao, P. Taephaisitphongse, A. West, P.C. Searson, J. Electrochem. Soc. 151 (2004) C369. [6] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, chapter IV, section 09.3 – Tantalum. [7] L.L. Shreier (Ed.), Corrosion, Newness-Butterworth, London, 1995. [8] A.J. Bard, Encyclopedia of Electrochemistry of the Elements, vol. 2, Marcel Dekker Inc., 1976 (chapter 3 – Tantalum). [9] S. Sapra, H. Li, Z. Wang, I.I. Suni, J. Electrochem. Soc. 152 (2005) B193. [10] J. Zhang, S. Li, P.W. Carter, J. Electrochem. Soc. 154 (2007) H109. [11] D.A. Vermilyea, J. Electrochem. Soc. 102 (1955) 655. [12] D. Tench, C. Ogden, J. Electrochem. Soc. 125 (1978) 194. [13] C. Ogden, D. Tench, J. Electrochem. Soc. 128 (1981) 539. [14] L. Magagnin, R. Maboudian, C. Carraro, Thin Solid Films 434 (2003) 100. [15] ASTM International Standards B571-97 (2003), Standard Practice for Qualitative Adhesion Testing of Metallic Coatings, Paints and Coatings, American Society for Testing and Materials (ASTM). [16] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, J. Electrochem. Soc. 153 (2006) C37. [17] H. Konno, M. Nagayama, Electrochim. Acta 23 (1978) 1001.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Seedless copper electroplating on Ta from a “single” electrolytic bath David Starosvetsky, Nina Sezin, Yair Ein-Eli ∗ Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
a r t i c l e
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Article history: Received 12 May 2009 Received in revised form 11 October 2009 Accepted 18 October 2009 Available online 25 October 2009 Keywords: Electroplating Seedless copper Ta barrier Single bath Oxide removal
a b s t r a c t An alternative approach for copper electroplating on Ta surface from a “single” injected bath is being described in this work. Copper electrodeposition over a thin TaN/Ta barrier was performed in a two-step process: (1) activation conducted by electrochemically reduction of Ta oxide from the TaN/Ta barrier at a negative potential of −2 V for a short period (“removal” step) and (2) copper electroplating performed in the invariable electrochemical bath by injecting a solution containing Cu-ions. Supplementary Cu plating is continued by shifting the applied potential to −1.2 V in the same electrolytic bath. It was also established that addition of low content (up to 10 ppm) dimercaptothiadiazole (DMcT) improves Cu nucleation and growth on Ta surface and allows a conformal features fillings. Copper layer deposited is characterized with an excellent adhesion to the Ta surface. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The progress in miniaturization ULSI design, achieving size features below 32 nm, requires new approaches in copper interconnect (IC) metallization. Traditional metallization process, which includes intermediate PVD formation of Cu seed layer on Ta barrier film prior to feature filling by Cu electroplating, cannot be effective once feature dimensions are comparable with seed layer thickness. One option for metallization of such narrow features (<32 nm), being thoroughly studied in recent years, is to eliminate PVD Cu seed layer deposition step, along with a reduction in Ta barrier film thickness, down to a several nanometers [1–5]. Thus, copper electroplating should be performed eventually directly over the thin Ta barrier film. A major setback of direct Cu plating on Ta based materials is associated with the passivation oxide film of tantalum pentoxide, T2 O5 , developed on the Ta electrode surface while exposing it to aqueous solutions [6–8]. Tantalum oxide surface is characterized by both poor wetting and adhesion of the electrodeposited metals. Therefore, efficient copper plating could be performed on a Ta surface only with a removal of the oxide film. It is absolutely clear that a complete oxide removal process from a thin tantalum film should be conducted without an additional thinning of the Ta barrier film. Current approaches to Ta oxides removal are based on either chemical etching in fluoride-ions containing acidic solutions [3–5], or anodic polarization in saturated alkaline solutions [7]. Chemical etching in fluoride containing acidic solutions is usually charac-
terized with a relatively high etching rate [6,8] that can lead to an excessive thinning of the Ta barrier film [9,10]. The perforation of the Ta film at certain surface sites cannot be excluded completely in fluoride acid solution’s etching, taking into account the inconsistency of wafer surface having trenches and vias with different dimensions and aspect ration, as well as certain irregularities presented in the Ta film itself. In addition, fluoride acid solutions’ etching does not exclude subsequent re-oxidation of the Ta surface via contact with air and/or washing in aqueous media. On the other hand, consideration of Ta oxide removal by anodic bias application seems most peculiar and undesired. It is well known that metal’s anodic polarization is usually associated with the opposite effect; oxide film formation and growth occurs under this condition, and Ta is not excluded [6–10]. In particular, anodic step process (anodizing) is used to develop Ta oxide film on tantalum, having a predetermined thickness in capacitors fabrication [11]. Additional problems associated with Ta surface oxidation are related to the subsequent Cu plating process. Since Ta surface rapidly re-oxidized by immersion and exposure to an aqueous solution, copper electrodeposition should be conducted (at least during the initial deposition steps), under specific conditions which would prevent oxide reformation and growth. Thus, the aim of the present work was to determine conditions for seedless copper electrodeposition over a thin Ta barrier film while removing the surface oxide, and avoiding any thinning in Ta barrier thickness and its re-oxidation during the initial steps of Cu deposition process. 2. Experimental
∗ Corresponding author. Tel.: +972 4 829 4588; fax: +972 4 829 5677. E-mail address: [email protected] (Y. Ein-Eli). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.10.044
Electrochemical measurements were conducted with a penciltype tantalum electrode, produced by mounting a pure tantalum
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Fig. 1. Potentiodynamic characteristics obtained from Ta electrode polarized in KOH solutions at scan rate of 5 mV/s: (a) 5, 10, 25 wt.% KOH at 25 ◦ C (inset: ECORR transient obtained from Ta electrode at OCP in KOH solutions); (b) 10 wt.% KOH, at T of 25, 40 and 60 ◦ C (inset: ECORR transient obtained from Ta exposure at OCP to different temperatures).
rod (99.99 wt.%, 3.5 mm diameter) in an epoxy resin. The electrode was freshly wet-abraded to a 1200 grit finish prior to each experiment. Commercial patterned Si wafer having 20–30 nm TaN/Ta barrier film prepared by PVD deposition were used in Cu electroplating experiments (Imec, Belgium). Patterned Si wafer electrodes (2.5 cm × 2.5 cm) were positioned in a Teflon holder (with a working area of 1 cm2 ) equipped with an O-ring and with an Ohmic front contact of InGa eutectic alloy. Electrochemical studies were conducted in 500-ml electrochemical cell equipped with a saturated calomel reference electrode (SCE, Luggin capillary) and platinum plate counter electrode. All potentials quoted in this study are vs. SCE potential. Copper deposition electrolytes were prepared from potassium pyrophosphate (K4 P2 O7 , Carlo Erba Reagents) and copper pyrophosphate (Cu2 P2 O7 , Alfa Aeasar) dissolved in de-ionized (DI) water (18 M, Millipore). Base solution was 0.3 M (100 g) K4 P2 O7 . Both low and high copper ions content in the pyrophosphate electrolytes were used: 0.03 M (2 g/l) Cu2+ , 0.3 M (100 g/l) K4 P2 O7 (pH 9.3) and 0.2 M (12 g/l) Cu2+ , 0.53 M (175 g/l) K4 P2 O7 (pH 8.5). Subsequent to the addition of 0.03 M Cu2+ as Cu2 P2 O7 to 0.3 M K4 P2 O7 the solution’s pH reduces from 10.1 to 9.3, as a result of copper pyrophosphate hydrolysis. The pH of the final copper plating solution (0.53 M K4 P2 O7 + 0.2 M Cu2 P2 O7 ) was 8.5. Electroplat-
ing was conducted in pyrophosphate solutions additive-free and with the addition of 2,5-dimercapto-1,3,4-thiadiazole, 98% (DMcT, AcrosOrganics). DMcT is one of the components in PY61-H brightener composition, developed for copper plating baths [12,13]. Some experiments were conducted also in potassium hydroxide (KOH) solution and potassium pyrophosphate solution. Potentiostat (PAR 2273) was used in the electrochemical studies and the electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were conducted with a Ta pencil-type electrode at 5 mV amplitude sinusoidal signals in the frequencies range between 0.1 Hz and 100 KHz. Scanning electron microscope (SEM, LEO-982 Geminate FEG-HRSEM) top view and a dual-beam focused ion beam (FIB, FEI Strata 400-S) cross-sectional images were used in order to monitor copper nucleation and deposition processes. The qualitative evaluation of adhesive characteristics of the deposited copper film to the surface of a Ta foil (5 mm × 30 mm × 1 mm) was conducted with the use of a bending, a heat-quench and peel-off tests, while copper adhesion characteristics to wafer samples was evaluated by the latter two tests. Procedures of these tests are described by Magagnin et al. [14] and in ASTM [15]. Ta foil (having a deposited copper film) bending test was performed as followed: bending to 180◦ followed by sample straitening to the initial state. Bent surface zone was examined by SEM prior and subsequent to the bending test. Peel-off test was conducted with adhesive scotch tape with an angle of about 90◦ . Heat-quench was conducted by thermally heating the samples (deposited Ta foil or a wafer) in a tube furnace at 300 ◦ C for 2 h under hydrogen atmosphere.
3. Results and discussion 3.1. Ta oxide “removal”
Fig. 2. Impedance spectra obtained from Ta electrode immersed in 10 wt.% KOH solution at different temperatures (in frequency range between 104 and 10−1 Hz) subsequent to 30 s exposure at OCP.
“Removal” studies of Ta oxide film from Ta surface were conducted with a Ta pencil-type electrode under a cathodic polarization in both KOH and K4 P2 O7 solutions (the latter will serve as the supporting electrolyte for further copper electroplating). Fig. 1a presents potentiodynamic characteristics (10 mV/s scan rate) obtained from tantalum polarized in 5, 10 and 25 wt.% KOH solutions at 25 ◦ C in a wide potential range (between −2 and +0.4 V). Corrosion potential transients obtained from Ta electrode exposure at OCP in KOH solutions are presented, as well (inset). The onsets of anodic current in KOH solutions are located in a potential range between −1.1 and −1.2 V (vs. SCE/V). Above this potential values (up to 0.4 V) Ta electrode remains passive. One can also observe
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that the anodic current density in the passivity region slightly increases with KOH concentration, indicating a minor reduction in Ta passivity. Fig. 1b presents the effect of temperature on the potentiodynamic characteristic of Ta electrode in 10 wt.% KOH solution having a pH value of 10.2. The ECORR transient obtained from Ta during OCP exposure is shown, as well (inset). The anodic current density in the passivity region markedly increases (approximately, in one order of magnitude) once the alkaline electrolyte temperature is increased from 25 to 60 ◦ C, indicating a significant decrease in Ta passivity. The effect of various parameters, such as temperature, solution composition and applied potential, on the state of Ta electrode interface in alkaline solutions was studied by EIS. EIS measurements were performed in accordance with Sapra et al. [9], who studied oxide removal from Ta surface in HF solution. Here, analysis of the charge-transfer resistance value (diameter of semicircle) was used for only a qualitative evaluation of Ta oxide removal from the electrode surface. Fig. 2 presents Niquist plots obtained from Ta electrode immersed in 10 wt.% KOH at temperatures of 25, 40 and 60 ◦ C subsequent to OCP exposure for 30 s. As can be seen, the charge-transfer resistance of the Ta electrode in 10% KOH at OCP significantly decreased with temperature. However, even at 60 ◦ C it remains quite high (∼2.5 K cm2 ), indicating the presence of Ta oxide film on the electrode surface. Electrochemical knowledge and a well-established practice point that cathodic reduction is a proper method for electrode surface cleaning from its oxides. This approach, for example, is effectively used for cathodic reduction of oxide film from surface of ruthenium barrier layers [16]. In this work, we also studied the efficiency of cathodic reduction of oxide film covered Ta electrode surface with the use of EIS. Fig. 3 presents Niquist plots obtained from Ta electrode exposed to a solution containing 10% KOH subsequent to 30 s exposure at different applied potentials, all of them below OCP. One can see that negative shift of the applied potential leads to a decrease in the charge-transfer resistance, indicating a reduction of Ta oxide film. Especially, the most significant reduction of Ta oxide occurred at potentials below −1.5 V. The charge-transfer resistance of tantalum electrode was decreased by approximately four orders of magnitudes by shifting the applied potential from −1.5 to −2.1 V. The value of charge-transfer resistance obtained at a potential of −2.1 V was substantially reduced. Thus, by application of an appropriate cathodic polarization one can achieve an effective “removal” of Ta oxide from the Ta electrode surface, by simply reducing the oxide film into metallic Ta layer.
Fig. 3. Impedance Niquist spectra obtained from Ta electrode immersed in a solution of 10 wt.% KOH (25 ◦ C) subsequent to 30 s potentiostatic exposure at different applied potentials: OCP, −1.3, −1.5, −1.7 V. Inset: EIS of Ta at potential of −1.9 and −2.1 V in the same solution.
Fig. 4. Impedance spectra obtained from Ta electrode immersed in a solution containing 0.3 M K4 P2 O7 (25 ◦ C) subsequent to 30 s potentiostatic exposure at different applied potentials: OCP, −1.3, −1.5. Inset: EIS of Ta in 0.3 M K4 P2 O7 at potentials of −1.7 and −1.9 V.
Fig. 5. FIB cross-sectional view of Si/TaN/Ta interface: (a) initial state: as received wafer prior to potential application; (b) subsequent to 2 h of exposure at a potential of −2.0 V.
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Fig. 4 presents data obtained from similar EIS measurements conducted once Ta electrode was immersed in a solution containing 0.3 M potassium pyrophosphate (K4 P2 O7 ) having a pH of 10.1. Similarly to the results obtained in KOH solution, potentiostatic exposure of Ta electrode in 0.3 M K4 P2 O7 solution at potentials below −1.5 V also leads to a remarkable decrease in the chargetransfer resistance. This indicates that a “removal” by a cathodic reduction of Ta oxide surface film in this potential range occurs, as well. However, interruption or suspension in the cathodic polarization process results in a rapid development and growth of a fresh new oxide layer. To ensure that the cathodic pretreatment does not reduce the thickness of the Ta barrier film, wafer samples having a barrier film were exposed for a deliberately extended time of 2 h, at an applied potential of −2.0 V. The thicknesses of the barrier film prior and subsequent to the long cathodic pretreatment were measured by FIB cross-sectional view of Si/TaN/Ta interface, as shown in Fig. 5. It can be seen that no thinning of the barrier film was detected subsequent to the extended cathodic pretreatment. 3.2. Copper electroplating Copper electroplating over a Ta electrode surface was conducted in Cu2+ containing alkaline pyrophosphate electrolytes prepared from K4 P2 O7 and Cu2 P2 O7 . Copper ion in alkaline pyrophosphate solutions is being presented as a complex ion [Cu(P2 O7 )2 ]6− . This specie undergoes a reduction process under cathodic conditions [17]: [Cu(P2 O7 )2 ]6− + 2e− → Cu + 2[P2 O7 ]4− Following our earlier results on Ta oxide “removal”, a copper electrodeposition over a Ta electrode surface was conducted immediately after Ta oxides cathodic reduction. This was performed by 30 s exposure of the Ta electrode in 0.3 M K4 P2 O7 solution (this will be served from now on as the copper bath supporting electrolyte) at a potential of −2 V. Thus, copper electroplating is being performed in a two-step process: a removal of oxide film and a subsequent copper electrodeposition. In the next series of studies, copper deposition (first electroplating step) was performed by shifting the applied potential to much higher potential values (≥−1.4 V) immediately after the cathodic pretreatment (without interruption of the polarization). Small portion (50 ml) of pyrophosphate solution was taken from the 1 L pyrophosphate solution and poured into a separate container, dissolving 5 g Cu2 P2 O7 . Thus, the content of Cu2+ ions in the bath was 0.03 M. Cathodic pretreatment was conducted in the remaining supporting electrolyte solution (950 ml), which was agitated by a magnetic stirrer. The solution containing 5 g Cu2 P2 O7 (in 50 ml solution) was poured back to the electrolyte at the end of the cathodic pretreatment. Copper deposition was initiated by simultaneously applying a potential and adding the small portion (50 ml) solution, containing dissolved copper in a pyrophosphate based solution (0.015 M Cu2 P2 O7 + 0.3 M K4 P2 O7 ) to the bath of the supporting electrolyte (950 ml). Cathodic behavior of Ta electrode in copper pyrophosphate solutions is illustrated in Fig. 6. Cathodic polarization characteristics of Ta electrode were measured in both electroplating solutions containing 0.03 and 0.2 M Cu2+ . This was performed subsequent to a cathodic pretreatment – potentiostatic exposure at −2 V for 30 s. For comparison, the inset graph presents the cathodic curve obtained from Ta electrode polarized in the supporting pyrophosphate electrolyte (0.3 M K4 P2 O7 ) subsequent to cathodic pretreatment. One can see that copper electrodeposition in electrolyte containing lower Cu2+ concentration (0.03 M) is initiated at more negative potentials (−1.1 vs. −0.8 V detected for the
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Fig. 6. Cathodic polarization characteristics of Ta electrode subsequent to oxide “removal” in two copper electroplating solutions: 0.03 M Cu2+ (2 g/L) + 0.3 M K4 P2 O7 and 0.2 M Cu2+ (12 g/L) + 0.53 M K4 P2 O7 . Inset: Polarization characteristic of Ta electrode in the absence of cooper ion (0.3 M K4 P2 O7 solution).
high copper ion solution) and is characterized with a lower current density values compared to copper deposition in electrolyte containing higher Cu2+ content (0.2 M). In a solution containing 0.03 M Cu2+ the increase in cathodic current density was observed by negative shift of the applied potential down to −1.25 V. Cathodic current density remained practically unaffected (∼8 mA/cm2 ) in a wide potential range below −1.25 V (between −1.25 and −1.5 V). Maximum cathodic current density value obtained in electrolyte containing 0.2 M Cu2+ , was 5 times higher (40 mA/cm2 ) than the value recorded in electrolyte containing 0.03 M Cu2+ . Increase in the cathodic current density displayed in both curves at potentials below −1.5 V is associated with acceleration of hydrogen evolution. Cathodic current density values measured potentiodynamically in 0.3 M K4 P2 O7 solution (Fig. 6, inset) were significantly smaller compared with values obtained in Cu2+ containing electrolytes. Features of copper deposition on Ta surface (foil 0.25 mm thickness, pre-polished to 1200 grid finish) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 electrolyte under the application of different potentials are shown in Figs. 7 and 8. The required potential was applied subsequent to 30 s of pre-exposure at a potential of −2 V. Current–time pro-
Fig. 7. Current–time transients obtained from Ta electrode polarized in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution (pH 9.3) under different applied potentials.
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Fig. 8. SEM images obtained from Ta surface presenting copper nucleus electrodeposited at −1.1 V (a) and −1.2 V (b) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3). Total charge accumulated was 100 mC/cm2 .
files presented in Fig. 7 evaluate nucleation and growth of copper on Ta surface under applied potentials of −1.0, −1.1 and −1.2 V. As can be seen in Fig. 7, the cathodic current density gradually increased during polarization in all applied potentials, indicating increase in copper deposition rate. Copper deposition rate is pronouncedly increased by a negative shift in the applied potential to −1.2 V, as the cathodic current density is significantly increased at this potential. These results are in agreement with SEM observation obtained from Ta surface subsequent to copper deposition at −1.1 and −1.2 V (Fig. 8). Copper deposition under each of these potentials was terminated once a total charge of 100 mC was accumulated. Size inconsistent, separation and irregular shape of copper crystallites distributed over Ta surface is observed in the −1.1 V sample. The number of nucleated crystallites increases by negatively shifting the applied potential to −1.2 V, in agreement with the electrochemical studies, presented in Fig. 7. In order to achieve a conformal copper deposition over TaN/Ta barrier surface the following studies were conducted with copper pyrophosphate electrolytes having dimercaptothiadiazole (DMcT) as an additive. It is known from copper electroplating onto Pt electrode [13] that DMcT/copper pyrophosphate electrolyte system involves two additive species, which are in dynamic equilibrium: DMcT monomer and dimmer. Monomer species form a complex compound with copper ions [13]. In such case, copper electrodeposition is accelerated, presumably, due to assistance in nucleation
of nodule copper crystallites randomly distributed over the surface. Unlike DMcT monomers, dimmer species hinder copper deposition rate by blocking nucleation surface sites [13]. This dual decelerating/accelerating behavior of DMcT on Pt results eventually in enhanced leveling of copper deposition from pyrophosphate electrolytic bath. Here, we describe the use of DMcT in copper pyrophosphate electrolytic bath while Ta serves as the electroplated electrode. Fig. 9 illustrates the effect of DMcT addition (1–10 ppm) on the cathodic potentiodynamic characteristics obtained from Ta electrode in K4 P2 O7 solution containing 0.03 M Cu2+ . As can be seen, the maximum value of cathodic current density, associated with copper cathodic reduction rate, is significantly decreased with increase in DMcT concentration. Further studies of copper electroplating solutions containing only 3–5 ppm DMcT were performed, since at this intermediate concentration the acceleration–inhibition effect of DMcT is well established. The effect of potential on copper nucleation and growth in a solution containing 3 ppm DMcT and copper ions–pyrophosphate system (0.03 M Cu2+ + 0.3 M K4 P2 O7 ) is illustrated in the current transient curves measured at different applied potentials (Fig. 10) and SEM images obtained from copper deposition on a cathodically (−2 V) pre-treated Ta surface subsequent to a short potentiostatic exposure (passed charge 100 mC) at potentials of −1, −1.1 and −1.2 V (Fig. 11). As can be seen, the current transient shape obtained from a potentiostatic exposure in the presence of
Fig. 9. Cathodic potentiodynamic (5 mV/s) profiles obtained from polarizing Ta electrode in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3) at different DMcT content.
Fig. 10. Current–time transients obtained from Ta electrode polarized in 0.03 M Cu2+ + 0.3 M K4 P2 O7 containing 3 ppm DMcT (pH 9.3) at different applied potentials.
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Fig. 11. SEM images obtained from Ta surface showing copper crystallites electrodeposited (100 mC charge was recorded) at potentials of −1.1 V (a) and −1.2 V (b) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 containing 3 ppm DMcT (pH 9.3).
3 ppm DMcT is completely different from the one obtained in an additive-free solution (Fig. 7). Cathodic current density, related to copper deposition rate, rapidly reached its maximum immediately after potential application and gradually decreased during further exposure. As the applied potential was more negative the current peak was higher and appeared earlier. It is reasonable to suggest that DMcT addition markedly increases copper nucleus formation rate, saturating all possible nucleation sites at the Ta electrode surface during the initial stages of copper deposition. Fig. 11 illustrates nucleation and growth of copper at the initial stages of copper deposition (accumulated charge of 100 mC was recorded) on the surface of cathodically pre-treated (−2 V) Ta at a potential of −1.1 and −1.2 V (Fig. 11a and b, respectively) in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution containing 3 ppm of DMcT, as an additive (pH 9.3). The presence of DMcT has a remarkable influence: copper crystallites density nucleated and developed under both applied potential of −1.1 and −1.2 V is markedly higher, compared with copper depositions obtained at the same potentials in DMcTfree solution (see Fig. 8). However, we should point out that despite this improvement, copper nucleation initiated by the addition of copper containing pyrophosphate solution subsequent to cathodic pretreatment at −2.0 V and positive shift of the applied potential cannot be considered as the most suitable one for a conformal filling of sub-micron trenches and vias: dimensions of nucleated copper
crystals are too large (>1 m) and density of nucleus remained quite small (the gap between them is higher than a few microns). In the following experiments, copper nucleation was initiated by adding a portion of a copper pyrophosphate (Cu2 P2 O7 ) solution into the supporting electrolyte (0.3 M K4 P2 O7 ) at the end of the cathodic pretreatment (without interrupting potentiostatic exposure at −2 V) and further exposure for 3–5 s under −2 V in Cu2+ containing electrolyte. Fig. 12 illustrates nucleation and growth of copper on the surface of Ta foil after 3 s exposure at −2.0 V in copper pyrophosphate solution (0.03 M Cu2+ + 0.3 M K4 P2 O7 ) without and with 3 ppm DMcT. One can see that even after 3 s exposure at −2.0 V in the copper pyrophosphate electrolyte with DMcT the surface of Ta foil was completely covered with fine Cu crystals. In the absence of DMcT the density of Cu nucleus is markedly lower while the size of the crystals is higher. Further study of copper deposition on Ta surface was conducted with commercial patterned Si wafers having 20–30 nm TaN/Ta barrier film. Identical copper pyrophosphate solution (0.03 M Cu2+ + 0.3 M K4 P2 O7 and 3 ppm DMcT) and an exact procedure of copper deposition was applied: 1. Activation stage: a cathodic reduction of Ta oxide via exposure to a supporting electrolyte, consisting of 0.3 M K4 P2 O7 solution at potential of −2 V for 30 s;
Fig. 12. SEM images obtained from of Ta foil surface presenting copper crystallites electrodeposited after 3 s exposure under applied potential of −2.0 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 (pH 9.3) without additive (a) and with 3 ppm DMcT (b).
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Fig. 13. Front view SEM images of coupon wafer surface covered with continuous copper layer electrodeposited on TaN/Ta barrier film for 500 s at −1.2 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution (first electroplating step) without DMcT (a) and with 3 ppm of DMcT (b).
2. Seeding stage: injection of 0.03 M Cu2+ (from a solution of Cu2 P2 O7 ) into the supporting electrolyte (0.3 M K4 P2 O7 ) without polarization interruption and further exposure for 3–5 s at a potential of −2 V constitutes the first plating stage; 3. Second plating stage: potential shift to values above −1.4 V and exposing the seeded electrode surface at this potential for a time length capable of obtaining a continuous copper film over the wafer surface; 4. Final plating stage: third electroplating stage is conducted in electrolyte bath containing 0.2 M Cu2+ and 0.53 M K4 P2 O7 . This is done in order to accelerate copper deposition.
Fig. 13 presents front view of a coupon wafer surface covered with continuous copper layer deposited over a TaN/Ta barrier film in 0.03 M Cu2+ + 0.3 M K4 P2 O7 solution without DMcT (Fig. 13a) and with 3 ppm of DMcT (Fig. 13b). Electroplating was conducted according to the above-described procedure with a 500 s expo-
sure time at a potential of −1.2 V. As can be seen in the SEM images, copper deposition over a coupon wafer surface, obtained in pyrophosphate solution in the absence of DMcT, is not uniform and is characterized with the formation of large crystals (Fig. 13a). In the presence of 3 ppm of DMcT a uniform copper deposition was obtained over the whole wafer coupon surface including centered and peripheral zones. FIB cross-sectional view of Si/TaN/Ta patterned wafer surface having a copper layer deposited in DMcT containing Cu pyrophosphate solution is shown in Fig. 14. Fig. 14a and b presents cross-section of wafers having a copper film (∼100 nm) deposited for 500 s at a potential of −1.2 V (matching deposition stage, as shown in Fig. 13). The filling of small features in the pyrophosphate solution can be described as a conformal process. Fig. 14c and d illustrates Si/TaN/Ta cross-section of copper filled features obtained at the final plating stage. The final plating stage was conducted at a potential of −1 V for 100 s in a fresh pyrophosphate electroplating bath with higher Cu2+ content (0.2 M Cu2+ (as Cu2 P2 O7 ), 0.53 M K4 P2 O7 and 5 ppm DMcT, pH 8.5, 25 ◦ C).
Fig. 14. FIB cross-sections of Si/TaN/Ta patterned wafer surface covered with copper film, electrodeposited on TaN/Ta barrier film in copper pyrophosphate electrolytes: (a, b) after first electroplating stage conducted for 500 s at −1.2 V in 0.03 M Cu2+ + 0.3 M K4 P2 O7 and 3 ppm DMcT (pH 9.3) solution at 25 ◦ C; (c, d) after a second (final) electroplating stage conducted for 100 s at −1.0 V in 0.2 M Cu2+ (as Cu2 P2 O7 ) + 0.53 M K4 P2 O7 solution containing 5 ppm DMcT, pH 8.5, 25 ◦ C.
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The thickness of the deposited copper layer after the final electroplating stage was 450 nm. Our studies indicate that the best adhesion of Cu to the Ta electrode surface was obtained when copper nucleation (first plating step) was conducted for 3–5 s under the applied potential of −2.0 V in pyrophosphate electrolyte containing 0.03 M Cu2+ (0.3 M K4 P2 O7 + 0.015 M Cu2 P2 O7 , pH 9.3) and 3 ppm DMcT. Copper film deposited on the surface of a patterned wafer was characterized with a very good adhesion to the thin TaN/Ta barrier film. As was noted above, the adhesion of the deposited copper film to Ta surfaces (foil and wafer) was qualitatively evaluated by bending (only with Ta foil), heat-quench and peel-off tests. No exfoliations of copper film from the tantalum surface were observed subsequent to the application of the test methods, indicating a good adhesion between the deposited copper film and the Ta surfaces. 4. Conclusions We present in this extensive work an alternative approach for seedless copper electroplating on Ta surface, being initially covered in pristine passive oxide layer. The method described in this study involves the introduction of a new electroplating bath along with a removal of the oxide passivating film. The method is a generic one and may also be applied to other passivated metals and alloys (Ru and Ru/Ta mixed metals), being currently considered as barrier films for future seedless integrated systems. The following conclusions can be drawn from this pioneering study: • Copper electrodeposition over a thin TaN/Ta barrier can be performed in a two-step process: activation of TaN/Ta barrier by a cathodic reducing of Ta oxide (“removal” step) and subsequent copper electroplating performed in the invariable electrochemical bath. • Ta oxide reduction (“removal”) is being performed in 0.3 M K4 P2 O7 solution under the application of a potential of −2 V for 30 s. At this potential, Ta oxide is being reduced to metallic Ta. • Copper plating is initiated at a potential of −2 V by injecting low copper content Cu2 P2 O7 solution, (0.03 M Cu2+ ) and 3 ppm DMcT, into the supporting K4 P2 O7 electrolyte, followed by 3–5 s exposure in this solution. It was established in this work that DMcT additive improves Cu nucleation and growth on Ta surface, providing a conformal features filling. • Supplementary Cu plating is continued by shifting the applied potential to −1.2 V in the same electrolytic bath, while the final plating process can be performed in high copper ion content pyrophosphate solution. • Copper layer deposited is characterized with an excellent adhesion to the Ta surface.
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It should be pointed out that copper filling with the use of Cu pyrophosphate chemistry is totally different from the acid-sulfate ones, being used in current IC metallization. While the wafer features in this study are well-filled with Cu from the pyrophosphate chemistry, it is yet to be demonstrated that the chemistry is capable to fill features that are much narrower (below 50 nm width). The organic additives, particular developed to the acid-based Cu plating chemistries, enable a rapid bottom-up fill, allowing a defect-free filling of narrow features. Thus, it is anticipated that pyrophosphate chemistry’s additives, to be developed in future research, may lead to enhanced nucleation densities and similar or even better narrow features fillings than current acid bath technology. Acknowledgements This research was financially supported by the Technion Research Foundation, a joint grant from the Center for Absorption in Science of the Ministry of Immigrant Absorption, and by the Committee for Planning and Budgeting of the Council for Higher Education under the framework of the KAMEA Program. The authors would like to thank Imec, Belgium, for patterned wafer supply. References [1] S. Kim, D. Duquette, J. Electrochem. Soc. 154 (2007) D195. [2] M. Zheng, J.J. Kelly, H. Deligianni, J. Electrochem. Soc. 154 (2007) D400. [3] H. Hui Hsu, C. Chan Hsieh, M. Hsian Chen, S. Jein Lin, J. Wei Yeh, J. Electrochem. Soc. 148 (2001) C590. [4] Z. Wang, H. Li, H. Shodiev, I. Ivar Suni, Electrochem. Solid-State Lett. 8 (2005) G283. [5] A. Radisic, Y. Cao, P. Taephaisitphongse, A. West, P.C. Searson, J. Electrochem. Soc. 148 (2001) C41; A. Radisic, Y. Cao, P. Taephaisitphongse, A. West, P.C. Searson, J. Electrochem. Soc. 151 (2004) C369. [6] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, chapter IV, section 09.3 – Tantalum. [7] L.L. Shreier (Ed.), Corrosion, Newness-Butterworth, London, 1995. [8] A.J. Bard, Encyclopedia of Electrochemistry of the Elements, vol. 2, Marcel Dekker Inc., 1976 (chapter 3 – Tantalum). [9] S. Sapra, H. Li, Z. Wang, I.I. Suni, J. Electrochem. Soc. 152 (2005) B193. [10] J. Zhang, S. Li, P.W. Carter, J. Electrochem. Soc. 154 (2007) H109. [11] D.A. Vermilyea, J. Electrochem. Soc. 102 (1955) 655. [12] D. Tench, C. Ogden, J. Electrochem. Soc. 125 (1978) 194. [13] C. Ogden, D. Tench, J. Electrochem. Soc. 128 (1981) 539. [14] L. Magagnin, R. Maboudian, C. Carraro, Thin Solid Films 434 (2003) 100. [15] ASTM International Standards B571-97 (2003), Standard Practice for Qualitative Adhesion Testing of Metallic Coatings, Paints and Coatings, American Society for Testing and Materials (ASTM). [16] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, J. Electrochem. Soc. 153 (2006) C37. [17] H. Konno, M. Nagayama, Electrochim. Acta 23 (1978) 1001.