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Seed layer corrosion of Damascene structures in copper sulfate electrolytes
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 225–233
Seed layer corrosion of Damascene structures in copper sulfate electrolytes Nicholas M. Martyak*, Pierre Ricou ATOFINA Chemicals, Inc., 900 First Avenue, King of Prussia, PA 19406, USA
Abstract ( seed layer The influence of sulfuric acid concentration in copper sulfate plating solutions on the stability of a 1000 A in Damascene structures is presented. Immersion of a seeded wafer into the plating solutions results in chemical dissolution, thinning the copper seed layer. Anodic current density steps initiate seed layer corrosion resulting in exposure of the underlying TaN barrier layer. Large applied current density steps (>300 mA/cm2) or high free sulfuric acid concentration causes very rapid dissolution of the seed layer. The anodic dissolution efficiency is greater than 100% indicating that chemical dissolution of copper is contributing to the disappearance of the copper seed layer. r 2003 Elsevier Ltd. All rights reserved. Keywords: Copper; Damascene; Corrosion; Seed layer
1. Introduction Copper electrodeposition has emerged as the most efficient way to fill sub-micron features in advanced semiconductor devices. The processing of such devices requires numerous steps including the deposition of a barrier layer on silicon or silicon oxide to prevent the diffusion of copper into silicon [1]. Several barrier layers may be used including titanium, titanium nitride, tantalum and tantalum nitride [2,3]. These barrier layers are typically deposited within semiconductor features such as trenches and vias using chemical vapor deposition (CVD) or physical vapor deposition (PVD) processes. The thickness of these barrier layers varies with each device generation and may be as thick as ( on 0.18 mm devices and as thin as 20 A ( on 100 nm 200 A features. However, this barrier layer must be uniform and continuous to ensure no copper diffusion into the underlying silicon. A copper seed layer is deposited on top of the barrier layer to allow for the subsequent electrolytic deposition of copper from an aqueous electrolyte [4,5]. Such a *Corresponding author. Tel.: +1-6108786730. E-mail address: nick.martyak@atofina.com (N.M. Martyak).
copper seed layer is necessary because it is very difficult to nucleate copper grains from an aqueous copper solution on silicon or refractory-type materials such as Ta or TaN [6]. This copper seed layer is usually deposited via a PVD method and its thickness also varies with device generation. It is important to deposit the seed layer at the bottom of the features to ensure both bottom-up fill of the electrodeposited copper and adequate adhesion of the bulk copper deposit to the barrier layer. A critical issue facing the semiconductor industry is adequate coverage of the copper seed layer at the base of the features particularly as the size of the vias or trenches decrease from 0.13 to 0.1 mm and smaller. The copper seed layer thickness profile was studied by Webb and co-workers who showed that the thickness may taper due to a buildup of the seed layer at the top edges of the features [7]. Mayer and co-workers [8] claim that the thickness of the PVD copper layer may only be 8–15% of the thickness on the sidewalls of a trench compared to the thickness in the field area such as seen in Figs. 1(a) and (b). The copper electroplating solutions currently used to metallize the Damascene structures are based on the chemistries used to plate copper onto printed wire boards. These solutions employ cupric sulfate and high concentrations of sulfuric acid, 100–200 g/l H2SO4.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.09.002
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with varying concentrations of sulfuric acid. Electrochemical methods were used to study the dissolution ( copper seed layer on a TaN barrier behavior of a 1000 A layer. Scanning electron (SEM) and atomic force microscopy (AFM) along with X-ray photoelectron spectroscopy (XPS) techniques were used to image the dissolution of the seed layer with time.
Fig. 1. (a) Uniform deposition of copper seed layer on barrier layer. (b) Non-uniform copper seed layer: very thin seed at bottom of via.
However, because of the vast differences in dimensions between the features on the printed circuit board and those on silicon devices, modifications in the plating chemistries have evolved to ensure void-free copper deposits in the advanced semiconductor features [9,10]. Additives such as suppressor (polyethylene glycol-type materials), accelerators (divalent sulfur compounds) and leveling agents are commonly added to modify the plating characteristics of the electrolyte. The development of new copper sulfate solution must take into account the fragile nature of the PVD seed layer. If the copper sulfate electrolytes chemically attack the seed layer, the thickness of the seed layer is diminished with a concomitant increase in the resistance of this layer leading to plating difficulties. Also, chemical dissolution of the seed layer due to the high free acid concentrations may produce discontinuities within this layer and lead to voids and other defects in the subsequent electrodeposited copper layer. This study was undertaken to understand the corrosion of the copper seed layer in cupric sulfate solutions
Fig. 2. (a) Eoc transition of the copper sulfate solutions on TaN. (b) Potential–pH diagram for the Ta–N–O system showing the Eoc values in Fig. 2(a) lies in the Ta2O5 domain. (c) SEM of barrier layer showing very smooth surface.
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2. Experimental Copper solutions were prepared by dissolving copper(II) oxide into an aqueous solution of sulfuric acid. The cupric ion concentration was held constant at 40 g/l.
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Once the copper oxide was dissolved, additional free sulfuric acid was added to the solution so the final free acid concentration was 5 or 30 g/l in each of the copper electrolytes. The solutions were filtered prior to testing and commercial additives, suppressor, accelerator and
Fig. 3. (a) Survey spectrum of barrier layer showing tantalum and nitrogen species. (b) Core spectrum of tantalum showing two species associated with TaN and Ta2O5. (c) Detailed core spectrum of oxygen in the barrier layer.
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leveler, obtained from Shipley (Maryborough, MA), were included in the copper solutions along with 50 mg/l of HCl. Electrochemical studies were done using an EG&G PAR 273 potentiostat and all potentials were recorded vs. a saturated calomel electrode (SCE). Open-circuit potential (Eoc) measurements were made on 1 cm2 of ( thick seed) TaN and copper seeded wafer (1000 A obtained from WaferNet (San Jose, CA) using the two copper electrolytes. Current density-step measurements were made on the copper seed wafer from 100 to 750 mA/ cm2 and the potential was followed with time. The surface morphology of the TaN and copper seed layer were studied using SEM, AFM and XPS. A Leo 1530 field-emission microscope SEM was used to study un-coated samples. The accelerating voltage was 4 kV. A Digital Nanoscope III was used to perform AFM analysis using tapping mode and height, deflection and phase images were recorded. A Kratos Axis 115 XPS instrument was used to study the surface composition of the various metallic coatings. Sample analysis was done using an aluminum target and spectra were
recorded from 0–1400 eV. Detailed spectra of the Cu2p(3/2), Ols, Ta4f(7/2), and Nls regions were recorded after 10 sweeps. Deconvolution of overlapping peaks was done using a mixed Gauassian(70%)–Lorenztian(30%) fit.
3. Results and discussion Eoc measurements of the copper sulfate electrolytes on the TaN barrier layer are shown in Fig. 2(a). The potentials quickly reached a limiting potential of about +0.21–+0.25 V vs. SCE in the more dilute copper sulfate solution whereas the solution containing the high free sulfuric acid required approximately 1700 s to attain a steady-state potential. The pH–potential couple [11] for these electrolytes shows the surface of the barrier layer is covered with a native oxide, Ta2O5, Fig. 2(b). 2Ta þ 5H2 O-Ta2 O5 þ 10Hþ þ 10e : The surface of the TaN barrier layer is very smooth as seen in Fig. 2(c).
Fig. 4. (a) Morphology of copper seed layer: copper seed B25–50 nm. (b) Eoc transition of the copper solutions on the copper seed layer. (c) E–pH diagram for the Cu–H2O system showing the predominant domains of stability.
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XPS studies on the barrier layer material after Eoc measurements are shown in Figs. 3(a)–(c). The survey spectra show tantalum, nitrogen, oxygen and small amounts of carbon and copper. The carbon is adventitious seen on all XPS spectra and the small amount of copper is due to residual surface contamination after immersion in the copper sulfate electrolyte. Detailed
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XPS spectra of the Ta4f(7/2) regions shows two main peaks centered between 18 to 23 eV and 23 eV to about 28 eV. The lower binding energy signals arise from tantalum in TaN and the higher binding energy peaks are associated with Ta2O5 in agreement with the Eoc measurements in Fig. 2(b). The intensities of the two peaks are approximately equal indicating that the
Fig. 5. (a) XPS survey scan of the copper seed layer. (b) Detailed core spectra of Cu2p(3/2) region showing various copper species. (c) O(1 s) Region of copper seed layer.
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tantalum nitride barrier layer is covered with an inherent oxide. The O(1 s) signal is deconvoluted into three separate regions. The strongest signal is due to the oxide on the barrier layer whereas the hydroxyl oxygen may be due to adsorbed water on the surface. Similar Eoc studies of the copper solutions on the ( copper seed layer are shown in Figs. 4(a) and (b). 1000 A Prior to immersion the copper sulfate solution, the seed layer is very uniform and continuous, Fig. 4(a). The potentials of both electrolytes are stable to at least 3600 s, reaching steady state after only 500 s of immersion in the copper sulfate solutions. The higher-free acid
Table 1 Measured transition times for dissolution of the copper seed layer Current density step (mA/cm2)
CuSO4:H2SO4 (5 g/l)
CuSO4:H2SO4 (30 g/l)
100 200 300 500 750
2400 1500 750 350 350
2100 1300 550 400 300
solution exhibits a slightly more active potential, +0.075 V, compared to the lower free acid solution, +0.060 V. These E–pH couples are in the Cu+2 region of the Pourbaix diagram indicating corrosion is of the copper seed layer occurring during this exposure time, Fig. 4(c). While the potential–pH couple of the CuSO4:H2SO4 electrolytes on the seeded layer indicates the domain of stability is cupric ion, Cu+2, the extent of the corrosion of this seed layer is difficult to know exactly because these E–pH diagrams show only the thermodynamic stability and give no indications about the kinetics of corrosion. Therefore, corrosion of the seed layer is occurring but not at a sufficient rate to corrode the seed layer to the underlying TaN barrier layer and thus shift the open-circuit potential into the anodic region seen in Fig. 2(a). XPS studies of the seed layer prior to corrosion are seen in Figs. 5(a)–(c). The survey spectrum shows the Cu2p(3/2), O(1 s) and trace amount of chloride. No barrier layer elements are seen in this spectrum. Detail regions of copper and oxygen are seen in Figs. 5(b) and (c), respectively. The total Cu2p(3/2) is centered at B932.0 eV and is due to either metallic copper or a Cu(I) oxide. The higher binding energy peak at B932 eV is due to a Cu(II) oxide on the seed layer surface. The
Fig. 6. (a) Potential–time transitions of the CuSO4:H2SO4 (5 g/l) Electrolyte. (b) Potential–time transitions of the CuSO4:H2SO4 (30 g/l) electrolyte. (c) Transition times to dissolve copper seed layer in copper sulfate electrolytes.
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intensities of the Cu(I) and Cu(II) signals are about 3:1 indicating most of the seed layer surface is metallic or covered with a reduced copper oxide species. The O(1 s) signal is also composed of three smaller signals with the largest contribution coming from the reduced copper oxide species, Cu2O. Comparison of the tow oxygen signals arising from the copper species confirms the
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surface of the barrier layer is covered with a reduced copper oxide coating. Stepping the current densities and monitoring the potentials with time initiates rapid corrosion of the seed layer in the two copper sulfate solutions. Table 1 and Figs. 6(a)–(c) show the transition times (t) in seconds for the potentials to change from a region were copper is
Fig. 7. (a) AFM top-view image and SEM image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), Cl (50 mg/l), 100 mA/cm2 step, 2400 s to +0.13 V vs. SCE. (b) XPS spectrum of corroded copper sulfate deposit showing exposed TaN barrier layer.
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fairly stable, yet slowly dissolving, to one characteristic of Eoc for the barrier layer. The onset of the barrier layer potential marks the time necessary to dissolve the seed layer exposing this underlying protective film. Low current density steps require significant time to completely dissolve the seed layer and expose the underlying barrier layer whereas a higher free acid concentration in the copper solution causes more rapid corrosion of the seed layer. The slope of the potential–time transitions is sharp indicating a uniform dissolution of the seed layer. If pitting or non-uniform corrosion of the seed layer occurred, small areas of the underlying barrier layer would be exposed first and the transition of the potential from about +0.075 V, characteristic of metallic copper, to about +0.25 V, that of the TaN layer, would occur slowly. The dissolution rate of the seed layer is greater than that expected from the electrochemical measurements. For example, the theoretical dissolution time to dissolve ( seed layer is 1 cm2 of 1000 A
Metallic copper can also corrode in the acid copper plating solution by dissolved oxygen Cu0 -Cuþ2 ; O2 þ 2Hþ þ 2e -H2 O:
Time to dissolve Cu Seed ¼ ðweight CuÞð#e-ÞðFaraday constantÞ= ðcurrent density stepÞðat: weight CuÞ; ( thick copper in 1 cm2 where the weight of 1000 A exposed area is ( Weight Cu ¼ ð1 cm2 exposed areaÞð1000 AÞ 8 ( g=cm3 Þ ð1 10 cm=AÞð8:9 ¼ 8:9 105 g Cu0 : Therefore, the theoretical time necessary to completely dissolve the copper seed layer using a current density step of 100 mA/cm2 is ¼ ð8:9 105 g Cu0 Þð2e-=molÞ ð96 487 C=molÞ=ð100 mA=cm2 Þ ( g=molÞ ¼ 2700 sÞ: ð1 106 A=mAÞð63:54 As seen in Figs. 6(a)–(c) and Table 1, the actual transition times for the 5 and 30 g/l H2SO4 electrolytes using the 100 mA/cm2 step are 2400 and 2100 s, respectively. Similar calculations were done for other current density steps. The data presented in Figs. 4(b) and (c) shows the stable copper species is Cu+2 and the XPS results in Fig. 5(b) shows the copper seed layer which covered with an oxide, likely both Cu(I) and Cu(II) oxides. Cuprous and cupric oxides are unstable in the acid sulfate electrolytes; both oxides decompose to cupric ions Cu2 O þ 2Hþ -Cuþ2 þ Cu0 þ H2 O; CuO þ 2Hþ -Cuþ2 þ H2 O:
Fig. 8. (a) AFM top-view image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), Cl (50 mg/l), 500 mA/cm2 step 425 s, to +0.15 V vs. SCE. (b) SEM image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), C (50 mg/l), 500 mA/cm2 step 425 s, to +0.15 V vs. SCE.
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The accelerated dissolution rates are likely due to a chemical attack, Figs. 4(b) and (c), of the seed layer by the free sulfuric acid and accounts for approximately 10% increase in the dissolution rate of the seed layer using 5 g/l free sulfuric acid and about a 21% increase in dissolution rate using 30 g/l free sulfuric acid. AFM and SEM images of the partially corroded seed layer are seen in Figs. 7 and 8. The PVD seed layer appears to agglomerate into larger, rounded structures after immersion in the copper solutions compared to the unexposed seed layer. Corrosion of the seed layer is general and defects (holes) appear in the seed layer. At a given current density step, the defects merge into one another resulting in large areas of the underlying barrier layer exposed. XPS analysis of the partially corroded seed layer seen in Fig. 7(b) shows signals for both the copper and the underlying barrier layer. At very high current density steps, Figs. 8(a) and (b), the seed layer is badly corroded and isolated islands of the seed layer remain. Plating on a highly corroded seed layer such as that in Fig. 8(b) is likely to result in defects in the electrodeposited copper layer. As the seed layer thickness decreases in future generation devices, the propensity for seed layer dissolution in acid copper sulfate solutions will increase, particularly if the concentration of free sulfuric acid is relatively high. Dissolution of the seed layer may lead to defects such as voids in the copper-filled features and poor adhesion of the electroplated copper to the barrier layer.
4. Conclusions ( Electrochemical studies on the corrosion of a 1000 A copper seed layer is shown to occur at an accelerated
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rate due to the free sulfuric acid in the copper electrolytes. A high free sulfuric acid concentration results in a more rapid dissolution of the seed layer exposing the underlying barrier layer. Continued exposure to the cupric sulfate–sulfuric acid electrolyte produces isolated islands of an agglomerated seed layer.
References [1] Hong SW, Park JW. Electrochem Solid State Lett, 2002;5:C107. [2] Nogami T, Romero J, Dubin V, Brown D, Adam E, Proceedings of IITC, 1998, p. 298. [3] Chin BL, Yao G, Ding P, Fu J, Chen L. Semiconductor International, 2001, p. 107. [4] Gilkes D, Ramappa D, Oh M, Merchant S, J. Electron Mater, 2002;31:1047. [5] Lee MK, Wang HD, Wang JJ. Solid State Electron, 1997;41:695. [6] Hoffmann PM, Radisic A, Searson PC. J Electrochem Soc, 2000;147:2576. [7] Webb E, Sukamto J, Andryushchenko T, Danek M, Klawuhn E, Rozbicki R, Alers G, Suwwan de Felipee T, Bhaskaran V, Frank A, Pfeifer K, Reid J. Proceedings of VMIC, 2001. [8] Mayer S, Contolini R, Jackson R, Reid J, Martin J, Morrissey D, Schetty R. Proceedings of Electrochemical Society, 1999. [9] Kobayashi T, Kawasaki J, Mihara K, Honma H. Electrochim Acta, 2001;47:85. [10] Yih P, Wang D, Chiao S. ACM Technical Bulletin, ACM Research, Fremont, CA. [11] Pourbaix M. Atlas of electrochemical equilibria in aqueous solutions. Oxford, England: Pergamon Press; 1966.
Materials Science in Semiconductor Processing 6 (2003) 225–233
Seed layer corrosion of Damascene structures in copper sulfate electrolytes Nicholas M. Martyak*, Pierre Ricou ATOFINA Chemicals, Inc., 900 First Avenue, King of Prussia, PA 19406, USA
Abstract ( seed layer The influence of sulfuric acid concentration in copper sulfate plating solutions on the stability of a 1000 A in Damascene structures is presented. Immersion of a seeded wafer into the plating solutions results in chemical dissolution, thinning the copper seed layer. Anodic current density steps initiate seed layer corrosion resulting in exposure of the underlying TaN barrier layer. Large applied current density steps (>300 mA/cm2) or high free sulfuric acid concentration causes very rapid dissolution of the seed layer. The anodic dissolution efficiency is greater than 100% indicating that chemical dissolution of copper is contributing to the disappearance of the copper seed layer. r 2003 Elsevier Ltd. All rights reserved. Keywords: Copper; Damascene; Corrosion; Seed layer
1. Introduction Copper electrodeposition has emerged as the most efficient way to fill sub-micron features in advanced semiconductor devices. The processing of such devices requires numerous steps including the deposition of a barrier layer on silicon or silicon oxide to prevent the diffusion of copper into silicon [1]. Several barrier layers may be used including titanium, titanium nitride, tantalum and tantalum nitride [2,3]. These barrier layers are typically deposited within semiconductor features such as trenches and vias using chemical vapor deposition (CVD) or physical vapor deposition (PVD) processes. The thickness of these barrier layers varies with each device generation and may be as thick as ( on 0.18 mm devices and as thin as 20 A ( on 100 nm 200 A features. However, this barrier layer must be uniform and continuous to ensure no copper diffusion into the underlying silicon. A copper seed layer is deposited on top of the barrier layer to allow for the subsequent electrolytic deposition of copper from an aqueous electrolyte [4,5]. Such a *Corresponding author. Tel.: +1-6108786730. E-mail address: nick.martyak@atofina.com (N.M. Martyak).
copper seed layer is necessary because it is very difficult to nucleate copper grains from an aqueous copper solution on silicon or refractory-type materials such as Ta or TaN [6]. This copper seed layer is usually deposited via a PVD method and its thickness also varies with device generation. It is important to deposit the seed layer at the bottom of the features to ensure both bottom-up fill of the electrodeposited copper and adequate adhesion of the bulk copper deposit to the barrier layer. A critical issue facing the semiconductor industry is adequate coverage of the copper seed layer at the base of the features particularly as the size of the vias or trenches decrease from 0.13 to 0.1 mm and smaller. The copper seed layer thickness profile was studied by Webb and co-workers who showed that the thickness may taper due to a buildup of the seed layer at the top edges of the features [7]. Mayer and co-workers [8] claim that the thickness of the PVD copper layer may only be 8–15% of the thickness on the sidewalls of a trench compared to the thickness in the field area such as seen in Figs. 1(a) and (b). The copper electroplating solutions currently used to metallize the Damascene structures are based on the chemistries used to plate copper onto printed wire boards. These solutions employ cupric sulfate and high concentrations of sulfuric acid, 100–200 g/l H2SO4.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.09.002
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with varying concentrations of sulfuric acid. Electrochemical methods were used to study the dissolution ( copper seed layer on a TaN barrier behavior of a 1000 A layer. Scanning electron (SEM) and atomic force microscopy (AFM) along with X-ray photoelectron spectroscopy (XPS) techniques were used to image the dissolution of the seed layer with time.
Fig. 1. (a) Uniform deposition of copper seed layer on barrier layer. (b) Non-uniform copper seed layer: very thin seed at bottom of via.
However, because of the vast differences in dimensions between the features on the printed circuit board and those on silicon devices, modifications in the plating chemistries have evolved to ensure void-free copper deposits in the advanced semiconductor features [9,10]. Additives such as suppressor (polyethylene glycol-type materials), accelerators (divalent sulfur compounds) and leveling agents are commonly added to modify the plating characteristics of the electrolyte. The development of new copper sulfate solution must take into account the fragile nature of the PVD seed layer. If the copper sulfate electrolytes chemically attack the seed layer, the thickness of the seed layer is diminished with a concomitant increase in the resistance of this layer leading to plating difficulties. Also, chemical dissolution of the seed layer due to the high free acid concentrations may produce discontinuities within this layer and lead to voids and other defects in the subsequent electrodeposited copper layer. This study was undertaken to understand the corrosion of the copper seed layer in cupric sulfate solutions
Fig. 2. (a) Eoc transition of the copper sulfate solutions on TaN. (b) Potential–pH diagram for the Ta–N–O system showing the Eoc values in Fig. 2(a) lies in the Ta2O5 domain. (c) SEM of barrier layer showing very smooth surface.
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2. Experimental Copper solutions were prepared by dissolving copper(II) oxide into an aqueous solution of sulfuric acid. The cupric ion concentration was held constant at 40 g/l.
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Once the copper oxide was dissolved, additional free sulfuric acid was added to the solution so the final free acid concentration was 5 or 30 g/l in each of the copper electrolytes. The solutions were filtered prior to testing and commercial additives, suppressor, accelerator and
Fig. 3. (a) Survey spectrum of barrier layer showing tantalum and nitrogen species. (b) Core spectrum of tantalum showing two species associated with TaN and Ta2O5. (c) Detailed core spectrum of oxygen in the barrier layer.
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leveler, obtained from Shipley (Maryborough, MA), were included in the copper solutions along with 50 mg/l of HCl. Electrochemical studies were done using an EG&G PAR 273 potentiostat and all potentials were recorded vs. a saturated calomel electrode (SCE). Open-circuit potential (Eoc) measurements were made on 1 cm2 of ( thick seed) TaN and copper seeded wafer (1000 A obtained from WaferNet (San Jose, CA) using the two copper electrolytes. Current density-step measurements were made on the copper seed wafer from 100 to 750 mA/ cm2 and the potential was followed with time. The surface morphology of the TaN and copper seed layer were studied using SEM, AFM and XPS. A Leo 1530 field-emission microscope SEM was used to study un-coated samples. The accelerating voltage was 4 kV. A Digital Nanoscope III was used to perform AFM analysis using tapping mode and height, deflection and phase images were recorded. A Kratos Axis 115 XPS instrument was used to study the surface composition of the various metallic coatings. Sample analysis was done using an aluminum target and spectra were
recorded from 0–1400 eV. Detailed spectra of the Cu2p(3/2), Ols, Ta4f(7/2), and Nls regions were recorded after 10 sweeps. Deconvolution of overlapping peaks was done using a mixed Gauassian(70%)–Lorenztian(30%) fit.
3. Results and discussion Eoc measurements of the copper sulfate electrolytes on the TaN barrier layer are shown in Fig. 2(a). The potentials quickly reached a limiting potential of about +0.21–+0.25 V vs. SCE in the more dilute copper sulfate solution whereas the solution containing the high free sulfuric acid required approximately 1700 s to attain a steady-state potential. The pH–potential couple [11] for these electrolytes shows the surface of the barrier layer is covered with a native oxide, Ta2O5, Fig. 2(b). 2Ta þ 5H2 O-Ta2 O5 þ 10Hþ þ 10e : The surface of the TaN barrier layer is very smooth as seen in Fig. 2(c).
Fig. 4. (a) Morphology of copper seed layer: copper seed B25–50 nm. (b) Eoc transition of the copper solutions on the copper seed layer. (c) E–pH diagram for the Cu–H2O system showing the predominant domains of stability.
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XPS studies on the barrier layer material after Eoc measurements are shown in Figs. 3(a)–(c). The survey spectra show tantalum, nitrogen, oxygen and small amounts of carbon and copper. The carbon is adventitious seen on all XPS spectra and the small amount of copper is due to residual surface contamination after immersion in the copper sulfate electrolyte. Detailed
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XPS spectra of the Ta4f(7/2) regions shows two main peaks centered between 18 to 23 eV and 23 eV to about 28 eV. The lower binding energy signals arise from tantalum in TaN and the higher binding energy peaks are associated with Ta2O5 in agreement with the Eoc measurements in Fig. 2(b). The intensities of the two peaks are approximately equal indicating that the
Fig. 5. (a) XPS survey scan of the copper seed layer. (b) Detailed core spectra of Cu2p(3/2) region showing various copper species. (c) O(1 s) Region of copper seed layer.
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tantalum nitride barrier layer is covered with an inherent oxide. The O(1 s) signal is deconvoluted into three separate regions. The strongest signal is due to the oxide on the barrier layer whereas the hydroxyl oxygen may be due to adsorbed water on the surface. Similar Eoc studies of the copper solutions on the ( copper seed layer are shown in Figs. 4(a) and (b). 1000 A Prior to immersion the copper sulfate solution, the seed layer is very uniform and continuous, Fig. 4(a). The potentials of both electrolytes are stable to at least 3600 s, reaching steady state after only 500 s of immersion in the copper sulfate solutions. The higher-free acid
Table 1 Measured transition times for dissolution of the copper seed layer Current density step (mA/cm2)
CuSO4:H2SO4 (5 g/l)
CuSO4:H2SO4 (30 g/l)
100 200 300 500 750
2400 1500 750 350 350
2100 1300 550 400 300
solution exhibits a slightly more active potential, +0.075 V, compared to the lower free acid solution, +0.060 V. These E–pH couples are in the Cu+2 region of the Pourbaix diagram indicating corrosion is of the copper seed layer occurring during this exposure time, Fig. 4(c). While the potential–pH couple of the CuSO4:H2SO4 electrolytes on the seeded layer indicates the domain of stability is cupric ion, Cu+2, the extent of the corrosion of this seed layer is difficult to know exactly because these E–pH diagrams show only the thermodynamic stability and give no indications about the kinetics of corrosion. Therefore, corrosion of the seed layer is occurring but not at a sufficient rate to corrode the seed layer to the underlying TaN barrier layer and thus shift the open-circuit potential into the anodic region seen in Fig. 2(a). XPS studies of the seed layer prior to corrosion are seen in Figs. 5(a)–(c). The survey spectrum shows the Cu2p(3/2), O(1 s) and trace amount of chloride. No barrier layer elements are seen in this spectrum. Detail regions of copper and oxygen are seen in Figs. 5(b) and (c), respectively. The total Cu2p(3/2) is centered at B932.0 eV and is due to either metallic copper or a Cu(I) oxide. The higher binding energy peak at B932 eV is due to a Cu(II) oxide on the seed layer surface. The
Fig. 6. (a) Potential–time transitions of the CuSO4:H2SO4 (5 g/l) Electrolyte. (b) Potential–time transitions of the CuSO4:H2SO4 (30 g/l) electrolyte. (c) Transition times to dissolve copper seed layer in copper sulfate electrolytes.
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intensities of the Cu(I) and Cu(II) signals are about 3:1 indicating most of the seed layer surface is metallic or covered with a reduced copper oxide species. The O(1 s) signal is also composed of three smaller signals with the largest contribution coming from the reduced copper oxide species, Cu2O. Comparison of the tow oxygen signals arising from the copper species confirms the
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surface of the barrier layer is covered with a reduced copper oxide coating. Stepping the current densities and monitoring the potentials with time initiates rapid corrosion of the seed layer in the two copper sulfate solutions. Table 1 and Figs. 6(a)–(c) show the transition times (t) in seconds for the potentials to change from a region were copper is
Fig. 7. (a) AFM top-view image and SEM image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), Cl (50 mg/l), 100 mA/cm2 step, 2400 s to +0.13 V vs. SCE. (b) XPS spectrum of corroded copper sulfate deposit showing exposed TaN barrier layer.
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fairly stable, yet slowly dissolving, to one characteristic of Eoc for the barrier layer. The onset of the barrier layer potential marks the time necessary to dissolve the seed layer exposing this underlying protective film. Low current density steps require significant time to completely dissolve the seed layer and expose the underlying barrier layer whereas a higher free acid concentration in the copper solution causes more rapid corrosion of the seed layer. The slope of the potential–time transitions is sharp indicating a uniform dissolution of the seed layer. If pitting or non-uniform corrosion of the seed layer occurred, small areas of the underlying barrier layer would be exposed first and the transition of the potential from about +0.075 V, characteristic of metallic copper, to about +0.25 V, that of the TaN layer, would occur slowly. The dissolution rate of the seed layer is greater than that expected from the electrochemical measurements. For example, the theoretical dissolution time to dissolve ( seed layer is 1 cm2 of 1000 A
Metallic copper can also corrode in the acid copper plating solution by dissolved oxygen Cu0 -Cuþ2 ; O2 þ 2Hþ þ 2e -H2 O:
Time to dissolve Cu Seed ¼ ðweight CuÞð#e-ÞðFaraday constantÞ= ðcurrent density stepÞðat: weight CuÞ; ( thick copper in 1 cm2 where the weight of 1000 A exposed area is ( Weight Cu ¼ ð1 cm2 exposed areaÞð1000 AÞ 8 ( g=cm3 Þ ð1 10 cm=AÞð8:9 ¼ 8:9 105 g Cu0 : Therefore, the theoretical time necessary to completely dissolve the copper seed layer using a current density step of 100 mA/cm2 is ¼ ð8:9 105 g Cu0 Þð2e-=molÞ ð96 487 C=molÞ=ð100 mA=cm2 Þ ( g=molÞ ¼ 2700 sÞ: ð1 106 A=mAÞð63:54 As seen in Figs. 6(a)–(c) and Table 1, the actual transition times for the 5 and 30 g/l H2SO4 electrolytes using the 100 mA/cm2 step are 2400 and 2100 s, respectively. Similar calculations were done for other current density steps. The data presented in Figs. 4(b) and (c) shows the stable copper species is Cu+2 and the XPS results in Fig. 5(b) shows the copper seed layer which covered with an oxide, likely both Cu(I) and Cu(II) oxides. Cuprous and cupric oxides are unstable in the acid sulfate electrolytes; both oxides decompose to cupric ions Cu2 O þ 2Hþ -Cuþ2 þ Cu0 þ H2 O; CuO þ 2Hþ -Cuþ2 þ H2 O:
Fig. 8. (a) AFM top-view image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), Cl (50 mg/l), 500 mA/cm2 step 425 s, to +0.15 V vs. SCE. (b) SEM image of seed layer corrosion: Cu++ (40 g/l), H2SO4 (5 g/l), C (50 mg/l), 500 mA/cm2 step 425 s, to +0.15 V vs. SCE.
ARTICLE IN PRESS N.M. Martyak, P. Ricou / Materials Science in Semiconductor Processing 6 (2003) 225–233
The accelerated dissolution rates are likely due to a chemical attack, Figs. 4(b) and (c), of the seed layer by the free sulfuric acid and accounts for approximately 10% increase in the dissolution rate of the seed layer using 5 g/l free sulfuric acid and about a 21% increase in dissolution rate using 30 g/l free sulfuric acid. AFM and SEM images of the partially corroded seed layer are seen in Figs. 7 and 8. The PVD seed layer appears to agglomerate into larger, rounded structures after immersion in the copper solutions compared to the unexposed seed layer. Corrosion of the seed layer is general and defects (holes) appear in the seed layer. At a given current density step, the defects merge into one another resulting in large areas of the underlying barrier layer exposed. XPS analysis of the partially corroded seed layer seen in Fig. 7(b) shows signals for both the copper and the underlying barrier layer. At very high current density steps, Figs. 8(a) and (b), the seed layer is badly corroded and isolated islands of the seed layer remain. Plating on a highly corroded seed layer such as that in Fig. 8(b) is likely to result in defects in the electrodeposited copper layer. As the seed layer thickness decreases in future generation devices, the propensity for seed layer dissolution in acid copper sulfate solutions will increase, particularly if the concentration of free sulfuric acid is relatively high. Dissolution of the seed layer may lead to defects such as voids in the copper-filled features and poor adhesion of the electroplated copper to the barrier layer.
4. Conclusions ( Electrochemical studies on the corrosion of a 1000 A copper seed layer is shown to occur at an accelerated
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rate due to the free sulfuric acid in the copper electrolytes. A high free sulfuric acid concentration results in a more rapid dissolution of the seed layer exposing the underlying barrier layer. Continued exposure to the cupric sulfate–sulfuric acid electrolyte produces isolated islands of an agglomerated seed layer.
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