Stress and Microstructure in Electrodeposited Copper Nanofilms by Substrate Curvature and In-situ Electrochemical AFM Measurements

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1 / The Electrochemical Society Stress and Microstructure in Electrodeposited Copper Nanofilms by Substrate Curvature and In-situ Electrochemical AFM Measurements S. Ahmed, T. T. Ahmed, M. O Grady, S. Nakahara and D. N. Buckley Department of Physics, Materials and Surface Science Institute University of Limerick, Ireland ABSTRACT Both AFM imaging and stress measurements were carried out insitu during potentiostatic electrodeposition of copper on gold in 0.05 mol dm -3 CuSO 4 in 0.1 mol dm -3 H 2 SO 4. In the absence of additives, compressive stress generally developed initially and films subsequently underwent a compressive-to-tensile (C-T) transition. The nucleation density measured by AFM increased from cm -2 at -75 mv to cm -2 at -300 mv. Very little coalescence of nuclei was observed at -75 mv but the rate of coalescence increased rapidly with increasing negative potential. The time for the C-T transition correspondingly decreased rapidly until, at -75 mv, none was observed. This is consistent with models that attribute the C-T transition to increasing tensile stress due to coalescence of nuclei. With a combination of Cl -, PEG and MPSA, compressive stress generally developed initially and was greater than in additive-free electrolyte. At less negative potentials, the stress continued to evolve in the compressive direction, even though the rate of coalescence of nuclei was rapid. At intermediate potentials (-90 mv to -150 mv), classical C-T-C behavior was observed; at more negative potentials the stress continued to evolve in the tensile direction. This enhancement of a compressive component of stress is attributed to incorporation of additive-derived impurities. INTRODUCTION Thin metal films deposited on a substrate are usually in a state of stress and this has been the subject of extensive experimental investigation and theoretical analysis Various studies 1-5,18-22,27,28 of the stress generated during the electrodeposition and aging of copper films have been reported. Haiss et al. 4 investigated the dependence of stress on overpotential for copper electrodeposition on Au(111) and examined the stress evolution during the first 25 monolayers of deposition. More recently Stafford et al. 1,2 extended this work and investigated stress evolution during electrodeposition of copper in sulphate electrolyte from the earliest stages up to 400 nm. Bath additives are used in electrodeposition technology for metallization to modify the deposition process so as to obtain desirable characteristics such as superfilling of trenches and vias in the Damascene process Combinations of additives such as chloride ion, polyethylene glycol (PEG) and 3-mercapto-1-propanesulfonic acid (MPSA), frequently 1

2 used in studies of copper electrodeposition can have a dramatic effect on film properties. Morphology and stress are closely related. The evolution of stress is strongly influenced by film morphology and, conversely, stress may induce morphology change Atomic force microscopy (AFM) enables in-situ monitoring of the morphology of films as they are deposited In this paper we compare film stress with the morphology observed using in-situ AFM during copper electrodeposition from an acidic CuSO 4 bath, both with and without common additives. EXPERIMENTAL The in-situ stress measurement system is based on a similar apparatus 1,2 developed in Stafford s group at the National Institute for Standards and Technology (NIST). It consists of an electrochemical cell (Starna 96/G/20) with a Teflon cover which holds a cantilever working electrode (gold-coated glass, see below). The cell has an optical flat at the front through which a laser beam can pass so that, as copper is deposited on the gold, any bending of the cantilever can be detected with high sensitivity by optical monitoring. The optical system basically consists of a helium-neon laser (JDS Uniphase, model 1108P), the output from which passes through a beam-splitter and is reflected from the cantilever working electrode in the cell. It then returns through the beam-splitter and impinges on a position-sensitive detector (PSD; DLS20, UTD Sensor Inc.). Thus, any bending of the cantilever causes a deflection of the laser spot on the PSD. The PSD output is fed to an amplifier and signal processing unit (OT-231DL, On-Track Photonics, Inc.) monitored by a data acquisition unit (Agilent 34970A) interfaced to a personal computer. The whole system is mounted on an optical bench for stability. The working electrode consisted of a 60 mm 3 mm mm glass cantilever (supplied by Präzisions Glas & Optik GmbH) on which 10 nm of titanium followed by 250 nm of gold were evaporated. It was annealed at 450ºC for 1 h in 5% hydrogen/argon and furnace cooled. The gold formed the substrate for electrodeposition and the laser beam was reflected from the metal-glass interface (i.e. from the back face of the electrode). A conventional three-electrode configuration with a copper-foil counter electrode and a copper-wire reference electrode (Cu/Cu 2+, to which all potentials are referenced) in a Luggin capillary was used. The cell was airtight, the electrolyte was deaerated before introduction into the cell and the gas volume in the cell was maintained air-free by flowing nitrogen gas. In-situ AFM imaging was carried out using a Veeco Enviroscope in contact mode with a gold-plated silicon nitride tip (Veeco DNP-20) in a three-electrode cell containing ~1 cm 3 of electrolyte. The cell was 18 mm in diameter and 6 mm in depth. The dry cell and electrodes were loaded into the AFM chamber which was then evacuated and back-filled with nitrogen. De-aerated electrolyte was then introduced into the cell via a special nitrogen-purged flow system. The substrate (working electrode) was the same as that used in the stress measurements; similarly a copper foil counter electrode and a copper wire reference electrode were used. 2

3 The electrolyte consisted of 0.05 mol dm -3 CuSO 4 in 0.1 mol dm -3 H 2 SO 4 with and without additives. Deionised water (~18 MΩ cm) was used for all solution preparation and rinsing. Electrolytes were de-aerated by purging with nitrogen for 3 h. In both in-situ stress measurements and in-situ AFM imaging experiments, the potential of the working electrode was controlled using a CH Instruments 650A workstation. Stoney s formula 15 relates the force per unit width F/w of the cantilever to its radius of curvature which, in turn, is related to the deflection d of the laser beam at the PSD by the geometry and optics of the system. Based on this, it can be shown 1,2 that 2 F Ez nad = (1) w 12( 1 ν ) LnwD where E, ν and z are the Young s modulus, Poisson ratio and thickness, respectively, of the glass cantilever, L is the length of cantilever above the laser spot immersed in the electrolyte, D is the distance from the cantilever to the PSD, n a is the refractive index of air and n w is the refractive index of water. The vendor-specified values of E ( N m -2 ) and ν (0.208) were used. RESULTS AND DISCUSSION In-Situ Stress Measurements in Additive-Free Electrolyte Electrodeposition of copper was carried out at constant potential on a gold-coated cantilever working electrode from additive-free electrolyte and the stress monitored in situ. In a typical experiment, the potential was stepped from +200 mv to -300 mv and both the current and the deflection of the laser beam due to cantilever bending were simultaneously monitored over time. The force per unit width of cantilever F/w was determined from the laser beam deflection using Equation (1). Typical plots of current and F/w (sometimes called the stress-thickness; see below) are shown in Fig. 1: positive values of F/w correspond to tensile force. We observe an initial cathodic current transient that decays rapidly to reach a relatively constant value of ~ -2.8 ma cm -2 after ~0.5 min. while F/w increases in the tensile direction. We can determine the average stress in the film at any given time by dividing F/w by the corresponding thickness, estimated coulometrically from the current-time curve. The incremental stress in the film at any given thickness can be obtained by (numerical) differentiation of F/w with respect to thickness. The values for average stress and incremental stress differ somewhat at any given thickness since the average stress contains components from each element of the deposited film. Ideally, if there were no time-variation of stress within the film, the incremental stress would represent the stress in the copper instantaneously deposited at the surface of a film at any given thickness. Generally that is not a good assumption. Thus, the interpretation of variations in either average or incremental stress is not straightforward. In this paper, therefore, we will generally discuss film stress directly in terms of F/w, which is the product of the average stress and the film thickness and is therefore called stress-thickness. 3

4 Current Density (ma cm -2 ) (a) (a) Current density (b) Force per unit width (b) Stress- Thickness (N m -1 ) Time (min) Fig. 1: (a) Current-time curve at -300 mv (Cu/Cu 2+ ) for a gold-coated cantilever electrode in a solution containing 0.05 mol dm -3 CuSO 4 and 0.1 mol dm -3 H 2 SO 4 following a potential step from +200 mv. (b) Corresponding force per unit width F/w obtained from the cantilever deflection. Plots of the stress-thickness against time are shown in Figs. 2 at a range of potentials from -75 mv to mv. With the exception of the curve at -75 mv, all of the curves show development of tensile stress in the time scale shown. More careful examination of the early stages of deposition (see expanded scale in Fig. 2b) shows that stress develops initially in the compressive direction. This eventually reaches a maximum value and subsequently decreases, i.e. it undergoes a transition from compressive direction to tensile direction (a C-T transition). This is in general agreement with the literature: a compressive-tensile-compressive (C-T-C) pattern of evolution with increasing film thickness has been reported both for physically vapor deposited (PVD) and for electrochemically deposited (ED) films. Initially, stress is compressive due to the surface stress in individual small nucleated grains. Eventually these begin to coalesce, leading to tensile stress due to grain boundary formation. A third stage is often observed, where the direction of stress evolution again becomes compressive. This third stage is not observed in Fig. 2 but it has been reported by Stafford et al. 1,2 at lower concentrations of Cu 2+ (10-2 mol dm -3 ). We have also observed this third (compressive) stage in the presence of additives as will be discussed later. In-Situ AFM Observations in Additive-Free Electrolyte Using in-situ AFM imaging, we investigated the deposition process on the same substrate used for the stress measurements and under the same set of deposition conditions. Typical results are shown in Figs. 3 and 4. At -75 mv it can be seen, Fig. 3, that the nucleation density is low. Small isolated grains can be observed in the images after 0.43 min and 2.17 min and even after 7.37 min the grains are still largely isolated from each other although some have now coalesced. In contrast, Fig. 4 shows that at -125 mv, 4

5 where the deposition current is much larger, the nucleation rate is much higher. Some grains have already grown together after 0.7 min and after 3.3 min the grains have largely coalesced. At higher potentials the grains coalesce at much shorter times and the scan rate of the AFM is not sufficiently fast to properly resolve the early nucleation stage mv mv -200 mv mv -200 mv Stress-Thickness (N m -1 ) mv -300 mv -150 mv -125 mv Stress-Thickness (N m -1 ) mv -175 mv -150 mv -100 mv -90 mv mv -90 mv -75 mv Time (min) mv Time (min) (a) (b) Fig. 2: (a) Time dependence of stress-thickness for copper electrodeposition at potentials ranging from -75 mv to -400 mv (Cu/Cu 2+ ) in additive-free 0.05 mol dm -3 CuSO 4 in 0.1 mol dm -3 H 2 SO 4 following a potential step from +200 mv. (b) Early stages of the curve in (a) shown with an expanded stress-thickness scale. Approximate values for the density of nucleation were determined from AFM images such as those in Figs. 3 and 4. The results are shown for various deposition potentials in Table 1. As expected, the nucleation density increases rapidly with potential from cm -2 at -75 mv to cm -2 at -300 mv. In time sequences of images (e.g. in Figs. 3 and 4) we observe that the grains eventually merge as they grow. The density of grain clusters was estimated from the AFM images at various times and, from this and the corresponding nucleation density, the average number of grains per cluster was obtained as a function of time. The results are shown in Fig. 5, for deposition potentials of -100 mv and -125 mv. Clearly the cluster size increases with time much more rapidly at -125 mv than at -100 mv. The C-T transition is generally attributed 4-9,42-47 to tensile stress generated when individual grains meet as they grow and form grain boundaries. These boundaries are regions of reduced density and consequently give rise to an attractive interaction between the grains, generating tensile stress. This tensile stress contribution is expected to be proportional to the total grain-boundary area. Thus, when the grain-boundary area is increasing the corresponding increasing contribution to tensile stress tends to counterbalance the increase in compressive stress due to the growth of individual grains. 5

6 A (a) 0.43 min A (a) 0.7 min (b) 2.17 min (b) 3.3 min (c) 7.37 min (c) 6.77 min Fig. 3: AFM images at (a) 0.43 min (b) 2.17 min and (c) 7.37 min obtained during electrodeposition of copper at -75 mv in additive-free electrolyte following a potential step from +200 mv (Cu/Cu 2+ ). The time refers to the end of the scan and is measured from the beginning of deposition (marked 'A'). The scan direction is shown by the arrow and the scan time for an image is 0.87 min. Fig. 4: AFM images at (a) 0.7 min (b) 3.3 min and (c) 6.77 min obtained during electrodeposition of copper at -125 mv in additive-free electrolyte following a potential step from +200 mv (Cu/Cu 2+ ). The time refers to the end of the scan and is measured from the beginning of deposition (marked 'A'). The scan direction is shown by the arrow and the scan time for an image is 0.87 min. 6

7 Table 1: Nucleation density at various potentials following a potential step from +200 mv (Cu/Cu 2+ ) in 0.05 mol dm -3 CuSO 4 in 0.1 mol dm -3 H 2 SO 4 with no additive. Potential / mv Nucleation Density / 10 6 cm No. of Nuclei per Cluster mv -100 m V Time (min) Fig. 5: Plot of Number of nuclei per cluster against deposition time for potentials of -100 mv and -125 mv (Cu/Cu 2+ ) in additive-free electrolyte. We note that a C-T transition clearly occurs in the -90 mv, -100 mv and -125 mv curves but not in the -75 mv curve. This is consistent with the AFM results which show that there is no significant coalescence of grains at -75 mv and consequently no significant tensile contribution to stress over the time-scale of the experiments (~7.37 min). Comparing the curves at -100 mv and -125 mv in Fig. 2 with the cluster size results in Fig. 5, it is clear that the number of nuclei per cluster is increasing with time both at -100 mv and at -125 mv. Thus the total area of grain boundaries is increasing with time and is consequently expected to make an increasing contribution to tensile stress. When this becomes sufficiently large to counterbalance the increase in compressive stress due to the growth of individual grains, the force curve is expected to show a C-T transition. This transition occurs at ~3.67 min on the -100 mv curve and at ~0.67 min on the -125 mv curve, Fig. 2b. This is consistent with the time-variation of cluster size, Fig. 5, since the rate of increase of cluster size, and consequently the tensile stress due to grain boundary formation, is considerably faster at -125 mv than at -100 mv. Similar observations may be made concerning the curves at higher potentials where C-T transitions occur at even 7

8 shorter times. In the case of the highest potentials examined, the initial compressive transient occurs too rapidly to be observed in the present experiments. Measurements in the Presence of Bath Additives A similar set of experiments was carried out in the presence of mol dm -3 Cl -, mol dm -3 3-mercapto-1-propanesulfonic acid (MPSA) and mol dm -3 polyethylene glycol (PEG) with an average molecular weight of The potential was stepped from +200 mv to potentials ranging from -50 mv to -400 mv and the current and cantilever deflection were simultaneously recorded. Typical plots obtained for stress-thickness against time are shown in Fig. 6. At higher potentials, the stress evolves in the tensile direction while at lower potentials it evolves in the compressive direction. However more careful examination of the early stages of deposition (see expanded scale in Fig. 6b) shows that, at intermediate potentials (-90 mv to -150 mv), the stress follows a typical C-T-C trend as discussed below. Stress-Thickness (N m -1 ) mv mv -200 mv mv mv -150 mv mv -90 mv -125 mv -50 mv 0-65 mv mv -75 mv Time (min) (a) Stress-Thickness (N m -1 ) mv -150 mv -140 mv Time (min) (b) -100 mv -90 mv -75 mv -50 mv Fig. 6: (a) Plot of stress-thickness against time for copper electrodeposition at potentials ranging from -75 mv to -400 mv (Cu/Cu 2+ ) in the presence of mol dm -3 Cl -, mol dm -3 PEG and mol dm -3 MPSA following a potential step from +200 mv. (b) Early stages of the curve in (a) shown with an expanded scale. Even though the same general pattern of stress evolution is observed in the presence of the additives as is observed in additive-free electrolyte, the actual quantitative evolution of stress is quite different. At lower potentials, a much larger compressive stress develops in the presence of the additives (see for example the curve for -75 mv in Fig. 6b). Similar behaviour has been previously observed, for example in electrodeposited 8

9 A A (a) 0.63 min (a) 0.67 min (b) 2.37 min (b) 2.4 min (c) 6.7 min (c) 6.73 min Fig. 7: AFM images at (a) 0.63 min (b) 2.37 min and (c) 6.7 min obtained during electrodeposition of copper at -75 mv in electrolyte containing additives ( mol dm -3 Cl -, mol dm -3 PEG and mol dm -3 MPSA) following a potential step from +200 mv (Cu/Cu 2+ ). The time refers to the end of the scan and is measured from the beginning of deposition (marked 'A'). The scan direction is shown by the arrow and the scan time for an image is 0.87 min. Fig. 8: AFM images at (a) 0.67 min (b) 2.4 min and (c) 6.73 min obtained during electrodeposition of copper at -125 mv in electrolyte containing additives ( mol dm -3 Cl -, mol dm -3 PEG and mol dm -3 MPSA) following a potential step from +200 mv (Cu/Cu 2+ ). The time refers to the end of the scan and is measured from the beginning of deposition (marked 'A'). The scan direction is shown by the arrow and the scan time for an image is 0.87 min. 9

10 nickel, 5 and attributed to the incorporation of additive-derived impurities into the deposit. Because of the low concentration of additives in the bath, the deposition rate of additiverelated impurities is expected to be diffusion limited while the deposition rate of metal increases with increasing negative potential. Thus the concentration of impurities in the deposit is higher at less negative potentials and consequently the development of compressive stress is more rapid. In-situ AFM imaging was also carried during deposition in the presence of the additives under the same conditions as in the in-situ stress measurements. Typical results obtained at -75 mv and -125 mv are shown in Figs. 7 and 8. As compared with the corresponding images obtained in additive-free electrolyte, it is clear that the rate of coalescence of nuclei is much higher in the presence of the additives. This is expected since the growth rate is also higher. However, the -75 mv curve in Fig. 6b shows that the stress continues to evolve in the compressive direction rather than undergoing a C-T transition. Similar results were obtained for potentials in the range -50 mv to -75 mv. This contrasts with the observation, described above, that deposits from the additive-free electrolyte with comparable rates of coalescence show a C-T transition and that the stress then evolves in the tensile direction due to the effect of increasing grain-boundary area. At potentials in the range -90 mv to -150 mv, typical C-T-C behavior is observed. This is exemplified by the -100 mv curves in Fig. 6. As can be seen from Fig. 6b, the stress initially evolves in the compressive direction undergoing a C-T transition at ~0.04 min (~4 nm) after which it increases in the tensile direction to reach a maximum, Fig. 6a, at ~1.5 min (~65 nm). Thereafter it again evolves in the compressive direction. This behaviour can be explained by a reduced rate of impurity incorporation due to the higher growth rate. As a result, the tensile stress due to increasing grain boundary area dominates at intermediate times (the T Region) but at longer times the compressive component again dominates. At potentials more negative than -150 mv there is no T-C transition, i.e. the force continues to increase in the tensile direction for the time scale of the experiment. This reflects the high deposition current at these potentials and the resulting lower concentration of impurities with a corresponding reduction in the compressive component of the stress so that the tensile component dominates at all times after the C-T transition. It is also noted that at higher potentials, no initial increase in compressive stress is observed, presumably because its time-scale is too short to be observed in the present experiments. CONCLUSIONS Stress measurements on copper films during electrodeposition in additive-free electrolyte generally showed that compressive stress developed initially and that the films subsequently underwent a C-T transition. The nucleation density measured by AFM increased from cm -2 at -75 mv to cm -2 at -300 mv. Very little coalescence of nuclei was observed at -75 mv but the rate of coalescence increased rapidly with increasing negative potential. The time for the C-T transition correspondingly decreased rapidly until, at -75 mv, none was observed. This is consistent with models that attribute the C-T transition to increasing tensile stress due to 10

11 coalescence of nuclei. With a combination of Cl -, PEG and MPSA, compressive stress generally developed initially and was greater than in additive-free electrolyte. At less negative potentials, the stress continued to evolve in the compressive direction, even though the rate of coalescence of nuclei was rapid. At intermediate potentials (-90 mv to -150 mv), classical C-T-C behavior was observed; at more negative potentials the stress continued to evolve in the tensile direction. This enhancement of a compressive component of stress is attributed to incorporation of additive-derived impurities. ACKNOWLEDGEMENTS We are grateful to Dr. G. R. Stafford (NIST) for valuable discussions and for sharing with us the detailed design of his stress measurement system on which the system used for this work is based. This material is based upon works supported by the Science Foundation Ireland under Grant No. 02/IN1/I217. REFERENCES 1. O. E. Kongstein, U. Bertocci and G. R. Stafford, J. Electrochem. Soc., 152, C116 (2005) 2. G. R. Stafford, O. E. Kongstein, and C. R. Beauchamp, ECS Trans., 2(6), 185 (2007) 3. G. G. Láng, M. Seo, J. Electroanal. Chem., 490, 98 (2000) 4. W. Haiss, R. J. Nichols, J-K Sass, Surf. Sci., 388, 141 (1997) 5. S. J. Hearne and J. A. Floro, J. Appl. Phys., 97, (2005) 6. C. Friesen, S. C. Seel and C. V. Thompson, J. Appl. Phys., 95, 1011 (2004) 7. R. C. Cammarata, T. M. Trimble and D. J. Srolovitz, J. Mater. Res., 15, 2468 (2000) 8. J. A. Floro, S. J. Hearne, J. A. Hunter and P. Kotula, E. Chason, S. C. Seel and C. V. Thompson, J. Appl. Phys., 89, 4886 (2001) 9. B. W. Sheldon, A. Rajamani, A. Bhandari, E. Chason, S. K. Hong and R. Beresford, J. Appl. Phys., 98, (2005) 10. R. Koch, J. Phys: Condenced Matter, 6, 9519 (1994) 11. R. W. Hoffman, in Physics of thin films, G. Hass, R. E. Thun, Editors, Vol. 3, p. 211, academic press, New York (1966). 12. R. Abermann, R. Kramer and J. Mäser, Thin Solid Films, 52, 215 (1978) 13. E. Chason, B. W. Sheldon and L. B. Freund, Phys. Rev. Lett., 88, (2002) 14. F. Spaepen, Acta Mater., 48, 31 (2000) 15. G. G. Stoney, Proc. R. Soc. London, Ser. A, 82, 172 (1909) 16. G. G. Láng, K. Ueno, M. Újvári and M. Seo, J. Phys. Chem. B, 104, 2785 (2000) 17. S. Cattarin, E. Pantano, F. Decker, Electrochem. Com., 1, 483 (1999) 18. K. Ueno and M. Seo, J. Electrochem. Soc., 146, 1496 (1999) 19. A. Brenner and S. Senderoff, U.S. Department of Commerce, National Bureau of Standards, Research Paper RP 1954, 42, 105 (1949) 20. R. Weil, Plating, 58, 137 (1971) 21. R. Weil, Plating, 57, 1231 (1970) 22. R. Weil, Plating, 58, 50 (1971) 23. P. Chaudhari, Vac Sci. Tech., 9, 520 (1971) 11

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