J. Phys. Chem. 1994, 98, 11246-11250
11246
Enhanced Electrochemical Deposition with an Atomic Force Microscope John R. LaGraff and Andrew A. Gewirth" Department of Chemistry and Materials Research Laboratory, University of Illinois at Urbana- Champaign, Urbana, Illinois 61801 Received: July 14, 1994; In Final Form: September 12, 1994@
In-situ atomic force microscopy (AFM) is shown to locally enhance the electrochemical deposition of copper (Cu) onto single-crystal Cu surfaces. The tip-sample interaction increases the growth rate of Cu, resulting in the localized formation of nanometer scale epitaxial deposits. The results are consistent with a heterogeneous nucleation and growth mechanism in which the tip-sample interaction creates surface defect sites in a passivating layer which are active toward the electrochemical adsorption of Cu species. This protectiondeprotection scheme enables precise control of feature sizes and allows this technique to be used for fabrication and constructive modification of solid-liquid interfaces.
Introduction Electrochemical deposition of Cu and other materials forms the basis for several important technological processes.' For example, there is currently a concerted effort to develop novel deposition, etching, and patterning processes for the formation of multilevel Cu interconnects of very large-scale integrated (VLSI) devices2 In practical electrodeposition schemes the Cu surface is conditioned andor modified through empirically developed plating solutions which control the initial nucleation and subsequent morphology of Cu deposits in ways which are not yet well understood on a nanometer level. In this paper we demonstrate that the AFM can modify the nucleation and growth of Cu on Cu single-crystal surfaces, hence yielding fundamental structural and chemical insight into the deposition process. In-situ scanning probe microscopy (SPM) has led to improved understanding of the solid-liquid interface by providing atomic and nanometer level resolution of surface morphology under well-controlled static and dynamic electrochemical conditions3 As one example, our observation of an oxide or hydroxide layer present on single-crystal Cu surfaces indicates that a passivating adlayer may be present on these materials in the pH regions where most aqueous deposition is perf0rmed.4.~ In practical deposition schemes the Cu surface is prepared and the passivating adlayer conditioned and/or modified through additives introduced into the plating solution. These additives modify the initial nucleation and subsequent morphology of the Cu deposits in ways which are not fully understood on a fundamental level. We investigate here the mechanisms of electrochemical Cu deposition on well-characterized single-crystal substrates and the possible role of tip-sample force in locally modifying these processes. Surface modification with SPM at the atomic and nanometer scale can be divided into two general categories, constructive or destructive corresponding to the addition or removal of material, respectively, by the probe tip. While reports of STM modifications have been of both types:-* surface modification with the AFlM has been destru~tive.~ Most surface modification studies have been carried out in air or v a ~ u u m . ~For - ~ example, a recent report' describes a two-step process in which the removal of protective layers with the STM created a lithographic mask, which upon subsequent exposure to a reactive gaseous
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, October 15, 1994.
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time (sec) Figure 1. Potential step chronocoulometry of the electrochemical deposition of Cu on Cu(ll0). (A) Deposition was initiated by the application of a -70 mV potential step at time, t = 0. (B) The corresponding current versus time plot which monitored the macroscopic deposition rate of Cu. Labeled arrows indicate the time regions over which the A M images in Figures 2 and 3 were collected.
species results in preferential adsorption on deprotected areas. In contrast, there are few examples of controlled surface modification in liquid environment^.'^-'^ STM modification of solid-liquid interfaces includes etching andor deposition on gallium arsenide,1° gold," and graphite.12 For example, Penner and co-workers12 have used the STM to form pits on graphite which served as nucleation sites for the electrochemical deposition of Ag or Cu pillars 10-30 nm in diameter. The AFM has been used to tear up polymer adlayers on graphite immersed in liquid13and to enhance the dissolution of A1 films,14 however, at the micron level. SPM-based lithography could have significant impact for in-situ surface modification of solidliquid interfaces because this environment precludes the use of competitive vacuum-based techniques such as electron beam lithography. In this paper we demonstrate that the AFM can modify the nucleation and growth of Cu on Cu single-crystal surfaces. In particular, we present in-situ AFM results which demonstrate for the first time that the tip-sample interaction locally enhances the deposition rate of Cu under controlled electrochemical conditions. These results are significant both as a fundamental example of locally controlled heterogeneous nucleation and growth and as an alternate route for constructively modifying materials surfaces. 0 1994 American Chemical Society
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Figure 2. A series of 100 x 100 nm AFM images following enhanced electrochemical deposition of epitaxial Cu on Cu( 1 IO). The deposition was carried out in a HC104 solution (pH = 2.45) and was initiated by the application of a large tip-sample force and a potential step of -70 mV (Figure I). (A) The Cu( 110) surface prior to deposition under open-circuit conditions imaged with a tip-sample force of 5 f 2 nN. (B) The force was first increased to 25 f 5 nN, and then a potential step (-70 mV) was applied midway through image collection as indicated by the arrow. This initiated the immediate nucleation of deposition features. (C, D) The initial and continued growth of a single large Cu deposit. Arrows indicate the slow scan direction during image collection.
Experimental Section
Results
In-situ AFM images of Cu( 110) crystals were collected in constant force repulsive mode on a Nanoscope I1 atomic force microscope (Digital Instruments) equipped with a 1 pm scanning head, microfabricated square-pyramidal Si3N4 tips, and a 0.2 mL fluid cell. The AFM was calibrated by imaging mica which has a known interatomic separation of 0.51 nm. During those experiments in which the bare Cu( 110) surface was observed, the known Cu-Cu separation served as an additional internal standard. Reference electrodes were formed from either a mercury/mercurous sulfate electrode (MSE; Hg/Hg2SO&S04(saturated)) or a Pt wire quasi-reference electrode (QRE). A 0.5 mm diameter gold wire served as the counter electrode. The working electrode was a Cu( 110) single crystal (Monocrystals Co., 99.999%) prepared by standard techniques.l5 As-prepared surfaces had roughness factors of approximately 1. The (1 10) in-plane crystal orientations-( 170) and (001)-were confirmed (within f 1 0 " of the collected images) by using Laue X-ray backscattering. Potential control was maintained by a BAS CV27 Voltammograph. Solutions were prepared with Millipore-Q water, H2SO4 (J.T. Baker, Ultrex), and HC104 (J.T. Baker, Ultrex). Cu was introduced into the solution either through the application of a small anodic potential (a few millivolts above open-circuit conditions) to the Cu single-crystal surface or by the direct addition of M Cu(C104)2. Deposition was induced by a cathodic overpotential step of 70 mV negative of the open-circuit potential E,, which was +280 mV versus the normal hydrogen electrode (NHE).
Figure 1 shows potential step chronocoulometry for the macroscopic deposition of Cu onto a Cu( 110) working electrode from a HC104 solution (pH = 2.45) containing M Cu2+. A potential step to 70 mV negative of Eo was applied to the surface at time t = 0, as shown in Figure 1A. In Figure lB, the corresponding deposition current decayed with a t- *I3 dependence as Cu was depleted from the solution. This behavior is typical of the electrodeposition of Cu and other metals.' The area under the current-time curve (Figure 1B) yields the amount of Cu deposited at any given time during the AFM experiments. Figure 2A shows a 100 x 100 nm AF'M image (collected during time region 2A in Figure 1B) which was typical of a Cu( 110) surface under open-circuit conditions (no applied potential) prior to Cu electrodeposition with a tip-sample force near 5 f 2 nN. The surface was smooth, exhibiting features no more than 5 nm in height. Atomic resolution of either the bare Cu( 110) lattice or the Cu(ll0) ( n x 1)-0 adlattice (n = 1, 2,3) has been obtained under these conditions.s Cu deposition, initiated and continued with the AFM tip retracted 15 p m above the surface, yielded images16 which showed uniform threedimensional deposits on the surface with lateral dimensions of 50-200 nm which were much greater than their heights (1 - 10 nm). Similar morphologies have been reported elsewhere. l7 However, when Cu is deposited while the tip is engaged and scanning, a completely different morphology develops. In Figure 2B (time region 2B in Figure lB), the tip-sample force was first increased from 5 f 2 to 25 f 10 nN, which yielded
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Letters the surface shown in the top part of the image just prior to deposition. Cu deposition was initiated midway through Figure 2B by the application of a potential step 70 mV negative of Eo. The potential step induced the immediate nucleation of islands on the surface which is apparent in the bottom of Figure 2B.'* With additional time, a single deposition feature developed extending about 40 nm above the surface (Figure 2C, D). Figure 3 shows long-range AFM images (850 x 850 nm) of the same process shown in Figure 2. Figure 3A is a typical Cu( 110) surface under open-circuit conditions prior to Cu deposition. Following deposition and high force AFh4 scanning of the 100 x 100 nm region marked in Figure 3A, the force was again minimized (ca. 5 nN) and the scan area increased to 850 x 850 nm to reveal a single large deposit in the center of the image possessing a "truncated-cone'' morphology with a height of 60 nm (Figure 3B). However, the observed feature is most likely a convolution of the actual feature shape and the AFM tip size and ge0met~y.l~ Continued AFM imaging of the whole 850 x 850 nm area in Figure 3B led to additional nucleation and growth of multiple deposition features as shown in Figure 3C. In Figure 3D, lattice-resolved AFM images obtained from the top of the central feature in Figure 3C reveal a rectangular structure with periodicities of 0.24 ZZ! 0.02 nm by 0.35 & 0.03 nm which is consistent with the Cu( 110) lattice. In addition, the lattice on top of the deposit was oriented to within 10" of the underlying Cu( 110) lattice vectors. Finally, the deposits shown in Figure 3C exhibited stable morphology to overpotentials approaching hydrogen evolution (11 > 400 mV) and redissolved under open-circuit or anodic conditions. These observations indicate that the central feature in Figure 3B,C was composed of Cu epitaxially grown atop the Cu( 110) substrate.
Discussion Enhanced Cu deposition through AFM scanning was very reproducible and has also been observed on the Cu( 111) and Cu(100) faces.16z20 Enhanced deposition was found to be independent of added Cu, electrolyte anion (perchlorate vs sulfate), and dissolved oxygen concentration.16 The growth rate and final height of the deposits were a function of both the tipsample force and the scanning time. The larger the tip force, the faster the deposition. Enhanced deposition could be induced in scanning areas down to 20 x 20 nm; however, the minimum dimensions of the deposits may ultimately depend on tip size. On a macroscopic scale, the integration of the current-time curve (Figure 1B) to 335 s indicates that 2.3(5) x 10l6Cu atoms/ cm2 or approximately 20 monolayers (4 nm) of Cu was deposited.21 The amount of Cu deposited in the central feature of Figure 3B, estimated from its height, corresponded to the local deposition of 300 monolayers, which is equivalent to 3.5(3) x 1017Cu atoms/cm2. This is an approximate 15-fold local enhancement over the average deposition rate. Results presented in Figures 2 and 3 indicate that the scanning force of the tip significantly increased the local deposition rate of Cu. With the STM, the tip inhibits deposition of Cu by acting as either a physical or electrostatic shield toward the diffusive flux of dissolved Cu ions to the deposition region directly under the tip." Apparently, the AFM tip did not significantly shield the Cu surface in the same manner as a STM tip. It is also known that the AFM tip-sample force, if sufficiently large, will etch the surfaces of materials by a destructive me~hanism.~ Under Cu deposition conditions, however, any possible material removal is far outweighed by locally enhanced metal deposition. The large central feature deposited under the AFM tip (Figure 2 and Figure 3) and observation of additional multiple nucleation
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Figure 4. Proposed mechanism for enhanced electrochemical deposition of Cu with the AFM tip-sample interaction. An oxide or hydroxide adlayer partially passivates the Cu surface. The scanning AFM tip deprotects the Cu surface by either removing or disrupting this adlayer, resulting in a local increase in the nucleation and growth
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events (Figure 3C) are consistent with a mechanism in which the AFM tip-sample interaction initiates the local heterogeneous nucleation and growth of Cu. Enhanced deposition was not observed {a) if the tip-sample force was sufficiently minimized (< ca. 5 nN) or (b) on bare Au( 111)16 and was not a function of scan rate. These observations indicate that convection was not the dominant mechanism for enhanced Cu deposition. Also, a mechanism controlled by tip charging appears unlikely owing to the lack of enhanced Cu deposition on bare Au(ll1) substrates.16 Deposition at low overpotentials (ca. 10 mV) is dominated by surface diffusion;' hence, nucleation and growth occur primarily at step edges and dislocation^.'^ The scanning force of the AFM tip here is sufficientto create alternativedefect sites electrochemically active toward the incorporation of Cu species. The active surface defects necessary for enhanced deposition are likely one or both of two types. First, the tip could create defects in the underlying Cu substrate which enhance the nucleation of Cu deposits. On Cu surfaces at opencircuit potentials, large tip-sample forces induce scanning artifacts wherein a large amount of material is removed by the physical interaction between tip and sample.16 This type of tipinduced etching has been observed for a variety of materia1s.9,13,14,16
The second possibility is that the tip is removing or modifying an intervening passivation layer composed of oxide or hydroxide. In the pH potential region studied here, bulk oxide formation on Cu is thermodynamically unstable;22however, we and others4,5,15J6s23 have reported the existence of oxide or hydroxide adlayers on Cu surfaces which exist down to low pH values (ca. 1). Hence, the normal growth mode for Cu deposition may be inhibited by an intervening passivation adlayer.' The role of some additives in plating solutions may thus be to remove or inhibit the formation of this passivating adlayer. Here, the AFM tip-sample force removes or alters the oxide adlayer, thus creating active sites for Cu adsorption. In pH = 2.45 solutions, at larger cathodic overpotentials (17 400 mV) where the oxide adlayer should be removed, enhanced deposition was less evident. In pH = 1.2-1.3 solutions, where again formation of the oxide adlayer is inhibited, some enhanced deposition was observed regardless of the magnitude of the overpotential (70 mV 7 < 450 mV). These observations are consistent with a mechanism wherein the AFM tip removes the passivating oxide or hydroxide adlayer, resulting in locally enhanced Cu deposition rates. Creation of surface defects by the AFM tip, either on the Cu surface over a passivation layer, is a demonstration of controlled heterogeneous nucleation and growth,24however, observed insitu on a nanometer length scale. In order to obtain enhanced electrodeposition, the passivated surface must be deprotected by the AFM tip-sample interaction (Figure 4). The deprotected sites react immediately with Cu species either in solution or on the surface. This protect-deprotect-react scheme is commonly utilized in synthetic chemistry,25but this is the first application
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in the electrochemical environment. The ability to locally deposit Cu epitaxially with precise feature sizes through the manipulation of electrochemical potential and tip-sample force demonstrates the feasibility of this technique as an alternative route for the nanometer scale surface modification of Cu as well as other materials.
Acknowledgment. We thank Jay Switzer for helpful comments. J.R.L. acknowledges a National Science Foundation Postdoctoral Fellowship in Chemistry (CHE-9302406). A.A.G. acknowledges a Presidential Young Investigator Award (CHE9027593) with matchingfunds provided by Digital Instruments, b Inc., and is an A. P. S l a n Foundation Fellow. This work was funded by the Department of Energy (DE-FG02-9 1ER45349) through the Materials Research Laboratory at the University of Illinois. References and Notes (1) West, J. M. Electrodeposition and Corrosion Processes; Van Nostrand Reinhold: New York, 1971. Winand, R. Trans. See. C Inst. Min. Metall. 1975, 84, 67. (2) See: Mater. Res. Soc. Bull. 1993, 18, 18-56. (3) Siegenthaler, H. In Scanning Tunneling Microscopy II; Wiesendanger, R., Guntherodt, H. J., Eds.; Springer-Verlag: New York, 1992; Vol. 28; Chapter 2. NATO Proceedings: Nanoscale Probes of SolidlLiquid Interface; Siegenthaler, H., Gewirth, A. A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, in press. (4) Cruickshank, B.; Sneddon, D. D.; Gewirth, A. A. Su$ Sei. 1993, 281, L308. ( 5 ) LaGraff, J. R.; Gewirth, A. A. Sui$ Sci. Lett., submitted. (6) Becker, R. S.; Golovchcnko, J. A,; Swartzentmber, B. S. Nature 1987, 325, 419. Lyo, I.; Avouris, P. Science 1991, 253, 173. Eigler, D. M.; Schweizer, E. K. Nature 1990, 344, 524. Huang, J. L.; Sung, Y.E.; Lieber, C. M. Appl. Phys. Lett. 1992,61,1528. Garfunkel, E.; etal. Science
Letters 1989,246,99. Sato, A.; Tsukamoto, Y. Nafure 1993,363,431. Kobayashi, A.; Grey, F.; Williams, R. S.; Aono, M. Science 1993, 259, 1724. (7) Schoer, J. K.; Ross, C. B.; Crooks, R. M.; Corbitt, T. S.; HampdenSmith, M. J. Langmuir 1994, IO, 615. (8) Staufer, U. In Scanning Tunneling Microscopy II; Wiesendanger, R., Guntherodt, H. J., Eds.; Springer-Verlag: New York, 1992; Vol. 28, Chapter 8. (9) Mamin, H. J.; Rugar, D. Appl. Phys. Lett. 1992,61, 1003. Leung, 0. M.; Goh, M. C. Science 1992,255,64. Delawski, E.; Parkinson, B. A. J. Am. Chem. SOC.1992,114, 1661. Kim, Y.; Lieber, C. M. Science 1992, 257, 375. (10) Lin, Ch. W.; Fan, F. R. F.; Bard, A. J. J. Electrochem. SOC.1987, 134, 1038. Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Appl. Phys. Lett. 1990, 57, 270. (11) Schneir, J.; Hansma, P. K. Langmuir 1987, 3, 1025. (12) Li, W.; Virtanen, J. A,; Penner, R. M. Appl. Phys. Lett. 1992, 60, 1181. Li, W.; Virtanen, J. A,; Penner, R. M. J. Phys. Chem. 1992, 96, 6529. (13) Brumfield, J. C.; Goss, C. A.; Irene, E. A.; Murray, R. W. Langmuir 1992, 8, 2810. (14) Chen, L.; Guay, D. J. Electrochem. SOC.1994, 141, L43. (15) Vilche, J. R.; Juttner, K. Electrochim. Acta 1987, 32, 1567. (16) LaGraff, J. R.; Gewirth, A. A. Manuscript in preparation. (17) Nichols, R. J.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1991, 313, 109. (18) Nucleation could also be initiated by first applying the potential step and then increasing the force. (19) Kepler, K. D.; Gewirth, A. A. Su$ Sci. 1994, 303, 101. (20) Deposits on the (111) and (100) surface often possessed similar epitaxial orientations to the underlying substrate. (21) This assumes layer-by-layer growth of copper with the complete transfer of two electrons. (22) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: New York, 1966; p 387. (23) Bradley, R. A.; et al. J. Electroanal. Chem. 1991, 309, 319. (24) Tiller, W. A. The Science of Crystallization; Cambridge University Press: New York, 1991. (25) Streitwieser, Jr., A.; Heathcock, C. H. Introduction to Organic Chemistry; Macmillian Publishing: New York, 1976.
