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Novel Approach for Thin Dense Nanoscale-Grained Metal Films Arvind Varma,* King L. Yeung,† Razima S. Souleimanova, and Alexander S. Mukasyan Department of Chemical Engineering and Center for Molecularly Engineered Materials, University of Notre Dame, Notre Dame, Indiana 46556
A novel approach to the synthesis of thin (∼1 µm) fully dense metallic films with nanoscale microstructure is presented. This technique is an unusual combination of two different phenomena: electroless plating and osmosis. It is remarkable that the method permits a systematic variation of film microstructure and thickness that allows for optimization of the film properties. Because the unique approach described herein can be used for synthesis of nanograined thin metal films of any desired composition, it can be employed in a variety of applications. Introduction Nanoscale-grained dense thin metallic films are of importance in a variety of scientific and technological fields, including microelectronics, optical devices, catalysis, and chemical and biological sensors.1 A number of techniques are used for the synthesis of these films, such as atomic layer epitaxy, magnetron sputtering, chemical vapor deposition, and electroless plating. The properties of these films depend significantly on their microstructure and thickness. However, the available synthesis techniques, although they yield different microstructures and thicknesses, do not permit a systematic variation of these parameters that would allow for optimization of film properties. In this paper, we describe a novel approach that overcomes this problem and synthesize thin (∼1 µm) fully dense film with nanoscale-grained microstructure. Results and Discussion We illustrate this approach using as an example the synthesis of dense metal films (e.g., Pd, Ag, Pd-Ag, etc.), which are good candidates for the separation of specific gases.2 However, to achieve widespread commercial applications, two antagonistic fundamental problems remain. On one hand, the films should have sufficient and stable mechanical-thermal properties, which, in principle, requires relatively large thicknesses. On the other hand, high gas permeability can be achieved only with thin films. Further, it is believed that films with finer-grained microstructures exhibit superior permeation properties.3 To obtain good mechanical and thermal stability while maintaining small metal film thicknesses, composite metal-substrate structures are synthesized. In this case, the metallic layer is deposited on a porous support (e.g., Vycor glass, alumina, stainless steel) that does not provide significant transport resistance. However, the critical problem that remains is to decrease the metal film thickness while ensuring that the synthesized film * To whom correspondence should be addressed. Tel.: 574631-6491. Fax: 574-631-8366. E-mail:
[email protected]. † Present address: Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
simultaneously remains fully dense so that infinite gas selectivity can be achieved. Why is it difficult to deposit thin dense films on a substrate? Consider this problem for the case of electroless deposition of palladium on two different porous inert supports: Vycor glass (25-mm diameter × 1.5-mm thickness; Corning Inc.) and stainless steel (25-mm diameter × 1-mm thickness; Mott Metallurgical Corporation) disks. The former has an average pore size of 4 nm and a relatively smooth surface (the variance of surface height, Ra, is less than 100 nm), whereas the latter is characterized by 0.2-µm pores and a rough surface with Ra ≈ 500 nm. The disks were cleaned, activated, and plated using procedures described elsewhere.4 The different pore sizes of the substrates require osmotic solutes of different molecular dimensions. Thus, during plating, different concentrations of sucrose osmotic solutions (0, 2, 6, and 9 M) were used for the Vycor glass, whereas a 3 × 10-7 M solution of poly(ethylene glycol) (molecular weight ) 7.2 × 106 g/mol) was used for the porous stainless steel. All membranes were plated until they became fully dense, i.e., until a H2/N2 permselectivity of greater than 107 was reached. To test for this end point, single-gas permeability tests were conducted on the membranes at temperatures up to 873 K and hydrogen and nitrogen pressures up to 827 kPa. To seal the membranes, Viton and graphite O-rings were used for Vycor glass and stainless steel, respectively. The volumetric flow rates of the permeated gases were measured using a bubble flowmeter. Experiments show that, to produce fully dense membranes, one must deposit an 8-µm-thick palladium film on smooth Vycor glass and about a 20-µm layer on the rougher stainless steel substrate. The main reason such relatively thick films are needed is related to the microstructure of the substrate surface itself, the socalled shading effect. Because of the initial microstructural nonuniformity of the surface, the probability of palladium grain growth on different nucleation sites located along the surface varies. Thus, sites located on top of the “hills” are more active for deposition than those settled in the “valley” regions or pores (see Figure 1a). Moreover, the growth mechanism has only a negative feedback loop; hence, the difference in growth rates between active and passive sites increases with time.
10.1021/ie0110080 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002
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Figure 1. Electroless plating of metal film on porous substrate. (a) Schematic representation of Vycor glass support with seeded palladium nucleii and typical initial support surface profile obtained by AFM. (b) Scheme of electroless plating deposition, with and without osmosis. (c) Typical AFM images of Pd film surfaces synthesized under different osmotic conditions and dependence of palladium grain size on applied osmotic flux (0, 2, 6, and 9 M sucrose solution).
This causes microstructural nonuniformity and the formation of micropores in the film during the early stages of deposition. Thus, many deposition events must occur (i.e., a long plating time is required) to heal defects caused by the nonideality of the support microstructure and create fully dense metal film, and this leads to relatively thick films. To overcome this problem, one can follow two approaches. The first is to use substrates with ideally smooth surfaces. However, such surfaces are not easy to obtain. More importantly, experiments show that, as a surface becomes smoother, adhesion between it and the deposited film decreases, leading to poor thermal and mechanical stability of the membrane. The second approach is to somehow equalize the probability of nucleation and grain growth along the “rough” surface of the substrate by enhancing grain growth in the valley-type regions. How can one equalize the probability of microstructure formation on the nanoscale, however? We showed that the well-known, but unusual for this field, phenomenon of osmosis is a powerful tool for achieving this goal.4 As noted above, porous substrates are used to produce composite membranes. Consider conditions such that a porous support separates the reaction volume into two parts (see Figure 1b, electroless plating with osmosis), with the upper part filled with aqueous palladium plating solution and the lower part filled with a concentrated solution of some substance (e.g., sucrose) having a molecular size larger than the characteristic pore size of the substrate. If this pore size is simultaneously large enough for the penetration of water
molecules, the concentration gradient of solute between the upper and lower sides of the semipermeable membrane leads, through osmosis, to the appearance of an additional water flux normal to the support surface. As a result, water passes from the plating solution side, through the porous network of deposited film and substrate, into the sucrose solution. Experiments have shown that this flux is several orders of magnitude higher than the typical bulk diffusion flux in this system. This osmotic flux carries the Pd2+-containing species to the substrate surface and through its porous network, thus increasing the probability of metal deposition in normally “passive” regions. This allows one to form fully dense films that are much thinner (e.g., ∼1-3 µm, as compared to 8-20 µm), in a shorter period of time (3 h instead of 15 h), than by using the conventional electroless plating technique in the absence of osmosis.5 Further, it is remarkable that the microstructure of the deposited film changes dramatically with the magnitude of the osmotic flux. A nanoscale grain size characterizes metallic films synthesized under osmosis. The characteristic microstructures determined by atomic force microscopy (AFM) for different samples are shown in Figure 1c. It can be seen that dense films plated with osmosis exhibit not only much smaller Pd grain size (less than 500 nm, as compared to 7 µm for conventional plating conditions), but also more uniform size distributions. Different properties of the thin Pd films have been tested to evaluate the influence of the film characteristics on performance. For example, hydrogen fluxes
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computed to isolate the effect of membrane microstructure. The results are shown in Figure 2b as a function of Pd grain size.5 It is evident that a finer deposited film microstructure leads to a higher hydrogen flux. These results, obtained with different membranes prepared by using the same technique (i.e., electroless plating), experimentally revealed the fundamental result that the finer-grained microstructure, because of the resulting larger hydrogen flux through the grain boundaries, enhances hydrogen transport through the palladium films. Concluding Remarks In this report, we have focused on the mechanism of nanoscale-grained thin metal film deposition by a novel technique involving electroless plating combined with osmosis. A detailed description of the experimental procedures, as well as extensive results related to the deposition features and hydrogen permeation and thermal stability characteristics of Pd films deposited on porous Vycor and stainless steel supports, can be found elsewhere.4,5 Finally, note that the novel deposition method described above can be used to produce thin metal films (with thicknesses approaching the roughness of the substrate) of desired composition (Pd, Ag, Cu, Pd-Ag, etc.) and uniform nanoscale microstructure on different porous supports, which can be used in a variety of applications. Acknowledgment It is a pleasure to participate in this celebration of Professor William Schowalter. We gratefully acknowledge financial support from the National Science Foundation (Grant CTS-9907321). Figure 2. Permeation properties of synthesized thin fully dense Pd films for T ) 573 K and ∆P ) 690 kPa. (a) Hydrogen flux through synthesized under different osmotic conditions; (b) Hydrogen flux (normalized to film thickness) through Pd films with different metal grain sizes. The SEM micrographs of films prepared using 9 M (left) and no (right) sucrose solution are shown.
through fully dense (H2/N2 permselectivity equal to infinity) membranes produced under different plating conditions were measured. The variation of this flux through Pd films as a function of the osmotic conditions during plating is shown in Figure 2a. It can be seen that up to a 4-fold permeation enhancement was reached when synthesis was conducted with osmosis. As mentioned above, membranes synthesized under different osmotic conditions have different metal film thicknesses and microstructures. Recognizing that the Pd film thickness varies with the osmotic flux, the hydrogen permeability normalized for thickness was
Literature Cited (1) Elshabini-Riad, A. A. R.; Barlow, F. D. Thin Film Technology Handbook; McGraw-Hill: New York, 1998. (2) Hsieh, H. P. Inorganic Membranes for Separation and Reaction; Elsevier Science: Amsterdam, The Netherlands, 1996. (3) Bryden, K. J.; Ying, J. Y. Nanostructured Palladium Membrane Synthesis by Magnetron Sputtering. Mater. Sci. Eng. A 1995, 204, 140. (4) Souleimanova, R. S.; Mukasyan, A. S.; Varma, A. Effects of Osmosis on Microstructure of Pd-composite Membranes Synthesized by Electroless Plating Technique. J. Membr. Sci. 2000, 166, 249. (5) Souleimanova, R.; Mukasyan, A. S.; Varma, A. Pd Membranes Formed by Electroless plating with Osmosis: H2 permeation studies. AIChE J. 2002, 48, 262.
Received for review December 14, 2001 Revised manuscript received April 1, 2002 Accepted April 6, 2002 IE0110080
