The Mechanical Behavior of Nanoporous Gold Thin Films

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The Mechanical Behavior of Nanoporous Gold Thin Films
Ye Sun, Jia Ye, Zhiwei Shan, Andrew M Minor, T John Balk. JOM. New York: Sep 2007. Vol. 59, Iss. 9; pg. 54, 5 pgs

Abstract (Summary)
A series of Au-Ag alloy films were deposited on two different types of substrate in order to produce suitable np-Au films.

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Copyright Minerals, Metals & Materials Society Sep 2007

[Headnote]
Thin films of nanoporous noble metals exhibit an interconnected, porous structure with ligament widths and pores on the order of 10 nm or higher. In this study, thin film stress measurements and in-situ nanoindentation in a transmission-electron microscope were performed to investigate the effects of nanoscale geometric confinement on the mechanical properties of metals and on dislocation-mediated plasticity. Although some films exhibit macroscopic cracking, the deformation of individual ligaments is completely ductile and clearly involves dislocation activity, even in 10 nm wide ligaments. The stresses generated in these films during thermal cycling correspond to bulk stresses that approach the theoretical strength of the metal. Film stress exhibits a dependence on film thickness, even though the ligament width is much smaller and would presumably govern deformation.

INTRODUCTION

Nanoporous noble metals possess chemical properties and a high surfaceto-volume ratio that make them promising candidates for a variety of applications, including actuators,' sensors,2 and catalysts^sup 3^ Fabrication of noble metals with a nanoporous structure proceeds via a process known as deailoying. during which the less noble atoms (e.g., silver) are chemically or electrochemically dissolved from a precursor alloy (e.g., Au-Ag). As the less noble atoms are leached from the alloy, the more noble atoms (e.g., gold) undergo surface diffusion and agglomeration, ultimately yielding a nanoscale. sponge-like structure consisting of nearly pure metal (gold) ligaments and open, interconnected pores.^sup 4,5^ An example of nanoporous gold (np- Au) from the current study is shown in the plan-view scanningelectron microscopy (SEM) image of Figure 1 . The length scale of the ligaments and pores within nanoporous metals can be readily tailored, from several nanometers up to micrometers in size, by varying the initial alloy composition, dealloying time, and/or applied potential, or by subsequent heat treatment.^sup 6^

Recently, the mechanical properties of np-Au have drawn increasing attention.^sup 7-9^ The strength of nanoscale gold ligaments measured by nanoindentation was reported to approach or even exceed the theoretical yield strength of bulk gold.^sup 7-10^ Some researchers found that the ligament strength followedaHall-Petchtype relationship with ligament size.11·12 Although much work on np-Au has focused on bulk material, several studies have addressed the behavior of thin film np-Au, revealing high strength levels compared to bulk gold. ^sup 13-15^

However, still unknown is which mechanism(s) govern deformation in nanoscale ligaments. Detailed studies of the behavior of dislocati ons and other defects are needed to understand the structure-property relationships that lead to the high equivalent strength levels of np-Au.

A recently developed in-situ nanoindentation technique, coupling nanoindentation and transmission electron microscopy (TEM), offers a unique possibility to observe the deformation microstructure within materials as they are indented.^sup 16-18^ With this technique, the nucleation and motion of dislocations can be observed while a load-displacement curve is simultaneously recorded, allowing direct correlation of measured mechanical behavior with microstructural changes. In this study. np-Au films of various thickness were tested by insitu nanoindentation. Additionally, the wafer curvature technique^sup 19^ was used to measure the biaxial stress in np-Au thin films on silicon substrates during thermal cycling.

NANOPOROUS GOLD THIN FILM SAMPLES

A series of Au-Ag alloy films were deposited on two different types of substrate in order to produce suitable np-Au films. For in-situ nanoindentation experiments, 30 at.% Au-70 at.% Ag alloy films of various thicknesses were magnetron sputtered (ORION system, AJA International, base pressure [asymptotically =]10^sup -6^ Pa) onto wedge-shaped silicon substrates that had been prepared using lithographic techniques.^sup 16^ To make plan-view TEM samples, Au-Ag films with the same composition were also sputtered onto 180 µm thick (100)-oriented silicon wafers (CrysTec GmbH, Germany) that had been coated with 10 nm of amorphous silicon oxide and 50 nm of amorphous silicon nitride. Prior to deposition of Au-Ag alloy films, a 10 nm tantalum interlayer and a 10 nm gold interlayer were sputtered onto each wafer to improve the adhesion of dealloyed np-Au films to their substrates. The as-sputtered Au-Ag films were dealloyed by free corrosion (i.e., with no applied voltage) in eoncentrated HNO3 (70% stock concentration) for 30 min., followed by rinsing in ethanol and slow air drying. In-situ nanoindentation of dealloyed np-Au on silicon wedges was performed with a Hysitron Picoindenter(TM) inside a JEOL 3010 TEM. Thermal cycling of np-Au films on flat silicon wafers was performed with a commercially available wafer curvature system (FLX-2320-S, Toho Technology Corporation).

Table I lists the average ligament width of np-Au films, after dealloying for 30 min., as a function of film thickness. This data was obtained from analysis of plan-view SEM images of np-Au films, such as that shown in Figure 1 . The ligament width was the same for all np-Au films, indicating that the length scale of pores/ligaments is primarily dependent on dealloying time, while film thickness does not have a significant effect. However, as described in the following, film thickness does appear to influence the mechanical behavior of np-Au films as measured by thermal cycling.

IN-SITU NANOINDENTATION IN THE TEM

Figure 2 presents TEM images of a 150nmthicknp-Au film before and after in-situ nanoindentation. The film shown here is completely crack-free, which is quite unusual for np-Au samples in either bulk or supported thin film form. The width of ligaments in Figure 2a is 10-20 nm, which matches well with the results shown in Table I. In-situ nanoindentation of this 150 nm np-Au film was performed in displacement control mode. The diamond indenter used here had a Berkovich geometry, with a radius of curvature [asymptotically =]100 nm. Figure 2b shows the indenter near the np-Au film, prior to indentation. Although the indenter vibrated during acquisition of Figures 2b and c, it was stable during the actual indentation experiment due to a damping effect upon contact. As shown in Figure 2c, the porous structure was compacted during indentation and became significantly denser, with a corresponding decrease in electron transparency.

Figure 3 presents the load-displacement curve corresponding to indentation of the 150 nm np-Au film portrayed in Figure 2. Two load drops were observed in the loading curve and reveal an interesting aspect of deformation of np-Au during indentation. In the initial stages of nanoindentation, np-Au deformed easily and the displacement increased monotonically with increasing load. During this time, only the outermost layer of ligaments was compacted by the indenter, while the rest of the underlying structure remained undeformed. Upon further indentation, the compaction front of deformed np-Au moved into the film, ahead of the indenter. successively collapsing each neighboring layer of ligaments. It was also observed during in-situ nanoindenlation tests that, after initially smooth and easy compaction, expansion of the zone of deformed np-Au under the indenter was punctuated by short, collective jumps in the nanostructure. A comparison of the real-time load-displacement curve with the in-situ video indicates that these compaction bursts appear to correlate with the load drops (e.g., those observed in Figure 3). In this particular case, local maxima in the loading curve are marked by arrows at 70 nm and 8 1 nm indentation depth. The distance between load drops is approximately 1 1 nm, roughly the same as the pore size (9-10 nm) calculated from the measured ligament width and relative density according to the Gibson-Ashby equations.^sup 20^ Numerous other indentation tests exhibited similar load-drop intervals. This suggests that the load drops result from collective collapse of a layer of pores (i.e., that a broad array of ligaments/struts collapse simultaneously or in a chain reaction that causes a noticeable decrease in indenter load).

Figure 4 presents TEM images recorded during in-situ nanoindentation of a 75 nm np-Au film using a cube corner indenter. For this test, indentation was performed manually (the indenter was moved forward in discrete steps until contact was made, instead of programming a set displacement ramp and letting it run to completion). In this way, ligaments were compressed only slightly and deformation was kept to a minimum. Comparing Figure 4a-c, it can be seen that the two np- Au ligaments marked by arrows deform locally, while the remainder of the porous structure was not affected by this indentation.

Upon contact, the ligament tips deformed easily and i n a ductile manner, as expected for gold. Although the global behavior of np-Au is often brittle and accompanied by extensive cracking, the deformation of individual ligaments is still ductile.

MOTION OF DISLOCATIONS IN GOLD LIGAMENTS

Figure 5 shows still images taken from in-situ video observation of the 75 nm np-Au film during nanoindentation. The time interval between images is 0.6 seconds. In both images, the indenter is in contact with the outer layer of np-Au ligaments (the white contrast between the apparent indenter edge and the np-Au ligaments masks the actual edge of the indenter). The outermost layer of ligaments moved slightly during continued motion of the indenter tip, accompanied by dislocations that were generated inside the ligament marked by a white arrow (compare Figure 5a and 5b). In Figure 5b. the dark lines corresponding to dislocations span the width of the ligament. The dislocations are straight, indicating that they do not experience a significant drag force from the surfaces of the ligament. In general, dislocations moved easily within ligaments during deformation. Dislocations spanned the ligament width andunderwent sequential glide to the ligament nodes, where they sometimes tangled with dislocations from other ligaments.

THERMAL CYCLING OF NANOPOROUS GOLD THIN FILMS

Thin film np-Au samples were also subjected to thermal cycling in order to study the stress evolution and to determine the maximum stresses that can be generated in the films. Figure 6 shows stress -tempe rature curves for three npAu films during cooling from 200°C to -70°C. Based on previous work with similar samples, it is known that the ligament width doubles (from [asymptotically =]15 nm to [asymptotically =]30 nm) during thermal cycling between 200°C and room temperature. Thus, for the curves shown in Figure 6, it is assumed that a constant ligament width of [asymptotically =]30 nm exists within all three films during cooling (no change in ligament width is expected during cooling from the maximum temperature). From Figure 6, it is apparent that thinner films carry higher stresses when subjected to the same thermal cycling temperature range.

Unlike solid metal films, for which stress varies inversely with film thickness due to geometric constraints that cause dislocation channeling,^sup 19^ the strength of nanoporous metal films was expected to depend on ligament size, as was found for bulk np-Au.^sup 7,12^ Overall, it is expected that the smallest geometric parameter (e.g., film thickness, grain size, ligament width, etc.) will most heavily influence the nueleation and motion of dislocations and hence the film strength. Thus, it would be expected that the three films shown in Figure 6, all of which should have a ligament width of [asymptotically =]30 nm, would exhibit similar stress-temperature behavior and the same maximum stress. But in this study, np-Au films were seen to behave more iike solid films, with an inverse relationship between film thickness and stress after thermal cycling.

Based on the curves shown in Figure 6. it appears that the np-Au films exhibit therm oelastic stress-temperature behavFor during the final stages of cooling to -70°C. In this case, the films would not have reached their yield points, and the unexpected trend of increasing film stress with decreasing film thickness could have a different explanation. From Gibson and Ashby's scaling law,^sup 20^ the yield strength of an open-cell foam is given by σ = C^sub 1^ σ^sub 2^ (ρ^sub np^ /ρ^sup S^ ) ^sup n^, where σ^sub s^ and ρ^sub s^ are the yield strength and density of solid gold and ρ^sub np^ is the density of np-Au. C 1 and n are empirical constants, withC ,=0.3 and n = 3/2. ρ ^sub np^/ρ^sub s^ istermed the relative density of np-Au and is equal to the volume/thickness percentage of gold in the precursor alloy (which in turn is taken to be the atomic percentage of gold in the Au-Ag alloy, due to the nearly identical lattice parameters of gold and silver). In the current study, the theoretical relative density is 30%, assuming no volume contraction during dealloying. However, based on TEM images of filmon-wedge samples for in-situ nanoindentation, which reveal the thickness of Ehe dealloyed np-Au layer, the actual relative density appears to be higher (36%) due to contraction of the film thickness.

Since all films appear to have undergone thermoelastic deformation and presumably never reached their yield points during cooling, it follows that the actual yield strength of np-Au films should be higher than the maximum biaxial stresses shown in the stresstemperature curves of Figure 6. Thus, it is possible to calculate a lower bound for the equivalent bulk yield strength of the gold ligaments for each film. The results of these calculations using the Gibson-Ashby equation are shown in Table II, along with the maximum film stress measured at the end of thermal cycling. These calculated stress levels. assumed to represent a lower limit for the equivalent yield strength, are already very high and approach the theoretical yield strength for bulk gold, in general agreement with other studies11 but still predicting lower strength levels.

For proper comparison with the theoretical shear strength, the calculated equivalent stress levels in Table II should be converted into resolved shear stresses. The thin films are highly {111}- textured and therefore have a Schmid factor of 0.27. Taking the maximum value of 2.0 GPa (using the corrected relative density of 36%) from Table II, the equivalent resolved shear stress at the end of thermal cycling is found to be 550 MPa. Estimates of the theoretical shear strength range from a maximum of µ/2π to a more realistic value =µ/30.^sup 21^ Using a shear modulus µ = 27 GPa for gold,^sup 21^ the theoretical shear strength is expected to lie between 900 MPa and 4.3 GPa. For the 75 nm np-Au film in this study, the calculated equivalent shear stress of 550 MPa corresponds to [asymptotically =]µ/49, which is approximately 60% of the realistic estimate of the theoretical shear strength for gold. Interestingly, this demonstrates that these nanoporous materials can support stress levels that approach the theoretical shear strength while simultaneously exhibiting dislocation activity.

ACKNOWLEDGEMENTS

The authors thank Ms. Sofie Burger for assistance with measurement of ligament widths. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for support of this research. TJB thanks the National. Center for Electron Microscopy (NCEM) for the NCEM Visiting Scientist Fellowship that allowed this TEM work to be carried out. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract NO.DE-AC02-05CH1123. The research was also supported in part by a U.S. Department of Energy Small Business Innovation Research grant (DE-FG0204ER83979) awarded to Hysilron, Inc., which does not constitute an endorsement by DOE of the views expressed in the article.

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Read this article on the 10M web site (www.tms.org/JOMPT) to see an in-situ movie recorded during the indentation of a nanoporous gold thin film.

[Reference]
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[Author Affiliation]
Ye Sun and T. John Balk are with the University of Kentucky, Chemical and Materials Engineering, Lexington, Kentucky; JiaYe.ZhiweiShan, and Andrew M. Minor are with the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California.Zhiwei Shan isalso with Hysitron Inc., Minneapolis, Minnesota. Prof. Balk can be reached at (859) 257-4582; e-mail balk@engr.uky. edu.

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