Effect of substrate surface roughness on electric current induced flow of liquid metals Santanu Talukder, Nalla Somaiah, and Praveen Kumar Citation: Appl. Phys. Lett. 102, 054101 (2013); doi: 10.1063/1.4790182 View online: http://dx.doi.org/10.1063/1.4790182 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i5 Published by the AIP Publishing LLC.
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APPLIED PHYSICS LETTERS 102, 054101 (2013)
Effect of substrate surface roughness on electric current induced flow of liquid metals Santanu Talukder,1 Nalla Somaiah,2 and Praveen Kumar2,a) 1
Center for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India
2
(Received 10 December 2012; accepted 17 January 2013; published online 4 February 2013) Electric current can induce long-range flow of liquid metals over a conducting substrate. This work reports on the effect of the substrate surface roughness on the liquid metal-front velocity during such a flow. Experiments were conducted by passing electric current through liquid gallium placed over 170 nm thick, 500 lm wide gold and platinum films of varying roughness. The ensuing flow, thus, resembles micro-fluidics behavior in an open-channel. The liquid-front velocity decreased linearly with the substrate surface roughness; this is attributed to the reduction in the effective C 2013 electric field along the liquid metal-substrate interface with the substrate surface roughness. V American Institute of Physics. [http://dx.doi.org/10.1063/1.4790182]
Electric current often passes through liquids, in particular, liquid metals, in several micro-and nano-scale devices, such as liquid metal batteries,1 ionic liquid channel field effect transistors,2 maskless coating of micro-patterned conductors,3 and other micro-fluidics-based devices.4–6 Recently, symmetric circular patterns of micro- and nanosized metallic beads were created over Si substrate by passing electric current through a very thin infinite metallic film using point electrical-probes;7 this process was shown to have several applications in the fields of optics and growth of patterned, catalytic structures. In many of these applications, the limited or long range movement of a liquid conductor over (or, through) a substrate is controlled through application of an electric field. Therefore, it is important to investigate the flow behavior of liquid conductors under an applied electric field. At small length-scales, the substrate surface roughness plays a very significant role on the flow behavior of the liquids; accordingly, this aspect of micro-fluidics has been extensively studied using both the experimental and the theoretical approaches.8–11 However, most of the studies dealing with the small length-scales address only the flow of liquids through closed-channels. Also, the dominant force driving the liquid flow in most of the previous studies has been one or a combination of more of the following: (i) electro-osmosis, (ii) capillary, and (iii) surface tension; and none of the studies addressed behavior of the electromigration driven flow in liquids. Furthermore, the effect of surface roughness on the electric current driven flow of liquid metals in the openchannel is not known. Therefore, the main objectives of this work are to (i) evaluate the effect of the substrate surface roughness on the electric current (i.e., electromigration) induced flow of the liquid metals, and (ii) present a general theoretical basis for analyzing flow of a conducting liquid over a rough substrate. The addressed flow behavior resembles micro-scale open-channel flow, which is important in several micro- and nano-scale applications, such as, self-assembly of a)
Author to whom correspondence should be addressed: Electronic mail:
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micro scale particles,12 field-flow fractionation (FFF),13 liquid electromigration (LEM) driven coating as well as creation of micro-patterned structures,3,7 microelectronics cooling,14 and several applications involving controlled transport of organic molecules in liquid mediums.15,16 Figure 1 shows a digital photograph of the experimental set-up. The set-up consisted of three components: (i) the substrate-sample, (ii) the liquid metal, and (iii) fixture to pass electric current through the liquid metal/substrate system. The substrate-sample was prepared by depositing 170 nm thick, 500 lm wide, and 3 cm long thin films of either Au or Pt over a 500 lm thick, p-type, h100i Si substrate. Prior to the deposition of the thin metallic films, the polished surface of the Si was wet-etched by a solution with 10 ml 25% tetra methyl amino hydroxide (TMAH), 20 ml iso-propyl alcohol (IPA), and 70 ml deionized water at a temperature of 82 6 1 C. IPA was added in the solution to reduce the etching rate and
FIG. 1. A digital photograph of the experimental set-up. All experiments were conducted in the ambient conditions. Typically, the thickness of the liquid metal was between 20 and 30 lm (enhanced online) [URL: http:// dx.doi.org/10.1063/1.4790182.1].
102, 054101-1
C 2013 American Institute of Physics V
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054101-2
Talukder, Somaiah, and Kumar
Appl. Phys. Lett. 102, 054101 (2013)
FIG. 2. Representative micrographs showing the (a) wet-etched surface of Si and (b) surface of the deposited thin metallic film (Pt). The Si surface was wet-etched for 4 min. (c) The effect of wet-etching period on the RMS roughness of sputter-coated Au and Pt thin films.
improve the surface quality.17 As shown in Figure 2(a), the surface of the Si became rough following the wet-etching, and the roughness of the Si surface increased with the etching time. Following wet-etching of the Si surface, the roughened Si surface was sputter-coated with 10 nm of Cr adhesion layer, followed by sputter-coating with either Au or Pt; these two sequential depositions were conducted without breaking the vacuum. Observation of the samples, before and after the thin metallic film depositions, under a scanning electron microscope (SEM) revealed that the film deposition was highly conformal to the asperities of the Si substrate (Figs. 2(a) and 2(b)). The roughness of the deposited metallic films (or, the substrate surface) was measured using an atomic force microscope (AFM); Figure 2(c) shows the root mean square (RMS) roughness of various Au and Pt thin films. To ensure the statistical accuracy, AFM scans from at least 10 different regions, each of size 10 10 lm2, were taken and the mean of the RMS roughness values was calculated. The RMS roughness of the substrate surface increased monotonically with the wet-etching period; the increase remained linear during shorter etching periods whereas the roughness increased quite rapidly once the etching period became greater than 3 min. Generally, the roughness of the Si surface increases linearly with time during wet-etching with TMAH solution;18 however, due to a continuous increase in the temperature of the etching solution during this study (where the etching solution was kept over a hot-plate), the etching rate rapidly increased once the etching period became large. After the substrate-sample was prepared, it was placed over a custom designed horizontal (less than 0.1 of tilt, as confirmed by a set of two mutually perpendicular glass levels) TeflonV platform. Two circular electrodes of Cu clamped the sample to the platform, while these electrodes directly touched the thin metallic film (Fig. 1). For better heat dissipation through the sample, glass slides were placed between the substrate sample and the TeflonV plate in such a fashion that the middle segment of the substrate-sample hanged in the air. As shown in Fig. 1, a small bead of solid Ga was placed over the metal film at almost halfway between the two electrodes; Ga was chosen primarily for its very low melting temperature (29.7 C). A constant electric current of either 0.2 or 0.6 A, corresponding to a current density through metallic films of R
R
2.3 108 or 7 108 A/m2, respectively, was flowed through Pt or Au thin films, respectively. As the electric current passed through the thin metallic film, the solid Ga bead quickly melted due to Joule heating, and it started to flow from the anode to the cathode (i.e., along the electric field) (Fig. 1, multimedia for a video) conformally coating the underlying metallic substrate. The flow of liquid Ga was directional and similar to the flow of liquid Bi over Cu thin film, as reported previously.3 A high-resolution optical camera, operating at 10 frames per second, was used to record video of the metal flow. Horn-Shunck method,19 which is an optical flow based algorithm, was implemented using the “video and image processing (VIP)” block-set of MATLABV to calculate liquid metal-front velocities from the recoded videos. The velocities of different samples were measured at the initial segment of the flow, as well as in the isochronous fashion; this ensured similar temperature rise in all samples. Furthermore, the temperature variation between different samples can be assumed to be insignificant as the tests were conducted in air and at relatively low current density (
