Solid-state dewetting of patterned thin filmsDonghyun Kim, Amanda L. Giermann, and Carl V. Thompson

Citation: Applied Physics Letters 95, 251903 (2009); doi: 10.1063/1.3268477 View online: http://dx.doi.org/10.1063/1.3268477 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quantitative analysis of anisotropic edge retraction by solid-state dewetting of thin single crystal films J. Appl. Phys. 113, 043512 (2013); 10.1063/1.4788822 Three-dimensional graphoepitaxial alignment resulting from solid-state dewetting of Au films on surfaces withmonoperiodic topography Appl. Phys. Lett. 101, 051904 (2012); 10.1063/1.4740277 Mechanisms of complex morphological evolution during solid-state dewetting of single-crystal nickel thin films Appl. Phys. Lett. 97, 071904 (2010); 10.1063/1.3480419 Nanopatterning by solid-state dewetting on reconstructed ceramic surfaces Appl. Phys. Lett. 94, 171114 (2009); 10.1063/1.3127442 Solid-state dewetting for ordered arrays of crystallographically oriented metal particles Appl. Phys. Lett. 86, 121903 (2005); 10.1063/1.1885180

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Solid-state dewetting of patterned thin filmsDonghyun Kim, Amanda L. Giermann, and Carl V. Thompsona�

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,Massachusetts 02139, USA

�Received 20 September 2009; accepted 2 November 2009; published online 22 December 2009�

Gold particle arrays have been produced through solid-state dewetting of patterned gold thin films.Patterns included rectangles and terminated lines of different widths, lengths, and film thickness.The particle spacing in long lines followed a Rayleigh-like dependence on the cross-sectional areaof the line. Shorter lines had lower numbers of particles, indicating line-end effects. In some cases,ordered particle arrays were obtained. The probability of forming ordered particle arrays was shownto depend on the relative magnitudes of the length, width, and thickness of patterned structures.These trends are captured in design maps. © 2009 American Institute of Physics.�doi:10.1063/1.3268477�

Solid-state dewetting of metallic thin films during an-nealing has been the subject of both theoretical and experi-mental studies.1–9 Continuous thin films are usually formedfar from equilibrium and are metastable or unstable with re-spect to forming discrete particles via dewetting. Dewettingis driven by a lowering of the total energy of the system withrespect to the initial energies of the surfaces of the film andparticles, as well as the energy of the interface between thefilm and the substrate. Dewetting occurs by atomic diffusionand thus the rate of dewetting increases with increasing tem-perature. However, dewetting generally occurs well belowthe melting temperature of the film so that the entire trans-formation occurs in the solid state. The particles that formthrough dewetting of metallic films can be used as catalystsfor growth of nanowires10–12 and carbon nanotubes,13–15 aswell as being applicable for use in bio sensors,16 photonicdevices,17,18 and magnetic memory applications.19 In theory,the particles formed during the dewetting processes are af-fected by the characteristic wavelength of a fingeringinstability7 and subsequent Rayleigh instability, which canlead to a characteristic spacing of the islands.20,21 However,in practice, the dewetting of polycrystalline thin films is af-fected by microstructural features such as grain boundariesand grain boundary triple junctions.9 As a result, solid-statedewetting of continuous polycrystalline films leads to par-ticles with uncontrolled relative positions.

In earlier work we have shown that films that dewet onsurfaces patterned with ordered surface topography can leadto formation of particles that are spatially ordered and thathave controlled size22–24 and crystallographic orientations.22

Zhao et al.25 have also used ion beam irradiation to causedewetting of metal films �Au, Pt� patterned into narrow metallines and observed formation of a single row of particleswith an average particle spacing consistent with a Rayleigh-like instability. It should be noted, though, that Zhao et al.interpreted dewetting as having occurred in the liquid state.Peterson and Mayr26 investigated the self-organization of Nithin films on a ripple patterned surface which served as atemplate for the solid-state dewetting process. Here we in-vestigate the effects of patterning polycrystalline films and

show that dewetting can lead to ordered particles at lengthscales that are less than those of the pattern.

Electron-beam evaporated polycrystalline Au films werepatterned through a lift-off process in which photoresist waspatterned before Au deposition and the Au on the photoresistwas subsequently removed to leave patterned Au on the sub-strate surface. The patterned Au shapes were rectangularpads �width: �4–20 �m, length: �4–20 �m�, 500 �mlong lines with 2–4 �m width and closed circular lines with4–6 �m width. The substrates were silicon covered with athermally grown oxide of thickness 200 nm to prevent diffu-sion of Au into the Si substrate during thermal annealing.Films of thickness 30, 60, 120, and 240 nm were deposited.The patterned films were annealed to induce solid-state dew-etting at 900 °C in air until dewetting was complete; 4, 8,and 16 h for the 60, 120, and 240 nm thick films, respec-tively. The annealed gold particles were characterized usingscanning electron microscopy �SEM�.

Films patterned into rectangles showed different dewet-ting behavior depending on their width and length, as well ason the initial film thickness. Figure 1�a� shows a region of acontinuous film that has dewet. Figure 1�b� shows an ex-ample in which a square pattern developed into a single par-ticle. Single rows and double rows of particles are shown inFigs. 1�c� and 1�d�, respectively. The tendency to form or-dered particles was observed to decrease with increasing pat-tern dimension, in either length or width.

Characteristics of dewetted particle arrays as a functionof pattern width and length as well as the film thickness areshown in Fig. 2. Figure 2�a� shows the particle spacing forsingle rows of particles that formed from very long �length=500 �m� or “infinitely” long lines �the closed circularlines�. These results are for a fixed film thickness, h, butvarying line width, w, and are plotted as a function of thesquare root of the product of w and h. Rayleigh showed thata cylinder of radius r is unstable and tends to decompose intoparticles with sizes and spacings that scale with r.20 To testfor a similar correlation in patterned films we have taken�wh�1/2 to be an analog for the radius of the initial line. If theunstable lines evolve in a Rayleigh-like way, a linear rela-tionship between the particle spacing and �wh�1/2 would beexpected. This appears to be supported by the results shownin Fig. 2�a�. However, in Fig. 2�b� we show the particle

a�Author to whom correspondence should be addressed. Electronic mail:cthomp@mit.edu. Tel.: �1-617-253-7652. FAX: �1-617-258-6749.

APPLIED PHYSICS LETTERS 95, 251903 �2009�

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spacing as a function of �wh�1/2 for films of different thick-ness and significantly short length �18.5 �m in Fig. 2�b�versus 500 �m in Fig. 2�a��. While the particle spacing gen-erally increases with �wh�1/2, there is an additional thicknessdependence such that, for a given h, the particle spacing isnearly independent of �wh�1/2. This suggests a higher degreeof ordering than would be expected from a simple instabilityof the sort that would occur in an infinitely long line orcylinder. As shown in Fig. 2�c�, we also find that the particlespacing depends on the length of the line, L, for relativelysmall L and fixed w and h. This also suggests an increased

tendency to order. For a fixed L and w the particle diameter,d, was found to be independent of h �Fig. 2�d��. In view ofthese trends, the clustering of nearly fixed particle spacingsat specific values as a function of �wh�1/2 in Fig. 2�b� isinterpreted to be an effect associated with the relatively shortlength of the lines.

The ordering seen in Figs. 1�c� and 1�d� and reflected inthe data of Fig. 2 is probabilistic in nature. Within a range offilm thicknesses, rectangles with short lengths and widthstend to form either one particle, a single row of two or threeparticles, or, in a limited number of cases, double rows oftwo or three particles. Larger rectangular patterns sometimesalso show ordering, however, the probability of ordering de-creases sharply with increasing rectangle dimensions. Toquantitatively characterize this phenomenon we defined thefollowing criteria for ordered particles:

�1� A line drawn through the centers of a set of particlesmust be within 10% of the initial pattern edge;

�2� Each pair of particles in a row or column has a spacingthat is within 10% of the average spacing in the row orcolumn; and

�3� Each particle has a diameter that deviates from theaverage diameter of all the particles by less than 10%.

Particles resulting from all rectangular patterns werecharacterized using these definitions. The investigated pat-terns covered widths between 3.6 and 18.5 �m, lengths be-tween 3.6 and 18.5 �m, and thicknesses of 60, 120, or240 nm. A statistical analysis was performed to determinethe probability of ordering as a function of pattern dimen-sions. The total number of measurements for a given w, L,and h was more than 10. Although double rows of two orthree particles sometimes formed, these were excluded fromour analysis due to their low numbers. The three cases con-sidered were therefore single particles �SP�, self-ordered par-ticles �SOP�, and randomly distributed particles.

A probability map for SOP and SP ordering as a functionof pattern width and pattern length scaled by the film thick-ness is shown in Fig. 3. The contours represent probabilitiesfrom 0 to 1. The measured probability data were smoothedthrough the correlation gridding method provided by Origin-Pro �OriginLab Co.�. From Fig. 3�a�, we find that SOP or-dering occurs with high probability at low values of w /h andfor L /h values of about 150. The probability deceases as w /hincreases and peaks for values of L /h of about 150. Theprobability of SP ordering shown in Fig. 3�b� is highest atlow values of L /h and w /h, and decreases with increasingvalues of w /h and more sharply decreases with increasingvalues of L /h. Figure 3 can be used to choose metal patterndimensions that lead to ordered island structures throughsolid-state dewetting.

In summary, patterning of polycrystalline films providesa means of exercising control over the characteristics of theparticles that result from solid-state dewetting. Polycrystal-line gold films patterned into narrow lines dewet to formlinear arrays of particles with an average spacing that de-pends on the cross-sectional area of the line in a way that isconsistent with formation through a Rayleigh-like process.Short lines dewet into a number of particles that is also af-fected by the line length, indicating that the ends of the linesaffect the instability that causes dewetting. Short lines andrelatively small rectangles generally have an increased like-

FIG. 1. SEM images of �a� continuous and �b�–�d� patterned Au films thathave undergone solid-state dewetting. �a� A continuous film dewets intoislands/particles with broadly distributed sizes and spacings. The film thick-ness is 30 nm. �b� A single Au particle developed when an 11 �m�11 �m�240 nm square pattern dewet. �c� A single row of three particlesdeveloped when a 4 �m�11 �m�60 nm rectangular pattern dewet. �d�Double rows of two particles developed when a 9.3 �m�9.3 �m�120 nm square pattern dewet. Scale bars=5 �m.

FIG. 2. Characteristics of dewetted particles as a function of pattern width,w, length, L, and film thickness, h. �a� The relationship between particlespacing and �wh�1/2 of 500 �m long lines and closed circular lines patternedfrom 120 nm thick films. �b� The relationship between particle spacing and�wh�1/2 for rectangles of length 18.5 �m. �c� Particle spacing vs. L withfixed w �5.5 �m� and h �120 nm�. �d� Average particle diameter as a func-tion of film thickness for 18.5�18.5 �m2 rectangles.

251903-2 Kim, Giermann, and Thompson Appl. Phys. Lett. 95, 251903 �2009�

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lihood to dewet into ordered arrays, though the probability ofordering increases for specific relative values of the geomet-ric characteristics of the patterns. While film patterning gen-erally led to an increased degree of order among the par-ticles, order was highly probabilistic in nature. This is likelyto be the result of varying local microstructural characteris-tics of the Au films. While we have established guidelines forpatterning to form ordered particle arrays, more completecontrol of the order resulting from solid-state dewetting willrequire better control of the initial microstructure of the pat-terned films.

This work was supported by the Singapore-MIT Alli-ance. D.K. was also supported by a Korea Research Founda-tion under Grant No. KRF-2007-357-D00135.

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FIG. 3. �Color online� Three-dimensional probability maps for �a� SOP and �b� SP ordering as a function of the pattern width to film thickness ratio and patternlength to film thickness ratio of rectangular patterns. The contours represent probabilities from 0 to 1.

251903-3 Kim, Giermann, and Thompson Appl. Phys. Lett. 95, 251903 �2009�

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