Microstructure evolution during dewetting in thin Au films

doi:10.1016/j.actamat.2010.07.021

Acta Materialia

Volume 58, Issue 18, October 2010, Pages 6035-6045

https://doi.org/10.1016/j.actamat.2010.07.021 Get rights and content

Abstract

Thin metal films can degrade into particles in a process known as dewetting. Dewetting proceeds in several stages, including void initiation, void growth and void coalescence. Branched void growth in thin Au films was studied by means of electron backscatter diffraction (EBSD). The holes were found to protrude into the film predominantly at high angle grain boundaries and the branched shape of the holes can be explained by surface energy minimization of the grains at the void boundaries. (1 1 1) Texture sharpening during dewetting was observed and quantified by EBSD and in situ X-ray studies.

Introduction

Thin metal films can disintegrate into an array of particles upon annealing. This process is referred to as solid state dewetting; it is driven by surface and strain energy minimization [1]. Dewetting starts with the formation of voids, which can take place by various mechanisms, e.g. thermal grooving at grain or twin boundaries [2], [3], [4], [5], nucleation of vacancies at the film–substrate interface [6], [7], [8] or at grain boundaries and grain boundary triple junctions in areas of high local stress [9]. In the latter case the cavity is expected to grow along grain boundaries, since this is the fastest diffusion path for vacancies. Furthermore, void formation due to dust particles or heterogeneities in the substrate is often observed. It is assumed that hole growth, i.e. the removal of material from the hole border, takes place by film surface diffusion [10], [11]. During hole growth a rim of piled-up material is formed around the hole. This rim has to be moved for further void growth, thus a decrease in hole growth rate for increasing hole size is expected if the hole remains circular. Jiran and Thompson [12] observed branched hole growth at the edges of thin gold lines and proposed a corrected model for hole growth kinetics which is based on branched growth. The model states that a rim is formed at the sides of the finger-shaped hole while at the tip of the hole the material can diffuse away in several directions and no pile-up of material is observed. According to this model hole growth preferably takes place at the tips of holes and no decrease in the void growth rate is expected. Kan and Wong [13] have shown that an instability in the rim at the edge of a film exists that leads to the growth of such regularly spaced fingers. Jiran and Thompson [12], [14] also attributed branching to capillary effects and stated that branching is not affected by the presence of grain boundaries; they found that voids grew across grain boundaries and grains alike. In contrast to their observations, several experimental studies have suggested that hole growth is taking place preferably along grain boundaries [9], [15], [16]. The presented study provides insights into the mechanism of branched hole growth based on the analysis of film microstructure by diffraction methods.

Dewetting of thin metal films is usually studied by analysis of the post-annealed film surface by electron microscopy or atomic force microscopy (AFM), but in situ observation in a microscope during annealing, conductivity/resistivity measurements of the film during annealing or X-ray diffraction (XRD)-based methods can also be used. The advantage of the in situ techniques is that they allow the investigation of the dewetting process in real time. There are two well-developed in situ characterization techniques that have been employed by various groups. The first technique is to monitor the surface of the sample by light microscopy. This is done in either reflection [17] or transmission [12], [14]. The method is based on evaluation of the coverage parameter, i.e. the area still covered by the film divided by the total sample area. The main disadvantage of this approach is that it is not possible to detect changes at the film–substrate interface. The second well-developed technique is to measure the sample resistivity. A typical resistivity measurement shows a sharp increase in resistivity as soon as the film disintegrates. This is a well-known phenomenon [18] and can be explained by a percolation model [19]. The main disadvantage of the resistivity technique is that it can track hardly any changes after the film becomes discontinuous. Sieradzki et al. [20] investigated both the resistance and the coverage evolution from SEM pictures during isothermal annealing. They showed a linear relationship between the resistance normalized to its initial value and the area fraction still covered by film normalized to the coverage at electrical failure. Another possibility for the investigation of dewetting is XRD. It has the potential to become an important technique for the characterization of dewetting because it can also provide information about texture, phase composition and grain size – information that would prove especially helpful in studying the dewetting process of alloys. To the authors’ knowledge a paper by Mizsei and Lantto [21] constitutes the only report in the literature on in situ XRD measurements applied to study dewetting. Mizsei and Lantto [21] studied the dewetting of thin silver films and found an increase in the intensity of the (1 1 1) reflection during annealing. They attributed this to an increase in particle height, however, no conclusive evidence was given to support this assumption. The true nature of the increase in (1 1 1) reflection intensity thus remains to be investigated.

Section snippets

Film deposition

Three inch p-type 380 μm thick Si(1 0 0)/SiO₂/SiN_x wafer substrates (Crystec Kristalltechnologie) were used as received. The thickness of the SiO₂ and SiN_x barrier layers was 50 nm each. A 99.99% pure gold wire (Umicore) and a Bal-Tec Med 020 Coating System were used to deposit thin gold films by thermal evaporation. A quartz crystal monitor was used to measure film thickness and deposition rate in situ. The deposition rate was 0.3 nm s⁻¹ for all films. Films of 10, 15, 20 and 30 nm thickness were

XRD measurements as a method to follow the dewetting process

An experiment analogous to the one reported by Mizsei and Lantto [21] was carried out using a 30 nm thick gold film. The film was annealed inside the XRD spectrometer at 600 °C in an N₂ atmosphere and measurements were carried out every 10 min. Fig. 1a shows an excerpt of the spectra obtained. As can be seen from Fig. 1, the (1 1 1) reflection of gold became increasingly sharp during annealing. After 1 h annealing the (1 1 1) peak was split into two narrower peaks corresponding to the K_α1 and K_α2 lines

XRD as an alternative to coverage studies

It can be seen in Fig. 1b that (1 1 1) peak intensity variations can be used to follow the dewetting of thin gold films. Fig. 8a shows the (1 1 1) peak intensity and normalized hole area as a function of annealing time. The normalized hole area data was measured from separate ex situ experiments in an oven, not from the sample investigated in situ by XRD. It is thus possible that the observed shift between the (1 1 1) peak intensity and the normalized hole area was caused by differences in sample

Conclusions

The pioneering work of Mizsei and Lantto [21] on characterization of dewetting by XRD measurements has been continued in this work. It has been shown that the increase in (1 1 1) peak intensity during dewetting is caused by both a height increase of the features on the sample and an increase in texture. The dewetting of gold films of 15–30 nm thickness was found to occur via branched hole growth. Holes were found to protrude into high angle grain boundaries. EBSD measurements have shown that the

Acknowledgements

C.M. thanks D. Spori for support during film deposition and B. Grant for help with the thickness determinations by AFM. Special thanks go to M. Seita for carrying out the EBSD measurements, to F. Mornaghini for FIB operation and to S. Frank, C. Borgia and D. Koziej for assistance during XRD measurements. Support by the Nonmetallic Inorganic Materials Group (SEM), the Surface Science and Technology Group (thermal evaporation), the Multifunctional Materials Group (XRD) and Electron Microscopy at

References (27)

F.Y. Génin et al.
Acta Metall Mater
(1992)
D.C. Agrawal et al.
Acta Metall
(1989)
C.M. Kennefick et al.
Acta Metall
(1989)
E. Shaffir et al.
Acta Mater
(2009)
N. Kristensen et al.
Thin Solid Films
(1991)
A.E.B. Presland et al.
Prog Surf Sci
(1972)
E. Jiran et al.
Thin Solid Films
(1992)
A.E.B. Presland et al.
Surf Sci
(1972)
V.A. Mazur et al.
Phys Lett A
(1989)
D.J. Srolovitz et al.
Thin Solid Films
(1986)

W.L. Winterbottom

Acta Metall

(1967)

D.T. Danielson et al.

J Appl Phys

(2006)

W.W. Mullins

J Appl Phys

(1957)

Cited by (105)

Effect of surface energy anisotropy on hole growth in a single-crystalline thin film: A phase-field study
2023, Scripta Materialia
Prediction of the temporal evolution of a pre-existing hole on a solid-state thin film has important implications for self-organized nanoscale pattern formation. In this work, we employ a three-dimensional phase-field model to elucidate the mechanism of corner instability during capillary-driven hole growth in a single-crystalline free-standing thin film. We perform a systematic study to demonstrate the effect of crystalline anisotropy in surface energy, surface diffusivity, and film thickness on hole growth. Our simulations reveal that the instability during hole growth arises due to saturation of the rim height at the corner of the hole that is directly related to the arc length of the corner, specified by the anisotropy in surface energy. Our investigation further reveals the presence of a time-invariant shape of the corner with the onset of corner instability.
The effect of cavities produced by focused ion beam milling on solid state dewetting of thin Au films
2023, Acta Materialia
The goal of this study was to explore the possibility to control the nucleation and expansion of holes during initial stages of solid state dewetting of thin polycrystalline Au film deposited on sapphire substrate. To this end, we fabricated isolated triangular cavities in the film employing Ga⁺ ion milling in the focused ion beam instrument. The following dewetting heat treatments indicated that the cavities do promote early nucleation and expansion of dewetting holes, yet these holes develop tortuous shapes dominated by a random distribution of nearby hillocks in the film. Deep holes exhibited a triangular patch of amorphous Al-Ga-O phase at their bottom which retained its size and morphology during subsequent thermal treatments. This amorphous phase caused some delay in holes expansion at high temperatures. We proposed a geometrical model of the retracting metal film edge pinning by the amorphous phase. The observed pinning phenomenon can be utilized for the design of patterned thin films with increased thermal stability.
Formation of CuO whiskers and facet-controlled oxidation during the oxidation of Au-Cu nanoparticles fabricated by solid-state dewetting
2023, Applied Surface Science
The fabrication of cupric oxide (CuO) nanowires from Cu particles via thermal oxidation provides a simple and scalable method to produce hierarchical structures. A stress-induced growth mechanism is believed to account for the nanowire formation while a slow oxidation rate is favored to sustain the driving force. Here, CuO whiskers are grown from Au-Cu nanoparticles because the formation of Au-Cu phases decreases the Cu diffusion rate and in turn slows down the oxidation rate. The driving force for the whisker growth is attributed to the compressive stress imposed by the CuO shell on the Au-Cu core, which is induced by the significant difference in their linear thermal expansion coefficients. The contribution of the compressive stress is proved by the calculation. Moreover, preferred oxidation is observed and it is related to the crystalline structures of different facets existing on the surface of Au-Cu nanoparticles. The more compact the plane, the slower the diffusion rate through the plane, resulting in the formation of thinner CuO on the relevant facet. The results open a cost-effect way to fabricate hybrid nanostructures consisting of Cu-based core–shell nanoparticles attached with CuO whiskers and bring new insights into the oxidation behaviors of Cu on different crystal planes.
Controllable preparation of single-crystal diamond nanopillar clusters by metal cyclic dewetting process
2023, Applied Surface Science
Diamond nanopillars can effectively improve the spontaneous emission efficiency and coupling-out efficiency of diamond-color-center single-photon sources. In this paper, a low-cost and controllable method for preparing diamond nanopillar clusters is proposed. By choosing the crystal planting method and adjusting the methane concentration of the grown diamonds, isolated single-crystal diamond particles with a flat surface were prepared. Etching masks with individually controllable diameters and spacings were prepared through the cyclic deposition and dewetting of metal films on the surface of microcrystalline diamond particles. Using inductively coupled reactive ion etching technology, diamond nanopillar clusters were prepared through oxygen plasma etching. The nanopillars were uniform in diameter and densely arranged. The single-crystal properties of the diamond nanopillars and their enhancement effect on the photoluminescence of diamond color centers were confirmed through the analysis of Raman and photoluminescence spectra. The results provide a foundation for the deviceization of diamond-color-center single-photon sources.
Solid state dewetting of Ni-Co bilayers on sapphire during slow heating and cooling
2022, Acta Materialia
Citation Excerpt :
For the lowest studied temperature (500 °C), uniformly distributed anisotropic holes with branches were observed in Ni (Fig. 2(a)). Such holes are typical for solid state dewetting of thin polycrystalline films and have been observed in several previous studies [18,34,35]. In the Ni-30Co film, only few isolated anisotropic holes with sections of rimless edges at very nascent stage of nucleation were present, as indicated by arrows in Fig. 2(b).
We studied the solid state dewetting of thin Ni films and Ni-Co bilayers deposited on sapphire substrate during slow heating to several target temperatures. We demonstrated that during initial stages of dewetting and bilayers intermixing, Ni atoms from the top layer diffuse along the interface between the bottom Co layer and sapphire, simultaneously with Co atoms diffusion on the surface of Ni layer. This alloying of the top surface layer with Co slows down the rate of thermal grain boundary grooving and leads to the increase of the thermal stability of the bilayers with increasing Co content. We also observed that during intermediate stages of dewetting the regions of the untransformed film with increased thickness are dotted with small surface pores of 20–30 nm in diameter reaching to the level of as-deposited film. We related the formation and extraordinary stability of these pores with the modalities of surface diffusion of metal atoms in the presence of segregated impurities. In particular, we emphasized the role of Ehrlich-Schwoebel barrier for adatoms attachment to the edge of (111) terrace in formation of the pores. Our results indicate that the fine features of surface diffusion and surface thermodynamics, together with the presence of impurities and defects in the film lead to a rich variety of film morphologies during solid state dewetting.
Solid-state dewetting of thin Au films on oxidized surface of biomedical TiAlV alloy
2022, Acta Materialia
Recent applications of solid-state dewetting for the functionalization of medical implants raise the question of the thin film dewetting kinetics on surfaces with natural roughness. In this work, we studied the initial stages of solid-state dewetting of thin nanocrystalline gold (Au) films deposited on an oxidized surface of a biomedical titanium-aluminum-vanadium (Ti-6Al-4V) alloy. The interrupted annealing technique was employed to study the evolution of holes in the film and film flattening. While a high number of dewetting holes was formed in the film at the early stages of dewetting at the temperature of 300°C, the nanoporous but continuous film morphology remained stable even after prolonged anneals at higher temperature of 500°C. We developed models describing Au thin film dewetting on surfaces with typical topographical features of oxidized Ti-6Al-4V. These models considered the Au diffusion along the grain boundaries as a controlling factor of the film flattening. The stabilization of nanoporous film structure was attributed to the slowdown of the dewetting holes expansion at the sharp edges of the substrate. Our work demonstrates that solid state dewetting of thin metal films deposited on the rough surface of oxidized metal can be utilized for producing continuous nanoporous metal coatings.

View all citing articles on Scopus

View full text

Microstructure evolution during dewetting in thin Au films

Abstract

Introduction

Section snippets

Film deposition

XRD measurements as a method to follow the dewetting process

XRD as an alternative to coverage studies

Conclusions

Acknowledgements

Acta Metall Mater

Acta Metall

Acta Metall

Acta Mater

Thin Solid Films

Prog Surf Sci

Thin Solid Films

Surf Sci

Phys Lett A

Thin Solid Films

Acta Metall

J Appl Phys

J Appl Phys