Dewetting near the glass transition: transition from a capillary force dominated to a dissipation dominated regime

Dynamics and corresponding morphology of dewetting of thin polystyrene films at temperatures close to the glass transition were investigated by measuring simultaneously dewetted distance and width of the rim. Comparing the opening of cylindrical holes with the retraction of a straight contact line revealed (i). a drastic influence of the geometry (planar or radial symmetry) on the dynamics at early stages, (ii). a new logarithmic dewetting regime, and (iii). transitions between four dewetting regimes clearly indicated by changes in the shape of the rim. The complete dewetting scenario can be understood as an initial dominance of capillary driving forces, which is progressively overtaken by dissipation related to the increasing size of the rim.     

Viscoelastic thin polymer films under transient residual stresses: two-stage dewetting on soft substrates

A nonmonotonic, two-stage dewetting behavior was observed for spin coated thin viscoelastic polymer films on soft elastic substrates. At times shorter than the relaxation time of the polymer (ttau_{rep}), dewetting accelerated, accompanied by an unstable rim. However, holes nucleated at t相似文献   

On dewetting dynamics of solid films of hydrogen isotopes and its influence on tritium ß spectroscopy

The dewetting dynamics of solid films of hydrogen isotopes, quench-condensed on a graphite substrate, was measured at various temperatures below desorption by observing the stray light from the film. A schematic model describing the dewetting process by surface diffusion is presented, which agrees qualitatively with our data. The activation energies of different hydrogen isotopes for surface diffusion were determined. The time constant for dewetting of a quench-condensed film at the working temperature of 1.86 K of the mainz neutrino mass experiment was extrapolated. Received 30 December 1999     

Visualization of nanosecond laser-induced dewetting,ablation and crystallization processes in thin silicon films

In the present work, nanosecond pulsed laser crystallization, dewetting and ablation of thin amorphous silicon films are investigated by time-resolved imaging. Laser pulses of 532 nm wavelength and 7 ns temporal width are irradiated on silicon film. Below the dewetting threshold, crystallization process happens after 400 ns laser irradiation in the spot central region. With the increasing of laser fluence, it is observed that the dewetting process does not conclude until 300 ns after the laser irradiation, forming droplet-like particles in the spot central region. At higher laser intensities, ablative material removal occurs in the spot center. Cylindrical rims are formed in the peripheral dewetting zone due to solidification of transported matter at about 500 ns following the laser pulse exposure.     

Persistent droplet motion in liquid-liquid dewetting

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.     

Interfacial reaction during dewetting of ultrathin silicon on insulator

We have studied the Si/SiO₂ interfacial reaction during solid-state dewetting of 7-nm-thick Si(001) ultrathin films on SiO₂ substrates. Immediately after formation of Si nanocrystals at the dewetting front, Si/SiO₂ interface depression occurs at the edge of the nanocrystal because of the interfacial reaction. By examining the Si/SiO₂ interface morphology for nanocrystals at different distances from the dewetting front, we found that the interface depth increases linearly with time. We also estimated that the effective activation energy for the interfacial depression is about 3.9 eV. Furthermore, we explain the effect of the interfacial reaction on the active morphological change involved in dewetting front propagation.     

Dewetting of thin polymer films at temperatures close to the glass transition

We present detailed studies on dewetting of thin polystyrene (PS) films which were deposited onto silicon wafers coated with a polydimethylsiloxane (PDMS) monolayer. Experiments were performed at temperatures close to the glass transition temperature of PS. Several significant deviations from the dewetting behaviour of Newtonian liquids were observed. The length of the PS molecules, and thus the viscosity, turned out to be of minor importance in determining the dewetting velocity, in particular for the later regimes. In stark contrast, the geometry of the drying spot had a striking influence on the dewetting velocity. Initially, dewetting from straight contact lines proceeded faster than the opening of circular holes. At later stages, the process slowed down significantly in both cases. Under the conditions at which our experiments were performed, PS cannot flow like a simple liquid. Thus, the observed dewetting has to be the consequence of plastic deformation induced by capillary forces. Our results indicate that under such conditions the energy dissipation process is strongly affected by geometry, which is not the case for viscous liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 68.60.-p Physical properties of thin films, nonelectronic - 61.41. + e Polymers, elastomers, and plastics - 68.55.-a Thin film structure and morphology - 83.50.-v Deformation and flow     

Modeling dewetting of ultra-thin solid films

We review some models for the dynamics of dewetting of ultra-thin solid films. We discuss the similarities and the differences between faceted and non-faceted systems. The faceting of the dewetting rim leads to corrections in the velocity of dewetting fronts both in flat and axisymmetric geometries. The faceting of the edge of the dewetting rim leads to a strong anisotropy of the dewetting instability. Faceting also induces novel dewetting regimes such as layer-by-layer dewetting, and monolayer dewetting.     

Dewetting of polymer films by ion implantation

We report dewetting of thermodynamically stable, thick (∼ 100 nm) polystyrene films by titanium ion implantation. The dynamic dewetting patterns in time evolution are recorded. The dewetting mechanism is determined to be heterogeneous nucleation, where the defects and Ti nanoparticles formed by ion implantation serve as the nuclei. In addition, we observe abundant rims with regular polygonal shapes in dewetting patterns. This is attributed to fingering instability, which results from the balance between the driving force arisen from thermally induced surface tension gradient and the resistive forces from the combination of friction force, Laplace pressure and long-range van der Waals interactions. Finally, a model based on mass conservation is used to qualitatively describe the transition from circular to polygonal shaped rims at a critical diameter for holes.     

Stabilization of discotic liquid organic thin films by ITO surface treatment

Discotic liquid crystals (LCs) are promising materials in the field of electronic components and, in particular, to make efficient photovoltaic cells due to their good charge transport properties. These materials generally exhibit a mesophase in which the disk-shaped molecules can self-assemble into columns, which favorize charge displacement, and may align themselves uniformly on surfaces to form well-oriented thin films. In order to orientate such a columnar thin film on an indium tin oxide (ITO) substrate, the film is heated up to the temperature range of the isotropic liquid phase and subsequently cooled down again. This treatment may lead not only to the desired alignment, but also to dewetting, which leads to an appreciable inhomogeneity in film thickness and to short circuits during the realization of photovoltaic cells. In this article, we describe how this dewetting and the film morphology can be influenced by ITO surface treatments. The chemical modifications of the surface by these treatments were studied by X-ray photoelectron spectroscopy (XPS). Such ITO treatments are shown to be efficient to prevent thin film dewetting when combined with rapid cooling through the isotropic-to-LC phase transition.     

Influence of the addition of silsesquioxane on the dewetting behavior of polystyrene thin film

A strategy to suppress the dewetting of polystyrene (PS) thin films by the addition of octacyclopentylsilsesquioxane (cPOSS) as a nanofiller was proposed. PS thin films with cPOSS were prepared by spin-coating. The bulk glass transition temperature of PS was not changed with an addition of the nanofiller up to 10 wt%. On the other hand, the addition of cPOSS to the PS thin films led to a great inhibition of dewetting. After annealing for 3 h at 373 K, no appreciable dewetting was observed by optical microscopy in the PS film with 15 wt% cPOSS, in contrast, the PS film without cPOSS was completely dewetted. Holes formed on the PS films with 10 wt% cPOSS. However, in that case, the growth of the holes stopped before reaching the final stage of the dewetting. This suggests that the increase of the cPOSS concentration per unit area at the rim of the holes prevents further growth of the holes, and this inhibition effect can be attributed to the interaction between cPOSS and the substrate accompanying modification of the PS-substrate interface.     

金属纳米薄膜在石墨基底表面的动力学演化

采用分子动力学方法研究了金属Au和Pt纳米薄膜在石墨(烯)基底表面的动力学演化过程,探讨了金属薄膜和石墨(烯)基底间的相互作用对金属纳米薄膜在固态基底表面的去湿以及脱附的动力学演化的影响.研究结果表明,在高温下,相同层数的Au和Pt纳米薄膜在单层石墨基底表面上存在不同的去湿现象,主要表现为厚度较小的Pt纳米薄膜在去湿过程中有纳米空洞形成,而同样厚度的Au薄膜在去湿过程中没有形成空洞.Au和Pt两种金属薄膜在高温下都去湿形成纳米液滴,这些液滴最终都以一定的速度脱离基底.在模拟的薄膜厚度范围内(0.2—2.3 nm),Au和Pt纳米液滴脱离基底的速度随厚度增加表现出不同的变化规律.Pt纳米液滴的脱离速度随薄膜初始厚度的增加先增加后减少,而Au脱离速度随厚度的增加先减少,达到一个临界厚度后脱离速度突然迅速增加.利用薄膜与基底间相互作用的不同导致去湿过程中的黏滞耗散不同,定性分析了这种变化规律的原因.此外,进一步研究还发现金属液滴的脱离时间与薄膜厚度和模拟温度的依赖关系,发现脱离时间随薄膜厚度的增加而增加,随模拟温度的升高而减小.这些研究结果可以为金属镀膜、浮选、表面清洁、器件表面去湿等工业生产过程提供理论指导.     

Dewetting of thin polymer films

We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.     

Molecular recoiling in polymer thin film dewetting

The molecular recoiling force stemming from nonequilibrium chain conformation was found to play a very important role in the dewetting stability of polymer thin films. Correct measurements and inclusion of this molecular force into thermodynamic consideration are crucial for analyzing dewetting phenomena and nanoscale polymer chain physics. This force was measured using a simple method based on contour relaxation at the incipient dewetting holes. The recoiling stress was found to increase dramatically with molecular weight and decreasing film thickness. The corresponding forces were calculated to be in the range from 9.0 to 28.2 mN/m, too large to be neglected when compared to the dispersive forces (approximately 10 mN/m) commonly operative in thin polymer films.     

Dewetting observations of ultrathin metallic films

Ultrathin metallic films like CoFe, Ta, Cu, Cr, and NiFe are widely used in magnetic devices such as magnetic random access memory (MRAM) and magnetic recording heads. Dewetting corrosions were often observed after O₂ plasma ashing in MRAM fabrications. The surface stability of these films was then examined. The results show that dewetting takes place when CoFe or Cu films are exposed to air after an O₂ plasma process. In contrast to the dewetting reported so far in organic or metallic liquid films on solid substrates, the observed dewetting does not occur in a liquid state but in a solid state. Several in situ and ex situ process methods were examined to control the dewetting. It is found that after ashing, the immediate immersion of wafer into acetone and ultrasonic cleaning some minutes after opening chamber can greatly suppress the occurrence of dewettings. Process examinations show that the heating is unimportant for the formation of the dewetting, while moisture in air may play an important role in the formation of the dewetting, acting as a necessary catalyst. Several dewetting patterns were observed, and the pattern shape depends not only on the thickness of the film, but also on the plasma parameters. Possible mechanisms responsible for the formation of these patterns are discussed.     

Disentanglement time of polymers determines the onset of rim instabilities in dewetting

Molecular relaxations determine the viscoelastic properties of polymers, which, in turn, control macroscopic processes like dewetting. Here, we demonstrate experimentally that the onset of rim instabilities is correlated with the longest relaxation ("reptation") time of the dewetting polymer. Conversely, such experiments allow us to determine the reptation time of polystyrene in thin films as a function of molecular weight. Our approach opens up new possibilities for testing rheological properties of polymers confined in thin films.     

Open questions and promising new fields in dewetting

This contribution summarizes the present understanding of dewetting focusing on three points that are either controversial or open. The first issue concerns the initial formation of holes, i.e. the film rupture. The second point concerns the unstable growth of holes, i.e. the transversal instability of the receding contact line. Finally, recent extensions towards dewetting on heterogeneous substrates are examined. In passing the long time evolution in dewetting and the coupling of dewetting with other effects are discussed.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 68.55.-a Thin film structure and morphology - 47.20.Ma Interfacial instability     

Dewetting as an investigative tool for studying properties of thin polymer films

Employing mass conservation, time-resolved dewetting experiments of thin polymer films allow to determine in real time the dynamic contact angle and the slippage length. Moreover, based on a systematic variation of interfacial properties of a polymer brush, dewetting makes it possible to calculate the force it needs to extract a single polymer chain from its own melt. In the visco-elastic regime close to the glass transition, the temperature and molecular weight dependence of the relaxation time of residual stresses resulting from film preparation by spin-coating can be obtained from the evolution of the shape of the dewetting rim. The presented examples demonstrate that dewetting represents a powerful approach for a sensitive characterization of rheological, frictional and interfacial properties of thin polymer films.     

Dynamics and instability of solid-state dewetting

Spontaneous dewetting of solid thin films proceeds by edge retraction of film edges and/or by heterogeneous void growth. Classical 1D and 2D continuous models of the evolution of a dewetting film, based on surface diffusion mechanisms, predict that in the long-time limit dewetting obeys universal scaling laws. In this paper, we review 1D and 2D predictions and recent experimental results. For this purpose, using Si(001)/SiO₂ and Ge(001)/SiO₂ single-crystalline thin films in different geometries, we have been able to compare theoretical predictions to experimental results obtained by combining in situ LEEM and ex situ AFM measurements. For dewetting from film edges, experimental results partially differ from continuous models predictions. More precisely, because of the crystallographic anisotropy: (i) the facetted edges remain stable during dewetting (they simply recede at constant shape) while poorly or un-facetted edges are unstable (they recede by finger formation); (ii) rim formation, induced by mass-conservation condition, proceeds in a layer-by-layer mode and is limited by 2D nucleation properties on the top of the rim; (iii) the island generation mechanism differs from the mass shedding behaviour predicted by 1D models. For dewetting mechanisms involving void growth, different behaviours are reported and discussed. For thin Si(001)/SiO₂ films, the corners of the opening square-shaped voids lead to a local destabilisation of the growing voids. For thin Ge(001)/SiO₂ films, the side of the voids invariably turns instable and forms tip dendrites whose branch density depends on the temperature and the initial film thickness. Finally, ultra-thin films, more sensitive to local fluctuations, dewet in a fractal geometry.     

The timescale of spinodal dewetting at a polymer/polymer interface

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface. Received 25 June 2001 and Received in final form 5 December 2001