In situ x-ray diffraction analysis of 2D crack patterning in thin films

doi:10.1016/j.actamat.2018.11.040

Acta Materialia

Volume 165, 15 February 2019, Pages 177-182

https://doi.org/10.1016/j.actamat.2018.11.040 Get rights and content

Abstract

In this work, the effect of the loading path on the multicracking of Nickel thin films on Kapton® substrate was studied thanks to an experimental set-up combining controlled biaxial deformation, x-ray diffraction and digital image correlation. Samples were biaxially stretched up to 10% strain following either a single equibiaxial path or a complex one consisting of loading successively along each of the axes of the cruciform specimen. While the first path leads to a mud-crack pattern (random), the second leads to a roman-bricks one (square). Moreover, the in situ x-ray diffraction experiments show that the stress field developed in the thin film during the multicracking is clearly dependent on the loading path. By combining the study of stresses and x-ray diffraction peaks linewidth, we evidenced mechanical domains related to initiation of cracks and their multiplication for each loading path. Moreover, stress evolution in the thin film during mud-crack pattern formation is significantly smoother than in the case of roman-bricks one as represented in the plane stress space.

Graphical abstract

Introduction

The field of stretchable and flexible electronics has been growing for many years [[1], [2], [3]]. The increasingly complex forms of electronic devices will benefit from the functionalities of these systems by their adaptability to non-planar surfaces (conformability) [4,5]. Other factors contributing to the growth of this market are the lightness and low cost of production compared to rigid substrates. From a general point of view, the main limitation of these flexible/stretchable systems is their often too weak durability [6]. If the substrates generally made of polymers are adapted to large strains, the inorganic films that carry the functionality are more prone to failure.

It has been known for a few decades that the cracking of thin films can lead to complex patterns that may be both harmful and useful depending on the intended applications [[7], [8], [9], [10]]. This cracking occurring for materials with significant mechanical contrast occurs both in nature at centimeter scale (deserts, crocodile skin [11]) or in the context of technological applications at micrometer scales [12]. In the latter case, uncontrolled cracking is generally an undesirable phenomenon that can lead to a modification or even a deterioration of the properties such as electrical [13,14] or magnetic [15,16]. However, multicracking can also be used to create patterns of material as can different lithography techniques with the possibility of arriving at non-equilibrium geometries [17]. In addition, this may be useful for substrates used in stretchable or flexible electronics such as polymers.

Regarding this last type of substrate, the most usual test is the uniaxial tensile test which induces a periodical array of cracks that is more or less straight depending on the mechanical properties (fragile or ductile) and the thickness of thin films [[18], [19], [20], [21]]. It is also known that the equibiaxial test leads to a random structure of cracks [22] as found in nature (sand or dried mud [23], animal skin [11]). Unlike rigid substrates that give way to very varied patterns, there are few alternatives to the straight cracks and mud-cracks. The formation of rectangular or square fragments is possible by creating a cavity network before biasing the film in equibiaxial traction [17,24], this of course requires a step upstream of microfabrication of the cavity network.

Among the works describing the multicracking of thin films on polymer substrates, a few groups have described the mechanisms by in situ x-ray diffraction and uniaxial tests [18,21,25,26]. Indeed, this technique allows measuring the residual stresses and the following evolution of thin film stresses during macroscopic deformation, which makes it unique. Hence, few researchers have found a correlation between the crack multiplication and in-plane stress relaxation after the initiation of cracks. However, this community has poorly explored the biaxial loading effects on the stress evolution during development of more complex crack patterns. Until now, Djaziri et al. have studied the effect of loading ratio during biaxial loading on the initiation of the cracking of W/Cu thin films [27], but without looking at the effect on resulting crack patterns. Moreover, the effect of complex loading have not been explored.

In this paper, we show how two different mechanical biaxial loading paths of thin films on stretchable substrates can lead to completely different two-dimensional crack patterns, namely mud-cracks or roman-bricks. These are obtained using a biaxial machine that is installed in a Synchrotron beamline (SOLEIL DiffAbs), which allows controlling independently the applied load along each axe. Therefore, this set-up allows monitoring in situ the stresses in the thin film by x-ray diffraction (XRD) during the development of these patterns in order to find relationships between stresses and multi-cracking micromechanisms.

Section snippets

Materials and experimental set-up

The Ni thin films were deposited onto 25-μm-thick polyimide (Kapton®) cruciform substrate and were produced at room temperature by physical vapor deposition (PVD) with an ${Ar}^{+}$ -ion-gun sputtering beam at 1.2 keV (Kaufman ion source) in a NORDIKO, Hampshire-3000 system. The base pressure of the deposition chamber was $7 \times 10^{- 5}$ Pa while the working pressure during films growth was approximately $1.4 \times 10^{- 2}$ Pa. Ni growing rates was previously calibrated by x-ray reflectivity and was found to be 0.05 $nm . s^{- 1}$

Loading effect on crack pattern

Two loading paths were adopted for this study: a first equibiaxial loading (Fig. 2) and a second complex one consisting of loading successively along each of the axis of the cruciform specimen (Fig. 3). Note that in the latter case, a slight transverse tension force has been applied in order to avoid the phenomenon of buckling of the thin film related to possible high compressive stresses [33]. The general goal is to achieve similar force values along both axes through totally different loading

Conclusion

We have biaxially tested Ni thin films (thickness of 50 nm) on Kapton® cruciform substrates (thickness of 25 $μ m$ ). Two main loading paths were explored, which resulted in two different crack patterns, i.e. mud-cracks for equibiaxial path and roman-bricks for complex one (by successively loading along each of the axes of the sample). In situ x-ray diffraction experiments have been combined with digital image correlation measurements at SOLEIL Synchrotron DiffAbs beamline in order to probe the

Acknowledgments

This work has been partly supported by the Region Ile-de-France in the framework of DIM Nano-K. Authors would like to thank Philippe Guérin from the PPRIME institute for sample preparation and acknowledge the support of the DiffAbs beamline team for technical help during the experiments described in this paper.

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Full length articleIn situ x-ray diffraction analysis of 2D crack patterning in thin films

Abstract

Graphical abstract

Introduction

Section snippets

Materials and experimental set-up

Loading effect on crack pattern

Conclusion

Acknowledgments

Nature

Int. Jour. Sol. Struct.

Fract. Mech.

Mech. Phys. Sol.

Nat. Commun.

Microelectron. Eng.

Acta Mater.

Scripta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng.

Acta Mater.

Surf. Coating. Technol.

Thin Solid Films

Acta Mater.

Thin Solid Films

Nat. Commun.

Nano Lett.

Appl. Phys. Rev.

Full length article
In situ x-ray diffraction analysis of 2D crack patterning in thin films