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# Selective crack suppression during deformation in metal films on polymer substrates using electron beam irradiation

## Abstract

While cracks are usually considered detrimental, crack generation can be harnessed for various applications, for example in ceramic materials, via directing crack propagation and crack opening. Here, we find that electron beam irradiation prompts a crack suppression phenomenon in a copper (Cu) thin film on a polyimide substrate, allowing for the control of crack formation in terms of both location and shape. Under tensile strain, cracks form on the unirradiated region of the Cu film whereas cracks are prevented on the irradiated region. We attribute this to the enhancement of the adhesion at the Cu–polyimide interface by electrons transmitted through the Cu film. Finally, we selectively form conductive regions in a Cu film on a polyimide substrate under tension and fabricate a strain-responsive organic light-emitting device.

## Introduction

Cracks should be avoided unless we can control them. Crack formation has been considered a fatal failure, and most studies on cracks have focused on suppressing crack generation. However, researchers have recently attempted to control crack formation and even utilize the beneficial effects of cracking1,2,3,4,5. For instance, cracks in thin ceramic materials can be controlled in paths with widths down to sub-nanometers by residual stress engineering, which can be used as nanofluid channels for lab-on-a-chip applications6,7. While most of the studies on crack control are for ceramic thin films, some studies have proposed techniques to suppress crack formation in metallic thin films on polymer substrates by enhancing the adhesion between the film and the substrate because major electrical circuit failure in flexible and stretchable electronics occurs at metallic interconnects or current collectors that are susceptible to cracking due to frequent and severe deformation such as bending or stretching8,9,10,11. However, the proposed processes are hardly applicable to submicron-thick metallic films because of the brittle nature of the adhesive interlayers and rough surfaces caused by surface modification12,13. Moreover, controlling the shape and area of crack formation in thin metallic films is even more challenging compared to ceramic thin films because the general crack formation behavior of metallic films is very different from the crack formation behavior of ceramic thin films due to completely different fracture mechanisms arising from plastic deformation in metals10. Consequently, possible applications of selective crack formation in metallic thin films have been beyond the conventional research scope.

In this paper, we introduce a method for controlling the crack formation in a metallic thin film on a polymer substrate by utilizing unique electron beam–matter interactions at the nanoscale. We found that when the electron beam (e-beam) irradiates the surface of a 100-nm-thick Cu film deposited on a polyimide (PI) substrate, the crack formation is significantly suppressed so that the e-beam-irradiated area is hardly cracked, even at a tensile strain of 30%. The controlled e-beam patterns can even generate a non-crack pattern of any shape in the metallic thin film upon applying a tensile load. Experiments and simulations have shown that transmitted electrons can alter and engineer the interface between the Cu thin film and the PI substrate, leading to the improved adhesion between the Cu thin film and the PI substrate, thereby suppressing crack formation during tensile deformation. By utilizing e-beam irradiation, we can not only achieve selective suppression of crack formation but also introduce a strain-responsive conductivity pattern in the metallic thin film because of the difference in electrical conductivity between the cracked and non-cracked areas. Hence, we further incorporate our non-destructive and mask-free crack patterning method into the fabrication of an organic light-emitting device (OLED). We have successfully fabricated a phosphorescent OLED on a Cu thin film with an e-beam irradiated pattern, which results in a strain-induced light emission pattern when a tensile load is applied. We believe that our technology can be applicable not only to enhancing crack resistance in flexible and stretchable devices but also to developing smart devices engineered by strain-responsive conductive patterning.

## Results

### Analyses of e-beam induced changes in the Cu film–PI system

If the ductility of the Cu film is not improved, how can we explain the crack suppression by e-beam irradiation? The most typical method for suppressing the formation of cracks in a metal thin film on a flexible substrate under tensile stress is to increase the adhesion between the metal film and the substrate10,11,16. The main mechanism of cracking in such a film is strain localization arising from its partial delamination from the substrate caused by tensile deformation. Since the delaminated film has the same behavior as a free-standing film, even a slight deformation results in deformation instability, generating cracks due to necking or void formation17. This crack formation can be avoided if the delamination of the metal film is prevented by increasing the adhesion. We performed a nanoscratch test on the two Cu  film–PI systems—as before, one unirradiated and the other irradiated. Specifically, the critical normal force (Lcr), which refers to the normal force at the Cu–PI interface at the onset of delamination, was obtained by the nanoscratch test, and the relative value of work of adhesion (W) was calculated from the following equation:

$$L_{{\mathrm{cr}}} = \frac{{d_{{\mathrm{cr}}}}}{{\nu\mu }}\sqrt {2tEW}$$
(1)

### Dependence of crack suppression on the acceleration voltage

Trajectories of electrons and their interaction with materials, such as elastic and inelastic scattering and energy absorption in materials, can be quantitatively described using a Monte Carlo simulation (CASINOTM software) to understand the electron–matter interactions in this thin film system19,20,21. Figure 3a shows the distribution of the amount of energy absorbed in a 100-nm-thick Cu film and a PI substrate, when an e-beam with a radius of 50 nm is irradiated with different VA. In each case, the e-beam, consisting of 1,000,000 electrons and centered at (r, z) = (0 nm, 0 nm), is directed along the positive z-direction, and the absorbed energy per unit volume is averaged over the azimuthal angle. When VA = 3 kV, the energy of the incident electrons is absorbed entirely in the Cu film. As VA increases, the inelastic scattering in the Cu film progressively decreases, resulting in a monotonic decrease of the energy absorbed in that region (upper panels, Fig. 3a). At the same time, the energy absorbed in the PI film (lower panels, Fig. 3a) monotonically increases with VA. This phenomenon is directly associated with the VA-dependence of the number of electrons transmitted through the 100-nm-thick Cu film, as shown in Fig. 3b (blue). The ratio of the number of electrons transmitted through the Cu thin film to that of the total incident electrons, starting at zero at VA = 3 kV, is only 0.03 at 5 kV, abruptly increases to 0.77 at 10 kV, and reaches a plateau of >0.92 when VA ≥ 15 kV, implying that the degree of crack suppression by e-beam is likely to have a strong VA-dependence.

Hence, we carried out the tensile tests on the Cu  film–PI systems irradiated with VA varying from 3 to 25 kV, I = 11 nA, and D = 4.87 × 103 µC cm−2. The SEM images of the Cu thin films obtained at ε = 30% shown in Fig. 3c clearly indicate that the crack density in the Cu thin film indeed decreases as VA increases. The dependence of crack suppression on VA can be more quantitatively seen in Fig. 3b (red), where the crack densities normalized by the crack density of the unirradiated Cu thin film are plotted. The crack densities were determined from the SEM images using an image analysis, the details of which are shown in Supplementary Fig. 6. Figure 3b shows that the degree of crack suppression indeed has a strong VA dependence, and for a relative crack density < ~0.2, VA needs to be larger than ~10 kV. This result indicates that the electrons are required to reach the Cu−PI interface with sufficient kinetic energy for effectively suppressing the crack formation, which is consistent with our rationalization that the enhancement of the Cu−PI interface adhesion by the transmitted e-beam is a key mechanism for the crack suppression. E-beam-induced radiolysis of PI, which is strongly suggested by the increases in C–O–C bonds in the irradiated PI substrates (Supplementary Fig. 5) and the increase in indentation hardness of the PI substrate (Supplementary Fig. 7), may have caused the adhesion enhancement: the radiolytic damage could have facilitated both the migration of Cu atoms and the formation of Cu2O. Alternatively, the e-beam-induced migration of the Cu atoms may have been caused by the successive momentum transfers from the irradiated electrons to the Cu atoms, analogous to the electromigration22,23,24. Further investigation is required to clarify the mechanism of the adhesion enhancement.

### Application to strain-responsive OLEDs

Our finding can be incorporated into a real device fabrication. Here, we fabricate a strain-responsive optoelectronic device based on the selective formation of a conductivity pattern by the crack suppression technique. The experimental process is schematically described in Fig. 4a. After a Cu layer deposited on a PI substrate was exposed to patterned e-beam irradiation (VA = 25 kV, I = 45 nA, D = 1.00 × 104 µC cm−2), small-molecule organic semiconductor layers and a top metal electrode, both being 1.5 × 1 mm in size and covering the patterned region of the Cu layer, were sequentially deposited to form a green phosphorescent OLED with a layer structure shown in Fig. 4b. The e-beam-irradiated area of the Cu layer was in the shape of a smiling face, corresponding to the OLED area that is to remain emissive upon the application of strain, and a DC bias of 8 V, with the positive bias applied on the neck of the smiling face (marked by a red star-shaped dot in Fig. 4a), was maintained during the stretching of the OLED. When ε, applied along the direction indicated by red arrows in Fig. 4a, was small (≤4%), the entire (1.5 × 1 mm) OLED was emissive, with small dark spots distributed throughout the device caused by imperfections such as particulates and scratches on the PI substrate (Fig. 4c, d). At ε = 5%, the pattern of light-emission intensity corresponding to that of the smiling face began to appear (Fig. 4e) and became increasingly clear as ε increased (Fig. 4f, g, in situ video provided in Supplementary Video 2). This intensity pattern is the result of cracks in the Cu layer derived only in the unirradiated area, which increased the electrical resistance of the Cu layer in that region. Consequently, the resistive voltage loss in the unirradiated Cu region was increased, which in turn decreases the values of the local electrical bias and therefore decreases the light intensities in the unirradiated region. This result suggests that with further development, such as the optimized design of the metal–flexible substrate system, the area-selective crack suppression by e-beam may be a versatile technique for patterning various electronic devices in addition to our strain-responsive devices.

In summary, we found that e-beam irradiation onto the surface of a 100-nm-thick Cu thin film deposited on a PI substrate significantly suppresses the crack formation so that the e-beam-irradiated area was nearly crack-free, even at a large tensile strain of 30%, whereas the unirradiated area started crack formation before a strain of 10%. Our experiments and simulations suggested that e-beam irradiation transmitted the Cu thin film and induced the migration of the Cu atoms near the Cu−PI interface, which improved the adhesion at that interface so that crack formation was suppressed during tensile deformation. Since we were able to control the crack formation region in a metallic thin film on a polymer substrate by e-beam patterning, we could also generate any shape of a non-crack pattern in the metallic thin film upon applying a tensile load. These strain-responsive conductivity patterns in a metallic thin film were further incorporated into the fabrication of an OLED, which unveiled a strain-induced light emissive pattern when a tensile load was applied.

## Methods

### Sample preparation

To deposit an array of Cu thin films (thickness: 100 nm, width and length: 100 μm) on a 125-μm-thick PI substrate (Kapton®, DuPont), thermal evaporation using a metallic shadow mask (Supplementary Fig. 1) was employed. A spherically shaped Cu pellet (purity of 99.99%) was used as a deposition source. The base pressure and deposition rate were 2 × 10−6 Torr and 8 Å s−1, respectively.

### Electron beam irradiation

Prior to e-beam irradiation and SEM observation, 10-nm-thick Pt films were deposited to prevent charging of the non-conducting PI substrate. E-beam irradiation was performed using an SEM (Inspect F or Quanta 3D FEG, FEI). Detailed e-beam conditions are summarized in Supplementary Table 1.

### Fabrication of OLED

The top-emitting OLED has the following structure: PI/100 nm Cu/10 nm 1, 4, 5, 8, 9, 11-hexaazatriphenylene hexacarbonitrile (HAT-CN)/40 nm N,N’-bis(naphthalene-1-yl)-N,N’-bis(phenyl)benzidine (NPB)/20 nm 4,4’-N,N’-dicarbazolebiphenyl (CBP) doped with 3 wt% fac-tris(2-phenylpyridine)iridium [Ir(ppy)3]/15 nm bathocuproine (BCP)/25 nm tris-(8-hydroxyquinoline) aluminum (Alq3)/20 nm Mg:Ag (1:2 mass ratio). All layers were deposited by thermal evaporation in vacuum at a pressure of ~10−7 Torr. The deposition rates for all layers were 1 Å s−1.

### Characterization

The tensile test was conducted using a microtensile machine (Microtest 200 N, DEBEN) mounted on a specimen holder in the chamber of the SEM, enabling in situ observation during tensile deformation. The Cu thin films on the PI substrate were elongated up to a strain of 30% at a strain rate of 0.05 min−1. High-voltage transmission orientation mapping in STEM was performed in an FEI TecnaiTM F20 S/TEM equipped with an ASTARTM unit. Accelerating voltage, aperture size for the nano-beam diffraction mode, and beam precession angle were 200 kV, 30 μm, and 1°, respectively. EELS spectra, HAADF and high-resolution TEM (HRTEM) images were acquired with a JEOL ARM200 under the accelerating voltage of 200 kV. Fast Fourier transforms (FFT) obtained from the HRTEM images were calculated using Gatan Digital MicrographTM (DM) software (version 3.5). XPS analysis was performed with a PHI 5000 VersaProbe (ULVAC PHI, Japan) using a monochromatized Al Kα source. The two ends of the OLED sample were fixed on a Microtest 200 N tensile stage in ambient, and both electrodes were electrically connected to a Keithley 2400 SourceMeter® to apply a DC bias of 8 V. Then, the tensile load was applied to the device with a strain rate of 0.05 min−1 until the sample was stretched 10% of its original length. A CCD camera (EO-0312c, Edmund Optics) was mounted above the device to record the OLED device performance in real time.

## Data availability

All relevant data are available from the authors upon request.

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## Acknowledgements

This research is funded by Engineering Research Center (ERC) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (NO. NRF-2015R1A5A1037627). Y.-C.J. appreciates the support of MOTIE (Ministry of Trade, Industry and Energy 10051601) for the development of future devices technology for the display industry. I.-S.C. acknowledges financial support through the NRF of Korea (2015R1A2A2A04006933 and 2019R1A2C2003430) and Creative-Pioneering Researchers Program and Research Resettlement Fund of Seoul National University.

## Author information

Authors

### Contributions

I.-S.C. conceived the concept. S.-Y.L., J.-H.L., Y.-C.J. and I.-S.C. conducted the e-beam irradiation and tensile testing experiments. S.-Y.L., C.-H.S., D.-I.K. and J.-P.A. performed EELS, TEM, HAADF, and the ASTARTM analysis. S.-G.K., H.N.H., E.-C.J. and I.-S.C. conducted the nanoindentation tests and the nanoscratch tests. S.-Y.L. and C.K. carried out the CASINO simulation. K.R.P. and C.K. fabricated the OLED devices. S.-Y.L., Y.-C.J., C.K. and I.-S.C. analyzed the data. Y.-C.J., C.K., and I.-S.C. supervised the project. S.-Y.L., I.-S.C. and C.K. wrote the paper. All authors discussed the results and commented on the paper.

### Corresponding authors

Correspondence to Young-Chang Joo, Changsoon Kim or In-Suk Choi.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Communications thanks Jagannathan Rajagopalan and the other, anonymous, reviewers for their contribution to the peer review of this work.

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Lee, SY., Park, K.R., Kang, Sg. et al. Selective crack suppression during deformation in metal films on polymer substrates using electron beam irradiation. Nat Commun 10, 4454 (2019). https://doi.org/10.1038/s41467-019-12451-8

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• DOI: https://doi.org/10.1038/s41467-019-12451-8

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