Crack Development and Electrical Degradation in Chromium Thin Films Under Tensile Stress on PET Substrates

Abstract

The mechanical and electrical deterioration of chromium (Cr) thin films sputtered onto polyethylene terephthalate (PET) substrates under tensile strain was studied. Understanding mechanical and electrical stability due to imposed strain is particularly important for device reliability, as the demand for flexible electronic devices increases. Cr thin films, widely spread across the field of electronic and sensor applications, face crack propagation with electrical degradation with tensile stress that can seriously compromise the performance. Accordingly, this study offers new findings on how Cr film thickness might influence crack formation and electrical resistance differently and also the general guidelines for flexible electronic component design with respect to long-term durability. Electrical resistances were measured while mechanically stretching 100- and 200 nm thin sheets. The study focused on crack development and propagation mechanisms in both film thicknesses and their effects on percentage change in electrical resistance (PCER). Scanning electronic microscopy (SEM) was used to characterize surface morphology and observe cracks as the strain rose. Early crack formation in 100 nm Cr films led to rapid PCER increases due to quick crack propagation and fast electrical degradation. Thicker 200 nm films, however, showed a more gradual PCER rise with fewer but deeper cracks, indicating a regulated strain response. Unlike the sharp PCER spike in 100 nm films, 200 nm samples were more variable, with three out of four showing a slight PCER decrease at the end, hinting at partial crack repair or conductive realignment before full failure. These results underscore the role of layer thickness in managing crack propagation and electrical stability, relevant for flexible electronics and strain sensors. This paper is aligned with the ninth goal of the United Nations Sustainable Development Goals, specifically Target 9.5: Enhance Research and Upgrade Industrial Technologies.

1. Introduction

Chromium, an atomic number 24 transition metal, has many oxidation states. Hard and steel-gray, it resists corrosion and tarnishing [1,2,3]. It is commonly utilized in surface coatings and alloying to increase material durability due to these properties [4]. Chromium improves the strength and corrosion resistance of stainless steel, one of its main uses [5,6,7,8,9]. This is because a protective oxide layer on the surface prevents environmental damage. Chromium is also perfect for automotive, aeronautical, and industrial equipment designed for high temperatures because of its reflective qualities and tolerance to high temperatures [10].

Cr thin films’ excellent conductivity, chemical stability, and ability to connect with numerous materials make them useful in electronics [10,11]. In semiconductor devices, chromium thin films are used as adhesion layers or contact materials. Their high adhesive qualities allow them to establish stable interfaces between materials, especially in multilayer constructions where they bind metals and insulating layers. Chromium thin films are widely utilized as contact or adhesion layers in electronics because they generate strong connections between materials [11,12,13,14]. They are used in optical coatings, microelectronics, and electronic device diffusion barriers. Chromium thin films are used in sensor technology and integrated circuit resistive element manufacturing due to their resistance.

Further, microelectronics uses chromium thin films as resistive elements in integrated circuits (ICs) [15,16]. Precision resistor networks require stability and durability; therefore, their variable resistivity is suitable. Chromium thin films also prevent atoms from migrating across layers in microelectronic systems, preserving their integrity and lifespan [17]. Chromium thin films also form anti-reflection and reflective optical coatings for photodetectors, optical sensors, and laser systems [18,19]. TFTs (thin-film transistors) and displays, which require electrical conductivity and optical performance, can employ them due to their reflecting qualities and strong temperature tolerance [20,21,22].

In flexible electronics, chromium thin films are also useful. Chromium thin films over flexible substrates like polyethylene terephthalate (PET) can create flexible electronic components [23,24,25]. For durability in wearable devices, flexible displays, and sensors, their mechanical behavior under bending, including fracture development and resistivity changes, must be continuously monitored. Chromium thin films are vital for sophisticated electronics, helping to create more robust, high-performance devices.

Hoffman and Thornton [26] examined the effect of high compressive stresses that might appear when sputtering takes place at low working pressure combined with a moderate deposition rate. If the deposition rate is increased with a corresponding rise in working pressure, this too serves to decrease the magnitude of the film’s stress. The most obvious effect concerns the increase in pressure, which first gives rise to a complete reversal of the stress from high compression to high tension and then a gradual reduction toward zero t of deposition rate, working pressure, and the nature of the working gas on the internal stresses present in chromium foils sprayed on glass using a magnetic post-cathode sprayer. It is suggested that the film stress in both cases results from the stress of the deposited film due to accelerating ions or neutral atoms. Takashi Kawanabe et.al [27] studied chromium films after preparing them using the facing targets sputtering (FTS) system. The morphology and crystallinity of the films were controlled by controlling the deposition conditions, particularly the argon gas pressure. It was found that the morphology of the chromium films depends greatly on the argon gas pressure. They also found a meshless surface morphology at low pressures of up to 0.2 mTorr and a clear column was found at high pressures of up to 10 mTorr. By continuous dc magnetron sputtering (dcMS) and modulated pulse power sputtering (MPP) techniques, Jianliang Lin et al. [28] studied chromium coatings in a closed-field unbalanced magnetron sputtering system. The MPP exhibited higher deposition rates than those in dcMS when the average target power density was above 14 W cm⁻² for chromium coating depositions. Their diagnostics confirmed a significant increase in the numbers of both target material ions (Cr) and gas (Ar) in MPP plasma compared to DC plasma. The enhanced ion flux bombardment from the highly ionized MPP plasma resulted in a denser microstructure and finer grain size in the MPP Cr coatings compared to the dcMS Cr coatings. In addition, the MPP volatile chromium coatings exhibited improved hardness and better adhesion strength.

Mechanical testing, such as cyclic bending and stretching of thin films placed on flexible substrates is necessary to determine their endurance and performance in flexible electronics applications. Flexible devices undergo repetitive bending, flexing, and stretching during operation, which these tests imitate. Cyclic bending tests regularly bend the flexible substrate to a given radius or angle, while stretching tests elongate it to assess tensile stress response [29]. Thin coatings like chromium on flexible substrates can split or delaminate during cyclic bending, changing their electrical characteristics, especially resistance. Stretching can propagate cracks and cause mechanical failure, compromising the thin film’s structural integrity and electrical continuity. These mechanical tests determine the material’s mechanical fatigue resistance, stress performance, and failure processes. These tests forecast the dependability and longevity of flexible electronic systems, because thin films are frequently mechanically deformed. Understanding thin films’ cyclic bending and stretching behavior is essential for building more durable materials for wearable electronics, flexible displays, sensors, and other new technologies. These tests discover material flaws, inform design modifications, and verify that final products can withstand real-world use.

2. Materials and Methods

Mechanical and electrical properties of chromium thin films sputtered onto PET substrates were studied using several well-defined experimental methods. First, PET substrates were created. PET sheets with 127 micrometers were carefully cut into 10 mm × 100 mm portions. The PET was sourced from Plasma Quest. A sharp knife was used to cut precisely to maintain sample consistency.

A chromium target-equipped sputtering chamber received the cleaned PET substrates. RF magnetron sputtering was used under strict conditions. The sputtering unit was acquired from Beijing Technol Science Co., Ltd. (Beijing, China), and the chromium target was sourced from MSE Supplies. The chamber was evacuated to a sub-2.5 × 10⁻³ Pa base pressure, and 29 sccm of argon gas and 250 W of RF power were used. For uniform film covering, substrates were rotated at 20 rpm during deposition. One batch acquired a 100 nm chromium coating after one hour of sputtering, whereas the other received 200 nm chromium after two hours. The thickness was measured using quartz crystal microbalance (QCM). The QCM is the most common companion device for most sputtering machines, and its use is to measure film thicknesses in situ. Since it operates by monitoring the change in frequency of a vibrating quartz crystal—as material is deposited from the sputtering process onto the crystal, the added mass lowers its vibration frequency—this shift in frequency is directly proportional to the thickness of the film.

Tensile testing was performed on chromium-coated PET samples after deposition. Each sample was put on an Instron tensile testing equipment according to a prescribed protocol. The 80 mm gauge machine jaws clasped the samples. Electrical connections were applied to each sample for tensile testing to determine electrical resistance. Copper tape was applied to the chromium coating 10 mm from each end, and tweezers connected to the tapes were coupled to an ohmmeter. The ohmmeter used was an NI myDAQ from National Instruments. The electrical resistance was measured once per second during testing and logged straight to a computer for analysis.

Tensile testing was done at 1 mm/s. To guarantee repeatability and dependability, four samples of each thickness (100 nm and 200 nm) were stretched to failure. Electrical resistance was measured and associated with the sample mechanical strain during testing.

Some samples were stretched to 3.75%, 5%, and 6.25% of their gauge length in addition to tensile tests. After the tensile tester, these samples were analyzed with scanning electronic microscopy (SEM). The SEM tool used was acquired from JEOL Ltd. (Tokyo, Japan). The imposed strain caused cracks, deformation, and other morphological changes on the chromium-coated surface, which were observed by SEM. Detailed surface characterization revealed sputtered film mechanical failure mechanisms.

Figure 1 shows the experimental setup, including the tensile testing equipment, electrical resistance measurement device, and post-deformation analysis SEM imaging arrangement.

3. Results

Figure 2 illustrates the crack propagation in a 100 nm thick chromium thin film sputtered on a PET substrate after being subjected to tensile stretching, extending the film to 3.75% of its original length. The image clearly reveals the formation of distinct cracks, which develop as a result of mechanical stress. These cracks propagate across the surface, indicative of the thin film’s mechanical degradation and its effect on the material’s structural integrity. The magnification and scale bar provided context for understanding the microstructural changes at the nanoscale level.

The cracks exhibit a predominantly irregular pattern, with many of them following a zigzagging path rather than straight lines. These cracks tend to branch off and intersect with each other, forming a network of irregular polygons. The non-uniformity in the crack shapes suggests that the mechanical stress is distributed unevenly across the film, leading to crack propagation in multiple directions. This irregular crack morphology is typical of thin films under strain, where localized stresses and weaknesses in the material contribute to the complex crack patterns observed. Further, the brittleness of chromium thin films leads to rapid crack initiation and propagation, contributing to both the crack intensity (how many cracks form) and the severity (depth and width) of those cracks. Thinner films may experience higher crack intensity due to their inability to resist stress, while thicker films, though still brittle, may show lower crack intensity but with deeper, more severe cracks.

Many crack patterns are reported in the literature. It is noticeable that the crack pattern of chromium is unique. The crack patterns observed in chromium thin films under tensile stress differ significantly from those seen in materials like aluminum (Al), indium tin oxide (ITO), poly (3,4-ethylenedioxythiophene) (PEDOT), and molybdenum (Mo) [1,2,3,4,5]. Chromium thin films exhibit irregular, zigzagging cracks that form a complex network, branching in multiple directions. This irregularity suggests inhomogeneous stress distribution and localized weaknesses, resulting in a more chaotic cracking process. In contrast, cracks in aluminum films tend to initiate at lower strains (around 2% for thicker films) and propagate more uniformly, primarily in the direction perpendicular to the applied tensile load, with lateral cracks appearing at higher strain levels to relieve internal stress [30]. ITO films also show more structured crack propagation, with primary cracks forming perpendicular to the tensile force at around 4% strain and secondary cracks developing perpendicular to the original ones as strain increases. These cracks in ITO are generally more uniform and linear compared to chromium films [31]. PEDOT films, on the other hand, demonstrate far greater flexibility and crack resistance, showing no significant cracks even at high strain levels, highlighting their superior mechanical robustness compared to both chromium and ITO [31]. Mo films, like chromium, exhibit more irregular and advanced crack patterns, particularly under higher strain rates, where secondary cracks form at angles (30° to 45°) to the primary cracks, reflecting anisotropic stress behavior. Mo films also experience edge delamination under extreme mechanical stress, a behavior not commonly observed in chromium films [32,33]. Thus, chromium films display more chaotic crack development compared to the relatively more structured and uniform cracking observed in Al, ITO, and Mo films, while PEDOT films stand out for their remarkable resistance to cracking.

Figure 3 and Figure 4 present the progressive crack formation in a 100 nm thick chromium thin film sputtered on a PET substrate as it is subjected to tensile strain. In Figure 3, the film has been stretched to 5% of its original length, while in Figure 4, the strain has increased to 6.25%. At 3.75% strain (as described earlier), the cracks in the chromium film began to appear irregular and branched, forming a network that is distributed across the film surface. As the strain progressed to 5% (Figure 3), the existing cracks further expanded and new cracks initiated. The crack network became more pronounced, and the cracks grew longer, often intersecting at various angles. The branching patterns also became more complex, with some cracks curving or following irregular paths rather than extending straight. This indicates the accumulation of localized stresses within the film. By the time the strain reached 6.25% (Figure 4), the crack formation had further intensified. The cracks not only lengthened but also increased in density, with more intersections and bifurcations. The main cracks, running perpendicular to the tensile direction, appeared to widen, and secondary cracks propagated from the original crack sites. This stage of crack development suggests that the material is experiencing greater mechanical failure, with the cracks propagating through areas of weakness, potentially leading to electrical failure as the cracks disrupt the film’s conductive paths.

Figure 5, Figure 6 and Figure 7 demonstrate crack formation in a 200 nm thick chromium thin film under tensile strain. Figure 5 shows the film cracked to 3.75%, Figure 6 to 5%, and Figure 7 to 6.25%. The film was thicker than the 100 nm film; however, all photos were obtained at the same magnification to compare crack formation.

In Figure 5, at 3.75% strain, initial cracks begin to form. These cracks are deeper due to the greater thickness of the film, but the overall crack intensity—the number of cracks and their density across the surface—is lower than in the 100 nm film at the same strain. The cracks are more linear, with fewer branches, indicating that the thicker film can initially withstand the applied stress better and delays the formation of complex crack networks.

At 5% strain (Figure 6), the cracks become more pronounced and longer. However, the crack intensity remains lower compared to the 100 nm film at the same strain level. There are fewer cracks per unit area, but they are deeper and tend to propagate in straighter lines, with limited branching. This suggests that while the thicker film is more resistant to widespread cracking, the cracks that do form penetrate deeper into the material.

In Figure 7, as the strain increases to 6.25%, the cracks further develop, becoming more interconnected and showing more branching. Although the cracks are still fewer and more isolated than in the thinner film, they are wider and deeper, reflecting the increased mechanical stress within the thicker film. The lower crack intensity compared to the 100 nm film is still evident, but the cracks that do appear are more severe in terms of depth and width, indicating the film is approaching a more advanced stage of mechanical failure.

Comparing this to Figure 2, Figure 3 and Figure 4, which show crack development in a 100 nm thick film, the thinner film exhibits a much higher crack intensity at every strain level. The cracks in the 100 nm film are more numerous, more branched, and appear at earlier stages of strain, showing that the thinner film is less capable of resisting the formation of cracks under tensile stress. In contrast, the 200 nm film develops fewer cracks, but they are deeper and straighter, indicating a different failure mechanism where the thicker film is able to withstand stress longer but experiences more pronounced localized damage when cracks do form.

The percentage change of electrical resistance (PCER) (Equation (1)) is a valuable metric for analyzing the relationship between mechanical strain and electrical resistivity in thin films, especially in brittle materials like chromium. PCER is defined as follows:

$$\mathrm{PCER}={\displaystyle \frac{\left(\mathrm{R}-\mathrm{Ro}\right)}{\mathrm{Ro}}}\times 100\%$$

(1)

where R is the current electrical resistance, and Ro is the initial resistance of the material. The use of PCER allows for the standardization of resistance measurements by accounting for the specimen’s initial resistance, which varies depending on the sample’s geometry (e.g., length, width). This allows for more consistency in sample comparison, since many samples may vary in dimension, normalizing the change in resistance to the initial resistance. Since electrical resistance is a function of length, directly proportional to sample length and inversely proportional to width, samples with different dimensional specifications may have different baseline resistances. The dimensional effects are further reduced by the percentage change in electrical resistance because it monitors relative changes, not magnitudes. The normalization makes PCER one of the proper methods to compare the effects of strain in various samples of thin films, since it gives an independent measure of electrical response with regard to tensile stress. The initial resistance is 13.96 ohms for the 100 nm thick sample and 25.28 ohms for the 200 nm thick sample.

Figure 8 displays the PCER versus strain for four different samples of 100 nm chromium thin films. All four samples follow a similar overall pattern, though there are slight variations. Initially, all samples show a minor decrease in PCER for a very short strain interval (around 0 to 0.02). This initial behavior, highlighted by the inset graph for Sample 1, as shown in Figure 9, suggests a microstructural adjustment or minor changes in the film’s surface conductivity, which temporarily improves electrical conduction before cracking begins. During this phase, although strain is applied, the cracks are either not yet visible or are in their earliest stages of formation. Following this, each sample experiences a gradual increase in PCER as the strain increases. This gradual rise reflects the slow development of cracks in the material, which progressively disrupt the film’s conductivity but not yet severely enough to cause catastrophic failure. In other words, as the strain increases beyond 2%, the PCER begins to increase slowly, indicating the onset of more substantial cracking, which is visible in Figure 2 at 3.75% strain and Figure 3 at 5% strain. Here, the cracks are more numerous and start to spread across the film, albeit with relatively low intensity compared to what is seen later. These cracks gradually widen, increasing the electrical resistance, as shown by the moderate rise in PCER in Figure 8. During this phase, the cracks propagate slowly, and the damage to the conductive pathways becomes progressively more severe, causing a corresponding slow increase in electrical resistance.

As strain continues to increase, each sample reaches a critical point where the PCER begins to rise sharply, indicating a more rapid disruption of electrical continuity. This sharp increase occurs at slightly different strain values for each sample, typically between 0.16 and 0.20 strain. This stage represents the rapid propagation of cracks through the film, leading to substantial breaks in the conductive paths. The increase in PCER during this phase happens over a short strain interval (less than 0.02), and this sharp rise is immediately followed by a sudden drop in PCER before complete electrical failure at the end of the curve. At this point, the conductivity is fully compromised as the cracks propagate to the extent that the material can no longer maintain electrical pathways.

Despite these similarities, there are notable differences between the samples. The PCER values at failure vary significantly. For instance, Sample 1 peaks at around 3500% PCER, while Sample 4 reaches the highest value, exceeding 9500%. This suggests variability in the quality or structural integrity of the films, which may be due to differences in deposition conditions, film thickness uniformity, or inherent microstructural flaws. Another key difference is the strain level at which the sharp increase in PCER begins, which varies slightly across the samples. This variation indicates that the films possess different levels of resilience to strain before catastrophic cracking begins.

The overall similarity in the behavior of all four samples suggests that the same underlying mechanism of electrical failure governs the response to strain in each case. The initial PCER drop, followed by a slow rise and eventual sharp increase, indicates a common process of crack initiation, propagation, and final failure. However, the differences in the PCER peak values and the exact strain at which electrical failure occurs highlight the individual variability among the films, which could be attributed to factors such as local defects or differences in microstructure.

In Figure 10, the PCER for the four 200 nm chromium thin film samples shows greater variability between samples compared to the more consistent behavior seen in the 100 nm films from Figure 8. Initially, the samples in Figure 10 display steady PCER values, followed by a small drop at low strain levels. This contrasts with the behavior observed in the 100 nm samples, where a sharper and more abrupt increase in PCER occurs. As the strain progresses, the increase in PCER for the 200 nm samples is gradual, without the steep rise seen in the thinner films. Furthermore, three out of the four samples in Figure 10 experience a slight drop in PCER toward the end of the strain curve, which suggests a distinct response of the thicker films to mechanical stress.

The initial drop in PCER in both figures can be attributed to microstructural adjustments within the film. As strain is applied, the films undergo slight reorganization at the atomic or microstructural level, temporarily improving surface conductivity before cracks form. In the 200 nm films, this drop is more pronounced, likely because the thicker films can redistribute mechanical stress more effectively. This improved stress distribution delays the onset of significant crack formation, which results in a more gradual increase in PCER compared to the sharper rise in the 100 nm films. The thicker films exhibit greater mechanical integrity and can withstand more strain before the conductive pathways are severely disrupted by cracks.

One of the most notable differences between Figure 8 and Figure 10 is the absence of a sharp increase in PCER in the 200 nm films. In the 100 nm films, cracks develop rapidly and extensively, leading to a sudden and steep rise in electrical resistance. In contrast, the thicker 200 nm films experience more gradual crack formation and propagation. The cracks in these thicker films are fewer but deeper, and they grow at a slower rate, which moderates the increase in PCER. This behavior is indicative of a more controlled crack development process, where the thicker films are better able to resist rapid crack propagation and maintain electrical continuity for longer periods under strain.

Toward the end of the strain curve, three of the four samples in Figure 10 show a small drop in PCER. This could be explained by partial realignment or re-bridging of the cracks, where some conductive paths temporarily reconnect before the film undergoes complete electrical failure. This behavior is not seen in the 100 nm films, where the cracks form earlier and are more numerous, leading to a faster and more permanent disruption of electrical pathways. The small PCER drop at the end of the 200 nm samples could reflect the ability of thicker films to exhibit some degree of strain-induced healing or structural adjustment before reaching catastrophic failure.

In summary, the differences between Figure 8 and Figure 10 highlight the distinct mechanical and electrical behaviors of chromium thin films based on their thickness. The 100 nm films experience early crack formation and a sharp increase in PCER, reflecting rapid electrical degradation. In contrast, the 200 nm films show greater resistance to crack propagation, with a more gradual increase in PCER and even small drops in resistance as strain approaches failure. This suggests that thicker films can distribute mechanical stress more evenly, delaying electrical failure and showing a more controlled crack development process.

4. Conclusions

This study showed that film thickness affects the mechanical and electrical performance of chromium thin films blasted onto PET substrates under tensile strain. Thinner chromium films (100 nm) are more prone to early crack initiation and rapid propagation, increasing electrical resistance sharply. Thin films are brittle; thus cracks occur quickly and form a dense network that interrupts conductive pathways, causing a considerable PCER.

PCER increased more slowly in thicker chromium films (200 nm), indicating a delayed crack propagation and more regulated crack growth. Though deeper and more severe, thicker coatings had fewer cracks. These films have more sample variation and a slower PCER rise than thinner films. The modest dip in PCER near the end of strain curves in some samples suggests partial crack healing or conductive route realignment, which was not found in 100 nm films.

These findings show that film thickness affects chromium thin film mechanical and electrical responses under tensile strain. Flexible electronics and reliable electrical performance under mechanical stress were better with 200 nm films due to their mechanical integrity and electrical stability. Future study should optimize the deposition technique and explore different film thicknesses to improve chromium thin film mechanical resilience and electrical conductivity in flexible electronic applications.