Study on microstructure and its correlation with sheet resistance of Zn-Al metallized film

Abstract

Zn-Al film was prepared on biaxially oriented polypropylene (BOPP) substrate via vacuum thermal evaporation. The microstructure and its correlation with sheet resistance of Zn-Al film were explored systematically. The results showed that the macromorphology of Zn-Al film was closely related to the surface topography of BOPP substrate. XRD and SEM analysis indicated that Zn-Al film displayed a columnar and strong textured structure of Zn (002). TEM results revealed that the film thickness was in the range of 20–25 nm, of which amorphous alumina was identified on/at the surface and interface, while Zn and a small amount of ZnO were detected in the middle. The amorphous alumina on the film surface prevented the further oxidation of Zn, and therefore helped to improve the stability of sheet resistance of the film. Nevertheless, the insulative alumina significantly decreased the film conductivity, which deteriorated the current handling ability of metallized film capacitors.

Keywords

Metallized film capacitor Zn-Al metallized film thermal evaporation microstructure sheet resistance

Introduction

Metallized film capacitors are widely applied in electronics, domestic appliances, communication industries, new energy vehicles, wind and solar power generation, etc. They consist of two polymer films where extremely thin metal layers (<100 nm) have been deposited, then the metallized polymer films are wound together around a hollow mandrel, followed by metal spraying at the end of the windings [1]. Compared with foil capacitors, the most distinctive advantage of metallized film capacitors is the excellent self-healing performance, which refers to the metal evaporation caused by a short duration high current when a localized breakdown of the dielectric occurs at the failure point, and the isolation between the two electrodes is re-established [2]. The self-healing characteristic ensures the running safety and reliability of the capacitors [3], however, it could result in the reduction of the capacity [4,5], and the capacitor lifetime is defined to be terminated when the capacitance loss exceeds a maximum threshold of 5% [6,7].

The main components of metallized film capacitor are the polymer dielectric, generally polypropylene (PP) or polyethylene terephthalate (PET), as well as the deposited conductive metal layer, which is typically composed of aluminium, or a type of alloy [2]. Among them, Zn-Al film is one of the most widely utilized metallized films. When working in AC circuit with high voltage and current, Zn film exhibits outstanding electrical properties due to its negligible loss of capacity and dissipation. To ensure a satisfactory adhesion to PP or PET substrate, a very thin Al layer is fabricated prior to the evaporation of Zn. In addition, the presence of Al can prevent the further oxidation of Zn because of the formation of Al₂O₃ on top of the Zn-Al film [8].

The microstructures of the metallized film, such as defect densities, film integrity, thickness uniformity, etc., play a crucial role in the capacitor performances. For instance, based on the self-healing mechanism [1], it is deduced that a large number of defects in the film could trigger frequent self-healing of the capacitor, which could significantly affect the lifetime of the capacitor. Besides, the discontinuity of top Al₂O₃ layer in Zn-Al film could not effectively hinder the further erosion of the moisture; On the contrary, the excessive of Al₂O₃ could seriously decrease the current handling capacity of the Zn-Al film due to its non-conducitve nature [5]. Therefore, detailed analyse of the metallized film microstructure makes significant sense to better understand the service behaviours of the capacitor. Unfortunately, few of researches have been focused on the microstructure investigation of the metallized films, especially for Zn-Al metallized film. The main reasons are that the metal film thickness is too thin and the dielectric substrate is soft and non-conductive, which make it difficult to reveal the microstructure features of the film. Among the limited literatures, Silvain et al. [9,10] reported that the evaporated Al film on PET possessed a columnar structure, and the film adhesion increased with the decrease in the Al grain size. Moreover, the metal–polymer interaction had a strong impact on the film microstructure. Na [11] found that Zn metallized film displayed a hexagonal morphology with a strong (001) preferred orientation, and relative change in sheet resistivity decreased with the reduction of the grain size. Godec et al. [12] studied the surface morphology and composition depth profile of Zn-Al metallized films with different Al contents. Preliminary results indicated that with the increase of Al contents, the corrosion resistance was improved, while the capacitor performance was decreased rather unexpectedly. Based on the researches mentioned above, it is evident that the metallized film microstructures, including composition, morphology, grain size, crystal orientation, etc., have a strong correlation with the capacitor performances, and more work should be done to further clarify the effects of film microstructure on the capacitor properties.

This work is aimed to explore the detailed microsturcture of Zn-Al metallized film fabricated by commercial evaporation processes, and the relationship between film microstructure and electrical properties is also discussed.

Materials and methods

Film deposition

A commercial isotactic BOPP film (Anhui Tongfeng Electronics CO., LTD, China) with a width of 300 mm and thickness of 6.8 μm was employed as the substrate in this experiment. The surface of BOPP film was processed by corona treatment before metal evaporation to increase the surface energy and therefore enhance the metal/BOPP interface adhesion.

An industrial roll-to-roll vacuum thermal evaporation facility (GB-M650S) equipped with a winding roll, an unwinding roll, a main roll (cooling roll) and several sets of back-up rolls was applied to deposit Zn-Al metal film. Al wires and Zn rods with the purity of 99.99% were used in this study. The two ends of BOPP film were passed through the main roll and back-up rolls and respectively fixed on the unwinding and winding rolls, as shown in Figure 1. The tensions of the winding and unwinding rolls were approximately 40 and 25 N, respectively. The Al evaporation source was placed beneath the cooling roll, while the Zn evaporation source was arranged beside the Al evaporation source on account of the specific structure of the metal layer utilized in metallized film capacitors. Prior to deposition, the chamber vacuum was evacuated to 3.5 × 10⁻⁴ mbar. At the same time, the evaporation boats of Al (made of boron nitride) and Zn (made of nickel-base superalloy) were heated up to 660°C and 540°C, respectively. The feeding speed of Al wires was about 280 mm/min. The temperature of the main roll was cooled down to −15°C so as to avoid any damage to BOPP film by the vapours of Al and Zn in the evaporation process. During deposition, BOPP film was driven by the unwinding and winding rolls at the speed of around 12 m s⁻¹. The shutters of Al and Zn evaporation sources were removed, and the evaporated Al and Zn atoms were deposited on the surface of BOPP substrate in sequence. The total length of BOPP film was about 25000 m, so the total deposition time was about 35 min.

Figure 1

The schematic of evaporation facility.

Film characterization

The phase structures of BOPP and Zn-Al film were identified using X-ray diffractometer (XRD, Rigaku UItima IV). The 2θ scanning range was 10–90°. The optical microscope (OM, Zeiss AxioLab.A1) was utilized to obtain the macromorphology of Zn-Al film. The surface and cross-section morphologies of BOPP and Zn-Al film were analysed by field emission scanning electron microscope (FESEM, Zeiss sigma 500 and Hitachi Regulus 8200). The cross-section SEM specimen was carefully prepared by ion beam milling system (Leica EM TIC 3X) to minimize the heat damage to the sample. The accelerating voltage was fixed at 4000 V, and the Ar⁺ ion energy was in the range of 1.8 ∼ 2 eV. Atomic force microscopy system (AFM, Bruker Icon and Nanosurf Easyscan 2), based on a force modulation technique, was applied to test the surface profile of BOPP and Zn-Al film, and the surface roughness was calculated as well. High-resolution transmission electron microscope (HRTEM, FEI Talos F200X) was employed to investigate the cross-section microstructure of Zn-Al film. The XTEM sample was prepared by conventional mechanical grinding and ion milling (Gatan 695).

Results and discussion

BOPP microstructure

The phase structure of BOPP substrate is shown in Figure 2. It can be seen that the BOPP substrate is composed of α phase (ICDD No. 50-2397) and γ phase (ICDD No. 47-1952). It is known that α crystal with monoclinic system is the most dominant crystal in PP [13], and γ crystallinity with orthorhombic structure can easily coexist with α phase due to the typical feature of α-γ lamellar branching in isotactic polypropylene [14].

Figure 2

XRD pattern of BOPP substrate.

Figure 3 displays the SEM surface morphology of BOPP substrate. A repeating ‘crater’ pattern with random distribution was clearly observed, and the average size of ‘crater’ was in the range of 100–200 μm, similar phenomenon was identified in Ref. [15]. ‘Crater’ pattern is a typical feature of BOPP. Tamura et al. [13] described the formation mechanism of crater in detail, which was highly related to the spherulite deformation as well as the collapse of BOPP during biaxial stretching process.

Figure 3

SEM surface morphology of BOPP substrate.

The AFM image of BOPP outside of the crater region is illustrated in Figure 4(a). A fiber-like network structure was detected, and the surface roughness Sa was 3.4 nm. Similar topography was reported in Ref. [16]. Figure 4(b) reveals the surface profile of BOPP at the crater region. A ridge-like pattern with the wide of 4–5 μm and the height of 400–600 nm was clearly found, and the calculated surface roughness Sa was 144.2 nm. The presence of ridge-like patterns is beneficial to enhance the friction between BOPP substrate and the rollers in the process of vacuum evaporation.

Figure 4

AFM surface topography of BOPP (a) Outside of the crater region (Performed by Bruker Icon) (b) At the crater region (Performed by Nanosurf Easyscan 2).

Zn-Al film microstructure

Figure 5 shows the XRD result of Zn-Al film. Apart from the BOPP substrate diffraction signals (c.f. Figure 2), only one peak located at 36.2° was detected, corresponding to either Zn (002) or ZnO (101) or both. Figure 6 describes the surface macromorphology of Zn-Al film conducted by OM. Apparent ‘crater’ pattern was discovered, and no significant difference was identified by comparing Figures 3 and 6. It is demonstrated that the macromorphology of Zn-Al film is highly influenced by the profile of BOPP substrate. The presence of ‘crater’ pattern results in a non-uniform surface of BOPP, which could probably affect the nucleation and growth process of the deposited film, therefore, the film microstructure and properties would be impacted. Unfortunately, little information has been published and this issue is still unclear so far.

Figure 5

XRD pattern of Zn-Al film.

Figure 6

Surface macromorphology of Zn-Al film.

The AFM surface topography of Zn-Al film is presented in Figure 7. Different from the fiber-like appearance in Figure 4(a), fine granular morphology was observed in Figure 7, and a polygon pattern with brighter contrast was identified as well. Figure 8 displays the cross-section morphology of Zn-Al film. The thickness of Zn-Al film ranged from 30 to 50 nm obtained from Figure 8(a), which is also a typical value for metal films utilized in capacitors [1]. Moreover, the Zn-Al film was firmly bonded with BOPP substrate, no cracks or voids were detected at the interface. The Zn-Al film exhibited a columnar structure, and the grain size was on the order of tens of nanometers as revealed in Figure 8(b). Silvain et al. [9] also reported columnar crystals of Al film fabricated on PET substrate.

Figure 7

AFM surface topography of Zn-Al film.

Figure 8

Cross-section FESEM morphology of Zn-Al film (a) Image conducted by thermal FESEM (Zeiss sigma 500) (b) Image conducted by cold FESEM (Hitachi Regulus 8200).

To further investigate the microstructure of Zn-Al film, cross-section HRTEM was performed and the results are illustrated in Figure 9. Figure 9(a) addresses the bright field image of the film and its corresponding Fast Fourier Transform (FFT) pattern. FFT photograph confirms the presence of single crystal Zn with (001) plane paralleling to surface. Note that the film thickness evaluated from Figure 9(a) was approximately 20–25 nm, which was thinner than that in Figure 8. The thickness differences could be due to the lower resolution of FESEM compared with HRTEM. Figure 9(b) depicts the high-angle annular dark filed (HAADF) image of Zn-Al film, and the corresponding element mapping and line scanning results are illustrated in Figure 9(c,d), respectively. It is known that the addition of Al in the Zn metallized film employed in capacitors improves the adhesion and oxidation resistance of the film, however, overdosing of Al could deteriorate the film electrical properties. Hence, the optimal Al content in the Zn-Al metallized film was reported to be 5–8 wt-% [8]. Based on the compositions in the framed zone in Figure 9(b), it is revealed that the Al content in the Zn-Al film reaches 6.84 wt-%, which is within the ideal range. It can be seen from Figure 9(c) that aluminium oxide was formed both on/at the surface and interface of the film. In addition, Zn and a small amount of ZnO were confirmed in the middle of the film. The line scanning results presented in Figure 9(d) are in good agreement with the conclusions in Figure 9(c). On the basis of the specific evaporation process in this work, Al atoms were first deposited on BOPP substrate, then followed by the deposition of Zn atoms. It can be seen from Figure 9(c,d) that remarkable diffusion of Al element from interface to surface was identified. When Al atoms arrived at the surface of the film, they were preferentially oxidized instead of Zn due to the higher affinity of Al with oxygen according to Ellingham-Richardson diagram [17,18]. The formation of aluminium oxide at the interface attributes to the oxygen adsorption on BOPP surface. After browsing the whole cross-section HRTEM images of Zn-Al film carefully (not shown here), the aluminium oxide on/at the surface and interface with the thickness of several nanometers was determined to be amorphous, which is a common form for the oxidation of Al under ambient conditions. It is revealed in Figure 9(c,d) that few of oxygen was detected in the middle of the film, which clearly states that the amorphous alumina on the film surface can effectively protect the further oxidation of Zn. However, alumina exhibits extremely high electrical resistivity (3 × 10¹⁵ Ω m [19]) compared with Al (2.67 × 10⁻⁸ Ω m, 20°C [20]) and Zn (5.96 × 10⁻⁸ Ω m, 20°C [20]), therefore, the film conductivity would be sacrificed unfortunately.

Figure 9

Cross-section HRTEM results of Zn-Al film (a) Bright field image of the film and the corresponding FFT pattern (b) HAADF image and element contents in the framed zone (c) Element mapping results (d) Element line scanning results.

Combined with the results of Figures 5 and 9, it is concluded that the diffraction peak located at 36.2° in Figure 5 was Zn (002) rather than ZnO (101). In spite of the limited film thickness (20–25 nm), significant diffraction signals of Zn (002) and no diffraction peaks from other crystal planes of Zn were observed. Therefore, it is deduced that Zn-Al film exhibits a strong texture feature; otherwise, no obvious diffraction signals could be detected due to extremely thin film thickness; or, several small diffraction peaks from other crystal planes of Zn could be identified.

Concerning the polygon morphology in Figure 7, by combining the XTEM conclusions and related literatures [11,12,21], it is speculated to be Zn or ZnO, either of which possesses hexagonal space lattice, probably be ZnO because Zn could be easily oxidized when exposure in atmosphere.

Sheet resistance of Zn-Al film

Numerous batches of Zn-Al films were fabricated using the identical evaporation parameters, and the sheet resistance is shown in Figure 10. It can be seen that the sheet resistance of the films was in the range of 6–9 Ω/□, and the average value was 7.47 with the standard deviation of 0.48. It is confirmed that the sheet resistance fluctuation is relatively small, indicating that the evaporation process is stable, which leads to the uniform film thickness and homogeneous microstructures.

Figure 10

Sheet resistance of Zn-Al film.

Based on the following equation of the sheet resistance: (1) R_s represents the sheet resistance, ρ is the electrical resistivity, and d means the film thickness. The resistivity of Zn was reported to be 5.96 × 10⁻⁸ Ω m [20], and d is assumed to be 22 nm derived from Figure 9. Therefore, the theoretical R_s is calculated to be 2.71 Ω/□, much smaller than the measured value in Figure 10. As a consequence, the presence of amorphous alumina seriously decreases the film conductive property.

The change of sheet resistance with the storage time under ambient condition is shown in Figure 11. It is demonstrated that the sheet resistance kept stable after storage up to 11 days. The stability of sheet resistance benefits from the amorphous alumina on the film surface, which prohibits the further erosion of oxygen to the film.

Figure 11

Sheet resistance of Zn-Al film under various storage time.

Based on the above results, it is revealed that the electrical properties of Zn-Al metallized film are closely related to the film microstructure. The Zn-Al film exhibits a columnar and strong textured structure. The perpendicular grain boundaries act as penetrative diffusion channels, which favours Al atoms transferring from interface to surface, and therefore contributes to the formation of alumina on the film surface. The existence of superficial amorphous alumina has two opposite effects on the film properties: on one hand, the amorphous oxide impedes the further oxidation of Zn effectively, and the sheet resistance of the film maintains almost unchanged over 10 days, which is beneficial for the storage and transport of the film; on the other hand, the amorphous alumina decreases the film conductivity obviously due to its insulative nature. Hence, a reasonable balance between film conductivity and sheet resistance stability should be considered seriously. Moreover, special attention should be paid on the accurate control of the thickness and integrity of the superficial oxides. The presence of interfacial alumina is detrimental to the film conductivity, therefore, appropriate pretreatment such as R.F. plasma modification is suggested to be conducted to remove or modify the adsorbed oxygen on BOPP substrate, related research work is under going and will be published later. In addition, to minimize the oxygen concentration at the interface of the film, the chamber pressure is recommended to be as low as possible.

Conclusion

The detailed microstructure of Zn-Al film fabricated on BOPP substrate via vacuum evaporation has been investigated, and its correlation with film performances has been explored as well. The main conclusions are as follows:

The macromorphology of Zn-Al film exhibits repeating ‘crater’ pattern, which is inherited from the surface profile of BOPP substrate.

The HRTEM results reveal a sandwich construction of the Zn-Al film: amorphous alumina with several nanometers located on/at the film surface and interface, Zn and a few of ZnO located in the middle of the film.

The electrical properties are closely relevant to the film microstructure. The columnar grain boundaries provide sufficient diffusion channels for Al atoms, and favour the formation of alumina on the film surface. The alumina layer blocks the penetration of the oxygen effectively, meanwhile, the film conductivity has been compromised.

The presence of alumina at the interface is considered to be harmful to the film performances, and feasible recommendations are provided to restrain the formation of interfacial alumina.

Footnotes

Acknowledgement

This work was supported by the University Synergy Innovation Program of Anhui Province [grant number GXXT-2019-016]; and the Key Research and Development Program of Anhui Province [grant number 202004a05020038].

Disclosure statement

No potential conflict of interest was reported by the authors.

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