SURFACE ENGINEERING
2021, VOL. 37, NO. 8, 1051–1058
https://doi.org/10.1080/02670844.2021.1918933
Study on microstructure and its correlation with sheet resistance of Zn-Al
metallized film
Ying Yanga,b, He Huanga,b, Fang Wanga,b, Jun Zhenga,b, Jinbing Wangc, Zebo Tangc, Feng Zhouc,
Mohan Chend and Qimin Wanga,b
a
Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials, Ministry of Education, Anhui University of
Technology, Ma’anshan, People’s Republic of China; bResearch Center of Modern Surface and Interface Engineering, Anhui University of
Technology, Ma’anshan, People’s Republic of China; cAnhui Saifu Electronics CO., LTD, Tongling, People’s Republic of China; dSchool of
Materials Science and Engineering, Pusan National University, Busan, Republic of Korea
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.
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
ARTICLE HISTORY
Received 12 March 2021
Revised 2 April 2021
Accepted 13 April 2021
KEYWORDS
Metallized film capacitor;
Zn-Al metallized film;
thermal evaporation;
microstructure; sheet
resistance
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 nonconducitve nature [5]. Therefore, detailed analyse of
CONTACT Qimin Wang
qmwang@gdut.edu.cn
Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials, Ministry
of Education, Anhui University of Technology, Ma’anshan 243002, People’s Republic of China; Research Center of Modern Surface and Interface EnginJzheng@ahut.edu.cn
Key Laboratory of Green
eering, Anhui University of Technology, Ma’anshan 243002, People’s Republic of China; Jun Zheng
Fabrication and Surface Technology of Advanced Metal Materials, Ministry of Education, Anhui University of Technology, Ma’anshan 243002, People’s
Republic of China; Research Center of Modern Surface and Interface Engineering, Anhui University of Technology, Ma’anshan 243002, People’s Republic of
China
© 2021 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute
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Y. YANG ET AL.
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
Figure 1. The schematic of evaporation facility.
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.
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
SURFACE ENGINEERING
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 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.
The AFM image of BOPP outside of the crater
region is illustrated in Figure 4(a). A fiber-like
Figure 2. XRD pattern of BOPP substrate.
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Figure 3. SEM surface morphology of BOPP substrate.
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 ridgelike patterns is beneficial to enhance the friction
between BOPP substrate and the rollers in the process
of vacuum evaporation.
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.
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Y. YANG ET AL.
(a)
(b)
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).
Figure 5. XRD pattern of Zn-Al film.
Figure 6. Surface macromorphology of Zn-Al film.
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.
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
SURFACE ENGINEERING
Figure 7. AFM surface topography of Zn-Al film.
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.
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
(a)
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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.
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
(b)
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).
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Y. YANG ET AL.
(a)
(b)
(b)
(c)
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.
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.
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
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
Figure 10. Sheet resistance of Zn-Al film.
SURFACE ENGINEERING
1057
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.
Figure 11. Sheet resistance of Zn-Al film under various storage time.
to the uniform film thickness and homogeneous
microstructures.
Based on the following equation of the sheet resistance:
RS =
r
d
(1)
Rs 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 Rs 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.
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
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:
(1) The macromorphology of Zn-Al film exhibits
repeating ‘crater’ pattern, which is inherited
from the surface profile of BOPP substrate.
(2) 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.
(3) 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.
(4) 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.
Acknowledgement
This work was supported by the University Synergy Innovation Program of Anhui Province [grant number GXXT2019-016]; 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.
Funding
This work was supported by the University Synergy Innovation Program of Anhui Province [grant number GXXT-
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Y. YANG ET AL.
2019-016]; and the Key Research and Development Program of Anhui Province [grant number 202004a05020038].
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