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MYU Tokyo
S & M 2275

Mechanical Fatigue Analysis of AlN/SUS430/AlN
Heterolayered Foils in Micromachined Devices
Qiu Zheng,* Le Van Minh, Hung Hoang Nguyen, and Hiroki Kuwano**
New Industry Creation Hatchery Center, Tohoku University
6-6-10 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan(Received March 9, 2020; accepted April 15, 2020)

Keywords: flexible device, AlN/SUS430/AlN heterolayered foils, micromachined specimen, fatigue
defects

The combination of piezoelectric aluminum nitride (AlN) films with flexible substrates
made of stainless steel (SUS) foils has been applied in low-frequency and robust microdevices,
especially in piezoelectric vibration energy harvesters (PVEHs). To find guides in the designof PVEHs, the fatigue properties of micromachined specimens fabricated using SUS430 foilsand AlN/SUS430/AlN heterolayered foils were investigated in this work. In particular, thethickness dependence of AlN films on the fatigue behavior of AlN/SUS430/AlN heterolayered
foils was investigated.   AlN films with different thicknesses of 1 and 2 μm were deposited
on 50-μm-thick SUS430 substrates.   The specimens for fatigue tests were prepared as
cantilevers by microfabrication processes. First, Young’s moduli were measured. Fatigue
tests were subsequently conducted by resonant bending. The fatigue behavior is discussed
in this paper on the basis of both the stress–number of cycle (S–N) curves and the fatigue
failures on the surfaces of the cantilevers.  The fatigue limits of SUS430 (50 μm), AlN (1 μm)/
SUS430 (50 μm)/AlN (1 μm), and AlN (2 μm)/SUS430 (50 μm)/AlN (2 μm) were estimated to
be 264, 291, and 368 MPa, respectively. The larger stiffness of the foils with a heterolayered
AlN/SUS430/AlN structure indicated their larger fatigue limit than the SUS430 foil. Fatigue
failure in the form of cracks was observed on the surface of SUS430 for AlN (1 μm)/SUS430/
AlN (1 μm) when vibrating at the maximum stress of 488 MPa for 2.9 × 107 cycles. Surface
defects of intrusions and slip bands rather than cracks were observed on the SUS430 substrate
for the heterolayered AlN (2 μm)/SUS430/AlN (2 μm) structure when vibrated at the maximum
stress of 605 MPa for 2.9 × 107 cycles, resulting in its higher fatigue limit than that of AlN(1 μm)/SUS430/AlN (1 μm).  No surface defects on AlN films were observed after undergoing
the aforementioned long-cycle vibration.

1.

Introduction

Micro-piezoelectric vibration energy harvesters (PVEHs) have prospective use as the powersource in microelectromechanical systems.(1,2) They can generate electricity from scavenging*

Corresponding author: e-mail: zheng@nanosys.mech.tohoku.ac.jp**Corresponding author: e-mail: hiroki.kuwano@nanosys.mech.tohoku.ac.jp
https://doi.org/10.18494/SAM.2020.2855

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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on-site ambient vibrations. To achieve robust and highly efficient devices under harsh ambient
vibrations at low frequencies, aluminum nitride (AlN)/stainless steel (SUS) heterolayeredmaterials were proposed to be used for PVEHs. AlN films were successfully deposited on 304
stainless steel (SUS304) foils with a thickness of 50 μm by electron cyclotron resonance (ECR)
sputtering and reactive radio frequency (RF) magnetron sputtering.(3,4)
The lifetime of PVEHs strongly depends on their resistance to dynamic vibrations since
they are exposed to cyclically dynamic stress during long-cycle vibration.   Defects in both
AlN and SUS materials may occur due to the vibration, resulting in a decrease in the output
power of AlN/SUS-based PVEHs and finally causing their failure. Hence, investigation of
the fatigue behavior of the materials is necessary for the design of PVEHs. Previous studies
showed the effects of the manufacturing process and material form (bulk or film) on material
properties of AlN.(5,6) There has been no investigation on the fatigue of sputtered AlN films
on flexible SUS430 foils although the fatigue of bulk AlN materials has been well studied.(7,8)
In addition, the downscaling of micro-PVEHs results in differences in mechanical properties
between the macro- and microscales for both metallic and nonmetallic materials.(9–13) Also,
the fatigue behavior of materials with heterolayered structures is affected by the material-layercomposition.(14,15)  A previous study on the fatigue of 1-μm-thick AlN films deposited on bothsides of a 50-μm-thick SUS304 foil showed that the fatigue limit of the materials in the AlN/
SUS304/AlN structure was higher than that of SUS304 foils.(15) Recently, metal foils with a low
coefficient of thermal expansion (CTE) close to that of piezoelectric films have become more
attractive for use as structural materials for micro-PVEHs. SUS430 foils with a lower CTE
than SUS304 and whose manufacturing can be scaled up as easily as SUS304 are the focus of
our interest.   Although the fatigue behaviors of SUS430 foils and 1 μm AlN films depositedon SUS430 were studied,(16) the fatigue property of the deposited AlN films is unknown when
their thickness is increased.
In this work, we investigated the fatigue of SUS430 foils and foils with an AlN/SUS430/
AlN heterolayered structure for micro-PVEHs. In particular, the effect of AlN thickness on
the fatigue behavior of the heterolayered materials was studied. AlN films with different
thicknesses of 1 and 2 μm were directly sputtered on 50-μm-thick SUS430 substrates.  
Cantilevers were microfabricated for the investigation. The Young’s moduli of SUS430 andthe sputtered AlN films were tested first. Then fatigue tests of SUS430 foils and AlN/SUS430/
AlN heterolayered materials were subsequently conducted by using a resonant fatigue testing
system.(15,17,18) The stress–number of cycle (S–N) curves, fatigue limit, and fatigue failure are
discussed in this paper.

2.

Experiment

2.1

Fabrication of specimens

Cantilever specimens for fatigue tests were manufactured by microfabrication as shown in
Fig. 1.(16)  The thickness of SUS430 foils was 50 μm.  AlN films were deposited on both sides
of a clean SUS430 substrate by alternative current magnetron reactive sputtering.(19) Two typesof heterolayered structure, AlN (1 μm)/SUS430 (50 μm)/AlN (1 μm) and AlN (2 μm)/SUS430

Sensors and Materials, Vol. 32, No. 7 (2020)

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(a)

(b)

(d)

(e)

(c)

Fig. 1. (Color online) Microfabrication of specimens: (a) cleaning SUS430 and depositing AlN films, (b) etching
AlN films, (c) etching SUS430 foil, (d) depositing Cr/Au, and (e) adding proof mass.(16)(50 μm)/AlN (2 μm), were prepared.  The thickness of the deposited AlN films was measured
using a film thickness measuring device (Lambda Å, Dainippon Screen MFG Co., Ltd., STM620J).   The average thicknesses were 1.05 and 2.02 μm.   Then, the films were wet-etchedwith tetramethylammonium hydroxide (TMAH 2.38%) in the temperature range of 35–38 °C[Fig. 1(b)].(15) After the SUS430 substrate was micromachined to from the cantilever shape byelectrochemical etching [Fig. 1(c)],(15,20) chromium (Cr) and gold (Au) films were deposited ona cantilever specimen for positioning when measuring the amplitude of the specimen during
fatigue tests [Fig. 1(d)]. Finally, the copper proof mass was added at the tip of the specimen[Fig. 1(e)]. The cantilever was designed to be 5.3 mm in length and 5.9 mm in width. The sizeof the proof mass was 2 × 6 mm2, and two different thicknesses, 1 and 0.5 mm, were used totune the resonant frequencies of the cantilevers. Figure 2 shows images of the microfabricatedspecimens. The cantilevers were successfully micromachined without any defect at the edge.
2.2

Experimental procedure

The resonant fatigue testing system developed in a previous study(15) was used. This system
can detect the amplitude of the vibrating specimen by using a laser displacement sensor (Keyence,
LK-H050).  The resonant spectra in the 1st vibration mode of the SUS430 (50 μm), AlN (1 μm)/
SUS430 (50 μm)/AlN (1 μm), and AlN (2 μm)/SUS430 (50 μm)/AlN (2 μm) cantilevers without
the proof mass were first tested by measuring the specimen amplitude varying with frequency.
Three cantilevers were tested for each material to ensure the measurement repeatability. The
Young’s moduli of SUS430 foils were determined from its relationship with the resonant
frequency as shown in Eq. (1) and then used for the calculation of the Young’s moduli of the
sputtered AlN films using Eq. (2).(15,16,21,22) To improve the accuracy of the calculation, theactual length of each cantilever was measured under a microscope (Hirox KH-7700).  Finally,
fatigue tests were conducted by measuring the deflection of the micromachined cantilevers2466

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(a)

(b)

(c)

(d)

Fig. 2. (Color online) Images of microfabricated specimens: (a) overall image of cantilever, (b) enlarged imagearound the corner, and (c) and (d) enlarged images of etched AlN films with thicknesses of 1 and 2 μm. (a), (b), and (c)are cited from Ref. 16.

under cyclic vibrations.(17) The fatigue of a material is discussed here on the basis of S–N
curves. The fatigue life was determined on the basis of a criterion proposed in a previous study.(15)The stress distribution in the cantilevers was calculated by finite element analysis (FEA) using
ANSYS Workbench 18.0.  The obtained Young’s moduli were input in the FEA.  The maximum
stress was used to plot S–N curves. The details of FEA calculation and measurement wereexplained in the previous study.(15)2

4π 2 f n2  L2 ⋅
ESUS = 12 ρ SUS ⋅ ,(kn L) 4  hSUS 
12 ρeq ⋅E AlN =

4π 2 f n2
4(1)

3⋅ VL3 − ESUS bhSUS( kn L),
3
2bhAlN + 6bhAlN (hSUS + 2hAlN ) 2(2)

where ESUS and EAlN are the Young’s moduli of the SUS430 foil and sputtered AlN film, respectively;
fn is the resonant frequency of the cantilever; L is the length of the cantilever; k n∙L is thecoefficient for a clamped-free boundary condition, under which k1∙L is equal to 1.875 in the 1st
vibration mode; hSUS and hAlN are the thicknesses of SUS layer and AlN layer, respectively; ρeq
is the equivalent density of the heterolayered AlN/SUS430/AlN structure;(23) b is the width ofthe cantilever; and V is the total volume of the cantilever.

Sensors and Materials, Vol. 32, No. 7 (2020)

3.

Results and Discussion

3.1

Young’s moduli of SUS430 foil and deposited AlN film

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Figure 3 shows the Young’s moduli of the SUS430 foils and sputtered AlN films calculatedusing Eqs. (1) and (2). As shown in Table 1, the average Young’s moduli are 207.7 and305.6 GPa for the SUS430 foils and AlN films, respectively. Compared with the measured
results for the SUS304 foils and AlN films deposited on SUS304 from the previous study,(15)the average Young’s modulus of SUS430 shows larger values while that of AlN films deposited

Fig. 3. (Color online) Young’s moduli of SUS430 foils and AlN films deposited on SUS430, and comparison withthose of SUS304 foils and AlN films deposited on SUS304 (from Ref. 15), in which Young’s moduli of SUS430 and
1 μm AlN deposited on SUS430 were taken from Ref. 16.

Table 1Young’s moduli of SUS, sputtered AlN films, and heterolayered AlN/SUS/AlN structure.
Material
Density (g/cm3) Young’s modulus (GPa) Bending stiffness (× 10 −5 N m 2)
(a)7.85 ± 0.08
184.9 ± 1.2
1.14
SUS3043.26(24)
342.9 ± 24.2—
AlN (1 μm) deposited on SUS304(a)7.68 ± 0.08(b)204.7 ± 2.7(b)1.41(b)
AlN (1 μm)/SUS304/AlN (1 μm)(a)SUS430(c)7.63 ± 0.08
207.7 ± 1.9
1.15
3.26(24)
326.4 ± 14.6—
AlN (1 μm) deposited on SUS430(c)284.8 ± 6.1—
AlN (2 μm) deposited on SUS430
3.26(24)
3.26(24)305.6 ± 24.9—
AlN deposited on SUS430(d)(c)
7.47 ± 0.08(b)223.7 ± 1.7(b)1.39(b)
AlN (1 μm)/SUS430/AlN (1 μm)
AlN (2 μm)/SUS430/AlN (2 μm)
7.32 ± 0.08(b)229.5 ± 1.4(b)1.60(b)(a)Corresponding values for SUS304 foils and AlN films deposited on SUS304, which are cited from Ref. 15.
Equivalent value for heterolayered AlN/SUS/AlN structure.
(c)
Cited from Ref. 16.
(d)Average value of AlN (1 and 2 μm) deposited on SUS430.  The density of SUS foils was measured using a balance scale(A&D, GR-120).
(b)

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on SUS430 shows lower values. However, considering the error range, the Young’s moduli
of the AlN films deposited on both SUS430 and SUS304 show similar variations. Comparedwith that of the AlN film deposited on the silicon substrate by pulsed direct current reactive
sputtering,(25) the Young’s moduli of the deposited AlN films in this study are lower.
To evaluate the resistance of the material against bending deformation, the bending stiffness(EI, where E is Young’s modulus and I is the second moment of the area) of each material is also
shown in Table 1, in which the equivalent bending stiffness of the heterolayered AlN/SUS/AlN
structure was calculated.(22) As the thickness of the sputtered AlN films increases, the bending
stiffness of the cantilever also increases. This is attributed to the stiffness of AlN being larger
than that of SUS.
3.2

Fatigue properties

S–N curves are first plotted to characterize the overall fatigue properties of the materials.
Since no fracture occurred in all the fatigue tests, the approach described in detail in theprevious study was used to determine the fatigue life and plot the S–N curves.(15) Then, thefatigue failure of the materials after long-cycle vibration is discussed.
3.2.1 S–N curves
Figure 4(a) shows the S–N curves of each material. Similar to the tendency of S–N curves of
SUS304 and AlN (1 μm)/SUS304/AlN (1 μm),(15) the S–N curve of SUS430 shows two stages,
while that of AlN/SUS430/AlN shows three stages. The fatigue life of the heterolayered foils
increases gradually with decreasing maximum stress when the applied maximum stress isrelatively high (350 MPa in the case of 1 μm AlN film and 450 MPa in the case of 2 μm AlN

(a)

(b)

Fig. 4. (Color online) S–N curves of (a) AlN/SUS430/AlN heterolayered foils with different thicknesses of AlN
films (0, 1, and 2 μm) and (b) comparison between AlN (1 μm)/SUS430/AlN (1 μm) and AlN (1 μm)/SUS304/
AlN (1 μm),(15) in which S–N curves of SUS430 and AlN (1 μm)/SUS430/AlN (1 μm) are from Ref. 16.

Sensors and Materials, Vol. 32, No. 7 (2020)

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film on SUS430). Then, the fatigue life increases rapidly with decreasing stress. Finally,
the fatigue life reaches more than 108 cycles for all the three foils at a relatively low stress.
At a stress higher than 450 MPa, the two heterolayered foils indicate similar fatigue lives.
This is considered to be due to the large deformation caused by the large deflection of bothmaterials. However, at a stress lower than 450 MPa, the fatigue life of the heterolayered foils
increases with increasing thickness of AlN films owing to the increased foil stiffness (Table 1).
The Basquin relationship was used to calculate the fatigue limit of SUS430,(26) while theleast-squares method was used to estimate the fatigue limit of the heterolayered materials. The
fatigue limits of SUS430, AlN (1 μm)/SUS430/AlN (1 μm), and AlN (2 μm)/SUS430/AlN (2 μm)
were estimated to be 264, 291, and 368 MPa, respectively, indicating an increasing tendencywith increasing thickness of the deposited AlN films. This is considered to be attributed to theincreased stiffness of the heterolayered foils with thicker AlN films. Similar results that thicker
AlN films exhibit a higher fracture strength were obtained in the study in Ref. 25.
Figure 4(b) shows the S–N curves of AlN (1 μm)/SUS430/AlN (1 μm) and AlN (1 μm)/
SUS304/AlN (1 μm).  The fatigue life of AlN (1 μm)/SUS430/AlN (1 μm) indicates its longer
cycle than that of AlN (1 μm)/SUS304/AlN (1 μm) when the maximum stress is higher than
400 MPa. When the applied stress decreases, the fatigue lives of these two materials become
close to each other. This is considered to be due to the reduced amplitude of the cantilever at alower stress. The large amplitude of the specimen at a higher stress causes a large deformation,
while the small amplitude results in a small elastic deformation. Thus, the fatigue lives of these
two heterolayered materials indicate a similar number of cycles at a stress lower than 400 MPa.
Owing to the smaller bending stiffness of AlN (1 μm)/SUS430/AlN (1 μm) (Table 1), its fatigue
limit is lower than that of AlN (1 μm)/SUS304/AlN (1 μm).
3.2.2 Fatigue failure
To evaluate the defect of the cantilevers after long-cycle vibration, surface changes wereobserved in the maximum-stress area [Fig. 5(a)] of the cantilevers.(15) Figures 5(b) and 5(c)show images of SUS430 foils after fatigue tests at two different stress levels. The surface shows
severer defects of intrusions when vibrating at 440 MPa than when vibrating at 275 MPa. Thisis caused by the long-time plastic deformation when vibrating at a higher stress level.
Regarding the heterolayered materials, no surface defects were observed on AlN films
after the fatigue tests at all stress levels up to 939 MPa. This is attributed to the high fracture
strength of the sputtered AlN thin films.(25) In addition, no fracture occurred in these films
after suffering a maximum cyclic stress of 1.28 GPa for 104 cycles.(25) However, surface
defects were observed on the SUS430 layer. Figures 5(d)–5(g) show the defects observed on
the heterolayered foils vibrating at different stress levels. Owing to the long-cycle vibration,
the fatigue failure of cracks was observed on the surface of SUS430 in AlN (1 μm)/SUS430/
AlN (1 μm) when vibrating at the maximum stress of 488 MPa for 2.9 × 107 cycles [Fig. 5(d)].
The faster initiation and propagation of intrusions inside the ductile material SUS430 are
considered to be the reason for the occurrence of cracks. However, no cracks were observed
for the long-cycle vibration at a lower stress of 292 MPa as shown in Fig. 5(e). In addition, the2470

Sensors and Materials, Vol. 32, No. 7 (2020)

(a)

(b)

(c)

(d)

(e)

(d)

(e)

Fig. 5. (Color online) Fatigue failures on SUS430 surface: (a) image of observation zone and mechanical loading
direction, (b) and (c) SUS430 after vibrating at the maximum stresses of 440 MPa for 3.0 × 107 cycles and 275 MPafor 1.5 × 108 cycles, (d) and (e) AlN (1 μm)/SUS430/AlN (1 μm) after vibrating at the maximum stresses of 488
MPa for 2.9 × 107 cycles and 292 MPa for 1.2 × 108 cycles, and (f) and (g) AlN (2 μm)/SUS430/AlN (2 μm) after
vibrating at the maximum stresses of 605 MPa for 2.9 × 107 cycles and 409 MPa for 8.3 × 107 cycles, in which (c), (d),and (e) are cited from Ref. 16.

Sensors and Materials, Vol. 32, No. 7 (2020)

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SUS430 surface in AlN (2 μm)/SUS430/AlN (2 μm) also showed no cracks after the long-cycle
vibration as shown in Figs. 5(f) and 5(g). In particular, for the same number of vibration cyclesof 2.9 × 107 for AlN (1 μm)/SUS430/AlN (1 μm) [Fig. 5(d)], the propagation of defects on the
SUS430 surface of AlN (2 μm)/SUS430/AlN (2 μm) is slower even when the stress is higher.  
This is due to the larger stiffness (Table 1) of the foils with thicker AlN films. Although nocracks occurred, the surface defects in forms of intrusions and slip bands were observed owing
to the strain localization that developed in this area.(15)4.

Conclusions

In this study, the fatigue behavior of micromachined specimens fabricated using SUS430 and
AlN/SUS430/AlN heterolayered foils was studied as guide in the design of PVEHs. The effectof AlN film thickness on the fatigue behavior of AlN/SUS430/AlN heterolayered foils was also
investigated.
AlN films with different thicknesses of 1 and 2 μm were successfully sputtered on the
SUS430 substrate with a thickness of 50 μm.   Cantilever specimens were microfabricatedfor use in fatigue tests. The Young’s moduli of the SUS430 foil and sputtered AlN film were
determined to be 207.7 and 305.6 MPa, respectively. Fatigue was discussed here on the basisof S–N curves and fatigue failures on the surfaces of the cantilevers. The fatigue limits of
SUS430 (50 μm), AlN (1 μm)/SUS430 (50 μm)/AlN (1 μm), and AlN (2 μm)/SUS430 (50 μm)/
AlN (2 μm) were estimated to be 264, 291, and 368 MPa, respectively.  The fatigue limit shows
an increasing tendency with increasing thickness of AlN films. Fatigue defects in forms of slip
bands, intrusions and extrusions, and cracks were observed on the surface of SUS430, while
no surface defects were found on AlN films. The fatigue failure of the heterolayered foils was
confirmed to be caused by the failure of SUS430 substrates rather than AlN films.

Acknowledgments
This work was supported by the A-STEP project of JST (AS2815014R) and JKA Social
Action Program.

References
1 S. Saadon and O. Sidek: Energ. Convers. Manage. 52 (2011) 500. https://doi.org/10.1016/j.enconman.2010.07.024
2 M. T. Todaro, F. Guido, V. Mastronardi, D. Desmaele, G. Epifani, L. Algieri, and M. D. Vittorio:
Microelectron. Eng. 183 (2017) 23. https://doi.org/10.1016/j.mee.2017.10.005
3 Z. Cao, J. Zhang, and H. Kuwano: Sens. Actuators, A 179 (2012) 178. https://doi.org/10.1016/j.sna.2012.02.026
4 N. Moriwaki, L. V. Minh, R. Ohigashi, O. Shimada, H. Kitayoshi, and H. Kuwano: PowerMEMS 2016, J.
Phys.: Conf. Series (2016) 012049. https://doi.org/10.1088/1742-6596/773/1/012049
5 Q. Li, Z. Wang, C. Wu, and X. Cheng: J. Alloy Compd. 640 (2015) 275. htt ps://doi.org/10.1016/
j.jallcom.2015.03.235
6 A. Ababneha, U. Schmidb, J. Hernandoc, J. L. Sánchez-Rojasc, and H. Seidel: Mater. Sci. Eng., B 172 (2010)
253. https://doi.org/10.1016/j.mseb.2010.05.026
7 T. Kawakubo and A. Goto: J. Soc. Mater. Sci. Jpn. 36 (1987) 1253. https://doi.org/10.2472/jsms.36.1253
8 G. Subhash and G. Ravichandran: J. Mater. Sci. 33 (1998) 1933. https://doi.org/10.1023/A:1004325926287
9 U. Engel and R. Eckstein: J. Mater. Process. Tech. 125–126 (2002) 35. https://doi.org/10.1016/S09240136(02)00415-6

2472

Sensors and Materials, Vol. 32, No. 7 (2020)

10 F. Vollertsen, D. Biermann, H. N. Hansen, I. S. Jawahir, and K. Kuzman: CIRP Annals-Manuf. Technol. 58 (2009),
566. https://doi.org/10.1016/j.cirp.2009.09.002
11 G.P. Zhang, K. Takashima, M. Shimojo, and Y. Higo: Mater. Lett. 57 (2003) 1555. https://doi.org/10.1016/
S0167-577X(02)01023-6
12 G.P. Zhang, K. Takashima, and Y. Higo: Mater. Sci. Eng., A 426 (2006) 95. https://doi.org/10.1016/
j.msea.2006.03.090
13 X. Zhang, C. Zhao, T. Yao, S. Zhou, J. Han, J. Li, T. Gao, X. Wang, K. Zheng, and B. Song: Nanotechnology
28 (2017) 275703. https://doi.org/10.1088/1361-6528/aa73e7
14 X. Yan, M. Kato, and K. Nakasa: J. Soc. Mater. Sci. Jpn. 50 (2001) 1335. https://doi.org/10.2472/jsms.50.1335
15 Q. Zheng, L. Van Minh, and H. Kuwano: Sens. Actuators, A 285 (2019) 173. https://doi.org/10.1016/
j.sna.2018.11.022
16 Q. Zheng, L. Van Minh, N. H. Hung, and H. Kuwano: PowerMEMS 2019 (accepted).
17 K. Kwak, M. Otsu, and K. Takashima: Sens. Mater. 22 (2010) 51. https://doi.org/10.18494/SAM.2010.627
18 Y. Shimizu, L. V. Minh, H. Kitayoshi, and H. Kuwano: PowerMEMS 2016, J. Phys.: Conf. Series 773 (2016)
012116. https://doi.org/10.1088/1742-6596/773/1/012116
19 L. V. Minh and H. Kuwano: IEEE 30th Int. Conf. Micro Electro Mechanical Systems (2017) 833. https://doi.
org/10.1109/MEMSYS.2017.7863537
20 T. Takahashi, L. Van Minh, K. Umeda, T. Fuji, and H. Kuwano: PowerMEMS 2017. https://doi.
org/10.1088/1742-6596/1052/1/012039
21 Y. Y. Kim: Measurement 115 (2018) 133. https://doi.org/10.1016/j.measurement.2017.10.029
22 J. Gryzagoridis, G. Oliver, and D. Findeis: Insight 57 (2015) 140. https://doi.org/10.1784/insi.2014.57.3.140
23 A. Khan, J. Philip, and P. Hess: J. Appl. Phys. 95 (2004) 1667. https://doi.org/10.1063/1.1638886
24 R. Andosca, T. G. McDonald, V. Genova, S. Rosenberg, J. Keating, C. Benedixen, and J. Wu: Sens. Actuators,
A 178 (2012) 76. https://doi.org/10.1016/j.sna.2012.02.028
25 E. Österlund, J. Kinnunen, V. Rontu, A. Torkkeli, and M. Paulasto-Krockel: J. Alloy. Comp. 772 (2019) 306.
https://doi.org/10.1016/j.jallcom.2018.09.062
26 E. Santecchia, A. M. S. Hamouda, F. Musharavati, E. Zalnezhad, M. Cabibbo, M. El Mehtedi, and S.
Spigarelli: Adv. Mater. Sci. Eng. 2016 (2016) 9573524. https://doi.org/10.1155/2016/9573524

About the Authors
Qiu Zheng received her B.S. and M.S. degrees in materials processing engineering from
Huazhong University of Science and Technology, China, in 2010 and 2013, respectively,
and her Ph.D. degree in human mechatronics systems from Tokyo Metropolitan University,
Japan, in 2015. She joined Hitachi, Ltd. as a researcher in 2015 and worked as a researcher of
New Industry Creation Hatchery Center, Tohoku University, Japan from 2017 to 2019. She iscurrently a researcher in the Department of Mechanical and Biofunctional Systems, Instituteof Industrial Science, The University of Tokyo, Japan. Her research interests are in materials
processing, microforming, and microfabrication. (zheng@nanosys.mech.tohoku.ac.jp)
Le Van Minh was born in Thanh Hoa, Vietnam, in 1983. He received his B.S. degree inengineering physics and his M.Sc. degree in materials science from Hanoi University of
Science and Technology, Vietnam, in 2006, and 2008, respectively, and his Ph.D. degree innanomechanics from Tohoku University, Sendai, Japan, in 2013. From 2013 to 2015, he wasa research fellow of Tohoku University, Japan. Since 2015, he has been an assistant professorof Tohoku University. His current research interests are in nano/microfabrication, nano/microenergy systems, lead-free piezoelectric materials, and polymer electrets.
(minhlv@nanosys.mech.tohoku.ac.jp)

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Hoang Hung Nguyen received his diploma in engineering physics from Hanoi University of
Science and Technology, Vietnam, in 2010, and Ph.D. degree in nanomechanics from Tohoku
University, Japan, in 2018. He is currently a postdoctoral fellow in the New Industry Creation
Hatchery Center (NICHe), Tohoku University, Japan. His research activities are focused onhigh-performance energy harvesters, especially lead-free-based vibration energy harvesters.
(hung@nanosys.mech.tohoku.ac.jp)
Hiroki Kuwano received his B.Eng. and M.Eng. degrees in mechanical engineering and his
Ph.D. degree in electrical engineering from Tohoku University, Sendai, Japan, in 1975, 1977,
and 1990, respectively. He was a member of Electrical Communication Laboratories of Nippon
Telephone and Telegraph Public Corporation. Since 2003, he has been a professor of Tohoku
University. He has authored or co-authored over 120 technical papers and books, and over
50 patents in microelectromechanical systems and particle beam processing. His researchinterests are in nanoenergy systems, including energy harvesting systems and sensor networks,
particularly, those used for safety and medical applications.
(hiroki.kuwano@nanosys.mech.tohoku.ac.jp)