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Adjustable response of PZT thin film based piezoelectric micro-actuator through DC bias pre-polarization
Solid State Electronics 163 (2020) 107675
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Adjustable response of PZT thin film based piezoelectric micro-actuator through DC bias pre-polarization Dongdong Gonga,b, Feng Qinc, Yichen Wanga,b, Yu Chena,b, Tingting Yanga,b, Xiangyu Suna,b,
T ⁎
a
Microsystem and Terahertz Research Center, China Academy of Engineering Physics, Chengdu, China Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, China c School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, China b
ARTICLE INFO
ABSTRACT
Keywords: PZT thin film Micro-actuator Pre-polarization treatment Properties improvement
PZT piezoelectric thin film based technology is promising in the field of micro-actuators. This paper discusses the effects of pre-polarization on the key properties of PZT thin film, and explores the improvement rules of prepolarization from the material to device level experiments. At the material level, the pre-polarization treatment increases the piezoelectric strain coefficient by 25%, and improves dielectric properties. Meanwhile, in ferroelectric properties the pre-polarization treatment increases the residual polarization by 50% and coercive field by 25%, respectively. At the device level, pre-polarization treatment greatly increases the output characteristics of the devices, such as maximum output displacement increasing by at least 45% and withstand voltage enhancing by 2 V, which is consistent with the material level enhancement. In addition, the optimal pre-polarization process conditions and the long-term stability of the performance improvement are investigated. The prepolarization treatment has proved to be meaningful for the improvement of the output capability of the piezoelectric thin film actuators.
1. Introduction Piezoelectric micro-electromechanical systems (piezo-MEMS) have attracted growing attentions due to its wide range of applications in future integrated micro systems. Unlike traditional sensing and actuating systems based on macro bulk devices, piezoelectric MEMS rely mainly on thin-film materials and micro-nano manufacturing processes, which serves better for miniaturizing and integrating application [1–3]. Among plenty of piezoelectric materials, PZT thin film has received extensive attention and research in the past few years owing to its high piezoelectric coefficient, high electromechanical coupling factor and adjustable dielectric constant, etc [4–6]. However, the structural quality and ferroelectric property of the fabricated PZT film have reached its physical limit in recent years [7–9]. It is more difficult to increase the output capability of piezoelectric devices by improving the properties of material itself [10–12]. Therefore, the performance optimization of PZT thin film by posttreatment process has become a new research hotspot [13,14]. It is well known that the polarization orientation of the domains in film is closely related to the piezoelectric performance of the material, and the disordered domain orientation of the freshly prepared film generally lead to poor piezoelectric properties. Even some highly oriented
⁎
polycrystalline films still have irregular internal polarization orientation so that they do not exhibit sufficient piezoelectric properties. Therefore, improving the polarization orientation of the PZT film by pretreatment such as electrical excitation plays an important role in improving the piezoelectric output capability of the material [15–17]. Previous researches have focused on the effect of polarization on ferroelectric response of PZT thin film itself [10,11]. Few studies have focused on the optimization transfer by polarization from the material level to the device level, even though the performance influence rules on device level make more sense for practical applications. In this paper, the piezoelectric PZT (lead zirconate titanate) film fabricated by magnetron sputtering is firstly pre-polarized by DC electric field. And a large number of tests are then conducted to evaluate the influences of pretreatment on performance optimization at both material level and device level. Experimental results demonstrate that pre-polarization treatment evidently improves performance of piezoelectric, dielectric and ferroelectric characteristics for PZT film, increases output capability of maximum out-of-plane displacement and withstand voltage for devices, which is of great significance for the research of piezoelectric film micro-actuators.
Corresponding author. E-mail address: [email protected] (X. Sun).
https://doi.org/10.1016/j.sse.2019.107675 Received 22 February 2019; Received in revised form 27 August 2019; Accepted 30 September 2019 Available online 04 October 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Electronics 163 (2020) 107675
D. Gong, et al.
crystallographic orientation of the PZT film is good which grows mainly along [1 0 0] and [2 0 0] orientation, especially better along the orientation of [2 0 0]. The characterization of PZT film by Atomic Force Microscope (AFM) shows that the roughness is only 5.0 nm in the area of 2 μm × 2 μm, indicating that the PZT film has a smooth surface. In the mode of piezoAFM as shown in Fig. 4, the in-plane displacement is non-uniform and a phase difference of 180° even exists between the blue region and the red region, indicating in-plane disordered domains (see Fig. 4(a)). While no obvious displacement or phase difference is observed in the out-ofplane response of the material at the testing area (see Fig. 4(b)). Therefore, it can be assumed that the PZT films have good orientation along the surface normal direction. Based on the above characterization of the PZT materials before pre-polarization, it can be concluded that the quality and properties of the obtained PZT samples by magnetron sputtering are pretty good.
Fig. 1. Characteristics of the PZT thin film.
3.2. Piezoelectric properties improvement
2. Pre-polarization design
The original samples have piezoelectric strain coefficient d33 of 173 pC/N. Multiple points on samples get good consistency and sustain a breakdown voltage of more than 20 V/μm. The samples are pre-polarized with different voltage excitation by the experimental schemes described above. The results show that after pre-polarization of positive voltages, the performances of the samples degrade and even lose their piezoelectricity. This indicates that the orientation of spontaneous polarization domain inside the material is in the opposite direction to the applied external electric field. Under a long-term positive electric field, the charged defects inside the materials migrate in a wide range and diffuse into the top and bottom electrodes However, piezoelectric performances of the samples improve significantly after the pre-polarization of negative voltages. After negative voltages pre-polarization of 30 min, the piezoelectric strain coefficient d33 of the samples increases by about 25%, reaching about 215 pC/N, as shown in Fig. 5. No significant effect on d33 appears after further increasing the voltage amplitude of pre-polarization. It indicates that there is a threshold on the increase of piezoelectric strain coefficient, which can be achieved from pre-polarization by enough time, rather than too much large voltage.
In order to explore the effect of polarization treatment on characteristics of PZT thin film material and actuator devices, wafer level PZT thin film was firstly obtained through MEMS processing of PZT sputtering and electrode deposition on commercially Silicon on Insulator (SOI) substrate with device layer of about 20 μm. The PZT film is deposited by simultaneous sputtering of both PbZrO3 and PbTiO3 ceramic at elevated temperature above 600 ℃. Atmosphere of Ar/O2 mixture is involved to enhance the reaction and impede the formation of oxygen vacancy. Ti/Pt metal electrodes with thickness of 100 nm are prepared separately at both sides of PZT film as top and bottom electrodes to ensure reliable electric connection as well as good adhesion between piezoelectric film and substrate, as shown in Fig. 1. Two kinds of PZT based actuators including piezoelectric cantilever and piezoelectric micro-actuator actuator are then fabricated through a set of complex micro machined processes. The experiment scheme of pre-polarization treatment is that a positive or negative strong DC electric field is applied to top and bottom electrodes of PZT film samples at room temperature. By adjusting magnitude, direction and polarized time of the polarized voltages, the best polarized conditions are obtained. The changes on the piezoelectric coefficients, dielectric constants, and ferroelectric curves of the material under different polarized conditions are tested. It is found that the pre-polarization treatment of the PZT film makes the spontaneous polarization domains within the piezoelectric material more ordered. This greatly improves the ferroelectric, piezoelectric and dielectric properties of the material. In order to investigate the optimization effect of pre-polarization treatment from material level to device level, the outof-plane displacement and withstand voltage are extracted as key indicators. The dependence of out-of-plane displacement and withstand voltage on polarized voltage and polarized time are explored. The performances of devices after pre-polarization treatment show a significant improvement, consistent with the material-level test results. In addition, the long-term stability after the pre-polarization treatment is also explored in this paper, showing extremely few performance degradation for one month. The schematic of the experimental design is shown in Fig. 2.
3.3. Dielectric properties improvement The dielectric properties characterize the degree of polarization of the dielectric medium under an external electric field. For piezoelectric materials, a smaller dielectric constant is desirable which corresponds to more spontaneous polarization. The dielectric properties of the material are characterized, exhibiting relatively stable dielectric properties over the frequency band of 100 kHz to 1 MHz as shown in Fig. 6. The consistency of dielectric parameters of different samples before prepolarization is fine with deviation less than 5%. The relative dielectric constant of original sample is high, reaching 1400 with a dielectric loss of 0.012. After the negative voltages pre-polarization for 30 min, the relative dielectric constant of the material decreases significantly. The relative dielectric constant decreases to about 980 with the dielectric loss to be about 0.003 after −25 V pre-polarization. Besides, as the prepolarization voltage further increasing, the change in the relative permittivity of the material has been saturated.
3. Effect of pre-polarization on material level
3.4. Ferroelectric properties improvement
3.1. Basic characteristics of materials
The ferroelectric hysteresis loop which indicates the hysteresis relationship of the polarization intensity with the changes of external electric field, is an important method for characterizing the ferroelectric properties of the film. The residual polarization and the coercive electric field of the PZT film can be obtained by measuring the P-E curve. As shown in Fig. 7, ferroelectric hysteresis loops are tested under the
Before exploration of pre-polarization on material properties, the basic properties of the film are first characterized. Fig. 3(a) shows the results of material composition by the Energy Dispersive X-ray Detector (EDX) analysis, indicating that the purity of PZT sample is very high. XRay Diffraction (XRD) analysis in Fig. 3(b) shows that the 2
Solid State Electronics 163 (2020) 107675
D. Gong, et al.
Material Level
Pre-polarization Treatment
Device Level
PZT film sample
Cantilever beams
Top Electrode
DC
-
PZT annular micromotor
+
Piezoelectric Property
Micromotor
Output Capability
Bottom Electrode
Dielectric Property
Voltage Performance
Ferroelectric Properties
Long-term Stability Fig. 2. The schematic of pre-polarization treatment experiment.
of the domains have deflected. Therefore, the symmetry and widening of the ferroelectric loops, with the residual polarization and coercive field increasing, indicate that after pre-polarization of negative voltages, the ferroelectric properties of the PZT film is significantly improved.
Intensity (cps)
(a) 5x10
5
4x10
5
3x10
5
2x10
5
1x10
5
O1s
Survey
4. Effect of pre-polarization on device level
Pb 4f Zr 3d
The pre-polarization test results of the material show that the prepolarization under negative voltages can significantly improve the piezoelectric, dielectric and ferroelectric properties of the materials. Material-level performance enhancements promote the feasibility of device performance optimization. For device-level applications, it is known from Gauss's theorem with dielectrics by Eq. (1),
C1S Ti 2p
0
0
100
200
300
400
500
600
700
800
Binding Energy (eV)
S
PZT(002/200)
Pt(111) PZT(111)
Si(200)
Intensity (a.u.)
PZT£¨001/100)
3.5 m PZT
20
30
40
=
S
D ·dS = Q0
(1)
The electrical displacement through an arbitrary surface is equal to the algebraic sum of the free charges contained within the closed surface. The introduction of the electric displacement is to simplify the calculation in case of complex polarized charges. Therefore, smaller electric displacement corresponds to less free charges and more polarized charges. According to the first type of piezoelectric equations by Eq. (2),
(b)
10
0 r E · dS
50
{D} = [eT ]·{E } + [d]·{T }
(2)
{S } = [d]T ·{E } + [s E ]·{T }
(3)
the decrease of the dielectric constant will reduce the electric displacement when the external force of the elastomer is zero or constant. The pre-polarization treatment reduces the dielectric constant, thus confirming the increase of polarized charges. From the piezoelectric equation by Eq. (3), it can be seen that in the process of inverse piezoelectric effect, the increase of the piezoelectric strain constant [d] corresponds with the increase of the piezoelectric strain in the piezoelectric elastic medium when the external electric field remains constant. Meanwhile, the increase of the coercive field allows piezoelectric materials to withstand higher voltages. From theoretical analysis, all performance improvements at the material level can be reflected at the device level, which greatly improves the device actuating performance, stability and reliability. In order to verify the above theoretical analysis, two typical types of actuators including cantilever beams of 3300 μm long ×300 μm wide, and annular micro-actuators of 4 mm diameter, are fabricated using PZT films under non-polarization versus pre-polarization.
60
2 (Deg.) Fig. 3. Basic characteristics of materials. (a) Material composition from EDX analysis; (b) Film crystallographic orientation from XRD.
applied external electric field with range of ± 80 V at 100 Hz and 1 kHz, respectively. The pristine samples show good ferroelectric properties with the coercive field about 20 V and the residual polarization about 50 μC/cm2. For the samples after negative voltages prepolarization treatment, ferroelectric loops under the two excitation frequencies have little difference overall. Only the curve saturation of ferroelectric loops is a little higher under 1 kHz than 100 Hz. The residual polarization increases from about 50 μC/cm2 to about 75 μC/ cm2, increasing by 50%. The coercive field increased to about 25 V, with an increase of 25%. Nevertheless, the improvement is not very obvious for polarized voltage range from −80 V to −100 V when most
4.1. Performances improvement Before pre-polarization treatment, the dynamic response of 3
Solid State Electronics 163 (2020) 107675
D. Gong, et al.
(a)
(b)
Fig. 4. Piezo-AFM response of PZT film. (a) In-plane displacement and phase difference; (b) Out-of-plane displacement and phase difference.
H 0 10 @
220
200 190
re su
H 1K
50
2
P ( C/cm )
Fig. 5. Improvement of pre-polarization treatment on piezoelectric properties.
1450 1400
0 -50
1350 Un-polarized -15V prepolarized -25V prepolarized -30V prepolarized
1300 1250 1200
80
z
100
30
40
@
25
0
re d
20
-40
su
15
-80
ea
0.0
Un-polarized -80 V prepolarized -100 V prepolarized
-100
Un-polarized Prepolarized
170
-50
M
180
0
M ea
2
210
d
50
Prepolarized Voltage (V)
Relative Dielectric Constant
z
100
P ( C/cm )
Piezoelectric Strain Constant d 33 (pC/N)
230
Un-polarized -80 V prepolarized -100 V prepolarized
-100 -80
-40
0
40
80
Voltage (V)
1150
Fig. 7. Improvement of ferroelectric properties by pre-polarization treatment.
1100 1050
cantilever beams are tested by Laser Doppler Vibrometer (LDV) at 500 Hz AC excitation. During the gradual increase of the excitation voltage, the displacement waveform at the end of the cantilever beam is stable and regular. When the excitation voltage exceeds 15 V, waveform distortion appears. The maximum undistorted displacement at 14.5 V is 22.675 μm. As a contrast, cantilever beam samples are taken for testing
1000 950
0.0
200.0k
400.0k
600.0k
800.0k
1.0M
Frequency (Hz) Fig. 6. Improvement of pre-polarization treatment on dielectric properties. 4
Solid State Electronics 163 (2020) 107675
D. Gong, et al.
35
Table 1 Comparison of improvement of cantilever beams and annular micro-actuators.
Displacement ( m)
30
Actuator type
Pre-polarized condition
Displacement increased by
Increase of withstand voltage
Cantilever beam Micro-actuator
−40 V, 30 min −25 V, 30 min
47.6% 46.0%
2V \
25 20 15
4.2. Optimum pre-polarized condition
10 Un-polarized Prepolarized
5 0
2
4
6
8
10
12
14
16
It can be seen from Fig. 9 that the dispalcement response of untreated sammple is smaller than all the other samples, indicating performance enhancement of all samples treated with different pre-polarized voltages. From the experiments, it is found that performance improvement of the devices after pre-polarization neither increases monotonously with the increase of the polarization voltage, nor increases monotonously with the increase of the pre-polarized time. The improvement of the performance has an optimal condition during the pre-polarization treatment. As the pre-polarized voltage increases, the piezo-response improves at first and then degrades. This is because excessive DC voltage interacts with the charged defects in PZT layer and continuous leakage current changes the charge distribution and lattice structure inside the material, and finally affects the piezo-response of PZT. With respect to the pre-polarized time, the performance also improves at beginning with pre-polarized time increasing. However, when the time is further extended, the device performance is no longer improved, but there is no downward trend either. Finally it tends to be stable as shown in Fig. 10. This indicates that when the pre-polarized time is enough to deflect most of the electric domains inside the material, there will be no more domain deflecting after extending the pre-polarized time. Therefore, there is an optimal pre-polarized voltage and minimum pre-polarized time for the PZT thin films-based device performance improvement.
18
Voltage (V) Fig. 8. Output displacement of cantilever beam before and after pre-polarization treatment.
after pre-polarization treatment of −40 V for 30 min. Under the same test conditions, the maximum undistorted voltage is 16.5 V with the out-of-plane displacement of 33.475 μm. Fig. 8 shows the voltage-displacement response of the cantilever beam samples. Under the same excitation conditions, the maximum displacement is increased by 47.63% after pre-polarization treatment. Meanwhile, the maximum undistorted voltage of the device has an increase of 2 V, which ascribed to the enhanced coercive voltage. Pre-polarization significantly improves the actuating performance and broadens the working range of devices. As for the annular micro-actuator, samples are pre-polarized at the polarized voltage of −15 V, −20 V, −25 V, and −30 V for 30 min, respectively. The results shown in Fig. 9 indicates that the micro-actuator achieved the best performance improvement under the −25 V pre-polarized condition with a maximum out-of-plane displacement increased by 46% at excitation of 9 V. Table 1 shows the comparison of improvement of device performances. In line with the performance improvement at the material level, the increase of the piezoelectric strain constant produces larger strains on device structures. And the increase of the coercive field in the ferroelectric loop allows the devices to withstand higher reverse voltages. Therefore, the improvements in piezoelectric and ferroelectric performance in the material characterization experiments by the prepolarization treatment, are accurately reflected on the device-level test results.
4.3. Long-term stability The longevity of the performance improvement by pre-polarization treatment will be an important concern. An experiment is conducted to verify the long-term stability after the pre-polarization treatment. The −20 V pre-polarized actuators are repeatedly tested after they are placed calmly for a different period of time as shown in Fig. 11. The performance of the pre-polarized devices show almost no attenuation compared to that of the just-prepolarized test, within a month-long period.
7
8
Un-polarized -15 V Prepolarized -20 V Prepolarized -25 V Prepolarized -30 V Prepolarized
6
Displacement ( m)
Displacement ( m)
7
5 4 3 2 1 0
1
2
3
4
Un-polarized 10 min 20 min 30 min 40 min
6
5
6
7
8
9
5 4 3 2 1 0
10
Voltage (V)
1
2
3
4
5
6
7
8
9
10 11
Voltage (V)
Fig. 9. Output displacement of annular micro-actuator under different pre-polarized voltages.
Fig. 10. Output displacement of annular micro-actuator under different prepolarized times. 5
Solid State Electronics 163 (2020) 107675
Displacement(um)
D. Gong, et al.
Platinized Silicon Wafers. Ferroelectrics 2010;405:242–8. [12] Chen X, Li A, Yao N, et al. Adjustable stiffness of individual piezoelectric nanofibers by electron beam polarization. Appl Phys Lett 2011;99:193102. [13] Adel S, Cherifa B. Effect of Cr2O3 and Fe2O3 doping on the thermal activation of unpolarized PZT charge carriers. Boletin De La Sociedad Espanola De Ceramica Y Vidrio 2017. [14] Parker DS, Herklotz A, Ward TZ, et al. Enhanced ferroelectric polarization and possible morphotrophic phase boundary in PZT-based alloys. Phys Rev B 2016;93:174307. [15] Xu H, Ono T, Zhang DY, et al. Fabrication and characterizations of a monolithic PZT microstage. Microsyst Technol 2006;12:883–90. [16] Mcgilly LJ, Feigl L, Dai X, et al. Polarization switching and domain wall motion in circular and ring capacitor structures in PZT thin films. Ferroelectrics 2015;480:58–64. [17] Zhang DY, Ono T, Esashi M. Piezoactuator-integrated monolithic microstage with six degrees of freedom. Sens Actuators, A Phys 2003;122:301–6.
Un-polarized 0 day 15 days 30 days
Dongdong Gong was born in 1990. He received the bachelor's degree from University of Electronic Science and Technology of China (UESTC) in 2013 and the master's degree in Materials Science and Engineering in 2017. He is a employee in Microsystems and THz Research Center now. His current research interests are focused on piezoelectric actuator.
Frequency(Hz) Fig. 11. Long-term stability of performances improvement after pre-polarization treatment.
5. Conclusion In this paper, PZT piezoelectric film samples are pre-polarized after produced by magnetron sputtering. Experiments on both material level and device level are conducted to explore the rules of the performance improvement by pre-polarization treatment. At the material level, the piezoelectric strain constant, residual polarization and coercive field of the material increases and the relative permittivity reduces after prepolarization treatment. At the device level, both the out-of-plane displacement and withstand voltage are improved after pre-polarization for typical actuators such as cantilever beams and micro-actuators. In addition, by investigating the optimal pre-polarized conditions, it is found that there is a relatively optimal value for the pre-polarized time and pre-polarized voltage, so that the performance improvement can be optimized to the maximum extent. Finally, the device performance improvements possess long-term stability after pre-polarization treatment. This paper proposes a feasible reference for improving the overall performance of PZT thin film-based piezoelectric micro-actuators.
Feng Qin received the B.E. degree in electronic science from Chengdu University of Technology, Chengdu, China in 2015.He is currently pursuing the Ph.D. degree in the Department of Integrated Circuits and Systems, School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, China. His research interests are focused piezoelectric behavior mechanism and application research on microelectromechanical system, including research on high dynamic characteristics of piezoelectric actuators, in-situ health prognostic and self-calibration of inertial sensors, as well as fabrication process, packaging technology and control circuit design for MEMS.
Yicheng Wang received his B.E. degree (2011) in Microelectronic Technology, Ph.D. degree (2016) in Microelectronics and Solid Electronics from the University of Electronic Science and Technology of China. He is currently working in Microsystem \& Terahertz Research Center as an assistant research fellow. He’s currently research field is MEMS technology.
References [1] Rezaeisaray M, El Gowini M, Sameoto D, et al. Wide-bandwidth piezoelectric energy harvester with polymeric structure. J Micromech Microeng 2015;25:015018. [2] Hosseini R, Hamedi M. Improvements in energy harvesting capabilities by using different shapes of piezoelectric bimorphs. J Micromech Microeng 2015;25:125008. [3] Williams RP, Kim D, Gawalt DP, et al. Surface micromachined differential piezoelectric shear-stress sensors. J Micromech Microeng 2017;27:015011. [4] Dezest D, Thomas O, Mathieu F, et al. Wafer-scale fabrication of self-actuated piezoelectric nanoelectromechanical resonators based on lead zirconate titanate (PZT). J Micromech Microeng 2015;25:035002. [5] Chidambaram N, Balma D, Nigon R, et al. Converse mode piezoelectric coefficient for lead zirconate titanate thin film with interdigitated electrode. J Micromech Microeng 2015;25:045016. [6] Benoit RR, Rudy RQ, Pulskamp JS, et al. Advances in piezoelectric PZT-based RF MEMS components and systems. J Micromech Microeng 2017;27:083002. [7] Lee N, Kim SJ. Effects of loading rate and temperature on domain switching and evolutions of reference remnant state variables during polarization reversal in a PZT wafer. Ceram Int 2012;38:1115–26. [8] Lin Y, Andrews C, Sodano HA. Enhanced piezoelectric properties of lead zirconate titanate (PZT) sol-gel derived ceramics using single crystal PZT cubes. J Appl Phys 2010;7644. 064108-064108-6. [9] Wang Z, Zhu W, Zhao C, et al. Dense PZT thick films derived from sol-gel based nanocomposite process. Mater Sci Eng, B 2003;99:56–62. [10] Bai Y, Wang ZJ, He B, et al. Enhancement of Polarization in Ferroelectric Films via the Incorporation of Gold Nanoparticles ACS. Omega 2017;2:9067–73. [11] Suchaneck G, Gerlach G. Polarization Fatigue of NiCr/PZT/BaPbO3 Capacitors on
Yu Chen received the B.E. degree and M.S degree in mechatronics engineering from the University of Electronic Science and Technology of China, in 2012 and 2015, respectively. He is currently working in China Academy of Engineering Physics. He’s currently research field is MEMS Packaging and MEMS integration technology.
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Solid State Electronics 163 (2020) 107675
D. Gong, et al. Tingting Yang was born in 1989. She received the B.S. degree from University of Electronic Science and Technology of China (UESTC) in 2012. Her Ph.D. degree was received in Materials Science and Engineering from Tsinghua University in 2017. She is an assistant research fellow in China Academy of Engineering Physics now. Her current research interests are focused on novel sensors, actuators and corresponding integration technology.
Xiangyu Sun received his B.E. degree (2011) in Microelectronic Technology, Ph.D. degree (2017) in Microelectronics and Solid Electronics from the University of Electronic Science and Technology of China. He is currently working in China Academy of Engineering Physics as an assistant research fellow. He’s currently research field is novel MEMS actuators and MEMS integration technology.
7
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Adjustable response of PZT thin film based piezoelectric micro-actuator through DC bias pre-polarization Dongdong Gonga,b, Feng Qinc, Yichen Wanga,b, Yu Chena,b, Tingting Yanga,b, Xiangyu Suna,b,
T ⁎
a
Microsystem and Terahertz Research Center, China Academy of Engineering Physics, Chengdu, China Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, China c School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, China b
ARTICLE INFO
ABSTRACT
Keywords: PZT thin film Micro-actuator Pre-polarization treatment Properties improvement
PZT piezoelectric thin film based technology is promising in the field of micro-actuators. This paper discusses the effects of pre-polarization on the key properties of PZT thin film, and explores the improvement rules of prepolarization from the material to device level experiments. At the material level, the pre-polarization treatment increases the piezoelectric strain coefficient by 25%, and improves dielectric properties. Meanwhile, in ferroelectric properties the pre-polarization treatment increases the residual polarization by 50% and coercive field by 25%, respectively. At the device level, pre-polarization treatment greatly increases the output characteristics of the devices, such as maximum output displacement increasing by at least 45% and withstand voltage enhancing by 2 V, which is consistent with the material level enhancement. In addition, the optimal pre-polarization process conditions and the long-term stability of the performance improvement are investigated. The prepolarization treatment has proved to be meaningful for the improvement of the output capability of the piezoelectric thin film actuators.
1. Introduction Piezoelectric micro-electromechanical systems (piezo-MEMS) have attracted growing attentions due to its wide range of applications in future integrated micro systems. Unlike traditional sensing and actuating systems based on macro bulk devices, piezoelectric MEMS rely mainly on thin-film materials and micro-nano manufacturing processes, which serves better for miniaturizing and integrating application [1–3]. Among plenty of piezoelectric materials, PZT thin film has received extensive attention and research in the past few years owing to its high piezoelectric coefficient, high electromechanical coupling factor and adjustable dielectric constant, etc [4–6]. However, the structural quality and ferroelectric property of the fabricated PZT film have reached its physical limit in recent years [7–9]. It is more difficult to increase the output capability of piezoelectric devices by improving the properties of material itself [10–12]. Therefore, the performance optimization of PZT thin film by posttreatment process has become a new research hotspot [13,14]. It is well known that the polarization orientation of the domains in film is closely related to the piezoelectric performance of the material, and the disordered domain orientation of the freshly prepared film generally lead to poor piezoelectric properties. Even some highly oriented
⁎
polycrystalline films still have irregular internal polarization orientation so that they do not exhibit sufficient piezoelectric properties. Therefore, improving the polarization orientation of the PZT film by pretreatment such as electrical excitation plays an important role in improving the piezoelectric output capability of the material [15–17]. Previous researches have focused on the effect of polarization on ferroelectric response of PZT thin film itself [10,11]. Few studies have focused on the optimization transfer by polarization from the material level to the device level, even though the performance influence rules on device level make more sense for practical applications. In this paper, the piezoelectric PZT (lead zirconate titanate) film fabricated by magnetron sputtering is firstly pre-polarized by DC electric field. And a large number of tests are then conducted to evaluate the influences of pretreatment on performance optimization at both material level and device level. Experimental results demonstrate that pre-polarization treatment evidently improves performance of piezoelectric, dielectric and ferroelectric characteristics for PZT film, increases output capability of maximum out-of-plane displacement and withstand voltage for devices, which is of great significance for the research of piezoelectric film micro-actuators.
Corresponding author. E-mail address: [email protected] (X. Sun).
https://doi.org/10.1016/j.sse.2019.107675 Received 22 February 2019; Received in revised form 27 August 2019; Accepted 30 September 2019 Available online 04 October 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Electronics 163 (2020) 107675
D. Gong, et al.
crystallographic orientation of the PZT film is good which grows mainly along [1 0 0] and [2 0 0] orientation, especially better along the orientation of [2 0 0]. The characterization of PZT film by Atomic Force Microscope (AFM) shows that the roughness is only 5.0 nm in the area of 2 μm × 2 μm, indicating that the PZT film has a smooth surface. In the mode of piezoAFM as shown in Fig. 4, the in-plane displacement is non-uniform and a phase difference of 180° even exists between the blue region and the red region, indicating in-plane disordered domains (see Fig. 4(a)). While no obvious displacement or phase difference is observed in the out-ofplane response of the material at the testing area (see Fig. 4(b)). Therefore, it can be assumed that the PZT films have good orientation along the surface normal direction. Based on the above characterization of the PZT materials before pre-polarization, it can be concluded that the quality and properties of the obtained PZT samples by magnetron sputtering are pretty good.
Fig. 1. Characteristics of the PZT thin film.
3.2. Piezoelectric properties improvement
2. Pre-polarization design
The original samples have piezoelectric strain coefficient d33 of 173 pC/N. Multiple points on samples get good consistency and sustain a breakdown voltage of more than 20 V/μm. The samples are pre-polarized with different voltage excitation by the experimental schemes described above. The results show that after pre-polarization of positive voltages, the performances of the samples degrade and even lose their piezoelectricity. This indicates that the orientation of spontaneous polarization domain inside the material is in the opposite direction to the applied external electric field. Under a long-term positive electric field, the charged defects inside the materials migrate in a wide range and diffuse into the top and bottom electrodes However, piezoelectric performances of the samples improve significantly after the pre-polarization of negative voltages. After negative voltages pre-polarization of 30 min, the piezoelectric strain coefficient d33 of the samples increases by about 25%, reaching about 215 pC/N, as shown in Fig. 5. No significant effect on d33 appears after further increasing the voltage amplitude of pre-polarization. It indicates that there is a threshold on the increase of piezoelectric strain coefficient, which can be achieved from pre-polarization by enough time, rather than too much large voltage.
In order to explore the effect of polarization treatment on characteristics of PZT thin film material and actuator devices, wafer level PZT thin film was firstly obtained through MEMS processing of PZT sputtering and electrode deposition on commercially Silicon on Insulator (SOI) substrate with device layer of about 20 μm. The PZT film is deposited by simultaneous sputtering of both PbZrO3 and PbTiO3 ceramic at elevated temperature above 600 ℃. Atmosphere of Ar/O2 mixture is involved to enhance the reaction and impede the formation of oxygen vacancy. Ti/Pt metal electrodes with thickness of 100 nm are prepared separately at both sides of PZT film as top and bottom electrodes to ensure reliable electric connection as well as good adhesion between piezoelectric film and substrate, as shown in Fig. 1. Two kinds of PZT based actuators including piezoelectric cantilever and piezoelectric micro-actuator actuator are then fabricated through a set of complex micro machined processes. The experiment scheme of pre-polarization treatment is that a positive or negative strong DC electric field is applied to top and bottom electrodes of PZT film samples at room temperature. By adjusting magnitude, direction and polarized time of the polarized voltages, the best polarized conditions are obtained. The changes on the piezoelectric coefficients, dielectric constants, and ferroelectric curves of the material under different polarized conditions are tested. It is found that the pre-polarization treatment of the PZT film makes the spontaneous polarization domains within the piezoelectric material more ordered. This greatly improves the ferroelectric, piezoelectric and dielectric properties of the material. In order to investigate the optimization effect of pre-polarization treatment from material level to device level, the outof-plane displacement and withstand voltage are extracted as key indicators. The dependence of out-of-plane displacement and withstand voltage on polarized voltage and polarized time are explored. The performances of devices after pre-polarization treatment show a significant improvement, consistent with the material-level test results. In addition, the long-term stability after the pre-polarization treatment is also explored in this paper, showing extremely few performance degradation for one month. The schematic of the experimental design is shown in Fig. 2.
3.3. Dielectric properties improvement The dielectric properties characterize the degree of polarization of the dielectric medium under an external electric field. For piezoelectric materials, a smaller dielectric constant is desirable which corresponds to more spontaneous polarization. The dielectric properties of the material are characterized, exhibiting relatively stable dielectric properties over the frequency band of 100 kHz to 1 MHz as shown in Fig. 6. The consistency of dielectric parameters of different samples before prepolarization is fine with deviation less than 5%. The relative dielectric constant of original sample is high, reaching 1400 with a dielectric loss of 0.012. After the negative voltages pre-polarization for 30 min, the relative dielectric constant of the material decreases significantly. The relative dielectric constant decreases to about 980 with the dielectric loss to be about 0.003 after −25 V pre-polarization. Besides, as the prepolarization voltage further increasing, the change in the relative permittivity of the material has been saturated.
3. Effect of pre-polarization on material level
3.4. Ferroelectric properties improvement
3.1. Basic characteristics of materials
The ferroelectric hysteresis loop which indicates the hysteresis relationship of the polarization intensity with the changes of external electric field, is an important method for characterizing the ferroelectric properties of the film. The residual polarization and the coercive electric field of the PZT film can be obtained by measuring the P-E curve. As shown in Fig. 7, ferroelectric hysteresis loops are tested under the
Before exploration of pre-polarization on material properties, the basic properties of the film are first characterized. Fig. 3(a) shows the results of material composition by the Energy Dispersive X-ray Detector (EDX) analysis, indicating that the purity of PZT sample is very high. XRay Diffraction (XRD) analysis in Fig. 3(b) shows that the 2
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Material Level
Pre-polarization Treatment
Device Level
PZT film sample
Cantilever beams
Top Electrode
DC
-
PZT annular micromotor
+
Piezoelectric Property
Micromotor
Output Capability
Bottom Electrode
Dielectric Property
Voltage Performance
Ferroelectric Properties
Long-term Stability Fig. 2. The schematic of pre-polarization treatment experiment.
of the domains have deflected. Therefore, the symmetry and widening of the ferroelectric loops, with the residual polarization and coercive field increasing, indicate that after pre-polarization of negative voltages, the ferroelectric properties of the PZT film is significantly improved.
Intensity (cps)
(a) 5x10
5
4x10
5
3x10
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2x10
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O1s
Survey
4. Effect of pre-polarization on device level
Pb 4f Zr 3d
The pre-polarization test results of the material show that the prepolarization under negative voltages can significantly improve the piezoelectric, dielectric and ferroelectric properties of the materials. Material-level performance enhancements promote the feasibility of device performance optimization. For device-level applications, it is known from Gauss's theorem with dielectrics by Eq. (1),
C1S Ti 2p
0
0
100
200
300
400
500
600
700
800
Binding Energy (eV)
S
PZT(002/200)
Pt(111) PZT(111)
Si(200)
Intensity (a.u.)
PZT£¨001/100)
3.5 m PZT
20
30
40
=
S
D ·dS = Q0
(1)
The electrical displacement through an arbitrary surface is equal to the algebraic sum of the free charges contained within the closed surface. The introduction of the electric displacement is to simplify the calculation in case of complex polarized charges. Therefore, smaller electric displacement corresponds to less free charges and more polarized charges. According to the first type of piezoelectric equations by Eq. (2),
(b)
10
0 r E · dS
50
{D} = [eT ]·{E } + [d]·{T }
(2)
{S } = [d]T ·{E } + [s E ]·{T }
(3)
the decrease of the dielectric constant will reduce the electric displacement when the external force of the elastomer is zero or constant. The pre-polarization treatment reduces the dielectric constant, thus confirming the increase of polarized charges. From the piezoelectric equation by Eq. (3), it can be seen that in the process of inverse piezoelectric effect, the increase of the piezoelectric strain constant [d] corresponds with the increase of the piezoelectric strain in the piezoelectric elastic medium when the external electric field remains constant. Meanwhile, the increase of the coercive field allows piezoelectric materials to withstand higher voltages. From theoretical analysis, all performance improvements at the material level can be reflected at the device level, which greatly improves the device actuating performance, stability and reliability. In order to verify the above theoretical analysis, two typical types of actuators including cantilever beams of 3300 μm long ×300 μm wide, and annular micro-actuators of 4 mm diameter, are fabricated using PZT films under non-polarization versus pre-polarization.
60
2 (Deg.) Fig. 3. Basic characteristics of materials. (a) Material composition from EDX analysis; (b) Film crystallographic orientation from XRD.
applied external electric field with range of ± 80 V at 100 Hz and 1 kHz, respectively. The pristine samples show good ferroelectric properties with the coercive field about 20 V and the residual polarization about 50 μC/cm2. For the samples after negative voltages prepolarization treatment, ferroelectric loops under the two excitation frequencies have little difference overall. Only the curve saturation of ferroelectric loops is a little higher under 1 kHz than 100 Hz. The residual polarization increases from about 50 μC/cm2 to about 75 μC/ cm2, increasing by 50%. The coercive field increased to about 25 V, with an increase of 25%. Nevertheless, the improvement is not very obvious for polarized voltage range from −80 V to −100 V when most
4.1. Performances improvement Before pre-polarization treatment, the dynamic response of 3
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(a)
(b)
Fig. 4. Piezo-AFM response of PZT film. (a) In-plane displacement and phase difference; (b) Out-of-plane displacement and phase difference.
H 0 10 @
220
200 190
re su
H 1K
50
2
P ( C/cm )
Fig. 5. Improvement of pre-polarization treatment on piezoelectric properties.
1450 1400
0 -50
1350 Un-polarized -15V prepolarized -25V prepolarized -30V prepolarized
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80
z
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@
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re d
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su
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ea
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-100
Un-polarized Prepolarized
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-50
M
180
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M ea
2
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d
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Prepolarized Voltage (V)
Relative Dielectric Constant
z
100
P ( C/cm )
Piezoelectric Strain Constant d 33 (pC/N)
230
Un-polarized -80 V prepolarized -100 V prepolarized
-100 -80
-40
0
40
80
Voltage (V)
1150
Fig. 7. Improvement of ferroelectric properties by pre-polarization treatment.
1100 1050
cantilever beams are tested by Laser Doppler Vibrometer (LDV) at 500 Hz AC excitation. During the gradual increase of the excitation voltage, the displacement waveform at the end of the cantilever beam is stable and regular. When the excitation voltage exceeds 15 V, waveform distortion appears. The maximum undistorted displacement at 14.5 V is 22.675 μm. As a contrast, cantilever beam samples are taken for testing
1000 950
0.0
200.0k
400.0k
600.0k
800.0k
1.0M
Frequency (Hz) Fig. 6. Improvement of pre-polarization treatment on dielectric properties. 4
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Table 1 Comparison of improvement of cantilever beams and annular micro-actuators.
Displacement ( m)
30
Actuator type
Pre-polarized condition
Displacement increased by
Increase of withstand voltage
Cantilever beam Micro-actuator
−40 V, 30 min −25 V, 30 min
47.6% 46.0%
2V \
25 20 15
4.2. Optimum pre-polarized condition
10 Un-polarized Prepolarized
5 0
2
4
6
8
10
12
14
16
It can be seen from Fig. 9 that the dispalcement response of untreated sammple is smaller than all the other samples, indicating performance enhancement of all samples treated with different pre-polarized voltages. From the experiments, it is found that performance improvement of the devices after pre-polarization neither increases monotonously with the increase of the polarization voltage, nor increases monotonously with the increase of the pre-polarized time. The improvement of the performance has an optimal condition during the pre-polarization treatment. As the pre-polarized voltage increases, the piezo-response improves at first and then degrades. This is because excessive DC voltage interacts with the charged defects in PZT layer and continuous leakage current changes the charge distribution and lattice structure inside the material, and finally affects the piezo-response of PZT. With respect to the pre-polarized time, the performance also improves at beginning with pre-polarized time increasing. However, when the time is further extended, the device performance is no longer improved, but there is no downward trend either. Finally it tends to be stable as shown in Fig. 10. This indicates that when the pre-polarized time is enough to deflect most of the electric domains inside the material, there will be no more domain deflecting after extending the pre-polarized time. Therefore, there is an optimal pre-polarized voltage and minimum pre-polarized time for the PZT thin films-based device performance improvement.
18
Voltage (V) Fig. 8. Output displacement of cantilever beam before and after pre-polarization treatment.
after pre-polarization treatment of −40 V for 30 min. Under the same test conditions, the maximum undistorted voltage is 16.5 V with the out-of-plane displacement of 33.475 μm. Fig. 8 shows the voltage-displacement response of the cantilever beam samples. Under the same excitation conditions, the maximum displacement is increased by 47.63% after pre-polarization treatment. Meanwhile, the maximum undistorted voltage of the device has an increase of 2 V, which ascribed to the enhanced coercive voltage. Pre-polarization significantly improves the actuating performance and broadens the working range of devices. As for the annular micro-actuator, samples are pre-polarized at the polarized voltage of −15 V, −20 V, −25 V, and −30 V for 30 min, respectively. The results shown in Fig. 9 indicates that the micro-actuator achieved the best performance improvement under the −25 V pre-polarized condition with a maximum out-of-plane displacement increased by 46% at excitation of 9 V. Table 1 shows the comparison of improvement of device performances. In line with the performance improvement at the material level, the increase of the piezoelectric strain constant produces larger strains on device structures. And the increase of the coercive field in the ferroelectric loop allows the devices to withstand higher reverse voltages. Therefore, the improvements in piezoelectric and ferroelectric performance in the material characterization experiments by the prepolarization treatment, are accurately reflected on the device-level test results.
4.3. Long-term stability The longevity of the performance improvement by pre-polarization treatment will be an important concern. An experiment is conducted to verify the long-term stability after the pre-polarization treatment. The −20 V pre-polarized actuators are repeatedly tested after they are placed calmly for a different period of time as shown in Fig. 11. The performance of the pre-polarized devices show almost no attenuation compared to that of the just-prepolarized test, within a month-long period.
7
8
Un-polarized -15 V Prepolarized -20 V Prepolarized -25 V Prepolarized -30 V Prepolarized
6
Displacement ( m)
Displacement ( m)
7
5 4 3 2 1 0
1
2
3
4
Un-polarized 10 min 20 min 30 min 40 min
6
5
6
7
8
9
5 4 3 2 1 0
10
Voltage (V)
1
2
3
4
5
6
7
8
9
10 11
Voltage (V)
Fig. 9. Output displacement of annular micro-actuator under different pre-polarized voltages.
Fig. 10. Output displacement of annular micro-actuator under different prepolarized times. 5
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Platinized Silicon Wafers. Ferroelectrics 2010;405:242–8. [12] Chen X, Li A, Yao N, et al. Adjustable stiffness of individual piezoelectric nanofibers by electron beam polarization. Appl Phys Lett 2011;99:193102. [13] Adel S, Cherifa B. Effect of Cr2O3 and Fe2O3 doping on the thermal activation of unpolarized PZT charge carriers. Boletin De La Sociedad Espanola De Ceramica Y Vidrio 2017. [14] Parker DS, Herklotz A, Ward TZ, et al. Enhanced ferroelectric polarization and possible morphotrophic phase boundary in PZT-based alloys. Phys Rev B 2016;93:174307. [15] Xu H, Ono T, Zhang DY, et al. Fabrication and characterizations of a monolithic PZT microstage. Microsyst Technol 2006;12:883–90. [16] Mcgilly LJ, Feigl L, Dai X, et al. Polarization switching and domain wall motion in circular and ring capacitor structures in PZT thin films. Ferroelectrics 2015;480:58–64. [17] Zhang DY, Ono T, Esashi M. Piezoactuator-integrated monolithic microstage with six degrees of freedom. Sens Actuators, A Phys 2003;122:301–6.
Un-polarized 0 day 15 days 30 days
Dongdong Gong was born in 1990. He received the bachelor's degree from University of Electronic Science and Technology of China (UESTC) in 2013 and the master's degree in Materials Science and Engineering in 2017. He is a employee in Microsystems and THz Research Center now. His current research interests are focused on piezoelectric actuator.
Frequency(Hz) Fig. 11. Long-term stability of performances improvement after pre-polarization treatment.
5. Conclusion In this paper, PZT piezoelectric film samples are pre-polarized after produced by magnetron sputtering. Experiments on both material level and device level are conducted to explore the rules of the performance improvement by pre-polarization treatment. At the material level, the piezoelectric strain constant, residual polarization and coercive field of the material increases and the relative permittivity reduces after prepolarization treatment. At the device level, both the out-of-plane displacement and withstand voltage are improved after pre-polarization for typical actuators such as cantilever beams and micro-actuators. In addition, by investigating the optimal pre-polarized conditions, it is found that there is a relatively optimal value for the pre-polarized time and pre-polarized voltage, so that the performance improvement can be optimized to the maximum extent. Finally, the device performance improvements possess long-term stability after pre-polarization treatment. This paper proposes a feasible reference for improving the overall performance of PZT thin film-based piezoelectric micro-actuators.
Feng Qin received the B.E. degree in electronic science from Chengdu University of Technology, Chengdu, China in 2015.He is currently pursuing the Ph.D. degree in the Department of Integrated Circuits and Systems, School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, China. His research interests are focused piezoelectric behavior mechanism and application research on microelectromechanical system, including research on high dynamic characteristics of piezoelectric actuators, in-situ health prognostic and self-calibration of inertial sensors, as well as fabrication process, packaging technology and control circuit design for MEMS.
Yicheng Wang received his B.E. degree (2011) in Microelectronic Technology, Ph.D. degree (2016) in Microelectronics and Solid Electronics from the University of Electronic Science and Technology of China. He is currently working in Microsystem \& Terahertz Research Center as an assistant research fellow. He’s currently research field is MEMS technology.
References [1] Rezaeisaray M, El Gowini M, Sameoto D, et al. Wide-bandwidth piezoelectric energy harvester with polymeric structure. J Micromech Microeng 2015;25:015018. [2] Hosseini R, Hamedi M. Improvements in energy harvesting capabilities by using different shapes of piezoelectric bimorphs. J Micromech Microeng 2015;25:125008. [3] Williams RP, Kim D, Gawalt DP, et al. Surface micromachined differential piezoelectric shear-stress sensors. J Micromech Microeng 2017;27:015011. [4] Dezest D, Thomas O, Mathieu F, et al. Wafer-scale fabrication of self-actuated piezoelectric nanoelectromechanical resonators based on lead zirconate titanate (PZT). J Micromech Microeng 2015;25:035002. [5] Chidambaram N, Balma D, Nigon R, et al. Converse mode piezoelectric coefficient for lead zirconate titanate thin film with interdigitated electrode. J Micromech Microeng 2015;25:045016. [6] Benoit RR, Rudy RQ, Pulskamp JS, et al. Advances in piezoelectric PZT-based RF MEMS components and systems. J Micromech Microeng 2017;27:083002. [7] Lee N, Kim SJ. Effects of loading rate and temperature on domain switching and evolutions of reference remnant state variables during polarization reversal in a PZT wafer. Ceram Int 2012;38:1115–26. [8] Lin Y, Andrews C, Sodano HA. Enhanced piezoelectric properties of lead zirconate titanate (PZT) sol-gel derived ceramics using single crystal PZT cubes. J Appl Phys 2010;7644. 064108-064108-6. [9] Wang Z, Zhu W, Zhao C, et al. Dense PZT thick films derived from sol-gel based nanocomposite process. Mater Sci Eng, B 2003;99:56–62. [10] Bai Y, Wang ZJ, He B, et al. Enhancement of Polarization in Ferroelectric Films via the Incorporation of Gold Nanoparticles ACS. Omega 2017;2:9067–73. [11] Suchaneck G, Gerlach G. Polarization Fatigue of NiCr/PZT/BaPbO3 Capacitors on
Yu Chen received the B.E. degree and M.S degree in mechatronics engineering from the University of Electronic Science and Technology of China, in 2012 and 2015, respectively. He is currently working in China Academy of Engineering Physics. He’s currently research field is MEMS Packaging and MEMS integration technology.
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Solid State Electronics 163 (2020) 107675
D. Gong, et al. Tingting Yang was born in 1989. She received the B.S. degree from University of Electronic Science and Technology of China (UESTC) in 2012. Her Ph.D. degree was received in Materials Science and Engineering from Tsinghua University in 2017. She is an assistant research fellow in China Academy of Engineering Physics now. Her current research interests are focused on novel sensors, actuators and corresponding integration technology.
Xiangyu Sun received his B.E. degree (2011) in Microelectronic Technology, Ph.D. degree (2017) in Microelectronics and Solid Electronics from the University of Electronic Science and Technology of China. He is currently working in China Academy of Engineering Physics as an assistant research fellow. He’s currently research field is novel MEMS actuators and MEMS integration technology.
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