Thick PZT Films Used to Develop Efficient Piezoelectric Energy Harvesters

By: Teresa Porter, Hong Goo Yeo, Susan Trolier-McKinstry

ABSTRACT

Piezoelectric energy harvesters using a bimorph structure are one way to eliminate the battery of small devices, including those used for medical applications. Thicker bimorph films consisting of piezoelectric Pb(Zr0.52Ti0.48)O3, also known as PZT, were evaluated to determine the efficiency of thicker films, specifically the surface condition and the generated power output. 5 – 6 micron thick PZT layers were deposited on a Ni foil substrate by rf magnetron sputter deposition; the film characteristics were assessed by low field dielectric characterization, x-ray diffraction, and an evaluation of the hysteresis loop corresponding to the sample. 3 μm thick PZT layers were grown at a time by increasing the deposition time during sputtering. After analysis, these thick samples showed strong {100} orientation, no pyrochlore second phase, and no micro- or macro-cracking within the microstructure.

INTRODUCTION

When designing any medical device worn by a patient, compliance of the user is critical. Willingness to wear any device for long times is likely to depend on weight and size of the device, its color or appearance, and especially the inconvenience of the device. A working device is not beneficial if the user will not utilize it. ASSIST (Advanced Self-Powered Systems of Integrated Sensors and Technology) is a National Science Foundation sponsored Nanosystems Engineering Research Center, and has as a main focus to eliminate the batteries for biosensor devices, allowing the devices to be more lightweight and hassle-free. The ASSIST’s vision is “To use nanotechnology to build miniature, self-powered, wearable and wireless sensing devices that can enable monitoring of personal health and personal environmental exposure and enable correlation of multimodal sensors” [1]. Piezoelectric energy harvesters are one potential energy harvesting methodology for such systems. There are several studies working towards improving the efficiency of the device, including its electromechanical coupling factor, efficient coupling of the human body, and even the location on the body where the device should be worn or installed.

PIEZOELECTRICITY What is Piezoelectricity?

Piezoelectricity is a term used to describe the transduction of energy between mechanical and electrical domains; it appears only in a limited number of materials. There are two different types of actions associated with piezoelectricity. Generator action converts mechanical energy into electrical energy. Sensor action occurs when electrical energy is converted into mechanical energy. There are several natural piezoelectric materials such as quartz, Rochelle salt, bone, cane sugar, and berlinite; man-made piezoelectric materials include barium titanate (BaTiO3), lead titanate (PbTiO3), and lead zirconate titanate (Pb(Zr, Ti)O3) among others [2].

HOW PIEZOELECTRIC MATERIALS WORK

When mechanical stress is applied to a piezoelectric material, dipoles are either created or changed within the material. Dipoles are the result of the separation of charges (positive and negative) within a unit cell and is typically due to the displacement of atoms. The resulting dipole moment per unit volume is known as the polarization. The amount of polarization change generated is directly related to the mechanical stress applied to the sample. The direction of the induced dipoles, which is a result of the type of mechanical loading, is also important. Both compression and tension of a material along a given axis will generate a change in polarization, however, the polarization will be opposite in sign.

Dipoles located near one another in a ferroelectric material tend to align together and form “Domains.” In a tetragonal perovskite, domains can orient their polarizations 90˚ or 180˚ from other domains [3]. For a ferroelectric film on a substrate of interest for the mechanical energy harvester, an out-of-plane polarization produces a greater output of energy than in-plane polarization. However, the fraction of out-of-plane polarization depends on the stress level applied to the sample by thermal expansion mismatch from the substrate. Samples with {001} orientation will have an out-of-plane polarization perpendicular to the film plane and in-plane polarization parallel to film plane.

Figure 1: Schematic of Domains in a

Polycrystalline Ferroelectric; the Direction of the Polarization is Denoted by an Arrow.

PZT, A PIEZOELECTRIC MATERIAL

Lead Zirconate Titanate (Pb (ZrxTi1-x) O3), also known as PZT, is a piezoelectric material. The value of x in this composition is typically set to the value that corresponds to the morphotropic phase boundary. For PZT, the morphotropic phase boundary occurs at x = 0.52, therefore, making the composition Pb(Zr0.52Ti0.48)O3. The morphotropic phase boundary separates the tetragonal and rhombohedral structures [4], [5]. The tetragonal and rhombohedral structures have different orientations for the dipole moments. The higher polarizability of this composition increases the power output for the energy harvester.

PZT can form two different structure types – perovskite and pyrochlore. Perovskite structures are piezoelectric, whereas pyrochlore phases are not piezoelectric. In order to produce pure perovskite structures without intermediate phase pyrochlore structures, specific conditions must be met during processing [6]. Perovskite structures contain large cations at the corners (Pb2+), smaller cations close to the body centers (Ti4+ and Zr), and anions near the center of the faces (O2-) [2]. The asymmetry of the perovskite structure creates a dipole moment, because the net positions for positive and negative charges from the cations and anions do not cancel out [3]. On the other hand, pyrochlore structures are centrosymmetric. Cations and anions are still present within the structure, but there is no net dipole.

PZT has a high induced polarization per unit applied mechanical stress, known as the piezoelectric charge constant. This means that under mechanical stress, PZT will generate larger changes in the dipole moments than piezoelectric material such as quartz. Another benefit of PZT is the large coupling factor [7]. The coupling factor reflects how effectively the mechanical energy can be converted into electrical energy [3].

Figure 2: PZT Perovskite Structure

The piezoelectric coefficient represents the amount of charge generated in a sample for a given applied stress. For thin films, the relevant piezoelectric coefficient is e31,f, meaning that the electric field applied to the sample is out-of-plane and the resultant stress is in-plane [8].

e , = (1)

In equation (1), d31 is the in-plane piezoelectric constant and sE

11 and sE12 are the elastic compliances.

Small mechanical stresses leading to large out-of-plane deformations yield an increase in power output. According to Trolier-McKinstry et al., {001} oriented films increase the piezoelectric coefficient and therefore enhance the power output [8]. ENERGY HARVESTING Batteries are useful in many applications for powering electronic systems, but there are challenges associated with batteries. They are heavy and require recharging, or in some cases, require replacement. When it comes to powering biosensor for health-care applications, energy harvesters are of interest. Energy harvesting, also known as energy scavenging, converts ambient energy to a useable form. Heat, fluid flow, and light are a few examples of potential energy sources. Within the human body, these sources include blood circulation, body movements, and muscle contractions [9]. Acceleration, applied force, vibration frequency, impedance matching, the mass of the device, the structural design, and materials all influence the performance of energy harvesters [10]. There are three basic steps to mechanical energy harvesting. The first step is to couple the harvester to the available AC stress. This stress is dependent on the location of the device relative to motion. For example, a harvester on the wrist, might undergo more rotational motion than compressive or tensile stresses. A sample applied at the elbow might

undergo compression when the elbow is flexed and tension when the elbow is extended.

Once the mechanical energy is coupled to the device, the next step is to convert the mechanical energy into electrical energy via a piezoelectric material. The effective transduction from mechanical energy to electrical energy is a result of the sample itself (i.e. the orientation of the sample, the thickness of the PZT, the amount of pyrochlore phase present, etc.). The final step to energy harvesting is the storage of the output energy [11].

PIEZOELECTRIC ENERGY HARVESTER Piezoelectric energy harvesters are of interest. When mechanical stress is applied to or released from a piezoelectric energy harvester, surface charges develop that can be collected and stored on a battery [12]. One of the main design parameters for an energy harvesters is the resonance frequency. The resonance frequency is the frequency which maximum deformation occurs.

ω = (2)

In Equation (2), keq is the equivalent spring constant, meq is the equivalent mass, and ω0 is the resonance frequency. The length of the sample, its stiffness, and mass all help dictate the resonance frequency. When the source vibrations match the resonance frequency, maximum power output occurs. However, even slight difference in the frequencies significantly drops the output power [13]. The PZT compliant mechanism structure reported previously was designed for a resonance frequency of 4.95 Hz [14]. Although this frequency is comparatively low, the frequencies of human body movements is around 1 Hz [15]. By changing the properties of the sample, the resonance frequency can be tuned to the desired magnitude. Increasing the mass or length of a sample, decreases the resonance frequency, whereas increasing the stiffness of a sample, increases the resonance frequency. On-going studies are trying to generate low resonance frequency samples that are also small and lightweight [7]. PIEZOELECTRIC COMPLIANT MECHANISM

One approach for a small, lightweight, low frequency harvester is a piezoelectric compliant mechanism (PCM) is used. The PCM consists of three parts: a PZT sample, two U-shaped forms with zero stiffness hinges, and a compliant hinge connecting the PZT beam to the frame with the proof mass [14]. On

excitation, the PZT is strained uniformly, which increases the power output. In contrast, a proof mass cantilever, in which the sample is fixed by a clamp on one side but is free to move on the other end, produces maximum deformation only near the clamped region. Overall, the PCM produced approximately 4 times more power than the proof mass cantilever and had a 100% mode shape efficiency compared to the efficiency of cantilevers [16, 14].

Figure 3: Top and Side View of Piezoelectric Compliance Mechanism from [14].

ONGOING PROBLEMS WITH PIEZOELECTRIC ENERGY HARVESTERS

There are several different problems associated with piezoelectric energy harvesters. First, motion from the human body is neither periodic nor constant over the course of a day. During a sports outing, an individual is consistently moving. However, while watching television, sitting in class, or even sleeping, the individual tends to be less active, and therefore produce less energy [16]. An irregular power source affects the use of the device.

Secondly, the output energy as a result of the piezoelectric energy harvesters is often lower than desired. For example, Roudy et al. report for their piezoelectric devices that the output voltage is around 3-8 V [7]. Higher voltages, and/or higher currents are preferred.

One way to increase the power output of piezoelectric energy harvesters is to grow thicker films. Just as a bimorph PZT structure outputs more usable energy than a unimorph structure, thicker films also provide more output energy for many structures [17]. As the thickness of the sample increases, the stiffness

of the sample also increases. This stiffness causes the PZT to have a decrease in deformation [18]. Since deformation has a direct relationship to the amount of generated power, one might think that thicker films would be less effective due to a decrease in deformation. In fact, although the samples is stiffer, thicker films increase the amount of generated power due to the increase in the volume of the piezoelectric phase [19].

One potential challenge is that thicker films could crack under mechanical loading. However, the limits in strain at which cracking is observed are strongly dependent on the details of the film microstructure. In this experiment, 5 to 6 μm thick PZT was grown on each side of Ni foil with the buffer layer, HfO2, and seed layer LNO. The performance and stability of the sample was analyzed.

Figure 4: Bimorph PZT Structure

EXPERIMENTAL METHODOLOGY

FABRICATION OF SAMPLE

FLEXIBLE PASSIVE LAYER The first step to fabricating piezoelectric

samples is to generate a flexible passive layer. This layer serves as the substrate onto which the thin-film is deposited, lowers the resonance frequency, and enables larger displacements [8]. Although the substrate could be a variety of materials, metal foils have several benefits over other substrates, such as silicon, including how easily metal can be machined and plastic, rather than brittle, responses to large stresses. In this work, Ni foil was chosen. Nickel foil has a larger coefficient of thermal expansion than silicon, producing more compressive stresses in the PZT film and resulting in out-of-plane polarization. This out-of-plane polarization helps to generate an increased figure of merit for energy harvesting [14, 8]. The passive layer also serves to shift the neutral axis of the sample. The neutral axis is a plane within the material, that when the material is bent, experiences no compression or tension. For a sample without a passive layer, the neutral axis is in the PZT itself, and the PZT experiences both compressive and tensile forces. These forces

produce opposite polarizations that cancel out, lowering the net polarization. The passive layer shifts the neutral axis out of the PZT. For this experiment, a 25 μm nickel foil passive layer was used.

SEED AND BUFFER LAYER

PZT is not thermodynamically stable in contact with Ni. Thus, a buffer layer can be used both to improve the quality of the crystallization and to minimize the diffusion of oxygen to the nickel foil during annealing [6, 14]. In this work, HfO2 deposited by atomic layer deposition (ALD) at 200˚C was used as the buffer [14, 20]. ALD deposits a continuous layer of HfO2 on the Ni foil. There is a tradeoff with the thickness of the buffer layer, as thicker HfO2 layers provide more of a barrier to the diffusion of oxygen, yet thicker layers are also more susceptible to cracking. This cracking is a result of a difference in the coefficient of thermal expansion between the Ni foil and HfO2 [21].

Although the passive elastic layer is beneficial to the performance of the sample, crystallization of the perovskite layer can be a challenge, especially at low temperatures. One way to overcome this obstacle is to provide a seed layer that initiates the nucleation. The desired seed layer should have an orienting structure similar to the structure of the thin film. One of the common seed layers for PZT is LaNiO3 (LNO), a perovskite metal oxide which often deposits on flat surfaces with a preferred {001} orientation. The seed layer was spin-coated on each side of the foil using a 0.2 M LaNiO3 precursor and crystallized by rapid thermal annealing (RTA) at 700˚C for 1 minute in air [14, 20]. SPUTTERING

Sputtering is one of the methods to produce thin films. The benefits of sputtering over other methods (see Table 1) include film composition control, high growth rates, uniformity, sample stability, and time-efficiency [22].

Sputtering is an effective process to grow thin film piezoelectric materials; however, there are numerous process parameters that can affect the quality of the resulting layers. These include substrate temperature, chamber pressure, oxygen flow, and the amount of excess lead. In this experiment, a Kurt Lesker CMS-18 rf magnetron sputtering tool was used; the deposition conditions are specified in Table 2 [14].

Substrate temperature significantly impacts the quality of the film. At low substrate temperatures, the evaporation of lead (Pb) atoms decreases, but both perovskite and pyrochlore phases are present. Pure

perovskite structures only occurs at temperatures greater than 450˚C [23]. Due to PZT’s crystallization temperature around 500˚C [24], a high temperatures step is required to grow pure perovskite PZT. Table 1: Advantages of rf magnetron sputtering at high temperatures [20]

Type of Deposition

Advantages Disadvantages

Chemical Solution Deposition (CSD)

Orientation, Dense

Need multiple spin-coating for > 3 μm PZT film

RF Magnetron Sputtering at Room Temperature and ex-situ crystallization

Large sample size or deposition on multiple smaller substrates at the same time

Hard to reduce porosity and to control nucleation sites; long processing time; cracking

RF Magnetron Sputtering at

High Temperature

for in-situ Crystallization

Orientation, Dense, thick films are possible

Optimization of conditions;

slow deposition rate (time-consuming)

Table 2: Old Deposition Conditions for rf magnetron sputtered PZT

Target: PZT(52/48) + 10% excess PbO

Buffer Layer: HfO2

Seed Layer: LaNiO3 (LNO) Substrate: LNO/HfO2/Ni/HfO2/LNO Chamber Pressure: 8 mtorr Gas: Ar +3% (10% O2 + 90%

Ar) RF Power: 88 Watts Time: 60,000 seconds for 2 μm Substrate Temperature 550-650 ˚C *

Figure 5: D.C. Sputtering (a) vs. Magnetron Sputtering (b)

One of the main goals during sputtering of PZT

is to reduce the amount of pyrochlore phase present. The pyrochlore phase decreases the performance of the sample and reduces energy output. Increasing the chamber pressure, decreasing the percent of oxygen present, and increasing the sputtering deposition rate are all ways in which the amount of pyrochlore phase present in the sample can be decreased [24, 22, 23]. For this experiment PZT (52/48) with 10% excess PbO was used as the target. The additional lead in the target ameliorates the problem of lead depletion during post-deposition annealing. The depletion of lead creates vacancies within the perovskite structure, and favors the formation of pyrochlore phases. With the additional lead in the sample, post-deposition annealing forms a single perovskite phase [5].

Since nickel and PZT are two different structures, they exhibit different coefficients of thermal expansion. As a result, when PZT was deposited on only one side of the nickel substrate, a curvature in the sample develops after crystallization. To reduce this curvature, the PZT should be deposited on both sides of the nickel substrate [14]. This is known as a bimorph structure.

In this experiment, 3 μm thick PZT layer was deposited onto the substrate during one deposition

step time. This is an increase from the 2 μm layer previously deposited by the Trolier-McKinstry research group. In practice, this was achieved by increasing the dwell time from 60,000 seconds to 90,000 seconds. This is reflected upon in the new deposition conditions specified in Table 3. Using a longer deposition time, each sample will only have to be sputtered a total of 4 times, 2 per side at a thickness of 3 μm thick each time. It was hypothesized that the film quality would improve if the substrate temperature was increased by 10-20˚C for each subsequent deposition [20].

Table 3: Old Deposition Conditions for rf magnetron sputtered PZT

Target: PZT(52/48) + 10% excess PbO

Buffer Layer: HfO2

Seed Layer: LaNiO3 (LNO) Substrate: LNO/HfO2/Ni/HfO2/LNO Chamber Pressure: 8 mtorr Gas: Ar +3% (10% O2 + 90%

Ar) RF Power: 88 Watts Deposition Time: 90,000 seconds for 3 μm Substrate Temperature 550-650 ˚C *

RAPID THERMAL ANNEALING (RTA)

When sputtering, at lower temperatures, the atoms will not have enough energy to move around and eventually find their low energy crystallization position. Therefore, in low temperature conditions, additional processes must be introduced in order to crystallize the film. There are two methods to fully crystallize the sample. First, after film growth, the sample can undergo rapid thermal annealing (RTA). During RTA, the substrate is heated to approximately 700˚C, enough that the atoms can move around [24]. After the sample is heated to high temperatures, the sample is crystallized. Thus, annealing improves ferroelectric properties and crystallization of the sample [4, 6]. RTA also rids the sample of impurities and space charges [22]. The second method to properly crystallize the film is to provide more energy during growth so that the atoms can find their low-energy crystallization position during growth itself.

POLING Poling is the process in which a ferroelectric

ceramic can be made piezoelectric. In order to do this, an electric field is applied when the material is below the Curie temperature. During this process, the electric field will cause domains aligned with the field to grow. Domains that are not aligned will shrink. This creates, a larger degree of polarization as shown in Figure 3 [3]. To increase power output of bimorph harvesters, the upper and lower PZT layers can be poled in opposite directions [14].

ANALYSIS OF THIN FILMS

FILM STRUCTURE ANALYSIS After sputtering, electron microprobe analysis

was used to examine the stoichiometry of the sputtered films. The results of this analysis revealed that some samples had a depletion of lead within the grown film. The resulting vacancies within the structure produce a pyrochlore structure instead of the desired perovskite structure.

To further analyze each sample, x-ray diffraction (XRD) (PANalytical XPert Pro MPD) and field-emission scanning electron microscope were used [24]. X-ray diffraction can be used to characterize the phases that are present. X-ray diffraction also helps assess the crystallographic orientation of the film; field-emission scanning electron microscopy was used to measure the thickness of the films, as well as the grain size [25].

HYSTERESIS ANALYSIS

For samples with top and bottom electrodes, the dielectric constant was analyzed by an impedance analyzer by measuring the capacitance and using Equation (3):

C = (3) In this equation, Cp represents the capacitance, ε0 is the dielectric constant of a vacuum, εr is the dielectric constant, A is the electrode area, and t is the thickness of the PZT.

Secondly, a Radiant Multiferroics tester was used to evaluate the polarization – electric field hysteresis loop, specifically the remanent polarization, maximum polarization, and coercive field. PZT films with an {001} orientation on Ni foil typically have a remanent polarization and coercive field around 35-40 μC/cm2 and 65 kV/cm, respectively [8, 14]. The tangent loss is also around 3% at 10 kHz [14]. These measurements can be used later to assess the quality of the piezoelectric sample.

Figure 6: Polarization – Electric Field Hysteresis Loop RESULTS AND DISCUSSION

Figure 7 shows an XRD pattern of a unimorph

sample with 6 μm thick PZT deposited under the original deposition conditions (60,000 second deposition time) This thick sample was grown by depositing 2 μm thick layers, a total of three times per side. Although this analysis shows that the films are highly oriented in the {100} direction, there is some unknown peaks, which may be from a pyrochlore phase. These peaks suggest that the sample was not heated to a high enough temperature during the deposition.

Figure 7: XRD Analysis of Unimorph Sample with 6 μm

Thick PZT In Figure 8, a bimorph sample had 4 μm thick PZT deposited on both sides of the Ni foil by the old deposition conditions. Two PZT layers of thickness 2 μm were deposited with a deposition time of 60,000 seconds each. After examining the XRD analysis, it is shown that there is very little pyrochlore phase present and the sample is highly oriented.

Figure 8: XRD Analysis of Bimorph Sample with 4 μm

PZT

A 5 μm thick PZT film was grown on both sides of Nickel foil. This film had two separate layers of PZT deposited. The first layer was deposited by the old deposition condition, in which the deposition time was 60,000 seconds. This layer had a thickness of 2 μm. A second PZT layer was deposited on this film by the new processing conditions with a deposition time of 90,000 seconds. This layer was 3 μm thick. After each PZT layer was added, the substrate temperature was increased to account for the additional film thickness. For this sample, the temperature was increased from 595˚C to 630˚C.Figure 9 show an XRD analysis of one of the samples after 5 μm thick PZT was deposited at 630˚C. This X-ray pattern shows that the films are highly oriented in the {100} direction. There is little, if any, pyrochlore second phase present in the sample.

Figure 9: XRD Analysis of 3 μm Thick PZT Layer

Additional samples were grown with the same increased deposition time and the corresponding XRD data are comparable to those in Figure 9.

Using a field-emission scanning electron microscope, the microstructure of the top surface of the samples was examined. The results are shown in Figure 10. Figure 10A represents a 6 μm thick PZT sample after depositing 2 μm thick layers three times according to the old deposition conditions. Figure 10B represents a 5 μm thick PZT sample under the new deposition conditions.

Sample A shows large grooves at the surface of the film. These grooves are problematic when it comes to applying an electric field to pole the energy harvesters. When an electric field is applied at the sharp points within a groove, dielectric breakdown is likely to occur. This would result in destruction of the film. The presence of the deep grooves could be an indication of volatilization of lead. In order to fix the grooving of this sample, sol-gel capping is required. This process will help smooth the top layer of the PZT and eliminate some of the grooving. Some micro-cracking is present within Sample A, but there are no macro-cracks evident.

Sample B shows little grooving within the sample and no evidence of micro- or macro- cracking. On many of the grains, a four-fold rotational symmetry is observed, which is consistent with {100} orientation. Overall, the surface morphology based on the FESEM is consistent with the XRD analysis of the samples.

Figure 10: Field-Emission Scanning Electron Microscope (FESEM) image shows grain size and porosity of a 6 μm

thick PZT sample (A), and a 5 μm thick sample under new deposition conditions (B).

Further analyses are required to determine if samples grown under the new deposition conditions lead to a greater output power. This information will be reported when it is available. ACKNOWLEDGEMENTS I would like to give a special thank you to my principal investigator, Dr. Susan Trolier-McKinstry, and my graduate student Hong Goo Yeo. This research was made possible by the ASSIST REU and SROP Program and was funded by the National Science Foundation under Grant No. EEC 1160483.

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