Double-beam Laser Interferometer Research Articles

Challenges in double-beam laser interferometry measurements of fully released piezoelectric films

When utilizing double-beam laser interferometry to assess the piezoelectric coefficient of a film on a substrate, probing both top and bottom sample surfaces is expected to correct the erroneous bending contribution by canceling the additional path length from the sample height change. However, when the bending deformation becomes extensive and uncontrolled, as in the case of membranes or fully released piezoelectric films, the double-beam setup can no longer account for the artifacts, thus resulting in inflated film displacement data and implausibly large piezoelectric coefficient values. This work serves to identify these challenges by demonstrating d33,f measurements of fully released PZT films using a commercial double-beam laser interferometer. For a 1 μm thick randomly oriented PZT film on a 10 μm thick polyimide substrate, a large apparent d33,f of 9500 pm/V was measured. The source of error was presumably a distorted interference pattern due to the erroneous phase shift of the measurement laser beam caused by extensive deformation of the released sample structure. This effect has unfortunately been mistaken as enhanced piezoelectric responses by some reports in the literature. Finite element models demonstrate that bending, laser beam alignment, and the offset between the support structure and the electrode under test have a strong influence on the apparent film d33,f.

Total Harmonic Distortion of a Piezoelectric MEMS Loudspeaker in an IEC 60318-4 Coupler Estimation Using Static Measurements and a Nonlinear State Space Model.

We propose a method to evaluate the Total Harmonic Distortion generated by a cantilever-based PZT loudspeaker inside an IEC 60318-4 coupler. The model is validated using experimental data of a commercial loudspeaker. Using the time domain equations of the equivalent electrical circuit of the loudspeaker inside the coupler and a state space formulation, the acoustic pressure response is calculated and compared to the measurement of the manufacturer. Next, the stiffness, transduction and capacitance nonlinear functions are evaluated with a Double-Beam Laser Interferometer (DBLI) and a nanoindenter on test devices and on the commercial loudspeaker. By introducing the nonlinear functions into the model as amplitude-dependent parameters, the THD generated by the loudspeaker is calculated and compared to the value provided by the manufacturer. The good agreement between the measurement and the simulation could allow for a rather quick simulation of the performance of similarly designed loudspeakers at the early stage of the design, by only estimating the static linearity of the main nonlinearity sources.

Studies on ferroelectric and nanomechanical response of single-layered PZT thick film for energy harvester applications

We have made an attempt to fabricate free standing warpage free PZT thick films of 100 µm thickness by tape casting method. The poly crystalline nature and nanoscale polarization switching of free standing thick film were confirmed using X-ray diffraction technique and piezo force microscopy, respectively. The piezoelectric coefficient (d33) of 100 µm free-standing Pb(Zr0.56Ti0.44)O3 [PZT] thick film was calculated using double beam laser interferometer and found to be 400 pm/V. From the results, it indicates that free-standing PZT thick films are suitable candidate for miniaturization of harvesting device by directly bonding them on microsystem components for microelectromechanical systems (MEMS) applications.

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 and pure ZrO2 films

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 (HZO) and pure ZrO2 films with a layer thickness of up to 390 nm prepared by chemical solution deposition (CSD) are investigated. The piezoelectric properties are measured using a double-beam laser interferometer (DBLI) and piezoresponse force microscopy. It is shown that for 100 nm thick films, the maximum remanent polarization is found for pure ZrO2 and reduces for the increasing hafnium content. A stable remanent polarization of 8 μC/cm2 is observed for ZrO2 film thicknesses between 195 and 390 nm. A piezoelectric coefficient of 10 pm/V is extracted from unipolar DBLI measurements. The observed thickness limitation for atomic layer deposition deposited HZO based ferroelectrics can be overcome by the CSD deposition technique presented in this work. Thick ZrO2 films are promising candidates for energy related applications such as pyroelectric and piezoelectric energy harvesting and electrocaloric cooling as well as for microelectromechanical systems.

Quantitative and high spatial resolution d33 measurement of piezoelectric bulk and thin films

A single beam laser interferometer based on a modified Mirau detection scheme with a vertical resolution of ∼5 pm was developed for localized d33 measurements on patterned piezoelectric films. The tool provides high spatial resolution (∼2 μm), essential for understanding scaling and processing effects in piezoelectric materials. This approach enables quantitative information on d33, currently difficult in local measurement techniques such as piezoresponse force microscopy. The interferometer is built in a custom microscope and employs a phase lock-in technique in order to detect sub-Angstrom displacements. d33 measurements on single crystal 0.67PbMg0.33Nb0.67O3-0.33PbTiO3 and bulk PbZrTiO3-5A ceramics demonstrated agreement within <3% with measurements using a double beam laser interferometer. Substrate bending contributions to out-of-plane strain, observed in thin continuous PbZr0.52Ti0.48O3 films grown on Si substrates is reduced for electrode diameters smaller than 100 μm. Direct scanning across room temperature and 150 °C poled 5 μm and 10 μm features etched in 0.5 μm thick PbZr0.52Ti0.48O3 films doped with 1% Nb confirmed minimal substrate contributions to the effective d33,f. Furthermore, enhanced d33,f values were observed along the feature edges due to partial declamping from the substrate, thus validating the application of single beam interferometry on finely patterned electrodes.

Electrode size dependence of piezoelectric response of lead zirconate titanate thin films measured by double beam laser interferometry

The electrode size dependence of the effective large signal piezoelectric response coefficient (d33,f) of lead zirconate titanate (PZT) thin films is investigated by using double beam laser interferometer measurements and finite element modeling. The experimentally observed electrode size dependence is shown to arise from a contribution from the substrate. The intrinsic PZT contribution to d33,f is independent of electrode size and is equal to the theoretical value derived assuming a rigid substrate. The substrate contribution is strongly dependent on the relative size of the electrode with respect to the substrate thickness. For electrode sizes larger than the substrate thickness, the substrate contribution is positive and for electrode sizes smaller than the substrate thickness, the substrate contribution is negative. In the case of silicon substrates, if the electrode size is equal to the substrate thickness, the substrate contribution vanishes, and the measured value of d33,f is equal to the theoretical value under the rigid substrate assumption.

High-rate sputtering of thick PZT thin films for MEMS

Crack and void free polycrystalline Lead Zirconate Titanate (PZT) thin films in the range of 5 µm to 10 µm have been successfully deposited on silicon substrates using a novel high rate sputtering process. The sputtered PZT layers show a high dielectric constant er between 1,000 and 1,800 with a moderate dissipation factor tan (δ) = 0,002 − 0,01 measured at f = 1 kHz, a distinct ferroelectric hysteresis loop with a remanent polarisation of 17 µC/cm2 and coercive field strength of 5.4 kV/mm. The piezoelectric coefficients d33,f = 80 pm/V are measured by using a Double Beam Laser Interferometer (DBLI). Based on this deposition process a membrane actuator mainly consisting of a SOI layer and a sputtered PZT thin film was prepared. The deflection of this membrane actuator depending on the driving voltage was measured with a white light interferometer and compared to the results of finite element analysis (FEA). With this approach a transverse piezoelectric coefficient of about e31 = −11.2 C/m2 was calculated, whereas all the other material parameters in the model were lent from PZT-5A.

Nonlinear Properties of the ZnO Thin Films and Their Experimentally Verification by Laser Interferometry

The paper is focused to the nonlinear electromechanical properties of piezoelectric thin films subjected to the high electric field. The influence of the electric filed on the piezoelectric body is presented by the change of the elastic and piezoelectric modules. Experimentally, the zinc oxide (ZnO) thin film samples of (002)-orientation were prepared by RF magnetron sputtering the Pt/Ti/SiO2/Si substrates. The thin film samples were investigated under high electric field excitations and corresponding strains. The samples exhibit frequency dispersion and nonlinear piezoelectric response, i.e. the nonlinear dependence of the mechanical displacement d 0 (or effective piezoelectric coefficient d 33,eff) on the electric field. In the frequency range (0.1 – 1.5) kHz, the remarkable frequency dispersion of the mechanical displacement d 0 was found on the ZnO samples. The characteristic nonlinearities and hysteresis are presented, using the data, measured on the PZT (60/40) (111) thin film sample. The nonlinear electromechanical properties of the ZnO thin films were investigated by original double beam laser interferometer at room temperature. The results are discussed and compared with already published data.

Electromechanical properties of lanthanum-doped lead hafnate titanate thin films for integrated piezoelectric MEMS applications

This paper focuses on the deposition and electromechanical characterization of lanthanum-doped lead hafnate titanate (PLHT) thin films as key material in piezoelectric microelectromechanical systems (pMEMS). PLHT (x/30/70) and PLHT(x/45/55) films with a thickness between 150 nm and 250 nm were deposited by chemical solution deposition (CSD). Thereby x varies between 0 and 10% La content. The electrical characterization shows that undoped (x=0) PLHT exhibit ferroelectric behavior similar to PZT of the same composition. La doping results in reduced ferroelectric properties and also affects the electromechanical properties. Measurements using a double beam laser interferometer yield a piezoelectric coefficient d 33 of 60 pm/V, which stays constant with an increasing electric field. This leads to a linear displacement compared to undoped PLHT or conventional PZT films used for MEMS applications.

Some Nonlinear Electromechanical Properties of Thin PZT and ZnO Compositions Deposited on the Si-Substrate

The paper deals with electromechanical properties of piezoelectric PZT (54/46) compositions near MPB, (001)-orientation, rhombohedral PZT (60/40) (111), and ZnO (002) thin films sputtered on the Pt/Ti/SiO2/Si substrates. The thin film samples were investigated under high electric excitations and corresponding strains. The samples exhibit frequency dispersion and nonlinear piezoelectric response (hysteresis), i.e. the nonlinear dependence of the mechanical displacement d 0 (or effective piezoelectric coefficient d 33,eff ) on the electric field. I the case of ZnO samples, the remarkable decrease of the mechanical displacement d 0 in the frequency range (0.1–1.5) kHz was found. These results are compared with data obtained on the PZT (60/40) (111) sample. The nonlinear electromechanical properties of the PZT and ZnO thin films were investigated using the original double beam laser interferometer at room temperature. The results are discussed and compared with the published data.

Thickness dependence of submicron thick Pb(Zr 0.3Ti 0.7)O 3 films on piezoelectric properties

The effects of thickness on the piezoelectric and electric properties of tetragonal composition, polycrystalline, (1 1 1)-textured Pb(Zr 0.3Ti 0.7)O 3 films are investigated. The effective piezoelectric coefficient d 33 is characterized by a double-beam laser interferometer and is measured to increase with film thickness although extrinsic contribution such as 90° domain wall motion is negligible from the nonlinearity of piezoelectric coefficient. Constituent parameters to affect piezoelectric coefficient such as polarization and dielectric properties were analyzed based on the semiempirical phenomenological equation. The effectiveness of poling was also evaluated as a function of film thickness. These results present that the increase of effective d 33 in these tetragonal PZT films can be mainly due to enhanced intrinsic contributions.

Thin Pb(ZrxTi1− x )O3 (PZT) Rhombohedral Compositions Deposited on the Si-Substrate and Its Non-Linear Piezoelectric Response

This paper presents some nonlinear piezoelectric properties and strain simulation in lead zirconate-titanate Pb(Zr x Ti1 − x )O 3 (PZT) thin films, sputtered on the Pt/Ti/SiO 2 /Si substrates. The samples PZT (60/40), (111) orientation, rhombohedral phase composition, and PZT (54/46), (001) near MPB were prepared. Thicknesses of the samples under test were estimated as 1.1, 1.3 and 2.2 μ m respectively. For first characterizations, X-ray diffraction (XRD), piezoelectric hysteresis, capacitance-voltage C(V) and strain vs. temperature dependencies were carried out. The effective piezoelectric coefficient d 33, eff ≈ 110 pm/V was found for PZT (60/40), and d 33, eff ≈ 70 pm/V for PZT (54/46) at room temperature, if the biasing electric field goes slowly from the one state to opposite state by 10 mHz, using the monitoring field of the amplitude 0.09 MV/m and frequency 1 kHz. The non-linear behaviour of the PZT thin film, as the effective d 33, eff vs. electric field was investigated by double beam laser interferometer and optical helium cryostat. Comparing our previously obtained data, the results are discussed. As consequences, the strain distribution in active thin PZT film area under top electrode was provided by FEM (ANSYS 8.0) and demonstrated in the contribution. Simulation of the dynamic deformation of the active thin PZT film area was presented during the conference ECAPD8.

Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices

We present a new measurement method to characterize piezoelectric thin films utilizing a four-point bending setup. In combination with a single- or a double-beam laser interferometer, this setup allows the determination of the effective transverse and longitudinal piezoelectric coefficients e31,f and d33,f, respectively. Additionally, the dielectric coefficient and the large signal electrical polarization are measured to add further important characteristics of the film. These data are essential for piezoelectric thin film process specification and the design and qualification of microelectromechanical systems devices.

Laser Interferometric Displacement Measurements of Multi-Layer Actuators and PZT Ceramics

This paper deals with the characterization of multi-layer actuators and bulk PZT ceramics under the influence of an applied electric field. The displacement profile of materials and devices with evaporated reflective dielectric layers was measured using a double-beam laser interferometer at room temperature. The displacements of 3 mm thick plates of bulk soft PZT were characterized to determine bulk material properties. The tested multi-layer actuator devices consisted of 20 layers, with each layer being 87 micrometers thick. The results of “point to point scanning” of displacement across piezoelectric materials are presented and the effective actuation area of the actuator displacement is determined. Comparisons are made of the behaviour of the bulk ceramics and the actuator. In addition to experimental measurement, representative coupled-field finite element model was constructed to compare with experimental x−y displacement data of multi-layer actuator.

Non-linear piezoelectric properties of the thin Pb(Zr xTi 1− x)O 3 (PZT) films deposited on the Si-substrate

In this paper some non-linear piezoelectric properties are investigated in lead zirconate-titanate Pb(Zr x Ti 1− x )O 3 (PZT) thin films, sputtered on the Si/SiO 2/Ti/Pt substrates. The thin PZT films were optimised by technology conditions (sputtering (Zr x Ti 1− x ) composition, PZT film thickness, buffer and seeding layers thickness). The significant piezoelectric response for PZT (60/40) and near MPB PZT (54/46) rhombohedral compositions, (1 1 1) and (0 0 1) orientations and thickness of 1.02–2.2 μm has been observed. The effective piezoelectric coefficient d 33 = 225 pC/N was found for high electric field of 10 MV/m and PZT (60/40) composition. The non-linear piezoelectric response, depending on electric field, frequency and temperature, was studied experimentally using an original double-beam laser interferometer and an optical cryostat. The temperature dependence of the thickness strain was investigated by laser interferometer in the temperature range −33 to 57 °C. The Pb(Zr x Ti 1− x )O 3-Si/SiO 2/Ti/Pt samples were prepared in the University of Valenciennes (France), and measured in the Laboratory of laser interferometry at the Technical University of Liberec (Czech Republic).

Effects of the top-electrode size on the piezoelectric properties (d33 and S) of lead zirconate titanate thin films

The effects of a decreasing top electrode size on the electric and piezoelectric properties of tetragonal Pb(ZrX,Ti1−X)O3 thin films are investigated. The effective piezoelectric small-signal coefficient d33,eff and the piezoelectric large signal-strain S are measured using a double-beam laser interferometer. Both properties are found to decrease rapidly with decreasing size of the used Pt top electrode for the investigated dimensions of 5mmto100μm edge length (square pads). While the loss of d33,eff is as high as 75%, the influence on the relative permittivity is only small. The source of the pad size effect on the measured piezoelectric properties is found to be the mechanics of the layered structure commonly used for piezoelectric measurements (Pt∕PZT∕Pt∕TiO∕SiO2∕Si), [PZT,Pb(Zrx,Ti1−x)O3] which is verified by finite element simulations.

Effects of ferroelectric switching on the piezoelectric small-signal response (d33) and electrostriction (M33) of lead zirconate titanate thin films

The effects of an increasing small signal amplitude on the piezoelectric small-signal response and electrostriction of tetragonal Pb(Zrx, Ti1−x)O3 thin films are investigated. The piezoelectric small-signal coefficient d33, piezoelectric large signal-strain S, and electrostriction coefficient M33 are measured using a double-beam laser interferometer. A continuously increasing influence of the small signal amplitude is found starting at very low values. In particular, the impact on the measured coercive field Ec is found to be stronger than the impact of the cycling frequency of the applied bias field. Also, unexpected electrostrictive behavior is investigated and explained by the influence of ferroelectric switching on the intrinsic piezoelectric lattice strain. Furthermore, the influence of an applied small-signal on the piezoelectric large-signal response is investigated.

Thickness Dependence of Piezoelectric Properties for PZT Thin Films with Regard to MEMS Applications

In this work we report on the investigation of the piezoelectric and ferroelectric properties of Pb(Zr,Ti)O3 (PZT) thin films with a composition of 45% zirconium and 55% titanium prepared by chemical solution deposition (CSD) regarding the major properties required in MEMS applications. Therefore we have measured the polarization and the piezoelectric coefficient d 33 and strain using a high-resolution double beam laser interferometer. We found that d 33 alike the strain is nearly independent of the sample thickness. To estimate the lifetime of MEMS we stressed the samples by applying switching and non-switching voltage pulses. In the non-switching case, no change of the material properties was observed whereas the use of switching voltage pulses resulted in a strong decrease of the piezoelectric properties and the polarization. Here we found a linear behavior between the material properties in the fatigued state and the sample thickness.

Short-time piezoelectric measurements in ferroelectric thin films using a double-beam laser interferometer

An evolution of the double-beam laser interferometer used for piezoelectric measurements in ferroelectric thin films is reported. Measuring the d33 hysteresis of a ferroelectric material using lock-in technique with large time constants requires a varying bias field to be applied to the sample over a long period of time. This long-term application leads to electrical stress during the measurement. We present a measurement technique using a different source for the applied field and a varied method for averaging the interferometric response. The measurement time for a complete d33 hysteresis will be shortened down to several seconds. Also, the cycle frequency becomes comparable to electrical hysteresis measurements. Experimental results on quartz and Pb(Zr(X),Ti(1−X))O3 are given to demonstrate the capabilities of the interferometer and the new measurement method.

Evaluation of Longitudinal Displacement for Lead Zirconate Titanate Films

An atomic force microscope (AFM) probing system was connected with a ferroelectric test system to measure the polarization hysteresis (P–E) curve and longitudinal displacement properties simultaneously with a large AC drive signal at 5 Hz, since this measurement system has the advantages of enabling dynamic evaluation of longitudinal displacement and direct comparison of the ferroelectric and piezoelectric properties. With decreasing the top electrode diameter from 500 to 30 µm, the longitudinal displacement direction changes from negative to positive since the longitudinal piezoelectric effect becomes dominant. The longitudinal piezoelectric constant calculated with a unipolar signal (+5 V, 5 Hz) was d33 =108 pm/V for a 30-µm-dia top electrode. This value is lower than that of bulk PZT but higher than that of reported data measured using the double-beam laser interferometer or the AFM with a small AC signal of about 1 kHz.