Laser Intensity Modulation Method Research Articles

Measurement setup to simultaneously explore the location and energy of trapped charges in thin polymer films

This research proposes a unique system that combines charge density measurements by the laser intensity modulation method with optically excited current acquisitions using the photo-stimulated discharge technique (PSD). The purpose of this setup is to investigate the relationship between space charge properties (such as density, spatial depth, and time evolution) and the photocurrent-associated energies in order to gain new insights into the trap population and detrapping mechanisms in thin polymer films. This paper presents a description of the technical principles of both methods as well as the whole combined system. The results on a 12 μm-thick polyethylene naphthalate film show pyroelectric currents, which after processing indicate the formation of homocharges whose magnitude and depth decrease after light irradiation. The PSD currents allow the identification of two major energy bands at 3.4 and 5.9 eV (360 and 207 nm, respectively), possibly related to charge detrapping. In addition, current transients during constant wavelength irradiation show that incident photons can interact differently with trapped charges depending on the applied field.

Measuring 3D pyroelectric distributions with high resolution in thin films by a laser scanning microscope.

A laser scanning microscope for measuring 3D pyroelectric distributions inside thin vinylidene fluoride-trifluoroethylene copolymer films using the Laser Intensity Modulation Method was developed. The setup consists of a laser unit, a laser driver, an xyz-stepper motor unit, a transimpedance amplifier, and a lock-in amplifier. The focus lens at the laser unit is fixed by magnetic levitation and can correct a defocusing of the system or a tilt of the sample surface. It has been demonstrated in different samples that the system has a lateral resolution of 1μm for measuring the topological surface structure or the pyroelectric distributions. The self-developed laser driver and transimpedance amplifier combined with a fast lock-in amplifier are able to measure small pyroelectric currents and their variation inside a pyroelectric sample in the range of some 1 pA. The maximum measure frequency of 4MHz and the fast lock-in make it possible to measure the 3D pyroelectric distributions with high resolution. A 3D scan of 30 different layers with depths of 100 nm-5μm inside the sample and 100 × 100 points in the xy-direction per layer is performed in 3 days.

Space charge effect and application in solid insulation dielectrics

<p indent="0mm">Space charge effect and its application in solid insulation dielectrics are long-term issues with continuous development. The study of space charge effect is of great significance to high-voltage direct current (HVDC) transmission. This paper summarizes the development history and research progress of space charges in solid dielectrics, from the aspects of measurement technologies, damage, suppression, application, and simulation, etc. Currently, the most widely used space charge distribution measurement technologies can be roughly divided into two categories, acoustic effect technique and thermal effect technique, whose spatial resolution and application range are different. For the acoustic effect technique, pressure wave propagation method and pulsed electro-acoustic method are most commonly used with proven technique. The spatial resolutions of these two are similar, both can reach an average of <sc>10 μm.</sc> With extremely narrow pulses, the highest spatial resolution can reach about <sc>1 μm.</sc> The thermal effect techniques, such as thermal pulse method and laser intensity modulation method, have more advantages in spatial resolution and practicability, especially for micron-thick film detection. Future research may focus on the more widely application of space charge distribution measurement technologies to electrical equipment in operation. For the trap parameters and charge storage properties of insulation dielectrics, there is still a lack of reliable and practical measurement technologies. The results of the isothermal decay method are more reliable and credible, but the required measurement time is too long. Therefore, it has been rarely used in practice in recent years. The most commonly used methods, thermal stimulated discharge current method and photo stimulated discharge method, each has its own disadvantages. And their results for the same sample are quite different for mutual corroboration. In recent years, some researchers are more focused on piezo-stimulated current method, but this method is still in a preliminary stage of experiment and application. In general, the space charge effect has pros and cons. On one hand, space charge accumulation in solid insulation dielectrics may cause electric field distortion and discharge damage, eventually lead to breakdown and destruction of insulation. In order to prevent these damages, many effective methods have been proposed, such as surface modification of insulation dielectrics and material modification by additives. However, these methods have not yet been practically used in industrial applications. On the other hand, space charge effect also has many efficient applications. One of the most important applications is electret, a dielectric material that quasi-permanently stores real charges in surface or volume traps. A large number of significant contributions have been published on preparation process, charge characteristics and applications of electrets. The simulation research of space charges can mainly be divided into physical model research of space charge behavior and data processing of experimental results. The development of space charge simulation is relatively backward compared to other research fields as the physical model of space charge behavior has not yet been built completely. The issues of space charge effect and its application are involved with multiple factors. Although some technical and theoretical developments have been achieved in recent years, there are still some uncertain questions remained to be explored.

Sub-micrometer pyroelectric tomography of AlScN films

We report on one- to three-dimensional characterization of the pyroelectric properties of aluminum scandium nitride. By means of the laser intensity modulation method, we reconstructed the in-depth distribution of the spontaneous polarization with sub-micrometer resolution. The reconstructed profiles of the spontaneous polarization indicate that the thermal diffusivity and its temperature-dependence differ significantly from what is reported for pure aluminum nitride, which we attribute to the dominant role of phonon-alloy scattering for the heat transfer.

Local polarization switching kinetics in thin-film P(VDF-TrFE) (76:24) studied by time-resolved LIMM

Extending the well-established laser intensity modulation method (LIMM) by means of sequential frequency modulation represents a new approach for obtaining polarization depth profiles at both high spatial (nanometer) and temporal (millisecond) resolution. Thin films of the archetypal, organic, ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) were investigated in this manner to assess local switching kinetics. For subcoercive field conditions of 5 MV/m, the central regions of 2000 nm thick samples show a “bulk”-like, uniform polarization switching behaviour at a time-scale of 250 ms. In contrast, a 100 nm thick region near the metal-polymer interface reveals clear deviations from bulk switching kinetics. This likely reflects local charge generation/injection and trapping effects.

Space charge deposition in tubular channel ferroelectrets: A combined fluorescence imaging/LIMM study with finite element analysis

Ferroelectrets, i.e., soft materials with electric charges deposited on the surfaces of internal voids, are well known for their potential in transducer applications and energy harvesting. Due to their regular geometry and optical transparency, tubular channel ferroelectrets (manufactured by laminating polymer films around a polytetrafluoroethylene template which is later removed) are well-suited for studying the process of charge deposition. Understanding how space charges are formed on the internal surfaces will lead to improvements in the charge density and in the piezoelectric performance of these films. In this work, the inception voltage for dielectric barrier discharges (and hence the onset of charge deposition) was measured using two independent techniques, fluorescence imaging and the laser intensity modulation method (LIMM). The results (around 1.4–1.7 kV, depending on the void height) are in agreement within ±50 V. The internal electric field distribution was calculated using finite element analysis (FEA). Combined with Paschen's law, these calculations explained the experimentally observed discharge patterns, starting from the channel edges in thick samples, but glowing more uniformly in films with void heights of 50 μm or less. A time-dependent FEA simulation of the LIMM measurement reproduced the observed thermoelastic resonances and their effect on the LIMM signal, and explained its seemingly erratic behavior. This approach has great potential for analyzing LIMM and thermal pulse data obtained in inhomogeneous materials.

Space charge formation in polyimide films and polyimide/SiO2 double-layer measured by LIMM

For Polyimides (PI) used as insulating inter-layers or passivation layers in microelectronics and power electronics, reliability and failure mechanisms are intimately linked to phenomena such as non-linear conduction, distortion of field distribution and electrical ageing that have all some relation to space charge processes. Knowledge about these space charge distributions is therefore of importance to optimize structures and designs involving PIs. However, in the thickness scale of interest for such applications, of the order of 10 μm, no results were reported for PI, and only scarce studies for other materials. Space charge measurements carried out on PI using the laser intensity modulation method (LIMM) are reported in this work. PI-containing metal-insulator-semiconductor structures were pre-stressed under DC fields up to 2 MV/cm. The impact of the doping type of the substrate (either n-type or, p-type silicon) has been evaluated to address the mechanisms of space charge formation. The nature of electrode substrate substantially impacts the measured charge amount and charge nature, pointing towards charge injection as the origin of space charge build-up. The interpretation is substantiated by results obtained using a SiO2 layer as a barrier against electronic injection from the substrate into the PI layer.

Contributed Review: Review of thermal methods for space charge measurement.

The space charge accumulation phenomenon has garnered great interest over the last two decades because of the increased use of direct current in high voltage electrical systems. In this context, a significant relevance has been achieved by the thermal methods, used for solid dielectrics. This paper presents a review of this non-destructive measurement system used for the measurement of space charge. The thermal pulse method, the thermal step method, and the laser intensity modulation method are described. For each configuration, the principle of operation, the thicknesses analyzed, and the spatial resolution are described, reporting also the main related applications.

Thermal-pulse Method for Life Monitoring of Integrated Piezoelectric Transducers

In this work, we apply the thermal-pulse method to evaluate the polarization state of piezoelectric transducers. The pyroelectric current is measured in time domain and Fourier-transformed. The results are compared to frequency spectra determined by the laser intensity modulation method.

Numerical extraction of electric field distribution from thermal pulse method based on Monte Carlo simulation

Thermal-pulse method is a powerful tool for measuring space charge distributions in polymer films. The data analysis for thermal-pulse method involves the Fredholm integral equation of the first kind, which requires an appropriate numerical procedure to obtain a solution. Various numerical techniques, including scale transformation and regulation method, are proposed. Of those numerical methods, the scale transformation (ST) is the simplest and the most widely used method. However, it presents a high spatial resolution only near the sample surface. Monte Carlo (MC) method is one of the recently proposed ways to solve the equation numerically and has been successfully applied to the analysis of laser intensity modulation method data, which also involves the Fredholm integral equation of the first kind. In this paper we attempt to analyze thermal-pulse data in frequency domain with the MC method and discuss its effectiveness based on some numerical simulations. The simulation results indicate that the electric field profiles can be effectively extracted by the MC method. The computed profiles by the MC method consist well with the supposed distributions in the entire thickness of the sample, while the profiles reconstructed by the ST method fit very well to the supposed one at the vicinity of the target surface and distort sharply along the direction of the thermal pulse propagation in the sample bulk. On the other hand, the oscillations in the computed results by the MC method could deteriorate its accuracy in this study. The influence of noise level on the analysis based on the MC method is also tested by the use of the simulated data. The results show that the computed profiles would become more fluctuant as the noise level increases. This problem can be solved by selecting a larger value of tolerance during the singular value decomposition procedure. Thus, the value of tolerance is considered to be one of the key parameters in this algorithm, which is actually hard to determine. Additionally, the experimental data obtained from a polypropylene film under applied electric field are analyzed to illustrate the feasibility of MC method to be applied to the thermal-pulse experimental data. The results also show that the spatial accuracy by the MC method in the entire sample thickness is higher than by the ST method, which verifies that the MC method is more suitable for detecting the electric field distribution in the deep bulk of the sample. Owing to noise and error, the accuracy of MC calculation depends on the chosen tolerance value, which is now considered to be an obstacle in applying this method to the practical thermal-pulse measurement.

Space charge distributions in insulating polymers: A new non-contacting way of measurement.

A new technique for the determination of space charge profiles in insulating polymers is proposed. Based on the evolution of an existing thermal wave technique called Focused Laser Intensity Modulation Method ((F)LIMM), it allows non-contact measurements on thin films exhibiting an internal charge to be studied. An electrostatic model taking into account the new sample-cell geometry proposed was first developed. It has been shown, in particular, that it was theoretically possible to calculate the internal charge from experimental measurements while allowing an evaluation of the air layer appearing between the sample and the electrode when non-contact measurements are performed. These predictions were confirmed by an experimental implementation for two thin polymer samples (25 μm-polyvinylidenefluoride and 50 μm-polytetrafluoroethylene (PTFE)) used as tests. In these cases, minimum air-layer thickness was determined with an accuracy of 3% and 20%, respectively, depending on the signal-to-noise ratio during the experimental procedure. In order to illustrate the reachable possibilities of this technique, 2D and 3D cartographies of a negative space charge implanted by electron beam within the PTFE test sample were depicted: like in conventional (F)LIMM, a multidimensional representation of a selectively implanted charge remains possible at a few microns depth, but using a non-contacting way of measurement.

Using High Frequency LIMM to characterize the poling state of piezoelectric ceramic thin films

ABSTRACTThe Laser Intensity Modulation Method, LIMM, has been used to investigate the poling state of ceramic piezoelectric thin films. The frequency of the system has been extended to 70MHz to enable films of thicknesses down to 100nm to be measured. A unique development has been to sweep the DC bias applied to the sample whilst performing the LIMM measurement, thus giving pseudo PE loops. These PE loops are unique in that they represent the polarization state within a distinct depth of the film, whereas normally PE loops are a result of the complete film. This allows us to investigate processes occurring within different regions of the film.

Inverse Problem Solution for Laser Intensity Modulated Method

The laser intensity modulation method is based on absorption of laser generated heat, which diffuses into a thin sample as temperature waves and creates a non-uniform time-varying temperature distr...

Space charge, conduction and photoluminescence measurements in gamma irradiated poly (ethylene-2,6-naphthalate)

Polyethylene naphthalate (PEN) thin films were subjected to gamma rays at different doses and changes in both the dielectric and photophysical properties were investigated. Samples were irradiated in air at room temperature by means of a 60Co gamma source at a dose rate of approximately 31Gy/min. Total doses of 650kGy (344h) and 1023kGy (550h) were adopted in this work. The space charge profiles and residual electric field of virgin material and material exposed to these representative doses, obtained after 30min of depolarization following field application in the range 40–200kV/mm, have been examined by Focused Laser-Intensity Modulation Method (FLIMM). The dependence of space charge density with electric field values and irradiation dose is analyzed. Under high DC voltages and after irradiation, PEN tends to accumulate space charge. Space charge versus electric field and current-field characteristics issued from FLIMM method and conduction measurements exhibit good agreement. The high radiation resistance of PEN film is highlighted.

A Laser Intensity Modulation Method for the Evaluation of the Polarization State of Embedded Piezoceramics

In this work we evaluate the polarization state of PZT plates and rods embedded into low temperature co-fired ceramics, polymers and epoxy resin by measuring the pyroelectric current spectrum as a result of temperature oscillations generated by illuminating the sample surface with a square-wave-modulated laser beam. Changes in pyroelectric current amplitude and phase produced by small periodic perturbations around the equilibrium are related to the polarization state of the embedded PZT and to thermal losses governing the return of the system to the equilibrium. The latter allows evaluating the thermal contact of PZT with the embedding material by means of transfer functions H(iω) describing amplitude attenuation and phase lag at each modulation frequency.

Effect of trapped space charge on mechanical deformation induced by electric field

An organic insulating film of polyethylene naphthalene 2,6-dicarboxylate (PEN) was characterized by two complementary techniques: the mechanical deformation of the sample surface, induced by the application of an electric field, was obtained by an image correlation technique named DIC (Digital Image Correlation) and the space charge distributions in the thickness direction were measured by FLIMM (Focused Laser Intensity Modulation Method). The validation of the DIC method, which consists of a following of the successive positions of the surface patterns of the sample, is presented. To validate the algorithms and evaluate the performance of the technique, it was applied to computer-simulated speckle images. In a second step, the experimental system was applied to measure the mechanical deformations of a PEN film previously charged by an electric field in order to have space charge accumulation. This space charge accumulation was quantified by FLIMM. The results show that the mechanical behavior of the sample is modified by the presence of space charge.

Conductivity and space charge in LDPE/BaSrTiO3nanocomposites

Nano-sized BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles and a dispersant were incorporated in samples of low density polyethylene (LDPE). The nanoparticle loading was 2% or 10% by weight, and the dispersant loading was 4 parts per hundred relative to BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . dc conductivity measurements were made in the temperature range 30-70°C in vacuum, and in air at 30 °C. The vacuum dc conductivity in LDPE with 0.4% dispersant was approximately five times larger than in LDPE, but in LDPE containing 0.4% dispersant and 10% nanoparticles it was approximately a factor of 6-7 smaller than in LDPE. Since the conductivity of the latter composite was smaller than that of its least conductive component, it may be that the interface regions between the nanoparticles and the LDPE control the dc conductivity. The dispersant and the nanoparticles both increased ε', the real part of the relative permittivity. The ε' values for the nanoparticles, implied by the Lichtenecker-Rother equation for a three-part composite, were of order several hundred, much smaller than the value of at least 10,000 quoted by the nanoparticle suppliers. Space charge measurements were made in air using the Laser Intensity Modulation Method (LIMM), and the profiles calculated using the LIMM Monte Carlo method. The maximum space charge density adjacent to the negative poling electrode was 600 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , for the pure LDPE and the LDPE with dispersant, and 200 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> near the positive electrode. The corresponding maxima in samples with dispersant and nanoparticles were approximately an order of magnitude smaller. The Monte Carlo method is more accurate than other methods of analyzing LIMM data, and this may account for the very large densities close to the electrodes.

Nondestructive Evaluation of Polarization in LTCC/PZT Piezoelectric Modules by Thermal Wave Methods

This work investigates the polarization behavior of low-temperature cofired ceramic (LTCC)/PZT sensor-actuator-modules fabricated by packaging of ceramic PZT plates between alumosilicate-corundum LTCC green layers and subsequent sintering at elevated temperatures. Polarization evaluation was done by applying the laser intensity modulation method (LIMM) where a thermal wave travelling into the ferroelectric PZT ceramic plate is generated by an intensity-modulated laser beam. The penetration depth of the thermal wave was varied with the modulation frequency allowing the separation of pyroelectric response contributions at different regions from the surface to the frequency-dependent penetration depth of the thermal wave.

Use of scanning LIMM (Laser Intensity Modulation Method) to characterise polarisation variability in dielectric materials

The Laser Intensity Modulation Method (LIMM) has traditionally been used to characterise the depth dependence of polarisation of piezoelectric materials1. Although the technique is simple, it is difficult to extract the polarisation / depth data from the measured pyroelectric current because of the complex mathematics pertaining to the physics of the technique. However, the laser probe may still be used as a comparative or qualitative tool in mapping out the polarisation across the surface of a material. A novel scanning LIMM system has been developed to map the variation in piezoelectric activity across a range of samples. The system has been upgraded with a galvanometer mirror scanner to increase speed and reduce sensitivity to acoustic noise. The improvements are discussed and tested on a range of case studies. The technique can be used to show differences in piezoelectric activity caused by features such as inhomogeneous material composition, porosity and mechanical damage. The method has application as a quality control tool for materials and device manufacturers.

A Comparison of Three Techniques for Solving the Fredholm Integral Equation of the Laser Intensity Modulation Method (LIMM)

The Laser Intensity Modulation Method (LIMM) is widely used for the determination of the spatial distribution of polarization in polar ceramics and polymers, and space charge in non-polar polymers. The analysis of experimental data requires a solution of a Fredholm integral equation of the 1st kind. This is an ill-posed problem that has multiple and very different solutions. A number of methods have been used to solve this problem. In this paper, a comparison is made among three different techniques: the Scale Transformation Method, the Polynomial Regularization L-Curve Method and the Monte Carlo Method. The accuracy and ease of use of the three techniques are compared using both simulated and experimental data.