Space charge cartography by FLIMM: a three-dimensional approach

Abstract

A technique for three-dimensional cartography of space charges profiles inside polymer insulating samples is proposed. Called focused laser intensity modulation method (FLIMM), it is derived from the well-known LIMM method, with an additional possibility of being able to focus the laser spot on the surface of the sample to be studied. The processed data is a short-circuited pyrolectric current collected between the electrodes and induced by the interaction of the charges with the periodic variations of temperature produced by the laser source. The focused aspect of our method requires a good three-dimensional modelling of the spatial evolution of this thermal gradient. Under these conditions, the treatment of the equation of heat propagation is carried out using simultaneously a double Fourier transform and Green functions. In association with the numerical simulations of this solution, a two-dimensional scanning of the beam on polyethylene test sample surfaces shows that one can get three-dimensional representations of space charge shapes with a lateral resolution lower than 10 μm and for a depth of analysis typically included in the range 1–100 μm.

Polarization imaging in ferroelectric polymer thin film capacitors by pyroelectric scanning microscopy

“…[20][21][22][23] PSM has also been used to image domains 14,24 thermally written polarization patterns, [25][26][27][28] and to follow polarization and domain dynamics. [29][30][31][32] Moreover, 3D polarization information can be obtained from crystals and thick films by combining 2D laser scanning with depth profiles obtained using either pulse time-of-flight methods, 33 or Laser Intensity Modulation Method (LIMM), [34][35][36][37][38] or both. 39 One key advantage of PSM over techniques based on atomic force microscopy (AFM) is that the optical probe is noninvasive and does not damage or alter the sample.…”

“…14,39 Optimizing the resolution of PFM requires careful attention to both optical and thermal limitations in conjunction with a 3D thermal model. [34][35][36]43 With these considerations in mind, we have designed a PSM system with much improved resolution by using a violet laser and high f-number microscope objective to reduce the optical spot size, and by operating at high modulation frequencies to reduce the thermal diffusion length. The results of these imaging studies agree well with the predictions of a thermal model implemented using finite element analysis (FEA).…”

“…4(a). 36 The calculations were made on a 5 nm to 50 nm size mesh, at four modulation frequencies from 0.1 MHz to 1.8 MHz, using the assumptions described above. The results were not sensitive to the mesh size.…”

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

“…27 Samples used are generally thin films of polymers metallized on both sides and placed in a cell for measurements and electromagnetic shielding. It is possible to add an external DC bias voltage V ht (positive or negative) in order to study its influence on the behavior of the internal space charge in various configurations (applied external electric field, effect of bias polarity, charge duration, etc.…”

Thermal-Pulse Tomography of Space-charge and Polarization Distributions in Electret Polymers

“…As laser beams can be brought to a tight focus by suitable optics, the focused LIMM method has the potential for high lateral resolution. However, its relatively slow data acquisition speed often required a compromise between full depth-resolution with a limited number of in-plane data points [13], or larger, high-resolution area maps at selected modulation frequencies [12,14]. Thermal-pulse measurements, however, can be carried out up to 50 times faster than a comparable LIMM scan, as was recently demonstrated in a direct comparison [15] of LIMM and TP which showed excellent agreement between the two techniques ( Fig.…”