Using High Frequency LIMM to characterize the poling state of piezoelectric ceramic thin films
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Using High Frequency LIMM to characterize the poling state of piezoelectric ceramic thin films Mark Stewart1 and Markys G Cain1 1 Functional Materials Group, National Physical Laboratory, Teddington, TW11 0LW, UK ABSTRACT The 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. INTRODUCTION The LIMM method was developed in the late 1980’s as a technique to measure the through thickness polarization of polymer films [1]. An intensity modulated laser is used to generate a thermal wave in the film and the resultant pyroelectric signal is measured using lock-in techniques. The depth sensitivity comes through varying the intensity modulation frequency and thus the penetration of the thermal wave. Although techniques such as PFM (Piezoresponse Force Microscopy) have excellent lateral x-y spatial resolution, the LIMM technique remains one of the few techniques with through thickness capability. Although initially developed for polymer films, the method has also been used on ceramic thick, [2], and thin films [3,4]. One of the challenges in using the technique on PZT thin films is that the thermal diffusivity, D, is higher compared with piezoelectric polymers such as PVDF, therefore the modulation frequency to achieve the same depth resolution increases. The penetration depth is given by x= √(2D/ω) [5] so to achieve a resolution of ~100nm PZT film, with a thermal diffusivity,[6,7] of ~4.5 x 107m2/s , frequencies of up to 15MHz are required. This poses severe constraints on the instrumentation, requiring lockin amplification into the 100’s of MHz range, high bandwidth and high sensitivity current amplifiers, and linear laser intensity modulation. In this work we have extended the frequency of the tests into the 10’s of MHz range to enable the characterization of films of thicknesses down to 100nm.
EXPERIMENT A series of thin PZT films were measured ranging in thickness from 100nm to 1000nm from three different sources. 100nm gold electrodes with a 10nm chromium adhesion layer were deposited using a shadow mask. Although gold is not the best electrode material choice for absorbing the laser radiation and thus introducing a thermal wave to the film, it was chosen because in practice it was easier to contact with the wire bonder. A less reflective electrode material such as a nickel-chrome thin film would increase the input [8], but the pyroelectric signal was sufficient to obtain a good signal to noise ratio using the gold electrodes. A 75mW, 785nm
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