Thin Film Capacitors Cut from Single Crystals Using Focused Ion Beam Milling
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Thin Film Capacitors Cut from Single Crystals Using Focused Ion Beam Milling M. M. Saad, N. J. Donnelly, R. M. Bowman and J. M. Gregg Department of Pure and Applied Physics Queens University Belfast Belfast, N. Ireland United Kingdom ABSTRACT The Focused Ion Beam Microscope (FIB) has been used to fabricate capacitors from single crystals of BaTiO3 and SrTiO3 with electrode areas ~200µm2, and thickness of single crystal dielectric between 2µm and 500nm. Cross-sectional transmission electron microscopy revealed that during capacitor fabrication, the FIB rendered around 20nm of dielectric at the electrodedielectric interface amorphous, associated with local gallium impregnation. Such a region would act electrically in series with the single crystal and would presumably have a considerable negative influence on dielectric properties. However, annealing prior to electrode deposition was found to fully recover the single crystal, and homogenise the gallium profile. Some subsequent dielectric testing of SrTiO3 was performed yielding a room temperature dielectric constant of ~150 and loss tangent of 0.015 at 100kHz. A technique has therefore been demonstrated which allows fabrication of capacitors in which size-effects in ‘thin-films’ can be studied, without the influence of grain boundaries, and other issues associated with conventional thin film growth. INTRODUCTION Ferroelectrics at submicron and nanoscale dimensions demonstrate significant changes in their functional behaviour compared to bulk [1]. For example, thin film ferroelectrics suffer significant suppression in low-field dielectric constant, and this becomes a catastrophic collapse when film thicknesses are reduced to below 100nm [2,3]. Equally importantly, for non-volatile memory applications, polarisation characteristics degrade on decreasing dimensions, with a general trend to an increase in coercive field [4] and a reduction of remanent polarisation [5]. While such changes can be generically attributed to ‘size effects’, in fact reductions in size are often associated with changes in many other aspects of film architecture, and it is commonly impossible to deconvolute the influences of, for example: film thickness, reduced grain size, altered domain size and changes in domain configurations. This is well-illustrated by the debate on the origin of the parasitic ‘interfacial capacitance’ that is associated with the dielectric collapse at film thicknesses less than ~100nm mentioned above. Traditionally, in this debate the influence of grain size has been ignored since microstructures of most thin film systems are columnar. Grain boundaries therefore act electrically in parallel with grains, while functional analysis shows dielectric collapse to be related to components acting electrically in series with each other [1]. However, recent work [6] has shown that because grain size is usually intimately linked to film thickness, there is a distinct possibility that grain boundaries acting in parallel could mimic series capacitor behaviour. It would clearly b
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