Femto-current Spreading Resistance Imaging

Spreading resistance imaging (SRI) is very productive SPM method used for various surface investigations like finding defects in conductive and weakly conductive layers, common characterization of investigated surfaces in terms of local resistance, getting I-V curves and "I-V volume"(I-V curve in each point of scan) and etc. This technique consists in using conductive cantilever and bias voltage applied to the tip. The result is simultaneous measuring of topography in Constant Force mode and current in each point of scanned area. Thus apart from surface topography the lateral Spreading Resistanse distribution is being obtained. The typical application of SRI technique is for example finding the real dimensions between source and drain of metal-insulator-semiconductor transistors, depth of occurrence of p-n junctions, finding the distribution of dopant concentration in semi-conducting stuctures.Also apart from semiconductors investigation this technique enables to measure local electrical properties of various conducting materials like metal alloys and different combined materials, finding of distribution of conductive and non-conductive regions on surface investigated (see Fig.1, Fig.2).The basic advantages of SRI are high resolution (nanometer scale) and the fact that this method as well as any SPM technigue is non-destructive way of getting high quality Spreading Resistanse distribution image of sample investigated.

Our company also produce and offer conductive cantilevers with Pt , TiN and W₂C coatings needed for such kind of measurements. Pt-coated cantilevers have better conductivity and wear resistance, but are less stable at high applied voltages (voltages < 1 V are recommended). TiN and W₂C coated tips can be used at voltages up to 10 V.

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Fig. 1 Topography and current pictures ( a) and b) accordingly ) obtained on Cu-CrN alloy with voltage applied U = 0.1V.

Fig. 2 Topography and current pictures accordingly obtained on TiSi2 film grains of conducting phase are clearly seen on SRI image. Voltage applied is 0.02V.

The newest NT-MDT development in SRI technique is low-current adjustment unit for Solver P47 multi-technique SPM system (see Fig.3) that allows measuring of ultra-low currents (from 30 fA up to 100 pA). Typical noise level is about 25 fA (RMS).

Fig. 3 AU020 low-current adjustment unit.

This new unit enables to carry out investigations of highly resistive materials (such as thin dielectric layers on semiconductors, DLC and piezo- films, conductive polymers, etc.) which were beyond reach of SRI before. Typical examples of characterisation of thin silicon oxide film inhomogenities are represented on Fig.4 and Fig.5.

Fig. 4 Low-current Spreading Resistance image of 2 nm thick SiO₂ layer showing localized electrical breakdown defect. Tip-sample bias voltage is 5V.Maximal current at defect point is about 0,6 pA.

Fig. 5 Low-current Spreading Resistance image of the same sample with low conductivity area. Difference of currents detected in defect and normal areas is about 100 fA.