Sensitivity and Resolution Limits in Scanning Capacitance Microscopy
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Sensitivity and Resolution Limits in Scanning Capacitance Microscopy
tefan Lányi Institute of Physics, Slovakian Academy of Sciences, Dúbravská cesta 9, Bratislava, Slovakia Miloslav Hrukovic Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Bratislava, Slovakia ABSTRACT
The capacitance detection techniques, applicable to Scanning Capacitance Microscopes, have been analyzed from the point of view of signal-to-noise ratio that finally affects the achievable lateral resolution. It was found that comparable sensitivities can be achieved from relatively low frequencies below 1 MHz up to the GHz region. On conducting surfaces, resolution better than 5 nm has been achieved. It is limited by the minimum probe-to-sample distance, below which large tunneling current would occur. Increasing the applied voltage above a few volts increases the sensitivity at the cost of lateral resolution, since, at high voltage and small probe-sample distance, field emission could occur. INTRODUCTION
Scanning Capacitance Microscopes (SCM) [1-4] are becoming interesting tools for imaging the morphology and analysis of structures on the surface of conductors, either free or coated with insulating films, and of buried structures in semiconductors and dielectrics. They are considered as most promising to fulfill the requirements of semiconductor industries on the analysis of next generations of integrated circuits [5]. Among others, their applications comprise the nanometerscale measurement of free carrier concentration in semiconductors [6] or the delineation of
pn
junctions in integrated circuits [7]. The SCM uses a sharp conducting tip, placed in the proximity of a conducting surface or a conducting substrate covered by a thin insulator, as a probe. Between the tip and the conducting substrate a small capacitor is formed. Its capacitance depends besides on geometry also on the dielectric constant of the material between the electrodes. By raster scanning the probe, an image of surface topography or of the topography modified by local changes of dielectric constant can be obtained. Three groups of capacitance sensors applied in microscopes can be distinguished. The most popular is the use of the RCA CED VideoDisc pickup[8]. It contains a strip-line resonator, coupled to an oscillator oscillating at 915 MHz, a frequency on the flank of the resonant curve of the resonator. It is fine-tuned by the tip/sample capacitance, modulating the RF voltage in it. A similar approach using lumped element circuits was employed in paper [1] at 90 MHz. In reference [4] phase-sensitive measurement of the current flowing through the tip/sample capacitance at 2 MHz was introduced, offering the possibility to separate the real and imaginary (loss) components of the capacitance. The third possibility is measuring the electrostatic force between the sample and a cantilever, to which a dc or ac voltage is connected [9]. The lateral resolution of capacitance microscopes was usually estimated from resolved features in im
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