Split-tip Scanning Capacitance Microscopy (SSCM): Special Techniques in Surface Characterization and Measurements

Abstract

There has been a flurry of activity in growth of nanostructures, but our ability to measure and characterize them has not kept pace. This work invents and develops a new technique for electrical, electro-optical, and topographical characterization at the nanoscale. Split-tip scanning capacitance microscopy (SSCM) offers some advantages over other scanning probe methods. The dependence of the measurements on sample characteristics is reduced, and the analysis is simplified by having both electrodes secured to the probe. SSCM differs from the related, single tip AFM based capacitance microscopy versions that use the sample as one electrode so the properties of the sample contribute to the signal and complicate analysis. SSCM allows the imaging of simultaneous topographic, optical, and electronic structures. This feature allows non-conducting, as well as conducting surfaces to be imaged without loss of optical or capacitance (conductivity) resolution. The newly developed split-tip is a dual electrode probe that allows measurements in a non-contact manner. SSCM allows surface measurements without destroying the sample of interest. It does not require special surface preparation. To develop this new technique, the project focused on the following: -shear-force feedback as an accurate tip-sample distance controller -imaging techniques for irregular sample surfaces -development of computational model for simulating split-tip measurements -split-tip integration into a conventional near-field scanning optical microscope -contrast modeling for simple surface structures -tip-sample approach capacitance measurements as a stringent test of SSCM.

We show that a non-linear tip sample interaction dominates the shear force feedback signal evidenced by a change in the resonance frequency as the tip approaches the sample. Shear force feedback relies on a decrease in the amplitude of the signal at the operating frequency. The relatively new tuning fork based oscillations have large quality factors (Q) and relatively low resonance frequencies. This makes their time response very slow. We present data and a numerical model describing the time response and how this nonlinear interaction can be used to speed up the response. The temporal data indicate that by appropriate choice of operating frequency, the feedback loop can exploit the intrinsic rapid response of this nonlinearity (resonance frequency shift) to enable wide bandwidth and stable distance regulation for these systems. The shift of the resonance decreases the signal at the operating frequency instead of waiting for the amplitude to change. We demonstrate the imaging of irregular surfaces such as paint samples and show the distribution of pigment quantified by the peak in the histogram of optical signal versus separation at the nano- to micron scale illuminates the length-scale of failure in paint samples. We compare a high quality paint sample with one that fails a standard quality control test based upon visual inspection. NSOM provides the required nanometer to micrometer mesoscopic regime resolution and range, combined with simultaneous topographic and optical information. Features such as pigment clumping and pigment density fluctuations are simultaneously analyzed. Good samples are distinguished by maximum fluctuations at a small, but nonzero length while bad samples peak at the longest lengths studied. Individual pigment particles are observed near the polymer surface of both samples.

We develop a split-tip model that yields the capacitance across the split-tip and also gives related insights into the origins of the features and behaviors via related calculated values such as charge and energy density. We elucidate these properties using computational finite element methods for several simple examples. The results are a qualitative agreement with a simple parallel plate model. The model yields insights into resolution and field enhancement effects near the probe edges.

We describe the fabrication of the novel split-tip optical nanoprobe that is used in the SSCM setup. The split-tip nanoprobe can be used to both orient molecules with a strong, localized electric field and deposit them (prior work by M. Taylor et al.), and to measure capacitance, energy density, and charge (B. Clark et al.). The process for mounting this probe for integration in SSCM is also described; this mounting process allows reliable contact to be made to each probe electrode while meeting the stringent requirements for shear-force feedback with the probe. Data is collected from the split-tip with the use of a capacitance bridge circuit integrated into the scanning probe setup.

Lastly experimental measurements with the SSCM tie the above results together. Split-tip capacitance measurements with respect to tip-sample distance provide a critical test of the models and instrument capabilities. Approach capacitance measurements show the ability to distinguish between different sample surfaces by measuring the capacitance between the probe electrodes and how it varies with respect to the distance from the sample surface. We present approach capacitance measurements made on a sample comprised of aluminum structures deposited on a silica substrate grating. The experimental data is compared with the finite element model to gain more insights on the localized edge effects caused by field enhancements just under the split-tip probe.