A scanning microwave microscope (SMM) for spatially resolved capacitance measurements in the attofarad-to-femtofarad regime is presented. The system is based on the combination of an atomic force microscope (AFM) and a performance network analyzer (PNA). For the determination of absolute capacitance values from PNA reflection amplitudes, a calibration sample of conductive gold pads of various sizes on a SiO(2) staircase structure was used. The thickness of the dielectric SiO(2) staircase ranged from 10 to 200 nm. The quantitative capacitance values determined from the PNA reflection amplitude were compared to control measurements using an external capacitance bridge. Depending on the area of the gold top electrode and the SiO(2) step height, the corresponding capacitance values, as measured with the SMM, ranged from 0.1 to 22 fF at a noise level of ~2 aF and a relative accuracy of 20%. The sample capacitance could be modeled to a good degree as idealized parallel plates with the SiO(2) dielectric sandwiched in between. The cantilever/sample stray capacitance was measured by lifting the tip away from the surface. By bringing the AFM tip into direct contact with the SiO(2) staircase structure, the electrical footprint of the tip was determined, resulting in an effective tip radius of ~60 nm and a tip-sample capacitance of ~20 aF at the smallest dielectric thickness.
The scanning microwave microscope is used for calibrated capacitance spectroscopy and spatially resolved dopant profiling measurements. It consists of an atomic force microscope combined with a vector network analyzer operating between 1–20 GHz. On silicon semiconductor calibration samples with doping concentrations ranging from 1015 to 1020 atoms/cm3, calibrated capacitance-voltage curves as well as derivative dC/dV curves were acquired. The change of the capacitance and the dC/dV signal is directly related to the dopant concentration allowing for quantitative dopant profiling. The method was tested on various samples with known dopant concentration and the resolution of dopant profiling determined to 20% while the absolute accuracy is within an order of magnitude. Using a modeling approach the dopant profiling calibration curves were analyzed with respect to varying tip diameter and oxide thickness allowing for improvements of the calibration accuracy. Bipolar samples were investigated and nano-scale defect structures and p-n junction interfaces imaged showing potential applications for the study of semiconductor device performance and failure analysis.
The Near-Field Scanning Microwave Microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip-geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips, and find them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius r sphere ) at the apex of the tip. The r sphere values used in the study ranged from 0.1 μm to 12.6 μm. Tips with larger r sphere show good signal contrast (as measured by the frequency shift (Δf) signal between tunneling height and 2 μm away from the sample) with NSMM. For example, the tips with r sphere = 8 μm give signal contrast of 1000 kHz compared to 85 kHz with a tip of r sphere = 0.55 μm. However, large r sphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sample interaction. The model a email address: anlage@umd.edu 1 quantitatively explains the measured change in quality factor (Q) as a function of height over bulk Copper and Silicon samples.
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