We have designed and tested a family of silicon nitride cantilevers ranging in length from 23 to 203 μm. For each, we measured the frequency spectrum of thermal motion in air and water. Spring constants derived from thermal motion data agreed fairly well with the added mass method; these and the resonant frequencies showed the expected increase with decreasing cantilever length. The effective cantilever density (calculated from the resonant frequencies) was 5.0 g/cm3, substantially affected by the mass of the reflective gold coating. In water, resonant frequencies were 2 to 5 times lower and damping was 9 to 24 times higher than in air. Thermal motion at the resonant frequency, a measure of noise in tapping mode atomic force microscopy, decreased about two orders of magnitude from the longest to the shortest cantilever. The advantages of the high resonant frequency and low noise of a short (30 μm) cantilever were demonstrated in tapping mode imaging of a protein sample in buffer. Low-noise images were taken with feedback at a rate of about 0.5 frames/s. Given proper setpoint adjustment, the sample was not damaged, despite this cantilever’s high spring constant of 1.3 N/m. Without feedback, images were taken at 1.5 frames/s.
Using a 26 μm cantilever with a resonant frequency of 100 kHz in water, we were able to obtain sequential images of calcite crystal steps growing from a screw dislocation. The small cantilever permitted acquisition of 250 nm images at scan rates of 104 lines/s (1.2 s/image). From this sequence we directly measured critical step lengths (the length of the shortest step that can advance) of 6–21 nm. These values provided a rough estimate of (0.25±0.13 J/m2) for the step energy per unit length per unit step height on the (104) face of calcite.
The scanning capacitance microscope (SCM) is capable of quantitative two-dimensional carrier and dopant density mapping with nanometer scale spatial resolution. The method can be applied to either the top or the cross-sectional surface of a silicon sample. The quantitative inversion of SCM data to carrier or dopant density is achieved using a quasi-one-dimensional model. Cross-sectional SCM measurements have been performed on samples that have abrupt dopant density steps. The dopant density in these samples systematically varies from 1017 to 1020 cm−3. The cross-sectional measurements provide a means to directly compare the SCM results with vertical secondary ion mass spectroscopy (SIMS) profiles. A spatial resolution of approximately 25 nm is achieved. A first order quantitative agreement between the SCM and the SIMS profiles is found.
We have applied a new generation of short cantilevers with high resonant frequencies to tapping mode atomic force microscopy of a process in situ. Crystal growth in the presence of protein has been imaged stably at 79 lines/s (1.6 s/image), using a 26 jim long cantilever with a spring constant of 0.66 N/rn at a tapping frequency of 90.9 kHz. This high scan speed nearly eliminated distortion in the step edge motion and allowed imaging of finer features along the step edges. Atomic force microscopy with short cantilevers therefore allows higher resolution imaging of crystal growth in space as well as time.
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