Effect of tip geometry on contrast and spatial resolution of the near-field microwave microscope

The near-field scanning microwave microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (lambda). 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 found 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 to 12.6 mu m. Tips with larger r(sphere) show good signal contrast [as measured by the frequency shift (Delta f) signal between tunneling height and 2 mu m away from the sample] with NSMM. For example, the tips with r(sphere)=8 mu m give signal contrast of 1000 kHz compared to 85 kHz with a tip of r(sphere)=0.55 mu m. 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 quantitatively explains the measured change in quality factor (Q) as a function of height over bulk copper and silicon samples. (c) 2006 American Institute of Physics.