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 find them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1 \mu m to 12.6 \mu m. Tips with larger rsphere 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 rsphere = 8 \mu m give signal contrast of 1000 kHz compared to 85 kHz with a tip of rsphere = 0.55 \mu m. However, large rsphere 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.