Near-Field Microwave Microscopy on nanometer length scales

TL;DR

This paper investigates a new length scale in near-field microwave microscopy caused by a tiny tip protrusion, affecting measurements at separations below 10nm, and models its impact on capacitance and frequency shift.

Contribution

It identifies and models a nanoscale protrusion at the tip end as a key factor influencing near-field microwave measurements at sub-10nm distances.

Findings

Protrusion size estimated at 3-5nm.

Frequency shift saturates at -1150 kHz near 1nm separation.

Protrusion causes deviation from logarithmic capacitance increase.

Abstract

The Near-Field Microwave Microscope (NSMM) can be used to measure ohmic losses of metallic thin films. We report on the presence of a new length scale in the probe-to- sample interaction for the NSMM. We observe that this length scale plays an important role when the tip to sample separation is less than about 10nm. Its origin can be modeled as a tiny protrusion at the end of the tip. The protrusion causes deviation from a logarithmic increase of capacitance versus decreasing height of the probe above the sample. We model this protrusion as a cone at the end of a sphere above an infinite plane. By fitting the frequency shift of the resonator versus height data (which is directly related to capacitance versus height) for our experimental setup, we find the protrusion size to be 3nm to 5nm. For one particular tip, the frequency shift of the NSMM relative to 2 micrometers away saturates at a value of about -1150 kHz at a height of 1nm above the sample, where the nominal range of sheet resistance values of the sample are 15 ohms to 150 ohms. Without the protrusion, the frequency shift would have followed the logarithmic dependence and reached a value of about -1500 kHz.