Effect of tip shape on surface roughness measurements from atomic force microscopy images of thin films

Abstract

The distortion of images by the finite radius of the tip is a significant artifact in atomic force microscopy (AFM) imaging of thin films. This article examines the effect of the finite radius of the AFM tip on the measurement of five surface roughness parameters: Ra and Rrms roughness, horizontal roughness, bearing ratio, and power spectral density. The effect of the finite size of the AFM tip on the measurement of these roughness parameters was studied using simulations. We found that the vertical roughness measures (Ra and Rrms) were the least affected by the tip induced distortion. For a spherical tip, the error in Ra and Rrms roughness was less than 15% as long as the radius of the major features in the AFM image (RAFM) was greater than twice the radius of the tip (Rtip). The horizontal roughness of the surface was sensitive to the distortion due to the tip. For distorted AFM images (RAFM/Rtip=5) the bearing ratio curve was similar in shape to the curve for the original surface. The bearing ratio curve of the severely distorted profile (RAFM/Rtip=1.5) was significantly different from the curve for the original trial surface. The general shape of the power spectral density (PSD) curves for the original and both distorted profiles were similar. However, the lower frequency peaks in the PSD curves for the distorted profiles differed from those in the PSD curve for the original surface.