Atomic force microscope tip radius needed for accurate imaging of thin film surfaces

This article discusses the distortion of atomic force microscope (AFM) images of columnar thin films caused by the finite size of the AFM tip. The amount of distortion in an image depends on the relative sharpness of the tip and the surface features. Two-dimensional numerical simulations were used to predict the effect of this distortion on the accuracy of AFM images. We propose that the ratio of the radius of the curvature of the features in an AFM image to the radius of the tip (RAFM/Rtip) provides an effective measure of the degree of tip induced distortion in AFM images of columnar thin films. Using simulations, we show that, for the distortion in an AFM image of columnar thin films to be undetectable by eye, the radius of the curvature of the features in the AFM image must be 10 times larger than the radius of the tip. At the other extreme, if the radius of the curvature features in the AFM image is less than twice the radius of the AFM tip, the images are severely distorted and not representative of the surface of the thin film. From a study of 23 different thin films we found that for only 6 (26%) thin films did the AFM images have distortion undetectable by eye. For 7 (30%) of the thin films, the AFM images were sufficiently distorted to be nonrepresentative of the surface of the thin film.