Apertureless scanning near-field infrared microscopy of a rough polymeric surface

Infrared near-field microscopy using an apertureless probe technique has been accomplished to study the surface of a cast copolymer film. Two basic models for the predicted signal and the experimental data are presented. The first model includes plane wave light scattering by a conductive sphere and an infinitely wide absorptive layer placed on a semi-infinite conductor. This model shows infrared signal dependence on the layer absorption and predicts topographic coupling into the infrared signal. The experimental data also indicate that a significant component in the infrared contrast arises from the probe following the sample's topography, and a method to eliminate the influence of topography following is demonstrated. The images corrected by such a procedure show spatial resolution of approximately lambda /80. A more complex model based on a three-dimensional finite difference time domain method was used to calculate scattering from a rough surface. Both constant tip-sample gap and constant tip-substrate height analyses were made, and it is found that constant height imaging is a preferred mode of operation. Calculations for dielectric and Lorentzian materials are reported. These calculations indicate that the near-field infrared signal attenuation for an absorptive object is larger than for a bare layer of the same thickness. This effect may be used to enhance chemical contrast in near-field imaging.