Model-based edge detection in height map images with nanometer resolution

The advances at the semiconductor fabrication cause a need for three-dimensional measurements within the micro- and nanometer range. A method to derive lateral measuring information from height profiles with nanometer resolution is shortly introduced. Thereby, measurements are taken with a high resolution, point-wise working laser sensor. The paper discusses in detail the analysis of the measuring signal as observed at different height standards. At height standards with discontinuities smaller than the coherence length of the laser the measuring signal overshoots. This effect is called batwing effect. The novel approach comprises the utilisation of the batwing effect to determine the edge position. The characteristic signal curve is modelled with an appropriate mathematical function. Thus, edge detection algorithms well known from optical precision measurements are modified and applied to calculate the location of the edge along the scan line of the laser sensor. Exemplary, an experimental standard deviation (k=2) of 16 nm of the width of a 69 nm high height standard is attained. Based on measurements with an AFM at the same height standards the accuracy of size of the proposed method is evaluated. The presented work results from the collaborative research centre (SFB) 622 supported by the German Research Foundation (DFG).