Uwe Nehse

Quality measures for depth-from-focus approaches

Besides the ongoing rapid development of processing power of computers new standardised interfaces are emerging. Thus, future measuring software will be able to process information of the quality of a measuring operation. This exceeds the current state-of-the-art. The originality of this research lies with the proposal of a novel method for computing a quality factor for each measuring point. The method is applicable for optical imaging sensors. However, similar regimes may be applied for other types of sensors. This paper presents a general methodology for computing a quality factor for focusing. Thereby different criteria such as unimodality, accuracy, reproducibility, definition range, general applicability and robustness are covered. The application of the proposed quality factor enables a more profound evaluation of a focusing process than the pure specification of the uncertainty of the focus position does. Within a closed quality loop it enables a higher level of control of the measuring process resulting in a significantly decreased measuring uncertainty and an increased robustness. Thereby the closed quality loop comprises the CAD process, inspection planning, measuring operations and the comparison between CAD data and measured geometry. The paper closes with some experimental results showing the soundness of the proposed method.

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 of 8.2 nm of the width of a 69 nm high height standard is attained. The measurements with the laser focus sensor are compared with AFM measurements at the same height standard. The presented work results from the collaborative research centre (SFB) 622 supported by the German Research Foundation (DFG).