A coordinate measuring device for the measurement of structures on substrates for the manufacture of wafers is described in the paper entitled “Pattern Placement Metrology for Mask Making”, presented by Dr. Carola Bläsing at the Semicon meeting, Education Program in Geneva, Switzerland on Mar. 31, 1998. For further functional details and details of the construction of the coordinate measuring apparatus please refer to the above mentioned publication and the commercially available models (currently LMS-IPRO II and III). In the context of the present application, the terms “sample”, “substrate” and the general term “object” are to be regarded as synonymous. In the production of semiconductor chips arranged on wafers, with ever increasing integration the structural widths of the individual structures become ever smaller. As a consequence, the requirements as to the specification of coordinate measuring devices used as measuring and inspection systems for measuring the edges and the positions of structures and for measuring structural widths become ever more stringent.
In the above-mentioned publication the structures are measured using so-called measuring windows or measuring fields. The fields are defined by the user of the coordinate measuring machine in the coordinate measuring system of the CCD camera or CCD chip of the camera. Subsequent adjustment to the actual conditions during the measuring process does not take place.
The measuring fields (ROI=region of interest), evaluated for the measurement, are as already mentioned defined in the coordinate system of the CCD camera. The structure to be measured is positioned under the lens assembly by the measuring stage of the coordinate measuring machine and imaged onto the CCD chip of the camera. The position of this image relative to the coordinate system of the CCD camera is, however, dependent on the actual position of the measuring stage. Due to the limited positioning accuracy of the measuring stage the structure is located at a slightly different position each time it is measured.
Assuming defects are found on a structure, for example edge roughness or dust particles, it is possible that the defect is located in the measuring field one time and not another depending on the actual position of the measuring stage. Thus, profiles are not reproducible and the reproducibility of the measurement suffers as a result. The accuracy of the coordinate measuring machine itself is also directly affected in this way. Another cause of inaccuracy is that the structure edges are not at right angles to the measuring direction or that the structure ends in the measuring window (ROI region of interest).
German Patent Application No. DE 101 29 818 A1 discloses a method for reading out a detector chip of an electronic camera. The detector chip is used in a coordinate measuring device to determine the position of an edge of a structure on a substrate. DE 101 29 818 A1 does not, however, in any way address the problem underlying the present invention. The positioning accuracy of the measuring stage is not taken into consideration with reference to the measuring accuracy of the coordinate measuring machine.
German Patent No. DE 10 2004 012 125 B3 discloses an ellipsometric measuring method with ROI based image correction. The ellipsometric imaging measuring method records a plurality of individual images in sequence, each of at least one part of the object. It must be noted, however, that an ellipsometric measuring method is not suitable for determining the position or the width of a structure on a substrate. The method proposed in D2 is used to determine position and distortion information from the partial images, i.e. from the image information contained in them. While the ROI can basically correspond to the entire individual image, it is advantageous if well definable, relatively small imaging areas are simply selected as the ROI. An area can be selected as the ROI, for example, that displays particular contrast to neighboring image areas. This area can then be analyzed with regard to form and position. A reference image and the first individual image can either differ or be identical. The latter is the case, for example, if the first individual image of a series of individual images recorded during measurement is used as the reference image. In a second individual image the same ROI is determined on the basis of the same parameter, i.e. each contrast. Due to optical distortion or mechanical position displacement this can, however, vary from the ROI of the first individual image in its form and/or position. Finally, readjustment of the entire image is carried out until the form and position of the ROIs is identical in both the first and second individual image.
German Patent Application No. DE 198 25 829 A1 discloses a method for determining the position of a structural element on a substrate. A measuring window, positioned over the substrate to be determined, records an image of the structure which can be transformed into an intensity profile. The position of a first edge of the structure and a second edge of the structure can then be determined on the basis of this intensity profile. The position of the structure is ultimately dependent on the positioning data of the measuring coordinate stage. Not once, however, in the entire document D3 is reference made to the fact that, when a plurality of images of the same structure are recorded using the same measuring window, errors can occur when, due to the positioning inaccuracy of the measuring stage, the measuring window comes to lie at a different position on the structure.