Measurement technique for determining the width of a structure on a mask

#1. Field of the Invention

#The present invention relates to a measurement technique for determining a width of a structure on a mask.

#Because of its higher resolution, increasing use is being made of the scanning electron microscope (SEM) in mask inspection in response to the increasing demands placed on the observation of the ever reducing structural dimensions on photomasks. In this case, 3-σ-values of three nanometers are currently required for the reproducibility of structural width measurements in the case of these microscopes. Such low values for system-induced errors are required in order to be able to detect unambiguously production-induced deviations of the structural widths from prescribed specifications of, currently, approximately 20 nanometers. It is even expected for 2002 that deviations of only 10 nanometers can be determined. The specifications can scarcely yet be achieved by current light-optical microscopes. In order to observe such low 3-σ-values in the case of scanning electron microscopes, it is necessary, in particular, to avoid inaccuracies in focusing. By feeding current into a magnetic lens, there is built up via a coil a magnetic field that focuses an electron beam, and acts as an objective and thereby reduces the electron beam at a distance denoted as a focus to a diameter of only 1-10 nm. Ideally, the surface, which is to be investigated, of the photomask is located at exactly this distance. Deviations from this distance are denoted as defocus. In the scanning electron microscope, the surface of the photomask is scanned by the electron beam by deflecting the electron beam line by line, the excited Auger electrons, which fall below the beam diameter, secondary electrons and, if appropriate, back-scattered electrons being picked up with the aid of a detector. Through shading effects, for example, the Auger electrons and secondary electrons, which amount to only a few electron volts, permit inferences relating to the surface topography of the photomask, or they also supply information on the first—viewed laterally—nanometers of the depth of the element structure, and at below the surface. Via a signal amplifier, the detected signal can, when plotted against the object scanning, be structured to form a substantially magnified image of the surface of the photomask. Any possible defocusing of the electron beam effects an expansion of the beam at the surface of the photomask, and thus a lower resolution of the individual surface structures.

#FIG. 1 is a flowchart of an exemplary embodiment in accordance with the invention;

#FIG. 2 is graph showing a fitting of mathematical functions for the purpose of determining a structural width;

#FIG. 3 is a block diagram of a configuration for applying the technique in accordance with the invention;

#Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown, by way of example, a sequence of the technique according to the invention. In this case, a maximum and a minimum value are fixed in advance for the focus, that is to say two values are determined for a current flow in a coil generating a magnetic lens, step 100. Furthermore, n−2 focal values that are likewise to be adjusted are determined in an equidistant fashion in the interval lying therebetween. n is the total number of the focal values to be traversed. First, the maximum focal value (minimum current in the coil) is adjusted.

#In FIG. 3, a scanning electron microscope is used to pick up the desired section of a surface having a structure 2 on a photomask 1 by a detector 10, amplified with the aid of a signal amplifier 11 and visualized on a display unit 15. In accordance with the basic principle of scanning electron microscopes, constructing the image is rendered possible by a raster generator 12 which carries out the synchronization with the signal coming from a deflecting unit 13 via a magnification control 14. The image taken can be evaluated by a connected evaluation unit 16 such that a structural width 5′, a contrast 7 and a slope 6 (as intensity gradient) are present for an adjusted focus 3. A multiplicity of techniques and standards are generally used in order to derive the actual width 5 of the structure 2 on the photomask 1 from the structural width 5′ of the measured intensity profile (FIG. 4B). A cross-sectional profile through the structure 2 to be measured on the photomask 1, as well as the intensity profile resulting therefrom in the scanning electron microscope are illustrated schematically and not to scale in FIGS. 4A and 4B. The intensity profile shown in FIG. 4B has at the rims of the structural edges overshooting profile curves which result from a particularly favored emission of excited secondary electrons at the upper edge corners, and/or from the reduced emission of the secondary electrons at the lower edge corners.

#According to the invention, in a loop the next focal value (steps 110-140), that is to say a current flow in the coil, relating to a renewed image taking, is adjusted for the purpose of measuring the structural width 5, the contrast 7 and the slope 6 for the new focus 3, as is to be seen in FIG. 1. For this purpose, a control unit 17, which is connected to the evaluation unit 16, controls a current source or voltage source 18 for the purpose of inducing a magnetic field in the magnetic lens generated by a coil 4, as is to be seen in FIG. 3. The control unit 17 has a focal traverse for all prescribed values of the focus 3 until, finally, the minimum focus (maximum current flow) is adjusted and the structural width, the contrast and the slope are measured.