Radiant energy – Inspection of solids or liquids by charged particles – Methods
Reexamination Certificate
2002-02-25
2003-11-11
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Methods
C250S311000
Reexamination Certificate
active
06646260
ABSTRACT:
BACKGROUND OF THE INVENTION
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-&sgr;-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-&sgr;-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.
Scanning electron microscopes used for mask inspection therefore use a multiplicity of focusing algorithms before the actual measurement of the respective structural width is undertaken. In a known method, a Fourier spectrum of the image determined is evaluated and a resolution-induced threshold frequency is used to determine the quality of the focus, this being followed by a focus correction with a new pick-up of the Fourier spectrum. The best focus is determined with the aid of an optimization method. Unfortunately, it is a common feature of all these focusing methods that the reproducibility of individual measurements is comparatively low, and this is caused, inter alia, by local spatial or temporal changes during measurement. This disadvantageously increases the three 3-&sgr;-values for the measurements of structural widths on photomasks, as a result of which a value, usually not to be exceeded, for this purpose, of 25% of the prescribed tolerance range for structural widths (critical dimension) is in many cases exceeded.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a measurement technique for determining the width of a structure on a mask which overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which the scattering of measured values, is reduced, in structural width determinations repeatedly carried out by scanning electron microscopy such that it is possible to measure smaller structural widths.
With the foregoing and other objects in view there is provided, in accordance with the invention, a measurement technique for determining a width of a structure on a photomask. The method includes the steps of:
a) bringing the photomask with the structure into an object chamber of a scanning electron microscope;
b) adjusting a first focus value of a magnetic lens of the scanning electron microscope;
c) measuring a first width of the structure;
d) measuring a first value for a further parameter characterizing the structure;
e) adjusting a second focus value of the magnetic lens of the scanning electron microscope;
f) measuring a second width of the structure;
g) measuring a second value for the further parameter characterizing the structure;
h) adjusting a third focus value of the magnetic lens of the scanning electron microscope;
i) measuring a third width of the structure;
j) measuring a third value for the further parameter characterizing the structure;
k) adapting a first function with exactly one extreme value to the first, second and third values for the further parameter as a function of the first, second and third focuses, respectively, such that deviations of the first, second and third values from the first function are minimized;
l) determining a focus value assigned to the extreme value of the first function as a best focus;
m) adapting a second mathematical function with exactly one extreme value to the first, second and third widths of the structure as a function of the first, second and third focuses, respectively, such that deviations of the first, second and third widths from the second function are minimized; and
n) determining the width of the structure in relation to a functional value of the second mathematical function which is assigned to the best focus.
The method proposed in accordance with the present invention is already known in a similar way from light-optical microscopes. In the case of the light-optical microscopes, the focus is adjusted as a rule mechanically or by piezoelectric effects by varying the distance between the wafer stage holding the mask and the microscope objective. In accordance with the present invention in the case of the scanning electron microscope the magnetic lens, which acts as the objective, is directly influenced preferably automatically by a control unit in such a way that the magnetic field focusing the electron beam changes due to a change in the controlled current flow for the purpose of adjusting various focusing strengths. The focus therefore does not, as in the light-optical case, describe the distance between the wafer stage and the objective, but the distance, determined by the strength of the magnetic field, of the strongest focusing of the beam from the magnetic lens. Ideally, the current flow in the coil generating the magnetic lens will generate a magnetic field such that the focus thus generated in the scanning electron microscope is equal to the distance of the surface of the photomask from the magnetic lens.
The invention is based on the finding that individual measurements cannot be reproduced by using different focusing algorithms. If a CD measurement is repeated by using a value for the focus adjusted with the aid of conventional algorithms, a scattering of the measured CD values results despite a constant focal value. The present invention circumvents this problem by virtue of the fact that the best focus is not fixed from the very start, but that use is made of further parameters that characterize the structure under consideration in an entirely local fashion—for example, the contrast or the slope of the structural edges in this case. The contrast describes the difference in intensity between the maximum measured value of the structure itself and the minimum measured value of the surroundings. The slope results from the intensity gradient at the location of the edge. The locally determined parameters directly supply the quality of the adjusted focus, and are therefore used in accordance with the present technique in order to determine the
Greenberg Laurence A.
Infineon - Technologies AG
Lee John R.
Leybourne James J.
Locher Ralph E.
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