Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2000-12-26
2003-01-07
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S364000, C356S609000, C356S614000, C356S326000
Reexamination Certificate
active
06504608
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This invention claims priority of a German filed patent application DE-P 199 63 345.2.
FIELD OF THE INVENTION
The invention refers to an optical measurement arrangement having an ellipsometer in which an incident beam of polarized light is directed at an angle &agr;≠90° onto a measurement location on the surface of a specimen, and information as to specimen properties, preferably as to layer thicknesses, is obtained from an investigation of the reflected return beam; and having a device for ascertaining and correcting directional deviations between the line normal to the specimen surface and the angle bisector between the incident and return beams. The invention further refers to a measurement arrangement and a method having an objective for illumination and imaging of a measurement location on a specimen, and a leveling device having an optical radiation source, a direction monitoring beam, and a spatially resolving detector.
BACKGROUND OF THE INVENTION
Optical measurement arrangements on the principle of ellipsometers and spectrophotometers, and their use for layer thickness measurement, are known from the existing art. They have been successfully used, in particular, for the measurement of thin layers, for example on patterns on wafer surfaces.
Since an effort is being made toward increasingly fine patterns and increasingly thin layers in wafer manufacture in particular, more and more stringent requirements are also being placed on the accuracy of the optical measurement arrangements with which the dimensional consistency of the patterns and layers is verified. In this context, it is important not only to ensure that no pattern edges are located in the measurement window (since the layer thickness measurement can thereby be falsified), but above all to guarantee that the specimen surface at the measurement point is oriented perpendicular to the measurement beam path, so that measurement errors can be ruled out.
Obliquities or undesired inclinations of the specimen surface occur, for example, if the specimen itself has an uneven surface, is not resting in tilt-free fashion on the specimen stage, or is distorted by suction onto the support surface. Such obliquities therefore must be identified and compensated for by way of suitable positioning systems. In addition, an accurate measurement also requires precise focusing, i.e. it must be ensured that the specimen surface lies in the focal plane and that, in the event of deviations, the specimen can be correspondingly aligned.
In order to allow even complex patterns and layer systems to be measured, both ellipsometers and spectrophotometers are often used in a combined arrangement for measurement. A high degree of measurement reliability is thereby obtained, but the requisite large number of optical assemblies results in space problems, since the assemblies must be coordinated and positioned with respect to one another in such a way that, if possible, the beam paths do not substantially influence each other. For example, it is usual for direct access to the measurement location to be already blocked by the measurement objective of the spectrophotometer.
Additional problems arise because of the variable measurement locations on the specimen surface, i.e. a change in specimen position relative to the measurement beam paths occurs during the measurement or between individual measurement steps; if the mechanical positioning devices provided for the purpose are insufficiently precise, this can result in defocusing and also in tilting of the specimen surface.
Because not only the demand for greater accuracy but also the effort toward increasing production volumes must be taken into account, it is necessary—for example in continuous production of wafers—to make measurements at ever shorter intervals and to check prior to each measurement that the prerequisites for the necessary high measurement accuracy are present. If such is not the case, that check must be followed by a rapid and, if possible, automatic correction of the specimen orientation.
A large number of publications regarding the orientation of wafers in wafer steppers and regarding leveling relative to the measurement beam path is already known in the existing art. U.S. Pat. No. 4,398,824, for example, describes a method for orienting a wafer in which local obliquities of the wafer and inhomogeneities in the photoresist can be compensated for. This method can only be applied, however, if portions of the wafer are configured as Fresnel zones. Since this is usually not the case, however, the method proposed here is not suitable for the most common wafer production equipment.
U.S. Pat. No. 5,218,415 describes a device for determining the obliquity of a wafer relative to the measurement beam in which an arrangement for illuminating a measurement location on the wafer, a device for receiving the reflected light beam, and means for modifying the size of the light beam are provided. In this context, a determination is made of the size or cross section of the light beam in the optically conjugated plane of the surface to be measured.
U.S. Pat. No. 4,595,829 discloses an arrangement for focusing a specimen surface with which it is possible to determine the focal plane and cause a change in the position of the sample in such a way that the specimen surface lies in the focal plane. It is not possible with this arrangement to ascertain and correct a tilt of the specimen surface relative to the measurement beam path, however, so that the prerequisites for extremely accurate measurements cannot be created.
U.S. Pat. No. 5,136,149 describes a method for the inspection of wafer surfaces which makes possible both focusing and a determination of the obliquity of the wafer surface. In this case a beam is directed through an objective onto the specimen surface, and the light reflected there is split into two beams. Of these, the first beam is recorded by a position-sensitive line receiver (CCD line), and a focus signal is generated with the aid of this receiver. The second partial beam strikes a two-dimensional position-sensitive detector and is used there to determine the obliquity. A substantial disadvantage here is the fact that the determinations of focus and obliquity, and thus the adjustment possibilities when correcting focus and obliquity, are not decoupled.
If the measurement and correction possibilities for focus and obliquity are dependent on one another in this fashion, it is time-consuming to meet the desired criteria for both the focus and obliquity of the wafer, since correcting the one variable always brings about a change in the other, and the approximation to the ideal state must be made iteratively. For example, if the focus is established first and then the obliquity is corrected, the obliquity correction causes the focus to drift out again as a result of the obliquity correction. The requirements in terms of obliquity have now been met, but the specimen surface is not adequately focused. If the focus is subsequently corrected, there is once again the risk of a change in the leveling or orientation of the wafer surface with respect to the measurement arrangement, and the leveling must once again be checked and, if necessary, corrected. This alternating adjustment until the desired result is achieved does not meet the need for a rapid inspection and production pace.
SUMMARY OF THE INVENTION
Proceeding therefrom, it is the object of the invention to develop an optical measurement arrangement of the kind cited initially in such a way that local inclinations and irregularities of the specimen surface are identified and a correction of the inclinational deviation of the specimen surface with reference to the optical axis of the measurement arrangement is made, said correction being performed with high accuracy and in a brief period of time, and being decoupled from any focusing of the specimen surface.
According to the present invention, the object is achieved by an optical measurement arrangement having
an ellipsometer in which an inci
Hallmeyer Klaus
Hoffmann Guenter
Wienecke Joachim
Foley & Lardner
Font Frank G.
Leica Microsystems Jena GmbH
Nguyen Sang H.
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