Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-07-10
2003-08-12
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
Reexamination Certificate
active
06606153
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention described in the present disclosure relates to a process and an assembly for the non-destructive inspection of surfaces, particularly for the measurement of small particles, defects, and inhomogeneities on and/or just below the surface of a test object, such particles, defects, and inhomogeneities collectively referred to herein as anomalies. It relates in particular to an instrument described below as the preferred embodiment for inspecting a silicon wafer, the instrument having a light source that generates a light beam, a beam deflector, an optical system that projects the incident beam on a light spot perpendicular to the test object, a photodetector to which the collected light is guided, and an assembly by which the test object is moved by a coordinated translational and rotary movement, so that the light spot scans the whole of the surface along a spiral path.
Such types of process and/or assembly can be used, for example in microelectronics, for the non-destructive checking and inspection of the surfaces of wafers, magnetic storage media, and/or substrates for optical applications, to determine the presence of any particles and/or defects.
The development of wafer-exposure processes has made it possible to manufacture wafer surfaces with ever finer structures parallel to this development, inspection systems that permit the detection of ever more minute defects and particles have become increasingly important. Apart from particles that account for about 75% of all waste in the manufacture of integrated circuits (ICS), inspection systems must be capable of detecting many other types of inhomogeneity, such as variations in the thickness of coatings, crystal defects on and below the surface, etc.
In the final inspection by wafer manufacturers and the inward-goods inspection by chip manufacturers, the unstructured, uncoated wafer must therefore be subjected to extremely searching examination for particle contamination, light-point crystal defects, roughness, polishing scratches, etc. If the test object has a rough surface, then a large amount of stray surface scattering will result. Thus, for this purpose, the test object has a well-polished surface that produces very little diffused light.
In chip manufacture it is usual to monitor each stage of the process, in order to recognize problems as early as possible and thus avoid undue waste. One method of process monitoring is to use so-called monitor wafers which remain unstructured but pass through some of the process stages. Comparison of two measurements, the first before the process stage and the other after it, can thus, for example, help determine the amount of particle contamination due to that process stage or indicate variations in the evenness of the process stage, for example the distribution of the coating thickness over the whole of the wafer. The surfaces subjected to inspection may be rough and metallized, and therefore, produce a great deal of diffused light, or they may be film-coated surfaces that cause interference-fringe effects. Thus, ideally the inspecting instrument has a wide dynamic range to permit defect and particle detection of a wide variety of surfaces.
Prior Art
For the type of inspection described above, so-called laser scanners are particularly suitable. An important feature of these is their high sensitivity to very small defects and the ability to determine the presence of these, and their high throughput. The main differences in the laser scanners now available are the type of scanning they use, their optical configuration, and the manner in which the results are processed.
For applications that require a high throughput and 100% inspection of the whole wafer surface, two processes are mainly used. In the first of these, for example as described in U.S. Pat. No. 4,314,763, the illuminating beam and the collecting optics are stationary, and the test object is scanned spirally by means of a coordinated translational and rotary movement of the test object itself. In the second process, for example as described in U.S. Pat. No. 4,378,159, a rotating or vibrating mirror moves the illuminating beam in one direction linearly back and forth across the wafer, and the whole of the wafer is scanned by virtue of a simultaneous translational movement of the test object perpendicular thereto.
Spiral scanning has the following advantages:
the optical system has no moving parts and thus is simpler;
the illuminated spot and the collector system's field remain constant during the whole of the measurement procedure, hence the system's sensitivity is homogeneous over the whole of the test object;
the system takes up less room, because the test object has to be moved only by the length of its radius; and
there is no need to alter the optical system for inspection of bigger objects, only the travel of the translational stage.
The advantages of moving the illuminating beam by means of a mirror or a set of mirrors are:
the test object has to be moved in one direction only, and this is simpler; and
as a rule, scanning is faster.
In the second scanning method, because the illuminating spot moves across the test object and thus the source of diffused light moves in relation to the optical collector system, it cannot ensure an even measuring sensitivity, nor does it permit a rotationally symmetrical arrangement of the collector optics. These are serious drawbacks in laser scanners configured in this manner.
Various optical configurations are known from prior art in the use of a laser scanner for spiral scanning as described above.
For example, U.S. Pat. No. 4,893,932 describes a system which has two differently polarized lasers and two corresponding detectors. The diffused-light intensity of spheres as a function of their diameter has oscillations for diameters within the range of the wavelength used and increases strictly monotonically for smaller diameters. The use of differently polarized light reduces the error in the attribution of diffused-light intensity to particle diameter for the spheres of polystyrene latex (PSL) spheres used for the calibration of laser scanners.
But in practice, the attribution of certain diffused-light intensities to particle diameters depends on so many factors, such as substrate material, films and coatings available, particle material, surface texture of particles, etc., that when the optics and calibration of the equipment are designed only for polystyrene-latex spheres, they tend to make interpretation of the results more difficult. A further major drawback of this method is that the oblique angle of incidence and linear polarization of the laser beam break the symmetry. The measured signal thus depends on the orientation of the defect.
Japanese Patent Application No. 63'14,830 describes collector optics made up of concentric rings, each having six fibre-optic light guides, which are directed to a photomultiplier. The drawbacks of this arrangement are that it fails to cover the central zone near the axis, and the discrete arrangement can achieve rotational symmetry only approximately.
EP-A-0,290,228 describes an arrangement whereby the diffused light is conducted to two detectors. The first detector collects light deflected by about 40 mrad to 100 mrad, the second collects light diffused by more than 100 mrad. Such an angle-resolving method of measurement by means of two detectors makes it possible to classify the defects, but because the collector angle is limited, the system cannot measure very small defects.
DE-A4,134,747 describes a similar solution that uses two detectors designed as arrays, one of which measures the radial and the other the azimuthal light distribution. In this system the test object rotates and the optical system moves linearly.
DD 250,850 also describes an angle-resolving method of measuring diffused light by means of fiber-optic light guides arranged in a circle.
Both the above methods have the drawback that the collector angle is much smaller and closer to specular than that described in the present
Altendorfer Hubert
Gross Kenneth P.
Kren George
Marxer Norbert
Kla-Tencor Corporation
Parsons Hsu & de Runtz LLP
Rosenberger Richard A.
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