The present invention relates to a method and apparatus for scanning a laser beam onto a semiconductor wafer to examine the surface of the semiconductor wafer prior to manufacturing a semiconductor device.
To detect microparticles (dust and the like) attached to the surface of the semiconductor wafer and any defects formed on the surface, the wafer surface is scanned by a focused laser beam having a small diameter, thereby detecting the light which is scattered by the micorparticles and defective portions. In this case, hitherto, a method of scanning the laser beam has been mainly classified into the following two kinds of methods. (1) In the first method, the optical system to irradiate a laser beam is fixed and, as shown in FIG. 1, a stage on which wafer 11 is mounted is gradually rectilinearly moved in constant direction X while rotating in rotational direction .theta., so that a laser beam is scanned on wafer 11 so as to draw spiral locus A. (2) In the second method, as shown in FIG. 2, the stage on which wafer 11 is mounted is fixed and a laser beam is raster scanned on wafer 11 in the X and Y directions. Or, the laser beam is scanned on wafer 11 in the X direction perpendicular to the Y direction while moving the stage in constant direction Y so as to eventually draw such a locus as shown by B in the diagram. The laser beam scanning can be realized by irradiating laser beam 1 on polygon mirror 2 and by using the reflection of the laser beam by this mirror.
In the conventional wafer surface examining technique, a spot diameter of the laser beam on the irradiating surface is set to about 100 .mu.m and the maximum detecting sensitivity is set to a value from 0.3 to 0.5 .mu.m.phi. (.phi. denotes a diameter of the detected particle). In the case of scanning the whole surface of the wafer by use of the laser beam having such a spot diameter, the problems of the measuring time (time required for scanning) and the design technique of the optical system can be relatively easily solved. On the other hand, the sizes of the dust particles and the defects to be detected have steadily decreased in association with the advance of the technique of finding a wafer pattern, and therefore, an increased detecting sensitivity is required. As the most effective method of satisfying such a requirement, there is a method whereby a diameter of a laser beam is converged into a small diameter to thereby increase the irradiation light intensity per unit area. For example, to obtain the detecting sensitivity of 0.1 .mu.m.phi., it is necessary to reduce the beam spot diameter to 10 .mu.m. However, in the foregoing conventional laser beam scanning methods, in the case of reducing the beam spot diameter, the following problems are caused. (1) In the method of spirally scanning the laser beam, the scan distance inevitably becomes long due to the reduction in the beam spot diameter and the measuring time, i.e., the scanning time becomes long, resulting in a loss of practical use efficiency. (2) In the case of scanning the beam on the whole surface of the wafer in the X and Y directions, the scanning distance, i.e., the swing width is increased. As a diameter of a wafer increases in the future, the swing width increases more and more. To reduce the beam spot diameter to 10 .mu.m, a demagnification optical system is needed. However, when the swing width is increased, the aberration also increases, so that it is difficult to reduce the beam spot diameter. On the other hand, the illuminance in the spot varies, depending on the irradiating position of the spot, resulting in a deterioration in the detecting accuracy. In addition, when the swing width is large, even if the beam spot is a true circle at the center of the wafer, the cross section of the beam becomes an ellipse at the edge portion of the wafer. This means that the detecting sensitivity differs depending on the measuring position on the wafer.
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