The present invention relates to a wafer defect inspection machine that captures an image of a pattern formed on a wafer and detects a defective portion by processing image signals. More particularly, the present invention relates to a structure of an illumination optical system.
Fixed patterns are formed repeatedly on semiconductor wafers, photomasks for semiconductor memories, liquid crystal display panels etc. In this case, optical images of these patterns are captured and defects of the patterns are detected by comparing neighboring patterns. If the result shows no difference between two patterns, it is judged that there is no defect in the two patterns, and if it shows a difference, it is judges that a defect exists in one pattern. As such a machine is generally called a wafer defect inspection machine, the term is also used here. Moreover, an example of a wafer defect inspection machine, for semiconductor wafers, that inspects for defects of patterns formed on a semiconductor wafer is described below. The present invention, however, is not limited to this but is applicable to a defect inspection machine for photomasks for semiconductor memories, for liquid crystal display panels etc.
FIG. 1 is a diagram that shows the rough structure of a defect inspection machine for semiconductor wafers. As shown in FIG. 1, the defect inspection machine for semiconductor wafers comprises a stage 1 that holds a semiconductor wafer 2, an objective lens 3 that projects the optical image of the surface of the semiconductor wafer 2, an image sensor 4 that converts the projected optical image of the surface of the semiconductor wafer 2 into electric image signals, an image signal processing circuit 5 that processes and converts the analog image signals output from the image sensor 4 into multi-valued digital image data, an image data processing circuit 6 that detects defects by processing the digital image data and comparing the same portion of patterns and an image data memory 7 that stores image data for data processing. The illumination optical system that illuminates the surface of the semiconductor wafer 2 comprises a light source 11, illumination lenses 12, 13 and 14, and a semi-transparent mirror (a beam splitter) 15 provided in the projection light path of the objective lens 3.
The illumination optical system in the defect inspection machine for semiconductor wafers is described below. In the defect inspection machine for semiconductor wafers, an illumination optical system of a metallographical microscope is used. As an illumination optical system of a metallographical microscope, a bright field illumination system as shown in FIG. 2A and a dark field illumination system as shown in FIG. 2B are known. In the bright field illumination system, the illumination light from a light source 21 is guided through a lens 22, an aperture stop 23, a lens 24, a field stop 25 and a lens 26, reflected toward the objective lens 3 by a semi-transparent mirror 27 provided in the projection path, and is directed to illuminate the surface of a specimen (wafer; through the objective lens 3. The lens 22 projects the image of the light source 22 on the position of the aperture stop 23 and the lenses 25 and 26 project the image on the position denoted by reference number 28. This position is the focal point of the objective lens 3 and uniform illumination without any unevenness in light quantity can be projected onto the surface of the wafer 2. Such illumination is called Koehler illumination. In the bright field illumination system, the surface of the wafer 2 is illuminated in the direction of the optical axis of the objective lens and the image of the regularly reflected light is captured.
In the dark field illumination system, on the other hand, the illumination light from a light source 31 is turned into an annular light flux by blocking its central portion and is further formed into an almost parallel light flux by the objective lens 3. This annular parallel light flux is directed into a perforated mirror 33 and formed into a light flux parallel to the optical axis of the objective lens 3. The perforated mirror 33 is an annular (more exactly, elliptic annular) reflecting mirror that allows light near the optical axis of the objective lens 3 to pass but reflects light of the peripheral portion. The annular illumination light reflected by the perforated mirror 33 enters a ring-shaped condenser lens 34 and illuminates the portion near the optical axis of the objective lens 3 of the wafer 2. In the dark field illumination system, an image of extremely high contrast can be obtained, but there is a problem that the image is not so bright because the illumination light regularly reflected by the surface of the wafer 2 cannot be captured.
There are metallographical microscopes equipped with both the bright field illumination system as shown in FIG. 2A and the dark field illumination system as shown in FIG. 2B. The bright portion and the dark portion, however, are opposite in the bright field illumination system and in the dark field illumination system and the images annihilate each other, therefore, one of the illumination systems is used according to the purpose. Devices such as a light source are, therefore, commonly used.
Moreover, there are dark field illumination systems in which the ring-shaped condenser lens, which is used in the dark field illumination system shown in FIG. 2B, is not used but means such as a reflecting mirror and optical fiber are used for illumination from the outside of the projection optical system of the objective lens, and such a dark field illumination system is used in a defect inspection machine for semiconductor wafers. It is, however, necessary to use an objective lens with a large NA (Numerical Aperture) in a defect inspection machine for semiconductor wafer because objects to be inspected have extremely fine patterns so, in the case of the illumination from the outside of the projection optical system, the incident angle of the illumination light becomes large. It is, therefore, impossible to illuminate the bottom part of the wafer pattern and a problem occurs that the defects in the pits of the patterns cannot be detected.
As described above, as the bright field illumination system and the dark field illumination system have both advantages and disadvantages, respectively, one of the illumination systems is used in a conventional defect inspection machine for semiconductor wafers according to the purpose of the inspection. Even if both the illumination systems are equipped, as described above, only one of them is used. U.S. Pat. No. 5,058,982 has disclosed a machine equipped with both the bright field illumination system and the dark field illumination system. FIG. 3 is a diagram that shows an example of the structure of the illumination optical system of the inspection machine disclosed in U.S. Pat. No. 5,058,982. As shown in FIG. 3, in the dark field illumination system, the illumination light from a fiber light source 41 is reflected by a cylindrical mirror 43, the illumination light from a fiber light source 42 is reflected by a cylindrical mirror 44, and the specimen 2 is illuminated obliquely. In the bright field illumination system, on the other hand, the illumination light from the fiber light source 42 is converged by a lens 46, reflected by a beam splitter 47, and is directed to illuminate the specimen 2 vertically. The surface image of the illuminated specimen 2 is projected by the objective lens 3. The simultaneous use, however, of both the illumination optical systems has not been stated in U.S. Pat. No. 5,058,982.
In the case of this inspection machine, as the bright field illumination system and the dark field illumination system are arranged between the objective lens 3 and the specimen 2, an objective lens of a large NA cannot be used and the projection magnification cannot be made large and, therefore, a problem occurs that this machine is not proper when inspecting defects in semiconductor wafer patterns.
As described above, a typical conventional defect inspection machine has a structure in which a dark field illumination system is provided outside the projection optical system, therefore, a problem occurs that it is not suitable as a defect inspection machine for semiconductor wafer that inspects fine patterns. This leads to a suggestion that the bright field illumination system shown in FIG. 2A and the dark field illumination system shown in FIG. 2B are provided in a defect inspection machine for semiconductor wafer. There occurs, however, a problem that the cost is increased if two illumination systems are provided and there is another problem particularly in the dark field illumination system that the cost is further increased because it is necessary to provide a ring-shaped condenser lens in addition to an objective lens.