1. Field of the Invention
This invention relates to a method and an apparatus for inspecting the state of a surface of an object and, more particularly, to a method and an apparatus for determining the existence/non-existence of defects and foreign particles on a reticle (photomask) and a pellicle, for protecting the reticle from foreign particles. The reticle and pellicle are used when a device, such as a semiconductor memory, a liquid crystal display, a solid-state image pickup device, or a magnetic head, is manufactured.
2. Description of the Related Art
FIG. 1 shows an example of an apparatus for inspecting foreign particles. A pellicle 3 attached to a pellicle frame 4 is provided under a lower surface 2a of a reticle 1 on which a circuit pattern is formed. The pellicle 3 serves to prevent foreign particles or the like from attaching to the lower surface 2a. The apparatus is arranged to examine foreign particles and defects on the lower surface 2a, the pellicle 3 and an upper surface 2b (blank) on which no pattern is formed.
The lower surface 2a, the pellicle 3 and the upper surface 2b are scanned with scanning light beams 5a and 5b from a scanning optical system (not shown) in a direction perpendicular to the projected plane of FIG. 1, while the reticle 1 is moved in the direction of arrow S.sub.1. Scattered light caused by foreign particles or the like on the lower surface 2b during this optical scanning is introduced into a photodetector 6a through a light receiving system 52 having a condenser lens (not shown). The scattered light caused by foreign particles or the like on the pellicle 3 during this optical scanning is introduced into a photodetector 6c through a light receiving system 51 having a cylindrical lens 51c and an optical fiber 10c. The scattered light caused by foreign particles or the like on the upper surface 2b during this optical scanning is introduced into a photodetector 6b through a light receiving system 53 having a cylindrical lens 53b and an optical fiber 10b.
FIG. 2 is a diagram showing a problem of the apparatus shown in FIG. 1. In FIG. 2, the same components as those shown in FIG. 1 are indicated by the same reference characters. Solid lines 60 indicate scattered light caused at the pellicle 3, dotted lines 61 indicate diffracted light caused by the circuit pattern, and reference characters F' and G' indicate regions irradiated by the scanning light. An angle .theta.i is formed between a center ray of scanning light 5a and the pellicle 3 (surfaces 2a, 2b), and an angle .theta.d' is formed between the optical axis of the light receiving system 51 and the pellicle 3 (surfaces 2a, 2b).
As can be understood from FIG. 2, when scanning light 5a reaches the pellicle frame 4, reflected light caused by the reflection of scanning light 5a upon the pellicle frame 4 travels to the lower surface 2a to cause diffracted light 61 from the irradiated region F', and the diffracted light 61 enters the light receiving system 51. Accordingly, if there is no foreign particle or defect in the irradiated region G', diffracted light from the irradiated region F' is erroneously detected as scattered light from some foreign particle or defect in the irradiated region G'.
Also in the apparatus shown in FIG. 1, when scanning light 5a, which is obliquely irradiating the reticle 1 from below reaches the pellicle frame 4 or the lower surface 2a, a part of scattered light from the pellicle frame 4 and diffracted light from the circuit pattern on the lower surface 2a enters the light receiving system 53 provided above the reticle 1. If there is no foreign particle or defect on the upper surface 2b, scattered light from the pellicle frame 4 or diffracted light from the circuit pattern on the lower surface 2a is erroneously detected as scattered light from some foreign particle or defect on the upper surface 2b.
FIG. 3 is a diagram showing another problem of the apparatus for examining a foreign particle or the like on a reticle. In FIG. 3, the same components as those shown in FIG. 1 are indicated by the same reference characters. Pellicles 3a and 3b, pellicle frames 4a and 4b, and light receiving systems 80 and 90, are for both receiving scattered light from the reticle upper surface 2b, and the pellicle 3b, which protects the upper surface 2b, and for introducing the scattered light into corresponding photodetectors 80 and 90. The light receiving systems 6a and 6b have cylindrical lenses 61a and 61b, lens barrels 62a and 62b, field stops 64a and 64b, and optical fibers 10a and 10b, respectively.
As can be understood from FIG. 3, when reflected light caused by the reflection on the upper surface 2b of scanning light 5b, which is obliquely irradiating the reticle 1 from above, reaches a point P on the pellicle frame 4b, scattered light from the point P enters the light receiving systems 80 and 90, and is erroneously detected as scattered light from some foreign particle or a defect on the upper surface 2b or the pellicle 3b, even though there is no foreign particle or defect on the upper surface 2b or the pellicle 3b.