The present invention relates to a focus-detecting method and device. More particularly, the present invention relates to a focus-detecting method and device used in a microscope having reflected illumination.
A conventional focus-detecting device used in a microscope with reflected illumination normally employs a detecting method in which a slit pattern is projected onto an object and a reflected image therefrom is detected by a photoelectric detecting device. Such a conventional arrangement is shown, for example, in U.S. Pat. No. 3,721,827, which is described hereinafter referring to FIGS. 9 and 10.
FIG. 9 shows an example with a focal objective lens. Illumination light from a light source 1 passes through a condenser 2 and illuminates a slit 3. One-half of the beam emitted from the slit 3 is blocked by an illumination-side shielding plate 4 which blocks the beam from 0 degrees to 180 degrees around the optical axis. The other half of the light is reflected by a half mirror 6 and passes through an objective lens 7 to reach an object 8. The light reflected from the object 8 passes back through the objective lens 7 and is transmitted through the half mirror 6 to reach a photodetecting device 14.
FIG. 10 shows a prior art example with an afocal objective lens. Illumination light from the light source 1 illuminates the slit 3 through the condenser 2. One-half of the beam emitted from the slit 3 is blocked by the illumination-side shielding plate 4. The other half of the light beam passes through a collimating lens 5 to form a parallel beam, is reflected by the half mirror 6, and passes through the objective lens 7' to reach the object 8. The light reflected from the object 8 passes back through the objective lens 7', is transmitted through the half mirror 6, and passes through a second objective lens 9 to reach the photodetecting device 14.
Both conventional examples shown in FIGS. 9 and 10 detect a focusing state by sensing the deviation of the center of gravity of the slit image formed on the photodetecting device 14. Not illustrated in FIGS. 9 and 10 are the eyepiece arrangement for viewing the object and the servomotor arrangement for automatically moving the object into the focal plane of the objective lens as a result of the image formed on the photodetecting device. These are conventional as shown in the aforementioned patent and are not illustrated in order to avoid unnecessarily complicating the drawings.
However, the above-mentioned conventional technology has the following problem. When deviations in light intensity and phase distribution of an object 8 are large, as seen in a wafer pattern, both diffracted light and scattered light from the object 8 reach the photodetecting device 14. This results in poor focal accuracy. Generally focal accuracy is excellent when the light intensity and phase distribution of an object 8 are small as seen in an object having a mirror surface. The light shown by the dotted lines in FIGS. 9 and 10 is diffracted light and scattered light.
To solve this problem, JP Kokai H6-3578 discloses a focus-detecting device in which a section for forming parallel rays is provided after forming an intermediate image between the objective lens and the photodetecting device. A detection-side shielding plate is formed asymmetrically around the optical axis and placed therein.
According to the technology disclosed in JP Kokai H6-3578, the diffracted light and scattered light which hinder focus detection are blocked. However, certain positioning of the detection-side shielding plate may block light which is necessary for focus detection. This makes it impossible to properly detect focusing.