Apparatus for measuring the dimensions of a minute object

#1. Field of the Invention

#This invention relates to an optical apparatus for measuring the dimensions of a minute object such as a minute pattern on the surface of a photomask for the manufacture of integrated circuits.

#2. Description of the Prior Art

#FIG. 1 is a perspective view schematically showing the construction of an embodiment of the present invention.

#FIG. 2 is an optical path view showing a state in which ghost images are created on a slit by a field lens.

#FIG. 3 is a plan view corresponding to FIG. 2 and showing the projected image and ghost images on a screen member.

#FIG. 1 schematically shows the construction of an embodiment of the present invention. The light beam from a light source 1 is condensed by a condenser lens 2 and illuminates an area of a sample 3 to be measured. The sample 3 is a transparent substrate having a minute pattern of contrast and is provided for movement in an x-y plane by a stage, not shown. The light beam passed through the sample 3 is imaged on a screen member 8 slidable in x direction, by an objective lens 4 and a field lens 7. An image rotating member 5 having a two-surface reflection prism 51 and a plane mirror 52 and a reflecting mirror 6 for bending the optical path are disposed between the objective lens 4 and the field lens 7. By the rotation of the image rotating member 5, the widthwise direction to be measured of the image formed on the surface of the screen member 8 parallel to an x-z plane is set so as to coincide with the direction of sliding of the screen member 8. A slit 81 elongated in z direction is formed on the screen member 8, and a measurement scale 82 is provided on one end of the screen member along the direction of sliding, and the amount of movement of the screen member 8 is detected by photoelectrically reading the measurement scale by a detector, not shown. Accordingly, the image formed on the surface of the screen member 8 is scanned by the slit 81 by the movement of the screen member. A photoelectric conversion element 9 responsive to the light beam passed through the slit 81 is disposed rearwardly of the slit 81, and the dimensions of the minute pattern on the sample 3 are measured by the state of the output from the photoelectric conversion element 9 relative to the amount of movement of the screen member measured by the detector. The surface of the screen member 8 which is opposed to the field lens 7 has a partial area thereof including the surroundings of the slit formed into a mirror surface, and the reflected light from this area is re-imaged on the focal plane of an eyepiece 12 by the field lens 7 and a relay objective lens 10 via a reflecting mirror 11, and the positional relation between the image on the screen member and the slit and the coincident state of the focus can be observed through the eyepiece 12.

#The field lens 7 is provided to increase the brightness of the image in the observation field. As will be seen from the foregoing description, both of the optical path from the reflecting mirror 6 to the screen member 8 and the optical path leading to the relay objective lens 10 after the reflection by the screen member 8 are along a plane perpendicular to the mirror surface and parallel to the slit, and they are proximate to each other at a slight angle. It is therefore necessary to take the following matters into consideration with respect to the arrangement of the field lens 7.

#Consider a case where, as shown in FIG. 2, a field lens 7' is disposed so that the optic axis thereof is perpendicular to the mirror surface of the screen member 8. The light beam from the sample 3 is reflected by the reflecting mirror 6, passes through the field lens 7' and is imaged on the screen member 8. At this time, at least two kinds of ghost images which hinder the measurement are created near the slit 81.