JPH10206741A - Optical microscope variable in numerical aperture of objective - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical microscope which is easily variable in the numerical aperture of the objective without replacing the objective. SOLUTION: The rear focus of the objective 7 is relayed to above an image forming lens 4. Namely, the conjugation point of the rear focus of the objective 7 is formed and a stop 2 is arranged at the position. The numerical aperture of the objective 7 can be varied by the stop 2. This one microscope makes it possible to observe a sample with a high step by increasing the depth of focus by the stop 2 without replacing the objective and also observe a sample with a low step which requires resolving power by detaching the stop.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical microscope having a variable numerical aperture of an objective lens.

[0002]

2. Description of the Related Art As semiconductor DRAMs become more highly integrated, process wafers tend to be bipolar with low steps due to flattening and high steps due to multilayering. As a device for inspecting the wafer, a minute positional displacement inspection device is used. This inspection apparatus has an optical microscope as a main component, and it is necessary to obtain an optimum contrast for both the low step and the high step wafers. The optical system is set so that the depth of focus is shallow and the resolution is high for wafers with low steps, while the resolution is slightly reduced but the depth of focus is set deep for wafers with high steps. .

[0003]

In this case, the numerical aperture of the objective lens of the optical microscope must be changed. However, conventionally, a single objective lens cannot be used, and it is necessary to replace the objective lens with the same magnification and a different numerical aperture. In general, a method of attaching an objective lens with a different numerical aperture to a revolver and switching can be considered. However, since there is a change in the optical axis when the revolver is switched, it cannot be used in the minute positional deviation detecting device. Also, many objective lenses having the same magnification but different numerical apertures are not supplied.

[0004] An object of the present invention is to solve the above-mentioned drawbacks and to provide an optical microscope which can easily change the numerical aperture of an objective lens without replacing the objective lens.

[0005]

In order to solve the above-mentioned problems, an optical microscope according to the present invention forms an optical conjugate point by moving a rear focal position of an objective lens toward an observation side by a relay lens. A stop is arranged at a conjugate point, and the numerical aperture of the objective lens is changed by the stop. In the above configuration, the relay lens is
It comprises an imaging lens arranged to face the objective lens and a relay lens arranged at a conjugate point formed by the imaging lens. Further, in the above configuration, a diaphragm attachment / detachment mechanism is provided at the conjugate point, and the aperture value is changed by attaching / detaching the aperture.

[0006]

According to the above arrangement, the numerical aperture of the objective lens used can be set to an arbitrary numerical aperture smaller than the numerical aperture of the objective lens used. In addition, it becomes possible to observe a sample with a low step requiring a resolving power by removing the stop. Also, when an autofocus system is incorporated in the optical system, the objective lens section has the numerical aperture of the lens itself even when the numerical aperture is reduced. This is larger than when a lens with a small numerical aperture is used, which is advantageous for detection.

[0007]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram for explaining how to determine the numerical aperture of a lens. When the angle at which the object point on the optical axis in the medium having the refractive index n looks into the radius of the entrance pupil is α, the numerical aperture (NA) is n × sin α.

[0008] The depth of focus is calculated by the following equation. Df = λ / (NA) ² where Df: depth of focus (μm) λ: wavelength of light beam (μm) NA: numerical aperture of lens Further, the resolving power is (when two bright points are close to each other on a dark background). , The minimum distance at which the two points can be distinguished as two points) is expressed by the following equation. d = 0.61λ / (NA) where d: resolution (μm) λ: wavelength of light beam (μm) NA: numerical aperture of lens

A comparison of the numerical aperture of the lens, the depth of focus, and the resolving power, taking into account specific numerical values, is as follows.

[Table 1]

FIG. 2 is a schematic view of an optical path for explaining a basic configuration of an optical microscope having a variable numerical aperture of an objective lens according to the present invention. The objective lens 7 faces the sample 8.
The imaging lens 4 is arranged above the objective lens 7. The imaging lens 4 is arranged at a position where the position twice the focal length f of the imaging lens 4 and the rear focal position of the objective lens 7 coincide. Further, a stop 2 is disposed above the imaging lens 4 at a position twice the focal length f of the imaging lens 4. The relay lens 3 is also arranged at this position. A CCD camera or the like is installed on the image plane 1 above the relay lens 3. The relay system lens includes an imaging lens 4 and a relay lens 3.

The imaging lens 4 relays the rear focus of the objective lens 7 to the position of the relay lens 3 at the same magnification. That is, the back focal point conjugate point is formed at the position of the relay lens 3. A semi-transmissive mirror 6 is inserted between the imaging lens 4 and the objective lens 7, and the illumination light 10 passes through the illumination diaphragm 9 and illuminates the surface of the sample 8 via the illumination lens 5. The aperture 2 allows the numerical aperture of the objective lens 7 to be changed. Here, assuming that the focal length of the relay lens is determined so as to have a predetermined total magnification, the numerical aperture of the objective lens used is 0.8, and the pupil diameter of the lens at this time is φ5.76, the lens should be inserted. The diameter of the stop can be determined by 5.76 ÷ 0.8 × (NA).

In the above description, an example in which the rear focus of the objective lens is relayed at the same magnification has been described.
If the conjugate point of the rear focal point of the objective lens is formed by a relay lens and a stop is arranged at that position, the numerical aperture can be similarly changed.

FIG. 3 is a schematic optical path diagram showing a first embodiment of an optical microscope in which the numerical aperture of an objective lens is variable according to the present invention. This example is a reflection type optical microscope, and has the same configuration as that of FIG. However, the distance from the arrangement position of the imaging lens 15 to the back focus of the objective lens 17 is 2f (1 ×).
Various distances can be taken, including the distance of. Illumination light 2
0 is condensed by the condenser lens 19 and reaches the sample 18 via the transflective mirror 16. A conjugate point is formed at the position of the relay lens 14 by the imaging lens 15 with respect to the rear focus of the objective lens 17. The aperture 13 can be attached and detached by an aperture attachment / detachment mechanism, and the numerical aperture of the objective lens 17 can be changed by removing the aperture or changing the aperture value. Position of the conjugate point with the relay lens 14 (aperture 13
Is set at the same position, the stop can be set at a position that does not interfere with the lens by changing the position of the principal point of the lens.

FIG. 4 is a schematic optical path diagram showing a second embodiment of the optical microscope according to the present invention, in which the numerical aperture of the objective lens is variable. This example is a transmission type optical microscope. An illumination light 28 and a condenser lens 2 are provided below a transmission type sample 26.
7 are arranged. The distance from the arrangement position of the imaging lens 24 to the back focal point of the objective lens 25 can take various distances including 2f. The illumination light 28 is collected by the condenser lens 27 and illuminates the sample 26. A conjugate point is formed at the position of the relay lens 23 by the imaging lens 24 with respect to the rear focus of the objective lens 25. 3, the aperture 22 can be attached and detached by an aperture attachment / detachment mechanism, and the numerical aperture of the objective lens 25 can be changed by removing the aperture or changing the aperture value.

As described above, the depth of focus and the resolving power of the optical microscope are determined by the numerical aperture (NA value) of the objective lens. In the present invention, by relaying the rear focus of the objective lens and setting an aperture there, it is possible to set an arbitrary numerical aperture equal to or less than the numerical aperture of the objective lens used. It enables observation of a thick sample by increasing the depth of focus, and observation of a sample requiring resolution by removing the aperture.

[0016]

As described above, according to the present invention,
Since the numerical aperture can be reduced with a single objective lens, the resolving power decreases, but the depth of focus can be increased, and observation is possible even with a thick sample. Also, if you remove the aperture,
The original numerical aperture of the objective lens is obtained, and the original function can be brought out for a sample having a small depth of focus but requiring a resolving power. Therefore, there is no need to prepare an objective lens having the same magnification and a different numerical aperture as in the related art, and there is an effect that the numerical aperture can be easily changed without replacing the objective lens.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining how to find a numerical aperture of an objective lens.

FIG. 2 is an optical path schematic diagram for explaining a basic configuration of an optical microscope having a variable numerical aperture of an objective lens according to the present invention.

FIG. 3 is a schematic optical path diagram showing a first embodiment of an optical microscope having a variable numerical aperture of an objective lens according to the present invention.

FIG. 4 is an optical path schematic diagram showing a second embodiment of an optical microscope having a variable numerical aperture of an objective lens according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Image surface 2,13,22 ... Aperture 3,14,23 ... Relay lens 4,15,24 ... Imaging lens 5 ... Illumination lens 6,16 ... Semi-transmissive mirror 7,17,25 ... Objective lens 8, 18, 26: Sample 9: Illumination diaphragm 10, 20, 28: Illumination light 12, 21: CCD camera 19, 27: Condenser lens

Claims (3)

[Claims] 1. A rear focus position of an objective lens is moved in a direction of an observation side by a relay lens to form an optical conjugate point, a stop is arranged at the conjugate point, and a numerical aperture of the objective lens is set by the stop. An optical microscope having a variable numerical aperture of an objective lens, wherein the numerical aperture is changed. 2. The relay system lens according to claim 1, wherein the relay lens includes an imaging lens arranged to face the objective lens, and a relay lens arranged at a conjugate point formed by the imaging lens. An optical microscope with a variable numerical aperture of the objective lens described. 3. The optical microscope according to claim 1, wherein an aperture attachment / detachment mechanism is provided at the conjugate point, and the aperture value is changed by attaching / detaching the aperture.