JPS6175206A - Apparatus for measuring minute step - Google Patents

Abstract

PURPOSE:To enable highly accurate measurement, by collecting the reflected beam from the surface of a specimen of wavelength changeable parallel luminous flux to make the same parallel and converting the beam passing through the pinhole at the focal position thereof to an electric signal. CONSTITUTION:The monochromatic beam of a monochrometer 9 having a wavelength controller 9' is divided by a beam splitter 4 through a collimator 10 and one of split beams is converged to the surface of a specimen 6 by a lens 1 while the reflected beam thereof is converged by a lens 2 through the lens 1 and the splitter 4 and a pinhole 3 having a diameter almost equal to that of the converged beam is arranged to the converging point of said lens 2 to receive the converged beam by a beam detector 5. In this case, the lens 1 is constituted of a lens capable of neglecting wavelength dispersion and a parallel flat plate having the wavelength dispersion of a refractive index and the other split beam of the splitter 4 is inputted to the intensity controller 8' of a while beam source 8 through a beam detector 11. Further, the wavelength of the meter 9 is changed to calculate a peak wavelength and the position of the specimen 6 is changed to similarily calculate a peak wavelength to calculate a step.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to movement of the focal point of light, and particularly to a minute step measuring device suitable for evaluating the roughness of the surface of an object.

[Background of the invention]

A typical known example of measuring the roughness or level difference of an object surface using light is explained in Fig. 1 (known documents: D, K,
) lamilton and T, li1ls
on: Appl, Phys.

827 (1982) 211-213).

(A) in Figure 1 is a confocal
) optical system. In this case, the optical system is focused at a position Z in the vertical direction of the surface of the object 6. At this time, the reflected light from the Z surface passes through the pinhole 3 and is converted into an electrical signal by the photodetector 5. Reflected light from positions Z and Z6, which are slightly away from Zl, cannot pass through the pinhole 3 and therefore hardly appears in the output signal of the photodetector. Therefore, the output signal of the photodetector when the object 6 is moved in the vertical direction (Z direction) is (
C). This means that the surface of the object is at the focal point of the lens 1 when the output signal I reaches its peak. Therefore, for example, when the object 6 has a step difference in a recess, the object 6 moves so that the recess is illuminated as shown in (B) in FIG. Further, the level difference is determined from the amount of movement when the object is moved upward to provide the peak of the output signal of the photodetector. In order to improve the accuracy of peak position detection, the point at which the differential signal of the output signal becomes zero is also detected.

However, in this method, since the object surface is moved to the focal position, there is a problem that errors are likely to occur due to optical path fluctuations caused by movement of the sample stage. In particular, when the sample is large and heavy, the accuracy improvement becomes a circular hole.

[Purpose of the invention]

An object of the present invention is to reduce optical path fluctuations caused by sample movement;
An object of the present invention is to provide a micro step measuring device capable of highly accurate measurement.

[Summary of the invention]

In order to achieve the above object, the present invention includes a light source that emits a wavelength-tunable parallel light beam, a light beam that is focused on the sample surface, the reflected light from the surface is made into a parallel light beam again, and a wavelength dispersion of the refractive index. an optical system with no wavelength dispersion of refraction that focuses the parallel beam of the reflected light; and a pinhole approximately the same size as the beam spot at the focal position of the reflected light focused by the optical system. a spatial filter consisting of;
The device is configured to include means for converting the light passing through the pinhole into an electrical signal.

[Embodiments of the invention]

The principle of the present invention is to measure the level difference by optically moving the focal point position using the wavelength dispersion of the lens. The effect of wavelength dispersion of the refractive index n of a lens is well known as chromatic aberration.

That is, when the wavelength of light is changed, the focal length f of the lens changes depending on the wavelength due to wavelength dispersion of the refractive index of the lens. The wavelength of light is λ, from λ2 (λ.

Suppose that when the refractive index changes to λ2), the refractive index changes from n to n2. Since Jn=nyo-n2.

The focus position movement Δf accompanying an is given by the following equation.

Generally, for a substance that absorbs in the ultraviolet region, as the wavelength in the visible region becomes longer, the refractive index decreases and the focal length of the lens increases. Therefore, the change in the focal length of the lens when the wavelength is changed is as shown in (a) in Figure 2.
Such wavelength dispersion has a remarkable effect only on lens 1,
It is assumed that lens 2 can be ignored.

Now, as shown in Figure 2 (b), if a signal peak is obtained at a wavelength λ1 for a certain sample surface, then a peak at another position on the sample is obtained at a wavelength λ2. Suppose that At this time, from the relationship shown in (a) in Figure 2, these two
The level difference If of the points is obtained from f, -f.

As an example, BK-7 is used as a lens material. From wavelength λ, = 435.8 nm m to λ, = 486.1n
The refractive index when changed up to m changes from n = 1.5267 to n = 1.5224. At this time Δn=
It is 0.0043. The focal length of the lens is 10m++
Then, Af=28 μm. Therefore, it is natural that the range of movement can be expanded by using zero fold flint glass, which can be used to measure steps or roughness of 20 μm or less, or other materials with large wavelength dispersion. Also,
It is also possible to expand the amount of movement by using light in a wavelength range with large wavelength dispersion. The wavelength of the light is determined by measuring the light from a white light source using a monochromator or an acousto-optic variable wavelength filter.
It is sufficient to make it monochromatic and wavelength variable.

In this way, in this method, when the focal point position is moved, the spatial arrangement between the optical system and the sample surface does not change.

FIG. 3 is a diagram showing an embodiment of the present invention. In the figure, light from a white light source indicated by 8 is converted into monochromatic light by a monochromator 9. Tuning of the wavelength is performed by a wavelength controller 9' of a monochromator. The monochromatic light is sent to the collimator 10.
It becomes a parallel beam of light, and is divided by beam splitter 4.
One of the lights is received by the photodetector 11 and converted into an electrical signal.

The other light is focused by lens 1 onto the surface of sample 6. The reflected light from the sample surface becomes a parallel beam of light at the lens 1, passes through the beam splitter 4, and is then focused at the lens 2. A pinhole 3 having a diameter approximately equal to the diameter of the focused beam is formed at the focal point of the reflected light by the lens 2. The light passing through the pinhole 3 is received by a photodetector 5 and converted into an electrical signal. The detection signal is input to the Y axis of the recorder 13. On the other hand, the signal detected by the photodetector 11 enters the intensity controller 8' of the light source 8, and limits the light intensity of the light source to the set value from the light amount setting signal generator 1z. An electrical signal corresponding to the wavelength of the monochromator is taken out from the wavelength controller 9' and input to the xi of the recorder 13. Therefore, when the wavelength of the monochromator is changed, the recorder records a peak at a certain wavelength λ1 at the 0th order, and then the movable stage 7 scans the wavelength at other positions on the sample surface. At this time, the wavelength at which a peak occurs in the recorder is assumed to be λ2. The focal length f corresponding to the wavelength λ of light is determined in advance, and fl and f corresponding to λ and λ2 are calculated in advance.
From the difference of 2, the step difference Δf between the two points can be found.

FIG. 4 is a diagram showing another embodiment of the present invention, in which a lens with negligible wavelength dispersion is used as lens 1, and a parallel plate 1' having wavelength dispersion of refractive index is inserted between lens 1 and the sample. This is what I did. Although only the lens 1 and the parallel plate are shown in FIG. 4, the other device configurations are the same as in FIG. 3. In this case, the change in the focal position with respect to the change in the refractive index is opposite to that in the case of a lens, so it is not effective to use an element that has wavelength dispersion at the same time as a lens and a parallel plate. In the case of a parallel plate, the shift in focal position due to the parallel plate having a refractive index n' and a thickness d is expressed by the following equation.

Δf= (1--) d (2)n' Therefore, the change in Jf with respect to the change in l is Δ/
It is given by d/n/2. Δn' is the change in refractive index when the wavelength is changed from λ1 to λ.

When a heavy windshield with a thickness of 2ffI11 is used, the movement of the focal point position with respect to a wavelength change of 436 nm to 486 nm is 14 μm.

〔Effect of the invention〕

As detailed above, according to the present invention, the focal position can be varied without moving the optical system such as a lens or the sample in the optical axis direction. Operational defects such as optical path disturbance due to tilt can be reduced,
Highly accurate measurements are possible.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining a conventional example.
) is a diagram showing the configuration of the optical system, (C) is a diagram showing an example of the output signal, and FIG. 2 is a diagram explaining the present invention in detail.
a) is a diagram showing the relationship between the amount of focus movement and wavelength, (b) is a diagram showing an example of an output signal, FIG. 3 is a diagram showing the configuration of an embodiment of the present invention, and FIG. FIG. 1... Light beam focusing lens with large wavelength dispersion of refractive index, 1'... Parallel plate with large wavelength dispersion of refractive index, 2.
...Light beam focusing lens, 3...Pinhole, 4...
・Beam splitter-15, 11...photodetector, 7・
...Sample moving stage for moving the sample in the optical axis and horizontal direction, 8
... Light source, 8'... Light intensity controller, 9... Monochromator, 9'... Monochromator wavelength controller, 10
... Beam collimator, 12... Light intensity of light source, etc.

Claims (1)

[Claims] a light source that emits a wavelength-variable parallel light beam, an optical system that focuses the light beam on the sample surface, converts the reflected light from the surface into a parallel light beam again, and has wavelength dispersion of refractive index, and a parallel light beam of the reflected light. An optical system with no wavelength dispersion of refraction to focus the
A spatial filter consisting of a pinhole approximately the same size as the beam spot is provided at the focal point of the reflected light focused by the optical system, and means for converting the light passing through the pinhole into an electrical signal. A device for measuring minute differences in height.