JPH10253315A - Optical interferometer and sample positioning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform multiple steps for sample positioning from rough adjustment of fine adjustment so as to realize high-speed and accurate positioning by constituting an optical wave interferometer using a light with short coherent distance so that the focal point of a condensing lens for sample positioning is set within an interferential range of a surface to be detected in a light flux of a light to be detected and the condensing lend obtains a plurality of different F values. SOLUTION: The diameter of luminous flux entering a condensing lens 9 is made small by using operture 11 in an initial stage and the condensing angle of collected luminous flux is made small, so that a light spot can be observed even when the moving speed of an object to be measured is high, and after the light spot is observed, the diameter of light flux entering the lens 9 is changed to be large by the aperture 11, so that the condensing angle of collected luminous flux is made large so as to detect optimal positioning position in high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an object positioning device for an optical interference device, and more particularly to a device for measuring the surface shape of a test surface of a thin plate-shaped object using light having a short coherence distance. The present invention relates to an improvement in an object positioning device of a light wave interference device used when positioning a test surface within a range in which interference is possible with a condenser lens.

[0002]

2. Description of the Related Art For example, in a Michelson-type interferometer, coherent parallel light is split into a reference light and a test light by a light splitting means, and the reference light and the test light are divided into a reference surface and a test light. After being reflected on the surface to be inspected and recombined by the light splitting means, interference fringes are formed on the observation surface, and by observing the interference fringes, the irregular shape of the surface to be inspected is formed. Evaluation can be performed.

When a laser interferometer is used as the interferometer, it is not necessary to accurately set the position of the test surface with respect to the reference surface because the interference distance of the laser light is long. In the case of a transparent thin plate such as glass, the reflected light from the back surface of the subject also interferes with the reflected light from the test surface and the reflected light from the reference surface, and noise components on the original interference fringes are generated. Interference fringes are superimposed.

For this reason, conventionally, when measuring thin glass or the like using a laser interferometer, it has been necessary to take measures such as applying a refractive index matching oil to the back surface where ghosts are generated. However, taking such measures requires a great deal of trouble, and there is also a problem such as contamination of the subject.

[0005] In addition, especially when the thickness of the test object is extremely thin, if a matching oil or the like is applied to the back surface, the test object is distorted due to the effect of the surface tension and accurate measurement of the test surface cannot be performed. Also occurs. Therefore, when measuring the surface shape of thin glass or the like, light having a short coherence distance (light having a coherence distance shorter than twice the thickness t of the subject) is used as the measurement light, and only the surface to be measured interferes. It is conceivable to set it in a possible range. However, in the above prior art, if the coherent distance of the light from the light source is S _CL , the interference possible range L where the test surface is to be disposed is L <S _CL / 2.

For example, when a light source having a coherence distance of about 30 μm such as a red light emitting diode (LED) is used as a light source, the interference possible range L is shorter than 15 μm. Unless the test surface of the subject is positioned within this narrow range, no interference fringe indicating the relative shape to the reference plate is generated.

Therefore, the inventor of the present application sets the condensing lens for positioning the subject within the light beam to be detected and the focal position thereof within the interference possible range of the surface to be detected. Developed a subject positioning device that observes the spot shape on the imaging surface formed by the return light from the test surface that has passed through the lens, and that allows the test surface to be easily positioned within the interference range. It has already been disclosed (Japanese Patent Application No. 7-35)
3871).

[0008]

However, the range of interference is extremely small, and when the surface to be measured is moved in the direction of the optical axis at a certain speed, a desired light spot is formed on the imaging surface. It is difficult to determine whether or not it has been formed. When the positioning of the subject is automatically performed, it is particularly difficult to recognize the light spot. Therefore, the surface to be inspected must be moved at an extremely low speed, and it is difficult to perform time-efficient positioning.

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a light wave interferometer using light having a short coherence distance, which can greatly reduce the operation time for setting the position of the surface to be detected in the interference possible range with a simple configuration. It is an object of the present invention to provide a subject positioning device for an interference device.

[0010]

The object positioning apparatus of the light wave interference apparatus according to the present invention divides light from a light source into two systems, one on a surface to be inspected and the other on a reference surface. Irradiating, observing interference fringes caused by interference between the test light from the test surface and the reference light from the reference surface, and measuring the surface shape of the test surface based on the observation result in a light wave interference device,
The coherence distance of the light is a distance shorter than twice the thickness of the subject, and the interference in the optical path between the split position of the light and the position where the interference is possible on the test surface. A condenser lens unit having a focal position within a possible range is provided, and the condenser lens unit is configured to be able to take a plurality of different F values.

Preferably, the plurality of different F values are obtained by changing the diameter of a light beam incident on the condenser lens using a variable stop disposed between the light source and the condenser lens. . Also, the plurality of different F values are
It is preferable that the plurality of condenser lenses are obtained by a plurality of condenser lenses having different focal lengths, and in this case, a condenser lens moving member for moving the plurality of condenser lenses so as to be sequentially inserted into the optical path is provided. Preferably.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view showing an object positioning apparatus according to an embodiment of the present invention, which is applied to a Michelson interferometer. This Michelson interferometer comprises a light source 1 and a collimator lens 2
, Beam splitter 3, reference plate 4, imaging lens 7
, A CCD element 8 and a subject holding drive mechanism (not shown).

The light source 1 is a light source for emitting light having a short coherent distance. The light 1a emitted from the light source 1 is collimated by a collimator lens 2 and then collimated by a beam splitter 3 with a reference light. It is divided into analysis and analysis.
The reference light and the test light are transmitted to the reference surface 4a of the reference plate 4 and the test object 5 supported by the test object holding drive mechanism.
After being reflected by the test surface 5a, the beam is split again by the beam splitter 3 to form an interference fringe on the imaging surface of the CCD 8 by the imaging lens 7.

In this interference apparatus, the back surface 5b of the subject 5 is placed on the interference fringe formed by the reflected light from the test surface 5a of the subject 5 and the reflected light from the reference surface 4a of the reference plate 4. A light source that emits light 1a having an extremely short interference distance is selected as the light source 1 so that interference fringe noise based on reflected light from the light source is not superimposed. When the light source that emits light with a short coherence distance is used in the interference device, the position of the test surface 5a in the optical axis direction is adjusted so that the optical path lengths of the reference light and the test light are equal to each other. It is necessary to adjust the distance of the test surface.

As shown in FIG. 1, if the coherent distance of the light 1a is S _CL , then L <S
_CL / 2. Moreover, the coherent distance _SCL is set to be extremely smaller than the thickness t of the subject 5.
For example, when the light source 1 is a red LED, the coherence length S _CL ≒ 30 μm. Then, the interference possible range L is 1
The value is smaller than 5 μm. Thereby, when the interference fringe based on the test surface 5a of the subject 5 is observed during the measurement of the interference fringe, the interference fringe based on the back surface 5b of the subject 5 is not observed. Then, the subject 5 is moved in the direction of arrow A by a subject holding and driving mechanism (not shown), and the subject surface 5a is positioned within the interference possible range. It is not easy to position the test surface 5a of the test object 5.

Therefore, in the above apparatus, as shown in the figure, a condenser lens 9 for positioning the subject 5 having a focal position F within the interference possible range 6 is provided in the optical path. . Then, when the test surface 5a is positioned within the interference possible range, the light beam from the condenser lens 9 is reflected so as to be substantially focused on the test surface 5a, and is substantially again reflected by the condenser lens 9. It is C
Since the light is incident on the CD 8, the light incident on the CCD 8 when the test surface 5a is not in the interference possible range is different from the state of incidence (divergent light or condensed light) on the CCD 8. Therefore, the observed spot pattern greatly differs depending on whether or not the test surface 5a is set in the interference possible range, and the current setting position of the test surface 5a can be clearly recognized.

However, even when such a configuration is employed, it is difficult to confirm the appearance of the interference fringes unless the rising speed of the test surface 5a is extremely slow. Therefore, in the above device, the aperture stop 11 that can arbitrarily change the diameter of the light beam incident on the condenser lens 9 is used.
Is provided so that the condensing lens 9 can take a plurality of different F values. In the initial stage of the positioning operation, the diameter of the light beam incident on the condensing lens 9 is set to be small (large F value). If a spot pattern is observed in this state, the aperture stop 11 is opened, and the diameter of the light beam incident on the condenser lens 9 is set to be large (small F value).
a is set to the optimum position.

The basic principle of the object positioning device using the condenser lens 9 will be described below with reference to FIGS. 2, 3 and 4. First, FIG. 2A shows that the test surface 5a of the test object 5 does not reach the coherent range 6 and the light from the condenser lens 9 is radiated and reflected on the test surface 5a in a divergent state. Show the case.

This condenser lens 9 is disposed in a parallel light beam emitted from the beam splitter 3 shown in FIG.
The test surface 5 is located at a position farther than the focal position F of the condenser lens 9.
When a exists, the light beam that has passed through the condenser lens 9 irradiates the surface 5a to be detected and is reflected, but the reflected light beam is located at a position F symmetrical to F with the surface 5a interposed therebetween. ′
It proceeds as if the light emitting point is present.

When this light beam passes through the condenser lens 9 again, as shown in FIG. 2A, the light beam returns to the beam splitter 3 as a converged light instead of a parallel light.
FIG. 2B shows that the test surface 5 a is located at the focal position F of the condenser lens 9.
Shows just reached. In this case, the test surface 5a
The light beam reflected by the light source becomes a light beam having a light emission point at the focal position F. This light beam is converted into a parallel light beam again by the condenser lens 9 and returns to the beam splitter 3.

FIG. 2C shows that the surface 5a to be detected is
Is shown after passing through the focal position F. In this case, a position F 'symmetrical to the focal point position F with the test surface 5a interposed therebetween becomes an actual point light source, and a divergent light beam from this point light source reenters the condenser lens 9. This position F '
Is inside the focal position F of the condenser lens 9, the light flux emitted from the condenser lens 9 and going backwards becomes divergent light and returns to the beam splitter 3.

In each of the cases shown in FIGS. 2A, 2B and 2C, the pattern formed on the CCD 8 is shown in FIG.
(B) and (c) are displayed. That is, FIG.
In the state shown in FIG. 3A, as shown in FIG.
2b is formed, and a bright spot 22a having a smaller size is formed in the spot 22b.

In the state shown in FIG. 2 (b), as shown in FIG. 3 (b), the entire diameter portion 21 of the condenser lens 9 becomes a spot 23 of uniform brightness.
Its brightness is equivalent to that of the periphery of this portion 21. Further, in the state shown in FIG. 2C, a slightly bright spot 25 is formed in a range larger than the size 21 of the diameter of the condenser lens 9 as shown in FIG. 3C. Further, a slightly dark spot 24 is formed corresponding to the size portion 21 of the diameter of the condenser lens 9.

As described above, the light beam that has traveled backward through the condenser lens 9 forms a spot of a predetermined size or a predetermined brightness on the CCD 8, and as shown in FIG. When the brightness is hardly distinguished from other areas in the area, it is possible to detect that the test surface 5a is positioned near the interference possible range 6.

The condensing lens 9 is arranged at the position of the peripheral light beam of the parallel light beam, so that the spot pattern generated by the converging lens 9 becomes the end of the observation screen which does not hinder the measurement when measuring the interference fringes. However, if it is desired to observe the spot pattern at the time of positioning in the center of the field of view 20 or if it is desired to completely eliminate the influence of this spot pattern from the interference fringe image,
At the time of measuring the interference fringes, a configuration may be employed in which the condenser lens 9 can be retracted out of the parallel light beam. Of course, it is also possible to automatically insert and retreat the condenser lens 9 into and out of the parallel light beam based on a signal that can recognize the difference between the positioning and the interference fringe measurement.

In this embodiment, FIG.
Based on the spot patterns shown in (a), (b), and (c), a CPU (not shown) automatically determines whether or not the current state is a state where the current state is positioned at a predetermined position. The subject holding and driving mechanism is controlled based on the determination result to stop and position the subject 5 at a predetermined position. In particular, when performing positioning by such automatic operation, it is difficult to detect the above-described state of FIG. 3B unless the speed of lifting the subject by the subject holding and driving mechanism is extremely slow. In this case, the efficiency of the positioning operation becomes extremely poor.

Therefore, in the apparatus according to the present embodiment, as shown in FIG. 1, the diameter of the parallel light beam incident on the condenser lens 9 is adjusted by the aperture stop 11, thereby adjusting the lens F value (F number). Then, the spot confirmable range and the intensity level of the peak value are switched. The F-number of a lens also indicates the form of an emitted light beam when a parallel light beam enters the lens. When a parallel light beam enters a convex lens having a large F value, the emitted light beam converges at the focal point f of the lens while gradually converging. That is, the change in the cross-sectional area of the light beam gradually occurs from the convex lens to the focal point f. This means that the intensity change on the optical axis from the convex lens to the focal point f also occurs gently.

Considering the condenser lens 9 shown in FIG. 2, if the F value of this lens is large, FIGS. 3 (a), (b), and (c)
The change of the obtained spot pattern shown in FIG. Further, there is a relationship between the F value and the diameter of the converging spot, that is, spot size S = F × θ (where F is the F value of the converging lens, and θ is the spread angle of the light beam incident on the converging lens). Yes, as the F value increases, the spot size increases in proportion.
The spot area changes in proportion to the square.

That is, when a convex lens having a large F value is used,
Both the change in the light beam diameter and the change in the intensity on the optical axis become gradual, and the central intensity of the condensed spot also decreases. In other words, the detection sensitivity of a sensor or the like can be reduced. The F value can be represented by F = f / D as described above. (Where f is the focal length and D is the effective beam diameter at the condenser lens).

Therefore, in order to lower the detection sensitivity, it is necessary to increase the F value, and it is necessary to increase the focal length f or reduce the light beam diameter D. For example, as shown in FIG. 4, the diaphragm aperture stop 11 to the state 11B, the beam diameter as a small diameter D _B, it is possible to gentle changes in beam diameter, in this manner, FIG. 3 As shown in (b), the range in which the spot 23 in which the brightness is hardly distinguished from other regions in the visual field 20 can be confirmed extends in the moving direction (vertical direction) of the subject 5. become.

However, when the F value increases, the change in the light beam diameter becomes gentler and the focused spot becomes larger, so that the change in the spot shown in FIG. , It is difficult to determine exactly.

FIG. 5 shows a schematic diagram of a change in light intensity on the optical axis across the focal point when light is condensed by lenses having different F values.
FIG. 5A shows a case where the F value is small, and FIG. 5B shows a case where the F value is large. In FIG. 5A, the desired position P
_O Although it can be determined with high accuracy, the rise of strength is steep, yet, it rises from P ₁ · P ₂ at the immediate vicinity of the intended position P _o. It is very difficult to detect this change while moving the subject 5 at high speed.

In FIG. 5B, the intensity gradually rises and rises from P ₃ and P _{4 at} a position distant from the expected position P ₀ . However, there is little change in light intensity near the expected position P ₀ , and P _{0 can be} accurately _detected.
The position cannot be determined. As described above, the case where the F value is increased and the case where the F value is decreased each have advantages and disadvantages with respect to the positioning. Therefore, when only one of them is used, highly accurate and efficient subject positioning is achieved. I can't do it.

Therefore, in the apparatus of the present embodiment, the positioning of the subject is performed efficiently and with high precision by setting the positioning operation to a two-stage operation of a coarse adjustment operation and a fine adjustment operation. That is, in the initial stage, by reducing the diameter of the light beam incident on the condenser lens 9 and increasing the F value, even if the moving speed of the subject is increased, it can be determined that the position is near the expected position Po. as the light spot can be observed, if the light spot is observed, for example, at the position P ₅ or P ₆ shown in FIG. 5 (C), the F value switches the light beam incident on the converging lens 9 to a larger diameter It is made small so that the expected position P ₀ can be detected with high accuracy.

As a result, the subject can be positioned with high speed and high accuracy. When the aperture diameter is large, the desired brightness of the condenser lens 9 is determined by the numerical aperture N.
When expressed by A, the numerical aperture NA <0.6.

In the above embodiment, in order to switch the converging angle of the light beam converged on the subject 5, the converging lens 9
Although the size of the diameter of the light beam incident on the lens is switched, the length of the focal length f may be switched instead. For example, as shown in FIG.
(F number = F _A ) and the long focal point converging lens 19B (F number = F _B ). At first, the coarse adjustment operation of the positioning is performed by using the long focal point converging lens 19B capable of reducing the converging angle of the light beam. Then, a fine adjustment operation of the positioning is performed using the short focal point converging lens 19A capable of increasing the converging angle of the light beam.

As a result, the subject can be positioned with high speed and high accuracy, as in the case where the diameter of the light beam incident on the condenser lens 9 is switched. When the desired brightness of the condensing lens 19A when the aperture diameter is large is expressed by the numerical aperture NA, the numerical aperture NA <0.6. Further, a plurality of condenser lenses 1 having different focal lengths are used.
The light spots 9A and 19B may be formed on the subject 5 at the same time, or the condensing lenses 19A and 19B
Are exchanged, and one condensing lens 19A, 1
Only the light spot of 9B may be sequentially formed.

As described above, when simultaneously forming a light spot by the plurality of condenser lenses 19A and 19B on the subject 5, it is necessary to switch the spot to be observed on the observation side. Further, as mentioned above,
Condensing lens 19 that sequentially forms a light spot on subject 5
When replacing A and 19B, the condenser lens 19A,
19B may be arranged on a reciprocating plate or a rotating disk, and these plates may be reciprocated or rotated to move desired condenser lenses 19A and 19B to predetermined positions.

Further, in the above-described embodiment, the case where the two-step adjustment is performed has been described. However, the aperture diameter of the aperture stop 11 may be changed in three or more steps or three or more different focal points. It is also possible to switch the lens having a distance to perform multi-stage adjustment of three or more stages from coarse adjustment to fine adjustment.

The method of switching the aperture of the aperture stop 11 in a plurality of stages can take various forms. For example, the aperture stop 11 can be formed by an iris stop, or the size can be changed sequentially. The plurality of openings may be provided on a slidable slide plate, or a slidable wedge-shaped plate that shields a part of the opening may be provided on the opening.

Further, as shown in FIG. 1, the shielding plate 10 is moved in the direction of arrow B, and inserted into the optical path of the reference light.
It may be possible to retreat and block the reference light from being incident on the reference plate 4 at the time of the above positioning. In this case, the S / N of the spot pattern in the observation visual field 20 is improved, and more accurate A positioning operation can be performed.

It should be noted that the object positioning device of the light wave interference device of the present invention is not limited to the above-described embodiment, but may be modified in various forms. For example, as the light receiving element, a line sensor or a single element sensor can be used in addition to the surface sensor such as the CCD. If a single element sensor is used,
The sensor may be provided so that only the change in the amount of light at the central portion of the sensor can be measured. The device of the present invention is not limited to a Michelson-type interferometer, and can be applied to an interferometer capable of making the optical path lengths of the test light and the reference light approximately equal distances, for example, a Mach-Zehnder interferometer. is there.

[0043]

As described above, according to the apparatus of the present invention, the light source for emitting light having a very short coherence distance is provided,
In the optical path of the light beam from the beam splitter, a condensing lens unit having a focal position within an interference possible range of the test surface is disposed,
When the test surface is located within the interference range,
The light beam from the condenser lens portion is reflected so as to be substantially focused on the surface to be measured, is converted into a substantially parallel light beam again by the condenser lens portion, and is incident on the light receiving element.
Further, the condensing lens portion is configured to be able to take a plurality of different F values. Therefore, in the initial stage of the positioning, the F value is increased, and the light spot observable range is increased so that the light spot on the object can be observed even when the object is moved at high speed. After the light spot is observed, the F value is reduced, and the intensity change of the light spot is increased so that the optimal positioning position of the subject can be detected with high accuracy. That is, by performing the positioning of the subject in a plurality of stages from the rough adjustment to the fine adjustment, the positioning of the subject can be performed at high speed and with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a subject positioning device of a light wave interference device according to the present invention.

FIG. 2 is a diagram for explaining a technique which is a premise of the embodiment.

FIG. 3 is a diagram for explaining a technique which is a premise of the embodiment.

FIG. 4 is an enlarged view showing a part of the embodiment.

FIG. 5 is a diagram showing the operation of the embodiment.

FIG. 6 is an enlarged view showing a part of an embodiment different from the above embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Collimator lens 3 Beam splitter 4 Reference plate 4a Reference surface 5 Subject 5a Test surface 6 Interferable range 7 Imaging lens 8 CCD 9 Condensing lens 10 Light shielding plate 11 Aperture stop 19A, 19B Condensing lens 20 Field of view

Claims (4)

[Claims] 1. A light from a light source is divided into two systems, one of which is irradiated on a test surface of a subject and the other is irradiated on a reference surface, and the test light from the test surface and the reference light are irradiated on the reference surface. In the light wave interference apparatus for observing interference fringes caused by interference of the reference light and measuring the surface shape of the test surface based on the observation result, the coherence length of the light is twice the thickness of the test object. A light-condensing lens portion having a focal position within the interferable range is disposed in an optical path between the light splitting position and the interferable range position of the test surface. Wherein the condensing lens unit is configured to be capable of taking a plurality of different F-numbers. 2. The method according to claim 1, wherein the plurality of different F-numbers are obtained by changing a diameter of a light beam incident on the condenser lens using a variable stop disposed between the light source and the condenser lens. The object positioning device for an optical interference device according to claim 1. 3. The apparatus according to claim 1, wherein the plurality of different F-numbers are obtained by a plurality of condenser lenses having different focal lengths. 4. A subject positioning apparatus for a light wave interference device according to claim 3, further comprising a converging lens moving member for moving said plurality of converging lenses so as to be sequentially inserted into said optical path. apparatus.