JP3072925B2 - Interferometer for transmitted wavefront measurement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a configuration of an interferometer such as a Twyman-Green type, a Michelson type and a Mach-Zehnder type used for measuring the performance of an optical component such as a lens on the transmitted wavefront of the optical component. It is about.

[0002]

2. Description of the Related Art Interferometers have been used for optical measurement for a long time, but after the emergence of a light source having good temporal and spatial coherency, such as a He-Ne (helium-neon) laser, has emerged. In particular, the range of application has been widened, and today it is indispensable for measuring and inspecting optical components.
Although there are various configurations of the interferometer, a Fizeau interferometer using a He-Ne laser as a light source is the most used at present. The reason is that there are many portions that become a common optical path between the reference optical path and the test optical path, the precision required for the optical components used in the interferometer is not strict, and it is strong against air disturbance and vibration.

When observing the transmitted wavefront of a lens as an interference fringe, a light source that generates light of an appropriate wavelength for the lens to be tested should be a light source for an interferometer. In many cases, a light source that generates light with sufficient coherency cannot be obtained. In that case, for example, a light source having an emission line spectrum of an appropriate center wavelength such as a Na (sodium) lamp or a mercury lamp is used as a quasi-monochromatic light source, and from the relation of coherence length, the configuration of the interferometer is Twyman-Green type. Alternatively, a Mach-Zender type is adopted.

[0004]

When a light source having an emission line spectrum having an appropriate center wavelength, such as a Na lamp or a mercury lamp, is used as a light source for an interferometer, temporal coherence, that is, coherence length (coherence length) is required. The problem is that the configuration of the interferometer was changed to Twyman-Green or Mach-
The problem can be solved by using a zender type. However, these lamp light sources have a certain size of a light-emitting point and poor spatial coherency. Therefore, when used as a light source for an interferometer, interference fringes with high contrast cannot be obtained, and optical components as test objects are not obtained. In some cases, there is a problem that measurement cannot be performed.

In order to effectively reduce the size of the light source,
Focus the light emission point of the lamp light source on the pinhole with the lens,
A method using light passing through this pinhole is also conceivable, but has a drawback that the obtained interference fringes are so dark that they cannot be observed. In recent years, in order to increase the resolution limit of an optical system, an excimer laser (XeF,
XeCl, KrF, ArF, etc.) have often been used as the light source. When an optical component of this optical system is measured with a transmitted wavefront, an appropriate alternative light source cannot be obtained in many cases, so that an excimer laser is used as a light source to constitute an interferometer. However, despite being a laser, excimer lasers also have poor spatial coherency, resulting in low contrast of interference fringes,
There is a problem that the required accuracy cannot be measured. As mentioned above, it is conceivable to image the excimer laser light source on the pinhole, but the high pulse energy of the excimer laser causes significant damage to the pinhole.
If the pulse energy of the excimer laser is reduced to prevent damage to the pinhole, it is a trade-off that interference fringes cannot be observed at the required brightness.

The above-mentioned problem is illustrated in FIGS. 6 to 10 by taking a Twyman-Green interferometer as an example. 6, reference numeral 1 denotes a light source having an emission line spectrum such as a Na lamp or a mercury lamp, and reference numeral 2 denotes a light emitting portion of the light source 1 having a certain size. Reference numeral 3 denotes a condenser lens, 4 denotes a pinhole having a predetermined hole diameter, 5 denotes a collimator lens, and 6 denotes a filter that passes only a bright line spectrum having a desired center wavelength. The light source 1 to the filter 6 constitute a light source system 7. Reference numeral 9 denotes a semi-transmissive mirror, 12 denotes a reference-side flat mirror, 13 denotes a lens, 14 denotes an observation surface, 15 denotes a test lens which is a test object, and 16 denotes a test-side spherical mirror. The semi-transparent mirror 9 is a rectangular or cubic prism beam splitter formed by joining the slopes of two right-angled triangular prisms with a semi-transparent film interposed therebetween, or a beam splitter in which a semi-transparent film is attached to one surface of a glass substrate having both flat surfaces. However, in order to make the discussion simple and easy here, a translucent film beam splitter having an extremely thin film thickness is used. The test-side spherical mirror 16 is arranged so that its spherical center coincides with the rear (exit-side) focal point of the test lens 15. The observation surface 14 is, for example, a photographic film, a TV camera imaging surface, frosted glass, or the like. When the wavelength of light emitted from the light source system 7 is in an ultraviolet region, the observation surface 14 may be a fluorescent surface that can be visualized.

In the Twyman-Green interferometer shown in FIG. 6, the light emitting section 2 is focused on the pinhole 4 by the condenser lens 3. The pinhole 4 is sufficiently large so that the interference fringe does not become insufficient in light amount. The light that has passed through the pinhole 4 is converted into a substantially parallel light beam by the collimator lens 5 and further passes through the filter 6 and enters the main body of the interferometer as quasi-monochromatic light. The substantially parallel light beam 8 of the quasi-monochromatic light emitted from the light source system 7 is transmitted through the semi-transparent mirror 9 to the reference light 10 and the test light 1.
The reference light 10 is reflected by the plane mirror 12 on the reference side, enters the semi-transparent mirror 9 again, passes through the lens 13, and reaches the observation surface 14. On the other hand, the test light 11 passes through the test lens 15, is reflected by the test-side spherical mirror 16, returns on the same optical path as the outward path, and is observed again through the test lens 15, the semi-transparent mirror 9, and the lens 13. The surface 14 is reached. As described above, the reference light 10 and the test light 11 passing through the respective optical paths are transmitted to the observation surface 1
4 overlaps to produce interference fringes. If the generated interference fringes have distortion, the distortion corresponds to the aberration of the lens 15 to be inspected. Therefore, by analyzing the distortion of the interference fringe, the inspection and measurement of the lens 15 to be inspected can be performed.

Here, a light beam emitted from the light source system 7 will be considered. In order to obtain interference fringes of the required brightness, the diameter of the pinhole 4 must be considerably large, and as a result, after passing through the collimator lens 5, the light beam having a certain divergence angle is transmitted to the interferometer body. However, this slightly spread light beam causes a decrease in contrast of interference fringes. When the light source is an excimer laser,
In general, an excimer laser has poor spatial coherency and an emission beam also has a certain divergence angle. Therefore, the same is true even when the entire light source system 7 is replaced with an excimer laser.

The reason why the contrast of interference fringes generated when a light beam having a diverging angle enters the main body of the interferometer is poor is shown in FIG.
This will be described in detail with reference to FIG. The configuration shown in FIG. 7 is basically the same as the configuration shown in FIG. 6, except that a light source system 7 ′ has a light emitting surface 17 having a certain size and a collimator lens in which the light emitting surface 17 is located at the front focal plane. 5 '. 1
Numeral 8 denotes an optical axis of the interferometer.
7 corresponds to the pinhole 4 in FIG. Now
Two light beams emitted from a point Q which is one point off the optical axis 18 on the light emitting surface 17 and reaching a point P which is an intersection of the observation surface 14 and the optical axis 18 via the reference optical path and the test optical path, respectively. Consider the ray of Reference numeral 19 denotes a light ray that emits the point Q and reaches the point P via the reference optical path, and reference numeral 20 denotes a light ray that emits the point Q and arrives at the point P via the test optical path. It is assumed that the reference-side flat mirror 12 is perpendicular to the optical axis 18 and the lens 15 and the spherical mirror 16 are not decentered. Ray 19 and ray 20
Are reflected by the reference-side plane mirror 12 and the test-side spherical mirror 16 and intersect at point C when reaching the translucent mirror 9 again.
Thereafter, the light reaches the point P via the same optical path. Since the light beam 19 and the light beam 20 are both light beams emitted from the same Q point and are coherent with each other, if the reference side optical path length is appropriately set with respect to the test side optical path length from the viewpoint of temporal coherence, That is, if the position of the reference side plane mirror 12 is appropriate, interference occurs at the point P. P due to this interference
The brightness at the point is determined by the phase difference between the light ray 19 and the light ray 20 at the point P.

The phase difference between the light beam 19 and the light beam 20 at the point P is as follows. The position of the reference-side plane mirror 12 is naturally determined from the viewpoint of temporal coherence once the configuration and position of the optical system to be measured are determined. On the other hand, looking at the optical system to be examined, an afocal optical system having an afocal ratio of 1 in consideration of reciprocation is formed, and a virtual plane mirror 21 is provided on the conjugate plane of the spherical mirror 16 to be inspected with respect to the lens 15 to be inspected. Ray 20 travels as if it were present. In FIG.
The optical path from the light source system 7 'to the observation surface 14 when viewed through the object side is shown in an expanded manner, ignoring the optical path from the virtual plane mirror 21 to the object side spherical mirror 16. FIG. 9 shows an expanded optical path from the light source system 7 ′ to the observation surface 14 when viewed through the reference side. As shown in FIGS. 8 and 9, the two optical paths generally appear to have a certain displacement (ΔZ). Since ΔZ also depends on the position of the reference-side plane mirror 12, ΔZ immediately changes from the virtual plane mirror 21 to the test-side spherical mirror 1.
The optical path length of the wave optical path does not always reach up to 6.

However, in order to make the discussion simple and easy, if the wave optical path length of the test path in monochromatic light of wavelength λ _s is equal to the wave optical path length of the reference path, ΔZ is The optical path length of the reciprocating wave optical path from the virtual plane mirror 21 to the test-side spherical mirror 16 is equal to the optical path length. FIG. 10 is an exploded view in which the case of passing through the subject side and the case of passing through the reference side under this condition are superimposed. Considering the reciprocation, the optical system to be inspected becomes an afocal optical system. Therefore, the wave optical optical path length of the optical path ignored in FIG. 8 is along the optical axis 18 from the virtual plane mirror 21 to the spherical mirror 16 to be inspected. It is equal to the reciprocating wave optical path length. Therefore, the wave optical path length of the light ray 20 'in FIG.
This is the actual wave optical path length of the light ray 20 (FIG. 10).
Therefore, the phase difference between the light beam 19 and the light beam 20 at the point P can be obtained from FIG.

Since the light beam 19 and the light beam 20 are both emitted from the same point Q, they have the same phase up to the points E and F. On the other hand, from the point G to the point P, since the light beam 19 and the light beam 20 travel on the same optical path, no phase difference occurs during this time. As a result, between the FG of the light ray 19 (ΔZ) and between the EG of the light ray 20 (Δ
Z ′) is the required phase difference (the phase difference between the light rays 19 and 20 at the point P) Δφ. That is,

Δφ = (2π / λ _s ) (ΔZ−ΔZ ′) = (2π / λ _s ) ΔZ (1-cos θ)・ ・ ・ ・ ・ ・ (1)

Here, θ is the spread angle of the light beam. From equation (1), it can be seen that the phase difference Δφ depends on the divergence angle θ of the light beam emitted from the light source system 7 ′, thereby lowering the contrast. For example, when the test lens 15 is large, such as a photographing lens for circuit pattern exposure, Δ
Z ranges from 1m to 2m. When the light source is KrF excimer laser, the center wavelength lambda _s of the spectrum 248 nm, divergence angle of the light beam θ is 1 mrad, [Delta] Z is calculated as 1 m, [Delta] [phi is about 2 [pi. That is, the divergence angle θ of the light beam is
Between 0 and 1 mrad, the phase will go around once,
Interference fringes with contrast are hardly obtained. So far, observation surface 1
Although the point P on the axis 4 has been described, the same result can be obtained for the off-axis observation point by the same method.

The present invention has been made in view of the above-mentioned problems involved in a transmitted wavefront measuring interferometer, and has been developed to reduce the brightness and contrast with which an optical component as a test object can be measured even if the spatial coherency of a light source used is somewhat poor. It is an object of the present invention to provide a transmitted wavefront measurement interferometer that generates interference fringes.

[0016]

An interferometer for measuring a transmitted wavefront according to the present invention divides a substantially parallel light beam emitted from a light source system into two, one of which is a reference light beam, and the other is a test light beam. to place a test object serving optical component composed of an optical material element, superposing the reference light beam passing through the reference optical system and the said test light beam that has passed through the target optical system, interference in a predetermined observation plane via the configured interferometer to form a fringe, and geometrical optics relationships between the light source system and the observation surface when viewed through the <br/> reference optical system, the target optical system to the so that the source system and the geometrical optics relationships between the observation surface is equivalent when viewed, in which is characterized in that constitute the reference optical system. Further, the present invention provides the above-mentioned reference light.
Make sure that the afocal optical system is
It is characterized by. Further, the present invention provides the above-mentioned reference optical system.
But with a plane mirror and a positive or negative or negative or positive power configuration
Afocal lens system
You. Further, the present invention relates to a method in which the configuration of an interferometer is
Lean type or Mach-Zehnder type
It is what it was.

[0017]

According to the above-described structure, even when the light source has a certain size and the spatial coherency is poor, of the light beams emitted from an arbitrary point of the light source, a certain light beam passing through the reference light path and the light Since the phase difference when a certain ray passing through the optical path and coherently superimpose at a certain point on the observation surface is aligned regardless of the position of the light emitting point on the light source, the contrast of the observed interference fringes increases, High-precision measurement and inspection become possible.

[0018]

FIG. 1 shows a first embodiment of the present invention, in which the present invention is applied to a Twyman-Green interferometer. The basic configuration is the same as the configuration described above with reference to FIGS.
In all the following descriptions, the parts related to the same function
The same symbols are used and will not be described again. Reference numeral 22 denotes an afocal optical system, which is provided as a correction optical system in a reference optical path. By providing the afocal optical system 22, the geometrical optics of the light source and the observation surface when viewed through the reference optical path can be changed without a change in the wave optical optical path length of the reference optical path determined by the requirement of temporal coherence. The relationship and the geometrical optical relationship between the light source and the observation surface when viewed through the optical path to be tested were made equivalent. More specifically, a virtual plane mirror 23 conjugate to the reference side plane mirror 12 and the substantially afocal optical system 22 having substantially no aberration is positioned on the observation side 14 on the reference side with respect to the position of the virtual plane mirror 21 on the test side. The afocal optical system 22 is attached to the reference optical path so as to be at an equivalent position when viewed from the point P. With this configuration,
Even if the divergence angle θ of the light beam exists, the ray 1 in FIG.
Since 9 coincides with the light ray 20, the phase difference Δφ is constant without depending on the spread angle θ of the light beam. Therefore, interference fringes with high contrast can be obtained.

In this embodiment, the position of the reference-side plane mirror 12 is naturally determined by the requirement of temporal coherence, but the afocal optical system 22 may move freely along the optical axis. At this time, since the position of the virtual mirror 23 on the reference side also moves, the optimum position can be easily selected while actually observing the interference fringes. The reason for the ease of operation is that the reference-side reflecting mirror is a plane mirror and is configured in combination with the afocal optical system. Note that the afocal optical system (correction optical system) 22 always has an afocal ratio of 1 when viewed as a reciprocation, so that the afocal ratio when viewed as one way may theoretically be any multiple. . Also, the afocal optical system (correction optical system) 22 must be aberration-free because it is attached to the reference optical path. However, in principle, only spherical aberration is a problem.
It is necessary to consider axial chromatic aberration in the range of the wavelength spectrum of the light emitted from the light source system.) However, in terms of design as well as production, the number of elements can be reduced with today's advanced precision optical element manufacturing technology. Can easily realize no aberration.

6 to 10, each group of the condenser lens 3, the collimator lenses 5, 5 ', the lens 13, the test lens 15, and the afocal optical system (correction optical system) 22 is a single lens. However, as a matter of course, it is actually composed of a plurality of lens elements as the case may require.

The second embodiment is applied to the case where the spatial coherency of the light source system 7 'in FIG. 1 is poor but the temporal coherency can be considered to be sufficiently good.
2 is arranged at the position of the virtual plane mirror 23. As a result, as described in the first embodiment, a decrease in interference fringe contrast due to spatial coherency is eliminated.

The third embodiment is also an example applied to a Twyman-Green interferometer. In the first embodiment, the test lens 15 in the test optical system is a convex lens as a whole, and the test-side spherical mirror 16 is a concave mirror. As another combination, there is a case where the test lens as a whole is a convex lens-the test-side spherical mirror is a convex mirror, and the test lens as a whole is a concave lens-the test-side spherical mirror is a concave mirror. The three types of combinations of the optical systems to be tested are referred to as (1), (2) and (3) in the order described above, and reference optical systems suitable for each of them and configured by applying the present invention are listed in FIG. This is shown in a table. However, these combinations are not always optimal. The reference optical system (a) includes a plane mirror and an afocal optical system (convex + convex, convex +
(B) is a spherical mirror (convex,
(Concave) and lens (convex, concave). Although not shown, the test optical system may be a combination of an afocal optical system and a plane mirror. With the configuration shown in FIG.
The combination of (1)-(a) has already been described as the first embodiment. In any case, the reference optical system is configured so that the virtual plane mirror on the subject side and the virtual plane mirror on the reference side come to equivalent positions. The effects of the reference optical system are the same for both (a) and (b), but the combination with the plane mirror of (a) is practical and convenient because the adjustment is easy as described above.

FIG. 3 shows a fourth embodiment of the present invention, in which a lens whose object point is at a finite position is a lens to be tested, and shows the configuration of a test optical system of a Twyman-Green interferometer. . Reference numeral 15 'denotes a test lens, and reference numeral 24 denotes a lens barrel lens.
The lens barrel 24 forms a spot at the object point position Q of the test lens 15 '. The lens barrel 24 needs to be substantially aberration-free. However, in principle, spherical aberration and, in some cases, axial chromatic aberration in the range of the wavelength spectrum of the light source may be considered. Astigmatism can be realized. If the lens barrel 24 and the test lens 15 'are combined and considered as the test lens 15, this corresponds to (3) in FIG. However, when the test lens 15 'is large, such as a projection lens for circuit pattern exposure, the reference optical system that can be used is (1)-(a) in FIG.
The combination of the convex-convex afocal optical system and the plane mirror is preferable.

FIG. 4 shows a fifth embodiment of the present invention, which is applied to measurement of an off-axis object point of a lens to be inspected, and shows a configuration of an optical system to be inspected of a Twyman-Green interferometer. Basically, the configuration is the same as that of the first embodiment shown in FIG. 1, but the test lens 15 is installed at a predetermined angle with respect to the optical axis 18. The test lens 15 is a thin lens, and its node is shown as being on the optical axis 18. In the case of a thick lens, the converging point of the lens 15 to be examined moves greatly unless the lens is tilted around the rear principal point (node). You need to move them together. In this state, the test-side spherical mirror 16
A straight line parallel to the optical axis 18 passes through the spherical center of
Is defined on a plane passing through a conjugate point (point J in the drawing) of the point with respect to the test lens 15 and perpendicular to the optical axis 18, and the above-described virtual plane mirror 21 'is considered. 1
To the third embodiment. Strictly speaking,
Not all rays that enter the test optical system at an angle to the optical axis 18 appear to be reflected by the virtual plane mirror 21 '. However, by configuring as in the present embodiment, a great improvement in practical use can be obtained as compared with the case where the present invention is not applied.

FIG. 5 shows a sixth embodiment of the present invention.
The configuration when applied to a zender interferometer is shown. 2
Reference numeral 6 denotes a test-side flat mirror, 27 denotes a second semi-transparent mirror, and 28 denotes a reference-side flat mirror. The substantially parallel light beam 8 emitted from the light source system 7 ′
The light is split into a reference light 10 and a test light 11 by a first semi-transparent mirror 9. The test light 11 is converged by a lens barrel 24 ′ having substantially no aberration to form a spot 25. The spot 25 becomes the object point of the lens 15 'to be inspected. The positions of the lens barrel 24 'and the lens 15' may be reversed. If the lens barrel 24 'and the lens 15' are combined to form the lens system 15, the lens system 15 is an afocal system. The light beam emitted from the test lens system 15 is reflected by the test-side flat mirror 26 and proceeds to the second semi-transparent mirror 27.

On the other hand, an afocal optical system 22 'as a correction optical system is provided on the reference optical path. The afocal optical system 22 'is an optical system that is geometrically equivalent to the lens system 15 to be inspected, and has the same afocal ratio. It is desirable that the position of the afocal optical system 22 ′ in the reference optical path be equivalent to the position of the lens system 15 to be inspected in the optical path to be inspected. However, in principle, it is not necessarily strictly equivalent. The afocal optical system 2
As the group configuration 2 ′, a configuration that is easily configured to be geometrically equivalent to the configuration of the test lens system 15 is selected. Reference light 10
Is transmitted through the afocal optical system 22 ′, reflected by the reference side plane mirror 28, and then proceeds to the second semi-transparent mirror 27, partially reflected and passed through the lens 13 to reach the observation surface 14, and
The light overlaps with the test light to form interference fringes reflecting the performance of the test lens 15 '. With this configuration, even if the light beam emitted from the light source system 7 'has a certain divergence angle with the same effect as described in the first embodiment, the contrast of the interference fringes is not impaired. Highly accurate measurement can be performed.

[0027]

As described above, in the transmitted wavefront measuring interferometer of the present invention, the spatial coherency of the light source is poor, and even if the luminous flux emitted from the light source has a certain divergence angle, a clear and high contrast can be obtained. Since interference fringes are obtained,
Inspection and measurement of optical components can be performed with high accuracy.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a first embodiment of a transmitted wavefront measuring interferometer according to the present invention.

FIG. 2 is a list of various aspects of a third embodiment of the transmitted wavefront measuring interferometer according to the present invention.

FIG. 3 is an explanatory view of a fourth embodiment of a transmitted wavefront measuring interferometer according to the present invention.

FIG. 4 is an explanatory view of a fifth embodiment of the transmitted wavefront measuring interferometer according to the present invention.

FIG. 5 is an explanatory view of a sixth embodiment of the transmitted wavefront measuring interferometer according to the present invention.

FIG. 6 is an explanatory diagram of an interferometer taking a Twyman-Green interferometer as an example.

FIG. 7 is an explanatory diagram of a problem taking a Twyman-Green interferometer as an example.

FIG. 8 is an explanatory diagram of a problem taking a Twyman-Green interferometer as an example.

FIG. 9 is an explanatory diagram of a problem taking a Twyman-Green interferometer as an example.

FIG. 10 is an explanatory diagram of a problem taking a Twyman-Green interferometer as an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 8 Substantially parallel light beam 10 Reference light 12 Reference side plane mirror 14 Observation surface 15 Test lens 22 Afocal optical system

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Claims (4)

(57) [Claims] A substantially parallel light beam emitted from a light source system is divided into two, one of which is a reference light beam and the other is a test light beam, and an optical component as a test object constituted by an optical material element is included in the test light beam. arrangement, and the reference light beam passing through the reference optical system and the passing through the target optical system superimposed a test light beam, in the configuration with interferometer to form an interference fringe at a predetermined observation plane, the <br / > the geometrical optics relationships between the light source system and the observation surface when viewed through the reference optical system, between the light source system and the observation surface when viewed through the target optical system Characterized in that the reference optical system is configured such that the geometrical optical relationship is equivalent to the above . 2. The reference optical system according to claim 1, wherein
The optical system according to claim 1, wherein the optical system is configured.
An interferometer for transmitted wavefront measurement as described. 3. The optical system according to claim 1, wherein the reference optical system includes a plane mirror,
Is an afocal lens system with positive / negative or negative / positive power
The transmitted wavefront according to claim 1, comprising:
Interferometer for measurement. 4. An interferometer having a Twyman-Green structure
Or Mach-Zehnder type
The transmitted wavefront measurement interferometer according to claim 1.