JP2002022606A - Method and equipment for measuring optical aberration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an equipment which can measure aberration even of an objective lens having a high numerical aperture of NA>1 by optical interference measurement. SOLUTION: When the optical aberration of an objective lens having a numerical aperture of NA>1 is measured by optical interference measurement, a hemispherical reference mirror is disposed such that the planar part thereof faces the bottom face of the objective lens and a matching oil is placed between the bottom face of the objective lens and the planar part of the hemispherical reference mirror. Optical axes of the objective lens and the hemispherical reference mirror are then aligned to constitute one optical path of an interference system. In case of an SIL or an SIM, incident light is totally reflected on the bottom face (recording/reproducing plane) and the light does not propagates to the outside of the lens. The hemispherical reference mirror is disposed such that the planar part thereof abuts on the bottom face in order to avoid total reflection conditions on the bottom face thus constituting a transmission optical system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solid immersion lens (SIL) and a solid immersion mirror (SIM).
The present invention relates to an optical aberration measuring method and an optical aberration measuring apparatus which can cope with an objective lens having a high numerical aperture as described above.

[0002]

2. Description of the Related Art Recording and reproduction of signals in an optical disk system are generally performed through an optical device called an optical head or an optical pickup. In such an optical head, as a means for condensing light on recording pits. An objective lens having a high numerical aperture (numerical aperture: NA) is used.

Conventionally, the resolution performance of an objective lens is NA <1.
However, in recent years, an optical disk system using the near-field principle has been proposed, and an optical configuration realizing NA> 1 has been proposed.
In addition, the configuration of the objective lens has been specifically proposed.

[0004]

By the way, Seidel (S) is used as one index indicating the optical performance of an objective lens.
Eidel) aberrations are commonly used,
As a specific measurement method, there is an optical interference measurement based on a fringe scan method.

[0005] In these measuring apparatuses, using a Mach-Zehnder interferometer or a Twyman-Green interferometer, interference generated between an optical path having a reference surface and an optical path having a test surface (optical path into which a lens to be measured is inserted). Since aberrations are classified by analyzing fringes, it is basically necessary to configure a transmission type optical system.

However, in the objective lens where NA> 1, the light converging point is located on the bottom surface of the objective lens, and the distance between this surface and the optical disk facing the surface is set to such an extent that an evanescent wave can be interposed (about 100 nm or less). Since signals are recorded / reproduced by approaching the lens, the light is not normally propagated outside the lens, which is a so-called total reflection condition. Therefore, in the objective lens for realizing such NA> 1, in principle, the above-described aberration measurement by the optical interference measurement cannot be performed.

The present invention has been proposed in view of such a conventional situation, and aims to make it possible to measure aberration by optical interference measurement even for an objective lens having a high numerical aperture of NA> 1. It is an object of the present invention to provide an optical aberration measuring method and an optical aberration measuring device for realizing this.

[0008]

In order to achieve the above-mentioned object, an optical aberration measuring method according to the present invention provides a method for measuring an optical aberration of an objective lens having a numerical aperture NA> 1 by optical interference measurement. The hemispherical reference mirror is placed so that the flat surface of the hemispherical reference mirror faces the bottom surface of the lens, and matching oil is interposed between the bottom surface of the objective lens and the flat surface of the hemispherical reference mirror. The alignment is performed so as to coincide with each other, and one optical path of the interference system is configured.

An optical aberration measuring apparatus according to the present invention is an optical aberration measuring apparatus for measuring the optical aberration of an objective lens having a numerical aperture NA> 1 by optical interference measurement. An optical system that constitutes one optical path, a reference plane mirror that constitutes the other optical path of the interferometer, and a laser beam from the laser light source.
It is characterized by comprising a beam splitter that separates and combines the optical paths corresponding to the two optical paths, and a detector that detects interference of return light from both optical paths.

For example, in the case of SIL, incident light is totally reflected on the bottom surface (recording / reproducing surface), and light does not propagate outside the lens.

Therefore, in the present invention, a transmission type optical system is configured by arranging the flat portion of the hemispherical reference mirror so as to be in contact with the bottom surface and avoiding the condition of total reflection at the bottom surface.

As described above, if a transmissive optical system is formed by using a hemispherical reference mirror, interference with an objective lens having a high numerical aperture (NA> 1) can be obtained by applying the conventional technology. It is possible to perform optical interference measurement for analyzing aberrations and performing aberration classification.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical aberration measuring method and an optical aberration measuring apparatus to which the present invention is applied will be described in detail with reference to the drawings.

Recording and reproduction of signals in an optical disk system are generally performed via an optical device called an optical head or an optical pickup. In such an optical head, a means for condensing light to a recording pit is used as a means. An objective lens having a numerical aperture (numerical aperture: NA) is used.

In particular, in recent years, an optical disk system [Near Field R] utilizing the near-field principle has been recently developed.
recording (NFR) optical disc system] has been proposed, and an optical configuration realizing NA>1;
And an objective lens configuration has been specifically proposed.

In the NFR optical disk system, a solid immersion lens (SIL: Soli) as shown in FIG.
d Immersion Lens) 1 or a solid immersion mirror (SIM: Solid) as shown in FIG.
An objective lens such as Immersion Mirror 2 is used, and light focused on the bottom surface of the lens is coupled to a recording pit (or recording mark) of an optical disk via an evanescent wave, thereby enabling signal recording or reproduction. Has become.

Here, the distance between the lens bottom surface and the optical disk surface is about 100 nm or less in order to realize good coupling by evanescent waves. Further, since the NA of the objective lens is NA> 1, all the light focused on the bottom surface is totally reflected on the same surface, and only the engineering vanescent wave controls the recording / reproducing process.

The optical aberration measuring method and optical aberration measuring apparatus of the present invention are for performing optical interference measurement on an objective lens having a high numerical aperture (NA> 1) used in such an NFR optical disk system. .

As one index indicating the optical performance of an objective lens, Seidel's aberration is generally and often used.
There is optical interference measurement based on the fringe scan method.

In these measuring apparatuses, a Mach-Zehnder interferometer or a Twyman-Green interferometer is used, and interference generated between an optical path having a reference surface and an optical path having a surface to be measured (optical path into which a lens to be measured is inserted). Since aberrations are classified by analyzing fringes, it is basically necessary to configure a transmission type optical system.

FIG. 3 shows a configuration of an aberration analyzing optical system using a Twyman-Green interferometer for measuring the aberration of an objective lens in which NA <1.

This aberration analyzing optical system is for analyzing aberration by optical interference measurement, and includes two optical paths constituting an interference system.

That is, a He—Ne laser 1 as a light source
The laser light emitted from 1 is expanded to a predetermined effective diameter via an expander 12 and then reflected by a reflection mirror 13 to enter a beam splitter 14.

In the beam splitter 14, a part of the incident light is reflected and irradiates the plane mirror 15 for reference.

The reflected light from the reference plane mirror 15 passes through the beam splitter 14 and is guided to a detector (for example, a CCD camera) 17 via a lens 16. This is one optical path of the interference system.

On the other hand, the laser light separated by the beam splitter 14 (laser light transmitted through the beam splitter 14)
Is incident on the objective lens 18 to be measured.

Behind the objective lens 18, a reference spherical mirror 19 is arranged so that the optical axis and the focal point coincide with the objective lens 18, so that the laser light transmitted through the objective lens 18 is reflected by the reference spherical surface. The light is reflected by the mirror 19, passes through the objective lens 18 as it is, and is returned to the beam splitter 14 as parallel light as in the outward path.

The return light from the objective lens 18 is reflected by the beam splitter 14 and enters the detector 17 via the lens 16. This is the other optical path of the interference system.

As described above, in the above-mentioned aberration analyzing optical system,
In order to properly observe the transmitted wavefront of the objective lens 18, a reference spherical mirror 19 is inserted into the optical path to be measured, and the propagating wavefront of the optical path composed of the reference spherical mirror 19 and the sample to be tested (the objective lens 18). Is caused to interfere with the reference wavefront reflected by the reference plane mirror 15, thereby converting the irregularities caused by the deviation of the lens shape (from the design value) into an optical phase difference, and analyzing the aberration by the fringe scan method. By performing the analysis according to, it is possible to derive Seidel aberration as an index of optical performance.

It is apparent from FIG. 3 that the above-described optical configuration is based on the premise that the objective lens 18 serving as a sample to be tested is a transmission type lens.

Therefore, the above-mentioned aberration analyzing optical system is
A major problem arises when trying to apply to L or SIM.

That is, for an objective lens with NA> 1,
The light converging point is on the bottom surface of the objective lens, and the propagating light is totally reflected on this surface. Therefore, the optical configuration shown in FIG. 3 cannot be directly employed.

The present inventors have conducted various studies, and as a result, by adopting a configuration that eliminates the condition of total reflection of the SIL or the SIM, the above-described optical configuration can be adopted also in the SIL or the SIM. I thought it was.

Here, in order to eliminate the condition of total reflection,
It is considered that a member having a refractive index close to those of the SIL or SIM should be arranged on the bottom surface of the SIL or SIM.

Therefore, for example, without using the spherical reference mirror shown in FIG. 3, for example, as shown in FIG.
It can be considered that the simplest configuration of the path to be tested is replaced with a configuration in which the plane mirror 22 is disposed in contact with the mirror. However, in this configuration, the main aberrations of Seidel aberration (astigmatism, coma, spherical Aberration), the coma aberration is canceled in principle, so that accurate aberration measurement cannot be performed.

In the present invention, considering these circumstances, N
It is a subject to provide an optical configuration that enables appropriate aberration measurement even for an objective lens in which A> 1.

FIG. 5 shows a basic optical configuration in the present invention. In the path to be tested, a hemispherical reference mirror 23, which is a hemispherical optical component, is inserted instead of the reference spherical mirror in FIG. 3, and the entrance surface of the mirror is a lens that realizes NA> 1 like SIL or SIM. (Here SIM2
It is installed so as to face the bottom surface of 1), and these are optically connected via an index matching oil 24. Then, align the optical axes so that they match,
This constitutes one path of the interferometer. The other configuration is the same as that shown in FIG. 3, and the description is omitted here.

In the above configuration, the SIM2 as the objective lens
1 and the hemispherical reference mirror 23 with matching oil 24
To be in contact with each other through the
Although there is a correction of the propagation angle due to the difference in refractive index according to the law of l), total reflection does not occur at the boundary surface, light propagates into the hemispherical reference mirror 23, and the surface has the curvature of the hemispherical reference mirror 23. The light is reflected and propagates to the detector 17 through the same path as the outward path.

Here, it is desirable that the refractive index of the objective lens (SIM 21), the refractive index of the hemispherical reference mirror 23, and the refractive index of the matching oil 24 be the same or almost the same. The above configuration applies the principle of a high-resolution optical microscope using an immersion objective lens to an interference measurement optical system.
Since an optical configuration equivalent to that of FIG. 3 is obtained in principle, coma is not canceled and good aberration measurement can be performed for an objective lens with NA> 1.

FIG. 6 shows an example in which an optical system for blocking unnecessary reflected light is added to the above basic configuration.

In this example, first, quarter-wave plates 31 and 32 are arranged between the beam splitter 14 and the reference plane mirror 15 and between the beam splitter 14 and the SIM 21, respectively. A deflecting plate 33 is arranged between the splitter 14 and the detector 17.

In the case of SIL or SIM, the return light of the optical path constituting the interference system is very weak, and if unnecessary light is mixed, the measurement is hindered.

For example, in the beam splitter 14, there is a slight reflection on the surface 14a and the surface 14b,
When this enters the detector 17, the relative intensity of the return light from the interference optical system decreases, and accurate measurement becomes difficult.

By interposing the quarter-wave plates 31 and 32, the direction of polarization of the interference system (reflected light from the reference plane mirror 15 and return light transmitted through the SIM 21) is changed by 90 °.
Can be converted. The reflected light from the surfaces 14a and 14b has the same polarization direction, and thus can be separated by the polarizing plate 33.

An aperture 34 is provided on the light incident side of the SIM 21 to prevent light from entering the SIM 21 from light other than the light source (He-Ne laser 11). .

Behind the hemispherical reference mirror 23, a CCD camera 35 and a lens 36 are arranged for aligning the SIM 21 and the optical axis of the hemispherical reference mirror 23.

When an actual sample was measured by using the above-described aberration analysis optical system, it was found that N was used for an optical disk system using the near-field principle.
It was possible to appropriately evaluate the optical aberration of the objective lens that satisfies A> 1.

[0048]

As is apparent from the above description, according to the present invention, it becomes possible to measure aberration by optical interference measurement for an objective lens having a high numerical aperture of NA> 1, and for example, the principle of the near field is used. It is also possible to appropriately evaluate the optical aberration of an objective lens used for an applied optical disk system.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an example of an SIL.

FIG. 2 is a schematic diagram illustrating an example of a SIM.

FIG. 3 is a schematic diagram illustrating a configuration example of an aberration analysis optical system for measuring aberration of an objective lens where NA <1.

FIG. 4 is a schematic diagram showing an example of a test path in which a plane mirror is arranged on a SIM.

FIG. 5 is a schematic diagram illustrating a configuration example of an aberration analysis optical system for performing aberration measurement of an objective lens where NA> 1.

FIG. 6 is a schematic diagram showing another configuration example of an aberration analysis optical system for performing aberration measurement of an objective lens in which NA> 1.

[Explanation of symbols]

11 He-Ne laser, 14 beam splitter, 1
5 Reference flat mirror, 21 SIM, 23 Hemisphere reference mirror, 24 Matching oil

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G11B 7/135 G11B 7/135 A F-term (Reference) 2G086 HH06 2H087 KA13 LA01 PA01 PA17 PB01 QA02 QA05 QA13 QA33 TA01 TA04 5D119 AA22 DA20 JA32 JA43 JA57 JB02 LB05

Claims (6)

[Claims] When measuring the optical aberration of an objective lens having a numerical aperture NA> 1, by optical interference measurement, the hemispherical reference mirror is disposed so that the flat portion of the hemispherical reference mirror faces the bottom surface of the objective lens. Matching oil is interposed between the bottom surface and the plane part of the hemispherical reference mirror, and the objective lens and the hemispherical reference mirror are aligned so that their optical axes coincide with each other to constitute one optical path of the interference system. Optical aberration measurement method. 2. The method according to claim 1, wherein the objective lens is a solid immersion lens (SIL) or a solid immersion mirror (SIM).
The method for measuring optical aberration according to claim 1, wherein 3. The other optical path of the interference system is constituted by a reference plane mirror, and a reference wavefront reflected by the reference plane mirror and a propagation wavefront of an optical path constituted by the objective lens and the hemispherical reference mirror. 2. The optical aberration measuring method according to claim 1, wherein 4. An optical aberration measuring apparatus for measuring the optical aberration of an objective lens having a numerical aperture NA> 1 by optical interference measurement, comprising: a laser light source; and an optical system forming one optical path of an interferometer including the objective lens. A reference plane mirror that constitutes the other optical path of the interferometer, a beam splitter that separates and combines the laser light from the laser light source corresponding to the two optical paths, and returns from both optical paths. An optical aberration measurement device comprising: a detector for detecting light interference. 5. A quarter-wave plate is disposed between the beam splitter and the reference plane mirror and between the beam splitter and the objective lens, and a deflection plate is disposed between the beam splitter and a detector. The optical aberration measuring device according to claim 4, wherein 6. The objective lens is a solid immersion lens (SIL) or a solid immersion mirror (SIM).
The optical aberration measuring device according to claim 4, wherein