JPH07168011A - Optical system including total reflection prism - Google Patents

Abstract

PURPOSE:To provide an optical system including a total reflection prism whereby incoming and outgoing beams of light are made to have the same polarization characteristic. CONSTITUTION:An optical system includes a total reflection prism 11 that provides a phase difference of 90 deg. between the P- and S-components of incoming and outgoing beams of light and a quarter wavelength plate 13 disposed opposite to the incidence plane 12 of the prism. When a plane-polarized laser beam 17 is made to impinge on the quarter wavelength plate 13 by properly adjusting the directions of the total reflection surfaces 14, 16 of the total reflection prism 11 and that of the crystal axis of the quarter wavelength plate 13, the outgoing beam emitted from the total reflection prism 11 also becomes plane-polarized; i.e., a laser beam of the same polarization characteristic as the laser beam 17 impinging on the quarter wavelength plate 13 is emitted from the plane 15 of emission of light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system including a total reflection prism used for changing the distance between two objective lenses of a photomask inspection apparatus while keeping the light receiving side fixed.

[0002]

2. Description of the Related Art In a photomask inspection apparatus for inspecting a photomask having a certain pattern for defects, light is applied to regions corresponding to each other of the photomask through two objective lenses to reflect or transmit the reflected light or the transmitted light. Defects are detected by finding the difference between the signals obtained from the light. In such a photomask defect inspection apparatus, the distance between the two objective lenses can be changed while the light receiving side is fixed so that the same portion of the subject can be detected at the same time.

FIG. 5 is a schematic configuration diagram of an optical system including a conventional total reflection prism. An optical system including this total reflection prism includes a first parallelogram total reflection prism 1 with a second parallelogram total reflection prism 2 and a third parallelogram total reflection prism 3 with a fourth parallelogram total reflection prism 3. The total reflection prisms 4 are attached to each other at right angles, and the first and third total reflection prisms 1 and 3 are centered on the optical axis of the light incident from the first total reflection prism to the third total reflection prism. I am trying to rotate with each other. An optical system including such a total reflection prism can be used to change the distance between the optical axes of the two objective lenses.

In recent years, a laser emitting linearly polarized light has been used as an irradiation light source. However, when the above-mentioned optical system is used, there is a problem that the polarization characteristics are affected by the rotation of the total reflection prism or the like. Has become. That is, when linearly polarized light is incident on the total reflection surface, the phase difference between the P-polarized component and the S-polarized component on one total reflection surface is determined by the incident angle and the refractive index on that surface. When the reflection surface rotates around the axis, the ratio of the amplitudes of the P-polarized component and the S-polarized component changes, and the phase difference of PS polarized light is constant, but for example, when the phase difference is 90 °, the outgoing light is circularly polarized light and circularly polarized light. It changes between polarizations. FIG. 6 is a schematic configuration diagram of an optical system in which two parallelogram total reflection prisms 1 and 3 of FIG. 5 are combined.
When the optical system in which two parallelogram prisms are combined as shown in FIG. 6 is relatively rotated around the axis G, the polarization characteristics of the axes A and C are not the same. In order to eliminate such inconvenience, in the optical system shown in FIG. 5, the second and fourth total reflection prisms are coupled to the first and third total reflection prisms 1 and 3, respectively, and the PS polarization components The phase difference of is canceled out.

[0005]

However, in an optical system including a total reflection prism as shown in FIG. 5, it is necessary to use four total reflection prisms, and since the number of reflection surfaces increases, the surface accuracy error is increased. There is a disadvantage that the resolution is reduced because the data is integrated. Also, due to various problems such as an offset amount of the optical axis and an adjustment error of mechanical dimensions such as twisting when attaching the total reflection prism, a deviation angle occurs between the incident light and the emitted light, and the desired optical characteristics are obtained. There is a drawback that cannot be obtained.

An object of the present invention is to make it possible to exactly and easily equalize the polarization characteristics of incident light and output light of a total reflection prism without causing the above-mentioned problems, and as a result, to obtain desired optical characteristics. It is intended to provide an optical system including a total reflection prism capable of obtaining

[0007]

An optical system including a total reflection prism of the present invention has at least one total reflection surface, and has a phase difference of 90 ° between the P component and the S component of incident linearly polarized light. It is equipped with a total reflection prism to be introduced and a 1/4 wavelength plate.
4 The crystal axis of the wave plate and the direction of the total reflection surface of the total reflection prism are adjusted so that the phase difference between the P component and the S component of the outgoing linearly polarized light becomes zero. It is a thing.

Further, the optical system including the total reflection prism of the present invention has an entrance surface and an exit surface that are parallel to each other, and first and second total reflection surfaces that are parallel to each other.
A first parallelogram total reflection prism that introduces a 90 ° phase difference between the S component and the S component, an incident surface and an emission surface that are parallel to each other, and a first and second total reflection surfaces that are parallel to each other. A second parallelogram total reflection prism for introducing a phase difference of 90 ° between the P component and the S component of the incident linearly polarized light, and the second parallelogram total reflection prism is arranged to face the incident surface of the first total reflection prism. A first quarter-wave plate and a second quarter-wave plate arranged to face the exit surface of the second total reflection prism. 1 total reflection surface and the first 1 /
4 The direction of the crystal axis of the wave plate and the direction of the crystal axis of the second total reflection surface of the second total reflection prism and the second 1/4 wave plate are between the P component and the S component of the outgoing linearly polarized light. Of the first total reflection prism and the optical axis between the second reflection surface and the emission surface of the first total reflection prism and the incident surface and the first reflection surface of the second total reflection prism. The optical axis between
It is characterized in that the first and second total reflection prisms are configured so as to be mutually rotatable about the optical axis.

[0009]

In the optical system including the total reflection prism of the present invention, when the total reflection prism that gives a phase difference of 90 ° between the P component and the S component of the linearly polarized light is used, the linearly polarized light is converted into the total reflection prism. When incident, circularly polarized light or elliptically polarized light is emitted as described above, and conversely, when circularly polarized light or elliptically polarized light is incident, linearly polarized light is emitted. The quarter-wave plate also has a similar function. Therefore, a total reflection prism and 1/4
The phase difference between the P and S components can be made zero by combining the wave plate with the direction of the total reflection surface and the direction of the crystal axis of the 1/4 wave plate, and the incident light and the outgoing light can be made zero. And the polarization characteristics can be made equal. That is, when linearly polarized light is made incident, linearly polarized light having the same polarization plane can be emitted.

Further, in the present technology, one total reflection prism can be manufactured with sufficient mechanical accuracy. Therefore, the decrease in resolution, the amount of offset of the optical axis and the total amount of reflection caused by using the above-mentioned multiple reflection surfaces. There is no problem of adjusting mechanical dimensions such as twisting when attaching the reflecting prism, and therefore the polarization characteristics of the outgoing light can be accurately and easily matched with the polarization characteristics of the incident light, and an optical system with good optical characteristics can be obtained. Can be realized.

[0011]

Embodiments of an optical system including a total reflection prism of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a first embodiment of an optical system including a total reflection prism of the present invention. In FIG. 1, an optical system including a total reflection prism includes a parallelogram total reflection prism 11 for introducing a phase difference of 90 ° between the P component and the S component of linearly polarized light, and an incident surface 12 of the total reflection prism. And a quarter-wave plate 13 arranged so as to face each other. In this example, the total reflection prism 11 is, for example,
It is for He-Ne laser (wavelength 632.8nm) and uses LLF1 as a glass material.
An antireflection film of 632.8 nm is formed. In addition, the angle θ ₁ between the incident surface 12 and the first reflecting surface 14 and the angle between the exit surface 15 and the second reflecting surface 14
The angle θ ₂ with the reflecting surface 16 is both 45.54 ± 0.03 °. In the present invention, in order to cancel the phase difference introduced by the total reflection prism 11, a quarter wave plate 13 is provided and its crystal axis is adjusted with respect to the directions of the total reflection surfaces 14 and 16. .

In the optical system of this example, the total reflection prism 1
The adjustment of the 1/4 wave plate 13 can be performed by various methods. First, the basic adjustment method will be described. First, the crystal axis of the quarter-wave plate 13 is adjusted to be inclined by 45 ° with respect to the polarization direction of the linearly polarized incident laser light 17.
For this reason, it is preferable to provide a mechanism for rotatably supporting the quarter-wave plate 13 in its plane. When adjusted in this way, the linearly polarized laser light 17 is emitted by the quarter-wave plate 13
The light is converted into circularly polarized light by and emitted. This circularly polarized light is transmitted through the incident surface 12 of the total reflection prism 11 to the first
Is incident on the total reflection surface 14 of the
And is emitted from the emission surface 15. As described above, this total reflection prism has a function of causing a 90 ° phase difference between the P-polarized light and the S-polarized light. However, when linearly polarized light is made incident on the incident surface and the total reflection surface, Circularly polarized light or elliptically polarized light is emitted depending on the relative positional relationship. Therefore, in this example, the total reflection prism 11 is adjusted so as to convert the incident circularly polarized light into the linearly polarized light, that is, so that the incident surface and the total reflection surface are orthogonal to each other. With this adjustment, the linearly polarized incident laser light 17 is 1/4
By transmitting through the wave plate 13 and the total reflection prism 11, the phase difference between the P component and the S component becomes zero and the amplitudes become equal, and total reflection is performed as linearly polarized light having the same polarization characteristics as the incident laser light 17. The light is emitted from the emission surface 15 of the prism 11.

Another example of the adjusting method of the optical system according to the present invention will be described. In this example, first, the position of the incident surface 14 of the total reflection prism 11 is fixed, then linearly polarized light is made incident, and the 1/4 wavelength plate 1 is made to emit linearly polarized light from the total reflection prism.
Adjust the rotation of 3 and fix it. With this adjustment, linearly polarized light will always be emitted regardless of how the optical system is set for incident linearly polarized light, unless the relative positions of the total reflection prism 11 and the quarter-wave plate 13 are changed. Become. Therefore, the polarization characteristics do not change even if the optical system is rotated about the incident optical axis A.

As described above, according to the present invention, by adjusting the total reflection prism 11 and the 1/4 wavelength plate 13 or the 1/4 wavelength plate, the polarization characteristics introduced by the total reflection prism 11 are The polarization characteristic opposite to that can be offset and removed by generating the quarter-wave plate 13 and the polarization characteristic of the emitted light can be made equal to the polarization characteristic of the incident light. In this case, the total reflection prism 11 and / or 1 /
4 The adjustment of the wave plate 13 can be made significantly easier and more accurate than in the case where the conventional total reflection prism is used.

FIG. 2 is a schematic configuration diagram of a second embodiment of an optical system including a total reflection prism of the present invention. In the embodiment shown in FIG. 1, linearly polarized light is made incident on the quarter-wave plate 13 and then made incident on the total reflection prism 11, but in this example, this relationship is reversed, and the linearly polarized light is first made on the total reflection prism 21. After being made incident, it is made incident on the quarter-wave plate 22. In this example, the incident linearly polarized light is converted into circularly polarized light or linearly polarized light by the total reflection prism 21.
Since the light is returned to the linearly polarized light by 2, it is possible to match the polarization characteristic of the emitted light with the polarization characteristic of the incident light as in the previous example. Also in this example, even if the total reflection prism 21 and the quarter-wave plate 22 are rotated together around the output optical axis B, the polarization characteristics of the output light are not affected.

FIG. 3 is a perspective view showing the structure of a third embodiment of an optical system including a total reflection prism of the present invention, and FIG. 4 is a front view thereof. The optical system of this example can be used as an optical system for adjusting the distance between the optical axes of two objective lenses in the above-described photomask defect inspection apparatus. In FIG. 4, the optical system including the total reflection prism is the same as the total reflection prism 11 of FIG.
And the second total reflection prisms 31 and 32, the first quarter-wave plate 34 arranged to face the entrance surface 33 of the first total reflection prism 31, and the emission of the second total reflection prism 32. Second quarter-wave plate 36 arranged opposite to the surface 35
And with. Further, the exit surface of the first total reflection prism 31 and the entrance surface of the second total reflection prism 32 are opposed to each other, and both prisms are coupled so as to be relatively rotatable about the axis C.

The total reflection surface of the first total reflection prism 31 and the crystal axis of the first quarter wave plate 34 are adjusted as described in the embodiment shown in FIG. Linearly polarized light on the wave plate 3
When 7 is incident, linearly polarized light is emitted from the emission surface of the first total reflection prism. Similarly, the total reflection surface of the second total reflection prism 32 and the crystal axis of the second quarter-wave plate 36 are adjusted as described in the embodiment shown in FIG. When the linearly polarized light is made incident on the incident surface, the second 1/4 wavelength plate 36 is made to emit the linearly polarized light.

Also in this example, when the linearly polarized light 37 is made incident on the first 1/4 wavelength plate 34, the second 1/4 wavelength plate 36 outputs linearly polarized light having the same polarization characteristics as the incident linearly polarized light. This relationship means that the first quarter-wave plate 34 and the first total reflection prism 31 are integrally rotated about the axis D, and the second quarter-wave plate 36 and the second quarter-wave plate 36 are rotated. It does not change even if the total reflection prism 32 is rotated around the axis E as a unit. That is, when the optical system of the present example is applied to the photomask defect inspection apparatus described above, the polarization characteristics do not change at all even if the optical system is rotated to change the distance between the optical axes of the objective lenses.

[0019]

As described above, according to the optical system including the total reflection prism of the present invention, the phase difference between the P polarization component and the S polarization component introduced by the total reflection prism is canceled by the 1/4 wavelength plate. Therefore, the polarization characteristics of the incident light and the outgoing light can be made equal to each other. In this case, the total reflection surface of the total reflection prism and the crystal axis of the quarter-wave plate can be adjusted very easily and accurately, so that desired optical characteristics can be easily obtained. Further, when the optical system of the present invention is used in, for example, a photomask inspection apparatus, the polarization characteristics do not change even if the distance between the optical axes of the two objective lenses is adjusted, so the optical characteristics of the entire apparatus change. Nothing.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a first embodiment of an optical system including a total reflection prism of the present invention.

FIG. 2 is a schematic configuration diagram of a second embodiment of an optical system including a total reflection prism of the present invention.

FIG. 3 is a schematic configuration diagram of a third embodiment of an optical system including a total reflection prism of the present invention.

FIG. 4 is a side view of a third embodiment of an optical system including a total reflection prism of the present invention.

FIG. 5 is a schematic configuration diagram of an optical system including a conventional total reflection prism.

FIG. 6 is a schematic configuration diagram of an optical system in which two parallelogram total reflection prisms are combined.

[Explanation of symbols]

 11, 21, 31, 32 Total reflection prism 12 Incident surface 13, 22, 34, 36 1/4 wavelength plate 14 First reflection surface 15, 35 Emission surface 16 Second reflection surface 17, 37 Laser light A, B, C , D, E axis

Claims (2)

[Claims] 1. A total reflection prism having at least one total reflection surface, which introduces a phase difference of 90 ° between the P component and the S component of incident linearly polarized light, and a quarter wavelength plate, This 1/4
The direction of the crystal axis of the wave plate and the direction of the total reflection surface of the total reflection prism are adjusted so that the phase difference between the P component and the S component of the outgoing linearly polarized light is zero. Optical system including a reflecting prism. 2. An incident surface and an exit surface which are parallel to each other and first and second total reflection surfaces which are parallel to each other, and a phase difference of 90 ° between the P component and the S component of the incident linearly polarized light. The first parallelogram total reflection prism to be introduced, the incident surface and the exit surface parallel to each other, and the first and second total reflection surfaces parallel to each other, the P component and the S component of the incident linearly polarized light Between 90
A second parallelogram total reflection prism that introduces a phase difference of °; a first quarter-wave plate facing the entrance surface of the first total reflection prism; and the second total reflection prism. A first quarter-wave plate of the first total reflection prism and a crystal axis of the first quarter-wave plate of the first total reflection prism. And the direction of the crystal axes of the second total reflection surface of the second total reflection prism and the second quarter-wave plate are such that the phase difference between the P component and the S component of the outgoing linearly polarized light is zero. And an optical axis between the second reflection surface and the emission surface of the first total reflection prism and an optical axis between the incidence surface and the first reflection surface of the second total reflection prism. And the first and second total reflection prisms are configured to be mutually rotatable about the optical axis. No optical system.