JPH10221010A - Interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a good interference fringe wherein adjustment of a light reflection amount from a specimen can be easily realized at a low cost and the contrast is the maximum. SOLUTION: A polarization plate 11 for converting a laser beam which is a light source beam into linear polarization, a half mirror 14, a reference plate 15, a specimen 16, a TV camera 25 and the like are provided. A movable polarization plate 17 is rotatably disposed between the reference plate 15 and the specimen 16, and a transmission light amount is changed by the rotation of the movable polarization plate 17. A reflection light amount from the specimen 16 is adjusted. The movable polarization plate 17 is preferably disposed to slightly be inclined, and a reflectance scale in which the rotation position of the movable polarization plate coinciding with the reflectance of the specimen 16 can be judged is given. Furthermore, the transmission wave face aberration of the movable polarization plate 17 is measured, and correction is preferable for the data of the test face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer apparatus for observing and analyzing interference fringes generated by reflected light from a reference plate and a subject using a Fizeau interferometer or the like. The present invention relates to a configuration for matching rates.

[0002]

2. Description of the Related Art FIG. 8 shows the configuration of a conventional Fizeau-type interferometer apparatus. As shown, a half (semi-transmissive) mirror 1 for transmitting a laser beam and a collimator lens 2 are arranged. And behind the collimator lens 2,
The reference plate 3 and the subject 4 are arranged. On the other hand, an imaging lens for forming an image of the test surface on the imaging surface of the TV camera 6 via the half mirror 1 is arranged.

According to such a configuration, the half mirror 1
Is partially reflected on the reference surface (rear surface) 3A of the reference plate 3 (which serves as reference light), and also reflected on the surface of the subject 4, and the reflected light causes interference. Streaks occur. These reflected lights enter the TV camera 6 via the half mirror 1, and the interference fringes captured by the TV camera 6 are displayed on a monitor or the like and observed, or used as analysis data.

[0004]

However, in the above-mentioned conventional interferometer apparatus, in order to obtain an interference fringe having a high contrast, the reference plate is formed so that the light intensity of both light beams becomes equal at the stage of forming the interference fringe. Since it is necessary to match the reflectance of the test object 3 with the reflectance of the subject 4, there is a problem that the adjustment of the reflectances of the two is complicated, and the observation of interference fringes in an optimal state may not be possible. Therefore, in general, a reference plate 3 having a high, medium, or low reflectance is prepared corresponding to the object 4 having a high, medium, or low reflectance, or the reference plate 3 having a low reflectance is prepared. ND between the reference plate 3 and the subject 4
The amount of light reflected from the reference plate 3 and the subject 4 is adjusted by inserting a filter, an attenuation plate, and the like.

[0005] However, for an object having a specific reflectance, good interference fringes that maximize the contrast can be obtained. However, as the reflectance differs, good interference fringes cannot always be obtained. Moreover, in the former solution, even though three kinds of expensive reference plates 3 are prepared, a large number of reference plates 3 having further different reflectivities must be prepared. In the latter solution, various ND filters and attenuators having different reflectances are required. Therefore, both of them are costly and involve cumbersome operations such as installation and removal.

The present invention has been made in view of the above problems, and an object of the present invention is to obtain a good interference fringe having a maximum contrast without replacing a reference plate or an attenuating plate due to the reflectance of an object. To provide an interferometer device that can be used.

[0007]

According to a first aspect of the present invention, a light source is provided to a reference plate and a subject, and the light is reflected by the reference plate and each surface of the subject. In an interferometer device that forms interference fringes, a light source unit that emits linearly polarized light to the reference plate and the subject by using a polarization conversion unit or a linearly polarized light source that converts the light source light into linearly polarized light; A movable polarizing plate that is rotatably disposed in an optical path between the reference plate and the subject, and that changes the amount of transmitted light by this rotation; and by rotating the movable polarizing plate, the light is reflected from the subject. It is characterized in that the amount of incoming light can be adjusted. The invention according to a second aspect is characterized in that the movable polarizing plate is arranged to be slightly inclined with respect to a plane orthogonal to the transmission optical axis. The invention according to a third aspect is characterized in that the transmitted wavefront aberration of the movable polarizing plate is measured, and the transmitted wavefront aberration is corrected corresponding to the rotational position of the movable polarizing plate. According to a fourth aspect of the present invention, by rotating an imaging element that captures an interference fringe formed from the reflected light from the reference plate and the reflected light from the subject in accordance with the rotation of the movable polarizing plate, It is characterized in that a measurement error due to the transmitted wavefront aberration of the movable polarizing plate is corrected.
The invention according to a fifth aspect is characterized in that the movable polarizing plate is attached to a reference plate holder that holds the reference plate or a sample holder that holds the subject. The invention according to a sixth aspect is characterized in that a reflectance scale is provided so that the rotational position of the movable polarizing plate that matches the reflectance of the object surface can be determined.

According to the above arrangement, for example, the light from the light source is converted into linearly polarized light by a polarization conversion means such as a polarizing plate, and this linearly polarized light is emitted to the reference plate and the test object. Interference fringes are formed by the reflected light from the light source.
The amount of light reflected from the test surface is adjusted by rotating a movable polarizer disposed between the reference plate and the subject. For example, when a low-reflectance reference plate is arranged on the front side and a high-reflectance sample is arranged on the rear side when viewed from the light source, the light intensity of both of the two members forming the interference fringes is reduced by reducing the light amount by the movable polarizing plate. And interference fringes with good contrast can be observed.

If the movable polarizing plate is disposed slightly inclined from a position perpendicular to the optical axis, light reflected directly from the polarizing plate can be prevented from affecting the imaging surface as noise.

If the movable polarizing plate has a transmitted wavefront aberration, the aberration affects interference fringes according to the rotation of the polarizing plate, resulting in a measurement error. Therefore, by measuring the transmitted wavefront aberration data in advance and storing it in a memory or the like, and correcting this aberration in accordance with the rotational position of the movable polarizing plate, it is possible to remove the influence of the aberration caused by the presence of the polarizing plate. it can. The correction for rotating the transmitted wavefront aberration can be corrected by rotating the imaging device for observation in accordance with the rotation of the movable polarizing plate.

Further, the movable polarizing plate can be rotatably mounted on a reference plate holder, a sample table, or the like. At this time, if a reflectance scale is provided on a support portion or the like of the movable polarizing plate, the reflectance of the subject can be improved. The rotational position of the movable polarizing plate can be set before the measurement, and when measuring an object whose reflectance is known, the setting of the reflectance adjustment becomes easy.

[0012]

1 to 5 show the structure of an interferometer device according to a first embodiment of the present invention.
The overall configuration will be described below. In FIG. 1, He-Ne
A polarizing plate 11 for converting laser light into linearly polarized light is disposed on the output side of the laser 10, and a divergent lens L1 and a pinhole section 12 are disposed on the rear side of the polarizing plate 11. A half (semi-transmissive) mirror 14 is provided behind the pinhole section 12, and a collimator lens L2 for converting the light transmitted through the half mirror 14 into a parallel light beam is disposed.

A reference plate 15 is disposed behind the collimator lens L2. The reference plate 15 has a low reflectance (for example, about 4%). Then, the subject 16 is disposed behind the reference plate 15. In the first example, between the reference plate 15 and the subject 16,
The movable polarizing plate 17 is provided in a rotatable state.

FIG. 2 shows the movable polarizing plate 17 arranged on the reference plate portion of the apparatus. In the reference plate section 18, the reference plate 15 is attached to an upper reference plate holder 19 via a buffer material 20 or the like, and the movable polarizing plate 17 is attached to a lower polarizing plate holder (contrast adjustment ring) 21.
Are attached via a spacer 22A and a cushioning material 22B. Then, the polarizing plate holder 21 is attached to the reference plate holder 19 by screw connection or the like so as to be rotatable, whereby the movable polarizing plate 17 rotates about the optical axis to perform a contrast adjusting function.

According to such a movable polarizing plate 17, when the polarization direction coincides with the polarization plane of the polarizing plate 11 on the light source side, the light source light (linearly polarized light) is transmitted while minimizing the decrease in the amount of light. When rotated clockwise or counterclockwise from this state, the amount of light transmission decreases according to the amount of rotation. Therefore, by changing the rotation angle of the movable polarizing plate 17, the intensity of the light reflected from the subject 16 can be adjusted, whereby the intensity of the light reflected from the reference plate 15 can be reduced. It can be adapted.

The movable polarizing plate 17 is not parallel to the plane F perpendicular to the center axis of the reference plate portion 18 as shown in the figure, but is slightly inclined by changing the thickness of the spacer 22A and the buffer material 22B. Are arranged. That is, when the movable polarizing plate 17 is arranged in parallel with the surface F, light directly reflected from the polarizing plate 17 enters an image pickup device to be described later and affects the interference fringe image (noise). The plate 17 is slightly inclined with respect to the vertical plane F so that the reflected light does not reach the image sensor.

In FIG. 1, an encoder 23 for detecting the rotation angle of the polarizing plate 17 is attached to the movable polarizing plate 17, and an angle detector 24 is connected to the encoder 23. In addition, the half mirror 1
An imaging lens L3 for forming an image of the surface to be inspected on the imaging surface of the TV camera 25 via the camera 4 is arranged. The TV camera 25 is connected to a TV monitor 26 for displaying interference fringes, and a computer 28 having a memory 27 and a display 29 for displaying analysis images and the like.

The computer 28 operates as the reference plate 1.
5 is moved slightly in the optical axis direction using a piezo element or the like (not shown).
The state of the surface of the subject 16 is analyzed from the interference fringes obtained in step (1), and the measurement error due to the transmitted wavefront aberration of the movable polarizing plate 17 is corrected. That is, since the movable polarizing plate 17 is disposed between the reference plate 15 and the subject 16,
The measurement result is obtained by adding the transmitted wavefront aberration of the movable polarizing plate 17 to the shape of the surface of the subject 16. Therefore, in the embodiment, the transmitted wavefront aberration of the movable polarizing plate 17 is measured in advance, this data is stored in the memory 27, and then the transmitted wavefront data in the memory 27 is subtracted from the actual measurement value. Correction improves measurement accuracy.

In order to correct the transmitted wavefront aberration, the data in the memory 27 may be simply subtracted if the movable polarizing plate 17 is in a fixed state.
Because of the rotation, accurate measurements cannot be made simply by subtraction. Therefore, in this example, although the details will be described later, the data of the transmitted wavefront aberration is also converted into a rotated state, and an operation of subtracting the data from the measured value is performed.

FIG. 3 shows the appearance of the reference plate portion 18 shown in FIG.
A reflectance scale 31 of the subject 16 such as 4, 5, 10 (%) is provided, and a polarizing plate holder (contrast adjusting ring) 21 is provided.
Can be provided with a triangular index 32 on the outer periphery of. According to this, by setting the index 32 to the reflectance scale 31 so as to be the reflectance of the surface to be observed / analyzed, the setting of the movable polarizing plate 17, that is, the contrast adjustment is simplified.

The relationship between the reflectance of the surface to be measured and the rotation angle is as follows. For example, the reflectance of the reference surface 15A is 0.04 (low-reflection reference plate), and the others are considered to be transmissive (the opposite surface of the reference surface 15A is usually provided with an antireflection coat), and the transmittance is 0.96. The angle between the polarization plane of the polarizer 11 and the polarization plane of the movable polarizer 17 is θ, the transmittance of the movable polarizer 17 when the angle θ is 0 degree is T, and the reflectance of the subject 16 is T Assuming R, at the time of observation (imaging), when the intensity of light reflected from the subject 16 and the intensity of light reflected from the reference surface 15A are equal and an interference fringe with good contrast is obtained, Holds.

[Formula 1] (0.96 × cos ² θ × T) ² × R = 0.04

Here, the angle θ = 0 of the movable polarizing plate 17
In this case, the loss of transmitted light at the movable polarizing plate 17 is ideally reduced to 0.
, The transmittance T = 1 (100%). Therefore, if T = 1, the above equation 1 becomes the following equation 2.

[Formula 2] θ = cos ⁻¹ ((0.04 ÷ R) ^1/2 ÷ 0.96) ^1/2

According to the above equation (2), it can be understood that interference fringes with good contrast can be obtained by adjusting the angle θ according to the reflectance of the surface of the subject 16. For example, when R = 0.05, the angle θ = 15 degrees, and when R = 0.1, the angle θ = 36.
Degree, when R = 0.2, the angle θ = 47 degrees, when R = 0.4, the angle θ = 55 degrees, and when R = 0.8, the angle θ = 61 degrees. However, in this case, when the above R is 0.043 or less, the angle θ must be adjusted to 0 degree, and the adjustment range for the reflectance R is about 4 to 100%.

The first example has the above configuration. According to such an interferometer device, the laser beam output from the laser 10 is converted into linearly polarized light by the polarizing plate 11 and the linearly polarized light is converted into the half mirror 14. Through the collimator lens L
2 and enters the reference plate 15 Also, this reference plate 1
5 is transmitted through the movable polarizing plate 17 to the subject 16.
And the reflected light from the reference surface 15A and the test surface is reflected by the half mirror 14, and the imaging lens L
3 to the TV camera 25. Therefore, the above T
In the V-camera 25, an interference fringe generated by light reflection by the reference surface 15A and the test surface of the subject 16 is imaged, and this interference fringe is displayed on the TV monitor 26 and interfered by the computer 28. An analysis of the fringes is performed.
This analysis result is displayed on the display 29.

In the above interference fringe observation, the contrast can be adjusted by rotating the movable polarizing plate 17. That is, since the reference plate 15 (reference surface 15A) having a reflectivity of 4% is used, when the object (test surface) 16 having a reflectivity of 4% is measured, the polarizing plate shown in FIG. The triangular index 32 of the holder 21 is set at 4% (θ
= 0), the intensity of light reflected from the test surface can be matched to the intensity of light reflected from the reference surface 15A, with the amount of light transmitted through the movable polarizing plate 17 being maximized.
Thereby, interference fringes with good contrast are observed.

Here, the light reflected from the test surface is reflected again by 4% on the reference surface 15A. Therefore, strictly speaking, the intensity of the light reflected from the test surface is slightly small, but it is practical. It is not a problem. This is why Equation 1.2 is an approximate equation. Therefore, the reference plate 1
In the case of using a medium with high reflectivity or a medium with high reflectivity, the above formula does not hold, and the adjustment range for the reflectivity R is reduced, which is not practical.

In this example, the transmitted wavefront aberration of the movable polarizing plate 17 is corrected by the operation shown in FIG.
In FIG. 4, first, in step 101, the movable polarizing plate 17
The transmitted wavefront aberration is measured. In this measurement, the polarization plane of the movable polarizing plate 17 is aligned with the polarization direction of the light beam to maximize the amount of light.
This is performed by arranging a reference object with high surface accuracy (for example, another reference plate) at the position of the object 16. The transmitted wavefront data at this time is stored in the memory 27 as correction data WA, and the angle of the movable polarizing plate 17 at the time of this measurement is set as the origin.

In this step 101, the measurement data using the reference plate 15 and the reference object is subtracted from the above-mentioned WA without disposing the movable polarizing plate 17 in the optical path, and this data is used as the correction data WA again. In this case, the measurement accuracy can be further improved. That is, the polarizing plate 1
When measuring the transmitted wavefront aberration of 7, the measurement is performed in a form that includes the measurement error of the apparatus, and such an error can be removed by the above-described operation.
, The amount of only the transmitted wavefront aberration is accurately extracted.

Next, in step 102, the subject 16 is arranged and the movable polarizing plate 17 is rotated to set it at a position where the contrast of the interference fringes becomes good.
The surface of the object 16 is measured, and the measured data is defined as WB.

In the next step 104, the rotated angle of the movable polarizing plate 17 is detected from the encoder 23, and the above-mentioned correction data WA is rotated by this rotation angle, thereby obtaining the rotation position correction data WAθ. That is, the point of the transmitted wavefront data WA that coincides with the rotation center of the movable polarizing plate 17 is rotated around the coordinate center, and this data is used as correction data WAθ at the rotation angle. Normally, since the measurement data obtained from the image data is in the XY coordinate system, the correction data WAθ
In such a case, some contrivance is required, such as using data near the rotation. Then, in step 105, the shape of the surface to be inspected is calculated by WB-WAθ = WX. In addition,
It is well known that the surface accuracy is corrected by the least square method.

FIG. 5 shows the concept of the correction of the transmitted wavefront aberration. As shown in FIG. 5A, in step 101, the polarization plane H1 of the light beam and the movable polarizing plate 17 are set.
Is assumed to coincide with the horizontal axis (X-axis), and the correction point data wa (one of WA) exists on a line 15 degrees from the rotation center O of the movable polarizing plate 17, for example.
Then, as shown in FIG.
Is rotated by 30 degrees (.theta.) To obtain good interference fringe contrast, correction point data wa.theta. (One of WA.theta.) At this 30.degree. Angle is obtained in step 104. Therefore, in step 105, the correction point data waθ is subtracted from the test surface data WB, and
Is corrected.

FIG. 6 shows a second example of the embodiment. In the second example, the image pickup device is rotated to correct the transmitted wavefront aberration. That is, if the image sensor is rotated in accordance with the rotation of the movable polarizing plate 17, the rotation of the transmitted wavefront aberration data of the movable polarizing plate 17 can be eliminated. Therefore, in this second example, T
The V camera 25 is rotatably supported, and the motor 34 and the gear 3
5A, 35B, etc., and the rotation angle of the TV camera 25 is detected by the encoder 36 and the angle detector 37.

According to the second example, the movable polarizing plate 1
By rotating the TV camera 25 in the rotation direction of the transmitted wavefront aberration by the same angle as the rotation angle of 7, the rotation of the transmitted wavefront aberration on the imaging surface can be eliminated. Accordingly, the correction data WA obtained in step 101 of FIG.
Is simply subtracted from the test surface data WB, the transmitted wavefront aberration is corrected, and accurate measurement becomes possible.

FIG. 7 shows a third example of the embodiment. In the third example, a movable polarizing plate is mounted on a sample table (subject table) side. As shown in FIG.
Is mounted in the sample stage support section 42 with a spring 40 and a horizontal position adjusting screw 41. The sample stage 39 is maintained horizontally by the horizontal position adjusting screw 41, and the inclination is adjusted. Is possible.

Then, a polarizing plate holder (contrast adjusting ring) 44 is rotatably attached to the sample stage supporting portion 42 by screw connection or the like.
In 4, the movable polarizing plate 17 is provided via a spacer 45A and a buffer material 45B. The movable polarizing plate 17 is also arranged so as to be oblique to the plane F perpendicular to the optical axis, as in the case of FIG. According to such a third example, similarly to the first example, the amount of reflected light from the subject 15 can be adjusted, and interference fringes with good contrast can be observed. In this case as well, the reflectance scale of the subject 16 can be provided on the outer periphery of the support portion 42.

In the above embodiment, the movable polarizing plate 17 is set on the reflectance scale of the subject 16, but the rotation angle corresponding to the reflectance can be directly set. That is, the reflectance of the subject 16 is stored in a memory with respect to the reflectance of the subject 16 in the memory, or an operation program for calculating the rotation angle from the reflectance is used to input the reflectance of the subject 16. By doing so, the movable polarizing plate 17
Find the rotation angle of. Then, the movable polarizing plate 17 may be set manually or automatically at the calculated rotation angle.

[0037]

As described above, according to the first aspect of the invention, the amount of light reflected from the subject is adjusted by rotating the movable polarizing plate using the linearly polarized light as the light source. Therefore, unlike the related art, there is no need to replace the reference plate according to the reflectance of the object, and continuous contrast adjustment of interference fringes can be performed. Therefore, even for various types of subjects having different reflectivities, the intensity of light reflected from the test surface can be easily adjusted to the intensity of light reflected from the reference surface, and interference with good contrast can be achieved. The stripes can be observed. In addition, the contrast can be adjusted at a lower cost than before.

In some cases, a surface treatment such as a reflection coat is performed on the subject. In this case, the surface condition before and after the surface treatment can be measured with the same apparatus and the same reference plate. It also has the advantage of being useful for evaluating differences.

According to the second aspect of the present invention, since the movable polarizing plate is disposed slightly inclined with respect to the plane perpendicular to the transmission optical axis, the reflected light from the polarizing plate surface causes noise in the observed image. Good interference fringes are obtained without any influence.

According to the third and fourth aspects of the invention,
Since the rotating transmitted wavefront aberration of the movable polarizing plate is corrected, it is possible to remove the influence of the aberration caused by the presence of the polarizing plate.

According to the sixth aspect of the invention, since the reflectance scale of the object is provided, the contrast of the interference fringes can be easily adjusted only by adjusting the movable polarizing plate to the reflectance scale. There is an advantage.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an interferometer device [FIG. 1A] and a movable polarizing plate [FIG. 1B] according to a first example of an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating an arrangement state of a movable polarizing plate in a reference plate unit of the first example.

FIG. 3 is an external view showing a configuration of an outer periphery of a reference plate portion of FIG. 2;

FIG. 4 is a flowchart illustrating an operation of correcting a transmitted wavefront aberration in the first example.

FIG. 5 is an explanatory diagram showing the concept of transmitted wavefront aberration correction of the first example.

FIG. 6 is a diagram illustrating a configuration of a second example of the embodiment.

FIG. 7 is a cross-sectional view illustrating a configuration of a third example of the embodiment.

FIG. 8 is a diagram showing a schematic configuration of a conventional interferometer device.

[Explanation of symbols]

 1,14 ... half mirror, 3,15 ... reference plate, 4,16 ... subject, 11 ... polarizing plate, 17 ... movable polarizing plate, 19 ... reference plate holder, 21 ... polarizing plate holder, 23, 36 ... encoder, 24, 37: Angle detector, 25: TV camera, 27: Memory, 28: Computer, 31: Reflectivity scale, 39: Sample stage.

Claims (6)

[Claims] 1. An interferometer apparatus for providing light from a light source to a reference plate and an object and forming interference fringes by light reflection on each surface of the reference plate and the object. A polarization converter for converting the light from the light source into linearly polarized light. A light source unit that emits linearly polarized light to the reference plate and the subject using a means or a linearly polarized light source, and is rotatably disposed in an optical path between the reference plate and the subject, and An interferometer device, comprising: a movable polarizing plate that changes the amount of transmitted light by rotating the movable polarizing plate, whereby the amount of light reflected from the subject can be adjusted by rotating the movable polarizing plate. 2. The interferometer apparatus according to claim 1, wherein said movable polarizing plate is disposed slightly inclined with respect to a plane perpendicular to a transmission optical axis. 3. The method according to claim 1, wherein the transmitted wavefront aberration of the movable polarizing plate is measured, and the transmitted wavefront aberration is corrected according to a rotational position of the movable polarizing plate. Interferometer device. 4. An imaging device for imaging interference fringes formed from the reflected light from the reference plate and the reflected light from the subject,
By rotating according to the rotation of the movable polarizing plate,
3. The interferometer device according to claim 1, wherein a measurement error due to a transmitted wavefront aberration of the movable polarizing plate is corrected. 5. The interference according to claim 1, wherein the movable polarizing plate is attached to a reference plate holder that holds the reference plate or a sample holding unit that holds the subject. Instrument. 6. The interferometer device according to claim 1, further comprising a reflectance scale for judging a rotational position of the movable polarizing plate corresponding to the reflectance of the object surface.