JP4869656B2 - Interferometer - Google Patents

Description

  The present invention relates to an interferometer using a phase shift method.

  An interferometer is known as an apparatus for measuring the surface shape of an object to be measured with high accuracy. The interferometer generates interference fringes by combining the reflected light (reference light) from the reference surface as a reference and the reflected light (measurement light) from the object to be measured, and analyzes the interference fringes to generate a reference surface. Measure the relative shape of the object to be measured. There are various types of interferometers. Among them, the Fizeau interferometer is known to have a simple configuration and high measurement accuracy because the reference light and the measurement light pass through a common optical path.

  A phase shift method is known as an analysis method of interference fringes obtained by an interferometer. The phase shift method is a method of calculating the shape of the object to be measured by acquiring a plurality of interference fringe images while shifting the phase of the interference fringes by a method such as displacing the reference surface in the optical axis direction. Since high accuracy is obtained, it is currently applied to many interferometers.

  However, it takes time to measure because of the method of acquiring interference fringes while displacing the reference surface. In addition, since it is necessary to stabilize the object to be measured while acquiring the interference fringes, it is necessary to perform measurement in a special environment that eliminates fluctuations such as vibration.

  On the other hand, for example, Patent Documents 1 and 2 disclose a method of simultaneously imaging interference fringes whose phases are optically shifted using a plurality of imaging elements. According to this method, the number of interference fringe images required for the arithmetic processing by the phase shift method can be instantaneously captured, so that the measurement speed can be increased. Also, measurement under vibration conditions is possible.

However, in this optical phase shift interferometer, since the polarization planes of the reference light and the measurement light need to be orthogonal to each other, a quarter wavelength plate is indispensable. However, in order to manufacture a quarter-wave plate, it is necessary to process a birefringent optical material such as synthetic quartz with high accuracy on the order of several μm. For this reason, the quarter-wave plate is generally expensive and causes an increase in the price of the optical phase shift interferometer. Large quarter-wave plates having a diameter of 50 mm or more are particularly difficult to obtain, and even if available, they are very expensive. For this reason, the measurement area of such an interferometer is limited to a limited small area, which is not desirable.
Japanese Patent Laid-Open No. 5-87541 JP 2000-329535 A

  It is an object of the present invention to provide an interferometer that does not require an expensive wave plate and that can have a large aperture and a wide measurement area.

In order to achieve the above object, a Fizeau interferometer according to the present invention transmits a first light having a polarization component in a first direction out of emitted light emitted from the light source and the first direction. Reference is made to the other light which reflects the second light having the polarization component in the second direction orthogonal to each other and reflects either the first light or the second light by the object to be measured to obtain the measurement light. A polarizing member that is reflected by a surface and combines the measurement light and the reference light as reference light to be combined light, a phase shift unit that shifts the phase of the combined light, and a plurality of different phase shifts by the phase shift unit And an imaging unit that captures an interference fringe image by the combined light, and the polarizing member is installed so that a reflection surface thereof is perpendicular to the emitted light, and the reflection surface is the reference surface Constructed to be combined Said first light, said after being reflected by the object to be measured, characterized in that it is with the synthesized light by the being through the polarization member combined with the second light.

  According to the present invention, the polarizing member causes the measurement light and the reference light to be light having polarization components that are orthogonal to each other. For this reason, it becomes possible to provide an interferometer that does not require an expensive wave plate and that can have a large aperture and a wide measurement area.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  First Embodiment FIG. 1 shows the overall configuration of a Fizeau interferometer according to a first embodiment of the present invention. For comparison, FIG. 8 shows the overall configuration of a conventional interferometer. The same constituent members will be described with the same reference numerals.

  First, the conventional interferometer of FIG. 8 will be described. In this interferometer, the emitted light emitted from the light source 101 is condensed by the condenser lens 102 and irradiated to the pinhole 103. Here, it is assumed that p-polarized light is emitted from the light source 101 to the incident surface of the beam splitter 104. Light from the point light source of the pinhole 103 passes through the beam splitter 104 and is collimated by the collimator lens 105. Part of this parallel light passes through the reference surface 106 and the quarter-wave plate 107 and reaches the object W to be measured. The rest is reflected by the reference surface 106 to become reference light (p-polarized light). The quarter wavelength plate 107 converts the p-polarized light passing therethrough into circularly polarized light. This circularly polarized light is reflected by the object W to be measured and used as measurement light. This measurement light passes through the quarter-wave plate 107 and is converted into s-polarized light. The s-polarized measurement light that has passed through the reference surface 106 is combined with the p-polarized reference light to be a combined light including a p-polarized component and an s-polarized component, but is synthesized because the polarization directions of both are perpendicular. However, no light interference occurs.

  The combined light is reflected by the beam splitter 104, passes through the quarter-wave plate 108, and is converted into light including a counterclockwise circularly polarized component and a clockwise circularly polarized component. The converted combined light is divided into three lights by the non-polarizing beam splitters 113A and 113B and the mirror 113C, and different phase shift amounts are given by the polarizing plates 114A, 114B and 114C. As shown in FIG. 9, the polarizing plates 114A-C are different from each other by 45 degrees in the transmission axis directions 0 °, 45 °, and 90 °, and the phases of the three divided lights that have passed through these are varied by 90 degrees ( 0 °, 90 °, 180 °). A plurality of interference fringe images given different phase shift amounts are picked up by the CCD image pickup devices 109A, 109B and 109C, respectively.

  In the interferometer of FIG. 8, the quarter wavelength plate 107 is necessary to make the polarization directions of the measurement light and the reference light orthogonal, but this quarter wavelength plate 107 is expensive and has a large size. Due to the difficulty, it was difficult to widen the measurement area.

  In contrast, the Fizeau interferometer according to the first embodiment of the present invention shown in FIG. 1 is provided with a polarizing plate 118 in place of the quarter-wave plate 107, thereby changing the polarization directions of the reference light and the measuring light. It is orthogonal. Further, since the reflection surface of the polarizing plate 118 functions as a reference surface and a Fizeau interferometer in which the polarizing plate 118 and the object to be measured W are arranged on the same optical path, it is not necessary to prepare a separate reference surface. . The polarizing plate 118 also functions as a combining member that combines the transmitted light as measurement light and the reflected light as reference light.

  For example, as shown in FIG. 2, the polarizing plate 118 includes a transparent glass substrate 18A and wire grids (metal wire lattices) 18B formed on the transparent glass substrate 18A at equal intervals. The wire grid 18B is made of, for example, aluminum, and can be formed with, for example, a line width of 65 nm, a thickness of 100 to 200 nm, and a pitch of 144 nm. The wire grid 18B can be generated by photolithography and etching used in the semiconductor manufacturing process, and can be manufactured at a lower cost than a quarter-wave plate. In the case of this method, a polarizing plate having a diameter of several hundreds of millimeters can be easily produced, and it is easy to cope with widening the measurement region. The wire grid type polarizing plate 118 configured as described above has a property of reflecting a polarization component in a direction parallel to the longitudinal direction of the wire grid 18B and transmitting a polarization component in a vertical direction. In this respect, it is different from a generally used polarizing plate produced by adsorbing iodine or the like on a base film and stretching and orienting it.

[Second Embodiment]
Next, an interferometer according to a second embodiment of the present invention will be described with reference to FIG. The same components as those of the interferometer of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The interferometer of this embodiment is common to the first embodiment in that a polarizing plate 118 ′ is provided and the quarter-wave plate 107 (FIG. 8) is unnecessary. However, the polarizing plate 118 ′ is installed obliquely with respect to the incident optical axis of the light from the light source 101, and the object to be measured W is installed on the transmission side, and the reference surface 120 is installed on the reflection side. This is different from the first embodiment. In this configuration, the p-polarized light reflected by the polarizing plate 118 ′ is reflected by the reference surface 120 and is reflected again by the polarizing plate 118 ′. On the other hand, the s-polarized light transmitted through the polarizing plate 118 ′ is reflected by the object to be measured W and is transmitted again through the polarizing plate 118 ′. The p-polarized light and s-polarized light are combined by the polarizing plate 118 ′ that also functions as a combining member, reflected by the beam splitter 104, collected by the condensing lens 121, and then phase-shifted by the polarizing plates 114 </ b> A to 114 </ b> C. Given a shift. Thereby, the interference measurement by the phase shift method can be performed in the same manner as in the first embodiment.

[Third Embodiment] Next, an interferometer according to a third embodiment of the present invention will be described with reference to FIG. In the interferometer of this embodiment, as in the first embodiment, the polarizing plate 118 is arranged perpendicular to the optical axis of the incident light, and the reflecting surface of the polarizing plate 118 functions as a reference surface. Is unnecessary. However, in this embodiment, the configuration of a quarter-wave plate and a polarizing plate for phase shift after the beam splitter 104 is different from that of the first embodiment. That is, in this embodiment, the quarter wavelength plate 108 between the beam splitter 104 and the beam splitter 113A is omitted, and the combined light of p-polarized light and s-polarized light is split as it is by the beam splitters 113A and 113B and the mirror 113C. The polarization members after branching by the beam splitters 113A to 113C are configured as shown in FIG. The transmission axis directions of the polarizing plates 114A, 114B, and 114C are set to angles α, α, and α + 90 °, respectively. That is, the transmission axis directions of the polarizing plates 114A and 114B are equal. However, a quarter-wave plate 122 is further disposed in front of the polarizing plate 114B. The fast axis direction and slow axis direction of the quarter-wave plate 122 are matched with the polarization directions of the reference light and the measurement light.
Thereby, the phase of the interference fringe imaged by the CCD image sensor 109B can be 90 ° different from the phase of the interference fringe imaged by the CCD image sensor 109A. On the other hand, since the transmission axis direction of the polarizing plate 114C is α + 90 °, a phase of 180 ° is formed between the interference fringe imaged by the CCD image sensor 109A and the interference fringe imaged by the CCD image sensor 109C. A shift occurs. For this reason, as in the first embodiment, it is possible to obtain interference fringes given a plurality of types of phase shifts.

  [Fourth Embodiment] Next, an interferometer according to a fourth embodiment of the present invention will be described with reference to FIG. The interferometer of this embodiment does not optically shift the phase of the interference fringes, but shifts the phase shift by moving the polarizing plate 118 functioning as a reference surface in the optical axis direction as shown in FIG. Cause it to occur. For this reason, the beam splitters 113A and 113B and the mirror 113C are omitted, and only one CCD image sensor 109 needs to be prepared, and the configuration can be simplified compared to the above embodiment.

  In addition, the interferometer of this embodiment includes a polarizing plate 131 and a polarizing plate 132 in order to adjust the intensity ratio between the reference light and the measuring light. The polarizing plate 131 is rotatably held so that its transmission axis direction can be changed.

The material of the object to be measured W is various, such as glass and metal, and has different reflectivities.
When the reflectance is small, the intensity of the measurement light is smaller than the intensity of the reference light. When this difference in intensity increases, the accuracy of interference measurement decreases. Therefore, in the present embodiment, the transmission axis direction of the polarizing plate 131 is changed in accordance with the reflectance of the object to be measured W, thereby changing the polarization direction of the light incident on the polarizing plate 118, thereby the reference light and the measuring light. The intensity ratio is adjusted. By doing so, the difference in intensity between the reference light and the measurement light can be reduced, and interference measurement with a high S / N ratio can be performed. Such a polarizing plate 131 can also be employed in an interferometer using an optical phase shift as in the first and second embodiments. Further, as shown in FIG. 7, the same effect can be obtained by omitting the polarizing plate 131 and rotating the polarizing plate 132 in accordance with the reflectance of the object W to be measured.

  Although the embodiments of the invention have been described above, the present invention is not limited to these embodiments, and various modifications and additions can be made without departing from the spirit of the invention. For example, an example in which a plurality of CCD cameras 209A-C are prepared has been described. However, the present invention is not limited to this, and it is also possible to perform imaging with only one CCD camera by switching with an optical path switching member. In the above embodiment, the Fizeau interferometer has been described in which the reference surface and the object to be measured are on the same optical path. However, the present invention is not limited to this, and the Michelson type interference in which both exist in different optical paths. It is also applicable to totals. When applied to a Fizeau interferometer, the polarizing plate 118 can function as a combining member that combines reference light and measurement light. However, in the case of a Michelson interferometer, a separate light splitting and combining member is prepared. By doing so, the reference light and the measurement light can be synthesized.

1 shows a configuration of an interferometer according to a first embodiment of the present invention. The structure of the polarizing plate 118 of FIG. 1 is shown. The structure of the interferometer which concerns on the 2nd Embodiment of this invention is shown. 3 shows a configuration of an interferometer according to a third embodiment of the present invention. The quarter wave plate 122 and polarizing plates 114A to 114C shown in FIG. 4 are shown. The structure of the interferometer which concerns on the 4th Embodiment of this invention is shown. The structure of the interferometer which concerns on the modification of 4th Embodiment is shown. The structure of the conventional interferometer is shown. The configuration of the polarizing plate 114A of the conventional interferometer of FIG. 8 will be described.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Light source, 102 ... Condensing lens, 103 ... Pinhole, 104 ... Beam splitter, 105 ... Collimator lens, 106 ... Reference surface, 107 ... 1/4 wavelength Plate, W ... object to be measured, 108 ... quarter wave plate, 113A, 113B ... non-polarizing beam splitter, 113 ... mirror, 114A, 114B, 114C ... polarizing plate, 109A, 109B, 109 ... CCD image sensor, 118, 118 '... polarizing plate, 121 ... lens, 18A ... transparent glass substrate, 18B ... wire grid (metal wire grid), 131, 132 ··Polarizer.

Claims (6)

A light source;
Of the outgoing light emitted from the light source, the first light having the polarization component in the first direction is transmitted, while the second light having the polarization component in the second direction orthogonal to the first direction is reflected, and the Either the first light or the second light is reflected by the object to be measured to be measured light, and the other is reflected by the reference surface, and the measurement light and the reference light are combined as reference light and combined. A polarizing member for light;
A phase shift unit for phase shifting the combined light;
An imaging unit that captures an interference fringe image with the combined light that has been given a plurality of different phase shifts by the phase shift unit, and
The polarizing member is configured so that a reflection surface thereof is perpendicular to the emitted light, and the reflection surface is also used as the reference surface,
The Fizeau interferometer, wherein the first light is reflected by the object to be measured, passes through the polarizing member, and is combined with the second light to be the combined light. 2. The Fizeau interferometer according to claim 1, wherein the polarizing member comprises a transparent substrate and a metal wire grating formed on the transparent substrate at equal intervals. 3. The Fizeau interferometer according to claim 2, wherein the metal line grating is formed on the transparent substrate by photolithography and etching. A light splitting means for splitting the combined light into a plurality of lights;
2. The Fizeau interferometer according to claim 1, wherein the phase shift unit gives different phase shifts to the plurality of lights divided by using an optical member. The Fizeau interferometer according to claim 1, wherein the phase shift unit gives a phase shift to the combined light by moving the reference surface. The polarization member of the present invention is provided with another polarization member between the polarization member and the light source in order to adjust the intensity ratio between the reference light and the measurement light by changing the polarization direction of the emitted light. 1. The Fizeau interferometer according to 1.

Images