JP2003322503A - High-speed shearing heterodyne interferometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform multipoint parallel data processing in order to measure the area degree of a wide area at a high speed with sensitivity of nanometer or sub-nanometer. <P>SOLUTION: The measurement is performed by a three-point shearing heterodyne method using an orthogonal two-frequency laser, and two beams are required therefor. This three-point shearing heterodyne interferometer having multi-scanning lines for performing parallel data processing is constituted by arranging the two beams in parallel and by forming many three-point pairs by using a linear multiple-beam divider (special diffraction grating). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring deviation from a true plane, that is, flatness, with sensitivity of nanometer to sub-nanometer, and at high speed by parallel processing of multipoint sequences of data.

[0002]

2. Description of the Related Art Three-point shearing using two polarization splitting elements is one of the methods for measuring the level difference of three points on a surface by heterodyne interferometry and measuring the flatness without using a reference plane. An interferometer was being developed.

However, the size of one point in the measurement plane is often several tens of micrometers square, and the total number of points becomes enormous. Therefore, a large number of sets of parallel data processing are desired. This is difficult to achieve, and one of the causes is the necessity of using many non-polarized beam splitters for multiplexing and splitting light, which increases the number of unnecessary ports (hereinafter referred to as dummy ports). The light utilization rate was declining.

[0004]

The first problem of the present invention is how to make three points into a series of point sequences, and the second is a dummy for emitting unnecessary light of the non-polarization beam splitter. It is an object to construct a three-point heterodyne interferometer capable of realistic parallel data processing by reducing the number of ports.

[0005]

In the present invention, a method of separating a two-frequency laser light into two light fluxes having different frequencies by a photoacoustic element to form a heterodyne light is incorporated in the basic method. Light with different frequencies of photoacoustic elements is always shifted from parallel. Now, assuming that the surfaces on which the optical elements are arranged are x and y surfaces, that is, in-planes, and the normal direction of the x and y surfaces is z, the directions of the light beams due to different order of the photoacoustic elements are different directions in the in-planes. However, when this is incident on a linear multi-beam splitting element having a dispersion direction in the z-axis direction, the phenomenon becomes complicated due to obliquely incident light. Therefore, the two optical paths of the present invention should be kept as parallel as possible in the in-plane. The splitting of the polarization splitting element occurs in the in-plane, and a real image of the linear multi-beam splitting element is formed at this splitting point. This image formation is performed in a plane including the z axis. Therefore, the lens becomes a cylindrical lens. A polarization prism will be used as the polarization splitting element. As the linear multi-beam splitting element, for example, "TELEDYNE BRO
WN's "MUTIBEAM SPLITTERS-
Linear patterns 3-13 spot
"s" is on sale, so I will use it. By using the linear multi-beam splitting element and the optical system as described above, a line of three points can be formed on the sample surface in the in-plane, and a multi-point line can be formed in the z-axis direction orthogonal to the in-plane, and parallel data processing can be performed by the number of lines. It will be possible.

On the other hand, as a method of reducing the number of dummy boats of the beam splitter, a method using an optical isolator of a type in which the reflected light from the sample can be taken out in an optical path different from the incident light and used is solved. However, since it is necessary to use a difficult-to-obtain element such as a large-sized polarization displacement element (calcite, rutile, etc.) or a Faraday element for the isolator, a form not using it is defined in claim 2.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view of an optical system in a plane including the z-axis. In the example in which the parallel light flux, which is the diffracted light of the linear multi-beam splitter 7, is imaged by a telescopic system having a focal length of 1: 2, a cylindrical lens is used.
At the branch point of the polarization splitting element, which is the conjugate point, the width of the light beam is 2
2 shows that the light flux becomes a parallel light flux with a cross chain angle of 1/2, and that this light flux forms a series of points along the z-axis on the sample surface by the lens 4 in FIG.

FIG. 2 shows an embodiment of the present invention written in an in-plane form. Light of the orthogonal dual-frequency laser light source 9 (f ₁ ,
f ₂ ) is frequency-shifted by the photoacoustic element 8. The shift amount of light whose diffraction orders differ by only the first order is equal to the driving frequency Δf of the photoacoustic element. Here, the light beam without shift is B, and the light beam whose frequency is shifted is A. The light flux A and the light flux B are made parallel to each other by a mirror or the like, and are dispersed in the z direction by the linear multi-beam splitter 7 (adjacent dispersion angles are 10 ⁻³ r
ad) and bent by a small width mirror 26 to enter the optical isolator. This isolator is already known, but will be described. The horizontal polarization of the orthogonal dual-frequency light, that is, the H polarization, becomes an extraordinary ray in the first polarization displacement element calcite 22 and obliquely travels (walk-off). Vertically polarized light, that is, V polarized light becomes an ordinary ray and goes straight. After that, the Faraday element rotated 45 °, and then the half-wave plate ( ^λ / ₂
In consideration of the rotation of 45 ° at 20) (azimuth 22.5 °), the exchange of ordinary rays and extraordinary rays occurs.
Enters the polarization displacing element 19 and its output becomes two light beams of two orthogonal frequencies. Regarding the reflected light, the ^λ / ₂ plate rotates −45 °, and the Faraday element additionally rotates 45 °, so that the return polarization is 90 ° in the forward polarization and the azimuth is 90 °.
different. Therefore, the first polarization displacement element has a walk-off as shown in the figure, and the two V-polarized light beams travel straight and H
The two polarized light beams largely walk off and return with the mirror 26 sandwiched therebetween. The H components of the returned luminous flux A and luminous flux B are P
As the transmitted light of the BS 23, the V polarized light of the light flux A and the light flux B is P
If combined as reflected light from the BS 23, both the light flux A and the light flux B return to the orthogonal dual frequency light and exit from the PBS 23. These two luminous fluxes are seen in the in-plane diagram, and when viewed in the plane including the z axis, that is, in the off-plane diagram, both the luminous flux A and the luminous flux B are composed of many parallel luminous flux groups having different traveling directions.
When they are referred to as a luminous flux group A and a luminous flux group B, the electric fields of the respective polarized light beams of both luminous flux groups are superposed by the polarizer 24, and the optical beat group of the luminous flux group A passes through the slit 13 optical fiber group 17 and the luminous flux B group. The optical beat group of the group is detected by the detectors 14 and 15 through the slit 12 optical fiber group 18, passes through the low pass filter group 29 for removing the frequency Δf of the photoacoustic element, and then the position of the frequency of the reference optical beat. The phase difference is
It is obtained by the phase meter group 30. The reference light beat is obtained by the detector 16 through the polarizer 25 by extracting the laser light from the glass plate 28. The beam splitter 3 has one dummy port. The three parallel light beams and the like that are branched and combined by the in-plane are converted into light beams whose central axes are parallel and converge on the sample surface by the condensing lens 4 whose front and rear focal points are the branch point and the sample surface 32. It

FIG. 2 is a diagram showing a measurement point matrix. The n-th detector group is the n-th information in the point row, that is, the optical path length differences a _n , b _n , and c _n when scanning in the S direction. Will be asked.

FIG. 4 shows an embodiment according to claim 2.
Elements 19 and 2 used in the optical isolator in FIG.
The embodiment is exactly the same as the embodiment of FIG. 3 except that the PBS 23 for combining the polarizations of the light fluxes 0, 21, 22 and the light fluxes A, B is eliminated and a beam splitter 30 is added instead. It is unavoidable that light escapes from the dummy port of the beam splitter 30 and causes a loss. Since the beam splitter 3 also has one dummy port, the number of dummy ports in this system is two.

[0011]

By increasing the number of traverse lines and performing parallel data processing at the same time, it is possible to measure nanometers and sub-nanometers having a flatness of a wide plane at a high speed.

[Brief description of drawings]

FIG. 1 is a diagram of an optical system for linear multi-beam splitting according to the present invention.

FIG. 2 is a diagram of a measurement point sequence of the present invention.

FIG. 3 is a diagram showing an embodiment of the present invention.

FIG. 4 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

1, 2 Rosyon prism 3, Beam splitter 4, Condensing lenses 5, 6 Cylindrical lens 7, Linear multi-beam splitter 8, Photoacoustic element 9, Orthogonal two-frequency laser light source 10, 11 Imaging lens 12, 13 Slit 14, 15 Detector group 16, Detector 17, 18 Optical fiber group 19, Second polarization displacing element 20, Half wave number 21, Faraday element 22, First polarization displacing element (calcite or rutile) 23 , PBS 24.25 Polarizer 26, small-width mirror 27, ROM 28, total reflection prism 29, low-pass filter group 30, retarder group 31, glass plate 32, sample surface a, b, c measurement point n, Column numbers f ₁ and f ₂ Light frequency Δf Frequency difference

Claims (2)

[Claims] 1. A light beam A and a light beam B having different frequencies by using orthogonal two-frequency laser light by a method such as a difference in diffraction order of a photoacoustic element.
Of the two light fluxes, the two light fluxes are arranged substantially in parallel, and the light fluxes arranged in parallel are split by a linear multi-beam splitting element (special diffraction grating) to form two sets of light flux groups. The light flux groups are made to pass through an optical isolator such as a first polarization displacement element, a Faraday rotation element, a half-wave plate, and a second polarization displacement element, and the light of each flux group is orthogonalized. After passing through an element having a function of branching into two traveling directions depending on the polarization direction for the polarized light to be polarized, for each luminous flux of the luminous flux group, A means for forming a total of three parallel light fluxes having different traveling directions obtained by combining so that the light flux of one polarization of two orthogonal polarizations of light coincides with each other, and the three parallel light fluxes with their central axes parallel to each other. Three convergences as there are And a sample plane at the convergent point (focus point) of the light beam, and the reflected light of the sample surface, for example, the light path of the light beam of the incident light up to the emission surface of the Faraday element or the like, that is, , The optical paths of the light flux A and the light flux B are reversed, and, for example, the light is incident on the first polarization displacement element or the like from the opposite direction, and the ordinary ray and the extraordinary ray of the light flux A and the light flux B are separated to separate the incident light. After doing
The polarized components of the separated luminous fluxes A and B, that is, the ordinary rays of the luminous fluxes A and B and the extraordinary rays are combined to form a luminous flux A and a luminous flux B of orthogonal two frequencies, and thereafter, Means for obtaining two sets of optical beat groups by superimposing electric fields to obtain a phase difference group between the two sets of optical beat groups and the optical beat of the light source, and a cylindrical lens system for the linear multi-beam splitting element. A high-speed shearing heterodyne for flatness measurement, characterized in that a splitting point and a splitting point of an element that splits in two traveling directions according to the polarization direction are conjugate point pairs in a plane including a splitting direction of a linear multi-beam splitting element. Interferometer 2. The optical isolator according to claim 1 is not used, and one beam splitter is used. Therefore, the polarized light in the light beam A (extraordinary ray) and the polarized light in the light beam B (extraordinary ray) are unnecessary. ) A high-speed shearing heterodyne interferometer for measuring flatness, which is formed by removing the means for overlapping each other and other polarized lights (ordinary rays).