JPH06307809A - Method and device for interference measurement - Google Patents

Abstract

PURPOSE:To sense accurately an interferential fringe finer than the picture element pitch in measuring the interference using a simple configuration. CONSTITUTION:The interferential fringe obtained by an interferometer is sensed by a detector 8 having elements installed in the two-dimensional array arrangement, where a mask means 7 provided with pinholes at the same pitch as the pitch, at which elements are arranged, and having a size smaller than the element is installed in front of the light receiving surface of a detector 8 or in any position equivalent thereto, and the mask means and detector are moved in the direction perpendicular to the optical axis while the specific relation is held, and thereby the sensing of interferential fringe is conducted at displacement intervals which is smaller than the element arrangement pitch.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometric measuring method and apparatus, and is particularly applicable to a light wave interferometer for high resolution measurement used for measuring the shape of an aspherical lens.

[0002]

2. Description of the Related Art In a conventional light wave interferometer, when an aspherical shape is to be measured with a resolution higher than a pixel pitch, a simple spherical wave is used as a reference wavefront.
E Vol. 645 (1986) p. 101 to 106. There was a measurement technique using two wavelengths as announced by Creath et al. According to this document, when two wavelength lights of λ ₁ and λ ₂ are separately incident on the interferometer and the measured phase distributions are φ ₁ and φ ₂ , respectively, the difference between the two sets of data φ = φ
_1- φ ₂ is the synthetic wavelength λ _eq expressed by the following equation (1)
By utilizing the fact that it can be regarded as the phase distribution measured in, the attempt is made to detect interference fringes finer than the pixel pitch.

Λ _eq = λ ₁ · λ ₂ / | λ ₁ −λ ₂ | (1) Further, in the same document, a pinhole is provided so that optical resolution phase detection by fringe scan can be performed even for fine pitch stripes. A mask having a system of 5 μm and a pitch of 60 μm is arranged in front of the array detector.

[0004]

However, in the above-mentioned conventional example, there is a problem that an optical member which suppresses aberrations with respect to light of two wavelengths is required, which makes it very difficult to manufacture an interferometer. Further, if the synthetic wavelength is increased to achieve higher resolution, the measurement S / N will deteriorate, and even if there is a slight noise or air fluctuation, there is a possibility that a measurement error will occur due to phase skipping or the like. It was

In view of the above-mentioned drawbacks of the conventional example, the present invention provides a measuring method and apparatus capable of performing high-resolution interferometric measurement with high accuracy even with a single wavelength and with less error. With the goal.

[0006]

In order to solve the above-mentioned problems, an interferometric measuring method of the present invention is designed to detect interference fringes obtained by an interferometer with a detector having an array of elements. The mask means, which has the same pitch as the arrangement pitch and has a transmissive portion smaller than the size of the element, is arranged at the front side of the light receiving surface of the detector or at a position equivalent to the position, and the mask means and the detector are predetermined. Therefore, the interference fringes are detected for each displacement smaller than the arrangement pitch of the elements by moving the element in the direction perpendicular to the optical axis.

Further, the interferometer of the present invention comprises an interferometer, a detector for detecting interference fringes obtained by the interferometer with elements arranged in an array, and the same pitch as the arrangement pitch of the elements and the elements. A mask means having a transmission part smaller than the size of the detector and arranged at the front position of the light receiving surface of the detector or at a position equivalent to the position.
It has a driving means for moving the mask means and the detector in a direction perpendicular to the optical axis in a predetermined relationship, and a measuring means for detecting an interference fringe for each displacement smaller than the arrangement pitch of the elements by the driving means. I am doing it.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a block diagram of an embodiment in which the present invention is applied to a Fizeau interferometer. In the figure, 1 is a laser light source, 2 is a lens for spreading light, 3 is a half mirror, 4 is a converging lens, 4'is a TS for emitting a measurement wavefront to the DUT and an emission side surface forming a reference wavefront. A lens, 5 is an object to be measured, 6 is an imaging lens, 7 is a pinhole array mask, 8
Is a CCD camera, 9 is an electrostrictive element for fringe scan, 1
0 is a driver for electrostrictive elements for fringe scan, 11 is an electrostrictive element for CCD camera scanning, 12 is a driver for electrostrictive elements for CCD camera scanning, 13 is a leaf spring mechanism for guiding the movement of CCD, and 14 is a computer. .

The coherent single-wavelength laser light emitted from the light source 1 becomes divergent light by the lens 2, passes through the half mirror 3, and enters the lens 4 and the TS lens 4 '. The lens 4 and the TS lens 4'are coated with an antireflection film according to the wavelength of the light source, but since only the emitting side surface of the TS lens 4'is not coated, a part of it is reflected as the reference light to the light source side. It returns, is reflected by the half mirror 3, and heads toward the CCD camera 8. On the other hand, TS lens 4 '
The light that has passed through is reflected by the surface to be measured of the object to be measured 5 as measurement light, passes through the TS lens 4 ′ and the lens 4 again, is reflected by the half mirror 3, passes through the imaging lens 6, and passes through the CCD.
Interference fringes are formed on the image pickup surface of the camera 8 by the interference with the reference light described above.

Generally, in an interference measuring apparatus, if the width of the phase difference π of the interference fringes becomes smaller than the interval between adjacent pixels, it becomes impossible to connect the phases between the pixels in the image processing. Therefore, in this apparatus, a pinhole array mask 7 as shown in FIG. 2 is arranged in front of the image pickup surface of the CCD camera 8. (Horizontal on the diagram, the vertical pitch of each p _x, p _y) two-dimensional array of the pinhole array mask CCD camera 8 at positions corresponding to the respective pixels 8a arranged, the size of the pixel (horizontal, vertical pitch p _x, p _y, respectively on the diagram) are formed at a smaller pinhole 7a is the same pitch of. The mask 7 is configured to move integrally with the CCD camera 8 in the direction perpendicular to the optical axis by the CCD camera scanning electrostrictive element 11.

FIG. 3 is an explanatory view of the operation principle of this apparatus.
(A) shows the relationship between the interference fringes and the pinhole 7a and the element 8a, and (b) shows an example of the output signal from one pixel (the horizontal axis is the position of the pixel in the x direction). FIG. 10A illustrates a case where vertical interference fringes arranged at a fine pitch in the horizontal direction are generated. In this case, in the x direction, the interference fringes in the range corresponding to the adjacent pixel interval are generated. The phase difference is close to 3π. In this state, it is impossible to connect the normal phases as described above. Therefore, in this device, the pixel pitch p is set in the x direction by the electrostrictive element 11 for CCD camera scanning.
_The CCD camera 7 and the pinhole array mask 7 are integrally moved in the direction perpendicular to the optical axis _{by an} amount of _x, and the output from the CCD camera 8 is captured at appropriate displacement intervals smaller than the arrangement pitch of the pixels 8a. There is. In this way, as shown in FIG. (B), the output from each pixel intensity changes according to the interference fringes each pixel crosses during displacement, and finally small interfering than the pixel pitch p _x The stripes can be detected as a change in the output of each pixel during movement. That is, it is possible to easily determine how much the phase difference is between adjacent pixels and which phase difference the pixel is advancing to.

In the above description, the case of displacing in the x direction by the CCD camera scanning electrostrictive element 11 has been described, but by displacing in the y direction by the same method, it is arranged at a fine pitch in the vertical direction in FIG. It is also possible to measure the horizontal interference fringes with high accuracy. That is, by combining displacements in both directions, it is possible to perform two-dimensional highly accurate interference fringe measurement.

For the initial phase at each point, the TS lens 4'is displaced by half the wavelength (λ / 2) in the optical axis direction by the fringe scan electrostrictive element 9, and the change in the interference fringes during this period is measured. It can be detected with high accuracy by the usual fringe scan method. That is, the interference fringe information at each point can be measured with high accuracy and high resolution by measuring the interference fringes based on the displacements of the CCD camera 8 and the mask 7 at each measurement time point in this fringe scan. The computer 14 performs the signal processing of the image signal obtained through the above process and the measurement of the surface profile of the object to be measured from the detected interference fringe information. In addition, the electrostrictive elements 9 and 11
The drive of is controlled by each driver 10 and 12 according to a command from the computer 14.

In the above description, the case where the pinhole array mask 7 is arranged in front of the CCD camera 8 has been described. However, the position of the pinhole array mask 7 may be the image plane conjugate position of the CCD camera 8 in the optical system, In that case, the size of the pinholes, the arrangement pitch, and the displacement amount of the mask 7 when the CCD camera 8 is moved are appropriately set according to the projection magnification of the mask 7 on the imaging surface of the CCD camera 8.

[0015]

As described above, according to the present invention, interference fringes having intervals smaller than the pixel pitch can be measured without difficulty, and high-precision and high-resolution interference measurement can be performed.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is a diagram illustrating the relationship between interference fringes, pinholes, and pixels.

FIG. 3 is an explanatory diagram of an operation principle of the device.

[Explanation of symbols]

 1 Laser Light Source 2 Lens 3 Half Mirror 4 Condensing Lens 4'TS Lens 5 Object to be Measured 6 Imaging Lens 7 Pinhole Array Mask 8 CCD Camera 9 Electrostrictive Element for Fringe Scanyan 10 Driver for Electrostrictive Element for Fringe Scanyan 11 CCD Electrostrictive element for camera scanning 12 CCD driver for electrostrictive element for camera scanning 13 Leaf spring mechanism 14 Computer

Claims (2)

[Claims] 1. When detecting interference fringes obtained by an interferometer with a detector having elements arranged in an array,
A mask means having a transmissive portion having the same pitch as the array pitch of the elements and smaller than the size of the elements is arranged at the front side of the light receiving surface of the detector or at a position equivalent to the position,
An interference measuring method characterized in that the mask means and the detector are moved in a direction perpendicular to the optical axis in a predetermined relationship, and an interference fringe is detected for each displacement smaller than the arrangement pitch of the elements. 2. An interferometer, a detector for detecting interference fringes obtained by the interferometer with elements arranged in an array, and a transmission part having the same pitch as the arrangement pitch of the elements and smaller than the size of the elements. Mask means formed and arranged at the front side of the light receiving surface of the detector or at a position equivalent to the position, and driving means for moving the mask means and the detector in a direction perpendicular to the optical axis,
An interference measuring apparatus comprising: a measuring unit that detects an interference fringe for each displacement smaller than the arrangement pitch of the elements by the driving unit.