JP2966568B2 - Interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for analyzing a wavefront by a fringe scan method, and more particularly to an improvement in an interferometer capable of accurately detecting a moving amount of a reference mirror.

[0002]

2. Description of the Related Art A fringe scanning method using an interferometer is one of the methods for measuring the shape of an object on the order of the wavelength of light or less. In the fringe scanning method, a light beam from a single light source is split into two beams and reflected by a surface to be measured and a reference surface, and these reflected lights are combined to generate interference fringes on the imaging surface of the television camera. Then, one of the measured surface and the reference surface is moved in the optical axis direction to analyze a change in the amount of light for each pixel of the camera, and the surface shape of the measured surface is measured.

For movement in the optical axis direction, a phase shift device such as a piezo element is used. However, since the accuracy of measurement is determined by the accuracy of movement, accurate control is required.

In the conventional interferometer system, the amount of movement of the reference surface is calculated from the applied voltage by considering the operation amount of the piezo element as a function of the applied voltage.

However, since the amount of operation of the piezo element varies not only with the applied voltage but also with disturbances such as temperature changes and vibrations, the amount of movement of the reference surface is calculated based on only the applied voltage as described above. However, there is a problem that the measurement accuracy is reduced due to the inclusion of disturbance.

[0006] Even if the phase shift cannot be performed accurately, the error can be canceled if an error from the set value of the real-time phase shift amount is found. 1
Proceedings of the Japan Society of Applied Physics Autumn 989, 777 page 29a-
ZE-3 mentions a method of forming a young fringe for shifter calibration on a part of an interference surface and calculating the phase in order to measure a phase shift amount in real time.

[0007]

However, the method described in the above-mentioned proceedings requires a special member for detecting a phase shift, such as a monitor mirror, in addition to the members constituting the interferometer. Since the diameter of the optical system is increased to include the monitor section and the sample section, and further, the monitor section and the sample section are separated from each other. The phase shift amount may be different.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and does not require any special members.
It is an object of the present invention to provide an interferometer capable of accurately detecting the amount of movement by a phase shift device.

[0009]

In order to achieve the above object, an interferometer according to the present invention comprises: an optical system for tilting and superimposing a wavefront from a reference surface and a surface to be measured; And a phase shift device that moves at least one of the surface to be measured in the optical axis direction, and interference fringes including spatial carriers due to tilt are measured multiple times, and the phase shift is performed by applying a spatial fringe scan method to each measurement result. Moving amount detecting means for detecting a moving amount by the device, first analyzing means for analyzing a wavefront including a spatial carrier by a time fringe scan method based on the moving amount and the measured interference fringes, and data of the first analyzing means And a second analyzing means for subtracting a space carrier by a tilt from the data and analyzing a shape of the surface to be measured.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.
1 and 2 show an embodiment of an interferometer according to the present invention. In the optical system shown in FIG. 1, divergent light emitted from a light source 1 is converted into a parallel light beam by a collimator lens 2 and a plane mirror 4 which is a surface to be measured by a half mirror 3.
And the reference mirror 5. The luminous flux reflected by each mirror is superimposed on the half mirror 3 and forms an image via the imaging lens 6 on the imaging surface of the CCD camera 7 arranged on the pupil plane conjugate with the pupil.

The reference mirror 5 is provided with a tilt with respect to the optical axis in advance, and is supported movably in the optical axis direction by a piezo element (PZT) 8 as a phase shift device. Therefore, fine interference fringes including spatial carriers due to tilt are formed on the imaging surface of the CCD camera 7.

The reference mirror 5 is provided to be tiltable so that the angle can be adjusted. In order to tilt the wavefront, a wedge-shaped prism may be inserted in the optical path between the half mirror 3 and the reference mirror 5 instead of tilting the reference mirror 5.

The analog image output of the CCD camera 7 is A
The data is converted into digital data by the / D converter 9 and stored in the image buffer 11 via the interface 10. The CPU 12 has a function of reading and processing image data, and a function of controlling the PZT 8 via the D / A converter 13 and the driver 14.

The image processing function of the CPU 12 is to detect a moving amount of the reference mirror 5 by the PZT 8 when each image data is fetched by applying the spatial fringe scanning method by reading image data one by one. A function as a means, a plurality of combinations of the movement amount and the image data, a time fringe scan method is applied, and a function as a first analysis means for analyzing an interference fringe including a spatial carrier is analyzed. The function can be divided into a function as a second analyzing means for analyzing the shape of the surface to be measured by subtracting the spatial carrier by the tilt from the stripe.

Next, the operation of the above-described interferometer will be described with reference to FIG. In step (indicated by "S." in FIG. 1), a plane mirror 4 as an object to be measured is set and an interferometer is aligned. In step 2, the reference mirror 5 is tilted to generate fine interference fringes on the CCD camera 7.

S. 3, the voltage applied to the PZT 8 is changed to move the reference mirror 5 in the optical axis direction. In step 4, the interference fringe image is sampled. Then, the spatial fringe scanning method is applied to the sampled interference fringe image, and an accurate movement amount of the reference mirror 5 is obtained.

S. In step S5, it is determined whether a predetermined number of image data has been sampled. 3, S. Step 4 is repeated. When a predetermined number of image data are sampled, based on the image data and the movement position of the reference mirror 5 at the time of sampling,
The phase of the wavefront including the spatial carrier due to the tilt is obtained by the processing of the time fringe scan method. And S. 7
Then, the surface shape of the plane mirror 4 is determined by subtracting the carrier by the tilt.

As described above, the moving position of the reference mirror 5 at the time of image sampling can be accurately obtained. The amount of error can be known and the analysis can be calibrated based on this. The calibration method is, for example, APPLIED OPTICSVo
l. 22, no. It is expressed by the expression (7) on the right column of page 21 of page 3422.

[0019]

As described above, according to the present invention, the amount of movement of the reference mirror by the phase shift device can be accurately measured without providing any special element or device. Accuracy measurements can be made.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of an interferometer according to the present invention.

FIG. 2 is a flowchart showing the operation of the interferometer of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Light source 4 ... Plane mirror (measured surface) 5 ... Reference mirror (reference surface) 7 ... CCD camera 8 ... PZT (phase shift device)

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01B 9/00-11/30 102

Claims (1)

(57) [Claims] An optical system for tilting and superimposing a wavefront from a reference surface and a surface to be measured for measurement, and a phase for moving at least one of the reference surface and the surface to be measured in an optical axis direction. A shift device, a plurality of times of measuring interference fringes including a spatial carrier due to tilt, a moving amount detecting means for detecting a moving amount by the phase shift device by applying a spatial fringe scanning method to each measurement result, and the moving amount A first analysis unit for analyzing a wavefront including a space carrier by a time fringe scan method based on the measured interference fringes and a space carrier due to tilt are subtracted from data of the first analysis unit to analyze a shape of a surface to be measured. Second
An interferometer, comprising: