JPH0448201A - Interference measuring instrument - Google Patents

Abstract

PURPOSE:To measure the surface accuracy of a surface to be inspected with high accuracy by measuring distortion that the condenser lens system and interference fringe detecting means of an interferometer and other internal optical elements have and correcting a measurement result with the measured values. CONSTITUTION:The instrument is equipped with a tilt angle adjusting part 4 which tilts a holding part 3 and an optical element 2 to be inspected in one body to the optical axis of the interferometer 1, an alignment controller 6 which controls the tilt angle and movement in the direction of the optical axis, an interference fringe detecting means 7 which detects interference fringes, etc. Then the distortion of the optical system of the interferometer 1 itself is measured and the measurement data of the optical element 2 to be inspected is corrected with the measurement data. The measurement data is based upon interference fringe data obtained while the optical element 2 to be inspected or a reference surface is tilted to the optical axis from the state wherein the center of curvature of a curved surface is aligned with the optical axis of the interference measuring instrument. Consequently, the surface accuracy of the object surface can be measured with extremely high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an interference measuring device that measures the surface shape of an optical element with high precision.

[Prior Art] Conventionally, interference measuring devices have been provided that measure the surface shape of an optical element to be tested using light interference.

In order to improve the measurement accuracy, these interference measurement devices are based on measurement data of polished surfaces, as shown in the optical measurement device of Japanese Patent Application Laid-Open No. 62-214305.
Either the aberrations of the internal optical system of the interferometric measurement device are determined and the measured values are corrected using this as correction data, or the interferometric measurement device is From multiple measurement data obtained by rotating the prototype around the optical axis or moving it in the optical axis direction, the distortion and inherent aberration of the reference plane of the interferometric measurement device are determined in advance, and this is used as correction data.
The surface shape of the test surface is measured by subtracting it from the measurement results of the test surface.

[Problem to be Solved by the Invention] As described above, correction of measurement data of the conventional interferometric measurement device is performed by determining in advance the aberration of the interference optical system of the reference surface of the interferometric measurement device by measuring the reference surface. However, these methods have the problem that distortion of the interference fringes due to distortion in the optical system of the interferometric measurement device is not measured, resulting in measurement errors that are superimposed on the measurement data of the surface to be measured. .

The present invention was made to solve the above problems, and measures the distortion of an interference measurement device, calculates a distortion correction value from this measurement value, and corrects the measurement data of the surface to be measured. The present invention aims to obtain an interference measuring device with higher precision.The present invention divides a light beam emitted from a light source into two, and uses one divided beam as a reference surface and the other divided beam as a test surface. An interferometer that measures the shape of the test surface from the interference fringes obtained by interfering with each reflected light beam, a holder for holding the test optical element, and adjusting the tilt of the test optical element. a measuring table having a tilt angle adjustment section for controlling the angle of interference; a measuring table having a support section for movably holding it in the direction of the optical axis; an alignment control device for controlling the tilt and movement in the direction of the optical axis; An interference fringe detection means for converting into a signal, and an A/A for converting an electric signal into a digital signal.
In an interference measurement device consisting of a D converter and an arithmetic unit for inputting and processing the obtained digital signal, the distortion of the optical system of the interferometer itself is measured, and the measurement data is used to measure the distortion of the optical system of the interferometer itself. It is characterized by correcting measurement data of optical elements.

In implementing the interferometric measurement device of the present invention, the above measurement data was obtained by tilting the optical element to be measured or the reference surface with respect to the optical axis from a state in which the center of curvature of the reference curved surface coincided with the optical axis of the interferometric measurement device. Measurement data is based on interference fringe data obtained under the following conditions.

FIG. 1 conceptually shows the interferometric measurement device of the present invention, which generates convergent light or parallel light, and separates the reflected light from the optical element 2 to be measured placed at a predetermined position and the reflected light from the reference surface. an interferometer 1 that forms interference fringes due to the interference of
A tilt angle adjustment unit 4 that integrally tilts the holding unit and the optical element to be tested with respect to the optical axis of the interferometer 1, an optical axis direction movement support unit 5 that moves parallel to the optical axis direction, and tilt angle and optical axis An alignment control device 6 that controls movement in the direction, an interference fringe detection means 7 that detects interference fringes, an A/D converter 8 that converts the output of the interference fringe detection means 7 into a digital signal, and an A/D converter 8 that converts the output of the interference fringe detection means 7 into a digital signal. In an interferometric measurement apparatus having an arithmetic unit 9 that calculates the shape of the surface to be measured based on the measured surface, a surface to be measured with relatively good surface accuracy is used as the optical element 2 to be measured, and a convergence point of the light beam of the interferometer, After adjusting the alignment with respect to the optical axis so that the centers of curvature of the test surface coincide and interference fringes on the test surface are not clearly observed, the alignment control device 6 changes the surface top position of the test optical element 2. Parallel and linear interference fringes are generated by tilting only a very small amount without causing any interference.

Originally, if there is no distortion in the optical system of the interferometer 1 itself and the interference fringe detection means 7, the interference fringes will be parallel, equally spaced straight lines, as shown in FIG. 2(a), or there will be distortions. This results in distorted interference fringes as shown in FIG. 2(b). These interference fringes are detected by the interference fringe detection means 7 and taken into the arithmetic unit 9 through the A/D converter 8. On the other hand, the phase of the test surface in the absence of distortion is determined from the amount of tilt of the test optical element, and from the difference between the phase of the test surface and the actually measured phase, the distortion of the internal optical system of this interferometer is determined. The measured distortion of the surface to be inspected is corrected based on the result of the calculation.

The method of determining the distortion inside the interferometer from the phase of the test surface measured when the test surface is tilted and calculating the amount of correction for the measurement data is based on the phase of the test surface measured when the test surface is tilted.
, Y, Z (Z: optical axis) coordinates, the X, Y coordinates are x, Y, on the observation plane of the interference fringe detection means.
Corresponds linearly with the y-coordinate.

Let the phase of the interference fringe on the surface to be measured be Φ(X, Y), the corresponding coordinates on the image plane of the interference fringe detection means be (x, y), and the phase on the image plane be Φ'(x, y). shall be. Since distortion is an axially symmetrical phenomenon with respect to the optical axis, a point on the X-axis of the test surface is mapped onto the X-axis of the image surface.

For the sake of simplicity, if we consider only the X (or The result is as shown in FIG. In other words, when the distortion is assumed to be O, the phase LO is Lo:Φ=αX (α: inclination of the surface to be tested), whereas the actual phase shift when there is a distortion is L Φ=α (x-D(x)).

In other words, the deviation D from the position where the original phase value Φ should be
(x) is the amount of distortion. This D(x)
is determined from the measured values. Here, α can be calculated and determined from the output of the alignment control device.

Next, the phase on the image plane measured in the shape measurement of the surface to be inspected is corrected in the -D(γ) radial direction. Here, r is the distance from the optical axis to the point to be corrected. The amount of distortion is a function D(
γ), and the position (x) on the image plane for any position (X, Y) on the object plane.

For y), the following relationship holds true (see Figure 4)
.

X=βX+D (7) cosθ y=βY+D (γ) cosθ That is, X=(1/β) (x−D (y) cosθ)Y=(
1/β) (y-D (7) sin θ)-(1) However, β・image magnification γ: (x2+y2) l/2 θ tan-'(y/x) To further increase accuracy, There is also a means to obtain detailed correction data by tilting the surface to be inspected not only around the Y axis (but also around a plurality of other axes orthogonal to the optical axis).

[Embodiments] Specific embodiments of the interference measurement apparatus according to the present invention will be described below.

FIG. 5 is an explanatory diagram showing the configuration of the first embodiment of the interference measuring device of the present invention. Coherent light source 11 such as laser light
The beam from the beam is expanded to a desired diameter by the beam expander 12, enters the beam splitter 13, passes through the condenser lens system 14, and enters the reference surface 15 at right angles. reflected by the beam splitter 13
is reflected back to the screen 19 which is an interference fringe detection means.
reach.

On the other hand, the light or line transmitted through the reference surface 15 is reflected by the test surface of the test optical element 16 whose alignment has been adjusted so that the center of curvature of the test surface coincides with the center of curvature of the reference surface, and is reflected again. The light passes through the reference surface 15 and the condensing lens 14 and is reflected by the beam splitter 13 and interferes with the previous light beam, so that interference fringes are observed on the screen 19. The interference fringes are converted into an electrical signal by an image sensor 20 serving as an interference fringe detection means, and converted into a digital signal by an A/D converter 21 and input into an arithmetic unit.

Next, while controlling the tilt angle and optical axis direction movement adjusting section 18 configured with a piezo element as an actuator so as not to change from the top position of the surface to be measured of the optical element 16 to be tested by the alignment control device 23, the holding section 17 is tilted slightly. This tilt can also be performed by issuing a command from the calculation device 22 to the alignment control device 23.

By tilting the surface to be inspected, parallel linear interference fringes appear on the screen 19. These interference fringes are taken in by the image pickup device 2D, converted into digital signals by the A/D converter 21, and then inputted to the calculation device 22 to calculate the surface distortion of the optical element to be tested.

Each X, Y incorporated into the calculation program for this measured value
A correction amount commensurate with the position is subtracted according to equation (1) to obtain true shape data of the surface to be inspected.

In this embodiment, in particular, the condensing lens system 14 has a
It is possible to correct both the distortion and the distortion of the imaging device 20.

Although the interferometer of the interference measurement apparatus of the first embodiment shown in FIG. 5 is a Fizeau type interferometer, the present invention is not limited to this type of interferometer.

FIG. 6 shows a second embodiment of the interference measuring device of the present invention. In this embodiment as well, an interference measuring device for measuring the aberration of the transmitted wavefront of the optical element 16 to be tested is constructed using a Fizeau interferometer similar to that described in the first embodiment.

In this example, in order to measure the transmitted wavefront of the optical element 16 to be tested, the optical element 16 is arranged so that the focal position of the optical element 16 coincides with the focal point of the condensing lens system 24 of the interferometer. A plane reflecting mirror 25 with good surface precision is provided in the tilt angle and optical axis direction movement adjustment section 18 to reflect the light beam transmitted through the element 16 and return it to the original direction.

In this example, parallel linear interference fringes are obtained by tilting the plane reflecting mirror 18 instead of tilting the optical element 16 to be tested, and from this interference fringe data, the internal aberration of the interferometer is calculated, and the measured value is calculated. Can be corrected.

According to the interferometric measurement apparatus of this embodiment, it is possible to measure the aberration of the transmitted wavefront of the optical element 16 to be measured in which the distortion of the interferometer 1 including the surface distortion of the plane reflecting mirror 25 has been corrected.

FIG. 7 shows a third embodiment of the interference measuring device of the present invention. In this example, a Twyman-Green type interferometer is used in place of the interferometer 1 of the first example to construct an interference measurement apparatus. In this Twyman-Green type interferometer, a reference mirror 26 is arranged in place of the reference surface 15, and the light reflected by the reference mirror 26 and the light reflected by the test surface of the test optical element 16 are made to interfere with each other. Interference fringes are formed on 19. In this type of interferometer, the alignment is adjusted so that the focal point of the condenser/zoom system 24 coincides with the center of curvature of the surface to be measured of the optical element 16, as described in the first embodiment. The wavefront aberration of the surface to be measured is measured from the interference fringes produced by tilting the optical element 16 to be tested, and the amount of distortion inherent in the interferometer can be determined by calculation. This calculated value is corrected using equation (1) when measuring the optical element to be tested.

According to the interference measurement device of this embodiment, it is possible to correct errors including errors caused by surface distortion of the reference mirror 26. Further, instead of providing the reference mirror 26 with a tilt angle and optical axis direction movement adjusting section to tilt the optical element 16 to be tested, it is also possible to measure distortion by tilting the reference mirror 26. In this case, the movable part can be built into the interferometer main body, which has the advantage that the holding part for the optical element to be tested can have a simple structure.

[Effects of the Invention] According to the above-mentioned interference measuring device of the present invention, the distortion of the condensing lens system of the interferometer, the interference fringe detection means, and other internal optical elements can be measured, and the measurement results can be obtained based on the measured values. Since the correction can be made, the surface accuracy of the surface to be inspected can be measured with extremely high precision.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram conceptually showing the basic configuration of the interferometric measurement device of the present invention, Fig. 2 (a) is an explanatory diagram showing interference fringes in the case without distortion, and Fig. 2 (b) is an explanatory diagram in the case with distortion. Fig. 3 is an explanatory diagram showing the relationship between the phase of interference fringes on the test surface and the phase on the image plane, and Fig. 4 shows the relationship between arbitrary positions on the object plane and the amount of distortion. An explanatory diagram showing the relationship with the position on the image plane, FIG. 5 is an explanatory diagram showing the configuration of the first embodiment of the interference measuring device of the present invention, and FIG. 6 is a diagram showing the configuration of the second embodiment of the interferometric measuring device of the present invention. FIG. 7 is an explanatory diagram showing the configuration of a third embodiment of the interference measuring device of the present invention. Interferometer Test optical element holder Tilt angle adjustment unit Optical axis direction movement support unit Alignment control device Interference fringe detection device A/D converter Arithmetic device Light source beam expander Beam splitter Converging lens system Reference plane Test optical element holder Tilt angle and optical axis direction movement adjustment unit Screen image pickup device A/D converter Arithmetic unit Alignment control means Condensing lens system Plane reflector Reference mirror

Claims (2)

[Claims] (1) Interference fringes obtained by dividing the light beam emitted from the light source into two, making one divided light beam incident on the reference surface and the other divided light beam on the test surface, and making each reflected light interfere. An interferometer that measures the shape of the surface to be measured, a holding part to hold the optical element to be measured, and a tilt angle adjustment part to adjust the tilt of the optical element to be measured, and is movable in the optical axis direction. A measurement table having a supporting part for holding, an alignment control device for controlling the tilt and movement in the optical axis direction, an interference fringe detection means for converting interference fringes into electrical signals, and an interference fringe detection means for converting the electrical signals into digital signals. In an interference measurement device consisting of an A/D converter and an arithmetic unit for inputting and processing the obtained digital signal, the distortion of the optical system of the interferometer itself is measured, and the measurement data is used to measure the distortion of the optical system of the interferometer itself. An interference measurement device characterized by correcting measurement data of an optical element. (2) The above measurement data is an interference pattern obtained by tilting the optical element or reference surface to the optical axis from a state in which the center of curvature of the reference curved surface coincides with the optical axis of the interference measurement device. The interference measurement device according to claim 1, wherein the measurement data is based on data.