JPH05141935A - Measuring method and measuring device for small displacement quantity - Google Patents

Abstract

PURPOSE:To provide the method and device capable of correctly measuring a small displacement quantity in the optical axis direction on a test surface. CONSTITUTION:The interferable light from a light source 1 is radiated to a test surface 7a and a reference surface 6a to form interference fringes 11 in an interference optical system, and the interference fringes 11 are image-formed on an area sensor 10 by a focusing optical system 9. When the distance between the reference surface 7a and the test surface 6a is continuously changed, the interference fringes 11 are moved in the length direction in response to the change. The area sensor 10 observes the moving state at two or more observation points P1, P2 having the phase difference of npi/2 between the interference fringes 11, where (n) is a natural number. When the inversion of contrast, the number of times, and the timing of the inversion at these observation points P1, P2 are detected, a small displacement quantity can be calculated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures a minute displacement amount based on unevenness of a measuring surface or movement of the measuring surface when measuring surface shape and surface accuracy such as a flat surface, a spherical surface, a toroidal surface, and a cylindrical surface. It is related to the technology.

[0002]

2. Description of the Related Art As a technique for forming interference fringes using a coherent light such as a laser beam and measuring the surface shape and surface accuracy of a toroidal surface with high accuracy of wavelength or less, the applicant of the present invention has
In Japanese Patent Application No. 2-126659, FIGS. 16 (a) and 16 (b)
We have proposed the toroidal surface measuring device shown in.

In the figure, reference numeral 1 is a light source, and a gas laser or a semiconductor laser having high coherence is used. 2a,
2b is a beam expander for expanding a narrow light beam from the light source 1 to an appropriate size. 3 is a spatial filter, which cuts off unnecessary light such as ghost light and reflected light. An optical isolator 4 is a beam splitter 4a, λ /
It has four plates 4b and a reflecting surface 4c.

The light beams expanded by the beam expanders 2a and 2b reach the toroidal surface 7a as the surface to be inspected of the subject 7 through the objective lens 6. This toroidal surface 7a
Has main radial lines AB and CD which are orthogonal to each other at its apex. One of these main radial lines CD is a bus line, which is formed by rotating along the other main radial line AB. G is the main diameter line, and the main diameter line AB orthogonal to this is R
We will call it the main diameter line.

The final surface of the objective lens 6 is a reference surface 6a as a semi-transparent mirror, and the center of curvature thereof is arranged at a position substantially coincident with the finished curvature center of the G main diameter line (CD) of the toroidal surface 7a. To be done. Further, the reference surface 6a or the toroidal surface 7a is arranged so as to be slightly shiftable and / or tiltable in the xz section.

Then, a part of the light incident on the objective lens 6 is reflected by the reference surface 6a, and the rest is transmitted to illuminate the toroidal surface 7a and is reflected.

Reference numeral 8 denotes a rotary base for fixing the subject 7, which has a rotary shaft coinciding with the center of curvature of the R main diameter line (AB) of the toroidal surface 7a and is driven by a DC servo motor or stepping motor (not shown). The toroidal surface 7a, which is the surface to be inspected, can be rotationally scanned along the R main radius line.

The coherent light beams reflected by the reference surface 6a and the toroidal surface 7a are returned and superposed on the incoming optical path, and the reference surface 6a.
The spherical surface and the toroidal surface 7a interfere with each other in a slit-shaped measurement portion or measurement cross section parallel to the G main diameter line that can be regarded as substantially parallel, and interference fringes 11 shown in FIG. An image is formed on the area sensor 10 by the focusing lens 9.

When the rotary table 8 is rotated along the R main radius line, the surface shape and surface accuracy of the entire toroidal surface 7a can be measured. In addition, it is possible to measure planes and spheres by making some modifications to the above device.

By the way, on the toroidal surface, the so-called donut type or normal type (hereinafter referred to as "NTS"), in which the busbar (G main diameter line) is short and the R main diameter line orthogonal to this is long, as described above. In addition, there is a barrel type (hereinafter referred to as “BTS”) or a saddle type (hereinafter referred to as “KTS”) having a long main line and a short R main diameter line.

Then, the above measuring method is applied to the BTS type and the K type.
When applied to the TS type toroidal surface, it is necessary to increase the diameter of the reference surface 6a in accordance with the busbar (G main diameter line),
The interference optical system becomes very expensive. So BT
As a technology to measure the toroidal surface of S type and KTS type,
The applicant is the Japanese Patent Application No. 3-050104, which is a prior application.
We have proposed the devices shown in (a) and (b).

Since this is the same as the apparatus shown in FIG. 16, the configuration will be described focusing on the different points. First, the test surface 7a is a BTS toroidal surface formed by rotating an arbitrary arc (including a straight line) around the rotation axis 12. It should be noted that the G and R main diameter lines are reversed from those of the subject 7 in FIG.

The next difference is that the translation table 13 in this example does not rotate, but instead supports the subject 7 and can perform parallel scanning in parallel with the rotation axis 12. And
The translation table 13 is driven by a DC servo motor, a stepping motor or the like (not shown).

FIG. 19 is a diagram showing the interrelationship between the toroidal surface 7a of the BTS, the reference surface 6a and the rotating shaft 12, where O is the intersection of the center of the subject 7 and the rotating shaft 12, and O'is the G main diameter. The center of curvature of the line is shown. By moving the translation table 13,
Although the reference surface 6a moves with respect to the toroidal surface 7a, the moving state is exemplarily shown in three places, that is, upper, lower, and middle. Further, the center of curvature O ″ of the reference surface 6a is always the rotation axis 12
The coherent light which overlaps the upper part and is emitted from the objective lens 6 is also irradiated so as to be focused on this point O ″.

Similar to the above-mentioned example, the coherent light from the light source 1 is reflected by the reference surface 6a and the surface 7a to be inspected, but among the coherent light reflected by the surface 7a to be inspected, interference fringes are present. The light rays involved in the formation of G are the light rays in which the incident light path and the reflected light path overlap, and as shown by the arrows at the three positions in FIG.
It is a ray traveling toward a point O'as the center of curvature of the principal radius line.
This light ray coincides with the optical axis of the objective lens 6 when the optical axis of the reference surface 6a passes through the point O, but the reference surface 6a
Scans from point O to h, this light beam also passes through the objective lens 6
It will be moved by H'from the optical axis of.

The reference wave and the test wave reflected by the reference surface 6a and the test surface 7a are superposed and interfere with each other in the measurement cross section parallel to the R main diameter line as described above, and the focusing lens 9 causes a slit-like shape. The interference fringes are imaged on the area sensor 10. This measurement cross section is scanned in the direction of the rotation axis 12, but since the focus point is always on the rotation axis 12 as shown in FIG. 19, it is possible to always perform scanning in a focused state.

[0017]

However, the above-mentioned BTS
In the case of the measurement, the cross section to be measured is displaced in the optical axis direction with the scanning of the translation table 13. However, when measuring the surface accuracy and the surface shape of the surface to be measured, accurate measurement of the displacement amount in the optical axis direction Was difficult. The present invention aims to solve this problem.
Provided is a method and apparatus capable of accurately measuring a minute displacement amount of a surface to be inspected in the optical axis direction in TS measurement, and also accurately measuring a minute displacement amount of a general surface to be inspected in the optical axis direction. The purpose is to do.

[0018]

In order to achieve the above object, the measuring method of the present invention irradiates coherent light from the same light source on a surface to be inspected and a reference surface which is a reference, and reflects from both surfaces. A step of forming interference fringes by superposing the reference wave and the test wave to be formed, a step of forming the interference fringes on the sensor, and a step of moving the test surface in the optical axis direction to continuously form the interference fringes. Process,
On the sensor, a step of measuring intensity signals at at least two designated observation points at which the phase difference of the interference fringes is approximately nπ / 2 (n is a natural number), and the interference fringes detected at these observation points And a step of calculating the displacement amount of the surface to be inspected in the optical axis direction from the inversion number of the intensity signal and the moving direction of the interference fringes.

Alternatively, a step of forming slit-shaped interference fringes on one measurement cross section of the surface to be inspected and forming an image on the area sensor by a focusing optical system, and scanning the surface to be inspected in a direction intersecting the interference fringes. A step of continuously forming slit-shaped interference fringes on almost the entire surface to be inspected, and on the area sensor,
The process of measuring the intensity of the interference fringes at two or more designated observation points where the phase difference of the interference fringes is approximately nπ / 2 (n is a natural number), and the inversion of the intensity signal of the interference fringes detected at each of these observation points. And a step of measuring the amount of displacement in the optical axis direction of the surface to be inspected due to scanning from the number and the moving direction of the interference fringes.

The interference fringes may be imaged on the photodiode array, and the change in the output of each element of the photodiode array corresponding to the change in the brightness of the interference fringes may be detected in parallel.

Next, the measuring device of the present invention irradiates the coherent light from the same light source onto the surface to be inspected and the reference surface as a reference, and superimposes the reference wave and the wave to be inspected reflected from both surfaces. A device for forming interference fringes, a sensor for forming an image of the interference fringes,
Detects the intensity signals of the interference fringes at at least two or more observation points on the parallel stage that scans the subject with the surface to be inspected and the sensor where the phase difference of the interference fringes is approximately nπ / 2 (n is a natural number). Observation point output measuring means, counting means for counting the number of inversions of the intensity signal at each point, scanning direction for detecting the moving direction of the interference fringes from the signal output of each observation point and determining the scanning direction of the surface to be inspected The present invention is characterized by a configuration including a detection unit and a calculation unit that receives a signal of the number of inversions and a moving direction and calculates a displacement amount of the surface to be inspected in the optical axis direction accompanying the scanning.

Further, the specified line reading means for reading the signals of a specified group of lines in the area sensor, and the line for outputting the interference fringe signal from the output of the specified line reading means are specified to obtain the interference fringe signal. An interference fringe signal extracting means for extracting, and a line designating means for instructing the designated line reading means so that the line for outputting the signal of the interference fringe detected by the interference fringe signal extracting means is located substantially at the center of the group of lines. It is desirable to have a configuration including.

Further, the area sensor is a photodiode array, means for measuring the output of each element of the photodiode array, PV value detecting means for obtaining the element with the largest output change among the outputs of each element, A switch array that is turned on / off by the PV value detecting means and a pulse counter that counts the number of changes in the output of the element may be used.

[0024]

The coherent light is applied to the surface to be inspected and the reference surface to form interference fringes, which are imaged on the area sensor by the focusing optical system. When the surface to be inspected is scanned in the direction intersecting the interference fringes or moved in the optical axis direction to continuously change the distance between the reference surface and the surface to be inspected, the interference fringes are Flow in the length direction. When this state of flow is observed at one point on the area sensor on which the interference fringes are imaged, it can be recognized as repeated light and dark at the observation point. That is, if the output of this observation point is detected, it is possible to easily detect the reversal of light and dark and the number of times. Since the wavelength of the coherent light is known, the absolute value of the displacement amount can be known.

However, if there is only one observation point, the direction of displacement cannot be known. Therefore, another such observation point is set at a position displaced by about half the width of the bright or dark fringes, generally at a position where the phase difference of the interference fringes is approximately nπ / 2 (n is a natural number), By examining the output change timings detected at these two observation points, it is possible to identify the moving direction of the interference fringes, that is, whether the surface to be inspected is closer or farther on the optical axis.

If a photodiode array is provided in place of the area sensor, parallel processing becomes possible and the speed can be further increased.

[0027]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 (a) shows an embodiment of the configuration of the measuring device according to the present invention. Most of the devices shown in FIG.
Since the apparatus is common to the apparatus described in 1), only the differences will be described.

Reference numeral 14 indicates an observation point output measuring means,
This is a means for detecting the intensity signal of the interference fringe from the area sensor for two or more designated observation points P ₁ and P ₂ on the interference fringe imaged on the area sensor shown in FIG. 1 (b). Is. The distance between the observation points P ₁ and P ₂ is a distance shifted by about half the width of the bright or dark fringes, generally a multiple of about 90 ° in the phase difference of the interference fringes, or nπ / 2 (n is Natural number)
It is only a distance away. Reference numeral 15 is a counting means for counting the number of times the signal output is inverted at any one or more of the observation points, that is, the number of times the interference fringes are moved in either the a direction or the b direction and the brightness is inverted. Is.

Reference numeral 16 is a scanning direction detecting means, which detects the moving direction of the interference fringes, that is, whether the surface to be inspected is closer or farther from the optical axis from the timing of the inversion of the signal output at the plurality of observation points. Is a means to do. Code 17
Is a calculation means, in which the wavelength of the coherent light is input in advance, to which the number of reversals from the counting means 15 and the movement direction signal from the scanning direction detecting means 16 are added, It is a means for calculating in what direction and how much 7a is displaced on the optical axis.

The distance scanned by the translating table 13 is input to the computing means 17, and the shape of the surface to be inspected can be obtained by obtaining the displacement amount corresponding to the scanning distance. Further, each of the means 14 to 17 described above is
It is executed by a computer and its software.

The measurement of the surface shape according to the present invention when the surface 7a to be measured is a flat surface will be described with reference to FIGS. 1 (b) and 2. The reference surface is also a plane, and the interferometer may be a well-known Fizeau type (Michelson type) having almost the same configuration as that in FIG. 1. However, the Fizeau type has a higher common path ratio than the Michelson type, and thus is stable. It is possible to perform the measurement. The interference fringes 11 are the area sensor 1
An image is formed on 0 as shown in FIG. 1 (b). After that, when the surface 7a to be inspected is moved in the optical axis direction, the arrow symbol a
Or, the interference fringes appear to move in the direction of b.

Here, paying attention to the observation point P ₁ shown in the figure,
Interference intensity signal I ₁ of the P ₁ point in accordance with the movement has changed temporally stripe, so that the dark line, the intensity signal I ₁ each time the changes from dark to bright lines inverted from bright line. Then, if the intensity signal I of the interference fringes is plotted on the vertical axis and the time t is plotted on the horizontal axis,
The graph shows the change of the intensity signal I ₁ at the observation point P ₁ with the passage of time t. Then, when the interference fringes 11 move in the a direction, it becomes as shown in FIG. 2 (a).
As can be seen from this graph, the intensity signal I ₁ draws a sine curve having one period (2π) from one bright line to the next bright line. Therefore, by counting the number of times the output is inverted, the wavelength λ of the coherent light is known in advance, so that the amount of displacement of the surface to be inspected in the optical axis direction can be calculated.

However, if there is only one observation point, it is not known whether the movement direction of the interference fringes is the a direction or the b direction. Therefore, since the moving direction of the subject is not known, accurate measurement cannot be performed when the traveling direction reverses midway.

[0034] Therefore, only half the width of the light or dark fringe, i.e., likewise intensity signal and P ₁ point of P ₂ points shifted by [pi / 2 in phase of the interference fringe from the observation point P ₁ as observation points When I ₂ is measured and a graph is drawn, it becomes as shown in Fig. 2 (b). if,
If the moving direction of the surface to be inspected is opposite, the interference fringes move in the direction of arrow mark b, so the graph of the intensity signal I ₂ is the reverse of that of (b) as shown in FIG. The direction can be detected.

Returning to the apparatus of FIG. 1, if the observation points P ₁ and P ₂ are determined and input to the observation point output measuring means 14, the means 14
Generates an output signal shown in FIGS. 2 (a) to 2 (c) for each observation point and inputs it to the counting means 15 and the scanning direction detecting means 16. The counting means 15 counts the number of inversions for one or more observation points.

The scanning direction detecting means 16 is as shown in FIG.
, The vertical axis is the observation point P₂Intensity signal I ₂Take and view on the horizontal axis
Station P₁Intensity signal I₁The strength of both observation points on the plane
Point P (I₁, I₂) On the display
Start. In the Lissajous waveform obtained in this way,
If the movement of the interference fringes is in the a direction, point P is clockwise.
If there is, it will be counterclockwise in the b direction, so the surface to be inspected
It is possible to easily determine the moving direction of the.

In FIG. 3, the displacement amount of the surface to be measured can be measured by calculating the number of rotations of the point P in either direction. Therefore, the moving direction means 16 counts the number of rotations of the point P. It may be configured to add a function.

The reversal number signal from the counting means 15 and the movement direction signal from the scanning direction means 16 are both inputted to the calculating means 17, where they are combined with the wavelength λ of the coherent light inputted in advance. The amount of movement in which direction is calculated as the displacement amount. In this case, the bright point P is not always
It is not necessary to display on the display as, and the position of the point P may be electrically detected by the synthesizing means, and the number of rotations may be counted by the pulse counter.

In the above measurement, the observation points are P ₁ , P ₂
, And the interval was π / 2, but the observation point was 0,
By increasing the number of π / 2, π, 3π / 2 ..., the SN ratio can be improved, which is a powerful method. Also, it will be easily understood that the intervals between the respective observation points do not have to be exactly π / 2 intervals, and may be approximately.

If the surface to be inspected is the NTS shown in FIG. 16, the interference fringes 11 are formed in the shape of slits for one measurement section, and the rotary table 8 is rotated to cause the object 7 to cross the interference fringes. The difference is that the scanning is performed in the same manner, but the displacement amount only indicates the surface shape of the surface to be measured, and the others are the same as in the case of a flat surface. However, in that case, the shape of the surface to be inspected is determined by inputting the scanning amount obtained from the rotation angle of the rotary table 8 into the calculating means 17 and associating the scanning distance with the displacement amount.

If the surface to be inspected is the BTS shown in FIG.
As described above, the interfering light is not necessarily the objective lens 6
4 does not coincide with the optical axis of FIG. 4, and the slit 7 is formed by scanning the subject 7 in parallel with the rotation axis 12.
As shown in, the area sensor 10 is moved left and right. Therefore, the observation point cannot be fixed as shown in Fig. 1 (b). Therefore, every time one interference fringe is imaged, the signal of the entire area sensor at that time is temporarily stored,
After that, the observation points P ₁ and P ₂ are determined and the data are extracted.

However, this method is very time consuming and
The moving speed that can be measured is limited. For example,
When using CCD image sensor of video camera, 1
The time to output the frame is 1/30 second. on the other hand,
It is necessary to output at least three frames for determining the movement of interference fringes. If it is a He-Ne laser light source, λ = 0.63
At 3 μm, λ / 2 corresponds to one fringe, so the maximum velocity of the subject is (λ / 2) / {(1/30) × 3} ≈3.2 μm / sec, It is difficult to measure when the subject moves more than that.

On the other hand, it is conceivable to use a line sensor instead of the area sensor, but in that case, the line sensor must be moved in accordance with the movement of the interference fringes, which also complicates the configuration.

FIG. 4 is an embodiment showing one solution to this problem. That is, in this embodiment, the two line sensors 18, 18 are provided so as to be orthogonal to the interference fringe 11. The distance between these line sensors 18 is
The phase of the interference fringes is approximately nπ / 2. Of course, the number is not limited to two, and a large number may be provided. According to this configuration, the observation points P ₁ and P ₂ always form an image on each line sensor, so that it suffices to monitor the outputs of these line sensors, and the detection speed can be increased. .. Although both the line sensor 18 and the area sensor 10 are used in this embodiment, the area sensor 10
Alternatively, the line sensor 18 alone may be provided on the image plane.

FIG. 5 also shows another solution. In this example, vertical focusing cylindrical lenses L1, L2 and L3, L4 are used as the anamorphic optical system for the focusing lens 9 for forming the interference fringes 11. According to this configuration, the image of the interference fringes is formed on the area sensor as an image of the focal line of the optical system regardless of the movement of the interference fringes due to the scanning. Therefore, the observation points P ₁ and P ₂ can be set to fixed points on the area sensor. In this case, it is possible to detect even if a line sensor is placed instead of the area sensor.

FIG. 6 shows an example using a plurality of slits. Reference numeral 19 indicates a slit plate placed at the image forming position of the interference fringes, in which slits 19a and 19b at intervals of nπ / 2 in the phase of the interference fringes are perpendicular to the movement direction of the interference fringes, that is, the paper surface. Are formed in the same direction (at the same position as the linear sensor 18 shown in FIG. 4). The image of the interference fringe is formed in a direction orthogonal to the slits 19a and 19b, and only the portion corresponding to the observation point passes through the slit and reaches the wedge lens 20. After the slit interval is enlarged by the wedge-shaped lens 20, the image of each slit is formed into a substantially point image by the condenser lens 21, and the two-segment photo is provided at the image forming position corresponding to the image of each slit. The signal strengths I ₁ and I ₂ at the observation points are incident on the respective elements of the diode 22.
Will be detected. When the slit spacing is wide, detection is possible without the wedge lens 21.

According to this structure, even if the interference fringes move due to the scanning of the surface to be inspected, the observation points are always located at the slits 19a, 19a.
Since the image is formed somewhere on b and is formed at a substantially constant position by the condenser lens 21, the intensity signals I ₁ and I ₂ at the observation point can be taken out by the photodiode 22. Further, since the photodiode is used for detection, high-speed detection is possible, and sufficient tracking can be performed even if the moving speed of the interference fringes is high.

FIG. 7 is a diagram showing an outline of a method in which an area sensor is arranged on the image forming plane and processing can be performed at high speed. The area sensor usually consists of multiple lines, but as shown in Fig. 7 (a), the output of only a group of lines (5 lines in the figure) centered on the interference fringes is measured and explained with reference to Figs. A small displacement amount of the surface to be inspected is calculated by the method. At this time, if the extracted line is shifted to the left as shown in Fig. 7 (b), it means that the interference fringes are moved to the left, so the output area is also shown in Fig. 7 (c). Shift to the left like this.

FIG. 8 is a block diagram of an apparatus for carrying out the method, which apparatus and its operation are actually carried out automatically by the computer and its software. Explaining each component, the reference numeral 23 indicates a designated line reading means, and this means reads a signal of a designated group of lines among the respective lines constituting the area sensor. Reference numeral 24 denotes an interference fringe signal extraction means, which determines which of a group of lines the signal of the interference fringes forms. When the line on which the signal of the interference fringes is placed is determined, the observation point output measuring means 14 extracts the signal intensities I ₁ and I ₂ at the observation points P ₁ and P ₂ shown in FIG. To do.

FIG. 9 is a flowchart of the apparatus shown in FIG. When the start instruction is given, the counter is first set to 0 to be in the initial state (# 1). Next, the line designation means 2
The initial value of the output area to be designated by the area sensor is set to 5 (# 2). The subject is moved (# 3) and the interference fringes are imaged on the area sensor (# 4). The designated line reading means 23 reads the signals of the designated group of lines and transfers them to the interference fringe signal extracting means 24 as an image memory (#
5). In the interference fringe signal extraction means 24, among the outputs of each line, the interference fringes are imaged on the line with the highest contrast, so the signal of that line is extracted and sent to the observation point output measurement means 14 (# 6). .. In the observation point output measuring means 14, the signal intensities for the observation points P ₁ and P ₂ designated in advance are taken out (# 7), and the number of inversions is counted by the counting means 15 and temporarily stored (# 8). .. next,
It is confirmed by the line designating means 25 whether or not the extraction line on which the interference fringes are present is in the center of the designated group of lines (# 9), and if it is not in the center, the output area of the area sensor is changed (# 10) When it comes to the center, it is checked whether or not the measurement is completed (# 11). If it is not completed, the number of reversals stored in the counting means 15 is output (#).
12), the displacement amount is calculated by the calculation unit 17 together with the output of the moving direction detection unit 16 (# 13). Hereinafter, # 3 to # 11 are repeatedly measured until the scan of the subject is completed.

FIGS. 10A and 10B show still another embodiment,
It shows a configuration of an apparatus capable of measuring at higher speed. The interferometer is almost the same as in FIG. Most of the devices shown in the figure are common to the above-mentioned devices, and therefore the differences will be described.

Reference numeral 34 is a beam splitter, which splits the interference light (in which the reference wave from the reference surface 6a and the test wave from the test surface 7a are superimposed) indicated by a dotted line into two optical paths.
The coherent light traveling to one of the two is detected by the area sensor 1
The interference fringe image 11 is imaged on 0, and the other coherent light is imaged on the photodiode array 37 through the cylindrical lens as the magnifying optical system 35 and the wedge prism as the optical path splitting means 36. To do.

FIG. 11 shows the magnifying optical system 35 shown in FIG.
And the beam splitter 1 without using the optical path splitting means 36.
4 shows the relationship between the interference fringe image 11 and the photodiode array 37 when the image is directly formed on the photodiode array 37 from 4. As shown in the figure, the photodiode array 37 is arranged in a direction substantially orthogonal to the interference fringe image 11.

In this state, if the surface 7a to be measured is continuously moved in the b direction by the translation table 13, the measurement cross section (FIG.
In (a), the intersection of the dotted line and the surface to be inspected 7a) is gradually separated from the optical axis.
1 moves in the y-axis direction. At the same time, the distance on the optical axis between the measurement cross section of the surface to be inspected 7a and the reference surface 6a changes, so the bright and dark fringes of the interference fringe image 11 flow in the x direction. Therefore, by counting the number of bright and dark fringes of the interference fringe image 11 crossing over the photodiode array 17, it is possible to measure the minute displacement on the optical axis of the surface 7a to be measured.

FIG. 12 is a diagram for explaining this in more detail. In the figure, the elements forming the photodiode array 37 are represented as P1, P2 ... Pi ... Pn. Reference numeral 38 denotes a current / voltage conversion means, and a plurality of current / voltage converters I / V1 ... I / Vn are arranged so as to correspond to the respective elements 1: 1. Reference numeral 39 denotes a high-pass filter means, in which a plurality of high-pass filters HF1 ... HFn are arranged so as to correspond to the respective current / voltage converters I / V1 ... I / Vn at a ratio of 1: 1. Reference numeral 40 denotes a PV value detecting means for detecting an element having a sine wave-shaped intensity amplitude maximum. 41 is a switch array arranged so as to correspond to each high-pass filter HF1 ...
42 is a pulse counter.

The interference fringe image 11 is assumed to be located on the i-th element of the photodiode array 17 shown in FIG. The elements P1, P2 ... Pi ... Pn generate currents according to the light intensity. Therefore, each element P1, P2 ... P
i ... Current from the Pn is converted into a current / voltage converter I / V1 ...
... I / Vi ... Converted to a voltage through I / Vn. The voltage signal that has passed through these is passed through the high-pass filter HF1 ...
By passing HFi ... HFn, only the elements before and after the HFi in which the interference fringe image 11 exists, that is, the element having a large intensity change with time, are output. High-pass filter HF1 ...
.. HFi ... The signal after passing through HFn is divided into two systems, one is input to the PV value detecting means 40 and the other is input to the switch array 41.

The PV value detecting means 40 is, by a known means, the output of the high-pass filter HFi in which the interference fringe image 11 exists and before and after the output, for example, as shown in FIG.
The outputs of i−1 and HFi + 1 are compared, and the switch array 41 is instructed to close the switch only for HFi having the maximum amplitude. Therefore, the pulse counter 22 has the element with the largest amplitude, that is, the interference fringe image 1
It is possible to receive a signal only from the element in which 1 is present.

After that, the number of stripes counted by the pulse counter 42 is input to the arithmetic means 43 composed of a CPU of a computer and the like, and a small displacement of the surface 7a to be inspected from the wavelength λ of the coherent light given in advance. The amount will be calculated.
With this method, even if the interference fringe image 11 moves in the y-axis direction, the position of the interference fringe image can be detected by following the movement and the number of fringes flowing in the x-axis direction can be counted.

In the above structure, it is important that the measurement for each element of the photodiode array 37 be processed in parallel. That is, if a CCD line sensor is used instead of the photodiode array, the CCD has a configuration in which the signals of the respective elements constituting the line sensor are sequentially read from one end to the other end, so that it is possible to perform one measurement. , Processing time required for one element ×
It takes as long as the number of elements. On the other hand, with the photodiode array, parallel processing can be performed, and one measurement can be performed in the processing time required for one element.

Furthermore, this method has an excellent feature that it is hardly affected by ghost light, ambient light, and the like. This is because these ghost light and ambient light do not change much over time. Therefore, when passing through the high-pass filter, these lights can be cut.

By the way, with the above configuration, even if the absolute value of the minute displacement amount of the surface 7a to be inspected is known, it is not possible to know whether it is approaching or far from the optical axis.

FIG. 14 solves this problem, and two photodiode arrays 37a and 37b are arranged in parallel instead of the line sensor 18 in FIG. Of course, the mutual interval is 9 due to the phase difference of the interference fringes.
They are arranged so that they are offset by 0 °. Although not shown, these two photodiode arrays 37a, 3a
7b for each of the current / voltage converters 3 of FIG.
8, a high pass filter 39, a PV value detecting means 40 and a switch array 41 are provided. The pulse counter 42
Instead of, a combining means (not shown) to which outputs from both switch arrays are input is provided. In this combining means, the output from the switch array corresponding to the photodiode array 37a is the A phase and the output from the switch array corresponding to the 37b is the B phase, and both outputs are combined and displayed on the display as in FIG. It can be observed that point P rotates in a Lissajous waveform.

Then, in FIG. 3, the displacement amount and direction of the surface to be inspected can be measured by calculating the number of rotations in which direction. In the above measurement, if the number of photodiode arrays is increased to 0, π / 2, π, 3π / 2 ...

If the surface to be inspected is the NTS shown in FIG. 16, there is a difference in that the rotary table 8 is rotated to scan the object 7, but the displacement amount changes the surface shape of the surface to be measured. It is only shown and is basically the same as the case of BTS.
However, in that case, the shape of the surface to be inspected is determined by inputting the scanning amount obtained from the rotation angle of the rotary table 8 into the calculating means 23 and associating the scanning distance with the displacement amount. It will be easily understood that the same measurement can be performed even when the surface to be inspected is a flat surface or a spherical surface.

In the embodiment shown in FIG. 14, it is necessary to previously adjust the distance between the photodiode arrays 37a and 37b in the x direction. The problem is that the photodiode array itself has some width in the x direction, and if the pitch of the interference fringes is small, it is difficult to bring the two photodiode arrays close to each other. As a solution to this problem, a method of increasing n of nπ / 2 can be considered, but in that case, other factors (surface waviness, etc.) of the surface to be inspected may be included. It is desirable to take out two adjacent points near the center.

Further, the photodiode arrays 37, 37
Unless the size of the elements a and 37b is sufficiently smaller than the width of one stripe, accurate measurement cannot be performed. for that reason,
In some cases, the interference fringe image must be enlarged to increase the fringe pitch.

FIGS. 15 (a) and 15 (b) show the structure of the main part of the embodiment proposed to achieve these objects. That is, this is an example in which a cylindrical lens is used as the magnifying optical system 35 described in FIG. 10 and a wedge prism is used as the optical path splitting means 36. Magnifying optical system 3
Although a concave lens having an ordinary spherical surface may be used as the lens 5, a cylindrical concave lens is used in the illustrated embodiment. This makes it possible to enlarge only in the x-axis direction, and it is possible to suppress a decrease in brightness due to image enlargement.

The wedge-shaped prism as the optical path splitting means 35 has a cross-sectional shape such that the wedges face each other in the vertical direction as shown in FIG. 15 (a), and the central portion is thin and both sides are thick. .. This is for dividing the interference fringe image and forming an image on the photodiode arrays 37a and 37b without changing the magnification of the interference fringe image 11. Figure 15
As shown in (b), the interference fringe image 11 is located at the top and bottom of the optical axis.
The distance between the two points Pa and Pb, which are divided like a and 11b and are 90 ° out of phase with each other, is greatly expanded by using the wedge prism, so that the photodiode arrays 37a and 37b can be easily arranged. Become.

Instead of the wedge prism 35,
It is also possible to use various optical path splitting means such as the beam splitter 14. Depending on the measurement conditions, either one of the magnifying optical system and the optical path dividing means may be used, or both may be used.

[0070]

As described above, according to claims 1 and 2,
According to the invention, it is possible to accurately measure a minute displacement amount of the surface to be inspected in the optical axis direction. In addition, claims 3, 5, 6,
According to the invention of 7, even if the surface to be inspected moves by scanning,
Accurate and fast measurement is possible without being affected by the measurement of the observation points on the interference fringes.

Furthermore, according to the invention of claim 4, the surface accuracy can be measured by the area sensor and the surface shape can be measured by the displacement amount measurement in the optical axis direction. According to the invention of claim 8,
In particular, it has a special effect that the amount of displacement in the optical axis direction can be automatically measured at high speed. In addition, claim 9 or 1
According to the first aspect of the invention, it is possible to accurately measure a minute amount of displacement of the surface to be inspected in the optical axis direction. In addition, claim 10 or 1
According to the second aspect, it is possible to accurately measure the minute displacement amount of the surface to be inspected and its direction. Further, according to the inventions of claims 13 to 16, even when the pitch of the interference fringes is small, it is possible to accurately measure the minute displacement amount of the surface to be inspected and its direction.

[Brief description of drawings]

FIG. 1A is a diagram showing a configuration of a measuring apparatus for a minute displacement amount according to the present invention, and FIG. 1B is a diagram showing an area sensor, an interference fringe, and an observation point.

2 (a) to (c) are graphs showing signal intensities I ₁ and I ₂ at observation points P ₁ and P ₂ of interference fringes.

FIG. 3 is a diagram illustrating a method of detecting a moving direction of a subject.

FIG. 4 is a diagram showing a configuration in which a line sensor detects movement of observation points P ₁ and P ₂ along with movement of interference fringes.

5A and 5B are diagrams in which a cylindrical lens is used in an optical system for forming an interference fringe, in which (a) is an xz plane view and (b) is a yz plane view.

FIG. 6 is a diagram showing a configuration of an apparatus for forming an image of an observation point on a two-divided photodiode by providing a slit on an image plane of interference fringes.

7 (a) to 7 (c) are diagrams for explaining a method of reading a designated group of lines on an area sensor and placing an interference fringe image at the center of the group.

8 is a block diagram showing a configuration of an apparatus for performing the method of FIG.

9 is a flow chart implementing the method of FIG.

10A and 10B are diagrams showing a configuration of a measuring apparatus for a minute displacement amount according to the present invention, in which FIG. 10A is a yz plane view and FIG. 10B is an xz plane view.

FIG. 11 is a diagram showing interference fringes formed on a photodiode array.

12 is a block diagram of the embodiment shown in FIG.

FIG. 13 is a graph showing output signals of HFi and elements on both sides thereof after passing through a high-pass filter.

FIG. 14 shows a photodiode array with a phase of interference fringes of 9
It is a figure which shows the state arrange | positioned 0 degree apart.

FIG. 15A is a diagram in which a cylindrical lens as a magnifying optical system and a wedge prism as an optical path dividing unit are used in an optical system for forming an interference fringe, and FIG. It is a figure which shows the state which was divided.

16 is a diagram showing an apparatus for measuring a toroidal surface of NTS in the example of the prior application, (a) is a yz plane view, and (b) is an xz plane view. FIG.

FIG. 17 is a diagram showing interference fringes formed on an area sensor.

FIG. 18 is a diagram showing a toroidal surface measuring device of a BTS in the example of the prior application, (a) is a yz-plane view, and (b) is an xz-plane view.

FIG. 19 is a diagram illustrating movement of interference fringes on a reference surface.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 light source 6 reference lens 6a reference surface 7 subject 7a subject surface 8 rotary table 9 focusing optical system 12 rotary shaft 13 translation table 14 observation point output measuring means 15 counting means 16 scanning direction detecting means 17 computing means 18 line sensor 19a, 19b Slit 21 Condenser lens 22 Photodiode 23 Designated line reading means 24 Interference fringe signal extraction means 25 Line designating means 35 Enlarging optical system (cylindrical lens) 36 Optical path splitting means (wedge prism) 37, 37a, 37b Photodiode array 38 Current / voltage conversion means 39 High-pass filter 40 PV value detection means 41 Switch array 42 Pulse counter 43 Arithmetic means P ₁ , P ₂ Observation points

Claims (16)

[Claims] 1. A step of irradiating a coherent light beam from the same light source on a test surface and a reference surface as a reference, and superimposing a reference wave and a test wave reflected from these both surfaces to form an interference fringe, A step of forming the interference fringes on the sensor; a step of moving the surface to be inspected in the optical axis direction to continuously form the interference fringes; and a phase difference of the interference fringes of approximately nπ / 2 on the sensor. (N is a natural number) A step of measuring the intensity signals at at least two designated observation points separated from each other, and the inversion number of the intensity signals of the interference fringes detected at these observation points and the moving direction of the interference fringes. And a step of calculating the displacement amount of the surface in the optical axis direction. 2. A step of irradiating a coherent light beam from the same light source on a test surface and a reference surface as a reference, and superimposing the reference wave and the test wave reflected from these both surfaces to form an interference fringe, The step of forming slit-shaped interference fringes on one measurement cross section of the surface to be inspected and forming an image on the area sensor by the focusing optical system, and scanning the surface to be inspected in the direction crossing the interference fringes A step of continuously forming slit-shaped interference fringes over the entire area, and a phase difference of the interference fringes of approximately nπ / 2 on the area sensor.
(N is a natural number) A step of measuring the intensity of the interference fringes at two or more designated observation points separated from each other, and scanning from the inversion number of the intensity signal of the interference fringes detected at each of these observation points and the moving direction of the interference fringes. A method of measuring a small amount of displacement, comprising a step of measuring the amount of displacement of the surface to be inspected in the optical axis direction. 3. In the step of forming an image of the interference fringes on a sensor or an area sensor, an anamorphic optical system is used as an image forming optical system to form an image of the interference fringes as a focal line of the optical system, The method for measuring a small amount of displacement according to claim 1 or 2, wherein an image is formed on a line sensor provided instead of the sensor or the area sensor. 4. A device for irradiating a coherent light beam from the same light source on a test surface and a reference surface as a reference, and superimposing a reference wave and a test wave reflected from these both surfaces to form an interference fringe, At least two points on the sensor where the image of the interference fringes is formed, the translation table that scans the subject with the surface to be inspected, and the phase difference of the interference fringes is approximately nπ / 2 (n is a natural number). Observation point output measuring means for detecting the intensity signal of the interference fringe at each observation point, counting means for counting the number of inversions of the intensity signal at each point, and detecting the moving direction of the interference fringe from the signal output at each observation point. A scanning direction detecting means for determining the scanning direction of the surface to be inspected, and an arithmetic means for receiving a signal of the inversion number and the moving direction to calculate a displacement amount of the surface to be inspected in the optical axis direction accompanying the scanning. A characteristic measuring device for minute displacements. 5. A plurality of slits extending in the scanning direction are provided in the vicinity of the image plane of interference fringes instead of the sensor,
The spacing between each slit is approximately nπ / 2 due to the phase difference of the interference fringes.
(N is a natural number), and a condensing lens for forming an image of each slit as a substantially point image and a photodiode provided at the image forming position corresponding to each slit are provided. The device for measuring a small amount of displacement according to claim 4. 6. An apparatus for measuring a small displacement amount according to claim 5, wherein an optical element for widening an image interval of the slits is inserted between the plurality of slits and the condenser lens. 7. A plurality of linear sensors, which are substantially parallel to the scanning direction, are used instead of the sensor, and the phase difference of interference fringes is approximately nπ /.
The measuring device for minute displacement according to claim 4, wherein the measuring devices are provided in parallel with each other at a distance of 2 (n is a natural number). 8. An apparatus for irradiating a coherent light beam from the same light source on a test surface and a reference surface as a reference, and superposing a reference wave and a test wave reflected from these both surfaces to form an interference fringe, A focusing optical system that forms an image of the interference fringes, an area sensor that is provided at the image formation position of the interference fringes and that has a plurality of reading lines that are parallel to the interference fringes, and a translation table that scans the subject with the surface to be examined. A specified line reading means for reading a signal for a specified group of lines in the area sensor, and an interference fringe for extracting a signal of the interference fringe by specifying a line for outputting a signal of the interference fringe from the output of the specified line reading means The signal extraction means and each intensity signal at two or more observation points, which are designated on the line for outputting the signal of the interference fringes and have a phase difference of the interference fringes that are substantially nπ / 2 (n is a natural number) apart, are used as the interference fringe signal. Observation point output measurement obtained from extraction means A step, counting means for counting the number of inversions of the intensity signal at each observation point, and scanning direction detection means for detecting the moving direction of the interference fringes from the signal output at each observation point to determine the scanning direction of the surface to be inspected. The calculation means for calculating the displacement amount in the optical axis direction of the surface to be inspected due to the scanning by receiving the signal of the inversion number and the moving direction, and the line for outputting the signal of the interference fringes detected by the interference fringe signal extracting means are A minute displacement amount measuring device comprising: a designated line reading unit that instructs a designated line reading unit so that the line is located substantially at the center of the group of lines. 9. A step of irradiating coherent light from the same light source on a surface to be inspected and a reference surface as a reference, and superposing the reference wave and the wave to be inspected reflected from both surfaces to form interference fringes. Forming an image of the interference fringes on a photodiode array, continuously forming the interference fringes by moving a surface to be inspected, and each element of the photodiode array corresponding to a change in brightness of the interference fringes Is detected in parallel and the output of the element having the largest change is taken out, and the number of changes in the output of the element is counted. 10. A step of irradiating a coherent light beam from the same light source onto a surface to be inspected and a reference surface as a reference, and superimposing a reference wave and a wave to be detected reflected from both surfaces to form interference fringes. , A step of forming an image of the interference fringes on at least two or more photodiode arrays arranged in parallel with each other with a phase difference of the interference fringes being substantially nπ / 2 (n is a natural number); The step of moving to continuously form the interference fringes and the output change of each element of each photodiode array corresponding to the change of the brightness of the interference fringes are detected in parallel, and the maximum output change in each photodiode array is detected. The process of extracting the output of each element, and the intensity signal of the interference fringes detected by each of these elements are combined to create a Lissajous waveform, and the displacement in the optical axis direction of the surface to be inspected based on the rotation direction and rotation speed of the Lissajous waveform. Calculate the amount A method for measuring a small amount of displacement, which comprises: 11. A device for irradiating a coherent light beam from the same light source on a test surface and a reference surface as a reference, and superimposing the reference wave and the test wave reflected from both surfaces to form an interference fringe. , The photodiode array provided at the image forming position of the interference fringe image, the translation table for scanning the surface to be inspected, the means for measuring the output of each element of the photodiode array, and the output of each element A minute displacement amount comprising a PV value detecting means for obtaining an element having a large output change, a switch array turned on / off by the PV value detecting means, and a pulse counter for counting the number of times of the output change of the element. Measuring device. 12. Coherent light from the same light source is applied to a test surface and a reference surface as a reference, and the reference wave and the test wave reflected from both surfaces are superposed to form slit-shaped interference fringes. Device for making, at least two or more photodiode arrays arranged in parallel at a phase difference of interference fringes of approximately nπ / 2 (n is a natural number), a parallel stage for scanning the surface to be inspected, and each photodiode array Means for measuring the output of each element, a PV value detecting means for obtaining the element having the largest output change in each photodiode array, a switch array turned on / off by the PV value detecting means, and each of these. A minute displacement characterized by comprising means for synthesizing a Lissajous waveform from changes in the output of the element, and arithmetic means for calculating the displacement amount in the optical axis direction of the surface to be measured from the rotation direction and rotation speed of the Lissajous waveform. Quantity measuring device. 13. An apparatus for measuring a small amount of displacement according to claim 12, wherein an optical path splitting means is provided in an optical path before forming an image on the photodiode array. 14. The device for measuring a small amount of displacement according to claim 12, wherein the optical path splitting means is a wedge prism. 15. The measurement of a minute displacement amount according to claim 12, wherein a magnifying optical system for magnifying an interference fringe image is inserted in an optical path before the image is formed on the line sensor. apparatus. 16. An apparatus for measuring a small amount of displacement according to claim 15, wherein the magnifying optical system for the interference fringe image is a cylindrical lens.