JPH046882B2 - - Google Patents

Description

[Detailed description of the invention] The present invention divides the light emitted from the light source into two,
The present invention relates to a surface shape measuring method that utilizes interference fringes when one of the optical path lengths is changed, and an apparatus for carrying out the method.

Conventional interference measurement devices, both Fizou type and Twyman type, simply use information about the shape of interference fringes or changes in interference pattern intensity when changing the optical path of the reference light or the measured object. The shape of the object to be measured was measured. This type of measurement method has the disadvantage that if there is a discontinuous part in the interference fringes (interference pattern), the relative surface positional relationship of multiple parts of the measured object separated by the discontinuous part cannot be determined. It was hot. That is, FIG. 1a shows the interference fringes 21, 22, 23 obtained by the conventional method for one surface, and FIG. It represents interference fringes 21', 22', 23', and 24' obtained by the same method for two surfaces, and the continuous curve of the interference fringes is an integral multiple of the wavelength λ (2π phase difference), that is, the step difference on the surface. In this case, there is an uncertain number that is an integer multiple of 1/2 of the wavelength λ, and it is impossible to know which fringe in Figure 1a, for example, the interference fringe 22, corresponds to which of Figure 1b, It was impossible to perform precise measurements over the entire surface of the object to be measured.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a measurement method that enables precise measurement of a surface shape even when discontinuous portions occur in interference fringes.

A second object of the invention is to provide an apparatus that makes it possible to carry out such a surface profile measurement method.

In order to achieve the above object, the surface profile measurement method of the type mentioned at the beginning according to the present invention selectively guides light emitted from a plurality of coherent light sources with different wavelengths into the same optical path, and uses a beam splitter to Interference where there is a discontinuous part is achieved by separating the beams, using one as a reference beam, and irradiating the other beam onto the object to be measured, and guiding the reflected or transmitted beams to the same optical path again by the beam splitter or a second beam splitter. The stripes are imaged for each wavelength, the information is stored, and the reference light and the other light (object light) are irradiated onto the object to be measured using light of one wavelength among the plurality of lights with different wavelengths. Measures the shape of multiple surfaces of the object to be measured separated by discontinuous parts of interference fringes that occur on the object from changes in interference fringes when the optical path length of one of the lights is changed. On the other hand, by calculating the relationship between discontinuous interference fringes from the above-mentioned stored information, it is possible to calculate the desired surface to be measured of the object to be measured, even for objects to be measured in which the interference fringes have discontinuous parts. The gist is to measure the surface shape over the entire surface.

In order to achieve the above second object, the surface profile measuring device according to the present invention includes a plurality of coherent light sources with different wavelengths, and a beam diameter adjustment for making the light emitted from the light sources a predetermined beam diameter. an optical system; an optical means for selectively guiding light emitted from the plurality of coherent light sources to the same optical path; splitting the light into two by a beam splitter and guiding one to a reference optical path;
The other beam is irradiated onto the object to be measured, and the light that has been reflected or transmitted through the object is guided to the same optical path again by the beam splitter or the second beam splitter, and the interference fringes generated are separated for each wavelength of light. means for capturing information on interference fringes obtained by the imaging means; means for changing either the reference optical path length or the optical path length of the object to be measured; and interference fringes produced by the change in the optical path length. means for storing changes in the interference fringes, and calculating the detailed shape of the surface of the object to be measured from the changes in the interference fringes, and calculating the detailed shape of the surface of the object to be measured from which discontinuous portions of the interference fringes occur based on the information of the separate interference fringes caused by the plurality of wavelengths of light. The present invention also includes means for calculating the surface shape of an object over the entire desired surface to be measured.

That is, in the present invention, two wavelengths different from each other are used.
Light emitted from multiple coherent light sources is used.
If the optical paths of these two or more lights are the same, the obtained interference fringes will be
As shown in b, for wavelength λ ₁ , as shown by the solid line, 21, 22...25; 20', 21',...2
4' is obtained, and for wavelength λ ₂ , as shown by the dotted line, 201, 202...204; 200', 20
1'...204' are obtained. As seen in FIG. 3, the fringes obtained for both wavelengths are nearly parallel, and the pitch of the fringes differs depending on the wavelength. If the fringes for two wavelengths are close to each other, the distance between the fringes is Δ(Measuring the λ ₂ fringes from the λ ₁ fringes +
j direction is positive), and the fringe pitch for light of λ ₁ is p. Similarly, we find Δ' and p' for the discontinuous Figure 3b, and find that interference fringes where Δ/p and Δ'/p' are almost equal are considered to be mutually continuous. I can judge. In other words, in the interference fringes shown in FIG. 1 using light of one wavelength, the first
It is unclear whether the stripe in Figure b is 22', 23' or 24', but in Figure 3 it can be seen that the stripe in b that is continuous with 21 in A is 21'. When the difference in level between the surfaces a and b is large, yet another wavelength λ ₃ may be used. As described above, an outline of the relative relationship between surfaces a and b can be grasped, but this method is already known as a method for measuring the distance between surfaces using a block gauge. In the present invention, it is possible to accurately measure the deviation of the surfaces a and b from the virtual plane (or virtual quadric surface) based on the positional relationship between a and b obtained in this way. It's tight. That is, as shown in Fig. 2, by using only one wavelength and slightly moving the wedge glass 6 in the reference optical path, the optical path length (phase) of the reference beam is changed, and the interference fringe intensity changes accordingly. is calculated over the entire measurement area, and the deviation from the plane on each surface is calculated (Patent application 1983-
No. 28066). This result gives accurate measurement results for each surface of a and b, but
The relative relationship of b is unknown because the fringes are discontinuous, so by using the measurement results using two or more wavelengths mentioned above, the entire measurement area (the entire surface of the object to be measured) can be measured.
It becomes possible to perform precise measurements over a period of time.

The present invention will be described in more detail below using Examples, but these are merely illustrative, and it goes without saying that various improvements and modifications may be made without going beyond the scope of the present invention.

FIG. 2 is a schematic diagram for explaining the method of measuring a discontinuous surface shape according to the present invention, and shows the configuration of the measuring device. Reference numeral 1 indicates an object to be measured having a discontinuous surface shape. We are trying to measure the flatness of the entire two top surfaces of a concave shape. 2 is an imaging surface for imaging an interference pattern, and has imaging pixels at addresses i and j as shown in FIG. 3 and 3' are wavelengths λ _{1 and} 3', respectively.
A light source of λ ₂ , 31 and 31' are shutters, 8 and 8' are collimator lenses, and 9 is a wavelength selective beam splitter, in which λ ₁ is transmitted and λ ₂ is reflected. 4 is a beam splitter, and 5 is a reference reflector. 10 is a mechanism for fanning (tilting) the object to be measured. 21 is an optical system that forms an image of the object to be measured on the imaging surface. 6
is a wedge glass, which is slightly moved as shown by the arrow,
The phase of the reference light can be changed. Information on interference fringes captured on the imaging plane is sent to a control circuit 7 having control storage and arithmetic processing functions.

A measuring method using the discontinuous surface shape measuring apparatus of the present invention shown in FIG. 2 will be described below.

Close the shutter 31', open the shutter 31,
Light of wavelength λ ₁ is used. Adjust the tilt mechanism 10,
As shown in FIG. 3, about 3 to 10 horizontally oriented stripes should appear in the left and right measurement areas.
Find the coordinates of the darkest (or brightest) part of the stripe. Let the coordinates be (i _lon , j _lon ). Here, l is 1, 2, (or 1, 2...) and represents the type of wavelength, so it is 1 in the case of wavelength λ ₁ .
n is 1, 2, 3, etc., representing the number of stripes from the top, and m is 1, 2, 3, etc., representing the number of the separated measurement area (see Figures 1 and 3). In case a
is 1 and b is 2). That is, the uppermost stripe 21 on the left at wavelength λ ₁ in Fig. 3 becomes (i ₁₁₁ , j ₁₁₁ ), and if this becomes as shown in Fig. 4 on the imaging plane, then (i ₁₁₁ , j ₁₁₁ ) ={(4,8), (5,8), (6,8), (7,8), (8,8), (9,7), (10,7), (11,7), (12, 7), (13, 7)} ...(1). Next, shutter 31' is opened, shutter 31 is closed, and light of wavelength λ ₂ is used. The tilting mechanism measures the fringes under the same conditions as the measurement at λ ₁ above. Similarly to above, (i _2o ′ _n ′, j _2o ′ _n ′) is obtained.

Once (i _1on , j _1on ) and (i _2o ′ _n ′, j _2o ′ _n ′) are found, the control circuit 7 calculates (i _1o1 , j _1o1 ) and (i _2o ′ ₁ , j _2o ′ ₁
)
Find the pair whose sign is positive and closest. That is, they are 21 and 201 in FIG. Similarly (i _1o2 ,
Find the closest pair with a positive sign of j _1o2 ) and (i _2o ′ ₂ , j _1o ′ ₂ ). That is, they are 21' and 201' in FIG. Once this is determined, the next coordinate is stored in the control circuit.

A=(i _1,o,1 , j _1,o,1 ) B=(i _2,o ′ _,1 , j _2,o ′ _,1 ) C=(i _1,o+1,1 , j _{1 ,o+1,1} ) D=(i _2,o ′ _+1,1 , j _2,o ′ _+1,1 ) E=(i _1,o,2 , j _1,o,2 ) F=( i _2,o ′ _,2 , j _2,o ′ _,2 ) G=(i _1,o+1,2 , j _1,o+1,2 ) H=(i _2,o ′ _+1,2 , j _2,o ′ _+1,2 ) ...(2) Here, A and E, B and F are consecutive stripes, and A
and B, and E and F are the closest pairs. C and G and D and H are the same continuous stripes, and C and G are A
and E are fringes of wavelength λ ₁ adjacent to each other, and D and H are fringes of wavelength λ ₂ adjacent to B and F.

Next, as shown in FIG. 2, shutter 31' is closed and 31 is opened to perform measurement at λ ₁ . In this case, the agitating mechanism 10 is agitated to widen the stripe spacing as much as possible. By moving the wedge glass 6 at a constant pitch, the interference fringe intensity I _k (i,j) on the imaging surface 2 is captured. here k
corresponds to the amount of movement of the wedge, so it is the amount of phase modulation of the reference light. Next, the strength of address (i, j) I _k (i,
Find k for which the intensity is the smallest among j). This calculation method is for the value k ₀ of k that minimizes I _k (i, j), 2N+1 values before and after k ₀ , that is, I _kN (i,
j), I _k-N+1 (i, J)...I _k (i, j)...I _k+N-1
Using the values of (i, j) and I _k+N (i, j), find the true minimum value I _k ′ ₀ (i, j) by interpolation. Let the amount of phase modulation corresponding to k′ ₀ be (i, j). All (i,
Find the phase modulation amount (wedge movement amount) (i, j) that minimizes the interference intensity for j). This value is from 0 to
If the value is up to 2π (k is 1 to K (maximum))
When there is a jump close to ±2π between adjacent picture elements, it is noted that the phases α and α±2π are the same phase, and the sum of ±2π is calculated so that (i, j) is smoothly connected. Perform calculation processing. The phase value ₀ (i, j) obtained in this way is the correct value within the left and right measurement parts, but since there is a phase difference of ±2nπ between the left and right, the two This n (integer) is determined using the results measured at two wavelengths. The method is shown below.

Find a, b, c, and d that maximize the next value G.

G= 〓 ^A,B { ₀ (i, j)−ai−bj−c} ² + 〓 ^E,F { ₀ (i, j)−ai−bj−d} ² + 〓 ^C,D { ₀ (i , j)−ai−bj−c+2π} ² + 〓 ^G,H { ₀ (i, j)−ai−bj−d+2π} ² …(3) However, in the above formula, for example, 〓 ^{A, B} is the formula (2) Indicates that the sum is calculated for the pair (i, j) shown by .
To find equation (3), partially differentiate G with respect to A, B, C, and D so that the value becomes 0, then a,
This becomes a four-dimensional linear equation regarding b, c, and d, which can be solved. For the obtained d and c, let n ₀ be the minimum value of |d-c-2πn| (n=0, ±1, ±2...), then '(i, j)= ₀ (i, j ) Left measurement part ₀ (i, j) + 2n ₀ ·π The right measurement part represents the surface shape covering the entire measurement area.

As explained above, by performing calculations using the results of measurements using two or more wavelengths for each of the left and right measurements, it is possible to accurately measure the surface shape over the entire measurement surface. In the above example, when determining the relationship between the left and right separation planes, all of the fringe information shown in equation (2) was used, but even if only a part of this information was used, that is, part of A, B to G, and H. It goes without saying that the relationship between c and d can be found using only the sample points of . In addition, by phase modulating the reference light and interpolating the minimum value of the interference fringe intensity, I _k (i, j),
The phase ₀ (i, j) (representing the surface shape) of each point is being determined, but I _k (i, j) k = 1, 2...K is fast Fourier transformed (FFT) and the absolute value of that value is calculated. ₀ (i, j) may be obtained from the phase value at the maximum value.

In the embodiment shown in FIG. 2, the optical system is used to measure deviation from a flat surface, but if a condensing lens (focus lens) is inserted between the beam splitter 4 and the object to be measured 1, it will be a spherical surface. It goes without saying that it is also possible to measure objects that are divided into two or more parts. In other words, the present invention is applicable to flat surfaces. Spherical surface.
Paraboloid (turns light emitted from one point into parallel light), hyperboloid, ellipse (light emitted from one point or light that is focused on one point becomes focused on another point or becomes a spherical wave emitted from another point) It is clear that this method can be applied to quadratic surfaces such as ). Furthermore, in the embodiment shown in Fig. 2, two types of light sources with different wavelengths are used, but even if there is a large step difference between the discontinuously resolved parts,
It is clear that more than two wavelengths can be used to precisely measure shapes.

According to the present invention, for the first time, it is possible to precisely measure the surface shape of an object to be measured over the entire measurement area, which was impossible to measure if there were discontinuous parts in the interference fringes when the shape was conventionally measured by interferometry. The effect is very large. For example, the magnetic disk slider surface is 2
Conventionally, it was impossible to measure the true surface accuracy between each slider because there are ~3 sliders, and they are located at completely separate positions. However, with the present invention, high accuracy (λ/
100) can now be measured without contact over the entire measurement surface.

[Brief explanation of drawings]

FIG. 1 shows interference fringes obtained by conventional measurement, FIG. 2 is a block diagram of the discontinuous surface shape measuring device of the present invention, and FIG. 3 shows interference fringes obtained by the method of the present invention. FIG. 4 is a diagram showing the measurement area and interference fringe positions on the imaging plane. DESCRIPTION OF SYMBOLS 1... Object to be measured having a discontinuous surface, 2... Imaging surface for imaging interference fringes, 3, 3'... Laser light source, 4... Beam splitter, 5... Reference reflector, 6... ...Wedge glass, 7...Control circuit, 8,
8'...Beam diameter adjustment system, 9...Wavelength selective mirror serving as means for guiding two optical paths to the same optical path, 10
...Tilt mechanism, 21...Optical system for forming an image of the object to be measured on the imaging surface, 31, 31'...Shutter.

Claims (1)

[Claims] 1. Each of the lights of different wavelengths emitted from a coherent light source is separated into a reference light and an object light by a separating optical element, and the separated object light is irradiated onto a measured object. The interference fringes generated by the reflected light from the object to be measured or the transmission hole and the reference light obtained by guiding the separated reference light to the reference optical path are captured by an imaging means for light of each wavelength. The interference occurring within the entire surface of the object to be measured is determined from the relative positional deviation between the detected interference fringes at each wavelength and the pitch information within the interference fringes at each wavelength. The relative shape relationship between the desired partial surfaces of the object to be measured separated by the discontinuous portions of the fringes is determined and memorized, and the memorized relative shape relationship and the interference fringes on the object to be measured are combined. A surface shape measuring method characterized by measuring the surface shape of an object to be measured from the intensity of interference fringes detected within each partial plane separated by a discontinuous portion. 2. A coherent light source that emits light with different wavelengths;
A separation optical element that separates light of different wavelengths emitted from the light source into a reference light and an object light, and a separation optical element that irradiates the object to be measured with the object light separated by the separation optical element. Interference fringes generated by the reflected light or transmitted light and the reference light obtained by guiding the reference light separated by the separation optical element to the reference optical path are detected for each wavelength of light. The desired surface of the object to be measured is determined from information on the relative positional deviation between the imaging means and the interference fringes of each wavelength detected by the imaging means and the pitch within the interference fringes of each wavelength. The relative shape relationship between the desired partial surfaces of the object to be measured, which are separated by the discontinuous portions of interference fringes that occur within the entire surface, is determined and stored.The relative shape between the surfaces separated by the discontinuous portions a shape relationship extraction means; an interference fringe intensity detection means for detecting the intensity of the interference fringes detected within each partial plane separated by the discontinuous portion of the interference fringes on the object to be measured; The relative shape relationship between the surfaces separated by the discontinuous portion obtained and stored by the relative shape relationship extraction means between the surfaces, and each portion separated by the discontinuous portion of the interference fringes on the object to be measured. The intensity of interference fringes detected in each partial plane separated by discontinuous parts of interference fringes on the object obtained from an interference fringe intensity detection means that detects the intensity of interference fringes detected in a plane. 1. A surface shape measuring device comprising: a measuring means for measuring a desired surface shape of an object to be measured.