JP2004045168A - Aspherical shape measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aspherical shape measuring method for reducing an error and accurately measuring although the number of data combined by an orbicular zone wavefront combining method, is increased. <P>SOLUTION: The measuring method includes a step (S1) for obtaining an interference pattern image having one color in its center, a step (S2) for obtaining a phase data D<SB>0</SB>of the center from the interference pattern image, a step (S3) for obtaining an interference pattern image having one color in its center and predetermined orbicular zones, steps (S4, S5) for obtaining phase data D<SB>i</SB>(i=1, 2 to m) of the center and the predetermined orbicular zones from the interference image, a step (S6) for correcting the phase data D<SB>i</SB>so as to overlap the data of the center of the phase data D<SB>i</SB>and the phase data D<SB>0</SB>, a step (S7) for determining 2nπ (n is a natural number) so as to make absolute value data ¾D<SB>i</SB>¾ and ¾D<SB>i-1</SB>¾ continuous and adding it to the corrected phase data, a step (S8) for adding a result of the step (S7) to the already combined data D<SB>0</SB>-D<SB>i-1</SB>and a step (S9) for implementing calculation from i=1, 2 to m. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an aspherical surface shape measuring method for measuring the shape of an aspherical surface to be measured by an annular wavefront synthesis method.
[0002]
[Prior art]
Conventionally, measurement of the shape of an aspheric surface using a spherical wave has been performed. FIG. 3 shows an example of a point diffraction interferometer used for such measurement.
This point diffraction interferometer has a wavelength stabilizing laser 1 and a polarization-maintaining fiber 2 in a light source section. The wavelength stabilized laser 1 is, for example, a light source using helium (He) -neon (Ne) as a laser medium, and emits light having a predetermined wavelength. The polarization preserving fiber 2 has a function of preserving the polarization state, and guides the light emitted from the wavelength stabilizing laser 1 to the interference optical system.
[0003]
On the upper stage 6, the objective lens 3, the pinhole substrate 4, the relay lens 9, and the like are installed. The objective lens 3 focuses the light L1 incident on the interference optical system on a pinhole 4a formed on the pinhole substrate 4. The pinhole 4a emits light incident on the pinhole as a spherical wave. A folding mirror 8 is provided on the optical path of the spherical wave emitted from the pinhole 4a. Half of the spherical wave emitted from the pinhole 5a is reflected from the turning mirror 8 and enters the relay lens 9 as a reference wave L4.
[0004]
A test object 5 having an aspheric test surface 5a is placed on the lower stage 7. The interferometer body 11 supports the upper stage 6 and the lower stage 7 and drives either one or both stages up and down to change the relative positions of both. Thereby, the distance between the pinhole 4a and the test surface 5a of the test object 5 changes. Further, the vibration damping table 12 supports the interferometer body 11 and absorbs external vibration, thereby stabilizing the optical system.
[0005]
The other half of the spherical wave (spherical wave L2) emitted from the pinhole 5a is reflected from the test surface 5a of the test object 5, becomes a test wave L3 including the wavefront characteristics of the same surface 5a, and becomes a pinhole. It returns to the substrate 4.
[0006]
Incidentally, the NA (numerical aperture) of the test wave L3 is equal to or less than half of the NA of the spherical waves L2 and L4 that have first passed through the pinhole 4a before division. Therefore, the test wave L3 returned to the pinhole substrate has a spot larger than the diameter of the pinhole. In particular, if the test surface 5a is an aspherical surface, the spot becomes a larger spot. The periphery of the pinhole 4a of the pinhole substrate 4 has light reflection characteristics. The test wave L3 is reflected from the pinhole substrate 4 to become a test wave L5, and further, is reflected from the folding mirror 8 and relayed. The light enters the lens 9.
[0007]
In FIG. 3, an imaging unit 10 is provided above the upper stage 6. The imaging unit 10 has a CCD (coupled charge device) as a light detection element.
[0008]
The reference wave L4 and the test wave L5 incident on the relay lens form an image on the CCD of the imaging unit 10 and interfere with each other. The imaging unit 10 processes the detection signal output from the CCD, and generates an interference fringe image generated by the interference.
[0009]
In the interference fringe image generated in this manner, the phase of the interference wave at the point of interest is determined by measuring the light intensity at the point of interest, and the optical path difference between the measurement wave and the reference wave can be determined. This optical path difference represents height information at the point of interest on the surface to be inspected. That is, the shape of the surface to be inspected can be obtained by performing arithmetic processing on the phase data obtained for each point.
[0010]
By the way, in the interference fringe image obtained for the aspherical surface, a region where the density of the interference fringes is low and a region where the fringes are densely crowded are observed. In the present application, a region of one stripe among regions having a low stripe density is referred to as a “one-color region”.
4 to 6 show interference fringe images including a one-color area. In FIG. 4, the central portion is one-color, and in FIGS. 5 and 6, the central portion and the ring zone away from the central portion are one-color. The change in the one-color ring band is based on the relationship between the wavefront of the spherical wave described below and the shape of the surface to be measured.
[0011]
Here, the type of the one-color area will be described in detail with reference to FIG. FIG. 7A shows the relationship between the position of the pinhole and the surface to be inspected (cross section), and FIG. 7B shows the relationship between the position of the pinhole shown in FIG. 3 schematically shows an interference fringe image. The test surface in this case is a surface which is axially symmetric and has a shape relatively similar to a spherical surface.
[0012]
First, when a spherical wave is emitted from the lower pinhole PH1 in FIG. 7A, that is, the distance between the pinhole and the center of the test surface (hereinafter referred to as the “distance between the pinhole and the test surface”). ) Is z ₁ , there is a region on the surface to be measured where the radius of curvature is approximately equal to z ₁ . That region, in (a) of FIG. 7, a region A ₁ slightly outwardly from the center of the test surface, this region approximates spherical locally radius z _1. Thus, in the area A _1, the relative difference in distance between the pinholes and the test surface be selected arbitrary point less, less change of the optical path difference between the reference wave and the test wave. For this reason, since the phase change of the interference light intensity is also within ± π, the area S ₁ on the interference fringe image corresponding to the area A ₁ becomes one color.
[0013]
Then, by moving the position of the pinhole PH2 above from PH1 below to increase the distance between the pinholes and the test surface in z _2. Then, again, since approximately equal area on the object surface radius of curvature and z ₂ are present, in response to such regions A _{2 (outside} the region in this case is the area A _1), area _{S 2} is a one-color.
[0014]
As described above, by adjusting the distance between the pinhole and the test surface, one color can be formed in a desired annular zone of the interference fringe image. Here, in FIG. 7 (b), has become a well one color center area S ₀ of the test surface. Central portion of the test surface, the radius of the area S ₀ corresponds to the distance z but varies, since the region where the distance between the pinhole PH is not much is always present, is always a one-color.
[0015]
When analyzing an interference fringe image, measurement data in such a one-color area is used. This is because, in a region where stripes are densely crowded, correct measurement cannot be performed because the region is affected by aberration, distortion, and the like of an optical system such as a relay lens. Further, in the case of aspherical surface measurement, stripes are often crowded and the number of stripes cannot be counted on the CCD.
[0016]
The analysis of the interference fringe image is performed by the following annular wavefront synthesis method. First, the distance between the pinhole and the surface to be inspected is adjusted, and as shown in FIG. 4, an interference fringe image is captured so that the central zone and the annular zone near the center become one color. From this interference fringe image, the light intensity of the one-color area C ₀ is measured, and the phase data D ₀ of the interference wave is calculated. Next, the distance between the pinhole and the test surface is changed, and for example, as shown in FIG. 5, the one-color area is moved outward to capture an interference fringe image. At this time, in two consecutive interference fringe images, the positions of the one-color areas slightly overlap. From this interference fringe image, it measures the light intensity of one color region C _1, calculates the phase data D ₁ of the interference wave. Similarly, the position of the one-color area is gradually moved outward by changing the distance between the pinhole and the surface to be measured, and each time an interference fringe image is taken, the light intensity of the one-color area is measured, and the interference wave is measured. Is obtained. Such measurement, as shown in FIG. 6, carried out until the outermost one color region C _m.
[0017]
By the way, the phase data obtained from each interference fringe image well reflects the information of the surface to be inspected in each measurement area, but includes an unknown element in relation to other measurement areas. That is, each phase data D _i includes an uncertainty of Δφ _i ± 2nπ with respect to the absolute value data | D _i |. Here, [Delta] [phi _i is -π ≦ Δφ ≦ π, the phase shift caused by varying the optical path length in order to change the measurement region. Further, n is a natural number and corresponds to the number of interference fringes that cannot be counted in a portion where the interference fringes are crowded. For this reason, when synthesizing data, Δφ _i and n must be determined so that the phases are continuous in adjacent regions.
[0018]
In principle, it is possible to determine n by counting the number of stripes from the center. However, when an aspheric surface is measured by a spherical wave, the difference between the curvature radii of the two becomes larger toward the outside of the surface to be measured. Therefore, even in the interference fringe image, since more fringes are densely crowded and picked up by the CCD as it goes to the outside, and the resolution exceeds the resolution of the CCD, it is impossible to identify each fringe.
[0019]
Therefore, conventionally, Δφ _i and n are determined so that the absolute value and the gradient (differential value) at the boundary coincide with each other so that the absolute value data | D _i−1 | and | D _i | I was This is sequentially performed for i = 1, 2,... To synthesize data.
[0020]
FIG. 8 schematically shows a graph of the composite data created as described above. In FIG. 8, the x-axis is a position coordinate in the radial direction from the center of the test surface, and the y-axis is a value obtained by converting the interference fringe phase data into the height of the test surface. The intermittent bold solid line indicates the synthesized data, and the broken line indicates the actual shape of the test surface. Note that thick solid lines are connected smoothly so that they do not actually intermittently. However, in this figure, the fact that there is an error is schematically shown intermittently.
[0021]
As shown in FIG. 8, according to the conventional phase connection method, when the number of phase data to be combined increases, errors at the boundaries of the phase data obtained from the respective interference fringe images are integrated. An error between the test surface and the measured value increases, and it is difficult to obtain sufficient accuracy.
[0022]
[Problems to be solved by the invention]
In view of the above, in the present invention, in measurement by the annular wavefront synthesis method of the shape of the test surface of the aspherical surface using a spherical wave, even if the number of data to be synthesized increases, the error does not increase, high accuracy The purpose is to make measurements.
[0023]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, an aspherical shape measuring method according to the present invention is a method for measuring the shape of a test surface having an aspherical shape by an annular wavefront synthesis method, wherein a center portion of the test surface is measured. It said surface of the spherical wave W ₀ of the approximate radius of curvature applied to said surface, and the test wave reflected from said surface, a part of the spherical wave (reference wave) and is by measuring the interference fringes generated by interference The interference wave phase data D ₀ corresponding to the shape of the center is obtained, and a spherical wave W ₁ having an approximate radius of curvature slightly outside the center of the surface is applied to the surface, and a test wave reflected from the surface is obtained. to obtain an interference wave phase data D ₁ corresponding to the shape of the part (reference wave) and is by measuring the interference fringes generated by interference said surface center and the slightly outer part of the spherical wave, successively similarly the spherical wave _W 2 of the approximate radius of curvature of the outer portion of the test _{surface, W 3, W 4, ···} , W i, ···, a _{W m} the Interference wave data _D 2 as measured to the outermost portion of said surface against the _{surface, D 3, D 4, ···} , D i, ···, give _{D m,} the measured charge wheel band _{D i the,} | | absolute value data | _{D i} D i | = determined with _{D i} + _{Δφ i} + _2nπ, where, n is a natural number, [Delta] [phi _i is the interfering optical system the center of the interfering optical system center data and _{D 0} of _{D i} a phase difference of less than 2π with the data, for [Delta] [phi _i, determined as overlapping and the interfering optical system center data of the interfering optical system center data and D _i of the D _0, for n is, D _{i- 1} and D _i are determined to be connected, and the respective data | D ₀ |, | D ₁ |, | D ₂ |, ..., | D _i |, ..., | D _m | The shape of the entire aspheric surface is measured.
[0024]
According to the present invention, when connecting the phases, the connection is always performed while referring not only to the data in the adjacent area but also to the data in the center, so that errors are not integrated even in an area far from the center. . Therefore, the aspherical surface can be measured with high accuracy using the spherical wave.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a flowchart illustrating an aspheric surface shape measuring method according to an embodiment of the present invention. The aspherical surface shape measuring method according to the present embodiment is a method for measuring the shape of a test surface having an aspherical surface shape, for example, by analyzing an interference fringe image obtained using a point diffraction interferometer. Done.
[0026]
First, in step S1, the spherical wave W ₀ irradiated to the test surface to obtain an interference fringe image one color region is formed in a center portion of the interference fringe image. When a point diffraction interferometer is used, the distance between the pinhole and the center of the test surface is adjusted to be equal to the approximate radius of curvature of the center of the test surface. Some of the spherical wave W _0, after being injected, is divided as a reference wave. Another part of the spherical wave W ₀ is reflected from the test surface, it becomes a test wave including information about the shape of the test surface and interferes with the reference wave. By taking an image of this interference wave, an interference fringe image is obtained.
[0027]
Then, obtained in step S2, by measuring the light intensity of one color region of the resultant interference fringe image in step S1, the phase data (measurement data) D ₀ of the interference wave. This data is used as a reference value in the annular wavefront synthesis method described later.
[0028]
Next, in step S3, the test surface is irradiated with the spherical wave W _i (i = 1) to obtain an interference fringe image. In step S3, the distance between the pinhole and the test surface is adjusted so that a one-color annular zone is formed outside the one-color area of the interference fringe image obtained in step S1. At this time, the distance between the pinhole and the test surface is set so that the one-color ring zone and the one-color area obtained in step S1 are slightly overlapped with each other so that no break occurs between them. Is preferably adjusted. As a result, an interference fringe image in which the central portion and the ring band slightly outside the central portion have one color is obtained.
[0029]
Next, in step S4, the light intensity of one color region of the resultant interference fringe image in Step S3 is measured to obtain the phase data (measurement data) D _i of the interference wave. Here, the measurement of the light intensity is performed for both the annular zone and the central portion, and the phase data _Di includes phase data (measurement data) DR _i obtained from the annular zone and phase data obtained from the central portion. (measurement data) are included and DC _i.
[0030]
This measurement is performed by changing the distance between the pinhole and the surface to be inspected (for example, by 0.1 mm each), thereby shifting the one-color ring band outward, i = 1, 2,. This is repeated for m (step S5). Here, the one-color ring band of the interference fringe image obtained earlier and the one-color ring band of the interference fringe image obtained next are slightly overlapped. This is performed until the one-color ring band comes to the outermost periphery (i = m) of the interference fringes.
[0031]
Next, in step S6 to S9, the annular wave field synthesis, phase data _{D 0, D 1, D 2} , ···, D i-1, D i, ···, to synthesize _{D m.}
Here, the data from phase data D ₀ to the phase data D _i-1 is synthesized, a method for combining the phase data D _i, be described with reference to FIG. 2A to 2E are graphs showing phase data at each position on the surface to be inspected, where the x-axis indicates the position coordinates in the radial direction of the surface to be inspected (cross section), and the y-axis indicates the phase. Is shown.
[0032]
In Figure 2 (a) is a graph created on the basis of the from the phase data D ₀ to the phase data D _i-1 to the combined data. Continuing overlapping phase data D _i in the graph is as shown in FIG 2 (b).
[0033]
Phase data _{D i} is already absolute value data are connected to the combined data _{D 0 ~D i-1 | D} i | respect, has uncertainty Δφ _i + 2nπ. Here, Δφ _i satisfies −π ≦ Δφ _i ≦ π, and can be considered to be caused by a phase shift at the center of the interference fringe image. Further, n is a natural number. Thus, the absolute value data | D _i | is expressed as | D _i | = DR _i + Δφ _i + 2nπ.
[0034]
Therefore, first, in step S6, to correct the phase shift of the center of the phase data D ₀ and phase data D _i. To this end, as shown in FIG. 2C, the entire measurement data D _i is moved in the y direction such that the measurement data DC _i at the center overlaps with the phase data D ₀ . That is, the correction value [Delta] [phi _i of less than 2π is obtained as Δφ _i = _D 0 -DC _i.
[0035]
Next, in step S7, an uncertain value 2nπ is determined. Since 2nπ the discrete values, it may be determined n as the data D _i-1 adjacent and D _i are continuous.
In step S8, the absolute value data | D _i | = D _i + Δφ _i + 2nπ thus obtained is added to the combined data D _{0 to} D _i−1 . Thus, as shown in FIG. 2 (d), smoothly connected combined data D ₀ to D _i is obtained.
[0036]
By performing such arithmetic processing for i = 1, 2,..., M (step S9), the composite data D _{0 to} D _m as shown in FIG.
Further, in step S10, the synthetic data D ₀ to D _m by converting the value of the height of the test surface, it is possible to obtain the shape of the surface.
[0037]
In the present embodiment, the method of obtaining the phase of the interference fringes from the intensity of the interference fringes has been described. However, other known methods can be used as the method of obtaining the phase of the interference fringes. . For example, it is more preferable to use the fringe scan method (phase shift method) because the error can be further reduced. In this method, the measurement is performed by changing the relationship between the wavefront and the test surface.
[0038]
Further, in the present embodiment, the case where an aspheric surface is measured using a point diffraction interferometer that does not use a wavefront correction element (null element) has been described. However, the present invention is not limited to the point diffraction The present invention can also be applied to a spherical Fizeau interferometer without using elements, a Twyman-Green interferometer, and the like.
[0039]
【The invention's effect】
As described above, according to the present invention, when measuring the shape of an aspheric surface using a spherical wave, high-precision measurement can be performed even if the distance between the aspheric surface and the aspheric surface is large.
[Brief description of the drawings]
FIG. 1 is a flowchart illustrating an aspherical surface shape measuring method according to an embodiment of the present invention. You.
FIG. 2 is a diagram for explaining an annular wavefront synthesis method performed in the aspherical shape measurement method according to one embodiment of the present invention.
FIG. 3 is a diagram illustrating the principle of a point diffraction interferometer.
FIG. 4 is a diagram showing an interference fringe image in which a central portion is one color.
FIG. 5 is a diagram showing an interference fringe image in which a central portion and a ring band have one color.
FIG. 6 is a diagram showing an interference fringe image in which a central portion and an outermost periphery are one color.
7A is a diagram showing a positional relationship between a pinhole and a surface to be inspected (cross section), and FIG. 7B is a diagram showing a positional relationship between the pinhole shown in FIG. The interference fringe image observed corresponding to the position is shown.
FIG. 8 is a diagram showing synthesized data obtained by a conventional annular wavefront synthesis method.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 wavelength stabilized laser 2 polarization preserving fiber 3 objective lens 4 pinhole substrate 4 a pinhole 5 test object 5 a test surface 6 upper stage 7 lower stage 8 folding mirror 9 relay lens 10 imaging unit 11 interferometer body 12 vibration isolation Table

Claims (1)

A method of measuring the shape of the test surface having an aspherical shape by an annular wavefront synthesis method,
Against the spherical wave W ₀ of the approximate radius of curvature of the center of該被interfering optical system to said surface, and the test wave reflected from said surface, occurs a portion of the spherical wave (see wave) interfere interfere the resulting interference wave phase data D ₀ corresponding to the shape of said surface center by measuring the fringe,
Said surface of the spherical wave W ₁ of the approximate radius of curvature of the slightly outer portion than the central portion against the said surface, and the test wave reflected from said surface, a part of the spherical wave (reference wave) and interferes The resulting interference fringes are measured to obtain interference wave phase data D ₁ corresponding to the shape of the center portion of the surface and the slightly outer portion,
Similarly, the spherical waves W ₂ , W ₃ , W ₄ ,..., W _i ,..., W _m having the approximate radius of curvature of the outer portion of the surface to be measured are sequentially applied to the surface, and the outermost periphery of the surface interference wave data _D 2 as measured up _part, D _3, D _4, resulting _···, D _{i, ···,} the _{D m,
The,} | | _{D i} | absolute value data of the measurement representative wheel band D _i D _i | = determined with _{D i} + _{Δφ i} + _2nπ,
Here, n is a natural number, [Delta] [phi _i is the phase difference of less than 2π with the interfering optical system center data of the interfering optical system center data and D ₀ of D _i,
The [Delta] [phi _i, determined as overlapping and the interfering optical system center data of the interfering optical system center data and D _i of the D _0, for n, determined to be connected and a D _i-1 and D _i ,
Each data _{| D 0 |, | D 1} |, | D 2 |, ···, | D i |, ···, | characterized in that combined to the measuring the aspherical overall shape | _{D m} Aspherical shape measurement method.

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