JPS62129707A - Method and apparatus for measuring surface configuration - Google Patents

Abstract

PURPOSE:To enhance measuring accuracy, by preliminarily storing the reference wavelength error and inherent aberration of an interference apparatus, which is based on a standard fixed thereto and a second rotary or freely movable standard, as values for correction. CONSTITUTION:The parallel laser beam from a laser beam source 1 is allowed to irradiate the first plane standard 7 fixed to an interference apparatus and the second plane standard 13 having the almost same accuracy as the standard 1 mounted to a specimen rotary stand 2 and the reflected beams from both standards 7, 13 interfere to form an interference fringe corresponding to the reference wavelength error and aberration of the interference apparatus and the image of said interference fringe is picked up by an image pickup apparatus 6. The image pickup signal from the apparatus 6 corresponding to the rotation of the stand 12 is subjected to averaging processing by a computer 21 to store error correction quantity in a data storage 20 and the interference fringe data by the standard 7 and the article to be measured mounted to the stand is corrected and a surface configuration not affected by the reference wavelength error and aberration of the interference apparatus is measured with high accuracy. Further, even if the second standard is moved to the direction crossing an optical axis at a right angle without being rotated, the similar result can be obtained.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a surface profile measurement method and apparatus that eliminates inherent aberrations of an interferometer and errors in a reference wavefront, thereby obtaining measurement accuracy higher than that of a prototype. .

[Conventional technology]

When measuring the surface shape of the object to be measured (plane 9 spherical, aspherical) or the wavefront aberration of an optical lens using various interferometers such as Fizeau, Shearing, and Twyman Green-ya, the measurement accuracy depends on the interferometer used. It is limited by the accuracy of the reference plane and the aberrations of the entire system of the interferometer itself, that is, the inherent aberrations.

Therefore, the measurement accuracy of currently commercially available interferometers is λ/
20 (where λ is the wavelength), which is lower than that of a stylus-type shape measuring device.

As a method for removing the inherent aberration of the interferometer itself, when it comes to measuring spherical shapes, there is a method called moving the object to be measured and subtracting the measurement results under three conditions.
nlng) et al. This method will be explained in detail according to FIG. As shown in Figure (a), the object to be measured N is placed so that the center of curvature of the surface S to be measured coincides with the focal position of the collimator lens C that condenses the light beam coming from the interferometer. Then, the interference measurement of the object to be measured N is performed. As a result, the obtained wavefront shape is designated as Wl. next,
As shown in the same figure (b), the object to be measured
Similar measurements were made using a 00 rotary system, and the resulting wavefront shape was designated as W2. In addition, as shown in the same figure (e), place the object N to be measured at the focal position of the collimator lens C,
The waveform shape W8 is measured. Then, by performing calculations as shown in the following equation from these three measurement results, the actual deviation of the shape of the surface to be measured of the object N to be measured from the spherical surface can be determined.

w0=-! -(w1+w2-w3-w8) However, "
The rotation represents a 180° rotation of the data itself.

[Problem that the invention seeks to solve]

However, in the conventional method of removing such inherent aberrations, (1) The accuracy of alignment in FIGS. 6(a) and 6(b) directly affects the accuracy of the measurement results. In other words,
The sample position in (b) of the same figure is exactly 18 mm with respect to the optical axis of the interferometer with respect to the position of the object to be measured N in (,) of the same figure.
0 must be in a rotated position and (a).

(b) In both figures, the central axis of the surface to be measured S and the optical axis must match, and the object N must be placed at the same position in the optical axis direction in both states.

(2) Three measurements are always required for one object to be measured;
Not only is it troublesome, but if the measurement time is extended over a long period of time, the influence of external disturbances cannot be ignored.

Due to the following drawbacks, it was not suitable for practical use.

[Failure to solve the problem]

The surface shape measuring method and device according to the present invention are designed to solve the above-mentioned problems, and the purpose thereof is to reduce the error and inherent aberration of the reference wavefront of the interference device into interference measurement results of the object to be measured. It is an object of the present invention to provide a method and apparatus for measuring the surface shape of a surface shape, which improves measurement accuracy.

Therefore, the surface profile measurement method according to the present invention uses a first prototype fixed to an interference device and a second prototype that can rotate around the optical axis or move in a direction perpendicular to the optical axis. By rotating or moving the second prototype, the error in the reference wavelength and the inherent aberration of the interferometer are determined in advance from the results of measurements at several points, and this is saved as data, and then the actual The shape of the surface to be inspected is measured by subtracting the data from the measurement results of the surface to be inspected.

Furthermore, the interference device according to the present invention includes a first prototype fixed to the interference device, a second prototype, and an object to be measured that are rotated around the optical axis of the interference device or in a direction perpendicular to the optical axis. The reference wavefront of the interference device and the reference wavefront of the interference device are determined from the results of measurements at each position of the rotating or moving device using a movable moving device, a device that controls its rotation or movement, and the second prototype. A device for determining the inherent aberration in advance and storing it as data, the object to be measured is attached to the rotating or moving device, and the surface shape of the surface to be measured is obtained by subtracting the data from the measurement result of the surface to be measured. It was designed to measure.

[Effect]

In the present invention, the error and inherent aberration of the reference wavefront of the interference device are measured in advance and subtracted from the actual measurement results, making it possible to measure the surface shape with an accuracy higher than that of the prototype. . In addition, after measuring the error and inherent aberration of the reference wavefront in advance, it is necessary to rotate the object to be measured relative to the optical axis and shift it in a direction perpendicular to the optical axis when measuring the object to be measured. The measurement is easy because it only needs to be measured once, and the measurement time can be shortened when measuring many objects of the same shape.

〔Example〕

Aberration correction in the present invention involves correcting and measuring the aberrations of the interference device itself or shape errors of the reference surface, storing the data as data in memory, and subtracting the data from the measurement results when actually measuring the surface to be measured. The purpose is to measure the test surface. In order to measure this aberration and the shape error of the reference surface, in addition to the first prototype fixed to the interference device, a second prototype with a flat or spherical surface with a certain degree of precision is prepared separately, and this is placed on the sample stand. By placing it on top of the optical axis and rotating it several times or moving it in a direction perpendicular to the optical axis, and averaging the data measured at several positions,
The aberrations of the reference surface (the reference surface of the first prototype) itself are measured in advance. In this method, while the reference surface itself is fixed to the interference device, the flat or spherical surface of the second prototype changes its position randomly, and data is collected for each position. By averaging the data, the shape error of the reference surface can be measured with higher accuracy. For this reason, it is necessary that the position of the plane or spherical surface of the second prototype in the surface direction changes during measurement at each position. However, other alignment, tilt (tilt of the plane or spherical surface with respect to the optical axis), and defocus (deviation in the axial direction from the optimal position when the spherical shape is used)
, it is necessary to install it in a few places, but these can be corrected to some extent by computer processing later, and this correction will prevent large deviations in the positional relationship between the data and the reference surface. Just install it within the range.

Further, after measuring the aberration of the thread and the shape error of the reference surface using this measurement method, the actual object to be measured is measured, and positional accuracy in the plane direction is not required at all for this object. This is because the shape error of the measured reference surface and the inherent aberration of the system have a one-to-one positional correspondence with the reference surface and the interferometer system.

Hereinafter, the present invention will be explained in detail based on embodiments shown in the drawings.

Fig. 1 is a diagram of an optical system showing an example of applying the present invention to a Fizeau interferometer, and Fig. 2 shows an example of an optical system that emits a highly coherent beam of sample supination, such as a He-Ne laser. It can be used. 2 is a diverging lens, 3 is a half mirror, 14 is a collimator lens, 5 is an imaging lens, 6 is a CC
7 is a flat prototype, 8 is an object to be measured, and 10 is a driving device such as a piezo element that moves the flat prototype (first prototype) 7 in the optical axis direction. Further, FIG. 2B shows the main parts of the optical system for measuring the spherical shape, and 11 is a spherical prototype (first prototype) integrally equipped with a collimator lens 4.

As shown in FIG. 2, the sample rotating table 12 on which the object to be measured 8 and a second prototype 13 (to be described later) are mounted rotates on the optical axis by transmitting the rotation of a pulse motor 14 via a worm 15. It is configured to be rotated with the direction as the rotation axis direction, and tilt and defocus (deviation from the focal position of the collimator lens) are automatically removed once after rotation.

In FIG. 1 (IL), the wavelength λ emitted from the light source 1
The monochromatic laser beam becomes a diverging beam by a diverging lens 2, is reflected by a half mirror 3, is made into a parallel beam again by a collimator lens 4, and then passes through a flat prototype 7 to the surface of the object to be measured 8, i.e. Irradiate the surface S to be inspected. At this time, a part of the light beam is reflected by the reference surface R of the flat prototype 7, and another part is reflected by the test surface S. The wavefronts of these reflected lights have shapes corresponding to the shapes of the reference surface R and the test surface S, respectively. The two reflected lights are superimposed on each other by returning along the same optical path, pass through the collimator lens 4 and the half mirror 3, and are imaged on the imaging surface of the imaging device 6 by the imaging lens 5. Form interference fringes based on interference.

In other words, when performing a plane measurement as shown in Figure 1 (, ), if the distance between the plane prototype 7 and the object to be measured 8 is d, the reference light reflected on the reference surface R and the surface to be measured S are There is an optical path difference of 2d in the round trip between the reflected light and the reflected light.

The intensity of the interference pattern created from this 2d optical path difference is I
"cos (2πλd+φ) (where φ is a constant). In other words, when the optical path difference 2d is an odd multiple of λ/2, 2d=λ/2(2n-1) <where n is an integer) υ
, they cancel each other out and become darker, and when they are an even multiple of 2/2, 2d=λ/2(2n), and they strengthen each other and become brighter. As a result, an interference image consisting of interference fringes based on mutual interference is formed, and the number, shape, width, etc. of the interference fringes, which correspond to equal knitting lines, are
However, if the flatness is measured only from the intensity of the stripes, the accuracy is only 2./2cm, so
The drive device 10 vibrates the flat prototype γ in the optical axis direction to vary the value of d near IJ, and the reading accuracy is improved from the phase of the change in brightness of the interference pattern at that time. This method is called fringe scanning.

In addition, in the case of the object to be measured 8 having a spherical test surface g as shown in FIG. What is necessary is to arrange the measurement object 8.

As described above, when the reading accuracy increases, the next problem is the accuracy of the reference surface R. Since this accuracy directly contributes to dl'i-, the accuracy of the reference plane R currently limits the accuracy of the interferometer itself. Therefore, the present invention is directed to the above-mentioned general
Measures the error of the reference surface R itself, stores this measurement value as data, and subtracts the data from the measurement result of the test surface s (s') to improve reading accuracy. It is.

The details will be explained below.

FIG. 3 shows a system diagram of the Fizeau interferometer described above.

In the figure, the sample rotating table 12 shown in FIG. 2 is shown. 21 is a computer, 22 is a nitrogen gas converter, 23 is a DC amplifier for the drive device 10, 24 is a controller for controlling the pulse motor 14, and 25 is a frame memory.

First, in order to measure the error of the reference wavefront of the flat prototype 7,
An object to be measured having the same degree of accuracy as the prototype 7 is placed on the sample rotating table 12 as a second flat prototype 13 . At this time, the measured result is the sum A+B of the shape B of the prototype 13 and the shape A of the reference surface R.

Next, measurement can be performed at a completely different position by rotating the second planar prototype 13 at an appropriate angle with the optical axis direction as the rotation axis direction, or by moving it in a direction perpendicular to the optical axis using an appropriate moving device. The data is stored in the data storage 20. If this process is performed n times and the measurement results are averaged on the same storage 20, the result will be A+
C1 (However, C = -(B1+B2...Bn months.) Now, if the error of B is random, the error of C will be reduced from the maximum error of B by the square root of the number of measurements n. In addition, this average value represents the deviation from the diagnostic surface R as the number of measurements increases.For this reason, if the second prototype S is a flat prototype with an accuracy of "/20" 7
If you measure 25 times using , the error of reference 1fiR is '/1
It can be measured with oo. Therefore, the data (average value)
is stored in the data storage 20. Next, as explained in Fig. 1, a general lateral measurement *8 is placed on the sample rotating table 12 and measured, and when the above-mentioned data is subtracted from this measurement result, the measurement of '/l OO is obtained. This makes it possible.

In addition, this data needs to be imported only when the flat prototype 7 is replaced or the optical arrangement of the interferometer itself is changed, which is very advantageous when measuring many objects of the same shape. Furthermore, since the object to be measured 8 only needs to be measured once, there is an advantage that the measurement can be carried out quickly.

It goes without saying that such measurements can also be applied to the spherical object 8 (FIG. 1(b)). In that case, the required error is the error of the reference surface R of the spherical prototype 11 plus the aberration of the lens between the reference surface R and the test surface g, and the measurement accuracy is the accuracy of the spherical prototype 11 plus the aberration of the lens between the reference surface R and the test surface g. The accuracy is multiplied by the square root of the number of times.

FIG. 4 shows an example in which the present invention is applied to a shearing interferometer. Components in the figure that are the same as those in FIG. 1 are designated by the same reference numerals.

30 is a condenser lens, 31.32 is a beam splinter, 3
3.34 is a total reflection mirror, 35 is a driving device such as a piezo element that moves the total reflection mirror 33 in the optical axis direction, and 36 is an imaging lens, and the structure itself is the same as that of a conventional shearing interferometer.

The light emitted from the light source 1 passes through the diverging lens 2 and beam splitter 31, passes through the collimator lens 4, becomes a parallel beam parallel to the optical axis, and is then condensed by the condensing lens 30 to illuminate the surface to be inspected. . The wavefront of the light beam reflected here has a shape that corresponds to the shape of the surface to be inspected. Then, this light beam passes through the condenser lens 30, collimator lens 4, and beam splitter 31 again, is split into two by the second beam splitter 32, and is partially transmitted and partially reflected.

The transmitted light is reflected by the total reflection mirror 33 and returns to the beam splinter 32 again. Further, the reflected light reflected by the beam splitter 32 is reflected by the total reflection mirror 34 and returns to the splitter 32 again. At this time, the total reflection mirror 34 is not orthogonal to the optical axis of the incident light, but is tilted by a small angle θ from this orthogonal direction, so that the incident light beam is reflected by the reflection mirror 34 as shown by the solid line. As shown by the broken line, the beam returns to the beam splinter 32, and there is a slight deviation from the incident light beam. Therefore, each total reflection mirror 33.3
4 and returns to the beam splinter 32, and the two light beams overlap again and are laterally shifted by a minute amount Δd, causing interference. The superimposed light beams are then imaged onto the imaging device 6 by the imaging lens 36 to form interference fringes.

In the case of a shearing interferometer, the wavefront to be measured is divided into two without using a standard reference surface, and the measurement result is caused by the inherent aberration of the entire system. have a direct impact on

Therefore, in a shearing interferometer, it is essential to cancel the inherent aberration of the entire system, but in the present invention, by directly adopting the method described above, this inherent aberration itself can be canceled, and the shearing interferometer It can be said that the application of is more useful.

In that case, prior to measuring the object to be measured 8, as in the case of the Fizeau interferometer, a spherical prototype is placed at the position of the object to be measured 8, and the measurement is performed several times while rotating the prototype. Average the data and save it in data storage.
Thereafter, the general object to be measured 8 may be measured, and the data may be subtracted from the measurement results.

FIG. 5 shows an embodiment in which the present invention is applied to a Twyman-Green interferometer. 40 is a reference surface mirror, and 41 is a drive device such as a piezo element that moves the reference surface mirror 41 in the optical axis direction.

The light emitted from the light source 1 is diverged by a diverging lens 2 and then deflected for 2 minutes by a beam splinter 31. One of the lights is condensed by a collimator lens 4 and then illuminates the surface of the object to be measured 8 to be measured. Then, this light is reflected by the test surface and passes through the original optical path again to the beam splitter 3.
Return to 1. On the other hand, the light reflected by the beam splinter 31 is reflected by the reference surface mirror 41, returns to the beam splinter 31 again, and is overlapped with the light from the test surface. The overlapping light beams are then imaged by the imaging lens 5 on the imaging surface of the imaging device 6 to form interference fringes.

In this case, the reference surface mirror 41 constitutes a first prototype, and prior to the measurement of the object to be measured 8, a second prototype having a spherical shape is set at the position of the object to be measured 8. , the shape error of the reference surface R of the reference surface mirror 41 is measured in advance, which is exactly the same as in the embodiment described above.

〔Effect of the invention〕

As explained above, according to the surface shape measuring method and device according to the present invention, the error and inherent aberration of the reference wavefront of the interferometer are measured, and this is stored as data, and the measurement of the actual surface to be measured is performed. Since the above data is subtracted from the measurement result, it is possible to perform highly accurate measurements that exceed the errors inherent in the device itself.Also, when measuring the surface to be measured, only one measurement is required, and it is possible to avoid rotating or shifting the surface. and many times fa1
It is not necessary to make one constant measurement, which is very advantageous when measuring many items of the same shape.

[Brief explanation of drawings]

Fig. 1 is a diagram of an optical system showing an example of applying the present invention to a Fizeau interferometer, Fig. 2 is a front view showing an example of a sample rotating table, and Fig. 3 is a system diagram of the Fizeau interferometer. , FIG. 4 is a diagram of an optical system showing an embodiment in which the present invention is applied to a shearing interferometer, and FIG. 5 is a diagram of an optical system in which the present invention is applied to a Twyman
FIG. 6 is a diagram showing an optical system according to an embodiment when applied to a Green interferometer, and FIG. 6 is a diagram showing a conventional method for removing inherent aberrations. 1...Light source, 6...Imaging device, T...Planar prototype (first prototype), 8...Object to be measured, 12...
・Sample turntable, 13...Second source! , 14...
Pulse motor, 20... Data storage, 24.
...Controller for pulse motor, R...Reference surface, S, Gi...Test surface.

Claims (2)

[Claims] (1) Using a first prototype fixed to the interference device and a second prototype that can rotate around the optical axis or move in a direction perpendicular to the optical axis, rotate or move the second prototype. Errors in the reference wavelength and inherent aberrations of the interference device are determined in advance from the results of measurements taken at several points, this is stored as data, and then the aforementioned data is subtracted from the measurement results of the actual surface to be inspected. A surface shape measuring method characterized in that the surface shape of a surface to be inspected is measured. (2) A first prototype fixed to the interference device, a device for rotating the second prototype and the object to be measured around the optical axis of the interference device or in a direction orthogonal to the optical axis; The reference wavefront and inherent aberration of the interference device are obtained in advance from the results of measurements at each position of several points of the rotation or movement device using the movement control device and the second prototype, and these are used as data. The object to be measured is attached to the rotating or moving device, and the surface shape of the surface to be measured is measured by subtracting the data from the measurement result of the surface to be measured. interference device.