JPS62129732A - Sharing interferometer - Google Patents

Abstract

PURPOSE:To enable highly accurate measurement of share value, by arranging a pattern for measuring a shift between two measuring wave faces split in two in an incident optical path to form an image of a shade of the pattern produced on the surface to be inspected on an imaging element. CONSTITUTION:The titled apparatus is so constructed that a luminous flux reflected by a surface S to be inspected is split in two to divide a measuring wave face in two so that these measuring wave faces will be shifted laterally from each other to make them interfere. A pattern 14 is arranged in the incident optical path to measure the shift between the two measuring surfaces and the pattern 14 is halved to be shifted sideways so that image overlapping one upon another is formed on an imaging element 13. The actual share value is calculated or measured from the overlapped image. This enables highly accurate measurement of the share value with a handy construction, thereby achieving a higher measuring accuracy of an interferometer.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention divides the light beam reflected by the surface to be measured into two, thereby dividing the measurement wavefront into two, and then shifting these measurement wavefronts laterally with respect to each other to generate interference. This is a shearing interferometer that can measure with high accuracy the force of two measurement wavefronts shifted by 7%, that is, the shear amount.

A shearing interferometer is an interferometer that does not use a standard reference surface, divides the wavefront to be measured into two, and causes interference by itself by applying a lateral shift, and does not require a reference standard. can reduce costs.

The number of interference fringes obtained can be adjusted by changing the amount of shear, and if the amount of shear is kept small, the spacing of interference fringes will not be crowded and measurement will not be possible. When scanning the fringe by moving it in steps of one-hundredth of a wavelength, the phase of the reference light and the test light changes according to the amount of movement of the total reflection mirror, so it is possible to obtain reading accuracy on the order of one-hundredth of a wavelength. It is particularly advantageous for measuring surface shapes 1, such as spherical and aspherical surfaces, of optical components such as lenses and mirrors that have relatively large aberrations.

Measure various objects using a shearing interferometer.

When determining the amount of shear, it is necessary to measure the amount of shear, and the accuracy of this measurement determines the accuracy of the interferometer itself. When measuring an aspheric surface with a large deviation from a spherical surface in spring.

The share amount must be reduced by /J%, which requires absolute accuracy in measuring the share amount. However, the amount of shear cannot be directly measured and is usually calculated from the tilted state of the mirror, but due to changes in the ambient temperature of the interferometer and errors in the installation of each optical element, the amount of shear is calculated based on the tilted state of the mirror. are not necessarily correlated, which makes the measurement of the shear amount inaccurate, making it difficult to know the actual shear amount of both divided measurement wavefronts, and potentially increasing measurement errors.

Therefore, as a method to directly measure the amount of shear, a position sensor is provided to measure the position of the two light beams that are superimposed on each other after being divided into two, and to measure the amount of deviation between the two divided measurement wavefronts. Sharing interferometer
-107507) has been proposed.

[Problem that the invention seeks to solve]

However, it is difficult to directly measure the amount of shear using a position sensor when there are no edges.

That is, shape measurement using a shearing interferometer is generally limited to mirror surfaces, and the reflective surface has less brightness and darkness than a general image. For this reason, when the object to be measured is within the measurement range as shown in Fig. 6<->, an edge is formed in the image due to the outer shape of the object, and by hiding one side of the light beam with the printer, the edge can be removed. Measure the amount of deviation and find out the amount of deviation. However, if it is desired to magnify and measure only the center of the object as shown in FIG. 6(b), there is no target that indicates a shift within the measurement range, making measurement difficult.

In addition, the area surrounded by a square in FIG. 6 indicates the measurement range.

However, direct measurement of the amount of share cannot be expected to be very accurate because the measurement accuracy depends on the pitch of the sensor. Conventional methods require the preparation of a special position sensor and the need to incorporate a shutter into the optical path. There were also disadvantages such as the complexity of the device.

[Means for solving problems]

The shearing interferometer according to the present invention was developed to solve the above-mentioned problems, and the measurement wavefront is divided into two by dividing the light beam reflected by the test surface into two, and these measurement wavefronts are mutually separated. In a shearing interferometer that causes interference by laterally shifting, a pattern for measuring the amount of deviation between two measurement wavefronts is placed in the incident optical path, and the oblique shadow of the pattern formed on the surface to be measured is projected onto the image sensor. It is designed to form an image.

[Effect]

In the present invention, since the oblique shadow of the pattern is imaged on the image sensor, this pattern is analyzed by a method using lateral shift, a method using an autocorrelation function, a method using Moiré fringe analysis, etc. The amount of deviation can be substantially measured.

〔Example〕

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

FIG. 1 is a diagram of an optical system showing an embodiment of a shearing interferometer according to the present invention. In this embodiment, from the light source 1 to the surface to be inspected S.
A pattern 14, which will be described later, is arranged at an appropriate position 2, for example, between the collimator lens 4 and the condensing lens 5, in the optical path up to, that is, in the incident optical path. 2 divergent lenses, 3 is a beam splitter, 6 is the object to be measured, 7 is a beam splitter that splits the beam from beam splitter 3 into two beams, 8 is a total reflection mirror, 9 is a total reflection mirror 8 in the optical axis direction (in the direction of the arrow) A drive device such as a piezo element to move Ic, 10 is a total reflection mirror, 11 and 12 are imaging lenses,
13 is an image sensor such as a CCD.

As the light source 1, a He-Nc laser, for example, is used as one that emits visible, infrared, or ultraviolet light beams with good coherence. The total reflection mirror 10 gives a horizontal shift to the reflected light reflected by the splitter 7 out of the two luminous fluxes split by the beam splitter 7, and the total reflection mirror 10 gives a horizontal shift to the reflected light reflected by the beam splitter 7, and the total reflection mirror 10 gives a horizontal shift to the reflected light reflected by the beam splitter 7. The device is tilted by a small angle θ from the orthogonal direction, and is configured to be tilted in two orthogonal directions by being held by an appropriate tilting mechanism (not shown) VC. .

In a shearing interferometer with such a configuration,
The light emitted from the light source 1 passes through the collimator lens 4 via the diverging lens 2 and the beam splitter 3, and is converted into parallel light rays parallel to the optical axis.Then, the light is focused by the condensing lens 5 to the focal point P, and Irradiate the surface S. The wavefront of the light beam reflected here has a shape according to the form of the surface S to be inspected. Then, this light flux is returned to the condensing lens 5. Collimator lens 4
The beam then passes through the beam splitter 3, is split into two by the second beam splitter 7, and is partially transmitted and partially reflected. The transmitted light is reflected by the total reflection mirror 8 and returns to the beam splitter 7 again.

Further, the reflected light reflected by the beam splitter 7 is reflected by the total reflection mirror 10 and returns to the splitter 7. At this time, since the total reflection mirror 10 is tilted by a small angle θ as described above, when the light beam incident on the total reflection mirror 10 as shown by the solid line is reflected by the reflection mirror 10, this reflected light beam is as shown by the broken line. Go ahead.

There will be a slight deviation from the light flux at the time of incidence. therefore,
The two beams are reflected by each total reflection mirror 8, 10, returned to the beam splitter 7, and superimposed again.
This will cause interference.

In other words, the beam splitter 7 splits the light beam from the test surface S into two, and the measurement wavefront of this light beam is divided into two, reflected by the total reflection mirror 8.10, and then superimposed again by the beam splitter T. Both measured wavefronts are 'd
is shared in the horizontal direction, and Δd between and is the amount of shearing. Then, the superimposed light beams are transferred to the imaging lens 11
．． 12 forms an image on the image sensor 13 to form interference fringes. After the interference fringes are converted into electrical signals as image information by the image sensor 13, the surface shape of the surface S to be inspected is measured by performing fringe analysis using a computer or the like.

In order to measure surface shapes such as 1-spherical and aspherical surfaces,
It is essential to measure the amount of shear in two orthogonal directions of both measurement wavefronts that are shared with each other.

In other words, as shown in Fig. 2, a certain wavefront W(,a) is
Let W(,r+Δd) be the wavefront that is laterally shifted by Δd in the direction. These two wavefronts W(r), W(r+Δd)
When superimposed, there is one phase difference Δgin) ΔWCx) = 1) -W (jC+Δd) -=-
- Interference fringes corresponding to song (1) are generated, and ΔW←) is obtained by observing and analyzing these interference fringes with the image sensor 13.

ΔW←) is differential information of W←), and by integrating this, the original wavefront W←) is obtained.

In addition, in FIG.

Dechiru.

As is clear from the above equation (1), the share amount Δd is ΔW
(, r). Therefore, Δ
In order to obtain W(r), it is necessary to measure Δd more accurately. However, conventionally, this value cannot be directly determined for the reasons mentioned above, resulting in inaccurate measurements.

Therefore, as described above, the present invention arranges the pattern 14 between the collimator lens 4 and the condensing lens 5, and images the oblique shadow of the pattern 14 formed on the surface S to be inspected on the imaging element 13. The actual deviation amount (Δd) between the two measured wavefronts is measured by analyzing this using a method described later.

As shown in FIG. 3, the patterns 14 include an edge pattern (figure a) and a random pattern (figure b).

A concentric regular pattern (Fig. 0) is used.

Methods for detecting the share amount Δd by analyzing P, #-, 714 include (1) a method by horizontally shifting the pattern;

Examples include (2) a method using an autocorrelation function, and (3) a method using moiré fringe analysis, and the methods, effects, etc. will be described below.

(1) Method using horizontal shift of the pattern The edge pattern 14 shown in FIG. ) and move it linearly in one direction.

The output of the image sensor 13 corresponding to the edge portion of the pattern 14 fluctuates as shown in FIG. Position a of the variation straight line is the position where the edge is on the image sensor 13, and 6 is the position where the edge disappears from the image sensor 13. Therefore,
If a and one point with respect to each of the two shifted patterns are detected with high accuracy, the actual amount of share of both measurement wavefronts can be calculated from the amount of shift of the position.

(2) Method using autocorrelation function The pattern 14 inserted into the incident optical path is the total reflection mirror 10
Similarly to the light flux reflected by the surface to be inspected, the light beams are shifted laterally by Δd, and after being superimposed again by the beam splitter 7, an image is formed on the image sensor 13. Therefore, when the random pattern shown in FIG. 3(b) is used as the pattern 14, this random pattern is uniformly shifted laterally by Δd, and the overlapping patterns are displayed on the image sensor 1.
Formed on 3. Therefore, by calculating the autocorrelation function of that pattern, it is possible to calculate the share number Δd with an accuracy of about 1/20 of the pitch.

(3) Moire fringe analysis method When a regular pattern is inserted into the incident optical path, moire fringes are formed on the image sensor 13 depending on the pattern depending on the amount of lateral shift. The roughness of the moire fringes changes depending on the fineness of the pattern, and coarse fringes occur with a small amount of shear. Therefore, if the concentric pattern 14 shown in Fig. 3(c) is used, Fig. 5(a), (b),
Moire fringes shown in (c) are obtained.

At this time, since the stripes created radially increase in proportion to the amount of misalignment, it is possible to measure minute shifts by counting them. Note that regular patterns are not limited to concentric circles. , radial and other patterns are also possible.

In this way, by implementing any one of the three methods described above using the pattern 14, it becomes possible to measure the share amount Δd with greater accuracy than the conventional technique. Further, since it is sufficient to simply arrange the pattern 14 in the incident optical path, the structure is also quite simple.

〔Effect of the invention〕

As explained above, the shearing interferometer according to the present invention arranges a pattern in the incident optical path, divides the pattern into two, shifts the pattern laterally, forms mutually overlapping images on the image sensor, Since the actual shear amount is calculated or measured from the image, the shear amount can be measured with high precision with a simple structure, and therefore the measurement accuracy of the interferometer can be improved.

[Brief explanation of drawings]

Fig. 1 is a diagram of an optical system showing an embodiment of a shearing interferometer according to the present invention, Fig. 2 is a diagram for explaining the shearing interferometry, and Fig. 3 (-), (b), ( c) is an example diagram of the pattern, and Figure 4 is the pattern transfer i1J'fA.
A diagram showing the relationship between and the light amount of the image sensor, FIG. 5(a),
(b). (C) is a diagram showing moire fringes, Figures 6 (a) and (b)
FIG. 2 is a diagram showing an interference fringe image for explaining inconveniences in specular surface measurement. 161@@Light source 3 m a * e beam splitter. 4-・Kenshi collimator lens, 5... Condensing lens,
6...Object to be measured 7 m@m s beam splitter, 8.10...Total reflection mirror, 13...Image sensor, S...Test surface, Δd...Share amount. Figure 3 (G)' (b) (C) Figure 4 Tsuma turn movement amount

Claims (1)

[Claims] In a shearing interferometer, the measurement wavefront is divided into two by dividing the light beam reflected by the test surface into two, and these measurement wavefronts are laterally shifted and interfere with each other. 1. A shearing interferometer characterized in that a pattern for measuring is arranged, and an oblique shadow of the pattern formed on the test surface is imaged on an image sensor.