JPH0447243B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface shape measuring method using interference, which allows the surface precision of any location on a surface to be measured to be measured with high precision.

Conventionally, repeating reflection interferometers such as Fabry-Perot interferometers have often been used to measure the surface accuracy of a surface to be inspected. In this type of interferometer, the air distance between the two surfaces represented by the interference fringes is a contour line of 1/2 the wavelength. In order to read interference fringes with high precision, it is necessary to detect even the slightest displacement of the interference fringes from a straight line. It is necessary to widen the interval between interference fringes. However, in this case, the shape of the region between the interference fringes cannot be measured, and even if the measurement accuracy is high, the measurement density is low.

Therefore, the present applicant has already proposed a planar measurement interference device as shown in FIG. 1 in order to increase the measurement density of the surface to be inspected. That is, the two reflecting surfaces 1 and 2 that can transmit light, a test surface and a reference surface that has been polished to a sufficiently high precision, are made to face each other and are fixed at a small angle. A laser source 3 is disposed on one side of the laser beam, and the emitted laser beam is guided as a point light source by a lens 4 to a collimator lens 5, and the collimator lens 5 causes the laser beam to enter the reflecting surfaces 1 and 2 as a parallel beam of light. Then, the laser beam is repeatedly reflected between the two reflective surfaces 1 and 2, passes through the telephoto optical system 6, and forms an image of a high finesse interference fringe pattern on the photosensitive material 7 for exposure. Next, while maintaining the relationship between the reflective surfaces 1 and 2, the reflective surfaces 1 and 2 are rotated in the direction of arrow A with respect to the optical axis of the laser beam, the optical system 6 is fixed, and the photosensitive material 7 is , 2 and arrow B in the opposite direction
The interference fringe pattern is superimposed on the photosensitive material 7 and recorded. By repeating this operation, a large number of interference fringes are exposed between the first recorded interference fringes, and for example, as shown in Figure 2, a pattern is formed as if interference fringes S were generated at every 1/20th of the wavelength. This allows for highly accurate reading of surface shapes. However, although this apparatus can measure the surface shape with high precision, it has drawbacks such as the trouble of performing multiple exposures in a short period of time and the time required for developing and analyzing the photosensitive material 7.

An object of the present invention is to provide a method for measuring a surface shape by interference, which eliminates the drawbacks of the conventional example described above, and at the same time makes it possible to measure the surface accuracy of any position on a test surface with high precision in real time. The gist of this is that measurement light, which is a parallel beam, is incident on a reference surface and a test surface that are arranged approximately parallel to each other from a direction that intersects the test surface, and while the air distance between the reference surface and test surface is fixed, the measurement light is The reference surface and the surface to be measured are rotated in a direction in which the inclinations of the reference surface and the surface to be measured with respect to the optical axis are changed, and the interference fringe output obtained by the transmitted light is projected onto the photosensitive surface. From the large rotation angle of the interference fringe intensity obtained on the photosensitive surface corresponding to the reference position and the large rotation angle of the interference fringe intensity obtained on the photosensitive surface corresponding to an arbitrary measurement position on the test surface, This method of measuring a surface shape by interference is characterized in that a difference in level between the measurement position and the reference position of the test surface is determined based on the distance from the reference position of the test surface to the reference surface.

Next, the present invention will be explained in detail based on the embodiments shown in FIG. 3 and below.

FIG. 3 is a block diagram of an apparatus for implementing the method according to the present invention, which has substantially the same configuration as the apparatus shown in FIG. 1. Note that the same reference numerals as those in FIG. 1 indicate the same members. Reflective surface 1, 2
is placed on the rotatable sample stage 8 while being maintained at a predetermined air interval as described above, so that the rotation angle of this sample stage 8 can be read. Further, a photosensitive surface 10 is placed on the photosensitive surface stage 9, and this photosensitive surface 10 is made of, for example, a photodiode array, and makes it possible to detect the light intensity at a large number of locations.

Here, in order to make it easier to understand, the measurement position _is
Assume that there is a minute step Δ on the surface of X ₂ as schematically shown in Figure 4, and the distance between the two reflective surfaces 1 and 2 at the reference position X ₁ on the reflective surface 1 is d. do. Now, consider two monochromatic lights I ₁ and I ₂ emitted from the light source 3 and incident on the reflection surface 1 at positions X ₁ and X ₂ at an angle θ with respect to the reflection surface 1. Monochromatic light I ₁ and
I ₂ is repeatedly reflected at a point X 1 at a distance d and at a point X ₂ at a distance d - Δ between the reflecting surfaces ₁ and 2, causing interference, and passes through the unit of the reflecting surfaces 1 and 2. When measuring the intensity of the interference light at this time while rotating the sample stage 8 in the direction of arrow A, that is, while changing the angle θ, it is found that since there is a step Δ on the reflecting surface 1, interference due to the monochromatic lights I ₁ and I ₂ occurs. The intensity changes of the lights S ₁ and S ₂ are not equal, and as shown in Figure 5, each rotation θ
The peak will shift to a different position.

Locations where the intensity of interference lights S ₁ and S ₂ changes rapidly,
For example, the rotation angle of the sample stage 8 when it reaches 1/2 of the tail of each peak is θ ₁ ,
If θ ₂ and the distance between the reflecting surfaces 1 and 2 along the optical axis of the monochromatic lights I ₁ and I ₂ at that time are l ₁ and l ₂ , then d=l ₁ cosθ ₁ ...(1) d- Δ=l ₂ cosθ ₂ ...(2), and the peaks occur together when l ₁ = l ₂ , so from equations (1) and (2), the relationship between the interval d and the step difference Δ is expressed by the following equation. It is expressed as

d/cosθ ₁ =(d−Δ)/cosθ ₂ (3) Therefore, the step difference Δ can be obtained as follows.

Δ=d(cosθ ₁ −cosθ ₂ )/cosθ ₁ ...(4) The above explanation is about the unevenness at the two points X ₁ and X ₂ on the reflective surface 1, but in the same way, the photosensitive surface 10
Measure many points at the same time and determine the reference surface position.
By calculating the amount of unevenness from X ₁ using equation (4), the shape of each point on the reflective surface 1, which is the surface to be inspected, can be measured. It has been confirmed that by doing so, the measurement accuracy can be improved to about 1/100 of the wavelength of the measurement light.

In addition, by rotating the sample stage 8,
Since distortion occurs in the image formed on the photosensitive surface 10, accurate measurement values can be obtained by correcting the distortion by rotating the photosensitive surface stage 9 by the same angle in the direction of arrow B opposite to the sample stage 8. be able to. If the rotation angle is small, the optical system 6 may be omitted, and in this case, the rotation of the photosensitive surface 10 is also unnecessary.

The shape of the surface to be measured can be measured with the simple operations explained above.
It is also possible to use a two-dimensional photodiode array as zero, input the output of each address of the diode array into a microcomputer, calculate using equation (4), and calculate the shape of the test surface in real time. . In addition, the distance d can be determined by rotating the sample stage 8, focusing on the specific case in which it is to be determined, and obtaining an angle θ from the two peaks of the interference lights S ₃ and S ₄ as shown in FIG. ₃ and θ ₄ , we get
The spacing d can be determined. That is, if the wavelength of the light source 3 is λ, the distance d can be obtained as follows: d=λ/{2(cosθ ₄ −cosθ ₃ )} (3).

Furthermore, in the conventional repeating interference measurement method, in order to form repeating interference fringes with high finesse, the surfaces of the reflective surfaces 1 and 2 had to be highly reflective and have good transmittance. However, in the present invention, if the part where the interference light S changes most rapidly, for example, 1/2 of the peak position as shown in FIG. In both cases, the angle can be measured with good accuracy even with a commonly used reflective film such as an aluminum film.

As explained above, the method of measuring a surface shape by interference according to the present invention rotates the test surface and the reference surface with respect to the optical axis of the measurement light while fixing the test surface and the reference surface substantially parallel to each other. The shape of the surface to be measured can be measured with high precision by a simple operation of detecting the change in light intensity at each position in comparison with the rotation angle and reading the angle that is, for example, 1/2 of the peak of the interference fringes. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a conventional example, FIG. 2 is an explanatory diagram of interference fringes obtained by a conventional measuring device, and FIG. FIG. 3 is a block diagram of an apparatus for implementing this method, FIG. 4 is a schematic layout of reflective surfaces, and FIGS. 5 and 6 are explanatory diagrams of the measuring method. Reference numerals 1 and 2 are reflective surfaces, 3 is a light source, 5 is a collimator lens, 6 is an imaging telephoto optical system, 8 is a sample stage, 9 is a photosensitive surface stage, and 10 is a photosensitive surface.

Claims (1)

[Scope of Claims] 1. Measurement light, which is a parallel beam, is incident on a reference surface and a test surface that are arranged substantially parallel to each other from a direction intersecting the test surface, and the air distance between the reference surface and the test surface is fixed. is rotated with respect to the optical axis of the measurement light in a direction that changes the inclination of the reference surface and the test surface with respect to the optical axis, and the interference fringe output obtained by the transmitted light is projected onto the photosensitive surface. A large rotation angle of the interference fringe intensity obtained on the photosensitive surface corresponding to the reference position on the surface and a large rotation angle of the interference fringe intensity obtained on the photosensitive surface corresponding to an arbitrary measurement position on the surface to be measured. A method for measuring a surface shape by interference, characterized in that, based on the distance from the reference position of the surface to be inspected to the reference surface, a step difference between the measurement position and the reference position is determined. 2 The distance from the reference position of the test surface to the reference surface is d, the step difference between the measurement position and the reference position is Δ, the rotation angle at which the interference fringe intensity obtained from the reference position is large is θ ₁ , and the interference obtained from the measurement position is 2. The method of measuring a surface shape by interference according to claim 1, wherein the step difference is determined as Δ=d( _cosθ1 - _cosθ2 )/ _cosθ1 , when the similar angle of the stripes is _θ2 . 3. The method of measuring a surface shape by interference according to claim 1, wherein distortion of the obtained interference fringes is removed by rotating the photosensitive surface in a direction opposite to that of the test surface and the reference surface.