WO2011138055A1

WO2011138055A1 - Method for determining the topography of a surface of an object

Info

Publication number: WO2011138055A1
Application number: PCT/EP2011/002309
Authority: WO
Inventors: Irina Lazareva; Andreas Nutsch
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V.
Priority date: 2010-05-07
Filing date: 2011-05-06
Publication date: 2011-11-10
Also published as: DE102010019668B4; DE102010019668A1

Abstract

The invention relates to a method for the unambiguous and reference-free determination of the topography of a surface (20) of an object (11), wherein a pattern is projected onto the surface (20) by a collimated beam of light and an image (26) of an intensity distribution of the reflected pencil of rays is detected in a plane (24) which is arranged at a distance from the surface (20). The method is characterised in that at least one further image (27a, 27b) of the intensity distribution of the pencil of rays that is reflected by the surface (20) is detected at a second distance or at a changed angle of inclination of the surface (20) with respect to the plane (24). Then the topography of the surface (20) is calculated from the comparison of the two images (26, 27a, 27b) taking into consideration the different distances or angles of inclination. The method does not require a reference surface and the adjustment problems associated therewith and can be carried out quickly and at low cost.

Description

Method for determining the topography of a surface of an object

Technical application

The present invention relates to a method for the clear and reference-free determination of

Topography of an optical radiation-reflecting surface of an object, in which a pattern with a collimated light beam is projected onto the surface and an intensity distribution of the light beam reflected by the surface is detected in a plane which is at a distance from the surface.

The determination of the surface topography of objects is mainly in technical areas

necessary in which the surface quality,

In particular, the flatness or structure of the surface plays an essential role for the application or for further processing steps. Examples include the surfaces of wafers in semiconductor technology or the surface quality of a workpiece after polishing.

State of the art

Topographies of reflective surfaces can be determined by means of plane wavefronts. For this purpose, the surface is illuminated with a flat wavefront. Deviations of the reflecting surface from an ideal plane are reflected in the reflection Wavefront introduced. The reflected wavefront is then analyzed for these changes.

In the so-called Makyoh method, an image of the intensity distribution of a parallel light beam reflected on the surface is detected in a plane which is at a distance from the surface. From this intensity distribution a deviation of the surface topography from an ideal plane can be determined. An example of such a technique is shown by F. Riesz "A quantitative approach to Makyoh (magic-mirror) topography", Journal of Crystal Growth 210 (2000), pages 370-374. US Pat. No. 6,545,764 B1 discloses a modified one

Technique for determining the topography of a surface is known in which two images of the intensity distribution at two different distances from the surface are detected and evaluated. The evaluation is carried out by a complex iterative process.

From DE 602 12 987 T2 another method for determining the topography of a surface of an object is known. In this technique, a pattern is projected onto the surface by means of a collimated light beam and the intensity distribution of the light beam reflected at the surface is detected at a distance from the surface. By using a flat reference surface instead of the surface of the object, another image is recorded in the same way. By comparing the two images, the surface topography of the object can then be determined on the basis of the distortion of the pattern. This is illustrated by the figure la, which the

Surface, the reference plane and the resulting beam path of a single light beam in the reflection shows. The dashed line represents the reference surface 21, the oblique solid line shows an example of an uneven surface 20 of the object to be examined. In the figure, an incident light beam 22 of the light beam is shown, which, for example, corresponds to a reference point of the pattern. After reflection at the surface, the intensity distribution of the light beam is detected in the plane 24 indicated in the figure, which has a distance L from the surface. The distance Δd between the black and white points in the figure represents the deviation of the reflected light beam from the perpendicularly incident light beam in the detection plane. If the viewed light beam corresponds to a reference point in the pattern, the distance Ad corresponds to the distance of the imaged reference points 23, 25 in the two pictures. With the aid of the distance L between the surface and the detection plane, the local inclination angle α of the surface of the object relative to the reference plane can then be determined:

ad

tgla =.

L

With the help of this equation can then

Topography profile of the surface of the object can be calculated.

However, the implementation of this method is not free of problems. Thus, the accuracy of the The topography is limited by the quality or evenness of the reference surface.

For optimal representation of the images, the surface of the object and the surface of the reference mirror must also be aligned exactly perpendicular to the incident light bundle. During the adjustment, however, there is often a slight deviation of the normals of the reference surface and the object surface from the incident light rays. Figure lb illustrates a case in which the reference surface 21 a

Inclination of a _Ref ^■ * ■ 0 has. The shift of the reference point between the two images is therefore smaller or greater by 5 _Ref than that shown in the ideal case of Figure la. This leads to a

Error of 5 _Ref / A when calculating the topography profile.

For an optimal calculation of the topography, the surface of the object and the reference

Surface also lie in the same plane. FIG. 1c illustrates a case in which the reference and object surfaces have different distances from the detection plane, that is to say they lie at different levels in height. This leads to a different distance L for the two measurements. The difference in the planes ÄL = LL _Ref changes the shift between the reference points in the images from the ideal case of Figure 1a to 5 _Ref - This also leads to an error of 5 _Ref / Ad in the calculation of the topography profile. Another problem with using only an image of the intensity distribution of a light beam reflected at the surface of the object is that ambiguities may arise due to the large amount of information in the image. FIG. 2 shows an example in which two different ones

Surface profiles 20a, 20b and four light beams 22, corresponding to four reference points, are shown. The upper part of the figure shows the position of the imaged reference points 25 in the captured image 26, which in terms of both surfaces with respect to. the location of the four reference points is identical. In this constellation, the two inner light beams each deviate from the vertical in such a way that they coincide after reflection for both surfaces 20a, 20b in the same point of the detection plane 24. Therefore, the inclination of the object surface can be misinterpreted from the image and thus the topography can be calculated incorrectly.

Both an undesirable tilting of the object surface or the reference surface and a different height of the object and reference surface can lead to errors in the calculation of the topography profile. Furthermore, the quality of the reference surface or of the mirror used for this purpose is a limiting factor with regard to the accuracy of the determination of the topography. A

Ambiguity in the picture can lead to a misinterpretation of the topography. To some extent, these problems can only be solved by using a flat reference surface with as little as possible

Deviations from an ideal level are used and the Adjustment of the object surface and the reference surface are each carried out with very high accuracy. However, this is time consuming and expensive. The object of the present invention is to provide a method for determining the topography of a surface of an object, with which these problems are largely avoided, so that the

Accuracy of the measurement no longer depends on the surface quality of a reference surface and as exact as possible adjustment and ambiguity in the evaluation can be avoided.

Presentation of the invention

The object is achieved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be the following

Description and the exemplary embodiments.

In the proposed method, a pattern with a collimated light beam to the

projecting surface of the object and an intensity distribution or an intensity profile of the light beam reflected at the surface is detected in a plane as an image having a first distance from the surface. The collimated light beam is preferably directed at least approximately perpendicular to the surface. The method is characterized in that at least a second image of the

Intensity distribution of the reflected light beam either in at least one further (second) distance from the surface, in particular by image acquisition in a further plane or by optical imaging of another plane having the second distance from the surface, or at least one

Angle of inclination or changed inclination angle of the surface to the plane is detected and from a

Comparison of the pictures taking into account the

Distances or tilt angle the topography of the

Surface is determined.

By acquiring at least two images of the object surface at different distances or at different inclination or tilting angles of the surface, no reference surface is needed for determining the topography. This avoids the problems of mutual adjustment of the two surfaces and improves the accuracy in determining the topography of the object surface. Furthermore, the clear calculation of the topography is possible by these at least two images.

The accuracy of determining the topography of the surface also depends on the structure of the pattern used. Preferably, a dot screen is used as

Pattern used, with the individual points of the

Point grid serve as reference points, the relative shift in the at least two images is determined in order to calculate the topography of the surface. Of course, however, other, in particular more complex, patterns or a

Combination of dots and other geometric

Use shapes as a pattern. The pattern will be there preferably generated via a structured mask, which is arranged in the beam path of the light beam.

The different distances for the detection of the intensity distribution of the light beam can, for example, mechanically by moving the object along the beam path or by moving the for the

Image capture unit used, for example, a CCD sensor or a camera set. In a further advantageous embodiment, a camera with a

Camera lens used. In this case, the different distances can be different

Focusing the camera lens are set. With the camera lens, a different plane is imaged on the image sensor at different focussing and thus the distance of the detection plane to the surface is changed. A different inclination of the surface can be achieved by gradually tilting the object. Here, for example, an image without tilting, d. H. in a plane parallel to the detection plane surface and the other image at a correspondingly inclined by a known angle surface. Of course, both images can be detected at a slightly inclined surface, but with different angles of inclination. The procedure is not based on the capture of two

Restrict images at two distances or two tilt angles. Rather, you can also have more pictures recorded at further distances or angles of inclination and used for the evaluation.

The proposed method is suitable for surface profiling of objects with reflective surfaces. By renouncing the

The use of a reference surface as well as the lower adjustment requirements simplify the procedure and improve the accuracy of determining the topography. In this case, all surfaces reflecting optical radiation, such as, for example, semiconductor wafers, reflective metal surfaces or mirrors, can be measured. The method is also suitable for the measurement of structured surfaces, such as.

Surfaces of integrated circuits or MEMS devices.

Brief description of the drawings

The proposed method will be explained in more detail using exemplary embodiments in conjunction with the drawings. Hereby show:

Fig. 1 is a schematic representation of

Conditions in the measurement according to the prior art;

FIG. 2 shows a further schematic representation of the conditions in the measurement according to the prior art; FIG.

Fig. 3 shows an example of the conditions in the proposed method;

4 shows another example of the conditions in the proposed method; 5 shows another example of the conditions in the proposed method;

6 shows an exemplary arrangement for carrying out the proposed method; and

Fig. 7 shows another example of the conditions in the proposed method.

Ways to carry out the invention

The problems occurring in the known method of the prior art have already been explained in more detail with reference to Figures 1 and 2 in the introduction to the description, so that these figures will not be discussed in more detail here.

In one embodiment of the proposed method, two images of the intensity distribution of the light beam reflected at the object surface are recorded at a different distance L _{1 #} L ₂ to the surface with a camera. By this technique, the ambiguity indicated in connection with the prior art method in FIG. 2 becomes

avoided. FIG. 3 shows the same situation with the two different surfaces 20a, 20b, as illustrated in connection with the prior art method in FIG. By recording two images of the same surface at different distances Li, L ₂ , each individual light beam of the light beam can be traced, as the

Difference between the two images shows the direction of displacement of the respective reflected light beam or the corresponding reference point. In The recorded images 26 of the two surfaces 20a, 20b at the distance L ₂ , the points of the light beams or reference points in the detection plane are identical, as indicated in the uppermost dashed line of Figure 3. In the two images 27a, 27b of the two surfaces, which were recorded at the distance Li, these points are in the

Entry level separated. This is indicated by the two underlying dashed lines, which represent the two images 27a, 27b at the distance Li. Thus, the determination of the topography is unique by the capture of two images at different distances L. A reference surface is not required.

The change of the distance L takes place here either by changing the focus of the camera lens or by changing the working distance, d. H. the distance between the surface and the camera lens or camera.

In the process alternative, in which the images are recorded at different distances to the object surface, can also be an undesirable

Tilting of the object or the surface of the object can be determined and eliminated. Such unwanted tilting of the surface 20 of the object is indicated in FIG. 4a. When comparing the two images with the different distances Li and L ₂ , the object tilting is shown as a systematic shift of all points of the reflected light beams or of the projected pattern in the detection plane. This systematic shift can be in known Identify patterns and thus in determining the

Outline topography. For this purpose, FIG. 4 a shows the tilting or tilt angle a 'and the respectively measurable displacement of the reference points between the two images by Ad ^' i, Ad ' ₂ and Ad' ₃ for the three light beams or reference points shown. Of the

Distance of the reference points was referred to in this figure with dzeiie. After determining the systematic shift of all points of the reflected light rays or reference points in the detection plane and after a correction of the data by the

Obj ectverkippung the topography profile can be calculated by the remaining shift of the reference points by the distances Adi, Äd ₂ and Äd ₃ , which is indicated in Figure 4b. The calculation of the topography profile is done with the following equations

These equations can be understood with reference to the schematic representation of FIG. Here, AL = L ₂ -L x and d _Ze ii _{e represent} the distance between two adjacent incident light beams or reference points. From the shift Äd of the points of the light beams and the reference points in the

Detection plane and the difference ÄL of the different distances of the detection plane to the surface, the angle between the vertically incident and the reflected light rays is calculated. This angle is twice the local tilt angle of the object surface a. With the aid of the determined local inclination angle α and the distance to the surface Li or L ₂ and the distance between two adjacent light beams or reference points d _Ze iie the topography profile h is calculated according to the above second equation.

The greater the difference AL of the detection distances of two images, the greater the shift Ad of the points of the light beams or of the reference points in the detection plane. The parameter Ad corresponds to the parameter Ad _P i _X also mentioned below. Both parameters differ only in their units (unit of Ad: meter, unit of Ad _P i _x : pixel). The parameter r _P i _x - _{m transforms} Ad _P i _x to Ad and back. The larger the parameter Ad, the smaller the relative error 5 _P i _x / Ad _pi

The parameter _{_δ} ρ ι χ = 0.5 pixels and Ad _pix correspond to the camera error, which is two times smaller than the maximum resolution of the camera d _ra = 1 pixel, and

Displacement of the points of light rays in the

Detection plane. The parameter r _P i _x - _m represents the

lateral calibration of the distances d on the object surface. Therefore, for the method proposed here, the greater the difference of the distances AL, the more accurate the measurement.

Figure 6 shows schematically an example of an arrangement for carrying out the proposed Process. In this case, the structure of a mask 2 is projected onto the surface of the object 11 with a light source 1, for example an LED or an LED array. The light beam emitted by the light source 1 is collimated by a collimator mirror 4 and falls perpendicular to the surface of the object 11, which is fixed in a corresponding holder 5. Via a beam splitter 3, the light beam reflected by the surface of the object 11 is directed onto a CCD camera 8, which has a camera lens 6. The captured with the camera images are processed in a computer 10. For this purpose, a suitable evaluation algorithm can be used, which on the one hand the

Identified reference points in the recorded images and on the other hand calculated from the displacement of these reference points in the respective images, the topography of the surface according to the above equations. The computer is connected to a controller 9, which controls the motorized holder 7 for the camera lens 6 in this example. The

Control takes place in such a way that, for a measurement of the surface of the object 11, the focus of the camera lens 6 is changed in order to capture at least two images at at least two different focus settings. The two images are then evaluated with the respective parameters Li and L ₂ known from the different focus settings and the local inclination angles of the surface are determined. Then a possible systematic tilt of the object surface from the data

eliminated. Subsequently, the topography of the

Surface calculated from the corrected data. This is done by the evaluation program used in the computer.

In an alternative embodiment, the

Object holder 5 driven to move the sample along the light beam. Again, be here

recorded at least two images with different distances of the surface to the camera 8. The two images are also evaluated here in the same way as was already explained in connection with changing the focus settings of the camera lens.

Such an arrangement can also be used in a second alternative method, in which the surface of the object is tilted stepwise, wherein at least two images at different angles of inclination, for example, inclination angle a _x and inclination angle a ₂ (where αι or a _{2 can} also be 0) to be recorded. For this purpose, the object holder 5 must allow a defined tilting of the object.

Such a stepwise tilting of the object holder 5 leads to a systematic displacement of the points of the light beams or the reference points in the detection plane. This shift in the two images is evaluated in the same way as when capturing the images with different ones

Intervals. Correspondingly small tilting of the surface of the object is necessary since otherwise no complete image is detected. The shift of the points of the light rays or reference points in the

Acquisition level is significantly lower than in the first alternative method. A motorized change of the focus of the focus lens and the Controller 9 are not required here. FIG. 7 shows schematically the conditions during the recording of the two images with the same distance Li but different tilting of the surface of the object.

The proposed method allows the

Measurement and determination of the topography of an object surface without a reference surface (eg

Mirror) . As a result, the accuracy of determining the topography is no longer limited by the quality of a reference surface. The accuracy of

Determination of the topography is - apart from the resolution of the camera used - only dependent on the quality of the optical components of the measuring arrangement and the accuracy of the determination of the instrumental parameters Li and L ₂ or a _x and a ₂ . In the first method alternative with different distances Li and L ₂ , the determination of the topography is more accurate, the greater the difference ΔL of the distances. Furthermore, errors caused by the quality of the optical components and the inaccuracy in the determination of the distances L, by two

compensated successive measurements. By

Eliminating the measurement of a reference surface, the process can be performed faster. Furthermore, in the first method alternative, an undesired inclination of the surface to the optical axis can be determined and corrected in a simple manner. LIST OF REFERENCE NUMBERS

I light source

2 mask

3 beam splitters

4 collimator

5 Object holder

6 camera obj ective

7 motorized holder

8 CCD camera

9 Camera Lens Controller

10 computers

II object

20, 20a, 20b surface of the object

21 reference surface

22 light beam

23 Reference point in the reference image

24 recording level

25 Reference point in the object image

26 image of both surfaces

27a Image of the first surface

27b Image of the second surface

Claims

claims

Method for determining the topography of a surface (20) of an object (11) in which a pattern is projected onto the surface (20) with a collimated light beam and an image (26) of an intensity distribution of the light beam reflected by the surface (20) in one Level (24) is detected, which has a first distance from the surface (20),

characterized,

in that at least one further image (27a, 27b) of the intensity distribution of the light bundle reflected by the surface (20) is arranged at at least one further distance from the surface (20) or at least one inclination angle or changed inclination angle of the surface (20) with respect to the plane (24 ) is detected,

and the topography from a comparison of the at least two images (26, 27a, 27b) below

Considering the distances or inclination angle is determined.

Method according to claim 1,

characterized ,

that a camera (8) with a camera lens (6) is used for the image capture and the change of the distance for the detection of the

Intensity distribution of the light beam reflected from the surface (20) by changing a Focusing the camera lens (6) takes place.

Method according to claim 1,

characterized,

in that the change in the distance for the detection of the intensity distribution of the light beam reflected by the surface (20) is effected by mechanical displacement of the object (11) and / or an image acquisition unit used for image acquisition.

Method according to one of claims 1 to 3,

characterized,

that in case of capturing images at

different distances an undesirable

Inclination of the surface (20) relative to the plane (24) by comparing the images (26, 27a, 27b) is determined and excluded in determining the topography.

Method according to one of claims 1 to 4,

characterized,

in that the projected pattern has reference points (25) and the topography is determined from a relative shift of the reference points (25) of the pattern in the images (26, 27a, 27b).