EP1402228A1

EP1402228A1 - Interferometer system, method for recording an interferogram and method for providing and manufacturing an object with a specified surface

Info

Publication number: EP1402228A1
Application number: EP02735425A
Authority: EP
Inventors: Stefan Schulte; Bernd DÖRBAND; Henriette Müller; Wolfgang KÄHLER
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2001-06-27
Filing date: 2002-06-26
Publication date: 2004-03-31
Also published as: WO2003002933A1; JP2004530905A; DE10130902A1; US20040190002A1; US7002694B2

Abstract

The invention relates to a interferometer system, comprising a reference surface, a mounting for an object which provides an object surface, a radiation source for emitting radiation of an adjustable frequency onto the reference and object surfaces, a positioning radiation detector and a control mechanism for adjusting a plurality of various frequencies of the radiation emitted by the radiation source. Said system also comprises an integrator for the location-dependent determination of the interference pattern which is superimposed on the radiation detector at various frequencies. The invention also relates to a method for recording an interferogram, a method for providing an object with a specified surface and a method for manufacturing the same.

Description

Interferometer system, method for recording an interferogram and method for providing and producing an object with a target surface

The present invention relates to an interferometer system and a method for recording an interferogram. The interferometer system and the method are preferably used to determine topological properties of an object surface from the interferogram by evaluating the recorded interferogram.

The invention further relates to a method for providing and producing an object with a target surface, deviations between the target surface and an actual surface of the object being determined from an interferogram and the object being made available or reworked depending on these deviations.

Interferometer systems are traditionally used, among other things, to determine the topological properties of an object surface. For this purpose, for example, a known reference surface and an object surface to be measured are illuminated with coherent radiation, and an object wave field reflected by the object surface and a reference wave field reflected by the reference surface are, for example, superimposed on a screen, so that an interference pattern arises there. Depending on the location, a difference in the optical path from reference surface to screen and object surface to screen can be determined from the interference pattern, from which topological differences between the object surface and the reference surface can then be determined. Two techniques are used to determine such path differences with an interferometer system:

A first approach is the so-called fringe pattern interferometry ("FPI"), in which an optical path difference between two wave fronts is determined from the positions of stripe centers of an interference pattern. For example, reference can be made here to R.A. Jones and P.L. Kadakia, "An Automated Interferogram Technique", Applied Optics, Vol. 7, pp. 1477-1482 (1968); Zanoni, U.S. Patent No. 4,159,522. June 26, 1979, and Zanoni, U.S. Patent No. 4,169,980, published October 2, 1979.

The other approach is so-called phase measuring interferometry ("PMI"), where the phase difference between the two wavefronts for each pixel of a detector is calculated from several interference patterns, the multiple interference patterns being recorded by generating different phase differences in them. For example, reference can be made to J.H. Brunning et al. , "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses", Applied Optics, Vol. 13, pp. 2693-2703 (1974); Gallagher et al., U.S. Patent No. 3,694,088. September 26, 1972, N. Balasubramanian, U.S. Patent No. 4,225,240. September 30, 1980; M. Schaham, Proceedings SPIE, Vol. 306, pp. 183-191 (1981); and H.Z. Hu, "Polarization heterodyne interferometry using a simple rotating analyzer. 1: Theory and error analysis", Applied Optics, Vol. 22, pp. 2052-2056 (1983).

From US 4,594,003 an interferometer system is known in which the frequency of the radiation source can be changed so that the stripes of the interference pattern can be shifted without an optical component of the interferometer system, such as a reference surface or an object surface, must be mechanically shifted. A change is provided over such a range that the stripes of the interference pattern can be displaced over a full stripe width. Four interference patterns are recorded, specifically at four different frequencies of the radiation source distributed in this area. A phase φ of the optical path difference is then calculated for each pixel of the detector using the following formula:

B (0) - B (2) φ (x, y) = arctan B (l) -B (3)

where B (0) to B (3) are the intensities of the individual images at the corresponding pixel.

This known method for determining path differences is less suitable if there is a further surface in the interferometer system which also reflects a wave field which interferes with the wave fields reflected by the reference surface and the object surface. The resulting interference pattern is then of a particularly complex nature. This case occurs, for example, when a surface of a transparent plate with two essentially plane-parallel surfaces is to be measured.

It is an object of the present invention to propose an interferometer system and a method for recording an interferogram which is less sensitive to interference reflections.

Furthermore, it is an object of the invention to specify a method for providing and producing an object with a target surface. The invention is based on an interferometer system which comprises a reference surface, an object surface, a radiation source for illuminating the reference surface and the object surface with radiation of an adjustable frequency and a spatially resolving radiation detector. The radiation source, the reference surface, the object surface and the detector are arranged in such a way that a reference wave field reflected by the reference surface overlaps with an object wave field reflected by the object surface to form an interference pattern with location-dependent intensity distribution, which is imaged on the detector. In this case, the interference pattern formed by superimposing the reference wave field and the object wave field is disturbed by an interference wave field also superimposed with these wave fields, which is reflected by an interference interference area which is illuminated by the radiation source together with the reference area or the object area.

The invention is characterized by an integrator in order to average a plurality of interference patterns, which are recorded at different frequencies of the radiation emitted by the radiation source.

The generated interferogram is created in such a way that for each location of the interferogram an average is formed from the intensities of the individual interference patterns at this location. The averaging is preferably a weighted averaging.

Values of the weights for the weighted averaging and / or values of the different radiation frequencies are preferably set as a function of the distance of the interference interference surface from the object surface or from the reference surface. These values are preferably set in such a way that an influence of the interference wave front on the interferogram is largely averaged out. It then has the middle The interferogram formed from a plurality of interference patterns has a shape or intensity distribution which roughly corresponds to how the wave fronts of the wave fronts reflected by the object surface and the reference surface would produce alone if the interference interference surface were not present in the interferometer system.

In this case, it is also advantageous to make the optical path difference between the reference surface and the object surface adjustable, since by means of a suitable choice these distances relative to one another an influence of the interference wave front on the interferogram can be further reduced.

Advantageously, the plurality of frequencies for generating the plurality of interference patterns are set in succession over a period of time that corresponds to an exposure time interval of a camera that records the interference patterns. This results in a particularly simple design of the integrator, since it is formed by the camera itself.

Embodiments of the invention are explained in more detail below with reference to drawings. Here shows:

FIG. 1 shows an embodiment of an interferometer system according to the invention,

FIG. 2 shows a diagram for explaining different frequencies of radiation emitted by a radiation source in FIG. 1 to generate interference patterns,

FIG. 3 shows a time dependency of the radiation emitted by the radiation source in FIG. 1, 4 shows a diagram which gives an interferogram intensity as a function of an optical path difference for an interference pattern generated by the interferometer system of FIG. 1 when the frequencies are set according to FIGS. 2 and 3,

FIG. 5 shows an interferogram modulation as a function of an optical path difference in the interferometer system of FIG. 1, as results from a further time-dependent setting of the frequencies of the radiation source,

FIG. 6 shows a frequency distribution corresponding to FIG. 2 of the radiation emitted by the radiation source,

FIG. 7 shows a representation of the interferogram intensity corresponding to FIG. 4 as a function of the optical path length difference when using the frequency distribution shown in FIG. 6,

FIG. 8 shows a partial view of a further embodiment of the interferometer system according to the invention and

FIG. 9 shows a partial view of yet another embodiment of the interferometer system according to the invention.

FIG. 1 shows a Fizeau interferometer system 1 for measuring a surface 5 of a plane-parallel plate 3. The plate 3 is held in a holder 4, which can be displaced relative to a reference surface 23 via a motor drive 6.

The interferometer system 1 comprises a light source 9 which emits a beam 11 of coherent light with adjustable wave length or frequency emitted. The light source 9 is a so-called ECDL source, a diode laser with an adjustable external cavity ("External Cavity Diode Laser").

Such an ECDL radiation source is described, for example, in the article "Widely Tunable External Cavity Diode Lasers" by Tim Day, Michael Brownell and I-Fan Wu. Corresponding sources can be obtained from New Focus, Inc., 1275 Reamwood Avenue, Sunnyvale, CA 94089, USA.

The beam 11 emitted by the source 9 is focused by a lens 13 onto a rotating screen or diffuser 15 to suppress spatial coherence of the radiation. The lens 15 rotates about an axis of rotation, not shown in Figure 1.

After passing through the focus in the region of the diffusing screen 15, the beam 11 'extending through it passes through a semitransparent mirror 17 and is then parallelized after sufficient processing by a collimator 19, which can comprise one or more lenses. The then parallelized beam 11 "passes through a glass plate 21, the surface 23 facing away from the collimator 19 forms the reference surface for measuring the surface 5 of the plane-parallel plate 3. The reference surface 23 is made as flat as possible. A surface 25 of the collimator 19 Plate 21 extends at an angle to the reference surface 23 so that radiation reflected by this surface 25 is not reflected back into itself and contributes to interference.

Radiation reflected back from the reference surface 23 is collimated again by the collimator 19, strikes the semitransparent mirror 17 and is imaged by this after passing through an aperture 27 and an eyepiece 29 onto a radiation-sensitive layer 31 of a CCD camera 33. One passing through the reference surface 23 Part of the beam 11 strikes the surface 5 of the plane-parallel plate 3 to be measured. The surface 5 to be measured is oriented as orthogonally as possible to the direction of the parallel beam 11 ″. A part of the radiation striking the surface 5 to be measured is in turn reflected back, passes through the plate 21 again, is likewise focused by the collimator 19 and is imaged on the radiation-sensitive surface 31. The radiation-sensitive layer 31 of the camera 33 thus forms a screen on which the radiation reflected by the reference surface 23 and the surface 5 to be measured are measured reflected radiation interferes.

One purpose of the interferometer arrangement 1 is to detect the interference pattern generated by the interfering superposition of the radiation reflected by the reference surface 23 and the radiation reflected by the surface 5 to be measured.

As already indicated above, the plate 3 is, however, a plane-parallel plate, that is to say the surface 5 of the plate 3 to be measured and one other rear surface 7 of the plate 3 opposite it extend essentially parallel to one another. As a result, part of the radiation 11, which passes through the area 5 to be measured, is likewise thrown back into itself by the rear surface 7 of the plate 3 and is imaged on the radiation-sensitive layer 31.

Thus, on the one hand, the radiation reflected by the reference surface 23 interferes with the radiation reflected by the surface 5 to be measured, whereby there is an optical path length difference 2-C _{Q between them} , on the other hand, the radiation interferes with the radiation-sensitive layer 31 radiation reflected from the reference surface 23 with the radiation from the back surface. 7 of the plane-parallel plate 3 reflects radiation, with an optical path length difference 2-C2 therebetween, and there further interferes with the radiation reflected from the surface 5 of the plate 3 to be measured with the radiation reflected from its rear surface 7, with an optical path length difference in between of 2-C _] _. The interference pattern which arises on the radiation-sensitive layer 31 is thus very complicated and difficult to evaluate.

The camera 33 supplies the data, which represent a radiation intensity distribution on the radiation-sensitive surface 31, to a computer 37 via a data line 35. The computer 37 in turn generates a representation of the interference pattern on the radiation-sensitive layer 31 on a display device 39, in which FIG 1, an interference pattern with a plurality of symbolic strips 40 is only indicated schematically. The computer 37 also stores the data and also carries out an evaluation of the interference pattern in order to determine from this level differences between the reference surface 23 and the surface 5 to be measured or the topology of the surface 5 to be measured.

The interferometer system 1 further comprises a controller 41, which is supplied and released by the computer 37 with a control line 43 with frequency data and which then, on the one hand, sets the frequency of the radiation 11 to be emitted by the source 9 as a function of time Trigger signal 48 received by the camera 33, which is also fed to the computer 37 via a line 47.

A method for operating the interferometer system 1 is described below, with a thickness of 74 mm being assumed for the plate 3, so that taking into account the refractive index of the glass of the plate 3 gives an optical path difference 2- ^ _ of 214.39mm.

The controller 41 first sets the frequency of the radiation source 9 to a first frequency with a value f-Δf via line 45 and starts the integration of the CCD camera 33 via line 47, so that the interference pattern, which is caused by the three Surfaces 23, 5 and 7 of reflected wave fronts are generated when illuminated with radiation of frequency f-Δf, onto which radiation-sensitive surface 31 of camera 33 falls and the corresponding radiation intensity is integrated there. After 3.75 msec, the controller 41 sets the source 9 to a second higher frequency f, so that the interference pattern generated at this frequency falls on the radiation-sensitive layer 31 and the corresponding radiation intensities as a second interference pattern during the integration time of the camera 33 there to be integrated to the intensities of the first interference pattern. After a further 7.5 ms, the controller 41 sets the frequency of the radiation source 9 to an even higher third

Frequency with the value f + Δf, so that the interference pattern generated at this third frequency also falls on the light collecting surface 31 within the integration time of the camera and the intensities of the third interference pattern are added to the intensities of the first and second interference patterns. The illumination with the third frequency f + Δf lasts 3.75 seconds, ^' and then the controller 41, via line 47, causes the integration time of the camera 33 to end, and the data which has fallen on the light collecting surface 31 during the integration time Represent light intensity depending on the location, are read out via line 35 to computer 37.

The integration time of the I5msec camera described above was selected in the exemplary embodiment in order to obtain a good quality image at the given laser power. Depending on the available laser energy and other boundary conditions, other integration times can also be set.

These data thus represent the sum of three different interference patterns, the first interference pattern being recorded with radiation of the frequency f-Δf, the second interference pattern being recorded with the frequency f and the third interference pattern being recorded with the frequency f + Δf. When integrating the three interference patterns, the interference pattern recorded with the middle frequency f is taken into account in comparison to the interference patterns recorded with the double weight at the other two frequencies f-Δf, f + Δf.

This weighted illumination with three different frequencies is illustrated again in FIGS. 2 and 3. FIG. 2 shows the spectral power density in arbitrary units as a function of the wave number k of the radiation from the source 9. It can be seen that the three different frequencies are illuminated with the relative weights 0.5, 1 and 0.5. This spectral power density distribution can be represented as a formula as follows:

F (k) = A - -δ (k - {k ₀ -Ak)) + δ (k -k ₀ ) + -δ (k - (k ₀ + Mc))

Equation (1)

Here is

δ the Dirac delta function,

2π k the wavenumber -, λ

2π o the output wave number - and λ ₀ Δk is the wave number change corresponding to the frequency change Δf.

In the present case, λo = 632.8nm was chosen. The radiation source 9 can be set to this value of the wavelength, and this setting has the advantage that, apart from the radiation source, a structure and components can be selected for the interferometer system as are known from interferometers operated with conventional He-Ne lasers.

The interferogram is the Fourier transform of the spectral power density

I (x) - F (k) ■ coskx - dx = A -— | COS (Ä: ₀ -ΔÄ) • x + cos {k ₀ + Ak) ■ xj + A ■ cos (k ₀ x) ^o 2

= A -COS & ₀ JC- (1 + COSΔ £ X)

Equation (2)

This results in a beat wave number of Δk for the interferogram. A course of the function I (x) is shown schematically for any point in the interferogram in FIG. 4. An envelope of the curves shown is also referred to as interference contrast or modulation. It follows that the modulation increases and decreases periodically as a function of the distance from the reference surface, the modulation decreasing to zero for certain distances.

The interferometer system 1 can be operated advantageously if the reflecting surfaces 23, 5, 7 are arranged relative to one another in such a way that the optical path difference 2-C] _ generated by the distance between the surface 5 to be measured and the rear surface 7 approximately coincides with the first minimum of the modulation, which is due to the distance between the reference surface 23 and the rear surface 7 of the Plate 3 generated path length difference 2-C2 coincides approximately with the second minimum of the modulation and the path length difference C _Q generated by the distance between reference surface 23 and surface 5 to be measured coincides approximately with the second maximum of the modulation. For this purpose, the frequency change Δf or wavenumber change Δk is first determined as follows:

First, l + cosΔk-C; _] _ = 0, which results in Δk-C ^ = π. Since the plate thickness C _] _ = 214, 139mm is assumed in the present example, Δk = 14.67m ^_1 results. Then the distance of the plate 3 from the reference surface 23 is adjusted via the drive 6 so that Δk-C2 = 3π. It should be noted here that the last-mentioned condition can only be met with comparatively low accuracy, since the modulation according to FIG. 4 has quadratic minima and these are therefore relatively insensitive to changes in the optical path difference.

With the described setting of Δk and the distance of the

The back surface 7 of the reference surface 23 then automatically results in the optical path length difference 2-C _Q SO that it coincides approximately with the second maximum of the modulation according to FIG. 4.

The interference caused by the rear surface 7 of the plate 3 is thus effectively averaged out by the weighted averaging carried out during the integration time of the camera 33, so that the interferogram obtained by averaging, apart from a constant light component, has only a stripe pattern, as is the case with the interference only the wavefront reflected by the reference surface 23 would arise with the wavefront reflected by the surface 5 to be measured. This comparatively simple and undisturbed interference pattern is then subjected to a known evaluation method for stripe patterns, to determine the topology of the area 5 to be measured.

The operation of the interferometer system 1 is not limited to driving the frequency of the radiation source 9 with the time scheme shown in FIG. 3. As a variant, the possibility is discussed below of changing the frequency of the radiation source 9 with a sinusoidal time dependency. For the interferogram intensity I, let us first assume:

I (x) = / ₀ - [l + V ■ cos {k ^■ x-Φ ₀ )],

Equation (3)

in which

k is the wave number of the radiation, which can be assumed to be approximately constant in this formula, x the optical path length difference, Φ _Q an interferogram phase and V an interference contrast

is.

The interferogram phase then results from the sinusoidal change in frequency

Φ ₀ = Φ ₀ (t) = Φ ₀ + A • sinωt,

Equation (4)

in which

ΦQ 'an average phase value, ω the angular velocity of the phase modulation and A a phase modulation amplitude is. Substituted in equation (3) we get

I (x, t) = J ₀ {1 + V • cos {k • x -Φ _Q -A • sin cot)

Equation (5)

The modulation period for the frequency change of the radiation is now set such that an integral multiple thereof corresponds to the integration time of the camera 33. The time-averaged interferogram is thus calculated

1 r «

I (x) ~ - I 1 ₀ - 1 + V -cos (kx-φ _Q ^' -A-smωt) -d (ωt)

9 TΓ J-π L J

~ I ₀ + I _Q -V -cosyk-x-Φύ) | cos ( _^ 4-sinω t) -d ^r (ωt)

+1 ₀ -V • sm (k ■ x -Φ I sin (i-sinωt) - (ωt)

2π ^J - ^π

I (x) + V ^■ cos (/ c • x -Φ _Q ) -J ₀ (A)

Equation (6)

where J _Q (A) is the 0 th order function of the phase modulation amplitude A. This function is shown in Figure 5.

It is now possible to find phase modulation amplitudes A for which interference between the wave fronts reflected by the surface 5 to be measured and the rear surface 7 of the plate 3 disappear. The frequency modulation amplitude of the radiation source 9 must therefore be set so that the phase modulation for the optical path length difference 2-C _j _ corresponds to the first zero of the Bessel function from equation (6). This is the case for A] _ = 0, 76547-π. By changing the distance between the reference surface 23 and the plate 3 it is further achieved that the optical path length difference 2-C2 corresponds to the second minimum of the Bessel function of equation (6), which is the case for A2 = 1.7571-π. The ratio of the optical path differences is thus given by the first two zeros of the Bessel function J ₀ (A):

- ^ = ^ = 2.2955. C, A equation (7)

On the other hand, C _Q = C2-C-J_, and the amplitude results for the path length difference C _Q to be measured

A ₀ = ^, = ^^, =, 99l6- π.

⁰ C, C, '

Equation (8)

At this point the Bessel function J _Q (A) has the value

> () = -0.297 «-0.3.

Equation (9)

In this arrangement, three partial beams interfere with approximately the same basic intensity, but in the weighted averaged bwz. integrated interferogram, only the stripe patterns of two interfering partial beams are visible. The other interferences are averaged out, but form a constant light component that reduces the contrast. The effective contrast adds up

Equation (10) This contrast is sufficient to determine the position of the strips 40 and to be able to infer the topology of the area 5 to be measured from the evaluation of the strip pattern. However, it should be noted that the setting of the radiation frequency according to the scheme shown in FIG. 3 leads to a higher effective contrast.

As a further exemplary embodiment, the case in which the radiation source 9 is controlled such that it has a Gaussian spectral power density shown in FIG. 6 will now be explained by way of example

Equation (11)

emitted, whereby

2π k the wave number ^■ -, λ

2π kO the center of gravity wave number ^■ - and λ ₀ σ the width of the Gaussian function

is.

A time-dependent control function for the radiation frequency must now be determined for this spectral distribution. Here is the connection

dk (t) dt A [k (t)]

Equation (12)

to consider. This equation can be solved numerically by the computer 37 in order finally to correspond to one of FIG. to obtain the appropriate time schedule for controlling the radiation frequency.

In the case of the spectral power density according to FIG. 6, the interferogram intensity depends on the path length difference, as shown in FIG. 7. It can be seen from this that high interference contrasts can be achieved at short distances from the reference surface 23, while the contrast drops sharply at larger distances from the reference surface 23. This drop in contrast is so great that when the plate 3 is arranged close to the reference surface 23, interferences caused by the rear side 7 are averaged out and only interferences caused by the surface 5 to be measured contribute to the stripe pattern of the averaged interferogram.

This corresponds to an interferogram with a radiation of constant frequency and a reduced coherence length which is shorter than the optical thickness C ^ _ of the plate 3. The time-dependent frequency change of a radiation source with a large coherence length thus has an effect of reducing the coherence over time for certain lengths. With reference to FIG. 4, this means that the time-dependent frequency change has resulted in the coherence of the radiation being destroyed in the regions of the modulation minima.

Further variants of the embodiments explained in FIGS. 1 to 7 are shown below. Components which correspond to one another with regard to their structure and function are identified by the reference numbers from FIGS. 1 to 7, but are provided with an additional letter to distinguish them. For explanation, reference is made to the entire preceding description in each case. Figure 8 shows a partial view of an interferometer system la, which is constructed similarly to the interferometer system shown in Figure 1. However, the interferometer system la is not used to measure a plane-parallel plate but a concentric meniscus lens 3a. In the beam path after a reference plate 21a with a reference surface 23a, an aplanar collimator 51 with a plurality of lenses 52 to 56 is provided, which focuses the parallel radiation 11 "a at a point 57, which at the same time is the center of curvature of surfaces 5a and 7a of the concentric ones Meniscus lens 3a.

The interferometer system 1a otherwise corresponds to the interferometer system shown in FIG. 1 and is operated according to a method as was explained in connection with the interferometer system of FIG. 1. This means that the frequency of the radiation source is controlled as a function of time in such a way that interference which is caused by a surface that has not been measured, in particular the surface 7a, the concentric meniscus lens 3a or by other optically active components in the beam path, largely averages out over time.

The areas 5a and 7a of the meniscus lens 3a can also be measured by reversing the lens, that is to say with its convex. Surface 7a facing collimator 51 and positioned in front of focus point 57 in the beam path.

FIG. 9 shows a variant of the interferometer system shown in FIG. 8. In contrast to this, in the interferometer system 1b according to FIG. 9 a reference surface 23a is not provided on a separate reference plate but on a precisely manufactured surface of the lens 56b of an aplanar collimator 51 facing the test object. The interferometer system lb is also used to measure a concentric meniscus lens. In addition to the time dependencies of the frequency of the radiation source for generating the interferogram described above, other time dependencies can also be selected which have proven to be favorable. It is important here that interference effects, which are caused by surfaces that are not to be measured, are at least partially averaged out over time.

The interferometer system has been previously described as a Fizeau interferometer. However, it is also possible to use alternative types of interferometers, such as a Michelson interferometer set-up or a Twyman Green interferometer set-up.

In the exemplary embodiments described above, the CCD camera was used as an integrator for weighted averaging of the interference patterns generated at different lighting frequencies. However, it is also possible to use other camera types which have an integration time to which the sequence of the lighting frequencies set one after the other is coordinated. Furthermore, it is possible to generate separate camera images for several lighting frequencies, to feed them to the computer and to carry out the integration or weighted averaging in the computer pixel by pixel. In the context of this application, a pixel is understood to mean a resolution unit of the digitized interference image, which is given, among other things, by the camera system. The averaging in the computer can in this case also for group-pen of pixels, that is, with a resolution, ^'the less be carried out as the camera resolution.

The interferometer system described above and the

A method for recording the interferogram is advantageously used in a method for providing of an object or in a method for producing an object with a predetermined target surface.

If, for example, the plane-parallel plate explained in connection with FIG. 1 is to be manufactured with high precision, it is placed in the beam path of the interferometer system and an interferogram is recorded using the method described above. From the interferogram, deviations of surface 5 from the planned target shape are determined. Postprocessing is planned from these deviations. In particular, these deviations are used to determine locations on the surface 5 where postprocessing, in particular by further material removal, is to take place. After the post-processing has been carried out, an interferogram may be recorded again and further post-processing may be carried out. If it emerges from the recorded interferogram that deviations between the shape of the surface 5 and the plan desired shape are less than a predetermined dimension, the plate is provided or delivered.

This provision or manufacturing method can be applied to any other object that is to have a predetermined surface. The application to a concentric meniscus lens has already been explained above. However, applications to any other objects are also conceivable.

Claims

claims

1. Interferometer system comprising: a reference surface (23), - a holder for an object surface (5) providing object (7), a radiation source (9) for emitting radiation of an adjustable frequency on the reference surface (23) and the Obj ect surface (5), and - a spatially resolving radiation detector (31),

wherein the radiation source (9), the reference surface (23), the holder and the radiation detector (31) are arranged such that a reference wave field reflected by the reference surface (23) with an object wave surface (5) reflected by the object surface superimpose an interference pattern imaged onto the radiation detector (31) with location-dependent intensity distribution,

wherein an interference interference surface (7) is provided which is irradiated together with the reference surface (23) and / or the object surface (5) by the radiation source (9), and

wherein the interferometer system further comprises a controller (37, 41) for setting a plurality of different frequencies (f, f + Δf, f-Δf) of the radiation emitted by the radiation source (9), characterized by an integrator (33) to control the at different frequencies on the radiation detector (31) superimposed interference pattern depending on location.

2. Interferometer system according to claim 1, wherein the radiation detector (31) is provided by a CCD camera (33).

3. Interferometer system according to claim 1 or 2, wherein the integrator is formed by the radiation detector (31).

4. Interferometer system according to claim 3, wherein the controller (41, 37) adjusts several, in particular all, of the different frequencies in succession during a time period which corresponds to an integration time of the detector (33).

5. A method of taking an interferogram comprising:

Illuminating a reference surface (23) and an object surface (5) with coherent radiation of a predetermined frequency,

Superimposing a reference wave field reflected by the reference surface (23) and an object wave field reflected by the object surface (5) in such a way that an interference pattern with a location-dependent radiation intensity distribution arises on a screen (31),

the illumination being carried out successively with a plurality of different radiation frequencies (f, f + Δf, f-Δf), so that interference patterns associated with the different radiation frequencies are produced in succession on the screen, and.

wherein the interferogram is formed in that intensities of the for each location of the interferogram

A plurality of interference patterns are weighted averaged at the corresponding location.

6. The method of claim 5, wherein the weights for the weighted. Means can be set by setting time periods of the illuminations with the different radiation frequencies.

7. The method according to claim 5 or 6, wherein at a distance from the object surface (5) and / and at a distance (C ₂ ) from the reference surface (23) a interference interference surface (7) is arranged, which together with the object surface (5 ) or the reference surface (23) is illuminated, and values of the different radiation frequencies and / or values of the weights for the weighted averaging are set as a function of the distance.

8. The method according to any one of claims 5 to 7, wherein a first path length difference (CQ) between an optical path from the reference surface (23) to the detector (31) and an optical path from the object surface (5) to the detector (31) there is a second path length difference (C) between an optical path from the reference surface (23) to the detector (31) and an optical path from the interference interference surface (7) to the detector (31), a third difference ( Cl) exists between the first and the second path length difference, the illumination being carried out with three different radiation frequencies,. namely a medium frequency (f), a higher frequency (f + Δf) and a lower frequency (f-Δf), the three frequencies having a frequency spacing (Δf) in pairs, the frequency spacing (Δf) being set such that the equation

Δk-C! = Π is fulfilled, where Δk is the change in wavenumber corresponding to the frequency spacing (Δf), the distance between the interference interference area and the detector being adjusted such that the equation

Δk-C ₂ = 3π is satisfied, and the weighting takes into account the interference pattern assigned to the low frequency and the interference pattern assigned to the higher frequency of a single weight and that of the interference frequency assigned to the middle frequency with a double weight.

9. A method for providing an object (3) with a target surface, comprising the steps:

Recording an interferogram with the method according to one of claims 5 to 8, wherein the object

(3) provides the object area (5),

Evaluating the interferogram and determining deviations between the object surface and the target surface as a function of the interferogram, - providing the object if the deviations are smaller than a predetermined threshold value and not providing the object if the deviations are larger than that predetermined threshold.

10. A method for producing an object with a target surface, comprising the steps:

Recording an interferogram with the method according to one of claims 5 to 8, wherein the object provides the object surface, - evaluating the interferogram and determining deviations between the object surface and the target surface as a function of the interferogram, removing surface areas of the object at locations , which depends on the deviations between the object surface and the target surface be tuned in order to adapt the object surface of the body to the target surface.

11. The method according to claim 9 or 10, wherein the body to be produced is a transparent plane-parallel plate or a concentric meniscus lens.