DE10321895A1 - Topography measuring sensor employs microscopic white light interferometry and has a variable refractive power imaging system in the plane of the test objective focal plane - Google Patents

Abstract

Sensor for measuring the topography of an object using a two beam interferometer with a beam splitter (3) for generating object and reference beam paths. At least a test objective (5) is provided for imaging an object (6). In the focal plane (F2) of the objective, which is distanced from the object, is, at least approximately, a variable refractive power imaging system (4).

Description

State of technology

The sequential acquisition of data from different depths of the object space through-focus plays in microscopic white light interferometry is known to have a functional role. There are notes on this in the following writings:

• [1.] Balasubramanian N: Optical system for surface topography measurement. U.S. patent. No. 4,340,306 (1982),
• [2.] Kino GS, Chim S: Mirau correlation microscope. Appl. Opt. 29 (1990) 3775-3783,
• [3.] Byron SL, Timothy CS: Profilometry with a coherence scanning microscope. Appl. Opt. 29 (1990) 3784-3788,
• [4.] Dresel Th, Häusler G, Venzke H: Three-dimensional sensing of rough sufaces by coherence radar. Appl. Opt. 31 (1992) 919-925.
• [5.] Deck L, de Groot P: High-speed noncontact profiler based on scanning white-light interferometry. Appl. Opt. 33 (1994) 7334-7338,
• [6.] Windecker R, Haible P, Tiziani H J: Fast coherence scanning interferometry for measuring smooth, rough and spherical surfaces. J. Opt. Soc. 42: 2059-2069 (1995).

It is in microscopic white light interferometry from Advantage, the information about to gain the object without moving mechanical parts. A first approach can be found at Hege G: speckle method for distance measurement. Dissertation, in reports from the Institute for Technical Optics. Vol. 4. 1984, pp. 20-25 [7]. However, there is only a single sharpness level here, so there are objects problems with sharpness with a certain depth can.

A possibility in microscopic white light interferometry without focusing on moving mechanical parts the wavelength-to-depth encoding technique by G. Li, P.-Ch. Sun, P.C. Lin and Y. Feinman in Optics Letters Oct. 15, 2000, Vol. 25, No. 20, pp. 1505 to 1507 with a diffractive lens in the object beam path proposed in conjunction with a tunable laser [8]. However, there is still room for improvement here the achievable depth measurement accuracy, since only the comparative broad coherence function is evaluated. Moreover the measurement setup used here is still quite complex overall. By varying the focal length of a lens in the object beam path changes to focus the image scale, too a color magnification error exists when mapping the object. This can be for high resolution 3D topometry a problem.

description the invention

The The goal is to further improve the achievable depth measurement accuracy and the lateral measurement accuracy in microscopic white light interferometry with a comparatively inexpensive sensor structure. In doing so especially inventive approaches described in which either sequential recording of data from different depths of the object space by means of focusing, or by the simultaneous recording of data, information from different Depths of the object space can be obtained. The latter leads to one highly dynamic measurement technology. It is intended according to the invention instead a tunable laser is also a comparatively inexpensive white light source can be used for lighting.

In order to the particular task to be solved is the lateral measurement accuracy to improve by changing the lateral magnification Light wavelength remains unaffected, i.e. no color magnification error, even as chromatic Queraberration known, exists when imaging the object. Furthermore, when optically probing the object surface in different Depths of the object space signals without the mechanical movement of Components are generated using a microscopic white light interferometer can. The signals obtained in this way are intended to be used to evaluate the Allow phase information.

It is in the inventive sensor for detecting the topography of it assumed that the to be checked Object, in all cases considered here, a single test lens is directly assigned. This is always used for lighting the object surface as well as the image of the object surface.

It can be a spectrally broadband source of electromagnetic radiation in the DUV, UV, VIS, NIR or MIR or also in the FIR area in Sensor be arranged. So it can also be a white light source in the visible spectral range with a share in the NIR or UV range be used. Possible is also the use of a tunable laser, for example a Ti: Sapphir laser or a multi-wavelength laser with frequency comb characteristics.

The object surface to be measured is at least approximate when it is optically probed wise and arranged at least in part in the focal plane of the test lens.

To 1st and 2nd: With a sensor for recording the topography with a two-beam interferometer with a beam splitter with one Object and a reference beam path with at least one Lens, the test lens, for lighting and imaging the object and a camera in the focal plane FPP of the test lens, which faces away from the object, a refractive variable, imaging System at least approximately with one of its main points (H ') arranged. Furthermore, the sensor for detecting the topography the refractive power variable imaging system is preferably chromatic be trained. That means the focal length of this imaging Systems is on the wavelength depending on the incoming electromagnetic radiation, where Usually light in the visible, in the near IR or UV range is used. The focal plane FPP of this test lens can coincide with the focal plane of a first lens, so that an afocal mapping level is formed. So that is also the location of the focus plane this imaging level in the object space depends on the light wavelength used. By the arrangement of the refractive power variable imaging system in the common focal plane of the afocal imaging level is the color magnification error at least approximately zero.

To 3 .: A sensor for detecting the topography is also proposed, in which the reference beam path of the two-beam interferometer at least approximately is designed so that there is no color magnification error in the same consists. This is due to the use of plane parallel plates and through chromatically complete corrected microscope objectives possible. So for the light of all wavelengths a high-contrast interference occurs in the detection plane, because here the light of all wavelengths in the reference beam path can be mapped to a reference mirror in the same way. Due to the longitudinal color effect of the refractive power variable imaging system in the object beam path the waves of different wavelengths of light in the detection plane the interference, however, is no longer in phase and so over the wavelength a waveform that is very similar to a white light interferogram. The wavelength of the Light source can also be scanned, for example when using a tunable laser. In this case, a gray value sensitive Camera are used. When using a white light source spectral splitting of the light must be carried out before detection become. The simplest case of spectral splitting is here the use of a commercially available Color camera. However, the three color channels are usually by far unsatisfactory. Therefore, when using a white light source A spectrometer is preferably arranged upstream of the gray-value-sensitive camera. When using a multi-wavelength laser with a frequency comb characteristic also a bigger optical Permitted path difference between the two arms of the interferometer , because such a blanking is made, which is in the manner of a stroboscopic blanking is comparable. With careful coordination of the optical path difference to the frequency spacing of the individual, narrow-band Considering laser lines the dispersion in the interferometer can be similar to the white light interferogram Signal are generated. The evaluation can be based on the known white light evaluation algorithms.

To 4 .: A sensor for detecting the topography is also proposed, where the lens is in the reference beam path of a Linnik interferometer is achromatic, so that none in the reference beam path Color magnification error exists. So the requirement is fulfilled um for Light of different wavelengths To be able to observe interference in the plane of the camera.

To 5 .: A sensor for detecting the topography is also proposed, in the reference beam path of a Michelson interferometer or Mirau interferometer a second refractive variable, imaging System, but arranged with opposite refractive power, so the color magnification error and the longitudinal color error of the first refractive power variable imaging system, at least approximately are compensated and so the light of everyone on the reference mirror wavelength is always focused in the same way.

6 .: A sensor for detecting the topography is also proposed, in which a plane parallel plate arrangement with at least one plane parallel plate made of dispersive material is arranged in the reference beam path of a Michelson interferometer or Mirau interferometer, so that the longitudinal color error of the first imaging system variable, refractive power, in the reference beam path is at least approximately compensated and so the light of all wavelengths is always focused in the same way on the reference mirror. In this case, a plane-parallel plate arrangement with at least approximately the same average optical thickness with at least one plane-parallel plate made of dispersive material is preferably arranged in the object beam path, the dispersion of this plane-parallel plate arrangement being significantly different. For example, the dispersion can be significantly smaller. For example, a positive longitudinal color error due to the refractive power variable system in the focal plane of the test lens is only slightly offset by a plane parallel plate made of comparatively weakly dispersive material weakens so that a desired longitudinal chromatic aberration for chromatically induced focusing remains in the object beam path. In contrast, a plane-parallel plate made of highly dispersive material is arranged in the reference beam path, which completely compensates for the longitudinal color error in the reference beam path. This can result in an additional phase response over the wavelength, which deforms the white light signal and is also known as chirping. This chirping effect must be corrected numerically.

To 7 .: A sensor for detecting the topography is also proposed, in which the refractive power variable imaging system in the object beam path a Linnik interferometer is arranged and in the reference beam path a plane parallel plate compensation system is arranged, which the same longitudinal color error like the refractive power variable imaging system in the object beam path has. So there is a corresponding adjustment of the reference arm and with identical lenses in the reference and in the object beam path and when sharply imaging an object point, the prerequisite that high-contrast interference for light within a limited Spectral range when imaging this point in the camera plane occurs.

To 8 .: But it is also possible that the refractive power variable, imaging system in the object beam path of a Linnik interferometer is arranged and a plane parallel plate compensation system in the reference beam path is arranged, which has a specially adapted longitudinal color error has in order to be able to scale the depth sensitivity of the sensor. So can also add a signal in the form of a white light interferogram desired Phase response over the wavelength be imprinted.

To 9 .: So far, only sensors with a refractive power have been considered of the refractive power variable imaging system from the wavelength of the used light depended. With a sensor for recording the topography can the refractive power variable imaging system also with a be electronically controllable, diffractive component. So is the refractive power of the diffractive, refractive power variable, imaging System depending changeable by the structure of the electronically controllable component. A monochromatic light source can also be used here, since the refractive power variation in this case does not take place over the light wavelength. On the other hand, a diffractive, Power-variable, imaging system in which the depth measuring range and the depth resolution of the sensor over the Adaptation of a diffractive lens adapted to the measuring task become.

At the Structure of the refractive power variable imaging system as an electronic controllable system can LCDs, LCOS displays or DMDs are used. So the refractive power dependent on be specifically changed by the structure of an electronically controllable array.

To 10 .: It is also a sensor for detecting the topography advantageous if preferably the main point (H ') of the refractive power variable, imaging Systems at least approximately with the pupil center PZD coincides in the imaging beam path. In in this case there is no lateral migration when focusing through of the image in the camera plane.

To 11th and 12th: There is also a sensor for detecting the topography of advantage if at least approximately for at least a wavelength the amount of the refractive power of the refractive power variable, imaging Systems results in zero. So a test lens can be used which is corrected to infinity. These lenses are inexpensive and in large Variety available. A refractive power of the imaging system variable in refractive power Reaching from zero can be achieved with a sensor for detecting the topography the refractive power imaging system preferably consists of two main components exist for a single wavelength of light at least approximately in are opposed to the refractive power.

Preferably at least one of these subsystems is chromatic. This system is then preferably diffractive or at least partially diffractive. But it can both systems should be chromatic. The second system can be dispersive. The refractive power variable is preferred Imaging system a focusing or diverging system, in any case a system with zoom properties and preferably rotationally symmetrical in its structure and in its function.

Re 13 .: Furthermore, with a sensor for detecting the topography, the camera can advantageously be designed as a color-sensitive camera. The camera can also have more than three spectral channels, for example also eight spectral channels. With a priori knowledge of the signal shape occurring, for example the mean ripple in the signal over the light wavelength, the center of gravity of the envelope and the phase at the center of gravity of the signal can be determined. The evaluation methods required for this have already been described in Applied Optics, Vol. 39, No. 8, March 10, 2000, pages 1290 to 1295 [9] and in Fringe'01: Proceedings of the 4th International Workshop on Automatic Processing of Fringe Patterns, Elsevier 2001, pp. 173–180 under "Generalized Signal Evaluation for White-light interferometry and Scanning fringe projection" [10] or in the technical paper "Signal processing for deep-scanning 3D sensors for new industrial applications" in the technical measurement journal 69 (2002) 5 [11] Such a camera can be constructed, for example, with four sensor chips and a beam splitter arrangement with a total of four Kösters prisms and four adapted color splitters, each of the four sensor chips taking two adjacent images of different focus wavelengths.

To 14 .: If the spectral splitting into eight spectral channels - especially if a larger depth measurement range required is - out establish the depth measurement accuracy is not sufficient for a sensor to record the topography, the camera also as a monochrome camera be trained. The surface of the object is then illuminated with a narrow band of light. Then this camera is a spectrometer module with an entrance slit, on which the light band is mapped becomes. This spectrometer module with internal imaging level can for example 100 spectral channels have by spectrally decomposing the light to 100 pixels and takes place perpendicular to the formation of the light band image on the camera surface. Consider a spectral range of 200 nm of around 450 up to 650nm the spectral resolution 2 nm. It is advantageous if the pixel pitch has about an eighth stripe equivalent. When an optical path difference of about 25 wavelengths is evaluated is, this corresponds approximately to a depth measuring range of approximately +/- 3 µm. The spectrometer module can be less than a straight view wedge or a grating spectrometer resolution be trained.

To 15 .: Furthermore, it is also possible that a modified Linnik interferometer after DE 3902591 A1 or US 4983042 is equipped with a triple reflector in the reference beam path. However, because of the larger optical path difference in the interferometer and / or the dispersion of the glass material, this generally requires a higher spectral splitting than with the classic Linnik interferometer with an objective in the reference beam path. When using this Linnik arrangement with a triple reflector, it is very advantageous to use modern multi-wavelength lasers with a frequency comb characteristic as the light source. With appropriate dimensioning of the interferometer and the frequency spacing of the individual laser lines and interpretation of the spectral resolution of the spectrometer, a signal that can be easily evaluated in the form of a white light interferogram over the wavelength can be obtained by blanking.

To 16. Furthermore, it is also possible that with a sensor for detecting the topography as a light source a multi-wavelength laser (MLL) with a frequency comb characteristic. is used, whereby this usually has far more than 10 individual wavelengths.

description the Figure

The The invention is described by way of example with reference to the figure.

The figure shows the effect of a refractive power-variable, imaging component in a modified Linnik interference microscope. The advantage of using a Linnik interferometer is that a comparatively large aperture, eg NA = 0.9, can be achieved. That from a slit-shaped white light source 1 outgoing light, with the gap positioned transversely to the plane of the drawing, enters a collimator lens 2 , The light source can consist of a conventional white light source, which is followed by a gap.

After the collimator lens 2 becomes the light beam on the beam splitter 3 split into a reference and an object beam path. That on the beam splitter 3 reflected light enters the object beam path and reaches the focal plane of the test lens 5 on the refractive power, imaging system 4 , which is designed here as a chromatic system. It consists of a dispersive diverging lens and a diffractive lens, which acts here as a converging lens. The dimensioning is carried out in such a way that the imaging system, which is variable in refractive power, has a light-scattering effect in blue light and a light-collecting effect in red light. The focal plane H'4 of the refractive power-variable imaging system 4 coincides with the focal plane which is the focal point F1 'of the collimator lens 2 and the focal point F2 of the test lens 5 contains. With a shorter wavelength of light, here with blue light, the refractive power of the imaging system is variable 4 negative and with red light the refractive power of the same is always positive. Depending on the light wavelength, the slit-shaped white light source is imaged at different depths in the object space 1 , For a wavelength of the white light source 1 there is a sharp image of a luminous point Q of the white light source in a viewed object point O. 1 , so that the pixel Q 'and the object point O coincide. The collimated light in the reference arm of the modified Linnik interference microscope passes through a plane parallel plate compensation system 7 , which has at least approximately the same longitudinal color error as the refractive power variable imaging system 4 in the object beam path. The subsequent microscopic standard lens 8th with focal point F3 is identical to the test lens 5 , On the reference mirror 9 the image of the white light source is created with a reflectivity of approximately 10% 1 and at the same time for the light of all colors. This is how the reference mirror appears 9 an image of a luminous point Q '' of the white light source 1 in exactly the color for which the object point O is illuminated with a sharp image of a light source point Q '. In the object beam path, the light backscattered from object point O passes the test objective again 5 , the refractive power variable imaging system 4 and the beam splitter 3 , The light is reflected in the reference beam path, and also that of the image Q '', and the light beam passes through the microscopic standard lens 8th and the plane parallel plate compensation system 7 , The light beam is on the beam splitter 3 reflected. The tube lens 10 forms the light bundles on a spectrometer slit 11 from. The blue light bundles originating from the two light source points Q 'and Q''arrive after passing through the imaging stage, which come from the lenses 12 and 14 exists, and after passing the light-separating double wedge 13 , consisting of a dispersive wedge and a diffractive element, on an area of the sensor chips of the camera 15 where there is interference between the two blue light beams. The light bundles of adjacent light wavelengths form, with the blue light bundles under consideration, a signal which looks at least approximately similar to a white light interferogram. Here, the asymmetry Δx, which is predetermined by the effective positions of the focal points F2 and F3, is intended, the axial displacement being caused by the imaging system variable in refractive power 4 and plane parallel plate compensation system 7 is calculated to be zero in the beam path. The focal points F2 and F3 are viewed from the beam splitter.

This Signal representing a white light interferogram looks at least approximately similar evaluated on the basis of the known algorithms, where appropriate Adjustments have to be made because there are non-linearities in the signal can. With this arrangement, the micro profile can be cut in a line by taking a single picture at high speed and accuracy can be determined.

Claims (16)

Topography sensor with a two-beam interferometer with a beam splitter ( 3 ) with an object and a reference beam path with at least one single objective, the test objective ( 5 ), for lighting and imaging the object ( 5 ) and a camera, characterized in that in the focal plane F2 of the test lens ( 5 ), which faces away from the object, a refractive power-variable, imaging system ( 4 ) is arranged at least approximately with one of its main points (H '). Sensor for detecting the topography according to claim 1, characterized in that the refractive power variable imaging system ( 4 ) is chromatic, so that its focal length is dependent on the wavelength of the incoming electromagnetic radiation. Sensor for detecting the topography according to claim 2, characterized in that the reference beam path of the two-beam interferometer at least approximately is achromatic, so that there is no color magnification error in the same consists. Sensor for detecting the topography according to claim 3, characterized in that the lens ( 8th ) is achromatic in the reference beam path of a Linnik interferometer, so that there is no color magnification error in the reference beam path. Sensor for detecting the topography according to claim 2, characterized in that a second refractive power-variable imaging system (S) is arranged in the reference beam path of a Michelson interferometer or Mirau interferometer, but with opposite refractive power, so that the color magnification error and the longitudinal color error of the first refractive power variable imaging system ( 4 ) is at least approximately compensated and so on the reference mirror ( 9 ) the light of all wavelengths is always focused in the same way. Sensor for detecting the topography according to claim 2, characterized in that in the reference beam path of a Michelson interferometer or Mirau interferometer with a plane parallel plate arrangement (PP) arranged at least one plane parallel plate made of dispersive material is, so the longitudinal color error of the first refractive power variable imaging system in the reference beam path at least approximately is compensated. Sensor for detecting the topography according to at least one of claims 1 to 4, characterized in that the refractive power-variable imaging system is arranged in the object beam path of a Linnik interferometer and a plane-parallel plate compensation system ( 7 ) is arranged, which is at least approximately the same longitudinal color error as the refractive power variable imaging system ( 4 ) in the object beam path. Topography sensor after at least one of claims 1 to 4, characterized in that the refractive power-variable imaging system is arranged in the object beam path of a Linnik interferometer and in the reference beam path a plane-parallel plate compensation system ( 7 ) is arranged, which has a specially adapted longitudinal color error. Sensor for detecting the topography according to at least one of claims 1 to 8, characterized in that the refractive power variable imaging system ( 9 ) is designed with an electronically controllable, diffractive component and thus the refractive power of the imaging system variable in refractive power ( 4 ) can be changed depending on the structure of the electronically controllable component. Sensor for detecting the topography according to at least one of claims 1 to 9, characterized in that the main point (H ') of the refractive power-variable imaging system ( 4 ) at least approximately coincides with the pupil center PZD in the imaging beam path. Sensor for detecting the topography according to at least one of Claims 1 to 10, characterized in that the magnitude of the refractive power of the refractive power-variable imaging system (at least approximately for at least one wavelength of the light) 4 ) results in zero. Sensor for detecting the topography according to at least one of claims 1 to 11, characterized in that the refractive power variable imaging system ( 4 ) consists of two main components that are at least approximately opposite in refractive power for a single light wavelength. Sensor to record the topography after at least one of the claims 1 to 12, characterized in that the camera as a color sensitive camera is trained. Sensor for detecting the topography according to at least one of claims 1 to 12, characterized in that the camera is designed as a monochrome camera and this camera has a spectrometer module ( 13 ) is upstream. Sensor to record the topography after at least one of the claims 1 to 3 and 7 to 14, characterized in that a modified Linnik interferometer is equipped with a triple reflector (T) in the reference beam path. Sensor to record the topography after at least one of the claims 1 to 15, characterized in that a multi-wavelength laser as the light source (MLL) with a frequency comb characteristic. is used, the has at least three different wavelengths.