DE10321895B4 - Sensor for detecting the topography with a two-beam interferometer - Google Patents

Abstract

Sensor for detecting the topography with a two-beam interferometer, with a light source (1), with a beam splitter (3) for splitting the light into an object and a reference beam path, with a test objective (5) in the object beam path for illumination and imaging of the object (6), with a camera (15) and with a refractive power variable, imaging system (4) whose refractive power variability allows the adaptation of depth measurement range and depth resolution of the sensor, characterized in that the refractive power variable, imaging system (4) at least approximately with one of his Principal points (H ') in the focal plane F2 of the Prüfobjektivs (5), which faces away from the object, is arranged.

Description

State of the art

• [1.] Balasubramanian N: Optical system for surface topography measurement. US 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 an 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. Modern Optics 42 (1995) 2059–2069.

The sequential recording of data from different depths of the object space by means of focussing is known to play a functional role in microscopic white light interferometry. There are hints in the following writings:

• [1.] Balasubramanian N: Optical system for surface topography measurement. US patent. No. 4340306 (1982)
• [2.] Cinema 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 HJ: Fast coherence scanning interferometry for measuring smooth, rough and spherical surfaces. J. Modern Optics 42 (1995) 2059-2069.

It However, in microscopic white light interferometry of Advantage, the information about to gain the object without moving mechanical parts. A first approach is found in Hege G: Speckleverfahren for distance measurement. Dissertation, in reports from the Institute of Technical Optics. Vol. 4. 1984, pp. 20-25 [7]. However, there is only one level of sharpness, so it's on objects give some sharpness problems with a certain depth can.

A possibility in microscopic white light interferometry without focusing on moving mechanical parts, was with the wavelength-to-depth-encoding technique of G. Li, P.-Ch. Sun, P.C. Lin and Y. Fainman in Optics Letters Oct. 15, 2000, Vol. No. 20, p 1505 to 1507 with a diffractive lens in the object beam path proposed in conjunction with a tunable laser [8]. However, there is room for improvement regarding this the achievable depth measurement accuracy, since only the comparatively broad coherence function is evaluated. Furthermore the measurement setup used here is still quite complicated on the whole. By varying the focal length of a lens in the object beam path to focussing changes also the magnification, because a color magnification error in the picture of the object. This can be for the high-resolution 3D topometry a problem.

Description of 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. It will be especially described inventive approaches at either with the sequential recording of data from different Depths of the object space by focussing, or by the simultaneous Recording of data, information from different depths of the object space be won. The latter leads to a highly dynamic measurement technology. It is intended according to the invention instead a tunable laser also a comparatively inexpensive white light source can be used for lighting.

In order to In particular, the task is to solve the lateral measuring accuracy to improve by the lateral magnification by changing the Light wavelength is unaffected, so no color magnification error, even as a chromatic Queraberration known in the image of the object exists. Furthermore, the optical surface of the object surface in different Depths of the object space Signals without the mechanical moving of Components are generated by means of a microscopic white light interferometer can. The signals obtained should be a useful evaluation of the Allow phase information.

It becomes in the inventive sensor for detecting the topography of it assumed that to be tested Object, in all cases considered here, a single test objective is assigned directly. This always serves the 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, in the UV, in the VIS, in the NIR or in the MIR or also in the FIR range in the Sensor be arranged. So it can also be a white light source in the visible spectral range with a share in the NW or UV range be used. Possible but is also the use of a tunable laser, for example a Ti: sapphire laser or a multi-wavelength laser with frequency comb characteristics.

The Object surface to be measured is at least approximately at the optical probing of the same and at least part of it in the focal plane of the test objective arranged.

In a sensor for detecting the topography with a two-beam interferometer with a Beam splitter with an object and a reference beam path with at least a single lens, the test object, for illumination and imaging of the object and a camera is in the focal plane FPP of the test object, which faces away from the object, a refractive power variable, imaging system, the refractive power variability of the adaptation of depth measuring range and depth resolution of the sensor, at least approximately arranged with one of its main points (H '). Furthermore, in the sensor for detecting the topography, the refractive-power-variable, imaging system may preferably be of chromatic design. That is, the focal length of this imaging system is dependent upon the wavelength of the incoming electromagnetic radiation, typically employing light in the visible, near IR or UV range. The focal plane FPP of this test objective can coincide with the focal plane of a first objective, so that an afocal imaging stage is formed. Thus, the position of the focal plane of this imaging stage in the object space is dependent on the wavelength of light used.

By the arrangement of the refractive power variable, imaging system in the common focal plane of the afocal imaging stage is the color magnification error at least approximately zero.

Farther a sensor for detecting the topography is proposed in the reference beam path of the two-beam interferometer at least approximately is formed so that in the same no color magnification error consists. This is due to the use of plane parallel plates and by chromatic completely corrected microscope lenses possible. So can for the light of all wavelengths a high-contrast interference occur 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 color longitudinal 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 on the other hand is no longer in phase and thus results over the wavelength a waveform 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 greyish-sensitive Camera to be used. When using a white light source must be made before the detection of a spectral splitting of the light become. The simplest case of spectral splitting is here the use of a commercial However, the three color channels are usually by far unsatisfactory. Therefore, when using a white light source preferably a spectrometer of the gray value sensitive camera upstream. When using a multi-wavelength laser with a frequency comb characteristic can also a larger optical Gap difference between the two arms of the interferometer allowed 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 thus be similar to the white light interferogram Signal are generated. The evaluation can be based on the known white light evaluation algorithms.

Farther a sensor for detecting the topography is proposed in the two-beam interferometer is a Linnik interferometer whose lens is in the reference beam path achromatisch is formed, so that in the reference beam path no Chromatic difference of magnification consists. Thus, the requirement is met to interfere with light of different wavelengths to be able to observe in the plane of the camera.

Farther a sensor for detecting the topography is proposed in the two-beam interferometer a Linnik interferometer is at which the refractive power variable, imaging System in the object beam path and in the reference beam path a plane parallel plate compensation system which has the same color longitudinal error as the refractive power variable, has imaging system in the object beam path. So insists corresponding adjustment of the reference arm and with identical lenses in the reference and object beam paths and in sharp imaging of an object point the requirement that high-contrast interference for light within a limited spectral range while mapping this Point in the camera level occur.

It but it is also possible that in a sensor for detecting the topography, the two-beam interferometer a Linnik interferometer is at which the refractive power variable, imaging System in the object beam path and in the reference beam path a plane parallel plate compensation system is arranged, which is a specially adapted longitudinal chromatic aberration to scale the depth sensitivity of the sensor. So may additionally input the signal in the form of a white light interferogram desired Phase transition over the wavelength imprinted become.

Furthermore, it is also possible that the two-beam interferometer after a Linnik interferometer DE 39 02 591 A1 or US 4,983,042 is that is modified with a triple reflector in the reference beam path. However, this usually requires because the here given greater optical path difference in the interferometer and / or the dispersion of the glass material, a higher spectral splitting than the classic Linnik interferometer with a lens in the reference beam path. Very advantageous in the use of this Linnik arrangement with a triple reflector is the use of modern multi-wavelength lasers with a frequency comb characteristic as a 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 can be obtained by blanking a well evaluable signal in the form of a white light interferogram over the wavelength.

Farther a sensor for detecting the topography is proposed in the two-beam interferometer a Michelson interferometer or Mirau interferometer is included in the reference beam path, a second refractive power variable, imaging System (S), but arranged with opposite refractive power, so that the color magnification error and the color longitudinal error of the first refractive power variable, imaging system at least approximately are compensated and so on the reference mirror the light of all wavelength always focused in the same way.

Farther a sensor for detecting the topography is proposed in the two-beam interferometer a Michelson interferometer or Mirau interferometer is included in the reference beam path a plane parallel plate arrangement (PP) with at least one plane parallel plate of dispersive material is arranged so that the longitudinal chromatic aberration of the first refractive power variable, imaging system in the reference beam path at least approximately is compensated and so on the reference mirror the light of all wavelengths always is focused in the same way. It is preferable in the object beam path a plane parallel plate arrangement with at least approximately the same average optical thickness with at least one plane parallel plate arranged of dispersive material, the dispersion of this Planparallelplatteanordnung is significantly different. For example the dispersion can be significantly smaller. So one becomes through the refractive power variable system in the focal plane of the test objective positive longitudinal chromatic aberration a plane parallel plate of comparatively weakly dispersive Material only slightly weakened, so that in the object beam path nor a desired longitudinal chromatic aberration remains to chromatic conditional focussing. On the other hand is in the reference beam path a plane parallel plate of highly dispersive Material arranged that the color longitudinal error in the reference beam path completely compensated. It can be an additional Phase transition over the wavelength arise, the white light signal deformed and also known as Chirping. This chirping effect must be corrected numerically.

So far only sensors were considered in which the refractive power of the refractive power variable, imaging system of the wavelength of the light used depended. In a sensor for detecting the topography, the refractive power variable, imaging system but also with one in their structure electronically be designed controllable, diffractive component. That's how it is Refractive power of the diffractive, refractive power variable, imaging system dependent on changeable by the structure of the electronically controllable component. Here also a monochromatic light source can be used since the refractive power variation in this case does not take place over the wavelength of light. On the other hand, even with a white light source, a diffractive, be arranged kraftkraftvariables, 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 measurement task become.

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

Farther is it advantageous for a topography sensor to if preferably the main point (H ') of the refractive power variable, imaging Systems at least approximately coincides with the pupil center PZD in the imaging beam path. In In this case, there is no lateral migration when focussing of the picture in the camera level.

Farther is an advantage for a topography sensor when at least approximately for at least a wavelength of light the amount of refractive power of the refractive power variable, imaging Systems to zero results. So a test lens can be used, which is corrected to infinity. These lenses are inexpensive and in big Diversity available. To a refractive power of the refractive power variable, imaging system reaching zero can be done with a sensor to capture the topography the refractive power variable imaging system preferably consists of two main components exist for a single wavelength of light at least approximately are opposite in refractive power.

Preferably, at least one of these subsystems is formed chromatically. Preferably, this system is then designed diffractive or at least partially diffractive. It can but also be chromatically formed both systems. The second system may be designed to be dispersive. Preferably, the refractive power variable imaging system is a focusing or diverging system, in each case a system having zoom characteristics and preferably rotationally symmetric in structure and function.

Farther can with a sensor for detecting the topography with advantage Camera be designed as a color-sensitive camera. This can be the camera also more than three spectral channels have, for example, eight spectral channels. So can with a priori knowledge of the occurring waveform, such as the mean ripple in the signal above the wavelength of light the focus of the envelope and determine the phase at the center of gravity of the signal. The to The necessary evaluation methods have already been used in Applied Optics, Vol. 39, No. 8, 10th of March 2000, pages 1290 to 1295 [9] as well as in Fringe'01: Proceedings of the 4th International Workshop on Automatic Processing of Fringe Patterns, Elsevier 2001, P. 173-180 under "Generalized Signal Evaluation for White-light Interferometry and Scanning Fringe projection "[10] or in the technical essay "Signal Processing Deep-Scanning 3D Sensors for New Industrial Applications " Technical Journal Technical Fairs 69 (2002) 5 [11]. Such a camera can, for example, with four sensor chips and a beam splitter arrangement with a total of four spoiler prisms and four matched Color dividers are built, with each of the four sensor chip two takes adjacent pictures of different centroid wavelengths.

If the spectral splitting into eight spectral channels - especially if a larger depth measuring range required is - off establish the depth measurement accuracy is insufficient, can with a sensor to capture the topography the camera also as a monochrome camera be educated. The illumination of the surface of the object then takes place with a narrow band of light. Then this camera is a spectrometer module preceded by an entrance slit onto which the light band is imaged becomes. This spectrometer module with internal imaging stage can for example, 100 spectral channels exhibit by the spectral decomposition of the light on 100 pixels and perpendicular to the formation of the light band image on the camera surface. Considering a spectral range of 200 nm of about 450 to 650 nm, so is the spectral resolution 2 nm. It is advantageous if the pixel pitch about an eighth strip equivalent. If an optical retardation of about 25 wavelengths is evaluated, so that corresponds approximately to a depth measuring range of about +/- 3 microns. The spectrometer module can be smaller than Geradsichtkeil or as a grating spectrometer resolution be educated.

Farther 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, wherein this usually has far more than 10 individual wavelengths.

Description of the figure

The Invention will be described by way of example with reference to FIG.

In the figure, the effect of a refractive power variable imaging component in a modified Linnik interference microscope is shown. In this case, when using a Linnik interferometer has the advantage that in this case a comparatively large aperture, z. B. NA = 0.9, is achievable. That of a slit-shaped white light source 1 Outgoing light, the gap is here positioned transversely to the plane, enters a collimator lens 2 , In this case, the light source consist of a conventional white light source, which is arranged downstream of a gap.

After the collimator lens 2 becomes the light beam at the beam splitter 3 split into a reference and an object beam path. The at the beam splitter 3 reflected light enters the object beam path, enters the focal plane of the test objective 5 on the refractive power variable, imaging system 4 , which is designed here as a chromatic system. It consists of a dispersive diverging lens and a diffractive lens, which acts as a convergent lens. The dimensioning is designed so that the refractive power variable, imaging system in a blue light diffuses light and in red light collects light. The main plane H'4 of the refractive power variable, imaging system 4 coincides with the focal plane representing the focal point F1 'of the collimator lens 2 and the focal point F2 of the test objective 5 contains. At a shorter wavelength of light, here in blue light, is the refractive power of the refractive power variable imaging system 4 negative and in red light its refractive power is always positive. Thus, depending on the wavelength of light in different depths of the object space, the image of the slit-shaped white light light source takes place 1 , For a wavelength of the white light source 1 a sharp image of a luminous point Q of the white light source takes place in a considered object point O. 1 such 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 at least approximately the same color longitudinal error as the refractive power variable map en System 4 possesses in the object beam path. The following microscopic standard objective 8th with focus F3 is identical to the test lens 5 , On the reference mirror 9 with a reflectivity of about 10%, the image of the white light source arises 1 at the same time for the light of all colors. So arises on the reference mirror 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 backscattered light from the object point O again passes the test objective 5 , the refractive power variable, imaging system 4 and the beam splitter 3 , In the reference beam path, the light is reflected, and also that of the image Q ", and the light beam passes through the standard microscope objective 8th and the plane parallel plate compensation system 7 , The light beam is at the beam splitter 3 reflected. The tube lens 10 forms the light bundles on a spectrometer gap 11 from. The blue light beams originating from the two light source points Q 'and Q''pass after passing through the imaging stage, which originates from the objectives 12 and 14 and after passing the light-decomposing double wedge 13 consisting of a dispersively acting wedge and a diffractive element, on a portion of the sensor chips of the camera 15 where there is interference between the two blue light bundles. The light bundles of adjacent light wavelengths form a signal with the considered blue light bundles, which at least approximately looks similar to a white light interferogram. In this case, the asymmetry Δx, which is determined by the effective positions of the foci F2 and F3, wherein the axial displacement by the refractive power variable, imaging system 4 and plane parallel plate compensation system 7 is included in the beam path zero. The focal points F2 and F3 are viewed from the beam splitter.

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

Claims (15)

Sensor for detecting the topography with a two-beam interferometer, with a light source ( 1 ), with a beam splitter ( 3 ) for splitting the light into an object and a reference beam path, with a test objective ( 5 ) in the object beam path for illumination and imaging of the object ( 6 ), with a camera ( 15 ) and with a refractive power variable, imaging system ( 4 ) whose refractive index variability enables the adaptation of the depth measuring range and depth resolution of the sensor, characterized in that the refractive power variable imaging system ( 4 ) at least approximately with one of its main points (H ') in the focal plane F2 of the test objective ( 5 ), which faces away from the object, is arranged. Sensor for detecting the topography according to claim 1, characterized in that the refractive power variable, imaging system ( 4 ) is formed chromatically, so that the focal length thereof 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 achromatisch is formed, so that in the same no color magnification error consists. Sensor for detecting the topography according to claim 3, characterized in that the two-beam interferometer is a Linnik interferometer whose lens ( 8th ) is formed achromatisch in the reference beam path, so that there is no color magnification error in the reference beam path. Sensor for detecting the topography according to one of claims 1 to 4, characterized in that the two-beam interferometer is a Linnik interferometer, wherein the refractive power variable, imaging system in the object beam path and in the reference beam path, a plane parallel plate compensation system ( 7 ), which has at least approximately the same color longitudinal error as the refractive-power-variable, imaging system ( 4 ) has in the object beam path. Sensor for detecting the topography according to one of claims 1 to 4, characterized in that the two-beam interferometer is a Linnik interferometer, wherein the refractive power variable, imaging system in the object beam path and in the reference beam path, a plane parallel plate compensation system ( 7 ), which has a specially adapted chromatic aberration. Sensor for detecting the topography according to one of claims 1 to 6, characterized in that the two-beam interferometer is a Linnik interferometer with a triple reflector (T) modified in the reference beam path. Sensor for detecting the topography according to claim 2, characterized in that the two-beam interferometer is a Michelson interferometer or Mirau interferometer, wherein in the reference A second refractive power variable, imaging system (S), but arranged with opposite refractive power, so that the color magnification error and the longitudinal chromatic aberration 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 the two-beam interferometer a Michelson interferometer or Mirau interferometer is at the in the reference beam path a plane parallel plate assembly (PP) with arranged at least one plane parallel plate of dispersive material is, so the color longitudinal error of the first refractive power variable, imaging system in the reference beam path at least approximately is compensated. A topographic detection sensor according to any one of claims 1 to 9, characterized in that the refractive power variable imaging system ( 9 ) is formed with a structurally electronically controllable, diffractive component and thus the refractive power of the refractive power variable, imaging system ( 4 ) is variable depending on the structure of the electronically controllable component. Topographically detecting sensor according to one of claims 1 to 10, characterized in that the main point (H ') of the refractive power variable imaging system ( 4 ) coincides at least approximately with the pupil center PZD in the imaging beam path. 12 sensor for detecting the topography according to one of claims 1 to 11, characterized in that at least approximately for at least one wavelength of the light, the amount of refractive power of the refractive power variable, imaging system ( 4 ) to zero. A topographically detecting sensor according to claim 12, characterized in that the refractive power variable imaging system ( 4 ) consists of two main components, which are at least approximately opposite in refractive power for a single wavelength of light. Sensor for detecting the topography according to one of claims 1 to 13, characterized in that the camera as a color-sensitive Camera is formed. Sensor for detecting the topography according to one of claims 1 to 13, characterized in that the camera is designed as a monochrome camera and this camera is a spectrometer module ( 13 ) is arranged upstream. Sensor for detecting the topography according to one of claims 1 to 15, characterized in that the light source is a multi-wavelength laser (MLL) with a frequency comb characteristic. is used, the has at least three different wavelengths.