WO2002082007A1 - Interferometric measuring device - Google Patents

Abstract

The invention relates to an interferometric measuring device for measuring the surface or boundary surface of an object of measurement (O). The aim of the invention is to prevent vibrations from influencing the result of measurement. To this end, a connecting unit (VE) is provided which directly links, in a mechanically rigid and detachable manner, the object of measurement (O) with at least one section of the measuring device associated with the object of measurement (O).

Description

 02/082007

Interferometric measuring device

The invention relates to an interferometric measuring device for measuring a surface or interface of a measurement object.

State of the art

Such an interferometric measuring device is identified as known in DE 1 97 2 1 843 C1, this known interferometric measuring device being constructed as a white light interferometer measuring device with a section designed as a modulation interferometer and a section designed as an imaging interferometer that is assigned to the measurement object. The imaging interferometer is designed in such a way that measurements can also be carried out in narrow cavities. It is suggested that a first 02/082007

teπ partial beam further into a reference partial beam and at least one measuring

Separate partial beam, with a further beam splitter and a reference mirror are arranged in a common measuring probe. Such a measuring probe can be inserted into cavities. E.g. in a manufacturing process during measurement, vibrations of the measuring device and / or the measurement object can result in inaccuracies in the measurement result.

The principle ^'of white light interferometry or short-coherence interferometry, for example, in P. de Groot, L. Deck, "Surface profiling by analysis of white- light interferogram in the spatial frequency domain" J. Mod. Opt., Vol. 42, No.

2, 389-401, 1 995 and T. Maack, G. Notni, W. Schreiber, W.-D. Prenzel, "Endoscopic 3D shape measuring system", in yearbook for optics and precision engineering, Ed. W.-D. Prenzel, Verlag Schiele and Schoen, Berlin, 231-240, 1 998.

Furthermore, the structure of an interferometric measuring device is also in DE

197 21 843 C2. In this known measuring device, a radiation generation unit, for example a light-emitting diode or superluminescent diode, emits a short-coherent radiation, which is split via a beam splitter into a first partial beam guided via object light and a second partial beam guided via a reference path. The reference light path is changed periodically by means of two deflector elements and a fixed diffraction grating arranged behind them by actuating the deflector elements in order to scan the object surface in the depth direction. If the object light path and the reference light path match, there is a maximum 02/082007

the interference contrast, which is detected by means of an evaluation device connected downstream of the photodetector device.

In a further white light interferometer measuring device specified in DE 41 08 944 A1, a moving mirror is used to change the light path in the reference beam path. Even with these procedures

Effects of vibrations of the measuring device and / or the measurement object on the measurement result are not excluded.

The invention has the object of providing an interferometric measurement device of the type mentioned to provide, with the impact of ^'

Shocks of the measuring device and / or the measurement object on the measurement result are excluded as completely as possible.

Advantages of the invention

This object is achieved with the features of claim 1 in that a connection unit is provided, by means of which at least one section of the measuring device associated with the measurement object can be directly mechanically rigidly and detachably connected to the measurement object.

Due to the mechanically rigid connection between the measurement object and the interferometric measurement device, movements during a vibration are communicated to the measurement object and the measurement device in the same way, so that they do not lead to a falsification of the measurement result. By means of the 02/082007

, 4

Connection unit is a mechanical adaptation between the measurement object and the measuring device for the measurement process. Then the connection can be released and a mechanically rigid coupling to the subsequent measurement object can be made, e.g. carry out a quality control in a series production.

Different design options consist in the fact that the connection unit is formed by a plug, clamp, tension, snap, clip or screw connection.

In principle, it is possible to implement these measures for vibration compensation in various interferometric measuring devices, e.g. classic interferometry and 2-wavelength π interferometry with appropriate training of data acquisition.

An advantageous embodiment of the interferometric measuring device is that it is designed as a white light interferometer measuring device with an object light path and a reference light path and that at least the reflective reference plane can be connected to the measuring object by means of the connecting unit.

A compact structure is achieved in that the object light path and the reference light path form an interconnected rigid unit consisting of object arm and reference arm. 02/082007

If it is provided that the white light interferometer has a modulation interferometer and an imaging interferometer assigned to the object, this results in a simplified operation since the generally more complex modulation interferometer with the device for changing the light path is separated from the imaging interferometer assigned to the measurement object, which corresponds to this - can be constructed simply and robustly and can be adapted to the respective measuring task.

In particular, a structure advantageous for handling z. B. characterized by a short-coherent radiation generating unit, a beam splitter for forming a first and a second partial beam, with the measurement changing the optical path length of the first partial beam relative to the optical path length of the second partial beam, the one formed by the beam splitter first partial beam is first directed to a fixed first mirror via a first arm, while the second partial beam is directed to the reflecting element via a second arm, the optical path difference between the first and second arms being greater than the coherence length of the radiation, wherein the radiation which comes from the first mirror 1 and the reflecting element and which is passed on together is guided by means of a further beam splitter partly via the object light path to the measurement object and partly via the reference path to a reference mirror, the reference mirror being in a s such a distance is arranged with respect to the measurement object that the path difference between the first mirror and the reflecting element is eliminated, and wherein the radiation incident on the reference mirror and that directed to the measurement object Radiation reflects superimposed and is recorded by a photodetector device with an image sensor.

A construction which is favorable for handling consists in that the reference light path is formed in a separate reference office or in an optical probe through which the radiation guided to the measurement object is also guided, the reference mirror reflecting the part of the radiation belonging to the reference light path and that part of the object light path that transmits the radiation. '

A simple measurement of different measuring surfaces is made possible by arranging at least one optical element and / or elements deforming the wavefront of the radiation in the object light path.

If it is provided that an optical probe with an optical arrangement for generating at least one optical intermediate image is provided in the object light path, a high lateral resolution of the measuring surface is made possible, as in the applicant's unpublished German patent application 1 00 47 495 executed.

Favorable configurations consist in that the at least one intermediate image is generated in the object light path and in that both the radiation leading to the measurement object and the radiation returning from it run through the optical probe. A favorable construction also results from the fact that the reference mirror is provided on a plane plate or a prism.

For handling it is advantageous that a fiber optic is arranged between the beam splitter and the further beam splitter.

For the construction and operation, advantages can still result in that in the Objektiichtweg for generating the ^{'intermediate} image is arranged an ene doskop.

Further favorable design options consist in the fact that a round vision optic or a superposition optic is arranged in the object light path and that it has several reference planes for scanning different measuring surfaces.

drawing

The invention is explained in more detail below using exemplary embodiments with reference to the drawings. Show it:

1 shows a first exemplary embodiment of an interferometric measuring device with a modulation interferometer and an imaging interferometer rigidly connected to an object, 02/082007

8th

Fig. 2 shows another embodiment in which the imaging interferometer has a Commoπ-Path structure and is rigidly connected to the object, and

Fig. 3 shows a further structure of the interferometric measuring device with a common reference and measuring light path (Cornmon Path

Construction) .

embodiment

Fig. 1 shows an interferometric measuring device with a modulation interferometer MI removed from an object D to be measured and an imaging interferometer AI assigned to the object O. The modulation interferometer MI has a short-coherent or broadband radiation-generating radiation unit SLD, such as a light-emitting diode or superluminescent diode, the radiation of which is divided by means of a beam splitter ST1 into a first partial beam T1 of a first arm and a second partial beam T2 of a second arm. The setup corresponds to a Michelson interferometer. In the second arm, the second partial beam T2 is reflected by a reference plane in the form of a reference mirror RSP1, the second arm periodically, for example, by moving the reference mirror RSP1 or by means of acousto-optical deflectors, as described in DE 1 97 21 842 C2 mentioned at the beginning will be changed. If the change in the light path s with two O 02/082007

made acousto-optical deflectors, there is no need for a mechanically moving reflective element, but instead a fixed element, in particular a grating, can be used.

The optical path difference between the arms thus formed is greater than the coherence length of the one generated by the radiation generation unit SLD

Radiation. From the two mirrors SP1 and RSP1, the reflected radiation is fed into the imaging interferometer via the beam splitter ST1, a fiber optic LF and a further beam splitter ST2.

An object light path with an optical probe OS and a reference light path with a further reference mirror RSP2 are formed in the imaging interferometer AI, which is likewise designed as a Michelson interferometer.

In the object light path, the radiation is coupled into the optical probe OS, so that the radiation is a surface of a measurement object to be measured

O illuminated. The object surface is imaged by the optical probe OS via one or more intermediate images, for example, on a photodetector device in the form of an image converter or image sensor BS, for example a CCD camera. The image of the measurement object O on the image sensor BS is overlaid with the reference wave of the second partial beam of the reference light path. In the image of the measurement object O, high interference contrast occurs when a path difference in the reference light path and the measurement light path is smaller than the coherence length. The measuring principle is based on white light interferometry (short-coherence interferometry), as described in more detail in the publications mentioned at the beginning. The length of the reference light path is varied over the entire measuring range for scanning in the depth direction of the surface to be measured by moving the mirror RSP1 in the second arm of the modulation interferometer MI, the length of the reference light path at which the highest interference contrast occurs being detected for each measuring point , The intermediate images make it possible to image the surface of the measurement object with a high lateral resolution over a distance that is large compared to the diameter of the imaging optics. The optical probe OS is similar to an endoscope or borescope, but the illumination and the return of the radiation coming from the measurement surface are carried out via the same optical arrangement via at least one intermediate image. In Fig. 1, some lenses L are shown schematically as further imaging elements. The actual intermediate images are generated in the optical probe OS.

For applications in which precise compensation of the influence of the imaging lenses of the optical probe OS is necessary, the same optical probe can also be integrated into the reference path between the beam splitter ST2 and the reference mirror RSP2 as in the object path between the beam splitter ST2 and the measurement object O.

In a modified construction according to FIGS. 2 and 3, the interferometric measuring device, specifically the imaging interferometer AI, can also be arranged with a common reference and measuring arm (common path 5 order). The interferometric measuring device is again illuminated with a short-coherent (broadband) radiation generation unit. The beam splitter ST1 divides the light into the first partial beam T1 and the second partial beam T2, the first partial beam T1 being fixed on the first mirror SP1 and the second partial beam T2 falling on the reflecting element RS P1 in the form 10 of the reference mirror.

, The optical path difference between the arms thus formed is in turn greater than the coherence length of the radiation generated by the radiation generating unit SLD. From the two mirrors SP1 and RSP1, the reflected radiation is fed into the optical probe OS via the beam splitter ST1 and the further beam splitter ST2. The peculiarity of this construction is that a reference mirror RSP2 is located in the object path or in the optical probe OS itself.

0 Part of the radiation is reflected at this reference mirror RSP2, while the other part of the radiation illuminates the surface to be measured. The reference mirror RSP2 can be applied on a flat plate or on a prism. By using a prism, the wavefront of the radiation illuminating the object surface, i.e. the object wave to the geometry

! 5 (e.g. inclination) of the surface to be measured. The measurement object O is in turn imaged on the image sensor BS by means of the optical probe OS and superimposed on the reference wave by means of one or more intermediate images. To obtain the height information, the reflecting element RSP1 is moved over the measuring range or the change in the light 02/082007

1 2

way, as described in the aforementioned DE 1 97 21 842 C2, made. In the image of the measurement object 0, high interference corrosion occurs when the path difference between the fixed mirror SP1 and the reflecting element RSP1 or the light paths of the two arms is exactly the optical path difference between the reference mirror RSP2 and the measurement object 0. Known methods for detecting the highest interference contrast in each image point (pixel) are used to obtain the 3D height profile. The advantage of this design is that the object and reference waves pass through almost the same optics, which largely compensates for aberrations. In addition, this arrangement is even more robust against mechanical vibrations.

For an even easier handling of the measuring device, the radiation from the beam splitter ST1 can also be transmitted to the further beam splitter ST1 by means of fiber optics LF, as shown in FIG. 2 shown. Alternatively, a free jet setup can be selected.

In all of the structures described, the imaging interferometer A is designed as a rigid mechanical unit, at least in the section of the object light path and the reference light path, which in turn is rigidly and detachably connected directly to the measurement object O during the measurement by means of a connection unit VE in a connection area VB , The connection unit is, for example, a screw connection with a union nut, a screw section of the imaging interferometer AI that can be screwed into a threaded bore of the measurement object O, a plug connection, a snap or clip 02/082007

13

Connection or a clamp or tension connection provided. If a common path structure is selected, the object light path or object arm and the reference light path or reference arm can be inserted together into the measurement object O.

Claims

Expectations 1 . Interferometric measuring device for measuring a surface or interface of a measurement object (O), characterized in that a connection unit (VE) is provided, by means of which at least one section of the measurement device associated with the measurement object (0) with the measurement object (0) is directly mechanically rigid and detachable is connectable. 2. Measuring device according to claim 1, characterized in that the connecting unit (VE) is formed by a plug, clamp, clamp, snap, clip or screw connection. 3. Measuring device according to claim 1 or 2, characterized in that 5 that it is designed as a white light interferometer measuring device with an object light path and a reference light path, and that at least the reflective reference plane (RSP2) can be connected to the measuring object (0) by means of the connection unit (VE). 4. Measuring device according to claim 3, characterized in that the object light path and the reference light path form an interconnected rigid unit consisting of object arm and reference office. 5 5. Messvorrichtuπg according to claim 3 or 4, characterized in that the white light interferometer has a modulation interferometer (MI) and an object (O) associated imaging interferometer (AI). O 6. Measuring device according to one of the preceding claims, characterized by a short-coherent radiation-emitting radiation generating unit (SLD), a beam splitter (ST1) for forming a first and a second partial beam (T1, T2), ; wherein the optical path length of the first partial beam (T1) is changed relative to the optical path length of the second partial beam (T2) for measurement, the first partial beam (T1) formed by the beam splitter (ST1) initially via a first arm to a fixed first S is directed (SPD), while the second partial beam (T2) is directed via a second arm onto the reflecting element (RSP 1), 5 wherein the optical path difference between the first and second Arm is greater than the coherence length of the radiation, the radiation which comes from the first mirror (SP1) and the reflecting element (RSP1) and is passed on together by means of a further beam splitter (ST2), in part via the object light path to the measurement object (0) and is guided in part via the reference light path to a reference mirror (RSP2), the reference mirror (RSP2) being arranged at such a distance with respect to the measurement object (0) that the path difference between the first mirror (SP1) and the reflecting element (RSP1) listed 5 is raised, and the radiation incident on the reference mirror (RSP2) and the radiation guided to the measurement object (0) are reflected and superimposed by a photodetector device with an image recorder (BS). O 7. Measuring device according to claim 6, characterized in that the reference light path is formed in a separate reference arm or in an optical probe (OS) through which the radiation guided to the measurement object s (0) is also guided, the reference mirror (RSP2 ) the Part of the radiation belonging to the reference light path is reflected and the part of the radiation belonging to the object light path is transmitted (common path arrangement). 8. Measuring device according to one of claims 3 to 7, characterized in that at least one optical element and / or the waveform of the radiation-deforming elements are arranged in the object path. 9. Measuring device according to one of claims 3 to 8, characterized in that an optical probe (OS) with an optical arrangement for generating at least one optical intermediate image is provided in the object path. 1 0. Measuring device according to claim 9, characterized in that the at least one intermediate image is generated in the object light path. 1 1. Measuring device according to claim 9 or 10, characterized in that both the radiation leading to the measurement object (O) and the radiation returning from it run through the optical probe (OS). 1 2. Measuring device according to one of claims 7 to 1 1, characterized in that the reference mirror (RSP2) is provided on a flat plate or a prism. 02/082007 1 8 1 3. Measuring device according to one of claims 6 to 1 2, characterized in that a fiber optic (LF) is arranged between the beam splitter (ST1) and the further beam splitter (ST2). 1 4. Measuring device according to one of claims 9 to 1 3, characterized in that an Eπ doscope is arranged in the object path for generating the intermediate image. 1 5. Measuring device according to one of claims 3 to 14, characterized in that an all-round optical system or a superposition optical system is arranged in the object path. 16. Measuring device according to one of claims 3 to 1 5, characterized in that it has a plurality of reference planes for scanning different measuring surfaces.