GB2168476A

GB2168476A - Interferometer, in particular for incremental scanning of variable interference structures

Info

Publication number: GB2168476A
Application number: GB8528756A
Authority: GB
Inventors: Gerd Jager; Hans Buchner; Wilfried Hirschfeld; Georg Ranft
Original assignee: SUHL FEINMESSZEUGFAB VEB
Current assignee: SUHL FEINMESSZEUGFAB VEB
Priority date: 1984-11-22
Filing date: 1985-11-22
Publication date: 1986-06-18
Also published as: FR2573525A1; DD229208A1; DD229208B1; GB2168476B; DE3540856C2; GB8528756D0; DE3540856A1

Abstract

An interferometer in which a diaphragm 3 is located between an interferometer divider 1 and a beam divider connected beyond it, and the beam passing through the diaphragm is divided in the beam divider into two partial beams, each of these beams being transferred to a discrete photoelectric detector. A first beam splitter 30 and a second beam splitter 34 positioned beyond it are disposed between the interferometer divider 1 and the beam divider. The beam split by the first beam splitter in the direction of the second beam splitter is divided in the second beam splitter into two partial beams 35,36, each of these beams falling on a linear arrangement of integrated photoelectric scanning elements 38,40 associated with it. The direction of the two lines of scanning elements are perpendicular to one another. <IMAGE>

Description

SPECIFICATION Interferometer, in particular for incremental scanning of variable interference structures The present invention is concerned with interferometers, particularly for use in the incremental scanning of variable interference structures.

Interferometers can be used to measure all those technical and physical dimensions whose influence causes a change in the optical path difference.

An interferometer is known from GDR Patent Specification No. 201 191 which allows interferometricincremental measurement of interference structures which vary during measurement. With this interferometer, it is necessary to exactly position the two photoelectric scanning elements required for incremental measuring signal acquisition relative to one another so that they coincide at one point when projected into the interference structure produced by the interferometer.

If it is not possible, or only partly possible, to produce the identity of the two scanning elements related to the interference structure, the range of application of this interferometer is limited. On the other hand, it is constructionally and technically difficult to release the scanning element in an exactly adjusted position from the adjustment device and to connect it to the interferometer components in such a way that there are no changes in position.

A further disadvantage of the known solution is that the error of measurement increases as a result of the admissible variability of the interference structure.

The general aim of the invention is to provide an interferometer, in particular for scanning variable interference structures, which can be manufactured on a simple technological level but which fulfils the highest measurement accuracy requirements.

In particular, the present invention seeks to provide an interferometer for incremental scanning of variable interference structures which is independent of the adjustment of the scanning elements, and enables the elimination of the measurement error resulting from the inclination of the measuring face.

The broad range of application of interfero-- metry is based on the principle of incremental acquisition of measuring signals. As is known, the incremental method requires two measuring signals which are phase-shifted by 90". In accordance with the state of the art, a grating is required which does not vary during measurement. For this reason, it has not been possible to use interferometric-incremental acquisition of measuring values in a whole series of important applications in which the invariability of the interference grating produced in the interferometer during measurement is not guaranteed.

This applies in particular to the contactless scanning of plane and curved surfaces because, in known interferometers, it must always be ensured that the angular positions of the measuring beam and reference beam of the reflecting surfaces do not change. If this condition is met, the invariability of the interference structure required for incremental measuring signal acquisition is ensured. For the known interferometers, this has the result that, at least in the measuring arm of the interferometer, tiltinvariable optical components such as triple prisms, triple reflectors or socalled cat's-eye reflectors must be used as reflectors. This precludes contactless scanning of a variety of objects to be measured.

One exception is provided by the device described in the above-mentioned patent specification, which, by introducing the principle of the locally identical scanning points relative to the interference structure, allows interferometric-incremental measuring signal acquisition with interference structures which alter during measurement. The above arrangement has, however, the disadvantage that this interferometer property is only achieved by positioning the two photoelectric scanning elements in relation to the scanned interference structure.

Although the adjustment of the detectors into this position is not problematic and may be carried out systematically and controllably, it must be assumed that when the scanning elements are detached from the adjusting device and connected to the components of the interferometer, new changes in position occur which can destroy the previously obtained adjusted position. Furthermore, it is not ensured that the scanning elements continue to maintain the position relative to the interference structure which they assume following adjustment, which fact also results in a deterioration in the expected data from the interferometer and/or makes their proper functioning for the intended measuring task questionable.

The incremental scanning of variable interference structures which is made possible by the introduction of the principle of locally identical scanning points, means that the measuring surface scanned in the measuring arm of the interferometer does not have to be perpendi cular to the arriving measuring beam, but that it can take up a position which varies, within certain limits, from the perpendicular. It is thus possible to do without tilt-invariable reflectors in the measuring arm of the interferometer and to scan plane and curved measuring surfaces directly with the measuring beam in an optically contactless manner.

However, if the measuring face is not perpendicular to the arriving measuring beam, but is at an angle alpha deviating from the perpendicular, the measuring beam is reflected by the angle 2 alpha and the result is an increased measurement error relative to the per pendicular angular position. It is one aim of the present invention to eliminate these errors.

In accordance with the present invention a diaphragm (stop) is located between the interferometer divider and the beam divider, and the beam passing through the diaphragm is divided in the beam divider into two partial beams, each of which is fed to a photoelectric detector. A first beam splitter and a second beam splitter disposed beyond it are located between the interferometer divider and the beam divider. The beam split by the first beam splitter in the direction of the second beam splitter is divided into two partial beams in the second beam splitter, and each of these partial beams falls on a linear arrangement of integrated photoelectric scanning elements associated with it.

A matrix-like arrangement of integrated photoelectric scanning elements may also be disposed at one of the two outlets of the first beam splitter. Reflecting, polarizational optical and optically birefringing elements as well as discretely disposed photoelectric scanning elements and integrated photoelectric scanning elements in linear or matrix form can be included.

By inserting the diaphragm between the interferometer divider and the beam divider, the scanning point which is identical for both scanning elements is represented by the common diaphragm, and it is no longer necessary to create an identical scanning point by adjusting the two scanning elements. An interference figure is created which is large in comparison to the diaphraqm diameter, which is actually scanned pointwise through the diaphragm. Thus the position of the scanning elements in relation to the interference structure is no longer of primary importance for the scanning procedure, rather it must merely be ensured that the radiation passing through the diaphragm hits the radiation-active part of the scanning element.This is relatively easy to implement, as the diameter of the diaphragm is generally considerably smaller than the diameter of the radiation-active surface of the scanning elements. As the diaphragm can be brought into an absolutely fixed position relative to the interference structure, changes in position between the diaphragm and the interference structure are not possible. The relative positions of the diaphragm and the photoelectric scanning elements are not critical either, as the position of the scanning elements no longer influences the phase position of the incremental measuring signal acquisition.Thus, as a result of the diaphragm, two optical signals are available which, completely independent of adjustment procedures and the constructional and technical problems of mounting the components, can have exactly the 90 phase shift required for incremental measuring data acquisition at any one moment.

The starting point for the correction of the error caused by the measuring surface being in a nonperpendicular angular position to the arriving measuring beam is provided by the interference figure itself, due to the fact that the scanned interference structure contains the information on the angular position sought. In the interference figure, this angular position is expressed by the thickness of the interference fringes existing at the scanning point. The thickness of the interference fringes can be ascertained if the interference structure is projected onto a plurality of closely adjacent photoelectric scanning elements, for example onto a CCD line. Each individual element in such an arrangement generates an electrical signal corresponding to the irradiance.Cyclic interrogation of the elements of the CCD line produces an image of the interference structure transformed into an electric quantity. This image can be used to ascertain those elements on which, for example, the maxima of the interference structure are located, and the known clearance of these elements and the wavelength of the monochromatic radiation used in the interferometer can be used to find the inclination of the measuring face in the measuring point concerned.

As mentioned initially, interferometers can be used to measure all those technical and physical dimensions whose influence causes a change in an optical path difference. The present invention can further be used for measurement tasks in which the object of measurement causes changes in the interference structure during measurement. This particular characteristic of the invention enables contactless interferometric-incremental measurement of the flatness of plane surfaces, for example reflectors, silicon wafers and disc stores. Furthermore, it allows scanning of flat reflectors without the reflectors having to be guided in strict parallel, as well as the optically contactless scanning of spherical and aspherical surfaces and the use of the interferometer in pressure measuring chambers in which the gas flowing into and out of the chamber causes turbulence in the interference structure.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows an interferometer provided with a diaphragm in accordance with the present invention; Fig. 2 shows an interferometer with diameter and CCD lines for ascertaining the angular position of the measuring surface; .Fig. 3 illustrates the interference structure with a modulated beam; Fig. 4 shows the natural, sinusoidal curve of the interference structure as produced on the elements of one CCD line; and Fig. 5 shows the overall arrangement of the interferometer.

The interferometer in Fig. 1 comprises an interferometer divider 1 and a beam divider 2, between which the diaphragm (stop) 3 is disposed. The linearly polarized monochromatic laser beam 4 passes through thepolarisation filter 5 and the lambda/4 plate 6 into the interferometer 1, in which it is divided at the divider layer 7 into a measuring beam 8 and a reference beam 9. The measuring beam 8 passes through the lambda/4 plate 10 and strikes the measuring reflector 11, which reflects it back into the interferometer divider 1. The measuring beam 8 and the reference beam 9 are reflected at the reference reflector 12.The measuring beam 8 and the reference beam 9 are superimposed on the divider layer 7, and the result is the interference structure 20 which, when the measuring reflector 11 and the reference reflector 12 are plane, is fringe-like, if the measuring mirror 11 and reference mirror 12 are not exactly perpendicular to one another. The part 13 of the interference structure passing through the diaphragm 3 is divided at the beam divider 2 into beams 14 and 15 which, after passing through the analyzers 16 and 17, strike the scanning elements 18 and 19. The 90" phase difference between beams 14 and 15 necessary for incremental measuring value acquisition is set by the angle which the transmission direction of the analyzers 16 and 17 form together.

The diaphragm 3 is suitably formed by vapour deposition or photolithqgraphic methods on the surfaces of the interferometer divider 1 or the beam divider 2, which are cemented together. These methods make it easy to produce circular diaphragm apertures with diameters of up to several micrometers. This is of importance for the present invention insofar as the smallest scannable grating constant of the interference structure is dependent upon the diaphragm diameter concerned. Thus, by selecting the diaphragm diameter, the interferometer can be exactly adapted to the present task.It can be assumed that evaluatable electric output signals can be generated at the discrete photoelectric detectors 18, 19 even when the diaphragm diameter is three quarters of the clearance of the grating of the interference structure, For example, with a diaphragm diameter of 20 micrometers, interference structures having grating constants < 26 micrometers can still be scanned. In this case the normal to the measuring surface at the measuring point has an angular position of 41' to the arriving measuring beam where lambda = 633 mm.

Fig. 2 shows an embodiment of the interferometer in which the angular position of the measuring surface to the arriving measuring beam can be ascertained, and the resulting measurement error corrected computationally.

For this purpose, two beam splitters 30 and 34 are connected between the interferometer divider 1 and the beam divider 2, which beam splitters have the task of projecting the beam 20 modulated with the interference structure onto two CCD lines 38 and 40. The beam 4 arriving from the monochromatic radiation source enters the interferometer divider 1, is divided at the divider layer 7 into a measuring beam 8 and a reference beam 9, whose recombination produces beam 20 which is modulated with the interference structure. This beam 20 enters a first beam splitter 30, at the divider layer 31 where it is divided into a partial beam 32 which is transferred to the diaphragm 3 which lets through a part 13 of the beam 32, which is further processed as described in Fig. 1, and into a second beam splitter 34 which splits the beam 3 into partial beams 35 and 36.Beam 34, after passing through an analyzer 37, strikes a CCD line 38, and beam 35 passes through the analyzer 39 and strikes the CCD line 40. The CCO line is disposed in such a way that its photoelectrically active elements lie in the plane of the drawing, whereas the photoelectrically active elements of the CCD line 40 are perpendicular to the plane of the drawing.

Fig. 3 shows the projection of the beam 20 modulated with the interference structure 50 onto the CCO line 38. It is assumed that the measuring surface is a plane reflector, so that the interference structure 50 consists of straight parallel fringes, which, triggered, are shown as a light-dark distribution. The lines of the maxima 51 or the minima 52 in this interference structure are excellent lines for the purposes of scanning. The natural sinusoidal curve of the inteiference structure 50 generates electrical signals at the photoelectric Ielements of the CCD line 38, as shown in Fig.

4. For example, a minimum of the interference structure falls on the elements 1=2 and 1=20.

As the clearances between the I-elements are known, the result is the grating constant of the interference structure 50 in the direction of the CCD line 38. However, in order to find the angular position of the measuring surface to the arriving measuring beam, the grating constant of the interference structure perpendicular to the direction of the interference fringes is required. This is obtained from a crossed arrangement of two CCD lines.

Fig. 5 shows the overall arrangement of the interferometer in Fig. 2, which is of importance for the acquisition of measuring signals, from the direction of beams 32, 35 and 36.

The diaphragm is transparent inside the circle and not transparent outside it. The line directions of the CCD lines 38 and 40 are perpendicular to one another and the interference structure 50 to be scanned is shown in projection onto the CCD lines 38 and 40 and the diaphragm 3. The CCD lines 38 and 40 represent a system of (x, y) co-ordinates, wherein CCD line 38 represents the x co-ordinate, CCD line 40 the y co-ordinate, and the point of intersection of the co-ordinate system is simultaneously the zero point of the co-ordinate system which coincides with the centre of the diaphragm 3. The grating spacing G of the interference structure is determined using the triangles AOB and COD. The paths AO, BO, CO, DO are the x or y co-ordinates of the intersection points of the adjacent maxima 51, 51' with the co-ordinate axes.These co-ordi nates are used to determine the height H1, H2 of the triangles whose sum is the required grating constant G.

A CCD line L 110 of the VEB WF Berlin is, for example, known, which has 256 integrated photoelectrically active elements on a path of 3.6 mm. The The interference structure can still be resolved if a cycle of the grating is scanned by 5 elements of the CCD line. Under this precondition, a smallest possible resolvable G=0.056 mm is obtained, to which, if lambda = 633 mm, a deviation of the angular position from the norm of 20' corresponds.

The main advantage of this arrangement is, on the one hand, that, by determining the angular position of the measuring surface with the same measurement error, considerably larger measuring ranges are admissible and, on the other, the angular position of the measuring surface is ascertained directly at the measuring point, that is in the diaphragm, which enables error-free measurement of objects with locally extremely varying curvatures. Of course, a matrix-like arrangement of integrated photoelectric scanning elements may be directly disposed instead of the two CCD lines 30 and 40 in the beam 33 following the first beam splitter.

Claims

1. An interferometer, in particular for incremental scanning of variable interference structures, comprising a monochromatic radiation source providing a radiation beam, an interferometer divider, a beam divider, a diaphragm located between the interferometer divider and the beam divider, the beam passing through the diaphragm being divided into two partial beams, a discrete photoelectric detector disposed in each partial beam path, a first beam splitter and a second beam splitter disposed beyond it which are located between the interferometer divider and the beam divider, the beam split by the first beam splitter being divided in the second beam splitter into two further partial beams, and respective linear arrangements of integrated scanning elements disposed in the paths of said further partial beams, the direction of the two lines of scanning elements being perpendicular to one another and the point of intersection of the lines being in the centre of the diaphragm.

2. An interferometer as claimed in claim 1, wherein a matrix-like arrangement of integrated photoelectric scanning elements is disposed on one of the two outlets of the first beam splitter and thus in the path of said further partial beams.

3. An interferometer substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.