DE10325601B3 - Switchable point light source array and its use in interferometry - Google Patents

Abstract

The present invention relates to a switchable point source array and its use for interferometric measurement of Prüflingsoberflächen. The point light source array consists of a liquid crystal display, a microlens array arranged behind the liquid crystal display, through which the input beam is split into individual partial beams after passing through the liquid crystal display, and a pinhole array. The pinhole array is designed and arranged behind the microlens array such that only a first or higher diffraction order of the partial beams impinging on the aperture opening of the pinhole array occurs due to diffraction at a diffraction pattern inscribed in the liquid crystal display. DOLLAR A By incorporating the point source array in an interferometer can be very well known wavefronts of different propagation direction generate reference waves, which can be switched independently and push in their phase.

Description

Technical application

The The present invention relates to a switchable point light source array, that of a coherent optical Input beam a variety of independently switchable Point light sources forms, as well as its use in an interferometer for the interferometric measurement of the wavefront of an object beam.

The interferometric measurement of strongly aspherical surfaces an unsatisfactorily solved task in optical metrology. In this measurement technique, an object beam of the interferometer on the surface to be measured reflected and with an undisturbed Superimposed reference beam, to reconstruct the object surface from the interference of the two rays. While this in the measurement of spherical surfaces by generating an object wave with a surface adapted to the surface to be measured spherical Wavefront usually without any problems is possible at the measurement of aspherical surfaces due to excessive deviations of the surface form of the test piece from the spherical one Object wavefront sections strong tilting of the reflected Object waves opposite the Reference wave occur too high, with the measuring sensors not more resolvable stripe densities in the interferogram to be evaluated.

to Avoiding this problem is for example from H. J. Tiziani et al., "Testing of aspheric surfaces ", Proc. SPIE, 4440, 2001, 109-119, known, computer-generated, lithographically crafted holograms in the object beam path, which is the wavefront of the object beam deform so that it impinges perpendicular to the surface to be tested. In this technique, also referred to as zero test shows the recorded Interferogram only the deviation of the surface to be tested from a nominal shape. Enable computer-generated holograms thus a fast and highly accurate measurement aspherical Examinees. However, this must be for every test piece shape a separate computer-generated hologram be made. This requires a considerable amount of time and money.

Out Y.-M. Liu et al., "Subaperture testing of aspheres with anular zone ", Appl. Opt., Vol.27, No.21, 1988, 4504-4513, or from M. Melozzi, et al., Testing aspheric surfaces using multipe anular interferogram ", Opt. Eng., 32 5, 1993, 1073-1079, is a technique for measuring the surface of rotationally symmetric specimens in the case of, for example, in a Twyman-Green interferometer several individual measurements with spherical Wavefront different radii are performed, with which the specimen is illuminated. This ensures that in each of these individual measurements different annular area of the DUT nearly is taken perpendicularly from the wavefront, so this area can be evaluated well interferometrically with the respective single measurement is. After this section-wise measurement of all surface areas of the test piece have to the results of the individual measurements taking into account propagation effects together to reconstruct the entire UUT surface. To produce the spherical Wavefronts of different radii will be between the individual measurements either the examinee or the interferometer optics, d. H. which is used to produce the spherical Wavefront lens inserted, mechanically shifted. The positioning accuracy However, the mechanical displacement units used thereby limited the measurement accuracy of this method. The stitching (stitching / sewing) the individual areas is as well in this case mathematically very complicated, because of the shift of the elements Propagation effects between the individual measurements considered Need to become.

Farther are interferometric curvature sensors known, which measure in sections the local curvature of the specimen, taking the form of the test piece is reconstructed by two-fold integration. The required here Pointwise surveying, however, is just as well known as the tactile one Measurement of the test object very time consuming with a coordinate measuring machine.

A furthermore known non-interferometric method for surveying aspherical Surfaces uses so-called Shack-Hartmann sensors for examining a specimen reflected from the specimen Wave front. However, with these sensors, the measuring point density is predetermined by the number of microlenses used and thus compared to areal interferometric measurement methods low.

G. Wernicke et al .: "The Digital Optical Laser System", in: Humboldt-Spektrum 1/2003, S. 54-57, be write applications of spatial light modulators, among other things, the use of a light modulator is referred to as a grid with switchable diffraction angle.

outgoing from this prior art, the object of the present Invention therein, an apparatus for use in the interferometric Measurement of surfaces specify a flexible adaptation to different aspherical UUT surfaces as well allows accurate measurement of these surfaces.

Presentation of the invention

The Task is with the switchable point light source array according to claim 1 solved. The claims 8 through 11 give preferred uses of this point light source array at. Advantageous embodiments of the point light source array are Subject of the dependent claims or can be the following description and the embodiments remove.

The Switchable according to the invention Point light source array forms from a coherent optical input beam a variety of independently switchable Point light sources, in addition can be postponed in their phase. The point light source array consists of a spatial Light modulator with a plurality of electrically addressable pixels, diffraction patterns in the partial regions of the light modulator Diffraction of the input beam can be generated, one behind the light modulator arranged microlens array, through the input beam after passing through the light modulator is split into individual sub-beams, and one behind the Microlens array arranged pinhole array. The pinhole array is designed and relatively arranged to microlens array that only a first or higher diffraction order the partial beams, by diffraction at one in the light modulator inscribed diffraction pattern arises on apertures of the pinhole array hits.

By this design and arrangement of the pinhole array is thus prevents the zeroth diffraction order of the partial beams or undeflected partial beams pass through the pinhole array. By writing a suitable diffraction pattern, preferably a stripe grid, into one with one of the microlenses of the microlens array congruent region of the light modulator can thus be the first (or higher) Diffraction order of the partial beam formed by the microlens pass through the pinhole array while the other sub-beams blocked by the pinhole array. The microlens assigned aperture in the pinhole array represents in this case an approximately punctiform light source In this way, by appropriate control of the Light modulator any further sub-beams and thus more Switch point light sources on or off. The pinhole array is located thereby preferably exactly in the focal plane of the microlens array, so that correspondingly small apertures are used in the pinhole array can be.

The Parameters of the generated diffraction pattern, in the case of the strip grating the strip spacing, are of course set so that the desired Diffraction order, preferably the first order, by the corresponding aperture passes. By moving the diffraction pattern can be in this way, the phase in which through the pinhole Moving sub-beam defined move.

When Light modulator, a liquid crystal display (LCD) is preferably used. However, it can also be other elements such as a Insert tilting mirror array, in the latter case, that of Kippspiegelarray reflected partial beams over a beam splitter and the microlens array on the pinhole array can be displayed. In the following, reference will be made to the use of a liquid crystal display, the designs Of course also for other light modulators apply.

The used microlens array preferably has a central microlens on, for example, rotationally symmetric or according to a square grid further rows of microlenses are arranged.

Of course, the liquid crystal display must be finely pixelated in such a way that corresponding diffraction patterns for diffracting the input beam can be generated in the individual subareas assigned to the microlenses. The parameters microlens size, micro lens focal length, wavelengths of the light used and the strip density of the diffraction pattern used must be matched to one another be that the diffraction orders in the focal plane of the microlenses are clearly separated from each other, so that the selection of a special diffraction order is possible through the pinhole array. Thus, for example, when using microlenses with a diameter of 2.5 mm and a focal length of 9 mm and a HeNe laser of wavelength 633 nm can be achieved with a liquid crystal display with a pixel pitch of 36 microns good results.

By the use of this switchable point light source array in the reference beam path of an interferometer assembly for measuring aspherical surfaces in conjunction with a the point light source array downstream Lens for collimating the partial beams by switching between the individual point light sources or partial beams differently across from the reference beam or the object wave tilted reference beams or generate reference waves. With these different tilted Reference waves can aspherical specimens random geometry can be measured in sections. The from the Interferograms of the individual measurements evaluated surface areas will be afterwards assembled to the surface of the test piece to reconstruct, as well as that in the introduction to the description cited Stitching / Sewing technique is known.

This applies equally to the use of the switchable point light source array in the beam path of the Object beam to successively different from the Reference beam to produce tilted object beams.

Farther is a use of the switchable point light source array in the beam path of the object beam of an interferometer possible to be different from the optical axis used for collimating the partial beams Lens tilted to produce object beams in which at least two differently tilted object beams generated simultaneously and be brought to interference. From the interference of different tilted object beams can be then also reconstruct a wavefront.

With the switchable invention Point light source array can thus be very well known wavefronts generate different propagation direction, which switch independently and have them postponed in their phase. By installation in an interferometer is the interferometric measurement of wavefronts with high Dynamics, such as B. in the reflection of strongly aspherical surfaces arise, possible. Across from the well-known surveying, in the wavefronts of different Radii for measuring the test object be used, on the one hand has the advantage that no mechanical Displacement of an optical element is performed so that thereby caused errors are avoided. On the other hand, there is the advantage that in the present technique, the individual measurements with interferometric Accuracy against each other can be calibrated, as in the execution example set out in more detail is. Because of this calibration, it is unproblematic that out different measurements obtained areas of the specimen with each other to combine and then how to handle a single measurement. Propagation effects must be there not considered become. Another significant advantage of using the present Switchable point light source arrays and the associated measurement method is that the process is not rotationally symmetric specimens limited is.

Short description of drawings

The present switchable point source array and its preferred Use for interferometric measurement of specimen surfaces below with reference to an embodiment in Connection with the drawings explained again.

in this connection demonstrate:

1 an example of an interferometer assembly for surveying specimen surfaces using the present point source array;

2 a section of the point light source array to illustrate the operation; and

3 an example of a construction with the point light source array, which replaces the displaceable end mirror in the reference beam path of a Twyman-Green interferometer.

Ways to execute the invention

1 shows an exemplary interferometer assembly for measuring a specimen surface using the present switchable point light source array 10 , The coherent laser beam emitted and expanded by a laser 8th In this interferometer construction, first of all, a beam splitter is encountered 7 through which he enters an object beam 8b and a reference beam 8a is split. The object beam 8b becomes after passing through another beam splitter 7 by means of a condenser lens 30 focused to one to the specimen surface 40 to produce a roughly adapted spherical wavefront. The from the UUT surface 40 The reflected wavefront of the object beam is finally transmitted through the beam splitter 7 with the reference beam 8a brought to interference. The imaging optics 50 forms the specimen surface 40 to the camera 60 from which the received interference pattern is recorded.

In the present case, the reference beam 8a via the switchable point light source array 10 directed to produce successively different tilted reference waves. For this purpose, successively different point sources of the point light source array 10 connected. 1 shows a schematic representation in which such a point light source is switched on. The light emanating from the point light sources is from the subsequent lens 20 , which is located at the focal distance from the pinhole array, collimated. By switching point light sources whose partial beams in different Lichem distance to the optical axis of the lens 20 run reference waves of different tilting are generated and superimposed with the object wave. This will cause different areas of the specimen surface 40 evaluable, which are then joined together (stitching). By suitable control of the point light source array 10 can be, as explained in more detail below, also the phases of the reference waves move defined. Thus, the phase shift algorithms customary in interferometry can also be carried out. It eliminates the need for an additional piezo or liquid crystal phase shifter, as used in particular in the end mirror of the reference arm known interferometer.

2 shows the structure and operation of the present switchable point light source array 10 , The point light source array 10 consists of a finely pixelated, electrically addressable liquid crystal display (LCD) 2 a microlens array 3 and a pinhole array 4 , The liquid crystal display 2 is collimated and coherent with an input beam 1 so lit up that behind the microlenses of the microlens array 3 initially arising foci not the apertures of the pinhole array 4 to meet. Only when a strip grid with a defined grid spacing is inscribed in the liquid crystal display in front of one of the microlenses does the first diffraction order arrive 5 of the sub-beam diffracted by the inscribed grating through the associated aperture. This is shown schematically in the figure, which only a section of the point light source array 10 represents. The remaining portions of the input beam 1 , In particular, the zeroth diffraction order shown in the figure and the remaining on the liquid crystal display 2 incident beam portions are transmitted through the pinhole array 4 blocked. In addition, there is the possibility of shifting the phase of the light in the first diffraction order in a defined manner by shifting the inscribed grating. Thus, the presented point light source array is a switchable phase shift point light source array.

An advantage of using this point light source array 10 In the interferometric measurement of specimen surfaces is that the individual measurements with interferometric accuracy can be calibrated against each other. For this purpose, the object arm of the interferometer is blocked. With the point light source array 10 two different reference waves are generated simultaneously, e.g. B. on the optical axis of the lens 20 extending non-tilted reference wave R ₀ and one of the N tilted reference waves R _n . They are brought to interference. One of the reference waves is phase shifted in phase and a phase function is calculated using a conventional phase shift algorithm. This yields the difference function D _n = R _n -R ₀ between the two reference waves in the plane of the camera chip. Subsequently, the specular wave O reflected by the specimen is measured with the different reference waves. The measured phase is then P ₀ = O-R ₀ for the untilted reference wave and P _n = O-Rn for the tilted reference waves. P _n = O - (R ₀ + D _n ) = P _{0 -Dn} , ie P ₀ = P _n + D _n . The different measurements can therefore be converted into each other. Therefore it is not a problem to combine ranges from different measurements and then treat them like a single measurement (eg P ₀ ). Propagation effects do not have to be considered here.

The present point light source array 10 Of course, it can also be used in other interferometric setups. Thus, for example, an existing Twyman-Green interferometer, as it is often used for the measurement of aspherical specimens, be upgraded with the present point light source array. For this purpose, the usual reference mirror with Piezophasenschieber by a Arrangement replaced, as in the 3 is shown. This arrangement consists of a beam splitter 7 and several deflecting mirrors 6 together, the one on the beam splitter 7 deflect appropriate incoming beam appropriately, wherein between the mirrors 6 the present switchable point light source array 10 as well as the lens 20 for collimating the point source array 10 outgoing partial beams is arranged. By appropriate connection of the different point light sources in this case the different tilted reference waves can be generated. By shifting the diffraction pattern of the liquid crystal display, the phase of the respective reference beam can be set in a defined manner so that a piezo-phase slider can be dispensed with.

LIST OF REFERENCE NUMBERS

Claims (11)

Switchable point light source array consisting of a coherent optical input beam ( 1 ) forms a plurality of independently switchable point light sources, comprising - a spatial light modulator ( 2 ) with a multiplicity of electrically addressable pixels, through which in partial regions of the light modulator ( 2 ) Diffraction pattern for diffracting the input beam ( 1 ) are producible, - one in an input beam direction behind the light modulator ( 2 ) arranged microlens array ( 3 ) through which the input beam ( 1 ) after passing through the light modulator ( 2 ) is split into individual sub-beams and - a pinhole array ( 4 ) formed in such a way and behind the microlens array ( 3 ) is arranged so that only one by diffraction at one in the light modulator ( 2 ) inscribed diffraction pattern first or higher diffraction order ( 5 ) of the partial beams on apertures of the pinhole array ( 4 ) meets. Switchable point light source array according to claim 1, characterized in that the light modulator ( 2 ) is a liquid crystal display. Switchable point light source array according to claim 1 or 2, characterized in that the pinhole array ( 4 ) in the focal plane of the microlens array ( 3 ) is arranged. Switchable point light source array according to one of claims 1 to 3, characterized in that in the input beam direction behind the pinhole array ( 4 ) a lens ( 20 ) is arranged for collimating the partial beams. Switchable point light source array according to one of claims 1 to 4, characterized in that a control electronics with the light modulator ( 2 ), which in one or more predeterminable areas of the light modulator ( 2 ), each congruent with one of the microlenses of the microlens array ( 3 ), a diffraction pattern with predefinable parameters is inscribed for a predeterminable period of time. Switchable point light source array according to claim 5, characterized in that the drive electronics are designed so that they the inscribed diffraction pattern within the range of the light modulator ( 2 ) can move defined. Switchable point light source array according to one of claims 1 to 6, characterized in that the microlens array ( 3 ) has a central microlens around which rotationally symmetric or according to a square grid further rows of microlenses are arranged. Use of Switchable Point Light Source Array ( 10 ) according to one of claims 1 to 7 in an interferometer for the interferometric measurement of a wavefront of an object beam ( 8b ), where the switchable point light source array ( 10 ) together with a lens ( 20 ) for collimating the partial beams in the beam path of a reference beam ( 8a ) of the interferometer is used to successively different from the object beam ( 8b ) tilted reference beams ( 8a ) and from the interference of the object beam ( 8b ) with the different tilted reference beams ( 8a ) to reconstruct the wavefront. Use of Switchable Point Light Source Array ( 10 ) according to one of claims 1 to 7 in an interferometer for the interferometric measurement of a wavefront of an object beam ( 8b ), where the switchable point light source array ( 10 ) together with a lens ( 20 ) for collimating the partial beams in the beam path of the object beam ( 8b ) of the interferometer is used to successively different from a reference beam ( 8a ) tilted object beams ( 8b ) and from the interference of the reference beam ( 8a ) with the differently tilted object beams ( 8b ) to reconstruct the wavefront. Use of Switchable Point Light Source Array ( 10 ) according to one of claims 1 to 7 in an interferometer for the interferometric measurement of a wavefront of an object beam ( 8b ), where the switchable point light source array ( 10 ) together with a lens ( 20 ) for collimating the partial beams in the beam path of the object beam ( 8b ) of the interferometer is used to be different than an optical axis of the lens ( 20 ) tilted object beams ( 8b ), wherein at least two object beams ( 8b ) are generated at the same time and made to interfere with each other in order to avoid the interference of the object beams ( 8b ) to reconstruct the wavefront. Use according to claim 8, 9 or 10 for measuring surfaces, in particular aspherical surfaces, in which the wavefront of the object beam ( 8b ) after reflection at the surface to be measured ( 40 ) is measured.