DE102019133009B4 - Device for illuminating a workpiece, method for its modification and method for measuring its surface - Google Patents

Abstract

Device for illuminating a workpiece with an interference pattern, comprising a two-grating interferometer (11, 12) which is aligned perpendicular to its optical axis and arranged at a distance from one another, namely an input-side (11) and an output-side diffraction grating (12). 10) for generating diffraction beams (21, 22) which run along different beam paths in a common diffraction plane and interfere with one another in an interference pattern plane (40) by bending an input beam (21) at the input side (11) and refracting the resulting diffraction beams (21, 22) ) on the diffraction grating (12) on the output side, with mirror-symmetrically tiltable, mirror-symmetrically tiltable, synchronized, mirror-symmetrically tiltable beam modification elements being arranged at mirror-symmetrical positions in the beam paths of the diffraction beams (21, 22), characterized in that the beam modes fication elements are designed as plane-parallel plates (31, 32) which are aligned perpendicular to the respective beam path in their basic position and which cause a parallel offset of the respective beam when tilted from this basic position.

Description

Field of invention

The invention relates to a device for illuminating a workpiece with an interference pattern, comprising a two-grating interferometer, which has two diffraction grids aligned perpendicular to its optical axis and spaced apart from one another, namely an input-side and an output-side diffraction grating, for generating along different beam paths Diffraction beams that run in a common diffraction plane and overlap in an interference pattern plane in an interfering manner by diffraction of an input beam in diffraction beams resulting from the diffraction grating on the input side and renewed diffraction of diffraction beams on the diffraction grating on the output side,
at mirror-symmetrical positions in the beam paths of the diffraction beams, beam modification elements of the same type, which are synchronized and mirror-symmetrically tiltable about tilting axes aligned perpendicular to the diffraction plane, are arranged.

• - Erzeugen von entlang unterschiedlicher Strahlengänge in einer gemeinsamen Beugungsebene verlaufenden Beugungsstrahlen durch Beugen eines gepulsten Eingangsstrahls am eingangsseitigen Beugungsgitter und
• - Erzeugen eines Interferenzmusters in einer Bearbeitungsebene des Werkstücks durch Überlagern der Beugungsstrahlen mittels ihres erneuten Beugens am ausgangsseitigen Beugungsgitter,

wobei der Eingangsstrahl in mit seinem Pulstakt synchronisierter Weise verschwenkt wird, um unterschiedliche Bereiche der Bearbeitungsebene mit dem Interferenzmuster zu beleuchten,
und wobei an spiegelsymmetrischen Positionen in den Strahlengängen der Beugungsstrahlen gleichartige Strahlmodifikationselemente um senkrecht zur Beugungsebene ausgerichtete Kippachsen in mit den Eingangsstrahl-Pulsen synchronisierter Weise spiegelsymmetrisch verkippt werden.The invention further relates to a method for modifying a workpiece by means of a two-grating interferometer with an input-side and an output-side diffraction grating, comprising the steps:

• - Generation of diffraction beams running along different beam paths in a common diffraction plane by diffraction of a pulsed input beam at the diffraction grating on the input side and
• - Generating an interference pattern in a processing plane of the workpiece by superimposing the diffraction beams by means of their renewed diffraction at the diffraction grating on the output side,

wherein the input beam is swiveled in a manner synchronized with its pulse rate in order to illuminate different areas of the processing plane with the interference pattern,
and wherein at mirror-symmetrical positions in the beam paths of the diffraction beams, beam modification elements of the same type are tilted mirror-symmetrically about tilt axes aligned perpendicular to the diffraction plane in a manner synchronized with the input beam pulses.

• - Erzeugen von entlang unterschiedlicher Strahlengänge in einer gemeinsamen Beugungsebene verlaufenden, zunächst auseinander und sodann wieder auf einen gemeinsamen geometrischen Schnittpunkt zulaufenden Beugungsstrahlen durch Beugen eines Eingangsstrahls am eingangsseitigen Beugungsgitter und erneutes Beugen resultierender Beugungsstrahlen am ausgangsseitigen Beugungsgitter,
• - Ermitteln des senkrechten Abstandes zwischen der Werkstückoberfläche und dem geometrischen Schnittpunkt der Beugungsstrahlen, wobei das Ermitteln besagten senkrechten Abstandes ein Variieren desselben umfasst,
• - Berechnen eines lokalen Oberflächenhöhenwertes aus dem ermittelten senkrechten Abstand,

wobei der Eingangsstrahl über die Werkstückoberfläche verschwenkt wird, um unter wiederholter Durchführung der vorgenannten Schritte für eine Mehrzahl von Punkten der Werkstückoberfläche die zugeordneten lokalen Oberflächenhöhenwerte zu erhalten.The invention finally relates to a method for measuring a workpiece surface by means of a two-grating interferometer with an input-side and an output-side diffraction grating, comprising the steps:

• - Generation of diffraction beams running along different beam paths in a common diffraction plane, initially diverging and then again converging towards a common geometric intersection point by bending an input beam at the entrance-side diffraction grating and renewed diffraction of the resulting diffraction rays at the exit-side diffraction grating,
• - Determining the perpendicular distance between the workpiece surface and the geometric intersection of the diffraction rays, wherein the determination of said perpendicular distance comprises varying the same,
• - Calculation of a local surface height value from the determined vertical distance,

wherein the input beam is swiveled over the workpiece surface in order to obtain the associated local surface height values for a plurality of points on the workpiece surface by repeating the aforementioned steps.

State of the art

A generic lighting device and a generic workpiece modification method are known from US Pat US 6 904 201 B1 .

From the DE 10 2006 032 053 A1 a so-called two-grating interferometer for illuminating a workpiece with an interference pattern and - with a suitable setting of the fluence of the interference pattern - for corresponding modification of the workpiece, in particular its surface, is known. A two-grating interferometer essentially consists of two parallel diffraction gratings which are spaced apart from one another and whose identically aligned grating lines are in a period relationship to one another known to the person skilled in the art and generally described in the aforementioned publication. An input beam impinging on the diffraction grating on the input side is diffracted into partial beams of different diffraction orders, each with a positive and negative sign. The two partial beams with different signs of a selected diffraction order diverge along diverging beam paths within a common plane, referred to here as the diffraction plane. So they hit the second diffraction grating, where they again experience diffraction in partial beams of different diffraction orders, each with a positive and a negative sign. In particular, partial beams are created along converging beam paths in the diffraction plane, which are at a distance behind the exit-side Diffraction grating, which corresponds to the distance between the diffraction gratings, superimpose on one another in an interfering manner. In a region of this plane, referred to here as the interference pattern plane, which is dependent on the beam width, an interference pattern arises with a periodic distribution of intensity maxima and minima. A workpiece lying in this interference pattern plane is illuminated accordingly. If the fluence of the interference pattern in the areas of its intensity maxima is above a material modification threshold of the workpiece material and if the fluence of the interference pattern in the areas of its intensity minima is below said material modification threshold, a correspondingly patterned modification of the workpiece takes place in the interference pattern plane, which in this context is then referred to as the processing plane can be. The material modification achieved can consist, for example, in an ablation. In this case, the workpiece surface is typically placed in the interference pattern or processing plane. In the case of transparent materials, however, it is also possible to place a deeper area of the workpiece at a finite distance from the workpiece surface in the interference pattern or processing plane. In these cases, the modification is often associated with a change in the refractive index of the material, for example due to a radiation-induced chemical reaction.

Such a case is described in the generic document mentioned at the beginning. Here, the workpiece to be modified is a light guide, in the core area of which a so-called Bragg grating is to be inscribed. In order to be able to inscribe Bragg gratings of different periodicity or a Bragg grating with varying periodicity without changing the diffraction grating, the cited publication suggests using a mirror-symmetrical pair of transparent prisms behind the diffraction grating on the output side, each around a tilt axis that is perpendicular to the diffraction plane stands, are tiltable. Due to the influence of these prisms, the periodicity of the interference pattern generated in the interference pattern plane changes as a function of the specifically selected tilt angle of the prisms. In order to provide a longer fiber section with a Bragg grating, the input beam can be swiveled according to the fiber orientation. A special feature of the two-grating interferometer is that the pattern formation and the position of the interference pattern plane are independent of the angle of incidence of the input beam. By means of a tilting of the prisms that is synchronized with the swiveling of the input beam and its pulse rate, a longer fiber section can be provided with a Bragg grating with a periodicity that varies over its length.

A problem that cannot be solved with the generic device arises when - regardless of the question of a varying periodicity of the interference pattern - no essentially one-dimensional workpiece, such as said light guide, but an extended workpiece surface is to be provided with the interference pattern. With a perfectly flat design of the workpiece surface and its alignment exactly perpendicular to the optical axis of the two-grating interferometer, the patterning of the large surface area can be achieved by corresponding two-dimensional pivoting of the input beam. However, if the surface is curved and / or the clamping is not perfectly perpendicular to the optical axis, the interference pattern is mapped in different surface areas above, below or (as optimally provided) exactly on the workpiece surface. This has a significant impact on the sharpness of the pattern written on the workpiece surface. With a known surface shape and / or known misalignment, the problem can theoretically be solved by corresponding tracking of the workpiece parallel to the optical axis. However, the accuracy of this tracking would typically have to be in the range of 10-20 µm, which is hardly possible in practice, especially with regard to the required speeds and weights to be moved.

A comparable problem also arises with the use of the two-grating interferometer, which is known in principle to the person skilled in the art, for measuring a curved or not ideally evenly clamped workpiece surface. It is known to illuminate the workpiece surface as described above, but the position of the interference pattern plane, ie that plane in which the geometric intersection of the overlapping diffraction rays lies, relative to the workpiece surface is initially unknown. The diffraction rays reflected back through the second and first diffraction grating along the reversed beam paths from the workpiece surface are then imaged with an image detector, the optics of the image detector being set so that the workpiece surface lies in its depth of field. The recorded image then shows two sharply mapped points, namely the two points of impact of the diffraction rays on the workpiece surface, the lateral distance between which is directly dependent on the absolute value of the distance between the workpiece surface and the geometric intersection of the diffraction rays. The latter can lie above or below the workpiece surface (in particular in the second case and with non-transparent workpiece material it is possible that it is not implemented as a real ray intersection). Variation of the vertical distance by mechanical lifting or lowering of the workpiece leads to a variation of the lateral distance between the mapped points. If the vertical distance is small, the depicted points merge into a single one. However, it is very difficult to determine the shape of this single depicted point so precisely that the exact superposition of the points of impact - corresponding to the coincidence of the geometric intersection point and workpiece surface - can be determined with sufficient accuracy. It is therefore customary to adjust the vertical distance, which results in two separately mapped points, the centers of which can be precisely determined by intensity measurements. The vertical distance is then varied by known, vertical, mechanical displacement of the workpiece. From the resulting lateral movement of the imaged points, knowing the geometry of the apparatus, the absolute value (in particular from the lateral point spacing) and the sign (in particular from the direction of movement) of the perpendicular distance between the geometric intersection of the diffraction rays and the workpiece surface are calculated. A major weak point of this approach, however, is the inadequate speed of the mechanical workpiece adjustment.

From the US 2007/0153250 A1 an interferometric lithography system is known in which a coherent input beam is split into different diffraction beams by means of a diffraction grating, which are then brought together again via deflecting mirrors. The diffraction rays are superimposed again on the surface of a workpiece to be machined and form a corresponding interference pattern. The cited publication also discloses the introduction of beam adjustment means in the beam path area between the diffraction grating and the deflecting mirrors. These are beam width, beam shape and beam angle adjustments, all of which cause an angle change in the diffraction beams and are therefore not suitable for solving the problems explained above.

With the surface lighting system of the DE 10 2013 007 524 A1 the partial beams generated by a single diffraction grating are first parallelized by means of prisms and then recombined, a converging lens being arranged between the prisms for beam shaping.

the DE 10 2015 214 960 A1 discloses beam splitting by means of a Wollaston prism and partial beam merging by means of prisms or deflecting mirrors and, if necessary, beam shaping by means of lenses.

the U.S. 5,995,224 A discloses a measuring device in which a measuring laser beam is split into partial beams by means of a partially transparent mirror, which are then recombined on a measurement surface via different partial beam paths predetermined by deflecting mirrors. The partial beams reflected by the measuring surface are again guided over different partial beam paths specified by deflecting mirrors and recombined on a detector. Lens systems are used for suitable focusing.

Task

It is the object of the present invention to develop a generic device in such a way that workpieces that are curved and / or clamped at an angle to the optical axis can also be precisely labeled with a continuous interference pattern, or to offer a corresponding workpiece modification method and a corresponding workpiece measurement method.

Statement of the invention

This object is achieved in connection with the features of the preamble of claim 1 in that the beam modification elements are designed as plane-parallel plates aligned perpendicular to the respective beam path in their basic position, which cause a parallel offset of the respective beam when tilted from this basic position.

The object is also achieved in connection with the features of the preamble of claim 5 in that the beam modification elements are designed as plane-parallel plates, the tilting of which sets a predetermined distance between the respectively illuminated area of the processing plane on the one hand and the workpiece surface on the other. In particular, this distance can be “zero”. I. E. The working plane and the workpiece surface coincide in the illuminated area.

The object is further achieved in connection with the features of the preamble of claim 11 in that said variation of the perpendicular distance between the workpiece surface and the geometric intersection of the diffraction beams takes place by mirror-symmetrical and synchronized plane-parallel plates arranged at mirror-symmetrical positions in the beam paths of the diffraction beams about tilt axes aligned perpendicular to the diffraction plane.

The plane-parallel plates, which in their basic position are perpendicular to the respective beam path in which they are introduced, cause tilting from this basic position out a parallel offset of the respective beam. In the diffraction plane, the diffraction rays run diamond-shaped, ie they diverge between the input-side and the output-side diffraction grating and again converge between the output-side diffraction grating and the interference pattern plane. The respective diffraction angles are predetermined by the properties of the respective diffraction grating. These angles therefore remain unchanged regardless of the tilted position of the plane-parallel plates. The parallel offset of the beams in the area between the diffraction gratings and / or behind the diffraction grating on the output side, however, leads to a migration of the intersection of the diffraction beams in the Z direction, ie in the direction of the optical axis of the two-grating interferometer. The intersection of the rays, however, defines the interference pattern plane in which said ray intersection lies and which in turn is perpendicular to the optical axis of the interferometer. The interference pattern forms in the illuminated area of this plane.

In the context of a machining process, as already explained above, the interference pattern plane corresponds to the machining plane of the workpiece. If a curved surface, e.g. the curved workpiece surface, is to be processed over a large area, the "drifting" of the desired processing plane that occurs when the workpiece is scanned from the interference pattern plane specified by the interferometer can be compensated for by tilting the plane-parallel plates accordingly and thus making changes the parallel offset of the diffraction beams is tracked so that it coincides exactly with the desired processing plane at least in the respectively illuminated area. Since, as explained above, the tilting of the plane-parallel plates is only associated with a parallel offset of the diffraction beams and not a change in the angle in the beam path, such tracking does not lead to a change in the period of the interference pattern. Rather, a workpiece can be covered with a continuous pattern projection of uniform periodicity along a curved machining surface, in particular along a curved surface, as was previously only possible in the prior art with exactly flat workpiece surfaces aligned exactly perpendicular to the optical axis.

Two first of the plane-parallel plates are preferably arranged in the area between the diffraction gratings. This position is particularly advantageous because the area behind the diffraction grating on the output side, in which the workpiece is arranged, remains free of additional elements, which generally facilitates the handling of the device. However, the effect according to the invention can also be achieved with first plane-parallel plates arranged behind the diffraction grating on the output side.

In a further development of the invention, which is particularly advantageous in the case of particularly short pulsed input beams, it is provided that two second plane-parallel plates are additionally arranged in the area between the diffraction grating on the output side and the interference pattern plane or the processing plane. It is known that ultrashort pulses are characterized by a broad spectrum of wavelengths. However, different wavelengths are diffracted to different degrees at the diffraction gratings, i.e. they strike the plane-parallel plates at different angles of incidence, which leads to wavelength-dependent parallel offsets there. Since the Z-shift of the interference pattern plane according to the invention is a direct consequence of this parallel offset, this angular dispersion leads to an undesirable wavelength dependence of the position of the interference pattern plane or, in the case of broadband light, to the formation of a "stack" of interference pattern planes according to wavelengths. However, this can be compensated for by the aforementioned arrangement of first plane-parallel plates between the diffraction gratings and second plane-parallel plates behind the diffraction grating on the output side. The first and second plane-parallel plates are preferably tilted by the same angle from their respective basic position. Due to the different alignment of the first and second plane-parallel plates to the respective upstream diffraction grating, those beam components which have experienced a larger parallel offset on the first plane-parallel plates, a smaller parallel offset on the second plane-parallel plates and vice versa. The parallel offset dispersion is thus compensated. On the other hand, the inventive effect of the interference pattern plane shift is increased by the double application of the parallel offset (namely both on the first and on the second plane-parallel plates).

Regardless of the explained dispersion advantage, this embodiment of the invention also has the advantage of producing a desired total parallel offset or a desired total displacement of the interference pattern plane with thinner plane-parallel plates or with plane-parallel plates of a less refractive material, which is the case with a special choice of the used Radiation source can be helpful.

In special cases in which the generation of a pattern with a uniform periodicity is not desired, but rather a change in the periodicity over the one made up of many individual lighting areas Processing area, in a further development of the invention, provision can be made for additional beam modification elements designed as prisms to be arranged at mirror-symmetrical positions in the beam paths of the diffraction beams of the same type, tilting axes aligned perpendicular to the diffraction plane, synchronized with mirror-symmetrical tilting. This type of targeted period adjustment is, as explained in the introduction to the description, from the US 6 904 201 B1 known.

In order to carry out precise Z tracking of the interference pattern or machining plane when machining a workpiece, precise knowledge of the surface shape and position of the workpiece is required in order to be able to control the tilting of the plane-parallel plates on this basis. Basically, the specific source of this surface data is irrelevant. However, the device according to the invention can itself be used to acquire such surface data.

The measuring method explained at the beginning is modified according to the invention in such a way that the variation of the vertical distance between the geometric intersection of the diffraction rays and the workpiece surface no longer takes place through a slow vertical displacement of the workpiece, but rather through the vertical displacement of said intersection by means of the tilting of the plane-parallel plates explained in detail above. In this way, the workpiece surface can be measured much faster than before. In particular, this makes it possible in an economically efficient way to have such a measurement interleaved in time with the workpiece processing: For each local processing area that is controlled by pivoting the input beam, a measurement step with an illumination fluence below the modification threshold of the workpiece material and then the the actual machining step is carried out with a fluence which at least partially exceeds the modification threshold of the workpiece material. Between these two steps, the processing plane is moved to the intended distance from the workpiece surface, in particular onto the workpiece surface. This is also done by precisely tilting the plane-parallel plates.

It is also conceivable that input beams from different beam sources are used for the measurement on the one hand and the processing on the other. They would only have to be guided to the same input beam path by suitable beam guiding means, which is possible, for example, by means of semitransparent mirrors or other optical elements known to the person skilled in the art. This variant is particularly useful when the laser wavelength used for material processing cannot be detected by means of the selected image detector.

Figure list

• 1: eine schematische Darstellung einer ersten Ausführungsform einer erfindungsgemäßen Beleuchtungsvorrichtung,
• 2: eine schematische Darstellung einer zweiten Ausführungsform einer erfindungsgemäßen Beleuchtungsvorrichtung und
• 3: eine schematische Darstellung des Schnittbereichs der Beugungsstrahlen zur Illustration des erfindungsgemäßen Vermessungsverfahrens.

Show it:

• 1 : a schematic representation of a first embodiment of a lighting device according to the invention,
• 2 : a schematic representation of a second embodiment of a lighting device according to the invention and
• 3 : a schematic representation of the intersection area of the diffraction rays to illustrate the measuring method according to the invention.

Description of preferred embodiments

Identical reference symbols in the figures indicate identical or analogous elements.

1 shows in a highly schematic representation the essential parts of the beam path in a two-grating interferometer according to the invention 10 . The interferometer 10 has two diffraction gratings 11 , 12th , namely an input-side diffraction grating 11 and a diffraction grating on the output side 12th on. The same oriented grid lines of the diffraction grating 11 , 12th are in a period ratio known to the person skilled in the art, as it is in general form in the DE 10 2006 032 053 A1 is described. The input beam 20th meets the input side diffraction grating on the input side 11 . In the representation of 1 is a normal incidence of the input beam 20th on the diffraction grating on the input side 11 shown. By means of pivoting optics (not shown), however, the input beam can also be incident on the input-side diffraction grating at different angles of incidence 11 be steered.

At the diffraction grating on the input side 11 becomes the input beam 20th diffracted in partial beams of different diffraction orders. For the sake of simplicity, in 1 only the symmetrical diffraction rays 21 a single, selected diffraction order shown. The partial beams of the other diffraction orders can, if they occur at all in a relevant manner, be blocked or deflected.

The diffraction rays 21 each meet first plane-parallel plates arranged at mirror-symmetrical positions in a mirror-symmetrical orientation 31 . The plane-parallel plates 31 are each shown in two positions. The respective basic position in which the first plane-parallel plates are shown is shown with dashed lines 31 exactly perpendicular to the respectively assigned diffraction beam 21 stand. In this basic position there is no change in the beam path, as is the case with the dotted continuation of the diffraction beams 21 behind the first plane-parallel plates 31 indicated.

The diffraction rays 21 then fall on the diffraction grating on the output side 12th and are again diffracted there in partial beams of different diffraction orders. A selected diffraction order that depends on the period ratio of the diffraction gratings 11 , 12th depends, produces partial beams of the two diffraction beams 21 that are in an interference pattern plane 40 cut, overlap there and form an interference pattern. If a machining plane of a workpiece lies in this interference pattern plane 40 and is the fluence of the diffraction rays 21 selected appropriately, the material of the workpiece can be used in this machining or

Interference pattern plane 40 modified accordingly. Such a workpiece is in 1 not shown.

The first plane-parallel plates can be used to shift the interference pattern plane 31 mirror symmetry around perpendicular to that of the diffracted rays 21 spanned diffraction plane (corresponding to the plane of the paper in 1 ) aligned tilt axes are tilted. The first plane-parallel plates located in such a tilted position 31 are in 1 shown with solid lines. In the in 1 The position shown is the right one of the first plane-parallel plates 31 in and the left of the first plane-parallel plates tilted counterclockwise.

This results in an oblique incidence of the diffraction rays 21 onto the first plane-parallel plates 31 , which results in a corresponding parallel offset of the diffraction rays 21 results. The path of the diffraction rays 21 with parallel offset is in 1 shown as a solid arrow line.

The parallel offset of the diffraction rays 21 through the first plane-parallel plates 31 , which in particular does not imply any change in the angle of the beam path, does not change anything in the previously described renewed diffraction of the diffraction beams 21 at the diffraction grating on the output side 12th . However, these are the points where the diffraction rays 21 onto the diffraction grating on the output side 12th impinge, compared to the situation with the first plane-parallel plates in the basic position 31 moved towards each other. This results in the diffraction rays 21 behind the diffraction grating on the output side 12th in another, in particular closer to the diffraction grating on the output side 12th lying interference pattern plane 40 meet and form the interference pattern there. By appropriately controlling the tilt position of the first plane-parallel plates 31 can therefore be the Z position of the interference pattern plane 40 and thus the machining plane for a workpiece can be set precisely. In particular, the processing plane can track a curved surface of the workpiece or be guided along a curved, predetermined processing surface when the workpiece is pivoted by pivoting the input beam 20th is scanned as part of a large-area processing.

2 shows in a representation similar to 1 a development of the lighting device according to the invention, wherein in addition to the between the diffraction gratings 11 , 12th arranged first plane-parallel plates 31 second plane-parallel plates 32 behind the diffraction grating on the output side 12th are arranged. The device of 2 is particularly suitable for cases where the input beam 20th is a pulsed laser beam with a short pulse duration.

As is known to the person skilled in the art, the necessary spectral bandwidth within a laser pulse is greater, the shorter its pulse duration. In the case of ultrashort pulses, the wavelength spectrum within the pulse is very broad. This can be problematic in the context of the present invention insofar as the diffraction gratings 11 , 12th have a clear angular dispersion, ie a dependence of the diffraction angle on the wavelength of the input beam. In the case of a broadband input beam, components of different wavelengths are bent at different angles, which is shown in 1 in a greatly exaggerated way by the double representation of diffraction rays 21 , 22nd is symbolized. The diffraction rays 21 , 22nd should be the beam paths of the shortest (diffraction beams 22nd ) and the longest wavelength (diffraction rays 21 ) within the input beam spectrum. The plane-parallel plates 31 , 32 are set in this example in such a way that the short-wave diffraction rays are in their basic position 22nd hit vertically.

This dispersion problem has no influence on the basic function of a two-grating interferometer, since in the absence of any plane-parallel plates 31 , 32 the effects on the diffraction gratings 11 , 12th fully compensate. However, this changes with the introduction of the first plane-parallel plates 31 . The extent of the parallel offset achieved by them is namely dependent on the angle of incidence of the diffraction rays 21 on their surface. As explained above, this is due to the angular dispersion of the diffraction grating on the input side 11 is different for different wavelengths, result behind the first plane-parallel plates 31 wavelength-dependent parallel offsets, which in a device according to 1 would lead to wavelength-dependent Z-positions of the respective interference pattern. This "smears" the machining plane for a workpiece.

However, as in 2 shown, additional, second plane-parallel plates 32 arranged behind the diffraction grating on the output side, the thickness and material of the first plane-parallel plates 31 correspond, and, as in 2 shown at the same tilt angle as their respective counterparts between the diffraction gratings 11 , 12th are tilted, the dispersion effect can be largely (in a linear approximation completely) compensated. Those beam portions of the diffraction beams that have a larger entrance angle on the first plane-parallel plates 31 are hit (long-wave diffraction rays 21 in 2 ), namely hit the second plane-parallel plates with a smaller angle of incidence 32 . These beam components are thus experienced at the first plane-parallel plates 31 a larger parallel offset and on the second plane-parallel plates 32 a smaller parallel offset. In comparison, those parts of the beam that have a smaller angle of incidence on the first plane-parallel plates experience 31 fall (short-wave diffraction rays 22nd ), there a smaller parallel offset and on the second plane-parallel plates 32 that they strike at a greater angle of incidence have a greater parallel offset. The result of this is that all of the spectral beam components of the diffraction beams are divided 21 , 22nd in the same interference pattern plane 40 meet and there form a precisely defined, sharp interference pattern. However, this approach neglects the effect of the dispersion caused by the plate material itself, which in most cases is entirely justifiable due to the small extent of the resulting error. Both dispersion effects can, however, be largely compensated for together by creating plane-parallel flaps for the first 31 on the one hand and the second plane-parallel plates 32 on the other hand, slightly different thicknesses (and / or slightly different materials with regard to their refractive index) can be used.

3 shows, likewise in a highly schematic manner, the region of intersection of the diffraction rays 21 when setting the two-grating interferometer according to the invention, which is otherwise not shown in detail 10 , in which the interference pattern plane 40 , in which the geometric intersection 23 of the diffraction rays 21 is a perpendicular distance 61 above a workpiece surface 50 is arranged. The person skilled in the art will also be able to imagine the reverse situation in which the geometric intersection point 23 below the workpiece surface 50 lies. In both cases the diffraction rays hit 21 at laterally spaced points of impact 24 on the workpiece surface 50 on. Your lateral distance 62 changes with the change in the vertical distance 61 . According to the invention, this is achieved by vertical displacement of the geometric intersection 23 by tilting the in 3 plane-parallel plates not shown 31 , 32 varies. The points of impact 24 are mapped onto an image detector and their movement is analyzed as a function of the plate tilt.

Of course, the embodiments discussed in the specific description and shown in the figures are only illustrative embodiments of the present invention. In the light of the disclosure here, a person skilled in the art is provided with a broad spectrum of possible variations.

List of reference symbols

10

Two-grating interferometer

11

diffraction grating on the input side of 10

12th

diffraction grating on the output side of 10

20th

Input beam

21

Diffraction rays

22nd

Diffraction rays

23

geometric intersection

24

Point of impact

31

first plane-parallel plates

32

second plane-parallel plates

40

Interference pattern plane

50

Workpiece surface

61

vertical distance

62

lateral distance

Claims (12)

Device for illuminating a workpiece with an interference pattern, comprising a two-grating interferometer (11, 12) which is aligned perpendicular to its optical axis and arranged at a distance from one another, namely an input-side (11) and an output-side diffraction grating (12). 10) for generating diffraction beams (21, 22) that run along different beam paths in a common diffraction plane and interfere with one another in an interference pattern plane (40) by bending an input beam (21) at the input side (11) and again bending the resulting diffraction beams (21, 22) ) on the diffraction grating (12) on the output side, whereby at mirror-symmetrical positions in the beam paths of the diffraction beams (21, 22) similar tilt axes aligned perpendicular to the diffraction plane are synchronized mirror-symmetrically tiltable beam modification elements are arranged, characterized in that the beam modification elements are designed as plane-parallel plates (31, 32) aligned perpendicular to the respective beam path in their basic position, which cause a parallel offset of the respective beam when tilted from this basic position. Device according to Claim 1 , characterized in that two first (31) of the plane-parallel plates are arranged in the area between the diffraction gratings (11, 12). Device according to Claim 2 , characterized in that two second (32) of the plane-parallel plates are additionally arranged in the area between the diffraction grating (12) on the output side and the interference pattern plane (40). Device according to one of the preceding claims, characterized in that at mirror-symmetrical positions in the beam paths of the diffraction beams, additional beam modification elements of the same type, synchronized and mirror-symmetrically tiltable around perpendicular to the diffraction plane and designed as prisms, are arranged. Method for modifying a workpiece by means of a two-grating interferometer (10) with an input-side (11) and an output-side diffraction grating (12), comprising the steps: - generating diffraction beams (21, 22) running along different beam paths in a common diffraction plane by bending a pulsed input beam (20) at the diffraction grating (11) on the input side and - generating an interference pattern in a processing plane of the workpiece by superimposing the diffraction beams (21, 22) by means of their renewed diffraction at the diffraction grating (12) on the output side, the input beam (20) is pivoted in a manner synchronized with its pulse rate in order to illuminate different areas of the processing plane with the interference pattern, and at mirror-symmetrical positions in the beam paths of the diffraction beams (21, 22) similar beam modification elements about tilt axes aligned perpendicular to the diffraction plane in mi t the input beam pulses are tilted mirror-symmetrically in a synchronized manner, characterized in that the beam modification elements are designed as plane-parallel plates (31, 32) aligned perpendicular to the respective beam path in their basic position, which cause a parallel offset of the respective beam when tilted from this basic position so that a predetermined distance between the respectively illuminated area of the processing plane on the one hand and the workpiece surface on the other hand is set by tilting it. Procedure according to Claim 5 , characterized in that two first (31) of the plane-parallel plates are arranged in the area between the diffraction gratings (11, 12). Procedure according to Claim 6 , characterized in that two second (32) of the plane-parallel plates are additionally arranged in the area between the diffraction grating (32) on the exit side and the processing plane. Procedure according to Claim 7 , characterized in that the second plane-parallel plates (32) are each tilted in the same direction as the first plane-parallel plates (31) by the same angular amount. Method according to one of the Claims 5 until 8th , characterized in that, at mirror-symmetrical positions in the beam paths of the diffraction beams, additional beam modification elements designed as prisms are tilted mirror-symmetrically about tilt axes aligned perpendicular to the diffraction plane in a manner synchronized with the input beam pulses. Method according to one of the Claims 5 until 9 , characterized in that the fluence of the interference pattern is set so that a material modification limit of the workpiece is exceeded in the areas of its illumination maxima and undershot in the areas of its illumination minima. Method for measuring a workpiece surface by means of a two-grating interferometer (10) with an input-side (11) and an output-side diffraction grating (12), comprising the steps: - Generating, first apart and then again, along different beam paths in a common diffraction plane diffraction beams (21, 22) converging on a common geometric intersection point by bending an input beam (20) at the input-side diffraction grating (11) and renewed bending of the resulting diffraction rays (21, 22) at the output-side diffraction grating (12), - determining the perpendicular distance between the workpiece surface and the geometric intersection of the diffraction beams (21, 22), wherein the determination of said perpendicular distance comprises varying the same, - calculating a local surface height value from the determined perpendicular distance, wherein the input beam (20) over the workpiece surface is pivoted in order to obtain the associated local surface height values for a plurality of points on the workpiece surface by repeating the aforementioned steps, characterized in that said varying of the perpendicular distance between the workpiece surface and the geometric intersection of the diffraction beams (21, 22) takes place by Plane-parallel plates (31, 32) arranged at mirror-symmetrical positions in the beam paths of the diffraction beams (21, 22), aligned in their basic position perpendicular to the respective beam path, which cause a parallel offset of the respective beam when tilted from this basic position, mirror-symmetrical and synchronized Tilting axes aligned perpendicular to the diffraction plane can be tilted. Procedure according to Claim 11 , characterized in that the calculated surface height values in the context of a method according to one of the Claims 5 until 10 can be used as a basis for controlling the tilting of the plane-parallel plates (31, 32).

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