GB2170005A - Interferometric multicoordinate measuring device - Google Patents

Abstract

A known interferometer for the incremental scanning of variable interference structures having a diaphragm 3 between the interferometer splitter 30 and the beam splitter 2, and CCD arrays 40 integrated into the interferometer, is provided for each coordinate direction such that a mutual angle of 90 DEG is achieved between the measuring beams IMX, IMY of the interferometers, and that, in the case of two-coordinate measuring systems, planar linear reflectors M???, M??? are associated with the measuring beams and, in the case of three-coordinate measuring systems, planar reflectors are associated with the measuring beams. An interferometric multicoordinate measuring device is described which, despite considerably simplified constructional and technical expenditure on the guide system or systems of the working means, guarantees an extremely high degree of measuring accuracy. Furthermore, the proposed solution enables for the first time ever interferometric-incremental measurement to be carried out in translatory moving drive systems but not in drive systems guided in separate cartesian coordinates. <IMAGE>

Description

SPECIFICATION Interferometric multicoordinate measuring device The present invention relates to an interferometric multicoordinate measuring device.

Multicoordinate measuring devices are used for measurement and control processes in many areas of production. Two-coordinate measuring devices have always been used for adjustable micro-optical and (x,y) co-ordinate measuring devices. Afurther area of use was added as a result of a development in microelectronics which required two-coordinate positioning devices for the repeat process with extremely accurate two-coordinate measuring devices. These devices may, of course, also be used for two-dimensional measurement of the structures of microelectronic circuits.

In the field of three-coordinate measurement technology, the development of incremental 3-D coordinate measuring machines has given rise to a new generation of measuring devices which can carry out complex measurements on workpieces. In this case, it is necessary to calibrate the 3-D measurement system to the finished measuring machine. Similar 3-D measurements are required when robot technology is used in the field of microelectronics and the precision-mechanical optical construction of precision apparatus. In this case, positioning of the robot hand is required in the micron (W) range, and this cannot be carried out, or can be carried out only insufficiently, in an open control system due to the play in the joints of the robot.In order to solve such measurement problems, or to calibrate the program flow, it is necessary to use an interferometric 3-D measurement system on the robot hand.

Furthermore, more recent developments on a surface motor are known in which an object is displaced in one plane by magnetic fields. In contrast to another (x, y) displacement devices, this surface motor does not have two separate cartesian (x, y) guides located in superimposed planes, but rather the object stage slides in a predominantly translatory manner on one plane and may be displaced in this plane at random. If the movement of the object stage is made to be interferometrically measurable, the object stage can be used as a rapid and highly accurate positioning device.

Interferometers for two-dimensional measure ment of length are already known, for example from German Patent Specification No.21 64 898, German Offenlegungsschrift No. 1 673843 and "Jen.Rund schau", 22 (1977), Sheet 4, Pages 159 - 166. In all interferometric two-coordinate measuring devices, the displacement of the object stage in the plane is derived from two translatory displacements perpen dicularto one another. These displacements are generated by two separate linear (x, y) guide sys tems temps located in superimposed planes.One interfero- meter for measuring displacement is provided for each displacement direction, and both are disposed in such a way that the measuring beam of the interferometerfalls in the guide direction and im pinges on a tilt-invariable (tilt insensitive) reflecting element, e.g. a triple reflector, and from there is returned into the interferometer. To effect precise measurement or positioning, it is necessary that the measuring beams of both interferometers are exactly perpendicular to one another.However, when such a two-coordinate measuring system is in the operating condition, it is difficult to check whether this prerequisite is being met, as the measuring beams are not freely accessible. Afurther disadvan tageofthissystem is that each mechanical guide is affected by guide clearance which allows tilting relative to the guide direction. In the present case, this means that tilts caused by movement of the upper sliding carriage lead to errors of the first order relative to the coordinate direction of the lower carriage.

In the "Jen.Rundschau", 22 (1977), No.4, pages 168 - 174, an (x, y) positioning device is described which consists of two drive carriages movable in the (x, y) coordinate direction respectively. In contrast to the above measuring systems, extremely planar angled reflector strips fastened to the work table are used as reflecting optical elements in the measuring arm of the interferometer for interferometric travel measurement. However, in order to avoid tilting of the planar measuring surface, which would inevitably lead to errors in incremental scanning of the interference structure produced in the interferometer, the work table and drive carriages must be "guided on one side in the horizontal and vertical directions by high-precision roller bearings".

An interferometer, in particular for the incremental scanning of variable interference structures, has already been proposed, in which a diaphragm is located between the interferometer splitter and the beam splitter, and the beam is split by the diaphragm in the beam divider into partial beams, and a photoelectric detector is disposed in each of the beam paths of these partial beams. Furthermore, a first beam splitter and a second beam splitter are located between the interferometer splitter and the beam splitter, and the beam entering the second beam splitter is split into partial beams and a linear arrangement of integrated photoelectric scanning elements is disposed in each of the beam paths of these partial beams and these two lines are perpendicularto one another.

It is an aim of the present invention to provide an interferometric multicoordinate measuring device which, despite considerably simplified constructional and technological expenditure on the guide systems, provides a high degree of measuring accuracy for the working means. Furthermore, the suggested solution enables for the first time interfer ometric-incremental measurement to be carried out in drive systems which move in a translatory manner but not in those which are guided in separate cartesian coordinates.

In accordance with the present invention, there is provided an interferometric multicoordinate measuring device, comprising a working means whose movement is primarily translatory, laser travel measuring systems comprising a monochro matic laser beam source, interferometers for in cremental scanning of variable interference struc tures, the interferometers having a diaphragm between an interferometer splitter and a beam splitter, and CCD arrays, beam-splitting and beam-deflecting optical components integrated into the interferometer, wherein an interferometer is provided for each coordinate direction, and a mutual angle of substantially 90 is obtained between the measuring beams, and planar longitudinal mirrors are associated with the measuring beams.

Thus, the present invention provides an interferometric multicoordinate measuring device in which measuring reflectors of a special configuration are used which can move angularly without putting the incremental acquisition of measured values out of phase. The permitted angular motion of the measuring reflectors should be of a magnitude which is easily achievable in production lines of average quality, thus obviating the need for cost-intensive precision techniques. Furthermore, the tilting error can be detected interferometrically in a guide system in order to correct for it computationally.

The defects in the known solutions are caused by the way in which the incremental grating used is scanned. As is already known, there are two ways of achieving incremental measuring signals in interferometric incremental methods. In the first way, a finite grating constant is produced and the grating is scanned at two points offset from one another In the second case, an extremely large grating constant is achieved in the interferometer such that the arrangement of scanning points is largely uncritical, and the 90 phase shift in the incremental measuring signals is produced using polarizing optical means.In both methods, the grating must remain unchanged throughout measurement For this reason, all interferometers used for technical purposes are provided with tilt-invariable triple prisms, triple mirrors or so-called "cat's eye" reflectors in the measuring arm in order to ensure maintenance of the grating constants.

Interferometers with triple reflectors in the measuring arm have, however, the disadvantage thatthetriple reflector can only be displaced in the measurement direction, but not perpendicular to it, as movement of the triple reflector perpendicularly to the measurement direction causes parallel displacement of the reflecting measuring beam relative to the incident measuring beam, thus, in extreme cases, causing the interference to be interrupted.

When applied to multicoordinate measurement technology, this means that use of triple prisms in interferometers is always coupled with the necessity to generate the resulting translatory motion of the working means from two separate translatory movements in cartesian coordinates.

This aim is achieved bythe invention in that, in a two-coordinate measuring device, each coordinate direction is provided with a known interferometerfor scanning variable interference structures, and a mutual angle of 90 is set between the measuring beams of these interferometers, and planar linear reflectors are associated with the measuring beams ofthese interferometers.

Furthermore, in a three-coordinate measuring device, each coordinate direction is provided with an interferometer for scanning invariable interference structures, and planar reflectors are associated with the measuring beams of these interferometers.

In an interferometer for scanning variable interfer encestructures, the insertion of a diaphragm between the interferometer splitter and the beam splitter allows the interference structure to be scanned pointwise,whereby changes in the grating constant of the interference structure during the measuring process do not affect the achievement of incremental measuring signals. It is thus possible to use planar reflectors as measuring reflectors in the measuring arm of the interferometer without having to guide them in strict parallelism during the measuring movement. In contrast to triple reflectors, planar reflectors can also be shifted perpendicularly to the direction of measurement within any limits without obstructing the measuring process.

As is known, a greater measurement error occurs in interferometric measurements as a result of tilting of the planar measuring reflector than if the measuring mirror is not tilted. In order to eliminate this, two CCD (charge coupled device) lines are integrated into each travel-measuring interferometer and serve to detect the angular position of the measuring reflector and the calculated correction of the position of the working means derived from it. At the same time, however, the CCD lines represent a spatially fixed angle reference system for the measuring reflectors with which the working means can, for example, be returned to the zero angle position following a technical defect.

By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which:- Figure 1 shows an embodiment of a twocoordinate measuring device according to the present invention; and Figure 2 shows an embodiment of a threecoordinate measuring device according to the present invention.

In Figure 1, planar linear reflectors Mx and My, disposed at an angle of 90 to one another, are fastened onto the angle 5 which is a part of the working means moving in the (x, y) direction. The interferometer lx for the x coordinates and the interferometer lyforthe y coordinates are located on a fixed block 6. Both interferometers are illuminated by the monochromatic ray bundle 4 arriving in particular from a laser, which bundle is split in a beam splitter cube 8 into part bundles 9 and 10 which enter the interferometers lx and I,. The interferometers lx and Iy. The interferometers lx and are of identical construction. The partial bundle 10 is split in the interferometer splitter 1 at the splitter layer 7 into a measuring beam IMY and a reference beam 1ny The reference beam IRy strikes the reference reflector 12, which is in the form of a planar reflector, and the measuring beam 1My strikes the measuring reflector My which is in the form of a planar linear reflector. Following reflection, the two beams are reunited on the splitter layer 7, thus producing the strip-shaped interference grating 20.

Tilting movements of the working means 5, which always occur when the working means moves, cause the angular position of the planar linear reflector My two change relative to the incident measuring beam 1My' SO that the measuring beam not shown - reflected at the measuring reflector My also returns to the interferometer Iy at an angular displacement. A diaphragm 3 is located between a first beam splitter 30 and the beam splitter 2, through which diaphragm the interference grating 20 is scanned pointwise. The bundle 13 passing through the diaphragm 3 in the interferometer iy is split in the beam splitter 2 into partial bundles 14 and 15, which are fed to photoelectric detectors (not shown).As a result of the pointwise scanning of the interference grating through the diaphragm 3, an invariance in measuring signal acquisition is obtained in relation to changes of any type in the interference grating 20. As a result, the optical signals are exactly in phase at any moment, regardless of any tilting movement of the working means 5.

The 90 phase shift required for forward-backward counting is obtained from the optical signals 14 and 15 with polarizing optical means (not shown). These optical signals are also exactly in-phase with regard to their 90 phase position, regardless of tilting movements of the working means 5. The beam 20 entering the first beam splitter 30 in the interferometer lx and modulated with the interference structure is split in the first beam splitter 30 into beams 32 and 33, and beam 33 enters a second beam splitter 34 at whose splitter layer it is split into beams 35 and 36. Beam 36 strikes the CCD array 38 and beam 35 the COD array 40. The linear arrangements of the photoelectric scanning elements on the COD arrays 38 and 40 are perpendicular to one another.As a result, the working means 5 may be tilted about the axes as required, interrogation of the two COD arrays 38 and 40 providing the resulting angle of tilt.

In another embodiment, Figure 2 shows a threecoordinate measuring device in accordance with the present invention. In this case, three planar reflectors Mx, My, Mz, each at an angle of 90 to the other, are used as measuring reflectors. These planar reflectors Mx, My, Mz thus cover a three-dimensional (x, y, z) coordinate system. Each surface mirror Mx, My, Mz has an associated interferometer lx, Iyl lz. The interferometers 1x Iyl lz are located together on a fixed block 6.Irradiation of the interferometers lx, I lz is effected by the monochromatic laser beam bundles 4x, 4y, 4z which are split in the interferometer divider 1 at the first splitter layer 7 into reference beams and measuring beams 1Mx, 1My' 1Mz and directed to the plane reference faces and measurement faces Mx, My, Mz. The measuring beams 1mix, 1My and IMZ strike the planar measuring faces Mx, Myl Mz at the points Px, Py and Pz and are reflected back from there into the interferometers lx, Iyl lz.At the same time, the measuring beam 1Mz passes through an opening in the block 9 and reaches the measuring face Mz. The beams reunited on the splitter layer 7 of each interferometer are scanned in a pointwise manner through the diaphragm 3. The beam bundle 13 passing through the diaphragm 3 is divided in the beam splitter 2 into partial bundles 14 and 15which,following polarizing optical treatment, are directed to the photoelectric detectors to produce the 90 phase shift.

Of course, it is also possible, in the case of three-coordinate measuring devices, to measure the angle of tilt of the measuring reflectors, as shown in Figure 1, by inserting a first beam splitter 30 between the interferometer divider 1 and the beam splitter 2 and a second beam splitter 34 behind the first beam splitter 30, as well as by disposing two crossed COD arrays 38 and 40 in the two outputs 35 and 36 of the second beam splitter.

List of reference numeral used 1 = interferometer splitter 2 = beam splitter 3 = diaphragm 4 = monochromatic laser beam 5 = working means 6 = fixed block 7 = splitter layer in interferometer splitter 8 = splitter cube 9 = partial bundle at output of splitter cube 8 10 = divider bundle at output of splitter cube 8 12 = reference reflector 13 = beam through diaphragm 3 14 = partial bundle in beam splitter 2 15 = partial bundle in beam splitter 2 20 = strip-shaped interference grating at output of interferometer splitter 1x 1y lz = interferometer for (x, y, z) coordinates 1Mx' IMYT lm2 = measuring beams of interferometers x, 1y lz IRy = reference beam of interferometer 1y x, y, z = coordinate directions Mx, My, M = measuring reflectors of interferometers lx, Iyl 30 = first beam splitter 32 = partial beam in first beam splitter 30 33 = partial beam in first beam splitter 30 34 = second beam splitter 35 = partial beam in second beam splitter 36 = partial beam in second beam splitter 38 = COD array 40 = COD array

Claims (4)

1. An interferometric multicoordinate measuring device, comprising a working means whose movement is primarily translatory, laser travel measuring systems comprising a monochromatic laser beam source, interferometers, for incremental scanning of variable interference structures, the interferometers having a diaphragm between an interferometer splitter and a beam splitter, and COD arrays, bemsplitter and beam-deflecting optical components integrated into the interferometer, wherein an interferometer is provided for each coordinate direction, and a mutual angle of substantially 90 is obtained between the measuring beams, and planar longitudinal mirrors are associated with the measuring beams.

2. An interferometric measuring device as claimed in claim 1, which is a two-coordinate measuring device and has an interferometer for each of the two co-ordinate directions.

3. An interferometric measuring device as claimed in claim 1 ,which is a three co-ordinate measuring device and has an interferometer for each ofthethree co-ordinate directions.

4. An interferometric multi-coordinate measur ing device substantially as herein described, with reference to, and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.