Abstract:
A measuring apparatus for determining relative positions of a positioning stage arranged in a moveable fashion in at least one direction by a predeterminable maximum traversing path. The measuring device comprises at least one interferometric measuring means and at least one interferometric correction means. An interferometric measuring means is operable with the laser light of a laser of at least one wavelength. Correction results can be generated with the interferometric correction means allowing conclusions to be drawn with respect to the actual wavelength of the laser light during a position determination of the positioning stage in order to take into account variations of the wavelength of the laser light, in particular due to ambient conditions, when evaluating the measuring results. The interferometric correction means is arranged proximate to the interferometric measuring means, and the proximity corresponds to a predeterminable portion of the maximum traversing path of the positioning stage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of German Patent Application No. 10 2005 023 085.7, filed May 13, 2005, and German Patent Application No. 10 2005 035 700.8, filed Jul. 27, 2005, which applications are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a measuring device for determining relative positions of a positioning stage configured to be moveable in at least one direction by a predeterminable maximum traversing path. The measuring device comprises at least one interferometric measuring means and at least one interferometric correction means. An interferometric measuring means is for obtaining measuring results to determine relative positions of the positioning stage for a movement along a movement direction. An interferometric measuring means is operable with laser light of a laser of at least one wavelength. The interferometric correction means is for obtaining correction readings allowing conclusions to be drawn with respect to the actual wavelength of the laser light during a position determination of the positioning stage in order to take into account a change of the wavelength of the laser light, in particular due to ambient influences, for evaluating the measuring results. The present invention also relates to methods for determining relative positions of a positioning stage moveable in at least one direction. 
   BACKGROUND OF INVENTION 
   Interferometric measuring means are often used for highly precise distance and position measurements. In a high-precision interferometric measurement the relative difference in distance between a measuring beam and a reference beam is usually measured with the beam path of an interferometric measuring means. A beam splitter of the interferometric measuring means is for dividing the light generated by a laser into a measuring beam and a reference beam. The measuring beam passes between a beam splitter of the interferometric measuring means and a measuring mirror, wherein the measuring mirror is arranged at a moveable measuring object, i.e., for example, on a positioning stage. The reference mirror is fixed with respect to the beam splitter of the interferometric measuring means. The measurement serves to determine how the phase of the light changes as the measuring object moves. The measuring accuracy is dependent, apart from the correct adjustment of the optical axes or the optics itself, on the signal processing (electronics, software) and the calibration of the overall system, also on the ambient conditions, such as the composition of the ambient air, moisture, temperature and pressure. The wavelength of the light beam is the basic measuring unit of the measurement. The relative difference in distance is therefore indicated in the unit “wavelength”. The current value of the wavelength of a light beam is a function of the refractive index of the medium passed by the light beam. The refractive index varies, for example, due to gradual variations or rapid fluctuations of the temperature, air pressure and moisture, or due to changes in the air composition. The fluctuations in the readings due to wavelength variations, at ±0.1 μm, are no longer negligible with respect to the structures to be measured in typical measurements, such as in a coordinate measuring device on wafers and masks, and are therefore no longer acceptable for the required measuring accuracy. To increase the measuring accuracy it is therefore necessary to take into account wavelength variations of the light beam by means of a continuous wavelength correction. 
   For high precision distance measurements, a coordinate measuring device, such as it is known from DE 198 19 492 could be operated in a climatic chamber. Herein, at least the temperature is maintained at a constant level, in some climatic chambers the moisture is also maintained at a constant level. The control accuracy of temperature and moisture has technical limits. With a reasonable expenditure of effort, it is impossible to create a hermetically sealed chamber to maintain a constant air pressure, in particular since in the example of a coordinate measurement device it is necessary that the measuring objects can be exchanged simply and rapidly. Even the operation of the loading door causes rapid pressure fluctuations. The ambient conditions are defined by a climatic chamber via the feedback of temperature sensors and humidity sensors, and via over-pressure adjustments. “Over-pressure” refers to a positive pressure differential of a few Pascal with respect to the ambient pressure of the climatic chamber. Fluctuations of the air pressure within the chamber are caused by the measuring structure itself and are therefore not entirely avoidable. Further tolerances for the chamber are: with respect to the temperature 1/100 degree (in time), 2/100 degree spatially and with respect to the humidity ±1%. The measuring signals are detected in the X and Y directions with a nominal detector accuracy of Δλ/2048˜0.3 nm (λ=laser wavelength=632.9 nm). As a result the interferometer wavelength must be continuously detected in a separate measurement. This can be done by measuring a wavelength calibration distance with a constant length, a “wavelength tracker”, or by measuring the influential factors, such as temperature, air moisture etc., while continuously calculating the current wavelength. One example of such an interferometric correction means is the commercially available laser interferometer HP 10702 of Agilent (formerly Hewlett Packard). 
   The light generated by a laser is divided into two partial beams by a beam splitter of the interferometric correction means. The two partial beams pass between the beam splitter of the interferometric correction means and a reference mirror fixed with respect to the beam splitter of the interferometric correction means. The two partial beams, also referred to as reference beams in the following, have optical paths of different lengths. Thus for a coordinate measuring device a wavelength tracker is usually provided which is arranged at a site on the coordinate measuring device that seems to be suitable for detecting changes in ambient conditions and where basically there is structural space available for it. It is necessary, however, to always point out possibilities for increasing the measuring accuracy of a coordinate measuring device or a measuring system. This can also be achieved by increasing the accuracy when determining changes in ambient conditions. 
   In a coordinate measuring device, the object to be measured is positioned or moved with the aid of a positioning stage relative to the detection optics in order to optically detect each area to be imaged by the optics of the usually planar surface of the object to be measured. Herein the surface of the usually square or circular shape of the object has a size of between about 10 cm×10 cm and about 30 cm×30 cm, or a diameter of about 10 cm to about 30 cm. In order to inspect the whole surface of the object with the aid of the coordinate measuring device, it should be possible to position the object with the positioning stage on a traversing path essentially corresponding to the size or the dimensions of the surface of the object. This is why the maximum traversing path of the moveable positioning stage is usually a function of the size of the surface of the object normally to be inspected by the coordinate measuring device. Similar considerations generally apply to measuring assemblies in which the most stringent requirements apply to the measuring accuracy. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide and further develop a measuring device for determining relative positions of a positioning stage at least moveable in one direction of the initially mentioned type, by which the above mentioned problems are overcome. In particular it should be possible to achieve increased accuracy in determining changes in the ambient influences. 
   The inventive measuring device of the initially mentioned type solves the above object and comprising at least one interferometric measuring means and at least one interferometric correction means, wherein measuring results for determining relative positions of the positioning stage for a movement along a movement direction are able to be generated by an interferometric measuring means, wherein an interferometric measuring means is operable with laser light of a laser of at least one wavelength, wherein correction results are able to be generated with the interferometric correction means, allowing conclusions to be drawn about the actual wavelength of the laser light during a position determination of the positioning stage in order to take into account a variation of the wavelength of the laser light, in particular due to ambient conditions, for an evaluation of the measuring results, wherein the interferometric correction means is proximate to the interferometric measuring means and the proximity corresponds to a predeterminable portion of the maximum traversing path of the positioning stage. 
   According to the present invention it has been found that, even if only varied slightly, the ambient conditions that have an effect on the interferometric measuring means can be corrected with a particularly high degree of accuracy if the interferometric correction means is arranged proximate to the interferometric measuring means. For most applications it is therefore sufficient for the interferometric correction means to be at a distance of no more than about 20 cm to about 30 cm from the interferometric measuring means. In this case local changes in the external ambient conditions can be detected directly where they arise or where they are created and therefore where they have an effect on the corresponding measurements. Basically the distance between the interferometric correction means and the interferometric measuring means is chosen such that a predeterminable accuracy is achievable in the detection of wavelength variations depending on the concrete application, the overall measuring assembly, and the ambient conditions of the measuring assembly. An indication herefore could be derived, for example, from an estimation according to Edlen describing the dependence of the refractory index of air n L  on the temperature and pressure:
 
 n   L =1+2.87755 10 −7   p*Func ( p,T )−2.610 −9   e   −0.0057627T    F   R ,
 
with
 
 Func ( p,T )=(1 +p (0.612−0.01010 −6   T ))/(1+0.0036610 T )
 
T=temperature in degrees Celsius
 
P=pressure in hPa
 
F R =the relative air moisture in %.
 
   In a coordinate measuring device this depends in particular on the characteristics of the climatic chamber, the flow properties and the position of the heat sources. Values for the portion of the maximum traversing path of the positioning stage could be any values ranging from 1 to 0. Preferably a smaller value is sought to be achieved for the portion of the maximum traversing path of the positioning stage, which means that the interferometric correction means is arranged as closely as possible to the interferometric measuring means so that a high correction quality can be achieved. As a result the correction data measured according to the present invention can be incorporated in the evaluation of the measuring results so that a measuring result of increased accuracy can be achieved overall. Unlike the prior art the interferometric correction means is not arranged at the location of a measuring system or on a coordinate measuring device which is relatively remote from the location of the interferometric measuring means. In such a case known from the prior art, the changes in the ambient influences are only measured with respect to this location and incorporated into the measuring result of one or more interferometric measuring means, even if possibly there were completely different ambient conditions, at least temporarily, at the site (or sites) of the interferometric measuring means. Since the varying ambient conditions, sometimes temporarily present, are now directly detected by the aid of the interferometric correction means at the location of the interferometric measuring means and are incorporated into the measuring results of the interferometric measuring means as a measuring correction, increased precision in the determination of the variation of the ambient conditions is achievable. This is how the correction quality and thus the measuring accuracy of the overall measuring system of the coordinate measuring device can be improved in a particularly advantageous way, all the more so, since the more climatic variations occur on the correction distance the more they have an effect on the measuring distance or arise on the measuring distance itself. Therefore, ideally, the correction distance and the measuring distance should overlap each other at least to a great extent. 
   An interferometric correction means could be arranged in an area traversed or crossed by the positioning stage and which could be positioned above or below the positioning stage. Such an arrangement of the interferometric correction means is suitable in particular with a positioning stage which is moveably arranged in two directions (X and Y directions). It is therefore conceivable to arrange one interferometric correction means essentially at the position of each interferometric measuring means, but above or below each interferometric measuring means with reference to the positioning stage. It is also conceivable to arrange at least one interferometric correction means above and/or below the positioning stage, which has its measuring direction or the path of its partial beams arranged essentially in a diagonal direction with respect to the positioning stage, which is usually of a rectangular configuration. 
   In a preferred embodiment an interferometric correction means is directly adjacent to an interferometric measuring means. If the measuring system has three interferometric measuring means for detection of an object movement in three different spatial directions, each interferometric measuring means of this measuring system has an interferometric correction means associated with it in a directly adjacent relationship. 
   For the variations of the ambient conditions to be measured with a precision which is as high as possible with an interferometric correction means associated with an interferometric measuring means, it is preferably provided for at least a portion of the beam path of the interferometric measuring means to be essentially parallel to at least a portion of the beam path of the interferometric correction means. Herein the parallel portions of the beam paths are suitably that portion of the interferometric measuring means which is also used for measuring. As a result the two parallel aligned portions of the beam paths are subject to almost the same localized variations of the ambient conditions. 
   In a concrete implementation, the interferometric correction means could be constructed according to the teaching of DE 101 31 898 A1. For a more detailed description of such interferometric correction means, reference is made to the entire disclosure of publication of patent application DE 101 31 898 A1, which is incorporated herein by reference in its entirety. 
   In a particularly preferred embodiment an interferometric correction means is incorporated in an interferometric measuring means. As a result the interferometric measuring means and the interferometric correction means form a common assembly. Compared to the approach according to which an interferometric correction means is provided for each interferometric measuring means, optical components can be saved in an advantageous way and therefore manufacturing costs can be reduced. Furthermore, common adjustment of the common assembly is necessary, which is why the interferometric measuring means need not be adjusted as well as the interferometric correction means. This serves to reduce assembly and manufacturing times and therefore manufacturing costs. It is almost impossible to arrange the interferometric correction means any more closely to the interferometric measuring means so that the accuracy achievable should be optimized in the present embodiment. The beam paths or at least a substantial portion of the beam paths of the interferometric measuring means and the interferometric correction means could be essentially aligned in parallel. As a result ultimately comparable measuring conditions are present for the beam paths of both the interferometric measuring means and the interferometric correction means. 
   Also, as an interferometric measuring means an interferometer may be suitable which is based on an operation according to the homodyne or heterodyne principle. The same also applies to the interferometric correction means. In a homodyne interferometer system the laser light is generated by a homodyne laser containing light not only from one frequency component. Homodyne interferometers are commercially available, for example, from Renishaw plc. The light generated by the homodyne laser is divided in a beam splitter in a fixed relationship (preferably 50:50) in two partial beams. One of the beams has its frequency shifted on reflection at a mirror on the measuring object due to the Doppler effect when the mirror and therefore the positioning stage is moved. The measuring value is derived from the frequency shift. For the interferometric correction means, a frequency shift of the light of the two partial beams is achieved based on wavelength variations due to the climate in a distance of predeterminable length which is traversed by only one of the partial beams. 
   An interferometric correction means could therefore be in the form of a heterodyne or homodyne system and could have a suitable laser and corresponding evaluation electronics. The same applies to an interferometric measuring means. The two systems could also be mixed, i.e., a measuring value could be determined using a homodyne system and another measuring value could be determined using a heterodyne system. 
   An interferometric measuring means based on a heterodyne system will be described in more detail together with the interferometric correction means incorporated in it, if any. It is operated with the laser light of a heterodyne laser. The light of such a laser has two components of a linearly polarized light. 
   The two components of linearly polarized light are oriented essentially vertically to each other. The laser light could have a wavelength of 632.9 nm or a frequency of 4.7 10 14  Hz. The two components are distinguished from each other at least slightly by their wavelengths or frequencies. The frequency difference of the two components is for example in the order of megahertz, in particular the frequencies of the light components differ by about 2 MHz. The laser light is divided into two partial beams on a beam splitting surface. Herein the beam splitting surface could be provided, for example, by a polarization beam splitter or a beam splitting cube. One of the two partial beams could be redirected on at least one beam redirecting means in a direction such that the two partial beams are essentially parallel, for example, in the moving direction of the positioning stage or in the measuring direction of the interferometric measuring means. The beam redirection means could be, for example, a mirror or a glass component, in which the light is internally totally reflected. 
   The above described functions of the beam splitting surface and the beam redirecting means could be provided, while using as few optical components as possible, in the form of two wedge prisms adjacent to each other. The two wedge prisms are arranged so that they are adjacent to each other, and at least one surface of the two adjacent surfaces of the wedge prisms is the beam splitting surface if the light is incident in a suitable way. At least one of the surfaces adjacent to each other of the wedge prisms is a beam unification surface for the returned light beams. A further surface of the wedge prism opposite the surface of a wedge prism serving as a beam splitter serves as the beam redirecting means. In a concrete implementation one of the wedge prisms has a base in the form of a parallelogram. The other wedge prism has a base with four corners wherein there are two further sides at right angles to a first side and wherein a fourth side faces the first side in an alignment which is not parallel to the first. 
   The measuring beam passes from the beam splitting surface to a reflection means arranged on the positioning stage. The reference beam passes from the beam splitting surface to a fixedly arranged reference reflection means. The reflection means arranged on the positioning stage comprises, in particular, a mirrored area of the positioning stage or a mirror fixed on the positioning stage. 
   In an interferometric correction means two reference beams pass from the beam splitting surface in each case to a fixedly arranged reference reflection means. The lengths of the optical paths of the two reference beams differ. 
   A lambda/4 plate is provided in each measuring beam and in each reference beam in order to limit or define the number of passes of the light beams in the interferometric measuring means or in the interferometric correction means. 
   It is particularly preferable that at least one further beam redirecting means is provided generating a parallel beam offset and with which a light beam exiting from one of the wedge prisms can be directed one more time into the interferometric correction means and/or into the interferometric measuring means. Herein it is provided, in particular, that a light beam can be directed one more time into the interferometric correction means and/or into the interferometric measuring means or into a wedge prism which comprises at least two reunified partial beams. The beam redirecting means could be, for example, two mirrors at right angles to each other or a prism having a base in the form of a right triangle. On the one hand these simple means therefore allow a beam offset to be created and on the other hand the light beam to be directed into the interferometer again, so that the effective measuring or reference distance can be doubled. 
   The interferometric correction means and the interferometric measuring means, in a preferred embodiment, are in the form of a multi-axis interferometer, preferably in the form of a biaxial interferometer. In a multi-axis interferometer, the light beam incident into the interferometer is multiplied with the aid of an optical component. This can be carried out, for example, with a glass parallelepiped or a prism having a base in the form of a parallelogram and using the multiple reflections therein. 
   Also provided is a detection means with which the intensity of the overlapping partial beams is detectable. The partial beams can be overlapped for example with the aid of a polarization filter or detected downstream of a polarization filter. 
   In a concrete implementation a measuring signal can be extracted from the intensity detected in this manner, wherein a movement of the positioning stage causes a wavelength shift in at least one of the two partial beams, due to the Doppler effect, as a function of the moving velocity of the positioning stage. With velocities of the positioning stage on the order of several mm/s, the frequency shift is in the kHz range. The intensity of the overlapping partial beams is modulated with a beating frequency depending on the wavelength shift and can be detected and processed with a detection means and with suitably configured electronics. 
   It is provided, with particular preference, that the reference beams of at least two interferometer axes or reference beams are overlapped and detected by a detector in order to enable a direct evaluation of a variation of the wavelength of the laser light which is caused, for example, by a variation of the ambient conditions. 
   Basically the measuring device of the present invention can be utilized in various applications. Preferably it is provided that relative positions of the positioning stage are determined for movements only along one movement direction using an interferometric measuring means. If, for example, the positioning stage is configured such that it can be moved in two directions perpendicular to each other, the movements of the positioning stage are detected by two interferometric measuring means having partial beam paths for measuring which are also at right angles to each other. Such an application is usually present in coordinate measuring devices in which high precision measurements of the coordinates of structures on substrates are carried out, such as on masks, wafers, flat panel displays, vapor deposited structures, semiconductor substrates, illumination masks or optical data carriers, in particular for transparent substrates. The coordinates are determined relative to a reference point with the precision of a few nanometers. This is why the interferometric measuring means must be arranged with, and aligned with respect to, the positioning stage in such a way that a large portion of the beam path of the interferometric measuring means, i.e., the portion of the beam path serving for the measurement, is essentially parallel to the movement direction of the positioning stage. Therefore, for a positioning stage moveable in three directions, an interferometric measuring means must be provided for each spatial direction in which the positioning stage is to be moved. 
   It is also conceivable for relative positions of a positioning stage to be movable in only one movement direction (linear displacement stage) to be determined. In this case it is sufficient to provide only one interferometric measuring means with which the relative positions of the positioning stage can be determined. The interferometric measuring means could be arranged relative to the positioning stage in such a way that a large portion of the beam path of the interferometric measuring means is essentially parallel to the movement direction of the positioning stage. 
   With respect to a method for determining relative positions of a positioning stage configured to be moveable in at least one direction by a predeterminable traversing path, the initially mentioned object is achieved by the features of claim  21 . In the method according to the present invention measuring results for determining relative positions of the positioning stage are generated by means of at least one interferometric measuring means for a movement along one movement direction. An interferometric measuring means is operated with the laser light of a laser at at least one wavelength. An interferometric correction means is for generating correction results allowing conclusions to be drawn on the actual wavelength of the laser light during a position determination of the positioning stage in order to take into account the variations of the wavelength of the laser light, in particular due to ambient conditions, when evaluating the measuring results. According to the invention, the interferometric correction means is proximate to the interferometric measuring means, and the proximity corresponds to a predeterminable portion of the maximum traversing path of the positioning stage. 
   A method of the present invention is suitable, in particular, for operating a measuring device and comprising the steps of:
         operating a measuring apparatus;   determining measuring results for relative positions of the positioning stage for a movement along a movement direction, wherein the measuring results are generated with at least one interferometric measuring means;   operating the interferometric measuring means with laser light of a laser at at least one wavelength; and,   generating correction results with the interferometric correction means, allowing conclusions to be drawn with respect to the actual wavelength of the laser light during a position determination of the positioning stage, in order to take into account a variation of the wavelength of the laser light, in particular due to ambient conditions, when evaluating the measuring results, wherein the interferometric correction means is proximate to the interferometric measuring means, and in that the proximity corresponds to a predeterminable portion of the maximum traversing path of the positioning stage.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There are various possibilities to improve upon and develop further the teachings of the present invention in an advantageous manner. Reference is made, in particular, to the patent claims depending on claim  1  and also to the following explanation of the preferred exemplary embodiments of the invention with reference to the drawings. In the context of the explanation of the preferred exemplary embodiments of the invention with reference to the drawings, generally preferred embodiments and further developments of the teaching are also explained. In the drawing, always as a schematic representation, 
       FIG. 1  shows a coordinate measuring device known from the prior art; 
       FIG. 2  shows the optical beam path of two interferometric measuring means and an interferometric correction means as used, for example, for determining the position of a positioning stage in a coordinate measuring device known from the prior art; 
       FIG. 3  shows a first embodiment of a measuring device according to the present invention; 
       FIG. 4  shows a second embodiment of a measuring device according to the present invention; and, 
       FIG. 5  shows the beam path of an interferometric measuring device into which the interferometric correction means is incorporated. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Components which are similar or the same are indicated with the same reference numerals throughout the figures.  FIG. 1  shows a coordinate measuring device  1  with which it is possible to optically inspect objects  2 . Object  2  shown in  FIG. 1  is a mask, which can be of quartz glass for example. Structures  3  are applied to the mask, which are inspected with the coordinate measurement device  1 . Coordinate measurement device  1  comprises two illumination beam paths  4  and  5 , wherein illumination beam path  4  is for the transmitted-light mode and illumination beam path  5  is for the incident-light mode of coordinate measurement device  1 . For the transmitted-light mode, a light source  6  is provided for emitting light in the near ultraviolet and which is reflected by mirror  7  towards an illumination optics configured in the form of a condenser  8 . The light of illumination beam path  4  passes through object  2  and is at least substantially collected by imaging optics  9  and imaged onto detector  10 . Detection beam path  11  therefore extends from object  2  to detector  10 , wherein the light coming from object  2  is almost entirely reflected by beam splitter  12  towards detector  10 . Imaging optics  9  can be moved by a focusing means, not shown in the figure, along the z direction indicated by a double arrow, whereby object  2  or structures  3  can be focused. Condenser  8  can also be moved along the z direction in the same way. 
   Coordinate measurement device  1  also has an incident-light mode. In this mode object  2  is illuminated with light from light source  13 , a portion of which passes through beam splitter  12  (e.g., a so-called 50:50 splitter) and illuminates object  2  via imaging optics  9 . The illumination light reflected on object  2  or on structures  3  in this mode passes in the reverse direction through imaging optics  9  and is reflected on beam splitter  12  towards detector  10 . Accordingly, illumination beam path  5  extends from light source  13  to object  2 . Illumination beam path  4  extends from light source  6  to object  2 . 
   Imaging optics  9  is a high-resolution, apochromatically corrected microscope optics adapted for light in the near ultraviolet. Detector  10  is in the form of a high-resolution CCD camera and is driven and read out by a computer evaluation and analysis system, not shown in  FIG. 1 . Reference numerals  16  and  17  indicate the optical axes of illumination beam paths  4 ,  5 . 
   Object  2  is supported on a positioning stage  18  and moveable along two different x and y directions indicated by the two double arrows. Positioning stage  18  has a frame in which object  2  is placed. Interferometric measuring means  14  is only schematically shown, with which the position of positioning stage  18  can be measured interferometrically via light beams  15 . The frame of positioning stage  18  is supported on an air cushion  19  and can be moved on granite block  20  in a substantially frictionless manner. Granite block  20  is supported on legs  21  in a vibration-damped way. 
     FIG. 2  is a schematic top view of positioning stage  18  with the optical beam path for determining the positions into which positioning stage  18  can be brought. Traversing path VX of positioning stage  18  in the x direction and traversing path VY of positioning stage  18  in the y direction are only schematically shown and essentially depend on each application, the dimensions of positioning stage  18 , and/or the dimensions of the object to be measured. Laser  22  emits laser light, wherein the laser light has two components of linearly polarized light which are vertical to each other. The laser light is divided by beam splitter  23  into two partial beams, wherein one partial beam is directed toward mirror  24  and interferometric measuring means  25 , and the other partial beam is directed toward interferometric measuring means  27  via mirror  26 . Interferometric measuring means  25  is for determining relative positions of positioning stage  18  in the x direction (as indicated by the arrow). Interferometric measuring means  27  is for determining relative positions of positioning stage  18  in the y direction (as indicated by the arrow). A beam splitter  28  is arranged between beam splitter  23  and mirror  26 , with which part of the laser light can be passed into an interferometric correction means  29 . The measuring results measured by interferometric correction means  29  are for correcting variations in the result measured by interferometric measuring means  25 ,  27  due to variations in the ambient conditions. 
   The laser light is divided into two partial beams  25   a ,  25   b  or  27   a ,  27   b  in each of interferometric measuring means  25 ,  27 , respectively. One of the partial beams  25   b  or  27   b  (i.e., each reference beam) is reflected twice by a fixed reference mirror  60  and  61 , respectively, and is made to overlap partial beam  25   a  or  27   a , respectively. The second partial beam  25   a  or  27   a  (i.e., each measuring beam) is also reflected twice on the reflection means of positioning stage  18  in the form of a mirrored area  37 ,  38 , respectively. Only one reflection is shown in  FIG. 2 . The two-fold reflection of the partial beams  25   a ,  27   a ,  25   b ,  27   b  is described in more detail in the portion of the description referring to  FIG. 5 . 
   In interferometric correction means  29  the laser light is divided into two partial beams  29   a ,  29   b . One of the partial beams  29   a  (i.e., the first reference beam) is reflected twice by a fixed reference mirror  62  and is made to overlap the second partial beam  29   b  (i.e., the second reference beam). The second partial beam  29   b  is also reflected twice by the fixed reference mirrors  63 . 
   The two reference mirrors  62 ,  63  are advantageously fixed to each other and arranged at different distances to the beam splitter (not shown in  FIG. 2 ). Partial beams  29   a ,  29   b  also have differing optical path lengths in interferometric correction means  29 . The precision of the correction measurement with the aid of interferometric correction means  29  depends on the path length difference of the two reflected partial beams  29   a ,  29   b . Moreover, the correction quality is the higher, the more the changes in climate on the correction distance correspond to those on the measuring distance. 
     FIG. 3  is a schematic top view of a positioning stage  18  and the optical beam path for determining positions into which positioning stage  18  can be brought. The optical beam path is shown in  FIG. 3  in a way essentially comparable to the optical beam path in  FIG. 2 . According to the present invention, however, an interferometric correction means  29  is arranged proximate to the interferometric measuring means  25 , i.e., in a directly adjacent or neighboring relationship. Between mirror  24  and beam splitter  23 , a beam splitter  30  is arranged, with the aid of which a portion of the light of laser  22  can be directed toward interferometric correction means  29 . A second interferometric correction means  32  is similarly arranged close to interferometric measuring means  27 , also in a directly adjacent relationship. Between mirror  26  and beam splitter  23 , a beam splitter  33  is provided, with which part of the light of laser  22  is directed toward second interferometric correction means  32  via beam splitter  33 . The two reference beams of this interferometric correction means  32  are indicated with reference numerals  32   a ,  32   b  in analogy to those of interferometric correction means  29 . It can only be schematically derived from  FIG. 3  that the portions of beam paths  35  used for measuring, in the concrete case measuring beam  25   a ,  27   a , or reference beams  25   b ,  27   b , of interferometric measuring means  25 ,  27 , respectively, are arranged or aligned in parallel to the portions of beam paths  36  relevant for detecting changes in the ambient conditions, i.e., reference beams  29   a ,  29   b ,  32   a ,  32   b , of interferometric correction means  29 ,  32 . The partial beams  25   a ,  27   a  of beam parts  35  of interferometric measuring means  25 ,  27 , which are used for measuring, pass from the laser to the mirrored areas  37 ,  38  of positioning stage  18 , from where they are reflected. As in  FIG. 2 , only one reflection is shown in  FIG. 3 . 
     FIG. 4  is a schematic top view of a positioning stage  18  and the optical beam path for determining the positions into which positioning stage  18  can be brought. The optical beam path in  FIG. 4  is basically comparable to the optical beam path in  FIG. 3 ; however, the interferometric correction means  29 ,  32  are incorporated into the interferometric measuring means  25 ,  27  in  FIG. 4 . In each case, they are implemented as a biaxial interferometer. As a result, beam splitters  30 ,  33  and the two individual interferometers  29 ,  32 , as indicated in  FIG. 3 , can be advantageously omitted. It is therefore no longer necessary to adjust these components during the manufacture of the coordinate measuring device. 
     FIG. 5  is a schematic top view of an exemplary embodiment of a measuring apparatus  39  according to the present invention, which has an interferometric measuring means and an interferometric correction means. Therefore the interferometric correction means is incorporated into the interferometric measuring means in the form of a biaxial interferometer. Laser light  40  coming from a light source not shown in  FIG. 5  and entering into measuring device  39  passes through prism  41  causing the incident light beam  40  to be divided into two partial beams  42 ,  43  due to a first partial internal reflection and a second internal total reflection. Laser light  40  from the light source is linearly polarized light having a wavelength of 632.9 nm or a frequency of 4.7×10 14  Hz. It has two components of light waves vertical to each other. One of the light waves has a very slight frequency shift, of about 2 MHz, with respect to the other light wave. 
   The beam path of partial beam  43 , which is used for the position measurement will initially be described. Partial beam  43  enters a first wedge prism  44 . The first wedge prism  44  has a base in the form of a parallelogram. Due to the orientation of the polarization directions of the two light waves vertical to each other of partial beam  43 , i.e., in the plane of the drawing and vertical to the plane of the drawing, partial beam  43  is divided into two further partial beams. The division into two further partial beams occurs at the interface between surface  45  of wedge prism  44  and surface  50  of the second wedge prism  46 . The two linearly polarized beams oriented vertical to each other are thereby divided according to their polarization directions and therefore their frequency. The light beams reflected at the interface  45  are internally reflected on surface  47  of wedge prism  44 , so that one of the two partial beams passes toward reference mirror  48   b . Reference mirror  48   b  is fixed. 
   The portion of partial beam  43  not reflected at interface  45  passes in the same direction through wedge prism  46  toward measuring mirror  49   b . Measuring mirror  49   b  is fixed on the moveable positioning stage  18  and directly follows the movements of positioning stage  18 , not shown in  FIG. 5 . The partial beams passing both between wedge prism  44  and reference mirror  48   b  and between wedge prism  46  and measuring mirror  49   b  pass λ/4 plates  51  and  52 , respectively, causing the linearly polarized light components of the partial beams to become circularly polarized light rotated in phase by about 45 degrees. 
   The partial beams are reflected on reference mirror  48   b  and measuring mirror  49   b , respectively, and pass through λ/4 plates  51 ,  52  again and cause the circularly polarized light to become linearly polarized light again, having a light rotated in phase by a further 45 degrees, which is now rotated by 90 degrees with respect to the original orientation. The light from reference mirror  48   b , after internal reflection on surface  47 , is now incident on interface  45 ,  50  of wedge prisms  44  and  46 , where, due to its rotated orientation by 90 degrees, it is not deflected but passes through wedge prism  46  in a straight line. The light from measuring mirror  49   b  is now reflected at the interface  50 ,  45  of wedge prisms  44  and  46  due to its rotated orientation, also by 90 degrees. These partial beams now pass through wedge prism  46  to the beam redirection means in the form of a prism  55  which directs the partial beams after twofold internal reflection with a parallel beam offset towards surface  50  of wedge prism  46 . Since the direction of polarization does not change at beam redirection means or prism  55 , the partial beams are transmitted or reflected as before on interface  50 ,  45  of wedge prisms  46 ,  44 , so that they are directed in the same direction towards mirrors  48   b ,  49   b , from where they came. After passing once again through the two λ/4 plates  51 ,  52 , the partial beams entering into wedge prisms  44 ,  46  have their polarization direction rotated again by 90 degrees. The partial beams internally reflected on surface  47  and passing through wedge prism  44  are reflected or transmitted on interface  45  toward a first detection means  56 . The light beams from measuring mirror  49   b  and entering into wedge prism  46  pass through wedge prism  44  also towards detection means  56  without being reflected. With the aid of a polarization filter, not shown, in detection means  56 , the interferometric overlap of the two divided and reunified partial beams can be detected, which have originally entered wedge prism  44  with partial beam  43 . 
   Similarly, partial beam  42  coming from prism  41  towards wedge prism  44  passes through wedge prisms  44 ,  46 , through λ/4 plates  51 ,  52  to reference mirror  48   a  and reference mirror  49   a , respectively. Reference mirror  49   a  is not moveable but advantageously fixedly connected with reference mirror  48   a , also fixed. After reflection at reference mirror  48   a  and reference mirror  49   a , respectively, the partial beams pass to the beam redirection means in the form of a prism  53  which also redirects the partial beams with a beam offset back into wedge prisms  46 ,  44 . After a further reflection on reference mirror  48   a  or reference mirror  49   a , respectively, the partial beams are again reflected at surface  45  or  50  towards further detection means  54 . Detection means  54  is for detecting the interferometric overlap of the two divided and reunified partial beams with the aid of a polarization filter, not shown, originally entering wedge prism  44  for the first time as partial beam  42 . 
   The lightwave resulting from the overlap still has the input frequency; it is modulated, however, with the beating frequency which is dependent on the frequency difference. The two frequency components are Doppler shifted by the ambient conditions and the component referred to as the measuring beam, as well as by the movement of the measuring stage. When positioning stage  18  and therefore measuring mirror  49   b  is moved, there is a Doppler shift of the frequency of the partial beam reflected on measuring mirror  49   b . In the overlapped light this results in the beating signal (about 2 MHz) which can be resolved by detection means  56  and downstream electronics and processed into a position signal. The beating frequency of the light beams reflected on the two fixed mirrors  48   a ,  49   a  and overlapped, which are detected by detection means  54 , is proportional to the change in wavelength and can be detected due to the different optical paths or paths of differing length up to mirrors  48   a  and  49   a.    
   In the present embodiment, fixed mirrors  48   a ,  48   b ,  49   a  are all in a different position with respect to the vertical of the mirror surfaces. Mirrors  48   a ,  48   b ,  49   a  could also be separately fixed on a coordinate measuring device, or, preferably, on a common support arranged on the coordinate measuring device. 
   Finally, it should be noted in particular that the above discussed exemplary embodiments only serve to describe the teaching claimed; however, they do not limit it to the exemplary embodiments.