Interferometric system having a reference surface including a mirrored zone

The present invention relates to an interferometric system having an illumination arm, including a light source and an illuminating optical system, for forming an illuminating beam; having an object arm, including a reference element for measuring an object having an object surface to be measured, for forming an image-rays path, the object to be measured having an object surface inaccessible to direct illumination; having a reference arm including a reference element; having a detector arm including a detector; and having a beam splitter.

The present invention further relates to a measuring method able to be carried out using the device.

The manufacturing of precision parts requires measuring methods for recording the geometry and the state of the parts in order to ensure the quality of the corresponding parts. Optical measuring methods such as image acquisition and image evaluation, interferometry, particularly white-light interferometry, makes an important contribution here.

The principle of the white-light interferometer is based on the fact that a short-coherent light source is used for the illumination of an imaging system. In addition to the normal imaging optics, the imaging system has a reference arm which is traversed by a portion of the irradiated light. If the transit path of the light Λ o in the object arm and the transit path in the reference arm Λ R now have a path difference that is less than the coherence length I c of the light, i.e.

|Λ R −Λ O |<I C   (1)

then the light fields brought together again can exhibit a measurable interference. This is utilized in that, during the measurement, the path difference of the light fields, defined by the shift of the object or the reference element along the optical axis, is altered. At the same time, the intensity of the reunited light fields is measured on a detector, usually a CCD camera, measuring in planar fashion. Since a constructive or destructive interference can only take place within the coherence length of the white-light source, the pixel-by-pixel evaluation of the intensity modulation produced by the interference, the intensity correlogram, supplies clear information concerning height for each individual pixel. Carried out for the entire pixel field, this results in complete height information for the object.

Commercial white-light interferometers typically have the following specifications:

The height resolution Δz is given by the average utilized wavelength of the light λ m , the coherence length I c and the type of correlogram evaluation algorithm. Typical parameters such as λ m =600 nm, I c =2 μm permit values of Δz=1 nm.

The lateral resolution δ is equal to that of a conventional imaging system and, in principle, is limited by λ m and the numerical aperture NA of the imaging optics.

δ≧0.61 λ m /NA   (2)

The maximum measurable total height difference z max is determined by the technical feasibility of producing a path difference in the reference arm and object arm that is guided precisely over the entire distance. Regulated piezosystems today permit values of z max ≧400 μm.

Conventional interferometers, particularly white-light interferometer systems, can be used for the tasks described above when the location to be measured is easily accessible and has a predominantly flat geometry. If this is not the case, interferometers are used which have special optics adapted to the object to be measured. However, these interferometers have the disadvantage that undercuts on the object to be measured lie in the shadow area of the illumination and therefore cannot be recorded. To measure these surfaces, the object must be dismounted and measured in a second measuring operation.

An object of the present invention is to provide an interferometric system that allows a 3-dimensional measurement of objects having hard-to-access surfaces, using a single mount.

This object of the present invention relating to the device may be achieved in that the reference element has one or more mirrored zones. The light beams thereby reach undercut surfaces, permitting them to be measured in the same measuring operation as the remaining surfaces. In particular, it is possible to determine the position of the undercut surfaces relative to the remaining surfaces.

Undercuts of any form, even those having surfaces which are not even, can be measured by adapting the mirrored zone to the object, to the effect that in each case, the mirrored zone is formed in the half angle to the perpendicular with respect to the optical axis of the image-rays path, like a partial surface to be measured on the object surface.

One specific example embodiment that is especially insensitive to environmental influences such as the influence of temperature provides that the mirrored zone is joined in one piece with the reference element.

If the mirrored zone is implemented as a separate unit and if it is joined mechanically to the reference element, e.g., by adhesive bonding or screwing, in a separate processing step, it may be adapted to the shape of the object surface and, if applicable, be reused in another reference element when working with a substantially identical object.

In one preferred specific embodiment, in addition to the mirrored zone, a second reference surface for measuring the object surface is formed in the reference element. It is thereby possible to determine the relative position of an object surface accessible for the customary measuring method and the surface that is usually not accessible.

If, in addition to the mirrored zone, at least one second reference surface for measuring at least one second object surface is formed in the reference element, in a depth scan, it is possible to determine the position of all object surfaces of interest in relation to the reference element.

The object of the present invention relating to the method may be achieved in that light beams reflected by an object surface are brought to interference with light beams reflected by an associated reference surface, while light beams reflected by the object surface inaccessible to direct illumination are additionally reflected via a mirrored zone and then brought to interference with light beams reflected by an associated reference surface. In this way, component parts having undercuts may be measured in a single measuring operation, and, in particular, it is possible to determine the relative position of the undercuts with respect to the remaining surfaces.

The present invention is explained in greater detail below with reference to exemplary embodiments shown in the figures.

FIG. 1 shows schematically a conventional white-light interferometer configuration.

FIG. 2 shows schematically an example interferometric system.

FIG. 3 shows schematically an interferometric system which is suitable for the simultaneous measurement of surfaces facing toward and away from the objective.

Interferometric system 1 of a white-light interferometer configuration according to the related art shown schematically in FIG. 1 includes an object arm 40 , in which the surface to be measured on an object 41 is located, and an illumination arm 20 which has a light source 21 and an illuminating optical system 22 , composed of one or more lenses, which form an illuminating beam path 60 . Situated orthogonally with respect to illumination arm 20 and object arm 40 , interferometric system 1 has a reference arm 10 having a reference element 11 that is coupled mechanically to an adjusting element 12 , usually a piezosystem. Diametrically opposite reference arm 10 is a detector arm 30 which has a detector 31 , usually a detector 31 measuring in planar fashion such as a CCD camera, as well as a lens 32 for imaging an intensity distribution to be evaluated in image-rays path 70 . An evaluation unit (not shown) is provided for the evaluation.

In this context, a beam splitter 50 splits up the various light beams and brings them together again, so that the light beams from reference arm 10 and those from object arm 40 are able to interfere in detector arm 30 at detector 31 in the manner described above.

The object may be scanned by shifting reference element 11 using adjusting element 12 or, alternatively, by shifting object 41 using a similar adjusting element.

The architecture of a conventional interferometric system 1 allows only the measurement of surfaces on object 41 which are directly accessible to the illumination. Undercuts in object 41 require that the object be dismounted, and that these surfaces as well as further object surfaces be measured in a second operation, in order to determine the position of the undercuts with respect to the object surfaces measured in the first cut.

In contrast, FIG. 2 shows schematically an interferometric system 1 in which, according to the present invention, object surfaces 45 on object 41 which are inaccessible to direct illumination are measured. To that end, object 41 is connected to a reference element 42 that has at least one mirrored zone 46 .

The light emerges from illumination arm 20 (not shown here) through a lens 48 in the direction of object 41 . The portion striking mirrored zone 46 is reflected in the direction of object surface 45 inaccessible to direct illumination. Reflected back from there and via mirrored zone 46 , it passes through lens 48 and is supplied via beam splitter 50 (not shown here) to detector 31 (likewise not shown). Used as reference for object surface 45 is a reference surface 47 whose reflected light traverses the same optical path length as that reflected by object surface 45 , and therefore produces the same interference pattern.

FIG. 3 shows a specific embodiment for interferometric system 1 which permits a measurement of a complete object 41 having undercuts. To that end, in object arm 40 , lens 48 is brought into at least one second position, represented here by lens 49 . This brings about a depth scan over object 41 . While in the position of lens 48 , object surface 43 and associated reference surface 44 contribute to interference, in the position of lens 49 , object surface 45 and associated reference surface 47 are measured, object surface 45 being measured with the aid of mirrored zone 46 . In this way, all surfaces of interest on object 41 may be measured relative to one another. Thus, given a suitable embodiment of reference element 42 having mirrored zones 46 , it is possible to reach all object surfaces of interest.