EP1929238A1

EP1929238A1 - Interferometric measurement apparatus

Info

Publication number: EP1929238A1
Application number: EP06793175A
Authority: EP
Inventors: Bernd Schmidtke; Ulrich Kallmann; Sebastian Jackisch; Hartmut Spennemann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-09-22
Filing date: 2006-09-04
Publication date: 2008-06-11
Also published as: KR20080046207A; WO2007033898A2; JP2009509150A; DE102006016131A1; US20090219515A1

Description

description

Interferometric measuring device

State of the art

The invention relates to a measuring of layer structures from a plurality of layers arranged one behind the other in the depth direction with a scanning device which scans them automatically in their depth direction, by means of which an interference plane is displaceable relative to the layer structure, with a white-light interferometer and / or a wavelength-scanning interferometer Interferometer, which is supplied to the measurement of an irradiation unit, an input radiation which is divided by a beam splitter and supplied to a reference arm via a reference beam as reference beam to a reference arm and the other part via an object beam path as an object beam to a measuring the layer structure exhibiting arm, with an imager which picks up the interfering radiation returning from the reference arm and the object arm and converts it into electrical signals, and with a downstream evaluation Device for providing the measurement results, as well as an interferometric measuring method.

Such an interferometric measuring device is specified in DE 101 31 779 A1. In this known interferometric measuring device, which operates according to the measuring principle of so-called white light interferometry, a surface structure of a measuring object in the depth direction (z direction) is scanned by means of a scanning device by changing the length of a reference light path relative to the length of an object light path, so that the interference plane, which results from the interaction of a reference beam guided by a reference arm and an object beam guided by an object beam, is displaced relative to the object surface. A special feature of this known interferometric measuring device is that at the same time several surface areas of the measurement object are detected and scanned can, for which purpose a special optics, namely a so-called superposition optics or optics with a sufficiently large depth of field or a multi-focal optics is arranged in the object arm, with which simultaneously the different surface areas can be detected. This results in correspondingly different beam paths in the object arm for the different surface areas to be measured, so that the surface areas are then measured with respect to their topographical surface structure by relatively changing the optical length of the object light path to the optical length of the reference light path, for example by adjusting a reference mirror in the depth scanning direction become. This structure is particularly suitable for scanning laterally adjacent surface areas, which may have different orientation or may be offset in the depth direction. Also a parallelism or thickness of the different surfaces can be measured. The spatially separated areas are always detected at the same time, so that an adaptation of the optics in the object arm, which takes into account the relative positional relationship of the two areas to be measured, must be provided.

Even with an interferometric measuring device shown in DE 197 21 843 C1, different surface areas of an object, in particular also in narrow boreholes, can be measured by means of a partially common object arm, object beam paths also being assigned to the different surface areas. Here, the measured surface areas are laterally separated from each other, so that e.g. the roundness of a cylinder bore can be checked. The different surface areas are distinguished on the basis of different polarization directions of the associated object beams. Again, the mapping of the different surface areas on the object arm is done simultaneously.

A further interferometric measuring apparatus shown in WO 01/38820 A1, which likewise is based on the principle of white-light interferometry, is constructed in such a way that thickness, distance and / or profile measurements are also carried out on successive layers, for example at Ophthalmological measurements Corneal thickness, anterior chamber depth, retinal layer thickness or retinal surface profile. For this purpose, different light paths are also formed in the object arm, which are assigned to different layers or interfaces in order to achieve the fastest possible measurement. For distinguishing and assigning the different measured surfaces or interfaces, the object beams (measuring beams) have different optical properties, such as different polarization direction or different wavelength; also a change in the detour of the various NEN object light paths in the object arm is possible, but leads to a loss of sensitivity, which is pointed out in this document.

More basic explanations of white light interferometry are given in T. Dresel, G. Häusler, H. Venke, "Three-dimensional sensing of rough surfaces by coherence radar", Applied Optics Vol. 31, 919, 1992 and in P. de Groot et al Deck, Journal of Modern Optics, "Surface profiling by analysis of white-light interferograms in the spatial frequency domain," Journal of Modern Optics, Vol. 42, 389-501, 1995. In Kieran G. Larkin "Efficient nonlinear algorithm for envelope Detection in white light interferometry ", J. Opt. Soc. Am. A, (4): 832-843, 1996 is carried out as described with a special algorithm of detected intensity values according to the so-called FSA method (fϊve-sample adaptive method) the modulation M of a correlogram can be determined Another approach for identifying and evaluating a correlogram is to consider the interference contrast.

The object of the invention is to provide an interferometric measuring device and a method for measuring layer structures, which supplies the most reliable measurement results possible with as little effort as possible.

ADVANTAGES OF THE INVENTION This object is achieved in the device having the features of claim 1 and in the method having the features of claim 21.

In the apparatus it is provided that the scanning device is formed in such a way that at the same reference beam path and object beam path of the associated Abtastweg is at least as large as the expected or determined in a pre-measurement distance of at least two successively arranged to be detected boundary surfaces of the layer structure optionally plus an expected depth structure of the interfaces, and that in the formation of the interferometer with the irradiation unit as Weißlichtferferometer the coherence length of the input radiation is chosen to be at most so large that the interference maxima of the depth sampling successive occurring correlograms are distinguishable at the interfaces to be detected, and / / or, if the interferometer part is formed with the irradiation unit as a wavelength-scanning interferometer, the irradiation unit with a narrow-band, tunable input terminal is formed, wherein the bandwidth of the input radiation is chosen so large that the smallest expected or by the pre-measurement abge- - A -

estimated distance of the successive interfaces to be detected is resolved and / or when forming the interferometer as a wavelength-scanning interferometer with spectrally broadband irradiation unit and a wavelength-scanning optical Spektrumanalysa- tor as detector the bandwidth of the input radiation is chosen so large that the smallest to be expected or estimated by the pre-measurement distance of the successive interface to be detected is resolvable.

In the method, it is provided that in the depth scanning of all layers to be measured and their boundary surfaces, the object beam is guided in a scanning cycle over the same object beam path and the reference beam is guided over the same reference beam path and that when applying the method of white light interferometry, the coherence length of in the interferometer coupled input radiation is chosen to be at most so large that the interference maxima of the depth scanning successively occurring at the interfaces to be detected correlograms are distinguished and the application of the method of wavelength-scanning interferometry, the bandwidth of the input radiation is chosen so large that the smallest expected or estimated by Vormessung distance of the interfaces to be detected is resolved.

With these measures, boundary surfaces of the layer structure including the outer boundary surface (surface) can be reliably detected and, if desired, precisely analyzed. In this case, it is also possible, for example, to detect transitions (boundary surfaces or boundary layers) in smaller areas of the layer structure during a passage of the scanning path and, if desired, to measure them more closely, since the correlograms are uniquely determined.

In principle, it would also be conceivable to detect a presence of several boundary layers due to deformations in superimposed correlograms; However, this approach would be more prone to error and inaccurate.

Larger lateral areas of the interfaces can be measured relatively quickly and also in relation to each other by virtue of the fact that the receiver has a surface resolution in the x / y direction which is higher than the mapping of the local height changes of the layer surface in the x / y direction , With these measures, it is also possible to detect and evaluate relative changes in the stratification courses in relation to each other. An advantageous embodiment for the detection and evaluation is that algorithms are programmed in the evaluation device, with which the individual layers can be detected separately from each other by an assignment by the sequence of occurring at the interfaces correlograms during a Tiefenabtastzyklus.

One possibility of using advance information on the layers in the evaluation is that there is an input unit operable by a user for entering the number of expected interfaces. On the basis of the number of interfaces, then e.g. the memory areas for the measured data to the interfaces are determined.

Another advantageous embodiment for the evaluation consists in that the evaluation device has a coarse value device with which the number of existing layers or boundary surfaces can be determined on the basis of a coarse definition of correlograms and that the evaluation device is designed in such a way that the number of determined layers is automatically taken over into an evaluation part or input by the user via the input unit.

If it is provided that the evaluation device has separate memory areas for the layers to be detected, to which data of the correlogram associated with the respective boundary surfaces can be assigned separately during a depth scan, the respective correlograms being related to their depth sampling position, then the result is relatively low Effort high efficiency in the evaluation and provision of measurement results.

The simple evaluation is thereby supported by the fact that the memory areas are designed as circulating memory areas.

A simple and rapid evaluation is further facilitated by the fact that for the recording of the interfacial correlograms there is a number of memory areas exceeding the number of interfaces by at least one, of which one memory area is an active memory area for writing current scan data during the depth scan and the others serve to record the determined correlogram data associated with the respective interfaces. The measures also contribute to a reliable detection of the correlograms and measurement of the boundary surfaces in that the evaluation device has an evaluation region which is designed to calculate the modulation of the intensity values obtained from the electrical signals during the depth scanning and to detect the respective boundary surfaces. the correlograms from the modulation is formed.

A favorable structure for more accurate measurements consists in that the evaluation device has an evaluation module, with which a fine measurement of a respective interface structure can be carried out.

In order to measure areal areas of the boundary surfaces, there are advantageous further measures in that the evaluation device is designed in such a way that during the depth scanning, the tracks extending in the direction of the depth scanning at different laterally in xy direction directly or indirectly adjacent interface areas in a corresponding way se are evaluable.

In order to obtain more detailed information, provision may also be made for the evaluation device to be designed in such a way that interface data of the interface areas adjacent in the x-y direction can be related to each other and evaluated with respect to each other for a respective depth scan.

For a reliable evaluation and accurate measurements, the measures are also advantageous in that, when the interferometer part is formed with a white light interferometer and wavelength-scanning interferometer, the evaluation device is designed in such a way that in a preliminary measurement the entire layer structure is involved in a depth scanning cycle for determining relevant regions the coarse scanning of the wavelength-scanning interferometer and in a subsequent measurement in a subordinate Tiefenabtastzyklus a fine measurement of the relevant regions with increased resolution.

A further advantageous embodiment of the measuring device is that the evaluation device for detecting streaks in the layer structure of an evaluation of the interference contrast or the phase shift at the caused by the streaks interfaces between Formed media different calculation indices, and further in that the evaluation device for detecting material changes or material transitions from an evaluation of the interference contrast or the phase shift at the caused by the material changes or material transitions interfaces or boundary layers, which are formed by different calculation indices, is formed, wherein Detecting the interference contrast change or the change of the phase shift laterally across the image field is included in the evaluation.

To an increased accuracy of the measurement results also contributes to the fact that when forming the interferometer as a white light interferometer dispersion compensation is carried out by the fact that in the reference arm corresponding layers are introduced as in the object arm.

In order to obtain reliable measurement results, the measures are also advantageous in that, when the interferometer is designed as a wavelength-scanning interferometer, software-assisted dispersion compensation of the measured data is provided in the evaluation device, which precedes the actual measurement data evaluation.

The evaluation is further facilitated by the fact that a variable optical attenuator is introduced into the reference arm, by means of which the light intensity can be controlled or regulated to the light intensity in the object arm.

The data acquisition is facilitated by the fact that in the object arm arranged with a control device optics is arranged, which causes an adjustment of the focus to the just scanned area during the depth scan.

Another favorable configuration for reliable detection of the interfaces is that the irradiation unit has optically pumped photonic crystal fibers and / or at least one superluminescent diode and / or at least one ASE light source.

drawings

The invention will be explained in more detail by means of embodiments with reference to the drawings. Show it: 1 is an interferometric measuring device for measuring layer structures in a schematic representation,

2 shows an intensity profile of an interference signal over the scanning path in the depth direction (z) as a correlogram and with the envelope in a schematic representation,

3 is a white light interferometer in a schematic representation with a multi-interface layer structure and associated correlograms,

Fig. 4 shows an intensity profile and an obtained therefrom with an F S A-algorithm

Modulation course over the scanning path in a basic representation,

5 different evaluation levels of an evaluation device in a modular structure,

6 is a schematic diagram for the measurement on a film having an oil film,

7 is a schematic diagram for writing intensity values into a circulating memory area,

8 shows an intensity profile and a modulation curve derived therefrom over the scanning path with two identified correlograms,

9 shows an intensity profile and an associated modulation curve over the scanning path with correlograms assigned to two separate memory areas,

10 shows a further illustration of an intensity profile and a modulation curve over the scanning path with correlograms written in two separate memory areas,

11 shows a further illustration of an intensity profile and a modulation curve over the scanning path with correlograms and extended scanning length assigned to two separate memory areas, 12 shows a representation of a modulation course over the scanning path with three associated correlograms written in separate memory areas,

13 shows a plurality of superluminescent diodes coupled via respective optical fibers to the

Bundling and coupling of their spectra in a white light interferometer in a schematic representation and

14 shows a further interferometric measuring device in combination with a tunable light source as a wavelength-scanning interferometer and with a

Laser pumped photonic crystal fiber as a white light interferometer in a schematic representation.

Disclosure of the invention

1 shows an interferometric measuring device which is designed to measure layer structures of a measurement object O with a plurality of layers lying one behind the other in the depth direction and transparent to an object beam OST. An interferometer part IT has a beam splitter ST, by means of which an input radiation EST, for example shown in FIG. 3, is split into the object beam OST guided by an object arm OA and a reference beam RST guided by a reference beam RST, in order to be superimposed by the superimposition of the reference beam RF Reference beam RST and the returned from the scanned layer structure of the object O object beam OST to generate an evaluable interference pattern, as known per se and described, for example, in the introductory cited documents. With regard to the object arm OA, the interference plane IE is arranged in the region of the measurement object O or the layer structure. In the depth scanning of the layer structure in the depth direction z, the interference plane IE is shifted relative to the layer structure in the depth direction z, whereby different interference patterns occur over the track of the depth scan. The depth scanning of the layer structure of the interference plane IE can take place in various ways, namely by changing the optical path length of the reference beam, in particular by moving the reference mirror, by movement of the measurement object O in the depth direction or by movement of the objective in the depth direction or by movement of the entire sensor relative to the measurement object O. In the exemplary embodiment shown in FIG. 1, an adjustment of the objective in the object arm OA is effected by means of an adjustment unit VE, for example a piezo stage, adjusted in discrete steps in the depth direction z. To measure the layer structure, the interference pattern is recorded with an image recorder BA and converted into corresponding electrical signals and evaluated in a subsequent evaluation device AW in order to obtain the measurement results to obtain information about the layer structures, in particular the interfaces. The image recorder BA is preferably a camera which has pixel-wise xy-directional image recording elements and dissolves the imaged interference pattern areally, so that at the same time several of the individual pixels associated tracks of the layer structure can be detected and evaluated in the depth scan.

The measurement of the interfaces can advantageously be carried out according to the principle of white-light interferometry. For this purpose, an irradiation unit or light source LQ (see Fig. 3) is used, which emits a short-coherent radiation, for example, one or more coupled superluminescent diodes SLDL ... SLD4 (see Fig. 13). Interference occurs only when the optical path length difference between the reference beam RST and the object beam OST lies within the coherence length Ic of the radiation emitted by the radiation unit LQ. The resulting interference signal is also referred to as correlogram in white-light interferometry. FIG. 2 shows a correlogram KG, which includes the intensity profile over the scanning path (in the direction of depth z), and the associated envelope G. As a characteristic parameter, a DC component GA, the position of the optical path adjustment Z ₀ , the phase .phi.o, are also entered Measure for the interference contrast IK as the difference between the maximum intensity I _max and minimum intensity I _mm, the central wavelength λo, the intensity I in arbitrary units and the path difference Δz between the object and reference arm as a double difference of sampling position z and Z ₀ in arbitrary units.

In Fig. 3, the interferometric measuring device is shown schematically, but wherein the scanning by displacement of the reference surface or in this case the reference plane RE is made. The object beam OST has the intensity Ii, the reference beam RST the intensity I ₂ and the image recorder BA results in an intensity I _d . The irradiation unit in the form of the light source LQ emits an input radiation EST with a spectral characteristic SV of short coherence length. On the measurement object there are two interfaces, namely the outer object surface OO and an underlying substrate surface SO, which when scanned in the depth direction z give the associated correlograms KG, which in the intensity profile I _d to the image sensor BA are included. For the exact detection and assignment of the correlograms KG to the respective interfaces, an evaluation based on the interference contrasts can take place. For better detection, however, the modulation M is determined in the present case, as shown in FIG. 4 together with the associated intensity profile over the scanning path z and is also shown clearly in FIG. 1 on a visual display. To determine the modulation M, the evaluation device AW is based on a special algorithm, namely the so-called FSA (fϊve-sample-adaptive) algorithm, which is based on the sampling of five successive intensity values of the interferogram and from which also the phase of the respective interferogram Scanning position in the scan z can be determined. For further details of the FS A algorithm, reference is made to the document cited (Larkin).

A peculiarity of the present interferometric measuring device and the measuring method lies in the fact that the scanning path z is chosen to be at least large enough to scan the entire area in which the boundary layers to be detected are present, the correlograms occurring at the various boundary surfaces being detected during the scanning to determine therefrom the presence of the interfaces by means of the evaluation device AW. In addition to the coarse detection of the boundary surfaces, it is also already possible to fine-tune the height structures of the individual boundary surfaces. The areal registration via the image recording elements of the image recorder BA or the camera K at the same time permits the acquisition of the height measurement data via a plurality of laterally adjacent tracks (in the depth direction z), so that 3D height information of the respective boundary surfaces can be obtained.

FIG. 5 schematically illustrates the procedure for determining the interfaces and, if desired, further information thereof. In the evaluation device AW, the evaluation is carried out in multiple stages, to which first the associated with the individual boundary layers

Correlograms KG can be identified from the calculation of the modulation M of the intensity profiles in a detection module EM. This is followed by a closer evaluation of the properties in an evaluation module AM of the evaluation device AW, if desired. Finally, there is still an assignment of the detected boundary surfaces and, if appropriate, closer properties thereof in an assignment module ZM, for example, the object surface OO and the substrate surface SO.

FIG. 6 shows, by way of example, a layer structure with a transparent layer in the form of an oil film on an underlying support. By scanning, for example, on Edge region of the oil film whose limit is detected. The height course can also be determined. If, for example, it is not known from the outset that there is contamination or intentional coating of a substrate, the property of the layer can be recognized on the basis of the measurement result obtained. The partial image a) shows the provided with an oil layer carrier, the partial image b) a measurement result in the edge region of the oil film on the support surface and the sub-image c) the calculated from the measurement result surface of the support under the oil film, while in the panel d) the clean Surface of the carrier is shown.

Another special feature of the present measuring device and the measuring method consists in the fact that in the evaluation device AW the measured data obtained during the depth scanning are stored in separate memory areas SB 1, SB 2, wherein a detection of the individual boundary surfaces takes place and an assignment to the different interfaces is made. This procedure is shown in FIGS. 7 to 12.

Fig. 7 shows the assignment of intensity values IW obtained during the depth scan via a track and their inscription in a circulating memory area SBO of a certain length. If the end of the memory area SBU is reached during the writing process of the intensity values IW, the first value is overwritten again and immediately. Thus, for each pixel x, y given by the picture elements of the image recorder BA, only a fixed number of successively sampled intensity values IW are stored in the memory and not the entire track. FIG. 7 shows by way of example a circulating memory area for eight intensity values IW. After the first eight values are written, the ninth value overwrites the value at the first memory address and the tenth sampled value overwrites the second. In particular, characteristic points of the layer structure, ie interfaces between the layers or within the layers, eg material inclusions or the like, are to be detected by way of the depth scan. This area of interest should then be written as completely as possible into the memory area. With the aid of the circulating memory areas SBU, it is possible to detect the areas of interest during the depth scanning of the layer structure quickly and reliably, whereby there is only a small storage space requirement in comparison with the totality of the data resulting from the track. The length of the circulating memory areas SBU is dimensioned according to the coherence length Ic and chosen to be so much larger that an associated correlogram is reliably detected over the so-defined scan length, as shown for example in FIGS. 8 to 12. The number of memory areas SB1, SB2... Is selected to be greater than the number of expected or pre-measured interfaces. Here are the circulating memory areas SBU into different memory areas, namely an active memory area and passive memory areas, one of which forms a reserve memory area. In the active memory area, the current intensity values IW are always written. Which memory address is currently assigned to the active memory area is decided by an algorithm when renaming the memory areas. The active memory area is always the memory area which contains the correlogram with the lowest modulation maximum of all memory areas SB 1, SB 2. If required, ie if the modulation maximum of the active memory area is exceeded during the depth scan, the sampled measurement data is also written into the reserve memory area parallel to the active memory area. For this write-in, the algorithm activates the spare memory area. In the reserve memory area, the correlogram with the second smallest modulation maximum is always stored. In the passive memory areas, the highest-modulation correlograms are saved. The active memory area is renamed after a successful acquisition of a modulation-rich correlogram in a passive memory area. If, therefore, a new complete correlogram is present in the active memory area, the writing to it is stopped and written further only in the reserve memory area. If another correlogram with stronger modulation follows, it is recorded in the reserve memory area. If it is completely captured, the other is rejected. Both memory areas exchange the function and the active memory area becomes the reserve memory area.

If a correlogram is completely detected, the depth scanning position assigned to it is stored and the memory areas are sorted according to maximum modulation. The area designated as reserve memory area becomes the new active memory area because it contains the weakest modulation correlogram at that time. From the remaining memory areas, the one with the next weaker maximum modulation is selected and set as the new reserve memory area. The method ensures that a weak modulation correlogram, which follows immediately one with stronger, is also completely detected within half the scan length, by the simultaneous detection in the reserve memory area.

For example, the number of memory areas is selected such that N + 1 memory areas are allocated at N correlograms to be detected. At N = 2 to be detected correlograms There is thus another passive memory area in addition to the active and the reserve memory area. In the passive memory areas, the corresponding, at N = 2 so only 1, correlograms are stored with the largest modulation maxima. In the reserve memory area, the correlogram with the second largest (N) and in the active memory area corresponding to the Kor- rogramme with the third largest (N + 1) modulation maximum is stored.

A counter saves the current progress of the acquisition. The counter is initialized as the start value with half the sampling length and counted down to 0 per sampling step. It is started at maximum modulation or reinitialized at a new maximum value and restarted. He has the task of ensuring that after reaching the maximum modulation, the remaining data is still written into the memory area. This ensures that sufficient values are symmetrical about the maximum of the envelope of the correlogram in the circulating memory area SBU and the correlogram is completely detected.

If the entire scanning path is scanned, the detection process ends with a data transfer. For the subsequent precise evaluation, the correlograms from the circulating memory areas are resorted in the middle of the memory area in the correct order of the sampling points with the modulation maximum. The active memory area is deleted during this copying process. In addition to the intensity values IW of the correlograms, the scanning position of the last sampling point is transmitted to the next module for each correlogram.

FIG. 8 shows the intensity and modulation curve over the scanning path z with two identified correlograms. FIG. 9 shows a memory state upon detection of two correlograms, wherein a correlogram with greater modulation occurs before a correlogram with lower modulation. Fig. 10 shows a memory state upon detection of two correlograms wherein a lower modulation correlogram occurs before a higher modulation correlogram.

FIG. 11 shows an adaptation of the scanning length with superimposed correlograms, as they occur on thin layers.

Fig. 12 shows a memory state upon detection of three correlograms. In order to obtain a larger usable optical spectral range and thus improved height resolution and delamination in the measurement of an n-layer structure with the white light interferometer, a plurality of fiber-coupled superluminescent diodes LD1, LD2, LD3, LD4... Or ASE light sources can be used different, overlapping optical spectra are advantageously combined in the near infrared spectral range in a fiber coupler FK to form a radiation unit for introducing the input radiation into the interferometer part IT, as FIG. 13 shows.

Another interferometric measuring device for scanning a layer structure in the depth direction is that instead of a white light interferometer, a wavelength-scanning interferometer WLSI is used. Wavelength-scanning interferometers in the sense of the present invention are characterized in that the optical spectrum of their spectrally narrow-band, variably tunable light source or irradiation unit LQ is chosen such that the layer structures to be examined are partially transparent or if the optical spectrum of the broadband spectral irradiation unit is chosen such that the layer structures to be examined are partially transparent. Accordingly, the detector is adapted to the light source LQ in order to obtain the highest possible sensitivity in the spectral range used. The choice of the light source LQ and of the detector or image recorder BA is therefore task-dependent as well as in the previously described white light interferometer WLI. In the near infrared spectral range (about 1000 nm to 1800 nm), an InGaAs CCD camera is used in the wavelength-scanning interferometer with spectrally narrow-band, variable tunable light source as an image sensor. During measurements in and through layers, the optics in the object-scanning arm OA of the wavelength-scanning interferometer are readjusted during the wavelength scan of the light source LQ (e.g., by a computer-controlled piezo unit) that the area of the object being scanned is always in focus of the optics. This ensures a sharp image on the image recorder BA or detector.

An alternative embodiment of a wavelength-scanning interferometer WLSI is that it is designed with a spectrally broadband irradiation unit and a wavelength-scanning optical spectrum analyzer as a detector. For the measurement, the bandwidth of the input radiation is selected to be so large that the smallest expected or pre-measured distance of the successive boundary surface to be detected can be resolved. Incidentally, it is advantageous, as is also the case with the interferometer part of the white light interferometer, to introduce a variable optical attenuator in the reference arm, for example in the form of a liquid crystal element, in order to control the light intensity Ii in the reference arm RA and via a closed control loop to match the light intensity I ₂ in the object arm OA, so that the contrast and the quality of the interference signal are increased.

By using application-specific adapted reference surfaces / layers in the interferometer part IT instead of a reference mirror or a reference plane RE, the quality of the measurement signal is increased, e.g. by compensation for aberrations or overexposure, and it is possible to measure specially shaped object shapes, such as e.g. curved surfaces or structured layer systems. The same also applies to the interferometer part IT of the above-described white light interferometer.

An exemplary embodiment of an interferometric measuring device consists of a combined measuring system comprising a white-light interferometer WLI and a wavelength-scanning interferometer WLSI corresponding to the embodiments described above. This combination is constructed as a measuring system by bringing the irradiation units or light sources LQ together via a fiber coupler FK (see Fig. 14) and introducing the light via a fiber into the interferometer part IT as shown in Fig. 14. The further elements of the interferometer part, such as the variable attenuator VA and a dispersion compensation DK, interfere with such a combination and a shared exploitation of the interferometer part IT in e.g. not consecutively connected in time. This makes it possible to combine the advantages of the white light interferometer WLI and the wavelength-scanning interferometer. For thin or non-highly dispersive layers or with experimental dispersion compensation, a measurement with the wavelength-scanning interferometer WLSI is generally faster and a measurement with the white-light interferometer WLI more accurate. In the case of dispersive layers and without experimental dispersion compensation, the dispersion in the data postprocessing can be compensated in the wavelength-scanning interferometer WLSI, so that the wavelength-scanning interferometer WLSI then supplies the more accurate measured data.

When designing the interferometric measuring device as a white light interferometer WLI, care must be taken to ensure that the optical spectrum of its broadband light source LQ is selected so that the layer structures to be examined at least up to a lower opaque carrier light source. Gersubstrat are partially transparent. Accordingly, the image recorder BA or detector is adapted to the radiation unit or light source LQ in order to obtain the highest possible sensitivity in the spectral range used. The choice of the light source LQ and the detector is therefore task-dependent. In the near infrared spectral range (about 1000 nm to 1800 nm), an InGaAs CCD camera is used as the detector in areal measuring white light interferometers WLI. As light sources LQ in the near infrared spectral range, so-called ASE (amplified spontaneous emission) light sources (eg laser-pumped Er fibers), laser-pumped photonic crystal fibers (PCF) or superluminescent diodes SLD are used. ASE light sources and superluminescent diodes are coupled into the white light interferometer WLI via a free jet or through an optical fiber. Photonic

Crystal fibers are connected directly to the Interfereometerteil the white light interferometer WLI.

To improve the height resolution and delamination in measurements in and through layers, an experimental dispersion compensation is introduced. Dispersion effects in the whitish interferometer WLI result from different optical paths in the object and reference arm. To compensate for these effects in measurements by layers, corresponding layers are also introduced into the beam path of the reference arm RA of the white light interferometer WLI. These layers are located at a distance from the reference surface RF, for example in the form of a reference mirror, in order to avoid overlapping multiple correlograms during the measurement. For example, such layers are introduced in a white light interferometer WLI in the Linnik structure between the beam splitter ST and the microscope objective.

The use of pumped photonic crystal fibers, superluminescent diodes and fiber-coupled, collimated superluminescent diodes, and experimental dispersion compensation are not limited to the near infrared spectral range.

The method performed with the interferometric measuring device can be applied to relatively uniform contiguous layers as well as to deformed layers having edges on an underlying layer, such as oil layers, surveying layered structures on substrates or under Si capped wafers, and also for surveying hidden structures within the layer structure can be used, including so for the measurement of wear protection layers, paint layers and semiconductors, as long as the inclination of the respective surface or boundary surface allows sufficient return reflection of the incident light wave in the image sensor BA.

In the detection of superimposed double correlograms on thin layers (for example <10 μm), algorithms are used in the evaluation device AW, which are e.g. allow adjustment of the scan length (range within the entire scan path, in particular of a correlogram of interest). Alternatively or additionally, in a prescan the expected number of correlograms can be determined and the number of memory areas can be adjusted accordingly.

As stated above, within the evaluation device AW, the acquisition module EM and the evaluation module AM are adjoined by an assignment module ZM and possibly also analysis modules for a more precise examination. The assignment module ZM ensures the assignment of distance values to the corresponding interfaces. This assignment is made in an oil-substrate arrangement according to the position of the correlations in the depth scan. For the assignment to the corresponding layers, a comparison of the position with the e.g. eight closest neighbors.

With the interferometric measuring device also streaks can be detected in layers, which also an interference contrast analysis is suitable. If a light beam hits an interface between two media with different refractive indices ni and n ₂ , a part of the light beam is reflected at the interface. The size of the reflected portion is determined by the two refractive indices. For vertical light incident applies

IR = Iin (n ₂ - ni) / (n ₂ + ni),

where I _{R is} the reflected intensity and Ii _{n is} the intensity of the incident light beam. The intensity detected in a white light interferometer WLI is composed of the reflected intensities of reference arm RA and object arm OA. Ideally, the reflected intensity from the reference arm RA is equal to the incident intensity I _1n . This is superimposed with the intensity I _R reflected from the object arm OA. The correlogram recorded during the measurement of an interface with the white light interferometer WLI in a picture element (pixel) has a maximum intensity value I _max = I _1n + I _R and a minimum intensity value I _mm = I _1n - I _R. Considering the interference _contrast I _Kθn , we get

Iκon = (Imax ^~ Imm) / (Imax + Imm) = IR / Iln = (n ₂ ^~ 1) I (n ₂ + ni)

ie that the interference contrast is determined by the two refractive indices ni, n ₂ . By evaluating the interference contrast of the correlogram of the white light interferometric measurement, it is thus possible to detect streaks with a refractive index n ₂ in a medium with a refractive index ni and to classify the type of inclusions by determining n ₂ with the aid of the interference contrast. At the same time, the position and size of the streaks are determined in the white light interferometric measurement.

Due to the different refractive indices at the interface between the two media, the streaks can also be detected on the basis of a phase consideration, since the light beam at the interface undergoes a phase shift ΔΦ, which is determined by the properties of the two media. In the white light interferometric measurement, such a phase shift in the object arm OA also affects the phase of the recorded correlogram KG. This phase shift in the measurement signal is used in phase evaluation in order to detect and classify streaks with refractive index n ₂ in a medium with refractive index ni. At the same time, the position and size of the streaks can also be determined with this evaluation method.

Furthermore, material changes and material transitions of the layer structure can also be detected by means of the interferometric measuring device, for which an evaluation of the interference contrast or an evaluation on the basis of a phase analysis are also suitable. As described above, the interference _contrast I _Kθn of the measurement signal obtained with the white light interferometer WLI is dependent on the two media which form the interface or boundary layer causing the signal. If the composition of the boundary layer changes over the image field of the white light interferometer WLI, the interference contrast also changes. In the measurement of hidden layers, a change in the boundary layer composition is detected and measured via the interference contrast evaluation, for example a transition between a conductor track and the SiO ₂ material under a Si cover layer.

In the phase analysis for the detection of material changes and material transitions of the layer structure, in turn, it is used that the phase shift of the light source with the white light interference Depending on the different refractive indices of the media that form the signal-causing interface or boundary layer. If the composition of the boundary layer changes over the image field of the white-light interferometer WLI, the phase relationship of the correlograms also changes over the image field. When surveying hidden layers, a change in the boundary layer composition can thus also be detected and measured via the phase change of the interference signal.

In the case of the wavelength-scanning interferometer WLSI, the dispersion which has already been mentioned is due to the wavelength-dependent refractive index profile. The dispersion effect worsens the measurement resolution of the wavelength-scanning interferometer WLSI with respect to closely spaced boundary layers, since the separation of successive correlograms is made more difficult. By the already mentioned software-based data processing in the evaluation before the actual data evaluation in the frequency space of the dispersion effect is compensated for known refractive index and the measurement resolution increases in measurements in and through the layers of the layer structure.

The method also allows investigations on thick layers (d »10μm). For measurements in thick layers, a preliminary measurement of the total layer thickness is first performed with the wavelength-scanning interleaver WLSI with reduced triggering. Due to the measuring principle of a wavelength-scanning interferometer WLSI, this measurement is much faster than a corresponding measurement with a white-light interferometer WLI. The subsequent measurement of the regions identified as relevant by the preliminary measurement is advantageously carried out with a white light interferometer WLI, for example, with a photonic crystal fiber (PCF) light source and, due to the measuring principle, yields a better resolution in terms of height and slice separation. Likewise, the downstream measurement can be carried out with a wavelength-scanning interferometer WLSI in non-equidistant scanning. Hereby, the regions identified as relevant in the pre-measurement are substantially more densely sampled (for example by a denser frequency scan) than non-relevant regions lying between them. Here, too, a better resolution results in terms of height and layer separation due to the measuring principle.

The above-described structures of the interferometric measuring device and the methods performed therewith enable non-destructive both point-like and areal measurements, in particular of interfaces in optically partially transparent for the radiation Layer systems of various types, whereby streaks and material changes as well as material transitions can be detected and identified. The measuring method can be followed by a production and process and / or quality control. It is then possible, for example, to check the tolerances directly after the processing of a functional area and to carry out a non-destructive process and / or quality control on relevant product parts.

Claims

A ccess

1. Interferometric measuring device for measuring layer structures comprising a plurality of layers arranged one behind the other in the depth direction with a scanning device which scans them automatically in their depth direction (Z), by means of which an interference plane (IE) is displaceable relative to the layer structure, with a white light interferometer (WLI) and / or a wavelength-scanning interferometer (WLSI) having interferometer part, for the measurement of an irradiation unit (LQ) an input radiation is supplied, which by means of a beam splitter (ST) divided and partly via a reference beam as a reference beam (RST) a Reference arm (RA) and the other part via an object beam path as an object beam (OST) is supplied to a measuring the layer structure having object arm (OA), with an image sensor (BA), the from the reference arm (RA) and the object arm (OA) accommodates returning interfering radiation and converted into electrical signals, as well as with a downstream evaluation device (AW) for providing the measurement results, characterized in that the scanning device is designed in such a way that at the same reference beam path and object beam path of the associated scan is at least as large as the expected or Distance determined in a preliminary measurement of at least two successively arranged interfaces of the layer structure to be detected, optionally plus an expected depth structure of the interfaces, and that a) when forming the interferometer part (IT) with the irradiation unit as white light interferometer (WLI), the coherence length (l _c ) the input radiation is selected to be at most large enough so that the interference maxima of the correlograms occurring successively in the depth scan can be distinguished at the interfaces to be detected, and / or b) when the interferometer part (IT) is formed the radiation unit is designed as a wavelength-scanning interferometer (WLSI), the irradiation unit with narrow-band, tunable input radiation, wherein the bandwidth of the input radiation is chosen so large that the smallest expected or estimated by the pre-measurement distance of the successive interfaces to be detected is resolvable . and / or c) when forming the interferometer part (IT) as a wavelength-scanning interferometer (WLSI) with a spectrally broadband irradiation unit and a wavelength-scanning optical spectrum analyzer as a detector, the bandwidth of the input radiation is chosen to be as small as possible or by pre-measurement estimated distance of the successive interfaces to be detected is resolvable.

2. Measuring device according to claim 1, characterized in that the receiver has a surface resolution in the x- / y-direction, which is higher than the mapping of the local height changes of the layer surface in the x- / y-direction.

3. Measuring device according to claim loder 2, characterized in that in the evaluation device (AW) algorithms are programmed with which the individual layers can be detected separately from each other by an assignment by the

The sequence of correlations occurring at the interfaces takes place during a depth scan cycle.

4. Measuring device according to claim 3, characterized in that for inputting the number of expected interfaces a user-operable input unit is present.

5. Measuring device according to one of the preceding claims, characterized in that the evaluation device (AW) has a coarse value device, with the basis of a coarse detection of correlograms, the number of existing layers or interfaces can be determined and that the evaluation device (AW) formed in the manner is that the number of detected layers automatically transferred to an evaluation part or can be entered by the user via the input unit.

6. Measuring device according to one of the preceding claims, characterized in that the evaluation device (AW) for the layers to be detected has separate memory areas (SB1, SB2...), to which data of the correlogram associated with the respective interfaces can be assigned separately during a depth scan, the respective correlograms having their depth scanning position (z) are related.

7. Measuring device according to claim 6, characterized in that the memory areas (SB1, SB2 ...) are designed as circulating memory areas (SBU).

8. Measuring device according to claim 6 or 7, characterized in that for the recording of the interfaces associated with the correlograms a number of interfaces by at least one exceeding number of memory areas (SBL, SB2 ...) is present, of which a memory area as active Memory area for writing current sample data during the depth scan is used and the rest are used to record the determined, associated with the respective interfaces correlogram data.

9. Measuring device according to one of the preceding claims, characterized in that the evaluation device (AW) has an evaluation range which is designed to calculate the modulation (M) of the electrical signals obtained during the depth sampling intensity values (I) and for detecting the correlograms belonging to the respective interfaces are formed from the modulation (M).

10. Measuring device according to one of claims 5 to 9, characterized in that the evaluation device (AW) has an evaluation module with which a fine measurement of a respective interface structure is feasible.

11. Measuring device according to one of claims 2 to 10, characterized in that the evaluation device (AW) is formed such that during the depth scanning simultaneously in the direction of the depth scan (Z) extending tracks at various laterally in the xy direction directly or indirectly adjacent boundary areas are evaluated in a corresponding manner.

12. Measuring device according to claim 11, characterized in that the evaluation device (AW) is designed in such a way that interface data of the adjacent x-y direction boundary surface areas for a respective Tiefenabtastung be related to each other and evaluable with respect to each other.

13. Measuring device according to one of the preceding claims, characterized in that when forming the interferometer part with white light interferometer (WLI) and wavelength-scanning interferometer (WLSI) the evaluation device (AW) is designed in such a way that in a preliminary measurement the entire layer structure at a depth scanning cycle for determining relevant regions is roughly measured with the wavelength-scanning interferometer (WLSI) and in a subsequent measurement at a downstream

Tiefenabtastzyklus a fine measurement of at least the relevant regions with increased resolution takes place.

14. Measuring device according to one of the preceding claims, characterized in that the evaluation device (AW) for detecting streaks in the layer structure from an evaluation of the interference contrast or the phase shift is formed at the caused by the streaks interfaces between media different calculation indices.

15. Measuring device according to one of the preceding claims, characterized in that the evaluation device (AW) for detecting material changes or material transitions is formed from an evaluation of the interference contrast or the phase shift at the interfaces or boundary layers caused by the material changes or material transitions, which result from different calculation indices, wherein the interference contrast change or the change of the Phase shift is included laterally over the image field in the evaluation.

16. Measuring device according to one of the preceding claims, characterized in that when forming the interferometer as a white light interferometer (WLI) a dispersion compensation is carried out in that in the reference arm (RA) corresponding layers are introduced as in the object arm (OA).

17. Measuring device according to one of the preceding claims, characterized in that when forming the interferometer as a wavelength-scanning interferometer (WLSI) in the evaluation device (AW) a software-based dispersion compensation of the measurement data is provided, which is upstream of the actual measurement data evaluation.

18. Measuring device according to one of the preceding claims, characterized in that in the reference arm (RA), a variable optical attenuator is introduced by the light intensity controlled or regulated to the light intensity in the object arm (OA) is passable.

19. Measuring device according to one of the preceding claims, characterized in that in the object arm (OA) coupled to a control device optics is arranged, which causes an adjustment of the focus to the just scanned area during the depth scan.

20. Measuring device according to one of the preceding claims, characterized the irradiation unit has optically pumped photonic crystal fibers and / or at least one superluminescent diode and / or at least one ASE light source.

21. A method for the interferometric measurement of layer structures of several in the depth direction (Z) successive layers, wherein an interference plane (IE), by the optical path length of an object beam path guided object beam and the optical path length of a guided in a reference beam reference beam is determined, is shifted for depth scanning of the layer structure in the depth direction (Z) relative to the position of the layer structure, generates an interference pattern using methods of white light interferometry or a wavelength-scanning interferometry and the interference pattern recorded by means of an image sensor (BA) and automatically evaluated by an evaluation device (AW) For example, in order to represent measurement results relating to the interfaces of the layer structure, it is characterized in that the depth scanning of all the layers to be measured and the layers limiting them

Boundary surfaces of the object beam in a scanning cycle over the same object beam path is performed and the reference beam is guided over the same reference beam path and that when using the method of white light interferometry, the coherence length (l _c ) of the coupled into the interferometer input radiation is chosen to be at most so large that the interference maxima of are distinguished in the depth scanning successive occurring at the interfaces to be detected correlograms and the application of the method of wavelength-scanning interferometry, the bandwidth of the input radiation is chosen so large that the smallest expected or pre-measured by the estimated distance of the interfaces to be detected is resolved.

22. The method according to claim 21, characterized in that the interfaces to be detected in the evaluation device (AW) separate memory areas (SBL, SB2 ...) are assigned and correlated during the depth scanning with the interfaces correlograms based on maximum modulation of The intensities resulting from the interference patterns are determined and assigned to the memory areas (SB1, SB2...).

23. The method according to claim 21 or 22, characterized in that when using the method of wavelength-scanning interferometry a software-based dispersion compensation is performed in the evaluation before the actual evaluation of the measured data takes place.