Device and method for reflection ellipsometry in the millimeter wave range

Abstract

Device for determining material properties by millimeter wave radiation, with a transmitting unit (11) which emits an arbitrarily polarized millimeter wave signal at a specific angle of incidence onto an object (15) to be examined, a receiving unit (12) that detects a signal (23, 23´) reflected from the object (15) under investigation with respect to intensity and / or phase, and a processing unit (20) which determines the permittivity and / or layer thickness of the investigated object (15) by means of an ellipsometric evaluation from a component of the reflected signal (23) polarized parallel to the plane of incidence and a component polarized perpendicular to the plane of incidence, wherein the transmitting and receiving unit (11, 12) are arranged in front of the object (15) to be examined such that they span an aperture area (10, 10'), characterized by that the transmitting and receiving unit (11, 12) has several transmitting and / or receiving antennas (41) forming a two-dimensional antenna array (40), wherein the antenna array (40) has an attenuation mat (48) in its center and the several transmitting and / or receiving antennas (41) are arranged around the attenuation mat (48).

Description

[0001] The invention relates to a device and a method for determining material properties by millimeter wave radiation and their ellipsometric evaluation according to the preamble of claim 1 or claim 11.

[0002] Millimeter-wave measurement techniques are increasingly used in security technology and non-destructive material testing. The millimeter-wave range offers the advantage that both metallic and non-metallic objects can be detected based on their material-specific dielectric properties. These dielectric properties are characterized by their permittivity.

[0003] In US 2006 / 0 164 104 A1, a measuring device for measuring the thickness or similar of a thin-film coating on a substrate surface such as a semiconductor waver is disclosed using microwave radiation.

[0004] US Patent 6 275 291 B1 discloses an ellipsometer, i.e., a combination of a polarized light source with a polarimeter for measuring the parameters of polarized light, which is used to measure the properties of thin films and surfaces.

[0005] In the article “Millimeter-wave imaging concepts: Synthetic Aperture Radar (SAR) and Digital Beam Forming (DBF)”, published in June 2009 in Frequenz, Vol. 63, No. 5-6, pages 106-110, F. Gumbmann et al. reveal an overview of short-range radar imaging for various safety and non-destructive material testing applications based on synthetic aperture radar (SAR) or digital beamforming (DBF).

[0006] In their article "Measurement of the Complex Refractive Index of Concrete at 57.5 GHz," published in January 1996 in IEEE Transactions on Antennas and Propagation, Vol. 44, No. 1, pages 35-40, K. Sato et al. describe a measurement of the reflection coefficient of a flat object with a homogeneous dielectric constant using millimeter waves at 57.5 GHz. A transmitting antenna alternately radiates signals polarized parallel and perpendicular to the plane of incidence onto the object. The signals are reflected at the interface between the surrounding air and the flat object with varying intensities, depending on the angle of incidence and the complex refractive index. A receiving antenna receives the magnitude of the reflected parallel-polarized signal and the magnitude of the reflected perpendicular-polarized signal at an angle of reflection corresponding to the angle of incidence.The object under test was measured at different angles between 6° and 75° in an incidence plane. The beamwidth of the transmitting / receiving antenna was approximately 5° in each case. The beam cross-section on the object was approximately 13 cm. From the dependence of the magnitude of the signal polarized parallel and perpendicular to the incidence plane, with precise knowledge of the object's layer thickness and using scalar transmission measurements, the complex refractive index and the complex reflection coefficient are calculated using Fresnel's formulas. From this, the permittivity of the object under test can be derived. Determining these data from the angular dependence of the magnitude of the reflected signals is called angle-varying ellipsometric analysis.

[0007] A disadvantage of ellipsometric evaluation with the described measuring setup is that only an average permittivity of the illuminated area of ​​the object can be determined. To distinguish different materials on any given surface, and especially to identify their contours, it is necessary to determine the material properties with spatial resolution.

[0008] The object of the invention is therefore to create a device and a method that enables the determination of material properties and thus anomaly detection, detection of isotropic, anisotropic, two- and multi-layered dielectric objects with high spatial resolution and a short measurement time.

[0009] The problem is solved by the device according to claim 1 and by the method according to claim 11. Advantageous embodiments of the device and method according to the invention are described in the dependent claims.

[0010] The device according to the invention comprises a transmitter unit that emits an arbitrarily polarized millimeter-wave signal at a specific angle of incidence onto an object under investigation. A receiver unit receives the signals reflected by the irradiated object and detects their intensity and / or phase. A processing unit determines the permittivity and / or the layer thickness of the object under investigation from a component of the reflected signal polarized parallel to the direction of incidence and a component polarized perpendicular to the direction of incidence by means of ellipsometric evaluation. The transmitter and receiver units are arranged in front of the object under investigation such that they define an aperture area. The transmitter and receiver unit has several transmitting and / or receiving antennas that form a two-dimensional antenna array.The antenna array has a damping mat in its center, and the multiple transmitting and / or receiving antennas are arranged around the damping mat.

[0011] The transmitting and receiving unit thus forms an aperture surface, i.e., a two-dimensional plane or curved transmitting and receiving surface. The size of the aperture surface determines the lateral resolution; a larger aperture surface improves the lateral resolution. The two-dimensional transmitting and receiving surface allows the focus point to be changed in three dimensions, so that the material properties of the object under investigation can be determined at any focus point and thus spatially resolved in the x, y, and z directions. Combining both measures is advantageous but not mandatory.

[0012] Advantageous, but not part of the invention, is the inclusion of at least one transmitting and at least one receiving antenna, arranged linearly and / or arbitrarily offset from one another, in the transmitting and receiving unit. The aperture area is defined by rotation about an axis and / or by sequential movement of the arrangement in any spatial direction. With such an antenna arrangement and its displacement, it is possible to create a large aperture area from only a few transmitting and receiving antennas. This allows for high spatial resolution. Such a linear transmitting and receiving unit is technically less complex and therefore cost-effective to purchase and operate.

[0013] It is also advantageous, though not part of the invention, if the transmitting and receiving unit has multiple transmitting and / or receiving antennas that form an arbitrarily arranged two-dimensional antenna array. This enables a very short measurement time and therefore its application to moving objects. The short measurement time allows for an increased throughput of objects being measured.

[0014] It is also advantageous if the receiving unit additionally receives the cross-polarized component of the reflected signal and feeds it to the processing unit for ellipsometric evaluation. The processing unit determines the electric field E from the ratio of the electric field reflected parallel to the plane of incidence. r p to the electric field E radiated perpendicular to the plane of incidence i s or the ratio of the electric field E reflected perpendicular to the plane of incidence r sto the electric field E radiated parallel to the plane of incidence i p the reflection coefficients R ps and R sp and performs anomaly detection using ellipsometric analysis. By analyzing cross-polarization, anisotropic media, which exhibit different permittivity in the x and y directions, can be measured. Measuring cross-polarization is particularly useful for detecting permittivity anomalies, allowing for more reliable identification of transitions between different materials and thus their contours.

[0015] It is advantageous if the transmitting or receiving unit emits or receives millimeter wave signals with different frequencies from a predetermined frequency range and / or in specific frequency steps, thus enabling spectrometric ellipsometry evaluation.

[0016] It is also advantageous if the transmitting unit emits millimeter-wave signals at different angles of incidence onto the object under investigation and / or if the receiving unit receives reflected millimeter-wave signals at different angles of reflection. This allows for angle-varying ellipsometry analysis. All of the aforementioned variants enable more precise anomaly detection, detection of dielectric layers, and more accurate determination of permittivity and / or layer thickness, thus increasing the accuracy of the device. These variants can be advantageously, but not necessarily, combined.

[0017] It is advantageous if the transmitting unit simultaneously or alternately emits a signal polarized perpendicular to the plane of incidence and a signal polarized parallel to the plane of incidence, and if the receiving unit simultaneously or alternately receives a reflected perpendicularly polarized signal and a reflected parallel-polarized signal. It is equally advantageous if the transmitting unit transmits an arbitrarily polarized signal with an electric field E. i , which can be decomposed into one polarization component perpendicular to E i s and parallel E i p is at the plane of incidence, emits simultaneously or sequentially, and the receiving unit receives a reflected signal with an electric field E r , which can be decomposed into one polarization component perpendicular to E r s and parallel E r pat the plane of incidence, receives simultaneously or sequentially, from which the processing unit derives a reflection coefficient R. ss of the reflected vertically polarized signal or a reflection coefficient R pp of the reflected parallel polarized signal, which is determined by the ratio of the reflected electric field E r s or E r p to the radiated electric field E i s or E i p are determined, as calculated, and from the ratio of the reflection coefficient R ss of the reflected perpendicularly polarized signal to the reflection coefficient R pp The dielectric layers are detected using the reflected parallel polarized signal, and the permittivity and / or layer thickness is determined.

[0018] The use of the aforementioned transmitted and received signals enables the direct measurement of the amplitude ratio and phase difference between the reflection coefficient of the reflected perpendicularly polarized signal and the reflection coefficient of the reflected parallel-polarized signal. This simplifies the evaluation algorithm for detecting dielectric layers and calculating the permittivity and layer thickness. This keeps processor requirements and computation time low. Furthermore, transmitting a signal polarized perpendicular to the plane of incidence and receiving a signal polarized parallel to the plane of incidence, and vice versa, allows for ellipsometric evaluation of anisotropic objects and detection of polarization-rotating objects. The device according to the invention thus remains cost-effective, and the processing time of the method is manageable.By measuring intensity ratios instead of intensities, the measurement is less susceptible to intensity fluctuations, which also contributes to high measurement reliability with short measurement durations.

[0019] It is also advantageous to reconstruct a focusing point by weighting the reflected signals with a phase term and summing them coherently, so that only those reflected signals originating from a specific object point contribute to the result. This allows known beamforming or aperture synthesis algorithms to be used to calculate the focusing point.

[0020] It is also advantageous if the focus can be adjusted by changing the phase term on the transmitting and / or receiving side, or if this adjustment is made during the process. The modification and weighting of the phase term for focusing can be implemented in hardware or software.

[0021] It is also advantageous if the transmitting and receiving units emit signals with different frequencies from a predetermined frequency range, and the processing unit uses this information to determine a focus in depth, i.e., in the direction of propagation of the microwave signals. By appropriately changing the wavelength of the signal, different levels of the object can be focused, and depth resolution can be achieved.

[0022] The aforementioned advantages apply analogously to the method according to the invention.

[0023] Exemplary embodiments of the device or method according to the invention are shown in the drawings and are explained in more detail below. The drawings show: Fig. 1 a schematic representation of the physical principle of ellipsometry measurement; Fig. 2 a schematic representation of the reconstruction principle of a focusing point; Fig. 3a A first example of a device with a flat aperture surface in schematic representation; Fig. 3b a second example of a device with a cylindrical aperture area in schematic representation; Fig. 4a a first example of a two-dimensional transmitting and receiving unit; Fig. 4b a second example of a two-dimensional transmitting and receiving unit; Fig. 4c an embodiment of a two-dimensional transmitting and receiving unit according to the invention; Fig. 5a an example of a measuring object with different materials attached; Fig. 5b a pictorial representation of a spatially resolved measurement using the inventive method or device, and Fig. 6 a flowchart of an embodiment of the method according to the invention.

[0024] Based on Fig. 1 and Fig. 2. First, the ellipsometry measurement principle and the focusing on a point of the object to be examined will be briefly explained.

[0025] Falls, as in Fig. Figure 1 shows an electromagnetic signal 5 with an arbitrarily polarized electric field E i ges , decomposable into one polarization component perpendicular to E i s and parallel E i pAt the plane of incidence 3, on a flat or curved surface of a sample 2, a portion of the signal 6 is reflected at an angle of reflection α0 that corresponds to the angle of incidence α0 of the incident signal 5. The component of the incident electric field E oriented perpendicular to the plane of incidence 3 i s This excites charge carriers in sample 3 to oscillate in a direction perpendicular to the plane of incidence, so that these in turn generate an electric field E r s radiate. The component of the incident electric field E oriented parallel to the plane of incidence 3. i p This causes charge carriers in sample 2 to vibrate in a direction parallel to the plane of incidence, so that these in turn generate an electric field E r pradiate. Depending on the electrical properties of the irradiated sample 2 and the angle of incidence α0, the magnitude of the reflected perpendicularly polarized component of the reflected electric field E changes. r s and the reflected parallel polarized component E r p . For single- or multi-layer dielectric samples 2 with a specific layer thickness d1, ..,d n An additional phase difference arises between the two polarization components. Thus, when a sample of a certain layer thickness d1, ...,d is irradiated, this would result in a phase difference of d1, ...,d. n and electrical properties with a circularly polarized electric field E i ges an elliptically polarized signal 6 with the electric field E r ges be reflected upon.

[0026] The reflection coefficient R pp , R ss , i.e., the ratio of the reflected electric field E rp or E r s to the radiated electric field E i p or E i s The Fresnel equations give the component of the signal parallel to and perpendicular to the plane of incidence 3. Likewise, the reflection coefficients R ps , R sp , i.e., the ratio of the reflected electric field E r p or E r s to the radiated electric field E i s or E i p , given by the Fresnel equations. The reflection coefficients R pp , R ss R ps and R sp are a function of the frequency, the angle of incidence α0, the complex permittivity of the surrounding medium, and the layer thickness d1, ..,d n and the relative complex permittivity ε1,.,ε n each individual layer of sample 2.

[0027] Reflection ellipsometry is based on measuring the change in polarization of an incident polarized electric field E. i ges after its reflection at a medium-sample surface at a fixed angle of incidence α0. Due to the different magnitude and phase of the Fresnel reflection coefficients R pp , R ss For the parallel and perpendicularly polarized signals 5, the polarization of the reflected electric field E changes. r ges of the reflected signal 6. The change in the polarization state can be described by the complex ratio ρ of the reflection coefficients R pp and R ss can be described. This results in: [EprEsr]=[RppRpsRspRss]⋅[EpiEsi] ρ=RppRss=|Rpp|⋅eiδpp|Rss|⋅eiδss=|RppRss|⋅ei(δpp−δss)=|ρ|⋅eiΔ

[0028] Automatic anomaly detection and the detection of dielectric layers can be directly derived from the measurement of the phase difference Δ. Likewise, the permittivities ε1,...ε can be determined using a numerical or analytical method with model-based optimization as material parameters. n and layer thicknesses d1, d n The properties of individual layers of sample 2 can be determined. Only electromagnetic radiation of a single selected frequency is required for the measurement, thus enabling a monofrequency determination of material parameters.

[0029] In addition to measuring the parallel or perpendicular polarized radiation E r p or E r s The reflection coefficients R can be determined by measuring the cross-polarization. ps , R sp their magnitude and phase can be determined using the reflection coefficients R. ps , R spAnisotropic materials, i.e., materials with inhomogeneous permittivity, can be measured. Cross-polarization measurements also provide information about permittivity transitions, such as those caused by the proximity of different materials.

[0030] To achieve spatially resolved material determination by reflection ellipsometric measurements, the object under investigation must be irradiated point by point and the reflected signal evaluated. According to the invention, this is achieved by a transmitting unit and a receiving unit, which are arranged in front of the object under investigation in such a way that they define an aperture area, and / or a spatially resolved image of the object under investigation is created from the received signals by changing the focus point. Both measures can be used in combination, but do not have to be.

[0031] In Fig. Figure 2 schematically depicts a transmitting unit 11 with several transmitting antennas 13 and a receiving unit 12 with several receiving antennas 14, which are arranged linearly in a direction of extension of an aperture surface 10 and are moved in a direction perpendicular to the linear arrangement, see arrow with reference numeral 16. In addition to the direction perpendicular to the linear arrangement, the arrangement can be rotated, e.g. about an axis through the center point M of the transmitting and receiving units 11, 12 in longitudinal extension, and / or move in any spatial direction to span an aperture surface.

[0032] The transmitting and receiving unit 11, 12 can also be designed as a two-dimensional antenna array, see Fig. 4a, Fig. 4b and Fig. 4c, in which a plurality of transmitting antennas 13 are arbitrarily distributed over an area, for example over a boundary region of the aperture area 10, 10', 10'', and a plurality of receiving antennas 14 are arbitrarily distributed over the entire aperture area 10, 10', 10''. The transmitting unit 11 alternately emits signals polarized perpendicular to the plane of incidence and signals polarized parallel to the plane of incidence onto the object 15 under investigation. The perpendicularly or parallel polarized radiation reflected by the object 15 under investigation is received and evaluated at each individual receiving antenna of the receiving unit 12.Likewise, the transmitting unit 11 can simultaneously or sequentially transmit an arbitrarily polarized signal, divisible into a polarization component perpendicular and parallel to the plane of incidence, onto the object 15 to be examined, and the receiving unit 12 can receive a signal, divisible into a polarization component perpendicular and parallel to the plane of incidence, simultaneously or sequentially.

[0033] To obtain spatial resolution, the received signals are weighted with a phase term and coherently summed, so that only the received data originating from a desired object point r0, or the focusing point 17, interfere constructively, while all received data from other object points interfere destructively. Thus, precisely the signal reflected from the focusing point 17 and received by each of the receiving antennas 14 can be considered and evaluated. This is also referred to as receiving-side focusing. The implementation of the phase term weighting can be hardware-based or software-based. This provides information about the material parameter of precisely this focusing point 17. A corresponding evaluation is then performed for each desired object point of the object 15.

[0034] Spatial resolution can also be achieved on the transmission side by weighting the transmitted signal with a phase term or by a combination of transmit and receive focusing. The implementation of the phase term weighting can be hardware-based or software-based.

[0035] The spatial resolution increases with the size of the aperture area. By evaluating the individual reflected signals as described, not only can spatial resolution in the x and y directions be achieved, but the distance between the aperture area 10, 10' and the object 15 under investigation can also be varied, thus achieving depth focusing. Similarly, by transmitting and receiving different frequencies from a predetermined frequency range, the separation of scattering centers or different material layers at depth can be achieved.

[0036] Fig. Figure 3a shows an object 15 to be examined, as well as a transmitting unit 11 and a receiving unit 12, in which several transmitting antennas 13 and receiving antennas 14, respectively, are arranged linearly one above the other in the y-direction. The transmitting and receiving antennas 13 and 14 are, for example, non-focusing antennas. Focusing is achieved subsequently by the previously described signal processing in a processing unit 20, for example, in digital form. In the y-direction, for example, beam focusing is achieved by digital beamforming.

[0037] The aperture area 10, 10', which is in Fig. 3a just, in Fig. 3b, cylindrical, which is arranged relative to the object 15 to be examined, is spanned by moving the transmitting and receiving unit 11, 12 perpendicular to the linear extent of the transmitting and receiving unit 11, 12 along arrow 16. The measurement described above is performed at a multitude of positions in direction 16, so that the transmitting and receiving unit forms a multistatic array. Focusing in the x-direction is carried out according to the same basic principle as described above. Fig. 2. One possible focusing method is aperture synthesis radar.

[0038] Focusing is performed in a processing unit 20. An optical representation of the measured object 15 with the determined material parameters is performed in a display unit 21.

[0039] To further increase the accuracy of material determination, ellipsometric measurements can be performed with signals (22, 22', 23, 23') of different frequencies from a frequency range, e.g., in predetermined frequency steps. For the same purpose, it is advantageous to perform measurements with incident signals (22, 22') at different angles of incidence α1, α2 and the corresponding reflected signals (23, 23') at the same angles of reflection β1 = α1 and β2 = α2. Likewise, for a specific angle of incidence α1, the reflected signal (23, 23') can be measured at different angles of reflection β1, β2, not equal to the angle of incidence, e.g., α1.

[0040] Fig. Figure 4a shows an example of a two-dimensional antenna array 25 arranged in an arbitrarily shaped aperture area 10''. Several transmitting and receiving antennas 13 are arranged, for example, statistically distributed within the aperture area 10''. The receiving antennas 14 arranged in this way enable the evaluation of the reflected signal at different angles of incidence β1, β2, thus yielding more accurate values ​​of the material parameters using ellipsometry. Such a two-dimensional antenna array 25 allows for the real-time acquisition of the reflected signals but requires significant computing power for signal processing.

[0041] Fig. Figure 4b shows a second example of a two-dimensional antenna array 30 forming a planar aperture surface 10. Several transmitting antennas 13 are arranged, for example, in the corners of the aperture surface 10. The transmitting unit 11 consists of a plurality of receiving antennas 14, which are, for example, evenly distributed over the aperture surface. In contrast, with the linear arrangement of the receiving and transmitting antennas, the following applies: Fig. 3a and Fig. 3b A measurement was performed at each position of the transmitting unit 11 or receiving unit 12. This requires more time to perform the numerous individual measurements, but is associated with lower computing power per individual measurement.

[0042] Fig. Figure 4c shows an embodiment of a two-dimensional antenna array 40 according to the invention with offset antenna elements 41. This plurality of antenna elements 41 is arranged in a strip at the edge of a printed circuit board assembly 43. The center of the printed circuit board is covered by an attenuation mat 48. Each antenna element 41 is connected either to a transmitter or to a receiver. For example, four antenna elements 41 arranged in the area 42 can each be connected to a transmitter, whereas, for example, four antenna elements 41 not located in the area 42 can each be connected to a receiver.

[0043] A control unit can specify that at any given time only one antenna element 41 connected to a transmitter emits an electromagnetic signal, and that simultaneously the reflected electromagnetic signal received by the antenna elements 41 connected to a receiver is evaluated by a processing unit 20. In the next step, the same process is repeated for another antenna element 41, which is also connected to a transmitter. Advantageously, several of the illustrated two-dimensional antenna arrangements 40 can be arranged adjacent to each other and operated together, thus forming a larger aperture area.

[0044] In Fig. Figure 5a shows an object to be examined in the form of a measuring mannequin 50, on which various objects made of different materials are arranged. Signals in any wavelength range are suitable for measurement. Signals in the wavelength range between 1 GHz and 1000 GHz, preferably between 50 GHz and 150 GHz, and particularly preferably between 75 GHz and 110 GHz, are used, as these ensure good lateral resolution and good depth resolution with sufficient penetration depth. The attached object 51 comprises a metallic component, object 52 is, for example, a wax strip, object 53 is, for example, made of a ceramic-plastic material, object 54 is a ceramic knife, and object 55 is a bottle containing a liquid.

[0045] In Fig. Figure 5b shows an image of the measuring mannequin 50 produced by the device or method according to the invention. The attached objects 51 to 55 are clearly identifiable as objects 51', 52', 53', 54', and 55'. The different permittivities of the various materials can be represented, for example, by different colors and allow for a good inference about the shape of the corresponding object. The measurements were taken with an antenna unit consisting of, for example, two transmitting antennas and approximately 300 receiving antennas, in a processing unit 20 (see Figure 5b). Fig. 3a or 3b evaluated and optically processed and displayed via an imaging unit 21.

[0046] Significant changes in permittivity between adjacent object points indicate the presence or contour of an object. This edge detection is further refined by analyzing the cross-polarization of the reflected signals. Additionally, dielectric layers can be visualized using ellipsometry. Determining the dielectric material properties allows conclusions to be drawn about the object's material and structure.

[0047] Fig.Figure 6 shows a schematic representation of an embodiment of the described measurement method 60. In the first method step 61, polarized millimeter-wave signals are emitted from the transmitting unit onto an object under investigation at an angle of incidence α0. In the second method step 62, polarized reflected signals, polarized perpendicular or parallel to the plane of incidence of the signal, are received by the receiving unit. In method step 63, the reflected signal from a focusing point on the object under investigation is determined by weighting the received signals using a phase term and coherent summation. This ensures that only the signals from the focusing point interfere constructively and thus contribute to the measurement. In method step 64, the signals incident and reflected at the focusing point are evaluated ellipsometrically, and the material parameter is determined.

[0048] In step 65, a check is performed to see if all desired object points have been treated as focal points. If not, an untreated object point is selected as the focal point, and steps 63 and 64 are repeated for this object point. If step 65 determines that all desired object points have been treated as focal points, and thus the object under investigation has been scanned two-dimensionally and its permittivity determined, the procedure is terminated in step 66.

[0049] All described and / or illustrated features can be advantageously combined within the scope of the invention. The invention is not limited to the described embodiments.

Claims

[1] Device for determining material properties by millimeter wave radiation, with a transmitting unit (11) which emits an arbitrarily polarized millimeter wave signal at a specific angle of incidence onto an object (15) to be examined, a receiving unit (12) that detects a signal (23, 23´) reflected from the object (15) under investigation with respect to intensity and / or phase, and a processing unit (20) which determines the permittivity and / or layer thickness of the investigated object (15) by means of an ellipsometric evaluation from a component of the reflected signal (23) polarized parallel to the plane of incidence and a component polarized perpendicular to the plane of incidence, wherein the transmitting and receiving unit (11, 12) are arranged in front of the object (15) to be examined such that they span an aperture area (10, 10'), characterized by , that the transmitting and receiving unit (11, 12) has several transmitting and / or receiving antennas (41) forming a two-dimensional antenna array (40), wherein the antenna array (40) has an attenuation mat (48) in its center and the several transmitting and / or receiving antennas (41) are arranged around the attenuation mat (48). [2] Device according to claim 1, characterized by , that the receiving unit (12) additionally receives the cross-polarized component of the reflected signal (23, 23') and supplies it to the processing unit (20) for ellipsometric evaluation. [3] Device according to claim 1 or claim 2, characterized by , that the transmitting or receiving unit (11, 12) emits or receives millimeter wave signals with different frequencies from a predetermined frequency range and / or in certain frequency steps, so that a spectrometric ellipsometric evaluation is carried out at different frequencies. [4] Device according to any one of claims 1 to 3, characterized by , that the transmitting unit (11) emits millimeter wave signals with different angles of incidence (α1, α2) onto the object (15) under investigation. [5] Device according to any one of claims 1 to 4, characterized by , that the receiving unit (12) receives reflected millimeter wave signals at different angles of incidence (β1, β2 ) and that an angle-varying ellipsometric evaluation is carried out in the processing unit (20). [6] Device according to any one of claims 1 to 5, characterized by that the millimeter wave signals have a frequency in a frequency range between 1GHz and 1000GHz, preferably between 50GHz and 150GHz and most preferably between 75GHz and 110GHz. [7] Device according to any one of claims 1 to 6, characterized bythat the transmitting unit (11) simultaneously or alternately emits a signal polarized perpendicular to the plane of incidence and a signal polarized parallel to the plane of incidence, that the receiving unit (12) simultaneously or alternately receives a reflected perpendicularly polarized signal and a reflected parallel-polarized signal, or that the transmitting unit (11) emits an arbitrarily polarized signal with an electric field E i , which can be decomposed into one polarization component perpendicular to E i s and parallel E i p to the plane of incidence, emits simultaneously or sequentially and the receiving unit (12) receives a reflected signal with an electric field E r , decomposable into one polarization component perpendicular to E r s and parallel to the plane of incidence E r p, receives simultaneously or sequentially, from which the processing unit (20) derives a reflection coefficient R pp of the reflected parallel polarized signal or a reflection coefficient R ss of the reflected vertically polarized signal and from the ratio of the reflection coefficient R pp of the reflected parallel polarized signal to the reflection coefficient R ss of the reflected vertically polarized signal, a detection of dielectric layers in conjunction with anomaly detection is performed and the permittivity and / or layer thickness of single-layer objects, as well as the permittivities and / or layer thicknesses of each individual layer of multi-layer objects under investigation, are determined. [8] Device according to any one of claims 1 to 7, characterized by, that a focusing point (17) is reconstructed by weighting the reflected signals (23, 23') with a phase term and summing them coherently such that only those reflected signals (23, 23') that were reflected from a considered focusing point (17) contribute. [9] Device according to claim 8, characterized by that the focus can be adjusted by changing the phase term on the transmitting and / or receiving side and / or that the weighting of the phase term is hardware-based or software-based. [10] Device according to claim 8, characterized by , that the transmitting or receiving unit (11, 12) emits millimeter wave signals with different frequencies from a predetermined frequency range and the processing unit (20) determines a focusing in depth, i.e. in the direction of propagation of the microwave signals. [11] Method for determining material properties by millimeter wave radiation, in which an arbitrarily polarized millimeter wave signal is emitted at an angle of incidence from a transmitting unit (11) directed towards an object (15) to be examined, a signal (23, 23') reflected from the object (15) to be examined is detected by a receiving unit (12) with respect to intensity and / or phase, and by means of an ellipsometric evaluation from a component of the reflected signal polarized parallel to the plane of incidence and a component polarized perpendicular to the plane of incidence, a permittivity and / or layer thickness of the object under investigation (15) is determined, wherein the transmitting and receiving unit (12) is arranged in front of the object under investigation (15) in such a way that it spans an aperture area (10), characterized by , that the transmitting and receiving unit (11, 12) has several transmitting and / or receiving antennas (41) forming a two-dimensional antenna array (40), wherein the antenna array (40) has an attenuation mat (48) in its center and the several transmitting and / or receiving antennas (41) are arranged around the attenuation mat (48). [12] Method according to claim 10 or 11, characterized by , that millimeter wave signals with different frequencies from a predetermined frequency range and / or in certain frequency steps are emitted or received by the transmitting or receiving unit (11, 12) so that a spectrometric ellipsometric evaluation is carried out at different frequencies. [13] Method according to any one of claims 10 to 12, characterized by , that millimeter wave signals with different angles of incidence (α1, α2) are emitted onto the object (15) under investigation by the transmitting unit (11). [14] Method according to any one of claims 10 to 13, characterized by , that the reflected millimeter wave signals are received by the receiving unit (12) at different angles of incidence (β1, β2) and an angle-varying ellipsometric evaluation is carried out. [15] Method according to any one of claims 11 to 14, characterized by , that a focusing point (17) is reconstructed by weighting the reflected signals with a phase term and summing them coherently, so that only those reflected signals that were reflected from a considered object point contribute. [16] Method according to claim 15, characterized by , that the focus is set by changing the phase term on the transmitting and / or receiving side and / or that the weighting of the phase term is hardware-based or software-based.

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