WO2013037833A1

WO2013037833A1 - Method and arrangement for measuring scattered light

Info

Publication number: WO2013037833A1
Application number: PCT/EP2012/067852
Authority: WO
Inventors: Gunther Notni; Alexander VON FINCK; Marcus Trost; Angela Duparré
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.; Friedrich-Schiller-Universität Jena
Priority date: 2011-09-14
Filing date: 2012-09-12
Publication date: 2013-03-21
Also published as: DE102011118607A1

Abstract

The present invention relates to a method and an arrangement for measuring scattered light. For this purpose a sample (17) is illuminated with light from at least one light source (1, 2, 3) and a portion of the light scattered on the sample (17) is detected as a measurement signal at a scattering angle by at least one detector (18). The light incident on the sample (17) comprises two light components, differing from one another in at least one parameter and modulated with different modulation signals. An output signal of the detector (18) is apportioned into several signal channels by filtering out components of the output signal correlated temporally with the different modulation signals, so that each of said light components is unambiguously assigned to at least one of the signal channels. It therefore becomes possible to carry out scattered light measurements with high dynamics and low crosstalk behaviour between the individual signal channels.

Description

Method and arrangement for scattered light measurement

The present invention relates to a method for scattered light measurement according to the preamble of the main claim and to a device for scattered light measurement according to the preamble of the independent claim.

Measurement methods which are based on an evaluation of light scattered on a sample, which is referred to as scattered light in the following, are generally used for the efficient analysis of surfaces, materials and coatings, wherein they are distinguished by high sensitivity. By analyzing scattered light measurements, it is possible to draw conclusions about sample-specific parameters such as roughness or root-mean-square (rms) roughness, a roughness spectrum (power spectral density function, PSD), scattering losses, Schich thickness error, roughness propagation in

Layer systems or a so-called subsurface defect analysis (SSD). Such a measurement setup for angle-resolved scattered-light measurement is disclosed, for example, by document WO 2010/127872 A1.

For a comprehensive characterization scattered light analyzes with different parameters are necessary. Almost all samples to be examined show a significantly different scattering behavior at different wavelengths and polarizations. For carrying out such comprehensive sample characterizations, therefore, different measurement configurations are used, with high sensitivities and dynamic ranges in particular being sought. In a sequential, so temporally staggered characterization of a sample with different measurement configurations or measurement parameters, such as different wavelengths or polarizations, this requires a time-consuming retooling of a measurement setup.

In previously used measuring systems, different methods are used to limit changeover times. Typically, such systems have powerful light sources with limited spectral bandwidth, generally lasers, to achieve high spectral power density. By means of manually or automatically movable mirrors of the measuring system, which can be introduced into or removed from an illumination beam path, light from individual sources is guided to the sample. Alternatively, white light sources with a detector-side spectral separation of the scattered light are used, but these are not sufficient Sensitivities and dynamic ranges for the characterization of, for example, nanostructures exhibit.

The present invention is therefore based on the object of proposing a method and a device for scattered light measurement, which avoid the disadvantages mentioned, which thus allow a scattered light measurement with different measurement parameters in the shortest possible time and without time-consuming retrofitting, at the same time a high sensitivity and the largest possible dynamic ranges to be achieved.

This object is achieved by a method according to claim 1 and an arrangement according to claim 9. Advantageous embodiments and further developments are described in the dependent claims.

The proposed method for carrying out a scattered light measurement, in which a sample is illuminated with light from at least one light source and a portion of the light scattered on the sample is detected as a measurement signal from at least one detector, makes use of the fact that the light emerging on the sample is at least two Includes light components that differ in at least one parameter. In order to allow a discrimination of proportions of a detected intensity or power of the scattered light according to the various light components to which these components are due, the light components are also modulated with different modulation signals. After the scattering of the light components on the sample and detection, an output signal of the detector is split into several signal channels by filtering out with time The different modulation signals correlated proportions of the output signal, so that each of said light components or each of the modulation signals is assigned to at least one of the signal channels unique.

The assignment between light components or modulation signals on the one hand and signal channels on the other hand consists in that the signal channel of the selective processing assigned to a light component or the modulation signal of a light component is a signal attributable to this light component and thus modulated or time-correlated even with the modulation signal of this light component Proportion of the output signal is used, the latter is thus guided by that signal channel. Scattering of the light at the sample should be understood to mean any conceivable scattering and, in particular, both a diffraction and a reflection, in particular a specular reflection in which the light has a vanishing azimuthal angle of reflection.

By this method, it is possible to record signals in parallel highly sensitive in several signal channels. A reliable separation of the signals to be processed is achieved by the different modulation of the correlated with the respective signals light components. By means of this modulation of the light components, scattered signals can be separated on the detector side with little effort, so that a simultaneous measurement of scattered light with different measurement parameters and a process-oriented use of the method is made possible. By a further processing in the individual signal channels, which can take place simultaneously in an advantageous manner, a sensitivity of the method is increased because an undesirable mutual interference of the signal channels is minimized by the reliable separation. Finally, with the proposed method, a dynamic range of at least 10 orders of magnitude per signal channel can be achieved.

The at least one parameter in which the light components differ may include an angle of incidence or a wavelength. Alternatively or additionally, the at least one parameter may include a different wavelength spectrum, a polarization, an intensity and / or a position at which the respective light component impinges on the sample for the different light components. Since scattering cross sections are sometimes wavelength or polarization dependent, or also dependent on the position on the sample at which the light is scattered, multiple information from the scattered light can be obtained in parallel by using these parameters.

It can be provided to perform an angle-resolved scattered light measurement, which is also referred to as angle-resolved-scattering (ARS). For this purpose, a scattering angle of the scattered portion of

Light for angle-resolved scattered light measurement can be varied. Alternatively or additionally, the light components can be irradiated onto the sample at different angles of incidence. As an ARS function, a power of the scattered light in a solid angle element divided by a product of the intensity of the incident light and the solid angle element into which scattered may be defined. In addition, a so-called bidirectional scattering distribution functi- which is defined as the quotient of the ARS function and a cosine of the scattering angle Preferably, the light components are integrated over a specific angle range on the detector side angle-resolved scattered light measurement can be obtained.

To filter out each of said components, the output signal in the respective signal channel can pass through a bandpass filter and / or a lock-in-Ve stronger. As a result, only the desired portions of the output signal, which thus have a suitable modulation signal or are temporally correlated with the appropriate modulation signal, fed to a further processing and in the case of the lock-in amplifier also improves the signal-to-noise ratio for further processing , Preferably, the bandpass filter and the lock-in amplifier comprise a low-pass filter, a

Average filter, a Chebyshev filter, a Bessel filter, a Butterworth filter or a combination of said filter so that the herauszufilternde proportion of the output signal are obtained with the desired sharpness and accuracy k ^ nn _*

An advantageous embodiment can provide that the components of the output signal remaining in the different signal channels are processed individually at the same time. Time-parallel processing minimizes the time required for the scattered light measurement and the analysis of this measurement. By splitting into several signal channels is the simultaneous, parallel processing of the in the individual signal channels guided signals easily possible.

Due to the extremely high dynamics of scattered light measurements, a low crosstalk behavior between the individual signal channels can be of decisive importance and should preferably be limited to between three and ten powers of ten for a reliable measurement. In order to reduce crosstalk between the signal channels, intensities of the light components can be selected or adjusted in such a way that scattered portions of all the light components mentioned with the detector have powers of the same order of magnitude. The same magnitude should be used to denote power or intensity values in the present specification if a difference between them is less than 50%, preferably less than 30%, particularly preferably less than 10%, of the larger of the stated values. Due to the approximately equal intensities of the individual light components none of these light components is detected as being too dominant and thus a result of the measurement distorting. This allows to obtain in all signal channels an optimal signal-to-noise ratio of the respective portion of the output signal.

To reduce the crosstalk between the signal channels and the individual light components can be passed through a polarization-dependent and / or wavelength-dependent division of the individual light components to different detectors.

This results in a spatial separation of the individual light components, which reduces the probability of mutual interference. In order to attenuate interference signals and / or portion of the output signal that leads to another of the signal channels, the output signal can be prefiltered electronically in at least one of the signal channels. Independently of optical filtering, the electronic signal in the respective signal channel can thus also be processed for further processing.

A further development provides that a modulation frequency of at least one of the modulation signals corresponds to a minimum of a frequency-dependent transmission function of a filter, which is used for filtering out a temporally correlated with another of the modulation signals portion of the output signal. Thus, only the desired portion of the output signal is filtered out and fed to further processing by the filter used, while unwanted portions of the output signal due to the coincidence of their modulation frequency with the minimum of the transfer function is not further transmitted or attenuated accordingly strong. This also allows crosstalk between the signal channels to be avoided or minimized.

The at least two light components can be combined to form a sample beam before impacting the sample. As a result, the beam guidance is facilitated and - what may be desirable - achieved that meet the different light components with the same angle of incidence on the sample.

The proposed arrangement for scattered light measurement comprises at least one light source for illuminating a sample with light and at least one detector for detecting a portion scattered on the sample of the light. Between the at least one light source and the sample are at least two different ones

Beam paths are provided for at least two light components differing in at least one parameter. The beam paths here run at least in sections differently. The light components can be modulated independently of each other in intensity. The arrangement also has a control unit which is set up to modulate the optical components with different modulation signals, and an evaluation unit which is set up to filter out in each of the signal channels in each case one portion of the output signal correlated in time with the different modulation signals, so that each of the said light components or each of the modulation signals is uniquely assigned to at least one of the signal channels.

For each of the light components or each of the modulation signals, there is thus at least one signal channel in each case in which a portion of the output signal attributable to this light component or modulated or modulated by this modulation signal is filtered out. As a result of the at least regional separation of the beam paths of the optical components, the light components can be modulated both independently of one another and also generated or manipulated in such a way that they differ from one another in the at least one parameter. A beam path is to be understood here as meaning any possible path between the light source and the sample. Due to the multiple signal channels of the evaluation unit, it is possible to simultaneously process signals carried in the signal channels. For generating the various light components, the arrangement may comprise a plurality of light sources. With regard to the at least one parameter in which the light components are to differ, the light sources may themselves already be arranged at different angles to the sample and / or designed to generate light of respectively different wavelengths or spectra and / or different polarizations. Due to several light sources with different properties, measurements with several measuring parameters can be carried out at the same time, so that the measuring time is reduced accordingly.

In a particularly advantageous manner, the angle of incidence between one of the light sources and the sample may be adjustable by a change in position of the respective light source and / or further components arranged in the beam path. Alternatively or additionally, the sample itself and / or the at least one detector can be adjustable in position, so that the angle of incidence and / or the scattering angle can be variably adjusted.

Alternatively or additionally, a wavelength-selective element and / or a polarizer can be arranged in at least one of the beam paths in order to give the light component guided by this beam path a different wavelength or spectral composition and / or polarization from at least one other of the light components. This also makes it possible to ensure that the light components are in the desired parameter (s) - ie z. B. wavelength or polarization - different. It can also be provided that an attenuation device, preferably a filter, for adjusting an intensity of the light or one of the light components is arranged in one of the beam paths located between the light source and the sample and / or in a beam path located between the sample and the detector , As a result, the scattered components of all light components detected by the detector can be normalized to powers of the same order of magnitude, thereby improving the signal-to-noise ratio and, in particular, minimizing tibersprechen between the signal channels.

The evaluation device may comprise at least one band-pass filter and / or at least one lock-in amplifier for filtering out the portions of the output signal correlated in time with the various modulation signals in each of the signal channels. As a result, filtering of the output signal can be performed efficiently. When using a lock-in amplifier is used as a reference signal to a reference input of this lock-in amplifier, preferably each exactly the modulation signal used to modulate the light component, the proportion of the output signal in this signal path is filtered out. The modulation signals can be z. B. generated in a control unit and time correlated to be passed both to the modulators or light sources and to the lock-in amplifier.

In addition, at least one optical component for beam shaping, beam folding, minimization of scattered light in the beam path or polarization adjustment can be arranged in at least one of the beam paths. Under "scattered light in the beam path" is intended here located in the beam paths, externally generated by ambient light or internally generated by unwanted scattering of optical elements of the beam path, unwanted light, which can adversely affect the measurement. Preferably, this optical component is a mirror, semipermeable or dichroic mirror, lens, spatial filter, aperture, polarizer, λ / plate, or λ / 2 plate, or comprises one or more of these elements. As a result, desired properties of the beam impinging on the sample, such as focusing or a geometric distribution of the beam, can be set and the beam paths can be guided in a suitable manner. In this case, mirrors have the advantage over lenses of having a smaller number of optical surfaces, which reduces scattered radiation. Finally, mirrors in polychromatic illumination are also color-defective.

For modulating the light components in at least one of the beam paths, a modulator controllable via the control unit can be arranged. Alternatively or additionally, the arrangement for at least one of the light components may have its own modulatable light source. The modulator preferably comprises a mechanical chopper or a

acousto-optic modulator. The modulatable light source preferably comprises a laser diode or a solid-state laser. By using the

modulated light source, the light component can already be provided with a modulation directly in their generation, so no additional components must be provided in the beam path and the arrangement can be carried out space-saving. In addition, this is a modulation with a sinusoidal shaped modulation signal, which is more favorable in terms of crosstalk, easier possible.

For carrying out the proposed method, the proposed arrangement may preferably be used.

Embodiments of the invention are illustrated in the drawings and are explained below with reference to Figures 1 to 6.

Show it:

Fig. 1 is a schematic view of an arrangement for carrying out an angle-resolved scattered light measurement;

Fig. 2 is a Fig. 1 corresponding representation <

further arrangement with a single light source,

Fig. 3 shows a modification of that shown in Fig. 1

Arrangement with an additional structure for a multiple modulation;

Fig. 4 a the Fign. 1 and 2 corresponding view of another arrangement for angle-resolved scattered light measurement, in the

Incident light components at different angles of incidence on the sample;

Fig. 5 a frequency-dependent transfer function of two filters and

6 is an exemplary diagram of the crosstalk Behavior for different ratios of interfering signal to signal to be measured.

In Fig. 1, an arrangement for angle-resolved scattered light measurement is shown in a schematic view. Three lasers 1, 2, 3 or laser diodes each generate a light component which is guided in a beam path to a sample 17. Overall, the arrangement shown just as many

Beams such as light sources on. The light components guided in the different beam paths differ in at least one parameter from each other, in their wavelength in the illustrated embodiment, since the lasers 1, 2, 3 each emit different wavelengths. The individual light components are combined by a mirror 13 and two dichroic mirrors 14 and 15 and passed to the sample 17 as a sample beam. In front of the dichroic mirror 15, however, the beam paths are guided differently. In the embodiment shown in Fig. 1, only the three lasers 1, 2, 3 are shown as light sources, but it can of course also be provided more than three light sources and a correspondingly higher number of beam paths. Instead, only a single light source could be provided and the light generated by this light source can be split, for example, by a beam splitter into several light components, which would then be guided again in each case a separate beam path. In particular, in the latter case, the light components by color filters in each case be given a different wavelength or spectral composition of the other light components.

In each of the beam paths is a modulator 7, 8, 9th contain, which modulates the respective light component independently of other light components in their intensity. The modulators 7, 8, 9 modulate the different light components with different modulation signals, with which the modulators 7, 8, and 9 are driven to and which can differ from each other in frequency, waveform and phase. The modulator 7 is an acousto-optic modulator in the illustrated embodiment, while the modulators 8 and 9 are mechanical choppers. In an alternative embodiment, the lasers 1, 2, 3 may themselves be modulated, for example by using modulatable laser diodes. The modulators 7, 8, 9 are each controlled independently of one another in FIG. 1 for reasons of clarity not shown control units. Alternatively, only a single control unit with several outputs for driving the modulators 7, 8, 9 may be provided. If the lasers 1, 2 and 3 themselves are externally or internally modulated, instead of the then eliminated modulators 7, 8 and 9, the laser 1, 2 and 3 are driven accordingly.

In each of the beam paths, an optical filter 4, 5, 6 is arranged, which ensures that the individual light components have an adjusted intensity such that the later detected scattered proportions of all light components have powers of the same order of magnitude. This serves to limit a crosstalk behavior to between three and ten orders of magnitude, in order to allow a reliable measurement. Typically, by using only the optical filters 4, 5, 6 as mechanical filters, crosstalk limitation is achievable to one to ten orders of magnitude, while Using only a lock-in amplifier to improve the crosstalk behavior typically the crosstalk behavior can be limited to three orders of magnitude. By a combination of both methods, the crosstalk behavior can be further improved so that the individual signal channels can have a signal difference of more than ten orders of magnitude. A required number of filter stages for achieving the dynamic range necessary for the scattered light measurement can also be reduced by improving the crosstalk behavior of the lock-in amplifier. Due to the extremely high dynamics of scattered light measurements, such a low crosstalk behavior between the individual signal channels is crucial for the success of the measurement.

In addition, in the beam paths in each case an optical component 10, 11, 12 included for beam shaping, which may also consist of several individual components, that can be realized by a group of components. In the illustrated embodiment, the optical components 10, 11, 12 each comprise a spatial filter for homogenizing the respective light component and a lens for adjusting a focus on the sample 17. About the mirror 13 and the dichroic mirrors 14 and 15 are the

Light components brought together and passed together as a sample beam to the sample 17. In this case, the sample beam passes through a further optical filter 16, which adjusts the intensity of the sample beam to a level not damaging the sample 17 and / or adjusts the intensity of the light scattered on the sample 17 in such a way that a signal-to-noise ratio of one for detection detector 18 is optimized and / or the intensities sität in the linear region of the detector 18 is located.

The optical filters 4, 5, 6 and 16 can be arranged at different positions in the respective beam paths and each have an adjustable attenuation ratio. Instead of the dichroic mirrors 14, 15 or additionally, a beam splitter, a grating, a prism, a waveguide and / or a light-conducting fiber can also be used to guide the beam paths. In particular, the sample beam can also be conducted to the sample 17 via a fiber, which makes possible a compact beam guidance and a compact design of the arrangement with precise superimposition of the light components and low sensitivity to mechanical instabilities in the case of long beam paths. For combining the light components, however, it is preferred to use mirrors or lenses with superpolymers, ie, low-scattering surfaces. If the optical components 10, 11, 12 do not comprise a spatial filter, preferably all of the mirrors and lenses used have super polishes, otherwise advantageously such elements with superpolicies are used only for mirrors and lenses arranged in the beam paths between the spatial filter and the sample 17 , Mirrors are generally preferable to lenses due to the lower number of optical surfaces and associated less unwanted scattered radiation.

The optical components 10, 11, 12 are also used to set the largest possible beam diameter and a divergence or homogenization of the individual light components as equal as possible. In addition, in the beam paths still in Fig. 1, not shown, spatial filters or diaphragms or components for adjusting a polarization such as polarizers or λ / 4 plates or λ / 2 plates. It may also, in particular when using polychromatic light sources, a wavelength-selective element such as a color filter may be provided to give the guided by the respective beam path light component of the light components of the other beam paths different wavelength or spectral composition.

The sample beam strikes the sample 17 at an angle of incidence Q ± and is scattered on the sample 17. The angle of incidence can be adjusted by tilting the sample 17. Alternatively or additionally, instead of the sample to tilt, the lasers 1, 2, 3 together with the optical filters 4, 5, 6, 16 can also be used for modu ^¬ lators 7, 8, 9 optical elements 10, 11, 12, the mirror 13 and the Dxchroitischen mirrors 14 and 15 are tilted. In this case, the detector 18 can be held stationary or also correspondingly

be tilted.

A portion of the light scattered and specularly reflected by the sample 17 is detected by the detector 18. The detector 18 includes a photomultiplier but may also include a photodiode or a matrix sensor. The detector 18 may be set at different angles to the sample 17 to detect portions of the light scattered at different scattering angles Q _s . The scattering angles Q _s can also be azimuthal scattering angles. It is also possible to provide a plurality of detectors for detecting the scattered light at different scattering angles, which results in a multiplication of a spatial frequency scan. made possible. The spatial frequency f as an in-plane component is over

, Isinö _j -s ^' 9i

'~

defines, where λ denotes the wavelength. By using multiple detectors with different spectral sensitivity but possibly the same field of view, broadening of the overall spectral sensitivity is possible.

In front of the detector 18, further optical components for setting a field of view, a detected solid angle and / or for setting a polarization, such as diaphragms, lenses, mirrors or polarization optics, may be arranged. In particular, by using polarization optics, such as correspondingly oriented λ / 4 plates or λ / 2 plates, or polarizers, polarization properties of the sample 17 can be characterized in a more comprehensive manner in order to be able to draw conclusions about different scattering causes. It is also possible to use optical fibers, waveguides or fibers in order to guide the scattered portion of the light to the detector 18, which in turn allows a compact construction of the arrangement.

The detector 18 converts the scattered portion of the light into an electrical signal, which is divided into a plurality of signal channels in an evaluation unit comprising three lock-in amplifiers 19, 20, 21. In each of the signal channels, one of the lock-in amplifiers 19, 20, 21 is arranged. Output signals of the lock-in amplifiers can be fed simultaneously for further processing. The evaluation unit thus has several parallel signal channels. At a reference input of each of the lock-in amplifier 19, 20, 21 is in each case a reference signal, the the modulation signal corresponds to one of the light components, wherein all the modulation signals in the present case in each case at exactly one of the lock-in amplifier 19, 20, 21 abut. In addition to the modulators 7, 8, 9, the respective reference signals are also transmitted by the control units to the lock-in amplifiers 19, 20, 21. The evaluation units 19, 20, 21 thereby filter out a portion of the output signal which is correlated in time with the respective modulation signal and which is then fed to further processing in the signal channels. Thus, each of the mentioned light components is uniquely assigned to one of the signal channels - in the case of modifications more than one signal channel could also be provided for each of the light components, for example when using a plurality of detectors whose analog signals are evaluated at the same time. Instead of the lock-in amplifiers 19, 20, 21, band-pass filters can also be used for filtering out the components of the output signal of the detector 18 attributable to the various light components. The arrangement shown in Fig. 1 has a dynamic range of more than 10 orders of magnitude and a noise level (ARS background level) of less than 10 ^-7 sr ^-1 due to the separate treatment of the individual components of the output signal in different signal channels. structures on surfaces, in layer systems and in the material volume with structure widths of less than 1 nm.

In Fig. 2 shows an embodiment of an arrangement for scattered light measurement with a single laser 1 is shown as a light source. Recurring features are provided with identical reference numerals here as well as in the following drawings. The in this figure illustrated arrangement corresponds to the arrangement shown in Fig. 1, however, only the laser 1 is used instead of several lasers and the light emitted by the laser 1 light is split by two dichroic mirrors 14 and 15 in the individual light components and by the dichroic mirrors 14 and 15 and the mirror 13 and directed to the modulators 7, 8 and 9 in the beam paths. In order to set different wavelengths, the filters 4, 5 and 6 are also wavelength-selective in this case, so that after passing through said filters each light component has a different wavelength.

In Fig. 3 a modification of the arrangement shown in Fig. 1 is shown, which has been extended by an additional structure for multiple modulation. In contrast to the arrangement shown in Fig. 1, the sample beam after passing through the further optical filter 16, but even before hitting the sample 17, again divided and modulated.

The individual light components are circularly polarized in the embodiment shown in FIG. Behind the further optical filter 16, the sample beam is divided by a polarizing beam splitter 25 into two sub-beams, wherein a first sub-beam has an s-polarization and is guided by a first additional modulator 27, while a second sub-beam has a p-polarization and by a second additional modulator 28 is performed. The said further modulators 27, 28 in turn modulate the two partial beams independently of each other in different ways. By a further polarizing beam splitter 26, the two partial beams finally combined with each other and passed to the sample 17. In further embodiments, the individual light components can also be linearly polarized and split by a respective λ / 4 plate in each of the partial beams into a right-circularly polarized component and a left-circularly polarized component.

Instead of the three lock-in amplifiers 19, 20, 21 shown in FIG. 1, the evaluation unit now comprises three further lock-in amplifiers 22, 23, 24, ie a total of six lock-in amplifiers. In each of the lock-in amplifiers 19, 20, 21, 22, 23 and 24, two demodulators are connected in series, so that a demodulation of the modulation signal of the modulators 7, 8, 9 arranged in the beam paths is present in all illustrated lock-in modes. Amplifiers 19, 20, 21, 22, 23, 24 and a demodulation of the modulation signal of the first further modulator 27 in the lock-in amplifiers 19, 20, 21 and a demodulation of the modulation signal of the second further modulator 28 in the lock-in Amplifiers 22, 23 and 24 takes place. In another embodiment, instead of using a lock-in amplifier with two demodulators, instead of the lock-in amplifiers 19, 20, 21, 22, 23, 24, two lock-in amplifiers can each be connected in series, so that based on the embodiment shown in FIG. 3, a total of twelve paired lock-in amplifiers are used. Thus, scattered signals of different wavelengths or spectra can each be reconstructed for s- or p-polarized light components. There are therefore a total of six different combination possibilities in the illustrated exemplary embodiment: s-polarized light component with a first wavelength, s-polarized light component. second wavelength, s-polarized light component having a third wavelength, p-polarized light component having a first wavelength, p-polarized light component having a second wavelength, p-polarized light component having a third wavelength.

In an alternative embodiment, two parallel lock-in amplifiers can also be used initially to separate the modulation signals of differently polarized light components, whereby the signals separated in this way are fed separately from each other to three further lock-in amplifiers, through which each of the signals after the modulation signals the first, second or third wavelength of the light component is split. Thus, a total of eight lock-in amplifiers are required in this embodiment. This arrangement of lock-in amplifiers is based on the fact that in the first stage n different parameters and in the second stage m different parameters are to be separated so that a total of n + n * m lock-in amplifiers are required for the demodulation. In the example described, n = 2 for two different polarizations and U7 = 3 for three different wavelengths.

A development of the exemplary embodiment illustrated in FIG. 3 provides for using a polarization-sensitive detector in order to separate the s and p polarized components directly at the detector. Such a detector may, for. B. provide an optical separation after the detector aperture in beams of different polarization via a polarizing beam splitter and detection via various sensors of the detector. Analogously, instead of a polarization-sensitive detector, it is also possible to use a wavelength-sensitive detector, for example for characterizing inelastic scattering or Raman scattering.

For this purpose, z. B. on a grid, the scattered proportion of the light are wavelength-dependent split on several sensors of the detector.

Instead of a detector with wavelength-dependent or polarization-dependent division of the light components onto a plurality of sensors of the detector, it is also possible to provide a plurality of detectors, the division into different wavelengths or different polarizations occurring even before the light components enter the detectors.

Fig. 4 shows in a FIGS. 1, 2 and 3 corresponding view another embodiment of an arrangement for angle-resolved scattered light measurement. In contrast to the in Figs. 1, 2 and 3, the individual light components are no longer combined with one another and passed to the sample 17 as a sample beam. The lasers 1, 2, 3 are arranged at different angles to the sample 17, so that the light components generated by them impinge on the sample 17 at a respectively different polar and / or azimuthal angle of incidence. The further optical filter 16 is now fixed directly in front of the detector 18 to protect it from damage by scattered radiation to high intensity, and is moved with this. The attenuation ratio of the filter 16 is also chosen so that the detector 18 can be operated in the linear range and the signal-to-noise ratio is optimized. Since the individual light components are already in their Distinguish different angles of incidence and thus given a larger adjustment range of Ortsf equenzen, the lasers 1, 2, 3 in this embodiment, all have a same wavelength or polarization, so that the light components may differ from each other here only in the parameter angle of incidence , In the exemplary embodiment shown in FIG. 4, all the light components strike the sample 17 at the same location, but it may of course also be provided that the individual light components impinge on the sample 17 at different positions.

Due to the extremely high dynamics of scattered light measurements (partly scattered powers are measured with the trillionth part of an incident power), the low crosstalk behavior between the individual signal channels is decisive. The crosstalk behavior must be limited to between three and ten powers of ten for a reliable measurement. It makes sense to use a combination of partially already presented techniques. This and the options described below apply equally to all embodiments.

As already discussed, the intensities of the light components can be equalized by the optical filters 4, 5, 6. In addition, in the signal channels an electronic prefiltering be made to attenuate noise and / or by another of the signal channels to leading portion of the output signal. This can be achieved by electronic low-pass or band-pass filters with a corresponding passband. An optical prefiltering has already been explained above by a complete or partial division of the individual light components onto different detectors or sensors of a detector.

As a further possibility, a modulation adaptation is proposed, which is to be explained using a simple example and FIG. 5. In the case of a rectangularly modulated input signal with an amplitude A _m and a modulation frequency über, which is superimposed with a rectangularly modulated interference signal of another signal channel with a frequency c _s and an amplitude A _s , the signals can be described by the following Fourier series U (t):

In this case, demodulation or filtering can be performed in one of the lock-in amplifiers 19, 20, 21, 22, 23, 24 by multiplying the Fourier series by an internally generated signal

be achieved with the same modulation frequency. For simplification, assume that the phase φ = 0.

For a signal X (t) after demodulation, the result is thus:

In a (time-independent) DC voltage component 2Α _ΙΗ / π is in this case a proportionality to the amplitude A _{M of} the input signal included. Higher frequency components are ideally completely eliminated by a subsequent low-pass filter, so that an output signal X _0ÜT (£) only depends on the amplitude _λ _{M of} the input signal:

For low-order analog filters, of which all orders are less than or equal to one. Order to be understood, for example, low-pass filter, this procedure works especially well for small stochastic noise signals very well, but encounters limits due to a limited slope of the low-pass filter with strong, non-random noise. A higher slope of the low-pass filter also leads to slower step responses, which has a negative effect on measurement times. It can also be seen that a sinusoidal modulation of the signals has a positive effect by eliminating the higher harmonics, ie omitting sum terms with i or j 2 in the Fourier series U (t). A demodulation or filtering of the type described here via a sinusoidal signal is accordingly just as advantageous.

An analog deep filter of first order and an integration time τ can be described in the frequency domain by the amount of the transfer function Η _ΤΡί1 (ω):

Digital lock-in amplifiers, on the other hand, offer the possibility of both digital low-pass filtering and moving-average filters, the latter being offer decisive advantages for the elimination of non-statistical interference signals with a known modulation frequency. A moving average filter over the time period 2τ of the signal X (t) can be described as a convolution of X (t) with a rect function m, i (t). For T _mr i (t), the following applies:

By means of a Fourier transformation, this results in a transfer function of the mean value filter

Calculate H _MW , _I (CO):

A multiple application N of the mean value filter thus corresponds to higher order mean value filters N with

5 shows in a diagram the course of the transfer function for the first-order analog lowpass filter (dashed line) and for the first order digital mean value filter (solid line). The ordinate represents the transfer function H and the abscissa the frequency f of the demodulated signal X (t), which is a function of the modulation frequency. For the integration time t, a value of 50 ms was assumed. Zeros as minima of the transfer function occur only in the digital average filter. At these frequencies, spurious signals are ideally completely attenuated. Thus, the modulation frequency is at least one of the modulation signals in front of in a manner suitable for filtering out a portion of the output signal correlated in time with another of the modulation signals so that it corresponds precisely to a zero position of the transfer function of the filter.

For the zeros of the mean value filter, the following relationship finally results: ω, _ ffi _j . f $

%

This results in the following relationship between the modulation frequency of the interference signal f _s , the modulation frequency of the measurement signal f _m and the integration time τ:

It is thus possible to selectively select modulation frequencies for the interference signals and / or another of the signal channels, which experience a very high attenuation during demodulation with comparatively low filter order (and thus faster measurement times). In order to suppress unwanted frequency components, other frequency filters, such as, for example, can be used in addition to the low-pass and average filters described here. B. Chebyshev, Bessel, Butterworth filter, etc. or a combination of different filters are used.

FIG. 6 shows by way of example the crosstalk behavior for different ratios of interference signal to signal to be measured in a double-logarithmic diagram. A quotient of the amplitude of the interference signal A ₃ and the amplitude of the signal A _m to be measured is plotted on the abscissa the ordinate is a quotient of the output signal

Xour (t) one of the lock-in amplifier and an output signal without portions of the interference signal X ₀ ur, o (t) plotted as a measure of the crosstalk behavior. For higher values on the abscissa, therefore, the interfering signal with respect to the modulation signal becomes stronger and stronger. A dotted curve plotted in the diagram denotes one of the modulation frequencies, which corresponds to a local maximum of the transfer function of the digital mean value filter shown in FIG. A curve shown by a solid line in FIG. 6 designates one of the modulation frequencies which corresponds to a local minimum of the transfer function of the digital signal shown in FIG

Average filter corresponds, so one of the frequencies at which interference signals are ideally mitigated as possible. In this case, the noise signal may be much stronger than in the aforementioned case before the output signal is negatively affected by the strong noise signal. If one of the frequencies is selected for the signal to be measured, which corresponds to one of the local minimums of the transfer function of the digital average filter shown in Fig. 5, interference signals, ie, for example, intensities of other signal channels, and signal to be measured by up to three powers of ten, without to negatively influence the crosstalk behavior.

Of course, only features disclosed in the embodiments of the various embodiments may be combined and claimed individually. In any case, with the measures described here, a large amount of information about a sample is obtained in a very short time, because their scattering behavior for the different light components and thus for light of very different eigenvalues can be studied simultaneously with a single structure.

Claims

claims

Method for scattered light measurement, wherein a sample (17) is illuminated with light from at least one light source (1, 2, 3) and a portion of the light scattered on the sample (17) is detected as a measurement signal from at least one detector (18) at a scattering angle .

characterized in that

the light incident on the sample (17) comprises at least two light components which differ in at least one parameter, the light components being modulated with different modulation signals and an output signal of the detector (18) by filtering out portions correlated temporally with the different modulation signals of the output signal is divided into a plurality of signal channels, so that each of said light components is uniquely associated with at least one of the signal channels.

A method according to claim 1, characterized in that the at least one parameter in which the light components differ, an angle of incidence and / or a wavelength

and / or a spectrum and / or a polarization and / or an intensity and / or a position at which the respective light component impinges on the sample. A method according to claim 1 or claim 2, characterized in that the output signal for filtering out each of said components each pass through a bandpass filter and / or a lock-in amplifier (19, 20, 21, 22, 23, 24).

Method according to one of the preceding claims, characterized in that to reduce crosstalk between the signal channels intensities of the light components are selected or adjusted so that with the detector (18) detected scattered portions of all said light components have powers of the same order of magnitude.

Method according to one of the preceding claims, characterized in that to reduce cross-talk between the signal channels, the individual light components are passed through a polarization-dependent and / or wavelength-dependent division of the individual light components to different detectors

Method according to one of the preceding claims, characterized in that the output signal is electronically prefiltered in at least one of the signal channels for attenuation of interfering signals and / or by another of the signal channels to leading portion of the output signal.

Method according to one of the preceding claims, characterized in that a modulation frequency of at least one of the modulation signals is a minimum of one frequency-dependent one Transfer function of a filter which is used to filter out a time correlated with another of the modulation signals proportion of the output signal.

Method according to one of the preceding claims, characterized in that the at least two light components are combined before striking the sample (17) to form a sample beam.

Arrangement for scattered light measurement, comprising at least one light source (1, 2, 3) for illuminating a sample (17) with light and at least one detector (18) for detecting a portion of the light scattered on the sample (17), characterized in that

between the at least one light source (1, 2, 3) and the sample (17) at least two different beam paths for at least two different in at least one parameter

Light components are provided which extend at least partially different, the light components are independently modulatable in intensity and the arrangement comprises a control unit which is adapted to modulate the light components with different modulation signals, and wherein the arrangement of the detector (18) downstream evaluation {19, 20, 21) having a plurality of signal channels arranged to filter out, in each of the signal channels, one portion of the output signal correlated in time with the different modulation signals, such that each of the two Light components of at least one of the signal channels is uniquely assigned.

Arrangement according to claim 9, characterized in that the arrangement for generating the different light components comprises a plurality of light sources (1, 2, 3), which are preferably arranged at different angles to the sample and / or for generating light of different wavelengths or spectra and / or or each of different polarizations are formed.

Method according to one of claims 9 or 10, characterized in that in at least one of the beam paths, a wavelength-selective element and / or a polarizer (25, 26) is arranged to the guided through this beam path light component of at least one other of the light components different wavelength or to give spectral composition and / or polarization.

Arrangement according to one of Claims 9 to 11, characterized in that in one of the beam paths located between the light source and the sample and / or in a beam path located between the sample and the detector, an attenuation device (4, 5, 6), preferably a filter , arranged for adjusting an intensity of the light or one of the light components,

Arrangement according to one of claims 9 to 12, characterized in that the evaluation device for filtering out the time with the different modulation signals correlated portions of the output signal in each of the signal channels comprises at least one bandpass filter and / or at least one lock-in amplifier.

14. Arrangement according to one of claims 9 to 13, characterized in that in at least one of the beam paths at least one optical component {10, 11, 12) for beam shaping, beam folding, minimization of scattered light in the beam path or polarization adjustment is arranged, preferably a Mirror, a lens, a spatial filter, a diaphragm, a polarizer, a λ / 4 plate or a λ / 2 plate.

15. Arrangement according to one of claims 9 to 14, characterized in that for modulating the light components in at least one of the beam paths via the control unit controllable modulator (7, 8, 9, 27, 28) is arranged and / or the arrangement for at least one of the light components has its own modulatable light source.