METHOD OF OPERATING A POLARIZATION MEASURING DEVICE AND POLARIZATION MEASURING DEVICE

Abstract

A polarization measuring device is operated by passing light having a predetermined input polarization state to a sample for a potentially polarization changing interaction and from the sample through a polarization selective analyzer and to an intensity detector. The method proceeds by varying an angle between the output polarization state of the light emanating from the sample and the analyzer. The wavelength of the light reaching the intensity detector is varied, and a plurality of intensity measurements are performed successively at different constellations of polarization. Spectral modulation states and corresponding intensity values are stored together with polarization and spectral values representing the corresponding constellation. The polarization modulation and the spectral modulation are performed simultaneously and continuously, and during a single, monotonic variation of the polarization modulation state, the spectral modulation state is varied plural times and during each spectral modulation period (?.sub.?) plural successive intensity measurements are performed.

Claims

1. A method of operating a polarization measuring device (10), comprising: passing light (14) having a predetermined input polarization state to a sample (16) for the purpose of a potentially polarization changing interaction with the sample (16), passing the light (14) from the sample (16) through a polarization selective analyzer (28) and at least partially to an intensity detector (20); within the framework of a polarization modulation, varying a relative angle between the output polarization state of the light (14) emanating from the sample (14) and the analyzer (28), and within the framework of a spectral modulation, varying a wavelength of the light (14) reaching the intensity detector (20), using the intensity detector (20) for successively performing a plurality of intensity measurements at different constellations of polarization and spectral modulation states and storing corresponding intensity values together with polarization and spectral values representing the corresponding constellation, and wherein the polarization modulation and the spectral modulation are performed simultaneously and continuously, the spectral modulation state is periodically varied a plurality of times during a single, monotonic variation of the polarization modulation state, and a plurality of successive intensity measurements are performed during each spectral modulation period (?.sub.?).

2. The method of claim 1, wherein the intensity measurements performed during each spectral modulation period (?.sub.?) immediately follow each other.

3. The method of claim 1, wherein the integration time underlying each intensity measurement is controlled as a function of the wavelength chosen in each case.

4. The method of claim 1, wherein the integration time underlying each intensity measurement is constant.

5. The method of claim 1, wherein the variation of the wavelength within each spectral modulation period (?.sub.?) is monotonic.

6. The method of claim 1, wherein the time period (?.sub.RC) over which the monotonic variation of the polarization modulation state occurs corresponds to an integer multiple of the spectral modulation period (?.sub.?).

7. The method of claim 1, wherein the polarization modulation, the spectral modulation and the intensity measurements are mechatronically synchronized.

8. The method of claim 1, wherein a polarization adjustment value representative of the respectively set polarization modulation state and/or a spectral adjustment value representative of the respectively set spectral modulation state are measured continuously and converted into the polarization values or spectral values, respectively, that are associated with the intensity values to be stored together with them.

9. The method of claim 8, wherein the polarization modulation is carried out by means of a polarization modulator that is arranged in the optical path in front of or behind the sample (16) and can be adjusted by a first servomotor.

10. The method of claim 9, wherein the polarization modulator is formed as a pair of a polarizer (22) and a compensator (24) positioned in the optical path between the light source and the sample, the compensator being rotatable by the first servomotor.

11. The method of claim 8, wherein the polarization adjustment value is a position value representative of the position of the first servomotor.

12. The method of claim 8, wherein the spectral modulation is carried out by means of a spectral modulator that is arranged in the optical path in front of or behind the sample and is adjustable by a second servomotor.

13. The method of claim 12, wherein the spectral adjustment value is a position value representative of the position of the second servomotor.

14. A polarization measuring device (10) comprising a light source (12) for generating light (16) intended to interact with a sample (16), a sample holder for positioning the sample (16) in the optical path of the light (14), an intensity detector (20) for detecting an intensity of the light (14) after its interaction with the sample (16), a polarization state generator disposed between the light source (12) and the sample (14) and adapted to provide a predetermined input polarization state to the light (14), a polarization-sensitive analyzer (28) disposed between the sample (16) and the intensity detector (20) and adapted to select light components of predetermined polarization states, a spectral modulator (30) adapted to vary the wavelength of the light (14) reaching the detector (20), a polarization modulator (24) adapted to vary a relative angle between the output polarization state of the light (14) emanating from the sample (16) and the analyzer (28), and a control unit adapted to drive the intensity detector (20), the polarization modulator (24), and the spectral modulator (30) according to a predetermined operating procedure, characterized in that the control unit is arranged to control the intensity detector (20), the polarization modulator (24) and the spectral modulator (30) according to the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1 is a schematic representation of a preferred embodiment of an imaging ellipsometer by means of which the method of the invention can be carried out,

[0043] FIG. 2 is a schematic representation of the process flow of the invention.

[0044] FIG. 3 is a further schematic representation of the process sequence of to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] Identical reference signs in the figures indicate identical or analogous elements.

[0046] FIG. 1 shows in a highly schematized representation a basically known imaging ellipsometer, which can be operated according to the rotating compensator principle (RC principle). A measuring beam 14 is transmitted from a light source 12 at an angle of incidence q onto a sample 16 and reflected from the latter through imaging optics 18 onto an imaging detector 20. On its way from the light source 12 to the sample 16, the measuring beam 14 passes a (linear) polarizer 22 and a compensator 24 arranged downstream of it, which together act as a polarization state generator (PSG). The polarizer 22 can be designed as a linear polarization filter. The compensator 24 can be designed as a A-quarter plate by means of which the light components polarized perpendicularly and parallel to the plane of incidence are phase-shifted with respect to each other. This results in a generally elliptical polarization of the measuring beam 14. By rotating the compensator 24 about the optical axis, i.e. the beam direction of the measuring beam 14, the ellipticity and axial orientation (angular position) of the polarization of the measuring beam 14 and thus its polarization state, in particular the input polarization state with which the measuring beam 14 impinges on the sample 16, can be varied. This is indicated by the rotation arrow 26 in FIG. 1.

[0047] The part of the sample 16 illuminated by the measuring beam 14 is imaged onto the imaging intensity detector 20 by means of the imaging optics 18. In operation, its light passes through an analyzer 28, which may be designed as a (linear) polarization filter. According to its orientation, the analyzer 28 allows only certain polarization components of the measuring beam 14 to pass, while others are suppressed. Further, in the embodiment shown, the measuring beam passes between the sample 16 and the imaging intensity detector 20 through an adjustable spectral filter 30 which allows onlyaccording to its settinglight components of selected wavelengths to pass and suppresses or deflects other light components to such an extent that they do not fall on the intensity detector 20. Instead of the spectral filter 30 positioned in the detection part of the optical path, this or an equivalent spectral variator, e.g. a monochromator, may be arranged in the illumination part of the optical path. In fact, the preferred embodiment in practice comprises a monochromator positioned between the light source 12 and the PSG 22/24. The illustration of the spectral filter 30 in the detection optical path in FIG. 1 is for clarity only.

[0048] In the course of carrying out the method according to the invention, the compensator 24 is continuously rotated from an initial position to an end position. This changes the input polarization state with which the measuring beam 14 falls on the sample 16 accordingly. The output polarization state with which the measuring beam 14 emanates from the sample 16 and interacts with the analyzer 28 also changes accordingly. Thus, the intensity of the light component incident on detector 20 varies with the angular position of compensator 24, and periodically with a period of 180? relative to the angular position of compensator 24. This means that the displacement of the compensator 24 can be limited to a maximum of 180? during a run of the method according to the invention, as is actually the case in the preferred embodiment.

[0049] During such a comparatively slow polarity modulation, according to the invention, a likewise continuous modulation of the wavelength of the light component impinging on the detector 20 is carried out several times, for example by corresponding multiple variation of the spectral filter 30 or an equivalent spectral modulator.

[0050] The time sequence of the process according to the invention is shown schematically and as an example in FIG. 2. With a period of Tic the compensator 24 is rotated by 180?. Meanwhile, the wavelength ? is tuned several times, in the embodiment shown six times, between a start wavelength ?.sub.s and an end wavelength ?.sub.e. In the preferred embodiment shown, the tuning is performed in a sawtooth fashion, i.e. during a spectral modulation period ?.sub.? the wavelength set on the spectral modulator is changed linearly in order to return to the initial state as instantaneously as possible at the end of the period. Corresponding control devices, for example a motor-driven grating of a monochromator, are known to the skilled person. Parallel to this polarization and spectral modulation, a clocked intensity measurement is performed by means of the detector 20. In the embodiment shown, the intensity measurement is performed with constant integration times ?.sub.I.

[0051] During an integration interval ?.sub.I, the angular position of the compensator 24 changes by an angular amount ?.sub.RC; the wavelength changes by a wavelength amount ?.sub.?. The corresponding intensity measurement can then be assigned a constellation of polarization and spectral state, which can be regarded, for example, as the mean value of the respective polarization or spectral interval ?.sub.RC or ?.sub.? with corresponding bandwidth (fuzziness).

[0052] FIG. 3 shows in a polarization/wavelength plane the positions of the individual intensity measurements, with the solid arrow line representing the time sequence of the intensity measurements. Note that for clarity, the plot of FIG. 3 shows only the intensity measurements at four different polarization intervals and three different spectral intervals (a procedure performed according to the scheme of FIG. 2 would result in intensity measurements at six different polarization intervals and four different spectral intervals).

[0053] From the schematic of FIG. 3 it can be seen that due to the method according to the invention both the spectral state and the polarization state change from one intensity measurement to the next intensity measurement, but in the result for three fixed wavelengths in each case four intensity measurements are recorded at different, essentially equally spaced polarization states. Thus, for each wavelength, predetermined polarimetric or ellipsometric target quantities (in the example shown, the ellipsometric parameters ? and ?) can be determined with equal accuracy. The required number of measuring points per wavelength depends, as known to the skilled person, on the target quantities of interest and the specific instrumental setup. As can be seen from FIG. 2, however, there are no dead times during data acquisition. Rather, the intensity detector runs at its maximum speed specified by the apparatus, so that the measurement is performed with maximum efficiency at the theoretically highest possible speed. At the same time, because the compensator 28 is varied only once, slowly and continuously, no vibrations are introduced that would interfere with the imaging quality of the detector 22. The consequent imaging-ellipsometric measurement results are thus not only obtained faster but also of higher quality and accuracy than has been possible in the prior art.

[0054] In the embodiment shown in FIG. 2, the spectral modulation, the polarization modulation and the intensity measurements are synchronized in such a way that a spectral period ?.sub.? corresponds to an integer multiple of the integration time of the detector 20 ?.sub.I and the polarization period ?.sub.RC corresponds to an integer multiple of the spectral period ?.sub.?. This synchronization can be achieved by appropriately synchronized control of the individual apparative components. Alternatively, the running speed of the individual components can be preset accordingly and their respective settings monitored and measured so that the positions of the individual intensity measurements in the polarization/wavelength plane (cf. FIG. 3) can be determined subsequently. In practice, a combination of these approaches will probably prevail, in which a rough mechatronic synchronization, i.e. a synchronized control, takes place and is corrected during the process execution by position measurement values recorded in parallel.

[0055] Of course, the embodiments discussed in the specific description and shown in the figures are only illustrative examples of embodiments of the present invention. The person skilled in the art is provided with a wide range of possible variations in light of the present disclosure. In particular, the method according to the invention can be used in both polarimetric and ellipsometric contexts. The polarimetric or ellipsometric target quantities selected for determination in each case also do not represent a limitation of the method. The number of intensity measurements to be recorded results in a manner recognizable to the person skilled in the art from the choice of the target quantities sought and the concrete apparative design, in particular the apparative nature of the polarization modulator. With regard to the concrete apparative design of the modulators, the skilled person is of course not limited to the embodiments with motor-driven, mechanical control elements described here as preferred. Electro-optical, magneto-optical, acousto-optical and other devices are already known to him which can be used as functional polarization or spectral modulators. Devices to be invented in the future will also be usable within the scope of the present invention. The same applies to the specific detection technique.

LIST OF REFERENCE SIGNS

[0056] 10 imaging ellipsometer [0057] 12 light source [0058] 14 measuring beam [0059] 16 sample [0060] 18 imaging optics [0061] 20 intensity detector [0062] 22 polarizer [0063] 24 compensator [0064] 26 rotation arrow [0065] 28 analyzer [0066] 30 spectral filter [0067] ?.sub.I integration time [0068] ?.sub.RC polarization modulation period [0069] ?.sub.RC polarization interval [0070] ?.sub.? spectral modulation period [0071] ?.sub.? spectral interval