DEVICE AND METHOD FOR MEASURING THE PROFILE OF FLAT OBJECTS COMPRISING UNKNOWN MATERIALS

Abstract

A method and device for measuring the profile of the surface of a flat object of unknown materials, including an interferometry measuring system, ellipsometry measuring system, beam splitter for splitting a light beam of a light source into an interferometry light beam and an ellipsometry light beam, and an analysis unit designed to ascertain the profile height in the measured region on the object surface from an analysis beam analyzed in a detector unit of the interferometry measuring system and a sensor beam received in an ellipsometry sensor. The interferometry measuring system includes a beam divider, reference mirror, and the detector unit, and the ellipsometry measuring system includes a polarizer for polarizing an ellipsometry light beam and transmitting same onto the measuring region on the object surface as well as the ellipsometry sensor, which includes a polarization filter in order to determine the polarization state of a received sensor beam.

Claims

1. A device for measuring the profile of an object surface of a flat object comprising unknown materials, comprising a beam splitter for splitting a light beam of a light source, an optical interferometry measuring system and an ellipsometry measuring system for simultaneously measuring a measurement region on the object surface, and an analysis unit, wherein: a) the optical interferometry measuring system comprises: a beam divider for dividing an interferometry light beam of a light source into a reference beam and a measuring beam; a reference mirror for reflecting the reference beam; a detector unit for receiving and analyzing an analysis beam of the interferometry measuring system; wherein the measuring beam is directed to a measurement region on the object surface for reflection and, after reflection, is directed as an object beam to the beam divider; the reference beam is reflected at the mirror and directed as a mirror beam to the beam divider; and the object beam and the mirror beam interfere after impinging on the beam divider and are fed as an analysis beam to the detector unit for analyzing; b) the ellipsometry measuring system comprises: a polarizer for polarizing an ellipsometry light beam and routing the ellipsometry light beam to the measurement region on the object surface; an ellipsometry sensor having a polarization filter configured to analyze the polarization state of a receiving sensor beam; wherein the ellipsometry light beam is directed to the measurement region on the object surface and, after reflection at the measurement region of the object surface, impinges on the ellipsometry sensor as a sensor beam; c) the beam splitter is designed for splitting a light beam of the light source into the interferometry light beam and the ellipsometry light beam; and d) the analysis unit is designed to simultaneously process the analysis beam analyzed in the detector unit and the sensor beam received in the ellipsometry sensor and to determine the profile height in the measurement region on the object surface without explicitly modeling the layer system to be measured.

2. A system for measuring the profile of an object surface of a flat object comprising unknown materials, comprising a light source for generating a monochromatic light beam; the optical device according to claim 1; and a movement unit for carrying out a relative movement between the device and the object to be measured.

3. The system according to claim 2, wherein the movement unit moves the device and/or the object, wherein during a movement of the object, a holder for receiving the object is provided, which holder is moved by the movement unit.

4. The device according to claim 1, wherein the beam divider and/or the beam splitter are/is a semi-transparent mirror.

5. The device according to claim 1, wherein a lens is provided in the beam path which directs light beams such that the polarized ellipsometry light beam impinges on the measurement region on the object surface at a predetermined angle and the measurement beam impinges on the measurement region on the object surface at a right angle.

6. The device according to claim 1, wherein mirrors are provided in the beam path of the ellipsometry light beam which mirrors direct the ellipsometry light beam perpendicularly onto the polarizer and/or direct the sensor beam perpendicularly onto the ellipsometry sensors.

7. The device according to claim 6, wherein tube optics are provided in the beam path upstream of the detector unit, which focus the analysis beam on the detector unit, and/or tube optics are provided in the beam path upstream of the ellipsometry sensor, which focus the sensor beam on the ellipsometry sensor.

8. The system according to claim 2, wherein the light source is a laser diode.

9. The system according to claim 2, wherein the light source has a monitor diode by means of which the output intensity of the light source is determined and monitored.

10. The system according to claim 2, wherein a plurality of light sources is provided for generating monochromatic light beams, the light beams of which have different wavelengths and no wavelength of a light beam is an integral multiple of the wavelength of another light beam and the light beams of which are bundled by means of beam-shaping optics.

11. The system according to claim 10, wherein the system comprises a plurality of ellipsometry sensors for determining the polarization state of a receiving sensor beam.

12. The device according to claim 1, wherein the device comprises an optical separating element for wavelength-specific separating of the sensor beam, which is arranged in such a manner that separating the sensor beam takes place before the sensor beams impinge on the ellipsometry sensors.

13. The system according to claim 2, wherein the ellipsometry sensor comprises a plurality of polarization filters such that for analyzing the polarization state of a received sensor beam a plurality of polarization directions are recorded and analyzed.

14. The or system according to claim 13, wherein the optical separating element is a dichroic mirror.

15. The device according to claim 1, wherein the detector unit comprises a time-delayed integration camera.

16. The device according to claim 1, wherein the ellipsometry sensor comprises a time-delayed integration camera.

17. The device according to claim 1, wherein a correction of the measurement results of the detector unit is carried out in the analysis unit by means of an effective refractive index and/or effective absorption coefficient determined with the ellipsometry measuring system of the object to be measured, and/or by means of an effective layer thickness of a substitute layer which combines the effective optical effect of one or more layers of the object.

18. A method for detecting the surface profile of an object surface of an object comprising unknown materials by means of an interferometric measurement and a simultaneous ellipsometry measurement, comprising the following steps: emitting a monochromatic light beam towards an optical device by means of a light source; splitting the light beam of the light source into an interferometry light beam and an ellipsometry light beam by means of a beam splitter; carrying out an interferometric measurement with the interferometry light beam at a measurement region of the object surface by means of an optical interferometry measuring system; carrying out a measurement of a monitor signal of the light source to determine the output intensity of the light source; simultaneously with the interferometric measurement, carrying out an ellipsometry measurement at the measurement region of the object surface by means of an optical ellipsometry measurement system; calculating correction parameters using the results of the ellipsometry measurement and taking into account the output intensity of the light source; correcting of the measurement values obtained with the interferometry measurement by means of the correction parameters; determining the profile height of the measurement region at the surface of the unknown material of the object taking into account the corrected measurement values by means of an analysis unit.

19. The method according to claim 18, wherein the correction parameters comprise an effective refractive index and/or an effective absorption coefficient and/or an effective layer thickness of a substitute layer which combines the effective optical effect of one or more layers of the object of the just measured material combination at each measuring point.

Description

[0104] The invention is described and explained in more detail below with reference to some selected exemplary embodiments in connection with the accompanying drawings. In the figures:

[0105] FIG. 1 shows a schematic illustration of a system according to the invention for interferometric distance measurement with simultaneous recording of ellipsometry data;

[0106] FIG. 2 shows a schematic illustration of the polarization conditions of a light wave with the aid of the polarization ellipse;

[0107] FIG. 3 shows a schematic illustration of the light reflection at a 3-layer system consisting of environment/transparent layer/substrate;

[0108] FIG. 4 shows a schematic diagram of a radiometric ellipsometry measuring system with fixed polarizer, sample and analyzer;

[0109] FIG. 5 shows an illustration of the method steps for preparing an interferometry measuring system for interferometric distance measurement;

[0110] FIG. 6 shows an illustration of the method steps for carrying out an interferometric distance measurement by means of the interferometry measuring system; and

[0111] FIG. 7 shows a schematic illustration of a system according to the invention for interferometric distance measurement with simultaneous recording of ellipsometry data using 3 light wavelengths.

[0112] First, the measurement setup and beam path of a system according to the invention are described with reference to FIGS. 1 to 4 and 7 before the procedure for preparing a measurement with a preferred embodiment of the invention and the procedure for carrying out a measurement in FIGS. 5 and 6 are discussed.

[0113] FIGS. 1 and 7 show two different embodiments of a system 1 according to the invention for measuring the profile of an object surface 40 of a flat object 20 comprising unknown materials. The system 1 comprises an optical device 2 for measuring the profile of the object surface 20, a light source 111 for generating a monochromatic light beam, and a movement unit 10 for moving the object 20 relative to the device 2. While the embodiment of the system 1 according to FIG. 1 comprises only one light source 111, the system 1 according to FIG. 7 comprises three light sources 111. Accordingly, the devices 2 of the respective embodiments differ.

[0114] In both embodiments, the device 2 comprises an interferometry measuring system 4 with a beam divider 91, a reference mirror 60 and a detector unit 250 for receiving and analyzing an analysis beam 490. The ellipsometry measuring system 5 comprises a polarizer 190 and an ellipsometry sensor 220 for analyzing the polarization state of a received sensor beam 520. An analysis unit determines the profile heights of the object surface 40 of the object 20, e.g., a wafer, from the results of the measurements with the interferometry measuring system 4 and the ellipsometry measuring system 5. In doing so, the measurement results of the interferometry measurement are corrected with the correction parameters of the ellipsometry measurement to obtain reliable measurement values that take into account different and locally different materials of the object surface.

[0115] Wafers or other flat objects 20 are moved sequentially relative to the system 1 according to the invention and a camera of the system 1. In the present exemplary embodiment, the camera is stationary and the objects 20 are passed through below the camera by means of the movement unit 10. In another exemplary embodiment, preferably the camera is moved. In an alternative exemplary embodiment, preferably the relative movement is divided between the camera and the object 20 such that, for example, the camera performs the movement in an axial direction while the object 20 can be moved in the direction perpendicular thereto. The movement between the camera and the object 20 is preferably continuous.

[0116] The exemplary embodiment in FIG. 1 uses a laser diode 110 as light source 111, a TDI line scan camera 251 (TDI=time delayed integration) as detector unit 250 for the interferometry signal (here analysis beam 490) and a TDI multi-channel line scan camera 221 with integrated polarization filters (not shown) as ellipsometry sensor 220 for a sensor beam 520 formed as an ellipsometry signal. However, a different number of light sources 111 can also be used. Likewise, a broader-band light source 111 can alternatively be used in combination with narrow band filters. Also, instead of TDI sensors, simple (multi-channel) line scan sensors (without TDI method) or area scan cameras or a set of multiple line scan cameras can be used.

[0117] In the exemplary embodiment in FIG. 1, the light from the laser diode 110 is supplied to the measurement arrangement (device 2) through a fiber coupler 100. The light emerges from the fiber coupler 100 as a light beam 84. The transmission via fiber optics serves only to decouple the laser 110 from the actual recording system, thus, the device 2. Instead of transmission via fiber optics, the (laser) light beams 84 can also be directed directly into the recording unit (device 2) via suitable optics, which reduces losses but requires a higher adjustment effort. Beam shaping optics 80 are used to shape the light beam 84 into a parallel light bundle (light beam 400) having a cross-section adapted to the receiving surface. The light beam 400 (light bundle) is split into the two sub-bundles, namely an interferometry light beam 410 and an ellipsometry light beam 420, by a beam splitter 82 which is designed as a 50% partly transparent mirror. The split need not be 50/50. It can also be in other proportions and can be adjusted according to the sensitivity requirements of the beam paths and sensors.

[0118] The interferometry light beam 410 is directed by a beam divider 91, which is designed as a 50% partly transparent mirror 90, partly as a sub-bundle via a lens 50 onto the object surface 40 to be measured. This sub-bundle is the measuring beam 430. The other 50% of the interferometry light beam 410 passes through the partly transparent mirror as reference beam 440. The reference beam 440 then impinges on a reference mirror 60 where it is reflected. The resulting returning sub-bundle (mirror beam 450) is again reflected by 50% at the partly transparent mirror 90 and travels as sub-bundle 460 to an interferometry detector 252, namely to the detector unit 250.

[0119] The measuring beam 430 directed onto the object surface 40 of the wafer is focused onto the imaging area by the lens 50. Due to the reflection at the object surface 40, the measuring beam 430 is reflected back through the lens 50 as the object beam 470. Half of the light from object beam 470 passes through the partly transparent mirror 90 and continues to travel as bundle 480 to interferometry detector 252. The light from both beams 460 and 480 interferes with each other to form analysis beam 490. The analysis beam 490 is now modulated in intensity by the interference of the light waves, corresponding to the difference in distance between partly transparent mirror 90 and reference mirror 60 on the one hand and partly transparent mirror 90 and object surface 40 on the other. The analysis beam 490 is focused by tube optics 260 as a bundle 495 onto the TDI line scan camera 251. The detector unit 250 thus picks up the distance-modulated interferometry signal.

[0120] The ellipsometry light beam 420 is directed here through two 100% mirrors 70 and 72, through the polarizer 190, which is adjustable in its angular position, as a bundle 500 and through the objective 50 onto the object surface 40 at an average angle of incidence Φ. The light bundle 510 reflected from the surface 40 passes through the lens 50 and via the mirrors 74 and 76 as a sensor beam 520 to the tube optics 230 and is focused by the latter onto the 4-channel ellipsometry sensor 220. In the present example, the ellipsometry sensor 220 is a 4-channel TDI line sensor that has a polarization filter mounted in front of each of the 4 TDI blocks. The polarization filters are suitably oriented such that the polarization state of the incoming light (sensor beam 520) can be fully analyzed (for example, at angles 0°, 45°, 90°, and −45°. Thus, in each of the 4 detector elements forming a column of the ellipsometry sensor 220, four ellipsometry signals can be recorded such that the polarization state of the incoming light in this column can be fully determined from these four signals.

[0121] To monitor the intensity of light source 111, the monitor diode 140 is used to continuously monitor and measure the laser output power.

[0122] FIG. 7 shows an alternative arrangement according to the present invention, in which three wavelengths are used to improve the robustness of the measurement and/or to extend the unique working range. The three wavelengths are generated by lasers 110, 120 and 130 (laser diode) and their intensity is monitored by means of monitor diodes 140, 150 and 160. By means of a preferably spliced fiber coupler 100, the light beams of the individual laser diodes are combined and preferably bundled into a light beam 84. Deviating from the arrangement in FIG. 1, a multi-color TDI line scan camera 255 is used here in FIG. 7 for the interferometry measurement in order to record the signals separately for the three wavelengths. Another deviation is that the ellipsometry signals are separated for measurement by means of a separating element 600. Preferably, the ellipsometry signals are separated into the three wavelength components by means of two dichroic mirrors 610 and 620 and passed through the tube optics 232, 234 and 230 as beams 522, 524 and 528 and finally measured by means of the ellipsometry sensors 220, 222 and 224.

[0123] This representation is a principal arrangement which can, of course, be arranged in many ways. For example, the order of beam splitting (610, 620) into the wavelength components and focusing by means of tube optics (232, 234 and 230) can be reversed.

[0124] With reference to FIGS. 5 and 6, a preferred embodiment of the method according to the invention is described, which preferably first comprises some preparatory steps.

[0125] According to the invention, the following preparatory steps are provided for preparing the measurement (see FIG. 5): [0126] 1) Measuring the dark signal with the light source 111 switched off to determine the dark signal of the detectors; [0127] 2) Setting the light source 111 to a predetermined working brightness; [0128] 3) Measuring the reflection at a 100% mirror as a sample to determine the device constant Θ in Eq. (8); [0129] 4) Measuring the reflection at a 100% mirror or at a known homogeneous material as a sample for determining the device constant Θ.sub.e for the ellipsometry measuring system 5.

[0130] To carry out the actual measurement, preferably by means of the system 1 according to the invention, the following steps per measurement point are provided according to the invention. [0131] 1) Simultaneously measuring the interferometry signal, the monitor signal and the ellipsometry signals at the desired measurement point of the object surface, thus receiving and analyzing the analysis beam 490 and the sensor beam 520; [0132] 2) calculating ρ.sub.s and ρ.sub.p from the 4 measured intensity values of the ellipsometry channel and the monitor value for the light source 111; [0133] 3) calculating effective parameters n.sub.eff, k.sub.eff and d.sub.eff from ρ.sub.s and ρ.sub.p using the angle of incidence Θ.sub.0 of the ellipsometry channel and the given parameters n.sub.subst, k.sub.subst of the substrate of the object 20; [0134] 4) calculating ρ.sub.s for the perpendicular incidence of light of the interferometry channel from n.sub.eff, k.sub.eff and d.sub.eff; [0135] 5) calculating R and φ from ρ.sub.s for perpendicular incidence of light; [0136] 6) calculating the distance difference Δz between each of the reference reflector and the sample surface to beam divider 91 (divider mirror); [0137] 7) calculating the profile height at the measurement point (with respect to the distance of the reference mirror).

[0138] Following the measurement with the device 2 and the interferometry measuring system 4 and ellipsometry measuring system 5 comprised therein, the signal analysis is carried out in the analysis unit 700.

[0139] The following explanation of the calculation of the surface profile is carried out with only one wavelength to simplify the representation. If multiple wavelengths are used, the equations apply separately for each wavelength in an analogous manner. Likewise, the representation is carried out only for one pixel set at a time, i.e., for one line position of the line scan cameras used advantageously. The equations apply in an analogous manner for each pixel set at the respective line position. A pixel set here denotes the pixels belonging to the same line position in the interferometry camera and in the ellipsometry camera.

[0140] It is therefore possible to determine a multiplicity of height points of the profile simultaneously, depending on the sensor size and arrangement. For a line scan camera with 16384 points per line available today, this means 16384 height values for each readout clock of the cameras. Furthermore, with the device and system 1 described here, the line scan cameras can be moved continuously relative to the wafer (object). In accordance with the clock speed of the cameras, a corresponding number of lines with height information per unit time is obtained. Thus, at a clock rate of the proposed multi-channel TDI line scan cameras 221 of, for example, 100 kHz, more than 1600 million height values per second are obtained. Such cameras are offered by different manufacturers (e.g. by Vieworks and by Dalsa Teledyne). The use of such TDI multi-channel line scan cameras is a particularly suitable variant, since very high measurement speeds can be achieved. These cameras contain multiple (usually 4) TDI blocks in one camera, which can be operated and read out simultaneously. When using such cameras, the recording of the used wavelengths, which belong to one line on the wafer surface, is done successively. This means, while the first TDI block records the line area at λ.sub.1, the second TDI block determines the signal at λ.sub.2 and the third TDI block the signal λ.sub.3. For the following calculation, this time offset is in principle irrelevant. It is only necessary to assign and analyze the signal images obtained in a chronologically staggered manner (corresponding to the spatial offset of the TDI blocks). This arrangement is particularly suitable for combining high signal quality (high-resolution and robust measurement) with high speed. Alternative arrangements are explained below.

[0141] The measurement procedure and its preparation are illustrated in FIGS. 5 and 6. For a correct determination of the height values, the measurement must be prepared by means of a dark signal measurement and a determination of the transfer function of the optics and the sensor system.

[0142] In the dark signal measurement, the signal d is measured at each camera pixel y of the two camera lines (ellipsometry sensor 220 and detector unit 250) with the light source switched off. This determines the so-called dark noise of the camera, which is an offset for each further measurement and is subtracted from the signal. This is done for both sensor arrays 220 (index z=1 to 4) and 250 (index z=5).

[0143] d.sub.yz=read out dark value of signal at pixel y of sensor z

[0144] To determine the optical and electrical transfer function of the arrangement, a bright signal measurement h is performed with a 100% mirror. For this purpose, the wafer surface or object surface 40 is replaced by a plane mirror with known reflection properties. Since the signal h at each sensor pixel is uniquely determined by the intensity of the light source 111 (signal value q), the transfer function M, the reflectance of the mirror R, the path difference I.sub.z of the two interfering light beams 460 and 480 (impinging on detector unit 250=sensor, z=5) and the dark signal d, the transfer function M can be determined for each pixel y in each of the sensor lines z if the values h, q, r, and d are known.

[0145] For the interferometry sensor, thus, the detector unit 250 (z=5), this is a function of the path difference I.sub.z. For the ellipsometry sensors 220 (z=1 to 4), M is independent of the sample height as long as it is within the focal range. The transfer function is generally different for each pixel y and each sensor z. It is determined by the sensitivity of each pixel, by illumination, material properties, coatings, and aberrations of the optics.

[0146] In order to check the output intensity of the laser light source 111 and to include it in the calculation as a correction or reference value, the signal q of the monitor diode 140 (on the side of the laser facing away from the output) usually installed in each laser module 110 is used directly.

[0147] The signal h of the brightness reference measurement for the interferometry sensor (detector unit 250) is:

h.sub.yz,href(l.sub.z)=q.sub.href*M.sub.yz(l.sub.z)*R.sub.href+d.sub.yz   (46)

[0148] Wherein:

[0149] h.sub.yz,href(l.sub.x)=measurement value of reference measurement at pixel y of sensor z

[0150] (interferometry sensor 250 [z=5], ellipsometry sensor 220 [=1 . . . 4])

[0151] as function of path difference l.sub.z

[0152] q.sub.href=light intensity of laser source during reference measurement

[0153] M.sub.yz(l.sub.z)=transfer function at pixel y of sensor z as function of path difference l.sub.z

[0154] R.sub.href=reflectance o f mirror in reference measurement (known material) Here, the argument I.sub.z denotes the path difference the two interfering bundles 460 and 480 (impinging on sensor 250, z=5) have to each other. The reflectance R can be taken as a real value since for the reference measurement, advantageously, a mirror is chosen which does not generate a phase shift due to layer references. If, for example, a mirror with a protective coating is to be used to improve long-term stability, the phase offset generated by the protective coating has no effect since the reference measurement is only used to determine the maximum of the transfer function. The following holds true for the ellipsometry sensor 220:

h.sub.yz,href=q.sub.hrefM.sub.yz*R.sub.href+d.sub.yz   (47)

[0155] For the ellipsometry channel, it should be noted that a phase offset at the mirror, e.g. due to a possible protective coating, must be taken into account mathematically in the determination of the transfer function.

[0156] To determine the distance of the wafer surface from the sensor arrangement z.sub.sample, the transfer function M(I) is split into a non-interfering factor M.sup.max and the interference effect. The factor M.sup.max is obtained by determining the maximum for each transfer function M(I). The intensity modulation due to the interference can then be determined directly from the values of the measurement sequence.

[0157] The following transfer function holds true for the sensor of the detector unit 250:

[00020]$\begin{array}{cc}{M}_{yz}\left(l_z\right)=M_{yz}^{\max }\star \left(1+\cos \left[\frac{2\pi }{\lambda /2}*\left(z_{\mathrm{mirror}}-z_{\mathrm{sample}}\right)\right]\right)& \left(48\right)\end{array}$

[0158] And for the ellipsometry sensor 220, M.sub.yz is constant within the focus range.

[0159] Wherein:

[0160] M.sub.yz(l.sub.z)=transfer function at pixel y of sensor z as a function of the path difference l.sub.z

[0161] M.sub.yz.sup.max=maximum value of the transfer function at pixel y of sensor z

[0162] z.sub.mirror=distance divider mirror—reference mirror of the interferometer

[0163] z.sub.sample=distance divider mirror—wafer surface

[0164] This results in the following for the interferometry sensor (detector unit 250):

[00021]$\begin{array}{cc}{M}_{yz}^{\max }=\frac{h_{\mathrm{yz},\mathrm{href}}\left(l_{zf\stackrel{.Math.}{u}r{h}_{\max }}\right)-d_{yz}}{q_{\mathrm{href}}*R_{\mathrm{href}}}& \left(49\right)\end{array}$

[0165] Wherein:

[0166] l.sub.z for h.sub.max=path difference, for which the signal at sensor z is a maximum=l.sub.z is an integer multiple of the wavelength λ

[0167] M.sub.max is determined by continuously changing the distance Z.sub.sample of the mirror used for the measurement and thus passing through a full wavelength period while recording the signal and determining the maximum.

[0168] In the measurement cycle with the unknown wafer surface (object surface 40) to be examined, the sensor signals i.sub.y1 to i.sub.y4 of the ellipsometry camera sensors 220 [z=1 . . . 4] and i.sub.y5 of the interferometry sensor (detector unit 250) [z=5] are now recorded simultaneously and the output intensities q of the laser light source 111 are measured by means of the installed monitor diode 140.

[0169] The following holds true for the signals at the sensor blocks in the camera (ellipsometry sensor 220):

i.sub.yz=q*M.sub.yz*r.sub.yz,wafer+d.sub.yz   (50)

[0170] Wherein:

[0171] i.sub.yz=measurement value ellipsometry/interferometry at pixel y of sensor z

[0172] q=irradiated light intensity

[0173] M.sub.yz=transfer function at pixel y of sensor z

[0174] r.sub.yz,wafer=reflection coefficient of the wafer at the location of pixel y at polarization angle z

[0175] d.sub.yz=read out dark signal value at pixel y of sensor z

[0176] According to equations (30) to (33), the reflection coefficients r.sub.y,wafer result here from the material properties of the sample and at the correspondingly selected polarization angles. The quantities n.sub.1,y=n.sub.eff,y, k.sub.1,y=k.sub.eff,y and d.sub.1,y=d.sub.eff,y describing the sample material are now calculated from these measurements with the aid of equations (34) to (37) and (13) to (19), wherein the intensity values I.sub.d,z =I.sub.d,polarization angle transformed with q=I.sub.0 are substituted into the system of equations (34) to (37):

[00022]$\begin{array}{cc}{I}_{d,z}/I_0=\frac{i_{yz}-d_{yz}}{M_{yz}}& \left(51\right)\end{array}$

[0177] The material values determined in this way can now be used according to equation (43) to determine p.sub.s,y5, the complex reflection coefficient for perpendicular light incidence at the location of pixel y of the interferometry sensor [z=5]. Finally, according to equations (40) and (41), the (real) reflectance R.sub.y and the phase offset φ.sub.y can be determined from ρ.sub.s,y5 for each pixel location y. The following thus holds true for the intensities measured at the detector unit 250:

[00023]$\begin{array}{cc}{i}_{y5}=q*R_y*M_{y5}^{\max }\star \left[1+\cos \left(\frac{2\pi }{\lambda /2}\left(z_{\mathrm{mirror},y}-z_{\mathrm{sample},y}\right)-{\phi }_y\right)\right]+d_{y5}& \left(52\right)\end{array}$

[0178] From this, finally, the sought quantity Δz.sub.y=z.sub.mirror,y−z.sub.sample,y=the difference of the distance of the sample surface 40 to the distance of the reference mirror 60 from the (divider) mirror 90 can be calculated:

[00024]$\begin{array}{cc}\Delta z_y={\phi }_y+\frac{\lambda /2}{2\pi }\mathrm{arc}\cos \left[\frac{i_{y5}-d_{y5}}{q*R_y*M_{y5}^{\max }}-1\right]& \left(53\right)\end{array}$

[0179] If, as shown in FIG. 7, multiple, e.g., preferably three wavelengths are used to increase the measurement region and/or improve the robustness of the measurement, the calculation shown up to this point must be performed separately for each wavelength. The distance difference values Δz.sub.xy are first determined in units of the respective half wavelength λ.sub.x from Eq. (51), where x is the index of the wavelengths. They can then be represented according to the so-called method of exact phase fractions in each case as the sum of integer fractions δ.sub.x and the fractional remainder f.sub.x:

[00025]$\begin{array}{cc}\Delta z_{yx}=\frac{{\lambda }_x}{2}*\left({\delta }_x+f_x\right)={\phi }_{xy}+\frac{{\lambda }_x/2}{2\pi }\mathrm{arc}\cos \left[\frac{i_{xy5}-d_{xy5}}{q_x*R_{xy}*M_{xy5}^{\max }}-1\right]& \left(54\right)\end{array}$

[0180] The sought distance difference Δz.sub.y is determined from the Δz.sub.yx by determining the triple δ.sub.x of integer fractions for which the mean deviation of the associated Δz.sub.yx from the respective mean value is minimal and then calculating the mean value for the 3 remaining fractional fractions.

[0181] By suitable selection of the wavelengths λ.sub.1, λ.sub.2 and λ.sub.3 for x=1, 2, 3, a working range of 0.5 mm with unique assignment of the intensity measurement value triples i.sub.xy5 to the distance difference Δz.sub.y can be established without difficulty, which is sufficient for a multiplicity of profile measurement tasks. This is explained in detail in the aforementioned publication by K. Meiners-Hagen, R. Schrödel, F. Pollinger and A. Abou-Zeid. In the arrangement disclosed therein, for example, the wavelengths 532, 632 and 780 nm are used and a working range of 0.6 mm with unique assignment of the distance difference is achieved.

[0182] Further preferred embodiments of the invention are described below. These alternative preferred embodiments are part of the invention without loss of generality.

[0183] Thus, the use of TDI technology in the proposed arrangement serves only to improve the signal-to-noise ratio and is not absolutely necessary for the principle according to the invention. Therefore, for simpler measurement configurations and requirements, instead of using TDI multi-channel camera line sensors as ellipsometry sensors 220, 222, 224, an arrangement of 4 independent line sensor cameras or of 4 independent, single-channel TDI line sensor cameras can also be selected.

[0184] Alternatively, common area scan cameras can also be used, or TDI single channel line scan cameras operated in area readout mode. In such an arrangement, the clock frequency is reduced accordingly to, e.g., a clock of 1 kHz (for the described area readout mode of a TDI single-channel line scan camera) and more than 16 million height values per second are still obtained.

[0185] It is understood that when using area scan cameras or TDI single channel line scan cameras in area readout mode, the three wavelengths λ.sub.1, λ.sub.2 and λ.sub.3 are mapped on the camera sensors to different lines x=1, 2, 3. The assignment of the obtained signal images to each other for the three wavelengths, which each look at the same point on the wafer surface 40, is carried out here by a spatial assignment of the areas of the camera sensor. For the analysis shown in principle above, it is irrelevant whether the assignment is made spatially (for area sensors) or in a chronologically staggered manner (for line sensors).

[0186] It should be noted that for applications of high lateral resolution (e.g. in the single-digit pm range), a lateral and rotational correction for the recorded signal images is necessary anyway due to the use of several cameras since an adjustment of the entire measuring arrangement to an offset of the camera of less than 1 μm is hardly achievable in a mechanical manner. Such mathematical correction can usually be made possible by capturing a reference pattern from which the exact location on the wafer viewed by each pixel can be determined.

[0187] In further possible embodiment variants, the number of wavelengths used can be adapted to the required working area. For particularly small working areas, a configuration with only one wavelength is possible, or with two wavelengths for small areas. For larger working areas or an improvement of the reliability by redundancy of the measurement, the extension to three or more wavelengths is suitable, which can be implemented in particular with the above mentioned multi-block TDI cameras (cameras with 7 TDI blocks have already been presented).

[0188] The illumination can suitably be provided by continuous monochromatic light sources. Lasers are suitable for this purpose, as are other broadband beam sources combined with appropriate interference filters. The only condition is that the coherence length of the light used is sufficiently large for the working range to be implemented.

[0189] The combination of the radiation when using different wavelengths can be achieved by spliced optical fibers or dichroic mirrors. Spliced optical fibers are optical fibers that are fused together in a certain section or are routed close to each other so that the light passes from one optical fiber to the other optical fiber. In the case of dichroic mirrors, only the light of a certain wavelength range is reflected while the light of at least one other wavelength range is transmitted. Such mirrors can be dimensioned for use at different angles. The most common use occurs at 45°. With a suitable arrangement, the outgoing reflected light and the transmitted light can thus have the same direction of propagation.

[0190] For the separation of light beams of different wavelengths in the ellipsometry beam path, one or more prisms, gratings or other spectral dispersive means can be used instead of dichroic mirrors. The spatial separation allows the signals to be recorded simultaneously, which allows a high measurement speed. However, temporal separation can also be done by carrying out the measurement sequentially at the different wavelengths, wherein a longer measurement time is accepted.

[0191] Likewise, instead of the beam divider 91 or a mirror 90, a glued prism with a square cross-section can be used, which is provided with a partially reflecting layer in the glued 45° surface. When such a prism is used, the reference arm can be created by vapor depositing a mirror coating directly onto the outer surface of the prism. Although this arrangement leads to a reduction of the interference contrast due to the significantly different dispersion in the reference and measuring arm of the interferometer, it is advantageous for simple requirement due to its robustness.

[0192] In particular, it can be provided that the optical arrangement for generating interference comprises a semi-transparent mirror which reflects part of the radiation at a first surface towards the object surface and transmits another part of the radiation. The reflected radiation can then be combined with the transmitted beam. This can be done, for example, by reflecting the transmitted beam back into itself at a mirror and coupling it into the reflected beam at the semi-transparent mirror. Such an arrangement is called a Michelson interferometer. However, other arrangements are also possible and usable, such as a so-called Mach-Zehnder interferometer, in which the irradiated light is split and recombined after reflection of a sub-beam at the sample surface.

[0193] Further preferred variants are already described in the patent application EP19188318, which is to be included herein in its entirety as a reference.

LITERATURE

[0194] (1) “Multi-Wavelength Interferometry for Length Measurements Using Diode Lasers”, K. Meiners-Hagen, R. Schrödel, F. Pollinger, A. Abou-Zeid, Measurement Science Review, Vol. 9, Sect. 3, Jan. 11, 2009 [0195] (2) “Handbook of Elliposmetry”, Harland G. Tompkins, Eugene A. Irene, Springer 2005, ISBN 0-8155-1499-9