Optical Thickness Measurement Device

Abstract

The invention relates to an optical thickness measurement device having a light source, a measurement head, an optical spectrometer with optical components for dispersing input light and a detector, and an evaluation device. The light source is connected to the measurement head and generates at least low-coherence measurement light and directs it to the measurement head, the measurement head is connected to a spectrometer and directs the measurement light onto a measurement object and directs reflected light resulting from two different surfaces of the measurement object as input light to the spectrometer, the spectrometer is connected to an evaluation device and generates a spectrum of the reflected light and sends the spectrum as an electrical signal to the evaluation device, the evaluation device being configured to measure the distance between the two surfaces, i.e. the thickness of the measurement object. According to the invention, the measurement light has a first and a second wavelength range, the spectrometer has two optical inputs of reflected light, which are spatially separated from each other, and both wavelength ranges of reflected light are dispersed by a common optical component due to the distance between the optical inputs and imaged onto the detector.

Description

[Technical Field]

[0001] The present invention relates to an optical thickness measuring device comprising a light source, a measuring head, an optical spectrometer with optical components for spectral splitting of the input light, a detector and an evaluation device. [Background technology]

[0002] In the manufacture of semiconductor wafers, the post-cutting grinding process must ensure that the wafers have an accurate absolute total thickness and a minimum required thickness distribution within the wafer. To check the grinding process, the wafer thickness is measured during grinding. Because thickness can decrease significantly during grinding, the measurement range covered by the measurement process is quite large.

[0003] Systems for measuring wafer thickness are known that measure wafer thickness during the grinding process using spectral interferometry. Such systems generally include a light source, a measurement head, and an optical spectrometer. The measurement head directs light from the light source onto the wafer to be measured and receives the light reflected from it. The reflected light is fed to the spectrometer, which splits it according to its wavelength components. This allows the optical spectrum of the reflected light to be measured. The measurement results are analyzed in an evaluation unit, and the wafer thickness is determined.

[0004] Existing systems are usually only capable of measuring either the initial thick wafer or the thin wafer produced after the grinding process. This is mainly due to the fact that, at the initial thickness of the wafer, the light from the light source must have a wavelength in the infrared range. Wafers (e.g., silicon) are almost opaque in the visible spectrum, or the visible spectrum has only a low penetration depth. At the same time, light sources emitting light in this spectrum are not broadband enough to provide sufficiently good accuracy for thinner layers, where measurement light in the visible range provides significantly higher accuracy.

[0005] This typically requires the use of two separate interferometer systems, with the associated higher costs and greater effort in terms of calibration and synchronization of the equipment.

[0006] For example, even if the evaluation unit or the measuring head is partially integrated, most components are still required in duplicate, which only partially solves the above-mentioned problems. Summary of the Invention

[0007] The object of the present invention is to at least alleviate the above-mentioned drawbacks and in particular to provide an optical thickness measuring device for measuring a wide range of coating thicknesses, which covers a large measuring range and at the same time has a compact and inexpensive design.

[0008] This object is solved by an optical thickness measurement device according to independent claim 1. Further embodiments of the invention are set out in the dependent claims.

[0009] The optical thickness measurement device according to the present invention comprises a light source, a measurement head, an optical spectrometer, an optical component for dispersing input light, and a detector, and an evaluation unit.

[0010] The light source is optically connected to the measurement head, e.g., by an optical waveguide, and is configured to generate at least low-coherence measurement light and transmit it to the measurement head, e.g., via the above-mentioned optical waveguide. The term "optically connected" is intended here and below to include both optical waveguide-based transmission of light, e.g., via a fiber, and free-beam-based transmission.

[0011] The measurement head is configured to direct the measurement light onto a measurement object, e.g., a wafer. This can be done, for example, as a free beam through air or a corresponding medium, such as water, oil, acid, or other liquid used in wafer processing. Furthermore, the measurement head is configured to collect light reflected from the measurement object originating from at least two different surfaces of the measurement object and transmit it as input light to the spectrometer, e.g., via an optical waveguide. The different surfaces can be, for example, the front and back surfaces of a wafer, or different optical interfaces in general.

[0012] The spectrometer is electrically connected to the evaluation device and configured to generate an optical spectrum of the interference of reflected light arising from at least two different optical interfaces using an optical component, convert it into an electrical signal using a detector, and transmit the electrical signal to the evaluation device.

[0013] The evaluation device is configured to measure the distance between at least two interfaces (e.g., the thickness of the object or layer of the object). The thickness is determined by evaluating the modulation of the interference caused by the difference in the optical path between the interfaces, for example, using a Fourier transform. The optical thickness thus determined is then calculated back to the geometric thickness using the known refractive index of the material.

[0014] According to the present invention, there is provided a spectrometer having measurement light having first and second wavelength ranges and two optical inputs for reflected light, such that the reflected light of the first wavelength range passes through the first optical input and the reflected light of the second wavelength range passes through the second optical input, the optical inputs being spatially separated such that both wavelength ranges are dispersed by a common arrangement and imaging fields on the detectors overlap in the direction of dispersion.

[0015] The wavelength range is preferably low coherent, i.e. polychromatic light.

[0016] In addition to the first and second wavelength ranges, a third or more wavelength ranges may also be used, so that the most appropriate wavelength range for measuring the interface distance can be used in each case.

[0017] In a preferred embodiment of the present invention, provision is made for switching wavelength ranges, in particular switching back and forth in a fixed cycle.

[0018] Switching between individual wavelength ranges can be performed at switching speeds in the kHz range, for example, between 0.5 kHz and 100 kHz. Such fast switching speeds allow quasi-simultaneous measurements in several wavelength ranges, and the change in distance between two measurements, especially between layers such as wafer thickness, is small compared to the measurement accuracy.

[0019] A common optical component that separates light can be, for example, a dispersive optical element. A dispersive optical element is an optical element whose optical properties important to its function, such as refractive index or diffraction angle, exhibit significant dispersion, and the dispersion is desirable for its function. Thus, an ordinary glass lens is not a dispersive optical element, even if its refractive power is somewhat wavelength-dependent. This differs from the use of a dispersive prism or diffraction grating, which exhibit strong dispersion and are designed to refract or diffract different wavelengths of light to different degrees.

[0020] During spectral splitting by optical components, diffraction, reflection, or refraction occurs depending on the wavelength, just as the incident point after focusing the diffracted / reflected / refracted light depends on the wavelength. Conversely, by selecting an appropriate incident point, the spatial shift caused by the two different wavelength ranges can be at least partially compensated for, allowing a single optical component and a single detector to be used for the two different wavelength ranges.

[0021] Preferably, one wavelength range is assigned to each of the optical inputs, which, as already mentioned, makes it possible to at least partially compensate for the different diffraction / reflection / refraction angles caused by the different wavelength ranges and to create at least a partial overlap of the spectroscopic beam paths.

[0022] This allows for a particularly compact spectrometer design, which on the one hand saves costs and allows particularly stringent installation space requirements to be met. On the other hand, such a compact design also benefits the spectrometer's accuracy: the smaller the dimensions of the spectrometer's optical elements (e.g., lens diameter), the easier it is to achieve virtually error-free images across the entire spectral range of both wavelengths. The better the imaging quality, the higher the modulation contrast and therefore the quality of the measurement results. In this way, the required space for the combining and separating optics is as similar as possible and therefore minimal. In addition, the overlapping spectra minimizes the required detector length or allows the splitting to be performed over more detector pixels, which improves resolution.

[0023] The term optical input here should not necessarily be understood as an input attached to the external housing, but rather as the point of entry of the respective light into the beam path of the spectrometer.

[0024] In a preferred embodiment, the light source comprises at least a first light source unit and a second light source unit.

[0025] Preferably, the measurement light generated by the two light source units is coupled to the measurement head via an optical connection, for example, an optical fiber. It is particularly preferred that the measurement light from each light source unit is coupled via its own optical fiber. It is particularly preferred to use different types of fiber for the two wavelength ranges. The fiber type can be adapted to the wavelength range in terms of its transmission characteristics, for example, single-mode or multimode fiber. The two optical fibers can be enclosed in a common cladding. Alternatively, the measurement light from both light source units is coupled via a common optical fiber.

[0026] Alternatively, a separate measurement head may be provided for each wavelength range, and each measurement head may be connected to a single light source unit via, for example, a single optical waveguide.

[0027] Regardless of the design of the separate light source units, the wavelength range can be in the visible and near-infrared range (VIS and NIR), particularly 400 nm to 1600 nm. For example, the first wavelength range can be 430 nm to 700 nm. The second wavelength range can be, for example, in the range of 700 nm to 1600 nm, particularly a subrange of about 830 nm to about 930 nm, about 870 nm to about 970 nm, or about 950 nm to about 1100 nm.

[0028] The distance measured in the above wavelength ranges can be, for example, 0.5 μm to 10 μm in the VIS range (visible range) and, in the NIR range, for example, up to a silicon thickness of 150 μm.

[0029] Preferably, the bandwidth of the first light source unit is different from the bandwidth of the second light source unit. In particular, the first wavelength range is broadband and the second wavelength range is relatively narrow. The narrower wavelength range enables measurement of thicker wafers, while the broadband wavelength range provides better accuracy for thin wafers.

[0030] In a preferred embodiment, the first light source unit is a light emitting diode (LED) and the second light source unit is a superluminescent diode (SLD).

[0031] The narrow wavelength range is preferably a long wavelength range, and the broad wavelength range is preferably a short wavelength range.

[0032] Instead of using two separate light sources as the light source unit, it is also possible to use a single light source whose spectral range spans both wavelength ranges, and separate the wavelength ranges by means of a filter or a dichroic beam splitter.

[0033] In one embodiment, the light source may be configured to generate measurement light in a first wavelength range alternately with measurement light in a second wavelength range. In this way, it is possible to quickly switch between the two measurement ranges. Therefore, the readout of the spectrometer or the evaluation of the electrical signal by the evaluation device can be performed synchronously, for example, in a fixed cycle.

[0034] In an advantageous embodiment, the first light source unit can be switched independently of the second light source unit.

[0035] It is also conceivable to use a first wavelength range in a first period, alternate between the first and second wavelength ranges in a second period, and use the second wavelength range in a third period, which provides the possibility of emitting only the corresponding wavelength range when measuring a thickness that is already optimally covered by a certain wavelength range.

[0036] In a measurement range that is equally covered by both wavelength ranges, the emission of the two wavelength ranges and the associated evaluation can be recorded alternately, which provides the possibility to calculate, for example, weighted thickness values ​​resulting from each other, thereby achieving a higher measurement accuracy than when using a single wavelength range.

[0037] If two spectra are generated for one thickness value during measurement, the thickness value can be calculated from the two partial spectra. If two measurement spectra with different bandwidths are used, the narrower spectrum for thicker wafers provides greater accuracy, and the broader spectrum for thinner wafers.

[0038] The thickness value calculation can, for example, provide a statistical weighting of two partial spectra: a narrowband spectrum provides greater accuracy for thicker wafers, and a broadband spectrum for thinner wafers. The calculable thickness value is always based on the partial spectrum that best suits the current thickness.

[0039] Preferably, this calculation is not based on a fixed threshold, but rather on a weighted average, for example. A transition range can be defined within which the weighting depends on the approximate position within the transition range in which the current thickness being measured is located, and / or the weighting can depend on the last calculated value and / or on the two measurements.

[0040] Alternatively or additionally, the weighting of two measurements for the same thickness can be performed based on the determined quality of the individual measurements: the measurement peak height (corresponding to the amplitude of the interferometric modulation) or any measure of the statistical noise of the values ​​(e.g., change over time) can be used as a measure of quality.

[0041] This object is also solved by the methods set forth in the independent claims.

[0042] The method according to the invention is used to measure the distance between two boundary surfaces of an object to be measured and comprises the following steps: The method includes generating measurement light having a first wavelength range, directing the measurement light onto a measurement object, collecting light reflected by the measurement object and generating a spectrum of the reflected light using interferometric modulation, repeating the above steps using measurement light of a second wavelength range in which the first and second wavelength ranges are at least partially different, measuring a first interface distance value using the spectrum of the measurement light in the first wavelength range, measuring a second interface distance value using the spectrum of the measurement light in the second wavelength range, and calculating an interface distance using the first and / or second interface distance values. The interface distance value is a measurement of the distance between two optical interfaces, particularly the thickness of a layer between the two optical interfaces. The first and second wavelength ranges can be evaluated one after the other, alternately or simultaneously.

[0043] In this way, the distance between two interfaces can be measured continuously with high accuracy over a wide range. High accuracy can also be achieved in distance ranges where the intensity or quality of one or both of the light sources is lower, but measurements from both measurement light ranges can be used.

[0044] In a preferred embodiment of the method, interference of reflected light occurs either between the reflected light of the interface of the object to be measured and a reference light, and / or between the reflected light of the first interface of the object to be measured and the reflected light of the second interface of the object to be measured. During the interference between the reflected light of the interface and the reference light that has traveled a known or at least time-constant path length, an absolute distance value can be calculated. In the case of interference between the reflected light of two interfaces, a distance value between the two interfaces can be calculated.

[0045] Averaging is performed when calculating the interface distance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0046] [Figure 1] 1 is a schematic diagram of an apparatus for measuring thickness according to the prior art; [Figure 2] 1 is a first embodiment of an optical thickness measurement device. [Figure 3] Various operating states of the optical thickness measurement device according to Figure 2. [Figure 4] Various operating states of the optical thickness measurement device according to Figure 2. [Figure 5] 2 shows a second embodiment of an optical thickness measurement device with a common measurement spot. [Figure 6] 3 shows a third embodiment of an optical thickness measurement device having an exclusively fiber-based light guide. [Figure 7] 4 shows a fourth embodiment of an optical thickness measurement device having a two-line detector. [Figure 8] 5 shows a fifth embodiment of an optical thickness measurement device having a reference arm. [Figure 9] An embodiment of the method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0047] Measurement principle and problem definition FIG. 1 shows a schematic diagram of a prior art measurement device 10. A measurement light source 12 generates measurement light 14, which is directed onto a measurement object 19 via a beam splitter device 16, e.g., a beam splitter cube or a fiber coupler, and via a measurement head 18. In FIG. 1, a portion of the measurement light 14 reflected by a first boundary surface 20 or a second boundary surface 22 of the measurement object 19 is indicated by a black arrow and is given the reference number 14′. The reflected measurement light 14′ is picked up by the measurement head 18 and directed by the beam splitter device 16 onto a spectrometer 24. The spectrometer 24 includes a dispersive optical element 26, which may be, for example, a diffraction grating or a dispersive prism.

[0048] Furthermore, the spectrometer 24 includes a detector 28 which includes a plurality of photosensitive cells 30, which are arranged along a line or a curve and are hereinafter referred to as pixels. The signals generated by the pixels are evaluated by an evaluation device 32 in order to calculate a distance value between the two surfaces 20, 22.

[0049] During measurement, the reflected measurement light 14' is deflected by a dispersive optical element 26, the deflection angle depending on the wavelength of the reflected measurement light 14'. In a measurement setup where the reflected measurement light 14' from one interface 20 interferes with the measurement light reflected from another interface 22, a broad spectrum is obtained on a spectrally modulated detector 28. The detector 28 then records a number of intensity maxima, and a modulation frequency is assigned to each distance between the first and second interfaces 20, 22. The desired distance can be calculated from the signal generated by the detector 28 by means of a Fourier transform, as is known per se in the prior art.

[0050] First embodiment 2 shows a schematic diagram of a first embodiment of an optical thickness measurement device 100. The thickness measurement device 100 comprises a light source 112, a measurement head 114, an optical spectrometer 116, and an evaluation device 118.

[0051] The light source 112 is configured to generate low-coherence light in at least two different wavelength ranges or frequency bands. At least one of the two wavelength ranges is advantageously broadband, i.e., the emitted light covers the entire continuous range of wavelengths, e.g., a range of 100 nm or more. To generate this light, the light source 112 in the embodiment shown in FIG. 2 comprises two light source units 120, 122. In the illustrated embodiment, one light source unit 120 comprises a light-emitting diode as a radiation source, and the other light source unit 122 comprises a superluminescent diode (SLD) as a radiation source. Exemplary wavelength ranges are 430 nm to 700 nm, 830 nm to 930 nm, 870 nm to 970 nm, or 950 nm to 1100 nm.

[0052] The light emitted by the light source units 120, 122 is guided to the measurement head 114 via two separate waveguides (first optical fiber 124 and second optical fiber 126 in FIG. 2). The measurement light coupled into the measurement head 114 is directed to the surface of the measurement object 130 via appropriate optics 128. Light of one wavelength range or one light source unit is guided into each of the fibers. Thus, the wavelength ranges are guided through separate fibers.

[0053] A portion of the measurement light is reflected from a first surface 132 of the measurement object 130, and a second portion is reflected from a second surface 134. To keep the illustration clear in Figure 2, the reflection method is shown only as an example on the first surface 132. A separate measurement spot is generated on the surface of the measurement object 130 for each wavelength range guided in the separate fibers.

[0054] A portion of the light reflected from the two surfaces 132 , 134 is then coupled into the measurement head 114 where it is coupled into one of the fibers 136 , 138 and thus reaches the spectrometer 116 .

[0055] Thereby, the measurement light originating from the first fiber 124 and reflected by one of the surfaces 132, 134 of the measurement object 130 is imaged again by the optical system 128 at the fiber end of the first fiber 124. Advantageously, the sensing head 114 comprises a beam splitter cube 129, by means of which the return light reflected from the measurement object 130 is at least partially deflected and imaged onto the end of another fiber 136, which is arranged conjugate to the end of the first fiber 124. This light is therefore only coupled into the fiber 136. The same applies to the measurement light originating from the second fiber 126 and reflected by the measurement object 130, which is coupled into a fiber 138, the end of which is arranged conjugate to the end of the second fiber 126.

[0056] Because fibers 124, 126 carry different wavelength ranges, the wavelength ranges coupled to fibers 136, 138 are also different, without the need for additional filtering or switching. Optical fibers 136, 138 are connected to spectrometer 116 such that two spatially separated optical inputs 140, 142 are provided to fibers 136, 138. In certain embodiments, the optical inputs can be spaced apart, for example, by 1-30 mm, preferably 15 mm. In spectrometer 116, the reflected light coupled into spectrometer 116 via the two optical inputs 140, 142 passes through the same spectrometer optics, shown here by optics 144, 146, and, by way of example, a reflective grating 148.

[0057] Instead of the reflective grating 148, a grating operating in transmission or a prism may also be provided.

[0058] The reflective grating 148 disperses the reflected light, and the result of the dispersion is imaged onto a detector 150. The detector 150 allows for position-dependent detection of the intensity distribution and may be, for example, in the form of a line, with, for example, cells or pixels as described above.

[0059] In an exemplary embodiment not shown for clarity, the diffraction grating may be positioned such that imaging of the light input to the diffraction grating and imaging from the diffraction grating to the detector are performed by the same optical system, i.e., optical systems 144 and 146 are coincident.

[0060] Detector 150 detects the measured light intensity as a function of position, and therefore as a function of wavelength due to splitting by an optical component such as reflective grating 148 .

[0061] As mentioned above, light from the two different optical waveguides 136, 138 passes through the same optical system of the spectrometer 116. The light sources 120, 122 are alternately switched on and off so that only light from a single wavelength range reaches the active surface of the detector 150. The detector 150 can be read out synchronously with the on / off switching of the light sources 120, 122, so that the spectrum detected in this way can be unambiguously assigned to the light sources 120, 122.

[0062] The detector 150 or a detector line thereof generates a corresponding signal from the light spectrum which is read out via the evaluation device 118. The evaluation device 118 is connected to the detector 150 via an electrical connection 152.

[0063] Figures 3 and 4 show schematic diagrams of a portion of Figure 2 to illustrate various operating conditions. In Figure 3, reflected light assigned to a first wavelength range (here, e.g., 430 nm to 700 nm) is directed to an optical input 142 via a waveguide 138. Starting from the input 142, the reflected light is collimated via a first optical system 144 and directed onto a reflective grating 148. From there, it is dispersed onto a second optical system 146, i.e., with a wavelength-dependent reflection angle, and imaged onto a line on a detector 150. As shown in Figure 3, depending on the wavelength of the reflected light, locally different intensities are present on the detector 150. The beam paths are shown (schematically) for two different wavelengths in the first wavelength range, with longer wavelengths shown as dashed lines. Light striking the detector 150 covers a specific region of the detector's active surface. Thus, a locally resolved spectrum of the reflected light appears on the detector line 150.

[0064] On the other hand, as shown in FIG. 4, if light in a second wavelength range (here, e.g., 830 nm to 930 nm) is supplied to optical input 140 via fiber 136, this reflected light also couples into optics 144, from which it is collimated and imaged onto reflective grating 148. Different emission positions of optical input 140 away from the emission position of optical input 142 result in different angles of incidence on reflective grating 148. This angle of incidence on reflective grating 148 is selected to compensate for the different emission angles resulting from the different wavelength ranges of the reflected light for the light dispersed by grating 148, and detector 150 can display the intensity distribution across the position of the detector line via optics 146 as a function of the wavelength distribution of the reflected light.

[0065] In FIG. 4, light in the 830-930 nm range is coupled into the spectrometer 116 via the second input 140. Due to its longer wavelength, the light is diffracted more strongly by the optical grating 148 than light in the 430-700 nm range. To compensate for this effect, the second input 140 is laterally offset relative to the first input 142, so that the reflected light strikes the optical grating 148 at a steeper angle. By selecting an appropriate lateral offset, it is possible to ensure that the areas on the active surface of the detector 150 reached by light from each spectrum at least partially overlap. This allows for a particularly compact design.

[0066] Second embodiment Figure 5 shows an alternative embodiment of a thickness measurement device 200 of the present invention. For all features described below, the same reference numerals as in Figure 2 are used, only with the addition of 100. Unless necessary, these will not be described again.

[0067] The optical thickness measurement device 200 comprises a light source 212 having two light source units 220, 222. In contrast to the embodiment of Figure 2, the different wavelength ranges of the light source 212 are fed to a common measurement spot 231 via a measurement head 214.

[0068] Light from light source units 220, 222 is fed to a dichroic beam splitter 229 via two fibers 224, 226. Light in a first wavelength range from light source unit 220 enters beam splitter 229 from first fiber 224, is transmitted, strikes measurement object 230 (or one of two interfaces 232, 234), is reflected from there, and re-enters fiber 224. Fiber 238 is connected to fiber 224 via a fiber coupler. This reflected light is directed to fiber 238 via this fiber coupler, which directs the light to optical input 242. Similarly, light in the other wavelength range from light source unit 222 enters second fiber 226 and, in the illustrated embodiment, laterally enters beam splitter 229, where it is reflected back toward the measurement head / measurement object, reflects off measurement object 230, and then re-enters fiber 226 and is directed from there via the fiber coupler to spectrometer 226 or associated optical input 240. Dichroic beam splitter 229 is selected to transmit as completely as possible the wavelength range of light provided via fiber 224 and to reflect as completely as possible the wavelength range of light provided via fiber 226 .

[0069] As an alternative to this embodiment, the beam splitter 229 may not be directly connected to the measurement head 214 but may exist as a separate element. In this case, the measurement light from the light source 212 may be guided to the measurement head 214 via a single fiber and only split off immediately before the spectrometer.

[0070] Third embodiment FIG. 6 shows a further embodiment of the thickness measurement device 300. In contrast to the previous embodiments of FIGS. 2 and 5, the light guide in this embodiment is entirely fiber-based, outside the measurement head 314 and spectrometer 316. As in the embodiment of FIG. 2, different wavelength ranges are coupled into the measurement head by separate fibers 324, 326 at different locations; therefore, the returning light is also re-imaged at the end of the corresponding fiber and only combined there. Via a fiber coupler, the returning light in fibers 324 and 326 is guided into fibers 338 and 336, and from there to the spectrometer.

[0071] The third embodiment corresponds mainly to the first embodiment, with the beam splitter replaced by a fiber coupler.

[0072] Conversely, it is also possible to perform beam steering entirely within the free beam.

[0073] In all of the above-described embodiments, it may be provided that the spatially separated coupling to the spectrometer 116, 216, 316 can be performed via ferrules that can be positioned adjacent to each other separately. Alternatively, the coupling can be performed via a double ferrule. The position of the partial spectrum in the detector plane can be set based on the position of the ferrule or the distance between the fibers in the double ferrule.

[0074] On the one hand, the separation of the partial spectra on the detector can be set up so that there is a large spatial overlap on the detector for both wavelength ranges, as previously mentioned, and the separation is achieved by timing the light source or light source unit.

[0075] Alternatively, spatial separation of the partial spectra on the detector can be achieved by choosing the spatial spacing of the spectrometer inputs so that the partial spectra of the reflected light split by the grating fall on two different detector lines, eliminating the need for timing.

[0076] The spacing of the input points of the spectrometer can also be selected or combined so that the detector rows are directly above each other, thus achieving a particularly compact configuration.

[0077] In a further alternative, the spatial spacing of the spectrometer's inputs can be such that the spectra do not overlap on the detector. In this case, to increase the readout speed, only a portion of the detector can be read out in synchronization with the switching of the light source. For example, the spectra can be arranged adjacent to each other in a line on the detector with a line configuration. The distance between the spectra, which is actually measured due to diffraction / refraction / reflection conditions, can be reduced by the spatial arrangement and alignment of the light inputs, thereby optimizing the use of the available detector area.

[0078] Fourth embodiment This is illustrated in Figure 7, which shows a schematic representation of part of the spectrometer 116. The dispersive optical element is recognizable, which is here designed as a transmission grating 449, mainly for reasons of better visualization. The transmission grating 449 is placed in the collimated beam path, similar to the case of the reflection gratings 148, 248, 348 shown in Figures 2, 5 and 6. As in the other embodiments, a focusing lens 444 focuses the diffracted light onto a detector 451.

[0079] In contrast to the embodiments described above, detector 451 has two pixel lines 453, 455 instead of just one. Along the first pixel line 453, through which axis A runs in the illustrated embodiment, first pixels 457 are arranged, which are only interested in light of a first wavelength range. Along the second pixel line 455, which is offset along the x-direction but runs parallel to the first pixel line 453, second pixels 459 are arranged, which are only interested in light of a second wavelength range. The division into two pixel lines avoids the need to switch light sources.

[0080] 7 as a solid beam bundle 460 falls on a dispersive optical element (transmission grating 449) along axis A. Axis A is, for example, tilted at a first angle α with respect to the z-axis. Because the diffractive structure of transmission grating 449 extends along the x-direction, light 460 is deflected in the plane subtended by axis A and the y-axis according to wavelength and directed by focusing lens 444 to one of first pixels 457 of first pixel line 453.

[0081] A collimated beam bundle 462 (shown by a dashed line in FIG. 7 ) is light of a second wavelength range and strikes the transmission grating 449 along a second axis, which in this embodiment is tilted with respect to axis A. As a result, the focusing lens 444 does not focus the light diffracted in the yz plane onto pixel 457 of the first pixel line 453, but rather onto one of the second pixels 459 of the second pixel line 455, which is offset in the x direction. Therefore, as a result of the different incident directions, light 460 of the first wavelength range and light 462 of the second wavelength range cannot be focused onto the same pixel.

[0082] At the same time, the direction of incidence of at least one of the two beam bundles 460, 462 relative to the z-axis is selected to at least partially compensate for wavelength-dependent diffraction, so that light of different wavelength ranges is deflected by the same dispersive element and also impinges on the detector 451. Specifically, in this embodiment, the direction of incidence of the light beam 462 is selected so that its direction of incidence on the dispersive element 449 includes an angle with the xz-plane. This angle is selected so that stronger or weaker deflections in the yz-plane caused by other wavelength ranges are "compensated." Thus, the dispersed light also impinges on the detector 451, but on one of the second detector lines 455 or pixels 459, as described above.

[0083] Therefore, in this embodiment, measurements in both wavelength ranges can be made simultaneously, and therefore this approach is particularly well suited for cases where the actual distance to be measured between two interfaces is at an unfavourable position between the wavelength ranges, and ideally measurements are made using both wavelength ranges simultaneously.

[0084] To enable light 460, 462 to be guided onto the dispersive optical element 449 from different directions, light of each wavelength can be guided through a separate fiber in a fiber-based configuration. In that case, the two ends of the fibers must be positioned adjacent to each other in the object plane of the spectrometer optics. In the embodiment shown in Figure 7, an offset of the fiber ends along the x-direction must be provided to control the two detector lines 453, 455, and an offset in the y-direction must be provided to adjust the dispersive effect of the grating 449 for different wavelength ranges.

[0085] Generally, in configurations with free beam propagation, adjustment of the beam propagation can be achieved, for example, by aligning apertures or by using wedge prisms.

[0086] The desired spatial separation of light having two different wavelength ranges on the detector can also generally be ensured by different incident directions of the respective light on the dispersive optical element. Alternatively, the light can be polarized differently, for example, orthogonal linearly polarized or oppositely circularly polarized. Then, with the aid of appropriate polarizing filters, for example, placed just before or on pixels 457, 459, it can be achieved that light having one wavelength falls only on pixels where light having the other wavelength cannot fall, and vice versa.

[0087] If two spectra are generated for one thickness value during measurement, the thickness value can be calculated from the two partial spectra. If two measurement spectra with different bandwidths are used, the narrower spectrum for thicker wafers provides greater accuracy, and the broader spectrum for thinner wafers.

[0088] The thickness value calculation can, for example, provide a statistical weighting of two partial spectra: a narrowband spectrum provides greater accuracy for thicker wafers, and a broadband spectrum for thinner wafers. The calculable thickness value is always based on the partial spectrum that best suits the current thickness.

[0089] Preferably, this calculation is not based on a fixed threshold, but rather on a weighted average, for example. A transition range can be defined within which the weighting depends on the approximate position within the transition range in which the current thickness being measured is located, and / or the weighting can depend on the last calculated value and / or on the two measurements.

[0090] Alternatively or additionally, the weighting of two measurements for the same thickness can be performed based on the determined quality of the individual measurements: the measurement peak height (corresponding to the amplitude of the interferometric modulation) or any measure of the statistical noise of the values ​​(e.g., change over time) can be used as a measure of quality.

[0091] Fifth embodiment As in the previous embodiment, not only can the distance between two interfaces be measured, but reference light can also be provided to obtain absolute distance measurements. Figure 8 shows an embodiment of a measurement device 500 that largely corresponds to the embodiment shown in Figure 6, except that a reference arm 570 with an end-side mirror 572 is additionally connected to a fiber coupler 574. Furthermore, for clarity, only one optical path for one optical wavelength is shown. In the reference arm 570, measurement light generated by a light source 512 is reflected by the mirror 572 and interferes in the fiber coupler 574 with measurement light reflected by one of the surfaces 532, 534 of the measurement object 530. The interference is detected by a spectrometer 516, generating a modulation spectrum on a detector 550. A fast Fourier transform (FFT) can be used to obtain modulation frequencies from the spectrum, each of which can be assigned a distance value. For further details, see the applicant's published patent application DE 10 2016005021 A1.

[0092] To be able to perform an FFT, the phase-dependent intensity P int (k i ) is first calculated for each pixel p i The intensity value P measured by int (p i ) The wave number k is related to the wavelength λ by the relationship k=n(λ) / λ Here, n(λ) represents the dispersion of the medium that constitutes the measurement target 530 and through which the measurement light can pass. The wavelength λ is calculated from the allocation table p i =p i (λ i ) to a pixel number p. The result is an assignment between wavenumber k and pixel number p, which is the pixel-dependent intensity P int (p i ) as the phase-dependent intensity P int (k i) For further details, see the applicant's DE 10 2017122689 A1. Depending on the application, several reference arms and / or length-adjustable reference arms can be used.

[0093] Sixth embodiment FIG. 9 illustrates one embodiment of a method according to the present invention. In the first (S1) and second (S2) steps, a first interface distance value (the distance between the first and second interfaces) is measured using measurement light in a first wavelength range, and a second interface distance value is measured using measurement light in a second wavelength range. For example, measurement light can be generated for this purpose in first and second wavelength ranges. As described above, the first wavelength range can be in the visible range, e.g., 430 nm to 700 nm. The second wavelength range can be, for example, in the range of 700 nm to 1600 nm, particularly in the subranges of about 830 nm to about 930 nm, about 870 nm to about 970 nm, or about 950 nm to about 1100 nm. Preferably, the bandwidth of the first light source is different from the bandwidth of the second light source. In particular, the first wavelength range is broadband, and the second wavelength range is relatively narrow. A narrower wavelength range provides higher accuracy for thick wafers, while a broadband wavelength range provides higher accuracy for thin wafers. In a preferred embodiment, the first light source unit is a light emitting diode (LED) and the second light source unit is a superluminescent diode (SLD). The narrowband wavelength range is particularly preferably a long-wavelength wavelength range, and the broadband wavelength range is preferably a short-wavelength wavelength range.

[0094] When measuring interface distance values, the measuring light can be rapidly switched between two wavelength ranges, for example, at frequencies in the kHz range, i.e., between 0.5 kHz and 100 kHz. In this way, it is possible to achieve a nearly continuous transition between the individual wavelengths and therefore between the measurement ranges. At the same time, in measurement ranges covered by the two wavelength ranges but where the individual measuring light provides a lower quality / intensity, a significantly better overall measurement signal can be achieved by averaging the results of the two measuring lights.

[0095] For this purpose, a weighted averaging can be carried out (S3), for example, with the weighting being based on the quality of the measurement signals. [Explanation of symbols]

[0096] 100 Measuring Device 112 Light source 114 Measuring Head 116 Optical Spectrometer 118 Evaluation Device 120, 122 light source unit 124, 126 Optical fiber 128 Optical system 130 Measurement Object 132 First Surface 134 Second Surface 136, 138 Optical fiber 140, 142 Optical input 144, 146 Optical system 148 Reflection grating 150 detectors

Claims

1. An optical thickness measurement device (100) comprising a light source (112), a measurement head (114), an optical spectrometer (116) having an optical component (148) for dispersing input light and a detector (150), and an evaluation device (118), a) the light source (112) is optically connected to the measurement head (114) and configured to generate measurement light and direct it to the measurement head (114); b) the measurement head (114) is optically connected to the spectrometer (116) and configured to direct measurement light onto the measurement object (130), collect reflected light arising from the two different interfaces (132, 134) therefrom, and direct the reflected light as input light to the spectrometer (116); c) the spectrometer (116) is electrically connected to the evaluation device (118) and configured to generate a spectrum of reflected light that originates from two different interfaces of the measurement object (130) and interferes with each other, and to transmit the spectrum as an electrical signal to the evaluation device (118); d) the evaluation device (118) is configured to measure the distance between the two interface surfaces (132, 134); moreover, e) the measurement light has at least first and second wavelength ranges; f) the spectrometer has two optical inputs (140, 142, 240, 242, 340, 342) for reflected light, the reflected light of a first wavelength range exiting the first optical input (140, 240, 340) and the reflected light of a second wavelength range exiting the second optical input (142, 242, 342); g) the optical inputs (140, 142, 240, 242, 340, 342) are spatially separated such that the reflected light of both wavelength ranges is dispersed by the common component (26) and the imaging areas on the detector (28) at least partially overlap in the dispersion direction; h) the evaluation device (118) is configured to generate first and second spectra of the first and second reflected light, respectively, using measurement light in the first and second wavelength ranges, measure the respective thicknesses from the respective spectra, and calculate weighted thickness values ​​for the respective thicknesses; Optical thickness measurement device.

2. The apparatus of claim 1 , wherein the light source (112) comprises a first light source unit (120) and a second light source unit (122).

3. The apparatus of claim 2 , wherein the bandwidth of the first light source unit (120) is different from the bandwidth of the second light source unit (122).

4. 4. The apparatus of claim 3, wherein the wavelength range of the first light source unit (120) is in the visible range and the wavelength range of the second light source unit (122) is in the near-infrared range.

5. The device according to any one of claims 2 to 4, wherein the first light source unit (120) is switchable independently of the second light source unit (122).

6. 5. The device according to claim 1, wherein the light source (112) is configured to be able to generate measurement light in a first wavelength range alternately with measurement light in a second wavelength range, preferably with a fixed clock.

7. 7. The device according to claim 6, wherein the spectrum is read out by the evaluation device (118) in synchronization with a clock of the light source circuit.

8. The device according to any one of claims 1 to 4, wherein the detector (451) comprises two lines (453, 455) that can be read out separately.

9. 9. The apparatus of claim 8, wherein the lines are shifted in a spatial direction across the spectrum, preferably directly above each other.

10. a reference arm (570) having a mirror (572) and a light splitting device (574) for splitting the measurement light; One of the split measurement beams is reflected by a mirror (572) and interferes with another of the split measurement beams reflected by one of the interfaces (532, 534) of the measurement object (530) in the beam splitting device (574); The device according to any one of claims 1 to 4, wherein the interfering measurement light is detected as input light by a spectrometer (516).

11. 5. The device according to claim 1, wherein the connection between the light source (112) and the measurement head (114) and / or between the measurement head (114) and the spectrometer (116) comprises two optical fibers (124, 126, 136, 138) respectively.

12. An apparatus according to any one of claims 1 to 4, wherein the calculation of the thickness value is based on a weighted average of the respective thickness values ​​measured from each spectrum.

13. An optical thickness measurement device (100) comprising a light source (112), a measurement head (114), an optical spectrometer (116) having an optical component (148) for dispersing input light and a detector (150), and an evaluation device (118), a) the light source (112) is optically connected to the measurement head (114) and configured to generate measurement light and direct it to the measurement head (114); b) the measurement head (114) is optically connected to the spectrometer (116) and configured to direct measurement light onto the measurement object (130), collect reflected light arising from the two different interfaces (132, 134) therefrom, and direct the reflected light as input light to the spectrometer (116); c) the spectrometer (116) is electrically connected to the evaluation device (118) and configured to generate a spectrum of reflected light that originates from two different interfaces (132, 134) of the measurement object (130) and interferes with each other, and to transmit the spectrum as an electrical signal to the evaluation device (118); d) the evaluation device (118) is configured to measure the distance between the two interface surfaces (132, 134); moreover e) the measurement light has at least first and second wavelength ranges; f) the light source (112) is configured to be able to generate measurement light in a first wavelength range alternately with measurement light in a second wavelength range, preferably with a fixed clock; g) the spectrum is read out by the evaluation device (118) in synchronization with the clock of the light source circuit; Optical thickness measurement device.

14. 14. The apparatus of claim 13, wherein only areas of the detector (150) in which wavelength ranges simultaneously generated for readout are imaged are read out.

15. A method for measuring a distance between two boundary surfaces of a measurement object using the device according to claim 13, comprising the steps of: a) generating at least low-coherence measurement light having a first wavelength range; b) directing the measurement light to the measurement object; c) collecting the reflected light and generating a spectrum of reflected light that is reflected at different interfaces and interferes with each other; d) repeating steps a to c using measurement light in a second wavelength range; e) measuring a first interface distance value using measurement light in a first wavelength range; f) measuring a second interface distance value using measurement light in a second wavelength range; g) calculating an interface distance using the first and / or second interface distance values; Measurement methods including:

16. 16. The method of claim 15, wherein the interference of the reflected light occurs either between the reflected light at the interface of the measurement object and the reference light, and / or between the reflected light at the first interface of the measurement object and the reflected light at the second interface of the measurement object.

17. 17. The method according to claim 15 or 16, wherein in the calculation of the interface distance an averaging, preferably a weighted averaging, of the first and second interface distance values ​​is performed.

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