Ellipsometer focusing system based on beam splitter

Abstract

The ellipsometer includes a focusing system that uses an image of a measurement spot to determine the optimal focus position for the ellipsometer. A focus signal is generated by separating the ellipsometer measurement spot before the signal is analyzed by a polarizer, thereby avoiding imaging the spot with modulated intensity. The focus signal is imaged onto a sensor array, and based on the position of the spot on the sensor array, the focus position of the ellipsometer can be determined. A single image can be used to determine the focus position of the ellipsometer, allowing real-time focus position measurement.

Description

[0001] Cross-references to Related Patent Applications

[0002] This patent application claims priority to U.S. non-provisional patent application No. 16 / 590,373, filed on October 1, 2019, entitled “BEAMSPLITTER BASED ELLIPSOMETER FOCUSING SYSTEM,” which is incorporated herein by reference in its entirety. Background Art

[0003] An ellipsometer is an optical metrology device that detects changes in the polarization state of light reflected from a sample's surface to measure the sample's properties. For example, Figure 1 A conventional spectroscopic ellipsometer 10 is shown that includes a broadband light source 12, a polarizer 14, and a lens 15 to focus the illumination light onto the surface of a sample 16 positioned on a stage 18. The ellipsometer 10 also includes an analyzer 22 after passing through a lens 20. After passing through the analyzer 22, the reflected light is focused by a lens system 24 onto a detector 26.

[0004] The ellipsometer 10 must be correctly focused on the sample. Some systems use a separate focusing system, i.e., a system that is attached to the ellipsometer but uses a separate optical path, to determine the position of the focusing system and, therefore, the ellipsometer, relative to the sample. However, such focusing systems require very precise alignment, which is expensive and difficult. Figure 1 An example of an integrated focusing system comprising a mirror 28 having an aperture 30 is shown. The mirror 28 reflects outer rays of the reflected light beam to a focus detector 32, while inner rays of the reflected light beam are transmitted through the aperture 30 and received by the ellipsometer detector 26. Typically, the focus detector 32 is a position sensitive device (PSD) or a "quad cell", neither of which images the detected light, but in principle adds all received light. Such devices are therefore prone to inaccuracies due to stray light. Other systems, such as the system described in US5,608,526, use a camera as the focus detector 32. However, such systems still suffer from inaccuracies due to systematic errors introduced by the mirror 28, since the detector 32 only samples the outer part of the light beam. Furthermore, focusing systems, such as the one described in 5,608,526, lack the precision required to produce small illumination spot sizes on the sample.

[0005] Therefore, there is a need for an improved focusing system for an ellipsometer. Summary of the Invention

[0006] The ellipsometer includes a focusing system that uses an image of the measurement spot to determine the optimal focus position for the ellipsometer. A focus signal is generated by separating the reflected measurement signal before it is analyzed by a polarizer, thereby avoiding modulated intensities in the imaged measurement spot. Additionally, separating the reflected measurement signal before the analyzer avoids relative movement of the imaged measurement spot from different samples, which is caused by differences in the distribution of the reflected elliptical polarization at the incident angle. The focus signal is imaged onto a sensor array, and based on the position of the spot on the sensor array, the focus position of the ellipsometer can be determined. A single image can be used to determine the focus position of the ellipsometer, thereby allowing real-time focus position measurement.

[0007] In one embodiment, the ellipsometer includes a source that emits light along a path; a polarizer that polarizes the light to produce a sample beam that interacts with a sample and is reflected to produce a reflected beam; a compensator that is disposed in the path of the sample beam or the reflected beam, the compensator inducing a phase delay in the polarization state of the light, wherein at least one of the polarizer and the compensator rotates about an axis parallel to a propagation direction of the light; a beam splitter that is positioned after the compensator and in the path of the reflected beam, the beam splitter being positioned to receive the reflected beam and direct a first portion of the reflected beam to a focusing system and a second portion of the reflected beam to an analyzer, wherein the first portion of the reflected beam and the second portion of the reflected beam each include an entire cross-section of the reflected beam; the focusing system is positioned The focusing system is positioned to receive a first portion of the reflected light beam from the beam splitter, the focusing system comprising: a lens system that receives the first portion of the reflected light beam, the lens system amplifying any deviation from the optimal focus position of the ellipsometer; and a first detector that is positioned to receive the first portion of the reflected light beam from the lens system, wherein the first detector comprises a two-dimensional sensor, and the lens system in the focusing system produces a spot on the two-dimensional sensor and the two-dimensional sensor produces an image of the spot; an analyzer that is positioned to receive a second portion of the reflected light beam from the beam splitter; a second detector that is positioned to receive the reflected light beam from the analyzer; and a processor that receives the image and is configured to obtain a position of the spot on the two-dimensional sensor in the image to determine a deviation from the optimal focus position of the ellipsometer.

[0008] In one embodiment, a method of focusing an ellipsometer includes generating light along a path; polarizing the light to generate a sample beam that interacts with a sample and is reflected to generate a reflected beam; inducing a phase delay in the polarization state of light in the sample beam or the reflected beam; rotating at least one of the polarization or the phase delay in the polarization state; separating the reflected beam and directing a first portion of the reflected beam along a first path and a second portion of the reflected beam along a second path, wherein the first portion of the reflected beam and the second portion of the reflected beam each include an entire cross-section of the reflected beam; focusing the first portion of the reflected beam into a spot on a two-dimensional sensor, wherein any deviation from an optimal focus position of the ellipsometer is amplified; generating an image of the spot; determining a position of the spot on the two-dimensional sensor in the image; determining a deviation from an optimal focus position using the position of the spot on the two-dimensional sensor; adjusting a focus position of the ellipsometer based on the deviation from the optimal focus position; analyzing the polarization state of a second portion of the reflected beam; and detecting the analyzed second portion of the reflected beam.

[0009] In one embodiment, the ellipsometer includes light emitted along a path; a polarizer that polarizes the light to produce a sample beam that interacts with a sample and is reflected to produce a reflected beam; a compensator disposed in the path of the sample beam or the reflected beam, the compensator inducing a phase delay in the polarization state of the light; an analyzer positioned in the path of the reflected beam after the compensator, wherein at least one of the polarizer, compensator, and analyzer rotates about an axis parallel to the direction of propagation of the light; and a focusing system positioned in the path of the reflected beam after the compensator and before the analyzer, the focusing system comprising: a beam splitter positioned to receive the incident beam and direct a first portion of the reflected beam to the lens system and a second portion of the reflected beam to the analyzer. , wherein the first portion of the reflected light beam and the second portion of the reflected light beam each include an entire cross-section of the reflected light beam; a lens system positioned to receive the first portion of the reflected light beam and direct the first portion of the reflected light beam to a first detector, the lens system amplifying any deviation from the optimal focus position of the ellipsometer; and a first detector positioned to receive the first portion of the reflected light beam from the lens system, wherein the first detector includes a two-dimensional sensor, the lens system in the focusing system produces a spot on the two-dimensional sensor and the two-dimensional sensor produces an image of the spot; a second detector positioned to receive the reflected light beam from the analyzer; and a processor that receives the image and is configured to obtain a position of the spot on the two-dimensional sensor in the image to determine a deviation from the optimal focus position of the ellipsometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 An ellipsometer with a conventional focusing system is shown.

[0011] Figure 2 An ellipsometer 100 with a high precision focusing system according to an embodiment of the present invention is shown.

[0012] Figure 3 A two-dimensional sensor of a camera in a focusing system is shown having a significantly smaller spot thereon.

[0013] Figure 4 is a flow chart illustrating the processing of sensor signals provided by the focusing system to automatically adjust the focus position of the ellipsometer.

[0014] Figure 5 Shown is the autofocus spot on the sensor without intensity modulation but with the wobble caused by the rotating optics.

[0015] Figure 6 An autofocus spot on a sensor with both intensity modulation and wobble due to rotating optics is shown.

[0016] Figure 7 is a flow chart illustrating a method of focusing an ellipsometer. DETAILED DESCRIPTION

[0017] A real-time focusing system for ellipsometers can determine the optimal focus position for the ellipsometer using an image of the ellipsometer's measurement spot. The focus signal is generated by separating the reflected measurement signal before analyzing it with a polarizer, thereby avoiding intensity modulation in the imaged measurement spot. Furthermore, separating the reflected measurement signal before the analyzer avoids relative movement of the imaged measurement spot from different samples, which is caused by differences in the reflected ellipsometry's angle of incidence distribution. Consequently, the ellipsometer's focus position can be determined using a single image, allowing real-time focus adjustment.

[0018] Figure 2An ellipsometer 100 is shown configured with a real-time high-precision focusing system 150. As shown, the ellipsometer 100 includes a light source 102, a polarization state generator 103, and a polarization state analyzer 115. The ellipsometer 100 can be monochromatic or spectral, and therefore, the light source 102 can be a narrowband or broadband light source. The polarization state generator 103 includes a polarizer 104 and can include a rotating compensator 105. In some embodiments, the compensator 105 can be located after the sample, as shown by the compensator 105' shown by the dashed line. The polarization state analyzer 115 includes another polarizer 112, generally referred to as an analyzer 112. If desired, the compensator 105 can be stationary and one or both of the polarizer 104 and analyzer 112 can be rotated, or alternatively, the compensator 105 and the polarizer 104 and analyzer 112 can be rotated. The lens system 106 focuses the illumination light 111 onto the surface of the sample 101 positioned on the stage 108. Incident illumination light 111 has a known polarization state due to polarization state generator 103. Sample 101 will alter the polarization state of the light, and the resulting light reflected by sample 101 is analyzed by polarization state analyzer 115, for example by passing reflected light 113 through analyzer 112 (and compensator 105', if located after sample 101), after the reflected light is received by another lens system 110. After passing through analyzer 112, reflected light 113 is focused by lens system 114 onto detector 116. Detector 116 detects the intensity of reflected light 113, which, along with the known positions of polarizer 104, analyzer 112, and compensator 105, can be used to determine elliptical polarization parameters ψ and Δ, from which various parameters of sample 101 can be determined, as is well known in the art.

[0019] In order to correctly measure the sample 101, the ellipsometer 100 is positioned at the optimal focus position relative to the sample 101. Therefore, the ellipsometer 100 includes an integrated autofocus system 150 that images the same light used by the ellipsometer 100 and additionally amplifies deviations from the optimal focus position. The focusing system 150 includes a beam splitter 152 that directs a portion of the reflected light 113 to a lens system 154, which focuses the light onto a detector 156.

[0020] like Figure 3 As shown, lens system 154 focuses light into a spot 302 that is significantly smaller than a two-dimensional sensor 304 used for detector 156. Sensor 304 can be, for example, a two-dimensional sensor array, such as a CCD. Spot 302 can be 1%-50% of the size of sensor 304, for example, 10% or less, which increases the useful autofocus range. The lenses of lens system 154 are arranged to amplify deviations from the optimal focus position, thereby providing greater measurement accuracy. For example, as Figure 3 As shown by arrow 306 in FIG, the movement of the position of spot 302 on sensor 304 provides an amplified indication of the deviation from the optimal focus position. As sample 101 is scanned through the focus range, the size of the spot on sensor 304 may vary slightly; however, this is a relatively minor effect to which the spot position calculation can be configured to be insensitive. The magnification produced by lens system 154 relative to the deviation from the optimal focus position may be 3x to 5x or greater, such as 10x. However, a reduction in spot size reduces focus accuracy and, therefore, represents a tradeoff between autofocus range and accuracy, as a smaller spot results in a less accurate spot position calculation but provides a higher spot intensity. Therefore, if desired, a larger spot, such as the size of sensor 304, may be generated, which can be used to provide a more accurate spot position calculation.

[0021] like Figure 2 As shown, the beam splitter 152 of the focusing system 150 reflects a portion of the reflected light 113 (e.g., 4% to 10% of the total light intensity) to the focusing system 150 and transmits the remaining portion of the reflected light 113 (e.g., 90% or more of the total light intensity) to the ellipsometer detector 116. Figure 2 In the configuration shown, the reflected portion is provided to the focusing system 150, but if desired, the transmitted portion of the reflected light 113 can be provided to the focusing system 150, wherein 4% to 10% of the reflected light intensity is transmitted. Figure 1 , the use of beam splitter 152, sometimes referred to as a "picking" beam splitter, is advantageous when the entire cross-section of reflected light 113 is sampled by focusing system 150. By sampling the entire beam of reflected light 113, focusing system 150 is less sensitive to systematic errors caused by sampling only a portion of reflected light 113.

[0022] The beam splitter 152 can be a pellicle beam splitter, which can have a thickness, for example 0.002 mm, which does not significantly affect the optical path length or aberrations in a converging beam. The effect on the optical path length and aberrations in a collimated beam will be even smaller, as shown in FIG. Figure 2 Using a pellicle beamsplitter will further minimize chromatic aberration and prevent imaging ghosts.

[0023] like Figure 2As shown, a detector 156 for the focusing system 150 is coupled to a computer 130, for example, via a frame capture board 157. Rotating optical elements, such as the compensator 105 or the polarizer 104, and the stage 108 may also be connected to the frame capture board 157, either directly or via a controller / driver, such as driver 105D. Detector 116 for the ellipsometer 100 may be coupled to the same computer 130 or a different computer, as desired. Computer 130 includes a processor 132 having a memory 134, and a user interface including, for example, a display 138 and an input device 140. Frame capture board 157 includes a processor 157p, which may be a field programmable gate array (FPGA), configured to determine a focus error, which is used to control the focus position of stage 108, for example, via stage servo controller 108cont, which receives focus error data from frame capture board 157 and controls actuators 109 in stage 108 accordingly. Thus, in one embodiment, the frame capture board 157 processes the focus error directly from the detector 156 and provides focus adjustments to the stage servo controller 108cont without input from the computer 130. Of course, if desired, the computer 130 can be used in part or in its entirety to process the focus error and instruct the stage servo controller 108cont. It should be understood that a processor, such as the processor 157p on the frame capture board 157, can include one or more separate processing units. For example, the processor 157p can include a first processor for image processing and a separate processor for focus error determination. In addition, one or more processors can be located elsewhere besides the frame capture board 157. For example, the processor 157p (or one or more processor units in the processor unit that includes the processor 157p) can be located in the detector 156 or elsewhere.

[0024] If processor 132 is, for example, a microprocessor that executes instructions of a computer program, then the data structures and software code for automatically performing one or more of the actions described in this detailed description can be implemented by one of ordinary skill in the art in accordance with the present disclosure and stored, for example, on a computer-readable storage medium, such as memory 134 or disk 142, which can be any device or medium that can store code and / or data for use by a computer system. Computer-readable storage media 134 / 142 can be, but are not limited to, magnetic and optical storage devices, such as disk drives, magnetic tape, optical disks, and DVDs (Digital Versatile Discs or Digital Video Discs). Communication port 144 can also be used to receive instructions for programming processor 132 to perform any one or more of the functions described herein and can represent any type of communication connection, such as a communication connection to the Internet or any other computer network. In addition, the functions described herein can be embodied in whole or in part in the circuitry of an application-specific integrated circuit (ASIC) or a programmable logic device (PLD), and these functions can be embodied in a computer-understandable descriptor language that can be used to create an ASIC or PLD that operates as described herein. For example, as discussed above, a field programmable gate array (FPGA) can be used. The FPGA can be located in the detector 156 or on a frame capture board 157 inside or outside the computer 130. In the case where the processor 132 is an FPGA, the computer-readable storage medium 134 / 142 can provide a programming file to embed the desired configuration in the processor 132, which can be executed once for the non-volatile FPGA or executed at power-up. By avoiding the need to use the main system CPU to perform the necessary calculations for autofocus, the CPU is not slowed down. In addition, a dedicated processor increases the image processing speed. Therefore, the stage servo controller 108cont can be directly coupled to the frame capture board 157, which can provide signals directly to the stage servo controller 108cont via a serial peripheral communication interface (SPI) channel.

[0025] As is known in the art, an ellipsometer (e.g., ellipsometer 100) for measuring properties and / or structure of a sample uses a polarization state generator 103 or a polarization state analyzer 115 to change the polarization state of light. For example, the polarization state of light can be changed by rotating at least one of the polarizer 104 and the compensator 105 around an axis parallel to the propagation direction of the light. Ideally, the rotating optical device (i.e., the polarizer 104 or the compensator 105) rotates around an axis completely parallel to the optical axis, in which case the position of the illumination spot on the sample 101 will not move. However, in reality, the motor and bearings that rotate the rotating optical device are not perfect, causing the rotating optical device to wobble. In addition, the input light beam to the rotating optical device and the output light beam after the rotating optical device will not be completely parallel. Due to these two effects, the illumination spot on the sample 101 moves, which will cause the spot imaged on the two-dimensional sensor of the detector 156 of the focusing system 150 to move as the rotating optical device rotates. Movement of the illumination spot in the ellipsometer 100 may be small, eg, less than 2 μm, but when present, may cause problems for the autofocus system 150. Therefore, the autofocus system 150 may be configured to compensate for movement of the illumination spot.

[0026] There are several different ways to change the polarization state of light, all of which are contemplated by the present disclosure. One way to change the polarization state is to continuously rotate the polarizer 104 about the optical axis while the analyzer 112, which transmits only one polarization state, is fixed. In this approach, there is no need to rotate the compensator 105. In the simplest case, where the sample does not change the polarization of the incident light, the result is a change in the intensity of the light after the analyzer 112. For example, the analyzer 112 can be set to transmit only horizontally polarized light and block vertically polarized light. With the polarizer 104 starting in a position where the illumination light 111 is horizontally polarized, the analyzer 112 will transmit 100% of the light. When the polarizer 104 is rotated 90 degrees about the optical axis, the light is now vertically polarized. Therefore, the analyzer 112 will block all light and there will be no signal at the detector 116. As the polarizer moves another 90 degrees (for a total of 180 degrees), the polarizer now transmits horizontally polarized light again and the analyzer again transmits 100% of the light. Thus, a graph of signal intensity versus time produces a sine wave where the intensity of the signal varies between 0% and 100%, and the frequency of the sine wave is twice the rotation frequency of the polarizer 104. Alternatively, the analyzer 112 may be rotated while the polarizer 104 remains fixed. Alternatively, as Figure 2 As shown, a rotating compensator 105 can be used to change the polarization state. The rotating compensator 105 will produce a substantially sinusoidal variation in intensity over time. As discussed above, the rotating compensator 105 can be located on either side of the sample 101.

[0027] In use, the sample under test will change the polarization state of the incident light, which will change the intensity and phase of the resulting signal received by detector 116. Using the changes in intensity and phase, the material properties of sample 101 can be determined, which is the essence of ellipsometry and is well known in the art. However, the continuous changes in intensity and phase due to rotating optics can cause problems for the autofocus system of the ellipsometer. For example, the signal intensity after analyzer 112 varies between 0% and 100% due to rotating optics, resulting in no or very little light being available to determine the focus position of the ellipsometer during parts of the signal cycle.

[0028] Therefore, if Figure 2 As shown, a beam splitter 152 for the focusing system 150 is in the optical path before the analyzer 112. By placing the beam splitter 152 before the analyzer 112, the intensity of the light is not modulated due to the rotation of the optics. Therefore, the beam splitter 152 directs a portion of the reflected light 113 to the detector 156 before the reflected light is modulated by the analyzer 112. Therefore, the reflected light 113 imaged onto the focal plane array of the focusing system 150, i.e., the detector 156, does not have a modulated intensity. However, the rotation of the optics (e.g., the polarizer 104 or the compensator 105) as the rotating optics rotates can still produce a wobble in the spot imaged on the detector 156 of the focusing system 150. Optionally, the analyzer 112 can be rotated and the polarizer 104 and compensator 105 remain stationary, which will avoid the wobble in the spot imaged by the detector 156 that is produced by the rotating optics. Additionally, if desired, the beam splitter 152 can be positioned in the beam path before the compensator 105' and one or both of the analyzer 112 and the compensator 105' can be rotated while the polarizer 104 remains stationary, which will again avoid wobble in the spot imaged by the detector 156 caused by the rotating optics.

[0029] Figure 4A flow chart of an autofocus process is shown, which can be performed in whole or in part by a detector 156 or frame capture board 157 external to the computer 130, such as an FPGA and DSP. As shown, an acquisition synchronization 400 step is performed, wherein the controller / driver 105D for rotating the compensator 105 (or polarizer 104) provides a synchronization (trigger) signal to the frame capture board 157 for compensating for spot movement on the sample, as discussed below. Automatic detection of an empty image can be performed (402). For example, an empty image can be detected by finding a threshold level to separate the spot from the background by automatic thresholding, which is well known in the art of image processing. If there is no spot (404), i.e., no region of interest, the process restarts acquisition synchronization 400. If there is a spot (404), a region of interest (ROI) can be extracted from the image (406). Optionally, for example, if the spot size is as large as the sensor 304 or if the processor is powerful enough, such as when an FPGA is used in the frame capture board 157, the ROI can be not extracted and the entire image can be calculated directly.

[0030] Most digital cameras, such as those used in conventional metrology systems, include internal automatic exposure control. However, the automatic exposure control of such cameras is insufficient for the high-precision focus system 150 of the present invention, which uses a single spot on the CCD. The automatic exposure control in conventional cameras attempts to adjust exposure for the entire CCD and, therefore, loses control when a single spot is present on the CCD. Furthermore, if the image is downloaded to the computer 130 to perform exposure control, there is excessive latency, resulting in unstable control. Therefore, the focus system 150 can use the camera exposure control I / O as a slave to a dedicated image processing processor, such as on the detector 156 or frame capture board 157, so that exposure control is well correlated with the requirements of image processing. Exposure is adjusted only relative to the spot on the CCD, which is, for example, 99% blank, rather than the entire CCD. Thus, once the spot is on the sensor 304 of the detector 156, an exposure error is calculated based on the local intensity of the spot, rather than across the entire sensor.

[0031] Additionally, some samples scatter unwanted light into the path of the autofocus system 150. The amount and pattern of unwanted light will vary depending on the sample. While the optical system of the ellipsometer 100 can be designed to minimize the reception of scattered light, the problem cannot be completely eliminated. If unwanted or scattered light is not excluded from the autofocus system's spot position calculation, errors will result. Therefore, the autofocus system 150 is configured to be insensitive to scattered light.

[0032] To extract the ROI (406), the image horizontal (X) and vertical (Y) can be summed into vectors and the maximum of the two vectors can be found in X and Y. Using the maximum in X and Y, the ROI can be located in the image data and extracted in the image data. The spot can then be determined based on the 2D data of the image in the ROI. Masking and thresholding (410) can be performed to filter noise from the signal. In this step, for example, a histogram showing the number of pixels at a given intensity can be generated. Most pixels receive little or no signal, so there will be a large peak near zero intensity. Pixels illuminated in the autofocus spot will produce a second peak. However, some pixels outside the autofocus spot may be illuminated due to background or scattered light that is to be removed. To determine whether any given pixel is part of the autofocus spot or part of the background, techniques such as a class-varying automatic thresholding algorithm can be employed. Pixels that are part of the autofocus spot are retained, while pixels that are determined to be outside the autofocus spot or isolated pixels, for example due to reflections, are eliminated or masked.

[0033] A blob position calculation (412) can then be performed for the pixels inside the mask. In one embodiment, the blob position can be determined based on the centroid of the blob. However, other techniques can be used to calculate the blob position. For example, the average x, y position of the pixels in the blob can be used to determine the location of the blob center, or a smoothing function can be used to smooth the points in the blob and the maximum value can be used as the location of the blob center. Alternatively, the center of the blob can be found using a large-scale optimization problem, for example by treating the perimeter of the blob as an ellipse and finding the center of the ellipse. Of course, other techniques or variations of the above can be used as needed. For example, the location of the blob can be calculated as the centroid based on the grayscale value (or alternatively, binary value) of the pixels whose intensity is greater than a threshold. A simple centroid calculation will assume that all pixels inside the mask are equally weighted, i.e., a binary centroid calculation. However, it has been found that when the blob has a Gaussian distribution, the brighter pixels near the center of the "blob" of the remaining pixels have less noise than the darker pixels at the edge of the blob. Therefore, a centroid calculation weighted by pixel intensity (also called grayscale) can be used to produce a grayscale centroid, where brighter pixels are more heavily weighted in the calculation based on their intensity, which improves focus accuracy. However, when the internal structure of the spot is not uniform and changes with variations across the wafer pattern, it may be advantageous to use a binary centroid calculation. The autofocus system should be insensitive to the wafer pattern, so a binary centroid calculation can be used in this case.

[0034] A phase locked loop (PLL) operation (414) can be used to synchronize with the angular position of the rotating optics so that wobble in the rotating optics can be compensated. Additionally, the PLL locked to the synchronization signal from the rotating optics can be used to compensate for alignment errors. For example, the center of the sensor 304 may not be perfectly aligned with the motion of the spot on the sensor. Additionally, the motion of the spot on the sensor 304 may not be linear, for example because the motion of the spot on the sensor 304 is a result of angular motion, even though the sensor 304 is linear. As discussed above, the controller / driver 105D for the rotating optics (i.e., the compensator 105 or the polarizer 104) outputs a signal indicating the angular position of the rotating optics, which is acquired during acquisition synchronization (400). Using the angular position signal as a "trigger" signal, the PLL can be used to lock to the trigger signal, and compensation for any wobble or alignment errors in the rotating optics can be performed using a lookup table (LUT) in an FPGA in the detector 156 or frame capture board 157 (or in the computer 130), where the values ​​in the lookup table are obtained through a calibration procedure. The system can be calibrated, for example, using a silicon wafer, where the rotating optics rotates one or more rotations. At each angular position of the rotating optics, the spot position can be measured for a plurality of different focus heights, and the average spot position for each focus height for each angular position of the rotating optics (i.e., each trigger angle) is determined and loaded into a LUT. The LUT can be obtained by stepping through the Z axis and recording the Z height or offset of the spot position relative to the desired focus position as measured on the CCD. The spot position can be recorded in terms of pixels in the CCD, and the focus error in micrometer or level encoder counts can be output from the LUT. Because wobble and alignment errors are hardware related, i.e., a property of the optical components of the ellipsometer, it is expected that this calibration will not be changed often.

[0035] Figure 5 The movement of the autofocus spot on the two-dimensional sensor 304 of the detector 156 due to the oscillation of the rotating optics is shown. Figure 5 , spot 552 represents the spot on the two-dimensional sensor 304 of the detector 156 during an eight-exposure sequence when the rotating optics (e.g., the rotating compensator 105 or the polarizer 104) rotates 360°, for example, based on a PLL trigger signal, even though the focus position of the ellipsometer 100 does not change. As shown, the exposure sequence can produce an ellipse 554 that describes the movement of the spot 552 as the rotating optics rotate. For comparison, spot 556 shows the center position where the spot 552 would be located if the rotating optics had no misalignment or mechanical wobble, and if the ellipsometer 100 was in the best focus position. Any movement from the center position provides an indication of deviation from the best focus position, as indicated by arrow 557.

[0036] The spot position can be calculated for each image frame (e.g., PLL trigger signal), and so in practice there are more than eight exposures for a 360° rotation of the rotating optics. Figure 5 As shown, the intensity of the spot 552 does not change in each exposure as the rotating optics rotate because the beam splitter 152 for the focusing system 150 is positioned before the analyzer 112. Furthermore, due to the difference in reflected elliptical polarization across the spread of incident angles, the position of the measurement spot does not vary between different samples because the focusing system 150 is positioned before the analyzer.

[0037] Ellipse 554 is a result of systematic mechanical errors, such as wobble of the rotating optics, and can be calibrated, as discussed above, so that focus measurements can be performed with a single image focus plane data acquisition. For example, image acquisition from detector 156 can be synchronized with the angular position of the rotating optics. The rotating optics hardware can be designed to send, for example, 13 trigger signals for every 360° rotation of the rotating optics. As discussed above, the use of a PLL allows the ellipse 554 to be divided into any desired number of angles. During calibration, as the ellipsometer 100 scans through different focus positions (Z heights), the XY position of the spot 552 on the sensor 304 for each focus position at each desired angular position of the rotating optics can be obtained and stored in a compensation LUT.

[0038] For example, during calibration, the ellipsometer 100 is placed over a blank silicon wafer in focus. The PLL will lock onto the trigger signal from the rotating optics signal and generate a camera trigger at a predefined acquisition angle. The detector 156 will capture an image at each specified angle (triggered by the PLL logic). For example, at Figure 5 In FIG, eight acquisition angles are shown, but additional (or fewer) acquisition angles may be used, for example 13 angles. The XY position of the spot for the focus position at each acquisition angle may be stored in a LUT. This process may be repeated at different focus heights. At runtime, the acquisition angles ( Figure 4 The acquisition angle is constant and can be repeated and processed by the PLL logic. Because the ellipsometer hardware (i.e., rotating optics) can have a fixed division of angle, such as 13, which may not be equally spaced, the PLL can synchronize only with the first trigger from the rotating optics in each rotation. The PLL can then generate its own trigger for itself and detector 156 for the desired division of the acquisition angle.

[0039] refer to Figure 4, the deviation from the best focus position can then be determined based on the acquisition angle of the rotating optics, the spot imaged at the detector 156, and the calibrated XY position of the spot for the acquisition angle (e.g., stored in a LUT) (416). For example, based on the acquisition angle determined, for example, based on the PLL triggering, the focus height can be determined based on the measured XY position of the spot on the two-dimensional sensor 304 according to the LUT. The LUT table can output the deviation from the best focus position in microns or, for example, in step encoder counts. The LUT table can alternatively output the current focus position, from which the deviation from the desired best focus position can be calculated and converted into step encoder counts.

[0040] The focus position of the ellipsometer 100 may then be adjusted accordingly ( 418 ), for example by sending the result to a servo controller for the actuator 109 to move the stage 108 to eliminate the determined focus position offset.

[0041] Due to the configuration of the focusing system 150, and in particular the location of the beam splitter 152 before the analyzer 112, the intensity of the spot imaged by the detector 156 does not change as the rotating optics rotate. Figure 5 As shown, the intensity of each spot 552 is constant at different acquisition angles. Therefore, deviations from the optimal focus position can be performed with a single focal plane data acquisition, i.e., using a single imaging spot 552 from any acquisition angle.

[0042] In contrast, if the analyzer 112 is positioned before the beam splitter 152, the intensity of the spot received by the focusing system will be modulated between 0% and 100% due to the rotation of the rotating optics. Figure 6An imaging spot 652 is shown in the focusing system positioned after the analyzer 112, showing the movement of the spot due to the wobble of the rotating optics and intensity variations caused by the rotation of the rotating optics. The different grayscales of the spot 652 represent different intensities. As shown, the spot 652 cycles through 0% and 100% intensity due to the rotation of the rotating optics. The presence of 0% intensity or close to 0% intensity at multiple positions during the rotation cycle of the rotating optics means that the focus position of the ellipsometer 100 cannot be measured at these acquisition angles, or multiple images must be acquired and integrated over a portion of the rotation period in order to generate a sufficient signal from which the center position 656 can be determined and used to measure the focus position offset. This limits the temporal bandwidth to about half of a complete mechanical cycle of the rotating element of the ellipsometer. In addition, for correct image acquisition, a proportional-integral-derivative controller (PID) controller may be required to predict the exposure time and / or gain of the detector in order to create a stable exposure for the spot following the sinusoidal variation of the spot intensity. Therefore, with such a configuration, single focal plane data acquisition, i.e., using a single imaging spot from any acquisition angle, cannot be used. As Figure 1 The illustrated configuration of the focusing system 150 with the beam splitter 152 preceding the analyzer 112 results in faster, ie, real-time, focus position measurements with fewer processing requirements.

[0043] Figure 7 is a diagram showing a focusing ellipsometer (e.g., Figure 2Flowchart 700 of a method for an ellipsometer (100) is shown. As shown, at block 702, light is generated along a path, and at block 704, the light is polarized to produce a sample beam, which interacts with a sample and is reflected to produce a reflected beam. At block 706, a phase delay is induced in the polarization state of light in the sample beam or the reflected beam. At block 708, at least one of the polarization or the phase delay of the polarization state is rotated. At block 710, the reflected beam is split into a first portion of the reflected beam directed along a first path and a second portion of the reflected beam directed along a second path, wherein the first portion of the reflected beam and the second portion of the reflected beam each include the entire cross-section of the reflected beam. For example, the reflected beam can be split, and the first portion of the reflected beam can be directed along the first path and the second portion of the reflected beam can be directed along the second path via a pellicle beam splitter. At block 712, the first portion of the reflected beam is focused onto a spot on a two-dimensional sensor, wherein any deviation from the optimal focus position of the ellipsometer is amplified. For example, any deviation from the optimal focus position of the ellipsometer can be amplified by at least 2x. Furthermore, the spot may be 50% or smaller than the two-dimensional sensor. At block 714, an image of the spot is generated. At block 716, the position of the spot on the two-dimensional sensor is determined in the image. At block 718, the deviation from the optimal focus position is determined using the position of the spot on the two-dimensional sensor. For example, a single image of the spot on the two-dimensional sensor may be used to determine the deviation from the optimal focus position. At block 720, the focus position of the ellipsometer is adjusted based on the deviation from the optimal focus position. At block 722, for example, by Figure 2 The analyzer 112 shown in FIG analyzes the polarization state of the second portion of the reflected light beam. At block 724, the analyzed second portion of the reflected light beam is detected.

[0044] In one specific implementation, rotating at least one of the polarization or the phase delay of the polarization state causes the position of the spot to move on the two-dimensional sensor, and the process may further include compensating for the movement of the position of the spot on the two-dimensional sensor. For example, the process may include providing an indication of an angular position of at least one of the polarization or the phase delay of the polarization state, wherein compensating for the movement of the position of the spot on the two-dimensional sensor may use the indication of the angular position of at least one of the polarization or the phase delay of the polarization state.

[0045] Although the present invention has been described in conjunction with specific embodiments for illustrative purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the present invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims

1. An ellipsometer comprising: a source that emits light along a path; a first polarizer that polarizes the light to produce a sample beam that interacts with a sample and is reflected to produce a reflected beam; a compensator disposed in a path of the sample beam or the reflected beam, the compensator inducing a phase delay in the polarization state of the light, wherein at least one of the first polarizer and the compensator rotates about an axis parallel to a propagation direction of the light; a beam splitter positioned after the compensator and in the path of the reflected beam, the beam splitter positioned to receive the reflected beam and direct a first portion of the reflected beam to a focusing system and a second portion of the reflected beam to a second polarizer for analyzing a polarization state of the second portion, wherein both the first portion of the reflected beam and the second portion of the reflected beam include an entire cross-section of the reflected beam; The focusing system is positioned to receive the first portion of the reflected light beam from the beam splitter, the focusing system comprising: a lens system that receives a first portion of the reflected light beam, the lens system amplifying any deviation from a best focus position of the ellipsometer; and a first detector positioned to receive a first portion of the reflected light beam from the lens system, wherein the first detector comprises a two-dimensional sensor and the lens system in the focusing system produces a spot on the two-dimensional sensor and the two-dimensional sensor produces an image of the spot; the second polarizer being positioned to receive a second portion of the reflected light beam from the beam splitter; a second detector positioned to receive the reflected light beam from the second polarizer; and A processor receives the image and is configured to obtain a position of the spot on the two-dimensional sensor in the image to determine a deviation from a best focus position of the ellipsometer.

2. The ellipsometer according to claim 1, wherein The processor is configured to determine a deviation from a best focus position of the ellipsometer using a single image of the spot on the two-dimensional sensor.

3. The ellipsometer according to claim 1, wherein Rotation of the at least one of the first polarizer and the compensator causes the position of the spot to move on the two-dimensional sensor as the at least one of the first polarizer and the compensator rotates, wherein the processor is configured to compensate for the movement of the position of the spot on the two-dimensional sensor caused by the rotation of the at least one of the first polarizer and the compensator.

4. The ellipsometer according to claim 3, wherein: The processor is coupled to receive an angular position signal indicating an angular position of at least one of the first polarizer and the compensator, wherein the processor is configured to use the angular position signal to compensate for movement of the position of the spot on the two-dimensional sensor caused by rotation of the at least one of the first polarizer and the compensator.

5. The ellipsometer according to claim 1 , further comprising an actuator for changing a focal position of the ellipsometer, wherein: The actuator changes the focal position of the ellipsometer based on a deviation from the optimal focus position.

6. The ellipsometer according to claim 1, wherein: The beam splitter is a pellicle beam splitter.

7. The ellipsometer according to claim 1, wherein: The lens system magnifies any deviation from the best focus position of the ellipsometer to at least 2x.

8. The ellipsometer according to claim 1, wherein The size of the spot focused by the lens system onto the two-dimensional sensor is 50% or less of the two-dimensional sensor.

9. A method for focusing an ellipsometer, the method comprising: Producing light along the path; polarizing the light to produce a sample beam that interacts with a sample and is reflected to produce a reflected beam; inducing a phase delay in the polarization state of light in the sample beam or the reflected beam; rotating at least one of the polarization or the phase delay of the polarization state; splitting the reflected light beam and directing a first portion of the reflected light beam along a first path and a second portion of the reflected light beam along a second path, wherein both the first portion of the reflected light beam and the second portion of the reflected light beam include an entire cross-section of the reflected light beam; focusing a first portion of the reflected light beam into a spot on a two-dimensional sensor, wherein any deviation from a best focus position of the ellipsometer is amplified; generating an image of the spot; determining, in the image, a position of the spot on the two-dimensional sensor; determining a deviation from the best focus position using the position of the spot on the two-dimensional sensor; adjusting a focal position of the ellipsometer based on a deviation from the optimal focus position; receiving a second portion of the reflected light beam using a polarizer; analyzing the polarization state of the second portion of the reflected light beam using a polarizer; and An analyzed second portion of the reflected light beam is detected.

10. The method according to claim 9, wherein: The deviation from the best focus position is determined using a single image of the spot on the two-dimensional sensor.

11. The method according to claim 9, wherein Rotating at least one of the polarization or the phase delay of the polarization state causes the position of the spot to move on the two-dimensional sensor, the method further comprising compensating for the movement of the position of the spot on the two-dimensional sensor.

12. The method of claim 11 further comprising providing an indication of the angular position of at least one of the polarization or the phase delay of the polarization state; and compensating for movement of the position of the spot on the two-dimensional sensor using the indication of the angular position of the polarization or the phase delay of the polarization state.

13. The method according to claim 9, wherein: Splitting the reflected light beam and directing a first portion of the reflected light beam along the first path and a second portion of the reflected light beam along the second path is performed by a pellicle beam splitter.

14. The method according to claim 9, wherein Any deviation from the best focus position of the ellipsometer was magnified to at least 2x.

15. The method according to claim 9, wherein The spot size is 50% or less of the two-dimensional sensor.

16. An ellipsometer comprising: a source that emits light along a path; a first polarizer that polarizes the light to produce a sample beam that interacts with a sample and is reflected to produce a reflected beam; a compensator disposed in a path of the sample beam or the reflected beam, the compensator inducing a phase delay in the polarization state of the light; a second polarizer positioned after the compensator in the path of the reflected light beam, wherein at least one of the first polarizer, the compensator, and the second polarizer rotates about an axis parallel to the direction of propagation of the light; a focusing system positioned after the compensator and before the second polarizer in the path of the reflected light beam, the focusing system comprising: a beam splitter positioned to receive the reflected light beam and direct a first portion of the reflected light beam to a lens system and a second portion of the reflected light beam to a second polarizer, wherein both the first portion of the reflected light beam and the second portion of the reflected light beam include an entire cross-section of the reflected light beam, and wherein the second polarizer is used to analyze a polarization state of the second portion; the lens system being positioned to receive the first portion of the reflected light beam and to direct the first portion of the reflected light beam to a first detector, the lens system amplifying any deviation from a best focus position of the ellipsometer; and the first detector being positioned to receive a first portion of the reflected light beam from the lens system, wherein the first detector comprises a two-dimensional sensor and the lens system in the focusing system produces a spot on the two-dimensional sensor and the two-dimensional sensor produces an image of the spot; a second detector positioned to receive the reflected light beam from the second polarizer; and A processor receives the image and is configured to obtain a position of the spot on the two-dimensional sensor in the image to determine a deviation from a best focus position of the ellipsometer.

17. The ellipsometer according to claim 16, wherein: The processor is configured to determine a deviation from a best focus position of the ellipsometer using a single image of the spot on the two-dimensional sensor.

18. The ellipsometer according to claim 16, wherein: Rotation of the at least one of the first polarizer and the compensator causes the position of the spot to move on the two-dimensional sensor as the at least one of the first polarizer and the compensator rotates, wherein the processor is configured to compensate for the movement of the position of the spot on the two-dimensional sensor caused by the rotation of the at least one of the first polarizer and the compensator.

19. The ellipsometer according to claim 18, wherein The processor is coupled to receive an angular position signal indicating an angular position of at least one of the first polarizer and the compensator, wherein the processor is configured to use the angular position signal to compensate for movement of the position of the spot on the two-dimensional sensor caused by rotation of the at least one of the first polarizer and the compensator.

20. The ellipsometer according to claim 16, further comprising an actuator for changing a focal position of the ellipsometer, wherein: The actuator changes the focal position of the ellipsometer based on a deviation from the optimal focus position.

21. The ellipsometer according to claim 16, wherein The beam splitter is a pellicle beam splitter.

22. The ellipsometer according to claim 16, wherein The lens system magnifies any deviation from the best focus position of the ellipsometer to at least 2x.

23. The ellipsometer according to claim 16, wherein: The size of the spot focused by the lens system onto the two-dimensional sensor is 50% or less of the two-dimensional sensor.

Images