Focusing systems for optical metrology equipment

Abstract

Light from an optical metrology device is focused into a measurement spot on a sample using a focusing system. The focusing system uses an image of light reflected from the measurement spot to determine an optimal focus position at a desired location on the sample. The focusing system selects a property of the reflected light, such as polarization state or wavelength, for focusing. The property of the reflected light selected for determining the focus position is affected differently by different parts of the sample. For example, light reflected from the top surface of a sample may have different properties than light reflected by underlying layers. The selected property of the reflected light is used by the focusing system to focus the measurement spot at the top surface or underlying layers of the sample.

Description

[0001] Cross-references to related patent applications

[0002] This patent application claims priority to U.S. non-provisional patent application No. 17 / 110,210, filed on December 2, 2020, entitled “FOCUS SYSTEM FOR OPTICALMETROLOGY DEVICE,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to optical metrology and, in particular, to focused optical metrology apparatus. Background Art

[0004] Semiconductor and other similar industries often use optical metrology equipment to provide non-contact evaluation of substrates during processing. With optical metrology, the sample to be measured is illuminated with light, for example, at a single wavelength or multiple wavelengths. After interacting with the sample, the resulting light is detected and analyzed to determine the desired characteristics of the sample.

[0005] Some optical metrology devices use light that is obliquely incident on a sample and is reflected or scattered from the sample before being detected. An example of an optical metrology device that uses an oblique angle of incidence is an ellipsometer. An ellipsometer is an optical metrology device that detects changes in the polarization state of light reflected from the surface of a sample to measure a property of the sample. Other types of optical metrology devices, such as reflectometers, may also use light with oblique angles of incidence.

[0006] Tilted optical metrology devices such as ellipsometers and reflectometers must be properly focused on the sample. Some systems use independent focusing systems, i.e., systems that are attached to the optical metrology device but use an independent light path to determine the focusing system and therefore the position of the optical metrology device relative to the sample. However, such focusing systems require very precise alignment, which is expensive and difficult. Integrated focusing systems sometimes use a mirror with an aperture, where the mirror reflects the outer rays of light reflected from the sample to a focused detector, while the inner rays of the reflected light are transmitted through the aperture and received by the detector of the optical metrology device. Therefore, such devices are prone to inaccuracies due to stray light. Summary of the invention

[0007] Light from an optical metrology device is focused into a measurement spot on a sample using a focusing system. The focusing system uses an image of the light reflected from the measurement spot to determine an optimal focus position at a desired location of the sample. The focusing system can be configured to select a characteristic of the light reflected from the sample to determine the focus position. The selected characteristic can be, for example, the polarization state or wavelength of the light. Light reflected from the top surface of some samples can have, for example, a different polarization state than light reflected by underlying layers. Similarly, light reflected from the top surface of a sample can have a different wavelength characteristic than light reflected by underlying layers. For example, the top surface can reflect light in both ultraviolet (UV) wavelengths and visible wavelengths, while the bottom surface can reflect light only in visible wavelengths. Thus, the focusing system can be configured to distinguish between reflections from the top surface of the sample and reflections from underlying layers based on a selected characteristic of the reflected light (e.g., polarization state or wavelength), and use the selected characteristic to focus the measurement spot at a desired level.

[0008] In one embodiment, a focusing system for determining a focus position of an optical metrology device may include a beam splitter positioned in an optical path of reflected light. The beam splitter is configured to direct a portion of the reflected light to a measurement detector and a remaining portion of the reflected light to a focus detector. The focusing system may also include a focus detector positioned to receive the remaining portion of the reflected light from the beam splitter. The focus detector receives an image of the reflected light on a detector array, wherein the image of the reflected light includes at least one of a first image of light reflected from a top surface of a sample and a second image of light reflected from one or more layers below the top surface, or a combination thereof, and generates a signal based on a location of the image and a characteristic of at least one of the first image and the second image of the reflected light identifying the reflected light. For example, the characteristic of the reflected light may be a polarization state or a wavelength (e.g., UV or visible-NIR) of the light. At least one processor is coupled to receive the signal from the focus detector and determine the focus position of the optical metrology device based on the signal.

[0009] In one embodiment, a method for determining a focal position of an optical metrology device may include directing a first portion of reflected light from a sample toward a measurement detector and directing a second portion of the reflected light toward a focus detector. An image of the reflected light is detected with the focus detector, wherein the image of the reflected light includes at least one of a first image of light reflected from a top surface of the sample and a second image of light reflected from one or more layers below the top surface, or a combination thereof. A portion of the reflected light is selected based on a characteristic of at least one of the first image and the second image of the reflected light to determine a focal position of the optical metrology device. For example, the characteristic of the reflected light may be a polarization state or a wavelength (e.g., UV or visible-NIR) of the light. The focal position of the optical metrology device is determined using the position of the image on the focus detector and the selected portion of the reflected light.

[0010] In one embodiment, a focusing system for determining a focal position of an optical metrology device may include a beam splitter positioned in an optical path of reflected light. The beam splitter may be configured to direct a portion of the reflected light to a measurement detector and a remaining portion of the reflected light to a focusing detector. The focusing detector may be positioned to receive the remaining portion of the reflected light. The focusing detector receives an image of the reflected light on a detector array, wherein the image of the reflected light includes at least one of a first image of light reflected from a top surface of a sample and a second image of light reflected from one or more layers below the top surface, or a combination thereof. The optical metrology device may also include a device for selecting a portion of the reflected light based on a characteristic of at least one of the first and second images of the reflected light to determine the focal position of the optical metrology device. For example, the characteristic of the reflected light may be a polarization state or a wavelength (e.g., UV or visible-NIR) of the light. The optical metrology device may also include a device for determining the focal position of the optical metrology device based on the image of the reflected light and the selected portion of the reflected light. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 A tilted optical metrology apparatus with a focusing system is shown.

[0012] Figure 2 A top view of a sensor array and a spot on the sensor array of a focus detector used to determine the focus position of an optical metrology device is shown.

[0013] Figure 3 A sample is shown including a film stack that produces multiple reflections on the sensor array of a focus detector.

[0014] Figure 4 A polarization detector is shown which can be used as a focus detector in a focusing system.

[0015] Figure 5 A sample is shown that includes a film stack that produces multiple reflections and a polarization detector that includes pixels that are sensitive to different reflections from the sample based on the polarization of the reflected light.

[0016] Figure 6 is a graph showing the wavelength sensitivity of light reflected from a film stack.

[0017] Figure 7 A portion of an optical metrology apparatus having a focusing system including one or more wavelength filters is shown.

[0018] Figure 8 A portion of an optical metrology apparatus having a focusing system including one or more wavelength filters is shown.

[0019] Fig. 9A sample is shown that includes a film stack that produces multiple reflections and a detector that is sensitive to different reflections from the sample based on the wavelength of the reflected light.

[0020] Fig.10 is a flow chart illustrating a method of focusing an optical metrology apparatus. DETAILED DESCRIPTION

[0021] The tilt optical metrology equipment including the tilt angle reflectometer and the ellipsometer needs to properly focus the light incident on the sample. Because multiple areas of the sample (e.g., semiconductor wafer) can be measured by the optical metrology equipment, the optical metrology equipment must be properly focused at each measurement position. In order to change the focus position of such optical metrology equipment, the distance between the optical system and the sample may be adjusted, which takes time. Therefore, it is desirable to not only accurately determine the focus position, but also to determine the focus position quickly.

[0022] An accurate real-time focusing system for tilting an optical metrology device can use an image of a measurement spot on a sample to determine the optimal focus position of the device. For example, light reflected from a sample can be split so that a portion of the light is received by a detector of the metrology device for measurement, while another portion of the reflected light is directed to a focusing system. The focusing system can receive an image of the measurement spot on the sample and use the image to determine and adjust the focus position of the optical metrology device. Using the measurement spot reflected from the sample enables accurate and rapid determination of the focus position.

[0023] However, some samples produce multiple reflections of the incident light. For example, some samples will produce one reflection from the top surface of the sample and another reflection from one or more layers located below the top surface. Examples of samples that produce multiple reflections are devices with many stacked layers or one or more thick layers that are transparent to the wavelength of light used by the optical metrology equipment. For example, 3D memory technology (such as vertical NAND flash memory) relies on stacking of multiple layers of memory cells. When tilting the metrology equipment to measure such a device, the focusing system may receive multiple images of the measurement spot, one from the top surface and one from the underlying layer, which may make it difficult to determine the exact focus position. If the desired focus is at the top surface of the sample, the imaged measurement spot produced by the reflected light from the bottom of the sample may cause inaccuracies or uncertainties in determining the focus position.

[0024] The focusing system can be configured to select light to be used to determine the focus position of the optical metrology device based on characteristics of the reflected light. For example, the focusing system can select light based on polarization or wavelength to determine the focus position of the optical metrology device. In addition, the focusing system can select light reflected from the top surface of the sample or light reflected from one or more layers below the top surface to use in order to determine the focus position of the optical metrology device. For example, the focusing system can be configured to distinguish between reflections from the top surface of the sample and reflections from the underlying layers based on characteristics of the reflected light.

[0025] In one embodiment, the focusing system can distinguish between the top surface and the underlying layers based on the polarization state of the reflected light. Light reflected from the top surface can have a different polarization state than light reflected from the underlying layers. The focusing system can include a polarizer to select light having a desired polarization state, i.e., a polarization state corresponding to light reflected from the top surface or the underlying layers. For example, the focusing system can use a polarization-sensitive detector that includes an array of micropolarizer pixels aligned with corresponding pixels in a detector array to select signals from pixels that receive light having a polarization state corresponding to light reflected from the top surface or the underlying layers.

[0026] In one embodiment, the focusing system can distinguish between the top surface and the underlying layers based on the wavelength of the reflected light. For example, light having a first wavelength range (such as ultraviolet wavelengths) may be more sensitive to surface reflections, while light having a second wavelength range (e.g., in the visible or infrared spectrum) may be more sensitive to underlying layer reflections. The focusing system may include one or more filters to select light having a desired wavelength range, thereby selecting light reflected from the top surface or the underlying layers. For example, the focusing system may use filters on at least a portion of the detector array to select light having a wavelength range corresponding to the top surface or the underlying layers.

[0027] Figure 1 An optical metrology apparatus 100 is shown with a focusing system 150. As shown, the optical metrology apparatus 100 includes a light source 102, which may be monochromatic or polychromatic, and thus may produce narrow-band or broadband light 111. An objective lens 106 focuses the light 111 onto the surface of a sample 101 held and positioned on a stage 108. The light 111 is incident on the sample 101 at an oblique angle of incidence. The light 111 interacts with and reflects from the sample 101 as reflected light 113, which is then received by another objective lens 110 that may be matched to the objective lens 106. The reflected light 113 from the objective lens 110 may be focused by another lens system 114 and received by a detector 116. The detector 116 may detect the intensity of the reflected light 113 at one or more wavelengths and may be used to determine one or more characteristics of the sample 101.

[0028] In some embodiments, the optical metrology device 100 may be a reflectometer that can use unpolarized light or polarized light. Thus, for example, the optical metrology device 100 may include a polarizer 104. In some embodiments, the optical metrology device 100 may be an ellipsometer and may include a polarization state generator 103 (PSG) and a polarization state analyzer 115. The polarization state generator is used to generate a known polarization state that may be static or variable in the light to be incident on the sample. The polarization state generator 103 includes a polarizer 104 and may include a rotating compensator 105. The polarization state analyzer is used to analyze the polarization state of the light after the light interacts with the sample. The polarization state analyzer 115 includes another polarizer 112, generally referred to as analyzer 112. In some embodiments, the compensator 105' may be located after the sample 101 in the polarization state analyzer 115, and the compensator 105 in the polarization state generator 103 may be removed. In other embodiments, both compensators 105 and 105' may be used. If desired, compensator 105 may be stationary and one or both of polarizer 104 and analyzer 112 may rotate, or alternatively, compensator 105 and polarizer 104 and analyzer 112 may rotate. In embodiments where optical metrology apparatus 100 is an ellipsometer, incident 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). Detector 116 detects the intensity of reflected light 113, which, along with the known positions of polarizer 104, analyzer 112, and compensator 105, may be used to determine ellipsometric parameters ψ and Δ, which may be used to determine various parameters of sample 101, as is well known in the art.

[0029] In order to properly measure the sample 101, the optical metrology device 100 is positioned at an optimal focus position relative to the sample 101. Therefore, the optical metrology device 100 includes an integrated autofocus system 150 that images the same light used by the optical metrology device 100. The focusing system 150 includes a beam splitter 152 and a focus detector 158 for focusing. The focusing system 150 may include additional optical elements for directing light from the beam splitter 152 to the focus detector 158. For example, as shown, the focusing system 150 may include folding mirrors 153 and 155 and lens systems 154 and 156. The beam splitter 152 of the focusing system 150 directs (e.g., reflects) a portion of the reflected light 113 (e.g., 4% to 10% of the total light intensity) to the focus detector 158 in the focusing system 150, and directs (e.g., transmits) the remaining portion of the reflected light 113 (e.g., 90% or more of the total light intensity) to the measurement detector 116. The use of beam splitter 152, sometimes referred to as a "pick-up" beam splitter, is advantageous because the entire cross-section of the beam of reflected light 113 is sampled by focusing system 150, unlike systems using apertures in mirrors where only a portion of the reflected light (e.g., external light) is sampled. 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. In some embodiments, beam splitter 152 can be used to select a characteristic of the light for determining the focus position of an optical metrology device. For example, beam splitter 152 can select a polarization state or wavelength to be used in focusing system 150.

[0030] like Figure 1 As shown, if the polarization state analyzer 115 is present, the beam splitter 152 of the focusing system 150 is located in the light path before the polarization state analyzer 115. By placing the beam splitter 152 before the analyzer 112, the intensity of the light received at the focused detector 158 is not modulated due to the rotation of the polarization optics. Therefore, the beam splitter 152 directs a portion of the reflected light 113 to the detector 158 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 158) does not have a modulated intensity.

[0031] As the rotating optics rotate, the rotation of the polarization optical element (such as the polarizer 104 or the compensator 105) on the polarization state generator 103 may still produce a wobble in the spot imaged on the detector 158 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 produced by the rotating optics in the spot imaged by the detector 158. In addition, if desired, the beam splitter 152 can be positioned in the beam path before the compensator 105', while one or both of the analyzer 112 and the compensator 105' rotate and the polarizer 104 remains stationary, which will again avoid the wobble produced by the rotating optics in the spot imaged by the detector 158.

[0032] The beam splitter 152 may be a pellicle beam splitter, which may have a thickness, such as 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, such as Figure 1 Use of a pellicle beamsplitter can further minimize chromatic aberration and prevent imaging ghosts.

[0033] Figure 2 A top plan view of a sensor 204 of the focus detector 158 and a spot 202 produced by the lens system 156 is shown. The sensor 204 may be, for example, a two-dimensional sensor array. The position of the spot 202 on the sensor 204, which may be moved as indicated by arrow 206, is used as an indication of the focus position of the optical metrology device 100. The spot 202 may be 1% to 50% of the size of the sensor 204, such as 10% or less, which increases the useful autofocus range. The lenses of the lens system 156 are arranged to amplify deviations from the best focus position, thereby providing greater measurement accuracy. For example, as Figure 2 As shown by arrow 206 in FIG. 1 , the movement of the position of spot 202 on sensor 204 provides an amplified indication of the deviation from the best focus position. The size of the spot on sensor 204 may vary slightly as sample 101 is swept through the focus range, however, this has a relatively small effect on the spot position calculation, which may be configured to be insensitive. The magnification produced by lens system 154 relative to the deviation from the best focus position may be 2x to 5x or greater, such as 10x. However, the reduction in spot size reduces the focus accuracy, and thus a trade-off is made between autofocus range and accuracy, as a smaller spot makes the spot position calculation less accurate, but provides a higher spot intensity. Therefore, if desired, a large spot, such as the size of sensor 204, may be produced, which may be used to provide a more accurate spot position calculation.

[0034] like Figure 11, the detector 158 for the focusing system 150 is coupled to a computer 170, for example, via a frame capture board 160. The rotating optical elements (if any, such as the compensator 105, polarizer 104, or analyzer 112) and the stage 108 may also be connected to the frame capture board 160, either directly or via a controller / driver (for example, as shown by driver 168). The detector 116 for the optical metrology device 100 may be coupled to the same computer 170 or a different computer, as desired. The computer 170 includes one or more processors 172 with a memory 174, and a user interface including, for example, a display 178 and an input device 180. The frame capture board 160 includes one or more processors 162 (which may be field programmable gate arrays (FPGAs)) configured to determine focus errors that are used to control the focus position of the stage 108, for example, via the stage servo controller 108cont, which receives focus error data from the frame capture board 160 and controls the actuator 109 in the stage 108 accordingly. Thus, in one embodiment, the frame capture board 160 processes the focus error directly from the detector 158 and provides focus adjustments to the stage servo controller 108cont without input from the computer 170. Of course, the computer 170 may be used in part or in whole to process the focus error and instruct the stage servo controller 108cont, as desired. It should be understood that a processor, such as the processor 162 on the frame capture board 160, may include one or more separate processing units, for example, the processor 162 may include a first processor for image processing and a separate processor for focus error determination. In addition, one or more processors may be located elsewhere besides the frame capture board 160. For example, processor 162 (or one or more of the processor units including processor 162) may be located in detector 158 or elsewhere.

[0035] If the processor 172 is, for example, a microprocessor that executes instructions of a computer program, then the data structures and software codes for automatically performing one or more actions described in this detailed description may be implemented by a person 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 174 or medium 182, which may be any device or medium that may store code and / or data for use by a computer system. The computer-readable storage medium 174 / 182 may be, but is not limited to, magnetic storage devices and optical storage devices, such as disk drives, tapes, optical disks, and DVDs (digital versatile disks or digital video disks). The communication port 184 may also be used to receive instructions for programming the processor 172 to perform any one or more of the functions described herein, and may represent any type of communication connection, such as to the Internet or any other computer network. In addition, the functions described herein may 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 may be embodied in a computer-understandable descriptor language that may be used to create an ASIC or PLD that operates as described herein. For example, as discussed above, a field programmable gate array (FPGA) may be used. The FPGA can be located in the detector 158 or on a frame capture board 160 that is internal or external to the computer 170. In the case where the processor 172 is an FPGA, the computer-readable storage medium 174 / 182 can provide a programming file to embed the desired configuration in the processor 172, which can be executed once or at power-up for the non-volatile FPGA. By avoiding the use of the main system CPU to perform the necessary calculations for autofocus, the CPU is not slowed down. In addition, the dedicated processor increases the image processing speed. Therefore, the stage servo controller 108cont can be directly coupled to the frame capture board 160, which can provide signals directly to the stage servo controller 108cont via a serial peripheral communication interface (SPI) channel.

[0036] Focusing a tilt angle optical metrology device such as metrology device 100 may be difficult if sample 101 produces multiple reflections from incident light. For example, reflected light 113 may include reflections from both the top surface of sample 101 and one or more layers below the top surface. If focusing system 150 receives multiple reflections from sample 101, it may be difficult to determine which reflection is from the desired focus position, i.e., the top surface of sample 101, resulting in inaccuracies in the focus position.

[0037] Figure 3By way of example, a sample 300 is shown that includes a film stack that can produce multiple reflections on a sensor array of a focus detector in an optical metrology device 100. Sample 300 can, for example, include a stack of a large number (e.g., 256) pairs 302 of layers such as silicon dioxide and tungsten. The layer stack can be located on a stack of oxide 304, polysilicon 306, metal 308 (e.g., tungsten), and oxide 310, which can be located on silicon (not shown). Light reflected from the film stack in sample 300 can be reflected from a top surface 301 of sample 300 and from a bottom of sample 300 (e.g., from metal layer 308). Reflections from layers between the top surface and metal layer 308 (e.g., layer 302) are weak and may not be visible (or desirable) in a focusing system. Therefore, Figure 3 Also shown is a detector image 320 that illustrates light that may be reflected from the sample 300. As shown, one spot 322 may be reflected from the top surface 301 of the sample 300, while another spot 324 may be reflected from a layer (e.g., metal layer 308) below the top surface 301 of the sample 300. The presence of multiple spots 322 and 324, which may partially overlap or may be separated, in the detector image 320 may create complications and inaccuracies when determining the focal position of the sample 300. For example, an average of the positions of the spots 322 and 324 may be used to determine the focal position rather than the actual position of the spots from the desired focal position (e.g., spot 322).

[0038] The film stack of the sample 300 may have different effects on the characteristics of the light reflected from the top surface 301 and the underlying layer 308. Therefore, the focusing system 150 may be configured to distinguish between the reflection from the top surface of the sample and the reflection from the underlying layer based on the characteristics of the reflected light. For example, the film stack of the sample 300 may produce different polarizations for the light reflected from the top surface 301 and the underlying layer 308. Therefore, the focusing system 150 may distinguish between the top surface and the underlying layer based on the polarization state of the reflected light. For example, in one embodiment, the focus detector 158 may distinguish between the reflection from the top surface of the sample and the reflection from the underlying layer based on the polarization state of the reflected light. In another example, the top surface 301 and the underlying layer 308 of the film stack of the sample 300 may reflect light of different wavelengths. Therefore, the focus detector 158 may distinguish between the reflection from the top surface 301 of the sample and the reflection from the underlying layer 308 based on the wavelength of the reflected light.

[0039] Figure 4 An example of a polarization detector 400 is shown, which can be used as a focus detector 158 in the focus system 150 to select a portion of the light reflected from the sample to be used to determine the focus position of the optical metrology device 100. If the polarization detector 400 is used as the focus detector 158, the optical metrology device 100 includes, for example, a polarizer 104 in a polarimetric reflectometer mode or an ellipsometer mode ( Figure 1104) to polarize light 111 incident on sample 101. In addition, because the polarization effect is used to distinguish light reflected from top surface 301 and underlying layer 308 using polarization detector 400, beam splitter 152 in focusing system 150 is located in the light path before analyzer 112 or compensator 105 (if present).

[0040] The polarization detector 400 may be an imaging polarimeter camera that may use, for example, an IMX250MZR sensor from Sony, such as sensors used in detectors produced by Imperx Inc, JAI Inc, FLIR Inc, and 4D Technology. The polarization detector 400 includes a micropolarizer array 402 that is aligned with a two-dimensional sensor 410. The micropolarizer array 402 is an array of wire grid polarizers 404 having a plurality of (e.g., four) polarization orientations. Each polarizer 404 in the micropolarizer array 402 is aligned with a pixel 412 in the two-dimensional sensor 410. In addition, the polarization detector 400 may include a microlens array 420, wherein each lens 422 is aligned with a separate polarizer 404 and a pixel 412.

[0041] Polarization detector 400 may be defined as an array of macropixels, where each macropixel includes a plurality of pixels having a wire grid polarizer 404 with different polarization states aligned with detector pixels 412. For example, Figure 4 As shown, macropixel 406 may be defined as a 2×2 pixel array having four different polarization states (e.g., 0°, 45°, −45°, and 90°). Polarization detector 400 may include a plurality of macropixels, such as a 227×227 macropixel array, to detect spots 322 and 324 (at Figure 3 ), and each macropixel detects four different polarization states.

[0042] Figure 5 A top plan view of the sample 300 and polarization detector 400 is shown, showing a plurality of macro-pixels including pixels sensitive to different reflections from the sample 300. As shown in macro-pixel 506, light reflected from the top surface 301 of the sample 300 has one polarization orientation and is detected by pixel 508, while light reflected from underlying layers 308 has a different polarization orientation and is detected by pixel 510. Thus, by using signals from pixels in each macro-pixel that are sensitive to reflections from a desired surface, reflections from undesired layers can be ignored and the focus position can be accurately determined. For example, assuming that the optical metrology device 100 is to be focused on the top surface 301 of the sample 300, only the signals from pixel 508 in each macro-pixel are used to determine the focus position.

[0043] In some embodiments, the optical metrology device 100 may include a rotating polarization element, such as the polarizer 104 or the compensator 105, that will change the polarization orientation of the light reflected from the sample 300. Thus, the pixels in the macropixel 506 that are sensitive to the polarization state of the reflected light from the surface on the sample will change as the rotating polarization element rotates. Thus, the polarization detector 400 will change the pixels in each macropixel that detect the reflection from the desired surface in synchronization with the rotating or rotating polarization element.

[0044] In some embodiments, the top surface 301 of the sample 300 and the layers below the top surface 301 can reflect light of different wavelengths. Figure 6 For example is a graph 600 showing the wavelength sensitivity of light reflected from a film stack. Figure 6 The normalized intensity of TE polarized light reflected from a film stack similar to sample 300 with respect to wavelength as measured by an optical metrology device such as optical metrology device 100 is shown. Graph 600 shows two curves, one curve 602 representing a film stack having 100 layer pairs, and the other curve 604 representing a film stack having 200 layer pairs. As can be seen in graph 600, both curves 602 and 604 have a large smooth peak 606 in the ultraviolet wavelength range of about 210nm to 240nm. Wavelengths longer than ultraviolet, such as wavelengths in the visible wavelengths greater than 400nm, illustrate high frequency oscillations. The peak 606 in the ultraviolet wavelength range is due to reflection from the top surface 301 of the sample, which is opaque to ultraviolet light, while the visible wavelength range is due to reflection from the top surface 301 and the surface below the top surface 301.

[0045] Thus, in embodiments where the light source 102 generates broadband light, the focusing system 150 can distinguish between the top surface and the underlying layers based on the wavelength of the reflected light. In some embodiments, the focusing system 150 can include a focus detector 158 capable of distinguishing between reflections from the top surface 301 of the sample and reflections from the underlying layers 308 based on the wavelength of the reflected light. In some embodiments, one or more filters can additionally or alternatively be used to select light having a desired wavelength range to distinguish between light reflected by the focus detector 158 from the top surface 301 of the sample and light reflected from the underlying layers 308 based on the wavelength of the reflected light.

[0046] Figure 7 1 shows a portion of the optical metrology apparatus 100, and in particular shows one embodiment of the detector arm 700 (e.g., located after the sample 101) of the optical metrology apparatus 100. It should be understood that the illumination arm (located before the sample) of the optical metrology apparatus 100 can be connected to the detector arm 700 of the optical metrology apparatus 100. Figure 1 The same as shown, for example, includes a light source 102, an objective lens 106 and an optional polarization state generator 103. Figure 7As shown, focusing system 150 may optionally be positioned after polarization state analyzer 115 (if present).

[0047] The focusing system 150 may include a UV sensitive focus detector 158. For example, the focus detector may be an ultraviolet camera having high sensitivity to wavelengths in at least the UV range, and in some embodiments extending to the visible and infrared ranges. For example, a back-illuminated CMOS camera, such as a UV CMOS camera, may be used. The UV sensitive focus detector 158 may receive reflected light and detect ultraviolet light. Additionally, optical components in the focusing system 150, such as the folding mirrors 153 and 155 and the lens systems 154 and 156, may be selected for UV performance. For example, the folding mirrors 153 and 155 may be UV enhanced aluminum coated fused silica mirrors, and the lens systems 154 and 156 may be fused silica lenses or another suitable type of lens that provides an appropriate response to the UV wavelengths of interest.

[0048] In some embodiments, one or more filters may be located before the focused detector 158, which may help distinguish ultraviolet light from visible light. Figure 7 As further shown in FIG. 1 , the focusing system 150 may also include a filter 159 prior to the focusing detector 158. The filter 159 may pass wavelengths corresponding to wavelengths reflected by a desired surface on the sample and may block other wavelengths. For example, if the top surface 301 of the sample 300 is the desired surface for focusing, the filter 159 may pass ultraviolet wavelengths and block longer wavelengths, such as visible wavelengths and infrared wavelengths. In some embodiments, as shown in the inset plan view of the detector 158 and the portions of the spots 322 and 324, the filter 159 may include an ultraviolet portion 159 that passes only ultraviolet light. UV and a visible portion 159 that passes only wavelengths greater than ultraviolet (eg, visible wavelengths and infrared wavelengths) VIS The detector 158 may, for example, have a UV portion 159 adapted to receive different wavelengths and to be coupled to a filter 159 corresponding thereto. UV and visible part 159 VIS Thus, the focused detector 158 can receive a spot 322 reflected from the top surface 301 of the sample 300 in a separate area, which includes ultraviolet wavelengths (in the portion 159 UV ) and visible and infrared wavelengths (shown in section 159 VIS ), and receiving a portion of the spot 324 reflected from the lower layer 308 of the sample 300, which has only wavelengths greater than ultraviolet, such as visible wavelengths or infrared wavelengths (in portion 159 VIS ).

[0049] Figure 8 Similar to Figure 7 1 and 2 show a portion of the optical metrology apparatus 100, and in particular another embodiment of the detector arm 800 (e.g., located after the sample 101) of the optical metrology apparatus 100. It should be understood that the illumination arm (located before the sample) of the optical metrology apparatus 100 may be connected to the detector arm 800 of the optical metrology apparatus 100. Figure 1 The same as shown, for example, includes a light source 102, an objective lens 106 and an optional polarization state generator 103. Figure 8 As shown, focusing system 150 may optionally be positioned after polarization state analyzer 115 (if present).

[0050] like Figure 8 As shown, the focus system 150 may include the filter 157 at a location other than immediately before the focus detector 158. For example, Figure 8 1, filter 157 can be positioned on mirror 155. In other embodiments, filter 157 can be positioned on beam splitter 152 or can be a beam splitter. Filter 157 can pass wavelengths corresponding to wavelengths reflected by a desired surface on the sample and can block other wavelengths. For example, if top surface 301 of sample 300 is the desired surface for focusing, filter 157 can pass ultraviolet wavelengths and block longer wavelengths, such as visible and infrared wavelengths. In some embodiments, as shown in the inset plan view of detector 158 and portions of spot 322 and spot 324, filter 157 can include a portion of detector 158 that passes only the portion of 157 that is reflected by the detector 158. UV The received ultraviolet light passes through the ultraviolet portion, and only the portion 157 at the detector 158 VIS The detector 158 may have a visible portion adapted to receive wavelengths greater than ultraviolet (eg, visible and infrared wavelengths) and pass through the visible portion. The detector 158 may, for example, have a plurality of portions adapted to receive different wavelengths and correspond to the ultraviolet portion 157. UV and visible part 157 VIS In another embodiment, the reflector 155 may be adjustable (as indicated by the double arrow 155). 旋转 301 of the sample 300 is the desired focus position, and only wavelengths greater than ultraviolet, such as visible wavelengths or infrared wavelengths, are reflected on the detector 158 when the reflection from the top surface 301 of the sample 300 is the desired focus position. Thus, the focused detector 158 can receive spots 322 reflected from the top surface 301 of the sample 300 in separate areas simultaneously or separately, which include ultraviolet wavelengths (in the portion 157 UV ) and visible and infrared wavelengths (shown in section 157 VIS), and receiving a portion of the spot 324 reflected from the lower layer 308 of the sample 300, which has only wavelengths greater than ultraviolet, such as visible wavelengths or infrared wavelengths (in portion 157 VIS ).

[0051] Fig. 9 A top plan view of a sample 300 and a focus detector 900 is shown, which may be Figure 7 The detector 158 shown has a filter 159 immediately preceding the detector 158, or a Figure 8 Filter 157 is shown elsewhere in the optical path of focusing system 150, such as on mirror 155. As shown, detector 158 includes a UV portion 902 that receives ultraviolet light and a VIS-NIR portion 904 that receives longer wavelengths, such as visible light or infrared light. Light reflected from top surface 301 of sample 300 will include ultraviolet light and visible light or infrared light and thus be received by UV portion 902 and VIS-NIR portion 904 as spot 322, and light reflected from underlying layers 308 of sample 300 will not include ultraviolet light but will only include long wavelengths, such as visible light or infrared light, and thus will be received by VIS-NIR portion 904 as only a portion of spot 324. In this way, by using signals from pixels in UV portion 902 and / or VIS-NIR portion 904, reflections from a desired surface of the sample can be detected while reflections from undesired layers are ignored or eliminated, so that the focus position can be accurately determined. For example, assuming that the optical metrology device 100 is to be focused on the top surface 301 of the sample 300, only the signal from the UV portion 902 is used to determine the focus position. On the other hand, assuming that the optical metrology device 100 is to be focused on the underlying layer 308 of the sample 300, the signal from the VIS-NIR portion 904 (including reflections from both the top and underlying surfaces) can be used, and the signal from the UV portion 902 can be used to identify and exclude the spots 322 generated by the top surface of the sample.

[0052] Fig.10 1 is a flow chart 1000 illustrating a method of focusing an optical metrology device (such as the optical metrology device 100 shown in 1) according to embodiments described herein. In some embodiments, the optical metrology device can direct light along a light path that is obliquely incident on a sample. The light can be narrowband (e.g., a single wavelength) or broadband. In some embodiments, the light that is obliquely incident on the sample can be obliquely polarized, for example, by a polarization state generator 103, which can include a polarizer 104 and can also include a compensator 105, one or more of which can be a rotating element. The means for directing light along a light path that is obliquely incident on the sample can be, for example, Figure 1An objective lens 106 is shown which directs light 111 generated by the light source 102 to be incident obliquely on the sample.

[0053] At block 1002, a first portion of reflected light from the sample is directed to a measurement detector and a second portion of the reflected light is directed to a focus detector. The reflected light from the sample may be directed to the focus detector before or after a polarization state analyzer, if present. By way of example, the polarization state analyzer, if present, may include an analyzer and may also include a compensator, one or more of which may be a rotating element. The means for directing the first portion of reflected light from the sample to the measurement detector and the second portion of reflected light to the focus detector may be, for example, Figure 1 A beam splitter 152 is shown which directs a portion of the reflected light 113 to a measurement detector 116 and another portion of the reflected light 113 to a focus detector 158 .

[0054] At block 1004, an image of the reflected light is detected with a focus detector, wherein the image of the reflected light includes at least one of or a combination of a first image of light reflected from a top surface of the sample and a second image of light reflected from one or more layers below the top surface. For example, the reflected light may be focused to form an image on the focus detector, wherein the position of the image on the focus detector provides an indication of a focus position of the optical metrology device. A plurality of images may be formed, wherein a first image is generated by reflected light from the top surface of the sample and a second image is generated by one or more layers below the top surface. The means for detecting an image of the reflected light with a focus detector, wherein the image of the reflected light includes at least one of or a combination of a first image of light reflected from a top surface of the sample and a second image of light reflected from one or more layers below the top surface, may be, for example, a focus detector 158, which receives one or more images generated by the reflected light.

[0055] At block 1006, a portion of the reflected light is selected based on a characteristic of the reflected light identifying at least one of the first image and the second image to determine a focus position of the optical metrology device. For example, in one embodiment where the light is obliquely incident on the sample, selecting the portion of the reflected light may include selecting or using a polarization state that is sensitive to light reflected from the top surface of the sample or one or more of the one or more layers below the top surface of the sample. In another embodiment, selecting the portion of the reflected light may include selecting or using a wavelength of light that is sensitive to light reflected from the top surface of the sample or one or more of the one or more layers below the top surface of the sample. For example, the top surface may reflect light in both ultraviolet (UV) wavelengths and visible to infrared wavelengths, while the bottom surface may only reflect light in visible to infrared wavelengths. Thus, an ultraviolet wavelength may be selected to focus the optical metrology device on the top surface, and a combination of ultraviolet light and visible and / or infrared light may be used to focus the optical metrology device on the lower surface, wherein the ultraviolet wavelength is used to exclude light reflected from the top surface. The means for selecting a portion of the reflected light based on a characteristic of the reflected light identifying at least one of the first image and the second image to determine a focus position of the optical metrology device may be, for example, one or more polarization elements (such as Figure 4 The polarization detector 400 shown or one or more wavelength filtering elements (such as filters 157 or 159) and a detector 158 capable of detecting wavelengths (such as ultraviolet wavelengths, visible wavelengths, infrared wavelengths or combinations thereof).

[0056] At block 108, a focus position of the optical metrology device is determined using the position of the image on the focus detector and the selected portion of the reflected light. The selected portion of the reflected light may be, for example, a polarization state or a wavelength. In some embodiments, the focus position of the optical metrology device may be changed based on the determined focus position. The apparatus for determining the focus position of the optical metrology device using the position of the image on the focus detector and the selected portion of the reflected light may include, for example, one or more processors 162 and / or 172 having dedicated hardware or implementing executable code or software instructions in memory 174 and / or media 182, such as Figure 1 The means for changing the focal position of the optical metrology device based on the determined focal position may be, for example, a stage servo controller 108cont and an actuator 109, such as Figure 1 shown.

[0057] References throughout this specification to "one example," "example," "some examples," or "exemplary embodiments" mean that a particular feature, structure, or characteristic described in conjunction with a feature and / or example may be included in at least one feature and / or example of the claimed subject matter. Thus, the phrases "in one example," "example," "in some examples," or "in some embodiments," or other similar phrases in various places in this specification do not necessarily all refer to the same features, examples, and / or limitations. Furthermore, particular features, structures, or characteristics may be combined in one or more examples and / or features.

[0058] Some parts of the detailed description included herein are presented according to the algorithm or symbolic representation of the operation on the binary digital signal stored in the memory of a specific device or a special computing device or platform. In the context of this specific specification, the term specific device, etc. includes a general-purpose computer that is once programmed to perform a specific operation according to instructions from program software. Algorithmic description or symbolic representation is an example of a technology used by ordinary technicians in signal processing or related fields to convey the essence of their work to other technicians in the field. The algorithm here is generally considered to be a self-consistent sequence of operations or similar signal processing sequences to obtain the desired results. In this context, operation or processing involves the physical manipulation of physical quantities. Usually, but not necessarily, such quantities can take the form of electrical or magnetic signals that can be stored, transmitted, combined, compared or otherwise manipulated. It has been proven that sometimes (mainly for reasons of customary usage) it is convenient to refer to such signals as bits, data, numerical values, elements, symbols, characters, items, numbers, digital numbers, etc. However, it should be understood that all these or similar terms will be associated with appropriate physical quantities and are only convenient labels. Unless otherwise specifically stated, as will be apparent from the discussion herein, it should be understood that terms such as "processing," "computing / calculating," "determining," etc., used throughout the specification discussion refer to the actions or processes of a specific device, such as a special-purpose computer, a special-purpose computing device, or a similar special-purpose electronic computing device. Thus, in the context of this specification, a special-purpose computer or a similar special-purpose electronic computing device is capable of manipulating or transforming signals, typically represented as physical electronic or magnetic quantities within a memory, register, or other information storage device, transmission device, or display device of a special-purpose computer or a similar special-purpose electronic computing device.

[0059] In the foregoing detailed description, numerous specific details have been set forth to provide a thorough understanding of the claimed subject matter. However, those skilled in the art will appreciate that the claimed subject matter may be practiced without these specific details. In other cases, methods and apparatus known to those of ordinary skill have not been described in detail in order to avoid obscuring the claimed subject matter.

[0060] As used herein, the terms "and," "or," and "and / or" may include multiple meanings that depend, at least in part, on the context in which such terms are used. Generally, if used in a list of associations, such as A, B, or C, "or" is intended to mean A, B, and C, used herein in an inclusive sense, and A, B, or C, used herein in an exclusive sense. Additionally, as used herein, the term "one or more" may be used to describe any feature, structure, or characteristic in the singular, or may be used to describe a plurality of features, structures, or characteristics, or other combinations thereof. It should be noted, however, that this is merely an illustrative example, and the claimed subject matter is not limited to this example.

[0061] While there has been illustrated and described what are presently considered to be exemplary features, it will be understood by those skilled in the art that various other modifications may be made and equivalents may be substituted without departing from the claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of the claimed subject matter without departing from the central concept described herein.

[0062] Although the present invention is shown 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. A focusing system for determining a focus position of an optical metrology device, the focusing system comprising: a beam splitter positioned in an optical path of the reflected light, the beam splitter being configured to direct a portion of the reflected light to a measurement detector and a remaining portion of the reflected light to a focus detector; the focus detector being positioned to receive the remaining portion of the reflected light from the beam splitter, the focus detector receiving an image of the reflected light on a detector array, wherein light used to determine the focus position of the optical metrology device is selected based on a characteristic of the reflected light to produce the image of the reflected light, the image of the reflected light comprising at least one of or a combination of a first image of light reflected from a top surface of a sample and a second image of light reflected from one or more layers below the top surface, and a signal is generated based on a position of the image and the characteristic of at least one of the first image and the second image identifying the reflected light; as well as At least one processor is coupled to receive the signal from the focus detector and determine the focus position of the optical metrology device based on the signal.

2. The focusing system according to claim 1, wherein: The at least one processor is configured to find a position of the image of the reflected light on the detector array to determine the focus position of the optical metrology device.

3. The focusing system according to claim 1, wherein: The image of the reflected light from the sample on the detector array includes both the first image of light reflected from the top surface of the sample and the second image of light reflected from the one or more layers below the top surface, wherein the characteristics of the reflected light distinguish the first image from the second image.

4. The focusing system according to claim 1, wherein: The characteristic of the reflected light is polarization.

5. The focusing system according to claim 4, wherein: The focus detector is a polarization detector comprising a plurality of macropixels, each macropixel comprising a micropolarizer array of micropolarizers aligned with corresponding pixels of the detector array, each micropolarizer in the macropixel having a discrete polarization orientation, wherein the at least one processor is configured to select the signal from the focus detector based on a polarization state, the polarization state being sensitive to light reflected from the top surface of the sample or one or more of the layers below the top surface of the sample.

6. The focusing system according to claim 5, further comprising: a rotating polarizer that polarizes light obliquely incident on the sample; Wherein the at least one processor is configured to select the signal from the focus detector in response to a change in polarization of the light produced by the rotating polarizer.

7. The focusing system according to claim 1, wherein: The optical metrology apparatus is an ellipsometer comprising a polarization state generator and a polarization state analyzer, and wherein the beam splitter is positioned in the optical path of the reflected light before the polarization state analyzer.

8. The focusing system according to claim 1, wherein: The characteristic of the reflected light is a wavelength range.

9. The focusing system according to claim 8, wherein: The focus detector includes a detector array that detects ultraviolet light reflected from the top surface of the sample.

10. The focusing system of claim 8, further comprising a filter positioned between the beam splitter and the detector array, the filter being configured to allow the remaining portion of the reflected light having a first wavelength range to be incident on a first set of pixels in the detector array and form the image of the reflected light thereon, wherein Light having the first wavelength range is sensitive to the top surface of the sample.

11. The focusing system according to claim 10, wherein: Light having a second wavelength range is reflected from the one or more layers below the top surface of the sample, wherein the remaining portion of the reflected light having the second wavelength range is incident on a second group of pixels in the detector array, and the at least one processor is further configured to: determine the focus position of the optical metrology device relative to the one or more layers below the top surface of the sample based at least on signals from the second group of pixels in the detector array that receive the reflected light having the second wavelength range.

12. The focusing system according to claim 10, wherein: The image of the reflected light from the sample on the detector array includes a first image of the light having the first wavelength range reflected from the top surface of the sample and a second image of light having a second wavelength range reflected from the one or more layers below the top surface, wherein the at least one processor is configured to find a position of at least one of the first image of light having the first wavelength range and the second image of light having the second wavelength range to determine the focus position of the optical metrology device.

13. The focusing system according to claim 10, wherein: The filter is on at least a portion of the detector array or on a reflective element that directs the reflected light from the beam splitter to the focus detector.

14. The focusing system of claim 1, further comprising an actuator for changing a focal position of the optical metrology device, wherein: The actuator changes the focal position of the optical metrology device based on the focal position determined by the at least one processor.

15. A method for determining a focus position of an optical metrology device, the method comprising: directing a first portion of reflected light from the sample to a measurement detector and directing a second portion of the reflected light to a focus detector; detecting the image of the reflected light with the focus detector, wherein the image of the reflected light includes at least one of or a combination of a first image of light reflected from a top surface of the sample and a second image of light reflected from one or more layers below the top surface; selecting a portion of the second portion of the reflected light based on a characteristic of the reflected light identifying at least one of the first image and the second image to determine the focus position of the optical metrology device; and The focus position of the optical metrology device is determined using the position of the image on the focus detector and the selected portion of the second portion of the reflected light.

16. The method according to claim 15, further comprising: polarizing light obliquely incident on the sample; Wherein selecting the portion of the reflected light comprises selecting a polarization state sensitive to light reflected from the top surface of the sample or one or more of the one or more layers below the top surface of the sample.

17. The method according to claim 15, wherein: Selecting the portion of the reflected light includes selecting a wavelength of light that is sensitive to light reflected from the top surface of the sample or one or more of the one or more layers below the top surface of the sample.

18. The method of claim 15, further comprising changing a focus position of the optical metrology device based on the determined focus position.

19. A focusing system for determining a focus position of an optical metrology device, the focusing system comprising: a beam splitter positioned in an optical path of the reflected light, the beam splitter being configured to direct a portion of the reflected light to a measurement detector and a remaining portion of the reflected light to a focus detector; the focus detector being positioned to receive the remaining portion of the reflected light, wherein the focus detector receives an image of the reflected light on a detector array, wherein the image of the reflected light includes at least one of or a combination of a first image of light reflected from a top surface of the sample and a second image of light reflected from one or more layers below the top surface; means for selecting a portion of the remaining portion of the reflected light to determine a focus position of the optical metrology device, the selecting being based on a characteristic of the reflected light that identifies at least one of the first image and the second image; and means for determining the focus position of the optical metrology device based on the image of the reflected light and a selected portion of the remaining portion of the reflected light.

20. The focusing system of claim 19, wherein: The means for selecting the portion of the reflected light selects a characteristic of the reflected light that is sensitive to light reflected from the top surface of the sample or one or more of the one or more layers below the top surface of the sample.

Images