Focusing system for optical metrology devices

Abstract

Light from the optical metrology device is focused onto a measurement spot on the sample using a focusing system. The focusing system uses an image of the light reflected from the measurement spot to determine the best focus position at a desired location on the sample. The focusing system selects characteristics of the reflected light, such as polarization state or wavelength, for use in focusing. The characteristics of the reflected light selected for use in determining the focus position are affected differently by different portions of the sample. For example, light reflected from the top surface of the sample may have different characteristics than light reflected by an underlying layer. The selected characteristics of the reflected light are used by the focusing system to focus the measurement spot onto the top surface or an underlying layer of the sample.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 17 / 110,210, entitled "FOCUS SYSTEM FOR OPTICAL METROLOGY DEVICE," filed December 2, 2020, which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE INVENTION The present invention relates to optical metrology, and more particularly to focusing optical metrology devices. [Background technology]

[0003] In the semiconductor and other similar industries, optical metrology devices are often used for non-contact evaluation of substrates during processing. In optical metrology, the sample under test 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 desired properties of the sample.

[0004] Some optical metrology devices use light that is incident on a sample at an oblique angle and is reflected or scattered from the sample before being detected. One 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 properties of the sample. Other types of optical metrology devices, such as reflectometers, can also use light at an oblique angle of incidence.

[0005] Inclination optical measurement devices, such as ellipsometers and reflectometers, must be properly focused on the sample. Some systems use a separate focusing system—i.e., a system that is attached to the optical metrology device but uses a separate optical path to determine the position of the focusing system, and therefore 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 that reflects the outer ray of light reflected from the sample to a focus detector, while the inner ray of the reflected light passes through the aperture and is received by the detector of the optical metrology device. However, such devices are susceptible to inaccuracies due to stray light. Summary of the Invention

[0006] Light from an optical measurement device is focused onto a measurement spot on the sample using a focusing system. The focusing system uses an image of the light reflected from the measurement spot to determine the best focus position at a desired location on 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 may be, for example, the polarization state or wavelength of the light. For example, light reflected from the top surface of some samples may have a different polarization state than light reflected by underlying layers. Similarly, light reflected from the top surface of the sample may have different wavelength characteristics than light reflected by underlying layers. For example, the top surface may reflect light in both ultraviolet (UV) and visible wavelengths, while the bottom surface may 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, such as polarization state or wavelength, of the reflected light and use the selected characteristic to focus the measurement spot to a desired level.

[0007] In one implementation, the focusing system for determining the focal position of the optical metrology device can include a beam splitter disposed in an optical path of the 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 can further include a focus detector disposed 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, the image of the reflected light including 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, and generates a signal based on a position of the image and a characteristic of the reflected light that identifies at least one of the first image and the second image. For example, the characteristic of the reflected light can be a polarization state or 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 focal position of the optical metrology device based on the signal.

[0008] In one implementation, a method for determining a focal position of an optical metrology device includes 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 using the focus detector, where 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 the reflected light that identifies at least one of the first image and the second image to determine a focal position of the optical metrology device. For example, the characteristic of the reflected light may be a polarization state or 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.

[0009] In one implementation, the focusing system for determining the focal position of the optical metrology device may include a beam splitter disposed in an optical path of the 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 focus detector. The focus detector may be positioned to receive the remaining portion of the reflected light. The focus detector receives an image of the reflected light on a detector array, the image of the reflected light including 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. The optical metrology device may further include means for selecting a portion of the reflected light based on a characteristic of the reflected light that identifies at least one of the first image and the second image to determine the focal position of the optical metrology device. For example, the characteristic of the reflected light may be a polarization state or wavelength (e.g., UV or visible-NIR) of the light. The optical metrology device may further include means 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 explanation of the drawings]

[0010] [Figure 1] 1 shows an angle beam optical metrology device with a focusing system. [Figure 2] 1 shows a top view of a sensor array of a focus detector and a spot of light on the sensor array used to determine the focus position of an optical measurement device. [Figure 3] 1 shows a sample containing a film stack that produces multiple reflections on the sensor array of a focus detector. [Figure 4] 1 shows a polarization detector that can be used as a focus detector in a focusing system. [Figure 5] 1 shows a sample 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. [Figure 6] 1 is a graph showing the wavelength sensitivity of light reflected from a film stack. [Figure 7]1 illustrates a portion of an optical metrology device having a focusing system that includes one or more wavelength filters. [Figure 8] 1 illustrates a portion of an optical metrology device having a focusing system that includes one or more wavelength filters. [Figure 9] A 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. [Figure 10] 1 is a flowchart illustrating a method for focusing an optical metrology device. DETAILED DESCRIPTION OF THE INVENTION

[0011] Oblique angle optical metrology devices, including oblique angle reflectometers and ellipsometers, require proper focusing of light incident on a sample. Because multiple regions of a sample (e.g., a semiconductor wafer) may be measured by the optical metrology device, the optical metrology device must be properly focused at each measurement location. Changing the focal position of such an optical metrology device may require adjusting the distance between the optics and the sample, which is time-consuming. Therefore, it is desirable not only to accurately determine the focal position, but also to quickly determine the focal position.

[0012] An accurate, real-time focus system for a tilted optical metrology device can use an image of a measurement spot on a sample to determine the best focus position of the device. For example, light reflected from the sample can be split so that a portion of the light is received by a detector in the metrology device for measurement and 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. The use of the measurement spot reflected from the sample provides an accurate and rapid determination of the focus position.

[0013] However, some samples produce multiple reflections of incident light. For example, some samples produce one reflection from the sample's top surface and another reflection from one or more layers below the top surface. One example of a sample that produces multiple reflections is a device with many stacked layers or one or more thick layers that are semi-transparent to the wavelengths of light used by optical metrology devices. For example, 3D memory technologies such as vertical NAND flash memory rely on stacking multiple layers of memory cells. When a tilt metrology device measures such a device, the focusing system may receive multiple images of the measurement spot—one from the top surface and one from the underlying layers—which can make accurate focus position determination difficult. When the desired focus is at the top surface of the sample, an imaged measurement spot produced by reflected light from the bottom of the sample can cause inaccuracies or uncertainties in the focus position determination.

[0014] The focusing system may be configured to select light used to determine the focal position of the optical metrology device based on characteristics of the reflected light. For example, the focusing system may select light based on polarization or wavelength to determine the focal position of the optical metrology device. Additionally, the focusing system may be used to select light reflected from the top surface of the specimen or light reflected from one or more layers below the top surface to determine the focal position of the optical metrology device. For example, the focusing system may be configured to distinguish between reflections from the top surface of the specimen and reflections from underlying layers based on characteristics of the reflected light.

[0015] In one implementation, the focusing system can distinguish between the top surface and the underlying layer 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 layer. The focusing system can include a polarizer that selects light having a desired polarization state, i.e., a polarization state corresponding to light reflected from the top surface or the underlying layer. For example, the focusing system can use a polarization-sensitive detector that includes an array of micropolarizer pixels aligned with corresponding pixels in the 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 layer.

[0016] In one implementation, the focusing system can distinguish between the top surface and the underlying layer based on the wavelength of the reflected light. For example, light having a first range of wavelengths, such as ultraviolet light, may be more sensitive to surface reflections, while light having a second range of wavelengths, for example, in the visible or infrared spectrum, may be more sensitive to reflections from the underlying layer. The focusing system can include one or more filters that select light having a desired wavelength range to select light reflected from the top surface or the underlying layer. For example, the focusing system can 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 layer.

[0017] FIG. 1 illustrates an optical metrology device 100 having a focusing system 150. As shown, the optical metrology device 100 includes a light source 102, which may be monochromatic or polychromatic and thus may generate narrowband or broadband light 111. An objective lens 106 focuses the light 111 onto the surface of a sample 101, which is 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 the sample 101 as reflected light 113, which is reflected therefrom and received by another objective lens 110, which may be compatible with 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 properties of the sample 101.

[0018] In some implementations, the optical metrology device 100 may be a reflectometer that can use unpolarized or polarized light. Thus, for example, the optical metrology device 100 may include a polarizer 104. In some implementations, 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, which may be static or variable, for light 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 it interacts with the sample. The polarization state analyzer 115 includes another polarizer 112, commonly referred to as analyzer 112. In some implementations, 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 implementations, both the compensators 105 and 105′ may be used. Optionally, the compensator 105 may be stationary, one or both of the polarizer 104 and the analyzer 112 may rotate, or the compensator 105 as well as the polarizer 104 and the analyzer 112 may rotate. In implementations in which the optical measurement device 100 is an ellipsometer, the incident light 111 has a known polarization state via the polarization state generator 103. The sample 101 changes the polarization state of the light, and the resulting light reflected by the sample 101 is analyzed by the polarization state analyzer 115, for example, by passing the reflected light 113 through the analyzer 112 (and the compensator 105′, if located after the sample 101). The detector 116 detects the intensity of the reflected light 113, which can be used along with the known positions of the polarizer 104, the analyzer 112, and the compensator 105 to determine ellipsometric parameters. From Ψ and Δ, various parameters of the sample 101 may be determined, as is well known in the art.

[0019] To properly measure the sample 101, the optical metrology device 100 is positioned at the best focus position relative to the sample 101. Therefore, the optical metrology device 100 includes an integrated autofocus system 150 that images the same light beam 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 to direct the 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 a focus detector 158 within 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 a measurement detector 116. The use of the beam splitter 152, sometimes referred to as a "pick-off" beam splitter, is advantageous because the entire cross-section of the reflected light 113 beam is sampled by the focusing system 150, as opposed to only a portion of the reflected beam, e.g., an external beam, as found in systems that use an aperture in a mirror. By sampling the entire beam of reflected light 113, the focusing system 150 is not susceptible to systematic errors caused by sampling only a portion of the reflected light 113. In some implementations, the beam splitter 152 can be used to select the characteristics of the light used to determine the focus position of an optical measurement device. For example, the beam splitter 152 can select the polarization state or wavelength used in the focusing system 150.

[0020] As shown in FIG. 1, if a polarization state analyzer 115 is present, a beam splitter 152 for the focusing system 150 is located in the optical 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 focal detector 158 is not modulated by 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 that is imaged onto the focal plane array of the focusing system 150, i.e., onto the detector 158, does not have a modulated intensity.

[0021] Rotation of the polarizing optical elements in the polarization state generator 103, such as the polarizer 104 or the compensator 105, can still produce a wobble in the spot imaged onto the detector 158 of the focusing system 150 as the rotating optics rotate. If desired, the analyzer 112 can be rotated and the polarizer 104 and compensator 105 can be held stationary, thereby avoiding wobble created by the rotating optics in the spot imaged by the detector 158. Furthermore, 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′ are rotated and the polarizer 104 is held stationary, which also avoids wobble in the spot imaged by the detector 158 produced by the rotating optics.

[0022] Beam splitter 152 may be a pellicle beam splitter, which may be as thin as 0.002 mm, for example, so as not to significantly affect the optical path length or aberrations in a converging beam. The effect on optical path length and aberrations is even smaller in a collimated beam, as shown in FIG. 1. The use of a pellicle beam splitter can further minimize chromatic aberrations and prevent imaging ghosts.

[0023] FIG. 2 shows a top view of the spot of light 202 generated by the sensor 204 and lens system 156 of the focus detector 158. The sensor 204 may be, for example, a two-dimensional sensor array. The position of the spot 202 on the sensor 204, which may move as indicated by the arrow 206, is used as an indicator of the focus position of the optical measurement device 100. The spot 202 may be 1% to 50%, e.g., 10% or less, of the size of the sensor 204, which increases the useful autofocus range. The lenses of the lens system 156 are positioned to magnify deviations from the best focus position, thereby providing greater measurement accuracy. For example, as indicated by the arrow 206 in FIG. 2, moving the location of the spot 202 on the sensor 204 provides a magnified indication of deviations from the best focus position. The size of the spot on the sensor 204 may change slightly as the sample 101 is scanned through the focus range, but this is a relatively small effect to which the spot position calculation can be configured to be insensitive. The magnification produced by lens system 154 with respect to deviation from best focus position may be 2x to 5x or more, for example 10x. However, a trade-off is made between autofocus range and accuracy, as reducing the spot size reduces focus accuracy, and therefore a smaller spot makes spot position calculations less accurate, but provides a higher spot intensity. Therefore, if desired, a larger spot, e.g., the size of sensor 204, can be produced that can be used to provide a more accurate spot position calculation.

[0024] As shown in FIG. 1 , the detector 158 for the focusing system 150 is coupled to a computer 170, for example, via a frame grabber board 160. Rotating optics, such as the compensator 105, polarizer 104, or analyzer 112, and the stage 108 may also be connected to the frame grabber board 160 directly or via a controller / driver, for example, as indicated by driver 168. If desired, the detector 116 of the optical measurement device 100 may be coupled to the same computer 170 or a different computer. The computer 170 includes one or more processors 172 with memory 174, as well as a user interface including, for example, a display 178 and input devices 180. The frame grabber board 160 includes one or more processors 162 (which may be field programmable gate arrays (FPGAs)) configured to determine a focus error, which is used to control the focus position of the stage 108, for example, via a stage servo controller 108 that receives focus error data from the frame grabber board 160 and controls an actuator 109 in the stage 108 accordingly. Thus, in one embodiment, frame grabber board 160 processes focus errors directly from detector 158 and provides focus adjustments to stage servo controller 108 cont without input from computer 170. Of course, if desired, computer 170 may be used for some or all of the focus error processing and instructions to stage servo controller 108 cont. It should be understood that a processor, such as processor 162 on frame grabber board 160, may include one or more separate processing units; for example, processor 162 may include a first processor for image processing and a separate processor for focus error determination. Furthermore, one or more processors may be located at other locations other than frame grabber board 160. For example, processor 162 (or one or more of the processor units that include processor 162) may be located at detector 158 or elsewhere.

[0025] If processor 172 is, for example, a microprocessor executing computer program instructions, data structures and software code for automatically implementing one or more acts described in this detailed description may be implemented by one of ordinary skill in the art in light of this disclosure and stored in a computer-readable storage medium, e.g., memory 174, medium 182, which may be any device or medium capable of storing code and / or data for use by a computer system. Computer-readable storage medium 174 / 182 may be, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tapes, compact disks, and DVDs (digital application disks or digital video disks). Communications port 184 may also be used to receive instructions used to program processor 172 to perform any one or more of the functions described herein and may represent any type of communications connection, such as the Internet or other computer network. Furthermore, the functions described herein may be embodied, in whole or in part, within the circuitry of an application-specific integrated circuit (ASIC) or programmable logic device (PLD), or the functions may be embodied in a computer-understandable description language that can be used to create an ASIC or PLD that operates as described herein. For example, as mentioned above, a field programmable gate array (FPGA) may be used. The FPGA may be within the detector 158 or may reside on the frame grabber board 160, either internal to the computer 170 or external to the computer. If the processor 172 is an FPGA, the computer-readable storage medium 174 / 182 may provide a programming file for embedding the desired configuration within the processor 172, which may be executed once for the non-volatile FPGA or may otherwise be executed at power-up. By avoiding the use of the main system CPU to perform the calculations required for autofocus, the CPU is not slowed down. Also, using a dedicated processor can improve image processing speed.Thus, the stage servo controller 108 cont may be directly coupled to the frame grabber board 160, which may provide signals directly to the stage servo controller 108 cont via a serial peripheral communications interface (SPI) channel.

[0026] Focusing an oblique-angle optical metrology device, such as metrology device 100, can be difficult when the sample 101 generates multiple reflections from the incident light. For example, the reflected light 113 may include reflections from both the top surface of the sample 101 and one or more layers below the top surface. When the focusing system 150 receives multiple reflections from the sample 101, it can be difficult to determine which reflection is from the desired focus position, i.e., the top surface of the sample 101, which can lead to inaccurate focus positions.

[0027] FIG. 3 illustrates, by way of example, a sample 300 including a film stack capable of generating multiple reflections on the sensor array of a focus detector in the optical metrology device 100. The sample 300 may include, for example, a stack of multiple, e.g., 256 pairs 302, of silicon dioxide and tungsten layers. The hierarchical stack may be on a stack of oxide 304, polysilicon 306, metal 308 (e.g., tungsten), and oxide 310, which may be on silicon (not shown). Light reflected from the film stack in the sample 300 may reflect from the top surface 301 of the sample 300 and from the bottom of the sample 300, e.g., from metal layer 308. Reflections from layers between the top surface and metal layer 308, e.g., layer 302, may be weak and not visible (or desirable) in the focusing system. Accordingly, FIG. 3 also illustrates a detector image 320 showing 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, such as metal layer 308, underlying the top surface 301 of the sample 300. The presence of multiple spots 322 and 324 in the detector image 320 may result in the spots partially overlapping or being separated, which can introduce complexity and inaccuracy into determining the focus position of the sample 300. For example, the average of the positions of spots 322 and 324 can be used to determine the focus position instead of the actual position of the spot, such as spot 322, from the desired focus position.

[0028] The film stack of the sample 300 may have different effects on the properties of light reflected from the top surface 301 and the underlying layers 308. Therefore, the focusing system 150 may be configured to distinguish between reflections from the top surface of the sample and reflections from the underlying layers based on the properties of the reflected light. For example, the film stack of the sample 300 may produce different polarizations of light reflected from the top surface 301 and the underlying layers 308. Therefore, the focusing system 150 may distinguish between the top surface and the underlying layers based on the polarization state of the reflected light. For example, in one implementation, the focus detector 158 may distinguish between reflections from the top surface of the sample and reflections from the underlying layers based on the polarization state of the reflected light. In another example, the top surface 301 and the underlying layers 308 of the film stack of the sample 300 may reflect different wavelengths of light. Therefore, the focus detector 158 may distinguish 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.

[0029] 4 shows an example of a polarization detector 400 that may be used as the focus detector 158 in the focusing system 150 to select a portion of light reflected from the sample that is used to determine the focus position of the optical metrology device 100. When the polarization detector 400 is used as the focus detector 158, the optical metrology device 100 includes a polarizer 104 (shown in FIG. 1 ), for example in polarized reflectometer mode or ellipsometry mode, to polarize the light 111 incident on the sample 101. Furthermore, because polarization effects are used to distinguish light reflected from the top surface 301 and the underlying layer 308 using the polarization detector 400, the beam splitter 152 in the focusing system 150, if present, is positioned in the optical path before the analyzer 112 or compensator 105′.

[0030] The polarization detector 400 may be an imaging polarimeter camera, which may use, for example, an IMX250MZR sensor from Sony, such as that used in detectors manufactured by Imperx, Inc., JAI, Inc., FLIR Inc., and 4D Technology. The polarization detector 400 includes a micropolarizer array 402 aligned with a two-dimensional sensor 410. The micropolarizer array 402 is an array of wire grid polarizers 404 having multiple, 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. Additionally, the polarization detector 400 may include a microlens array 420, with each lens 422 aligned with a separate polarizer 404 and pixel 412.

[0031] Polarization detector 400 can be defined as an array of macropixels, each including several pixels with wire grid polarizers 404 having different polarization states aligned with detector pixels 412. For example, as shown in Figure 4, macropixel 406 can be defined as a 2x2 array of pixels with four different polarization states, e.g., 0°, 45°, -45°, and 90°. Polarization detector 400 may include several macropixels, e.g., a 227x227 macropixel array, to image spots 322 and 324 (shown in Figure 3) across multiple macropixels, each detecting four different polarization states.

[0032] FIG. 5 shows a top view of the sample 300 and polarization detector 400, illustrating multiple macropixels, each containing pixels sensitive to different reflections from the sample 300. As shown by macropixel 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 the underlying layer 308 has a different polarization orientation and is detected by pixel 510. Thus, by using the signal from the pixel within each macropixel sensitive to reflections from the desired surface, reflections from undesired layers can be ignored and the focus position can be accurately determined. For example, assuming the optical measurement device 100 is focused on the top surface 301 of the sample 300, only the signal from pixel 508 within each macropixel is used to determine the focus position.

[0033] In some implementations, the optical measurement device 100 can include a rotating polarization element, such as a polarizer 104 or compensator 105, that changes the polarization orientation of light reflected from the sample 300. Thus, the pixels within the macropixel 506 that are sensitive to the polarization state of light reflected from a surface on the sample change as the rotating polarization element rotates. Thus, the polarization detector 400 changes the pixels within each macropixel that are used to detect reflections from the desired surface in synchronization with the rotation or rotating polarization element.

[0034] In some implementations, the top surface 301 of sample 300 and the layers below top surface 301 can reflect different wavelengths of light. For example, FIG. 6 is a graph 600 illustrating the wavelength sensitivity of light reflected from a film stack. FIG. 6 shows the normalized intensity of TE-polarized light reflected from a film stack similar to sample 300 versus wavelength, as measured by an optical metrology device, such as optical metrology device 100. Graph 600 shows two curves: curve 602 represents a film stack with 100 layer pairs, and curve 604 represents a film stack with 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 from approximately 210 nm to 240 nm. Wavelengths longer than ultraviolet, such as visible wavelengths greater than 400 nm, exhibit high-frequency vibrations. 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 as well as surfaces below the top surface 301 .

[0035] Thus, in implementations in which the light source 102 produces 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 implementations, the focusing system 150 can include a focus detector 158 that can distinguish between reflections from the top surface 301 of the specimen and reflections from the underlying layers 308 based on the wavelength of the reflected light. In some implementations, one or more filters can additionally or alternatively be used to select light having a desired wavelength range, allowing the focus detector 158 to distinguish between light reflected from the top surface 301 of the specimen and light reflected from the underlying layers 308 based on the wavelength of the reflected light.

[0036] 7 shows portions of the optical metrology device 100, and in particular, for example, one embodiment of a detector arm 700 after the sample 101 of the optical metrology device 100. It should be understood that the illumination arm (before the sample) of the optical metrology device 100 may be the same as that shown in FIG. 1, including, for example, the light source 102, the objective lens 106, and the optional polarization state generator 103. As shown in FIG. 7, the focusing system 150, if present, may optionally be positioned after the polarization state analyzer 115.

[0037] Focusing system 150 can include UV-sensitive focus detector 158. For example, the focus detector can be an ultraviolet camera with high sensitivity to wavelengths at least in the UV range, and in some implementations, extending to the visible and infrared ranges. For example, a back-illuminated CMOS camera, such as a UV CMOS camera, can be used. UV-sensitive focus detector 158 can receive reflected light and detect ultraviolet light. Additionally, optical components within focusing system 150, such as folding mirrors 153 and 155 and lens systems 154 and 156, can be selected for UV performance. For example, folding mirrors 153 and 155 can be UV-enhanced aluminum-coated fused silica mirrors, and lens systems 154 and 156 can be fused silica lenses or another suitable type of lens that provides an appropriate response at the UV wavelengths of interest.

[0038] In some implementations, one or more filters can be placed before focus detector 158, which can help distinguish ultraviolet light from visible light. For example, as further shown in FIG. 7 , focusing system 150 can further include filter 159 before focus detector 158. Filter 159 can pass wavelengths corresponding to wavelengths reflected by a desired surface on the sample and block other wavelengths. For example, if top surface 301 of sample 300 is the desired surface for focusing, filter 159 can pass ultraviolet wavelengths and block longer wavelengths, such as visible and infrared wavelengths. In some implementations, as shown in the inset plan view of detector 158 and a portion of spots 322 and 324, filter 159 can filter only ultraviolet portion 159, which passes only ultraviolet light. UV and a visible portion 159 that transmits only wavelengths longer than ultraviolet light, such as visible light and infrared light. VIS The detector 158 may, for example, be adapted to receive different wavelengths and filter corresponding ultraviolet portions 159 of the filter 159. UV and visible portion 159 VIS Thus, the focus detector 158 detects the spot 322 reflected from the top surface 301 of the sample 300 (portion 159). UV (shown in part 159) VIS 300) and may be received in separate regions, including both visible and infrared wavelengths (shown in portion 159 VIS 3. Light may be received in portions of spot 324 having wavelengths greater than ultraviolet, for example, only visible or infrared wavelengths (shown in FIG. 3).

[0039] Figure 8 is similar to Figure 7 and shows portions of the optical metrology device 100, and in particular shows another embodiment of a detector arm 800 of the optical metrology device 100, e.g., after the sample 101. It should be understood that the illumination arm (before the sample) of the optical metrology device 100 may be the same as that shown in Figure 1, including, e.g., the light source 102, the objective lens 106, and the optional polarization state generator 103. As shown in Figure 8, the focusing system 150, if present, may optionally be positioned after the polarization state analyzer 115.

[0040] As shown in FIG. 8 , focusing system 150 may include filter 157 in a location other than immediately before focus detector 158. For example, as shown in FIG. 8 , filter 157 may be located on mirror 155. In other implementations, filter 157 may be located on or be beam splitter 152. Filter 157 may pass wavelengths corresponding to wavelengths reflected by a desired surface on the sample and block other wavelengths. For example, if top surface 301 of sample 300 is the desired surface for focusing, filter 157 may pass ultraviolet wavelengths and block longer wavelengths, such as visible and infrared wavelengths. In some implementations, filter 157 may be located near portion 157 of detector 158, as shown in the inset plan view of portions of detector 158 and spots 322 and 324. UV a portion 157 of the detector 158 that passes only the ultraviolet light received on the detector 158; VIS The detector 158 may be adapted to receive different wavelengths and transmit the corresponding ultraviolet portion 157, for example, a visible portion that transmits only visible and infrared light. UV and visible portion 157 VISIn another implementation, the mirror 155 may be adjustable (as indicated by the double arrow 155 ) to reflect only ultraviolet light onto the detector 158 when reflection from the top surface 301 of the sample 300 is at the desired focal position, and to reflect only wavelengths greater than ultraviolet light, such as visible or infrared light, onto the detector 158 when reflection from the layer 308 below the sample 300 is at the desired focal position. 回転 Therefore, the focus detector 158 detects the ultraviolet wavelength portion (157 UV ) and visible and infrared wavelengths (part 157 VIS 3, a portion of spot 324 (portion 157) reflected from layer 308 below sample 300 includes both spot 322 reflected from top surface 301 of sample 300 and only wavelengths greater than ultraviolet, e.g., visible or infrared wavelengths. VIS ) can be received simultaneously or separately in separate areas.

[0041] 9 shows a top view of sample 300 and a focusing detector 900, which may be detector 158 with filter 159 immediately prior to detector 158, as shown in FIG. 7, or filter 157 elsewhere in the optical path of focusing system 150, such as on mirror 155, as shown in FIG. 8. 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 or infrared light. Light reflected from top surface 301 of sample 300 includes ultraviolet and visible or infrared light and is therefore received by UV portion 902 and VIS-NIR portion 904 as spot 322, while light reflected from layer 308 below sample 300 includes only longer wavelengths, such as visible or infrared light, but does not include ultraviolet light, and is therefore received only by VIS-NIR portion 904 as part of spot 324. Thus, by using signals from pixels in the UV portion 902 and / or the VIS-NIR portion 904, reflections from the desired surface of the sample can be detected while reflections from undesired layers can be ignored or filtered out, allowing for an accurate determination of the focus position. For example, assuming the optical metrology device 100 is 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 the optical metrology device 100 is focused on the underlying layer 308 of the sample 300, the signal from the VIS-NIR portion 904, which includes reflections from both the top surface and the underlying layers, can be used, and the signal from the UV portion 902 can be used to identify and filter out the spot 322 produced by the top surface of the sample.

[0042] FIG. 10 is a flowchart 1000 illustrating a method for focusing an optical metrology device, such as the optical metrology device 100 shown in FIG. 1 , according to embodiments described herein. In some implementations, the optical metrology device may direct light along an optical path that is obliquely incident on the sample. The light may be narrowband, e.g., a single wavelength, or broadband. In some implementations, the light that is obliquely incident on the sample may be obliquely polarized, for example, by a polarization state generator 103, and may include a polarizer 104, and may further include a compensator 105, one or more of which may be rotation elements. The means for directing light that is obliquely incident on the sample along an optical path may be, for example, the objective lens 106 shown in FIG. 1 that directs light 111 generated by the light source 102 to be obliquely incident on the sample.

[0043] In block 1002, a first portion of the 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. For example, the polarization state analyzer, if present, may include an analyzer and may further include a compensator, one or more of which may be rotation elements. The means for directing the first portion of the reflected light from the sample to the measurement detector and the second portion of the reflected light to the focus detector may be, for example, beam splitter 152 shown in FIG. 1 , which directs a portion of reflected light 113 to measurement detector 116 and another portion of reflected light 113 to focus detector 158.

[0044] In block 1004, an image of the reflected light is detected using a focus detector, the image of the reflected light including at least one of a first image of light reflected from the top surface of the specimen and a second image of light reflected from one or more layers below the top surface, or a combination thereof. For example, the reflected light may be focused to form an image on the focus detector, and the position of the image on the focus detector provides an indication of the focus position of the optical metrology device. Multiple images may be formed, with a first image generated by the light reflected from the top surface of the specimen and a second image generated by one or more layers below the top surface. The means for detecting the image of the reflected light with the focus detector, including at least one of a first image of light reflected from the top surface of the specimen and a second image of light reflected from one or more layers below the top surface, or a combination thereof, may be, for example, focus detector 158 that receives the one or more images generated by the reflected light.

[0045] In block 1006, a portion of the reflected light is selected based on characteristics of the reflected light that identify at least one of the first image and the second image to determine a focal position of the optical metrology device. For example, in one implementation in which 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 underlying the top surface of the sample. In another implementation, 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 underlying the top surface of the sample. For example, the top surface may reflect both ultraviolet (UV) wavelengths and visible to infrared wavelengths of light, while the bottom surface may reflect only visible to infrared wavelengths of light. Thus, ultraviolet wavelengths may be selected to focus the optical metrology device on the top surface, or a combination of ultraviolet light and visible and / or infrared light may be used to focus the optical metrology device on the underlying surface, with ultraviolet wavelengths used to reject light reflected from the top surface. The means for selecting a portion of the reflected light based on a characteristic of the reflected light that identifies at least one of the first image and the second image and determining the focus position of the optical metrology device may be, for example, one or more polarizing elements, such as polarization detector 400 shown in FIG. 4, or one or more wavelength filtering elements, such as filters 157 or 159, and detector 158 capable of detecting wavelengths such as ultraviolet wavelengths, visible wavelengths, infrared wavelengths, or combinations thereof.

[0046] In block 108, a 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. The selected portion of the reflected light may be, for example, a polarization state or a wavelength. In some implementations, the focal position of the optical metrology device may be changed based on the determined focal position. The means for determining the focal 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 implementing executable code or software instructions in memory 174 and / or medium 182 shown in FIG. 1 . The means for changing the focal position of the optical metrology device based on the determined focal position may be, for example, the stage servo controller 108 and actuator 109 shown in FIG. 1 .

[0047] References throughout this specification to "one example," "an example," "a particular example," or "an exemplary implementation" mean that a particular feature, structure, or characteristic described with respect to a feature and / or example may be included in at least one feature and / or example of the claimed subject matter. Thus, appearances of the phrases "in one example," "an example," "a particular example," or "in a particular implementation," or other similar phrases in various places throughout 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 characteristics.

[0048] Some portions of the detailed descriptions contained herein are presented in terms of algorithms or symbolic representations of operations on binary digital signals stored within a memory of a particular apparatus or special purpose computing device or platform. In the context of this particular specification, the term particular apparatus or the like includes a general purpose computer that has been programmed to perform particular operations pursuant to instructions from program software. Algorithmic descriptions or symbolic representations are examples of techniques used by those skilled in the signal processing or related arts to convey the substance of their work to others skilled in the art. An algorithm is here, and generally, considered to be a self-consistent sequence of operations or similar signal processing leading to a desired result. In this context, operations or processing involve physical manipulations of physical quantities. Typically, though not necessarily, such quantities may take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, or otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to such signals as bits, data, values, elements, symbols, characters, terms, numbers, numerals, or the like. It should be understood, however, that all of these or similar terms are to be associated with appropriate physical quantities and are merely convenient labels. Unless otherwise indicated, and as will be apparent from the description herein, it should be understood that throughout this specification, descriptions utilizing terms such as "processing," "computing," "calculating," "determining," and the like refer to the operations or processes of a particular apparatus, such as a special purpose computer, a special purpose computing apparatus, or a similar special purpose electronic computing device. Thus, in the context of this specification, a special purpose computer or similar special purpose electronic computing device may manipulate or transform signals that are typically represented as physical electronic or magnetic quantities within a memory, register, or other information storage, transmission, or display device of the special purpose computer or similar special purpose electronic computing device.

[0049] In the foregoing detailed description, numerous specific details are set forth to provide a thorough understanding of the claimed subject matter. However, it will be understood by those skilled in the art that the claimed subject matter may be practiced without these specific details. In other instances, methods and apparatuses that would be known by those skilled in the art have not been described in detail so as not to obscure the claimed subject matter.

[0050] As used herein, the terms "and," "or," and "and / or" can have a variety of meanings that are expected to depend, at least in part, on the context in which such terms are used. Typically, when "or" is used to link a list, such as A, B, or C, it is intended to mean A, B, and C, which are used herein in an inclusive sense, as well as A, B, or C, which are 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 or other combination of features, structures, or characteristics. However, it should be noted that this is merely an illustrative example and claimed subject matter is not limited to this example.

[0051] While what are presently considered to be exemplary features have been illustrated and described, it would 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.

[0052] While the present invention has been illustrated in connection with specific embodiments for purposes of illustration, the invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Accordingly, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims

1. A focusing system for determining a focal position of a tilted optical metrology device, the focusing system comprising: a beam splitter disposed in the optical path of the reflected light, the beam splitter 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; a focus detector 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, the image of the reflected light including at least one of a first image of light reflected from a top surface of the specimen and a second image of light reflected from one or more layers below the top surface, or a combination thereof, and generating a signal based on a position of the image and a characteristic of the reflected light that identifies at least one of the first image and the second image; at least one processor coupled to receive the signal from the focus detector and to determine the focus position of the tilted optical metrology device based on the signal.

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

3. 10. The focusing system of claim 1, wherein the image of 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, and the characteristics of the reflected light distinguish between the first image and the second image.

4. 2. The focusing system of claim 1, wherein the property of the reflected light is polarization, the focus detector is a polarization detector comprising a plurality of macropixels, each macropixel comprising a micropolarizer array of micropolarizers aligned with a corresponding pixel of the detector array, each micropolarizer within a macropixel having a discrete polarization orientation, and the at least one processor is configured to select the signal from the focus detector based on a polarization state sensitive to light reflected from one or more of the top surface of the sample or layers underlying the top surface of the sample.

5. further comprising a rotating polarizer that polarizes the light that is obliquely incident on the sample; The focusing system of claim 4 , 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.

6. 2. The focusing system of claim 1, wherein the tilted optical measurement device is an ellipsometer comprising a polarization state generator and a polarization state analyzer, and the beam splitter is positioned before the polarization state analyzer in the optical path of the reflected light.

7. 10. The focusing system of claim 1, wherein the characteristic of the reflected light is a wavelength range, and the focus detector comprises the detector array that detects ultraviolet light reflected from the top surface of the sample.

8. 8. The focusing system of claim 7, further comprising a filter disposed between the beam splitter and the detector array and configured to allow the remaining portion of the reflected light having a first range of wavelengths to be incident on a first set of pixels in the detector array to form the image of the reflected light, wherein light having the first range of wavelengths is sensitive to the top surface of the sample.

9. 10. The focusing system of claim 8, wherein light having a second range of wavelengths is reflected from the one or more layers below the top surface of the specimen, and the remaining portion of the reflected light having the second range of wavelengths is incident on a second set of pixels in the detector array, and the at least one processor is further configured to determine the focal position of the tilted optical metrology device relative to the one or more layers below the top surface of the specimen based on at least the signals from the second set of pixels in the detector array that receive the reflected light having the second range of wavelengths.

10. 9. The focusing system of claim 8, wherein the images of reflected light from the sample on the detector array include a first image of the light having the first range of wavelengths reflected from the top surface of the sample and a second image of light having a second range of wavelengths reflected from the one or more layers below the top surface, and the at least one processor is configured to locate the position of at least one of the first image of light having the first range of wavelengths and the second image of light having the second range of wavelengths to determine the focal position of the tilted optical metrology device.

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

12. A method for determining a focal position of a tilted optical metrology device, comprising: directing a first portion of the reflected light from the sample to a measurement detector and a second portion of the reflected light to a focus detector; detecting an image of the reflected light using 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 specimen and a second image of light reflected from one or more layers below the top surface, or a combination thereof; selecting a portion of the second portion of the reflected light based on a characteristic of the reflected light that identifies at least one of the first image and the second image to determine the focal position of the tilt optical metrology device; and determining the focus position of the tilted optical metrology device using the position of the image on the focus detector and the selected portion of the second portion of the reflected light.

13. further comprising polarizing the light obliquely incident on the sample; 13. The method of claim 12, wherein selecting the portion of the reflected light comprises selecting 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 underlying the top surface of the sample.

14. 13. The method of claim 12, wherein selecting the portion of the reflected light comprises selecting wavelengths of light that are sensitive to light reflected from the top surface of the sample or one or more of the one or more layers underlying the top surface of the sample.

15. The method of claim 12 , further comprising: changing a focus position of the tilted optical metrology device based on the determined focus position.

Images