Patent Publication Number: US-2013242303-A1

Title: Dual angles of incidence and azimuth angles optical metrology

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical measurement instrument, and more particularly to a metrology tool with dual angles of incidence and azimuth angles. 
     BACKGROUND 
     The semiconductor industry, as well as other complex nanotechnology process industries, requires very tight tolerances in process control. As dimensions of chip continue to shrink, the tolerance requirements continue to become tighter. Accordingly, new more precise ways of measuring very small dimensions, e.g., on the order of a few nanometers, is desired. At this scale, typical microscopies, such as optical microscopy, or Scanning Electron Microscopy, are not suitable to obtain the desired precision, or to make quick, non-invasive measurements, which are also desirable. 
     Optical metrology techniques have been presented as a solution. The basic principle of optical metrology techniques is to reflect and/or scatter light from a target, and measure the resulting light. The received signal can be based simply on the reflectance of the light from the sample, or the change in polarization state (Psi, Del) of the light caused by the sample. The light may be modeled to retrieve the geometries or other desired parameters of the illuminated sample. Continued improvements in optical metrology, however, are desirable. 
     SUMMARY 
     A dual optical metrology system includes a first metrology device and a second metrology device, each producing light at different oblique angles of incidence on the same spot of a sample from different azimuth angles. The dual optical metrology system further includes a rotating stage that is capable of rotating the sample so the first and second metrology devices can measure the same spot on the sample at different orientations. Thus, the first and second metrology devices generate first and second sets of optical metrology data, respectively, at a first orientation with respect to the sample. After the sample is rotated, the first and second metrology devices generate third and fourth sets of optical metrology data. The first, second, third, and fourth sets of data can then be used to determine one or more parameters of the sample. 
     In one implementation, a method includes generating a first set of optical metrology data using a first light source that produces a first beam of light that is obliquely incident on a spot on a sample at a first angle of incidence with respect to the sample and at a first azimuth angle; generating a second set of optical metrology data using a second light source that produces a second beam of light that is obliquely incident on the spot at a second angle of incidence with respect to the sample and at a second azimuth angle, wherein the second angle of incidence is different than the first angle of incidence and the second azimuth angle is different than the first azimuth angle; altering the orientation of the first beam of light with respect to the sample and the second beam of light with respect to the sample; generating a third set of optical metrology data after altering the orientation by producing a third beam of light that is obliquely incident on the spot at the first angle of incidence with respect to the sample and at a third azimuth angle; generating a fourth set of optical metrology data after altering the orientation by producing a fourth beam of light that is obliquely incident on the spot at the second angle of incidence with respect to the sample and at a fourth azimuth angle, wherein the third azimuth angle is different than the fourth azimuth angle; and using the first set of optical metrology data, the second set of optical metrology data, the third set of optical metrology data, and the fourth set of optical metrology data together to determine at least one parameter of the sample. 
     In one implementation, an apparatus includes a first light source that produces a first beam of light that is obliquely incident on a spot on a sample at a first angle of incidence with respect to the sample; a first detector that detects the first beam of light after interacting with the sample; a second light source that produces a second beam of light that is obliquely incident on the spot at a second angle of incidence with respect to the sample, wherein the second angle of incidence is different than the first angle of incidence and wherein the first light source and the second light source are positioned at different angles with respect to the sample to produce the first beam of light and the second beam of light with different azimuth angles; a second detector that detects the second beam of light after interacting with the sample; means for altering the orientation of the first beam of light with respect to the sample and the second beam of light with respect to the sample; and a processor coupled to receive data from the first detector and data from the second detector and coupled to control the means for altering the orientation, the processor configured to control the means for altering the orientation to produce a first orientation of the first beam of light with respect to the sample while the first detector generates and provides to the processor a first data set based on the first beam of light interacting with the sample at a first azimuth angle and at the first angle of incidence, and to produce a second orientation of the second beam of light with respect to the sample while the second detector generates and provides to the processor a second data set based on the second beam of light interacting with the sample at a second azimuth angle and at the second angle of incidence, wherein the first azimuth angle and the second azimuth angle are different, the processor being further configured to control the means for altering the orientation to produce a third orientation of the first beam of light with respect to the sample so the first detector generates and provides to the processor a third data set based on the first beam of light interacting with the sample at a third azimuth angle and at a third angle of incidence, and to produce a fourth of the second beam of light with respect to the sample while the second detector generates and provides to the processor a fourth data set based on the second beam of light interacting with the sample at a fourth azimuth angle and at a fourth angle of incidence, wherein the third azimuth angle and the fourth azimuth angle are different, the processor being further configured to determine at least one parameter of the sample using the first data set, the second data set, the third data set, and the fourth data set together and to store the parameter of the sample. 
     In one implementation, a method includes producing a first beam of light having a first angle of incidence with respect to a sample, the first beam of light being obliquely incident on a target on the sample at a first azimuth angle with respect to the target; detecting the first beam of light after interacting with the target at the first azimuth angle to produce a first set of data; producing a second beam of light having a second angle of incidence with respect to the sample, the second beam of light being obliquely incident on the target at a second azimuth angle with respect to the target, wherein the second angle of incidence is different than the first angle of incidence and the second azimuth angle is different than the first azimuth angle; detecting the second beam of light after interacting with the target at the second azimuth angle to produce a second set of data; rotating a stage holding the sample; producing the first beam of light to be obliquely incident on the target at the first angle of incidence and at a third azimuth angle with respect to the target; detecting the first beam of light after interacting with the target at the third azimuth angle to produce a third set of data; producing the second beam of light to be obliquely incident on the target at the second angle of incidence and at a fourth azimuth angle with respect to the target, wherein the fourth azimuth angle is different than the third azimuth angle; detecting the second beam of light after interacting with the target at the fourth azimuth angle to produce a fourth set of data; and using the first set of data, the second set of data, the third set of data, and the fourth set of data together to determine a parameter of the sample. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a dual beam metrology system that uses beams having different azimuth angles φ and different angles of incidence. 
         FIGS. 1B and 1C  illustrate a dual beam metrology system that is similar to that shown in  FIG. 1A  but includes flip mirrors. 
         FIG. 2  is a flow chart of a method of using the dual beam metrology system to determine a characteristic of a sample. 
         FIG. 3 , by way of example, illustrates an ellipsometer that may be used as the first metrology device and/or the second metrology device in the dual beam metrology system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a dual beam metrology system  100  that uses beams having different azimuth angles φ and different angles of incidence θ. The dual beam metrology system  100  is illustrated with a first metrology device  110  having a light source  112  and a detector  114  and which has an azimuth angle φ 1  shown with respect to an arbitrary reference line  101  on a sample  102  and an angle of incidence θ 1  with respect to surface normal n to the sample  102 . The sample  102  may be, e.g., a semiconductor wafer or flat panel display or any other substrate. A second metrology device  120  is illustrated with a light source  122  and a detector  124  and with an azimuth angle φ 2  with respect to arbitrary reference line  101  and an angle of incidence θ 2  with respect to surface normal n. The azimuth angles φ 1  and φ 2  are different, and in one implementation may differ by 90°, but may differ by other amounts if desired. The angles of incidence θ 1  and θ 2  are also different and may vary between, e.g., 10° to 80°. The amount that the angles of incidence θ 1  and θ 2  differ may be, e.g., 10° to 70°. 
     In addition, the dual beam metrology system  100  further includes a rotating stage  104  that can rotate the sample  102 , as illustrated by arrow R, as well as translate in the X and Y coordinates, as illustrated. The stage  104  may rotate the sample  102  between measurements of a target  103  so that data can be collected by the first metrology device  110  and second metrology device  120  from the target  103  at different azimuth angles. Thus, for example, where the azimuth angles φ 1  and φ 2  differ by 90°, the rotating stage  104  may rotate between a first measurement and a second measurement by the dual metrology system  100  so that measurements are produced at four different beam orientations, i.e., at (φ 1 , θ 1 ) and (φ 2 , θ 2 ) by the first metrology device  110  and the second metrology device  120 , respectively, prior to rotating the stage, and at (φ 2 , θ 1 ) and (φ 1 , θ 2 ) by the first metrology device  110  and the second metrology device  120 , respectively, after rotating the stage  104  by 90°. If desired, the azimuth angles between the first metrology device  110  and the second metrology device  120  may differ by an amount other than 90°. In such an implementation, the rotating stage  104  may rotate twice to produce the same four beam orientations, i.e., (φ 1 , θ f ), (φ 2 , θ f ), (φ 2 , θ 2 ), and (φ 1 , θ 2 ). If desired, other combinations of beam orientations may be produced by appropriate rotation of the stage  104  and/or selection of azimuth angles of the first metrology device  110  and the second metrology device  120 . Moreover, if desired, the metrology system  100  may include additional metrology devices, which may use angles of incidence with oblique angles or normal incidence beams. If desired, the number of times that the stage rotates may be more than once, which produces more than four sets of data for analysis. 
     Thus, the dual beam metrology system  100  produces four optical metrology data sets: two data sets from the first metrology device  110 , with beam orientations (φ 1 , θ 1 ), (φ 2 , θ 1 ) and two data sets from the second metrology device  120 , with beam orientations (φ 2 , θ 2 ), (φ 1 , ∝ 2 ). The detectors  114  and  124  are coupled to provide the four data sets to a computer  130 , which includes a processor  132  with memory  134 , as well as a user interface including e.g., a display  138  and input devices  140 . A non-transitory computer-usable medium  142  having computer-readable program code embodied may be used by the computer  130  for causing the processor to control the device  100  and to perform the functions including the analysis described herein. The data structures and software code for automatically implementing one or more acts described in this detailed description can be implemented by one of ordinary skill in the art in light of the present disclosure and stored, e.g., on a computer readable storage medium  142 , which may be any device or medium that can store code and/or data for use by a computer system such as processor  132 . The computer-usable medium  142  may be, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, compact discs, and DVDs (digital versatile discs or digital video discs). A communication port  144  may also be used to receive instructions that are used to program the computer  130  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. Additionally, the functions described herein may be embodied in whole or in part within the circuitry of an application specific integrated circuit (ASIC) or a programmable logic device (PLD), and the functions may be embodied in a computer understandable descriptor language which may be used to create an ASIC or PLD that operates as herein described. 
       FIG. 2  is a flow chart of a method of using the dual beam metrology system to determine a characteristic of a sample. As illustrated in  FIG. 2 , a first set of optical metrology data is generated using a first beam of light that is obliquely incident on a spot on a sample at a first angle of incidence and at a first azimuth angle ( 202 ). Optical metrology data may be generated by producing a beam of light from a light source and detecting the beam of light after it interacts with the sample. A second set of optical metrology data is generated using a second beam of light that is obliquely incident on the same spot at a second angle of incidence and at a second azimuth angle ( 204 ). The second angle of incidence is different than the first angle of incidence and the second azimuth angle is different than the first azimuth angle. By way of example, the first set of optical metrology data may be produced by the first metrology device  110 , shown in  FIG. 1A , and the second set of optical metrology data may be produced by the second metrology device  120 . The orientation of the first beam of light with respect to the sample and the second beam of light with respect to the sample is then altered ( 206 ). For example, a relative rotation may be produced between the sample and the first light source that produces the first beam of light and between the sample and the second light source that produces the second beam of light. As shown in  FIG. 1A , the stage  104  may be rotated with respect to the light sources  112  and  122 . Alternatively, if desired, the first metrology device  110  and the second metrology device  120  may be rotated with respect to the sample  102 . Alternatively, rather than rotating the stage, flip mirrors may be used to alter the orientation, i.e., the angle of incidence and/or the azimuth angle, of the first beam of light and the second beam of light.  FIG. 1B , by way of example, illustrates a metrology system  100 ′ that is the same as metrology system  100 , shown in  FIG. 1A , like designated elements being the same, except that metrology system  100 ′ includes flip mirrors  113   a ,  113   b  and  115   a ,  115   b  to alter the angle of incidence of the first light beam from light source  112  and includes flip mirrors  123   a ,  123   b  and  125   a ,  125   b  to alter the angle of incidence of the second light beam from light source  122  without rotating the stage  104 .  FIG. 1C  illustrates a metrology system  100 ″ that is the same as metrology system  100 , shown in  FIG. 1A , like designated elements being the same, except that metrology system  100 ″ includes flip mirrors  113   a ′,  113   b ′ and  115   a ′,  115   b ′ to alter the azimuth angle of the first light beam from light source  112  and includes flip mirrors  123   a ′,  123   b ′ and  125   a ′,  125   b ′ to alter the angle of incidence of the second light beam from light source  122  without rotating the stage  104 . Additionally, and/or alternatively, the azimuth angle of the first light beam from light source  112  and the azimuth angle of the second light beam from second source  122  may both be altered using flip mirrors. 
     A third set of optical metrology data is generated after altering the orientation ( 206 ) using a third beam of light that is obliquely incident on the same spot at the first angle of incidence and at a third azimuth angle ( 208 ). The third beam of light may be from the same light source as the first beam of light, e.g., as illustrated in  FIGS. 1A and 1C  or from the same light source as the second beam of light, e.g., as illustrated in  FIG. 1B . A fourth set of optical metrology data is generated after producing the relative rotation using a fourth beam of light that is obliquely incident on the spot at the second angle of incidence and at fourth azimuth angle ( 210 ). The fourth beam of light may be from the same light source as the second beam of light, e.g., as illustrated in  FIGS. 1A and 1C  or from the same light source as the first beam of light, e.g., as illustrated in  FIG. 1B . The third azimuth angle may be different than the fourth azimuth angle. By way of example, the third set of optical metrology data may be produced by the first metrology device  110 , shown in  FIG. 1A , and the third set of optical metrology data may be produced by the second metrology device  120 . The first set of optical metrology data, the second set of optical metrology data, the third set of optical metrology data, and the fourth set of optical metrology data are used together to determine at least one parameter of the sample ( 212 ). For example, the optical metrology data may be compared to a reference database that includes a plurality of functions, where each of the functions corresponds to one or more parameters of the sample and the optical metrology data, which may be combined or compared to the reference database separately. Once the one or more parameters are obtained, the one or more parameters are stored, e.g., in memory  134  and maybe used in wafer process monitoring, closed-loop control or focus-exposure control in photolithography, or other desired processes. 
     The first metrology device  110  and second metrology device  120  of the dual beam metrology system  100  may employ any desired type of optical metrology, including ellipsometry, spectroscopic ellipsometry, Mueller Matrix ellipsometry, polarized reflectometry, or any other appropriate type of optical metrology. Moreover, the first metrology device  110  and second metrology device  120  may employ the same or different types of optical metrology. Further, the data collection from the first metrology device  110  and second metrology device  120  may be sequential or substantially simultaneous, e.g., within the capabilities of the processor  132 . 
       FIG. 3 , by way of example, illustrates an ellipsometer  300  that may be used as the first metrology device  110  and/or the second metrology device  120 . Ellipsometer  300  is illustrated as a rotating compensator ellipsometer that includes a light source in the form of a polarization state generator (PSG)  302  and detector in the form of a polarization state detector (PSD)  312 . The PSG  302  produces light having a known polarization state and is illustrated as including light source  304  and  306 , e.g., a Xenon Arc lamp and a Deuterium lamp, respectively, to produce light with a range of 200-3100 nm. A beam splitter  308  combine the light from the light sources  304 ,  306  and a polarizer  310  produces the known polarization state. It should be understood that additional, different, or fewer light sources may be used if desired. 
     The PSD  312  includes a polarizing element, referred to as an analyzer  314 , a spectrometer  316  and a detector  318 , which may be, e.g., a cooled CCD array, which is illustrated as coupled to the computer  130 . The analyzer  314  is illustrated as being coupled to the spectrometer  316  and detector  318  via a fiber optic cable  320 . It should be understood that other arrangements are possible, such as directly illuminating the spectrometer  316  from the analyzer  314  without the fiber optic cable  320 . 
     The ellipsometer  300  is illustrated with two rotating compensators  322  and  324  between the PSG  302  and PSD  312 . If desired, the ellipsometer  300  may use a single rotating compensator  322  or  324 , e.g., between the PSG  302  and the sample  301  or between the sample  301  and the PSD  312 , respectively. In another embodiment, the ellipsometer may use a rotating polarizer or analyzer configuration to generate the ellipsometric signals. In these cases, the compensator is not needed. The ellipsometer  300  may further include focusing elements  326  and  328  before and after the sample  301 . The focusing elements may be, e.g., refractive or reflective lenses. 
     The ellipsometer  300  obliquely illuminates the sample  301 , e.g., at a non-zero value of θ with respect to surface normal n. For example, the ellipsometer  300  may illuminate the sample  301  at an angle between 10° to 80°, for example at 65°, but other angles may be used if desired. By way of example, the ellipsometer may be a M2000 ellipsometer produced by JA Woollam Co., Inc. 
     As described above, other types of metrology devices may be used as one or both of the first metrology device  110  and second metrology devices  120 . Moreover, additional metrology device may be used with the dual beam metrology system  100 . For example, a normal incidence polarized reflectometer, or other similar instruments may be used with the dual beam metrology system  100  if desired. 
     The process of analyzing the metrology data obtained by the first metrology device  110  and the second metrology device  120  may vary depending, e.g., on the type of parameter or parameters being measured and the configuration of the sample. In one embodiment, for example, the sample may include a diffracting structure, where the optical metrology data is obtained by detecting the zeroth order diffraction from the diffracting structure. If desired, however, additional orders of the diffraction, e.g., the ±1st orders, may also be detected. The optical metrology data may be analyzed in the original data format or other converted or transformed formats, for example, linear combination, principal components, etc. The sample structure may include two dimensional lines or three dimensional structures. The parameters of the diffracting structure may include, e.g., a shape of lines, holes or islands, linewidth or line length, height, and wall angle of the diffracting structure and overlay shifts. Alternatively, the parameters may be, e.g., at least one of an optical index and film thickness of one or more films on the sample. 
     In one embodiment, the analysis of the optical metrology data may use real time regression, where the measurement data is compared to calculated data by real time calculations and parameters are determined by a nonlinear regression method, an optimization process to minimize the mean square error (MSE) between measurement data and the calculated data. The real time calculation may include Rigorous Couple Wave Analysis (RCWA), finite element, finite difference and machine learning methods. In another embodiment, the analysis of the optical metrology data may be done by a database or library method. The optical metrology data may be compared to a database or library that includes a plurality of functions, where each of the functions corresponds to the one or more parameters of the sample and the optical metrology data, which may be combined or compared to the reference database or library separately. 
     There may be different ways to determine the parameters in data analysis. In one case, all the parameters are determined in one step analysis, where all the metrology data are analyzed simultaneously. In another case, multiple steps of analysis may be used and for each step, partial parameters may be determined by partial data set. The data analysis may also include using different weights for different metrology data set to enhance the parameter sensitivity or reduce parameter correlation. 
     Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.