Patent Publication Number: US-7595869-B1

Title: Optical metrology system optimized with a plurality of design goals

Description:
BACKGROUND 
   1. Field 
   The present application generally relates to the design of an optical metrology system to measure a structure formed on a workpiece, and, more particularly, to a method of optimizing the design of an optical metrology system to meet a plurality of design goals. 
   2. Related Art 
   Optical metrology involves directing an incident beam at a structure on a workpiece, measuring the resulting diffraction signal, and analyzing the measured diffraction signal to determine various characteristics of the structure. The workpiece can be a wafer, a substrate, photomask or a magnetic medium. In manufacturing of the workpieces, periodic gratings are typically used for quality assurance. For example, one typical use of periodic gratings includes fabricating a periodic grating in proximity to the operating structure of a semiconductor chip. The periodic grating is then illuminated with an electromagnetic radiation. The electromagnetic radiation that deflects off of the periodic grating are collected as a diffraction signal. The diffraction signal is then analyzed to determine whether the periodic grating and, by extension, whether the operating structure of the semiconductor chip has been fabricated according to specifications. 
   In one conventional system, the diffraction signal collected from illuminating the periodic grating (the measured diffraction signal) is compared to a library of simulated diffraction signals. Each simulated diffraction signal in the library is associated with a hypothetical profile. When a match is made between the measured diffraction signal and one of the simulated diffraction signals in the library, the hypothetical profile associated with the simulated diffraction signal is presumed to represent the actual profile of the periodic grating. The hypothetical profiles, which are used to generate the simulated diffraction signals, are generated based on a profile model that characterizes the structure to be examined. Thus, in order to accurately determine the profile of the structure using optical metrology, a profile model that accurately characterizes the structure should be used. 
   With increased requirement for throughput, decreasing size of the test structures, smaller spot sizes, and lower cost of ownership, there is greater need to optimize design of optical metrology systems to meet several design goals. Characteristics of the optical metrology system including throughput, range of measurement capabilities, accuracy and repeatability of diffraction signal measurements are essential to meeting the increased requirement for smaller spot size and lower cost of ownership of the optical metrology system. 
   SUMMARY 
   Provided is a method of designing an optical metrology system for measuring structures on a workpiece where the optical metrology system is configured to meet two or more design goals. The design of the optical metrology system is optimized by using collected design goal data in comparison to the set two or more design goals. In one embodiment, the optical metrology system is used for standalone metrology systems. In another embodiment, the optical metrology system is integrated with a fabrication cluster in semiconductor manufacturing. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  is an architectural diagram illustrating an exemplary embodiment where an optical metrology system can be utilized to determine the profiles of structures formed on a semiconductor wafer. 
       FIG. 1B  depicts an exemplary optical metrology system in accordance with embodiments of the invention. 
       FIG. 2  depicts an exemplary flowchart for designing a metrology system for extracting structure profile parameters and controlling a fabrication process. 
       FIG. 3  depicts an exemplary flowchart for a system for extracting structure profile parameters using measurements from the optical metrology system. 
       FIG. 4  depicts an exemplary flowchart for optimizing the design of an optical metrology system based on two or more design goals. 
       FIG. 5  depicts a graph of a reflectance as a function of wavelength for a simulated diffraction signal and a measured diffraction signal. 
       FIG. 6A  depicts a graph of an illumination beam intensity as a function of distance from the center of a uniform beam whereas  FIG. 6B  depicts a graph of an illumination beam intensity from the center of an asymmetrical non-uniform beam. 
       FIG. 7  is an exemplary block diagram of a system to optimize the design of an optical metrology system using two or more design goals. 
       FIG. 8  depicts an exemplary flowchart for optimizing the design of an optical metrology system using primary components and a selected plurality of design goals. 
   

   DETAILED DESCRIPTION 
   In order to facilitate the description of the present invention, a semiconductor wafer may be utilized to illustrate an application of the concept. The systems and processes equally apply to other workpieces that have repeating structures. The workpiece may be a wafer, a substrate, disk, or the like. Furthermore, in this application, the term structure when it is not qualified refers to a patterned structure. 
     FIG. 1A  is an architectural diagram illustrating an exemplary embodiment where optical metrology can be utilized to determine the profiles or shapes of structures fabricated on a semiconductor wafer. The optical metrology system  40  includes a metrology beam source  41  projecting a metrology illumination beam  43  at the target structure  59  of a wafer  47 . The metrology beam  43  is projected at an incidence angle θ (label  45  in  FIG. 1A ) towards the target structure  59 . The diffracted detection beam  49  is measured by a metrology beam receiver  51 . A measured diffraction signal  57  is transmitted to a processor  53 . The processor  53  compares the measured diffraction signal  57  against a simulator  60  of simulated diffraction signals and associated hypothetical profiles representing varying combinations of critical dimensions of the target structure and resolution. The simulator can be either a library that consists of a machine learning system, pre-generated data base and the like (e.g., this is a library system), or on demand diffraction signal generator that solves the Maxwell equation for a giving profile (e.g., this is a regression system). In one exemplary embodiment, the diffraction signal generated by the simulator  60  instance best matching the measured diffraction signal  57  is selected. The hypothetical profile and associated critical dimensions of the selected simulator  60  instance are assumed to correspond to the actual cross-sectional shape and critical dimensions of the features of the target structure  59 . The optical metrology system  40  may utilize a reflectometer, an ellipsometer, or other optical metrology device to measure the diffraction beam or signal. An optical metrology system is described in U.S. Pat. No. 6,913,900, entitled “GENERATION OF A LIBRARY OF PERIODIC GRATING DIFFRACTION SIGNAL”, issued on Sep. 13, 2005, which is incorporated herein by reference in its entirety. 
   Simulated diffraction signals can be generated by applying Maxwell&#39;s equations and using a numerical analysis technique to solve Maxwell&#39;s equations. It should be noted that various numerical analysis techniques, including variations of rigorous coupled-wave analysis (RCWA), can be used. For a more detail description of RCWA, see U.S. Pat. No. 6,891,626, entitled “CACHING OF INTRA-LAYER CALCULATIONS FOR RAPID RIGOROUS COUPLED-WAVE ANALYSES”, filed on Jan. 25, 2001, issued May 10, 2005, which is incorporated herein by reference in its entirety. 
   Simulated diffraction signals can also be generated using a machine learning system (MLS). Prior to generating the simulated diffraction signals, the MLS is trained using known input and output data. In one exemplary embodiment, simulated diffraction signals can be generated using an MLS employing a machine learning algorithm, such as back-propagation, radial basis function, support vector, kernel regression, and the like. For a more detailed description of machine learning systems and algorithms, see U.S. patent application Ser. No. 10/608,300, entitled “OPTICAL METROLOGY OF STRUCTURES FORMED ON SEMICONDUCTOR WAFERS USING MACHINE LEARNING SYSTEMS”, filed on Jun. 27, 2003, which is incorporated herein by reference in its entirety. 
     FIG. 1B  shows an exemplary block diagram of an optical metrology system in accordance with embodiments of the invention. In the illustrated embodiment, an optical metrology system  100  can comprise a lamp subsystem  105 , and at least two optical outputs  106  from the lamp subsystem can be transmitted to an illuminator subsystem  110 . At least two optical outputs  111  from the illuminator subsystem  110  can be transmitted to a selector subsystem  115 . The selector subsystem  115  can send at least two signals  116  to a beam generator subsystem  120 . In addition, a reference subsystem  125  can be used to provide at least two reference outputs  126  to the beam generator subsystem  120 . The wafer  101  is positioned using an X-Y-Z-theta stage  102  where the wafer  101  is adjacent to a wafer alignment sensor  104 , supported by a platform base  103 . 
   The optical metrology system  100  can comprise a first selectable reflection subsystem  130  that can be used to direct at least two outputs  121  from the beam generator subsystem  120  on a first path  131  when operating in a first mode “LOW AOI” (AOI, Angle of Incidence) or on a second path  132  when operating in a second mode “HIGH AOI”. When the first selectable reflection subsystem  130  is operating in the first mode “LOW AOI”, at least two of the outputs  121  from the beam generator subsystem  120  can be directed to a first reflection subsystem  140  as outputs  131 , and at least two outputs  141  from the first reflection subsystem can be directed to a high angle focusing subsystem  145 , When the first selectable reflection subsystem  130  is operating in the second mode “HIGH AOI”, at least two of the outputs  121  from the beam generator subsystem  120  can be directed to a low angle focusing subsystem  135  as outputs  132 . Alternatively, other modes in addition to “LOW AOI” and “HIGH AOI” may be used and other configurations may be used. 
   When the metrology system  100  is operating in the first mode “LOW AOI”, at least two of the outputs  146  from the high angle focusing subsystem  145  can be directed to the wafer  101 . For example, a high angle of incidence can be used. When the metrology system  100  is operating in the second mode “HIGH AOI”, at least two of the outputs  136  from the low angle focusing subsystem  135  can be directed to the wafer  101 . For example, a low angle of incidence can be used. Alternatively, other modes may be used and other configurations may be used. The optical metrology system  100  can comprise a high angle collection subsystem  155 , a low angle collection subsystem  165 , a second reflection subsystem  150 , and a second selectable reflection subsystem  160 . 
   When the metrology system  100  is operating in the first mode “LOW AOI”, at least two of the outputs  156  from the wafer  101  can be directed to the high angle collection subsystem  155 . For example, a high angle of incidence can be used. In addition, the high angle collection subsystem  155  can process the outputs  156  obtained from the wafer  101  and high angle collection subsystem  155  can provide outputs  151  to the second reflection subsystem  150 , and the second reflection subsystem  150  can provide outputs  152  to the second selectable reflection subsystem  160 . When the second selectable reflection subsystem  160  is operating in the first mode “LOW AOI” the outputs  152  from the second reflection subsystem  150  can be directed to the analyzer subsystem  170 . For example, at least two blocking elements can be moved allowing the outputs  152  from the second reflection subsystem  150  to pass through the second selectable reflection subsystem  160  with a minimum amount of loss. 
   When the metrology system  100  is operating in the second mode “HIGH AOI”, at least two of the outputs  166  from the wafer  101  can be directed to the low angle collection subsystem  165 . For example, a low angle of incidence can be used. In addition, the low angle collection subsystem  165  can process the outputs  166  obtained from the wafer  101  and low angle collection subsystem  165  can provide outputs  161  to the second selectable reflection subsystem  160 . When the second selectable reflection subsystem  160  is operating in the second mode “HIGH AOI” the outputs  162  from the second selectable reflection subsystem  160  can be directed to the analyzer subsystem  170 . 
   When the metrology system  100  is operating in the first mode “LOW AOI”, high incident angle data from the wafer  101  can be analyzed using the analyzer subsystem  170 , and when the metrology system  100  is operating in the second mode “HIGH AOI”, low incident angle data from the wafer  101  can be analyzed using the analyzer subsystem  170 . 
   Metrology system  100  can include at least two measurement subsystems  175 . At least two of the measurement subsystems  175  can include at least two detectors such as spectrometers. For example, the spectrometers can operate from the Deep-Ultra-Violet to the visible regions of the spectrum. 
   The metrology system  100  can include at least two camera subsystems  180 , at least two illumination and imaging subsystems  182  coupled to at least two of the camera subsystems  180 . In addition, the metrology system  100  can also include at least two illuminator subsystems  184  that can be coupled to at least two of the imaging subsystems  182 . 
   In some embodiments, the metrology system  100  can include at least two auto-focusing subsystems  190 . Alternatively, other focusing techniques may be used. 
   At least two of the controllers (not shown) in at least two of the subsystems ( 105 ,  110 ,  115 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145 ,  150 ,  155 ,  160 ,  165 ,  170 ,  175 ,  180 ,  182 ,  190 , and  195 ) can be used when performing measurements of the structures. A controller can receive real-signal data to update subsystem, processing element, process, recipe, profile, image, pattern, and/or model data. At least two of the subsystems ( 105 ,  110 ,  115 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145 ,  150 ,  155 ,  160 ,  165 ,  170 ,  175 ,  180 ,  182 , and  190 ) can exchange data using at least two Semiconductor Equipment Communications Standard (SECS) messages, can read and/or remove information, can feed forward, and/or can feedback the information, and/or can send information as a SECS message. 
   Those skilled in the art will recognize that at least two of the subsystems ( 105 ,  110 ,  115 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145 ,  150 ,  155 ,  160 ,  165 ,  170 ,  175 ,  180 ,  182 ,  190 , and  195 ) can include computers and memory components (not shown) as required. For example, the memory components (not shown) can be used for storing information and instructions to be executed by computers (not shown) and may be used for storing temporary variables or other intermediate information during the execution of instructions by the various computers/processors in the metrology system  100 . At least two of the subsystems ( 105 ,  110 ,  115 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145 ,  150 ,  155 ,  160 ,  165 ,  170 ,  175 ,  180 ,  185 , and  190 ) can include the means for reading data and/or instructions from a computer readable medium and can comprise the means for writing data and/or instructions to a computer readable medium. The metrology system  100  can perform a portion of or all of the processing steps of the invention in response to the computers/processors in the processing system executing at least two sequences of at least two instructions contained in a memory and/or received in a message. Such instructions may be received from another computer, a computer readable medium, or a network connection. In addition, at least two of the subsystems ( 105 ,  110 ,  115 ,  120 ,  125 ,  130 ,  135 ,  140 ,  145 ,  150 ,  155 ,  160 ,  165 ,  170 ,  175 ,  180 ,  182 , and  190 ) can comprise control applications, Graphical User interface (GUI) components, and/or database components. 
   It should be noted that the beam when the metrology system  100  is operating in the first mode “LOW AOI” with a high incident angle data from the wafer  101  all the way to the measurement subsystems  175 , (output  166 ,  161 ,  162 , and  171 ) and when the metrology system  100  is operating in the second mode “HIGH AOI” with a low incident angle data from the wafer  101  all the way to the measurement subsystems  175 , (output  156 ,  151 ,  152 ,  162 , and  171 ) is referred to as diffraction signal(s). 
     FIG. 2  depicts an exemplary flowchart for designing an optical metrology system for extracting structure profile parameters and controlling a fabrication process for semiconductors (or integrated circuits, or electronic devices). In this exemplary embodiment, the optical metrology system is integrated in a fabrication cluster. It is understood that the optical metrology system may be in a standalone metrology system with or without automated equipment delivering and retrieving work pieces from the optical metrology system. In step  204 , an optical metrology system coupled to a fabrication cluster is designed to meet two or more design goals. The fabrication cluster may be a lithography, etch, cleaning, chemical-mechanical polishing fabrication cluster, deposition cluster, or the like. The optical metrology system includes an optical metrology tool such as a spectroscopic reflectometer, spectroscopic ellipsometer, hybrid optical device, and the like. The detail steps for designing the optical metrology system are included in the description associated with the flowchart in  FIG. 4 . 
   Referring to  FIG. 2 , the two or more design goals may include: accuracy of the measured diffraction signals assessed by comparing the profile parameter determined from the measured diffraction signal to the profile parameter determined using a reference tool such as scanning electron microscope (SEM); repeatability of the measured diffraction signals, either static or dynamic repeatability or both, typically measured as statistical variation from mean; range of spot sizes of illumination beam that can be measured by the optical metrology system; range of sizes of the measurement spot; throughput in the number of workpieces measured per unit time; and types and range of applications measured. Types and range of applications measured may include line and space or other one dimensional repeating structures, complex transistor structures, two dimensional repeating structures such as vias, contact holes, posts, and trenches, structures with surface, edge, or shape roughness, irregularly shaped structures such as structures having peanut-shaped islands, conical structures, structures with convex or concave surfaces, structures in multiple layers such as overlay and chemical-mechanical polishing (CMP) applications, and other complex structures such as double patterning structures, multiple pitch structures or iso-dense structures. In alternate embodiments, design goals may also include tool-to-tool matching ranges to a similar tool or to a reference tool or to a fleet of tools, reliability of the optical metrology system expressed as up time or mean time between failures, time needed to develop libraries for extracting profile parameters or time needed to train machine learning systems for extracting profile parameters, or cost of ownership measured in dollars needed for service, repair cost, and maintenance costs, and the like. In one embodiment, the optical metrology system may either be integrated in a process tool such as an etcher or be part of a stand alone metrology module. 
   Still referring to  FIG. 2 , in step  208 , a structure is measured with the designed optical metrology system generating a diffraction signal. As mentioned above, the workpiece may be a wafer, a substrate, disk, photomask or the like. In step  212 , at least one profile parameter of the structure is extracted from the measured diffraction signal using one or more systems, such as the regression system, the library system or the machine learning system described above. In step  216 , at least one extracted profile parameter of the structure is transmitted to the fabrication cluster. Extracted profile parameters may include critical dimensions such as bottom width, top width or sidewall angle of the structure. In other embodiments, extracted profile parameters may include any profile parameter of the structure. In step  220 , at least one process parameter or equipment setting of the fabrication cluster is adjusted based on the at least one transmitted profile parameters. 
     FIG. 3  depicts an exemplary flowchart for a system for extracting profile parameters using measurements from the optical metrology system. In step  254 , an optical metrology model is developed using the profile model of the structure and the designed optical metrology system. As mentioned above, the profile of the structure may be a simple line and space grating or a more complex group of repeating structures such as posts, contact holes, vias, or combinations of different shapes structures in a repeating pattern of unit cells. For a detailed description of modeling two-dimensional repeating structures, refer to U.S. patent application Ser. No. 11/061,303, entitled “OPTICAL METROLOGY OPTIMIZATION FOR REPETITIVE STRUCTURES”, by Vuong, et al., filed on Apr. 27, 2004, and is incorporated in its entirety herein by reference. The optical metrology model includes characterization of the illumination beam that is used to illuminate the structure and characterization of the detection beam diffracted from the structure. 
   In step  258 , a regression algorithm is developed to extract the profile parameters of the structure profile using measured diffraction signals. Typically, the regression algorithm compares a series of simulated diffraction signals generated from a set of profile parameters where the simulated diffraction signal is matched to the measured diffraction signal until the matching criteria are met. For a more detailed description of a regression-based process, see U.S. Pat. No. 6,785,638, entitled “SYSTEM AND SYSTEM FOR DYNAMIC LEARNING THROUGH A REGRESSION-BASED LIBRARY GENERATION PROCESS”, filed on Aug. 6, 2001, which is incorporated herein by reference in its entirety. 
   In step  262 , a library of pairs of simulated diffraction signals and profile parameters of the structure are developed. For a more detailed description of an exemplary library-based process, see U.S. Pat. No. 6,943,900, entitled “GENERATION OF A LIBRARY OF PERIODIC GRATING DIFFRACTION SIGNALS”, issued on Sep. 13, 2005, which is incorporated herein by reference in its entirety. In step  266 , an MLS trained using pairs of simulated diffraction signals and profile parameters are developed. The trained MLS is configured to generate a set of profile parameters as output based on an input measured diffraction signal. For a more detailed description of a generating and using a trained MLS, see U.S. Pat. No. 7,280,229, entitled “EXAMINING A STRUCTURE FORMED ON A SEMICONDUCTOR WAFER USING MACHINE LEARNING SYSTEMS”, filed on Dec. 3, 2004, which is incorporated herein by reference in its entirety. In step  270 , at least one profile parameter of the structure profile is determined using the regression algorithm, the library, and/or the trained MLS. It should be noted that the steps described above, ( 254 ,  258 ,  262 ,  264 ,  268 , and  270 ), apply to an optical metrology system in a fabrication cluster or to a standalone optical metrology system. 
     FIG. 4  depicts an exemplary flowchart for optimizing the design of an optical metrology system based on achieving two or more design goals. In step  300 , the range of capabilities of the optical metrology system is determined. The range of capabilities of the optical metrology system may include the types of workpieces or in the case of semiconductor workpieces, wafer applications that can be measured which in turn determines the number and type of measurement beams and optical paths, the range of illumination angles of incidence, number of measurement sites per wafer, the number of measurements per site, and the like. For example, if an optical metrology system is designed to measure one-dimensional repeating structures comprising lines and spaces, two measurement beams may be specified and the illumination beam of incidence would more likely use a fixed angle of incidence. If an optical metrology system is designed to measure both one-dimensional repeating structures and complex two-dimensional repeating structures, two or more measurement beams with a range of illumination beam angles of incidence may be specified. Furthermore, the range of capabilities determined for the optical metrology system may require different types of illumination devices, types of beam focusing optics, types of polarization of the beams, types of collection beam equipment, detectors, and processors. 
   In step  304 , an initial design of the optical metrology system is developed based on the range of capabilities determined in the step  300 . The initial design includes components of the optical metrology system comprising light sources, a homogenizer to produce a uniform light spot, focusing optics for the illumination beams and coating specification for the focusing optics, polarizers for the illumination beams and detection beams, a motion control system for moving the workpiece during pattern recognition and diffraction signal measurement, collecting optics for the detection beams, at least two detectors for measuring the diffraction signals and efficiency of the detector gratings, use of a nitrogen-purged system, a first processor for converting the measured diffraction output to diffraction data, data storage for storing profile parameter extraction algorithms, libraries, or trained machine learning systems, and a second processor for extracting at least one parameter of the structure from the diffraction signal. Furthermore, the measured diffraction signal may be processed to increase the signal to noise ratio by using the first processor and algorithms for minimizing systematic noise from the signal. 
   Referring to  FIG. 4 , in step  308 , two or more design goals for optical metrology system are set. As mentioned above, the design goals can include accuracy of the measured diffraction signals compared to a reference tool such as a SEM, an AFM (atomic force microscope) and the like. Accuracy may also be measured in comparison to a previously calibrated scatterometry tool. Another design goal is repeatability of the measured diffraction signals, either static or dynamic repeatability or both, typically measured as statistical variation from the mean. Another design goal is the range of spot sizes of illumination beam that can be measured by the optical metrology system as well as the range of sizes of the measurement spot. The optical metrology system may be designed to be able to measure a spot size of 32 by 32 micron or smaller. The throughput in the number of workpieces measured per unit time is another design goal. The throughput can be number of workpieces measured per unit time, such as an hour. As mentioned above, the optical metrology system may either be integrated in a process tool such as an etcher or be part of a stand alone metrology module. Another design goal is the type of applications that can be measured by the optical metrology system. As mentioned above, types of applications measured may include one dimensional repeating structures, complex transistors structures, two dimensional repeating structures, structures with surface, edge, or shape roughness, irregular shapes such as peanut shaped islands, convex or concave surfaces, multiple layers structures such as overlay and structures prior to chemical-mechanical polishing, complex structures such double patterning structures, multiple pitch structures or iso-dense structures. Further design goals may also include tool-to-tool matching ranges to a similar tool or to a reference tool or to a fleet of tools, reliability of the optical metrology system expressed as up time or mean time between failures, time needed to develop libraries for extracting profile parameters or time needed to train machine learning systems for extracting profile parameters, or cost of ownership measured in dollars needed for service, repair cost, and maintenance costs, and the like. 
   In step  312 , a metrology model for the optical metrology system is developed. The metrology model includes components of the optical metrology system that have a functional association with set two or more design goals. Assume the two or more design goals include a throughput rate of at least 200 wafers per hour and an accuracy of structure measurement of 3 nanometers (nm) or less compared to cross-section SEM. For the throughput rate, the metrology model can include components associated with the time budget of measuring a wafer for a given application, the number of measurement sites, the number of measurements per site, speed of moving the wafer or the measurement optics to the site, alignment of the site, focusing of the beam, collection of the diffraction signal, processing of the diffraction signal, extraction of profile parameters such critical dimension (CD), sidewall angle, or width of the structure. For a detailed discussion of an optical metrology model designed to optimize an operating time budget, refer to U.S. patent application Ser. No. 12/050,053, entitled “METHOD OF DESIGNING AN OPTICAL METROLOGY SYSTEM OPTIMIZED FOR OPERATING TIME BUDGET”, by Tian, et al., filed on Mar. 18, 2008 and is incorporated in its entirety herein by reference. 
   Referring to  FIG. 4 , step  312 , the metrology model for the example will also include components of the optical metrology system that have a functional association with the accuracy of the measurement as expressed in the difference between the profile parameter extracted using the measured diffraction signal and one measured using a cross-section SEM, where the difference is 3 nm or less. One operating characteristic of the optical metrology system that is correlated with accuracy of the measurement includes intensity of the illumination and detection beams from the one or more light sources up to and including the detection subsystem. Another operating characteristic of the optical metrology system that is correlated to accuracy of the measurement includes signal to noise ratio (SNR) of the illumination signal, the detection signal or both. The SNR is affected by the type of optical components used and the environment in which the optical components operate in, such as the use of an optical environment purged with nitrogen gas, use of a homogenizer to provide highly uniform illumination, use of temperature control subsystems to keep the temperature of the light sources within a narrow temperature range, use of a motion control system that minimizes vibrations of the chuck and workpiece, and the like. In other embodiments, the SNR may also be enhanced using software algorithms to reduce the noise in the measured diffraction signal. Use of algorithms to reduce the noise in the measured diffraction signal are described in U.S. patent application Ser. No. 12/018,028, entitled “NOISE-REDUCTION METROLOGY MODELS”, by Li, et al., filed on Jan. 22, 2008 and U.S. patent application Ser. No. 11/371,752, entitled “WEIGHTING FUNCTION TO ENHANCE MEASURED DIFFRACTION SIGNALS IN OPTICAL METROLOGY”, by Vuong, et al., filed on Mar. 8, 2006, and are incorporated in their entirety herein by reference. 
   In step  316  of  FIG. 4 , a prototype of the optical metrology model is developed. The prototype may include two or more of the metrology components coupled so as to simulate the actual connections and settings in the actual devices in production. For example, the prototype may comprise the light sources and optical components up to and including the focusing optics in the illumination optical path. In another embodiment, the prototype may comprise the light sources, focusing optical components, polarizers, and other optics in the illumination optical path, collection optical components, polarizers, and other optics in the detection optical path. In still another embodiment, the prototype may comprise all optical components from the light sources all the way to the detectors. In still another embodiment, the prototype may include a fully assembled optical metrology system. 
   In step  320 , the data associated with the two or more design goals are collected. For example, assume the two or more design goals include a throughput rate of at least 200 wafers per hour and an accuracy of 3 nanometers (nm) or less compared to cross-section SEM measurements. As mentioned above, the data collected to determine the throughput rate comprises the total time budget needed to complete the measurement of all the sites for the wafer. Time budget applies to metrology steps that cannot be overlapped with other steps in the metrology cycle and the total time budget for each wafer is converted to the equivalent throughput rate, such as wafers per hour. For a detailed discussion of an optical metrology model designed to optimize an operating time budget, refer to U.S. patent application Ser. No. 12/050,053, entitled “METHOD OF DESIGNING AN OPTICAL METROLOGY SYSTEM OPTIMIZED FOR OPERATING TIME BUDGET”, by Tian, et al., filed on Mar. 18, 2008, and is incorporated in its entirety herein by reference. Also, as mentioned above, accuracy of measurement in comparison to a reference metrology tool is a function of the intensity of the light sources, illumination beams, and detection beams. Accuracy is also affected by the signal to noise ratio of the metrology beams. For a detailed discussion of an optical metrology system optimized using operating criteria such as signal intensity and signal to noise ratio, refer to U.S. patent application Ser. No. 12/057,316, entitled “DESIGNING AN OPTICAL METROLOGY SYSTEM OPTIMIZED WITH SIGNAL CRITERIA”, by Tian, et al., filed on Mar. 27, 2008 and is incorporated in its entirety herein by reference. 
   In step  324 , collected design goal data are compared to the set two or more design goals. If the set two or more design goals are not met, in step  328 , the design of the optical metrology system is modified and steps  312 ,  316 ,  320 ,  324 , and  328  are iterated until the two or more design goals are met. In step  328 , modification of the design of the optical metrology system depends on the set two or more design goals. As in the example above, the two or more design goals include measurement of a spot size of 32 by 32 microns or less and a repeatability of measurements with a 3-sigma variance of 2 nanometers or less. For the spot size design goal, modification of the design can include changing the numerical aperture of a set of optical components by substituting different optics and/or using a different optics vendor. Other modifications of the design can include changing the aperture to allow more or less light or changing the shape of the aperture. Other embodiments include design modifications that includes use of apodizers, changing the light source to use different kinds of bulbs, changing the light source or using a different light source from a different vendor, using different size of lenses, changing the aperture shape, changing the angle of incidence of the illumination beam closer to normal or vice versa, using a combination of large aperture optics and a small aperture slit, and the like. For the repeatability design goal, modification of the design can include the same design changes to increase the SNR mentioned above. In addition, design modifications can include keeping the light source in a narrow range of temperature, increasing the accuracy of the auto focus subsystem, increasing the precision of the polarizer rotating mechanism, reducing the electronic noise in the detection system, increasing the loading and positioning accuracy of the workpiece loading mechanism, increasing the wafer alignment accuracy, and the like. 
   Other design modifications can include selecting two or more light sources utilizing different ranges of wavelengths instead of utilizing one light source, illuminating the structure at substantially the same spot with the two or more beams from the two or more light sources at the same time, measuring the two or more diffraction signals off the structure and using one or more detectors for each of the two or more diffraction signals; selecting an off-axis reflectometer wherein the angle of incidence of the illumination beam is substantially around 28 degrees instead of a normal or near normal angle of incidence; selecting an off-axis reflectometer wherein the angle of incidence of the illumination beam is substantially around 65 degrees instead of a near normal reflectometer or instead of 28 degrees; or reducing the number of optical components needed to implement the design. 
   Still referring to step  328 , modification of the design of the of the optical metrology system can also include using a selectable angle of incidence for the illumination beam to optimize accuracy of the diffraction measurement instead of a fixed angle of incidence of the illumination beams, higher efficiency grating and higher efficiency signal detector, configurable numerical aperture for the focusing optics, light source, and the like. In other embodiments, modification of the design of the of the optical metrology system can include selecting a first polarizer in the illumination path and a second polarizer (or analyzer) in the detection path, wherein the first and second polarizers are configured to increase the signal to noise ratio of the illumination and detection beams respectively instead of regular polarizers or substituting the first polarizer and the second polarizer with polarizers from another vendor, replacing mirrors and focusing optics with different quality coatings, replacement of diffractive optic with reflective optics, and the like. 
   In one embodiment mentioned above, one of the two or more design goals may be used to qualify the accuracy of the measurement by comparing to a reference measurement such as an AFM or a cross-section SEM measurement. Modification of the design can include minimizing the effect of system noise and artifacts in the measurement process. Referring to  FIG. 5 , a reflectance graph  500  is depicted as a function of wavelength. In the reflectance graph  500 , a simulated reflectance signal  504  and a measured diffraction signal  508  are provided. The simulated reflectance signal  504  is based on a model of the optical metrology system, the model including a profile model of the structure on the workpiece and the model of the optical metrology tool. Simulation of the reflectance signal is performed, for example, using a numerical analysis solution of the Maxwell equations on electromagnetic diffraction such as RCWA. The measured diffraction signal  508  tracks the simulated reflectance signal  504  except for artifacts  512 ,  514 , and  518 . The artifacts  512 ,  514 , and  518  may be due to a variety of causes such as polarizer leakage due to additional or residual polarization or polarization caused by imperfections or contamination of the structure surface, scattering of the detection beam due to surface roughness of the structure, misalignment of an optical component, and the like. Modifications of the design of the optical metrology system may include replacement of the polarizer, calibration of optical components to minimize the effect of deviations in the specified tolerance and/or imperfections of the components during manufacture, or changes to the structure model to incorporate the surface roughness of the structure. 
   Still referring to  FIG. 5 , another cause for artifacts of the measured diffraction signal can be due to lack of symmetry or uniformity in the intensity of the illumination beam. Referring to  FIG. 6A , the graphical illustration  600  of the illumination beam intensity as a function of the distance from a center  604  of the illumination beam. An intensity graph  608  is substantially a Gaussian distribution where the intensity of the illumination beam diminishes from the center  604  of the beam. Referring to  FIG. 6B , a graphical illustration  650  of the illumination beam intensity as a function of the distance from a center  654  of the illumination beam shows irregularity of an intensity graph  658  where the intensity of the illumination beam dips in around the center  654  of the illumination beam. If the illumination beam included in the optical metrology tool model assumed a symmetrical and uniform intensity beam, the design change to increase accuracy of measurements may include changing the model to incorporate the unevenness of the illumination intensity, doing additional calibration of the light sources and optical components to minimize the residual effects of misalignment, imperfections during manufacture, and/or contamination during handling, or replacing the light source or using a different light source model or another light source vendor, or using a beam homogenizer. 
     FIG. 7  is an exemplary block diagram of a system  700  to optimize the design of the optical metrology system to meet two or more design goals. The system  700  comprising an optical metrology system model  704 , a design goal data collector  708 , a prototype  712 , and a design goal analyzer  716  are coupled to collect and optimize the optical metrology system of a particular design based on the set of two or more design goals. The optical metrology system model  704  is based on an initial design of the optical metrology system. The initial design is driven by the range of capabilities of the optical metrology system. As mentioned above, the range of capabilities includes types of applications measured and may include one dimensional repeating structures, complex transistors structures, two dimensional repeating structures, and complex structures such double patterning structures, multiple pitch structures or iso-dense structures. Included in the optical system model  704  are components of the optical metrology system that arc functionally associated with the two or more design goals. The prototype  712  comprises optical metrology system components that are configured to simulate the performance of the actual optical metrology system. The prototype  712  for an optical metrology system, two or more of the actual metrology components are utilized to test out the optical path and connections between mechanical and electronic components. For example, the prototype may include a motion control subsystem (not shown) programmed to position the wafer to the selected measurement sites, focusing subsystems in the illumination and detection optical paths, and a pattern recognition subsystem (not shown) to determine the orientation of the wafer, where the pattern recognition subsystem is coupled to the motion control subsystem. 
   Referring to  FIG. 7 , design goal data  721 , for example, signal intensity or SNR at different points in the optical path measured in the prototype  712  are transmitted to the design goal data collector  708 . In addition, design goal data from the vendors or historical design goal data  731  for similar optical components are input into the design goal data collector  708 , and collections of design goal data  723  are further sent to the optical metrology system model  704 . The collections of design goal data  723  are processed by the optical metrology system model  704  to generate design goal data for each design goal  725  and are transmitted to the design goal analyzer  716 . The design goal analyzer  716  compares the two or more design goals collected and assembled from all the sources to corresponding input two or more design goals  729 . Based the results of the comparison in the design goal analyzer  716 , modifications to the optical metrology system design  727  are determined and transmitted to and implemented in the prototype  712 . 
   As mentioned above, modification of the design of the optical metrology system depends on the set two or more design goals. As in the example above, assume the two or more design goals include measurement of a spot size of 32 by 32 microns or less and a repeatability of measurements with a 3-sigma variance of 2 nanometers or less. For the spot size design goal, modification of the design can include changing the numerical aperture of a set of optical components by substituting different optics, and/or using a different optics vendor. Other modifications of the design can include changing the slit to allow more or less light or changing the shape of the slit. Other embodiments include design modifications including use of apodizers, changing the light source to use different kinds of bulbs, changing the light source or using a different light source from a different vendor, using different size of lenses, changing the angle of incidence of the illumination beam closer to normal or vice versa, using a combination of large aperture optics and a small aperture slit, and the like. For the repeatability design goal, modification of the design can include the same design changes to increase the SNR mentioned above. In addition, design modifications can include keeping the light source in a narrow range of temperature, increasing the accuracy of the auto focus subsystem, increasing the precision of the polarizer rotating mechanism, reducing the electronic noise in the detection system, increasing the loading and positioning accuracy of the workpiece loading mechanism, increasing the wafer alignment accuracy, and the like. Other modification of the design of the optical metrology system can also include using a selectable angle of incidence for the illumination beam to optimize accuracy of the diffraction measurement instead of a fixed angle of incidence of the illumination beams; higher efficiency grating and higher efficiency signal detector, configurable numerical aperture for the focusing optics, light source, and the like. 
   In the other embodiment mentioned above, one of the two or more design goals may be accuracy of the measurement compared to a reference measurement such as an AFM or a cross-section SEM. Modification of the design can include changing the model of the optical metrology tool included in the optical metrology system model  704 . Steps to increase accuracy can include minimizing the effect of system noise and artifacts in the measurement process discussed in connection with  FIG. 5 . Also, as mentioned above, modifications of the design of the optical metrology system may include replacement of the polarizer, calibration of optical components to minimize the effect of deviations in the specified tolerance and/or imperfections of the components during manufacture. 
   Referring to  FIG. 7 , the changes to the design of the prototype  712  are also incorporated in the design goal data collector  708  and into the optical metrology system model  704 . The set of design goal data  721  from the prototype  712  and from vendor data  731  are input into the design goal data collector  708  and transmitted to the optical metrology system model  704 , generating new design goal data to compare to the set two or more design goals. Changes to the design of the prototype  712 , updates to the design goal data collector  708  and to the optical metrology system model  704 , and processing in the design goal analyzer  716  are iterated until the two or more design goals are met. 
     FIG. 8  depicts an exemplary flowchart for optimizing the design of an optical metrology system using primary components and a selected plurality of design goals. In step  800 , a range of capabilities of the optical metrology system is determined. As mentioned above, the range of capabilities includes types of applications measured and may include one dimensional repeating structures, two dimensional repeating structures, and complex structures such double patterning structures, multiple pitch structures or iso-dense structures. In step  804 , a plurality of design goals is selected. In step  808 , the primary components of the optical metrology system for the selected plurality of design goals are determined. Primary components are those devices or optical components that have significant impacts on a design goal. Assume the plurality of design goals include throughput, accuracy of measurement, repeatability, and measurement spot size. For the throughput design goal, the speed of the motion control system in loading and unloading a wafer has a significant impact on throughput. Similarly, the light intensity output of the bulb and consistency of the output intensity of a xenon light source have significant impact to the accuracy and repeatability of the measurements. The numerical aperture of a lens and the size of the light source bulbs impact the spot size that can be measured by the optical metrology system. For each design goal in the plurality of selected design goals, the primary components are determined either empirically or obtained based on knowledge from previous optical metrology systems or from industry and/or academic resources. Alternatively, the primary components associated with a design goal may be determined using the optimization method described in connection with  FIG. 4  selecting only one design goal. 
   In yet another embodiment, multivariate analysis may be used to determine the correlations of primary components to a design goal of the selected plurality of design goals. For example, assume one design goal is accuracy of the measurement of the diffraction signal. Accuracy is a function of multiple variables including the intensity and temperature stability of the light sources, the intensity of the illumination and detection beams, the signal to noise ratio at various points on the optical paths, presence of a beam homogenizer, vibrations in the motion control system, residual polarization or leakage and the like. Based on comparisons of the measured diffraction signal and the simulated diffraction signal for a known structure and floating values of the variables, the variables can be correlated as to effect on the design goal of accuracy. Formalized multivariate analysis techniques can be used such as linear analysis or nonlinear analysis. Additionally, multivariate analysis can include Principal Components Analysis (PCA), Independent Component Analysis, Cross Correlation Analysis, Linear Approximation Analysis, and the like. In the example above, the intensity and temperature stability of the light sources, the intensity of the illumination and detection beams, the signal to noise ratio at various points on the optical paths, presence of a beam homogenizer, vibrations in the motion control system, residual polarization or leakage may be selected as the principal components affecting the accuracy design goal using PCA. For a detailed description of a method of applying multivariate analysis to determine the primary components or variables affecting optical metrology, refer to U.S. patent application Ser. No. 11/349,773, entitled “TRANSFORMING METROLOGY DATA FROM A SEMICONDUCTOR TREATMENT SYSTEM USING MULTIVARIATE ANALYSIS”, by Vuong, et al., filed on May 8, 2006, and is incorporated herein in its entirety. 
   In step  812 , the initial design of the optical metrology system is developed based on the determined range of capabilities, the selected plurality of design goals, and primary components determined for the selected plurality of design goals. If the range of capabilities include measurement of one and two dimensional repeating structures and the design goals include throughput, accuracy of measurement, and a range of spot sizes to be measured, then the primary components determined empirically or obtained from experience or from industry and industry resources or determined using multivariate analysis will be used to develop the initial design of the optical metrology system. For example, as stated above, a motion control system for loading and aligning the wafer with the appropriate specifications, a dual light source comprising a xenon and a deuterium light sources, specific intensity bulbs for the xenon and deuterium light sources, specific beam homogenizers, specific polarizers, focusing mirrors, and use of an optical path purged with nitrogen may be utilized in the initial design. 
   Still referring to  FIG. 8 , step  816 , a prototype of the optical metrology system is developed based on the initial design and selected plurality of design goals. As mentioned above, the prototype comprises two or more metrology components coupled to simulate the optical, electrical, and mechanical connections in a complete optical metrology tool. In other embodiments, the prototype is a manufactured optical metrology tool such as a reflectometer, ellipsometer, and the like. In still another embodiment, the prototype is a completely manufactured optical metrology system. In step  820 , design goal data is collected using the prototype. The collected design goal data is compared to the corresponding selected plurality of design goals in step  824 . If the plurality of the design goals are not met, the initial design of the optical metrology system is modified, in step  828 , and the development of a prototype, collection of design goal data, comparison of the new collected design goal data, and modification of initial design of the optical metrology system, steps  816 ,  820 ,  824 , and  828 , are iterated until the selected plurality of design goals are met. Modifications to the initial design performed in step  828  are similar to those mentioned above in connection with step  8  of  FIG. 4 . 
   Although exemplary embodiments have been described, various modifications can be made without departing from the spirit and/or scope of the present invention. For example, although throughput, spot size, accuracy, and repeatability of measurements were primarily used to describe the embodiments of the invention, other design goals may also be used. For automated process control, the fabrication clusters may be a track, etch, deposition, chemical-mechanical polishing, thermal, or cleaning fabrication cluster. Furthermore, the elements required for the design of the optical metrology system are substantially the same whether the optical metrology system is integrated in a fabrication cluster or used in a standalone metrology setup. Therefore, the present invention should not be construed as being limited to the specific forms shown in the drawings and described above.