Abstract:
A metrology recipe includes dynamic instructions that allow a metrology tool to perform a secondary metrology operation on a test wafer when previous measurement data indicates a process issue with that test wafer. The metrology recipe can instruct the metrology tool to perform an efficient default metrology operation on all test wafers, and perform a more in-depth secondary metrology operation on only those wafers that warrant additional scrutiny. In this manner, critical metrology data can be captured with a minimum of effect on metrology throughput. The metrology data used to determine whether or not the secondary metrology operation is to be performed can be generated from default metrology operations within the same tool, or can be generated by measurements taken by a completely different tool. Such “external” metrology data can be received via a communications network, either directly or from a server on the network for processing the metrology data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of semiconductor metrology, and in particular, to a method and system for efficiently dealing with process issues indicated by metrology data. 
     2. Related Art 
     As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Tools used in such characterization efforts are commonly described as metrology tools. For example, ellipsometry tools, scatterometry tools, x-ray fluorescence tools, x-ray reflectometry tools, and electron microprobe analysis tools are all types of metrology tools used to evaluate the properties of the semiconductor, dielectric, and metal layers that form semiconductor devices. 
     A metrology tool is typically controlled by an instruction set (called a “recipe”) that defines measurement parameters for that tool. Those measurement parameters can include the particular test wafers to be measured (e.g., wafers from slots 3, 7, and 12 of a wafer cassette), what type of measurement to perform (e.g., refractive index, film thickness), what type of film to expect, what locations on a wafer to measure, and any other information related to the operation of the tool. 
     Conventional recipes specify a static set of metrology parameters that are applied to all test wafers in a particular test group (typically one or more cassettes of wafers). Therefore, all the test wafers are measured in the same manner. The results of this testing are then reviewed by an operator to determine the appropriate response. 
     The goal of a metrology tool is to monitor the performance of a process tool (or set of process tools) by evaluating structures on a test wafer processed by the process tool. Specifically, the output of a metrology tool is used to detect process excursions (i.e., process results that are outside the acceptable output range), so that appropriate corrective measures can be taken. 
     However, because of the static nature of conventional recipes, the detection of process excursions can sometimes occur too late for optimal response. For example, if one or more of the test wafers from a cassette exhibit process excursions, it would be desirable to perform additional metrology operations on those wafers (for example, to confirm the problem, to determine the extent of the problem, or to precisely characterize the problem). Unfortunately, because of the lag between the metrology operation and the manual review of the results, the test wafers on which the process excursions have been detected have often moved on to the next process step before any additional metrology can be performed. Consequently, any opportunity to “debug” the process is lost. 
     Furthermore, even if the problematic wafers are caught before any further processing is performed, performing the additional metrology operations to determine the scope of the problem can result in significant (expensive) production delays. Once a problem is detected, a new static recipe must be loaded into the metrology tool and the new metrology operation must be performed. The production line is typically shut down during this reconfiguration and restarting of the process tool, thereby resulting in significantly reduced fab output even if the problem is ultimately found to be of no consequence. 
     Accordingly, it is desirable to provide a system and method for detecting and evaluating process excursions without significantly delaying the overall process flow. 
     SUMMARY OF THE INVENTION 
     Conventional metrology recipes specify a static set of metrology parameters, thereby forcing the same metrology operations to be applied to each test wafer in a test group. Consequently, timely detection of process excursions can be difficult, and effective analysis can be time consuming and expensive. To overcome these problems, a metrology recipe or metrology tool operation can be based on dynamic instructions that allow different metrology operations to be performed depending on prior measurement results. 
     In one embodiment, a metrology recipe includes an instruction for performing a default metrology operation on a test wafer and an instruction for performing a secondary metrology operation on the test wafer in response to an error indicator. The error indicator can be generated based on the results of the default metrology operation, or can be based on prior measurements taken by a different tool. This dynamic operation allows different metrology parameters to be used with different wafers. For example, a default (e.g., standard, high speed) metrology operation can be applied to most wafers, while a secondary (e.g., supplemental, in-depth) metrology operation can be applied to those wafers that exhibit indications of process excursions. Because the secondary metrology operations can be applied immediately and without reconfiguring the metrology tool (i.e., without loading a new recipe), supplemental metrology can be performed both in a timely manner and before the opportunity for such supplemental metrology is lost. 
     In various embodiments, the default metrology operation(s) and supplemental metrology operation(s) can include different measurement maps and measurement types. For example, in one embodiment, the supplemental metrology operation can be similar to the default metrology operation, except performed at a larger number of locations on the test wafer. In another embodiment, the supplemental metrology operation can comprise a completely different metrology type than the default metrology operation. 
     In another embodiment, a metrology tool can be operated such that the tool applies a default metrology operation to a test wafer, and subsequently applies a secondary metrology operation to the test wafer if the results of the default metrology operation indicate a process excursion. Thus, an efficient default metrology operation can be applied to most wafers (i.e., those not exhibiting problematic measurements), while a more in-depth secondary metrology operation can be applied to potentially problematic wafers, thereby maximizing the effectiveness of the metrology tool while minimizing the decrease in metrology throughput. 
     According to another embodiment, a metrology tool can be operated such that the tool applies a default metrology to a test wafer unless imported metrology data (i.e., measurement results from a different metrology tool) indicates a problem with the test wafer. In various embodiments of the invention, the imported metrology data can be provided by a networked metrology tool, and can be received either directly from the networked tool, or can be received from a central server that compiles and processes the output of the networked tool. 
     In another embodiment, a first metrology tool and a second metrology tool (and optional any number of additional metrology tools) are coupled via a communications network to allow the second metrology tool to perform dynamic metrology operations based on measurement data from the first metrology tool. In one embodiment, the first metrology tool and the second metrology tool communicate directly. In another embodiment, the first metrology tool and the second metrology tool are coupled to a server that processes data passed between the tools. In one embodiment, the second metrology tool can perform default metrology operations on a set of test wafers unless prior measurements on those test wafers by the first metrology tool (received as imported data by the second metrology tool) indicate a problem with one or more of the test wafers, or if the prior measurements on those test wafers indicate a problematic trend or pattern. If the imported data does indicate a process issue, the second metrology tool can apply a secondary metrology operation to the identified test wafers (or even all of the test wafers). 
     In another embodiment, a metrology recipe includes an instruction for measuring a default number of wafers from a test group of wafers, an instruction for performing a default metrology operation on test wafers from the test group of wafers, and an instruction for increasing the number of wafers being measured if any of the default measurements indicates a process excursion (either a wafer-specific excursion or a problematic trend). In this manner, the number of wafers tested can be increased if a problem is detected for improved process monitoring. Otherwise, the minimum number of wafers are measured to maximize metrology throughput. 
     The invention will be more fully understood in view of the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram for a metrology recipe/tool operation process that incorporates dynamic response to prior measurements. 
         FIGS. 2A and 2B  are exemplary measurement maps for default and secondary (supplemental) measurement operations, respectively, in a dynamic metrology process. 
         FIG. 3A  is a block diagram of a grouping of metrology tools incorporating communications capabilities to enable dynamic metrology operations. 
         FIG. 3B  is a flow diagram for a metrology recipe/tool operation process that incorporates dynamic response to imported measurements. 
     
    
    
     DETAILED DESCRIPTION 
     Conventional metrology recipes specify a static set of metrology parameters, thereby forcing the same metrology operations to be applied to each test wafer in a test group. Consequently, timely detection of process excursions can be difficult, and effective analysis can be time consuming and expensive.  FIG. 1  depicts an embodiment of a metrology process flow diagram  100  that overcomes these limitations. Note that flow diagram  100  can be applied to any type of metrology operation/tool (e.g., spectroscopic ellipsometry, single wavelength ellipsometry, x-ray reflectometry, electron microprobe analysis, x-ray fluorescence, and scatterometry, among others). Note further that each step in  FIG. 1  can represent an operation by a metrology tool, or an instruction (or set of instructions) in a metrology recipe. 
     In a “LOAD WAFER” step  110 , a test wafer is loaded into the metrology tool. Then, in a “DEFAULT MEASUREMENT” step  120 , a standard metrology operation is performed on the test wafer. This standard metrology operation can comprise any basic measurement designed for process monitoring. Typically, the default measurement will be a relatively quick metrology operation designed to catch major problems. 
     If a review of the default measurement results in a subsequent “EXCURSION/TREND?” step  130  does not turn up any process excursions, the test wafer is unloaded from the metrology tool in an “UNLOAD WAFER” step  150 , and the process loops back to step  110  for the next test wafer. However, if a process excursion is detected in step  130 , an error indicator EIN causes a secondary measurement to be performed on the test wafer in a “SECONDARY MEASUREMENT” step  140 . The secondary measurement is a metrology operation that is designed to provide enhanced information about process excursion results detected in step  130 . 
     Note that in one embodiment, the default measurement performed in step  120  can be repeated in step  140  to confirm the process excursion noted in step  130 . However, the secondary measurement performed in step  140  will typically be a more detailed or more sensitive metrology operation that provides additional information about the process excursion detected in step  130 . 
     For example, in one embodiment, the default measurement in step  120  can take measurements at a first number of measurement locations, while the secondary measurement in step  140  can take measurements at a larger number of measurement locations. Exemplary measurement maps for the default and secondary measurements are shown in  FIGS. 2A and 2B , respectively. 
       FIG. 2A  shows a sample default measurement map  200 A that can be used in step  120  of  FIG. 1 . Measurement map  200 A specifies five measurement locations  210  for each test wafer  211  (shown as a dotted line for reference). The relatively small number of default measurement locations  210  allows the default measurement operation of step  120  to be relatively fast, thereby maximizing metrology throughput. 
     In contrast,  FIG. 2B  shows a sample secondary measurement map  200 B that can be used in step  140  of  FIG. 1 . Measurement map  200 B specifies thirty-seven measurement locations  220  for a test wafer  221  (shown as a dotted line for reference). The larger number of secondary measurement locations  220  allows the secondary measurement operation of step  140  to provide a detailed and accurate indication of the process quality. Note that in one embodiment, secondary measurement locations  220  can include default measurement locations  210 , thereby enabling direct comparison between the results of the default and secondary measurement operations. 
     Returning to  FIG. 1 , once the secondary measurement of step  140  is completed, the test wafer is removed from the metrology tool (e.g., placed back in to the wafer cassette) in step  150 , and the process loops back to step  110 , where the next test wafer is loaded into the metrology tool. This looping continues until all the test wafers in the test group (e.g., a single wafer cassette) have been measured. Note that in one embodiment, the number of test wafers from a test group to be measured can be increased based on error indicator EIN (e.g., while only three wafers from a cassette may be measured during normal testing, if a problematic trend is detected in step  130 , the number of wafers to be tested may be increased to 6). 
     Note that “SECONDARY MEASUREMENT” step  140  (and even “DEFAULT MEASUREMENT” step  120 ) can include any number of different measurement operations, and can also include any number of different types of metrology operations. For example, the metrology tool being used could include multiple measurement technique capabilities, such as a combination x-ray reflectometry (XRR) and x-ray fluorescence (XRF) tool, as described in co-owned, co-pending U.S. patent application Ser. No. 10/094,537. The default measurement performed in step  120  could comprise an initial XRR measurement at the default measurement locations to make an initial assessment of film thickness. Then, if any excessively thick or thin measurements are detected in step  130 , additional XRR and XRF measurements could be taken in step  140 . The supplemental XRR measurements could then be used to evaluate overall film thickness, while the additional XRF measurements could be used to determine if the process excursion is due to a reaction chemistry issue. 
     Note further that in another embodiment, application of “SECONDARY MEASUREMENT” step  140  can be applied based on trend analysis of previous metrology results. For example, if the data from “DEFAULT MEASUREMENT” step  120  for a series of wafers in a test group indicate a problematic trend when analyzed in step  130 , an error indicator EIN may be issued that instructs that the next test wafer receiver receive the more rigorous testing in step  140 . In one embodiment, a test wafer so identified by error indicator EIN can bypass step  120  completely, as indicated by the dotted arrows. In this manner, application of “SECONDARY MEASUREMENT” step  140  can be triggered by prior metrology results, rather than data from the current wafer. 
     Note further that in various other embodiments, a dynamic recipe can instruct a metrology tool to perform supplemental/more exacting measurement operations on test wafers based on imported metrology results; i.e., measurements not made using the current metrology tool. For example,  FIG. 3A  shows a pair of metrology tools  320  and  330  that are connected via a communications network  310 . Communications network  310  can comprise any type of communications path, including, for example, a server  311  and a hub  312  for managing network traffic between metrology tools  320  and  330  and server  311 . Optionally, metrology data DATA from tools  320  and  330  can be compiled and processed by error-detection software  311 -S in server  311 , with software  311 -S generating initial error indicators IEI(S) upon detecting process excursions or problematic trends/patterns. Server  311  can then send error indicators IEI (S) to either of tools  320  and  330  for use with the dynamic recipes controlling those tools. Alternatively, either or both of metrology tools  320  and  330  could generate error indicators IEI (S) based on their own metrology results. 
     In one embodiment, each of metrology tools  320  and  330  can operate according to the flow diagram shown in  FIG. 1  for any given test group of wafers. However, in another embodiment, the operation of metrology tool  330  can be based in part on prior measurements (of the same wafers) by metrology tool  320 . For example, assume a metrology flow in which metrology tool  320  performs a first measurement operation on a test group of wafers, after which metrology tool  330  performs a second measurement operation on the same test group of wafers. During or after the operation of metrology tool  320  on the test group of wafers, error detection software  311 -S in server  311  compiles the resulting metrology data DATA and identifies any potential problems (e.g., process excursions, increasing error trends, or unusual process variations). Server  311  then issues an appropriate error indicator IEI(S) to metrology tool  330 , which applies the appropriate supplemental metrology to the identified test wafers. In one embodiment, in response to metrology data taken from metrology tool  320 , metrology tool  330  might test more wafers or apply more rigorous testing/testing of a different type. In another embodiment, metrology tool  330  might only perform metrology operations on the test group of wafers if the data from metrology tool  320  indicates an excursion or problematic trend. 
     In another embodiment, the operation of metrology tool  330  can be based in part on prior measurements of a different set of test wafers. For example, assume a metrology flow in which metrology tool  320  performs a first measurement operation on a first test group of wafers, after which metrology tool  330  performs a second measurement on a second test group of wafers. If the first test group and the second test group are related (e.g., two cassettes processed by the same tool), server  311  could issue an appropriate error indicator IEI(S) to metrology tool  330  to instruct metrology tool  330  to apply an appropriate metrology operation to the second test group based on the data from the first test group. 
     Note that while only two metrology tools ( 320  and  330 ) are shown for exemplary purposes, any number of metrology tools can be networked to enable dynamic metrology operations based on shared measurement data. Note further that initial error identifier signal IEI(S) can either identify specific test wafers (e.g., identifying specific wafers exhibiting process excursions), or can indicate that the entire test group requires additional measurement testing (e.g., identifying process results trending in a problematic direction). 
       FIG. 3B  shows a flow diagram  300  of a dynamic metrology recipe (or metrology tool operation flow in response to the dynamic metrology recipe) that includes input from prior metrology operations. Flow diagram  300  is substantially similar to flow diagram  100  shown in  FIG. 1 , except that flow diagram  300  includes an additional “ID WAFER(S)” step  115 . In step  115 , information related to prior metrology operations (e.g., error indicators IEI(S) in  FIG. 3A ) is evaluated with respect to the current test group. If the prior metrology information indicates process excursions (or potential process excursions), the metrology operation can bypass “DEFAULT MEASUREMENT” step  120  and progresses directly to “SECONDARY MEASUREMENT” step  140  (indicated by error indicator IEI 1 ), where an appropriate metrology operation(s) is performed in light of the indicated process excursions. In another embodiment, if the prior metrology information indicates process excursions (or potential process excursions), the metrology operation can perform the “DEFAULT MEASUREMENT” step  120  and then move on to the “SECONDARY MEASUREMENT” step  140  (indicated by error indicator IEI 2 ). In this manner, potential process problems can be quickly identified and evaluated. 
     Although the invention has been described in connection with several embodiments, it is understood that the invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to one of ordinary skill in the art. Thus, the invention is limited only by the following claims and their equivalents.