Abstract:
System and method for implementing a VM APC platform are described. In one embodiment, the VM APC system comprises a process tool for processing a plurality of wafers, a metrology tool for measuring a sample wafer of the plurality of wafers and generating actual metrology data therefor, and a VM model for predicting metrology data for each of the plurality of wafers. The actual metrology data is received from the metrology tool and used to update the VM model. Key variables of the virtual metrology model are updated only in response to a determination that the VM model is inaccurate and parameters of the VM model are updated responsive to receipt of the actual metrology data for the sample wafer of the plurality of wafers. The system also includes an APC controller for receiving the predicted metrology data and the actual metrology data and controlling an operation of the process tool based on the received data

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to Advanced Process Control (“APC”) as applied to semiconductor fabrication and, more particularly, to system and method for implementing a Virtual Metrology (“VM”) APC platform. 
         [0002]    APC has become an essential component in semiconductor fabrication for enabling continued improvement of device yield and reliability at a reduced cost. Significant elements of APC include integrated metrology, fault detection and classification, and run-to-run (“R2R”) control. APC aids in reducing process variation as well as production costs. A key requirement for effective APC is that metrology tools must be available to measure key parameters within an acceptable time frame. Additionally, methods must be provided for analyzing and interpreting measurement data produced by the metrology tools. In practice, APC requires rich in-line measurements because the manufacturing processes are usually subjected to disturbance and drift. In the past, one of the primary limitations on APC at the wafer-to-wafer (“W2W”) level has been the nonavailability of timely metrology data at that level. VM techniques have alleviated this problem to some degree. 
         [0003]    The basic theory of operation for VM is that, in a development flow, a range of production runs is used to develop an empirical prediction model that correlates to actual measurements from metrology tools with the process trace data that was present at the time. The model is refined until the metrology values it predicts show a reasonable correlation to the actual measurement data. Once the model is developed, the VM system goes “live,” or “online,” in the production flow and is used during execution of a process runs to estimate metrology values for wafers being processed. Variations from the desired targets are used to update recipe parameters in a traditional R2R control fashion. These updates, or adjustments, can be made on a W2W basis without requiring a separate measurement step to be performed. It will be recognized that actual metrology measurements are still made in the production flow; however, their role is primarily that of calibrating/updating the prediction model, rather than as the primary control checkpoint for the process. 
       SUMMARY 
       [0004]    One embodiment is a VM APC system comprising a process tool for processing a plurality of wafers, a metrology tool for measuring a sample wafer of the plurality of wafers and generating actual metrology data therefor, and a VM model for predicting metrology data for each of the plurality of wafers. The actual metrology data is received from the metrology tool and used to update the VM model. Key variables of the virtual metrology model are updated only in response to a determination that the VM model is inaccurate and parameters of the VM model are updated responsive to receipt of the actual metrology data for the sample wafer of the plurality of wafers. The system also includes an APC controller for receiving the predicted metrology data and the actual metrology data and controlling an operation of the process tool based on the received data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0006]      FIG. 1A  illustrates application of VM principles to an individual manufacturing process in accordance with the prior art. 
           [0007]      FIG. 1B  illustrates application of VM principles to an individual manufacturing process in accordance one embodiment. 
           [0008]      FIG. 2  illustrates a flow diagram of a method for implementing a VM APC platform in accordance with one embodiment. 
           [0009]      FIG. 3  illustrates a process for performing a lot trend adaptive update in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present disclosure relates generally to APC as applied to semiconductor fabrication and, more particularly, to system and method for implementing a VM APC platform. It is understood, however, that specific embodiments are provided as examples to teach the broader inventive concept, and one of ordinary skill in the art can easily apply the teachings of the present disclosure to other methods and systems. Also, it is understood that the methods and systems discussed in the present disclosure include some conventional structures and/or steps. Since these structures and steps are well known in the art, they will only be discussed in a general level of detail. Furthermore, reference numbers are repeated throughout the drawings for the sake of convenience and example, and such repetition does not indicate any required combination of features or steps throughout the drawings. 
         [0011]      FIG. 1A  illustrates VM principles as applied to an individual process in accordance with the prior art. A module  100  illustrates construction of a process model via a design of experiments (“DOE”) process. As will be recognized by those of ordinary skill in the art, DOE is a structured, organized method for determining a relationship between factors affecting a process (x) and the output of that process (y). As illustrated in  FIG. 1  in connection with the module  100 , for each process run, a process tool  102  is used to process the wafers comprising the run in accordance with process inputs, or recipe settings, u(n). In one embodiment, the process tool  102  may be any one of, for example, an etch tool, a deposition tool, a CMP tool, a coating tool, a developing tool, or a thermal treatment tool. As each of the wafers of the run is processed, quality variables, x k (t) are provided to a Fault Detection and Classification (“FDC”) module  104 . As will be recognized by one of ordinary skill in the art, the FDC module  104  monitors and analyzes variations in tool and/or process data to detect anomalies and determine the cause of a fault. This data is then stored in a database of the FDC module  104 . In one embodiment, process output data is acquired by the FDC module  104  once every second, although it is understood that different sampling rates may be used. The FDC data v(n), or V, out put from the FDC module  104  is applied to a VM Model Building module  110  for purposes that will be described below. For each process run, one or more quality control (“QC”) sample wafers are measured after processing by one or more metrology tools, such as a metrology tool  112 . The metrology tool generates measurement data y(k), or Y. This measurement data is also applied to the VM model building module  110  for purposes that will be described in greater detail below. 
         [0012]    Once the DOE has been completed, process inputs U, FDC data V, and historical and current measurement data Y prev  and Y are input to a VM modeling module  114 , of the module  110 , which uses the inputs thereto to construct a VM model  116  in a conventional manner The VM model  116  is applied in a system  118  comprising online implementation of VM with wafer-to-wafer (“W2W”) control. In the system  118 , the VM model  116  is used during execution of a process run to estimate metrology values for the wafers being processed and variations from the desired targets are used to update recipe parameters in traditional R2R control fashion. However, using VM, such adjustments can be made on a W2W basis by a W2W APC controller  120  without requiring a separate measurement step to be performed either internally or externally to the process tool  102 . It will be noted that while actual metrology measurements are still made, these measurements are used to calibrate and update the VM model  116 , rather than serving as the primary control checkpoint for the system  118 . 
         [0013]      FIG. 1B  illustrates a system  150  comprising online implementation of VM with wafer-to-wafer (“W2W”) control in accordance with embodiments described herein. The system  150  is similar in many respects to the system  118  of  FIG. 1A , in that the system  150  includes a process tool  152  for processing wafers as described with respect to process tool  102  ( FIG. 1A ), quality variables from which are provided to an FDC module  154 , which monitors and analyzes variations in tool and/or process data to detect anomalies and determine the cause(s) of a fault and stores data in a database  155  therein. As previously noted, for the purposes of example, it will be assumed that process output data from the process tool  152  is acquired by the FDC module  154  once every second, although it is understood that different sampling rates may be used. As also described above, For each process run, or lot, one or more quality control (“QC”) sample wafers are measured after processing by one or more metrology tools, such as a metrology tool  162 , which generates measurement data y(k), or Y. As previously noted, the metrology data is applied used to calibrate and update a VM model  166 , which is used during execution of a process run to estimate metrology values for the wafers being processed and variations from the desired targets are used to update recipe parameters in traditional R2R control fashion. In accordance with features of the embodiments described herein, the VM model  166  is unique from the VM model  116  in several respects, including the fact that it includes a Y2Y adjustment unit  168  for purposes that will be described in detail below. The adjusted prediction data output from the VM model  166  is applied to a W2W APC controller  170 , which adjusts the operation of the process tool in accordance with the data. 
         [0014]    Referring now to  FIG. 2 , illustrated therein is a method for implementing a VM APC platform in accordance with one embodiment. In accordance with one preferred embodiment, and as will be described in greater detail below, in one aspect, key variables of a VM model, such as the VM model  166 , are updated less frequently than parameters of the model. As used herein, “key variables” refers to important VM model inputs extracted and selected from various sources, such as an FDC, an APC, or an SPC. In contrast, “parameters” refers to weights, constants, or any other tuning variables used in a VM modeling algorithm. In particular, assuming for the sake of example that a VM model may be represented as follows: 
         [0000]    
       
      
       Y=C 
       1 
       X 
       1 
       +C 
       2 
       X 
       2 
       + . . . C 
       n 
       X 
       n  
      
     
         [0000]    key variables of the VM model (X 1 -X n ) are updated on an “as-needed” basis, as will be described, whereas the model parameters (C 1 -C n ) are updated on an L2L basis as new metrology data becomes available. In step  200 , a moving window is applied to historical data records, comprising measured input and output data of QC sample wafers measured over time, to develop a historical training data set for building and training the VM model  166 . For example, in one embodiment, the historical training data set comprises data records for the most recent 60 wafers. It will be assumed for the sake of example herein that each lot comprises 25 wafers and that for each lot, two wafers are sampled by a metrology tool, such as the tool  162 . After each lot is sampled and metrology data for the two QC sample wafers is made available, the historical training data set is updated to include the two new records and to discard the two oldest records. Additionally in step  200 , FDC data may be preprocessed. If this is the initial setup of the VM model  166 , execution proceeds to step  202 , in which an initial set of key variables, such as, for example, temperature and process time, is selected using the historical data training set developed in step  200 . The selection of key variables is generally performed by an engineer or technician familiar with the process being monitored. 
         [0015]    Additionally in step  200 , FDC data may be preprocessed. In accordance with one embodiment, preprocessing of FDC data is accomplished using a time-averaging-based technique to process the time-series FDC data. For example, assuming there are 10 variables reported by the process tool to the FDC (“SVIDs”) and recorded each second and further assuming that the process executes for three minutes, there will exist  180  records for each of the SVIDs for a total of 1800 records for a single wafer. As these records are typically treated as 1800 variables, so that a data matrix can be formed with each row representing a different wafer and each column representing a different variables, without some sort of preprocessing, there will be 1800 input candidates for key variable selection. This can result in two problems. First, 1800 candidates it too large to perform stepwise regression described below for selecting key variables due to the large amount of time it would take to run such a process on that many candidates. Second, every single “variable” is actually just a second-based record of an SVID. The resolution of the data is too high, resulting in low system information and high noise susceptibility; therefore, it is physically not suitable for model input. The data may be prepossessed in two ways. First, given a fixed averaging time, for example, 10 seconds, the data is averaged into a 10-second based manner to piecewise the time series data. For each SVID, 180/10 (seconds) is equal to 18, so there will be 180 pieces of data (18*10 SVIDs) per wafer. Second, given a fixed step-piece number, such as 3, each step is divided into three pieces equally. So if a recipe contains 10 steps, 10*3=30 pieces for each SVID and 30*3=20 pieces of data per wafer. The “pieces” are treated as variables for key variable selection. It will be recognized that this preprocessing can be performed either in the FDC module  154  or in the VM model  166 . 
         [0016]    In step  202 , an initial set of key variables, such as, for example, temperature and process time, is selected using the historical data training set developed in step  200 . The selection of key variables is generally performed by an engineer or technician familiar with the process being monitored. 
         [0017]    In step  204 , on-line build-up and training of the VM model  166  is performed using the historical training data set developed in step  200  and the key variables determined in step  202 . Any one of a number of well-known methods may be used for performing the build-up and training of the VM model  166  in step  204 ; therefore, the process will not be described in further detail. In step  205 , the VM model  166  is used in processing a next lot of wafers. 
         [0018]    After step  205  if this is the initial setup of the VM model  166 , or immediately after step  200  if this is not the initial setup of the VM model, execution proceeds to step  206 . In step  206 , the VM model  166  is tested for accuracy. This is generally accomplished by performing a statistical test, such as a Z-test or T-test, on the VM model, on the 20 and 60 most recent wafers. In particular, because the most recent 20 wafers are used to update the parameters, there must be some means for ensuring that they are consistent with the 60 wafers that are used to select key input variables. In particular, the statistical test is used to determine whether any meaningful change has occurred to the system or tool. If so, the VM must reselect its inputs to ensure that it remains faithful to the system. If the model  166  fails to pass the test, the model is determined to be inaccurate and execution proceeds to step  210 . In step  210 , new key variables are selected for the VM model using the 60 historical data points. In one embodiment, the step  210  may be performed using stepwise regression to select the new key variables. Execution then proceeds to step  212 . If in step  206 , the VM model passes the test and is determined to be accurate, execution proceeds directly to step  212 . 
         [0019]    In step  212 , the parameters of the VM model parameters  166  are updated using the most recent historical data points. As various methods for performing step  212  will be recognized by those of ordinary skill in the art, particular methods of updating the VM model parameters in accordance with the embodiments described herein will not be further described. It will be noted that, in the embodiment described herein, VM model parameters are updated with a higher frequency (e.g., on a per-lot basis) than key variables of the VM model, thereby ensuring that the model is synchronized with the system  150  at all times. In contrast, key variables are updated less frequently (e.g., on an as-needed basis), as key variables represent characteristics of the process that by their nature do not change frequently. Lower key variable update frequency results lower computation loadings for online considerations. 
         [0020]    In step  214 , a lot trend adaptive update, or Y2Y update, of the VM model predictions is performed for each of the wafers of the current lot. In particular, as noted above, the VM model  166  includes a Y2Y adjustment unit  168  which adjusts the prediction values Y predicted , output by the VM model for each of the j non-sampled wafers of the current lot using i historical output measurements (Y historical, i ) in a weighted manner. The Y2Y adjustment unit  168  adjusts the predictions made by the VM model  166  using historical measurements of Y to capture the L2L tendencies and to smooth the results, filter noises, and maintain predictions close to actual metrology data.  FIG. 3  illustrates this process in greater detail. As represented in  FIG. 3 , historical output data Y historical, i , from the metrology tool  162  and maintained in the VM model  166  is obtained for each of i most recent wafers  300 . It will be assumed for the sake of example herein that i=20, although different quantities may be used. Additionally, output data for a QC sample wafer  302  of a current lot  304  is measured using the metrology tool  162  to obtain Y metrology . In one embodiment, an Exponentially Weighted Moving Average (“EWMA”) is applied to the output data Y historical, i , and Y metrology  to Y determine a Y2Y adjustment for the wafers of the current lot  304 . The following equation illustrates application of an EWMA to the historical Y data: 
         [0000]        Y   EWM,i ,=(1−ω)* Y   EWMA,i-1)   +ω*Y   historical,i , for  i= 1-20 
         [0021]    It will be noted that the initial value of Y EWMA, 0  is equal to Y EWMA, 1  and that ω is a weight between 0 and 1. It will be assumed for the sake of example that ω=0.5. 
         [0022]    Then a weighted value of Y for the current lot (Y EWMA     —     current     —     lot ), for the current lot is calculated as follows: 
         [0000]        Y   EWMA     —     current     —     lot =(1−ω)* Y   EWMA,20   +ω*Y   metrology    
         [0023]    Finally, the Y2Y adjusted output values for each of the j wafers of the current lot  404  are calculated as follows: 
         [0000]        Y 2 Y   adjusted,j =(1−ω)* Y   EWMA     —     current     —     lot   +ω*Y   predicted,j    
         [0000]    where Y predicted, j  is the output value predicted by the VM for wafer j. 
         [0024]    Referring again to  FIG. 2 , in step  216 , W2W control is performed by the module  170  in a conventional manner using the Y2Y adjusted values output from the VM model  166 . 
         [0025]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. In particular, it is understood that the historical training data set and the historical update data set may include more or fewer than 60 and 20 data records, respectively. Moreover, it is understood that the EWMA may be performed using historical data for more or fewer than 20 wafers. Additionally, each of the FDC  154 , the VM model  166 , and the W2W APC controller module  170  may be implemented on separate computing devices or one or more of those elements may be implemented on a single computing device. Each such computing device may include one or more of processing capabilities, memory and/or storage capabilities, input/output capabilities, and display capabilities. The Y2Y adjustment unit  168  may be implemented as part of the VM model  166  or the module  170 . 
         [0026]    It is understood that various different combinations of the above-listed embodiments and steps can be used in various sequences or in parallel, and there is no particular step that is critical or required. Furthermore, features illustrated and discussed above with respect to some embodiments can be combined with features illustrated and discussed above with respect to other embodiments. Accordingly, all such modifications are intended to be included within the scope of this invention.