Patent Publication Number: US-6985847-B2

Title: System and method for process matching

Description:
RELATED APPLICATIONS 
   This application is a Continuation of U.S. Ser. No. 09/460,565 filed on Dec. 14, 1999, now issued as U.S. Pat. No. 6,463,403 on Oct. 8, 2002, which is a Divisional of U.S. Ser. No. 09/031,397 filed on Feb. 26, 1998, now issued as U.S. Pat. No. 6,033,814 on Mar. 7, 2000. These applications are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to photolithography. More particularly, this invention relates to the changing of photolithographic processes in the fabrication of integrated circuit parts. 
   BACKGROUND OF THE INVENTION 
   Photolithography is one method used to produce integrated circuit parts for semiconductor chips. Photolithography consists of processing substrate wafers with stepper machines that have been loaded with photomasks. This processing fabricates hundreds of integrated circuit parts on each wafer. The microscopic detail of the circuits created on the wafers makes photolithography a very exacting manufacturing method. Just a small change in the setup of the equipment or in one of the operating variables can drastically alter the resulting wafers. 
   However, in the course of production, one may wish to change the process. For example, although a current process may be providing for yields with good critical dimension behavior, the depth of focus may be unacceptable. A new process, using a different illumination setup may be desired. Altering the illumination may improve the depth of focus. The alteration may, however, cause a decline in critical dimension behavior. Other behaviors may also be of concern, including those caused by the photoresist process, etch process, or mask process. 
   What is needed is a process matching method which would allow the photolithography process to be changed without introducing variances within the resulting wafer images. The wafer image from the new process should be the same as the initial wafer image. In such a system, the results of the primary process are matched to the results of a secondary process. 
   One useful tool for improving a wafer image is optical proximity effect correction (“OPC”). OPC is a misnomer since the term now encompasses not only optical proximity-effect correction, but also etch proximity effect correction and all other forms of corrections to proximity effects. In OPC, a correction is made to a photomask design to compensate for dimensional and positional inaccuracies resulting from optical or etch effects. It is well known that there are several ways to accomplish OPC. One way is by a rule-based system. In such a system, a pattern for a photomask is analyzed by a series of rules, which isolate and adjust the problematic edges. A wafer printed from the OPC-corrected pattern should result in a flat printed “critical dimension vs. environment” response based on proximity. 
   A model-based system is a second type of OPC. A model is a mathematical description of a system. In a photolithography system, a model can describe optical, optical through resist, or other system. Regardless of the type of system described, there are generally two types of models. A theoretical model relies on the pure science involved in the system. Often, a theoretical model will be used to describe optical systems. Otherwise an empirical model can be used. In an empirical model, copious measurements are compiled and are then used to generate a set of formulae describing the behavior of the system. 
   In OPC, first, a simulation is made by applying the photomask pattern to the model. The resulting value is a prediction on how the pattern will print on the wafer. The prediction is the intensity return value. If normalized to a scale of 1, an intensity return value of 0 indicates chrome and a value of 1 indicates quartz. After obtaining the resulting value, then the factor of change needed to compensate for the proximity effect is calculated. In general, a threshold is determined, which is the intensity that is required to print on the wafer. For positive tone resist, above threshold equals clear substrate, below threshold equals resist. Then the intensity return value from the simulation is subtracted from the threshold value to find the intensity difference. This difference is then used to find the physical offset necessary to correct the pattern. This OPC method is repeated at close intervals throughout the pattern, resulting in a corrected photomask pattern. 
   SUMMARY OF THE INVENTION 
   A computer-implemented method for matching parameters of outputs generated by a first and second process. The first process generates a first output having a characteristic measurable by a first parameter, and the second process generates a second output having the characteristic measurable by a second parameter. A computer having a processing unit and storage is provided. The computer generates a first model of the first parameter for the first process and a second model of the second parameter for the second process. The computer generates a first simulated output of the first process using the first model. A correction, which is a function of the second model and which compensates for the effect of the second process on the second parameter, is applied to the first simulated output to obtain a corrected output. The second process is applied to the corrected output to generate with the computer thereby a third output matching the first parameter of the first output. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flowchart of two processes. 
       FIG. 2  is a flowchart showing a method of process matching. 
       FIG. 3  is an illustration of the matching of the wafer image after a process change by simulation and proximity effect correction. 
       FIG. 4  is a block diagram of a computer system which produces process matching. 
   

   DETAILED DESCRIPTION 
   In the following Detailed Description, reference is made to the accompanying drawings which form a part hereof and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice and to use the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following Detailed Description is, therefore, not to be taken in a limiting sense and the scope of the invention is defined by the appended claims. In the figures, elements having the same number perform essentially the same functions. 
   The present invention permits a change in process while maintaining the same wafer image, thus allowing a migration from one process to another with wafer image continuity. Process matching is the method of achieving a desired output of a process based on the another process which achieves the desired output. For example,  FIG. 1  is a flowchart showing two processes, process —   1   105  and process —   2   110 . Process —   1   105  accepts input —   1   115  and generates output —   1   125 . Similarly, process —   2   110  accepts input —   2   120  and generates output —   2   130 . These processes can be symbolized with the notation:
         OUTPUT —   1 =PROCESS —   1 (INPUT —   1 )   OUTPUT —   2 =PROCESS —   2 (INPUT —   2 ).       

   A process&#39;s effectiveness can be gauged by the values of any of a number of parameters applied to the output produced for a given characteristic. In  FIG. 1 , process —   1 &#39;s effectiveness can be gauged by parameters A —   1   135  and B —   1   145 . Likewise, the effectiveness of process —   2  is measured by the parameters A —   2   140  and B —   2   150 . In a situation where parameter A —   1   135  is satisfactorily achieved in output —   1   125  by process —   1   105 , and where there is a second process (process —   2   110 ), which results in a desired level of parameter B —   2   150 , but not A —   2   140  (assuming input —   1   115  and input —   2   120  are equal), one of the goals of the present invention is to modify the input to process —   2   110  so that output —   2   130  results in an A —   2   140  parameter as good as A —   1   135 . The matching of these processes can be written as: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               OUTPUT — 1 = PROCESS — 1(INPUT — 1): 
               A — 1 = good 
             
             
                 
                 
               B — 1 = bad 
             
             
                 
               OUTPUT — 2 = PROCESS — 2(INPUT — 2): 
               A — 2 = bad 
             
             
                 
                 
               B — 2 = good 
             
             
                 
               OUTPUT — 2 = PROCESS — MATCHING 
               A — 2 ≈ A — 1 = good 
             
             
                 
               (INPUT — 2): 
               B — 2 = good 
             
             
                 
                 
             
          
         
       
     
   
   Although there are several situations in which process matching is useful, one exemplary use is in the field of semiconductor fabrication by photolithography. While building semiconductor wafers, a change in setup variables, such as a change in the illuminator, causes the wafer image produced from a photomask or reticle to vary. However, sometimes such a change will improve the value of one parameter while decreasing the value of another. The present invention allows a process to be introduced without causing a change in the wafer image with respect to a particular parameter. In addition to supporting a photolithographic process, other processes supported by the present invention include, but are not limited to the following: an ion projection process, an x-ray projection process and an e-beam projection process. 
   Two parameters of a wafer image which can be used to measured the effectiveness of the manufacturing process are critical dimension behavior and depth of focus. Process —   1   105  could be a set-up using on-axis illumination. The output —   1   125  of this process is the wafer image. This wafer image could have a good critical dimension behavior but a poor depth of focus (i.e., A —   1 =good, B —   1 =bad). However, the wafer image output resulting from the use of off-axis illumination (process —   2   110 ) might improve the depth of focus (i.e., B —   2 =good) but not provide for quality critical dimension behavior (A —   2 =bad). By using process —   2   110  instead of process —   1   105 , the favorable A —   1  parameter  135  would be lost. The goal of process matching in this example is to transform input —   2   120  so that the behavior of process —   2   110  is good, i.e., so that the use of off-axis illumination improves the depth of focus (B —   2   150 ) without degrading the critical dimension behavior parameter (A —   2   140 ). 
   Process matching is accomplished through the unconventional uses of OPC and simulation. Traditionally, simulation is used upon input data, such as a photomask pattern, to predict how the pattern will print on the wafer based on a given model. Then OPC corrects the photomask pattern by adjusting the edges so that when the pattern is printed on the wafer, the proximity effect is mitigated. Symbolically, by using the SIM-subscript to denote “simulation”, simulation is:
         OUTPUT —   1   SIM =PROCESS —   1   SIM (INPUT —   1 ).       

   At any point in the pattern, the offset value amounting to the physical offset can be calculated. OPC involves adjusting the pattern by the offset value so that the wafer image reflects the photomask pattern correctly. For example, suppose a simulated point (“X1”) on an edge in the pattern has an intensity return of 0.30. If the threshold is 0.32, then the intensity difference is 0.02 (e.g., 0.32−0.30=0.02). If the contour shows that at point X1 there is 300 nanometers of slope per intensity value, then the physical offset value is 6 (e.g., 300×0.02=6). This indicates that although it is desired that the edge falls at X1, because of the offset, the edge is really falling at (X1−6). The offset value is 6 here and thus the OPC compensation is to move the original edge of the pattern up by 6. 
     FIG. 2  is a flow chart showing process matching applied to FIG.  1 &#39;s process —   1   105  and process —   2   110 . In  FIG. 2 , at step  210 , a simulation model of process —   1  is applied to input —   1   115 . The simulation produces output —   1   SIM    215  and indicates where the edge is actually printing. The simulated output can be a three-dimensional image for a given contour level. Instead of correcting the input data to obtain the desired output data, as is usually done in OPC, the output data is stored without correction. Then at step  220 , instead of using the original pattern (i.e., input —   2   120 ) as input for process —   2   110 , the method uses the output generated from the model of process —   2  with OPC (from step  220 ) resulting in output —   2   130 . As an optional step, the simulated output —   2   225  can be generated for verification purposes. This output —   2   130  is the same as the output —   2   130  from  FIG. 1 , except both parameters A —   2   140  and B —   2   150  are good. 
   Another way to formulate this method of process matching is by:
     OUTPUT —   2 =PROCESS —   2  [PROCESS —   2   SIM  [PROCESS —   1   SIM (INPUT —   1 )] −1 ];   THEREFORE: IF (PROCESS —   2 ≈PROCESS —   2   SIM )
       THEN (OUTPUT —   2 ≈PROCESS —   1   SIM (INPUT —   1 )).   
       

     FIG. 3  illustrates the use of this process matching in photolithography. Here, process —   1   105  uses mask pattern  305  to produce wafer image  310 . Thus, compared to  FIG. 1 , mask pattern  305  is input —   1   115  and wafer image  310  is output —   1   125 . 
   A model of process —   1   210  can be created. This model can be used to generate a simulated wafer image  315  from the mask pattern  305  at a given contour level. The contour level ranges from 0 (representing chrome) to 1 (representing quartz). Then, in order to reduce the amount of data and to ease later corrections, a simulation approximation  325  may be generated of simulated wafer image  315 . 
   When a new process —   2   110  is introduced, models of process —   2  can be generated  220 . The simulation approximation data  325  is used as input to an OPC correction (e.g., process —   2   SIM    330 ). The output from this step  335  is used as input to process —   2   110  to gain a new wafer image  340 . This new wafer image  340  is equivalent to the original wafer image  310 , but it has the desired parameters values found in wafer images produced by process —   1   105  as well as the desired parameter values found in wafer images produced by process —   2   110 . 
     FIG. 4  shows a computer system  405  which can implement the process matching method previously described. In  FIG. 4 , computer system  405  includes a central processing unit  410  and memory  420 . Memory  420  can be RAM, disk storage, CD-ROM storage, or other type of memory. Within memory  420 , computer system  410  has access to its operating system  430  and user software  440 . To implement the method of the present invention, user software  440  can include software from Precim Company along with user-modified scripts. Or, user software  440  can be user-developed software, or software from some other vendor. Computer system  410  accepts input from input devices  450 . Wafer image  310  can also be generated by software, such as FAIM from Vector Technologies (Brookline, Mass.), DEPICT from Technology Modeling Associates, Inc. (Sunnyvale, Calif.), and SPLAT by the University of California (Berkeley, Calif.). User software  440  also includes software which generates models (theoretical or empirical). 
   Other embodiments of the present invention are possible. For example, in addition to photolithography masks, the present invention can be used with x-ray masks, stencil masks for ion projection lithography, or electron beam projection lithography masks. In addition to photolithography, the present invention&#39;s method can be used for other processes for which output can be evaluated based upon one or more types of parameters.