Abstract:
A method of controlling lithographic overlay offsets in the manufacture of semiconductor devices from wafers, comprising the steps of forming a lithographic pattern on a wafer layer with a lithographic tool, processing the wafer after the pattern is formed to enable fabrication of a semiconductor device, predicting overlay offset corrections based on one or more factors involved in the processing of the wafer, and utilizing the predicted overlay offset corrections to positionally control the lithographic tool.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates generally to semiconductor lithography, and particularly to controlling overlay offset errors.  
         [0002]     A semiconductor device is typically built up of a number of levels which overlie each other. In the usual case, the registration of one layer with the next is not perfect, i.e., there is some overlay offset between successive layers. For any given semiconductor device, there is a parameter corresponding to the total amount of offset which can be tolerated for the device as a whole, known as an overlay budget.  
         [0003]     In the prior art, it is known to provide offset error control which compensates for those factors which are involved in the lithography operation itself. However, effects which are external to the lithography operation can also affect offset, and the prior art has not addressed such problem. In particular, effects which occur in the processing of the wafer which takes place after the lithography operation is completed can affect overlay offsets. By way of a specific illustrative example, a chemical-mechanical polishing (CMP) tool having a rotating pad may be used to flatten and polish a level before the next level is built. It has been observed that such tool may add a rotational offset to the level polished, which if not compensated for will adversely affect the placement of each successive layer. There are actually many processing factors which can affect overlay offset, and by way of non-limitative examples these include non-uniform etching, the specific type of film being processed, and the state of a sputtering tool which is used to accomplish metal deposition.  
       SUMMARY OF THE INVENTION  
       [0004]     In accordance with the present invention, overlay offset errors due to processing factors are addressed by forming a lithographic pattern on a wafer layer with a lithographic tool, processing the wafer after the lithographic pattern is formed to enable the fabrication of a semiconductor device, predicting overlay offset corrections based on one or more factors involved in the processing of the wafer, and utilizing the predicted overlay offset corrections to positionally control the lithographic tool. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention will be better understood by referring to the accompanying drawings wherein:  
         [0006]      FIG. 1  is a block diagram showing an embodiment of a system which may be used to implement the present invention.  
         [0007]      FIG. 2  is a flow chart depicting an embodiment of the method of the invention.  
         [0008]      FIG. 3  illustrates an overlay problem.  
         [0009]      FIG. 4  is a graphical representation of the overlay problem of  FIG. 3 .  
         [0010]      FIG. 5  is a graphical representation of the improved results achieved with the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]      FIG. 1  is a block diagram of an example of a semiconductor manufacturing system in which the present invention can be implemented. A work station  2  is provided on which semiconductor wafer  4  disposition is applied. A lithography tool  6  is provided which is used to accomplish coating, alignment, exposure and development steps, with the result that a pattern is formed on a layer on wafer  4 . Post-lithography processing equipment  8  is also provided, which may include systems for accomplishing etching, doping, material deposition and metallization, and may also include a system for accomplishing chemical-mechanical polishing (CMP).  
         [0012]     Also present in  FIG. 1  are systems for measuring overlay offset between respective levels of the wafer. These include overlay metrology part  10  for taking overlay measurements due to the lithography factors, and overlay metrology part  12  for taking overlay measurements due to post-lithography processing. A computer  14  is provided for controlling the operations performed on the wafer and for mathematically processing the overlay offset data for producing appropriate overlay offset corrections. Thus, it is seen that computer  14  communicates with both lithography tool  6  and post-lithography processing equipment  8  for controlling the operations performed on the wafers, and that it communicates with overlay metrology parts  10  and  12  for controlling the measurement of overlay offsets and for processing the measurements to produce corrections, which are fed to lithography tool  6  for positionally controlling the tool. While communication between the computer and the other components would typically be via a bus, direct connections are shown in  FIG. 1  for clarity of illustration.  
         [0013]     The operator input block  16  is an operator interface through which an operator would input necessary information to computer  14  for initiating and controlling a wafer fabrication process. Diskette  17  may contain a set of instructions for the computer for performing method routines in accordance with the present invention.  
         [0014]     Referring now to  FIG. 2 , which is a flow chart depicting an embodiment of a method in accordance with the invention, it is seen that in step  18  a level of the wafer is coated, aligned and exposed, and developed. These operations result in the formation of a pattern on the wafer and would be performed with lithography tool  6  shown in  FIG. 1 . Thus, a “manufacturing” symbol appears next to step  18  in  FIG. 2 .  
         [0015]     After a pattern is formed, the overlay offset between the level being worked on and the preceding layer is measured in the overlay metrology step  20 , which would be performed by overlay metrology part  10  in  FIG. 1 . The usual “box in the box” marks may be used to determine offsets. However, step  20  results in raw X-Y data, and this must be converted to recognized offset categories, such as translation, rotation, magnification, and skew by processing the raw data with an alignment offset model in step  22 . The modeled alignment offset information is fed back to effect positional control of the lithography tool to eliminate or minimize the error due to factors involved in the lithography process. This feedback is accomplished with feedback modeling step  24  which applies a weighting factor based on information obtained for the current lot as well as information about past lots. At step  26 , the actual optimized positional settings for the lithography tool are determined, which are utilized in the lithography performed in step  18 . The modeling steps  22  and  24  and the optimization step  26  are performed in computer  14  of  FIG. 1 .  
         [0016]     Referring again to the flow chart of  FIG. 2 , the post processing is accomplished in step  30 , and typically may comprise a number of operations including etching, doping, material deposition, and metallization. As mentioned above, the post processing may introduce offset errors which have nothing to do with the lithography process. For example, if the etching is non-uniform, the profile of etched pattern elements may not be perpendicular to the wafer. When material is deposited into etched regions its placement will not be correct, as there will be a translation. The error will be carried forward to the next level, and the pattern formed in the next level will not be correctly placed. It is thus desirable to characterize the offset error caused by etching immediately after the etching step.  
         [0017]     Thus, step  32 , measurement of post processing offset would occur after each process step which may produce an offset error. The step is performed in the same manner as step  20  described above and may be performed by overlay metrology part  12  shown in  FIG. 1 . In fact, although two overlay metrology parts  10  and  12  are shown in  FIG. 1  for purposes of illustration, it is possible that there may be only one measuring part performing offset measurements for both lithography and non-lithography induced errors.  
         [0018]     The measurement in step  32  is not typically taken for every lot of wafers, which is why the step appears in dotted lines. A bifurcated sampling scheme may be used with a greater amount of sampling being done while data is acquired to produce a predictive model, then is done after the model is established. By way of illustrative example, the sampling scheme may be 100% lot sampling for 3 wafers/lot for two to three months while data is acquired to produce a feed forward predictive bias model. After the bias model is established sampling may occur at a predetermined frequency for purposes of model validation only, e.g., three lots per week.  
         [0019]     Alignment offset modeling step  34  is similar to step  22  described above in that it converts the raw X-Y data to meaningful offset information. In step  36 , the offset information is correlated as a function of pre-process conditions to produce a bias model. For example, the pre-process condition being considered may be the age of a rotatable pad on a chemical-mechanical polishing (CMP) tool. As mentioned above, such pads have been known to impart a rotation offset to the wafer and to the alignment and overlay marks, and the degree of the offset has been found to be related to the age of the pad. Thus, offset data is accumulated as the pad ages, and in step  36  a model is created, which correlates the offset to the age of the pad. Then, when the age of a current pad to be used is inputted to the model, the degree of offset due to the pad may be predicted. Since physical and optical distortion to overlay and alignment marks may be due to a multiplicity of factors, the bias model may be a correlative function of a number of pre-process conditions. Such conditions may include the degree of non-uniformity of etch produced by an etching tool, the age of a target used in a sputter system for metal deposition, the state of a particular tool or tool chamber component, the batch, the process time, and whatever other process parameter is determined to be related to the measurement effect. The bias model produced in step  36  may include the use of a database and application server.  
         [0020]     Pre-processing of the wafer as is necessary occurs in step  40 , and it is at this point where the current pre-process conditions are determined. The current pre-process condition information is utilized in step  38  where it is incorporated in the bias model to determine the feed forward predictive target offset corrections which correspond to the current pre-process conditions. It is possible for each target offset to be represented as a polynomial a 0 +a 1 x+a 2 x 2 +a 3 x 3 +a 4 x 4 +a 5 x 5 + . . . +a n x n  where x corresponds to a specific offset. In step  26 , the actual optimized settings for the lithography tool which correct the offset errors due to processing are determined from the information outputted by the feed forward model, and the appropriate corrections are utilized in lithography step  18 .  
         [0021]     It is noted that there may also a statistical process control (SPC) step  28  which occurs after each of offset modeling steps  22  and  34 . In step  28 , the offsets are compared against norms to see whether they are within acceptable limits. If not, the wafer may be re-worked or if the problem is severe, the entire system may be shut down. Such options are represented by the “Process Disposition” symbol in  FIG. 2 .  
         [0022]     In accordance with a further aspect of the invention, after the lithography tool is positionally controlled to compensate for an offset error, the target offset is calibrated out of the overlay measurement so that zero offset overlay targets and standard disposition methods and rework procedures can be maintained. Because of such calibration, it is always the same “zero line” which is keyed to in the alignment process.  
         [0023]      FIG. 3  depicts an overlay problem which may be encountered in semiconductor fabrication. A wafer  50  is depicted, and the top part of the Figure shows the wafer at the lithography stage. The spacing between the two vertically extending broken lines represents the apparent overlay which is seen at post development. Elements  52  and  54  are tungsten, while element  56  is an aluminum-copper (AlCu) layer which has been deposited thereon. Elements  62  are photoresist.  
         [0024]     When following the broken lines to below the top part of the Figure, the center line shift due to the AlCu deposition is seen. The lower part of the Figure depicts the wafer after the etching step, where the center line shift due to to AlCu deposition is manifested. Elements  58  and  60  are what remain of the AlCu after etch. The significant overlay present here is represented by the spacing between the broken lines.  
         [0025]     In the example of  FIG. 3 , the overlay being considered is of the rotation type.  FIG. 4  is a graphical representation of interfield-rotation bias on the ordinate and date/time of exposure on the abscissa in which the process bias is characterized for the example of  FIG. 3 . The problem is shown by the substantial number of data points which are far from the zero line.  
         [0026]      FIG. 5  is a graphical representation similar to  FIG. 4  at the lithography step after a bias correction provided in accordance with the method of the present invention is introduced. It is seen that most data points are clustered relatively close to the zero line.  
         [0027]     It has been found that more accurate semiconductor devices may be manufactured with the process and system of the invention, functional yield may be increased, and frequent downstream overlay measurements to maintain accurate overlay performance may no longer be necessary.  
         [0028]     It should be understood that while the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that such modifications and variations of the invention be covered provided they come within the scope of the appended claims and their equivalents.