Patent Publication Number: US-6215127-B1

Title: Method of using critical dimension mapping to qualify a new integrated circuit fabrication tool set

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to techniques which are used during the fabrication of semiconductor devices. More specifically, the invention relates to a technique which uses multiple mappings of critical dimensions of selected features formed on a wafer during the process of forming integrated circuits, to enable tool set calibration/qualification in an efficient manner and with particular regard to mitigating effects which are encountered during the various steps which are carried during the constructive processes. 
     2. Description of the Related Art 
     When a new tool set, which should be understood throughout the disclosure to mean at least the combination of a stepper, a wafer track an associated apparatus for moving the wafers to and from the various production stages and locating the wafer in each of the same, is introduced into an IC production line, a great deal of set-up and adjustment is necessary before the arrangement is ready for regular production. For example, calibration or qualifying of the stepper is necessary before production can begin. It is also necessary to calibrate the operation of the wafer track and associated robotics in order to determine that the wafer is being moved between and disposed in, the stepper and processing chambers (e.g. etching chambers) in an optimally correct manner, and thus assure that the wafer is reproducibly set on the table of the stepper in a correctly oriented and located position. 
     That is to say, during the fabrication of an IC it is necessary to imprint images on resist coating and to etch, deposit, implant or the like, any number of times before the devices on the wafer are completed. It is therefore necessary to ensure that hardware which is used to move the wafer(s) back and forth, manipulate and to photolithograph, is operating in a manner wherein each and every wafer undergoes the same manipulations/operations during each and every stage of production. For example, accurate reproducible location of the wafer in the stepper is necessary. U.S. Pat. No. 5,392,361 issued on Feb. 21, 1998 in the name of Imaizumi et al., discloses the use of a mark on the wafer and a mark position detecting method and apparatus which uses fuzzy logic to improve alignment accuracy. 
     For further examples of the type of arrangements which are associated with the tool set reference can be had to U.S. Pat. No. 4,641,071 issued in February 1987 in the name of Tazawa et al, and U.S. Pat. No. 4,719,357 issued in January 1998 in the name of Ayata et al. 
     However, no matter what measures are taken, in the final analysis, the only way of determining if all of the necessary adjustments have been made in an optimal manner, is to make a test run and to examine the end product (viz., conduct empirical testing). However, this technique tends to leave it to chance as to which adjustment or setting needs to be fine tuned in order to bring this highly complex arrangement into truly optimal operational status. That is to say, the setting and arrangement of the reticle which is set in the optics of the stepper must be carefully examined in order to determine if adjustments to this vital piece of apparatus is necessary to correct some less than desirable outcome of the IC production. 
     This is a kind of feedback type of approach. An adjustment to the stepper operation the robotics which move the wafers from the wafer track to the stepper table, the position to which the wafer track moves the wafers prior to transfer, in combination with a possible change in the reticle or even in a resist or etching recipe, may, even though it would appear contrary to what might be conventionally considered to be correct and/or appropriate, enable the end result to be improved and, hence, would be highly beneficial. 
     Nevertheless, without some form of sophisticated analysis which can be carried out in a reliable and reproducible manner, the above types of adjustment and changes in technique amount to nothing more than guess work. Accordingly, there remains a need for a reliable technique via which a new tool set can be introduced and qualified in a manner which identifies the problems which need to be addressed in order to achieve set-up quickly and relatively inexpensively. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for implementing a type of feedback control in a manner that enables the calibration or qualification of a new tool set. The underlying technique is based on sequential mappings which are carried out at each of a select number of production stages, and wherein critical dimension (CD) data, accumulated during each of the mappings, is examined, compared and used to determine what adjustments should be made to ensure that the closest possible adherence to the design requirements is achieved. 
     In other words, the present invention enables a kind of feedback control data base. For example, mapping of results of the etching are examined. If it is found that a line width or corner is too great or too small, or the configurations of given features are not as good as is required to assure the best performance of the device (e.g., features necessary to optimize the speed and performance of a microprocessor for example), it is possible to determine what adjustments can be made to the various pieces of apparatus which go to make up the tool set so as to influence the processes at each of the stages which are involved in the process, and to instigate changes/adjustments which will enable improvements to be made and for a better product to be realized. 
     In particular, the inventive technique enables the qualifying or calibrating a new tool set be to checked/modified so as to achieve the best possible results prior to actually beginning production. Once the operation of the stepper and associated apparatus, such as a wafer track, are modified/adjusted and performance assured, the amount of mapping which is used during actual production runs can be reduced to conventional levels or more preferably that which is necessary to determine that the process is running properly and that the new reticle is functioning optimally. 
     More specifically, a first aspect of the present innovation resides in a method of improving IC fabrication comprising the steps of: mapping the critical dimensions of a predetermined plurality of features at each of a predetermined number of stages of production of an IC; comparing the data collected at each of the mappings; and determining, from the comparison, what changes are required in the operation(s) of a tool set which is used in the production stages, to bring the critical dimension of the predetermined features into agreement with a predetermined set of design critical dimension. These predetermined features comprise ring oscillators, turning forks, test transistors, and wafer electrical testing-purpose (WET) transistors. 
     Another aspect of the invention resides in a method of qualifying an apparatus used in the production of integrated circuits, the method comprising the steps of: disposing a reticle in a stepper and using the stepper to move a wafer, which has been set on a table of the stepper using robotic apparatus, in a predetermined manner with respect to the reticle and to sequentially expose a predetermined number of exposure fields on the wafer; mapping the critical dimension of all features that impact integrated circuit speed performance, including ring oscillators. turning forks, test transistors, and wafer electrical testing-purpose (WET) transistors, which are located in the exposure field, which are contained in a selected group of the predetermined number of exposure fields; comparing the mapped critical dimension with a set of corresponding prerequisite critical dimension values; and adjusting the operation of at least one of the stepper and the robotic apparatus in order to bring the critical dimension which are derived using the mapping into accordance with a difference between the mapped critical dimension and the prerequisite critical dimension values. 
     A further aspect of the invention resides in a method of qualifying a tool set used in the production of integrated circuitry, said tool set including a stepper, a wafer track and robotic apparatus for selectively moving a wafer between at least the stepper and a reaction chamber and for reproducibly locating the wafer in each of same, the method comprising the steps of: mapping predetermined features of a reticle to determine a first set of critical dimension data; mounting the reticle in a stepper and operating the stepper to move the substrate into a predetermined position with respect to the reticle; impressing an image produced by the reticle onto a layer of photo resist formed on the wafer a plurality of times to form a corresponding plurality of exposure fields; removing the portion of the photo resist effected by the image impression to leave a photo resist mask pattern: mapping the predetermined features as they are formed in the photo resist mask pattern for each of selected exposure fields selected from among the plurality of exposure fields, to determine a second predetermined set of critical dimension data for the pattern; etching the wafer through the photo resist mask pattern; removing the photo resist mask pattern to reveal an etched pattern formed in the water; mapping the predetermined features in the etched pattern corresponding to each of the selected exposure fields and recording a third set of critical dimension data; repeating the steps of impressing, removing, mapping, etching, removing and mapping, in at least one subsequent fabrication stage; comparing the first, and at least one of the second and third sets of critical dimension data with each other and/or a predetermined set of standard critical dimension data values; and determining an adjustment to at least one of the stepper, the wafer track and the robotic apparatus, which is required to reduce a difference between the third set of critical dimension data and the predetermined set of critical dimension data. 
     In the above technique, the step of mapping comprises mapping a predetermined plurality of exposure fields which are clustered at a center portion of the wafer. The predetermines features which are mapped are ring oscillators, turning forks, test transistors, and WET transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various aspects and advantages of the present invention will become better understood as a description of the preferred embodiments is given with reference to the appended drawings in which. 
     FIG. 1 is a schematic perspective view of a stepper, wafer track, reaction/process chamber, and scanning station, wherein the stepper includes an optical system in which a reticle is mounted and a moveable table on which a wafer is carried; 
     FIG. 2 is an enlarged view of an exposure field showing the features which are mapped in accordance with the present invention; 
     FIG. 3 is a plan view of a wafer showing the positions of the exposure fields in which mapping according to the present invention is carried out; 
     FIG. 4 is a table showing the results which are graphed in FIG. 4; and 
     FIG. 5 is a graph showing an example of the results derived using the mapping technique. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows a stepper  100  which includes a table  102  adapted to support and hold wafers  104  thereon, and an optical system  106  which includes a source of radiant energy  108  and a reticle  10  for determining the pattern which is imprinted on the wafer. A wafer track which is use to move the wafers to and from the stepper, is denoted by the numeral  112 . In this figure, the wafer track  112  is shown as being arranged to transport the wafer  104  between the stepper  100 , a processing unit  114  in which etching (for example) can be carried out, and a mapping station  116  in which the features which are formed, can be measured via an electron microscope or the like device  118 . Robotics which are associated with the wafer track and which are used to move the wafers from one device to another are designated by the numerals  120 A,  120 B and  120 C. 
     As will be appreciated, this view is highly schematic and omits all but the outlines of the basic elements of the stepper  100 , wafer track  112 , processing unit or chamber  114 , and mapping station  116 . Further, only a limited number of chip outlines are shown on the wafer  104  and the overall arrangement is not drawn to scale nor is intended to accurately represent the actual structures of the respective devices. For further details pertaining to the construction, control and operation of steppers reference may be had to U.S. Pat. No. 5,392,361 issued on Feb. 21, 1995 in the name of Imaizumi et al. For details pertaining to wafer tracks and associated apparatus, reference may be had to U.S. Pat. No. 5,685,588 issued to Wong et al. on Nov. 11, 1997. 
     In accordance with the present invention, before the stepper  100  is used to imprint the image which is produced by the reticle  110 , repeatedly and sequentially over the surface of a resist covered wafer  104 , a predetermined number of key features which are present on the reticle are mapped and a first set of data is obtained. 
     FIG. 2 shows an example of the locations of the multiple features which are mapped in accordance with the present invention. As will be appreciated from this figure, a tuning fork feature TF is located at each of the four corners of the exposure field. A fifth tuning fork feature TF is formed at the center of the field at a location wherein the four quadrants K6-1, K6-2, K6-3 and K6-4 of the field, intersect with one another. Each of these features has a horizontal element and a vertical element. 
     In addition to the above mentioned elements, four ring oscillators RO and four test transistors TR are located in the illustrated locations. In this illustration, a single WET transistor spans the boarder of the K6-1 and K6-2 quadrants of the exposure field. It will be understood from the following data that the test transistors, which are subjected to measurement in accordance with the present invention, have both vertical and horizontal elements which are subject to CD measurement. 
     Table I below clarifies the element/function relationship which exists in accordance with a preferred embodiment of the invention. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Poly Gate C.D. Map Features and Functions 
               
            
           
           
               
               
            
               
                 Feature 
                 Function 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 (1) 
                 Ring Oscillator 
                 DENSE 
                 K6 1/2/3/4 
                 Directly Impacting Speed 
               
               
                   
                 Horizontal 
                 DENSO 
                 K6 1/2/3/4 
                 Performance and Sort Yield 
               
               
                   
                 (R) 
                 ISO 
                 K6 1/2/3/4 
               
            
           
           
               
               
               
               
            
               
                 (2) 
                 Tuning Fork 
                 DENSO, C (CENTER) 
                 No Electrical Value, 
               
               
                   
                 Vertical 
                 DENSO, LL (LOWER LEFT) 
                 For Measurement of Stepper 
               
               
                   
                 (TF) 
                 DENSO, LR (LOWER RIGHT) 
                 Lens Field CD Control 
               
               
                   
                   
                 DENSO, UL (UPPER LEFT) 
                 Uniformity Only 
               
               
                   
                   
                 DENSO, UR (UPPER RIGHT) 
               
            
           
           
               
               
               
               
               
            
               
                 (3) 
                 Test Transistor 
                 DENSE 
                 K6 1/2/3/4 
                 Directly Impacting Speed 
               
               
                   
                 Horizontal 
                 DENSO 
                 K6 1/2/3/4 
                 Performance and Sort Yield 
               
               
                   
                 (TR) 
                 ISO 
                 K6 1/2/3/4 
               
               
                   
                 Vertical 
                 DENSO 
                 K6 1/2/3/4 
               
               
                 (4) 
                 WET Transistor 
                 DENSO 
                   
                 For Process Electrical 
               
               
                   
                 Vertical 
                   
                   
                 Evaluation Only 
               
               
                   
                 (WET) 
                   
                   
                 (Electrical CDs) 
               
               
                   
               
            
           
         
       
     
     The mapping according to a preferred embodiment of the invention is carried out on the cluster of five center exposure fields which are shown in fine hatching at the center of the wafer  104  in FIG.  3 . 
     FIG. 4 shows an example of the data which is collected using a mapping which is carried out in accordance with the present invention while FIG. 5 shows this data in graphical form. 
     Embodiments of the present invention include optimizing by conducting more than one mapping to determine the best effect/modification which can be made to a tool set which is being qualified or calibrated in preparation for production. That is to say, the effect of an etching step, for example, which is carried out between the imprinting of the image on the resist layer and the final mapping which is carried out, can have a significant effect on the decision as to what modifications to the positioning operations that are carried out by the stepper which need to be made in order to rectify the process and to ensure that the CDs of the final product are as close to the design specifications as possible. 
     Accordingly, after the CD data for the features listed in the above table are measured via a mapping of the reticle at the mapping station  116 , the reticle  110  is set in the stepper  100  and a photo resist covered wafer is sequentially imprinted to form a plurality of exposure fields. Following this, the wafer  104  is transferred to the reaction chamber  114 , wherein portions of the resist layer are removed to reveal the etch mask. The wafer  104  is then removed and the CDs of the same features which were mapped on the reticle are again mapped at the mapping station  116  and the data recorded along with that for the reticle. 
     The wafer  104  is then moved back along the wafer track  114  to the reaction chamber  114  wherein etching is carried out. This etching can take the form of dry etching, wet etching, plasma etching or the like. The etching mask is then removed by a well known technique such as by dissolution in a solvent or by oxidation. Inasmuch as each of these process are very well known in the art, further explanation of this stage will be omitted for brevity. 
     The etched wafer  104  is then removed from the chamber  114  and transported back to the mapping station  116  and each of the features which have been previously mapped are again mapped This data is then stored. 
     This process of coating the upper surface of the wafer with resist, exposing surface to form a plurality of exposure fields, the removal of portions of the resist to form an etch or implantation mask, the mapping of the further features which have been formed in the mask and the subsequent etching (for example) and the mapping of the same, can be carried one or more times while collecting pertinent data at each of the mappings. 
     The three sets of data are then compared to determine if the process is proceeding in a manner which is satisfactory and that the final product, i.e. the etched wafer is being produced with the features in such a condition that the speed performance and the like of the device, which is being constructed using the above mentioned steps, will be as good as desired. This comparison can reveal a lot of information and provide guidance as to how to adjust one or more stages of the process to improve the end result. However, while the resist recipe, the etch recipe/parameters, the operation of the stepper and or wafer track, can have a profound influence on the IC which is produced, it is the effect that the settings of the stepper  100 , the wafer track  112  and associated robotics, which is the center of attention in this instance. 
     It is deemed essentially self-evident to the person skilled in the art of producing IC circuits, how the multiple mapping of the invention can be used to show how the process proceeds and what feedback adjustments need to be made to the various piece of apparatus which go to make up the tool set in order to achieved the desired end. Examples of the manner in which the above type of data can be interpreted and used, is given in U.S. Pat. No. 5,646,870, issued on Jul. 8, 1997, in the name of Krivokapic et al. This reference deals with process simulation and statistical analysis of CD values. The content of this reference is hereby incorporated by reference thereto. 
     More specifically, in the case wherein data is gathered via the mapping of poly gate CD on a specific stepper, if the collected data show the lens field CD uniformity (see table 1 feature 2 and FIG. 5) does not meet poly gate CD specification (i.e. +/−10 nm), the stepper or tool set is disqualified until requirements are met; if the lens field CD uniformity meets the gate CD requirement, but the R (ring oscillator) and TR (test transistor) (see table 1 feature 1 and 3) biases between iso/dense/denso features are too high (&gt;20 nm), the partial coherence of the stepper and/or the reticle CD can be adjusted to reduce the biases. 
     Only the preferred embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. However, it is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications with the scope of the inventive concept expressed herein.