Abstract:
A method for monitoring mask focus includes measuring profile asymmetries in a target feature including sub-resolution assist features and deriving a focus response based on a known correlation between the profile and focus of a corresponding mask. A computer system in a lithographic process may adjust mask focus based on such derived information to conform to a desired fabrication process.

Description:
PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/656,330, filed Jun. 6, 2012, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed generally toward semiconductor wafer fabrication and more particularly to maintaining focus during a lithographic process. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices. 
         [0004]    Metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control one or more semiconductor layer processes. For example, during lithography, maintaining sharp focus between the reticle and the resist is vital. Sub-resolution assist features (SRAF) are especially useful where lithographic targets are particularly sensitive to focus. Sub-resolution assist features (SRAF) are very small mask features designed to improve processes margins and enhance resolution for isolated features through additional constructive or destructive interference, but are not intended to be printed on the resulting wafer. Existing lithographic technology requires deconvolution of dose and focus using some form of CD metrology data and separation of data to determine focus response. 
         [0005]    While sub-resolution assist features are useful for assisting focus during lithography, existing sub-resolution assist features do not offer a reliable mechanism for maintaining focus. Consequently, it would be advantageous if an apparatus existed that is suitable for monitoring focus of a mask in a lithographic process. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, the present invention is directed to a novel method and apparatus for monitoring focus of a mask in a lithographic process. 
         [0007]    In at least one embodiment, a lithographic mask includes sub-resolution assist features to produce asymmetry in profile or image placement or both. Asymmetries enhance or deteriorate image log slope. Profile asymmetry may be altered via focus change, and such changes can be measured during processing. 
         [0008]    In another embodiment, a computer system in a lithographic process monitors profile asymmetries of one or more target features. Changes in profile asymmetry are used to derive changes in focus and thereby continually monitor focus during processing. 
         [0009]    In another embodiment, a method for monitoring mask focus includes measuring profile asymmetries in a target feature and deriving focus based on a known correlation between the profile and focus of a corresponding mask. Mask focus may be adjusted based on such derived information to conform to a desired fabrication process. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
           [0012]      FIG. 1  shows a block diagram of a computer system useful for implementing at least one embodiment of the present invention; 
           [0013]      FIG. 2  shows a representation of mask elements including sub-resolution assist features for producing asymmetric profiles according to at least one embodiment of the present invention; 
           [0014]      FIG. 3  shows a representation of asymmetric profiles produced by sub-resolution assist features in a lithographic mask; 
           [0015]      FIG. 4  shows a representation of mask elements including sub-resolution assist features for producing asymmetric profiles in dual corresponding designs according to another embodiment of the present invention; and 
           [0016]      FIG. 5  shows a flowchart of a method for utilizing sub-resolution assist features for monitoring and maintaining focus during a lithographic process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
         [0018]    Referring to  FIG. 1 , a block diagram of a computer system useful for implementing at least one embodiment of the present invention is shown. The computer system may include a processor  100 , memory  102  connected to the processor  100  and a measuring device  106  configured to measure a profile asymmetry connected to the processor  100 . In a lithographic process, a lithographic mask  108  having a plurality of main features and at least one sub-resolution assist feature is used to produce elements on a semiconductor wafer  110 . 
         [0019]    During a lithographic process, the processor  100  receives a first projection profile through the measuring device  106  and a second projection profile through the measuring device  106 . The first projection profile and the second projection profile are asymmetrical such that profile asymmetries may be measured to indicate a focus of a scanner used in the lithographic processes. 
         [0020]    Additionally, measurement device  106  may measure a reference overlay marker so that the processor  100  may determine an overlay error. Based on the overlay error, the processor  100  may determine an image placement error. Unlike overlay error, image placement error is a function of focus. Once an image placement error is calculated, the processor  100  may use the image placement error to determine a focus error. 
         [0021]    Profile Asymmetry and Image Placement Error are two separate focus responses on the same target. The processor  100  may combine both profile asymmetry and image placement error to determine a focus response. Based on the focus response, the processor  100  may alter a scanner focus to produce a desired lithographic image. Embodiments of the present invention do not require deconvolution of dose or focus. Furthermore, using embodiments of the present invention, CD metrology measurement is not required; dose is relatively independent of the focus changes; and focus and overlay data can be obtained from the same measurement. 
         [0022]    Referring to  FIG. 2 , a representation of mask elements including sub-resolution assist features for producing asymmetric profiles according to at least one embodiment of the present invention is shown. A mask may include main features  200 ,  202 ,  204 ,  208  that are intended to produce elements on a semiconductor wafer. The mask may also include one or more sub-resolution assist features  206 ,  210 . Sub-resolution assist features  206 ,  210  produce or alter constructive or destructive interference to alter the shape or size of corresponding elements produced by main features  200 ,  202 ,  204 ,  208 , and such alteration may be variable as a function of the focus of the mask. For example, a first Sub-resolution assist feature  206  associated with a third main feature  204  may alter the size of the area of a semiconductor illuminated during lithographic processing. Furthermore, such alteration may vary such that the size of the area increases as the focus of the mask varies. 
         [0023]    Sub-resolution assist features  206 ,  210  may enhance or deteriorate image log slope and change profile asymmetry of the projected feature as compared to features without corresponding sub-resolution assist features  206 ,  210  by focus change to a scanner. Profile asymmetry may be used for focus monitoring and can be measured by means known in the art. 
         [0024]    The resulting target may contain embedded asymmetries and focus sensitivity can be controlled by properties of the sub-resolution assist features  206 ,  210  such as the number or size of such sub-resolution assist features  206 ,  210 . Also, the distance between the sub-resolution assist features  206 ,  210  and corresponding main features  204 ,  208  may be used to control the focus sensitivity. 
         [0025]    The design of sub-resolution assist features  206 ,  210  is dependent on the lithography process. Metrology tools may measure sub-resolution assist features  206 ,  210  according to means known in the art. 
         [0026]    Referring to  FIG. 3 , a representation of asymmetric profiles produced by sub-resolution assist features in a lithographic mask is shown. The profiles  300 ,  302  represent a cross-section of the projected images produced by a mask having main features and sub-resolution assist features. An unassisted profile  300  may be produced by a main feature. The unassisted profile  300  may be substantially the same at a first focal distance  304  and a second focal distance  306 . 
         [0027]    An assisted profile  302  may be produced by a main feature and a corresponding sub-resolution assist feature. The assisted profile  302  may vary as a function of distance from the projecting mask such that the size of the projected image of the assisted profile  302  at a first focal distance  304  is smaller than the size of the projected image at a second focal distance  306 . 
         [0028]    The unassisted profile  300  and assisted profile  302  are asymmetrical. During lithographic imaging, the focus of a scanner may be monitored by comparing an image corresponding to the unassisted profile  300  with an image corresponding to the assisted profile  302 . Alternatively, profile asymmetry between the unassisted profile  300  and the assisted profile  302  may be measured by means known in the art. Furthermore, placement error of a sub-resolution assist feature may be measured using overlay techniques. 
         [0029]    Referring to  FIG. 4 , a representation of mask elements including sub-resolution assist features for producing asymmetric profiles in dual corresponding designs according to another embodiment of the present invention is shown. A mask may include a first design with first design main features  400 ,  402 ,  404 ,  408  that are intended to produce elements on a semiconductor wafer. The first design may also include one or more first design sub-resolution assist features  406 ,  410 . First design sub-resolution assist features  406 ,  410  produce or alter constructive or destructive interference to alter the shape or size of corresponding elements produced by first design main features  400 ,  402 ,  404 ,  408 , and such alteration may be variable as a function of the focus of the mask. For example, a first Sub-resolution assist feature  406  of the first design associated with a third main feature  404  of the first design may alter the size of the area of a semiconductor illuminated during lithographic processing. Furthermore, such alteration may vary such that the size of the area increases as the focus of the mask varies. 
         [0030]    The mask may also include a second design with second design main features  412 ,  414  and one or more second design sub-resolution assist features  416 . The second design main features  412 ,  414  may comprise a plurality of elements intended to constructively or destructively interference with each other to produce a projected image. A first sub-resolution assist feature  416  of the second design associated with a second main feature  414  of the second design may alter the size of the area of a semiconductor illuminated during lithographic processing. 
         [0031]    Sub-resolution assist features  406 ,  410 ,  416  may enhance or deteriorate image log slope and change profile asymmetry of the projected feature as compared to features without corresponding sub-resolution assist features  406 ,  410 ,  412  by focus change to a scanner. Profile asymmetry may be used for focus monitoring and can be measured by means known in the art. 
         [0032]    The resulting target may contain embedded asymmetries and focus sensitivity can be controlled by properties of the sub-resolution assist features  406 ,  410 ,  416  such as the number or size of such sub-resolution assist features  406 ,  410 ,  416 . Also, the distance between the sub-resolution assist features  406 ,  410 ,  416  and corresponding main features  404 ,  408 ,  414  may be used to control the focus sensitivity. 
         [0033]    The design of sub-resolution assist features  406 ,  410 ,  416  is dependent on the lithography process. Metrology tools may measure sub-resolution assist features  406 ,  410 ,  416  according to means known in the art. 
         [0034]    In an embodiment including more than one corresponding design, focus can be measured by individual asymmetry results. For example, the second design may report one focus result and the first design can report another focus result. Differentiated results may also be used to measure focus. 
         [0035]    Referring to  FIG. 5 , a flowchart of a method for utilizing sub-resolution assist features for monitoring and maintaining focus during a lithographic process is shown. During a lithographic process, a first image having a first projection profile may be projected  500  on a semiconductor through a mask. A second image having a second projection profile may also be projected  502  on a semiconductor through a mask. The first projection profile and the second projection profile are asymmetrical such that profile asymmetries may be measured  504  to indicate a focus of a scanner used in the lithographic processes. 
         [0036]    Additionally, measurement tools used in the lithographic process may measure  506  a reference overlay marker to determine  508  an overlay error. Based on the overlay error, an image placement error may be calculated  510 . Unlike overlay error, image placement error is a function of focus. Once an image placement error is calculated, it may be used to determine  512  a focus error. 
         [0037]    Profile Asymmetry and Image Placement Error are two separate focus responses on the same target. Both or either can be used but Profile Asymmetry is preferred because it is a singular solution. Both profile asymmetry and image placement error may be combined to determine  514  a focus response. Based on the focus response, a scanner focus may be altered  516  to produce a desired lithographic image. Embodiments of the present invention do not require deconvolution of dose or focus. Furthermore, using embodiments of the present invention, CD metrology measurement is not required; dose is relatively independent of the focus changes; and focus and overlay data can be obtained from the same measurement. 
         [0038]    Utilizing embodiments of the present invention, detection of focus deviation or variation which is normally done on a tool basis, can be done on a wafer basis allowing for real time feedback and thereby improvement in process control and yield. Embodiments of the present invention may also utilize processes with reduced process windows in focus, thereby allowing production of chips with more aggressive lithography processes, possibly avoiding the high costs in other resolution enhancement techniques including advanced scanners, masks and resists. 
         [0039]    It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description of embodiments of the present invention, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.