Patent Publication Number: US-2007108368-A1

Title: Focus masking structures, focus patterns and measurements thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a divisional of patent application Ser. No. 11/084,556 filed Mar. 16, 2005, which is a continuation of patent application Ser. No. 10/291,181 filed Mar. 16, 2005 now U.S. Pat. No. 6,884,552, which claims priority of U.S. Provisional No. 60/338,204 filed on Nov. 9, 2001, all of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates generally to photolithography methods and systems, and more specifically to improved techniques for determining focus error.  
      Integrated circuits are made by photolithographic processes, which use photolithographic masks and an associate light or radiation source to project a circuit image onto a wafer. Referring to  FIG. 1 , for example, a simplified diagram of a lithography system  2  is shown. By way example, the lithography system may correspond to a stepper or scanning system. The lithography system  1  typically includes a light or radiation source  3  and a first set of optics  4  that illuminate a mask  5  having a circuit pattern  6  disposed thereon. In order to form the circuit pattern, the mask  6  may include opaque portions and/or transmissive portions. As generally well known, the opaque portions block the light from pass through the mask  6  while the transmissive portions allow the light to pass therethrough. In some cases, the transmissive portions may constitute phase shifted areas. Phase shifted areas tend to alter the phase of the light or radiation passing through the mask. The lithography system  2  also includes a second set of optics  7  that pick up the transmitted light or radiation and focuses (or images) it onto a surface  9  of a semiconductor wafer  8  thus writing the pattern of the mask  6  onto the surface  9  of the semiconductor wafer  8 . In most cases, the semiconductor wafer  8  includes a layer of photoresist that when exposed to the patterned light or radiation forms the pattern of the mask onto the wafer.  
      One problem that has been encountered during lithographic processes is the misfocus found between the surface of the wafer being patterned on and the ideal focus plane. Referring back to  FIG. 1 , the light or radiation follows an optical path that corresponds to the Z axis. The first and second set of optics as well as the mask and the wafer are thus positioned orthogonal to the optical path in different X&amp;Y planes. With this in mind, the second set of optics generally focuses the light on a specific X&amp;Y plane (not shown) positioned along the Z axis. This plane is generally referred to as the ideal focus plane. When the system is in focus, the ideal focus plane generally coincides with the surface of the wafer. When the system is out of focus, the ideal focus plane is offset relative to the surface of the wafer. That is, there is Z axis displacement between the ideal focus plane and the surface of the wafer being written on and thus there is misfocus. As should be appreciated, misfocus generally has a sign and magnitude corresponding to the Z axis displacement. The sign corresponds to the direction of the displacement (e.g., positive or negative), and the magnitude corresponds to the amount of displacement (e.g., the actual distance between planes). The displacement may be caused by many factors. For example, the second set of optics and/or the wafer may be mis-aligned (e.g., tilted) or they may be positioned in the wrong plane along the Z axis.  
      Unfortunately, misfocus may adversely effect the printed pattern on the wafer. For example, misfocus may cause increases or decreases in the width of the lines printed on the wafer, i.e., linewidth is a function of focus. The linewidth generally determines the speed and the timing across the circuit and thus misfocus may cause one portion of the chip to run faster or slower than another portion of the chip. In most cases, the chip is clocked to the slowest portion thereby reducing the selling price of the chip. In addition, misfocus may cause open or shorted circuits such that the chip must be discarded or reworked. Presently, focus is determined by exposing a pattern through a range of focus settings, and then inspecting the resultant patterns for the best looking images or by using an aerial image monitor to determine the spatial location of the best focus.  
      In view of the foregoing, there is a desire for improved techniques for determining focus error, as for example, the direction (i.e., the positive or negative Z-axis translation of the wafer) and the magnitude (i.e., offset displacement) of misfocus.  
     SUMMARY OF THE INVENTION  
      The invention relates, in one embodiment, to a focus masking structure used to determine the focus quality of a photolithographic pattern or a photolithographic system. The focus masking structure is disposed on a substrate. The focus masking structure includes a plurality of source lines. The focus masking structure also includes a plurality of phase shift zones configured to separate the plurality of source lines. The plurality of phase shift zones alternating between a first phase shift zone and a second phase shift zone thereby creating alternating first and second phase transitions. The first and second phase shift zones having phases that are not equal to the phase of the substrate, and a phase difference therebetween that is not equal to 0 or 180 degrees or integer multiples of 180 degrees.  
      The invention relates, in another embodiment, to a focus masking structure used to determine the focus quality of a photolithographic pattern or a photolithographic system. The focus masking structure is disposed on a substrate. The focus masking structure includes a plurality of source lines. The focus masking structure also includes a plurality of phase shift zones configured to separate the plurality of source lines. The plurality of phase shift zones alternating between a first phase shift zone and a second phase shift zone thereby creating alternating first and second phase transitions, a phase difference between the first and second phase shift zones not being equal to 0 or 180 degrees or integer multiples of 180 degrees. The plurality of source lines and the phase shift zones cooperate to produce a focus pattern that includes at least two periodic structures that shift relative to one another with changes in focus. The shift is based on the sign and magnitude of defocus and the sign and magnitude of the phase difference.  
      The invention relates, in another embodiment, to a method of determining best fit focus masking structure. The method includes selecting a process for creating focus patterns with focus masking structures. The method also includes selecting a technique for measuring position shift of focus patterns. The method further includes selecting or optimizing characteristics of focus masking structure based on process parameters associated with the process or the measurement technique.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is illustrated by way of example, and not by way of limitation.  
       FIG. 1  is a simplified example of a lithography system.  
       FIG. 2  is a simplified perspective view of a focus masking structure and a focus pattern, in accordance with one embodiment of the present invention.  
       FIG. 3A  is a simplified perspective diagram of a focus pattern with shifted structures, in accordance with one embodiment of the present invention.  
       FIG. 3B  is a simplified perspective diagram of a focus pattern with shifted structures, in accordance with one embodiment of the present invention.  
       FIG. 3C  is a simplified perspective diagram of a focus pattern with shifted structures, in accordance with one embodiment of the present invention.  
       FIG. 4  is a top view of a focus masking structure, in accordance with one embodiment of the invention.  
       FIG. 5  is a broken away side view, in cross section, of a focus masking structure, in accordance with one embodiment of the invention.  
       FIG. 6  is a top view of a focus pattern, in accordance with one embodiment of the invention.  
       FIGS. 7A and 7B  are perspective views of a focus masking system, in accordance with one embodiment of the invention.  
       FIG. 8  is a top view of a trim masking structure, in accordance with one embodiment of the invention.  
       FIG. 9  is a top view of a focus pattern, in accordance with one embodiment of the invention.  
       FIG. 10  is a top view of a focus masking system and a focus pattern, in accordance with another embodiment of the invention.  
       FIG. 11  is a flow diagram of focus processing, in accordance with one embodiment of the present invention.  
       FIG. 12  is a top view of a focus masking system and a focus pattern, in accordance with one embodiment of the present invention.  
       FIG. 13  is a top view of a focus masking system and a focus pattern, in accordance with one embodiment of the present invention.  
       FIGS. 14A-14K  are side views showing various embodiments of a focus masking structure with weak phase shifting components, in accordance with one embodiment of the present invention.  
       FIG. 15  illustrates a focus masking system that may be used to produce focus pattern with finely segmented elements, in accordance with one embodiment of the present invention.  
       FIG. 16  is a flow diagram of a focus calibration procedure, in accordance with one embodiment of the present invention.  
       FIG. 17  is a flow diagram of a calibration procedure, in accordance with one embodiment of the present invention.  
       FIG. 18  is a flow diagram of a focus monitoring procedure, in accordance with one embodiment of the present invention.  
       FIGS. 19A-19D  are broken away side views, in cross section, of various focus masking structures, in accordance with one embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention.  
       FIG. 2  is a simplified perspective view of a focus masking structure  10 , in accordance with one embodiment of the present invention. The focus masking structure  10  is generally provided to produce a focus pattern  12  corresponding to the focus quality of a photolithographic pattern or a photolithographic system. The focus pattern  12  may be used to improve the focus control of subsequent lithographic patterning and to determine whether the quality of the photolithographic pattern meets specified requirements. The focus masking structure  10  may be suitable for a wide variety of photolithography processes, as for example, photolithographic processes pertaining to semiconductor manufacturing, optical device manufacturing, micro-mechanical device manufacturing, magnetic recording data storage manufacturing and the like.  
      Although not shown, one or more of the focus masking structures  10  may be positioned on a photolithographic mask. In one embodiment, the focus masking structure is positioned on a product mask configured for producing a circuit pattern on a wafer. In this embodiment, the focus masking structure is generally positioned in a region of the product mask that is not used by a circuit pattern. For example, the focus masking structure may be positioned in a region that corresponds to the area of the wafer used for sawing (e.g., saw street). Any number of focus masking structures may be positioned on the product mask. Generally speaking, about 1 to about 10 focus masking structures may be used for focus monitoring on a mask used for a product. In another embodiment, the focus masking structure is positioned on a test mask for producing test patterns on a wafer. A test mask is generally used for testing the lithography equipment rather than for generating a product. Any number of focus masking structures may be positioned on a test mask. For example, 1000&#39;s of focus masking structures may be used on the test mask.  
      In general, the focus masking structure  10 , which is located on a substrate  14 , is used to project the focus pattern  12  onto a surface  16  of a workpiece  18 . In most cases, the focus pattern  12  is printed on the surface  16  of the workpiece  18  when light or radiation  20  is made to travel through the focus masking structure  10 . By way of example, the surface  16  may include a layer of photoresist for capturing the image produced by the focus masking structure  10  when light  20  is passed therethrough. That is, the light or radiation induces chemical and physical changes in the photoresist that can be developed into a structural pattern.  
      The focus masking structure  10  is generally configured to produce a focus pattern  12  that changes with changes in focus. In general, the focus masking structure  10  transforms actual focus information in the Z-direction into measurable focus information in a direction orthogonal to the Z-axis, as for example, in the X and/or Y directions. For example, the measurable focus information is contained in the focus pattern  12 , which is positioned in an X &amp; Y plane. In one embodiment, the focus masking structure  10  is configured to produce a focus pattern  12  having multiple structures that form measurable shifts therebetween corresponding to system defocus. That is, the focus masking structure  10  creates patterns having positions that are sensitive to focus. For example, the focus masking structure  10  may produce a focus pattern  12  having a plurality of periodic structures formed by parallel printed lines that shift when the system is out of focus. Periodic structures increase the amount of information that may be used to determine focus. Periodic structures may be widely modified to diminish the impact of certain processes on the focus measurements. For example, the size and pitch of the printed lines may be arranged to coincide with the size of the lines of the circuit pattern so that the structures are more correlated with what is happening to the circuit pattern during processing.  
      In one implementation, the periodic structures of the focus pattern are interposed or interlaced with one another such that their parallel lines alternate from one to the other along the same axis. For example, as shown in  FIG. 2 a  first periodic structure  22 , which includes a plurality of parallel lines  26  (shown by cross hatching), is interlaced with a second periodic structure  24 , which includes a plurality of parallel lines  28  (shown by shading). In another implementation, the periodic structures of the focus pattern are spatially distinct relative to one another. For example, as shown in  FIG. 7B , all of the periodic structures  118  are separated from one another rather than being interlaced therewith. In yet another implementation, the periodic structure of the focus pattern may be a combination of the two implementations above. That is, a first portion of the periodic structures may be interposed or interlaced while a second portion is spatially separated (see  FIGS. 12 and 13 ). In any of the implementations, the periodic structures of the focus pattern shift relative to one another when the system is out of focus. In most cases, the periodic structures are arranged to shift both positively and negatively in the X direction when the system is out of focus.  
      The shifting periodic structures  22 ,  24  are produced by a focus masking structure  10  that includes a plurality of source lines  30  (shown by cross hatching) and  32  (shown by shading) that are separated by alternating phase shift zones  34  and  36 . As should be appreciated, source lines  30  generally correspond to the printed lines  26  of periodic structure  22  and source lines  32  generally correspond to the printed lines  28  of periodic structure  24 . In one embodiment, the phase shift zones  34 ,  36  are configured to adjust the placement of the periodic structures  22 ,  24  and thus the lines  26 ,  28  in accordance with the sign and magnitude of the focus. This is generally accomplished by using different phases, Ph 1  and Ph 2 , respectively, that are not equal to the phase Phs of the substrate  14 . If there is a phase difference (e.g., phase difference not equal to an integer multiple of 180), the position of the dark/bright zones shift left or right in accordance with focus. In one implementation, the substrate  14  represents a non-phase shifted area and the phase shift zones  34  and  36  represent first and second phase shifted areas. Alternatively, one of the phases Ph 1  or Ph 2  may be equal to the phase Phs of the substrate.  
      In addition, the alternating phase shift zones  34  and  36  are constructed so that the phase difference (Ph 1 -Ph 2 ) therebetween is not 0 or 180 degrees or integer multiples of 180 degrees. It has been found that if (Ph 1 -Ph 2 ) does not equal 0 or 180 then the position of periodic structures  22  and  24  shift as a function of focus, i.e., the positions of lines  26  and  28  change with changes in focus. Conversely, if the phase difference (Ph 1 -Ph 2 ) equals 0 or 180 then the position of periodic structures  22  and  24  are not a function of focus, i.e., the positions of lines  26  and  28  do not change with changes in focus. Alternatively,  
      When projected onto the surface  16  of the workpiece  18 , the quality of focus may be determined by comparing the relative positions of the periodic structures  22 ,  24 . For instance, the position of the first periodic structure  22  may be compared with the position of second periodic structure  24  in the X direction to determine the focus in the Z direction. In general, the periodic structures are configured to shift in opposite directions. For example, the first periodic structure may shift in the positive X direction and the second periodic structure may shift in the negative X direction (or vice versa) thus producing a displacement that corresponds to misfocus. The shift displacement (e.g., the distance the structures shifted) may relate to the magnitude of misfocus and the shift direction (e.g., the direction the structures shifted) may relate to the sign of misfocus.  
      In one embodiment, the spacings between the lines of the shifting structures are used to determine focus errors. Referring to the focus pattern  12 , adjacent lines  26  and  28  of periodic structures  22 ,  24  form a first spacing  44  (from lines  26  to  28 ) and a second spacing  46  (from lines  28  to  26 ) therebetween. These spacings change as a function of focus. That is, the spacings  44 ,  46  change when the periodic structures  22 ,  24  shift in opposite directions in accordance with defocus. The change in spacings  44  and  46  are generally inverse to one another. That is, as spacing  44  gets larger, spacing  46  gets correspondingly smaller (or vice versa).  
      Referring to  FIGS. 3A and 3B , the spacings will be described in greater detail. In order to simplify the discussion, the focus pattern  12  includes interlaced periodic structures  22  and  24 . As shown in  FIG. 3A , for example, a negative change in focus (e.g., the difference between the ideal focus plane  48  and the surface  16  of the workpiece  18  in the negative Z direction) may correspond to a decrease in spacing  44  and an increase in spacing  46 . In addition, as shown in  FIG. 3B , a positive change in focus (e.g., the difference between the ideal focus plane  48  and the surface  16  of the workpiece  18  in the positive Z direction) may correspond to an increase in spacing  44  and a decrease in spacing  46 . Furthermore, the amount of change (e.g., distance) in either direction may correspond to the magnitude of focus.  
      It should be noted that the  FIGS. 3A and 3B  are shown by way of example and not by way of limitation and thus the directional change of the spacings may be reversed. For example, a negative change in focus may correspond to an increase in spacing  44  and a decrease in spacing  46  while a positive change in focus may correspond to a decrease in spacing  44  and an increase in spacing  46 . The manner in which the spacings change generally depends on the configuration of the focus masking structure.  
      In one implementation, the sign as well as the magnitude of defocus can be determined by comparing the actual spacings relative to their corresponding spacing at best focus. For example, the actual first spacing may be compared with the first spacing at best focus and/or the actual second spacing may be compared with the second spacing at best focus to determine focus errors. Alternatively, in another implementation, the sign as well as the magnitude of defocus can be determined by comparing the first spacing relative to the neighboring second spacing. For example, the first spacing may be compared with the second spacing to determine focus errors.  
      In another embodiment, the centers of symmetry of the shifting structures are used to determine focus errors. Referring to  FIG. 3C , the centers of symmetry will be described in greater detail. In order to simplify the discussion, the focus pattern  12  includes a plurality of spatially separated periodic structures  22  and  24 . As shown in  FIG. 3C , for example, the first periodic structures  22  may form a first center of symmetry  38  and the second periodic structure  24  may form a second center of symmetry  40 . The centers of symmetry represent the center of their respective periodic structures, i.e., the centers of symmetry are imaginary lines parallel to the actual shifting lines with equal number of actual lines on either side. The centers of symmetry are typically determined by averaging the position of the lines contained within the periodic structures. As shown, the centers of symmetry form a spacing  42  therebetween.  
      Similar to the spacings between lines  26  and  28 , the spacing  42  between centers of symmetry change as a function of focus. That is, the position of the centers  38 ,  40  and thus the center spacing  42  generally depends on the focus quality, i.e., the centers  38 ,  40  move further apart or closer together relative to the amount of misfocus, and they move in different directions relative to the direction of misfocus. As such, a spacing that is small or large compared with the spacing at best focus may indicate a positive or negative change in focus (i.e., the positive or negative Z-axis translation of the workpiece) and the distance between spacings may correspond to the magnitude of the focus change (i.e., offset displacement along the Z-axis).  
      Additionally, in the case where multiple periodic structures are used as shown in  FIG. 3C , the centers of symmetry  38  A&amp;B for each of the periodic structures  22  may be averaged, and the centers of symmetry  40  A&amp;B for each of the periodic structures  40  may be averaged so as to produce a resultant center of symmetry  43  and  45 , respectively, for each of the periodic structure groups  22  and  24 . Much like above, the resultant centers of symmetry form a spacing  47  therebetween that changes as a function of focus.  
      In one implementation, the sign as well as the magnitude of defocus can be determined by comparing the center spacing  42  relative to the center spacing at best focus. For example, the focus may be determined by measuring the periodic structures  22  and  24 , calculating the centers of symmetry  38 ,  40  for each of the periodic structures  22  and  24 , and comparing the spacing  42  therebetween with the spacing at best focus (e.g., no shift).  
      In one embodiment, a measurement tool is used to measure the positional changes of the focus pattern. The measurement tool may be an in situ or ex situ measurement tool. That is, the measurement tool may be integrated with the lithography tool or it may be a stand alone device. With regards to the integrated measurement tool, the measurement tool may cooperate with the lithography process tool to change focus in real time or while the wafer is still in the lithography tool. That is, the measurement tool may provide a feedback control signal to the lithography tool so as to correct the focus. By way of example, the control signal may be used to control some aspect of the optics and/or wafer to achieve and maintain the best possible focus. For example, the position of the optics and/or the wafer may be adjusted so that the ideal focus plane substantially coincides with the surface of the wafer. Alternatively, the focus information may be displayed to an operator so that the operator may make adjustments.  
      The shifts associated with the focus pattern may be measured using suitable methods for measuring positional changes in the X or Y directions, as for example, the relative displacement (motion) of the lines  26  and  28  relative to each other, or the relative displacement between each of the periodic structures  22  and  24 . In one embodiment, methods used for measuring overlay error may be used. That is, the focus masking structure is configured to create a focus pattern that can be measured using overlay measurement techniques. As is generally well known, overlay pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it or to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Any shift between separately generated patterns is known as overlay error.  
      Any suitable overlay measuring technique may be used. By way of example, scatterometry, scanning, imaging and other suitable techniques may be used. Each of these techniques is configured to generate a signal that can be directly or indirectly converted into focus information, i.e., direction and magnitude of focus. In one embodiment, a conversion algorithm or calibration curve is used to convert the signal to focus information that can be used by the system. Conversion algorithms and calibration curves generally provide predetermined focus data related to a given signal, i.e., this signal corresponds to this much change in focus. Conversion algorithms and calibration curves are well know in the art and for the sake of brevity will not be discussed in greater detail. In one implementation, the signal is converted into displacement information in accordance with a first calibration curve (i.e., this signal corresponds to this much change in position) and the displacement information is subsequently converted into focus information in accordance with a second calibration curve (this change in position corresponds to this much change in focus). In another implementation, the measured signal is directly converted to focus information (this signal corresponds to this much change in focus) without converting to displacement information.  
      In one embodiment, known overlay tools such as those capable of measuring the overlay error for separately generated overlay patterns may be modified to measure the focus dependent shifts and/or displacements of the components parts of the focus patterns described herein.  
       FIG. 4  is a top view of a focus masking structure  50 , in accordance with one embodiment of the invention. By way of example, the focus masking structure  50  may generally correspond to the focus masking structure  10  shown in  FIG. 2 . In one embodiment, the focus masking structure  50  is constructed on a photolithographic mask. By way of example, the focus masking structure  50  may be part of a photolithographic mask used to create a circuit pattern on a wafer, or it may be part of a photolithographic mask used for testing certain attributes or characteristics (e.g., focus or optical aberrations) of a photolithographic system.  
      As shown, the focus masking structure  50  is positioned on a transparent substrate  52 . By way of example, the transparent substrate may be formed from materials such as quartz or glass. The focus masking structure  50  generally consists of parallel source lines  54  and  56 , which are separated by alternating phase shift zones  58  and  60 . The source lines and phase shift zones may be widely varied. The source lines  54  and  56  generally represent areas of the masking structure configured for preventing the transmission of light. As such, the source lines  54  and  56  may be formed from a suitable light blocking material. By way of example, the source lines  54 ,  56  may be formed from metals such as chromium and chromium alloys that are deposited on the surface of the substrate  52 . The phase shift zones  58  and  60 , on the other hand, generally represent areas of the masking structure configured for allowing the transmission of light (albeit with a phase shift). As such, the phase shift zones  58  and  60  may be formed from suitable phase shifting materials disposed above or within the substrate  52 . By way of example, the phase shifting zones  58 ,  60  may be formed by a portion of the substrate (e.g., openings, trenches and the like etched into the substrate) or they may be formed from other materials deposited above the surface of the substrate or a combination of the two.  
      It should be noted that sources may be formed from other materials besides metals. For example, the source lines may also be formed from a portion of the substrate or from other light effecting materials (e.g., chromeless phase shift mask).  
      In one embodiment, the alternating zones are constructed so that the optical phase difference (Ph 1 -Ph 2 ) between the light transmitted by the neighboring zones  20  and  22  is not 0 or 180 degrees. Any suitable phase difference (Ph 1 -Ph 2 ) may be used. For example, a phase difference (Ph 1 -Ph 2 ) between about 10 degrees to about 170 degrees, and more particularly about 90 degrees may be used. The phase difference (Ph 1 -Ph 2 ) is generally determined by the mask making capabilities of the process used to make the mask and the amount of sensitivity desired. It should be noted that for good focus sensitivity the phase difference should be closer to 90 degrees. In one implementation, the phase difference is configured to be as close as possible to 90 degrees without changing the mask making process. In another implementation, a dedicated process may be used to create the focus masking structure separate from the rest of the mask manufacturing process. For example, a separate patterning and etch process may be used (e.g., one that is similar to forming the circuit pattern). Alternatively, focus ion beam micro machining, atomic force microscope micro-mechanical machining, focused high intensity pulsed laser micro machining and the like may be used. In general, adding steps to the mask making process will increase costs.  
      The line widths of the source lines  54  and  56  are generally designated Wa and Wb, respectively while the zone widths of the phase shift zones  58  and  60  are generally designated Wc and Wd, respectively. The widths Wa, Wb, Wc and Wd may be widely varied. For example, they may or may not be equal to each other. In the illustrated embodiment, the widths Wa and Wb are configured to be equal to one another while the widths Wc and Wd are configured to be not equal to one another. It is generally believed that lines having equal widths Wa and Wb provides a measurement that is more similar to device features and therefore provides better correlation to device (e.g., circuit) performance parameters. In some methods of detection (e.g., scatterometry), it is believed that in an ideal phase shift mask there is low (or zero) sensitivity to positive and negative changes in the focus when Wc=Wd and therefore it is preferable to make them not equal. That is, it is difficult to determine the sign of focus when Wc=Wd. Furthermore, the widths Wc and Wd are generally configured to be larger than widths Wa and Wb.  
      The pitch of the source lines  54  and  56  are generally designated Pa and Pb, respectively while the pitch of the phase shift zones  58  and  60  are generally designated Pc and Pd, respectively. As should be appreciated, pitch is generally defined as the distance between specific lines or zones. The pitches Pa, Pb, Pc and Pd may be widely varied. In the illustrated embodiment, the pitches Pa, Pb, Pc and Pd are equal to one another. One advantage of equal pitches is simplicity of calculating reference signals.  
      The focus masking structure  50  may be a single or it may be used with other focus masking structures. In one embodiment, a combination of different focus masking structures may be used on a single photolithographic mask in order to increase the measurement range over which the defocus can be uniquely determined. For example, the focus masking structures may have different focus characteristics, i.e., best focus spacings and/or different relative phase shifts.  
       FIG. 5  is a broken away side view, in cross section (taken along line  5 - 5 ′ as shown in  FIG. 4 ), of the focus masking structure  50 , in accordance with one embodiment of the invention. In this embodiment, the source lines  54  and  58  are deposited on a front surface  66  of the substrate  52  and the phase shift zones  58  and  60  are etched into the substrate  52 . By etching both of the phase shift zones  58  and  60 , the light transmission therethrough is more similar.  
      As shown, the phase shift zones  58  and  60  are etched to different depths so as to produce different phase shifts. That is, the first phase shift zone  58  is etched to a first depth D 1  relative to the surface  54  of the substrate and the second phase shift zone  60  is etched to a second depth D 2  relative to the surface  54  of the substrate. Any suitable arrangement of depths may be used (e.g., deep/shallow, shallow/deep). In the illustrated embodiment, the second depth D 2  is deeper than the first depth D 1 . The phase shift zones thus produce different path lengths L 1  and L 2  through the substrate that change the phase of light coming through the substrate. That is, when light  70  is made incident on a back surface  68  of the substrate  66 , light  72  going through phase shift zone  58  travels the distance L 1  through the substrate  66  thus producing a first phase Ph 1 . In addition, light  74  going through phase shift zone  60  travels the distance L 2  through the substrate  66  thus producing a second phase Ph 2 . This arrangement produces a zones of minimum light intensity that make more distinct shadows and that move to the left or right according to the sign and magnitude of the system defocus and the phase difference Ph 1 -Ph 2  (e.g., positive sign moves in one direction, and negative sign moves in the opposite direction).  
      To elaborate, Ph 1 =L 1 (n mask )+D 1 (n air ) and Ph 2 =L 2 (n mask )+D 2 (n air ), where n=the index of refraction for the mask and air, respectively. Furthermore, Ph 1 =T(n mask )+D 1 (n air −n mask ) and Ph 2 =T(n mask )+D 2 (n air −n mask ) where T=the total path distance (either L 1 +D 1  or L 2 +D 2 ).  
       FIG. 6  is a top view of a focus pattern  80 , in accordance with one embodiment of the invention. By way of example, the focus pattern  80  may generally correspond to the focus pattern  14  shown in  FIG. 2  and may generally be produced by the focus masking structure  50  shown in  FIG. 4 . In general, the focus pattern  80  is printed on the surface  82  of a workpiece  83  when light is made to travel through the masking structure. By way of example, the surface  82  may represent an exposed layer of photoresist, a developed layer of photoresist, an underlying layer of the wafer (e.g., etched into metal or dielectric) and/or the like. In one embodiment, the workpiece is a semiconductor wafer. The focus pattern  80  generally includes a first periodic structure  84  of parallel printed lines  86  (shown by cross hatching) and a second periodic structure  88  of parallel printed lines  90  (shown by shading). The first set of parallel printed lines  86  correspond to the Ph 1  to Ph 2  transition, while the second set of printed parallel lines corresponds to the Ph 2  to Ph 1  transition.  
      As shown, the first periodic structure  84  is generally interposed or interlaced with the second periodic structure  88  such that the printed lines  86  alternate with the printed lines  90  along an axis  93 , i.e., the lines of the periodic structures are parallel to one another so as to provide position information in a single direction (e.g., X, Y, etc.). Furthermore, as shown, adjacent lines  86  and  90  form a first spacing  92  (left to right from  86  to  90 ) and a second spacing  94  (left to right from  90  to  86 ). As the system goes in and out of focus these lines either move apart or together thus changing the first and second spacings. For example, one focus value may move the lines further apart while another focus value may move the lines closer together. As should be appreciated, the change in  86  to  90  and  90  to  86  provide measurable information that can be converted to actual focus information, i.e., changes in these distances will indicate the focus value.  
      The number of lines  86 ,  90  inside each periodic structure  84 ,  88  may be varied to meet the specific needs of each focus pattern. From the perspective of the minimum number of lines that is needed for operation, that number is two. In the embodiment shown, each of the periodic structures includes  10  coarsely segmented lines. In some cases, it may even be desirable to have periodic structures having a different number of lines, i.e., a first periodic structure having 5 lines and a second periodic structure having 10 lines. Furthermore, the line configuration of each of the periodic structures, i.e., pitch, linewidths and spacings, may be varied to meet the specific needs of each focus pattern. In the illustrated embodiment, each of the periodic structures  84 ,  88  has the same pitch, linewidths and spacings. That is, each of the periodic structures  84 ,  88  has equal linewidths and equal spacings therebetween.  
      The shifts or relative displacement between interlaced periodic structures, as for example the periodic structures shown in  FIG. 6 , may measured using a variety of techniques.  
      In one embodiment, the shifts between interlaced periodic structures are measured via scatterometry (e.g., reflectometry, spectroscopic ellipsometry, multiwavelength reflectometry or angle resolved scatterometry). In scatterometry, a measurement signal(s) corresponding to the shifts between periodic structures is acquired with one or more light or radiation beams that are made incident on the pattern and one or more detectors that detect the scattered, reflected and/or diffracted beams emanating from the pattern. As should be appreciated, the scattered, reflected and/or diffracted light changes in accordance with the shifting structures and thus the scattered, reflected and/or diffracted light may be used to determine the relative displacement between the shifting structures. For example, when the spacing changes between lines, the scattered, reflected and/or diffracted light may be less or more intense at a certain wavelength or polarization and/or the optical phase may be altered. The modifying (e.g., scattering, reflecting and/or diffracting) of light from the printed pattern may be determined by many factors including but not limited to: the linewidths, line spacings, the pitch, the optical properties (n(lambda), k(lambda), the shapes and profiles (e.g., critical dimensions), height, sidewall angle, sidewall roughness, T-toppings, footing, features, volume, etc. Scatterometry may also be used to determine the shapes of the printed lines. For example, printed in individual separate grating structures of purely  86  or purely  90 , i.e., specular reflection direction. This information can be fed-forward into the analysis of the signal so as to enable a better or faster determination of focus quality.  
      The beam(s) is generally detected at the same angle that it was brought in at, however, it may also be detected at other angles. The measurement may be performed in a fixed angle, sprectroscopic ellipsometry mode, in an angle scanning single or multiple wavelength mode, or in a multiple angle, multiple wavelength mode. That is, the beam(s) may be brought in at a single or multiple angles and they may be brought in at a single wavelength or at multiple wavelengths. In addition, the beams may be detected at a single angle or multiple angles and they may be detected at single or multiple wavelengths. Furthermore, the intensity, polarization and/or optical phase of the beams may be detected at different angles and/or different wavelengths. By way of example, scatterometry techniques that may be used are described in greater detail in a pending U.S. patent application Ser. No. 09/833,084, titled “Periodic Patterns and Technique to Control Misalignment Between Two Layers,” filed on Apr. 10, 2001, and which is herein incorporated by reference.  
      In another embodiment, the shifts between interlaced periodic structures are measured via scanning techniques. In scanning, a signal corresponding to the shifts between periodic structures is acquired with a scanning light or electron beam and one or more detectors that detect the reflected or scattered beam in one or more scattering directions. By way of example, representative scanning techniques that may be used are described in greater detail in U.S. Pat. Nos. 6,023,338 and 6,079,256 issued to Bareket, which are herein incorporated by reference.  
       FIGS. 7A and 7B  are perspective views of a focus masking system  100 , in accordance with one embodiment of the invention. The focus masking system  100  is configured to utilize a focus masking structure  102  and a trim masking structure  104  to produce a focus pattern  106  on a wafer  108 . By way of example, the focus masking structure  102  may correspond to the focus masking structure  50  shown in  FIG. 4 . In general, the focus masking structure  102  determines the displacement of the periodic structures  118  in the focus pattern  106 , and the trim masking structure  104  determines what part of the focus pattern can be measured. In most cases, the wafer  108  includes a layer of photoresist  110 , which when exposed to the light, captures the latent image of the focus pattern  106  therein. The focus masking system  100  is configured to produce the focus pattern  106  in two steps: a first exposure (as shown in  FIG. 7A ) and a second exposure (as shown in  FIG. 7B ). In the first exposure, light  112  is made to travel through the focus masking structure  102  to produce an initial focus pattern  114  on the wafer  108 , i.e., some portions of the photoresist are exposed and some portions are not exposed. By way of example, the initial focus pattern  114  may correspond to the focus pattern  80  shown in  FIG. 6 . In the second exposure, light  116  is made to travel through the trim masking structure  104  to change the initial focus pattern  114  into the final focus pattern  106 , i.e., select portions of the non-exposed portions of the photoresist are exposed. In essence, the trim masking structure  104  allows select portions of the focus pattern  114  to be removed therefrom so as to create a different focus pattern  106 .  
      In the illustrated embodiment, the trim masking structure  104  is arranged to form a focus pattern  106  having spatially distinct and multiple periodic structures  118 A-D, which include a plurality of coarsely segmented lines  119 . By constructing patterns with spatially distinct periodic structures, it is possible to implement a broader range of focus measurement algorithms that maximize the benefits of higher information density in the pattern. That is, the plurality of coarsely segmented lines  119  increase the amount of information that may be used for focus measurements. In addition, it is generally believed that by distributing the periodic structures to more points within the perimeter of the pattern, the more likely they are to balance out the non-uniformities caused by the process.  
      In one embodiment, the focus masking structure and the trim masking structure are disposed on the same substrate. In this embodiment, a stepper may move the substrate to a first position so as to make an exposure with the focus masking structure and thereafter move the substrate to a second position so as to make a second exposure with the trim masking structure. That is, the stepper may be programmed to select which portion of the mask is used at one time. In another embodiment, the focus masking structure and the trim masking structure are disposed on different substrates. In this embodiment, a stepper may move a focus masking structure on a first substrate into an exposure position so as to make a first exposure, and thereafter move a trim masking structure on a second substrate into the exposure position to make a second exposure.  
       FIG. 8  is a top view of a trim masking structure  120 , in accordance with one embodiment of the invention. By way of example, the trim masking structure  120  may generally correspond to the trim masking structure shown in  FIG. 7B . In one embodiment, the trim masking structure  120  may be used along with the focus masking structure  50  of  FIG. 4  to produce a focus pattern with spatially distinct and multiple periodic structures. The trim masking structure  120  generally includes a plurality of masking zones  122 A-D, which are disposed on a transparent substrate  124 . Each of the masking zones  122 A-D is configured to produce a separate periodic structure.  
      As shown, each of the masking zones  122 A-D includes a plurality of masking lines  126 . The masking lines  126  are configured to cover select portions of a pre existing focus pattern so as to prevent further exposure thereto. That is, the trim masking lines protect the areas where the pattern is desired, and the open areas therebetween let light through thereby erasing the latent pattern in the areas that are not desired. In most cases, the masking lines  126  are used to cover select portions of the printed lines of the initial focus pattern produced by the focus masking structure. In cases such as these, the masking lines  126  generally have a linewidth larger than the linewidth of the printed lines. In addition, the masking lines generally have a pitch equal to the pitch of the printed lines for which they are configured to cover. The masking lines  126  are generally formed from a light blocking material. By way of example, the masking lines  126  may be formed from light blocking materials that are deposited on the surface of the substrate  124 .  
      Although, the order of patterning the focus pattern is described using the focus masking structure first and the trim masking structure second, it should be noted that this is not a limitation. For example, the order in which the trim masking structure and focus masking are applied (to create the focus pattern) may be reversed.  
      Any suitable arrangement of the masking zones may be used. In the illustrated embodiment, the first masking zone  122 A is positioned in an upper left portion of the substrate  124 , the second masking zone  122 B is positioned in an upper right portion of the substrate  124 , the third masking zone  122 C is positioned in a lower left portion of the substrate  124  and the fourth masking zone  122 D is positioned in the lower right portion of the substrate  124 . Furthermore, any suitable number of masking lines  126  may be used. In general, the number of masking lines corresponds to the number of printed lines that need to be covered to produce the desired periodic structure. In the illustrated embodiment, each of the masking zones includes five masking lines.  
      Referring to FIGS.  6  an and  8 , during the second exposure, the first masking zone  122 A is configured to cover an upper left portion of the initial focus pattern  80  so as to produce a first periodic structure in the upper left portion of the final focus pattern. In particular, the masking lines  126 A of the first masking zone  122 A are configured to cover a first group of exposed lines so as to prevent exposure thereto. By group of printed lines, it is generally meant a group of the printed lines  86  or a group of the printed lines  90 . Furthermore, each of the masking lines  126 A is configured to cover an individual printed line from the group of printed lines. In the illustrated embodiment, the first group of lines corresponds to the first five printed lines  86  (from left to right). As such, the first masking line  126 A covers the first printed line  86 , the second masking line  126 A covers the second printed line  86 , and so forth. In one implementation, the masking lines  126 A are configured to partially cover the printed lines  86 . For example, as shown, the masking lines  126 A are arranged to cover an upper left portion of some of the printed lines  86 .  
      The second masking zone  122 B is configured to cover an upper right portion of the initial focus pattern  80  so as to produce a second periodic structure in the upper right portion of the final focus pattern. In particular, the masking lines  126 B of the second masking zone  122 B are configured to cover a second group of printed lines so as to prevent exposure thereto. By group of printed lines, it is generally meant a group of the printed lines  86  or a group of the printed lines  90 . Furthermore, each of the masking lines  126 B is configured to cover an individual printed line from the group of printed lines. In the illustrated embodiment, the second group of lines corresponds to the last five printed lines  90  (from left to right). As such, the first masking line  126 B covers the 6th printed line  90 , the second masking line  126 B covers the 7th printed line  90 , and so forth. In one implementation, the masking lines are configured to partially cover the printed lines. For example, as shown, the masking lines  126 B are arranged to cover an upper portion of the printed line  90 .  
      The third masking zone  122 C is configured to cover a lower left portion of the initial focus pattern  80  so as to produce a third periodic structure in the lower left portion of the final focus pattern. In particular, the masking lines  126 C of the third masking zone  122 C are configured to cover a third group of printed lines so as to prevent exposure thereto. By group of printed lines, it is generally meant a group of the printed lines  86  or a group of the printed lines  90 . Furthermore, each of the masking lines  126 C is configured to cover an individual printed line from the group of printed lines. In the illustrated embodiment, the third group of lines corresponds to the first five printed lines  90  (from left to right). As such, the first masking line  126 C covers the first printed line  90 , the second masking line  126 C covers the second printed line  90 , and so forth. In one implementation, the masking lines are configured to partially cover the printed lines. For example, as shown, the masking lines  126 C are arranged to cover a lower portion of the printed line  90 .  
      The fourth masking zone  122 D is configured to cover a lower right portion of the initial focus pattern  80  so as to produce a second periodic structure in the lower right portion of the final focus pattern. In particular, the masking lines  126 D of the second masking zone  122 D are configured to cover a fourth group of printed lines so as to prevent exposure thereto. By group of printed lines, it is generally meant a group of the printed lines  86  or a group of the printed lines  90 . Furthermore, each of the masking lines  126 D is configured to cover an individual printed line from the group of printed lines. In the illustrated embodiment, the fourth group of lines corresponds to the last five printed lines  86  (from left to right). As such, the first masking line  126 D covers the 6th printed line  86 , the second masking line  126 D covers the 7th printed line  86 , and so forth. In one implementation, the masking lines are configured to partially cover the printed lines. For example, as shown, the masking lines  126 D are arranged to cover a lower portion of the printed line  86 .  
       FIG. 9  is a top view of a focus pattern  140 , in accordance with one embodiment of the invention. In one embodiment, the focus pattern  140  is produced by the focus masking structure  50  of  FIG. 4 , and the trim masking structure of  FIG. 8 , in accordance with the focus masking system of  FIGS. 7A and 7B . The focus pattern  140  includes a plurality of periodic structures  142 A-D that generally correspond to the configuration of the masking zones  122 A-D. Periodic structures  142 A and  142 D are formed from a first set of parallel printed lines  144  (shown by cross hatching) while periodic structures  142 B and  142 C are formed from a second set of parallel printed lines  146  (shown by shading). The parallel printed lines  144 ,  146  generally correspond to the configuration (i.e., pitch, linewidths and spacings) of the parallel printed lines  86  and  90 , respectively.  
      As shown, the periodic structures  142 A-D are spatially separated from one another so that they do not overlap portions of an adjacent periodic structure (i.e., each of the periodic structures represents a different area of the pattern). Further, the periodic structures  142 A and  142 D are offset relative to periodic structures  142 B and  142 C in the X-direction such that the lines of the periodic structures do not line up. That is, the lines of periodic structures disposed below or above one another do not align. In the illustrated embodiment, the periodic structures  142 A and  142 D are offset to the left of periodic structures  142 B and  142 C.  
      Further still, periodic structure  142 A is diagonally opposed to periodic structures  142 D, and periodic structures  142 B is diagonally opposed to periodic structures  142 C. Moreover, periodic structures  142 A is displaced from periodic structures  142 D, and periodic structures  142 B is displaced from periodic structures  142 D. For example, the center of periodic structures  142 D is positioned below and to the right of the center of periodic structure  142 A, and the center of periodic structures  142 C is positioned below and to the left of the center of periodic structures  142 B. As should be appreciated, these cross-positioned structures form an “X” shaped pattern.  
      In an alternate embodiment, the configurations of the focus masking structure and the trim masking structure may be reversed. That is, the focus masking structure may include a plurality of spatially distinct periodic regions and the trim masking structure may include a plurality of parallel masking lines. When at 0 defocus, this arrangement may be used to produce a focus pattern having a plurality of periodic structures that are aligned rather than offset as shown in  FIG. 9 . That is, the periodic structures are not offset or shifted relative to an adjacent periodic structure disposed above or below it. For example, the parallel lines of the first periodic structure may be positioned in line with the parallel lines of the third periodic structure. When the system is out focus, the locations of the printed lines may change from their aligned position thus permitting measurement of the system defocus from the relative displacement of the lines and their corresponding structure.  
      To facilitate discussion,  FIG. 10  illustrates a focus masking system  160 , in accordance with another embodiment of the invention. The focus masking system  160  is configured to utilize a focus masking structure  162  and a trim masking structure  164  to produce a focus pattern  166 . Although shown in top views, the focus masking system  160  may generally correspond to the focus masking system  100  shown in  FIG. 7 .  
      In this embodiment, the focus masking structure  162  includes a plurality of spatially distinct regions  168 A-D, which are disposed on a transparent substrate  169 . Each of the regions  168 A-D is configured to produce a separate and spatially distinct periodic structure. Each of the regions  168 A-D includes parallel source lines  170  (shown by cross hatching) and  172  (shown by shading), which are separated by alternating phase shift zones  174  and  176 .  
      Any suitable arrangement of the regions  168 A-D may be used. In the illustrated embodiment, the first region  168 A is positioned in an upper left portion of the substrate  169 , the second region  168 B is positioned in an upper right portion of the substrate  169 , the third region  168 C is positioned in a lower left portion of the substrate  169  and the fourth region  168 D is positioned in the lower right portion of the substrate  169 . Furthermore, the first region  168 A is offset relative to the third region  168 C and the second region  168 B is offset relative to the fourth region  168 D. In order to generate a final focus pattern  166  having nominally aligned periodic structures  180 A-D, the parallel source lines  170  of the first periodic region  168 A are configured to be aligned with the parallel source lines  172  of the third periodic region  168 C and the parallel source lines  172  of the second periodic region  168 B are configured to be aligned with the parallel source lines  170  of the fourth periodic region  168 D. The lines are nominally aligned when at zero focus error depending on the phase difference Ph 1 -Ph 2 .  
      The trim masking structure  164  may be used along with the focus masking structure  162  to produce the focus pattern  166  with spatially distinct and multiple periodic structures  180 . The trim masking structure  164  generally includes a pair of masking zones  182  and  184 , which are disposed on a transparent substrate  186 . Each of the masking zones  182  and  184  is configured to cooperate with the regions  168  of the focus masking structure  162  to produce a focus pattern  166  with a plurality of periodic structures  180 . As shown, each of the masking zones  182  and  184  includes a plurality of masking lines  188 . The masking lines  188  are configured to cover select portions of a pre existing focus pattern so as to prevent exposure thereto. In most cases, the masking lines  188  are used to cover select portions of the printed lines of an initial focus pattern.  
      Any suitable arrangement of the masking elements may be used. In the illustrated embodiment, the first masking zone  182  is positioned on a left portion of the substrate  186 , and the second masking zone  184  is positioned on a right portion of the substrate  186 . Furthermore, any suitable number of masking lines  188  may be used. In general, the number of masking lines  188  corresponds to the number of printed lines that need to be masked to produce the desired periodic structure (masked from the second exposure). In the illustrated embodiment, each of the masking elements includes five masking lines. During the second exposure, the first masking element  182  is configured to cover the aligned printed lines formed by the aligned first and third regions  168  A and C of the focus masking structure  162 . The second masking element  184 , on the other hand, is configured cover the aligned printed lines formed by the aligned second and fourth regions  168  B and D of the focus masking structure  162 . When the second exposure is completed, a focus pattern such as the focus pattern  166  may be produced.  
      The focus pattern  166  includes a plurality of periodic structures  180 A-D that generally correspond to the configuration of the regions  168 A-D. Periodic structures  180 A and  180 D are formed from a first set of parallel printed lines  190  (shown by cross hatching) while periodic structures  180 B and  180 C are formed from a second set of parallel printed lines  192  (shown by shading). The parallel printed lines  190 ,  192  generally correspond to the configuration (i.e., pitch, linewidths and spacings) of the parallel printed lines formed by the focus masking structure  162 .  
      As shown, the periodic structures  180 A-D are spatially separated from one another so that they do not overlap portions of an adjacent periodic structure (i.e., each of the periodic structures represents a different area of the pattern). Further, periodic structures  180 A and  180 D are aligned relative to periodic structures  180 B and  180 C. As such, the printed lines  190  of periodic structure  180 A line up with the printed lines  192  of the third periodic structure  180 C, and the printed lines  192  of the second periodic structure  180 B line up with the printed lines  190  of the fourth periodic structure  180 D.  
      Further still, periodic structures  180 A and  180 D, which are formed from the first set of printed lines  190 , are positioned opposite one another at a first vertical angle while periodic structures  180 B and  180 C, which are formed from the second set of printed lines  192 , are positioned opposite one another at a second vertical angle. That is, periodic structures  180 A is diagonally opposed to periodic structures  180 D, and periodic structures  180 B is diagonally opposed to periodic structures  180 C. Moreover, periodic structures  180 A is displaced from periodic structures  180 D, and periodic structures  180 B is displaced from periodic structures  180 D. For example, the center of periodic structures  180 D is positioned below and to the right of the center of periodic structures  180 A, and the center of periodic structures  180 C is positioned below and to the left of the center of periodic structures  180 B. As should be appreciated, these cross-positioned structures form an “X” shaped pattern.  
      The shifts or relative displacement between spaced apart periodic structures, as for example the periodic structures of the focus patterns shown in  FIGS. 8 and 9 , may measured using a variety of techniques.  
      In one embodiment, the shifts between spaced apart periodic structures are measured via imaging techniques. In imaging a measurment signal corresponding to the shifts between periodic structures is acquired by capturing an image of the focus pattern with a microscope and a camera. In one implementation, the imaging technique includes selecting, from the captured image, at least one working zone (e.g., periodic structures) from each phase shift group (e.g. Ph 1  to Ph 2  and Ph 2  to Ph 1 ), forming representative signals for each of the selected working zones, and comparing the signal from the first phase shift group (e.g. Ph 1  to Ph 2 ) to the second phase shift group (Ph 2  to Ph 1 ) to determine the relative shift between different phase shift groups. The resulting relative shift (analogous to an overlay signal) is converted to a focus measurement or focus error measurement. By way of example, representative imaging techniques that may be used are described in greater detail in a pending U.S. patent application Ser. No. 09/894,987, titled “OVERLAY MARKS, METHODS OF OVERLAY MARK DESIGN AND METHODS OF OVERLAY MEASUREMENTS,” filed on Jun. 27, 2001, which is herein incorporated by reference.  
      In another embodiment, the shifts between spaced apart periodic structures are measured via scanning techniques. In scanning, a signal corresponding to the shifts between periodic structures is acquired with a scanning light or electron beam and one or more detectors that detect the reflected or scattered beam in one or more scattering directions. By way of example, representative scanning techniques that may be used are described in greater detail in U.S. Pat. Nos. 6,023,338 and 6,079,256 issued to Baraket, which are herein incorporated by reference.  
      In another embodiment, the shifts between spaced apart periodic structures are measured via phase based techniques. In phase based techniques, a measurement signal corresponding to the shifts between periodic structures is acquired by measuring an interference signal from two or more coherent light beams reflected or scattered from the different periodic structures. In particular, it is preferable to measure the +1 or −1 diffraction orders from the coherent light beams.  
      By way of example, representative phase base techniques that may be used are described in greater detail in a pending U.S. patent application Ser. No. 09/639,495, titled “Metrology System Using Optical Phase,” filed on Aug. 14, 2001, which is herein incorporated by reference.  
       FIG. 11  is a flow diagram of focus processing  200 , in accordance with one embodiment of the present invention. Prior to starting the process  100  conventional process steps may be performed. For example, a mask having a focus or trim masking structure and/or a wafer having a photoresist layer disposed thereon may be loaded into a lithography system.  
      The process  200  generally begins at block  202  where a focus pattern is formed on a wafer. The focus pattern generally includes a plurality of periodic structures having parallel lines, which shift in accordance with system defocus. In one embodiment, the focus pattern is a portion of an exposed resist layer disposed on the surface of the wafer (latent image). This is generally accomplished by shining light or radiation through a focus or trim masking structure so as to expose select portions of the resist layer to the light. In one implementation, the focus pattern is produced by a focus masking structure (single exposure). By way of example, the focus pattern may be produced by the focus masking structure  50  shown in  FIG. 4 . In another implementation, the focus pattern is produced by a focus masking structure and a trim masking structure (dual exposure). By way of example, the focus pattern may be produced by the focus masking structure  50  shown in  FIG. 4  and the trim masking structure  120  shown in  FIG. 8  or the focus masking structure  162  and trim masking structure  164  shown in  FIG. 10 .  
      In another embodiment, the focus pattern is a portion of a developed resist layer disposed on the surface of the wafer.  
      In yet another embodiment, the focus pattern is a portion of an etched underlayer of the wafer. For example, the focus pattern may be an etched metal layer, etched semiconducting layer, an etched dielectric layer or an etched substrate layer (e.g., silicon wafer). This is generally accomplished by etching through the developed resist pattern, which serves as an etch mask, using well known etching techniques.  
      Following block  202 , the process proceeds to block  204  where focus information from the focus pattern is obtained. This is generally accomplished by detecting the relative shift between periodic structures of the focus pattern, generating a displacement signal based on the shift, and converting the displacement signal to focus information. The shift may be detected using any suitable measuring technique (e.g., scatterometry, scanning, imaging, phase based techniques and the like). In some cases, the displacement signal corresponds to the displacement between spacings of alternating parallel lines. In other cases, the displacement signal corresponds to the displacement between the periodic structures themselves (e.g., centers of symmetry). Once the displacement signal is generated by the measuring system, the displacement signal can be converted into focus information (e.g., direction and magnitude of focus, optical aberrations, and the like).  
      The focus information may be determined from the displacement signal or from a signal derived from the displacement signal. In one embodiment, the displacement signal is converted to an overlay signal and the overlay signal is converted to a focus measurement or focus error measurement. In another embodiment, the displacement signal is converted to a focus measurement or focus error measurement.  
      Both embodiments may be accomplished using a reference signal, calibration curve or conversion algorithm. That is, the displacement signal may be converted into displacement information (e.g., overlay) in accordance with a first calibration curve (i.e., this signal corresponds to this much change in position) and the displacement information may be subsequently converted into focus information in accordance with a second calibration curve (this change in position corresponds to this much change in focus). Alternatively, the displacement signal may be directly converted to focus information (this signal corresponds to this much change in focus) using a single calibration curve and without converting to displacement information (overlay). The reference signal, calibration curve and conversion algorithms may be contained in a database.  
      After obtaining the focus information at block  204 , the process flow proceeds to block  206  where a focus control signal is generated in accordance with the focus information. The focus control signal may be generated automatically via a control computer or manually via an operator. In most cases, a determination is made to how much to adjust the stepper focus control. For instance, the focus information may indicate that the system is positively or negatively out of focus by a certain distance. That is, the image plane of the focussed image may be offset relative to its optimum position. By way of example, the optimum position may be the surface of the wafer or positions close to the surface of the wafer. Based on this information, a control signal can be produced so as to move the image plane to the optimum position so as to produce optimum focus. By way of example, the focussing optics or the wafer may be moved along the z axis in order to compensate for the offset.  
      After generating a focus control signal at block  208 , the focus processing  200  is complete and ends. It should be noted, however, that the focus processing can be repeatedly performed throughout wafer processing.  
      In accordance with an alternate embodiment of the invention, the focus pattern may include periodic structures that are both interlaced and spaced apart. To facilitate discussion,  FIG. 12  illustrates a top view of a focus masking system  220  capable of producing a focus pattern  221  with interlaced and spaced apart periodic structures. As shown, the focus masking system  220  includes a focus masking structure  222  and a trim masking structure  224 . The focus masking structure  222  includes a plurality of parallel source lines  226  (shown by cross hatching) and  228  (shown by shading), which are separated by alternating phase shift zones  230  and  232 . The focus masking structure  222  may be widely varied. In the illustrated embodiment, the focus masking structure  222  corresponds to the focus masking structure shown in  FIG. 4 . The trim masking structure  224 , on the other hand, includes a first masking element  234  for producing the interlaced periodic structures and a second masking element  236  for producing spaced apart periodic structures.  
      As shown, the first masking element  234  is configured to cover a substantial portion of the initial focus pattern produced by the focus masking structure, as for example the printed lines  86  and  90  of  FIG. 6 . In the illustrated embodiment, the first masking element covers a middle portion of all of the printed lines  86  and  90 . Furthermore, the second masking element  236  includes a plurality of masking lines  238  that extend from the first masking element  234 . The masking lines  238  are configured to cover individual printed lines of the initial focus pattern. In the illustrated embodiment, a first set of masking lines  238 A extends from a top portion of the first masking element  234  while a second set of masking lines  238 B extends from a bottom portion of the first masking element  234 . The first set of masking lines  238 A is arranged to cover an upper portion of a first group of printed lines while the second set of masking lines is arranged to cover a lower portion of a second group of printed lines. In the illustrated embodiment, the first group of printed lines corresponds to the printed lines  86  while the second group of printed lines corresponds to the printed lines  90 .  
      Referring now to the focus pattern  221 , the focus pattern  221  includes a first periodic structure  240  having a plurality of coarsely segmented lines  244  and a second periodic structure  242  having a plurality of coarsely segmented lines  246 . The first periodic structure  240  includes a spaced apart portion  248  and an interlaced portion  250 . In a similar fashion, the second periodic structure  242  includes a spaced apart portion  252  and an interlaced portion  254 . As shown, the interlaced portion  250  of the first periodic structure  240  is interlaced with the interlaced portion  254  of the second periodic structure  242  such that their printed lines alternate along an axis. Furthermore, the spaced apart portion  248  of the first periodic structure  240  is spaced apart from the spaced apart portion  252  of the second periodic structure  242 .  
      In an alternate embodiment, the spaced apart portions of the first and second periodic structures may be separated from the interlaced portions. This is generally accomplished by separating the second masking element from the first masking element of the trim masking structure. By way of example,  FIG. 13  illustrates a top view of a focus masking system  260  capable of producing a focus pattern  261  with interlaced periodic structures that are separated from spaced apart periodic structures.  
      As shown, the focus masking system  260  includes a focus masking structure  262  and a trim masking structure  264 . The focus masking structure  262  includes a plurality of parallel source lines  266  (shown by cross hatching) and  268  (shown by shading), which are separated by alternating phase shift zones  270  and  272 . The focus masking structure  262  may be widely varied. In the illustrated embodiment, the focus masking structure  262  corresponds to the focus masking structure shown in  FIG. 4 . The trim masking structure  264 , on the other hand, includes a first masking element  274  for producing the interlaced periodic structures and a second masking element  276  for producing spaced apart periodic structures. The first masking element  274  is spaced apart from the second masking elements  276  such that there is a gap  277  therebetween. The gap  277  may be widely varied to meet specific requirements.  
      As shown, the first masking element  274  is configured to cover a substantial portion of the initial focus pattern produced by the focus masking structure, as for example the printed lines  86  and  90  of  FIG. 6 . In the illustrated embodiment, the first masking element covers a middle portion of all of the printed lines  86  and  90 . Furthermore, the second masking element  276  includes a plurality of masking lines  278  that are configured to cover individual printed lines of the initial focus pattern. In the illustrated embodiment, a first set of masking lines  278 A is arranged to cover an upper portion of a first group of printed lines while a second set of masking lines  278 B is arranged to cover a lower portion of a second group of printed lines. In the illustrated embodiment, the first group of printed lines corresponds to the printed lines  86  while the second group of printed lines corresponds to the printed lines  90 . With regards to the focus pattern  261 , the focus pattern  261  includes spaced apart periodic structures  280  and  282  and interlaced periodic structures  284  and  286 . Each of the periodic structures  280 - 286  includes a plurality of coarsely segmented lines.  
      In accordance with an alternate embodiment of the invention, the focus masking structure can be designed to incorporate partially transmitting materials in addition to mostly opaque and transparent materials. Phase shift masks that incorporate partially transmitting materials are sometimes referred to as weak phase shift masks, attenuated phase shift masks, or tri-tone phase shift masks. Partially transmitting materials generally have a transmission or attenuation associated therewith that can effect the phase shift of the phase shift zones. In most cases, the partially transmitting material is disposed between a substrate and the source lines of the focus masking structure. The partially transmitting material may be widely modified. For example, the material properties, widths, thickness and/or position of the partially transmitting material may be adjusted to produce the desired phase shifts for each of the phase shift zones in the focus masking structure. The partially transmitting materials may also cooperate with other phase shifting techniques to produce the desired phase shifts. For example, the partially transmitting material may be used along with openings etched into the substrate.  
      To facilitate discussion  FIGS. 14A  to  14 K show various embodiments of a focus masking structure with weak phase shifting components. Similar to the embodiments shown above, the focus masking structures  290  of  FIGS. 14A  to  14 K include a plurality of source lines  292  separated by alternating phase shift zones  294 . However, unlike the preceding embodiments, in  FIGS. 14A  to  14 K, the phase shift of the phase shift zones are altered by the addition of an attenuating phase shifter  296 . As shown, the attenuating phase shifter  296  is disposed on or above the substrate  298 , and the source lines  292  are disposed on or above the attenuating phase shifter  296 . In most cases, the source lines  292  are formed from chrome, however, other opaque (or nearly opaque materials) may be used. The attenuating phase shifter  296 , on the other hand, is generally formed from a partially transmissive material. The configuration of the attenuating phase shifter may be widely varied to produce phase shift zones with different phases.  
      In accordance with an alternate embodiment of the invention, finely segmented elements may be used to form the coarsely segmented printed lines of the periodic structures of the focus pattern. Finely segmented elements allow the focus pattern to facilitate focus measurements that more accurately represent the degree of misfocus. That is, the finely segmented elements serve to provide focus information that more closely matches the focus used to form the integrated circuits. Each of these finely segmented elements are about the same size and separation as structures of the actual integrated circuits. By forming each of the periodic structures with sub-structures that are sized closer to the size of the actual circuits, a more accurate measurement of any focus error in such circuits is obtained.  
      The configuration of the finely segmented elements may be widely varied.  
      For example, the finely segmented elements may be composed of lines, squares, rectangles, triangles, polygons, circles, ovals, arbitrary shapes and the like. As should be appreciated, the finely segmented elements may not have perfectly symmetrical shapes since they are typically formed via lithographic and pattern transfer processes.  
      In addition, the periodic structures of the focus pattern may or may not contain the same form or finely segmented elements. For example, one periodic structure may contain linearly shaped elements while a different periodic structure may contain circularly shaped elements. In addition, one periodic structure may contain circularly shaped elements and a different periodic structure may contain square shaped elements. Moreover, one periodic structure may contain linearly shaped elements and a different periodic structure may contain square shaped elements. Even further variations include some periodic structures, which are composed of finely segmented elements and others, which are not composed of finely segmented elements, but rather single solid lines.  
      Moreover, individual periodic structures may or may not contain the same form or finely segmented elements. For example, a single periodic structure may include one coarsely segmented line that is formed by linearly shaped elements and another coarsely segmented line that is formed by square shaped elements.  
      Representative configurations that may used in focus patterns may be found in co-pending patent application Ser. No. 09/654,318, titled “Improved Overlay Alignment Measurement Mark,” filed on Sep. 1, 2000, and U.S. patent application Ser. No. 09/894,987, titled “OVERLAY MARKS, METHODS OF OVERLAY MARK DESIGN AND METHODS OF OVERLAY MEASUREMENTS,” filed on Jun. 27, 2001, both of which are herein incorporated by reference.  
      To facilitate discussion,  FIG. 15  illustrates a focus masking system  320  that may be used to produce a focus pattern  321  with finely segmented elements. Although the components of this system appear to be similar to the components described above, it should be noted that this is an enlarged view and that the individual lines represent finely segmented elements rather than coarsely segmented lines. As shown, the focus masking system includes a focus masking structure  322  and a trim masking structure  324 . The focus masking structure  322  includes a plurality of source lines  326 ,  328  separated by alternating phase shift zones  330 ,  332 . The trim masking structure  324  includes a plurality masking zones  334  having masking elements  336  configured to cover portions of the printed lines of the initial focus pattern produced by the focus masking structure  322 . The focus pattern  321  includes a plurality of periodic structures  340  having coarsely segmented printed lines  342  that are formed by a plurality of finely segmented lines  344 .  
      The focus and trim masking structures  322 ,  324  may be widely varied to produce the desired focus pattern  321 . For example, the focus and trim masking structures may be designed to produce a focus pattern with a certain number of periodic structures, coarsely segmented lines and finely segmented lines. Furthermore, the focus and trim masking structures may designed to produce a specific configuration such as interlaced periodic structures or spatially separated periodic structures.  
      In general, the trim masking structure  324  includes a plurality of masking zones  334  so as to produce a plurality of periodic structures  340 . Each of the masking zones  334  includes a plurality of masking elements  336  so as to produce a plurality of coarsely segmented lines  342 . Furthermore, each of the masking elements  336  includes a plurality of masking lines  338  so as to produce the plurality of finely segmented lines  344 . The configuration of these components may be widely varied. In the illustrated embodiment, the trim masking structure  324  includes four spaced apart masking zones  334 A-D that produce four periodic structures  340 A-D that are spatially distinct. Each of the masking zones  334 A-D includes two masking elements  336 A-D so as to produce two coarsely segmented lines  342 A-D, and each of the masking elements  336 A-D includes two masking lines  338 A-D so as to produce two finely segmented lines  344 A-D. It should be noted that this configuration is not a limitation and is shown simplified for ease of discussion. For example, it is generally preferred to have more than two coarsely segmented lines, and more than two finely segmented lines. It is generally believed that the greater number of lines produces a better focus information.  
      In most cases, the dimensions of the coarsely segmented lines of  FIG. 15  correspond to the dimensions of the printed lines shown in  FIG. 6 . The width of the coarsely segmented lines generally range between about 0.2 to about 2.0, and the line spacing therebetween generally ranges between about 0.2 to about 2.0. The finely segmented lines, on the other hand, generally have widths that range between about 0.01 um and about 0.25 um, and spacings that range between about 0.1 and about 2. In most cases, the coarsely segmented lines widths are generally 3 to 50 times the widths of the finely segmented lines.  
       FIG. 16  is a flow diagram of a focus calibration procedure  400 , in accordance with one embodiment of the present invention. The process  400  generally begins at block  402  where a known or nominal phase difference Ph 1 -Ph 2  of a focus masking structure is provided. The phase difference Ph 1 -Ph 2  may be measured using a separate measurement technique or it may be supplied by the phase shift mask manufacturer. Following block  402 , the process flow proceeds to block  404  where the position shift of the focus patterns of the focus masking structure as a function of focus error is calculated based on lithography models (e.g., position shifts that are due to different focus errors). This data may be used for example to form a first calibration curve. Following block  404 , the process flow proceeds to block  406  where one or more test wafers are exposed and developed with different stepper focus settings using the focus masking structure. That is, different fields and/or different resist coated wafers are exposed with different focus settings. After exposure the wafers are processed to develop the pattern in the exposed resist. Following block  406 , the process flow proceeds to block  408  where the position shift of the focus patterns formed on the test wafer are measured and compared to the stepper focus settings. This data may be used for example to form a second calibration curve (e.g., empirical calibration curve). Following block  408 , the process flow proceeds to an optimizing block  410  where the data from block  404  is compared with the data from block  408 . This information may be used for example to form a third optimized calibration curve.  
       FIG. 17  is a flow diagram of a calibration procedure  450 , in accordance with one embodiment of the present invention. The process  450  generally begins at block  452  where one or more focus patterns are formed on one or more test wafers using different focus settings. After block  452 , the process flow proceeds to block  454  where the position shift of the focus patterns are measured and compared with the focus settings in order to form calibration data. The calibration data may be used in subsequent processes to determine the focus value of a stepper system (see.  FIG. 18 ). By way of example, the shifts of the focus patterns may be analyzed as a function of nominal stepper focus (or the stepper focus measured by another calibrated method) to determine the calibration of the focus pattern shifts vs. stepper focus. For example, the slope or linear fit to the plot of focus pattern shift vs. stepper focus may be used to calculate the stepper correction given a measured focus pattern shift and a known focus pattern shift corresponding to the condition required for best device performance. The slope of the focus pattern shift vs. stepper focus can also be analyzed using lithography modeling techniques to determine the phase difference Ph 1 -Ph 2  for the focus masking region.  
       FIG. 18  is a flow diagram of a focus monitoring procedure  500 , in accordance with one embodiment of the present invention. The process  500  generally begins at block  502  where calibration data is provided. By way of example, the calibration data may be formed using the method described in  FIG. 17 . Following block  502 , the process flow proceeds to block  504  where the position shift of one or more focus patterns on a wafer, as for example, a production wafer, is measured. Following block  504 , the process flow proceeds to block  506  where the measured position shift of the focus pattern is converted into a measured focus value using the calibration data. Following block  506 , the process flow proceeds to block  508  where the measured focus value is compared with the desired focus value. Following block  508 , the process flow proceeds to block  510  where the difference between the measured focus value and the desired focus value are fed into a control system in order to correct the focus of the stepper system used to produce the production wafer.  
      With regards to  FIGS. 16-18 , it may be necessary to calibrate the focus masking structures on each mask separately to account for variation in the manufacture of the masks or variation of the manufacture of the focus masking structure. It may also be necessary to determine the value of the focus measurement (determined from measurements of the focus pattern) which yields the best device performance, best predicted device performance, or acceptable range of predicted device performance. The focus control system would then be set up to maintain focus at best focus for device performance or within a range of acceptable device performance. The best focus for device performance may be determined with lithography and/or device modeling or with measurements of device structures that have known correlations to device performance (for instance resist line width, transistor gate speed, electrical test parameters, etc.) for device structures that have been created with a known variation of focus conditions. The device structures may be measured at different stages in the device manufacturing process including incomplete, non-functional devices with developed resist patterns or etched device features to completed functional devices.  
      While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents, which fall within the scope of this invention. For example, although the focus masking system was described in terms of producing a shift in the X direction, it should be noted that this is not a limitation and the focus masking system may also be configured to produce shifts in the Y direction or at some other angle in the X and Y plane. Furthermore, a single direction is not a limitation. For example, the focus masking system may be configured to produce shifts in multiple directions, as for example in the X and Y directions. This may be accomplished using multiple focus masking systems or using a single focus masking system with structures that are positioned in multiple directions, as for example 90 degrees from the other. Moreover, with regards to spatially separated periodic structures, it should be noted the “X” configuration is shown by way of example and not by way of limitation, i.e., the size, shape and distribution of the periodic structures may vary according to the specific needs of each mark. Examples of other patterns, as well as patterns that have both X and Y components, may be found in co-pending U.S. patent application Ser. No. 09/894,987, titled “OVERLAY MARKS, METHODS OF OVERLAY MARK DESIGN AND METHODS OF OVERLAY MEASUREMENTS,” filed on Jun. 27, 2001, which is herein incorporated by reference.  
      In addition, the masking structure may be configured to produce a focus pattern that partially shifts, i.e., one of the periodic structures moves laterally with changes in focus while another one of the periodic structures does not move laterally with changes in focus (remains stationary). This embodiment may or may not require multiple masking and exposure steps to correctly generate the correct exposed pattern for a layer. Furthermore, the measurement of this type of focus pattern may be conducted in a similar manner as above by measuring the relative displacement between the shifted structure and the non-shifted structure.  
      Moreover, although it has been described herein that the effective phase difference is effected by the thickness and index of refraction of the phase shift zones, it should be noted that the width of the phase shift zones, the surface properties of the phase shift zones, and the like may also effect the phase shift zones in a non-trivial manner. These types of parameters may be used to correct or compensate the phase of the phase shift zones or they may be used to induce a desired phase of the phase shift zones.  
      It should also be noted that sources (source lines) may be formed from other materials besides metals. For example, the source lines may also be formed from a portion of the substrate or from other light effecting materials (e.g., chromeless phase shift mask). FIGS.  19 A-D show various examples of focus masking structures where the chrome width of one or more of the source lines is zero. For ease of explanation, these Figures are variations of  FIG. 5 . These examples can also be considered a combination of a chromeless alternating phase shift mask (for one phase transition, e.g. Ph 1  to Ph 2  from left to right) and a conventional chrome-based alternating phase shift mask (for the other phase transition, e.g. Ph 2  to Ph 1 ). Alternatively, the chrome width may be non-zero but smaller than the minimum width required to print a line or feature on the developed wafer. Referring to  FIG. 19A , the line width corresponding to width  56  in  FIG. 5  is zero. Referring to  19 B, the line width corresponding to width  54  in  FIG. 5  is zero. Referring to  19 C, the line width corresponding to width  56  in  FIG. 5  is zero (here Ph 1  is equal to Phs). Referring to  19 D, the line width corresponding to width  54  in  FIG. 5  is zero (here Ph 1  is equal to Phs).  
      The focus masking structure may also be constructed with combinations of regions with different widths, for instance a focus masking structure similar in layout to  162  in  FIG. 10  may be constructed with the quadrants  168 A and  168 D with one structure with characteristic widths and  168 B and  168 C with a different structure with different feature widths. For instance, the width of the chrome line for the Ph 1  to Ph 2  transition may be (or small) for quadrants  168  A and  168  D and the width of the Ph 2  to Ph 1  transition may be zero (or small) for quadrants  168  B and  168  C. In the case where the zero (or small width) transition does not print on the developed wafer, the developed focus pattern will have lines corresponding to the Ph 2  to Ph 1  transition in quadrants corresponding  180 A and  180 D and lines corresponding to the Ph 1  to Ph 2  transition in quadrants corresponding  180 B and  180 C. In this case a trim mask  164  and corresponding second exposure is not required. These lines will move in opposite directions thus the relative position will depend in part on the sign and magnitude of the stepper focus error. The resulting focus pattern can be measured with scanning, imaging, or phase-based measurement techniques described elsewhere in this patent.  
      It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. For example, detailed analysis of focus errors at multiple locations in the stepper image plane (stepper field on the wafer) can be used to determine the optical aberrations of the photolithography system. Optical aberrations in the stepper lens system can alter the intensity and/or the phase of the radiation differently at different locations in the image plane or differently for different feature pitches or widths. Methods of analyzing multiple focus error measurements to derive the optical aberrations of the stepper lens system are well known to those skilled in the art. Optical aberrations may be described in terms of the Zernike coefficients. Zernike coefficients are described in greater detail in References: Principles of Optics, 6th edition, Max Born and Emil Wolf, Permagon Press, 1980; Handbook of Optics, Vol. 1, p 1.98, Optical Society of America, McGraw Hill, 1995, and Handbook of Optics Vol. 3, p 1.12-1.15, Optical Society of America, McGraw Hill, 2001, all of which are herein incorporated by reference. Optical aberrations can be described with other methods understood by those skilled in the art. Alternatively, the shape information from either or both sets of the printed lines can be analyzed to determine the optical aberrations of the stepper. One important feature is that some lines were created with PH 1 -Ph 2  transitions and other lines were created with Ph 2 -Ph 1  transitions. The patterns produced by different transitions are affected differently by optical aberrations. That is, the lines from different phase shift groups (Ph 1  to Ph 2 ) and (Ph 2  to Ph 1 ) may have different characteristic shapes, wall angles, rounding, asymmetric tilts, etc. that may change with focus.  
      As mentioned above, the use of multiple measurements of the focus masking structure may be used to determine the aberrations of a stepper lens system. The stepper lens aberration information obtained with this method may be used to qualify a stepper for production. The stepper lens aberration information obtained with this method may be used to determine the best stepper to use for lithographic patterning of certain critical device features. The stepper lens aberration information obtained with this method may be used to determine the stepper overlay matching. The stepper lens aberration information obtained with this method may be used to determine the stepper control settings to improve the device overlay. The stepper lens aberration information obtained with this method may be used to determine the stepper control settings to improve the device line width characteristics. The stepper lens aberration information obtained with this method may be used to repair, rework, or minimize the effects of stepper lens aberrations. The stepper lens aberration information obtained with this method may be used to determine the stepper control settings to correct for wafer tilt.  
      The use of multiple focus measurements within a field may also be used to determine the stepper control settings to correct for mask tilt. The use of multiple focus measurements within a field may also be used to determine the stepper control settings to correct for wafer tilt. The use of multiple focus measurements within a field may also be used to determine the stepper control settings to correct for wafer stage tilt. The use of multiple focus measurements within a field may also be used to determine the effects of wafer topography on local pattern focus. The wafer topography information obtained in this manner may be used to optimize device design and layout to reduce the topography variation within the device or stepper field.  
      It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.