Abstract:
Methods and apparatus utilizing a posteriori adjustments to a projection imaging machine used in manufacturing of semiconductor integrated circuits are described. Measurements of degradation in a lithography processing of a projection imaging tool or machine are provided. The operation of the projection imaging tool is adjusted in response to the measurements. The projection imaging machine or tool can be adjusted in a single adjustment, or in a series of sub-adjustments. Likewise, only a single machine may be adjusted, or multiple machines used in the manufacture of semiconductor integrated circuits may be adjusted. The measurements may be made using in-situ metrology tests. A database including historical and current machine states may be utilized in determining degradation of the lithography processing.

Description:
REFERENCE TO PRIORITY DOCUMENT  
       [0001]     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/578,909, entitled “Process for Manufacture of Semiconductor Chips Utilizing a Posteriori Corrections to Machine Control System and Settings”, filed Jun. 11, 2004. Priority of the filing date of Jun. 11, 2004 is hereby claimed, and the disclosure of U.S. Provisional Patent Application Ser. No. 60/578,909 is hereby incorporated by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     Relates to manufacturing of semiconductors and specifically to photolithography.  
       BACKGROUND  
       [0003]     Roadmaps for semiconductor lithography that project continual shrinkage of feature size have been developed and indicate a need for increased manufacturing capability (see, for example, “International Technology Roadmap for Semiconductors, 2001 Edition, Lithography”, Semiconductor Industry Association, 2001). Some portion of this increased manufacturing capability will come from reduced wavelength (λ) EUV (see, H. Chapman et al., “First Lithographic Results from the Extreme Ultraviolet engineering Test Stand”,  J. Vac. Scie. Technology , B 19(6), November/December 2001) or 157 nm optical lithography and increased projection imaging optic (lens) numerical aperture (NA). Immersion particularly increases NA (see, “Development of ArF Immersion Stepper for Mass-Production Applications”, Nikon Press Release available at www.nikon.co.jp/main/eng/news/2004immersion_e — 04.htm, 2004, pp. 1-3). However, decreased process variation and machine-to-machine matching in the lithography cell is required to achieve the projected small process windows (see, “International Technology Roadmap for Semiconductors, 2001 Edition, Lithography, supra). Increased use of system on chip (SOC) architectures push application specific integrated circuit (ASIC) and processor overlay requirements into the dynamic Random Access memory (DRAM) domain (see, “ASC9: 90 nm CMOS Process Technology The Industry&#39;s First Practical 90 nm Embedded DRAM Process”, Sony Corporation, pp. 1-4).  
         [0004]     Some recent techniques for process control that utilize structures on product masks and wafers are capable of allowing manufacturers to adjust gross focus or focal plane tilt, dose and low order distortion (see C. Ausschnitt et al., “Method for Overlay Control System”, U.S. Pat. No. 5,877,861, issued Mar. 2, 1999 and C. Ausschnitt et al, “Process for Controlling Exposure Dose or Focus Parameters Using Tone Reversing Pattern”, U.S. Pat. No. 5,976,740, issued Nov. 2, 1999).  
         [0000]     Memory Manufacturing Steps  
         [0005]     DRAM and other memory manufacturing processes are generally discussed in “ASC9: 90 nm CMOS Process Technology The Industry&#39;s First Practical 90 nm Embedded DRAM Process”, supra, “EE141-Fall 2003 Digital Integrated Circuits”, EE141—Lecture 28 Memory Perspectives, 2003, and J. Binder, “Micro, Nano and Below”, CeBIT 2003, Hannover, 2003, pp. 1-11.  FIG. 1  shows plan view of a portion of a DRAM memory cell structure. A partial cross-section AA is shown in  FIG. 2  with vias, V, and metal bit lines, B. Transverse (X and Y) alignment of one layer to another is typically the chip parameter with the lowest process capability (C PK , see, NIST “Engineering Statistics Handbook”, October 2002). The effect of transverse misalignment of the bit lines to the vias V or contact holes is shown in  FIG. 3  where bit line B is shifted an amount ΔX from via V large enough to cause device failure.  
         [0000]     ASIC or Microprocessor Manufacturing Steps  
         [0006]     In ASIC or microprocessor manufacturing, the variation of the gate linewidth of individual transistors is the chip parameter with the lowest C PK  and thus the most difficult to achieve. D. Chesebro et al., “Overview of Gate Linewidth Control in the Manufacture of CMOS Log Chips”,  IBM J. Res. Development , January/March 1995, pp. 189-200 discusses the overall impact of gate linewidth in VLSI MOSFET transistors.  FIG. 4  is a schematic cross-section of a generic prior art MOS integrated circuit transistor up through the first metal layer, M. Tungsten, W, vias connect source, gate, and drain to M. The effect of transverse misalignment ΔX on metal layer M is shown in  FIG. 5 . There, a large enough transverse misalignment will cause patterned metal layer M to not overlap at all with Tungsten via plug W resulting in complete circuit failure. This is just one example of failure or yield loss through misalignment. Turning to  FIG. 6 , we see the prior art process flow for fabricating a MOS integrated circuit transistor from a bare silicon (Si) wafer up through the first metal layer. As can be appreciated from this explicit recitation of the process flow, there are numerous instances where a transverse error can impact device yield. In the following, we refer to the collection of prior art process steps for making the integrated circuit as the process of reference (POR). As such,  FIG. 6  is only a partial recitation of a complete POR.  
         [0000]     Process Variation  
         [0007]     A simple example of process variation is shown in  FIGS. 7 and 8 . In  FIG. 7  the projected image field (PIF) of a particular machine, MM 1  (not shown), and 2 field points (A and B) separated in the X direction is shown. If there is a systematic focus error at field points A and B on MM 1  (lens field curvature is one cause), then this will be added to the common random causes of process variation such as field to field focus and dose error to produce the separate process variation windows A′ and B′ of  FIG. 8 . In  FIG. 8 , gate linewidth, a critical performance parameter, is plotted versus focus, F, at several exposure dose (E 1 , E 2 , E 3 ) values. Region A′ depicts the total centroid and variation in focus and dose encountered at field point A while B′ corresponds to field point B. B′ is shifted in focus because of systematic (e.g., repeatable) focal error between field points A and B. Thus, this process on machine MM 1  has a process variation in focus (PVF in  FIG. 8 ) larger than it could be (IPVF in  FIG. 8 ) because of the systematic focus variation. It would be desirable to have a technique for realizing focus variation IPVF since this would improve machine MM 1 &#39;s process capability on chip layers it is currently qualified for and possibly qualify MM 1  for chip layers it is currently unqualified for.  
         [0008]     For ease of illustration, the previous discussion was restricted to transverse misalignment and focus. The influence of lens aberrations on DRAM feature placement is discussed in R. Holscher et al., “Effect of Lens Aberrations on Pattern Placement Error”,  Proc. Of SPIE , Vol. 4000, March 2000 and N. Seong et al., “Pattern Displacement Error Under Off Axis Illumination”,  Jpn J. Applied Physics , Vol. 37, December 1998, pp. 6695-6697, while their influence on process variability is discussed in J. Gortych et al., “Effects of Higher-Order Aberrations on the Process Window”,  Proc. Of SPIE , Vol. 1463, 1991, pp. 368-381. The effect of the illumination source (e.g., light source and combined scrambling and condenser optics) on process variability and feature shifts is in D. Peters, “The Effects of an Incorrect Condenser Lens Setup on Reduction Lens Printing Capabilities”,  Interface  85,  Kodak Publ ., No. G-154, 1985 pp. 66-72, and Y. Borodovsky, “Impact of Local Partial Coherence Variations on Exposure Tool Performance”,  Proc. of SPIE, Vol.  2440, 1995, pp. 750-770. N. Sullivan, “Semiconductor Pattern Overlay”,  Proc. Of SPIE Critical Reviews , Vol. CR52, pp. 160-188 contains a discussion of various transverse overlay error sources.  
         [0009]     The time scale for the causes of process variation is significant when we wish to correct the causes; weekly or semi-weekly measurement of a machine setting we adjust once a month will improve a manufacturing process while the same metrology/adjustment schedule for causes that vary from wafer to wafer (˜1 minute time difference) will degrade it. Table 1 lists some process variation causes and their typical variation period. Most of these have variation periods of more than one week. The ones relating to reticle or wafer stage motion will have a repeatable part (typically determined as the average) that varies on a time scale of more than one day. Because of the relative constancy of these process variation causes, it would be desirable to directly measure and adjust the machines on a fab floor to minimize these sources of process variation.  
                             TABLE 1                           Some process variation causes, their typical variation period       and some reasons for slow variation in the causes.                Typical               Variation   Some reasons for slow       Process Variation Cause   Period   variation               static/dynamic lens   &gt;1 week   Glass compaction/       aberrations       rarefaction.               Creep in lens mounting.       source   &gt;1 week   Laser beamtrain optical               component degradation. Laser               wavelength calibration drift.       exit pupil   &gt;1 week   Absorbing film buildup and/               or anti-reflection coating               degradation of projection               optics.       static lens distortion   &gt;1 week   Glass compaction/               rarefaction. Creep in lens               mounting. Drift in lens               manipulators.       dynamic lens distortion   &gt;1 week   Glass compaction/               rarefaction. Creep in lens               mounting. Drift in lens               manipulators.       dynamic scan   &gt;1 day   Interferometer drift.       synchronization (average)       Reference frame drift of               reticle and wafer               interferometer subsystems.       wafer stage grid &amp; yaw   &gt;1 day   Interferometer drift.       (average)       static lens field   &gt;1 week   Glass compaction/       curvature       rarefaction. Creep in lens               mounting. Drift in lens               manipulators.       dynamic lens field   &gt;1 week   Glass compaction/       curvature       rarefaction. Creep in lens               mounting. Drift in lens               manipulators.       dynamic scan field   &gt;1 day   Interferometer drift.       curvature (average)                  
 
       SUMMARY  
       [0010]     Methods and apparatus utilizing a posteriori adjustments to a projection imaging machine used in manufacturing of semiconductor integrated circuits are described. Methods include providing measurements of degradation in a lithography processing of a projection imaging tool or machine. The operation of the projection imaging tool is adjusted in response to the measurements. Then, a substrate is exposed using the adjusted projection imaging tool. Examples of types of degradation include lens aberrations, illumination source variation, variation in transmission as a function of transverse ray direction, scan synchronization error, lens distortion in a dynamically scanned field, static lens distortion, static lens field curvature as a function of field position, dynamic lens field curvature as a function of cross scan direction field position, dynamic height and roll error as a function of scan position, and wafer stage grid and yaw error.  
         [0011]     The projection imaging machine or tool can be adjusted in a single adjustment. The projection imaging machine can also be adjusted in a series of sub-adjustments. Likewise, only a single machine may be adjusted, or multiple machines used in the manufacture of semiconductor integrated circuits may be adjusted. The measurements may be made using in-situ metrology tests.  
         [0012]     A projection imaging tool may include a controller configured to receive measurements of degradation in lithography processing of the projection imaging tool. The controller can then adjust operation of the projection imaging tool in response to the received measurements. The adjusted projection imaging tool may then be used to expose a substrate.  
         [0013]     Another method of semiconductor manufacture includes emulating a projection imaging machine utilizing a database including historical and current machine states. The projection imaging machine process capability is evaluated. It is then determined if the machine needs adjustment, and if it does determining what adjustments are desired. The adjustments may be made in a single adjustment, or as a series of sub-adjustments.  
         [0014]     Another method of semiconductor manufacture includes emulating a plurality of projection imaging machines utilizing a database including historical and current states of the plurality of machines. Machine-to-machine processing capabilities are evaluated. Then it is determined if any of the machines need adjustment, and if any do then determining what adjustments are desired. The adjustments can be made in a single adjustment or as a series of sub-adjustments. In addition, a set of machines can be adjusted in a single step and other machines adjusted in a series of sub-adjustments. For example, machines that have been offline, or are new, may be adjusted in a single step and other machines adjusted in a series of sub-adjustments. Likewise, machines that have been offline, or are new, may be adjusted in a series of sub-adjustments and other machines adjusted in a single step. In other words, any combination of machines may be adjusted in a single step and any other combination of machines may be adjusted in a series of sub-adjustments.  
         [0015]     Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows the plan view of portion of DRAM memory cell structure.  
         [0017]      FIG. 2  shows the AA cross section of DRAM memory cell showing vias, V, and bit lines.  
         [0018]      FIG. 3  shows the AA cross section of  FIG. 1  showing bit lines B misaligned by ΔX from vias, V.  
         [0019]      FIG. 4  shows the cross section of generic prior art MOS integrated circuit transistor with metal, M, conductors making contact with Tungsten vias connected to source, drain, and gate.  
         [0020]      FIG. 5  shows the cross section of MOS integrated circuit transistor showing misalignment of metal, M, layer from vias.  
         [0021]      FIG. 6  shows prior art process steps for fabricating MOS integrated circuit transistor of  FIG. 4  through 1 st  metal layer.  
         [0022]      FIG. 7  shows the projected image field in machine and two field points, A &amp; B, separated in the X-direction.  
         [0023]      FIG. 8  shows the Bossung or ISO—dose contours for gate linewidth and process variation windows at points A &amp; B.  
         [0024]      FIG. 9  illustrates a block diagram showing exemplary steps for utilizing a posteriori measurement to adjust a projection tool.  
         [0025]      FIG. 10  shows exemplary steps for running an in-situ interferometer.  
         [0026]      FIG. 11  shows exemplary steps for in-situ measurement of source radiant intensity.  
         [0027]      FIG. 12  shows exemplary steps for in-situ measurement of an exit pupil transmission.  
         [0028]      FIG. 13  shows exemplary steps for in-situ measurement of transverse scanning synchronization error.  
         [0029]      FIG. 14  shows exemplary steps for in-situ measurement of dynamic lens distortion.  
         [0030]      FIG. 15  shows exemplary steps for in-situ measurement of static lens field curvature.  
         [0031]      FIG. 16  shows exemplary steps for in-situ measurement of static lens field curvature.  
         [0032]      FIG. 17  shows exemplary steps for measurement of dynamic lens field curvature.  
         [0033]      FIG. 18  shows exemplary process for determination of dynamic scan Z and roll synchronization error.  
         [0034]      FIG. 19  shows an exemplary process for in-situ measurement of wafer stage grid and Yaw.  
         [0035]      FIG. 20  is a block diagram of an example of a projection imaging tool. 
     
    
     DETAILED DESCRIPTION  
       [0036]      FIG. 9  illustrates exemplary steps for utilizing a posteriori measurements to adjust a projection imaging tool. Each step is discussed in detail below.  
         [0000]     Provide Process of Reference  
         [0037]     First, a process of reference (POR) for manufacturing a chip is provided. In abbreviated and truncated form,  FIG. 6  is one example. Of greatest relevance to the present invention are the specific machine settings utilized. Table 2 shows an example of the relevant information extracted from the POR. Knowledge of machine setting indentifiers is required for running the in-situ metrology tests.  
                                                   TABLE 2                       Lithographic   Machine                       FIELD               Layer   Model   XPOTYPE   LAID   SID   XID   TRID   SIZE/LAYOUT   F nom     E                   Implant   S2071   dynamic   abb1   iid2   iid2   8943   size = 24 × 30 mm{circumflex over ( )}2/   200   40                                   pitch = 24.2 × 30.2/                                   fields = 4, 6, 6, 6, 4       Implant   AT900.1   dynamic   LADJ_11   CON_0.6   NA_0.8   8943   size = 24 × 30 mm{circumflex over ( )}2/   200   40                                   pitch = 24.2 × 30.2/                                   fields = 4, 6, 6, 6, 4       Source/   S2071   dynamic   abb1   iid1   iid1   2612   size = 24 × 30 mm{circumflex over ( )}2/   230   60       Drain                           pitch = 24.2 × 30.2/       Contacts                           fields = 4, 6, 6, 6, 4       Source/   AT900.1   dynamic   LADJ_11   ANN_0.5_0.7   NA_0.8   2612   size = 24 × 30 mm{circumflex over ( )}2/   230   60       Drain                           pitch = 24.2 × 30.2/       Contacts                           fields = 4, 6, 6, 6, 4       Gate   S2071   dynamic   abb1   iid1   iid1   2612   size = 24 × 30 mm{circumflex over ( )}2/   190   75       Definition                           pitch = 24.2 × 30.2/                                   fields = 4, 6, 6, 6, 4       •   •   •   •   •   •   •   •       •   •   •   •   •   •   •   •       •   •   •   •   •   •   •   •                  
 
         [0038]     In Table 2, the photographic layer is the name of the lithographic layer in process. The machine model is the machine model (not specific identity) used. The XPOTYPE (exposure type) is the static/dynamic for stepper/scanner models. The LAID (lens aberration identifier) is the keyword typed into or selected on the machine in the recipe or at exposure time that sets the lens manipulators or adjusters to a specific state. The SID (source identifier) similar to LAID but specifies the source configuration utilized. The XID (exit pupil identifier) similar to LAID but specifies the exit pupil numerical aperture utilized. The TRID (track identifier) similar to LAID but specifies the photoresist coat/develop recipe utilized. The FIELD SIZE/LAYOUT is the description of the exposure field size and pitch on wafer; the last entry describes layout; can also contain scanner direction and scan speed.  
         [0039]     The F nom  is the nominal machine focus in nanometers. The E is the nominal exposure dose in MJ/cm 2 .  
         [0000]     Run In-Situ Metrology Test  
         [0040]     For each lithographic layer of the process deemed as critical (low C PK ) we run some or all of the in-situ metrology tests outlined and referenced in Table 3. As an example, Table 2 defines the lens manipulator settings (LAID) required for the Gate Definition Layer of process 80065. Machine model (not serial number or specific machine ID (MAID)) S2071 is qualified for that layer and the user would set or type in “abb1” on the machine console at exposure time or have it specified within the job deck for this process and layer on all machines of model S2071. To measure the lens aberrations, we would run each machine of model S2071 at LAID=“abb1” but with SID, XID, TRJD and field size/layout as described in U.S. Pat. No. 5,978,085, supra; the exposure portions of the process being detailed in  FIG. 12A  of that reference. Measuring the box-in-box patterns, we could then reconstruct the higher order (focus and above) lens aberrations. The resulting Zernike coefficients provide the basis for the next step.  
                             TABLE 3                           Causes of lithography cell process degradation and       in-situ measurement techniques for measuring causes            Process   Resulting           Variation   Process   Exemplary In-Situ Measurement       Cause   Degradation   Techniques of Cause               static/   contrast loss,   A. Smith et al., “Apparatus,       dynamic   ACLV, image   Method of Measurement and       lens   shift   Method of Data Analysis for       aberrations       Correction of Optical System”,               U.S. Pat. No. 5,828,455, issued               Oct. 27, 1998; A. Smith et al.,               “Apparatus, Method of               Measurement and Method of Data               Analysis for Correction of Optical               System”, U.S. Pat. No.               5,978,085, issued Nov. 2,               1999;               A. Smith et al., “In-Situ               Interferometer Arrangement”, U.S.               Patent Application No.               10/623,364, filed Jul. 18, 2003; B.               McArthur et al., “Method and               Apparatus for Proper Ordering of               Registration Data”, U.S. Patent               Application No. 10/039,426, Filed               Jan. 4, 2002       source   image shift,   B. McArthur et al., “In-Situ           contrast loss   Source Metrology Instrument and               Method of Use”, U.S. Pat. No.               6,356,345, issued Mar. 12, 2002;               B. McArthur et al., “In-Situ               Source Metrology Instrument and               Method of Use”, U.S. Pat. No.               6,741,338, issued May 25, 2004;               A. Smith et al., “Apparatus and               Method for High Resolution n-Situ               Illumination Source Measurement               in Projection Imaging Systems”,               U.S. Patent Application No.               10/828,579, filed Apr. 20, 2004.       exit pupil   image shift,   A. Smith et al., “Method and           resolution loss   Apparatus for Measurement of               Exit Pupil Transmittance”, U.S.               Patent Application No.               11/105,799, filed Apr. 13, 2005       static lens   image shift,   A. Smith et al., “Method &amp;       distortion   layer to layer   Apparatus for Self-Referenced           misalignment   Projection Lens Distortion               Mapping”, U.S. Pat. No.               6,573,986, issued Jun. 2, 2003; A.               Smith et al., “Reference Wafer and               Process for Manufacturing Same”,               U.S. Pat. No. 6,699,627, issued               Mar. 2, 2004,               M. Hamatani et al., “Method of               Forming and Adjustin Optical               System and Exposure Apparatus,               and for Determining Specification               thereof and Related Computer               System”, European Patent               Application No. EP 1 231 516 A2,               2002       dynamic lens   image shift,   A. Smith et al., “Method and       distortion   layer to layer   Apparatus for Self-Referenced           misalignment   Dynamic Step and Scan Intra-               Field Lens Distortion”, U.S. Patent               Application No. 10/252,020, filed               Sep. 20, 2002; EP 1 231 516               A2, supra       dynamic scan   image shift,   A. Smith et al., “Method and       synchronization   layer to layer   Apparatus for Self-Referenced           misalignment   Dynamic Step and Scan Intra-               Field Scanning Distortion”, U.S.               Patent Application No.               10/252,021, filed Sep. 20,               2002; EP 1 231 516 A2, supra       wafer stage   image shift,   A. Smith et al., “Method and       grid &amp; yaw   layer to layer   Apparatus for Self-Referenced           misalignment   Wafer Stage Positional Error               Mapping”, U.S. Pat. No.               6,734,971, issued May 11, 2004;               EP 1 231 516 A2, supra       static lens   contrast loss,   A. Smith et al., “Apparatus and       field   image shift   Process for the Determination of       curvature       Static Lens Field Curvature”, U.S.               Patent Application No.               10/844,939, filed May 12, 2004       dynamic lens   contrast loss,   A. Smith et al., “Apparatus and       field   image shift   Process for Determination of       curvature       Dynamic Lens Field Curvature”,               U.S. Patent Application No.               10/833,557, filed Apr. 28, 2004       dynamic scan   contrast loss,   according to A. Smith et al.,       field   image shift   “Apparatus and Process for       curvature       Determination of Dynamic Scan               Field Curvature”, U.S. Patent               Application No. 10/833,781, filed               Apr. 28, 2004                  
 
         [0041]     All of the patents and patent applications listed in Table 2 are incorporated by reference herein their entirety.  
         [0000]     Adjust Machines  
         [0042]     At this point we assess the current state of adjustment of our machines and correct them. The preferred method for assessing the current state of machine adjustment is a direct machine emulation that utilizes a database containing the history and current machine state (see, A. Smith et al., “Method of Emulation of Lithographic Projection Tools”, U.S. patent application Ser. No. 11/111,302, filed Apr. 20, 2005). Since we are concerned with corrections to slowly varying (&gt;1 day) machine characteristics in the emulator, we turn off rapid (&lt;1 day) machine variations and look at individual or machine-to-machine process capability. Based on these results, we then determine which machines require adjustment and by what amount. Continuing the gate layer/lens aberration example above, using the nominal focus and exposure dose, we calculate the gate linewidth as a function of focus and exposure at a collection of field points representative of the exposure field size. Including random focus errors at each field point gives us the focus process variation. From this information we can determine whether the machine requires adjustment and by what amount.  
         [0043]     Having decided how much we need to adjust each machine, there are a number of ways to implement the adjustment, some being: 1) adjust all machines at once, 2) gradually adjust machines into compliance, and 3) a combination of 1) and 2), immediately adjust machines outside the general distribution and gradually adjust the balance.  
         [0000]     Adjust Machines  
         [0044]     This provides the simplest and shortest schedule for machine improvement. Minimizing yield loss to WIP fabricated on an out of adjustment machine precludes this strategy in all cases. We are thinking particularly of transverse, layer-to-layer misalignments that can be temporarily exacerbated by relatively large adjustments to critical machines. However, once the method of the present invention is adopted on the factory floor, large adjustments of critical machines should be exceptional and their (and all other) machine adjustments can be done at once. It is typically only on the introduction of this method to the factory floor that we generally cannot adjust all machines at once.  
         [0000]     Phase In Adjustment  
         [0045]     On introduction of this method, to minimize the transient yield loss to WIP, the adjustments should be phased in over a number of steps and time. First, each setting of each machine is scored according to its deviation from the target value, Q tar , relative to upper (USL) and lower (LSL) specification limits according to: 
 
 S= 4| Q   max   −Q   tar |/( USL−LSL )  Equation 1 
 
 where: 
    Q max =maximum deviation of quantity over machine printing field,     S=score, 
 
 The use of maximum is justified since we are looking at deterministic, explainable causes, not random ones. 
 
 The total number of adjustment phases is then:  
               N   adj     =     ceiling   ⁡     (           max   ⁡     (   S   )                 machine   ,   process   ,             layers         )               Equation   ⁢           ⁢   2             
 
 where the maximum is taken over all machines, process and layers and ceiling is the next integer above the fractional part e.g., ceiling (3.1)=ceiling (3.9)=4. 
   
 
         [0048]     The first adjustment is then made to machine settings with scores, S, falling within the range 
 
 N   adj −1&lt; S≦N   adj   Equation 3 
 
 Before the next adjustment we should wait a time period  
               Δ   ⁢           ⁢   T     =     6   *       T   fab       N   layer                 Equation   ⁢           ⁢   4             
 
 where: 
        T fab =average start to finish chip manufacturing time.     N layer =number of lithographic layers (photoresist deposition and definition steps). 
 
 After elapsed time ΔT, machine settings with scores within the range: 
 
 N   adj −2&lt; S≦N   adj —1  Equation 5 
 
 are adjusted. This process of selective machine adjustment and waiting then continues to completion after a total of N adj  cycles. 
 
 Mixed Adjustment 
       
 
         [0051]     Mixed adjustment is a combination of phased in and immediate adjustment. New machines or existing machines that have been offline (≧ΔT time period) will be immediately adjusted to the factory standard. Phase-in adjustment will be applied to the balance. Note, that in a well controlled factory, the phase-in adjustment will typically require only a single step (N adj =1) and in that case, is equivalent to immediate adjustment.  
         [0000]     Print Lithographic Layers  
         [0052]     In a continuously run factory, product will be emerging as we follow the above steps. So, lithographic layers are defined in photoresist and developed.  
         [0000]     1 st  Variation In-Situ Interferometer  
         [0053]     In this variation, an in-situ interferometer of the type described in U.S. Pat. Nos. 5,978,085 and 5,828,455, supra and U.S. patent application Ser. No. 10/623,364, supra may be used to measure the projection lens aberrations. Exemplary steps for running the in-situ interferometer are outlined in  FIG. 10  of the present application. After getting the Zernike coefficients, the machine lens manipulators could be adjusted according to European Patent Application No. EP 1 231 516 A2, supra.  
         [0000]     2 nd  Variation In-Situ Source Metrology  
         [0054]     In this variation, an in-situ source metrology instrument of the type described in Reference U.S. Pat. No. 6,741,338, supra and U.S. patent application Ser. No. 10/828,579, supra may be used to measure the effective source radiant intensity, or effective source.  FIG. 11  of the present application shows exemplary steps for performing this operation.  
         [0000]     3 rd  Variation, In-Situ Exit Pupil Mapping  
         [0055]     In this variation, an in-situ exit pupil transmission mapper of the type described in U.S. patent application Ser. No. 11/105,799, supra may be used to measure the transmission as a function of transverse ray direction cosine T(nx,ny). From this, quantities such as the overall numerical aperture, NA as a function of field position can be assessed.  FIG. 12  of the present application lays out exemplary steps for measuring the exit pupil transmission map.  
         [0000]     4 th  Variation, In-Situ Transverse Scanning Synchronization Error  
         [0056]     In this variation, a special reticle as described in U.S. patent application Ser. No. 10/252,021, supra and wafer with predisposed alignment marks may be used to determine the scanning synchronization error in X, Y and Yaw.  FIG. 13  of the present application illustrates exemplary steps used to determine the scanning synchronization error.  
         [0000]     5 th  Variation of Main Embodiment, In-Situ Dynamic Lens Distortion (Dynamic a2, a3)  
         [0057]     In this variation, the substantially similar reticle and wafer of the fourth variation are used to determine the contribution of lens distortion in a dynamically scanned field to overall overlay error. An exemplary technique for determining lens distortion is described in U.S. patent application Ser. No. 10/252,020.  FIG. 14  of the present application illustrates exemplary steps for determining lens distortion.  
         [0000]     6 th  Variation, In-Situ Measurement of Static Lens Distortion (Static a2, a3)  
         [0058]     In this variation, a substantially similar reticle and wafer of the fourth variation may be used to determine the static lens distortion or tilt Zernike coefficients a2 and a3. An exemplary technique for determining this is described in Reference U.S. Pat. No. 6,573,986 B2.  FIG. 15  of the present application illustrates exemplary steps for determining the static lens distortion.  
         [0000]     7 th  Variation, In-Situ Measurement of Static Lens Field Curvature  
         [0059]     In this variation, a focusing fiducial reticle may be used to determine the static lens field curvature. U.S. patent application Ser. No. 10/844,939, supra describes an exemplary technique for performing this. The result is the static lens field curvature or focus Zernike (a4) as a function of field position. By using the method of U.S. patent application Ser. No. 10/844,939, supra, the final result is independent of wafer height variations.  FIG. 16  of the present application illustrates exemplary steps for determining the static lens field curvature.  
         [0000]     8 th  Variation, In-Situ Measurement of Dynamic Lens Field Curvature  
         [0060]     In this variation, a focusing fiducial reticle is provided and may be used to determine dynamic lens field curvature. U.S. patent application Ser. No. 10/833,557, supra illustrates an exemplary technique for determining lens field curvature. The result is the dynamic lens field curvature focus Zernike (a4) as a function of cross scan (X) direction field position. Using this technique, results independent of wafer height are obtained.  FIG. 17  of the present application illustrates exemplary steps for determining the dynamic lens field curvature.  
         [0000]     9 th  Variation, In-Situ Measurement of Dynamic Z and Roll Synchronization Error  
         [0061]     In this variation, a focusing fiducial reticle is provided and dynamic Z and roll synchronization error is determined. U.S. patent application Ser. No. 10/833,781, supra illustrates an exemplary technique for making these measurements. The result is the dynamic height (Z) and roll error as a function of the scan position (Y).  FIG. 18  of the present application illustrates exemplary steps for determining dynamic Z and roll synchronization error.  
         [0000]     10 th  Variation, In-Situ Measurement of Wafer Stage Grid and Yaw Errors  
         [0062]     In this variation, a reticle is provided and exposed on a wafer to determine wafer stage grid and yaw errors illustrates an exemplary technique for making these measurements. U.S. Pat. No. 6,734,971, supra. The result is the wafer stage grid and Yaw stepping error.  FIG. 19  of the present application illustrates exemplary steps for determining these errors.  
         [0000]     Manufacturing of Integrated Circuits  
         [0063]      FIG. 20  is a schematic diagram of an example of a projection imaging tool or machine (MA) that can be used in the manufacturing of semiconductor integrated circuits. As shown in  FIG. 20 , the MA includes a light source S, a reticle stage RS, imaging objective IMO, wafer stage WS, and a controller C. The light source can include an illumination source S 1  that outputs illumination light IL and an illumination conditioning S 2  that conditions the light IL. The IMO includes an upper imaging objective IMO 1 , a lower imaging objective IMO 2 , and an aperture stop AS.  
         [0064]     The controller C can be configured to receive measurements of degradation in lithography processing of the projection imaging tool. The controller can then adjust the operation of the projection imaging tool in response to the received measurements. For example, the controller can adjust the operation of the reticle stage RS, the illumination source S, the wafer stage WS, or other operations of the projection imaging tool. The adjusted imaging tool may then be used to expose a substrate, for example, a substrate that is positioned by the wafer stage WS at the output of the imaging objective IMO for a semiconductor integrated circuit manufacturing process.  
         [0065]     The present invention has been described above in terms of a presently preferred embodiment so that an understanding of the present invention can be conveyed. There are, however, many configurations for semiconductor manufacturing not specifically described herein but with which the present invention is applicable. The present invention should therefore not be seen as limited to the particular embodiments described herein, but rather, it should be understood that the present invention has wide applicability with respect to semiconductor manufacturing generally. All modifications, variations, or equivalent arrangements and implementations that are within the scope of the attached claims should therefore be considered within the scope of the invention.