Abstract:
A substrate is moved in a linear direction simultaneously between a processing face of an upper proximity head and a processing face of a lower proximity head. As the substrate is moved, a first meniscus of processing liquid is generated between the processing face of the upper proximity head and a top surface of the substrate, and a second meniscus of processing liquid is generated between the processing face of the lower proximity head and a bottom surface of the substrate. The first meniscus has a meniscus protrusion extending in the linear direction in which the substrate is moved and positioned on a trailing side of the first meniscus relative to the linear direction in which the substrate is moved. The meniscus protrusion is centered on the substrate relative to a diameter of the substrate as measured perpendicular to the linear direction in which the substrate is moved.

Description:
CLAIM OF PRIORITY 
     This application is a divisional application of U.S. patent application Ser. No. 12/725,422, filed on Mar. 16, 2010, entitled “Reduction of Entrance and Exit Marks Left by a Substrate-Processing Meniscus,” issued as U.S. Pat. No. 8,317,932, on Nov. 27, 2012, which is a divisional application of U.S. patent application Ser. No. 11/612,868, filed on Dec. 19, 2006, entitled “Reduction of Entrance and Exit Marks Left by a Substrate-Processing Meniscus,” issued as U.S. Pat. No. 7,703,462, on Apr. 27, 2010, which is a continuation-in-part application of U.S. patent application Ser. No. 11/537,501, filed on Sep. 29, 2006, entitled “Carrier For Reducing Entrance And/Or Exit Marks Left By A Substrate-Processing Meniscus,” issued as U.S. Pat. No. 7,946,303, on May 24, 2011. Each of the above-identified patent applications is incorporated herein by reference in its entirety. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present Application is related to the following U.S. patents and U.S. patent applications, all of which are incorporated herein by reference in their entirety: U.S. Pat. No. 6,488,040, issued on Dec. 3, 2002 to De Larios, et al. and entitled, “Method And Apparatus For Drying Semiconductor Wafer Surfaces Using A Plurality Of Inlets And Outlets Held In Close Proximity To The Wafer Surfaces;” U.S. patent application Ser. No. 10/330,843, filed on Dec. 24, 2002 and entitled, “Meniscus, Vacuum, IPA Vapor Drying Manifold;” U.S. patent application Ser. No. 10/330,897, also filed on Dec. 24, 2002, entitled, “System For Substrate Processing With Meniscus, Vacuum, IPA Vapor, Drying Manifold;” U.S. patent application Ser. No. 10/404,692, filed Mar. 31, 2003 and entitled, “Methods And Systems For Processing A Substrate Using A Dynamic Liquid Meniscus;” and U.S. patent application Ser. No. 10/817,620, which was filed on Apr. 1, 2004, entitled, “Substrate Meniscus Interface And Methods For Operation.” 
    
    
     BACKGROUND 
     In the semiconductor chip fabrication industry, it is necessary to clean and dry a substrate after a fabrication operation has been performed that leaves unwanted residues on the surfaces of the substrate. Examples of such a fabrication operations include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP). In CMP, a substrate is placed in a holder that pushes a substrate surface against a polishing surface. The polishing surface uses a slurry which consists of chemicals and abrasive materials. Unfortunately, the CMP process tends to leave an accumulation of slurry particles and residues on the substrate surface. If left on the substrate, the unwanted residual material and particles may defects. In some cases, such defects may cause devices on the substrate to become inoperable. Cleaning the substrate after a fabrication operation removes unwanted residues and particulates. 
     After a substrate has been wet cleaned, the substrate must be dried effectively to prevent water or cleaning fluid, (hereinafter, “fluid”) remnants from leaving residues on the substrate. If the cleaning fluid on the substrate surface is allowed to evaporate, as usually happens when droplets form, residues or contaminants previously dissolved in the fluid will remain on the substrate surface after evaporation and can form spots. To prevent evaporation from taking place, the cleaning fluid must be removed as quickly as possible without the formation of droplets on the substrate surface. In an attempt to accomplish this, one of several different drying techniques are employed such as spin-drying, IPA, or Marangoni drying. All of these drying techniques utilize some form of a moving liquid/gas interface on a substrate surface, which, if properly maintained, results in drying of a substrate surface without the formation of droplets. Unfortunately, if the moving liquid/gas interface breaks down, as often happens with all of the aforementioned drying methods, droplets form and evaporation occurs resulting in contaminants being left on the substrate surface. 
     In view of the foregoing, there is a need for improved cleaning systems and methods that provide efficient cleaning while reducing the likelihood of marks from dried fluid droplets. 
     SUMMARY 
     Broadly speaking, the present invention fills these needs by providing various techniques for reduction of entrance and exit marks caused by dried fluid droplets left by a substrate-processing meniscus. 
     It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below. 
     In one embodiment, a proximity head for generating and maintaining a meniscus for processing a substrate is provided. The proximity head includes a plurality of meniscus nozzles formed on a face of the proximity head, the nozzles being configured to supply liquid to the meniscus, a plurality of vacuum ports formed on the face of the proximity head, the vacuum ports being arranged to completely surround the plurality of meniscus nozzles. A plurality of main gas nozzles may be formed on the face of the proximity head, the main gas nozzles at least partially surrounding the vacuum ports. The proximity head further includes means for reducing a size and frequency of entrance and/or exit marks at a leading edge and a trailing edge on the substrate by aiding and encouraging liquid from the meniscus to evacuate a gap between the substrate and the carrier. 
     In another embodiment, a method for processing a substrate using a meniscus formed by upper and lower proximity heads is provided. In the method the substrate is placed on a carrier, which is passed through a meniscus generated between upper and lower proximity heads. The carrier has an opening sized for receiving the substrate and a plurality of support pins for supporting the substrate within the opening, the opening being slightly larger than the substrate such that a gap exists between the substrate and the opening. Each of the upper and lower proximity heads include a plurality of meniscus nozzles formed on a face of the proximity head, the nozzles being configured to supply liquid to the meniscus; a plurality of vacuum ports formed on the face of the proximity head, the vacuum ports being arranged to completely surround the plurality of meniscus nozzles; and a plurality of main gas nozzles formed on the face of the proximity head, the main gas nozzles at least partially surrounding the vacuum ports. The method further includes a step for reducing a size and frequency of at least one of entrance or exit marks on substrates by encouraging liquid from the meniscus to evacuate the gap using the upper and lower proximity heads. 
     Since introduction by the present Assignee of the use of a moving meniscus generated by a proximity head for use in cleaning, processing, and drying semiconductor wafers, it has become possible to wet and dry a substrate with a very low risk of droplets forming on the substrate surface. This technology has been very successful at preventing any droplets from being left on the active device region of the wafer after the meniscus is removed. However, the meniscus does occasionally tend to leave a small droplet at the entrance and exit points at the leading and trailing edges of the substrate on the exclusion zone as the substrate passes through the meniscus. The exclusion zone is a margin that extends from the active device region to the edge of the substrate, where microelectronic structures are not formed. On occasion, entrance and exit marks can become mains surface marks, especially on hydrophilic wafers. Therefore, it is preferable that instances of such entrance and exit marks are reduced or eliminated. 
     The advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements. 
         FIG. 1  shows a perspective view of an exemplary implementation of a proximity head apparatus. 
         FIG. 2  shows a schematic representation of an upper proximity head. 
         FIGS. 3A ,  3 B,  3 C, and  3 D illustrate a substrate exiting a meniscus generated by upper and lower proximity heads. 
         FIG. 4  shows a perspective view of a gap between a carrier and a substrate. 
         FIG. 5  shows a cross section view of a meniscus as it is completing transition onto a carrier. 
         FIG. 6A  shows a top view of the lower proximity head having means for reducing entrance and exit mark size and frequency. 
         FIGS. 6B ,  6 C, and  6 D illustrate various meniscus protrusion sizes and shapes. 
         FIG. 7  shows a plan view of a carrier, substrate, and meniscus perimeter. 
         FIGS. 8A ,  8 B, and  8 C show an exemplary embodiment of a proximity head having means for reducing entrance and exit mark size and frequency. 
         FIGS. 9A and 9B  show an exemplary embodiment of a proximity head having means for reducing entrance and exit mark size and frequency. 
         FIG. 10  shows another embodiment of a proximity head having means for reducing entrance and exit mark size and frequency. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well known process operations and implementation details have not been described in detail in order to avoid unnecessarily obscuring the invention. The term, “meniscus,” as used herein, refers to a volume of liquid bounded and contained in part by surface tension of the liquid. The meniscus is also controllable and can be moved over a surface in the contained shape. In specific embodiments, the meniscus is maintained by the delivery of fluids to a surface while also removing the fluids so that the meniscus remains controllable. Furthermore, the meniscus shape can be controlled by precision fluid delivery and removal systems that are in part interfaced with a controller a computing system, which may be networked. 
       FIG. 1  is a perspective view of an exemplary implementation of a proximity head apparatus  100 . In this example, substrate  160  is positioned within a carrier  150  which passes between upper proximity head  110  and lower proximity head  120  in the direction of arrow  152 . Upper and lower proximity heads  110 ,  120 , form a meniscus of fluid between them. Carrier  150  may be connected to some apparatus (not shown) for causing carrier  150  to move between upper and lower proximity heads  110 ,  120  in the direction of arrow  166 . In one embodiment, a substrate  160  is deposited on carrier  150  at a first location on one side of proximity heads  110 ,  120 , and removed when carrier  150  arrives at a second location on an opposite side of proximity heads  110 ,  120 . Carrier  150  may then pass back through proximity heads  110 ,  120 , or over, under, or around proximity heads  110 ,  120 , back to the first location, where a next substrate is deposited, and the process is repeated. 
     It should be noted that, while in the example shown in  FIG. 1 , the substrate moves through proximity heads  110 ,  120  in the direction of arrow  152 , it is also possible for the substrate to remain stationary while the proximity heads  110 ,  120 , pass over and under the substrate, so long as the substrate moves with respect to the proximity heads. Furthermore, the orientation of the substrate as it passes between the proximity heads is arbitrary. That is, the substrate is not required to be oriented horizontally, but can instead be vertically oriented or at any angle. 
     In certain embodiments, a controller  130 , which may be a general purpose or specific purpose computer system whose functionality is determined by logic circuits or software, or both, controls the movement of carrier  150  and the flow of fluids to upper and lower proximity heads  110 ,  120 . 
       FIG. 2  shows a schematic representation of upper proximity head  110 . Proximity head includes a plurality of central meniscus nozzles  116  formed in face  111  of proximity head  110  through which a liquid is supplied that forms meniscus  200 . The liquid may be deionized water, a cleaning solution, or other liquid designed to process, clean, or rinse substrate  160 . A plurality of ports  114  apply a vacuum at a perimeter of meniscus  200 . Vacuum ports  114  aspirate liquid from meniscus  200  and surrounding fluid, such as air or other gas supplied by main gas nozzles  112 . In certain embodiments, main gas nozzles  112  surround vacuum ports  114  and supply isopropyl alcohol vapor, nitrogen, a mixture thereof, or other gas or gases or gas/liquid fluids. Depending on the implementation, main gas nozzles  112  and fluid supplied therefrom may be provided to aid in maintaining a coherent liquid/gas interface at the surface of meniscus  200 . In one embodiment, main gas nozzles  112  are absent or are not used. In another embodiment, main gas nozzles  112  supply carbon dioxide (CO 2 ) or a mixture of N 2  and isopropyl alcohol (IPA) vapor. The lower proximity head  120 , not shown in  FIG. 2 , may be provided as a minor image to the upper proximity head, and may operate in a similar manner. More details relating to proximity head structure and operation are incorporated by reference in the Cross Reference to Related Art section above. In particular, U.S. patent application Ser. Nos. 10/261,839, 10/330,843, and 10/330,897 are referenced for additional details relating to proximity head structure and operation. 
       FIGS. 3A through 3D  illustrate a substrate  160  exiting meniscus  200  generated by upper and lower proximity heads  110 ,  120 . In  FIG. 3A , substrate  160  extends all the way through meniscus  200  such that its leading edge  162  and trailing edge  164  lie on opposite sides of meniscus  200 . It should be noted that, typically, substrate  160  will be circular and while carrier  150  is shown outside of meniscus  200 , parts of carrier  150  may be in contact with meniscus  200 , although not visible in this Figure. 
     In  FIG. 3B , meniscus  200  is transitioning from substrate  160  to carrier  150 . Carrier  150  may be slightly thicker in cross section than substrate  160 . For example, substrate  160  may be about 0.80 mm thick whereas the carrier may be about 1.5 mm thick. Thus, as meniscus  200  transitions onto carrier  150 , a certain amount of meniscus liquid is displaced by carrier  150 . 
     In  FIG. 3C , meniscus  200  is transitioning completely off substrate  160  and onto carrier  150 . At this time, the trailing edge of meniscus  200  is still in contact with trailing edge  164  of substrate  160 . Forces acting on meniscus  200  at this point in time is described with reference to  FIG. 5  below. 
     In  FIG. 3D , the meniscus has completely transitioned off of substrate  160 , leaving a small droplet  202  of meniscus liquid on the exclusion zone of substrate  160  at the trailing edge  164  of substrate  160 . Droplet  202 , if allowed to dry can leave a spot formed of dissolved or entrained elements, the spot being referred to herein as an exit mark. If the substrate surface is hydrophilic, droplet  202  can migrate to the active device region of the substrate, which can cause defects in devices formed thereon. A number of factors are believed to contribute to the presence and size of small droplet  202  at the trailing edge  164  of substrate  160 . It should be noted that an entrance mark at leading edge  162  can be formed in a similar manner as leading edge  162  exits meniscus  200 . 
       FIG. 4  shows a perspective view of a gap  158  between a carrier  150  and a substrate  160 . Meniscus perimeter  204  shows the area of contact of the meniscus with carrier  150  and substrate  160 . The meniscus is traveling in the direction indicated by arrow  208 . As the meniscus transitions off of waver  160 , meniscus fluid in gap  158  is swept by the edge of the meniscus along the direction of arrows  206 . As the trailing edge  210  of meniscus perimeter  204  reaches trailing edge  164  of substrate  160 , fluid is directed toward a point  212  in gap adjacent to the substrate&#39;s trailing edge  164 . It should be recognized that fluid in gap  158  is constantly flowing out of gap  158  as the meniscus transitions onto carrier  150 . Therefore, the fluid is not expected to literally follow arrows  206 , but rather that a vector component of the direction of flow lies on arrows  206 , resulting in a build-up of fluid at point  212 . 
       FIG. 5  shows a cross section view of meniscus  200  at point  212  as it is completing transition onto carrier  150 . At this point, meniscus  200  is still attached to trailing edge  164  of substrate  160 . Carrier  150  may be somewhat thicker than substrate  160 , inhibiting the flow of liquid away from substrate  160  along arrows  214 . Vacuum ports  114  draw fluid including meniscus liquid and surrounding gas, exerting a force on meniscus liquid indicated by arrows  216 . An additional force may be exerted by fluid exiting main gas nozzles  112  ( FIG. 2 ) which, if provided, push inward against the gas/liquid interface of meniscus  200  as shown by arrows  218 . A gravitational force  221  is also exerted against meniscus  200 . And, if substrate  160  is hydrophilic, then an attraction to meniscus liquid can cause hydrogen bonding forces to pull water back onto substrate  160 , as represented by arrows  222 . 
       FIG. 6A  shows a face  121  of lower proximity head  120  having means for reducing entrance and exit mark size and frequency by enhancing the flow of meniscus liquid from the carrier-substrate gap as the meniscus transitions off of the substrate. As described above with reference to  FIG. 2 , lower proximity head  120  includes a plurality of centrally disposed meniscus nozzles  116  for supplying meniscus liquid, a plurality of vacuum ports  114  disposed so that they completely surround meniscus nozzles  116 . Vacuum ports  114  aspirate meniscus liquid and surrounding gas. Optionally, a plurality of main gas nozzles  112  at least partially surround vacuum ports  114  and supply a gas or gas/liquid mixture to help maintain the integrity of the gas/liquid interface of the meniscus. Meniscus nozzles  116 , vacuum ports  114 , and, optionally, main gas nozzles  112  are formed in face  121  of proximity head  120 . In one embodiment, a means for reducing the size and frequency of entrance and exit marks is provided by a centrally located protrusion on the trailing side  122  of the meniscus. The arrangement of vacuum ports  114  determines the shape of the meniscus.  FIG. 6  shows lower proximity head  120  forming the centrally located protrusion by positioning vacuum ports  114  and adjacent main gas nozzles  112  farther from an axis defined by meniscus nozzles  116  at a center portion  125  of lower proximity head  120 . A corresponding central protrusion is formed on an upper proximity head  110  ( FIG. 1 ) so that they both generate a centrally located meniscus protrusion on the trailing side of the meniscus. 
       FIG. 6B  shows an outline of exemplary meniscus configuration formed by the proximity head of  FIG. 6A . The meniscus includes a main portion  225  and a protrusion  220 . The protrusion  220  may have various shapes. For example, as shown in  FIG. 6C , the protrusion  220 A has straight leading, the protrusion  220 B has two convex leading edges extending smoothly from the meniscus to a central point, the protrusion  220 C has concave leading edges, and the protrusion  220 D has a complex leading edge shape. In one embodiment, the leading edges are concave as presented in  220 C, with the leading edges having the same radius of curvature as the substrate  160  ( FIG. 7 ). 
     In  FIG. 6C , a minimum protrusion size is shown having a protrusion extension  224  equal to the distance between two adjacent vacuum ports and a protrusion width  226  equal to the distance between three adjacent vacuum ports. However, the protrusion can be of any suitable size. In one embodiment, for example, the protrusion has an extension equal to a length defined by the distance of 6 adjacent vacuum ports and a width defined by the distance of 12 vacuum ports, with each vacuum port being about 0.06 inches in diameter and having a 0.12 inch pitch (center-to-center spacing). 
       FIG. 7  shows a plan view of carrier  150 , substrate  160 , and meniscus perimeter  204  at a first position  230  and at a second position  232 . Carrier  150  moves in a direction indicated by arrow  166  and/or meniscus moves in a direction indicated by arrow  208 . Carrier  150  includes a plurality of support pins  153 , each having substrate support and centering features (not shown), to ensure a uniform substrate-carrier gap  158  between substrate  160  and carrier  150 . In one embodiment, carrier  150  has sloped edges at the leading side  154  and trailing side  156  to prevent abrupt changes in the volume of meniscus liquid as carrier  150  enters and exits the meniscus. For example, carrier  150  has six sides with two leading edges  155  each at an angle θ from transverse and together forming a centrally-located point, and corresponding trailing edges  159  each forming an angle θ to the transverse direction and together forming a centrally-located point. In one embodiment, θ is about 15 degrees. Other shapes that don&#39;t result in a rapid displacement of meniscus liquid are also possible, such as a trapezoid or parallelogram, wherein leading and trailing edges are at an angle other than a right angle to the direction of travel of the carrier or are at an angle to (i.e., not parallel with) the leading and trailing edges of the meniscus. 
     At a certain point in time, the meniscus is located at position  230  and traveling in a direction  208  with respect to carrier  150 . At a later time, the meniscus is located at position  232 . At position  232 , protrusion  220  extends across gap  158 . Because of protrusion  220 , the trailing edge  210  of meniscus perimeter  204  is not a straight line tangent to gap  158 . As a result, fluid exiting point  212  ( FIG. 4 ) has additional time to escape from gap  158  due to the presence of protrusion  220 . Since the fluid has additional time to escape from gap  158 , it is less likely to remain attached to substrate  160  and leave a mark. 
     Protrusion  220  can also be effective in reducing an entrance mark formed at a leading edge  162  of substrate  160 . In one embodiment, a centrally-located indentation is formed on the leading edge of the meniscus (not shown) to further reduce instances of entrance marks. It should be noted that the shape of protrusion  220 , including the width of protrusion  220  and the depth of protrusion  220 , that is the amount of extension of protrusion  220 , may vary depending on implementation, but in one embodiment, is sufficiently narrow and provides sufficient extension to improve the flow of meniscus liquid from the carrier-substrate gap. 
       FIGS. 8A ,  8 B, and  8 C show an exemplary embodiment of a proximity head  250  having means for reducing entrance and exit mark size and frequency by enhancing the flow of meniscus liquid from the carrier-substrate gap as the meniscus transitions off of the substrate. In particular, proximity head  250 , which can be an upper or lower proximity head, includes centrally disposed gap evacuation gas nozzles  252  formed on face  251  to provide additional supply of gas to push against meniscus  200  as it transitions completely off of substrate  160 , as best seen in  FIG. 8C . As mentioned above with reference to  FIG. 5 , the meniscus touching substrate  160  and substrate-carrier gap  158  is subject to competing forces, including surface tension forces that both draw the meniscus onto the substrate and inhibit transitioning onto the carrier, suction forces drawing the meniscus off of both the substrate and carrier, and gravity, which pulls the meniscus liquid into substrate-carrier gap  158 . In addition, a gas flow can exert positive pressure on meniscus  200 , and therefore help counter surface tension forces which can result in entrance and exit marks. Main gas nozzles  112  deliver carbon dioxide or nitrogen and/or isopropyl alcohol vapor to the meniscus to aid in meniscus containment and wafer drying. However, main gas nozzles  112  cannot readily be used to aid in entrance and exit mark elimination because main gas nozzles  112 , which are arranged around at least a portion of vacuum ports  114 , affect the entire meniscus, or a substantial volume thereof. One or more gap evacuation gas nozzles  252 , on the other hand, provide a localized “fan,” or “curtain” of gas flow, which can be independently controlled, and therefore be selectively applied to the substrate/meniscus interface only in the areas of entrance and exit mark formation. By applying additional pressure against meniscus  200  just as the trailing edge of meniscus  200  transitions on or off of substrate  160  using gap evacuation gas nozzles  252  meniscus liquid exiting gap  158  is pushed back into the meniscus so that it does not stick to the wafer, thereby reducing the size and likelihood of entrance and exit mark formation, and prevent wafer defects. 
     In certain embodiments, a plurality of central fluid port zones provide for additional pressure during just prior and during transition by the trailing edge of the meniscus between substrate  160  and carrier  150 . In this embodiment, one or more centrally disposed gap evacuation gas nozzles  252  are surrounded by one or more additional zones tertiary nozzles  254 , visible in  FIGS. 8A and 8B . Any number of zones can be provided, each being independently controlled by controller  130  ( FIG. 1 ) to supply gas pressure against meniscus  200  at appropriate times during meniscus transition. Controller  130  may include a mechanical or computer initiated timing mechanism. For example, a mechanical timing mechanism may employ a proximity sensor (not shown) to activate gap evacuation gas nozzles  252  (or gap evacuation gas nozzles  252  and secondary gap evacuation nozzles  254 ), wherein the proximity sensor responds to a position of carrier  150  with respect to upper and lower proximity heads  110 ,  120  ( FIG. 1 ). A computer initiated timing mechanism may include position information from a robotic actuating mechanism (not shown) used to convey carrier  150  through the meniscus. 
       FIGS. 9A and 9B  show an exemplary embodiment of a proximity head  260  having means for reducing entrance and exit mark size and frequency by enhancing the flow of meniscus liquid from the carrier-substrate gap as the meniscus transitions off of the substrate. In particular, proximity head  260  includes a partitioned vacuum manifold to provide a plurality of vacuum port zones, each connected to an independent vacuum source. By providing an independent vacuum source to a central zone  262 , which has vacuum ports formed into face  216  at a location corresponding to the trailing edge of the meniscus, vacuum shunting is minimized, which enhances flow of meniscus liquid exiting the carrier-substrate gap  158 . 
     Vacuum shunting occurs when a few vacuum ports are overrun with liquid. In such cases, the pressure drop across faceplate  265  ( FIG. 9A ) may be insufficient to clear the liquid, causing gases and fluid from the meniscus to divert to nearby vacuum ports. When a significant volume of water is trapped between the wafer and carrier, vacuum shunting can occur just as the meniscus is transitioning completely off the substrate. This reduces the available suction at the exact spot where is most needed: at the trailing edge of the substrate as the substrate exits the meniscus. This reduced availability of vacuum can therefore reduce the removal of liquid from the substrate-carrier gap, which can lead to exit mark formation. A similar effect can occur at the leading edge of the substrate, which can cause an entrance mark to form. 
     By providing an independent vacuum source to a central zone  262  of vacuum ports centrally positioned on the trailing edge of the meniscus, vacuum shunting is minimized, which enhances flow of meniscus liquid exiting the carrier-substrate gap  158 . 
     Referring to  FIGS. 9A and 9B , a plurality of meniscus nozzles  116  formed into face  261  provide meniscus liquid to the meniscus  200  ( FIG. 2 ). Surrounding meniscus nozzles  116  are vacuum ports  114 , which aspirate a mixture of meniscus liquid and surrounding gas. At least partially surrounding vacuum ports  114 , main gas nozzles  112  may be provided to supply gas or gas/liquid mixture, which can aid in maintaining meniscus integrity. In one embodiment, vacuum ports  114  are divided into a plurality of zones, including a first zone  262  centrally disposed along the trailing edge  204  of proximity head  260  and at least one additional zone  263 ,  264  comprising the remaining vacuum ports  114 . The additional zone(s) can include a secondary zone  264  comprising a plurality of vacuum ports  114  on either side of first zone  262 , and tertiary zone  263  comprising the remaining vacuum ports  114 . Each zone  263 ,  262 ,  264  has a corresponding dedicated, independent manifold  271 ,  272 ,  274 , and each manifold  271 ,  272 ,  274 , is in fluid communication with a corresponding connection  281 ,  282 ,  284  to a vacuum source  280 . 
       FIG. 10  shows another embodiment of a proximity head having means for reducing entrance and exit mark size and frequency by enhancing the flow of meniscus liquid from the carrier-substrate gap as the meniscus transitions off of the substrate. Proximity head  290  includes a plurality of meniscus nozzles  116  formed into face  291  for supplying meniscus liquid. Surrounding meniscus nozzles  116  on face  291  is a plurality of vacuum ports  114  for aspirating meniscus liquid and surrounding gases. Vacuum ports  114  includes a first row  293  of vacuum ports  114  that completely surrounds meniscus nozzles  116  and row  295  of vacuum ports  114  adjacent a central portion of the trailing edge  298  of proximity head  290 . Vacuum ports  114  lying in first row  293  are connected to a common manifold as described above for zone  263  in  FIG. 9A . Vacuum ports  114  lying in second row  295  are connected to one of one or more additional manifolds. In one embodiment, ports  114  in row  295  at zone  292  centrally disposed on the trailing side of proximity head  290  are connected to one manifold (not shown), and ports  114  in row  295  in zones  294  are connected to another manifold (not shown). Each manifold includes an independent connection to a vacuum source as described above with reference to  FIG. 9A . 
     It should be noted that either or both the upper and lower proximity heads and the carrier can be controlled by a computer system such that the rate of travel of the carrier with respect to the proximity heads may be constant or vary depending on the position of the carrier with respect to the proximity heads. In some embodiments, for example, the rate of travel of the carrier may be slower as the meniscus transitions on and off the substrate, thereby providing additional time for meniscus liquid to be flow out of the carrier-substrate gap. In addition, the gas flow through gas nozzles  112 ,  252  ( FIG. 8C ) and vacuum supplied to vacuum ports  114  ( FIGS. 9A-10 ) can be mechanically and/or computer controlled, either to time the activation/deactivation of suction, or to vary the flow rates depending on the relative position of the carrier with respect to the proximity heads. The computer control can be implemented using hardware logic or in conjunction with a multipurpose computer processor, using a computer program written to control the movement and/or application of suction. In certain embodiments, a computer program also controls the volume and/or constituents of fluid supplied to the meniscus. Therefore, the computer program can define fluid recipes specifically tailored to each of a plurality of given applications. 
     With the above embodiments in mind, it should be understood that the invention can employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared and otherwise manipulated. Further, the manipulations performed are often referred to in terms such as producing, identifying, determining, or comparing. 
     Any of the operations described herein that form part of the invention are useful machine operations. The invention also relates to a device or an apparatus for performing these operations. The apparatus can be specially constructed for the required purpose, or the apparatus can be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general-purpose machines can be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. 
     The invention can also be embodied as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data, which can be thereafter be read by a computer system. The computer readable medium also includes an electromagnetic carrier wave in which the computer code is embodied. Examples of the computer readable medium include hard drives, network attached storage (NAS), read-only memory, random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network-coupled computer system so that the computer readable code is stored and executed in a distributed fashion. 
     Embodiments of the present invention can be processed on a single computer, or using multiple computers or computer components which are interconnected. A computer, as used herein, shall include a standalone computer system having its own processor(s), its own memory, and its own storage, or a distributed computing system, which provides computer resources to a networked terminal. In some distributed computing systems, users of a computer system may actually be accessing component parts that are shared among a number of users. The users can therefore access a virtual computer over a network, which will appear to the user as a single computer customized and dedicated for a single user. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.