Abstract:
The present invention is directed to an immersion lithography proximity sensor having a nozzle shroud with a flow curtain. The immersion lithography proximity sensor includes a shroud that affixes to the nozzle. A plenum is located inside the shroud that holds a shroud liquid, which is fed into the plenum through one or more intake holes. The shroud liquid is emitted out through a series of openings, such as holes or slots, along a bottom surface of the shroud in a direction away from the nozzle. The shroud liquid that is emitted forms a curtain around the nozzle to prevent cross currents from impacting the flow of liquid out of the nozzle.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application is a divisional of and claims priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 11/005,246, entitled  Proximity Sensor Nozzle Shroud with Flow Curtain , filed on Dec. 7, 2004, which is hereby expressly incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to lithography, and more particularly, to immersion lithography proximity gauges.  
         [0004]     2. Background of Invention  
         [0005]     Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.  
         [0006]     The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.  
         [0007]     Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in photolithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.  
         [0008]     U.S. Pat. No. 4,953,388, entitled  Air Gauge Sensor , issued Sep. 4, 1990 to Andrew Barada (“&#39;388 Patent”), and U.S. Pat. No. 4,550,592, entitled  Pneumatic Gauging Circuit , issued Nov. 5, 1985 to Michel Deschape (“&#39;592 Patent”), disclose an alternative approach to proximity sensing that uses an air gauge sensor. U.S. Pat. Nos. 4,953,388 and 4,550,592 are incorporated herein in their entireties. Furthermore, principles of pneumatic gauging are discussed in Burrows, V. R.,  The Principles and Applications of Pneumatic Gauging , FWP Journal, October 1976, pp. 31-42, which is incorporated herein in its entirety. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers. Earlier versions of air gauge sensors, however, often do not meet today&#39;s lithography requirements for precision.  
         [0009]     Co-pending, commonly owned U.S. patent application, Ser. No. 10/322,768, entitled  High Resolution Gas Gauge Proximity Sensor , filed Dec. 19, 2002 by Gajdeczko et al., (“&#39;768 Patent Application”) describes a high precision gas gauge proximity sensor that overcomes some of the precision limitations of earlier air gauge proximity sensors. The 768 Patent Application, which is incorporated herein in its entirety, describes a gas gauge proximity sensor that provides a high degree of accuracy. Similarly, co-pending, commonly owned U.S. patent application, Ser. No. 10/683,271, entitled  Liquid Flow Proximity Sensor for Use in Immersion Lithography , filed Oct. 14, 2003, by Violette, Kevin, (“&#39;271 Patent Application”) describe a high precision immersion lithography proximity sensor that provides a high degree of precision in an immersion lithography application.  
         [0010]     While the sensors disclosed in the &#39;768 and &#39;271 patent applications provide a high degree of precision, the precision can be impacted by cross flows of gas or liquid that intersect the stream of gas or liquid that is being emitted from a measurement channel of the proximity sensor. Specifically, purging gases, for example, can exhibit local cross winds with velocities of the order of a few meters per second. Cross-winds or cross-flows will cause gauge instability and drift, introducing non-calibratable errors within proximity sensors.  
         [0011]     What is needed is an apparatus to neutralize these cross-flows, and in the case of immersion lithography, cross currents, to improve the accuracy of proximity sensors.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is directed to an immersion lithography proximity sensor having a nozzle shroud with a flow curtain. The immersion lithography proximity sensor includes a shroud that affixes to the nozzle. A plenum is located inside the shroud that holds a shroud liquid, which is fed into the plenum through one or more intake holes. The shroud liquid is emitted out through a series of openings, such as holes or slots, along a bottom surface of the shroud in a direction away from the nozzle. The shroud liquid that is emitted forms a curtain around the nozzle to prevent cross currents from impacting the flow of liquid out of the nozzle.  
         [0013]     Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention are described in detail below with reference to accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]     The invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawing in which an element first appears is indicated by the left-most digit in the corresponding reference number.  
         [0015]      FIG. 1  is a schematic diagram showing the functional components of a proximity sensor.  
         [0016]      FIG. 2  is a diagram of a proximity sensor nozzle having a shroud with a flow curtain, according to an embodiment of the invention.  
         [0017]      FIG. 3  is a diagram of a bottom view of a proximity sensor nozzle having a shroud that produces a flow curtain, according to an embodiment of the invention.  
         [0018]      FIG. 4  is a diagram of a cross sectional view of a proximity sensor nozzle having a shroud that produces a flow curtain, according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility  
         [0020]      FIG. 1  illustrates gas gauge proximity sensor  100 , according to an embodiment of the present invention. Gas gauge proximity sensor  100  includes mass flow controller  106 , central channel  112 , measurement channel  116 , reference channel  118 , measurement channel restrictor  120 , reference channel restrictor  122 , measurement probe  128 , reference probe  130 , bridge channel  136  and mass flow sensor  138 . Gas supply  102  injects gas at a desired pressure into gas gauge proximity sensor  100 .  
         [0021]     Central channel  112  connects gas supply  102  to mass flow controller  106  and then terminates at junction  114 . Mass flow controller  106  maintains a constant flow rate within gas gauge proximity sensor  100 . Gas is forced out from mass flow controller  106  through a porous snubber  110 , with an accumulator  108  affixed to channel  112 . Snubber  110  reduces gas turbulence introduced by the gas supply  102 , and its use is optional. Upon exiting snubber  110 , gas travels through central channel  112  to junction  114 . Central channel  112  terminates at junction  114  and divides into measurement channel  116  and reference channel  118 . Mass flow controller  106  injects gas at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired pneumatic noise. Likewise, the system geometry can be appropriately sized to maintain the laminar flow characteristics established by mass flow controller  106 .  
         [0022]     Bridge channel  136  is coupled between measurement channel  116  and reference channel  118 . Bridge channel  136  connects to measurement channel  116  at junction  124 . Bridge channel  136  connects to reference channel  118  at junction  126 . In one example, the distance between junction  114  and junction  124  and the distance between junction  114  and junction  126  are equal.  
         [0023]     All channels within gas gauge proximity sensor  100  permit gas to flow through them. Channels  112 ,  116 ,  118 , and  136  can be made up of conduits (tubes, pipes, etc.) or any other type of structure that can contain and guide gas flow through sensor  100 . The channels do not have sharp bends, irregularities or unnecessary obstructions that may introduce pneumatic noise, for example, by producing local turbulence or flow instability. The overall lengths of measurement channel  116  and reference channel  118  can be equal or in other examples can be unequal.  
         [0024]     Reference channel  118  terminates into reference nozzle  130 .  
         [0025]     Likewise, measurement channel  116  terminates into measurement nozzle  128 . Reference nozzle  130  is positioned above reference surface  134 . Measurement nozzle  128  is positioned above measurement surface  132 . In the context of photolithography, measurement surface  132  is often a semiconductor wafer or stage supporting a wafer. Reference surface  134  can be a flat metal plate, but is not limited to this example. Gas injected by gas supply  102  is emitted from each of the nozzles  128 ,  130  and impinges upon measurement surface  132  and reference surface  134 . As stated above, the distance between a nozzle and a corresponding measurement or reference surface is referred to as a standoff.  
         [0026]     Measurement channel restrictor  120  and reference channel restrictor  122  serve to reduce turbulence within the channels and act as a resistive element. In other embodiments, orifices can be used. Although orifices will not reduce turbulence.  
         [0027]     In one embodiment, reference nozzle  130  is positioned above a fixed reference surface  134  with a known reference standoff  142 . Measurement nozzle  128  is positioned above measurement surface  132  with an unknown measurement standoff  140 . The known reference standoff  142  is set to a desired constant value representing an optimum standoff. With such an arrangement, the backpressure upstream of the measurement nozzle  128  is a function of the unknown measurement standoff  140 ; and the backpressure upstream of the reference nozzle  130  is a function of the known reference standoff  142 . If standoffs  140  and  142  are equal, the configuration is symmetrical and the bridge is balanced. Consequently, there is no gas flow through bridging channel  136 . On the other hand, when the measurement standoff  140  and reference standoff  142  are different, the resulting pressure difference between the measurement channel  116  and the reference channel  118  induces a flow of gas through mass flow sensor  138 .  
         [0028]     Mass flow sensor  138  is located along bridge channel  136 , preferably at a central point. Mass flow sensor  136  senses gas flows induced by pressure differences between measurement channel  116  and reference channel  118 . These pressure differences occur as a result of changes in the vertical positioning of measurement surface  132 . For a symmetric bridge, when measurement standoff  140  and reference standoff  142  are equal, the standoff is the same for both of the nozzles  128 ,  130  compared to surfaces  132 ,  134 . Mass flow sensor  138  will detect no mass flow, since there will be no pressure difference between the measurement and reference channels. Differences between measurement standoff  140  and reference standoff  142  will lead to different pressures in measurement channel  116  and reference channel  118 . Proper offsets can be introduced for an asymmetric arrangement.  
         [0029]     Mass flow sensor  138  senses gas flow induced by a pressure difference or imbalance. A pressure difference causes a gas flow, the rate of which is a unique function of the measurement standoff  140 . In other words, assuming a constant flow rate into gas gauge  100 , the difference between gas pressures in the measurement channel  116  and the reference channel  118  is a function of the difference between the magnitudes of standoffs  140  and  142 . If reference standoff  142  is set to a known standoff, the difference between gas pressures in the measurement channel  116  and the reference channel  118  is a function of the size of measurement standoff  140  (that is, the unknown standoff in the z direction between measurement surface  132  and measurement nozzle  128 ).  
         [0030]     Mass flow sensor  138  detects gas flow in either direction through bridge channel  136 . Because of the bridge configuration, gas flow occurs through bridge channel  136  only when pressure differences between channels  116 ,  118  occur. When a pressure imbalance exists, mass flow sensor  138  detects a resulting gas flow, and can initiate an appropriate control function. Mass flow sensor  138  can provide an indication of a sensed flow through a visual display or audio indication. Alternatively, in place of a mass flow sensor, a differential pressure sensor may be used. The differential pressure sensor measures the difference in pressure between the two channels, which is a function of the difference between the measurement and reference standoffs.  
         [0031]     Proximity sensor  100  is provided as one example of a device with a nozzle that can benefit from the present invention. The invention is not intended to be limited to use with only proximity sensor  100 . Rather the invention can be used with other types of proximity sensors, as well as other nozzles that emit gases or liquids in which the flow of the emitted gas or liquid needs to be protected from cross winds or cross currents.  
         [0032]      FIG. 2  is a diagram of a vertical cross section of proximity sensor measurement nozzle  128  having shroud  210  with a flow curtain, according to an embodiment of the invention. Measurement nozzle  128  includes channel  229  and opening  228 . As discussed above, gas will flow through channel  229  and exit measurement nozzle  128  through opening  228 . The gas impinges on measurement surface  132 , and based on the amount of backpressure within proximity sensor  100  a measurement of the standoff  140  can be estimated.  
         [0033]     When cross-winds flow through the area around standoff  140 , the cross-winds will impact the amount of backpressure and degrade the precision of proximity sensor  100 . Typically, cross-winds only impact measurement standoffs, as reference standoffs are often sheltered by enclosed area that eliminates cross-winds. Thus, the invention focuses on using a shroud on a measurement nozzle. However, the invention is not limited to this case. The shroud can be used on any type of nozzle in which protection against cross-winds or cross currents of fluids need to be reduced.  
         [0034]     Measurement nozzle  128  is surrounded by shroud  210 . Shroud  210  is made of materials that are suitable for a lithography environment, or other environment in which the shroud is being used. The specific types of acceptable materials will be known to individuals skilled in the relevant arts based on the teachings herein.  
         [0035]     In embodiments shroud  210  can be affixed to measurement nozzle  128  by use of a fastener, glue, epoxy or the like. In an embodiment shroud  210  substantially circumscribes measurement nozzle  128 . In another embodiment, measurement nozzle  128  and shroud  210  can be machined as a single structure. In a further embodiment, shroud  210  can be snapped around measurement nozzle  128  and held in place by a small latch on shroud  210 .  
         [0036]     Shroud  210  includes plenum  220  that serves as a reservoir to hold a shroud gas. In the case of an immersion lithography proximity sensor, plenum  220  holds a shroud liquid. Shroud  230  includes an intake hole  230 , which allows shroud gas to be emitted into plenum  220 . A series of holes, such as holes  242  and  244  exist along a lower surface of shroud  210 , such that the holes emit the shroud gas in a direction away from measurement nozzle  120  to form a gas curtain. In an alternate embodiment, slots can be used in place of the holes. Parameters, such as the number of holes, angle of the holes, diameter of the holes and velocity of gas being emitted are adjusted, such that an integrally continuous gas curtain is created around the lower portion of shroud  210 .  
         [0037]     The holes project cones of shroud gas having an arrival velocity at point  264 , such that the horizontal components of the shroud gas flow arrival velocity is equal to or greater than the horizontal components of cross winds. Arrival point  264  represents the intersection of the centerline of a shroud gas cone, such as shroud gas cone  252  and  254 , and measurement surface  132 .  FIG. 2  illustrates shroud gas cone  254 , which is emitted from hole  244  and shroud gas cone  252 , which is emitted from hole  242 .  
         [0038]      FIG. 3  is a diagram of a bottom view of measurement nozzle  128  and shroud  210 . Nozzle opening  228  appears at the center of the diagram, surrounded by measurement nozzle  128 . Interface  222  represents the interface between measurement nozzle  128  and shroud  210 . In this example, shroud  210  includes eight holes, such as holes  242  and  244 .  
         [0039]      FIG. 4  is a diagram of a cross sectional view of shroud  210  and measurement nozzle  128 . The cross section illustrates that plenum  220  fully encircles shroud  210 . In other embodiments, multiple plenums can be used within shroud  210 .  
         [0040]     While the discussion above has focused primarily on the use of shroud  210  with a gas gauge proximity sensor, shroud  210  can also be used with an immersion lithography proximity sensor, such as, for example, the one disclosed in the &#39;271 Patent Application. Additionally, the invention can be used with other types of nozzles that emit a gas or liquid in which the flow of the emitted gas or liquid needs to be protected from cross winds or cross currents.  
         [0041]     When used in immersion lithography, plenum  220  would contain a shroud liquid. The shroud liquid would be emitted through holes  242  and  244  to form a liquid curtain that shields the flow of liquid from a measurement nozzle from cross-currents of liquid that may be occurring that would degrade performance. The specific location of holes, number of holes, angles of the holes, velocity of shroud liquid would be a function of a particular design application, as can be determined by individuals skilled in the relevant arts, based on the teachings herein.  
       CONCLUSION  
       [0042]     Exemplary embodiments of the present invention have been presented. The invention is not limited to these examples. These examples are presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the invention.