Abstract:
An apparatus and method for precisely detecting very small distances between a measurement probe and a surface, and more particularly to a proximity sensor using a constant liquid flow and sensing a liquid mass flow rate within a bridge to detect very small distances. Within the apparatus the use of a flow restrictor and/or snubber made of porous material and/or a liquid mass flow rate controller enables detection of very small distances in the nanometer to sub-nanometer range. A further embodiment wherein a measurement channel of a proximity sensor is connected to multiple measurement branches.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation in part of application Ser. No. 10/322,768, filed Dec. 19, 2002, which is incorporated herein in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing with liquid flow.  
           [0004]    2. Related Art  
           [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. Nos. 4,953,388 and 4,550,592 disclose an alternative approach to proximity sensing that uses an air gauge sensor. 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.  
           [0009]    Furthermore, as imaging requirements within lithography methods become more challenging, one alternative approach being used is immersion lithography. Within immersion lithography the gap between the last lens in the projection optic box and a wafer is filled with a liquid to enhance system performance. Such an approach supports printing of smaller feature sizes. In these systems, a wafer to be worked is surrounded by a pool of the liquid. Air gauge sensors, such as those disclosed in U.S. Pat. Nos. 4,953,388 and 4,550,592, would be ineffective in an immersion lithography system.  
           [0010]    Immersion lithography systems are generating considerable interest within the microlithography community. The technology enables the index of refraction in the image space, and thus the numerical aperture of the projection system to be greater than unity. As a result, the technology has the potential to extend 193 nm tools used in lithography down to 45 nm, and possibly below. For an immersion lithography system to be effective, however, the index of refraction of the liquid surrounding the work surface must remain constant. Such variables as bubbling and temperature changes in the liquid can effect the index of refraction. A proximity sensor, therefore, must not induce bubbling or temperature changes, and ideally can reduce these effects.  
           [0011]    What is needed is a liquid flow proximity sensor that can precisely sense distances in an immersion lithography system.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a high-resolution liquid flow proximity sensor and method that can precisely sense distances in an immersion lithography system. The liquid flow proximity sensor determines proximity by detecting a difference in measurement and reference standoffs. A standoff is the distance or gap between a nozzle of the proximity sensor and the surface beneath the nozzle.  
           [0013]    To determine the standoff difference, a flow of liquid with a constant liquid mass flow rate is metered with a liquid mass flow controller and is forced through two channels—a measurement channel and a reference channel. According to the present invention, porous restrictors are used in the reference channel and measurement channel. The porous restrictors introduce no turbulence, while performing a resistive function required for proper operation of the sensor. In alternate embodiments of the present invention, a porous snubber is placed within the proximity sensor following the liquid mass flow controller and before the proximity sensor bifurcates into the reference and measurement channel. The porous snubber quiets turbulence and reduces possible noise propagated through the channels, and enhances the proximity sensor&#39;s precision.  
           [0014]    Each channel has a probe on the distal end that is positioned above a surface. A liquid is forced through the channels and emitted through nozzles against respective measurement and reference surfaces. A bridge channel between the reference and measurement channels senses liquid mass flow between the two channels that is induced by differences in the liquid pressure in the reference and measurement channel. The sensed liquid mass flow rate is representative of the difference in reference and measurement standoffs. In other words, the sensed liquid mass flow across the bridge is representative of any difference between a reference standoff of a reference probe and reference surface in the reference channel and a measurement standoff of a measurement probe and a measurement surface in the measurement channel. The liquid flow proximity sensor can provide an indication and invoke a control action based on the sensed mass flow rate.  
           [0015]    According to a further aspect of the present invention, different nozzle types can be used as measurement and reference probes. These nozzles enable the sensor to be readily adapted for different types of work surfaces.  
           [0016]    According to a further aspect of the present invention, a liquid flow proximity sensor can contain a measurement channel connected to a switching device that connects to multiple measurement branches. Each of the measurement branches has characteristics that are the same as those of a measurement channel in a device that does not contain measurement branches. Multiple measurement branches enhance the ability of a proximity sensor to measure standoffs over a larger region of a measurement surface.  
           [0017]    According to a further embodiment of the present invention, a method is provided for liquid flow proximity sensor with a single measurement channel. The method includes steps of distributing liquid flow into measurement and reference channels, and restricting liquid flow evenly across cross-sectional areas of each channel.  
           [0018]    According to a further embodiment of the present invention, a method is provided for a liquid flow proximity sensor with multiple measurement branches. The method includes steps of distributing liquid flow into a measurement branch and a reference channel, restricting liquid flow evenly across cross-sectional areas of the reference channel or a measurement branch, and switching between measurement branches. An additional method describes the use of a liquid flow proximity sensor with multiple measurement branches to map the topography of a measurement surface.  
           [0019]    Through the use of porous restrictors, a liquid mass flow controller, and/or snubbers, embodiments of the present invention allow measurement of distances based on liquid flow at a high-resolution with nanometer accuracy. The present invention is especially advantageous in immersion photolithography systems and tools. In photolithography systems it is increasingly desired to determine a distance between a suitable geometrical reference of a lithography production tool and semiconductor wafers at high-resolution. Using a high-resolution liquid flow proximity sensing technique further provides independence of wafer proximity measurements from the physical parameters of wafer materials and materials deposited on wafers during semiconductor fabrication at high-resolution performance.  
           [0020]    Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention are described in detail below with reference to accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]    The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.  
         [0022]    [0022]FIG. 1A is a diagram of a liquid flow proximity sensor, according to an embodiment of the present invention.  
         [0023]    [0023]FIG. 1B is a diagram of a liquid flow proximity sensor with multiple measurement branches, according to an embodiment of the present invention.  
         [0024]    [0024]FIG. 1C is a diagram of reference and measurement probes of a liquid flow proximity sensor coupled to a immersion liquid supply system, according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 2 is a diagram that provides a cross sectional view of a restrictor, according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 3A is a diagram that shows the basic characteristics of a nozzle.  
         [0027]    [0027]FIG. 3B is a diagram that shows a perspective view of a nozzle that may be used in a reference probe or a measurement probe, according to an embodiment of the present invention.  
         [0028]    [0028]FIG. 3C is a diagram that shows a cross sectional view of the nozzle illustrated in FIG. 3B, according to an embodiment of the present invention.  
         [0029]    [0029]FIG. 3D is a diagram that shows a perspective view of a shower-head nozzle that may be used in a reference probe or a measurement probe, according to an embodiment of the present invention.  
         [0030]    [0030]FIG. 3E is a diagram that shows a cross sectional view of the nozzle illustrated in FIG. 3D, according to an embodiment of the present invention.  
         [0031]    [0031]FIG. 4 is a flowchart diagram that shows a method for using a liquid flow proximity sensor to detect very small distances and perform a control action, according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    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 present invention would be of significant utility.  
         [0033]    A. Liquid Flow Proximity Sensor  
         [0034]    [0034]FIG. 1A illustrates liquid flow proximity sensor  100 , according to an embodiment of the present invention. Liquid flow proximity sensor  100  includes liquid 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 liquid mass flow sensor  138 . Liquid supply  102  injects a liquid at a desired pressure into liquid flow proximity sensor  100 . The sensor liquid used in liquid supply  102  can be any liquid suitable for liquid immersion lithography, such as de-ionized water, cyclo-octane, and Krytox. The sensor liquid should be compatible with the liquid used as the immersion liquid surrounding the work surface.  
         [0035]    Central channel  112  connects liquid supply  102  to liquid mass flow controller  106  and then terminates at junction  114 . Liquid mass flow controller  106  maintains a constant flow rate within liquid flow proximity sensor  100 . In an alternative embodiment, a pressure regulator can be used in place of liquid mass flow controller  106 . Liquid is forced out from liquid mass flow controller  106  through a porous snubber  110 , with an accumulator  108  affixed to channel  112 . Snubber  110  reduces turbulence introduced by the liquid supply  102 , and its use is optional. Upon exiting snubber  110 , liquid 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 . Liquid mass flow controller  106  injects liquid at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired hydraulic noise.  
         [0036]    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.  
         [0037]    All channels within liquid flow proximity sensor  100  permit liquid 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 liquid flow through sensor  100 . The channels do not have sharp bends, irregularities or unnecessary obstructions that may introduce hydraulic 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.  
         [0038]    Reference channel  118  terminates into reference probe  130 . Likewise, measurement channel  116  terminates into measurement probe  128 . Reference probe  130  is positioned above reference surface  134 . Measurement probe  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. Both reference probe  130  and measurement probe  128  are positioned such that the bore in which liquid flows out of is submerged beneath the pool of immersion liquid  144  that covers the wafer being worked. Liquid injected by liquid supply  102  is emitted from each of the probes  128 ,  130  and impinges upon measurement surface  132  and reference surface  134 . Nozzles are provided in measurement probe  128  and reference probe  130 . Example nozzles are described further below with respect to FIGS.  3 A- 3 E. As stated above, the distance between a nozzle and a corresponding measurement or reference surface is referred to as a standoff.  
         [0039]    In one embodiment, reference probe  130  is positioned above a fixed reference surface  134  with a known reference standoff  142 . Measurement probe  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 probe  128  is a function of the unknown measurement standoff  140 ; and the backpressure upstream of the reference probe  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 liquid 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 liquid through liquid mass flow sensor  138 .  
         [0040]    Liquid mass flow sensor  138  is located along bridge channel  136 , preferably at a central point. Liquid mass flow sensor  138  senses liquid 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 probes  128 ,  130  compared to surfaces  132 ,  134 . Liquid mass flow sensor  138  will detect no liquid 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.  
         [0041]    Liquid mass flow sensor  138  senses liquid flow induced by a pressure difference or imbalance. A pressure difference causes a liquid flow, the rate of which is a unique function of the measurement standoff  140 . In other words, assuming a constant flow rate into liquid gauge  100 , the difference between liquid 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 liquid 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 probe  128 ).  
         [0042]    Liquid mass flow sensor  138  detects liquid flow in either direction through bridge channel  136 . Because of the bridge configuration, liquid flow occurs through bridge channel  136  only when pressure differences between channels  116 ,  118  occur. When a pressure imbalance exists, liquid mass flow sensor  138  detects a resulting liquid flow, and can initiate an appropriate control fluction. Liquid mass flow sensor  138  can provide an indication of a sensed flow through a visual display or audio indication. Alternatively, in place of a liquid 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.  
         [0043]    The control function may be to calculate the exact gap differences. In another embodiment, the control function may be to increase or decrease the size of measurement gap  140 . This is accomplished by moving the measurement surface  132  relative to measurement probe  128  until the pressure difference is sufficiently close to zero, which occurs when there is no longer a difference between the standoffs from measurement surface  132  and reference surface  134 .  
         [0044]    [0044]FIG. 1A illustrates at least three elements of the present invention that limit liquid turbulence and other hydraulic noise to enable the present invention to achieve nanometer accuracy. These elements, liquid mass flow rate controller  106 , snubber  110  and restrictors  120 ,  122 , may all be used within an embodiment of the present invention or in any combination depending on the sensitivity desired. For example, if an application required very precise sensitivity, all elements may be used. Alternatively, if an application required less sensitivity, perhaps only snubber  110  would be needed with porous restrictors  120  and  122  replaced by orifices. As a result, the present invention provides a flexible approach to cost effectively meet a particular application&#39;s requirements.  
         [0045]    [0045]FIG. 1B illustrates liquid flow proximity sensor  150 , according to an embodiment of the present invention. Liquid flow proximity sensor  150  includes many of the same components as liquid flow proximity sensor  100  with similar principles of operation. The difference between the two sensors is that liquid flow proximity sensor  150  has three measurement branches which are comparable to the one measurement channel included within liquid flow proximity sensor  100 . Three measurement branches are shown for ease of illustration, and the present invention is not limited to three measurement branches. Any number of measurement branches from two or more may be used.  
         [0046]    Liquid flow proximity sensor  150  includes liquid mass flow controller  153 , central channel  156 , reference channel  158 , reference channel restrictor  166 , reference probe  174 , bridge channel  190  and liquid mass flow sensor  192 . In addition, liquid flow proximity sensor  150  includes measurement channel  159 . Measurement channel  159  divides into three measurement branches  163 ,  164  and  165 . Measurement branch  163  includes measurement branch restrictor  167  and measurement probe  175 . Measurement branch  164  includes measurement branch restrictor  168  and measurement probe  176 . Measurement branch  165  includes measurement branch restrictor  169  and measurement probe  177 . Finally, liquid flow proximity sensor  150  includes measurement channel switching device  160 , bridge channel switching device  161 , and switching device lever  162 .  
         [0047]    Liquid supply  151  injects liquid at a desired pressure into liquid flow proximity sensor  150 . As in the case of liquid flow proximity sensor  100 , sensor liquid used in liquid supply  151  can be any liquid suitable for liquid immersion lithography, such as de-ionized water, cyclo-octane, and Krytox. The sensor liquid should be compatible with the liquid used as the immersion liquid surrounding the work surface.  
         [0048]    Central channel  156  connects liquid supply  151  to liquid mass flow controller  153  and then terminates at a junction  157 . Liquid mass flow controller  153  maintains a constant flow rate within liquid flow proximity sensor  150 . Liquid mass flow controller  153  injects liquid at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired hydraulic noise. In an alternative embodiment, a pressure regulator can be used in place of liquid mass flow controller  153 . Liquid is forced out from liquid mass flow controller  153  through porous snubber  155 , with accumulator  154  affixed to channel  156 . Snubber  155  reduces liquid turbulence introduced by the liquid supply  151 , and its use is optional. Upon exiting snubber  155 , liquid travels through central channel  156  to junction  157 . Central channel  156  terminates at junction  157  and divides into measurement channel  159  and reference channel  158 .  
         [0049]    Measurement channel  159  terminates into measurement channel switching device  160 . Measurement channel switching device  160  can be a scanning valve or other type of switching device that serves to switch a measurement channel to one of several measurement branches that are also connected to measurement channel switching device  160 . The physical characteristics of a measurement branch are the same as the physical characteristics of a measurement channel. Measurement channel switching device  160  is operated by switching device lever  162 . Switching device lever  162  controls which measurement branch  163 ,  164  or  165  is connected to the measurement channel  159  through measurement channel switching device  160 .  
         [0050]    Bridge channel  190  is coupled between reference channel  158  and one of the three measurement branches  163 ,  164  or  165  through bridge channel switching device  161 . Bridge channel  190  connects to reference channel  158  at junction  170 . Bridge channel  190  terminates in bridge channel switching device  161 . Bridge channel switching device  161  can be a scanning valve or other type of switching device that serves to switch a bridge channel to one of the measurement branches. In one example shown in FIG. 1B, three measurement branches  163 ,  164  and  165  are connected to bridge channel switching device  161  at junctions  171 ,  172 , and  173  respectively. Switching device lever  162  controls which measurement branch  163 ,  164 , or  165  is connected to the bridge channel through bridge channel switching device  161 . Switching lever  162  controls both measurement channel switching device  160  and bridge channel switching device  161 , such that the same measurement branch will be connected to both measurement channel  159  and bridge channel  190 . Alternatively, two independent switching levers can be used.  
         [0051]    In one example, the distance between junction  157  and junction  170  and the distance between junction  157  and junction  171 ,  172  or  173  are equal.  
         [0052]    All channels and branches within liquid flow proximity sensor  150  permit liquid to flow through them. Channels  156 ,  158 ,  159 , and  190 , and branches  163 ,  164 , and  165  can be made up of conduits (tubes, pipes, etc.) or any other type of structure that can contain and guide liquid flow through sensor  150 . The channels and branches do not have sharp bends, irregularities or unnecessary obstructions that may introduce hydraulic noise, for example, by producing local turbulence or flow instability. The overall lengths of reference channel  158  and measurement channel  159  plus one of measurement branches  163 ,  164  or  165  can be equal or in other examples can be unequal.  
         [0053]    Reference channel  158  terminates into reference probe  174 . Likewise, measurement branches  163 ,  164  and  165  terminate into measurement probes  175 ,  176  and  177  respectively. Reference probe  174  is positioned above reference surface  178 . Measurement probes  175 ,  176  and  177  are positioned above measurement surface  179 . In the context of photolithography, measurement surface  179  is often a semiconductor wafer or stage supporting a wafer. Reference surface  178  can be a flat metal plate, but is not limited to this example. Liquid injected by liquid supply  151  is emitted from reference probe  174  and impinges upon reference surface  178 . Likewise, liquid injected by liquid supply  151  is emitted from one of the three measurement probes  175 ,  176  or  177  and impinges on measurement surface  179 . All reference probe and measurement probes are positioned such that the bore in which liquid flows out of is submerged beneath the pool of immersion liquid  194  that covers the wafer being worked. The position of switching device lever  162  determines from which measurement probe liquid is emitted. Nozzles are provided in probes  174 ,  175 ,  176  and  177 . Example nozzles are described further below with respect to FIGS.  3 A- 3 E. As stated above, the distance between a nozzle and a corresponding measurement or reference surface is referred to as a standoff.  
         [0054]    In one embodiment, reference probe  174  is positioned above a fixed reference surface  178  with a known reference standoff  180 . Measurement probes  175 ,  176  and  177  are positioned above measurement surface  179  with unknown measurement standoffs  181 ,  182  and  183 . Measurement standoffs  181 ,  182  and  183  may be equal or they may be unequal where the topography of a measurement surface varies from region to region. The known reference standoff  180  is set to a desired constant value representing an optimum standoff. With such an arrangement, the backpressure upstream from measurement probe  175 ,  176  or  177  that is in use is a function of the unknown measurement standoff  181 ,  182  or  183  respectively; and the backpressure upstream of the reference probe  174  is a function of the known reference standoff  180 . If reference standoff  180  and the measurement standoff  181 ,  182  or  183  that is being used are equal, the configuration is symmetrical and the bridge is balanced. Consequently, there is no liquid flow through bridge channel  190 . On the other hand, when the reference standoff  180  and the measurement standoff  181 ,  182  or  183  corresponding to the measurement branch in use is different, the resulting pressure difference between the reference channel  158  and the measurement branch  163 ,  164  or  165  that is being used induces a flow of liquid through bridge channel  190 .  
         [0055]    Liquid mass flow sensor  192  is located along bridge channel  190 , preferably at a central point. Liquid mass flow sensor  192  senses liquid flows induced by pressure differences between reference channel  158  and the measurement branch  163 ,  164  or  165  that is being used. These pressure differences occur as a result of changes in the vertical positioning of measurement surface  179 . For a symmetric bridge, when reference standoff  180  and a measurement standoff  181 ,  182  or  183  corresponding to the measurement branch that is being used are equal, liquid mass flow sensor  192  will detect no liquid mass flow, since there will be no pressure difference between the measurement branch in use and the reference channel. Differences between the reference standoff  180  and a measurement standoff  181 ,  182  or  183  corresponding to the measurement branch in use will lead to different pressures in reference channel  158  and the measurement branch  163 ,  164  or  165  being used. Proper offsets can be introduced for an asymmetric arrangement.  
         [0056]    Liquid mass flow sensor  192  senses liquid flow induced by a pressure difference or imbalance. A pressure difference causes a liquid flow, the rate of which is a unique function of a measurement standoff  181 ,  182  or  183 . In other words, assuming a constant flow rate into liquid gauge  150 , the difference between liquid pressures in a measurement branch  163 ,  164  or  165  and reference channel  158  is a function of the difference between reference standoff  180  and a measurement standoff  181 ,  182  or  183  corresponding to the measurement branch that is being used. If reference standoff  180  is set to a known standoff, the difference between liquid pressures in a measurement branch  163 ,  164  or  165  that is being used and reference channel  158  is a function of the size of a measurement standoff (that is, the unknown standoff in the z direction between measurement surface  179  and a measurement probe  175 ,  176  or  177  that is being used).  
         [0057]    Liquid mass flow sensor  192  detects liquid flow in either direction through bridge channel  190 . Because of the bridge configuration, liquid flow occurs through bridge channel  190  only when pressure differences occur between reference channel  158  and a measurement branch  163 ,  164  or  165  that is being used. When a pressure imbalance exists, liquid mass flow sensor  192  detects a resulting liquid flow, and can initiate an appropriate control function. Liquid mass flow sensor  192  can provide an indication of a sensed flow through a visual display or audio indication. Alternatively, in place of a liquid mass flow sensor, a differential pressure sensor may be used. The differential pressure sensor measures the difference in pressure between the reference channel and a measurement branch, which is a function of the difference between a measurement standoff and the reference standoff.  
         [0058]    The control function may be to calculate the exact gap differences. In another embodiment, the control function may be to increase or decrease the size of a measurement standoff  181 ,  182  or  183 . This is accomplished by moving the measurement surface relative to a measurement probe until the pressure difference is sufficiently close to zero, which occurs when there is no longer a difference between the standoffs from a measurement surface and reference surface  178 .  
         [0059]    In an alternative embodiment liquid flow proximity sensor  150  has multiple reference branches that can be used as the reference channel, but only one measurement branch for use as the measurement channel. This is the opposite arrangement of liquid flow proximity sensor  150 , which has multiple measurement branches, but only one reference branch. The design of a liquid flow proximity sensor with multiple reference branches can be determined by an individual skilled in the relevant arts based on the teachings herein. The reference standoffs of each of the reference branches can be adjusted to different heights. When the reference standoffs are set to different heights, the measurement standoff can also be easily adjusted to different heights depending on the sensitivity required. In this way, in cases where additional clearance is needed between a measurement probe and measurement surface, the measurement probe can easily be raised.  
         [0060]    [0060]FIG. 1B illustrates at least three elements of the present invention that reduce liquid turbulence and other hydraulic noise to enable the present invention to achieve nanometer accuracy. These elements, liquid mass flow rate controller  153 , snubber  155  and restrictors  166 ,  167 ,  168  and  169  may all be used within an embodiment of the present invention or in any combination depending on the sensitivity desired. For example, if an application required very precise sensitivity, all elements may be used. Alternatively, if an application required less sensitivity, perhaps only snubber  155  would be needed with porous restrictors  166 ,  167 ,  168 , and  169  replaced by orifices. As a result, the present invention provides a flexible approach to cost effectively meet a particular application&#39;s requirements.  
         [0061]    [0061]FIG. 1C illustrates a diagram of reference and measurement probes of a liquid flow proximity sensor coupled to an immersion liquid supply system, according to an embodiment of the present invention. Measurement probe  128  and reference probe  130  are submerged in immersion liquid  144 . Immersion chamber  143  provides a chamber to contain immersion liquid  144  around measurement surface  132  and reference surface  134 . Immersion liquid supply  146  provides a flow of immersion liquid into immersion chamber  143  through one or more entry points. Likewise, immersion liquid pump  148  removes immersion liquid from the immersion chamber  143  through one or more entry points. Control sensors and circuitry are used to maintain a constant temperature and volume of immersion liquid  144  around measurement surface  132 .  
         [0062]    1. Flow Restrictors  
         [0063]    According to one embodiment of the present invention and referring to liquid flow proximity sensor  100 , measurement channel  116  and reference channel  118  contain restrictors  120 ,  122 . Each restrictor  120 ,  122  restricts the flow of liquid traveling through the respective measurement channel  116  and reference channel  118 . Measurement channel restrictor  120  is located within measurement channel  116  between junction  114  and junction  124 . Likewise, reference channel restrictor  122  is located within reference channel  118  between junction  114  and junction  126 . In one example, the distance from junction  114  to measurement channel restrictor  120  and the distance from junction  114  to reference channel restrictor  122  are equal. In other examples, the distances are not equal. There is no inherent requirement that the sensor be symmetrical, however, the sensor is easier to use if it is geometrically symmetrical.  
         [0064]    According to a further feature of the present invention, each restrictor  120 ,  122  consists of a porous material, such as polyethylene or sintered stainless steel. FIG. 2 provides a cross-sectional image of restrictor  120  having porous material  210  through which a liquid flow  200  passes. Measurement channel restrictor  120  and reference channel restrictor  122  have substantially the same dimensions and permeability characteristics. Restrictors typically range in length from 2 to 15 mm, but are not limited to these lengths. Measurement channel restrictor  120  and reference channel restrictor  122  restrict liquid flow evenly across the cross-sectional areas of the channels  116 ,  118 . Within liquid flow proximity sensor  150 , porous restrictors  166 ,  167 ,  168  and  169  with above mentioned characteristics are also used to achieve these advantages.  
         [0065]    The restrictors serve two key functions. First, they mitigate the pressure and flow disturbances present in liquid flow proximity sensor  100 , most notably disturbances generated by liquid mass flow controller  106  or sources of acoustic pick-up. Second, they serve as the required resistive elements within the bridge.  
         [0066]    Exemplary embodiments of a liquid flow proximity sensor have been presented. The present invention is not limited to this example. This example is 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 present invention.  
         [0067]    2. Snubber  
         [0068]    According to one embodiment of the present invention and referring to liquid flow proximity sensor  100 , channel  112  contains snubber  110 . Similar to the operation of a restrictor, snubber  110  reduces liquid turbulence introduced by liquid supply  102  and isolates the liquid mass flow sensor from acoustic pick-up in the upstream part of the liquid flow proximity sensor. Snubber  110  is located within channel  112  between accumulator  108  and junction  114 . According to a further feature of the present invention, snubber  110  consists of a porous material, such as polyethylene or sintered stainless steel. Snubber  155  used within liquid flow proximity sensor  150  has the same characteristics as Snubber  110  and is used to achieve the same benefits.  
         [0069]    3. Nozzles  
         [0070]    In liquid flow proximity sensor  100 , different types of nozzles may be used as reference probe  130  and measurement probe  128  depending on a particular application. Similarly different types of nozzles may be used in liquid flow proximity sensor  150  for reference probe  174  and measurement probes  181 ,  182  and  183 . The choice of nozzle type, among other things, depends on the footprint (measurement area) that is required.  
         [0071]    The basic configuration of the liquid flow proximity sensor nozzle  300  is characterized by a flat end surface that is parallel to the surface of the measurement surface, as shown in FIG. 3A. The geometry of a nozzle is determined by the gauge standoff, h, and the inner diameter, d. Generally, the dependence of the nozzle pressure drop on the nozzle outer diameter D is weak, if D is sufficiently large.  
         [0072]    [0072]FIGS. 3B and 3C illustrate a nozzle  310  that may be used as a reference probe or measurement probe, according to an embodiment of the present invention. Nozzle  310  includes front surface  312 , liquid bore front opening  314 , and liquid bore rear opening  315 .  
         [0073]    Nozzle  310  is affixed to both measurement channel  116  and reference channel  118 . In one embodiment, two identical nozzles  310  serve as measurement probe  128  and reference probe  130 . In principle, the nozzles do not need to be identical. Nozzle  310  is affixed to measurement channel  116 . Front surface  312  should be parallel to measurement surface  132 . Liquid travelling through measurement channel  116  enters nozzle  310  through liquid bore rear opening  315  and exits through liquid bore front opening  314 . Similarly, nozzle  310  is affixed to reference channel  118 . Front surface  312  is parallel to reference surface  134 . Liquid travelling through reference channel  118  enters nozzle  310  through liquid bore rear opening  315  and exits through liquid bore front opening  314 . The diameter of liquid bore front opening  314  can vary depending upon a particular application. In one example, the inner diameter of liquid bore front opening  314  is between approximately 0.5 and 2.5 millimeters (mm).  
         [0074]    [0074]FIGS. 3D and 3E illustrate shower-head nozzle  350  that may be used as the reference and measurement probes, according to an embodiment of the present invention. Shower-head nozzle  350  includes front surface  355 , a plurality of liquid bore front openings  360 , and a liquid bore rear opening  365 .  
         [0075]    The multiple liquid bore front openings distribute pressure across a wider area of measurement surface  132  than nozzle  310 . A shower-head nozzle is principally used for lowering spatial resolution to evenly integrate proximity measurements over a wider spatial area. An alternative approach would be to use a nozzle that contains a porous filter.  
         [0076]    A shower-head nozzle  350  is affixed to both measurement channel  116  and reference channel  118 . In one embodiment, two identical shower-head nozzles  350  serve as measurement probe  128  and reference probe  130 . In principle, the nozzles do not need to be identical. Shower-head nozzle  350  is affixed to measurement channel  116 . Front surface  355  is parallel to measurement surface  132 . Liquid travelling through measurement channel  116  enters shower-head nozzle  350  through liquid bore rear opening  365  and exits through a plurality of liquid bore front openings  360 . Similarly, shower-head nozzle  350  is affixed to reference channel  118 . Front surface  355  is parallel to reference surface  134 . Liquid travelling through reference channel  118  enters shower-head nozzle  350  through liquid bore rear opening  365  and exits through a plurality of liquid bore front openings  360 . The use of nozzles has been explained with reference to liquid flow proximity sensor  100  for ease of illustration. Each of the nozzle types may also be used with liquid flow proximity sensor  150 , wherein the nozzles would be affixed to each of the measurement branch probes and the reference channel probe.  
         [0077]    Exemplary embodiments of different types of nozzles have been presented. The present invention is not limited to these examples. The 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 present invention.  
         [0078]    B. Methods  
         [0079]    The process illustrated in FIG. 4 presents a method  400  for using liquid flow to detect very small distances and perform a control action (steps  410 - 470 ). For convenience, method  400  is described with respect to liquid flow proximity sensor  100 . However, method  400  is not necessarily limited by the structure of sensor  100 , and can be implemented with liquid flow proximity sensor  150  or a sensor with a different structure.  
         [0080]    The process begins in step  410 . In step  410 , an operator or mechanical device places a reference probe above a reference surface. For example, an operator or mechanical device positions reference probe  130  above reference surface  134  with known reference standoff  142 . Alternatively, the reference standoff can be arranged within the sensor assembly, that is, internal to the sensor assembly. The reference standoff is pre-adjusted to a particular value, which typically would be maintained constant. In step  420 , an operator or mechanical device places a measurement probe above a measurement surface. For example, an operator or mechanical device positions measurement probe  128  above measurement surface  132  to form measurement gap  140 .  
         [0081]    In step  430 , liquid is injected into a sensor. For example, a measurement liquid is injected into liquid flow proximity sensor  100  with a constant liquid mass flow rate. In step  440 , a constant liquid flow rate into a sensor is maintained. For example, liquid mass flow controller  106  maintains a constant liquid flow rate. In step  450 , liquid flow is distributed between measurement and reference channels. For example, liquid flow proximity sensor  100  causes the flow of the measurement liquid to be evenly distributed between measurement channel  116  and reference channel  118 .  
         [0082]    In step  460 , liquid flow in the measurement channel and the reference channel is restricted evenly across cross-sectional areas of the channels. Measurement channel restrictor  120  and reference channel restrictor  122  restrict the flow of liquid to reduce hydraulic noise and serve as a resistive element in liquid flow proximity sensor  100 .  
         [0083]    In step  470 , liquid is forced to exit from a reference and measurement probe. For example, liquid flow proximity sensor  100  forces liquid to exit measurement probe  128  and reference probe  130 . In step  480 , a flow of liquid is monitored through a bridge channel connecting a reference channel and a measurement channel. In step  490 , a control action is performed based on a pressure difference between the reference and measurement channel. For example, liquid mass flow sensor  138  monitors liquid mass flow rate between measurement channel  116  and reference channel  118 . Based on the liquid mass flow rate, liquid mass flow sensor  138  initiates a control action. Such control action may include providing an indication of the sensed liquid mass flow, sending a message indicating a sensed liquid mass flow, or initiating a servo control action to reposition the location of the measurement surface relative to the reference surface until no liquid mass flow or a fixed reference value of liquid mass flow is sensed.  
         [0084]    The above method may be adapted to use with a sensor that has multiple measurement branches, such as liquid flow proximity sensor  150 . When liquid flow proximity sensor  150  is used, an additional step may be incorporated that includes switching from the use of one measurement branch to another measurement branch.  
         [0085]    The use of a liquid flow proximity sensor  150  can also better facilitate the mapping of the topography of a measurement surface. This mapping may be accomplished through the principles described in the above method, wherein topography measurements are taken over a particular region of a work surface using one of the measurement branches. If a topography mapping is desired of a different region, the flow of liquid may be switched to a different measurement branch to map the topography of a different region. Because of limitations that may exist in the ability to move a measurement surface, a proximity sensor with multiple branches can be used in some instances to more readily map the topography of a measurement surface than a proximity sensor with only one measurement channel.  
         [0086]    For example, in one embodiment a method for mapping the topography includes injecting liquid into a proximity sensor, such as liquid flow proximity sensor  150 , and measuring the topography of a region of a measurement surface by taking a series of measurements using one of the measurement branches. Upon completing the mapping of the region that can be mapped by a particular measurement branch, the proximity sensor would be switched to a different measurement branch to repeat the mapping process for the region reached by that measurement branch. The process would be repeated until the surface for which a topography mapping is desired is completed. The measurement surface may be a semiconductor wafer or other measurement surface for which a topography mapping is desired.  
         [0087]    Additional steps or enhancements to the above steps known to person skilled in the relevant art(s) form the teachings herein are also encompassed by the present invention.  
         [0088]    The present invention has been described with respect to FIGS.  1 - 4  with reference to a liquid. In one embodiment the liquid is water. The invention is not limited to the use of water. The choice of a particular liquid to use will be based on the liquid in which a wafer is submerged in as part of the immersion lithography process. In many cases, the liquid used within the liquid flow proximity sensor will be the same as the liquid used for the immersion lithography process, however, this may not always be the case.  
         [0089]    D. Conclusion  
         [0090]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.  
         [0091]    The present invention has been described above with the aid of method steps illustrating the performance of specified functions and relationships thereof. The boundaries of these method steps have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.