Abstract:
Provided are a methods and systems for determining a topography of an object. In an embodiment, a system includes a reference probe configured to measure a surface of a reference surface and to generate a reference signal, a measuring probe configured to measure a surface of an object and to generate a measurement signal, a sensor configured to sense a position of the measuring probe and to generate a sensor signal, and a combiner configured to receive the sensor signal and the measurement signal and to generate a combination signal therefrom. A desired distance between the measuring probe and the object is substantially maintained by adjusting the position of the measuring probe based on the measurement signal. A topography of the object is determined based at least on a comparison of the reference signal and the combination signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. Non-Provisional application Ser. No. 11/011,435, filed Dec. 15, 2004, now allowed, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to pressure sensors, more particularly, to air gauge devices used in lithography systems. 
         [0004]    2. Background Art 
         [0005]    Conventional style air gauges are used to measure the location of a wafer surface in a number of lithography tools. These conventional air gauges include a bridge having a measurement nozzle located near the wafer&#39;s surface. The conventional air gauges typically also include a separate reference nozzle located near, or in the same environment as, measurement components. As a gap between the wafer and the measurement nozzle changes, the flow rate through the measurement nozzle is altered, and a change in differential pressure or flow in the bridge is detected. 
         [0006]    In general, although the measurement nozzle may be retractable, its position is fixed during the measurement process. Likewise, the gap between a reference nozzle and its target may be adjustable, but remains fixed during the measurement process. The gap measurements made by these conventional air gauges are most accurate when the wafer surface is at the nominal gap where the flow through the bridge is nearly balanced, and becomes less accurate as the measurement gap moves away from the nominal value. Off null, the air gauge becomes sensitive to changes and ambient pressure, and the relationship between gap and sensed differential flow or pressure is non-linear. 
         [0007]    The air gauge can be used at typical standoffs of less than approximately 0.150 millimeters (mm). At the physical scales of interest to wafer surface sensing, a substantial increase in an air gauge standoff value (H) is not possible, as the measurement sensitivity drops quite drastically, approximately to H −3.3 . At such small standoffs, there is a possibility of a collision between the air gauge nozzle and, for example, a wafer surface. Also, to the extent that the air gauge is required to accurately measure a range of wafer positions, its accuracy is limited. 
         [0008]    What is needed, therefore, is a method and system for facilitating measurements where the air gauge will always be operated at a favorable standoff, maximizing its performance and useful measurement range. More specifically, what is needed is a gauging device that will minimize the risk of a collision between the air gauge nozzle and the surface of the wafer. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    One embodiment of the present invention provides a system that includes a system includes a reference probe configured to measure a surface of a reference surface and to generate a reference signal, a measuring probe configured to measure a surface of an object and to generate a measurement signal, a sensor configured to sense a position of the measuring probe and to generate a sensor signal, and a combiner configured to receive the sensor signal and the measurement signal and to generate a combination signal therefrom. A desired distance between the measuring probe and the object is substantially maintained by adjusting the position of the measuring probe based on the measurement signal. A topography of the object is determined based at least on a comparison of the reference signal and the combination signal. 
         [0010]    In a further embodiment, the system further includes an actuator configured adjust the position of the measuring probe so that the desired distance between the measuring probe and the object is substantially maintained. 
         [0011]    In a further embodiment, the system further includes a controller configured to generate a control signal. The actuator is configured to adjust the position of the measuring probe based on the control signal. 
         [0012]    In a further embodiment, the measuring probe is a self-gapping measuring probe configured to self-adjust its position to substantially maintain the desired distance between the measuring probe and the object. 
         [0013]    In another embodiment, a method includes measuring a distance to a reference surface, measuring a distance to an object using a measuring probe, adjusting a position of the measuring probe used to measure the distance to the object, such that a desired distance between the measuring probe and the object is substantially maintained, sensing the position of the measuring probe, generating a combined signal based on the measured distance to the object and the sensed position, and determining a topography of the object based at least on the combined signal. 
         [0014]    In a further embodiment, adjusting a position of the measuring probe includes generating a control signal based on which an actuator is configured to adjust the position of the measuring probe. 
         [0015]    In a further embodiment, adjusting a position of the measuring probe includes adjusting at least one of internally produced force of the measuring probe or a preload force of a spring to adjust the position of the measuring probe. 
         [0016]    Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0017]    The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description given above and the detailed description of the embodiment given below, serve to explain the principles of the present invention. In the drawings: 
           [0018]      FIG. 1  is a block diagram illustration of a gas proximity sensing apparatus; 
           [0019]      FIG. 2  is a block diagram illustration of a gauging device constructed in accordance with an embodiment of the present invention and used in the apparatus of  FIG. 1 ; 
           [0020]      FIG. 3  is a block diagram illustration of a gauging apparatus constructed in accordance with a further embodiment of the present invention; 
           [0021]      FIG. 4  is a block diagram illustration of a gauging apparatus constructed in accordance with yet another embodiment of the present invention; and 
           [0022]      FIG. 5  is a flowchart of an exemplary method of practicing an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The following detailed description of the present invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other embodiments are possible, and modifications may be made to the embodiments within the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims. 
         [0024]    It would be apparent to one skilled in the art that the present invention, as described below, may be implemented in many different embodiments of hardware, software, firmware, and/or the entities illustrated in the drawings. Any actual software code with the specialized controlled hardware to implement the present invention is not limiting of the present invention. Thus, the operation and behavior of the present invention will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail presented herein. 
         [0025]      FIG. 1  illustrates a conventional gas gauge proximity sensor  100 , according to an embodiment of the present invention. The gas gauge proximity sensor  100  can include a mass flow controller  106 , a central channel  112 , a measurement channel  116 , a reference channel  118 , a measurement channel restrictor  120 , a reference channel restrictor  122 , a measurement probe  128 , a reference probe  130 , a bridge channel  136 , and a mass flow sensor  138 . A gas supply  102  can inject gas at a desired pressure into gas gauge proximity sensor  100 . 
         [0026]    The central channel  112  connects the gas supply  102  to the mass flow controller  106  and then terminates at a junction  114  (e.g., a gas dividing or directing portion). The mass flow controller  106  can maintain a constant flow rate within the gas gauge proximity sensor  100 . Gas is forced out from the mass flow controller  106  through a porous snubber  110 , with an accumulator  108  affixed to the channel  112 . The snubber  110  can reduce gas turbulence introduced by the gas supply  102 , and its use is optional. 
         [0027]    Upon exiting the snubber  110 , gas travels through the central channel  112  to the junction  114 . The central channel  112  terminates at the junction  114  and divides into the measurement channel  116  and the reference channel  118 . In one embodiment, the mass flow controller  106  can inject gas at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired pneumatic noise. 
         [0028]    A bridge channel  136  is coupled between the measurement channel  116  and the reference channel  118 . The bridge channel  136  connects to the measurement channel  116  at the junction  124 . The bridge channel  136  connects to the reference channel  118  at the junction  126 . In one embodiment, the distance between the junction  114  and the junction  124  and the distance between the junction  114  and the junction  126  are equal. It is to be appreciated that other embodiments are envisioned with different arrangements. 
         [0029]    All channels within the gas gauge proximity sensor  100  can permit gas to flow through them. The channels  112 ,  116 ,  118 , and  136  can be made up of conduits (e.g., tubes, pipes, etc.) or any other type of structure that can contain and guide gas flow through the sensor  100 , as would be apparent to one of ordinary skill in the art. In most embodiments, the channels  112 ,  116 ,  118 , and  136  should not have sharp bends, irregularities, or unnecessary obstructions that can introduce pneumatic noise. This noise can result from the production of local turbulence or flow instability, as an example. In various embodiments, the overall lengths of the measurement channel  116  and the reference channel  118  can be equal or unequal. 
         [0030]    The reference channel  118  terminates adjacent a reference probe  130 . Likewise, the measurement channel  116  terminates at an adjacent measurement probe  128 . The reference probe  130  is positioned above a reference surface  134 . The measurement probe  128  is positioned above a measurement surface  132 . In the context of photolithography, the measurement surface  132  can be substrate (e.g., a wafer, a flat panel, print head or the like) or stage supporting a substrate. The reference surface  134  can be a flat metal plate, but is not limited to this example. 
         [0031]    Nozzles are provided in the measurement probe  128  and the reference probe  130 . An example nozzle is described further below with respect to  FIGS. 2-4  below. Gas injected by the gas supply  102  is emitted from nozzles in the probes  128  and  130 , and impinges upon the measurement surface  132  and the reference surface  134 . 
         [0032]    As described above, the distance between a nozzle and a corresponding measurement or reference surface can be referred to as a standoff. 
         [0033]    In one embodiment, the reference probe  130  is positioned above a fixed reference surface  134  with a known reference standoff  142 . The measurement probe  128  is positioned above the measurement surface  132  with an unknown measurement standoff  140 . The known reference standoff  142  is set to a desired constant value, which can be at 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 . 
         [0034]    If the standoffs  140  and  142  are equal, the configuration is symmetrical and the bridge is balanced. Consequently, there is no gas flow through the bridging channel  136 . On the other hand, when the measurement standoff  140  and the 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 the mass flow sensor  138 . 
         [0035]    The mass flow sensor  138  is located along the bridge channel  136 , which can be at a central point. The mass flow sensor  138  senses gas flow induced by pressure differences between the measurement channel  116  and the reference channel  118 . These pressure differences occur as a result of changes in the vertical positioning of measurement surface  132 . 
         [0036]    In an example where there is a symmetric bridge, the measurement standoff  140  and the reference standoff  142  are equal. The mass flow sensor  138  will detect no mass flow because there will be no pressure difference between the measurement and the reference channels  116  and  118 . On the other hand, any differences between the measurement standoff  140  and the reference standoff  142  values can lead to different pressures in the measurement channel  116  and the reference channel  118 . Proper offsets can be introduced for an asymmetric arrangement. 
         [0037]    The 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 the 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 the standoffs  140  and  142 . If the 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 the measurement standoff  140  (that is, the unknown standoff along a vertical (Z) axis between the measurement surface  132  and the measurement probe  128 ). 
         [0038]    The mass flow sensor  138  detects gas flow in either direction through the bridge channel  136 . Because of the bridge configuration, gas flow occurs through the bridge channel  136  only when pressure differences between the channels  116  and  118  occur. When a pressure imbalance exists, the mass flow sensor  138  detects a resulting gas flow, and can initiate an appropriate control function, which can be done using an optional controller  150  that is coupled to appropriate parts of the system  100 . The mass flow sensor  138  can provide an indication of a sensed flow through a visual display and/or audio indication, for example, which can be done through use of an optional output device  152 . 
         [0039]    Alternatively, in place of a mass flow sensor, a differential pressure sensor (not shown) can be used. As well understood by those of skill in the art, a differential pressure sensor is designed to detect a change in pressure as a difference between two applied pressures. 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. 
         [0040]    The control function in the optional controller  150  can be to calculate the exact gap differences. In another embodiment, the control function may be to increase or decrease the size of the measurement standoff  140 . This is accomplished by moving the measurement surface  132  relative to the measurement probe  128  until the pressure difference is sufficiently close to zero. This occurs when there is no longer a difference between the standoffs from the measurement surface  132  and the reference surface  134 . 
         [0041]    It is to be appreciated that the mass flow rate controller  106 , the snubber  110 , and the restrictors  120  and  122  can be used to reduce gas turbulence and other pneumatic noise, which can be used to allow the present invention to achieve nanometer accuracy. These elements can all be used within an embodiment of the present invention or in any combination depending on the sensitivity desired. 
         [0042]    For example, if an application required very precise sensitivity, all elements can be used. Alternatively, if an application required less sensitivity, perhaps only the snubber  110  would be used with the porous restrictors  120  and  122  replaced by orifices. As a result, the present invention provides a flexible approach to cost effectively meet the requirements of a particular application. 
         [0043]    Porous restrictors  120  and  122  are also used. The porous restrictors  120  and  122  can be used instead of saphire restrictors when pressure needs to be stepped down in many steps, and not quickly. This can be used to avoid turbulence. 
         [0044]    The measurement channel  116  and the reference channel  118  contain restrictors  120  and  122 . Each of the restrictors  120  and  122  restricts the flow of gas traveling through their respective measurement channel  116  and the reference channel  118 . The measurement channel restrictor  120  is located within the measurement channel  116  between the junction  114  and the junction  124 . 
         [0045]    Likewise, the reference channel restrictor  122  is located within the reference channel  118  between the junction  114  and the junction  126 . In one example, the distance from the junction  114  to the measurement channel restrictor  120  and the distance from the junction  114  to the 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. 
         [0046]      FIG. 2  is an illustration of a gauging apparatus  200  constructed in accordance with an embodiment of the present invention. The exemplary gauging apparatus  200  of  FIG. 2  can be used, for example, to supplement and/or replace the measurement probe  128 , shown in  FIG. 1 . More specifically, output control signals  201  produced by the gauging apparatus  200  provide an extended air gauge reading. This extended air gauge reading is analogous to control signals output from the measurement probe  128 , and forwarded along a feedback path  154 . 
         [0047]    According to the present invention, many of the limitations of conventional air gauge sensors can be overcome by replacing the conventional air gauge sensors with devices that use alternative sensing techniques. The exemplary gauging apparatus  200  is one such device. 
         [0048]    The gauging device  200 , of the present invention, essentially extends the measurement range of conventional gas proximity sensors by maintaining a constant gap between the sensor and a target, such as a wafer surface. This constant gap is maintained by either servoing the position of the sensor or servoing the target to reduce the sensitivity of the gauge to error, thus improving performance. 
         [0049]    For example, although conventional air gauges are fairly accurate, their accuracy is restricted to relatively short distances. That is, air gauges typically have fairly short working distances, and much shorter measurement ranges than working distances. For example, a requirement may exist to measure a distance of 10 micrometers (μm). A conventional air gauge sensor, however, may have an accurate measurement range of only 1 μm. 
         [0050]    By using the present invention, the air gauge is maintained at a constant gap and is restricted to measuring very miniscule changes (e.g., on the order of several nanometers) in the distance between the target and the air gauge. Then, for example, in one embodiment of the present invention, the air gauge can be moved or servoed as the distance between the air gauge and the target changes. 
         [0051]    In being restricted to measuring small distances, the air gauge is only relied upon to measure the miniscule changes in distance between the air gauge and the target. Another sensing device is subsequently used to measure the movement of the air gauge. A combiner is then used to add the measured distance of the air gauge device with the measured distance of the second sensing device to produce a significantly more accurate combined measurement reading. 
         [0052]    As noted above, the gauging apparatus  200  of the present invention produces a more accurate (i.e., extended) air gauge reading. This more accurate reading is represented by output control signals  201 . More specifically, the output control signals  201  more accurately represent the distance between an air gauge and a target, such as a wafer surface. 
         [0053]    In the embodiment of  FIG. 2 , for example, the gauging apparatus  200  can be used to measure distances associated with a wafer  202  mounted on a movable wafer stage  204 . In practice, the wafer stage  204  can be moveable in six degrees of freedom. However, for purposes of illustration only, the present invention will focus on measuring movement in only two degrees of freedom, along a vertical (Z) axis to a horizontal surface of the wafer stage  204 . 
         [0054]    The gauging apparatus  200  includes a metrology frame  206 . In the present invention, the term “metrology frame” is used to denote an isolated frame of reference, which can be mechanically isolated from its associated measurement apparatus. Conventional metrology frames include sensitive components such as interferometers and other position sensors, which are isolated from vibration and other movements within the structure of the metrology frame. In the embodiment of  FIG. 2 , the metrology frame  206  includes an air gauge  208  and a sensor  210 . The sensor  210  can include an interferometer, a cap gauge, an encoder, or the like. The sensor  210  measures a distance  211  to the wafer stage  204 . 
         [0055]    Also included in the gauging apparatus  200  is a motion generating machine  212 , and a combiner  214 . The present application is focused on movement in two degrees of freedom, i.e., along the vertical (Z) axis. The motion machine  212  can be an actuator, a motor, a controller, or any other device capable of producing motion. The gauging apparatus  200  is used to accurately measure a distance  216  between the air gauge  208  and the wafer  202 . 
         [0056]    In the example of  FIG. 2 , the distance  216  is maintained at a substantially constant gap. That is, the wafer  202  is desirably mounted to the wafer stage  204 . During a measurement session, the distance  216  can change at least slightly, for example, due to changes in topography of the wafer  202 . In the embodiment of  FIG. 2 , however, although the topography of the wafer  202  may change, the air gauge  208  is maintained in a substantially fixed position. 
         [0057]    The wafer  202 , mounted to the wafer stage  204 , is moved along the (Z) axis by the motion machine  212 . The purpose of the movement along the (Z) axis is to make any adjustments necessary to maintain the distance  216  at a substantially constant value. That is, the motion machine  212  produces drive signals  218  that move the wafer stage  204  along the (Z) axis whenever slight changes occur in the distance  216 . The distance  216  can be a preprogrammed based upon user requirements. 
         [0058]    As the distance  216  changes, these changes are sensed by the air gauge  208 . Correspondingly, measurement signals representative of any changes in the distance  216  are communicated to the motion machine  212 . 
         [0059]    In response, the motion machine  212  produces the drive signals  218  to move the wafer stage  204  along the (Z) axis by an amount necessary to readjust the distance  216  to the predetermined value. At the same time, air gauge gap error signals forwarded along a feedback path  220  are also communicated to the combiner  214 . As the wafer stage  204  moves in accordance with the drive signals  218 , its movement in the direction (Z) is measured by the sensor  210 . 
         [0060]    The measurement by the sensor  210  of the movement (in one direction) of the wafer stage  204  is forwarded along a path  221  to the motion machine  212 . In response, the motion machine  212  produces the drive signals  218  to move the wafer stage  204  back, in the opposite direction. The movements produced by the motion machine  212  are quantified, and this quantified value is forwarded to the combiner  214  along a path  222 . The combiner  214  then adds the values forwarded along the paths  220  and  222  to produce the combined measurement distance  201 . 
         [0061]    The combined measurement distance  201  produced by the embodiment shown in  FIG. 2  can be used to increase the accuracy of the proximity of a proximity sensor, such as the measurement probe  128  of  FIG. 1 . In the system of  FIG. 1 , for example, the combined measurement distance  201  can be forwarded along the path  156  as a more accurate reading of the distance  140 . 
         [0062]      FIG. 3  is an illustration of a block diagram of a gauging apparatus  300  constructed in accordance with another embodiment of the present invention. In the embodiment of  FIG. 3 , an air gauge is moved or served while a target is maintained in a substantially stationary position. More specifically, in the example of  FIG. 3 , the gauging apparatus  300  is used to measure distances associated with the wafer  202  of  FIG. 2 . In the embodiment of  FIG. 3 , however, the wafer  202  is mounted on a substantially stationary wafer stage  304 . 
         [0063]    The gauging apparatus  300  of  FIG. 3  can include many of the components used in the gauging apparatus  200  of  FIG. 2 . For example, the gauging apparatus  300  includes a metrology frame  306 , which comprises the air gauge  208 , the sensor  210 , the motion machine  212 , and the combiner  214  from the gauging device  200  of  FIG. 2 . In  FIG. 3 , however, the metrology frame  304  also includes an actuator  306 . 
         [0064]    During operation, the motion machine  212  adjusts the position of the air gauge  208  to minimize the amount of any air gap errors. For example, during a measurement session, as the wafer stage  302  moves along a horizontal direction (substantially stationary along the vertical (Z) axis), the air gauge  208  maintains a distance  308  from the wafer  202 , at a substantially constant value. That is, as the wafer  202  moves along in the horizontal direction, and changes in a topography of the wafer  202  occur, the air gauge  208  is servoed along the vertical (Z) axis. The actuator  306  moves the air gauge  208  along the (Z) axis. 
         [0065]    As the air gauge  208  moves, this movement is sensed and measured by the sensor  210 . This movement is quantified and communicated to the motion machine  212  and the combiner  214 , in the form of an air gauge gap movement signal along a feedback path  314 . At the same time, an air gauge gap error signal is communicated to the combiner  214  along an error path  312 . 
         [0066]    The motion machine  212  then readjusts the position of the air gauge  208  via the actuator  306 , in order to maintain the distance  308  at a substantially constant value. Finally, the combiner  214  combines the air gauge gap error signal and the air gauge movement signal  313  to produce an extended air gauge reading  316 . 
         [0067]    The extended air gauge reading  316  can be applied to the measurement probe  128  of  FIG. 1 . Particularly, the extended reading  316  can be forwarded along the path  156  to increase the overall accuracy of systems such as the proximity gauge sensor  100 . 
         [0068]      FIG. 4  is an illustration of a gauging apparatus  400  constructed in accordance with yet another embodiment of the present invention. The gauging apparatus  400  of  FIG. 4  operates in a manner similar to the gauging apparatus  300  of  FIG. 3 . However, in the exemplary embodiment of  FIG. 4 , a metrology frame  402  includes a self-gapping air gauge  404 , which replaces the air gauge  208  of  FIG. 3 . As understood by persons having ordinary skill in the art, self-gapping air gauges include air bearings and operate based on the principles of aerostatic and aerodynamic design. 
         [0069]    In the apparatus  400  of  FIG. 4 , the self-gapping air gauge  404  acts as an air bearing to sense a distance to an object. More specifically, in the gauging apparatus  400  of  FIG. 4 , the motion machine  212  and the actuator  308 , shown in  FIG. 3 , can be eliminated. Their elimination is possible since the movement of the self-gapping air gauge  404  is self-maintained. For example, a preload force  406  applied by a spring (not shown) facilitates automatic readjustment of the self-gapping air gauge  404 . 
         [0070]    During operation, internally produced aerodynamic forces and the preload force  406  cooperate to maintain the distance  308  at a substantially constant value. As the air gauge  404  moves, its position is sensed by, for example, the position sensor  210 , which subsequently forwards an air gauge movement signal  408  to the combiner  214 . At the same time, and air gauge error signal  410  is forwarded along a feedback path  410  to the combiner  214 . 
         [0071]    As the self-gapping air gauge  404  moves, due for example to changes in the topography of the surface of the wafer  202 , the preload force  406  readjusts the position of the air gauge in an attempt to maintain a constant air gap. In this manner, the gauging apparatus  400  of  FIG. 4  is able to maintain a constant distance or gap  308  without any direct feedback from the sensor  210 . The air gauge movement signal  408  and the air gauge error signal  410  are combined, within the combiner  214 , to produce an extended air gauge topography measurement signal  414 . 
         [0072]      FIG. 5  is a flowchart of an exemplary method  500  of practicing an embodiment of the present invention. In  FIG. 5 , the gauging apparatus is used to sense a distance to a surface of an object, as indicated in step  502 . Next, the gauging apparatus will measure at least one from a group including a relative position of an air gauge and the relative position of the surface of the object, as indicated in step  504 . In step  506 , the sensed distance and the measurement are combined to produce an extended air gauge measurement. 
       CONCLUSION 
       [0073]    The present invention provides techniques, for example, whereby the position of a wafer substrate is controlled in a classical negative feedback loop. Using this feedback loop, a difference between the air gauge reading and a programmable set point value can be used to keep a measurement gap constant. Thus, while scanning a wafer, the air gauge maintains a known constant preprogrammed distance from the wafer surface. 
         [0074]    By using the present invention, all of the desired characteristics of the air gauge can be preserved, while perfectly linear readings can be maintained. Additionally, programmability of the standoff can be improved. The air gauge can be operated at a more favorable standoff, maximizing its performance, and useful measurement range. At the same time, the risk of a collision between the air gauge nozzle and the wafer can essentially be eliminated. 
         [0075]    The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks 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. 
         [0076]    Any such alternate boundaries are thus within the scope and spirit of the claimed invention. Persons having ordinary skill in the art will recognize that these functional building blocks can be implemented by analog and/or digital circuits, discrete components, application-specific integrated circuits, firmware, processor executing appropriate software, and the like, or any combination thereof. 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. 
         [0077]    The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art (including the contents of the references cited herein), readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted in light of the teachings and guidance presented herein, in combination with the knowledge of one of ordinary skill in the art.