Abstract:
A system for inspecting a depth relative to a layer using a sensor with a fixed focal plane. A focus sensor senses the surface of the substrate and outputs focus data. In setup mode the controller scans a first portion of the substrate, receives the focus data and XY data, and stores correlated XYZ data for the substrate. In inspection mode the controller scans a second portion of the substrate, receives the focus data and XY data, and subtracts the stored Z data from the focus data to produce virtual data. The controller feeds the virtual data plus an offset to the motor for moving the substrate up and down during the inspection, thereby holding the focal plane at a desired Z distance.

Description:
FIELD 
     This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to auto focus mechanisms, such as are used during the optical inspection of integrated circuits. 
     INTRODUCTION 
     The optical inspection of integrated circuits requires very precise control of the desired focal plane. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, light emitting diode arrays, and other substrates containing multiple repeating three dimensional electrical circuitry structures. 
     Modern integrated circuits often exhibit a sculpted topography. As various layers are deposited and partially removed and new layers are added on top, a surface topography of mesas and valleys (so to speak) develops across the surface of the integrated circuit. Thus, a given process layer of the integrated circuit may or may not exist at any particular X/Y point on the surface of the device. A plane that defines the Z location of this layer might then cut through the mesas and over the valleys. 
     Unfortunately, the auto focus systems of current inspection tools tend to get confused by one or more of a variety of different factors that are present during integrated circuit inspection. For example, the range of heights and depths in the Z axis of the sculpted surface topography as described above tends to confuse an auto focus mechanism, causing the focal plane to shift from the desired layer (that exists on a single plane) to the ever-shifting level of the surface topography of the integrated circuit (or elsewhere). In addition, noise that is introduced by the movement of the motor and the chuck that moves the substrate relative to the inspection optics can cause the auto focus mechanism to move away from the desired inspection plane. Further, bow across the substrate and vibration that is external to the tool introduce more variables that tend to shift the focal plane away from the desired inspection plane. 
     What is needed, therefore, is a system that reduces the substrate topography response while generally retaining the ability to respond in real time to other Z disturbances such as substrate bow and vibration. 
     SUMMARY OF THE CLAIMS 
     The above and other needs are met by an optical inspection system for inspecting a substrate at a constant layer depth relative to a particular device layer. The inspection system has an image sensor with a fixed focal plane. A focus sensor senses Z distance in regard to the surface topography of the substrate and outputs the Z distance in a focus data stream. The focus sensor and the image sensor are disposed in a known relationship. An XY stage moves the substrate in an XY plane relative to the image sensor and the focus sensor, and a Z motor moves the substrate in a Z dimension relative to the image sensor and the focus sensor. A controller selectively operates the optical inspection system in one of a setup mode and an inspection mode. 
     In the setup mode the controller controls XY movement of the substrate using the XY stage so as to scan a first portion of the substrate under the focus sensor. The controller receives the focus data stream from the focus sensor, concurrently receives XY data from the XY stage, and stores correlated XYZ data for the first portion of the substrate in a memory. In the inspection mode the controller controls XY movement of the substrate using the XY stage so as to scan a second portion of the substrate under the focus sensor and the image sensor. The controller receives the focus data stream from the focus sensor, concurrently receives XY data from the XY stage, and subtracts the Z distance in the memory from the focus data stream of the focus sensor to produce a virtual data stream, where the Z distance from the memory is correlated with the XY data from the stage. The controller feeds the virtual data stream plus an offset to the Z motor for moving the substrate up and down during the inspection, thereby holding the focal plane at a desired Z distance, regardless of the surface topography of the substrate. 
     In this manner, the fixed focal plane of the image sensor is held at a desired layer of the integrated circuit, regardless of the differences in the surface topography of the integrated circuit at any given position. Further, the setup mode can be accomplished for a given repeating pattern of the integrated circuits on the substrate, such as for a single die or reticle field. The inspection mode can then be applied to all of the die or reticle fields on the substrate (and for similar substrates) without repeating the setup procedure. 
     In various embodiments, the first portion is the entire substrate. In other embodiments the first portion is one or more reticle fields of the substrate. In some embodiments the second portion is the entire substrate. In other embodiments the second portion is one or more reticle field of the substrate. In some embodiments the first portion is a subset of the second portion. In other embodiments the first portion is identical to the second portion. In some embodiments the offset is a value that holds the focal plane above the virtual data stream. In other embodiments the offset is a value that holds the focal plane below the virtual data stream. 
     According to another aspect of the invention there is described a method for inspecting a substrate at constant layer depth relative to a particular device layer of the substrate by controlling XY movement of the substrate so as to scan a first portion of the substrate under a focus sensor, sensing XY position of the substrate during the XY movement, concurrently sensing Z distance in regard to the surface topography of the substrate with the focus sensor, storing correlated XYZ data for the first portion of the substrate, controlling XY movement of the substrate so as to scan a second portion of the substrate under the focus sensor and an image sensor, where the focus sensor and the image sensor are disposed in a known relationship, sensing XY position of the substrate during the XY movement, concurrently sensing Z distance in regard to the surface topography of the substrate with the focus sensor, subtracting the stored Z distance from the sensed Z distance to produce a virtual data stream, where the stored Z distance and the sensed Z distance are correlated by the XY position, moving the substrate up and down relative to the image sensor as directed by the virtual data stream plus an offset, while scanning the second portion of the substrate, thereby holding a focal plane of the image sensor at a desired Z distance, regardless of the surface topography of the substrate, and inspecting the second portion of the substrate at the desired Z distance with the image sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1A  is a cross sectional diagram of an integrated circuit at a particular process step, showing surface topography and a coplanar inspection layer and focal plane. 
         FIG. 1B  is a top plan view of a substrate with a repeating matrix of patterns (reticle fields). 
         FIG. 2  is a functional block diagram of an apparatus for measuring the topography information of an integrated circuit and for optically inspecting an integrated circuit while maintaining a focal plane to be coplanar with a desired layer of the integrated circuit according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 1A , there is depicted a cross sectional diagram of a portion of an integrated circuit  100 , showing surface topography  102  and a desired inspection layer  104 . It is appreciated that the depiction of  FIG. 1  is not intended to represent any specific (or real) integrated circuit  100 , but rather just to exemplify different layers having different thicknesses, residing at different depths under the surface, with mesas and valleys etched between portions of a given layer. 
     Optical inspection tools according to the various embodiments of the present invention hold the focal plane  106  coplanar with the desired inspection layer  104  at all times, regardless of any factors that might be present during the inspection process. For example, factors such as the topography  102  of the integrated circuit  100 , floor vibration, chuck bumps, and substrate bow do not cause the focal plane  106  to move away from the desired layer  104 . Thus, a focused image of the desired layer  104  is maintained at all times, regardless of such factors. 
     With reference now to  FIG. 1B  there is depicted a substrate  214  with a repeating matrix of patterns (reticle fields)  112  on the substrate  214 . These reticle fields  112  represent, for example, individual die on the substrate  214 , where the circuit patterns, such as circuit  100  as depicted in  FIG. 1A , repeat from one die to the next. These repeating patterns  112  have a constant XY offset from one die pattern to the corresponding portion of the next die pattern. It is appreciated that the example of  FIG. 1B  is extremely simplified so as to not unnecessarily encumber the drawing with insignificant details. 
     With reference now to  FIG. 2 , there is depicted a functional block diagram of a processor-based inspection tool  200  for inspecting a layer  104  of an integrated circuit  100  on a substrate  214  according to an embodiment of the present invention. In a standard mode of operation, the tool  200  scans the substrate  214  relative to an image sensor  210 . A focus sensor  208  determines the height of the top surface  102  within the particular field of view. The controller  202  takes the height information and uses it to move the z motor  218  so that the top surface  102  the substrate  214  at that particular location is moved toward the focal plane  106 . 
     However, this standard mode of operation continually shifts the focal plane  106  to keep the output of the focus sensor  208  constant. Since the focus sensor  208  is typically sampling a large area of the substrate  214 , the actual layer inspected depends upon the topographic content of the particular autofocus field of view being sampled at any given point in time. Further, the response of the focus sensor  208  may be sensitive to the electrical or optical properties of the various layers within the autofocus field of view, thereby further confusing the response of the system  200  and making the actual plane of inspection difficult to determine. 
     Because of the repeating nature of the reticle fields  112 , equivalent XY reticle field positions can be inspected at the same Z position relative to some reference surface. However, if it is desired to keep a specific layer  104  in focus, where the layer  104  does not reside at all locations on the substrate  214  at a set depth relative to the upper surface as described in regard to  FIG. 1 , then such a simplistic focusing mechanism is insufficient. 
     Thus, in an advanced mode of operation, the tool  200  senses the topography information  102  from the substrate  214  during a setup process, and compensates for the topography information  102  based on XY location in a feed-forward manner during an inspection process. This process produces what can be thought of as a virtualized surface for the substrate  214 . In this manner, setting the focal plane  106  to a given offset from the virtualized surface keeps the focal plane  106  at the desired layer  104 , as depicted in  FIG. 1 . 
     Thus, the tool  200  compensates for the topography of the substrate  214  using a feed-forward method. However, the tool  200  can still dynamically compensate for variable influences such as Z vibration and the bow of the substrate  214  in a feed-back manner. 
     Setup Process 
     The substrate  214  is mounted to a chuck  216  which is mounted to a Z motor  218  which is mounted to an XY stage  204 . The XY stage  204  scans the substrate  214  in the XY plane at a fixed height as measured by the chuck sensor  212 , so that the focus sensor  208  can detect the surface topography  102  of the substrate  214  at given discrete XY locations of the substrate  214 , thereby developing an XYZ map of the topography  102  of the substrate  214 . Alternately, only a portion of the substrate  214  is scanned, such as a single reticle field  112 . 
     This map of the topography  102  of the substrate  214  (or reticle field  112 ) is then further processed to identify topography that is common to all identical reticle field  112  locations across the substrate  214 . Topographic features that are not common to all identical reticle field locations  112  across the substrate  214  are mathematically removed from the map data and an averaged reticle field topography map is constructed and stored in a reticle field position offset table  206 , which can be located either in the tool  200  or in some accessibly location external to the tool  200 . 
     In some embodiments, the topography map is measured and stored only once for a given substrate containing an integrated circuit  100  at a particular process step, and then is used thereafter during the inspection of all equivalent types of substrates of integrated circuits  100  at the same process step. In other embodiments, mathematical models of the integrated circuit  100  are used to create the topography map, such as might be developed from the design files for the integrated circuit  100 . In other embodiments, the topography map is acquired by keeping the autofocus sensor output constant during the XY mapping process and reading the chuck sensor position at each discrete XY location. 
     Thus, a map of the reticle field  112  topography is constructed, but not of the substrate  214  topography. 
     Inspection Process 
     The substrate  214  topography includes both the reticle field  112  topography and other things like the bow of the substrate  214  and the bow of the chuck  216 , bumps on the chuck  216 , and so forth. Only the reticle field  112  map is played back (subtracted from the focus sensor  208  output signal) during the inspection process. The key to the playback is that the current XY stage  204  position is used as the memory address for the memory bank containing the reticle field  112  topography map. In some embodiments there is no dynamic focusing element for the imaging optics  210 . The only thing that is moved to control the image focus is the stage Z, using the Z motor  218 . This keeps the optics for the image sensor  210  focused at a given level, regardless of the surface topography. In other embodiments the moving element is not the Z stage with the Z motor  218 , but rather a focusing element in the optical path. 
     In some embodiments, the topography map that is stored in the position offset table  206  is played back during the inspection process in the opposite polarity, so as to cancel the integrated circuit topography response of the auto focus sensor  208 . This topography cancellation signal is based on the XY location of the field of view of the image sensor  210 . Using this feed-forward method, the integrated circuit  100  topography  102  is no longer tracked up and down by the image sensor  210  optics, and the ideal “planar” response  106  is obtained while still maintaining the ability of the system  200  to track out chuck contamination Z disturbances. 
     The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.