Abstract:
A substrate inspection system of a type that receives substrates disposed within a cassette and inspects a planar surface of the substrates with a read head, where the substrates are inspected while they are disposed within the cassette, and the read head is of a size to fit between adjacent substrates within the cassette. In this manner, the substrates do not need to be removed from the cassette, and no robotic arm is required to do so.

Description:
FIELD 
   This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to inspecting the substrates on which integrated circuits are fabricated. 
   BACKGROUND 
   Integrated circuits are often formed on substrates, such as substrates of semiconducting material. Such substrates can hold as few as one or many as thousands of the integrated circuits. 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, and charge coupled devices. 
   Integrated circuits are typically formed in a series of process steps, where materials are first added to the substrate in layers, then patterned, and finally etched or otherwise altered before another layer is added. Typically it is extremely difficult to go back and fix a layer that has been improperly formed, after another layer has been formed on top of the malformed layer. For this and other reasons, the substrates are usually given frequent and thorough inspections, such as optical inspections. 
   A substrate is typically inspected by removing the substrate from the cassette in which it and other substrates are held, and moving the substrate into an inspection system. This substrate handling is usually performed by a robot arm that has been fitted with a head that can engage the substrate. The inspection system typically includes a motorized stage that moves the substrate around underneath the inspection optics. The inspection optics remain in one place, because they tend to include large glass lenses that—because of their weight, size, and delicate nature—cannot be moved as easily as the substrate. Once the inspection is complete, the robot arm removes the substrate from the stage and places it back into the cassette, from which another substrate is removed, and the inspection process is repeated. 
   Unfortunately, the robot arms required to move the substrates tend to be rather expensive. Further, moving the substrates around in this manner tends to require an appreciable amount of time. 
   What is needed, therefore, is a system that overcomes problems such as those described above, at least in part. 
   SUMMARY 
   The above and other needs are met by a substrate inspection system of a type that receives substrates disposed within a cassette and inspects a planar surface of the substrates with a read head, where the substrates are inspected while they are disposed within the cassette, and the read head is of a size to fit between adjacent substrates within the cassette. In this manner, the substrates do not need to be removed from the cassette, and no robotic arm is required to do so. 
   In various embodiments according to this aspect of the invention, relative movement between the substrate and the read head produces scan lines that are assembled into an inspection image of the substrate. In some embodiments, movement of only the substrate relative to the read head produces the scan lines that are assembled into an inspection image of the substrate. In other embodiments, movement of only the read head relative to the substrate produces the scan lines that are assembled into an inspection image of the substrate. In yet other embodiments, movement of both the substrate and the read head produces the scan lines that are assembled into an inspection image of the substrate. An air bearing is disposed on the read head in some embodiments, where the air bearing rides along the surface of the substrate and sets a working distance between the substrate and the read head. Some embodiments have multiple read heads, where the substrate inspection system is operable to simultaneously inspect more than one substrate while the substrates are disposed within the cassette. 
   According to another aspect of the invention there is described a substrate inspection system of a type that receives substrates disposed within a cassette and inspects a planar surface of the substrates at a working distance with a read head, where the working distance is set with an air bearing disposed on the read head, and the air bearing rides along the surface of the substrate. 
   According to yet another aspect of the invention there is described a substrate inspection system of a type that receives substrates disposed within a cassette and inspects a planar surface of the substrates with a read head, where the substrate is inspected by scanning the read head across the surface of the substrate while the substrate remains stationary. 

   
     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. 1  is a functional block diagram of an inspection system according to an embodiment of the present invention. 
       FIG. 2  is a representation of motion between a substrate under inspection and a read head of an inspection system according to an embodiment of the present invention. 
       FIG. 3  is a representation of scan lines in an inspection pattern on a substrate according to a first embodiment of the present invention. 
       FIG. 4  is a representation of scan lines in an inspection pattern on a substrate according to a second embodiment of the present invention. 
       FIG. 5  is a representation of scan lines in an inspection pattern on a substrate according to a third embodiment of the present invention. 
       FIG. 6  is a representation of scan lines in an inspection pattern on a substrate according to a fourth embodiment of the present invention. 
       FIG. 7  is a representation of an inspection system according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   According to various embodiments of the present invention, the substrate to be inspected remains in the cassette during the inspection process. In prior art inspection systems, such an arrangement would be impossible because of the size of the inspection optics. The means by which this is accomplished in the present system are described in more detail hereafter. 
   With reference now to  FIG. 1 , there is depicted a functional block diagram of an inspection system  10  according to an embodiment of the present invention. The inspection system  10  illustrated includes a substrate handling subsystem  12 . The substrate handling system  12  preferably does not remove the substrate from the cassette  40  (depicted in  FIG. 7 ) in which it is initially disposed. Rather, the substrate handling system  12  has one component that engages and retains the cassette  40 , and in some embodiments a second component that engages and retains the substrate within the cassette  40 . 
   For example, the first component that engages and retains the cassette  40  is, in one embodiment, a cassette  40  elevator. Substrates are typically disposed within slots that are formed in the cassette  40 , in an orientation where the front of a first substrate is facing the back of the next substrate, and so on, as generally depicted in  FIG. 7 . Cassettes  40  may hold any number of substrates, such as twenty-five. An elevator is a device that engages and retains a cassette  40 , typically with the substrates held in a horizontal orientation, and can then move up and down in steps, so as to index the position of the cassette  40  and dispose each substrate in turn at a given elevation. This is referred to as movement in the Z direction. In some embodiments the first component is operable to translate the cassette  40  in an X direction. Also, in some embodiments the first component is operable to translate the cassette  40  in a Y direction. 
   The second component that is present in some embodiments has one or more different functions. In one embodiment the second component holds the substrate within the cassette  40  and prevents it from moving. In another embodiment the second component rotates the substrate, either in only one direction, or selectively in either direction. In some embodiments the second component is not provided at all, such as when gravity alone is used to keep the substrate in place within the cassette  40 . 
   The first and second components of the relative motion means  16  are used in one embodiment to align the substrates to a given orientation prior to being inspected. In other embodiments, this alignment is accomplished using other equipment prior to placing the cassette  40  in the inspection system  10 . In yet other embodiments, no physical prealignment of the substrates  40  is performed at all. In some of these embodiments, a virtual alignment of the substrate  26  is performed, such as after an image of the substrate  26  is formed, as described in more detail hereafter. 
   A read head  14  is provided to inspect the substrate. The read head  14  is disposed on an armature  24  (as depicted in  FIG. 2 ), which in one embodiment has a length that is sufficient to dispose the read head  14  at any desired point across the surface of the substrate  26 . Further, both the read head  14  and the armature  24  in one embodiment are sized such that they fit between adjacent substrates  26  as they reside within the cassette  40 . In this manner, the read head  14  can be used to inspect a substrate  26  without removing the substrate  26  from the cassette  40 . 
   Means  16  are provided in some embodiments to provide relative motion between the substrate  26  and the read head  14 . This may take the form of moving one or both of the read head  14  and the substrate  26 , as depicted in  FIG. 2 . In various embodiments, the relative motion system  16  provides one or more of the motions indicated in  FIG. 2 , which include translation of the read head  14  in the Y direction  28 , translation of the read head  14  in the X direction  30 , rotation or sweeping back and forth of the read head  14  in a rotary manner  32 , translation of the substrate  26  in the Y direction  38 , translation of the substrate  26  in the X direction  36 , and rotation of the substrate  26  in either one or both directions in a rotary manner  38 . 
   By moving one or both of the substrate  26  and the read head  14  relative to other, while the substrate  26  remains within the cassette  40 , a desired portion of the substrate  26  can be inspected, where the desired portion can include anything from a single point on the substrate  26  up to the entire surface of the substrate  26 . 
     FIGS. 3-6  illustrate various scan patterns that are possible with the inspection system  10 . For example,  FIG. 3  depicts a horizontal linear scan pattern. This scan pattern could be formed by a variety of different embodiments of the system  10 . For example, the scan pattern could be formed by moving the read head  14  in and out of the cassette  40 , indexing the position of the read head  14  as each scan line is completed, and while holding the substrate  26  motionless or allowing it to just reside within the cassette  40  without any additional means to hold it motionless. 
   In another embodiment, the cassette  40  could be moved back and forth in a similar manner, while the read head  14  is held motionless. In yet another embodiment, the read head  14  is moved in an out, while the cassette  40  is translated in the X direction  24  to provide the index between the scan lines. Thus, it is appreciated that the scan pattern could be provided in a great variety of different ways. 
     FIG. 4  depicts a vertical scan pattern, which could be formed in much the same manner as the pattern described in regard to  FIG. 3 . Once again, motion for the scan pattern could be provided entirely by the read head  14 , entirely by the substrate  26  as it resides within the cassette  40 , or by a combination of the two. 
   Yet another scan pattern is depicted in  FIG. 5 , where concentric circles of scan lines are used to form the scan pattern. This scan pattern could be formed, for example, by rotating the substrate  26  in a desired direction  38  while it resides within the cassette  40 , and as one circumference is completed, either translating the cassette  40  in direction  36  or  38  so as to index the scan pattern, or translating the read head  14  in direction  28  or  30 , or sweeping  32  the read head  14  across the surface of the substrate  26 , so as to index the scan pattern. In some embodiments only one of these motions is used, and in other embodiments a combination of these motions are used to produce the scan pattern. 
     FIG. 6  depicts another scan pattern, which can be formed such as by sweeping  32  the read head  14  back and forth across the surface of the substrate  26  as is resides within the cassette  40 , and then translating either the cassette  40  or the read head  14  to create the indexing between the scan lines of the scan pattern. As before, the desired motion can be created in a great variety of different ways. 
   In some embodiments only the read head is operable to move in the X and Y directions, such as by translations  28  and  30  and sweeps  32 , while the cassette  40  only moves during inspection operations in the Z direction, such as to step from one substrate  26  to another. In other embodiments, this motion in the Z direction is accomplished by the read head  14 , and the cassette  40  doesn&#39;t move at all during inspection operations, although it may move during operations whereby it is loaded and loaded from the inspection system. In some of these embodiments, the substrates  26  and cassette  40  are retained in the system  10  by gravity alone. 
     FIG. 7  depicts a simple embodiment of the inspection system  10 , where the read head  14  is moved during the inspection operations. The embodiment of  FIG. 7  also depicts an embodiment of the inspection system  10  that has multiple read heads  14 , which can be used to inspect more than one substrate at a time. In some embodiments, the read head  14  has multiple channels, such as multiple sensors, which can simultaneously sense either multiple properties of the substrate  26  or can sense the same property separately. 
   The rate of movement is, in some embodiments, in a range of from several cycles (scan lines) per second to several tens of cycles per second, in order to scan the entire substrate  26  within from about one minute to about five minutes, which is dependent at least in part upon the size of the substrates  26  to be scanned. The movement in some embodiments is also carefully controlled so as to not run off the edge of the substrate  26  or hit the sides of the cassette  40 . 
   The read head  14  and armature  24 , in one embodiment, have properties similar to a hard disk drive head slider. The read head  14  in this and some other embodiments contains the imaging optics. It sweeps back and forth across the substrate  26  to build up an image of the surface of the substrate  26 . 
   In one embodiment the read head  14  moves across the surface of the substrate  26  on an air bearing  42  (depicted in  FIG. 2 ) that maintains a constant distance between the read head  14  and the surface of the substrate  26 , and thereby maintains focus control. The fly height of the read head  14  can, in this embodiment, be adjusted by adjusting the air bearing  42  pressure, thus providing a control on the focus. The lens of the read head  14  can be similar to that used in a CD or DVD player. 
   In one embodiment, light is coupled in to and out from the read head  14  through fiber optic cables. The optics can be set up in a bright field mode, such as for macro-inspection, or in a dark field mode, such as for particle inspection. For higher throughput, a multi-channel design can be employed, such as by using multiple fiber channels, with either one larger lens or multiple mini-lenses on the read head  14 . Other applications, such as layer thickness monitoring, could be enabled, such as through spectroscopic analysis. 
   A signal processing unit  18  receives the electrical or optical signals from the read head  14 , and provides image data to an image processing unit  20 . The image processing unit  20  assembles the incoming data stream into an X-Y image and, in some embodiments, provides a coordinate transform to convert the image from its original form, such as circular, polar, or curvilinear coordinates, to X-Y Cartesian coordinates. This information is provided to a control and analysis unit  22 , which controls the various functions of the inspection system  10 , and performs various comparisons (such as die to die, substrate to substrate, or absolute gray level) to detect defects of interest or process issues. Depending on the resolution and data rate of the inspection system  10 , the image processing system  20  and the control and analysis system  22  may be a single processing node, such as a personal computer, or for higher resolutions and throughput, a cluster of processing nodes. 
   By leaving the substrate  26  within the cassette  40 , the various embodiments of the present invention save the time that is otherwise required for transporting and aligning the substrate  26  within the inspection system  10 , along with the associated cost of the robotics. Using miniaturized lenses in the read head  14  and controlling the working distance with an air bearing  42  reduces the cost and complexity of the optics and associated supporting mechanics, such as a Z-focus stage. 
   As introduced above, various embodiments of the inspection system  10  are amenable to a variety of applications, such as (but not limited to) particle inspection, pattern inspection, overlay metrology, critical dimension metrology or scatterometry, film thickness measurement, and surface flatness measurements. 
   The foregoing description of preferred 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 the best 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.