Abstract:
A system for simultaneously inspecting the frontsides and backsides of semiconductor wafers for defects is disclosed. The system rotates the semiconductor wafer while the frontside and backside surfaces are generally simultaneously optically scanned for defects. Rotation is induced by providing contact between the beveled edges of the semiconductor wafer and roller bearings rotationally driven by a motor. The wafer is supported in a tilted or semi-upright orientation such that support is provided by gravity. This tilted supporting orientation permits both the frontside and the backside of the wafer to be viewed simultaneously by a frontside inspection device and a backside inspection device.

Description:
This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/101,400, filed on Sep. 22, 1998, pursuant to 35 U.S.C. Sections 111 and 119(e). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to contamination inspection for semiconductor wafers and the like and in particular to a system which inspects both the frontside and backside of a semiconductor wafer without manual or automatic inversion of the wafer. 
     2. Description of the Related Art 
     Tools used in the semiconductor wafer manufacturing process must periodically be checked to determine whether they must be replaced or are still in usable condition. The condition of a tool is checked by inspecting wafers processed by that tool for defects. Bare wafers are typically routed through the process tool with the frontside facing up, and wafer defects detected optically by illuminating portions of the wafer and measuring the amount of illuminating light scattered by defects on the wafer surface. 
     Previously, systems which performed inspection of wafers did so in two discrete stages. First, the frontside of the wafer was scanned for contamination caused by the process tool. If the defect rate on the frontside of the wafer was acceptable, the wafer was then turned over to inspect the backside for further particle contamination and other defects. 
     The process tool was considered usable if the defect rate on the backside of the wafer was also acceptable. 
     Inspection of both sides of a wafer by these procedures accordingly required time for inspection of one side, examination of the one side, inverting the wafer without excessively damaging the wafer, scanning the reverse side, and examining the results of the second side scan. In addition to this excessive amount of time required for examination, the process of flipping the semiconductor wafer had a tendency to contaminate the edges of the wafer due to surface or edge contact with a gripping device. In some processes, when the wafer was flipped over to inspect the backside, the front side of the wafer could be contaminated by the flipping process. 
     The resulting contamination of the frontside of the wafer tends to render the wafer unsuitable for further processing. 
     Thus, all test wafers were usually scrapped after each inspection, reducing overall productivity and increasing per unit cost. 
     Edge handling of wafers has also complicated the problem. As wafers tend to suffer from contamination or other degradation when handled by wafer orientation systems, the handling of a wafer requires special care. Although previous wafer orientation systems have included multiple drive rollers, radially inwardly-biased contact rollers, and a tiltable wafer-supporting table with an air-bearing mechanism, each of these handling methods have benefits and drawbacks. Systems without multiple drive rollers and radially inwardly-biased or spring-loaded contact rollers cannot maintain steady wafer rotation rate during the portion of a cycle in which the drive roller is not in contact with the round edge of the wafer because the drive roller loses traction along the wafer edge. 
     In inspection equipment, it is important to maintain steady rates of wafer rotation to avoid errors in defect detection, such as errors in detecting defects where none exist, or simply failing to detect defects. Previous systems which supported semiconductor wafers through direct contact with a solid surface present special problems during inspection since contact with the support surface may increase contamination or move defects from one location to another in ways that render the wafer unsuitable for future processing. 
     It is therefore an object of the current invention to provide a system for minimizing the time required for full inspection of both the front side and back side of a wafer. 
     It is another object of the current invention to provide an arrangement which minimizes overall wafer contamination during the inspection process, particularly when inspecting both front and back sides of the wafer. 
     It is a further object of the current invention to minimize edge handling concerns, such as contamination, during the inspection of the front side and back side of a wafer. 
     It is still a further object of the current invention to minimize the number of defects missed or falsely detected by the inspection system. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an apparatus that simultaneously inspects the frontsides and backsides of semiconductor wafers for defects. The inventive system disclosed herein may also read tracking information imprinted on the backsides of the semiconductor wafers. 
     The invention rotates the semiconductor wafer while the frontside and backside surfaces are generally simultaneously optically scanned for defects. Rotation is induced by providing contact between the beveled edges of the semiconductor wafer and roller bearings rotationally driven by a motor. 
     In the present invention, a semiconductor wafer is supported such that the semiconductor wafer lays flat during the inspection process. The surface is large enough to accommodate the wafer as well as the rollers for rotating the wafer and the means for holding the wafer. The wafer is preferably supported in a tilted or semi-upright orientation such that support is provided by gravity. This tilted supporting orientation permits both the frontside and the backside of the wafer to be viewed simultaneously by a frontside inspection device and a backside inspection device. The backside of the wafer for purposes of this invention is the side of the semiconductor wafer by which the wafer is being supported. Simultaneous dual-side inspection of the front side and back side of the wafer effectively doubles the throughput of inspection equipment and eliminates the need to turn the semiconductor wafer over during the inspection process, thereby reducing the opportunity for edge contamination of the inspected wafer. 
     The wafer is rotated by multiple motor-driven roller bearings. These drive rollers are positioned at the circumference of the wafer and are angled such that the roller pads contact the wafer only along the beveled edge. This periphery positioning and rotation coupled with angular contact between the rollers and wafer edge and surface permits inspection of the entire surface and significantly reduces the potential for contamination of the surface resulting from edge contact, or contact with the roller pads. 
     The drive rollers are spaced apart such that at least one of the two drive rollers spaced farthest apart contacts the round edge of the wafer throughout the rotation cycle. This constant contact feature ensures that the rotation rate of the wafer is suitably steady during defect inspection. Also, the steady rotation rate minimizes the number of defects missed or falsely detected by the inspection system. 
     The wafer rotation rate is such that roller contact does not damage the wafer edge. Furthermore, defects are not carried or transported from one part of the edge to another. Moreover, the rate should be controlled so as to minimize slip between the roller and the wafer edge. The present invention is intended for use at wafer rotation rates on the order of 400 revolutions per minute. Unlike previous systems, the present invention does not exhibit excessive vibration for defect inspection purposes at these rotation rates. The increased wafer rotation rate also increases the throughput of inspection equipment. 
     The semiconductor wafer is held against the drive rollers by pressure using a set of undriven roller bearings (contact rollers) or alternatively simply using gravitational force by tilting the wafer and inspection surface. This pressure ensures that the drive rollers hold traction on the beveled wafer edge so that a steady rotation rate can be maintained. All contact rollers thereby maintain contact with the edge of the semiconductor wafer throughout the rotation cycle. 
     Prior to inspection, the system locates the edge registration feature, commonly called the “flat”. The system detects the specific position of the wafer using the edge registration feature either by measuring the position of the contact rollers, or by connecting the contact rollers to switches which are turned on when the contact rollers are touching a flat registration edge calibration switch. Once the flat registration edge or notch is located, the system rotates the wafer to desired orientations for inspection purpose by controlling the drive rotors. 
     Other objects, features, and advantages of the present invention will become more apparent from a consideration of the following detailed description and from the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of the preferred embodiment of the invention in an unloaded state; 
     FIG. 2 presents a perspective view of the preferred embodiment of the invention loaded with a semiconductor wafer to be inspected; 
     FIG. 3 is a perspective view of the preferred embodiment of the invention with the semiconductor wafer loaded and the table surface tilted in the scan position; 
     FIG. 4 is a plan view of an arrangement including the loaded semiconductor wafer, roller bearings and scan head elements; 
     FIG. 5 presents a perspective view of the scan head CCD detector elements arranged in relation to the surface of the semiconductor wafer during backside inspection; and 
     FIG. 6 illustrates a perspective view of an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 present various views of the invention in the loaded and unloaded states. From FIG. 1, the background contamination inspection device is initially in its unloaded state, or without a semiconductor wafer located thereon. The semiconductor wafer is supported by a substantially flat table surface  101 . The substantially flat table surface  101  is equipped with an air-bearing mechanism  102  upon which the semiconductor wafer may be floatably supported to eliminate contamination of the backside by contact with the table surface  101 . The table surface  101  is mounted to a fixed base  103  such that the table surface  101  can tilt about an axis  104  defined at a side edge of the table surface  101 . Four wafer load pins  105   a - 105   d  are mounted on the table surface  101  such that they can retract and temporarily maintain the wafer. The wafer load pins  105   a-d  are located in a circular pattern concentric with the air-bearing mechanism  102  and semiconductor wafer which is to be loaded. Furthermore, the wafer load pins  105   a-d  are located proximate the round edge of the semiconductor wafer to be loaded. 
     Roller bearings  106   a-d  are rotatably mounted on the table surface  101  in an orientation substantially equivalent to the angle or axis  104  about which the table surface  101  is tilted. Roller bearings  106   a-d  are further arranged in a circular pattern having substantially the same center as the air-bearing mechanism  102  and the semiconductor wafer to be loaded such that the radius of the smallest circle simultaneously tangent to all of the roller bearings  106   a-d  is equal in length to the radius of the semiconductor wafer to be loaded. As shown in FIG. 1, roller bearings  106   a  and  106   b  are driven by motors (not shown) and are separated by such a distance that both cannot simultaneously contact the flat, or registration edge, in the semiconductor wafer. Thus roller bearings  106   a-d  provide continuous driving of the wafer when loaded thereon. 
     Prior to inspection, the system locates the edge registration feature, commonly called the “flat”. The system detects the specific position of the wafer using the edge registration feature either by measuring the position of the contact rollers, or by connecting the contact rollers to switches which are turned on when the contact rollers are touching a flat registration edge calibration switch. Once the flat registration edge or notch is located, the system rotates the wafer to desired orientations for inspection purpose by controlling the drive rotors. 
     The scan head  107  is situated within the table channel  108 . Table channel  108  passes completely through the top and bottom surfaces of table surface  101 . The table channel  108  is symmetric about the radius of the semiconductor wafer and is of such length that the scan head  107  may travel from a position directly beneath the center of the semiconductor to a position directly under the outer edge of the wafer. The preferred scan head is shown in greater detail in FIG.  4 . 
     FIG. 2 shows the preferred embodiment of the invention having the semiconductor wafer  201  loaded thereon. The semiconductor wafer  201  is floatably supported by the air-bearing mechanism  202 . During the loading process, the wafer load pins  206   a-d  hold and center the semiconductor wafer  201  over the air-bearing mechanism  202 . Once the operator or software determines that the semiconductor wafer  201  is centered over the table surface  201 , the wafer load pins  206   -d  are partially retracted and no longer contact the edge of the semiconductor wafer  201 . 
     FIG. 3 shows the background contamination inspection device in scan position. The table surface  101  in FIG. 3 has been tilted to a predetermined angle about axis  304 . The driven roller bearings  306   a  and  306   b  are continuously kept in contact with the wafer edge by the gravitational force acting on the semiconductor wafer  301  due to tilting. The tilting of the semiconductor wafer  301  permits high speed rotation of the semiconductor wafer and minimizes the amount of pressure exerted on the edge of the wafer  301  while still ensuring that at least one drive roller maintains contact and traction along the edge of the wafer throughout the wafer rotation cycle. Edge contact is therefore minimized since no undriven contact rollers are needed. 
     The wafer loading pins  305   a-d  are fully retracted when the invention is in the scan position and thus only contact the semiconductor wafer during the loading phase of the inspection. The wafer loading pins  305   a-d  do not contact the wafer during rotation or while the system is in the inspection phase. 
     Once the semiconductor wafer  301  has been loaded onto the table surface  301 , the wafer loading pins  305   a-d  are retracted, the table surface  301  tilted as shown in FIG. 3, and the drive rollers  306   a  and  306   b  are turned to rotate the semiconductor wafer  301 . The semiconductor wafer  301  is rotated by the motor (not shown) turning the drive rollers  306   a  and  306   b . Positioned within the table surface  301  is the scan head  307  (not shown) which traverses in a linear manner to scan the backside of the semiconductor wafer  301 , i.e. the side of the wafer adjacent to the table surface  301 . The scan head  307  is positioned within the table surface channel  308  such that the orientation of the scan head  307  does not change relative to the semiconductor wafer  301  as the table surface  301  is tilted to the position shown in FIG.  3 . During rotation of the table surface  301 , the scan head  307  translates linearly within table surface channel  308  in a parallel orientation with respect to the bottom surface of the semiconductor wafer  301 . While the semiconductor wafer  301  rotates adjacent to the wafer table  301  using drive rollers  306   a  and  306   b , the scan head  307  translates within the table surface channel  308 , moving from the edge of the semiconductor wafer  301  to the center thereof, or vice versa. 
     Various tilting angles may be employed in the current system while still within the scope of the present invention. The current desired tilting angle for the table surface is 45 degrees, but higher angles may be used successfully depending on the speed of the rotation of the semiconductor wafer  301  and the size and particularly weight of the wafer  301 . For example, an excessively high angle between the table surface  301  and the horizontal may cause the wafer  301  to fall away from the table surface, while a relatively small angle between the table surface  301  and the horizontal may cause the wafer  301  to lose contact with the drive rollers  306   a  and  306   b . It is therefore preferable to maintain the angle of tilt within the range of 15 degrees from horizontal to 75 degrees from horizontal. 
     FIG. 4 illustrates the backside inspection process. Backside inspection is preferably performed using the double-dark field method. Roller bearings  404  are rotated by a drive motor (not shown) to induce rotation of the semiconductor wafer  401 . The roller bearings  404  illustrated in FIG. 4 represent an alternate orientation of the roller bearings from those shown in FIGS. 1-3. The roller bearings  404  of FIG.  4  and the undriven roller bearings  405  may be originally oriented away from the table surface (not shown) for purposes of loading the wafer  401  onto the table surface, and then the driven and undriven roller bearings may be repositioned adjacent the wafer  401  to provide sufficient but not excessive contact between the bearings  404  and  405  and the wafer  401 . The orientation of the elements illustrated in FIG. 4 contemplates a horizontal and untilted arrangement of the wafer and bearings, but the optical elements of FIG. 4 may be used in the tilted orientation of the invention illustrated in FIGS. 1-3. 
     In FIG. 4, the wafer  401  maintains contact with both the driven roller bearings  404  and the undriven roller bearings  405 . During operation, as semiconductor wafer  401  rotates, the scan head  407  (not shown), including laser illuminator  402  and sensor  403 , travels along the table surface channel (not shown) in close proximity to the surface being scanned. The sensor  403  may include one or more CCD detector elements. The laser illuminator  402  projects an elongated illuminating beam onto an area roughly 50 μm×10 mm in size, illustrated by the illuminated patch  406  in FIG. 4, on the surface of the semiconductor wafer  401  at a non-normal angle of incidence. 
     FIG. 5 shows the arrangement of the CCD detector elements relative to the semiconductor wafer  501 . The illuminator (not shown) projects collimated beam  502  through cylindrical lens  503  onto illuminated patch  504  on  4 the surface of the semiconductor wafer  501 . CCD detector elements  505  are symmetrically located on either side of and parallel to the incident plane (the plane formed by the intersection of the wafer surface normal and the illumination path). The CCD detector elements  505  are linear and produce a serial read-out which corresponds to the amount of scattered light received by the detector. This output is used to determine whether a defect exists at the particular section of the wafer being examined. Using this information, the system determines whether the wafer  501  may be used in further processing. If the system determines that the wafer  501  is not usable, the process tool must be replaced and the wafer  501  is scrapped. If the wafer  501  is usable, the defect location information for the particular wafer is stored with its tracking number. The wafer  501  is then placed back in the processing stream and the process tool is not replaced. 
     FIG. 6 illustrates an alternate, stand-alone embodiment of the present invention. In this embodiment, the table surface  601  is affixed to base  603 . Base  603  is mounted to support legs  602  such that the base  603  may be rotated about axis  604 . Scan head  607  is fixedly mounted to arm  605 , and arm  605  is attached to turning screw  606 . Turning screw  606  is rotationally coupled to a motor (not shown). 
     Rotation of turning screw  606  causes arm  605  and scan head  607  to move laterally along the table surface channel  608 , parallel to the backside of semiconductor wafer  611  in its tilted state (as shown) or untilted state. This motion of the scan head  607  permits scanning of the back side of the semiconductor wafer  611 . The semiconductor wafer  611  is rotated by contact with roller bearings  609  which are driven by a motor (not shown). The semiconductor wafer  611  also maintains contact with roller bearing  610  (second roller bearing not shown), which is undriven. The contact with undriven roller bearing  610  is due to gravitational force being exerted on the semiconductor wafer  611 . Thus the orientation of the wafer, as shown, is in constant contact with the rollers and may be inspected on both front and back sides. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within known and customary practice within the art to which the invention pertains.