Abstract:
A ring chuck that holds a wafer with a vacuum uses a vacuum trough that contacts the entire outer edge of the wafer. The chuck has a base having a top surface equal to or slightly smaller than a water to be tested with vacuum channels in the base. The base provides the mechanism to connect the chuck to a measurement instrument and a vacuum source. An annulus of non-contaminant material that has concentric rings extending upward from its outer edge is fixed to the base top surface with the trough between the concentric rings connected to the vacuum channels. The vacuum trough holds the wafer securely to the chuck and minimizes vibrations when the wafer is rotated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. §120 to prior U.S. application Ser. No. 09/934,038 filed Aug. 21, 2001, entitled “RING CHUCK TO HOLD 200 AND 300 MM WAFER”, as well as under 35 U.S.C. §119(e) of U.S. provisional application No. 60/227,071, filed Aug. 22, 2000, the disclosures of which are incorporated herein by reference. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    N/A  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates generally to semiconductor test equipment and, more specifically, to chucks used to hold wafers to be tested. Semiconductor manufacturers test semiconductor wafers throughout the manufacturing process but especially before they are processed to produce integrated circuits. The flattest wafers with the smoothest surface have been shown to yield the highest number of acceptable integrated circuits. Therefore, there is an increasing demand to produce test equipment that can detect microscopic defects in the wafers.  
           [0004]    Integrated circuits are predominantly manufactured from 200 mm or 300 mm semiconductor wafers. The wafers incorporate a fiducial mark, either a flat in the normally circular edge or a small notch in the edge. These fiducials provide a reference point for identifying the position of measurements on the wafer surface. Typically the outer edge of the wafer surface, approximately a 3 mm wide exclusion band, is an area from which integrated circuits are not manufactured. The fiducials are contained within this exclusion band.  
           [0005]    The desire to measure wafers with more precision and predictability leads to the need to support a wafer with minimal and predictable deformation while maximizing stiffness. Wafers are fragile, and can only be touched by materials that will not transfer contaminants to the surface. In addition, the equipment handling the wafer must not scratch it or permanently damage it. Vacuum chucks are well suited to the purpose, but the wafer is thin enough that an image of a chuck touching one surface will be discernible on the other surface.  
           [0006]    Prior art test equipment has held wafers to be tested using chucks with a number of vacuum ports spaced beneath the wafers to hold them securely. Chucks designed for minimal vibration have included types displaying full backside contact or having pin contacts spaced regularly across the back. The chucks must be designed to introduce minimal errors into the wafer production and testing process. Errors may be introduced by contamination from chuck material contacting the wafer. A second error source is particle transfer from the surface of the chuck. In addition, particles already on the surface of the wafer may be forced into the surface by the contacting surfaces of the chuck. The force of the vacuum on the wafer will cause an effect termed print-through where the image of the contacting points of the chuck are detectable by measurement equipment on the opposite surface of the wafer. Prior art chucks have not succeeded in holding wafers securely without sacrificing a measure of contact contamination.  
           [0007]    Once the wafer is securely held by the chuck, the test equipment rotates the wafer beneath a sensor to test the entire surface. The accuracy of measurements taken this way is limited by the stiffness of the wafer, the vibration of the wafer, the contamination of the surfaces, the amount of print through from the chuck and the instrument noise that cannot be factored out of the measurements taken. Very slow rotation speeds, that limit the test throughput, have been necessary to limit the effects of vibration.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    A ring chuck holds a wafer in a horizontal or any other orientation with a vacuum that supports the entire outer edge of the wafer. The chuck is mounted to a measurement instrument and has a base having a top surface equal to or slightly smaller than the wafer to be tested with vacuum channels in the base. An annulus of non-contaminant material that has a plurality of concentric rings extending upward from its top surface close to its outer edge is affixed to the base and the trough between the concentric rings is connected to the vacuum channels. The vacuum trough holds a wafer securely to the chuck and minimizes vibrations when the wafer is rotated. When the plurality of concentric rings are contained within the wafer exclusion band, the print through onto the tested area is minimized. In one implementation, the tops of the concentric rings are located close to the top surface of the base and provide a sealed volume to support the interior portion of the wafer.  
           [0009]    The tops of the concentric rings are made very narrow to minimize contamination of the wafer and to restrict the transfer of particles between the chuck and the wafer. In addition, the narrow rings limit the number of particles already on the back side of the wafer that are forced onto the wafer. The consistent support provided by the ring chuck simplifies modeling of the chuck/wafer system, improving the ability to remove the instrument signature from measurements.  
           [0010]    Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0011]    The invention will be understood from the following detailed description in conjunction with the drawings, of which:  
         [0012]    [0012]FIG. 1 is a perspective drawing of a prior art chuck to hold a wafer;  
         [0013]    [0013]FIG. 2 is a top view of a chuck according to the invention;  
         [0014]    [0014]FIG. 3 is a perspective view of a chuck according to the invention;  
         [0015]    [0015]FIG. 4 is a detail cutaway of the chuck of FIG. 3;  
         [0016]    [0016]FIG. 5 is a cross section of the chuck of FIG. 3;  
         [0017]    [0017]FIG. 6 is an alternate version of the chuck according to the invention; and  
         [0018]    [0018]FIG. 7 is a block diagram of equipment utilizing the capabilities of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    A prior art chuck  2  is shown in FIG. 1. The chuck  2  consists of a base  4  with three vacuum chuck columns  6  projecting from the base. Each column  6  has a sealing ring  5  on the upper perimeter of the column  6  and a depression  7  with a vacuum port within the sealing ring  5 . A wafer (not shown) is placed above the chuck  2  and is held to the chuck  2  by the suction exerted by the vacuum columns  6 . The force of the vacuum must be sufficient to hold the wafer securely and therefore print-through of the sealing rings  5  and vacuum depressions  7  of the column heads is expected. The centering of the wafer on the vacuum columns  6  must be repeatable in order to allow data analysis to remove some of the print-through artifact from the measured data. In addition, the unsupported parts of the wafer vibrate, further distorting the measurements and increasing the demand for repeatable positioning. Modeling of the chuck/wafer system can be attempted to identify some of the error sources, but empirical data is required to cope with the complexities in the system to define a partial instrument error signature.  
         [0020]    One measurement that needs to be performed in production due to the decreasing width of the etched lines in integrated circuits is a measurement of the smoothness of a wafer&#39;s surface. This is referred to as nano-topology. A means of achieving this is to record the slope data of a wafer scan, and integrate to construct the nano-topology. This scanning method allows for the high throughput required of production measurements. To measure the entire wafer surface, the wafer must be scanned, usually by rotating under a sensor, quickly. A system that is sensitive enough to discern the nano-topology will also be sensitive to surface irregularities, surface vibration and print through effects.  
         [0021]    The ring chuck  70  of the invention, illustrated in FIG. 2, is well adapted to these new requirements. The ring chuck  70  is comprised of a base  72 , as is known in the industry, and a hoop or annulus  74  of low contaminant material affixed to the top surface  75  of the base  72 . In one implementation, hoop  74  is affixed at the outer edge of top surface  75 . The base  72  is formed, as is known in the industry, with channels (not shown) for vacuum delivery, and mounting facilities to secure it to a measurement instrument. The top surface  75  of the base  72  may be essentially solid or may be shaped to aid the operation of the chuck. FIG. 2 illustrates mounting holes  78  to provide a means to fasten the chuck  70  to the instrument.  
         [0022]    The base  72  and hoop  74  are sized to correspond to the wafer to be tested. The ring chuck design is readily adaptable to wafers with diameters of 200 mm or 300 mm as well as other sizes. A number of concentric raised rings  76  circle the outer circumference of the surface of hoop  74  just in from the edge. Troughs  80  are formed in the space between the concentric rings. A vacuum source is introduced into troughs  80 . After a wafer (not shown) is placed on the ring chuck  70 , a relatively uniform vacuum is formed in trough  80  to hold the wafer to the ring chuck  70  around the entire circumference of the wafer. This mechanism holds the wafer and provides consistent support to all portions of the edge of the wafer.  
         [0023]    [0023]FIG. 3 illustrates a cutaway view of the ring chuck  70 . A channel vacuum distribution channel  82  is formed in the base  72  and through internal pathways (not shown) connects to a vacuum source (not shown). Further unshown channel passageways between the vacuum distribution channel  82  and the vacuum trough  80  are contained within the hoop  74 . The base  72  is machined or cast as is known in the industry to provide a seating area  92  to contain the hoop  74 .  
         [0024]    [0024]FIG. 4 is a cutaway drawing of the edge of a ring chuck  70  that uses two rings  76  showing a vacuum channel  82  within the base  72 . Distribution channels  100  and  102  bring the vacuum to trough  80  from the vacuum channel  82 . The width of the rings  76  is significantly narrower than the trough. In some implementations, the ratio of the width of the rings to the width of the trough is approximately  1 : 4 . The outer ring  76  is sufficiently spaced inward from the rim of the wafer  8  that a notch fiducial (not shown) does not break the integrity of the vacuum within trough  80 .  
         [0025]    [0025]FIG. 5 is a cutaway drawing of the chuck  70  with a wafer  8  where the scale of the circumferential detail is enlarged relative to the radius of the wafer  8 . The concentric rings  76  circumscribe the hoop  74  and are formed having peaks that are narrow in order to limit the contact of the rings with the wafer  8 . The hoop  74  has a height  94  that may be modified as needed to improve operation. The ring chuck  70  may be formed minimizing the height of the space between the base  72  and the wafer  8  by modifying the height of the hoop  74  and concentric rings  76 . By limiting this height, the volume of air trapped in the space dampens the vibration across the wafer and the wafer presents a more stable surface for test equipment inspection. The tapered cross section of the rings  76  is illustrated in FIG. 5. The concentric rings  76  make contact with the wafer only at the top of the rings  96 . The narrow tips  96  of the rings  76  are the only part of the hoop  74  making contact with the wafer  8  thereby minimizing contact between the chuck  70  and the wafer  8 . The possible contamination and area for transfer of particulate matter is minimized. It is desirable to have the total area of the rings contacting the wafer be significantly less than the area of the wafer. In particular, in an advantageous implementation with two rings  76 , the area of the ring tips  96  touching the wafer  8 , may be as small as 0.4% of the area of the wafer  8 .  
         [0026]    The vacuum that is maintained in trough  80  is proportional to the height and separation of the rings  76 . The vacuum area  80  between the concentric rings  76  is larger than the area of the contacting rings. This allows a greater vacuum to be maintained within the vacuum trough  80 . The area of the concentric ring tips  96  less than ½ of the area of the troughs.  
         [0027]    The vacuum and contact provided by the ring chuck  70  are readily modeled using ANSYS finite element analysis. The models make it easier to identify the error patterns in the data that are caused by the chuck. Because the contact between ring chuck  70  and wafer  8  is so regular and so close to the edge of the wafer  8 , the wafer  8  is held rigid with minimal vibration along the edge. The symmetricity and uniformity of the contact and vacuum make the interaction between the contact points and the wafer easy to model. Such models have been verified by experimentation to determine the instrument signature that is subtracted from the wafer measurement to improve the results of the readings. While interruptions in the concentric rings may seem desirable for purposes such as allowing paddle access to the wafer, the models for interrupted rings are significantly more complex and show more variation and vibration of the wafer. Experimental studies confirm that breaks significantly increase the vibration and complexity of the error data.  
         [0028]    [0028]FIG. 6 illustrates an alternate implementation of a ring chuck  70 ′ adapted to support a wafer (not shown) with a flat-edged fiducial. In this chuck, the concentric structures  76 ′ are circles with a flat side corresponding to the wafer flat. The trough  80 ′ still provides a continuous vacuum around the periphery of the wafer. The wafer must be placed on the chuck in an orientation that aligns the fiducial with the flat of the trough  80 ′. By placing the concentric rings  76 ′ and vacuum trough  80 ′ within the exclusion zone, the area for building IC&#39;s is freed from print through and other artifacts from the mounting.  
         [0029]    [0029]FIG. 7 illustrates an angle resolve scatter system  50  that is practical because of the increased stiffness of a wafer supported by the ring chuck of the invention. The accuracy of measuring the wafer&#39;s surface topography increases substantially using this design. This system can perform surface quality measurements (SQM) which measure the nano-typography of the wafer  8 . Whereas the prior art had too much vibration to allow a reflected light beam received by a detector to be quantitatively measured, the stillness of the wafer  8  using the ring chuck allows use of a quadrant detector  110  as a light sensor. The collimated, typically low power laser beam  22  is directed into a beam conditioning optic assembly  24 , which produces a telecentric, relatively short scan line  26 , directed onto the wafer surface  9  at an angle. The specular reflection  28  of this beam is collected by a receiving optic assembly  30 , which includes a lens and a quadrant detector  110 . The optical system is such that the quadrant detector  110  is at the Fourier Transform Plane (FTP) of the optical system. This results in a focused spot on the center of the detector  110 . By definition of the FTP, the spot will displace on the detector  110 , only as a function of a change in the angular orientation of the wafer surface  9  at the incident point. By differentially comparing the signals from the 4 quadrants of the detector  110 , the offset, and therefore the angular change is measured, corresponding to two orthogonal tilt axes of the wafer  8 . The optical system is insensitive to wafer surface height variations, or lateral displacement of the scanning beam  26 . A stage system  44 , coordinated with the short laser scan, is used to scan the entire surface of the test wafer  8 . The output of the quadrant detector  110  is used to generate a slope map of the surface  9  of the wafer. These slopes can be integrated to create a topographical map of the wafer, ranging from very fine surface characteristics to very gross surface characteristics.  
         [0030]    The electrical output of the quadrant detector  110  is pre-amplified  34 , then passed to a Digital Deflection Board  42 , which also accepts position signals  36 ,  38  from the stage system  44  and the laser scanning system  24 . The resulting information, which describes  2  axes of tilt for every position on the wafer  8 , is supplied to the PC  40 . Within the PC, algorithms are applied to convert slope data into topography data, and additional software modules provide the desired graphic and statistical presentation of the data.  
         [0031]    Embodiments described have positioned the ring chuck concentric rings and vacuum troughs within the exclusion band. Even when the fiducial utilizes some part of the exclusion zone, the ring chuck can be placed just inside this feature and still within the band. In alternate implementations, where the ring chuck mechanism is positioned inside the exclusion band, the print through artifacts can be readily removed from the measured data because the rings and troughs are regular structures.  
         [0032]    With contact rings having significantly less area in contact with the wafer than the vacuum area, the vacuum force that can be provided through the vacuum troughs can be increased from the industry typical force of 15 psi to higher levels. With these levels of force, the chuck and wafer may be spun up to vary high speeds, even beyond those sustainable by the wafer itself. Currently many measurements are taken at low rpm. More wafers can be tested with the greater testing capacity provided by using the higher rotational speeds.  
         [0033]    Hoops on the ring chuck may be manufactured of non-conductive, non-contaminating materials as are known in the semiconductor industry. In particular, hoops made of Poly-ethyl-ethly-ketone (PEEK) are considered to exhibit less contamination than others do and are well accepted in the semiconductor industry.  
         [0034]    In one implementation, the vacuum trough has been moved outboard such that the outer concentric ring is approximately 1.5 mm from the edge of the wafer. The vacuum trough area has been increased to increase the clamping force. Backside vacuum contact for this implementation applied to a 200 mm wafer is nearly 0.4%. This chuck can be used on scanning stations where the wafer is rotated at higher speed. The sag due to gravity is less compared to edge grip because of the contact throughout the edge of the wafer.  
         [0035]    To further limit the sag and vibration of the central portion of the wafer  8 , the volume between the top surface  75  of the base  74  and the wafer  8  is constructed so that the wafer/vacuum contact acts as a seal. The contained volume of air supports the interior of the wafer, dampening vibrations and limiting sag. In a further implementation, the volume between the base  74  and wafer  8  is pressurized after the vacuum seal has been established. By pressurizing slightly above atmospheric pressure, the weight of the wafer is supported by the volume of air. The pressurized air provides a predictable and uniform support to the wafer  8 .  
         [0036]    The uniform support and sealing capacity are enhanced when the annulus is manufactured of a slightly deformable material. When the vacuum is applied, the ring tips  96  deform against the backside of the wafer just enough to smooth out any small variations in the height of the tips  96 . This smoothed surface provides a good seal that minimizes vacuum leakage and maximizes the clamping force. It is known in the industry to select materials that will maintain the essential shape of the rings  76  while deforming slightly to improve the contact.  
         [0037]    Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.