Abstract:
Evaluating irregularities in surfaces of objects such as semiconductor wafers using a thickness profile of a surface section and analyzing the profile to obtain information of an irregularity start position, magnitude, and span along with surface slope and height information.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    N/A 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    N/A 
       BACKGROUND OF THE INVENTION 
       [0003]    This application claims all rights and priority on prior pending U.S. patent application Ser. No. 11/789,037 filed 2007.04.23. The present invention relates to the measurement of surface topology of semiconductor wafers prior to their fabrication by semiconductor manufacturers into various micro circuits and other similar devices. 
         [0004]    Semiconductor wafers are expensive real estate in that the manufacturers of semiconductor products need to know the quality of the surface area of the wafers, which can be as large as 300 cm across, before committing time, equipment and materials to process them into finished semiconductor products. If there is a sufficient perturbation in the surface profile of a portion of a semiconductor which could prevent its being processed into a finished product, that information needs to be known before the manufacturer proceeds so that that portion can be eliminated from processing. This problem is particularly severe in the edge areas of semiconductors where there tends to be a slope roll-off so that traditional thickness measurements by themselves are unlikely to reveal the presence of all or many bumps that affect the processing of finished semiconductor products. 
         [0005]    Current processing and sensing techniques for a wafer surface profile are inadequate to accurately reveal the presence of a bump which exists as a significant variation from the local surface profile. Present techniques use a proximity probe which may, in fact, be a laser interferometer, to measure thickness of a wafer. This information is presently processed to provide a curvature profile by taking the second derivative of the curve representing data obtained by current probing technologies. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention aims at providing additional information about the bump (or depression anomaly) characteristics and in particular information about its start point radially from the semiconductor wafer central axis, the span of the bump to its maximum height and the actual height or departure from local surface level. That level is typically tilted with respect to a plane perpendicular to the wafer axis. Current semiconductor manufacturers need this additional information in order to accurately assess the suitability of all regions of a semiconductor wafer for processing purposes. The fact that a wafer may slope or roll off towards its edge does not necessarily disqualify it for purposes of producing finished semiconductor processes but a departure from the local plane of curvature could be a significant impediment. 
         [0007]    The present invention provides additional signal processing which allows or provides the values of start point, peak point and distance therebetween which are essential to industry users. This data is obtained from the profile or second derivative curve by a process of integration of the profile value from the point where a rise in the local plane is first detected to outwardly extending radial points. A single integral will provide slope information about the wafer surface as a function of radius. The invention further provides for a double integral of the profile of curvature which will, in fact, provide surface normalized height information showing bumps and other perturbations above the local surface plane. Further, processing allows the maximum in the height information to be determined which can be used by manufacturers to judge the severity of the local departure from norm or bump. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    These and other features of the present invention are illustrated in the figure, and accompanying detailed description wherein: 
           [0009]      FIG. 1  illustrates a wafer surface condition evaluation system with which the invention is operative and which provides data reflecting profiles of the wafer front surface, back surface, or thickness; 
           [0010]      FIG. 2  illustrates data acquisition points on a semiconductor wafer surface as practiced in the invention; 
           [0011]      FIG. 3  illustrates a typical profile obtained from utilization of the measuring apparatus of  FIG. 1 ; 
           [0012]      FIG. 4  illustrates a typical thickness profile; 
           [0013]      FIG. 5  illustrates a corresponding ZDD profile as practiced in the invention; 
           [0014]      FIG. 6  illustrates processing of the data of  FIG. 5  according to the present invention; 
           [0015]      FIG. 7  illustrates further processing of the data of  FIG. 5  according to the invention; 
           [0016]      FIG. 8  illustrates graphically wafer anomaly data from processing according to the invention; 
           [0017]      FIG. 9  illustrates the difference in detected surface anomalies between thickness data and anomaly measurements according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    As seen in  FIG. 1 , the present invention operates in the environment of testing of a semiconductor wafer  10  for surface characteristics which is typically accomplished by spinning the wafer about its center on an axis  12  with a sensor or wafer measurement tool  14  of a type known in the art to measure distance to the wafer  10 . The sensor output is processed by a system  20  in conjunction with a microprocessor  22  to develop a profile, such as thickness or front/back surface distance, of the wafer  10  as it spins. Output information representing thickness or other parameters can be provided to a user interface  24  for use by I/O devices  26 . 
         [0019]    In  FIG. 2 , a top view of the wafer  10  is shown illustrating part of typical processing of the thickness information provided by the sensor  14  and associated electronics. In particular, in a sector  30  the sensor head  14  makes a plurality of measurements  32 , typically of thickness. The sector  30  may span a distance of 5° or more along the edge  34  of the wafer  10 . The sector  30  typically includes data taken within that angle over a distance between and 1 and 5-10 mm from the edge  34 . In typical application, the data in the segment  30  is averaged so as to provide thickness information along a radius vector  36 . While the invention is particularly useful at an edge, it may also be used over the whole wafer or other parts thereof. 
         [0020]    Typical processing of information from the sensor  14 , as described with respect to  FIG. 2 , will yield a profile curve  40  in  FIG. 3 , in this case a thickness profile. Such a profile  40  typically has a plurality of small surface unevennesses. A larger anomaly would be classified as a bump  44 , or possibly a void, or inverted bump. It is with information about such anomalies  44  that the invention is concerned. The profile  40  of thickness illustrates a gradual roll-off or reduction in thickness illustrated in  FIG. 3 , as the edge  34  of the wafer is reached. 
         [0021]      FIG. 4  illustrates an expanded view of a thickness profile in the range of 140 to 150 mm from wafer center as a curve  60 . When an anomaly  62  such as a bump is present, the change in the (2 nd  slope derivative) of curve  60  will go from negative to positive. This point marks a bump start radius (BSR). Second derivative processing converts the curve  60  of  FIG. 4  to a ZDD profile  70  in  FIG. 5 . The profile  70  is typically referred to as the ZDD profile or metric. See Document No. 4209 of Semiconductor Equipment and Materials International, 3081 Zanker Road, San Jose, Calif. 95134-2127. That document bears a publication date of Nov. 6, 2006 and is incorporated herein by reference. 
         [0022]    While both curves  60  and  70  illustrate the presence of respective bumps  62  and  72 , the start position of the bump in  FIG. 4  is rather indeterminate from mere thickness measurement given the roll-off nature of the wafer at its edge. In  FIG. 5  however, because of the second derivative properties, the beginning of the bump (BSR), or void, is identifiable as the zero crossing in the profile curve, or second derivative curve,  70 . 
         [0023]    The demands of modern semiconductor processing and the cost of semiconductor real estate demands even better information about bumps or other anomalies be provided to the semiconductor manufacturer from testing of the raw semiconductor wafers. In order to accomplish that, the processing of  FIGS. 6 and 7 , typically by processor  22 , which has provided the curve  70  of  FIG. 5 , is employed. 
         [0024]    The processor  22  acts on the profile data of  FIG. 5  to perform the integration of  FIG. 6  in which r represents the radius as shown in  FIG. 1  and is the variable of integration. The integration is from the bump start radius (BSR) out to the actual radius r. This provides a slope database. Height information is found using the integration of  FIG. 7  in which a double integral is formed first from BSR to y using the integration variable p followed by the integral from BSR to radius r using variable γ. The function C represents the curvature or ZDD profile  70  of  FIG. 5 . From the integration h b , a function of radius r, the bump peak radius (BPR) is calculated as the radius of the peak value bump height, of the double integral. A void would be a minimum. The bump height is calculated as the peak value of the distance between that point and BSR is calculated as the radial span of the bump or anomaly. This information calculated by processor  22  provides the bump start point, the bump height and the radial distance between them, valuable information for the semiconductor manufacturers. 
         [0025]      FIG. 8  illustrates these effects of the processing of the invention. A curve  80  corresponds to the profile  70  of Fig. S. Curve  82  illustrates the results of the first integral processing of  FIG. 6  to provide the slope S b  information. The curve  84  illustrates the h b  bump height information from the double integral of  FIG. 7 . This information clearly allows the calculation of the bump peak radius or maximum height (BPR) in curve  84 . The span is the distance between BSR and BPR and the height as given from the processor  22  evaluation of the maximum value of curve  84  using known technologies. 
         [0026]    The wafer tends to slope downward at the edges so that simple thickness information cannot readily be used to identify a variation in height relative to the near value of the slope throughout the edge region. This is illustrated in  FIG. 9  which shows a gap  94  in minimum height between the thickness curve  90  and the bump height curve  92 . 
         [0027]    The above description is illustrative only and the inventions scope is provided by the following claims.