The flatness of wafers used to manufacture integrated circuits needs to be controlled within strict tolerances to help ensure that the entire wafer is flat enough for photolithography processing. Wafer flatness is characterized by measuring bow, warp, and total thickness variation (TTV). A low value indicates better wafer flatness. Alfa Chemistry can provide customers with accurate wafer flatness testing services to verify process tolerances and eliminate expensive waste wafers. Our non-contact capacitive sensing is a highly accurate and cost-effective method for measuring TTV/WARP/BOW of wafers.
Total Thickness Variation (TTV)
TTV is the difference between the maximum and minimum thickness encountered during a scan pattern or series of point measurements. Place a wafer of a known thickness (Tw) between the probes for calibration, and obtain an upper probe-to-wafer gap (A) and a lower probe-to-wafer gap (B). The total gap between the upper and lower probes: Gtotal = A + B + Tw. When the sample wafer is placed between the probes during measurement, new values of A and B will be obtained, and Tw = Gtotal -(A + B). During the automatic scanning of the wafer, a series of points are measured and stored. After the scan is completed, TTV = Tmax - Tmin.
Bow is the deviation of the center point of the center surface of a free, unclamped wafer from the center surface reference plane. When measuring and calculating the curvature, the position of the wafer must be known to the center surface. The bow value is calculated by measuring the position of the middle surface of the wafer center and determining the distance from the three-point reference plane. A positive bow indicates that the center point of the middle surface is above the three-point reference plane. Negative means that the center point of the intermediate surface is below the three-point reference plane.
Warp is the difference between the maximum and minimum distances of the median surface of a free, unclamped wafer from a reference place. Warp uses the entire middle surface of the wafer, which can provide a more useful measurement of true wafer shape. There are two options for the reference plane: one is the same three-point plane around the edge of the wafer. The other is to perform least squares fitting calculation on the median surface data obtained during the measurement scan. Then the warp age is calculated by the maximum deviation (RPD max) and the minimum difference (RPD min) from the reference plane.
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