![]() ![]() ![]() In this example RPDmax is 1.5 and is shown as the maximum distance of the median surface above the reference plane. RPD min is the largest distance below the reference plane and is a negative number.įigure above is an illustration of the warp calculation. RPD max is defined as the largest distance above the reference plane and is a positive number. Warp is then calculated by finding the maximum deviation from the reference plane (RPD max) and the minimum differentiation from the reference plane (RPD min). The other is by performing a least squares fit calculation of median surface data acquired during the measurement scan. One is the same three point plane around the edge of the wafer. For warp determination, there are two choices for construction of the reference plane. The location of the median surface is calculated exactly as it is for bow and shown above. By looking at the entire wafer, warp provides a more useful measurement of true wafer shape. ![]() Warp, however, uses the entire median surface of the wafer instead of just the position at the center point. Like bow, warp is a measurement of the differentiation between the median surface of a wafer and a reference plane. The differences between the maximum and minimum distances of the median surface of a free, unclamped wafer from a reference place. Negative denotes the center point of the median surface is below the three point reference plane. Positive denotes the center point of the median surface is above the three point reference plane. Note that bow can be a positive or negative number. The value of bow is then calculated by measuring the location of the median surface at the center of the wafer and determining it’s distance from the reference plane. Since bow is measured at the center point of the wafer only, a three (3) point reference plane about the edge of the wafer is calculated. Solving both equations for Z, the value can be determined simply by: To determine the value of Z at any location on the wafer, there are two equations: Z = Distance between wafer median surface and the point halfway between the upper and lower probe (D/2).B = Distance from lower probe to bottom wafer surface. ![]()
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