Patent ID: 6938505

Claim:
Method of detecting a position status of a wafer in a wafer chamber, said chamber including a plurality of lift pins for supporting the wafer, the method comprising the steps: sensing respective loads exerted by the wafer upon each of the plurality of lift pins, wherein the plurality of pins numbers three distributed at the apexes of an equilateral triangle; and determining from said respective loads a position status of the wafer wherein said step of determining the position status of the wafer includes the step of determining whether the wafer is acceptably positioned, wherein the wafer is determined to be unacceptably positioned when the respective loads exerted by the water upon each of the plurality of lift pins are not substantially equivalent, and determining the location of the wafer by performing the step of calculating the location of the wafer from the distribution of the respective loads exerted by the wafer upon each of the plurality of lift pins, wherein the location of the wafer is defined by a coordinate pair location (x,y) of the wafer geometric center on a Cartesian coordinate system wherein a first one of the three pins corresponds to the coordinate system origin, a line internecting the origin and a second one of the three pins corresponds to an X-axis of the coordinate system, a line intersecting the origin perpendicular to the X-axis corresponds to the Y-axis of the coordinate system, wherein x = M 1 × 0.5 ⁢ t + M 3 × t M 1 + M 2 + M 3 y = ( 3 / 2 ) × t × M 1 M 1 + M 2 + M 3 t-an equivalent distance from any one of the three lift pins to the other two lift pins, M1-load at a third one of the three pins, M2-load at the first one of the three pins, and M3-load at the second one of the three pins.