US Patent 7667159 Resolving thermoelectric potentials during laser trimming of resistors

Thermoelectric effects that occur during laser trimming of resistors (20) are resolved by taking voltage measurements. The voltage attributed to laser heating on a resistor (20) during a low-power simulated trim (10) is used to determine a relatively thermal-neutral location (18) on the resistor (20). A trimming-to-value operation can then be performed on all like resistors (20). Voltage measurements can also be taken before and after every pulse in a trimming operation to establish thermal deviation information that can be used to offset the desired trim value against which resistor measurement values are compared. Spatially distant or nonadjacent resistors (20) in a row or column can also be trimmed sequentially to minimize heating effects that might otherwise distort resistance values on adjacent or nearby resistors (20).