A methodology was developed to study the residual stress distribution of thin-film patterns residing on a silicon wafer. The Si underlying the patterns studied was thinned down so that the deformation caused by residual stress in the microstructure could be detected by a Twyman-Green interferometer. A procedure called "numerical etching" was implemented to simulate the Si etching process, which linked the stress state of the microstructure on a regular wafer to that on a Si diaphragm. An initial stress field on the pattern was assumed, and its effect on the bending of the Si diaphragm beneath was calculated and compared to the measured value. The discrepancy between them was used to modify the initially assumed stress field and repeated until a satisfactory match was achieved. We believe that the approximated stress field sufficiently reflects the real stress distribution in the patterned structure under investigation. The stress distribution in an electroless Ni pad residing on Si for a flip-chip packaging application is used as an example.