A simple technique for measuring the stress at the Si/SiO2 interface is described. This involves etching a groove in the silicon, using the SiO2 layer as a mask. The SiO2 overhang, resulting from etch undercutting, is not constrained by the Si/SiO2 bond and assumes a periodic shape. Measurement of the amplitude and period gives the strain required to conform the SiO2 to the silicon. This allows calculation of the interface stress and the strain in the silicon. For 280 nm of SiO2 on (110) Si, the SiO2 strain is 9×10−3, the interface stress is 6×109dyne/cm2, and the silicon strain 3×10−3, in agreement with previously reported values.