Stress distribution under electroless nickel bumps extracted using arrays of 7×7 piezo-FETs

This paper presents CMOS-based, high density arrays of 7 × 7 n- and p-type piezoresistive field effect transistor (piezo-FET) based stress sensors with a pitch of 23 μm for extracting the distribution of the in-plane normal stress difference σ xx - σ yy and the in-plane shear stress σ xy under electroless Ni bumps. For the first time, pre-deposition stress caused by openings in the passivation, stress induced by the electroless Ni bump deposition, and stress redistributions during annealing processes between 50°C and 200°C are presented. Typical values of σ xx - σ yy = ±25 MPa are introduced by bump deposition. These values are further increased by up to 60% during annealing steps of up to 200°C. The in-situ monitoring of the mechanical stress redistribution during an anneal at 115°C shows a relaxation of the material compound by σ xx - σ yy = ±2.5 MPa over 180 min. The change of the stress components is found to be linear with temperature during thermal cycling resulting in an almost stress-free state at the deposition temperature.