Experimental Determination of Stress Distributions Under Electroless Nickel Bumps and Correlation to Numerical Models

This paper presents high-density arrays of 7 × 7 n- and p-type piezoresistive field effect transistor (piezo-FET)-based stress sensors realized with a pitch of 23 μm using a commercial complementary metal-oxide semiconductor (CMOS) technology. The sensor elements make it possible to extract the distribution of the in-plane normal stress difference σ_xx-σ_yy and the in-plane shear stress σ_xy under electroless nickel (eNi) bumps. For the first time, pre-deposition stress caused by openings in the passivation, stress induced by the eNi bump deposition, and stress redistribution during anneals between 50°C and 260°C are presented. Typical values of σ_xx-σ_yy of ±25 MPa are introduced by bump deposition. These values are further increased by up to 160% during anneals up to 260°C. The in situ monitoring of the mechanical stress redistribution during annealing and thermal cycling is studied. At the deposition temperature, the system is found to be almost stress free. The stress due to bump deposition is numerically modeled. The unknown adhesion strength between nickel bump and silicon nitride (SiN) passivation is taken into account using an adjustable Young´s modulus of SiN. For an optimized model, the correlation between measured and simulated spatial stress distributions is found to be high, while the magnitude of the stress values is underestimated in the model by about 35%.