Stress Mapping Below Flip-Chip Bumps With High Spatial Resolution Using Piezoresistive CMOS Sensors

This paper presents a CMOS stress sensor chip including arrays of piezoresistive sensor elements with high spatial resolution sensitive to the in-plane stress components σ_xx – σ_yy and σ_xy, to the out-of-plane stress σ_xz and σ_yz, and to the normal stress sum σ_Σ = (σ_xx + σ_yy)/2 − σ_zz. For the first time, an application of novel vertical stress sensors is presented, measuring the mechanical stress distributions below electroless nickel (eNi) bumps subject to lateral shear forces and vertical compression. All measured stress values are linearly proportional to the applied forces. The vertical shear stress sensors resolve residual vertical shear stresses of up to 51 MPa in the shear experiments. An adjustable numerical model is established assuming two different Young’s moduli of silicon nitride (SiN) emulating the adhesion between the SiN and eNi. Qualitative agreement of the in-plane stress distributions between experiment and numerical simulation is found in the shear and compression experiments, while good correlation for σ_Σ is found only for temperature uncompensated stress values in the compression test. The modeling of the absolute values shows differences to the experimental data of about ±30%.