From stress sensor towards back end of line embedded thermo-mechanical sensor

• A novel approach is proposed to investigate thermo-mechanical properties of BEoL.
• Design of a metallic stress sensor is modified to address electrical polarization.
• Experiment is performed in situ SEM using electrical nano-probing.
• Results are compared to analytical modeling and Finite Element Method (FEM).
• Stress, coefficient of thermal expansion and thermal conductivity are identified.

In the context of Back-End of Line (BEoL) roadmap, dimension reduces, density integration increases and new materials are introduced. These points associated to manufacturing thermal budget could induce mechanical failures. Thus, a metallic in situ sensor was developed to study residual stress on a single metal level: using standard CMOS BEoL processing on 8″ silicon wafer, aluminum thin film is patterned on dielectric layer. The sensor is composed by arms and a flexible beam that are fixed to anchors. As the structure is released from its surrounding layer, the relaxation of residual stress induces a displacement of flexible beam. Therefore, the measurement of this displacement allows determining the initial residual stress. Using this structure, the purpose of this paper is not only to determine the residual stress state, but also the thermo-mechanical properties: coefficient of thermal expansion and thermal conductivity. For that reason, new designs are released to address electrical polarization and thus to locally heat this sensor by Joule effect. Due to thermal expansion, the flexible beam will move. The thermo-mechanical properties were determined by coupling SEM electrical nano-probing (displacement of flexible beam and electrical resistance as a function of applied current) with analytical modeling and Multi-physics Finite Element Method (FEM). As a result, a tensile stress state of 190 MPa in arm direction is identified in the aluminum thin film. The coefficient of thermal expansion of 22.5 × 10−6 K−1 and thermal conductivity of 190 W/(K m) were identified, in agreement with literature.

Finite element modeling of “H” stress sensor after release (a – displacement) and under applied current of 78 mA (b – displacement and c – temperature) Back End of Line embedded.Figure optionsDownload as PowerPoint slide