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3D Coaxial Through-Strata-Via (TSV) Evaluation and Modeling
Keywords: Coaxial TSV, 3D integration, Wideband modeling
Through-strata-via (TSV) is well accepted as a critical component in 3D integration that would extend Moore's Law. Since its electromagnetic fields that carry the signal exist only between the inner conductor and outer shell, the coaxial TSV offers small coupling, reduced loss and high density to meet rigorous specifications. The published results also show that the signal integrity of coaxial TSVs is better than that of other TSV configurations, while lack of detailed analysis and modeling for TSV physical design guidelines. In comparison to other TSV configurations, there are some advantages in fabrication of tungsten-based coaxial TSVs, although the fabrication could be challenging and complex for copper-based coaxial TSVs. This paper examines the performance of various coaxial TSV physical designs using a 3D electromagnetic field solver and SPICE simulator. Based on the analysis in both frequency domain and time domain, we report a number of design notes (as listed below) to avoid undesired energy consumption and potential logic malfunction: * Compared with a signal-ground TSV pair, the proposed coaxial TSV reduces the signal loss by more than half (~53%). * The electrical performance of copper-based coaxial TSV is very close to that of tungsten-based one. * 3D bathtub curves suggest good timing characteristics of coaxial TSV at high data rates. * The impact of the thickness of the coaxial TSV outer ground shell is insignificant. * Keeping the outer ground shell radius (that determines the I/O density), an optimal value of the inner signal TSV conductor radius exists (i.e., neither big nor small is good). * A thick tubular dielectric medium (i.e., large spacing) between the inner and outer conductors favors the performance remarkably. * This tubular dielectric medium with a small dielectric constant and tangent loss can suppress the signal attenuation. * If the tubular dielectric material is silicon or poly-silicon, low doping is preferred. * The desired thickness of the isolation liner, which isolates the metals, depends on the material properties of the tubular dielectric medium. * By converting fullwave scattering (S) matrix to the corresponding impedance (Z), admittance (Y) and transmission (ABCD) matrices, frequency-dependent RLGC of the coaxial TSV are analytically extracted based on 'T', 'Pi' and 'lossy transmission-line' models, showing a good agreement. * A SPICE broadband modeling approach is applied to accurately fit the amplitude and phase of insertion loss S11 and return loss S21 from 100 MHz to 100 GHz in the proposed coaxial TSV. This work can help designers optimize the coaxial TSV physical design, hence improving electrical performance and maximizing the benefits of 3D integration/packaging.
Zheng Xu, Student
Rensselaer Polytechnic Institute
Troy, NY

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