Structural Analysis of Stencil Printed Cu for Applications in Semiconductor Industry
Barbara Eichinger
Infineon Austria, KFU
15:10 - 15:50 Friday 10 May 2019 KFU HS 05.12

Structural Analysis of Stencil Printed Cu for Applications in Semiconductor Industry

The current trend in bond materials for high power die attach applications is going towards cheap, lead-free, high-conductivity and high-reliability sinter interconnects being able to withstand high operating temperatures. [1] Pure-Cu sinter interconnects match the demand for increasing current densities and have been shown to surpass Ag sinter pastes regarding cost efficiency, electro-migration stability and high-temperature reliability. [2, 3]
Thus, this work investigates microstructural, mechanical and electrical properties of Cu sinter pastes consisting of coated and dispersed nano- and micro-particles, mainly targeted at die-attach applications in semiconductor industry. The sinter pastes are deposited on wafer level by stencil printing and undergo subsequent thermal conditioning. By pressure sintering of the Cu pastes in reducing atmosphere at elevated temperatures, a die-attach can be formed on various Cu surfaces, e.g. Cu-sputtered Si or Cu-plated DCB (see Fig. 1, right).
The resulting Cu metallization layer is characterized by means of Scanning Electron Microscopy (SEM), Auger Spectroscopy, Micro Computer Tomography (µCT), Laser Ultrasound Spectroscopy (LUS), four-point probing and Electron Backscatter Diffraction (EBSD). The sinter point of various Cu pastes during processing is determined using in-situ four-point probing and factor-effect correlations between the experimental variables and the measured material properties are postulated.
In addition, the pressure sintering process for die-attach formation is systematically investigated. A range of parameters is screened using Scanning Electron Microscopy (SEM), Scanning Acoustic Microscopy (SAM), roll and shear testing as well as Thermal Shock Treatment (TST). Fig. 1, left shows a seamless Cu sinter interconnect formed between two printed wafers at two different sinter pressures. Ultimately, it is shown that the investigated Cu sinter pastes are suited to form pure-Cu interconnects with comparable strength and thermal shock durability as standard Ag sinter pastes currently used for die-attach applications.

Find reference article:
• B. Eichinger, T. Behrendt, S.N. Ohm, F. Craes, M. Mischitz, R. Brunner, “Cu Sinter Pastes for Pure-Cu Die-Attach Applications of Power Modules”, 2018 IEEE 20th Electronics Packaging Technology Conference (EPTC), p.26-30

[1] T. Furukawa, M. Shiraishi, Y. Yasuda, K. Akitoyo, M. Mori, T. Morita, S. Watanabe, T. Arai, M. Nakamura und D. Kawase, „High power density Side-gate HiGT Modules with Sintered Cu Having Superior High-temperature Reliability to Sintered Ag,“ in Proceedings of the 29th International Symposium on Power Semiconductor Devices & ICs, Sapporo, 2017.
[2] J. Zhao, M. Yao und N.-C. Lee, „Nano-Cu Sintering Paste for High Power Devices Die Attach Applications,“ in IEEE 68th Electronic Components and Technology Conference, 2018.
[3] L. Del Carro, J. Zürcher, S. Gerke , T. Brunschwiler, T. Wildsmith und G. Ramos, „Morphology of low-temperature all-copper interconnects formed by dip transfer,“ in IEEE 67th Electronic Components and Technology Conference, 2017.