Stress engineering to reduce mechanical loading in Cu-based metallizations

Imane Souli

Research output: ThesisDoctoral Thesis

Abstract

The repetitive stress accumulations induced in electronic devices may result in thermo-mechanical damage and thus dysfunction of the device. Tailoring the physical properties of Cu metallizations already in the design stage is a promising approach to enhance their lifetime. Within this work, DC magnetron sputter deposited Cu-based films have been grown on Si substrates without and with WTi interlayers at different growth condition. Thus, the combined impact of thermal- and kinetic-energy activation of film growth on structure and properties of the films and retarding Cu-Si inter-diffusion is illuminated to enable the optimization of the overall stress distribution within the metallization. An alloyed functionally graded multilayer is designed to overcome the Cu limitations and to engineer stresses and stress gradients. Thus, alloying with Mo, at first as monolithic films and subsequently within a functionally graded architecture, was investigated for films in the as-deposited state and upon annealing. For the monolithic Cu-Mo films, the metastable Cu(Mo) solid solution observed for low Mo contents provides remarkable thermal stability and improves the mechanical properties with a minimized impact on the electrical resistivity. In contrast, the Cu- and Mo-rich dual-phase structures offer the possibility to benefit from the thermo-mechanical properties of both the Cu and Mo within the miscibility gap; however, they are characterized by a reduced thermal stability. Furthermore, for the functionally graded architecture, a coupled predictive thermo-mechanical finite element model has been set up in order to design a Cu-Mo based adaptation layer with steps in the chemical composition from pure Mo to pure Cu of ⁓10 at.%. Subsequently an improved-graded Cu/Mo multilayer with a stress profile optimized according to finite element calculations was designed and synthesized. The multilayer films were investigated by means of scanning electron microscopy, X-ray diffraction and position-resolved synchrotron X-ray nanodiffraction. The in-plane residual stress-depth distribution across the film thickness showed the effectiveness of the improved functionally graded Cu/Mo multilayer film. Finally, the evolution of the films’ microstructure and mechanical properties upon thermal cycling between 170 and 400 °C confirmed the thermal stability of the predicted improved Cu/Mo multilayer.
Translated title of the contributionEigenspannungsdesign für Cu-basierte Metallisierungen zur Reduktion mechanischer Belastungen
Original languageEnglish
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Keckes, Jozef, Assessor B (internal)
  • Mitterer, Christian, Assessor A (internal)
Publication statusPublished - 1800

Bibliographical note

embargoed until 04-02-202026

Keywords

  • Thin films
  • Copper (Cu)
  • Silicon (Si)
  • Molybdenum (Mo)
  • sputter deposition
  • stress
  • Cu/Mo graded multilayer
  • Finite element analysis

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