In-situ TEM observations on void formation and growth have been performed in passivated Al-lines in order to collect detailed microstructural data such as void number density, void size distribution, and growth rates as a basis for modelling stress relaxation and electromigration (EM) effects.
The samples were 0.5mm long lines of 200nm thick Al with different widths (0.8, 1.6 and 2.4μm) passivated with 200μm thick SiNx. Additional experimental parameters were average grain size (0.5μm to more than 3μm, i.e. nearly perfect bamboo structure in the lines), relaxation time (0.1 to 20h) and temperature (20 – 300°C) and electromigration current density (without current up to ∼2MA/cm2).
Measurements without current reveal that stress induced voids
– develop after a temperature step of 50°C during cooling from 450°C and above. Upon heating from RT they start shrinking at 150°C and disappear completely at 350°C.
– decrease in number and total relative volume but have constant mean void size with increasing number n of temperature cycles (n < 4)
– show a strong dependence of their total relative volume on the microstructure (grain size, bamboo structure) which is strongly dependent on the anealing temperature for T ≥450° C.
– show growth rates during isothermal annealing which are very dependent on relaxation time and temperature.
Additional measurement with an applied electrical current, i.e. during an EM-test, show that nearly all the preexisting stress-induced voids
– change their shapes and positions. This is dependent on electrical current density and time. Their frequently observed motion along the line (against the electron wind) was not neccessarily stopped by so-called blocking grains, i.e. grains without boundaries parallel to the line.
– slightly change their total number but nearly maintain their total relative volume which is independent of current density or time.
The collected statistical microstructural data are in good agreement with macroscopic stress relaxation results by wafer curvature and X-ray diffraction and recent in-situ stress measurement during an EM-test.