Annealing of copper electrodeposits

The properties of electroplated copper film have been investigated as a function of annealing temperatures together with the diffusion barrier performance. Electroplated copper films on copper and tungsten seed materials were annealed from 300 to 700 °C in N2 atmosphere for grain growth study. The average grain size of the as-deposited copper films was found to be different; larger copper grains (0.6 μm) were formed on the W seed layer compared to those formed on Cu (0.2 μm). The copper films also started to recrystallize at 300 °C and grain growth occurred from 400 °C onward. Regardless of the initial grain size of the electroplated copper films, the final grain size was found to be similar on both seed materials. After 600 and 700 °C annealing, the average grain sizes became 1.00 and 1.20 μm, respectively. The driving force for grain growth is from the elimination of grain boundaries where the surface energy is being released to achieve the equilibrium state. The annealed films produced a layered microstructure, together with the presence of pinholes and cavities. The average grain size of the electroplated copper increased in proportion to the square root of the annealing time and increasing temperature. The activation energies of grain growth were 0.31 eV from 400 to 700 °C on the W seed material and 0.62 eV from 400 to 500 °C and 0.28 eV from 500 to 700 °C on the Cu seed material. With annealing, the tensile stress decreased from 202 to 105–149 MPa while the strain at break increased from 7.6% to 17.7%–20.7%. The resistivity of the electroplated copper films was reduced from 2.25 to 1.87 μΩ cm after high temperature annealing. X-ray diffraction results show that only the ionized metal plasma (IMP) TaN survived as the diffusion barrier between copper and silicon up to 700 °C while chemically vapor deposited TiN, physically vapor deposited TiN and IMP Ta failed at lower temperature.