Thermal annealing of sputtered Al-Si-Cu-N films

This article reports on stress, structure and thermal stability of Al-Si-Cu-N films prepared by DC reactive magnetron sputtering. The films were deposited under the following conditions: discharge current I_d=1A, substrate bias U_s=-100V, substrate ion current density i_s=0.6 and 0.9mA/cm², substrate temperature T_s=300°C and 500°C, substrate-to-target distance d_s-t=60mm, partial pressure of nitrogen p_N2 ranging from 0.06 to 0.14Pa and constant total pressure p_T=p_Ar+p_N2=0.5Pa. It was found that [1] the macrostress σ increases with increasing p_N2 and [2] the films sputtered at very low values of p_N2 (i) are polycrystalline films consisting of Al and small amounts of Al₂Cu, (ii) are in tension and (iii) have a mat appearance due to a rough surface. According to the changes in structure during annealing up to 700°C, the Al-Si-Cu-N films can be divided into two groups: (a) metastable films which are characterized by a crystalline structure and (b) stable films which exhibit an X-ray amorphous structure. The metastable films change their structure upon thermal annealing. On the contrary, the stable films remain amorphous even after annealing at T_a=700°C for 30min. The stable films contain a high (approx. 8at%) amount of copper and exhibit the lowest thermal expansion coefficient α_c≈4.2×10^-6K^-1 compared to the metastable films with α_c≈10-12×10^-6K^-1. The experiments described in this paper show that the thermal expansion coefficient of the films strongly depends on their structure.