Stress and stress relaxation study of sputtered PZT thin films for microsystems applications

Residual stresses in thin films play an important role in the mechanical behaviour of MEMS. In this paper we present a study of the stress and its relaxation for the PZT films, and associated electrodes, deposited on oxidized silicon substrates. The stresses were calculated from the bending plate method and the Stoney's equation. The radius of curvature were measured by optical profilometry before and after films deposition. The substrates (180 mum Si + 0.66 mum thermal SiO2) were coated with sputtered Ti (20 nm) and Pt (200 nm) used as bottom electrode. The global stress in the Ti/Pt layer was found compressive (-1.29 GPa) after deposition and tensile (465 MPa) after annealing (400 degreesC, 30s, Ar). A 0.55 mum thick PZT layer was RF-magnetron sputtered and crystallized by a RTA (700 degrees, 30s, Air). The as-deposited PZT films exhibited a little tensile stress of 43 MPa. After annealing, a tensile stress value of 363 MPa was found. Finally, we observed that the stress of the whole multilayer showed a decrease as a function of time. In order to explain this phenomenon, depth profile of each component of the PZT layer were obtained by Secondary Ion Mass Spectrometry (SIMS). This time-dependent stress relaxation was then correlated to a lead and oxygen migration across the PZT layer.