Lead zirconate titanate (PZT) is the ferroelectric material which has the perovskite structure. PZT thin films are applicable to micro electrical mechanical system (MEMS) devices because of their superior piezoelectric properties. For the application of PZT films to MEMS devices, it is necessary to fabricate the PZT films which have superior electrical properties and solve the problems that the residual stress in PZT films deforms MEMS devices.
The sol-gel derived PZT films have been mainly used for fabrication of MEMS devices. The disadvantages of the sol-gel derived PZT films are poor step coverage and high residual stress. Therefore, it is necessary to fabricate the PZT films by sputtering method is needed. However, the sputtered PZT films have poor electric properties compared with sol-gel derived PZT films.
In this study, PZT films were fabricated by DC magnetron multi-target reactive sputtering. $RuO_2$ was used as bottom electrode since the residual stress of $RuO_2$ is lower than that of Pt. The superior electrical properties of PZT films were obtained by deposition at high temperature using double seed layer depositon method since the depositon at high temperature reduces defects density in films by thermal annealing effect.
In this study, the residual stress of PZT films fabricated on $RuO_2$ by double seed layer deposition method was tensile stress as low as 80-110MPa and hardly dependent on the film thickness. Therefore, it is expected not to deform MEMS devices when PZT films were applicable to MEMS devices. However, although the residual stress of our PZT films is not high enough to deform the MEMS devices, we attempted to remove the low residual stress in our PZT films and fabricate the stress free PZT films. In general, the residual stress in films could be controlled by changing incident ion flux and energy (atomic peening effect). In this study, incident ion energy and flux were controlled by substrate bias and electron cyclotron resonance (ECR) plasma.
To increase incident ion energy, we used substrate bias. As a result, collisions between PZT films and high energy ions damaged films and degraded ferroelectric properties of films. However, the residual stress in PZT films was not changed. It is thought that incident ion flux was insufficient to arise compressive stress.
To increase incident ion flux, we generated $O_2$ ECR plasma. As a result, the superior electrical properties of PZT films were not changed as ECR plasma power increased. However, the residual tensile stress in PZT films was increased slightly, which may be attributed to grain boundary relaxation through enhancement in grain growth by oxygen radical in $O_2$ ECR plasma.