The Residual Stress Measurement Of Polycrystalline Ferroelectric Thin Films Based On The Orientation Average Method

For the specific physical properties, such as pyroelectric properties, ferroelectricproperties, piezoelectric properties, electrostrictive properties, nonlinear optical properties andelectro-optical properties in the ferroelectric thin film, it has been more and more widely usedin many areas of technology and science, and drawn a lot of attention to the research of theferroelectric thin film. The mechanical properties present important influence in thedelamination, brittle fracture, and fatigue degradation of the ferroelectric thin film/substratestructures, which may cause poor reliability that is one of the major problems in MEMSdesign. Residual stress is inevitable for the structural misfit, thermal misfit and the processfrom high temperature to low temperature during deposition of thin film. In this case, theinvestigation of the residual stress is very important for the fabrication of the ferroelectric thinfilm and ferroelectric thin film device. In the paper, we discuss several commonly usedmethods for residual stress measurement in thin films, and then focus on the use of thenon-destructive X-ray diffraction （XRD） method on the measurement of residual stresses forferroelectric thin film. Moreover, two methods based on XRD were proposed to evaluate theresidual stress in ferroelectric thin film, taking account of the intrinsic properties offerroelectric thin films, such as isotropy or transversely isotropy, piezoelectricity and grainorientation. Investigations are presented on the evaluation and analysis of residual stresses in(Pb(Zr_0.52Ti_0.48)O₃（PZT）) and (Na_1-xK_x)_0.5Bi_0.5TiO₃（NBT-KBT-100x） thin films by XRD. Themain contents are presented as follow:1. The transversely isotropy, piezoelectricity and grain orientations of the thin film aretaken account in the orientation average method, it’s a comprehensive method. The method’svalidity is verified by comparing the residual stresses in three kinds of the previous PZT thinfilms evaluated by different method. The orientation average method on the specificdiffraction peak is only appropriate for polycrystalline ferroelectric thin film with randomorientation, and the orientation average method on all the diffraction peaks is appropriate forpolycrystalline ferroelectric thin film with not only random orientation but also preferredorientation.2. NBT-KBT-100x thin films with different potassium content were deposited onLaNiO₃（LNO）/SiO₂/Si（100） substrate by using metal-organic decomposition （MOD）, and themicrostructure, dielectric, piezoelectric and mechanical properties were investigated.Furthermore, their residual stresses are evaluated by the conventionalsin2method. In casethat the grain orientation is not taken account in the conventional method, theorientation average method is employed to evaluate the residual stresses as well. Comparing with the conventionalsin2method, the comprehensive information of grain orientation isconsidered from XRD patterns for the orientation average method. Therefore, it can be betterable to reflect the reality state of the polycrystalline ferroelectric thin film. The results showthat the residual stresses are related with the grain size.3. Based on the strain of a grain, the interface and surface residual stress can beevaluated for ferroelectric thin film via a grain strain model. The interface residual at thefilm/substrate interface are particularly discussed by the thermodynamic theory for epitaxialPZT thin films. In case of very thin films, the results from the grain strain model are close tothose from the thermodynamic theory, and there are some deviations for thick films since thestructural relaxation mechanisms for interface residual stress are neglected in thethermodynamic theory. Furthermore, the surface residual stresses evaluated from the grainstrain model are discussed with those of the orientation average method, the analysis of theorigin of residual stress and thermodynamic theory. They are close to each other excepted theresults for the thermodynamic theory, and it indicates that the consideration of structuralrelaxation mechanisms on interface residual stress is reasonable in the grain strain model.