The Study On A Surface Stability And Spontaneous Delamination Of Piezoelectric Thin Films

With the development of micro-electronic technology and the trend of highly integrated,piezoelectric thin films have been paid more and more attention. The structure and material properties of piezoelectric thin film and substrate are different, it is easy to cause crack and fracture of piezoelectric film, and piezoelectric film delamination from the substrate, to reduce the reliability of the device, so to study the failures form is urgent. The surface stability of the piezoelectric thin film is very important to the device application, and mismatch strain in the film would cause the piezoelectric film delamination, to damage on the device. So it is important significant to study the surface stability and interfacial delamination of the piezoelectric thin films.In this work, the chemical potential and stability of the piezoelectric thin films surface and the behavior of crack growth at the interface between the film and the substrate are studied, it could provide reliable theoretical basis for the piezoelectric thin films in practical application.（1）Firstly, the chemical potential of the piezoelectric thin films surface is put forward and studied. The surface stability of the piezoelectric thin films under the action of stress-electric coupling and small amplitude perturbation is analyzed. The critical disturbance wavelength of balance of the piezoelectric thin film which is between with the surface stress field and electric field and the surface energy of the piezoelectric thin film is derived.（2）Then, the basic theory of cracks in the piezoelectric material is introduced, the singularity of the interface crack tip of the piezoelectric material/elastic substrates is described. The stress, displacement, stress intensity factors and electric displacement intensity factors and the energy density factor of interfacial cracks under the effect of the mismatch strains are derived.（3）Finally, a finite element model of the piezoelectric/conductor is established, to simulate the crack growth behavior of the crack tip, to analyze in detail the effect of the mismatch strain on the delamination of the piezoelectric thin film. Theoretically the critical film thickness of spontaneous delamination of the piezoelectric thin film is obtained. The stress field and electric displacement field of the interface crack tip are analyzed, stress intensity factor K₁, K₂ and electric displacement intensity factor K_D and energy density factor S are calculated. The effect of thickness and crack length on crack growth and discriminates the direction of crack growth by using energy density factor are analyzed.This work analyzes the piezoelectric thin films under only misfit strain, it is very important to guide manufacturing of the piezoelectric thin films.