Ion-beam-induced thin film stress in lithium niobate

Abstract

The dominating modification of crystalline solids by energetic ions is the formation of lattice defects, which accumulate with ongoing irradiation. Many materials exhibit a phase transition from crystalline to the amorphous state at higher ion fluence. However, this ion-beam-induced structural modification involves the formation of mechanical stress, which is generally disadvantageous for the successful application of ion irradiation in the micro-device technology. Hence, a fundamental understanding of the ion-beam-induced stress evolution is crucial for the effective use of ion beam technology. Lithium niobate (LiNbO3) is a promising candidate for the application of integrated photonic structures. However, for the fabrication of such structures in LiNbO3 ion irradiation is indispensable. In order to get a fundamental and comprehensive understanding of the ion-beam-induced defect and stress evolution in LiNbO3, irradiations with varying parameters (ion energy and irradiation temperature) over a wide range of ion fluence for different crystallographic orientations were performed. The ion-beam-induced defect and stress evolution were studied by means of in situ Rutherford backscattering spectrometry and laser reflection technique, respectively.The investigations demonstrate that ion-beam-induced defect and stress evolution in LiNbO3 is highly anisotropic. Moreover, a complex stress evolution is observed, i.e. with increasing ion fluence different stress formation and stress relaxation processes occur. It will be demonstrated that effects such as radiation-induced viscosity or anisotropic deformation that were suggested by previous models cannot explain the stress evolution in LiNbO3.This work presents a new approach that describes the anisotropic stress and defect evolution in LiNbO3 by a complex defect formation mechanism, i.e. the presence of different defect types and their transformation into each other. Each defect type strains the surrounding crystal matrix and leads to a macroscopic deformation. The total stress is the superposition of the individual stress caused by different defect types.