Analysis of Competing Failure Mechanisms in Layer Transferred Thin Films

In this chapter, we develop the criterion for successful layer transfer from a thin film mechanics standpoint. To insure proper thin film exfoliation, samples of lithium niobate were implanted with hydrogen and helium based on the criterion developed in Chapter 3. The analysis of transverse cracks, often observed in thin films obtained by the layer transfer technique, is done for films in a state of tensile and compressive stress. The level of stress imposed on the film during heating due to the mismatch in thermal expansion coefficients between the substrate and thin film is shown to be the dominent factor in determining the quality of the transferred layer. In particular, it is shown that if the film is submitted to a tensile stress, the microcracks produced by ion implantation are not stable and deviate from the plane of implantation, causing the layer transfer process to fail. Conversely, if the transfered layer is in compressive stress exceeding a threshold value, then the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress σm which is determined by the temperature to which the samples are heated must be within the range −σc < σm < 0 to produce an intact thin film where σc depends on the interfacial fracture energy and the size of defects at the interface between film and substrate. 1 This chapter is based on work done with Dr. Laurent Ponson who was the lead author on this paper. The theory