Origin of undesirable cracks during layer transfer

TL;DR

This paper analyzes how thermal stress during layer transfer causes undesirable cracks, showing that controlling stress within a specific range prevents crack formation and ensures film integrity.

Contribution

It identifies the critical role of thermal stress in crack formation during layer transfer and establishes the stress thresholds for avoiding cracks and delamination.

Findings

Tensile stress destabilizes microcracks, preventing successful transfer.

Compressive stress above a threshold causes buckling and transverse cracks.

Optimal stress range is between -σ_c and 0 to ensure crack-free transfer.

Abstract

We investigate the origin of undesirable transverse cracks often observed in thin films obtained by the layer transfer technique. During this process, two crystals bonded to each other containing a weak plan produced by ion implantation are heated to let a thin layer of one of the material on the other. The level of stress imposed on the film during the heating phase due to the mismatch of thermal expansion coefficients of the substrate and the film is shown to be the relevant parameter of the problem. 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 their straight trajectory making the layer transfer process impossible. However, if the compressive stress exceeds a threshold value, after layer transfer, the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress \sigma_m - or equivalently the heating temperature - must be within the range -\sigma_c < \sigma_m < 0 to produce an intact thin film where \sigma_c depends on the interfacial fracture energy and the size of defects at the interface between film and substrate.