Origin of undesirable cracks during layer transfer

Kavli Affiliate: Harry Atwater

| First 5 Authors: Laurent Ponson, Kenneth Diest, Harry Atwater, Guruswami Ravichandran, Kaushik Bhattacharya

| Summary:

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.

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