UCLA

Exfoliation is a technique used to remove a thin, uniform layer of material from the bulk that involves the annealing of hydrogen ion-implanted materials in order to initiate defect nucleation and growth leading to guided crack propagation. This study presents an investigation into the annealing process required to initiate blistering (an essential precursor to exfoliation) in (0001) sapphire implanted at room temperature with hydrogen ions.

Triple axis x-ray diffraction was used to characterize the evolution of the implanted layer for single crystal (0001) sapphire substrates implanted at room temperature at 360 keV with either a 5x1016 cm-2 or 8x1016 cm-2 dose of hydrogen ions. A simulation of the ion distribution in TRIM estimated that the projected range and thickness of the implanted layer for both doses was approximately 2.2 �m.

Following implantation, the implanted sapphire was annealed using a two-step annealing procedure. The first step was performed at a lower temperature, ideally to nucleate and coarsen defects. Temperatures investigated ranged from 550 – 650 �C. The second step was performed at a higher temperature (800 �C) to induce further defect coarsening and surface blistering. After all annealing steps, triple axis ω/2θ and ω scans were taken to observe any changes in the diffraction profile – namely, any reduction in the amplitude and shift in the location of the fringes associated with strain in the crystal – which would correlate with defect growth and nucleation. It was found that significant strain fringe reduction first occurred after annealing at 650 �C for 8 hours for both doses; however, it was not clear whether or not this strain reduction was due primarily to hydrogen diffusion or to recovery of other defects induced during the ion implantation. The ω/2θ curves were then fit using Bede RADS in order to quantify the strain within the crystal and confirm the reduction of the strained layer within the crystal. Finally, Nomarski optical images of the sample surfaces were taken after each step to observe any visual changes or blistering that might have occurred. These optical images showed that the strain reduction observed using XRD did not correlate to blistering, as no blisters were observed in any of the optical images.

Experimental results showed that at temperatures below 650 �C, no significant strain reduction occurs in hydrogen ion implanted (0001) sapphire. It has also been determined that for (0001) sapphire implanted at room temperature, it was not possible to produce surface blistering after a two-step annealing process at 650 �C and 800 �C, although significant strain reduction did occur, and ω scans showed peak broadening with subsequent annealing, indicating increasing mosaicity and potential defect nucleation. This was in contrast to previous findings that asserted that for sapphire annealed at 650 �C, surface blistering was observable.[7] As previous findings were based on sapphire implanted at elevated temperatures, this may imply that the sapphire substrate reaches a higher temperature than expected during such implantation processes, which may account for the capability for surface blistering at a lower temperature. Conversely, for room temperature ion implantation, temperatures greater than 800 �C may be necessary to first nucleate hydrogen platelet defects and then produce surface blistering.