Ion Implantation and Synthesis of Materials - Studium

11.4 The Mechanisms Behind the Ion-Cut Process 149The ways hydrogen interacts with acceptors was further demonstrated byhydrogen in-diffusion experiments into c-Si with a buried layer of high boronconcentration. The experiments showed significant trapping of hydrogen withinthe boron-doped layer. The amount of H atoms trapped was found to far exceedthe amount of boron atoms present in the layer. Hydrogen in p-type material is in aH + charge state and will be attracted to the negatively charged acceptor, B − .Under these conditions, the hydrogen–boron capture radius is large (Coulombinteraction), and the boron trap site can pull multiple H + ions that can react toform molecular H, thereby enabling the trapping of 8–12 hydrogen atoms by oneboron atom in the silicon crystal (Borenstein et al. 1993).11.4 The Mechanisms Behind the Ion-Cut ProcessThe process of hydrogen-induced silicon surface layer exfoliation has beeninvestigated by a variety of spectroscopic methods to determine how hydrogeninteracts with silicon at the atomic level. The general belief has been that thesilicon cleavage takes place at the peak in the H implantation concentrationprofile. However, as was shown in Chap. 7, the implantation of ions alwaysproduces some level of lattice damage, which influences the interaction betweenthe H implant and the Si host atoms. The resulting distribution of ions and latticedamage that occurs from a 175 keV H implant of 5 × 10 16 H cm −2 in Si is shown inFig. 11.4.The nominal H ion implantation dose in the Ion-Cut application for theproduction of SOI is 5 × 10 16 cm −2 . At this implantation dose, cleavage of theH-implanted silicon wafer can be accomplished easily by annealing at around400°C for several minutes, depending on the conductivity type of the silicon andthe dopant concentration.11.4.1 The Ion-Cut DepthFigure 11.5 shows the ion implantation damage distribution in Si following the175 keV H implantation at a dose of 5 × 10 16 H cm −2 . The data is obtained fromion channeling experiments. The implantation damage peak is located at a depthof 1.41 µm.Figure 11.6 shows the hydrogen concentration in the sample as a function ofsample depth as determined by ERD (Tesmer and Nastasi 1995). The H concentrationpeaks at a depth of 1.51 µm, somewhat deeper than the implantationdamage peak. The ratio of the depth of the H concentration peak to the ionimplantation damage peak is 1.06, consistent with the SRIM Monte Carlosimulations presented in Fig. 11.4.After bonding and annealing to produce the complete delamination of thehydrogen-implanted wafer, the thickness of the transferred layer is measured by