Surface hydrogen incorporation and profile broadening caused by sheath expansion in hydrogen plasma immersion ion implantation

Hydrogen plasma immersion ion implantation (PIII) in conjunction with ion-cut is an efficient and economical technique to synthesize silicon-on-insulator (SOI) substrates. Unlike beam-line ion implantation, the PIII hydrogen profile usually exhibits multiple peaks because of different implanted species, such as H⁺, H₂ ⁺, and H₃ ⁺. In addition, a certain amount of adsorbed hydrogen exists near the surface and the hydrogen in-depth distribution is broader than that of a beam-line implant also as a result of a low-energy component. For the ion-cut process, the broadened hydrogen profile and surface hydrogen can decrease the efficiency of the blistering process, induce uneven exfoliation, and degrade the interfacial quality of the bonded wafer. Hydrogen can adsorb on the wafer surface during the `off-cycle' of the sample voltage pulse and consequently be driven in by ion mixing or diffusion. In order to reduce surface hydrogen incorporation, the implantation time must be short, and this requires an efficient cooling mechanism on the sample stage because a high ion current is needed to implant a high dose in a short time (less than 5 min). Another mechanism of profile broadening is that the expanding sheath creates low-energy ions during PIII. Our experimental and simulation data disclose that profile broadening is less severe for a shorter sample voltage pulsewidth and that good blistering characteristics can be achieved using a long pulse, in spite of a relatively long implantation time of 1 h.