Paper PS-MoA2
Analyses of Deposition/Etching Regimes during Selective Etching of HfO₂ on Silicon in BCl₃ Plasmas: Impact of Chamber Walls

Monday, October 15, 2007, 2:20 pm, Room 607

With the continuous scaling down of CMOS devices to ensure higher speed and density, the thickness of the SiO₂ gate dielectric is expected to be reduced down to 1nm for the 45 and 32nm technological nodes. This thickness reduction brings some serious issues such as increased gate leakage current and reduced oxide reliability. Therefore, high-k metal oxides, and more particularly HfO₂ have been considered as alternative materials to provide substantially thicker dielectric layers for reduced leakage current and increased gate capacitance. The present work focuses on the understanding of HfO₂, SiO₂ and Si etching mechanisms in BCl₃ based plasmas. BCl₃ seems to be a promising gas providing high etch selectivity between HfO₂ and Si substrates. The 200mm wafers are etched in an industrial ICP reactor, and then transferred under vacuum into an X-ray Photoelectron Spectroscopy (XPS) analysis chamber to investigate surface modifications induced by plasma exposure. XPS experiments help us in understanding the mechanism driving the etch selectivity between HfO₂ and Si-containing substrates. The role of Boron is fundamental since Boron by reacting with Silicon and forming Si-B bonds favour the growth of BCl_x polymer on Silicon surfaces slowing down Silicon etching. On the other hand, on HfO₂ surfaces Boron is directly involved in the etching by helping the formation of volatile BOCl etch products. The ionic bombardment plays also a key role since it controls the BCl deposition rate. The ion energy threshold which controls the transition between etching and deposition is lower on HfO₂ than on Si and SiO₂ wafers, implying that infinite etch selectivity between HfO₂ and Silicon can be obtained if the ion energy is well adjusted. In-situ kinetic ellipsometric measurements were also carried out on HfO₂, SiO₂ and Si substrates to monitor in real time the etching/deposition transition during BCl₃ plasma exposure. These experiments have revealed that the etch or deposition rate is linear with time only after a transient regime of about 10s and that during the10 first seconds, HfO₂, Silicon and SiO₂ show very different kinetic behaviors. We also observed that reactor wall conditioning plays a key role in controlling BCl_x deposition on the wafer and that infinite selectivity can be obtained by coating the reactor walls with carbon layer prior etching in BCl₃.