SiO₂ microstructure evolution during plasma deposition analyzed via ellipsometric porosimetry

Abstract

The evolution of SiO2 microstructures, deposited from hexamethyldisiloxane (HMDSO) and oxygen gas mixtures by two different low pressure plasma sources, namely an inductively coupled plasma (ICP process) at 3 Pa and a microwave plasma (MW process) at 100 Pa, is evaluated and compared. The microstructure is monitored using ellipsometric porosimetry (EP) applying three different solvent molecules (water, ethanol, and toluene) to probe the different adsorption and absorption mechanisms as well as the pore sizes. Both plasma processes are adjusted so that an equivalent oxygen atom contribution to the growth flux is established and that an equivalent specific energy per molecule is dissipated in the process. The major difference is the partial pressure of the HMDSO precursor molecules, which is 0.04 Pa in the ICP process and 1 Pa in the MW process. The porosimetry analysis indicates that the SiO2 films originating from the MW process are more porous than those from the ICP process. The pore sizes are typically in the range of 0.3 nm for films deposited from both plasma processes. This is explained by assuming that the gas phase polymerization in the MW process is much stronger due to the higher HMDSO partial pressure and, therefore, the SiO2 films are deposited from larger HMDSO fragments in the MW process compared with smaller HMDSO fragments in the ICP process. This difference in the main growth species becomes visible in the different microstructures. Consequently, a plasma process using smaller precursor partial pressures seems to be optimal.

Special issue: Plasmas for microfabrication

“…For example, two review papers by Laermer and Urban and Bittner et al deal with MEMS, two others, by Pranda et al and Marchack et al with lithography. The recently emerging fields of atomic layer deposition and etching are also well represented by Kim et al, Dallorto et al, and Leick et al, while packaging, interconnects and yield enhancement are addressed in a review by Miyajima et al, Park et al, Hirata et al, and Eriguchi et al Buschhaus and von Keudell describe the microstructure evolution of SiO 2 films deposited using different plasma sources; finally, Thomas et al present an update on the exciting topic of microscale plasma printing with nano inks, a subject first reviewed in this journal some years ago by the same group.…”