AVS 59th Annual International Symposium and Exhibition
    Plasma Science and Technology Thursday Sessions
       Session PS1-ThM

Paper PS1-ThM2
Damage Free Cryogenic Etching of Porous Organosilica Ultralow-k Film

Thursday, November 1, 2012, 8:20 am, Room 24

Session: Plasma Processing for Disruptive Technologies (NVM, TSV, etc.)
Presenter: L. Zhang, IMEC, Belgium
Authors: L. Zhang, IMEC, Belgium
R. Ljazouli, GREMI CNRS/Université d'Orléans, France
T. Tillocher, GREMI CNRS/Université d'Orléans, France
P. Lefaucheux, GREMI CNRS/Université d'Orléans, France
R. Dussart, GREMI CNRS/Université d'Orléans, France
Y. Mankelevich, Moscow State University, Russia
J.-F. de Marneffe, IMEC, Belgium
S. de Gendt, IMEC, Belgium
M.R. Baklanov, IMEC, Belgium
Correspondent: Click to Email
Porous organosilicates (OSG) are popular candidates as low dielectric constant materials for interconnect application. However, the integration of this class of materials remains challenging, especially for k≤ 2.3 materials with a large open porosity. Due to carbon depletion and surface modification, porous low-k materials suffer from plasma induced damage (PID) that leads to k value degradation and high leakage current. In this paper, we report a cryogenic low-k etching approach that benefits from a sidewall surface protection effect. It is known that, at cryogenic temperature, an SiOxFy passivation layer forms more easily on the surface of the material being etched, providing efficient sidewall protection.In this work, cryogenic etching is applied to porous organosilicate ultralow-k materials with k=2.3 and k=2.0, which are prepared by Spin-coating and PECVD methods respectively. Influence of the chuck temperature and bias power with different plasma chemistries has been investigated in an ICP chamber. Plasma induced damage is evaluated by means of spectroscopic ellipsometry (SE), Fourier-transformed infra-red spectroscopy (FTIR) and time of flight secondary ion mass spectroscopy (TOF-SIMS). Blanket low-k films are exposed to pure SF6 plasma with different chuck temperatures, ranging from 20oC to -120° C. Almost no carbon depletion is observed when the chuck temperature is below a threshold point (-70oC and-120oC for k=2.3 and k=2.0 materials respectively). By addition of SiF4 and O2 to the gas discharge, etching rate is reduced as a result of enhanced SiOxFy polymerization. In order to evaluate the damage depth, an equivalent damage layer (EDL) is calculated based on the measurement of FTIR methyl group loss. The EDL decreases by increasing the proportion of SiF4/O2 in gas discharge, which confirms the protection effect of the SiOxFy passivation layer. In addition, the formation of alkyl alcohol polymer during cryogenic etching is observed from ex-situ FTIR, which is a major etch by-product at cryogenic temperature due to the lack of activation energy for complete oxidation of the methyl groups into CO2 and H2O. Our experiments also show that this polymer can be easily removed by high temperature anneal without additional damage to low-k film. This incomplete oxidation of Carbon-based etch reaction products reveals the mechanism for reduced Carbon depletion during cryogenic etching. A protection model is proposed, for porous SOG etching at cryogenic temperature, based on SiOxFy growth and incomplete oxidation. Although the etch by-products cannot plug the interconnected pores, they condense on their surface and retard the oxidation of methyl groups.