Formation Mechanism of Metal-Oxides on Plasma-Exposed Wsi/Poly Si Gate Stacks
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ABSTRACT The formation mechanism of deformed tungsten oxides (WO,, x=2,3) on WSix/poly Si gate stacks has been investigated. It was revealed that plasma etching process introduced oxygen impurity into WSix surface and caused the WO, formation during the subsequent thermal oxidation. WSix films preferred corientation were found to be stable in the oxidation process after plasma etching process.
INTRODUCTION Tungsten disilicide (WSi 2)/poly Si has been widely used as a gate electrode in large scale integrated circuit device. It, therefore, is necessary to develop techniques for covering the gate stacks with an insulating layer of good quality. WSix (x_2) is generally thought to be stable in an oxidizing ambient resulting in the formation of a metal-free SiO2 overlayer [1]. However, deformed oxide layers including metal-oxides such as W0 2 and W0 3, have also been observed on WSix [2,3]. Since these metal-oxides are conductive and volatile, their formation creates a rough and porous oxide layer and leads to gate leakage through the Si0 2 covering insulating layer. Hence understanding the formation mechanism of WO, and founding a solution to solve this problem are very important. A great number of articles dealing with
the oxidation behavior of WSix films have focused on the kinetics of the thermal oxidation, diffusion nature, interface effects and industry process factors [2-6]. But as yet no feasible method for improving the oxidation stability of WSix films has been indicated. On the other hand, as plasma-assisted etching has become a key etching process in microfabrication technology, plasma-induced surface damage has attracted great attention and efforts are being made at microanalysis of the degraded surface [7,8]. It is required to develop techniques for improving the interaction between materials and plasma irradiation. Recently we found that plasma damage of WSix films induced the formation of WO, and resulted in the deformation of the SiO2 insulating layer grown on these WSix films. In this paper we investigate the formation mechanism of WO, on WSix films, and suggest that growing c-oriented WSix films is able to improve the stability of this material in oxidation.
EXPERIMENT WSix films were deposited on phosphorus doped polysilicon layers by the sputtering process. The thicknesses of WSix films and polysilicon layers were 120nm and I00nm, respectively. A SiO 2 (1)film was deposited on top of the WSix films. Silicon wafers covered with the thermally-grown SiO, were used as 189 Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
substrates. The process dependence on oxidation behaviors of WSix films was investigated as WSix deposition by co-sputteringschematically illustrated in Fig. 1. The SiO, (1) I [post annealing layer deposited on WSix surface was removed by I etching of CF,+CHF, plasma (sample 1)and by layer growth [ dipping film in buffered HF (sample 2). After removal of the SiO2(I) layer the surface of the Ss-io5, -drylyrem-Oved ]ISiO, wetly removed I I WSix films was cleaned us
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