Evolution of the mechanical stress on PECVD silicon oxide films under thermal processing

Abstract

Plasma enhanced chemical vapor deposited (PECVD) silicon oxides obtained from SiH4 and N2O as reactant gases are being extensively studied nowadays. This interest is based on potential uses in microelectronics, integrated optics technology [1, 2] and as a basic material for fundamental studies about the luminescent properties of silicon-based nanostructures and silicon oxide defect centers [3]. The composition and physical properties of these oxides strongly depend on the deposition parameters, the precursor gas flow ratio, R= [N2O]/[SiH4], and the deposition temperature, td, being the most important of them [4]. During deposition, impurities such as hydrogen, nitrogen and water are incorporated into the a-SiOX network. Furthermore, moisture can be absorbed from the atmosphere after the deposition process, thus enhancing bond absorption and transmission losses. Thermal processing to achieve impurity effusion will often cause structural changes, significantly changing the film stress and affecting the film integrity. In a previous work [5] we studied the effect of rapid thermal annealing (RTA) at 900 ◦C on the stress values of PECVD silicon oxide films deposited at 300 ◦C as a function of flow ratio. The differences between the curves obtained before and after RTA were explained on the basis of the layer composition (oxygen content) and the hydrogen incorporation during the deposition process. No differences were established between the effect of different impurities as, for example, Si-H and Si-OH groups. It was also demonstrated by infrared spectroscopy (FTIR) that for the as-deposited films a sort of complementary effect exists, i.e., for low R-values the films are Si-rich with a high concentration of Si-H bonds and as R increases more stoichiometric films are obtained with lower bonded hydrogen content. To clarify the role of the different impurities in the stress behavior during thermal processing of the as-deposited oxides, thermal cycles up to 300 ◦C followed by RTA at 900 ◦C in inert atmospheres were accomplished and the preliminary results are presented here. Samples obtained with two extreme R-values (5.5 and 20.7) and different deposition temperatures, ranging from 200 to 350 ◦C, were used. A similar analysis