Comparison of stress in single and multiple layer depositions of plasma-deposited amorphous silicon dioxide

The stress in a single-layer continuous deposition of amorphous silicon dioxide (SiO 2 ) film is compared with the stress within multiple-layer intermittent or 'stop–start' depositions. The films were deposited by helicon activated reactive evaporation (plasma assisted deposition with electron beam evaporation source) to a 1 µm total film thickness. The relationships for stress as a function of film thickness for single, two, four and eight layer depositions have been obtained by employing the substrate curvature technique on a post-deposition etch-back of the SiO 2 film. At film thicknesses of less than 300 nm, the stress–thickness relationships clearly show an increase in stress in the multiple-layer samples compared with the relationship for the single-layer film. By comparison, there is little variation in the film stress between the samples when it is measured at 1 µm film thickness. Localized variations in stress were not observed in the regions where the 'stop–start' depositions occurred. The experimental results are interpreted as a possible indication of the presence of unstable, strained Si–O–Si bonds in the amorphous SiO 2 film. It is proposed that the subsequent introduction of a 'stop–start' deposition process places additional strain on these bonds to affect the film structure. The experimental stress–thickness relationships were reproduced independently by assuming a linear relationship between the measured bow and film thickness. The constants of the linear model are interpreted as an indication of the density of the amorphous film structure.