Residual stress in thin silicon dioxide films has been studied as a function of storage time in air. Films of varying microstructre and impurity content were deposited by plasma-enhanced chemical vapor deposition. For all the deposited films, we measured the electrical properties of breakdown electrical field ($E_b$) and electrical current density ($J_L$) for the application of interlayer dielectric (ILD) in semiconductor devices. The evolution of residual stress was monitored for the films exhibited $E_b ＞ 5 MV/cm$ and $J_L ＜ 10^{-6} A/cm^2$. The films with high compressive stress deposited at low $SiH_4$ flow rate exhibited stable stress behavior with exposure time to air. However, the films deposited at high $SiH_4$ flow rate initially showed low compressive stress, the magnitude of compressive stress was found to increase rapidly with time for the first few hours after exposure to air. The stress instability in chemical vapor deposited silicon dioxide film is important issues in ILD application for semiconductor devices due to the reliability problems associated with stress-related failures. In this work, the stress instability of silicon dioxide was investigated in a viewpoint of the chemical bonding nature and pore in silicon dioxide, which were known to affect the mechanical stress of silicon dioxide films.
The change in the film stress with respect to the initial value was monitored as a function of storage time under different environment such as vacuum, oxygen or humidity conditions. The film stress did not change in vacuum and oxygen atmosphere. A considerable change in stress was observed in the film upon exposure to air, and the larger variation of residual stress is observed for higher relative humidity. This implies that the moisture in air is responsible for higher relative humidity. This implies that the moisture in air is responsible for the stress instability of PECVD $SiO_2$ films.
For the evolution of chemical bonds in $SiO_2$, the IR spectra of Si-O-Si, Si-H, and Si-OH absorption were monitored with increasing exposure time in air. The characteristic behavior of Si-O-Si stretching peak position (ν) shifted to higher wavenumber with increasing exposure time for all PECVD films exhibited stress instability in air. However, Si-OH absorption showed different evolution behavior with different initial bonding configuration of as-deposited $SiO_2$ films. The $SiO_2$ film with Si-H bond showed negligible evolution behavior in Si-OH absorption band. However, the $SiO_2$ film without Si-H bond exhibited considerable increase in Si-OH absorption band.
The stress change in $SiO_2$ film was monitored as a function of exposure time under different relative humidity conditions. We exposed the sample to different relative humidity conditions in air, and measured the stress in successive cycles of exposure between high (85% RH) and low humidity (23% RH) conditions. In successive humidity cycles, the film stress was oscillated with amplitude ($C_r$). However, the preserved stress ($C_p$) is observed to be constant irrespective of the humidity cycles. Thus, it can be noted that the stress instability in air is consisted of two components of $C_r$ and $C_p$. And, we can conclude that the $C_r$ is attributed to pores and $C_p$ is due to evolution of chemical bonds in $SiO_2$.
ECR oxygen plasma treatment was attempted to remove bonded hydrogen in $SiO_2$ films. It is noticeable that ECR oxygen plasma treatment could completely eliminate bonded hydrogen like Si-H from the deposited films and stabilize the Si-O-Si network structure, thereby, obtaining the stress stability in air with additional improvement in electrical properties. PECVD $SiO_2$ films with low stress and high stability in air were able to obtain by the ECR plasma treatment in the conditions of microwave power 600 W, pressure 2 mTorr, $N_2O$ flow rate 30 sccm, and treatment time 5 min.
