Stress of SiO2 Films Deposited by APCVD on 200 mm Heavily As-Doped Silicon Wafers

SiO2 film deposited by APCVD on the back-surface of the polished wafer was used to prevent self-doping，and also used as insulating material in some electronic device manufacturing. The structure of this film was amorphous crystal. The pore and surface structure in film would affect the stress of film. In this study，the stress of the silicon dioxide film deposited by APCVD under different process conditions，the change of film stress after heat treatment，and the change of film stress with different storage time were tested. In the experiment，scanning electron microscope（SEM）and transmission electron microscope（TEM）were used to observe the surface and internal structure of the film. The initial stress of SiO2 film grown on 200 mm silicon substrate by APCVD was compressive stress. This stress came from the structure of SiO2 film. The film structure was very loose due to the rapid deposition of APCVD and the lack of grain formation process. During the film deposition，some gas would be sealed in the pores of the film，and the pore shrinkage would be happened after the cooling of the film deposition，so that the film formed compressive stress on the surface of wafer. In this study，the wafer thickness was 745 μm，the initial bow value of SiO2 film with a thickness of 600 nm was 25 μm，and the stress was about 120 MPa. The film stress increased with the increase of the thickness. When the film thickness reached 3.8 μm，the film cracking was caused by excessive stress and substrate deformation，and the film stress decreased instantaneously. The stress performance of SiO2 films with the same thickness deposited on silicon substrates with different thickness was different. In this study，it was found that the thickness of deposited films was 600 nm and the thickness of substrates was ≤620 μm，the membrane stress calculated was about 278 MPa，while the thickness was ≥745 μm and the stress was about 141 MPa. The change was no longer obvious with the increase of thickness. The reason was that the service conditions of deformation method calculation were not met，that was，the deformation displacement was far less than the substrate thickness. The heat treatment process would affect the internal structure of SiO2 film，which was mainly reflected in the reduction of Si-Si bond，Si-H bond and Si=O bond of unstable bond in the film，as well as the separation of -OH on the film surface，resulting in the shrinkage and collapse of pores. When the heat treatment temperature was below 500 ℃，the stress of the film would not change significantly，because the energy provided by high temperature was not enough to open the unstable chemical bond，and the thermal stress was not obvious at this time. When the heat treatment temperature was greater than 500 ℃，with the progress of pore collapse and the -OH dissociation，the compressive stress of the film tended to increase. At this time，Thermal stress began to manifest. Because the thermal expansion coefficient of SiO2 was much smaller than that of silicon single crystal，the film began to reflect tensile stress. After heat treatment，the film itself would also change，including film thickness，refractive index，density and acid corrosion rate. The changes of film thickness，refractive index and density were related to pore collapse at high temperature，and there was a correlation between film thickness decrease，refractive index increase and density increase. The decrease of acid corrosion rate indicated not only the decrease of pores，but also the decrease of the number of -OH. The film would release during storage，which was related to the temperature，humidity and pressure of the storage environment. The stress generated by the deposition of 600 nm film by APCVD was obviously released within 48 h，and then tended to be stable. The reason for stress release was that -OH on the surface and in the pores would adsorb H2O molecules in the surrounding environment，which was equivalent to filling the pores in the film，resulting in the gradual reduction of film stress. After heat treatment，the stress release effect changed due to the collapse of pores and the reduction of -OH. For example，after heat treatment at 300~500 ℃，the residual stress after stress release was larger，while after heat treatment above 900 ℃，the stress release was no longer obvious. The stress of the films deposited under different processes was different. In the reaction temperature range from 400 to 460 ℃，the film stress decreased gradually with the increase of temperature，which was related to the decomposition of SiH4 at 420 ℃. The increase of Si-Si bond and the decrease of pores reduced the gas sealed in the film，and the film stress decreased accordingly. In addition，the removal of edge oxide film had little effect on the film stress，mainly because the bevel area of the substrate edge was very narrow，which was not enough to produce enough stress to change the trend of film stress on the substrate plane. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.