Low-k dielectrics materials are becoming increasingly attractive as device geometries shrink below the 65 nm technology node. Several thin films have been investigated as replacement of SiO2. Important properties of such new interlayer dielectrics are e.g. the thermal stability, as the processing conditions call for temperatures up to 450°C. Another important feature is the negative bias temperature instability (NBTI), which is mainly responsible for the conductivity breakdown due to the heat generated in semiconductor device. High concentration of hydrogen is normally present in CVD dielectric films, and hydrogen migration at elevated temperatures is thought to be one of the causes for a shift in device threshold voltage. Hence the possibility of hydrogen release during fabrication process is of interest for understanding device reliability, especially the device degradation caused by NBTI.
In the current study, various low-k dielectric films, grown by CVD on silicon substrates, were subjected to thermal annealing in N2 ambient atmosphere at temperatures that are generally used for device fabrication. Their stability was investigated after heat treatment up to 600°C. Rutherford backscattering spectrometry (RBS) and Elastic recoil detection analysis (ERDA) were used to monitor the compositional changes and hydrogen redistribution in the dielectric films.
The hydrogen retention ability varies between the different films. According to our results, hydrogen release from the surface region was evident in films of phosphorus doped silicon glass (PSG) and plasma-enhanced tetraethylorthosilicate (PETEOS), even after heat treatment at 300°C for 30 minutes. In thin films of organosilicate glass (OSG), silicon nitride (SiN), silicon oxynitride (SiON) and silicon carbide (SiC), the hydrogen content was stable at temperatures up to 500°C. However, as temperature rises above 500°C, significant hydrogen release was observed in SiC, OSG, PETEOS and PSG, whereas the hydrogen content still remained constant in SiN and SiON. There were only minor compositional changes of the heavier elements up to 400°C except in the case of OSG, where a major change in the stoichiometry was seen. |