Elastic properties of tensile nitrogen-plasma-treated multilayer silicon nitride films

• A picosecond ultrasonic method is used to measure the Young's modulus in thin films.
• The Young's modulus is related to the silicon nitride film structure and chemistry.
• A simple model allows describing the Young's modulus increase during N2 plasma treatment.

Highly stressed silicon nitride thin films are used in gate first complementary metal oxide semiconductors to improve mobility in the silicon channel. Compressive stresses improve hole mobility in p-type MOS transistors while tensile stresses increase electron mobility in n-type MOS devices. High levels of compressive stress are easily reached in plasma enhanced chemical vapor deposited films by using the plasma power setting at a temperature compatible with the integration flow. Tensile stresses are more difficult to obtain with a plasma process because of the low temperatures required. Nevertheless, some post-treatments have been developed based on desorption of hydrogen that has been incorporated during the deposition step. The present study concerns one of those treatments consisting in a sequential deposition/nitrogen plasma treatment of elementary layers. Both nano-indentation and picosecond ultrasonic methods are used to measure the Young's modulus of the obtained silicon nitride thin films. The effect of the plasma treatment on the change in elastic modulus is investigated through the relationship with other properties like mass density and the concentration of SiN bonds.